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arXiv · 1011.0685

From pre- to young Planetary Nebulae: radio continuum variability

L. Cerrigone·C. Trigilio·G. Umana·C. S. Buemi·P. Leto
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Abstract

Searching for variability, we have observed a sample of hot post-AGB stars and young Planetary Nebulae 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 ionising their circumstellar envelopes and an increase in radio flux with time can be expected as a result of the progression of the ionisation front. Such a behaviour has been found in IRAS 18062+2410, whose radio modelling has allowed us to determine that its ionised mass has increased from 10^{-4} to 3.3 10^{-4} M_sun in 8 years 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 (10^{15} cm) with an electron density of 2 10^6 cm^{-3}. IRAS 22568+6141 and 17516-2525 have decreased their radio flux densities of about 10% per year over 4 years. While a linear increase of the flux density with time points out to the progression of the ionisation 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 ionising photons.

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L. Cerrigone, C. Trigilio, G. Umana, C. S. Buemi, P. Leto. 2010-11-02. From pre- to young Planetary Nebulae: radio continuum variability. https://doi.org/10.1111/j.1365-2966.2010.17968.x

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