T le bertre e g rard paris observatory and j m winters iram
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T. Le Bertre, E. Gérard Paris Observatory and J. M. Winters IRAM. AGB mass-loss and recycling. The Dusty and Molecular Universe, Paris, 27-29 October 2004. AGB outflows. Slow winds : V exp ~ few to 20 km s -1 Massive winds :

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AGB mass-loss and recycling

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T. Le Bertre, E. Gérard

Paris Observatory

and

J. M. Winters

IRAM

AGB mass-loss and recycling

The Dusty and Molecular Universe, Paris, 27-29 October 2004


AGB outflows

Slow winds :

Vexp~ few to 20 km s-1

Massive winds :

Mdot~ 10-8 to a few 10-4 Msol yr-1

AGB stars are therefore surrounded by expanding circumstellar shells that can be traced through their emission from dust and gas in the infrared and radio domains.


  • through mass loss they contribute to the replenishment of the ISM

  • they also contribute to the chemical evolution of the ISM, in particular to its enrichment in carbon

    + Mass loss affects the star evolution

    (luminosity, nucleosynthesis)

    ----> role of stellar mass loss in galactic evolution

    (in particular as compared to infall)


Sedlmayr (1994, IAU coll. 146, 163)

but dependence with galactic location and with time

+ galactic infall


The outflows on the AGB are variable on timescales that may be short compared to stellar evolution

IRC +10216

(Mauron & Huggins 2000, A&A 359, 707)

  • CO rotational transitions from J=2-1 to J=7-6 indicate mass loss variations on the same timescales (Kemper et al. 2003, A&A 407, 609)

  • Mass loss variations may be so large that one observes a “detached shell” (e.g. Olofsson et al. 2000, A&A 353, 583)


TT Cyg (Olofsson et al. 2000, A&A 353, 583)CO 1-0Vlsr +/- 2 km/s

Mdot (in.) ~ 3 10-8 Msol yr-1

Mdot (out.) ~ a few 10-5 Msol yr-1

---> meaning of Mdot ?

We may have large differences on the estimates of Mdot when we use different tracers sensitive to different zones of the circumstellar shells (in addition to the abundance problem)


---> necessity to combine these different tracers

  • Need to understand better the mass loss process and its history

  • Molecular lines are useful in particular because of the high spectral resolution available with heterodyne techniques, but they probe a limited extent of the circumstellar shells.

  • The dust emission at long wavelengths may help to trace the circumstellar shells on large distances, but cannot provide velocity information.

Izumiura et al. 1996, A&A 315, L221

ISOPHOT 90 µm


A special mention may be done to the atomic line of hydrogen at 21 cm that up to now has not been much used.

  • optically thin in most situations

  •  = 1.4 GHz ==> h/kT << 1 ==> TB  NH

  • circumstellar HI should be protected by the surrounding ISM

  • ~70 % of mass in hydrogen

    Glassgold & Huggins (1983, MNRAS 203, 517):

    if Teff > 2500 K, all hydrogen should be atomic

    if Teff < 2500 K, H2 should bephotodissociated at ~ 1017 cm

    (lines at 28 µm….)


EP Aqr (Winters et al. 2003, A&A 409, 715)

Le Bertre & Gérard 2004,

A&A 419, 549

Complementarity between H I and CO :

same multiple components, but the relative intensities are different

(the zones which are probed are distinct)


EP Aqr (Le Bertre & Gérard 2004, A&A 419, 549)

4’ x 22’ or 0.16pc x 0.86pc (at 135 pc)

Complex spatial and dynamic structures

The HI emission is very extended (~ 1 pc)

Mtot ~ 0.07 Msol


EP Aqr

Winters et al. 2003, A&A 409, 715

CO : Vlsr = -34 km s-1

Le Bertre & Gérard 2004, A&A 419, 549

HI : Vlsr = -31 km s-1

Gonzalez Delgado et al. 2003, A&A 411, 123

SiO : Vlsr = -32 km s-1

(thermal SiO, v=0, J=2-1)

==> need to resolve spatially these emissions with

a spectral resolution corresponding to 1 km s-1,or better


Y CVn (Le Bertre & Gérard 2004, A&A 419, 549)

Knapp et al. 1998, ApJS 117, 209

H I can be traced out

to the ISM

(Mtot ~ 0.06 Msol)

Izumiura et al. 1996, A&A 315, L221 --->

(ISOPHOT 90 µm : 12x8 arcmin2)


The matter in the shell is the sum of slowed down circumstellar material and accelerated external (ISM ?) material

from Chevalier & Tiret (2004)

==> need to separate the 2 phases

spectroscopy ? dust properties ?

high spatial resolution over a large f.o.v.


Other difficulties

  • non-sphericity

    bipolar flows, e.g. X Her (Kahane & Jura 1996)

    V Hya (Sahai et al. 2003)

    clumpiness, e.g. Mira (Lopez et al. 1997)

    IRC+10216 (Weigelt et al. 2002)

    ----> clumpiness might be an effect of the self-amplifying nature of the dust formation process (Woitke, this conference)

    ===> importance imaging with high spatial resolution

  • distance

    -in general, mass loss rate estimates depend on the adopted distance

    -needed to locate the sources in the Galaxy

    ===> importance of GAIA


A further complication

The atmospheric composition of AGB stars depends on

initial mass, initial abundances, multiplicity, … ,

and changes as a function of time.

Standard dichotomy :

C/O < 1 ---> O-rich, silicate grains, ….

C/O > 1 ---> C-rich, carbon grains, ….

==> the composition of the gas and grains injected into the ISM is variable


Dependence of the nature of the presently

injected material on galacto-centric distance

Thin line : M+C AGB starsThick line : C only IRTS data(Le Bertre et al. 2003, A&A 403, 943)

IR surveys should be useful to evaluate this effect in different locations of the Galaxy


Summary

  • CO, HI, SiO, …, radio lines and dust emission are complementary probes of the expanding circumstellar shells and of the zone where they interact with the ISM.

  • Infrared surveys are useful to evaluate the contribution of the population of AGB stars to the replenishment of the ISM.


Simulations of the HI emission from a spherical source

Unresolved source with V = const.

Resolved source with V = const.


Unresolved source with decreasing V

Resolved source with decreasing V


Y CVn (Le Bertre & Gérard 2004, A&A 419, 549)


Woitke (this conference)


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