Star exploded, survived, and exploded again

[beecee]

https://phys.org/news/2017-11-star-survived-years.html

Star exploded, survived, and exploded again more than 50 years later

November 8, 2017

It's the celestial equivalent of a horror movie villain—a star that wouldn't stay dead.

An international team of astronomers including Carnegie's Nick Konidaris and Benjamin Shappee discovered a star that exploded multiple times over a period of 50 years. The finding, published by Nature, completely confounds existing knowledge of a star's end of life, and Konidaris' instrument-construction played a crucial role in analyzing the phenomenon.

In September 2014, the intermediate Palomar Transient Factory team of astronomers detected a new explosion in the sky, iPTF14hls.

The light given off by the event was analyzed in order to understand the speed and chemical composition of the material ejected in the explosion.

This analysis indicated that the explosion was what's called a type II-P supernova, and everything about the discovery seemed normal. Until, that is, a few months later when the supernova started getting brighter again.

Type II-P supernovae usually remain bright for about 100 days. But iPTF14hls remained bright for more than 600! What's more, archival data revealed a 1954 explosion in the exact same location.

Read more at: https://phys.org/news/2017-11-star-survived-years.html#jCp

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the paper:

https://www.nature.com/articles/nature24030

Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star

 

Abstract

Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining1. Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability2,3,4,5. That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.

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Anyone with any ideas? 

Edited November 9, 2017 by beecee

[T. McGrath]

The location of the 1954 supernova appears to be coincidental.  Based upon the spectrum of the 1954 supernova in NGC 4214, it was not the same star that produced iPTF14hls.  The iPTF14hls supernova was rich in hydrogen, and the 1954 supernova lacked hydrogen in its spectra and was dominated primarily by helium.  They were not entirely sure how to classify the 1954 supernova, but they were leaning towards a type I supernova, and iPTF14hls is classified as a Type II-P supernova.

The 1954 supernova lasted between 45 and 50 days after peak brightness. It was also suggested that the progenitor may have been an OB-type star, except that it lacked the hydrogen lines and has abundant helium in its spectra.

As far as ideas are concerned, could iPTF14hls be a Thorne–Żytkow object?

I know that when a neutron star siphons material from a companion it becomes an X-ray pulsar (Be X-ray binary) during that period, but what if the neutron star periodically shed this outer layer of material from its companion in a similar manner as a white dwarf that has exceeded the Chandrasekhar limit? Normally there is nothing left of the white dwarf after a Type Ia supernova, but would that process also apply to a neutron star?

Quote

This supernova appears to have been unique among those recorded. No other of type I has shown definite absorption-like features. No supernova of type II has had an absorption spectrum dominated by helium. The star seems more closely allied with type I than with type II, however. The last spectrogram, on August 20, had the broad emission about λ 4650 relatively more conspicuous, as in type I.

Source: The Spectrum of the Supernova of 1954 in NGC 4214 - Publications of the Astronomical Society of the Pacific, Volume 75, Number 443, April 1963

Edited November 11, 2017 by T. McGrath