40,213 research outputs found
A CO Line and Infrared Continuum Study of the Active Star-Forming Complex W51
We present the results of an extensive observational study of the active
star-forming complex W51 that was observed in the J=2-1 transition of the 12CO
and 13CO molecules over a 1.25 deg x 1.00 deg region with the University of
Arizona Heinrich Hertz Submillimeter Telescope. We use a statistical
equilibrium code to estimate physical properties of the molecular gas. We
compare the molecular cloud morphology with the distribution of infrared (IR)
and radio continuum sources, and find associations between molecular clouds and
young stellar objects (YSOs) listed in Spitzer IR catalogs. The ratios of CO
lines associated with HII regions are different from the ratios outside the
active star-forming regions. We present evidence of star formation triggered by
the expansion of the HII regions and by cloud-cloud collisions. We estimate
that about 1% of the cloud mass is currently in YSOs.Comment: 18 pages, 29 figures; accepted for publication in ApJ
Genetic Stratigraphy of Key Demographic Events in Arabia
The issue of admixture in human populations is normally addressed by genome-wide (GW) studies, and several approaches have been developed to date admixture events [1,2,3,4,5]. Admixed populations bear chromosomes with segments of DNA from all contributing source groups, the size of which decreases over successive generations until recombination renders them undetectably short. Several algorithms attempt to date admixture events by inferring the size of the nuclear ancestry segments, and these can work well when dating recent episodes in human history, such as the sub-Saharan African input into the New World [6], but they fail to detect several known episodes that took place at earlier times, such as the African input into Iberia [1] and genetic exchanges across the Red Sea [7]. Simulations with the suite of methods available at the ADMIXTOOLS package indicated that these methods could detect admixture events as early as 500 generation ago, but real data did not allow the tracing of such old events [8]. A recent improved algorithm, called GLOBETROTTER, has been used to tackle the detection of the co-occurrence of several mixture events by decomposing each chromosome into a series of haplotypic chunks and then analysing each chunk independently [3], but the problem of detecting ancient events remains. Its application to the systematic screening of worldwide admixture events was able to reveal around 100 events, but all occurring over only the past 4,000 years [3
A direct measurement of the baryonic mass function of galaxies & implications for the galactic baryon fraction
We use both an HI-selected and an optically-selected galaxy sample to
directly measure the abundance of galaxies as a function of their "baryonic"
mass (stars + atomic gas). Stellar masses are calculated based on optical data
from the Sloan Digital Sky Survey (SDSS) and atomic gas masses are calculated
using atomic hydrogen (HI) emission line data from the Arecibo Legacy Fast ALFA
(ALFALFA) survey. By using the technique of abundance matching, we combine the
measured baryonic function (BMF) of galaxies with the dark matter halo mass
function in a LCDM universe, in order to determine the galactic baryon fraction
as a function of host halo mass. We find that the baryon fraction of low-mass
halos is much smaller than the cosmic value, even when atomic gas is taken into
account. We find that the galactic baryon deficit increases monotonically with
decreasing halo mass, in contrast with previous studies which suggested an
approximately constant baryon fraction at the low-mass end. We argue that the
observed baryon fractions of low mass halos cannot be explained by reionization
heating alone, and that additional feedback mechanisms (e.g. supernova blowout)
must be invoked. However, the outflow rates needed to reproduce our result are
not easily accommodated in the standard picture of galaxy formation in a LCDM
universe.Comment: Accepted to ApJ. 45 pages (aastex), 19 figures; added references and
updated fig.18 for version
OB Stars & Stellar Bowshocks in Cygnus-X: A Novel Laboratory Estimating Stellar Mass Loss Rates
We use mid-IR images from the Spitzer Cygnus~X Legacy Survey to search for
stellar bowshocks, a signature of early type "runaway" stars with high space
velocities. We identify ten arc-shaped nebulae containing centrally located
stars as candidate bowshocks. New spectroscopic observations of five stars show
that all are late O to early B dwarfs. Our morphologically selected sample of
bowshock candidates encompasses diverse physical phenomena. Three of the stars
appear to be pre-main-sequence objects on the basis of rising SEDs in the
mid-IR, and their nebulae may be photon-dominated regions (PDRs). Four objects
have ambiguous classification. These may be partial dust shells or bubbles. We
conclude that three of the objects are probable bowshocks, based on their
morphological similarity to analytic prescriptions. Their nebular morphologies
reveal no systematic pattern of orientations that might indicate either a
population of stars ejected from or large-scale hydrodynamic outflows from Cyg
OB2. The fraction of runaways among OB stars near Cyg OB2 identified either by
radial velocity or bowshock techniques is ~0.5%, much smaller than the 8%
estimated among field OB stars. We also obtained a heliocentric radial velocity
for the previously known bowshock star, BD+43\degr3654, of -66.2+/-9.4 km/s,
solidifying its runaway status and implying a space velocity of 77+/-10 km/s.
We use the principles of momentum-driven bowshocks to arrive at a novel method
for estimating stellar mass loss rates. Derived mass loss rates range between
10^-7 and few x10^-6 solar masses/yr for the three O5V -- ~B2V stars identified
as generating bowshocks. These values are at the upper range of, but broadly
consistent with, estimates from other methods. (Abridged)Comment: 49 pages, 19 figures; Accepted for publication in ApJ;
full-resolution color figure version available at
http://physics.uwyo.edu/~chip/Papers/CygXBowshocks; comments invite
Mid-Infrared IRS Spectroscopy of NGC 7331: A First Look at the SINGS Legacy
The nearby spiral galaxy NGC 7331 was spectrally mapped from 5-38um using all
modules of Spitzer's IRS spectrograph. A strong new dust emission feature,
presumed due to PAHs, was discovered at 17.1um. The feature's intensity is
nearly half that of the ubiquitous 11.3um band. The 7-14um spectral maps
revealed significant variation in the 7.7 and 11.3um PAH features between the
stellar ring and nucleus. Weak [OIV] 25.9um line emission was found to be
centrally concentrated in the nucleus, with an observed strength over 10% of
the combined neon line flux, indicating an AGN or unusually active massive star
photo-ionization. Two [SIII] lines fix the characteristic electron density in
the HII regions at n_e < ~200 cm^-3. Three detected H_2 rotational lines,
tracing warm molecular gas, together with the observed IR continuum, are
difficult to match with standard PDR models. Either additional PDR heating or
shocks are required to simultaneously match lines and continuum.Comment: 6 pages, 5 figures, accepted for publication in ApJS Spitzer Special
Issu
The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. III. A quintuple stellar population in NGC2808
In this study we present first results from multi-wavelength Hubble Space
Telescope (HST) observations of the Galactic globular cluster GC NGC2808 as an
extension of the Hubble Space Telescope UV Legacy Survey of Galactic GCs
(GO-13297 and previous proprietary and HST archive data). Our analysis allowed
us to disclose a multiple-stellar-population phenomenon in NGC2808 even more
complex than previously thought. We have separated at least five different
populations along the main sequence and the red giant branch (RGB), that we
name A, B, C, D and E (though an even finer subdivision may be suggested by the
data). We identified the RGB bump in four out of the five RGBs. To explore the
origin of this complex CMD, we have combined our multi-wavelength HST
photometry with synthetic spectra, generated by assuming different chemical
compositions. The comparison of observed colors with synthetic spectra suggests
that the five stellar populations have different contents of light elements and
helium. Specifically, if we assume that NGC2808 is homogeneous in [Fe/H] (as
suggested by spectroscopy for Populations B, C, D, E, but lacking for
Population A) and that population A has a primordial helium abundance, we find
that populations B, C, D, E are enhanced in helium by Delta Y~0.03, 0.03, 0.08,
0.13, respectively. We obtain similar results by comparing the magnitude of the
RGB bumps with models. Planned spectroscopic observations will test whether
also Population A has the same metallicity, or whether its photometric
differences with Population B can be ascribed to small [Fe/H] and [O/H]
differences rather than to helium.Comment: 49 pages, 19 figure, accepted for publication in Ap
How coherent are the vortices of two-dimensional turbulence?
We use recent developments in the theory of finite-time dynamical systems to
objectively locate the material boundaries of coherent vortices in
two-dimensional Navier--Stokes turbulence. We show that these boundaries are
optimal in the sense that any closed curve in their exterior will lose
coherence under material advection. Through a detailed comparison, we find that
other available Eulerian and Lagrangian techniques significantly underestimate
the size of each coherent vortex.Comment: revised versio
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