The study on Li abundances of solar-like stars

1. The study on Li abundances of solar-like stars Li Tanda Beijing Nomal University 2010.4.2

2. Contents • Introduction • Observations of Lithium anomalies • Possible mechanisms for Lithium anomalies • Future work

3. Introduction • Lithium anomalies has been a problem for a long time. For solar-like stars, two important observational facts need to be explained: the high dispersion in Li abundance for stars of similar temperature, age and metallicity (Pasquini et al. 1994) and the large Li deficiency in the Sun. These observations were inconsistent with theoretic models.To explain these phenomena, new mechanisms should be added in stellar evolution models.

4. Aims • To explain the Li anomalies of G dwarfs in the solar neighbourhood. • To obtain new information on stellar evolution

5. Li depletion in the sun • The large Li depletion in the Sun The initial abundance of Li for the sun is 3.31±0.04dex. (1989, Geochim. Cosmochim. Acta, 53,197.) Now ,it is 1.03±0.04dex (2005,The Astronomical Journal, 130:2318–2325 ;)

6. Li depletion in the sun(2) • The existence of strong Li depletion in the Sun is inconsistent with classical models. • To explain the observations, lithium must be transported to the hot layers where the temperature is more than 2.5×106 K. • The location of the base of convective zone of the sun is 0.713±0.003R☉(Inferred from helioseismology by Christensen-Dalsgaard et al 1991),not hot enough to burn Li.

7. Dispersion in solar-like stars • Large dispersion in Li abundance for solar-like stars We selected 7 G dwarfs who have similar properties with the sun. As the table showed, although their properties are very closed to each other, the abundances of lithium are dispersive. In stellar evolution models, the surface abundance of Li was the function of mass, age and composition. The large dispersion found in G dwarfs came into conflict with theoretics.

8. Table 1. Li dispersion of 7 G dwarfs

9. Lithium in stars with exoplanets • What interested us most was the relationship between lithium and exoplanets. • A comparative study on lithium with and without planets was reported by Y. Q. Chen and G. Zhao(2006). Their work indicated that stellar photospheric abundances of lithium in stars with planets may be somewhat affected by the presence of planets. • Besides, G. Israelian(2004) suggested that there is a possible excess of Li depletion in cool planets-host stars with effective temperatures in the range 5600–5850 K.

10. Fig.4 of Y. Q. Chen and G. Zhao(2006). The figure shows Li abundances versus temperatures for all stars with (filled circles) and without (open circles) planets. Upper limits of Li abundances are indicated by downward directed arrows.

11. The possible mechanisms of lithium deleption • Stars with and without exoplanets have different initial rotation velocity due to the conservation of angular momentum by the protoplanetary disk. • Several works have been done to discuss rotation-induced mixing in stars and the sun. (e.g. Brian Chaboyer, et. al.1995;M.H.Pinsonealt,et.al. 1998) • In this study, we will focus on the lithium-rotation correlation.

12. Effect of changing the initial rotation velocity on the surface velocity V and 7Li abundance of the solar model. • The 7Li depletion occurs earlier in models with higher initial rotation velocities, although not by a measurable amount. Fig. 8 in Brian Chaboyer, et. al.1995.

13. Rotation-induced mixing(1) • Three cases for mixing not present in standard stellar models. • Meridional circulation (Eddington(1925)) • GSF instability (Goldreich and Schubert(1967),Fricke(1968)) • Shear instability(ZAHN(1974)) • The three mechanisms induced by rotation will cause the transport of angular momentum and material from surface to interior.

14. Rotation-induced mixing(2) • Following Endal & Sofia (1978), the transport of angular momentum and the associated material transport caused by those three mechanisms can be described by coupled diffusion equations: where I is the moment of inertia per unit mass, D is the diffusion coefficient, and fc is an efficiency factor relating the diffusion coefficient for material transport to that for mixing.

15. Future work • Establishing rotational models for solar-like stars to reproduct Lithium depletion in observations. • Looking for other physical parameters related to surface abundance of lithium besides Mass, age and composition.

16. Thanks

17. log(NLi)+12=__dex • D~v (characteristic velocity of each circulation)