Carbon rich dust grains Amorphous Carbon (AC) SiC PAH
	Oxygen rich dust grains Olivine Pyroxene
	Other issues

Carbon rich dust grains Amorphous Carbon (AC) (back to top) Amorphous silicate was found in the disk of a bipolar PN, the Ant nebula, Hen 2-154. It means the disk is still young. (Chesneau et al.,  2007A&A...473L..29C) SiC (back to top) sub-classes:  alpha-SiC: hexagonal and rhomboedric types, very stable up to a temperature of 2700 deg. beta-SiC: cubic typs. It's more stable than alpha-SiC above a temperature of  1600 deg. The two classes of SiC are similar in structure and thermaldynamical property, can be  differentiated only through crystallography and IR spectrum. SiC feature in IRAS LWS spectra can be divided into 3 classes (a), (b) and (c), with each class characterized by the width of 11.3um feature and the presence (or no) of secondary features at ~ 8.6um, ~11.7um and ~ 12.8um. With the secondary features subtracted, class(a) matches purest and smallest grains of alpha-SiC, while class (c) matches polluted, coarse grains of alpha-SiC. 24 M type stars are associated with class (c) SiC feature: 00186+5940, 01030-3157, 01150+5732, 02169+5645, 05027-2158, 05208-0436, 05374+3153, 08555+1102, 09057+1325, 11011-6651, 13244-5904, 15298+0348, 15361+2441, 15492+4837, 16521-2153, 17072+1844, 17398-4344, 18401+2854, 21581+5707, 23420+5618, 00245-0652, 20111-4708, 21321+0136, 21377-0200. Many optical spectral type S stars are associated with class (c). Secondary features might be carbonaceous dust responsible for IR continuum excess. (from Papoular, 1988A&A...204..138P) (figures: class (a), (b), (c) with LRS type 4n (1st rwo) and 1n (2nd row) respectively)                                                                       (figure: 6 sub- classes of class (a): with different strength at 11.3um) Laboratory measurement were made for beta-SiC grains with two different shapes: one mainly  in small spherical shape (diameter = 27nm) and the other in ellipsoidal aggregates (size of  100-500nm) of small crystallites (diameter = 15-20nm).  Laboratory spectral measurements  showed that, the spherical grains show double peak features (at 11.0 and 12.1 um) with the redder  peak stronger, while ellipsoidal aggregates show a single peak feature near 11.4um. The different  types of observed 11.3um (IRAS LRS) features can be explained by linear combination of the  two types of grain shapes, but can not be interpreted in terms of different polytypes. (from  Papoular et al., 1998A&A...329.1035P) (figure: left is 6 types of observed 11.3um feature profiles--IRAS LRS; right is different combination of the  two shapes: 1--ellipsoid; 2--0.78 ellipsoid + 0.39 sphere; 3--0.58 ellipsoid + 0.77 sphere.) Alpha-SiC is stable up to 2700 deg. Beta-SiC is also very stable but less thermodynamically favored  than alpha-SiC. Beta-SiC is favored when condensation takes place in vacuum. Beta-SiC is more stable than alpha-SiC under 2000 deg. But beta-SiC will transform into alpha-SiC when the temperature is higher than 2100 deg while the reverse reaction is very difficult.  Several important facts about SiC grains in C-stars or ISM:   Carbon stars contribute roughly 1/3 of mass to ISM (Tielens, 1990). 11.3 um feature found in some C stars which could be produced by SiC grains, e.g., Hackwell (1972), Treffers and Cohen (1974);  some optically thick C stars envelope show very weak or no 11.3 um feature. Whittet, Duley and Martin (1990MNRAS.244..427W). ISM extinction curve doesn't show 11.3um feature, where have SiC grains from carbon stars gone?  Stephens (1980), Whittet, Duley and Martin (1990). Laboratory measurements showed that different alpha-SiC grain size distributions or impurities do not alter the width of the 11.3um feature, instead, they affect the  position of the peak position. Larger grains produce weaker 11.3um feature. Fiedemann et all. (1981). They found a peak at 11.4 um in their experiment. Laboratory measurements showed that alpha-SiC grains have a feature near 11.4 um, while beta-SiC grains have a feature near 11.0um. The feature positions of pure alpha- or beta-SiC grains do not change with grain sizes. But alpha-SiC with 11% of impurities (carbon, silicon, metallic ion, SiO2) does show a red shift of emission peak with larger grain sizes. Borghesi et al. (1985). small particles of a single shape produce several narrow emission features between the longitudinal and transverse vibrational wavelengths at 10.2um and 12.8um. Particles of different shapes would produce features at different wavelengths between these limits. A continuous distribution of shapes will produce smooth feature with cut-on and cut-off wavelengths corresponding to the longitudinal and transverse vibrational modes of SiC. (Treffers and Cohen 1974, Kozasa et al., 1996) alpha-SiC is the best candidate to reproduce the strongest 11.3um features observed in IRAS LRS data, which means alpha-SiC is very similar only to the 11.3um features observed in optically thin carbon stars whose underlying continuum is the hotest. For cooler carbon stars, the discrepancy becomes larger.  Large size grains formed in the disk of Red Rectangle are mostly beta-SiC. (Jura et al. 1997) AFGL 3068 was the first C stars found to show 11.3um absorption  feature. (Jones et al., 1978) SiC feature is weaker in cooler carbon stars and companioned by an emission feature around 8.5um which may be attributed to alpha:C-H on the surface of SiC grains. (Baron et al., 1987; Geobel et al., 1995) Several facts about SiC grains in meteorites:   SiC grains found in meteorites are all beta-SiC (with 0.1-1.0 um size. Bernatowicz et al., 1987, Geobel et al., 1995). Grains in Murchison meteorite appear to be hexagonal (alpha-SiC) in scanning electron microscope, but show Raman scpectroscopy between 9.5-16.7um of beta-SiC. Hoppe et al., 1994. Meteorites are lack of SiC grains. The reason is unknown yet. (Tang et al., 1989) Isotopic composition analysis indicated that SiC in meteorites may originate from four different types of stars, including nova. (Tang et al., 1989) From the analysis of C and N isotopic data, four distinct source of the two elements in meteorites had been suggested (three of them are CNO cycle (produce 13C, deplete 15N), He-burning (produce 13C and 15N), and H-burning). (Anders and Zinner, 1993) beta-SiC appear in large size grains in meteorites. (Anders and Zinner, 1993) Given that it is geologically unlike to form SiC through processing of solar nebula products in parent bodies of meteorites, all beta-SiC grains in meteorites should have a pre-solar origin. Because alpha-SiC is difficult to transform into beta-SiC, the beta-SiC in meteorites must be directly created . Ground based IR spectral observations discovered 4 carbon stars with SiC absorption feature (including previously found AFGL 3068, another three: IRAS 02408+5458, AFGL 2477, AFGL 5625). All carbon stars with SiC emission or absorption features can be best fitted by self-absorption model of alpha-SiC. The role of self-absorption may indicate that we don't need coating of SiC grains with alpha:C-H to explain the diminution of SiC in cooler stars. (from Speck et al., 1997MNRAS.288..431S) Majority of C stars were found to contain mainly alpha-SiC grains, contrary to meteorites which contain  contain mainly beta-SiC. (from Speck, et al., 2000IAUS..177..578S) Laboratory measurement of the IR extinction curves of mixtures of amorphous  carbon (AC) and SiC. (from Orofino et al., 1991A&A...252..315O) (figure: a--AC with 25% alpha-SiC (by mass, solid line) and 13% alpha-SiC (dashed line); b--AC with 25% beta-SiC (solid line) and 10% beta-SiC (dashed line)) (figure: a--AC with 25% alpha-SiC in KBr (solid line) and in vacuum (dashed line); b--AC with 25% beta-SiC in KBr (solid line) and in vacuum (dashed line)) Radiative transfer model fitting of C-stars showed that, beside the continuum emission from  amorphous carbon dust grains, alpha-SiC grains are needed for most of the sources to  reproduce the 11.3um feature, while beta_SiC is also occasionally needed for some sources.  The presence of SiC grains is not correlated with optical thickness or any other physical or  geometrical properties of the circumstellar envelope. However, the strength of the 11.3um  SiC feature is correlated with mass loss rate (confirmed old results). (from Blanco et al., 1998A&A...330..505B)  (Here is an example of the RT model fit of IRAS 06331+3829. Dots are data, line is model.)   If SiC is the carrier of 21um feature, the 21um/11.3um strength ratio requires too strong 21um   resonance that pure SiC can not supply. Large impurity of C could be able to produce enhanced 21um. (need more laboratory work on this idea.) (from Jiang et al., 2005ApJ...630L..77J) PAHs (back to top) IR emission spectrum of a mixture of Amorphous silicates and graphite grains and varying amount of PAH particles. Only one-photon heating is considered. They present updated IR absorption properties of PAHs. They give a grid of IR emission models with various compositions that agree to Spitzer observations of the Milky Way galaxy. (from Draine & Li, 2007ApJ...657..810D) (figure: left -- Absorption cross sections of neutral and ionized PAH; right -- absorption cross section of ionized carbonaceous grains)

 

Oxygen rich dust grains Olivine (back to top) Formula: Mg2xFe2(1-x)SiO4, with 0 <= x <= 1 denotes magnesium content. when x=0, it is called fayalite (Fe2SiO4) when x=1, it is called forsterite (Mg2SiO4) amorphous (classy) olivine mainly show broad features around 9.8um (Si-O stretching mode) and 18um (O-Si-O bending mode). crystalline olivine  Pyroxene (back to top) Formula: MgxFe1-xSiO3, with 0 <= x <= 1 denotes magnesium content. when x=0, it is called Ferrosilite (FeSiO3) when x=1, it is called enstatite (MgSiO3) amorphous (glassy) pyroxene mainly show broad features around 10um (Si-O stretching mode) and 20um (O-Si-O bending mode). Both are similar but red shifted a little than the features of olivine grains. crystalline pyroxene

Other issues (back to top) A radiative transfer code was used to fit the observed IR SED of two OH/IR stars IRAS 17004-4119 and IRAS 17411-3154. They concluded that the 9.8 and 17.5um absorption features are due to amorphous silicates, the at 11.2um absorption shoulder and 33.6um emission are due to crystalline forsterite, the 3.09um absoprtion and 43um emission features are due to crystalline water ice, the 40.5um emission is due to crystalline diopside (MgCaSi2O6), the 43um emission band is also due to crystalline enstatite.  The crystalline silicates represent 35% and 25% of amorphous dust mass in the above two stars respectively. About 10% of olivines and 65-100% of pyroxenes were found to be crystalline. (from Demyk et al., 2000A&A...364..170D) (figure: mass absorption coefficients of amorphous and crystalline silicates used in the paper. Upper panel -- solid line is amorphous olivine, dashed line is amorphous pyroxene, dotted line is FeO, dot-dashed line is crystalline water ice; Lower panel -- solid line is crystalline forsterite, dashed line is crystalline enstatite, dot-dashed line is diopside.)

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