Infrared photometry and spectroscopy covering a time span of a quarter century are presented for HD 31648 (MWC 480) and HD 163296 (MWC 275). Both are isolated Herbig Ae stars that exhibit signs of active accretion, including driving bipolar flows with embedded Herbig-Haro (HH) objects. HD 163296 was found to be relatively quiescent photometrically in its inner disk region, with the -3exception of a major increase in emitted flux in a broad wavelength region centered near 3 µm in 2002. In contrast, HD 31648 has exhibited sporadic changes in the entire 3-13 µm region throughout this span of time. In both stars the changes in the 1-5 µm flux indicate structural changes in the region of the disk near the dust sublimation zone, possibly causing its distance from the star to vary with time. Repeated thermal cycling through this region will result in the preferential survival of large grains, and an increase in the degree of crystallinity. The variability observed in these objects has important consequences for the interpretation of other types of observations. For example, source variability will compromise models based on interferometry measurements unless the interferometry observations are accompanied by nearly-simultaneous photometric data.

Digital image reconstruction is a robust means by which the underlying images hidden in blurry and noisy data can be revealed. The main challenge is sensitivity to measurement noise in the input data, which can be magnified strongly, resulting in large artifacts in the reconstructed image. The cure is to restrict the permitted images. This review summarizes image reconstruction methods in current use. Progressively more sophisticated image restrictions have been developed, including (a) filtering the input data, (b) regularization by global penalty functions, and (c) spatially adaptive methods that impose a variable degree of restriction across the image. The most reliable reconstruction is the most conservative one, which seeks the simplest underlying image consistent with the input data. Simplicity is context-dependent, but for most imaging applications, the simplest reconstructed image is the smoothest one. Imposing the maximum, spatially adaptive smoothing permitted by the data results in the best image reconstruction.

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We report on the results of a number of infrared spectra (0.8-2.5, 2.1-4.6, and 3-14 m) of V838 Monocerotis, taken from a short time after discovery in 2002 January to about 14 months later, in early 2003. The spectrum evolved dramatically, changing from a quasi-photospheric stellar spectrum with weak atomic emission lines (some with P Cygni profiles) to one showing a wide range of deep absorption features indicative of a cool, extended atmosphere with a circumstellar dust shell. The early spectra showed lines of s-process elements, such as Sr ii and Ba i. The later spectra showed absorption by gaseous H 2 O, CO, AlO, TiO, SiO, SO 2 , OH, VO, and SH, as well as a complex of emission near 10 m reminiscent of silicate emission, with a central absorbing feature at 10:3 m. Thus, V838 Mon appears to be oxygen-rich. A simple, spherically symmetric model of the system involving a central star with a two-component expanding circumstellar shell is presented that is able to explain the major molecular features and spectral energy distribution in the object's late stages. The derived shell mass and distance are 0.04 M and 9.2 kpc, respectively.

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In this paper we describe the optimal image model, maximum residual likelihood method (OptMRL) for image reconstruction. OptMRL, like maximum entropy methods (ME), is a Bayesian iipage reconstruction technique for removing point-spread function blurring. Like ME, OptMRL uses both a goodness-of-fit criterion (GOF) and an "image prior," i.e., a function which quantifies the a priori probability of the image. However, unlike standard ME techniques which typically reconstruct the image on the data pixel grid, OptMRL varies the image model in order to find the optimal functional basis with which to represent the image. In this regard, our method is similar to the multichannel ME methods proposed by Weir. In this paper, we show how an optimal basis for image representation can be selected and in doing so, develop the concept of the "pixon" which is a generalized image cell from which this basis is constructed. By allowing both the image and the image representation to be variable, the OptMRL method greatly increases the volume of solution space over which the image is optimized. Hence the likelihood of the final reconstructed image is greatly increased. For the goodness-of-fit criterion, OptMRL uses the maximum residual likelihood probability distribution introduced previously by Pina and Puetter. This GOF probability distribution, which is based on the spatial autocorrelation of the residuals, has the advantage that it ensures spatially uncorrelated image reconstruction residuals.

Aims. We studied the distribution of mid-infrared thermal emission from Neptune to determine the spatial variability of temperatures and the distribution of trace constituents, allowing us to determine the relative strengths of radiation and dynamics in its atmosphere. Methods. Mid-infrared images of the planet were taken at the Very Large Telescope on 1-2 September 2006. Results. These images reveal strong inhomogeneities in thermal emission. 17.6 and 18.7-µm images exhibit strong seasonally elevated south polar temperatures near Neptune's tropopause. These high temperatures allow tropospheric methane, elsewhere cold-trapped at depth, to escape into the stratosphere. Poleward of 70• S, 8.6-and 12.3-µm emission from stratospheric methane and ethane is enhanced, and a distinct, warm stratospheric feature near 65-70• S latitude is rotating with the neutral atmosphere. This feature may result from a localized wave propagating upward from the troposphere.