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Guo B.,China Institute of Atomic Energy | Li Z.-H.,China Institute of Atomic Energy | Lugaro M.,Monash University | Buntain J.,Monash University | And 23 more authors.
Yuanzineng Kexue Jishu/Atomic Energy Science and Technology | Year: 2014

13C(α,n)16O is the main neutron source reaction of slow neutron capture (s) process in asymptotic giant branch (AGB) stars. The 6.356 MeV 1/2+ subthreshold resonance in 17O plays an important role in the 13C(α,n)16O reaction. The 13C(11B, 7Li)17O angular distributions were measured using HI-13 tandem accelerator and Q3D magnetic spectrograph, and then used to determine the α-width of the subthreshold resonant state which is currently the most uncertain parameter for determining the 13C(α,n)16O reaction rate. By using the present α-width, the astrophysical S-factor and the stellar rate of the 13C(α,n)16O reaction were derived. This shed some light on the existing discrepancies up to a factor of 25 in S-factors derived from different authors. The simulation of s-process in AGB stars shows that the new reaction rate produces 25% more Pb than the CF88 compilation. Source


Cioni M.-R.L.,University of Hertfordshire | Cioni M.-R.L.,Ludwig Maximilians University of Munich | Clementini G.,National institute for astrophysics | Girardi L.,National institute for astrophysics | And 31 more authors.
Astronomy and Astrophysics | Year: 2011

Context. The new VISual and Infrared Telescope for Astronomy (VISTA) has started operations. Over its first five years it will be collecting data for six public surveys, one of which is the near-infrared YJKs VISTA survey of the Magellanic Clouds system (VMC). This survey comprises the Large Magellanic Cloud (LMC), the Small Magellanic Cloud, the Magellanic Bridge connecting the two galaxies and two fields in the Magellanic Stream. Aims. This paper provides an overview of the VMC survey strategy and presents first science results. The main goals of the VMC survey are the determination of the spatially-resolved star-formation history and the three-dimensional structure of the Magellanic system. The VMC survey is therefore designed to reach stars as faint as the oldest main sequence turn-off point and to constrain the mean magnitude of pulsating variables such as RR Lyrae stars and Cepheids. This paper focuses on observations of VMC fields in the LMC obtained between November 2009 and March 2010. These observations correspond to a completeness of 7% of the planned LMC fields. Methods. The VMC data are comprised of multi-epoch observations which are executed following specific time constraints. The data were reduced using the VISTA Data Flow System pipeline with source catalogues, including astrometric and photometric corrections, produced and made available via the VISTA Science Archive. The VMC data will be released to the astronomical community following the European Southern Observatory's Public Survey policy. The analysis of the data shows that the sensitivity in each wave band agrees with expectations. Uncertainties and completeness of the data are also derived. Results. The first science results, aimed at assessing the scientific quality of the VMC data, include an overview of the distribution of stars in colour-magnitude and colour-colour diagrams, the detection of planetary nebulae and stellar clusters, and the Ks band light-curves of variable stars. Conclusions. The VMC survey represents a tremendous improvement, in spatial resolution and sensitivity, on previous panoramic observations of the Magellanic system in the near-infrared, providing a powerful complement to deep observations at other wavelengths. © 2011 ESO. Source


Vlajic M.,Leibniz Institute for Astrophysics Potsdam | Vlajic M.,University of Sydney | Bland-Hawthorn J.,University of Sydney | Freeman K.C.,Mount Stromlo Observatory
Astrophysical Journal | Year: 2011

Studies of outer regions of spirals disks are fundamental to our understanding of both the process of galaxy assembly and the subsequent secular evolution of galaxies. In an earlier series of papers, we explored the extent and abundance gradient in the outer disk of NGC300 and found an extended purely exponential disk with a metallicity gradient which flattens off in the outermost regions. We now continue the study of outskirts of pure disk spirals with another Sculptor Group spiral, NGC7793. Using the Gemini Multi Object Spectrograph camera at Gemini South, we trace the disk of NGC7793 with star counts out to ∼9 scale lengths, corresponding to 11.5kpc at our calibrated distance of 3.61 ± 0.53Mpc. The outer disk of NGC7793 shows no evidence of a break in its light profile down to an effective surface brightness of ∼30 mag arcsec-2 (∼3 mag arcsec-2 deeper than what has been achieved with surface photometry) and exhibits a non-negative abundance gradient within the radial extent of our data. © 2011. The American Astronomical Society. All rights reserved. Source


Offringa A.R.,University of Groningen | De Bruyn A.G.,University of Groningen | Zaroubi S.,University of Groningen | Van Diepen G.,ASTRON | And 102 more authors.
Astronomy and Astrophysics | Year: 2012

Aims. This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations. Methods. We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30-78 MHz with low-band antennas and 115-163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz/1 s resolution. Results. We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFAR's nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution. Conclusions. Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills. © 2012 ESO. Source


Norris R.P.,CSIRO | Hopkins A.M.,Australian Astronomical Observatory | Afonso J.,University of Lisbon | Brown S.,CSIRO | And 62 more authors.
Publications of the Astronomical Society of Australia | Year: 2011

EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The primary goal of EMU is to make a deep (rms ∼10Jy/beam) radio continuum survey of the entire Southern sky at 1.3GHz, extending as far North as +30 declination, with a resolution of 10arcsec. EMU is expected to detect and catalogue about 70million galaxies, including typical star-forming galaxies up to z∼1, powerful starbursts to even greater redshifts, and active galactic nuclei to the edge of the visible Universe. It will undoubtedly discover new classes of object. This paper defines the science goals and parameters of the survey, and describes the development of techniques necessary to maximise the science return from EMU. © Astronomical Society of Australia 2011. Source

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