A significant proportion of masive white dwarfs are the result of merging of two stars.
"A-type" stars are the major type of stars that would eventually form "white dwarf" stars. Also, about 45% of A-type stars around our Sun are found in star systems comprised of two or more stars. It is safe to assume that different types of stars have been forming in our galaxy in somewhat similar proportions. Therefore, it is expected that about 45% white dwarf stars that we observe should be a part of binary or multiple star systems. But the data from stars about 80 light years around our Sun show that only 25% of the white dwarf stars are a part of binary or multiple star systems. A recent study suggests that this might be because many of the white dwarfs might actually be a product of merging of two stars.
A white dwarf star
[Image source : https://www.nasa.gov/wp-content/uploads/2023/03/139459main_image_feature_468_ys_full.jpg]
A white dwarf , that is a product of merging, is expected to be massive than the average white dwarf stars which are about the mass of our Sun. The researchers observed the data from 25 of the ultra-massive white dwarf stars situated within about 80 light years from our solar system. These were all about 1.3 times or more massive than the Sun.
The properties that differentiate the product of a merger event from a normal white dwarf are its rotational speed, the magnetic strength and kinematics properties.
Rotational speed
A normal white dwarf star with an average mass of about 0.6 solar mass rotates once in about 24 hours. However the white dwarf merger products are expected to rotate in about 10 to 20 minutes. Five of the stars observed by the scientists had rotational speed from 1 minute to 1 hour.
Magnetism
White dwarfs acquire strong magnetic properties as a result of crystallization of carbon and oxygen in their cores. The crystallization causes separation of carbon and oxygen from other gases present which form a convective current. But this phenomenon cannot produce magnetic fields of strength that is observed in some of the white dwarfs formed after merging of two white dwarfs.
Kinematics
White dwarf which is a product of a merging appears to be hotter and younger than what it would appear if it had evolved as a single star.
The observations on the 25 white dwarfs show that several of these stars have one or more of the above abnormal properties. The scientists suggest that 14 of the 25 stars observed were a result of a merger event between two white dwarf stars. Although, the number they got was higher than the previously estimated number of about 10-30%, the authors hope to have more robust estimates of the merger events with new larger data from Gaia and Vera Rubin Observatory's Legacy Survey of Space and Time (LSST).
Original article: https://academic.oup.com/mnras/article/518/2/2341/6795949