3,063 research outputs found
B709: An Evaluation of the Potential for Maine Raised Oysters
The primary purpose of the study reported here was to investigate the feasibility of developing profitable markets for Maine raised Ostrea edulis. More specifically the objective were to (1) evaluate trend in oyster supplies, including landings, imports, and exports in the U.S., Canada and France; (2) analyze trends in oyster consumption in the U.S., Canada, and France and develop a predictive equation based on factors influencing consumption in each of these areas; (3) evaluate the economic feasibility of developing a half-shell oyster market for Maine raised oysters.https://digitalcommons.library.umaine.edu/aes_bulletin/1071/thumbnail.jp
Benchmarking of 3D space charge codes using direct phase space measurements from photoemission high voltage DC gun
We present a comparison between space charge calculations and direct
measurements of the transverse phase space for space charge dominated electron
bunches after a high voltage photoemission DC gun followed by an emittance
compensation solenoid magnet. The measurements were performed using a
double-slit setup for a set of parameters such as charge per bunch and the
solenoid current. The data is compared with detailed simulations using 3D space
charge codes GPT and Parmela3D with initial particle distributions created from
the measured transverse and temporal laser profiles. Beam brightness as a
function of beam fraction is calculated for the measured phase space maps and
found to approach the theoretical maximum set by the thermal energy and
accelerating field at the photocathode.Comment: 11 pages, 23 figures. submitted to Phys Rev ST-A
The Spitzer c2d Survey of Nearby Dense Cores. IX. Discovery of a Very Low Luminosity Object Driving a Molecular Outflow in the Dense Core L673-7
We present new infrared, submillimeter, and millimeter observations of the
dense core L673-7 and report the discovery of a low-luminosity, embedded Class
0 protostar driving a molecular outflow. L673-7 is seen in absorption against
the mid-infrared background in 5.8, 8, and 24 micron Spitzer images, allowing
for a derivation of the column density profile and total enclosed mass of
L673-7, independent of dust temperature assumptions. Estimates of the core mass
from these absorption profiles range from 0.2-4.5 solar masses. Millimeter
continuum emission indicates a mass of about 2 solar masses, both from a direct
calculation assuming isothermal dust and from dust radiative transfer models
constrained by the millimeter observations. We use dust radiative transfer
models to constrain the internal luminosity of L673-7, defined to be the
luminosity of the central source and excluding the luminosity from external
heating, to be 0.01-0.045 solar luminosities, with 0.04 solar luminosities the
most likely value. L673-7 is thus classified as a very low luminosity object
(VeLLO), and is among the lowest luminosity VeLLOs yet studied. We calculate
the kinematic and dynamic properties of the molecular outflow in the standard
manner, and we show that the expected accretion luminosity based on these
outflow properties is greater than or equal to 0.36 solar luminosities. The
discrepancy between this expected accretion luminosity and the internal
luminosity derived from dust radiative transfer models indicates that the
current accretion rate is much lower than the average rate over the lifetime of
the outflow. Although the protostar embedded within L673-7 is consistent with
currently being substellar, it is unlikely to remain as such given the
substantial mass reservoir remaining in the core.Comment: 19 pages, 14 figures. Accepted by Ap
Can Protostellar Outflows Set Stellar Masses?
The opening angles of some protostellar outflows appear too narrow to match
the expected core-star mass efficiency SFE = 0.3-0.5 if outflow cavity volume
traces outflow mass, with a conical shape and a maximum opening angle near 90
deg. However, outflow cavities with paraboloidal shape and wider angles are
more consistent with observed estimates of the SFE. This paper presents a model
of infall and outflow evolution based on these properties. The initial state is
a truncated singular isothermal sphere which has mass 1 ,
free fall time 80 kyr, and small fractions of magnetic, rotational,
and turbulent energy. The core collapses pressure-free as its protostar and
disk launch a paraboloidal wide-angle wind. The cavity walls expand radially
and entrain envelope gas into the outflow. The model matches SFE values when
the outflow mass increases faster than the protostar mass by a factor 1 - 2,
yielding protostar masses typical of the IMF. It matches observed outflow
angles if the outflow mass increases at nearly the same rate as the cavity
volume. The predicted outflow angles are then typically 50 deg as they
increase rapidly through the stage 0 duration of 40 kyr. They increase
more slowly up to 110 deg during their stage I duration of 70 kyr.
With these outflow rates and shapes, model predictions appear consistent with
observational estimates of typical stellar masses, SFEs, stage durations, and
outflow angles, with no need for external mechanisms of core dispersal.Comment: Accepted for publication by The Astrophysical Journal; 47 pages, 10
figure
The RMS Survey: Ammonia and water maser analysis of massive star forming regions
The Red MSX Source (RMS) survey has identified a sample of ~1200 massive
young stellar objects (MYSOs), compact and ultra compact HII regions from a
sample of ~2000 MSX and 2MASS colour selected sources. We have used the 100 m
Green Bank telescope to search for 22-24 GHz water maser and ammonia (1,1),
(2,2) and (3,3) emission towards ~600 RMS sources located within the northern
Galactic plane. We have identified 308 H2O masers which corresponds to an
overall detection rate of ~50%. Abridged: We detect ammonia emission towards
479 of these massive young stars, which corresponds to ~80%. Ammonia is an
excellent probe of high density gas allowing us to measure key parameters such
as gas temperatures, opacities, and column densities, as well as providing an
insight into the gas kinematics. The average kinetic temperature, FWHM line
width and total NH3 column density for the sample are approximately 22 K, 2
km/s and 2x10^{15} cm^{-2}, respectively. We find that the NH3 (1,1) line width
and kinetic temperature are correlated with luminosity and finding no
underlying dependence of these parameters on the evolutionary phase of the
embedded sources, we conclude that the observed trends in the derived
parameters are more likely to be due to the energy output of the central source
and/or the line width-clump mass relationship. The velocities of the peak H2O
masers and the NH3 emission are in excellent agreement with each other, which
would strongly suggest an association between the dense gas and the maser
emission. Moreover, we find the bolometric luminosity of the embedded source
and the isotropic luminosity of the H2O maser are also correlated. We conclude
from the correlations of the cloud and water maser velocities and the
bolometric and maser luminosity that there is a strong dynamical relationship
between the embedded young massive star and the H2O maser.Comment: 17 pages and 17 figures and 8 tables. Tables\,2 and 5 and full
versions of Figs. 3 and 7 are only available in electronic form at the CDS
via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or via
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A
Recommended from our members
The Spitzer c2d Survey Of Nearby Dense Cores. X. Star Formation In L673 And Cb188
L673 and CB188 are two low-mass clouds isolated from large star-forming regions that were observed as part of the Spitzer Legacy Project "From Molecular Clouds to Planet Forming disks" (c2d). We identified and characterized all the young stellar objects (YSOs) of these two regions and modeled their spectral energy distributions (SEDs) to examine whether their physical properties are consistent with values predicted from the theoretical models and with the YSO properties in the c2d survey of larger clouds. Overall, 30 YSO candidates were identified by the c2d photometric criteria, 27 in L673 and 3 in CB188. We confirm the YSO nature of 29 of them and remove a false Class III candidate in L673. We further present the discovery of two new YSO candidates, one Class 0 and another possible Class I candidate in L673, therefore bringing the total number of YSO candidates to 31. Multiple sites of star formation are present within L673, closely resembling other well-studied c2d clouds containing small groups such as B59 and L1251B, whereas CB188 seems to consist of only one isolated globule-like core. We measure a star formation efficiency (SFE) of 4.6%, which resembles the SFE of the larger c2d clouds. From the SED modeling of our YSO sample we obtain envelope masses for Class I and Flat spectrum sources of 0.01-1.0 M-circle dot. The majority of Class II YSOs show disk accretion rates from 3.3 x 10(-10) to 3 x 10(-8) M-circle dot yr(-1) and disk masses that peak at 10(-4) to 10(-3) M-circle dot. Finally, we examined the possibility of thermal fragmentation in L673 as the main star-forming process. We find that the mean density of the regions where significant YSO clustering occurs is of the order of similar to 10(5) cm(-3) using 850 mu m observations and measure a Jeans Length that is greater than the near-neighbor YSO separations by approximately a factor of 3-4. We therefore suggest that other processes, such as turbulence and shock waves, may have had a significant effect on the cloud's filamentary structure and YSO clustering.University of SouthamptonNASA 1279198, 1288806, 1365763Jet Propulsion Laboratory, California Institute of TechnologyAstronom
Design for validation: An approach to systems validation
Every complex system built is validated in some manner. Computer validation begins with review of the system design. As systems became too complicated for one person to review, validation began to rely on the application of adhoc methods by many individuals. As the cost of the changes mounted and the expense of failure increased, more organized procedures became essential. Attempts at devising and carrying out those procedures showed that validation is indeed a difficult technical problem. The successful transformation of the validation process into a systematic series of formally sound, integrated steps is necessary if the liability inherent in the future digita-system-based avionic and space systems is to be minimized. A suggested framework and timetable for the transformtion are presented. Basic working definitions of two pivotal ideas (validation and system life-cyle) are provided and show how the two concepts interact. Many examples are given of past and present validation activities by NASA and others. A conceptual framework is presented for the validation process. Finally, important areas are listed for ongoing development of the validation process at NASA Langley Research Center
Vortex wake alleviation studies with a variable twist wing
Vortex wake alleviation studies were conducted in a wind tunnel and a water towing tank using a multisegmented wing model which provided controlled and measured variations in span load. Fourteen model configurations are tested at a Reynolds number of one million and a lift coefficient of 0.6 in the Langley 4- by 7-Meter Tunnel and the Hydronautics Ship Model Basin water tank at Hydronautics, Inc., Laurel, Md. Detailed measurements of span load and wake velocities at one semispan downstream correlate well with each other, with inviscid predictions of span load and wake roll up, and with peak trailing-wing rolling moments measured in the far wake. Average trailing-wing rolling moments are found to be an unreliable indicator of vortex wake intensity because vortex meander does not scale between test facilities and free-air conditions. A tapered-span-load configuration, which exhibits little or no drag penalty, is shown to offer significant downstream wake alleviation to a small trailing wing. The greater downstream wake alleviation achieved with the addition of spoilers to a flapped-wing configuration is shown to result directly from the high incremental drag and turbulence associated with the spoilers and not from the span load alteration they cause
- …