14 research outputs found
Double-periodic blue variables in the Magellanic Clouds
We report the discovery, based on an inspection of the OGLE-II database, of a
group of blue variables in the Magellanic Clouds showing simultaneously two
kinds of photometric variability: a short-term cyclic variability with typical
amplitude 0.05 mag and period between 4 and 16 days and
a sinusoidal, long-term cyclic oscillation with much larger amplitude 0.2 mag with period in the range of 150-1000 days. We find that
both periods seems to be coupled through the relationship = 35.2
0.8 . In general, the short term variability is reminiscent of those
shown by Algol-type binaries. We propose that the long-term oscillation could
arise in the precession of a elliptical disc fed by a Roche-lobe filling
companion in a low mass ratio Algol system.Comment: submitted to A&A letter
Eclipsing Binaries in the Young LMC Cluster NGC 1850
I present light curves for two detached eclipsing binary stars in the region
of the LMC cluster NGC 1850, which is possibly a young globular cluster still
in formation. One, a likely spectral type O star, is a newly detected eclipsing
binary in the region of the very young subcluster NGC 1850A. This binary is
among a small number of highly massive O-type stars in binary systems found in
LMC clusters. These two eclipsing binaries are the first discovered in the well
studied NGC 1850, and the O-type star is the first eclisping binary found in
NGC 1850A. Light curves for two NGC 1850 region Cepheid variables are also
shown. Discovering two eclipsing binaries in the young globlular-like cluster
NGC 1850 is discussed in terms of the importance of the binary fraction to
globular cluster evolution.Comment: 27 pages, 11 figures; Accepted for publication in December 2004 PASP.
(No changes in paper from original submitted version; only this comment
reflecting acceptance by the Proceedings of the Astronomical Society of the
Pacific has been added.
Seismology of triple-mode classical Cepheids of the Large Magellanic Cloud
We interpret the three periods detected in OGLE LMC Cepheids SC3-360128 and
SC5-338399 as corresponding to the first three overtones of radial pulsations.
This interpretation imposes stringent constraints on parameters of the stars
and on their evolutionary status, which could only be the first crossing of the
instability strip. Evolutionary models reproducing measured periods exist only
in a restricted range of metallicities (Z=0.004-0.007). The models impose an
upper limit on the extent of overshooting from the convective core. Absolute
magnitude of each star is confined to a narrow interval. This allows to derive
a new estimate of the distance to the LMC. We obtain m-M ranging from 18.34mag
to 18.53mag, with a systematic difference between the two stars of about
0.13mag. The rates of period change predicted by the models are formally in
conflict with the derived observational limits, though the uncertainities of
measured dP/dt may be underestimated. If the discrepancy is confirmed, it would
constitute a significant challenge to the stellar evolution theory.Comment: 9 pages, 2 figures, accepted for publication in A&
Forty eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance
OGLE-2003-BLG-238: Microlensing Mass Estimate of an Isolated Star
Microlensing is the only known direct method to measure the masses of stars that lack visible companions. In terms of microlensing observables, the mass is given by M=(c^2/4G)\tilde r_E \theta_E and so requires the measurement of both the angular Einstein radius, \theta_E, and the projected Einstein radius, \tilde r_E. Simultaneous measurement of these two parameters is extremely rare. Here we analyze OGLE-2003-BLG-238, a spectacularly bright (I_min=10.3), high-magnification (A_max = 170) microlensing event. Pronounced finite source effects permit a measurement of \theta_E = 650 uas. Although the timescale of the event is only t_E = 38 days, one can still obtain weak constraints on the microlens parallax: 4.4 AU < \tilde r_E < 18 AU at the 1 \sigma level. Together these two parameter measurements yield a range for the lens mass of 0.36 M_sun < M < 1.48 M_sun. As was the case for MACHO-LMC-5, the only other single star (apart from the Sun) whose mass has been determined from its gravitational effects, this estimate is rather crude. It does, however, demonstrate the viability of the technique. We also discuss future prospects for single-lens mass measurements