Naked T Tauri stars and post T Tauri stars

Abstract

It has become evident during the past few years that there is more to low mass pre-main sequence (PMS) stellar populations than the classical T Tauri stars (TTS). If all low mass stars pass through the T Tauri phase, and star formation continues for a dynamical timescale, then in a typical star formation region the post-T Tauri (PTT), PMS stars should vastly outnumber the TTS (Herbig 1978). That they are not seen can be attributed to the extreme youth of most T associations, as well as the intrinsic difficulty in discovering these relatively (to the TTS) inactive stars. X-ray observations of regions of star formation with the EINSTEIN Observatory have revealed many stellar X-ray sources which are not classical TTS (cf Feigelson 1984, Walter 1984). Mundt et al. (1983) studied 5 late K PMS stars in some detail. Here we report further studies of 13 non-T Tauri, PMS stars. We comment on their relation to the TTS, and present a report on progress in a search for more members of this population. The original sample of PMS, non-T Tauri stars in Taurus consists Of the 5 late K dwarfs studied by Mundt et al. (1983) and the G giant HD283572. Full details of the current observations are given by Walter (1985) for the former and Walter et al. (1985) for HD283572. We obtained spectrophotometry in the blue (~3700-4200) and red (~6200-7000) with the IIDS on the KPNO 2~1 telescope, and ultraviolet observations with the IUE at low dispersion(6-8 ~ resolution). The late K (K7-M0) stars have strong Ca II H and K resonance emission, and variable H-Balmer emission(H6-Hl3). The Ca II K/H ratio is similar to that observed in main sequence stars and in dMe stars; the Ca II H/HE ratio is similar to that observed in dMe stars. The absolute surface fluxes in Ca II are larger than observed in main sequence stars, and less than observed in most TTS. Qualitatively, P4 (nomenclature from Mundt et al. 1983; = V830 Tau) resembles the relatively inactive T Tauri star DN Tau, while P1 (V819 Tau), P2 (V826Tau), and P3(V827 Tau) are less active. PS(V836 Tau), the most extreme (the strongest Balmer emission), is the only one with a significant infrared excess. The mid-G giant HD283572 is a PMS star on a radiative track for M ~ 2 MO. The rotation period is i@5; the stellar radius is 3 R@. The