378 research outputs found
A Near-Infrared Spectroscopic Study of the Accreting Magnetic White Dwarf SDSS J121209.31+013627.7 and its Substellar Companion
The nature of the excess near-infrared emission associated with the magnetic
white dwarf commonly known as SDSS 1212 is investigated primarily through
spectroscopy, and also via photometry. The inferred low mass secondary in this
system has been previously detected by the emission and variation of H,
and the m spectral data presented here are consistent with the
presence of a late L or early T dwarf. The excess flux seen beyond 1.5 m
in the phase-averaged spectrum is adequately modeled with an L8 dwarf
substellar companion and cyclotron emission in a 7 MG magnetic field. This
interesting system manifests several observational properties typical of
polars, and is most likely an old interacting binary with a magnetic white
dwarf and a substellar donor in an extended low state.Comment: 28 pages, 5 figures, Accepted to Ap
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
The Spitzer Atlas of Stellar Spectra
We present the Spitzer Atlas of Stellar Spectra (SASS), which includes 159
stellar spectra (5 to 32 mic; R~100) taken with the Infrared Spectrograph on
the Spitzer Space Telescope. This Atlas gathers representative spectra of a
broad section of the Hertzsprung-Russell diagram, intended to serve as a
general stellar spectral reference in the mid-infrared. It includes stars from
all luminosity classes, as well as Wolf-Rayet (WR) objects. Furthermore, it
includes some objects of intrinsic interest, like blue stragglers and certain
pulsating variables. All the spectra have been uniformly reduced, and all are
available online. For dwarfs and giants, the spectra of early-type objects are
relatively featureless, dominated by Hydrogen lines around A spectral types.
Besides these, the most noticeable photospheric features correspond to water
vapor and silicon monoxide in late-type objects and methane and ammonia
features at the latest spectral types. Most supergiant spectra in the Atlas
present evidence of circumstellar gas. The sample includes five M supergiant
spectra, which show strong dust excesses and in some cases PAH features.
Sequences of WR stars present the well-known pattern of lines of HeI and HeII,
as well as forbidden lines of ionized metals. The characteristic flat-top shape
of the [Ne III] line is evident even at these low spectral resolutions. Several
Luminous Blue Variables and other transition stars are present in the Atlas and
show very diverse spectra, dominated by circumstellar gas and dust features. We
show that the [8]-[24] Spitzer colors (IRAC and MIPS) are poor predictors of
spectral type for most luminosity classes.Comment: Accepted by ApJS; Atlas contents available from:
http://web.ipac.caltech.edu/staff/ardila/Atlas/index.html;
http://irsa.ipac.caltech.edu/data/SPITZER/SASS/; 70 PDF pages, including
figure
Nova-like Cataclysmic Variables in the Infrared
Novalike cataclysmic variables have persistently high mass transfer rates and prominent steady state accretion disks. We present an analysis of infrared observations of twelve novalikes obtained from the Two Micron All Sky Survey, the Spitzer Space Telescope, and the Wide-field Infrared Survey Explorer All Sky Survey. The presence of an infrared excess at >3-5 microns over the expectation of a theoretical steady state accretion disk is ubiquitous in our sample. The strength of the infrared excess is not correlated with orbital period, but shows a statistically significant correlation (but shallow trend) with system inclination that might be partially (but not completely) linked to the increasing view of the cooler outer accretion disk and disk rim at higher inclinations. We discuss the possible origin of the infrared excess in terms of emission from bremsstrahlung or circumbinary dust, with either mechanism facilitated by the mass outflows (e.g., disk wind/corona, accretion stream overflow, and so on) present in novalikes. Our comparison of the relative advantages and disadvantages of either mechanism for explaining the observations suggests that the situation is rather ambiguous, largely circumstantial, and in need of stricter observational constraints.Peer reviewe
Observations of the SW Sextantis star DW Ursae Majoris with the Far Ultraviolet Spectroscopic Explorer
We present an analysis of the first far-ultraviolet observations of the SW
Sextantis-type cataclysmic variable DW Ursae Majoris, obtained in November 2001
with the Far Ultraviolet Spectroscopic Explorer. The time-averaged spectrum of
DW UMa shows a rich assortment of emission lines (plus some contamination from
interstellar absorption lines including molecular hydrogen). Accretion disk
model spectra do not provide an adequate fit to the far-ultraviolet spectrum of
DW UMa. We constructed a light curve by summing far-ultraviolet spectra
extracted in 60-sec bins; this shows a modulation on the orbital period, with a
maximum near photometric phase 0.93 and a minimum half an orbit later. No other
periodic variability was found in the light curve data. We also extracted
spectra in bins spanning 0.1 in orbital phase; these show substantial variation
in the profile shapes and velocity shifts of the emission lines during an
orbital cycle of DW UMa. Finally, we discuss possible physical models that can
qualitatively account for the observed far-ultraviolet behavior of DW UMa, in
the context of recent observational evidence for the presence of a
self-occulting disk in DW UMa and the possibility that the SW Sex stars may be
the intermediate polars with the highest mass transfer rates and/or weakest
magnetic fields.Comment: accepted by the Astronomical Journal; 36 pages, including 12 figures
and 4 table
A Self-Occulting Accretion Disk in the SW Sex Star DW UMa
We present the ultraviolet spectrum of the SW Sex star and nova-like variable
DW UMa in an optical low state, as observed with the Space Telescope Imaging
Spectrograph on board the Hubble Space Telescope (HST). The data are well
described by a synthetic white dwarf (WD) spectrum with T_eff = 46,000 +/- 1000
K, log g = 7.60 +/- 0.15, v*sin(i) = 370 +/- 100 km/s and Z/Z_solar = 0.47 +/-
0.15. For this combination of T_eff and log g, WD models predict M_WD = 0.48
+/- 0.06 M_solar and R_WD = (1.27 +/- 0.18) * 10^9 cm. Combining the radius
estimate with the normalization of the spectral fit, we obtain a distance
estimate of d = 830 +/-150 pc.
During our observations, DW UMa was approximately 3 magnitudes fainter in V
than in the high state. A comparison of our low-state HST spectrum to a
high-state spectrum obtained with the International Ultraviolet Explorer shows
that the former is much bluer and has a higher continuum level shortward of
1450 A. Since DW UMa is an eclipsing system, this suggests that an optically
thick accretion disk rim blocks our view of the WD primary in the high state.
If self-occulting accretion disks are common among the SW Sex stars, we can
account for (i) the preference for high-inclination systems within the class
and (ii) their V-shaped continuum eclipses. Moreover, even though the emission
lines produced by a self-obscured disk are generally still double-peaked, they
are weaker and narrower than those produced by an unobscured disk. This may
allow a secondary line emission mechanism to dominate and produce the
single-peaked, optical lines that are a distinguishing characteristic of the SW
Sex stars.Comment: 9 pages, including 2 figures; accepted for publication in
Astrophysical Journal Letters; New version matches version in press (footnote
added to discussion section; figures now use color
First Spitzer Space Telescope Observations of Magnetic Cataclysmic Variables: Evidence for Excess Emission at 3--8 microns
We present the first observations of magnetic cataclysmic variables with the
Spitzer Space Telescope. We used the Infrared Array Camera to obtain photometry
of the polars EF Eri, GG Leo, V347 Pav, and RX J0154.0-5947 at 3.6, 4.5, 5.8,
and 8.0 m. In all of our targets, we detect excess mid-infrared emission
over that expected from the component stars alone. We explore the origin of
this IR excess by examining bremsstrahlung, cyclotron emission, circumbinary
dust, and L/T brown dwarf secondary stars. Bremsstrahlung and cyclotron
emission appear unlikely to be significant contributors to the observed fluxes.
At present, the most likely candidate for the excess emission is dust that is
probably located in a circumbinary disk with an inner temperature near 800 K.
However, a simple dust disk plus any reasonable low mass or brown dwarf-like
secondary star is unable to fully explain the observed flux densities in the
3--8 m region.Comment: Accepted to ApJ Letter
The system parameters of DW Ursae Majoris
We present new constraints on the system parameters of the SW Sextantis star
DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the
Hubble Space Telescope. Our data were obtained during a low state of the
system, in which the UV light was dominated by the hot white dwarf (WD)
primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set
firm limits on the masses and radii of the system components and the distance
between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014,
M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5
the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's
mass-period relation for CV secondaries, our estimates for the system
parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001,
M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82
\pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral
type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160
pc, from time-resolved I- and K-band photometry. Finally, we have repeated
Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa
in order to account for the higher surface gravity indicated by our eclipse
analysis. In this way we obtained a second estimate for the distance, d = 590
\pm 100 pc, which allows us to obtain a second estimate for the spectral type
of the secondary, M7 \pm 2.0. We conclude that the true value for the distance
and spectral type will probably be in between the values obtained by the two
methods.Comment: 23 pages including 5 figures and 3 tables. Accepted for publication
in Ap
Boron Isotope Effect in Superconducting MgB
We report the preparation method of, and boron isotope effect for MgB, a
new binary intermetallic superconductor with a remarkably high superconducting
transition temperature (B) = 40.2 K. Measurements of both
temperature dependent magnetization and specific heat reveal a 1.0 K shift in
between MgB and MgB. Whereas such a high transition
temperature might imply exotic coupling mechanisms, the boron isotope effect in
MgB is consistent with the material being a phonon-mediated BCS
superconductor.Comment: One figure and related discussion adde
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