Issue 
A&A
Volume 578, June 2015



Article Number  L8  
Number of page(s)  6  
Section  Letters  
DOI  https://doi.org/10.1051/00046361/201526041  
Published online  11 June 2015 
Online material
Appendix A: Supplementary figure
Fig. A.1
Spitzer IRS spectrum of GoHam (continuous black line) and fit (diamonds) using the PAHTAT model. The midIR optical depth derived with the model is shown in red. The positions and bandwidths of the VISIR PAH1 and PAH2 filters are indicated. This spectrum includes GoHam a and b, the absorption is dominated by the disk midplane. 

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Appendix B: Model
Appendix B.1: Model description
We modeled GoHam b as a spherical clump of constant density n_{b} and radius r_{b}. The presence of this clump results in a localized excess of column density with a profile along the cut as a function of position p (Fig. 3) of parabolic form: (B.1)where p_{b} is the position of GoHam b in the cross cut, that is, 4.25′′. The resulting midIR optical depth τ_{b} due to GoHam b is then (B.2)where C_{ext} is the dust crosssection per unit of H atom. The midIR emission along the cut, corrected for the extinction by GoHam b, can then be recovered by (B.3)where I_{IR}(p) is the observed midIR emission profile shown in Fig. 3. In addition, we computed the emission in the ^{13}CO(2−1) line as a function of p: (B.4)where Δv is the width of the line measured to be 1.7 km s^{1}, Ω_{ff} is the beamfilling factor equal to , where a and b are the minor and major axis of the beam, that is, a = 1.14′′ and b = 1.52′′. K is a Gaussian kernel of width equal to b, which is the angular resolution along the cut shown in Fig. 3 (since the major axis of the beam is almost aligned northsouth, see Bujarrabal et al. 2009, as is our cross cut). is the brightness temperature of the ^{13}CO(2−1) line that is equal to zero for p<p_{b} − r_{b} and p>p_{b} + r_{b}. For the other values of p, the brightness is equal to (optically thick line) (B.5)where T_{ex} is the excitation temperature, which we fixed at 16 K, that is, the temperature of the disk midplane derived by Bujarrabal et al. (2009). T_{ex} could be higher than this value in the internal parts of the clump, but this is not critical in the estimation of r_{b} , which is a function of the square root of T_{ex}. Overall, the parameters of the model are the density of GoHam b n_{b}, the radius of GoHam b r_{b}, and the dust crosssection C_{ext}. For interstellar dust, the cross section is typically C_{ext} = 2.5 × 10^{23} cm^{2} per H atom at 8.6 and 11.2 μm (Weingartner & Draine 2001). In disks, however, this value is expected to decrease significantly due to grain growth, typically by a factor of 10 in the midIR (Andrews et al. 2009; D’Alessio et al. 2001). We therefore considered these two extreme cases and their effects on the parameters derived by the model.
Appendix B.2: Adjustment of the parameters n_{b} and r_{b}
First, r_{b} was adjusted so as to reproduce the observed emission profile of I_{CO} as shown in Fig. 3. Once the value of r_{b} is adjusted, the following step consists of adjusting the parameter n_{b} so as to obtain a symmetric profile, which is what is expected in a disk without any clump. The result of this procedure is shown in Fig. 3. From the adjusted values of n_{b} and r_{b}, we can derive the clump mass, (B.6)where μ is the mean molecular weight and m_{H} the proton mass. The parameters used in the model and those derived from the fit for the two values of C_{ext} are summarized in Table B.2.
Main physical parameters of the GoHam disk.
Main physical parameters of the GoHam b candidate protoplanet in the model.
© ESO, 2015
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