[Paper Review] Gaia Data Release 3: Hot-star radial velocities

R. Blomme, Y. Fremat+39 more|arXiv (Cornell University)|Jun 11, 2022

Astronomy and Astrophysical Research4 citations

TL;DR

This paper presents the first radial velocity measurements for hot stars (7000–14,500 K) in Gaia Data Release 3, achieved through a specialized pipeline module that corrects systematic biases caused by template mismatch between cool-star templates and hot-star spectra. The key contribution is a magnitude-dependent correction formula that reduces radial velocity offsets, improving accuracy for 543,017 hot stars with grvs_mag ≤ 12 mag.

ABSTRACT

The second Gaia data release, DR2, contained radial velocities of stars with effective temperatures up to Teff = 6900 K. The third data release, Gaia DR3, extends this up to Teff = 14,500 K. We derive the radial velocities for hot stars (i.e. in the Teff = 6900 - 14,500 K range) from data obtained with the Radial Velocity Spectrometer (RVS) on board Gaia. The radial velocities were determined by the standard technique of measuring the Doppler shift of a template spectrum that was compared to the observed spectrum. The RVS wavelength range is very limited. The proximity to and systematic blueward offset of the calcium infrared triplet to the hydrogen Paschen lines in hot stars can result in a systematic offset in radial velocity. For the hot stars, we developed a specific code to improve the selection of the template spectrum, thereby avoiding this systematic offset. With the improved code, and with the correction we propose to the DR3 archive radial velocities, we obtain values that agree with reference values to within 3 km/s (in median). Because of the required S/N for applying the improved code, the hot star radial velocities in DR3 are mostly limited to stars with a magnitude in the RVS wavelength band <= 12 mag.

Motivation & Objective

• To extend radial velocity measurements in Gaia DR3 to hot stars with effective temperatures up to 14,500 K, beyond the previous limit of 7000 K.
• To address systematic radial velocity offsets caused by using cool-star templates for hot stars, which misalign calcium infrared triplet lines with Paschen hydrogen lines.
• To develop and implement a dedicated module in the radial velocity pipeline to improve template selection and reduce systematic errors.
• To provide a correction formula for magnitude-dependent residual offsets in radial velocities for hot stars with grvs_mag ≤ 12 mag.
• To ensure reliable radial velocity data for hot stars by removing 20,470 stars previously assigned incorrect templates due to classification mismatches.

Proposed method

• Developed a specialized module within the Gaia radial velocity pipeline to improve template selection for hot stars, particularly addressing spectral line misalignment between Ca II triplet and Paschen lines.
• Re-evaluated 20,470 hot stars previously assigned cool-star templates, removing those with systematic velocity offsets due to template mismatch.
• Applied a signal-to-noise (S/N) threshold of grvs_mag = 12 mag to ensure reliable radial velocity measurements, filtering out fainter, lower-S/N sources.
• Calibrated a magnitude-dependent correction using reference data, deriving the formula: RV_corrected = radial_velocity - 7.98 + 1.135 × grvs_mag.
• Used synthetic spectra and astrophysical parameters from CU8 (e.g., T_eff, log g, [M/H]) to refine template matching and improve spectral fitting.
• Validated results against external reference data, identifying and correcting systematic biases in the initial pipeline output.

Experimental results

Research questions

• RQ1 What causes systematic radial velocity offsets when using cool-star templates for hot stars in Gaia DR3?
• RQ2 How can the radial velocity pipeline be improved to accurately measure velocities for hot stars with T_eff > 7000 K?
• RQ3 What magnitude threshold ensures reliable radial velocity measurements for hot stars in the GRVS spectra?
• RQ4 Can a magnitude-dependent correction formula effectively reduce residual systematic offsets in hot-star radial velocities?
• RQ5 To what extent do template parameters (T_eff, log g, [M/H]) in DR3 accurately reflect the true spectral type of hot stars?

Key findings

• Gaia DR3 includes radial velocities for 543,017 hot stars with effective temperatures between 7000 K and 14,500 K, marking the first such release.
• A systematic radial velocity offset of up to ~8 km/s was observed when using cool-star templates for hot stars due to misalignment between Ca II triplet and Paschen lines.
• After reprocessing with the improved template module, 20,470 stars previously assigned incorrect templates were removed from the DR3 archive.
• The magnitude-dependent correction formula RV_corrected = radial_velocity - 7.98 + 1.135 × grvs_mag effectively reduces residual offsets for stars with 6 ≤ grvs_mag ≤ 12 mag.
• The correction was derived from median offsets between DR3 radial velocities and reference data, with individual stars showing variable behavior.
• The final radial velocity measurements are reliable only for stars with grvs_mag ≤ 12 mag, due to the S/N requirement for accurate line profile fitting.