The Ratio of Total to Selective Absorption in the RHO Ophiuchi Cloud

The ratio of total to selective absorption, |R=A_ v/EB-V| , is determined for the ρ Oph dust cloud by the variable extinction method, using published data for stars in the Sco OB-2 association. The calibration of absolute magnitude in terms of spectral type is discussed and compared with results from H α and H β photometry. A least squares fit to the |EB-V/(V-Mᵥ)| plot gives an overall result of R = 3.7 ± 0.3. The stars associated with nebulosity give R = 4.2 ± 0.5, while a ‘normal’ value of R = 3.3 ± 0.3 is found for the remainder. The mean distance modulus of the association is deduced to be 6.1 ± 0.1, which agrees well with the kinematical value.