Discovery of Luminous Star Formation in PMN1452-5910/IRAS14482-5857: the Pterodactyl Nebula

We present sensitive 1-3 GHz ATCA radio continuum observations of the hitherto unresolved star forming region known as either IRAS14482-5857 or PMN1452-5910. At radio continuum frequencies, this source is characterised by a "filled-bubble" structure reminiscent of a classical HII region, dominated by three point sources, and surrounded by low-surface-brightness emission out to the $3'\times4'$ source extent observed at other frequencies in the literature. The infrared emission corresponds well to the radio emission, with polycyclic aromatic hydrocarbon emission surrounding regions of hot dust towards the radio bubbles. A bright 4.5 $\mu$m point source is seen towards the centre of the radio source, suggesting a young stellar object. There is also a linear, outflow-like structure radiating brightly at 8 and 24 $\mu$m towards the brightest peak of the radio continuum. In order to estimate the distance to this source, we have used Mopra Southern Galactic Plane CO Survey $^{12}$CO(1-0) and $^{13}$CO(1-0) molecular line emission data. Integrated-intensity, velocity at peak intensity and line-fitting of the spectra all point towards the peak centred at $v_{LSR}$=-1.1 km/s being connected to this cloud. This infers a distance to this cloud of ~12.7 kpc. Assuming this distance, we estimate a column density and mass towards IRAS14482-5857 of ~$1.5\times10^{21}$ cm$^{-2}$ and $2\times10^4$ $M_\odot$, implying that this source is a site of massive star formation. Reinforcing this conclusion, our broadband spectral fitting infers dust temperatures of 19 and 110K, emission measures for the sub-pc radio point-source of emission measure $EM\sim10^{6-7}$ pc cm$^{-6}$, electron densities of $n_e\sim10^3$ cm$^{-3}$ and photon ionisation rates of $N_{Ly}~10^{46-48}$ s$^{-1}$. The evidence strongly suggests that IRAS14482-5857 is a distant, and hence intense site of massive star-formation.Comment: 11 pages, 12 figures, accepted for publication in the Astronomical Journa

The Hall effect in star formation

Magnetic fields play an important role in star formation by regulating the removal of angular momentum from collapsing molecular cloud cores. Hall diffusion is known to be important to the magnetic field behaviour at many of the intermediate densities and field strengths encountered during the gravitational collapse of molecular cloud cores into protostars, and yet its role in the star formation process is not well-studied. We present a semianalytic self-similar model of the collapse of rotating isothermal molecular cloud cores with both Hall and ambipolar diffusion, and similarity solutions that demonstrate the profound influence of the Hall effect on the dynamics of collapse. The solutions show that the size and sign of the Hall parameter can change the size of the protostellar disc by up to an order of magnitude and the protostellar accretion rate by fifty per cent when the ratio of the Hall to ambipolar diffusivities is varied between -0.5 <= eta_H / eta_A <= 0.2. These changes depend upon the orientation of the magnetic field with respect to the axis of rotation and create a preferred handedness to the solutions that could be observed in protostellar cores using next-generation instruments such as ALMA. Hall diffusion also determines the strength and position of the shocks that bound the pseudo and rotationally-supported discs, and can introduce subshocks that further slow accretion onto the protostar. In cores that are not initially rotating Hall diffusion can even induce rotation, which could give rise to disc formation and resolve the magnetic braking catastrophe. The Hall effect clearly influences the dynamics of gravitational collapse and its role in controlling the magnetic braking and radial diffusion of the field merits further exploration in numerical simulations of star formation.Comment: 22 pages, 10 figures, accepted by MNRA

The Mopra Southern Galactic Plane CO Survey-data release 4-complete survey

We present observations of the Mopra carbon monoxide (CO) survey of the Southern Galactic Plane, covering Galactic longitudes spanning l = 250◦ (−110◦) to l = 355◦ (−5◦), with a latitudinal coverage of at least |b| 210 deg2. These data have been taken at 0.6 arcmin spatial resolution and 0.1 km s−1 spectral resolution, providing an unprecedented view of the molecular gas clouds of the Southern Galactic Plane in the 109–115 GHz J = 1 − 0 transitions of 12CO, 13CO, C18O, and C17O.K. O. Cubuk ... G. Rowell ... et al

Gravitational Collapse and Disk Formation in Magnetized Cores

We discuss the effects of the magnetic field observed in molecular clouds on the process of star formation, concentrating on the phase of gravitational collapse of low-mass dense cores, cradles of sunlike stars. We summarize recent analytic work and numerical simulations showing that a substantial level of magnetic field diffusion at high densities has to occur in order to form rotationally supported disks. Furthermore, newly formed accretion disks are threaded by the magnetic field dragged from the parent core during the gravitational collapse. These disks are expected to rotate with a sub-Keplerian speed because they are partially supported by magnetic tension against the gravity of the central star. We discuss how sub-Keplerian rotation makes it difficult to eject disk winds and accelerates the process of planet migration. Moreover, magnetic fields modify the Toomre criterion for gravitational instability via two opposing effects: magnetic tension and pressure increase the disk local stability, but sub-Keplerian rotation makes the disk more unstable. In general, magnetized disks are more stable than their nonmagnetic counterparts; thus, they can be more massive and less prone to the formation of giant planets by gravitational instability.Comment: Chapter 16 in "Magnetic Fields in Diffuse Media", Springer-Verlag, eds. de Gouveia Dal Pino, E., Lazarian, A., Melioli,

A deep spectromorphological study of the γ\gamma-ray emission surrounding the young massive stellar cluster Westerlund 1

Young massive stellar clusters are extreme environments and potentially provide the means for efficient particle acceleration. Indeed, they are increasingly considered as being responsible for a significant fraction of cosmic rays (CRs) accelerated within the Milky Way. Westerlund 1, the most massive known young stellar cluster in our Galaxy is a prime candidate for studying this hypothesis. While the very-high-energy $\gamma$-ray source HESS J1646-458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified. We aim to identify the physical processes responsible for the $\gamma$-ray emission around Westerlund 1 and thus to better understand the role of massive stellar clusters in the acceleration of Galactic CRs. Using 164 hours of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the $\gamma$-ray emission of HESS J1646-458. We furthermore employed H I and CO observations of the region to infer the presence of gas that could serve as target material for interactions of accelerated CRs. We detected large-scale ($\sim 2^\circ$ diameter) $\gamma$-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with $\gamma$-ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and is uniform across the entire source region. We did not find a clear correlation of the $\gamma$-ray emission with gas clouds as identified through H I and CO observations. We conclude that, of the known objects within the region, only Westerlund 1 can explain the bulk of the $\gamma$-ray emission. Several CR acceleration sites and mechanisms are conceivable, and discussed in detail. (abridged)Comment: 15 pages, 9 figures. Corresponding authors: L. Mohrmann, S. Ohm, R. Rauth, A. Specoviu

SEDIGISM: Structure, excitation, and dynamics of the inner Galactic interstellar medium

The origin and life-cycle of molecular clouds are still poorly constrained, despite their importance for understanding the evolution of the interstellar medium. Many large-scale surveys of the Galactic plane have been conducted recently, allowing for rapid progress in this field. Nevertheless, a sub-arcminute resolution global view of the large-scale distribution of molecular gas, from the diffuse medium to dense clouds and clumps, and of their relationshipto the spiral structure, is still missing. Aims. We have carried out a systematic, homogeneous, spectroscopic survey of the inner Galactic plane, in order to complement the many continuum Galactic surveys available with crucial distance and gas-kinematic information. Our aim is to combine this data set with recent infrared to sub-millimetre surveys at similar angular resolutions. © 2017 ESO

The Hall effect in accretion flows

Magnetic diffusion in accretion flows changes the structure and angular momentum of the accreting material. We present two power-law similarity solutions for flattened accretion flows in the presence of magnetic diffusion: a secularly evolving Keplerian disc and a magnetically diluted free fall on to the central object. The influence of Hall diffusion on the solutions is evident even when this is small compared to ambipolar and Ohmic diffusion, as the surface density, accretion rate and angular momentum in the flow all depend upon the product eta H(B. Omega ), and the inclusion of Hall diffusion may be the solution to the magnetic braking catastrophe of star formation simulations.8 page(s

Star Formation and the Hall Effect

1 page(s