Optical spectroscopy of Galactic field Classical Be stars

TL;DR

This study analyzes emission lines in 118 Galactic Be stars, re-estimates extinction using Gaia data, and explores the properties and formation regions of various emission lines, revealing insights into their circumstellar discs.

Contribution

It provides a comprehensive spectroscopic analysis of classical Be stars, incorporating Gaia-based extinction correction and investigating emission line characteristics and formation regions.

Findings

Approximately 20% of stars have optically thick circumstellar discs.

Hα emission equivalent width below 10 Å is necessary for Fe II emission.

Emission line intensities peak near spectral types B1-B2.

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 \AA. 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 $D_{34}$ $\geq$ 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.