The Crab Nebula's Composition and Precursor Star Mass

We present results of new photoionization calculations for investigating gaseous regions that represent potentially expected stages of nuclear processing in the Crab Nebula supernova remnant. In addition to gas resulting from CNO-processing and oxygen-burning, as previously reported, a large component of the nebula appears to be carbon-rich. These results suggest that the precursor star had an initial mass of 9.5 solar masses or more.Comment: manuscript in AASTeX, 6 figures in .eps, submitted to Astronomical Journa

Possible High-Redshift, Low-Luminosity AGN Activity in the Hubble Deep Field

In the Hubble Deep Field (HDF), twelve candidate sources of high-redshift (z > 3.5) AGN activity have been identified. The color selection criteria were established by passing spectra of selected quasars and Seyfert galaxies (appropriately redshifted and modified for "Lyman forest" absorption), as well as stars, observed normal and starburst galaxies, and galaxy models for various redshifts through the filters used for the HDF observations. The actual identification of AGN candidates also involved convolving a Laplacian-of-Gaussian filter with the HDF images, thereby removing relatively flat galactic backgrounds and leaving only the point-like components in the centers. Along with positions and colors, estimated redshifts and absolute magnitudes are reported, with the candidates falling toward the faint end of the AGN luminosity function. One candidate has been previously observed spectroscopically, with a measured redshift of 4.02. The number of sources reported here is consistent with a simple extrapolation of the observed quasar luminosity function to magnitude 30 in B_Johnson. Implications for ionization of the intergalactic medium and for gravitational lensing are discussed.Comment: 10 pages LaTex plus 2 separate files (Table 1 which is a two-page landscape LaTex file; and Figure 6 which is a large (0.7 MB) non-encapsulated postscript file). Accepted for publication in the Astronomical Journa

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. 5: Variability of the ultraviolet continuum and emission lines of NGC 3783

We report on the results of intensive ultraviolet spectral monitoring of the Seyfert 1 galaxy NGC 3783. The nucleus of NGC 3783 was observed with the International Ultraviolet Explorer satellite on a regular basis for a total of 7 months, once every 4 days for the first 172 days and once every other day for the final 50 days. Significant variability was observed in both continuum and emission-line fluxes. The light curves for the continuum fluxes exhibited two well-defined local minima or 'dips,' the first lasting is less than or approximately 20 days and the second is less than or approximately 4 days, with additional episodes of relatively rapid flickering of approximately the same amplitude. As in the case of NGC 5548 (the only other Seyfert galaxy that has been the subject of such an intensive, sustained monitoring effort), the largest continuum variations were seen at the shortest wavelengths, so that the continuum became 'harder' when brighter. The variations in the continuum occurred simultaneously at all wavelengths (delta(t) is less than 2 days). Generally, the amplitude of variability of the emission lines was lower than (or comparable to) that of the continuum. Apart from Mg II (which varied little) and N V (which is relatively weak and badly blended with Ly(alpha), the light curves of the emission lines are very similar to the continuum light curves, in each case with a small systematic delay or 'lag.' As for NGC 5548, the highest ionization lines seem to respond with shorter lags than the lower ionization lines. The lags found for NGC 3783 are considerably shorter than those obtained for NGC 5548, with values of (formally) approximately 0 days for He II + O III), and approximately 4 days for Ly(alpha) and C IV. The data further suggest lags of approximately 4 days for Si IV + O IV) and 8-30 days for Si III + C III). Mg II lagged the 1460 A continuum by approximately 9 days, although this result depends on the method of measuring the line flux and may in fact be due to variability of the underlying Fe II lines. Correlation analysis further shows that the power density spectrum contains substantial unresolved power over timescales of is less than or approximately 2 days, and that the character of the continuum variability may change with time

Binary systems and their nuclear explosions

Peer ReviewedPreprin

Three-Dimensional Fabry-Perot Imaging Spectroscopy of the Crab Nebula, Cassiopeia A, and Nova GK Persei

Three-dimensional spatial models of the line-emitting gas in the Crab Nebula, Cassiopeia A, and the nova remnant of GK Persei have been developed from imaging spectroscopy taken with the Goddard Fabry-Perot Imager mounted on the McGraw-Hill 1.3 m and Hiltner 2.4 m telescopes of the Michigan-Dartmouth-MIT Observatory. The spatial model of the Crab Nebula in [O III] λ5007 emission reveals the thin, annular morphology of the high-helium torus, the association of this torus and other bright filament loops with north-south bipolar asymmetry of the remnant shell, the structure of the filamentary shell at the boundary with the inner synchrotron cavity, and holes in the distribution of the brightest filaments at the ends of the major optical axis. Images of the fast-moving knots in Cas A in [S II] λλ6716, 6731 emission extend deeper than previously published broadband images, providing a more complete view of the limb-brightened, spherical edge of the remnant. The bright northern arch is imaged as two continuous rings of material distinct in velocity space. The larger ring includes the most highly redshifted material detected in Cas A, and incorporates many of the newly formed [O III] knots. The most blueshifted material occurs in the patchy nebulosity in the southeastern region of the remnant, and a large velocity asymmetry is seen in the radial and spatial velocities of the most highly blueshifted and redshifted optical knots. The data cube for the quasi-stationary flocculi in Cas A in Hα+[N II] λλ6548, 6583 emission reveals more diffuse nebulosity than previously detected and shows a distribution contained within an elliptical region on the sky which extends well beyond the spherical distribution of FMKs. Isophotes of GK Per in Hα+[N II] λλ6548, 6583 emission demonstrate the boxiness of the remnant as projected on the sky, show a lack of emission in the northern front and eastern central portions of the nebula, suggest the presence of a bulge-like structure in the south-southeast, and may indicate recent strengthening of emission in the northeastern limb

The large, bright QSO survey. III - QSOs in six equatorial fields

Results from six new UK Schmidt Telescope equatorial fields are presented in which 335 QSOs have been found in a total area of 190 square degrees. The detection of 29 new QSOs in the four fields in Virgo, discussed in a previous paper, is also reported. Coordinates, magnitudes, redshifts, and spectra of moderate resolution and signal-to-noise ratio are presented for all 365 QSOs and for nine additional extragalactic objects which fail to meet the absolute magnitude criterion as QSOs.The author was affiliated with Institute of Astronomy, Madingley Road, Cambridge, England when the paper was published