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 Å. We re-estimated the extinction parameter (AV) for our sample stars using the newly available data from Gaia DR2 and suggest that it is important to consider AV while measuring the Balmer decrement (i.e. D34 and D54) values in classical Be stars. Subsequently, we estimated the Balmer decrement values for 105 program stars and found that ≈ 20% of them show D34 ≥ 2.7, implying that their circumstellar disc are generally optically thick in nature. One program star, HD 60855 shows Hα in absorption- indicative of disc-less phase. From our analysis, we found that in classical Be stars, Hα emission equivalent width values are mostly lower than 40 Å, which agrees with that present in literature. Moreover, we noticed that a threshold value of ∼ 10 Å of Hα emission equivalent width is necessary for Fe ii emission to become visible. We also observed that emission line equivalent widths of Hα, P14, Fe ii 5169 and O i 8446 Å 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 ii emission lines around the circumstellar disc of classical Be stars. We suggest the possibility that Ca 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.

Decoding the X-Ray Flare from MAXI J0709–159 Using Optical Spectroscopy and Multiepoch Photometry

“…It is known that He I emission lines are found rarely in Be stars (Banerjee et al 2021) and are produced in hightemperature regions with T ∼15,000 K (Kwan & Fischer 2011). Bahng & Hendry (1975) proposed that these lines may either be a temporary phenomenon or non-LTE effects are responsible for their selective excitation.…”

“…A Be star is a special class of massive B-type main-sequence (MS) star, surrounded by a geometrically thin, equatorial, gaseous, decretion disk (Meilland et al 2007). Be stars are primarily identified by means of a Hα emission feature formed in the ionized decretion disk of Be stars, by means of the recombination process (e.g., Banerjee et al 2021;Mathew et al 2012).…”

Multiple giant eruptions and X-ray emission in the recoiling AGN/LBV candidate SDSS1133

“…The rest-frame velocity widths are 386.45, 172.67, and 170.43 km s −1 for the SDSS and two LRIS spectra (the later two are spectrally-unresolved), respectively, suggesting that the line widths of these lines are variable. The emission strength ratio of Ca II triplets is roughly 1:1:1 rather than the theoretically predicted value of 1:9:5, suggesting that the line-forming region is optically-thick (e.g., Apparao & Tarafdar 1988;Banerjee et al 2021). The resemblance between the Ca II and Fe II line widths in the SDSS spectrum implies that Ca II and Fe II emission lines are originated from the same, high density photo-ionization region.…”

Optical Spectroscopy of Classical Be Stars in Old Open Clusters

“…They are main sequence stars having luminosity classes III-V. An in depth overview of CBe star studies carried out on different aspects is given in Rivinius et al (2013) and Porter & Rivinius (2003). Being a subset of massive B-type stars, these CBe stars are rapid rotators and are characterized by emission lines of different elements in their spectra (e.g., Mathew & Subramaniam 2011;Paul et al 2012;Aguayo et al 2017;Shokry et al 2018;Banerjee et al 2021), in addition to an infrared excess in the spectral energy distribution (Gehrz et al 1974;Hartmann & Cassinelli 1977). These emission lines and infrared excess originate from a surrounding circumstellar disk, which is an equatorial, gaseous, geometrically thin decretion disk that orbits the central star (Meilland et al 2007).…”

“…Hence, several spectroscopic surveys have been performed until date, both in optical (e.g., Andrillat & Fehrenbach 1982;Dachs et al 1986;Hanuschik 1986;Koubský et al 2012;Arcos et al 2017;Klement et al 2019) and in near-infrared (e.g., Clark & Steele 2000;Steele & Clark 2001;Granada et al 2011) wavelength bands for characterizing CBe star disks and better understanding the Be phenomenon. Focusing on similar aspects, numerous additional spectroscopic surveys have also been done for CBe stars situated in different environments, such as fields (Banerjee et al 2021), open clusters (Mathew et al 2008) and extragalactic regimes such as the Magellanic Clouds (Paul et al 2012).…”