Surface spots on cool giants probed byspectro-astrometry

摘要

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