The GALAH team is pleased to announce our second data release on 18 April 2018!

GALAH is a large-scale stellar spectroscopic survey project exploring the history of star formation, chemical evolution, stellar migration and minor mergers in the Milky Way. One of the main science goals of GALAH is to use chemical tagging to identify groups of stars that formed at the same place and time, even though they have dispersed throughout the Galaxy, using the similarity of their chemical abundance patterns. Using the HERMES spectrograph at the Anglo-Australian Telescope, we collect high-resolution (R~28,000) spectra for 360 stars simultaneously. These stars are selected simply in magnitude (12<V<14), Galactic latitude (|b|>10) and declination (dec<+10) to maximise our ability to learn about the underlying structure of the Galaxy.

There are 342,682 unique stars in DR2, observed between January 2014 and January 2018. For these stars, we have derived radial velocities, stellar parameters, and the abundances of up to 23 elements per star. For full details on DR2, as well as search access, visit https://docs.datacentral.org.au/galah/. The table can also be queried using TAP at https://datacentral.org.au/vo/tap.

GALAH DR2 expands the range of publicly available GALAH data dramatically, in terms of position in the Galaxy and detail per star. The majority of dwarfs are located within 1 kpc, but the giants extend to 8 kpc, sampling a radial range from the Galactic centre past the edge of the thick disk. GALAH DR2 provides radial velocity, stellar parameters and abundances of up to 23 elements per star.

• We determine radial velocity in two ways, by cross-correlation against a grid of synthetic spectra and by constructing median spectra in small well-populated regions in Teff - log(g) - [Fe/H] parameter space and then cross-correlating against those. The latter radial velocities also include corrections for atmospheric convection and gravitational redshift. Typical errors on the velocity measurements are small: for the first method, 50% of errors are below 140 m/s and 95% of errors are below 600 m/s. For the second method, 50% of errors are below 75 m/s and 95% of errors are below 240 m/s.

• The median spectra generated for RV determination form a high-quality observational spectroscopic library that spans the well-populated regions of the H-R diagram. They can be downloaded from the "Additional data" section of the GALAH page at AAO Data Central.

• Stellar parameters and abundances are determined in a two-step process, in which a classical analysis for a subset of stars is used to train the data-driven algorithm The Cannon, which then calculates parameters and abundances for all GALAH stars based on their spectra. Full details of GALAH data analysis and the quality of the results is given in the data release paper.

The 23 elements included in DR2 are:

• Li, C, O (light elements)
• Na, Al, K (light odd-Z elements)
• Mg, Si, Ca, Ti (alpha elements)
• Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn (iron peak elements)
• Y, Ba, La (s process elements)
• Eu (r process element)

GALAH team members have written a number of papers using the DR2 data set. Three of them have been published recently:

• Kos et al. 2018a, "The GALAH survey: chemical tagging of star clusters and new members in the Pleiades"
• Duong et al. 2018, "The GALAH survey: properties of the Galactic disc(s) in the Solar neighbourhood"
• Quillen et al. 2018, "The GALAH survey: Stellar streams and how stellar velocity distributions vary with Galactic longitude, hemisphere and metallicity"

An additional papers are being made available on the arXiv preprint server on 18 April:

• Buder et al. 2018a, "The GALAH survey: Second data release"
• Buder et al. 2018b, "The GALAH survey: Chemical compositions, ages, and kinematics of the GALAH + TGAS sample"
• Simpson et al. 2018a, "The GALAH survey: co-orbiting stars and chemical tagging"
• Zwitter et al. 2018, "The GALAH survey: Accurate radial velocities and library of observed stellar template spectra"
• Simpson et al. 2018b, "The GALAH and TESS-HERMES surveys: high-resolution spectroscopy of luminous supergiants in the Magellanic clouds and bridge"
• Kos et al. 2018b, "Holistic spectroscopy: complete reconstruction of a wide-field, multi-object spectroscopic image using a photonic comb"
• Gao et al. 2018, "The GALAH survey: Verifying abundance trends in the open cluster M67 using non-LTE spectroscopy"

The press release about GALAH DR2 can be found at this link

Questions about GALAH DR2? Email s.martell -at- unsw.edu.au