Lithium Enrichment in a Subgiant Star with a Brown Dwarf Companion: A Planetary Engulfment Candidate

This study identifies the subgiant star TOI-5882 as a strong candidate for planetary engulfment, evidenced by its significant lithium enrichment which modeling suggests could result from the ingestion of a super-Earth to Neptune-mass planet.

The Gist

The Big Picture: A Star with a "Sweet Tooth"

Imagine a star as a giant, aging soup pot. As the star gets older and evolves (moving from its main life stage to a "subgiant" stage), the soup inside starts to churn. Usually, this churning brings hot, deep ingredients to the surface that destroy a fragile ingredient called Lithium. So, most old stars have very little Lithium left on their surface; it's like a soup that has been boiled so long the salt has completely dissolved and vanished.

However, the star in this study, TOI-5882, is different. It has a surprisingly high amount of Lithium on its surface. The astronomers wanted to know: Did this star just happen to keep its Lithium, or did it accidentally swallow a planet that was rich in Lithium, "seasoning" its soup?

The Suspect: A Brown Dwarf Neighbor

TOI-5882 has a massive neighbor, a Brown Dwarf (a "failed star" that is too heavy to be a planet but too light to be a star). This Brown Dwarf is huge (about 22 times the mass of Jupiter) and orbits very close to the star.

Think of the Brown Dwarf as a bully on a playground. Because it is so massive and close, it constantly tugs on anything else in the neighborhood. The astronomers believe this bully is shaking up the inner solar system, knocking smaller planets off their safe orbits and sending them crashing into the star.

The Investigation: Three Steps to Solve the Mystery

The team didn't just guess; they followed a three-step detective process:

1. Checking the Evidence (The Lithium Test)
They took a very detailed "snapshot" of the star's light using a powerful telescope spectrograph (TRES). They measured the Lithium line and found it was extremely strong.

• The Comparison: They compared this star to a "control group" of 61 other similar stars (stars of the same age, temperature, and size).
• The Result: TOI-5882 was in the 98.4th percentile. Imagine a room full of 61 people; this star is the only one holding a giant bucket of Lithium, while everyone else has almost none. This proves the Lithium is real and unusual.

2. Ruling Out Other Culprits
Before blaming a planet, they had to rule out other reasons for the extra Lithium:

• Is the star young? No. Young stars naturally have Lithium, but TOI-5882 shows no signs of youth (no rapid spinning, no infrared glow). It is definitely an older star.
• Did the star make its own Lithium? No. Stars usually make Lithium deep inside only when they are much older and larger (Red Giants). TOI-5882 is only a "subgiant," so it hasn't reached the stage where it can cook up its own Lithium.

3. Calculating the "Swallowed" Mass
If the star didn't make it and didn't keep it, it must have eaten it. The team ran computer simulations to figure out how much of a planet the star would need to eat to get that much Lithium.

Here is where they found a twist in the story:

• Old Assumption (The "Solar" Guess): If you assume the planet was made of the same stuff as our Sun (mostly hydrogen gas), the math says the star would have had to eat a 5.6 Jupiter-mass object. That's a huge planet, almost a star itself.
• New, Realistic Assumption (The "Rocky" Guess): We now know planets are usually "enriched" with heavy metals (like rocks and iron), much more than the Sun is. If the swallowed planet was rocky or metal-rich (like a Super-Earth or Neptune), it would be packed with Lithium.
  • The Result: With this realistic assumption, the star only needed to eat a planet between 9 and 95 Earth masses. That's the size of a Super-Earth or a Neptune.

The Conclusion

The paper concludes that TOI-5882 is a strong candidate for having engulfed a planet.

• The Mechanism: The massive Brown Dwarf neighbor likely acted as a gravitational bully, knocking a smaller planet out of its orbit.
• The Crash: That planet spiraled into the star. Because the star is in a specific phase of life where its outer "soup" (convective zone) is deep enough to mix things up but not so deep that the Lithium gets destroyed immediately, the Lithium from the planet is now visible on the surface.
• The Size: The evidence suggests the star ate a planet roughly the size of Neptune or a large Super-Earth, not a giant gas ball.

Why This Matters

This study is like finding a fossil record of a violent event in a star's past. It shows that even when we can't see the planets anymore (because they were eaten), we can still see the "scars" (the extra Lithium) on the star's surface. It confirms that massive neighbors can destabilize planetary systems and lead to dramatic, planet-eating events.

Technical Summary

Technical Summary: Lithium Enrichment in a Subgiant Star with a Brown Dwarf Companion

Problem and Motivation
Theoretical models predict that subgiant stars within a specific mass and evolutionary regime can retain detectable lithium (Li) enrichment signatures resulting from the engulfment of planetary companions. While lithium is typically depleted as stars evolve off the main sequence due to deepening convective envelopes mixing surface material into hotter interior layers, a small fraction of evolved stars exhibit anomalously high lithium abundances. Previous work by M. Soares-Furtado et al. (2021) identified a narrow evolutionary window in subgiants where the convective zone is massive enough to dissociate an inspiraling planet but shallow enough that the base temperature remains below the lithium-burning threshold, preserving the enrichment signature.

This study investigates TOI-5882, a subgiant star selected to test these predictions. The system is notable for two reasons: (1) it occupies the theoretically favorable evolutionary window (mass $\approx 1.33 M_\odot$, early subgiant phase), and (2) it hosts a massive brown dwarf companion ($22 M_J$, $P=7.1$ days) capable of driving dynamical instability in an inner planetary system through secular perturbations, potentially leading to planetary engulfment. The authors aim to determine if TOI-5882 exhibits significant lithium enhancement relative to similar subgiants, whether planetary material could be deposited in its convective zone, and if the required engulfed mass falls within a plausible range for planetary accretion.

Methodology
The study employs a multi-faceted approach combining high-resolution spectroscopy, statistical comparison, and stellar modeling:

1. Spectroscopic Observations: The team obtained 12 optical spectra of TOI-5882 using the Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5-m telescope at Fred Lawrence Whipple Observatory. The data were co-added to achieve a signal-to-noise ratio (SNR) of 58 per pixel.
2. Lithium Measurement: Using the Specutils package, the authors measured the equivalent width (EW) of the Li I resonance doublet at 6707.8 Å. They applied corrections for the blending iron (Fe I) feature at 6707.4 Å and converted the EW to a logarithmic lithium abundance, $A(\text{Li})$, using curves of growth from E. Franciosini et al. (2022) and stellar parameters derived from the TRES Stellar Parameter Classification (SPC) pipeline.
3. Control Sample Construction: To establish a baseline, the authors curated a control sample of 61 field subgiants from the GALAH DR4 survey. Selection criteria ensured the control stars matched TOI-5882 in metallicity ($[\text{Fe/H}]$), surface gravity ($\log g$), effective temperature ($T_{\text{eff}}$), and absolute magnitude. The authors rigorously screened for youth indicators (e.g., H$\alpha$ emission, infrared excess, rapid rotation) and removed a young contaminant to ensure the sample represented evolved stars.
4. Statistical Analysis: The lithium metrics of TOI-5882 were compared against the control sample using percentile ranking and tail probability analysis. A non-parametric bootstrap analysis with 100,000 iterations was performed to assess the robustness of the results against measurement uncertainties.
5. Stellar and Engulfment Modeling: Using the Modules for Experiments in Stellar Astrophysics (MESA), the authors modeled TOI-5882 to estimate the mass and depth of the convective zone. They then calculated the mass of an engulfed companion required to reproduce the observed lithium enhancement under two scenarios:
   • Proto-solar composition: Assuming the planet has a lithium abundance similar to the proto-solar value.
   • Realistic heavy-element enrichment: Assuming the planet follows core accretion theory, possessing significant metallicity enhancements (modeled using CI chondritic lithium abundances).

Key Results

• Lithium Enhancement: TOI-5882 exhibits a Li I equivalent width of $75.39 \pm 3.58$ mÅ and an abundance of $A(\text{Li}) = 2.49 \pm 0.12$ dex. Compared to the control sample, TOI-5882 ranks in the 98.4th percentile for both metrics. Only one control star showed a higher lithium equivalent width, yielding an empirical tail probability of $P = 0.016$ (1.6%). Bootstrap analysis confirmed a median percentile rank of 99.09% (95% CI: $95.45\% - 100\%$).
• Convective Zone Deposition: MESA modeling indicates a convective zone mass of $\approx 0.035 M_\odot$ ($\approx 37 M_J$) and a depth of $\approx 0.42 R_\odot$. The authors estimate that planets with densities $\rho_p \lesssim 5\rho_\star$ (typical of super-Earths to Neptune-mass planets) would be tidally disrupted within or above the convective zone, allowing their material to mix into the observable photosphere.
• Engulfed Mass Estimates:
  • Under proto-solar assumptions, the required engulfed mass is $\approx 5.6 M_J$.
  • Under realistic heavy-element assumptions (incorporating CI chondritic Li abundances and metal enrichment), the required mass drops significantly. For a terrestrial-like composition ($Z_p=1$), the mass is $9.4^{+10.7}_{-8.9} M_\oplus$. For a giant-planet-like composition ($Z_p=0.1$), the mass is $94.6^{+127.3}_{-90.1} M_\oplus$.
  • Both realistic estimates fall within the $7 M_\oplus \lesssim M_p \lesssim 1 M_J$ range where tidal disruption is expected to deposit material in the convective zone.

Significance and Claims
The paper claims that TOI-5882 is a compelling candidate for a planetary engulfment event. The authors argue that internal lithium production (Cameron-Fowler mechanism) is ruled out by the star's early subgiant evolutionary state, and primordial retention is excluded by the absence of youth indicators.

The primary significance of the work lies in demonstrating that the assumed composition of the engulfed planet critically alters the inferred mass of the event. By moving away from proto-solar assumptions and adopting a core-accretion framework with heavy-element enrichment, the authors show that the observed lithium excess can be explained by the ingestion of a super-Earth to Neptune-mass planet rather than a massive Jupiter-like object. This finding aligns with the dynamical architecture of the system, where the massive brown dwarf companion could perturb inner planets into star-grazing orbits.

The authors conclude that while the lithium excess is a strong indicator of engulfment, confirming the scenario requires further evidence, such as a homogeneous survey of subgiants to refine the baseline distribution and differential analysis of refractory elements (e.g., Ti, V, Mn, Ni) to distinguish planetary accretion from other enrichment pathways. TOI-5882 serves as a test case for using spectroscopic signatures to reconstruct the dynamical history and ultimate fate of planetary systems.