Dust shells

Stars further along in their life cycle are often cooler and redder than younger stars. Shells of dust that has been condensed from material ejected from these cool stars often surround them. Such circumstellar dust shells, heated by the stars, emit strongly in the infrared with a spectrum characteristic of absorption bands in the dust the emissivity of a small particle is equal to its absorption efficiency (see Section 4.7). An excellent review of circumstellar dust has been given by Ney (1977). 

Figure 14.5 Emission spectra of dust shells around a carbon star (top) and around an oxygen star (bottom). From Treffers and Cohen (1974).

The R CrB variables and related hydrogen-deficient carbon stars show strong carbon features (except CH) and very weak Balmer lines. The R CrB stars are surrounded by circumstellar dust shells, and continue to eject puffs of new circumstellar material on time scales of a month or two mass loss rates of order 10- Mo yr- are reported by Walker 1986. The compositions of the R CrB stars and hydrogen-deficient carbon stars are were compiled by Lambert (1986). Hydrogen is extremely deficient in both groups (H/He 10 J-10 °), and C/Fe is enriched by typically an order of magnitude over the solar ratio. C/0 is typically 2 (more than is measured in the AGB stars, and [N/Fe] 1. R CrB itself contains a strong lithium... 

The surface distribution of M stars is studied by differentiating them according to whether they show a circumstellar dust shell (CS) or not. Analysis shows that galactic latitudinal and longitudinal distributions are not determined by spectral subclasses alone. The study also indicates that the M type stars with CS have higher intrinsic luminosities in the K band than those without CS. The M stars used in the study are obtained from the Two Micron Sky Survey catalogue (IRC) which is an unbiased sample with respect to the interstellar extinction. The CS feature is identified by the ratio of flux densities at 12 and 25 m in the IRAS point source catalog. 

Fig. 1s Structure of a dust driven wind mean grain radius , hydrodynamioal velocity v, optical depth of the dust shell at X = Ip, degree of condensation f.

The eigenvalue problem is solved for M which results to be 3-5 10 5 Mayr 1. Fig. 1 shows the structure of the dust shell as it has been discussed in former papers (Gail, Sedlmayr, 1985, 1987a). We only want to point out two facts ... 

Silicon carbide is thought to be an important component of the dust shells surrounding carbon-rich stars it is likely, therefore, that astrophysical studies of the SiC radical will be significant in the future. 

Already the first infrared observations of late-type giant stars have revealed that many of them are indeed surrounded by thick dust shells (Woolf Ney 1969). These were rapidly found to consist of carbonaceous dust (some kind of soot) if the stellar spectrum indicates the star to be carbon-rich, and to be silicate dust (olivine, pyroxene) if the star is oxygen-rich (Gilman 1969). Since this dust is mixed into the interstellar medium due to mass loss by stellar winds, it was then assumed that silicate and carbon particles are abundant dust components in the interstellar medium. 

With improved possibilities for infrared spectroscopy, broad extinction bands around 9.7 pm and 18 pm have been detected, which were ascribed to the stretching (Woolf Ney 1969) and bending (Treffers Cohen 1974) modes in the SiC>4 tetrahedron forming the building block of silicates, because they correspond to known absorption bands seen in all terrestrial silicates. These bands are also seen in the emission from dust shells around O-rich stars. This gave the first observational hints on the mineralogy of the silicate dust. The smooth, structureless nature of the bands indicated that the silicates in the ISM and in circumstellar dust shells are amorphous. 

Since jqZq seems to set the lower limit for efficient dust formation,4 planetary systems are only expected to form around stars with at least this metallicity. The relative abundances of rock-forming elements in such systems are essentially the same as in the Solar System. Only the total amount of such elements may vary considerably, depending on the birthplace and birth-time of such systems. Planetary systems with unusual elemental compositions, e.g. like those observed in many highly evolved stars, are not expected to exist in particular, oxygen is always more abundant than carbon such that for planetary systems there is no counterpart to the carbon-rich circumstellar dust shells. 

Figure 2.9 Hubble Space Telescope image of the Cat s Eye Nebula. The evolved AGB star (center) ejected its mass in a series of pulses at 1500 year intervals which created concentric dust shells. Photo credit ESA, NASA, the Hubble European Information Centre and the Hubble Heritage Team, STScI/AURA.

Carbonates are common in hydrous meteorites and hydrous IDPs, where they are believed to have formed by parent-body aqueous processing. Since simple models of cometary evolution involve no aqueous processing, carbonates were generally presumed not to occur in comets. However, carbonates have also been detected by infrared spectroscopy in the dust shell around evolved stars and in protostars, where liquid water is not expected (Ceccarelli et al. 2002 Kemper et al. 2002). Indeed, Toppani et al. (2005) have performed experiments that indicate that carbonates can be formed by non-equilibrium condensation in circumstellar environments where water is present as vapor, not as liquid. Detections of carbonates in other exosolar systems are reported by Ceccarelli et al. (2002) and Chiavassa et al. (2005). 

Grain formation occurs when temperatures in the expanding envelope of red giants (RGs) or in SN ejecta are low enough for the condensation of minerals. Many late-type stars are observed to be surrounded by dust shells of grains whose mineral compositions reflect the major chemistry of the gas (e.g., Little-Marenin, 1986). The study of morphological features of pristine grains, of internal... 

In addition to their irregular fadings thought to be due to directed mass-ejections, R CrB stars show low-amplitude variability with periods of 30 - 60 d and associated with pulsations. All show a substantial infrared excess attributed to a warm dust shell, while a few show evidence of a more extended nebula. It will be seen that these stars are extraordinarily rich in exotic behaviour [132]. 

Fig. 10,1. Left Hubble Space Telecsope image of tlie Eskimo nebula. The filaments visible above consist of material ejected by the central star (now a white dwarf) during its AGB phase. Right Speckle interferometry of the carbon star IRC 10216 at 1.125 /im. The structure of the dust shell around the star can be clearly seen. Credit J. Hester, P. Scowen (ASU), HST, NASA...

In this spirit we investigated models where the dust density n fell off as r°, r, or r . The grain emission (and absorption) efficiency was taken as a three part power law in X. a for X < lOOoX e a X for lOOOA < X < 20y and e a X" for X > 20y. This crude form is consistent with expectations for H2O and silicate grains (Irvine and Pollack 1968 and Knacke and Thomson 1973). Once the dependence of dust opacity on radius from the star and on wavelength have been adopted, the only remaining critical parameter is the absolute value of the optical depth at lOOy, T] ooy measured from the outer radius of the dust shell through to the star. The adopted inner and outer radii at which one truncates the dust shell are easily seen to be unimportant provided the cloud is not optically thin (T] QQy O.l) and provided the outer layers do not contribute substantial optical depth respectively. 

At each radius in the dust shell the grain temperature is calculated from the condition of radiative equilibria in the radiation of both the star and the other dust. Neglecting for the moment the secondary radiation, the equilibrium implies that... 

Abstract. Observations of CO emission in Nova Sco 1992 are discussed. Observations of CO in this and other novae, notably NQ Vul (Nova Vul 1976) suggest that CO formation is a necessary precursor to the formation of an optically thidc dust shell. 

The observational evidence is not compelling, but this is largely because there are so few novae with complete data. Theoretical considerations (Rawlings and Williams 1989) also suggest that dust formation is preceded by molecule formation (required for formation of grain nuclei). Thus, the following sequence ought to be observed in all novae that form thick dust shells ... 

Abstract. We resolved dust shells at 10.5 and 12.5 pm around the post asynq>totic pant branch stars HD 161796 and AFGL 2343, two oxygen-ridi proto-planetary nebulae with very cold IRAS colors. The shell of AFGL 2343 appears as a drculax ring with 4" — 5" diameter, while HD 161796 appears bipolar with 2" diameter, bmer shell radii indicate that both stars left the asymptotic giant brand about 200 years ago. 

We modelled the dust shells of HD 161796 and AFGL 2343 to derive physical properties of the shells. Li the models, we fit the inner shell radii and optical and infrared spectra to the observed values. For HD 161796, we used a radiative transfer code due to Haisdi (1979). We derive a distance of 1200 ( 400) pc to HD 161796, a stellar luminosity of3600 ( 1800 X0), and a mass loss rate in the shell of 3 ( 1) x 10 M yr . Our derived luminosity clearly casts HD 161796 as a p op H object, in accord with its low wind velocity and low metal abundance. Farther details of the observations and modelling of HD 161796 are given in Skinner et al. (1993). 

We can estimate the time elapsed since each star left the AGB from the observed inner radii of the dust shells. The shell expansion velocity is obtsdned from published CO observations. For HD 161796, with D=1.2 kpc d Vexp = 12 km sec, the observed inner radius of 0.5" implies that the star stopped losing mass and left the AGB 240 years ago. For AFGL 2343, the relevant parameters are D=1.5 kpc, Vexp = 33 km sec, and 0.75" inner radius, meaning that the star left the AGB 170 years ago. Therefore, if AFGL 2343 is a proto-planetary nebula, it is lazier than HD 161796, whidi left the AGB about the same time as AFGL 2343, but already has a hotter central star (F2-5) than AFGL 2343 (G5). 

The 21fjan PPNe are a class of PPNe unto themselves of which there are almost a dozen known to date. They are considered PPNe because they have double peaked spectral energy distributions (SEDs), one peak in the optical from the star and the second in the infrared due to the drcumstellar dust shell (Pottasdi Parthasarathy 1988 Kwok, Volk and Hrivnak 1989). In fact, both IRAS 22272 and IRAS 07134 have well known optical counter-... 

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