Modelling anomalous extinction using nanodiamonds

Abstract

The modelling of extinction along anomalous/non‐Cardelli, Clayton & Mathis sightlines, which are characterized by a broad 217.5‐nm bump and steep far‐ultraviolet (FUV) rise, is reported. The extinction along these sightlines, namely HD 210121, HD 204827, HD 29647 and HD 62542, is difficult to reproduce using standard silicate and graphite grains. A very good match with the observed extinction is obtained by considering a nanodiamond component as part of the carbonaceous matter. Most of these sightlines are rich in carbon and are invariably backed by a young hot stellar object. Nanodiamond is taken as a core within amorphous carbon and graphite. These core–mantle particles, taken as additional components along with graphite and silicates, lead to a reduction in the silicate requirement. The abundance of carbonaceous matter is not affected, as a very small fraction of nanodiamond is required. Extinction along sightlines that show steep FUV is also reported, demonstrating the importance of the nanodiamond component in all such regions.

Interstellar extinction and interstellar polarization: Old and new models

“…A large difference in extinction for the nearby stars observed through single clouds was first noted by Kre lowski and Wegner [21]. It should be mentioned that the major part of anomalous or peculiar extinction sightlines studied so far are related to not very distant stars [22,23,24,25,26].…”

“…EMT-Mie: silicate + amorphous Mathis (1996) [46] carbon + iron + voids Zubko et al (1998,2004) [47,49] EMT [24] sp 3 -carbon Rai and Rastogi (2012) [26] nanodiamonds coated by amorpous carbon or graphite can be characterized by its porosity P (0 ≤ P < 1)…”

“…The models discussed in this Section were first applied to interpretating the average Galactic extinction curve. Modelling of extinction in several sightlines has been also performed [22,23,26,31,48,49,61,67,68,69,70]. The models include multi-component mixtures of bare particles with a rather complicated size distribution function or/and inhomogeneous particles and PAHs and often took into account abundance restrictions.…”

“…Especially popular is the direction to the halo star HD 210121 with very high UV extinction. The extinction for this star has been modeled by Li and Greenberg [69] with a mixture of silicate core-organic mantle spheres, bare spheres and PAHs, by Larson et al [70] and by Clayton et al [71] with two-component (silicate, graphite) and three-component (silicate, graphite, amorphous carbon) models, by Weingartner and Draine [31] who considered the mixture of carbonaceous and silicate grains with the size distribution given in Table 1, and by Rai and Rastogi [26] who used a silicate-graphite mixture and nanodiamonds coated by carbon. This list illustrates the non-uniquiness of parameters of dust grains obtained from modelling.…”

Dust Growth in Molecular Cloud Envelopes: A Numerical Approach

“…The other characteristic feature of the UV extinction curve is the steep slope at far ultraviolet (FUV) wavelengths. This feature is mainly due to the effective absorption and scattering of UV photons by very small dust grains (Rai & Rastogi 2012), and its parametrization is simpler than that of the bump (Fitzpatrick & Massa 2007).…”