Title: Carbon enrichment in APOGEE disk stars as evidence of mass transfer in binaries
Authors: Steve Foster, Ricardo P. Schiavon, Denise B. de Castro, Sara Lucatello, Christine Daher, Zephyr Penoyre, Adrian Price-Whelan, Carles Badenes, José G. Fernández-Trincado, Domingo Aníbal García-Hernández , Jon Holtzman, Henrik Jönsson, and Matthew Shetrone
First Author’s Institution: Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
Status: Published to ApJ, July 2024
Background
One of the most essential aspects of stars is that they are, broadly speaking, fossil records of the molecular cloud that they formed from. This property allows astronomers to study the chemical composition of the Milky Way billions of years ago by simply looking at stars at a certain age! This is how we know so much about the chemical evolution of galaxies. It also allows us, in indirect ways such as , to study how stellar chemistry changes during a star’s lifetime.
One of the most interesting potential changes is in s-process, or “slow-neutron capture” elements”, or elements like Barium and Cerium. These elements are formed in the upper atmospheres of some evolved stars known as asymptotic giant-branch (AGB) stars and are usually blown away by stellar winds. However, interesting things can happen if an AGB star is in a binary.
Figure 1: A periodic table colored by how different elements are formed in the universe. All the elements colored in blue, cyan, and green are largely expected to be invariant in stars during their lifetime. However, s-process elements here are colored in yellow, and these are elements expected to change as stars ascend the asymptotic branch. Notably, there is no way for a star to be extra enriched in s-process material prior to the AGB phase. So, if a red giant star were to be found with unusually high amounts of s-process elements, that implies it was enriched externally, likely through mass transfer from a higher mass companion. Figure from Jennifer Johnson, Ohio State University, and SDSS.
What Did They Do?
This brings us to today’s paper! The authors set out to test this binary mass transfer enrichment theory by constraining the correlation between binary fraction and s-process enhancement. To accomplish this, they used data from the Sloan Digital Sky Survey’s APOGEE 2 mission, which took medium-resolution near-infrared spectra of over 700,000 stars in the Milky Way. From these spectra, the APOGEE team calculated chemical abundances for ~20 elements, one of which is an s-process element: Cerium.
Figure 2: Here, we show the sample of stars that the authors studied. As you can see, the entire sample consists of stars on the red giant branch. The selection of these particular stars is essential as they span a large set of ages, metallicities, and galactic positions. However, they are all bright enough that APOGEE is able to measure good Cerium abundances. Figure 1 in the paper.
Because Cerium is a challenging element to measure (due to having very weak absorption lines), the authors focused on studying only giant stars in the Milky Way disk. They then used existing catalogs of known binaries, identified using the radial-velocity method, to separate their sample into single stars and binary stars. The authors then binned their sample by metallicity ([Fe/H]), carbon abundance ([C/Fe]), and cerium abundance ([Ce/Fe]). Within these bins, they were able to calculate the expected values for each abundance, as well as the standard deviation. Finally, from these values, they could characterize how anomalously enriched each star was in Cerium and how that links to binarity.
So What Did They Find?
Figure 3: A plot showing the number of standard deviations away from the median [Ce/Fe] compared to the binary fraction for that sample. From this plot, it’s clear that as the amount of [Ce/Fe] enhancement increases, so does the binary fraction! Adding credibility to the claim that s-process enhancement is caused by binary mass transfer. Figure 12 in the paper.
The authors found that, as expected, the amount of Ce enhancement was very strongly linked to the binary fraction! This result adds strong observational credibility to the theory that as massive stars evolve, they can dump parts of their atmospheres onto their companion, thereby enriching them. It also strengthens our theory that s-process elements are primarily formed in AGB stars, an important constraint for properly modeling galactic chemical evolution. However there is still work to do! Corroborating this finding with other s-process elements like Barium, or measuring the amount of enrichment in relation to the binary members separation are important next steps for understanding the efficacy of this process. Regardless, as always with astronomy, answering one question only raises three more!
Astrobite edited by Nathalie Korhonen Cuestas
