Journal Article

From pre- to young planetary nebulae: radio continuum variability

L. Cerrigone, C. Trigilio, G. Umana, C. S. Buemi and P. Leto

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 412, issue 2, pages 1137-1144
Published in print April 2011 | ISSN: 0035-8711
Published online March 2011 | e-ISSN: 1365-2966 | DOI:
From pre- to young planetary nebulae: radio continuum variability

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Searching for variability, we have observed a sample of hot post-asymptotic giant branch stars and young planetary nebula candidates with the Very Large Array at 4.8, 8.4 and 22.4 GHz. The sources had been previously detected in the radio continuum, which is a proof that the central stars have started ionizing their circumstellar envelopes, and an increase in radio flux with time can be expected as a result of the progression of the ionization front. Such a behaviour has been found in IRAS 18062+2410, whose radio modelling has allowed us to determine that its ionized mass has increased from 10−4 to 3.3 × 10−4 M in 8 yr and its envelope has become optically thin at lower frequencies.

Different temporal behaviours have been found for three other sources. IRAS 17423−1755 has shown a possibly periodic pattern and an inversion of its radio spectral index, as expected from a varying stellar wind. We estimate that the radio flux arises from a very compact region around the central star (∼1015 cm) with an electron density of 2 × 106 cm−3. IRAS 22568+6141 and IRAS 17516−2525 have decreased their radio flux densities by about 10 per cent per year over 4 yr.

While a linear increase in the flux density with time points to the progression of the ionization front in the envelope, decreases as well as quasi-periodic patterns may indicate the presence of unstable stellar winds/jets or thick dusty envelopes absorbing ionizing photons.

Keywords: stars: AGB and post-AGB; circumstellar matter; radio continuum: stars

Journal Article.  6326 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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