Astronomers using NASA’s Hubble Space Telescope have captured snapshots of fledging white dwarfs beginning their slow-paced, 40-million-year migration from the crowded center of the ancient star cluster 47 Tucanae (also known as NGC 104) to its less populated suburbs.
The heart of the giant globular star cluster 47 Tucanae in the Hubble image at left reveals the glow of 200,000 stars. The green box outlines the cluster’s crowded core, where Hubble spied a parade of young white dwarfs starting their slow-paced 40-million-year journey to the less populated suburbs. The stellar relics are too faint to be seen clearly in visible light, as shown in the Hubble image at top right. But in ultraviolet light the stars glow brightly because they are extremely hot, as shown in the image at bottom right. The green circles in the image outline the brightest of the young white dwarfs spied by Hubble. Image credit: NASA / ESA / H. Richer & J. Heyl, University of British Columbia / J. Mack, STScI / G. Piotto, University of Padova.
White dwarfs are the burned-out relics of ancient stars that rapidly shut down their nuclear furnaces, cooling down and losing mass at the end of their active lives.
As these stellar carcasses age and shed mass, they are expelled from the densely packed center of the globular cluster and migrate to wider orbits. Whilst astronomers knew about this process, they had never seen it in action, until now.
A team of astronomers led by Dr Jeremy Heyl of the University of British Columbia in Canada used Hubble to watch the exodus of nearly 3,000 white dwarfs in 47 Tucanae, a globular cluster located in the southern constellation Tucana approximately 16,700 light-years away.
“We’ve seen the final picture before: white dwarfs that have migrated and settled into more distant orbits outside the core, determined by their mass,” said Dr Heyl, who is the first author of a paper submitted for publication in the Astrophysical Journal (arXiv.org preprint).
“But in this study, which comprises about a quarter of all the young white dwarfs in the cluster, we’re actually catching the stars in the process of moving outward and distributing themselves appropriately according to mass.”
Using the capabilities of Hubble’s Wide Field Camera 3, the astronomers traced populations of white dwarfs with a range of ages.
One group of 6-million-year-old stars has just begun its journey from the dense cluster center.
Another population is around 100 million years old and has already arrived at its new position, roughly 1.5 light-years from its starting point, and far from the cluster center.
“Before becoming white dwarfs, the migrating stars were among the most massive in the cluster, roughly as massive as the Sun,” said co-author Dr Elisa Antolini of the Università degli Studi di Perugia, Italy.
“We knew that as they lost mass we would see a migration to the outskirts; that wasn’t a surprise. But, what did surprise us was that the youngest white dwarfs were only just embarking on their journey. This could be evidence that the stars shed much of their mass at a later stage in their lives than we once thought, which is an exciting find.”
About 100 million years before stars evolve into white dwarfs, they swell up and become red giant stars. Many astronomers thought that stars lost most of their mass during this phase. However, if this were the case, the stars would already have been expelled from the center of the cluster at the red giant stage.
“Our observations with Hubble found white dwarfs that are just beginning their migration to wider orbits,” said co-author Dr Harvey Richer of the University of British Columbia.
“This reveals that the migration of the stars from the center – and the loss in their mass that has caused it – begins later in the star’s life than once thought. These white dwarfs are losing a large amount of mass just before they become white dwarfs and not during the earlier red giant phase.”
The results imply that the stars actually lose 40 to 50 percent of their bulk just 10 million years before completely burning out as white dwarfs.
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Jeremy Heyl et al. 2015. When do stars in 47 Tucanae lose their mass? ApJ, submitted for publication; arXiv: 1502.07306
