The bright long-lived Type II SN 2021irp powered by aspherical circumstellar material interaction (I): Revealing the energy source with photometry and spectroscopy

TL;DR

This work analyzes the luminous, long-lasting Type II SN 2021irp using comprehensive photometry and spectroscopy to identify its energy source. The evidence points to extensive interaction with a massive, highly asymmetric circumstellar medium (CSM) as the dominant power source, with dust forming in the interaction region around 250–300 days and an early IR echo from pre-existing dust likely contributing at the earliest times. The observed line-profile asymmetries, flat optical colours followed by redward evolution, and a growing IR excess collectively indicate a patchy, non-spherical photosphere created by CSM interaction and dust processing. The results imply a substantial, structured CSM environment around the progenitor and highlight the role of dust formation in shaping late-time observables in luminous Type II SNe.

Abstract

Some core-collapse supernovae (CCSNe) are too luminous and radiate too much total energy to be powered by the release of thermal energy from the ejecta and radioactive-decay energy from the synthesised $^{56}$Ni/$^{56}$Co. A source of additional power is the interaction between the supernova (SN) ejecta and a massive circumstellar material (CSM). This is an important power source in Type IIn SNe, which show narrow spectral lines arising from the unshocked CSM, but not all interacting SNe show such narrow lines. We present photometric and spectroscopic observations of the hydrogen-rich SN 2021irp, which is both luminous, with $M_{o} < -19.4$ mag, and long-lived, remaining brighter than $M_{o} = -18$ mag for $\sim$ 250 d. We show that an additional energy source is required to power such a SN, and determine the nature of the source. We also investigate the properties of the pre-existing and newly formed dust associated with the SN. Photometric observations show that the luminosity of the SN is an order of magnitude higher than typical Type II SNe and persists for much longer. We detect a infrared excess attributed to dust emission. Spectra show multi-component line profiles, an Fe II pseudo-continuum, and a lack of absorption lines, all typical features of Type IIn SNe. We detect a narrow (< 85 kms$^{-1}$) P-Cygni profile associated with the unshocked CSM. An asymmetry in emission line profiles indicates dust formation occurring from 250-300 d. Analysis of the SN blackbody radius evolution indicates asymmetry in the shape of the emitting region. We identify the main power source of SN 2021irp as extensive interaction with a massive CSM, and that this CSM is distributed asymmetrically around the progenitor star. The infrared excess is explained with emission from newly formed dust although there is also some evidence of an IR echo from pre-existing dust at early times.

Figures (16)

• Figure 1: Combined $Bri$ images of the field of SN 2021irp obtained with the NOT+ALFOSC. We used our best quality images: the $B$ and $i$ images are taken after the SN has entirely faded, while the $r$ image is a late time (499 d) deep image in which the SN is still visible and indicated by the tick marks. The faint line on the right side of the image is a saturation artefact from a bright star.
• Figure 2: Top panel: The NaI D absorption feature in our highest S/N spectra. In the +271 d spectrum, the individual narrow components of an absorption doublet at the host redshift can be seen. The resolving powers of the two spectra are listed. Middle panel: The Hei$\lambda$5876 / Na i D emission feature for SN 2021irp and SN 2017hcc at a similar epoch, normalised at the emission line peak. Velocity is with respect to the midpoint of the Na doublet. Both SNe display an absorption feature superimposed in the emission profile. The narrower absorption at $\sim-6000$km s$^{-1}$ is the MW Na absorption. Bottom panel: The absorption line in the spectrum of SN 2021irp associated with Ca H has a very similar width and blueshift to the Na D feature.
• Figure 3: Optical and IR data of SN 2021irp. All data is corrected both for MW and host galaxy extinction. Vertical ticks represent the timing of spectral observations. Downward triangles are $3\sigma$ upper limits. Where there are multiple observations on the same night, the individual observations are shown with transparent symbols, and the binned measurement is shown with a solid symbol with a black outline. The solid lines show the results of Gaussian processing of the measured fluxes in both time and wavelength simultaneously, with the shaded region showing the associated uncertainties.
• Figure 4: Colour evolution for SN 2021irp. Upper panel: Optical colour evolution. Lower panel: IR colour evolution.
• Figure 5: Results from blackbody fitting. The blue line indicates fitting to the interpolated optical light curves. The red crosses represent the fit parameters for the optical blackbody shown in Fig. \ref{['fig:SED_fitting_IR']}. The diamonds indicate parameters associated with the IR blackbody, with dark green indicating fitting to NEOWISE MIR photometry and lighter green fitting to NOTCam NIR photometry. Top panel: Pseudo-bolometric luminosities implied from the blackbody fitting. Luminosities were calculated from the Stefan-Boltzmann law. The total luminosity is the sum of the IR and optical luminosities. At later times where we have no optical data, the total luminosity is simply the IR luminosity. Middle panel: Evolution of the temperature for the optical and IR blackbodies. Bottom panel: Evolution of the radius for the optical and IR blackbodies.