AB Doradus
AB Doradus ( AB Dor abbreviated ) is about 49 light years distant quadruple star system in the constellation of swordfish (Latin Dorado ), located in the southern sky. It contains a rapidly rotating main sequence star (AB Doradus A) in just twelve hours turns despite its hundred times Earth's diameter around its own axis, and three red dwarf star (AB Doradus Ba, AB Doradus Bb and AB Doradus C). AB Doradus C is the lightest known star, its mass is only slightly above the edge of the brown dwarf.
The apparent brightness of AB Doradus A varies with a period that is equal to its rotational period. So the star belongs to the rotational variables. Its variability is attributed to large star spots, which are related to its complex magnetic field. Observations showed a like between about 0.01 and 0.05 varying amplitude of the change in brightness.
The brightness variations of AB Doradus A gave the system its name: The first part of the name "AB" follows the rules for naming variable stars and states that AB Doradus is the 56th star in the constellation swordfish, to the variability could be detected. The second part of the name " Doradus " is the genitive of the Latin name of the constellation. The third part "A" of the main component was added later to distinguish it from later discovered smaller companions.
AB Doradus is only 50 million years old and namesake of the AB Doradus moving cluster that is explored, inter alia, to further clarify the star formation.
- 4.1 AB Doradus A-C 4.1.1 AB Doradus A 4.1.1.1 rotation
- 4.1.1.2 Further Development
- 4.1.2.1 discovery
- 4.1.2.2 Further Development
- 4.2.1 discoveries
- 4.2.2 Age determination
Finding in the night sky
For observers in the southern hemisphere AB Doradus is almost all year to see. However, the star itself is not observable from the southern parts of Europe.
Due to its low apparent brightness of the star is not visible to the naked eye. Even a small telescope or binoculars is sufficient, however, to observe it can. Near AB Doradus is the Large Magellanic Cloud (the largest satellite galaxy of the Milky Way ) and the constellations network ( reticulum ) and Table Mountain ( Mensa ).
Position
Distance
The distance from AB Doradus can be determined relatively accurately at 49 light years due to its strong self-motion and the relatively large parallax. [A 1]
The 500 star closest to the Sun are (with correct parallax) is known, in an environment of 33 light years. This gives (with assumed constant star density ) by simple calculation that the components of AB Doradus to the 1600-1700 sun next stars belong. [A 2]
AB Doradus appears to be a part of the sky standing next to him the Large Magellanic Cloud. However, this is at a distance of 160,000 light years, about 3,000 times farther from the Sun than AB Doradus.
Proper motion
A comparison of the proper motion with the other stars showed some striking similarities. The 50 million year old star system AB Doradus became the namesake of the AB Doradus moving cluster. He is a young movement with a loose pile group of stars that are not characterized by a spatial concentration around a cluster center, but by a common movement towards a convergence or vanishing point. [A 3]
Joint development with the Pleiades?
During the investigation of the age of AB Doradus a marked similarity in the proper motion of the cluster motion with the Pleiades was found. The Pleiades are also a young open cluster of stars can be observed as the " Seven Sisters " in the constellation Taurus. This finding led to an investigation of the space velocity or movement within the Milky Way. With it should be determined whether the two groups are formed in the same gas cloud.
Currently there are AB Doradus and the Pleiades 475 light -years apart. Using the observation data, the movement of the two groups of stars could be simulated in the last million years. The result indicates that the distance between AB Doradus and the Pleiades 125 million years ago (ie at the time as to light up the first stars of the movement pile started ) with 850 light-years was significantly greater. [A 3] [A 4] This was refutes the hypothesis AB Doradus had arisen in the same (probably 160 light-years wide ) molecular gas cloud as the Pleiades. However, do not include the proper motions of a larger formation region.
Construction of the system
AB Doradus is a multiple star system consisting of two binary systems, which are at a distance of about 135 astronomical units, which corresponds to 135 times the distance of Earth to the Sun, mutually orbiting around a common barycenter.
The less massive binary star system contains two star AB Doradus Ba and AB Doradus Bb, which orbit with a period of about 135 days. Contrast, the heavier double star system called " AB Doradus A- C" and thus contains the components AB Doradus A and AB Doradus C, which revolve at a distance of 2.3 AU all 11.75 years. Because of its higher mass of the distance from the common center of mass AB Doradus is about half as large as that of the lighter double star AB Doradus B.
Components of the system
AB Doradus A-C
AB Doradus A
AB Doradus A, the glowing orange main star is a sun -like star of spectral type K2 Vk. The numeric designation ranges from 0 ( hottest ) to 9 ( coolest ) star within the spectral class K; with K2 AB Doradus A belongs Thus, as α Centauri B or ε Eridani to the hotter K- stars. The luminosity class V indicates that it belongs to the main sequence stars. Due to his young age, AB Doradus A frequently than pre-main -sequence star ( " pre-main sequence star " ) is referred to, so as a star that has not yet reached the main sequence or just now. The suffix "k " stands for interstellar absorption lines.
The mass of AB Doradus A is three quarters of the Sun's mass. He is by far the heaviest component of the quadruple star system. As earlier K- rating it has a surface temperature of 4900 K and is therefore cooler than the sun only by less than 900 K. However, this slight difference in the effective temperature makes a big difference in terms of luminosity. Although AB Doradus A with 0.9 times solar diameter has a relatively large surface area, results from just over a third of the solar luminosity. In addition, the sun radiates in the visible light almost entirely due to its surface temperature of about 5800 K, while at the AB Doradus A proportion of the infrared radiation is significantly higher. Thus, accounts for only about 85 % of the total luminosity of AB Doradus A to visible light. This weaker visual luminosity is the cause that the orange dwarf, despite the relatively short distance of nearly fifty light years with an apparent magnitude of 6.93 like it with the naked eye is no longer recognizable.
Rotation
In the 1990s, could be established that AB Doradus A rapidly rotating about its own axis, which has bulges at the equator, a much more complex magnetic field and temperature variations on the surface result by measuring the width of its spectral lines. This is the reason why AB Doradus A different spectral types assigned.
It has been found that a full rotation axis occurs at the equator in approximately twelve hours. That corresponds to the fifty times angular velocity of the sun, which takes 25 days for a full turn, or twice the angular velocity of the earth, even though their diameter is only about a hundredth of AB Doradus A. Thus, the rotational period of the orange star is one of the shortest of all known stars; similar to high rotational speeds are also Vega, Altair or Achernar. However, AB Doradus A is still far from falling apart because of the occurring centrifugal forces. This limit would be exceeded probably only from an equatorial rotational velocity of 450 km / s.
Additionally eruptions were detected on the surface of AB Doradus A; Here, the weak hydrogen plasma is heated to up to fifteen million degrees and, included in the magnetic fields, the spherical bulge on the star surface. This plasma glows in X-rays. Thus AB Doradus A is due to its small distance one of the brightest stellar X-ray sources in the sky at all. Equatorial outbreaks rotate due to differential rotation is much faster than those at the poles, but the difference in the years 1988 and 1996 was only half as great as if by 1992 until 1995. During these years, the star appeared much flattened.
Further development
Currently is AB Doradus A still at the beginning of his life. The fusion of hydrogen into helium in the core mainly through the proton-proton chain occurs, as in all Sun-like stars, which does not generate a steep temperature gradient. Thus, the thermal radiation dominates in the interior of solar-like stars. In the outer part against the prevailing convection, since the star is cool enough so that the hydrogen is neutral and therefore impermeable to ultraviolet photons (see also Article stellar structure ).
Because not fusionable helium ash accumulates in the nucleus, the reduction of hydrogen per unit mass leads to a gradual reduction in the rate of nuclear fusion within this mass. To compensate, core temperature and pressure slowly increase, which causes an increase in the total fusion rate. This results in a steady increase in the luminance and the radius of AB Doradus A over time. For example, the luminosity of the young Sun was only about 70 % of its current value, the luminosity increase thus gradually changes over time, the position of the star in the Hertzsprung -Russell diagram.
In about 22 billion years, the hydrogen storage [A 5] will be exhausted in the core of AB Doradus A. Residue is then taken back by the loss of energy of the gravitational collapse. The core surrounds the hydrogen reaches the necessary temperature and pressure to fuse. Thus, a hydrogen- burning shell around the helium core is formed. As a result of these changes, the outer shell expands, the temperature drops and the star turns into a red giant.
From this point, the star leaves the main sequence and reaches the giant branch. The helium core of the star is contracted to continue until it is stopped by the so-called degenerate electron pressure - a quantum mechanical effect, which limits the extent to which matter can be compressed.
Since AB Doradus A is a star with more than half the solar mass, the core may reach a temperature at which it is possible that carbon is generated from helium through the three alpha - process. At the end of this process AB Doradus A will repel its outer shells and form planetary nebulae. What remains of extinct core as a white dwarf, the only lights in the form of heat radiation and cools slowly. The rest star is reddish, until it finally disappears entirely as a black dwarf in the visible range.
AB Doradus C
AB Doradus C is the smaller component of the subsystem AB Doradus AC. The red dwarf belongs to the spectral class M8 and is thus classified as later M- dwarf star. With 2600 Kelvin its surface temperature is correspondingly cool while the sun is more than twice as high. Similarly low is its radius, which accounts for only about one-sixth of AB Doradus A.
With 8.9% of the solar mass (about 93 Jupiter masses) applies AB Doradus C than the lightest known star at all. If he were just a little less massive, it would be a brown dwarf. Such objects occupy a special position between planets and stars. You can temporarily gain small amounts of energy from the fusion of deuterium before they cool. In this initial phase, they must be distinguished from normal stars barely.
While the sun and AB Doradus A are as average main sequence stars below the middle of the main sequence in the Hertzsprung -Russell diagram, AB Doradus C marked as masseärmster all stars, unless the cooler brown dwarfs are not considered, the end.
Discovery
Already in the 1990s, the small companion of AB Doradus A was known until 2004, he could only be detected due to its gravitational effect by a " wobbling " of the circled AB Doradus A, which completely outshines him. This is due to the shaking rotary movement about the common center of mass. The new component was AB Doradus C called because the name AB Doradus B was already taken. Even the name AB Doradus From resigned because otherwise the orange main star, who had already made due to its peculiarities with respect to its rotation a name that would have renamed AB Doradus Aa must be.
Since AB Doradus C could not be detected optically by the Hubble Space Telescope, it was only in 2005 recorded visually using the instrument NACO SDI of the Very Large Telescope of the European Southern Observatory. Equipped with adaptive optics camera took them through different filters four images of AB Doradus A-C simultaneously. Thus, the lighter AB Doradus A disappeared and the cooler companion was visible, the images were subtracted from each other. Here, the fact was used that the two stars have their maximum brightness in different spectral ranges, AB Doradus A in visible light, however, AB Doradus C in the infrared wavelength range.
Using the new observations was found out that AB Doradus A is surrounded by his companion in a highly eccentric orbit. This means that the center of the orbit of AB Doradus C is outside the common barycenter of the two stars, and they are thus apart in a varying distance.
It was also found that AB Doradus C cooler by 400 K 2600 K was as expected. This calls into question models for the calculation of stellar masses of luminous intensities for small celestial objects so far in question, as they have to be adapted for small stars to explain the observation result.
The below pictures show the ESO- System AB Doradus A-C before application of the new method of observation and after. During the low luminosity component is still outshines completely on the left picture, she is on the right picture, favored by the "disappearance" of AB Doradus A, clearly visible.
Further development
As in Sun-like stars fused a red dwarf AB Doradus C as the proton-proton reaction hydrogen to helium. Since this class of stars but also has low core temperatures prevailing in the entire star in front of the convection, it is regarded as " vollkonvektiv " while in severe and average main sequence stars, the heat radiation is part of the energy transfer in stellar structure outward.
In larger stars helium accumulates over time in the core. Instead, this is not the case for the fully convective red dwarfs. Therefore, they can merge a higher percentage of hydrogen before leaving the main sequence, which their enormous lifetime justified. The smaller the mass of the star, the longer it is on the main sequence. Thus, AB Doradus C is longer than any other star known as the lightest star, possibly even ten trillion years linger on the main sequence.
That AB Doradus C will eventually reach this enormous age, is still uncertain. If not stabilize its orbit to AB Doradus A in the next billion years, it will come during the phase of AB Doradus A as a red giant to a mass transfer between the two stars. Then flows of matter from the Roche limit border giant star to the small component. The growing mass of AB Doradus C would then favor a higher reaction activity, whereby the fuel would be consumed quickly and the once small red dwarf would go out after a few billion years.
Due to its low mass, AB Doradus C itself does not swell to a red giant. Instead, the star, once the hydrogen is depleted in the star shrink and allow the released gravitational energy the last fusion processes. Finally, AB Doradus C reached the stage of the white dwarf.
AB Doradus Ba and Bb
According to the results, let AB Doradus Ba and AB Doradus Bb with correct results only one percent of the luminosity of AB Doradus A from.
In AB Doradus Ba is, with an estimated 13 to 20 % of the solar mass, the heavier component of the AB Doradus B system. First, the star was classified brighter, but was later the spectral type M5 V are assigned. Its surface temperature is 3145-3305 degrees Kelvin. The luminosity of the weaker, 11 to 18 % of the solar mass making up the AB Doradus Bb, however, was determined to be 3080 K to 3240 K. This makes it one of the cooler spectral type M5.5 V.
As AB Doradus C AB Doradus Ba and AB Doradus Bb AB Doradus A, the main component of a multiple survive significantly and eventually also end up as white dwarfs. [A 5]
Discoveries
Component B of AB Doradus was discovered by Richard A. Rossiter, who received the double star as number 137 in his catalog; Accordingly, the component B is now known as Rossiter 137 B or short Rst 137 B. It was originally assumed that it is the ten seconds of arc AB Doradus A remote little companion to a single star thirteenth magnitude of spectral type is M3.5 Ve. "E" suffix stands for emission lines.
However, since the detection of small AB Doradus C could be detected using the new methods of observation that AB Doradus B consists of two red dwarf stars, which is at a distance comparable to each other circle with the distance of the sun to the earth. The new components were given in accordance with the terms AB Doradus Ba and AB Doradus Bb means of the mass-luminosity law can determine a total mass of the two stars of one third of the mass of the sun from it. [A 6]
Age determination
Using the determined physical properties of AB Doradus Ba and AB Doradus Bb the age of the system AB Doradus should be more accurately determined. An exact result had not been possible due to the unusual properties of AB Doradus A and AB Doradus C.
The two of them orbiting small red stars were with three other red dwarfs of spectral type M3 of the AB Doradus moving the pile in terms of their luminosity with other M- dwarfs from the more than 100 million years old Pleiades and the existing since 50 to 60 million years open star cluster IC 2391, the region around ο Velorum in the constellation of sails of the ship, are compared. On the diagram indicated the similarities of the studied stars in the direction of the younger cluster IC 2391st Thus the age of AB Doradus was determined to at least 50 million years.
Possibility of planet formation
With the discovery of three super-Earths around HD 40307 which has been theorized that there could be planets as well as to each star. With the discovery of a planet in binary star system γ Cephei AB Doradus came to the fore of the possible candidate for harboring a potential planetary system. The habitable zones of the individual components can be calculated from the luminosity of the stars and their size. [A 7] This life zone marks the distance range in which a planet must stop by his central star to allow water permanently, in liquid form as a prerequisite for life can be comparable with terrestrial conditions, on the surface.
Theoretically, a planet to AB Doradus A at a distance of Venus to the sun could get enough energy to retrieve liquid water. However, the main problem, which significantly limits the formation of planets is the fact that it is a double - double star system in AB Doradus. While individual stars can form dust disks easily, the stars of a binary system often hinder each other. To ensure stable orbits of the planets, the two stars of a system must revolve around either so close that a planet its orbit moves to the center of mass of the star, or the stars revolve around one another at such a large distance that the orbits of the planets around the individual stars are not disturbed.
So AB Doradus A has indeed on the sonnenähnlichsten properties, but could possible planets by the enormous eccentricity of the orbit of AB Doradus C in its orbit does not remain stable. In addition, the high X-ray and magnetic activity of the rapidly rotating AB Doradus A, reducing the chance of planet formation in a shorter distance to the orange star is also equal to zero. Similarly, the low luminosity red dwarf is hardly suitable as a fictional planet would have to move to an extremely small orbit around the star. In addition, the varying proximity of the bright stronger AB Doradus A would limit the climatic conditions vary widely.
Thus, only the two red dwarf star AB Doradus Ba and AB Doradus Bb remain as a possible central planets extrasolar planets. Since their respective life zones make up only one twenty-fifth of the distance of the two stars to low-mass, ie terrestrial planets could form, quite, without being subjected to the forces of the second star helpless. Exclude Certainly so gas giants like Jupiter and Saturn, due to the gravitational perturbations in a binary system can not be formed.
The habitability of planets red dwarfs is the subject of some debate. Despite their frequent occurrence and the long lifetime of these stars there are several factors that could make life on such planets difficult. Transferred to AB Doradus Ba and AB Doradus Bb likely a planet orbiting at a distance of only 10 % of the distance of the planet Mercury to the Sun at one of the two stars. When a planet orbits so close to a star that would be set by the tidal forces synchronous rotation. One side of the surface would be facing permanently the star, thus the red sun would always be seen in the same place in the sky. Even if the rotation would remain unrestricted, no seasons could develop as a circulation of a planet within the habitable zone would take a few days.
In addition, since at red dwarfs usually Flareausbrüche occur, this life would hardly allow. Within a few minutes, the luminosity of the star could double or even triple. These flares could be the atmosphere of any planet located in the habitable zone destroy.
Wings of AB Doradus
Although the AB Doradus system is only 50 light years away, the sky appears to an observer for the most part completely changed. Only some constellations as ship's keel and dragon look almost unchanged. The constellation swordfish acts but completely equalized. In contrast, the sun is as inconspicuous star sixth size in the dragon, near Aldhibah ( ζ Draconis ), antipodal ( in the opposite direction ) to the seen from the Earth position of AB Doradus, ie at coordinates α = 17284517h 28m 45s and δ = 2,652,655 65 ° 26 ' 55 ".
The sun stand closer stars like Sirius, Procyon and Alpha Centauri are to be seen in clearly shifted and approximate positions. For example, Sirius is not the same brightness of -1.46 likes to, he's just a star of the second magnitude, comparable to the other parts of the large dog as Aludra (2.42 mag) or Wezen (1.78 mag). Instead, Canopus shines as the brightest and only the -1 mag- border border point of light, followed by Achernar and the two main stars of Orion, Betelgeuse and Rigel. In addition, only eight light years distant HD 40307 forms with an apparent Pollux orange double star. Also, the system Zeta Reticuli appears as only 14 light years distant third yellow double size in the sky.
The faint neighbors from the motion pile of AB Doradus adorn the sky as orange-reddish, weakly recognizable stars, which itself never appear striking in appearance and hardly change their position in the sky due to their identical proper motion.