They are often eclipsed by more attractive topics, like black holes or exoplanets. Even the name itself is less than sensational: brown dwarfs. But Viki Joergens and her colleagues from the Max Planck Institute for Astronomy in Heidelberg have gained fascinating insights in this research field. PHYSICS & ASTRONOMY_Brown Dwarfs
TEXT THOMAS BÜHRKE
he Milky Way probably has it. As Japanese theoreticians Chu-shiro Stars are formed when individual re- roughly as many brown dwarfs Hayashi and Takenori Nakano discov- gions in the interior of a large cloud of as it has planets. Astronomers ered back in 1963, this process requires gas and dust contract under the effects don’t know the precise num- a star to have at least 7 to 8 percent of of gravity. The core of such a cloud ro- ber, as these celestial bodies the solar mass, corresponding to 75 tates and forms a disk. The material in T are small and very dim, and thus diffi- times the mass of the planet Jupiter. the cloud center becomes more and cult to observe. They have, however, re- But bodies that are just below this more compressed until hydrogen fusion ally shaken our tried and tested defini- threshold should initially still be suffi- begins. The young star is then stable. tions of the terms “star” and “planet” ciently hot to fuse heavy hydrogen Dust particles collide with each oth- of which we have grown so fond. Spots (deuterium) to helium-3 and thus gen- er in the disk that still surrounds them, form on their surface like the spots on erate energy. However, the raw materi- clump together and finally grow into the Sun, and clouds form as if they were al deuterium is present only in small asteroids and planets. Earth and the gas planets. “This is one of the most impor- quantities, so that the fire is extin- planet Jupiter were also formed in this tant characteristics of our research field: guished after just a few million years. way. And the terms “star” and “planet” there are countless overlaps with the From then on, the celestial body slow- are defined in accordance with this sce- properties of both planets and stars,” ly cools down. “Eventually, all brown nario: a star is a stable ball of gas that explains Viki Joergens, who has been dwarfs are approximately the size of Ju- generates energy in its interior through investigating these celestial bodies for piter,” says Joergens. nuclear fusion; planets aren’t able to do more than ten years now. this, are smaller, and orbit their sun. Back in 1962, Shiv Kumar, who was THE HYDROGEN FUSION STARTS But which formation path do brown then working as a post-doc at the God- DEEP IN THE INTERIOR dwarfs choose? dard Space Flight Center of the US “Since young stars are initially still space agency NASA, set the ball rolling. In order for this deuterium burning to surrounded by a disk of dust, the ob- He asked himself how small a star can start, the body must have at least 13 vious thing was to look at whether actually be, and what properties bod- times the mass of Jupiter. This repre- brown dwarfs can also be surrounded ies that are just below this threshold sents the lower mass threshold of by a disk,” explains Viki Joergens. The possess. The fundamental characteris- brown dwarfs according to the current Heidelberg-based astronomers used tic of stars consists in the multi-step definition of the International Astro- Herschel, the space telescope of the Eu- fusion of hydrogen to helium in their nomical Union. These objects thus ropean Space Agency (ESA), for this central region. form the link between planets and project. Equipped with a 3.5-meter di- This process releases energy that, in stars in a range of around 13 to 75 Ju- ameter main mirror, it was the largest the form of heat, creates an outward piter masses. telescope ever launched into space. It pressure against the gravity acting in- Astronomers spent more than 30 was in operation from June 2009 for ward. If these two pressures balance, years looking in vain for these failed stars nearly four years, at which point the the star is stable. Our Sun has been in until they finally found the first repre- helium for cooling the instruments was this phase for around 4.5 billion years. sentative of this class of objects in 1995. exhausted. To reach this state in the first place, Approximately 2,000 brown dwarfs Herschel observed exclusively in the the celestial body must have a certain have since been identified – some with region of mid- to far infrared at wave- minimum mass; otherwise, the pressure surprising properties. “One of the most lengths ranging from 70 to 500 mi- and temperature aren’t sufficient to ig- topical questions concerns their birth,” crometers. “This is where the thermal nite the hydrogen fusion and maintain says Joergens. radiation of cool dust, among other
View of a brown dwarf: These linking elements between stars and planets have been keeping astronomers busy for over half a century. Graphic: ESO/Ian Crossfield
3 | 14 MaxPlanckResearch 47 above: Presentation in the team: Viki Joergens things, can be observed, as we expect it planets such as Jupiter to form in the (standing) and her colleagues Ian Crossfield, to be emitted by the disks of brown disks of brown dwarfs, but smaller Niall Deacon and Esther Buenzli (from left). dwarfs,” says Joergens. rocky planets may very well do so. But below: The cosmos in art: The object PSO J318.5-22 (top) has around seven times the mass PACS, one of the three instruments to date none have been discovered. of Jupiter and travels through space alone, aboard Herschel, was built under the di- Surprisingly, brown dwarfs follow a meaning without a parent sun. The picture of rection of the Max Planck Institute for law that was discovered for young stars: OTS44 (bottom) illustrates that this object, just Extraterrestrial Physics with crucial the disks of dust always have around 1 two million years young, formed in a similar contributions from the astronomers in percent of the stellar mass. Viki Joer- way to stars, namely from a disk of gas and dust. Even today, considerable quantities of matter Heidelberg. In return, they received gens’ observations show that this ap- still fall onto OTS44. guaranteed observation time with plies down to a central mass of only 12 PACS. Working with colleagues from Jupiter masses. Thus, in this respect, the University of Texas, they com- brown dwarfs don’t differ from their menced a program to search for dust big brothers. They are apparently also disks around brown dwarfs – with formed in the same way and not like great success. planets. If this relation also existed for “For 36 of 47 carefully selected our Sun, it is possible to conclude from brown dwarfs, we found infrared emis- this that only around 10 percent of the sions that originate from such disks,” dust was converted into planets. says Viki Joergens. And the initiator of Since brown dwarfs appear to over- the project, Max Planck Director Thom- lap seamlessly with the range of the as Henning, adds: “We therefore suc- planets at the lower threshold of around ceeded in carrying out the first survey 13 Jupiter masses, the Max Planck as- for such disks in the infrared region, tronomers from Heidelberg searched and in narrowing down their masses.” through the Herschel observations to “We find that disks around brown see whether any of the representatives dwarfs have masses ranging from just with the lowest mass had a disk – and below one Earth mass up to one Jupiter found a celestial body at a distance of mass,” says Joergens; Jupiter, on the 530 light-years with the designation other hand, has 300 times more mass OTS44. The intensity of its far-infrared than Earth. However, not all the disk radiation had to originate from a disk material is used to form planets. It of at least 10 Earth masses. In addition,
therefore isn’t possible for large gas the researchers discovered that the ob- Graphics: MPI for Astronomy/V. Ch. Quetz (top) – A. M. (bottom); photo: Thomas Hartmann
48 MaxPlanckResearch 3 | 14 Graphic: V. Joergens (editor), 50 Years of Brown Dwarfs, Springer-Verlag, Heidelberg 2014 it isapproximately thesizeofJupiter. shrinking any further. This isthecasewhen counter-pressure andprevent thebodyfrom the interiorbuildupaquantummechanical untilthefreely movingelectrons in further extinguishes inabrown dwarf, itcontracts (from left)to scale.Ifthedeuteriumfusion star,a red dwarf three brown andJupiter dwarfs Size comparison: The imageshows theSun, in ayoungsystemwithseveralplanets. nearby, oralsoasaresultofinstabilities the gravitationaleffectofastarpassing oretically, thiscantakeplacethrough The- ejected fromthesystemafterward. PSO J318.5-22wasbornasaplanetand star formation,soitisstillpossiblethat ed anyprocessesthatareknownfrom like astar? without aparentstar. Diditalsoform years istravellingalonethroughspace PSO J318.5-22atadistanceof80light- The objectwiththecatalognumber only seventimesthemassofJupiter. my discoveredacelestialbodythathad the MaxPlanckInstituteforAstrono- mers thatincludedNiallDeaconfrom At theendof2013,agroupastrono- the Pan-STARRS1 telescopeonHawaii: This wasshownbyafindingmadewith OVER TIME THE SIGNATURE CHANGES isn’t knownin allcases. mation history,” saysJoergens.Butthis should bedrawnwiththeaidoffor- line betweenbrowndwarfsandplanets there isnodifferencehere,becausethe as afree-floatingplanet.Inmyview, low-massbrowndwarfor ther asavery mation. “OTS44canbedesignatedei- of informationfortheoriesstarfor- moves freelythroughspace–akeypiece ever, theobjectdoesn’t orbitastar, but classical massrangeoftheplanets.How- 12 Jupitermassesandisthusstillinthe young starsdo. still pickingupmatterfromthedisk,as ject, onlytwomillionyearsyoung,was But thescientistshaven’t yetdetect- Interestingly, OTS44hasonlyaround a browndwarfisparticularlytricky The importantmassdeterminationof cause itispartofastarformationregion from models. dwarfs inordertothendeducethemass somehow determinetheageofbrown ample. Theastronomersmusttherefore years oldwith15Jupitermasses,forex- temperature asabodyonly200million with 70Jupitermasseshasthesame age: abrowndwarfonebillionyearsold old, massiveonewithoutknowingits a young,low-massbrowndwarforan mine fromitstemperaturewhetheritis isn’t possible tounequivocallydeter- ter itwasduringitsformation.Thus, changes. they cool,andtheirspectralsignature light becomesweakerandas of gasandcooldownovertime.Their brown dwarfs:Theyformashotballs of stableburning,thisisn’t thecasefor and themassofastarduringphase between themeasurableluminosity Whereas thereisaclearrelationship old, familiarrulesofstellarphysics. task andcan’t bemanagedwiththe With OTS44,thiswaspossible be- The heavierabrowndwarf,thehot- 5800 K Sun G 2 -star Gliese Red dwarf 60K2600K 3600 K 229 A PHYSICS & ASTRONOMY Young brown Teide dwarf new findingsconcerningbrowndwarfs tional datamoredifficult. makes theinterpretationofobserva- such fortunatecircumstances, andthis the researchers can’t always hopefor around 12to21millionyearsago.But group ofyoungstarsthatformed ascertained thatitusedtobelonga motion inspace.Intheprocess,they astronomers wereabletomeasurethe PSO J318.5-22,ontheotherhand, stellation inthesouthernsky. With of knownageintheChameleoncon- by dynamicinteractionsinagroupof rupted starformation.Thiscouldoccur are theconsequenceofarudelyinter- on theassumptionthatbrowndwarfs different explanations.Mostarebased under theirowngravity. massive enoughtobeablecontract lieve thatthecloudfragmentsaren’t of gasanddust,sometheoreticiansbe- stars inthecenterofacollapsingcloud cate thatbrowndwarfsareformedlike currentlyindi- results ofobservations mation remainunanswered.Whileall in thepast,questionsabouttheirfor- Although therehavebeenmany The researchers therefore devised 1 Gliese Old brown dwarf 5 0 125K 300K 950 K 229 B 3 | 14 MaxPlanckResearch MaxPlanckResearch brown dwarf Ultra-cold WISE _Brown Dwarfs 1828 Jupiter Planet 49 PHYSIK & ASTRONOMIE 50 radiation couldthenvaporize thegas cinity ofaformingstar. ItsintenseUV there isafurther, hotstarinthevi- very fore ithasreachedadulthood. members isejectedfromthis systembe- stars thatformedtogetherifoneofthe with the Very Large Telescope (ESO). oftheEuropean SouthernObservatory imageofthebrown Luhman reconstructed thissurface dwarf ofastrange world: below: Panoramic chart Astronomers inIanCrossfield’s team ofbrownproperties dwarfs. Niall Deacon,EstherBuenzliandJoshuaSchlieder(from left)investigate thephysical above: Focusonexotic bodiesinspace: Viki Joergens, IanCrossfield, Amelia Bayo, MaxPlanckResearch MaxPlanckResearch It wouldalsobeconceivable that 3 | 14
_Braune Zwerge data, however, Viki Joergensconcludes in thetruemeaningofword. scenarios, browndwarfsare failed stars newcomer of“food.”According tothese still collectinggas,anddeprive the cloud fromwhichtheemergingsunis From the previous observational From thepreviousobservational 16 B from datatheyobtained that thereisnoneedtodeviseaspecial However, thetendencytoward togeth- by Joergens’survey, amongothers. only 10to20percent, aswasshown With browndwarfs,thisproportionis ormultiplesystems. exist inbinary time thataroundtwo-thirds ofallstars mine infull. velocity orbittheywereabletodeter- browndwarfs,whoseradial few binary what theyhavefoundisoneofthevery found abrowndwarfwithplanet, Although theresearchers haven’t yet spectrum byvirtueofaDopplershift. these shouldbecomeevidentinthe inChile; an SouthernObservatory the Very LargeTelescope of theEurope- with oneofthe8-metertelescopes to scanbrowndwarfsforcompanions search forplanets. method, whichisknownfromthe toapplythis scopes arenecessary ions ofbrowndwarfs.Very largetele- tion spectroscopytolookforcompan- was oneofthefirsttousehigh-resolu- teresting inthisrespect.Theresearcher tems, whichsheinvestigates,isalsoin- sys- dwarfs. Thefrequencyofbinary mechanism fortheformationofbrown Astronomers haveknownforsome Joergens andhercolleaguesstarted
Graphic (bottom): ESO/Ian Crossfield; photo Thomas Hartmann PHYSICS & ASTRONOMY_Brown Dwarfs
erness seems generally to decrease er than those at higher latitudes, and the atmosphere: the seven wavelength with decreasing mass. Astronomers in this can also be seen in the spectrum. ranges very probably correspond to Heidelberg discovered a few years ago Crossfield analyzed these changing different atmospheric layers with dif- that only 25 percent of stars with very spectral signatures with a computer fering temperature. low mass, so-called M-dwarfs, are in bi- program and used them to create two- “The cloud structure varies depend- nary systems. In this sense, brown dimensional surface maps. “It’s proba- ing on how deep one looks into the at- dwarfs follow the trend set by the stars bly a non-uniform cloud cover – not mosphere – this means there is definite- – they prefer life as a single with de- unlike that of the planet Jupiter,” says ly more than just one cloud layer,” says creasing mass. the scientist. Biller. For the first time, theoreticians Brown dwarfs only ever appear as can now compare their models for the featureless points, even on images tak- RESEARCHERS ARE LOOKING cloud structure of brown dwarfs with en with the largest telescopes. The first DEEP INTO THE ATMOSPHERE observations. surface map of a brown dwarf is there- These fascinating new findings on fore a sensation. It belongs to a binary Beth Biller and her colleagues didn’t the formation of brown dwarfs, the system whose discovery by American use a spectrograph in their work, but weather in their atmospheres, and free- astronomer Kevin Luhman from Penn- recorded light variations in the two floating planets open up new prospects sylvania State University in March 2013 brown dwarfs simultaneously in sev- for research. And not least, the Max caused a stir. It’s located at a distance of en different wavelength ranges. Al- Planck astronomers in Heidelberg have 6.5 light-years from the Sun – only two though the two-dimensional informa- also succeeded in moving the brown other stellar systems are closer. tion was lost with this method, the dwarfs slightly out of the shadow of the Despite the proximity of the brown researchers were able to look deep into black holes and into the limelight. dwarfs, called Luhman 16A and 16B, it isn’t possible to observe any features on their surfaces directly. With the aid of a clever technique, two internation- TO THE POINT al teams working with Ian Crossfield ● Brown dwarfs are a sort of halfway house between star and planet. At a mass and Beth Biller from the Max Planck below 75 Jupiter masses, no atomic fire burns in their interior. Institute for Astronomy have succeed- ● Brown dwarfs apparently form in gas and dust clouds and are therefore born ed for the first time in drawing up in the same way as stars. something like a weather chart of one ● Brown dwarfs prefer to be single. While around two-thirds of all stars exist in of the two celestial bodies. Clouds binary or multiple star systems, the proportion for brown dwarfs is only form in the atmosphere despite the 10 to 20 percent. high temperatures of more than 1,000 ● Max Planck astronomers recently succeeded in mapping the surface of a brown degrees Celsius. These clouds obvious- dwarf for the first time. This weather chart shows clouds in the atmosphere; ly don’t consist of water, like on Earth, but temperatures of more than 1,000 degrees Celsius mean they consist, not but of heavy elements such as iron of water, but of heavy elements, such as iron and minerals. and minerals. Crossfield applied a method called Doppler imaging, which works as fol- GLOSSARY lows: Initially, the light of the brown Doppler effect: When a celestial body moves toward us or away from us, its light dwarf is dispersed into its spectral col- spectrum shifts to shorter (blue) or longer (red) wavelengths, respectively. Sound ex- ors. Lines that originate from the sub- hibits the same phenomenon: when a police car is moving toward us, the pitch of the stances contained in the atmosphere siren is higher (shorter acoustic wavelength) than when it’s moving away from us. then appear in this spectrum. Since the Spectroscopy: One of the most important methods to determine the physical state brown dwarf rotates, one half is always and the chemical composition of a distant star. With the aid of an optical device moving toward us and the other half (a prism, for example) the radiation of an object is dispersed according to its energy. Spectrometry provides quantitative data on the concentration of elements, away from us. This is evident as a Dop- pressure and electric or magnetic fields, for example. pler shift in the spectrum. Furthermore, the regions near the equator rotate fast-
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