Файловый архив студентов. Файловый архив студентов.
1259 вузов, 4593 предметов.
/ Регистрация
FAQ Обратная связь Вопросы и предложения
Добавил:
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Сумский государственный университет
Предмет:
Английский язык
Файл:
Dictionary of Geophysics, Astrophysic, and Astronomy.pdf
Скачиваний:
122
Добавлен:
10.08.2013
Размер:
5.66 Mб
Скачать
☆
►Содержание►

supernovae, 1987A

supergranulation Large scale of convection on the sun comprising the tops of large convection cells. Supergranules are irregular in shape and have diameters ranging from 20,000 to 50,000 km in size with horizontal motions0.3 to 0.4 km s−1 and vertical motions, at the edges of the convective cells, 0.1 to 0.2 km s−1. Individual supergranules last from 1 to 2 days.

superior conjunction

See conjunction.

superior mirage A spurious image of an object formed above its true position by abnormal atmospheric refraction, when the temperature increases with height. Then light rays are bent downward as they propagate horizontally through the layer, making the image appear above its true position. See inferior mirage.

superluminal source A radio source showing plasma apparently flowing at transverse velocity larger than the speed of light. Superluminal sources are discovered by comparing two high resolution radio maps obtained at different epochs with Very Long Baseline Interferometers, and the difference is seen as an angular position charge α > cd t where c is the speed of light, d is the distance to the object, and t is the time interval between observations. Thus, simple geometry suggests motion faster than c, the speed of light. Superluminal motion is found usually in core dominated radio galaxies and quasars, and is made possible by the presence of highly relativistic motion and by a favorable orientation. The apparent transverse velocity is

vtrans = v sin θ/(1 − (v/c) cos θ) ,

where v is the velocity of the radiating particles, and θ the angle between the jet and the line of sight. For example, the radio quasar 3C 273 shows blobs of gas moving out along the jet at an angular speed of ≈ 0.67 milli-arcsec per year. If the radio jet points a few degrees from the line of sight, then the observed apparent velocity is 6.2 times the speed of light. About 70 extragalactic superluminal sources were catalogued by late 1993. In 1995, a galactic superluminal source was identified by Mirabel and Rodriguez, and as of early 1998, two galactic superluminal sources are known.

supermassive black hole A black hole of mass 106 − 109 solar masses. Supermassive black holes are believed to be present in the nuclei of quasars, and possibly, of most normal galaxies. A black hole of mass as large as 108 solar masses would have a gravitational radius ≈ 3×108 km and thus, linear dimension comparable to distance of the Earth from the sun. Evidence supporting the existence of supermassive black holes rests mainly on the huge luminosity of quasars and on the perturbations observed in the motion of stars in the nuclei of nearby galaxies that are probably accelerated by the central black hole gravity. If a massive dark object is present in the nucleus of a galaxy, the dispersion in the velocity of stars σ(r) is expected to rise toward the center according to a Keplerian law i.e., σ(r) r−1/2. Such a rise has been detected, for example, in Messier 87 and suggests a central dark mass there of 5 × 109 solar masses.

supernova A stellar explosion, triggered either by the collapse of the core of a massive star (Type II) or degenerate ignition of carbon and oxygen burning in a white dwarf (Type I). The former leaves a neutron star or black hole remnant; the latter disrupts the star completely. The appearance of the spectrum of the event and of the changes of brightness with time are rather similar for the two types. Supernovae occur at a rate of a few per century in a moderately large galaxy like the Milky Way. Ones in our galaxy, visible to the naked eye, occurred in 1054 (Crab Nebula), 1572 and 1604 (Tycho’s and Kepler’s supernovae) and a few other times in the past two thousand years. Many others undoubtedly have been hidden by galactic dust and gas from our sight. See supernova rates, supernovae, classifi- cation.

supernovae, 1987A SN 1987A is the most studied supernova of the 20th century. Due to its close proximity, in the Large Magellanic Clouds, and the recent developments of CCD and neutrino detectors, it has become the Rosetta stone of Type II supernovae. Observations of the Large Magellanic Clouds prior to the supernova explosion recorded the progenitor star before its death, affirming that the progenitors of Type II supernovae were indeed massive su-

© 2001 by CRC Press LLC

supernovae, 1991bg

pergiant stars. The neutrino signal (the first supernova neutrino detection) agreed well with the predicted flux from core-collapse models.

However, SN 1987A brought with it many more puzzles than it did answers. SN 1987A peaked twice at much lower magnitudes than most Type II supernovae. The progenitor was indeed a supergiant, but it was a blue supergiant, not a red supergiant as was predicted by theorists. The neutron star which should have formed in the collapse mechanism has yet to be detected. In addition, images and spectra of SN 1987A revealed the presence of interstellar rings, likely to be caused by asymmetric mass loss from the progenitor star due to a binary companion. The progenitor of SN 1987A may have been a merged binary which provides an explanation for the rings and for the fact that it was a blue supergiant.

supernovae, 1991bg A peculiar Type Ia supernova. Although most Type Ia supernovae exhibit very little scatter in their peak luminosity (0.3 to 0.5 magnitudes in V and B), the peak luminosity for SN 1991bg was extremely subluminous (1.6 magnitudes less in V, 2.5 magnitudes less in B with respect to normal Type Ia supernovae). The late-time light curve decay was consistent with an explosion ejecting only 0.1M of nickel. In addition, its expansion velocity (10, 000 km s−1) was slightly lower than typical Type Ia supernovae. SN 1991bg, and a growing list of additional low-luminosity (low nickel masses) supernovae (e.g., 1992K,1997cn), may make up a new class of supernovae which are better explained by alternative Type Ia mechanisms (sub-Chandrasekhar thermonuclear explosions or accretion induced collapse of white dwarfs).

supernovae, 1993J Supernova 1993J’s early-time spectra had hydrogen lines and hence, is officially a Type II supernova. However, its light curve peaked early, dipped, and increased again, marking it as a peculiar supernova. Its late-time spectra exhibited strong oxygen and calcium lines with little hydrogen, very similar to the late-time spectra of Type Ib supernovae. This transition of supernova 1993J (and the similar 1987K) from Type II to Type Ib spectra suggests that Type II and Type Ib supernovae

are caused by a common core-collapse mechanism. The progenitor of 1993J was probably a massive star that went into a common envelope phase with a binary companion, removing most of its hydrogen envelope.

supernovae, accretion induced collapse (AIC)

Rapidly accreting C/O white dwarfs and most OMgNe white dwarfs, which accrete sufficient material to exceed the Chandrasekhar limit, collapse into neutron stars. The collapse proceeds similarly to the core collapse mechanism of Type Ib/Ic and Type II supernovae. These collapses eject up to a few tenths of a solar mass and may explain the low-luminosity Type Ia supernovae such as Supernova 1991bg. Like the corecollapse of massive stars, AICs produce neutron star remnants, but at a rate < 1% that of corecollapse supernovae. However, in special cases, such as globular cluster where it is difficult to retain neutron stars from Type II supernovae, AICs may form most of the neutron star population.

supernovae, classification Supernovae are classified in two major groups: those with hydrogen lines in their spectra (Type II) and those without hydrogen lines (Type I). Type I supernovae are further subclassified by their spectra: Type Ia supernovae have strong silicon lines (Si II) whereas Type Ib/c supernovae do not. Type Ib supernovae exhibit helium lines (He I) which are absent in Type Ic supernovae. Type II supernovae are differentiated by their light curves: the luminosity of Type II-Linear supernovae peak and then decay rapidly whereas Type II-Plateau supernovae peak, drop to a plateau where the luminosity remains constant for 100 days and then resume the light curve decay. Type Ia are more luminous than Type II supernovae ( 3 magnitudes) and occur in all galaxies. Type Ib/c and Type II supernovae do not occur in ellipticals. Type Ia are thought to be the thermonuclear explosions of white dwarfs, whereas Type Ib/c and Type II supernovae are caused by the collapse of massive stars.

This classification scheme collapses the 5 class Zwicky system (Zwicky 1965) to these two separate classes (the Zwicky III, IV, and V explosions are all placed in the category of peculiar Type II supernovae). In addition, supernovae 1987K and 1993J exhibited hydrogen features

© 2001 by CRC Press LLC

supernovae, expanding photosphere method

in their early spectra, but their final spectra more closely resembled Type Ib supernovae suggesting a common link between Type Ib and Type II supernovae.

supernovae, core collapse mechanism Any supernova mechanism which involves the collapse of a massive star’s core into a neutron star. Massive stars ( 10M ) ultimately produce iron cores with masses in excess of the Chandrasekhar limit. The density and temperature at the center of this core eventually become large enough to drive the dissociation of iron and the capture of electrons onto protons. These processes lower the pressure support in the core and initiate its collapse. As the core collapses, the density and temperature increase, accelerating the rate of electron capture and iron dissociation and, hence, the collapse itself. The collapse process runs away, and very quickly (on a free-fall timescale), the core reaches nuclear densities. Nuclear forces halt the collapse. Only a small fraction (1%) of the potential energy released in this collapse is required to drive off the outer portion of the star and power the observed explosion. Several mechanisms have been proposed to harness this energy (See supernovae, prompt shock mechanism, supernovae, delayed neutrino mechanism). Core collapse supernovae leave behind neutron star (or black hole) remnants. Type II and Type Ib/Ic supernovae are thought to be powered by the core collapse mechanism.

supernovae, delayed neutrino mechanism

Neutrinos carry away most of the gravitational potential energy released during the core collapse of a massive star (see supernova, core collapse mechanism). Roughly 1% of the energy of these neutrinos must be converted into kinetic energy to drive a supernova explosion. This 1% efficiency is achieved by absorbing neutrinos just above the proto-neutron star surface where densities are high, and the optical depth to neutrinos is also high. The neutrino-heated material convects upward and expands (and cools) before it can lose too much energy by the reemission of neutrinos. The convection takes place after the prompt shock stalls (see supernova, prompt shock mechanism), and is thus a “delayed” mechanism.

supernovae, M15/Phillips relation The scatter in peak magnitudes of Type Ia supernovae can be correlated to the decay of the light curve. The relation championed by Phillips (1993), M15 is defined as the amount (in magnitudes) that the B light curve decays in the first 15 days after maximum. This value can then be used to determine the absolute magnitude of any Type Ia supernova. This correction is similar to a technique used to correct for composition effects in the Cepheid Variable period-luminosity relation. The scatter in peak magnitudes typically lowers from 0.3 – 0.5 magnitudes to less than 0.1 magnitudes, making Type Ia very effective standard candles. A similar method using synthetic light curves, the light curve shape (LCS) method, has been developed with comparable results (Riess 1995). Like most standard candles, Type Ia supernova must be calibrated at low redshifts (typically with Cepheid Variables).

supernovae, distance indicators The high peak luminosities of supernovae and the homogeneity of Type Ia supernovae in particular, make them ideal standard candles for cosmological studies at high redshift. In addition, the scatter of Type Ia supernovae can be further reduced by correcting the peak luminosities using the relation between peak magnitude and light-curve decay (see supernovae, M15/Phillips relation). The absolute magnitudes of Type II-Plateau supernovae can be theoretically calculated from the colors and expansion velocities of the supernovae, placing them as one of the few standard candles which do not need to be calibrated at low-redshifts (see supernovae, expanding pho- tosphere method).

supernovae, expanding photosphere method

The expanding photosphere method (EPM) provides a physical model to calculate the absolute magnitudes of Type II-Plateau supernovae without any calibration. The expanding photosphere method combines observations of the photospheric radii and temperature to get a supernova luminosity (also known as the Baade– Wesselink method). Assuming the supernova emits as a blackbody, this luminosity is sim-

ply: L photosphere = 4πRphotosphereσ Tphotosphere4

where Rphotosphere = V photospheret, and the tem-

perature is determined by the observed colors.

© 2001 by CRC Press LLC

supernovae, fallback

In practice, the blackbody assumption is not valid, and the luminosity is calculated using models of Type II supernovae.

This mechanism does not require calibration at low redshifts and is not limited by the uncertainties of the calibrator. Unfortunately, Type II supernovae are dimmer than Type Ia supernovae, and the low observed numbers limit their use as cosmological candles.

supernovae, fallback Supernova fallback is that material that, during a core collapse supernova explosion, does not receive enough energy to escape the gravitational potential of the neutron star and eventually “falls back” onto the neutron star. Many physical effects can produce fallback. For instance, when the supernova shock slows down as it moves through the shallow density gradients of the exploding star’s hydrogen envelope, a reverse shock is produced which drives material back onto the neutron star. The decay of 56Ni also can reduce the velocity of some material to below the escape velocity, and this material will eventually fall back onto the neutron star. Neutron rich material is formed near the proto-neutron star surface due to the high neutrino emission and absorption rates. This material is nearest to the proto-neutron star and hence, most likely to fall back.

supernovae, gravitational waves In the delayed neutrino mechanism, neutrino heating drives vigorous convection and ultimately powers the supernova explosion. This convection causes oscillations in the mass distribution which lead to the production of gravitational waves. The gravitational wave amplitude can be estimated from the time variation in the mass distribution and suggests detectability in current detectors (LIGO, Virgo, Geo) at tens of Mpc. Simulations of rotating core-collapses can produce wave amplitudes nearly 2 orders of magnitude higher. However, the gravitational wave amplitudes from supernova are all over an order of magnitude smaller than that of merging compact objects which will be detectable at hundreds of Mpc in current detectors.

supernovae, light curves The time evolution of the luminosities (light curves) of Type Ia supernovae are dominated by 56Ni decay and

are remarkably homogeneous (with some exceptions: see supernovae, 1991bg). Type II supernovae light curves are characterized by an early-time peak as the material becomes optically thin and photons escape and a late-time

exponential decreased determined by the decay of 56Co to 56Fe. The light curves of Type II su-

pernovae vary dramatically (peak luminosities range over 4 magnitudes). Type II supernovae are classified roughly into two groups based on the light curve: Type II-Plateau supernovae whose decay after peak flattens for roughly 30 to 100 days, and Type II-Linear supernovae which do not exhibit any flattening.

supernovae, neutrino detectors Supernova 1987A initiated the first supernova neutrino detectors. Both the Kamiokande and Irvine– Michigan–Brookhaven proton decay detectors observed Cherenkov radiation from relativistic particles accelerated by neutrinos. Current detectors fit into 3 basic categories based on the instrument design: water detectors, heavy water detectors, and scintillation detectors. Kamiokande, its successor, Super Kamiokande, and the Irvine–Michigan–Brookhaven detectors are all water detectors. Water detectors rely primarily upon anti-neutrino absorption by protons or neutrino scattering which then produces relativistic particles and Cherenkov radiation. Heavy water detectors, in addition to the processes in water detectors, also detect the dissociation of neutrons by neutrino scattering. Scintillation detectors rely upon emission from the decay of atoms excited by neutrino scattering.

supernovae, neutrino-driven wind mechanism After the launch of a core collapse supernova explosion, material continues to blow off the hot proto-neutron star. The cooling neutron star emits copious neutrinos 20 s after the initial explosion. The ejecta of this wind is neutron rich and thought to be a source of r-process nucleosynthesis products. However, the neutron fraction of the ejecta depends sensitively upon the relative absorption of the electron neutrino and anti-neutrinos and the true nucleosynthetic yield is difficult to determine.

The neutrino-driven wind mechanism is often confused with the delayed-neutrino mechanism. In the delayed neutrino mechanism, a convec-

© 2001 by CRC Press LLC

Соседние файлы в предмете Английский язык
Помощь Обратная связь Вопросы и предложения Пользовательское соглашение Политика конфиденциальности