3 research outputs found
Secular changes in the quiescence of WZ Sge: the development of a cavity in the inner disk
We find a dimming during optical quiescence of the cataclysmic variable WZ
Sge by about half a magnitude between superoutbursts. We connect the dimming
with the development of a cavity in the inner part of the accretion disk. We
suggest that, when the cavity is big enough, accretion of material is governed
by the magnetic field of the white dwarf and pulsations from the weakly
magnetic white dwarf appear. The time scale of forming the cavity is about a
decade, and it persists throughout the whole quiescent phase. Such a cavity can
be accommodated well by the proposed magnetic propeller model for WZ Sge, where
during quiescence mass is being expelled by the magnetic white dwarf from the
inner regions of the accretion disk to larger radii.Comment: 10 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics; following referee report, many textual changes, figures
improved, more historic data added, conclusions unchange
Secular Evolution in Mira Variable Pulsations
Stellar evolution theory predicts that asymptotic giant branch stars undergo
a series of short thermal pulses that significantly change their luminosity and
mass on timescales of hundreds to thousands of years. Secular changes in these
stars resulting from thermal pulses can be detected as measurable changes in
period if the star is undergoing Mira pulsations. The American Association of
Variable Star Observers (AAVSO) International Database currently contains
visual data for over 1500 Mira variables. Light curves for these stars span
nearly a century in some cases, making it possible to study the secular
evolution of the pulsation behavior on these timescales. In this paper, we
present the results of our study of period change in 547 Mira variables using
data from the AAVSO. We find non-zero rates of period change, dlnP/dt, at the
2-sigma significance level in 57 of the 547 stars, at the 3-sigma level in 21
stars, and at the level of 6-sigma or greater in eight of the 547. The latter
eight stars have been previously noted in the literature, and our derived rates
of period changes largely agree with published values. The largest and most
statistically significant dlnP/dt are consistent with the rates of period
change expected during thermal pulses on the AGB. A number of other stars
exhibit non-monotonic period changes on decades-long timescales, the cause of
which is not yet known.Comment: 37 pages, with 9 figures and 1 table. The complete electronic version
of Table 1 is available from the authors upon request. Accepted for
publication in The Astronomical Journa