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The star S2 moves according to Einstein’s Relativity

Do sub-relativistic meteors exist?

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BI-MONTHLY MAGAZINE OF SCIENTIFIC AND TECHNICAL INFORMATION The star S2 moves according to Einstein’s Relativity FREELY AVAILABLE THROUGH Observations made with ESO’s Very Large Telescope (VLT) have revealed for the first time that a star or- THE INTERNET biting the supermassive black hole at the centre of the Milky Way moves just as predicted by Einstein’s general theory of relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by... May-June 2020 4

Biofluorescent planets The closest potentially habitable rocky exoplanets all orbit around red dwarfs. These types of stars are particularly suitable for hosting Earth-sized planets, but they are also characterized by surface phenom- 10 ena very harmful to life as we know it. Some organisms, however, may be able to adapt to intense...

ESO telescope observes exoplanet where it rains iron Researchers using ESO’s Very Large Telescope (VLT) have observed an extreme planet where they suspect it rains iron. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees 22 Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where...

How newborn stars prepare for the birth of planets An international team of astronomers used two of the most powerful radio telescopes in the world to create more than three hundred images of planet-forming disks around very young stars in the Orion 24 Clouds. These images reveal new details about the birthplaces of planets and the earliest stages of star... English edition of the magazine lA’ STROFILO The best evidence for elusive mid-size black hole New data from the NASA/ESA Hubble Space Telescope have provided the strongest evidence yet for Editor in chief mid-sized black holes in the Universe. Hubble confirms that this “intermediate-mass” black hole dwells Michele Ferrara 28 inside a dense star cluster. Intermediate-mass black holes (IMBHs) are a long-sought “missing link” in... Scientific advisor Prof. Enrico Maria Corsini ALMA spots metamorphosing aged star Publisher Astro Publishing di Pirlo L. An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) Via Bonomelli, 106 captured the very moment when an old star first starts to alter its environment. The star has ejected 25049 Iseo - BS - ITALY high-speed bipolar gas jets which are now colliding with the surrounding material; the age of the... email [email protected] 32 Internet Service Provider Aruba S.p.A. Do sub-relativistic meteors exist? Via San Clemente, 53 24036 Ponte San Pietro - BG - ITALY Estimates produced from satellite data and extrapolations from meteor falls indicate that, every day, a few to a few hundred tons of dust and rock enter the Earth’s atmosphere from outer space. Nobody Copyright knows the precise amount, but it is assumed that all of that material formed in our own Solar System... All material in this magazine is, unless 34 otherwise stated, property of Astro Publishing di Pirlo L. or included with permission of its author. Reproduction or retransmission of the materials, in The strange orbits of ‘Tatooine’ planetary disks whole or in part, in any manner, with- Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found striking orbital out the prior written consent of the geometries in protoplanetary disks around binary stars. While disks orbiting the most compact binary copyright holder, is a violation of copy- star systems share very nearly the same plane, disks encircling wide binaries have orbital planes that are... right law. A single copy of the materi- 44 als available through this course may be made, solely for personal, noncom- mercial use. Users may not distribute such copies to others, whether or not The impact of satellite constellations on astronomical observations in electronic form, whether or not for Astronomers have recently raised concerns about the impact of satellite mega-constellations on scientific a charge or other consideration, with- research. To better understand the effect these constellations could have on astronomical observations, out prior written consent of the copy- ESO commissioned a scientific study of their impact, focusing on observations with ESO telescopes in... right holder of the materials. The 46 publisher makes available itself with having rights for possible not charac- terized iconographic sources. ALMA and Rosetta map the journey of phosphorus Advertising - Administration Phosphorus, present in our DNA and cell membranes, is an essential element for life as we know it. But Astro Publishing di Pirlo L. how it arrived on the early Earth is something of a mystery. Astronomers have now traced the journey Via Bonomelli, 106 of phosphorus from star-forming regions to comets using the combined powers of ALMA and the... 25049 Iseo - BS - ITALY 50 email [email protected] chronicles EN_l'Astrofilo 27/04/2020 16:39 Page 4

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The star S2 moves according to Einstein’s Relativity

by ESO

bservations made with ESO’s wards in the plane of motion. This Very Large Telescope (VLT) famous effect — first seen in the Ohave revealed for the first orbit of the planet Mercury around time that a star orbiting the super- the Sun — was the first evidence in massive black hole at the centre of favour of General Relativity. One the Milky Way moves just as pre- hundred years later we have now dicted by Einstein’s general theory detected the same effect in the mo- of relativity. Its orbit is shaped like tion of a star orbiting the compact a rosette and not like an ellipse as radio source Sagittarius A* at the predicted by Newton’s theory of centre of the Milky Way. This obser- gravity. This long-sought-after re- vational breakthrough strengthens sult was made possible by increas- the evidence that Sagittarius A* ingly precise measurements over must be a supermassive black hole nearly 30 years, which have enabled of 4 million times the mass of the scientists to unlock the mysteries of Sun,” says Reinhard Genzel, Direc- the behemoth lurking at the heart tor at the Max Planck Institute for of our galaxy. “Einstein’s General Relativity pre- his artist’s impression illustrates dicts that bound orbits of one object T the precession of the star’s orbit, with the effect exaggerated for eas- around another are not closed, as in ier visualisation. [ESO/L. Calçada] Newtonian Gravity, but precess for-

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his simulation shows the orbits T of stars very close to the super- massive black hole at the heart of the Milky Way. One of these stars, named S2, orbits every 16 years and is passing very close to the black hole in May 2018. This is a perfect laboratory to test gravitational physics and specifically Einstein’s general theory of relativity. [ESO/L. Calçada/spaceengine.org]

Extraterrestrial Physics (MPE) in Garching, Germany and the archi- tect of the 30-year-long pro- gramme that led to this result. Located 26 000 light-years from the Sun, Sagittarius A* and the dense cluster of stars around it provide a unique laboratory for testing physics in an otherwise unexplored and extreme regime of gravity. One of these stars, S2, sweeps in to- wards the supermassive black hole to a closest distance less than 20 bil- lion kilometres (one hundred and twenty times the distance between the Sun and Earth), making it one of the closest stars ever found in orbit around the massive giant. At its closest approach to the black hole, S2 is hurtling through space at almost three percent of the speed of light, completing an orbit once every 16 years. “After following the star in its orbit for over two and a half decades, our exquisite meas- urements robustly detect S2’s Schwarzschild precession in its path around Sagittarius A*,” says Stefan Gillessen of the MPE, who led the analysis of the measurements pub- lished today in the journal Astron- omy & Astrophysics. Most stars and planets have a non- circular orbit and therefore move closer to and further away from the object they are rotating around. S2’s orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that

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his visible light wide-field view T shows the rich star clouds in the constellation of Sagittarius (the Archer) in the direction of the centre of our Milky Way galaxy. The entire image is filled with vast numbers of stars — but far more remain hidden behind clouds of dust and are only revealed in in- frared images. This view was cre- ated from photographs in red and blue light and forming part of the Digitized Sky Survey 2. The field of view is approximately 3.5 degrees x 3.6 degrees. [ESO and Digitized Sky Survey 2. Acknowledgment: Davide De Martin and S. Guisard]

the next orbit is rotated with re- measured for a star around a super- invisible material, such as distrib- gard to the previous one, creating massive black hole. uted dark matter or possible smaller a rosette shape. General Relativity The study with ESO’s VLT also helps black holes, is present around Sagit- provides a precise prediction of scientists learn more about the tarius A*. This is of great interest how much its orbit changes and the vicinity of the supermassive black for understanding the formation latest measurements from this re- hole at the centre of our galaxy. and evolution of supermassive black search exactly match the theory. “Because the S2 measurements fol- holes,” say Guy Perrin and Karine This effect, known as Schwarzschild low General Relativity so well, we Perraut, the French lead scientists precession, had never before been can set stringent limits on how much of the project.

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This result is the culmination of 27 years of observations of the S2 star using, for the best part of this time, a fleet of instruments at ESO’s VLT, located in the Atacama Desert in Chile. The number of data points marking the star’s position and ve- locity attests to the thoroughness and accuracy of the new research: the team made over 330 measure- ments in total, using the GRAVITY, SINFONI and NACO instruments. Be- cause S2 takes years to orbit the su- permassive black hole, it was crucial to follow the star for close to three decades, to unravel the intricacies of its orbital movement. The research was conducted by an his schematic illustration shows S2’s orbit around Sagitarius A*, the super- international team led by Frank T massive black hole at the centre of the Milky Way. The precession movement Eisenhauer of the MPE with collab- is exaggerated for easier viewing. [ESO/L. Calçada] orators from France, Portugal, Ger- many and ESO. The team make up telescope (with a resolution equiv- being stretched to longer wave- the GRAVITY collaboration, named alent to that of a telescope 130 me- lengths as the star passed close to after the instrument they devel- tres in diameter). The same team Sagittarius A*. “Our previous result oped for the VLT Interferometer, reported in 2018 another effect has shown that the light emitted which combines the light of all four predicted by General Relativity: from the star experiences General 8-metre VLT telescopes into a super- they saw the light received from S2 Relativity. Now we have shown that the star itself senses the effects of General Relativity,” says Paulo Gar- cia, a researcher at Portugal’s Cen- tre for Astrophysics and Gravitation and one of the lead scientists of the GRAVITY project. With ESO’s upcoming Extremely Large Telescope, the team believes that they would be able to see much fainter stars orbiting even closer to the supermassive black hole. “If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole,” says An- dreas Eckart from Cologne Univer- sity, another of the lead scientists of the project. This would mean as- his zoom video sequence starts with a broad view of the Milky Way. We tronomers would be able to meas- T then dive into the dusty central region to take a much closer look. There ure the two quantities, spin and lurks a 4-million solar mass black hole, surrounded by a swarm of stars orbiting mass, that characterise Sagittarius rapidly. We first see the stars in motion, thanks to 26 years of data from ESO’s A* and define space and time around telescopes. We then see an even closer view of one of the stars, known as S2, it. “That would be again a com- passing very close to the black hole in May 2018. The final part shows a simula- tion of the motions of the stars. [ESO/GRAVITY Collaboration] pletely different level of testing rel- ativity,” says Eckart. !

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Biofluorescent planets

by Michele Ferrara revised by Damian G. Allis NASA Solar System Ambassador

The closest potentially habitable rocky exoplanets all orbit around red dwarfs. These types of stars are particularly suitable for hosting Earth- sized planets, but they are also characterized by surface phenomena very harmful to life as we know it. Some organisms, however, may be able to adapt to intense stellar radiation and also be detectable from the Earth thanks to the fascinating chemistry of biofluorescence.

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ccording to two Cornell A University researchers, fluorescent organisms similar to these could cover the shal- low ocean floors of planets orbiting the habitable zones of some red dwarfs.

hen the next generation of super- telescopes (the Extremely Large W Telescope and James Webb Space Telescope first among them) are completed and become operational, it will finally be possible to observe and study the first po- tentially habitable exoplanets directly. For this reason, some teams of astrobiologists are currently engaged in modeling the most

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favorable scenarios in order to define the ied first by the upcoming super-telescopes? raphical rep- most promising targets in the search for life Definitely those closest to our Solar System Gresentation beyond Earth. Knowing what to look for and − the easiest ones to separate from their of the structure where it is most likely to be found will allow stars. Better still are those nearby systems of the Extremely us to make the most of both the time and where the stars are both smaller than the Large Telescope. When this gigan- the telescopes that will be assigned to these Sun and not very bright, allowing the re- tic instrument observations, excluding a priori targets that flected light coming from their planets to enters service offer fewer chances of successful discovery. emerge more distinctly. If we consider the (perhaps in 2025), Today, only a handful of known exoplanets surroundings of the Sun, say up to a distance it will be the flag- are of astrobiological interest. The numbers of 5 parsecs (16.3 light-years), we find about ship telescope in do increase, and others will be added to this 50 star systems, composed of over 70 stars, the search for life list thanks to the TESS (Transiting Exoplanets of which most are red dwarfs (also called M on other planets. Survey Satellite) mission, which has so far stars). For reasons related to their formation [ESO/H. Zodet] identified nearly 1800 exoplanet candidates, and evolution, these small stars are very of which only about 40 have been officially likely to host rocky planets in their habitable confirmed. Dozens of exoplanets as big as zones. It is, therefore, not surprising that Earth are expected to be discovered during most of the potentially habitable planets this mission, with some of them in orbit in closest to us are in orbit around red dwarfs. the habitable zone of their stars. Other exo- In recent years, astronomers have repeat- planets will be found directly by ground- edly discussed the usefulness of looking for based telescopes and added to the growing indicators of possible life (biosignatures) in list. Which of these new worlds will be stud- the light of these planets.

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he James It is, in fact, known that T Webb Space red dwarfs show violent Telescope, slated surface activity for several to launch in billion years after their March 2021, will birth, an activity that man- be the most pow- ifests itself with frequent erful instrument available in the flares that pour high-en- search from space ergy UV radiation (in ad- of biosignatures dition to less abundant originating in X-rays) into their surround- other planetary ing space, well beyond systems. [NASA] their habitable zones. As red dwarfs age, explosive activity decreases signifi- cantly but remains, on av- erage, higher than that of the Sun. We know very lit- tle about the atmospheres of planets orbiting red dwarfs, but we can Astronomical Society), according to which infer that the intense and prolonged stellar planets might exist around red dwarfs with radiation can erode them so deeply as to lifeforms capable of defending themselves generate a surface environment totally un- from UV rays that reach the surface, and suitable for the appearance of life as we these defense mechanisms could possibly know it. We cannot, however, ever rule out be used by us to recognize their existence the existence of lifeforms able to adapt to from Earth. When, about 3.9 billion years those environments inhospitable to us. ago, the processes that led to the appear- Among those who support this hypothesis ance of life began on our planet, UV fluxes are Lisa Kaltenegger and Jack T. O’Malley- reaching the ground were more intense James (Carl Sagan Institute, Cornell Univer- than those which typically reach the habit- sity, Ithaca NY). For some years now, these able zones of red dwarfs. Nonetheless, life two researchers have developed a very tan- blossomed. If we add to this that, according talizing hypothesis (published in several ar- to recent studies, UV radiation would be ticles in the Monthly Notices of the Royal necessary for prebiotic chemistry, the work he Transiting T Exoplanets Survey Satellite, the successor to the Kepler Space Telescope, has the task of dis- covering new candidate Earth- sized planets or- biting the hab- itable zones of their stars. [NASA]

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of the two Cornell researchers becomes Let’s suppose that, in the habitable zones n this illustra- even more relevant. We already know that of some nearby red dwarfs, there are plan- I tion, flares from lifeforms would be able to survive and per- ets that have managed to maintain a suffi- a red dwarf acti- haps proliferate in a hostile environment, ciently thick atmosphere (observations go vate the biofluo- such as that on Mars (where UV rays con- in this direction) to prevent possible surface rescence of life- forms that cover tinuously hit the surface), simply by shelter- oceans from evaporating completely. In this the shallow ocean ing under rocks or within the ravines along case, there could be aquatic lifeforms that floors of a planet, the surface. That said, such lifeforms are defend themselves from UV rays just by re- elevating the not remotely detectable. maining immersed and reacting through planet-star con- What Kaltenegger and O’Malley-James pro- biofluorescence to that part of the incom- trast. [adapted pose is something different − something vis- ing radiation not shielded by the water. from NASA ible from Earth. Biofluorescence is a mechanism by which image]

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the red dwarfs inhabited, even if their at- mospheres have been depleted by stellar activity. Very extensive colonies of biofluo- rescent organisms could defend against UV fluxes produced by stellar flares through the production of visible radiation, thus causing a temporary increase in the bright- ness of the planetary surface.

ight, Lisa certain organisms prevent or mitigate the RKaltenegger harmful effects caused by the most ener- and Jack T. O'Mal- getic radiation that reaches the Earth’s sur- ley-James, the two face. To do this, these organisms use par- researchers from ticular proteins and pigments as anti-UV the Carl Sagan In- screens. The task of these biological screens stitute who, for some years, have is to absorb UV light and to almost instantly been considering re-emit radiation at longer wavelengths, the possibility of dissipating any residual energy in the form discovering fluo- of heat. In an aquatic environment with rescent extrater- shallow waters, biofluorescence might also restrial life. play a crucial role in keeping the planets of

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Since biofluorescence would solely be cor- and color models for Earth-like planets or- related with UV fluxes triggered by solar biting in these systems. By doing this, flares and independent of the visible light they took into account the different fluxes from the star, the planet-star contrast characteristics of the candidate could increase significantly in the visible planet’s surface, atmospheric spectrum compared to any non-biofluores- absorption, and cloud cover. cent state. On the emerged surface of the The most optimistic of the Earth, the biofluorescence of vegetation modeled scenarios sug- produces an effect that can be detected gests that, in the case from orbit but is too weak to be recognized of a transparent at- from another planetary system. More inter- mosphere, active esting is the biofluorescence produced in b i o f l u o r e s c e n c e shallow ocean floors by corals and organ- could produce a isms in symbiosis with them. The fluores- temporary in- cence efficiency of these aquatic biological crease in visible structures is much higher than that of sur- spectrum bright- face vegetation (some coral species re-emit ness of two orders more than 70% of the incident radiation) of magnitude over and high-resolution observations from orbit the planetary bright- can distinguish the signals they emit. How- ness during the ever, these are fractions of a percentage non-fluorescing point of the total signal emitted by all at- state. All this, mospheric and surface biosignatures. This however, is does not rule out that, on an Earth-like provided that planet orbiting the habitable zone of a par- the entire sur- ticularly active red dwarf, fluorescent life- face is covered forms can proliferate and spread to the by a biosphere point of producing a signal observable from characterized another planetary system. by highly-effi- Kaltenegger and O’Malley-James investi- cient fluorescent gated this possibility, modeling many sce- processes − an ide- narios based essentially on what we know alized scenario that about the fluorescence in coral structures is highly unlikely to and about the planets of some of the clos- be observed in reality, est red dwarfs: Proxima Centauri, Ross 128, at least among exoplan- TRAPPIST-1, and LHS 1140. The two re- ets at relatively short dis- searchers used the emission and absorption tances from our planetary properties of fluorescent proteins and pig- system. Countless variables can ments of common corals to create spectra negatively impact the chances for

pectral diagram Sshowing the different types of UV rays and the colors visible to us. Aquatic fluo- rescent life reacts to the former by emitting visible light almost in- stantly.

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considered in the modeling, are totally unknown to us. Fur- thermore, in order for bioflu- orescence to be perceived by other systems, very high surface coverage by extraterrestrial corals is required − close to 100%. To meet this condition, the distri- bution of shallow ocean floors must be equally high, a scenario to be identified and then verified. By comparison, fluo- rescent corals on Earth cover only 0.2% of the ocean floor, reaching down to about 60 meters deep. For a moment, let us try to be opti- mistic and accept that, around one of the closest red dwarfs, there is a planet with a global distribution of fluorescent corals immersed in a crys- talline ocean, above which there is a trans- parent atmosphere. For geometric reasons re- lated to planetary orbit, we will never have the opportunity to see much more than half a hemi- sphere. Since we observe these planets as they transit their star, we will not be able n example promising scenarios. We do not know, for to see the surplus of light due to the bios- A of coral flu- example, whether biofluorescence can de- phere either during the transit or half an orescence, by velop in biospheres other than our own on orbit later, when the planet is either com- which proteins Earth, and we can only speculate on the ex- pletely hidden or too close to the star to be absorb near-UV istence of extraterrestrial coral structures. observed separately. We can, in fact, hope and blue light and re-emit light Even assuming their existence, the fluores- to capture the light of the corals only near at longer wave- cence efficiency of proteins and pigments the maximum elongations of the planet – lengths. [Mon- of terrestrial corals depends on the environ- when only half of the lit hemisphere is fac- terey Bay Aquar- ments in which they live, environments ing us. According to the calculations of ium, California] that, for planets around the red dwarfs Kaltenegger and O’Malley-James, once bio-

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fluorescence is triggered by the UV radia- tion, the entire process might last a few hours, but the maximum peak (the reaction associated with the peak of the flare) would last only about a quarter of an hour, too short a time to hope to record an event that started with the planet near the con- junctions with the star. Now that it is clear that we will be able to observe only half of the planetary hemi- sphere or a little more, it is necessary that the red dwarf be particularly active in order to have a greater chance of observing bio- fluorescence. Among those examined by the Cornell researchers, only Proxima Cen- tauri and TRAPPIST-1 are considered active enough to guarantee at least some daily flares. Considering that only a small frac- it is also not foreseeable when this might oss 128 above) tion of these flares will pour any UV flux on happen, it will not be easy for researchers R and Proxima the part of the planetary surface visible to obtain with ELT and Webb a sufficient Centauri (with its from time to time from the Earth and that amount of time-telescope to observe the orbit around hypothetical biological Alpha Centauri, on the left) are signal. two red dwarfs Even while we want to be that host poten- overly optimistic about tially habitable the possibility of observ- planets. They ing biofluorescence from could be interest- exoplanets, there is an ing targets to test evolutionary characteris- the hypothesis of tic of red dwarfs that Kaltenegger and poses a perhaps insur- O’Malley-James. mountable challenge. [Sloan Digital Sky When these little stars go Survey, P. Kervella through their slow pre- (CNRS/U. of Chile/ Observatoire de main sequence phase, Paris/LESIA), ESO/ which can last up-to DSS2, D. De Mar- about 2 billion years, they tin/M. Zamani] are much brighter and warmer than they will be after reaching the main sequence, and this is be- cause they are still con- tracting under their own gravity (a process that heats the core). There- fore, the planets we see today orbiting in the hab- itable zones of Proxima Centauri (almost 5 billion years), Ross 128 (about 10 billion years), TRAPPIST-1 (about 8 billion years),

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he other two T red dwarfs taken into consid- eration in the Kaltenegger and O’Malley-James models: LHS 1140 (in the center of the image on the right) and TRAP- PIST-1 (below). [ESO, Sloan Digi- tal Sky Survey]

LHS 1140 (over 5 billion years), and many likely, but it is true that exploiting biofluo- other red dwarfs have all been subjected to rescence may be the only way to discover extreme conditions in their first billion life around the red dwarfs, which account years, probably sufficient to make the plan- for about 75 percent of known stars. We ets sterile forever. end with the conclusions of Kaltenegger Let’s add that, inevitably, the innermost and O’Malley-James, as reported in their ar- planets of the aged red dwarfs have rota- ticle Lessons from early Earth: UV surface tion and orbital periods tidally locked and, radiation should not limit the habitability therefore, are always presenting the same of active M star systems: “While a multitude hemisphere to their star, with easily imagi- of factors ultimately determine an individ- nable consequences. That situation makes ual planet’s habitability, our results demon- the prospects for finding any lifeforms un- strate that high UV radiation levels may not be a limiting factor. The compositions of the atmospheres of our nearest habitable exoplanets are currently unknown; how- ever, if the atmospheres of these worlds re- semble the composition of Earth’s atmos- phere through geological time, UV surface radiation would not be a limiting factor to the ability of these planets to host life. Even for planets with eroded or anoxic atmos- pheres orbiting active, flaring M stars the surface UV radiation in our models remains below that of the early Earth for all cases modelled. Therefore, rather than ruling these worlds out in our search for life, they provide an intriguing environment for the search for life and even for searching for al- ternative biosignatures that could exist under high-UV surface conditions.” !

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images by Massimo Vesnaver chronicles EN_l'Astrofilo 27/04/2020 16:38 Page 22

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ESO telescope observes exoplanet where it rains iron

by ESO

esearchers using ESO’s Very in the evening, except it rains iron,” its parent star, its cooler night side Large Telescope (VLT) have says David Ehrenreich, a professor remaining in perpetual darkness. R observed an extreme planet at the University of Geneva in Like the Moon on its orbit around where they suspect it rains iron. Switzerland. He led a study, pub- the Earth, WASP-76b is ‘tidally The ultra-hot giant exoplanet has a lished today in the journal Nature, locked’: it takes as long to rotate day side where temperatures climb of this exotic exoplanet. Known as around its axis as it does to go above 2400 degrees Celsius, high WASP-76b, it is located some 640 around the star. enough to vaporise metals. light-years away in the constella- On its day side, it receives thousands Strong winds carry iron vapour to tion of Pisces. of times more radiation from its par- the cooler night side where it con- This strange phenomenon happens ent star than the Earth does from denses into iron droplets. “One because the ‘iron rain’ planet only the Sun. It’s so hot that molecules could say that this planet gets rainy ever shows one face, its day side, to separate into atoms, and metals like iron evaporate into the atmosphere. his illustration shows a night-side view of the exoplanet WASP-76b. The The extreme temperature difference T ultra-hot giant exoplanet has a day side where temperatures climb above between the day and night sides re- 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry sults in vigorous winds that bring iron vapour to the cooler night side where it condenses into iron droplets. To the iron vapour from the ultra-hot the left of the image, we see the evening border of the exoplanet, where it transitions from day to night. [ESO/M. Kornmesser] day side to the cooler night side, where temperatures decrease to

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he ultra-hot giant exoplanet WASP-76b orbits the star WASP-76, T located some 390 light-years away in the constellation of Pisces. This video shows the orbit of this strange planet around its host star. [ESO/L.Calçada/spaceengine.org]

around 1500 degrees Celsius. Not “The observations show that iron Portugal, Italy, Switzerland, Spain only does WASP-76b have different vapour is abundant in the atmos- and ESO. day-night temperatures, it also has phere of the hot day side of WASP- ESPRESSO — the Echelle SPectro- distinct day-night chemistry, accord- 76b,” adds María Rosa Zapatero graph for Rocky Exoplanets and Sta- ing to the new study. Using the new Osorio, an astrophysicist at the ble Spectroscopic Observations — ESPRESSO instrument on ESO’s VLT Centre for Astrobiology in Madrid, was originally designed to hunt for in the Chilean Atacama Desert, the Spain, and the chair of the Earth-like planets around Sun-like astronomers identified for the first ESPRESSO science team. “A fraction stars. However, it has proven to be time chemical variations on an ultra- of this iron is injected into the much more versatile. “We soon re- hot gas giant planet. night side owing to the planet’s ro- alised that the remarkable collecting They detected a strong signature of tation and atmospheric winds. power of the VLT and the extreme iron vapour at the evening border There, the iron encounters much stability of ESPRESSO made it a that separates the planet’s day side cooler environments, condenses prime machine to study exoplanet from its night side. “Surprisingly, and rains down.” atmospheres,” says Pedro Figueira, however, we do not see the iron This result was obtained from the ESPRESSO instrument scientist at vapour in the morning,” says Ehren- very first science observations done ESO in Chile. “What we have now is reich. The reason, he says, is that “it with ESPRESSO, in September 2018, a whole new way to trace the cli- is raining iron on the night side of by the scientific consortium who mate of the most extreme exoplan- this extreme exoplanet.” built the instrument: a team from ets,” concludes Ehrenreich. !

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How newborn stars prepare for the birth of planets

by ALMA Observatory

n international team of as- LMA and the VLA observed more than 300 protostars and their young proto- tronomers used two of the Aplanetary disks in Orion. This image shows a subset of stars, including a few bi- A most powerful radio tele- naries. The ALMA and VLA data compliment each other: ALMA sees the outer disk scopes in the world to create more structure (visualized in blue), and the VLA observes the inner disks and star cores (or- than three hundred images of ange). [ALMA (ESO/NAOJ/NRAO), J. Tobin; NRAO/AUI/NSF, S. Dagnello; Herschel/ESA]

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planet-forming disks around very young stars in the Orion Clouds. These im- ages reveal new de- tails about the birthplaces of plan- ets and the earliest stages of star forma- tion. Most of the stars in the Universe are accompanied by planets. These plan- ets are born in rings of dust and gas, called protoplane- tary disks. Even very young stars are sur- rounded by these disks. Astronomers want to know ex- actly when these disks start to form, and what they look like. But young stars are very faint, and there are dense clouds of dust and gas surrounding them in stellar nurs- eries. Only highly sensitive radio tele- scope arrays can spot the tiny disks his image shows the Orion Molecular Clouds, the target of the VANDAM survey. Yellow dots are the locations of the around these infant T observed protostars on a blue background image made by Herschel. Side panels show nine young protostars imaged stars amidst the by ALMA (blue) and the VLA (orange). [ALMA (ESO/NAOJ/NRAO), J. Tobin; NRAO/AUI/NSF, S. Dagnello; Herschel/ESA] densely packed ma- terial in these clouds. For this new tion of a star is when these dense and observed hundreds of proto- research, astronomers pointed both clouds collapse due to gravity. As stars and their disks in various stages the Atacama Large Millimeter/sub- the cloud collapses, it begins to spin of their formation. millimeter Array (ALMA) and the – forming a flattened disk around “This survey revealed the average National Science Foundation’s Karl the protostar. Material from the mass and size of these very young G. Jansky Very Large Array (VLA) to disk continues to feed the star and protoplanetary disks,” said John a region in space where many stars make it grow. Eventually, the left- Tobin of the National Radio Astron- are born: the Orion Molecular over material in the disk is expected omy Observatory (NRAO) in Char- Clouds. This survey, called VLA/ to form planets. lottesville, Virginia, and leader of ALMA Nascent Disk and Multiplicity Many aspects about these first the survey team. “We can now com- (VANDAM), is the largest survey of stages of star formation, and how pare them to older disks that have young stars and their disks to date. the disk forms, are still unclear. But been studied intensively with ALMA Very young stars, also called proto- this new survey provides some miss- as well.” stars, form in clouds of gas and dust ing clues as the VLA and ALMA What Tobin and his team found, is in space. The first step in the forma- peered through the dense clouds that very young disks can be similar

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his schematic shows a proposed pathway (top row) for the formation of protostars, based on four very young protostars T (bottom row) observed by VLA (orange) and ALMA (blue). Step 1 represents the collapsing fragment of gas and dust. In step 2, an opaque region starts to form in the cloud. In step 3, a hydrostatic core starts to form due to an increase in pressure and temperature, surrounded by a disk-like structure and the beginning of an outflow. Step 4 depicts the formation of a class 0 protostar inside the opaque region, that may have a rotationally supported disk and more well-defined outflows. Step 5 is a typical class 0 protostar with outflows that have broken through the envelope (making it optically visible), an ac- tively accreting, rotationally supported disk. In the bottom row, white contours are the protostar outflows as seen with ALMA. [ALMA (ESO/NAOJ/NRAO), N. Karnath; NRAO/AUI/NSF, B. Saxton and S. Dagnello]

in size, but are on average much stars to propose a schematic path- a lot of mass, but just started its more massive than older disks. way for the earliest stages of star outflow to lose angular momentum “When a star grows, it eats away formation. “We are not entirely sure to be able to keep growing,” ex- more and more material from the how old they are, but they are prob- plained Karnath. “This is one of the disk. This means that younger disks ably younger than ten thousand smallest outflows that we have seen have a lot more raw material from years.” and it supports our theory of what which planets could form. Possibly To be defined as a typical (class 0) the first step in forming a protostar bigger planets already start to form protostar, stars should not only have looks like.” around very young stars.” a flattened rotating disk surround- The exquisite resolution and sensi- Among hundreds of survey images, ing them, but also an outflow – tivity provided by both ALMA and four protostars looked different spewing away material in opposite the VLA were crucial to understand than the rest and caught the scien- directions – that clears the dense both the outer and inner regions of tists’ attention. “These newborn cloud surrounding the stars and protostars and their disks in this sur- stars looked very irregular and makes them optically visible. This vey. While ALMA can examine the blobby,” said team member Nicole outflow is important, because it pre- dense dusty material around proto- Karnath of the University of Toledo, vents stars from spinning out of con- stars in great detail, the images from Ohio (now at SOFIA Science Center). trol while they grow. But when the VLA made at longer wave- “We think that they are in one of exactly these outflows start to hap- lengths were essential to under- the earliest stages of star forma- pen, is an open question in astron- stand the inner structures of the tion and some may not even have omy. youngest protostars at scales smaller formed into protostars yet.” One of the infant stars in this study, than our solar system. “The com- It is special that the scientists found called HOPS 404, has an outflow of bined use of ALMA and the VLA has four of these objects. “We rarely only two kilometers (1.2 miles) per given us the best of both worlds,” find more than one such irregular second (a typical protostar-outflow said Tobin. “Thanks to these tele- object in one observation,” added of 10-100 km/s or 6-62 miles/s). “It is scopes, we start to understand how Karnath, who used these four infant a big puffy sun that is still gathering planet formation begins.” !

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Science in School aims to promote inspir- ing science teaching by encouraging communica- tion between teachers, scientists, and everyone else involved in European sci- ence education. Science in School is published by EIROforum, a collaboration between eight European intergovernmen- tal scientific research organisations, of which ESO is a member. The journal addresses science teaching both across Europe and across disciplines: highlighting the best in teaching and cutting-edge research. Read more about Science in School at: http://www.scienceinschool.org/ chronicles EN_l'Astrofilo 27/04/2020 16:38 Page 28

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The best evidence for elusive mid-size black hole

by NASA/ESA

ew data from the NNASA/ESA Hubble Space Tele- scope have provided the strongest evidence yet for mid-sized black holes in the Universe. Hub- ble confirms that this “inter- mediate-mass” black hole dwells

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inside a dense star cluster. Interme- stars. This new black hole is over his artist’s impression depicts a diate-mass black holes (IMBHs) are 50,000 times the mass of our Sun. star being torn apart by an in- T a long-sought “missing link” in IMBHs are hard to find. “Intermedi- termediate-mass black hole (IMBH), black hole evolution. There have ate-mass black holes are very elu- surrounded by an accretion disc. been a few other IMBH candidates sive objects, and so it is critical to This thin, rotating disc of material found to date. They are smaller than consists of the leftovers of a star carefully consider and rule out alter- which was ripped apart by the tidal the supermassive black holes that native explanations for each candi- forces of the black hole. [ESA/Hub- lie at the cores of large galaxies, but date. That is what Hubble has ble, M. Kornmesser] larger than stellar-mass black holes allowed us to do for our candidate,” formed by the collapse of massive said Dacheng Lin of the University

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of New Hampshire, ide-field principal investigator W image of the study. around the Lin and his team field of used Hubble to fol- J2150−0551 low up on leads from (ground-based NASA’s Chandra X- view), in ray Observatory and which an in- the European Space termediate- Agency’s X-ray Multi- mass black Mirror Mission hole named ( X M M - N e w t o n ) , 3XMM which carries three J215022.4−055 high-throughput X- 108 has been detected. ray telescopes and an [NASA, ESA, optical monitor to Digitized Sky make long uninter- Survey 2. Ac- rupted exposures knowledge- providing highly sen- ment: Davide sitive observations. De Martin] “Adding further X- ray observations allowed us to un- Surprisingly, the X-ray source, Deep, high-resolution imaging con- derstand the total energy output,” named 3XMM J215022.4−055108, firmed that the X-rays emanated not said team member Natalie Webb of was not located in the centre of a from an isolated source in our the Université de Toulouse in France. galaxy, where massive black holes galaxy, but instead in a distant, “This helps us to understand the normally reside. This raised hopes dense star cluster on the outskirts of type of star that was disrupted by that an IMBH was the culprit, but another galaxy — just the sort of the black hole.” first another possible source of the place astronomers expected to find In 2006 these high-energy satellites X-ray flare had to be ruled out: a evidence for an IMBH. Previous Hub- detected a powerful flare of X-rays, neutron star in our own Milky Way ble research has shown that the but it was not clear if they origi- galaxy, cooling off after being more massive the galaxy, the more nated from inside or outside of our heated to a very high temperature. massive its black hole. Therefore, galaxy. Researchers attributed it to a Neutron stars are the extremely this new result suggests that the star star being torn apart after coming dense remnants of an exploded star. cluster that is home to 3XMM too close to a gravitationally power- Hubble was pointed at the X-ray J215022.4−055108 may be the ful compact object, like a black hole. source to resolve its precise location. stripped-down core of a lower-mass dwarf galaxy that has his is an animation been gravitationally T of a rare and exotic and tidally disrupted intermediate-mass black by its close interactions hole at the centre of a with its current larger star cluster, similar to galaxy host. the one thought to be IMBHs have been par- at the centre of globular ticularly difficult to cluster Messier 15. find because they are Studying these unusual smaller and less active black holes could tell us about how such objects than supermassive grow and evolve within black holes; they do both star clusters and not have readily avail- galaxies. [NASA, ESA, able sources of fuel, and M. Kornmesser] nor do they have a gravitational pull that

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sources to find strong evidence for this one IMBH candidate. Once found, the X- ray glow from the shredded star al- lowed astronomers to estimate the black hole’s mass. Confirming one IMBH opens the door to the possibility that many more lurk undetected in the dark, waiting to be given away by a star passing too close. Lin plans to continue this meticulous detec- tive work, using the methods his team has proved successful. “Studying the origin and evolution of the intermediate mass black holes will finally give an answer as to how the supermas- sive black holes that we find in the centres of massive galaxies came to exist,” added Webb. Black holes are one of the most extreme en- vironments humans are aware of, and so his Hubble Space Telescope image identified the location of an intermediate-mass black hole they are a testing T (IMBH), weighing over 50,000 times the mass of our Sun (making it much smaller than the su- ground for the laws permassive black holes found in the centres of galaxies). The location of the black hole, named of physics and our un- 3XMM J215022.4− 055108, is indicated by the white circle. This elusive type of black hole was first identified via a telltale burst of X-rays emitted by hot gas from a star as it was captured and de- derstanding of how stroyed by the black hole. Hubble was needed to pinpoint the black hole’s location in visible light. the Universe works. Hubble’s deep, high-resolution imaging shows that the black hole resides inside a dense cluster of Does a supermassive stars that is far beyond our Milky Way galaxy. The star cluster is in the vicinity of the galaxy at the black hole grow from centre of the image. Much smaller images of distant background galaxies appear sprinkled around an IMBH? How do the image, including a face-on spiral just above the central foreground galaxy. This photo was taken IMBHs themselves with Hubble’s Advanced Camera for Surveys. [NASA, ESA, and D. Lin (University of New Hampshire)] form? Are dense star clusters their is strong enough for them to be con- red-handed in the relatively rare act favoured home? With a confident stantly drawing in stars and other of gobbling up a star. Lin and his conclusion to one mystery, Lin and cosmic material and producing the colleagues combed through the other black hole astronomers find tell-tale X-ray glow. Astronomers XMM-Newton data archive, search- they have many more exciting ques- therefore have to catch an IMBH ing hundreds of thousands of tions to pursue. !

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ALMA spots metamorpho aged star

by ALMA Observatory

n international team of as- lae are formed. Sun-like stars evolve vestigate the inner-workings of the tronomers using the Atacama to puffed-up Red Giants in the final process. To tackle this problem, a A Large Millimeter/submillime- stage of their lives. Then, the star team of astronomers led by Daniel ter Array (ALMA) captured the very expels gas to form a remnant called Tafoya in Chalmers University of moment when an old star first starts a planetary nebula. There is a wide Technology, Sweden, pointed ALMA to alter its environment. The star variety in the shapes of planetary at W43A, an old star system in the has ejected high-speed bipolar gas nebulae; some are spherical, but constellation Aquila, the Eagle. jets which are now colliding with others are bipolar or show compli- Thanks to ALMA’s high resolution, the surrounding material; the age of cated structures. Astronomers are the team obtained a very detailed the observed jet is estimated to be interested in the origins of this vari- view of the space around W43A. less than 60 years. These are key ety, but the thick dust and gas ex- “The most notable structures are its features to understand how the pelled by an old star obscure the small bipolar jets,” says Tafoya, the complex shapes of planetary nebu- system and make it difficult to in- lead author of the research paper

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LMA image of the old star system W43A. The high velocity bipolar jets ejected Afrom the central aged star are seen in blue, low velocity outflow is shown in green, and dusty clouds entrained by the jets are shown in orange. [ALMA (ESO/NAOJ/NRAO), Tafoya et al.]

published by The Astrophysical Jour- panion pours onto the core of the nal Letters. The team found that the dying star, and a portion of this velocity of the jets is as high as 175 new gas forms the jets. Therefore, km per second, which is much higher whether or not the old star has a than previous estimations. Based on companion is an important factor to this speed and the size of the jets, determine the structure of the re- the team calculated the age of the sulting planetary nebula. jets to be less than a human life- “W43A is one of the peculiar so span. “Considering the youth of the called ‘water fountain’ objects,” says jets compared to the overall lifetime Hiroshi Imai at Kagoshima University, of a star, it is safe to say we are Japan, a member of the team. witnessing the ‘exact moment’ that “Some old stars show characteristic the jets have just started to shove radio emissions from water mole- through the surrounding gas,” ex- cules. We suppose that spots of these plains Tafoya. “When the jets carve water emissions indicate the inter- through the surrounding material in face region between the jets and the some 60 years, a single person can surrounding material. We named watch the progress in their life.” them ‘water fountains,’ and it could rtist’s impression of AW43A based on the In fact, the ALMA image clearly maps be a sign that the central source is a ALMA observation results. the distribution of dusty clouds en- binarity system launching a new jet.” Diffuse spherical gas was trained by the jets, which is telltale “There are only 15 ‘water fountain’ emitted from the star in the evidence that it is impacting on the objects identified to date, despite past. W43A has just started surroundings. the fact that more than 100 billion ejecting bipolar jets which The team assumes that this entrain- stars are included in our Milky Way entrain the surrounding ma- ment is the key to form a bipolar- Galaxy,” explains José Francisco terial. Bright spots in radio shaped planetary nebula. In their Gómez at Instituto de Astrofísica de emissions from water mole- scenario, the aged star originally Andalucía, Spain. “This is probably cules are distributed around ejects gas spherically and the core of because the lifetime of the jets is the interface of the jets and the diffuse gas. [NAOJ] the star loses its envelope. If the star quite short, so we are very lucky to has a companion, gas from the com- see such rare objects.” !

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Do sub-relativist meteors exist?

by Michele Ferrara revised by Damian G. Allis NASA Solar System Ambassador

Estimates produced from satellite data and extrapolations from meteor falls indicate that, every day, a few to a few hundred tons of dust and rock enter the Earth's atmosphere from outer space. Nobody knows the precise amount, but it is assumed that all of that material formed in our own Solar System and therefore moves at a relatively slow speed. But is it really so?

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eteor showers are certainly one hile waiting for someone to photo- of the best-known phenomena W graph a sub-relativistic meteor, we in the night sky. It is difficult to present a running series of the most spec- M tacular images of ordinary meteors, cap- imagine that there are people who, in tured in the night skies of some famous their lifetime, have not seen at least one astronomical observatories. Let’s start with “falling star” (but, sadly, those people this one, taken in 2015 at ESO La Silla, which probably do exist). The tradition of shows a multitude of different objects, in- “making a wish” when a meteor is seen cluding Comet Lovejoy. [P. Horálek/ESO]

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falling is quite widespread, and it is by now a custom for ama- teur associations to organize public events on the occasion of the peaks of the most fa- mous meteor showers. Although they are familiar to almost everyone, and although astronomers and meteorolo- gists know nearly everything bright meteor A rushes over the Atacama Large Millimeter/sub- millimeter Array. [ESO/C. Malin, 2014] On the left, two Perseids in the vibrant sky of the Himalayan Chandra Telescope at 4500 meters elevation. The light from Mars is reflected in the dome. [Chirag Upreti, 2018] Below, another Perseid, captured this time in the night sky above the four Very Large Telescope domes. [ESO/ S. Guisard, 2010]

about this phenomenon, there may be an exotic type of me- teor that so far no one has ever seen: the sub-relativistic me- teor. As you can guess from the name, the original meteoric material would travel through interstellar space at very high speed, measurable in non-neg- ligible fractions of the speed of light (the conditional here is a must). Occasionally, very small agglomerations of this material would strike the atmosphere of our planet, producing effects different from those typically generated by ordinary meteors. subrelativistic meteors EN_l'Astrofilo 27/04/2020 16:35 Page 37

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n the right, This bizarre possibility O many Geminids was recently discussed in photographed a paper published in The above the Teide Ob- Astrophysical Journal by servatory, Tenerife, Amir Siraj and Abraham Canary Islands. Loeb (Department of As- [Flickr/StarryEarth, tronomy, Harvard Uni- 2013] Below, a versity, Cambridge, MA). shower of Gemi- The two researchers de- nids, the sum of five hours of many ex- veloped a hydrodynamic posures. In the fore- and radiative model to ground, the two determine whether it is principal domes of possible to detect sub- the Las Campanas relativistic meteors gen- Observatory. [Yuri erated by solid particles Beletsky, Carnegie, 1 mm in size or larger. TWAN, 2015] The assumptions in Siraj and Loeb’s study are reasonable based on the results of previous work. Back in the middle of the last century, Lyman Spitzer theoretically demonstrated that the radia- tion pressure of a supernova could acceler- ate dust grains to relativistic speeds. In the early 1970s, Satio Hayakawa had suggested that particles of such dust could be respon- sible for ultrahigh-energy cosmic rays. In the decades that have followed, other re- searchers further developed these topics, studying the processes that influence the origin and survival of relativistic dust, all the way to showing that, in our galaxy, there may exist dust grains that move at nearly the speed of light. We also have the cer- tainty that, in the past, the Earth has been targeted by matter fired by the explosion of relatively close supernovae. Evidence of this is found in the enrichment of iron-60, dis- covered at the end of the last century in rocks within the depths of the Pacific Ocean. This radioactive iron isotope is formed in na- ture by the explosion of supernovae, has a half-life of 2.6 million years, and decays into cobalt-60 (which in turn decays much more rapidly into nickel-60). The abundance of iron-60 found in the first 2 cm of the ocean floor, corresponding to the last 13.4 million years, suggests that a supernova could have exploded less than 100 light-years away from Earth in the last 5 million years. It is estimated that, in the geological period cov- ered by that thin layer of crust, twenty su- pernovae may have exploded within 1000

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light-years of Earth. Although no other of low-density ejecta have been observed in event has left clear traces, this frequency, the remnants of several supernovae, which extrapolated to the entire Milky Way, makes could at least in part be made up of the tiny acceptable the hypothesis that interstellar solid bodies considered in the study. space is crossed by a non-negligible quan- The two researchers estimate that if just tity of very small agglomerations of matter 0.01% of the dust ejected from a supernova endowed with sub-relativistic speeds. The were contained in particles measurable in value of the speed depends, of course, on millimeters, we could expect to observe, on the mass of matter accelerated by a given average, one sub-relativistic meteor per supernova. Currently, we do not know if, in month. This estimate takes into account the the ejecta rain that supernovae pour into in- average supernovae rate estimated for the terstellar space, there are also solid particles Milky Way (~1 every 50 years), the average more massive than the dust grains. In their mass of dust released by a supernova (~0.1 study, Siraj and Loeb are optimistic in this re- solar masses), and our distance from the gard, relying on the fact that concentrations galactic center (~26,000 light-years).

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Siraj and Loeb have applied their model to hypothetical sub-relativistic meteoroids with diameters between 1 mm and 100 mm, which enter the atmosphere perpendicular to the surface and produce effects essen- tially starting from a height of 10 km. The researchers’ goal was to track the evolution of the plasma generated by sub-relativistic meteors in order to determine what type of signal they might produce, then provide as- tronomers with information on what to look for. The results indicate that a large part of a meteoroid’s energy would be re- leased into the upper atmosphere and would feed the adiabatic expansion of a

Perseid shower in the sky over the Na- A tional Astronomical Observatory of Japan, the sum of images taken with the Moon illuminating the horizon. [NAOJ, 2017] subrelativistic meteors EN_l'Astrofilo 27/04/2020 16:35 Page 40

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boiling plasma cylinder. In other words, the exceptionally high speed of the meteoroid would cause the column of air being passed through by the meteor to be instantly ionized along its entire length, but the surrounding pressure from the rest of the atmosphere would thermally isolate this column. There would, therefore, be no heat exchange with the gas outside the plasma cylinder, as is the case in ordinary meteors. Being much slower (20-30 km/s), ordinary meteors move slowly enough to allow cold air (ions and electrons again united) to reset behind them before the end of the glowing phenome- non. It is for this reason that we can appreciate the movement of the meteor trajectory without the possibility of swarm of meteors through the sky. recognizing its beginning and end in the at- A Lyrids pho- In the case of sub-relativistic meteors, how- mosphere. According to Siraj and Loeb, the tographed above ever, the speeds would be hundreds of times duration of the event should not exceed some structures of the Las Cam- faster (thousands of km/s), the result being one-tenth millisecond, but on the other panas Observa- hand, the brightness would be very high, an instantaneous manifestation of the boil- tory, with the ing plasma phenomenon along the entire comparable at least to that of a typical fire- dome of the Opti- cal Gravitational Lensing Experi- ment (OGLE) tele- scope in the fore- ground. [Y. Belet- sky, Carnegie, TWAN, 2017] Alongside, sev- eral Geminids captured at the Kitt Peak National Observatory. In the foreground, the open dome of the Bok Telescope; just behind the dome of the Mayall telescope. [David A. Harvey, 2010]

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brilliant A fireball pho- tographed “near” the domes of the Observatoire de Saint-Véran, in the French Alps, near the Italian border, at almost 3000 meters ele- vation. [Mikael De Ketelaere, 2019] Below, the sum of many images showing a storm of over fifty Gem- ball. Potentially, the human eye can observe optical detectors separated by distances of inids above the such a rapid and bright phenomenon, but up to 1000 km (with a surface area of just 1 Xinglong Obser- no evidence of such an event has been cm2 each) would be sufficient to reveal any vatory, China. recorded thus far. sub-relativistic meteors produced by 1 mm [Steed Yu and The two researchers predict that a global diameter particles. Even the shock wave NightChina.net] network of several hundred small ultrafast generated in the atmosphere could be recorded with the help of infra- sound microphones. A part of the structures necessary for this research already exist and it would be possible to implement their experimental design, but going beyond the theoretical phase might not be so simple. Indeed, there seem to be some critical issues in the conclusions reached by Siraj and Loeb. A first doubt concerns the ex- trapolation to macroscopic di- mensions of phenomena that we know today only take place at the microscopic level, and therefore are based on different theoretical models. In this par- ticular instance, iron-60 atoms, dust grains, and agglomerations of matter as large as a grain of sand can all be accelerated at extreme speeds by a supernova. That said, they remain very dis- tant from one another on the size scale, with differences measurable in millions of times both in mass and in volume. To suggest that a grain of sand be- haves like a grain of dust or an atom when subjected to the thrust of the explosion and radi-

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bright, green-toned meteor pho- A tographed above the La Silla Ob- servatory, Chile. By magnifying the meteor wake, you can easily notice its sinuous shape, produced by the rota- tion of the meteoroid during the fall. [ESO/B. Tafreshi (twanight.org)] subrelativistic meteors EN_l'Astrofilo 27/04/2020 16:35 Page 43

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ation pressure of a supernova is only a working hypothesis or little more. A second doubt concerns the preservation of sub-relativistic speed by the grains with diameters on the order of 1 mm or larger. These grains would be mainly produced in the central regions of the galaxy, where the density of the interstellar medium exceeds by thousands of times the average density of one atom per cm3 adopted to establish the potential frequency of sub-relativistic meteors. The doubt arises because a body of a certain mass, moving at very high speed in a medium, slows down significantly by coming into contact with a total mass of matter comparable to its own. Conse- quently, since only bodies of very small mass can be accelerated at relativistic or sub-rel- ativistic speeds, and since these bodies de- celerate rather quickly through a medium with relatively high density, the prediction of observing an average sub-relativistic me- teor per month appears quite optimistic. Another point of Siraj and Loeb’s hypothesis that does not seem convincing is the appar- ent brightness of the sub-relativistic mete- ors. Their calculations show that if the progenitor body had a diameter of 1 mm, it could generate about a billion photons of visible light in 0.1 milliseconds, a much higher flow than that coming from the brightest stars in the sky. In the case of a meteoroid with a diameter of 10 mm, the brightness of the meteor produced could exceed 10 million times that of Vega. Finally, a body placed at the maximum limit of the size scale considered by the researchers, 100 mm, would produce a Sun-like flash. To our knowledge, no one has ever re- ported observations of this kind – and one would imagine a record of such an event existing, considering that even the weakest flashes of light would still be much brighter than Venus. And what about the atmospheric traces left by the passage of hyper-fast meteors? The authors of the study don’t address this topic, but we can imagine that, for at least a few seconds or perhaps for a longer time, the section of the atmosphere af- fected by the phenomenon would be no- ticeably altered. ! chronicles EN_l'Astrofilo 27/04/2020 16:38 Page 44

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The strange orbits of ‘Ta

by ALMA Observatory

stronomers using the Ata- ments. Astronomers already know rtist impression of a double cama Large Millimeter/sub- that the orbits of binary stars can Asunset on a ‘Tatooine’ exo- A millimeter Array (ALMA) warp and tilt the disk around them, planet forming in a circumbinary have found striking orbital geome- resulting in a circumbinary disk mis- disk that is misaligned with the tries in protoplanetary disks around aligned relative to the orbital plane orbits of its binary stars. binary stars. While disks orbiting of its host stars. For example, in a [NRAO/AUI/NSF, S. Dagnello] the most compact binary star sys- 2019 study led by Grant Kennedy of tems share very nearly the same the University of Warwick, UK, their circumbinary disks are mis- plane, disks encircling wide binaries ALMA found a striking circumbinary aligned is strongly dependent on have orbital planes that are severely disk in a polar configuration. the orbital period of the host stars. tilted. These systems can teach us “With our study, we wanted to The shorter the orbital period of the about planet formation in complex learn more about the typical binary star, the more likely it is to environments. geometries of circumbinary disks,” host a disk in line with its orbit. In the last two decades, thousands said astronomer Ian Czekala of the However, binaries with periods of planets have been found orbiting University of California at Berkeley. longer than a month typically host stars other than our Sun. Some Czekala and his team used ALMA misaligned disks. of these planets orbit two stars, data to determine the degree of “We see a clear overlap between just like Luke Skywalker’s home alignment of nineteen protoplane- the small disks, orbiting compact Tatooine. Planets are born in proto- tary disks around binary stars. “The binaries, and the circumbinary planetary disks – we now have high resolution ALMA data was planets found with the Kepler mis- wonderful observations of these critical for studying some of the sion,” Czekala said. thanks to ALMA – but most of the smallest and faintest circumbinary Because the primary Kepler mission disks studied so far orbit single disks yet,” said Czekala. lasted 4 years, astronomers were stars. ‘Tatooine’ exoplanets form in The astronomers compared the only able to discover planets disks around binary stars, so-called ALMA data of the circumbinary around binary stars that orbit each circumbinary disks. disks with the dozen ‘Tatooine’ other in fewer than 40 days. And Studying the birthplaces of planets that have been found with all of these planets were aligned ‘Tatooine’ planets provides a unique the Kepler space telescope. To their with their host star orbits. A linger- opportunity to learn about how surprise, the team found that the ing mystery was whether there planets form in different environ- degree to which binary stars and might be many misaligned planets

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Tatooine’ planetary disks

that Kepler would have a hard time stellar hosts,” added Czekala. Still, finding methods like direct imaging finding. “With our study, we now based on this finding, the as- and microlensing. (NASA’s Kepler know that there likely isn’t a large tronomers conclude that mis- mission used the transit method, population of misaligned planets aligned planets around wide binary which is one of the ways to find a that Kepler missed, since circumbi- stars should be out there and that planet.) nary disks around tight binary stars it would be an exciting population Czekala now wants to find out why are also typically aligned with their to search for with other exoplanet- there is such a strong correlation be- tween disk (mis)alignment and the binary star orbital period. “We want to use existing and coming facilities like ALMA and the next generation Very Large Array to study disk struc- tures at exquisite levels of preci- sion,” he said, “and try to under- stand how warped or tilted disks af- fect the planet formation environ- ment and how this might influence the population of planets that form within these disks.” “This research is a great example of how new discoveries build on pre- vious observations,” said Joe Pesce, National Science Foundation Pro- wo examples of aligned and misaligned protoplanetary disks around binary gram Officer for NRAO and ALMA. T stars (circumbinary disks), observed with ALMA. Binary star orbits are added “Discerning trends in the circumbi- for clarity. Left: in star system HD 98800 B, the disk is misaligned with inner bi- nary disk population was only made nary stars. The stars are orbiting each other (in this view, towards and away possible by building on the founda- from us) in 315 days. Right: in star system AK Sco, the disk is in line with the tion of archival observational pro- orbit of its binary stars. The stars are orbiting each other in 13.6 days. [ALMA (ESO/NAOJ/NRAO), I. Czekala and G. Kennedy; NRAO/AUI/NSF, S. Dagnello] grams undertaken by the ALMA community in previous cycles.” !

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The impact of satellite constellations on astronomical observations

by ESO

stronomers have recently is more pronounced for long expo- US National Science Foundation’s raised concerns about the sures (of about 1000 s), up to 3% of Vera C. Rubin Observatory (not an A impact of satellite mega-con- which could be ruined during twi- ESO facility) would be “severely af- stellations on scientific research. To light, the time between dawn and fected”, depending on the time of better understand the effect these sunrise and between sunset and year, the time of night, and the sim- constellations could have on astro- dusk. Shorter exposures would be plifying assumptions of the study. nomical observations, ESO commis- less impacted, with fewer than 0.5% Mitigation techniques that could be sioned a scientific study of their of observations of this type af- applied on ESO telescopes would impact, focusing on observations fected. Observations conducted at not work for this observatory al- with ESO telescopes in the visible other times during the night would though other strategies are being and infrared but also considering also be less affected, as the satellites actively explored. Further studies are other observatories. The study, would be in the shadow of the Earth required to fully understand the sci- which considers a total of 18 repre- and therefore not illuminated. De- entific implications of this loss of ob- sentative satellite constellations pending on the science case, the im- servational data and complexities in under development by SpaceX, pacts could be lessened by making their analysis. Wide-field survey tel- Amazon, OneWeb and others, to- changes to the operating schedules escopes like the Rubin Observatory gether amounting to over 26 thou- of ESO telescopes, though these can scan large parts of the sky sand satellites, has now been changes come at a cost. On the in- quickly, making them crucial to spot accepted for publication in Astron- dustry side, an effective step to mit- short-lived phenomena like super- omy & Astrophysics. igate impacts would be to darken novae or potentially dangerous as- The study finds that large telescopes the satellites. teroids. Because of their unique like ESO’s Very Large Telescope (VLT) The study also finds that the greatest capability to generate very large and ESO’s upcoming Extremely impact could be on wide-field sur- data sets and to find observation tar- Large Telescope (ELT) will be “mod- veys, in particular those done with gets for many other observatories, erately affected” by the constella- large telescopes. For example, up to astronomy communities and fund- tions under development. The effect 30% to 50% of exposures with the ing agencies in Europe and else-

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his annotated image shows the night sky at ESO’s Paranal Observatory around twilight, about 90 minutes before sunrise. T The blue lines mark degrees of elevation above the horizon. A new ESO study looking into the impact of satellite con- stellations on astronomical observations shows that up to about 100 satellites could be bright enough to be visible with the naked eye during twilight hours (magnitude 5–6 or brighter). The vast majority of these, their locations marked with small green circles in the image, would be low in the sky, below about 30 degrees elevation, and/or would be rather faint. Only a few satellites, their locations marked in red, would be above 30 degrees of the horizon — the part of the sky where most as- tronomical observations take place — and be relatively bright (magnitude of about 3–4). For comparison, Polaris, the North Star, has a magnitude of 2, which is 2.5 times brighter than an object of magnitude 3. The number of visible satellites plum- mets towards the middle of the night when more satellites fall into the shadow of the Earth, represented by the dark area on the left of the image. Satellites within the Earth’s shadow are invisible. [ESO/Y. Beletsky/L. Calçada]

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where have ranked wide-field survey telescopes as a top priority for future developments in astronomy. Professional and amateur as- tronomers alike have also raised concerns about how satellite mega- constellations could impact the pris- tine views of the night sky. The study shows that about 1600 satellites from the constellations will be above the horizon of an observatory at mid-latitude, most of which will be low in the sky — within 30 de- grees of the horizon. Above this — the part of the sky where most as- tronomical observations take place — there will be about 250 constella- tion satellites at any given time. his image shows the night sky above the construction site of ESO’s Extremely While they are all illuminated by the T Large Telescope, which is planned to start operating in late 2025. A laser guide Sun at sunset and sunrise, more and from the nearby Very Large Telescope is seen in the background. [ESO/M. Zamani] more get into the shadow of the Earth toward the middle of the of which would be higher than 30 lites visible in the night sky to the night. The ESO study assumes a degrees of elevation. All these num- naked eye above 30 degrees. brightness for all of these satellites. bers plummet as the night gets These numbers do not include the With this assumption, up to about darker and the satellites fall into the trains of satellites visible immedi- 100 satellites could be bright shadow of the Earth. Overall, these ately after launch. Whilst spectacu- enough to be visible with the naked new satellite constellations would lar and bright, they are short lived eye during twilight hours, about 10 about double the number of satel- and visible only briefly after sunset or before sunrise, and — at any given time — only from a very lim- ited area on Earth. The ESO study uses simplifications and assumptions to obtain conser- vative estimates of the effects, which may be smaller in reality than calculated in the paper. More so- phisticated modelling will be neces- sary to more precisely quantify the actual impacts. While the focus is on ESO telescopes, the results apply to similar non-ESO telescopes that also operate in the visible and infrared, with similar instrumentation and science cases. Satellite constellations will also have an impact on radio, millimetre and his 2D diagram illustrates that an observer at mid-latitude would see only a submillimetre observatories, includ- T fraction of the constellation satellites orbiting the Earth. To be visible, satel- ing the Atacama Large Millimeter/ lites need to be above the observer’s horizon and be illuminated by the Sun. submillimeter Array (ALMA) and the Most satellites would be below the horizon and/or hidden by the Earth’s shadow which, for a given observer, covers increasingly more of the sky as night ad- Atacama Pathfinder Experiment vances. [ESO/L. Calçada] (APEX). This impact will be consid- ered in further studies. !

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ALMA and Rosetta map the journey of phosphorus

by ESO

hosphorus, present in our DNA and cell 5142, astronomers could pinpoint where phos- membranes, is an essential element for life phorus-bearing molecules, like phosphorus Pas we know it. But how it arrived on the monoxide, form. New stars and planetary sys- early Earth is something of a mystery. As- tems arise in cloud-like regions of gas and dust tronomers have now traced the journey of in between stars, making these interstellar phosphorus from star-forming regions to clouds the ideal places to start the search for comets using the combined powers of ALMA life’s building blocks. and the European Space Agency’s probe The ALMA observations showed that phospho- Rosetta. Their research shows, for the first time, rus-bearing molecules are created as massive where molecules containing phosphorus form, stars are formed. Flows of gas from young mas- how this element is carried in comets, and how sive stars open up cavities in interstellar clouds. a particular molecule may have played a crucial Molecules containing phosphorus form on the role in starting life on our planet. cavity walls, through the combined action of “Life appeared on Earth about 4 billion years shocks and radiation from the infant star. The ago, but we still do not know the processes astronomers have also shown that phosphorus that made it possible,” says Víctor Rivilla, the monoxide is the most abundant phosphorus- lead author of a new study published today in bearing molecule in the cavity walls. the journal Monthly Notices of the Royal As- After searching for this molecule in star-form- tronomical Society. The new results from the ing regions with ALMA, the European team Atacama Large Millimeter/Submillimeter Array moved on to a Solar System object: the now- (ALMA), in which the European Southern Ob- servatory (ESO) is a partner, and from the his infographic shows the key results from ROSINA instrument on board Rosetta, show T a study that has revealed the interstellar thread of phosphorus, one of life’s building that phosphorus monoxide is a key piece in the blocks. [ALMA (ESO/NAOJ/NRAO), Rivilla et al.; origin-of-life puzzle. ESO/L. Calçada; ESA/Rosetta/NAVCAM; Mario With the power of ALMA, which allowed a de- Weigand, www.SkyTrip.de] tailed look into the star-forming region AFGL

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ALMA and ROSINA data has revealed a sort of chemical thread during the whole process of star formation, in which phosphorus monoxide plays the dominant role,” says Rivilla, who is a researcher at the Arcetri Astro- physical Observatory of INAF, Italy’s National Institute for Astrophysics. “Phosphorus is essential for life as we know it,” adds Altwegg. “As comets most probably delivered large amounts of organic com- pounds to the Earth, the phosphorus monoxide found in comet 67P may twegg, the Principal Investigator for strengthen the link between comets his wide-field view shows the Rosina and an author in the new and life on Earth.” T region of the sky, in the constel- lation of Auriga, where the star- study, got a clue about what this This intriguing journey could be doc- forming region AFGL 5142 is located. molecule could be after being ap- umented because of the collabora- This view was created from images proached at a conference by an as- tive efforts between astronomers. forming part of the Digitized Sky tronomer studying star-forming “The detection of phosphorus Survey 2. [ESO/Digitized Sky Survey 2] regions with ALMA: “She said that monoxide was clearly thanks to an phosphorus monoxide would be a interdisciplinary exchange between famous comet 67P/Churyumov– very likely candidate, so I went back telescopes on Earth and instruments Gerasimenko. The idea was to follow to our data and there it was!” in space,” says Altwegg. the trail of these phosphorus-bear- This first sighting of phosphorus Leonardo Testi, ESO astronomer and ing compounds. If the cavity walls monoxide on a comet helps as- ALMA European Operations Man- collapse to form a star, particularly a tronomers draw a connection be- ager, concludes: “Understanding our less-massive one like the Sun, phos- tween star-forming regions, where cosmic origins, including how com- phorus monoxide can freeze out and the molecule is created, all the way mon the chemical conditions get trapped in the icy dust grains to Earth. “The combination of the favourable for the emergence of life that remain around the are, is a major topic of new star. Even before modern astrophysics. the star is fully formed, While ESO and ALMA those dust grains come focus on the observa- together to form peb- tions of molecules in dis- bles, rocks and ulti- tant young planetary mately comets, which systems, the direct ex- become transporters of ploration of the chemi- phosphorus monoxide. cal inventory within our ROSINA, which stands Solar System is made for Rosetta Orbiter Spec- possible by ESA missions, trometer for Ion and like Rosetta. The synergy Neutral Analysis, col- between world leading lected data from 67P for ground-based and space two years as Rosetta or- facilities, through the bited the comet. As- collaboration between tronomers had found ESO and ESA, is a power- hints of phosphorus in his video starts by showing a wide-field view of a region of ful asset for European the ROSINA data before, T the sky in the constellation of Auriga. It then zooms in to show researchers and enables the star-forming region AFGL 5142, recently observed with ALMA. but they did not know transformational discov- [ALMA (ESO/NAOJ/NRAO), Rivilla et al.; Mario Weigand, www.Sky- what molecule had car- Trip.de; ESO/Digitized Sky Survey 2; Nick Risinger (skysurvey.org).] eries like the one re- ried it there. Kathrin Al- ported in this paper.” !

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www.newmoontelescopes.com [email protected] cover EN.qxp_l'astrofilo 27/04/2020 16:36 Page 54

www.northek.it www.facebook.com/northek.it [email protected] phone +39 01599521

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