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 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 in large-scale computational experiments. Observational 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 theory of general relativity. Confirming these stars in the visible Universe. The interaction Foundation’s CAREER Award, Ramirez-Ruiz ripples are indeed created by the movement of of stars reveals physical properties that could is also an experienced teacher and public compact objects like neutron star binaries would never be gauged in a lone star. With such a lecturer, and the youngest scientist ever have profound implications for the scientific rich variety of existing cases, coupled with elected to the Mexican Academy of Sciences. understanding of the Universe – and the 2010 72 INTERNATIONAL INNOVATION INTELLIGENCE COMPACT OBJECTS IN THE ERA OF CELESTIAL CINEMATOGRAPHY By running complex simulations, the astrophysicists can build a predictive OBJECTIVES framework to inform future observation and draw firm conclusions about To combine high-quality numerical simulations with analytical techniques in order to address the properties of these extreme forms of matter basic questions in high-energy astrophysics. KEY COLLABORATORS study points observers in what is almost role as an educator and is known within UCSC certainly the right direction to look for them. as a particularly effective mentor of both Joshua Bloom; Daniel Kasen, University of undergraduate and doctoral students. He was California, Berkeley, USA • Fabio De Colle; STELLAR HOMICIDE responsible for establishing the University’s William Lee, Universidad Nacional Autónoma supercomputer lab for undergraduates de México, Mexico • Elena D’Onghia, University Another compelling aspect of Ramirez-Ruiz’s (SLUG). Now in its fourth year, the SLUG is of Wisconsin-Madison, USA • Mark Krumholz; research programme is what he refers to as maintained with the aim of providing physics Camille Liebler; Doug Lin; Phillip Macias; ‘crime scene investigation for stellar homicides’. and astrophysics majors with a space in Morgan Macleod; Anna Rosen; Xavier Supermassive black holes are unobservable on which to develop their work as independent Prochaska, University of California, Santa Cruz, their own because they actually draw in light, scientists. “I have invested in generating USA • Laura Lopez, Massachusetts Institutue but they are big – in fact some of them can dwarf a unique culture for my students; enabling of Technoogy, USA • James Guillochon; Jill the entire Solar System. While most stars die as discovery,” he reflects – and the investment Naiman, Harvard University, Massachusetts, their nuclear fuel burns out after tens of billions of has certainly paid off. Students working in USA • Martin Rees; Michele Trenti, University years of life, some are forcibly dismantled by the the SLUG have consistently achieved entry of Cambridge, UK • Luke Roberts, California tidal forces of a supermassive black hole. Studying to the best graduate programmes and his Institute of Technology, USA • Stephan Rosswog, these ‘crime scenes’ has required the development doctoral students are routinely awarded Stockholm University, Sweden • Ilya Mandel, of novel computational tools, but could ultimately the top postdoctoral fellowships and University of Birmingham, UK • Jens Hjorth, Niels pave the way to a clearer understanding of these faculty positions. Bohr Institute, Denmark • Jason Kalirai, Space mysterious and terrifying objects. Telescope Science Institute, Maryland, USA • Neil Interestingly, Ramirez-Ruiz advocates a Gehrels, NASA, Maryland, USA As an example of his work, the California scientist standard education programme in astrophysics CONTACT together with collaborator Stephan Rosswog has consisting of training in all three elements of proposed a new form of supernova caused when contemporary physics: theory, experiment Dr Enrico Ramirez-Ruiz a white dwarf encounters a mid-size black hole. and simulation. He argues that establishing Principal Investigator Their simulations show that the resulting tidal a simulation contains elements of both Department of Astronomy & Astrophysics force would compress the dense gas of the star, theoretical and experimental physics, and University of California, Santa Cruz reheating it to the point where fusion could be that simulation is, in the words of preeminent 1156 High Street, ignited. This process would have its own unique physicist and Nobel laureate Ken Wilson, the Santa Cruz signature distinguishable from other supernovae, third paradigm of science. Ramirez-Ruiz is California 95064, USA which the researchers will be watching out for in deeply committed to providing opportunities future surveys. for the astrophysicists of the future: “In the T +1 831 459 3400 end, I hope my greatest contribution to E [email protected] science is not any particular discovery but the STELLAR STUDENTS creation of a new way of thinking that will www.ucolick.org/~enrico/home.html In addition to his work as a researcher, enable a great multitude of students to make Ramirez-Ruiz places strong emphasis on his their own discoveries,” he concludes. ENRICO RAMIREZ-RUIZ is Professor of Astronomy and Astrophysics at the University of California, Santa Cruz (UCSC). He is also Director of the Theoretical Astrophysics Santa Cruz Institute and Sophie and Visiting Professor at the Niels Bohr Institute. After studying at the University of Cambridge, UK, he became the John Bahcall Fellow at the Institute for Advanced Study in Princeton. Since joining the UCSC faculty in 2007, Ramirez- Ruiz has won a number of national awards for his research, including a Packard Fellowship for Science and Engineering and the National Science Foundation CAREER Award. He is also a highly regarded teacher and public lecturer. In 2010, he was elected a member of the Mexican Academy of Sciences. Then aged 34, he was the youngest scientist ever elected to the academy.

The merger of two neutron stars resulting, after a few milliseconds, in a spinning black hole orbited by a torus of neutron-density matter. WWW.RESEARCHMEDIA.EU 73