Universe of Spectroscopy

10Iodine molecules, G-type giant star spectra, and the search for extrasolar planets

The question of Earth's existence in this vast universe is one that is perhaps generally held by mankind. To answer this question, extensive research is required on related topics such as the types of planetary systems that exist around stars other than the Sun, conditions necessary for planetary system formation, and origin of life.

Ever since the first extrasolar planet candidate was discovered near 51 Pegasi in 1995, research on extrasolar planets has increased like the gold rush. In Japan as well, many researchers in various disciplines have been working around the clock on the study of extrasolar planets. Number of different techniques exists for detecting these planets, one of which is the use of the radial velocity method, also known as Doppler spectroscopy, which was first implemented at Okayama Astrophysical Observatory / National Astronomical Observatory of Japan (OAO/NAOJ).

In this method, a large number of electron transition frequency rotational lines are used as high-precision wavelength standards. This technique enables high-precision radial-velocity measurement, which is impossible to achieve through conventional high-dispersion spectroscopic observation.

In actual, the precise computer modeling of the measured spectral data is absolutely essential, as is the patience of the researcher in persevering with observations and measurements. When the above conditions were met in 2003, right after the completion of the construction of the High Dispersion Echelle Spectrograph (HIDES), the first planetary candidate, the G-type giant star HD104985, was detected. In 2007, it was determined that the G-type giant star εTau, with approximately three solar masses, had a planet candidate. This discovery marked the first case for a star belonging to an open cluster and forged a path for assigning an age to the planetary system. Until January 2011, 14 potential planets and brown dwarfs have been found at Okayama. As data continues to accumulate, the detection of multi-planetary systems and planetary systems extending beyond the frost line is anticipated for the future.

Column “The G-type giant star and A-type dwarf star”

In 2009, intense discussion centered around the use of direct imaging for the detection of planet candidates for two A-type dwarfs. G-type giant stars, the target planet detection at OAO, are the future forms of A-type dwarf stars; thus, an interesting comparison can be made with planetary systems of G-type giant stars and those of A-type dwarf stars. Moreover, using high-precision coronagraphic measurements to study the G-type giant stars detected by Doppler spectroscopy, we can comprehensively examine their planetary systems. Thus, entire structures can be clarified for planetary systems that revolve around stars with masses 2-3 times that of the Sun.

εTau of the Taurus Constellation
Image 1: εTau of the Taurus Constellation, the first star in an open star cluster determined to have a planet candidate. εTau belongs to the Hyades Cluster located near the head section of the Taurus Constellation. The stars of this open cluster are believed to have formed at approximately the same time, which in the case of the Hyades was approximately 600 million years ago. This discovery marks the first time a relatively precise age was applied to a planetary candidate.
Spectrum image of wavelengths at approximately 5500 A obtained by HIDES
Image 2: Spectrum image of wavelengths at approximately 5500 Å obtained by the High Dispersion Echelle Spectrograph (HIDES) through an iodine absorption cell and a continuous light lamp. A large amount of iodine-molecule (I2) electron transition frequency rotational lines are apparent. In actual observation, this information is recorded onto the star's spectrum.
One example showing, from top to bottom, the absorption line spectrum of iodine molecules
Image 3: One example showing, from top to bottom, the absorption line spectrum of iodine molecules obtained from an ultra-high-resolution spectrograph in the laboratory, a star's spectrum used as a reference, and a star spectrum obtained by the High Dispersion Echelle Spectrograph (HIDES) through an iodine absorption cell. Dots indicate observation data; the line indicates the model data. (Sato et al. 2002, PASJ, 54, 873). The bottom section shows differences between the model and observation.


March 4, 2013
High Dispersion Spectrograph HIDES + Iodecell + High-precision numerical modelling + Fortitude