Analysis of B and Be Star Populations of the Double Cluster h and χ Persei

Presented at the meeting of the Working Group on Active B Stars during the
27th IAU General Assembly in Rio de Janeiro, Brazil on 2009 August 6

Amber N. Marsh¹, M. Virginia McSwain¹, and Thayne Currie²

¹ Lehigh University
e-mail: anm506@lehigh.edu, mcswain@lehigh.edu
² Harvard - Smithsonian Center for Astrophysics
e-mail: tcurrie@cfa.harvard.edu

Received: 2009 October 29; Accepted: 2010 February 5

1  Introduction

NGC 869 and NGC 884 (h and χ Persei, respectively) are a well known double cluster rich in massive B-type stars, and have been the focus of many studies over the years. Recent studies show that NGC 869 and NGC 884 have nearly identical ages of ∼13–14 Myr, common distance moduli of dM ∼ 11.85, and common reddenings of E(B-V) ∼ 0.5–0.55 (Currie et al. 2009; Slesnick et al. 2002; Bragg & Kenyon 2005).

Currie et al. (2008) (hereafter C08) identified two populations of NGC 869 and NGC 884 stars with detected Spitzer MIPS-24 μm excess emission: 20 A and F-type stars with luminous debris disk emission and 57 brighter, earlier stars with weaker excess emission. They identify most of the latter group as candidate Be stars. However, only 21 were previously listed as Be stars (e.g. Bragg & Kenyon (2002); Slesnick et al. (2002)). Currie et al. (2010) find evidence for some variable reddening across the clusters however, this does not deviate substantially from the median reddening values they adopt for the cluster cores; E(B-V) = 0.55 for NGC 869 and E(B-V) = 0.52 for NGC 884. Their spectroscopic analysis does not indicate a separate population of cluster members which have an extinction that differs significantly from the rest of the population.

In this study, we analyze blue optical spectra of 92 early-type cluster members, including 16 candidate Be stars from C08, and investigate their near-to-mid infrared (IR) excesses. With continued monitoring of these stars in the both the optical and IR regimes, we hope to explore these excesses as a reasonable means for identifying potential Be stars within clusters, as well as to investigate the transient natures of the disks surrounding the known Be stars in NGC 869 and NGC 884.

2  Overview

Blue optical spectra of 92 members of NGC 869 and NGC 884 were taken on 2005 Nov. 14–15 using the WIYN 3.5m telescope with the Hydra spectrograph. The observed spectra cover the wavelength range 4250–4900 Å. Model spectral fits were used to measure values for V sin i, T_eff, and log g for B-type stars. V sin i was determined by comparing the He I λλ4387, 4471, 4713, and Mg II λλ4481 lines with the Kurucz ATLAS9 models (Kurucz 1994) and taking a weighted average of these four values. For stars having T_eff ≥ 15000 K, the TLUSTY BSTAR2006 models (Lanz & Hubeny 2007) were used to find T_eff and log g using the Hγ line. For stars having T_eff ≤ 15000 K, the Kurucz ATLAS9 models were used (Kurucz 1994). The method of Huang & Gies (2006) (hereafter HG06) was used to determine log g_polar. For Be stars, Strömgren photometry available from the WEBDA database was used to derive T_eff and log g_polar based on the methods of McSwain et al. (2008). The masses and radii for all stars were determined from the Schaller et al. (1992) evolutionary tracks, which are shown plotted with T_eff and log g_polar in Figure 1.

Figure 1. For both NGC 869 (left) and NGC 884 (right), T_eff and log g_polar are plotted with the evolutionary tracks of Schaller et al. (1992). The ZAMS mass of each evolutionary track is labeled along the bottom. Normal B-type stars are shown as open diamonds while Be stars are filled diamonds.

3  Results

Sixteen Be candidates from C08 are present in our sample or that of HG06. Three of these 16 stars (Nos. 869-566, 869-1162, 884-2468) show no evidence of circumstellar emission in our spectra, though all have been observed to be Be stars in the past (Keller et al. 2001). Ten of the C08 Be candidates in our spectra do show emission. Stellar parameters for the remaining 3 candidates are found in HG06, thus we cannot comment on the presence of emission. In addition, we find Be emission in one star (No. 1772) that was not observed by C08, and we present results for one additional star (No. 1268) identified as a Be star by Keller et al. (2001). These results are summarized in Table 1.

Table 1: Measured physical parameters for Be stars

Notes: 1 Identification numbers from the WEBDA database. 2 Be candidate in C08. 3 Stars not showing emission in our observations are likely transient Be stars. 4 Identified as Be star by Keller et al. (2001).

Spectral energy distributions (SEDs) for two stars in NGC 884 and one star in NGC 869 are displayed in Figure 2. UBV magnitudes are from the WEBDA database, JHK_s are from the 2MASS survey, and Spitzer [8] and [24] μm are from C08. These magnitudes were then converted to fluxes via the methods detailed in Bessell et al. (1998); Cohen et al. (1996), Colina et al. (1996); Reach et al. (2005); Rieke et al. (2008). Assuming a constant E(B-V) = 0.52 for NGC 869 and NGC 884 (Bragg & Kenyon 2005; Slesnick et al. 2002), reddened blackbody curves have been overlaid with these plots to investigate their near-to-mid IR excesses. All three stars shown in Figure 2 are proposed Be candidates (C08), with NGC 884-2138 and NGC 869-49 having emission present in our optical spectra and observed near-to-mid IR excess. NGC884-2165 is not in our spectroscopic sample but has previously been identified as a Be star and has observed IR excess (Keller et al. 2001).

4  Conclusions and Further Work

We have measured the physical parameters of 77 B-type stars and 15 Be stars in NGC 896 and NGC 884. Sixteen Be candidates from C08 are present in our sample or that of HG06. Of these 16 Be candidates, 3 stars show no evidence of emission in our optical data and are likely transient Be stars. Ten of these Be candidates do show emission in our spectra. Those Be candidates without emission in our spectra should be monitored in the future to further investigate their transient nature.

In the future, IRAC 3.6–5.8 μm data will be combined with the optical and IR fluxes used here to investigate the observed SEDs. We will fit the new SEDs using modern flux models rather than blackbody curves. Modifications accounting for variable reddening throughout the clusters will also be made. These new SED fits can then be used to model the Be disk sizes and temperatures.

5  Acknowledgments

We would like to thank the referee, Carol Jones, for her insightful comments which greatly improved this manuscript. We are grateful for travel support provided by the American Astronomical Society and the International Astronomical Union. We would like to thank Yale University for providing access to the WIYN telescope at KPNO. Institutional support was provided by Lehigh University. This work was also supported by NASA DPR number NNX08AV70G.

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