COMICS Detects Silicate Crystals Around a Sun-like Star
October 1, 2003
Mid-infrared spectra from the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on Subaru telescope show the first direct evidence of crystalline silicates in a proto-planetary disk surrounding a Sun-like star. COMICS is the first large format mid-infrared instrument to go into operation on an 8-meter class telescope. Although astronomers suspected the presence of crystalline silicates in proto-planetary disks surrounding young Sun-like stars, previous generations of mid-infrared instrument and telescope combinations were not sensitive enough to detect it. COMICS's success ushers in a new era in mid-infrared astronomy and the study of planet formation.
Most of Earth's crust is made of silicates, various minerals composed of silicon and oxygen. Silicates come in two forms: crystalline and amorphous. Crystalline silicates have some form of symmetry in its structure; Amorphous minerals don't . Silicates naturally found on Earth are usually crystalline because they have been processed by heat from Earth's tectonic activity. In outer space, amorphous silicates are more common.
Laboratory experiments show that both crystalline and amorphous silicates have distinguishing characteristics in the mid-infrared light they emit. Amorphous silicates have smooth spectra with a characteristic peak at 9.8 microns. The spectra from crystalline silicates have more peaks.
Previous astronomical observations suggested that sometime during the formation of stars and planets, amorphous silicates turn into crystalline silicates. Interstellar matter, the repository of raw ingredients for stars and planets, and proto-planetary disks, disks of matter surrounding young stars, both contain silicates mostly in amorphous form. Comets, made of matter left over from the formation of our solar system, contain crystalline silicates.
Until now, crystalline silicates have only been detected around young stars much heavier than the Sun. These stars have large proto-planetary disks which shine bright in the mid-infrared. Proto-planetary disks of proto-stars and young stars with lower mass are smaller and fainter, and thus much more challenging to observe.
COMICS is the first of the large format mid-infrared observing instrument to go into operation on a 8-meter class telescope. To test COMICS's capabilities, the development team, consisting of researchers from the University of Tokyo, the National Astronomical Observatory of Japan, the Institute of Space and Astronautical Science, and Kitasato University, decided to study the evolution of silicates around stars with masses comparable to our own Sun. Their specific targets were young stars 5 to 10 million years old called T-Tauri stars because of their similarity to the proto-typical young star in the constellation Taurus, T-Tauri.
The team succeeded in the first ever detection of mid-infrared spectral signatures of crystalline silicates from the proto-planetary disk surrounding a young Sun-like star called Hen3-600A. Hen3-600A belongs to a group of young stars called the TW Hydrae association. The association consists of about two dozen stars 5 to 10 million years old - not new born but not yet mature stars - within 120 light years of each other. Because it is only 160 light years from the Sun, it is an ideal place to study star and planet formation in detail.
The detection confirms that crystalline silicates can form around Sun-like stars when they are young. Because it takes temperatures of 600 degrees Celsius to from crystalline silicates, this detection implies the presence of a mechanism that heats the proto-planetary disk to such a temperature. "Now that we know that the crystalline silicate exists, our next challenge is to observe how the crystalline silicate is distributed in space," says Mitsuhiko Honda, a graduate student at the University of Tokyo and a leader of the project. "We want to understand the processes taking place in the proto-planetary disk that lead to the formation of crystalline silicates in the first place".
Scientific Reference: Honda et al. 2003, ApJ, 585, L59-63
Figure 1: An 8.8 micron image of Hen3-600A taken by COMICS in imaging mode. The fainter dot to the right is a companion star.
Figure 2: The spectra of Hen3-600A taken by COMICS in spectral mode. The peaks indicate the presence of various forms of crystalline silicate. The red peaks at 9.2 and 12.5 microns are from silica (SiO2, silicon dioxide), the blue peak at 10.9 microns is from enstatite (MgSi03, a magnesium silicate), and the green peaks at 10.1, 10.5 and 11.2 microns are from forsterite (Mg2SiO4, a magnesium silicate). Silica or silicon dioxide comes in many forms including all forms of quartz. Fosterite is a form of olivine. As a gemstone, it is called peridot.