Hubble Reveals Stellar Jets

Contacts:
Lia Unrau
Phone: (713) 831-4793
E-mail: unrau@rice.edu

Patrick Hartigan
Phone: (713) 527-8101, ext. 2245
E-mail: hartigan@sparky.rice.edu

HUBBLE REVEALS STELLAR JETS: MULTIPLE EJECTIONS AND A STAR BIRTH WIGGLE

New images from the Hubble Space Telescope
provide the deepest space-based images to date of the powerful jets
of gas ejected in the violent process of star birth, showing a 1,000
year history of massive, erratic ejections and a wiggle in their
direction of flow, a Rice University astronomer announced today.

Analysis of photos taken by the Hubble Space Telescope (HST)
reveals delicate filamentary arcs and dramatic ejections produced by
the passage of shock waves in the hypersonic outflows from young
stars, says Patrick Hartigan, Rice University assistant professor of
space physics and astronomy, who reported the findings at a poster
session today at the American Astronomical Society meeting in
Madison, Wis. The findings also indicate that the source of the jet
emissions changes angle, producing a wiggle in the jet stream flow.

Hartigan and his team analyzed the hydrogen and sulfur emissions
of two jets of gas, known as Herbig-Haro jets (named for the
astronomers who discovered them), spewing from stellar nurseries
some 1,460 light years away. Spanning over half a light-year, the
jet known as HH47 streams out at the edge of the Gum Nebula in the
southern constellation Vela the Sails. Another jet named HH111 is
located in the Orion constellation, one of the most active known
starbirthing regions.

“We are seeing the guts and details of how stars form, a very
violent process that involves both accreting material onto the star
and ejecting material at high speeds into space,” Hartigan says.”These images show more detail of stellar jets than any other so far
seen by man.”

Stars form out of spinning disks of gas and dust, which become
unstable and collapse inward under the influence of gravity. Powered
by the infall of material from the disk onto the star, a column of
energetic wind-the jets of gas-ejects from the star. However, the
exact workings of this process remain a mystery.

“As material falls onto the star, it makes `hot spots,’ and
somehow this process of infall converts into outflow,” Hartigan
says. “These jets probably provide enough energy to prevent the star-forming material from collapsing back on itself. It’s nature’s way
of regulating the size of newborn stars.”

The star jettisons a powerful column of gas at speeds greater
than 2 million miles per hour, creating a strong shock wave in front
of it. The amount of material and the velocity of the flow vary with
time, so additional shock waves are generated as faster material
overtakes the material in front, much like fast-moving cars
sometimes collide with slower ones and pile up on the freeway. Each
year these jets spit out an amount of gas roughly 10 times the mass
of the moon.

“By analyzing the Hubble data, we’ve been able to study how this
material interacts with the surrounding medium and powers the much
larger outflows observed with radio telescopes,” says Hartigan.

Multiple bow-shaped features, or arcs of gas, define the shocked
areas in the jet stream-the points where collisions take place
between older, slower gas and faster gas more recently ejected. In
most cases, Hartigan and his team were able to see both the bow
shock, which accelerates the slow material, and the Mach disk, a
shock that decelerates the fast material. As many as a dozen
episodes of fast and slow material appear along the jets, the
furthest of which began its journey from the newborn star almost
1,000 years ago.

It appears both HH47 and HH111 spit out material in intervals
and change the angle of
ejection to produce the telltale skewed bow shocks seen in the
Hubble images, Hartigan says.
The images also enabled the team to trace the direction of flow,
and it appears that whatever is ejecting the material wiggles, as
though the source changes the angle of ejection, Hartigan theorizes.
It is evident that the jets of gas are not perfectly straight,
and as the angle of emission changes, subsequent shocks drive the
earlier shocks into the surrounding interstellar medium. This is
similar to a car rear-ending another car, not straight on, but at an
angle, so that the first car is pushed into another lane.

“The HST has given us an incredibly clear view of this,”
Hartigan says. “We’re able to get a detailed structure of what
happens between the shocks. We can put together a real picture of
how the material is flowing.”

Implications of the apparent wiggle is currently a source of
debate among astronomers.
Other team members include Bo Reipurth of European Space Agency,
Jon Morse and John Bally of the University of Colorado, Steve
Heathcote of Cerro Tololo Interamerican Observatory, Richard
Schwartz of the University of Missouri, and Jim Stone of the
University of Maryland.

A paper authored by the team has been accepted by Astronomical
Journal and will be published later this year.

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