A new method of measuring center-of-mass velocities of radially pulsating stars from high-resolution spectroscopy

TL;DR

This paper introduces a novel spectroscopic method to accurately measure the center-of-mass velocities of pulsating stars, using line asymmetry analysis and synthetic LSD profile grids, applicable even with limited observations.

Contribution

It presents a new technique combining line profile asymmetry analysis with LSD to determine gamma velocities from minimal data, improving accuracy over traditional methods.

Findings

Gamma velocities determined with ±10 km/s accuracy.

Line asymmetry varies with pulsation phase, affecting projection factor.

Synthetic LSD profile grids covering all pulsation phases are provided.

Abstract

We present a radial velocity analysis of 20 solar neighborhood RR Lyrae and 3 Population II Cepheids variables. We obtained high-resolution, moderate-to-high signal-to-noise ratio spectra for most stars and obtained spectra were covering different pulsation phases for each star. To estimate the gamma (center-of-mass) velocities of the program stars, we use two independent methods. The first, `classic' method is based on RR Lyrae radial velocity curve templates. The second method is based on the analysis of absorption line profile asymmetry to determine both the pulsational and the gamma velocities. This second method is based on the Least Squares Deconvolution (LSD) technique applied to analyze the line asymmetry that occurs in the spectra. We obtain measurements of the pulsation component of the radial velocity with an accuracy of $\pm$ 3.5 km s$^{-1}$. The gamma velocity was determined with an accuracy $\pm$ 10 km s$^{-1}$, even for those stars having a small number of spectra. The main advantage of this method is the possibility to get the estimation of gamma velocity even from one spectroscopic observation with uncertain pulsation phase. A detailed investigation of the LSD profile asymmetry shows that the projection factor $p$ varies as a function of the pulsation phase -- this is a key parameter which converts observed spectral line radial velocity variations into photospheric pulsation velocities. As a byproduct of our study, we present 41 densely-spaced synthetic grids of LSD profile bisectors that are based on atmospheric models of RR Lyr covering all pulsation phases.