49 research outputs found
VI Photometry of Globular Clusters NGC6293 and NGC6541: The Formation of the Metal-Poor Inner Halo Globular Clusters
We present VI photometry of the metal-poor inner halo globular clusters
NGC6293 and NGC6541 using the planetary camera of the WFPC2 on board HST. Our
color-magnitude diagrams of the clusters show well-defined BHB populations,
consistent with their low metallicities and old ages. NGC6293 appears to have
blue straggler stars in the cluster's central region. We discuss the
interstellar reddening and the distance modulus of NGC6293 and NGC6541 and
obtain E(B-V) = 0.40 and (m-M)_0 = 14.61 for NGC6293 and E(B-V) = 0.14 and
(m-M)_0 = 14.19 for NGC6541. Our results confirm that NGC6293 and NGC6541 are
clearly located in the Galaxy's central regions (R_{GC} < 3 kpc). We also
discuss the differential reddening across NGC6293. The interstellar reddening
value of NGC6293 appears to vary by Delta E(B-V) ~ 0.02 -- 0.04 mag within our
small field of view.
The most notable result of our study is that the inner halo clusters NGC6293
and NGC6541 essentially have the same ages as M92, confirming the previous
result from the HST NIC3 observations of NGC6287.Comment: AJ, accepte
Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy
HIRES on the Keck I telescope has been used to measure the first radial
velocities for stars belonging to eleven, heavily-reddened globular clusters in
the direction of the inner Galaxy. The question of kinematic substructuring
among the Galactic globular cluster system is investigated using an updated
catalog of globular cluster distances, metallicities and velocities. It is
found that the population of metal-rich globular clusters shows significant
rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc
of the Galactic center, the measured rotation velocity and line-of-sight
velocity dispersion are similar to those of bulge field stars. We investigate
claims that the metal-rich clusters are associated with the central Galactic
bar by comparing the kinematics of the innermost clusters to that of the atomic
hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich
and metal-poor clusters bears a remarkable similarity to that of the gas,
including the same non-circular motions which have traditionally been
interpreted as evidence for a Galactic bar, or, alternatively, a
non-axisymmetric bulge. However, uncertainties in the existing
three-dimensional Galactocentric positions for most of the clusters do not yet
allow an unambiguous discrimination between the competing scenarios of
membership in a rigidly rotating bar, or in a bulge which is an oblate
isotropic rotator. We conclude that the majority of metal-rich clusters within
the central 4 kpc of the Galaxy are probably associated with the bulge/bar, and
not the thick disk. (ABRIDGED)Comment: 18 pages, including 7 of 13 postscript figures. Figures 1-6 available
at http://astro.caltech.edu/~pc. Accepted for publication in the Astronomical
Journa
X-ray and Optical Study of Low Core Density Globular Clusters NGC6144 and E3
We report on the Chandra X-ray Observatory and Hubble Space Telescope
observation of two low core density globular clusters, NGC6144 and E3. By
comparing the number of X-ray sources inside the half-mass radius to those
outside, we found 6 X-ray sources within the half-mass radius of NGC6144, among
which 4 are expected to be background sources; 3 X-ray sources are also found
within the half-mass radius of E3, of which 3 is expected to be background
source. Therefore, we cannot exclude that all our sources are background
sources. However, combining the results from X-ray and optical observations, we
found that 1-2 sources in NGC6144 and 1 source in E3 are likely to be
cataclysmic variables and that 1 source in NGC6144 is an active binary, based
on the X-ray and optical properties. The number of faint X-ray sources in
NGC6144 and E3 found with Chandra and HST is higher than a prediction based on
collision frequency, but is closer to that based on mass. Our observations
strongly suggest that the compact binary systems in NGC6144 and E3 are
primordial in origin.Comment: 28 pages, 9 figures, 6 tables, Accepted for publication in Ap
A population of gut epithelial enterochromaffin cells is mechanosensitive and requires Piezo2 to convert force into serotonin release
Enterochromaffin (EC) cells constitute the largest population of intestinal epithelial enteroendocrine (EE) cells. EC cells are proposed to be specialized mechanosensory cells that release serotonin in response to epithelial forces, and thereby regulate intestinal fluid secretion. However, it is unknown whether EE and EC cells are directly mechanosensitive, and if so, what the molecular mechanism of their mechanosensitivity is. Consequently, the role of EE and EC cells in gastrointestinal mechanobiology is unclear. Piezo2 mechanosensitive ion channels are important for some specialized epithelial mechanosensors, and they are expressed in mouse and human EC cells. Here, we use EC and EE cell lineage tracing in multiple mouse models to show that Piezo2 is expressed in a subset of murine EE and EC cells, and it is distributed near serotonin vesicles by superresolution microscopy. Mechanical stimulation of a subset of isolated EE cells leads to a rapid inward ionic current, which is diminished by Piezo2 knockdown and channel inhibitors. In these mechanosensitive EE cells force leads to Piezo2-dependent intracellular Ca(2+) increase in isolated cells as well as in EE cells within intestinal organoids, and Piezo2-dependent mechanosensitive serotonin release in EC cells. Conditional knockout of intestinal epithelial Piezo2 results in a significant decrease in mechanically stimulated epithelial secretion. This study shows that a subset of primary EE and EC cells is mechanosensitive, uncovers Piezo2 as their primary mechanotransducer, defines the molecular mechanism of their mechanotransduction and mechanosensitive serotonin release, and establishes the role of epithelial Piezo2 mechanosensitive ion channels in regulation of intestinal physiology
The Chemical Enrichment History of the Fornax Dwarf Spheroidal Galaxy from the Infrared Calcium Triplet
Near infrared spectra were obtained for 117 red giants in the Fornax dwarf
spheroidal galaxy with the FORS1 spectrograph on the VLT, in order to study the
metallicity distribution of the stars and to lift the age-metallicity
degeneracy of the red giant branch (RGB) in the color-magnitude diagram (CMD).
Metallicities are derived from the equivalent widths of the infrared Calcium
triplet lines at 8498, 8542, and 8662 A, calibrated with data from globular
clusters, the open cluster M67 and the LMC. For a substantial portion of the
sample, the strength of the Calcium triplet is unexpectedly high, clearly
indicating that the main stellar population of Fornax is significantly more
metal-rich than could be inferred from the position of its RGB in the CMD. We
show that the relative narrowness of the RGB in Fornax is caused by the
superposition of stars of very different ages and metallicities. The
metallicity distribution in Fornax is centered at [Fe/H]= -0.9, with a
metal-poor tail extending to [Fe/H] ~= -2. While the distribution to higher
metallicities is less well determined by our observations, the comparison with
LMC data indicates that it extends to [Fe/H] ~ -0.4. By comparing the
metallicities of the stars with their positions in the CMD, we have derived the
complex age-metallicity relation of Fornax. In the first few Gyr, the metal
abundance rose to [Fe/H] ~ -1.0 dex. The enrichment accelerated in the past ~
1-4 Gyr to reach [Fe/H] ~ -0.4 dex. More than half the sample is constituted of
star younger than ~ 4 Gyr, thus indicating sustained recent star formation in
Fornax. These results indicate that the capacity of dwarf spheroidal galaxies
to retain the heavy elements that they produce is larger than expected.
(Abridged)Comment: 36 pages, 19 figures, to appear in Astronomical Journal, January 200
A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
The (rest-frame) near-IR domain contains important stellar population
diagnostics and is often used to estimate masses of galaxies at low as well as
high redshifts. However, many stellar population models are still relatively
poorly calibrated in this part of the spectrum. To allow an improvement of this
calibration we present a new database of integrated near-infrared JHKs
magnitudes for 75 star clusters in the Magellanic Clouds, using the 2-Micron
All-Sky Survey (2MASS). The majority of the clusters in our sample have robust
age and metallicity estimates from color-magnitude diagrams available in the
literature, and populate a range of ages from 10 Myr to 15 Gyr and a range in
[Fe/H] from -2.17 to +0.01 dex. A comparison with matched star clusters in the
2MASS Extended Source Catalog (XSC) reveals that the XSC only provides a good
fit to the unresolved component of the cluster stellar population. We also
compare our results with the often-cited single-channel JHK photometry of
Persson and collaborators, and find significant differences, especially for
their 30"-diameter apertures up to ~2.5 mag in the K-band, more than 1 mag in
J-K, and up to 0.5 mag in H-K. Using simulations to center apertures based on
maximum light throughput (as performed by Persson et al, we show that these
differences can be attributed to near-IR-bright cluster stars (e.g., Carbon
stars) located away from the true center of the star clusters. The wide age and
metallicity coverage of our integrated JHKs photometry sample constitutes a
fundamental dataset for testing population synthesis model predictions, and for
direct comparison with near-IR observations of distant stellar populations.Comment: AJ August 2006 issue, 67 pages, 8 tables, 17 figure
A Comparison of Optical and Near-Infrared Colours of Magellanic Cloud Star Clusters with Predictions of Simple Stellar Population Models
We present integrated JHK_s 2MASS photometry and a compilation of
integrated-light optical photoelectric measurements for 84 star clusters in the
Magellanic Clouds. These clusters range in age from ~200 Myr to >10 Gyr, and
have [Fe/H] values from -2.2 to -0.1 dex. We find a spread in the intrinsic
colours of clusters with similar ages and metallicities, at least some of which
is due to stochastic fluctuations in the number of bright stars residing in
low-mass clusters. We use 54 clusters with the most reliable age and
metallicity estimates as test particles to evaluate the performance of four
widely used SSP models in the optical/NIR colour-colour space. All models
reproduce the reddening-corrected colours of the old (>10 Gyr) globular
clusters quite well, but model performance varies at younger ages. In order to
account for the effects of stochastic fluctuations in individual clusters, we
provide composite B-V, B-J, V-J, V-Ks and J-Ks colours for Magellanic Cloud
clusters in several different age intervals. The accumulated mass for most
composite clusters are higher than that needed to keep luminosity variations
due to stochastic fluctuations below the 10% level. The colours of the
composite clusters are clearly distinct in optical-NIR colour-colour space for
the following intervals of age: >10 Gyr, 2-9 Gyr, 1-2 Gyr, and 200 Myr-1 Gyr.
This suggests that a combination of optical plus NIR colours can be used to
differentiate clusters of different age and metallicity.Comment: 30 pages, 21 figures, 10 tables, accepted for publication at MNRAS, a
full-resolution version of the manuscript is available at
http://www.astrosci.ca/users/puziat/HIA/Home_files/ms.pd
Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud
One hundred and forty six long-period red variable stars in the Large
Magellanic Cloud (LMC) from the three year MOA project database were analysed.
A careful periodic analysis was performed on these stars and a catalogue of
their magnitudes, colours, periods and amplitudes is presented. We convert our
blue and red magnitudes to band values using 19 oxygen-rich stars. A group
of red short-period stars separated from the Mira sequence has been found on a
(log P, K) diagram. They are located at the short period side of the Mira
sequence consistent with the work of Wood and Sebo (1996). There are two
interpretations for such stars; a difference in pulsation mode or a difference
in chemical composition. We investigated the properties of these stars together
with their colour, amplitude and periodicity. We conclude that they have small
amplitudes and less regular variability. They are likely to be higher mode
pulsators. A large scatter has been also found on the long period side of the
(log P, K) diagram. This is possibly a systematic spread given that the blue
band of our photometric system covers both standard B and V bands and affects
carbon-rich stars.Comment: 19 pages, 19 figures, accepted for publication in MNRA
The Palomar Testbed Interferometer Calibrator Catalog
The Palomar Testbed Interferometer (PTI) archive of observations between 1998
and 2005 is examined for objects appropriate for calibration of optical
long-baseline interferometer observations - stars that are predictably
point-like and single. Approximately 1,400 nights of data on 1,800 objects were
examined for this investigation. We compare those observations to an
intensively studied object that is a suitable calibrator, HD217014, and
statistically compare each candidate calibrator to that object by computing
both a Mahalanobis distance and a Principal Component Analysis. Our hypothesis
is that the frequency distribution of visibility data associated with
calibrator stars differs from non-calibrator stars such as binary stars.
Spectroscopic binaries resolved by PTI, objects known to be unsuitable for
calibrator use, are similarly tested to establish detection limits of this
approach. From this investigation, we find more than 350 observed stars
suitable for use as calibrators (with an additional being
rejected), corresponding to sky coverage for PTI. This approach
is noteworthy in that it rigorously establishes calibration sources through a
traceable, empirical methodology, leveraging the predictions of spectral energy
distribution modeling but also verifying it with the rich body of PTI's on-sky
observations.Comment: 100 pages, 7 figures, 7 tables; to appear in the May 2008ApJS, v176n