1,091 research outputs found
From infall to rotation around young stellar objects: A transitional phase with a 2000 AU radius contracting disk?
Evidence for a transitional stage in the formation of a low-mass star is
reported, intermediate between the fully embedded and the T Tauri phases.
Millimeter aperture synthesis observations in the HCO+ J=1-0 and 3-2, HCN 1-0,
13CO 1-0, and C18O 1-0 transitions reveal distinctly different velocity fields
around two embedded, low-mass young stellar objects. The 0.6 M(sun) of material
around TMC 1 (IRAS 04381+2517) closely follows inside-out collapse in the
presence of a small amount of rotation (~3 km/s/pc), while L1489 IRS (IRAS
04016+2610) is surrounded by a 2000 AU radius, flared disk containing 0.02
M(sun). This disk shows Keplerian rotation around a ~0.65 M(sun) star and
infall at 1.3 (r/100 AU)^-0.5 km/s, or, equivalently, sub-Keplerian motions
around a central object between 0.65 and 1.4 M(sun). Its density is
characterized by a radial power law and an exponential vertical scale height.
The different relative importance of infall and rotation around these two
objects suggests that rotationally supported structures grow from collapsing
envelopes over a few times 10^5 yr to sizes of a few thousand AU, and then
decrease over a few times 10^4 yr to several hundred AU typical for T Tauri
disks. In this scenario, L1489 IRS represents a transitional phase between
embedded YSOs and T Tauri stars with disks. The expected duration of this phase
of ~5% of the embedded stage is consistent with the current lack of other known
objects like L1489 IRS. Alternative explanations cannot explain L1489 IRS's
large disk, such as formation from a cloud core with an unusually large
velocity gradient or a binary companion that prevents mass accretion onto small
scales. It follows that the transfer and dissipation of angular momentum is key
to understanding the formation of disks from infalling envelopes.Comment: Accepted ApJ. 33 pages, including 10 B/W figures and 1 color figure.
Uses AASTe
Patients First:Toward a Patient-Centered Instrument to Measure Impact of Chronic Pain
Background. Numerous instruments are available to measure the impact of chronic pain, yet most have been developed with little or no patient involvement. This study seeks to start bridging that gap by determining which health aspects or attributes (to be included in a future instrument) are considered most important by people with chronic pain. Objective. The goal of this study was to reveal which attributes reflecting impact of chronic pain are considered most important by people with chronic pain and to analyze differences in importance according to gender, age categories, diagnostic subgroups, and pain intensity categories. Design. This study used a sequential explanatory mixed-methods design: literature search, focus group meetings, and online survey. Methods. First, a literature search was performed to identify the attributes in existing instruments. In 68 instruments meeting inclusion criteria, 155 unique attributes were identified, 85 of which remained after applying the exclusion criteria. Second, 2 focus group meetings, with 6 and 4 patients, respectively, were held to verify that no attributes had been missed. Three attributes were subsequently added. Third, individuals with chronic pain were then sent an online survey through several patient organizations. Results. A total of 939 patients were asked to select the 8 attributes they deemed most important, which resulted in the following list: fatigue, social life, cramped muscles, sleeping, housekeeping, concentration, not being understood, and control over pain. The importance assigned to these 8 attributes varied slightly according to age, gender, and diagnostic subgroup. Limitations. Participation rate could not be established because of the online survey. Conclusions. Attributes reflecting impact of chronic pain deemed most important by patients are revealed. Importance of impact differs according to subgroups. The "patients-first" methodology used here revealed attributes that were not comprehensively covered in currently available instruments for measuring the impact of chronic pain
CO isotopolog line fluxes of viscously evolving disks: cold CO conversion insufficient to explain observed low fluxes
Protoplanetary disks are thought to evolve viscously, where the disk mass -
the reservoir available for planet formation - decreases over time as material
is accreted onto the central star. Observations show a correlation between dust
mass and the stellar accretion rate, as expected from viscous theory. However,
the gas mass inferred from 13CO and C18O line fluxes, which should be a more
direct measure, shows no such correlation. Using thermochemical DALI models, we
investigate how 13CO and C18O J=3-2 line fluxes change over time in a viscously
evolving disk. We also investigate if the chemical conversion of CO through
grain-surface chemistry combined with viscous evolution can explain the
observations of disks in Lupus. The 13CO and C18O 3-2 line fluxes increase over
time due to their optically thick emitting regions growing in size as the disk
expands viscously. The C18O 3-2 emission is optically thin throughout the disk
for only a subset of our models (Mdisk (t = 1 Myr) < 1e-3 Msun). For these
disks the integrated C18O flux decreases with time, similar to the disk mass.
The C18O 3-2 fluxes for the bulk of the disks in Lupus (with Mdust < 5e-5 Msun)
can be reproduced to within a factor of ~2 with viscously evolving disks in
which CO is converted into other species through grain-surface chemistry driven
by a cosmic-ray ionization rate zeta_cr ~ 5e-17 - 1e-16 s^-1. However,
explaining the stacked C18O upper limits requires a lower average abundance
than our models can produce and they cannot explain the observed 13CO fluxes,
which, for most disks, are more than an order of magnitude fainter than what
our models predict. Reconciling the 13CO fluxes of viscously evolving disks
with the observations requires either a combination of efficient vertical
mixing and a high zeta_cr or low mass disks (Mdust < 3e-5 Msun) being much
thinner and/or smaller than their more massive counterparts.Comment: 21 pages, 14 figures, accepted in A&
Envelope structure of deeply embedded young stellar objects in the Serpens Molecular Cloud
Aperture synthesis and single-dish (sub) millimeter molecular lines and
continuum observations reveal in great detail the envelope structure of deeply
embedded young stellar objects (SMM1, SMM2, SMM3, SMM4) in the densely
star-forming Serpens Molecular Cloud. Resolved millimeter continuum emission
constrains the density structure to a radial power law with index -2.0 +/- 0.5,
and envelope masses of 8.7, 3.0, and 5.3 M_sol for SMM1, SMM3, and SMM4. The
core SMM2 does not seem to have a central condensation and may not have formed
a star yet. The molecular line observations can be described by the same
envelope model, if an additional, small amount of warm (100 K) material is
included. This probably corresponds to the inner few hundred AU of the envelope
were the temperature is high. In the interferometer beam, the molecular lines
reveal the inner regions of the envelopes, as well as interaction of the
outflow with the surrounding envelope. Bright HCO+ and HCN emission outlines
the cavities, while SiO and SO trace the direct impact of the outflow on
ambient gas. Taken together, these observations provide a first comprehensive
view of the physical and chemical structure of the envelopes of deeply embedded
young stellar objects in a clustered environment on scales between 1000 and
10,000 AU.Comment: 46 pages, incl. 12 postscript figures, uses ApJ latex and psfig
macro
First principles modelling of magnesium titanium hydrides
Mixing Mg with Ti leads to a hydride Mg(x)Ti(1-x)H2 with markedly improved
(de)hydrogenation properties for x < 0.8, as compared to MgH2. Optically, thin
films of Mg(x)Ti(1-x)H2 have a black appearance, which is remarkable for a
hydride material. In this paper we study the structure and stability of
Mg(x)Ti(1-x)H2, x= 0-1 by first-principles calculations at the level of density
functional theory. We give evidence for a fluorite to rutile phase transition
at a critical composition x(c)= 0.8-0.9, which correlates with the
experimentally observed sharp decrease in (de)hydrogenation rates at this
composition. The densities of states of Mg(x)Ti(1-x)H2 have a peak at the Fermi
level, composed of Ti d states. Disorder in the positions of the Ti atoms
easily destroys the metallic plasma, however, which suppresses the optical
reflection. Interband transitions result in a featureless optical absorption
over a large energy range, causing the black appearance of Mg(x)Ti(1-x)H2.Comment: 22 pages, 9 figures, 4 table
Nervousness and Performance Characteristics as Predictors of Peer Behavior Towards Socially Anxious Adolescents
Social anxiety in adolescents has frequently been linked to negative outcomes from social interactions. The present study investigated whether socially anxious adolescents are treated negatively by their classmates and which characteristics of socially anxious adolescents could explain negative social responses. Classroom observations of class behavior were made during oral presentations of 94 students (60% females) in the ages of 13–18 years. Speakers’ social performance, speech quality, and nervousness during the presentation were also rated. Findings showed that the social performance of socially anxious students was a predictor of class behavior, whereas their overt nervousness was not. Surprisingly, the quality of their speech was negatively related to class behavior. Implications of these findings for the treatment of socially anxious adolescents are discussed
A Chandra observation of the long-duration X-ray transient KS 1731-260 in quiescence: too cold a neutron star?
After more than a decade of actively accreting at about a tenth of the
Eddington critical mass accretion rate, the neutron-star X-ray transient KS
1731-260 returned to quiescence in early 2001. We present a Chandra/ACIS-S
observation taken several months after this transition. We detected the source
at an unabsorbed flux of ~2 x 10^{-13} erg/cm^2/s (0.5-10 keV). For a distance
of 7 kpc, this results in a 0.5-10 keV luminosity of ~1 x 10^{33} erg/s and a
bolometric luminosity approximately twice that. This quiescent luminosity is
very similar to that of the other quiescent neutron star systems. However, if
this luminosity is due to the cooling of the neutron star, this low luminosity
may indicate that the source spends at least several hundreds of years in
quiescence in between outbursts for the neutron star to cool. If true, then it
might be the first such X-ray transient to be identified and a class of
hundreds of similar systems may be present in the Galaxy. Alternatively,
enhanced neutrino cooling could occur in the core of the neutron star which
would cool the star more rapidly. However, in that case the neutron star in KS
1731-260 would be more massive than those in the prototypical neutron star
transients (e.g., Aql X-1 or 4U 1608-52).Comment: Accepted for publicaton in ApJ letters, 13 September 200
Genome-Wide Identification of Expression Quantitative Trait Loci (eQTLs) in Human Heart.
In recent years genome-wide association studies (GWAS) have uncovered numerous chromosomal loci associated with various electrocardiographic traits and cardiac arrhythmia predisposition. A considerable fraction of these loci lie within inter-genic regions. The underlying trait-associated variants likely reside in regulatory regions and exert their effect by modulating gene expression. Hence, the key to unraveling the molecular mechanisms underlying these cardiac traits is to interrogate variants for association with differential transcript abundance by expression quantitative trait locus (eQTL) analysis. In this study we conducted an eQTL analysis of human heart. For a total of 129 left ventricular samples that were collected from non-diseased human donor hearts, genome-wide transcript abundance and genotyping was determined using microarrays. Each of the 18,402 transcripts and 897,683 SNP genotypes that remained after pre-processing and stringent quality control were tested for eQTL effects. We identified 771 eQTLs, regulating 429 unique transcripts. Overlaying these eQTLs with cardiac GWAS loci identified novel candidates for studies aimed at elucidating the functional and transcriptional impact of these loci. Thus, this work provides for the first time a comprehensive eQTL map of human heart: a powerful and unique resource that enables systems genetics approaches for the study of cardiac traits
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