THE LIGHT CURVE OF THE PLATEAU TYPE II SN 1983K

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1 Publications of the Astronomical Society of the Pacific 102: , March 1990 THE LIGHT CURVE OF THE PLATEAU TYPE II SN 1983K M. M. PHILLIPS AND MARIO HAMUY Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories* Casilla 603, La Serena, Chile J. MAZA AND M.T. RUIZ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile BRUCE W. CARNEYt Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina AND J. A. GRAHAM Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC Received 1989 December 2 ABSTRACT Optical photometry of the plateau Type II supernova 1983K extending for nearly a year after outburst is presented. At early epochs the blue luminosity of SN 1983K was approximately two magnitudes greater than that of the prototype of the plateau class, SN 1969L, and was five magnitudes greater than that of SN 1987A in the Large Magellanic Cloud. However, by the onset of the exponential tail phase a few months after outburst, the luminosities of all three supernovae were very similar, implying that nearly identical amounts of 56 Ni were produced through explosive nucleosynthesis. The huge range in initial luminosities observed is most likely due to different preexplosion radii, with the progenitor of 1983K having been the most extended of the three supernovae. Hence, it is unlikely that the progenitor of SN 1983K was a Wolf-Rayet star, in spite of the observed similarity of the premaximum spectrum to such stars. Key words: supernovae: photometry 1. Introduction The Type II SN 1983K was discovered in the Sab galaxy NGC 4699 on a plate taken on 1983 June 6 (UT) as part of the supernova search program of the Cerro Calán Observatory (see Maza et al. 1981). Maximum light was not reached until approximately 2 l k weeks later, providing a rare opportunity to observe the premaximum spectral evolution of a Type II supernova. Optical spectra obtained 6-9 days before maximum showed high-ionization N ill and He II emission lines atop a strong blue continuum (Niemela, Ruiz, and Phillips 1985). Within two days of maximum these emission lines had disappeared, leaving a strong continuous spectrum with weak and relatively narrow absorption lines due to H I, He I, and Ca II. *Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. fvisiting Observer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories. Approximately 25 days after maximum the spectrum had developed the broad P Cygni-like features of H I and Fe II which typify the postmaximum spectra of Type II supernovae. However, the width of the Ha emission at this phase was rather narrow in comparison with observations of other Type II supernovae obtained at similar epochs. The slow initial rise of the light curve and the narrow absorption line spectrum observed around maximum together suggested that the progenitor of SN 1983K had an extensive circumstellar shell. Based on the strong nitrogen emission lines seen before maximum, Niemela et al. (1985) speculated that the progenitor may have been either a Wolf-Rayet star or a red supergiant. In thist>aper we present BV photometry of SN 1983K extending for nearly a full year after outburst. A subset of these data covering the first three months was illustrated and briefly discussed by Niemela et al. (1985), who pointed out that the light-curve morphology of SN 1983K at these early epochs most closely resembled that of the plateau class of Type II supernovae. This classification 299

2 300 PHILLIPS ETAL. is confirmed in the present paper, and a close comparison is made with the absolute B light curve of the prototype plateau supernova, 1969L, as well as that of SN 1987A in the Large Magellanic Cloud. Although these three Type II supernovae showed a huge range in initial brightness, with SN 1983K having been five magnitudes brighter in the blue than SN 1987A at maximum, these differences nearly completely disappeared by the onset of the exponential tail phase approximately three months later. We conclude that the amount of 56 Ni synthesized in all three explosions was very nearly the same but that the progenitor of SN 1983K must have been the most extended (or the least compact) of the three. 2. Observations 2.1 Photoelectric and CCD Photometry Photoelectric photometry of SN 1983K was carried out with the Lowell 0.6-m telescope at Cerro Tololo Inter- American Observatory (CTIO) on a single night in June 1983 and on four nights in August Direct CCD observations were obtained at the prime focus of the CTIO 4-m telescope on five nights from August 1983 to May Additional CCD images were acquired on a single night in May 1984 at the Cassegrain focus of the 1.5-m telescope. Photoelectric and CCD observations were also made on several of these nights of a photometric sequence of nine stars with V magnitudes ranging from 10.8 to Finder charts for the sequence stars are reproduced in Figure 1 while the results of the photometry are listed in Table 1. The sequence was used to calibrate some of the CCD observations as well as a number of photographic plates obtained of the supernova (see Section 2.2). Final results for the photoelectric and CCD photometry of SN 1983K are given in Table Photographic Photometry A series of photographic observations of SN 1983K was obtained with both the Cerro Roble Observatory 0.7-m Maksutov and the CTIO 0.6-m Curtis Schmidt telescopes. The plates were unfiltered and employed either Ila-O (0.7-m Maksutov) or 103a-O (0.6-m Curtis Schmidt) emulsions. Iris photometry of the supernova and stars A-G of the photometric sequence was later carried out at the CTIO headquarters in La Serena. The first step in reducing these measurements was to compute photographic magnitudes for the sequence stars. We adopted the conversion formula used by Rust (1974), m pg = B (B -V), (1) which is essentially identical to that given by Arp (1957) for main-sequence stars in the color range 0.34 < (B V) < For each plate the photographic magnitude of the supernova was then derived by fitting a second-order polynomial to measurements of the sequence stars made with an iris photometer. Finally, the photographic magnitudes were converted to B magnitudes using equation (1). The supernova (B V) values needed for this calculation were computed from a secondorder polynomial fit to the photoelectric and CCD (B V) measurements given in Table 2. The resulting magnitudes are given in Table 3. Although this is clearly an approximate procedure, the errors in the final supernova B magnitudes were dominated by the measurement errors intrinsic to the iris photometry rather than uncertainties in the supernova (B V) color. 2.3 SIT Vidicon Spectrophotometry Spectra of SN 1983K covering the wavelength range 3800 Â-7300 Â were obtained on 1983 July 17 and 19 (UT) with a SIT Vidicon detector on the Cassegrain spectrograph of the CTIO 1.5-m telescope. Slit widths of 3.7 and 9.2 arc sec were employed, with the wider one providing absolute spectrophotometry. The flux calibration for both nights were derived through wide-slit (9.2 arc sec) observations of the Stone and Baldwin (1983) standard stars CD , LTT 377, LTT 1020, and LTT (See Fig. 2 of Niemela et al (1985) for a plot of TABLE 1 Photometric Sequence Star B-V V-R R-I A B C D E F G H I 11.27(02) 13.14(04) 13.43(02) 14.90(03) 14.51(03) 17.90(04) 17.52(04) 20.20(06) 18.35(04) 0.47(01) 0.54(05) 0.69(01) 0.98(03) 1.06(02) 0.89(02) 0.54(02) 0.91(05) 0.95(03) 0.28(01) 0.31(03) 0.39(03) 0.44(05) 0.66(03) 0.31(04) 0.32(04) 0.43(02) 0.68(07) Note: Estimated errors are given in parentheses in units of 0.01 mag. U.T. Date 1983 Jun Aug Aug Aug Aug Aug Dec Jan Mar May May TABLE 2 Photoelectric and CCD Photometry of SN 1983K J.D (04) (04) 13.65(04) 0.41(04) 0.20(04) 13.78(04) 0.45(04) 0.21(04) 13.76(04) 0.52(04) 0.22(04) 13.77(06) 0.48(06) _ 13.81(03) 0.50(02) 18.86(04) 1.08(04) 18.99(04) 0.89(04) 19.41(04) 20.08(07) 0.66(08) 20.09(04) Note: Estimated errors are given in parentheses in units of 0.01 mag. (P)hotoelectric or (C)CD?

3 LIGHT CURVE OF SN 1983K 301 Fig. l-(a) Reproduction of the O print of the National Geographic-Palomar Sky Survey plate showing the field around NGC Stars A-G of the photometric sequence are identified. The rectangle indicates the area covered by the CCD image displayed in Figure 1(b). (b) B image of SN 1983K obtained with an RCA CCD at the prime focus of the CTIO 4-m telescope on 1983 December (UT). Stars F-I of the photometric sequence are also identified. the average of the two resulting supernova spectra.) In order to obtain B and V magnitudes from these spectra, synthetic photometry calculations were performed similar to those described by Phillips et al. (1988) for SN 1987A. The zero points for the supernova magni- tudes were found by comparing synthetic magnitudes derived for the standards with broad-band photometry of the same stars (A. R. Walker, private communication). Final results are listed in Table 4.

4 302 PHILLIPS ETAL. TABLE 3 TABLE 4 Photographic Photometry of SN 1983K SIT Vidicon Spectrophotometry of SN 1983K J.D. U.T. Date m pg B a Telescope Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jul Jul Jul Aug Sep (21) 17.49(21) M 13.31(20) 13.67(20) M 12.65(08) 13.00(08) S 12.47(12) 12.81(12) S 12.60(11) 12.94(11) M 12.48(04) 12.82(04) S 12.42(13) 12.76(13) S 12.27(19) 12.61(19) S 12.42(15) 12.72(15) S 12.35(13) 12.65(13) S 12.50(07) 12.80(07) S 13.04(17) 13.30(17) M 13.29(06) 13.52(06) S 13.08(15) 13.31(15) S 13.52(13) 13.72(13) M 13.82(18) 14.04(18) M a Computed assuming the following relationships: B = im (B-V) B-V = (J.D ) x1o- 4 (J.D ) 2. b M = Cerro Roble 0.7m Maksutov; S = CTIO 0.6m Curtis Schmidt Note: Estimated errors are given in parentheses in units of 0.01 mag. 3. Results and Discussion The B and V light curves of SN 1983K are illustrated in Figure 2, along with the evolution of the (B V) color. Figure 3 shows the photometry for the first three months plotted on an expanded time scale. On outburst the B light curve climbed fairly rapidly to a broad peak which was then followed by a phase of nearly constant brightness lasting for at least two months. The latter property clearly identifies SN 1983K as a plateau Type II supernova (Barbon, Ciatti, and Rosino 1979). Sometime between 70 and 170 days after maximum, a fairly rapid decrease in brightness must have taken place. This was then followed by a phase of slow exponential decline, similar to that observed in other Type II supernovae, which continued at least through day 340. A detailed comparison of the absolute B and V light curves of SN 1983K with those of the prototype plateau supernova 1969L and the recent Type II supernova 1987A in the Large Magellanic Cloud (LMC) is illustrated in Figure 4. Table 5 gives the distance moduli and reddenings assumed for each supernova as well as the morphology of the host galaxy and the projected radius of the supernova from the host galaxy nucleus. Note that SN 1983K and SN 1969L were located at such very large distances from the nuclei of their respective host galaxies that, in both cases, the contribution of the host galaxy itself to the total extinction was most likely negligible J.D. U.T. Date B B-V 1983 Jul (10) 0.28(10) Jul (10) 0.22(10) Note: Estimated errors are given in parentheses in units of 0.01 mag. (Niemela et al. 1985; Tammann 1974). Therefore, we have corrected the photometry for these two supernovae for only the small amount of extinction produced by our own Galaxy. In order to compare the light curves it was also necessary to assume the date of outburst for each supernova. In the case of 1987A we have used the time of the neutrino burst observed by the IMB and Kamiokande II detectors (Bionta et al. 1987; Hirata et al. 1987), while for SN 1983K we adopted the time of the discovery plate taken at Cerro Roble. The corresponding date for SN 1969L was found by aligning the shape of the plateau in the B light curve to fit that of SN 1983K. Although the resulting outburst times for the latter two supernovae may be in error by several days, this has a negligible effect on the conclusions reached below. Figure 4 shows that, for the first month or so after outburst, SN 1983K was brighter than SN 1969L and SN 1987A in blue light by 2.5 and 5 magnitudes, respectively. According to current thinking such a huge range in initial brightness most likely reflects differences in the radii of the progenitor stars (see Young and Branch 1989 and references therein). The implication is that the progenitor of SN 1983K had a large radius. Indeed, with Mg = 19.5 at maximum light, SN 1983K is the most luminous plateau Type II supernova yet to be observed (Young and Branch 1989). It therefore seems unlikely in spite of the strong N ill and He II emission lines seen in the premaximum spectrum that the progenitor of this supernova could have been a Wolf-Rayet star, since such stars are considerably more compact than red supergiants. We note, parenthetically, that low metallicity has been suspected by many authors as a possible explanation for why the progenitor of SN 1987A, Sk , exploded as a blue supergiant (see Arnett et al and references therein). However, as noted earlier, both SN 1983K and SN 1969L were located at very large distances from the centers of their respective parent galaxies where the metallicity is also expected to be low. SN 1983K was particularly extreme, having been situated at a projected

5 LIGHT CURVE OF SN 1983K 303 TABLE 5 Parameters for SN 1969L, SN 1983K, and SN 1987A Host Proj. Rad. SN Galaxy Morphology 3 (m-m) 0 Source of SN A B Source 1969L NGC 1058 Sc(s)II-III kpc K NGC 4699 Sab(sr) or Sa kpc A LMC SBmlll a Revised Shapley-Ames Catalog (Sandage and Tammann 1987) Sources: (1) Tully (1988); mean of 17-1 group (2) Tully (1988) (3) Tully (1988); mean of group (4) Suntzeff and Bouchet (1990) radius of 23 kpc from the nucleus of NGC The oxygen abundance of H II regions at this radius in a galaxy of similar morphological type, M31, is O/H ~3 X 10-4 (Blair, Kirshner, and Chevalier 1982), which is comparable to typical H ii region abundances in the LMC. Nevertheless, the evidence is quite strong from the light curves that the progenitor of SN 1983K was not a blue supergiant. Thus, it would appear that a low metallicity environment does not exclude red supergiant progenitors for at least some Type II supernovae. Figure 4 shows that, by the onset of the final exponential decline phase, these huge brightness differences had disappeared, with the visual light curves of all three supernovae tracking at essentially the same magnitude Fig. 2-B and V photometry of SN 1983K plotted as a function of the Julian Date.

6 304 PHILLIPS ET AL J.D Fig. 3-Same as Figure 2, but showing only the first three months of data. and decline rate. The luminosity of Type II supernovae at this stage is derived entirely from energy released by the radioactive decay of 56 Co to 56 Fe (Weaver and Woosley 1980). Thus, if the relative distances assigned in Table 5 are correct, the amount of 56 Ni synthesized in all three explosions must have been very nearly identical. A similar conclusion was reached by Hamuy et al. (1988) on the basis of a comparison of the absolute V light curves for a somewhat larger sample of Type II supernovae. These authors suggested that the exponential tail phase might serve as a useful distance indicator, but this viewpoint has been challenged by Young and Branch (1989). It should be pointed out, however, that Young and Branch restricted their study to B light curves only. A glance at Figure 4 shows that the agreement is actually better in the V band (which is closer to the peak of the bolometric flux) than in B. More importantly, Young and Branch did not correct for extinction in the supernova parent galaxies, which can be substantial at blue wavelengths. Thus, the evidence at this point would seem to support the idea that, for whatever reason, the amount of 56 Ni produced in Type II supernovae does not vary much. Clearly, however, there is a need for more photometry of Type II supernovae at late epochs to further test this conclusion. REFERENCES Arnett, W. D., Bahcall, J., Kirshner, R. P., and Woosley, S. E. 1989, Ann. Rev. Astr. Ap., 27, 629.

7 LIGHT CURVE OF SN 1983K Days Since Outburst Fig. 4 Comparison of the absolute B and V light curves of SN 1983K, SN 1969L, and SN 1987A. Shown at the bottom is the evolution of the extinction-corrected (B V) color. The photometry for SN 1969L was taken from Ciatti, Rosino, and Bertola (1971) while that for SN 1987A is from Hamuy et al. (1988) and Suntzeff et al. (1988). Arp, H. 1957, A.]., 62, 129. Barbon, R., Ciatti, F., and Rosino, L. 1979, Astr. Ap., 72, 287. Bionta, R. M., et al. 1987, Phys. Rev. Letters, 58, Blair, W. P., Kirshner, R. P., and Chevalier, R. A. 1982, Ap./., 254, 50. Ciatti, F., Rosino, L., and Bertola, L. 1971, Mem. Soc. Astr. Italy, 42, 163. Hamuy, M., Suntzeff, N. B., Gonzalez, R., and Martin, G. 1988, A.J., 95, 63. Hirata, R., etal. 1987, Phys. Rev. Letters, 58, Maza, J., Wischnjewsky, M., Torres, C. Gonzalez, I., Costa, E., and Wroblewski, H. 1981, Pub. A.S.P., 93, 239. Niemela, V. S., Ruiz, M. T., and Phillips, M. M. 1985, Ap.J., 289, 52. Phillips, M. M., Heathcote, S. R., Hamuy, M., and Navarrete, M. 1988, A./., 95, Rust, B. W. 1974, Ph.D. dissertation, University of Illinois. Sandage, A., and Tammann, G. A. 1987, A Revised Shapley-Ames Catalog of Bright Galaxies (Washington, DC: Carnegie Institution). Stone, R. P. S., and Bladwin, J. A. 1983, M.N.R.A.S., 204, 347. Suntzeff, N. B., and Bouchet, P. 1990, A.]., in press. Suntzeff, N. B., Hamuy, M., Martin, G., Gómez, A., and González, R. 1988, A./., 96, Tammann, G. A. 1974, in Supernovae and Supernova Remnants, ed. C. B. Cosmovici (Dordrecht: Reidel), p Tully, R. B. 1988, Nearby Galaxies Catalog (Cambridge: Cambridge University Press). Weaver, T. A., and Woosley, S. E. 1980, Ann. NY Acad. Sei., 336, 335. Young, T. R., and Branch, D. 1989, Ap. J. (Letters), 342, L79.