Appendix B: Deriving the distribution of $A_{\bf V}$ from star counts

We used the USNOFS Image and Catalogue Archive to derive the distribution of the visual extinction $A_{\rm V}$ in a field of $1\hbox{$^\circ$ }\times 1\hbox{$^\circ$ }$ containing L 1340. We counted the stars on overlapping circles of 3 $\hbox{$^\prime$ }$ in diameter, the centres of which were distributed on a regular grid with step of 30 $\hbox{$^{\prime\prime}$ }$. We removed from the stellar list all known candidate pre-main sequence stars associated with the cloud, and all identified foreground stars. We derived $A_{\rm V}$ from R star counts using the method described by Dickman (1978). The extinction-free reference area was a field 20 $\hbox{$^\prime$ }\times~20\hbox{$^\prime$ }$ centred on RA(2000) = 1 $^{{\rm h}}52^{{\rm m}}$, Dec(2000) = +73$^\circ$15$^\prime$. $A_{\rm V}$ values obtained in this manner saturate at 6 mag.

We estimated the contribution of the foreground diffuse matter to $A_{\rm V}$ with the aid of the neutral hydrogen spectra (see Fig. A.1). Assuming optically thin radiation we used the relationship A.1 to derive hydrogen column densities from the spectra, and regarded as foreground all the H I gas at velocities $v_{{\rm LSR}} > -6$ km s^-1, as well as half of the gas at -20 km s $^{-1} < v_{{\rm LSR}} < -$6 km s^-1. Taking the average of four H I spectra covering the face of L 1340, and using the relationship N(H I)  $\approx 2\times10^{21}$ [cm^-2/mag] A $_{{\rm V}}$(Spitzer 1978) we obtained $A_{{\rm V},f} \approx$ 0.55 mag for the foreground extinction to be subtracted from the $A_{\rm V}$ values derived from the star counts.