CCD UBVRI photometry is presented for type IIb SN 2011dh for about 300 days. The main photometric parameters are derived and the comparison with SNe of similar types is reported. The light curves are similar to those for SN IIb 2008ax, but the initial flash is stronger and very short, and there are humps on the light curves in and at the onset of linear decline. Preliminary modeling is carried out, and the results are compared to the quasi-bolometric light curve and to the light curves in UBVRI bands.
Peremennye Zvezdy 32, No.3, 2012 Variable Stars 32, No.3, 2012
PHOTOMETRIC OBSERVATIONS AND PRELIMINARY MODELING
OF TYPE IIB SUPERNOVA 2011dh
D.YU. TSVETKOV, I.M. VOLKOV, E.I. SOROKINA, S.I. BLINNIKOV, N.N. PAVLYUK, G.V. BORISOV Sternberg Astronomical Institute, Lomonosov Moscow State University, University Ave. 13, 119992 Moscow, Russia Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranska Lomnica, Slovak Republic Institute for Theoretical and Experimental Physics, Bol’shaya Cheryomushkinskaya Str. 25, 117259 Moscow, Russia Crimean Laboratory of Sternberg Astronomical Institute, Lomonosov Moscow State University, Nauchnyi, Crimea, Ukraine
On May 31, 2011 a supernova exploded in bright nearby spiral galaxy M51 (NGC5194, 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 SN, 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 range 18-21 . 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). Vinko et al. (2012) presented optical spectroscopy and photometry of SN 2011dh and applied the EPM method to derive the distance of 8.4 Mpc for M51.
Observations and reductions
On June 1 a sequence of unfiltered images of M51 was obtained with the 192-mm telescope (hereafter C19), equipped with FLI PL16803 CCD camera, at Crimean Observatory of Sternberg Astronomical Institute (SAI). 53 frames were obtained in the period 19:25–20:31 UT. 5 days later we started regular monitoring of SN 2011dh and continued observations until 2012 April 4. CCD images in Johnson-Cousins UBVRI bands were obtained with the following instruments: the 15-cm and 60-cm telescopes of 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 Crimean Observatory of SAI (C60) with Apogee AP-47p camera; the 70-cm reflector of SAI in Moscow (M70) with Apogee AP-7p camera; 60-cm reflector of Simeiz Observatory (K60) with VersArray F512 camera, and 1-m reflector of Simeiz Observatory, equipped with either VersArray F512, or VersArray B1300 cameras (K100a, K100b).
The standard image reductions and photometry were made using IRAF.222IRAF is distributed by the National Optical Astronomy Observatory, which is operated by AURA under cooperative agreement with the National Science Foundation
The galaxy background around SN 2011dh is quite smooth, nevertheless we applied image subtraction for all of the frames obtained later then 2011 September 20. The template images were constructed from frames obtained at C60 while carrying out observations of SN 2005cs.
The 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 the images with larger field of view we also used more distant comparison stars, also from Pastorello et al. (2009).
The results of observations of the SN are presented in Table 1. We did not detect significant variations of brightness on the sequence of images obtained on June 1, so the averaged values, calibrated by magnitudes, are reported in Table 1.
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 SN declined very fast. At the phase 15 days past maximum the magnitude declined by 1.64 mag. The fast drop continued for about 21 days, and at about JD 2455753 the onset of the linear decline is observed (K-point). The rates of decline in the period JD 2455780-2456010 are (in mag/day): 0.016 in , 0.020 in , 0.019 in , 0.021 in .
Comparison with SN 2008ax (Pastorello et al., 2008; Tsvetkov et al., 2009) reveals good match of the light curves at the main peak. After K-point the agreement is good in the , and bands, while in the and the luminosity decline of SN 2011dh is slower. In the band there is even a slight increase of brightness after K-point, and in the band a protrusion on the light curve can be noticed.
Fig. 3 shows the light curves for the first 30 days after outburst. We plotted our data and the observations by Vinko et al. (2011), Arcavi et al. (2011a), and results of amateur astronomers, taken from ”Latest supernovae” site222www.rochesterastronomy.org/supernova.html
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 as the time of explosion. The initial flash was very fast: the rise to first peak with brightness of about 12.8 mag took only about 0.4 days, and atfer 2.6 days the local minimum was reached on JD 2455716, at about 15 mag.
We compare the early light curves of SN 2011dh with 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 coinside at the estimated moment 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 for SN 2008ax it was very weak.
The color curves are shown in Fig. 5. The evolution of colors , and is similar. SN 2011dh quickly reddens until K-point, and then becomes bluer. The color remains nearly constant after K-point. The comparison with type IIb SNe 1993J and 2008ax reveals diversity of the color curves, both in shape and the values of colors. Good match is observed only for 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 distance of 8.4 Mpc (Vinko 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 absolute peak magnitude of mag SN 2011dh appears to be quite typical among SNe of similar classes. It is little fainter than SNe IIb 1993J, 2008ax and SN Ib 1999ex, have nearly the same luminosity as SN Ic 2002ap and is significantly brighter than SN IIb 1996cb.
Modeling the light curves
We derived quasi-bolometric light curve for SN 2011dh, integrating the flux in UBVRI bands. On the dates when observations in some bands were missing, we used the color curves to estimate the color of 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 UBVRI 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 M main sequence star that lost most of its hydrogen envelope to a nearby companion. We present 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 M. 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 show the worst agreement with observational data. Models 3,4 and 5 have good agreement with observed curve at late stages. Figs. 8,9 show the computed UBVRI light curves for the models 3 and 6, which fit better the quasi-bolometric light curves. 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.
|Model||Radius,||Mass,||Energy, 10 erg/s|
Acknowledgements. We thank N.P.Ikonnikova who made some of the observations.
The work is supported partly by the grant of the Government of the Russian Federation (No 11.G34.31.0047), by RFBR grants 10-02-00249a, 10-02-01398a, 11-02-01213a, by RF Sci. Schools 3458.2010.2 and 3899.2010.2, by the grant IZ73Z0-128180/1 of the Swiss National Science Foundation (SCOPES), and by SAIA – scholarship (Slovakia).
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