Stellar Astrophysics at Penn State

Introduction

At Penn State, the study of stars and their immediate surroundings includes stars of all ages, from pre-main sequence stars to stellar remnants. It also includes a wide range of interesting topics, such as accretion in pre-main sequence and cataclysmic variable star disks, asteroseismology, magnetic activity, and planet detection, to name a few. Our observational data span the range from x-ray to radio wavelengths.

Professors Robin Ciardullo, Eric Feigelson, John Nousek, Larry Ramsey, Richard Wade, and Alex Wolszczan, are the department members currently active in stellar research. The following sections provide brief overviews of their research efforts. For more information please follow the hypertext links to their individual pages.

Pre-Main Sequence Stars

Feigelson and colleagues have been leaders in the study of magnetic activity of low-mass pre-main sequence stars. For example, he recently participated in a spectroscopic and photometric study of several dozen X-ray luminous stars identified in his ROSAT observations of the nearby Camaeleon I star forming cloud. This effort has produced one of the largest and most complete samples of T Tauri stars available, with 80 of 117 cloud members placed on the HR diagram, and to insights into the population and evolution of low-mass PMS stars.

Feigelson and graduate student Lee Carkner has also led a collaboration with colleagues at Caltech, Tokyo, and Saclay to study X-ray emitting T Tauri stars in the L1551 cloud. They used the ROSAT and ASCA satellites and detected 38 X-ray sources, including several new WTTS detections. They observed high amplitude variability in the young CTTS XZ Tau, and a powerful flare in the WTTS V826 Tau.

Ramsey and Wolszczan plan to use the power and unique capabilities of the Hobby*Eberly Telescope to search for low-mass companions orbiting PMS stars. They will use precise radial velocity measurements to detect the stellar motion due to the unseen companions. They will also monitor systems known to be binary from various imaging and lunar occultation surveys. Accurate determination of masses of these stars is a major goal.

Flare Stars

AD Leo, a single dM4e flare star, is a popular target for activity studies because of its frequent flares. Nousek used ASCA to observe AD Leo for a full day on 1993 May 24 in hopes of detecting a flare, and obtained simultaneous ground observations. Although no optical flare and no X-ray variation by more than a factor of two was seen during 24,000 seconds of observing, the ASCA data allow us to make the best available X-ray spectra of a flare star.

The total spectrum can be explained by a combination of two Raymond-Smith coronal plasmas with temperatures of 0.3 and 0.8 keV. If the data are divided into halves, according to the brightness of the star, the fainter data are consistent with the same low temperature component as the brighter data (kT=0.26 +- 0.02 keV), but the high temperature component increases from 0.83+-0.02 keV to 1.0+-0.02 keV. We interpret this result as indicating that even without dramatic flaring the high temperature plasmas are significantly variable. Hence even flare star `quiescent' intervals are merely periods of reduced micro-flaring activity.

Evolved Active Stars

Ramsey has been studying the peculiar rapidly rotating giant FK Com. Their ROSAT PSPC observations indicated an X-ray flare, but temporal coverage was insufficient to allow detailed study. In a series of 51 FOE spectra from 1989 they discovered a periodic variation of linewidth, which they interpreted as evidence for non-radial pulsations. Other less extensive FOE datasets reveal different patterns of linewidth variation. In 1994 May they obtained 403 FOE spectra to study this phenomenon in greater detail. Analysis is ongoing.

Ramsey and his graduate students and research associates have accumulated a database of several thousand FOE spectra of active stars (RS CVn & W UMa systems, FK Com-type stars, & PMS stars) since 1987. Dozens of journal articles and several Ph.D. theses based on those data have not exhausted the potential of those data.
Contact Ramsey (lwr@astro.psu.edu) or Welty (welty@stsci.edu) to inquire about access to the database.

Interacting Binary Stars

Wade and collaborators carried out a study of the ultraviolet spectra of accretion disks in cataclysmic variables, with emphasis on limb darkening. Limb darkening has a strong effect on the spectra of flat objects such as accretion disks, since no averaging over emergent angles occurs for disks (unlike stars). As with stars, disk limb darkening is especially important in the UV, and at a given wavelength is more important for low temperature atmospheres. They studied the effect in 925-1750 Å spectra of accretion disks in cataclysmic variables, using a grid of models and spectra of disk atmospheres. UV surface brightness distributions show the expected temperature and wavelength dependences. At fixed effective temperature and wavelength, the behavior of limb darkening is similar in stars and in the disks studied (which are optically thick). Limb darkening correction factors for the integrated disk brightness at 1448 Å were derived for the grid, a large representative set of disk parameters and orbital inclinations.

Disk line profiles, which show strong kinematic broadening, are also highly dependent on the view angle (i.e., inclination). Wade's team computed these effects both for full disks and for disks in which the inner part of the disk (close to the white dwarf) is artificially removed. In the latter case, there are noticeable changes in the shapes of lines from highly ionized species and in the ratio of lines formed at high and low temperature, so high quality line profiles obtained in the UV may serve as a diagnostic of the structure of the disk. This would be a valuable check on the suggestion that in some cataclysmic variables the inner disk is in fact missing.

While there are many difficulties and traps remaining in the interpretation of accretion disk spectra, the art of modeling the structure of disks and the radiative transfer within them has advanced to the stage where reliance on blackbodies and stellar atmospheres can in many cases be abandoned.

Eracleous and collaborators have been studying the periodic oscillations and large aperiodic flares of the cataclysmic variable AE Aquarii using ultraviolet spectra obtained with the HST. AE Aquarii harbors the most rapidly spinning white dwarf known in any cataclysmic variable and its flares are unlike any other flaring phenomena observed in cataclysmic variables. Theory suggests that the flares are the result of the interaction of a feeble accretion flow with the magnetosphere of the rapidly-rotating white dwarf which acts as a magnetic propeller, expelling from the system most of the matter transferred from the secondary star. The HST observations support this scenario and allow an estimate of the mass transfer rate, hence permitting us to verify the energy budget of the system. The same data also show that the white dwarf is an ultraviolet pulsar, the pulsed light most likely originating in the magnetic polar caps which are heated by X-rays from the accretion columns. AE Aquarii represents the missing link in an evolutionary scenario which calls for the rapid loss of angular momentum from the white dwarf via the expulsion of matter from the system.

More recently Eracleous and collaborators have embarked on a search for an accretion-disk wind from the cataclysmic variable DQ Herculis. The magnetic white dwarf of DQ Herculis interrupts the flow in the accretion disk at a distance of a few white dwarf radii from the center and prevents the formation of a disk-white dwarf boundary layer. Thus, the detection of an accretion disk wind in such a system is an important test of models of wind formation some of which require a hot boundary layer in order to accelerate a wind, while others suggest that the entire disk participates in this process. HST spectroscopy of DQ Herculis through eclipse has shown that a wind is present, favoring the latter class of models.

Pulsars

Wolszczan has completed timing analysis of four years of pulse arrival time data from the planets pulsar, PSR B1257+12. Currently the best timing model for this pulsar includes three planets, gravitational perturbations between planets B and C, and a significant second derivative of the pulsar spin period. A need to include this parameter in the model is probably indicates a fourth, more distant planet in the system. Alternatively, timing noise due to the pulsar's low-level rotational instability may be responsible.

Wolszczan and collaborators have obtained reliable timing models for two extremely faint millisecond pulsars they discovered in the globular cluster M5 (NGC 5904) in 1989. Based on their extensive pulse arrival time data they conclude that one is a solitary 5.5-ms pulsar, and the other is a 7.9-ms pulsar in a 7-day binary system with a significant orbital eccentricity of 0.14. The measured orbit precession, if due to relativistic effects, constrains the companion's mass to be 0.45-5.5 M_sun, i.e., the companion may be a black hole.

A collaboration including Wolszczan detected X-ray emission from a 147-ms pulsar in ROSAT All-Sky Survey data. The pulsar is in the supernova remnant S147 (G180.0-1.7) recently discovered by the Penn State/NRL group. Preliminary analysis constrains its blackbody temperature to be about 0.1 keV, consistent with the pulsar spindown age of 6 x 105years.

Pavlov, Stringfellow, and Córdova analyzed HST Faint Object Camera observations of nearby radio pulsars. Each of the deep images of fields around pulsars PSR B0656+14, B1929+10, and B0950+08 obtained with a broad band UV filter contains only one point source whose position is compatible with the radio position of the pulsar. A comparison with ROSAT data and theoretical models shows that the radiation detected from the middle-age PSR B0656+14 is mainly of a nonthermal origin. The UV-optical radiation from the old pulsars PSR B1929+10 and B0950+08 probably is emitted from the neutron star surfaces, with temperatures 1-2 x 105 and 7±1 x 104 K, respectively.

Planetary Nebula Central Stars

Ciardullo has been using the Hubble Space Telescope to investigate the fundamental properties of planetary nebulae. By imaging over 100 PNe, identifying those with wide-binary companions, and measuring their photometric properties, Ciardullo has almost doubled the sample of Galactic PNe with known distances. These new data strongly suggest that the long-running controversy over the Galactic planetary nebula distance scale is due to selection effects in the calibration samples. The extraordinary HST images also are also facilitating investigations into many other areas of planetary nebula science.

Department Home Page Department Directory Faculty Publications 1998 Annual Report

Web page by Alan D. Welty (welty@stsci.edu)
Last update: 1996 September 9