Searching for Disks in the Magellanic Clouds

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

K.S. Bjorkman,¹ J.P. Wisniewski,¹ and A.M. Magalhães²

¹ Ritter Observatory, Dept. of Physics & Astronomy, University of Toledo, Toledo, OH 43606
email: karen@astro.utoledo.edu, jwisnie@physics.utoledo.edu
² Instituto Astronomico e Geofisico, Universidade de São Paulo, Caixa Postal 3386, SP 01060-970 São Paulo, Brazil
email: mario@astro.iag.usp.br

Received: 2004 August 6; Accepted: 2004 August 9.

While rotation is clearly the dominant effect resulting in the differences between Be stars and normal B stars, recent work on the nature of the Be phenomenon has led to several testable suggestions about possible additional underlying causes for the presence of circumstellar disks in Be stars. Fabregat & Torrejon (2000) have suggested, on the basis of fractions of Be stars in older and younger clusters, that the development of Be characteristics may be correlated with later stages of the main-sequence lifetime of B stars. Grebel et al. (1992, 1996; Grebel 1997; Dieball & Grebel 1998; Grebel & Chu 2000), Keller et al. (1999), and others (Mermilliod et al. 1982) have used photometric surveys in UBVRI and H to suggest that the fraction of Be stars is higher in young star clusters in the Large (LMC) and Small (SMC) Magellanic Clouds compared with the fraction in clusters in our Milky Way Galaxy (MWG). These results, together with some theoretical arguments, suggest that metallicity also may play a role in the ability to form disks.

One of the problems of addressing questions of the relative fractions of Be vs. B stars in clusters and their surrounding regions is that using H excess as the sole criterion for finding Be stars makes the assumption that only Be stars (i.e., classical disk-like systems) will show H excess. However, other types of objects, including pre-main-sequence Herbig Ae/Be (HAeBe) stars, OB supergiants, B[e] stars, and even candidate LBV's, may well "contaminate" such a selected sample. What is needed is a way to further separate the H-selected sample to pick out the true classical Be (circumstellar disk) systems.

Polarization observations provide a useful means to try to do this. A net polarization arises from electron scattering in the circumstellar disk, modified across the spectrum by emission and absorption processes in the disk. This causes classical Be stars to show intrinsic polarization with characteristics different from that seen in other classes of H emitting stars. Furthermore, the wavelength-dependent nature of this polarization can be predicted and has been tested. Thus, the combination of multi-wavelength polarization measurements (either UBVRI broad-band or spectro-polarimetry) with BVRH photometric measurements should, in principle, provide a more accurate test of the true fraction of Be stars in clusters.

We are carrying out such a study, using a polarimeter module in combination UBVRI imaging capabilities at the 1.5m telescope at Cerro-Tololo Inter-American Observatory (CTIO). Here we report on the initial stages of the project, and illustrate that preliminary results do confirm the usefulness of the technique. Figure 1 shows a sample CCD imaging polarimetry image from one of our clusters. Note the double images for each star - the polarization module splits each image into an ordinary and extraordinary image. To measure the polarization, images like this are made at each of 8 polarization waveplate positions. Comparison between the ordinary and extraordinary images at each position are then used to determine the polarization level and its position angle for each star. Figure 2 shows an example UBVRI polarization measurement for one star from this cluster, a Be candidate identified from the photometry. This candidate did turn out to be a classical Be star disk system.

Figure 1. A CCD image of the LMC cluster NGC 1948 taken using the polarizing module with the waveplate at one of the eight measured positions.

Figure 1. UBVRI polarization results for one Be star candidate in the cluster. Note the characteristic shape of the wavelength dependence of the polarization, which clearly shows a polarization Balmer jump. This is certainly a classical Be disk system.

The present status of the project is that we now have complete UBVRI polarimetry data for 5 LMC and 5 SMC clusters, and partial coverage (not all 5 filters) for for 1 additional LMC cluster and 1 additional SMC cluster. For 2 LMC clusters we have limited polarimetry coverage (1-2 filters). For the photometry, we have 8 SMC and 5 LMC clusters, and 3 MWG clusters, with full BVRH filter photometry. Our photometry data are supplemented with data on additional clusters from the literature. For the combined project, we have 4 SMC and 2 LMC clusters with a full BVRH complement of photometry and UBVRI polarimetry. We are also following up on candidates identified from the polarimetry, using spectroscopy in the optical and near-IR to determine disk properties. Full details of our results will be presented in forthcoming papers. As a side benefit, several other results will come from these efforts, including polarization catalogs, interstellar polarization measurements, analysis of magnetic fields in the regions of the clusters, and polarization observations of many other types of objects found in and around the clusters of interest.

Acknowledgements. This work has been supported in part by NASA LTSA grant NAG5-8054 and NASA GSRP fellowship NGT5-50469 to the University of Toledo. KSB is a Cottrell Scholar of the Research Corporation, and gratefully acknowledges their support. We thank the CTIO TACs for granting observing time for this project, and the NOAO for providing travel support for JPW to CTIO. KSB and AMM thank FAPESP and IAG-USP for partial travel support. AMM is also supported by CNPq.

References:

Dieball & Grebel 1998, A&A, 339, 773

Fabregat & Torrejon 2000, A&A, 347 451

Fullerton et al. 2000, ApJ, 538, L43

Grebel et al. 1992, A&A, 254, L5

Grebel et al. 1996, A&A, 311, 470

Grebel 1997, A&A, 317, 448

Grebel & Chu 2000, AJ, 119, 787

Keller et al. 1999, A&AS, 134, 489

Maeder 1999, A&A, 347, 185

Maeder et al. 1999, A&A, 346, 459

Mazzali et al. 1996, A&A, 316, 173

Mermilliod 1982, A&A, 109, 48

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