Generalized Transport Equation of The Autocovariance Function of the Density Field and Mass Invariant in Star-forming Clouds

Abstract In this Letter, we study the evolution of the autocovariance function of density-field fluctuations in star-forming clouds and thus of the correlation length l c (ρ) of these fluctuations, which can be identified as the average size of the most correlated structures within the cloud. Generalizing the transport equation derived by Chandrasekhar for static, homogeneous turbulence, we show that the mass contained within these structures is an invariant, i.e., that the average mass contained in the most correlated structures remains constant during the evolution of the cloud, whatever dominates the global dynamics (gravity or turbulence). We show that the growing impact of gravity on the turbulent flow yields an increase of the variance of the density fluctuations and thus a drastic decrease of the correlation length. Theoretical relations are successfully compared to numerical simulations. This picture brings a robust support to star formation paradigms where the mass concentration in turbulent star-forming clouds evolves from initially large, weakly correlated filamentary structures to smaller, denser, more correlated ones, and eventually to small, tightly correlated, prestellar cores. We stress that the present results rely on a pure statistical approach of density fluctuations and do not involve any specific condition for the formation of prestellar cores. Interestingly enough, we show that, under average conditions typical of Milky-Way molecular clouds, this invariant average mass is about a solar mass, providing an appealing explanation for the apparent universality of the IMF in such environments.

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L1495 revisited: a ppmap view of a star-forming filament

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2234 ◽

2019 ◽

Vol 489 (1) ◽

pp. 962-976 ◽

Cited By ~ 2

Author(s):

A D P Howard ◽

A P Whitworth ◽

K A Marsh ◽

S D Clarke ◽

M J Griffin ◽

...

Keyword(s):

Critical Line ◽

Kinetic Temperature ◽

Line Density ◽

Star Forming ◽

Physical Conditions ◽

Fitting Procedure ◽

Different Types ◽

Prestellar Cores ◽

Different Temperatures ◽

Average Profile

ABSTRACT We have analysed the Herschel and SCUBA-2 dust continuum observations of the main filament in the Taurus L1495 star-forming region, using the Bayesian fitting procedure ppmap. (i) If we construct an average profile along the whole length of the filament, it has FWHM $\simeq 0.087\pm 0.003\, {\rm pc};\,\,$ but the closeness to previous estimates is coincidental. (ii) If we analyse small local sections of the filament, the column-density profile approximates well to the form predicted for hydrostatic equilibrium of an isothermal cylinder. (iii) The ability of ppmap to distinguish dust emitting at different temperatures, and thereby to discriminate between the warm outer layers of the filament and the cold inner layers near the spine, leads to a significant reduction in the surface-density, $\varSigma$, and hence in the line-density, μ. If we adopt the canonical value for the critical line-density at a gas-kinetic temperature of $10\, {\rm K}$, $\mu _{{\rm CRIT}}\simeq 16\, {\rm M_{\odot }\, pc^{-1}}$, the filament is on average trans-critical, with ${\bar{\mu }}\sim \mu _{{\rm CRIT}};\,\,$ local sections where μ > μCRIT tend to lie close to prestellar cores. (iv) The ability of ppmap to distinguish different types of dust, i.e. dust characterized by different values of the emissivity index, β, reveals that the dust in the filament has a lower emissivity index, β ≲ 1.5, than the dust outside the filament, β ≳ 1.7, implying that the physical conditions in the filament have effected a change in the properties of the dust.

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Velocity fluctuations in a low-Reynolds-number fluidized bed

Journal of Fluid Mechanics ◽

10.1017/s0022112007009652 ◽

2008 ◽

Vol 596 ◽

pp. 467-475 ◽

Cited By ~ 10

Author(s):

SHANG-YOU TEE ◽

P. J. MUCHA ◽

M. P. BRENNER ◽

D. A. WEITZ

Keyword(s):

Reynolds Number ◽

Relaxation Time ◽

Fluidized Bed ◽

Correlation Length ◽

Volume Fraction ◽

Low Reynolds Number ◽

Density Fluctuations ◽

Velocity Fluctuations ◽

Similarities And Differences ◽

Low Reynolds

The velocity fluctuations of particles in a low-Reynolds-number fluidized bed have important similarities and differences with the velocity fluctuations in a low-Reynolds-number sedimenting suspension. We show that, like sedimentation, the velocity fluctuations in a fluidized bed are described well by the balance between density fluctuations due to Poisson statistics and Stokes drag. However, unlike sedimentation, the correlation length of the fluctuations in a fluidized bed increases with volume fraction. We argue that this difference arises because the relaxation time of density fluctuations is completely different in the two systems.

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ALMA observations of the young protostellar system Barnard 1b: Signatures of an incipient hot corino in B1b-S

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731955 ◽

2018 ◽

Vol 620 ◽

pp. A80 ◽

Cited By ~ 22

Author(s):

N. Marcelino ◽

M. Gerin ◽

J. Cernicharo ◽

A. Fuente ◽

H. A. Wootten ◽

...

Keyword(s):

Ethyl Formate ◽

High Energy ◽

Source Model ◽

Mass Star ◽

Line Profiles ◽

Stellar Objects ◽

Star Forming ◽

Compact Component ◽

Prestellar Cores ◽

Low Mass

The Barnard 1b core shows signatures of being at the earliest stages of low-mass star formation, with two extremely young and deeply embedded proto-stellar objects. Hence, this core is an ideal target to study the structure and chemistry of the first objects formed in the collapse of prestellar cores. We present ALMA Band 6 spectral line observations at ~0.6″ of angular resolution towards Barnard 1b. We have extracted the spectra towards both protostars, and used a local thermodynamic equilibrium (LTE) model to reproduce the observed line profiles. B1b-S shows rich and complex spectra, with emission from high energy transitions of complex molecules, such as CH3OCOH and CH3CHO, including vibrational level transitions. We have tentatively detected for the first time in this source emission from NH2CN, NH2CHO, CH3CH2OH, CH2OHCHO, CH3CH2OCOH and both aGg′ and gGg′ conformers of (CH2OH)2. This is the first detection of ethyl formate (CH3CH2OCOH) towards a low-mass star forming region. On the other hand, the spectra of the FHSC candidate B1b-N are free of COMs emission. In order to fit the observed line profiles in B1b-S, we used a source model with two components: an inner hot and compact component (200 K, 0.35″) and an outer and colder one (60 K, 0.6″). The resulting COM abundances in B1b-S range from 10−13 for NH2CN and NH2CHO, up to 10−9 for CH3OCOH. Our ALMA Band 6 observations reveal the presence of a compact and hot component in B1b-S, with moderate abundances of complex organics. These results indicate that a hot corino is being formed in this very young Class 0 source.

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The highly variable time evolution of star-forming cores identified with dendrograms

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2253 ◽

2020 ◽

Vol 497 (4) ◽

pp. 4517-4534

Author(s):

Rachel A Smullen ◽

Kaitlin M Kratter ◽

Stella S R Offner ◽

Aaron T Lee ◽

Hope How-Huan Chen

Keyword(s):

Time Evolution ◽

Mass Function ◽

Density Field ◽

Initial Mass Function ◽

Stellar Content ◽

Core Mass ◽

Structure Variation ◽

Star Forming ◽

The Core ◽

Dense Cores

ABSTRACT We investigate the time evolution of dense cores identified in molecular cloud simulations using dendrograms, which are a common tool to identify hierarchical structure in simulations and observations of star formation. We develop an algorithm to link dendrogram structures through time using the three-dimensional density field from magnetohydrodynamical simulations, thus creating histories for all dense cores in the domain. We find that the population-wide distributions of core properties are relatively invariant in time, and quantities like the core mass function match with observations. Despite this consistency, an individual core may undergo large (>40 per cent), stochastic variations due to the redefinition of the dendrogram structure between time-steps. This variation occurs independent of environment and stellar content. We identify a population of short-lived (<200 kyr) overdensities masquerading as dense cores that may comprise $\sim\!20$ per cent of any time snapshot. Finally, we note the importance of considering the full history of cores when interpreting the origin of the initial mass function; we find that, especially for systems containing multiple stars, the core mass defined by a dendrogram leaf in a snapshot is typically less than the final system stellar mass. This work reinforces that there is no time-stable density contour that defines a star-forming core. The dendrogram itself can induce significant structure variation between time-steps due to small changes in the density field. Thus, one must use caution when comparing dendrograms of regions with different ages or environment properties because differences in dendrogram structure may not come solely from the physical evolution of dense cores.

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Small-angle X-ray scattering study on correlation length and density fluctuations in a supercritical CO2–water mixture

Fluid Phase Equilibria ◽

10.1016/s0378-3812(01)00767-1 ◽

2002 ◽

Vol 198 (2) ◽

pp. 251-256 ◽

Cited By ~ 8

Author(s):

Jianling Zhang ◽

Juncheng Liu ◽

Liang Gao ◽

Xiaogang Zhang ◽

Zhenshan Hou ◽

...

Keyword(s):

Correlation Length ◽

Small Angle ◽

Supercritical Co2 ◽

Density Fluctuations ◽

Water Mixture ◽

X Ray ◽

X Ray Scattering ◽

Scattering Study ◽

Ray Scattering

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Episodic accretion in binary protostars emerging from self-gravitating solar mass cores

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732076 ◽

2018 ◽

Vol 614 ◽

pp. A53 ◽

Cited By ~ 2

Author(s):

R. Riaz ◽

S. Vanaverbeke ◽

D. R. G. Schleicher

Keyword(s):

Molecular Cloud ◽

Accretion Rate ◽

Solar Mass ◽

Multiple Systems ◽

Average Mass ◽

Triple Systems ◽

Large Spread ◽

Low Mass ◽

Over Time

Observations show a large spread in the luminosities of young protostars, which are frequently explained in the context of episodic accretion. We tested this scenario with numerical simulations that follow the collapse of a solar mass molecular cloud using the GRADSPH code, thereby varying the strength of the initial perturbations and temperature of the cores. A specific emphasis of this paper is to investigate the role of binaries and multiple systems in the context of episodic accretion and to compare their evolution to the evolution in isolated fragments. Our models form a variety of low-mass protostellar objects including single, binary, and triple systems in which binaries are more active in exhibiting episodic accretion than isolated protostars. We also find a general decreasing trend in the average mass accretion rate over time, suggesting that the majority of the protostellar mass is accreted within the first 105 years. This result can potentially help to explain the surprisingly low average luminosities in the majority of the protostellar population.

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A centrally concentrated sub-solar-mass starless core in the Taurus L1495 filamentary complex

Publications of the Astronomical Society of Japan ◽

10.1093/pasj/psz051 ◽

2019 ◽

Vol 71 (4) ◽

Cited By ~ 3

Author(s):

Kazuki Tokuda ◽

Kengo Tachihara ◽

Kazuya Saigo ◽

Phillipe André ◽

Yosuke Miyamoto ◽

...

Keyword(s):

Star Formation ◽

Initial Conditions ◽

Volume Density ◽

Mass Star ◽

Brown Dwarf ◽

Solar Mass ◽

Star Forming ◽

The Core ◽

Order Of Magnitude ◽

Low Mass

Abstract The formation scenario of brown dwarfs is still unclear because observational studies to investigate its initial condition are quite limited. Our systematic survey of nearby low-mass star-forming regions using the Atacama Compact Array (aka the Morita array) and the IRAM 30-m telescope in 1.2 mm continuum has identified a centrally concentrated starless condensation with a central H2 volume density of ∼106 cm−3, MC5-N, connected to a narrow (width ∼0.03 pc) filamentary cloud in the Taurus L1495 region. The mass of the core is $\sim {0.2\!-\!0.4}\, M_{\odot }$, which is an order of magnitude smaller than typical low-mass pre-stellar cores. Taking into account a typical core to star formation efficiency for pre-stellar cores (∼20%–40%) in nearby molecular clouds, brown dwarf(s) or very low-mass star(s) may be going to be formed in this core. We have found possible substructures at the high-density portion of the core, although much higher angular resolution observation is needed to clearly confirm them. The subsequent N2H+ and N2D+ observations using the Nobeyama 45-m telescope have confirmed the high-deuterium fractionation (∼30%). These dynamically and chemically evolved features indicate that this core is on the verge of proto-brown dwarf or very low-mass star formation and is an ideal source to investigate the initial conditions of such low-mass objects via gravitational collapse and/or fragmentation of the filamentary cloud complex.

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21 cm Line Study of Large Scale Density Fluctuations in the Taurus Molecular Complex

Symposium - International Astronomical Union ◽

10.1017/s0074180900094900 ◽

1987 ◽

Vol 115 ◽

pp. 67-68

Author(s):

W.L.H. Shuter ◽

R. L. Dickman ◽

C. Klatt

Keyword(s):

Correlation Length ◽

Molecular Complex ◽

Preliminary Analysis ◽

Large Scale ◽

Power Spectra ◽

Density Fluctuations ◽

Central Point ◽

Spectral Peak ◽

Autocorrelation Functions ◽

Power Spectral

21-cm spectra on a 41 × 31 grid, centered at 1950: RA 04h30m; DEC 27d00m, at points separated by a true angle of 0.25 degrees, were observed using the Arecibo telescope in October 1985. The identical grid had previously been observed in 13CO by Kleiner and Dickman (1984) with the FCRAO mm wave telescope. In this preliminary analysis we determined autocorrelation functions and power spectra for 21-cm self absorption “intensities”, for a cross passing through the central point. Both arms of the cross, aligned parallel to RA and DEC, show a power spectral peak at a frequency of 0.312 reciprocal degrees, corresponding to a period of 3.2 degrees on the sky. Assuming that the Taurus complex is at a distance of 140 pc, this corresponds to a correlation length of 7.8 pc, which is about a factor of two smaller than the value of 14 pc found by Kleiner and Dickman for 13CO.

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Super-rotating protostellar jets

10.21203/rs.3.rs-82280/v1 ◽

2020 ◽

Author(s):

Yuko Matsushita ◽

Satoko Takahashi ◽

Shun Ishii ◽

Kohji Tomisaka ◽

Paul Ho ◽

...

Keyword(s):

Angular Momentum ◽

Star Formation ◽

Formation Process ◽

Rotation Velocity ◽

Solar Mass ◽

Large Rotation ◽

Star Forming ◽

Disk Wind ◽

Star Forming Regions ◽

Protostellar Jets

Abstract Protostellar jets are most striking phenomena in star-forming regions and considered to be an essential ingredient in the star formation process. Stars form in gravitationally collapsing clouds. The mass of protostar at its birth is equivalent to Jovian mass or 0.1 percent of the solar mass. After the birth, the protostar acquires its mass by accreting material from a surrounding rotation disk embedded in an infalling envelope that is a remnant of the natal cloud of the star. Protostellar jets are believed to expel the excess angular momentum from the circumstellar region that allows accretion on to the star. Here, we report the detection of super-rotating jets driven from a protostar FIR 6b (HOPS 60) in Orion Molecular Cloud-2. The jet rotation velocity exceeds 20kms-1 and the specific angular momentum of the jet is as large as ∼ 1022cm2s-1, which hitherto are the largest that have been observed in protostellar jets. The extraordinary large rotation velocity and specific angular momentum can be explained by a magnetohydrodynamic disk wind. This is clear evidence that magnetic fields play a crucial role for protostellar evolution and that angular momentum is removed by protostellar jets.

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