The variation of interstellar extinction in the ultraviolet

Monthly Notices of the Royal Astronomical Society

2008

The large majority of extinction curves in our own Galaxy obey to a simple relation depending on one parameter, the total-to-selective extinction coefficient, R V . Different values of R V are able to match the whole extinction curve through different environments. However, anomalous curves still exist in our own Galaxy. In this paper, we aim at analysing the behaviour of some anomalous extinction curves in order to shed light into both the properties of the dust and the physical properties of their environments. From the catalogue of Savage et al., by adding the infrared data from Two-Micron All-Sky Survey catalogue, 84 lines of sight (i.e. 11 per cent) have been selected which deviate at the 2σ level from the normal extinction law corresponding to their best-fitting R V value. We single out a subsample of 20 lines of sight to analyse their ultraviolet (UV) deviations, in particular, at λ = 0.22 and 0.15 μm, which correspond to the bump and the far-UV rise. The lines of sight with bump lower and far-UV rise higher than their normal extinction law are defined as type A anomalous lines, and those with bump higher and far-UV rise lower as type B anomalous lines. The results here discussed are derived from extinction curve models. A anomalous types require small-to-large grainsize ratios larger than the normal curves both for silicates and carbonaceous grains; B curves also require larger ratios for carbonaceous ones but lower ratios for silicates grains. In order to interpret these results, the environments of type A curves require low-velocity shocks in order to make effective the grain-grain collisions, which increase the number of both small silicate and carbonaceous grains. However, type B lines of sight would be characterized by relatively high-velocity shocks, implying a sputtering process which sensibly destroys small silicate grains while it produces only a partial destruction of the carbonaceous ones with the consequence of increasing the number of these smaller particles.

Section: The Fm Parametrizationsupporting

confidence: 54%

Section: The Fm Parametrizationmentioning

confidence: 97%

Dust properties along anomalous extinction sightlines

Monthly Notices of the Royal Astronomical Society

2008

“…Moreover another parameter, c 1 , is not relevant, as Jenniskens & Greenberg (1993) out of a sample of 115 extinction curves have found that c 1 and c 2 are linearly correlated. A similar result was also obtained by Carnochan (1986), Fitzpatrick & Massa (1986 and CCM.…”

Section: The Anomalous Sightlinessupporting

confidence: 88%

Classification and properties of UV extinction curves

Morbidelli³

et al. 2001

A&A

Abstract. The catalog of Savage et al. (1985) reporting colour excesses of 1415 stars from ANS photometry offers the opportunity to deeply investigate the characteristics of UV extinction curves which differ from the standard extinction of the diffuse interstellar medium. To this aim we have selected a sample of 252 curves, which have been compared with the relations derived by Cardelli et al. (1989; CCM in the following) for a variety of RV values in the range 2.4-5 and have been classified as normal if they fit at least one of the CCM curves or anomalous otherwise. We find that normal curves with small RV are just as numerous as those with large RV . The anomalous objects are arranged into two groups according to the strength of the bump at 0.217 µ. For a given value of c2 this increases along the sequence: type A anomalous, normals and type B anomalous, suggesting that this sequence should correspond to an increase of the amount of small grains along the sightline. Considerations concerning the environmental characteristics indicate that the anomalous behaviour is not necessarily tied to the existence of dense gas clouds along the line of sight.

“…11, A and B curves outline the lower and upper limits of the region where C curves are superimposed on normal ones. The correlation occurring separately for A and B types suggests that some fraction of the linear rise is associated with the bump (Carnochan 1986) but in a different proportion for these types, as discussed in Paper I. The C type curves, characterized by properties of their grain populations of values in-between those of A and B types, as outlined in the previous section, do not have the same proportion of grains contributing to the bump and to the linear rise, thus do not correlate.…”

Section: The Fm Parameterizationmentioning

confidence: 98%

Dust properties along anomalous extinction sightlines

2011

A&A

Context. Studies of the dust extinction in the Galaxy have found that the large majority of sight lines analyzed obey a simple relation depending on one parameter, the total-to-selective extinction coefficient, R V . Different values of R V are able to match the whole extinction curve in different environments, thus characterize normal extinction curves. However anomalous curves i.e, curves which deviate strongly from this simple behavior have been observed in the Galaxy as well as external galaxies. Aims. More than sixty curves with large ultraviolet deviations from their best-fit one-parameter curve are analyzed. The extinction curves are fitted with dust models to shed light on the properties of the grains along selected lines of sight, the processes affecting them, and their relations with the environmental characteristics. Methods. The extinction curve models are developed using the latest prescriptions for grain-size distributions capable of describing one-parameter curves for R V values from 3.1 to 5.5. These models, extended here down to R V = 2.0, allow us to compare the resulting properties of our deviating curves with those of normal curves in a self-consistent framework, and thus to recover the relative trends overcoming the modeling uncertainties.Results. In addition to 20 anomalous curves extracted from the same sample for study in a previous paper and revised here to account for the latest updates, these curves represent the largest, homogeneous sample of anomalous curves studied so far with dust models. Our results show that the ultraviolet deviations are driven by a larger amount of small grains than predicted for lines of sight where extinction depends on only one parameter. Moreover, the dust-to-gas ratios of anomalous curves are lower than the same values for no deviating lines of sight. Conclusions. Both shocks and grain-grain collisions should destroy dust grains, hence reduce the amount of the dust trapped in the grains, modify the size distribution of the dust, and increase the small-to-large grain size ratio. Therefore, the extinction properties derived should correspond to sight lines where shocks and high velocity flows perturb the physical state of the interstellar medium and leave their signature on the dust properties.