Probing the inner wind of AGB stars: Interferometric observations of SiO millimetre line emission from the oxygen-rich stars R Dor and L2Pup

Autor(en)

Fredrik L. Schöier, Hans Olofsson, Tony Wong, M Lindqvist, Franz Kerschbaum

Abstrakt

High angular resolution Australia Telescope Compact Array (ATCA) observations of SiO "thermal" millimetre line emission towards the two oxygen-rich, low mass loss rate AGE stars R Dor and L2 Pup are presented. In both cases the emission is resolved with an overall spherical symmetry. Detailed radiative transfer modelling of the SiO line emission has been performed, and the comparison between observations and models are conducted in the visibility plane, maximizing the sensitivity. The excitation analysis suggests that the abundance of SiO is as high as 4 ×10-5 in the inner part of the wind, close to the predicted values from stellar atmosphere models. Beyond a radius of ˜l × 1015 cm the SiO abundance is significantly lower, about 3 × 10-6, until it decreases strongly at a radius of about 3 × 10-6 cm. This is consistent with a scenario where SiO first freezes out onto dust grains, and then eventually becomes photodissociated by the interstellar UV-radiation field. In these low expansion velocity sources the turbulent broadening of the lines plays an important role in the line formation. Microturbulent velocity widths in the range 1.1-1.5 km s-1 result in a very good reproduction of the observed line shapes even if the gas expansion velocity is kept constant. This, combined with the fact that the SiO and CO lines are well fitted using the same gas expansion velocity (to within 5-10%), suggest that the envelope acceleration occurs close to the stellar photosphere, within ?20-30 stellar radii.

Organisation(en)

Institut für Astrophysik

Externe Organisation(en)

Chalmers University of Technology, Stockholm Observatory, University of Illinois at Urbana-Champaign

Journal

Astronomy & Astrophysics

Band

422

Seiten

651-663

Anzahl der Seiten

13

ISSN

0004-6361

Publikationsdatum

2004

Peer-reviewed

Ja

ÖFOS 2012

103003 Astronomie

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/63dab9fd-123b-4677-b067-a4752a1fa98e