A Study of Be Stars in the Magellanic Clouds 3
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A Mon. Not. R. Astron. Soc. 000, 1–10 (2013) Printed 27 November 2017 (MN LTEX style file v2.2) A study of Be stars in the Magellanic Clouds S. Iqbal1⋆ and S. C. Keller1† 1Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia. Received 2013 August ABSTRACT We present the results of a photometric survey for Be stars in eleven young clusters in the Large Magellanic Cloud and fourteen young clusters in the Small Magellanic Cloud. B stars with hydrogen in emission are identified on the basis of their R-Hα colour. We find that Be star fraction in clusters decreases with cluster age, and also decreases with the metallicity. Key words: early-type stars: emission-line, Be galaxies: Magellanic Clouds 1 INTRODUCTION pare the predictions of evolutionary models for early-type stars for NGC 2004 and the N 11 region on the LMC, and The transient nature of emission lines, especially those of NGC 330 and NGC 346 in the SMC. They find that their the Balmer series of hydrogen, exhibited in the spectra of nitrogen abundances are inconsistent with those predicted some B-type stars is known as the ‘Be phenomenon’. These for stars that spend most of the main-sequence lifetimes ro- spectral changes are attributed to a disk of gaseous material tating close to their critical velocity. In particular, they find surrounding the central star, the origins of which are still similar estimated nitrogen enrichment for Be and B type unclear. While recent observational data has allowed for the stars, and postulate that either Be stars rotate faster than elimination of some theoretical models (such as the wind B stars, but not at critical velocity, or that Be stars only compressed disc models of Bjorkman & Cassinelli 1993), no spend a short period (less than 10 %) of their main-sequence satisfactory mechanism for injecting material into the disk life times rotating close to critical velocity. has been proposed to date. It is likely that the formation of Photometric surveys for Be stars have been conducted this disk may be attributed to a number of factors, including, in the Galaxy but are limited by the relative paucity of Be but not limited to, non-radial pulsations, rapid rotation, and stars (e.g. McSwain & Gies 2005; Keller et al. 2001). On the the magnetic properties of the central star. other hand, the Magellanic Clouds provide large samples The viscous decretion model suggests that a B star ro- of Be stars in different environments and the opportunity tating at less than critical velocity ejects material into a cir- to examine the effect of cluster age and metallicity on the cumstellar disk through an unknown mechanism. The mate- Be phenomenon. Early searches for Be stars in the Magel- rial then settles into a Keplarian orbit and the radial struc- arXiv:1309.1224v1 [astro-ph.GA] 5 Sep 2013 lanic Clouds have been made by Feast (1972) using objec- ture of the disk is governed by viscosity. It was first proposed tive prisms, while later surveys, such as Keller et al. (1999), by Lee et al. (1991) and developed by Bjorkman (1997), Wisniewski & Bjorkman (2006) and Martayan et al. (2010), Okazaki et al. (2002) and Bjorkman & Carciofi (2005). have relied on photometry achieved through a narrow-band The Be phenomenon has always been tied to the rapid Hα filter. Photometric techniques are advantageous as they rotation of a B star. The initial rotational velocity of a B allow for efficient selection of Be stars, especially within star must be at least 70% of its critical velocity for it to dense clusters where spectroscopy is difficult. exhibit Be emission (Porter 1996; Ekstr¨om et al. 2008). Maeder et al. (1999) summarise the content of Be stars To study the possible evolution of B stars into Be stars, within Large Magellanic Cloud (LMC), Small Magellanic rapidly rotating stars have been modelled by Endal & Sofia Cloud (SMC) and Milky Way (MW) to disentangle age and (1979) and Endal (1982). Endal & Sofia show that stars that metallicity effects of the Be phenomenon. Within their sam- have low rotation at birth are unlikely to develop into Be ple there is a clear decrease in Be star fraction with increas- stars, while those born rotating at 59% - 76% of their break ing metallicity. Similarly, Wisniewski & Bjorkman (2006) up velocity are highly likely to become Be stars. use photometry to determine the Be star content of eight However, Cranmer (2005) suggests that a subset of clas- clusters in the SMC, five in the LMC, and three in the MW, sical Be stars could could have rotations as low as 40% - and find that the Be phenomenon is enhanced in low metal- 60% of their critical velocity. Dunstall et al. (2011) com- licity environments. Additionally, Ekstr¨om et al. (2008) and Keller (2004) find that the fraction of fast rotators on the ⋆ E-mail: [email protected] zero-age main-sequence is higher at lower metallicities. † E-mail: [email protected] This paper examines the effects of metallicity and age c 2013 RAS 2 S. Iqbal and S. C. Keller on Be star formation through a photometric study of clusters in the Magellanic Clouds. In Section 2 we outline the obser- vations undertaken for this study and the data reduction steps used to extract photometry. We describe the selection criteria used to identify Be star candidates in Section 3 and compare our results to previous photometric searches for Be stars in Section 4. We outline the results obtained through the photometry in Section 5. Finally we present our conclu- sions in Section 6. 2 OBSERVATIONS AND DATA REDUCTION Target clusters were chosen from the OGLE catalog (Paczynski 1996) satisfying 7 < log age < 8, allowing us to probe the Be star content of young clusters. We note that this criterion excludes early-type Be stars from our sample. To identify B stars exhibiting hydrogen emission we im- aged through a narrow-band Hα filter (λc = 6563 A,˚ ∆λ = Figure 1. The difference in V magnitudes in our work and those 300 A)˚ and compared it with an image taken in the Cousins in the MCPC for field stars (I < 17) around NGC 330. We noted R band. Stars with strong hydrogen emission have an R−Hα an offset of 0.02 magnitudes is observed, which we considered in colour that is larger than stars without emission at a given our analysis. V − I. The observations were obtained using the Faulkes South Hα emission. Stars with strong Hα emission were selected Telescope at Siding Spring Observatory from July to Novem- from these plots. Examples are shown on the left hand side × ber in 2010. Each image is 10.04 10.04 arc minutes in size, of Figures 2 and 3. with a pixel scale of 0.3 arc seconds per pixel. Exposure times for images using the g, i and R filters were 90 seconds each, In each cluster we restrict our definition of a B star to stars and three 300 second exposures were taken with the Hα still on the main-sequence, with V magnitudes between the filter. While the typical seeing for the entirety of the observ- Main Sequence Turn Off (MSTO) and V = 18. Blue super- ing period was between 2” and 3”, for each cluster the value giant stars lie directly above the main-sequence stars, with varied between 1.83” (ESO 86SC2) and 3.87” (NGC 1902). V < MSTO. A star displaying Be characteristics lying above The photometry of the CCD fields was carried out using the MSTO is likely to be a supergiant B[e] star: a B type Source Extractor (Bertin & Arnouts 1996). After excluding star with forbidden emission lines in its optical spectrum sources with abnormal Full-Width at Half-Maximum mea- (Lamers et al. 1998). Since we are only interested in iden- surements and saturated sources, the stars with good pho- tifying classical (non-supergiant) Be stars in our study, we tometry across all four filters were matched. The g and exclude the stars above the MSTO from our search. i magnitudes were transformed to Bessell V and I us- As mentioned in Section 2, the R − Hα colour has an ing the Magellanic Clouds Photometric Catalog (MCPC; arbitrary zero point. A zero-point for this colour was selected Zaritsky et al. 2004), but the R and Hα magnitudes were such that the main-sequence stars lie at R−Hα = 0 for each not calibrated and the R − Hα colour has an arbitrary zero- cluster. Thus in R − Hα,V − I main-sequence stars form a point. tight clump near the origin, while nearly all the cooler stars Figure 1 shows the difference in V magnitudes in our form a horizontal band. These stars do not exhibit hydrogen work and those in the MCPC for field stars around NGC 330. emission. Only the brightest stars (I < 17) are used to calibrate our The group of stars that extend up and slightly to the data with the MCPC for each cluster. An offset of 0.02 mag- red of the main-sequence clump clearly show significant Hα nitudes is observed, which was taken into consideration in emission, and these are the stars of interest to us, shown on our analysis, while the standard deviation is 0.07 magni- the plots as (purple) squares. Occasionally, cooler giant stars tudes. also exhibit strong Hα emission. Cool objects with excess In order to determine the cluster members, we find the hydrogen emission are shown as (red) circles on the plot, radius at which the stellar density drops to a value indis- isolated as potentially interesting objects.