Are inner disc misalignments common? ALMA reveals an isotropic outer disc inclination distribution for young dipper stars

ABSTRACT Dippers are a common class of young variable star exhibiting day-long dimmings with depths of up to several tens of per cent. A standard explanation is that dippers host nearly edge-on (id ≈ 70°) protoplanetary discs that allow close-in (<1 au) dust lifted slightly out of the mid-plane to partially occult the star. The identification of a face-on dipper disc and growing evidence of inner disc misalignments brings this scenario into question. Thus, we uniformly (re)derive the inclinations of 24 dipper discs resolved with (sub-)mm interferometry from ALMA. We find that dipper disc inclinations are consistent with an isotropic distribution over id ≈ 0−75°, above which the occurrence rate declines (likely an observational selection effect due to optically thick disc mid-planes blocking their host stars). These findings indicate that the dipper phenomenon is unrelated to the outer (>10 au) disc resolved by ALMA and that inner disc misalignments may be common during the protoplanetary phase. More than one mechanism may contribute to the dipper phenomenon, including accretion-driven warps and ‘broken’ discs caused by inclined (sub-)stellar or planetary companions.

The Galactic distribution of SNRs

It is not straightforward to determine the distribution of supernova remnants (SNRs) in the Galaxy. The two main difficulties are that there are observational selection effects that mean that catalogues of SNRs are incomplete, and distances are not available for most remnants. Here I discuss the selection effects that apply to the latest catalogue of Galactic SNRs. I then compare the observed distribution of ‘bright’ SNRs in Galactic longitude with that expected from models in order to constrain the Galactic distribution of SNRs.