ALBERT

Description: We present early results from the JCMT (James Clerk Maxwell Telescope) Plane Survey (JPS), which has surveyed the northern inner Galactic plane between longitudes  = 7° and  = 63° in the 850-μm continuum with SCUBA-2 (Submm Common-User Bolometer Array 2), as part of the JCMT Legacy Survey programme. Data from the  = 30° survey region, which contains the massive-star-forming regions W43 and G29.96, are analysed after approximately 40 per cent of the observations had been completed. The pixel-to-pixel noise is found to be 19 mJy beam –1 after a smooth over the beam area, and the projected equivalent noise levels in the final survey are expected to be around 10 mJy beam –1 . An initial extraction of compact sources was performed using the FellWalker method, resulting in the detection of 1029 sources above a 5 surface-brightness threshold. The completeness limits in these data are estimated to be around 0.2 Jy beam –1 (peak flux density) and 0.8 Jy (integrated flux density) and are therefore probably already dominated by source confusion in this relatively crowded section of the survey. The flux densities of extracted compact sources are consistent with those of matching detections in the shallower APEX (Atacama Pathfinder Experiment) Telescope Large Area Survey of the Galaxy (ATLASGAL) survey. We analyse the virial and evolutionary state of the detected clumps in the W43 star-forming complex and find that they appear younger than the Galactic-plane average.

Description: We report the results of 870-μm continuum observations, using the Large APEX Bolometer Camera, towards 77 class-II, 6.7-GHz methanol masers identified by the Methanol MultiBeam (MMB) survey to map the thermal emission from cool dust towards these objects. These data complement a study of 630 methanol masers associated with compact dense clumps identified from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) survey. Compact dust emission is detected towards 70 sources, which implies a dust-association rate of 99 per cent for the full MMB catalogue. Evaluation of the derived dust and maser properties leads us to conclude that the combined sample represents a single population tracing the same phenomenon. We find median clump masses of a few 10 3  M and that all but a handful of sources satisfy the mass–size criterion required for massive star formation. This study provides the strongest evidence of the almost ubiquitous association of methanol masers with massive, star-forming clumps. The fraction of methanol-maser associated clumps is a factor of ~2 lower in the outer Galaxy than the inner Galaxy, possibly a result of the lower metallicity environment of the former. We find no difference in the clump-mass and maser-luminosity distributions of the inner and outer Galaxy. The maser-pumping and clump formation mechanisms are therefore likely to be relatively invariant to Galactic location. Finally, we use the ratio of maser luminosity and clump mass to investigate the hypothesis that the maser luminosity is a good indicator of the evolutionary stage of the embedded source, however, we find no evidence to support this.

Description: We present ammonia maps of portions of the W3 and Perseus molecular clouds in order to compare gas emission with submillimetre continuum thermal emission which are commonly used to trace the same mass component in star-forming regions, often under the assumption of local thermodynamic equilibrium (LTE). The Perseus and W3 star-forming regions are found to have significantly different physical characteristics consistent with the difference in size scales traced by our observations. Accounting for the distance of the W3 region does not fully reconcile these differences, suggesting that there may be an underlying difference in the structure of the two regions. Peak positions of submillimetre and ammonia emission do not correlate strongly. Also, the extent of diffuse emission is only moderately matched between ammonia and thermal emission. Source sizes measured from our observations are consistent between regions, although there is a noticeable difference between the submillimetre source sizes with sources in Perseus being significantly smaller than those in W3. Fractional abundances of ammonia are determined for our sources which indicate a dip in the measured ammonia abundance at the positions of peak submillimetre column density. Virial ratios are determined which show that our sources are generally bound in both regions, although there is considerable scatter in both samples. We conclude that sources in Perseus are bound on smaller scales than in W3 in a way that may reflect their previous identification as low- and high-mass, respectively. Our results indicate that assumptions of local thermal equilibrium and/or the coupling of the dust and gas phases in star-forming regions may not be as robust as commonly assumed.

Description: Using spectral line observations of HNCO, N 2 H + , and HNC, we investigate the kinematics of dense gas in the central ~250 pc of the Galaxy. We present scouse (Semi-automated multi-COmponent Universal Spectral-line fitting Engine), a line-fitting algorithm designed to analyse large volumes of spectral line data efficiently and systematically. Unlike techniques which do not account for complex line profiles, scouse accurately describes the { l , b , v LSR } distribution of Central Molecular Zone (CMZ) gas, which is asymmetric about Sgr A* in both position and velocity. Velocity dispersions range from 2.6 km s –1 〈 〈 53.1 km s –1 . A median dispersion of 9.8 km s –1 , translates to a Mach number, $\mathcal {M}_{\rm 3D}\ge 28$ . The gas is distributed throughout several ‘streams’, with projected lengths ~100–250 pc. We link the streams to individual clouds and sub-regions, including Sgr C, the 20 and 50 km s –1 clouds, the dust ridge, and Sgr B2. Shell-like emission features can be explained by the projection of independent molecular clouds in Sgr C and the newly identified conical profile of Sgr B2 in { l , b , v LSR } space. These features have previously invoked supernova-driven shells and cloud–cloud collisions as explanations. We instead caution against structure identification in velocity-integrated emission maps. Three geometries describing the 3D structure of the CMZ are investigated: (i) two spiral arms; (ii) a closed elliptical orbit; (iii) an open stream. While two spiral arms and an open stream qualitatively reproduce the gas distribution, the most recent parametrization of the closed elliptical orbit does not. Finally, we discuss how proper motion measurements of masers can distinguish between these geometries, and suggest that this effort should be focused on the 20 km s –1 and 50 km s –1 clouds and Sgr C.

Description: By matching infrared-selected, massive young stellar objects (MYSOs) and compact H ii regions in the Red MSX Source survey to massive clumps found in the submillimetre ATLASGAL (APEX Telescope Large Area Survey of the Galaxy) survey, we have identified ~1000 embedded young massive stars between 280° 〈  〈 350° and 10° 〈  〈 60° with | b | 〈 1 $_{.}^{\circ}$ 5. Combined with an existing sample of radio-selected methanol masers and compact H ii regions, the result is a catalogue of ~1700 massive stars embedded within ~1300 clumps located across the inner Galaxy, containing three observationally distinct subsamples, methanol-maser, MYSO and H ii -region associations, covering the most important tracers of massive star formation, thought to represent key stages of evolution. We find that massive star formation is strongly correlated with the regions of highest column density in spherical, centrally condensed clumps. We find no significant differences between the three samples in clump structure or the relative location of the embedded stars, which suggests that the structure of a clump is set before the onset of star formation, and changes little as the embedded object evolves towards the main sequence. There is a strong linear correlation between clump mass and bolometric luminosity, with the most massive stars forming in the most massive clumps. We find that the MYSO and H ii -region subsamples are likely to cover a similar range of evolutionary stages and that the majority are near the end of their main accretion phase. We find few infrared-bright MYSOs associated with the most massive clumps, probably due to very short pre-main-sequence lifetimes in the most luminous sources.

Description: Using the 870 μm APEX Telescope large area survey of the Galaxy, we have identified 577 submillimetre continuum sources with masers from the methanol multibeam survey in the region 280° 〈  〈 20°; | b | 〈 1 ${^{\circ}_{.}}$ 5. 94 per cent of methanol masers in the region are associated with submillimetre dust emission. We estimate masses for ~450 maser-associated sources and find that methanol masers are preferentially associated with massive clumps. These clumps are centrally condensed, with envelope structures that appear to be scale-free, the mean maser position being offset from the peak column density by 0 ± 4 arcsec. Assuming a Kroupa initial mass function and a star formation efficiency of ~30 per cent, we find that over two-thirds of the clumps are likely to form clusters with masses 〉20 M . Furthermore, almost all clumps satisfy the empirical mass–size criterion for massive star formation. Bolometric luminosities taken from the literature for ~100 clumps range between ~100 and 10 6  L . This confirms the link between methanol masers and massive young stars for 90 per cent of our sample. The Galactic distribution of sources suggests that the star formation efficiency is significantly reduced in the Galactic Centre region, compared to the rest of the survey area, where it is broadly constant, and shows a significant drop in the massive star formation rate density in the outer Galaxy. We find no enhancement in source counts towards the southern Scutum–Centaurus arm tangent at  ~ 315°, which suggests that this arm is not actively forming stars.

Description: The line of sight through the Galactic plane between longitudes l  = 37 ${^{\circ}_{.}}$ 83 and 42 ${^{\circ}_{.}}$ 50 allows for the separation of Galactic Ring Survey molecular clouds into those that fall within the spiral arms and those located in the interarm regions. By matching these clouds in both position and velocity with dense clumps detected in the mm continuum by the Bolocam Galactic Plane Survey, we are able to look for changes in the clump formation efficiency (CFE), the ratio of clump to cloud mass, with Galactic environment. We find no evidence of any difference in the CFE between the interarm and spiral-arm regions along this line of sight. This is further evidence that, outside the Galactic Centre region, the large-scale structures of the Galaxy play little part in changing the dense, potentially star-forming structures within molecular clouds.

Description: Near-infrared H - and K -band spectra are presented for 247 objects, selected from the Red MSX Source (RMS) survey as potential young stellar objects (YSOs). 195 (~80 per cent) of the targets are YSOs, of which 131 are massive YSOs ( L BOL  〉 5  x 10 3 L , M  〉 8 M ). This is the largest spectroscopic study of massive YSOs to date, providing a valuable resource for the study of massive star formation. In this paper, we present our exploratory analysis of the data. The YSOs observed have a wide range of embeddedness (2.7 〈 A V  〈 114), demonstrating that this study covers minimally obscured objects right through to very red, dusty sources. Almost all YSOs show some evidence for emission lines, though there is a wide variety of observed properties. The most commonly detected lines are Br, H 2 , fluorescent Fe  ii , CO bandhead, [Fe  ii ] and He  i 2–1 1 S– 1 P, in order of frequency of occurrence. In total, ~40 per cent of the YSOs display either fluorescent Fe  ii 1.6878 μm or CO bandhead emission (or both), indicative of a circumstellar disc; however, no correlation of the strength of these lines with bolometric luminosity was found. We also find that ~60 per cent of the sources exhibit [Fe  ii ] or H 2 emission, indicating the presence of an outflow. Three quarters of all sources have Br in emission. A good correlation with bolometric luminosity was observed for both the Br and H 2 emission line strengths, covering 1 〈 L BOL  〈 3.5  x 10 5 L . This suggests that the emission mechanism for these lines is the same for low-, intermediate- and high-mass YSOs, i.e. high-mass YSOs appear to resemble scaled-up versions of low-mass YSOs.

Description: We have used the well-selected sample of ~1750 embedded, young, massive stars identified by the Red MSX Source (RMS) survey to investigate the Galactic distribution of recent massive star formation. We present molecular line observations for ~800 sources without existing radial velocities. We describe the various methods used to assign distances extracted from the literature and solve the distance ambiguities towards approximately 200 sources located within the solar circle using archival H i data. These distances are used to calculate bolometric luminosities and estimate the survey completeness (~2 10 4  L ). In total, we calculate the distance and luminosity of ~1650 sources, one third of which are above the survey's completeness threshold. Examination of the sample's longitude, latitude, radial velocities and mid-infrared images has identified ~120 small groups of sources, many of which are associated with well-known star formation complexes, such as G305, G333, W31, W43, W49 and W51. We compare the positional distribution of the sample with the expected locations of the spiral arms, assuming a model of the Galaxy consisting of four gaseous arms. The distribution of young massive stars in the Milky Way is spatially correlated with the spiral arms, with strong peaks in the source position and luminosity distributions at the arms’ Galactocentric radii. The overall source and luminosity surface densities are both well correlated with the surface density of the molecular gas, which suggests that the massive star formation rate per unit molecular mass is approximately constant across the Galaxy. A comparison of the distribution of molecular gas and the young massive stars to that in other nearby spiral galaxies shows similar radial dependences. We estimate the total luminosity of the embedded massive star population to be ~0.76 10 8  L , 30 per cent of which is associated with the 10 most active star-forming complexes. We measure the scaleheight as a function of the Galactocentric distance and find that it increases only modestly from ~20–30 pc between 4 and 8 kpc, but much more rapidly at larger distances.