The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) spectra of 10 Be stars are presented. It can be seen that the Be stars show a diversity in their ISO SWS01 spectral classi fi cations by Kraemer et al., from naked stars, stars associated with dust, stars with warm dust shells, stars with cool dust shells to very red sources. In addition, the Br α /HI(14-6) line fl ux ratio derived for the sample stars is compared with that of P Cyg, and it is found that the line ratio of Be stars which were investigated show not only lower values as suggested by Waters et al., but also larger values. Therefore, the line ratio cannot be used to judge whether a star is a Be star or not.

http://iopscience.iop.org/article/10.1088/1674-4527/9/7/005/pdf

An infrared study of Be stars based on ISO SWS01 spectra

The IR spectra of Be stars display numerous hydrogen recombination lines, constituting a great resource for obtaining information on the physical and dynamic structures of different regions within the circumstellar envelope. Nevertheless, this spectral region has not been analysed in depth, and there is a lack of synthetic spectra with which to compare observations. Therefore, we computed synthetic spectra with the HDUST code for different disc parameters. Here, we present our results on the spectral region that includes lines of the Brackett series. We discuss the dependence of the line series strengths on several parameters that describe the structure of the disc. We also compared model line profiles, fluxes, and EWs with observational data for two Be stars (MX Pup and π Aqr). Even though the synthetic spectra adequately fit our observations of both stars and allow us to constrain the parameters of the disc, there is a discrepancy with the observed data in the EW and flux measurements, especially in the case of MX Pup. It is possible that by including Brackett lines of higher terms or adding the analysis of other series, we may be able to better constrain the parameters of the observed disc. 

Infrared Spectroscopy of Be Stars: Influence of the Envelope Parameters on Brackett-Series Behaviour

This paper emphasizes the importance of spectroscopy techniques and the need of additional assumptions in analyzing stellar spectra, when experimental data are mapped in terms of a few parameters. Collisionally broadened profiles of emission lines originated in circumstellar material surrounding rotating stars are analyzed. Theoretical relationships between the ratio of the central flux intensity to the equivalent width of the emission line Fij,0/Wij, and the half-width at half of the maximum HWHMij are found. The plane shows general trends of the measurements and imposes bounds to the plasma parameters and the rotation velocities of the stars. With some educated guesses, it can be inferred whether the circumstellar material is spherically distributed or has a disc-like structure. As an example, an observational evaluation is made from data of gamma Cass.

Relations between observable parameters of collisionally broadened emission lines of rotating stars: a useful plot

We present the 2.4 45 m ISO-SWS spectrum of the Be star Cas (B0.5 IVe). The spectrum is characterised by a thermal continuum which can be well fit by a power- law S / 0:99 over the entire SWS wavelength range. For an isothermal disc of ionized gas with constant opening angle, this correponds to a density gradient (r) / r 2:8 . We report the de- tection of the Humphreys (6-1) bound-free jump in emission at 3.4 m. The size of the jump is sensitive to the electron temper- ature of the gas in the disc, and we find T 9 000 K, i.e. much lower than the stellar effective temperature (25 000-30 000 K). The spectrum is dominated by numerous emission lines, mostly from Hi, but also some Hei lines are detected. Several spectral features cannot be identified. The line strengths of the H i emis- sion lines do not follow case B recombination line theory. The line strengths and widths suggest that many lines are optically thick and come from an inner, high density region with radius 3-5 R and temperature above that of the bulk of the disc mate- rial. Only the , and transitions of the series lines contain a contribution from the outer regions. The level populations de- viate significantly from LTE and are highly influenced by the optically thick, local (disc) continuum radiation field. The inner disc may be rotating more rapidly than the stellar photosphere.

/pdf/the-infrared-spectrum-of-the-be-star-gamma-cassiopeiae-3ijmfh46u0.pdf

The infrared spectrum of the Be star gamma Cassiopeiae

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The infrared spectrum of the Be star gamma Cassiopeiae