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Astron. Astrophys. 325, 1077-1082 (1997)

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1. Introduction

In an effort to identify early-type stars in their pre-main sequence (PMS) stages of evolution, Herbig (1960) compiled a list of 26 Ae and Be stars, using the criteria of early spectral type and strong emission lines, location in an obscured region, and illumination of nearby nebulosity. Since then his selection criteria have been used to choose stars for spectroscopic and photometric study, and the term "Herbig Ae/Be star" (HAeBes) is employed to identify a possible PMS object of moderate mass. Individual HAeBes have been studied photometrically (Bastien et al. 1983; Cohen 1973a, 1973b, 1974; Cohen & Kuhi 1979; Herbst et al. 1982; Herbst et al. 1983;), spectroscopically (Böhm & Catala 1994; Finkenzeller & Mundt 1984; Finkenzeller & Jankovics 1984; Grinin et al. 1994; Grinin et al. 1996; Strom et al. 1972) and polarimetrically (Garrison & Anderson 1978; Grinin 1991).

However, after more than thirty years of investigations it is still difficult to define a unique set of observable quantities that can distinguish intermediate-mass PMS objects from more evolved stars. Moreover, some alternative hypotheses were suggested concerning the location of HAeBes on the Hertzsprung-Russell diagram (HRD). Herbst et al. (1982) have proposed that these objects are ordinary Be-stars associated with dark cloud material, rather than being PMS objects. Their location above the main sequence is commonly interpreted as due to rapid rotation, an effect which seems to explain the shift of Be-stars above the main sequence (Collins & Sonneborn 1977). Thé et al. (1994) also believed that a certain fraction of HAeBes is not PMS stars. This point of view is supported by Davies et al. (1990). Thus the validity of the assumption that all HAeBes as defined by Herbig are young stars is now questioned by some authors.

There is a small subgroup of variable Ae-stars with unpredicted Algol-like minima. Most of them demonstrate no obvious connection with bright or dark nebulosities, as well as star-formation regions, and part of them was previously called by Grinin "isolated HAeBes" (Grinin et al. 1994). We conventionally call them ALIVARS-Alfa Line (or Algol Like) Irregular VARiable Stars.

The object of our investigation is a small group of HAeBes with non-periodic light fadings. They are rapid irregular variables of Is(A)-type according to (Kholopov et al. 1985). Their light variations is expressed as algol-like minima of 1- [FORMULA] that last a few days. Owing to the specific form of their light curve they are sometimes called "anti-flare stars" and we will denote these stars hereafter as ALIVARS. Analysis of photometric UBVR observations has shown that on the HR-diagram ALIVARS occupy the region on the both sides of the giant branch (Pugach & Kovalchuk 1991). This fact as well as a photometric resemblance of ALIVARS and fairly evolved RCB stars support an assumption that ALIVARS are post-main sequence stars rather than pre-main sequence ones. The space distribution of the most of ALIVARS: VX Cas, BH Cep, BO Cep, SV Cep, V517 Cyg, BN Ori, UX Ori, EO Per, XY Per, DD Ser, CQ Tau - has revealed no obvious connection with star-formation regions, although some of them seem to be surrounded by nebulae (Thé et al. 1994). A small part of ALIVARS (UX Ori, WW Wul, BF Ori, CQ Tau) are considered to be the so-called "isolated HAeBes" (Grinin et al. 1994), or may be considered (V351 Ori, RZ Psc) as isolated ones. It remains to be shown whether the ALIVARS are young objects if most of them are located outside star formation regions?

This investigation is devoted to determination the gravity prameter lg g - a fundamental physical characteristic of ALIVARS. The results of the investigation allow us to give some conclusions about the evolutionary status of ALIVARS.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998