Interaction Between The Broad-lined Type Ic Supernova 2012ap and Carriers of Diffuse Interstellar Bands

TLDR: The diffuse interstellar bands (DIBs) are absorption features observed in optical and near-infrared spectra that are thought to be associated with carbon-rich polyatomic molecules in interstellar gas as discussed by the authors.

Abstract: The diffuse interstellar bands (DIBs) are absorption features observed in optical and near-infrared spectra that are thought to be associated with carbon-rich polyatomic molecules in interstellar gas. However, because the central wavelengths of these bands do not correspond with electronic transitions of any known atomic or molecular species, their nature has remained uncertain since their discovery almost a century ago. Here we report on unusually strong DIBs in optical spectra of the broad-lined Type Ic supernova SN 2012ap that exhibit changes in equivalent width over short (~30 days) timescales. The 4428 and 6283 Angstrom DIB features get weaker with time, whereas the 5780 Angstrom feature shows a marginal increase. These nonuniform changes suggest that the supernova is interacting with a nearby source of the DIBs and that the DIB carriers possess high ionization potentials, such as small cations or charged fullerenes. We conclude that moderate-resolution spectra of supernovae with DIB absorptions obtained within weeks of outburst could reveal unique information about the mass-loss environment of their progenitor systems and provide new constraints on the properties of DIB carriers.

Figures

Figure 1. Optical spectra of the broad-lined Type Ic SN 2012ap during its first ∼ 40 days after explosion. Three prominent DIBs around 4428 Å, 5780 Å, and 6283 Å are highlighted with dashed lines. SALT and Keck data have a 4–6 Å FWHM resolution. MMT data have 7 Å FWHM. Symbols show telluric absorptions in the SALT spectra that have not been corrected. Spectra have been corrected for a redshift of z = 0.0121 measured from narrow H ii region lines of [O iii] λλ4959, 5007, Hα, and [N ii] λλ6548, 6583 observed near the location of the supernova.

Figure 3. Model spectrum of known DIB absorption features compared to early-time spectra of SN 2012ap, SN 2009bb (Pignata et al. 2011), and SN 2008D (Modjaz et al. 2009). SN 2009bb and SN 2008D have been corrected for redshifts of z = 0.010 and z = 0.007, respectively. All SNe exhibit conspicuous absorptions having central wavelengths of well-known DIBs highlighted with vertical dashed lines. Time is with respect to maximum light. The model DIB spectrum was created from a catalog retrieved online at http://leonid.arc.nasa.gov/DIBcatalog.html.

Figure 2. Enlarged DIB absorption features of SN 2012ap. The left and middle columns show select epochs to illustrate possible changes in EW, and the right column shows all EW measurements of narrow absorptions. Phase is with respect to B-band maximum on February 18.2 UT. The EW of the Na i D lines of the host galaxy shows no measurable change. To further illustrate that the changes are not related to instrumental setups, spectra from the same spectrograph using identical configurations are plotted.