166 research outputs found
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
The White Dwarfs within 20 Parsecs of the Sun: Kinematics and Statistics
We present the kinematical properties, distribution of spectroscopic
subtypes, stellar population subcomponents of the white dwarfs within 20 pc of
the sun. We find no convincing evidence of halo white dwarfs in the total 20 pc
sample of 129 white dwarfs nor is there convincing evidence of genuine thick
disk subcomponent members within 20 parsecs. Virtually the entire 20 pc sample
likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc
sample is 1.6, a manifestation of deepening envelope convection which
transforms DA stars with sufficiently thin H surface layers into non-DAs. The
addition of 5 new stars to the 20 pc sample yields a revised local space
density of white dwarfs of M_{\sun}/yr and a
corresponding mass density of M_{\sun}/pc.
We find that at least 15% of the white dwarfs within 20 parsecs of the sun (the
DAZ and DZ stars) have photospheric metals that possibly originate from
accretion of circumstellar material (debris disks) around them. If this
interpretation is correct, this suggests the possibility that the same
percentage have planets or asteroid-like bodies orbiting them.Comment: Accepted for publication in The Astronomical Journa
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
Discovery of a Bright, Extremely Low Mass White Dwarf in a Close Double Degenerate System
We report the discovery of a bright (V ~ 13.7), extremely low-mass white dwarf in a close double degenerate system. We originally selected GALEX J171708.5+675712 for spectroscopic follow-up among a group of white dwarf candidates in an ultraviolet-optical reduced proper-motion diagram. The new white dwarf has a mass of 0.18 M_solar and is the primary component of a close double degenerate system (P=0.246137 d, K_1 = 288 km/s) comprising a fainter white dwarf secondary with M_2 ~ 0.9 M_solar. Light curves phased with the orbital ephemeris show evidence of relativistic beaming and weaker ellipsoidal variations. The light curves also reveal secondary eclipses (depth ~ 8 mmag) while the primary eclipses appear partially compensated by the secondary gravitational deflection and are below detection limits. Photospheric abundance measurements show a nearly solar composition of Si, Ca, and Fe (0.1-1 solar), while the normal kinematics suggest a relatively recent formation history. Close binary evolutionary scenarios suggest that extremely low mass white dwarfs form via a common-envelope phase and possible Roche-lobe overflow
Know Your Neighborhood: A Detailed Model Atmosphere Analysis of Nearby White Dwarfs
We present improved atmospheric parameters of nearby white dwarfs lying
within 20 pc of the Sun. The aim of the current study is to obtain the best
statistical model of the least-biased sample of the white dwarf population. A
homogeneous analysis of the local population is performed combining detailed
spectroscopic and photometric analyses based on improved model atmosphere
calculations for various spectral types including DA, DB, DC, DQ, and DZ stars.
The spectroscopic technique is applied to all stars in our sample for which
optical spectra are available. Photometric energy distributions, when
available, are also combined to trigonometric parallax measurements to derive
effective temperatures, stellar radii, as well as atmospheric compositions. A
revised catalog of white dwarfs in the solar neighborhood is presented. We
provide, for the first time, a comprehensive analysis of the mass distribution
and the chemical distribution of white dwarf stars in a volume-limited sample.Comment: 104 pages, 22 figures, 2 tables, accepted for publication in the
Astrophysical Journal Supplemen
K-band spectroscopy of pre-cataclysmic variables
Aims. There exists now substantial evidence for abundance anomalies in a number of cataclysmic variables (CVs), indicating that the photosphere of the secondary star incorporates thermonuclear processed material. However, the spectral energy distribution in CVs is usually dominated by the radiation produced by the accretion process, severely hindering an investigation of the stellar components.
On the other hand, depending on how the secondary star has acquired such material, the above mentioned abundance anomalies could also be present in pre-CVs, i.e. detached white/red dwarf binaries that will eventually evolve into CVs, but have not yet started mass transfer, and therefore allow for an unobstructed view on the secondary star at infrared wavelengths.
Methods. We have taken K-band spectroscopy of a sample of 13 pre-CVs in order to examine them for anomalous chemical abundances. In particular, we study the strength of the 12CO and 13CO absorption bands that have been found diminished and enhanced, respectively, in similar studies of CVs.
Results. All our systems show CO abundances that are within the range observed for single stars. The weakest 12CO bands with respect to the spectral type are found in the pre-CV BPM 71214, although on a much smaller scale than observed in CVs. Furthermore there is no evidence for enhanced 13CO. Taking into account that our sample is subject to the present observational bias that favours the discovery of young pre-CVs with secondary stars of late spectral types, we can conclude the following: 1) our study provides
observational proof that the CO anomalies discovered in certain CVs are not due to any material acquired during the common envelope phase, and 2) if the CO anomalies in certain CVs are not due to accretion of processed material during nova outburst, then the progenitors of these CVs are of a significantly different type than the currently known sample of pre-CVs
Spectroscopic confirmation of UV-bright white dwarfs from the Sandage Two-Color Survey of the Galactic Plane
We present spectroscopic observations confirming the identification of hot
white dwarfs among UV-bright sources from the Sandage Two-color Survey of the
Galactic plane and listed in the Lanning (Lan) catalog of such sources. A
subsample of 213 UV bright Lan sources have been identified as candidate white
dwarfs based on the detection of a significant proper motion. Spectroscopic
observations of 46 candidates with the KPNO 2.1m telescope confirm 30 sources
to be hydrogen white dwarfs with subtypes in the DA1-DA6 range, and with one of
the stars (Lan 161) having an unresolved M dwarf as a companion. Five more
sources are confirmed to be helium white dwarfs, with subtypes from DB3 to DB6.
One source (Lan 364) is identified as a DZ 3 white dwarf, with strong lines of
calcium. Three more stars are found to have featureless spectra (to within
detection limits), and are thus classified as DC white dwarfs. In addition,
three sources are found to be hot subdwarfs: Lan 20 and Lan 480 are classified
as sdOB, and Lan 432 is classified sdB. The remaining four objects are found to
be field F star interlopers. Physical parameters of the DA and DB white dwarfs
are derived from model fits.Comment: To appear in the Astronomical Journa
Two new hot subdwarf binaries in the GALEX survey
We report the discovery of two new hot, hydrogen-rich subdwarfs (sdB) in
close binary systems. The hot subdwarfs, GALEX J0321+4727 and GALEX J2349+3844,
were selected from a joint optical-ultraviolet catalogue of hot sub-luminous
stars based on GSC2.3.2 and the Galaxy Evolution Explorer all-sky survey. Using
high-dispersion spectra of the Halpha core obtained using the 2m telescope at
Ondrejov Observatory we measured the radial velocities of the sdB primaries and
determined orbital periods of 0.26584+/-0.00004 days and 0.46249+/-0.00007 days
for GALEX J0321+4727 and GALEX J2349+3844, respectively. The time series
obtained from the Northern Sky Variability Survey with an effective wavelength
near the R band show that GALEX J0321+4727 is a variable star (Delta m=0.12
mag) while no significant variations are observed in GALEX J2349+3844. The
period of variations in GALEX J0321+4727 coincides with the orbital period and
the variability is probably caused by a reflection effect on a late-type
secondary star. Lack of photometric variations in GALEX J2349+3844 probably
indicates that the companion is a white dwarf star. Using all available
photometry and spectroscopy, we measured the atmospheric properties of the two
sdB stars and placed limits on the mass and luminosity of the companion stars.Comment: Accepted for publication in MNRA
Modeling of the Lyman gamma satellites in FUSE spectra of DA white dwarfs
We present new theoretical calculations of the line profile of Lyman gamma
that include transitions in which a photon is absorbed by a neutral hydrogen
atom while it interacts with a proton. Models show that two absorption features
located near 992 A and 996 A are due to H-H^+ collisions. These quasi-molecular
satellites are similar to those that were identified in the wings of Lyman
alpha and Lyman beta lines of hydrogen-rich white dwarfs. We compute synthetic
spectra that take account of these new theoretical profiles and compare them to
the spectra of four DA white dwarfs that were observed with FUSE. The models
predict the absorption features that are observed in the wing of Lyman gamma
near 995 A, and confirm that these features are quasi-molecular satellites.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
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