2,212 research outputs found
A survey of UV-excess AGNs in the South Galactic Pole
Spectra, position, magnitudes and colors are presented for 485 faint (B<20.5)
emission line objects selected with the ultraviolet-excess (UVX) criterion on a
area of 24.6 sq. deg in the South Galactic Pole. The objects were selected from
the analysis of pixel-to-pixel stacking of COSMOS scans of UKST U, J and R
plates. The candidates were observed with the Meudon-ESO Fiber Optics System
(MEFOS) at the ESO 3.6m telescope. 429 type 1 AGNs have been identified (373 in
the redshift range 0.3<z<2.2). This sample has allowed the measure of a
difference on the QSO clustering evolution in comparison with that found for
galaxies (La Franca et al 1998). The region is part of the ESO Imaging Survey
(EIS) and of the 2dF QSO redshift survey.Comment: 32 pages, 19 figures. To appear on A&AS, revised after minor comment
Dust from AGBs: relevant factors and modelling uncertainties
The dust formation process in the winds of Asymptotic Giant Branch stars is
discussed, based on full evolutionary models of stars with mass in the range
MMM, and metallicities .
Dust grains are assumed to form in an isotropically expanding wind, by growth
of pre--existing seed nuclei. Convection, for what concerns the treatment of
convective borders and the efficiency of the schematization adopted, turns out
to be the physical ingredient used to calculate the evolutionary sequences with
the highest impact on the results obtained. Low--mass stars with MM produce carbon type dust with also traces of silicon carbide. The
mass of solid carbon formed, fairly independently of metallicity, ranges from a
few M, for stars of initial mass M, to
M for MM; the size of dust
particles is in the range mm. On the contrary,
the production of silicon carbide (SiC) depends on metallicity. For the size of SiC grains varies in the range m, while the mass of SiC formed is
. Models of
higher mass experience Hot Bottom Burning, which prevents the formation of
carbon stars, and favours the formation of silicates and corundum. In this case
the results scale with metallicity, owing to the larger silicon and aluminium
contained in higher--Z models. At Z= we find that the most
massive stars produce dust masses M, whereas models of
smaller mass produce a dust mass ten times smaller. The main component of dust
are silicates, although corundum is also formed, in not negligible quantities
().Comment: Paper accepted for publication in Monthly Notices of the Royal
Astronomical Society Main Journal (2014 January 4
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