1,859 research outputs found
High Accretion Rate during Class 0 Phase due to External Trigger
Recent observations indicate that some class 0 sources have orders of
magnitude higher accretion rates than those of class I. We investigated the
conditions for the high accretion rates of some class 0 sources by numerical
calculations, modelling an external trigger. For no external trigger, we find
that the maximum value of the accretion rate is determined by the ratio
of the gravitational energy to the thermal one within a flat inner
region of the cloud core. The accretion rate reaches \sim 10^{-4} M_{\sun}
yr^{-1} if the cloud core has . For an external trigger we find
that the maximum value of the accretion rate is proportional to the momentum
given to the cloud core. The accretion rate reaches > 10^{-4} M_{\sun}
yr^{-1} with a momentum of \sim 0.1 M_{\sun} km s^{-1} when the initial
central density of the cloud core is . A comparison
between recent observational results for prestellar cores and our no triggered
collapse model indicates that the flat inner regions of typical prestellar
cores are not large enough to cause accretion rates of \sim 10^{-4} M_{\sun}
yr^{-1}. Our results show that the triggered collapse of the cloud core is
more preferable for the origin of the high accretion rates of class 0 sources
than no triggered collapse.Comment: 7 pages, 8 figures, accepted for publication in MNRA
A Survey for Low-mass Stars and Brown Dwarfs in the Upper-Scorpius OB Association
The Upper-Scorpius association is the OB association nearest to the Sun (145
pc). Its young age (5 Myr) makes it an ideal place to search for low-mass stars
and brown dwarfs, as these objects should be relatively bright. We have
performed a photometric search for the low-mass members of the association,
using the R, I, and Z filters. The completeness limit is I=18.5 and the
saturation limit is I=13. We obtain 138 candidate members, covering nearly the
entire M spectral type range. We find an excess of brown dwarf candidates over
the number predicted by a Miller-Scalo Initial Mass Function. In addition, we
have performed infrared imaging and low resolution optical spectroscopy of
selected candidates. We find that the infrared observations confirm the
spectral types obtained with the optical photometry. Furthermore, we find
H_alpha in emission in 20 of the 22 objects observed spectroscopically. As
H_alpha is an indicator of youth, we believe that these 20 objects may belong
to the association. One of them, UScoCTIO 128 has a very strong and constant
H_alpha line (equivalent width: -130 A), and its position in the color
magnitude diagram suggests that it is a brown dwarf with mass equal to 0.02
msun. Confirmation of this and the other candidates will have to wait for
higher resolution observations that can reveal spectroscopic mass indicators
like Li I and gravity indicators, such as K I and the subordinate lines of Na
I.Comment: 20 pages, 7 figures, 3 tables, accepted in the Astronomical Journa
Mid-infrared Imaging of a Circumstellar Disk Around HR 4796: Mapping the Debris of Planetary Formation
We report the discovery of a circumstellar disk around the young A0 star, HR
4796, in thermal infrared imaging carried out at the W.M. Keck Observatory. By
fitting a model of the emission from a flat dusty disk to an image at
lambda=20.8 microns, we derive a disk inclination, i = 72 +6/-9 deg from face
on, with the long axis of emission at PA 28 +/-6 deg. The intensity of emission
does not decrease with radius as expected for circumstellar disks but increases
outward from the star, peaking near both ends of the elongated structure. We
simulate this appearance by varying the inner radius in our model and find an
inner hole in the disk with radius R_in = 55+/-15 AU. This value corresponds to
the radial distance of our own Kuiper belt and may suggest a source of dust in
the collision of cometesimals. By contrast with the appearance at 20.8 microns,
excess emission at lambda = 12.5 microns is faint and concentrated at the
stellar position. Similar emission is also detected at 20.8 microns in residual
subtraction of the best-fit model from the image. The intensity and ratio of
flux densities at the two wavelengths could be accounted for by a tenuous dust
component that is confined within a few AU of the star with mean temperature of
a few hundred degrees K, similar to that of zodiacal dust in our own solar
system. The morphology of dust emission from HR 4796 (age 10 Myr) suggests that
its disk is in a transitional planet-forming stage, between that of massive
gaseous proto-stellar disks and more tenuous debris disks such as the one
detected around Vega.Comment: 9 pages, 4 figures as LaTex manuscript and postscript files in
gzipped tar file. Accepted for publication in Astrophysical Journal Letters.
http://upenn5.hep.upenn.edu/~davidk/hr4796.htm
The perception of bicycle crashes with and without motor vehicles : which crash types do older and middle-aged cyclists fear most?
Common variants underlying cognitive ability: further evidence for association between the SNAP-25 gene and cognition using a family-based study in two independent Dutch cohorts.
The synaptosomal associated protein of 25 kDa (SNAP-25) gene, located on chromosome 20 p12-12p11.2 encodes a presynaptic terminal protein. SNAP-25 is differentially expressed in the brain, and primarily present in the neocortex, hippocampus, anterior thalamic nuclei, substantia nigra and cerebellar granular cells. Recently, a family-based genetic association was reported between variation in intelligence quotient (IQ) phenotypes and two intronic variants on the SNAP-25 gene. The present study is a follow-up association study in two Dutch cohorts of 371 children (mean age 12.4 years) and 391 adults (mean age 36.2 years). It examines the complete genomic region of the SNAP-25 gene to narrow down the location of causative genetic variant underlying the association. Two new variants in intron 1 (rs363043 and rs353016), close to the two previous reported variants (rs363039 and rs363050) showed association with variation in IQ phenotypes across both cohorts. All four single nucleotide polymorphisms were located in intron 1, within a region of about 13.8 kbp, and are known to affect transcription factor-binding sites. Contrary to what is expected in monogenic traits, subtle changes are postulated to influence the phenotypic outcome of complex (common) traits. As a result, functional polymorphisms in (non)coding regulatory sequences may affect spatial and temporal regulation of gene expression underlying normal cognitive variation. © 2007 The Authors
Discovery of a Wide Low-mass Binary System in Upper Scorpius
Using the near-infrared spectrometer SpeX and its slit-viewing camera at the
IRTF, I have resolved a low-mass member of the Upper Scorpius OB association
into a double star. From K-band images of the pair, DENIS-P J161833.2-251750.4
A and B, I measure a separation of 0.96" and a magnitude difference of dK=0.42
mag. I present resolved 0.8-2.5 micron spectroscopy of the two objects, both of
which exhibit signatures of youth in the shape of their H- and K-band continua,
demonstrating that both are members of Upper Scorpius rather than field stars.
In addition, through a comparison to optically-classified pre-main-sequence
objects, I derive a spectral type near M5 for each component, corresponding to
a mass of ~0.15 Msun with the evolutionary models of Chabrier and Baraffe. The
probability that this pair is composed of unrelated M-type members of Upper
Scorpius is ~10^-5. When added to the recent discoveries of other wide, easily
disrupted low-mass binaries, this new system further establishes that the
formation of low-mass stars and brown dwarfs does not require ejection from
multiple systems. These observations also indicate that wide low-mass binaries
can form in OB associations as well as in smaller clusters where the previously
known wide pairs have been found. Thus, the available data show no perceptible
effect of star-forming environment on the prevalence of loosely bound low-mass
systems.Comment: 5 page
Photometric Accretion Signatures Near the Substellar Boundary
Multi-epoch imaging of the Orion equatorial region by the Sloan Digital Sky
Survey has revealed that significant variability in the blue continuum persists
into the late-M spectral types, indicating that magnetospheric accretion
processes occur below the substellar boundary in the Orion OB1 association. We
investigate the strength of the accretion-related continuum veiling by
comparing the reddening-invariant colors of the most highly variable stars
against those of main sequence M dwarfs and evolutionary models. A gradual
decrease in the g band veiling is seen for the cooler and less massive members,
as expected for a declining accretion rate with decreasing mass. We also see
evidence that the temperature of the accretion shock decreases in the very low
mass regime, reflecting a reduction in the energy flux carried by the accretion
columns. We find that the near-IR excess attributed to circumstellar disk
thermal emission drops rapidly for spectral types later than M4. This is likely
due to the decrease in color contrast between the disk and the cooler stellar
photosphere. Since accretion, which requires a substantial stellar magnetic
field and the presence of a circumstellar disk, is inferred for masses down to
0.05 Msol we surmise that brown dwarfs and low mass stars share a common mode
of formation.Comment: 37 pages, 14 figures, accepted by A
Triggered Star Formation by Massive Stars
We present our diagnosis of the role that massive stars play in the formation
of low- and intermediate-mass stars in OB associations (the Lambda Ori region,
Ori OB1, and Lac OB1 associations). We find that the classical T Tauri stars
and Herbig Ae/Be stars tend to line up between luminous O stars and
bright-rimmed or comet-shaped clouds; the closer to a cloud the progressively
younger they are. Our positional and chronological study lends support to the
validity of the radiation-driven implosion mechanism, where the Lyman continuum
photons from a luminous O star create expanding ionization fronts to evaporate
and compress nearby clouds into bright-rimmed or comet-shaped clouds. Implosive
pressure then causes dense clumps to collapse, prompting the formation of
low-mass stars on the cloud surface (i.e., the bright rim) and
intermediate-mass stars somewhat deeper in the cloud. These stars are a
signpost of current star formation; no young stars are seen leading the
ionization fronts further into the cloud. Young stars in bright-rimmed or
comet-shaped clouds are likely to have been formed by triggering, which would
result in an age spread of several megayears between the member stars or star
groups formed in the sequence.Comment: 2007, ApJ, 657, 88
Methodological considerations in quantification of oncological FDG PET studies
Contains fulltext :
87741.pdf (publisher's version ) (Closed access)
Contains fulltext :
87741-1.pdf (postprint version ) (Open Access)PURPOSE: This review aims to provide insight into the factors that influence quantification of glucose metabolism by FDG PET images in oncology as well as their influence on repeated measures studies (i.e. treatment response assessment), offering improved understanding both for clinical practice and research. METHODS: Structural PubMed searches have been performed for the many factors affecting quantification of glucose metabolism by FDG PET. Review articles and references lists have been used to supplement the search findings. RESULTS: Biological factors such as fasting blood glucose level, FDG uptake period, FDG distribution and clearance, patient motion (breathing) and patient discomfort (stress) all influence quantification. Acquisition parameters should be adjusted to maximize the signal to noise ratio without exposing the patient to a higher than strictly necessary radiation dose. This is especially challenging in pharmacokinetic analysis, where the temporal resolution is of significant importance. The literature is reviewed on the influence of attenuation correction on parameters for glucose metabolism, the effect of motion, metal artefacts and contrast agents on quantification of CT attenuation-corrected images. Reconstruction settings (analytical versus iterative reconstruction, post-reconstruction filtering and image matrix size) all potentially influence quantification due to artefacts, noise levels and lesion size dependency. Many region of interest definitions are available, but increased complexity does not necessarily result in improved performance. Different methods for the quantification of the tissue of interest can introduce systematic and random inaccuracy. CONCLUSIONS: This review provides an up-to-date overview of the many factors that influence quantification of glucose metabolism by FDG PET.01 juli 201
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