Starspots on weak-lined T Tauri stars in the sigma Orionis cluster

Abstract

The light curves and spectra of T Tauri stars indicate the presence of cool starspots covering large areas of their photospheres. Heavily spotted stars should have a larger mid-infrared flux than the equivalent single-temperature photospheres. To test this prediction we first fit single and two-temperature models to optical spectra of young stars in the 4 Myr-old sigma Orionis cluster and a control set of older field stars. The fits to the young stars favour two temperatures while the field stars show no preference. Independent tests of our models for nine probable single K-type weak-lined T Tauri stars using infrared photometry confirms the presence of large cool components. We consider and rule out as explanations of the two temperature photospheres: binary stars for all but one WTTS; accretion, as the observed H-alpha equivalent width is too small; gravity darkening as the predicted temperature differences are too small. This leaves cool areas akin to starspots covering 30-95 per cent of the surface as the plausible explanation. Our results show no significant difference in the cool temperature for surface fractions between 0.30-0.75, implying a common mechanism is responsible. These cool surface fractions would lead to significant overestimates in age and mass when fitting unspotted isochrones in colour-magnitude diagrams. The dispersion in cool surface fraction could also explain the dispersion in lithium equivalent width seen in the spectra of coeval zero-age main sequence stars.