Type IIb supernova SN 2011dh: Spectra and photometry from the ultraviolet to the near-infrared

Marion, G. H. and Vinkó, József and Kirshner, R. P. and Foley, R. J. and Berlind, P. and Bieryla, A. and Bloom, J. S. and Calkins, M. L. and Challis, P. and Chevalier, R. A. and Chornock, R. and Culliton, C. and Curtis, J. L. and Esquerdo, G. A. and Everett, M. E. and Falco, E. E. and France, K. and Fransson, C. and Friedman, A. S. and Garnavich, P. and Leibundgut, B. and Meyer, S. and Smith, N. and Soderberg, A. M. and Sollerman, J. and Starr, D. L. and Szklenár, Tamás and Takáts, Katalin and Wheeler, J. C. (2014) Type IIb supernova SN 2011dh: Spectra and photometry from the ultraviolet to the near-infrared. ASTROPHYSICAL JOURNAL, 781 (2). ISSN 1538-4357

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Abstract

We report spectroscopic and photometric observations of the Type IIb SN 2011dh obtained between 4 and 34 days after the estimated date of explosion (May 31.5 UT). The data cover a wide wavelength range from 2,000 ˚A in the ultraviolet (UV) to 2.4 µm in the near-infrared (NIR). Optical spectra provide line profiles and velocity measurements of H I, He I, Ca II and Fe II that trace the composition and kinematics of the SN. NIR spectra show that helium is present in the atmosphere as early as 11 days after the explosion. A UV spectrum obtained with the Space Telescope Imaging Spectrograph reveals that the UV flux for SN 2011dh is low compared to other SN IIb. Modeling the spectrum with SYNOW suggests that the UV deficit is due to line blanketing from Ti II and Co II. The H I and He I velocities in SN 2011dh are separated by about 4,000 km s−1 at all phases. A velocity gap is consistent with models for a pre-explosion structure in which a hydrogen-rich shell surrounds the progenitor. We estimate that the H-shell of SN 2011dh is ≈ 8 times less massive than the shell of SN 1993J and ≈ 3 times more massive than the shell of SN 2008ax. Light curves (LC) for twelve passbands are presented: UVW2,UVM2,UVW1,U,u’,B,V,r’,i’,J,H and Ks. In the B-band, SN 2011dh reached peak brightness of 13.17 mag at 20.0±0.5 days after the explosion. The maximum bolometric luminosity of 1.8±0.2×1042 erg s−1 occurred ≈ 22 days after the explosion. NIR emission provides more than 30% of the total bolometric flux at the beginning of our observations and the NIR contribution increases to nearly 50% of the total by day 34. The UV produces 16% of the total flux on day 4, 5% on day 9 and 1% on day 34. We compare the bolometric light curves of SN 2011dh, SN 2008ax and SN 1993J. The LC are very different for the first twelve days after the explosions but all three SN IIb display similar peak luminosities, times of peak, decline rates and colors after maximum. This suggests that the progenitors of these SN IIb may have had similar compositions and masses but they exploded inside hydrogen shells that that have a wide range of masses. SN 2011dh was well observed and a likely progenitor star has been identified in pre-explosion images. The detailed observations presented here will help evaluate theoretical models for this supernova and lead to a better understanding of SN IIb.

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