583 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
Characterization of extrasolar planetary transiting candidates II
We present a second paper of fully characterization of a sample of stars
whose low-depth transits were discovered by the OGLE-III campaign in order to
select the most promising candidates for spectroscopic confirmation, following
the same analysis done in Gallardo et al. (2005). We present new optical and
near-IR photometry, deriving physical parameters like effective temperature
(Teff), distance (d), the stellar radii (R*) and the companion radii (Rc). We
selected eight M (2800 K \le Teff \le 3850 K) or K (3850 K \le Teff \le 5150 K)
spectral type stellar objects as potential candidates to host exoplanets, even
though, considering the radii of their companions, only stars OGLE-TR-61,
OGLE-TR-74, OGLE-TR-123 and OGLE-TR-173 are the most promising M-type transit
candidates to host planets. Confirmation of the planetary nature of any of
these objects will yield another transiting extrasolar planet orbiting a M-type
star, or even more interesting, the first extrasolar planets orbiting a late
M-type dwarf of effective temperature about 2900 K.Comment: 6 pages, 2 figures, 2 tables, accepted for publication in Astronomy
and Astrophysic
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
CCD photometry of distant open clusters NGC 2425, Haffner 10 and Czernik 29
We present BVI photometry for poorly known southern hemisphere open clusters:
NGC 2425, Haffner 10 and Czernik 29. We have calculated the density profile and
established the number of stars in each cluster. The colour-magnitude diagrams
of the objects show a well-defined main sequence. However, the red giant clump
is present only in NGC 2425 and Haffner 10. For these two clusters we estimated
the age as 2.5 +/- 0.5 Gyr assuming metallicity of Z=0.008. The apparent
distance moduli are in the ranges 13.2<(m-M)_V<13.6 and 14.3<(m-M)_V<14.7,
while heliocentric distances are estimated to be 2.9<d<3.8 kpc and 3.1<d<4.3
kpc, respectively for NGC 2425 and Haffner 10. The angular separation of 2.4
deg (150 pc at mean distance) may indicate a common origin of the two clusters.Comment: 5 pages, 9 figures, accepted for publication in 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
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