iPTF14hls is an unusual supernova star that has erupted continuously for about 1,000 days before becoming a remnant nebula. It had previously erupted in 1954. None of the theories nor proposed hypotheses fully explain all the aspects of the object.
Supernova iPTF14hls before and after detection
Epoch J2000 Equinox
|Right ascension||09h 20m 34.30s|
|Declination||+50° 41′ 46.80″|
|Apparent magnitude (V)||17.716 (R)|
|Distance||156,200,000 pc (509,000,000 ly) pc|
The star iPTF14hls was discovered in September 2014 by the Intermediate Palomar Transient Factory, and it was first made public in November 2014 by the CRTS survey as CSS141118:092034+504148. Based on that information it was confirmed as an exploding star in January 2015. It was thought then that it was a single supernova event (Type II-P) that would dim in about 100 days, but instead, it continued its eruption for about 1,000 days while fluctuating in brightness at least five times. The brightness varied by as much as 50%, going through five peaks. Also, rather than cooling down with time as expected of a Type II-P supernova, the object maintains a near-constant temperature of about 5000–6000 K. Checks of photographs from the past found one from 1954 showing an explosion in the same location. Since 1954, the star has exploded six times.
The principal investigator is Iair Arcavi. His international team used the Low Resolution Imaging Spectrometer (LRIS) on the Keck I telescope to obtain the spectrum of the star's host galaxy, and the Deep Imaging and Multi-Object Spectrograph (DEIMOS) on Keck II to obtain high-resolution spectra of the unusual supernova itself.
The host galaxy of iPTF14hls is a star-forming dwarf galaxy, implying low metal content, and the weak iron-line absorption seen in the supernova spectra are consistent with a low metallicity progenitor. The study estimates that the star that exploded was at least 50 times more massive than the Sun. The researchers also remark that the debris expansion rate is slower than any other known supernova by a factor of 6, as if exploding in slow-motion. However, if this were due to relativistic time dilation then the spectrum would be red-shifted by the same factor of 6, which is inconsistent with their observations. In 2017 the expansion speed was constrained to approximately 1,000 km/s.
Arcavi's team continue monitoring the object in other bands of the spectrum in collaboration with additional international telescopes and observatories. These facilities include the Nordic Optical Telescope and NASA's Swift space telescope, the Fermi Gamma-ray Space Telescope, while the Hubble Space Telescope began to image the location in December 2017.
iPTF14hls is an ongoing event, and after about 1,000 days, its light displayed a dramatic drop, but the supernova is still visible, and by November 2018 its spectra had become a remnant nebula. A high-resolution image of this latest phase was obtained with the Hubble Space Telescope.
Current theory predicts that the star would consume all its hydrogen in the first supernova explosion, and depending on the initial size of the star, the remnants of the core should form a neutron star or a black hole, so Iair Arcavi argued in 2017 that there was a novel unidentified phenomenon happening. At that time, no known theory explained all the observations. None of the hypotheses published before early 2018 — the first three listed below — could explain the continued presence of hydrogen or the energetics observed. According to Iair Arcavi, this discovery requires refinement of existing explosion scenarios, or the development of a new scenario, that can:
One hypothesis involves burning antimatter in a stellar core; this hypothesis holds that massive stars become so hot in their cores that energy is converted into matter and antimatter, causing the star to become extremely unstable, and undergo repeated bright eruptions over periods of years. Antimatter in contact with matter would cause an explosion that blows off the outer layers of the star and leaves the core intact; this process can repeat over decades before the large final explosion and collapse to a black hole.
Another hypothesis is the pulsational pair-instability supernova, a massive star that may lose about half its mass before a series of violent pulses begins. On every pulse, material rushing away from the star can catch up with earlier ejected material, producing bright flashes of light as it collides, simulating an additional explosion (see Supernova impostor). However, the energy released by the iPTF14hls supernova is more than the theory predicts.
Jennifer E Andrews and Nathan Smith hypothesised that the observed light spectrum is a clear signature of shock interaction of ejected material with dense circumstellar material (CSM). They proposed that a typical explosion energy, with "enveloped" or "swallowed" CSM interaction — as seen in some recent supernovae, including SN 1998S, SN 2009ip, and SN 1993J — could "explain the peculiar evolution of iPTF14hls."
In December 2017, a team using the Fermi Gamma-ray Space Telescope reported that they may have detected in iPTF14hls, for the first time, high energy gamma-ray emission from a supernova. The gamma-ray source appears ∼ 300 days after the explosion of iPTF14hls, and is still observable, but more observations are needed to verify that iPTF14hls is the exact source of the observed gamma-ray emission. If the association between the gamma-ray source and iPTF14hls is real, there are difficulties to model its gamma-ray emission in the framework of particle acceleration in supernova ejecta produced shock. The energy conversion efficiency needs to be very high, so it is suggested that a jet (anisotropic emission) from a close companion may be necessary to explain some of the observed data. No X-ray emissions have been detected, which makes the interpretation of the gamma-ray emission a difficult task.
This hypothesis suggests common envelope jets supernova (CEJSN) impostors resulting from a neutron star companion. It proposes "a new type of repeating transient outburst initiated by a neutron star entering the envelope of an evolved massive star, accreting envelope material and subsequently launching jets which interact with their surroundings." The ejecta could reach velocities of 10000 km/s despite not being a supernova.
2017 in science
A number of significant scientific events occurred in 2017. The United Nations declared 2017 the International Year of Sustainable Tourism for Development.Eta Carinae
Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around 7,500 light-years (2,300 parsecs) distant in the constellation Carina. Previously a 4th-magnitude star, it brightened in 1837 to become brighter than Rigel marking the start of the Great Eruption. Eta Carinae became the second-brightest star in the sky between 11 and 14 March 1843 before fading well below naked eye visibility after 1856. In a smaller eruption, it reached 6th magnitude in 1892 before fading again. It has brightened consistently since about 1940, becoming brighter than magnitude 4.5 by 2014. Eta Carinae is circumpolar from latitudes south of latitude 30°S, and it is never visible north of about latitude 30°N.
The two main stars of the Eta Carinae system have an eccentric orbit with a period of 5.54 years. The primary is a peculiar star similar to a luminous blue variable (LBV) that was initially 150–250 M☉ of which it has lost at least 30 M☉ already, and is expected to explode as a supernova in the astronomically near future. This is the only star known to produce ultraviolet laser emission. The secondary star is hot and also highly luminous, probably of spectral class O, around 30–80 times as massive as the Sun. The system is heavily obscured by the Homunculus Nebula, material ejected from the primary during the Great Eruption. It is a member of the Trumpler 16 open cluster within the much larger Carina Nebula.
Although unrelated to the star and nebula, the weak Eta Carinids meteor shower has a radiant very close to Eta Carinae.List of supernovae
This is a list of supernovae that are of historical significance. These include supernovae that were observed prior to the availability of photography, and individual events that have been the subject of a scientific paper that contributed to supernova theory.Pulsational pair-instability supernova
A pulsational pair-instability supernova is a supernova impostor event that generally occurs in stars at around 100 to 130 solar mass (M☉), as opposed to a typical pair-instability supernova which occurs in stars of 130 to 250 M☉. Like pair-instability supernovae, pulsational pair-instability supernovae are caused by draining of a star's energy in the production of electron-positron pairs but, whereas a pair-instability supernova completely disrupts the star in a massive supernova, the star's pulsational pair-instability eruption sheds 10–25 M☉. This generally shrinks it down to a mass of less than 100 M☉, too small for electron-positron pair creation, where it then undergoes a core-collapse supernova or hypernova. It is possible that this is what occurred during the 1843 eruption of the primary star of the Eta Carinae star system although there is no substantial evidence supporting this.Supernova
A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a transient astronomical event that occurs during the last stellar evolutionary stages of the life of a massive star, whose dramatic and catastrophic destruction is marked by one final, titanic explosion. This causes the sudden appearance of a "new" bright star, before slowly fading from sight over several weeks or months or years.
Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by Walter Baade and Fritz Zwicky in 1931.
Only three Milky Way, naked-eye supernova events have been observed during the last thousand years, though many have been observed in other galaxies. The most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but the remnants of recent supernovae have also been found. Observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, and that any galactic supernova would almost certainly be observable with modern astronomical telescopes.
Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion, completely disrupting the star. In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics have been established and accepted by most astronomers for some time.
Supernovae can expel several solar masses of material at speeds up to several percent of the speed of light. This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, sweeping up an expanding shell of gas and dust observed as a supernova remnant. Supernovae are a major source of elements in the interstellar medium from oxygen through to rubidium. The expanding shock waves of supernova can trigger the formation of new stars. Supernova remnants might be a major source of cosmic rays. Supernovae might produce strong gravitational waves, though, thus far, the gravitational waves detected have been from the merger of black holes and neutron stars.