IK Pegasi (or HR 8210) is a binary star system in the constellation Pegasus. It is just luminous enough to be seen with the unaided eye, at a distance of about 154 light years from the Solar System.
The primary (IK Pegasi A) is an A-type main-sequence star that displays minor pulsations in luminosity. It is categorized as a Delta Scuti variable star and it has a periodic cycle of luminosity variation that repeats itself about 22.9 times per day. Its companion (IK Pegasi B) is a massive white dwarf—a star that has evolved past the main sequence and is no longer generating energy through nuclear fusion. They orbit each other every 21.7 days with an average separation of about 31 million kilometres or 0.21 astronomical units (AU). This is smaller than the orbit of Mercury around the Sun.
IK Pegasi B is a progenitor for a near-Earth supernova, although it is not the nearest supernova progenitor, which is Wolf 1130. When the primary begins to evolve into a red giant, it is expected to grow to a radius where the white dwarf can accrete matter from the expanded gaseous envelope. When the white dwarf approaches the Chandrasekhar limit of 1.4 solar masses (), it may explode as a Type Ia supernova.
Observation This star system was catalogued in the 1862 Bonner Durchmusterung ("Bonn astrometric Survey") as BD +18°4794B. It later appeared in Pickering's 1908 Harvard Revised Photometry Catalogue as HR 8210. The designation "IK Pegasi" follows the expanded form of the variable star nomenclature introduced by Friedrich W. Argelander.
Examination of the spectrographic features of this star showed the characteristic absorption line shift of a binary star system. This shift is created when their orbit carries the member stars toward and then away from the observer, producing a doppler shift in the wavelength of the line features. The measurement of this shift allows astronomers to determine the relative orbital velocity of at least one of the stars even though they are unable to resolve the individual components.
In 1927, the Canadian astronomer William E. Harper used this technique to measure the period of this single-line spectroscopic binary and determined it to be 21.724 days. He also initially estimated the orbital eccentricity as 0.027. (Later estimates gave an eccentricity of essentially zero, which is the value for a circular orbit.) The velocity amplitude was measured as 41.5 km/s, which is the maximum velocity of the primary component along the line of sight to the Solar System.
The distance to the IK Pegasi system can be measured directly by observing the tiny parallax shifts of this system (against the more distant stellar background) as the Earth orbits around the Sun. This shift was measured to high precision by the Hipparcos spacecraft, yielding a distance estimate of 150 light years (with an accuracy of ±5 light years). The same spacecraft also measured the proper motion of this system. This is the small angular motion of IK Pegasi across the sky because of its motion through space.
The combination of the distance and proper motion of this system can be used to compute the transverse velocity of IK Pegasi as 16.9 km/s. The third component, the heliocentric radial velocity, can be measured by the average red-shift (or blue-shift) of the stellar spectrum. The General Catalogue of Stellar Radial Velocities lists a radial velocity of −11.4 km/s for this system. The combination of these two motions gives a space velocity of 20.4 km/s relative to the Sun.
An attempt was made to photograph the individual components of this binary using the Hubble Space Telescope, but the stars proved too close to resolve. Recent measurements with the Extreme Ultraviolet Explorer space telescope gave a more accurate orbital period of . The inclination of this system's orbital plane is believed to be nearly edge-on (90°) as seen from the Earth. If so it may be possible to observe an eclipse.
IK Pegasi A
The Hertzsprung–Russell diagram (HR diagram) is a plot of luminosity versus a color index for a set of stars. IK Pegasi A is currently a main sequence star—a term that is used to describe a nearly linear grouping of core hydrogen-fusing stars based on their position on the HR diagram. However, IK Pegasi A lies in a narrow, nearly vertical band of the HR diagram that is known as the instability strip. Stars in this band oscillate in a coherent manner, resulting in periodic pulsations in the star's luminosity.
The pulsations result from a process called the κ-mechanism. A part of the star's outer atmosphere becomes optically thick due to partial ionization of certain elements. When these atoms lose an electron, the likelihood that they will absorb energy increases. This results in an increase in temperature that causes the atmosphere to expand. The inflated atmosphere becomes less ionized and loses energy, causing it to cool and shrink back down again. The result of this cycle is a periodic pulsation of the atmosphere and a matching variation of the luminosity.
Stars within the portion of the instability strip that crosses the main sequence are called Delta Scuti variables. These are named after the prototypical star for such variables: Delta Scuti. Delta Scuti variables typically range from spectral class A2 to F8, and a stellar luminosity class of III (giants) to V (main sequence stars). They are short-period variables that have a regular pulsation rate between 0.025 and 0.25 days. Delta Scuti stars have an abundance of elements similar to the Sun's (see Population I stars) and between 1.5 and . The pulsation rate of IK Pegasi A has been measured at 22.9 cycles per day, or once every 0.044 days.
Astronomers define the metallicity of a star as the abundance of chemical elements that have a higher atomic number than helium. This is measured by a spectroscopic analysis of the atmosphere, followed by a comparison with the results expected from computed stellar models. In the case of IK Pegasus A, the estimated metal abundance is [M/H] = +0.07 ± 0.20. This notation gives the logarithm of the ratio of metal elements (M) to hydrogen (H), minus the logarithm of the Sun's metal ratio. (Thus if the star matches the metal abundance of the Sun, this value will be zero.) A logarithmic value of 0.07 is equivalent to an actual metallicity ratio of 1.17, so the star is about 17% richer in metallic elements than the Sun. However the margin of error for this result is relatively large.
The spectrum of A-class stars such as IK Pegasi A show strong Balmer lines of hydrogen along with absorption lines of ionized metals, including the K line of ionized calcium (Ca II) at a wavelength of 393.3 nm. The spectrum of IK Pegasi A is classified as marginal Am (or "Am:"), which means it displays the characteristics of a spectral class A but is marginally metallic-lined. That is, this star's atmosphere displays slightly (but anomalously) higher than normal absorption line strengths for metallic isotopes. Stars of spectral type Am are often members of close binaries with a companion of about the same mass, as is the case for IK Pegasi.
Spectral class-A stars are hotter and more massive than the Sun. But, in consequence, their life span on the main sequence is correspondingly shorter. For a star with a mass similar to IK Pegasi A (1.65 ), the expected lifetime on the main sequence is 2–3 billion years, which is about half the current age of the Sun.
In terms of mass, the relatively young Altair is the nearest star to the Sun that is a stellar analogue of component A—it has an estimated 1.7 . The binary system as a whole has some similarities to the nearby system of Sirius, which has a class-A primary and a white dwarf companion. However, Sirius A is more massive than IK Pegasi A and the orbit of its companion is much larger, with a semimajor axis of 20 AU.
IK Pegasi B The companion star is a dense white dwarf star. This category of stellar object has reached the end of its evolutionary life span and is no longer generating energy through nuclear fusion. Instead, under normal circumstances, a white dwarf will steadily radiate away its excess energy, mainly stored heat, growing cooler and dimmer over the course of many billions of years.
Evolution Nearly all small and intermediate-mass stars (below about 8~9 ) will end up as white dwarfs once they have exhausted their supply of thermonuclear fuel. Such stars spend most of their energy-producing life span as a main-sequence star. The time that a star spends on the main sequence depends primarily on its mass, with the lifespan decreasing with increasing mass. Thus, for IK Pegasi B to have become a white dwarf before component A, it must once have been more massive than component A. In fact, the progenitor of IK Pegasi B is thought to have had a mass between 5 and .
As the hydrogen fuel at the core of the progenitor of IK Pegasi B was consumed, it evolved into a red giant. The inner core contracted until hydrogen burning commenced in a shell surrounding the helium core. To compensate for the temperature increase, the outer envelope expanded to many times the radius it possessed as a main sequence star. When the core reached a temperature and density where helium could start to undergo fusion this star contracted and became what is termed a horizontal branch star. That is, it belonged to a group of stars that fall upon a roughly horizontal line on the H-R diagram. The fusion of helium formed an inert core of carbon and oxygen. When helium was exhausted in the core a helium-burning shell formed in addition to the hydrogen-burning one and the star moved to what astronomers term the asymptotic giant branch, or AGB. (This is a track leading to the upper-right corner of the H-R diagram.) If the star had sufficient mass, in time carbon fusion could begin in the core, producing oxygen, neon and magnesium.
The outer envelope of a red giant or AGB star can expand to several hundred times the radius of the Sun, occupying a radius of about (3 AU) in the case of the pulsating AGB star Mira. This is well beyond the current average separation between the two stars in IK Pegasi, so during this time period the two stars shared a common envelope. As a result, the outer atmosphere of IK Pegasi A may have received an isotope enhancement.
Some time after an inert oxygen-carbon (or oxygen-magnesium-neon) core formed, thermonuclear fusion began to occur along two shells concentric with the core region; hydrogen was burned along the outermost shell, while helium fusion took place around the inert core. However, this double-shell phase is unstable, so it produced thermal pulses that caused large-scale mass ejections from the star's outer envelope. This ejected material formed an immense cloud of material called a planetary nebula. All but a small fraction of the hydrogen envelope was driven away from the star, leaving behind a white dwarf remnant composed primarily of the inert core.
Composition and structure The interior of IK Pegasi B may be composed wholly of carbon and oxygen; alternatively, if its progenitor underwent carbon burning, it may have a core of oxygen and neon, surrounded by a mantle enriched with carbon and oxygen. In either case, the exterior of IK Pegasi B is covered by an atmosphere of almost pure hydrogen, which gives this star its stellar classification of DA. Due to higher atomic mass, any helium in the envelope will have sunk beneath the hydrogen layer. The entire mass of the star is supported by electron degeneracy pressure—a quantum mechanical effect that limits the amount of matter that can be squeezed into a given volume.