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We study the formation of photospheric emission lines in O stars and show that the rectangular profiles, sometimes double peaked, that are observed for some stars are a direct consequence of rotation, and it is unnecessary to invoke an enhanced density structure in the equatorial regions. Emission lines, such as N IV 4058 and the N III 4634-4640-4642 multiplet, exhibit non-standard "limb darkening" laws. The lines can be in absorption for rays striking the center of the star and in emission for rays near the limb. Weak features in the flux spectrum do not necessarily indicate an intrinsically weak feature -- instead the feature can be weak because of cancellation between absorption in "core" rays and emission from rays near the limb. Rotation also modifies line profiles of wind diagnostics such as He II 4686 and Halpha and should not be neglected when inferring the actual stratification, level and nature of wind structures.

Rotation can markedly impact the interpretation of spectral diagnostics from hot stars with winds. Using a newly developed formal solution code, and a semianalytic model for the effects of rotation on the wind structure, we investigate the effects of rotation on the observed spectrum. Nonphotospheric lines formed in extended, optically thick atmospheres do not show just the simple classical Doppler line broadening. In such cases the traditional method of convolving the line profile with a rotation profile is insufficient. To handle these situations, we start with a sophisticated one-dimensional model of a stellar atmosphere and use a two-dimensional observer's-frame code to calculate the observed effect on line profiles under various assumptions about the two-dimensional velocity field and density distribution. Because it should generally be a small effect, rotational distortion of the stellar surface is currently ignored. Gravity darkening is also not handled at this time. Importantly, the code handles arbitrary line profiles and line blends, and lines formed at any depth in the stellar photosphere or wind. We apply the code to models that were used to investigate the Small Magellanic Cloud O7 Iaf+ supergiant AV 83. We find that the distortions in the velocity field caused by rotation can have a significant influence on the profile shape, while modifications to the density structure primarily influence the line strength. These affect the quality of the spectroscopic fits to the data, and consequently, the derived velocity field and other stellar parameters might be significantly biased for rapidly rotating stars with extended winds. The code will made available with CMFGEN and can be used to examine the influence of rotation on spectroscopic fitting and derived stellar parameters.

We present a new formulation of the short characteristic method for solving the transfer equation in axially symmetric systems. The radiation field is solved for on a grid in r and the spherical polar angle β, with radiation coordinates chosen to take into account the spherical nature of the underlying source. Such a coordinate system is advantageous because it takes into account the inherent discontinuities and symmetries due to the forward-peaking nature of the radiation field. An important consequence of this is that the determination of the boundary condition for the intensity along a short characteristic does not involve an interpolation across a discontinuity. A parabolic approximation for the source function provides the best combination of accuracy, stability, and speed. Systematic errors introduced by using a weighted trapezoidal rule for quadrature in the radiation coordinate μ were overcome by using a monotonic cubic polynomial to approximate integrands. The code has been successfully used to solve the polarized transfer equation and employs both an approximate operator iteration and Ng acceleration to improve convergence. Comparison with an accurate long characteristic code shows that a tremendous saving in computation time can be realized with a minimal loss of accuracy.

We present a catalog of intervening MgII quasar absorption-line systems in the redshift interval 0.36 17,000 measured MgII doublets. We also present data on the ~44,600 quasar spectra which were searched to construct the catalog, including redshift and magnitude information, continuum-normalized spectra, and corresponding arrays of redshift-dependent minimum rest equivalent widths detectable at our confidence threshold. The catalog is available on the web. A careful second search of 500 random spectra indicated that, for every 100 spectra searched, approximately one significant MgII system was accidentally rejected. Current plans to expand the catalog beyond DR4 quasars are discussed. Many MgII absorbers are known to be associated with galaxies. Therefore, the combination of large size and well understood statistics makes this catalog ideal for precision studies of the low-ionization and neutral gas regions associated with galaxies at low to moderate redshift. An analysis of the statistics of MgII absorbers using this catalog will be presented in a subsequent paper., AJ, in press

We present multi-color light-curves of the optical afterglow of GRB 000926. Beginning ~1.5 days after the burst, the light-curves of this GRB steepen measurably. The existence of such achromatic breaks are usually taken to be an important observational signature that the ejecta are not expanding isotropically, but rather have a collimated jet-like geometry. If we interpret the data in this context, we derive an opening angle of 5 degrees, which reduces the energy release compared to an isotropic model by a factor of ~275, to 1.7 x 10^51 erg. To fit the data with a simple jet model requires extinction along the line of sight. The derived A_V is in the range 0.11 - 0.82 mag, depending on the adopted extinction law and whether the electrons giving rise to the optical emission are undergoing synchrotron cooling or not. Since this is in excess of the expected extinction from our Galaxy, we attribute this to the GRB host. We note that this extinction is typical of a galactic disk, and therefore the event likely took place in the disk of its host., 4 pages, 2 figures, 2 tables. Accepted for publication in ApJ Letters