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University of Southern California

Dr. Geraldine Peters

Research Professor
Building: SHS 271
Mail Code: 1341
Department: Space Sciences Center
Division: LAS
Telephone: (213) 740-6336
Fax: (213) 740-6342
Email: gjpeters@mucen.usc.edu
Updated: Nov 02, 2015

Dr. Peters


   Geraldine (Gerrie) Peters specializes in stellar astrophysics. Her research is focused on the early-type stars that is supported by several grants from NASA and HST/STScI. Most of her studies have been of B stars, which have surface temperatures that are 2-5 times that of our sun and masses of ∼3-15 Msun. She studies B stars that are single and those that are part of a close binary system. Her long-term interest has been in the circumstellar (CS) material about these stars and in determining the causes for their mass loss, or in the case of the close binaries the physics of the ongoing mass transfer. The latter is important for understanding the evolution of massive stars, since most of these objects are formed in binary or multiple star systems, and it has become quite clear during the past two decades that accreting B stars lose a substantial amount of mass before their cores end up as a compact object such as a white dwarf, neutron star, or black hole.


   Recently Peters has been devoting much of her research time to determining the abundances of the Fe group elements in B stars in the nearby field, close OB associations, and the Magellanic Cloud galaxies that reside just below our galaxy. This endeavor in fundamental stellar astrophysics is providing for the first time reliable stellar abundances for Ti, V, Cr, Mn, Fe, Co, and Ni, which are important for computing opacities for stellar evolution calculations and for inferring the rate of supernova activity in various locations in our galaxy and the Magellanic Clouds. It is through supernova explosions that heavy elements, that are synthesized in the cores of massive stars, are delivered to the interstellar medium (ISM) and provide enriched material for a new-generation stars. This is a marker for the chemical evolution of our galaxy. Whereas the light elements are delivered to the ISM by SNe II, the Fe group elements are believed to come from mostly low/intermediate mass binaries containing white dwarfs that undergo SNe Ia explosions. A single SNe Ia can deliver 0.5 solar masses of pure Fe (and perhaps Mn) to the ISM compared with about 0.07 solar masses from a SN II. The older stellar populations were formed from an ISM that was enriched primarily by SNe II, but the Fe group elements in B stars probably owe their origin mostly to SN Ia. In collaboration with T. Lanz (Observatoire de la Coté d’Azur) and several co-Is, Peters was recently awarded 35 orbits of time on the Hubble Space Telescope to study the chemical yields in the low metallicity environments of the Magellanic Clouds.


   Peters is continuing a long-standing study of the detailed physics of mass transfer in interacting binaries with B-A type primaries and a Roche-lobe filling cooler (A-G) star. That mass transfer exists in close binaries has been know for more than 50 years, but details of the dynamical process remains uncertain. The physics of mass transfer is deduced from observations, mostly those in the UV spectral region. In collaboration with several colleagues Peters has confirmed the presences of an accretion hot spot of ~100,000 K in the close binary U Cep from FUSE observations. The dynamics of mass transfer in Algol binaries located in the field-of-view of the Kepler/K2 spacecraft is being studied in collaboration with R. E. Wilson (Univ. of Florida) and T. J. Vaccaro (St. Cloud State Univ.) Continuous photometric data taken at the long cadence (30 min) are being analyzed with the 2015 version of the Wilson-Devinney (WD) program that includes the treatment of both hot accretion and cool magnetic spots on either star that can migrate or change with time. These observations have been supplemented with ground-based spectroscopy obtained with the 4m Mayall Telescope at Kitt Peak National Observatory that provide needed radial velocity and spectral type information. Four years of Kepler observations revealed a class of Algol-type binaries in which the relative brightness of the quadrature light varies from > 1 to <1 on a time scale of about 100 to more than 1400 days. Flux reversals can happen quickly, within one month or less. The behavior pattern is quasi-periodic. We call these systems L/T (leading hemisphere/ trailing hemisphere) variables. Although L/T inequality in eclipsing binaries has been noted from ground-based photometry by several observers since the early 1950s, the regular or quasi-regular switching between maxima is new. Twenty L/T systems have so far been found in the Kepler/K2 databases and at least three classes of L/T behavior have been identified. The Kepler/K2 photometry allows us to characterize the variability and derive systemic and star spot parameters for these systems. The prototype L/T variable is WX Draconis (A8V + K0IV, P=1.80 d) which shows L/ T light variations of 2-3%. Preliminary analysis of the WX Dra data suggests that the L/T variability can be fit with either an accretion hot spot on the primary (T = 2.3 T_phot) that jumps in longitude or a magnetic cool spotted region on the secondary. If the latter model is correct the dark region must occupy about 20-40% of the surface of the facing hemisphere of the secondary. In both hot and cool spot scenarios magnetic fields must play a role in the activity.


   In collaboration with D. Gies, Georgia State University, Peters is continuing an investigation of Be-type interacting binaries whose secondaries might be O-type subdwarfs. These hot subdwarfs are the stripped-down remnants of stars that were originally more massive than the present primary star. Presumably the mass transfer process added angular momentum to the mass gainer (typically a very rapid rotator of ∼0.8 Vcrit ) which then became a Be star (it is believed that rapid rotation is important in producing the Be phenomenon). We confirmed from HST and IUE data that one long-period (126d) system, φ Per, indeed has an O subdwarf companion (Gies et al. 1998, ApJ, 493, 440). Later we combined spectra of the Be star FY CMa taken with the IUE spacecraft and KPNO Coudé Feed Telescope over the course of 15-20 years to confirm that this star is also a binary system with an O subdwarf companion (Peters, et al. 2008, ApJ, 686, 1280). In a similar study we further confirmed that the well-known bright Be star 59 Cyg also harbors an sd-O secondary (Peters, et al. 2013, ApJ, 765, A2).

    website last updated 11/02/2015