Stellar Classification and Radial Velocity Measurement

BANZAI-NRES uses cross-correlation methodology in order to measure stellar parameters and radial velocities. In all cases we cross-correlate the normalized spectra with a Phoenix model spectrum (Husser et al. 2013, DOI: 10.1051/0004-6361/201219058).

Stellar Classification

BANZAI-NRES obtains an initial guess for the stellar effective temperature by querying the Gaia catalog (Gaia Collaboration et al. 2016, DOI: 10.1051/0004-6361/201629272; Gaia Collaboration et al. 2021, arXiv:) at the coordinates of the observation. It then cross-correlates the spectra with models from a Phoenix model grid in steps of \(T_{eff}\), \(\log g\), [Fe/H], and [\(\alpha\)/Fe] around the initial guess. Accounting for the effects of stellar rotation is a potential future development. The set of parameters resulting in the highest cross-correlation peak is taken as the stellar parameter estimate and the corresponding Phoenix model is used as the cross-correlation template to generate the RV measurement.

The stellar classifications are stored in the BANZAI-NRES database. If the star is observed again, it will not be re-classified and the same template will be used for all RV measurements.

The following header keywords store the stellar classification:

  • ‘TEFF’: Stellar effective temperature (K).

  • ‘LOGG’: Stellar surface gravity (cgs units).

  • ‘FEH’: Stellar metallicity [Fe/H] (dex).

  • ‘ALPHA’: Stellar alpha abundance [alpha/Fe] (dex).

  • ‘CLASSIFY’: Equals 1 if this spectrum was classified, or 0 if the classification was taken from an previous spectrum of this target.

Radial Velocity Measurement

BANZAI-NRES measures the radial velocity of the target by cross-correlating the best Phoenix template found by the stellar classification step with the normalized spectra. This cross-correlation is performed on an order-by-order basis over 5140-6276 Angstroms, a range that has high signal-to-noise but is not affected by telluric absorption. It first computes the CCFs on a coarse grid with a wide velocity range, selects the highest peak, and then uses a fine velocity grid around that peak. The peak of each fine CCF is taken as the per-order velocity. The RV measurement is the sigma-clipped mean of the per-order velocities. We compute the barycentric correction using Astropy, and correct the RVs to the barycentric frame per Wright & Eastman (2014), DOI: 10.1086/678541.

The following header keywords contain the RV value and associated parameters:

  • ‘RV’: Radial velocity measurement in barycentric frame (m / s)

  • ‘RVERR’: Radial velocity formal uncertainty (m / s)

  • ‘BARYCORR’: Barycentric velocity correction (m / s)

  • ‘TCORR’: Mid-exposure time barycentric Julian date (BJD_TDB)

Telluric Lines

Our telluric spectrum was generated with https://github.com/kgullikson88/Telluric-Fitter . We exclude regions of the spectrum with strong telluric absorption.