485 research outputs found
Period Changes of the LMC Cepheids determined from the Harvard and OGLE/ASAS data
Observations of Cepheids in the Large Magellanic Cloud, made over last several decades, allow us to search for evolutionary period changes. None of the Cepheid from our sample of 378 stars stopped pulsating. Also none of them showed a large period change which could indicate mode switching. However for Cepheids with log P > 0.9 we found significant period changes, positive as well as negative. A comparison between the observed period changes and theoretical predictions shows moderate agreement with some models (Bono et al. 2000), and a very large disagreement with others (ABHA - Alibert, Baraffe, Hauschildt, Allard 1999). The large differences between the models are likely caused by the very high sensitivity of stellar evolution during core helium burning phase to even small changes in the input physics, as discovered by Lauterborn, Refsdal and Weigert (1971)
St 2-22 - another symbiotic star with high-velocity bipolar jets
We report finding high-velocity components in the H{\alpha} emission wings of
St 2-22 spectra, obtained in 2005. This discovery have encouraged us to start
the present study, aiming to show that this little studied object is a jet
producing symbiotic system. We used high-resolution optical and low-resolution
near infrared spectra, as well as available optical and infrared photometry, to
evaluate some of the physical parameters of the St 2-22 components and the
characteristics of the jets. The evaluated parameters of the components
confirmed that St 2-22 is a S-type symbiotic star. Our results demonstrate that
an unnoticed outburst of St 2-22, similar to those in classical symbiotics,
occurred in the first half of 2005. During the outburst, collimated, bipolar
jets were ejected by the hot component of St 2-22 with an average velocity of
about 1700 km/s.Comment: 18 pages, 7 figures, 4 tables, accepted for publication in Acta
Astronomic
Large Magellanic Cloud Cepheids in the ASAS data
A catalog of Cepheids in the Large Magellanic Cloud (LMC) from the ASAS
project is presented. It contains data on 65 fundamental mode pulsators with
periods longer than about 8 days. The period-luminosity (PL) relation in the
V-band does not significantly differ from the relation determined by Soszynski
et al. (2008) from the OGLE data extended toward longer periods but with much
larger spread. For objects with periods longer than 40 days there is an
evidence for a shallower PL relation. The rates of long-term period variations
significant at 3 sigma level are found only for 7 objects. The rates for 25
objects determined with the 1 sigma significance are confronted with the values
derived from stellar evolution models. The models from various sources yield
discrepant predictions. Over the whole data range, a good agreement with
measurements is found for certain models but not from the same source.Comment: 17 pages, 10 figures, accepted for publication in Acta Astronomic
Identification of V735 Sgr as an Active Herbig Ae/Be Object
V735 Sgr was known as an enigmatic star with rapid brightness variations.
Long-term OGLE photometry, brightness measurements in infrared bands, and
recently obtained moderate resolution spectrum from the 6.5-m Magellan
telescope show that this star is an active young stellar object of Herbig Ae/Be
type.Comment: published in Acta Astronomic
Absolute parameters of AE For -- a highly active detached binary of late K type
We present photometric and spectroscopic analysis of AE For -- a detached
eclipsing binary composed of two late K dwarfs. The masses of the components
are found to be 0.6314 +- 0.0035 and 0.6197 +- 0.0034 Msun and the radii to be
0.67 +- 0.03 and 0.63$ +- 0.03 Rsun for primary and secondary component,
respectively. Both components are significantly oversized compared to
theoretical models, which we attribute to their high activity. They show
Halpha, Hbeta, Hgamma, Ca H and Ca K lines in emission, and are heavily
spotted, causing large variations of the light curve.Comment: Accepted by MNRA
Cluster AgeS Experiment (CASE): Dwarf Novae and a Probable Microlensing Event in the Globular Cluster M22
We report the identification of a new cataclysmic variable (denoted as CV2)
and a probable microlensing event in the field of the globular cluster M22. Two
outbursts were observed for CV2. During one of them superhumps with
P_sh=0.08875 d were present in the light curve. CV2 has an X-ray counterpart
detected by XMM-Newton. A very likely microlensing event at a radius of 2.3
arcmin from the cluster center was detected. It had an amplitude of
Delta_V=0.75 mag and a characteristic time of 15.9 days. Based on model
considerations we show that the most likely configuration has the source in the
Galactic bulge with the lens in the cluster. Two outbursts were observed for
the already known dwarf nova CV1.Comment: 12 pages, 8 figures, accepted for publication in Acta Astronomic
A Low-Resolution Spectroscopic Exploration of Puzzling OGLE Variable Stars
We present the results of a spectroscopic follow-up of various puzzling
variable objects detected in the OGLE-III Galactic disk and bulge fields. The
sample includes mainly short-period multi-mode pulsating stars that could not
have been unambiguously classified as either delta Sct or beta Cep type stars
based on photometric data only, also stars with irregular fluctuations
mimicking cataclysmic variables and stars with dusty shells, and periodic
variables displaying brightenings in their light curves that last for more than
half of the period. The obtained low-resolution spectra show that all observed
short-period pulsators are of delta Sct type, the stars with irregular
fluctuations are young stellar objects, and the objects with regular
brightenings are A type stars or very likely Ap stars with strong magnetic
field responsible for the presence of bright caps around magnetic poles on
their surface. We also took spectra of objects designated OGLE-GD-DSCT-0058 and
OGLE-GD-CEP-0013. An estimated effective temperature of 33,000 K in
OGLE-GD-DSCT-0058 indicates that it cannot be a delta Sct type variable. This
very short-period (0.01962 d) high-amplitude (0.24 mag in the I-band) object
remains a mystery. It may represent a new class of variable stars. The spectrum
of OGLE-GD-CEP-0013 confirms that this is a classical Cepheid despite a
peculiar shape of its light curve. The presented results will help in proper
classification of variable objects in the OGLE Galactic Variability Survey.Comment: published in Acta Astronomic
Discovery of an Outbursting 12.8 Minute Ultracompact X-Ray Binary
We report the discovery of OGLE-UCXB-01, a 12.8 minute variable object
located in the central field of Galactic bulge globular cluster Djorg 2. The
presence of frequent, short-duration brightenings at such an ultrashort period
in long-term OGLE photometry together with the blue color of the object in
Hubble Space Telescope images and the detection of moderately hard X-rays by
Chandra observatory point to an ultracompact X-ray binary system. The observed
fast period decrease makes the system a particularly interesting target for
gravitational-wave detectors such as the planned Laser Interferometer Space
Antenna.Comment: full OGLE light curve is available at
http://www.astrouw.edu.pl/ogle/ogle4/OGLE-UCXB-01
Searching for Potential Mergers among 22 500 Eclipsing Binary Stars in the OGLE-III Galactic Bulge Fields
Inspired by the discovery of the red nova V1309 Sco (Nova Scorpii 2008) and
the fact that its progenitor was a binary system with a rapidly decreasing
orbital period, we have searched for period changes in OGLE binary stars. We
have selected a sample of 22 462 short-period (P_orb < 4d) eclipsing binary
stars observed toward the Galactic bulge by the OGLE-III survey in years
2001-2009. This dataset was extended with photometry from OGLE-II (1997-2000)
and the first six years of OGLE-IV (2010-2015). For some stars, the data were
supplemented with OGLE-I photometry (1992-1995). After close inspection of the
whole sample we have found 56 systems with realistic period decrease and 52
systems with realistic period increase. We have also recognized 35 systems with
cyclic period variations. The highest negative period change rate of -1.943
x10^-4 d/y has been detected in detached eclipsing binary OGLE-BLG-ECL-139622
with P_orb = 2.817 d, while all other found systems are contact binaries with
orbital periods mostly shorter than 1.0 d. For 22 our systems with decreasing
orbital period the absolute rate is higher than the value reported recently for
eclipsing binary KIC 9832227. Interestingly, there is an excess of systems with
high negative period change rate over systems with positive rate. We cannot
exclude the possibility that some of the contact binaries with relatively long
orbital period and high negative period change rate will merge in the future.
However, our results rather point to the presence of tertiary companions in the
observed systems and/or spot activity on the surface of the binary components
OGLE Atlas of Classical Novae. II. Magellanic Clouds
The population of classical novae in the Magellanic Clouds was poorly known
because of a lack of systematic studies. There were some suggestions that nova
rates per unit mass in the Magellanic Clouds were higher than in any other
galaxy. Here, we present an analysis of data collected over 16 years by the
OGLE survey with the aim of characterizing the nova population in the Clouds.
We found 20 eruptions of novae, half of which are new discoveries. We robustly
measure nova rates of yr (LMC) and yr
(SMC) and confirm that the K-band luminosity-specific nova rates in both Clouds
are 2-3 times higher than in other galaxies. This can be explained by the star
formation history in the Magellanic Clouds, specifically the re-ignition of the
star formation rate a few Gyr ago. We also present the discovery of the
intriguing system OGLE-MBR133.25.1160 which mimics recurrent nova eruptions.Comment: ApJS, in pres
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