Circumstellar envelopes and Asymptotic Giant
Branch stars |
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Authors: | HJ Habing |
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Institution: | (1) Sterrewacht Leiden, 2300 RA Leiden, The Netherlands , NL |
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Abstract: | Summary.
Red giants are sometimes surrounded by envelopes, the
result of the ejection of stellar matter at a large rate
(/yr)
and at a low velocity (10 km/s). In this review
the envelopes are discussed and the relation between stars and
envelope: what stars combine with what envelopes?
The envelope emits radiation by various processes and has been
detected at all wavelengths between the visual and the microwave
range. I review the observations of continuum radiation emitted by
dust particles and of rotational transitions of molecules, where
these molecules have been excited by thermal or by non–thermal
(“maser”) processes. I discuss mainly the oxygen–rich
stars, those of spectral type M, and only briefly the closely related
carbon–rich stars.
By and large the density in the envelope is well described by
spherically symmetric outflow at a constant velocity; on the time
scale needed to flow from stellar surface to the outermost layers,
i.e.
yr, the loss of mass is sometimes interrupted suddenly
after which the envelope becomes “detached” from the star. The
temperature decreases when moving outward; heat input is by
friction between dust particles and gas and cooling occurs by line
radiation by various molecules, especially by
HO. The molecular
composition is determined by formation in an equilibrium process
deep in the atmosphere and by destruction in the outer parts of the
outflow by interstellar UV radiation
(H, CO, HO) or by
depletion due to condensation on dust grains (SiO); dust particles
of silicate material solidify where the radiation temperature is
decreased to about 1000 K, and this is at a few stellar radii.
The various continuum spectra produced by the dust particles in
different stars are well modelled by a simple model of the density
and dust temperature distribution plus the assumption that the
particles consist of “dirty silicate”, i.e. silicate with Fe and Al
ions added. A large range of optical depths,
, is observed:
from 0.01 to 10. In envelopes with large optical depth the star
itself can no longer be detected directly. Model calculations also
show that the momentum in the outflow, i.e.
is
provided by radiation pressure on the dust particles followed by
the complete transfer of this momentum to the gas. The mass–loss
rate itself,
, is not determined by radiation pressure but by
dynamic processes in the region below the dust condensation layer.
When
is sufficiently large its measurement, that of the
stellar luminosity,
and that of the outflow velocity,
,
permit the determination of
, i.e. the total
outflow rate, without making assumptions about the abundance of the dust
particles or of the molecular gases. Detached envelopes have been
seen in a few cases.
Thermal molecular radiation is faint compared to the maser emission
but has been measured in distant stars, e.g. in stars near the
galactic center. Different molecules outline different “spheres”
around the star. The largest sphere (a radius of 0.1 pc) is
outlined by an emission line belonging to the
CO()
transition. Higher rotational transitions of CO give smaller
diameters. A comparison of CO
() and () fluxes
in stars with very thick envelopes leads to the conclusion that an
abrupt decrease in the mass–loss rate occurred some ten thousand
years ago.
Three molecules produce each several maser lines: SiO,
HO and
OH. Several new
HO lines have recently been discovered; their
exploration has hardly been started. The high intensity of the
maser lines makes interferometry possible and hence detailed
mapping. The SiO lines are formed deep in the envelope, below the
dust condensation layer. OH maser lines are produced farthest out,
HO lines in
between. The excitation mechanisms for most maser
lines is understood globally, but detailed models are lacking,
largely because the problem is non–linear and the solution of the
radiative transfer equation requires a highly anisotropic geometry.
The geometrical and kinematical properties of the 1612 MHz OH
maser, which in many objects is very strong, are explained by a
thin shell of OH; because the angular diameter of the shell can be
measured directly and the linear diameter can be determined from
the difference in the time of maximum flux of blue and red maser
peaks, the distance of the shell and of the star can be measured.
The presence or absence of individual maser lines appears to depend
on the value of
and is well described by a sequence called
“Lewis' chronology”.
The central star is a long–period variable with a period of 300
days or longer and with a large luminosity amplitude
().
Evidence is given that each star has the
maximum luminosity it will reach during its evolution and that it
is a thermally–pulsing Asymptotic–Giant–Branch star (TP–AGB)
with a main–sequence mass between 1 and 6
. Stars of the same
main–sequence mass,
,
have different mass–loss rates, in
some cases by a factor of 10. The mass–loss rate probably
increases with time, and the highest mass–loss rates are reached
toward the end of the evolution. Stars with higher
ultimately
reach higher mass–loss rates. The calibration of the
main–sequence mass is reviewed. Most Mira variables with mass loss
have a mass between 1.0 and 1.2
. OH/IR stars with periods
over 1000 days have no counterparts among the carbon stars and thus
have .
Stars as discussed in this review have been
found only in the thin galactic disk and in the bulge.
Finally I review several recently proposed scenarios for TP–AGB
evolution in which mass loss is taken into account. These scenarios
represent the observations quite well; their major short–coming is
the lack of an explanation why the central stars are always
large–amplitude, long–period variables and why such stars are the
ones with high mass–loss rates.
Received: 10 January 1996 |
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Keywords: | |
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