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.

IMAGE CAPTIONS:

[Figure 1:
http://subarutelescope.org/Latestnews/200201/AO_IRCS/fig1-e.jpg (55KB)]
Mechanism of Adaptive Optics (AO). Copyright (c) Subaru Telescope,
National Astronomical Observatory of Japan (NAOJ)

[Figure 2:
http://subarutelescope.org/Latestnews/200201/AO_IRCS/fig2.jpg (137KB)]
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.
Copyright (c) Subaru Telescope, National Astronomical Observatory of
Japan (NAOJ)

[Figure 3:
http://subarutelescope.org/Latestnews/200201/AO_IRCS/fig3.jpg (17KB)]
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. Copyright (c) Subaru Telescope,
National Astronomical Observatory of Japan (NAOJ)

[Figure 4:
http://subarutelescope.org/Latestnews/200201/AO_IRCS/fig4.gif (100KB)]
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. Copyright
(c) Subaru Telescope, National Astronomical Observatory of Japan (NAOJ)

[Figure 5:
http://subarutelescope.org/Latestnews/200201/AO_IRCS/fig5.gif (15KB)]
Spectra of HD 130958B & C in H (top panel) and K (bottom panel) bands.
The deep absorption bands by water are clearly seen. Copyright (c)
Subaru Telescope, National Astronomical Observatory of Japan (NAOJ)

[Figure 6:
http://subarutelescope.org/Latestnews/200201/AO_IRCS/fig6.gif (14KB)]
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.
Copyright (c) Subaru Telescope, National Astronomical Observatory of
Japan (NAOJ)