Young Star Probably Ejected From Triple System
Astronomers analyzing nearly 20 years of data from the
National Science Foundation’s
Very Large Array radio
telescope have discovered that a small star in a multiple-star
system in the constellation Taurus probably has been ejected
from the system after a close encounter with one of the
system’s more-massive components, presumed to be a
compact double star. This is the first time any
such event has been observed.
“Our analysis shows a drastic change in the orbit of
this young star after it made a close approach to
another object in the system,” said Luis Rodriguez of the
Institute of Astronomy of the National Autonomous
University of Mexico (UNAM).
“The young star was accelerated to a large velocity by
the close approach, and certainly now is in a very
different, more remote orbit, and may even completely
escape its companions,” said Laurent Loinard, leader
of the research team that also included Monica Rodriguez
in addition to Luis Rodriguez. The UNAM astronomers
presented their findings at the American Astronomical
Society’s meeting in Seattle, WA.
The discovery of this chaotic event will be important
for advancing our understanding of classical dynamic
astronomy and of how stars evolve, including possibly
providing an explanation for the production of the
mysterious “brown dwarfs,” the astronomers said.
The scientists analyzed VLA observations of T Tauri,
a multiple system of young stars some 450 light-years from
Earth. The observations were made from 1983 to 2001.
The T Tauri system includes a “Northern” star, the famous
star that gives its name to the class of young visible
stars, and a “Southern” system of stars, all orbiting each
other. The VLA data were used to track the orbit of
the smaller Southern star around the larger Southern
object, presumed to be a pair of stars orbiting each
other closely.
The astronomers’ plot of the smaller star’s orbit shows
that it followed an apparently elliptical orbit around
its twin companions, moving at about 6 miles per second.
Then, between 1995 and 1998, it came within about 200
million miles (about two times the distance between the
Sun and the Earth) of its companions. Following that
encounter, it changed its path, moving away from its
companion at about 12 miles per second, double its
previous speed.
“We clearly see that this star’s orbit has changed
dramatically after the encounter with its larger companions,”
said Luis Rodriguez. “By watching over the next five years
or so, we should be able to tell if it will escape completely,”
he added.
“We are very lucky to have been able to observe this
event,” said Loinard. Though studies with computer
simulations long have shown that such close approaches and
stellar ejections are likely, the time scales for these
events in the real Universe are long — thousands
of years. The chance to study an actual ejection of a
star from a multiple system can provide a critical test
for the dynamical theories.
If a young star is ejected from the system in which it
was born, it would be cut off from the supply of gas and
dust it needs to gain more mass, and thus its development
would be abruptly halted. This process, the astronomers
explain, could provide an explanation for the very-low-mass
“failed stars” called brown dwarfs.
“A brown dwarf could have had its growth stopped by being
ejected from its parent system,” Loinard said.
The VLA observations were made at radio frequencies of
8 and 15 GHz.
T Tauri, the “Northern” star in this system,
is a famous variable star, discovered in October
of 1852 by J.R. Hind, a London astronomer using a 7-inch
diameter telescope. At its brightest, it is some 40 times
brighter than when at its faintest. It has been studied
extensively as a nearby example of a young stellar system.
While readily accessible with a small telescope, it is
not visible to the naked eye. The observed orbital changes
took place in the southern components of the system, displaced
from the visible star by about one hundred times the distance
between the Sun and the Earth.
The
National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated
under cooperative agreement by
Associated Universities, Inc.
