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The external environments surrounding molecular clouds vary widely across galaxies such as the Milky Way, and statistical samples of clouds from surveys are required to understand them. We present the Perseus Arm Molecular Survey (PAMS), a James Clerk Maxwell Telescope (JCMT) survey of $^{13}$CO and C$^{18}$O ($J$=3$-$2) of several molecular cloud complexes including W5 and NGC 7538 in the outer Perseus spiral arm situated at $\ell \approx 110^{\circ}$ and $\ell \approx 135^{\circ}$, with a total survey area of $\sim$6 deg$^2$. The PAMS data have an effective resolution of 17.2 arcsec, and rms sensitivity of $T_\rm{mb} = 0.7$ K in 0.3 km/s channels. We present a first look at the data, and compare the PAMS regions in the Outer Galaxy with Inner Galaxy regions from the CO Heterodyne Inner Milky Way Plane Survey (CHIMPS), incorporating archival $^{12}$CO (3$-$2) data. By comparing the various CO data with maps of H$_2$ column density from $\textit{Herschel}$, we find that the CO-to-H$_2$ column density $X$-factors do not vary significantly between Galactocentric radii of 4 and 10 kpc, and present representative values of $X_{^{12}\rm{CO} 3-2}$ and $X_{^{13}\rm{CO} 3-2}$. We find that the emission profiles, size-linewidth and mass-radius relationships of $^{13}$CO-traced structures are similar between the Inner and Outer Galaxy. Although PAMS sources are more massive than their Inner Galaxy counterparts for a given size scale, the discrepancy can be accounted for by the Galactic gradient in gas-to-dust mass ratio, uncertainties in the $X$-factors, and selection biases. We have made the PAMS data publicly available, complementing other CO surveys targeting different regions of the Galaxy in different isotopologues and transitions., Comment: Currently under review at MNRAS

High-mass stars have an enormous influence on the evolution of the interstellar medium in galaxies, so it is important that we understand how they form. We examine the central clumps within a sample of seven infrared-dark clouds (IRDCs) with a range of masses and morphologies. We use 1 pc-scale observations from NOEMA and the IRAM 30-m telescope to trace dense cores with 2.8 mm continuum, and gas kinematics in C$^{18}$O, HCO$^+$, HNC, and N$_2$H$^+$ ($J$=1$-$0). We supplement our continuum sample with six IRDCs observed at 2.9 mm with ALMA, and examine the relationships between core- and clump-scale properties. We have developed a fully-automated multiple-velocity component hyperfine line-fitting code called mwydyn which we employ to trace the dense gas kinematics in N$_2$H$^+$ (1$-$0), revealing highly complex and dynamic clump interiors. We find that parsec-scale clump mass is the most important factor driving the evolution; more massive clumps are able to concentrate more mass into their most massive cores - with a log-normally distributed efficiency of around 9% - in addition to containing the most dynamic gas. Distributions of linewidths within the most massive cores are similar to the ambient gas, suggesting that they are not dynamically decoupled, but are similarly chaotic. A number of studies have previously suggested that clumps are globally collapsing; in such a scenario, the observed kinematics of clump centres would be the direct result of gravity-driven mass inflows that become ever more complex as the clumps evolve, which in turn leads to the chaotic mass growth of their core populations., Comment: Accepted by MNRAS

We present a study of the physical and chemical properties of the Extended Green Object (EGO) G19.01$-$0.03 using sub-arcsecond angular resolution Atacama Large Millimeter/submillimeter Array (ALMA) 1.05mm and Karl G. Jansky Very Large Array (VLA) 1.21cm data. G19.01$-$0.03 MM1, the millimetre source associated with the central massive young stellar object (MYSO), appeared isolated and potentially chemically young in previous Submillimeter Array observations. In our $\sim0.4''$-resolution ALMA data, MM1 has four low-mass millimetre companions within 0.12pc, all lacking maser or outflow emission, indicating they may be prestellar cores. With a rich ALMA spectrum full of complex organic molecules, MM1 does not appear chemically young, but has molecular abundances typical of high-mass hot cores in the literature. At the 1.05mm continuum peak of MM1, $\mathrm{N}(\mathrm{CH}_{3}\mathrm{OH})=(2.22\pm0.01)\times10^{18}$cm$^{-2}$ and $T_{\mathrm{ex}} = 162.7\substack{+0.3 \\ -0.5}$K based on pixel-by-pixel Bayesian analysis of LTE synthetic methanol spectra across MM1. Intriguingly, the peak CH$_{3}$OH $T_{\mathrm{ex}}=165.5\pm0.6$ K is offset from MM1's millimetre continuum peak by $0.22''\sim880$au, and a region of elevated CH$_{3}$OH $T_{\mathrm{ex}}$ coincides with free-free VLA 5.01cm continuum, adding to the tentative evidence for a possible unresolved high-mass binary in MM1. In our VLA 1.21cm data, we report the first NH$_{3}$(3,3) maser detections towards G19.01$-$0.03, along with candidate 25GHz CH$_{3}$OH $5(2,3)-5(1,4)$ maser emission; both are spatially and kinematically coincident with 44GHz Class I CH$_{3}$OH masers in the MM1 outflow. We also report the ALMA detection of candidate 278.3GHz Class I CH$_{3}$OH maser emission towards this outflow, strengthening the connection of these three maser types to MYSO outflows., Comment: 24 pages, 15 figures, 7 tables, Accepted for publication in MNRAS

Hub-filament systems (HFSs) are potential sites of protocluster and massive star formation, and play a key role in mass accumulation. We report JCMT POL-2 850 $\mu$m polarization observations toward the massive HFS SDC13. We detect an organized magnetic field near the hub center with a cloud-scale "U-shape" morphology following the western edge of the hub. Together with larger-scale APEX 13CO and PLANCK polarization data, we find that SDC13 is located at the convergent point of three giant molecular clouds (GMCs) along a large-scale, partially spiral-like magnetic field. The smaller "U-shape" magnetic field is perpendicular to the large-scale magnetic field and the converging GMCs. We explain this as the result of a cloud-cloud collision. Within SDC13, we find that local gravity and velocity gradients point toward filament ridges and hub center. This suggests that gas can locally be pulled onto filaments and overall converges to the hub center. A virial analysis of the central hub shows that gravity dominates magnetic and kinematic energy. Combining large- and small-scale analyses, we propose that SDC13 is initially formed from a collision of clouds moving along the large-scale magnetic field. In the post-shock regions, after the initial turbulent energy has dissipated, gravity takes over and starts to drive the gas accretion along the filaments toward the hub center., Comment: 34 pages, 25 figures, 3 tables

Using the Atacama Large Millimetre/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA), we observed the Extended Green Object (EGO) G19.01$-$0.03 with sub-arcsecond resolution from 1.05 mm to 5.01 cm wavelengths. Our $\sim0.4''\sim1600$ AU angular resolution ALMA observations reveal a velocity gradient across the millimetre core MM1, oriented perpendicular to the previously known bipolar molecular outflow, that is consistently traced by 20 lines of 8 molecular species with a range of excitation temperatures, including complex organic molecules (COMs). Kinematic modelling shows the data are well described by models that include a disc in Keplerian rotation and infall, with an enclosed mass of $40-70 \mathrm{M}_{\odot}$ (within a 2000 AU outer radius) for a disc inclination angle of $i=40^{\circ}$, of which $5.4-7.2 \mathrm{M}_{\odot}$ is attributed to the disc. Our new VLA observations show that the 6.7 GHz Class II methanol masers associated with MM1 form a partial ellipse, consistent with an inclined ring, with a velocity gradient consistent with that of the thermal gas. The disc-to-star mass ratio suggests the disc is likely to be unstable and may be fragmenting into as-yet-undetected low mass stellar companions. Modelling the centimetre--millimetre spectral energy distribution of MM1 shows the ALMA 1.05 mm continuum emission is dominated by dust, whilst a free-free component, interpreted as a hypercompact HII region, is required to explain the VLA $\sim$5 cm emission. The high enclosed mass derived for a source with a moderate bolometric luminosity ($\sim$10$^{4} \mathrm{L}_{\odot}$) suggests that the MM1 disc may feed an unresolved high-mass binary system., Comment: 15 pages, 8 figures, 5 tables, Accepted for publication in MNRAS

The physical processes behind the transfer of mass from parsec-scale clumps to massive-star-forming cores remain elusive. We investigate the relation between the clump morphology and the mass fraction that ends up in its most massive core (MMC) as a function of infrared brightness, i.e. a clump evolutionary tracer. Using ALMA 12 m and ACA we surveyed 6 infrared-dark hubs in 2.9mm continuum at $\sim$3" resolution. To put our sample into context, we also re-analysed published ALMA data from a sample of 29 high mass-surface density ATLASGAL sources. We characterise the size, mass, morphology, and infrared brightness of the clumps using Herschel and Spitzer data. Within the 6 newly observed hubs, we identify 67 cores, and find that the MMCs have masses between 15-911 $\mathrm{M}_{\odot}$ within a radius of 0.018-0.156 pc. The MMC of each hub contains 3-24% of the clump mass ($f_\mathrm{MMC}$), becoming 5-36% once core masses are normalised to the median core radius. Across the 35 clumps, we find no significant difference in the median $f_\mathrm{MMC}$ values of hub and non-hub systems, likely the consequence of a sample bias. However, we find that $f_\mathrm{MMC}$ is $\sim$7.9 times larger for infrared-dark clumps compared to infrared-bright ones. This factor increases up to $\sim$14.5 when comparing our sample of 6 infrared-dark hubs to infrared-bright clumps. We speculate that hub-filament systems efficiently concentrate mass within their MMC early on during its evolution. As clumps evolve, they grow in mass, but such growth does not lead to the formation of more massive MMCs., Comment: 15 pages, 9 figures

Converging networks of interstellar filaments i.e. hubs, have been recently linked to the formation of stellar clusters and massive stars. The goal is to study the kinematic and density structure of the SDC13 hub at high angular resolution to determine what drives its evolution and fragmentation. We have mapped SDC13, a 1000Msun infrared dark hub, in NH3(1,1) and NH3(2,2) emission lines, with both the JVLA and GBT down to 0.07pc. The mass-per-unit-lengths of all four hub filaments are thermally super-critical, consistent with the presence of tens of gravitationally bound cores along them. These cores exhibit regular separation of 0.37 +/- 0.16 pc suggesting gravitational instabilities running along these super-critical filaments are responsible for their fragmentation. The observed local increase of the dense gas velocity dispersion towards starless cores is believed to be a consequence of such fragmentation process. We see velocity gradient peaks towards 63% of the cores as expected during the early stages of filament fragmentation. The most massive cores are located at the filament junctions, where the velocity dispersion is the largest. We interpret this as the result of the hub morphology generating the largest acceleration gradients near the hub centre. We propose a scenario for the evolution of the SDC13 hub in which filaments first form as post-shock structures in a supersonic turbulent flow. Then gravity takes over and starts shaping the evolution of the hub, both fragmenting filaments and pulling the gas towards the centre of the gravitational well. By doing so, gravitational energy is converted into kinetic energy in both local (cores) and global (hub centre) potential well minima. Furthermore, the generation of larger gravitational acceleration gradients at the filament junctions promotes the formation of more massive cores. [abridged], Comment: 25 pages, 26 figures, 3 tables, abridged abstract, Accepted for publication in A&A