Simultaneous Polarimetry and Photometry of the Young Stellar Object R Monocerotis

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Abstract
Linear polarization and flux of the young stellar object R Mon were observed simultaneously at seven bands in the optical wavelength from 0.36 to 0.76 μm in 1991–1997. During the observational period, the V magnitude of R Mon changed by about 0.7 mag in a diaphragm of 18''. We have found a strong positive correlation between the degree of linear polarization p and the V magnitude and have found that the coefficient of correlation is 0.92 for the data in 1993–1997. The values of p and V in 1991–1992 did not follow this correlation. The colors B-V, V-RC, V-RJ, and RC - IC are also found to increase with the V magnitude during the whole observational period. However, in 1991 when the object was in the faintest phase, only the color U-B decreased. These correlations are similar to those observed in other Herbig Ae/Be stars. The position angle θ of the linear polarization also shows significant variation with time, though the correlation between θ and other quantities is weak. The observed correlations between magnitude, colors, and polarization degree can be explained by the combination of extinction by the clouds orbiting around R Mon and scattering by diffusely surrounding medium, as originally proposed by Grinin for Herbig Ae/Be stars. Since the timescale of variation is short (≈10 days), the orbiting clouds should be in the vicinity, that is, 10 AU, of the star. The extinction by those clouds less depends on wavelength than that observed in the diffuse interstellar medium. The size of grains in the clouds may be larger than those in the diffuse interstellar clouds. Our Mie calculations show that the radius of grains may be in the range from 0.08 to 0.50 μm. Alternatively, if the clouds are dense and opaque, "gray" extinction may occur and explain the observation.