Astron. Astrophys. 324, L33-L36 (1997)

1. Introduction

In a historical paper Herbig (1960) was the first to realize that the "Be and Ae stars associated with nebulosity" are in fact stars of intermediate mass still in their pre-main sequence (PMS) phase of evolution: stars which have lost most of their envelope of infalling gas and dust, but are not yet fusing hydrogen into helium and whose energy is mainly supplied by gravitational contraction. Criteria for membership of this class of stars, now more commonly known as Herbig Ae/Be (HAeBe) stars, are (Thé et al. 1994): (1) spectral type earlier than F8; (2) presence of emission lines; (3) presence of IR-excess in the spectral energy distribution; (4) location in or near a probable star formation region.

Later, stars were found which are not near any plausible star formation region, but show all other characteristics of HAeBe stars. Whether these so-called isolated Herbig Ae/Be stars are young stellar objects remains controversial. Moreover, the membership criteria outlined above do not uniquely select intermediate-mass pre-main sequence stars, but will also select evolved massive stars (Davies et al. 1990) which are still located in the region where they formed. Therefore one has to be careful in identifying Herbig stars with pre-main sequence objects (Herbig 1992).

In this letter we will derive astrophysical parameters of a sample of 10 Herbig Ae/Be candidates as well as 3 non emission-line A and B-type stars in star formation regions for which the Hipparcos mission resulted in 3 detections of the parallax. An attempt will be made to construct an evolutionary scenario for the circumstellar environment of HaeBe stars. In a forthcoming paper (van den Ancker et al. 1997b; Paper II) we will study the photometric behaviour of the Herbig Ae/Be stellar group using the photometric data on all Herbig stars provided by Hipparcos.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

helpdesk.link@springer.de