Peremennye Zvezdy (Variable Stars) 32, No. 6, 2012 Received 28 August; accepted 3 September.

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Photometric observations and preliminary modeling of type IIb supernova 2011dh

D.Yu. Tsvetkov¹, I.M. Volkov^1,2, E.I. Sorokina¹, S.I. Blinnikov^3,1, N.N. Pavlyuk¹, G.V. Borisov⁴

1. Sternberg Astronomical Institute, Lomonosov Moscow State University, University Ave. 13, 119992 Moscow, Russia

2. Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranska Lomnica, Slovak Republic

3. Institute for Theoretical and Experimental Physics, Bol'shaya Cheryomushkinskaya Str. 25, 117259 Moscow, Russia

4. Crimean Laboratory of Sternberg Astronomical Institute, Lomonosov Moscow State University, Nauchnyi, Crimea, Ukraine

1. Introduction

On May 31, 2011, a supernova exploded in the bright nearby spiral galaxy M51 (NGC 5194, Whirlpool galaxy). The outburst, designated SN 2011dh, was promptly discovered independently by several amateurs and the Palomar Transient Factory (see CBET No. 2736 and Arcavi et al. 2011b for details). The early discovery of such a nearby SN facilitated numerous follow-up studies. Early spectra and light curve indicated SN 2011dh to belong to the class of stripped-envelope core-collapse SNe, designated as type IIb (Arcavi et al. 2011a, 2011b). The progenitor or progenitor system was identified in archival images obtained by the HST (Li and Filippenko 2011), although its nature remains controversial. Maund et al. (2011) suggest it was a yellow supergiant with initial mass about 13, while Van Dyk et al. (2011) prefer higher mass, in the 18-21 range. The variability of the candidate progenitor was reported by Szczygiel et al. (2012). Multi-wavelength follow-up observations in the radio, millimiter, X-ray, and gamma-ray bands suggest a compact progenitor with cm, which is inconsistent with the radius of the yellow supergiant, so this star may be a binary companion of presupernova or even unrelated to the SN (Soderberg et al. 2012). Radio observations were reported also by Krauss et al. (2012), Marti-Vidal et al. (2011), and Bietenholz et al. (2012). Vinkó et al. (2012) presented optical spectroscopy and photometry of SN 2011dh and applied the Expanding Photosphere Method to derive the distance of 8.4 Mpc for M51.

2. Observations and reductions

On June 1, 2011, a sequence of unfiltered images of M51 was obtained with the 192-mm telescope (hereafter C19), equipped with an FLI PL16803 CCD camera, at Crimean Observatory of the Sternberg Astronomical Institute (SAI). 53 frames were obtained in the 19:25-20:31 UT time interval. 5 days later we started regular monitoring of SN 2011dh and continued observations till 2012 April 4. CCD images in the Johnson-Cousins bands were obtained with the following instruments: the 15-cm and 60-cm telescopes of the Astronomical Institute of Slovak Academy of Sciences at Tatranska Lomnica (S15, S60), equipped, respectively, with SBIG ST-10XME and Princeton Instruments VersArray F512 CCD cameras; the 60-cm reflector of the SAI Crimean Observatory (C60) with an Apogee AP-47p camera; the 70-cm SAI reflector in Moscow (M70) with an Apogee AP-7p camera; the 60-cm reflector of Simeiz Observatory (K60) with a VersArray F512 camera; and the 1-m reflector of Simeiz Observatory, equipped either with VersArray F512 or with VersArray B1300 cameras (K100a, K100b).

The standard image reductions and photometry were made using IRAF.¹

The galaxy background around SN 2011dh is quite smooth; nevertheless, we applied image subtraction for all of the frames obtained later than 2011 September 20. The template images were constructed from frames obtained at C60 while carrying out observations of SN 2005cs.

Magnitudes of the SN were derived by PSF fitting relative to a sequence of local standard stars. The image of SN 2011dh and comparison stars is shown in Fig. 1.

The magnitudes of these stars were taken from Pastorello et al. (2009). On images with larger field of view, we used several more distant comparison stars, also from Pastorello et al. (2009).

The results of our observations of the SN are presented in Table 1. We did not detect significant variations of brightness in the sequence of images obtained on June 1, so the averaged values, calibrated by magnitudes, are reported in Table 1.

3. Light and color curves

The light curves of SN 2011dh are shown in Fig. 2. The premaximum rise and the main peak have good coverage by observations, and we can determine the dates and magnitudes of maximum light in different bands: . After the maximum, the brightness of the SN declined very fast. At the phase of 15 days past maximum, the magnitude declined by 164. The fast drop continued for about 21 days, and the onset of the linear decline (K-point) is observed at about JD 2455753. The rates of decline in the JD 2455780-2456010 time interval, in mag/day, are: 0.016 in , 0.020 in , 0.019 in , 0.021 in .

Comparison to SN 2008ax (Pastorello et al. 2008; Tsvetkov et al. 2009) reveals a good match of the light curves at the main peak. After the K-point, the agreement is good in the , and bands, while in the and bands, the luminosity decline of SN 2011dh is slower. In the  band, there is even a slight increase of brightness after the K-point, and in the  band, a protrusion on the light curve can be noticed.

Figure 3 shows the light curves for the first 30 days after the outburst. We plotted our data and the observations by Vinkó et al. (2012), Arcavi et al. (2011a), as well as results of amateur astronomers, taken from the "Latest supernovae" site.²

The last image of M51 with no SN visible (mag 18) was obtained on JD 2455712.86, and the first detection was on JD 2455713.34. We assume JD 2455712.9 to be the time of explosion. The initial flash was very fast: the rise to the first peak with brightness of about 128 took only about 04, and after 2.6 days, a local minimum was reached on JD 2455716, at about 15.

We compare the early light curves of SN 2011dh to those for SNe IIb 1993J and 2008ax (Richmond et al. 1996; Pastorello et al. 2008; Tsvetkov et al. 2009). The light curves were shifted in time to coincide at the estimated time of explosion, and the shift in magnitudes was applied to match the main peak brightness. The difference between the objects is evident: the initial peak for SN 1993J was the widest and strongest among these objects, while it was very weak for SN 2008ax .

The color curves are shown in Fig. 5. The evolution of the , , and colors is similar. SN 2011dh quickly reddens till the K-point and then becomes bluer. The color remains nearly constant after the K-point. A comparison to the type IIb SNe 1993J and 2008ax reveals diversity of the color curves, both in shape and the values of colors. A good match is observed only for the and colors between SNe 2011dh and 2008ax. SN 2011dh is significantly redder in and than the other two objects.

The absolute -magnitude light curves of SN 2008ax and several SNe of types IIb, Ib, and Ic are compared in Fig. 5.

For SN 2011dh, we adopted the distance of 8.4 Mpc (Vinkó et al. 2012) and extinction . The light curves of other SNe are taken from Richmond et al. (1996), Qiu et al. (1999), Stritzinger et al. (2002), Foley et al. (2003), Pastorello et al. (2008), Tsvetkov et al. (2009). With its absolute peak magnitude of , SN 2011dh appears to be quite typical among SNe of similar classes. It is a little fainter than SNe IIb 1993J, 2008ax and SN Ib 1999ex, has nearly the same luminosity as SN Ic 2002ap, and is significantly brighter than SN IIb 1996cb.

4. Modeling the light curves

We derived a quasi-bolometric light curve for SN 2011dh integrating its flux in the bands. On the dates when observations in some of the bands were missing, we used the color curves to estimate the color of the SN at that date and then calculated the missing magnitudes. We attempted to model the quasi-bolometric light curve as well as the light curves in the bands using our code STELLA, which incorporates implicit hydrodynamics coupled to a time-dependent multi-group non-equilibrium radiative transfer (Blinnikov et al. 1998). The specific model employed here was Model 13C of Woosley et al. (1994). This model was derived from a 13  main sequence star that lost most of its hydrogen envelope to a nearby companion. We present the results for 6 variants of the model with different values of radius, explosion energy, and ejected mass, which are reported in Table 2. The mass of Ni was fixed at 0.07 . The results are presented in Figs. 6-9.

The influence of changing main parameters on the shape of resulting light curve can be seen in Figs. 6,7. The reduction of radius leads to shortening of the primary flash, but at the same time it becomes weaker. Model 5 with increased energy of explosion shows the worst agreement with observational data. Models 3, 4, and 5 have a good agreement with the observed curve at late stages. Figures 8,9 show the computed light curves for the models 3 and 6, which fit the quasi-bolometric light curve better. The main maximum in the and bands is reproduced satisfactorily, but the computed duration of the initial flash is longer, and its lumonosity is lower than observed. The agreement in other bands is worse. We may conclude that, although our models reproduce main features of the observed light curves, the agreement is not satisfactory. We continue the search for models which will give better fits. The results and more detailed discussion of the properties of the models and their impact on the possible evolution of the progenitor will be published in a subsequent paper.

Acknowledgements. We thank N.P. Ikonnikova who made some of the observations.

The work was supported partly by the grant of the Government of the Russian Federation (No. 11.G34.31.0047); by grants 10-02-00249a, 10-02-01398a, 11-02-01213a of the Russian Foundation for Basic Research; by the grants 3458.2010.2 and 3899.2010.2 from the Program of State Support to Leading Scientific Schools of Russia; by the grant IZ73Z0-128180/1 of the Swiss National Science Foundation (SCOPES); and by the SAIA scholarship (Slovakia).

References:

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Table 1. Observations of SN 2011dh

JD 2450000+											Tel.
5714.32					13.54	0.03					C19
5714.35					13.52	0.03					C19
5719.34	14.67	0.06	14.54	0.02	13.94	0.02	13.64	0.01	13.61	0.01	M70
5723.37	13.85	0.04	13.79	0.02	13.17	0.02	12.89	0.01			K100a
5724.37	13.83	0.04	13.70	0.02	13.03	0.02	12.77	0.01			K100a
5725.31	13.65	0.04	13.60	0.02	12.92	0.02	12.65	0.01	12.56	0.01	K100a
5726.31	13.62	0.04	13.55	0.02	12.87	0.02	12.65	0.01	12.49	0.01	K100a
5727.28	13.60	0.04	13.53	0.02	12.79	0.02	12.51	0.01	12.40	0.01	K100a
5729.31	13.52	0.04	13.42	0.02	12.67	0.02	12.39	0.01	12.26	0.01	K100a
5730.31	13.53	0.04	13.42	0.02	12.64	0.02	12.34	0.01	12.22	0.01	K100a
5731.30	13.53	0.04	13.38	0.02	12.60	0.02	12.30	0.01	12.17	0.01	K100a
5731.32	13.49	0.07	13.32	0.02	12.60	0.02	12.31	0.01	12.17	0.01	K100a
5733.31	13.59	0.04	13.41	0.02	12.55	0.02	12.27	0.01	12.11	0.01	K100a
5734.29	13.66	0.04	13.42	0.02	12.55	0.02	12.21	0.01	12.07	0.01	K100a
5734.37	13.63	0.08	13.36	0.02	12.60	0.04	12.26	0.03	12.09	0.04	C60
5735.27	13.76	0.04	13.48	0.02	12.57	0.02	12.22	0.01	12.06	0.01	K100a
5735.33	13.78	0.06	13.43	0.02	12.60	0.02	12.25	0.03	12.08	0.04	C60
5736.28	13.90	0.04	13.59	0.02	12.61	0.02	12.24	0.01	12.07	0.01	K100a
5737.28	14.16	0.04	13.72	0.02	12.66	0.02	12.27	0.01	12.08	0.01	K100a
5741.28	15.15	0.05	14.28	0.02	13.00	0.02	12.49	0.01	12.25	0.01	K100a
5743.28	15.44	0.04	14.57	0.02	13.19	0.02	12.61	0.01	12.33	0.01	K100a
5744.32	15.71	0.05	14.79	0.02	13.27	0.02	12.67	0.01			K100a
5744.33			14.63	0.05	13.35	0.02	12.72	0.02	12.43	0.03	C60
5746.28	15.95	0.04	14.89	0.02	13.41	0.02	12.79	0.01	12.47	0.01	K100a
5747.27	16.06	0.06	14.99	0.02	13.48	0.02	12.82	0.01	12.48	0.01	K100a
5749.30	16.36	0.12	15.07	0.02	13.59	0.02	12.91	0.01	12.57	0.01	K100a
5749.34			14.99	0.02	13.65	0.02	12.95	0.02	12.60	0.03	C60
5750.29	16.37	0.11	15.11	0.02	13.64	0.02	12.96	0.01	12.62	0.01	K100a
5751.26	16.41	0.04	15.21	0.02	13.69	0.02	13.00	0.01	12.63	0.01	K100a
5752.35	16.46	0.04	15.24	0.02	13.75	0.02	13.05	0.01	12.67	0.01	K100a
5755.35			15.34	0.02	13.94	0.02	13.19	0.02	12.79	0.03	C60
5757.34			15.31	0.04	14.00	0.02	13.26	0.02	12.84	0.02	C60
5783.28	16.19	0.17	15.68	0.03	14.53	0.04	13.85	0.01	13.41	0.02	C60
5786.31			15.66	0.03	14.60	0.04	13.91	0.02	13.48	0.02	C60
5788.28			15.74	0.05	14.62	0.04	13.95	0.03	13.53	0.03	C60
5789.26			15.69	0.03	14.64	0.02	13.97	0.02	13.55	0.03	C60
5790.24			15.78	0.04	14.62	0.02	13.99	0.01	13.56	0.02	C60
5808.24			15.94	0.02	14.98	0.02	14.39	0.02	13.99	0.02	C60
5811.22			15.98	0.03	15.03	0.02	14.45	0.02	14.07	0.02	C60
5817.22			16.01	0.03	15.16	0.02	14.62	0.02	14.07	0.03	K100b
5818.22	16.39	0.06	16.06	0.02	15.20	0.02	14.61	0.02	14.09	0.03	K100b
5819.21	16.71	0.07	16.10	0.02	15.23	0.01	14.66	0.01	14.10	0.03	K100b
5820.21	16.57	0.05	16.12	0.02	15.22	0.02	14.66	0.02	14.11	0.03	K100b
5821.21	16.62	0.08	16.08	0.02	15.22	0.02	14.68	0.02	14.13	0.03	K100b
5822.21	16.66	0.08	16.14	0.02	15.24	0.02	14.69	0.02	14.17	0.03	K100b
5823.24	16.70	0.09	16.15	0.02	15.27	0.02	14.73	0.03	14.16	0.02	K100b
5825.20	16.63	0.07	16.17	0.03	15.34	0.02	14.75	0.02	14.25	0.03	K100b
5830.21	16.82	0.37	16.24	0.03	15.41	0.04	14.85	0.04	14.37	0.05	K60
5831.20	16.47	0.18	16.27	0.04	15.41	0.04	14.91	0.03	14.39	0.04	K60
5832.20	16.88	0.12	16.32	0.04	15.46	0.04	14.90	0.04	14.45	0.04	K60
5835.19			16.50	0.05	15.52	0.03	14.90	0.02			K100a
5853.21					15.84	0.12	15.29	0.10	14.62	0.20	S15
5853.22					15.95	0.06	15.31	0.04			S60
5854.22					15.92	0.06	15.48	0.04	14.97	0.12	S60
5856.20					15.82	0.13	15.43	0.09	14.87	0.07	S15
5856.21			16.77	0.06	15.96	0.05	15.42	0.04	14.97	0.10	S60
5880.68			17.43	0.21	16.66	0.12	16.09	0.10			S15
5880.69					16.56	0.08					S60
5953.46			18.28	0.17	17.64	0.20	17.12	0.07	16.99	0.11	M70
5955.66			18.45	0.05	18.22	0.06	17.29	0.04			S60
5987.58							17.91	0.07			S60
6022.39					19.44	0.12	18.35	0.04			K100b

Table 2. Model parameters

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