The lithium-rich giant star problem

Transcript

1. Andy Casey The Lithium-Rich Giant Star Problem And a simple solution. ( @astrowizicist)
2. Difficult to produce, easy to destroy. Lithium 1. Lithium produced by BBN 2. Convection brings Li into stellar interior 3. High interior temperatures destroy Li 4. Most conditions that can produce Li will also destroy it Andy Casey
3. The problem is that they exist, when we think they shouldn’t. The Li-Rich Giant Problem Ruchti+ (2014) Luminosity Lithium abundance Andy Casey Expectations from stellar evolution Unexpected reality (~1% of FGK stars)
4. The problem is that they exist, when we think they shouldn’t. The Li-Rich Giant Problem Ruchti+ (2014) Luminosity Lithium abundance Andy Casey Expectations from stellar evolution Unexpected reality (~1% of FGK stars) No model has yet to explain their ensemble properties.
5. Li-Rich Giant Stars They’re mysterious and hard to find No anomalous photometry No other common chemical signature Casey et al. (2016b)
6. The Gaia-ESO Survey A vehicle for finding Li-rich giant stars ~100,000 stars with high-resolution spectra ~450 co-investigators (PIs: Gilmore, Randich) 20 Li-rich giant stars in iDR4 (one of the largest samples to date) Andy Casey
7. Li-Rich Stars in Gaia-ESO Little doubt that they’re Li-rich Wavelength Effective temperature Surface gravity Metallicity Andy Casey Casey et al. (2016b)
8. Plausible Explanations 1. preserve their existing lithium, 2. produce more lithium, or 3. they have to accrete lithium from an external source. Preservation / Production / Accretion Some stars need to either: Andy Casey
9. Plausible Explanations Extremely model-dependent (most are implausible) Cannot explain any stars with Li exceeding Big-Bang nucleosynthesis predictions Preservation might contribute, but it can’t be the whole story Preservation scenarios (keep existing Lithium): Andy Casey Preservation / Production / Accretion
10. Production by internal mixing (Cameron-Fowler mechanism): Plausible Explanations “Goldilocks” conditions required: Hot enough to produce 7Be Quickly move 7Be away to cooler region Allow for 7Li production Andy Casey Preservation / Production / Accretion
11. Effective temperature Surface gravity Metallicity Plausible Explanations Preservation / Production / Accretion
12. Effective temperature Surface gravity Metallicity Plausible Explanations Production and mixing can only occur past here But most Li-rich giants are before here Incompatible with most of the observations. Preservation / Production / Accretion
13. Plausible Explanations Introduced to explain Li-rich giants all across the RGB/AGB Preservation / Production / Accretion Accretion scenario: 1. Reservoir of unburnt Li 2. Angular momentum transfer Two effects contribute: Andy Casey
14. Plausible Explanations Introduced to explain Li-rich giants all across the RGB/AGB Accretion scenario: Incompatible with what we now know about exoplanets: most planets aren’t found around RGB or AGB stars. Andy Casey Preservation / Production / Accretion
15. Where and why the hell are they? Johnson+ (2010) Andy Casey Occurrence rates of hot Jupiters
16. Occurrence rates of hot Jupiters Where and why the hell are they? Andy Casey Schlaufman & Winn (2013) Sub-giant hosts Main- sequence hosts
17. Why do sub-giants host fewer planets? Different disk dissipation timescales There are two (main) possibilities Convective envelope expands at the sub-giant phase Different stellar masses? Tidal destruction of close-in planets? Higher mass stars are more evolved and have fewer planets Any nearby planets are tidally destroyed (Stellar masses) (Stellar ages)
18. Why do sub-giants host fewer planets? Different stellar masses, or different stellar ages? Schlaufman & Winn (2013) Andy Casey (Sub-giant hosts)
19. Why do sub-giants host fewer planets? Andy Casey Schlaufman & Winn (2013) Different stellar masses, or different stellar ages? (Main-sequence hosts)
20. Why do sub-giants host fewer planets? Different disk dissipation timescales There are two (main) possibilities Convective envelope expands at the sub-giant phase Andy Casey Different stellar masses? Tidal destruction of close-in planets? Higher mass stars are more evolved and have fewer planets Any nearby planets are tidally destroyed This is the fate of the Earth and Solar system
21. What happens to the star? (After eating a close-in planet) Well we know what happens: 1. Reservoir of fresh Li 2. Angular momentum transfer induces mixing (Both effects are required!) Andy Casey
22. A plausible solution? That predicts the ensemble properties. 1. Li-rich giant stars should occur preferentially before the luminosity bump 2. They should occur more frequently around metal-rich stars Effective temperature Surface gravity Metallicity Andy Casey Casey et al. (2016b)
23. A plausible solution? That predicts the ensemble properties. These two lines of evidence actually predict the existence of lithium-rich giant stars, and their ensemble properties! Models of planet accretion onto the host star Kinematic evidence of tidal destruction of close-in giant planets + Andy Casey
24. Conclusions One model isn’t enough; two sizes fits all Stars destroy close-in planets at the sub-giant stage, and some of them become Li-rich giant stars Planet accretion unlikely for evolved or metal-poor giants, but these can be explained by internal mixing (1) (2) Andy Casey
25. Thank you for your attention Andy Casey Questions?