IK Pegasi

IK Pegasi (HR 8210 ) is an approximately 150 light-years distant binary star in the constellation Pegasus. The two stars can not be resolved as individual objects, but it is a spectroscopic binary, that is, they are identifiable only by their spectrum as a double star. Like with an apparent brightness of 6.1, the object can be just perceived at very good observing conditions even with the naked eye.

The primary star ( IK Pegasi A) is a main-sequence star of spectral type A, which shows a slight pulsation in its luminosity, which is repeated 22.9 times per day. These pulsations are generated primarily by instabilities in the hydrogen convection that lead alternately to the expansion and contraction of the atmosphere. Among the pulsating variables IK Pegasi A belongs to the Delta Scuti stars.

His companion ( IK Pegasi B) is a white dwarf and thus a star that has the majority of its development phase already behind you and now no longer is able to generate energy through nuclear fusion. Both revolve each other every 21.7 days at an average distance of about 31 million kilometers or 0.21 astronomical units (AU ). This distance corresponds to about the distance of Mercury to our sun.

IK Pegasi B is the closest to us known candidate for a supernova of type Ia. At such an event occurs when the main star beginning to reach the stage of development of a red giant. In this case, its radius grows to such an extent that the adjacent white dwarf matter from the expanding gaseous envelope accreted. As soon as the white dwarf approaches the Chandrasekhar limit of 1.44 solar masses, it is expected that it will explode as a Type Ia supernova.

Observation history

The star system was first cataloged Bonner Durchmusterung BD 18 ° under the entry 4794 B in the star catalog published in 1862. Later, it was under the name HR 8210 mention in Pickering in 1908 issued Bright Star Catalogue. The term " IK Pegasi " is based on the extended form of the naming of variable stars, which was introduced by Friedrich W. Argelander.

In studies of the spectrometric properties of this star, characteristic absorption line shifts, showing clear signs of a double star system showed. Such a shift occurs when both partners are moving away at their mutual orbit to the observer or of him, creating a periodic Doppler shift in the wavelength of the spectral lines occurs. The measurement of this shift, in turn, allows astronomers to determine the relative orbital velocity of at least one of the stars, even without that the objects can be individually resolved.

In 1927, the Canadian astronomer William E. Harper used this technique to measure the period of the spectrometric shift of this binary system, in which he calculated a distance of 27.724 days between the two phases. He also estimated for the eccentricity of the orbit, a value of 0.027; subsequent assessments showed an eccentricity of almost zero, which is equivalent to a circular orbit. The maximum deflection of the radial velocity of the main star was determined here at 41.5 km / s.

The distance from IK Pegasi to Earth, you can still determine by a parallax. The shift was ultimately measured by the Hipparcos probe with a high precision and the removal of this double star 150 light-years, with an accuracy of ± 5 light years determined. By means of these spacecraft also the proper motion of the system was determined, ie the small angular movement, the IK Pegasi takes as it moves across the sky, during which it moves through space.

The combination of distance and motion of the system could in turn be used to determine a cross rate of IK Pegasi of 16.9 km / s. [A 3] The third component, the heliocentric radial velocity can be based on the average redshift ( or blueshift ) of the star spectrum can be determined. In the General Catalogue of Stellar Radial Velocities (General Catalogue of radial velocities of stars ) is specified on this machine, a radial velocity of -11.4 km / s. From these two movements can turn a space velocity inferred that the sun of 20.4 km / s corresponds to a value relative. [A 4]

It has already been attempts to resolve the individual components of this binary system with the help of photographs from the Hubble Space Telescope, but the distance between the two stars has proven to be too low, as they would have been identify separately. With the Space Telescope Extreme Ultraviolet Explorer ( EUVE ) now current measurements were performed, allowing for double stars now a more accurate orbital period of 21.72168 ± 0.00009 days could be determined. The orbital inclination of the orbital plane of the system, when the object is observed from the earth is apparently close to 90 °. Under these circumstances it would be possible to observe an occultation of the larger object through the smaller white dwarf, which would be manifested by a noticeable drop in brightness.

IK Pegasi A

In its current state, IK Pegasi A is a star that within the Hertzsprung- Russell diagram ( HR diagram ) is counted to the main series. The term main-sequence stars are summarized, which release their radiation energy by burning hydrogen in its core. However, IK Pegasi A is located in a narrow, almost vertical strip of the HR diagram, known as the instability strip. Stars in this band oscillate in a coherent way, so that they have a regular variation in brightness.

The pulsations resulting from a process that is referred to as κ mechanism. A part of the outer atmosphere of the star appears optically dense, which is triggered by partial ionization of certain elements. These atoms lose an electron by the pressure and temperature conditions within the atmospheric layer, the probability increases that energy is absorbed by them. This leads to an increase in temperature, which in turn causes the atmospheric expanded. The bloated atmosphere is less ionized and loses energy, so it cools and shrinks again. The result of these cycles is a regular pulsation of the atmosphere that brings a corresponding variation of brightness with it. Such pulsating variables stars, which are located in the vicinity of the crossing point of the main sequence and instability strip in the HR diagram are referred to as Delta Scuti stars. With them are stars with a short-cycle luminosity change that have a regular pulse rate of 0.025 to 0.25 days. In their construction they have the same frequency of heavy elements like the sun (also see Population I ), however, have the 1.5 - to have up to 2.5 times the solar mass. In astronomy, the metallicity of a star is defined as the frequency of chemical elements within it which have a higher atomic weight than helium. This frequency is determined by a spectral analysis of the atmosphere, the result is then compared with the results that would be expected according to the calculated by computer modeling results. In the case of IK Pegasus A solar metallicity is estimated to be [M / H] = 0.07 ± 0.20. This value describes the logarithm of the ratio between metals (M ) [A 5] to hydrogen ( H) minus the logarithm of the ratio corresponding to the value of our sun. ( Had thus a star exactly the metallicity of the sun, so would the value of its metallicity equal to zero. ) A logarithmic value of 0.07 corresponds to an actual Metallizitäsverhältnisses of 1.17, which means that the star about 17% richer in metallic elements than our sun. However, the error rate for this result is relatively large. The pulse rate of IK Pegasi A was measured at 22.9 cycles per day, which is a radiation pulse corresponds exactly all 0,044 days.

In the spectrum of an A - class star like IK Pegasi A is further strong Balmer lines of hydrogen, together with absorption lines of ionized metals reveal, including a K- line on ionized calcium (Ca II) at a wavelength of 393, 3 nm suggesting. The spectrum of IK Pegasi A can thus be classified as marginal On, which means that it shows on the one hand features a spectral class A, on the other hand has a marginal row metal. The reason for this is that the metallic isotopes are recognized in the atmosphere of this star, as compared to normal stars, slightly different, but noticeably higher absorption line strengths. Star of spectral type Am are often members of binary systems, which, as IK Pegasi, a very close companion of about the same mass.

Star of spectral type A are hotter and more massive than the sun. However, this in turn means that their life is correspondingly shorter on the main sequence. ( Comprising of about 1.65 solar masses ) for a star with a mass similar to that of IK Pegasi A, is the expected lifetime on the main sequence 2-3 × 109 years, which corresponds to about half the current age of our Sun.

In terms of the mass of the relatively young Altair is the nearest star to the sun, which can be called as a stellar counterpart to the A component of IK Pegasi, because it has an estimated 1.7 times the sun's mass. Overall, the binary star system of IK Pegasi instead possesses some similarities to the nearby system of Sirius, which also consists of a Class A primary star and a white dwarf companion. However, Sirius A has a much greater mass than IK Pegasi A and the orbit of its companion is, with a semimajor axis of 20 AU, compared with far greater.

IK Pegasi B

The companion IK Pegasi of A is a dense white dwarf. This category of stellar objects has already reached the evolutionary end of its life and is no longer able to maintain energy through nuclear fusion. Under normal circumstances, it radiates its excess energy, which is mainly composed of heat stored in the sequence continuously, making it increasingly cooler and darkening of a few billion years more and more in the course.

Previous development

To understand how it had come to this stage of development, one must look back at the past few million years. As the hydrogen in the core of the progenitor star of IK Pegasi B was used up, its inner core pulled together until it came in a shell around its helium core around to a new hydrogen burning, resulting in a temperature increase in the interior of the star. To compensate for the increase in temperature, the outer casing extended to a multiple of the radius of a normal main sequence star. The now greatly enlarged shell cooled off and so was the visible red light outer shell, which characterizes a red giant. Once the core had reached a temperature and density at which there was a fusion of helium, the star pulled even further together and belonged now to a group of stars that is located on an approximately horizontal line on the HR diagram. By helium fusion is an inner core of carbon and oxygen formed. Finally, as the helium was exhausted in the core, was in addition to the outer shell in the burning of hydrogen, another shell in the now displaced helium burning. The star shifted within the HR- diagram in an area that astronomers call the Asymptotic Giant Branch (English asymptotic giant branch, AGB ). Decreed the progenitor star of IK Pegasi B have enough mass, so it was at its core with time to a carbon burning, and oxygen, neon and magnesium emerged.

In general, it is a case that expands the outer shell of a red giant or AGB star on the several hundred times the solar radius, ( The pulsating AGB star Mira, for example, reaches a radius of about 5 × 108 km (3 AE). ) why this may be assumed also for IK Pegasi B. The expansion of the casing had it towers over the distance that the two stars of IK Pegasi today have an average, so both had to share a common envelope during this time. This in turn means that an increased number of isotopes was fed at this stage the outer atmosphere of IK Pegasi A.

Some time after an internal carbon-oxygen or oxygen - magnesium -neon core had formed, there was a nuclear fusion in two concentric shells around the core region around. In this case, hydrogen was burned at the outermost of the two shells, while the helium fusion took place around the inner core. However, such a double -shell phase is unstable, leading to so-called thermal pulses, which involve a large mass emission of the outer envelope by itself. This material was eventually repelled in a huge cloud of material as a planetary nebula. Except for a small part of the entire hydrogen coat was cut off from the star, leaving behind a white dwarf, which consisted of the remains of the inner core in the first place.

Composition and structure

The core of IK Pegasi B is, as with most white dwarfs, probably entirely of carbon and oxygen with a coat of hydrogen and helium. If its progenitor star for carbon combustion had been able, but there is also the possibility that its core is composed of oxygen and neon, which is surrounded by a sheath of carbon and oxygen. Due to the higher atomic mass must how any helium in the envelope to fall below the hydrogen layer, which is why you can expect that the outer shell of IK Pegasi B is surrounded by an atmosphere of almost pure hydrogen, so the star of spectral type DA can be assigned. The total mass of the star is now supported only by the degeneracy pressure of the electrons, a quantum mechanical effect, which limits the number of particles of matter that can be squeezed into a given volume.

With an estimated 1.15 solar masses is IK Pegasi B is classified as a highly massive white dwarf. [A 6] Although its extent has not yet been directly observed, it is possible to him, based on known theoretical relationships between the mass and the radius of other white dwarfs estimate. Here, a size of 0.6 % of the solar radius is assumed for him. ( Other sources assume a value of 0.72 %, which is some uncertainty remains. ) This means in other words that this star with a mass greater than that of the sun, in a volume of about the size earth fits what gives an impression of the extreme density that has this property [A 7]

The massive and compact nature of a white dwarf, a strong surface gravity is generated. Astronomers enter this value with the decimal logarithm of the gravity in cgs units, or log g. For IK Pegasi B, a log g of 8.95 is assumed. In comparison, the log of the earth g is 2,99. In other words, is the gravity on the surface of IK Pegasi the more than 900,000 times the force of gravity on earth. [A 8]

The effective surface temperature of IK Pegasi B is estimated at about 35,500 ± 1500 K, which makes this celestial body to a strong source of UV radiation. [A 9] Under normal conditions cools such a white dwarf during the next more than a billion years further, while its radius essentially remains unchanged.

Development forecasts

In its elaboration in 1993 identified David Wonnacott, Barry J. Kellett and David J. Stickland this system as a candidate for the development of a supernova of type Ia or a cataclysmic variable star. At a distance of 150 light years, this system is thus the erdnächste known candidate of a supernova precursor. However, this variant is only one of several scenarios that may take the development of such a double star. Basically, it can be assumed that IK Pegasi A have been used up to a certain point, the hydrogen in its core and a development is traversed away from the main sequence to become a red giant. The surface of this red giant will then grow so that their dimension exceeds its original radius around a hundred times or more. Eventually, the outer envelope of IK Pegasi A has extended so far that it exceeds the Roche limit of his companion and a gaseous accretion disk around the white dwarf can occur. This gas, which is composed primarily of hydrogen and helium, leads to an increase in the scope of his companion. Based on observations of similar objects can be assumed that the two stars, triggered by the mass exchange, are closer to each other constantly.

According to the probable development forecast, the accreted gas is compressed on the surface of the white dwarf, whereupon it heats up until the accumulated gas from a certain point has the necessary prerequisites for hydrogen fusion. Thus, a thermal reaction is caused, in turn, removed as a consequence of a portion of the gas from the surface. While increasing its mass, only a part of the accreted gas can be dropped, so that with each cycle, the mass of the white dwarf will increase continuously. As usual with a recurring nova, the surface would increase even with IK Pegasus B. Thus arise ( continuous) Nova explosions that are typical of a cataclysmic variable star. During these phases, the brightness of the white dwarf for a period of several days or months will increase rapidly to several Magnitudengrößen. An example of such a star system RS Ophiuchi is a binary star system, which is also composed of a red giant and a white dwarf as a companion. For RS Ophiuchi at least six times was a recurring ( recurring ) Nova observed 1898-2006. Every time the white dwarf had reached the critical value of his the collected mass of hydrogen, it led to a renewed explosive thermal reaction, which could then be observed as a Nova.

However, various binaries through an alternative development model in which succeeds the white dwarf, steadily picking up mass without causing a nova event. Such close binary systems are commonly referred to as Type Super Soft X - Ray Source (CBSS ) ( X-ray source with extremely soft radiation). For these objects, the transfer rate of the mass to their near- white - dwarf companion is low enough that a continuous fusion can be maintained without the incoming hydrogen is burned on the surface in a nuclear fusion into helium. The class of super soft X -ray source comprises all highly massive white dwarfs with a very high surface temperature ( 0.5 × 10 6 to 1 × 10 6 K. ).

The continuous recording of mass, such a white dwarf approaches the Chandrasekhar limit at some point of 1.44 solar masses, from which the degeneracy pressure of the electron gas to the gravitational pressure can no longer compensate and it must come to a collapse. In the event that the core is composed mainly of oxygen, neon, and magnesium, this means that the collapsing white dwarf usually only a fraction of its mass is blown off and the residue finally coincides to a neutron star. If the core of carbon and oxygen on the other hand, the increasing pressure and increasing temperature are re- carbon fusion in the center initiate before the Chandrasekhar limit is reached. This would result in an unstoppable nuclear fusion reaction, which consumes a significant part of the stellar mass in a short time and eventually sufficient to tear apart the star in a massive Type Ia supernova explosion.

But will have to this system reaches a state where it could lead to a supernova explosion, the two objects are at a considerably greater distance from the Earth, as it is highly unlikely that the primary star, IK Pegasi A, in the immediate future become a red giant. For a supernova event poses a serious threat to life on earth, it must be held to the ground within a distance of about 26 light years. Within this radius, it is possible that the biosphere of the planet affected and in an extreme case, the ozone layer of the earth could be destroyed. The velocity of this star is currently 20.4 km / s relative to the Sun, which corresponds to an increase in the distance of one light-year every 14,700 years. After 5 million years ago this star will thus be removed from the sun more than 500 light-years. A distance that is far enough outside the radius within which a Type Ia supernova would pose a threat to our solar system.

Following such a supernova explosion, the rest of the donor star ( IK Pegasus A) It moves forward at the speed he had as a member of the binary system once. The resulting relative velocity to the galactic environment can be up to 100-200 km / s, which would make these heavenly bodies into one of the fastest objects in our galaxy. The supernova explosion itself leaves nurmehr a remnant of expanding material that eventually enters into the surrounding interstellar matter.

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