Subaru Telescope, NAOJ

First Spectroscopic Observations with Subaru's Adaptive Optics

The Adaptive Optics (AO) system, installed at the Cassegrain focus of Subaru Telescope, corrects star light affected by atmospheric turbulence and delivers a high quality image close to the theoretical limits of the telescope. Since its first light in December 2000, we have adjusted the AO system with test observations and the spectroscopic observations with AO and Infrared Camera and Spectrograph (IRCS) were recently successful. These results make the most of Subaru's capabilities.

The summit of Mauna Kea is one of the best sites for astronomical observations because of the settled weather, stable atmosphere, and dark night sky. However, turbulence in the atmosphere prevents Subaru from achieving its theoretical image quality (0.02 arcsec in visible light and 0.06 arcsec in the near-infrared). Instead, typical images have a size of 0.6 arcsec, which makes it hard to resolve fine structure.

The role of the AO system (Figure 1), which is installed between the telescope and instruments (IRCS and Coronagraphic Imager with Adaptive Optics (CIAO)), is to measure the rapidly-changing wavefront produced by the atmospheric turbulence, and to quickly correct it with a special mirror (called a bimorph deformable mirror, Figure 2) . As a result, we can obtain sharp images close to the diffraction limit in the infrared (Figure 3). Subaru's AO divides the light of a guide star near the target object into 36 elements, and measures the turbulence with a wavefront curvature sensor that includes a high sensitivity photon-counting module. Other AO systems on Mauna Kea divide the star light into more elements (e.g., the Keck telescope divides the star light into 240 elements), but this limits the faintness of the guide stars they can work with. Subaru's AO system appears to offer the best image quality when faint guide stars are used. Since fainter stars are far more numerous than brighter ones, this greatly increases the area of sky over which the AO system can be used to produce high-resolution images, compared to other telescopes.

The AO system is also beneficial to spectroscopic observations, in which the light gathered by the telescope passes through a narrow slit, and is dispersed to different locations on a detector according to its wavelength. If we use a narrower slit, the wavelength resolution is increased, allowing us to see more detail in the spectrum. However, if the slit is narrower than the size of the object, we lose the light outside the slit. With the AO system, we can make the star light sharp and use a narrow slit to obtain high wavelength resolution without losing any light. Moreover, the high spatial resolving power of AO enables us to get spectra for a very fine structure of objects. Subaru and Keck are the only telescopes at Mauna Kea that can make spectroscopic observations with the AO system.

Subaru observed a binary system composed of two stars with very low masses (called brown dwarfs) with AO and IRCS. The binary system (HD 130948B & C) was discovered by astronomers from the University of Hawaii with Gemini's AO system in February 2001, and is only 2.6 arcsec away from a bright star (HD 130948A; 5.9 magnitude in visible light). Since the separation between the two brown dwarfs is only 0.13 arcsec, we cannot confirm it is a binary system without the AO system.

We successfully observed HD 130948B & C separately with the AO system (Figure 4) and made spectroscopic observations with IRCS. The spectra of HD 130948B & C (Figure 5) show the existence of a huge amount of water vapor, indicating that the atmospheric temperature is cooler (1500 - 1700 degrees Celsius) than the decomposition temperature of water molecules. Furthermore, it is clear that HD 130948B & C are brown dwarfs because they have lower masses than the limit of ordinary stars, assuming they are the same age as HD 130948A (0.5 - 1 billion years, estimated from its X-ray activity) (Figure 6). Only a few examples of such close brown dwarf binaries are known, and this is only the second example of AO spectroscopic observations. These observations are an essential technique for understanding the evolution and physical/chemical characteristics of low mass stars.

This work was done in collaboration with researchers at the Institute for Astronomy of the University of Hawaii.

Figure 1: Mechanism of Adaptive Optics (AO)

Figure 2: Backside of the deformable mirror of the Subaru's AO system. We can see the electrodes which control the shape of the 36 element deformable mirror. The surface of the mirror bends in proportion to the voltage put on the electrodes and corrects the distortion of the images caused by atmospheric turbulence. The mirror is 110 millimeters in diameter.

Figure 3: Example images with Subaru's AO system (rge binary system HR1852). Left: without AO, right: with AO. K band (2.2 micron). The separation between the two stars is 0.31 arcsec.

Figure 4: An image of the brown dwarf binary system with IRCS (HD 130948). Upper left: with AO. Upper right: without AO. Lower left: a magnified image of HD 130948B & C. Lower right: the spectra around 2 microns.

Figure 5: Spectra of HD 130958B & C in H (top panel) and K (bottom panel) bands. The deep absorption bands by water are clearly seen.

Figure 6: The relation between masses and ages of HD 130958B & C. Their ages are assumed to be equal to that of HD 130958A (0.5 - 1 billion years), estimated from its X-ray activity. The effective temperature is estimated from the depth of the absorption lines by water. The evolutionary model is taken from Baraffe et al. 1998, A&A, 337, 403.

January 16, 2002