Mass Loss at the Tip of the AGB

What do we know and

what do we wish we knew!

Mechanism for dusty winds

Poster: What happens when planets orbit in this dynamical atmosphere/wind

Dust enhances the mass loss and increases the momentum in the wind - but mass loss can occur without

dust

Mass loss rate and metallicityTwo factors separate high and

low Z stars:

1. Low Z stars are smaller at the same L

2. Low Z stars don’t make dust

Therefore lower Z stars survive to higher L (for a given M)

3 4 5

= logL

Characteristics of AGB mass loss

• Mass loss rates are very sensitive to stellar (and model) parameters

• =>• the main mass loss “event” is short-lived, lasting

only about 200,000 years.• AND• the mass loss rate is subject to modulation

- in time AND in space

Dependence of M on L and M

Use the evolutionary track, R = a LbM-cZ-de, to eliminate R dependence. This works as long as the star stays “on track”.

3 4 5 logL

Where M/M = L/L - an approximation to the “cliff”

Power law fits: M=ALyM-z with 11<y<16, 15<z<20.

Steep mass loss law => lemming diagram: Stars evolve over a cliff

-10 -8 -6

-4

logM=

0.7

1

1.4

2

2.8

4

core mass

Chandrasekhar limit

0.6

0.4

0.2

0.0

-0.2

logM

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 logL

the cliff

Stars near the cliff are

Miras

0.7

1

1.4

22.8

4

2.2 2.4 2.6 2.8 logP

5

4

5

4

3

logL

logL

(Hipparcos distances to Miras are not very good - R ~ AU => angular diameter parallax)

Fit with NO parameter adjustment

Selection effects dominate empirical relations

7.06.86.66.46.26.05.85.6-8

-7

-6

-5

-4

logLR/M

log(M

dot)

cliff stars withM/Sun indicated

Reimers' formula

10xcliffM

0.1xcliff M

0.71.0

1.42.0

2.84.0

Fit with NO parameter adjustment

or fail to provide information on the dependence on mass

Note: The uncertainty in P is very small => the spread in Mdot is large

+/- 1 dex

Bowen models compared with VW relation:

Note: The uncertainty in P is very small => the spread in Mdot is large

Fit with NO parameter adjustment

Shell flashing modulates L, P, and Mdot

2.8

2.9

3

3.1

3.2

3.3

3.4

3.5

3.6

0 1 105

2 105

3 105

4 105

5 105

6 105

logL

t-t0

Shell flashing modulates L, P, and Mdot

0

50

100

150

200

250

0 1 105 2 105 3 105 4 105 5 105 6 105

P

time

Shell flashing modulates L, P, and Mdot

-18

-16

-14

-12

-10

-8

-6

0 1 105

2 105

3 105

4 105

5 105

6 105

logMdot bowenlinear

t-t0

Peak to trough - 5 orders of magnitude!

Mira masses are near Mi while the shaping occurs near Mf

-10 -8 -6

-4

logM=

0.7

1

1.4

2

2.8

4

core mass

Chandrasekhar limit

0.6

0.4

0.2

0.0

-0.2

logM

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 logL

OH-IR stars

The shaping occurs near the dotted line

Sources of asphericityAngular momentum from a companion (star, planet)?

/crit ~ 10 (Mcompanion/Menvelope) (k/0.1) √(a/R*)

where Ienvelope = k M R*2 and a = initial orbit of companion

General magnetic field (?without rotation??)

Global convection flow?

Shell flashes with non-spherical symmetry?

Note flash time scale << time scale of surface modulation of L

Movies on the web: www.lcse.umn.edu and www.astro.uu.seå

Mcomp >0.1 Menv

(Porter & Woodward) (B. Freytag)

What do the products tell us?

We expect that we should be able to learn something about the mass loss law from the distribution of stellar remnant masses and the Minitial-Mfinal relation.

If we assume that L = c1(Mcore-c2) and that the mass loss rate evolves with L according toMdot = A LyM-z on a given evolutionary track,

Then the curvature of Mi vs. Mf depends on (z+1)/y and the zero-point depends on A, c1, c2, and y.

(project carried out by Agnes Kim)

Initial-final mass relation

From Weidemann V., 2000, A&A, 363, 647

Evolution with mass loss and standard core mass - luminosity relations don’t fit.

Mass loss pre-AGB tip or ??

There is a deeper problem

P=>L=>Mcore for Miras

dndlogP

200 400 600 days

0.56 0.60 0.64 0.72 0.85

Nearly all Miras have L such that we’d expect Mcore > 0.6 solar masses.0.7 1 21.4 2.8

Their fate is to be white dwarf stars

Nearly all WD have masses < 0.6 solar masses.

Paradox? OR:

Core mass - L relation is wrong?Deep mixing can keep Mcore low while L increases.

Miras are all in He flash peak?Unlikely given how common they are - life time of

several times 105 years is not consistent with a reasonable number of shell flash peaks each lasting 1000 years.

Only high mass stars (leaving higher mass WD) go through a Mira stage?

Unlikely given the match in numbers and lifetimes and other constraints that all suggest the typical progenitor mass is not much more than 1 solar mass.

Conclusions• Mass loss rates increase steeply with increasing L

and decreasing M - exponents are 11-16 in L for fixed M and 15-20 for M at fixed L.

• Observed luminosities imply core masses > observed remnant masses so remnant masses do NOT provide a useful constraint on AGB mass loss (yet).

• The APN shaping event takes place whenM ~ Mremnant plus “a little”, but the mass loss process we understand is for Miras with M ~ initial AGB mass.

Spherical Planetary Nebula Abell 39Credit & Copyright: George Jacoby (WIYN Obs.) et al., WIYN, AURA, NOAO, NSF

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