Spectroscopic Observations and Analysis of the Unusual Type Ia SN 1999ac

G. Garavini (1) , G. Aldering , A. Amadon , R. Amanullah , P. Astier (1) , C. Balland (2) , G. Blanc , A. Conley , T. Dahlén , S.E. Deustua , R. Ellis , S. Fabbro , V. Fadeyev , X. Fan , G. Folatelli , B. Frye , E.L. Gates , R. Gibbons , G. Goldhaber , B. Goldman , A. Goobar , D.E. Groom , J. Haissinski (3) , D. Hardin (1) , I. Hook , D.A. Howell , S. Kent , A.G. Kim , R.A. Knop , M. Kowalski , N. Kuznetsova , B.C. Lee , C. Lidman , J. Mendez , G.J. Miller , M. Moniez (3) , M. Mouchet (4, 5) , A. Mourão , H. Newberg , S. Nobili (1) , P.E. Nugent , R. Pain (1) , O. Perdereau (3) , S. Perlmutter , R. Quimby , N. Regnault (1) , J. Rich , G.T. Richards , P. Ruiz-Lapuente , B.E. Schaefer , K. Schahmaneche (1) , E. Smith , A.L. Spadafora , V. Stanishev , R.C. Thomas , N.A. Walton , L. Wang , W.M. Wood-Vasey

1 LPNHE - Laboratoire de Physique Nucléaire et de Hautes Énergies
2 APC - Cosmologie
3 LAL - Laboratoire de l'Accélérateur Linéaire
4 APC (UMR_7164) - AstroParticule et Cosmologie
5 LUTH (UMR_8102) - Laboratoire Univers et Théories

We present optical spectra of the peculiar Type Ia supernova (SN Ia) 1999ac. The data extend from -15 to +42 days with respect to B-band maximum and reveal an event that is unusual in several respects. Prior to B-band maximum, the spectra resemble those of SN 1999aa, a slowly declining event, but possess stronger SiII and CaII signatures (more characteristic of a spectroscopically normal SN). Spectra after B-band maximum appear more normal. The expansion velocities inferred from the Iron lines appear to be lower than average; whereas, the expansion velocity inferred from Calcium H and K are higher than average. The expansion velocities inferred from SiII are among the slowest ever observed, though SN 1999ac is not particularly dim. The analysis of the parameters v_10, R(SiII), dv(SiII)/dt, and d_m15 further underlines the unique characteristics of SN 1999ac. We find convincing evidence of CII 6580 in the day -15 spectrum with ejection velocity v > 16,000 km/s, but this signature disappears by day -9. This rapid evolution at early times highlights the importance of extremely early-time spectroscopy.

Mots clés: supernovae: general; supernovae: individual (SN 1999ac)

Domaines: Astrophysique [astro-ph]; Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Cosmologie et astrophysique extra-galactique [astro-ph.CO]

G. Garavini, G. Aldering, A. Amadon, R. Amanullah, P. Astier, et al.. Spectroscopic Observations and Analysis of the Unusual Type Ia SN 1999ac. The Astronomical Journal, 2005, 130, pp.2278-2292. ⟨10.1086/444595⟩. ⟨in2p3-00025098⟩