Characteristic Structure of Star-Forming Clouds

Myers, Philip C.

doi:10.1088/0004-637x/806/2/226

Cited by 31 publications

(37 citation statements)

References 66 publications

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“…In summary, we consider the H 2 column densities from CO to be correct within a factor of two and the ones from CS and N 2 H + within a factor of a few. Note that similar calculations can be found in Federrath & Klessen (2013), Fischera et al (2014), and Myers (2015.…”

Section: Cs and N 2 Hsupporting

confidence: 69%

“…Peretto et al 2013;Schneider et al 2010Schneider et al , 2015b) but the gas we see in filaments (and cores) can account for power-law N-PDFs if it has the right power-law radial-density structure, even in hydrostatic balance with no net inward motion. The observed N-PDF slopes (s = −1.4 to −2.6, leading to α f between 1.4 and 1.7) are consistent with self-gravitating but noncollapsing filament models (Myers 2015;Toci & Galli 2015). In this case, filamentary gas that has a sufficient mass per length ratio is centrally concentrated and self-gravitating, but does not need to be globally collapsing.…”

Section: Slopes Of the N-pdfssupporting

confidence: 64%

“…For spherical geometry, representing a single core or a core ensemble (Girichidis et al 2014), we use α c = 1−2/s (Federrath & Klessen 2013). For singular polytropic cylinders, which portray filaments, we use α f = 1−1/s (see Toci & Galli (2015) for regular polytropic cylinder models and Fischera (2014) and Myers (2015) for the corresponding N-PDFs). The values we obtain for α (α c or α f ) vary between 1.4 and 2.4.…”

Section: Slopes Of the N-pdfsmentioning

confidence: 99%

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Understanding star formation in molecular clouds

Schneider

Bontemps

Motte

et al. 2016

A&A

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The probability distribution function of column density (N-PDF) serves as a powerful tool to characterise the various physical processes that influence the structure of molecular clouds. Studies that use extinction maps or H 2 column-density maps (N) that are derived from dust show that star-forming clouds can best be characterised by lognormal PDFs for the lower N range and a power-law tail for higher N, which is commonly attributed to turbulence and self-gravity and/or pressure, respectively. While PDFs from dust cover a large dynamic range (typically N ∼ 10 20−24 cm −2 or A v ∼ 0.1−1000), PDFs obtained from molecular lines -converted into H 2 column density -potentially trace more selectively different regimes of (column) densities and temperatures. They also enable us to distinguish different clouds along the line of sight through using the velocity information. We report here on PDFs that were obtained from observations of 12 CO, 13 CO, C 18 O, CS, and N 2 H + in the Cygnus X North region, and make a comparison to a PDF that was derived from dust observations with the Herschel satellite. The PDF of 12 CO is lognormal for A v ∼ 1-30, but is cut for higher A v because of optical depth effects. The PDFs of C 18 O and 13 CO are mostly lognormal up to A v ∼ 1-15, followed by excess up to A v ∼ 40. Above that value, all CO PDFs drop, which is most likely due to depletion. The high density tracers CS and N 2 H + exhibit only a power law distribution between A v ∼ 15 and 400, respectively. The PDF from dust is lognormal for A v ∼ 3-15 and has a power-law tail up to A v ∼ 500. Absolute values for the molecular line column densities are, however, rather uncertain because of abundance and excitation temperature variations. If we take the dust PDF at face value, we "calibrate" the molecular line PDF of CS to that of the dust and determine an abundance [CS]/[H 2 ] of 10 −9 . The slopes of the power-law tails of the CS, N 2 H + , and dust PDFs are −1.6, −1.4, and −2.3, respectively, and are thus consistent with free-fall collapse of filaments and clumps. A quasi static configuration of filaments and clumps can also possibly account for the observed N-PDFs, providing they have a sufficiently condensed density structure and external ram pressure by gas accretion is provided. The somehow flatter slopes of N 2 H + and CS can reflect an abundance change and/or subthermal excitation at low column densities.

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Section: Cs and N 2 Hsupporting

confidence: 69%

Section: Slopes Of the N-pdfssupporting

confidence: 64%

Section: Slopes Of the N-pdfsmentioning

confidence: 99%

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Understanding star formation in molecular clouds

Schneider

Bontemps

Motte

et al. 2016

A&A

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“…There is a broad consensus among observers and theorists that the PDF of molecular clouds are characterized by a log-normal peak and by a "power-law tail" towards the high column-densities (e.g., Lombardi et al 2008Lombardi et al , 2010Kainulainen et al 2009;Goodman et al 2009;Froebrich & Rowles 2010;Schneider et al 2011;Beaumont et al 2012;Kainulainen et al 2013;Alves et al 2014;Schneider et al 2015;Abreu-Vicente et al 2015). Log-normal PDFs for column density maps of turbulent clouds were first predicted from theoretical work (Vazquez-Semadeni 1994;Padoan et al 1997;Scalo et al 1998;Ostriker et al 2001) and have been used as promising tools to compare numerical simulations with observations and theory (e.g., Federrath et al 2010;Renaud et al 2013;Ward et al 2014;Myers 2015;Donkov et al 2017). If observed, log-normal PDFs would offer tremendous insight into cloud physics, allowing hard-to-measure parameters such as turbulent driving, Mach number, and magnetic pressure to be quantified (e.g., Nordlund & Padoan 1999;Federrath et al 2008Federrath et al , 2010Molina et al 2012).…”

Section: Introductionmentioning

confidence: 99%

The shapes of column density PDFs

Alves

Lombardi

Lada

2017

A&A

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The probability distribution function of column density (PDF) has become the tool of choice for cloud structure analysis and star formation studies. Its simplicity is attractive, and the PDF could offer access to cloud physical parameters otherwise difficult to measure, but there has been some confusion in the literature on the definition of its completeness limit and shape at the low column density end. In this letter we use the natural definition of the completeness limit of a column density PDF, the last closed column density contour inside a surveyed region, and apply it to a set of large-scale maps of nearby molecular clouds. We conclude that there is no observational evidence for log-normal PDFs in these objects. We find that all studied molecular clouds have PDFs well described by power laws, including the diffuse cloud Polaris. Our results call for a new physical interpretation of the shape of the column density PDFs. We find that the slope of a cloud PDF is invariant to distance but not to the spatial arrangement of cloud material, and as such it is still a useful tool for investigating cloud structure.

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“…The data is binned with a uniform spacing of ∆ log(Σ/Σ 0 ) 0.02. For the piecewise function (see also Myers 2015;Pokhrel et al 2016) we consider a combination of a lognormal and a power law:…”

Section: Fitting Functionsmentioning

confidence: 99%

The Transition from a Lognormal to a Power-law Column Density Distribution in Molecular Clouds: An Imprint of the Initial Magnetic Field and Turbulence

Auddy

Basu

Kudoh

2019

ApJL

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We introduce a theory for the development of a transitional column density Σ TP between the lognormal and the power-law forms of the probability distribution function (PDF) in a molecular cloud. Our turbulent magnetohydrodynamic simulations show that the value of Σ TP increases as the strength of both the initial magnetic field and turbulence increases. We develop an analytic expression for Σ TP based on the interplay of turbulence, a (strong) magnetic field, and gravity. The transition value Σ TP scales with M 2 0 , the square of the initial sonic Mach number, and β 0 , the initial ratio of gas pressure to magnetic pressure. We fit the variation of Σ TP among different model clouds as a function of M 2 0 β 0 , or equivalently the square of the initial Alfvénic Mach number M 2 A0 . This implies that the transition value Σ TP is an imprint of cloud initial conditions and is set by turbulent compression of a magnetic cloud. Physically, the value of Σ TP denotes the boundary above which the mass-to-flux ratio becomes supercritical and gravity drives the evolution.

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Characteristic Structure of Star-Forming Clouds

Cited by 31 publications

References 66 publications

Understanding star formation in molecular clouds

Understanding star formation in molecular clouds

The shapes of column density PDFs

The Transition from a Lognormal to a Power-law Column Density Distribution in Molecular Clouds: An Imprint of the Initial Magnetic Field and Turbulence

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