Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999arep...43..167y&link_type=abstract
Astronomy Reports, Volume 43, Issue 3, March 1999, pp.167-175
Astronomy and Astrophysics
Astronomy
Scientific paper
We perform an in-depth analysis of the infrared variability of the symbiotic Mira V407 Cyg resulting both from its pulsations and from a secular trend in its mean brightness. The two effects are shown to be similar in nature and are attributable to a change in the bolometric luminosity of the Mira in combination with a change in its temperature and/or in the optical depth of its dust envelope. In the last fourteen years, the mean bolometric luminosity has risen by ~30%, which must have been reflected in an increase of the pulsation period by delta P/P >= 0.1. The optical depth tau(J) of the dust envelope decreased by ~0.06, or the temperature of the Mira rose by ~100 K. Both of these effects result in approximately the same displacement of the star in the (J-L, K) diagram, which is most suitable for estimating variations in the parameters (L, T, tau). The changes in the shape of the energy distribution observed during pulsations of the Mira can be explained in terms of the dust-envelope model in which a new dust layer condenses during the pulsation minimum at a distance of ~3.5 R*. It consists of a mixture of silicate and graphite grains with a total mass of ~7 x 10^-9 M_solar. At this time, more than half of the optical depth of the entire dust envelope is concentrated in this layer, which changes by a factor of ~2 in half the pulsation period (~745d). Near the pulsation maximum, tau(J) ~0.25 and the inner radius of the dust envelope is R_in ~ 5 R*. Over the past ~10^4 years, the Mira has lost its mass at an approximately constant rate of ~5 x 10^-7 M_solar yr^-1 for M_gas/M_dust ~200. The dust envelope may be composed either of small (~0.01 micron) or large (~0.1 micron) grains. In the latter case, it should be kept in mind that these grains rather than medium-sized (~0.05 micron) ones determine its optical depth. We analyze the dust-envelope models in detail in terms of their possible observational verification.
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