The conversion of gas into stars is a fundamental process in astrophysics and cosmology. Stars are known to form from the gravitational collapse of dense clumps in interstellar molecular clouds, and ...it has been proposed that the resulting star formation rate is proportional to either the amount of mass above a threshold gas surface density, or the gas volume density. These star formation prescriptions appear to hold in nearby molecular clouds in our Milky Way Galaxy's disc as well as in distant galaxies where the star formation rates are often much larger. The inner 500 pc of our Galaxy, the Central Molecular Zone (CMZ), contains the largest concentration of dense, high-surface density molecular gas in the Milky Way, providing an environment where the validity of star formation prescriptions can be tested. Here, we show that by several measures, the current star formation rate in the CMZ is an order-of-magnitude lower than the rates predicted by the currently accepted prescriptions. In particular, the region 1° < l < 3°.5, |b| < 0°.5 contains ∼107 M of dense (> several 103 cm−3) molecular gas - enough to form 1000 Orion-like clusters - but the present-day star formation rate within this gas is only equivalent to that in Orion. In addition to density, another property of molecular clouds must be included in the star formation prescription to predict the star formation rate in a given mass of molecular gas. We discuss which physical mechanisms might be responsible for suppressing star formation in the CMZ.
Increasing evidence indicates that combined mechanical and oral antibiotic bowel preparation reduces the infectious complications of colorectal surgery. Anecdotal evidence suggests the combination is ...rarely used in the UK and Europe.
To establish colorectal surgeons' current use and awareness of the benefits of such bowel preparation, and to identify decision-making influences surrounding preoperative bowel preparation.
An electronic survey was emailed to all members of the Association of Coloproctology of Great Britain and Ireland, and promoted via Twitter.
A total of 495 respondents completed the survey: 413 (83.2%) UK, 39 (7.9%) other European, 43 (8.7%) non-European. Respondents used oral antibiotics for 12–20% of cases. Mechanical bowel preparation (MBP), phosphate enema, and no preparation, respectively, ranged between 9 and 80%. Combined MBP and oral antibiotic bowel preparation ranged between 5.5 and 18.6%.
Fifty-three percent (260/495) agreed that combined mechanical and oral antibiotic bowel preparation reduces surgical site infection; 32% (157/495) agreed that the combination reduces risk of anastomotic leak.
Kappa statistics between 0.06 and 0.27 indicate considerable incongruity between surgeons' awareness of the literature, and day-to-day practice.
Twenty-four percent (96/495) believed MBP to be incompatible with enhanced recovery after surgery (ERAS); 41% (204/495) believed that MBP delays return to normal intestinal function.
Few UK and European colorectal surgeons use mechanical and oral antibiotic bowel preparation, despite evidence of its efficacy in reducing infectious complications. The influence of ERAS pathways and UK and European guidelines may explain this. In contradiction to the UK and Europe, North American guidelines recommend incorporating combined mechanical and oral antibiotic bowel preparation into ERAS programmes. This study suggests that future UK and European guidelines incorporate combined mechanical and oral antibiotic bowel preparation into the ERAS pathway.
Abstract
The inner few hundred parsecs of the Milky Way harbours gas densities, pressures, velocity dispersions, an interstellar radiation field and a cosmic ray ionization rate orders of magnitude ...higher than the disc; akin to the environment found in star-forming galaxies at high redshift. Previous studies have shown that this region is forming stars at a rate per unit mass of dense gas which is at least an order of magnitude lower than in the disc, potentially violating theoretical predictions. We show that all observational star formation rate diagnostics – both direct counting of young stellar objects and integrated light measurements – are in agreement within a factor two, hence the low star formation rate (SFR) is not the result of the systematic uncertainties that affect any one method. As these methods trace the star formation over different time-scales, from 0.1 to 5 Myr, we conclude that the SFR has been constant to within a factor of a few within this time period. We investigate the progression of star formation within gravitationally bound clouds on ∼parsec scales and find 1–4 per cent of the cloud masses are converted into stars per free-fall time, consistent with a subset of the considered ‘volumetric’ star formation models. However, discriminating between these models is obstructed by the current uncertainties on the input observables and, most importantly and urgently, by their dependence on ill-constrained free parameters. The lack of empirical constraints on these parameters therefore represents a key challenge in the further verification or falsification of current star formation theories.
Super star clusters are the end product of star formation under the most extreme conditions. As such, studying how their final stellar populations are assembled from their natal progenitor gas clouds ...can provide strong constraints on star formation theories. An obvious place to look for the initial conditions of such extreme stellar clusters is gas clouds of comparable mass and density, with no star formation activity. We present a method to identify such progenitor gas clouds and demonstrate the technique for the gas in the inner few hundred pc of our Galaxy. The method highlights three clouds in the region with similar global physical properties to the previously identified extreme cloud, G0.253 + 0.016, as potential young massive cluster (YMC) precursors. The fact that four potential YMC progenitor clouds have been identified in the inner 100 pc of the Galaxy, but no clouds with similar properties have been found in the whole first quadrant despite extensive observational efforts, has implications for cluster formation/destruction rates across the Galaxy. We put forward a scenario to explain how such dense gas clouds can arise in the Galactic Centre environment, in which YMC formation is triggered by gas streams passing close to the minimum of the global Galactic gravitational potential at the location of the central supermassive black hole, Sgr A*. If this triggering mechanism can be verified, we can use the known time interval since closest approach to Sgr A* to study the physics of stellar mass assembly in an extreme environment as a function of absolute time.
We propose that bound, young massive stellar clusters form from dense clouds that have escape speeds greater than the sound speed in photo-ionized gas. In these clumps, radiative feedback in the form ...of gas ionization is bottled up, enabling star formation to proceed to sufficiently high efficiency so that the resulting star cluster remains bound even after gas removal. We estimate the observable properties of the massive proto-clusters (MPCs) for existing Galactic plane surveys and suggest how they may be sought in recent and upcoming extragalactic observations. These surveys will potentially provide a significant sample of MPC candidates that will allow us to better understand extreme star-formation and massive cluster formation in the Local Universe.
Abstract
In this paper we provide a comprehensive description of the internal dynamics of G0.253+0.016 (a.k.a. ‘the Brick’); one of the most massive and dense molecular clouds in the Galaxy to lack ...signatures of widespread star formation. As a potential host to a future generation of high-mass stars, understanding largely quiescent molecular clouds like G0.253+0.016 is of critical importance. In this paper, we reanalyse Atacama Large Millimeter Array cycle 0 HNCO J = 4(0, 4) − 3(0, 3) data at 3 mm, using two new pieces of software that we make available to the community. First, scousepy, a Python implementation of the spectral line fitting algorithm scouse. Secondly, acorns (Agglomerative Clustering for ORganising Nested Structures), a hierarchical n-dimensional clustering algorithm designed for use with discrete spectroscopic data. Together, these tools provide an unbiased measurement of the line-of-sight velocity dispersion in this cloud, $\sigma _{v_{\mathrm{ los}}, {\rm 1D}}=4.4\pm 2.1$ km s−1, which is somewhat larger than predicted by velocity dispersion-size relations for the central molecular zone (CMZ). The dispersion of centroid velocities in the plane of the sky are comparable, yielding $\sigma _{v_{\mathrm{ los}}, {\rm 1D}}/\sigma _{v_{\mathrm{ pos}}, {\rm 1D}}\sim 1.2\pm 0.3$. This isotropy may indicate that the line-of-sight extent of the cloud is approximately equivalent to that in the plane of the sky. Combining our kinematic decomposition with radiative transfer modelling, we conclude that G0.253+0.016 is not a single, coherent, and centrally condensed molecular cloud; ‘the Brick’ is not a brick. Instead, G0.253+0.016 is a dynamically complex and hierarchically structured molecular cloud whose morphology is consistent with the influence of the orbital dynamics and shear in the CMZ.
Context.
Recent surveys of the Galactic plane in the dust continuum and CO emission lines reveal that large (≳50 pc) and massive (≳10
5
M
⊙
) filaments, know as giant molecular filaments (GMFs), may ...be linked to Galactic dynamics and trace the mid-plane of the gravitational potential in the Milky Way. Yet our physical understanding of GMFs is still poor.
Aims.
We investigate the dense gas properties of one GMF, with the ultimate goal of connecting these dense gas tracers with star formation processes in the GMF.
Methods.
We imaged one entire GMF located at
l
~ 52–54° longitude, GMF54 (~68 pc long), in the empirical dense gas tracers using the HCN(1–0), HNC(1–0), and HCO
+
(1–0) lines, and their
13
C isotopologue transitions, as well as the N
2
H
+
(1–0) line. We studied the dense gas distribution, the column density probability density functions (N-PDFs), and the line ratios within the GMF.
Results.
The dense gas molecular transitions follow the extended structure of the filament with area filling factors between 0.06 and 0.28 with respect to
13
CO(1–0). We constructed the N-PDFs of H
2
for each of the dense gas tracers based on their column densities and assumed uniform abundance. The N-PDFs of the dense gas tracers appear curved in log–log representation, and the HCO
+
N-PDF has the flattest power-law slope index. Studying the N-PDFs for sub-regions of GMF54, we found an evolutionary trend in the N-PDFs that high-mass star-forming and photon-dominated regions have flatter power-law indices. The integrated intensity ratios of the molecular lines in GMF54 are comparable to those in nearby galaxies. In particular, the N
2
H
+
/
13
CO ratio, which traces the dense gas fraction, has similar values in GMF54 and all nearby galaxies except Ultraluminous Infrared Galaxies.
Conclusions.
As the largest coherent cold gaseous structure in our Milky Way, GMFs, are outstanding candidates for connecting studies of star formation on Galactic and extragalactic scales. By analyzing a complete map of the dense gas in a GMF we have found that: (1) the dense gas N-PDFs appear flatter in more evolved regions and steeper in younger regions, and (2) its integrated dense gas intensity ratios are similar to those of nearby galaxies.
ABSTRACT
The evolution of molecular clouds in galactic centres is thought to differ from that in galactic discs due to a significant influence of the external gravitational potential. We present a ...set of numerical simulations of molecular clouds orbiting on the 100-pc stream of the Central Molecular Zone (the central ${\sim }500\,{{\rm pc}}$ of the Galaxy) and characterize their morphological and kinematic evolution in response to the background potential and eccentric orbital motion. We find that the clouds are shaped by strong shear and torques, by tidal and geometric deformation, and by their passage through the orbital pericentre. Within our simulations, these mechanisms control cloud sizes, aspect ratios, position angles, filamentary structure, column densities, velocity dispersions, line-of-sight velocity gradients, spin angular momenta, and kinematic complexity. By comparing these predictions to observations of clouds on the Galactic Centre ‘dust ridge’, we find that our simulations naturally reproduce a broad range of key observed morphological and kinematic features, which can be explained in terms of well-understood physical mechanisms. We argue that the accretion of gas clouds on to the central regions of galaxies, where the rotation curve turns over and the tidal field is fully compressive, is accompanied by transformative dynamical changes to the clouds, leading to collapse and star formation. This can generate an evolutionary progression of cloud collapse with a common starting point, which either marks the time of accretion on to the tidally compressive region or of the most recent pericentre passage. Together, these processes may naturally produce the synchronized starbursts observed in numerous (extra)galactic nuclei.
We search the lambda = 1.1 mm Bolocam Galactic Plane Survey for clumps containing sufficient mass to form ~10 super(4) M sub(middot in circle) star clusters. Eighteen candidate massive proto-clusters ...are identified in the first Galactic quadrant outside of the central kiloparsec. This sample is complete to clumps with mass M sub(clump) > 10 super(4) M sub(middot in circle) and radius r <, ~ 2.5 pc. The overall Galactic massive cluster formation rate is CFR(M sub(cluster) > 10 super(4)) <, ~ 5 Myr super(-1), which is in agreement with the rates inferred from Galactic open clusters and M31 massive clusters. We find that all massive proto-clusters in the first quadrant are actively forming massive stars and place an upper limit of tau sub(starless) < 0.5 Myr on the lifetime of the starless phase of massive cluster formation. If massive clusters go through a starless phase with all of their mass in a single clump, the lifetime of this phase is very short.
We present new JVLA observations of the high-mass cluster-forming region W51A from 2 to 16 GHz with resolution θfwhm ≈ 0.3−0.5″. The data reveal a wealth of observational results: (1) Currently ...forming, very massive (proto-O) stars are traced by o - H2CO21,1−21,2 emission, suggesting that this line can be used efficiently as a massive protostar tracer; (2) there is a spatially distributed population of ≲mJy continuum sources, including hypercompact H ii regions and candidate colliding wind binaries, in and around the W51 proto-clusters; and (3) there are two clearly detected protoclusters, W51e and W51 IRS2, that are gas-rich but may have most of their mass in stars within their inner ≲0.05 pc. The majority of the bolometric luminosity in W51 most likely comes from a third population of OB stars between these clusters. The presence of a substantial population of exposed O-stars coincident with a population of still-forming massive stars, together with a direct measurement of the low mass loss rate via ionized gas outflow from W51 IRS2, implies that feedback is ineffective at halting star formation in massive protoclusters. Instead, feedback may shut off the large-scale accretion of diffuse gas onto the W51 protoclusters, implying that they are evolving toward a state of gas exhaustion rather than gas expulsion. Recent theoretical models predict gas exhaustion to be a necessary step in the formation of gravitationally bound stellar clusters, and our results provide an observational validation of this process.
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