9 research outputs found
A first-overtone RR Lyrae star with cyclic period changes
A detailed light curve analysis is presented for a first-overtone RR Lyrae
star, MACHO* J050918.712-695015.31, based on MACHO and OGLE-III observations.
As a foreground object of the Large Magellanic Cloud, it gives an extraordinary
opportunity to study an almost continuous, 12-year long dataset of a relatively
bright (V~15.0 mag) RRc star with rapid period change. Cyclic period modulation
is suggested by the O-C method, where the cycle length is about 8 years. With
the available unique dataset, we could draw strong limits on other light curve
changes that may be associated to the period modulation. We could exclude both
multiple periodicity and amplitude modulation unambiguously. Any theoretical
model should reproduce the observed lack of photometric modulations. Simple
arguments are also given for possible hydromagnetic effects.Comment: 6 pages, 6 figures, accepted for publication in MNRA
The all-sky GEOS RR Lyr survey with the TAROT telescopes. Analysis of the Blazhko effect
We used the GEOS database to study the Blazhko effect of galactic RRab stars.
The database is continuously enriched by maxima supplied by amateur astronomers
and by a dedicated survey by means of the two TAROT robotic telescopes. The
same value of the Blazhko period is observed at different values of the
pulsation periods and different values of the Blazhko periods are observed at
the same value of the pulsation period. There are clues suggesting that the
Blazhko effect is changing from one cycle to the next. The secular changes in
the pulsation and Blazhko periods of Z CVn are anticorrelated. The diagrams of
magnitudes against phases of the maxima clearly show that the light curves of
Blazhko variables can be explained as modulated signals, both in amplitude and
in frequency. The closed curves describing the Blazhko cycles in such diagrams
have different shapes, reflecting the phase shifts between the epochs of the
brightest maximum and the maximum O-C. Our sample shows that both clockwise and
anticlockwise directions are possible for similar shapes. The improved
observational knowledge of the Blazhko effect, in addition to some
peculiarities of the light curves, have still to be explained by a satisfactory
physical mechanism.Comment: 13 pages, 12 figures, accepted for publication in Astronomical
Journa
Metallicities for Double Mode RR Lyrae in the Large Magellanic Cloud
Metallicities for six double mode RR Lyrae's (RRd's) in the Large Magellanic
Cloud have been estimated using the DeltaS method. The derived [Fe/H] values
are in the range [Fe/H] = -1.09 to -1.78 (or -0.95 to -1.58, adopting a
different calibration of [Fe/H] vs DeltaS). Two stars in our sample are at the
very metal rich limit of all RRd's for which metal abundance has been
estimated, either by direct measure (for field objects) or on the basis of the
hosting system (for objects in globular clusters or external galaxies). These
metal abundances, coupled with mass determinations from pulsational models and
the Petersen diagram, are used to compare the mass-metallicity distribution of
field and cluster RR Lyrae variables. We find that field and cluster RRd's seem
to follow the same mass-metallicity distribution, within the observational
errors, strengthening the case for uniformity of properties between field and
cluster variables At odds to what is usually assumed, we find no significative
difference in mass for RR Lyrae's in globular clusters of different metallicity
and Oosterhoff types, or there may even be a difference contrary to the
commonly accepted one, depending on the metallicity scale adopted to derive
masses. This ``unusual'' result for the mass-metallicity relation is probably
due, at least in part, to the inclusion of updated opacity tables in the
computation of metal-dependent pulsation models.Comment: 33 pages, 7 figures, uses aastex, accepted for publication in A
Cyclic variations in O−C
This paper presents an extensive overview of known and proposed RR Lyrae
stars in binaries. The aim is to revise and extend the list with new Galactic
field systems. We utilized maxima timings for eleven RRab type stars with
suspicious behaviour from the GEOS database, and determined maxima timings from
data of sky surveys and our own observations. This significantly extended the
number of suitable maxima timings. We modelled the proposed Light Time Effect
(LiTE) in O-C diagrams to determine orbital parameters for these systems. In
contrast to recent studies, our analysis focused on decades-long periods
instead of periods in the order of years. Secondary components were found to be
predominantly low-mass objects. However, for RZ Cet and AT Ser the mass of the
suspected companion of more than one solar mass suggests that it is a massive
white dwarf, a neutron star or even a black hole. We found that the semi-major
axes of the proposed orbits are between 1 and 20 au. Because the studied stars
belong to the closest RR Lyraes, maximal angular distances between components
during orbit should at least be between 1 and 13 mas and this improves the
chance to detect both stars using current telescopes. However, our
interpretation of the O-C diagrams as a consequence of the LiTE should be
considered as preliminary without reliable spectroscopic measurements. On the
other hand our models give a prediction of the period and radial velocity
evolution which should be sufficient for plausible proof of binarity.Comment: accepted for publication in MNRAS, 18 pages, 18 figure