Evidence of Dissipation of Circumstellar Disks from L-band Spectra of Bright Galactic Be Stars

Abstract

We present L-band spectra of the Be stars γ Cas, f Per, 28 Tau, θ CrB, 66 Oph, o Her, and 28 Cyg, obtained through use of the CID-InSb spectrograph with the 2.1-m telescope at OAN/UNAM San Pedro Martir Observatory. This is the first report of L-band spectra of o Her and θ CrB, and of the data obtained with this spectrograph. We obtain flux ratios of hydrogen lines for these stars, finding that they have optically thin envelopes, except by 66 Oph and θ CrB, which do not show evidence of a circumstellar disk. γ Cas and f Per have flux ratio values of hydrogen lines closer to the optically thick case than the other stars. We use the line flux ratio diagram and optical spectra reported in the literature to study the life cycles of the disks. We find clear evidence of the dissipating process of the envelopes of 66 Oph and 28 Cyg, i.e., they are decaying stars. 28 Tau seems to have passed by a similar process. γ Cas and f Per are stable stars because their circumstellar disks do not show notorious changes for many years. Finally, the stars in a build-up phase, whose envelopes are generated after a decaying phase or for the first time, have not yet been observed in the L-band. It would be useful to monitor more Be stars to observe this class of stars that probably change from a very tenuous envelope to an optically thick circumstellar disk. The line flux ratio diagram seems to confirm that late Be stars have more tenuous disks than early-type Be stars, as they tend to be separated at the left bottom and the top right parts of the diagram, respectively. Larger samples of Be stars are needed to confirm this hypothesis through a statistical analysis.

Be and Bn stars: Balmer discontinuity and stellar-class relationship

“…This spectra was taken almost four months before our low-resolution spectrum with a sBD in absorption. Considering the spectral variability of this object (Mennickent et al 2009;Vieira et al 2017;Sabogal et al 2017), it is possible that the small emission in the Hα line occurred owing to the development of a new CE that was present when the sBD was observed.…”

Multi-epoch L-band Spectroscopy of the Be Star μ Centauri Prior to Outburst

“…in a Be star disk (see also M09 and Sabogal et al 2017). Our measurements illustrated in Figure 5 show that the position of μ Centauri varies along the diagonal of the graph by moving from an optically thin zone to an optically thick one.…”

“…Thus, studying the time evolution of near-IR L-band lines that form in the innermost region of the disk is of particular interest to understand the formation and dissipation processes observed in Be stars. Sabogal et al (2017) presented new L-band spectra of a number of Be stars and studied the long-term variability of these objects plus others available in the literature. They found some objects exhibiting disk dissipation, but none exhibiting disk formation.…”

Estructura y propiedades físicas de envolturas circunestelares en estrellas Be

“…Además, en muchas de ellas la contribución fotosférica resulta prácticamente despreciable. En consecuencia, estas líneas aportan información valiosa sobre las propiedades físicas, morfológicas y dinámicas de las envolturas de estas estrellas (Hony et al, 2000;Steele & Clark, 2001;Lenorzer et al, 2002a;Mennickent et al, 2009;Granada et al, 2010;Sabogal et al, 2017).…”

“…Esta región espectral se encuentra menos estudiada que el rango óptico, ya que recién en las últimas décadas se ha logrado contar con una instrumentación adecuada que brinde datos con alta resolución espectral (espectrógrafos del IR cercano y medio en grandes telescopios) y espacial (interferometría óptica). Esta región espectral ha demostrado ser de gran utilidad para los fines que buscamos (Lenorzer et al, 2002b;Mennickent et al, 2009;Granada et al, 2010;Sabogal et al, 2017).…”