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.