We present high resolution images and high dispersion spectra of the planetary nebula IC 4593. Benefiting from superb seeing (0″.4), the narrowband images reveal the existence of a complex system of collimated knots in the inner 15″ of the nebula.The knots, which morphologically resemble outward facing bow-shocks, are located at different position

We have tested current models for the atmospheres (including photosphere and low chromosphere) of late-type stars using the D resonance lines of neutral sodium as a diagnostic. To this end, we have measured the equivalent widths of the D lines for a sample of 39 dwarf and 45 giant late-type stars observed with high spectral resolution. We

The Universe at Large presents a unique survey of key questions outstanding in contemporary astronomy and cosmology. In this timely volume, eleven of the world's greatest living astronomers and cosmologists present personal views of what problems must be addressed by future research. Allan Sandage presents a 23-point plan to reach a full

We present a critical analysis of linearization and preconditioning, the two most used approaches proposed for achieving the required linearity in the iterative solution of the multilevel transfer problem. By distinguishing from the outset between the response of the radiation field to the source function and opacity perturbations, we are able to

We report Keck high-resolution echelle spectroscopic observations of two very cool field dwarfs discovered by the near-IR photometric survey DENIS. DENIS-P J1228.2-1547 shows a conspicuous LiI resonance line that confirms it as an incontrovertible brown dwarf (BD). From the presence of Li, and its low surface temperature, we estimate from

In the previous papers of this series, we introduced the implicit integral method (IIM) to solve those radiative transfer (RT) problems in which the source function depends on an integral of the specific intensity of the radiation field over directions and frequencies. The IIM rests upon a forward-elimination, back-substitution scheme naturally