54 research outputs found
Some Systematics of Galactic Globular Clusters
The global properties of all known Galactic globular clusters are examined.
The relationship between the luminosities and the metallicities of Galactic
globular clusters is found to be complex. Among luminous clusters there is a
correlation in the sense that the oldest clusters are slightly more metal
deficient than are younger clusters. However, no such clear-cut relationship is
found among the faintest globular clusters. The central concentration index C
of globular clusters is seen to be independent of metallicity. The dependence
of the half-light radii of globular clusters on their Galactocentric distances
can be approximated by the relation . Clusters with
collapsed cores are mostly situated close to the Galactic nucleus. For kpc the luminosities and the radii of clusters appear to be uncorrelated.
The Galaxy differs from the LMC and the SMC in that it appears to lack highly
flattened luminous clusters. Galactic globular clusters with ages 13.0
Gyr are all of Oosterhoff type II, whereas almost all of those with ages
13.0 Gyr have been assigned to Oosterhoff type I. Globular clusters with ages
11.5 Gyr are all located in the outer Galactic halo, have below-average
luminosities and above-average radii. On the other hand the very old globular
cluster NGC 6522 is situated close to the Galactic nucleus.Comment: PASP, in pres
Variable Stars in the Field of the Globular Cluster NGC 3201
We report on the discovery and analysis of 14 short-period variable stars in
the field of the southern globular cluster NGC 3201, located within roughly two
magnitudes on either side of the main-sequence turnoff. 11 of these variable
stars are eclipsing binaries, one is an RR Lyrae, and two are thus far
unclassified systems. Among the eclipsing binary stars, nine are of the W Ursa
Majoris (W UMa) type, one an Algol (EA) system, and one a detached system.
Using spectroscopic follow-up observations as well as analysis of the
variables' locations in the color-magnitude diagram of the cluster, we find
that only one variable star (a W UMa type blue straggler) is actually a member
of NGC 3201. We present the phased photometry lightcurves for all the variable
star systems as well as their locations in the field-of-view and in the
color-magnitude diagram.Comment: 18 pages, 8 figures, 3 tables; accepted for the January 2002 issue of
AJ. For full resolution versions of this paper, go to:
http://www.astro.lsa.umich.edu/users/kaspar/html/vars3201.ps.gz or
http://www.astro.lsa.umich.edu/users/kaspar/html/vars3201.pd
BVI Photometric Variability Survey of M3
We have conducted a three band (BVI) variability survey of the globular
cluster M3. This is the first three band survey of the cluster using modern
image subtraction techniques. Observations were made over 9 nights in 1998 on
the 1.2m telescope at the F.L. Whipple Observatory in Arizona. We present
photometry for 180 variable stars in the M3 field, of which 12 are newly
discovered. New discoveries include six SX Phe type variables which all lie in
the blue straggler region of the color magnitude diagram, two new first
overtone RR Lyrae, a candidate multi-mode RR Lyrae, a detached eclipsing
binary, and two unclassified variables. We also provide revised periods for 52
of the 168 previously known variables that we observe. The catalog and
photometry for the variable stars are available via anonymous ftp at
ftp://cfa-ftp.harvard.edu/pub/kstanek/M3/Comment: 35 pages, 11 figures. Movies associated with this paper at
http://www-cfa.harvard.edu/~jhartman/M3_movies.html Accepted for publication
in the Astronomical Journal. This replacement contains a few minor revision
Diameters of Open Star Clusters
The present paper presents a tabulation of data on all 600 Galactic open
clusters for which it is presently possible to calculate linear diameters. As
expected, the youngest `clusters' with ages < 15 Myr, contain a significant
(greater than or equal to 20%) admixture of associations. Among
intermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, the median
cluster diameter is found to increase with age. Small compact clusters are rare
among objects with ages > 1.5 Gyr. Open clusters with ages > 1 Gyr appear to
form what might be termed a `cluster thick disk', part of which consistst of
objects that were probably captured gravitationally by the main body of the
Galaxy.Comment: Astronomical Journal, in pres
RR Lyrae Variables in the Globular Cluster M55. The First Evidence for Non Radial Pulsations in RR Lyr Stars
We present the results of a photometric study of RR Lyrae variables in the
field of the globular cluster M55. We have discovered nine new RR Lyrae stars,
increasing the number of known variables in this cluster to 15 objects. Five of
the newly discovered variables belong to Bailey type RRc and two to type RRab.
Two background RRab stars are probable members of the Sagittarius dwarf galaxy.
Fourier decomposition of the light curves was used to derive basic properties
of the present sample of RR Lyrae variables. From an analysis of the RRc
variables we obtain a mean mass of , luminosity , effective temperature K, and helium
abundance . Based on the colors, periods and metallicities
of the RRab stars we estimate the value of the color excess for M55 to be equal
to . Using this value we derive the colors of the blue and
red edges of the instability strip in M55. The blue edge lies at
mag and the red edge lies at mag. We estimate the values of the
visual apparent and dereddened distance moduli to be and
, respectively. The light curves of three of the RRc variables
exhibit changes in amplitude of over 0.1 mag on the time scale of less than a
week, rather short for the Blazhko effect, but with no evidence for another
radial pulsational frequency. However we do detect other periodicities which
are clearly visible in the light curve after removing variations with the first
overtone radial frequency. This is strong evidence for the presence of
non-radial pulsations, a behavior common for Scuti stars but not yet
observed among RR Lyr variables.Comment: submitted to Astronomical Journal, 33 pages with 11 figure
Variable Stars in the Globular Cluster M5. Application of the Image Subtraction Method
We present -band light curves of 61 variables from the core of the
globular cluster M5 obtained using a newly developed image subtraction method
(ISM). Four of these variables were previously unknown. Only 26 variables were
found in the same field using photometry obtained with DoPHOT software. Fourier
parameters of the ISM light curves have relative errors up to 20 times smaller
than parameters measured from DoPHOT photometry. We conclude that the new
method is very promising for searching for variable stars in the cores of the
globular clusters and gives very accurate relative photometry with quality
comparable to photometry obtained by HST. We also show that the variable V104
is not an eclipsing star as has been suggested, but is an RRc star showing
non-radial pulsations.Comment: submitted to MNRAS, 9 pages, 4 figure
An Erupting Classical Nova in a Globular Cluster of M87
Only one certain classical nova eruption has ever been detected inside a
globular cluster - nova 1860 A.D. (T Sco) in M80. During a survey of M87 we
have detected an erupting star coincident (to within 0.08 pixels) with a
globular cluster of that giant elliptical galaxy. We are able to discount
variables in the foreground or background of M87. The light curve and color of
the erupting star match those expected for a nova at the distance of M87. The
chance superposition of an M87 field nova on the globular cluster is very
unlikely but cannot be completely ruled out.Our detection hints at a globular
cluster nova frequency novae/cluster/year, much higher than
previous observations have suggested
M75, a Globular Cluster with a Trimodal Horizontal Branch. II. BV Photometry of the RR Lyrae Variables
We present new BV CCD photometry, light curves, and ephemerides for 9
previously known, 29 newly detected RR Lyrae variables and one newly detected
variable of an unknown type in the globular cluster M75. The photometry used
for the detection of the additional variables was obtained with the image
subtraction package ISIS. The data were acquired on an observing run in July
1999 and range over 7 observing nights. Estimates of fundamental photometric
parameters are presented including intensity- and magnitude-averaged B and V
magnitudes, magnitude-averaged colors, pulsation periods, and pulsation
amplitudes. The mean period of the RRab variables, = 0.5868 d, and the
number fraction of RRc stars, N_c/N_RR = 0.342, are both large for an
Oosterhoff type I (OoI) globular cluster, suggesting that M75 may be
Oosterhoff-intermediate. Possible conflicts between Oosterhoff-type
determination based on the A_V - logP and A_B - logP diagrams are discussed.
The physical parameters of the RRc and RRab variables, as obtained from Fourier
decomposition of their light curves, do not show any clear deviation from
normal OoI behavior.Comment: 26 pages, 10 figures, uses emulateapj5 and apjfonts style files
(included). The Astronomical Journal, in pres
A New Bayesian Test to Test for the Intractability-Countering Hypothesis
We present a new test of hypothesis in which we seek the probability of the
null conditioned on the data, where the null is a simplification undertaken to
counter the intractability of the more complex model, that the simpler null
model is nested within. With the more complex model rendered intractable, the
null model uses a simplifying assumption that capacitates the learning of an
unknown parameter vector given the data. Bayes factors are shown to be known
only up to a ratio of unknown data-dependent constants--a problem that cannot
be cured using prescriptions similar to those suggested to solve the problem
caused to Bayes factor computation, by non-informative priors. Thus, a new test
is needed in which we can circumvent Bayes factor computation. In this test, we
undertake generation of data from the model in which the null hypothesis is
true and can achieve support in the measured data for the null by comparing the
marginalised posterior of the model parameter given the measured data, to that
given such generated data. However, such a ratio of marginalised posteriors can
confound interpretation of comparison of support in one measured data for a
null, with that in another data set for a different null. Given an application
in which such comparison is undertaken, we alternatively define support in a
measured data set for a null by identifying the model parameters that are less
consistent with the measured data than is minimally possible given the
generated data, and realising that the higher the number of such parameter
values, less is the support in the measured data for the null. Then, the
probability of the null conditional on the data is given within an MCMC-based
scheme, by marginalising the posterior given the measured data, over parameter
values that are as, or more consistent with the measured data, than with the
generated data.Comment: Accepted for publication in JAS
The Globular Cluster Omega Centauri and the Oosterhoff Dichotomy
CCD observations obtained by the OGLE team for 128 RR Lyrae variables in
Omega Cen have been analysed. The P-L and P-A plots indicate that, in addition
to fundamental (RRab) and first overtone (RRc) pulsators, the Omega Cen RR
Lyrae population seems to include second overtone (RRe) and possibly third
overtone pulsators. The mean periods derived for the RRab and RRc stars are
typical values for an Oosterhoff type II (OoII) cluster. Nevertheless, the P-A
plot plot also shows that some of the RR Lyrae variables have `Oosterhoff type
I' (OoI) characteristics. Relative luminosities derived for the RRc variables
from Fourier coefficients correlate with the observed apparent magnitudes.
Masses for the RRc stars have been calculated from Fourier coefficients. A
comparison of the derived masses for RRc stars in the four OoII clusters Omega
Cen, M15, M55 and M68 indicates that the masses of the RRc stars in M15 and M68
are almost 0.2 solar masses greater than those in the other two. Since M15 and
M68 have a high frequency of RRd stars among their first overtone pulsators,
while none have been identified in Omega Cen or M55, this suggests that the
double-mode pulsation phenomenon may be associated with mass. Among the RRc
variables in Omega Cen, the OoII variables have lower derived masses and higher
luminosities than the OoI variables. An application of the period-density law
to pairs of OoI and OoII RRab stars selected according to their position in the
P-A plot also indicates that the OoII variables in general have lower masses
and higher luminosities. These findings support the hypothesis that RR Lyrae
variables in OoII systems are evolved HB stars that spend their ZAHB phase on
the blue side of the instability strip.Comment: 52 pages, 10 figures, to appear in the AJ (November 2000
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