We report on performance studies for wavelength calibration using a laser frequency comb and the fiber-fed HIgh Dispersion Echelle Spectrograph (HIDES-F) on the Okayama 188cm telescope. We use a laser frequency comb system that has been recently developed and reported. The comb is based on an erbium-doped fiber-based femtosecond laser and can generate comb-shaped laser modes with a wavelength range of 350nm - 408nm, 453nm - 543nm, and 664nm - 873nm with a mode spacing of 30GHz. The comb has been installed in a room of the Okayama 188cm telescope dome and has been in operation since 2020. The comb spectra were obtained during observations for precision radial velocity (RV) measurements with an iodine absorption for about two years. Using the spectra of the comb and other wavelength calibrators, we have measured instrument shifts of HIDES-F and evaluated its effects on wavelength calibration for precise RV measurements.
The InfraRed Doppler (IRD) spectrograph can provide high-resolution (R > 70,000) spectra over 1000-1700 nm with stable wavelength calibrations thanks to a dedicated laser frequency comb. Since the first science operation on the Subaru 8.2-m Telescope in 2018, IRD has been extensively used for observations, in which the main field is exoplanet but some studies cover the fields of Galaxy evolution and compact object. One of the main outputs is the discovery of a super-Earth close to the habitable zone of cool M dwarf Ross 508. IRD was also used to constrain the masses of many planets identified with TESS, and to identify the atomic/molecular features of transiting planets. Recently, the extreme adaptive optics system, SCExAO, can be combined with IRD to directly characterize a substellar companion with high-contrast and high-resolution spectroscopy. We here highlight and summarize the outputs obtained via the six-year operation of IRD.
GAOES-RV (Gunma Astronomical Observatory Echelle Spectrograph for Radial Velocimetry) is a high-dispersion echelle spectrograph for the 3.8 m Seimei Telescope at Okayama Observatory, Kyoto University. It covers the wavelength band from 516 to 593 nm and provides reciprocal resolution of 65,000. To maintain both the throughput and wavelength resolution of the observing system, starlight is collected in a 2.4 arc-second diameter field of view at the Nasmyth focus with a multimode optical fiber, and an image slicer is used at the other end of the fiber link. An iodine absorption cell is used for precise radial velocity measurements, and currently a precision of about 2 m/s can be achieved for bright solar-type stars. GAOES-RV has been in operation since July 2023, and is widely used for a variety of scientific observations, including the detection and characterization of exoplanets, stellar abundance analysis, and research on active stars.
We assessed the impact of Earth’s atmospheric absorption lines, known as telluric contamination, on near-infrared radial velocity (RV) measurements using IRD/Subaru. We focused on the telluric removal process implemented in the RV pipeline for IRD data, which works in two phases: the creation of a stellar template spectrum and the measurement of RVs through a forward modeling approach. Our analysis revealed that discrepancies of approximately 1% exist between the observed telluric standard star’s spectra and theoretical telluric spectra, both used within the RV pipeline. These discrepancies are particularly significant in regions with strong water vapor absorption. Additionally, we investigated the impacts of residual tellurics on RV measurements through mock spectrum analysis. By comparing RV values derived from mock spectra made with either theoretical or observed tellurics, we found that residual tellurics can introduce an additional scatter of at least 1 m/s in RV measurements. Our findings highlight the necessity for improved telluric removal methods in the near-infrared spectrum to achieve precise RV measurements critical for detecting small-mass planets.
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