Distances of absorbing features in the LDN 1622 direction

With Tycho-2 BT and VT photometry of Hipparcos stars with π \(>\) 8.0 mas, $\sigma _{\rm \pi}$/π \(<\) 15% and with spectral type and luminosity classification from the Michigan Catalogues we have a presumably unreddened sample for establishing a ($B_{\rm T}-V_{\rm T})_{\rm 0}- M_{V_{\rm T}}$ main sequence relation in the range from A0 to G5. We discuss the location of the median main sequence relation relative to published ZAMS relations for the A0-G5 range in some detail since the applications of this relation will have to assume that the evolutionary status of the calibration sample is representative of the local disk. If the ZAMS is defined as the lower envelope of the class V stars the published versions we discuss seem to be too bright by about half a magnitude in the F0-G5 range. The resulting intrinsic color and absolute magnitude estimates have errors of the mean of 0$\fm$03 and 0$\fm$3–0$\fm$4 respectively. From this relation any given star with Tycho-2 photometry, spectral classification and luminosity class V can have its color excess $E(B_{\rm T} - V_{\rm T})$ and distance modulus estimated. This means that the local interstellar extinction may be estimated for the ~50% of the sky where Michigan classification presently is available. The individual color excesses and distance moduli may not be particularly accurate but we propose that they may be used to indicate extinction discontinuities caused by interstellar material at well defined distances. We have applied this new technique on stars from the Michigan Catalogue in the direction of LDN 1622 $(l, b) = (204\fdg7, -11\fdg8)$ supposed to be associated to the Orion B region 400–500 pc away but color excesses $E(B_{\rm T} -V_{\rm T})$ exceeding $0\fm15$ start appearing already in the distance slot 160–200 pc. Presently we cannot decide whether this nearby dust is related to LDN 1622 or whether this cloud is associated to Orion B. In a final comparison we study the color excess – distance variation in a 4° region centered on LDN 1622 from Hipparcos – Tycho-1 – Michigan data and the dust at ~160 pc is confirmed