Walker M.A.,Manly Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013
We consider the survival of solid H2 in the diffuse interstellar medium, with application to grains which are small enough to qualify as dust. Consideration of only the thermal aspects of this problem leads to the familiar conclusion that such grains sublimate rapidly. Here we show that charging plays a critical role in determining the sublimation rate, because an electric field helps to bind molecules to the grain surface. A key aspect of the charging process is that the conduction band of solid hydrogen lies above the vacuum free-electron energy level, so low-energy electrons cannot penetrate the solid. But they are attracted by the dielectric and by positive ions in the matrix, so they become trapped in vacuum states just above the surface. This charge-separated configuration suppresses recombination and permits overall neutrality, while supporting large electric fields at the surface. Charging ceases when the potential energy just outside the electron layer coincides with the conduction band minimum. By that stage the heat of sublimation has increased tenfold, effecting a huge reduction in the sublimation rate. Consequently, hydrogen grains may be able to survive indefinitely in the diffuse interstellar medium. There are good prospects for identifying H2 grains, if they exist, as fully localized surface electrons should exhibit discrete energy levels, with a corresponding spectral line signature. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
Lin C.Y.,Australian National University |
Gilbert A.T.B.,Australian National University |
Walker M.A.,Manly Astrophysics
Astrophysical Journal | Year: 2011
We consider the possibility that solid molecular hydrogen is present in interstellar space. If so cosmic-rays and energetic photons cause ionization in the solid leading to the formation of H+6. This ion is not produced by gas-phase reactions and its radiative transitions therefore provide a signature of solid H2 in the astrophysical context. The vibrational transitions of H+6 are yet to be observed in the laboratory, but we have characterized them in a quantum-theoretical treatment of the molecule; our calculations include anharmonic corrections, which are large. Here we report on those calculations and compare our results with astronomical data. In addition to the H+6 isotopomer, we focus on the deuterated species (HD)+3 which is expected to dominate at low ionization rates as a result of isotopic condensation reactions. We can reliably predict the frequencies of the fundamental bands for five modes of vibration. For (HD)+3 all of these are found to lie close to some of the strongest of the pervasive mid-infrared astronomical emission bands, making it difficult to exclude hydrogen precipitates on observational grounds. By the same token these results suggest that (HD)+3 could be the carrier of the observed bands. We consider this possibility within the broader picture of interstellar medium photo-processes and we conclude that solid hydrogen may indeed be abundant in astrophysical environments. © 2011. The American Astronomical Society. All rights reserved.
Walker M.A.,Manly Astrophysics
Astrophysical Journal | Year: 2016
In light of evidence for a high ionization rate due to low-energy cosmic rays (LECR) in diffuse molecular gas in the solar neighborhood, we evaluate their heat input to the warm ionized medium (WIM). LECR are much more effective at heating plasma than they are at heating neutrals. We show that the upper end of the measured ionization rates corresponds to a local LECR heating rate sufficient to maintain the WIM against radiative cooling, independent of the nature of the ionizing particles or the detailed shape of their spectrum. Elsewhere in the Galaxy the LECR heating rates may be higher than those measured locally. In particular, higher fluxes of LECR have been suggested for the inner Galactic disk, based on the observed hard X-ray emission, with correspondingly larger heating rates implied for the WIM. We conclude that LECR play an important and perhaps dominant role in the thermal balance of the WIM. © 2016. The American Astronomical Society. All rights reserved.
Walker M.A.,Manly Astrophysics |
Demorest P.B.,U.S. National Radio Astronomy Observatory |
Van Straten W.,Swinburne University of Technology
Astrophysical Journal | Year: 2013
Cyclic spectroscopy is a signal processing technique that was originally developed for engineering applications and has recently been introduced into the field of pulsar astronomy. It is a powerful technique with many attractive features, not least of which is the explicit rendering of information about the relative phases in any filtering imposed on the signal, thus making holography a more straightforward proposition. Here we present methods for determining optimum estimates of both the filter itself and the statistics of the unfiltered signal, starting from a measured cyclic spectrum. In the context of radio pulsars these quantities tell us the impulse response of the interstellar medium (ISM) and the intrinsic pulse profile. We demonstrate our techniques by application to 428 MHz Arecibo data on the millisecond pulsar B1937+21, obtaining the pulse profile free from the effects of interstellar scattering. As expected, the intrinsic profile exhibits main- and inter-pulse components that are narrower than they appear in the scattered profile; it also manifests some weak, but sharp, features that are revealed for the first time at low frequency. We determine the structure of the received electric field envelope as a function of delay and Doppler shift. Our delay Doppler image has a high dynamic range and displays some pronounced, low-level power concentrations at large delays. These concentrations imply strong clumpiness in the ionized ISM, on AU-size scales, which must adversely affect the timing of B1937+21. © 2013. The American Astronomical Society. All rights reserved..
Hancock P.J.,University of Sydney |
Roberts P.,CSIRO |
Kesteven M.J.,CSIRO |
Ekers R.D.,CSIRO |
And 18 more authors.
Experimental Astronomy | Year: 2011
The Australia Telescope 20 GHz (AT20G) survey is a large area (2π sr), sensitive (40 mJy), high frequency (20 GHz) survey of the southern sky. The survey was conducted in two parts: an initial fast scanning survey, and a series of more accurate follow-up observations. The follow-up survey catalog has been presented by Murphy et al. (MNRAS 402:2403, 2010). In this paper we discuss the hardware setup and scanning survey strategy as well as the production of the scanning survey catalog. © 2011 Springer Science+Business Media B.V.