Lisa A. Wells

Constraints on Cosmological Models from Hubble Space Telescope Observations of High-z Supernovae

We have coordinated Hubble Space Telescope (HST) photometry with ground-based discovery for three supernovae: Type Ia supernovae near z ≈ 0.5 (SN 1997ce, SN 1997cj) and a third event at z = 0.97 (SN 1997ck). The superb spatial resolution of HST separates each supernova from its host galaxy and leads to good precision in the light curves. We use these light curves and relations between luminosity, light-curve shape, and color calibrated from low-z samples to derive relative luminosity distances that are accurate to 10% at z ≈ 0.5 and 20% at z = 1. When the HST sample is combined with the distance to SN 1995K (z = 0.48), analyzed by the same precepts, we find that matter alone is insufficient to produce a flat universe. Specifically, for Ωm+ΩΛ = 1, Ωm is less than 1 with more than 95% confidence, and our best estimate of Ωm is -0.1±0.5 if ΩΛ = 0. Although this result is based on a very small sample whose systematics remain to be explored, it demonstrates the power of HST measurements for high-redshift supernovae.

SN 1991bg - A type Ia supernova with a difference

We present 13 spectra and 31 photometric observations covering the first 150 days of SN 1991bg in NGC 4374 (M 84). Although SN 1991bg was a type Ia supernova displaying the characteristic Si II absorption at 6150 A near maximum and the Fe emission lines at late phases, it varied from the well-defined norm for SNe Ia in several important respects. The peculiarities include faster declines in the B and V light curves after maximum, a distinct color evolution, a very red B−V color near maximum, relatively faint peak luminosity, a distinct spectral evolution, and a short peak phase

SN 1991T : further evidence of the heterogeneous nature of type Ia supernovae

Optical spectra of SN 1991T obtained approximately one week before B maximum showed no evidence of the Si, Ca, and S absorption lines which normally dominate the spectra of type Ia supernovae at early phases. Nevertheless, within two weeks after maximum, the spectrum of SN 1991T had evolved to resemble closely that of other type Ia events. These observations suggest that the abundances of Si, Ca, and S in the outer ejecta of SN 1991T were unusually low. The B and V light curves of SN 1991T resembled those of other type Ia supernovae, but the slope of the initial decline following maximum was less steep than normal in both colors

The type Ia supernova 1989B in NGC 3627 (M66)

We report extensive optical photometry and spectroscopy of the Type Ia supernova 1989B. Maximum light in B occurred approximately seven days after discovery on JD 2447565.3 +/- 1.0 (1989 February 7.8 +/- 1.0) at a magnitude of 12.34 +/- 0.05. The UBV light curves of this supernova were very similar to those of other well observed Type Ia events such as SN 1981B and SN 1980N. From a comparison of the UBVRIJHK photometry, we derive an extinction for SN 1989B of E(B-V) = 0.37 +/- 0.03 mags relative to the unobscured Type Ia SN 1980N. The properties of the dust responsible for the reddening of SN 1989B appear to have been similar to those of normal dust in the Milky Way. In particular, we find no evidence for an unusually low value of the ratio of the total to selective absorption. We derive a distance modulus of delta mu(sub 0) = -1.62 +/- 0.03 mag relative to the Type Ia SN 1980N. We present optical spectra which provide essentially continuous coverage of the spectral evolution of SN 1989B over the first month following B maximum. These data show the transition from the maximum-light spectrum, in which lines of elements such as Ca, Si, S, Mg, and O are most prominent, to the Fe-dominated spectrum observed a few weeks after maximum. This transition occurred quite smoothly over a two-week period following B maximum. Comparison of the spectra of SN 1989B with data for two other well observed Type Ia supernovae -- 1981B and 1986G -- reveals subtle differences in the relative strengths of the S II and Si II absorption lines at maximum light. However, these differences disappeared within a week or so after maximum with the onset of the Fe-dominated phase.

The evolution and classification of postoutburst novae spectra

The first results are presented of a long-term CTIO study of the postoutburst spectra of novae, obtained with CCDs over the wavelength interval 3200-1 microns. For certain novae, the temporal coverage has been sufficiently complete that successive stages are observed in the spectral evolution, suggesting a method by which the spectra may be classified. A classification system is proposed which defines phases in evolution in terms of the characteristics of the stronger emission lines. These phases are related to physical conditions in the shell through the ionizing radiation field and mean gas density. Reliable abundance determinations of nova ejecta must incorporate condensations in the analyses. The observations also indicate that the spectral evolution of neon novae may differ from that of other novae in not developing coronal emission lines. 12 refs.

SN 1991T: Reflections of past glory

We have obtained photometry and spectra of SN 1991T which extend more than 1000 days past maximum light, by far the longest of SN Ia has been followed. Although SN 1991T exhibited nearly photometric behavior in the first 400 days following maximum, by 600 days its decline had slowed, and by 950 days the supernova brightness was consistent with a constant apparent magnitude of m(sub B) = 21.30. Spectra near maximum showed minor variations on the SN Ia theme which grew less conspicuous during the exponential decline. At 270 days the nebular spectrum was composed of Fe and Co lines common to SN Ia. However, by 750 days past maximum light, these lines had shifted in wavelength, and were superposed on a strong blue continuum. The luminosity of SN 1991T at 950 days is more than (9.0 x 10(exp 38)(D/13 Mpc)(2) ergs/s with a rate of decline of less than 0.04 mag per 100 days. We show that this emission is likely to be light that was emitted by SN 1991T near maximum light which has reflected from foreground dust, much like the light echos observed around SN 1987A.

The 1990 Calán/Tololo supernova search

We have started a search for supernovae as a collaboration between the University of Chile and the Cerro Tololo Inter-American Observatory, with the aim of producing a moderately distant (0.01<z<0.10) sample of Type Ia and Type II supernovae suitable for cosmological studies. The project began in mid-1990 and continues to the present. This paper reports on the Calan/Tololo discoveries in the course of 1990, and on the spectroscopic and photometric observations gathered for these objects. All of these observations were obtained with CCDs, with the extensive collaboration of visiting astronomers. Great care was exercised in the reduction of the light curves in order to properly correct for the background light of the host galaxy of each supernova

Two views of the Andromeda Galaxy H-alpha and far infrared

A complete H-alpha image of the Andromeda Galaxy (M31) is presented allowing the first direct measurement of the total H-alpha luminosity which is (7.3 +/- 2.4) x 10(exp 6) solar luminosity. The H-alpha emission is associated with three morphologically distinct components; a large scale star-forming ring, approximately 1.65 deg in diameter, contributing 66% of the total H-alpha emission, a bright nucleus contributing 6% of the total H-alpha emission with the remaining 28% contributed by a previously unidentified component of extended and filamentary H-alpha emission interior to the star forming ring. The correspondence between the H-alpha image and the Infrared Astronomy Satellite (IRAS) far-infrared high resolution image is striking when both are convolved to a common resolution of 105 arcsec. The close correspondence between the far-infrared and H-alpha images suggests a common origin for the two emissions. The star-forming ring contributes 70% of the far-infrared luminosity of M31. Evidence that the ring emission is energized by high mass stars includes the fact that peaks in the far-infrared emission coincide identically with H II regions in the H-alpha image. In addition, the far-infrared to H-alpha luminosity ratio within the star-forming ring is similar to what one would expect for H II regions powered by stars of spectral types ranging between O9 and B0. The origin of the filamentary H-alpha and far-infrared luminosity interior to the star-forming ring is less clear, but it is almost certainly not produced by high mass stars.

K-CORRECTIONS FOR TYPE IA SUPERNOVAE

The usefulness of type Ia supernovae as distance indicators is a metter of current study and discussion. The avaialbel data suggest that the intrinsic dispersion in the absolute B magnitudes of SNe Ia at maximum light is less than 0.4 mag. Hence, SNe Ia offer considerable potential for cosmological studies out to redshifts of z~0.1 or greater. At these distances, however, the effects of the redshift on the observed magnitudes are not negligible and must be taken into account in order to compare the magnitudes and light curves of objects of different redshifts. In this paper we make use of the best spectra avaialbe for type Ia supernovae in order to calculate the effect of theredshift on the observed B and V magnitudes. We present K-temrs in the redshift range z=0.005-0.50 from a collection of spectra of SNe 1990N, 1991T, and 1992A covering from day -14 to day +76, counted from the time of maximum light in the blue band. The K-terms calculated for the V band from the different spectra show a well defined trend as a function of time. In the B band the K-terms show some scatter before maximum light due to the spectroscopic differences displayed by the trhee SNe selected. These spectral differences tend ti disappear for later epochs and, consequently the K-term curves in the B band after maximum light are much tighter. Our calculations enable us to interpolate K-terms for SNe Ia at maximum light, which should prove useful to study the Hubble diagram for these objects.

Photometric and Spectroscopic Observations of SN 1990E in NGC 1035: Observational Constraints for Models of Type II Supernovae

We present 126 photometric and 30 spectral observations of SN 1990E spanning from 12 days before B maximum to 600 days past discovery. These observations show that SN 1990E was of type II-P, displaying hydrogen in its spectrum, and the characteristic plateau in its light curve. SN 1990E is one of the few SNe II which has been well observed before maximum light, and we present evidence that this SN was discovered very soon after its explosion. In the earliest spectra we identify, for the first time, several N II lines. We present a new technique for measuring extinction to SNe II based on the evolution of absorption lines, and use this method to estimate the extinction to SN 1990E, A(V) = 1.5 +/- 0.3 mag. From our photometric data we have constructed a bolometric light curve for SN 1990E and show that, even at the earliest times, the bolometric luminosity was failing rapidly. We use the late-time bolometric light curve to show that SN 1990E trapped a majority of the gamma rays produced by the radioactive decay of Co-56, and estimate that SN 1990E ejected an amount of Ni-56 virtually identical to that of SN 1987A.