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A.M. Watson (Instituto de Astronom{\'i}a UNAM), K.R. Stapelfeldt (JPL), J.E. Krist (STScI), C.J. Burrows (STScI & ESA)
Hubble Space Telescope images from five old and seven new epochs show continuing variability in HH 30, an edge-on circumstellar disk around a young stellar object. The lateral brightness asymmetry first seen strongly in 1998 is present in all of the new images, and is observed to shrink and then re-extend itself. The images suggest that persistent illumination patterns, generated at or near the central object, sweep across the outer disk.
The asymmetry in the upper nebula is strong, is similar in continuum color to the nebula, but is weaker in emission lines than in the continuum. There is no strong correlation between the strength of the asymmetry in the upper nebula and the brightness of the nebula as a whole. Possible periods lie between 0.66 and 17 days and 289 and 344 days.
Two models have been suggested: illumination by asymmetric stellar hot-spots and shadowing by the inner disk. We have constrained the models by requiring that they qualitatively and quantitatively reproduce the magnitude and color of HH 30, the strength of the asymmetry, and the light-curve of the asymmetry in both the upper and lower nebulae. The absence of a strong correlation between the asymmetry in the upper and lower nebulae allow us to rule out point-symmetric hot-spot, midplane-symmetric shadowing, and certain point-symmetric shadowing models. Severely asymmetric hot-spots and certain point-symmetric shadowing models are still allowed. However, the hot-spot models require a large and unsustainable accretion rate.
Modeling of the scattered-light images suggests that the ratio of the opacity between 0.8 and 2.0 microns in the dust in the disk is similar to that in interstellar dust and that the dust may not be homogeneously mixed with the gas, but may have settled towards the midplane of the disk at smaller radii.
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