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Aerosols in Astrophysics Stars Complete Their Cycle of Existence, There R.E.Stencel, C.A

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Aerosols in Astrophysics Stars Complete Their Cycle of Existence, There R.E.Stencel, C.A Aerosols in Astrophysics stars complete their cycle of existence, there R.E.Stencel, C.A. Jurgenson & is a wholesale return of modified material T.A. Ostrowski-Fukuda back to the interstellar medium, affecting the The Observatories, University of Denver, next generation of star and planet formation. Department of Physics & Astronomy, Denver, Colorado 80208, USA Submitted: Evolved stars are characterized by surface 2002December30, Email: [email protected] temperatures low enough to allow molecules and solid-phase particles to exist in their ABSTRACT: atmospheres. In contrast to the nearly 6000K surface temperature of the Sun, the Solid phase material exists in low density kinetic temperatures of evolved stars may astrophysical environments, from stellar range between 2000K near their “surfaces” atmospheres to interstellar clouds, from to a few hundred K in their circumstellar current times to early phases of the universe. envelopes [CSE] at a few stellar radii We consider the origin, composition and distance. Time-dependent phase transitions evolution of solid phase materials in the from ionized plasma to cold, solid state are particular case of stellar outer atmospheres, observed, spectroscopically, to exist. as representative of many conditions. Also considered are the dynamics and interactions Some terminology may be helpful for the in circumstellar and galactic environments. reader not already acquainted with stellar Finally, new observational prospects for evolution. Solar mass stars are predicted to spectroscopy, polarimetry and leave the hydrogen-fusing “main sequence” interferometry are discussed. after approximately ten billion years, and progress through a rapid series of internal changes during what is known as the red 1.0 INTRODUCTION giant and asymptotic giant branch [RGB, AGB] phases, lasting one percent of the Solid phase material can be found nearly main sequence lifetime. These internal everywhere in space. In the astronomical changes are driven by interior literature, these tiny solids are referred to as thermodynamics, as helium fusion is “dust” and/or “grains” in discussions of the initiated and exhausted as a primary energy most important opacity source at source, past the main sequence. The wavelengths longer than the Lyman limit for important phenomenon during these latter ionizing hydrogen (91 nanometers). These phases is the high rate of evaporation (mass solids play key roles in molecular clouds loss) of the outer layers, which during peak and star formation – enabling cooling, times can rival the internal evolutionary shielding from radiation and energetic timescales. The outflows are estimated to particles, and forming the basis for alter the chemistry of the interstellar planetessimal formation in primitive solar medium and ultimately, the galaxy. The systems. The spectral energy distributions evaporation rate is modulated by the of all of these astronomical objects are quantity and composition of molecular and strongly modified by solid particles, which solid material in the outflow. Hence, Carl Sagan once characterized as having the understanding the role of solids in size and average composition of bacteria. In circumstellar environments is key to this review, we focus on the role that solids correctly predicting the evolutionary paths play in the most evolved stars. As these of stars of diverse initial masses and categories: thermal-driven, radiation-driven, compositions. wave-driven, and shock-driven (pulsation, radiation and dust). An excellent summary 1.1 Aerosols of the relevant physics was presented by Holzer and MacGregor (1985). Most studies treat solids and the surrounding gas as a two fluid system. While this has Two of the most important parameters been useful in advancing the subject, describing a stellar wind are derivable from modern computing power may allow us to observation: the mass loss rate [the amount consider the system in combination as an of mass lost by the star per unit time] and “aerosol”. An aerosol is defined in its the terminal velocity [the velocity of the simplest form as a collection of solid or stellar wind at a large distance from the liquid particles suspended in a gas (Hinds, star]. Stellar mass loss rates vary from 1999). Aerosols are two phase systems, present solar wind levels (10-14 solar masses consisting of the particles and gas in which per year) to AGB and post-AGB “super- they are suspended (cf . Seinfeld and Pandis wind” phases (up to 10-4 solar masses per 1998). This definition admits a range of year), deduced from spectroscopically sizes from molecular clusters (tens of observed density and velocity. The spectral nanometers) to solids (micron-sized), and lines from stellar winds can often be can include neutrals as well as ions – exactly distinguished from the photospheric lines the mix found in circumstellar environments due to their large width or Doppler shift (CSE). Terrestrial atmospheric densities and produced by the outflowing motion of the temperatures range from 10-3 g cm-3 (1019.4 gas in the wind. Wind lines can appear in cm-3, 288K) at the surface, to roughly six emission, in absorption, or as a combination orders of magnitude less density in the upper of the two [P Cygni profile], where stratosphere (~200K). While the observable absorption and emission lines temperatures resemble circumstellar ones, show a Doppler shift due to the outflow, terrestrial densities are orders of magnitude even with relatively small mass loss rates. larger. Micron sized terrestrial aerosols Stellar wind speeds range from 10-20 km float in a molecular nitrogen and oxygen per second for an AGB star, to ~500 km per gas, while circumstellar aerosols form and second for the solar wind near earth, to over move in a mostly hydrogen gas medium. 3,000 km per second (1% of the speed of Therefore, it is timely to review recent light!) for an early type star, such as an O6 progress in astrophysical descriptions of star. Although the velocity structure of the CSE, along with advances in terrestrial wind varies as it progresses out from the atmospheric aerosol chemistry and rarified photosphere, for a typical AGB wind gas dynamical theory and experiment, velocity, the crossing time for a wind applied to an astrophysical context. traveling out 1000 AU from the star would be about 50 years. 1.2 Mass Loss Mechanisms Stellar winds require heat input or It long been known that red giant and momentum input to become transonic, supergiant stars are losing mass at high rates varying from subsonic (typically, a few in the latter stages of life in an observable km/sec) to supersonic with height. The expansion called the stellar wind. Stellar energy per unit mass of an atmosphere, wind theories can be grouped into four main which is gravitationally bound, must be negative. The energy of an outflow, which photosphere, radiation force on the dust escapes the gravitational potential well of grains might drive massive outflows from the star, is positive at large distances. This the star. requires that energy must be added to the gas in order for it to escape. The energy can (c) Wave-driven winds involve the transport be added in the form of heat input or work of a mechanical energy flux through the done by an outward directed force stellar atmosphere and takes into account (momentum input). Four general wind only small amplitude waves for energy driving mechanisms have been discussed in transport, including weak shock waves. detail (cf. Holzer & MacGregor 1985): When a magnetic field is present the small thermally-driven, line-driven, wave-driven amplitude waves will either be compressive and shock-driven winds. or non-compressive, corresponding to acoustic or Alfven waves. However, the (a) The concept of thermally-driven winds compressive waves tend to steepen very was developed in the early 1960's mainly by rapidly into weak shocks and are dissipated Eugene Parker. In this model for the wind, within a few pressure scale-heights of the the only important outward force exerted on base of the atmosphere. Therefore, although the expanding atmosphere would be the acoustic waves can levitate material to a thermal pressure gradient. Thermally driven small height in the atmosphere they are not winds require high (coronal) temperatures, strong enough to lift a massive wind out of and exhibit much larger radiative fluxes, the stars' gravitational well. With sufficient especially in the ultraviolet and extreme- magnetic field strength relative to the ultraviolet. Unlike the sun, red giant and thermal energy, Alfven waves show promise supergiant stars do not have hot coronae, for driving outflows, particularly in the solar effectively ruling out this mechanism. case. Questions related to energy dissipation in these waves remain (b) For line-radiation driven winds, the unresolved. electromagnetic radiation field of a star provides a reservoir of momentum that can (d) Shock-driven winds (combination of be used to drive a stellar wind. This pulsational ‘levitation’ and outward mechanism works when there is a coupling radiation forces on dust) arise from the of the radiation field to the gas/dust in the coupling of the stellar radiation field to a atmosphere. Radiation-driven winds work sufficiently dusty atmosphere and CSE well for hot stars when the atmosphere already expanded by the effect of pulsations. exhibits many strong UV resonance lines Variable stars, such as the ones of interest in and the radiative flux is substantial in the this study, exhibit somewhat regular ultraviolet. The coupling comes from the pulsations that produce large-amplitude stellar radiation and the atmospheric opacity. shock waves propagating outward through Stellar radiation for cool stars peaks in the the photosphere and into the CSE. This red or near-infrared, and the strong allows for the stellar material to be lifted to resonance lines of the atoms occur in the sufficient height where dust will form and visible and ultraviolet. Therefore, this where the escape velocity is substantially mechanism acting alone can be ruled out for lowered.
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