350 research outputs found
The Role of Column Density in the Formation of Stars and Black Holes
The stellar mass in disk galaxies scales approximately with the fourth power
of the rotation velocity, and the masses of the central black holes in galactic
nuclei scale approximately with the fourth power of the bulge velocity
dispersion. It is shown here that these relations can be accounted for if, in a
forming galaxy with an isothermal mass distribution, gas with a column density
above about 8 Msun/pc^2 goes into stars while gas with a column density above
about 2 g/cm^2 (10^4 Msun/pc^2) goes into a central black hole. The lower
critical value is close to the column density of about 10 Msun/pc^2 at which
atomic gas becomes molecular, and the upper value agrees approximately with the
column density of about 1 g/cm^2 at which the gas becomes optically thick to
its cooling radiation. These results are plausible because molecule formation
is evidently necessary for star formation, and because the onset of a high
optical depth in a galactic nucleus may suppress continuing star formation and
favour the growth of a central black hole.Comment: Accepted by Nature Physic
Acute myopathy secondary to oral steroid therapy in a 49-year-old man: a case report
<p>Abstract</p> <p>Introduction</p> <p>Acute myopathy caused by oral corticosteroids is rare. We present a case of myopathy occurring after two doses of methylprednisolone. Typically, acute steroid myopathy occurs with therapy using intravenous corticosteroids at high doses. Acute myopathy developing very early in the course of treatment with oral corticosteroids has been reported only once in the literature. Corticosteroid therapy may be complicated by myopathy, usually chronic, after prolonged high-dose therapy. Acute myopathy caused by exogenous corticosteroids is rare, usually with intravenous corticosteroids at high doses.</p> <p>Case presentation</p> <p>A 49-year-old Caucasian man developed acute myopathy after taking oral methylprednisolone for only two days, 24 mg on day 1 and 20 mg on day 2. He discontinued the medication because of new-onset myalgias and lethargy on day 3 and was seen in our clinic four days after beginning therapy. He completely recovered in four weeks by discontinuing the corticosteroids.</p> <p>Conclusion</p> <p>Among the many complications of corticosteroid therapy, acute myopathy is very rare. It requires prompt recognition and adjustment of therapy.</p
A Substantial Population of Low Mass Stars in Luminous Elliptical Galaxies
The stellar initial mass function (IMF) describes the mass distribution of
stars at the time of their formation and is of fundamental importance for many
areas of astrophysics. The IMF is reasonably well constrained in the disk of
the Milky Way but we have very little direct information on the form of the IMF
in other galaxies and at earlier cosmic epochs. Here we investigate the stellar
mass function in elliptical galaxies by measuring the strength of the Na I
doublet and the Wing-Ford molecular FeH band in their spectra. These lines are
strong in stars with masses <0.3 Msun and weak or absent in all other types of
stars. We unambiguously detect both signatures, consistent with previous
studies that were based on data of lower signal-to-noise ratio. The direct
detection of the light of low mass stars implies that they are very abundant in
elliptical galaxies, making up >80% of the total number of stars and
contributing >60% of the total stellar mass. We infer that the IMF in massive
star-forming galaxies in the early Universe produced many more low mass stars
than the IMF in the Milky Way disk, and was probably slightly steeper than the
Salpeter form in the mass range 0.1 - 1 Msun.Comment: To appear in Natur
Tunable magnetic exchange interactions in manganese-doped inverted core/shell ZnSe/CdSe nanocrystals
Magnetic doping of semiconductor nanostructures is actively pursued for
applications in magnetic memory and spin-based electronics. Central to these
efforts is a drive to control the interaction strength between carriers
(electrons and holes) and the embedded magnetic atoms. In this respect,
colloidal nanocrystal heterostructures provide great flexibility via
growth-controlled `engineering' of electron and hole wavefunctions within
individual nanocrystals. Here we demonstrate a widely tunable magnetic sp-d
exchange interaction between electron-hole excitations (excitons) and
paramagnetic manganese ions using `inverted' core-shell nanocrystals composed
of Mn-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe.
Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the
band-edge exciton that, surprisingly, are tunable in both magnitude and sign.
Effective exciton g-factors are controllably tuned from -200 to +30 solely by
increasing the CdSe shell thickness, demonstrating that strong quantum
confinement and wavefunction engineering in heterostructured nanocrystal
materials can be utilized to manipulate carrier-Mn wavefunction overlap and the
sp-d exchange parameters themselves.Comment: To appear in Nature Materials; 18 pages, 4 figures + Supp. Inf
A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
Massive stars are very rare, but their extreme luminosities make them both
the only type of young star we can observe in distant galaxies and the dominant
energy sources in the universe today. They form rarely because efficient
radiative cooling keeps most star-forming gas clouds close to isothermal as
they collapse, and this favors fragmentation into stars <~1 Msun. Heating of a
cloud by accreting low-mass stars within it can prevent fragmentation and allow
formation of massive stars, but what properties a cloud must have to form
massive stars, and thus where massive stars form in a galaxy, has not yet been
determined. Here we show that only clouds with column densities >~ 1 g cm^-2
can avoid fragmentation and form massive stars. This threshold, and the
environmental variation of the stellar initial mass function (IMF) that it
implies, naturally explain the characteristic column densities of massive star
clusters and the difference between the radial profiles of Halpha and UV
emission in galactic disks. The existence of a threshold also implies that
there should be detectable variations in the IMF with environment within the
Galaxy and in the characteristic column densities of massive star clusters
between galaxies, and that star formation rates in some galactic environments
may have been systematically underestimated.Comment: Accepted for publication in Nature; Nature manuscript style; main
text: 14 pages, 3 figures; supplementary text: 8 pages, 1 figur
Particulate Matter (PM) Research Centers (1999â2005) and the Role of Interdisciplinary Center-Based Research
Objective: The U.S. Environmental Protection Agency funded five academic centers in 1999 to address the uncertainties in exposure, toxicity, and health effects of airborne particulate matter (PM) identified in the âResearch Priorities for Airborne Particulate Matterâ of the National Research Council (NRC). The centers were structured to promote interdisciplinary approaches to address research priorities of the NRC. In this report, we present selected accomplishments from the first 6 years of the PM Centers, with a focus on the advantages afforded by the interdisciplinary, center-based research approach. The review highlights advances in the area of ultrafine particles and traffic-related health effects as well as cardiovascular and respiratory effects, mechanisms, susceptibility, and PM exposure and characterization issues. Data sources and synthesis: The collective publications of the centers served as the data source. To provide a concise synthesis of overall findings, authors representing each of the five centers identified a limited number of topic areas that serve to illustrate the key accomplishments of the PM Centers program, and a consensus statement was developed. Conclusions: The PM Centers program has effectively applied interdisciplinary research approaches to advance PM science
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-
SingleâColumn Model Simulations of Subtropical Marine BoundaryâLayer Cloud Transitions Under Weakening Inversions
Results are presented of the GASS/EUCLIPSE singleâcolumn model intercomparison study on the subtropical marine lowâlevel cloud transition. A central goal is to establish the performance of stateâofâtheâart boundaryâlayer schemes for weather and climate models for this cloud regime, using largeâeddy simulations of the same scenes as a reference. A novelty is that the comparison covers four different cases instead of one, in order to broaden the covered parameter space. Three cases are situated in the NorthâEastern Pacific, while one reflects conditions in the NorthâEastern Atlantic. A set of variables is considered that reflects key aspects of the transition process, making use of simple metrics to establish the model performance. Using this method, some longstanding problems in lowâlevel cloud representation are identified. Considerable spread exists among models concerning the cloud amount, its vertical structure, and the associated impact on radiative transfer. The sign and amplitude of these biases differ somewhat per case, depending on how far the transition has progressed. After cloud breakup the ensemble median exhibits the wellâknown âtoo few too brightâ problem. The boundaryâlayer deepening rate and its state of decoupling are both underestimated, while the representation of the thin capping cloud layer appears complicated by a lack of vertical resolution. Encouragingly, some models are successful in representing the full set of variables, in particular, the vertical structure and diurnal cycle of the cloud layer in transition. An intriguing result is that the median of the model ensemble performs best, inspiring a new approach in subgrid parameterization
The importance of plasma apolipoprotein E concentration in addition to its common polymorphism on inter-individual variation in lipid levels: results from Apo Europe
The ApoEurope group, collaborating centres, and their associated investigators: Portugal: Unidade de QuĂmica ClĂnica, Instituto Nacional de SaĂșde, Lisboa: Maria do Carmo Martins, Maria Odete Rodrigues, Maria Isabel Albergaria, Maria Liseta AlpendreInterindividual variation in the concentration of plasma lipids which are associated with coronary artery disease (CAD) risk is determined by a combination of genetic and environmental factors. This study investigates the effects of apoE genotype and plasma concentration on cholesterol and triglycerides (TG) levels in subjects from five countries: Finland, France, Northern Ireland, Portugal, and Spain. Age and sex significantly influenced serum cholesterol, TG and apoE concentrations. The age effect differs in males and females. The allele frequencies of the apoE gene, one of the most widely studied CAD susceptibility genes, were determined: the epsilon2 allele frequency and the apoE concentration showed a north-south increasing gradient while the epsilon4 allele frequency showed the reverse. ApoE plays an important role in lipid metabolism. Total cholesterol and TG concentrations were significantly dependent on apoE genotype in both sexes. These differences in lipids between genotypes were more pronounced when plasma apoE concentrations were taken into account
Massive star formation in 100,000 years from turbulent and pressurized molecular clouds
Massive stars (with mass m_* > 8 solar masses) are fundamental to the
evolution of galaxies, because they produce heavy elements, inject energy into
the interstellar medium, and possibly regulate the star formation rate. The
individual star formation time, t_*f, determines the accretion rate of the
star; the value of the former quantity is currently uncertain by many orders of
magnitude, leading to other astrophysical questions. For example, the variation
of t_*f with stellar mass dictates whether massive stars can form
simultaneously with low-mass stars in clusters. Here we show that t_*f is
determined by conditions in the star's natal cloud, and is typically ~10^5 yr.
The corresponding mass accretion rate depends on the pressure within the cloud
- which we relate to the gas surface density - and on both the instantaneous
and final stellar masses. Characteristic accretion rates are sufficient to
overcome radiation pressure from ~100 solar mass protostars, while
simultaneously driving intense bipolar gas outflows. The weak dependence of
t_*f on the final mass of the star allows high- and low-mass star formation to
occur nearly simultaneously in clusters.Comment: 9 pages plus 2 figures, Nature, 416, 59 (7th March 2002
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