The role of convection on the RGB modelling

P. Ventura

F. D’Antona

R. Carini

M. Di Criscienzo

Observatory of Rome

courtesy by M. Di Criscienzo

Two local models for convection

MLT (Vitense 1953)

=d=Hp

FST (Canuto & Mazzitelli 1991) All eddies

=z+Hp

Canuto & Mazzitelli 1991

Canuto & Mazzitelli 1991

ΔT ~ 100K for Δ=100%ΔT ~ 250K for Δ=24%

0.8MSun

Z=10-4

=z+Hp

Log P

Δ

ad

Δ

-

0.8 Msun

FST more efficient in the interior!

Overadiabaticity peakhigher and narrower

FST more efficie

nt

MLT more efficient

Canuto & Mazzitelli 1992

Photometry of NGC 6791 (Kalirai et al. 1997)

[Fe/H]=+0.3-0.4

1.2MSun

Z=0.04

FST (=0.2)MLT (=1.9)

1.2MSun

different slopes!

Parallel paths

500K !FSTMLT

1.2MSun

FSTMLT

FST moreefficient

Log P

ΔΔ-

ad

Δ Δrad ad

-

Log P

FSTMLT

Log P

Log T

FSTMLT

Δad

Log P

recombination

=10

FST ATMOSPHERE NEEDED!

Distribution of clump stars in NGC 1866 (Testa et al. 1999)

NB ~ NR

The extensions of the loops are similar, but the diffusivescheme predicts a blue clump more populated

Synthetic diagrams(no dispersion!) using isochrones where mixing was treated in the instantaneousand diffusion-like modality

Time

Log

Teff

Time

YC

In the diffusivecase helium isconsumed moreslowly in the core

In the instantaneous mixing scheme the helium is consumed faster, which favours a quick return tothe red

Conclusions* The morphology of the RGB of high Z clusters depend on the treatment of convection modelling

* At any luminosity, the MLT model is cooler in the interior, enters the region of molecular recombination, with the consequent lowering of the adiabatic gradient, and the increase in the overadiabaticity: the track results much cooler than the FST counterpart * The distribution of stars in the clump of relatively young clusters (150-200 Myr) depend on the modality with which overshooting is modelled: a diffusive approach favours longer staying in the blue