[논문 리뷰] Gaia Data Release 2: Specific characterisation and validation of all-sky Cepheids and RR Lyrae stars

G. Clementini, V. Ripepi외 22명|arXiv (Cornell University)|2018. 05. 05.

Stellar, planetary, and galactic studies참고 문헌 80인용 수 85

한 줄 요약

본 논문은 Gaia DR2 SOS Cep&RRL pipeline를 수정하여 Cepheids와 RR Lyrae 별에 대한 모든 하늘 Gaia G, G_BP, G_RP 시계열을 처리하고, 다중 밴드 광도측정, 분류 및 유래 항성 매개변수를 대규모 전하늘 샘플에 대해 제공하는 내용을 설명한다.

ABSTRACT

The Gaia second Data Release (DR2) presents a first mapping of full-sky RR Lyrae stars and Cepheids observed by the spacecraft during the initial 22 months of science operations. The Specific Object Study (SOS) pipeline, developed to validate and fully characterise Cepheids and RR Lyrae stars (SOS Cep&RRL) observed by Gaia, has been presented in the documentation and papers accompanying the Gaia first Data Release. Here we describe how the SOS pipeline was modified to allow for processing the Gaia multiband (G, G_BP and G_RP) time series photometry of all-sky candidate variables and produce specific results for confirmed RR Lyrae stars and Cepheids that are published in the DR2 catalogue. The SOS Cep&RRL processing uses tools such as the period-amplitude and the period-luminosity relations in the G band. For the analysis of the Gaia DR2 candidates we also used tools based on the G_BP and G_RP photometry, such as the period-Wesenheit relation in (G,G_RP). Multiband time series photometry and characterisation by the SOS Cep&RRL pipeline are published in Gaia DR2 for 150,359 such variables (9,575 classified as Cepheids and 140,784 as RR Lyrae stars) distributed all over the sky. The sample includes variables in 87 globular clusters and 14 dwarf galaxies. To the best of our knowledge, as of 25 April 2018, variability of 50,570 of these sources (350 Cepheids and 50,220 RR Lyrae stars) is not known in the literature, hence likely they are new discoveries by Gaia. An estimate of the interstellar absorption is published for 54,272 fundamental-mode RR Lyrae stars from a relation based on the G-band amplitude and the pulsation period. Metallicities derived from the Fourier parameters of the light curves are also released for 64,932 RR Lyrae stars and 3,738 fundamental-mode classical Cepheids with period shorter than 6.3 days.

연구 동기 및 목표

• Describe how the SOS Cep&RRL pipeline was adapted from DR1 to DR2 for all-sky processing of Gaia G, G_BP, G_RP time-series.
• Publish and validate multiband photometry, pulsation properties, and derived parameters (e.g., metallicity, absorption) for confirmed Cepheids and RR Lyrae stars in DR2.
• Assess limitations, misclassifications, and biases in the DR2 Cepheid/RRL samples and outline paths for improvement in future releases.

제안 방법

• Process Gaia DR2 time-series photometry (G, G_BP, G_RP) for sources pre-classified as Cepheid or RR Lyrae candidates.
• Apply Lomb-Scargle period search within RR Lyrae and Cepheid period ranges to determine P_SOS.
• Model light curves with truncated Fourier series to extract pulsation parameters and Fourier coefficients.
• Use region-specific relations (PL, PW, Bailey diagrams) and Fourier parameters to classify pulsation mode and type (including multi-mode cases).
• Derive stellar parameters such as photometric [Fe/H] from Fourier parameters (phi31 for RR Lyrae; R21/R31 for Cepheids) and estimate G-band absorption A(G) for RR Lyrae ab-type, using Gaia- and Johnson-Cousins-based transformations.
• Publish DR2 results for confirmed RR Lyrae (including RRab, RRc, RRd) and Cepheids, with caveats about parallaxes and reddening at DR2 quality.

실험 결과

연구 질문

• RQ1 What are the characteristics and classifications of all-sky Cepheids and RR Lyrae stars in Gaia DR2?
• RQ2 How can multiband Gaia time-series data (G, G_BP, G_RP) be used to validate and characterize Cepheids and RR Lyrae stars with the SOS pipeline?
• RQ3 What stellar parameters (e.g., metallicity, absorption) can be robustly estimated from Gaia photometry for these variables in DR2?
• RQ4 What are the limitations, misclassifications, and biases in the DR2 SOS Cep&RRL results, and how can they be mitigated in future releases?

주요 결과

• Gaia DR2 delivers multiband time-series photometry and characterisation for 150,359 variable stars, including 9,575 Cepheids and 140,784 RR Lyrae stars, across the sky.
• A substantial portion of the sample (50,570 sources) are new discoveries not previously known in the literature as of 25 April 2018.
• Photometric metallicities [Fe/H] are estimated for 64,932 RR Lyrae stars and 3,738 fundamental-mode Cepheids with P < 6.3 days, using Fourier parameters.
• G-band absorption A(G) is estimated for fundamental-mode RR Lyrae stars from light-curve and color information, after transforming to Johnson-Cousins passbands.
• The pipeline provides period, amplitude, mean magnitudes, and Fourier parameters in G, G_BP, G_RP, enabling CMD placement and period-amplitude analyses (Bailey diagrams).
• The DR2 processing included limitations due to preliminary parallaxes, reddening, and non-activation of some validation tools, leading to known misclassifications that are documented.