15 research outputs found
Comparing the Ca II H and K Emission Lines in Red Giant Stars
Measurements of the asymmetry of the emission peaks in the core of the Ca II
H line for 105 giant stars are reported. The asymmetry is quantified with the
parameter V/R, defined as the ratio between the maximum number of counts in the
blueward peak and the redward peak of the emission profile. The Ca II H and K
emission lines probe the differential motion of certain chromospheric layers in
the stellar atmosphere. Data on V/R for the Ca II K line are drawn from
previous papers and compared to the analogous H line ratio, the H and K spectra
being from the same sets of observations. It is found that the H line V/R value
is +0.04 larger, on average, than the equivalent K line ratio, however, the
difference varies with B-V color. Red giants cooler than B-V = 1.2 are more
likely to have the H line V/R larger than the K line V/R, whereas the opposite
is true for giants hotter than B-V = 1.2. The differences between the Ca II H
and K line asymmetries could be caused by the layers of chromospheric material
from which these emission features arise moving with different velocities in an
expanding outflow.Comment: 36 pages, 12 figures, 2 tables. Accepted to PASP. Corrected a typo in
Table
Sizes and Shapes of Young Star Cluster Light Profiles in M83
We measure the radii and two-dimensional light profiles of a large sample of
young, massive star clusters in M83 using archival HST/WFC3 imaging of seven
adjacent fields. We use GALFIT to fit the two-dimensional light profiles of the
clusters, from which we find effective (half-light) radii, core radii, and
slopes of the power-law (EFF) profile (). We find lognormal distributions
of effective radius and core radius, with medians of 2.5 pc and
1.3 pc, respectively. Our results provide strong evidence for a
characteristic size of young, massive clusters. The average effective radius
and core radius increase somewhat with cluster age. Little to no change in
effective radius is observed with increasing galactocentric distance, except
perhaps for clusters younger than 100 Myr. We find a shallow correlation
between effective radius and mass for the full cluster sample, but a stronger
correlation is present for clusters 200-300 Myr in age. Finally, the majority
of the clusters are best fit by an EFF model with index . There is
no strong evidence for change in with cluster age, mass, or
galactocentric distance. Our results suggest that clusters emerge from early
evolution with similar radii and are not strongly affected by the tidal field
of M83. Mass loss due to stellar evolution and/or GMC interactions appear to
dominate cluster expansion in the age range we study.Comment: 34 pages, 11 figures, 3 tables, accepted by MNRAS. Machine-readable
table attached (full version of Table 3). To obtain, download the source file
from the "Other formats" link abov
Measuring Young Stars in Space and Time -- I. The Photometric Catalog and Extinction Properties of N44
In order to better understand the role of high-mass stellar feedback in
regulating star formation in giant molecular clouds, we carried out a Hubble
Space Telescope (HST) Treasury Program "Measuring Young Stars in Space and
Time" (MYSST) targeting the star-forming complex N44 in the Large Magellanic
Cloud (LMC). Using the F555W and F814W broadband filters of both the ACS and
WFC3/UVIS, we built a photometric catalog of 461,684 stars down to
mag and mag,
corresponding to the magnitude of an unreddened 1 Myr pre-main-sequence star of
at the LMC distance. In this first paper we describe
the observing strategy of MYSST, the data reduction procedure, and present the
photometric catalog. We identify multiple young stellar populations tracing the
gaseous rim of N44's super bubble, together with various contaminants belonging
to the LMC field population. We also determine the reddening properties from
the slope of the elongated red clump feature by applying the machine learning
algorithm RANSAC, and we select a set of Upper Main Sequence (UMS) stars as
primary probes to build an extinction map, deriving a relatively modest median
extinction mag. The same procedure applied to
the red clump provides mag.Comment: 29 pages, 15 figures, accepted for publication in A
Measuring Young Stars in Space and Time -- II. The Pre-Main-Sequence Stellar Content of N44
The Hubble Space Telescope (HST) survey Measuring Young Stars in Space and
Time (MYSST) entails some of the deepest photometric observations of
extragalactic star formation, capturing even the lowest mass stars of the
active star-forming complex N44 in the Large Magellanic Cloud. We employ the
new MYSST stellar catalog to identify and characterize the content of young
pre-main-sequence (PMS) stars across N44 and analyze the PMS clustering
structure. To distinguish PMS stars from more evolved line of sight
contaminants, a non-trivial task due to several effects that alter photometry,
we utilize a machine learning classification approach. This consists of
training a support vector machine (SVM) and a random forest (RF) on a carefully
selected subset of the MYSST data and categorize all observed stars as PMS or
non-PMS. Combining SVM and RF predictions to retrieve the most robust set of
PMS sources, we find candidates with a PMS probability above 95%
across N44. Employing a clustering approach based on a nearest neighbor surface
density estimate, we identify 18 prominent PMS structures at
significance above the mean density with sub-clusters persisting up to and
beyond significance. The most active star-forming center, located
at the western edge of N44's bubble, is a subcluster with an effective radius
of pc entailing more than 1,100 PMS candidates. Furthermore, we
confirm that almost all identified clusters coincide with known H II regions
and are close to or harbor massive young O stars or YSOs previously discovered
by MUSE and Spitzer observations.Comment: 29 pages, 21 figures, accepted for publication in A
An Open System for Social Computation
Part of the power of social computation comes from using the collective intelligence of humans to tame the aggregate uncertainty of (otherwise) low veracity data obtained from human and automated sources. We have witnessed a surge in development of social computing systems but, ironically, there have been few attempts to generalise across this activity so that creation of the underlying mechanisms themselves can be made more social. We describe a method for achieving this by standardising patterns of social computation via lightweight formal specifications (we call these social artifacts) that can be connected to existing internet architectures via a single model of computation. Upon this framework we build a mechanism for extracting provenance meta-data across social computations
Legacy ExtraGalactic UV Survey with The Hubble Space Telescope: Stellar Cluster Catalogs and First Insights Into Cluster Formation and Evolution in NGC 628
We report the large effort that is producing comprehensive high-level young star cluster (YSC) catalogs for a significant fraction of galaxies observed with the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. We present the methodology developed to extract cluster positions, verify their genuine nature, produce multiband photometry (from NUV to NIR), and derive their physical properties via spectral energy distribution fitting analyses. We use the nearby spiral galaxy NGC 628 as a test case for demonstrating the impact that LEGUS will have on our understanding of the formation and evolution of YSCs and compact stellar associations within their host galaxy. Our analysis of the cluster luminosity function from the UV to the NIR finds a steepening at the bright end and at all wavelengths suggesting a dearth of luminous clusters. The cluster mass function of NGC 628 is consistent with a power-law distribution of slopes and a truncation of a few times 105. After their formation, YSCs and compact associations follow different evolutionary paths. YSCs survive for a longer time frame, confirming their being potentially bound systems. Associations disappear on timescales comparable to hierarchically organized star-forming regions, suggesting that they are expanding systems. We find mass-independent cluster disruption in the inner region of NGC 628, while in the outer part of the galaxy there is little or no disruption. We observe faster disruption rates for low mass (â€104) clusters, suggesting that a mass-dependent component is necessary to fully describe the YSC disruption process in NGC 628. © 2017.A.A.
acknowledges partial support from the Swedish Royal Academy.
G.A. acknowledges support from the Science and Technology
Facilities Council (ST/L00075X/1 and ST/M503472/1). C.D.
acknowledges funding from the FP7 ERC starting grant
LOCALSTAR (no. 280104). M.F. acknowledges support by
the Science and Technology Facilities Council (grant number
ST/L00075X/1). D.A.G. kindly acknowledges financial support
by the German Research Foundation (DFG) through program
GO 1659/3-2. A.H. thanks the Spanish MINECO for grant
AYA2015-68012-c2-1
A Study of Two Dwarf Irregular Galaxies with Asymmetrical Star Formation Distributions
Two dwarf irregular galaxies, DDO 187 and NGC 3738, exhibit a striking pattern of star formation: intense star formation is taking place in a large region occupying roughly half of the inner part of the optical galaxy. We use data on the H i distribution and kinematics and stellar images and colors to examine the properties of the environment in the high star formation rate (HSF) halves of the galaxies in comparison with the low star formation rate halves. We find that the pressure and gas density are higher on the HSF sides by 30%â70%. In addition we find in both galaxies that the H i velocity fields exhibit significant deviations from ordered rotation and there are large regions of high-velocity dispersion and multiple velocity components in the gas beyond the inner regions of the galaxies. The conditions in the HSF regions are likely the result of large-scale external processes affecting the internal environment of the galaxies and enabling the current star formation there