Cataclysmic variable - Astronomy News

Blobrana

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Posts: 131433

Date: Aug 7 09:11 2007

J0644+3344	Permalink

Title: Discovery of a bright eclipsing cataclysmic variable Authors: D. K. Sing, E. M. Green, S. B. Howell, J. B. Holberg, M. Lopez-Morales, J. S. Shaw, G. D. Schmidt We report on the discovery of J0644+3344, a bright deeply eclipsing cataclysmic variable (CV) binary. Spectral signatures of both binary components and an accretion disk can be seen at optical wavelengths. The optical spectrum shows broad H I, He I, and He II accretion disk emission lines with deep narrow absorption components from H I, He I, Mg II and Ca II. The absorption lines are seen throughout the orbital period, disappearing only during primary eclipse. These absorption lines are either the result of an optically-thick inner accretion disk or from the photosphere of the primary star. Radial velocity measurements show that the H I, He I, and Mg II absorption lines phase with the primary star, while weak absorption features in the continuum phase with the secondary star. Radial velocity solutions give a 150 ±4 km/s semi-amplitude for the primary star and 192.8 ±5.6 km/s for the secondary. The individual stellar masses are 0.63-0.69 Mdot for the primary and 0.49-0.54 Mdot for the secondary. The bright eclipsing nature of this binary has helped provide masses for both components with an accuracy rarely achieved for CVs. This binary most closely resembles a nova-like UX UMa or SW Sex type of CV. J0644+3344, however, has a longer orbital period than most UX UMa or SW Sex stars. Assuming an evolution toward shorter orbital periods, J0644+3344 is therefore likely to be a young interacting binary. The secondary star is consistent with the size and spectral type of a K8 star, but has an M0 mass. Read more (312kb, PDF) __________________

Blobrana

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Posts: 131433

Date: Aug 2 12:27 2007

SDSS J150722.30+523039.8	Permalink

Title: SDSS J150722.30+523039.8: a CV formed directly from a detached white dwarf/brown dwarf binary? Authors: S. P. Littlefair, V. S. Dhillon, T. R. Marsh, B. T. Gaensicke, I. Baraffe, C. A. Watson We present high-speed, three-colour photometry of the eclipsing cataclysmic variable SDSS J150722.30+523039.8 (hereafter SDSS J1507). This system has an orbital period of 66.61 minutes, placing it below the observed ''period minimum'' for cataclysmic variables. We determine the system parameters via a parameterised model of the eclipse fitted to the observed lightcurve by chi² minimisation. We obtain a mass ratio of q = 0.0623 ± 0.0007 and an orbital inclination i = 83.63 ± 0.05 degrees. The primary mass is M_w = 0.90 ± 0.01 M_sun. The secondary mass and radius are found to be M_r =0.056 ± 0.001 M_sun and R_r = 0.096 ± 0.001 R_sun respectively. We find a distance to the system of 160 ± 10 pc. The secondary star in SDSS J1507 has a mass substantially below the hydrogen burning limit, making it the second confirmed sub-stellar donor in a cataclysmic variable. The very short orbital period of SDSS J1507 is readily explained if the secondary star is nuclearly evolved, or if SDSS J1507 formed directly from a detached white dwarf/brown dwarf binary. Given the lack of any visible contribution from the secondary star, the very low secondary mass and the low HeI(6678AA)/Halpha emission line ratio, we argue that SDSS J1507 probably formed directly from a detached white dwarf/brown dwarf binary. If confirmed, SDSS J1507 will be the first such system identified. The implications for binary star evolution, the brown-dwarf desert and the common envelope phase are discussed. Read more (353kb,PDF) __________________

Blobrana

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Posts: 131433

Date: May 10 00:03 2007

Novae	Permalink

ESAs X-ray observatory XMM-Newton has revealed a new class of exploding stars where the X-ray emission lives fast and dies young. The identification of this particular class of explosions gives astronomers a valuable new constraint to help them model and understand stellar explosions. Exploding stars called novae remain a puzzle to astronomers. Modelling these outbursts is very difficult - Wolfgang Pietsch of the Max Planck Institut für Extraterrestrische Physik. Now, ESAs XMM-Newton and NASAs Chandra space-borne X-ray observatories have provided valuable information about when individual novae emit X-rays. Between July 2004 and February 2005, the X-ray observatories watched the heart of the nearby galaxy, Andromeda, also known to astronomers as M31. During that time, Pietsch and his colleagues monitored novae, looking for the X-rays. They detected that eleven out of the 34 novae that had exploded in the galaxy during the previous year were shining X-rays into space. Expand (254kb, 825 x 751) This is an optical H-alpha image overplotted with contours from Chandra observations. The optical image of the central area of M31 was taken with the 4-metre Kitt Peak National Observatory telescope during the Local Group Survey. The overlying contours are taken from merged observations of the Chandra High Resolution Camera in the imaging mode (HRC-I) carried out from December 2004 to February 2005. Credits: W. Pietsch (MPE Garching, Germany), P. Massey (Lowell Observatory, USA), NASA/Chandra Read more __________________

Blobrana

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Posts: 131433

Date: Apr 10 10:18 2007

Dwarf Novae	Permalink

Title: FUSE Observations of the Dwarf Novae UU Aql, BV Cen, and CH UMa in Quiescence Authors: Edward M. Sion, Patrick Godon, Fuhua Cheng, Paula Szkody We report on FUSE spectra of three U Gem-type, long period, dwarf novae, UU Aql, BV Cen and CH UMa taken during their quiescence intervals. We discuss the line identifications in their spectra and attempt to characterise the source(s) of their FUV flux distribution. Archival IUE spectrum of CH UMa and BV Cen in quiescence were identified as having a matching flux level with the FUSE spectra and these were combined with each FUSE spectrum to broaden the wavelength coverage and further constrain model fits. Multi-component synthetic spectral fits from our model grids, consisting of single temperature white dwarfs, two-temperature white dwarfs, accretion disks and white dwarfs plus accretion disks, were applied to the FUSE spectra alone and to the combined FUSE + IUE spectra. We present the results of our model analyses and their implications. Read more (143kb, PDF) __________________

Blobrana

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Posts: 131433

Date: Jan 24 05:11 2007

Cataclysmic Variables	Permalink

Title: Cataclysmic Variables Authors: Robert Connon Smith Cataclysmic variables are binary stars in which a relatively normal star is transferring mass to its compact companion. This interaction gives rise to a rich range of behaviour, of which the most noticeable are the outbursts that give the class its name. Novae belong to the class, as do the less well known dwarf novae and magnetic systems. Novae draw their energy from nuclear reactions, while dwarf novae rely on gravity to power their smaller eruptions. All the different classes of cataclysmic variable can be accommodated within a single framework and this article will describe the framework, review the properties of the main types of system and discuss models of the outbursts and of the long-term evolution. Read more (282kb, PDF) __________________

Blobrana

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Posts: 131433

Date: Oct 19 14:53 2006

1RXS J092737.4-191529	Permalink

A 5' X 5' finding chart of the optical counterpart of 1RXS J092737.4-191529. This was made using a digitised ESO Schmidt telescope R-band plate. North is at the top and east to the left of the figure. The system is near minimum light in this image. __________________

Blobrana

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Posts: 131433

Date: Oct 19 11:19 2006

Cataclysmic variable	Permalink

Title: Discovery of a new cataclysmic variable through optical variability and X-ray emission Authors: Magaretha L. Pretorius, Christian Knigge Aims: We present discovery observations of the new cataclysmic variable star (CV) 1RXS J092737.4-191529, as well as spectra and photometry of SY Vol. The selection technique that turned up these two CVs is described; it should be efficient for finding dwarf novae with high outburst duty cycles. Methods: Two very common observational features of CVs, namely optical variability and X-ray emission, are combined to select targets for follow-up observations. Long-slit spectra were taken to identify CVs in the sample. Results: Two out of three objects selected in this way are CVs. One of these is the known dwarf nova SY Vol, while the second system, 1RXS J092737.4-191529, is a new discovery. We present medium resolution spectra, UBVRI magnitudes, and high-speed photometry for both these CVs. Rapid flickering in the light curve of 1RXS J092737.4-191529 confirms the mass transferring binary nature of this object; it is probably a dwarf nova that was in quiescence during our observations. Read more (238kb, PDF) __________________

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