The Carina Nebula and Gum 31 molecular complex: II. The distribution of the atomic gas revealed in unprecedented detail

We report high spatial resolution observations of the HI 21cm line in the Carina Nebula and the Gum 31 region obtained with the Australia Telescope Compact Array. The observations covered $\sim$ 12 deg$^2$ centred on $l= 287.5\deg,b = -1\deg$, achieving an angular resolution of $\sim$ 35 arcseconds. The HI map revealed complex filamentary structures across a wide range of velocities. Several "bubbles" are clearly identified in the Carina Nebula Complex, produced by the impact of the massive star clusters located in this region. An HI absorption profile obtained towards the strong extragalactic radio source PMN J1032--5917 showed the distribution of the cold component of the atomic gas along the Galactic disk, with the Sagittarius-Carina and Perseus spiral arms clearly distinguishable. Preliminary calculations of the optical depth and spin temperatures of the cold atomic gas show that the HI line is opaque ($\tau \gtrsim$ 2) at several velocities in the Sagittarius-Carina spiral arm. The spin temperature is $\sim100$ K in the regions with the highest optical depth, although this value might be lower for the saturated components. The atomic mass budget of Gum 31 is $\sim35 \%$ of the total gas mass. HI self absorption features have molecular counterparts and good spatial correlation with the regions of cold dust as traced by the infrared maps. We suggest that in Gum 31 regions of cold temperature and high density are where the atomic to molecular gas phase transition is likely to be occurring.Comment: 20 pages, 1 table, 16 Figures, Accepted for Publication in the Monthly Notices of the Royal Astronomical Society Journa

Lord of the Rings: A Kinematic Distance to Circinus X-1 from a Giant X-Ray Light Echo

Circinus X-1 exhibited a bright X-ray flare in late 2013. Follow-up observations with Chandra and XMM-Newton from 40 to 80 days after the flare reveal a bright X-ray light echo in the form of four well-defined rings with radii from 5 to 13 arcminutes, growing in radius with time. The large fluence of the flare and the large column density of interstellar dust towards Circinus X-1 make this the largest and brightest set of rings from an X-ray light echo observed to date. By deconvolving the radial intensity profile of the echo with the MAXI X-ray lightcurve of the flare we reconstruct the dust distribution towards Circinus X-1 into four distinct dust concentrations. By comparing the peak in scattering intensity with the peak intensity in CO maps of molecular clouds from the Mopra Southern Galactic Plane CO Survey we identify the two innermost rings with clouds at radial velocity ~ -74 km/s and ~ -81 km/s, respectively. We identify a prominent band of foreground photoelectric absorption with a lane of CO gas at ~ -32 km/s. From the association of the rings with individual CO clouds we determine the kinematic distance to Circinus X-1 to be $D_{Cir X-1} = 9.4^{+0.8}_{-1.0}$ kpc. This distance rules out earlier claims of a distance around 4 kpc, implies that Circinus X-1 is a frequent super-Eddington source, and places a lower limit of $\Gamma \gtrsim 22$ on the Lorentz factor and an upper limit of $\theta_{jet} \lesssim 3^{\circ}$ on the jet viewing angle.Comment: 20 pages, 21 figures, Astrophysical Journal, in prin

Star formation and the Hall Effect

"June 2011"Thesis (PhD) -- Macquarie University, Faculty of Science, Dept. of Physics and Astronomy, 2011.Bibliography: p. 211-228.1. Star formation -- 2. Self-similar gravitational collapse -- 3. The inner asymptotic solutions -- 4. Collapse without the Hall Effect -- 5. Collapse with the Hall Effect -- 6. Discussion and conclusions.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. This thesis describes a semianalytic self-similar model of the collapse of rotating isothermal molecular cloud cores with both Hall and ambipolar diffusion, presenting similarity solutions that demonstrate that the Hall effect has a profound influence on the dynamics of collapse. Two asymptotic power law similarity solutions to the collapse equations on the inner boundary are derived. The first of these represents a Keplerian disc in which accretion is regulated by the magnetic diffusion; with an appropriate value of the Hall diffusion parameter a stable rotationally-supported disc forms, but when the Hall parameter has the opposite sign disc formation is suppressed by the strong diffusion. The second solution describes the infall when the magnetic braking is so efficient at removing angular momentum from the core that no disc forms and the matter free falls onto the protostar. The full similarity 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 accretion rate onto the protostar by 1.5 x 10 [superscript] -6 M [subscript circle with dot] yr [superscript] -1 when the ratio of the Hall to ambipolar diffusion parameters moves between the extremes of -0.5 [le] tilde eta H / tilde eta A [le] 0.2. These variations (and their dependence upon the orientation of the magnetic field with respect to the axis of rotation) 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 of the magnetic diffusion and centrifugal 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 would be worth exploring in future numerical simulations of star formation.Mode of access: World Wide Web.xi, 263 p. il

Fun and feedback at the press of a button

A common phenomenon across disciplines and universities is that students complain that they do not receive enough feedback, even when student evaluation forms indicate satisfaction in other areas such as teacher competency and enthusiasm. On the other side, but less considered, is the lack of feedback that teachers receive as they struggle to get students to participate and engage with the learning process. While technology does not offer an automatic solution, keypad-based automatic response systems do offer the potential to let both parties know how well the learning outcomes are being achieved in a timely and cost-effective manner. We have just completed two years of pilot trialling of such technology at our university in the Computing and Physics Departments. This paper reports our experiences together with the findings of others.11 page(s

Mopra Southern Galactic Plane CO Survey - Data Release 3

We present observations of fifty square degrees of the Mopra carbon monoxide (CO) survey of the Southern Galactic Plane, covering Galactic longitudes l = 300-350&deg; and latitudes |b| &le; 0.5&deg;. These data have been taken at 0.6 arcminute spatial resolution and 0.1 km/s spectral resolution, providing an unprecedented view of the molecular clouds and gas of the Southern Galactic Plane in the 109-115 GHz J = 1-0 transitions of 12CO, 13CO, C18O and C17O. Available here is the full data release with each file containing a 1 square degree field of the plane centred at the longitude listed in the filename (e.g. G321.5-12CO_Vcrop.tar.gz covers l=321 to 322&deg;). Each isotopologue is available separately, and for ease of downloading two velocity ranges are available. The 'Vcrop' velocity range covers from -150 &le; VLSR &le; +50 km/s in all isotopologues, which is the range in which emission is detected above a 2&sigma; level; while the larger 'Vfull' cubes cover a velocity range of -1000 &le; VLSR &le; +1000 km/s in 12CO, and smaller ranges for the other lines (the Vfull range is also smaller between l=323-332&deg;; see the paper for more information). Each data cube is packaged with a FITS map of the Mopra system temperature, the beam coverage and sigma RMS noise statistics to assist in interpretation of these data. Please note that the survey data presented here are in units of T*A K and have not been corrected for the Mopra extended beam efficiency, &eta;XB = 0.55 (see our paper for further information).<br /