The members of the Astronomy and Astrophysics group are among the leaders of their chosen areas of research. We are one of the largest astrophysics groups within a physics department in the country. The research conducted in our group is interwoven and dynamic, with five focus areas that are complementary to each other.
Supernovae. Supernovae are the explosions of dying stars. Our group is among the top few internationally in supernova research. Baron and Branch are interested in understanding the systematics of how supernovae explode and what kinds of stars lead to what type of supernovae. Baron uses supernovae as a ``laboratory" to study the properties of extremely dense matter, neutrino physics, and the initial conditions of progenitor stars. Baron and Branch study the spectra of the expanding supernova atmosphere to determine physical conditions and chemical abundances in the ejecta. Romanishin observes the brightness of a supernova as a function of time, which gives clues to the type of star that exploded, its distance, and the amount of radioactive nickel that was produced in the explosion. Cowan studies the expanding remnants of supernova explosions many years after the explosion has occurred by using the radio emission given off by the expanding shell. Henry studies the chemical composition of such supernova remnants. objects which represent the remnants of the very supernova explosions. We are setting up a ``supernova spectrum repository'' -- a website at which any astronomer can view all of the supernova spectra that we gather from observers, and from which any of the SN spectrum data files can be extracted in a common format. This will make us the ``headquarters'' of supernova spectra.
Cosmology. The cosmological research in our group is anchored in observational data, and aims at gaining a deep understanding of our universe. Wang's research focuses on using various independent cosmological data sets (cosmic microwave background anisotropy, galaxy redshift surveys, supernovae) to determine the cosmological parameters that describe our universe, to probe the physics of the very early universe, and to constrain the dark energy in the universe. Baron and Branch study the use of supernovae as distance indicators to remote galaxies in order to determine the age, size, and fate of the universe. Wang, Baron, and Branch study the systematic uncertainties of supernovae as cosmological probes.
Extragalactic Astronomy. Cowan studies properties of radio emitting galaxies such as our own, which may contain a black hole at its center. Henry studies the distribution of chemical elements in spiral galaxies such as our own, in order to study their origin and evolution. Leighly studies Active Galactic Nuclei (AGN). The ultimate power source for AGNs is thought to be accretion onto a supermassive blackhole. Her extensive program involves both observations in the X-ray and theoretical modeling. Romanishin studies various types of AGN as well, including quasars- the most powerful objects in the universe.
Nucleosynthesis. Henry studies the chemical abundances of a variety of emission line objects with the goal of understanding stellar production rates and subsequent cosmic accumulation of elements such as C, N, O, Ne, S, and Ar. Cowan studies cosmochronology and the chemical evolution of the Galaxy. He uses the history of radioactive elements in the oldest stars to study the age of our galaxy and to set limits on the age of the universe.
Observational Astronomy. Romanishin studies the Solar System, in addition to AGN and quasars. He is involved in a program to obtain colors and other photometric properties for faint minor bodies in the outer solar system, including Kuiper Belt Objects and irregular satellites of the Jovian planets. Leighly has an extensive background in techniques and methodology of X-ray observational astronomy. She has been awarded observational time on state of the art X-ray satellites. She has recently expanded her observational work to the optical as well. Cowan conducts observational studies of supernovae and supernova remnants. Branch and Baron are on key observational teams of supernovae. On the basis of scientific merit, our astronomers have significant access to the major ground and space observatories, including the Hubble Space Telescope and the Keck. We use our campus computer-controlled 16 inch (0.4 meter) telescope for student training and certain research projects.
Astronomy continues to be an exciting field with new ideas and new facilities emerging in the dawning year of the new millennium. We welcome the chance to work with motivated and qualified graduate students.
Edward A. BaronI am interested in the physics of supernova explosions, as well as properties of neutron stars and galaxy formation. My research focuses on carrying out detailed theoretical models of the formation of supernovae, the nucleosynthesis that occurs in the supernova explosion, and the transport of radiation in the fast-moving supernova atmosphere. The tools of this research are detailed numerical calculations of both hydrodynamic and radiation transport,as well as detailed models of the nuclear, atomic, and weak-interaction physics that is needed as input for these calculations. Primarily I am interested in understanding the detailed systematics of how a supernova works, what types of stars lead to what types of supernovae, what is the variation in the energy of the explosion, and what are the characteristics of the object that is left behind. I am also interested in using supernovae as cosmological probes, to study the nature of the universe. Supernovae are fascinating systems to study, since all fields of physics are important to their understanding, and one is forever learning new things. E.Baron, D. Branch, P. H. Hauschildt, A. V. Filippenko, and R. P. Kirshner, Spectral Models of the Type Ic SN 1994I in M51, Ap. J., (1999), 527, 739-745.
Associate Professor
B.A. 1980 Pennsylvania
Ph.D. 1985 SUNY Stony Brook
E. J. Lentz, E.Baron, D. Branch, P. H. Hauschildt, and P. Nugent, Metallicity Effects in NLTE Model Atmospheres of Type Ia Supernovae Ap. J., 530, (2000), 966-976.
A. Schweitzer, P. H. Hauschildt, and E.Baron, Non-LTE Treatment of Molecules in the Photospheres of Cool Stars, Ap. J., , (2000), submitted.
K. Hatano, D. Branch, E. Lentz, E.Baron, A. V. Filippenko, and P. Garnavich, On the Spectroscopic Diversity of Type Ia Supernova, Ap. J., (2000), submitted.
E.Baron, D. Branch, et al., Preliminary Spectral Analysis of the Type II Supernova 1999em, Ap. J., (2000), in press.
E. J. Lentz, E.Baron, D. Branch, et al., Analysis of the Type IIn Supernova 1998S: Effect of Circumstellar Interaction on Observed Spectra, Ap. J., (2000), submitted.
E. J. Lentz, E.Baron, D. Branch, and P. H. Hauschildt, SN 1984A and Delayed Detonation Models of Type Ia Supernovae Ap. J. (Letters), (2000), submitted.
P. H. Hauschildt and E.Baron, Numerical Solution of the Expanding Atmosphere Problem, Jour. of Computational and Applied Mathematics, 109, (1999) 41-63.
David R. Branch
George Lynn Cross Research Professor
B.S. 1964 Rensselaer
Ph.D. 1969 Maryland
I am working on the interpretation of the spectroscopic, photometric, and statistical properties of supernovae. One goal is to learn how to infer the physical conditions of the ejected matter - the temperature, density, velocity, chemical composition, and mass. By comparing this information with the predictions of theoretical explosion models, we try to find out which kinds of stars produce the various observed supernova types, and how they explode. A related goal is to use supernovae as distance indicators, to measure the expansion rate (Hubble constant), geometry, and expansion rate of the universe.
D. Branch, D. J. Jeffery, M. Blaylock, & K. Hatano, ``Supernova Resonance-Scattering Profiles in the Presence of External Illumination", Publications of the Astronomical Society of the Pacific 112, 217 (2000)
K. Hatano, D. Branch, A. Fisher, J. Millard, & E. Baron, ``Ion Signatures in Supernova Spectra", Astrophysical Journal Supplement 121, 233 (1999)
D. Branch,``Type Ia Supernovae and the Hubble Constant", Annual Review of Astronomy and Astrophysics 36, 17 (1998).
K. Hatano, D. Branch, A. Fisher, & S. Starrfield, ``New Insight into the Spatial Distribution of Novae in M31", Astrophysical Journal 487, L45 (1997).
John J. Cowan
Professor
B.A. 1970 George Washington
Ph.D. 1976 Maryland
I continued my theoretical studies in nucleosynthesis, cosmochronology and the chemical evolution of the Galaxy. Working with one of my students, Debra Burris, we have used data from the Hubble Space Telescope, the Keck Telescope and Kitt Peak National Observatory to make a number of studies of the the formation of heavy elements in explosive environments such as supernovae and how these abundances have changed with time throughout the history of the Galaxy. Also, by comparing the observed and predicted abundances of the long-lived radioactive element in the oldest stars, we are determining the age of the Galaxy, and setting limits on the age of the universe.
I also continued my observational studies of supernovae and supernova remnants. Working with Chris Eck, another student, we have used the Very Large Array (VLA) radio telescopes to detect radio emission from decades-old, extragalactic supernovae, such as SN 1923A in the galaxy M83. This study of supernovae led to a project with another student, Chris Stockdale. We have used the ROSAT satellite to identify X-ray emission coming from the center of the galaxy NGC 7331. This X-ray source is associated with a previously detected central radio source, and may indicate the presence of a massive black hole (MBH) at the center of this galaxy.
C. Stockdale, W. Romanishin and J. J. Cowan, ``Discovery of a Nuclear X-ray Source in NGC 7331: Evidence for a Massive Black Hole,'' Astrophys. J. Letters, 508, L33 (1998)
J. J. Cowan, B. Pfeiffer, K.-L. Kratz, F.-K. Thielemann, C. Sneden, S. Burles, D. Tytler and T. C. Beers, ``R-Process Abundances and Chronometers in Metal-Poor Stars,'' Astrophys. J., 521, 194 (1999)
J. Cowan, ``Supernova Birth for a Black Hole,'' Nature, 401, 124 (1999)
D. L. Burris, C. A. Pilachowski, T. Armandroff, C. Sneden, J. J. Cowan, and H. Roe, ``Neutron-Capture Elements in the Early Galaxy: Insights from a Large Sample of Metal-Poor Giants,'' Astrophys. J., in press (2000)
J. Westin, C. Sneden, B. Gustafsson and J. J. Cowan, ``The r-Process Enriched Low-Metallicity Giant HD 115444,'' Astrophys. J., 530, 783 (2000)
C. Sneden, J. J. Cowan, I. I. Ivans, G. Fuller, S. Burles, T. C. Beers, and J. E. Lawler, ``Evidence of Multiple r-Process Sites in the Early Galaxy: New Observations of CS 22892-052,'' Astrophys. J. Letters, 533, L139 (2000)
C. Sneden, G. H. Smith, J. Johnson, R. P. Kraft, J. J. Cowan, and M. S. Bolte, ``Neutron-Capture Element Abundances in the Globular Cluster M15,'' Astrophys. J. Letters, 536, L85 (2000)
Richard C. Henry
Professor
B.A. 1977 Kansas
Ph.D. 1983 Michigan
My research emphasis is the study of the chemical evolution of spiral galaxies. Currently, I am most interested in the abundances of carbon, nitrogen, neon, sulfur, and argon, all as a function of time within spirals. The probes used for this work are planetary nebulae and HII regions. One focus of the planetary nebula studies is the determination of carbon and nitrogen abundances in planetaries, as this provides useful information for assessing the contributions that PN progenitor stars make to the galactic evolution of these two elements. Another focus involves the use of planetary nebulae as abundance probes to map the galactic distribution of elements such as Ne, Ar, and S, elements which are not processed by PN progenitors. Collaborators in these areas include Karen Kwitter (Williams College), Bruce Balick (U. of Washington), and Jackie Milingo (Gettysburg College). Projects using extragalactic H II regions as probes are intended to study heavy element distributions in external galaxies. On this topic work with Mike Edmunds (Cardiff University) is designed to identify and evaluate the important cosmic synthesis sites of the elements carbon and nitrogen, both of which may be forged in intermediate-mass as well as massive stars.
R.B.C. Henry and M.G. Edmunds, On The Cosmic Origins of Carbon and Nitrogen, Astrophysical Journal, Sept. 20 issue (2000).
R.B.C. Henry, K.B Kwitter, and J.A. Bates A New Look At Carbon Abundances In Planetary Nebulae IV: Implications For Stellar Nucleosynthesis, Astrophysical Journal 531, 928 (1998).
R.B.C. Henry and Guy Worthey, The Distribution of Heavy Elements in Spiral and Elliptical Galaxies, Invited Review, Publications of the Astronomical Society of the Pacific, 111, 919 (1999).
R.B.C. Henry, K.B. Kwitter, and R.J. Dufour Morphology and Composition of the Helix Nebula, Astrophysical Journal, 517, 782 (1999).
Karen Leighly
Assistant Professor
B.S. 1983 NMIMT
Ph.D. 1991 Montana State University
Active Galactic Nuclei (AGN), recognized by their intense emission that vastly outshines the light of the stars in their host galaxy, are the most luminous persistently emitting object in the universe. My research has focused on a fundamental problem: I seek to understand the origin of their intense emission.
AGN are nearly universally believed to be powered by mass accretion onto a supermassive black hole. AGN emit nearly the same power per logarithmic energy band from infrared to X-rays, and observations at all wavelengths are valuable. My research has primarily focused on X-ray observations since both theoretical and observational evidence indicates that most of the X-rays are emitted very close to the black hole event horizon and therefore the X-rays tell us the most about the central engine of the AGN.
Recently, my work has focused on a subset of AGN known as Narrow-line Seyfert 1 galaxies (NLS1s). These objects were recently recognized to have X-ray emission properties distinct from those of ordinary AGN. The most promising explanation to date for their characteristic behavior is that NLS1s have a higher rate of accretion for their black hole mass compared with ordinary AGN. Since all AGN are thought to be powered by accretion, study of NLS1s, characterized by extremely large rates of accretion, may help us understand AGN emission in general.
One of the interesting things that we have found is that correlations among the X-ray spectral and variability properties of NLS1s exist. On one end of the correlations, typified by very high amplitude X-ray variability, are ``extreme'' NLS1s. Recently we have discovered that the UV spectroscopic properties of these extreme NLS1s are also characteristic. The observations suggest that in these objects the high-ionization UV emission lines are dominated by emission from an outflow from the nucleus. We postulate that the extreme NLS1s have the highest accretion rates among AGN.
K. M. Leighly and J. P. Halpern, ``HST STIS Ultraviolet Spectral Evidence of Outflow in Extreme Narrow-line Seyfert 1 Galaxies'', The Astrophysical Journal, submitted.
D. Grupe, K. M. Leighly, H.-C. Thomas and S. A. Laurent-Muehleisen, ``The enigmatic soft X-ray AGN RX J0134.2-4258'', Astronomy & Astrophysics 356 11 (2000)
K. M. Leighly, ``A Comprehensive Spectral and Variability Study of Narrow-line Seyfert 1 Galaxies observed by ASCA. I. Observations and Time Series Analysis'', The Astrophysical Journal Supplement Series 125 297 (1999)
K. M. Leighly, ``A Comprehensive Spectral and Variability Study of Narrow-line Seyfert 1 Galaxies observed by ASCA. II. Spectral Analysis and Correlations'', The Astrophysical Journal Supplement Series 125 317 (1999)
K. M. Leighly, J. P. Halpern, H. Awaki, M. Cappi, S. Ueno and J. Siebert, An RXTE Observation of NGC 6300: A New, Bright, Compton Reflection-dominated Seyfert 2 Galaxy'', The Astrophysical Journal 522 209 (1999)
William J. Romanishin
Associate Professor
B.S. 1974 Harvard
Ph.D. 1980 Arizona
My research involves the application of optical CCD imaging of astronomical objects using various large and small telescope, along with associated image processing techniques, to a variety of astronomical topics.
Currently, my main topics of interest are: studies of the colors and other photometric properties of minor bodies in the outer solar system, including Kuiper Belt Objects and irregular satellites of the Jovian planets; the accurate measurement of the brightness of active nuclei in active galaxies, particularly those with low luminosity nuclei, where it is difficult to disentangle the light of the nucleus from the light of the host galaxy; study of photometric and astrometric properties of bright asteroids (using the campus telescope); and ``target of opportunity" observations of supernovae, using telescope time scheduled for other projects, as well as the campus telescope. A common theme of these projects is to obtain accurate measurements of the observed brightnesses of various astronomical objects, frequently in the presence of contaminating background or foreground light sources.
S. C. Tegler and W. Romanishin, ``Red Snowballs on the Ragged Edge of the Solar System,'' NATURE , submitted July 2000.
W. Romanishin and S. C. Tegler ``Rotation Rates of Kuiper-belt Objects from Their Light Curves,'' NATURE 398, 129 (1999).
S. C. Tegler and W. Romanishin, ``Two Distinct Populations of Kuiper Belt Objects,'' NATURE 392, 49 (1998).
W. Romanishin, S.C. Tegler, J. Levine, and N. Butler, ``BVR Photometry of Centaur Objects 1995 GO and 1993 HA2", Astronomical Journal 113, 1893 (1997).
W. Romanishin, T. J. Balonek, R. Ciardullo, H. R. Miller, B. M. Peterson, ``The Galaxy Component and Nuclear Flux Measurements of NGC 5548 from Direct Imaging", Astrophysical Journal 455, 516 (1995).
Yun Wang
Assistant Professor
B.S. 1985 Tsinghua Univ., P.R. China
Ph.D. 1991 Carnegie-Mellon
I am a theoretical cosmologist. My research is focused on using various independent cosmological data sets to gain a deep understanding of our universe. I have engaged myself in this through three complementary projects:
Cosmic microwave background anisotropy and large scale structure as probes of cosmology. The cosmic microwave background anisotropies (CMB) are signatures of the primordial seeds (matter density fluctuations) imprinted when photons decoupled from matter. The large scale structure in the distribution of galaxies is a direct consequence of the power spectrum of the primordial density fluctuations. I have explored how we can use the upcoming CMB data from the Microwave Anisotropy Probe (MAP) and the large scale structure data from the Sloan Digital Sky Survey (SDSS) to accurately measure the basic cosmological parameters, and the spectrum of primordial density fluctuations. My proposal of parametrizing the primordial power spectrum in a model-independent way will enable us to probe physics in the early universe, and to reliably extract the cosmological parameters simultaneously. The main question I will address in my future research is, how well can we reliably probe the early universe physics (in particular, inflation and phase transitions)?
Type Ia supernovae as probe of cosmology. Supernovae are our best candidates for cosmological distance indicators, they provide a unique probe of the dark energy in the universe (the nature of most of the energy in the universe is unknown). I have studied how we can more reliably and efficiently use type Ia supernovae as cosmological distance indicators to measure the cosmological parameters and to constrain the dark energy in the universe. My goal is to establish the potentials and limitations of the use of SNe Ia in cosmology.
Gravitational lensing as probe of cosmology. The images of background galaxies can be distorted by foreground distributions of mass into coherent arclets via gravitational lensing. This weak lensing of galaxies is a powerful tool for mapping the mass distribution in the universe. I am interested in using the weak lensing of galaxies to constrain fundamental cosmological models.
I believe that our understanding of the universe will be significantly enhanced in the next several years. I am very excited to be an active participant in the very dynamic field of cosmology.
Yun Wang and Peter Garnavich, ``Measuring Time-Dependence of Dark Energy Density from Type Ia Supernova Data'', ApJ submitted (2000).
Yun Wang, ``Flux-averaging Analysis of Type Ia Supernova Data'', ApJ 536, 531 (2000).
Yun Wang, ``Supernova Pencil Beam Survey'', ApJ 531, 676 (2000).
Yun Wang, D. N. Spergel, and M. Strauss, ``Cosmology in the Next Millennium: Combining MAP and SDSS Data to Constrain Inflationary Models'', ApJ, 510, 20 (1999).