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تسجيل مستخدم جديدOptical spectroscopy of Classical Be stars in open clusters
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We present a spectroscopic study of 150 Classical Be stars in 39 open clusters using medium resolution spectra in the wavelength range 3800 - 9000 AA. One-third of the sample (48 stars in 18 clusters) has been studied for the first time. All these candidates were identified from an extensive survey of emission stars in young open clusters using slitless spectroscopy (Mathew et al. 2008). This large data set covers CBe stars of various spectral types and ages found in different cluster environments in largely northern open clusters, and is used to study the spectral characteristics of CBe stars in cluster environments. About 80% of CBe stars in our sample have H-alpha equivalent width in the range -1 to -40 AA. About 86% of the surveyed CBe stars show Fe II lines. The prominent Fe II lines in our surveyed stars are 4584, 5018, 5169, 5316, 6318, 6384, 7513 and 7712 AA. We have identified short and long-term line profile variability in some candidate stars through repeated observations.
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In this study, we analyze the emission lines of different species present in 118 Galactic field classical Be stars in the wavelength range of 3800 - 9000 AA. We re-estimated the extinction parameter (A$_V$) for our sample stars using the newly availa
ble data from Gaia DR2 and suggest that it is important to consider A$_V$ while measuring the Balmer decrement (i.e. $D_{34}$ and $D_{54}$) values in classical Be stars. Subsequently, we estimated the Balmer decrement values for 105 program stars and found that $approx$ 20% of them show $D_{34}$ $geq$ 2.7, implying that their circumstellar disc are generally optically thick in nature. One program star, HD 60855 shows H$alpha$ in absorption -- indicative of discless phase. From our analysis, we found that in classical Be stars, H$alpha$ emission equivalent width values are mostly lower than 40 AA, which agrees with that present in literature. Moreover, we noticed that a threshold value of $sim$ 10 AA~of H$alpha$ emission equivalent width is necessary for Fe{sc ii} emission to become visible. We also observed that emission line equivalent widths of H$alpha$, P14, Fe{sc ii} 5169 and O{sc i} 8446 AA~for our program stars tend to be more intense in earlier spectral types, peaking mostly near B1-B2. Furthermore, we explored various formation regions of Ca{sc ii} emission lines around the circumstellar disc of classical Be stars. We suggest the possibility that Ca{sc ii} triplet emission can originate either in the circumbinary disc or from the cooler outer regions of the disc, which might not be isothermal in nature.
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Rapid rotation is a fundamental characteristic of classical Be stars and a crucial property allowing for the formation of their circumstellar disks. Past evolution in a mass and angular momentum transferring binary system offers a plausible solution
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