Dalim (Alpha Fornacis)

Dalim, Alpha Fornacis A (α For A), is the primary component in a triple star system located 45.66 light-years from the Sun. With a combined apparent magnitude of 3.85, it is the brightest star in the southern constellation of Fornax (the Furnace). It is a subgiant star with a radius twice that of the Sun.

Star system

Alpha Fornacis is a triple star system composed of the subgiant star Alpha Fornacis A and the binary system Alpha Fornacis B. The two visual components take 269 years to complete an orbit around a common centre of mass. They are separated by 4 arcseconds.

The primary component, Alpha Fornacis A, is a white subgiant star of the spectral type F8IV. It has a mass of 1.33 solar masses and a radius 2.04 times that of the Sun. With a surface temperature of 6,240 K, it is 4.87 times more luminous than the Sun. The evolved star spins at 3.9 km/s. It has an estimated age of 2.9 billion years.

Alpha Fornacis B consists of a blue straggler, Alpha Fornacis Ba, and a white dwarf, Alpha Fornacis Bb.

Blue stragglers are stars that are bluer and more luminous than expected for their age and evolutionary phase. These stars are typically found in star clusters. They have higher effective temperatures than other cluster members, even though they formed at around the same time. Blue stragglers are also more massive than their neighbours. Astronomers believe that these stars are the product of interactions between two or more stars in tightly packed clusters.

Alpha Fornacis Ba is a field blue straggler with a mass 78% that of the Sun. The star’s high X-ray luminosity has led scientists to conclude that it had interacted with another star only a few tens of millions of years ago, resulting in mass transfer from the companion to Alpha Fornacis Ba.

The companion, Alpha Fornacis Bb, was discovered in 2016. It is believed to be a white dwarf, based on the barium enrichment detected in the component Alpha Fornacis Ba.

Dalim (Alpha Fornacis), image: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Facts

Dalim emits an infrared excess, possibly indicating the presence of a circumstellar debris disk.

Dalim is a high proper motion star. Around 351,200 years ago, it made a close approach to Nu Horologii, an A-type main sequence star in the constellation Horologium (the Pendulum Clock). During the close encounter, the two stars came within only 0.081 parsecs (0.264 light-years) of each other, which may have disrupted their hypothetical Oort clouds and directly perturbed both stars’ circumstellar disks.

The binarity of Alpha Fornacis was discovered in 1835.

In Chinese astronomy, Dalim was part of an asterism called 天苑 (Tiān Yuàn), meaning Celestial Meadows, formed by the relatively faint stars in the constellations Fornax, Eridanus, and Cetus. The asterism was part of the Hairy Head mansion, which represented the body of the White Tiger.

Shining at magnitude 3.85, Dalim is one of the fainter lucidae (brightest stars in constellations) in the sky. However, it is not the faintest. The lucidae of 14 constellations are even fainter: Alpha Antliae (mag. 4.25), Alpha Caeli (mag. 4.456), Beta Camelopardalis (mag. 4.02), Alpha Chamaeleontis (mag. 4.06), Beta Comae Berenices (mag. 4.26), Alpha and Beta Coronae Australis (mag. 4.10), Kitalpha (Alpha Equulei, mag. 3.9), Alpha Horologii (mag. 3853), Alpha Mensae (mag. 5.09), Gamma Microscopii (mag. 4.68), Gamma² Normae (mag. 4.02), Alpha Sculptoris (mag. 4.30), Alpha Sextantis (mag. 4.49), and Anser (Alpha Vulpeculae, mag. 4.40).

Name

The name Dalim (pronunciation: /ˈdeɪlɪm/) is derived from the Arabic al-ẓalīm, meaning “ostrich.” In Arabic astronomy, the name applied to Achernar (Alpha Eridani) and Fomalhaut (Alpha Piscis Austrini). It was later transferred to Acamar (Theta Eridani) and then to Alpha Fornacis.

The International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) approved the name on September 5, 2017. The name formally applies only to the primary component in the system, Alpha Fornacis A.

Location

Dalim is an inconspicuous star in the faint constellation of Fornax. It can be found using the stars of the neighbouring constellation Eridanus. A line drawn from Epsilon Eridani (Ran, mag. 3.736) through Tau² Eridani (Angetenar, mag. 4.78) points in its direction. Epsilon Eridani, one of the nearest visible stars to Earth, can be found using the brighter stars of Lepus and Orion.

At declination – 28° 59’, Dalim never rises for observers north of the latitude 60° N.

Dalim location, image: Stellarium

Constellation

Dalim is located in the constellation Fornax. Representing the Chemical Furnace, the constellation was created by the French astronomer Nicolas-Louis de Lacaille in the 18th century. It stretches across 398 square degrees of the southern sky and is the 41st largest of the 88 constellations.

Fornax is one of the fainter southern constellations. Its three brightest stars – Dalim (Alpha Fornacis), Beta and Nu Fornacis – form a flattish triangle that can be spotted in the bend of the celestial River (Eridanus) from areas without too much light pollution. Dalim is the only star in Fornax brighter than magnitude 4.0.

Other notable stars in Fornax include the yellow giants Beta and Rho Fornacis, the variable hot blue giant Nu Fornacis, the blue giant Delta Fornacis, the binary systems Omega and Pi Fornacis, the multiple star system Kappa Fornacis, the Mira variable R Fornacis, and the binary system HD 20781, which hosts a total of five known exoplanets. Fornax also hosts the red dwarf UDF 2457, one of the most distant stars known in the Milky Way galaxy, located around 59,000 light-years away.

Fornax constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

Deep sky objects in Fornax include the planetary nebula NGC 1360 (Robin’s Egg Nebula), the barred spiral galaxy NGC 1097, the Fornax Dwarf Galaxy, and the globular cluster NGC 1049, which lies in the Fornax Dwarf. The constellation hosts most of the Fornax Cluster of galaxies, whose brighter members include NGC 1316 (Fornax A), NGC 1380, NGC 1399, and the Great Barred Spiral Galaxy (NGC 1365).

The best time of the year to see the stars and deep sky objects in Fornax is during the month of December, when the constellation appears higher above the horizon in the early evening. The entire constellation is visible from locations south of the latitude 50° N.

The 10 brightest stars in Fornax are Dalim (Alpha For, mag. 3.85), Beta Fornacis (mag. 4.46), Nu Fornacis (mag. 4.69), Omega Fornacis (mag. 4.95), Delta Fornacis (mag. 5.0), Phi Fornacis (mag. 5.13), Kappa Fornacis (mag. 5.19), Mu Fornacis (mag. 5.27), Pi Fornacis (mag. 5.360), and Gamma² Fornacis (mag. 5.389).

Dalim – Alpha Fornacis