Stellar ages and lithium abundances. The study: “The Gaia-ESO Survey: empirical estimates of stellar ages from lithium equivalent widths (EAGLES)” of R. D. Jeffries (Keele University) appeared on MNRAS
Estimating stellar ages can prove to be a challenging task. The methods typically employed for estimating stellar ages rely on our understanding of how fundamental stellar properties that can be observed change as stars age. This often results in measurements that lack precision, particularly for stars in the main sequence, which is a stable evolutionary phase during which stellar properties remain relatively constant over time. In this context, stellar clusters hold significance as valuable datasets. Stellar clusters, in fact, provide abundant collections of stars with varying properties but the same age. This generally enables astronomers to determine stellar ages with greater accuracy, even though these estimations always depend on evolutionary stellar models.
Over the past few decades, a method has been developed to estimate stellar ages through the measurement of lithium abundance. This chemical element is rapidly depleted within the stellar interior, specifically in layers where the temperature exceeds 2.5 million degrees. This leads to a quick reduction in lithium abundance, particularly during the pre-main sequence phase. The rate of lithium consumption is determined by the structure of the stellar interior, and consequently, by the stellar mass. In fact, low-mass stars remain fully convective, resulting in rapid lithium consumption. Conversely, more massive stars rapidly develop a radiative core during their pre-main sequence phase, which diminishes the extent of the convective zone. If the internal temperature at the base of the convective zone falls below 2.5 million degrees, lithium burning ceases. This complex interplay gives rise to intricate relationships between the timescale of lithium burning and stellar properties, including mass and temperature. The complexity is even larger for main sequence stars, where a wide range of lithium abundances is observed. This suggests that additional factors, such as stellar rotation and magnetic activity, might exert a significant influence. As an example, the lithium abundance in the Sun is two orders of magnitude lower than that in other main sequence stars of solar type. Presently, current stellar models can not explain this discrepancy and the wide range of lithium abundances observed in old stars.
The team of researchers led by astrophysicist R. D. Jeffries from the Astrophysics Group at Keele University has developed an model that enables the determination of stellar ages based on lithium abundances. The model is empirical, in order to explain the evidence that theoretical models are not able to explain, and it has been calibrated using measurements of lithium abundances in stars (excluding giants) of 52 star clusters. This extensive collection of stellar samples covers an age range spanning from 2 million to 6 billion years, temperatures ranging between 3000 and 6500 K, and a wide spectrum of metallicity (representing the abundance of heavy elements compared to hydrogen). The data used were obtained from the Gaia-ESO Survey (GES), which was conducted using the FLAMES spectrograph mounted on the Very Large Telescope at the European Southern Observatory (ESO). GES provided spectra from over 100000 stars, which were uniformly analyzed.
The model effectively predicts the ages of M-type stars ranging from 10 to 100 million years, K-type stars from 30 to 300 million years, and G-type stars from 100 million to 1 billion years. The accuracy of predicted ages diminishes for older stars, due to both their lower lithium abundances and the stability of their physical properties. For these stars, however, the model can still provide limits which can be used in combination with other methods that provide stellar ages.
The model has been applied to a sample of ten stars known to host exoplanets and recognized as being younger than 300 million years. This application confirmed the youth of these stars, with the exception of KOI-7913 and Kepler 16343. Additionally, the model was used for six moving stellar groups, affirming their youth age. These findings are detailed in the paper titled “The Gaia-ESO Survey: empirical estimates of stellar ages from lithium equivalent widths (EAGLES)” recently published in the Monthly Notices of the Royal Astronomical Society (MNRAS). Among the coauthors is astronomer L. Prisinzano from INAF – Astronomical Observatory of Palermo.
The figure (click here to view it in full) displays two spectra of stars in the 25 Ori cluster (panels a and b). These two stars exhibit distinct effective temperatures and rotational velocities. The vertical blue lines delineate the spectral region employed for measure the equivalent width of the lithium absorption line at 6708 Å, a value that astronomers utilize to determine the lithium abundance. Panel c) illustrates a scatter plot showing the relationship between lithium abundances and stellar effective temperatures within the star clusters of the training set, which have been utilized for model calibration.
