The GALAH Survey: Second Data Release (1804.06041v1)

Abstract: The Galactic Archaeology with HERMES (GALAH) survey is a large-scale stellar spectroscopic survey of the Milky Way and designed to deliver chemical information complementary to a large number of stars covered by the $Gaia$ mission. We present the GALAH second public data release (GALAH DR2) containing 342,682 stars. For these stars, the GALAH collaboration provides stellar parameters and abundances for up to 23 elements to the community. Here we present the target selection, observation, data reduction and detailed explanation of how the spectra were analysed to estimate stellar parameters and element abundances. For the stellar analysis, we have used a multi-step approach. We use the physics-driven spectrum synthesis of Spectroscopy Made Easy (SME) to derive stellar labels ($T_\mathrm{eff}$, $\log g$, $\mathrm{[Fe/H]}$, $\mathrm{[X/Fe]}$, $v_\mathrm{mic}$, $v \sin i$, $A_{K_S}$) for a representative training set of stars. This information is then propagated to the whole survey with the data-driven method of $The~Cannon$. Special care has been exercised in the spectral synthesis to only consider spectral lines that have reliable atomic input data and are little affected by blending lines. Departures from local thermodynamic equilibrium (LTE) are considered for several key elements, including Li, O, Na, Mg, Al, Si, and Fe, using 1D MARCS stellar atmosphere models. Validation tests including repeat observations, Gaia benchmark stars, open and globular clusters, and K2 asteroseismic targets lend confidence in our methods and results. Combining the GALAH DR2 catalogue with the kinematic information from $Gaia$ will enable a wide range of Galactic Archaeology studies, with unprecedented detail, dimensionality, and scope.

• The paper presents a comprehensive catalog offering stellar parameters and 23-element abundances for 342,682 Milky Way stars.
• The study employs a hybrid method combining physics-driven synthesis with data-driven modeling, validated against benchmark stars and Gaia data.
• The paper underscores the catalog's value for Galactic archaeology by providing actionable insights into the chemical evolution and dynamics of the Milky Way.

The GALAH Survey: Second Data Release

The second data release (DR2) from the GALAH survey offers an extensive catalog of stellar parameters and chemical abundances for 342,682 stars within the Milky Way. This effort provides a robust spectroscopic counterpart to the astrometric data from the Gaia mission, enhancing our understanding of the Galaxy's stellar population. Employing data from the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) at the Anglo-Australian Telescope, DR2 incorporates sophisticated spectral analysis techniques to determine properties for each star, including abundances for up to 23 chemical elements ranging from Lithium (Li) to Europium (Eu).

Methodology

The analysis pipeline for GALAH DR2 uses a hybrid approach, combining physics-driven spectrum synthesis with data-driven modeling to extend the determination of stellar properties to this vast cohort of stars. Initially, a representative set of training data is established through spectrum synthesis using the Spectroscopy Made Easy (SME) code, where careful attention is given to reliable atomic data and minimizing blending in spectral lines. This yields stellar parameters such as effective temperature (T_eff), surface gravity (log g), metallicity ([Fe/H]), and microturbulence (v_mic) for the training set. Non-LTE corrections are considered for key elements to enhance the accuracy of the derived abundances.

Subsequently, the data-driven model The Cannon propagates this information onto the full survey through quadratic models, enabling robust determination of both stellar parameters and individual element abundances. The approach is supplemented by careful masking of spectral lines and utilizes a flexible framework to address potential systematic biases, particularly with attention to validation against known benchmark stars, clusters, and asteroseismic measurements.

Key Results

The paper reports several significant findings stemming from DR2:

• Stellar Parameters and Abundances: The survey provides effective temperatures, gravities, metallicities, and abundances for 23 elements with notable precision. The data show that the derived metallicity and abundance scales align well with existing literature and datasets, although certain discrepancies, such as for specific elements at extreme metallicities, underline the importance of continued model developments.
• Challenges and Validation: Validation against known benchmarks and external datasets indicates good consistency for most parameters, though some systematic trends are noted, particularly for hot stars and specific elemental abundances. These are attributed to the complexities inherent in spectral modeling and the limitations of current methodologies, illustrating areas for future refinement.
• Scientific Applications: Coupled with Gaia kinematics, the GALAH data provide an unprecedented opportunity to paper Galactic Archaeology, offering insights into the Galaxy's assembly history, disk dynamics, and chemical evolution. The potential to chemically tag stars to their birth environments opens substantial avenues for constructing detailed maps of stellar populations and dynamics.

Furthermore, GALAH DR2 equips researchers with valuable data facilitating investigations beyond these immediate findings, highlighting its role as a critical resource for Galactic studies.

Implications and Future Work

The GALAH DR2 catalog stands as a testament to the powerful synergy of spectroscopic and astrometric data in modern astrophysics, serving as a foundation for investigating the Milky Way's structure and evolution. Future work may focus on refining spectral analysis techniques, addressing systematic biases, and expanding the survey's scope by incorporating additional data and non-LTE corrections for more elements. Moreover, the continued acquisition of high-resolution spectra will augment this rich dataset, advancing our exploration of the chemical and dynamic landscapes of our Galaxy.

In conclusion, the GALAH DR2 is a significant step forward in the domain of Galactic astronomy, offering comprehensive stellar data that enhance both theoretical and observational frameworks, facilitating a deeper understanding of the Galaxy and its evolutionary processes.