Astronomy and Astrophysics – Astrophysics
Scientific paper
1996-01-02
Astronomy and Astrophysics
Astrophysics
20 pages, latex, 8 figures, (uses styles: crckapb.sty and psfig.sty) tarred and gzipped. A PostScript version is available at
Scientific paper
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 crucial for the measurement of H_0, but it is for the measurement of q_0 and of parent-galaxy peculiar 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 galaxies, (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 Ni, firmly indicate that the value of H_0 is low, less than or about 60 km/s/Mpc. Some issues regarding the determination of q_0 by means of SNe Ia are discussed briefly. Finally, we conjecture that even if q_0 = 0.5, there probably is no cosmic age problem.
Branch David
Fisher Andrew
Nugent Peter
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