GK Workshop - E.K. Grebel et al. (P1)

Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

Be Stars and the IMF of Young Clusters

Eva K. Grebel¹, Wolfgang Brandner², and Andrea Dieball³

¹Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
²Caltech - JPL/IPAC, Mail Code 100-22, Pasadena, CA 91125, USA
³Sternwarte der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany

Received 04th March 1998

Abstract. Since early-type Be stars tend to be systematically brighter than ordinary B stars, they may appear as more massive stars when unrecognized and thus affect the IMF determination. We investigated this effect through Monte-Carlo simulations and show that an IMF uncorrected for Be stars becomes shallower with increasing Be star fraction, decreasing age, and increasing metallicity.

1. Introduction

Be stars are non-supergiant B stars with Balmer emission and infrared excess originating in circumstellar disks. Be stars are often rapid rotators and in many cases members of binary systems.

What causes B stars to become Be stars is not yet understood. The proposed mechanisms include the wind-compressed disk model of Bjorkman & Cassinelli (1993), non-radial pulsations, magnetic activity, and binarity.

Also the evolutionary state of Be stars is under debate. Recent studies suggest that Be star fractions are similar for luminosity classes III to V and that conditions during the formation of B stars determine their becoming Be stars soon thereafter (Zorec & Briot 1997).

Among field B stars approximately 18% are Be stars, while the percentage in young clusters can be significantly higher (Grebel 1997). Systematic studies of Be stars in young clusters, where stars share a common origin, are coeval, and of the same metallicity, help to better constrain properties and origins of the Be phenomenon. Similarly, knowledge of the Be star content may be essential in deriving the correct IMF for massive stars.

2. How can Be stars affect the IMF?

In the visible wavelength range, Be stars are overluminous due to the presence of their circumstellar envelope. The earlier the spectral type, the more pronounced the overluminosity (Zorec & Briot 1997). Furthermore, the highest frequency of Be stars occurs among the earliest B types.

Thus, in young clusters with Be stars the luminosity function may seem to contain too massive stars, leading to an artificially top-heavy IMF.

We have conducted Monte Carlo simulations to evaluate this effect using B and Be star properties as parameterized by Zorec & Briot (1997) and models from the Geneva group. We ran 30 simulations (each containing 20 000 stars) for different metallicities (Z=0.004, 0.008, 0.02), ages (13, 20, 32 Myr), and Be star fractions (10, 20, 30, 40, 50, 60%).

We always used a Salpeter IMF with a slope of 2.3. Be stars were simulated at random by accounting for their overluminosity, total fraction, and subfraction per magnitude and spectral subclass. Afterwards, the IMF was determined from the luminosity function derived from the resulting color-magnitude diagram. Results are shown in the tables below.

Be fraction	IMF without / with Be stars
Z=0.008, log(t)=7.1, 30 × 20 000 stars
20%	-2.32±0.04	-2.16±0.04
50%	-2.33±0.03	-2.08±0.04
Z=0.008, log(t)=7.3, 30 × 20 000 stars
10%	-2.32±0.04	-2.25±0.04
20%	-2.33±0.04	-2.24±0.04
30%	-2.33±0.04	-2.21±0.04
40%	-2.32±0.03	-2.17±0.04
50%	-2.33±0.03	-2.14±0.04
60%	-2.33±0.04	-2.11±0.04
Z=0.008, log(t)=7.5, 30 × 20 000 stars
20%	-2.32±0.05	-2.30±0.04
50%	-2.32±0.04	-2.14±0.04
Z=0.020, log(t)=7.3, 30 × 20 000 stars
20%	-2.33±0.04	-2.15±0.03
50%	-2.33±0.04	-2.04±0.03

3. Conclusions

Our simulations show that an IMF that is not corrected for Be stars becomes shallower with increasing Be star fraction, decreasing age, and increasing metallicity.

Be star fractions may vary considerably from cluster to cluster (e.g., Grebel 1997). Our simulations demonstrate also that the IMF is affected only slightly for young clusters with Be star fractions of up to ∼30%.

IMF determinations based on ground-based observations are often affected by relatively large errors (∼±0.2), especially when dealing with fairly sparse clusters. In these cases the observational errors will mask effects from Be stars (e.g., Dieball & Grebel 1998). Corrections become more important for accurate HST analyses.

Acknowledgments. EKG acknowledges financial support from Dennis Zaritsky through NSF AST-9619576, and AD is supported through the Graduiertenkolleg ``The Magellanic Clouds and other dwarf galaxies''.

References

Dieball A., Grebel E.K., 1998, A&A, submitted
Grebel E.K., 1997, A&A 317, 448
Bjorkman J.E., Cassinelli J.P., 1993, ApJ 409, 429
Zorec J., Briot D., 1997, A&A 318, 443

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First version:	18th	March,	1998
Last update:	29th	September,	1998