Early Optical Spectroscopy of SN 2011dh in M51

ATel #3428; G. S. Stringfellow (University of Colorado, Boulder), S. B. Howell (NASA ARC), R. Probst (NOAO), and S. Seebode (SMHS)
on 12 Jun 2011; 20:17 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Guy S. Stringfellow (Guy.Stringfellow@Colorado.edu)

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We obtained optical spectroscopy of SN 2011dh (PTF11eon) in M51 (ATEL #3398; CBET #2736) with the Palomar Hale 5m on UT 2011 June 5.18 using the Double Spectrograph. The red and blue cameras were set to provide a resolving power of R~4700 with a wavelength range covering roughly 6170-6850 A and R~3000 spanning 3500-5050 A, respectively. The spectra were heliocentric corrected and shifted to the rest frame of M51 using 465 km/s (Falco et al. 1999, PASP, 111, 438).

Our red spectrum shows a broad H-alpha line with a peak near 6492 A (blueshifted by 3200 km/s) and a broad P-Cygni profile. The blueshifted minimum in the H-alpha absorption extends out to at least 14,300 km/s relative to the peak, though our spectrum does not contain much of a rise to the blue below 6200 A to determine this precisely. The flux ratio between the peak in H-alpha and the absorption minimum is ~2.2, quite similar to that seen in the 1987 August 7 spectrum of SN 1987K (see Figure 4 in Filippenko 1988, AJ, 96, 1941). Pastorello et al. (2008, MNRAS, 389, 955) identify this stage for SN 1987K as Type IIb (see their Figure 5). No other absorption features are seen to the red of the peak in H-alpha, out to 6850 A. There are no prominent emission lines indicating that SN 2011dh is surrounded by an HII region, though such emission could be swamped by that of the SN. However, our long-slit spectrum does indicate the presence of HII regions at distances farther from SN 2011dh. The Type IIb SN 2008ax lacks distinct H-alpha emission seen in our spectrum of SN 2011dh. SN 2008ax also shows early signs of He I 6678 A absorption developing in the +10 day post-shock-breakout spectrum (Figure 4 of Pastorello et al. 2008), which is absent in our spectrum of SN 2011dh.

The blue spectrum has a continuum that rises toward the red with prominent absorption minima identified as Ca II, H-delta, H-gamma, and H-beta, with several Fe II lines present beyond 4700 A. In the spectral regions of overlap, the line structure closely resemble both SN 1993J and SN 1987K. There are some modest differences in line strengths below 4200 A, between 4750-5100 A, and in the slope of the overall continuum. In SN 1993J the slope of the continuum turns over from red to blue at wavelengths longer than H-beta, while SN 2011dh is still rising to the red out to 5000 A. The overall blue+red spectral continuum shape resembles the Day 16 spectrum of SN 1993J (Figure 1 of Matheson et al. 2000, AJ, 120, 1487) rather than SN 1987K.

Silverman et al. (2011, ATEL #3398) report a Type II spectrum obtained on June 2 (3 days earlier than ours) that has a "relatively blue continuum". Comparative reevaluation of the same Silverman et al. spectrum by Arcavi et al. (2011, ATEL #3413) reveal similarity to SN 1993J on 1993 April 13 (Day 17 in Matheson et al. 2000). That spectrum still reflects a blue continuum at wavelengths longer than 4700 A, but the comparison would then also imply a drop in flux at shorter wavelengths. Our blue spectrum has a red slope, rising continuously from 3500 A to 5000 A, though not steeply. Barring visual inspection of the SN 2011dh spectra at the two epochs, both spectra of SN 2011dh appear generally consistent with a Type IIb event (at this stage), and are not grossly disparate with one another. Differences between the two epochs may arise from simple, rapid time evolution of the supernova. There is no conclusive evidence yet of a transitional spectrum, though continued spectral monitoring will decide this. We will continue to follow the spectral evolution of SN 2011dh.