NGC 300-OT
On a CCD image obtained on May 14, 2008, amateur astronomer L.A.G. Berto Monard discovered an interesting bright optical transient (OT) in NGC 300 that is designated NGC 300-OT. It is located at RA: 00h 54m 34.552s and DEC: -37° 38′ 31.79″ in a spiral arm containing active star formation. Its broad-band magnitude was 14.3 in that image. An earlier image (from April 24, 2008), taken just after NGC 300 reemerged from behind the sun, evidenced an already brightening OT at ~16.3 magnitude. No brightening was detected on a February 8, 2008 image or on any earlier ones. The transient's peak measured magnitude was 14.69 on May 15, 2008.
At discovery, the transient had an absolute magnitude of MV ≈ −13, making it faint in comparison to a typical core-collapse supernova but bright in comparison to a classical nova. Additionally, the photometric and spectroscopic properties of the OT imply that it is not a luminous blue variable either. Since its peak, brightness dropped smoothly through September 2008 while becoming continuously redder. After September 2008, brightness continued to fall at a lower rate in the optical spectrum but with strong Hα emissions. Further, the optical spectrum is mostly made up of fairly narrow Hydrogen Balmer and Ca II emission lines coupled with strong Ca II H&K absorption. Research into historical Hubble images provide an accurate upper bound on the progenitor star's brightness. This suggested a low-mass main sequence star as progenitor with the transient resulting from a stellar merger similar to red Galactic nova V838 Monocerotis. Analysis of historical images of the area of the OT suggest with 70% certainty that the progenitor formed in a burst of stars around 8–13 Myr ago and implies the progenitor's mass to be 12–25 M⊙ assuming the OT is due to an evolving massive star.
However, in 2008 a bright mid-infrared progenitor to the transient was discovered in historical Spitzer data. This was a star that was obscured by dust, with energy distribution analogous to a black-body of R ≈ 300 AU and radiating at T ≈ 300 K with Lbol ≈ ×106 L⊙. This demonstrated that the transient was associated with an energetic explosion of a low-mass ≈ 10 M⊙ star. The transient's low luminosity as compared to typical core-collapse supernova, combined with its spectral attributes and dust covered properties, make it nearly identical to NGG 6946's SN 2008S.
The spectrum of NGC 300-OT observed with Spitzer shows strong, broad emission features at 8 μm and 12 μm. Such features are also seen in Galactic carbon-rich protoplanetary nebulae.
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