A Nearby Supernovae Search : Eros 2

Type Ia supernovae (SNIa) have been used as approximate standard candles to measure cosmological parameters such as the Hubble constant and the deceleration parameter. These measurements rely on empirical correlations between peak luminosities and other features that can be observed in the supernovae spectra and their light curves. Such correlations deserve further study since they have been established from small samples of nearby SNIa. Two years ago, the EROS2 collaboration launched an automated search for supernovae with the 1m Marly telescope operating at La Silla. In all, 57 SNe have been discovered in this EROS2 search and spectra have been obtained for 26 of them. We found that 75% were of type Ia and 25% of type II. Using this sample, a preliminary SN explosion rate has been obtained. Our most recent observation campaign took place in February and March 99. It was performed in the framework of a large consortium led by the Supernova Cosmology Project. The aim of this intensive campaign was to provide an independent set of high quality light curves and spectra to study systematic effects in the measurement of cosmological parameters. We will briefly describe our search procedure and present the status of our ongoing analysis. 1 Type Ia supernovae and cosmology Supernovae are classified in different subtypes, according to their spectral features. Type Ia supernovae (SNIa) are believed to be explosions of carbon-oxygen white dwarfs. SNIa progenitors are likely to be binary systems, composed of a red giant and an old C/O white dwarf. The latter accretes matter from its companion until it reaches the Chandrasekar mass (∼ 1.4M ⊙), and then becomes unstable. This process leads to the total thermonuclear explosive burning of the white dwarf. Thus, the total energy released should be nearly constant from one SNIa to another. These objects may therefore be used as standard candles. Indeed, photometric and spectroscopic studies have shown that SNIa compose an homogeneous sample and their peak magnitudes present a small scatter (∼ 20%) in all colors. Furthermore, these objects are very luminous — they have been detected up to redshifts z ∼ 1.2 (Aldering et al. 2). Thus they constitute powerful cosmological distance indicators. It has been shown that the absolute maximum luminosities of SNIa correlate with other observables, like the post maximum decline rate, the color at maximum, the SN spectral features , or the galaxy type. When corrections based on such correlations …