Resolved OH megamaser emission in the nuclear region of the ultraluminous infrared galaxy Markarian 273     

J. A. Yates  A. M. S. Richards  M. M. Wright  J. L. Collett  M. D. Gray  D. Field  R. J. Cohen 《Monthly notices of the Royal Astronomical Society》2000,317(1):28-36

We present high angular resolution MERLIN observations of the 18-cm OH maser and continuum emission associated with the active core of the ultraluminous infrared galaxy Markarian 273. The continuum emission comes from three distinct regions in the central arcsecond of the galaxy. The brightest region of emission has a double-peaked structure which is spatially coincident with similar structures observed at 6 cm and 2.2 μm. The peak of the OH maser emission is spatially coincident with the peak in the continuum. For the first time the maser emission is spatially resolved, allowing us to measure the gas motion within the central 100 pc of the galaxy. Maser emission is found in both the 1665- and 1667-MHz lines, with no systematic offset found in the spatial locations of the two lines. The brighter component of the maser emission shows ordered motion and is aligned along the axis of the double-peaked structure in the brightest continuum region. The gas motion enables us to estimate the central mass density to be 850±50 M_⊙ pc⁻³, which corresponds to a total mass of ≈1.5×10⁸ M_⊙.  相似文献   

Polarization models of young stellar objects – II. Linear and circular polarimetry of R Coronae Australis     

Stuart Clark  Alan McCall  Antonio Chrysostomou  Tim Gledhill  Jeremy Yates  James Hough 《Monthly notices of the Royal Astronomical Society》2000,319(2):337-349

Near-infrared linear imaging polarimetry of the young stellar objects R CrA and T CrA in the J , H and K n bands, and circular imaging polarimetry in the H band, is presented. The data are modelled with the Clark and McCall scattering model. The R CrA and T CrA system is shown to be a particularly complex scattering environment. In the case of R CrA there is evidence that the wavelength dependence of polarization changes across the nebula. MRN dust grain models do not explain this behaviour. Depolarization by line emission is considered as an alternative explanation. The dust grain properties could also be changing across the nebula.
Although surrounded by reflection nebulosity, there is a region of particularly low polarization surrounding R CrA that is best modelled by the canonical bipolar outflow being truncated by an evacuated spherical cavity surrounding the star. The symmetry axis of the nebula appears inclined by 50° to the plane of the sky.
The H -band circular polarimetry of R CrA clearly shows a quadrupolar structure of positive and negative degrees of circular polarization that reach peak magnitudes of ∼5 per cent within our limited map. It is shown that spherical MRN grains are incapable of producing this circular polarization given the observed linear polarization of the R CrA system. Instead, scattering from aligned non-spherical grains is proposed as the operating mechanism.
T CrA is a more archetypical bipolar reflection nebula, and this object is modelled as a canonical parabolic reflection nebula that lies in the plane of the sky. The wavelength independence of linear polarization in the T CrA reflection nebula suggests that the scattering particles are Rayleigh sized. This is modelled with the MRN interstellar grain size distribution.  相似文献   

Assessing the role of alkaline soils on the carbon cycle at a playa site     

Emma L. Yates  Angela M. Detweiler  Laura T. Iraci  Brad M. Bebout  Christopher P. McKay  Kathleen Schiro  Edwin J. Sheffner  Cheryl A. Kelley  Jovan M. Tadić  Max Loewenstein 《Environmental Earth Sciences》2013,70(3):1047-1056

Alkaline soils occupy approximately 5 % of the Earth’s land surface (7 million km), and this may increase due to the global trend towards increasing desertification, yet the extent to which these soils modulate carbon dynamics on regional and global scales is inadequately studied and poorly understood. Railroad Valley (RRV) playa (Nevada, USA) is a semi-arid playa with highly alkaline soils (pH > 10) and no vegetation. The extreme, alkaline environment and absence of vascular vegetation make RRV an ideal site to investigate the role of physiochemical processes of soil-atmosphere CO₂ exchange. Both field and laboratory investigations were conducted. This work shows how the atmospheric CO₂ mixing ratio decreases at nighttime at RRV playa to a value well below the average global background CO₂ concentration. Laboratory investigations using soil samples collected at RRV playa confirmed that CO₂ uptake by RRV playa soils occurs when temperatures are decreased. Both field and laboratory studies suggest that the alkaline RRV soil acts as a CO₂ reservoir during colder periods, such as at nighttime. These results highlight the importance of investigating carbon dynamics in previously understudied environments. Given how little information is available on the CO₂ flux in desert and semi-arid alkaline ecosystems lacking vegetation, our findings draw attention to these environments as becoming increasingly important for carbon fluxes on regional and global scales.  相似文献   

MERLIN polarimetry of the OH masers in OH17.7–2.0     

I. Bains  T. M. Gledhill  J. A. Yates  A. M. S. Richards 《Monthly notices of the Royal Astronomical Society》2003,338(2):287-302

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Sub-au imaging of water vapour clouds around four asymptotic giant branch stars     

I. Bains  R. J. Cohen  A. Louridas  A. M. S. Richards  D. Rosa-González  J. A. Yates 《Monthly notices of the Royal Astronomical Society》2003,342(1):8-32

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OH maser mapping of the evolved star HD 179821: evidence for interacting outflows     

T.M. Gledhill  J.A. Yates  A.M.S. Richards 《Monthly notices of the Royal Astronomical Society》2001,328(1):301-310

The evolved star HD 179821 continues to be the subject of much debate as to whether it is a nearby     post-asymptotic giant branch (post-AGB) star or a distant     high initial mass     post-red supergiant. We have mapped the OH maser emission around HD 179821 in the 1612- and 1667-MHz lines with the MERLIN interferometer array at a resolution of 0.4 arcsec and 0.35 km s⁻¹. The OH emission lies in a thick shell with inner and outer radii of 1.3 and         and expansion velocity of 30 km s⁻¹. Although we find some evidence for acceleration and for deviations from spherical symmetry, the bulk of the maser emission is consistent with a constant-velocity spherical shell. The extent of the shell agrees with H₂O and OH dissociation models and supports a distance estimate of 6 kpc. However, the shell is incomplete and appears to have been disrupted by more recent collimated outflow activity within the last 1500 yr. We suggest that this activity is also responsible for the active envelope chemistry (in particular the presence of HCO⁺) and for the apparent offset of the star from the centre of the shell. The luminous yellow hypergiant star IRC +10420 also shows signs of recent outflows, and HD 179821 may be at a similar, perhaps slightly earlier, phase of evolution. We suggest that the SiO thermal emission arises from the same detached envelope as the OH maser emission as in IRC +10420. If so then this would strengthen the connection between these two stars and probably rule out a post-AGB status for HD 179821.  相似文献   

Co-propagation of maser emission at 1720 and 4765 MHz     

M. D. Gray  R. J. Cohen  A. M. S. Richards  J. A. Yates  D. Field 《Monthly notices of the Royal Astronomical Society》2001,324(3):643-647

MERLIN observations are presented of OH 4765-MHz and OH 1720-MHz masers in the massive star-forming region W3(OH). Two of the three intense spots of maser emission at 4765 MHz are spatially coincident with two similar spots at 1720 MHz in both left-hand circular (LHC) and right-hand circular (RHC) polarizations, to an accuracy of 15 mas. The spots also overlap in velocity when allowance is made for Zeeman splitting of the 1720-MHz line. We conclude that we have found two examples of masers in different rotational levels of OH which are co-propagating through the same column of gas and experiencing competitive gain effects. The third 4765-MHz maser spot was found to have no overlapping counterpart amongst the 1720-MHz masers.  相似文献   

Assessing stone degradation using an integrated database and geographical information system (GIS)     

Rob Inkpen  Bill Duane  John Burdett  Tim Yates 《Environmental Geology》2008,56(3-4):789-801

An integrated database and geographical information system (GIS) for the recording and monitoring of stone degradation are outlined. GIS requirements for stone degradation are summarized and a simple classification system for weathering forms identified and applied. The potential use of the system for identifying change in weathering forms using historic imagery is illustrated using an example from Oxford. Combining information from imagery of different buildings at different time periods, it is possible to put forward a possible scenario of weathering form evolution.  相似文献   

Sharks caught in the KwaZulu-Natal bather protection programme,South Africa. 13. The tiger shark Galeocerdo cuvier     

ML Dicken  G Cliff  H Winker 《African Journal of Marine Science》2016,38(3):285-301

The current study provides long-term catch rate and biological data for tiger sharks Galeocerdo cuvier caught in the KwaZulu-Natal bather protection programme. Between 1978 and 2014, 1 760 G. cuvier were caught in nets and between 2007 and 2014, 108 G. cuvier were caught on drumlines. Standardised catch rates increased significantly over time (p < 0.001) for both small (≤180 cm precaudal length, PCL) and large sharks (>180 cm). There was also a significant temporal increase in the mean size of sharks across gear types (p < 0.001). A quasi-Poisson generalised additive mixed model showed that the deployment of drumlines had no significant effect on the catch rate of sharks in nets. The nets, however, caught significantly larger sharks (mean 184.5 cm, SD 39) than did drumlines (mean 138.6 cm, SD 36.5; p < 0.001). The size frequency of the catch was unimodal and females significantly outnumbered males in both gear types. Few young-of-the-year (0.8%) or mature sharks (1.8%) were caught. Only 23 (4.7%) of the 486 sharks tagged and released were recaptured, with the majority (87.0%) of those recaptured <150 km from their original tagging locality. The results from this study suggest an increasing local population trend in G. cuvier along the KwaZulu-Natal coast.  相似文献   

Geographic characteristics of a network interdiction problem     

Irene Casas  Eric Delmelle  Justin Yates 《GeoJournal》2016,81(1):37-53

The protection of critical infrastructure from natural and intentional events is a key component of any national security agenda. Protection schemes need to be readily identifiable and adaptable to complex changing environments. In this paper, we identify strategic geographic characteristics that impact the location of detection resources (e.g. sensors) towards the defense of regional critical infrastructure. Specifically, we seek to estimate the relationship between the results of a variation of the traditional shortest path network interdiction problem and geographical characteristics of the transportation infrastructure and the urban environment. Experiments conducted on three distinct transportation networks of different shapes and granularities (New York City—grid, Houston—radial, Boston—hybrid) underline the importance of geographic characteristics such as the proximity to resource location, attacker entry points as well as network coverage. Insights gained from this work are relevant to policy and decision makers to facilitate the development of analytical and decision-support tools capable of identifying resource allocation strategies. We discuss a heuristic-based framework that prioritizes the selection of detection resources, reflecting the importance of geographic characteristics. The findings underline the importance of geographical characteristics for the allocation of resources in a regional setting.  相似文献