20 research outputs found
Long period variables in 47 Tuc: direct evidence for lost mass
We have identified 22 new variable red giants in 47 Tuc and determined
periods for another 8 previously known variables. All red giants redder than
V-I_c=1.8 are variable at the limits of our detection threshold, which
corresponds to delta V ~ 0.1 mag. This colour limit corresponds to a luminosity
log L/L_sun=3.15 and it is considerably below the tip of the RGB at log
L/L_sun=3.35. Linear non-adiabatic models without mass loss on the giant branch
can not reproduce the observed PL laws for the low amplitude pulsators. Models
that have undergone mass loss do reproduce the observed PL relations and they
show that mass loss of the order of 0.3 M_sun occurs along the RGB and AGB. The
linear pulsation periods do not agree well with the observed periods of the
large amplitude Mira variables, which pulsate in the fundamental mode. The
solution to this problem appears to be that the nonlinear pulsation periods in
these low mass stars are considerably shorter than the linear pulsation periods
due to a rearrangement of stellar structure caused by the pulsation. Both
observations and theory show that stars evolve up the RGB and first part of the
AGB pulsating in low order overtone modes, then switch to fundamental mode at
high luminosities.Comment: 11 pages, accepted for publication in A&
A study of bright Southern Long Period Variables
In this paper we present radial velocity curves of AGB variables that exhibit
various kinds of anomalies: Semiregular variables (SRVs) with typical mira
periods, SRVs exceeding the mira 2.5 mag amplitude limit, miras with secondary
maxima in their light curves, and a SRV with a long secondary period. The stars
with reliable Hipparcos parallaxes from this and from previous studies are
plotted in a logP--diagram. Our objects nicely follow the
logP--relations determined for the LMC. This allows the pulsation mode
to be identified. While all miras fall on the fundamental mode sequence, the
SRVs fall on both the first overtone and fundamental mode sequences. The SRVs
on the fundamental mode sequence occur at both high and low luminosities, some
of them being more luminous than larger amplitude miras. This demonstrates
observationally that some parameter other than luminosity affects the stability
of long period variables, probably mass. First overtone pulsators all show
velocity amplitudes around 4 km/s. For the fundamental mode pulsators, the
velocity amplitude shows a correlation with light amplitude. The two miras R
Cen and R Nor, known for their double-peaked light curves, have velocity curves
that are quite different. The R Nor velocity curve shows no evidence of the
double peaks, meaning that the true pulsation period is the time between
alternate minima or maxima. There is slight evidence for a double bump in the R
Cen velocity curve. It is suggested that these stars are relatively massive
(3-5 Msun).Comment: 13 pages, accepted for publication in A&