SN 2012au: A GOLDEN LINK BETWEEN SUPERLUMINOUS SUPERNOVAE AND THEIR LOWER-LUMINOSITY COUNTERPARTS

SN 2012au was discovered on UT 2012 March 14 by the CRTS SNHunt project (Howerton et al. 2012). Figure 1 (left) shows the region around the SN and its host galaxy NGC 4790 (D = 23.5 ± 0.5 Mpc; Theureau et al. 2007). SN 2012au is located at coordinates $\alpha = 12^{\rm h}54^{\rm m}52\buildrel{\mathrm{s}}\over{.}18$ and δ = −10°14'502 (J2000.0), which is less than 600 pc away in projection from the center of NGC 4790's bright central nucleus.

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Ultraviolet (UV) and optical observations with the Swift-UVOT instrument began on 2012 March 15 and continued through to 2012 April 21 (PI: P. Brown). Data were acquired using six broadband filters and have been analyzed following standard procedures. The details of these Swift-UVOT observations are provided in Table 1.

Two epochs of late-time r'-band photometry of SN 2012au were obtained. A sequence of 3 × 300 s dithered images was taken on 2013 January 23 using the 2.4 m Hiltner telescope at MDM Observatory with the OSMOS instrument⁸ and MDM4k detector, and a sequence of 3 × 120 s dithered images was taken on 2013 February 18 using the 6.5 m MMT with the MMTcam instrument.⁹ Images were bias-subtracted, flat-fielded, and stacked following standard procedures using the IRAF¹⁰ software.

Emission from SN 2012au dropped ≲ 4 mag between maximum light and our first late-time measurement 308 days later. This decline is considerably slower than other SNe Ib/c; e.g., SN 1998bw declined by ≈6.5 mag over the same time period (Patat et al. 2001). The longevity of the brightness made it impossible to obtain subtraction templates that could completely remove contaminating emission from the underlying host galaxy. Photometric measurements were thus made using the sextractor software (Bertin & Arnouts 1996) and checked manually using point-spread function fitting techniques.

We constructed a pseudo-bolometric light curve using the Swift-UVOT photometry. Observations were corrected assuming R_V = A_V/E(B − V) = 3.1 and a total reddening of E(B − V) = 0.063 mag. This estimate combines reddening due to the Milky Way, E(B − V)_mw = 0.043 mag (Schlafly & Finkbeiner 2011), and host internal extinction of E(B − V)_host = 0.02 ± 0.01 mag determined from measuring the equivalent width (EW) of Na i D absorption in our optical spectra (Section 2.3) and using the relationship between EW and E(B − V) described in Poznanski et al. (2012).

The pseudo-bolometric light curve incorporates two values from Howerton et al. (2012), one of which is a detection of the SN on 2012 March 4 that constrains the rise time from explosion to peak brightness to be ≳16 days. We summed the flux emitted in the uvw2 through v bands by means of a trapezoidal interpolation and estimated the missing flux from the RI passbands and the near-infrared around the time of maximum light, t_max. Using values observed in other SNe Ib/c (see Valenti et al. 2008), we assumed that for −5 < Δt_max < 30 days, the RI-band flux contribution varies from 25% to 40% and the near-infrared band contribution varies from 15% to 45%.

The pseudo-bolometric light curve was modeled using assumptions in Arnett (1982) and following procedures in Valenti et al. (2008) to determine the SN's total nickel mass, M_Ni, explosion kinetic energy, E_K, and total ejecta mass M_ej. A constant optical opacity k_opt of 0.05 cm² g⁻¹ was adopted (see Drout et al. 2011 for details). Our best-fit model shown in Figure 1 (right) uses the explosion date 2012 March 3.5, and a rise time of 16.5 ± 1.0 days. The peak bolometric luminosity is ∼6 × 10⁴² erg s⁻¹, and the estimates of the explosion parameters are M_Ni ≈ 0.3 M_☉, M_ej ≈ 3–5 M_☉, and E_K ∼ 1 × 10⁵² erg.

We break the degeneracy between E_K and M_ej using an estimate of the ejecta velocity measured from our optical spectra around t_max to be ≈1.5 × 10⁴ km s⁻¹ (Section 2.3). The value of M_Ni is consistent with independent estimates derived from the late-time photometry under the assumption of full gamma-ray trapping (Figure 1, right inset). These explosion parameters are comparable to those estimated for the hypernovae SN 1998bw (M_Ni ∼ 0.4 M_☉ and E_K ∼ 2 × 10⁵² erg; Maeda et al. 2006) and SN 1997ef (M_Ni ∼ 0.16 M_☉ and E_K ∼ 1 × 10⁵² erg; Iwamoto et al. 2000).

We note that SN 2012au may exhibit properties that are not strictly incorporated in the Arnett (1982) model we have used. Opacities as high as 0.2 cm² g⁻¹ could apply that increase the M_Ni/M_ej ratio to values that run contrary to model assumptions, and optical spectra (Section 2.4) suggest possible deviations from the model's spherically symmetric conditions. Future work may look to apply more sophisticated models to address these additional factors.

Low-resolution optical spectra of SN 2012au were obtained from 2012 March through 2013 February. Many of these observations were made with the F. L. Whipple Observatory 1.5 m telescope mounted with the FAST instrument (Fabricant et al. 1998). Additional observations were obtained with the MMT 6.5 m telescope using the Blue Channel instrument (Schmidt et al. 1989), and the 2.4 m MDM telescope using the OSMOS instrument.

Standard procedures using the IRAF software were followed and flux calibrations were made using our own IDL routines. A recession velocity of 1295 km s⁻¹ determined from overlapping nebular Hα emission has been removed from all spectra. Line identifications and estimates of expansion velocities of the photospheric spectra were made with the SN spectrum synthesis code SYN++ (Thomas et al. 2011).

Four epochs of our optical spectra of SN 2012au are plotted in Figure 2 (top). The reported phase is with respect to the v-band maximum on 2012 March 21. Our earliest spectrum obtained on 2012 March 17 (Δt_max = −4 days) shows features associated with He i, Fe ii, Si ii, Ca ii, Na i, and O i exhibiting velocities of (1.8–2.0) × 10⁴ km s⁻¹. There is weak evidence of Hα absorption.

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We show a slightly later spectrum obtained on 2012 March 27 (Δt_max = +6 days) in Figure 2 (top), illustrating how the identified ions are generally consistent with a Type Ib classification (e.g., SN 2008D) though the velocities in SN 2012au are relatively high. Approximately two months later on 2012 May 17 (Δt_max = +57 days), the same ions are detected but the Type Ib classification is less appropriate. The observed velocities (∼7000 km s⁻¹) remain above average relative to other SNe Ib/c at this epoch, the P-Cyg absorptions broaden, and the spectrum resembles those of SN 1997dq and SN 1997ef (Matheson et al. 2001).

The defining properties of SN 2012au are seen in its nebular spectrum (Figure 2, top). Emissions from SNe Ib/c at these epochs are normally optically thin and dominated by [O i] λλ6300, 6364, [Ca ii] λλ7291, 7324, and Mg i] λ4571. In addition to these emissions, however, SN 2012au shows persistent P-Cyg absorptions attributable in part to Fe ii at ≲ 2000 km s⁻¹, as well as unusually strong emissions from Ca ii H&K, Na i D, and a feature around 7775 Å we identify as O i λ7774.

Also seen in the nebular spectrum is a "plateau" of emission between 4000 and 5600 Å. This emission is due largely to iron-peak elements (primarily [Fe ii], and some [Fe iii] and [Co ii]), which is usually only observed with this strength in the late-time spectra of SNe Ia. Unlike SNe Ia, however, here the [Fe iii] lines are weak.

In Figure 3 (left), emission line profiles of SN 2012au's most prominent late-time spectral features are shown. There are real differences among the emission line profiles across elements and their ions. Specific features of note include (1) the [Ca ii], Na i, and Mg i] lines are not symmetric about zero velocity and exhibit higher expansion velocities than [O i], and (2) the O i and [O i] lines indicate two distinct velocity regions in the ejecta.

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The relative strengths of narrow lines from coincident host galaxy emission in the nebular spectra were measured to estimate the explosion site metallicity using the method described in Sanders et al. (2012). From the N2 diagnostic of Pettini & Pagel (2004), we measure an oxygen abundance of log (O/H) + 12 = 8.9 with uncertainty 0.2 dex. Adopting a solar metallicity of log (O/H)_☉ + 12 = 8.7 (Asplund et al. 2005), the measurement indicates that SN 2012au exploded in an environment of super-solar metallicity around Z ∼ 1–2 Z_☉. This metallicity is significantly higher than any of the broad-lined SN Ic host galaxies from untargeted surveys (Sanders et al. 2012).

Three epochs of low-resolution, near-infrared spectra spanning 0.82–2.51 μm were obtained using the FoldedPort Infrared Echellette (FIRE) spectrograph on the 6.5 m Magellan Baade Telescope (Simcoe et al. 2008). The low dispersion prism used in combination with a 06 slit yielded a spectral resolution R ∼ 500 in J band. Data were reduced following standard procedures (see, e.g., Hsiao et al. 2013) using a custom-developed IDL pipeline (FIREHOSE).

The reduced near-infrared spectra are plotted in Figure 2 (bottom). In our Δt_max = +31 day spectrum, absorptions around both the He i λ1.083 μm and λ2.05 μm lines support the identification of He i made in the optical spectra. The minima of these absorptions shift toward longer wavelengths in the later epochs, and the absorption around 1 μm grows in strength as the 2 μm absorption fades. This suggests that the He i strength diminishes as other ions possibly including Si i and C i gradually dominate the 1 μm absorption as time passes.

Between Δt_max = +81 and +318 days, the FWHM of strong emission features associated with the Mg i λ1.503 μm and O i λ1.317 μm lines narrowed from approximately 5700 km s⁻¹ to 2000 km s⁻¹. The presence of these lines and the similarity of their velocity distribution to the feature around 7775 Å support our identification of it being associated with O i λ7774 (Figure 3, left).

Our UV/optical and near-infrared observations of SN 2012au show it to be a slow-evolving energetic SN with a number of rarely observed late-time emission properties. SN 2012au's plateau of iron emission lines, intermediate-width O i λ7774 emission, persistent P-Cyg absorption features, and prolonged brightness almost one year after explosion are not observed in the majority of late-time spectra of SNe Ib/c (Figure 4, top).

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However, a handful of objects including SN 1997dq and 2007bi do share SN 2012au's rare blend of late-time properties. Aside from some difference in the strength and velocity widths of the Fe and Mg emissions below ≈5600 Å, above ≈5600 Å emissions from these SNe are almost indistinguishable (Figure 4, bottom). Like SN 2012au, these objects exhibited slow spectroscopic evolution and slowly declining light curves (see Mazzali et al. 2004; Gal-Yam et al. 2009; Young et al. 2010).

Mazzali et al. (2004) interpreted the long duration of the photospheric phase in SN 1997dq and SN 1997ef to be the consequence of a sharply defined two-component ejecta distribution made of an inner, high-density region, located inside a much lower-density region of high-velocity ejecta. This was a scenario that had been previously modeled by Maeda et al. (2003). Mazzali et al. concluded that a significant fraction of Ni-rich material was associated with velocities below the normal mass cutoff imposed in one-dimensional explosion models and that this signaled the presence of explosion asymmetries.

A similar explanation may be applicable to SN 2012au. The persistent P-Cyg absorptions and asymmetries between elements and their ions in the emission line profiles are consistent with expectations of a moderately aspherical explosion. Moreover, the absence of [Fe iii] in the iron plateau region (Figure 4) indicates that the density of the Fe-rich region is high and likely clumped. Thus, it is possible that this asphericity was jet-driven, since models have demonstrated that jetted explosions can significantly alter density profiles and ⁵⁶Ni distributions, especially in the central region (Maeda & Nomoto 2003).

The presence of the density-sensitive line O i λ7774 and its velocity width is especially noteworthy. In the cases of SN 2012au and SN 2007bi, the center of the O i distribution sits in the middle of an emission gap of the [O i] λλ6300, 6364 profile (Figure 3, right). This, along with the Mg i 1.503 μm feature in the near-infrared, is suggestive of an O- and Mg-rich region with densities that are collisionally quenching some of the [O i] λλ6300, 6364 emission.

On the one hand, it seems unlikely that SNe with very different peak luminosities and light curves can share progenitor systems and explosion mechanisms (Figure 1, right). SN 2007bi was a superluminous (M_B ≈ −21 mag) explosion that ejected some ∼3–5 M_☉ of radioactive ⁵⁶Ni, and was associated with a 200 M_☉ progenitor (zero-age main sequence) located in a low-metallicity dwarf galaxy (Gal-Yam et al. 2009; Young et al. 2010). These properties are very different from SN 2012au, which was far less luminous (M_B ≈ −18 mag), produced an order of magnitude less ⁵⁶Ni, and was located in a nearby flocculent spiral we estimate to have super-solar metallicity. Moreover, we find that a comparison of Geneva stellar evolution models (Ekström et al. 2012) to results from a preliminary analysis of pre-explosion Hubble Space Telescope images of the region encompassing SN 2012au (Van Dyk et al. 2012) places loose limits on the progenitor star as likely being <80 M_☉.

Despite these differences, however, the combination of observed optical properties shared between these SNe is still suggestive of a unified explosion mechanism. If a connection between these objects exists, then one of the three currently favored energy mechanisms of SLSNe powering their slow light curves should extend to SN 2012au. One late-time mechanism is interaction with circumstellar material. Although no narrow emission lines (FWHM ≲ 100 km s⁻¹) have been detected, interaction with H-poor circumstellar shells from pulsational pair-instability SNe could potentially be applicable (Chatzopoulos & Wheeler 2012). Favoring against this, however, is that SN 2012au's radio light curves and spectral energy distribution evolution are most consistent with blast wave interaction with a steady, homogeneous wind (A. Kamble et al., in preparation). Another mechanism is the injection of energy from a magnetar, but the presence of strong iron emission, as observed in SN 2012au, could be problematic for this scenario (Kasen & Bildsten 2010).

A straightforward understanding of SN 2012au's light curve is radioactive ⁵⁶Ni. The rise and fall of the light curve at photospheric epochs, and the subsequent slow decline of the light curve at a rate of ≈0.01 mag day⁻¹ up until ∼300 days, reasonably follow predictions of Ni–Co decay (Figure 1). Possible origins for the above average ⁵⁶Ni production may be either a pair-instability explosion or Fe core collapse of a massive progenitor. A pair-instability explosion is unlikely given that the progenitor mass and metallicity we estimate for SN 2012au are outside theoretical limits (see Langer et al. 2007). Hence, Fe core collapse is the prime candidate. SN 2012au's exceptionally high energies suggest that the explosion may have been aided by magnetohydrodynamic jets brought about by rapid rotation (Burrows et al. 2007).