Abstract
Aims. SPIRou is a near-infrared (nIR) spectropolarimeter at the CFHT, covering the YJHK nIR spectral bands (980−2350 nm). We describe the development and current status of the SPIRou wavelength calibration in order to obtain precise radial velocities (RVs) in the nIR. Methods. We make use of a UNe hollow-cathode lamp and a Fabry-Pérot étalon to calibrate the pixel-wavelength correspondence for SPIRou. Different methods are developed for identifying the hollow-cathode lines, for calibrating the wavelength dependence of the Fabry-Pérot cavity width, and for combining the two calibrators. Results. The hollow-cathode spectra alone do not provide a sufficiently accurate wavelength solution to meet the design requirements of an internal error of < 0.45 m s−1, for an overall RV precision of 1 m s−1. However, the combination with the Fabry-Pérot spectra allows for significant improvements, leading to an internal error of ∼0.15 m s−1. We examine the inter-night stability, intra-night stability, and impact on the stellar RVs of the wavelength solution.
Highlights
The spectroscopic method of exoplanet detection, known as the radial velocity (RV) method, has proven itself extremely productive, with over 700 exoplanets discovered by this method1
Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT), which is operated from the summit of Maunakea by the National Research Council of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii
We found that fixing the input calibrations introduces large drifts between the wavelength solutions for different nights, of the order of ∼10−50 m s−1; they are calibrated out by subtracting the night-to-night median RV difference
Summary
The spectroscopic method of exoplanet detection, known as the radial velocity (RV) method, has proven itself extremely productive, with over 700 exoplanets discovered by this method. Two main instrumental approaches have been used for wavelength calibration: iodine cells and hollow-cathode (HC) lamps. Each of these calibrators has advantages and disadvantages. Baranne et al 1996) The advantage of these lamps is that the thorium lines cover a much larger wavelength domain than the iodine lines, and – as they are not superimposed on the stellar spectrum – fainter targets can be observed. An overview of the main spectrographs operating in the visible, their wavelength calibrators, and measurement precisions, is given in Fischer et al (2016). Fabry-Pérot étalons provide lines that are evenly spaced in frequency, but whose wavelengths need to be derived by anchoring to an absolute calibrator, such as an HC lamp
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