Discovery of a new photometric sub-class of faint and fast classical novae

arXiv:1003.1720v1 [astro-ph.SR] 8 Mar 2010 Draft version March 9, 2018 Preprint typeset using LATEX style emulateapj v. 11/10/09 DISCOVERY OF A NEW PHOTOMETRIC SUB-CLASS OF FAINT AND FAST CLASSICAL NOVAE M. M. Kasliwal1, S. B. Cenko2, S. R. Kulkarni1, E. O. Ofek1, R. Quimby1, A. Rau3 Draft version March 9, 2018 ABSTRACT We present photometric and spectroscopic follow-up of a sample of extragalactic novae discov- ered by the Palomar 60-inch telescope during a search for “Fast Transients In Nearest Galaxies” (P60-FasTING). Designed as a fast cadence (1-day) and deep (g <21 mag) survey, P60-FasTING was particularly sensitive to short-lived and faint optical transients. The P60-FasTING nova sample in- cludes 10 novae in M 31, 6 in M 81, 3 in M 82, 1 in NGC 2403 and 1 in NGC 891. This significantly expands the known sample of extragalactic novae beyond the Local Group, including the first dis- coveries in a starburst environment. Surprisingly, our photometry shows that this sample is quite inconsistent with the canonical Maximum Magnitude Rate of Decline (MMRD) relation for classical novae. Furthermore, the spectra of the P60-FasTING sample are indistinguishable from classical no- vae. We suggest that we have uncovered a sub-class of faint and fast classical novae in a new phase space in luminosity-timescale of optical transients. Thus, novae span two orders of magnitude in both luminosity and time. Perhaps, the MMRD, which is characterized only by the white dwarf mass, was an over-simplification. Nova physics appears to be characterized by quite a rich four-dimensional parameter space in white dwarf mass, temperature, composition and accretion rate. Subject headings: stars: novae,cataclysmic variables, galaxies: starburst, galaxies: individ- ual(M81,M82,M31,NGC2403,NGC891), techniques: spectroscopic, techniques: photometric, surveys 1. INTRODUCTION Since the discovery of classical novae, astronomers have pursued their use as standard candles to determine dis- tances (see Hubble 1929). Zwicky (1936) first noticed some regularity in nova light curves and termed this the “life-luminosity” relation. Arp (1956) undertook a com- prehensive search for novae in M31, discovering thirty no- vae in 290 nights, and found a clear relation — luminous novae evolve faster than less luminous novae. The mod- ern name for this observation is the maximum-magnitude rate-of-decline relation (MMRD relation). The MMRD relation has attracted considerable theo- retical attention (e.g. Livio 1992). The basic idea is that the relation is entirely due to the mass of the accreting white dwarf. The more massive the white dwarf, the higher the surface gravity, the higher the pressure at the base of envelope and stronger the thermonuclear runaway (and hence, higher the peak luminosity). Also, the more massive the white dwarf, the smaller the envelope mass to attain the critical pressure for thermonuclear runaway (TNR) and hence, faster the decline. In more recent times, della Valle & Livio (1995) used a sample of novae in M31 and LMC to propose an arc- tangent relation between the peak luminosity and rate of decline. Downes & Duerbeck (2000) used a sample of Galactic novae to propose a linear relation between the same two parameters. Darnley et al. (2006) used a score of novae in M31 from POINT AGAPE survey and claimed their observations were consistent with the mansi@astro.caltech.edu 1 Astronomy Department, California Institute of Technology, 105-24, Pasadena, CA 91125, USA 2 Department of Astronomy, University of California at Berke- ley, Berkeley, CA 94720, USA 3 Max-Planck Institut fuer Extraterrestrische Physik, 85748 Garching, Germany della Valle & Livio (1995) formulation of the MMRD. In comparison to supernovae, classical novae are not very luminous. Hence, searches have traditionally fo- cussed only on the Milky Way and its nearest neigh- bors (Andromeda and the Large Magellanic Cloud). Hornoch et al. (2008) looked into archival data and found 49 nova candidates4 in M 81 in the past 20 years — un- fortunately, these candidates neither have light curves nor spectra. Ferrarese et al. (2003) undertook a search for novae using 24 orbits of the Hubble Space Telescope and found nine nova candidates in M49. Even with their sparsely sampled light curves for nine novae, they con- cluded that novae are not good standard candles. An- other survey, CFHT-COVET5 (aimed at finding tran- sients in the gap between novae and supernovae) found a dozen nova candidates in many galaxies in the Virgo su- percluster, including some in the far outskirts of galaxies (Kasliwal et al 2010, in prep). Here, we report on novae discovered in high cadence monitoring observations of a representative collection of galaxies with distance less than that of the Virgo cluster. The original motivation of this search – P60-FasTING6 was to explore rapid transients (those which last less than a couple of nights) in the nearest galaxies. A strong spec- troscopic follow-up effort w

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