Probing the Role of Carbon in the Interstellar Ultraviolet Extinction

Abstract

We probe the role of carbon in the ultraviolet (UV) extinction by examining the relations between the amount of carbon required to be locked up in dust [C/H] dust with the 2175Å extinction bump and the far-UV extinction rise, based on an analysis of the extinction curves along 16 Galactic sightlines for which the gas-phase carbon abundance is known and the 2175Å extinction bump exhibits variable strengths and widths. We derive [C/H] dust from the Kramers-Kronig relation which relates the wavelength-integrated extinction to the total dust volume. This approach is less model-dependent since it does not require the knowledge of the detailed optical properties and size distribution of the dust. We also derive [C/H] dust from fitting the observed UV/optical/near-infrared extinction with a mixture of amorphous silicate and graphite. We find that the carbon depletion [C/H] dust tends to correlate with the strength of the 2175Å bump, while the abundance of silicon depleted in dust shows no correlation with the 2175Å bump. This supports graphite or polycyclic aromatic hydrocarbon (PAH) molecules as the possible carrier of the 2175Å bump. We also see that [C/H] dust shows a trend of correlating with 1/R V , where R V is the total-to-selective extinction ratio, suggesting that the far-UV extinction is more likely produced by small carbon dust than by small silicate dust.

The global dust modelling framework THEMIS

“…Their work supports the suggestion that big grains form in dense clouds and that they are then progressively fragmented into smaller grains in lower density regions, which is consistent with the gradual decrease in the FUV extinction in the transition to the lower density diffuse ISM. In general agreement with the results of Parvathi et al (2012), Mishra & Li (2015) find that carbon depletion into dust tends to correlate with 1/R V , indicating that the FUV extinction is most likely due to small carbon grains. However, in their study they find that the strength of the UV bump does tend to correlate with the depletion of carbon into dust, in contrast to the results of Parvathi et al (2012).…”

“…That significant dust variations are related to variations in the carbon depletion in the ISM are supported by observational studies (Parvathi et al 2012;Mishra & Li 2015).…”

Interstellar Silicon Depletion and the Ultraviolet Extinction

“…For conducting, graphitic grains of moderately elongated shapes, Mishra & Li (2015) derived F C ≈ 1.25 (see their Figure 1). …”

“…In Figures 1-5 we show the UV/optical/near-IR extinction curves of 39 sightlines constructed as above. The extinction curves of the other seven sightlines have already been constructed and modeled previously (see Mishra & Li 2015).…”

“…Being highly processed in the ISM by supernovae shocks through sputtering and shattering (Draine 2003), silicate dust is expected to have a range of sizes. Without knowing the silicate dust size distribution, Mishra & Li (2015) applied the Kramers-Kronig (KK) relation of Purcell (1969) to estimate that silicate dust contributes to ∼ 48% of the total wavelength-integrated interstellar extinction which is obtained by integrating the observed interstellar extinction over wavelength from the far-UV to the far infrared (IR). Indeed, all modern interstellar dust models assume amorphous silicate to be a major dust species (e.g., Mathis et al 1977, Draine & Lee 1984, Duley et al 1989, Désert et al 1990, Siebenmorgen & Krügel 1992, Mathis 1996, Li & Greenberg 1997, Weingartner & Draine 2001, Li & Draine 2001a, Zubko et al 2004, Jones et al 2013.…”