Candidate Pre-Mainsequence F Stars with Circumstellar Dust Identified Using Combined 2MASS and uvby Data

We propose a method that uses near-infrared plus uvby photometry to identify potentially extensive circumstellar dusty environment about F and A stars. The method has been applied to a sample of ~900 metal rich reddened F stars with 2MASS and uvby data, suggesting the presence of circumstellar dust emitting in the near infrared for ~70 stars. The log T_e - M_V diagram suggests that most, if not all, of them are likely pre-mainsequence (PMS). They seem to be consistent with being a continuation of the class of Herbig Ae/Be PMS stars into the spectral type F. Their number drops sharply downward of log T_e ~ 3.84 (spectral types later than ~F5), which may provide new clues to the PMS evolution of stars with 1 to 2 solar mass. We present a list of 21 most conspicuous candidate stars with circumstellar dust. About half of them are associated with the extended star-forming region around rho Oph. The brightest of these 21 stars, with V < 7.5, turn out to be IRAS sources, suggesting the presence of heated dust emitting in the far infrared. Also in this list, HD 81270 is reported as a very unusual star moving away from the Galactic plane at a projected speed of 70 km/sec.