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

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

2005-07-12

arxiv.org/abs/astro-ph/0507288v1

Astron.J.130:2278-2292,2005

Astronomy and Astrophysics

Astrophysics

40 pages, 24 figures, accepted for publication in The Astronomical Journal

Scientific paper

10.1086/444595

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.

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