3,687 research outputs found
Protostellar half-life: new methodology and estimates
(Abridged) Protostellar systems evolve from prestellar cores, through the
deeply embedded stage and then disk-dominated stage, before they end up on the
main sequence. Knowing how much time a system spends in each stage is crucial
for understanding how stars and associated planetary systems form, because a
key constraint is the time available to form such systems. Equally important is
understanding what the spread in these time scales is. The most commonly used
method for inferring protostellar ages is to assume the lifetime of one
evolutionary stage, and then scale this to the relative number of protostars in
the other stages, i.e., assuming steady state. This method does not account for
the underlying age distribution and apparent stochasticity of star formation,
nor that relative populations are not in steady state. To overcome this, we
propose a new scheme where the lifetime of each protostellar stage follows a
distribution based on the formalism of sequential nuclear decay. The main
assumptions are: Class 0 sources follow a straight path to Class III sources,
the age distribution follows a binomial distribution, and the star-formation
rate is constant. The results are that the half-life of Class 0, Class I, and
Flat sources are (2.4+/-0.2)%, (4.4+/-0.3)%, and (4.3+/-0.4)% of the Class II
half-life, respectively, which translates to 47+/-4, 88+/-7, and 87+/-8 kyr,
respectively, for a Class II half-life of 2 Myr for protostars in the Gould
Belt clouds with more than 100 protostars. The mean age of these clouds is
1.2+/-0.1 Myr, and the star formation rate is (8.3+/-0.5)x10^-4 Msun/yr. The
critical parameters in arriving at these numbers are the assumed half-life of
the Class II stage, and the assumption that the star-formation rate and
half-lives are constant. This method presents a first step in moving from
steady-state to non-steady-state solutions of protostellar populations.Comment: Accepted for publication in A&
A Systematic Search for Molecular Outflows Toward Candidate Low-Luminosity Protostars and Very Low Luminosity Objects
We present a systematic single-dish search for molecular outflows toward a
sample of 9 candidate low-luminosity protostars and 30 candidate Very Low
Luminosity Objects (VeLLOs; L_int < 0.1 L_sun). The sources are identified
using data from the Spitzer Space Telescope catalogued by Dunham et al. toward
nearby (D < 400 pc) star forming regions. Each object was observed in 12CO and
13CO J = 2-1 simultaneously using the sideband separating ALMA Band-6 prototype
receiver on the Heinrich Hertz Telescope at 30 arcsecond resolution. Using
5-point grid maps we identify five new potential outflow candidates and make
on-the-fly maps of the regions surrounding sources in the dense cores B59,
L1148, L1228, and L1165. Of these new outflow candidates, only the map of B59
shows a candidate blue outflow lobe associated with a source in our survey. We
also present larger and more sensitive maps of the previously detected L673-7
and the L1251-A IRS4 outflows and analyze their properties in comparison to
other outflows from VeLLOs. The accretion luminosities derived from the outflow
properties of the VeLLOs with detected CO outflows are higher than the observed
internal luminosity of the protostars, indicating that these sources likely had
higher accretion rates in the past. The known L1251-A IRS3 outflow is detected
but not remapped. We do not detect clear, unconfused signatures of red and blue
molecular wings toward the other 31 sources in the survey indicating that
large-scale, distinct outflows are rare toward this sample of candidate
protostars. Several potential outflows are confused with kinematic structure in
the surrounding core and cloud. Interferometric imaging is needed to
disentangle large-scale molecular cloud kinematics from these potentially weak
protostellar outflows.Comment: 42 pages, 19 figures, Accepted for publication in the Astronomical
Journa
An experiment in software reliability
The results of a software reliability experiment conducted in a controlled laboratory setting are reported. The experiment was undertaken to gather data on software failures and is one in a series of experiments being pursued by the Fault Tolerant Systems Branch of NASA Langley Research Center to find a means of credibly performing reliability evaluations of flight control software. The experiment tests a small sample of implementations of radar tracking software having ultra-reliability requirements and uses n-version programming for error detection, and repetitive run modeling for failure and fault rate estimation. The experiment results agree with those of Nagel and Skrivan in that the program error rates suggest an approximate log-linear pattern and the individual faults occurred with significantly different error rates. Additional analysis of the experimental data raises new questions concerning the phenomenon of interacting faults. This phenomenon may provide one explanation for software reliability decay
An empirical study of flight control software reliability
The results of a laboratory experiment in flight control software reliability are reported. The experiment tests a small sample of implementations of a pitch axis control law for a PA28 aircraft with over 14 million pitch commands with varying levels of additive input and feedback noise. The testing which uses the method of n-version programming for error detection surfaced four software faults in one implementation of the control law. The small number of detected faults precluded the conduct of the error burst analyses. The pitch axis problem provides data for use in constructing a model in the prediction of the reliability of software in systems with feedback. The study is undertaken to find means to perform reliability evaluations of flight control software
Hague Evidence Convention: A Practical Guide to the Convention, United States Case Law, Convention - Sponsored Review Commissions (1978 and 1985), and Responses of Other Signatory Nations: with Digest of Cases and Bibliography
Low-speed wind-tunnel tests of an advanced eight-bladed propeller
As part of a research program on advanced turboprop aircraft aerodynamics, a low-speed wind-tunnel investigation was conducted to document the basic performance and force and moment characteristics of an advanced eight-bladed propeller. The results show that in addition to the normal force and pitching moment produced by the propeller/nacelle combination at angle of attack, a significant side force and yawing moment are also produced. Furthermore, it is shown that for test conditions wherein compressibility effects can be ignored, accurate simulation of propeller performance and flow fields can be achieved by matching the nondimensional power loading of the model propeller to that of the full-scale propeller
Cold electron beams from cryo-cooled, alkali antimonide photocathodes
In this letter we report on the generation of cold electron beams using a
Cs3Sb photocathode grown by co-deposition of Sb and Cs. By cooling the
photocathode to 90 K we demonstrate a significant reduction in the mean
transverse energy validating the long standing speculation that the lattice
temperature contribution limits the mean transverse energy or thermal emittance
near the photoemission threshold, opening new frontiers in generating
ultra-bright beams. At 90 K, we achieve a record low thermal emittance of 0.2
m (rms) per mm of laser spot diameter from an ultrafast (sub-picosecond)
photocathode with quantum efficiency greater than using a
visible laser wavelength of 690 nm
The role of periarticular soft tissues in persistent motion loss in a rat model of posttraumatic elbow contracture
Effect of solidity and inclination on propeller-nacelle force coefficients
A series of wind tunnel experiments were conducted to study the effect of propeller solidity and thrust axis inclination on the propeller normal force coefficient. Experiments were conducted in the Langley 14 by 22 foot Subsonic Tunnel with a sting mounted, counterrotation, scale model propeller and nacelle. Configurations had two rows of blades with combinations of 4 and 8 blades per hub. The solidity was varied by changing the number of blades on both rows. Tests were conducted for blade pitch setting of 31.34 deg, 36.34 deg, and 41.34 deg over a range of angle of attack from -10 deg to 90 deg and range of advance ratio from 0.8 to 1.4. The increase in propeller normal force with angle of attack is greater for propellers with higher solidity
Revealing The Millimeter Environment of the New FU Orionis Candidate HBC722 with the Submillimeter Array
We present 230 GHz Submillimeter Array continuum and molecular line
observations of the newly discovered FUor candidate HBC722. We report the
detection of seven 1.3 mm continuum sources in the vicinity of HBC722, none of
which correspond to HBC722 itself. We compile infrared and submillimeter
continuum photometry of each source from previous studies and conclude that
three are Class 0 embedded protostars, one is a Class I embedded protostar, one
is a Class I/II transition object, and two are either starless cores or very
young, very low luminosity protostars or first hydrostatic cores. We detect a
northwest-southeast outflow, consistent with the previous detection of such an
outflow in low-resolution, single-dish observations, and note that its axis may
be precessing. We show that this outflow is centered on and driven by one of
the nearby Class 0 sources rather than HBC722, and find no conclusive evidence
that HBC722 itself is driving an outflow. The non-detection of HBC722 in the
1.3 mm continuum observations suggests an upper limit of 0.02 solar masses for
the mass of the circumstellar disk. This limit is consistent with typical T
Tauri disks and with a disk that provides sufficient mass to power the burst.Comment: 12 pages, 7 figures, accepted by Ap
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