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Advanced stellar spectroscopy in the Gaia era

Final Report Summary - GAIASPECTROSCOPY (Advanced stellar spectroscopy in the Gaia era)

This document summarizes the progress made and actions taken by the grant holder (Lind) from the beginning of the project duration (13/03/2014) until the present day. It is divided into subcategories for convenience. Concluding remarks and a publication list are given at the end.

Galactic archaeology with stellar surveys

With >400 co-investigators, the Gaia-ESO Public Spectroscopic Survey has grown to dominate the research field of Galactic archaeology in Europe, now more than half-way through its planned 5-year duration. Lind is heavily involved in the data analysis of this unprecedented survey and was awarded so called “builder” status for her efforts in 2013. Since March 2014, she has co-authored 13 Gaia-ESO publications and first-authored one. In addition to data-release and calibration, they describe how survey data have been used to conduct chemo-dynamical studies of clusters and field stars in the Galactic disk (e.g. Bergemann et al. 2014, Recio-Blanco et al. 2014, Magrini et al. 2014, Ruchti et al. 2015, Kordopatis et al 2015).
The publication lead by Lind describes the discovery of the first halo star with strong anomaly in its Mg and Al abundances, a tell-tale signature of a globular cluster origin (Lind et al. 2015).
Combining the chemical patterns with backwards orbit-integration, the star was tentatively suggested as an escapee from the massive cluster Omega Centauri – a prediction that, if confirmed by more detailed measurements, would be the first practical realisation of the concept of “chemical tagging” (linking long-lost stellar debris to a progenitor cluster). The spectrum analysis method developed by Lind for the Gaia-ESO Survey has proven highly successful in the competition with other leading analysis teams in Europe. Recently, Lind singlehandedly analysed spectra for ~40 000 stars included in the fourth internal data release, more than any other of the thirteen analysis nodes currently involved. Further, the stellar parameters resulting from Lind’s pipeline have been assessed to have the smallest systematic errors and therefore selected as reference scale for the survey homogenization work (Figure 1).
Lind has recently adapted her method to enable analysis of spectra of the GALAH survey of half a million bright stars in the solar neighborhood (De Silva et al. 2015). Her pipeline will be exclusively used for abundance analysis of all targets. Finally, she has taken lead of the work package (~35 people) for parameter and abundance analysis of all Galactic sources observed by 4MOST, a next generation multi-object spectrograph that will see first light in ~2020 (de Jong et al. 2014). During her time as a Marie-Curie Fellow Lind has participated in several conferences and workshops for the Gaia-ESO Survey, the GALAH Survey and the 4MOST project.

NLTE radiative transfer

One of the main goals of the projects is to construct a model atom library for spectral-line formation calculations in non-local thermodynamic equilibrium (NLTE). Lind is co-supervising two PhD students at Uppsala University whose thesis projects involve such calculations. Yeisson Osorio successfully defended his thesis in June 2015. He has constructed a new model atom for magnesium and demonstrates the impact on abundance determination of cool stars in two publications published in A&A (Osorio et al. 2015). Osorio will continue with calcium. Thomas Nordlander is conducting a similar study for aluminum and a first publication is expected soon.
Both Mg and Al are astrophysically crucial elements and the new atomic models are free from the large uncertainties in collisional data that plague earlier work.
Lind is further collaborating with PhD student Anish Amarsi at the Australian National University. Amarsi has calculated oxygen NLTE line formation in 3D radiation-hydrodynamical model atmospheres. During Amarsi’s research visit to Uppsala in April 2015, significant improvements were made to the MPI-parallelised 3D, NLTE code such that calculations even for extremely complex species (e.g. neutral iron) are now numerically feasible. Lind has also developed a method to simplify complex atoms, while preserving NLTE behavior, to increase numerical efficiency. These results are being written up for publication in MNRAS.
The situation is now such that Lind and her colleagues have well-constrained model atoms for almost all of the astrophysically important lighter elements (Li, Na, O, Mg, Al, Si, Ca). Lind is preparing a publication presenting a new model atom library for all iron peak elements, Z=21-30, i.e. Sc-Zn, aiming for a publication 2016. The library makes use of last few years’ progress in the calculations of energy levels and transition probabilities (Kurucz 2007-2014) as well as collisional cross sections (Barklem et al. in prep). Once finalized, the model atoms will be used for NLTE analysis of large survey data sets, which is currently done only for a few elements.
Largely because of her expertise in NLTE line formation calculations, Lind has previously established close collaborations with colleagues at Australian National University. Since the start of the project in March 2014, this has resulted in two co-authored publications on various aspects of multiple populations in metal-poor globular clusters (Marino et al., 2014, 2015) and four publications on the abundances patterns of the most metal-poor stars in the halo (Jacobsson et al. 2014, Bessel et al. 2015) and the bulge (Howes et al. 2014, 2015). Lind was also asked to perform NLTE calculations for the Sun for Na, Mg, and Ca, for the latest revision of the solar chemical composition (Scott et al. 2014).

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