Holmberg IX V1


Holmberg IX V1, also known as EQ J095737+690211 is a W Ursae Majoris type eclipsing contact binary star system in the constellation of Ursa Major. The binary system is located in the dwarf galaxy Holmberg IX, roughly 13 million light years away. The binary system has a maximum visual magnitude of 20.7, and is one of most extreme and rarest systems discovered.

Characteristics

Holmberg IX V1 was first discovered by the Large Binocular Telescope on Mt. Graham, Arizona, between January and October 2007. In the same variability survey, Prieto and his team also found a similar contact binary system designated SMC R47 in the Small Magellanic Cloud.

Physical Properties

The binary system is made up of two yellow supergiant stars referred to as Holmberg IX V1 A and Holmberg IX V1 B. These stars are almost identical in their physical properties such as their luminosity, mass, and age. The binary system is important because it helps us figure out the progenitors of unusual supernovae such as SN 2004et and SN 2006ov.
The two yellow supergiant stars in this system are both estimated to be approximately 15 to 20 times the mass of the Sun, and are predicted to be 10 to 15 million years old. The two stars in this binary system are classified as G-type yellow supergiants, and they both were originally O-type main-sequence stars with initial masses of 30 times more massive than the Sun.
The metallicity of both stars in the binary system is Z = 0.004, equivalent to roughly 3 times less metal-rich than the Sun.
Due to the two stars in this binary system equal in properties, the two stars likely appear a peanut shape, which is a common way of describing contact binaries like Holmberg IX V1. The binary system is similar to other supergiant contact binary systems such as SMC R47 and BM Cassiopeiae.
The primary star, called Holmberg IX V1 A has a diameter of 337 times larger than the Sun. This was estimated using the Stefan-Boltzmann law, based on a temperature of 4,800 K, and an absolute luminosity of 54,450 times more luminous than the Sun.
The secondary star, termed as Holmberg IX V1 B has a diameter of 306 time larger than the Sun. This was found using the Stefan-Boltzmann law, based on a temperature of 5,040 K, and an absolute luminosity of 54,450 times more luminous than the Sun.

Orbit

The two stars orbit each other every 270.7 days, on an orbit with zero eccentricity. The binary stars also eclipse each other, typical for contact binaries, and classifying it as a W Ursae Majoris variable.

Cepheid Variable Hypothesis

The data from the Large Binocular Telescope for Holmberg IX V1 could also fit a massive long-period Cepheid variable with a variable period of 135 days with a absolute luminosity of 49,659 times more luminous than the Sun. However, the contact binary theory is more likely.