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Polarimetry of Li-rich giants
Context: .Protoplanetary nebulae typically present non-sphericalenvelopes. The origin of such geometry is still controversial. There areindications that it may be carried over from an earlier phase of stellarevolution, such as the AGB phase. But how early in the star's evolutiondoes the non-spherical envelope appear? Aims.Li-rich giants show dustycircumstellar envelopes that can help answer that question. We study asample of fourteen Li-rich giants using optical polarimetry in order todetect non-spherical envelopes around them. Methods.We used the IAGPOLimaging polarimeter to obtain optical linear polarization measurementsin {V} band. Foreground polarization was estimated using the field starsin each CCD frame.Results.After foreground polarization was removed,seven objects presented low intrinsic polarization (0.19-0.34)% and two(V859 Aql and GCSS 557) showedhigh intrinsic polarization values (0.87-1.16)%. This intrinsicpolarization suggests that Li-rich giants present a non-sphericaldistribution of circumstellar dust. The intrinsic polarization level isprobably related to the viewing angle of the envelope, with higherlevels indicating objects viewed closer to edge-on. The correlation ofthe observed polarization with optical color excess gives additionalsupport to the circumstellar origin of the intrinsic polarization inLi-rich giants. The intrinsic polarization correlates even better withthe IRAS 25 μ m far infrared emission. Analysis of spectral energydistributions for the sample show dust temperatures for the envelopesthat tend to be between 190 and 260 K. We suggest that dust scatteringis indeed responsible for the optical intrinsic polarization in Li-richgiants.Conclusions.Our findings indicate that non-spherical envelopesmay appear as early as the red giant phase of stellar evolution.
| Li-rich giants: A survey based on IRAS colours
In a previous work we studied the IRAS colours of known Li-rich redgiants and showed that they have flux ratiosF12/F25 and F25/F60 in welldefined ranges. By using this result as a selection criterion, weprepared a list of 280 IRAS Point Source candidates to be Li-rich giantstars. Up to the present we have obtained spectra for 57% of our targetlist. We identified five stars showing a strong LiI 670.079 nm line andsix ones with a Li line of medium strength. Most of the candidates showfeatures typical of normal giants having circumstellar dust, asindicated by their IRAS colours. Observations collected at theLaboratorio Nacional de Astrofisica - LNA, Brazil; Observatoire de HauteProvence - OHP, France; European Southern Observatory - ESO, Chile.
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