X-ray sources in regions of star formation. III - Naked T Tauri stars associated with the Taurus-Auriga complex

Abstract

We report results from a multispectral investigation of stellar counterparts of x-ray sources in the general region of the Taurus-Auriga star-formation complex. We discuss 28 new low-mass pre-main-sequence "naked T Tauri" stars, bringing the census of naked T Tauri stars to 44 in the Tau-Aur region. We discuss the optical spectra and the optical and near-IR colors. We show that these stars are basically normal stars with ages of 1-40 X10 6 yr. There is no evidence for IR or UV excesses. Chromos-pheric and coronal surface fluxes are comparable to those of the most active main-sequence stars and the RS CVn systems. We show that the chromospheric fluxes do not increase for stellar ages < 10 8 yr, confirming the postulated exponential decay of stellar activity. There is a population of PMS stars about 10° west of the Tau-Aur star-formation region, far from the gas and dust which generally accompanies star formation. Within an area of about 400 sq. deg near the Tau-Aur dark clouds, the naked T Tauri stars outnumber the classical T Tauri stars by about 10:1. We suggest that the naked T Tauri stars, and not the classical T Tauri stars, show how low-mass stars evolve towards the zero-age main sequence. The existence of the naked T Tauri stars with ages of order 10 6 yr provides strong constraints on the timescales involved in the dissipation of circumstellar disks and the formation of planets.