Star Facts: Al Tarf

Beta Cancri
Image Credit: Torsten Bronger

Despite its “beta” designation the giant star Al Tarf (Beta Cancri) is the most luminous star in the zodiacal constellation of Cancer, although with an apparent magnitude of just +3.5 it is still a challenging naked-eye object to see in the night sky. The star is actually a binary system consisting of an orange K-type giant about 61 times bigger than the Sun, and a much fainter 14th magnitude companion that orbit each other once every 76,000 years. The stars’ traditional name derives from the Arabic word for “edge”, or “end” referring to its position along one the crab’s legs.

Quick Facts

• Constellation: Cancer
• Coordinates: RA 08h 16m 30.9206s|Dec. +09° 11′ 07.961″
• Distance: 290 light years
• Star Type: K4III Ba1
• Mass: Undetermined
• Radius: 61 sol
• Apparent Magnitude: +3.50 to +3.58
• Luminosity: 871 sol
• Surface Temperature: 3,990K
• Rotational Velocity: 6.9 km/sec
• Age: 2 billion years
• Other Designations: ß Cancri, Al Tarf, Tarf, 17 Cancri, HR 3249, HD 69267, BD+09°1917, FK5 312, HIP 40526, SAO 116569, GC 11254, ADS 6704, CCDM 08165+0911

Visibility

The constellation of Cancer is visible from latitudes of +90 to -60 degrees, and can be found occupying an area between Leo to the east, and Gemini to the west. In the northern hemisphere, this constellations is a regular feature of the autumn to spring night sky, although it is best seen at around 9 PM (Local Time) during the month of March. Look for Al Tarf at the bottom right “foot” of the inverted “Y” that marks out the “back” of the Crab.

Physical Properties

Binary System

Beta Cancri is a binary star whose main component is an orange giant situated 290 light years distant that has 61 times the Sun’s size and 660 times its brightness. It is also known to have a 14th magnitude red dwarf companion that has an angular separation from Al Tarf of 29 seconds of arc, which translates into a real distance of about 2,600 astronomical units. According to a paper published in the journal Astronomy & Astrophysics on May 12, 2014, there is a planet orbiting Al Tarf every 605 days, based upon radial velocity data, that has an estimated 7.8 times the mass of Jupiter.

Barium Star

The most notable aspect of Al Tarf is that it is a barium star, a class of cool giant stars that show pronounced abundances of the element barium. Stars of this type typically fall into the G to K-spectral classes, which invariably show an overabundance of s-process elements through the presence of Ba II, which is singly-ionized barium at a wavelength of 455.4 nanometers. Additionally, the spectra of barium stars also exhibit enhanced carbon lines, and more specifically, in the bands of CH, CN, and C2 molecules.

Barium stars are thought to be the result of a process of mass transfer between the now-observed star, in this case Al Tarf, when it was still on the main sequence, and a companion that was a carbon star on the asymptotic giant branch (of the HR diagram). In essence, it is believed that elements such as carbon (that was produced in the s-process in the donor star), were dredged up in convection currents to reach the donor stars’ surface, and were transferred to the recipient star when the donor star blew off its upper layers as it evolved into a white dwarf.

NOTE: The term “s-process” refers to a specific series of chemical reactions that occur in stars that mainly fall onto the asymptotic giant branch. This process accounts for about 50% of all atomic nuclei that are more massive than iron. However, we are now observing these systems long after the mass transfer event, at a time when the white dwarf has long been a white dwarf, and long enough after the event for the recipient star to have evolved into its red-giant phase. Nonetheless, studies have shown that barium stars will at times be cooler and bigger than the normally accepted limits that generally apply to G or K-type stars. During these times, a barium star will appear as an M-type star, although its s-process abundances will clearly show up in its spectrum as a spectral peculiarity.

Put in another way, its spectrum will identify such a star as a peculiar M-type star (even though its temperature may fall within the normal range for M-type stars), since the spectrum of such a star will exhibit the presence of typical s-process elements such as zirconium, and zirconium oxide, which do not normally appear in “standard” M-type stars.

It should be noted that the formation of barium stars had posed a conundrum to investigators for a long time, since standard stellar evolutionary models could not account for the presence of s-process elements in normal G and K-type stars. It was only when it was discovered that all known barium stars occur in binary systems that it became possible to attribute the presence of s-process elements to the white dwarf component, since white dwarfs are known to produce these elements. The mass-transfer theory predicts that main-sequence stars showing s-process related peculiarities should exist, and one such star, designated HR 107, is in fact known. From a historical perspective, it is believed that Population II, and CH stars (a particular class of carbon stars), are the much older and metal-poor analogues of the “modern” barium stars that are observed today.

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