The Evolutionary Status of Be Stars in Clusters and in the Galactic Field

Presented at the meeting of the Working Group on Active B Stars during the 25th IAU General Assembly in Sydney, Australia on 2003 July 16

J. Fabregat and J. Gutierréz-Soto

Observatorio Astronómico, Universidad de Valencia, 46100 Burjassot, Spain
email: juan.fabregat@uv.es, juan.gutierrez-soto@uv.es

Received: 2003 October 8; Accepted: 2003 November 10.

Modern studies on the Be star population of young open clusters point towards the presence of an evolutionary enhancement of the Be phenomenon at the second half of the B stars main sequence lifetime. Fabregat & Torrejón (2000) show that the Be star frequency reach a maximum in clusters within the age interval of 14-30 Myr, and there are almost no Be stars in clusters younger than 10 Myr. Keller et al. (2000, 2001) found most of the Be stars close to the turn-off of the clusters they observed. Fabregat (2003) studied the Be star frequency as a function of the spectral subtype for galactic and Magellanic Cloud clusters in the 14-30 Myr age interval, and found that Be stars of the earlier subtypes are significantly more frequent than in the galactic field, and late Be stars are scarce or inexistent.

On the other hand, Zorec & Briot (1997) show that the galactic field Be stars are equally present in luminosity classes V to III. If we consider luminosity classes as related to the evolutionary status, this result provides evidence of no evolutionary trend.

In the B star range, all class V to class III stars are main sequence objects. It is assumed that BV stars are young objects close to the ZAMS, and BIII are evolved ones close to the TAMS, with a dividing line placed somewhere between 0.4 and 0.6 times the main sequence lifetime. To the best of our knowledge this assumption has not been observationally checked so far.

With this aim, we will study the same sample that Zorec & Briot (1997): stars with B spectral type from the Bright Star Catalogue (BSC). We have selected among them those with HIPPARCOS parallaxes with relative error lower than 20%, and with Strömgren uvby photometry in the Hauck & Mermilliod (1998) catalogue. We have calculated the reddening of each star individually. Absolute magnitudes have been computed from the dereddened V₀ and the HIPPARCOS parallax.

Figure 1. HR diagram of B stars in the Bright Star Catalogue.

The observational M_{V - c0} HR diagram is presented in Fig. 1. Different symbols are used for each luminosity class. We have used the luminosity classification from the BSC, and the same grouping criteria that Zorec & Briot (1997). Solid lines represent the ZAMS and the TAMS. Most stars fall between them, clearly defining the main sequence. It is apparent that there is no segregation between class V and class III stars, being both groups evenly distributed along the whole main sequence. If we assume the position in the HR diagram as representative of the evolutionary status, results in Fig. 1 clearly imply that there is no relation between the evolutionary status of a B star and its luminosity class.

We have done a further check with the same techniques and a different sample, the B star population of the h and  Per clusters. We selected stars with Strömgren photometry from Capilla & Fabregat (2002) and spectral classification from Slesnick et al. (2002). Results are presented in Fig. 2.

Figure 2. HR diagram of B stars in the Perseus double cluster.

The clusters' sequence is clearly defined in the figure, and hence the evolutionary status of each star is unambiguously determined from its position in the HR diagram. As in the previous case, there is no segregation between classes V and III. Several class V stars are found in the cluster turn-off, right on the TAMS. In the same way, a significant number of class III stars are in the unevolved part of the cluster sequence, right on the ZAMS. Once again the lack of relation between luminosity class and evolutionary status is apparent.

We conclude that luminosity classes in the B star range are not related with the evolutionary status of the stars. As a consequence, the similar frequencies of Be stars for different luminosity classes does not argue against the evolutionary enhancement of the Be phenomenon which is apparent in the open clusters population.

References:

Capilla, G. and Fabregat, J. 2002, A&A 394, 488

Fabregat, J. 2003, ASP-CS 292, 65 (also astro-ph/0210653)

Fabregat, J. and Torrejón J.M. 2002, A&A 357, 451

Hauck, B. and Mermilliod, M. 1998, A&AS 129, 431

Keller, S.C., Bessell, M.S. and da Costa, G.S. 2000, ASP-CS 214, 75

Keller, S.C., Grebel, E.K., Miller G.J. and Yoss, K.M. 2001, AJ 122, 248

Slesnick, C.L., Hillenbrand, L.A. and Massey, P. 2002, ApJ 576, 880

Zorec, J. and Briot, D. 1997, A&A 318, 443

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