It seems that evolved giant stars which are no longer burning hydrogen (via fusion) unexpectedly contain significant amounts of lithium, defying the theoretical prediction that lithium should be depleted over their lifetimes due to processes like convection or mixing. Trying to understand where this mystery lithium comes from is one of the open questions in astrophysics, motivating the authors of this paper to investigate a possible mechanism which may be responsible for enriching evolved massive stars with lithium—namely mass transfer from a binary companion. If binary interaction is an important enrichment channel, then we should technically see lithium-rich stars in binary systems at a higher rate than their non-enriched counterparts.

To get a sense of how often lithium-rich stars interact with a companion, the authors pulled together a sample of low-mass giant stars with known lithium levels and looked for signs of binary interaction. One way to investigate this is by tracking changes in a star's radial velocity or its velocity along the line of sight. If a star and its companion are orbiting about their centre of mass, then it's likely to experience periods where it's travelling towards the observer (negative radial velocity), away from the observer (positive radial velocity), or perpendicular to the observer's line of sight (zero radial velocity). Thus if a star's radial velocity is changing with time, then that's a pretty good sign that it's interacting with some other companion. If the interacting stars tend to have higher levels of lithium than solitary stars, then it's possible that the interaction may account for at least some of the unexplained lithium we're observing.

The authors arranged their sample consisting of all red giant stars with radial velocity and lithium measurements from the Gaia, GALAH, and RAVE surveys, totalling 1418 stars. They then calculated these parameters labelled w_{ij} and w_{t} for each star in their sample to determine whether it's likely found in a binary. w_{ij} describes the variation between radial velocity measurements from two different surveys (i.e., Gaia, GALAH, and RAVE), and w_{t} is the sum of the individual values of w_{ij}, with higher w values indicating a star is more likely to be in a binary. From the calculated values, we can assign a classification to each star; classes 5a and 5b indicate that a star is very likely to be interacting with a binary, and classes 1 - 4 are for stars which show little evidence of such. The authors then looked at what fraction of lithium-rich stars fell into classes 5a and 5b compared to the lithium-normal population.

Since classes 5a and 5b indicate interaction through radial velocity variation, a higher fraction of lithium-rich stars in these classes would suggest they are more often found in binaries. Looking at the figure above, it's clear that when the entire population is considered (left), there is no significant difference between the lithium-rich and lithium-normal samples in regard to the fraction of stars classed as 5a and 5b. However, when only the red giant branch stars are considered, the fraction of 5a and 5b stars is greater than the fraction for lithium-normal stars, although the large uncertainty makes the difference between the populations marginal as it stands.

So, given that radial velocity variability isn't more common in lithium-rich stars vs lithium-normal stars when considering the entire population, the authors suggest that binary interaction is not the primary mechanism responsible for the observed lithium enrichment. However, their analysis does allow for the possibility that these binary interactions are a contributing factor for stars on the red giant branch. They argue that these red giants may have a different origin to the other lithium-rich giant stars that do not show higher rates of variability. For now, it remains unclear as to where these stellar giants hijacked their lithium from.

published: 08/11/24 by kaan evcimen