431 research outputs found
The Optical Gravitational Lensing Experiment. Variable Stars in Globular Clusters - IV. Fields 104A-E in 47 Tuc
Five fields located close to the center of the globular cluster NGC 104=47
Tuc were surveyed in a search for variable stars. We present V-band light
curves for 42 variables. This sample includes 13 RR Lyr stars -- 12 of them
belong to the Small Magellanic Cloud (SMC) and 1 is a background object from
the galactic halo. Twelve eclipsing binaries were identified -- 9 contact
systems and 3 detached/semi-detached systems. Seven eclipsing binaries are
located in the blue straggler region on the cluster color-magnitude diagram
(CMD) and four binaries can be considered main-sequence systems. One binary is
probably a member of the SMC. Eight contact binaries are likely members of the
cluster and one is most probably a foreground star. We show that for the
surveyed region of 47 Tuc, the relative frequency of contact binaries is very
low as compared with other recently surveyed globular clusters. The sample of
identified variables also includes 15 red variables with periods ranging from
about 2 days to several weeks. A large fraction of these 15 variables probably
belong to the SMC but a few stars are likely to be red giants in 47 Tuc. VI
photometry for about 50 000 stars from the cluster fields was obtained as a by
product of our survey.Comment: Latex file, l-aa style, 10 pages, 7 ps figures included. Submitted to
A&A. Figure 8 available from
ftp://sirius.astrouw.edu.pl/pub/jka/47Tuc/fig8.ps.
Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low Mass White Dwarfs
We present the first detailed study of the properties (temperatures,
gravities, and masses) of the NGC 6791 white dwarf population. This unique
stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~
0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that
exhibits both a red giant clump and a much hotter extreme horizontal branch.
Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS
spectra, suggests that most of these stars are undermassive, = 0.43 +/-
0.06 Msun, and therefore could not have formed from canonical stellar evolution
involving the helium flash at the tip of the red giant branch. We show that at
least 40% of NGC 6791's evolved stars must have lost enough mass on the red
giant branch to avoid the flash, and therefore did not convert helium into
carbon-oxygen in their core. Such increased mass loss in the evolution of the
progenitors of these stars is consistent with the presence of the extreme
horizontal branch in the CMD. This unique stellar evolutionary channel also
naturally explains the recent finding of a very young age (2.4 Gyr) for NGC
6791 from white dwarf cooling theory; helium core white dwarfs in this cluster
will cool ~3 times slower than carbon-oxygen core stars and therefore the
corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well
measured main-sequence turnoff age. These results provide direct empirical
evidence that mass loss is much more efficient in high metallicity environments
and therefore may be critical in interpreting the ultraviolet upturn in
elliptical galaxies.Comment: 15 pages, 9 figures, 2 tables. Accepted for publication in Astrophys.
J. Very minor changes from first versio
SALT long-slit spectroscopy of LBQS 2113-4538: variability of the Mg II and Fe II component
The Mg II line is of extreme importance in intermediate redshift quasars
since it allows us to measure the black hole mass in these sources and to use
these sources as probes of the distribution of dark energy in the Universe, as
a complementary tool to SN Ia. Reliable use of Mg II requires a good
understanding of all the systematic effects involved in the measurement of the
line properties, including the contamination by Fe II UV emission. We performed
three spectroscopic observations of a quasar LBQS 2113-4538 (z = 0.956) with
the SALT telescope separated in time by several months and we analyze in detail
the mean spectrum and the variability in the spectral shape. We show that even
in our good-quality spectra the Mg II doublet is well fit by a single
Lorentzian shape. We tested several models of the Fe II pseudo-continuum and
showed that one of them well represents all the data. The amplitudes of both
components vary in time, but the shapes do not change significantly. The
measured line width of LBQS 2113-4538 identifies this object as a class A
quasar. The upper limit of for the contribution of the Narrow Line Region
(NLR) to Mg II may suggest that the separation of the Broad Line Region (BLR)
and NLR disappears in this class of objects.Comment: 10 pages, 8 figures, accepted to A&
Planets in Stellar Clusters Extensive Search. I. Discovery of 47 Low-amplitude Variables in the Metal-rich Cluster NGC 6791 with Millimagnitude Image Subtraction Photometry
We have undertaken a long-term project, Planets in Stellar Clusters Extensive
Search (PISCES), to search for transiting planets in open clusters. As our
first target we have chosen NGC 6791 -- a very old, populous, metal rich
cluster. In this paper we present the results of a test observing run at the
FLWO 1.2 m telescope. Our primary goal is to demonstrate the feasibility of
obtaining the accuracy required for planetary transit detection using image
subtraction photometry on data collected with a 1 m class telescope. We present
a catalog of 62 variable stars, 47 of them newly discovered, most with low
amplitude variability. Among those there are several BY Dra type variables. We
have also observed outbursts in the cataclysmic variables B7 and B8 (Kaluzny et
al. 1997).Comment: 15 pages LaTeX, including 8 PostScript figures and 3 tables. More
discussion added on the implications for transit detection. Accepted for
publication in the Astronomical Journal. Version with full resolution figures
available through ftp at
ftp://cfa-ftp.harvard.edu/pub/bmochejs/PISCES/papers/1_N6791
The Clusters AgeS Experiment (CASE). IV. Analysis of the Eclipsing Binary V69 in the Globular Cluster 47 Tuc
We use photometric and spectroscopic observations of the eclipsing binary
V69-47 Tuc to derive the masses, radii, and luminosities of the component
stars. Based on measured systemic velocity, distance, and proper motion, the
system is a member of the globular cluster 47 Tuc. The system has an orbital
period of 29.5 d and the orbit is slightly eccentric with e=0.056. We obtain
Mp=0.8762 +- 0.0048 M(Sun), Rp=1.3148 +-0.0051 R(Sun), Lp=1.94 +- 0.21 L(Sun)
for the primary and Ms=0.8588 +- 0.0060 M(Sun), Rs=1.1616 +- 0.0062 R(Sun),
Ls=1.53 +- 0.17 L(Sun) for the secondary. These components of V69 are the first
Population II stars with masses and radii derived directly and with an accuracy
of better than 1%. We measure an apparent distance modulus of (m-M)v=13.35 +-
0.08 to V69. We compare the absolute parameters of V69 with five sets of
stellar evolution models and estimate the age of V69 using mass-luminosity-age,
mass-radius-age, and turnoff mass - age relations. The masses, radii, and
luminosities of the component stars are determined well enough that the
measurement of ages is dominated by systematic differences between the
evolutionary models, in particular, the adopted helium abundance. By comparing
the observations to Dartmouth model isochrones we estimate the age of V69 to be
11.25 +- 0.21(random) +- 0.85(systematic) Gyr assuming [Fe/H]=-0.70,
[alpha/Fe]=0.4, and Y=0.255. The determination of the distance to V69, and
hence to 47Tuc, can be further improved when infrared eclipse photometry is
obtained for the variable.Comment: 49 pages, 15 figures, submitted to A
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