119 research outputs found
Boundary-Layer Similar Solutions for Equilibrium Dissociated Air and Application to the Calculation of Laminar Heat-Transfer Distribution on Blunt Bodies in High-Speed Flow
No abstract availabl
The effect of real-air properties upon aerodynamic forces, moments, and heat transfer rates for reentry vehicles
Spectrophotometrically Identified stars in the PEARS-N and PEARS-S fields
Deep ACS slitless grism observations and identification of stellar sources
are presented within the Great Observatories Origins Deep Survey (GOODS) North
and South fields which were obtained in the Probing Evolution And Reionization
Spectroscopically (PEARS) program. It is demonstrated that even low resolution
spectra can be a very powerful means to identify stars in the field, especially
low mass stars with stellar types M0 and later. The PEARS fields lay within the
larger GOODS fields, and we used new, deeper images to further refine the
selection of stars in the PEARS field, down to a magnitude of mz = 25 using a
newly developed stellarity parameter. The total number of stars with reliable
spectroscopic and morphological identification was 95 and 108 in the north and
south fields respectively. The sample of spectroscopically identified stars
allows constraints to be set on the thickness of the Galactic thin disk as well
as contributions from a thick disk and a halo component. We derive a thin disk
scale height, as traced by the population of M4 to M9 dwarfs along two
independent lines of sight, of h_thin = 370 +60/-65 pc. When including the more
massive M0 to M4 dwarf population, we derive h_thin = 300 +/- 70pc. In both
cases, we observe that we must include a combination of thick and halo
components in our models in order to account for the observed numbers of faint
dwarfs. The required thick disk scale height is typically h_thick=1000 pc and
the acceptable relative stellar densities of the thin disk to thick disk and
the thin disk to halo components are in the range of 0.00025<f_halo<0.0005 and
0.05<f_thick<0.08 and are somewhat dependent on whether the more massive M0 to
M4 dwarfs are included in our sample
A Uniform CO Survey of the Molecular Clouds in Orion and Monoceros
We report the results of a new large scale survey of the Orion-Monoceros
complex of molecular clouds made in the J = 1->0 line of CO with the
Harvard-Smithsonian 1.2m millimetre-wave telescope. The survey consists of
52,288 uniformly spaced spectra that cover an area of 432 square degrees on the
sky and is the most sensitive large-scale survey of the region to date.
Distances to the constituent molecular clouds of the complex, estimated from an
analysis of foreground and background stars, have provided information on the
three dimensional structure of the entire complex.Comment: Accepted for publication in Astronomy and Astrophysics. 19 pages with
17 colour figures - 39 if you count the sub-figures separately. The figures
here have been bit-mapped with some loss of quality and beauty. The paper
version in A&A will be in greyscale with the on-line version in colour. In
the meantime the colour version can be obtained by following links at
http://www.star.bris.ac.uk/mrwm . The 9MB PostScript is recommended if you
have appropriate bandwidth or otherwise the 2.3MB PDF is usabl
A Search for Mid-Infrared Molecular Hydrogen Emission from Protoplanetary Disks
We observed the Herbig Ae/Be stars UX Ori, HD 34282, HD 100453, HD 101412, HD
104237 and HD 142666, and the T Tauri star HD 319139 and searched for H2 0-0
S(2) emission at 12.278 micron and H2 0-0 S(1) emission at 17.035 micron with
VISIR, ESO-VLT's high-resolution MIR spectrograph. None of the sources present
evidence for H2 emission. Stringent 3sigma upper limits to the integrated line
fluxes and the mass of optically thin warm gas in the disks are derived. The
disks contain less than a few tenths of Jupiter mass of optically thin H2 gas
at 150 K at most, and less than a few Earth masses of optically thin H2 gas at
300 K and higher temperatures. We compare our results to a Chiang and Goldreich
(1997, CG97) two-layer disk model. The upper limits to the disk's optically
thin warm gas mass are smaller than the amount of warm gas in the interior
layer of the disk, but they are much larger than the amount of molecular gas in
the surface layer. We present a calculation of the expected thermal H2 emission
from optically thick disks, assuming a CG97 disk structure, a gas-to-dust ratio
of 100 and Tgas = Tdust. The expected H2 thermal emission fluxes from typical
disks around Herbig Ae/Be stars (10^-16 to 10^-17 erg/s/cm2 at 140 pc) are much
lower than the detection limits of our observations (5*10^-15 erg/s/cm2). H2
emission levels are very sensitive to departures from the thermal coupling
between the molecular gas and dust. Additional sources of heating of gas in the
disk's surface layer could have a major impact on the expected H2 disk
emission. In the observed sources the molecular gas and dust in the surface
layer have not significantly departed from thermal coupling (Tgas/Tdust< 2) and
that the gas-to-dust ratio in the surface layer is very likely lower than 1000.Comment: 16 pages, 9 figures, accepted by A&A. v2: typo in footnote **
corrected, v3: corrections of the A&A language editor included, typo in title
of Fig. 1. correcte
The Hubble Space Telescope Wide Field Camera 3 Early Release Science data: Panchromatic Faint Object Counts for 0.2-2 microns wavelength
We describe the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) Early
Release Science (ERS) observations in the Great Observatories Origins Deep
Survey (GOODS) South field. The new WFC3 ERS data provide calibrated, drizzled
mosaics in the UV filters F225W, F275W, and F336W, as well as in the near-IR
filters F098M (Ys), F125W (J), and F160W (H) with 1-2 HST orbits per filter.
Together with the existing HST Advanced Camera for Surveys (ACS) GOODS-South
mosaics in the BViz filters, these panchromatic 10-band ERS data cover 40-50
square arcmin at 0.2-1.7 {\mu}m in wavelength at 0.07-0.15" FWHM resolution and
0.090" Multidrizzled pixels to depths of AB\simeq 26.0-27.0 mag (5-{\sigma})
for point sources, and AB\simeq 25.5-26.5 mag for compact galaxies.
In this paper, we describe: a) the scientific rationale, and the data taking
plus reduction procedures of the panchromatic 10-band ERS mosaics; b) the
procedure of generating object catalogs across the 10 different ERS filters,
and the specific star-galaxy separation techniques used; and c) the reliability
and completeness of the object catalogs from the WFC3 ERS mosaics. The
excellent 0.07-0.15" FWHM resolution of HST/WFC3 and ACS makes star- galaxy
separation straightforward over a factor of 10 in wavelength to AB\simeq 25-26
mag from the UV to the near-IR, respectively.Comment: 51 pages, 71 figures Accepted to ApJS 2011.01.2
Hubble Space Telescope WFC3 Early Release Science: Emission-Line Galaxies from Infrared Grism Observations
We present grism spectra of emission-line galaxies (ELGs) from 0.6-1.6
microns from the Wide Field Camera 3 on the Hubble Space Telescope. These new
infrared grism data augment previous optical Advanced Camera for Surveys G800L
0.6-0.95 micron grism data in GOODS-South from the PEARS program, extending the
wavelength covereage well past the G800L red cutoff. The ERS grism field was
observed at a depth of 2 orbits per grism, yielding spectra of hundreds of
faint objects, a subset of which are presented here. ELGs are studied via the
Ha, [OIII], and [OII] emission lines detected in the redshift ranges 0.2<z<1.4,
1.2<z<2.2 and 2.0<z<3.3 respectively in the G102 (0.8-1.1 microns; R~210) and
G141 (1.1-1.6 microns; R~130) grisms. The higher spectral resolution afforded
by the WFC3 grisms also reveals emission lines not detectable with the G800L
grism (e.g., [SII] and [SIII] lines). From these relatively shallow
observations, line luminosities, star-formation rates, and grism spectroscopic
redshifts are determined for a total of 48 ELGs to m(AB)~25 mag. Seventeen
GOODS-South galaxies that previously only had photometric redshifts now have
new grism-spectroscopic redshifts, in some cases with large corrections to the
photometric redshifts (Delta(z)~0.3-0.5). Additionally, one galaxy had no
previously-measured redshift but now has a secure grism-spectroscopic redshift,
for a total of 18 new GOODS-South spectroscopic redshifts. The faintest source
in our sample has a magnitude m(AB)=26.9 mag. The ERS grism data also reflect
the expected trend of lower specific star formation rates for the highest mass
galaxies in the sample as a function of redshift, consistent with downsizing
and discovered previously from large surveys. These results demonstrate the
remarkable efficiency and capability of the WFC3 NIR grisms for measuring
galaxy properties to faint magnitudes and redshifts to z>2.Comment: Accepted for publication in AJ. Updated to include referee comments.
Updated sample using improved reduction contains 23 new galaxies (Table 1;
Figures 2 & 3
The weak-line T Tauri star V410Tau I. A multi-wavelength study of variability
We present the results of an intensive coordinated monitoring campaign in the
optical and X-ray wavelength ranges of the low-mass, pre-main sequence star
V410Tau carried out with the aim to study the relation between various
indicators for magnetic activity that probe emission from different atmospheric
layers: optical photometric star spot (rotation) cycle, chromospheric Halpha
emission, and coronal X-rays. Two X-ray pointings were carried out with the
Chandra satellite simultaneously with the optical observations, and centered
near the maximum and minimum levels of the optical lightcurve. A relation of
their different count levels to the rotation period of the dominating spot is
not confirmed by a third Chandra observation carried out some months later,
during another minimum of the 1.87d cycle. Similarly we find no indications for
a correlation of the Halpha emission with the spots' rotational phase. The
extraordinary stability of the largest spot is confirmed by long-term
photometric and radial velocity measurements. Joining our optical photometry
with previous data we provide a new estimate for the dominant periodicity of
V410Tau. This updated value removes systematic offsets of the time of minimum
observed in data taken over the last decade. Furthermore, the combination of
the new data with published measurements taken during the last decade allows us
to examine long-term changes in the mean light level of the photometry of
V410Tau. A variation on the timescale of 5.4yr is suggested. Assuming that this
behavior is truely cyclic V410Tau is the first pre-main sequence star on which
an activity cycle is detected.Comment: 16 pages, accepted for publication in A&
Exo--Zodiacal Dust Levels for Nearby Main Sequence Stars
The Keck Interferometer Nuller (KIN) was used to survey 25 nearby main
sequence stars in the mid-infrared, in order to assess the prevalence of warm
circumstellar (exozodiacal) dust around nearby solar-type stars. The KIN
measures circumstellar emission by spatially blocking the star but transmitting
the circumstellar flux in a region typically 0.1 - 4 AU from the star. We find
one significant detection (eta Crv), two marginal detections (gamma Oph and
alpha Aql), and 22 clear non-detections. Using a model of our own Solar
System's zodiacal cloud, scaled to the luminosity of each target star, we
estimate the equivalent number of target zodis needed to match our
observations. Our three zodi detections are eta Crv (1250 +/- 260), gamma Oph
(200 +/- 80) and alpha Aql (600 +/- 200), where the uncertainties are 1-sigma.
The 22 non-detected targets have an ensemble weighted average consistent with
zero, with an average individual uncertainty of 160 zodis (1-sigma). These
measurements represent the best limits to date on exozodi levels for a sample
of nearby main sequence stars. A statistical analysis of the population of 23
stars not previously known to contain circumstellar dust (excluding eta Crv and
gamma Oph) suggests that, if the measurement errors are uncorrelated (for which
we provide evidence) and if these 23 stars are representative of a single class
with respect to the level of exozodi brightness, the mean exozodi level for the
class is <150 zodis (3-sigma upper-limit, corresponding to 99% confidence under
the additional assumption that the measurement errors are Gaussian). We also
demonstrate that this conclusion is largely independent of the shape and mean
level of the (unknown) true underlying exozodi distribution
The complex circumstellar environment of HD142527
The recent findings of gas giant planets around young A-type stars suggest
that disks surrounding Herbig Ae/Be stars will develop planetary systems. An
interesting case is HD142527, for which previous observations revealed a
complex circumstellar environment and an unusually high ratio of infrared to
stellar luminosity. Its properties differ considerably from other Herbig Ae/Be
stars. This suggests that the disk surrounding HD142527 is in an uncommon
evolutionary stage. We aim for a better understanding of the geometry and
evolutionary status of the circumstellar material around the Herbig Ae/Be star
HD142527. We map the composition and spatial distribution of the dust around
HD142527. We analyze SEST and ATCA millimeter data, VISIR N and Q-band imaging
and spectroscopy. We gather additional relevant data from the literature. We
use the radiative transfer code MCMax to construct a model of the geometry and
density structure of the circumstellar matter, which fits all of the
observables satisfactorily. We find that the disk of HD142527 has three
geometrically distinct components separated by a disk gap running from 30 to
130 AU. There is a geometrically flat inner disk running from 0.3 AU up to 30
AU; an optically thin halo-like component of dust in the inner disk regions;
and a massive self-shadowed outer disk running from 130 AU up to 200 AU. We
derived a total dust mass in small grains of 10^-3 M0 and a vertical height of
the inner wall of the outer disk of h = 60 AU. ....Comment: 13 pages, 17 figures. Accepted by A&A 14 January 201
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