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Post Info TOPIC: White / Red Dwarf Systems


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RE: White / Red Dwarf Systems
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Title: DE CVn: A bright, eclipsing red dwarf - white dwarf binary
Authors: E.J.M. van den Besselaar, R. Greimel, L. Morales-Rueda, G. Nelemans, J.R. Thorstensen, T.R. Marsh, V.S. Dhillon, R.M. Robb, D.D. Balam, E.W. Guenther, J. Kemp, T. Augusteijn, P.J. Groot

Close white dwarf - red dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarf - red dwarf binary with a relatively short (~8.7 hours) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters which we discuss in the frame work of common-envelope evolution. Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average low-resolution spectrum of DE CVn we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system. The derived ephemeris is HJD_min = 2452784.5533(1) + 0.3641394(2) x E. The red dwarf in DE CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8000 K. The inclination of the system is 86 (+3, -2) deg and the mass and radius of the red dwarf are 0.41 0.06 M_sun and 0.37 (+0.06, -0.007) R_sun, respectively, and the mass and radius of the white dwarf are 0.51 (+0.06, -0.02) M_sun and 0.0136 (+0.0008, -0.0002) R_sun, respectively. We found that the white dwarf has a hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a common-envelope phase, we can reconstruct its evolution and we find that the progenitor of the white dwarf was a relatively low-mass star (M <= 1.6M_sun). The current age of this system is 3.3-7.3 x 10^9 years, while it will take longer than the Hubble time for DE CVn to evolve into a semi-detached system.

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Magnetic cataclysmic variables
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Title: Spitzer Space Telescope observations of magnetic cataclysmic variables: possibilities for the presence of dust in polars
Authors: C. S. Brinkworth, D. W. Hoard, S. Wachter, S. B. Howell, D. R. Ciardi, P. Szkody, T. E. Harrison, G. T. van Belle, A. A. Esin

We present Spitzer Space Telescope photometry of six short-period polars, EF Eri, V347 Pav, VV Pup, V834 Cen, GG Leo, and MR Ser. We have combined the Spitzer Infrared Array Camera (3.6 -8.0 microns) data with the 2MASS J, H, K_s photometry to construct the spectral energy distributions of these systems from the near- to mid-IR (1.235 - 8 microns). We find that five out of the six polars have flux densities in the mid-IR that are substantially in excess of the values expected from the stellar components alone. We have modelled the observed SEDs with a combination of contributions from the white dwarf, secondary star, and either cyclotron emission or a cool, circumbinary dust disk to fill in the long-wavelength excess. We find that a circumbinary dust disk is the most likely cause of the 8 micron excess in all cases, but we have been unable to rule out the specific (but unlikely) case of completely optically thin cyclotron emission as the source of the observed 8 micron flux density. While both model components can generate enough flux at 8 microns, neither dust nor cyclotron emission alone can match the excess above the stellar components at all wavelengths. A model combining both cyclotron and dust contributions, possibly with some accretion-generated flux in the near-IR, is probably required, but our observed SEDs are not sufficiently well-sampled to constrain such a complicated model. If the 8 micron flux density is caused by the presence of a circumbinary dust disk, then our estimates of the masses of these disks are many orders of magnitude below the mass required to affect CV evolution.

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Polars
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"This discovery points to a new mechanism for the generation of stellar activity by forces outside of the star itself, a phenomenon that we have dubbed 'hyperactivity'" - co-author Steve B. Howell of NOAO and the WIYN observatory.

Over the past two years, a team of astronomers consisting of Kafka, Howell, Kent Honeycutt (Indiana University), Fred Walter (State University of New York), Thomas Harrison (New Mexico State University) and Jeff Robertson (Arkansas Tech University) have carefully observed four polars (in particular, EF Eridanus and ST Leo Minor) using the 2.1-metre, 4-metre and WIYN 3.5-metre telescopes at Kitt Peak National Observatory, the Magellan 6.5-metre telescope and the ESO Very Large Telescope in Chile, for more than 20 nights of observing.

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Astronomers studying highly energetic binary stars called polars have obtained the first observational evidence that the intense magnetic fields produced by the white dwarf half of the interacting pair can induce flares, sunspots and other explosive activity in its otherwise low-wattage, low-mass partner.

Like Dr. Frankenstein zapping an inert corpse, the white dwarfs in these systems produce very strong electrical currents inside the bodies of their partner star, which can create violent eruptions where there otherwise would be very little if any. These transitory phenomena occur on human timescales, lasting from minutes to years - Stella Kafka, an astronomer at the National Optical Astronomy Observatory (NOAO) and lead author of one of two related poster papers presented today in Seattle at the 209th meeting of the American Astronomical Society.

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White / Red Dwarf Systems
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Title: White Dwarf - Red Dwarf Systems Resolved with the Hubble Space Telescope: I. First Results
Authors: J. Farihi, D.W. Hoard, S. Wachter

First results are presented for a Hubble Space Telescope Advanced Camera for Surveys snapshot study of white dwarfs with likely red dwarf companions. Of 48 targets observed and analysed so far, 27 are totally or partially resolved into two or more components, while an additional 15 systems are almost certainly unresolved binaries. These results provide the first direct empirical evidence for a bimodal distribution of orbital separations among binary systems containing at least one white dwarf.

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