The consistency of chemical clocks among coeval stars

The consistency of chemical clocks among coeval stars

Francisca Espinoza Rojas, a master student at PUC, gets her paper submitted!

Most of the chemical elements in the Universe are produced inside stars, and spread in the interstellar medium once these stars die, mixing with the existing material to form new stars. This translates in the fact that abundance ratios evolve with time, at a rate that depends on the star formation history, among other complex physical processes influencing the production and recycling of chemical abundances.

Using abundance ratios of long-lived stars and how they change with ages and positions across the Galaxy can give us hints about these different physical processes. More specifically, by finding stars that have same abundance ratios and ages can we characterise the building blocks of the Galaxy. This concept is currently called “chemical tagging”. 

There are two big challenges in chemical tagging. The first one is that some abundance ratios have strong correlations with other abundance ratios, and so the chemical dimension and uncertainties on the abundance measurements is not high enough to distinguish one building block from the other one. The second one is that, when abundance ratios are measured with very high precision in stellar groups that were born together (e.g. open clusters or wide binaries), their abundance ratios are not fully homogeneous. 

In order to address these challenges, with Caroline Soubiran and Laia Casamiquela from the Laboratoire d’Astrophysique de Bordeaux, France, and with Julio Chaname from the Pontificia Universidad Católica (PUC) de Chile, we obtained funding for a collaborative grant called ECOS-Anid, to visit each other, train students, and work on this problem. While in France Caroline and Laia have been analysing open clusters, in Chile with Julio we have mainly trained Francisca Espinoza Rojas, a master student from PUC, to look into wide binaries. 

Francisca learnt to analyse stellar spectra and determine abundances of wide binaries. These binaries have been form from the same material but are apart from each other such that they haven’t interacted. We know their chemical compositions are more consistent than random pairs, although there are exceptions, which haven’t been understood yet.

In parallel to this, previous literature work have shown that some abundance ratios have very strong correlations with stellar ages, suggesting they could be used to trace chemical enrichment history instead of ages, which is normally very difficult to determine accurately. These abundances are dubbed as “chemical clocks”.  Francisca considered this to find that chemical clocks are very consistent in her sample of wide binaries, with no exception. Furthermore, they are significantly more consistent than in random pairs, offering very interesting perspectives of using chemical clocks for chemical tagging. 

This work was recently submitted to the Astrophysical Journal, and I am proud. When Francisca started getting acquainted with stellar spectral analysis, there was an historical social unrest in Chile, making us unable to concentrate and work in person for a few months. Laia visited in the frame of ECOS-Anid, despite the protests going on in the city. We met wherever and whenever possible, and tried to help and guide Francisca to keep progressing with her work at any cost. 

Once she got her results on chemical abundances, we had to make a scientific interpretation. That was difficult once more, this time because of the Pandemic.  We had to accept to do this online, without traveling to France with the ECOS-Anid grant as initially planned, without having in-person meetings, without any of that.  

Francisca struggled  in this scenario like every student who is working alone from home.  But her drive to understand the scientific problem she was working kept her motivated. Her enthusiasm was spread in my group. Indeed, the picture was taken in October 2020, and Francisca is discussing with me and Danielle de Brito Silva, a PhD student from UDP, the draft of her paper. Working with her on this project, despite the uncomfortable conditions, has given me strength and motivation to keep doing my best, regardless of the situation. 

More information of paper here

The last work on chemical clocks of open clusters lead by Laia Casamiquela in France can be also found here.


Project funded by ECOS-Anid 180049 and Fondecyt Iniciación 11170174