Surface spots on cool giants probed by spectro-astrometry

Astronomy and Astrophysics – Astronomy

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Giant And Subgiant Stars, Stellar Activity, Star Formation

Scientific paper

Spectro-astrometry enables us to study phenomena on milli-arcsecond spatial scales. It exploits the variation with wavelength of the centroid position of a structured source in a longslit spectrum, predominantly near spectral absorption/emission lines. The centroid can be measured with sub-diffration-limited precision, depending mostly on the S/N. Our goal is to identify surface features on cool giant stars and to constrain their size and temperature constrast. We obtained VLT/CRIRES 2.3 um-CO-spectra (R~90000 S/N~1000-5000) of TW Oph and RS Vir. We find spectro-astrometric signatures at a 2 mas amplitude for TW Oph. The best-fitting models suggest a mid-lattitude spot covering 10% of the visible stellar disk with ΔT~500 K. In the case of RS Vir, we derive upper limits on spot coverage.

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