CONCLUSIONES Y RECOMENDACIONES

We use a MRN distribution to describe the particle size distribution. The distribution with particles sizes ranging between 0.005µmand0.25µmimplies that most of the mass is in the large particles, while most of the surface area is in the small particles. MRN offers a simple parameterization, useful in this study. It is beyond the scope of this work to test more sophisticated models such as Weingartner and Draine (2001) and Zubko et al. (2004). We note, however, that these size distributions have a maximum size cutoff around0.25µmand the silicate-type distributions do not differ dramatically from one another.

In Figure 2.6, it can be observed that there is a feature between6.8Å and the edge at6.7Å. The MRN distribution predicts a large amount of small particles in our models. These particles do not add significantly to the scattering features of the extinction profiles. The model does not fit the data in the area around6.7Å, well, which may indicate the presence of larger particles along the line of sight than present in our model (Van de Hulst, 1958). This section therefore contains an investigation of the effect of the particle size distribution and focuses, in particular, on the presence of particles larger than0.25µm. In order to model the enhancement of the scattering peak around6.7Å we introduce a range of particle sizes:0.05−0.5µm. The effect of a change in the size distribution is shown in Figure 2.9. The olivine Si K-edge with a MRN distribution with particle sizes of0.005−0.25µmis shown in red and in blue the same edge is shown but now with a MRN size distribution that has a particle range of0.05−0.5µm.

The edge is fitted again with models of the same compounds, that contain the new particle distribution. We use the same parameters as in the first fit in Section 2.6.2. The results are shown in Figure 2.10. The reducedC2_{value of the best fit is: 1.08. There is a small contribution}

(of less than2%of the total amount of dust) of hypersthene (sample 6) in this fit in addition to amorphous pyroxene (sample 5) and crystalline olivine (sample 1). The contribution is so small that it does not significantly change any of the results derived above. The silicon abundance remains at4.0±0.4×10−5per H atom. The scattering feature before the edge is better fitted using a model with larger particles, which leads to the possibility of the presence of particles

2.7Discussion 57

Figure 2.9:The effect of a change in the size distribution on the extinction. The olivine Si K-edge with an MRN distribution with particle sizes of0.005−0.25µmmicron is shown in red. The blue line shows the same edge, but now the particle sizes range between0.05−0.5µm.

larger than0.25µm. It is possible that in environments such as the Galactic center region we observe a substantial amount of particles larger than0.25µm(Ossenkopf et al., 1992; Ormel et al., 2009, 2011). In our analysis, we assume that the dust particles are solid spheres, while it is more likely that large particles in dense environments are grown by coagulation of dust particles (Jura, 1980). Porous dust particles show an enhanced extinction profile when com- pared to solid particles of the same mass due to their larger surface area. Solid particles are too massive to be abundant enough to cause the enhanced scattering. Another indicator that dust particles are non-spherical is the observation of polarized starlight. Spheroidal grain models are able to reproduce these observations (Kim and Martin, 1995; Draine, 2009) and allow for larger particles in the size distribution (Draine, 2009). The effects of large porous grains in X-rays have also been studied by Hoffman and Draine (2016). They conclude that grains with a significant porosity produce narrower forward scattering peaks than equal-mass non- porous grains. Chiar and Tielens (2006) analyzed the possibility of the presence of solid and porous spherical dust particles along sight lines toward four Wolf Rayet stars. They conclude that a mixture of solid and porous silicates fits the9.7µmand18µmabsorption features. The presence of large porous dust particles along the line of sight toward GX 5-1 could be an explanation for the observed prominent scattering features in the Si K-edge. The presence of larger grains is also derived from studies of the mid-infrared extinction law. The extinction curve in the diffuse ISM is represented byRV = 3.1, while higher values ofRV (i.e., 4-6) are observed for dense clouds, which may indicate the presence of larger grains (Weingartner and Draine, 2001). Xue et al. (2016) calculate the intrinsic mid-infrared color excess from the stellar effective temperatures in order to determine the mid-infrared extinction. They find that the extinction curve is consistent with theRV = 5.5model curve and agrees well with the WD01 (Weingartner and Draine, 2001) interstellar dust model. The sight line toward GX 5-1 traverses the molecular ring and likely probes a mixture of diffuse and dense medium.

Figure 2.10:The upper panel shows a fit of the Si K-edge with particles of sizes in a range of0.1− 0.5µm. The best fitting mixture consist out of the same compounds as the fit in Section 2.6.2: sample 1 (crystalline olivine) and sample 5 (amorphous pyroxene) with a small addition of sample 6 (hypersthene). The lower panel shows the models residuals of the fit in terms of the standard deviationσ.

The dense region may be associated with the molecular ring, characterized by larger grains (Ormel et al., 2009, 2011).