A multistep scenario which describes a plausible mass loss mechanism associated with red giant and related stars is outlined. The process involves triggering a condensation instability in an extended chromosphere, leading to the formation of cool, dense clouds which are conducive to the formation of molecules and dust grains. Once formed, the dust can be driven away from the star by radiation pressure. Consistency with various observed phenomena is discussed

Stencel, R. E.

English

NASA Technical Reports Server

Paper presented at the February 1985 NASA/Wye Grain Workshop 
Chromospheric Dust Formation, Stellar Masers and Mass Loss 
Robert E. Stencel 
Astrophysics Division, NASA Headquarters 
ABSTRACT 
I outline a multi-step scenario which describes a plausible mass loss mecha- 
nism associated with red giant and related stars. The process in~volves trig- 
gering a condensation instability in an extended chromosphere, leading to the 
formation of cool, dense clouds which are conducive to the formation of mole- 
cules and dust grains. Once formed, the dust can be driven away from the star 
by radiation pressure. Consistency with various observed phenomena is dis- 
cussed. 
New View of Red Giant Chromospheres 
The analysis of cool stars has benefited from reference to the Sun as a well 
studied archetype. However, in the past few years, it has been recognized 
that the solar analogy must be applied with caution, especially in the case of 
much lower surface gravity. Quantitative analyses of red giant chromospheres 
use ultraviolet lines of singly ionized carbon, to derive tempera~ture (Brown 
and Carpenter 1984), densities and physical extent (Carpenter, Brown and 
Stencel 1985; cf. Hartmann and Avrett 1984). 
These analyses combined with other multispectral studies indicate that red 
giants and supergiants altogether lack solar-like coronae, hut instead fill 
comparable volumes with extended warm gas having densities of order 10E7- 
10E8/cm3 at temperatures of order 8,000 degrees Kelvin. Chromospherie radial 
dimensions are at least several stellar radii; approximately 10E14 cm for the 
supergiants. Maintenance of this extended warm region can be accomplished by 
absorption ionization of UV and EUV photons. In what follows, I argue that 
cooling can occur in two co-existing thermal regimes: chromospheric UV line 
emission in "warm" regions, and via molecular masers and thermal emission from 
dust in "cool" regions. This thermal bistability may contribute to the 
observed mass loss and other properties of such stars (cf. Goldberg, 1983). 
Cooling via Condensation Instabilitv 
Key point: if you compress a chromospheric gas (8000K, lE7/cm3), a condensa- 
tion instability occurs because as the density rises, the radiative cooling 
increases with the density squared. Because this happens on the very steep 
portion of the radiative power loss curve (cooling rate, ergs/cm3/sec-vs- 
temperature, cf. Raymond et al. 1976), there is a dethermalizing runaway to 
very low temperatures. 
https://ntrs.nasa.gov/search.jsp?R=19860014049 2020-03-20T15:33:40+00:00Z
An original application of this physics was described by Field et al. (1969) 
for the two-phase interstellar medium, where they derived "two thermally 
stable gas phases that coexist in pressure equilibrium, one at 10,000K and the 
other cooler than 300K." Their model was derived for interstellar gas heated 
by low energy cosmic rays. The resulting partition cooled 75% of the gas. 
Lepp et al. (1984) have more recently confirmed this.basic result for matter 
in the presence of radiation fields. By including high energy photons, they 
also derived an inverse Compton term and thus a coronal gas phase. Although a 
steady state solution, this model replicates the three-phase interstellar 
medium and makes predictions for quasar clouds. The coronal result does not 
apply to stellar chromospheres which lack a source of hard photons. The range 
of pressures and heating/cooling channels described by Lepp et al. imply 
applicability to the lower temperature physics of the putative two-phase 
chromospheres. 
Instability Trigger Mechanisms 
Basic stellar structure suggests that red giants possess large convective 
envelopes in response to steep temperature gradients Schwarzschild (1975). 
Convective motion provides a source of "acoustic noise" that will give rise to 
pressure perturbations in an overlying atmosphere. The density enhancement 
can trigger the condensation instability. To avoid shock waves which could 
produce high temperature emission lines, either the input energy varies slowly 
or the density profile with height is less steep than exponential. I prefer 
the latter, anticipating a quasi-isobaric extended chromosphere. Many cool, 
evolved stars pulsate. Shock phenomena may dominate those atmospheres, but it 
is possible that this scenario plays a role in regions where, or at times 
when, shocks are less important. 
Formation of Molecules and Dust 
The condensation instability yields conditions appropriate for the formation 
of molecules and grains. As a result of the rapid cooling and enhanced 
densities, molecules are formed in excited states and then cool via maser 
emission. &cent work by Elitzur and Cooke (1985) states that H20 masers 
occur for densities at and above 1E9,cm3, are collisionally pumped and corre- 
spond to a lOOOK excitation temperature. This is exactly the higher density, 
lower temperature condition expected from the condensation instability. VLBI 
studies show SiO masers occuring within a few stellar radii of dusty objects, 
like IRC+10215 (Lane, 1982; Johnston, et al. 1985). While the detailed 
chemistry is still to be elaborated (Kozasa et al. 1984; Maciel 1973, 19761, 
we may be seieing a manifestation of the condensation instability process. 
Interferometric mapping of the 10 micron emission associated with silicate 
dust grains indicates a lack of such emission inside of approximately 10 stel- 
lar radii in many dusty objects (cf. Sutton et al. 1977). However, molecular 
clusters and less-annealed silicates with spectral features at other than 10 
microns could be forming within this inner dust radius. SiO masers within 10 
stellar radii are probably associated with this chemistry. Chromospheric and 
envelope expansion velocities require months - years for material to move 
several stellar radii. This places temporal constraints on cluster formation 
and subsequent dust nucleation (Donn and Nuth 1985). High spatial resolution 
observations of radial and azimuthal spectral variations may reveal the entire 
process. 
Quenching chromospheric Radiation/Accelerating Dust 
Extended stellar chromospheres are optically thick to the usual radiative loss 
channels (hydrogen lines, Ca +, Mg +). The chromospheric gas is filled with 
photons of the Lyman series, scattered endlessly and effectively trapped 
(Wilson 1960). This radiation field could act on the newly condensed mole- 
cules and grains to provide acceleration and eventual expulsion. Jura (1984; 
1985) has already concluded that once grains form near these stars, radiation 
pressure can accelerate them to infinity. 
An extended chromosphere could trap part of the star's bolometric luminosity 
in endlessly scattered Lyman transitions. A 10E-7 Mo/yr mass loss at 10 
km/sec requires an energy content which is comparable to less than IOE-4 of 
the bolometric luminosity. If we "quench" these chromospheric photons, trans- 
ferring their energy to mass motion of grains, the net effect will be a lack 
of, or reduction of observed ultraviolet chromospheric emission, as discovered 
by Hagen, Carpenter and Stencel (1985). The radiative transfer applicable to 
this reduction of resonance line flux was discussed by LJehrse and Kalkofen 
(1985), where factors of five in flux reduction were derived. In light of the 
detection of UV chromospheric emission from dusty stars, this scenario pro- 
vides an alternative to the quenched chromosphere idea of Jennings and Dyck 
(1972), which was based on the lack of Ca I1 emission in dusty stars. 
Further Work 
To further investigate the validity of this scenario, several lines of work 
are required. First, the timescale for the growth of the instability needs to 
be evaluated, against the constraint of the "fully formed" 10 micron silicate 
dust emission features outside of ten stellar radii. Second, an observational 
test involving high spatial resolution spectral imaging is needed to monitor 
radial and azimuthal changes in the infrared dust emission profiles, in 
response to nucleation, growth and processing of dust grains in the circum- 
stellar environment. Finally, the role of this process in the atmospheres of 
cool, pulsating stars deserves study. I am happy to acknowledge useful dis- 
cussions with Joseph Nuth and Leo Goldberg in the preparation of this report. 
References: 
Brown, A. and Carpenter, K. 1984 Ap. J. L43. 
Carpenter, K., Brown, A. and Stencel, R. 1985 Ap. J. 289, 676. 
Donn, B. and Nuth, J. 1985 Ap. J. 288, 187. 
Elitzur, M. and Cooke, B. 1985 B.A.A.S. 16, 941. 
Field, G. 1965 Ap. J. 142, 531. 
Field, G., Goldsmith, D. and Habing, H. 1969 Ap. J.  155, L149. 
Goldberg, L. 1983 in "Cool Stars, Stellar Systems and the Sun: Lecture Notes 
in Physics Vol. 193", eds. S. Baliunas and L. Hartmann (Berlin; 
Springer-Verlag), p. 333. 
Hagen, W,, Carpenter, K. and Stencel, R. 1985 B.A.A.S. 16, 895. 
Hartmann, L. and Avrett, E. 1984 Ap. J. 284, 238. 
Jennings, M. and Dyck, H. 1972 Ap. J. 177, 427. 
Johnston, R., Spencer, J .  and Bowers, P. 1985 Ap. J. 290, 660. 
Jura, M. 1984 Ap, J. 282, 200. 
Jura, M, a ~ d  Morris, M. 1985 Ap. J. 292, 487. 
Kozasa, T., Hasegawa, H. and Seki, J. 1984 Astrophys. and Space Sci. 98, 61. 
Lane, A ,  P,  1982 B.A.A.S. 14, 895. 
Lepp, S., McCray, R., Shull, J., Woods, D. and Kallman, T. 1985 Ap. J. 288, 
58. 
Maciel, W. J. 1973 Astrophys. Letters 15, 177 (grains). 
Maciel, W. J. 1976 A&A 48, 27 (molecules). 
Raymond, J . ,  Cox, D. and Smith, B. 1976 Ap. J. 204, 290. 
Schwarzschild, M. 1975 Ap. J. 195, 137, 
Sutton, E., Storey, J., Betz, A., Townes, C. and Spears, D. 1979 Ap. J. 230, 
L105. 
Wehrse, R. and Kalkofen, W. 1985 A&A in press (CfA preprint 2090). 
Wilson, 0. C. 1960 Ap. J.  131, 75. 
PRESSURE 
PERTURBATIONS 
IN EXTENDED 
CHROMOSPHERE 
CONDENSATION 
INSTABILITY/ 
COOLING 
EXPELLED DUST 
CSE Thermal instability 
Mass Loss 
Scenario 
SiO Masers 
R/R,: 1 
t (sec) : 


Chromospheric dust formation, stellar masers and mass loss

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