Confined Dense Circumstellar Material Surrounding a Regular Type II Supernova: The Unique Flash-Spectroscopy Event of SN 2013fs

Abstract: With the advent of new wide-field, high-cadence optical transient surveys, our understanding of the diversity of core-collapse supernovae has grown tremendously in the last decade. However, the pre-supernova evolution of massive stars, that sets the physical backdrop to these violent events, is theoretically not well understood and difficult to probe observationally. Here we report the discovery of the supernova iPTF13dqy = SN 2013fs, a mere ~3 hr after explosion. Our rapid follow-up observations, which include multiwavelength photometry and extremely early (beginning at ~6 hr post-explosion) spectra, map the distribution of material in the immediate environment (<~ 10^15 cm) of the exploding star and establish that it was surrounded by circumstellar material (CSM) that was ejected during the final ~1 yr prior to explosion at a high rate, around 10^-3 solar masses per year. The complete disappearance of flash-ionised emission lines within the first several days requires that the dense CSM be confined to within <~ 10^15 cm, consistent with radio non-detections at 70--100 days. The observations indicate that iPTF13dqy was a regular Type II SN; thus, the finding that the probable red supergiant (RSG) progenitor of this common explosion ejected material at a highly elevated rate just prior to its demise suggests that pre-supernova instabilities may be common among exploding massive stars.

• The paper demonstrates rapid flash-spectroscopy detection of dense, confined circumstellar material around SN 2013fs.
• It utilizes multiwavelength observations from several telescopes, capturing spectra as early as 6 hours post-explosion.
• Numerical models indicate a high mass-loss rate and pre-supernova instability, challenging traditional red supergiant evolution models.

An Analysis of Pre-Explosion Circumstellar Material in SN2013fs: Implications for Type II Supernova Dynamics

The discovery and rapid follow-up observations of supernova iPTF13dqy (SN2013fs), a regular Type II supernova in NGC7610, have provided unprecedented insights into the nature and dynamics of circumstellar material (CSM) around progenitor stars in the final stages before explosion. The study, focusing on a meticulously documented sequence of spectroscopic and photometric data captured within hours after the supernova's explosion, reveals a complex interaction between the supernova ejecta and dense CSM. The findings have implications on our understanding of mass loss processes and pre-supernova instability in massive stars.

Observational Insights and Methodology

The supernova was detected by the intermediate Palomar Transient Factory (iPTF) merely 3 hours post-explosion. Follow-up observations using several telescopes yielded multiwavelength photometry and a series of spectra, beginning as early as 6 hours after the explosion. The spectral data—the earliest ever captured for a supernova—exhibited strong high-ionization lines, including those of oxygen that disappeared rapidly, signifying their association with flash-ionized CSM. The narrow P-Cygni profiles transformed into typical Type II features over a few days, aligning with both the light-curve plateau and the blue-shift of spectral lines characteristic of Type II-P supernovae.

Numerical Results and Modeling

Quantitative analysis through CMFGEN models underscores a high pre-explosion mass-loss rate of 10^-3 solar masses per year from the red supergiant (RSG) progenitor. The CSM appears confined within 5-10^15 cm, consistent with radio non-detections at 70-100 days post-explosion. The models suggest an average effective temperature within the CSM of about 53kK, with the progenitor's wind velocities possibly reaching 100 km/s, suggesting an atypical episodic mass-loss mechanism distinct from normal stellar winds. Despite accounting for different wind speeds, a dense CSM was found necessary to explain the data, pointing to the mass-loss being a discrete episode rather than continuous.

Implications and Future Directions

The confinement of CSM close to the progenitor, indicated by rapid disappearance of flash-ionized lines, suggests that such enhanced mass ejections possibly result from pre-supernova instabilities. This observation challenges current stellar evolution models that do not predict such pronounced instabilities and mass ejection episodes in typical RSGs. Theoretical models should explore potential causal relationships between core-collapse triggers and pre-supernova outbursts. Moreover, the estimated low progenitor radius and the explosion's moderate energy are indicative of variability in progenitor conditions that could be factored into computational models.

The paper provides robust, observational evidence supporting the occurrence of enhanced pre-explosion mass-loss among massive stars, a finding hitherto confirmed only in rare supernova subtypes. It further implies that many Type II supernovae, which constitute a significant percentage of core-collapse supernovae, may indeed unfold with similar dynamical features. As our capability to observe and analyze supernova events continues to improve, especially with advanced flash spectroscopy, the consistency and ubiquity of such phenomena across a broader sample of Type II supernovae should be evaluated. This expanded data set would significantly aid astrophysicists in refining models of stellar evolution, core-collapse mechanisms, and subsequent supernova energetics.