TILARA: Template-Independent Line-by-line Algorithm for Radial velocity Analysis

I. Description of the code and application to a Sun-like star

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

^★ Corresponding author.

Received: 13 November 2025
Accepted: 4 March 2026

Abstract

Context. Precise radial velocities (RVs) are commonly derived through cross-correlation or template-matching methods, both of which rely on a reference spectrum that can introduce biases when the data are variable, contaminated, or sparsely sampled. Line-by-line methods offer an alternative way to compute RVs but generally still rely on template creation, and therefore share its inherent limitations.

Aims. We introduce TILARA, a template-independent, line-by-line RV extraction code designed to derive line-by-line RVs and operate effectively even when spectral template construction is not recommended. While originally motivated by future Paranal solar ESPRESSO Telescope (PoET) disk-resolved solar observations, TILARA has been built with the flexibility to work with different stellar spectral types and instruments.

Methods. A curated list of individual absorption lines is used as a reference to automatically measure line centers with via Gaussian fitting with ARES. Then, using the reference lines list, and the lines measured with ARES on the spectra of the target star, TILARA computes the RVs and applies configurable outlier rejection through sigma-clipping or down-weighting methods.

Results. We tested different configurations of the code, RV uncertainty estimation methods, and line selection criteria. The code was applied to 520 ESPRESSO observations of the Sun-like star HD 102365 to evaluate its performance. TILARA was then tested against other RV extraction methods. Both in its sigma-clipping and its down-weighting mode, TILARA provided resulting RV time series with similar standard deviation and error bars as the ones derived using existing methods that follow different approaches.

Conclusions. Our results demonstrate that TILARA can deliver precise, template-free RV measurements and effectively reduce the impact of spectral outliers. Its flexibility makes it well suited for both current ultraprecise spectrographs and future applications to spatially resolved solar observations. Additionally, the ability to easily modify the line list allows TILARA to be adapted to explore how various spectral features or physical phenomena affect RV measurements.