Appendix F: Dust mass from NIR data  

In order to evaluate the dust content from the NIR data, we use the color excess $E(B-\lambda)$. Since the color excess is related to absorption, we estimated the dust grain column density and the optical depth using the model of Cardelli et al. (1989, hereafter CCM) which uses the mean extinction curve

 

$$\langle A(\lambda)/A(V)\rangle = a(\lambda) + b(\lambda)/R_V$$ (F1)

where R_V = A(V)/E(B-V) is the only free parameter. The coefficients $a(\lambda)$ and $b(\lambda)$ were derived as in CCM. Taking into account the relation between the color excess and the absorption we can write

$$E(B-\lambda) = A(V)\lbrack R_V^{-1} + 1 - F_{\rm CCM}(\lambda) \rbrack,$$ (F2)

being $F_{\rm CCM}(\lambda)$ the right hand term of Eq. (11). Assuming R_V=3.1 and the derived value of $F_{\rm CCM}(\lambda)$, we got A(V). We estimated a visual absorption $A_V \simeq 0.3$ mag. Since A_V is proportional to the optical depth, a dust grain model has to be introduced to derive both the dust column density and the dust mass:

$$M_{\rm d} = {\frac 43} {\frac{a \rho_{\rm d}} {Q_\lambda}} {\frac{A_\lambda}{1.086} A_{\rm Gal}},$$ (F3)

a value which depends on the dust grains radius a, on the grain density $\rho_{\rm d}$, on the extinction efficiency $Q_\lambda$, on the absorption $A_\lambda$ and on the area of the galactic region interested by the absorption features $A_{\rm Gal}$.

Using the composite dust grain model of Mathis & Whiffen (1989) the dust mass turns out to be $6\ 10^6$ $M_{\hbox{$\odot$}}$. It is the dust mass excess associated to the North semiaxis.