1,201 research outputs found
Low-disturbance wind tunnels
During the past years, there was an extensive program under way at the Langley Research Center to upgrade the flow quality in several of the large wind tunnels. This effort has resulted in significant improvements in flow quality in these tunnels and has also increased the understanding of how and where changes in existing and new wind tunnels are most likely to yield the desired improvements. As part of this ongoing program, flow disturbance levels and spectra were measured in several Langley tunnels before and after modifications were made to reduce acoustic and vorticity fluctuations. A brief description of these disturbance control features is given for the Low-Turbulence Pressure Tunnel, the 4 x 7 Meter Tunnel, and the 8 Foot Transonic Pressure Tunnel. To illustrate typical reductions in disturbance levels obtained in these tunnels, data from hot-wire or acoustic sensors are presented. A concept for a subsonic quiet tunnel designed to study boundary layer stability and transition is also presented. Techniques developed at Langley in recent years to eliminate the high intensity and high-frequency acoustic disturbances present in all previous supersonic wind tunnels are described. In conclusion, the low-disturbance levels present in atmospheric flight can now be simulated in wind tunnels over the speed range from low subsonic through high supersonic
Upper limit for the D2H+ ortho-to-para ratio in the prestellar core 16293E (CHESS)
The H3+ ion plays a key role in the chemistry of dense interstellar gas
clouds where stars and planets are forming. The low temperatures and high
extinctions of such clouds make direct observations of H3+ impossible, but lead
to large abundances of H2D+ and D2H+, which are very useful probes of the early
stages of star and planet formation. The ground-state rotational ortho-D2H+
111-000 transition at 1476.6 GHz in the prestellar core 16293E has been
searched for with the Herschel/HIFI instrument, within the CHESS (Chemical
HErschel Surveys of Star forming regions) Key Program. The line has not been
detected at the 21 mK km/s level (3 sigma integrated line intensity). We used
the ortho-H2D+ 110-111 transition and para-D2H+ 110-101 transition detected in
this source to determine an upper limit on the ortho-to-para D2H+ ratio as well
as the para-D2H+/ortho-H2D+ ratio from a non-LTE analysis. The comparison
between our chemical modeling and the observations suggests that the CO
depletion must be high (larger than 100), with a density between 5e5 and 1e6
cm-3. Also the upper limit on the ortho-D2H+ line is consistent with a low gas
temperature (~ 11 K) with a ortho-to-para ratio of 6 to 9, i.e. 2 to 3 times
higher than the value estimated from the chemical modeling, making it
impossible to detect this high frequency transition with the present state of
the art receivers.Comment: Accepted in A&
The 1.2 Millimeter Image of the beta Pictoris Disk
We present millimeter imaging observations in the 1200 micron continuum of
the disk around beta Pictoris. With the 25 arcsec beam, the beta Pic disk is
unresolved perpendicularly to the disk plane (< 10 arcsec), but slightly
resolved in the northeast-southwest direction (26 arcsec). Peak emission is
observed at the stellar position. A secondary maximum is found 1000 AU along
the disk plane in the southwest, which does not positionally coincide with a
similar feature reported earlier at 850 micron. Arguments are presented which
could be seen in support of the reality of these features. The observed
submm/mm emission is consistent with thermal emission from dust grains, which
are significantly larger than those generally found in the interstellar medium,
including mm-size particles, and thus more reminiscent of the dust observed in
protostellar disks. Modelling the observed scattered light in the visible and
the emission in the submm/mm provides evidence for the particles dominating the
scattering in the visible/NIR and those primarily responsible for the thermal
emission at longer wavelengths belonging to different populations.Comment: 6 pages, 3 postscript figures, accepted for publication in Astronomy
and Astrophysic
Young Stars and Protostellar Cores near NGC 2023
We investigate the young (proto)stellar population in NGC 2023 and the L 1630
molecular cloud bordering the HII region IC 434, using Spitzer IRAC and MIPS
archive data, JCMT SCUBA imaging and spectroscopy as well as targeted BIMA
observations of one of the Class 0 protostars, NGC 2023 MM1. We have performed
photometry of all IRAC and MIPS images, and used color-color diagrams to
identify and classify all young stars seen within a 22'x26' field along the
boundary between IC 434 and L 1630. For some stars, which have sufficient
optical, IR, and/or sub-millimeter data we have also used the online SED
fitting tool for a large 2D archive of axisymmetric radiative transfer models
to perform more detailed modeling of the observed SEDs. We identify 5
sub-millimeter cores in our 850 and 450 micron SCUBA images, two of which have
embedded class 0 or I protostars. Observations with BIMA are used to refine the
position and characteristics of the Class 0 source NGC 2023 MM 1. These
observations show that it is embedded in a very cold cloud core, which is
strongly enhanced in NH2D. We find that HD 37903 is the most massive member of
a cluster with 20 -- 30 PMS stars. We also find smaller groups of PMS stars
formed from the Horsehead nebula and another elephant trunk structure to the
north of the Horsehead. We refine the spectral classification of HD 37903 to B2
Ve. Our study shows that the expansion of the IC 434 HII region has triggered
star formation in some of the dense elephant trunk structures and compressed
gas inside the L 1630 molecular cloud. This pre-shock region is seen as a
sub-millimeter ridge in which stars have already formed. The cluster associated
with NGC 2023 is very young, and has a large fraction of Class I sources.Comment: 21 pages, 11 figures. Accepted for publication in A&A Replaced with
higher resolution figure
Ultraviolet number counts of galaxies from Swift UV/Optical Telescope deep imaging of the Chandra Deep Field South
Deep Swift UV/Optical Telescope (UVOT) imaging of the Chandra Deep Field
South is used to measure galaxy number counts in three near ultraviolet (NUV)
filters (uvw2: 1928 A, uvm2: 2246 A, uvw1: 2600 A) and the u band (3645 A).
UVOT observations cover the break in the slope of the NUV number counts with
greater precision than the number counts by the Hubble Space Telescope (HST)
Space Telescope Imaging Spectrograph (STIS) and the Galaxy Evolution Explorer
(GALEX), spanning a range from 21 < m_AB < 25. Number counts models confirm
earlier investigations in favoring models with an evolving galaxy luminosity
function.Comment: 20 pages, 6 figures, accepted to Ap
Self-similar solutions of viscous and resistive ADAFs with thermal conduction
We have studied the effects of thermal conduction on the structure of viscous
and resistive advection-dominated accretion flows (ADAFs). The importance of
thermal conduction on hot accretion flow is confirmed by observations of hot
gas that surrounds Sgr A and a few other nearby galactic nuclei. In this
research, thermal conduction is studied by a saturated form of it, as is
appropriated for weakly-collisional systems. It is assumed the viscosity and
the magnetic diffusivity are due to turbulence and dissipation in the flow. The
viscosity also is due to angular momentum transport. Here, the magnetic
diffusivity and the kinematic viscosity are not constant and vary by position
and -prescription is used for them. The govern equations on system have
been solved by the steady self-similar method. The solutions show the radial
velocity is highly subsonic and the rotational velocity behaves sub-Keplerian.
The rotational velocity for a specific value of the thermal conduction
coefficient becomes zero. This amount of conductivity strongly depends on
magnetic pressure fraction, magnetic Prandtl number, and viscosity parameter.
Comparison of energy transport by thermal conduction with the other energy
mechanisms implies that thermal conduction can be a significant energy
mechanism in resistive and magnetized ADAFs. This property is confirmed by
non-ideal magnetohydrodynamics (MHD) simulations.Comment: 8 pages, 5 figures, accepted by Ap&S
Millimeter and submillimeter high angular resolution interferometric observations: dust in the heart of IRAS 18162-2048
The GGD27 complex includes the HH 80-81-80N system, which is one of the most
powerful molecular outflows associated with a high mass star-forming region
observed up to now. This outflow is powered by the star associated with the
source IRAS 18162-2048. Here we report the detection of continuum emission at
sub-arcsec/arcsec resolution with the Submillimeter Array at 1.36mm and
456microns, respectively. We detected dust emission arising from two compact
cores, MM1 and MM2, separated by about 7" (~12000AU in projected distance). MM1
spatially coincides with the powerful thermal radio continuum jet that powers
the very extended molecular outflow, while MM2 is associated with the protostar
that drives the compact molecular outflow recently found in this region.
High angular resolution obervations at 1.36mm show that MM1 is unresolved and
that MM2 splits into two subcomponents separated by ~1". The mass of MM1 is
about 4Msun and it has a size of <300AU. This is consistent with MM1 being
associated with a massive and dense (n(H2)>10^9cm-3) circumstellar dusty disk
surrounding a high-mass protostar, which has not developed yet a compact HII
region. On the other hand, the masses of the two separate components of MM2 are
about 2Msun each. One of these components is a compact core with an
intermediate-mass young protostar inside and the other component is probably a
pre-stellar core.
MM1 is the brigthest source at 1.36mm, while MM2 dominates the emission at
456microns. These are the only (sub)millimeter sources detected in the SMA
observations. Hence, it seems that both sources may contribute significantly to
the bolometric luminosity of the region. Finally, we argue that the
characteristics of these two sources indicate that MM2 is probably in an
earlier evolutionary stage than MM1.Comment: Accepted in AJ (Oct 31, 2010
The dusty environment of HD 97300 as seen by Herschel and Spitzer
Aims. We analyze the surroundings of HD 97300, one of two intermediate-mass
stars in the Chamaeleon I star-forming region. The star is known to be
surrounded by a conspicuous ring of polycyclic aromatic hydrocarbons (PAHs).
Methods. We present infrared images taken with Herschel and Spitzer using 11
different broad-band filters between 3.6 um and 500 um. We compare the
morphology of the emission using cuts along different position angles. We
construct spectral energy distributions, which we compare to different dust
models, and calculate dust temperatures. We also derive opacity maps and
analyze the density structure of the environment of HD 97300.
Results. We find that HD 97300 has no infrared excess at or below 24 um,
confirming its zero-age main-sequence nature. The morphology of the ring is
very similar between 3.6 um and 24 um. The emission at these wavelengths is
dominated by either PAH features or PAH continuum. At longer wavelengths, only
the northwestern part of the ring is visible. A fit to the 100-500 um
observations suggests that the emission is due to relatively warm (~26 K) dust.
The temperature gradually decreases with increasing distance from the ring. We
find a general decrease in the density from north to south, and an approximate
10% density increase in the northeastern part of the ring.
Conclusions. Our results are consistent with the theory that the ring around
HD 97300 is essentially a bubble blown into the surrounding interstellar matter
and heated by the star.Comment: 6 pages, 3 figures, accepted for publication in A&
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
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