135 research outputs found
Mapping the Recent Star Formation History of the Disk of M51
Using data acquired as part of a unique Hubble Heritage imaging program of
broadband colors of the interacting spiral system M51/NGC 5195, we have
conducted a photometric study of the stellar associations across the entire
disk of the galaxy in order to assess trends in size, luminosity, and local
environment associated with recent star formation activity in the system.
Starting with a sample of over 900 potential associations, we have produced
color-magnitude and color-color diagrams for the 120 associations that were
deemed to be single-aged. It has been found that main sequence turnoffs are not
evident for the vast majority of the stellar associations in our set,
potentially due to the overlap of isochronal tracks at the high mass end of the
main sequence, and the limited depth of our images at the distance of M51. In
order to obtain ages for more of our sample, we produced model spectral energy
distributions (SEDs) to fit to the data from the GALEXEV simple stellar
population (SSP) models of Bruzual and Charlot (2003). These SEDs can be used
to determine age, size, mass, metallicity, and dust content of each association
via a simple chi-squared minimization to each association's B, V, and I-band
fluxes. The derived association properties are mapped as a function of
location, and recent trends in star formation history of the galaxy are
explored in light of these results. This work is the first phase in a program
that will compare these stellar systems with their environments using
ultraviolet data from GALEX and infrared data from Spitzer, and ultimately we
plan to apply the same stellar population mapping methodology to other nearby
face-on spiral galaxies.Comment: 13 pages, 3 figures, 1 table. Accepted to The Astronomical Journa
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The LTHE LUMINOSITY, MASS, AND AGE DISTRIBUTIONS OF COMPACT STAR CLUSTERS IN M83 BASED ON HUBBLE SPACE TELESCOPE/WIDE FIELD CAMERA 3 OBSERVATIONS
The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near-ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function (LF) for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL vprop L α, with α = –2.04 ± 0.08, down to MV ≈ –5.5. We test the sensitivity of the LF to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in α. We estimate ages and masses for the clusters by comparing their measured UBVI, Hα colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power law, dN/dτ vprop τγ, with γ = –0.9 ± 0.2, for M gsim few × 103 M sun and τ lsim 4 × 108 yr. This indicates that clusters are disrupted quickly, with ≈80%-90% disrupted each decade in age over this time. The mass function of clusters over the same M-τ range is a power law, dN/dM vprop M β, with β = –1.94 ± 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, MC , or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e., mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters
Absolute Magnitudes and Colors of RR Lyrae stars in DECam Passbands from Photometry of the Globular Cluster M5
We characterize the absolute magnitudes and colors of RR Lyrae stars in the
globular cluster M5 in the ugriz filter system of the Dark Energy Camera
(DECam). We provide empirical Period-Luminosity (P-L) relationships in all 5
bands based on 47 RR Lyrae stars of the type ab and 14 stars of the type c. The
P-L relationships were found to be better constrained for the fundamental mode
RR Lyrae stars in the riz passbands, with dispersion of 0.03, 0.02 and 0.02
magnitudes, respectively. The dispersion of the color at minimum light was
found to be small, supporting the use of this parameter as a means to obtain
accurate interstellar extinctions along the line of sight up to the distance of
the RR Lyrae star. We found a trend of color at minimum light with pulsational
period that, if taken into account, brings the dispersion in color at minimum
light to < 0.016 magnitudes for the (r-i), (i-z), and (r-z) colors. These
calibrations will be very useful for using RR Lyrae stars from DECam
observations as both standard candles for distance determinations and color
standards for reddening measurements.Comment: Accepted for publication in A
Age and Mass for 920 LMC Clusters Derived from 100 Million Monte Carlo Simulations
We present new age and mass estimates for 920 stellar clusters in the Large
Magellanic Cloud (LMC) based on previously published broad-band photometry and
the stellar cluster analysis package, MASSCLEANage. Expressed in the generic
fitting formula, d^{2}N/dM dt ~ M^{\alpha} t^{\beta}, the distribution of
observed clusters is described by \alpha = -1.5 to -1.6 and \beta = -2.1 to
-2.2. For 288 of these clusters, ages have recently been determined based on
stellar photometric color-magnitude diagrams, allowing us to gauge the
confidence of our ages. The results look very promising, opening up the
possibility that this sample of 920 clusters, with reliable and consistent age,
mass and photometric measures, might be used to constrain important
characteristics about the stellar cluster population in the LMC. We also
investigate a traditional age determination method that uses a \chi^2
minimization routine to fit observed cluster colors to standard infinite mass
limit simple stellar population models. This reveals serious defects in the
derived cluster age distribution using this method. The traditional \chi^2
minimization method, due to the variation of U,B,V,R colors, will always
produce an overdensity of younger and older clusters, with an underdensity of
clusters in the log(age/yr)=[7.0,7.5] range. Finally, we present a unique
simulation aimed at illustrating and constraining the fading limit in observed
cluster distributions that includes the complex effects of stochastic
variations in the observed properties of stellar clusters.Comment: Accepted for publication in The Astrophysical Journal, 37 pages, 18
figure
How do parents of preverbal children with acute otitis media determine how much ear pain their child is having?
The objective of this study was to determine how parents of preverbal children determine whether their child is having otalgia. We constructed 8 cases describing a 1-year-old child with acute otitis media (AOM) using various combinations of the following 6 observable symptoms: fussiness, ear tugging, eating less, fever, sleeping difficulty, and playing less. Parents of children with a history of AOM presenting for well or sick appointments to an ambulatory clinic were asked to assign a pain level to each case on a visual analog scale. Sixty-nine parents participated in the study. Each of the 6 behaviors was associated with increased pain levels (P < .0001). Ear tugging and fussiness had the highest impact on the assigned pain levels. Higher level of parental education and private insurance were associated with higher reported pain levels (P = .007 and P = .001, respectively). Because interpretation of symptoms appears to be influenced by socioeconomic status, we question the utility of using an overall pain score from a 1-item parent scale as an outcome measure in clinical trials that include preverbal children. Perspective: Parents of preverbal children with acute otitis media use observable behaviors to determine their child's pain level. Interpretation of symptoms, however, appears to be influenced by socioeconomic status. Thus, we question the utility of using a 1-item parental pain scale in clinical trials that include preverbal children. © 2010 by the American Pain Society
Variable stars in the field of the Hydra II ultra-faint dwarf galaxy
We report the discovery of one RR Lyrae star in the ultra--faint satellite
galaxy Hydra II based on time series photometry in the g, r and i bands
obtained with the Dark Energy Camera at Cerro Tololo Interamerican Observatory,
Chile. The RR Lyrae star has a mean magnitude of which
translates to a heliocentric distance of kpc for Hydra II; this
value is larger than the estimate from the discovery paper based on
the average magnitude of several blue horizontal branch star candidates. The
new distance implies a slightly larger half-light radius of pc
and a brighter absolute magnitude of , which keeps this
object within the realm of the dwarf galaxies. The pulsational properties of
the RR Lyrae star ( d, mag) suggest Hydra II may be
a member of the intermediate Oosterhoff or Oosterhoff II group. A comparison
with other RR Lyrae stars in ultra--faint systems indicates similar pulsational
properties among them, which are different to those found among halo field
stars and those in the largest of the Milky Way satellites. We also report the
discovery of 31 additional short period variables in the field of view (RR
Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid). However,
given their magnitudes and large angular separation from Hydra II, they must be
field stars not related to Hydra II.Comment: Revised version after comments from the referee. Accepted for
publication in A
HII Region Luminosity Function of the Interacting Galaxy M51
We present a study of HII regions in M51 using the Hubble Space Telescope ACS
images taken as part of the Hubble Heritage Program. We have catalogued about
19,600 HII regions in M51 with Ha luminosity in the range of L = 10^{35.5}
erg/s to 10^{39.0} erg/s. The Ha luminosity function of HII regions (HII LF) in
M51 is well represented by a double power law with its index alpha=-2.25\pm0.02
for the bright part and alpha=-1.42\pm0.01 for the faint part, separated at a
break point L= 10^{37.1} erg/s. This break was not found in previous studies of
M51 HII regions. Comparison with simulated HII LFs suggests that this break is
caused by the transition of HII region ionizing sources, from low mass clusters
(with ~ 10^3 M_sun, including several OB stars) to more massive clusters
(including several tens of OB stars). The HII LFs with L < 10^{37.1} erg/s are
found to have different slopes for different parts in M51: the HII LF for the
interarm region is steeper than those for the arm and the nuclear regions. This
observed difference in HII LFs can be explained by evolutionary effects that
HII regions in the interarm region are relatively older than those in the other
parts of M51.Comment: 36 pages, 12 figures, 2 tables, accepted to Ap
Hubble Space Telescope Pixel Analysis of the Interacting Face-on Spiral Galaxy NGC 5194 (M51A)
A pixel analysis is carried out on the interacting face-on spiral galaxy NGC
5194 (M51A), using the HST/ACS images in the F435W, F555W and F814W (BVI)
bands. After 4x4 binning of the HST/ACS images to secure a sufficient
signal-to-noise ratio for each pixel, we derive several quantities describing
the pixel color-magnitude diagram (pCMD) of NGC 5194: blue/red color cut, red
pixel sequence parameters, blue pixel sequence parameters and blue-to-red pixel
ratio. The red sequence pixels are mostly older than 1 Gyr, while the blue
sequence pixels are mostly younger than 1 Gyr, in their luminosity-weighted
mean stellar ages. The color variation in the red pixel sequence from V = 20
mag arcsec^(-2) to V = 17 mag arcsec^(-2) corresponds to a metallicity
variation of \Delta[Fe/H] ~ 2 or an optical depth variation of \Delta\tau_V ~ 4
by dust, but the actual sequence is thought to originate from the combination
of those two effects. At V < 20 mag arcsec^(-2), the color variation in the
blue pixel sequence corresponds to an age variation from 5 Myr to 300 Myr under
the assumption of solar metallicity and \tau_V = 1. To investigate the spatial
distributions of stellar populations, we divide pixel stellar populations using
the pixel color-color diagram and population synthesis models. As a result, we
find that the pixel population distributions across the spiral arms agree with
a compressing process by spiral density waves: dense dust \rightarrow
newly-formed stars. The tidal interaction between NGC 5194 and NGC 5195 appears
to enhance the star formation at the tidal bridge connecting the two galaxies.
We find that the pixels corresponding to the central active galactic nucleus
(AGN) area of NGC 5194 show a tight sequence at the bright-end of the pCMD,
which are in the region of R ~ 100 pc and may be a photometric indicator of AGN
properties.Comment: 27 pages, 20 figures, accepted for publication in Ap
The Luminosity, Mass, and Age Distributions of Compact Star Clusters in M83 Based on HST/WFC3 Observations
The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope
has been used to obtain multi-band images of the nearby spiral galaxy M83.
These new observations are the deepest and highest resolution images ever taken
of a grand-design spiral, particularly in the near ultraviolet, and allow us to
better differentiate compact star clusters from individual stars and to measure
the luminosities of even faint clusters in the U band. We find that the
luminosity function for clusters outside of the very crowded starburst nucleus
can be approximated by a power law, dN/dL \propto L^{alpha}, with alpha = -2.04
+/- 0.08, down to M_V ~ -5.5. We test the sensitivity of the luminosity
function to different selection techniques, filters, binning, and aperture
correction determinations, and find that none of these contribute significantly
to uncertainties in alpha. We estimate ages and masses for the clusters by
comparing their measured UBVI,Halpha colors with predictions from single
stellar population models. The age distribution of the clusters can be
approximated by a power-law, dN/dt propto t^{gamma}, with gamma=-0.9 +/- 0.2,
for M > few x 10^3 Msun and t < 4x10^8 yr. This indicates that clusters are
disrupted quickly, with ~80-90% disrupted each decade in age over this time.
The mass function of clusters over the same M-t range is a power law, dN/dM
propto M^{beta}, with beta=-1.94 +/- 0.16, and does not have bends or show
curvature at either high or low masses. Therefore, we do not find evidence for
a physical upper mass limit, M_C, or for the earlier disruption of lower mass
clusters when compared with higher mass clusters, i.e. mass-dependent
disruption. We briefly discuss these implications for the formation and
disruption of the clusters.Comment: 36 pages, 13 figures, 1 table; accepted for publication in the
Astrophysical Journa
Age distributions of star clusters in spiral and barred galaxies as a test for theories of spiral structure
We consider models of gas flow in spiral galaxies in which the spiral
structure has been excited by various possible mechanisms: a global steady
density wave, self-gravity of the stellar disc and an external tidal
interaction, as well as the case of a galaxy with a central rotating bar. In
each model we estimate in a simple manner the likely current positions of star
clusters of a variety of ages, ranging from ~ 2 Myr to around 130 Myr,
depending on the model. We find that the spatial distribution of cluster of
different ages varies markedly depending on the model, and propose that
observations of the locations of age-dated stellar clusters is a possible
discriminant between excitation mechanisms for spiral structure in an
individual galaxy.Comment: 10 pages, 4 figures, accepted for publication in MNRA
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