56 research outputs found
The highly variable time evolution of star-forming cores identified with dendrograms
We investigate the time evolution of dense cores identified in molecular
cloud simulations using dendrograms, which are a common tool to identify
hierarchical structure in simulations and observations of star formation. We
develop an algorithm to link dendrogram structures through time using the
three-dimensional density field from magnetohydrodynamical simulations, thus
creating histories for all dense cores in the domain. We find that the
population-wide distributions of core properties are relatively invariant in
time, and quantities like the core mass function match with observations.
Despite this consistency, an individual core may undergo large (>40%),
stochastic variations due to the redefinition of the dendrogram structure
between timesteps. This variation occurs independent of environment and stellar
content. We identify a population of short-lived (<200 kyr) overdensities
masquerading as dense cores that may comprise ~20% of any time snapshot.
Finally, we note the importance of considering the full history of cores when
interpreting the origin of the initial mass function; we find that, especially
for systems containing multiple stars, the core mass defined by a dendrogram
leaf in a snapshot is typically less than the final system stellar mass. This
work reinforces that there is no time-stable density contour that defines a
star-forming core. The dendrogram itself can induce significant structure
variation between timesteps due to small changes in the density field. Thus,
one must use caution when comparing dendrograms of regions with different ages
or environment properties because differences in dendrogram structure may not
come solely from the physical evolution of dense cores.Comment: 20 pages, 17 figures. Submitted to MNRA
The Green Bank Ammonia Survey (GAS): First Results of NH3 mapping the Gould Belt
We present an overview of the first data release (DR1) and first-look science
from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green
Bank Telescope to map all Gould Belt star-forming regions with
mag visible from the northern hemisphere in emission from NH and other key
molecular tracers. This first release includes the data for four regions in
Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and
Orion A North in Orion. We compare the NH emission to dust continuum
emission from Herschel, and find that the two tracers correspond closely.
NH is present in over 60\% of lines-of-sight with mag in
three of the four DR1 regions, in agreement with expectations from previous
observations. The sole exception is B18, where NH is detected toward ~ 40\%
of lines-of-sight with mag. Moreover, we find that the NH
emission is generally extended beyond the typical 0.1 pc length scales of dense
cores. We produce maps of the gas kinematics, temperature, and NH column
densities through forward modeling of the hyperfine structure of the NH
(1,1) and (2,2) lines. We show that the NH velocity dispersion,
, and gas kinetic temperature, , vary systematically between
the regions included in this release, with an increase in both the mean value
and spread of and with increasing star formation activity.
The data presented in this paper are publicly available.Comment: 33 pages, 27 figures, accepted to ApJS. Datasets are publicly
available: https://dataverse.harvard.edu/dataverse/GAS_DR
Droplets I: Pressure-Dominated Sub-0.1 pc Coherent Structures in L1688 and B18
We present the observation and analysis of newly discovered coherent
structures in the L1688 region of Ophiuchus and the B18 region of Taurus. Using
data from the Green Bank Ammonia Survey (GAS), we identify regions of high
density and near-constant, almost-thermal, velocity dispersion. Eighteen
coherent structures are revealed, twelve in L1688 and six in B18, each of which
shows a sharp "transition to coherence" in velocity dispersion around its
periphery. The identification of these structures provides a chance to study
the coherent structures in molecular clouds statistically. The identified
coherent structures have a typical radius of 0.04 pc and a typical mass of 0.4
Msun, generally smaller than previously known coherent cores identified by
Goodman et al. (1998), Caselli et al. (2002), and Pineda et al. (2010). We call
these structures "droplets." We find that unlike previously known coherent
cores, these structures are not virially bound by self-gravity and are instead
predominantly confined by ambient pressure. The droplets have density profiles
shallower than a critical Bonnor-Ebert sphere, and they have a velocity (VLSR)
distribution consistent with the dense gas motions traced by NH3 emission.
These results point to a potential formation mechanism through pressure
compression and turbulent processes in the dense gas. We present a comparison
with a magnetohydrodynamic simulation of a star-forming region, and we
speculate on the relationship of droplets with larger, gravitationally bound
coherent cores, as well as on the role that droplets and other coherent
structures play in the star formation process.Comment: Accepted by ApJ in April, 201
The Green Bank Ammonia Survey: A Virial Analysis of Gould Belt Clouds in Data Release 1
We perform a virial analysis of starless dense cores in three nearby
star-forming regions : L1688 in Ophiuchus, NGC 1333 in Perseus, and B18 in
Taurus. Our analysis takes advantage of comprehensive kinematic information for
the dense gas in all of these regions made publicly available through the Green
Bank Ammonia Survey Data Release 1, which used to estimate internal support
against collapse. We combine this information with ancillary data used to
estimate other important properties of the cores, including continuum data from
the James Clerk Maxwell Telescope Gould Belt Survey for core identification,
core masses, and core sizes. Additionally, we used \textit{Planck} and
\textit{Herschel}-based column density maps for external cloud weight pressure,
and Five College Radio Astronomy Observatory CO observations for
external turbulent pressure. Our self-consistent analysis suggests that many
dense cores in all three star-forming regions are not bound by gravity alone,
but rather require additional pressure confinement to remain bound. Unlike a
recent, similar study in Orion~A, we find that turbulent pressure represents a
significant portion of the external pressure budget. Our broad conclusion
emphasizing the importance of pressure confinement in dense core evolution,
however, agrees with earlier work.Comment: 35 pages, 8 tables, and 14 figures consisting of 16 .pdf files.
Accepted for publication in the Astrophysical Journa
Adenovirus Type 21–Associated Acute Flaccid Paralysis during an Outbreak of Hand-Footand-Mouth Disease in Sarawak, Malaysia
A three gene DNA methylation biomarker accurately classifies early stage prostate cancer
Background: We identify and validate accurate diagnostic biomarkers for prostate cancer through a systematic evaluation of DNA methylation alterations. Materials and methods: We assembled three early prostate cancer cohorts (total patients = 699) from which we collected and processed over 1300 prostatectomy tissue samples for DNA extraction. Using real-time methylation-specific PCR, we measured normalized methylation levels at 15 frequently methylated loci. After partitioning sample sets into independent training and validation cohorts, classifiers were developed using logistic regression, analyzed, and validated. Results: In the training dataset, DNA methylation levels at 7 of 15 genomic loci (glutathione S-transferase Pi 1 [GSTP1], CCDC181, hyaluronan, and proteoglycan link protein 3 [HAPLN3], GSTM2, growth arrest-specific 6 [GAS6], RASSF1, and APC) showed large differences between cancer and benign samples. The best binary classifier was the GAS6/GSTP1/HAPLN3 logistic regression model, with an area under these curves of 0.97, which showed a sensitivity of 94%, and a specificity of 93% after external validation. Conclusion: We created and validated a multigene model for the classification of benign and malignant prostate tissue. With false positive and negative rates below 7%, this three-gene biomarker represents a promising basis for more accurate prostate cancer diagnosis
The core mass function in the Orion Nebula Cluster region : What Determines the Final Stellar Masses?
Funding: European Research Council (ERC) via the ERC Synergy Grant ECOGAL (grant 855130) (R.S.K.).Applying dendrogram analysis to the CARMA-NRO C18O (J = 1–0) data having an angular resolution of ∼8'', we identified 692 dense cores in the Orion Nebula Cluster region. Using this core sample, we compare the core and initial stellar mass functions in the same area to quantify the step from cores to stars. About 22% of the identified cores are gravitationally bound. The derived core mass function (CMF) for starless cores has a slope similar to Salpeter's stellar initial mass function (IMF) for the mass range above 1 M⊙, consistent with previous studies. Our CMF has a peak at a subsolar mass of ∼0.1 M⊙, which is comparable to the peak mass of the IMF derived in the same area. We also find that the current star formation rate is consistent with the picture in which stars are born only from self-gravitating starless cores. However, the cores must gain additional gas from the surroundings to reproduce the current IMF (e.g., its slope and peak mass), because the core mass cannot be accreted onto the star with 100% efficiency. Thus, the mass accretion from the surroundings may play a crucial role in determining the final stellar masses of stars.Peer reviewe
HIV and adolescents: focus on young key populations
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138345/1/jia20076.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138345/2/jia20076-sup-0001.pd
Recommended from our members
The Green Bank Ammonia Survey: First Results of NH3 Mapping of the Gould Belt
We present an overview of the first data release (DR1) and first-look science from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green Bank Telescope to map all Gould Belt star-forming regions with {A}{{V}}≳ 7 mag visible from the northern hemisphere in emission from NH3 and other key molecular tracers. This first release includes the data for four regions in the Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and Orion A North in Orion. We compare the NH3 emission to dust continuum emission from Herschel and find that the two tracers correspond closely. We find that NH3 is present in over 60% of the lines of sight with {A}{{V}}≳ 7 mag in three of the four DR1 regions, in agreement with expectations from previous observations. The sole exception is B18, where NH3 is detected toward ̃40% of the lines of sight with {A}{{V}}≳ 7 mag. Moreover, we find that the NH3 emission is generally extended beyond the typical 0.1 pc length scales of dense cores. We produce maps of the gas kinematics, temperature, and NH3 column densities through forward modeling of the hyperfine structure of the NH3 (1, 1) and (2, 2) lines. We show that the NH3 velocity dispersion, {σ }v, and gas kinetic temperature, T K, vary systematically between the regions included in this release, with an increase in both the mean value and the spread of {σ }v and T K with increasing star formation activity. The data presented in this paper are publicly available (https://dataverse.harvard.edu/dataverse/GAS_DR1).Astronom
- …