Late-time spectroscopy of Type Iax Supernovae

Abstract

We examine the late-time (t 200 days after peak brightness) spectra of Type Iax supernovae (SNe Iax), a low-luminosity, low-energy class of thermonuclear stellar explosions observationally similar to, but distinct from, Type Ia supernovae. We present new spectra of SN 2014dt, resulting in the most complete published late-time spectral sequence of a SN Iax. At late times, SNe Iax have generally similar spectra, all with a similar continuum shape and strong forbidden-line emission. However, there is also significant diversity where some SN Iax spectra display narrow P-Cygni features from permitted lines and a continuum indicative of a photosphere at late times in addition to strong narrow forbidden lines, while others have no obvious P-Cygni features, strong broad forbidden lines, and weak narrow forbidden lines. Finally, some SNe Iax have spectra intermediate to these two varieties with weak P-Cygni features and broad/narrow forbidden lines of similar strength. We find that SNe Iax with strong broad forbidden lines also tend to be more luminous and have higher-velocity ejecta at peak brightness. We find no evidence for dust formation in the SN ejecta or the presence of circumstellar dust, including for the infrared-bright SN 2014dt. Late-time SN Iax spectra have strong [Ni ii] emission, which must come from stable Ni, requiring electron captures that can only occur at the high densities of a (nearly) Chandrasekhar-mass WD. Therefore, such a star is the likely progenitor of SNe Iax. We estimate blackbody and kinematic radii of the late-time photosphere, finding the latter an order of magnitude larger than the former for at least one SN Iax. We propose a two-component model that solves this discrepancy and explains the diversity of the late-time spectra of SNe Iax. In this model, the broad forbidden lines originate from the SN ejecta, similar to the spectra of all other types of SNe, while the photosphere, P-Cygni lines, and narrow forbidden lines originate from a wind launched from the remnant of the progenitor white dwarf and is driven by the radioactive decay of newly synthesised material left in the remnant. The relative strength of the two components accounts for the diversity of late-time SN Iax spectra. This model also solves the puzzle of a long-lived photosphere and slow late-time decline of SNe Iax.

Wait for It: Post-Supernova Winds Driven by Delayed Radioactive Decays

“…The large optical depths of the relatively massive SN Iax bound envelopes result in very extended photospheres and low effective temperatures that cool to 1000−2000 K by ∼ 400 d. The IR detection of SN 2014dt at 300−400 d (Fox et al 2016) may be a common feature of SNe Iax and may be due to the low effective temperature of the photosphere, as opposed to, or in addition to, dust formation (Foley et al 2016).…”

Light-curve and spectral properties of ultrastripped core-collapse supernovae leading to binary neutron stars

“…8), but it still seems to fail to reproduce crowded, unblended line features. In addition, the spectra of ultrastripped SNe evolve quickly to the nebular regime, while the spectra of SN 2002cx-like SNe do not (e.g., Sahu et al 2008;Foley et al 2016). Table 2 summarizes the results of the comparison between our ultra-stripped SN models and possible observational counterparts.…”

Type Iax Supernovae

“…Around 250 days past maximum, the spectra of SN 2002cx (a "bright" SN Iax; Jha et al 2006) and SN 2010ae (one of the faintest) are nearly identical (see Figure 12 Foley et al (2013) and show clear evidence of He I emission in their post-maximum spectra, something never seen in normal SN Ia. White et al (2015) argue that these objects may be type-IIb supernovae instead, though Foley et al (2016) counter that claim and call PTF 09ego and PTF 09eiy into question as SN Iax. In the end, there are a handful of objects for which the classification may be ambiguous.…”

“…It is at late times that the spectra of SN Iax radically diverge from SN Ia, and indeed, almost all other supernovae of any type Sahu et al 2008;Foley et al 2010aFoley et al , 2016. SN Iax never truly enter a fully "nebular" phase in which broad forbidden lines dominate the optical spectrum ( Figure 3) [Ca II] are also usually present, and in some cases with narrow widths down to < 500 km s −1 (McCully et al 2014b;Stritzinger et al 2015).…”

“…SN Iax never truly enter a fully "nebular" phase in which broad forbidden lines dominate the optical spectrum ( Figure 3) [Ca II] are also usually present, and in some cases with narrow widths down to < 500 km s −1 (McCully et al 2014b;Stritzinger et al 2015). The linewidths and relative strengths of the forbidden and permitted lines seem to vary significantly among different SN Iax (Yamanaka et al 2015;Foley et al 2016). The late-time linewidth variation, for example, approaches nearly an order of magnitude, from a few hundred to ∼3000 km s −1…”