The
positions and trajectories of 20 hypervelocity stars as reconstructed from data
acquired by the Gaia satellite, overlaid on top of an artistic view of the
Milky Way. Credit: ESA (artist's impression and composition); Marchetti et al
2018 (star positions and trajectories); NASA/ESA/Hubble (background galaxies),
CC BY-SA 3.0 igo.
Hypervelocity
stars have, since the 1920s, been an important tool that allows astronomers to
study the properties of the Milky Way galaxy, such as its gravitational
potential and the distribution of matter. Now astronomers from China have made
a large-volume search for hypervelocity stars by utilizing a special class of
stars known for their distinct, regular, predictable pulsation behavior that
makes them useful as distance indicators.
Their research is published in The
Astrophysical Journal.
The escape velocity of any planet, star
or galaxy is the velocity required for a mass, leaving the object's surface, to
coast completely and exactly out of the planet's gravitational well, going to
infinity. Earth's escape velocity is 11.2 kilometers per second (km/s).
Any mass that leaves the surface having
that immediate initial speed will, without further energy, leave Earth's
gravitational grasp. Examples are rocks ejected from Earth by a colliding
incoming asteroid (as happened with rocks exchanged between Earth and Mars) or
the possible escape of a steel lid covering a blast hole from a 1957
underground nuclear explosion in Nevada (unless the lid vaporized as it
ascended towards space at an estimated six times Earth's escape velocity).
The escape velocity from the sun is 618
km/s (but only 42 km/s from Earth's position), and about 550 km/s from the
sun's position in the Milky Way. Hypervelocity stars (HVSs) have tangential
speeds of 1,000 km/s or more, making them gravitationally unbound from the
Milky Way.
A prominent way HVSs come about is from
a gravitational slingshot interaction with the supermassive black hole,
Sagittarius A*, at the Milky Way's center.
The Hills mechanism, first proposed by
astronomer Jack Hills in 1988, has one star of a binary pair captured by a
black hole while the other is flung away from the black hole at a high speed.
Such a jettisoned star was first
observed in 2019, traveling away from the core of the Milky Way at 1,755
km/s—0.6% the speed of light—which is greater than the escape velocity of the
galactic center. Such stars also provide direct evidence for the supermassive
black holes in galactic centers and their properties.
Moreover, by tracing back the trajectories of the runaway stars, scientists can map the gravitational potential of
the Milky Way—how masses interact within the galaxy—including the distribution
of dark matter in the halo, the huge spherical volume that surrounds a galaxy's
disk.
With these motivations, three
astronomers from Beijing scientific institutions, with lead author Haozhu Fu of
Peking University, looked for HVSs by starting with RR Lyrae stars (RRLs).
These are old, giant stars that pulse with periods of 0.2 to one day, found in
the thick disk and halo of the Milky Way galaxy and often in globular clusters.
(The Milky Way contains more than 150 globular clusters, with about a third of
them arranged in a nearly spherical halo around the Milky Way's center.)
The intrinsic luminosity of these
RRLs—their total energy output—is relatively well-determined from a
relationship that connects their pulsing period, their absolute magnitude and
their metallicity (the abundance of elements heavier than hydrogen and helium,
which to astronomers are "metals"). Knowing their absolute energy
output and their energy received at Earth enables their distance to be
calculated from the inverse-square distance relationship.
One
published star catalog contained 8,172 RRLs from the Sloan Digital Sky Survey and an extended catalog held 135,873 RRLs with
metallicity and distance estimated from Gaia photometry, which are measurements
of the brightness of stars as observed by the Gaia satellite launched by the
European Space Agency in 2013.
Looking for hypervelocity RRLs, they
eliminated almost all that did not have properties needed for their search,
especially spectroscopic measurements that gave radial velocities (away from
the galactic center) with sufficiently low uncertainties. This reduced the
relevant dataset drastically, to 165 hypervelocity RRLs.
The group then looked at each star's light curve, selecting Doppler shifts for 87 such stars that were
the most reliable hypervelocity stars. (Of these, seven had a tangential
velocity above 800 km/s.) These divided into two groups: one that was
concentrated towards the Milky Way's galactic center, and the other localized
around the Magellanic Clouds, Large and Small, two irregular dwarf galaxies
located near the Milky Way.
Their locations and concentrations
suggested they had reached hypervelocity status through the Hills (or similar)
mechanism. Many had movements that exceeded the Milky Way's escape velocity,
probably ejected from their host systems.
The team suspects that future Gaia satellite observations and spectroscopic analysis will shed light on the origins of these ejections. Identifying runaway stars in this way allows the properties of the Milky Way halo to be studied further, hopefully shedding light on its dark matter, still one of the deepest mysteries in all of modern physics.
Source: Finding runaway stars to help map dark matter in the Milky Way
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