Further information on the super-Jupiter and the recording technique

November 19, 2012

How do the stars and planets get their names?
For bright stars like Kappa Andromedae (κ And), the astronomical naming convention follows that of the 17th century German astronomer Johann Bayer: the second part is the abbreviation of the Latin name of the constellation (here And = Andromeda), while the first part, a lower-case Greek letter, denotes the star itself. The letters are usually conferred in order of descending brightness. Since “kappa” is the tenth letter of the Greek alphabet, κ And is relatively low in brightness.

The planets around a star are named in order of discovery with lower-case Latin letters appended to the star’s name, starting with b. Consequently, the first planet discovered around Kappa Andromedae has the name κ  And b.

What are the basic characteristics of the system?
Kappa Andromedae is a massive, hot young star (spectral type B9; estimated mass between 2.4 and 2.5 solar masses; the effective temperature is 10,000 degrees Celsius) in the Andromeda constellation. The star is around 170 light years from Earth.

Its companion κ And b has an estimated mass roughly 12.8 times that of Jupiter. Considering this mass alone, the object could either be a massive planet or a very lightweight brown dwarf. However, comparative measurements of the brightness of κ And b at four different infrared wavelengths reveal an infrared colour similar to that of the handful of comparable planets, also gas giants which orbit other stars. Also, planet formation models predict that the probability of larger gas giants increases with the mass of the star; since the mass of κ  And is between 2.4 and 2.5 solar masses, the large planetary mass of the super-Jupiter is not surprising.

For planet formation models it is of minor interest whether κ  And b is located below or beyond the mass limit between planets and brown dwarfs. Joseph Carson and his colleagues define the term “super-Jupiter” in such a way that it can include both types of object - as long as they form in a similar way to the planets in our own solar system.

Kappa Andromedae b appears to orbit its parent star at a distance of at least 8 billion kilometres. Kappa Andromedae is believed to be a member of the Columba stellar moving group – an association of stars that formed as a group an estimated 30 million years ago and are still orbiting through our galaxy on somewhat similar orbits. Another member of this group is the high mass star HR 8799, whose planets are some of the first extrasolar planets which astronomers have been able to image directly.  Although HR 8799 is not nearly as massive as κ  And, this planetary system hosts several gas giants whose masses and infrared colour are similar to those of κ And b. 

How was the image of the planet recorded?
In a single infrared snapshot, the tiny point of light that is κ And b is completely lost amid the glare of the parent star. The astronomers were able to filter out the planet’s light only by using a clever combination of temporal sequences of individual images, i.e. by Angular Differential Imaging (ADI). The method exploits the fact that the telescope changes its orientation relative to the sky sector selected during longer observations; this allows stray light and the light from the planet to be separated.

κ And b is one of only 30 planets where astronomers have taken direct images. In this observation, the super-Jupiter was detected in independent observations in January and July 2012 at four different wavelengths.  A comparison of the relative positions at these two times showed that κ And b and its companion change their position  in the night sky in precisely the same way i.e. they exhibit a common proper motion in the sky - around two hundred thousandths of a degree per year. This is convincing evidence that the objects are indeed gravitationally bound. 

Young planets are comparatively hot as a consequence of their formation. κ And b has a surface temperature of around 1400 degrees Celsius. The fact that the planet thus shines comparatively brightly in the infrared makes young star systems suitable candidates for the search for planets which can be imaged directly.

Which telescopes and instruments were used for the observations?
The observations were made with the SUBARU Telescope, the flagship of the National Astronomical Observatory of Japan (NAOJ). Its mirror is 8.2 metres in diameter. »SUBARU« is Japanese for the Pleiades, perhaps the best-known open star cluster.

The IRCS infrared camera and HiCIAO were used. The latter is specially designed for the observation of faint objects near stars, such as exoplanets and protoplanetary disks. HiCIAO uses state-of-the-art adaptive optics, a technique which compensates for the interfering »twinkling« of stars. Moreover, HiCIAO uses a mechanism which physically blocks out large parts of a bright star’s light and thus allows neighbouring, fainter objects to be investigated.


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