CORE Metadata, citation and similar papers at core.ac.uk Provided by CERN Document Server A Comparative Study of the Absolute-Magnitude Distributions of Supernovae Dean Richardson, David Branch, Darrin Casebeer, Jennifer Millard, R. C. Thomas and E. Baron Dept. of Physics and Astronomy, University of Oklahoma, Norman, OK 73019; [email protected] ABSTRACT The Asiago Supernova Catalog is used to carry out a comparative study of super- nova absolute-magnitude distributions. An overview of the absolute magnitudes of the supernovae in the current observational sample is presented, and the evidence for sub- luminous and overluminous events is examined. The fraction of supernovae that are underluminous (MB > 15) appears to be higher (perhaps much higher) than one − fifth but it remains very uncertain. The fraction that are overluminous (MB < 20) − is lower (probably much lower) than 0.01. The absolute-magnitude distributions for each supernova type, restricted to events within 1 Gpc, are compared. Although these distributions are affected by observational bias in favor of the more luminous events, they are useful for comparative studies. We find mean absolute blue magnitudes (for H = 60) of 19:46 for normal Type Ia supernovae (SNe Ia), 18:04 for SNe Ibc, 17:61 0 − − − and 20:26 for normal and bright SNe Ibc considered separately, 18:03 for SNe II-L, − − 17:56 and 19:27 for normal and bright SNe II-L considered separately, 17:00 for − − − SNe II-P, and 19:15 for SNe IIn. − Subject headings: supernovae: general — catalogs 1. Introduction The absolute-magnitude distributions of the various supernova (SN) types provide vital infor- mation for determining SN rates, for advancing our knowledge of the stellar progenitors and their explosion mechanisms, and for planning future ground- and space-based SN searches. More than a decade ago, ?)MB90;][]mb90 used the Asiago Supernova Catalog (?, the ASC;)]barbon89 to carry out a comparative study of absolute-magnitude distributions. At the end of 1989, the ASC listed only 687 SNe, and many of those were of unknown type. By June, 2001 (updates are available at http://merlino.pd.astro.it/~supern/), the number of events in the ASC had increased to 1910. Almost all of the SNe of the 1990’s had been assigned types, and some of the pre-1990 data had been improved. We therefore decided that an updated study of the absolute-magnitude distributions would be timely. It should be acknowledged that the characteristic uncertainty of the apparent magnitudes listed in the ASC, perhaps 0.2 or 0.3 mag, is not negligible for this purpose. Thanks to the tremendous –2– interest in using SNe Ia as distance indicators for cosmology, studies of the SN Ia absolute-magnitude distribution now can be based on carefully selected samples of events for which both the apparent magnitudes and the relative distances are known to high accuracy (?, e.g.)]hamuy96. The absolute- magnitude scatter in these samples can be further reduced in two ways. First, the host galaxy extinction can be taken into account for each SN using the SN’s B V color. Next, the fact that − some SNe Ia are intrinsically dimmer than others can be accounted for by using a correction to the light-curve width such as the ∆m15 parameter (Phillips et al. 1999) or the method of multicolor light-curve shapes (?, MLCS,)]riess96. With these corrections the dispersion in MB for SNe Ia can be reduced to 0.11 (Phillips et al. 1999), making them extremely useful as distance indicators for cosmology. Our present study, which has little to add to our knowledge of the SN Ia absolute-magnitude distribution, is directed more at the absolute-magnitude distributions of the other SN types. As far as the other SN types are concerned, either the number of events for which accurate peak apparent magnitudes have been reported remains small (SNe Ib, Ic, II-L, IIn) or the intrinsic dispersion in the peak absolute magnitude is large (SNe II-P). For these types a study such as this one, based on all available data, can be useful. For comparison with the other types, SNe Ia are included as well. 2. Data Most of the SN types and apparent magnitudes are taken from the ASC (as of June, 2001). Several changes in the spectral types are made after examining spectra available to us. Some additional data (mostly for recent SNe Ia) are included from Perlmutter et al. (1999), Riess et al. (1999), Riess et al. (1998), Leonard et al. (2001), Clocchiatti et al. (2000) Macri et al. (2001), Matheson et al. (2000), van den Bergh (1994), Iwamoto et al. (2000) and Hatano et al. (2001). Mag- nitudes from Perlmutter et al. include K-corrections. Several changes in the apparent magnitudes are made after examining the Atlas of Light Curves of SNe I by Leibundgut et al. (1991). Where possible we use blue magnitudes. Photographic magnitudes are converted to blue magnitudes us- ing the usual relation B = mpg +0:3. When necessary we assume that B = V = R (except for the K-corrected events). The apparent magnitudes are corrected for Galactic extinction following Schlegel, Finkbeiner & Davis (1998) with the help of NED1. When possible, we use a Cepheid distance to the parent galaxy or to a member of the same galaxy group as the parent galaxy. The Cepheid distances are obtained from Freedman et al. (2001), Macri et al. (2001), Ferrarese et al. (2000) and Saha et al. (2001a,b). If a Cepheid distance is not available we use distances from the Nearby Galaxies Catalog (?, NGC;)]tully88, which incorporates 1The NASA/IPAC Extragalactic Database (NED) is operated by the Jet Propulsion Laboratory, California Insti- tute of Technology, under contract with the National Aeronautics and Space Administration. –3– a Virgocentric infall model to relate galaxy recession velocity and distance. The errors in the relative distances to these events contribute significantly to the absolute-magnitude scatter in our 1 1 figures. We rescale the distances from the H0 =75kms− Mpc− of the NGC to our choice of H0 = 60. If a Cepheid distance or an NGC distance is not available then the luminosity distance is calcu- lated Kantowski, Kao & Thomas (2000), provided that the SN parent galaxy has a Galactocentric 1 recession velocity of cz > 2000 km s− . The luminosity distance is calculated assuming H0 = 60, Ωm =0:3, and ΩΛ =0:7. Errors in the relative distances to these events should be small. A different choice of H0 would simply rescale the absolute magnitudes. Different choices of Ωm or ΩΛ would change our plots only slightly (at large distances). A few SNe whose parent galaxies have 1 cz < 2000 km s− but do not have a measured Cepheid distance and are not listed in the NGC are excluded from the study. The Large Magellanic Cloud, the site of SN 1987A, is assigned a distance modulus of 18.50. 3. Overview of Supernova Absolute Magnitudes In Figure 1, absolute magnitude is plotted against distance modulus for all 297 SNe for which we have an estimate of the apparent magnitude at the time of maximum light; we will refer to these as maximum-light SNe. Also shown, at the left side of Figure 1, are estimates by Schaefer (1996) of the absolute magnitudes of six Galactic SNe of the last millennium that occurred within about 4 kpc of the Earth. Schaefer’s estimates actually are of MV , rather than MB, and his estimates of the distance moduli of these events range from 11.4 to 12.7, off-scale to the left of Figure 1. Figure 2 is like Figure 1, but for the 1078 SNe for which we have only lower limits to the peak absolute brightness because the apparent magnitude at the time of maximum light is unknown; we will refer to these as limit SNe. Both figures contain some straight lines to guide the eye. The horizontal line at MB = 19:5 is regarded as characteristic of normal SNe Ia and we will refer to − this as the SN Ia ridge line. The horizontal line at MB = 15 and the vertical lines at µ =30and − µ = 35 will be referred to below. The slanted lines correspond to apparent magnitudes of B =16 and B = 25. Figure 1 shows that so far most of the maximum-light SNe have had peak apparent magnitudes brighter than B = 16, while Figure 2 shows that most of the limit SNe were fainter than B = 16 at their brightest observation. An indication of the dramatic observational progress that has taken place during the 1990s is that MB90 also plotted the B = 16 line, and only four maximum-light SNe were to the right of it. MB90 did not consider the limit SNe. When viewing Figures 1 and 2 it is helpful to think in terms of several different distance regimes. First come the Galactic SNe, then SN 1987A and SN 1885A in the Local Group. Next come SNe in “nearby galaxies”, say within 10 Mpc (µ 30). The interval from µ =30toµ =35 1 ≤ (cz = 6000 km s− , 100 Mpc) includes many SNe of the Local Supercluster, with those of the Virgo cluster being concentrated near µ =31:2. From µ = 35 (or less) to µ 40 (cz 60; 000 km s 1, ' ' − z 0:2) is the “Hubble flow”, where luminosity distance depends on H but only slightly on Ωm ' 0 –4– and ΩΛ. As will be seen below, most of the maximum-light events in the Hubble flow are SNe Ia; this is due to the observational bias in favor of luminous events.