the effects of mass loss on the evolution of chemical abundances in fm stars

The Effects of Mass Loss on the Evolution of Chemical

Abundances in Fm Stars

Mathieu Vick1,2 Georges Michaud1

(1) Département de physique, Université de Montréal, Canada (2) GRAAL / UMR5024, Université Montpellier II, France

Basic Physical Properties

• Pop.I, MS stars• 7000 K <Teff< 10 000 K• Non magnetic• Abundance anomalies => Slow

rotators• Binaries

Typical abundance patternsunderabundances :

Li, CNO, Ca, Scoverabundances :

Iron peak elements (2-5) Rare earths (10 or more)

• Fm’s are expected to have the same patterns

Gebran et al., 2007 (poster 05)

The Basic Model

• Michaud (1970): separation in radiative zone leads to observed abundance anomalies

• Anomalies predicted by purely diffusive models are larger than those observed

• Other processes?

1.4M : Diffusion only (black), mass loss (red, blue,

green), turbulence (orange).

Transport Processes

Mass

Loss

• Competition between g and grad approx. determines movement of elements

• Position of BSCZ and g = grad (vdrift= 0)

• Large scale effects can hinder diffusion

• Diffusion time scales grow with increasing density

Models with Turbulence

Richer et al (2000): • Sirius A:

– 1 free parameter (mixed mass)

– 12 of 16 elements observed are well reproduced

Other papers: Richard et al. (2001)Michaud et al. (2005)

Can mass loss do the same?

Implementation of Mass Loss

Physical considerations:

1. diff >> conv Homogeneous abundances in CZ Convective overshoot mixes the atmosphere and

links H-He CZ (Latour et al. ,1981)

The mass loss rates considered are: • chemically homogenous (with the same composition as

the SCZ) • spherically symmetrical• weak enough not to influence nuclear burning in the

core or the stellar structure

Implementation of Mass Loss

• Can’t simply add to total velocity field

(many numerical problems encountered) • But with simple hypotheses these problems can be

avoided:

(1) homogeneous CZ

(2) Mass lost has same composition as SCZ• Mechanism is not important

cSccDt

c)(Uln nuc

rww evU ˆ

U

Implementation of Mass Loss

• where:

rww ev

Uˆ

0

cSSccDt

c)()(ln wnucw

UU

rw

w evU

ˆ

0 In SCZ

Under SCZw

SCZ SMM

0

Models with Mass Loss

• The evolutionary calculations take into detailed account time-dependant abundance variations of 28 chemical species and include all effects of atomic diffusion and radiative accelerations.

• These are the first fully self-consistent evolutionary models which include mass loss.

• Models were calculated for 1.35, 1.40, 1.45 and 1.50 M.

• All the models have evolved from the homogenous pre-main sequence phase with a solar metallicity (Z=0.02).

• The mass loss rates considered varied from 1 x 10-14 to 3 x 10-13 Myr-1.

Results: 1.5 M model

• Observation: UMa (Hui-Bon-Hoa, 2000)Age~500 Myr, Teff~7000 K

• Turbulence and mass loss have slightly different effect on certain elementsFe convection zone

appears naturally!

Results (cont.)

• Anomalies appear with decreasing importance down to stars of 1.35M.

• Reasonable mass loss rates can reduce anomalies to the desired levels

Conclusions

• With a mass loss rate of the order of the solar mass loss rate we can successfully reproduce the observed anomalies of UMa.

• It is shown that turbulence and mass loss affect anomalies differently. It is thus possible that additional observations (and more massive models) could help constrain the relative importance of each process.

• Observations of elements between Al and Ar could allow us to determine if there is separation between the Fe and H-He convection zones.

• In any case, it is seen that mass loss can effectively reduce the predicted anomalies to observed levels.