The Transitional Stripped-Envelope SN 2008ax: Spectral Evolution and Evidence for Large Asphericity

Understand

Supernova (SN) 2008ax in NGC 4490 was discovered within hours after shock breakout, presenting the rare opportunity to study a core-collapse SN beginning with the initial envelope-cooling phase immediately following shock breakout.

• We present an extensive sequence of optical and near-infrared spectra, as well as three epochs of optical spectropolarimetry.
• Our initial spectra, taken two days after shock breakout, are dominated by hydrogen Balmer lines at high velocity.
• However, by maximum light, He I lines dominated the optical and near-infrared spectra, which closely resembled those of normal Type Ib supernovae (SNe Ib) such as SN 1999ex.