1. Giants reveal what dwarfs conceal:
Li abundance in lower RGB stars
as diagnostic of the primordial Li
Alessio Mucciarelli
(Department of Astronomy, University of Bologna, Italy)
Mucciarelli, Salaris & Bonifacio, 2012, MNRAS, 410,2195

2. The Li problem …
LITHIUM
WMAP
Dwarf stars
2.72

3. Pietrinferni et al.(2006)
RGB Bump
FDU
The FDU ends when the convective envelope attains its maximum depth
Deepening of the convective
envelope

4. Another plateau … Mucciarelli et al.(2011) Gratton et al.(2000)
Spite et al.(2006)
Palacios et al.(2006)
Lind et al.(2008)

5. Why lower RGB stars?
- They are sensitive to the total amount of Li left in the star after the MS
- They are very weakly affected by atomic diffusion
The surface Li abundance at the end of FDU is only 0.07 dex lower when diffusion is fully efficient.
NO DIFFUSION
DIFFUSION
Depletion starting from A(Li)=2.72

6. Why lower RGB stars?
- They are sensitive to the total amount of Li left in the star after the MS
- They are very weakly affected by atomic diffusion
Depletion starting from A(Li)=2.72
In MS stars the effect of diffusion reaches several tenths of dex
The (uncertain) effect of diffusion is minimized !!!

7. A very robust constraint for the efficiency of the turbulent mixing
Why lower RGB stars?
- They are sensitive to the total amount of Li left in the star after the MS
- They are very weakly affected by atomic diffusion
SPITE PLATEAU
Lower RGB PLATEAU
A very robust constraint for the efficiency of the turbulent mixing
Mucciarelli et al.(2011) M4

8. 17 lower RGB Halo stars … (UVES + ELODIE spectra) Li @ 6708 A
We employed 3 Teff scales:
- 2 IRFM using (J-K) colours
(Alonso et al. 1999, Gonzalez Hernandez & Bonifacio 2009)
- Excitation equilibrium
FDU
FDU-0.01 dex
FDU-0.10 dex
Mucciarelli et al.(2012)

9. Primordial Li abundance from lower RGB …
Mucciarelli et al.(2012)
Field Halo stars
NGC 6397 (GIRAFFE) Standard
Diffusion
NGC 6752 (UVES) Standard
Diffusion 
M4 (GIRAFFE) Standard
Diffusion

10. Lower RGB stars: some advantages …
The lower RGB are the only possibility to infer the initial A(Li)
in stars more distant than those usually observed to investigate
the Spite Plateau
To enlarge the sample of field
and GC stars within our Galaxy
To assess whether the Li problem
exists also in extragalactic systems
(but see also Monaco et al. 2010)

11. to observe the lower RGB stars in
Lower RGB stars: some advantages …
The lower RGB are the only possibility to infer the initial A(Li)
in stars more distant than those usually observed to investigate
the Spite Plateau
30 hours with FLAMES
to observe the lower RGB stars in
M54 + Sgr
(PI: Mucciarelli, 089.D-0341 )

12. Conclusions…
The lower RGB stars are significantly less sensitive to the efficiency
of atomic diffusion (compared with the dwarf stars)
(2) The unique route to derive A(Li) in distant stellar systems
(alternative to the Spite Plateau)
(3) A robust constraint for the efficiency of turbulent mixing
(complementary to the Spite Plateau)