WASHINGTON — Two of NASA’s great observatories, the Spitzer and
Hubble space telescopes, have teamed up to uncover a mysterious
infant star that behaves like a strobe light.
Every 25.34 days, the object, designated LRLL 54361, unleashes a burst
of light. Although a similar phenomenon has been observed in two
other young stellar objects, this is the most powerful such beacon
seen to date.
The heart of the fireworks is hidden behind a dense disk and envelope
of dust. Astronomers propose the light flashes are caused by periodic
interactions between two newly formed stars that are binary, or
gravitationally bound to each other. LRLL 54361 offers insights into
the early stages of star formation when lots of gas and dust is being
rapidly accreted, or pulled together, to form a new binary star.
Astronomers theorize the flashes are caused by material suddenly being
dumped onto the growing stars, known as protostars. A blast of
radiation is unleashed each time the stars get close to each other in
their orbits. This phenomenon, called pulsed accretion, has been seen
in later stages of star birth, but never in such a young system or
with such intensity and regularity.
“This protostar has such large brightness variations with a precise
period that it is very difficult to explain,” said James Muzerolle of
the Space Telescope Science Institute in Baltimore, Md. His paper
recently was published in the science journal Nature.
Discovered by NASA’s Spitzer Space Telescope, LRLL 54361 is a variable
object inside the star-forming region IC 348, located 950 light-years
from Earth. Data from Spitzer revealed the presence of protostars.
Based on statistical analysis, the two stars are estimated to be no
more than a few hundred thousand years old.
The Spitzer infrared data, collected repeatedly during a period of
seven years, showed unusual outbursts in the brightness of the
suspected binary protostar. Surprisingly, the outbursts recurred
every 25.34 days, which is a very rare phenomenon.
Astronomers used NASA’s Hubble Space Telescope to confirm the Spitzer
observations and reveal the detailed stellar structure around LRLL
54361. Hubble observed two cavities above and below a dusty disk. The
cavities are visible by tracing light scattered off their edges. They
likely were blown out of the surrounding natal envelope of dust and
gas by an outflow launched near the central stars. The disk and the
envelope prevent the suspected binary star pair from being observed
directly. By capturing multiple images over the course of one pulse
event, the Hubble observations uncovered a spectacular movement of
light away from the center of the system, an optical illusion known
as a light echo.
Muzerolle and his team hypothesized the pair of stars in the center of
the dust cloud move around each other in a very eccentric orbit. As
the stars approach each other, dust and gas are dragged from the
inner edge of a surrounding disk. The material ultimately crashes
onto one or both stars, which triggers a flash of light that
illuminates the circumstellar dust. The system is rare because close
binaries account for only a few percent of our galaxy’s stellar
population. This is likely a brief, transitory phase in the birth of
a star system.
Muzerolle’s team next plans to continue monitoring LRLL 54361 using
other facilities including the European Space Agency’s Herschel Space
Telescope. The team hopes to eventually obtain more direct
measurements of the binary star and its orbit.
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