Optical spectroscopy of Galactic field Classical Be stars

Abstract

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 Ã . We re-estimated the extinction parameter (A${}_V$) for our sample stars using the newly available 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 $D_{34}$ $\ge$ 2.7, implying that their circumstellar disc are generally optically thick in nature. One program star, HD 60855 shows H$\hat{I}\pm$ in absorption -- indicative of discless phase. From our analysis, we found that in classical Be stars, H$\hat{I}\pm$ emission equivalent width values are mostly lower than 40 Ã , which agrees with that present in literature. Moreover, we noticed that a threshold value of $\sim$ 10 Ã ~of H$\hat{I}\pm$ emission equivalent width is necessary for Fe{\sc ii} emission to become visible. We also observed that emission line equivalent widths of H$\hat{I}\pm$, P14, Fe{\sc ii} 5169 and O{\sc i} 8446 Ã ~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.

20 pages, 9 figures, accepted for publication in MNRAS