Compact Objects in the Era of Celestial Cinematography

A merging white dwarf binary. Mass transfer in these systems proceeds directly onto the massive white dwarf companion (depicted here), which could spark a thermonuclear explosion that might destroy the binary. DR ENRICO RAMIREZ-RUIZ Astronomically significant Celebrated astrophysicist and educator Dr Enrico Ramirez-Ruiz describes the role of dense objects in the visible Universe, and his own work towards simulating and analysing stellar cataclysmic events Gravity waves are ripples in space generated to be employed to study the deformations and by extreme cosmic events such as colliding exchanges of energy and angular momentum, stars, black holes and supernova explosions, and perhaps mass transfer or even a complete which carry vast amounts of energy at the merger between the stars. While stellar speed of light. Compact objects can generate dynamics and stellar evolution each have a copious gravitational waves when they collide history of half a century of simulations, the and merge. Because of this, simulations of hydrodynamics of stellar encounters is much such encounters play an essential role in less developed. This is an area where there is ongoing efforts to detect gravitational waves. plenty of room for basic breakthroughs. In general, all these phenomena, from extreme matter to event horizons and gravitational Could you describe the novel techniques waves, cannot be created in a laboratory. established to characterise the transient Instead, we have to create virtual laboratories Universe over a range of temporal scales? on Earth to simulate the relevant physics in large-scale computational experiments. Observational astronomy is entering a period of metamorphosis with large-scale, systematic How can the properties of these extreme exploration replacing the small sample, forms of matter be better understood? piecemeal studies of the past. The Universe Why is it important to monitor the is now being explored in a panchromatic What methodologies do you intend to use interplay between black holes and other way, over a range of temporal scales leading to observe and analyse celestial activity in objects in a dense environment? towards a more complete and less biased the Universe, given that experiments of this understanding of its constituents. Exotic nature cannot be recreated in laboratories? According to the best-tested theory of gravity and transient astrophysical phenomena will we have – general relativity, the matter reveal the violent and capricious nature of the Empirically, astrophysical objects are not in a black hole is compressed in a central Universe. Wide-field and all-sky monitoring amenable to experimental studies; we rely on singularity. As a consequence, effects that are has already led to a wealth of discoveries such careful observations rather than laboratory observationally accessible have to occur outside as pulsars and gamma-ray bursts that probe experimentation to obtain data. Theoretically, the event horizon. Observations of phenomena extreme realms of physics and yet were not astrophysical objects are rarely amenable just outside the horizon are still mostly anticipated prior to their discovery. to simple physical models; we rely more unexplored, and would form a very welcome and more on computational methods to addition to our experience – especially in What are your plans for the future of your understand the complicated physics that testing our most basic theory of gravitation. astrophysics research? What lies ahead for governs the phenomena we observe. With the the field as a whole? advancement of computer technology and Are there challenges associated with numerical algorithms, complex astrophysical simulating such encounters? To high-energy astrophysicists, the calm beauty phenomena such as supernova explosions, of the night sky is the world’s grandest illusion. the accretion of material onto stars, stellar Stellar dynamics is perfectly adequate in In fact, it contains some of the wildest, oddest pulsations and the granular patterns of modelling the motions of stars as point and most elusive creatures in the Universe. solar convection are now accessible via masses moving under the influence of gravity These phenomena are unique in their extreme simulation almost as if they were subject to unless individual stars approach each other to physical conditions, not faced by astronomers laboratory experiments. within a few stellar radii. When that happens, before, and understanding them requires both the distorted internal structure of the stars has imagination and physical intuition. My long- Why is an understanding of the to be taken into account, and we must switch term goal is to help extend our views of the gravitational waves created when black to a hydrodynamical description to follow the Universe as a site of impressive cataclysmic holes and neutron stars collide significant? encounter. Hydrodynamical calculations have events, and begin to look ahead to a truly fascinating period in the history of astronomy. WWW.RESEARCHMEDIA.EU 71 DR ENRICO RAMIREZ-RUIZ Computer simulation of the disruption of a star by a black hole shows the formation of an ‘accretion disk’ of stellar material spiralling into the black hole. The compact cosmos Space scientists at the University of California, Santa Cruz have been investigating the behaviours of compact stellar remnants such as black holes and neutron stars with the hope of learning more about the properties of these extreme forms of matter THE CUTTING EDGES of physics are found intricate computer modelling and simulation His current research focuses on high-energy in the unimaginably tiny realm of particle tools, astrophysicists can essentially achieve astrophysics and compact stellar remnants, and quantum physics, and the ungraspably the productivity of experimentation without encompassing some of the most spectacular vast arena of astrophysics. In this latter field, the need for laboratories in which to create and dynamic events in the cosmic theatre. comparatively small objects such as the such objects. neutron star can often be difficult to detect CLOSE ENCOUNTERS – despite the fact they represent a structure THE DEATH OF STARS roughly the same size as Chicago. Even white The recent work of Ramirez-Ruiz and his dwarfs, which have a similar radius to Earth, Such practices are especially important when colleagues in California has hinged primarily produce little radiation and are challenging to it comes to compact objects, which are either on modelling stellar encounters and binary spot. The reason physicists find these extremes small like neutron stars and white dwarfs, or stars, usually composed of one black hole, at so fascinating is, of course, that their detection dark like black holes. These extreme forms of least one neutron star or white dwarf or two in isolation is at the limits of experimental matter, the remnants of deceased stars, have of each. By running complex simulations, the technology. What is more, the quantities no analogue within the lab, so their behaviour astrophysicists can build a predictive framework involved are too large to effectively manipulate can only be observed in situ. Stars are the basic to inform future observation and draw firm in a laboratory setting. The problem is related building blocks of the Universe, but the issue conclusions about the properties of these to the weakness of the gravitational force, of how their fate is determined – whether they extreme forms of matter. For example, in 2010 which implies we have to add quite a number become black holes, white dwarfs or neutron Ramirez-Ruiz worked alongside undergraduate of particles before gravity can dominate. stars – is still open. What is more, multiple and Luke Kelley to predict the locations of neutron Therefore, scientists wishing to study them binary star systems also provide the perfect star mergers in the local galactic neighborhood, must resort to more creative methods. environment for discovering more about other allowing observers to more clearly trace where aspects of stellar evolution, the structure of such events might occur. Because neutron star Interestingly, despite the obvious difference individual stars and high-energy phenomena binaries are moving through the Galaxy at very in scale between particle physics and such as supernovae and γ-ray bursts. high speeds, this is not as straightforward as it astrophysics, there are two observational first sounds. approaches that are equally effective in both One astrophysicist investigating these cases. One is to examine the particles as systems is Dr Enrico Ramirez-Ruiz, Professor This study, like many others conducted by they collide, and the other is to watch their of Astronomy and Astrophysics at the Ramirez-Ruiz and his students, also has behaviour when they exist in a bound state, University of California, Santa Cruz (UCSC) implications in the hunt for gravitational waves. with two or more in close proximity. This is just and Director of the Theoretical Astrophysics As they are so minute these ripples in the fabric as applicable to the study of stars and stellar Santa Cruz Institute. The recipient of of space-time have only ever been observed remnants as it is to basic particles, which numerous awards for his work in the field, indirectly, but circumstantial evidence for their is lucky, since binaries and stellar collisions including a Packard Fellowship for Science existence is strong, and supported by Einstein’s may account for the majority of compact and Engineering and the National Science

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