Search

The literature on the $\lambda$ Boo stars has grown to become somewhat heterogenous, as different authors have applied different criteria across the UV, optical and infrared regions to determine the membership status of $\lambda$ Boo candidates. We aim to clear up the confusion by consulting the literature on 212 objects that have been considered as $\lambda$ Boo candidates, and subsequently evaluating the evidence in favour of their admission to the $\lambda$ Boo class. We obtained new spectra of $\sim$90 of these candidates and classified them on the MK system to aid in the membership evaluations. The re-evaluation of the 212 objects resulted in 64 members and 103 non-members of the $\lambda$ Boo class, with a further 45 stars for which membership status is unclear. We suggest observations for each of the stars in the latter category that will allow them to be confidently included or rejected from the class. Our reclassification facilitates homogenous analysis on group members, and represents the largest collection of confirmed $\lambda$ Boo stars known., Comment: 40 pages, 2 tables (spanning 4 pages), 0 figures. Accepted for publication in PASA. (The MN style file was used for convenience in manuscript preparation, only.)

Far-ultraviolet (FUV), Ha, and H I observations of dwarf galaxy Holmberg II are used to investigate the means by which star formation propagates in galaxies lacking global internal triggering mechanisms such as spiral density waves. The observations trace the interaction between sites of massive star forma- tion and the neutral and ionized components of the surrounding ISM in this intrinsically simple system. Both local and large-scale triggering mechanisms related to massive star formation are seen, suggesting that feedback from massive stars is a microscopic process operating in all galaxies to a certain degree. The data emphasize the importance of local conditions in regulating star formation from evidence such as massive stars inside ionized shells, compact H II regions surrounding aging clusters, and stars formed in chains of progressing age. Surface brightness pro—les show that current activity correlates with the time-averaged level of past star formation at a given radius demonstrating a reliance on local conditions. Large-scale triggering by H I shells is supported by observations of progenitor populations as well as secondary sites of star formation associated with their dense rims. Analysis of the energy available from massive stars inside H I shells indicates that energy deposited into the ISM from supernovae and stellar winds is sufficient to account for the H I morphology. Ages of individual star-forming regions are derived using B ,H a, and FUV photometry and show both older, diUuse FUV regions and younger, compact H II regions. The distribution of ages is reconciled with the H I morphology, showing a clear preference of young regions for areas of dense H I and old regions for H I voids. Global kinematical properties may also play a role in the star formation process since diUerences in the rotation characteristics of the neutral gas disk correlate with diUerences in triggering mechanisms. Large-scale feedback from massive stars is shown to operate in regions that lack diUerential shear in the gas disk. Subject headings: galaxies: dwarfgalaxies: ISMgalaxies: individual (DDO 50) ¨ stars: formationultraviolet: galaxies

Since the IRAS discovery of Vega’s large thermal infrared excess over the expected photospheric flux, dust disks have been found around a large fraction of main‐sequence A‐type stars. While dust in debris disks has been studied extensively with Spitzer, relatively little is known about their gas content. We have carried out detailed studies of the circumstellar environments of all nearby A‐type stars. We have studied both thermal emission from their circumstellar dust and absorption from circumstellar gas. With high‐resolution and high signal‐to‐noise visible spectra, we identified about a dozen main‐sequence A‐type stars with circumstellar gas through a volume‐limited survey. Although these spectra confirm the existence of the circumstellar gas and serve as a probe of the gas dynamics, they do not allow us to determine the gas density and temperature. Fortunately, there are many absorption lines in the UV and FUV range that are sensitive to the gas density and temperature. Through our FUSE, IUE, HST, and...

In this manuscript, we further develop our concepts for the free-flying occulter space-based mission, the Umbral Missions Blocking Radiating Astronomical Sources (UMBRAS). Our optical simulations clearly show that an UMBRAS-like mission designed around a 4-m telescope and 10-m occulter could directly image terrestrial planets. Such a mission utilizing existing technology could be built and flown by the end of the decade. Moreover, many of the other proposed concepts for Terrestrial Planet Finder (TPF) could significantly benefit by using an external occulter. We present simultations for an optical design comprising a square aperture telescope plus square external occulter. We show that the entire diffraction pattern, which is propagated from occulter to telescope and through telescope to focal plane, may be characterized by two parameters, the Fresnel number and the ratio of the telescope diameter to the occulter width. Combining the effects of a square occulter with apodization provides a much more rapid roll-off in the PSF intensity between the diffraction spikes than may be achieved with an unapodized telecope aperture and occulter. We parameterize our results with respect to wavefront quality and compare them against other competing methods for exo-planet imaging. The combination of external occulter and apodization yields the required contrast in the region of the PSF essential for exo-planet detection. An occulter external to the telescope (i.e., in a separate spacecraft, as opposed to a classical coronagraph with internal occulter) reduces light scatter within the telescope by approximately 2 orders of magnitude. This is due to less light actually entering the telescope. Reduced scattered light significantly relaxes the constraints on the mirror surface roughness, especially in the mid-spatial frequencies critical for planet detection. This study, plus our previous investigations of engineering as well as spacecraft rendezvous and formation flying clearly indicates that the UMBRAS concept is very competitive with, or superior to, other proposed concepts for TPF missions.

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.

We present a novel coronagraphic imaging technique and design for space-based telescopes. The Umbral Mission Blocking Radiating Astronomical Sources (UMBRAS) is a space mission design consisting of a free flying occulter, the Solar Powered Ion Driven Eclipsing Rover (SPIDER), and possibly one or two metrology platforms. The UMBRAS spacecraft operate in conjunction with a space-based telescope. The size of the occulting SPIDER is dictated by the size of the telescope with which it will work. The goal of UMBRAS is to provide "paleolithic" (i.e., non-focal plane) coronagraphic capability to enable direct imaging of extrasolar Jovian planets and other bright substellar companions such as brown dwarfs. We discuss two aspects of the operation of a free flying occulter: acquisition of targets and station keeping. Target acquisition is modeled after the onboard schemes used by Hubble Space Telescope (HST) science instruments. For UMBRAS, the onboard commanding sequences would include imaging the field using instruments on the telescope, locating the target and the occulter in the field, and accurately positioning the occulter over the target. Station keeping consists of actively maintaining the occulter position in the telescope line of sight to the target. Velocity matching of the c)cculter with the space-based telescope is essential to mission performance. An appropriate combination of solar electric and cold gas thrusters provide the ability to match velocities using position information derived from communication and from ranging data between telescope, occulter and any metrology stations. The accuracy requirements for target acquisition and station keeping depend upon the science requirements, the occultation geometry, and the sensitivity of the science to changes in occultation geometry during an exposure sequence. Observing modes other than the ideal centered occultation of a target will be discussed.

Direct imaging of terrestrial and Jupiter-size planets about other stars is a major goal of NASA's Origins Program and should be as well for the next generation of spaceborne telescopes. In this paper, we discuss a free-flying occulter to augment the design and imaging capability of space-based telescopes. The Umbral Mission Blocking Radiating Astronomical Sources (UMBRAS) space mission would consist of a Solar- Powered Ion-Driven Eclipsing Rover (SPIDER) and possibly one or two metrology platforms. The UMBRAS spacecraft would be semi-autonomous, with their own propulsion systems, internal power (solar cells), communications, and navigation capability. The spacecraft (the telescope, SPIDER, and any metrology platform) would define a reference frame for aligning the telescope and the SPIDER with the observed target. When stationed at distances of 1,000 to 15,000 km from a telescope, the occulter will enable an 8 m telescope to image very faint sources as close as 0.15' from the target stars. Three of the Doppler-detected planets about nearby stars are at this separation and could be directly imaged with this observing technique. It would be possible to image giant planets as close as 5 Au from parent stars at distances from the Sun as great as 30 pc. With this technique, terrestrial- size planets could be detected around nearby stars within the next decade. We briefly discuss the diffraction effects caused by the occulter and a preliminary proof-of-concept design for the UMBRAS spacecraft. Finally, we suggest types of observations other than planet finding that could be performed with UMBRAS.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on our search for possible planetary system candidates in a volumelimited sample of 62 nearby A stars. Since the evolutionary lifetimes of A stars (≤ 109 yrs) roughly correspond to the era of planet formation and subsequent “heavy bombardment” in our solar system, our study could provide valuable insight into the origin of our own Solar System. From our ground-based visual and IUE high-resolution spectroscopy of all the northern nearby A stars, we have identified at least 12 stars with circumstellar gas. Combining these results with our previous IRAS survey we are probing the link between stars with circumstellar gas and those showing circumstellar dust disks. Our aim is not just to identify stars with gas, or stars with both gas and dust, but to identify systems with dynamic spectral activity similar to s Pic, a well known proto-planetary system candidate. By measuring the gas dynamics in the disks of these s Pic-like stars, we can begin to study the physics of accretion disks of young evolving systems.