References:Carbon measurements
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[edit] Bakes & Tielens (1994)
1) The structure of ISM depends to a large extent on the heating and cooling sources of the interstellar gas (Goldsmith, Habing, & Field 1969; de Jong 1977; McKee & Ostriker 1977; Draine 1978; Ferriere, Zweibel, & Shull 1988).
2) The photoelectric effect on interstellar dust grains is thought to be the dominanat heating source of the neutral atomic interstellar gas.
3) Absorption of a far-ultra violet (FUV) photon by a dust grain can lead to the ejection of an electron which carries some of the photon energy away in the form of kinetic energy. This excess kinetic energy leads to heating of the gas.
4) The dominant cooling lines in the ISM are the far-IR (FIR) atomic fine structure lines of [C II] and [O I].
5) Photodissociation regions (PDR) are UV illuminated regions and are generally associated with H II regions, planetary nebulae and normal and active galactic nuclei.
6) In PDRs about 0.001-0.01 of the available UV energy is converted into gas heating (Hollenbach 1989).
7) PAH modelcules with 
50 C atoms is an important component of the interstellar gas. These are the most abundant interstellar molecules after H2 and CO.
8) Study of grain charging in the ISM is done by Draine & Sutin (1987).
9) Classical grains (radius > 0.1μm); very small grains (radius << 0.01μm).
10) Photoelectric ionization yield is measured in photoelectrons per absorbed photon.
11) The UV absorption properties of PAHs and small aromatic gains are dominated by the π electron system as for graphite.
12) For bulk graphite the electron affinity remains tied to the energy of the Fermi level whereas the electron affinity decreases with radius for small clusters.
13) Charged particles adhering to a grain surface remove energy from the surrounding gas.
14) The interstellar grain size distribution as derived from optical and UV extinction measurements is given by n(a)~a − 3.5.
15) Recombination cooling is important for warm intercloud medium. At low temperatures the photoelectic effect dominates, while at high temperatures cooling is more efficient. Heating and cooling balance at about 15000K.
16) The recombination cooling acts as a thermostat keeping the temperature below ~ 104K.
[edit] Cartledge et al. (2004)
1. They have analyzed the interstellar 1356
O I line and 1216 H I line along 36 sightlines and analyzed the variations in interstellar O/H abundance ratios along 56 paths towards stellar objects observed by GHRS, STIS and FUSE. Molecular hydrogen abundances were determined using J=0 and 1 transitions in the FUSE spectral interval 1040-1120 since in general nearly 100% of the molecular hydrogen along a sightline populates these two rotational levels.
2. The Kr/H ratios along sightlines toward stars with types as late as B3 is uniform. Therefore the interstellar krypton abundance can be used as a secondary estimate of the total hydrogen column density for a sightline along which it can be measured. Krypton is a reliable indicator of the total interstellar hydrogen abundance for sightlines lacking accurate and direct determinations.
3. Decreasing gas phase abundances or increasing inferred depletions onto dust are positively correlated with increasing sightline mean of total hydrogen column density, < nH > . Interstellar oxygen, however, did not exhibit the same dependence as other elements. Majority of UV absorption studies to date have found a remarkable O/H ratio constancy within their samples. But given the large amounts of CO and other oxygen bearing molecules observed in dense clouds and conditions that are favourable for the growth of dust grains, it has been expected that oxygen depletion must be enhanced as the particle density increases to values achieved in molecular clouds.
4. Although density vay in a continuous fashion in the natural ISM, the appearance of a plateau in a plot of abundance versus < n(H) > would indicate that an elements's depletion is fairly constant over a range of real densities. Oxygen depletion remains roughly fixed in two distinct interstellar density intervals. Cartledge et al. (2004) have also performed an uncertainty weighted fit of the Boltzmann function to the data and simultaneously measured reliable warm and cold ISM depletion levels. Using the full oxygen data sample, the values derived from the entire sample correspond to levels of 
for warm, low < n(H) > sightlines and 
along paths dominated by cooler and denser material with the transition centered at 
.
5. The sightlines examined in this paper will not contain significant amounts of ice. Any depletion of an element, threrefore, should be in to grains. The most likely oxygen carriers in dust are silicates and oxides. Thus, the large cosmic abundances and heavy depletions of magnesium and iron suggest that the majority of the O-bearing grains will be associated with these two elements. Thus, silicon, magnesium and iron depletions can be used to place limits on the amount of oxygen incorporated into grains.
6. The author's discussion of dust composition assumes that solar photospheric abundances well represent the total (gas+dust) abundances in the ISM. In their study they use the solar abundances of silicon, magnesium and iron from Holweger (2001) [Holweger, H. 2001, in AIP Conf. Proc. 598, Solar and Galactic Composition, ed. R. F. Wimmer-Schweingruber ( Berlin: Springer), 23].
7. Mg bearing silicates better fit interstellar spectral features than Fe bearing silicates (Whittet 2003) [Whittet, D. C. B. 2003, Dust in the Galactic Environment ( Bristol: Institute of Physics)]. This suggests that silicon and magnesium are usually incorporated into the same grains. An average halo like region will have upto 
oxygen atoms incorporated into dust per million hydrogen atoms in the gas, while for typical cool disk regions the value would rise to 
.
8. Two recent measurements of the solar photospheric oxygen abundance are 
(Holweger 2001) and 
(Asplund et al. 2004) [Asplund, M., Grevesse, N., Sauval, A. J., Allende Prieto, C., & Kiselman, D. 2004, A&A, 417, 751].
9. The large inferred oxygen dust abundance for high < n(H) > sightlines might be influenced by an apparent spatial effect. If the GHRS and STIs low density ( < n(H) > < 1.0cm − 3) samples are devided into groups shorter and longer than 800pc, the weighted mean O/H abundance ratios diverge [
and 
].
10. Orion region is oxygen poor.
[edit] Cartledge et al. (2006)
1. Interstellar elemental abundance measurements derived from STIS echelle observations of 47 sightlines extending upto 6.5kpc through the galactic disk.
2. The current data is coupled with GHRS data from 17 additional sightlines and corresponding FUSE and Copernicus observations of H2 absorption features. Magnesium, phosphurus, manganese, nickel, copper and germanium gas phase abundance variations as a function of < n(H) > is explored.
3. S/N ratio from GHRS detector is 
(Meyer et al. 1994)
4. Spectral windows of GHRS are 5.5-9 and 8.5-17 in the echelle A and B modes.
5. STIS bandpasses are > 200. STIS high resolution R 114,000. STIS medium resolution R 45,800.
6. Transitions analyzed :
1. Mg II (λλ 1239, 1240)
2. P II (λ 1302) 3. Mn II (λ 1197)
4. Ni II (λ 1317) 5. Cu II (λ 1358) 6. Ge II (λ 1237)
7. Targets are Galactic O and B type stars.
8. The three STIS setups are :
1.aperture and the E140H grating centered at 1271
2.aperture and the E140H grating centered at 1271
3.
9. Column densities are independently determined using two methods: apparent optical depth analysis (Savage & Sembach 1991; Jenkins 1996) and profile fitting (Welty et al. 1991; Mar & Bailey 1995).
[edit] Draine 1989
1) The λ2175 feature is quite well correlated with visual extinction.
2) The feature strength (relative to the underlying continuum extinction) can vary by as much as a factor of two away from the mean.
3) Both diffuse and dense clouds have the λ2175 feature appearing with comparable strength.
4) The possible candidates for the λ2175 extinction feature are C, Mg, Si and Fe.
5) The carrier of the λ2175 feature must not have associated strong extinction eatures.
6) Evidence suggesting scattering on the long wavelength shoulder of the feature is reported. 
the carrier is located in grains with radii 
100 , since smaller particles or molecules are ineffective at scattering at 
2300.
7) Alternatively, the scattering may actually be fluorescence in which case large particle sizes are not required.
8) The only published data on possible polarization in the λ2175 feature are the upper limits of Gehrels (1974).
9) The optics of "surface plasmon" absorption have been thoroughly treated in Gilra (1972) and Bohren & Huffman (1986).
10) As the particle size increases, the extinction profile tends to shift toward longer wavelengths as a result of scattering which contributes predominantly on the long wavelength side of the absorption peak.
11) The observed near-constancy of the central wavelength suggests that a) the absorbing grains are small enough to be in the Rayleigh limit (a
50); b) the shape distribution of the absorbers probably varies little from one sightline to another; c) the absorbing material is probabaly free of significant substrates or coatings.
12) The possible explanations for the discrepancy between the measured profile and the theoretical cross sections for spheres are a) a broad range of shapes and sizes among the smoke particles, as noted by Day and Huffman (1973); b) incomplete graphitization of the particles; and c) small graphite particles may not be accurately described by the bulk dielectric function.
13) Hecht (1986) proposed that the λ2175 feature is produced by small a50 spherical graphite grains with the variations in FWHM being due to variations in the temperature and size distribution. Larger carbon grains are assumed to be hydrogenated and non-graphitic and therefore would not contribute to the λ2175 feature.
14) For a100 graphite particles, the peak wavelength is nearly independent of particle size, but shifts toward longer wavelengths as the particle size is increased to ~200.
15) With just one adjustable parameter (axial ratio of randomly oriented oblate graphite spheroid) the graphite hypotheris is able to reproduce two observed quantities: λ0 and γ.
16) Coatings will cause a shift in the central wavelength (Gilra, 1972). For thin coatings the shift is only slight (Hecht, 1981).
17) Lack of associated extinction features is consistent with the graphite hypothesis.
18) Kroto and McKay (1988) have suggested that a quasi-icosahedral shape may be favored for small carbon particles, with nearly-spherical "graphitic" sheets.
19) Wright (1988) has used the discrete dipole approximation to calculate the expected ultraviolet absorption by such a particle.
20) Using a plasma discharge in methane, Sakata et al. (1983) have produced a material which they refer to as "quenched carbonaceous composite" (QCC). X-ray diffraction studies of QCC indicate the presence of fine graphitic microcrystals containing some hydrogen.
21) Absorption spectra of QCC shows a peak near 2200; however the peak is considerably broader than the observed interstellar λ2175 feature.
22) Amorphous carbon particles exhibit an absorption peak at approximately 2500 (Stephens, 1980; Borghesi et al. ,1985; Maggipinto et al., 1985; Bussoletti et al., 1987).
23) Amorphous carbon particles can account for the circumstellar extinction around various carbon stars (Hecht et al. 1984).
24) Bussoletti, Colangeli and Orofino (1987) have suggested that in very small (a10) amorphous carbon grains the peak may be shifted to match the λ2175 profile.
25) Sakata et al. (1977) reported evidence for an absorption feature at 2200 in organic extract from the Murchison carbonaceous chondrite.
26) There is as yet no evidence indicating a suitable λ2175 band in any non-graphitic carbonaceous material.
27) measured yield = luminescence photos per incident photons
efficiency = emitted photons per absorbed photon.
28) Free polycyclic aromatic hydrocarbons (PAHs) have been proposed as an explanation for the infrared emission bands observed from a number of reflection nebulae, HII regions, and planetary nebulae (Leger and Puget, 1984).
29) In order to account for the strength of the IR emission features, these molecules would contain 
6% of the total carbon abundance (Leger and Puget, 1984; Puget et al. 1985; Leger and d'Hendecourt 1986).
30) Those PAH for which UV absorption has been measured cannot contribute significantly to the λ2175 feature.
[edit] Draine 2003
1) The wavelength dependence of extinction strongly constrains the grain size distibution and spectral features reveal the chemical composition.
2) "Pair method" determinations of the reddening law for many sightlines indicate that Rv3.1 for the average extinction law for diffuse regions in the local Milkyway (Savage & Mathis 1979, Cardelli et al. 1989).
3) Sightlines intersecting clouds with larger extinction per cloud tend to have larger values of Rv; the larger Rv values may indicate grain growth by accretion and coagulation.
4) The sightlines in the SMC bar that lack the 2175 extinction feature can be reproduced by models that lack carbonaceous grains with radii a0.02μm.
[edit] Duley et al. (1981)
1) Infrared spectroscopy has always been an important diagnostic technique for the identification of chemical species in laboratory systems.
2) All objects show spectral features at 3.3 and/or 3.4μm in either absorption or emission. Such features are characteristic of the resonances within CH bonds in organic material (Duley & Williams 1979).
3) As stated by Snoeyink & Weber (1972) "nearly every type of functional group known in organic chemistry has been suggested as being present on the surface of microcrystalline carbon".
4) Puri (1970) has shown that amorphous carbons typically contain 1-10 percent H by number and that these H atoms are bound at surface C sites as either CH, CH2 or CH3 gourps.
5) The surface functional groups may also influence the chemical and optical properties of carbon dust in the ISM.
6) The surface chemistry of carbon (Boehm 1966; Snoeyink & Weber 1972; Puri 1970) indicates that under a wide range of physical conditions various chemical species can be bound to reactive C atom sites.
7) The dominant surface species in diffuse clouds may be different from those on gains in emission regions.
8) It appears from the infrared data available that methyl groups may be abundant in C grains in diffuse clouds.
9) Emission at 3.3 and 11.3μm would tend to be limited to the boundary between cold cloud material and the H II region.
10) The frequent occurance of weak emission at 3.4μ reflects the chemical balance between CH and CH3 groups on grain surfaces.
11) Aromatic CH groups dominate on grains at elevated temperature while aromatic CH3 groups are important in diffuse clouds. The balance between these two species is controlled by chemical reactions involving C and H.
12) A variety of other chemical groups are likely including -OH, -CHO and -NH2.
[edit] Gnacinski (2000)
1) The continuum reconstruction method is applied to the 1334 caron absorption line to determine the carbon column density for a number of stars.
2) The carbon abundance was then compared to various extinction curve parameters for individual lines of sight, as well as to the fractional abundance of molecular hydrogen.
3) Continuum reconstruction - one has to know the velocities and the doppler spread parameters for clouds in the investigated direction.
4) There is a UV Bright Star Spectroscopic Catalogue by Jamar et al. (1976).
5) The Aλ extinction is directly proportional to the optical depth τλ. So it is more physical than Aλ / AV or Eλ − V / EB − V extinction curve.
6) The absolute visual magnitudes for population I stars are available in Lang (1992).
7) For standard stars, 
.
8) The extinction curves were calculated from star fluxes at 10pc with the formula 
.
9) The bump area is given by Abump = (πC3) / (2γ).
10) According to Cardelli et al. (1996) cosmic abundance of carbon remains nearly constant for different lines of sight.
11) Gas phase C/H ratio and integrated optical depth normalized to the observed hydrogen column density are correlated.
12) Gas phase C/H have been known to remain constant over the observed range of f(H2). This lack of variations suggested that additional depletion of gas-phase carbon into dust grains does not occur in dense, molecule rich clouds.
13) Contradicts the hypothesis made by Duley et al. (1989) that bump arises from absorption made by small silicate grains.
14) Variations of C/H associated with physical conditions in interstellar gas (indicated by f(H2)) were found. Two groups of stars can be distinguished.
[edit] Jenkins & Tripp (2001)
1) In the ISM, the abundace of carbon relative to hydrogen is about 
over a broad range of average densities.
2) The amount of sulphur in the ISM is generally very close to its solar abundance ratio relative to hydrogen, S/H = 
[edit] Jenniskens & Greenberg (1993)
1) Shape of the extinction curve in the FUV is constant for diffuse medium lines of sight, which implies that there is no scattering component that affects the shape of the curve.
2) The classical difference between various lines of sight is the reddening per unit distance (d).
3) Circumstellar dust has different extinction properties than interstellar dust.
4) In some C rich outflows a weak bump is found at about 0.24μm, offset from the interstellar medium position, which is characteristic of amorphous carbon grains.
5) There is a weak correlation between bump width, bump height and FUV non-linear rise. In environments where the bump is broad, the bump height is small and the FUV non-linear rise is strong.
6) In H II regions, but not in bubbles, the bump (c3) is systematically weak. From individual cases, the preferential weakening of the bump in H II regions was noted by Savage (1975) and Borgman et al. (1975).
7) The bump strength does not vary with decreasing steepness (linear rise).
8) The FUV non linear rise is in no way dependent on environment conditions except for a weak dependence on E(B-V)/d, most notably in H II regions.
9) The decrease of bump strength in H II regions is apparent in the C3 : γ correlation, where many of these lines of sight fall below the general trend. 2σ deviations occur for HD 37061 and HD 37903. These are located in H II regions. While bump strength decreases, the bump width remains the same.
10) In H II regions broader bumps are shifted to longer wavelengths.
11) The correlation between the wavelength of maximum polarization and Rv suggests that the big grains grow when the small grains are depleted.
12) The decrease of linear rise in dense media is consistent with an accretion of small grains on big grains. This does not affect very much the extinction at long wavelength. Large values of c2 were observed in lines of sight for which shock destruction may have occurred. In diffuse medium c2 is usually large.
13) The stronger the radiation field, the weaker the bump. The bump carrier is brought back in the gas phase continuously, as for the small gas phase molecules. The bump height correlates with 60 over 100 micron flux ratio.
14) The bump carriers are significantly less abundant in most H II regions.
[edit] Joblin et al. (1992)
1) PAHs are likely ionized in strongly irradiated regions such as reflection nebulae, planetary nebulae and active galaxies (Allamandola, Tielens & Barker 1985).
2) For the diffuse interstellar medium, PAHs are expected to be mostly neutral (Verstraete et al. 1991).
3) An important test for the PAH model is to compare the absorption spectra of these molecules with the interstellar extinction curve.
4) Each individual PAH molecule involved in a mixture can affect the 2100 band in position, width and intensity. As a result, the exact band will depend on the exact composition of the mixture.
5) Assumed physical components are a) a large grain component which produces most of the visible absorption and b) a small grain component which is purely absorbing and responsible for the 2175 bump and the far-UV rise.
6) Component (b) is assumed to include small grains with radii a<100 which undergo temperature fluctuations.
7) Component (b) includes PAHs which emit at 12μm and the so-called very small grains (VSGs) which are responsible for the 25μm emission and part of the 60μm emission (Ryter, Puget & Perault 1988).
8) Large grains (a>500) are assumed to be responsible for the whole extinction at wavenumbers upto 2μm − 1 (5000) where laboratory PAHs start to absorb.
9) The above hypothesis leads to neglect the effect of the medium size grains (100<a<500).
10) SMC, where there is no bump but a strong FUV rise observed, shows no evidence of PAHs (no 12μm emission, Rice et al. 1988).
11) Exact contribution of PAHs to the bump is expected to vary, both in position and intensity, with the exact composition of the mixture.
12) Bump characteristics would be determined by the exact composition of both the PAH mixture and the graphite grain population.
13) FUV rise would result from a contribution of both PAHs and the VSGs regardless of their composition.
14) The exact contribution of PAH molecules to the bump would be better known by attempting to correlate the bump strength with the 12μm IR data.
[edit] Mathis (1996)
1) Interstellar polarization clearly shows that large interstellar grains (producing the visual and near-infrared (NIR) extinction) are not spherical.
[edit] Savage et al. (1985)
1) It is common to find low far-UV extinction in dense nebular environments such as Orion, Sco-Oph, M8 and Cep OB3.
2) Very high far-UV extinction may be a characteristic of shocked or turbulent interstellar regions.
3) Extinction produced by circumstellar dust can be completely different from extinction produced by general interstellar medium.
4)
[edit] Valencic et al. (2004)
1) Factors influencing the bump width are different from those influencing the carrier of the FUV rise.
2) C3 and γ are truly related.
3) The width of the bump shows real variation and environmental dependence. The average γ for dense and diffuse sightlines does not differ significantly.
4) The narrowest bump tend to be found along sightlines with Av/d < 0.9 mag kpc − 2, the diffuse subset.
5) The grains that produce the bump may form a separate population from those that are responsible for variations in Rv (CCM).
6) Hydrogenation of grains in dense regions broadens the bump, without affecting x0.
7) C3 and C4 correspond to the strength of the bump and the curvature of the FUV rise.
8) FM fit is not reliable longward of 2700.
9) The spectra are cut at the blue end at 1250 in order to exclude the Lyα feature at 1215.
[edit] Whittet et al. (1997)
1) There is a general agreement that the dust responsible for extinction must include both carbon-rich and oxygen-rich materials.
2) Infrared spectroscopy provides a protentially powerful and direct technique for investigating grain composition.
3) Carbon rich dust has been identified by means of absorptions at 3.4 and 6.8μm, attributed to CH2 and CH3 groups in saturated aliphatic hydrocarbons (Sandford et al. 1991; Pendleton et al. 1994; Tielens et al. 1995).
4) Infrared Space Observatory (ISO)'s Short Wavelength Spectrometer (SWS) covers the range 2.4-45μm.
5) Cygnus OB2 No.12 is more reddened than other members of the association; this might be interpreted as evidence for excess extinction arising in a dusty circumstellar shell or disk (Reddish 1967).
6) The above possibility appears to be ruled out by the absence of dust emission (Persi & Ferrari-Toniolo 1982).
7) The unusually high degree of extinction arises because of an uneven spatial distribution of intracluster material within the Cyg OB2 association (McMillan & Tapia 1977).
8) The optical properties of the dust appear to be typical of the diffuse ISM.
9) Phyllosilicates (or layer-lattice silicates) commonly consist of planar silicon-oxygen networks that contain OH groups in the center of hexagonal arrangement of Sio4 tetrahedra (Hurlbut & Klein 1977).
10) McFadzean et al. (1989) argued that the total extinction to the Galactic center has two components: a spacially variable molecular cloud component producing 3.0μm ice absorption and a constant diffuse cloud component producing 3.4μm absorption due to hydrocarbons.
11) Chiar et al. (1995) found a reasonable close correlation between N(H2O) and Av in molecular clouds in the solar neighbourhood.
12) As silicates account for virtually all of the available Si and Mg atoms in the ISM, the best possibility for a further oxygen-rich grain component of significant abundance would appear to be oxides of iron. (e.g., Jones 1990).
