375 research outputs found
RR Lyrae Variables in M33. I. Evidence For a Field Halo Population
We present observations of RR Lyrae variables in the Local Group late-type
spiral galaxy M33. Using the Advanced Camera for Surveys on the Hubble Space
Telescope, we have identified 64 ab-type RR Lyraes in M33. We have estimated
reddenings for these stars based on their minimum light V-I colors and
metallicities based on their periods. From the distributions of these
properties, we conclude that the RR Lyraes belong to two populations - one
associated with the halo of M33 and the other with its disk. Given that RR
Lyraes are produced by populations older than ~10 Gyr, this suggests that not
only does the field halo of M33 contain an old component, but so does its disk.
This is one of the best pieces of evidence for the existence of a halo field
component in M33. Using a relation between RR Lyrae absolute magnitude and
metallicity (Mv(RR) = 0.23[Fe/H] + 0.93), we estimate a mean distance modulus
of = 24.67 +/- 0.08 for M33. This places M33 approximately 70 kpc
beyond M31 in line-of-sight distance.Comment: 20 pages, 17 figures, accepted for publication in The Astronomical
Journa
HST Colour-Magnitude Diagrams of Six Old Globular Clusters in the LMC
We report on HST observations of six candidate old globular clusters in the
Large Magellanic Cloud: NGC 1754, NGC 1835, NGC 1898, NGC 1916, NGC 2005 and
NGC 2019. Deep exposures with the F555W and F814W filters provide us with
colour-magnitude diagrams that reach to an apparent magnitude in V of ~25, well
below the main sequence turnoff. These particular clusters are involved with
significantly high LMC field star densities and care was taken to subtract the
field stars from the cluster colour-magnitude diagrams accurately. In two cases
there is significant variable reddening across at least part of the image, but
only for NGC 1916 does the differential reddening preclude accurate
measurements of the CMD characteristics. The morphologies of the colour-
magnitude diagrams match well those of Galactic globular clusters of similar
metallicity. All six have well-developed horizontal branches, while four
clearly have stars on both sides of the RR Lyrae gap. The abundances obtained
from measurements of the height of the red giant branch above the level of the
horizontal branch are 0.3 dex higher, on average, than previously measured
spectroscopic abundances. Detailed comparisons with Galactic globular cluster
fiducials show that all six clusters are old objects, very similar in age to
classical Galactic globulars such as M5, with little age spread among the
clusters. This result is consistent with ages derived by measuring the
magnitude difference between the horizontal branch and main sequence turnoff.
We also find a similar chronology by comparing the horizontal branch
morphologies and abundances with the horizontal branch evolutionary tracks of
Lee, Demarque, & Zinn (1994). Our results imply that the LMC formed at the same
time as the Milky Way Galaxy.Comment: 23 pages, 18 PostScript figures, LaTeX, accepted by MNRAS. Uses
mn.sty and epsf.sty. Requires ols.sty (included
Positive Changes in Safety Perception Among Blacks with HIV and Comorbidities: Assessment of Social Determinants of Health During COVID‑19
Purpose
This study aimed to examine the impact of the COVID-19 lockdown on social determinants of health (SDOH) among Blacks with HIV and a comorbid diagnosis of hypertension or type 2 diabetes mellitus (T2DM). Methods
This was a longitudinal survey study. The inclusion criteria were adults ≥ 18 years and the presence of hypertension and/or diabetes, along with a positive HIV diagnosis. This study enrolled patients in the HIV clinics and chain specialty pharmacies in the Dallas-Fort Worth (DFW) area. A survey of ten questions examining SDOH was conducted before, during, and after the lockdown. A proportional odds mixed effects logistic regression model was applied to assess differences between time points. Results
A total of 27 participants were included. Respondents felt significantly safer in their living place post-lockdown than in the pre-lockdown period (odds ratio = 6.39, 95% CI [1.08–37.73]). No other statistically significant differences in the responses were found over the study timeframe. However, borderline p values indicated better SDOH status post-lockdown as compared to pre-lockdown. Conclusion
Study participants feel safer one year after lockdown compared to pre-lockdown. The CARES Act and the moratorium on rent and mortgage are among the factors that may explain this increase. Future research should include designing and evaluating interventions for social equity enhancement
N-body simulations of the Magellanic Stream
A suite of high-resolution N-body simulations of the Magellanic Clouds --
Milky Way system are presented and compared directly with newly available data
from the HI Parkes All-Sky Survey (HIPASS). We show that the interaction
between Small and Large Magellanic Clouds results in both a spatial and
kinematical bifurcation of both the Stream and the Leading Arm. The spatial
bifurcation of the Stream is readily apparent in the HIPASS data, and the
kinematical bifurcation is also tentatively identified. This bifurcation
provides strong support for the tidal disruption origin for the Magellanic
Stream. A fiducial model for the Magellanic Clouds is presented upon completion
of an extensive parameter survey of the potential orbital configurations of the
Magellanic Clouds and the viable initial boundary conditions for the disc of
the Small Magellanic Cloud. The impact of the choice of these critical
parameters upon the final configurations of the Stream and Leading Arm is
detailed.Comment: Accepted by MNRAS, 07 Jun 2006. 14 pages, 14 figures, 3 tables. LaTeX
(mn2e.sty). File with decent resolution images (strongly recommended)
available at http://astronomy.swin.edu.au/~tconnors/publications/ .
References added; distance and HI-LOres difference figures added; clearer
figures; discussion added to, but conclusions unchange
The Stellar Populations of M33's Outer Regions IV: Inflow History and Chemical Evolution
We have modelled the observed color-magnitude diagram (CMD) at one location
in M33's outskirts under the framework of a simple chemical evolution scenario
which adopts instantaneous and delayed recycling for the nucleosynthetic
products of Type II and Ia supernovae. In this scenario, interstellar gas forms
stars at a rate modulated by the Kennicutt-Schmidt relation and gas outflow
occurs at a rate proportional to the star formation rate (SFR). With this
approach, we put broad constraints on the role of gas flows during this
region's evolution and compare its [alpha/Fe] vs. [Fe/H] relation with that of
other Local Group systems. We find that models with gas inflow are
significantly better than the closed box model at reproducing the observed
distribution of stars in the CMD. The best models have a majority of gas inflow
taking place in the last 7 Gyr, and relatively little in the last 3 Gyr. These
models predict most stars in this region to have [alpha/Fe] ratios lower than
the bulk of the Milky Way's halo. The predictions for the present-day SFR, gas
mass, and oxygen abundance compare favorably to independent empirical
estimates. Our results paint a picture in which M33's outer disc formed from
the protracted inflow of gas over several Gyr with at least half of the total
inflow occurring since z ~ 1.Comment: 22 pages, 12 figures, accepted to MNRA
The dependence of HII region properties on global and local surface brightness within galaxy discs
Using B, R, and H-alpha images of roughly equal-sized samples of low surface
brightness (LSB) and high surface brightness (HSB) galaxies (~40 galaxies
apiece), we have explored the dependence of HII region properties on local and
global disc surface brightness. We have done this by constructing co-added HII
region luminosity functions (LFs) according to local and central disc surface
brightness and fitting Schechter functions to these LFs. The results show that
the shape of the HII region LF within LSB galaxies does not change noticeably
as different limiting (i.e., mu>mu_lim) local surface brightness values are
used. However, the LFs for HSB galaxies have larger values of L_* and are less
steep at the faint-end than those of LSB galaxies for limiting B-band local
surface brightness values as faint as mu_B,lim~23-24. Both the LFs and the data
for individual HII regions show that luminous (log L>39 ergs/s) HII regions are
much more common within HSB discs than within LSB discs, implying that the
newly formed star clusters are also larger. Taking this into account along with
the results of Monte Carlo simulations, the shapes of the LFs imply that the
regions within LSB discs and those within the LSB areas of HSB discs are
relatively old (~5 Myr) while the regions within HSB discs for mu_B<24 are
significantly younger (<1 Myr). Since the majority of the LSB galaxies do not
have noticeable spiral arms and the majority of the HSB galaxies do, this may
indicate a transition within HSB discs from spiral arm-driven star formation to
a more locally driven, possibly sporadic form of star formation at mu_B~24, a
transition that does not appear to occur within LSB discs.Comment: Accepted to MNRA
Small RNA Profile in Moso Bamboo Root and Leaf Obtained by High Definition Adapters
Moso bamboo (Phyllostachy heterocycla cv. pubescens L.) is an economically important fast-growing tree. In order to gain better understanding of gene expression regulation in this important species we used next generation sequencing to profile small RNAs in leaf and roots of young seedlings. Since standard kits to produce cDNA of small RNAs are biased for certain small RNAs, we used High Definition adapters that reduce ligation bias. We identified and experimentally validated five new microRNAs and a few other small non-coding RNAs that were not microRNAs. The biological implication of microRNA expression levels and targets of microRNAs are discussed
A study of HI-selected galaxies in the Hercules cluster
The present study is aimed at a sample of 22 galaxies detected in the blind
VLA HI survey of the Hercules cluster by Dickey (1997), 18 of which were
selected on an HI line width smaller than 270 km/s and 4 others with only
tentative optical counterparts on the Palomar Sky Survey. Sensitive single-dish
HI line spectra were obtained for 20 of them, and for one (47-154) the VLA
detection was not confirmed. Optical surface photometry was obtained of 10
objects, for 8 of which optical spectroscopy was obtained as well. Based on
various selection criteria, two (ce-143 and ne-204) can be classified as
dwarfs. The objects of which optical observations were made show star formation
properties similar to those of published samples of actively star forming
galaxies, and approximately half of them have properties intermediate between
those of dwarf galaxies and low-luminosity disc galaxies. No optical redshifts
could be obtained for two of the galaxies (sw-103 and sw-194) and their
physical association with the HI clouds detected at their positions therefore
remains uncertain. Unique among the objects is the Tidal Dwarf Galaxy ce-061 in
a tail of the IC 1182 merger system.Comment: 20 pages, 19 figures, accepted for publication in A&
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