Fred C. Bruhweiler

Star formation in NGC 4449: MAMA-detector UV imagery and Fabry-Perot Balmer-line imagery

Using far-ultraviolet (FUV) and Balmer-line imagery, we investigate the star formation history of 22 large OB complexes in the Magellanic irregular galaxy NGC 4449. The FUV luminosity of NGC 4449 is comparable to those of late-type spirals and is greater than that of the LMC by approximately 2.4 mag, indicating substantial star formation in the last 10(exp 8) yr. FUV data were taken using a sounding-rocket telescope with a Multianode Microchannel Array (MAMA) detector, and Balmer-line data were taken using the Goddard Fabry-Perot Imager. The resulting imagery shows bright, roughly coincident FUV and H alpha sources throughout the extent of the visible galaxy. We model these sources using cluster-evolution codes. Although all sources are a few Myr old, clear age differences are found. In particular, several of the most recently active star formation regions are located together in the galaxys northern periphery, which is apparently coincident with a large H I reservoir. The brightest and most massive OB complexes are found along the northeast-southwest surface brightness ridgeline (the bar). Over the entire galaxy, star formation rates are consistent on timescales of 10(exp 6), 10(exp 8), and 10(exp 9) yr. A history of recent star formation is suggested with two main episodes, one predominantly in the bar ending approximately 5 Myr ago, and an ongoing one associated with an observed H I cloud.

Ionization, Extinction, and Spiral Structure in the Inner Disk of NGC 1068*

We present our analysis of Hubble Space Telescope/Planetary Camera narrow-bandpass and broadband imagery of the inner 3 kpc region of NGC 1068. Our analysis of F160BW and F547M broadband continuum imagery suggests that roughly 40% of the scattered active galactic nucleus (AGN) continuum emission originates from an unobscured single cloud complex largely free from dust with total number densities typical of diffuse clouds in our own Milky Way. The net emission-line fluxes are extracted from continuum-subtracted narrow-bandpass imagery for Ha + [N ], Hβ, S II λλ6717, 6731, and [O III] λ5007. Although the [O ]/(Hα + [N ]) flux ratio shows a sharp drop-off at distances beyond ~4 northeast of the nucleus, the [O ]/Hb ratio indicates no such decrease. This implies that the ionization of these species is not strongly influenced by shocks associated with the expanding radio lobes as inferred from a previous study. The sharp drop-off seen in the [O ]/(Ha + [N ]) appears to be due to high interstellar reddening immediately beyond the scattering bright clouds near the nucleus, as further evidenced by the high Ha/Hb ratio in this region. The presence of a faint inner spiral arm interior to the sharply defined star formation ring, possibly driven by an outer-inner Linblad resonance, may provide a means of fueling the central AGN, as recently suggested by Yuan and Kuo. Because IR observations do not support the presence of a true AGN torus in NGC 1068, we present a qualitative model in which the radio ejecta has carved out an ionization cone in the high concentration of dense molecular clouds surrounding the nucleus. This picture also implies that the extended conical emission region to the northeast of the nucleus lies near the galactic plane and is surrounded by lower density ambient gas than that surrounding the highly ionized gas southwest of the nucleus.

STIS Spectral Imagery of the OB Stars in NGC 604. II. The Most Luminous Stars

We present results using two-dimensional spectral imagery and photometry obtained with the Hubble Space Telescope (HST) for the starburst H II region NGC 604, in the nearby galaxy M33. The spectral imagery was acquired with the Space Telescope Imaging Spectrograph (STIS) using the MAMA-G140L configuration, which provided wavelength coverage spanning 1170–1730 A. From a single 1720 s STIS exposure, we have extracted spectra for 49 stars and derived individual UV spectral types for 40 stars in the crowded 25 × 2 stellar field sampled by the STIS aperture. These stars represent a significant fraction of the young, luminous O and B stars in NGC 604. Three objects have pronounced He II λ1640 emission, the signature of Wolf-Rayet (W-R) or luminous Of stars. By combining UV fluxes with HST WF/PC-1 and WFPC2 photometry at visible wavelengths, we derive the extinction curve for NGC 604. We use this extinction curve, together with the available accurate distance for M33, derived UV spectral types, and HST photometry, to determine positions of the luminous stars in the upper Hertzsprung-Russell diagram for NGC 604. The revision to the O star effective temperature scale by Martins et al., based on non-LTE, line-blanketed model atmospheres, is essential in obtaining reliable positions in the log L*–log Teff plane. These stars are quite young, with a characteristic age of ~3 Myr. The spectra and photometry indicate that three objects are exceedingly luminous. Their inferred locations in the H-R diagram relative to theoretical evolutionary tracks indicate stellar masses ≥120 M⊙. High spatial resolution HST imagery provides no evidence of multiple stars composing these objects. Still, we cannot eliminate the possibility that these objects are not unresolved multiple stars of lower mass, possibly W-R stars. Simple tests demonstrate that the 10 most luminous stars predominantly determine the UV spectral features seen in the total light of NGC 604. We conclude that the interpretation of spectral fitting of more distant starburst galaxies, where individual stars are not resolved, must be done with extreme care.

UMBRAS: a matched occulter and telescope for imaging extrasolar planets

We describe a 1-meter space telescope plus free-flying occulter craft mission that would provide direct imaging and spectroscopic observations of Jovian and Uranus-sized planets about nearby stars not detectable by Doppler techniques. The Doppler technique is most sensitive for the detection of massive, close-in extrasolar planets while the use of a free-flying occulter would make it possible to image and study stellar systems with planets comparable to our own Solar System. Such a mission with a larger telescope has the potential to detect earth-like planets. Previous studies of free-flying occulters reported advantages in having the occulting spot outside the telescope compared to a classical coronagraph onboard a space telescope. Using an external occulter means light scatter within the telescope is reduced due to fewer internal obstructions and less light entering the telescope and the polishing tolerances of the primary mirror and the supporting optics can be less stringent, thereby providing higher contrast and fainter detection limits. In this concept, the occulting spot is positioned over the star by translating the occulter craft, at distances of 1,000 to 15,000 kms from the telescope, on the sky instead of by moving the telescope. Any source within the telescope field-of-view can be occulted without moving the telescope. In this paper, we present our current concept for a 1-m space telescope matched to a free-flying occulter, the Umbral Missions Blocking Radiating Astronomical Sources (UMBRAS) space mission. An UMBRAS space mission consists of a Solar Powered Ion Driven Eclipsing Rover (SPIDER) occulter craft and a matched (apodized) telescope. The occulter spacecraft would be semi-autonomous, with its own propulsion systems, internal power (solar cells), communications, and navigation capability. Spacecraft rendezvous and formation flying would be achieved with the aid of telescope imaging, RF or laser ranging, celestial navigation inputs, and formation control algorithms.

Detection of β Pictoris-like Gaseous Infall in 2 Andromedae*

Ultraviolet observations of 2 Andromedae with the Goddard High Resolution Spectrograph on board the Hubble Space Telescope (HST) clearly reveal the existence of circumstellar gas, in addition to previously known anomalous interstellar features, in the line of sight toward this nearby star. We also detect varying, redshifted Ca II absorptions, which we interpret as gaseous infalls. These variable Ca II absorption features and the presence of circumstellar absorptions from Al III λλ1854.716 and 1862.790 and from excited fine-structure levels of Fe II near 2600 A seen in 2 And are similar to those observed in spectra of β Pic.

Imaging planets about other stars with UMBRAS II

In this paper we discuss operational considerations for the free-flying occulter. Operations consist of maneuvering the Solar-Powered Ion-Driven Eclipsing Rover (SPIDER) between targets, alignment with the space-based telescope line of sight to the target, and stationkeeping target-to-target maneuvers need to be optimized to conserve propellant. A reasonable balance needs to be determined between target observation rate and the number of targets that are observable during mission lifetime. Velocity matching of the SPIDER with the telescope is essential to mission performance. An appropriate combination of solar electric and cold-gas thrusters provides the ability to match velocities using positional information derived from comminution and ranging between telescope, occulter and any metrology stations. Desirable features of using an external coronagraphic vehicle include the ability to obtain coronagraphic data with any instrument on the telescope-- imaging, spectroscopic, or interferometric.

Discovery of a cool expanding shell at -1200 kilometers per second around V471 Tauri

High-resolution IUE spectra of V471 Tauri reveal the presence of a very-high-velocity cool expanding gas in the line of sight to the binary system with an expansion velocity of -1200 km/s. The summed strength of the coadded absorption is 125 mA + or - 25 mA, with FWHM = 30 km/s. It is suggested that the observed absorption may be related to the narrow coadded absorption at -590 km/s noted by Bruhweiler and Sion (1966). The large expansion velocity suggests a possible association with an ancient nova outburst. 12 refs.

STIS SPECTRAL IMAGERY OF THE OB STARS IN NGC 604. I. DESCRIPTION OF THE EXTRACTION TECHNIQUE FOR A CROWDED STELLAR FIELD

We have developed a data reduction procedure to extract multiple spectra from a single two-dimensional Space Telescope Imaging Spectrograph (STIS) image of a crowded stellar field. This paper provides a description of our new technique, utilizing a STIS ultraviolet spectral image, acquired with the G140L grating and the 52 × 2 aperture, sampling a concentration of O and B stars in the central region of the NGC 604 starburst in M33. The software routines can disentangle and produce reliable ultraviolet spectra of stars with angular separations as small as 0055. Use of the extraction slit, based on our model of the spectral cross-dispersion profile, generates spectra with slightly higher resolution than the STScI standard processing. Our results clearly show that the spectral imaging capability of STIS represents a powerful tool for studying luminous stars in the star-forming regions of the Local Group.

UMBRAS: DESIGN OF A FREE-FLYING OCCULTER FOR SPACE TELESCOPES

A free-flying occulter used with a space-based telescope can enhance the contrast in the region close to a star, allowing extrasolar planet searches. An occulting spacecraft design, emphasizing configuration, control, propulsion, mass estimates, and power requirements, is presented requiring no extension of existing technology or exotic engineering solutions. The design is scalable for use with telescopes having 1 to 10 metre apertures. Several innovations are employed to block starlight and suppress scattered sunlight. A station keeping control method is possible using various thruster types. With scientifically interesting rates at which different stars are surveyed, solarelectric propulsion allows the occulting craft to be packaged on existing launchers with no on-orbit assembly.

A search for ultraviolet circumstellar gas absorption features in alpha Piscis Austrinus (Fomalhaut), a possible Beta Pictoris-like system

Archival high-dispersion International Ultraviolet Explorer (IUE) spectra have been used to search for circumstellar gas absorption features in alpha PsA (A3 V), a nearby (6.7 pc) proto-planetary system candidate. Recent sub-millimeter mapping observations around the region of alpha PsA indicate a spatially resolved dust disk like the one seen around Beta Pic. To determine how closely this putative disk resembles that of Beta Pic, we have searched for signatures of circumstellar gaseous absorption in all the available IUE high-dispersion data of alpha PsA. Examination of co-added IUE spectra shows weak circumstellar absorptions from excited levels in the resonance multiplet of Fe II near 2600 A. We also conclude that the sharp C I feature near 1657 A, previously identified as interstellar absorption toward alpha PsA, likely has a circumstellar origin. However, because the weakness of these absorption features, we will consider the presence of circumstellar gas as tentative and should be verified by using the Goddard High-Resolution Spectrograph aboard the Hubble Space Telescope. No corresponding circumstellar absorption is detected in higher ionization Fe III and Al III. Since the collisionally ionized nonphotospheric Al III resonance absorption seen in Beta Pic is likely formed close to the stellar surface, its absence in the UV spectra of alpha PsA could imply that, in contrast with Beta Pic, there is no active gaseous disk infall onto the central star. In the alpha PsA gaseous disk, if we assume a solar abundance for iron and all the iron is in the form of Fe II, plus a disk temperature of 5000 K, the Fe II UV1 absorption at 2611.8743 A infers a total hydrogen column density along the line of sight through the circumstellar disk of N(H) approximately equals 3.8 x 10(exp 17)/cm.