1,292 research outputs found
The Mass Function of Newly Formed Stars (Review)
The topic of the stellar "original mass function" has a nearly 50 year
history,dating to the publication in 1955 of Salpeter's seminal paper. In this
review I discuss the many more recent results that have emerged on the initial
mass function (IMF), as it is now called, from studies over the last decade of
resolved populations in star forming regions and young open clusters.Comment: 9 pages, 1 figure; to appear in "The Dense Instellar Medium in
Galaxies -- 4'th Cologne-Bonn-Zermatt-Symposium" editted by S. Pfalzner, C.
Kramer, C. Straubmeier and A. Heithausen, Springer-Verlag (2004
Clinical and radiological studies in PSP and related conditions
This thesis examines clinical and radiological aspects of Progressive Supranuclear Palsy (PSP) and related conditions. Significant milestones occur sooner in pathologically confirmed PSP than multiple system atrophy (MSA); older age of onset and shorter duration to first milestone are associated with worse prognosis in both; in PSP, the Richardson’s syndrome phenotype and male gender and in MSA, early autonomic failure and the female gender are also predictive of poorer prognosis. Using objective measurements of bradykinesia we found progressive bradykinesia and hypokinesia in Parkinson’s disease (PD) which correlates with disability and responds to levodopa but hypokinesia without decrement in PSP. Using conventional MRI 72.7% of PSP and 76.9% of MSA are correctly identified. The ‘hummingbird sign’ was highly specific for PSP, but sensitvity was 68.4%. A simple measurement of the midbrain < 9.35mm had 100% specificity for a pathological diagnosis of PSP. In a clinically diagnosed PSP 90.5% had a measurement of < 9.35mm. Using high field 9.4 Tesla MRI, the anatomy of the subthalamic nucleus is clearly defined when compared to histology in post mortem material. The anteromedial portion was hypointense in correlation with Perls stain and there was variability in the volume, shape and location of its borders. The nigrosomes within the substantia nigra were visibile as high intensity bands which correlated with calbindin poor zones on immunohistochemical stains. The volume and anatomy were preserved in PD but not PSP. Multimodal 3 Telsla MRI during life revealed distinct patterns of atrophy in PSP and MSA using voxel-based morphometry. Tract-based spatial statistics revealed abnormalities in the frontal and parieto-occipital white matter changes in PSP more than MSA. Midbrain atrophy and frontal white matter increased mean diffusivity were associated with increasing PSP rating scale score, and frontal white matter reduced fractional anisotropy with disease duration
XMM-Newton observations of the Small Magellanic Cloud: X-ray outburst of the 6.85 s pulsar XTE J0103-728
A bright X-ray transient was seen during an XMM-Newton observation in the
direction of the Small Magellanic Cloud (SMC) in October 2006. The EPIC data
allow us to accurately locate the source and to investigate its temporal and
spectral behaviour. X-ray spectra covering 0.2-10 keV and pulse profiles in
different energy bands were extracted from the EPIC data. The detection of 6.85
s pulsations in the EPIC-PN data unambiguously identifies the transient with
XTE J0103-728, discovered as 6.85 s pulsar by RXTE. The X-ray light curve
during the XMM-Newton observation shows flaring activity of the source with
intensity changes by a factor of two within 10 minutes. Modelling of
pulse-phase averaged spectra with a simple absorbed power-law indicates
systematic residuals which can be accounted for by a second emission component.
For models implying blackbody emission, thermal plasma emission or emission
from the accretion disk (disk-blackbody), the latter yields physically sensible
parameters. The photon index of the power-law of ~0.4 indicates a relatively
hard spectrum. The 0.2-10 keV luminosity was 2x10^{37} with a contribution of
~3% from the disk-blackbody component. A likely origin for the excess emission
is reprocessing of hard X-rays from the neutron star by optically thick
material near the inner edge of an accretion disk. From a timing analysis we
determine the pulse period to 6.85401(1) s indicating an average spin-down of
~0.0017 s per year since the discovery of XTE J0103-728 in May 2003. The X-ray
properties and the identification with a Be star confirm XTE J0103-728 as
Be/X-ray binary transient in the SMC.Comment: 5 pages, 4 figures, submitted to A&A on 21 Dec. 200
Compulsive versifying after treatment of transient epileptic amnesia
Compulsive production of verse is an unusual form of hypergraphia that has been reported mainly in patients with right temporal lobe seizures. We present a patient with transient epileptic amnesia and a left temporal seizure focus, who developed isolated compulsive versifying, producing multiple rhyming poems, following seizure cessation induced by lamotrigine. Functional neuroimaging studies in the healthy brain implicate left frontotemporal areas in generating novel verbal output and rhyme, while dysregulation of neocortical and limbic regions occurs in temporal lobe epilepsy. This case complements previous observations of emergence of altered behavior with reduced seizure frequency in patients with temporal lobe epilepsy. Such cases suggest that reduced seizure frequency has the potential not only to stabilize or improve memory function, but also to trigger complex, specific behavioral alterations
Chandra observations of Cygnus OB2
Cygnus OB2 is the nearest example of a massive star forming region,
containing over 50 O-type stars and hundreds of B-type stars. We have analyzed
two Chandra pointings in Cyg OB2, detecting ~1700 X-ray sources, of which ~1450
are thought to be members of the association. Optical and near-IR photometry
has been obtained for ~90% of these sources from recent deep Galactic plane
surveys. We have performed isochrone fits to the near-IR color-magnitude
diagram, deriving ages of 3.5(+0.75,-1.0) and 5.25(+1.5,-1.0) Myrs for sources
in the two fields, both with considerable spreads around the pre-MS isochrones.
The presence of a second population in the region, somewhat older than the
present-day O-type stars, has been suggested by other authors and fits with the
ages derived here. The fraction of sources with inner circumstellar disks (as
traced by the K-band excess) is found to be very low, but appropriate for a
population of age ~5 Myrs. We measure the stellar mass functions and find a
power-law slope of Gamma = -1.09 +/- 0.13, in good agreement with the global
mean value estimated by Kroupa. A steepening of the mass function at high
masses is observed and we suggest this is due to the presence of the previous
generation of stars that have lost their most massive members. Finally,
combining our mass function and an estimate of the radial density profile of
the association suggests a total mass of Cyg OB2 of ~30,000 Msun, similar to
that of many of our Galaxy's most massive star forming regions.Comment: 6 pages, 4 figures, conference proceedings for JENAM 2010: Star
Clusters in the Era of Large Surveys, Editors: A.Moitinho and J. Alve
XMM-Newton survey of the Local Group galaxy M 33
In an XMM-Newton raster observation of the bright Local Group spiral galaxy M
33 we study the population of X-ray sources (X-ray binaries, supernova
remnants) down to a 0.2--4.5 keV luminosity of 10^35 erg/s -- more than a
factor of 10 deeper than earlier ROSAT observations. EPIC hardness ratios and
optical and radio information are used to distinguish between different source
classes. The survey detects 408 sources in an area of 0.80 square degree. We
correlate these newly detected sources with earlier M 33 X-ray catalogues and
information from optical, infra-red and radio wavelengths. As M 33 sources we
detect 21 supernova remnants (SNR) and 23 SNR candidates, 5 super-soft sources,
and 2 X-ray binaries (XRBs). There are 267 sources classified as hard, which
may either be XRBs or Crab-like SNRs in M 33 or background AGN. The 44
confirmed and candidate SNRs more than double the number of X-ray detected SNRs
in M 33. 16 of these are proposed as SNR candidates from the X-ray data for the
first time. On the other hand, there are several sources not connected to M 33:
five foreground stars, 30 foreground star candidates, 12 active galactic
nucleus candidates, one background galaxy and one background galaxy candidate.
Extrapolating from deep field observations we would expect 175 to 210
background sources in this field. This indicates that about half of the sources
detected are sources within M 33.Comment: 14 pages, 6 figures, accepted for publication in A&A, the images of
Figs. 1,2,3,4,6 are available in jpg format, a full version of the paper is
available at ftp://ftp.xray.mpe.mpg.de/people/fwh/docs/M33_AA0068.p
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
On the Disruption of Star Clusters in a Hierarchical Interstellar Medium
The distribution of the number of clusters as a function of mass M and age T
suggests that clusters get eroded or dispersed in a regular way over time, such
that the cluster number decreases inversely as an approximate power law with T
within each fixed interval of M. This power law is inconsistent with standard
dispersal mechanisms such as cluster evaporation and cloud collisions. In the
conventional interpretation, it requires the unlikely situation where diverse
mechanisms stitch together over time in a way that is independent of
environment or M. Here we consider another model in which the large scale
distribution of gas in each star-forming region plays an important role. We
note that star clusters form with positional and temporal correlations in giant
cloud complexes, and suggest that these complexes dominate the tidal force and
collisional influence on a cluster during its first several hundred million
years. Because the cloud complex density decreases regularly with position from
the cluster birth site, the harassment and collision rates between the cluster
and the cloud pieces decrease regularly with age as the cluster drifts. This
decrease is typically a power law of the form required to explain the mass-age
distribution. We reproduce this distribution for a variety of cases, including
rapid disruption, slow erosion, combinations of these two, cluster-cloud
collisions, cluster disruption by hierarchical disassembly, and partial cluster
disruption. We also consider apparent cluster mass loss by fading below the
surface brightness limit of a survey. In all cases, the observed log M - \log T
diagram can be reproduced under reasonable assumptions.Comment: ApJ vol. 712, March 20, 2010, 33 pages 15 figure
The fractal dimension of star-forming regions at different spatial scales in M33
We study the distribution of stars, HII regions, molecular gas, and
individual giant molecular clouds in M33 over a wide range of spatial scales.
The clustering strength of these components is systematically estimated through
the fractal dimension. We find scale-free behavior at small spatial scales and
a transition to a larger correlation dimension (consistent with a nearly
uniform distribution) at larger scales. The transition region lies in the range
500-1000 pc. This transition defines a characteristic size that separates the
regime of small-scale turbulent motion from that of large-scale galactic
dynamics. At small spatial scales, bright young stars and molecular gas are
distributed with nearly the same three-dimensional fractal dimension (Df <=
1.9), whereas fainter stars and HII regions exhibit higher values (Df =
2.2-2.5). Our results indicate that the interstellar medium in M33 is on
average more fragmented and irregular than in the Milky Way.Comment: 18 pages including 4 figures. Accepted for publication in Ap
- …