Circumstellar H-alpha emission from SN 1992ad^*

Robert J. Cumming^1,2,3, W. Peter S. Meikle^1,2

¹ Royal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ;
² Imperial College of Science Technology and Medicine, Blackett Laboratory, Prince Consort Road, London SW7 2BZ;
³ Stockholm Observatory, AlbaNova University Center, SE-106 91 Stockholm, Sweden.

^* Based on an unpublished poster presented at the 34th Herstmonceux conference, Circumstellar Media in the Late Stages of Stellar Evolution, Cambridge, July 12-16, 1993.

Abstract

We report the probable detection of narrow circumstellar H-alpha emission associated with the normal Type II supernova SN 1992ad at ~210 d after explosion.

Introduction

Our aim is to test stellar evolution theory by studying the progenitors of supernovae, in particular by attempting to detect their circumstellar media. The EUV burst from shock breakout and X-rays from ejecta/CSM interaction can ionize the wind material, producing narrow optical emission lines.

SN 1992ad, one of 1992's brightest supernovae, was found in NGC 4411B in the Virgo cluster on 1992 July 1 (McNaught et al. 1992). It was a normal Type II supernova, showing P Cygni profiles in the Balmer lines (McNaught et al. 1992), suggesting that it was probably a Type II-P (plateau) event.

Observations

We observed SN 1992ad on a service night at the 4.2-m William Herschel Telescope (1993 February 16, ~210d after the SN exploded), using ISIS at resolution about 8000, in excellent conditions (seeing 1 arcsec).

The figure shows part of our long-slit spectrum around H-alpha, with broad SN emission subtracted. There's H II region emission adjacent to the supernova, but the H-alpha from the supernova position is narrower than the H II region emission, and slightly blueshifted relative to it.

Results

The change in velocity and width of the H-alpha emission at the supernova position lead us to believe that what we're seeing here is narrow emission associated with the supernova. Calibrating relative to a low-resolution spectrum taken the same night, we measured the H-alpha flux. We don't see [N II] 6583Å, so we have derived an upper limit.

For H-alpha, we find

• f(H-alpha) ~ 9×10^-17 erg s^-1 cm^-2
• L(H-alpha) ~ 4×10³⁵(D_Mpc/15.6)² erg s^-1
• line unresolved, FWHM < 38 km s^-1
• velocity shift relative to H II region ~12 km s^-1
• flux ratio f(H-alpha)/f([N II] 6583) > 4

Discussion and conclusions

The emitting gas seems to be slow-moving circumstellar material. The narrow H-alpha luminosity is about the same as detected from SN 1987A a year after it exploded. The H-alpha/[N II] 6583 ratio could be due to high density, high ionization state, or perhaps low N abundance.

This may have been the the first detection of late narrow circumstellar emission from a normal Type II supernova. Other evidence has also pointed to circumstellar interaction in SN 1992ad: it since has been observed in the radio (Van Dyk et al. 1996) and in 0.5-2 keV X-rays (Bregman et al. 2003).

References

• Bregman, J, N., Houck, J. C., Chevalier, R. A., Roberts, M. S. 2003, ApJ, 596, 323 [ADS]
• McNaught, R. H., Evans, R., Spyromilio, J., Taylor, K., Hawkins, M., Veron, P. 1992, IAU Circ. 5552
• Van Dyk, S. D., Sramek, R. A., Weiler, K. W., Montes, M. J., Panagia, N. 1996, IAU Circ. 6378

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Robert Cumming
Updated 2003-12-05.