Sunday, July 27, 2014

An LBV Masquerading as a Cool Hypergiant

The search for supernovae (plural for supernova) has led to the discovery of a population of “supernova impostors”. These outbursts appear like supernovae, but exhibit much lower luminosities and ejecta velocities. A study by Mauerhan et al. (2014) presents observations of a supernova impostor identified as SN Hunt 248. The outburst associated with SN Hunt 248 was observed in May to June 2014 and it occurred in two stages. Between May 21and June 3, the source brightened slowly. On June 4, it began to brighten rapidly, reaching a peak on June 16. The source then plateaued for ~10 days at peak brightness before fading away.

Analysis of the photometric and spectroscopic data indicates that SN Hunt 248 is consistent with an outburst from a massive star. Archival images from the Hubble Space Telescope between 1997 and 2005 reveal that the precursor star is a cool hypergiant with ~400,000 times the Sun’s luminosity and ~32 times the Sun’s mass. SN Hunt 248 is believed to be the first outburst observed from a cool hypergiant that is similar to the giant eruptions typical for luminous blue variable stars (LBVs).

Figure 1: Artist’s impression of a hypergiant.

Figure 2: Light curve of SN Hunt 248 compared with other supernova impostors, including SN 1997bs, SN 2006jc, SN 2002bu, SN 2009ip and SN 2008S. The vertical axis indicates the absolute magnitude and the horizontal axis indicates the number of days relative to peak brightness. Mauerhan et al. (2014).

The slow rise in brightness followed by an episode of rapid brightening indicates that something sudden took place. LBVs and cool hypergiants are known to have strong winds that launch stellar material off them, causing them to be surrounded by circumstellar material of their own. In the case for SN Hunt 248, the initial phase of slow brightening is from the initial outburst. Subsequently, ejecta from the initial outburst collide into the circumstellar material around the cool hypergiant, resulting in the sudden conversion of kinetic energy into radiation. This explains the rapid brightening following the initial phase of slow brightening. Rough estimates suggest an ejecta mass of ~1 times the Sun’s mass and an ejecta velocity of ~1000 km/s.

In fact, the precursor star of SN Hunt 248 is probably a LBV masquerading as a cool hypergiant. This is because there seems to be an absence of LBVs with effective temperatures in the range between 15,000 K to 21,000 K (Figure 3). It has been proposed that at these temperatures, the stellar winds emanating from LBVs become considerably denser. The density can be high enough to produce an opaque pseudo-photosphere around the LBV, allowing the LBV to masquerade as a cool hypergiant with an effective temperature of less than ~8,500 K. If such a connection between LBVs and cool hypergiants is true, then one might expect some cool hypergiants to also exhibit giant outbursts like LBVs do.

Figure 3: Hertzsprung-Russell (HR) diagram for LBVs and related stars, including SN Hunt 248 (purple square). The diagonal and vertical grey strips illustrate the regions of the S Doradus instability strip and the minimum temperature strip for classical LBVs near visual maximum. Mauerhan et al. (2014).

Mauerhan et al. (2014), “SN Hunt 248: a super-Eddington outburst from a massive cool hypergiant”, arXiv:1407.4681 [astro-ph.SR]