Kilonova

A kilonova (also called a macronova or r-process supernova) is a transient astronomical event that occurs in a compact binary system when two neutron stars or a neutron star and a black hole merge into each other. Kilonovae are thought to emit short gamma-ray bursts and strong electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected fairly isotropically during the merger process.[1]

The term kilonova was introduced by Metzger et al. in 2010[2] to characterize the peak brightness, which they showed reaches 1000 times that of a classical nova. They are ​110 to ​1100 the brightness of a typical supernova, the self-detonation of a massive star.[3]

The first kilonova to be found was detected as a short gamma-ray burst, SGRB 130603B, by instruments on board the Swift Gamma-Ray Burst Explorer and KONUS/WIND spacecrafts and then observed using the Hubble Space Telescope 9 and 30 days after burst.[1]

In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be analogous to the historic GW170817, a gravitational wave event detected in 2017, and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are considered "striking", and this remarkable resemblance suggests the two separate and independent events may both be the result of the merger of neutron stars, and both may be a hitherto-unknown class of kilonova transients. Kilonova events, therefore, may be more diverse and common in the universe than previously understood, according to the researchers.[4][5][6][7]

This artist's impression video shows how two tiny but very dense neutron stars merge via gravitational wave radiation and then explode as a kilonova.

Theory

The inspiral and merging of two compact objects are a strong source of gravitational waves (GW).[2] Kilonovae are thought to be progenitors of short gamma-ray bursts[2] (GRB) and the predominant source of stable r-process elements in the Universe.[1] The basic model for neutron star mergers was introduced by Li-Xin Li and Bohdan Paczyński in 1998.[8]

Observations

Hubble observes first kilonova
First kilonova observations by the Hubble Space Telescope.[9]

The first observational suggestion of a kilonova came in 2008 following the short-hard gamma-ray burst GRB 080503,[10] where a faint object appeared in optical and infrared light after one day and rapidly faded. Another kilonova was suggested in 2013, in association with the short-duration gamma-ray burst GRB 130603B, where the faint infrared emission from the distant kilonova was detected using the Hubble Space Telescope.[1]

On October 16, 2017, the LIGO and Virgo collaborations announced the first simultaneous detections of gravitational waves (GW170817) and electromagnetic radiation (GRB 170817A, SSS17a) of any phenomena,[11] and demonstrated that the source was a kilonova caused by a binary neutron star merger.[12] This short GRB was followed by a longer transient visible for weeks in the optical electromagnetic spectrum (AT 2017gfo) located in a relatively nearby galaxy, NGC 4993.[13]

See also

References

  1. ^ a b c d Tanvir, N. R.; Levan, A. J.; Fruchter, A. S.; Hjorth, J.; Hounsell, R. A.; Wiersema, K.; Tunnicliffe, R. L. (2013). "A 'kilonova' associated with the short-duration γ-ray burst GRB 130603B". Nature. 500 (7464): 547–549. arXiv:1306.4971. Bibcode:2013Natur.500..547T. doi:10.1038/nature12505. PMID 23912055.
  2. ^ a b c Metzger, B. D.; Martínez-Pinedo, G.; Darbha, S.; Quataert, E.; Arcones, A.; Kasen, D.; Thomas, R.; Nugent, P.; Panov, I. V.; Zinner, N. T. (August 2010). "Electromagnetic counterparts of compact object mergers powered by the radioactive decay of r-process nuclei". Monthly Notices of the Royal Astronomical Society. 406 (4): 2650. arXiv:1001.5029. Bibcode:2010MNRAS.406.2650M. doi:10.1111/j.1365-2966.2010.16864.x.
  3. ^ "Hubble captures infrared glow of a kilonova blast". spacetelescope.org. 5 August 2013. Retrieved 28 February 2018.
  4. ^ University of Maryland (16 October 2018). "All in the family: Kin of gravitational wave source discovered - New observations suggest that kilonovae -- immense cosmic explosions that produce silver, gold and platinum--may be more common than thought". EurekAlert!. Retrieved 17 October 2018.
  5. ^ Troja, E.; et al. (16 October 2018). "A luminous blue kilonova and an off-axis jet from a compact binary merger at z = 0.1341". Nature Communications. 9 (1): 4089. arXiv:1806.10624. Bibcode:2018NatCo...9.4089T. doi:10.1038/s41467-018-06558-7. PMC 6191439. PMID 30327476.
  6. ^ Mohon, Lee (16 October 2018). "GRB 150101B: A Distant Cousin to GW170817". NASA. Retrieved 17 October 2018.
  7. ^ Wall, Mike (17 October 2018). "Powerful Cosmic Flash Is Likely Another Neutron-Star Merger". Space.com. Retrieved 17 October 2018.
  8. ^ Li, L.-X.; Paczyński, B.; Fruchter, A. S.; Hjorth, J.; Hounsell, R. A.; Wiersema, K.; Tunnicliffe, R. (1998). "Transient Events from Neutron Star Mergers". The Astrophysical Journal. 507 (1): L59–L62. arXiv:astro-ph/9807272. Bibcode:1998ApJ...507L..59L. doi:10.1086/311680.
  9. ^ "Hubble observes source of gravitational waves for the first time". www.spacetelescope.org. Retrieved 18 October 2017.
  10. ^ Perley, D. A.; Metzger, B. D.; Granot, J.; Butler, N. R.; Sakamoto, T.; Ramirez-Ruiz, E.; Levan, A. J.; Bloom, J. S.; Miller, A. A. (2009). "GRB 080503: Implications of a Naked Short Gamma-Ray Burst Dominated by Extended Emission". The Astrophysical Journal. 696 (2): 1871–1885. arXiv:0811.1044. Bibcode:2009ApJ...696.1871P. doi:10.1088/0004-637X/696/2/1871.
  11. ^ Abbott, B. P.; Abbott, R.; Abbott, T. D.; Acernese, F.; Ackley, K.; Adams, C.; Adams, T.; Addesso, P.; Adhikari, R. X.; Adya, V. B.; et al. (LIGO Scientific Collaboration & Virgo Collaboration) (16 October 2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral". Physical Review Letters. 119 (16): 161101. arXiv:1710.05832. Bibcode:2017PhRvL.119p1101A. doi:10.1103/PhysRevLett.119.161101. PMID 29099225.
  12. ^ Miller, M. Coleman (16 October 2017). "Gravitational waves: A golden binary". Nature. News and Views (7678): 36. Bibcode:2017Natur.551...36M. doi:10.1038/nature24153.
  13. ^ Berger, E. (16 October 2017). "Focus on the Electromagnetic Counterpart of the Neutron Star Binary Merger GW170817". Astrophysical Journal Letters. Retrieved 16 October 2017.

External links

Cosmic distance ladder

The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs) to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung.

Egglescliffe School

Egglescliffe School & Sixth Form College is a large mixed comprehensive school and sixth form in Eaglescliffe, Stockton-on-Tees, County Durham for students aged 11 to 18.

GRB 150101B

GRB 150101B is a gamma-ray burst (GRB) that was detected on 1 January 2015 at 15:23 UT by the Burst Alert Telescope (BAT) on board the Swift Observatory Satellite, and at 15:23:35 UT by the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope. The GRB was determined to be 1.7 billion light-years (0.52 Gpc) from Earth near the 2MASX J12320498-1056010 host galaxy in the Virgo constellation. Observations of the GRB 150101B event demonstrates remarkable similarities to the historic GW170817 event, that involved the merger of neutron stars, according to astronomers.

GW170817

GW170817 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, originating from the shell elliptical galaxy NGC 4993. The GW was produced by the last minutes of two neutron stars spiralling closer to each other and finally merging, and is the first GW observation which has been confirmed by non-gravitational means. Unlike the five previous GW detections, which were of merging black holes not expected to produce a detectable electromagnetic signal, the aftermath of this merger was also seen by 70 observatories on seven continents and in space, across the electromagnetic spectrum, marking a significant breakthrough for multi-messenger astronomy.

The discovery and subsequent observations of GW170817 were given the Breakthrough of the Year award for 2017 by the journal Science.The gravitational wave signal, designated GW170817, had a duration of approximately 100 seconds, and shows the characteristics in intensity and frequency expected of the inspiral of two neutron stars. Analysis of the slight variation in arrival time of the GW at the three detector locations (two LIGO and one Virgo) yielded an approximate angular direction to the source. Independently, a short (~ 2 seconds duration) gamma-ray burst, designated GRB 170817A, was detected by the Fermi and INTEGRAL spacecraft beginning 1.7 seconds after the GW merger signal. These detectors have very limited directional sensitivity, but indicated a large area of the sky which overlapped the gravitational wave position. It has long been theorized that short gamma-ray bursts are caused by neutron star mergers.

An intense observing campaign then took place to search for the expected emission at optical wavelengths. An astronomical transient designated AT 2017gfo (originally, SSS17a) was found, 11 hours after the gravitational wave signal, in the galaxy NGC 4993 during a search of the region indicated by the GW detection. It was observed by numerous telescopes, from radio to X-ray wavelengths, over the following days and weeks, and was shown to be a fast-moving, rapidly-cooling cloud of neutron-rich material, as expected of debris ejected from a neutron-star merger.

In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be analogous to GW170817. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are considered "striking", and this remarkable resemblance suggests the two separate and independent events may both be the result of the merger of neutron stars, and both may be a hitherto-unknown class of kilonova transients. Kilonova events, therefore, may be more diverse and common in the universe than previously understood, according to the researchers.

Gamma-ray burst

In gamma-ray astronomy, gamma-ray bursts (GRBs) are extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several hours. After an initial flash of gamma rays, a longer-lived "afterglow" is usually emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).The intense radiation of most observed GRBs is thought to be released during a supernova or superluminous supernova as a high-mass star implodes to form a neutron star or a black hole.

A subclass of GRBs (the "short" bursts) appear to originate from the merger of binary neutron stars. The cause of the precursor burst observed in some of these short events may be the development of a resonance between the crust and core of such stars as a result of the massive tidal forces experienced in the seconds leading up to their collision, causing the entire crust of the star to shatter.The sources of most GRBs are billions of light years away from Earth, implying that the explosions are both extremely energetic (a typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime) and extremely rare (a few per galaxy per million years). All observed GRBs have originated from outside the Milky Way galaxy, although a related class of phenomena, soft gamma repeater flares, are associated with magnetars within the Milky Way. It has been hypothesized that a gamma-ray burst in the Milky Way, pointing directly towards the Earth, could cause a mass extinction event.GRBs were first detected in 1967 by the Vela satellites, which had been designed to detect covert nuclear weapons tests; this was declassified and published in 1973. Following their discovery, hundreds of theoretical models were proposed to explain these bursts, such as collisions between comets and neutron stars. Little information was available to verify these models until the 1997 detection of the first X-ray and optical afterglows and direct measurement of their redshifts using optical spectroscopy, and thus their distances and energy outputs. These discoveries, and subsequent studies of the galaxies and supernovae associated with the bursts, clarified the distance and luminosity of GRBs, definitively placing them in distant galaxies.

Gamma-ray burst progenitors

Gamma-ray burst progenitors are the types of celestial objects that can emit gamma-ray bursts (GRBs). GRBs show an extraordinary degree of diversity. They can last anywhere from a fraction of a second to many minutes. Bursts could have a single profile or oscillate wildly up and down in intensity, and their spectra are highly variable unlike other objects in space. The near complete lack of observational constraint led to a profusion of theories, including evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, supernovae, hypernovae, and rapid extraction of rotational energy from supermassive black holes, among others.There are at least two different types of progenitors (sources) of GRBs: one responsible for the long-duration, soft-spectrum bursts and one (or possibly more) responsible for short-duration, hard-spectrum bursts. The progenitors of long GRBs are believed to be massive, low-metallicity stars exploding due to the collapse of their cores. The progenitors of short GRBs are thought to arise from mergers of compact binary systems like neutron stars, which was confirmed by the GW170817 observation of a neutron star merger and a kilonova.

Gravitational wave

Gravitational waves are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light. They were proposed by Henri Poincaré in 1905 and subsequently predicted in 1916 by Albert Einstein on the basis of his general theory of relativity. Gravitational waves transport energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation. Newton's law of universal gravitation, part of classical mechanics, does not provide for their existence, since that law is predicated on the assumption that physical interactions propagate instantaneously (at infinite speed) – showing one of the ways the methods of classical physics are unable to explain phenomena associated with relativity.

Gravitational-wave astronomy is a branch of observational astronomy that uses gravitational waves to collect observational data about sources of detectable gravitational waves such as binary star systems composed of white dwarfs, neutron stars, and black holes; and events such as supernovae, and the formation of the early universe shortly after the Big Bang.

In 1993, Russell A. Hulse and Joseph H. Taylor, Jr. received the Nobel Prize in Physics for the discovery and observation of the Hulse-Taylor binary pulsar, which offered the first indirect evidence of the existence of gravitational waves.On 11 February 2016, the LIGO and Virgo Scientific Collaboration announced they had made the first direct observation of gravitational waves. The observation was made five months earlier, on 14 September 2015, using the Advanced LIGO detectors. The gravitational waves originated from a pair of merging black holes. After the initial announcement the LIGO instruments detected two more confirmed, and one potential, gravitational wave events. In August 2017, the two LIGO instruments and the Virgo instrument observed a fourth gravitational wave from merging black holes, and a fifth gravitational wave from a binary neutron star merger. Several other gravitational wave detectors are planned or under construction.In 2017, the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry Barish for their role in the direct detection of gravitational waves.

Las Cumbres Observatory

Las Cumbres Observatory (LCO) is a network of astronomical observatories run by a non-profit private operating foundation directed by the technologist Wayne Rosing. Its offices are in Goleta, California. The telescopes are located at both northern and southern hemisphere sites distributed in longitude around the Earth. For some astronomical objects, the longitudinal spacing of telescopes allows continuous observations over 24 hours or longer. The operating network currently consists of two 2 meter telescopes, nine 1 meter telescopes, and seven 40 cm telescopes, placed at six astronomical observatories. The network operates as a single, integrated, observing facility, using a software scheduler that continuously optimizes the planned observing schedule of each individual telescope.

NGC 4993

NGC 4993 (also catalogued as NGC 4994) is a lenticular galaxy located about 140 million light-years away in the constellation Hydra. It was discovered on 26 March 1789 by William Herschel and is a member of the NGC 4993 Group.NGC 4993 is the site of the first astronomical event detected in both electromagnetic and gravitational radiation, the collision of two neutron stars, a discovery given the Breakthrough of the Year award for 2017 by the journal Science. Detecting a gravitational wave event associated with the gamma-ray burst provided direct confirmation that binary neutron star collisions produce short gamma-ray bursts.

Neil Gehrels Swift Observatory

The Neil Gehrels Swift Observatory, previously called the Swift Gamma-Ray Burst Mission, is a NASA space telescope designed to detect gamma-ray bursts (GRBs). It was launched on November 20, 2004, aboard a Delta II rocket. Headed by principal investigator Neil Gehrels, NASA Goddard Space Flight Center, the mission was developed in a joint partnership between Goddard and an international consortium from the United States, United Kingdom, and Italy. The mission is operated by Pennsylvania State University as part of NASA's Medium Explorers program (MIDEX).

Neutron star

A neutron star is the collapsed core of a giant star which before collapse had a total mass of between 10 and 29 solar masses. Neutron stars are the smallest and densest stars, not counting black holes, hypothetical white holes, quark stars and strange stars. Neutron stars have a radius on the order of 10 kilometres (6.2 mi) and a mass lower than 2.16 solar masses. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.

Once formed, they no longer actively generate heat, and cool over time; however, they may still evolve further through collision or accretion. Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons (subatomic particles with no net electrical charge and with slightly larger mass than protons); the electrons and protons present in normal matter combine to produce neutrons at the conditions in a neutron star. Neutron stars are partially supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle, just as white dwarfs are supported against collapse by electron degeneracy pressure. However neutron degeneracy pressure is not sufficient to hold up an object beyond 0.7M☉ and repulsive nuclear forces play a larger role in supporting more massive neutron stars. If the remnant star has a mass exceeding the Tolman–Oppenheimer–Volkoff limit, it continues collapsing to form a black hole.

Neutron stars that can be observed are very hot and typically have a surface temperature of around 600000 K. They are so dense that a normal-sized matchbox containing neutron-star material would have a weight of approximately 3 billion metric tons, the same weight as a 0.5 cubic kilometre chunk of the Earth (a cube with edges of about 800 metres). Their magnetic fields are between 108 and 1015 (100 million to 1 quadrillion) times stronger than Earth's magnetic field. The gravitational field at the neutron star's surface is about 2×1011 (200 billion) times that of Earth's gravitational field.

As the star's core collapses, its rotation rate increases as a result of conservation of angular momentum, hence newly formed neutron stars rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as pulsars. Indeed, the discovery of pulsars by Jocelyn Bell Burnell in 1967 was the first observational suggestion that neutron stars exist. The radiation from pulsars is thought to be primarily emitted from regions near their magnetic poles. If the magnetic poles do not coincide with the rotational axis of the neutron star, the emission beam will sweep the sky, and when seen from a distance, if the observer is somewhere in the path of the beam, it will appear as pulses of radiation coming from a fixed point in space (the so-called "lighthouse effect"). The fastest-spinning neutron star known is PSR J1748-2446ad, rotating at a rate of 716 times a second or 43,000 revolutions per minute, giving a linear speed at the surface on the order of 0.24 c (i.e. nearly a quarter the speed of light).

There are thought to be around 100 million neutron stars in the Milky Way, a figure obtained by estimating the number of stars that have undergone supernova explosions. However, most are old and cold, and neutron stars can only be easily detected in certain instances, such as if they are a pulsar or part of a binary system. Slow-rotating and non-accreting neutron stars are almost undetectable; however, since the Hubble Space Telescope detection of RX J185635−3754, a few nearby neutron stars that appear to emit only thermal radiation have been detected. Soft gamma repeaters are conjectured to be a type of neutron star with very strong magnetic fields, known as magnetars, or alternatively, neutron stars with fossil disks around them.Neutron stars in binary systems can undergo accretion which typically makes the system bright in X-rays while the material falling onto the neutron star can form hotspots that rotate in and out of view in identified X-ray pulsar systems. Additionally, such accretion can "recycle" old pulsars and potentially cause them to gain mass and spin-up to very fast rotation rates, forming the so-called millisecond pulsars. These binary systems will continue to evolve, and eventually the companions can become compact objects such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through ablation or merger. The merger of binary neutron stars may be the source of short-duration gamma-ray bursts and are likely strong sources of gravitational waves. In 2017, a direct detection (GW170817) of the gravitational waves from such an event was made, and gravitational waves have also been indirectly detected in a system where two neutron stars orbit each other.

In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817 and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.

Neutron star merger

A neutron star merger is a type of stellar collision. It occurs in a fashion similar to the rare brand of type Ia supernovae resulting from merging white dwarfs. When two neutron stars orbit each other closely, they spiral inward as time passes due to gravitational radiation. When the two neutron stars meet, their merger leads to the formation of either a more massive neutron star, or a black hole (depending on whether the mass of the remnant exceeds the Tolman–Oppenheimer–Volkoff limit). The merger can also create a magnetic field that is trillions of times stronger than that of Earth in a matter of one or two milliseconds. These events are believed to create short gamma-ray bursts. The mergers are also believed to produce kilonovae, which are transient sources of fairly isotropic longer wave electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected during the merger process.

Nial Tanvir

Professor Nial Tanvir of the University of Leicester is a professional astronomer who has made important contributions to the extra-galactic distance scale and galaxy evolution. Perhaps most significant is his research in and around Gamma ray bursts. Tanvir has featured in various TV programs, including The Sky at Night hosted by Sir Patrick Moore, which is an astronomically focused TV program, and a BBC Horizon documentary about gamma-ray bursts (GRBs).

In 2002 he was a member of the research group which won the European Union Descartes Prize for their pioneering work on gamma-ray bursts.

Tanvir headed the international team that discovered the infrared afterglow of GRB 090423 (detected 2009 April 23), the most distant event recorded to date.

In 2013 he led a team that discovered so-called kilonova emission accompanying GRB 130603B, which provided the first direct evidence that short-duration gamma-ray bursts are created by merging compact sources, either two neutron-stars or a neutron-star and black-hole.

R-process

The rapid neutron-capture process, or so-called r-process, is a set of nuclear reactions that in nuclear astrophysics is responsible for the creation of approximately half of the atomic nuclei heavier than iron; the "heavy elements". The other half are produced by the p-process and s-process. The r-process usually synthesizes all of the two most neutron-rich stable isotopes of each heavy element.

The heavy elements typically have six to ten stable isotopes. Chemical elements are defined by the number of protons in their atomic nucleus, e.g. all xenon atoms have 54 protons. But all elements also have neutrons in their atomic nucleus. Each isotope is characterized by the number of neutrons that it contains, e.g. xenon can have 70, 72, 74, 75, 76, 77, 78, 80, and 82 neutrons, and thus has 9 stable isotopes. The r-process contributes to the abundances of the heaviest four isotopes: 131Xe, 132Xe, 134Xe and 136Xe, and is solely responsible for the heaviest two of those. The s-process contributes to xenon's middle five isotopes: 128Xe, 129Xe, 130Xe, 131Xe, and 132Xe. The lightest two isotopes, 124Xe, and 126Xe, are produced by other processes.

The r-process can typically synthesize the heaviest four isotopes of every heavy element, and the two heaviest isotopes, which are referred to as r-only nuclei, can only be created via the r-process. The r-process abundances peak near atomic weights A = 82 (elements Se, Br and Kr), A = 130 (elements Te, I, and Xe) and A = 196 (elements Os, Ir and Pt).

The r-process entails a succession of rapid neutron captures (hence the name) by one or more heavy seed nuclei, typically beginning with nuclei in the abundance peak centered on 56Fe. The captures must be rapid in the sense that the nuclei must not have time to undergo radioactive decay (typically via β- decay) before another neutron arrives to be captured. This sequence can continue up to the limit of stability of the increasingly neutron-rich nuclei (the neutron drip line) to physically retain neutrons as governed by the short range nuclear force. The r-process therefore must occur in locations where there exist a high density of free neutrons. Early studies theorized that 1024 free neutrons per cm3 would be required, for temperatures about 1GK, in order to match the waiting points, at which no more neutrons can be captured, with the atomic numbers of the abundance peaks for r-process nuclei. This amounts to almost a gram of free neutrons in every cubic centimeter, an astonishing number requiring extreme locations. Traditionally this suggested the material ejected from the reexpanded core of a core-collapse supernova, as part of supernova nucleosynthesis, or decompression of neutron-star matter thrown off by a binary neutron star merger. The relative contributions of these sources to the astrophysical abundance of r-process elements is a matter of ongoing research.A limited r-process-like series of neutron captures occurs to a minor extent in thermonuclear weapon explosions. These led to the discovery of the elements einsteinium (element 99) and fermium (element 100) in nuclear weapon fallout.

The r-process contrasts with the s-process, the other predominant mechanism for the production of heavy elements, which is nucleosynthesis by means of slow captures of neutrons. The s-process primarily occurs within ordinary stars, particularly AGB stars, where the neutron flux is sufficient to cause neutron captures to recur every 10–100 years, much too slow for the r-process, which requires 100 captures per second. The s-process is secondary, meaning that it requires pre-existing heavy isotopes as seed nuclei to be converted into other heavy nuclei by a slow sequence of captures of free neutrons. The r-process scenarios create their own seed nuclei, so they might proceed in massive stars that contain no heavy seed nuclei. Taken together, the r- and s-processes account for almost the entire abundance of chemical elements heavier than iron. The historical challenge has been to locate physical settings appropriate for their time scales.

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. 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.

Supernova nucleosynthesis

Supernova nucleosynthesis is a stellar evolution theory about the origin of the natural abundances of the chemical elements as created in supernovae. It explains how the nucleosynthesis of elements, from carbon to nickel, are made in the cores of massive stars. Knowledge of this process was a major stepping-stone in understanding how supernovae deposit these elements into the surrounding environment by enriching the interstellar medium and providing recycling materials for the birth of new stars. Supernova nucleosynthesis was first postulated by Fred Hoyle in 1954.

Timothy Beers

Timothy C. Beers (born June 24, 1957) is an American astrophysicist. Beers teaches at the University of Notre Dame in the Department of Physics (2014–present), where he holds the Notre Dame Chair in Astrophysics. He is a co-founder of the Physics Frontier Center Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements. Prior to coming to Notre Dame, Beers was Director of Kitt Peak National Observatory (2011-2014), and for 25 years was a professor in the Department of Physics and Astronomy at Michigan State University (1986-2011), retiring from that position as University Distinguished Professor.

Beers has published more than 425 refereed papers, and has received multiple Highly Cited Author awards. Beers is a Fellow of the American Physical Society, and was recognized with a Humboldt Senior Research award in 2009 by the Alexander von Humboldt Foundation of Germany. In 2017 he was presented a Distinguished Alumnus award from the Purdue University College of Science.

For decades, Beers has designed and executed large-scale surveys of stars in the Milky Way, efficiently sifting through literally millions of individual stars to find objects that have recorded the chemical history of the Universe in their atmospheres. These rare objects, known as metal-poor stars, are the most chemically primitive stars known, and are among the first generations of stars born in our galaxy, the Milky Way. They provide crucial information on the astrophysical nucleosynthesis sites of the chemical elements, and are powerful tracers of the assembly and evolution of large spiral galaxies.

Beers’ discoveries include: (1) The identification of the first metal-poor stars with measured abundances of Uranium, enabling the determination of a radioactive decay age limit on the Universe, (2) a class of stars known as carbon-enhanced metal-poor (CEMP) stars, a subset of which are thought to reflect the nucleosynthesis products of the very first stars in the Universe, and (3) The first large-scale chronographic (age) maps of the halo of the Milky Way, which astronomers can compare with simulations of the formation of the galaxy. In 2017, Beers and his graduate students were part of a team that identified the characteristic signature of the astrophysical r-process in the kilonova associated with a neutron star merger. This site is thought to be responsible for the production of over half of the elements in the periodic table heavier than iron.

Beers earned his PhD. in astronomy in 1983 from Harvard University. He also holds a master's degree in astronomy from Harvard University (1980), as well as bachelor's degrees of science in physics and in Metallurgical Engineering (1979), both from Purdue University.

Tolman–Oppenheimer–Volkoff equation

In astrophysics, the Tolman–Oppenheimer–Volkoff (TOV) equation constrains the structure of a spherically symmetric body of isotropic material which is in static gravitational equilibrium, as modelled by general relativity. The equation is

Here, r is a radial coordinate, and ρ(r0) and P(r0) are the density and pressure, respectively, of the material at r = r0. The quantity m(r0), the total mass within r0, is discussed below.

The equation is derived by solving the Einstein equations for a general time-invariant, spherically symmetric metric. For a solution to the Tolman–Oppenheimer–Volkoff equation, this metric will take the form

where ν(r) is determined by the constraint

When supplemented with an equation of state, F(ρ, P) = 0, which relates density to pressure, the Tolman–Oppenheimer–Volkoff equation completely determines the structure of a spherically symmetric body of isotropic material in equilibrium. If terms of order 1/c2 are neglected, the Tolman–Oppenheimer–Volkoff equation becomes the Newtonian hydrostatic equation, used to find the equilibrium structure of a spherically symmetric body of isotropic material when general-relativistic corrections are not important.

If the equation is used to model a bounded sphere of material in a vacuum, the zero-pressure condition P(r) = 0 and the condition eν = 1 − 2Gm/rc2 should be imposed at the boundary. The second boundary condition is imposed so that the metric at the boundary is continuous with the unique static spherically symmetric solution to the vacuum field equations, the Schwarzschild metric:

William Elliott Whitmore

William Elliott Whitmore (born May 11, 1978) is an American blues, country, folk singer and musician. He plays roots-folk music that is often inspired by his life on the family farm in the hills of southeastern Iowa.

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