461 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&
Irradiated accretion discs in post novae
We derive approximations for th
Detection of the white dwarf and the secondary star in the new SU UMa dwarf nova HS 2219+1824
We report the discovery of a new, non-eclipsing SU UMa-type dwarf nova, HS 2219+1824. Photometry obtained in
quiescence (V â 17.5) reveals a double-humped light curve from which we derive an orbital period of 86.2 min. Additional photometry obtained during a superoutburst reaching V 12.0 clearly shows superhumps with a period of 89.05 min. The optical spectrum contains double-peaked Balmer and He I emission lines from the accretion disc as well as broad absorption troughs of Hβ, HÎł, and Hδ from the white dwarf primary star. Modelling of the optical spectrum implies a white dwarf temperature of 13 000 K < âź
T eďŹ < âź 17 000 K, a distance of 180 pc < âź d <
âź 230 pc, and suggests that the spectral type of the donor star is later than M 5. Phase-resolved spectroscopy obtained during quiescence reveals a narrow HÎą emission line component which has a radial velocity amplitude and phase consistent with an origin on the secondary star, possibly on the irradiated hemisphere facing the white dwarf. This constitutes the first detection of line emission from the secondary star in a quiescent SU UMa star
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
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
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&
Thermal emission from low-field neutron stars
We present a new grid of LTE model atmospheres for weakly magnetic
(B<=10e10G) neutron stars, using opacity and equation of state data from the
OPAL project and employing a fully frequency- and angle-dependent radiation
transfer. We discuss the differences from earlier models, including a
comparison with a detailed NLTE calculation. We suggest heating of the outer
layers of the neutron star atmosphere as an explanation for the featureless
X-ray spectra of RX J1856.5-3754 and RX J0720.4-3125 recently observed with
Chandra and XMM.Comment: 8 pages A&A(5)-Latex, 6 Figures, A&A in press. The model spectra
presented here are available as XSPEC tables at
http://www.astro.soton.ac.uk/~btg/outgoing/nsspec
Constraints on the Lifetimes of Disks Resulting from Tidally Destroyed Rocky Planetary Bodies
Spitzer IRAC observations of 15 metal-polluted white dwarfs reveal infrared
excesses in the spectral energy distributions of HE 0110-5630, GD 61, and HE
1349-2305. All three of these stars have helium-dominated atmospheres, and
their infrared emissions are consistent with warm dust produced by the tidal
destruction of (minor) planetary bodies. This study brings the number of
metal-polluted, helium and hydrogen atmosphere white dwarfs surveyed with IRAC
to 53 and 38 respectively. It also nearly doubles the number of metal-polluted
helium-rich white dwarfs found to have closely orbiting dust by Spitzer. From
the increased statistics for both atmospheric types with circumstellar dust, we
derive a typical disk lifetime of log[t_{disk} (yr)] = 5.6+-1.1 (ranging from
3*10^4 - 5*10^6 yr). This assumes a relatively constant rate of accretion over
the timescale where dust persists, which is uncertain. We find that the
fraction of highly metal-polluted helium-rich white dwarfs that have an
infrared excess detected by Spitzer is only 23 per cent, compared to 48 per
cent for metal-polluted hydrogen-rich white dwarfs, and we conclude from this
difference that the typical lifetime of dusty disks is somewhat shorter than
the diffusion time scales of helium-rich white dwarf. We also find evidence for
higher time-averaged accretion rates onto helium-rich stars compared to the
instantaneous accretion rates onto hydrogen-rich stars; this is an indication
that our picture of evolved star-planetary system interactions is incomplete.
We discuss some speculative scenarios that can explain the observations.Comment: 20 pages, 9 figures, accepted to be published in Ap
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
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