292 research outputs found
The nature of the high Galactic latitude O-star HD93521: new results from X-ray and optical spectroscopy
Owing to its unusual location and its isolation, the nature of the high
Galactic latitude O9.5Vp object HD93521 is still uncertain. We have collected
X-ray and optical observations to characterize the star and its surroundings.
X-ray images and spectra are analyzed to search for traces of a recent star
formation event around HD93521 and to search for the signature of a possible
compact companion. Optical echelle spectra are analysed with plane-parallel
model atmosphere codes, assuming either a spherical star or a gravity darkened
rotationally flattened star, to infer the effective temperature and surface
gravity, and to derive the He, C, N and O abundances of HD93521. The X-ray
images reveal no traces of a population of young low-mass stars coeval with
HD93521. The X-ray spectrum of HD93521 is consistent with a normal late O-type
star although with subsolar metallicity. No trace of a compact companion is
found in the X-ray data. In the optical spectrum, He and N are found to be
overabundant, in line with the effect of rotational mixing in this very fast
rotator, whilst C and O are subsolar. A critical comparison with the properties
of subdwarf OB stars, indicates that, despite some apparent similarities,
HD93521 does not belong to this category. Despite some ambiguities on the
runaway status of the star, the most likely explanation is that HD93521 is a
Population I massive O-type star that was ejected from the Galactic plane
either through dynamical interactions or a result of a supernova event in a
binary system.Comment: Accepted for publication in Astronomy & Astrophysic
Top-heavy integrated galactic stellar initial mass functions (IGIMFs) in starbursts
Star formation rates (SFR) larger than 1000 Msun/ yr are observed in extreme
star bursts. This leads to the formation of star clusters with masses > 10^6
Msun in which crowding of the pre-stellar cores may lead to a change of the
stellar initial mass function (IMF). Indeed, the large mass-to-light ratios of
ultra-compact dwarf galaxies and recent results on globular clusters suggest
the IMF to become top-heavy with increasing star-forming density. We explore
the implications of top-heavy IMFs in these very massive and compact systems
for the integrated galactic initial mass function (IGIMF), which is the
galaxy-wide IMF, in dependence of the star-formation rate of galaxies. The
resulting IGIMFs can have slopes, alpha_3, for stars more massive than about 1
Msun between 1.5 and the Salpeter slope of 2.3 for an embedded cluster mass
function (ECMF) slope (beta) of 2.0, but only if the ECMF has no low-mass
clusters in galaxies with major starbursts. Alternatively, beta would have to
decrease with increasing SFR >10 Msun/ yr such that galaxies with major
starbursts have a top-heavy ECMF. The resulting IGIMFs are within the range of
observationally deduced IMF variations with redshift.Comment: Accepted for publication in MNRAS, reference adde
Comparison of two independent systematic reviews of trials of recombinant human bone morphogenetic protein-2 (rhBMP-2) : The Yale Open Data Access Medtronic Project
Background: It is uncertain whether the replication of systematic reviews, particularly those with the same objectives and resources, would employ similar methods and/or arrive at identical findings. We compared the results and conclusions of two concurrent systematic reviews undertaken by two independent research teams provided with the same objectives, resources, and individual participant-level data. Methods: Two centers in the USA and UK were each provided with participant-level data on 17 multi-site clinical trials of recombinant human bone morphogenetic protein-2 (rhBMP-2). The teams were blinded to each other's methods and findings until after publication. We conducted a retrospective structured comparison of the results of the two systematic reviews. The main outcome measures included (1) trial inclusion criteria; (2) statistical methods; (3) summary efficacy and risk estimates; and (4) conclusions. Results: The two research teams' meta-analyses inclusion criteria were broadly similar but differed slightly in trial inclusion and research methodology. They obtained similar results in summary estimates of most clinical outcomes and adverse events. Center A incorporated all trials into summary estimates of efficacy and harms, while Center B concentrated on analyses stratified by surgical approach. Center A found a statistically significant, but small, benefit whereas Center B reported no advantage. In the analysis of harms, neither showed an increased cancer risk at 48 months, although Center B reported a significant increase at 24 months. Conclusions reflected these differences in summary estimates of benefit balanced with small but potentially important risk of harm. Conclusions: Two independent groups given the same research objectives, data, resources, funding, and time produced broad general agreement but differed in several areas. These differences, the importance of which is debatable, indicate the value of the availability of data to allow for more than a single approach and a single interpretation of the data. Systematic review registration: PROSPERO CRD42012002040and CRD42012001907
Building galaxies by accretion and in-situ star formation
We examine galaxy formation in a cosmological AMR simulation, which includes
two high resolution boxes, one centered on a 3 \times 10^14 M\odot cluster, and
one centered on a void. We examine the evolution of 611 massive (M\ast >
10^10M\odot) galaxies. We find that the fraction of the final stellar mass
which is accreted from other galaxies is between 15 and 40% and increases with
stellar mass. The accreted fraction does not depend strongly on environment at
a given stellar mass, but the galaxies in groups and cluster environments are
older and underwent mergers earlier than galaxies in lower density
environments. On average, the accreted stars are ~2.5 Gyrs older, and ~0.15 dex
more metal poor than the stars formed in-situ. Accreted stellar material
typically lies on the outskirts of galaxies; the average half-light radius of
the accreted stars is 2.6 times larger than that of the in-situ stars. This
leads to radial gradients in age and metallicity for massive galaxies, in
qualitative agreement with observations. Massive galaxies grow by mergers at a
rate of approximately 2.6% per Gyr. These mergers have a median (mass-weighted)
mass ratio less than 0.26 \pm 0.21, with an absolute lower limit of 0.20, for
galaxies with M\ast ~ 10^12 M\odot. This suggests that major mergers do not
dominate in the accretion history of massive galaxies. All of these results
agree qualitatively with results from SPH simulations by Oser et al. (2010,
2012).Comment: 18 pages, 12 figures, submitted to MNRA
The central dark matter content of early-type galaxies: scaling relations and connections with star formation histories
We examine correlations between the masses, sizes, and star formation
histories for a large sample of low-redshift early-type galaxies, using a
simple suite of dynamical and stellar populations models. We confirm an
anti-correlation between size and stellar age, and survey for trends with the
central content of dark matter (DM). An average relation between central DM
density and galaxy size of ~ Reff^-2 provides the first clear
indication of cuspy DM haloes in these galaxies -- akin to standard LCDM haloes
that have undergone adiabatic contraction. The DM density scales with galaxy
mass as expected, deviating from suggestions of a universal halo profile for
dwarf and late-type galaxies. We introduce a new fundamental constraint on
galaxy formation by finding that the central DM fraction decreases with stellar
age. This result is only partially explained by the size-age dependencies, and
the residual trend is in the opposite direction to basic DM halo expectations.
Therefore we suggest that there may be a connection between age and halo
contraction, and that galaxies forming earlier had stronger baryonic feedback
which expanded their haloes, or else lumpier baryonic accretion that avoided
halo contraction. An alternative explanation is a lighter initial mass function
for older stellar populations.Comment: 24 pages, 23 figures. MNRAS, submitted with minor modifications
following referee report
Volume-based referral for cardiovascular procedures in the United States: a cross-sectional regression analysis
BACKGROUND: We sought to estimate the numbers of patients affected and deaths avoided by adopting the Leapfrog Group's recommended hospital procedure volume minimums for coronary artery bypass graft (CABG) surgery and percutaneous coronary intervention (PCI). In addition to hospital risk-adjusted mortality standards, the Leapfrog Group recommends annual hospital procedure minimums of 450 for CABG and 400 for PCI to reduce procedure-associated mortality. METHODS: We conducted a retrospective analysis of a national hospital discharge database to evaluate in-hospital mortality among patients who underwent PCI (n = 2,500,796) or CABG (n = 1,496,937) between 1998 and 2001. We calculated the number of patients treated at low volume hospitals and simulated the number of deaths potentially averted by moving all patients to high volume hospitals under best-case conditions (i.e., assuming the full volume-associated reduction in mortality and the capacity to move all patients to high volume hospitals with no related harms). RESULTS: Multivariate adjusted odds of in-hospital mortality were higher for patients treated in low volume hospitals compared with high volume hospitals for CABG (OR 1.16, 95% CI 1.10–1.24) and PCI (OR 1.12, 95% CI 1.05–1.20). A policy of hospital volume minimums would have required moving 143,687 patients for CABG and 87,661 patients for PCI from low volume to high volume hospitals annually and prevented an estimated 619 CABG deaths and 109 PCI deaths. Thus, preventing a single death would have required moving 232 CABG patients or 805 PCI patients from low volume to high volume hospitals. CONCLUSION: Recommended hospital CABG and PCI volume minimums would prevent 728 deaths annually in the United States, fewer than previously estimated. It is unclear whether a policy requiring the movement of large numbers of patients to avoid relatively few deaths is feasible or effective
The stellar and sub-stellar IMF of simple and composite populations
The current knowledge on the stellar IMF is documented. It appears to become
top-heavy when the star-formation rate density surpasses about 0.1Msun/(yr
pc^3) on a pc scale and it may become increasingly bottom-heavy with increasing
metallicity and in increasingly massive early-type galaxies. It declines quite
steeply below about 0.07Msun with brown dwarfs (BDs) and very low mass stars
having their own IMF. The most massive star of mass mmax formed in an embedded
cluster with stellar mass Mecl correlates strongly with Mecl being a result of
gravitation-driven but resource-limited growth and fragmentation induced
starvation. There is no convincing evidence whatsoever that massive stars do
form in isolation. Various methods of discretising a stellar population are
introduced: optimal sampling leads to a mass distribution that perfectly
represents the exact form of the desired IMF and the mmax-to-Mecl relation,
while random sampling results in statistical variations of the shape of the
IMF. The observed mmax-to-Mecl correlation and the small spread of IMF
power-law indices together suggest that optimally sampling the IMF may be the
more realistic description of star formation than random sampling from a
universal IMF with a constant upper mass limit. Composite populations on galaxy
scales, which are formed from many pc scale star formation events, need to be
described by the integrated galactic IMF. This IGIMF varies systematically from
top-light to top-heavy in dependence of galaxy type and star formation rate,
with dramatic implications for theories of galaxy formation and evolution.Comment: 167 pages, 37 figures, 3 tables, published in Stellar Systems and
Galactic Structure, Vol.5, Springer. This revised version is consistent with
the published version and includes additional references and minor additions
to the text as well as a recomputed Table 1. ISBN 978-90-481-8817-
Gas kinematics in powerful radio galaxies at z ∼2: Energy supply from star formation, AGN, and radio jets⋆
We compare the kinetic energy and momentum injection rates from intense star
formation, bolometric AGN radiation, and radio jets with the kinetic energy and
momentum observed in the warm ionized gas in 24 powerful radio galaxies at z~2.
These galaxies are amongst our best candidates for being massive galaxies near
the end of their active formation period, when intense star formation, quasar
activity, and powerful radio jets all co-exist. All galaxies have VLT/SINFONI
imaging spectroscopy of the rest-frame optical line emission, showing
emission-line regions with large velocity offsets (up to 1500 km/s) and line
widths (typically 800-1000 km/s) consistent with very turbulent, often
outflowing gas. As part of the HeRGE sample, they also have FIR estimates of
the star formation and quasar activity obtained with Herschel/PACS and SPIRE,
which enables us to measure the relative energy and momentum release from each
of the three main sources of feedback in massive, star-forming AGN host
galaxies during their most rapid formation phase. We find that star formation
falls short by factors 10-1000 of providing the energy and momentum necessary
to power the observed gas kinematics. The obscured quasars in the nuclei of
these galaxies provide enough energy and momentum in about half of the sample,
however, only if these are transfered to the gas relatively efficiently. We
compare with theoretical and observational constraints on the efficiency of the
energy and momentum transfer from jet and AGN radiation, which advocates that
the radio jet is the main driver of the gas kinematics.Comment: A&A accepted. Comments welcom
Star forming dwarf galaxies
Star forming dwarf galaxies (SFDGs) have a high gas content and low
metallicities, reminiscent of the basic entities in hierarchical galaxy
formation scenarios. In the young universe they probably also played a major
role in the cosmic reionization. Their abundant presence in the local volume
and their youthful character make them ideal objects for detailed studies of
the initial stellar mass function (IMF), fundamental star formation processes
and its feedback to the interstellar medium. Occasionally we witness SFDGs
involved in extreme starbursts, giving rise to strongly elevated production of
super star clusters and global superwinds, mechanisms yet to be explored in
more detail. SFDGs is the initial state of all dwarf galaxies and the relation
to the environment provides us with a key to how different types of dwarf
galaxies are emerging. In this review we will put the emphasis on the exotic
starburst phase, as it seems less important for present day galaxy evolution
but perhaps fundamental in the initial phase of galaxy formation.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy
Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon,
September 2010, Springer Verlag, in pres
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