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  • 1
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    Resolving structure of the disc around HD100546 at 7 mm with ATCA (2015)
    Oxford University Press
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    Publication Date: 2015-08-15
    Description: There is much evidence that planet formation is occurring in the disc around the Herbig Be star HD100546. To learn more about the processes occurring in this disc, we conducted high-resolution imaging at 43/45 GHz with the Australia Telescope Compact Array. Multiple array configurations were used, providing a best spatial resolution of ~0.15 arcsec, or 15 au at HD100546's distance of ~100 pc. Significant structure is revealed, but its precise form is dependent on the u – v plane sampling used for the image reconstruction. At a resolution of ≤30 au, we detected an inner gap in the disc with a radius of ~25 au and a position angle approximately along the known disc major axis. With different weighting, and an achieved resolution of ~15 au, emission appears at the centre and the disc takes on the shape of an incomplete ring, much like a horseshoe, again with a gap radius of ~25 au. The position angle of the disc major axis and its inclination from face-on are determined to be 140° ± 5° and 40° ± 5°, respectively. The ~25 au gap radius is confirmed by a null in the real part of the binned visibilities at 320 ± 10 k, whilst the non-axisymmetric nature is also confirmed through significant structure in the imaginary component. The emission mechanism at the central peak is most likely to be free–free emission from a stellar or disc wind. Overall our data support the picture of at least one, but probably several, giant planets orbiting HD100546 within 25 au.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    Ammonia excitation imaging of shocked gas towards the W28 gamma-ray source HESS J1801-233 (2016)
    Oxford University Press
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    Publication Date: 2016-08-04
    Description: We present 12 mm Mopra observations of the dense (〉10 3  cm –3 ) molecular gas towards the north-east of the W28 supernova remnant (SNR). This cloud is spatially well matched to the TeV gamma-ray source HESS J1801–233 and is known to be an SNR-molecular cloud interaction region. Shock-disruption is evident from broad NH 3 (1,1) spectral linewidths in regions towards the W28 SNR, while strong detections of spatially extended NH 3 (3,3), NH 3 (4,4) and NH 3 (6,6) inversion emission towards the cloud strengthen the case for the existence of high temperatures within the cloud. Velocity dispersion measurements and NH 3 ( n , n )/(1,1) ratio maps, where n = 2, 3, 4 and 6, indicate that the source of disruption is from the side of the cloud nearest to the W28 SNR, suggesting that it is the source of cloud-disruption. Towards part of the cloud, the ratio of ortho to para-NH 3 is observed to exceed 2, suggesting gas-phase NH 3 enrichment due to NH 3 liberation from dust-grain mantles. The measured NH 3 abundance with respect to H 2 is ~(1.2 ± 0.5) x 10 –9 , which is not high, as might be expected for a hot, dense molecular cloud enriched by sublimated grain-surface molecules. The results are suggestive of NH 3 sublimation and destruction in this molecular cloud, which is likely to be interacting with the W28 SNR shock.
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    Topics: Physics
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  • 3
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    Scaled up low-mass star formation in massive star-forming cores in the G333 giant molecular cloud (2016)
    Oxford University Press
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    Publication Date: 2016-04-04
    Description: Three bright molecular line sources in G333 have recently been shown to exhibit signatures of infall. We describe a molecular line radiative transfer (RT) modelling process which is required to extract the infall signature from Mopra and Nanten2 data. The observed line profiles differ greatly between individual sources but are reproduced well by variations upon a common unified model where the outflow viewing angle is the most significant difference between the sources. The models and data together suggest that the observed properties of the high-mass star-forming regions such as infall, turbulence and mass are consistent with scaled-up versions of the low-mass case with turbulent velocities that are supersonic and an order of magnitude larger than those found in low-mass star-forming regions. Using detailed RT modelling, we show that the G333 cores are essentially undergoing a scaled-up version of low-mass star formation. This is an extension of earlier work in that the degree of infall and the chemical abundances are constrained by the RT modelling in a way that is not practical with a standard analysis of observational data. We also find high velocity infall and high infall mass rates, possibly suggesting accelerated collapse due to external pressure. Molecular depletion due to freeze-out on to dust grains in central regions of the cores is suggested by low molecular abundances of several species. Strong evidence for a local enhancement of 13 C-bearing species towards the outflow cloud cores is discussed, consistent with the presence of shocks caused by the supersonic motions within them.
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    Topics: Physics
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  • 4
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    Aligned grains and inferred toroidal magnetic fields in the envelopes of massive young stellar objects (2013)
    Oxford University Press
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    Publication Date: 2013-10-12
    Description: Massive young stellar objects (YSOs), like low-mass YSOs, are thought to be surrounded by optically thick envelopes and/or discs and are observed to have associated regions that produce polarized light at near-infrared wavelengths. These polarized regions are thought to be lower density outflows along the polar axes of the YSO envelopes. Using the 0.2 arcsec spatial resolution of Near-Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope, we are examining the structure of the envelopes and outflow regions of massive YSOs in star-forming regions within a few kpc of the Sun. Here, we report on 2 μm polarimetry of Mon R2-IRS3, S140-IRS1 and AFGL 2591. All three sources contain YSOs with highly polarized monopolar outflows, with Mon R2-IRS3 containing at least two YSOs in a small cluster. The central stars of all four YSOs are also polarized, with position angles perpendicular to the directions of the outflows. We infer that this polarization is due to scattering and absorption by aligned grains. We have modelled our observations of S140-IRS1 and AFGL 2591 as light scattered and absorbed both by spherical grains and by elongated grains that are aligned by magnetic fields. Models that best reproduce the observations have a substantial toroidal component to the magnetic field in the equatorial plane. Moreover, the toroidal magnetic field in the model that best fits AFGL 2591 extends a large fraction of the height of the model cavity, which is 10 5 au. We conclude that the massive YSOs in this study all show evidence of the presence of a substantial toroidal magnetic field.
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  • 5
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    6.7-GHz methanol maser associated outflows: an evolutionary sequence (2015)
    Oxford University Press
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    Publication Date: 2015-03-20
    Description: We present a continuing study of a sample 44 molecular outflows, observed in 13 CO lines, closely associated with 6.7-GHz methanol masers, hence called methanol maser associated outflows (MMAOs). We compare MMAO properties with those of outflows from other surveys in the literature. In general, MMAOs follow similar trends, but show a deficit in number at low masses and momenta, with a corresponding higher fraction at the high end of the distributions. A similar trend is seen for the dynamical time-scales of MMAOs. We argue that the lack of relatively low mass and young flows in MMAOs is due to the inherent selection-bias in the sample, i.e. its direct association with 6.7-GHz methanol masers. This implies that methanol masers must switch on after the onset of outflows (hence accretion), and not before a sufficient abundance of methanol is liberated from icy dust mantles. Consequently the average dynamical age of MMAOs is older than for the general population of molecular outflows. We propose an adjusted evolutionary sequence of outflow and maser occurrence in the hot core phase, where methanol masers turn on after the onset of the outflow phase.
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  • 6
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    Methanol maser associated outflows: detection statistics and properties (2014)
    Oxford University Press
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    Publication Date: 2014-08-20
    Description: We have selected the positions of 54 6.7 GHz methanol masers from the Methanol Multibeam Survey catalogue, covering a range of longitudes between 20° and 34° of the Galactic plane. These positions were mapped in the J = 3-2 transition of both the 13 CO and C 18 O lines. A total of 58 13 CO emission peaks are found in the vicinity of these maser positions. We search for outflows around all 13 CO peaks, and find evidence for high-velocity gas in all cases, spatially resolving the red and blue outflow lobes in 55 cases. Of these sources, 44 have resolved kinematic distances, and are closely associated with the 6.7 GHz masers, a subset referred to as Methanol Maser Associated Outflows (MMAOs). We calculate the masses of the clumps associated with each peak using 870 μm continuum emission from the ATLASGAL survey. A strong correlation is seen between the clump mass and both outflow mass and mechanical force, lending support to models in which accretion is strongly linked to outflow. We find that the scaling law between outflow activity and clump masses observed for low-mass objects, is also followed by the MMAOs in this study, indicating a commonality in the formation processes of low-mass and high-mass stars.
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  • 7
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    ISM gas studies towards the TeV PWN HESS J1825-137 and northern region (2016)
    Oxford University Press
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    Publication Date: 2016-03-31
    Description: HESS J1825–137 is a pulsar wind nebula (PWN) whose TeV emission extends across ~1 $\deg$ . Its large asymmetric shape indicates that its progenitor supernova interacted with a molecular cloud located in the north of the PWN as detected by previous CO Galactic survey (e.g. Lemiere, Terrier & Djannati-Ataï). Here, we provide a detailed picture of the interstellar medium (ISM) towards the region north of HESS J1825–137, with the analysis of the dense molecular gas from our 7 and 12 mm Mopra survey and the more diffuse molecular gas from the Nanten CO(1–0) and GRS 13 CO(1–0) surveys. Our focus is the possible association between HESS J1825–137 and the unidentified TeV source to the north, HESS J1826–130. We report several dense molecular regions whose kinematic distance matched the dispersion measured distance of the pulsar. Among them, the dense molecular gas located at (RA, Dec.) = (18 h 421h,–13 $_{.}^{\circ}$ 282) shows enhanced turbulence and we suggest that the velocity structure in this region may be explained by a cloud–cloud collision scenario. Furthermore, the presence of a H α rim may be the first evidence of the progenitor supernova remnant (SNR) of the pulsar PSR J1826–1334 as the distance between the H α rim and the TeV source matched with the predicted SNR radius R SNR ~ 120 pc. From our ISM study, we identify a few plausible origins of the HESS J1826–130 emission, including the progenitor SNR of PSR J1826–1334 and the PWN G018.5–0.4 powered by PSR J1826–1256. A deeper TeV study however, is required to fully identify the origin of this mysterious TeV source.
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    Topics: Physics
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  • 8
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    Erratum: 3 to 12 millimetre studies of dense gas towards the western rim of supernova remnant RX J1713.7-3946 (2013)
    Oxford University Press
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    Publication Date: 2013-03-16
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  • 9
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    New detections of HC5N towards hot cores associated with 6.7 GHz methanol masers (2014)
    Oxford University Press
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    Publication Date: 2014-08-01
    Description: We present new detections of cyanodiacetylene (HC 5 N) towards hot molecular cores, observed with the Tidbinbilla 34 m radio telescope (DSS–34). In a sample of 79 hot molecular cores, HC 5 N was detected towards 35. These results are counter to the expectation that long chain cyanopolyynes, such as HC 5 N, are not typically found in hot molecular cores, unlike their shorter chain counterpart HC 3 N. However, it is consistent with recent models which suggest HC 5 N may exist for a limited period during the evolution of hot molecular cores.
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    Topics: Physics
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  • 10
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    Interstellar gas towards CTB 37A and the TeV gamma-ray source HESS J1714-385 (2013)
    Oxford University Press
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    Publication Date: 2013-08-22
    Description: Observations of dense molecular gas towards the supernova remnants CTB 37A (G348.5+0.1) and G348.5–0.0 were carried out using the Mopra and Nanten2 radio telescopes. We present CO(2–1) and CS(1–0) emission maps of a region encompassing the CTB 37A TeV gamma-ray emission, HESS J1714–385, revealing regions of dense gas within associated molecular clouds. Some gas displays good overlap with gamma-ray emission, consistent with hadronic gamma-ray emission scenarios. Masses of gas towards the HESS J1714–385 TeV gamma-ray emission region were estimated, and were of the order of 10 3 –10 4 M . In the case of a purely hadronic origin for the gamma-ray emission, the cosmic ray flux enhancement is ~80–1100 times the local solar value. This enhancement factor and other considerations allow a discussion of the age of CTB 37A, which is consistent with ~10 4 yr.
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    Topics: Physics
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