506 research outputs found
A High Resolution Study of the Slowly Contracting, Starless Core L1544
We present interferometric observations of N2H+(1--0) in the starless, dense
core L1544 in Taurus. Red-shifted self-absorption, indicative of inward
motions, is found toward the center of an elongated core. The data are fit by a
non-spherical model consisting of two isothermal, rotating, centrally condensed
layers. Through a hybrid global-individual fit to the spectra, we map the
variation of infall speed at scales ~1400AU and find values ~0.08 km/s around
the core center. The inward motions are small in comparison to thermal,
rotational, and gravitational speeds but are large enough to suggest that L1544
is very close to forming a star.Comment: 11 pages, 2 figures Accepted for publication in Astrophysical Journal
Letter
Envelope Structure of Starless Core L694-2 Derived from a Near-Infrared Extinction Map
We present a near-infrared extinction study of the dark globule L694-2, a
starless core that shows strong evidence for inward motions in molecular line
profiles. The J,H, and K band data were taken using the European Southern
Observatory New Technology Telescope. The best fit simple spherical power law
model has index p=2.6 +/- 0.2, over the 0.036--0.1 pc range in radius sampled
in extinction. This power law slope is steeper than the value of p=2 for a
singular isothermal sphere, the initial condition of the inside-out model for
protostellar collapse. Including an additional extinction component along the
line of sight further steepens the inferred profile. Fitting a Bonnor-Ebert
sphere results in a super-critical value of the dimensionless radius xi_max=25
+/- 3. The unstable configuration of material may be related to the observed
inward motions. The Bonnor-Ebert model matches the shape of the observed
profile, but significantly underestimates the amount of extinction (by a factor
of ~4). This discrepancy in normalization has also been found for the nearby
protostellar core B335 (Harvey et al. 2001). A cylindrical density model with
scale height H=0.0164+/- 0.002 pc viewed at a small inclination to the cylinder
axis provides an equally good radial profile as a power law model, and
reproduces the asymmetry of the core remarkably well. In addition, this model
provides a basis for understanding the discrepancy in the normalization of the
Bonnor-Ebert model, namely that L694-2 has prolate structure, with the full
extent (mass) of the core being missed by assuming symmetry between the
profiles in the plane of the sky and along the line-of-sight. If the core is
sufficiently magnetized then fragmentation may be avoided, and later evolution
might produce a protostar similar to B335.Comment: 38 pages, 7 figures, accepted to Astrophysical Journa
Infall, Outflow, Rotation, and Turbulent Motions of Dense Gas within NGC 1333 IRAS 4
Millimeter wavelength observations are presented of NGC 1333 IRAS 4, a group
of highly-embedded young stellar objects in Perseus, that reveal motions of
infall, outflow, rotation, and turbulence in the dense gas around its two
brightest continuum objects, 4A and 4B. These data have finest angular
resolution of approximately 2" (0.0034 pc) and finest velocity resolution of
0.13 km/s. Infall motions are seen from inverse P-Cygni profiles observed in
H2CO 3_12-2_11 toward both objects, but also in CS 3-2 and N2H+ 1-0 toward 4A,
providing the least ambiguous evidence for such motions toward low-mass
protostellar objects. Outflow motions are probed by bright line wings of H2CO
3_12-2_11 and CS 3-2 observed at positions offset from 4A and 4B, likely
tracing dense cavity walls. Rotational motions of dense gas are traced by a
systematic variation of the N2H+ line velocities, and such variations are found
around 4A but not around 4B. Turbulent motions appear reduced with scale, given
N2H+ line widths around both 4A and 4B that are narrower by factors of 2 or 3
than those seen from single-dish observations. Minimum observed line widths of
approximately 0.2 km/s provide a new low, upper bound to the velocity
dispersion of the parent core to IRAS 4, and demonstrate that turbulence within
regions of clustered star formation can be reduced significantly. A third
continuum object in the region, 4B', shows no detectable line emission in any
of the observed molecular species.Comment: LateX, 51 pages, 9 figures, accepted by Ap
Inner Structure of Protostellar Collapse Candidate B335 Derived from Millimeter-Wave Interferometry
We present a study of the density structure of the protostellar collapse
candidate B335 using continuum observations from the IRAM Plateau de Bure
Interferometer made at wavelengths of 1.2mm and 3.0mm. We analyze these data,
which probe spatial scales from 5000 AU to 500 AU, directly in the visibility
domain by comparison to synthetic observations constructed from models that
assume different physical conditions. This approach allows for much more
stringent constraints to be derived from the data than from analysis of images.
A single radial power law in density provides a good description of the data,
with best fit power law index p=1.65+/-0.05. Through simulations, we quantify
the sensitivity of this result to various model uncertainties, including
assumptions of temperature distribution, outer boundary, dust opacity spectral
index, and an unresolved central component. The largest uncertainty comes from
the unknown presence of a centralized point source. A point source with 1.2mm
flux of F=12+/-7 mJy reduces the density index to p=1.47+/-0.07. The remaining
sources of systematic uncertainty, the most important of which is the
temperature distribution, likely contribute a total uncertainty of < 0.2. We
therefore find strong evidence that the power law index of the density
distribution within 5000 AU is significantly less than the value at larger
radii, close to 2.0 from previous studies of dust emission and extinction.
These results conform well to the generic paradigm of isolated, low-mass star
formation which predicts a power law density index close to p=1.5 for an inner
region of gravitational free fall onto the protostar.Comment: Accepted to the Astrophysical Journal; 27 pages, 3 figure
Protostar Mass Due to Infall and Dispersal
The mass of a protostar is calculated from the infall and dispersal of an
isothermal sphere in a uniform background. For high contrast between peak and
background densities and for short dispersal time t_d, the accretion is
"self-limiting": gas beyond the core is dispersed before it accretes, and the
protostar mass approaches a time-independent value of low mass. For lower
density contrast and longer dispersal time, the accretion "runs away": gas
accretes from beyond the core, and the protostar mass approaches massive star
values. The final protostar mass is approximately the initial gas mass whose
free-fall time equals t_d. This mass matches the peak of the IMF for gas
temperature 10 K, peak and background densities 10^6 and 10^3 cm^-3, and for
t_d comparable to the core free-fall time t_core. The accretion luminosity
exceeds 1 L-Sun for 0.1 Myr, as in the "Class 0" phase. For t_d/t_core=0.4-0.8
and temperature 7-50 K, self-limiting protostar masses are 0.08-5 M-Sun. These
protostar and core masses have ratio 0.4 +- 0.2, as expected if the core mass
distribution and the IMF have the same shape.Comment: 60 pages, 8 figures, accepted by Astrophysical Journa
Comparative Distribution and Metabolism of Xylem-Borne Amino Compounds and Sucrose in Shoots of Populus deltoides
Clinical Outcomes of Molecular Tumor Boards: A Systematic Review
PURPOSE: We conducted this systematic review to evaluate the clinical outcomes associated with molecular tumor board (MTB) review in patients with cancer.
METHODS: A systematic search of PubMed was performed to identify studies reporting clinical outcomes in patients with cancer who were reviewed by an MTB. To be included, studies had to report clinical outcomes, including clinical benefit, response, progression-free survival, or overall survival. Two reviewers independently selected studies and assessed quality with the Quality Assessment Tool for Before-After (Pre-Post) Studies with No Control Group or the Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies depending on the type of study being reviewed.
RESULTS: Fourteen studies were included with a total of 3,328 patients with cancer. All studies included patients without standard-of-care treatment options and usually with multiple prior lines of therapy. In studies reporting response rates, patients receiving MTB-recommended therapy had overall response rates ranging from 0% to 67%. In the only trial powered on clinical outcome and including a control group, the group receiving MTB-recommended therapy had significantly improved rate of progression-free survival compared with those receiving conventional therapy.
CONCLUSION: Although data quality is limited by a lack of prospective randomized controlled trials, MTBs appear to improve clinical outcomes for patients with cancer. Future research should concentrate on prospective trials and standardization of approach and outcomes
Mass distributions of stars and cores in young groups and clusters
We investigate the relation of the stellar initial mass function (IMF) and
the dense core mass function (CMF), using stellar masses and positions in 14
well-studied young groups. Initial column density maps are computed by
replacing each star with a model initial core having the same star formation
efficiency (SFE). For each group the SFE, core model, and observational
resolution are varied to produce a realistic range of initial maps. A
clumpfinding algorithm parses each initial map into derived cores, derived core
masses, and a derived CMF. The main result is that projected blending of
initial cores causes derived cores to be too few and too massive. The number of
derived cores is fewer than the number of initial cores by a mean factor 1.4 in
sparse groups and 5 in crowded groups. The mass at the peak of the derived CMF
exceeds the mass at the peak of the initial CMF by a mean factor 1.0 in sparse
groups and 12.1 in crowded groups. These results imply that in crowded young
groups and clusters, the mass distribution of observed cores may not reliably
predict the mass distribution of protostars which will form in those cores.Comment: 48 pages, 17 figures, accepted for publication in Astrophysical
Journa
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