Detailed Spectral Analysis of the Type Ib Supernova 1999dn. Paper I: Hydrogen-free Models

We present spectral fits to five epochs of the typical Type Ib supernova 1999dn using the generalized, non-LTE, stellar atmospheres code PHOENIX. Our goal is threefold: to determine basic physical properties of the supernova ejecta, such as velocity, temperature, and density gradients; to reproduce He I absorption lines by invoking non-thermal excitation; and, to investigate possible spectral signatures of hydrogen, especially a feature around 6200 Angstrom, which has been attributed to high velocity $H_\alpha$. Our models assume an atmosphere with uniform composition devoid of any hydrogen. Our model spectra fit the observed spectra well, successfully reproducing most of the features, including the prominent He I absorptions. The most plausible alternative to $H_\alpha$ as the source of the 6200 Angstrom feature is a blend of Fe II and Si II lines, which can be made stronger to fit the observed feature better by increasing the metallicity of the ejecta. High-metallicity models fit well at early epochs, but not as well as solar-metallicity models after maximum light. While this blend of metal lines is a reasonable explanation of the source of the 6200 Angstrom feature, it is still important to investigate hydrogen as the source; therefore, a second paper will present models that include a thin shell of hydrogen around the main composition structure.Comment: 24 pages, 11 figures, 2 tables, submitted to Ap

Comparative Direct Analysis of Type Ia Supernova Spectra. V. Insights from A Larger Sample and Quantitative Subclassification

A comparative study of optical spectra of Type Ia supernovae (SNe Ia) is extended, in the light of new data. The discussion is framed in terms of the four groups defined in previous papers of this series: core normal (CN); broad line (BL); cool (CL); and shallow silicon (SS). Emerging features of the SN Ia spectroscopic diversity include evidence (1) that extreme CL SN 1991bg-likes are not a physically distinct subgroup and (2) for the existence of a substantial number of SN 1999aa-like SSs that are very similar to each other and distinguishable from CN even as late as three weeks after maximum light. SN 1999aa-likes may be relatively numerous, yet not a physically distinct subgroup. The efficacy of quantitative spectroscopic subclassification of SNe Ia based on the equivalent widths of absorption features near 5750 A and 6100 A near maximum light is discussed. The absolute magnitude dispersion of a small sample of CNs is no larger than the characteristic absolute magnitude uncertainty.Comment: 32 pages including 14 figures and 1 table, accepted by PAS

Type Ia Supernovae: Toward the Standard Model?

In this short review I suggest that recent developments support the conjecture that Type Ia supernovae (SNe Ia) are the complete disruptions of Chandrasekhar-mass carbon-oxygen white dwarfs in single-degenerate binary systems. The causes of the observational diversity of SNe Ia within the context of this standard model, and the implications of the model for young remnants of SNe Ia, are briefly discussed.Comment: 9 pages, text of invited review talk presented at the 11th Annual Astrophysics Conference in Maryland, to appear in Young Supernova Remnants, ed. by S. S. Holt and U. Hwang (AIP

Low Hubble Constant from Type Ia Supernovae by van den Bergh's Method

An interesting way to calibrate the absolute magnitudes of remote Type Ia supernovae (SNe Ia) that are well out in the Hubble flow, and thus determine the value of the Hubble constant, H_0, has been introduced by van den Bergh. His approach relies on calculations of the peak absolute magnitudes and broad--band colors for SN Ia explosion models. It does not require any corrections for extinction by interstellar dust, and no SNe Ia are excluded on grounds of peculiarity. Within the last few years distances have been determined to the parent galaxies of six SNe Ia by means of Cepheid variables. Cepheid--based distances also have become available for three other SNe Ia if one is willing to use the distance to a galaxy in the same group in lieu of the distance to the parent galaxy itself. Here we determine the value of H_0 in a way that is analogous to that of van den Bergh, but now using Cepheid--based distances instead of calculated light curves. We obtain H_0 = 55 km/s/Mpc. This value, with Lambda=0 and Omega=1, corresponds to a cosmic expansion time of 12 Gyr, which is consistent with several recent determinations of the ages of globular clusters.Comment: Latex, 4 pages, 1 table, 1 figure, Submitted to Nature March 28, 1996. PostScript version available at http://www.nhn.ou.edu/~nugent

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

Biracial Children Research Project

The purpose of the Biracial Children Research Project is to determine and develop elementary school social workers\u27 awareness of the unique needs and issues related to biracial children. Sixty-seven Minneapolis elementary school social workers were surveyed in order to assess their awareness level of issues related to African American and White racially mixed children. Issues that were identified by the Biracial Children Research Project dealt with; 1) school and community; 2) psycho-social; 3) racial identity development; 4) racial categorization; and 5) social workers use of referrals to agencies that serve biracial children. The Biracial Children Research Project is a study aimed at adding to the knowledge base of social workers, so that they may more effectively work with this growing population of children with unique needs. The Project\u27s findings indicate that this is area of development for elementary school social workers in the Minneapolis Public School system