Showing papers by "Peter Nugent published in 1997"

Discovery of a Supernova Explosion at Half the Age of the Universe and its Cosmological Implications

Abstract: The ultimate fate of the universe, infinite expansion or a big crunch, can be determined by measuring the redshifts, apparent brightnesses, and intrinsic luminosities of very distant supernovae. Recent developments have provided tools that make such a program practicable: (1) Studies of relatively nearby Type Ia supernovae (SNe Ia) have shown that their intrinsic luminosities can be accurately determined; (2) New research techniques have made it possible to schedule the discovery and follow-up observations of distant supernovae, producing well over 50 very distant (z = 0.3 -- 0.7) SNe Ia to date. These distant supernovae provide a record of changes in the expansion rate over the past several billion years. By making precise measurements of supernovae at still greater distances, and thus extending this expansion history back far enough in time, we can distinguish the slowing caused by the gravitational attraction of the universe's mass density Omega_M from the effect of a possibly inflationary pressure caused by a cosmological constant Lambda. We report here the first such measurements, with our discovery of a Type Ia supernova (SN 1997ap) at z = 0.83. Measurements at the Keck II 10-m telescope make this the most distant spectroscopically confirmed supernova. Over two months of photometry of SN 1997ap with the Hubble Space Telescope and ground-based telescopes, when combined with previous measurements of nearer SNe Ia, suggests that we may live in a low mass-density universe. Further supernovae at comparable distances are currently scheduled for ground and space-based observations.

Synthetic Spectra of Hydrodynamic Models of Type Ia Supernovae

Abstract: We present detailed non-LTE synthetic spectra of hydrodynamic supernovae (SNe) Ia models. We make no assumptions about the form of the spectrum at the inner boundary. We calculate both Chandrasekhar-mass deflagration models and sub-Chandrasekhar {open_quotes}helium detonators.{close_quotes} Gamma-ray deposition is handled in a simple, accurate manner. We have parameterized the storage of energy, which arises from the time-dependent deposition of radioactive decay energy, in a reasonable manner that spans the expected range. We find that the Chandrasekhar-mass deflagration model W7 of Nomoto, Thielemann, & Yokoi shows good agreement with the observed spectra of SN 1992A and SN 1994D, particularly in the UV, where our models are expected to be most accurate. The sub-Chandrasekhar models do not reproduce the UV deficit observed in normal SNe Ia. They do bear some resemblance to subluminous SNe Ia, but the shapes of the spectra (i.e., the colors) are opposite that of the observed ones, and the intermediate-mass element lines (such as SiII and CaII) are extremely weak, which seems to be a generic difficulty of the models. Although the sub-Chandrasekhar models have a significant helium abundance (unlike Chandrasekhar-mass models), helium lines are not prominent in the spectra near maximum light and thus do not actmore » as a spectral signature for the progenitor. {copyright} {ital 1997} {ital The American Astronomical Society}« less

Evidence for a High-velocity Carbon-rich Layer in the Type IA SN 1990N

Abstract: We use a parameterized spectrum-synthesis code to make a direct analysis of a high quality spectrum of the Type Ia SN 1990N that was obtained by Leibundgut et al. 14 days before the time of optical maximum. We suggest that the absorption feature observed near 6040 A, which has been attributed to blueshifted λ6355 of Si II, actually was produced by blueshifted λ6580 of C II, in a high-velocity (v > 26, 000 km s-1) carbon-rich region. Implications for SN Ia explosion models are briefly discussed.

Snapshot Distances to Type IA Supernovae -- All in 'One' Night's Work

Abstract: We present an empirical method that measures the distance to a Type Ia supernova (SN Ia) with a precision of ~10% from a single night's data. This method measures the supernova's age and luminosity/light-curve parameter from a spectrum and the extinction and distance from an apparent magnitude and color. We are able to verify the precision of this method from error propagation calculations, Monte Carlo simulations of well-sampled SNe Ia, and the Hubble diagram of sparsely observed supernovae. With the reduction in telescope time needed, this method is 3-4 times more efficient for measuring cosmological parameters than conventional light-curve-based distance estimates.

Type ia supernovae as extragalactic distance indicators

Abstract: Because Type Ia supernovae (SNe Ia) are not perfect standard candles, it is important to be able to use distance–independent observables (DIOs) to define subsets of SNe Ia that are “nearly standard candles” or to correct SN Ia absolute magnitudes to make them nearly homogeneous (“standardized candles”). This is not cruci al for the measurement of H o but it is for the measurement of q o and of parent–galaxy peculi ar velocities. We discuss the use of various photometric and spectroscopic SN Ia DIOs, and a parent–galaxy DIO, for this purpose. We also discuss the status of the absolute–magnitude calibration of SNe Ia. We find that SNe Ia, whether calibrated by means of (1) Cepheids in their parent g al axies, (2) fitting their optical –ultraviolet spectra with detailed non-LTE model atmosphere calculations, or (3) by considering that the light curve is powered by the decay of radioactive 56Ni, firmly indicate that the value of H o is low, less than or about 60 km s-1 Mpc-1. Some issues regarding the determination of q o by means of SNe Ia are discussed briefly. Finally, we conjecture that even if q o=0.5, there probably is no cosmic age problem.

The Hubble constant, supernova light curves and spectra, and radiation transport

Abstract: As luminous events that can be physically modeled, supernovae provide an attractive route to the value of the Hubble constant. The modeling involves radiation transport through matter undergoing homologous expansion with velocity gradient on the order of 10−6 s−1. For supernovae of type Ia, which are thermonuclear disruptions of mass accreting or coalescing carbon–oxygen white dwarfs, one wants to be able to calculate the light curve (luminosity in some optical passband versus time), which is powered by the radioactivity decay chain 56Ni→56Co→56Fe. For all kinds of supernovae, including those of types II, Ib, and Ic, which result from the gravitational collapse of the cores of massive stars, the goal is to accurately calculate the emergent ultraviolet–optical–infrared spectra, as a function of time. Local-thermodynamic-equilibrium (LTE) light-curve calculations for type Ia supernovae by Hoflich and co-workers, and our spectrum calculations based on a fully relativistic non-LTE radiative transfer code, are...