89 research outputs found
The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Centauri and NGC 2808
Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch
(EHB) are found in the UV color-magnitude diagrams of omega Cen and NGC 2808.
Such stars are unexplained by canonical HB theory. In order to explore the
origin of these subluminous stars, we evolved a set of low-mass stars from the
main sequence through the helium-core flash to the HB for a wide range in the
mass loss along the red-giant branch (RGB). Stars with the largest mass loss
evolve off the RGB to high effective temperatures before igniting helium in
their cores. Our results indicate that the subluminous EHB stars, as well as
the gap within the EHB of NGC 2808, can be explained if these stars undergo a
late helium-core flash on the white-dwarf cooling curve. Under these conditions
the flash convection will penetrate into the stellar envelope, thereby mixing
most, if not all, of the envelope hydrogen into the hot helium- burning
interior. This phenomenon is analogous to the "born-again" scenario for
producing hydrogen-deficient stars during a very late helium-shell flash.
"Flash mixing" greatly enhances the envelope helium and carbon abundances and,
as a result, leads to an abrupt increase in the HB effective temperature. We
argue that the EHB gap in NGC 2808 is caused by this theoretically predicted
dichotomy in the HB morphology. Using new helium- and carbon-rich stellar
atmospheres, we show that the flash-mixed stars have the same reduced UV flux
as the subluminous EHB stars. Moreover, we demonstrate that models without
flash mixing lie, at most, ~0.1 mag below the EHB and hence fail to explain the
observations. Flash mixing may also provide a new evolutionary channel for
producing the high gravity, He-rich sdO and sdB stars.Comment: 8 pages, 5 figures, to appear in "Omega Centauri: a Unique Window
into Astrophysics" (Cambridge, August, 2001), ASP Conf. Ser., edited by F.
van Leeuwen, G. Piotto, and J. Hughe
Iron in Hot DA White Dwarfs
We present a study of the iron abundance pattern in hot hydrogen-rich (DA)
white dwarfs. The study is based on new and archival far ultraviolet
spectroscopy of a sample of white dwarfs in the temperature range 30,000 K <
T_eff < 64,000 K. The spectra obtained with the Far Ultraviolet Spectroscopic
Explorer along with spectra obtained with the Hubble Space Telescope Imaging
Spectrograph and the International Ultraviolet Explorer sample FeIII to FeVI
absorption lines enabling a detailed iron abundance analysis over a wider range
of effective temperatures than previously afforded. The measurements reveal
abundance variations in excess of two orders of magnitude between the highest
and the lowest temperatures probed, but also show considerable variations (over
one order of magnitude) between objects with similar temperatures and surface
gravities. Such variations in cooler objects may be imputed to accretion from
unseen companions or so-called circumstellar debris although the effect of
residual mass-loss and selective radiation pressure in the hottest objects in
the sample remain dominant.Comment: Accepted for publication in Ap
The Discovery of Pulsating Hot Subdwarfs in NGC 2808
We present the results of a Hubble Space Telescope program to search for
pulsating hot subdwarfs in the core of NGC 2808. These observations were
motivated by the recent discovery of such stars in the outskirts of omega Cen.
Both NGC 2808 and omega Cen are massive globular clusters exhibiting complex
stellar populations and large numbers of extreme horizontal branch stars. Our
far-UV photometric monitoring of over 100 hot evolved stars has revealed six
pulsating subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of
2.0 to 6.8%. In the UV color-magnitude diagram of NGC 2808, all six of these
stars lie immediately below the canonical horizontal branch, a region populated
by the subluminous "blue-hook" stars. For three of these six pulsators, we also
have low-resolution far-UV spectroscopy that is sufficient to broadly constrain
their atmospheric abundances and effective temperatures. Curiously, and in
contrast to the omega Cen pulsators, the NGC 2808 pulsators do not exhibit the
spectroscopic or photometric uniformity one might expect from a well-defined
instability strip, although they all fall within a narrow band (0.2 mag) of
far-UV luminosity.Comment: 5 pages, 1 table, 2 color and 2 grayscale figures. Accepted for
publication in The Astrophysical Journal Letter
Hubble Space Telescope Imaging Spectrograph Observations of the Hot White Dwarf in the Close Binary Feige 24
We obtained and analyzed two Hubble Space Telescope Imaging Spectrograph spectra of the white dwarf in the DA plus dMe binary Feige 24. The spectra, obtained at orbital quadratures, provide new estimates of the white dwarf motion and gravitational redshift resulting in revised white dwarf parameters. An analysis of interstellar absorption lines reveals the presence of two clouds (+3.1 km s-1, +17.6 km s-1) in the line of sight toward Feige 24; one of these clouds (+17.6 km s-1) is identified with the local cloud. A study of the Lyα H I and D I interstellar medium lines shows that the deuterium-to-hydrogen abundance ratio (D/H = 1.3 × 10-5) is consistent with other measurements supporting a relative constancy of this ratio throughout the local interstellar medium. The total hydrogen column density measured with Lyα (log nH = 2.95 × 1018 cm-2) is in agreement with EUV Lyman continuum flux measurements. Finally, we present a complete abundance pattern for the white dwarf, demonstrating the predominance of iron and nickel over lighter elements. Residual ionization imbalance in the case of several elements, most notably in the case of O IV/O V, which cannot be explained by temperature or surface gravity variations, may indicate the presence of other atmospheric constituents, inhomogeneous stratification of oxygen in the photosphere, and/or remaining inaccuracies in the treatment of model atoms. The abundance patterns in Feige 24 and in the hot DA white dwarf G191-B2B are remarkably similar, indicating that the same processes are operating equally in both stars
First Evidence of Circumstellar Disks around Blue Straggler Stars
We present an analysis of optical HST/STIS and HST/FOS spectroscopy of 6 blue
stragglers found in the globular clusters M3, NGC6752 and NGC6397. These stars
are a subsample of a set of ~50 blue stragglers and stars above the main
sequence turn-off in four globular clusters which will be presented in an
forthcoming paper. All but the 6 stars presented here can be well fitted with
non-LTE model atmospheres. The 6 misfits, on the other hand, possess Balmer
jumps which are too large for the effective temperatures implied by their
Paschen continua. We find that our data for these stars are consistent with
models only if we account for extra absorption of stellar Balmer photons by an
ionized circumstellar disk. Column densities of HI and CaII are derived as are
the the disks' thicknesses. This is the first time that a circumstellar disk is
detected around blue stragglers. The presence of magnetically-locked disks
attached to the stars has been suggested as a mechanism to lose the large
angular momentum imparted by the collision event at the birth of these stars.
The disks implied by our study might not be massive enough to constitute such
an angular momentum sink, but they could be the leftovers of once larger disks.Comment: Accepted by ApJ Letters 10 pages, 2 figure
On The Age Estimation of LBDS 53W091
The recent spectral analysis of LBDS 53W091 by Spinrad and his collaborators
has suggested that this red galaxy at z=1.55 is at least 3.5 Gyr old. This
imposes an important constraint on cosmology, suggesting that this galaxy
formed at z > 6.5, assuming recent estimates of cosmological parameters. We
have performed chi^2 tests to the continuum of this galaxy using its UV
spectrum and photometric data (RJHK). We have used the updated Yi models that
are based on the Yale tracks. We find it extremely difficult to reproduce such
large age estimates, under the assumption of the most probable input
parameters. Using the same configuration as in Spinrad et al. (solar abundance
models), our analysis suggests an age of approximately 1.4 -- 1.8 Gyr. The
discrepancy between Spinrad et al.'s age estimate (based on the 1997 Jimenez
models) and ours originates from the large difference in the model integrated
spectrum: the Jimenez models are much bluer than the Yi models and the Bruzual
\& Charlot (BC) models. Preliminary tests favor the Yi and BC models. The
updated age estimate of LBDS 53W091 would suggest that this galaxy formed
approximately at z=2-3.Comment: LaTeX, 18 eps files Accepted for publication in ApJ (Feb 10, 2000,
vol 530), uses emulateapj.st
- …