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The Chemistry in Circumstellar Envelopes of Evolved Stars: Following the Origin of the Elements to the Origin of Life

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The Chemistry in Circumstellar Envelopes of Evolved Stars: Following the Origin of the Elements to the Origin of Life The chemistry in circumstellar envelopes of evolved stars: Following the origin of the elements to the origin of life Lucy M. Ziurys† Departments of Astronomy and Chemistry, Arizona Radio Observatory, and Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 Edited by William Klemperer, Harvard University, Cambridge, MA, and approved May 19, 2006 (received for review March 21, 2006) Mass loss from evolved stars results in the formation of unusual the H shell continues to burn. The fusion of helium produces 12C, chemical laboratories: circumstellar envelopes. Such envelopes are and alpha capture on 12C yields 16O in the core. Eventually, the found around carbon- and oxygen-rich asymptotic giant branch central helium is exhausted, leaving an oxygen͞carbon core stars and red supergiants. As the gaseous material of the envelope surrounded by He-burning and H-burning shells. The appear- flows from the star, the resulting temperature and density gradi- ance of this two-shell structure marks the beginning of the AGB ents create a complex chemical environment involving hot, ther- phase (4). The primary source of luminosity in an AGB star is modynamically controlled synthesis, molecule ‘‘freeze-out,’’ shock- the H-burning shell, but periodically the He shell ignites for short initiated reactions, and photochemistry governed by radical periods of time (5). The energy released in this so-called mechanisms. In the circumstellar envelope of the carbon-rich star ‘‘thermal pulse’’ creates a convective zone extending from shell IRC؉10216, >50 different chemical compounds have been identi- to shell, which mixes nuclear products from the He-burning zone fied, including such exotic species as C8H, C3S, SiC3, and AlNC. The to the stellar surface. This process, called the ‘‘third dredge-up,’’ chemistry here is dominated by molecules containing long carbon brings carbon and other elements into the detaching envelope. chains, silicon, and metals such as magnesium, sodium, and alu- The thermal pulses also aid in driving the outer material from the minum, which makes it quite distinct from that found in molecular star, creating a circumstellar shell (1, 2). In fact, AGB stars lose clouds. The molecular composition of the oxygen-rich counterparts up to 80% of their original mass in the form of an envelope, with Ϫ Ϫ Ϫ is not nearly as well explored, although recent studies of VY Canis typical rates of 10 6 to 10 4 MJ⅐yr 1 (1, 2, 4). They therefore play ؉ Majoris have resulted in the identification of HCO ,SO2, and even a major role in the chemical evolution of the galaxy (7). NaCl in this object, suggesting chemical complexity here as well. As RSGs evolve from massive O and B stars (M Ϸ8–30 MJ) that these envelopes evolve into planetary nebulae with a hot, exposed have left the main sequence. These objects experience intense Ϫ Ϫ Ϫ central star, synthesis of molecular ions becomes important, as mass loss (Ϸ10 5 to 10 3 MJ⅐yr 1) for Ϸ103 to 104 yr before they ؉ indicated by studies of NGC 7027. Numerous species such as HCO , become supernovae, although there are many uncertainties HCN, and CCH are found in old planetary nebulae such as the Helix. connected with their evolution (2). They typically have surface This ‘‘survivor’’ molecular material may be linked to the variety of temperatures of T Ϸ 3,500–4,000 K. compounds found recently in diffuse clouds. Organic molecules in dense interstellar clouds may ultimately be traced back to carbon- The Chemical Environment of Circumstellar Shells rich fragments originally formed in circumstellar shells. Circumstellar shells exhibit a range of physical conditions, as shown in Fig. 1. In Fig. 1, a simplified cross section of a typical envelope asymptotic giant branch stars ͉ astrochemistry ͉ for an AGB star is displayed, plotted on a scale of log r (r ϭ distance molecular line observations from star). These objects are quite large, extending 104 to 105 stellar radii (R*). Near the stellar photosphere, the envelope material (all ircumstellar envelopes of evolved stars are among the most gaseous) is warm (T Ϸ1,500 K) and dense (n Ϸ1010 cmϪ3). As the Cremarkable chemical laboratories in the universe. These matter flows away from the star, it expands and cools, typically with Ϫ1 Ϫ2 envelopes are created by extensive mass loss in the later stages T ϰ r and n ϰ r (8, 9). Dust begins to form at Ϸ5–15 R*, and of stellar evolution, caused by thermal pulses in the star interior shortly thereafter the maximum outflow velocity is achieved. Be- and radiation pressure on dust (1, 2). Because of the low yond the dust formation zone, the temperature and density con- temperature of the central star, and the long time scales for mass tinue to decrease such that at the outer edges of the shell, T Ϸ25 loss, molecules and dust form in the envelope, and are then K and n Ϸ105 cmϪ3 (8, 9). gently blown into the interstellar medium (ISM). The material The chemistry varies substantially throughout the envelope. In lost from such envelopes is thought to account for nearly 80% (by the inner shell, the temperature and density are sufficiently high mass) of the ISM (3). that thermodynamic equilibrium governs molecular abundances. Such local thermodynamic equilibrium (LTE) chemistry was The Origin of Circumstellar Envelopes predicted in the early models of Tsuji (10) and incorporated into Chemically rich circumstellar shells are found around asymptotic later calculations (e.g., refs. 11 and 12). The LTE predictions giant branch (AGB) stars and red supergiants (RSGs). AGB have been borne out by observations (13, 14). Infrared spec- stars are objects of low and intermediate initial mass (Ϸ1–8 MJ) troscopy has demonstrated that stable, closed-shell species such with effective temperatures near T Ϸ2,000–3,500 K (4–6). These objects occur on an evolutionary track that begins at the end of the main sequence. At this stage, all hydrogen in the stellar core Conflict of interest statement: No conflicts declared. has been converted to helium, and the core itself starts to This paper was submitted directly (Track II) to the PNAS office. gravitationally contract. This contraction ignites hydrogen in a Abbreviations: AGB, asymptotic giant branch; RSG, red supergiant; LTE, local thermody- shell around the core, and the star enters the so-called red giant namic equilibrium; PN, planetary nebula(e); PPN, protoplanetary nebula(e); VYCMa, VY branch. As the core continues to contract, the envelope of the Canis Majoris. red giant expands and becomes convectively unstable. Given †E-mail: [email protected]. sufficient contraction, the helium core will eventually ignite as © 2006 by The National Academy of Sciences of the USA 12274–12279 ͉ PNAS ͉ August 15, 2006 ͉ vol. 103 ͉ no. 33 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0602277103 Downloaded by guest on September 28, 2021 Table 1. Confirmed molecular identifications in IRC؉10216 CO CCH HC3N CCS SiO NaCl SPECIAL FEATURE CS C3HHC5NC3S SiS AlCl CN C3OHC7NC3N SiC KCl HCN C4HHC9NC5N SiN AlF HCCH C5HH2C4 HC4N SiC2 MgNC HNC C6HH2C6 c-C3H2 SiC3 MgCN H2CCH2 C7HHC2NCH3CN SiCN AlNC CH4 C8HC3 CP SiC4 KCN NH3 C2 C5 PN SiH4 NaCN H2S Fig. 1. Cross section of a circumstellar envelope, plotted in terms of log r, where r is the distance from the central star. Regions of LTE chemistry, dust formation, and UV-induced photochemistry are shown. suggested that during the PPN phase, a second, more violent mass loss event occurs with high velocity winds that partially destroy the remnant AGB envelope (e.g., refs. 21–23). For as CS, NH3, HCN, and HCCH have high abundances in the inner Ͻ ϩ Ϸ Ϫ3 Ϫ6 example, CRL 2688, the Egg Nebula, exhibits both low ( 20 envelope of IRC 10216 ( 10 to 10 , relative to H2). As ⅐ Ϫ1 Ͼ ⅐ Ϫ1 molecules flow outward, some abundances freeze out at inter- km s ) and high velocity ( 100 km s ) outflows (24, 25). A mediate radii, HCN and CO, for example (15, 16). At the outer more evolved object is CRL 618; here, the star, which has lost regions, photons and cosmic rays initiate an active photochem- its dusty shroud, is producing sufficient UV radiation to create istry that creates radicals (11, 12). As shown in interferometer a small, nearby HII region (26, 27). The envelopes of RSGs also have large temperature and maps (17, 18), radicals such as CN, CCH, C3H, and C4H exist in similar shell-like structures near the envelope edge (Fig. 2). They density gradients, a dust formation zone, and significant molec- are the daughter species of parent molecules such as HCN and ular abundances, substantiated by various observations (e.g., HCCH produced at LTE. In addition, shock waves in the shell, refs. 3, 13, 28, and 29). However, RSG stars have shorter lifetimes than their AGB counterparts (a few hundred thousand caused by thermal pulses, may influence abundances (19, 20). ASTRONOMY There are additional complications to this scheme. For exam- years), and their mass loss appears to be more sporadic and ple, some shells are carbon-rich (C type), whereas others are asymmetric, characterized by jets and high-density streams (30, oxygen-rich (M type), and S types have C Ϸ O. AGB envelopes 31). The impact of these differences on the chemistry of RSG are thought to initially start with more oxygen than carbon; they shells has yet to be evaluated. become C-enriched during the third dredge-up (4, 5). Shocks and pulsations associated with the third dredge-up may also facilitate Distinct Chemical Aspects: Carbon, Silicon, and Metals the chemistry, because C-rich envelopes appear to have the Circumstellar shells are principally studied by infrared and greatest variety of chemical compounds.
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