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A NEWBORN STAR OFFERS A VIEW TO OUR PAST
Evidence that small dust grains are agglomerating into larger blocks inside a
persistent shell of gas and dust around a young, nearby star is giving a team of
astronomers a rare glimpse into the process that likely formed our solar system.
The star, designated HD100546, is estimated to be a relatively young 10
million years old (if stars existed for a typical human lifespan of 70 years,
this star would be about one year old). It is surrounded by a disk of gas and
dust, out of which planetesimals, the building blocks of planets, may be
forming. Additionally, it is the first system in which astronomers have evidence
that large dust grains in the disk are forming from the agglomeration of smaller
grains, a necessary step to build planetesimals.
HD 100546 is in our cosmic neighborhood at about 335 light years away, in the
direction of the southern constellation Musca (one light year is the distance
light travels in a year, about six trillion miles). Like our Sun, it is an
isolated, single star.
"The envelope of infalling matter surrounding a star as old as HD 100546
is not expected by models of star formation," said Dr. Carol Grady of the
National Optical Astronomy Observatories, who is stationed at NASA's Goddard
Space Flight Center, Greenbelt, Md. "It was thought that when a star begins
to radiate energy via thermonuclear reactions in its core, it would blast this
spherical cloud away. Since we still see the cloud, our models of star formation
are probably too simple. There is an ongoing debate among astronomers about the
features present around a newborn star - spherical envelopes vs. flattened
disks. Our observations of HD 100546 and other young stars tell us that the
answer is yes, all of the above, some of the time."
The team used multiple observatories for its research, including the Blanco 4
meter telescope at the Cerro Tololo Inter-American Observatory (CTIO), located
near La Serena, Chile, and the European Southern Observatory (ESO), La Silla,
Chile, in addition to the Hubble Space Telescope. At ESO the team used the
Adaptive Optics Near Infrared System (ADONIS), while at CTIO the team used the
University of Florida Observatory Spectrometer Camera for the InfraRed (OSCIR).
The Hubble observations were made using the Space Telescope Imaging Spectrograph
(STIS) instrument in its coronagraphic imaging mode, where an occulting wedge
artificially eclipses the star. We see the disk and envelope of HD 100546 by the
starlight reflected by dust grains.
These photos are a composite of two observations made with the STIS. The
colored polygons are produced by the coronagraph on STIS, which blocks direct
light from the star so the much fainter surrounding material can be seen. The
central disk is seen as a dark swirl of material where the coronagraph polygons
intersect. The darker part of the disk is seen from about 4.6 billion miles (7.3
billion km) to 32.5 billion miles (52 billion km) from the star. STIS cannot
view regions closer to the star due to the presence of the coronagraph. The
fainter cloud-like areas around the dark disk are the outer regions of the disk
and a surrounding cloud of gas and dust. The gas and dust disk of HD 100546 can
be detected out to 46.5 billion miles (almost 73 billion km, or 500 AU) in both
the ADONIS and STIS data. Beyond 26 billion miles (41.6 billion km or 278 AU)
the disk becomes much fainter in the near-infrared, but not in the optical,
indicating that the grains in the outer disk are smaller than those closer to
the star.
Imagine sitting in a dark movie theater and throwing a handful of popcorn
into the projector beam. Next, imagine blowing a cloud of smoke into the beam.
The light reflected from the smoke cloud is brighter because the millions of
tiny smoke particles collectively have a much greater surface area to reflect
the beam than a few big pieces of popcorn. Similarly, smaller dust grains have
more surface area and reflect starlight better than large grains. If more,
smaller grains are found in the outer regions, their greater reflectivity would
cause the more gradual decrease in the brightness of the disk.
The tenuous outer envelope of HD 100546 is only seen in the STIS data and
continues out to 93 billion miles (149 billion km, or 1000 AU).
The observations with the OSCIR and ADONIS instruments reveal what is
happening closer to the star, where STIS can't see. Infrared data from OSCIR
shows a lot of dust at 200 degrees Kelvin (minus 100 degrees Fahrenheit),
becoming colder as distance from the star increases. The absence of warmer dust
indicates that the innermost regions, where it is warmest, are clear.
"There are probably things bigger than dust grains running around in
this disk, something that's preventing the outer disk material from reaching the
inner disk," said Grady. Previous studies of HD 100546 have estimated that
there could be 100 billion planetesimals the size of comet Hale-Bopp in the
inner regions of the disk.
"This star's proximity, age, and wealth of interesting features make it
an important system for understanding how planets form," Grady said.
"HD100546 will be studied in great detail by every instrument that can be
brought to bear on it." Grady is the lead author of a paper describing this
research to be submitted to the Astronomical Journal.
Image Credit: NASA/National Science Foundation/European Southern
Observatory.
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