Planetary engulfment may explain the chemical paradox in the binary system HD 81809

Star system: HD 81809 A | Distance from the Sun: 101 light years

The binary star system HD 81809 presents a significant chemical mystery, as its two components exhibit a striking difference in metallicity — the abundance of elements heavier than hydrogen and helium. While the primary star has a low metallicity, the secondary star, HD 81809B, shows a metallicity closer to that of our Sun. Scientists are investigating whether this anomaly could be the result of the secondary star having engulfed one or more planets during its evolution, which would increase its surface metal content.

To test this hypothesis, researchers utilized the MESA stellar evolution software to simulate accretion events where the star consumes material of varying masses and compositions. The simulations showed that to reach the observed iron abundance in HD 81809B, the star would need to have engulfed between 25 and 75 Earth masses of material enriched in metals. These models were able to successfully match the observed effective temperature of the star, reinforcing the plausibility of a past planetary engulfment.

However, the study also highlights a remaining challenge involving the element lithium. If the star were to accrete enough mass to explain the high metal content, the models predict an over-enrichment of lithium compared to what is actually observed. To match the measured lithium abundance, the accretion would need to be limited to less than 6 Earth masses. These conflicting results suggest that while planetary engulfment is a likely explanation for the unusual chemistry of HD 81809B, the composition of the consumed planetary material might be more complex than currently assumed, emphasizing the need to account for such external events in models of chemically unusual binary systems.

Key findings
- The binary star HD 81809B shows an anomalously high metallicity compared to its companion star.
- Stellar evolution models suggest that a substantial planetary engulfment event of 25–75 Earth masses of metals could explain the star's current iron abundance.
- Discrepancies in predicted versus observed lithium levels indicate that the composition of the accreted material remains an open question.
- This study highlights the importance of accounting for external factors like planetary engulfment when analyzing chemically unusual stars in binary systems.

Scientific study

Chemical paradox in a binary system: Exploring metal enrichment in HD 81809B (external link)