The following document lists the file abstract/MGERBALD_PROP_1.abs
from catalogue VI/111.
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Among A-type stars only a small group, known as lambda Boo, are affected by metal deficiencies. These deficiencies are the result of an interaction between the star and its environment (Waters et al., 1992 A.A. 262, L37). The analysis of optical and UV spectra have proven that lambda Boo are Pop I stars with atmospheric abundance patterns similar to those of the ISM (Venn and Lambert, 1990 Ap.J. 363, 234). An IR excess has been detected by IRAS for 3 out of 25 known lambda Boo and has been associated with thermal emission of a dusty envelope. These observations together with complementary information on the position in the HR diagram (of lambda Boo with known parallaxes) lead us (Gerbaldi et al. 1993 ASP Conf. 44, 413) to interpret the characteristics of lambda Boo as a sign of the very young age of these stars which are just arriving on the ZAMS. Waters et al. (1992) suggested that the observed abundance anomalies are the result of the separation of gas and dust in a disc surrounding the star during the last phases of accretion and this disc being the remnant of the material from which the star was formed. According to this scenario, young A-type stars should present an IR excess which could have been detected only partly by IRAS due to its limited sensitivity. The main goal of this proposal is to point out the IR properties of the stellar environment for stars in the spectral range B9-A3 (wherein we find the lambda Boo known up to now). We want to observe a sample of stars which belong to young open clusters or associations, since the lambda Boo, being field stars, are not suitable to establish a relation between the presence of an IR excess and the stellar age. That is why we selected clusters of various ages less than 8 to 10E7 years in order to assure the time scale related to the stellar environment.