[Paper Review] Gaia Data Release 3: Properties of the line broadening parameter derived with the Radial Velocity Spectrometer (RVS)

Y. Frémat, F. Royer+39 more|arXiv (Cornell University)|Jun 22, 2022

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper presents the first comprehensive analysis of the line broadening parameter (vbroad) derived from Gaia Data Release 3 using the Radial Velocity Spectrometer (RVS), measuring broadening from 3500 K to 14,500 K down to magnitude 12. It finds that vbroad estimates are generally consistent with other surveys but suffer from biases at high temperatures (>7500 K) and faint magnitudes (G_RVS > 10), primarily due to template mismatches and blending of hydrogen Paschen and Ca II triplet lines.

ABSTRACT

The third release of the Gaia catalogue contains the radial velocities for 33,812,183 stars having effective temperatures ranging from 3100 K to 14,500 K. The measurements are based on the comparison of the observed RVS spectrum (wavelength coverage: 846--870 nm, median resolving power: 11,500) to synthetic data broadened to the adequate Along-Scan Line Spread Function. The additional line-broadening, fitted as it would only be due to axial rotation, is also produced by the pipeline and is available in the catalogue (field name gaia_source:vbroad). To describe the properties of the line-broadening information extracted from the RVS and published in the catalogue, as well as to analyse the limitations imposed by the adopted method, wavelength range, and instrument. We use simulations to express the link existing between the line broadening measurement provided in Gaia Data Release 3 and Vsin(i). We then compare the observed values to the measurements published by various catalogues and surveys (GALAH, APOGEE, LAMOST, ...). While we recommend being cautious in the interpretation of the vbroad measurement, we also find a reasonable global agreement between the Gaia Data Release 3 line broadening values and those found in the other catalogues. We discuss and establish the validity domain of the published vbroad values. The estimate tends to be overestimated at the lower vsini end, and at $T_\mathrm{eff}>7500\,\mathrm{K}$ its quality and significance degrade rapidly when $G_\mathrm{RVS}>10$. Despite all the known and reported limitations, the Gaia Data Release 3 line broadening catalogue contains the measurements obtained for 3,524,677 stars with $T_\mathrm{eff}$\ ranging from 3500 to 14,500 K, and $G_\mathrm{RVS}<12$. It gathers the largest stellar sample ever considered for the purpose, and allows a first mapping of the \Gaia\ line broadening parameter across the HR diagram.

Motivation & Objective

• To characterize the properties and reliability of the line broadening parameter (vbroad) derived from Gaia's Radial Velocity Spectrometer (RVS) in Data Release 3.
• To identify systematic biases in vbroad measurements, particularly at high temperatures and faint magnitudes.
• To assess the impact of spectral template mismatches—especially for hot stars—on the accuracy of vbroad estimates.
• To evaluate the influence of unresolved binary stars on vbroad measurements through spectral variability and profile distortions.
• To provide guidance for users on the reliability of vbroad values by defining domains where vbroad is expected to be consistent with Vsin i.

Proposed method

• The vbroad parameter is derived from the cross-correlation function (CCF) computed over the entire RVS wavelength range (847–874 nm), integrating broadening effects not captured by synthetic spectra.
• The method uses a single, global CCF to estimate the total line broadening, including contributions from Vsin i, macroturbulence, and instrumental effects.
• The analysis compares vbroad values with external surveys and compilations to validate consistency and identify discrepancies.
• Spectral quality and variability are assessed through visual inspection of RVS spectra, particularly for stars on the main sequence with suspected binarity.
• The impact of temperature, magnitude, and metallicity on vbroad accuracy is quantified using color-magnitude diagrams and HRD distributions.
• The study evaluates the sensitivity of vbroad to template mismatches by analyzing the dependence on effective temperature and G_RVS magnitude.

Experimental results

Research questions

• RQ1 How accurate and precise are the vbroad measurements derived from Gaia DR3 RVS data across the full stellar temperature and magnitude range?
• RQ2 What are the dominant sources of bias in vbroad estimates, particularly for hot stars (T_eff > 7500 K) and faint targets (G_RVS > 10)?
• RQ3 To what extent do unresolved binary systems contribute to spurious or inflated vbroad values?
• RQ4 How does the choice of spectral template affect vbroad measurements, especially when the template-metallicity does not match the observed star?
• RQ5 In which regions of the HR diagram can vbroad be reliably interpreted as a proxy for Vsin i?

Key findings

• The Gaia DR3 vbroad catalogue provides the largest homogeneous survey of line broadening down to magnitude 12 and across a temperature range from 3500 K to 14,500 K.
• For stars hotter than 7500 K, vbroad accuracy and precision degrade rapidly with increasing magnitude, particularly above G_RVS = 10, due to blending of Paschen and Ca II triplet lines.
• A significant fraction (17%) of main-sequence stars in the G_BP - G_RP range 1.1–1.4 show residual vbroad estimates despite post-processing cleaning, indicating persistent contamination from unresolved binaries.
• Spectral variability and profile distortions in unresolved binaries lead to a statistical overestimation of vbroad, as confirmed by visual inspection of RVS spectra.
• The HRD shows a median vbroad variation consistent with magnetic braking in cool stars (around F5), but also hints at metallicity mismatch effects in the template-fitting process.
• Table 3 provides a reliable domain for vbroad where values are expected to be consistent with Vsin i, particularly for stars with T_eff < 7500 K and G_RVS ≤ 10.