517 research outputs found
Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
Trends observed in galaxies, such as the Gao \& Solomon relation, suggest a
linear relation between the star formation rate and the mass of dense gas
available for star formation. Validation of such relations requires the
establishment of reliable methods to trace the dense gas in galaxies. One
frequent assumption is that the HCN (--0) transition is unambiguously
associated with gas at densities . If so,
the mass of gas at densities could be inferred from
the luminosity of this emission line, . Here we
use observations of the Orion~A molecular cloud to show that the HCN (--0)
line traces much lower densities in cold sections of
this molecular cloud, corresponding to visual extinctions
. We also find that cold and dense gas in a cloud like
Orion produces too little HCN emission to explain
in star--forming galaxies, suggesting that galaxies might contain a hitherto
unknown source of HCN emission. In our sample of molecules observed at
frequencies near 100~GHz (also including , ,
, CN, and CCH), is the only species clearly
associated with rather dense gas.Comment: accepted to A&A Letter
Polarisation Observations of HO 620.701 GHz Maser Emission with Herschel/HIFI in Orion KL
Context. The high intensities and narrow bandwidths exhibited by some
astronomical masers make them ideal tools for studying star-forming giant
molecular clouds. The water maser transition at
620.701 GHz can only be observed from above Earth's strongly absorbing
atmosphere; its emission has recently been detected from space. Aims. We sought
to further characterize the star-forming environment of Orion KL by
investigating the linear polarisation of a source emitting a narrow 620.701 GHz
maser feature with the heterodyne spectrometer HIFI on board the Herschel Space
Observatory. Methods. High-resolution spectral datasets were collected over a
thirteen month period beginning in 2011 March, to establish not only the linear
polarisation but also the temporal variability of the source. Results. Within a
uncertainty, no polarisation was detected to an upper limit of
approximately 2%. These results are compared with coeval linear polarisation
measurements of the 22.235 GHz maser line from
the Effelsberg 100-m radio telescope, typically a much stronger maser
transition. Although strongly polarised emission is observed for one component
of the 22.235 GHz maser at 7.2 km s, a weaker component at the same
velocity as the 620.701 GHz maser at 11.7 km s is much less polarised.Comment: Accepted for publication in A&
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Studies on control of diacetyl off-flavor in beer
Sporadic outbreaks of diacetyl off-flavor in beer are a serious
economic problem to the brewing industry. Studies were carried out
in an attempt to improve the understanding of the problem and to experiment
with new ways of controlling this defect.
The Owades and Jakovac method of diacetyl determination as
modified by Pack was further refined to increase its sensitivity to the
low diacetyl levels encountered in beers. A survey of alcoholic
beverages showed diacetyl levels of all samples tested to be below
threshold levels for organoleptic detection. Comparing yeast strains
on a per cell basis, a 2.5-fold difference was found to exist between
yeast strains in their ability to produce diacetyl. Also, corn steep
liquor addition to wort resulted in increased diacetyl production
during the subsequent fermentation.
Diacetyl removal from beer was studied using both live whole
cells and crude enzyme extracts. Cells of Streptococcus diacetilactis 18-16 destroyed diacetyl from solutions at a rate almost equal to that
achieved by the addition of live, whole yeast cells. Yeast cells impregnated
in a diatomaceous earth filter bed were found capable of
destroying all of the diacetyl from solutions percolated through the
bed. Undialyzed crude enzyme extracts from yeast cells removed
diacetyl very slowly from beer at its normal pH. When attempted at
a pH of 5.0 or higher, rapid diacetyl removal was achieved. Dialyzed
crude enzyme extracts from yeast cells were found to destroy diacetyl
in a manner quite similar to that of diacetyl reductase from
Aerobacter aerogenes, and both the bacterial extract and the yeast
extract were stimulated significantly by the addition of NADH.
Diacetyl reductase was studied, and it was found that at least
three strains of A. aerogenes were better sources of the enzyme than
strain 8724, the strain generally studied. Gel electrophoresis results
indicated that at least three different NADH oxidases were
present in crude extracts of diacetyl reductase. Sephadex gel filtration
was found to be an excellent method for separating NADH oxidase
activity from diacetyl reductase activity. It was also noted that
alcohol concentrations approximately equivalent to that found in beer
were quite inhibitory to diacetyl reductase activity
Distribution of Water Vapor in Molecular Clouds
We report the results of a large-area study of water vapor along the Orion
Molecular Cloud ridge, the purpose of which was to determine the
depth-dependent distribution of gas-phase water in dense molecular clouds. We
find that the water vapor measured toward 77 spatial positions along the
face-on Orion ridge, excluding positions surrounding the outflow associated
with BN/KL and IRc2, display integrated intensities that correlate strongly
with known cloud surface tracers such as CN, C2H, 13CO J =5-4, and HCN, and
less well with the volume tracer N2H+. Moreover, at total column densities
corresponding to Av < 15 mag., the ratio of H2O to C18O integrated intensities
shows a clear rise approaching the cloud surface. We show that this behavior
cannot be accounted for by either optical depth or excitation effects, but
suggests that gas-phase water abundances fall at large Av. These results are
important as they affect measures of the true water-vapor abundance in
molecular clouds by highlighting the limitations of comparing measured water
vapor column densities with such traditional cloud tracers as 13CO or C18O.
These results also support cloud models that incorporate freeze-out of
molecules as a critical component in determining the depth-dependent abundance
of water vapor
GGD 37: An Extreme Protostellar Outflow
We present the first Spitzer-IRS spectral maps of the Herbig-Haro flow GGD 37 detected in lines of [Ne III], [O IV], [Ar III], and [Ne v]. The detection of extended [O IV] (55 eV) and some extended emission in [Ne v] (97 eV) indicates a shock temperature in excess of 100,000 K, in agreement with X-ray observations, and a shock speed in excess of 200 km s(-1). The presence of an extended photoionization or collisional ionization region indicates that GGD 37 is a highly unusual protostellar outflow.Jet Propulsion Laboratory, under NASA 1407NASA 1257184Jet Propulsion Laboratory (JPL) 960803University of Rochester 31419-5714Astronom
Detection of Extended Hot Water in the Outflow from NGC 2071
We report the results of spectroscopic mapping observations carried out
toward a ~1 min x 1 min region within the northern lobe of the outflow from NGC
2071 using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope.
These observations covered the 5.2-37 um spectral region and have led to the
detection of a number of ionic, atomic, and molecular lines, including
fine-structure emission of Si+, Fe+, S++, S, the S(0)-S(7) pure rotational
lines of H2, the R(3) and R(4) transitions of HD, and at least 11 transitions
of H2O. In addition, the 6.2, 7.4, 7.6, 7.9, 8.6 and 11.3 um PAH emission bands
were also observed and several transitions of OH were tentatively detected.
Most of the detected line transitions were strong enough to map including, for
the first time, three transitions of hot H2O. We find that: (1) the water
emission is extended; (2) the extended emission is aligned with the outflow;
and, (3) the spatial distribution of the water emission generally follows that
observed for H2. Based on the measured line intensities, we derive an HD
abundance relative to H2 of 1.1-1.8 10^-5 and an H2O number density of 12-2
cm^3. The H2 density in the water-emitting region is not well constrained by
our observations, but is likely between 3 10^4 and 10^6 cm^3, yielding an H2O
abundance relative to H2 of between 2 10^-5 and 6 10^-4. Future observations
planned for the Herschel Space Observatory should greatly improve the density
estimate, and thus our knowledge of the H2O abundance, for the water-emitting
regions reported here. Finally, we note a possible departure from the H2O
ortho-to-para ratio of 3:1 expected for water formed in hot post-shocked gas,
suggesting that a significant fraction of the water vapor we detect may arise
from H2O sputtered from cold dust grains.Comment: 35 pages, 15 figures, 4 tables, accepted for publication in Ap
Precise Wavefront Correction with an Unbalanced Nulling Interferometer for Exo-Planet Imaging Coronagraphs
Very high dynamical range coronagraphs targeting direct exo-planet detection
(10^9 - 10^10 contrast) at small angular separation (few lambda/D units)
usually require an input wavefront quality on the order of ten thousandths of
wavelength RMS. We propose a novel method based on a pre-optics setup that
behaves partly as a low-efficiency coronagraph, and partly as a
high-sensitivity wavefront aberration compensator (phase and amplitude). The
combination of the two effects results in a highly accurate corrected
wavefront. First, an (intensity-) unbalanced nulling interferometer (UNI)
performs a rejection of part of the wavefront electric field. Then the
recombined output wavefront has its input aberrations magnified. Because of the
unbalanced recombination scheme, aberrations can be free of phase singular
points (zeros) and can therefore be compensated by a downstream phase and
amplitude correction (PAC) adaptive optics system, using two deformable
mirrors. In the image plane, the central star's peak intensity and the noise
level of its speckled halo are reduced by the UNI-PAC combination: the
output-corrected wavefront aberrations can be interpreted as an improved
compensation of the initial (eventually already corrected) incident wavefront
aberrations. The important conclusion is that not all the elements in the
optical setup using UNI-PAC need to reach the lambda/10000 rms surface error
quality.Comment: Accepted for publication in A&
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