Radial velocity and atmospheric parameter calculations for the GaiaNIR spectrograph

The upcoming GaiaNIR mission is currently planning to add a near-infrared spectrograph to its payload in order to enhance its scientific return, particularly for mapping the dust-obscured regions of the Milky Way. We identify the optimal wavelength region between 800 and 2300 nm for the proposed GaiaNIR spectrograph to maximize the precision for radial velocities and atmospheric parameters. To determine its spectral range, we generated 10,000 synthetic spectra from the BOSZ library across a wide range of stellar parameters, with resolutions varying from 5000 to 20,000. By cross-correlating these mock observations with ideal templates, we assessed the statistical scatter of the velocity residuals to isolate six candidate windows for further testing of the atmospheric parameters. Our analysis found that the 1926 - 1968 nm window at R = 16100 - 20100 in the K band is the preferred strategic choice because it has the potential to reach a radial velocity precision for the brightest FGKM stars of about 160 - 260 depending on the resolution while providing a precision of the atmospheric parameters close to that of the Gaia Radial Velocity Spectrometer. We also identified a second region between 1158 and 1202 nm (R = 9300 - 11600) that has a slightly lower radial velocity precision, but at a wider temperature range than the K band. Both regions enable the derivation of abundances of ten species at these resolutions: O, Na, Mg, Si, Ca, Ti, V, Cr, Mn, and Ni in the K band and Mg, Si, K, Ca, Ti, V, Cr, Mn, Co, and Ni between 1158 - 1202 nm. The K band delivers sufficiently precise measurements for the primary cool star targets of the mission while taking advantage of significantly lower interstellar extinction to enable the mapping of the dust-obscured regions of the Milky Way.