76 research outputs found
FUSE and HST/STIS far-ultraviolet observations of AM Herculis in an extended low state
We have obtained FUSE and HST/STIS time-resolved spectroscopy of the polar AM
Herculis during a deep low state. The spectra are entirely dominated by the
emission of the white dwarf. Both the far-ultraviolet (FUV) flux as well as the
spectral shape vary substantially over the orbital period, with maximum flux
occurring at the same phase as during the high state. The variations are due to
the presence of a hot spot on the white dwarf, which we model quantitatively.
The white dwarf parameters can be determined from a spectral fit to the faint
phase data, when the hot spot is self-eclipsed. Adopting the distance of
79+8-6pc determined by Thorstensen, we find an effective temperature of
19800+-700K and a mass of Mwd=0.78+0.12-0.17Msun. The hot spot has a lower
temperature than during the high state, ~34000-40000K, but covers a similar
area, ~10% of the white dwarf surface. Low state FUSE and STIS spectra taken
during four different epochs in 2002/3 show no variation of the FUV flux level
or spectral shape, implying that the white dwarf temperature and the hot spot
temperature, size, and location do not depend on the amount of time the system
has spent in the low state. Possible explanations are ongoing accretion at a
low level, or deep heating, both alternatives have some weaknesses that we
discuss. No photospheric metal absorption lines are detected in the FUSE and
STIS spectra, suggesting that the average metal abundances in the white dwarf
atmosphere are lower than 1e-3 times their solar values.Comment: ApJ in press, 12 pages, 11 figure
Phase-resolved HST/STIS spectroscopy of the exposed white dwarf in the high-field polar AR UMa
Phase-resolved HST/STIS ultraviolet spectroscopy of the high-field polar AR
UMa confirms that the WD photospheric Ly alpha Zeeman features are formed in a
magnetic field of ~200 MG. In addition to the Ly alpha pi and sigma+
components, we detect the forbidden hydrogen 1s0->2s0 transition, which becomes
``enabled'' in the presence of both strong magnetic and electric fields. Our
attempt in fitting the overall optical+UV low state spectrum with single
temperature magnetic WD models remains rather unsatisfactory, indicating either
a shortcoming in the present models or a new physical process acting in AR UMa.
As a result, our estimate of the WD temperature remains somewhat uncertain,
Twd=20000+-5000K. We detect a broad emission bump centered at ~1445A and
present throughout the entire binary orbit, and a second bump near ~1650A,
which appears only near the inferior conjunction of the secondary star. These
are suggestive of low harmonic cyclotron emission produced by low-level
(M-dot~1e-13 Msun/yr) accretion onto both magnetic poles. However, there is no
evidence in the power spectrum of light variations for accretion in gas blobs.
The observed Ly alpha emission line shows a strong phase dependence with
maximum flux and redshift near orbital phase phi~0.3, strongly indicating an
origin on the trailing hemisphere of the secondary star. An additional Ly alpha
absorption feature with similar phasing as the Ly alpha emission, but a
\~700km/s blueshift could tentatively be ascribed to absorption of WD emission
in a moderately fast wind. We derive a column density of neutral hydrogen of
NH=(1.1+-1.0)1e18 cm**-2, the lowest of any known polar.Comment: 26 pages, 10 figures, AAS TeX 5.0, accepted for publication in the
Astrophysical Journa
Anomalous cooling of the massive white dwarf in U Geminorum following a narrow dwarf nova outburst
We obtained Hubble GHRS medium resolution (G160M grating) phase-resolved
spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf
nova quiescence, 13 days and 61 days following the end of a narrow outburst.
The spectral wavelength ranges were centered upon three different line regions:
N V (1238\AA, 1242\AA), Si III (1300\AA) and He II (1640\AA). All of the
quiescent spectra at both epochs are dominated by absorption lines and show no
emission features. The Si III and He II absorption line velocities versus
orbital phase trace the orbital motion of the white dwarf but the N~V
absorption velocities appear to deviate from the white dwarf motion. We confirm
our previously reported low white dwarf rotational velocity, V sin i= 100 km/s.
We obtain a white dwarf orbital velocity semi-amplitude K1=107 km/s. Using the
gamma-velocity of Wade (1981) we obtain an Einstein redshift of 80.4 km/s and
hence a carbon core white dwarf mass of ~1.1 Msun. We report the first subsolar
chemical abundances of C and Si for U Gem with C down by 0.05 with respect to
the Sun, almost certainly a result of C depletion due to thermonuclear
processing. This C-depletion is discussed within the framework of a weak TNR,
contamination of the secondary during the common envelope phase, and mixing of
C-depleted white dwarf gas with C-depleted matter deposited during a dwarf nova
event. Remarkably the Teff of the white dwarf 13 days after outburst is only
32,000K, anomalously cooler than previous early post-outburst measurements.
Extensive cooling during an extraordinarily long (210 days) quiescence followed
by accretion onto an out-of-equilibrium cooled degenerate could explain the
lower Teff.Comment: 16 pages AAS-Latex, 4 Figures, accepted for publication in Ap
Chandra High Resolution X-ray Spectroscopy of AM Her
We present the results of high resolution spectroscopy of the prototype polar
AM Herculis observed with Chandra High Energy Transmission Grating. The X-ray
spectrum contains hydrogen-like and helium-like lines of Fe, S, Si, Mg, Ne and
O with several Fe L-shell emission lines. The forbidden lines in the spectrum
are generally weak whereas the hydrogen-like lines are stronger suggesting that
emission from a multi-temperature, collisionally ionized plasma dominates. The
helium-like line flux ratios yield a plasma temperature of 2 MK and a plasma
density 1 - 9 x10^12 cm^-3, whereas the line flux ratio of Fe XXVI to Fe XXV
gives an ionization temperature of 12.4 +1.1 -1.4 keV. We present the
differential emission measure distribution of AM Her whose shape is consistent
with the volume emission measure obtained by multi-temperature APEC model. The
multi-temperature plasma model fit to the average X-ray spectrum indicates the
mass of the white dwarf to be ~1.15 M_sun. From phase resolved spectroscopy, we
find the line centers of Mg XII, S XVI, resonance line of Fe XXV, and Fe XXVI
emission modulated by a few hundred to 1000 km/s from the theoretically
expected values indicating bulk motion of ionized matter in the accretion
column of AM Her. The observed velocities of Fe XXVI ions are close to the
expected shock velocity for a 0.6 M_sun white dwarf. The observed velocity
modulation is consistent with that expected from a single pole accreting binary
system.Comment: 6 figures, AASTEX style, accepted for publication in Ap
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
FUSE Observations of the Dwarf Nova SW UMa During Quiescence
We present spectroscopic observations of the short-period cataclysmic
variable SW Ursa Majoris, obtained by the Far Ultraviolet Spectroscopic
Explorer (FUSE) satellite while the system was in quiescence. The data include
the resonance lines of O VI at 1031.91 and 1037.61 A. These lines are present
in emission, and they exhibit both narrow (~ 150 km/s) and broad (~ 2000 km/s)
components. The narrow O VI emission lines exhibit unusual double-peaked and
redshifted profiles. We attribute the source of this emission to a cooling flow
onto the surface of the white dwarf primary. The broad O VI emission most
likely originates in a thin, photoionized surface layer on the accretion disk.
We searched for emission from H_2 at 1050 and 1100 A, motivated by the
expectation that the bulk of the quiescent accretion disk is in the form of
cool, molecular gas. If H_2 is present, then our limits on the fluxes of the
H_2 lines are consistent with the presence of a surface layer of atomic H that
shields the interior of the disk. These results may indicate that accretion
operates primarily in the surface layers of the disk in SW UMa. We also
investigate the far-UV continuum of SW UMa and place an upper limit of 15,000 K
on the effective temperature of the white dwarf.Comment: 21 Pages, 3 figures, to be published in Ap
Far Ultraviolet Observations of the Dwarf Nova VW Hyi in Quiescence
We present a 904-1183 A spectrum of the dwarf nova VW Hydri taken with the
Far Ultraviolet Spectroscopic Explorer during quiescence, eleven days after a
normal outburst, when the underlying white dwarf accreter is clearly exposed in
the far ultraviolet. However, model fitting show that a uniform temperature
white dwarf does not reproduce the overall spectrum, especially at the shortest
wavelengths. A better approximation to the spectrum is obtained with a model
consisting of a white dwarf and a rapidly rotating ``accretion belt''. The
white dwarf component accounts for 83% of the total flux, has a temperature of
23,000K, a v sin i = 400 km/s, and a low carbon abundance. The best-fit
accretion belt component accounts for 17% of the total flux, has a temperature
of about 48,000-50,000K, and a rotation rate Vrot sin i around 3,000-4,000
km/s. The requirement of two components in the modeling of the spectrum of VW
Hyi in quiescence helps to resolve some of the differences in interpretation of
ultraviolet spectra of VW Hyi in quiescence. However, the physical existence of
a second component (and its exact nature) in VW Hyi itself is still relatively
uncertain, given the lack of better models for spectra of the inner disk in a
quiescent dwarf nova.Comment: 6 figures, 10 printed page in the journal, to appear in APJ, 1 Sept.
2004 issue, vol. 61
ORFEUS II Far-UV Spectroscopy of AM Herculis
Six high-resolution (\lambda/\Delta\lambda ~ 3000) far-UV (\lambda\lambda =
910-1210 \AA) spectra of the magnetic cataclysmic variable AM Herculis were
acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM
Her was in a high optical state at the time of the observations, and the
spectra reveal emission lines of O VI \lambda\lambda 1032, 1038, C III \lambda
977, \lambda 1176, and He II \lambda 1085 superposed on a nearly flat
continuum. Continuum flux variations can be described as per Gansicke et al. by
a ~ 20 kK white dwarf with a ~ 37 kK hot spot covering a fraction f~0.15 of the
surface of the white dwarf, but we caution that the expected Lyman absorption
lines are not detected. The O VI emission lines have narrow and broad component
structure similar to that of the optical emission lines, with radial velocities
consistent with an origin in the irradiated face of the secondary and the
accretion funnel, respectively. The density of the narrow- and broad-line
regions is n_{nlr} ~ 3\times 10^{10} cm^{-3} and n_{blr} ~ 1\times 10^{12}
cm^{-3}, respectively, yet the narrow-line region is optically thick in the O
VI line and the broad-line region is optically thin; apparently, the velocity
shear in the broad-line region allows the O VI photons to escape, rendering the
gas effectively optically thin. Unexplained are the orbital phase variations of
the emission-line fluxes.Comment: 15 pages, 6 Postscript figures; LaTeX format, uses aaspp4.sty;
table2.tex included separately because it must be printed sideways - see
instructions in the file; accepted on April 17, 1998 for publication in The
Astrophysical Journa
Variable emission from a gaseous disc around a metal-polluted white dwarf
We present the discovery of strongly variable emission lines from a gaseous disc around the DA white dwarf SDSS J1617+1620, a star previously found to have an infrared excess indicative of a dusty debris disc formed by the tidal disruption of a rocky planetary body. Time series spectroscopy obtained during the period 2006–2014 has shown the appearance of strong double-peaked Ca II emission lines in 2008. The lines were weak, at best, during earlier observations, and monotonically faded through the remainder of our monitoring. Our observations represent unambiguous evidence for short-term variability in the debris environment of evolved planetary systems. Possible explanations for this extraordinary variability include the impact on to the dusty disc of either a single small rocky planetesimal, or of material from a highly eccentric debris tail. The increase in flux from the emission lines is sufficient that similar events could be detected in the broad-band photometry of ongoing and future large-area time domain surveys
A Far Ultraviolet Study of the Nova-like V794 Aquilae
We present a spectral analysis of the dereddened FUSE and HST/STIS spectra
separately and combined together assuming E(B-V)=0.1 & 0.2. Overall, we find
that the model fits are in much better agreement with the dereddened spectra
when E(B-V) is large, as excess emission in the longer wavelengths render the
slope of the observed spectra almost impossible to fit, unless E(B-V)=0.2 .
The best fit accretion disk model is obtained for E(B-V)=0.2 . A single white
dwarf model leads to a rather hot temperature (30,000K < Twd < 55,000K
depending on the assumptions) but does not provide a fit as good as the
accretion disk model. A combination of a white dwarf plus a disk does not lead
to a better fit. The same best fit disk model is consistently obtained when
fitting the FUSE and HST/STIS spectra individually and when combined together,
implying therefore that the disk model is the best fit not only in the least
chi2 sense, but also as a consistent solution across a large wavelength span of
observation. This is not the case with the single white dwarf model fitting
which leads to a different (and therefore inconsistent) temperature for each
different spectrum FUSE, STIS and FUSE+STIS
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