Highly Host-Extinguished Type Ia Supernova SNF20080720-001

ATel #1624; The Nearby Supernova Factory: G. Aldering, S. Bongard, M. Childress, S. Loken, P. Nugent, S. Perlmutter, P. Ripoche, K. Runge, R. C. Thomas (Lawrence Berkeley National Laboratory, Berkeley, CA); P. Antilogus, S. Bailey, R. Pain, R. Pereira, C. Wu (Laboratoire de Physique Nucleaire et de Haute Energies de Paris); C. Buton, Y. Copin, E. Gangler, G. Smadja, C. Tao (Institut de Physique Nucleaire de Lyon); G. Rigaudier, E. Pecontal (Centre de Recherche Astronomique de Lyon); C. Baltay, J. Jerke, D. Rabinowitz, R. Scalzo (Yale University, New Haven, CT); and D. Birchall (University of Hawaii)
on 22 Jul 2008; 01:28 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: R. C. Thomas (rcthomas@lbl.gov)

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The Nearby Supernova Factory reports its discovery and classification of supernova SNF20080720-001 (J2000.0 coordinates RA 01:05:27.20 Dec +37:51:11.05). The supernova was discovered using images obtained Jul 20.5 UTC with the QUEST-II camera on the Palomar Oschin 48-inch telescope operated by the Palomar-QUEST Consortium, at a magnitude of 16.6, calibrated to SDSS i-band. A spectrum (range 320-1000 nm) obtained Jul 21.6 UTC with the SuperNova Integral Field Spectrograph (SNIFS) on the University of Hawaii 2.2-meter telescope reveals it to be a Type Ia SN. The following link includes the SNIFS acquisition and IFU images, and SNIFS spectrum. The host galaxy is an edge-on spiral. The SN was discovered near the core of its host, in projection, sitting above a prominent dust lane. Schlegel et al. 1998, ApJ, 500, 525 predict a Milky Way reddening of E(B-V) = 0.071 along the line of sight.

http://snfactory.lbl.gov/snf/data/SNF20080720-001/

The SN spectrum is most striking. While the relative strengths and line profiles seem quite typical for a normal SN Ia, the overall shape of the continuum is very red. Additionally, three narrow absorption features are visible, centered at 401.6, 405.1, and 601.7 nm (observer frame) with corresponding equivalent widths (EW) 1.8, 1.7, and 7.1 AA. The wavelengths are consistent with Ca II H&K and Na I D respectively if the host galaxy redshift is z = 0.021. K I absorption lines are not detected.

The redshift derived from the narrow absorptions is consistent with a result derived from a cross-correlation of the SN spectrum with a library of SN spectra (via SNID, see Blondin & Tonry 2007, ApJ, 666, 1024). The cross-correlation indicates that the SN is almost at maximum light. The Ca II IR triplet feature seems to include a high-velocity component extending above 20,000 km/s, though this feature is not as strong as in the case of say, SN 2001el (Wang et al. 2003 ApJ, 591, 1110). Otherwise, the features are spectroscopically normal.

SNF20080720-001 represents an opportunity to make progress on the vexing interrelated issues of SN Ia intrinsic colors and dust extinction from host galaxies or immediate SN environment. The Na I D EW is greater than in other notable reddened SNe Ia --- 5.3 AA for SN 2003cg (Elias-Rosa et al. 2006, MNRAS, 369, 1880), 3.6 AA for SN 1986G (Rich 1987, AJ, 94, 651), and 1.7 AA for SN 2006X (Wang, X. et al. 2008, ApJ, 675, 626). The relationship of Munari & Zwitter (1997, A&A, 318, 269), which attempts to account for saturation of the Na I D line, predicts E(B-V) ~ 2.5, or A_V ~ 8 mag. As the reddening and brightness of SNF20080720-001 do not support such a large dust extinction, the gas/dust ratio along this line of sight must be quite abnormal. This raises the possibility that at least some of the atomic absorption is occurring in the immediate vicinity of the SN. Further, a preliminary analysis suggests that the R_V parameter (Cardelli, Clayton, & Mathis 1989, ApJ, 345, 245) may be lower than the standard Milky Way value of 3.1.

Therefore, the Nearby Supernova Factory urges observations of this interesting object with emphasis on high-resolution spectroscopy (especially to detect any evolution in the atomic absorption features that may indicate they form in proximity to the SN), as well as infrared and space-based ultraviolet photometry.

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