[Paper Review] The carbon-rich type Ic supernova 2016adj in the iconic dust lane of Centaurus A: signatures of interaction with circumstellar hydrogen?

M. Stritzinger, E. Baron+25 more|arXiv (Cornell University)|Sep 10, 2023

Gamma-ray bursts and supernovae4 citations

TL;DR

This study reclassifies SN 2016adj in Centaurus A as a carbon-rich Type Ic supernova, not a Type Ib/IIb, based on optical and NIR spectroscopy. It presents evidence of circumstellar interaction with a hydrogen-rich shell, detected via H Paschen and Brackett emission lines at +97 days, suggesting ejecta-shock interaction. The analysis infers 0.5 M☉ of ⁵⁶Ni and 4.2 M☉ of ejecta, though uncertainties remain due to reddening and light echo effects.

ABSTRACT

We present a comprehensive data set of supernova (SN) 2016adj located within the central dust lane of Centaurus A. SN 2016adj is significantly reddened and after correcting the peak apparent $B$-band magnitude ($m_B = 17.48\pm0.05$) for Milky Way reddening and our inferred host-galaxy reddening parameters (i.e., $R_{V}^{host} = 5.7\pm0.7$ and $A_{V}^{host} = 6.3\pm0.2$), we estimate it reached a peak absolute magnitude of $M_B \sim -18$. Detailed inspection of the optical/NIR spectroscopic time-series reveals a carbon-rich SN Ic and not a SN Ib/IIb as previously suggested in the literature. The NIR spectra shows prevalent carbon-monoxide formation occurring already by +41 days past $B$-band maximum, which is $\approx 11$ days earlier than previously reported in the literature for this object. Interestingly around two months past maximum, the NIR spectrum of SN~2016adj begins to exhibit H features, with a +97~d medium resolution spectrum revealing both Paschen and Bracket lines with absorption minima of $\sim 2000$ km/s, full-width-half-maximum emission velocities of $\sim 1000$ km/s, and emission line ratios consistent with a dense emission region. We speculate these attributes are due to circumstellar interaction (CSI) between the rapidly expanding SN ejecta and a H-rich shell of material formed during the pre-SN phase. A bolometric light curve is constructed and a semi-analytical model fit suggests the supernova synthesized 0.5 solar masses of $^{56}$Ni and ejected 4.2 solar masses of material, though these values should be approached with caution given the large uncertainties associated with the adopted reddening parameters, possible CSI contamination, and known light echo emission. Finally, inspection of Hubble Space Telescope archival data yielded no progenitor detection.

Motivation & Objective

• Reclassify SN 2016adj from Type Ib/IIb to carbon-rich Type Ic using multi-epoch optical and NIR spectroscopy.
• Investigate signs of circumstellar interaction (CSI) with hydrogen-rich material via late-time H-line emission in NIR spectra.
• Construct a bolometric light curve and fit a semi-analytical model to infer ⁵⁶Ni mass and ejecta mass, accounting for dust reddening and light echo contamination.
• Search for the progenitor star in pre-explosion Hubble and VLT+NACO images using difference imaging and artificial star tests.
• Assess the host-galaxy reddening (RV_host = 5.7 ± 0.7, AV_host = 6.3 ± 0.2) and its impact on photometric and spectroscopic interpretation.

Proposed method

• Acquired a comprehensive optical and NIR spectroscopic time-series of SN 2016adj using ground- and space-based telescopes.
• Used MCMC-based modeling to fit CO emission features in the NIR spectra, determining formation timescale and excitation conditions.
• Constructed a bolometric light curve from multi-band photometry and applied a semi-analytical model to estimate ⁵⁶Ni mass and ejecta mass.
• Performed difference imaging between pre-explosion HST WFPC2/F555W and late-time WFC3/F555W images to search for progenitor disappearance.
• Conducted artificial star tests on NACO AO images to determine limiting magnitudes and assess progenitor detectability.
• Calibrated photometry using standard stars and modeled PSF variations across the NACO field to derive reliable zeropoints and depth limits.

Figure 1: Colored image of Centaurus A (NGC 5128) constructed from RGB images obtained with the Swope telescope located at the Las Campanas Observatory. SN 2016adj occurred within the central dust lane, just South-East of a very bright, and typically saturated foreground star.

Experimental results

Research questions

• RQ1 Is SN 2016adj a carbon-rich Type Ic supernova, and does it exhibit signs of circumstellar interaction with hydrogen-rich material?
• RQ2 What is the nature and origin of the H Paschen and Brackett emission lines observed at +97 days in the NIR spectrum?
• RQ3 What are the inferred ⁵⁶Ni mass and ejecta mass of SN 2016adj, and how do reddening and light echo effects affect these estimates?
• RQ4 Can the progenitor star of SN 2016adj be detected in pre-explosion Hubble and VLT+NACO images, and what are the constraints on its luminosity?
• RQ5 How do the host-galaxy reddening parameters (RV_host = 5.7 ± 0.7, AV_host = 6.3 ± 0.2) influence the interpretation of photometric and spectroscopic data?

Key findings

• SN 2016adj is reclassified as a carbon-rich Type Ic supernova based on optical and NIR spectroscopy, with CO formation detected as early as +41 days post-maximum.
• H Paschen and Brackett emission lines appear at +97 days, with absorption minima at ~2000 km s⁻¹ and FWHM emission velocities of ~1000 km s⁻¹, indicating dense circumstellar interaction.
• The emission line ratios are consistent with a dense, photoionized region, supporting the presence of a H-rich circumstellar shell formed during the pre-SN phase.
• The bolometric light curve fit suggests a ⁵⁶Ni mass of 0.5 M☉ and an ejecta mass of 4.2 M☉, though these values carry large uncertainties due to reddening and light echo contamination.
• No progenitor star is detected in pre-explosion HST or VLT+NACO images, with absolute magnitude limits of F814W > -5.7 mag and Ks > -7.5 mag, making detection of typical WR stars unlikely.
• The host-galaxy reddening is exceptionally high (AV_host = 6.3 ± 0.2 mag, RV_host = 5.7 ± 0.7), significantly affecting photometric corrections and spectral interpretation.

Figure 2: Optical spectra of SN 2016adj from $-$ 7 d to $+$ 156 d. Phase and telescope facility are indicated on the right of each spectrum. Prevalent spectral features are marked with vertical dashed lines and labeled. Telluric absorption features are labeled with Earth symbols, while for the +43 d