Showing papers by "Nathan Smith published in 2000"

Large-Scale Structure of the Carina Nebula.

Abstract: Observations obtained with the Midcourse Space Experiment (MSX) satellite reveal for the first time the complex mid-infrared morphology of the entire Carina Nebula (NGC 3372). On the largest size scale of ~100 pc, the thermal infrared emission from the giant H II region delineates one coherent structure: a (somewhat distorted) bipolar nebula with the major axis perpendicular to the Galactic plane. The Carina Nebula is usually described as an evolved H II region that is no longer actively forming stars, clearing away the last vestiges of its natal molecular cloud. However, the MSX observations presented here reveal numerous embedded infrared sources that are good candidates for sites of current star formation. Several compact infrared sources are located at the heads of dust pillars or in dark globules behind ionization fronts. Because their morphology suggests a strong interaction with the peculiar collection of massive stars in the nebula, we speculate that these new infrared sources may be sites of triggered star formation in NGC 3372.

Randomised controlled trial comparing effectiveness of touch screen system with leaflet for providing women with information on prenatal tests.

Abstract: Objective: To compare the effectiveness of touch screen system with information leaflet for providing women with information on prenatal tests. Design: Randomised controlled trial; participants allocated to intervention group (given access to touch screen and leaflet information) or control group (leaflet information only). Setting: Antenatal clinic in university teaching hospital. Subjects: 875 women booking antenatal care. Interventions: All participants received a leaflet providing information on prenatal tests. Women in the intervention arm also had access to touch screen information system in antenatal clinic. Main outcome measures: Women9s informed decision making on prenatal testing as measured by their uptake of and understanding of the purpose of specific tests; their satisfaction with information provided; and their levels of anxiety. Results: All women in the trial had a good baseline knowledge of prenatal tests. Women in the intervention group did not show any greater understanding of the purpose of the tests than control women. However, uptake of detailed anomaly scans was significantly higher in intervention group than the control group (94% (351/375) v 87% (310/358), P=0.0014). Levels of anxiety among nulliparous women in intervention group declined significantly over time (P Conclusions: The touch screen seemed to convey no benefit over well prepared leaflets in improving understanding of prenatal tests among the pregnant women. It did, however, seem to reduce levels of anxiety and may be most effective for providing information to selected women who have a relevant adverse history or abnormal results from tests in their current pregnancy.

η Carinae: Testing a Binary Orbit Model with the Hubble Space Telescope/Space Telescope Imaging Spectrograph

Abstract: Ground-based spectroscopy of η Car shows periodic changes in some emission-line wavelengths. These variations have been cited as strong evidence that this object is a 5.5 yr binary system and have been used to produce specific orbit models. High spatial resolution data obtained with the Hubble Space Telescope/Space Telescope Imaging Spectrograph, however, do not confirm the predicted velocity behavior; therefore, the published orbit models are almost certainly invalid. Wavelength fluctuations seen at ground-based spatial resolution most likely result from other effects, which we describe. If this object is a binary system (which has not been proven), then the parameters of the secondary star and of the orbit remain largely unknown.

Recent Changes in the Near-Ultraviolet and Optical Structure of η Carinae

Abstract: Multiepoch HST/WFPC2 images of η Carinae are used to investigate the relationship between the photometric variability of the circumstellar nebula and the variability of the central star. In the past few years, the central star has brightened considerably, and the response of the reflection nebula to this brightening has been surprisingly complex. While the central star has brightened by a factor of 2 at near-ultraviolet and optical wavelengths, bright dust condensations in the bipolar lobes have increased by only a factor of ~1.3, and dark lanes between these dust condensations have brightened by factors of 1.5–2. Certain regions of the nebula have brightened much more than the star itself (as much as a factor of 8), and others have actually faded, despite the brightening of the star. Some of the anomalous fading can be attributed to contributions of intrinsic [S III] and [N II] line emission. The variations of the equivalent width of [S III] λ6312 and [N II] λ6583 as measured in the WFPC2 F631N and F658N filters follow the same trend of other high-excitation lines observed in ground-based spectra during η Car's 5.5 yr spectroscopic cycle, and the amplitude of the change accounts for the total change measured in ground-based spectra for these same lines. The WFPC2 images indicate, however, that these high-excitation lines are emitted by circumstellar gas at distances of a few hundred to a few thousand AU from the star, probably located in the equatorial plane. Thus, periodic ionization of gas at large distances from the central star appears to dominate the spectroscopic changes that define the 5.5 yr cycle. The fact that the variable high-excitation emission is extended places important constraints on models for η Car's 5.5 yr spectroscopic variability.

Recent Changes in the Near-Infrared Structure of η Carinae*

Abstract: Near-infrared imaging at two phases in the 5 yr spectroscopic cycle of η Carinae reveals changes in the spatial structure of the inner core that may be related to recently reported near-infrared photometric variability. The central source changed from a pointlike object to a more extended, bipolar or shell structure. This behavior is reminiscent of changes observed in the radio continuum. NICMOS images show a toroidal distribution of dust and gas around the central star and confirm the morphology of several other structures observed in ground-based images of the Homunculus. The morphological variations appear to be confined to the central core of the nebula.

Thermal infrared imaging of ultracompact H II regions in W49A

Abstract: Several compact radio continuum sources in W49A show detectable 8-20 μm emission in MIRAC2 images obtained at the IRTF. In general, the infrared morphologies of these sources closely resemble the radio continuum emission. Spectral energy distributions indicate an infrared continuum excess above the level expected from free-free emission, consistent with thermal emission from dust grains heated to a few hundred K. The bright radio continuum sources concentrated at the western end of the ring of ultracompact H II regions are not detected in the mid-infrared, while those at other positions in the ring are detected. This could be due to a localized region of high extinction along the line of sight. In addition, there are a few new infrared sources with no radio continuum counterparts. Finally, several infrared sources show strong 12.8 μm [Ne II] emission, yielding neon abundances that are typically a few percent of the cosmic abundance of neon but are high considering the expected Ne++/Ne+ ratios for the range of spectral types of the ionizing sources. We conclude that the [Ne II] emission must come from shells around the ultracompact H II regions, where the neon is able to survive as Ne+ rather than Ne++ because the radiation field has been softened by absorption of hard UV photons within the H II regions.

A massive cool dust torus around η Carinae

Abstract: The star η Carinae, the most luminous known in the Milky Way galaxy, is a puzzling object. It is surrounded by a cloud of glowing gas that shows a distinct ‘pinched waist’ appearance, whose origin has been controversial. Morris et al.1 proposed that a massive but very compact torus of gas encircles the star, and that this caused the bipolar shape by preventing gas ejected by the star from expanding in the equatorial plane. Here we show that the small size of the torus (diameter of 5 arcsec) is insufficient to produce the observed infrared flux that inspired the conjecture. The dust giving rise to the 17-μm emission must be distributed over an area at least ten times the size of the torus. Such an extended structure of gas, however, would probably be incapable of generating the pinched waist.

ASCA and other contemporaneous observations of the blazar B2 1308+326

Abstract: The high redshift (z=0.997) blazar B2 1308+326 was observed contemporaneously at x-ray, optical and radio wavelengths in June 1996. The x-ray observations were performed with ASCA. The ASCA results were found to be consistent with reanalysed data from two earlier ROSAT observations. The combined ASCA and ROSAT data reveal an x-ray spectrum that is best fit by a broken power law with absorber model. The break in the x-ray spectrum is interpreted, to be the emerging importance of inverse Compton (IC) emission which dominates the ASCA spectrum. The faint optical state reported for these observations (m_V=18.3+/-0.25) is incompatible with the high synchrotron flux previously detected by ROSAT. The IC emission detected by both ROSAT and ASCA was not significantly affected by the large change in the synchrotron component. MgII emission was detected with an equivalent width (EW) significantly different from previously reported values. Absorption at a level of in excess of the Galactic value was detected, indicating the possible presence of a foreground absorber. A gravitational microlensing scenario cannot therefore be ruled out for this blazar. B2 1308+326 could be a typical radio-selected BL Lac in terms of peak synchrotron frequency and optical and radio variability but its high bolometric luminosity, variable line emission and high Doppler boost factor make it appear more like a quasar than a BL Lac. It is suggested that B2 1308+326 be considered as the prototype of this class of composite source.

Wide-Field Thermal Imaging of the Orion Nebula at High Spatial Resolution

Abstract: We describe multi-wavelength (8--20 micron), diffraction-limited, mid-infrared images of the OMC-1 cloud core in Orion, covering an approximately two arcminute area around the Trapezium and BN/KL regions We have detected mid-infrared emission at the locations of a subset of the previously identified proplyds in the Orion Nebula along with two new infrared sources The Ney-Allen nebula surrounding the OB star theta 1 Ori d exhibits a ring or toroidal structure at the longest wavelengths The BN/KL complex appears as an extended, butterfly-shaped structure with significant bipolar symmetry which is bifurcated by a dust lane at the longer wavelengths The infrared sources IRc3, 4, and 5 give the appearance of a ring-like structure with a possible jet-like protrusion from its center along a line from IRc2 Derived color temperature and dust opacity maps suggest that IRc3, 4, and 5 may not be self-luminous objects