169 research outputs found
The ACS Survey of Galactic Globular Clusters. IX. Horizontal Branch Morphology and the Second Parameter Phenomenon
The horizontal branch (HB) morphology of globular clusters (GCs) is most
strongly influenced by metallicity. The second parameter phenomenon
acknowledges that metallicity alone is not enough to describe the HB morphology
of all GCs. In particular, the outer Galactic halo contains GCs with redder HBs
at a given metallicity than are found inside the Solar circle. Thus, at least a
second parameter is required to characterize HB morphology. Here we analyze the
median color difference between the HB and the red giant branch (RGB), d(V-I),
measured from HST ACS photometry of 60 GCs within ~20 kpc of the Galactic
Center. Analysis of this homogeneous data set reveals that, after the influence
of metallicity has been removed, the correlation between d(V-I) and age is
stronger than that of any other parameter considered. Expanding the sample to
include HST photometry of the 6 most distant Galactic GCs lends additional
support to the correlation between d(V-I) and age. This result is robust with
respect to the adopted metallicity scale and the method of age determination,
but must bear the caveat that high quality, detailed abundance information is
not available for a significant fraction of the sample. When a subset of GCs
with similar metallicities and ages are considered, a correlation between
d(V-I) and central luminosity density is exposed. With respect to the existence
of GCs with anomalously red HBs at a given metallicity, we conclude that age is
the second parameter and central density is most likely the third. Important
problems related to HB morphology in GCs, notably multi-modal distributions and
faint blue tails, remain to be explained. (Abridged)Comment: Accepted for publication in ApJ; 49 pages, 19 figure
Tidal Signatures in the Faintest Milky Way Satellites: The Detailed Properties of Leo V, Pisces II and Canes Venatici II
We present deep wide-field photometry of three recently discovered faint
Milky Way satellites: Leo V, Pisces II, and Canes Venatici II. Our main goals
are to study the structure and star formation history of these dwarfs; we also
search for signs of tidal disturbance. The three satellites have similar
half-light radii ( pc) but a wide range of ellipticities. Both Leo
V and CVn II show hints of stream-like overdensities at large radii. An
analysis of the satellite color-magnitude diagrams shows that all three objects
are old ( 10 Gyr) and metal-poor ([Fe/H] ), though neither the
models nor the data have sufficient precision to assess when the satellites
formed with respect to cosmic reionization. The lack of an observed younger
stellar population (\la 10 Gyr) possibly sets them apart from the other
satellites at Galactocentric distances \ga 150 kpc. We present a new
compilation of structural data for all Milky Way satellite galaxies and use it
to compare the properties of classical dwarfs to the ultra-faints. The
ellipticity distribution of the two groups is consistent at the
2- level. However, the faintest satellites tend to be more
aligned toward the Galactic center, and those satellites with the highest
ellipticity (\ga 0.4) have orientations () in the range
. This latter
observation is in rough agreement with predictions from simulations of dwarf
galaxies that have lost a significant fraction of their dark matter halos and
are being tidally stripped.Comment: 25 pages, 7 figures, ApJ accepted; version updated to match ApJ
accepte
The Metallicity Distribution Functions of SEGUE G and K dwarfs: Constraints for Disk Chemical Evolution and Formation
We present the metallicity distribution function (MDF) for 24,270 G and
16,847 K dwarfs at distances from 0.2 to 2.3 kpc from the Galactic plane, based
on spectroscopy from the Sloan Extension for Galactic Understanding and
Exploration (SEGUE) survey. This stellar sample is significantly larger in both
number and volume than previous spectroscopic analyses, which were limited to
the solar vicinity, making it ideal for comparison with local volume-limited
samples and Galactic models. For the first time, we have corrected the MDF for
the various observational biases introduced by the SEGUE target selection
strategy. The SEGUE sample is particularly notable for K dwarfs, which are too
faint to examine spectroscopically far from the solar neighborhood. The MDF of
both spectral types becomes more metal-poor with increasing |Z|, which reflects
the transition from a sample with small [alpha/Fe] values at small heights to
one with enhanced [alpha/Fe] above 1 kpc. Comparison of our SEGUE distributions
to those of two different Milky Way models reveals that both are more
metal-rich than our observed distributions at all heights above the plane. Our
unbiased observations of G and K dwarfs provide valuable constraints over the
|Z|-height range of the Milky Way disk for chemical and dynamical Galaxy
evolution models, previously only calibrated to the solar neighborhood, with
particular utility for thin- and thick-disk formation models.Comment: 70 pages, 25 figures, 7 tables. Accepted by The Astrophysical Journa
The Fifth Data Release of the Sloan Digital Sky Survey
This paper describes the Fifth Data Release (DR5) of the Sloan Digital Sky
Survey (SDSS). DR5 includes all survey quality data taken through June 2005 and
represents the completion of the SDSS-I project (whose successor, SDSS-II will
continue through mid-2008). It includes five-band photometric data for 217
million objects selected over 8000 square degrees, and 1,048,960 spectra of
galaxies, quasars, and stars selected from 5713 square degrees of that imaging
data. These numbers represent a roughly 20% increment over those of the Fourth
Data Release; all the data from previous data releases are included in the
present release. In addition to "standard" SDSS observations, DR5 includes
repeat scans of the southern equatorial stripe, imaging scans across M31 and
the core of the Perseus cluster of galaxies, and the first spectroscopic data
from SEGUE, a survey to explore the kinematics and chemical evolution of the
Galaxy. The catalog database incorporates several new features, including
photometric redshifts of galaxies, tables of matched objects in overlap regions
of the imaging survey, and tools that allow precise computations of survey
geometry for statistical investigations.Comment: ApJ Supp, in press, October 2007. This paper describes DR5. The SDSS
Sixth Data Release (DR6) is now public, available from http://www.sdss.or
The Visual Orbit of the 1.1-day Spectroscopic Binary \sigma^2 Coronae Borealis from Interferometry at the CHARA Array
We present an updated spectroscopic orbit and a new visual orbit for the
double-lined spectroscopic binary \sigma^2 Coronae Borealis based on radial
velocity measurements at the Oak Ridge Observatory in Harvard, Massachusetts
and interferometric visibility measurements at the CHARA Array on Mount Wilson.
\sigma^2 CrB is composed of two Sun-like stars of roughly equal mass in a
circularized orbit with a period of 1.14 days. The long baselines of the CHARA
Array have allowed us to resolve the visual orbit for this pair, the shortest
period binary yet resolved interferometrically, enabling us to determine
component masses of 1.137 \pm 0.037 M_sun and 1.090 \pm 0.036 M_sun. We have
also estimated absolute V-band magnitudes of MV (primary) = 4.35 \pm 0.02 and
MV(secondary) = 4.74 \pm 0.02. A comparison with stellar evolution models
indicates a relatively young age of 1-3 Gyr, consistent with the high Li
abundance measured previously. This pair is the central component of a
quintuple system, along with another similar-mass star, \sigma^1 CrB, in a ~
730-year visual orbit, and a distant M-dwarf binary, \sigma CrB C, at a
projected separation of ~ 10 arcmin. We also present differential proper motion
evidence to show that components C & D (ADS 9979C & D) listed for this system
in the Washington Double Star Catalog are optical alignments that are not
gravitationally bound to the \sigma CrB system.Comment: 40 pages, 14 figures. Accepted by Ap
The Seventh Data Release of the Sloan Digital Sky Survey
This paper describes the Seventh Data Release of the Sloan Digital Sky Survey
(SDSS), marking the completion of the original goals of the SDSS and the end of
the phase known as SDSS-II. It includes 11663 deg^2 of imaging data, with most
of the roughly 2000 deg^2 increment over the previous data release lying in
regions of low Galactic latitude. The catalog contains five-band photometry for
357 million distinct objects. The survey also includes repeat photometry over
250 deg^2 along the Celestial Equator in the Southern Galactic Cap. A
coaddition of these data goes roughly two magnitudes fainter than the main
survey. The spectroscopy is now complete over a contiguous area of 7500 deg^2
in the Northern Galactic Cap, closing the gap that was present in previous data
releases. There are over 1.6 million spectra in total, including 930,000
galaxies, 120,000 quasars, and 460,000 stars. The data release includes
improved stellar photometry at low Galactic latitude. The astrometry has all
been recalibrated with the second version of the USNO CCD Astrograph Catalog
(UCAC-2), reducing the rms statistical errors at the bright end to 45
milli-arcseconds per coordinate. A systematic error in bright galaxy photometr
is less severe than previously reported for the majority of galaxies. Finally,
we describe a series of improvements to the spectroscopic reductions, including
better flat-fielding and improved wavelength calibration at the blue end,
better processing of objects with extremely strong narrow emission lines, and
an improved determination of stellar metallicities. (Abridged)Comment: 20 pages, 10 embedded figures. Accepted to ApJS after minor
correction
Minimum pricing of alcohol versus volumetric taxation:which policy will reduce heavy consumption without adversely affecting light and moderate consumers?
Background We estimate the effect on light, moderate and heavy consumers of alcohol from implementing a minimum unit price for alcohol (MUP) compared with a uniform volumetric tax. Methods We analyse scanner data from a panel survey of demographically representative households (n = 885) collected over a one-year period (24 Jan 2010–22 Jan 2011) in the state of Victoria, Australia, which includes detailed records of each household's off-trade alcohol purchasing. Findings The heaviest consumers (3% of the sample) currently purchase 20% of the total litres of alcohol (LALs), are more likely to purchase cask wine and full strength beer, and pay significantly less on average per standard drink compared to the lightest consumers (A2.21 [95% CI 2.10–2.31]). Applying a MUP of A393.60 [95% CI 374.19–413.00]) and beer (552.46 [95% CI 530.55–574.36] and $163.92 [95% CI 152.79–175.03], respectively). Both a MUP and uniform volumetric tax have little effect on changing the annual cost of wine and beer for light and moderate consumers, and likewise little effect upon their purchasing. Conclusions While both a MUP and a uniform volumetric tax have potential to reduce heavy consumption of wine and beer without adversely affecting light and moderate consumers, a MUP offers the potential to achieve greater reductions in heavy consumption at a lower overall annual cost to consumers
Low Frequency Vibrations Disrupt Left-Right Patterning in the Xenopus Embryo
The development of consistent left-right (LR) asymmetry across phyla is a fascinating question in biology. While many pharmacological and molecular approaches have been used to explore molecular mechanisms, it has proven difficult to exert precise temporal control over functional perturbations. Here, we took advantage of acoustical vibration to disrupt LR patterning in Xenopus embryos during tightly-circumscribed periods of development. Exposure to several low frequencies induced specific randomization of three internal organs (heterotaxia). Investigating one frequency (7 Hz), we found two discrete periods of sensitivity to vibration; during the first period, vibration affected the same LR pathway as nocodazole, while during the second period, vibration affected the integrity of the epithelial barrier; both are required for normal LR patterning. Our results indicate that low frequency vibrations disrupt two steps in the early LR pathway: the orientation of the LR axis with the other two axes, and the amplification/restriction of downstream LR signals to asymmetric organs
The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III
The Sloan Digital Sky Survey (SDSS) started a new phase in August 2008, with
new instrumentation and new surveys focused on Galactic structure and chemical
evolution, measurements of the baryon oscillation feature in the clustering of
galaxies and the quasar Ly alpha forest, and a radial velocity search for
planets around ~8000 stars. This paper describes the first data release of
SDSS-III (and the eighth counting from the beginning of the SDSS). The release
includes five-band imaging of roughly 5200 deg^2 in the Southern Galactic Cap,
bringing the total footprint of the SDSS imaging to 14,555 deg^2, or over a
third of the Celestial Sphere. All the imaging data have been reprocessed with
an improved sky-subtraction algorithm and a final, self-consistent photometric
recalibration and flat-field determination. This release also includes all data
from the second phase of the Sloan Extension for Galactic Understanding and
Evolution (SEGUE-2), consisting of spectroscopy of approximately 118,000 stars
at both high and low Galactic latitudes. All the more than half a million
stellar spectra obtained with the SDSS spectrograph have been reprocessed
through an improved stellar parameters pipeline, which has better determination
of metallicity for high metallicity stars.Comment: Astrophysical Journal Supplements, in press (minor updates from
submitted version
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