COMPARISON OF 2 METHODS FOR DETERMINING THE INTERSTELLAR EXTINCTION OF PLANETARY-NEBULAE

We have compared the extinction of planetary nebulae as derived from the Balmer decrement and from the ratio of radio and Hbeta fluxes. This was done after a compilation of all the relevant radio and optical measurements and a serious selection of the best data, resulting in a sample of about 130 objects. We have shown that the extinction determined from the Balmer decrement is systematically larger than the one derived from the radio data, the slope of the (C(opt) C(rad)) relation being about 1.2. It does not seem that this effect can be entirely attributed to observational errors. It is opposite to the well known effect observed in extragalactic H II regions, which has been explained by the presence of large amounts of dust mixed with the ionized gas or unevenly distributed in front of it. It is also opposite to what would be produced by deviations from the standard extinction law, when the line of sight crosses dense molecular clouds. We argue that, for most of distant planetary nebulae, the total to selective extinction, R(V), is significantly lower than 3, the value corresponding to the standard extinction law for the diffuse interstellar medium. This, being due to a different size distribution of dust particles outside the spiral arms, would also apply to other distant objects.