Strange trail suggests presence of galactic interloper

CHAMPAIGN, Ill. — Scientists have discovered what looks like a jet contrail,
possibly left behind by a dwarf star traveling through interstellar space.

As reported in the September issue of The Astronomical Journal, astronomer
Peter R. McCullough at the University of Illinois at Urbana-Champaign and
research scientist Robert Benjamin at the University of Wisconsin at Madison
found a straight and narrow filament of ionized gas stretching 2.5 degrees
across the sky near the Big Dipper in the constellation Ursa Major.

“We believe the gas trail was produced by the radiation from a white dwarf
or some other low-luminosity source zipping through the local interstellar
medium and leaving behind an ionized wake,” McCullough said. “The problem
is that we have not yet identified the source.”

While other possible explanations were considered — such as a jet of
low-density stellar radiation or a linear wisp of gas associated with some
nearby nebula — they are not favored because the filament’s properties
are so different from other examples of those types of objects, McCullough
said.

The filament is roughly Y-shaped. The vertical segment of the Y is about
1.2 degrees long and about 20 arcseconds wide. The full width of the two
diagonal segments is about 5 arcminutes. The distance to the gas trail is
not known, but it is suspected to be approximately 300 light-years from
Earth.

“We know that white dwarfs — hot, dense stars not much bigger than a large
planet — can leave these kinds of trails, but they will be very faint,”
Benjamin said. Such trails had been predicted to exist by two Harvard
astronomers in the early 1980s, but had never been seen. “This could be
the brightest trail visible from Earth and therefore the first one found.”

If that turns out to be the case, astronomers might locate other such
trails by photographing candidate white dwarfs whose distance and direction
of motion are accurately known.

The object was first photographed in January 1997 with a small camera
equipped with a hydrogen-alpha filter. Additional observations were made
in April and May 1999 with a different filter mounted on the UI’s 40-inch
reflecting telescope at Mount Laguna Observatory in southern California.
The researchers also detected the object with the Wisconsin Hydrogen-Alpha
Mapper (WHAM), confirming that the source was not from beyond our galaxy.
The research was funded in part by the National Science Foundation.

“The filament’s large angular size also suggests it is nearby, and therefore
we should be able to identify what created it,” McCullough said. If the
source can be identified and studied, astronomers could use its properties
to probe interesting parameters of the local interstellar medium — such as
the density of the ambient gas and the level of turbulence in interstellar
space.

“The culprit could be sitting right under our noses and we don’t recognize
it,” McCullough said. Additional observations with other telescopes may
solve this cosmic whodunit.

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