The work of Poul Erik Nissen from Aarhus University in Denmark has been an important source of inspiration for my research. Here some examples
The first paper I’ve intensively used is the one entitled "Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics” published in 2010. At that time I was finishing my PhD thesis on understanding the timescales for the formation of the Galactic halo, finding that indeed the halo was dominated by one very old population, formed probably very rapidly. We didn’t have much information about distances and chemical compositions of halo stars at the time, and this paper opened interesting possibilities to answer fundamental questions about the formation of the halo.
Stars in the halo could have different abundance patterns which could reflect different formation histories. We could study which stars formed in the Milky Way and which formed outside and accreted later on into the Galaxy. How many of these formed outside? How can we best find them? When did they form? How do they help us constraining the accretion history of the Milky Way? Ever since that publication I have been working, with students and colleagues, on understanding the difference between in-situ and accreted halo stars. We’ve studied the age difference of these populations, and found other key chemical elements that would help us selecting these objects from large spectroscopic surveys. That has helped us to study several properties of accreted vs in situ stars now because we have better distances and motions from Gaia for many stars. We have been observing more of these stars with Chilean facilities at Las Campanas, to characterise their full chemical patterns and so get detailed properties.
Five years after that publication, in 2015, P.E. Nissen published a revolutionary paper analysing near-by stars. That work, entitled "High-precision abundances of elements in solar twin stars. Trends with stellar age and elemental condensation temperature " showed how stars like the Sun, when studied with very high precision by a comparative analysis of their spectra with respect to the solar spectrum, could be used to determine very precise properties such as ages and chemical patterns. With that precision, it became possible to measure the rate of enrichment of different chemical elements. That sample had everything I needed to test my theory of stellar phylogenies, because I knew the differences in chemical compositions were due to chemical evolution and hence inherited between stellar generations. We made our first phylogenetic tree with this precious sample, defining a brand-new and powerful way to study the evolution of the Milky Way and finding the chemical elements that are most evolutionarily informative in the disk.
Five years later, in 2020, continuing using the technique of high precision abundance analysis, P.E. Nissen published another key paper on disk stars entitled "High-precision abundances of elements in solar-type stars. Evidence of two distinct sequences in abundance-age relations”. The age-metallicity relation has been for decades one of the most fundamental diagrams to study chemical evolution in the disk. It shows a high scatter that is normally explained by the effects of stellar radial migration, namely that stars formed in different parts of the disk which trace different chemical enrichment histories migrate to the solar neighbourhood. If precision in ages however increase, it is possible to find structure in the age-metallicity relation, and so constrain better the effects of migrations and star formation rates across the disk.
I was once more blown by that result, and thought of taking advantage of large datasets to see if such structures in the age-metallicity relation could be identified. Since ages aren’t as precise as in Nissen’s work, I came up with an alternative of using other abundance ratios which can relate to the age, finding the dual sequences in the solar neighbourhood! I worked on this last paper, alone, from home, trying to focus my mind on what I found scientifically exciting while forgetting about the craziness of the Pandemic and the pressure of other online academic commitments. I'm super excited to share these results with the community now!
The impact of this work remains to be seen over the next 5 years, until P.E Nissen publishes one more inspirational paper that will drive my scientific mind towards new avenues. Thanks Poul Erik!