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doi: 10.1086/379058We present a 2.218 lm image from the Hubble Space Telescope/Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and a 55 lm image from ISOPHOT of the dust ring surrounding the luminous blue variable (LBV) candidate HD 168625, together with new temperature and optical depth maps derived from mid-IR images. The shell is detached from the star in the near-IR, and substructure in the overall toroidal shell is visible. The far-IR image constrains the extent of the dust shell to  2500 in diameter, providing an upper radius limit for modeling. The temperature maps and the NICMOS image show evidence for very smalltransiently heated dust grains in the shell. The opacity maps show higher optical depth in the limbs, consistent with interpretation of the dust shell as an equatorially enhanced torus inclined  60  with respect to theobserver. An overall trend in the dust emission location with wavelength is observed and interpreted as a variation with respect to location in the nebula of either the dust grain size distribution or gas-to-dust massratio. Radiative transfer calculations using 2-Dust indicate that a mass-loss event occurred  5700 yr ago with a rate of ð1:9   0:1Þ   10 4 M  yr 1, creating a dust torus that currently has a  V   0:22 in the equatorial plane and a dustmass of ð2:5   0:1Þ   10 3 M .Using published values for the gas mass, we find a gas-to-dust mass ratio of 840, which is  4 times higher than current estimates for the interstellar medium. In addition to a high equator-to-pole density ratio ( 31) torus, an ellipticalmidshell is needed to reproduce the appearance andspectral energy distribution of the dust. Therefore, HD 168625 is an excellent example of proposed models of LBV nebulae in which a stellar wind interacts with a preexisting density contrast and creates a blowout in thepolar direction perpendicular to the equatorial ring. The circumstellar shell is much lower in mass than that of LBV   Carinae, suggesting thatHD168625 had a lowermass progenitor.M. Meixner, T. B. O'Hara, A. K. Speck, and T. Ueta were partially supported by NSF CAREER grant AST 97-33697. A. K. Speck was also supported by NASA JPL 961504 and NASA STI 7898.02-96A. M. Bobrowsky was supported byNASA through grant HST-GO-07378.02-A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555

The Dust Ring of Luminous Blue Variable Candidate HD 168625: Infrared Observations and Model Calculations