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Rubin R.H.,NASA | Simpson J.P.,Search for Extraterrestrial Intelligence Institute | Colgan S.W.J.,NASA | Dufour R.J.,Rice University | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2016

Using the short-high module of the Infrared Spectrograph on the Spitzer Space Telescope, we have measured the [S IV] 10.51, [Ne II] 12.81, [Ne III] 15.56, and [S III] 18.71-μm emission lines in nine HII regions in the dwarf irregular galaxy NGC 6822. These lines arise from the dominant ionization states of the elements neon (Ne++, Ne+) and sulphur (S3+, S++), thereby allowing an analysis of the neon to sulphur abundance ratio as well as the ionic abundance ratios Ne+/Ne++ and S3+/S++. By extending our studies of HII regions in M83 and M33 to the lower metallicity NGC 6822, we increase the reliability of the estimated Ne/S ratio. We find that the Ne/S ratio appears to be fairly universal, with not much variation about the ratio found for NGC 6822: the median (average) Ne/S ratio equals 11.6 (12.2±0.8). This value is in contrast to Asplund et al.'s currently best estimated value for the Sun: Ne/S = 6.5. In addition, we continue to test the predicted ionizing spectral energy distributions (SEDs) from various stellar atmosphere models by comparing model nebulae computed with these SEDs as inputs to our observational data, changing just the stellar atmosphere model abundances. Here, we employ a new grid of SEDs computed with different metallicities: solar, 0.4 solar, and 0.1 solar. As expected, these changes to the SED show similar trends to those seen upon changing just the nebular gas metallicities in our plasma simulations: lower metallicity results in higher ionization. This trend agrees with the observations. © 2016 The Authors. Source


Dilday B.,Rutgers University | Dilday B.,University of Chicago | Smith M.,University of Cape Town | Smith M.,University of Portsmouth | And 40 more authors.
Astrophysical Journal | Year: 2010

We present a measurement of the volumetric TypeIa supernova (SN Ia) rate based on data from the Sloan Digital Sky Survey II (SDSS-II) Supernova Survey. The adopted sample of supernovae (SNe) includes 516 SNe Ia at redshift z ≲ 0.3, of which 270(52%) are spectroscopically identified as SNe Ia. The remaining 246 SNe Ia were identified through their light curves; 113 of these objects have spectroscopic redshifts from spectra of their host galaxy, and 133 have photometric redshifts estimated from the SN light curves. Based on consideration of 87 spectroscopically confirmed non-Ia SNe discovered by the SDSS-II SN Survey, we estimate that 2.04+1.61 -0.95% of the photometric SNeIa may be misidentified. The sample of SNe Ia used in this measurement represents an order of magnitude increase in the statistics for SN Ia rate measurements in the redshift range covered by the SDSS-II Supernova Survey. If we assume an SN Ia rate that is constant at low redshift (z < 0.15), then the SN observations can be used to infer a value of the SN rate of rV= (2.69+0.34+0.21 -0.30-0.01)×10-5SNe yr-1Mpc-3(H0/(70kms-1Mpc-1))3at a mean redshift of 0.12, based on 79 SNe Ia of which 72 are spectroscopically confirmed. However, the large sample of SNe Ia included in this study allows us to place constraints on the redshift dependence of the SN Ia rate based on the SDSS-II Supernova Survey data alone. Fitting a power-law model of the SN rate evolution, rV(z) = Ap× ((1 + z)/(1 + z0))ν, over the redshift range 0.0 < z < 0.3 with z0= 0.21, results in Ap= (3.43+0.15 -0.15) × 10-5SNe yr-1Mpc-3(H0/(70kms-1Mpc-1))3and ν = 2.04+0.90 -0.89. © 2010. The American Astronomical Society. All rights reserved.. Source

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