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1996Aj 111. . 32 7R the Astronomical Journal 7R 32 . THE ASTRONOMICAL JOURNAL VOLUME 111, NUMBER 1 JANUARY 1996 111. UBVRI PHOTOMETRY OF THE TYPE Ic SN 19941 IN M51 Michael W. Richmond Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544 1996AJ Electronic mail: [email protected] Schuyler D. Van Dyk, Wynn Ho, Chien Peng, Young Paik, Richard R. Treffers, and Alexei V. Filippenko1 Astronomy Department, University of California, Berkeley, California 94720 Javier Bustamante-Donas Department of Ethics and Sociology, Universidad Complutense de Madrid, Madrid 28040, Spain Michael Moeller and Claus Pawellek Luebeck Public Observatory, Luebeck, Germany Heather Tartara and Melody Spence Oil City High School, Oil City, Pennsylvania 16301 Received 1995 July 31; revised 1995 October 10 ABSTRACT We present optical photometry for the type Ic SN 19941 in M51 (NGC 5194) from 1994 March 31 to August 6, UT starting eight days before Æ-band maximum and ending three months later. We estimate the extinction to SN 19941 was considerable and quite uncertain, A 1.4±0.5 mag, making a detailed comparison to other supemovae difficult. Using a distance modulus /j=29.6±03 mag to M51, we calculate absolute magnitudes and a quasibolometric light curve for the supernova. It appears that SN 19941 was less luminous than “normar’ type la SNe. There is strong evidence that the ejecta of SN 19941 were much less massive than those of other SNe. © 1996 American Astronomical Society. 1. INTRODUCTION The position of the SN is slightly uncertain: Rupen et al. (1994) used the VLA to measure a radio position h m 0 SN 19941 in the face-on Sc galaxy M51 (NGC 5194, a= 13 27 47í731, ¿=+47 26'57'.'84) equinox 1950.0, corre- “The Whirlpool”) was discovered on 1994 April 2, UT in- spending to a position in the J2000.0 system of dependently by at least four sets of observers: Tim Puckett a= l 3h29m54?06, 5=+47° 11'29:96. Morrison & Argyle and Jerry Armstrong, Wayne Johnson and Doug Millar, Ri- (1994) report that optical measurements with a meridian chard Berry, and Reiki Kushida (Puckett et al. 1994). Its circle yield a=13h29m54?072, <?=+47oll'30'.’50, equinox spectrum caused some confusion in the first few days after J2000.0. The small difference may be due to the strong gra- discovery, being interpreted as similar to that of a type II dient in light from the nucleus of M51 at the position of the (Schmidt & Kirshner 1994a), type lb (Filippenko et al. supernova, as Morrison and Argyle state. 1994), type Ic (Schmidt et al. 1994), and peculiar type la We initiated a program of multicolor photometry at (Turatto & Zanin 1994); after a week, however, a consensus Leuschner and Lick Observatories, and report here our re- grew that it was a type Ic event (Clocchiatti et al. 1994; sults. In addition, we have collected several very early im- Kirshner et al. 1994). Detailed spectroscopic studies by ages taken at other sites and reduced them in the same man- Wheeler et al. (1994) and Filippenko et al. (1995) confirmed ner as our own. this classification, which is largely the end of a process of Section 2 contains a description of the observations. Sec- elimination: type Ic SNe shows no lines of hydrogen (hence tion 3 discusses the “template subtraction” method of pho- are not type II), lack the deep Si II absorption trough near tometry we used to extract the light of SN 19941 from its 6150 Â (hence are not type la), and also lack absorption lines environs in the disk of M51. Our calibration of the measure- of He in the optical (hence are not type lb). Note, however, ments onto the standard Johnson-Cousins system appears in that Filippenko et al. did detect the He I XI0830 line in SN Sec. 4. In Sec. 5 we present light curves of SN 19941. Since 19941, showing that at least a small amount of helium must this event occurred close to the nucleus of M51, it is not have been present in its atmosphere. surprising that there are several indications of a large amount of extinction, which we describe in Sec. 6. In Sec. 7, we lAlso associated with the Center for Particle Astrophysics, University of discuss the distance to M51, and calculate absolute magni- California at Berkeley. tudes in all passbands for our adopted extinction. In addition, 327 Astron. J. Ill (1), January 1996 0004-6256/96/1 ll(l)/327/13/$6.00 © 1996 Am. Astron. Soc. 327 © American Astronomical Society • Provided by the NASA Astrophysics Data System 328 RICHMOND ETAL: SN 19941 328 7R 32 . Table 1. Johnson-Cousins magnitudes of comparison stars. star U B V R I source 111. A 14.14* 14.05 13.44 13.08 12.81 Corwin 1994 A — 14.005* ± 0.004 13.420* ± 0.002 13.067* db 0.003 12.730* ± 0.004 Lick B — 16.57: 15.09: 14.39: 13.65: Corwin 1994 B — 16.339 ±0.017 15.096 ±0.007 14.331 ±0.009 13.680 ± 0.005 Lick 1996AJ C — 15.751 ±0.017 15.212 ±0.010 14.857 ± 0.013 14.497 ±0.010 Lick D — 16.168 ±0.016 15.266 ±0.007 14.746 ± 0.009 14.200 ± 0.005 Lick 'Adopted as primary comparison star values. telescope and an SBIG ST-4 CCD camera without filters to obtain exposures of 160 and 180 s on April 2 UT. The plate scale is 377 ±072 per pixel, and stellar images appear to have FWHM about 1.5 pixels. Comparing the raw, instrumental differences in magnitude between stars A, B, C, and D with those listed in Table 1, we find the response of this system to be closest to the Cousins R band. Specifically, we find in- strumental differences (B-A)=1.32, (C-A)=1.73, and (D —A)=1.49. Defining r to be the magnitude of a star in the Fig. 1. V-band image of M51 (=NGC 5194) and SN 19941 taken 1994 April instrumental system, we can write R = r+fcr(V—R) + C, 2, with comparison stars marked. North is up and east to the left. The field where C is an arbitrary zero-point offset. We can find a value of view is about 53X53. SN 19941 is about 19" from the nucleus of M51. for kr using the instrumental magnitudes and our adopted V and R magnitudes for the comparison stars listed in Table 1. we calculate a quasibolometric light curve by summing the We find fcr=-0.41 ±0.25. We will designate these data “Æ.” observed optical fluxes. In Sec. 8, we compare the light MM and CP used a 49 cm //4.2 telescope at the Luebeck curves of SN 19941 with those of the type Ic SN 1987M. We Public Observatory in Germany, equipped with an SBIG place our conclusions in Sec. 9. An Appendix lists details of ST-6 CCD camera and no filter, to acquire three images of the equations used to place our observations onto the stan- SN 19941 on March 31 UT. The scale is 276 per pixel. All dard Johnson-Cousins photometric system. objects show trails of about 4 pixels in the north-south di- rection, and a width of about 1.5 pixels. The exposure times 2. OBSERVATIONS were 10, 15, and 10 s. Although a correction was made for dark count, no flat fielding was applied to the images. Once Figure 1 shows SN 19941, its host galaxy M51, and ref- again using the instrumental magnitude differences between erence stars A, B, C, and D. We report on observations ob- stars A, B, C, and D, we find these images to have a response tained mainly by the two telescopes at Leuschner Observa- closest to the Johnson-Cousins V band. In this case, the tory and one at Lick Observatory, with several very early instrumental differences are (Æ—A)=1.76, (C—A) = 1.72, observations from telescopes at other sites. All data were and (D-A) = 1.77. Asserting that V= v + kv(V— R) + C, we measured from CCD images, and reduced to a common mag- calculate kv = —0.26±0A2. We will use the term “V” when nitude sequence of comparison stars. referring to these data. At Leuschner Observatory, we used a 50 and a 76 cm telescope, both reflectors equipped with CCD cameras (Rich- mond, et al 1993; Richmond et al. 1994, hereafter referred to as R94). The full width at half maximum (FWHM) of 3. EXTRACTING INSTRUMENTAL MAGNITUDES stellar images was usually 3"-4". Our exposure times varied from about 200 s (in the R and / bands) to 1500 s (in the U Light from SN 19941 was surrounded by light from the and, at late time, B bands). We created bias frames by taking galaxy’s nucleus, making photometry a difficult task. In or- the median of five images each afternoon, and flatfield der to extract instrumental magnitudes from each image, we frames from the median of five exposures of the twilight sky, attempted to use the “rotsub” technique described in Rich- we then applied them to the raw images in the usual fashion. mond et al (1995, hereafter referred to as R95), but found Lick Observatory’s 1 m Nickel telescope is equipped with that the strong spiral features of M51 led to significant re- a 2048X2048 pixel CCD camera, which we operated in 2X2 siduals at the position of the SN (however, we did use mea- binned mode, yielding a plate scale of 0737 per pixel.
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