A theoretical study of the R. Coronae Borealis group of variable stars
Abstract
A theoretical study of the R Coronae Borealis group of variables is
presented. A modified Christy code is used for the non-linear
calculations and Dr Worrell's codes are used for the linear
calculations. These codes are used in conjunction with both Carson
and Los Alamos opacity tables, following the work of Saio on the
linear analysis of Hydrogen deficient Carbon stars. 630 linear models
are presented in 3 surveys along with 10 non-linear models of RCB
stars and 20 incidental models used for comparison purposes.
A study of the observations, given in the literature showed that
the masses and luminosities of these stars are poorly defined. A
linear analysis of 7 of these stars, in conjunction with observations
given in literature, has allowed luminosity limitations to be placed
on seven of these stars, and for the two stars which also have
spectroscopic data a rough mass was calculable. The spectroscopic
mass and luminosity limits of R CrB are (0.96 +1- 0.07)M☉ and
(9,500 - 16,500)L☉
and for BY Sgr are (0.95 +1- 0.06)M☉
and
(9,000 - 13,000)L☉. A non-linear model of RY Sgr using the best mass
and luminosity produced a light curve that agreed well with the
overall characteristics of the observations, i.e., period, amplitude
m
and semi-regularity. Another of the non-linear models showed a 7ᵐ
drop in its luminosity curve, which lasted for about twenty days.
This is greatly short of the months to years seen in the observations,
but could be suggestive of the initiation of the 'deep minimum'
phenomenon. It was followed by the rapid outward movement of all the
outer zones, which could indicate a method of ejecting carbon grains.
Further research needs to be done before any conclusions can be drawn.
The non-linear analysis of the 5,000K and 6,000K models may
indicate that the effective temperatures given in the literature for
these stars are too low, as a value nearer 7,000K seems to give better
results. From this analysis, it seems that the majority of the ReB
group of stars have masses in the range (0.8 - 1.2)M☉, which is
consistent with the evolutionary analysis of Weiss.
Type
Thesis, PhD Doctor of Philosophy
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