1,564 research outputs found

    Panel: Cricket or Baseball?

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    Characterisation of the Mopra Radio Telescope at 16--50 GHz

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    We present the results of a programme of scanning and mapping observations of astronomical masers and Jupiter designed to characterise the performance of the Mopra Radio Telescope at frequencies between 16-50 GHz using the 12-mm and 7-mm receivers. We use these observations to determine the telescope beam size, beam shape and overall telescope beam efficiency as a function of frequency. We find that the beam size is well fit by λ\lambda/DD over the frequency range with a correlation coefficient of ~90%. We determine the telescope main beam efficiencies are between ~48-64% for the 12-mm receiver and reasonably flat at ~50% for the 7-mm receiver. Beam maps of strong H2_2O (22 GHz) and SiO masers (43 GHz) provide a means to examine the radial beam pattern of the telescope. At both frequencies the radial beam pattern reveals the presence of three components, a central `core', which is well fit by a Gaussian and constitutes the telescopes main beam, and inner and outer error beams. At both frequencies the inner and outer error beams extend out to approximately 2 and 3.4 times the full-width half maximum of the main beam respectively. Sources with angular sizes a factor of two or more larger than the telescope main beam will couple to the main and error beams, and therefore the power contributed by the error beams needs to be considered. From measurements of the radial beam power pattern we estimate the amount of power contained in the inner and outer error beams is of order one-fifth at 22 GHz rising slightly to one-third at 43 GHz.Comment: Accepted for publication in PAS

    ATLASGAL - environments of 6.7 GHz methanol masers

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    Using the 870 micron APEX Telescope large area survey of the Galaxy, we have identified 577 submillimetre continuum sources with masers from the methanol multibeam survey in the region 280deg lt ell lt 20deg; |b| lt 1deg.5 94 per cent of methanol masers in the region are associated with submillimetre dust emission. We estimate masses for tilde450 maser-associated sources and find that methanol masers are preferentially associated with massive clumps. These clumps are centrally condensed, with envelope structures that appear to be scale-free, the mean maser position being offset from the peak column density by 0 plusmn 4 arcsec. Assuming a Kroupa initial mass function and a star formation efficiency of tilde30 per cent, we find that over two-thirds of the clumps are likely to form clusters with masses gt20 M?_?. Furthermore, almost all clumps satisfy the empirical mass-size criterion for massive star formation. Bolometric luminosities taken from the literature for tilde100 clumps range between tilde100 and 106^6 L?_?. This confirms the link between methanol masers and massive young stars for 90 per cent of our sample. The Galactic distribution of sources suggests that the star formation efficiency is significantly reduced in the Galactic Centre region, compared to the rest of the survey area, where it is broadly constant, and shows a significant drop in the massive star formation rate density in the outer Galaxy. We find no enhancement in source counts towards the southern Scutum-Centaurus arm tangent at ell tilde 315deg, which suggests that this arm is not actively forming stars.Peer reviewe

    The RMS Survey: Ammonia and water maser analysis of massive star forming regions

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    The Red MSX Source (RMS) survey has identified a sample of ~1200 massive young stellar objects (MYSOs), compact and ultra compact HII regions from a sample of ~2000 MSX and 2MASS colour selected sources. We have used the 100 m Green Bank telescope to search for 22-24 GHz water maser and ammonia (1,1), (2,2) and (3,3) emission towards ~600 RMS sources located within the northern Galactic plane. We have identified 308 H2O masers which corresponds to an overall detection rate of ~50%. Abridged: We detect ammonia emission towards 479 of these massive young stars, which corresponds to ~80%. Ammonia is an excellent probe of high density gas allowing us to measure key parameters such as gas temperatures, opacities, and column densities, as well as providing an insight into the gas kinematics. The average kinetic temperature, FWHM line width and total NH3 column density for the sample are approximately 22 K, 2 km/s and 2x10^{15} cm^{-2}, respectively. We find that the NH3 (1,1) line width and kinetic temperature are correlated with luminosity and finding no underlying dependence of these parameters on the evolutionary phase of the embedded sources, we conclude that the observed trends in the derived parameters are more likely to be due to the energy output of the central source and/or the line width-clump mass relationship. The velocities of the peak H2O masers and the NH3 emission are in excellent agreement with each other, which would strongly suggest an association between the dense gas and the maser emission. Moreover, we find the bolometric luminosity of the embedded source and the isotropic luminosity of the H2O maser are also correlated. We conclude from the correlations of the cloud and water maser velocities and the bolometric and maser luminosity that there is a strong dynamical relationship between the embedded young massive star and the H2O maser.Comment: 17 pages and 17 figures and 8 tables. Tables\,2 and 5 and full versions of Figs. 3 and 7 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A

    Introduction to a Biological Systems Science

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    Biological systems analysis and biodynamic modelling of physiological and biological interrelationships in human body and mammal

    The statistics of triggered star formation: an overdensity of massive YSOs around Spitzer bubbles

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    We present a detailed statistical study of massive star formation in the environment of 322 Spitzer mid-infrared bubbles by using the RMS survey for massive Young Stellar Objects (YSOs). Using a combination of simple surface density plots and a more sophisticated angular cross-correlation function analysis we show that there is a statistically significant overdensity of RMS YSOs towards the bubbles. There is a clear peak in the surface density and angular cross-correlation function of YSOs projected against the rim of the bubbles. By investigating the autocorrelation function of the RMS YSOs we show that this is not due to intrinsic clustering of the RMS YSO sample. RMS YSOs and Spitzer bubbles are essentially uncorrelated with each other beyond a normalised angular distance of two bubble radii. The bubbles associated with RMS YSOs tend to be both smaller and thinner than those that are not associated with YSOs. We interpret this tendency to be due to an age effect, with YSOs being preferentially found around smaller and younger bubbles. We find no evidence to suggest that the YSOs associated with the bubbles are any more luminous than the rest of the RMS YSO population, which suggests that the triggering process does not produce a top heavy luminosity function or initial mass function. We suggest that it is likely that the YSOs were triggered by the expansion of the bubbles and estimate that the fraction of massive stars in the Milky Way formed by this process could be between 14 and 30%.Comment: 12 pages, 8 figures. Accepted by MNRAS. This version incorporates minor suggestions by the referee. A version with higher resolution Figure 1 is available upon reques

    ATLASGAL-selected massive clumps in the inner Galaxy: VI. Kinetic temperature and spatial density measured with formaldehyde

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    We aim to directly determine the kinetic temperature and spatial density with formaldehyde for the ∼\sim100 brightest ATLASGAL-selected clumps at 870 μ\mum representing various evolutionary stages of high-mass star formation. Ten transitions (JJ = 3-2 and 4-3) of ortho- and para-H2_2CO near 211, 218, 225, and 291 GHz were observed with the APEX 12 m telescope. Using non-LTE models with RADEX, we derive the gas kinetic temperature and spatial density using the measured p-H2_2CO 321_{21}-220_{20}/303_{03}-202_{02}, 422_{22}-321_{21}/404_{04}-303_{03}, and 404_{04}-303_{03}/303_{03}-202_{02} ratios. The gas kinetic temperatures derived from the p-H2_2CO 321_{21}-220_{20}/303_{03}-202_{02} and 422_{22}-321_{21}/404_{04}-303_{03} line ratios are high, ranging from 43 to >>300 K with an unweighted average of 91 ±\pm 4 K. Deduced TkinT_{\rm kin} values from the JJ = 3-2 and 4-3 transitions are similar. Spatial densities of the gas derived from the p-H2_2CO 404_{04}-303_{03}/303_{03}-202_{02} line ratios yield 0.6-8.3 ×\times 106^6 cm−3^{-3} with an unweighted average of 1.5 (±\pm0.1) ×\times 106^6 cm−3^{-3}. A comparison of kinetic temperatures derived from p-H2_2CO, NH3_3, and the dust emission indicates that p-H2_2CO traces a distinctly higher temperature than the NH3_3 (2,2)/(1,1) transitions and the dust, tracing heated gas more directly associated with the star formation process. The H2_2CO linewidths are found to be correlated with bolometric luminosity and increase with the evolutionary stage of the clumps, which suggests that higher luminosities tend to be associated with a more turbulent molecular medium. It seems that the spatial densities measured with H2_2CO do not vary significantly with the evolutionary stage of the clumps. However, averaged gas kinetic temperatures derived from H2_2CO increase with time through the evolution of the clumps.Comment: Accepted for publication in A&
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