52 research outputs found
The anomalous Cepheid XZ Ceti
XZ Ceti is the only known anomalous Cepheid in the Galactic field. Being the
nearest and brightest such variable star, a detailed study of XZ Ceti may shed
light on the behaviour of anomalous Cepheids whose representatives have been
mostly detected in external galaxies. CCD photometric and radial velocity
observations have been obtained. The actual period and amplitude of pulsation
were determined by Fourier analysis. The long time scale behaviour of the
pulsation period was studied by the method of the O-C diagram using the
archival Harvard photographic plates and published photometric data. XZ Ceti
differs from the ordinary classical Cepheids in several respects. Its most
peculiar feature is cycle-to-cycle variability of the light curve. The radial
velocity phase curve is not stable either. The pulsation period is subjected to
strong changes on various time scales including a very short one. The ratio of
amplitudes determined from the photometric and radial velocity observations
indicates that this Cepheid performs an overtone pulsation, in accord with the
other known anomalous Cepheid in our Galaxy, BL Boo (V19 in the globular
cluster NGC 5466). Continued observations are necessary to study the deviations
from regularity, to determine their time scale, as well as to confirm binarity
of XZ Ceti and to study its role in the observed peculiar behaviour.Comment: 7 pages, 4 figures. accepted for Astron. Astrophy
V371 Per - A Thick-Disk, Short-Period F/1O Cepheid
V371 Per was found to be a double-mode Cepheid with a fundamental mode period
of 1.738 days, the shortest among Galactic beat Cepheids, and an unusually high
period ratio of 0.731, while the other Galactic beat Cepheids have period
ratios between 0.697 and 0.713. The latter suggests that the star has a
metallicity [Fe/H] between -1 and -0.7. The derived distance from the Galactic
Plane places it in the Thick Disk or the Halo, while all other Galactic beat
Cepheids belong to the Thin Disk. There are indications from historical data
that both the fundamental and first overtone periods have lengthened.Comment: Accepted for publication in MNRA
A dwarf nova in the globular cluster M13
Dwarf novae in globular clusters seem to be rare with only 13 detections in
the 157 known Galactic globular clusters. We report the identification of a new
dwarf nova in M13, the 14th dwarf nova identified in a globular cluster to
date. Using the 2m Faulkes Telescope North, we conducted a search for stars in
M13 that show variability over a year (2005-2006) on timescales of days and
months. This led to the detection of one dwarf nova showing several outbursts.
A Chandra X-ray source is coincident with this dwarf nova and shows both a
spectrum and variability consistent with that expected from a dwarf nova, thus
supporting the identification. We searched for a counterpart in Hubble Space
Telescope ACS/WFC archived images and found at least 11 candidates, of which we
could characterize only the 7 brightest, including one with a 3 sigma H-alpha
excess and a faint blue star. The detection of one dwarf nova when more could
have been expected likely indicates that our knowledge of the global Galactic
population of cataclysmic variables is too limited. The proportion of dwarf
novae may be lower than found in catalogs, or they may have a much smaller duty
cycle in general as proposed by some population synthesis models and recent
observations in the field.Comment: Accepted for publication in ApJ, 12 pages, 12 figures, 5 tables (v2
contains corrections of the proofs
New periodic variable stars coincident with ROSAT sources discovered using SuperWASP
We present optical lightcurves of 428 periodic variable stars coincident with ROSAT X-ray sources, detected using the first run of the SuperWASP photometric survey. Only 68 of these were previously recognised as periodic variables. A further 30 of these objects are previously known pre-main sequence stars, for which we detect a modulation period for the first time. Amongst the newly identified periodic variables, many appear to be close eclipsing binaries, their X-ray emission is presumably the result of RS CVn type behaviour. Others are probably BY Dra stars, pre-main sequence stars and other rapid rotators displaying enhanced coronal activity. A number of previously catalogued pulsating variables (RR Lyr stars and Cepheids) coincident with X-ray sources are also seen, but we show hat these are likely to be misclassifications. We identify four objects which are probable low mass eclipsing binary stars, based on
their very red colour and light curve morphology
Light Curves and Period Changes of Type II Cepheids in the Globular Clusters M3 and M5
Light curves in the B, V, and I_c passbands have been obtained for the type
II Cepheids V154 in M3 and V42 and V84 in M5. Alternating cycle behavior,
similar to that seen among RV Tauri variables, is confirmed for V84. Old and
new observations, spanning more than a century, show that V154 has increased in
period while V42 has decreased in period. V84, on the other hand, has shown
large, erratic changes in period that do not appear to reflect the long term
evolution of V84 through the HR diagram.Comment: 28 pages, 12 figure
The Behavior of Novae Light Curves Before Eruption
In 1975, E. R. Robinson conducted the hallmark study of the behavior of
classical nova light curves before eruption, and this work has now become part
of the standard knowledge of novae. He made three points; that 5 out of 11
novae showed pre-eruption rises in the years before eruption, that one nova
(V446 Her) showed drastic changes in the variability across eruptions, and that
all but one of the novae (excepting BT Mon) have the same quiescent magnitudes
before and after the outburst. This work has not been tested since it came out.
We have now tested these results by going back to the original archival
photographic plates and measuring large numbers of pre-eruption magnitudes for
many novae using comparison stars on a modern magnitude scale. We find in
particular that four out of five claimed pre-eruption rises are due to simple
mistakes in the old literature, that V446 Her has the same amplitude of
variations across its 1960 eruption, and that BT Mon has essentially unchanged
brightness across its 1939 eruption. Out of 22 nova eruptions, we find two
confirmed cases of significant pre-eruption rises (for V533 Her and V1500 Cyg),
while T CrB has a deep pre-eruption dip. These events are a challenge to
theorists. We find no significant cases of changes in variability across 27
nova eruptions beyond what is expected due to the usual fluctuations seen in
novae away from eruptions. For 30 classical novae plus 19 eruptions from 6
recurrent novae, we find that the average change in magnitude from before the
eruption to long after the eruption is 0.0 mag. However, we do find five novae
(V723 Cas, V1500 Cyg, V1974 Cyg, V4633 Sgr, and RW UMi) that have significantly
large changes, in that the post-eruption quiescent brightness level is over ten
times brighter than the pre-eruption level.Comment: 91 pages (preprint), AJ accepte
Variable stars in the globular cluster M13
Results of a search for variable stars in the central region of the globular
cluster M13 are presented. Prior to this study, 36 variable and suspected
variable stars were known in this cluster (Osborn 2000; Clement et al. 2001).
Of these stars, five were not observed by us. We find v3, v4, v10, v12, and v13
to be constant in light. Surprisingly, only two out of the ten variable star
candidates of Kadla et al. (1980) appear to be variable. Both are RRc
variables. Additionally, three RR Lyrae stars and one SX Phoenicis variable are
discovered. Three close frequencies are detected for an RRc star v36. It
appears that this variable is another multi-periodic RR Lyrae star pulsating in
non-radial modes. Light curves of the three known BL Herculis stars and all
known RR Lyrae stars are presented. The total number of known RR Lyrae stars in
M13 is now nine. Only one is an RRab star. The mean period of RRc variables
amounts to 0.36 +/- 0.05 d, suggesting that M13 should be included in the group
of Oosterhoff type II globular clusters. Mean V magnitudes and ranges of
variation are derived for seven RR Lyrae and three BL Herculis variables.
Almost all observed bright giants show some degree of variability. In
particular, we confirm the variability of two red giants announced to be
variable by Osborn (2000) and in addition find five new cases.Comment: 10 pages, 9 figures, 5 tables, accepted for publication in AA
Variable Stars in Galactic Globular Clusters
Based on a search of the literature up to May 2001, the number of known
variable stars in Galactic globular clusters is approximately 3000. Of these,
more than 2200 have known periods and the majority (approximately 1800) are of
the RR Lyrae type. In addition to the RR Lyrae population, there are
approximately 100 eclipsing binaries, 120 SX Phe variables, 60 Cepheids
(including population II Cepheids, anomalous Cepheids and RV Tauri) and 120
SR/red variables. The mean period of the fundamental mode RR Lyrae variables is
0.585, for the overtone variables it is 0.342 (0.349 for the first-overtone
pulsators and 0.296 for the second-overtone pulsators) and approximately 30%
are overtone pulsators. These numbers indicate that about 65% of RR Lyrae
variables in Galactic globular clusters belong to Oosterhoff type I systems.
The mean period of the RR Lyrae variables in the Oosterhoff type I clusters
seems to be correlated with metal abundance in the sense that the periods are
longer in the more metal poor clusters. Such a correlation does not exist for
the Oosterhoff type II clusters. Most of the Cepheids are in clusters with blue
horizontal branches.Comment: 45 pages, 10 figures, to be published in AJ November 200
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