Photon-weighted barycentric correction and its importance for precise radial velocities

ABSTRACT When applying the barycentric correction to a precise radial velocity measurement, it is common practice to calculate its value only at the photon-weighted mid-point time of the observation instead of integrating over the entire exposure. However, since the barycentric correction does not change linearly with time, this leads to systematic errors in the derived radial velocities. The typical magnitude of this second-order effect is of order 10 cm s−1, but it depends on several parameters, e.g. the latitude of the observatory, the position of the target on the sky, and the exposure time. We show that there are realistic observing scenarios, where the errors can amount to more than 1 m s−1. We therefore recommend that instruments operating in this regime always record and store the exposure meter flux curve (or a similar measure) to be used as photon-weights for the barycentric correction. In existing data, if the flux curve is no longer available, we argue that second-order errors in the barycentric correction can be mitigated by adding a correction term assuming constant flux.

Lessons from the curious case of the ‘fastest’ star in Gaia DR2

Gaia DR2 5932173855446728064 was recently proposed to be unbound from the Milky Way based on the $-614.3\pm 2.5\, \mathrm{km}\, \mathrm{s}^{-1}$ median radial velocity given in Gaia DR2. We obtained eight epochs of spectroscopic follow-up and find a very different median radial velocity of $-56.5 \pm 5.3\, \mathrm{km}\, \mathrm{s}^{-1}$. If this difference were to be explained by binarity, then the unseen companion would be an intermediate-mass black hole; we therefore argue that the Gaia DR2 radial velocity must be in error. We find it likely that the spectra obtained by Gaia were dominated by the light from a star $4.3\, \mathrm{arcsec}$ away, and that, due to the slitless, time delay integration nature of Gaia spectroscopy, this angular offset corresponded to a spurious $620\, \mathrm{km}\, \mathrm{s}^{-1}$ shift in the calcium triplet of the second star. We argue that such unanticipated alignments between stars may account for 105 of the 202 stars with radial velocities faster than $500\, \mathrm{km}\, \mathrm{s}^{-1}$ in Gaia DR2 and propose a quality cut to exclude stars that are susceptible. We propose further cuts to remove stars where the colour photometry is suspect and stars where the radial velocity measurement is based on fewer than four transits, and thus produce an unprecedentedly clean selection of Gaia radial velocities for use in studies of Galactic dynamics.

Error Analysis of SDSS/LAMOST Stellar Radial Velocity Measurement

We select super high quality spectra of spectral type A, F, G, K, M stars from SDSS C DR7. We mix our spectra with noise and then measure their radial velocity. The analysis shows that the accuracy of K, M type stars is much better than A, F stars. With our relationship between the signal-to-noise and accuracy of radial velocity measurement of different spectral type, we provide a good reference for stellar study related to radial velocity.

The Radial Velocity Measurement and Error Analysis for Low Resolution Spectra

Radial velocity error of the spectra always be given by the widths of the cross-correlation function of the target spectrum and template spectrum. Radial velocity error affected by many inevitable factors, caused by instrument, observation weather, etc.. Another important error comes from calibration, which should be reduced as much as possible. This paper analysis the error based on different temperatures for different types of spectral. The effect of the calibration error to the radial velocity error is also presented.