87 research outputs found
The dynamical properties of dense filaments in the infrared dark cloud G035.39-00.33
Infrared Dark Clouds (IRDCs) are unique laboratories to study the initial
conditions of high-mass star and star cluster formation. We present
high-sensitivity and high-angular resolution IRAM PdBI observations of N2H+
(1-0) towards IRDC G035.39-00.33. It is found that G035.39-00.33 is a highly
complex environment, consisting of several mildly supersonic filaments
(sigma_NT/c_s ~1.5), separated in velocity by <1 km s^-1 . Where multiple
spectral components are evident, moment analysis overestimates the non-thermal
contribution to the line-width by a factor ~2. Large-scale velocity gradients
evident in previous single-dish maps may be explained by the presence of
substructure now evident in the interferometric maps. Whilst global velocity
gradients are small (<0.7 km s^-1 pc^-1), there is evidence for dynamic
processes on local scales (~1.5-2.5 km s^-1 pc^-1 ). Systematic trends in
velocity gradient are observed towards several continuum peaks. This suggests
that the kinematics are influenced by dense (and in some cases, starless)
cores. These trends are interpreted as either infalling material, with
accretion rates ~(7 \pm 4)x10^-5 M_sun yr^-1 , or expanding shells with
momentum ~24 \pm 12 M_sun km s^-1 . These observations highlight the importance
of high-sensitivity and high-spectral resolution data in disentangling the
complex kinematic and physical structure of massive star forming regions.Comment: 25 pages, 23 figures, accepted for publication in MNRA
A probable Keplerian disk feeding an optically revealed massive young star.
The canonical picture of star formation involves disk-mediated accretion, with Keplerian accretion disks and associated bipolar jets primarily observed in nearby, low-mass young stellar objects (YSOs). Recently, rotating gaseous structures and Keplerian disks have been detected around several massive (M > 8 M ) YSOs (MYSOs) , including several disk-jet systems . All the known MYSO systems are in the Milky Way, and all are embedded in their natal material. Here we report the detection of a rotating gaseous structure around an extragalactic MYSO in the Large Magellanic Cloud. The gas motion indicates that there is a radial flow of material falling from larger scales onto a central disk-like structure. The latter exhibits signs of Keplerian rotation, so that there is a rotating toroid feeding an accretion disk and thus the growth of the central star. The system is in almost all aspects comparable to Milky Way high-mass YSOs accreting gas from a Keplerian disk. The key difference between this source and its Galactic counterparts is that it is optically revealed rather than being deeply embedded in its natal material as is expected of such a massive young star. We suggest that this is the consequence of the star having formed in a low-metallicity and low-dust content environment. Thus, these results provide important constraints for models of the formation and evolution of massive stars and their circumstellar disks. [Abstract copyright: © 2023. The Author(s).
A probable Keplerian disk feeding an optically revealed massive young star
The canonical picture of star formation involves disk-mediated accretion,
with Keplerian accretion disks and associated bipolar jets primarily observed
in nearby, low-mass young stellar objects (YSOs). Recently, rotating gaseous
structures and Keplerian disks have been detected around a number of massive (M
> 8 solar masses) YSOs (MYSOs) including several disk-jet systems. All of the
known MYSO systems are located in the Milky Way, and all are embedded in their
natal material. Here we report the detection of a rotating gaseous structure
around an extragalactic MYSO in the Large Magellanic Cloud. The gas motions
show radial flow of material falling from larger scales onto a central
disk-like structure, the latter exhibiting signs of Keplerian rotation, i.e., a
rotating toroid feeding an accretion disk and thus the growth of the central
star. The system is in almost all aspects comparable to Milky Way high-mass
young stellar objects accreting gas via a Keplerian disk. The key difference
between this source and its Galactic counterparts is that it is optically
revealed, rather than being deeply embedded in its natal material as is
expected of such a young massive star. We suggest that this is the consequence
of the star having formed in a low-metallicity and low-dust content
environment, thus providing important constraints for models of the formation
and evolution of massive stars and their circumstellar disks.Comment: 20 pages, 9 page
Multicomponent kinematics in a massive filamentary IRDC
To probe the initial conditions for high-mass star and cluster formation, we
investigate the properties of dense filaments within the infrared dark cloud
G035.39-00.33 (IRDC G035.39) in a combined Very Large Array (VLA) and the Green
Bank Telescope (GBT) mosaic tracing the NH3 (1,1) and (2,2) emission down to
0.08 pc scales. Using agglomerative hierarchical clustering on multiple
line-of-sight velocity component fitting results, we identify seven extended
velocity-coherent components in our data, likely representing spatially
coherent physical structures, some exhibiting complex gas motions. The velocity
gradient magnitude distribution peaks at its mode of 0.35 km/s/pc and has a
long tail extending into higher values of 1.5 - 2 km/s/pc, and is generally
consistent with those found toward the same cloud in other molecular tracers
and with the values found towards nearby low-mass dense cloud cores at the same
scales. Contrary to observational and theoretical expectations, we find the
non-thermal ammonia line widths to be systematically narrower (by about 20%)
than those of N2H+ (1-0) line transition observed with similar resolution. If
the observed ordered velocity gradients represent the core envelope solid-body
rotation, we estimate the specific angular momentum to be about 2 x 10^21
cm^2/s, similar to the low-mass star-forming cores. Together with the previous
finding of subsonic motions in G035.39, our results demonstrate high levels of
similarity between kinematics of a high-mass star-forming IRDC and the low-mass
star formation regime.Comment: 22 pages, 14 figures, accepted to Ap
Dynamic competition and resource partitioning during the early life of two widespread, abundant and ecologically similar fishes
Competition and resource partitioning can have profound implications for individuals, populations and communities, and thus food webs, ecosystems and the management of biota and environments. In many species, the impacts of competition and resource partitioning are believed to be most severe during early life, but our understanding of the mechanisms and implications is incomplete. This study revealed short-term variations in both the occurrence and direction of competition during the early life of roach Rutilus rutilus and common bream Abramis brama, two of the most widespread and abundant fish species in Europe. There was also evidence of resource partitioning when small taxa dominated the zooplankton, but not when larger taxa were more abundant. In spite of the differences in foraging ecology, there were no significant differences in growth or nutritional condition in allopatry and sympatry. Similar to the concept of condition-specific competition, when competitive abilities vary along environmental gradients, the impacts of interspecific interactions on foraging ecology, growth and condition are dynamic and likely vary according to temporal fluctuations in prey availability. This is important because short-term incidences of competition could have cascading effects on food webs, even when no impacts on growth rates or condition are detected
Local Gamete Competition Explains Sex Allocation and Fertilization Strategies in the Sea
Within and across taxa, there is much variation in the mode of fertilization, that is, whether eggs and/or sperm are released or kept inside or on the surface of the parent’s body. Although the evolutionary consequences of fertilization mode are far-reaching, transitions in the fertilization mode itself have largely escaped theoretical attention. Here we develop the first evolutionary model of egg retention and release, which also considers transitions between hermaphroditism and dioecy as well as egg size evolution. We provide a unifying explanation for reported associations between small body size, hermaphroditism, and egg retention in marine invertebrates that have puzzled researchers for more than 3 decades. Our model, by including sperm limitation, shows that all these patterns can arise as an evolutionary response to local competition between eggs for fertilization. This can provide a general explanation for three empirical patterns: sperm casters tend to be smaller than related broadcast spawners, hermaphroditism is disproportionately common in sperm casters, and offspring of sperm casters are larger. Local gamete competition also explains a universal sexual asymmetry: females of some species retain their gametes while males release theirs, but the opposite (“egg casting”) lacks evolutionary stability and is apparently not found in nature
The complex multiscale structure in simulated and observed emission maps of the proto-cluster cloud G0.253+0.016 (\u27the Brick\u27)
The Central Molecular Zone (the central ∼500 pc of the Milky Way) hosts molecular clouds in an extreme environment of strong shear, high gas pressure and density, and complex chemistry. G0.253+0.016, also known as \u27the Brick\u27, is the densest, most compact, and quiescent of these clouds. High-resolution observations with the Atacama Large Millimetre/submillimetre Array (ALMA) have revealed its complex, hierarchical structure. In this paper we compare the properties of recent hydrodynamical simulations of the Brick to those of the ALMA observations. To facilitate the comparison, we post-process the simulations and create synthetic ALMA maps of molecular line emission from eight molecules. We correlate the line emission maps to each other and to the mass column density and find that HNCO is the best mass tracer of the eight emission lines within the simulations. Additionally, we characterize the spatial structure of the observed and simulated cloud using the density probability distribution function (PDF), spatial power spectrum, fractal dimension, and moments of inertia. While we find good agreement between the observed and simulated data in terms of power spectra and fractal dimensions, there are key differences in the density PDFs and moments of inertia, which we attribute to the omission of magnetic fields in the simulations. This demonstrates that the presence of the Galactic potential can reproduce many cloud properties, but additional physical processes are needed to fully explain the gas structure
JWST reveals widespread CO ice and gas absorption in the Galactic Center cloud G0.253+0.016
We report JWST NIRCam observations of G0.253+0.016, the molecular cloud in
the Central Molecular Zone known as The Brick, with the F182M, F187N, F212N,
F410M, F405N, and F466N filters. We catalog 56,146 stars detected in all 6
filters using the crowdsource package. Stars within and behind The Brick
exhibit prodigious absorption in the F466N filter that is produced by a
combination of CO ice and gas. In support of this conclusion, and as a general
resource, we present models of CO gas and ice and CO ice in the F466N,
F470N, and F410M filters. Both CO gas and ice may contribute to the observed
stellar colors. We show, however, that CO gas does not absorb the Pf and
Hu lines in F466N, but that these lines show excess absorption,
indicating that CO ice is also present and contributes to observed F466N
absorption. The most strongly absorbed stars in F466N are extincted by 2
magnitudes, corresponding to 80\% flux loss. This high observed absorption
requires very high column densities of CO, requiring total CO column that is in
tension with standard CO abundance and/or gas-to-dust ratios. There is
therefore likely to be a greater CO/H ratio (X) and more
dust per H molecule () in the Galactic Center than the Galactic
disk. Ice and/or gas absorption is observed even in the cloud outskirts,
implying that additional caution is needed when interpreting stellar photometry
in filters that overlap with ice bands throughout our Galactic Center. The
widespread CO absorption in our Galactic Center hints that significant ice
absorption is likely present in other galactic centers.Comment: Submitted to ApJ. Revised after second referee report. 16 pages, 11
figure
PHANGS CO kinematics: disk orientations and rotation curves at 150 pc resolution
We present kinematic orientations and high resolution (150 pc) rotation
curves for 67 main sequence star-forming galaxies surveyed in CO (2-1) emission
by PHANGS-ALMA. Our measurements are based on the application of a new fitting
method tailored to CO velocity fields. Our approach identifies an optimal
global orientation as a way to reduce the impact of non-axisymmetric (bar and
spiral) features and the uneven spatial sampling characteristic of CO emission
in the inner regions of nearby galaxies. The method performs especially well
when applied to the large number of independent lines-of-sight contained in the
PHANGS CO velocity fields mapped at 1'' resolution. The high resolution
rotation curves fitted to these data are sensitive probes of mass distribution
in the inner regions of these galaxies. We use the inner slope as well as the
amplitude of our fitted rotation curves to demonstrate that CO is a reliable
global dynamical mass tracer. From the consistency between photometric
orientations from the literature and kinematic orientations determined with our
method, we infer that the shapes of stellar disks in the mass range of log()=9.0-10.9 probed by our sample are very close to circular
and have uniform thickness.Comment: 19 figures, 36 pages, accepted for publication in ApJ. Table of
PHANGS rotation curves available from http://phangs.org/dat
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