Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986a%26a...159...75d&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 159, no. 1-2, April 1986, p. 75-89.
Astronomy and Astrophysics
Astrophysics
19
B Stars, Far Ultraviolet Radiation, Stellar Color, Stellar Luminosity, Stellar Spectra, Visible Spectrum, Periodic Variations, Photosphere, Radiant Flux Density, Spectrum Analysis, Stellar Atmospheres, Stellar Envelopes, Stellar Mass Ejection, Stellar Models, Stellar Spectrophotometry
Scientific paper
In this paper, hereafter called Paper II, we analyse and interpret the observations, presented in Paper I, of the luminosity variations, from the far-UV to the visual region, of 88 Her as it changes from the quasi-normal B phase to the Be-shell phase. These observations show two remarkable facts: (i) The luminosity decreases, in all the observed wavelengths, when the star changes from the quasi-normal B phase to the beginning of the Be-shell phase; and (ii) the luminosity increases, in all the observed wavelengths, as the Be-shell spectrum develops. In Paper I, we showed that the basic assumption of traditional Be star modelling, which attributes all the observed luminosity variations to changes in the physical conditions of the exophotospheric cool envelope - while the photosphere remains invariable - did not agree with the observations. In this Paper II, we analyze the data without any preconceived assumptions on what are the atmospheric regions producing the luminosity changes; we admit that the observed luminosity variations may arise from changes of the structure and thermodynamic state of both the photosphere and the exophotospheric cool envelope. We present an iterative approach to such disguostics of the comparative roles of the photosphere and envelope: each higher- order in the iteration adds one more atmospheric region and/or one more kind of absorption/emission process in the envelope. We show that different spectral regions respond quite differently, in amplitude of change, to changes in photospheric temperature, and in absorption/emission by the cool envelope. We emphasize that the sensitivity of our diagnostic approach comes from our focus on luminosity changes which accompany phase changes, and on our analysis of such luminosity changes and their wavelength dependence over a broad wavelength range - from the far-UV to the visual region. We conclude from our analysis: 1) The major cause which produces the luminosity drop, when the star varies from the quasi-normal B phase to the beginning of the Be-shell phase, is a decrease in photospheric temperature by about 1000 K. However, in order to adequately represent the wavelength dependence of these luminosity changes, it is necessary to also take into account the effect of the envelope absorption and emission. 2) Apparently, this decrease in photospheric thermal energy is associated with that increase in mass-outflow, which produces that increase in mass-content of the envelope, which is observed as a strengthening of the shell-spectrum. 3) The strengthening of the shell-spectrum, which reflects the increase in mass-content, and presumably the size of the envelope, is associated with a monotonic increase in luminosity which, according to our diagnostics, demands either an increase in photospheric radius with little change in photo spheric temperature or a mild increase in photo spheric temperature and in envelope radius but no change in photospheric radius. The observations are inadequate for a definitive choice between the two alternatives; although we favor the latter, from considerations based on physical consistency and observations of other stars. 4) These results leave us with a choice between searching either for a direct way of converting thermal energy of the photosphere into mass-outflow, or for a subphotospheric diversion of some energy from photospheric thermal energy into nonthermal energy modes which produce and amplify mass-outflow.
Barylak Michael
Doazan Vera
Thomas Nicholas R.
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