Astronomers using the National Science Foundation’s (NSF) 140-foot radio
telescope at the National Radio Astronomy Observatory (NRAO) in Green
Bank, W.Va., have discovered a highly unusual, massive interstellar
cloud that appears poised to begin a burst of star formation. The
cloud may be the first ever to be detected in the transition between
atomic and molecular states.

NRAO scientists Felix J. Lockman and Anthony H. Minter presented
their findings at the American Astronomical Society meeting in
Pasadena, Calif.

Radio Image of G28.17+0.05

The scientists discovered the cloud, identified as G28.17+0.05, lying
along the inner plane of the Milky Way Galaxy, approximately 16,300
light-years from Earth. Observations of the cloud indicate that it is
near one of the Galaxy’s sweeping spiral arms, which are outlined by
young stars and the massive clouds that form them. Lockman and Minter
speculate that as the interstellar cloud slams into the Galactic arm,
the resulting shock wave may be precipitating the conversion of the
neutral hydrogen atoms into heavier molecules, which could herald the
onset of star formation.

“These may be the first observations of a cloud that is in the
transition between the neutral atomic hydrogen and molecular phases,”
said Lockman. “This provides astronomers a unique opportunity to study
the chemistry of very young interstellar clouds, which could give us
significant insights into the early stages of star formation and the
structure of the Galaxy.”

Interstellar clouds that contain neutral atomic hydrogen, called HI
(H-one) clouds, are thought of as giant, cold blobs of gas. Researchers
study these objects because they offer intriguing glimpses of the
composition of our Galaxy and the cosmos, and reveal much about how
stars and planets are born. Hydrogen atoms in these clouds give off
natural signals (at the 21-cm wavelength), which can be detected only by
radio telescopes.

The scientists discovered that this HI cloud was unusual in many
respects. First, it was uncharacteristically massive, about 500 light-
years across and containing nearly 100,000 times the mass of the sun in
atomic hydrogen. The gas in clouds this large and massive has typically
undergone the transition to the molecular phase, and has begun making
stars. The size and mass of this cloud indicate that it is
gravitationally bound, which means that it should be collapsing and
forming new stars.

“When you find a cloud that is as massive as the one we detected, and
one that is gravitationally bound as this structure indicates, then you
would expect to see areas of star formation,” said Lockman. The
scientists were able to identify a few indicators of star formation, but
not at the rate that one would expect. “We think we have caught
something in a special state.” Lockman said, “It could be one of the
missing links in the cycle of star formation.”

The core of the cloud also gives off radio signals at 1720 MHz from the
molecule OH in an unusual state of excitation. Since other astronomers
have detected similar signals throughout the Galactic plane, the
researchers believe that these emissions may be an indication that this
previously undetected type of cloud may turn out to be fairly common.

“We suspect that this cloud may be the first example of an object that
may be fairly common in the inner Galactic plane,” said Lockman, “but
has not been recognized. That is, a cloud that is observed while
entering a spiral shock and is in the transition between atomic to
molecular hydrogen.”

The scientists caution, however, that additional research is needed to
confirm their speculations. “The presence of anomalous OH through the
Galactic plane does suggest that other clouds of this nature can be
detected,” said Lockman, “and it would be particularly valuable if a
similar cloud could be detected entering the ‘spiral shock’ on the
opposite side of the Galactic center.” The patterns of velocities of
atomic and molecular gas should be reversed there, due to the difference
in galactic rotation. Such a discovery could help to validate the
possible interaction among the spiral shock, atomic hydrogen, and star

The NSF’s 140-foot radio telescope now is decommissioned after a long
and highly productive career. Research will continue on the newly
Robert C. Byrd Green Bank Telescope
, which is the world’s
largest fully steerable radio telescope. “Though the 140-foot telescope
enabled us to make remarkable observations,” commented Minter, “we
anticipate that the new Green Bank Telescope, with its increased
sensitivity and better resolution, will enable us to see more clearly
the nature of this peculiar object.”

In addition to Minter and Lockman, other astronomers involved in this
research include Glen I. Langston, NRAO; and Jennifer A. Lockman who was
a student from the College of Charleston, S.C., at the time the research
was conducted.

National Radio Astronomy Observatory
is a facility of the
National Science Foundation, operated
under cooperative agreement by
Associated Universities, Inc