The explosion energy of the type IIP supernova SN 2013fs with a confined dense circumstellar shell

Abstract: The recent study of SN 2013fs flash spectrum suggests enormous for SN IIP explosion energy, far beyond possibilities of the neutrino mechanism. The issue of the explosion energy of SN 2013fs is revisited making use of effects of the early supernova interaction with the dense circumstellar shell. The velocity of the cold dense shell between reverse and forward shocks is inferred from the analysis of the broad \heii\,4686\,\AA\ on day 2.4. This velocity alongside with other observables provide us with an alternative energy estimate of $\sim1.8\times10^{51}$\,erg for the preferred mass of $\sim10$\msun. The inferred value is within the range of the neutrino driven explosion.

• The paper reevaluates SN 2013fs's explosion energy by modeling the interaction between supernova ejecta and a confined dense circumstellar shell.
• It deduces a cold dense shell velocity from the 4686 Å emission line at 2.4 days, resulting in an explosion energy estimate of ~1.8 × 10^51 erg.
• Observational constraints from H-alpha luminosity and light curve fitting support a neutrino-driven mechanism within a moderate ejecta mass range.

The Explosion Energy of SN 2013fs: A Reevaluation

The study under review addresses the explosion energy of the Type IIP supernova SN 2013fs, specifically considering its interaction with a confined dense circumstellar (CS) shell. Initially, SN 2013fs was reported to exhibit unusually high explosion energy based on early multiband photometry—a value far exceeding typical supernovae of this class and challenging the plausibility of the neutrino mechanism as its driving force. This paper revisits those findings by exploring the dynamics of the early supernova-CS shell interaction.

Key Findings and Methodology

The study introduces a dynamical model based on the interaction of the supernova ejecta with the dense CS shell. It derives the velocity of a cold dense shell (CDS) situated between reverse and forward shocks, from the analysis of the 4686 Å emission line 2.4 days post-explosion. The deduced CDS velocity, along with other observational data, is employed to estimate the explosion energy. The result suggests an energy of approximately $1.8 \times 10^{51}$ erg—within the range plausible through the neutrino mechanism and notably lower than previous estimates reaching $5 \times 10^{51}$ erg.

Implications and Observational Constraints

Strong observational constraints guide the study, particularly the CDS velocity and photospheric radius. The paper argues that models positing an energy exceeding $2 \times 10^{51}$ erg are incompatible with observed H-alpha luminosities. The established upper-bound on energy arises from consistent CDS velocities and radial expansion matches with observations, bolstered by emission measure calculations for the H-alpha line, indicative of the confined CS gas's density.

The research extends the discussion to fit alternative mass scenarios of the SN ejecta, adjusting the explosion energy estimates accordingly. It finds that varying the ejecta mass leads to proper fit light curves when considering diffusion and cooling wave luminosities separately. These results corroborate the derived explosion energy while offering a moderate mass range estimate for the supernova ejecta.

Speculations on the Mechanism of Mass Loss

The existence of a dense confined CS shell around SN 2013fs raises questions about the mechanisms driving such substantial mass loss shortly before supernovae events. The paper tentatively suggests possibilities related to stellar rotation or specific progenitor conditions that could trigger intense mass loss. This aspect is critical as only a subset of Type II supernovae display evidence of such dense shells.

Practical and Theoretical Outlook

The outcomes of this research have substantial implications for the theoretical understanding of core-collapse supernovae and the conditions leading to mass loss in progenitor stars. The study reinforces the potential need to revise models of pre-supernova stellar evolution, particularly for those leading to higher than average explosion energies.

Moving forward, further observational and theoretical work is needed to elucidate the particular conditions that lead to the formation of dense CS shells and verify the neutrino-driven explosion energies in supernova contexts fully. Future supernovae observations with comparable early emission line data will be crucial for testing the generalizability of these findings.

In conclusion, this reevaluation of SN 2013fs's explosion energy underlines the importance of considering complex progenitor environments when interpreting supernova observations and challenges us to refine the theoretical frameworks that underpin our understanding of stellar evolution and explosion mechanisms.