Stellar Content of Young Open Clusters - Part Two - Be-Stars

Details Stellar Content of Young Open Clusters - Part Two - Be-Stars Stellar Content of Young Open Clusters - Part Two - Be-Stars

May 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982a%26a...109...48m&link_type=abstract

Astronomy and Astrophysics, Vol.109, P. 48, 1982

Astronomy and Astrophysics

Astrophysics

67

Scientific paper

Absolute magnitudes and dereddened colours have been calculated for 94 Be stars in 34 open clusters. Analysis of the resulting composite MV, (U-B)0 diagram, based on the age group concept, yielded the following results:
1. The distribution of the Be stars as a function of spectral types (or colours) has well-marked maxima at types B t - B 2 and B 7- B 8, confirming earlier results. The positions of these maxima are identical to those of gaps already noticed in the U - B vs B-V plane at exactly B1-B2 and B7-B8.
2. The distribution of the Be stars as a function of the age of the parent cluster is not uniform. The frequency presents a strong maximum for clusters with turn up types at B I - B 2, and then decreases with increasing age. The Be stars become rare in clusters older than 108 yr (turn up at B 8.5 and later). This contrasts with the constant percentage found by Abt over the same age interval.
3. The Be stars occupy the whole main sequence band (from the ZAMS to the TAMS) and occur in very different evolutionary states, from the nearly unevolved case (dwarfs) to the fully evolved one (giants). Consequently, they are not confined to the termination of the main sequence (core contraction phase).
4. Classification of the Be stars has been achieved by taking into account the age and evolutionary state of each star, which is only possible because most Be stars occur on the evolved part of the main sequence. Comparison with the spectroscopic groups showed that there is a rather close relation between the two classification schemes and that properties of the Be stars probably depend on their position in the HR diagram.
Examination of the energy distribution and position of the early Be stars in the UBV plane resulted in the identification of two kinds of Be stars: the first one (70 % of the early Be) contains those stars which lie within the band formed by the luminosity classes V and Ia mean loci in the U - B, B - V plane. These stars do not show infrared excesses. The second kind (30 %) contains the stars which lie redder than the Ia locus (which forms a sharp boundary). They show the infrared excess feature observed by Schild. Extreme Be stars correspond in most cases to the second kind of Be stars. However, they are not all confined to the very end of the main sequence and existing V sin i data indicate that they are not all viewed pole-on.

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