Could the Ultra Metal-poor Stars be Chemically Peculiar and Not Related to the First Stars?

Chemically peculiar stars define a class of stars that show unusual elemental abundances due to stellar photospheric effects and not due to natal variations. In this paper, we compare the elemental abundance patterns of the ultra metal-poor stars with metallicities [Fe/H] $\sim -5$ to those of a subclass of chemically peculiar stars. These include post-AGB stars, RV Tauri variable stars, and the Lambda Bootis stars, which range in mass, age, binarity, and evolutionary status, yet can have iron abundance determinations as low as [Fe/H] $\sim -5$. These chemical peculiarities are interpreted as due to the separation of gas and dust beyond the stellar surface, followed by the accretion of dust depleted-gas. Contrary to this, the elemental abundances in the ultra metal-poor stars are thought to represent yields of the most metal-poor supernova and, therefore, observationally constrain the earliest stages of chemical evolution in the Universe. The abundance of the elements in the photospheres of the ultra metal-poor stars appear to be related to the condensation temperature of that element; if so, then their CNO abundances suggest true metallicities of [X/H]~ -2 to -4, rather than their present metallicities of [Fe/H] < -5.Comment: Accepted for ApJ. 17 pages, 10 figure

A Keplerian Disk around the Herbig Ae star HD169142

We present Submillimeter Array observations of the Herbig Ae star HD169142 in 1.3 millimeter continuum emission and 12CO J=2-1 line emission at 1.5 arcsecond resolution that reveal a circumstellar disk. The continuum emission is centered on the star position and resolved, and provides a mass estimate of about 0.02 solar masses for the disk. The CO images show patterns in position and velocity that are well matched by a disk in Keplerian rotation with low inclination to the line-of-sight. We use radiative transfer calculations based on a flared, passive disk model to constrain the disk parameters by comparison to the spectral line emission. The derived disk radius is 235 AU, and the inclination is 13 degrees. The model also necessitates modest depletion of the CO molecules, similar to that found in Keplerian disks around T Tauri stars.Comment: 10 pages, 2 figures, accepted by A

Pulsed Doppler lidar for the detection of turbulence in clear air

A pulsed C02 Doppler lidar system is described and demonstration tests in ground-based and airborne flight operations are discussed. As a ground-based system, it can detect wind shears in thunderstorm gust fronts to a range of 6 km. When in the airborne configuration, the lidar can detect clear air turbulence in advance of the aircraft encountering clear air turbulence. The data provided by the lidar included turbulence location and intensity with intensity being indicated by the measured spectral width which is proportional to the wind gust velocity

Warm Gas in the Inner Disks around Young Intermediate Mass Stars

The characterization of gas in the inner disks around young stars is of particular interest because of its connection to planet formation. In order to study the gas in inner disks, we have obtained high-resolution K-band and M-band spectroscopy of 14 intermediate mass young stars. In sources that have optically thick inner disks, i.e. E(K-L)>1, our detection rate of the ro-vibrational CO transitions is 100% and the gas is thermally excited. Of the five sources that do not have optically thick inner disks, we only detect the ro-vibrational CO transitions from HD 141569. In this case, we show that the gas is excited by UV fluorescence and that the inner disk is devoid of gas and dust. We discuss the plausibility of the various scenarios for forming this inner hole. Our modeling of the UV fluoresced gas suggests an additional method by which to search for and/or place stringent limits on gas in dust depleted regions in disks around Herbig Ae/Be stars

Atomic and molecular interstellar absorption lines toward the high galactic latitude stars HD~141569 and HD~157841 at ultra-high resolution

We present ultra-high resolution (0.32 km/s) spectra obtained with the 3.9m Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF), of interstellar NaI D1, D2, Ca II K, K I and CH absorption toward two high galactic latitude stars HD141569 and HD157841. We have compared our data with 21-cm observations obtained from the Leiden/Dwingeloo HI survey. We derive the velocity structure, column densities of the clouds represented by the various components and identify the clouds with ISM structures seen in the region at other wavelengths. We further derive abundances, linear depletions and H2 fractional abundances for these clouds, wherever possible. Toward HD141569, we detect two components in our UHRF spectra : a weak, broad component at - 15 km/s, seen only in CaII K absorption and another component at 0 km/s, seen in NaI D1, D2, Ca II K, KI and CH absorption. In the case of the HD157841 sightline, a total of 6 components are seen on our UHRF spectra in NaI D1, D2 Ca II K, K I and CH absorption. 2 of these 6 components are seen only in a single species.Comment: 16 pages, Latex, 4 figures, ps files Astrophysical Journal (in press

Lunar elemental composition and ivestigations with D-CIXS x-ray mapping spectrometer on SMART-1

The D-CIXS Compact X-ray Spectrometer on ESA SMART-1 successfully launched in Sept 2003 can derive 45 km resolution images of the Moon with a spectral resolution of 185 eV, providing the first high-resolution global map of rock forming element abundances

Dust Migration and Morphology in Optically Thin Circumstellar Gas Disks

We analyze the dynamics of gas-dust coupling in the presence of stellar radiation pressure in circumstellar gas disks, which are in a transitional stage between the gas-dominated, optically thick, primordial nebulae, and the dust-dominated, optically thin Vega-type disks. Dust undergo radial migration, seeking a stable equilibrium orbit in corotation with gas. The migration of dust gives rise to radial fractionation of dust and creates a variety of possible observed disk morphologies, which we compute by considering the equilibrium between the dust production and the dust-dust collisions removing particles from their equilibrium orbits. Sand-sized and larger grains are distributed throughout most of the gas disk, with concentration near the gas pressure maximum in the inner disk. Smaller grains (typically in the range of 10 to 200 micron) concentrate in a prominent ring structure in the outer region of the gas disk (presumably at radius 100 AU), where gas density is rapidly declining with radius. The width and density, as well as density contrast of the dust ring with respect to the inner dust disk depend on the distribution of gas. Our results open the prospect for deducing the distribution of gas in circumstellar disks by observing their dust. We have qualitatively compared our models with two observed transitional disks around HR 4796A and HD 141569A. Dust migration can result in observation of a ring or a bimodal radial dust distribution, possibly very similar to the ones produced by gap-opening planet(s) embedded in the disk, or shepherding it from inside or outside. We conclude that a convincing planet detection via dust imaging should include specific non-axisymmetric structure following from the dynamical simulations of perturbed disks.Comment: 27 pages, 16 figures, submitted to Ap

PAH emission from Herbig AeBe stars

We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained with the Infrared Spectrograph on the Spitzer Space Telescope. All but one of the Herbig stars show emission from polycyclic aromatic hydrocarbons (PAHs) and seven of the spectra show PAH emission, but no silicate emission at 10 microns. The central wavelengths of the 6.2, 7.7--8.2, and 11.3 micron emission features decrease with stellar temperature, indicating that the PAHs are less photo-processed in cooler radiation fields. The apparent low level of photo processing in HAeBe stars, relative to other PAH emission sources, implies that the PAHs are newly exposed to the UV-optical radiation fields from their host stars. HAeBe stars show a variety of PAH emission intensities and ionization fractions, but a narrow range of PAH spectral classifications based on positions of major PAH feature centers. This may indicate that, regardless of their locations relative to the stars, the PAH molecules are altered by the same physical processes in the proto-planetary disks of intermediate-mass stars. Analysis of the mid-IR spectral energy distributions indicates that our sample likely includes both radially flared and more flattened/settled disk systems, but we do not see the expected correlation of overall PAH emission with disk geometry. We suggest that the strength of PAH emission from HAeBe stars may depend not only on the degree of radial flaring, but also on the abundance of PAHs in illuminated regions of the disks and possibly on the vertical structure of the inner disk as well.Comment: 52 pages, 12 figure

Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps

We have identified four circumstellar disks with a deficit of dust emission from their inner 15-50 AU. All four stars have F-G spectral type, and were uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence stars. Modeling of the spectral energy distributions indicates a reduction in dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU, but with massive gas-rich disks at larger radii. This large contrast between the inner and outer disk has led us to use the term `cold disks' to distinguish these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux density ratio as a new diagnostic for identifying cold disks. The mechanisms for dust clearing over such large gaps are discussed. Though rare, cold disks are likely in transition from an optically thick to an optically thin state, and so offer excellent laboratories for the study of planet formation.Comment: 13 pages, 3 figures, accepted to ApJ

Discovery of Reflection Nebulosity Around Five Vega-like Stars

Coronagraphic optical observations of six Vega-like stars reveal reflection nebulosities, five of which were previously unknown. The nebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, and HD 49662 resemble that of the Pleiades, indicating an interstellar origin for dust grains. The reflection nebulosity around HD 123160 has a double-arm morphology, but no disk-like feature is seen as close as 2.5 arcsec from the star in K-band adaptive optics data. We demonstrate that uniform density dust clouds surrounding HD 23362, HD 23680 and HD 123160 can account for the observed 12-100 micron spectral energy distributions. For HD 4881, HD 26676, and HD 49662 an additional emission source, such as from a circumstellar disk or non-equilibrium grain heating, is required to fit the 12-25 micron data. These results indicate that in some cases, particularly for Vega-like stars located beyond the Local Bubble (>100 pc), the dust responsible for excess thermal emission may originate from the interstellar medium rather than from a planetary debris system.Comment: The Astrophysical Journal, in press for March, 2002 (32 pages, 13 figures