As a class, compact objects consist of white dwarfs, neutron stars, and black holes. As the endpoint states of normal stellar evolution, they represent fundamental constituents of the physical Universe. The most telling observations of compact objects come from dense stellar systems, where stars are crowded close enough to each other to undergo frequent interactions. This interplay between compact objects and other stars yields observations which when compared to simulations provides firm conclusions about the properties of these extreme forms of matter. Under this award, Dr. Enrico Ramirez-Ruiz (University of California - Santa Cruz, UCSC) will build complete models for a wide variety of stellar encounters. Only through full-scale simulations will come the ability to critically test our understanding of the physics of stellar encounters and their aftermath, with a direct comparison with observations. A comprehensive model database of transient events will be developed and disseminated to the astronomical community. This aspect of the work is particularly important as astronomy moves toward an era of celestial cinematography where the entire accessible sky is imaged to unprecedented depth, with near-real-time detection and classification of variable sources. The prediction and, hence, understanding of the observable characteristics of stellar encounters is one of the main goals of this modeling program.
As a part of this project, Dr. Ramirez-Ruiz will join forces with the faculty, and staff at Hartnell Community College to effectively identify potential transfer students who are promising researchers at the beginning of their careers and also members of groups who are traditionally underrepresented in the University of California. Key components of the program include intensive research instruction through summer internships, academic counseling and mentoring, and workshops that integrate family members. The selected summer interns will work closely with Dr. Ramirez-Ruiz on a research project and also be integrated into one of the many existing student support programs at UCSC, such as the California Alliance for Minority Participation program. This will provide the interns with a student community as well as the opportunity to develop additional skills to advance their education and career. Family involvement in the program is considered essential. For this reason, as part of the public outreach effort, Dr. Ramirez-Ruiz will develop a Family Astronomy Night lecture series. The value of the lecture series will be twofold. First, it will communicate aspects of the proposed research and the thrill of discovery to a diverse group of community college students and their parents. Second, it will help ensure that parents have the information needed to support their child's academic success.
My research over the duration of the grant has mainly focused on the hydrodynamics of tidal encounters, mass transfer, and collisions between astrophysical bodies, especially those involving white dwarfs, neutrons stars and black holes, and the luminous transient signatures produced by these events. Black holes, neutron stars and white dwarfs present extreme forms of matter that cannot be created in terrestrial laboratories. Instead, we have to observe and analyze the experiments that are ongoing in the Universe to draw firm conclusions about the properties of these extreme forms of matter, through comparisons with simulations. The art of modeling stellar encounters through computer simulations forms the main theme of my research program. Compact objects can generate copious gravitational waves when they collide and merge. Because of this, simulations of such encounters play an essential role in the ongoing efforts at detections of gravitational waves. In general, all these phenomena, from extreme matter to event horizons and gravitational waves, cannot be created in a laboratory. Instead, we have no choice but to create virtual laboratories on Earth, in order to simulate all the relevant physics in large-scale computational experiments. This requires a realistic treatment of nuclear physics, radiation transport, general relativity and hydrodynamics. None of these treatments form part of the standard tool set of stellar evolution or stellar hydrodynamics, which have been well developed. The hydrodynamics of stellar collisions and tidal interactions in comparison, is much less developed. Over the past few years I have developed a research program in this area, pushing the frontiers of this exciting new field. Throughout this grant, I have been interested in constructing complete first principle models for a wide variety of explosive phenomena. For certain astrophysicists, including myself, crossing several disciplines (stellar evolution, nuclear physics, general relativity, observational astronomy, particle physics, and computations) while challenging, brings profound insights. It is rewarding to see that many of my main contributions to the field have grown out of such cross-discipline efforts. Some of the questions which have occupied my thoughts and time are: What is the identity of the progenitors of gamma-ray bursts, type Ia supernova, tidal disruption flares and other exotic transients? What is the nature of the triggering mechanism, the transport of the energy and the timescales involved? And, if so, can we decide between the various alternative ways of producing the explosion? Consequently, a large body of my theoretical work has been directed towards describing the possible formation channels for these systems, evaluating those which are likely to produce a viable central engine, and trying to determine how the progenitor and the environment can affect their observable characteristics. During the duration of the grant I have produced over 60 refereed publications on the subject and supervised 12 undergraduate stduents and 7 graduate students. During the review period, I have also been engaged in an ambitious education and public outreach (EPO) program in the Salinas Valley of California in collaboration with educators at Hartnell Community College. The principal goal of my EPO program, is to increase the retention and graduation rates for Hispanics already enrolled in college through inquiry-based learning of concepts related to astronomy, and aspects of the proposed research. Too often, effective advising and constructive encouragement for students to pursue an academic major and to transfer to a four- year college are lacking. Key components of the program include intensive research instruction through summer internships, academic counseling and mentoring, and workshops that integrate family members. Inspired by F'elix Galaviz's efforts more than two decades ago at a community college in the San Francisco Bay Area to serve mainly a Hispanic-student clientele, early last year I started the Lamat Program with the financial help of a UCSC Diversity Fund Fellowship. The Lamat (which means star in Mayan) program is a collaboration between the Department of Astronomy and Astrophysics, CfAO, and Hartnell Community College, which I hope to transform into a long lasting and healthy EPO program with the help of my NSF CAREER grant. Hartnell Community College is a federally designated Hispanic Serving Institution with a 72% minority student body. Hispanics compromise 60% of the total, and non-native English speakers make up 44% of the students. Among minority student population, 82% are first generation college students. The Lamat initiative is fundamentally an academic enhancement and enrichment program that has three components: an intensive summer internship, a mentoring program, and a public outreach program. The core aspects of the program are to develop academic skills in physics and astronomy and to prepare students at the undergraduate level for careers in engineering and science. During the review period, I have advised twelve Hartnell students over the summer.
