Spaceflight Now: Breaking News

Newborn star offers insight into our solar system's past
Posted: January 13, 2000

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 disk and environment around HD100546. Photo: C.A. Grady (NOAO,GSFC), the STIS Investigation Definiation Team, NASA
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.

Detail of the inner disk of HD100546. Photo: C.A. Grady (NOAO,GSFC), the STIS Investigation Definiation Team, NASA
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.