White Dwarfs Migrating from Globular Cluster 47 Tucanae's Core
Using NASA's Hubble Space Telescope, astronomers have captured for the
first time snapshots of fledgling white dwarf stars beginning their
slow-paced, 40-million-year migration from the crowded center of an
ancient star cluster to the less populated suburbs.
White dwarfs are the burned-out relics of stars that rapidly lose
mass, cool down, and shut off their nuclear furnaces. As these glowing
carcasses age and shed weight, their orbits begin to expand outward
from the star cluster's packed downtown. This migration is caused by a
gravitational tussle among stars inside the cluster. Globular star
clusters sort out stars according to their mass, governed by a
gravitational billiard-ball game where lower mass stars rob momentum
from more massive stars. The result is that heavier stars slow down and
sink to the cluster’s core, while lighter stars pick up speed and move
across the cluster to the edge. This process is known as "mass
segregation." Until these Hubble observations, astronomers had never
definitively seen the dynamical conveyor belt in action.
Astronomers used Hubble to watch the white-dwarf exodus in the
globular star cluster 47 Tucanae, a dense swarm of hundreds of
thousands of stars in our Milky Way galaxy. The cluster resides 16,700
light-years away in the southern constellation Tucana.
"We've seen the final picture before: white dwarfs that have already
sorted themselves out and are orbiting in a location outside the core
that is appropriate for their mass," explained Jeremy Heyl of the
University of British Columbia (UBC), Vancouver, Canada, first author
on the science paper. The team's results appeared in the May 1 issue of
The Astrophysical Journal.
"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 segregating themselves according to mass,"
Heyl said. "The entire process doesn't take very long, only a few
hundreds of millions of years, out of the 10-billion-year age of the
cluster, for the white dwarfs to reach their new home in the outer
suburbs."
"This result hasn't been seen before, and it challenges some ideas
about some of the details of how and when a star loses its mass near
the end of its life," added team member Harvey Richer of UBC.
Using the ultraviolet-light capabilities of Hubble's sharp-eyed Wide
Field Camera 3, the astronomers examined 3,000 white dwarfs, tracing
two populations with diverse ages and orbits. One grouping was 6
million years old and had just begun their journey. Another was around
100 million years old and had already arrived at its new homestead far
away from the center, roughly 1.5 light-years, or nearly 9 trillion
miles, away.
Only Hubble can detect these stars because ultraviolet light is
blocked by Earth's atmosphere and therefore doesn't reach ground-based
telescopes. The astronomers estimated the white dwarfs' ages by
analyzing their colors, which gives them the stars' temperatures. The
hottest dwarfs shine fiercely in ultraviolet light.
The dwarfs were tossed out of the rough-and-tumble cluster center due
to gravitational interactions with heftier stars orbiting the region.
Stars in globular clusters sort themselves out by weight, with the
heavier stars sinking to the middle. Before flaming out as white
dwarfs, the migrating stars were among the most massive in the cluster,
weighing roughly as much as our Sun. The more massive stars burned out
long ago.
The migrating white dwarfs, however, are not in a hurry to leave.
Their orbits expand outward at about 30 miles an hour, roughly the
average speed of a car traveling in the city. The dead stars will
continue this pace for about 40 million years, until they reach a
location that is more appropriate for their mass.
Although the astronomers were not surprised to see the migration,
they were puzzled to find that the youngest white dwarfs were just
embarking on their journey. This discovery may be evidence that the
stars shed much of their mass at a later stage in their lives than once
thought.
About 100 million years before stars evolve into white dwarfs, they
swell up and become red giant stars. Many astronomers thought that
stars lose most of their mass during this phase by blowing it off into
space. But the Hubble observations reveal that the stars actually dump
40 percent to 50 percent of their bulk just 10 million years before
completely burning out as white dwarfs.
"This late start is evidence that these white dwarfs are losing a
large amount of mass just before they become white dwarfs and not
during the earlier red giant phase, as most astronomers had thought,"
said Richer.
"That's why we are seeing stars still in the process of
moving slowly away from the center of the cluster. It's only after they
lose their mass that they get gravitationally pushed out of the core.
If the stars had shed most of their weight earlier in their lives, we
wouldn't see such a dramatic effect between the youngest white dwarfs
and the older ones that are 100 million years old."
Although the white dwarfs have exhausted the hydrogen fuel that makes
them shine as stars, these stellar relics are among the brightest
stars in this primordial cluster because their brilliant hot cores have
been exposed, which are luminous largely in ultraviolet light. "When a
white dwarf forms, they've got all this stored-up heat in their cores,
and the reason we can see a white dwarf is because over time they
radiate their stored thermal energy slowly into space," Richer
explained. "They're getting cooler and less luminous as time goes on
because they have no nuclear sources of energy."
After making it through the gauntlet of gravitational interactions
within the crowded 1.5-light-year-wide core, the traveling white dwarfs
encounter few interactions as they migrate outward, because the
density of stars decreases. "A lot of action happens when they're 30
million to 40 million years old, and continues up to around 100 million
years, and then as they get older the white dwarfs still evolve but
less dramatically," Heyl said.
The 47 Tucanae cluster is an ideal place to study the mass
segregation of white dwarfs because it is nearby and has a significant
number of centrally concentrated stars that can be resolved by Hubble's
crisp vision.
Contact
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Felicia Chou
NASA Headquarters, Washington, DC
202-358-0257
felicia.chou@nasa.gov
Jeremy Heyl
University of British Columbia, Vancouver, BC, Canada
604-822-0995
heyl@phas.ubc.ca
Source: HubbleSite
