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

2019 ◽  
Vol 881 (1) ◽  
pp. L15 ◽  
Author(s):  
Sayantan Auddy ◽  
Shantanu Basu ◽  
Takahiro Kudoh
Keyword(s):  
Magnetic Field ◽  
Density Distribution ◽  
Power Law ◽  
Molecular Clouds ◽  
Column Density ◽  
Initial Magnetic Field

scholarly journals Effects of grain alignment efficiency on synthetic dust polarization observations of molecular clouds

2019 ◽  
Vol 490 (2) ◽  
pp. 2760-2778 ◽  
Author(s):  
Patrick K King ◽  
Che-Yu Chen ◽  
L M Fissel ◽  
Zhi-Yun Li
Keyword(s):  
Magnetic Field ◽  
Power Law ◽  
Molecular Clouds ◽  
Column Density ◽  
Local Magnetic Field ◽  
Continuum Emission ◽  
Field Orientation ◽  
Local Conditions ◽  
Grain Alignment ◽  
Polarization Fraction

ABSTRACT It is well known that the polarized continuum emission from magnetically aligned dust grains is determined to a large extent by local magnetic field structure. However, the observed significant anticorrelation between polarization fraction and column density may be strongly affected, perhaps even dominated by variations in grain alignment efficiency with local conditions, in contrast to standard assumptions of a spatially homogeneous grain alignment efficiency. Here we introduce a generic way to incorporate heterogeneous grain alignment into synthetic polarization observations of molecular clouds (MCs), through a simple model where the grain alignment efficiency depends on the local gas density as a power law. We justify the model using results derived from radiative torque alignment theory. The effects of power-law heterogeneous alignment models on synthetic observations of simulated MCs are presented. We find that the polarization fraction-column density correlation can be brought into agreement with observationally determined values through heterogeneous alignment, though there remains degeneracy with the relative strength of cloud-scale magnetized turbulence and the mean magnetic field orientation relative to the observer. We also find that the dispersion in polarization angles-polarization fraction correlation remains robustly correlated despite the simultaneous changes to both observables in the presence of heterogeneous alignment.

scholarly journals Cloud formation in the atomic and molecular phase: H I self absorption (HISA) towards a giant molecular filament

2020 ◽  
Vol 634 ◽  
pp. A139 ◽  
Author(s):  
Y. Wang ◽  
S. Bihr ◽  
H. Beuther ◽  
M. R. Rugel ◽  
J. D. Soler ◽  
...  
Keyword(s):  
Interstellar Medium ◽  
Power Law ◽  
Molecular Clouds ◽  
Column Density ◽  
Cloud Formation ◽  
Neutral Medium ◽  
Molecular Component ◽  
Density Peaks ◽  
Log Normal ◽  
High Column

Molecular clouds form from the atomic phase of the interstellar medium. However, characterizing the transition between the atomic and the molecular interstellar medium (ISM) is a complex observational task. Here we address cloud formation processes by combining H I self absorption (HISA) with molecular line data. Column density probability density functions (N-PDFs) are a common tool for examining molecular clouds. One scenario proposed by numerical simulations is that the N-PDF evolves from a log-normal shape at early times to a power-law-like shape at later times. To date, investigations of N-PDFs have been mostly limited to the molecular component of the cloud. In this paper, we study the cold atomic component of the giant molecular filament GMF38.1-32.4a (GMF38a, distance = 3.4 kpc, length ~ 230 pc), calculate its N-PDFs, and study its kinematics. We identify an extended HISA feature, which is partly correlated with the 13CO emission. The peak velocities of the HISA and 13CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s−1 is found on the western side. The sonic Mach number we derive from the linewidth measurements shows that a large fraction of the HISA, which is ascribed to the cold neutral medium (CNM), is at subsonic and transonic velocities. The column density of the CNM part is on the order of 1020 to 1021 cm−2. The column density of molecular hydrogen, traced by 13CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H I emission (the warm and cold neutral medium), and 13CO (molecular component) are well described by log-normal functions, which is in agreement with turbulent motions being the main driver of cloud dynamics. The N-PDF of the molecular component also shows a power law in the high column-density region, indicating self-gravity. We suggest that we are witnessing two different evolutionary stages within the filament. The eastern subregion seems to be forming a molecular cloud out of the atomic gas, whereas the western subregion already shows high column density peaks, active star formation, and evidence of related feedback processes.

scholarly journals Molecular clouds have power-law PDFs (not log-normal)

2015 ◽  
Vol 11 (A29B) ◽  
pp. 706-707
Author(s):  
João Alves ◽  
Marco Lombardi ◽  
Charles Lada
Keyword(s):  
Power Law ◽  
Molecular Clouds ◽  
Column Density ◽  
Functional Form ◽  
Power Laws ◽  
Common Belief ◽  
Normal Functions ◽  
Complex Models ◽  
Log Normal

AbstractContrary to common belief, the column density PDFs of molecular clouds are not described well by log-normal functions, but are instead power-laws with exponents close to two. We argue that the intrinsic functional form of the PDF cannot be securely determined below AK ~ 0.1-0.2 mag, limiting our ability to investigate more complex models for the shape of the cloud PDF.

scholarly journals Relationship between the column density distribution and evolutionary class of molecular clouds as viewed by ATLASGAL

2015 ◽  
Vol 581 ◽  
pp. A74 ◽  
Author(s):  
J. Abreu-Vicente ◽  
J. Kainulainen ◽  
A. Stutz ◽  
Th. Henning ◽  
H. Beuther
Keyword(s):  
Density Distribution ◽  
Molecular Clouds ◽  
Column Density

scholarly journals THE “TRUE” COLUMN DENSITY DISTRIBUTION IN STAR-FORMING MOLECULAR CLOUDS

2009 ◽  
Vol 692 (1) ◽  
pp. 91-103 ◽  
Author(s):  
Alyssa A. Goodman ◽  
Jaime E. Pineda ◽  
Scott L. Schnee
Keyword(s):  
Density Distribution ◽  
Molecular Clouds ◽  
Column Density ◽  
Star Forming

scholarly journals Helical magnetic fields in molecular clouds?

2018 ◽  
Vol 614 ◽  
pp. A100 ◽  
Author(s):  
M. Tahani ◽  
R. Plume ◽  
J. C. Brown ◽  
J. Kainulainen
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Molecular Clouds ◽  
Column Density ◽  
Measure Data ◽  
Line Of Sight ◽  
Small Scale ◽  
Rotation Measure ◽  
Data Points

Context. Magnetic fields pervade in the interstellar medium (ISM) and are believed to be important in the process of star formation, yet probing magnetic fields in star formation regions is challenging. Aims. We propose a new method to use Faraday rotation measurements in small-scale star forming regions to find the direction and magnitude of the component of magnetic field along the line of sight. We test the proposed method in four relatively nearby regions of Orion A, Orion B, Perseus, and California. Methods. We use rotation measure data from the literature. We adopt a simple approach based on relative measurements to estimate the rotation measure due to the molecular clouds over the Galactic contribution. We then use a chemical evolution code along with extinction maps of each cloud to find the electron column density of the molecular cloud at the position of each rotation measure data point. Combining the rotation measures produced by the molecular clouds and the electron column density, we calculate the line-of-sight magnetic field strength and direction. Results. In California and Orion A, we find clear evidence that the magnetic fields at one side of these filamentary structures are pointing towards us and are pointing away from us at the other side. Even though the magnetic fields in Perseus might seem to suggest the same behavior, not enough data points are available to draw such conclusions. In Orion B, as well, there are not enough data points available to detect such behavior. This magnetic field reversal is consistent with a helical magnetic field morphology. In the vicinity of available Zeeman measurements in OMC-1, OMC-B, and the dark cloud Barnard 1, we find magnetic field values of − 23 ± 38 μG, − 129 ± 28 μG, and 32 ± 101 μG, respectively, which are in agreement with the Zeeman measurements.

scholarly journals Molecular clouds in galaxies as seen from NIR extinction studies

2006 ◽  
Vol 2 (S237) ◽  
pp. 204-207
Author(s):  
João Alves
Keyword(s):  
Mass Spectrum ◽  
Power Law ◽  
Molecular Cloud ◽  
Molecular Clouds ◽  
Column Density ◽  
Large Scale ◽  
Near Infrared ◽  
Mass Function ◽  
Core Mass ◽  
Dense Cores

AbstractNear infrared dust extinction mapping is opening a new window on molecular cloud research. Applying a straightforward technique to near infrared large scale data of nearby molecular complexes one can easily construct density maps with dynamic ranges in column density covering, 3σ~ 0.5 < AV< 50 mag or 1021<N<1023 cm−2. These maps are unique in capturing the low column density distribution of gas in molecular cloud complexes, where most of the mass resides, and at the same time allow the identification of dense cores (n~104cm−3) which are the precursors of stars. For example, the application of this technique to the nearby Pipe Nebula complex revealed the presence of 159 dense cores (the largest sample of such object in one single complex) whose mass spectrum presents the first robust evidence for a departure from a single power-law. The form of this mass function is surprisingly similar in shape to the stellar IMF but scaled to a higher mass by a factor of about 3. This suggests that the distribution of stellar birth masses (IMF) is the direct product of the dense core mass function and a uniform star formation efficiency of 30%±10%, and that the stellar IMF may already be fixed during or before the earliest stages of core evolution. We are now extending this technique to extra-galactic mapping of Giant molecular Clouds (GMCs), and although a much less straightforward task, preliminary results indicate that the GMC mass spectrum in M83 and Centaurus A is a power-law characterized by α~−2 unlike CO results which suggest α~−1.

scholarly journals Jet Impingement Heat Transfer of Confined Single and Double Jets with Non-Newtonian Power Law Nanofluid under the Inclined Magnetic Field Effects for a Partly Curved Heated Wall

2021 ◽  
Vol 13 (9) ◽  
pp. 5086
Author(s):  
Fatih Selimefendigil ◽  
Hakan F. Oztop ◽  
Ali J. Chamkha
Keyword(s):  
Magnetic Field ◽  
Heat Transfer ◽  
Particle Size ◽  
Aspect Ratio ◽  
Power Law ◽  
Volume Fraction ◽  
Inclined Magnetic Field ◽  
Magnetic Field Effects ◽  
Power Law Nanofluid ◽  
Power Law Index

Single and double impinging jets heat transfer of non-Newtonian power law nanofluid on a partly curved surface under the inclined magnetic field effects is analyzed with finite element method. The numerical work is performed for various values of Reynolds number (Re, between 100 and 300), Hartmann number (Ha, between 0 and 10), magnetic field inclination (γ, between 0 and 90), curved wall aspect ratio (AR, between 01. and 1.2), power law index (n, between 0.8 and 1.2), nanoparticle volume fraction (ϕ, between 0 and 0.04) and particle size in nm (dp, between 20 and 80). The amount of rise in average Nusselt (Nu) number with Re number depends upon the power law index while the discrepancy between the Newtonian fluid case becomes higher with higher values of power law indices. As compared to case with n = 1, discrepancy in the average Nu number are obtained as −38% and 71.5% for cases with n = 0.8 and n = 1.2. The magnetic field strength and inclination can be used to control the size and number or vortices. As magnetic field is imposed at the higher strength, the average Nu reduces by about 26.6% and 7.5% for single and double jets with n greater than 1 while it increases by about 4.78% and 12.58% with n less than 1. The inclination of magnetic field also plays an important role on the amount of enhancement in the average Nu number for different n values. The aspect ratio of the curved wall affects the flow field slightly while the average Nu variation becomes 5%. Average Nu number increases with higher solid particle volume fraction and with smaller particle size. At the highest particle size, it is increased by about 14%. There is 7% variation in the average Nu number when cases with lowest and highest particle size are compared. Finally, convective heat transfer performance modeling with four inputs and one output is successfully obtained by using Adaptive Neuro-Fuzzy Interface System (ANFIS) which provides fast and accurate prediction results.

The influence of magnetic field on wall shear stress in power law fluid flow of blood

2021 ◽  
Author(s):  
Amira Husni Talib ◽  
Ilyani Abdullah ◽  
Nik Nabilah Nik Mohd Naser
Keyword(s):  
Magnetic Field ◽  
Shear Stress ◽  
Fluid Flow ◽  
Wall Shear Stress ◽  
Power Law ◽  
Power Law Fluid ◽  
Wall Shear
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