The Type IIb SN 2011dh: Two years of observations and modelling of the lightcurves

Creators: Ergon, M.; Jerkstrand, A.; Sollerman, J.; Elias-Rosa, N.; Fransson, C.; Fraser, M.; Pastorello, A.; Kotak, R.; Taubenberger, S.; Tomasella, L.; Valenti, S.; Benetti, S.; Helou, G.; Kasliwal, M. M.; Maund, J.; Smartt, S. J.; Spyromilio, J.

Abstract

We present optical and near-infrared (NIR) photometry and spectroscopy as well as modelling of the lightcurves of the Type IIb supernova (SN) 2011dh. Our extensive dataset, for which we present the observations obtained after day 100, spans two years, and complemented with Spitzer mid-infrared (MIR) data, we use it to build an optical-to-MIR bolometric lightcurve between days 3 and 732. To model the bolometric lightcurve before day 400 we use a grid of hydrodynamical SN models, which allows us to determine the errors in the derived quantities, and a bolometric correction determined with steady-state non-local thermodynamic equilibrium (NLTE) modelling. Using this method we find a helium core mass of 3.1^(+0.7)_(-0.4) M⊙ for SN 2011dh, consistent within error bars with previous results obtained using the bolometric lightcurve before day 80. We compute bolometric and broad-band lightcurves between days 100 and 500 from spectral steady-state NLTE models, presented and discussed in a companion paper. The preferred 12 M⊙ (initial mass) model, previously found to agree well with the observed spectra, shows a good overall agreement with the observed lightcurves, although some discrepancies exist. Time-dependent NLTE modelling shows that after day ~600 a steady-state assumption is no longer valid. The radioactive energy deposition in this phase is likely dominated by the positrons emitted in the decay of ^(56)Co, but seems insufficient to reproduce the lightcurves, and what energy source is dominating the emitted flux is unclear. We find an excess in the K and the MIR bands developing between days 100 and 250, during which an increase in the optical decline rate is also observed. A local origin of the excess is suggested by the depth of the He I 20 581 Å absorption. Steady-state NLTE models with a modest dust opacity in the core (τ = 0.44), turned on during this period, reproduce the observed behaviour, but an additional excess in the Spitzer 4.5 μm band remains. Carbon-monoxide (CO) first-overtone band emission is detected at day 206, and possibly at day 89, and assuming the additional excess to bedominated by CO fundamental band emission, we find fundamental to first-overtone band ratios considerably higher than observed in SN 1987A. The profiles of the [O I] 6300 Å and Mg I] 4571 Å lines show a remarkable similarity, suggesting that these lines originate from a common nuclear burning zone (O/Ne/Mg), and using small scale fluctuations in the line profiles we estimate a filling factor of ≲0.07 for the emitting material. This paper concludes our extensive observational and modelling work on SN 2011dh. The results from hydrodynamical modelling, steady-state NLTE modelling, and stellar evolutionary progenitor analysis are all consistent, and suggest an initial mass of ~12 M⊙ for the progenitor.

Additional Information

© 2015 ESO. Article published by EDP Sciences. Received: 11 July 2014; Accepted: 9 January 2015; Published online 21 August 2015. This work is based on observations obtained with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofísica de Canarias; the German-Spanish Astronomical Center, Calar Alto, jointly operated by the Max-Planck-Institut für Astronomie Heidelberg and the Instituto de Astrofísica de Andalucía (CSIC); the United Kingdom Infrared Telescope, operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK; the William Herschel Telescope and its service programme (proposals SW2011b21 and SW2012a02), operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias; the Copernico 1.82m Telescope and Schmidt 67/92 Telescope operated by INAF – Osservatorio Astronomico di Padova at Asiago, Italy; by the 3.6m Italian Telescopio Nazionale Galileo operated by the Fundación Galileo Galilei – INAF on the island of La Palma; the Liverpool Telescope, operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias with financial support from the UK Science and Technology Facilities Council; the AlbaNova telescope operated by the Department of Astronomy at Stockholm University and funded by a grant from the Knut and Alice Wallenberg Foundation; the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma. We acknowledge the exceptional support we got from the NOT staff throughout this campaign, we thank the Calar Alto Observatory for the allocation of the director's discretionary time and we thank Philip Dufton, Paul Dunstall, Darryl Wright and Lindsay Magill for assistance with the WHT observations. The Oskar Klein Centre is funded by the Swedish Research Council. J.S. acknowledge support by the Swedish Research Council. S.J.S. thanks European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no [291222] and STFC. A.P., L.T. and S.B. are partially supported by the PRIN-INAF 2011 with the project "Transient Universe: from ESO Large to PESSTO". M.F. acknowledges support by the European Union FP7 programme through ERC grant number 320360. N.E.R. acknowledges the support from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 267251 "Astronomy Fellowships in Italy" (AstroFIt). R.K. acknowledges observing time at the LT, NOT, TNG, and WHT via programme CCI-04. M.M.K. acknowledges generous support from the Hubble Fellowship and Carnegie-Princeton Fellowship. We thank Melina Bersten for providing the post-explosion density profile for the He4R270 model (B12), for inspiration and a great contribution to the understanding of SN 2011dh. We thank Peter Meikle and Dan Milisavljevic for providing spectra on SN 1993J and SN 2008ax, respectively.

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