Weak and Post–T Tauri Stars around B-Type Members of the Scorpius-Centaurus OB Association

Abstract

I report medium-resolution (FWHM = 1.9 A) spectroscopic observations of six Hα emission-line stars proposed in 1993 by Meyer, Wilking, &Zinnecker to be T Tauri stars formed in the vicinity of the B1 giant σ Sco, a bright member of the Sco-Cen OB association. Using spectroscopic criteria (spectral types, Hα and Li I equivalent widths), which are distance-independent, I classify these stars in different pre–main-sequence (PMS) classes. Taking data from the literature, a number of stars detected by X-ray observations around other B-type members of Sco-Cen are also classified. The current census of bona fide low-mass PMS stars identified in about 9 deg2 in Sco-Cen includes two classical T Tauri stars, 18 weak T Tauri stars (WTTSs), and 10 post–T Tauri stars (PTTSs). The presence of a mixture of T Tauri and post–T Tauri stars implies that previous results based on isochrone fitting that indicated an extremely young age (~1 Myr) for the Sco-Cen PMS low-mass population are incorrect. A distance of about 125 pc for Sco-Cen, instead of the 160 pc used in previous works, is consistent with the Hipparcos parallaxes for many of the B-type stars and would lead to older H-R diagram ages. Taking into account that PTTSs are generally fainter and harder to identify than WTTSs, I argue that the WTTS-to-PTTS ratio in Sco-Cen may be of order unity. This result suggests that the low-mass stars of the OB association span an age range similar to the B-type members (5–15 Myr), i.e., the low- and high-mass star populations are essentially coeval. Sco-Cen appears to be indeed a promising place to find many PTTSs in future surveys.