IK Pegasi

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IK Pegasi

Observation data Equinox J2000
Constellation	Pegasus
Right ascension	21h 26m 26.6624s
Declination	+19° 22′ 32.304″
Apparent magnitude (V)	6.078
Characteristics
Spectral type	A8m:[1]/DA
U-B color index	0.06/unknown
B-V color index	0.22/unknown
Variable type	Delta Scuti
Astrometry
Radial velocity (Rv)	-11.4 km/s
Proper motion (μ)	RA: 80.23 mas/yr Dec.: 17.28 mas/yr
Parallax (π)	21.72 ± 0.78 mas
Distance	150 ± 5 ly (46 ± 2 pc)
Absolute magnitude (MV)	2.762
Details
Mass	1.65[2]/1.15 M☉
Radius	1.6[2]/0.0074[2] R☉
Luminosity	? L☉
Temperature	7,700[3]/? K
Metallicity	115/? % Sun
Rotation	?
Age	5–60 × 107[2] years
Other designations
V* IK Peg, HR 8210, BD +18°4794B, HD 204188, SAO 107138, HIP 105860.
Database references
SIMBAD	data

IK Pegasi is a spectroscopic binary star system in the constellation Pegasus. The primary (IK Pegasi A) is a main sequence, spectral class A star that displays minor pulsations in luminosity.^[3] The companion (IK Pegasi B) is a high-temperature white dwarf; a star that has evolved past the main sequence and is no longer generating energy through nuclear fusion. The two stars are separated from each other by about 31 million kilometres, or 0.21 Astronomical units: 21% of the average distance between the Earth and the Sun. They orbit around their common barycenter every 21.7 days.

This is the nearest known supernova candidate to the Solar System. When the primary begins to evolve into a red giant, it is expected to share a common gaseous envelope with the white dwarf. The later star will begin to accrete matter from the primary, and, when it approaches the Chandrasekhar limit of 1.44 solar masses, it will explode as a Type Ia supernova.^[4]

Contents 1 Observation 2 Component A 3 Component B 4 References 5 External links

[edit] Observation

This star system was catalogued in the 1862 Bonn Durchmusterung ("Bonn astrometric Survey") as BD +18°4794B. It later appeared in Pickering's 1908 Harvard Revised Photometry Catalogue as HR 8210.^[5] 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 that characteristic absorption line shift of a binary star system. This shift is created when their orbit carries the members stars toward and then away from the observer, producing a shift in the wavelength of the line features. (See redshift.) The measurement of this shift allows astronomers to determine the relative orbital velocity of at least one of the stars, even when 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 estimated the orbital eccentricity as 0.0, 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.^[6]

The distance to the IK Pegasi system can be measured directly by observing the tiny parallax shifts of this system against the more distant 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.^[7] The same spacecraft also measured the proper motion of this system; the small angular motion of IK Pegasi across the sky due to its motion through space.

The combination of the distance and proper motion of this system gives the space velocity of IK Pegasi in the direction perpendicular to the line of sight. 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.^[8]

An attempt was made to photograph the individual components of this binary using the Hubble Space Telescope, but, unsurprisingly, the stars proved too close to resolve.^[9]

[edit] Component A

The Hertzsprung-Russell diagram is a plot of luminosity versus surface temperature for a set of stars. 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.^[10]

The pulsations result from a process called the κ-mechanism. A part of the star's atmosphere becomes optically thick due to partial ionization of certain elements, which causes the atmosphere to expand in compensation. The inflated atmosphere becomes less ionized and looses 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.^[10]

Stars within the portion of the instability strip that crosses the main sequence are called Delta Scuti variables. (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 (subgiants) 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 (Population I stars), and between 1.5 and 2.5 solar masses.^[11]

The spectrum star 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, the atmosphere of this star displays slightly (but anomalously) higher than normal absorption line strengths for elements heavier than Lithium.^[1] 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.^[12]

Astronomers define the metallicity of a star as the abundance of elements that are heavier in mass than hydrogen and 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:

This notation givens the logarithm of the ratio of metal elements (M) to hydrogen (H), minus the logarithm of the Sun's metal ratio. That is, if the star matches the metal abundance of the Sun, this value will be zero. A logarithmic value of 0.06 is equivalent to an actual metallicity ratio of 1.15, so the star is about 15% richer in metallic elements than the Sun.^[2] However the margin of error for this result is relatively high.

Spectral class A stars are hotter and more massive than the Sun. But, in consequence, their life span on the main sequence is correspondingly brief. For a star with a mass similar to IK Pegasi A, the expected lifetime on the main sequence is 2–3 × 10⁹ years, which is about half the current age of the Sun.^[13]

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 times the solar mass. The binary system as a whole has been compared to Sirius, which has a type-A primary and a white dwarf companion. Sirius A, however, is more massive than IK Pegasi A and the orbit of its companion is larger, with a semimajor axis is 20 A.U.

[edit] Component B

The companion star is a dense white dwarf star consisting primarily of carbon and oxygen. 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 gradually radiate away its remaining energy, growing cooler and dimmer over the course of many billions of years.

Nearly all small and intermediate-mass stars (below eight solar masses) will end up as a white dwarf, once they have exhausted their supply of hydrogen and helium fuel. Such stars spend most of their energy-producing life span as a main sequence star. (This term is used to describe a grouping of hydrogen-fusing stars based on a plot of their luminosity versus surface temperature.) The amount of time that a star spends on the main sequence depends primarily on its mass, with the lifespan decreasing as a function of increasing mass. Thus, for IK Pegasi B to become a white dwarf before component A, this star must have once possessed the greater mass of the two.

As its hydrogen fuel at the core became consumed, this star evolved into a red giant. The inner core contracted until it reached a temperature and density where helium could star to undergo fusion. To compensate for the temperature increase, the outer envelope expanded to many times the radius it possessed as a main sequence star. The fusion of helium began to form an inert core of carbon and oxygen.

The outer envelope of a red giant star can expand to several hundred times the radius of the Sun, occupying a radius of about 5 × 10⁸ km (3 A.U.) in the case of the red giant star Mira.^[14] This is well beyond the current average separation between the two stars in this system, so during this period the two stars shared a common envelope.

Shortly after the oxygen-carbon core formed, the star began to pulsate due to an instability in the rate of fusion. Helium fusion is very sensitive to temperature changes, so a slight increase in heat results in an significant increase in energy output. This causes an expansion, which lowers the temperature again. This feedback loop results in large oscillations of energy production.

The pulsations of the star caused the outer hydrogen-rich envelope to be ejected from the surface over a period of time, forming an immense cloud of material called a planetary nebula. All but a small fraction of the hydrogen is driven away from the star, leaving behind a remnant composed primarily of the carbon-oxygen core.^[15]

[edit] References

[edit] External links

• IK Pegasi. Alcyone. Retrieved on January 18, 2007.