741 research outputs found
On the Radial Distribution of White Dwarfs in the Globular Cluster NGC 6397
We have examined the radial distribution of white dwarfs over a single
HST/ACS field in the nearby globular cluster NGC 6397. In relaxed populations,
such as in a globular cluster, stellar velocity dispersion, and hence radial
distribution, is directly dependent on stellar masses. The progenitors of very
young cluster white dwarfs had a mass of ~0.8 solar masses, while the white
dwarfs themselves have a mass of ~0.5 solar masses. We thus expect young white
dwarfs to have a concentrated radial distribution (like that of their
progenitors) that becomes more extended over several relaxation times to mimic
that of ~0.5 solar mass main-sequence stars. However, we observe young white
dwarfs to have a significantly extended radial distribution compared to both
the most massive main sequence stars in the cluster and also to old white
dwarfs.Comment: 13 pages including 1 table and 3 figures. Accepted for publication in
the MNRAS Letter
Late-Type Stars in M31. I.: A Photometric Study of AGB Stars and Metallicity Gradients.
We have imaged five 7\arcmin \x 7\arcmin\ fields in M31 spanning
galactocentric radii from 4 to 32 kpc along the SW-major axis. The fields were
observed through two broad-band (\V\ and \I) and two narrow-band (\CN\ and
\TiO) filters. The broad-band data were used to construct \IvsVI\
color-magnitude diagrams (CMDs) and, in some of our fields, we found
significant numbers of stars in the Cepheid instability strip. A distance
modulus for the Cepheids in the middle field was found that agreed well with
other values in the literature values. The width of the giant branch (GB) in
the \IvsVI\ CMD of all 5 fields was investigated, and we show that in four of
the fields a likely explanation for the GB width is a combination of {\it both}
metallicity and mass variations. Using the broad-band data, the asymptotic
giant branch (AGB) luminosity functions (LFs) were measured in the five fields,
and we show that differences exist between these LFs. We speculate on how the
different star forming histories in the fields may lead to the observed AGB LFs
and GB widths. Using the narrow-band data along with the broad-band data we
separated the AGB stars into carbon-rich (C) and oxygen-rich (M) types. The
carbon stars LFs were used to obtain an estimate for the distance modulus of
M31 which agrees with the value derived from Cepheids. The ratio of C- to
M-stars (C/M) is believed to be an indicator of gaseous chemical abundance at
the time of formation of these stars. We show that the C/M ratio increases
smoothly with galactocentric distance, suggesting an inverse correlation with
metallicity. This is the first demonstration of this effect within a single
extragalactic system. We find that differences in the width of the GB and the
AGB LFs do not significantly affect the C/M ratio. We consider the effect of
the increasing C/M ratio on the ISM in M31, and cite evidence in favor of a
model where the grain composition in M31 is a function of galactocentric
distance.Comment: UUencoded compressed postscript, 3 Figs. available on request.
(Contact [email protected]
A Cluster of Compact Radio Sources in NGC 2024 (Orion B)
We present deep 3.6 cm radio continuum observations of the H II region NGC
2024 in Orion B obtained using the Very Large Array in its A-configuration,
with angular resolution. We detect a total of 25 compact radio
sources in a region of . We discuss the nature of these sources
and its relation with the infrared and X-ray objects in the region. At least
two of the radio sources are obscured proplyds whose morphology can be used to
restrict the location of the main ionizing source of the region. This cluster
of radio sources is compared with others that have been found in regions of
recent star formation.Comment: 21 pages, 7 figure
The white dwarf cooling sequence in the old open cluster NGC 188
We develop the white dwarf luminosity function (LF) of the old open cluster
NGC 188 in order to determine a lower limit to the age of the cluster by using
the faint end of the cooling sequence.
To produce an extensive sequence of the cooling white dwarfs we imaged four
contiguous HST-WFPC2 fields in the center of the cluster in the F555W and F814W
filters. After imposing selection criteria on the detected objects we found a
white dwarf cooling sequence (down to V ~26.5) including 28 candidate white
dwarfs in the cluster. The exposures are not deep enough to reach the end of
this sequence, but the results of our analysis allow us to establish a lower
limit to the age of the cluster independently of the isochrone fit to the
cluster turnoff.
The most ancient white dwarfs found are ~4 Gyr old, an age that is set solely
by the photometric limit of our data. Classical methods provide an estimate of
\~7 Gyr (Sarajedini et al., 1999).Comment: 6 pages, 5 figures, accepted on Astronomy and Astrophysic
NGC 2419, M92, and the Age Gradient in the Galactic Halo
The WFPC2 camera on HST has been used to obtain deep main sequence photometry
of the low-metallicity ([Fe/H]=-2.14), outer-halo globular cluster NGC 2419. A
differential fit of the NGC 2419 CMD to that of the similarly metal-poor \
standard cluster M92 shows that they have virtually identical principal
sequences and thus the same age to well within 1 Gyr. Since other
low-metallicity clusters throughout the Milky Way halo have this same age to
within the 1-Gyr precision of the differential age technique, we conclude that
the earliest star (or globular cluster) formation began at essentially the same
time everywhere in the Galactic halo throughout a region now almost 200 kpc in
diameter. Thus for the metal-poorest clusters in the halo there is no
detectable age gradient with Galactocentric distance. To estimate the absolute
age of NGC 2419 and M92, we fit newly computed isochrones transformed through
model-atmosphere calculations to the (M_V,V-I) plane, with assumed distance
scales that represent the range currently debated in the literature.
Unconstrained isochrone fits give M_V(RR) = 0.55 \pm 0.06 and a resulting age
of 14 to 15 Gyr. Incorporating the full effects of helium diffusion would
further reduce this estimate by about 1 Gyr. A distance scale as bright as
M_V(RR) = 0.15 for [Fe/H] = -2, as has recently been reported, would leave
several serious problems which have no obvious solution in the context of
current stellar models.Comment: 32 pages, aastex, 9 postscript figures; accepted for publication in
AJ, September 1997. Also available by e-mail from [email protected]
An Empirical Measure of the Rate of White Dwarf Cooling in 47 Tucanae
We present an empirical determination of the white dwarf cooling sequence in
the globular cluster 47 Tucanae. Using spectral models, we determine
temperatures for 887 objects from Wide Field Camera 3 data, as well as 292
objects from data taken with the Advanced Camera for Surveys. We make the
assumption that the rate of white dwarf formation in the cluster is constant.
Stellar evolution models are then used to determine the rate at which objects
are leaving the main sequence, which must be the same as the rate at which
objects are arriving on the white dwarf sequence in our field. The result is an
empirically derived relation between temperature () and time () on
the white dwarf cooling sequence. Comparing this result to theoretical cooling
models, we find general agreement with the expected slopes between 20,000K and
30,000K and between 6,000K and 20,000K, but the transition to the Mestel
cooling rate of is found to occur at hotter
temperatures, and more abruptly than is predicted by any of these models.Comment: 10 pages, 16 figures, accepted for publication in Ap
An Age Difference of 2 Gyr between a Metal-Rich and a Metal-Poor Globular Cluster
Globular clusters trace the formation history of the spheroidal components of
both our Galaxy and others, which represent the bulk of star formation over the
history of the universe. They also exhibit a range of metallicities, with
metal-poor clusters dominating the stellar halo of the Galaxy, and higher
metallicity clusters found within the inner Galaxy, associated with the stellar
bulge, or the thick disk. Age differences between these clusters can indicate
the sequence in which the components of the Galaxy formed, and in particular
which clusters were formed outside the Galaxy and later swallowed along with
their original host galaxies, and which were formed in situ. Here we present an
age determination of the metal-rich globular cluster 47 Tucanae by fitting the
properties of the cluster white dwarf population, which implies an absolute age
of 9.9 (0.7) Gyr at 95% confidence. This is about 2.0 Gyr younger than inferred
for the metal-poor cluster NGC 6397 from the same models, and provides
quantitative evidence that metal-rich clusters like 47 Tucanae formed later
than the metal-poor halo clusters like NGC 6397.Comment: Main Article: 10 pages, 4 figures; Supplementary Info 15 pages, 5
figures. Nature, Aug 1, 201
Star counts in NGC 6397
I-band CCD images of a large area of the nearby globular cluster NGC~6397
have been used to construct a surface density profile and two luminosity and
mass functions. The surface density profile extends out to 14\arcm from the
cluster center and shows no sign of a tidal cutoff. The inner profile is a
power-law with slope -0.8 steepening to -1.7 outside of 1\arcm. The mass
functions are for fields at 4\arcm\ and 11\arcm from the cluster center and
confirm the upturn in the mass function for stars less massive than about 0.4
M\solar. There appears to be an excess of low-mass stars over higher-mass stars
in the outer field with respect to the inner, in qualitative agreement with
expectations for mass segregation.Comment: 16 pages + 7 pages of tables, LaTeX using AASTeX macros, 11 figures
available by request, IoA preprin
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