scholarly journals The turbulent environment of low-mass dense cores

2006 ◽  
Vol 2 (S237) ◽  
pp. 24-30 ◽  
Author(s):  
E. Falgarone ◽  
P. Hily-Blant ◽  
J. Pety ◽  
G. Pineau des Forêts
Keyword(s):  
Turbulent Energy ◽  
Injection Rate ◽  
Solar Neighborhood ◽  
Low Velocity ◽  
Gas Condensation ◽  
Dense Cores ◽  
Gas Cooling ◽  
Low Mass ◽  
Impulsive Heating ◽  
Turbulent Cascade

AbstractThe signatures of intermittent dissipation of turbulent energy have been sought in the translucent environment of a low-mass dense core. Molecular line observations reveal a network of narrow filamentary structures, found on statistical grounds to be the locus of the largest velocity shears. Three independent properties of these structures make them the plausible sites of intermittent dissipation of turbulence: (1) gas there is warmer and more diluted than average, (2) it bears the signatures of a non-equilibrium chemistry triggered by impulsive heating due to turbulence dissipation, and (3) the power that these structures radiate in the gas cooling lines (mostly H2) is so large that it balances the total energy injection rate of the turbulent cascade, for a volume filling factor of only a few percents, consistent with other observations in the Solar Neighborhood. These filamentary structures may act as tiny seeds of gas condensation in diffuse molecular gas. They do not exhibit the properties of steady-state low-velocity magneto-hydrodynamic (MHD) shocks, as presently modelled.

scholarly journals Inside Out and Upside-Down: The Roles of Gas Cooling and Dynamical Heating in Shaping the Stellar Age-Velocity Relation

2021 ◽  
Author(s):  
Jonathan C Bird ◽  
Sarah R Loebman ◽  
David H Weinberg ◽  
Alyson M Brooks ◽  
Thomas R Quinn ◽  
...  
Keyword(s):  
Milky Way ◽  
Velocity Dispersion ◽  
Solar Neighborhood ◽  
Disk Galaxies ◽  
Low Velocity ◽  
Velocity Relationship ◽  
Gas Cooling ◽  
Multi Phase ◽  
Inside Out ◽  
Over Time

Abstract Kinematic studies of disk galaxies, using individual stars in the Milky Way or statistical studies of global disk kinematics over time, provide insight into how disks form and evolve. We use a high-resolution, cosmological zoom-simulation of a Milky Way-mass disk galaxy (h277) to tie together local disk kinematics and the evolution of the disk over time. The present-day stellar age-velocity relationship (AVR) of h277 is nearly identical to that of the analogous solar-neighborhood measurement in the Milky Way. A crucial element of this success is the simulation’s dynamically cold multi-phase ISM, which allows young stars to form with a low velocity dispersion (σbirth∼6 − 8 km s−1) at late times. Older stars are born kinematically hotter (i.e., the disk settles over time in an “upside-down” formation scenario), and are subsequently heated after birth. The disk also grows “inside-out”, and many of the older stars in the present-day solar neighborhood are present because of radial mixing. We demonstrate that the evolution of σbirth in h277 can be explained by the same model used to describe the general decrease in velocity dispersion observed in disk galaxies from z ∼ 2 − 3 to the present-day, in which the disk evolves in quasi-stable equilibrium and the ISM velocity dispersion decreases over time due to a decreasing gas fraction. Thus, our results tie together local observations of the Milky Way’s AVR with observed kinematics of high z disk galaxies.

scholarly journals Turbulent heating in an inhomogeneous magnetized plasma slab

2018 ◽  
Vol 84 (3) ◽  
Author(s):  
Michael Barnes ◽  
P. Abiuso ◽  
W. Dorland
Keyword(s):  
Mean Free Path ◽  
Turbulent Energy ◽  
Magnetized Plasma ◽  
Mass Ratio ◽  
Wave Instability ◽  
Astrophysical Plasmas ◽  
Plasma Slab ◽  
Turbulent Heating ◽  
Ion Species ◽  
Turbulent Cascade

Observational evidence in space and astrophysical plasmas with a long collisional mean free path suggests that more massive charged particles may be preferentially heated. One possible mechanism for this is the turbulent cascade of energy from injection to dissipation scales, where the energy is converted to heat. Here we consider a simple system consisting of a magnetized plasma slab of electrons and a single ion species with a cross-field density gradient. We show that such a system is subject to an electron drift wave instability, known as the universal instability, which is stabilized only when the electron and ion thermal speeds are equal. For unequal thermal speeds, we find from quasilinear analysis and nonlinear simulations that the instability gives rise to turbulent energy exchange between ions and electrons that acts to equalize the thermal speeds. Consequently, this turbulent heating tends to equalize the component temperatures of pair plasmas and to heat ions to much higher temperatures than electrons for conventional mass-ratio plasmas.

scholarly journals A binary star formation mechanism through the fragmentation of prolate dense cores rotating end over end

1991 ◽  
Vol 147 ◽  
pp. 526-528
Author(s):  
Hans Zinnecker
Keyword(s):  
Star Formation ◽  
Formation Mechanism ◽  
Binary Stars ◽  
Binary Star ◽  
Initial Structure ◽  
Eccentric Orbit ◽  
Dense Cores ◽  
Molecular Core ◽  
Low Mass

I propose and briefly elaborate on a major new mechanism for the formation of wide, low-mass binary stars: the fragmentation of a collapsing, initially elongated dense molecular core rotating end over end. This initial structure will develop into two independent gravitationally bound stellar condensations orbiting each other in a rather eccentric orbit.

scholarly journals A binary star formation mechanism through the fragmentation of prolate dense cores rotating end over end

1991 ◽  
Vol 147 ◽  
pp. 526-528
Author(s):  
Hans Zinnecker
Keyword(s):  
Star Formation ◽  
Formation Mechanism ◽  
Binary Stars ◽  
Binary Star ◽  
Initial Structure ◽  
Eccentric Orbit ◽  
Dense Cores ◽  
Molecular Core ◽  
Low Mass

I propose and briefly elaborate on a major new mechanism for the formation of wide, low-mass binary stars: the fragmentation of a collapsing, initially elongated dense molecular core rotating end over end. This initial structure will develop into two independent gravitationally bound stellar condensations orbiting each other in a rather eccentric orbit.

scholarly journals A Radial velocity survey of spatially resolved young, low-mass binaries

2018 ◽  
Vol 618 ◽  
pp. A5 ◽  
Author(s):  
Stephen Durkan ◽  
Markus Janson ◽  
Simona Ciceri ◽  
Wolfgang Brandner ◽  
Joshua Schlieder ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Solar Neighborhood ◽  
Activity Level ◽  
Activity Levels ◽  
Old Field ◽  
Triple Systems ◽  
Chromospheric Activity ◽  
Low Mass ◽  
Orbital Periods ◽  
Moving Groups

The identification and characterisation of low-mass binaries is of importance for a range of astrophysical investigations. Low-mass binaries in young (∼10–100 Myr) moving groups (YMGs) in the solar neighborhood are of particular significance as they provide unique opportunities to calibrate stellar models and evaluate the ages and coevality of the groups themselves. Low-mass M-dwarfs have pre-main sequence life times on the order of ∼100 Myr and therefore are continually evolving along a mass-luminosity track throughout the YMG phase, providing ideal laboratories for precise isochronal dating, if a model-independent dynamical mass can be measured. AstraLux lucky imaging multiplicity surveys have recently identified hundreds of new YMG low-mass binaries, where a subsample of M-dwarf multiples have estimated orbital periods less than 50 yr. We have conducted a radial velocity survey of a sample of 29 such targets to complement the astrometric data. This will allow enhanced orbital determinations and precise dynamical masses to be derived in a shorter timeframe than possible with astrometric monitoring alone, and allow for a more reliable isochronal analysis. Here we present radial velocity measurements derived for our sample over several epochs. We report the detection of the three-component spectroscopic multiple 2MASS J05301858-5358483, for which the C component is a new discovery, and forms a tight pair with the B component. Originally identified as a YMG member, we find that this system is a likely old field interloper, whose high chromospheric activity level is caused by tidal spin-up of the tight BC pair. Two other triple systems with a tight pair exist in the sample, 2MASS J04244260-0647313 (previously known) and 2MASS J20163382-0711456, but for the rest of the targets we find that additional tidally synchronized companions are highly unlikely, providing further evidence that their high chromospheric activity levels are generally signatures of youth.

scholarly journals Structure function measurements of the intermittent MHD turbulent cascade

1997 ◽  
Vol 4 (3) ◽  
pp. 185-199 ◽  
Author(s):  
T. S. Horbury ◽  
A. Balogh
Keyword(s):  
Energy Transfer ◽  
High Speed ◽  
Solar Wind Plasma ◽  
Turbulent Energy ◽  
Structure Functions ◽  
Order Structure ◽  
Data Sets ◽  
Magnetic Field Data ◽  
Spacecraft Data ◽  
Turbulent Cascade

Abstract. The intertmittent nature of turbulence within solar wind plasma has been demonstrated by several studies of spacecraft data. Using magnetic field data taken in high speed flows at high heliographic latitudes by the Ulysses probe, the character of fluctuations within the inertia] range is discussed. Structure functions are used extensively. A simple consideration of errors associated with calculations of high moment structure functions is shown to be useful as a practical estimate of the reliability of such calculations. For data sets of around 300 000 points, structure functions of moments above 5 are rarely reliable on the basis of this test, highlighting the importance of considering uncertainties in such calculations. When unreliable results are excluded, it is shown that inertial range polar fluctuations are well described by a multifractal model of turbulent energy transfer. Detailed consideration of the scaling of high order structure functions suggests energy transfer consistent with a "Kolmogorov" cascade.

The X-Ray View of the Low-Mass Stars in the Solar Neighborhood

1995 ◽  
Vol 450 ◽  
pp. 392 ◽  
Author(s):  
Juergen H. M. M. Schmitt ◽  
Thomas A. Fleming ◽  
Mark S. Giampapa
Keyword(s):  
Solar Neighborhood ◽  
Low Mass Stars ◽  
X Ray ◽  
Low Mass

The Unsteady Behavior of Diffuser Stall in a Centrifugal Compressor With Vaneless Diffuser

2020 ◽  
Author(s):  
Hiroshi Miida ◽  
Kenta Tajima ◽  
Nobumichi Fujisawa ◽  
Yutaka Ohta
Keyword(s):  
Mass Flow ◽  
Centrifugal Compressor ◽  
Rotational Speed ◽  
Flow Region ◽  
Cfd Analysis ◽  
Vaneless Diffuser ◽  
Low Velocity ◽  
Flow Angle ◽  
Boundary Separation ◽  
Low Mass

Abstract The unsteady diffuser stall behavior in a centrifugal compressor with a vaneless diffuser was investigated by experimental and computational analyses. The diffuser stall generated as the mass flow rate decreased. The diffuser stall cell rotated at 25–30% of the impeller rotational speed, with diffuser stall fluctuations observed at 180° from the cutoff. The diffuser stall fluctuation magnitude gradually increased near the cutoff. Based on diffuser inlet velocity measurements, the diffuser stall fluctuations generated near both the shroud and hub sides, and the diffuser stall appeared at 180° and 240° from the cutoff. According to the CFD analysis, the mass flow fluctuations at the diffuser exit showed a low mass flow region, rotating at approximately 25% of the impeller rotational speed. They began at 180° from the cutoff and developed as this region approached the cutoff. Therefore, the diffuser stall could be simulated by CFD analysis. First, the diffuser stall cell originated at 180° from the cutoff by interaction with boundary separation and impeller discharge vortex. Then, the diffuser stall cell further developed by boundary separation accumulation and the induced low velocity area, located at the stall cell center. The low velocity region formed a blockage across the diffuser passage span. The diffuser stall cell expanded in the impeller rotational direction due to boundary separation caused by a positive flow angle. Finally, the diffuser stall cell vanished when it passed the cutoff, because mass flow recovery occurred.

Comparison of Pathogens Dispersion in an Aircraft Cabin Using Gas Injection Source Versus a Coughing Manikin

2020 ◽  
Author(s):  
Seif Mahmoud ◽  
James S. Bennett ◽  
Mohammad H. Hosni ◽  
Byron Jones
Keyword(s):  
Gas Injection ◽  
Longitudinal Direction ◽  
Injection Rate ◽  
Sampling Location ◽  
Tracer Gas ◽  
Low Velocity ◽  
Co2 Gas ◽  
Gas Concentration ◽  
Continuous Injection ◽  
Injection Location

Abstract The dispersion characteristics of airborne pathogens were investigated in a Boeing 767 mockup cabin containing 11 rows with 7 seats per row, using two tracer gas source methods: continuous injection at low velocity and a coughing manikin. Both the injection source and the coughing manikin were located on the same seat in the sixth row. The injection source utilized CO2 gas at an injection rate of 5.0 liters per minute mixed with helium at a rate of 3.07 liters per minute to neutralize buoyancy. The manikin coughed approximately once every 75 seconds, with a volume of 4.2 liters of CO2 per cough. To ensure sufficient data were collected at each sampling location, each coughing manikin test was run for 6 coughs and each injection source test for 30 minutes of continuous injection. In both test methods, the tracer gas concentration was measured using CO2 gas analyzers at seated passenger breathing height of 1.2 m and radially up to 3.35 m away from the gas injection location, representing approximately four rows of a standard B767 aircraft. The collected data obtained from each tracer method was then normalized to provide a suitable comparison basis that is independent of tracer gas introduction flowrate. The results showed that both tracer source methods gave similar dispersion trends in diagonal and lateral directions away from the injection location. However, the tracer gas concentration was higher along the longitudinal direction in the coughing manikin tests due to the cough momentum. The results of this work will help researchers analyze different experimental and numerical approaches used to determine contaminant dispersion in various environments and will provide a better understanding of the associated transport phenomena.

Sign in / Sign up

Export Citation Format

Share Document