Research Byte
Published in the RSAA Lunations
Vol1 Issue28 1–31 May 2022
Galactonomics: young and poor, old and rich, mergers and acquisitions.
As stars change their densities through their lifetimes, the measurement of sound waves propagating through their structure opens up the possibility of determining stellar ages to great precision. This is the idea at the core of asteroseismology, the study of stellar oscillations. In the last decade, this field has evolved from a pioneering technique to a well established field. This revolution has been driven by ultra-precise space photometry which now is available for several tens of thousands of stars. Particularly interesting to age-date are red giant stars. These intrinsically bright objects can be observed across the Galaxy and with typical masses around that of the Sun, these long-lived stars provide an unprecedented fossil record of different epochs of star formation. Traditional age-dating techniques relying on the position of red giants on the HR diagram are plagued by strong degeneracies. By contrast, asteroseismic ages are finally enabling us to use red giants in a fashion not possible even with the exquisite distances from the Gaia satellite alone.
Stellar ages however are only one third of the story. To use red giants as fossils from the history of the Milky Way, we also need to know their chemistry and kinematic. In the framework where stars are born from gas that is chemically enriched by different stellar generations, old stars are expected to be preferentially metal poor while young stars are metal rich. A surprise from asteroseismic ages has been the discovery of a non negligible number of very-young metal poor and old super-metal rich stars. Is this because stars from different generations migrate through the Galaxy more than we thought? Are some of these stars the telltale of galactic mergers? Or stellar mergers? Is stellar mass-loss muddling the ages we derive? Or is asteroseismology not as trustworthy as we believe? The existence and nature of these anomalous stars lies at the cross-road of stellar and Galactic astrophysics and demand answers.
This topic is the subject of the PhD thesis of Arthur Alencastro Puls, who has used data obtained through ANU Keck time to carry out one of the most detailed chemical inventories of anomalous asteroseismic targets selected from the SAGA survey. To improve upon the precision of his analysis, Arthur has combined spectroscopy with photometry, state of the art asteroseismic grid-based modelling and dynamical analysis. In the paper dedicated to the metal poor stars, Arthur has shown that stars selected due to their anomalously young ages and metal poor content are likely evolved blue stragglers, i.e. the product of stellar mergers. Asteroseismic ages can be trusted (that is, if surface effects are included in grid-based modelling) and half of the remaining metal poor stars are from the likely acquisitions of stars that the Milky Way did some 10 billion years ago at the expense of other galaxies. The exquisite sensitivity of asteroseismology to stellar ages has also shown that published Gaia distance errors are underestimated by a couple of percent.
For the remainder of the sample, Arthur is now uncovering nucleosynthetic trends against stellar ages and other parameters. This includes the firm presence of old super-solar metallicity stars, telltales of stellar migration and of the prompt chemical enrichment of the early Milky Way. The hunt for more answers is now on, with bigger samples coming from ESPRESSO@VLT and HDS@Subaru time!
Arthur's team at the ANU includes his supervisors David Yong and Luca Casagrande, PhD student Stephanie Monty and Ken Freeman. Their study is published here.
Luca Casagrande
