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

Abstract

I report medium-resolution (FWHM = 1.9 Å) 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 sigma 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 deg^2 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. Based on observations made with the Isaac Newton Telescope, operated by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, and on data obtained with the IAC-80 Telescope at the Observatorio del Teide, Tenerife, Spain.