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.