Mid-infrared interferometry on spectral lines. II. Continuum (dust) emission around IRC+10216 and VY canis majoris

The University of California Berkeley Infrared Spatial Interferometer has m easured the mid-infrared visibilities of the carbon star IRC +10216 and the red supergiant VY CMa. The dust shells around these sources have been prev iously shown to be time variable, and these new data are used to probe the evolution of the dust shells on a decade timescale, complementing contempor aneous studies at other wavelengths. Self-consistent, spherically symmetric models at maximum and minimum light both show the inner radius of the IRC +10216 dust shell to be much larger (150 mas) than expected from the dust-c ondensation temperature, implying that dust production has slowed or stoppe d in recent years. Apparently, dust does not form every pulsational cycle ( 638 days), and these mid-infrared results are consistent with recent near-i nfrared imaging, which indicates little or no new dust production in the la st 3 yr. Spherically symmetric models failed to fit recent VY CMa data, imp lying that emission from the inner dust shell is highly asymmetric and/or t ime variable.