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1993Aj 106.1101F the Astronomical THE ASTRONOMICAL JOURNAL VOLUME 106, NUMBER 3 SEPTEMBER 1993 106.1101F CCD PHOTOMETRY OF THREE TYPE la SUPERNOVAE: V, R, AND / LIGHT CURVES Charles H. Ford1 and William Herbst1 Van Vleck Observatory, Wesleyan University, Middletown, Connecticut 06459 1993AJ Electronic mail: [email protected] Michael W. Richmond,2 Michael L. Baker,3 Alexei V. Filippenko,4 Richard R. Treffers, and Young Paik Department of Astronomy, and Center for Particle Astrophysics, University of California, Berkeley, California 94720 Priscilla J. Benson Department of Astronomy, Wellesley College, Wellesley, Massachusetts 02181 Received 1993 March 12; revised 1993 April 13 ABSTRACT Cousins system VRI photometry has been obtained for the Type la supernova 1992G with CCDs attached to small telescopes at Leuschner Observatory (LO), Van Vleck Observatory (WO), and Whitin Observatory (WO). Similar equipment, observing and reduction procedures have been adopted at all three sites and there is generally good agreement among the data sets. Two other Type la Supernovae, 1991M and 199 IT, were observed only at WO. The shapes of the light curves of all three Supernovae, including the spectroscopically peculiar SN 199IT, are quite similar, and the combined light curve in V matches Leibundgut’s [Ph.D. thesis, University of Basel (1988)] template reasonably well. Since very little R and / photometry of supernovae has been done previously, our data provide the first real definition of SN la templates in those bands. The I template is of particular interest since it shows a secondary maximum 23 ± 1 days after the primary peak, similar to what is seen in normal Type la supernovae at J, H, and K wavelengths. The behavior in R is intermediate between V and /. The existence of substantial structure in the R and / light curves of normal Type la supernovae (and its absence in the peculiar Type la SN 199 Ibg) enhances their utility as cosmological probes and provides an observational challenge to models of the event. The observed colors of SN 1991M and SN 199 IT as a function of time are quite similar, indicating that they suffer about the same amount of reddening. SN 1992G is clearly more heavily reddened than these two, by about 0.1 mag in R—I. The intrinsic colors of these Type la supernovae at Fmax are V—R=0.0 and R—I= —0.3, with an uncertainty of around 0.05 mag, if the reddening of SN 1991T is E(B— V) =0.13. If Type la supemovae are assumed to be good standard candles, then NGC 4527, the parent galaxy of SN 199 IT, must have a peculiar velocity of 1200± 150 km s-1 relative to the Hubble flow and directed away from the Galaxy. If the peculiar velocity of NGC 4527 is less than this, then SN 199 IT must be an overluminous SN la. Tully & Shaya (1984) regard NGC 4527 as a near-side member of the Virgo cluster falling in at approximately 1000 -1 -1 -1 km s . Our data alone yield a Hubble constant of 75 ± 15 km s Mpc if Mv at maximum light is —18.7 mag, and 50± 10 km s_1 Mpc-1 if it is —19.6 mag. 1. INTRODUCTION without warning and fade on a time scale of months, mak- ing it virtually impossible to schedule sufficient telescope Obtaining high quality, well sampled light curves for time for this work at a shared facility. One generally needs supernovae is not a trivial task. The classical techniques of to combine data from more than one site to get good tem- photographic and photoelectric photometry are not well poral coverage. The required transformations are compli- suited to the job of extracting a stellar brightness from the cated by the fact that a supernova changes color dramati- galactic background. Supemovae are generally faint, occur cally during its initial evolution and has a determinedly nonstellar spectrum at all times. Unless one has very closely matched instrumental systems it can be difficult to Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which are operated by the Association of Uni- achieve consistency in the photometry at the level of better versities for Research in Astronomy, Inc. under contract with the Na- than 0.1 mag (e.g., Hamuy et al 1990). tional Science Foundation. Several years ago supernova monitoring programs were 2Present address: Department of Astrophysical Sciences, Princeton Uni- versity, Princeton, NJ 08544. launched independently on opposite coasts of the U.S. by 3A1so at Department of Physics, Curry College. groups at Van Vleck Observatory (WO) on the campus of Presidential Young Investigator. Wesleyan University, and at Leuschner Observatory 1101 Astron. J. 106 (3), September 1993 0004-6256/93/106(3)/! 101/12/$0.90 © 1993 Am. Astron. Soc. 1101 © American Astronomical Society • Provided by the NASA Astrophysics Data System 1102 FORD ET AL. : PHOTOMETRY OF 3 SNs 1102 106.1101F (LO), which is associated with the Berkeley campus of the light curves, it is both desirable and likely that future work University of California. Wesleyan is a member of the will concentrate more on longer wavelength observations. Keck Northeast Astronomy Consortium (KNAC)5 and The Johnson/Cousins UBVRI system described by Bessell several KNAC institutions participate in the east coast ( 1990) has been adopted at both Berkeley and the KNAC 1993AJ program, including Wellesley College, with its campus fa- institutions. The advantages of this system are that it has cility, Whitin Observatory (WO). Partly by good fortune broad bandpasses which allow us to reach relatively faint and partly as a result of communication and planning, the objects with small telescopes and that there is a large set of instrumental systems at KNAC schools and at Berkeley photometric standards available (Landolt 1983, 1992). are nearly identical, and a rather uniform observing and One disadvantage is that the R and / filters have very long reduction procedure has been adopted, as described in Sec. tails to the red and it is possible that small variations in 2. It is hoped that this approach will enhance the utility of CCD quantum efficiencies and atmospheric transparencies the data obtained at each site. in the red could couple with the complex and evolving There are, of course, a variety of reasons why high qual- spectra of supernovae to produce systematic differences in ity, well sampled light curves of all types of supemovae are photometry obtained at different sites (even when similar interesting. While our observing program is not restricted equipment and common standards are employed). On ac- to any particular type, in this paper we discuss only results count of the small telescope sizes and relatively poor blue for SNe la. It has long been argued that these extremely and ultraviolet sensitivity of the CCD chips used, data on luminous objects could be used as standard candles for the majority of supernovae from these telescopes will be determining the distance scale in the Universe, setting lim- restricted to VRI, with only the brighter objects observed its on the deceleration parameter, and addressing the fun- in B as well. damental issue of whether the redshift is indeed produced Leibundgut (1988) has reviewed the extant photometry by the expansion of the Universe (e.g., Branch & Tam- of Type la supernovae and argued that “template” light mann 1992; Branch & Miller 1993). The central question curves in UBV and JHK can be formed to which virtually is whether or not SNe la really are standard candles (e.g., all SNe la adhere. Very little work has been done on su- Pskovskii 1977; Leibundgut 1988; Leibundgut & Tam- pernovae at or / wavelengths. Lee et al ( 1972) obtained mann 1990; Miller & Branch 1990; Boisseau & Wheeler UBVRI photometry on the Johnson system (which differs 1991; van den Bergh & Pazder 1992). It is now clear that substantially from the Cousins system at R and I) for the there are both spectroscopically and photometrically pecu- Type la SN 1972E. Buta & Turner (1983) obtained liar SNe la, such as SN 1986G (Phillips et al 1987; Frogel i£-band photometry on the Cousins system for SN 198IB. et al 1987; Christiani eia/. 1992) and SN 1991bg (Filip- Hamuy et al ( 1991 ) observed SN 1980N in R and / but with significant gaps in their temporal coverage. Balonek penko et al 1992b; Leibundgut et al 1993) which cannot et al ( 1991 ) presented data on SN 1991T in VRI, some of be used as standard candles. However, it appears likely which are included in this paper. Filippenko et al that these abnormal objects are rather uncommon and can (1992b), in the first published report of the photometry easily be identified by their observed properties. What is programs at Berkeley and Wesleyan, presented VRI pho- perhaps more worrisome are SNe la having less obvious tometry of the subluminous Type la SN 1991bg. What has and less persistent peculiarities such as SN 199IT (Filip- been lacking, however, is enough photometry of normal penko et al 1992a; Phillips et al 1992; Ruiz-Lapuente Type la supernovae to establish the form of the template et al 1992). A good deal of careful photometry and spec- light curves in R and I. That is one of the principal con- troscopy of bright nearby supernovae is needed to ascertain tributions of this paper. In Sec. 2 our observing and reduc- how common these objects are and how unusual their pho- tion techniques are described (see also Filippenko et al tometric properties (particulary their absolute magni- 1992b) . VRI data and light curves for three Type la su- tudes) may be. pemovae are given in Sec. 3. These data are used in Sec. 4 Owing to the blue sensitivity of photographic and pho- to discuss the issue of whether the Type la supernovae in toelectric detectors, almost all supernova photometry has our sample are good standard candles and, in Sec.
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