L-band spectra of 13 outbursting Be stars
R.E. Mennickent, B. Sabogal, A. Granada, L. Cidale

TL;DR
This study presents L-band spectra of 13 outbursting Be stars, classifying their circumstellar environments and analyzing emission line properties to understand disk evolution during outbursts.
Contribution
It provides the first detailed L-band spectral classification of outbursting Be stars and links emission line features to disk optical depth changes.
Findings
Humphreys decrements follow Menzel case-B in optically thin conditions.
Transition from optically thin to thick occurs around 3.65--3.75 μm.
Infrared lines show larger broadening than optical lines.
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 Br and Pf 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 Br and Pf lines stronger than Humphreys lines, the Humphreys decrement roughly follow the Menzel case-B for optically thin conditions. For the group showing comparable Br, Pf and Humphreys emission line strengths, the Humphreys decrements 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…
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