11,971 research outputs found
Hydrogen in Type Ic Supernovae?
By definition, a Type Ic supernova (SN Ic) does not have conspicuous lines of
hydrogen or helium in its optical spectrum. SNe Ic usually are modelled in
terms of the gravitational collapse of bare carbon-oxygen cores. We consider
the possibility that the spectra of ordinary (SN 1994I-like) SNe Ic have been
misinterpreted, and that SNe Ic eject hydrogen. An absorption feature usually
attributed to a blend of Si II 6355 and C II 6580 may be produced by H-alpha.
If SN 1994I-like SNe Ic eject hydrogen, the possibility that hypernova (SN
1998bw-like) SNe Ic, some of which are associated with gamma-ray bursts, also
eject hydrogen should be considered. The implications of hydrogen for SN Ic
progenitors and explosion models are briefly discussed.Comment: Accepted by PASP. Several significant changes including one
additional figur
Submillimeter Atmospheric Transparency at Maunakea, at the South Pole, and at Chajnantor
For a systematic assessment of submillimeter observing conditions at
different sites, we constructed tipping radiometers to measure the broad band
atmospheric transparency in the window around 350 m wavelength. The
tippers were deployed on Maunakea, Hawaii, at the South Pole, and in the
vicinity of Cerro Chajnantor in northern Chile. Identical instruments permit
direct comparison of these sites. Observing conditions at the South Pole and in
the Chajnantor area are better than on Maunakea. Simultaneous measurements with
two tippers demonstrate conditions at the summit of Cerro Chajnantor are
significantly better than on the Chajnantor plateau.Comment: Accepted by PAS
Recommended from our members
Cousins Photometry and Temperatures for the Hyades, Coma, NGC 752, Praesepe, and M67
In this paper, new Cousins VRI data are presented for NGC 752 and Praesepe, and new and extant data are combined into an augmented database for M67. For those three clusters, catalogs containing Cousins VRI photometry, reddening-corrected values of (V -K)(J), and temperatures are produced. The same is done for Coma by using both previously published and newly derived Cousins photometry. An extant set of catalogs for the Hyades is updated to include V magnitudes and values of (R -I)(C) that were published after the original catalogs appeared. Finally, M67 V magnitudes published previously by Sandquist are corrected for an effect that depends on location on the face of the cluster. The corrected data and values of (V -I)(C) given by Sandquist are then set out in a supplementary catalog. Data files containing all of these catalogs are deposited in the CDS archives. To assess the quality of the data in the catalogs, the consistency of extant Cousins VRI databases is tested by performing analyses with the following features: (1) quantities as small as a few millimags are regarded as meaningful; (2) statistical analysis is applied; (3) no use is made of data other than VRI measurements and comparable results; (4) no inferences are drawn from color-magnitude comparisons; (5) pertinent data that have not been included previously are analyzed; and (6) results based on direct comparisons of stellar groups at the telescope are featured. In this way, it is found that our updated M67 color data and those of Sandquist are on the E region zero point. In contrast, values of (V -I)(C) from Montgomery and collaborators are found to be too red by 27 +/- 3 mmag, with an even larger offset being likely for unpublished data from Richer and his collaborators. Zero-point tests of our Cousins VRI colors for Coma, Praesepe, and NGC 752 are also satisfactory. Scale factor tests of the M67 colors are performed, and a likely scale factor error in the Montgomery et al. colors is found. However, it appears at present that the scale factors of our M67 colors and those of Sandquist are satisfactory. For the most part, zero-point tests of the assembled V magnitudes are also satisfactory, although it is found that further work on the V magnitudes for Praesepe and M67 would be useful. To put these results in perspective, it is pointed out that photometric tests that are satisfactory at the few-millimag level have been published for some two decades and so are not appearing for the first time in this paper.Astronom
Industry mix and lending environment variability: what does the average bank face
Diversification opportunities for banks may be greater today because of the lessening of geographic restrictions. In addition, regional economies have undergone vast transformations, with relatively volatile industries often assuming a diminished role. To assess whether these changes have resulted in a more stable lending environment, Jeff Gunther and Ken Robinson form industry portfolios for banks based on their presence in different states and the mix of economic activity found in those states. The authors find that the risk underlying banks' lending environments declined from 1985 to 1996 because of both a geographic restructuring of the banking system and increasing industrial diversification of state economies.Banks and banking ; Financial institutions
Intercomparison of soil pore water extraction methods for stable isotope analysis
Funded by NSERC Discovery Grant U.S. Forest Service U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies OfficePeer reviewedPostprin
Supernova Resonance-Scattering Profiles in the Presence of External Illumination
We discuss a simple model for the formation of a supernova spectral line by
resonance scattering in the presence of external illumination of the
line-forming region by light from circumstellar interaction (toplighting). The
simple model provides a clear understanding of the most conspicuous toplighting
effect: a rescaling or, as we prefer, a ``muting'' of the line profile relative
to the continuum. This effect would be present in more realistic models, but
would be harder to isolate. An analytic expression for a muting factor for a
P-Cygni line is derived that depends on the ratio E of the toplighting specific
intensity to the specific intensity from the supernova photosphere. If E<1, the
line profile is reduced in scale or ``muted''. If E=1, the line profile
vanishes altogether. If E>1, the line profile flips vertically: then having an
absorption component near the observer-frame line center wavelength and a
blueshifted emission component.Comment: accepted for publication in PAS
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