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Type Ia supernovae (SN Ia) are explosions of white dwarfs whose distances can be measured to a precision of ~5% using luminosity information that is encoded in the light curve shape. This property has been very successfully exploited to measure the history of cosmic expansion and to infer the presence of dark energy. But to learn the properties of dark energy and determine whether it is different from the cosmological constant demands higher precision and better accuracy than optical light curves alone can provide. The largest systematic uncertainties come from light curve fitters, photometric calibration errors, and from poor knowledge of the scattering properties of dust along the line of sight. Efforts to use SN Ia spectra as luminosity indicators have had some success, but have not produced a big step forward. Fortunately, observations of SN Ia in the near infrared (NIR), from 1 to 2 microns, offer a very promising path to better knowledge of the Hubble constant, improved constraints on dark energy, and, possibly, a route to discriminating the progenitor paths for SN Ia explosions.
Supernovae and the discovery of the expanding universe
Supernovae have been firmly woven into the fabric of cosmology from the very beginning of modern understanding of the expanding, and now accelerating universe. Today's evidence for cosmic acceleration is just the perfection of a long quest that goes right back to the foundations of cosmology. In the legendary Curtis-Shapley debate on the nature of the nebulae, the bright novae that had been observed in nebulae suggested to Shapley (1921) (see Trimble, 1995) that the systems containing them must be nearby. Otherwise, he reasoned, they would have unheard-of luminosities, corresponding to M = −16 or brighter. Curtis (1921) countered, concluding, “the dispersion of the novae in spirals and in our galaxy may reach ten magnitudes … a division into two classes is not impossible.” Curtis missed the opportunity to name the supernovae, but he saw that they must exist if the galaxies are distant. Once the distances to the nearby galaxies were firmly established by the observation of Cepheid variables (Hubble, 1925), the separation of ordinary novae and their extraordinary, and much more luminous super cousins, became clear.
A physical explanation for the supernovae was attempted by Baade and Zwicky (1934). Their speculation that supernova energy comes from the collapse to a neutron star is often cited, and it is a prescient suggestion for the fate of massive stars, but not the correct explanation for the supernovae that Zwicky and Baade studied systematically in the 1930s.
The Supernova Working Group was re-established at the IAU XXV General Assembly in Sydney, 21 July 2003, sponsored by Commissions 28 (Galaxies) and 47 (Cosmology). Here we report on some of its activities since 2005.
Knowledge of the size and age of the Universe depends on understanding supernovae. The direct geometric measurement of the circumstellar ring of SN 1987A using IUE spectra and HST images provides an independent test of the Cepheid distance scale to the Large Magellanic Cloud. Understanding the details of the mass distribution in the circumstellar matter is important to improving the precision of this distance. Type la supernovae have a narrow distribution in absolute magnitude, and new Cepheid distances to IC 4182 (the site of SN 1937C) and to NGC 5253 (the site of SN 1972E) obtained with HST by Sandage and his collaborators allow that absolute magnitude to be calibrated. Comparison with more distant SNIa gives H0 = 56 ± 8 km s-1 Mpc-1. Recent work in supernova spectroscopy and photometry shows that the apparent homogeneity of SNIa is not quite what it seems, and a deeper understanding of these variations is needed to use the SNIa to best advantage. The Expanding Photosphere Method (EPM) allows direct measurement to each Type II supernova that has adequate photometry and spectroscopy. There are now 18 such objects. The sample of EPM distances from 4.5 Mpc to 180 Mpc indicates H0 = 73±6 (statistical) ±7 (systematic) km s-1 Mpc-1. Better understanding of supernova atmospheres can reduce the systematic error in this approach, which is completely independent of all other astronomical distances.
We have measured proper motions for fast, oxygen-rich knots in Puppis A, which we demonstrate are probably uncontaminated ejecta from the progenitor star’s core. Typical fast knots show motions of 0.1-0.2 arcsec yr-1 diverging from a point 4’ northeast of the center of the radio shell. A model assuming constant expansion fits the data well and gives an age of 3700 ± 300 yr for Puppis A. We also present new spectra which indicate the presence of neon along with oxygen in the fast knots.
Observing supernova remnants provides important clues to the nature of supernova explosions. Conversely, the late stages of stellar evolution and the mechanism of supernova explosions affect supernova remnants through circumstellar matter, stellar remnants, and nucleosynthesis. The elements of supernova classification and the connection between supernova type and remnant properties are explored. A special emphasis is placed on SN 1987a which provides a unique opportunity to learn the connection between the star that exploded (whose name we know) and the remnant that will develop in our lifetimes.
Mass loss from the B3 Ia progenitor star for SN 1987A is revealed by the recent emergence of narrow ultraviolet emission lines. The emitting gas is nitrogen-rich, has low velocity, and may be located a light-year from the supernova. This gives every sign of having been ejected from the SK −69 202 progenitor when it was a red supergiant, prior to its brief and ultimately violent life as a blue supergiant. Changes in the hydrogen line profiles during the early evolution provide a way to estimate the density distribution in the supernova atmosphere, and the mass of hydrogen it contains. A preliminary estimate is that the power-law index of density in the envelope goes as V−11 and the mass that lies above a velocity of 6, 000 km s−1 is between 1 and 6 solar masses.
Existing catalogues of supernova remnants (SNRs) in external galaxies are very incomplete. Potentially however, such samples are of great importance in understanding SNRs, since the distances to objects in a given sample are essentially the same and since absorption is small (compared to galactic SNRs). We have recently obtained Hα+[NII], Hβ, [SII], [OIII], and 6100 Å continuum CCD images of nine selected areas in M33 using the KPNO 4m. In addition to the six SNRs already known to exist in the fields we have surveyed, we have identified 21 other nebulae with [SII]:Hα+[NII] ratios which may be SNRs. Spectra of seven of these nebulae were obtained subsequently and show that the majority are indeed SNRs. A more detailed analysis of regions containing significant HII region contamination and a search for very small diameter remnants is currently underway.
Observed properties of supernovae and of very young supernova remnants provide important clues to answer the question, “which stars become supernovae?” There are three general lines of evidence (l) the statistics of supernovae, (2) the physical state of supernovae near maximum light, and (3) the chemistry of young supernova remnants. These lines of evidence appear to be converging on the view that all the supernova explosions we see, whether Type I or Type II, arise from the destruction of young massive stars.
It is important to note that supernovae fall into two distinct spectroscopic classes: Type I which does not show strong hydrogen lines, and Type II in which hydrogen emission and absorption is prominent (Oke and Searle, 1974). It is possible that the stellar progenitors of these two types are very different, but recent evidence suggests that the similarities in the properties of the two classes are more important.
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