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  • 1
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    HCN hyperfine ratio analysis of massive molecular clumps (2016)
    Oxford University Press
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    Publication Date: 2016-12-16
    Description: We report a new analysis protocol for HCN hyperfine data, based on the PySpecKit package, and results of using this new protocol to analyse a sample area of seven massive molecular clumps from the Census of High- and Medium-mass Protostars (CHaMP) survey, in order to derive maps of column density for this species. There is a strong correlation between the HCN integrated intensity, I HCN , and previously reported $I_{\rm HCO^{+}}$ in the clumps, but $I_{\rm N_{2}H^{+}}$ is not well correlated with either of these other two ‘dense gas tracers’. The four fitted parameters from PySpecKit in this region fall in the range of V LSR = 8–10 km s –1 , V = 1.2–2.2 km s –1 , T ex = 4–15 K, and = 0.2–2.5. These parameters allow us to derive a column density map of these clouds, without limiting assumptions about the excitation or opacity. A more traditional (linear) method of converting I HCN to total mass column gives much lower clump masses than our results based on the hyperfine analysis. This is primarily due to areas in the sample region of low I , low T ex , and high . We conclude that there may be more dense gas in these massive clumps not engaged in massive star formation than previously recognized. If this result holds for other clouds in the CHaMP sample, it would have dramatic consequences for the calibration of the Kennicutt–Schmidt star formation laws, including a large increase in the gas depletion time-scale in such regions.
    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.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    The dust properties and physical conditions of the interstellar medium in the LMC massive star-forming complex N11 (2015)
    Oxford University Press
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    Publication Date: 2015-12-26
    Description: We combine Spitzer and Herschel data of the star-forming region N11 in the Large Magellanic Cloud (LMC) to produce detailed maps of the dust properties in the complex and study their variations with the interstellar-medium conditions. We also compare Atacama Pathfinder EXperiment/Large APEX Bolometer Camera (APEX/LABOCA) 870 μm observations with our model predictions in order to decompose the 870 μm emission into dust and non-dust [free–free emission and CO(3–2) line] contributions. We find that in N11, the 870 μm can be fully accounted for by these three components. The dust surface density map of N11 is combined with H  i and CO observations to study local variations in the gas-to-dust mass ratios. Our analysis leads to values lower than those expected from the LMC low-metallicity as well as to a decrease of the gas-to-dust mass ratio with the dust surface density. We explore potential hypotheses that could explain the low ‘observed’ gas-to-dust mass ratios (variations in the X CO factor, presence of CO-dark gas or of optically thick H  i or variations in the dust abundance in the dense regions). We finally decompose the local spectral energy distributions (SEDs) using a principal component analysis (i.e. with no a priori assumption on the dust composition in the complex). Our results lead to a promising decomposition of the local SEDs in various dust components (hot, warm, cold) coherent with that expected for the region. Further analysis on a larger sample of galaxies will follow in order to understand how unique this decomposition is or how it evolves from one environment to another.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    Stochastic non-circular motion and outflows driven by magnetic activity in the Galactic bulge region (2015)
    Oxford University Press
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    Publication Date: 2015-10-19
    Description: By performing a global magnetohydrodynamical simulation for the Milky Way with an axisymmetric gravitational potential, we propose that spatially dependent amplification of magnetic fields possibly explains the observed noncircular motion of the gas in the Galactic centre region. The radial distribution of the rotation frequency in the bulge region is not monotonic in general. The amplification of the magnetic field is enhanced in regions with stronger differential rotation, because magnetorotational instability and field-line stretching are more effective. The strength of the amplified magnetic field reaches 0.5 mG, and radial flows of the gas are excited by the inhomogeneous transport of angular momentum through turbulent magnetic field that is amplified in a spatially dependent manner. In addition, the magnetic pressure-gradient force also drives radial flows in a similar manner. As a result, the simulated position–velocity diagram exhibits a time-dependent asymmetric parallelogram-shape owing to the intermittency of the magnetic turbulence; the present model provides a viable alternative to the bar-potential-driven model for the parallelogram shape of the central molecular zone. This is a natural extension into the central few 100 pc of the magnetic activity, which is observed as molecular loops at radii from a few 100 pc to 1 kpc. Furthermore, the time-averaged net gas flow is directed outward, whereas the flows are highly time dependent, which we discuss from a viewpoint of the outflow from the bulge.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Isolating signatures of major cloud-cloud collisions using position-velocity diagrams (2015)
    Oxford University Press
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    Publication Date: 2015-04-16
    Description: Collisions between giant molecular clouds are a potential mechanism for triggering the formation of massive stars, or even super star clusters. The trouble is identifying this process observationally and distinguishing it from other mechanisms. We produce synthetic position–velocity diagrams from models of cloud–cloud collisions, non-interacting clouds along the line of sight, clouds with internal radiative feedback and a more complex cloud evolving in a galactic disc, to try and identify unique signatures of collision. We find that a broad bridge feature connecting two intensity peaks, spatially correlated but separated in velocity, is a signature of a high-velocity cloud–cloud collision. We show that the broad bridge feature is resilient to the effects of radiative feedback, at least to around 2.5 Myr after the formation of the first massive (ionizing) star. However for a head-on 10 km s –1 collision, we find that this will only be observable from 20 to 30 per cent of viewing angles. Such broad–bridge features have been identified towards M20, a very young region of massive star formation that was concluded to be a site of cloud–cloud collision by Torii et al., and also towards star formation in the outer Milky Way by Izumi et al.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Isolating signatures of major cloud-cloud collisions - II. The lifetimes of broad bridge features (2015)
    Oxford University Press
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    Publication Date: 2015-10-08
    Description: We investigate the longevity of broad bridge features in position–velocity diagrams that appear as a result of cloud–cloud collisions. Broad bridges will have a finite lifetime due to the action of feedback, conversion of gas into stars and the time-scale of the collision. We make a series of analytic arguments with which to estimate these lifetimes. Our simple analytic arguments suggest that for collisions between clouds larger than R  ~ 10 pc the lifetime of the broad bridge is more likely to be determined by the lifetime of the collision rather than the radiative or wind feedback disruption time-scale. However, for smaller clouds feedback becomes much more effective. This is because the radiative feedback time-scale scales with the ionizing flux N ly as $R^{7/4}N_{{\rm ly}}^{-1/4}$ so a reduction in cloud size requires a relatively large decrease in ionizing photons to maintain a given time-scale. We find that our analytic arguments are consistent with new synthetic observations of numerical simulations of cloud–cloud collisions (including star formation and radiative feedback). We also argue that if the number of observable broad bridges remains ~ constant, then the disruption time-scale must be roughly equivalent to the collision rate. If this is the case, our analytic arguments also provide collision rate estimates, which we find are readily consistent with previous theoretical models at the scales they consider (clouds larger than about 10 pc) but are much higher for smaller clouds.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Membrane-induced alteration of the secondary structure in the SWAP-70 pleckstrin homology domain (2012)
    Oxford University Press
    Details
    Publication Date: 2012-04-25
    Description: Differences in the conformation of the pleckstrin homology (PH) domain of switch-associated protein-70 (SWAP-70) in solution and at the lipid bilayer membrane surface were examined using CD, fluorescence and NMR spectroscopy. Intracellular relocalization of SWAP-70 from the cytoplasm to the plasma membrane and then to the nucleus is associated with its cellular functions. The PH domain of SWAP-70 contains a phosphoinositide-binding site and a nuclear localization signal, which localize SWAP-70 to the plasma membrane and nucleus, respectively. CD and fluorescence spectra showed that a significant conformational alteration involving formation of disordered structure occurs when the PH domain binds to d - myo -phosphatidylinositol 3,4,5-trisphosphate or d - myo -phosphatidylinositol 4,5-bisphosphate embedded in lipid bilayer vesicles. NMR spectra indicate that Ala and Trp residues located in the C-terminal α-helix of the PH domain undergo conformational alterations to form a disordered structure at the vesicle surface. These conformational alterations were not induced by association with inositol 1,3,4,5-tetrakisphosphate in solution or coexistence of phosphatidylcholine vesicles. Interaction with the plane of the lipid bilayer via association with the phosphoinositides is required for the unfolding of the C-terminal α-helix of the PH domain. The unwinding of the C-terminal α-helix could regulate the functions of SWAP-70 at the plasma membrane surface.
    Print ISSN: 0021-924X
    Electronic ISSN: 1756-2651
    Topics: Biology , Chemistry and Pharmacology
    Published by Oxford University Press on behalf of The Japanese Biochemical Society (JBS).
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  • 8
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    ISM gas studies towards the TeV PWN HESS J1825-137 and northern region (2016)
    Oxford University Press
    Details
    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.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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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
    Details
    Publication Date: 2013-03-16
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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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.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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