281 research outputs found
VLT/X-shooter observations and the chemical composition of cool white dwarfs
We present a model atmosphere analysis of cool hydrogen-rich white dwarfs
observed at the Very Large Telescope (VLT) with the X-shooter spectrograph. The
intermediate-dispersion and high signal-to-noise ratio of the spectra allowed
us to conduct a detailed analysis of hydrogen and heavy element line profiles.
In particular, we tested various prescriptions for hydrogen Balmer line
broadening parameters and determined the effective temperature and surface
gravity of each star. Three objects (NLTT 1675, 6390 and 11393) show the
presence of heavy elements (Mg, Al, Ca, or Fe). Our abundance analysis revealed
a relatively high iron to calcium ratio in NLTT 1675 and NLTT 6390. We also
present an analysis of spectropolarimetric data obtained at the VLT using the
focal reducer and low dispersion spectrograph (FORS) and we established strict
upper limits on the magnetic field strengths in three of the DAZ white dwarfs
and determined the longitudinal magnetic field strength in the DAZ NLTT 10480.
The class of DAZ white dwarfs comprises objects that are possibly accreting
material from their immediate circumstellar environment and the present study
contributes in establishing class properties.Comment: Accepted for publication in Astronomy and Astrophysic
A new extremely low-mass white dwarf in the NLTT catalogue
We report on the discovery of the extremely low-mass, hydrogen-rich white
dwarf, NLTT 11748. Based on measurements of the effective temperature
(8540+/-50 K) and surface gravity (log g = 6.20+/-0.15) obtained by fitting the
observed Balmer line profiles with synthetic spectra, we derive a mass of
0.167+/-0.005 M_solar. This object is one of only a handful of white dwarfs
with masses below 0.2 M_solar that are believed to be the product of close
binary evolution with an episode of Roche lobe overflow onto a degenerate
companion (neutron star or white dwarf). Assuming membership in the halo
population, as suggested by the kinematics and adopting a cooling age of 4.0 -
6.3 Gyrs for the white dwarf, we infer a progenitor mass of 0.87 - 0.93
M_solar. The likely companion has yet to be identified, but a search for radial
velocity variations may help constrain its nature.Comment: Accepted for publication in A&A Letter
Dynamic Thermal Analysis of a Power Amplifier
This paper presents dynamic thermal analyses of a power amplifier. All the
investigations are based on the transient junction temperature measurements
performed during the circuit cooling process. The presented results include the
cooling curves, the structure functions, the thermal time constant distribution
and the Nyquist plot of the thermal impedance. The experiments carried out
demonstrated the influence of the contact resistance and the position of the
entire cooling assembly on the obtained results.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Optical observations and orbital parameters of the close DA plus dMe binaries BPM 71214, EUVE J0720-31.7, BPM 6502, and EC 13471-1258
We have obtained photometric and spectroscopic observations of the close DA plus dMe binaries BPM 6502 and EC 13471-1258, and spectroscopic observations of the close binaries BPM 71214 and EUVE J0720-31.7. We have updated the ephemerides of BPM 6502 and EUVE J0720-31.7 with the spectroscopic measurements and obtained periods of 0.20162 and 0.15074 days for BPM 71214 and EC 13471-1258, respectively, by measuring the shift in Hα emission. Photometric variations in R and I due to reprocessing of incoming radiation from the hot white dwarf were observed in BPM 6502. We have also observed eclipses and ellipsoidal variations in B and R for EC 13471-1258, suggesting that the secondary is nearly filling its Roche lobe
The heavily polluted atmosphere of the DAZ white dwarf GALEX J193156.8+011745
We report on the discovery of a new heavily polluted white dwarf. The DAZ
white dwarf GALEX J193156.8+011745 was identified in a joint GALEX/GSC survey
of ultraviolet-excess objects. Optical spectra obtained at ESO NTT show strong
absorption lines of magnesium and silicon and a detailed abundance analysis
based on VLT-Kueyen UVES spectra reveal super-solar abundances of silicon and
magnesium, and near-solar abundances of oxygen, calcium, and iron. The overall
abundance pattern bears the signature of ongoing accretion onto the white dwarf
atmosphere. The infrared spectral energy distribution shows an excess in the H
and K bands likely associated with the accretion source.Comment: Accepted for publication in MNRAS Letter
Orbital Parameters and Chemical Composition of Four White Dwarfs in Post-Common Envelope Binaries
We present FUSE observations of the hot white dwarfs in the post-common
envelope binaries Feige 24, EUVE J0720-317, BPM 6502, and EUVE J2013+400. The
spectra show numerous photospheric absorption lines which trace the white dwarf
orbital motion. We report the detection of C III, O VI, P V, and Si IV in the
spectra of Feige 24, EUVE J0720-317 and EUVE J2013+400, and the detection of C
III, N II, Si III, Si IV, and Fe III in the spectra of BPM 6502. Abundance
measurements support the possibility that white dwarfs in post-common envelope
binaries accrete material from the secondary star wind. The FUSE observations
of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We used the FUSE
spectra to measure the radial velocities traced by the white dwarf in the four
binaries, where the zero-point velocity were fixed using the ISM velocities in
the line of sight of the stellar systems. For BPM 6502 we determined a white
dwarf velocity semi-amplitude of K_WD = 18.6+/-0.5km/s, and with the velocity
semi-amplitude of the red dwarf companion (K_RD = 75.2+/-3.1 km/s), we estimate
the mass ratio to be q = 0.25+/-0.01. Adopting a spectroscopic mass
determination for the white dwarf, we infer a low secondary mass of M_RD =
0.14+/-0.01 M_solar. For EUVE J2013+400 we determine a white dwarf velocity
semi-amplitude of K_WD = 36.7+/-0.7 km/s. The FUSE observations of EUVE
J0720-317 cover approximately 30% of the binary period and combined with the
HST GHRS measurements (Vennes et al. 1999, ApJ 523, 386), we update the binary
properties. FUSE observations of Feige 24 cover approximately 60% of the orbit
and we combine this data set with HST STIS (Vennes et al. 2000, ApJ, 544, 423)
data to update the binary properties.Comment: Accepted for publication in Ap
A new EUVE-detected flare star (EUVE J0613-23.9B)
Extreme Ultraviolet Explorer (EUVE) observations have provided detailed spectroscopic and timing studies of several flare stars, including AU Mic (Cully et al., 1993), AD Leo (Hawley et al., 1995) and EQ Peg (Monsignori-Fossi et al., 1995).
In this bulletin, we present the EUVE and optical follow-up observations of a newly detected flare star. This star was serendipitously discovered during an EUVE observation of the G star HD 43162 as part of the analysis for the 3rd EUVE Right Angle Program Catalog (Christian, 2002). Analysis of the EUVE spectra obtained during the largest flare (Fe XIX-XXIV emission and a strong 300-650 Å continuum) have been presented elsewhere (Christian et al., 2003). We present optical spectroscopy and results obtained at MT Stromlo Observatory to identify the optical counterpart in § 1, and the long-term EUVE Deep Survey light-curves in § 2
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