1,102 research outputs found
HST Spectra of GW Librae: A Hot Pulsating White Dwarf in a Cataclysmic Variable
We have obtained Hubble Space Telescope UV spectra of the white dwarf in GW
Lib, the only known non-radially pulsating white dwarf in a cataclysmic
variable, and the first known DAZQ variable. The UV light curve reveals large
amplitude (10%) pulsationsin the UV with the same periods (646, 376 and 237 s)
as those seen at optical wavelengths, but the mean spectrum fits with an
average white dwarf temperature (14,700K for a 0.6M_{odot} white dwarf) that is
too hot to be in the normal instability strip for ZZ Ceti stars. A better fit
is achieved with a dual temperature model (with 63% of the white dwarf surface
at a temperature of 13300K and 37% at 17100K), and a higher mass (0.8M_{odot})
white dwarf with 0.1 solar metal abundance. Since the blue edge of the
instability strip moves to higher temperature with increasing mass, the lower
temperature of this model is within the instability strip. However, the
presence of accretion likely causes abundance and atmospheric temperature
differences in GW Lib compared to all known single white dwarf pulsators, and
the current models that have been capable of explaining ZZ Ceti stars may not
apply.Comment: 13 pages, 4 figure
White Dwarfs: Cosmological and Galactic Probes
The emphasis on white dwarf stars and cosmology arises from the most recent advances in cosmological and galactic structure research in which white dwarf stars are playing a very prominent role. Examples are Type Ia supernovae (i.e. white dwarf supernovae), the origin and evolution of the universe, the age of the galactic disk, cosmochronology using white dwarfs in globular clusters and galactic clusters, and the physics of accretion onto compact (very dense) stars. As an assisting guide to the reader, we have included, by invitation, comprehensive review articles in each of the four major areas of the book, white dwarf supernovae, cosmology, accretion physics and galactic structure. The reviews include introductory material that they build upon. The book is suitable and most useful to advanced undergraduates, graduate students and scientific professionals (e.g. astronomers, astrophysicists, cosmologists, physicists)
ST/STIS Spectroscopy of the White Dwarfs in the Short-Period Dwarf Novae LL And and EF Peg
We present new HST/STIS observations of the short-period dwarf novae LL And
and EF Peg during deep quiescence. We fit stellar models to the UV spectra and
use optical and IR observations to determine the physical parameters of the
whitedwarfs in the systems, the distances to the binaries, and the properties
of thesecondary stars. Both white dwarfs are relatively cool, having T_{eff}
near 15000K, and consistent with a mass of 0.6 M-sun. The white dwarf in LL And
appears to be of solar abundance or slightly lower while that in EF Peg is near
0.1-0.3 solar. LL And is found to be 760 pc away while EF Peg is closer at 380
pc. EF Peg appears to have an ~M5V secondary star, consistent with that
expected for its orbital period, while the secondary object in LL And remains a
mystery.Comment: Accepted in Ap
Development of a Critical Path Method Specification and a Training Program for use of CPM for KyTC
The Critical Path Method scheduling technique is outlined as it applies to KYTC projects. Special Provision 82 (94) and its requirements are outlined
Detection of Coronal Mass Ejections in V471 Tauri with the Hubble Space Telescope
V471 Tauri, an eclipsing system consisting of a hot DA white dwarf (WD) and a
dK2 companion in a 12.5-hour orbit, is the prototype of the pre-cataclysmic
binaries. The late-type component is magnetically active, due to its being
constrained to rotate synchronously with the short orbital period. During a
program of UV spectroscopy of V471 Tau, carried out with the Goddard High
Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope, we
serendipitously detected two episodes in which transient absorptions in the Si
III 1206 A resonance line appeared suddenly, on a timescale of <2 min. The
observations were taken in a narrow spectral region around Ly-alpha, and were
all obtained near the two quadratures of the binary orbit, i.e., at maximum
projected separation (~3.3 Rsun) of the WD and K star.
We suggest that these transient features arise when coronal mass ejections
(CME's) from the K2 dwarf pass across the line of sight to the WD. Estimates of
the velocities, densities, and masses of the events in V471 Tau are generally
consistent with the properties of solar CME's. Given our detection of 2 events
during 6.8 hr of GHRS observing, along with a consideration of the restricted
range of latitudes and longitudes on the K star's surface that can give rise to
trajectories passing in front of the WD as seen from Earth, we estimate that
the active V471 Tau dK star emits some 100-500 CME's per day, as compared to
1-3 per day for the Sun. The K dwarf's mass-loss rate associated with CME's is
at least (5-25) x 10^{-14} Msun/yr, but it may well be orders of magnitude
higher if most of the silicon is in ionization states other than Si III.Comment: 24 pages AASTeX, 4 figures. Accepted by Astrophysical Journa
Ultraviolet observations of LMC nova 1988
The IUE obtained ultraviolet spectra of a nova in an external galaxy. The spectral features do not seem unusual for a nova at maximum but it is hoped to be able to follow it for a long enough time to be able to study the high ionization lines that appear when the density drops to lower values (the nebular stage). A high dispersion spectrum was also obtained to assist in the line identification and to study the line of sight to the LMC 1 deg of arc away from SN 1987A
Composite Accretion Disk and White Dwarf Photosphere Analyses of the FUSE and HST Observations of EY Cygni
We explore the origin of FUSE and HST STIS far UV spectra of the dwarf nova,
EY Cyg, during its quiescence using \emph{combined} high gravity photosphere
and accretion disk models as well as model accretion belts. The best-fitting
single temperature white dwarf model to the FUSE plus HST STIS spectrum of EY
Cygni has TK, log , with an Si abundance of 0.1 x
solar and C abundance of 0.2 x solar but the distance is only 301 pc. The
best-fitting composite model consists of white dwarf with TK,
log , plus an accretion belt with TK covering 27% of
the white dwarf surface with V km/s. The accretion belt
contributes 63% of the FUV light and the cooler white dwarf latitudes
contribute 37%. This fit yields a distance of 351 pc which is within 100 pc of
our adopted distance of 450 pc. EY Cyg has very weak C {\sc iv} emission and
very strong N {\sc v} emission, which is atypical of the majority of dwarf
novae in quiescence. We also conducted a morphological study of the
surroundings of EY Cyg using direct imaging in narrow nebular filters from
ground-based telescopes. We report the possible detection of nebular material^M
associated with EY Cygni. Possible origins of the apparently large N {\scv}/C
{\sc iv} emission ratio are discussed in the context of nova explosions,
contamination of the secondary star and accretion of nova abundance-enriched
matter back to the white dwarf via the accretion disk or as a descendant of a
precursor binary that survived thermal timescale mass transfer. The scenario
involving pollution of the secondary by past novae may be supported by the
possible presence of a nova remnant-like nebula around EY Cyg.Comment: To appear in AJ, Oct. 2004. 5 figures, including 2 color ones (2D
pictures
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