5,062 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&
Charge carrier induced lattice strain and stress effects on As activation in Si
We studied lattice expansion coefficient due to As using density functional
theory with particular attention to separating the impact of electrons and
ions. Based on As deactivation mechanism under equilibrium conditions, the
effect of stress on As activation is predicted. We find that biaxial stress
results in minimal impact on As activation, which is consistent with
experimental observations by Sugii et al. [J. Appl. Phys. 96, 261 (2004)] and
Bennett et al.[J. Vac. Sci. Tech. B 26, 391 (2008)]
Detailed pressure distribution measurements obtained on several configurations of an aspect-ratio-7 variable twist wing
Detailed pressure distribution measurements were made for 11 twist configurations of a unique, multisegmented wing model having an aspect ratio of 7 and a taper ratio of 1. These configurations encompassed span loads ranging from that of an untwisted wing to simple flapped wings both with and without upper-surface spoilers attached. For each of the wing twist configurations, electronic scanning pressure transducers were used to obtain 580 surface pressure measurements over the wing in about 0.1 sec. Integrated pressure distribution measurements compared favorably with force-balance measurements of lift on the model when the model centerbody lift was included. Complete plots and tabulations of the pressure distribution data for each wing twist configuration are provided
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
Cosmic background explorer (COBE) navigation with TDRSS one-way return-link Doppler in the post-helium-venting phase
A navigation experiment was performed which establishes Ultra-Stable Oscillator (USO) frequency stabilized one way return link Doppler TDRSS tracking data as a feasible option for mission orbit determination support at the Goddard Space Center Flight Dynamics Facility. The study was conducted using both one way and two way Tracking and Data Relay Satellite System (TDRSS) tracking measurements for the Cosmic Background Explorer (COBE) spacecraft. Tracking data for a 4 week period immediately follow the depletion of the helium supply was used. The study showed that, for both definitive orbit solution and short term orbit prediction (up to 4 weeks), orbit determination results based on one way return link Doppler tracking measurements are comparable to orbit determination results based on two way range and two way Doppler tracking measurements
The People\u27s Poets: Literature Born of the Texas Singer-Songwriter Movement of the Last Forty Years
The People’s Poets of Texas: Literature Born Within the Singer/Songwriter Tradition of the Last Forty Years is a creative nonfiction exploration of the poetry found within the songs of multiple generations of modern Texas singer/songwriters and a case for the consideration of their work as a genuine regional literature. Studying the roots of Texas music, the musicality of Texan manners of speech and storytelling, and re-examining the Austin, Texas music scene of the 1970s that brought a national focus to the organic, reciprocal manner in which Texas music is traditionally experienced, radically altered the ways in which the songs were written, recorded, and marketed. An examination of this phenomenon allows us to understand that, first, a proliferation of Texas singer/songwriters of unprecedented quality has emerged in recent decades and that, second, a legitimate people\u27s literature is emerging from their song-craft
Applying machine learning to categorize distinct categories of network traffic
The recent rapid growth of the field of data science has made available to all fields opportunities to leverage machine learning. Computer network traffic classification has traditionally been performed using static, pre-written rules that are easily made ineffective if changes, legitimate or not, are made to the applications or protocols underlying a particular category of network traffic. This paper explores the problem of network traffic classification and analyzes the viability of having the process performed using a multitude of classical machine learning techniques against significant statistical similarities between classes of network traffic as opposed to traditional static traffic identifiers.
To accomplish this, network data was captured, processed, and evaluated for 10 application labels under the categories of video conferencing, video streaming, video gaming, and web browsing as described later in Table 1. Flow-based statistical features for the dataset were derived from the network captures in accordance with the “Flow Data Feature Creation” section and were analyzed against a nearest centroid, k-nearest neighbors, Gaussian naïve Bayes, support vector machine, decision tree, random forest, and multi-layer perceptron classifier. Tools and techniques broadly available to organizations and enthusiasts were used. Observations were made on working with network data in a machine learning context, strengths and weaknesses of different models on such data, and the overall efficacy of the tested models.
Ultimately, it was found that simple models freely available to anyone can achieve high accuracy, recall, and F1 scores in network traffic classification, with the best-performing model, random forest, having 89% accuracy, a macro average F1 score of .77, and a macro average recall of 76%, with the most common feature of successful classification being related to maximum packet sizes in a network flow
Vacancy assisted arsenic diffusion and time dependent clustering effects in silicon
We present results of kinetic lattice Monte Carlo (KLMC) simulations of
substitutional arsenic diffusion in silicon mediated by lattice vacancies.
Large systems are considered, with 1000 dopant atoms and long range \textit{ab
initio} interactions, to the 18th nearest lattice neighbor, and the diffusivity
of each defect species over time is calculated. The concentration of vacancies
is greater than equilibrium concentrations in order to simulate conditions
shortly after ion implantation. A previously unreported time dependence in the
applicability of the pair diffusion model, even at low temperatures, is
demonstrated. Additionally, long range interactions are shown to be of critical
importance in KLMC simulations; when shorter interaction ranges are considered
only clusters composed entirely of vacancies form. An increase in arsenic
diffusivity for arsenic concentrations up to is
observed, along with a decrease in arsenic diffusivity for higher arsenic
concentrations, due to the formation of arsenic dominated clusters. Finally,
the effect of vacancy concentration on diffusivity and clustering is studied,
and increasing vacancy concentration is found to lead to a greater number of
clusters, more defects per cluster, and a greater vacancy fraction within the
clusters.Comment: 22 pages, 16 figure
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