The Hyades star cluster, situated a mere 153 light-years away in the Taurus constellation, is easily observable with the naked eye due to its close proximity. This proximity has made it a favored target for professional astronomers, as it provides a unique opportunity to study a cluster with hundreds of stars of similar age, metallic composition, and motion through space.
However, there's a curious absence within the Hyades cluster: white dwarfs. Surprisingly, the cluster hosts only eight white dwarfs in its core. This scarcity of white dwarfs has puzzled scientists and created a notable anomaly within the Hyades. But recent research titled “An Extremely Massive White Dwarf Escaped From the Hyades Star Cluster,” led by David Miller from the University of British Columbia, sheds new light on this mystery.
Open clusters like the Hyades are loosely held together, and over time, they lose stars due to interactions with gas clouds, other clusters, and interactions among cluster members. Miller and his team used the absence of white dwarfs in the Hyades as a key to reconstructing the cluster's history.
Fortuitously, the European Space Agency's Gaia spacecraft has been diligently tracking over a billion stars in the Milky Way, providing a treasure trove of data for Miller and his colleagues to investigate. Their investigation identified three ultra-massive white dwarfs whose motion suggests they might have originated from the Hyades cluster. While two of them were ruled out due to their mass, the third one appeared to be a likely escapee from the cluster.
White dwarfs, which are about as massive as the Sun but compacted to Earth's size, represent the final stage in the life cycle of approximately 97% of Milky Way stars. These stellar remnants are governed by the Chandrasekhar Limit, capping their mass at around 1.44 solar masses. If a white dwarf accrues more mass, usually by siphoning it from a binary companion, it can explode as a Type 1a supernova, dispersing its entire mass into space.
The Hyades' escaped white dwarf is classified as an ultra-massive white dwarf, possessing a mass of 1.317 solar masses, which is notably higher than the typical white dwarf mass of approximately 0.6 solar masses. These high-mass white dwarfs usually originate from binary star systems where one white dwarf accumulates material from its companion, increasing its mass.
The unique aspect of the Hyades ultra-massive white dwarf is its mass and age consistency with a single progenitor. This finding makes it a remarkable and significant addition to the study of white dwarfs, as it's potentially the most massive white dwarf resulting from a single progenitor star and the most massive single progenitor star closely associated with an open cluster.
In light of a single-stellar evolution formation scenario, the researchers have calculated a 97.8% probability that the identified candidate is indeed a genuine escapee from the Hyades cluster. This star carries significance because typically, the formation of such a massive white dwarf involves a binary system, where one star accretes material from the other.
This discovery serves as a crucial observational milestone, illustrating that white dwarfs originating from single progenitor stars can reach masses that closely approach the Chandrasekhar limit, challenging previous assumptions.
Although the Hyades cluster might appear unremarkable in many aspects, its close proximity to Earth enables astronomers to detect older, cooler white dwarfs and investigate their origins with remarkable precision. This in-depth study of the cluster has broader implications, potentially extending to our understanding of open star clusters in general.
The unassuming nature of the Hyades, coupled with the advantages of its proximity, suggests that open star clusters might be more prolific in producing ultramassive white dwarfs, including those approaching the Chandrasekhar limit, than previously anticipated, as the authors conclude.
Locating the Hyades in the night sky is relatively straightforward. Orion's Belt conveniently points directly to it. For those interested in observing, the best time for viewing is during November, December, and January, particularly from Northern Latitudes. With the naked eye, it resembles a distinctive “V” shape formed by approximately 20 stars, and the use of binoculars can reveal even more, totaling around 100 stars.
Source: Universe Today