4.7
Conclusions
This Chapter presented the infrared analysis of 29 YSOs, already found to be variable in optical, and it has shown that most of them are also variable in infrared. The variability was assessed using two methods: the S.I. and the χ2, and both confirmed that variations are more pronounced innear infrared, at 3.4µm and 4.6µm, rather than inmid infrared, at 11µm and 22µm. In particular, according to the χ2 threshold, 79% are variable at 3.4µm, 93% at 4.6µm, 38% at 11µm and only 17% at 22µm.
Correlations with optical variability are very weak, which may be explained as due to different causes. Simple models were tested to ascertain the effect of a hot spot and of additional disc emission on the total emission. The infrared component of hot spots is too faint to affect the variability, even at 3.4µm. It seems more likely that infrared variations arise in the disc, in the inner region, compatible with a distance within 0.11 AU from the star, for fluxes at 3.4-4.6µm. Despite the large error bars in the estimate of the emission areas, the model provides an indication of a feasible scenario. An eclipsing scenario, instead, like the one presented by Bouvier et al. (1999) to explain optical variations in AA Tau, does not seem to apply in infrared, because the same sample should be accompanied by periodic optical variations, which are not observed on a regular basis.
On the whole, the current results would confirm the idea developed by Flaherty & Muzerolle (2010) and Flaherty et al. (2012, 2013) that the inner disc, traced by infrared radiation, is a rapidly evolving structure, whose changes in height would cause variations in the absorption and emission of light. Another model which involves similar changes is the one proposed by Bans & K¨onigl (2012), where fluctuations would be caused by disc wind instead. This model was not tested on the current sample, but could offer another interpretation of the data.
5
Measurement of dust growth:
observations and models
The previous Chapters showed the role played in discs by dust, or aggregates of dust, in determining the luminosity changes observed both in optical and infrared. This Chap- ter focusses more on the analysis of dust, from both an observational and a theoretical perspective: measurement of dust grains from observed SEDs, and how different dust opacities can affect the mm slope of the SED.
After an overview on the historical background of dust models (Section 5.1), followed by the presentation of the data sample (Section 5.3) and by the description of properties of dust grains (Section 5.2), the Chapter is structured in two parts. The former is the observational part, which uses the slope of the SEDs in the mm-cm range to infer the dust grain size, after correcting for wind emission (Sections 5.4 and 5.5). A further correction for optical depth effect is then applied (Section 5.6) before deriving the final results (Sec- tion 5.7). This part of the Chapter makes also an in depth comparison with the literature,
highlighting the differences among a number of procedures adopted by authors who used the SED slope method.
The latter part of the Chapter utilises a radiative transfer model (BETAgrid) to analyse the optical depth effects on the slope of the SED from a theoretical perspective, comparing models to observations. The BETAgrid is also used to explore at which wavelengths the medium can be considered optically thin or thick (Section 5.8). By means of another radiative transfer model, the analysis is then extended to longer wavelengths than in the BETAgrid and aims also to investigate the impact of a number of disc and dust parameters on the SEDs (Section 5.9). This last Section is part of a paper (Woitke et al., 2016) where I am co-author. In this Chapter the analysis described in the paper has been redone using different wavelength intervals in order to adapt it to the current study.
5.1
Dust: historical background
This Section is based on the reviews presented in the books by Whittet (2002) and Li & Greenberg (2003), and provides an overview of dust models developed for the last century. The presence of dust obscuring starlight was evident already at the beginning of the 20th century, but the origin of dust and its composition were still unknown. Extensive work on interstellar reddening and its dependence on optical wavelength were carried out by Trumpler (1930) and Rudnick (1936).
In the beginning of the research area, dust particles were expected to have a compo- sition similar to meteors and to be made of small metallic particles, but this model was discarded when it was understood that meteors do not have an interstellar origin. In the 1940s it was already known that atoms like H, O, C and N existed in space, leading van de Hulst (1946) to propose the “dirty ice model”, where the condensation of gas on dust particles would create ices made of H2O, CH4 and NH3. The dirty component referred
to the internal core, upon which ices would condense, which was still unknown. Even if predicted by Kamijo (1963), it was only in the 1970s that infrared observation allowed the presence of silicate to be inferred, through the characteristic aborption line at 10µm (e.g. Draine, 2003b). Graphite was also proposed to explain the high dust polarization observed. Its presence was partly confirmed by the detection in 1965 of the extinction feature in UV, at 2175 ˚A, but more recently other molecules were considered responsible
5.1. Dust: historical background
for that absorption line, for example Polycyclic Aromatic Hydrocarbons (PAHs) (Li & Draine, 2001).
Since the 1970s, a number of other models have been proposed. Greenberg (1982) developed the core-mantel model, where a core of silicates would be covered with a mantel of complex organic molecules. Subsequently, the model was improved to take into account both graphite and PAHs in the core (Li & Greenberg, 1997). A completely different model, proposed initially by Mathis et al. (1977) and developed further by Draine & Lee (1984) considers two separate populations of grains, made of either silicate or carbonaceous particles, without any mantel. The carbonaceous component is made of PAHs for particles having radii smaller than 50 ˚A, and of graphite for larger ones. In this model there are spherical particles, having radii between 0.005µm and 0.25µm, with a power-law size distribution ∝ r−3.5. Zubko et al. (2004) argued that this model requires too large an
amount of silicon, magnesium and iron, which is inconsistent with the accepted solar abundances of these elements. They propose a mixture of dust types, where some contain only PAHs, graphite and silicate grains like in Li & Draine (2001), others have amorphous carbon instead of graphite, or are totally devoid of carbon apart from PAHs. Unlike all these models, Freund & Freund (2006) presented a solid solution model, where the organic mineral components in grains become physically inseparable, after undergoing thermodynamical solid state processes. The authors point out that their model can address issues not previously well resolved, especially concerning the emission lines observed in infrared. Another very satisfactory model, included also in the DIANA project, is the one proposed by Min et al. (2005). Based on the Mie theory of spherical particles (Mie, 1908) and astronomical silicate described by Draine & Lee (1984), in this model the carbonaceous composition is again made of PAHs, graphite and organics, whereas the silicate component is richer in Mg (Min et al., 2007). The innovation in this theory is the introduction of hollow spheres, which can better model the observables, like the 10µm silicate feature.
5.1.1 Dust in protoplanetary discs
As seen in Section 1.6, dust is an important component of protoplanetary discs, because it forms the building blocks of planet formation. However, the actual composition and size of dust grains in discs is still under debate; initially it was assumed to be similar to that of the interstellar medium, i.e. amorphous silicates of 0.1µm size, but through
satellite observations it was discovered that dust in protoplanetary discs is mostly in cristalline state and in larger size, up to 10µm. Cristallinity shows that dust experiences high temperatures (800-1000K) during disc evolution (Henning & Meeus, 2009). Grains seems to be mainly formed of silicates, bound to magnesium or to a lesser extent iron, although the Mg/Fe ratio is not well known yet. In the former case, the composition may be either forsterite (Mg2SiO4) or enstatite (Mg2SiO3). Regarding iron, other authors
propose different Fe species, for example FeS (Min et al., 2011). This difference affects not only the composition but also the dust opacity, which is strongly influenced by iron (Henning & Meeus, 2009). The carbonaceous component, especially PAHs, could then depend on the host star, because they were mostly found around Herbig stars rather than T Tauri (Henning & Meeus, 2009).