L-Band Spectra of 13 Outbursting Be Stars

Abstract

We present new L-band spectra of 13 outbursting Be stars obtained with ISAAC at the ESO Paranal observatory. These stars can be classified in three groups depending on the presence/absence of emission lines and the strength of Brα and Pfγ emission lines relative to those of Humphreys lines from transitions 6-14 to the end of the series. These groups are representative of circumstellar envelopes with different optical depths. For the group showing Brα and Pfγ lines stronger than Humphreys lines, the Humphreys decrement roughly follow the Menzel case-B for optically thin conditions. For the group showing comparable Brα, Pfγ and Humphreys emission line strengths, the Humphreys decre-ments moves from an optically thin to an optically thick regime at a transition wavelength which is characteristic for each star, but typically is located around 3.65-3.75 µm (transitions 6-19 and 6-17). Higher order Humphreys lines probe optically thin inner regions even in the optically thicker envelopes. We find evidence of larger broadening in the infrared emission lines compared with optical lines, probably reflecting larger vertical velocity fields near the star. The existence of the aforementioned groups is in principle consistent with the proposed description by de Wit et al. (2006) for Be star outbursts in terms of the ejection of an optically thick disk that expands and becomes optically thin before dissipation into the interstellar medium. Time resolved L-band spectroscopy sampling the outburst cycle promises to be an unique tool for testing Be star disk evolution.

Search for Southern Galactic Be Star Candidates

“…Mennickent et al (2009) and Granada, Arias, & Cidale (2010) analyzed independently L-band spectra of different samples of outbursting Galactic Be stars. They found changes in the optical density of the CDDs that could be consistent with the model proposed by de Wit et al (2006).…”

K- and L-band spectroscopy of Be stars

“…Their profiles are sensitive to the physical properties and dynamical structure of the line-forming regions and thus become useful probes of circumstellar environments. An inspection of the L-band hydrogen spectra of Be stars allows a classification in three groups following Mennickent et al (2009): Group I contains the stars with Brα and Pfγ equally intense as Hu lines, Group II consists of those with Brα and Pfγ more intense than Hu lines, and Group III is made up of those with no detected emission. This classification scheme reflects the optical depth conditions in the Be star envelope.…”

“…We describe the behaviour of IR hydrogen emission lines of a sample of Be stars and discuss the physical properties of the circumstellar envelopes of Be stars classified in Groups I and II (Mennickent et al 2009). We find that while Humphreys and Pfund lines of Group I stars form in an optically thick envelope/disk, Group II stars show Pfund lines that form in an optically thick medium and Humphreys lines originating in optically thinner regions.…”

A spectroscopic study of Be-like variable stars in the Small Magellanic Cloud

“…The same kind of variable stars were found in the Large Magellanic Cloud (Sabogal et al 2005) and in the direction of the Galactic Bulge (Sabogal et al 2008). Samples of Type-1 stars were studied by de Wit et al (2006) and by Mennickent et al (2009). These studies showed that Type-1 stars could be stars with an optically thick circumstellar envelope that evolves in an optically thin one before to dissipate into the interstellar medium.…”