395 research outputs found
Evidence of Rocky Planetesimals Orbiting Two Hyades Stars
The Hyades is the nearest open cluster, relatively young and containing
numerous A-type stars; its known age, distance, and metallicity make it an
ideal site to study planetary systems around 2-3 Msun stars at an epoch similar
to the late heavy bombardment. Hubble Space Telescope far-ultraviolet
spectroscopy strongly suggests ongoing, external metal pollution in two remnant
Hyads. For ongoing accretion in both stars, the polluting material has
log[n(Si)/n(C)] > 0.2, is more carbon deficient than chondritic meteorites, and
is thus rocky. These data are consistent with a picture where rocky
planetesimals and small planets have formed in the Hyades around two
main-sequence A-type stars, whose white dwarf descendants bear the scars. These
detections via metal pollution are shown to be equivalent to infrared excesses
of Lir/L* ~ 1e-6 in the terrestrial zone of the stars.Comment: 7 pages, 3 figures, 2 tables, accepted to MNRA
Rapid variability of accretion in AM Herculis
We present the last pointed observation of AM Her carried out during the life
of the BeppoSAX satellite. It was bright at the beginning of the observation,
but dropped to the lowest X-ray level ever observed so far. The X-ray emission
during the bright period is consistent with accretion occurring onto the main
pole of the magnetized white dwarf. The rapid change from the active state to
the low deep state indicates a drop by a factor of 17 in the accretion rate and
hence that accretion switched-off. The short timescale (less than one hour) of
this variation still remains a puzzle. Optical photometry acquired
simultaneousy during the low state shows that the white dwarf remains heated,
although a weak emission from the accretion stream could be still present.
Cyclotron radiation, usually dominating the V and R bands, is negligible thus
corroborating the possibility that AM Her was in an off-accretion state. The
X-ray emission during the inactive state is consistent with coronal emission
from the secondary late type star.Comment: 6 pages A&A-Latex, 6 Figures, accepted for publication in A&
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
Echoes of a decaying planetary system: the gaseous and dusty disks surrounding three white dwarfs
We have performed a comprehensive ground-based observational program aimed at
characterizing the circumstellar material orbiting three single white dwarf
stars previously known to possess gaseous disks. Near-infrared imaging
unambiguously detects excess infrared emission towards Ton 345 and allows us to
refine models for the circumstellar dust around all three white dwarf stars. We
find that each white dwarf hosts gaseous and dusty disks that are roughly
spatially coincident, a result that is consistent with a scenario in which
dusty and gaseous material has its origin in remnant parent bodies of the white
dwarfs' planetary systems. We briefly describe a new model for the gas disk
heating mechanism in which the gaseous material behaves like a "Z II" region.
In this Z II region, gas primarily composed of metals is photoionized by
ultraviolet light and cools through optically thick allowed Ca II-line
emission.Comment: 43 pages, 9 tables, 9 figures. Accepted to Ap
Eclipse Mapping of the Accretion Stream in UZ Fornacis
We present a new method to map the surface brightness of the accretionstreams in AM Herculis systems from observed light curves. Extensive tests ofthe algorithm show that it reliably reproduces the intensity distribution ofthe stream for data with a signal-to-noise ratio >5. As a first application, wemap the accretion stream emission of Civ lambda 1550 in the polar UZ Fornacisusing HST FOS high state spectra. We find three main emission regions along theaccretion stream: (1) On the ballistic part of the accretion stream, (2) on themagnetically funneled stream near the primary accretion spot, and (3) on themagnetically funneled stream at a position above the stagnation region
HS 2237+8154 : on the onset of mass transfer or entering the period gap?
We report follow-up observations of a new white dwarf/red dwarf binary HS 2237+8154, identified as a blue variable star from the Hamburg Quasar Survey. Ellipsoidal modulation observed in the R band as well as the radial velocity variations measured from time-resolved spectroscopy determine the orbital period to be Porb = 178.10 +- 0.08 min. The optical spectrum of HS 2237+8154 is well described by a combination of a Teff = 11500 +- 1500 K white dwarf (assuming log g = 8) and a dM 3.5 +- 0.5 secondary star. The distance implied from the flux scaling factors of both stellar components is d = 105 +- 25 pc. Combining the constraints obtained from the radial velocity of the secondary and from the ellipsoidal modulation, we derive a binary inclination of i = 50-70 and stellar masses of and Mwd = 0.47-0.67 M and Msec = 0.2-0.4 M. All observations imply that the secondary star must be nearly Roche-lobe filling. Consequently, HS 2237+8154 may be either a pre-cataclysmic variable close to the start of mass transfer, or - considering its orbital period - a cataclysmic variable that terminated mass transfer and entered the period gap, or a hibernating nova
Analysis of cool DO-type white dwarfs from the Sloan Digital Sky Survey Data Release 10
We report on the identification of 22 new cool DO-type white dwarfs (WD)
detected in Data Release 10 (DR10) of the Sloan Digital Sky Survey (SDSS).
Among them, we found one more member of the so-called hot-wind DO WDs, which
show ultrahigh excitation absorption lines. Our non-LTE model atmosphere
analyses of these objects and two not previously analyzed hot-wind DO WDs,
revealed effective temperatures and gravities in the ranges Teff=45-80kK and
log g= 7.50-8.75. In eight of the spectra we found traces of C (0.001-0.01, by
mass). Two of these are the coolest DO WDs ever discovered that still show a
considerable amount of C in their atmospheres. This is in strong contradiction
with diffusion calculations, and probably, similar to what is proposed for DB
WDs, a weak mass-loss is present in DO WDs. One object is the most massive DO
WD discovered so far with a mass of 1.07 M_sun if it is an ONe-WD or 1.09 M_sun
if it is a CO-WD. We furthermore present the mass distribution of all known hot
non-DA (pre-) WDs and derive the hot DA to non-DA ratio for the SDSS DR7
spectroscopic sample. The mass distribution of DO WDs beyond the wind limit
strongly deviates from the mass distribution of the objects before the wind
limit. We address this phenomenon by applying different evolutionary input
channels. We argue that the DO WD channel may be fed by about 13% by
post-extreme-horizontal branch stars and that PG1159 stars and O(He) stars may
contribute in a similar extent to the non-DA WD channel.Comment: 13 pages, accepted for publication in A&
An in-depth study of the pre-polar candidate WX Leonis Minoris
Optical photometry, spectroscopy, and XMM-Newton ultraviolet and X-ray observations with full phase coverage are used for an in-depth study of WXLMi, a system formerly termed a low-accretion rate polar. We find a constant low-mass accretion rate, ËM ⌠1.5 Ă 10â13 M yrâ1, a peculiar accretion geometry with one spot not accessible via Roche-lobe overflow, a low temperature of the white dwarf, Teff < 8000 K, and the secondary very likely Roche-lobe underfilling. All this lends further support to the changed
view on WXLMi and related systems as detached binaries, i.e. magnetic post-common envelope binaries without significant Rochelobe overflow in the past. The transfer rate determined here is compatible with accretion from a stellar wind. We use cyclotron spectroscopy to determine the accretion geometry and to constrain the plasma temperatures. Both cyclotron spectroscopy and X-ray
plasma diagnostics reveal low plasma temperatures below 3 keV on both accretion spots. For the low-mË , high-B plasma at the accretion spots in WXLMi, cyclotron cooling dominates thermal plasma radiation in the optical. Optical spectroscopy and X-ray timing reveal atmospheric, chromospheric, and coronal activity at the saturation level on the dM4.5 secondary star
Monte Carlo simulations of post-common-envelope white dwarf + main sequence binaries: The effects of including recombination energy
Detached WD+MS PCEBs are perhaps the most suitable objects for testing
predictions of close-compact binary-star evolution theories, in particular, CE
evolution. The population of WD+MS PCEBs has been simulated by several authors
in the past and compared with observations. However, most of those predictions
did not take the possible contributions to the envelope ejection from
additional sources of energy (mostly recombination energy) into account. Here
we update existing binary population models of WD+MS PCEBs by assuming that a
fraction of the recombination energy available within the envelope contributes
to ejecting the envelope. We performed Monte Carlo simulations of 10^7 MS+MS
binaries for 9 different models using standard assumptions for the initial
primary mass function, binary separations, and initial-mass-ratio distribution
and evolved these systems using the publicly available BSE code. Including a
fraction of recombination energy leads to a clear prediction of a large number
of long orbital period (>~10 days) systems mostly containing high-mass WDs. The
fraction of systems with He-core WD primaries increases with the CE efficiency
and the existence of very low-mass He WDs is only predicted for high values of
the CE efficiency (>~0.5). All models predict on average longer orbital periods
for PCEBs containing C/O-core WDs than for PCEBs containing He WDs. This effect
increases with increasing values of both efficiencies. Longer periods after the
CE phase are also predicted for systems containing more massive secondary
stars. The initial-mass-ratio distribution affects the distribution of orbital
periods, especially the distribution of secondary star masses. Our simulations,
in combination with a large and homogeneous observational sample, can provide
constraints on the values of the CE efficiencies, as well as on the
initial-mass-ratio distribution for MS+MS binary stars.Comment: 11 pages, 10 figures, accepted for publication in A&
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