SIMBAD references

A study of the extinction law in the UV-visual-IR spectral regions has been made towards the variable Herbig Ae star HR 5999 at three brightness levels V=6.9mag, 7.2mag, and 7.3mag. For comparison we have made an equivalent study of its proper motion non-variable companion HR 6000, which is embedded in the same dark cloud. The extinction law towards HR 6000 appears to be normal in the considered wavelength range. This means that the origin of any anomaly in the extinction law towards HR 5999 should be caused by the dust grains in the circumstellar envelope of this star. The circumstellar extinction law of HR 5999, characterized by the R_V value, has been studied using the extinction calculations by Steenman & The (1991), in which it is assumed that the small grains are depleted. We found that for the spectral region considered the value of R_V is larger than 5.8. For the UV alone we found for R_V^UV a range between 3.3 to 3.9. Above mentioned law has also been studied quantitatively using the multiple scattering model based on the Monte Carlo method of Voshchinnikov, Molster & The (1996). We have found that the extinction curve near the state of maximum brightness (V=6.9mag) of HR 5999 can be produced by a nearly spherical (A/B=1.5) shell, seen edge-on, and of which the dust grains of sizes smaller than about 0.01µm are depleted. For the fainter brightness states, which we have also studied, additional extinction due to circumstellar dust clumps, obscuring the star, is assumed. From these conditions, we have found that the properties of the dust particles in the clumps differ from those in the shell. Among others, the smaller dust grains in the clumps are more depleted than in the above mentioned shell. The R_V^cs values found in the Monte Carlo calculations range from 2.5 to 3.2. We suggest that the depletion of small grains is due to the sweeping effect of the radiation field of the central star.