Circumstellar envelopes

MacKay D. D. S. and Chamley S. B. (2001). Phosphorus in circumstellar envelopes. Monthly Notices of the Royal Astronomical Society 325 545-549... 

In order to investigate the gas phase chemistry of the circumstellar envelopes around these peculiar objects, we have observed radio molecular lines of H20, SiO, HCN, and CO towards three of them BM Gem (C5, 4J), V778 Cyg (C4, 5J), and EU And (C4, 4). 

Water maser emission has been detected towards V778 Cyg (Fig. 1) in the present observation. It is another evidence of the oxygen-rich nature of the circumstellar envelope around V778 Cyg, because water vapor has always been found around M stars. Thermochemical calculations also support the idea that the water molecules are present in the oxygen-rich environment but not in the carbon-rich one. The positional coincidence of the water maser with the optical star was found to be less than 0.5 arcsec (Deguchi et al. 1987), (Fig. 2). Unfortunately no other emission has been found, though Benson and Little-Marenin (1987) observed a water maser in EU And. Our negative detection of maser emission in EU And was probably due to the intensity variation of the object. 

Abstract A molecular line search in the range between 85 and 89 GHz has been performed in the circumstellar envelopes of 11 evolved stars. Emissions of 29SiO J=2-l,28SiO J=2-l, HCN J=l-0, H13CN J=l-0, HC5 N J=33-32, HCO+ J=l-0 transitions and other transitions of C2 H, C4 H, and C3 N have been observed in 11 stars. We have detected the ground state 29SiO J=2-l maser in several stars. We have also detected HCN emission in VY CMa. A narrow H13CN spike feature near the central velocity has been found in the spectrum of CRL 2688. 

In regard to this object, I would point out that its spectrum varies with phase, shows the effect of a veiling at some phase interval and that the circumstellar envelope around the fainter, larger mass component is quite opaque, at least it is not completely transparent to the stellar radiation. 

The successful models for the ultraviolet spectra of previous SN II consider the emission from a substantial circumstellar layer (Fransson et al. 1984), such as might be expected around a red supergiant with a slow dense wind. The absence of these features makes it unlikely that SK -69 202 had such a wind immediately before the explosion, although It does not rule out a wind at an earlier stage of evolution. The picture in which SN 1987a has a low density circumstellar envelope is consistent with the weak, brief radio emission (Turtle et al 1987) and the absence of early X-ray flux (Makino 1987) as described by Chevalier and Fransson (1987). 

Infrared and radio observations of the circumstellar envelope have been exploited very successfully. The most commonly studied molecule is CO hundreds of nearby stars are known CO sources (Nyman et al. 1992). Additionally, over 50 different species have now been detected in various circumstellar envelopes (Olofsson 1992). 

To date, neither PAH emission nor absorption has been detected in the circumstellar envelope around a cool carbon star PAH emission has only been seen in carbon-rich environments where there is substantial energy density of ultraviolet radiation. This correlation could simply be an excitation effect the carbon features are only excited by the presence of ultraviolet radiation. However, it could also be that carbon particles are eroded into PAHs in the environment where ultraviolet penetrates either directly by the ultraviolet radiation or indirectly by shocks that accompany the radiation. 

In contrast to the dark cloud chemistry, the molecules in circumstellar envelopes (IRC -f 10216) seem to be created continuously in a small, high temperature high density layer- which allow fast thermodynamic equilibrium- and subsequently expelled into the lower density cool envelope. There they are observed with an... 

Figure 10 Phase diagram for ice as a function of temperature and F, the deposition rate of water vapor. PSN, Ce, and MC denote the primordial solar nebula, circumstellar envelope, and molecular cloud (source Kouchi et al, 1994).

Millar, T.J. Herbst, E. A new chemical-model of the circumstellar envelope surrounding IRC-l-10216. Astron. Astrophys. 1994, 288, 561-571. 

Doty, S.D. Leung, C.M. Detailed chemical modeling of the circumstellar envelopes of carbon stars Application to IRC-l-10216. Astrophys. J. 1998, 502, 898-908. 

In this chapter, we reviewed the methods and results of chemical equilibrium calculations applied to solar composition material. These types of calculations are applicable to chemistry in a variety of astronomical environments including the atmospheres and circumstellar envelopes of cool stars, the solar nebula and protoplanetary accretion disks around other stars, planetary atmospheres, and the atmospheres of brown dwarfs. The results of chemical equilibrium calculations have guided studies of elemental abundances in meteorites and presolar grains and as a result have helped to refine nucleosynthetic models of element formation in stars. 

This chapter briefly introduces the chemistry in circumstellar envelopes (CSE) around old, mass-losing stars. The focus is on stars with initial masses of one to eight solar masses that evolve into red giant stars with a few hundred times the solar radius, and which develop circumstellar shells several hundred times their stellar radii. The chemistry in the innermost circumstellar shell adjacent to the photosphere is dominated by thermochemistry, whereas photochemistry driven by interstellar UV radiation dominates in the outer shell. The conditions in the CSE allow mineral condensation within a few stellar radii, and these grains are important sources of interstellar dust. Micron-sized dust grains that formed in the CSE of red giant stars have been isolated from certain meteorites and their elemental and isotopic chemistry provides detailed insights into nucleosynthesis processes and dust formation conditions of their parent stars, which died before the solar system was bom 4.56 Ga ago. 

Figure 1. Approximate conditions in a circumstellar envelope around an AGB star of 500 Rq (3.5x10 cm) and IMq...

---