TYPE IIb SUPERNOVA SN 2011dh: SPECTRA AND PHOTOMETRY FROM THE ULTRAVIOLET TO THE NEAR-INFRARED

Abstract

We report spectroscopic and photometric observations of the Type IIb SN\n2011dh obtained between 4 and 34 days after the estimated date of explosion\n(May 31.5 UT). The data cover a wide wavelength range from 2,000 Angstroms in\nthe UV to 2.4 microns in the NIR. Optical spectra provide line profiles and\nvelocity measurements of HI, HeI, CaII and FeII that trace the composition and\nkinematics of the SN. NIR spectra show that helium is present in the atmosphere\nas early as 11 days after the explosion. A UV spectrum obtained with the STIS\nreveals that the UV flux for SN 2011dh is low compared to other SN IIb. The HI\nand HeI velocities in SN 2011dh are separated by about 4,000 km/s at all\nphases. We estimate that the H-shell of SN 2011dh is about 8 times less massive\nthan the shell of SN 1993J and about 3 times more massive than the shell of SN\n2008ax. Light curves (LC) for twelve passbands are presented. The maximum\nbolometric luminosity of $1.8 \\pm 0.2 \\times 10^{42}$ erg s$^{-1}$ occurred\nabout 22 days after the explosion. NIR emission provides more than 30% of the\ntotal bolometric flux at the beginning of our observations and increases to\nnearly 50% of the total by day 34. The UV produces 16% of the total flux on day\n4, 5% on day 9 and 1% on day 34. We compare the bolometric light curves of SN\n2011dh, SN 2008ax and SN 1993J. The LC are very different for the first twelve\ndays after the explosions but all three SN IIb display similar peak\nluminosities, times of peak, decline rates and colors after maximum. This\nsuggests that the progenitors of these SN IIb may have had similar compositions\nand masses but they exploded inside hydrogen shells that that have a wide range\nof masses. The detailed observations presented here will help evaluate\ntheoretical models for this supernova and lead to a better understanding of SN\nIIb.\n