PASJ: Publ. Astron. Soc. Japan 54, 865–871, 2002 December 25 c  2002. Astronomical Society of Japan. Development of Iodine Cells for the Subaru HDS and the Okayama HIDES: I. Instrumentation and Performance of the Spectrographs Eiji KAMBE, 1 Bun’ei SATO, 2,3 Yoichi TAKEDA, 4 Hiroyasu ANDO, 3 Kunio NOGUCHI, 3 Wako AOKI, 3 Hideyuki I ZUMIURA, 3 Setsuko WADA, 5 Seiji MASUDA, 3 Norio OKADA, 3 Yasuhiro SHIMIZU, 3 Etsuji WATANABE, 3 Michitoshi YOSHIDA, 3 Satoshi HONDA, 3 and Satoshi KAWANOMOTO 3 1 Department of Earth and Ocean Sciences, National Defense Academy, Yokosuka, Kanagawa 239-8686 2 Department of Astronomy, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033 3 National Astronomical Observatory, Mitaka, Tokyo 181-8588 4 Komazawa University, Komazawa, Setagaya, Tokyo 154-8525 5 The University of Electro-Communications, Chofu, Tokyo 182-8585 kambe@nda.ac.jp (Received 2002 January 30; accepted 2002 June 26) Abstract We have developed iodine (I 2 ) cells for the High Dispersion Spectrograph (HDS) on the Subaru telescope and for the HIgh Dispersion Echelle Spectrograph (HIDES) on the Okayama 1.88 m reflector. The cell is put into a compact vacuum vessel, which fits into a small space behind the entrance slit of the HDS. The cell assembly is designed to minimize heat losses from the cell, which is heated during observations, to keep I 2 vapor from condensation. The amount of I 2 in the cell is determined to be best suited for radial-velocity measurements of solar-type stars. We also report on some performances of the HDS and HIDES as well as their instrumental profiles based on tests of our I 2 cells. Lastly, we discuss future improvements of our instruments and data-analysis softwares for I 2 observations, and also describe the scientific goals of our project. Key words: instrumentation: spectrographs — stars: oscillations — stars: planetary systems — techniques: radial velocities 1. Introduction Precise measurements of radial velocity of stars recently turned out to be a powerful technique in the search for extra- solar planets around stars (Mayor, Queloz 1995; Marcy, Butler 1998; Marcy et al. 2000; Butler et al. 2002) and for de- tecting solar-type oscillations (Bedding et al. 2001; Queloz et al. 2001). Since its discovery (Mayor, Queloz 1995), about 80 candidates of giant planets have been found so far by measuring the tiny wobble of stars in radial velocity due to the existence of planets. Extensive searches for extra- solar planets are currently on-going and/or planned around the globe (Butler et al. 2002; Cumming et al. 1999; Tinney et al. 2001; Queloz et al. 2000; Latham 2000; Nisenson et al. 1999; Cochran et al. 1999; Endl et al. 2001; Pepe et al. 2000a). Applications of precise radial-velocity mea- surement techniques to other astronomical objects also attract our attention, such as measuring the atmospheric motions in Cepheids (Butler 1998) and detecting very small-amplitude stellar oscillations in rapidly oscillating Ap (roAp) stars, β Cephei stars, and so on. Thus, motivated by the construction of the new echelle-type High Dispersion Spectrograph for the Subaru telescope (HDS, Noguchi et al. 1998, 2002) and of the new HIgh Dispersion Echelle Spectrograph for the Okayama 1.88 m reflector (HIDES, Izumiura 1999), we decided to de- velop instruments for precise measurements of the radial ve- locity of stars. For such purposes, two methods, though not mutually ex- clusive, are extensively used in high dispersion spectroscopy at present. One is to adopt the technique of simultaneous cal- ibration (e.g., CORALIE, Queloz et al. 2000; AFOE, Brown et al. 1994; HARPS, Pepe et al. 2000b). A stellar spectrum is usually obtained, flanked by the typical hollow-cathode lamp (Th–Ar lamp) spectra as wavelength calibration references. The merits of the technique are that the whole observed wave- length range can be used to estimate the radial velocity of stars, which might overcome the ultimate accuracy of the iodine (I 2 ) technique described below, and that the stellar spectrum re- mains uncontaminated, which facilitates its further inspection. However, to implement the simultaneous calibration capabil- ity on an existing spectrograph requires a modification of the slit area: the introduction of an image scrambler to cancel the effect of guiding errors, as well as calibration fibers adjacent to the output of the scrambler. Also, devices for object acquisition and on-slit guiding might need to be modified. Another method is to use gas filters, like HF and I 2 (Campbell et al. 1988; Walker et al. 1995; Butler 1987; Marcy, Butler 1992). In the I 2 technique, numerous I 2 molecular ab- sorption lines are superposed on a stellar spectrum, which can be used both as a wavelength standard and to correct for any local or time-dependent instrumental profile (IP) of a spectro- graph at each position on a detector. It is in fact a merit of the I 2 technique that we can essentially compensate for changes of IP due to guiding errors or distortions of the spectrographs. The I 2 cell can be added to existing spectrographs without many modifications to them. Since both spectrographs were in the final stage of construction when we started our precise radial- velocity measurement project, we decided to introduce I 2 cells. Downloaded from https://academic.oup.com/pasj/article-abstract/54/6/865/1551395 by guest on 28 May 2020