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Astron. Astrophys. 325, 1083-1087 (1997)

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1. Introduction

In the last decades there are growing evidences that the structure of the Herbig Ae/Be (HAeBes) group is not so homogenuous as it was previously thought. Because of lack of adequate definitions, many emission A-stars having different morphological, physical and evolutional characteristics, were prematurely included in the HAeBes group. HAeBes show a vast variety of emission features which makes a strict classification of the HAeBes in a frame of unified conception a very problematic task. In spite of the fact that more and more complex system and criteria of classification have appeared during the last decade (Bastian et al. 1983; Hamann & Person 1992; Thé et al. 1994) a strict universal criterion is not yet elaborated and an ambiguety still remains.

There is a small subgroup of HAeBes showing significant photometric variations. The light variability of these stars is governed by drastic non-periodic light fadings, of which the amplitude ranges up to 3 magnitudes in several days or weeks (Kolotilov et al. 1977; Pugach 1981; Kardopolov & Philipjev 1985; Bibo & Thé 1991). The stars are indicated as Is(A)-type stars (rapid irregular variables with Algol-like minima) in the General Catalogue of Variable Stars (Kholopov et al. 1985). Owing to the specific form of their light curves the stars are sometimes called "antiflare stars". We shall conventionally call them ALIVARS - Alpha Line (or Algol Like) Irregular VARiable Stars.

The ALIVARS, being members of a larger sample of the HAeBes group, differ from "classic" HAeBes by photometrically as well as spectral characteristics. The emission spectrum of ALIVARS is poorer than that of HAeBes. There are numerous emission lines of CaII, FeI, [FeII], OI and others elements, which are clearly seen in the near infrared spectrum of HAeBes. However these lines are absent in the ALIVARS (see for example spectra of XY Per, HK Ori, VV Ser to be presented in paper of Hamann & Persson 1992).

Spectral differences between ALIVARS and classic T Tau-stars are more pronounced owing to noticeable differences in their temperatures. However, an intermediate temperature interval 6000 K  [FORMULA] 7000 K exists where these two group of stars intersect each other. Such stars with the intermediate temperatures as CO Ori, CQ Tau, EZ Ori and others may often be considered as HAeBes (Herbig & Bell 1988) and T Tau-stars simultaneously (Herbst et al. 1994). The spectra of HAeBes, ALIVARS and T Tau-stars are different, however, they have in common a special feature, namely the emission line [FORMULA]. For this reason, the examination of this line would provide significant informations for diverse types of emission stars in order to be classified more strictly. However, ALIVARS being mostly of spectral class A, have in their spectra a strong absorption [FORMULA] -line [FORMULA] which distorts the real form and the equivalent width of the emission profiles. The influence of the underlaying absorption line should be accurately taken into account before the correct value of the emission line equivalent width can be determined.

In the present article the true parameters of the [FORMULA] -emission luminosity of a representative group (16 objects) of ALIVARS are obtained after the contribution of the absorption profile was accounted for.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998