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Type Stars (type + star)

Distribution by Scientific Domains
Physics and Astronomy100%

Selected Abstracts

The remarkable properties of the symbiotic star AE Circinus

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008
R. Mennickent
ABSTRACT We present new optical spectroscopy and photometry, Two-Micron All-Sky Survey (2MASS) infrared observations and 24 yr of combined American Association of Variable Star Observers (AAVSO) and Association Francaise des Observateurs d'Etoiles Variables (AFOEV) photometry of the symbiotic star candidate AE Cir. The long-term light curve is characterized by outbursts lasting several years and having a slow decline of ,2 × 10,4 mag d,1. The whole range of variability of the star in the V band is about 4 mag. The periodogram of the photometric data reveals strong signals at ,342 and 171 d. The presence of the emission feature at ,6830 Å at minimum and the detection of absorption lines of a ,K5 type star confirm the symbiotic classification and suggest that AE Cir is a new member of the small group of s-type yellow symbiotic stars. We estimate a distance of 9.4 kpc. Our spectrum taken at the high state shows a much flatter spectral energy distribution, the disappearance of the ,6830 Å emission feature and the weakness of the He ii 4686 emission relative to the Balmer emission lines. Our observations indicate the presence of emission-line flickering in time-scales of minutes in 2001. The peculiar character of AE Cir is revealed in the visibility of the secondary star at the high and low state, the light curve resembling a dwarf nova superoutburst and the relatively short low states. The data are hard to reconciliate with standard models for symbiotic star outbursts. [source]

Magnetic fields in white dwarfs and stellar evolution

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2004
Christopher A. Tout
ABSTRACT The surface magnetic field strengths observed in the magnetic Ap, Bp stars (100,20 000 G) and in the high field magnetic white dwarfs (106,109 G) cover many decades but nevertheless the range of magnetic fluxes observed in each of these stellar groups is similar. An evolutionary link between them therefore appears plausible. For both groups of stars there is also information on field complexity. The magnetic white dwarfs in general show non-dipole field structures which can be best modelled if we assume contributions from higher order multipoles. The field structures of the Ap and Bp stars are similarly complex. We investigate the hypothesis that the magnetic fields of the white dwarfs could be fossil remnants from the main-sequence phase by focussing on the problem of how field complexity may arise and be maintained during evolution to the compact star state. We also address the question of to what extent magnetic fields seen in the early type stars could be fossil remnants from the pre-main-sequence phases of stellar evolution dating back perhaps to the time of star formation. [source]

SSS in young stellar populations and the "prompt" component of Type Ia supernovae

ASTRONOMISCHE NACHRICHTEN, Issue 2 2010
T. Nelson
Abstract We present the results of a search for UV and optical counterparts of the SSS population in M 31. We find that out of the 56 sources we included in our search, 16 are associated with regions of ongoing or recent star formation. We discuss two particularly interesting sources that are identified optically as early type stars, one of which displayed long term X-ray evolution similar to that observed in classical novae. We discuss the physical origin of supersoft X-rays in these and the other SSS in young regions, and their possible link to the so-called "prompt" component of the Type Ia supernova population (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The empirical upper limit for mass loss of cool main sequence stars

ASTRONOMISCHE NACHRICHTEN, Issue 4 2008
A. Lednicka
Abstract The knowledge of mass loss rates due to thermal winds in cool dwarfs is of crucial importance for modeling the evolution of physical parameters of main sequence single and binary stars. Very few, sometimes contradictory, measurements of such mass loss rates exist up to now. We present a new, independent method of measuring an amount of mass lost by a star during its past life. It is based on the comparison of the present mass distribution of solar type stars in an open cluster with the calculated distribution under an assumption that stars with masses lower than Mlim have lost an amount of mass equal to ,M. The actual value of ,M or its upper limit is found from the best fit. Analysis of four clusters: Pleiades, NGC 6996, Hyades and Praesepe gave upper limits for ,M in three of them and the inconclusive result for Pleiades. The most restrictive limit was obtained for Praesepe indicating that the average mass loss rate of cool dwarfs in this cluster was lower than 6 × 10,11 M,/yr. With more accurate mass determinations of the solar type members of selected open clusters, including those of spectral type K, the method will provide more stringent limits for mass loss of cool dwarfs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Can variable meridional flows lead to false exoplanet detections?

ASTRONOMISCHE NACHRICHTEN, Issue 10 2007
J.M. Beckers
Abstract The search for habitable exoplanets centers on planets with Earth-like conditions around late type stars. Radial velocity searches for these planets require precisions of 1 m/s and better. That is now being achieved. At these precisions stellar surface motions might lead to false detections. Of particular interest are variable meridional flows on stellar surfaces. I review the available observations of solar surface meridional flows using both Doppler shift and local helioseismology techniques. Interpretation in terms of Doppler shifts in integrated starlight leads to estimates of the likelihood of false detections. It is unlikely that these false detections occur in the habitability zones of exoplanets. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]