We report on our discovery and multi-wavelength follow-up observations of the Pan-STARRS1 supernova (SN) PS1-12sk, a transient with atypical ejecta and host environment properties that together challenge popular progenitor system scenarios.
The optical spectra of PS1-12sk are dominated by intermediate-width and time variable He I emission at z=0.054, reminiscent of the spectra of the archetypal Type Ibn SN 2006jc and commonly interpreted as the signature of a massive star (17-100 M⊙) explosion within a He-enriched circumstellar medium.
However, unlike other SNe Ibn, PS1-12sk is associated with a galaxy cluster, RXC J0844.9+4258, with the most likely host being the brightest cluster galaxy, CGCG 208-042, located at a projected separation of ~28 kpc from the SN. Moreover, our optical spectroscopy reveals no evidence for star formation at the explosion site of PS1-12sk to sensitive limits.
If the progenitor of PS1-12sk is a massive star, it may be a representative of low-level ongoing star formation associated with a cluster cooling flow or tidally-stripped dwarf galaxy. This is statistically unlikely given the infrequency of common core-collapse SNe (Type II) observed in red sequence galaxies coupled with the low rate of SN Ibn relative to SN II.
PS1-12sk represents either a fortuitous and statistically unlikely discovery, evidence for a top-heavy IMF in galaxy cluster cooling flow filaments, or the first clue suggesting an alternate progenitor channel for Type~Ibn SNe.
PS1-12sk was discovered on March 11th, 2012 with the Pan-STARRS1 (PS1) telescope with this z-band image from the PS1 Medium Deep Survey:
In total, the SN was imaged 17 times in 5 filters (grizy) with PS1 and we obtained additional optical imaging with the Swift Space Telescope and Liverpool Telescope and infrared imaging with the Canada France Hawaii Telescope and PAIRITEL.
Optical spectra of PS1-12sk show a strong similarity to the Type Ibn SN 2006jc, particularly the presence of luminous, intermediate-width He I emission features.
We track the evolution of the SN through 5 spectra obtained during the first month after peak brightness.
PS1-12sk is the first Type Ibn supernova that was not found in association with a detected star-forming galaxy. Instead, PS1-12sk was discovered in the halo of the brightest cluster elliptical galaxy RXC J0844.9+4258, whose brightest cluster galaxy, CGCG 208-042. Below are 3'x3' color composite images of the host galaxies of previously reported SNe Ibn and PS1-12sk.
|SN 1999cq (DSS)||SN 2000er (DSS)||SN 2002ao (SDSS)|
|SN 2006jc (SDSS)||SN 2011hw (DSS)||PS1-12sk (SDSS)|
We stack 71 hours of pre-explosion PS1 imaging to study the optical properties of the host environment in more detail. The figure below shows the stacked pre-explosion imaging on the left, with contours overlayed for radio emission imaged with the Jansky Very Large Array (black) and NVSS (red) and X-ray observations from XMM-Newton (blue). The middle-panel shows PS1 imaging on March 13th, 2012, shortly after the discovery of PS1-12sk. The panel on the right shows the pre-explosion stacked imaging after subtracting off a model for the galaxy CGCG 208-042. From this imaging, it can be seen that there is no dwarf galaxy source detected at the position of the SN (marked in red), and we perform statistical tests identifying CGCG 208-042 as the most likely host galaxy for the SN.
We conclude that our data suggest no evidence of ongoing star formation at the explosion site of PS1-12sk. However, we cannot rule out the possibility of low-level star formation possibly associated with a cooling flow in the host cluster or a tidal tail stripped from the nearest (~2 kpc distant) dwarf host galaxy candidate visible in the image above.
We discuss the constraints our observations place on possible progenitor channels for SNe Ibn.
We consider the massive star progenitor models put forward for SN 2006jc and past SNe Ibn, calling for WR or LBV stars with main sequences masses of ~17-100 solar masses. Such models seek to explain the extreme mass loss of the SN progenitor evidenced by the dense CSM and the LBV-like eruption which preceded by two years SN 2006jc. However, SNe commonly associated with massive stars (Type II core-collapse SNe) are extremely infrequent in red sequence cluster galaxies (only one is confirmed, discovered by the MENeaCS survey). This infrequency is compounded by the relative rates of SNe Ibn to SNe II (~1:100), which make it unlikely to find a SN Ibn in a red sequence cluster galaxy if their progenitors are very massive stars.
We therefore explore possible double degenerate progenitor channels for PS1-12sk that could produce SNe Ibn within an older stellar population. The bolometric energy radiated by the SN, if powered primarily by circumstellar interaction, suggests that as much as ~0.05 solar masses of He were ejected by the progenitor star within a few years before explosion. Additionally, the detection of an intermediate-width hydrogen emission feature in the SN suggests a significant fraction of H in the circumstellar material. These factors make it challenging to conceive a physically plausible degenerate progenitor model, constraining the mass range of the binary consituents and requiring evolution on the viscous timescale (a few years).