near-UV Imaging Research Articles

Detection and Photometry of Hot Horizontal Branch Stars in the Core of M32

We present the deepest near-UV image of M32 to date, which for the first time resolves hot horizontal branch (HB) stars in an elliptical galaxy. Given the near-solar metallicity of M32, much larger than that of globular clusters, the existence of an extended horizontal branch is a striking example of the second parameter effect, and, most importantly, provides direct evidence that hot HB stars and their progeny are the major contributors to the UV upturn phenomenon observed in elliptical galaxies. Our image, obtained with the Space Telescope Imaging Spectrograph (STIS), detects approximately 8000 stars in a 25'' × 25'' field, centered 77 from the galaxy nucleus. These stars span a range of 21-28 mag in the STMAG system, and in the deepest parts of the image, our catalog is reasonably complete (>25%) to a magnitude of 27. The hot HB spans a magnitude range of 25-27 mag at effective temperatures hotter than 8500 K. We interpret this near-UV luminosity function with an extensive set of HB and post-HB evolutionary tracks. Although the UV-to-optical flux ratio in M32 is weak enough to be explained solely by the presence of post-asymptotic giant branch (post-AGB) stars, our image conclusively demonstrates that it arises from a small fraction (5%) of the population passing through the hot HB phase. The production of these hot HB stars does not appear to rely upon dynamical mechanisms—mechanisms that may play a role in the HB morphology of globular clusters. The majority of the population presumably evolves through the red HB and subsequent post-AGB phases; however, we see far fewer UV-bright stars than expected from the lifetimes of canonical hydrogen-burning low-mass post-AGB tracks. There are several possible explanations: (1) the transition from AGB to Teff > 60,000 K could be much more rapid than previously thought; (2) the vast majority of the post-AGB stars could be evolving along helium-burning tracks; (3) the post-AGB stars could be surrounded by circumstellar dust during the transition from the AGB to Teff > 60,000 K.

Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models

Motivated by new sounding-rocket wide-field polarimetric images of the Large Magellanic Cloud (reported simultaneously by Cole et al.), we have used a three-dimensional Monte Carlo radiation transfer code to investigate the escape of near-ultraviolet photons from young stellar associations embedded within a disk of dusty material (i.e., a galaxy). As photons propagate through the disk, they may be scattered or absorbed by dust. Scattered photons are polarized and tracked until they escape the dust layer, allowing them to be observed; absorbed photons heat the dust, which radiates isotropically in the far-infrared where the galaxy is optically thin. The code produces four output images: near-UV and far-IR flux, and near-UV images in the linear Stokes parameters Q and U. From these images we construct simulated UV polarization maps of the LMC. We use these maps to place constraints on the star+dust geometry of the LMC and the optical properties of its dust grains. By tuning the model input parameters to produce maps that match the observed polarization maps, we derive information about the inclination of the LMC disk to the plane of the sky and about the scattering phase function g. We compute a grid of models with i = 28°, 36°, and 45°, and g = 0.64, 0.70, 0.77, 0.83, and 0.90. The model that best reproduces the observed polarization maps has i = 36° and g ≈ 0.7. Because of the low signal-to-noise in the data, we cannot place firm constraints on the value of g. The highly inclined models do not match the observed centrosymmetric polarization patterns around bright OB associations or the distribution of polarization values. Our models approximately reproduce the observed ultraviolet photopolarimetry of the western side of the LMC; however, the output images depend on many input parameters and are nonunique. We discuss some of the limitations of the models and outline future steps to be taken; our models make some predictions regarding the polarization properties of diffuse light across the rest of the LMC.

Synoptic measurements of Martian winds using the Hubble Space Telescope

We used the Hubble Space Telescope to make wind speed and wind direction measurements of the Martian atmosphere during early northern summer (Ls=97°). Our strategy utilized a series of near‐UV images with optimal temporal sampling to track observable cloud features and to determine the direction and speed of cloud motion. Several techniques were used to identify similar features between images, including Fourier transform and cross‐correlation. In these measurements, which span 19–75° N lat and 319° E to 135° W Ion, we find average wind speeds of 26.2 ± 10.1 m/s and a range of wind directions from 51° to 162° with an average error of ±23°.

Deep UV Imaging of Galaxies in the Fornax Cluster

view Abstract Citations (8) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Deep UV Imaging of Galaxies in the Fornax Cluster O'Neil, Karen ; Bothun, G. D. ; Smith, Eric P. ; Stecher, T. P. Abstract We compare the Astro-1 Ultraviolet Imaging Telescope (UIT) deep image of the Fornax cluster with deep optical surveys for purposes of determining the efficacy of vacuum ultraviolet imaging in terms of the recovery of very faint and/or diffuse galaxies. This field covers approximately 1/3 of a square degree and contains 15 galaxies with B_t_ brighter than 18.0 mag. A 1000 s near-UV image detected ten of these galaxies. These ten detections include three luminous ellipticals, two bonafide dwarf ellipticals, and five galaxies of small angular size that may or may not be members of the Fornax cluster. If these optically faint galaxies are members of the Fornax cluster, then the VUV imaging did sample an appreciable range in the galaxy luminosity function. For the dwarf ellipticals, a comparison of their optical and UV fluxes indicates the UV light is not dominated by any hot population but rather represents the general UV characteristics of the underlying stellar population. In particular, we can place an upper limit of 25 O3-5 stars that are present due to possible low level star formation. Furthermore, the UV-optical colors of the dEs are significantly redder than Galactic globular clusters which indicates that these dEs do not have a significant population of extreme horizontal branch stars. With the failure of the near-UV camera aboard the Astro-2 mission, the opportunity for performing a deeper near-UV survey has been lost. We conclude, however, that such a survey would have good sensitivity to the detection of optically faint and/or diffuse galaxies and would provide credible information on the galaxy luminosity function. Publication: The Astronomical Journal Pub Date: August 1996 DOI: 10.1086/118026 Bibcode: 1996AJ....112..431O Keywords: ULTRAVIOLET: GALAXIES; GALAXIES: CLUSTERS: INDIVIDUAL: FORNAX full text sources ADS | data products NED (17) SIMBAD (11) MAST (1)

Ultraviolet Imaging Telescope images - Limits on recent star formation in Holmberg IX

Ultraviolet Imaging Telescope near-UV and far-UV images of Holmberg IX show no OB associations. Relative fluxes measured for the galaxy as a whole in the UV and optical B and V bands are consistent with models in which about 0.6-0.7 of the V flux is from an old population of age 10 Gyr, while the remainder is from stars of age 20-200 Myr, which also contribute 0.80 of the near-UV flux and 0.99 of the far-UV flux. Individual stars measured in B and V appear to be evolved stars of mass about 12 solar masses and age about 20 Myr belonging to the youngest population in the galaxy.