Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...305l..61g&link_type=abstract
Astronomy and Astrophysics, v.305, p.L61
Astronomy and Astrophysics
Astrophysics
12
Circumstellar Matter, Stars: Individual: Irc +10216, Stars: Mass Loss, Stars: Agb, Post-Agb
Scientific paper
Drake et al. (1991) observed the carbon star IRC +10216 with the VLA at 2cm and determined the FWHM size of the emission region to be 80x59mas with a position angle of 41deg. In principle the emission is due to stellar, dust and free-free emission. Based on a detailed dust model that fits the spectral energy distribution and various other constraints it is shown that dust emission at 2cm is negligible. From a radiative transfer calculation follows that free-free emission is optically thin (τ_2cm_=0.03), and that the FWHM of the brightness distribution should equal the diameter of the central star. The observed size and shape of the 2cm emission region are therefore interpreted as due to an a-spherical central star. The position angle is in reasonable agreement with that characterising the circumstellar shell. It is suggested that the a-spherical circumstellar shell observed in IRC +10216 is due to an a-spherical central star. Three stars have previously been identified as being a-spherical (ο Cet, R Cas and χ Cyg). I discuss the available observations in the literature and conclude that in all three cases there is evidence that the circumstellar shells of these stars are a-spherical as well. These three stars all have binary companions (while none is known for IRC +10216). I conclude that non-radial pulsations (NRP), or a (as yet unidentified) binary component which has spun up the central star, are the most likely explanations for the a-sphericity of IRC +10216. Monitoring the a-sphericity in time should constrain the possible effect of NRP. Finally, the 0.5-2cm region is an ideal wavelength region to study the shapes and sizes of (nearby) stars. Even for stars with high mass loss rates, dust emission is relatively small in synthesized apertures <1", while free-free emission is also relatively unimportant.
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