137 research outputs found
Spectroscopy of the enigmatic short-period cataclysmic variable IR Com in an extended low state
We report the occurrence of a deep low state in the eclipsing short-period
cataclysmic variable IR Com, lasting more than two years. Spectroscopy obtained
in this state shows the system as a detached white dwarf plus low-mass
companion, indicating that accretion has practically ceased. The spectral type
of the companion derived from the SDSS spectrum is M6-7, somewhat later than
expected for the orbital period of IR Com. Its radial velocity amplitude,
K_2=419.6+-3.4 km/s, together with the inclination of 75-90deg implies
0.8Msun<Mwd<1.0Msun. We estimate the white dwarf temperature to be ~15000K, and
the absence of Zeeman splitting in the Balmer lines rules out magnetic fields
in excess of ~5 MG. IR Com still defies an unambiguous classification, in
particular the occurrence of a deep, long low state is so far unique among
short-period CVs that are not strongly magnetic.Comment: Revised version, MNRAS Letters in pres
Magnetic white dwarfs in the Early Data Release of the Sloan Digital Sky Survey
We have identified 7 new magnetic DA white dwarfs in the Early Data Release
of the Sloan Digital Sky Survey. Our selection strategy has also recovered all
the previously known magnetic white dwarfs contained in the SDSS EDR,
KUV03292+0035 and HE0330-0002. Analysing the SDSS fibre spectroscopy of the
magnetic DA white dwarfs with our state-of-the-art model spectra, we find
dipole field strengths 1.5<=B_d<=63MG and effective temperatures
8500<=Teff<=39000K. As a conservative estimate, we expect that the complete
SDSS will increase the number of known magnetic white dwarfs by a factor 3.Comment: 7 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Irradiated atmospheres of accreting magnetic white dwarfs with an application to the polar AM Herculis
We present a pilot study of atmospheres of accreting magnetic white dwarfs irradiated by intense fluxes at ultraviolet to infrared wavelengths. The model uses a standard LTE stellar atmosphere code which is expanded by introducing an angle-dependent external radiation source. The present results are obtained for an external source with the spectral shape of a 10 000 K blackbody and a freely adjustable spectral flux. The model provides an explanation for the observed largely filled-up Lyman lines in the prototype polar AM Herculis during its high states. It also confirms the hypotheses (i) that irradiation by cyclotron radiation and other radiation sources is the principle cause for the large heated polar caps surrounding the accretion spots on white dwarfs in polars and (ii) that much of the reprocessed light appears in the far ultraviolet and not in the soft X-ray regime as suggested in the original simple theories. We also briefly discuss the role played by hard X-rays in heating the polar cap
The ultra-compact binary candidate KUV 23182+1007 is a bright quasar
KUV 23182+1007 was identified as a blue object in the Kiso UV Survey in the
1980s. Classification-dispersion spectroscopy showed a featureless continuum
except for a strong emission line in the region of He II 4686 A. This is a
hallmark of the rare AM CVn class of cataclysmic variable star, so we have
obtained a high-S/N blue spectrum of this object to check its classification.
Instead, the spectrum shows a strong quasar-like emission line centred on 4662
A. Comparison with the SDSS quasar template spectra confirms that KUV
23182+1007 is a quasar with a redshift of z = 0.665.Comment: 4 pages, 2 figures, to appear in IBVS. Data are available from
http://www.astro.keele.ac.uk/~jkt
The cool end of the DZ sequence in the SDSS
We report the discovery of cool DZ white dwarfs, which lie in the SDSS (u-g)
vs. (g-r) two-color diagram across and below the main sequence. These stars
represent the extension of the well-known DZ sequence towards cooler
temperatures.Comment: To appear in the proceedings of the "17th European Workshop on White
Dwarfs", Tuebingen, Germany, August 16-20, 201
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