8 research outputs found
Ultra-Low Amplitude Cepheids in the Large Magellanic Cloud
The MACHO variables of LMC Field 77 that lie in the vicinity of the Cepheid
instability strip are reexamined. Among the 144 variables that we identify as
Cepheids we find 14 that have Fourier amplitudes <0.05 mag in the MACHO red
band, of which 7 have an amplitude <0.006 mag : we dub the latter group of
stars ultra-low amplitude (ULA) Cepheids. The variability of these objects is
verified by a comparison of the MACHO red with the MACHO blue lightcurves and
with those of the corresponding OGLE LMC stars. The occurrence of ULA Cepheids
is in agreement with theory. We have also discovered 2 low amplitude variables
whose periods are about a factor of 5--6 smaller than those of F Cepheids of
equal apparent magnitude. We suggest that these objects are Cepheids undergoing
pulsations in a surface mode and that they belong to a novel class of Strange
Cepheids (or Surface Mode Cepheids) whose existence was predicted by Buchler et
al. (1997).Comment: 4 pages, 5 figures, slightly revised, to appear in ApJ Letter
Variability-selected quasars behind the Small Magellanic Cloud
We present followup spectroscopic observations of quasar candidates in the
Small Magellanic Cloud selected by Eyer from the OGLE database. Of twelve
observed objects identified as "QSO Candidate", five are confirmed quasars,
with the emission redshifts ranging from 0.28 to 2.16. Two of those quasars
were also recently identified independently in the MACHO database by Geha et
al. We discuss the prospects of using variability-based selection technique for
quasar searches behind other dense stellar fields. An additional criterion
utilizing the color-color diagram should reduce the number of stars in the
candidate lists.Comment: Revised version, AASTeX, 11 pages, 3 EPS figures, one table, accepted
14 Nov 2002 for publication in the Astronomical Journal, March 2003 issu
Post Main Sequence Orbital Circularization of Binary Stars in the Large and Small Magellanic Clouds
We present results from a study of the orbits of eclipsing binary stars (EBs)
in the Magellanic Clouds. The samples comprise 4510 EBs found in the Large
Magellanic Cloud (LMC) by the MACHO project, 2474 LMC EBs found by the OGLE-II
project (of which 1182 are also in the MACHO sample), 1380 in the Small
Magellanic Cloud (SMC) found by the MACHO project, and 1317 SMC EBs found by
the OGLE-II project (of which 677 are also in the MACHO sample); we also
consider the EROS sample of 79 EBs in the bar of the LMC. Statistics of the
phase differences between primary and secondary minima allow us to infer the
statistics of orbital eccentricities within these samples. We confirm the
well-known absence of eccentric orbit in close binary stars. We also find
evidence for rapid circularization in longer period systems when one member
evolves beyond the main sequence, as also found by previous studies.Comment: 37 pages, 16 figures, accepted for publication in ApJ. Added a new
reference and updated information on on line materia
New X-ray quasars behind the Small Magellanic Cloud
We present five X-ray quasars behind the Small Magellanic Cloud, increasing
the number of known quasars behind the SMC by ca. 40%. They were identified via
follow-up spectroscopy of serendipitous sources from the Chandra X-ray
Observatory matched with objects from the OGLE database. All quasars lie behind
dense parts of the SMC, and could be very useful for proper motion studies. We
analyze X-ray spectral and timing properties of the quasars. We discuss
applications of those and other recently discovered quasars behind the SMC to
the studies of absorption properties of the Cloud, its proper motion, and for
establishing the geometrical distance to the SMC.Comment: Revised version, AASTeX, 20 pages, 7 figures, accepted for
publication in the Astronomical Journal, August 2003 issu
Discovery of four X-ray quasars behind the Large Magellanic Cloud
We present the discovery of four X-ray quasars (z_em = 0.26, 0.53, 0.61,
1.63) located behind the Large Magellanic Cloud; three of them are located
behind the bar of the LMC. The quasars were identified via spectroscopy of
optical counterparts to X-ray sources found serendipitously by the Chandra
X-ray Observatory satellite. All four quasars have archival VI photometry from
the OGLE-II project; one of them was found by OGLE to be variable. We present
the properties of the quasars and discuss their possible applications.Comment: AASTeX, 8 pages, 1 ps figure, accepted for publication in
Ap.J.Letter
Evidence for Companion-Induced Secular Changes in the Turbulent Disk of a Be Star in the LMC MACHO Database
The light curve of a blue variable in the MACHO LMC database (FTS ID
78.5979.72) appeared nearly unvarying for ~4 years (quasi-flat segment) but
then rapidly changed to become periodic with noisy minima for the remaining 4
years (periodic segment); there are no antecedent indications of a gradual
approach to this change. Lomb Periodogram analyses indicate the presence of two
distinct periods of ~61 days and 8 days in both the quasi-flat and the periodic
segments. Minima of the periodic segment cover at least 50% of the orbital
period and contain spikes of light with the 8-day period; maxima do not show
this short period. The system typically shows maxima to be redder than minima.
The most recent OGLE-III light curve shows only a 30-day periodicity. The
variable's V and R magnitudes and color are those of a Be star, and recent sets
of near infrared spectra four days apart, secured during the time of the
OGLE-III data, show H-alpha emission near and at a maximum, confirming its Be
star characteristics. The model that best fits the photometric behavior
consists of a thin ring-like circumstellar disk of low mass with four obscuring
sectors orbiting the central B star in unison at the 61-day period. The central
star peers through the three equi- spaced separations between the four sectors
producing the 8-day period. The remainder of the disk contains hydrogen in
emission making maxima appear redder. A companion star of lower mass in an
inclined and highly eccentric orbit produces an impulsive perturbation near its
periastron to change the disk's orientation, changing eclipses from partial to
complete within ~ 10 days.Comment: 42 pages, 14 figures, and 2 tables Submitted to AJ v3: Title changed,
figures added, model modifie