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

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.

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Atomic and molecular gas properties during cloud formation

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038449 ◽

2020 ◽

Vol 642 ◽

pp. A68 ◽

Cited By ~ 1

Author(s):

J. Syed ◽

Y. Wang ◽

H. Beuther ◽

J. D. Soler ◽

M. R. Rugel ◽

...

Keyword(s):

Column Density ◽

Emission Spectra ◽

Transition Process ◽

Cloud Formation ◽

Neutral Medium ◽

Molecular Gas ◽

Eastern Region ◽

Density Peaks ◽

Order Of Magnitude ◽

Log Normal

Context. Molecular clouds, which harbor the birthplaces of stars, form out of the atomic phase of the interstellar medium (ISM). To understand this transition process, it is crucial to investigate the spatial and kinematic relationships between atomic and molecular gas. Aims. We aim to characterize the atomic and molecular phases of the ISM and set their physical properties into the context of cloud formation processes. Methods. We studied the cold neutral medium (CNM) by means of H I self-absorption (HISA) toward the giant molecular filament GMF20.0-17.9 (distance = 3.5 kpc, length ~170 pc) and compared our results with molecular gas traced by 13CO emission. We fitted baselines of HISA features to H I emission spectra using first and second order polynomial functions. Results. The CNM identified by this method spatially correlates with the morphology of the molecular gas toward the western region. However, no spatial correlation between HISA and 13CO is evident toward the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with 13CO within the whole filament. The column densities of the CNM probed by HISA are on the order of 1020 cm−2 while those of molecular hydrogen traced by 13CO are an order of magnitude higher. The column density probability density functions (N-PDFs) of HISA (CNM) and H I emission (tracing both the CNM and the warm neutral medium, WNM) have a log-normal shape for all parts of the filament, indicative of turbulent motions as the main driver for these structures. The H2 N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of H I column density is observed at ~25 M⊙ pc−2. Conclusions. We conjecture that different evolutionary stages are evident within the filament. In the eastern region, we witness the onset of molecular cloud formation out of the atomic gas reservoir while the western part is more evolved, as it reveals pronounced H2 column density peaks and signs of active star formation.

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

Proceedings of the International Astronomical Union ◽

10.1017/s174392131600644x ◽

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.

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Deterministic Self-Propagating Star Formation

International Astronomical Union Colloquium ◽

10.1017/s0252921100024337 ◽

1989 ◽

Vol 120 ◽

pp. 518-523

Author(s):

Jan Palouš

Keyword(s):

Star Formation ◽

Simple Model ◽

Molecular Cloud ◽

Molecular Clouds ◽

Large Scale ◽

Galactic Evolution ◽

Cloud Formation ◽

Rotating Disks ◽

Atomic Gas ◽

High Column

AbstractThe evolution of large scale expanding structures in differentially rotating disks is studied. High column densities in some places may eventually lead to molecular cloud formation and initiate also star-formation. After some time, multi-structured arms evolve, where regions of intensive star-formation are separated from each other by regions of atomic gas or molecular clouds. This is due to the deterministic nature and to the coherence of this process. A simple model of galactic evolution is introduced and the different behaviour of Sa, Sb, and Sc galaxies is shown.

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A two-step gravitational cascade for the fragmentation of self-gravitating disks

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab738 ◽

2021 ◽

Author(s):

Noé Brucy ◽

Patrick Hennebelle

Keyword(s):

Power Law ◽

Column Density ◽

Smooth Transition ◽

Scale Free ◽

Gravitational Stress ◽

Particle Hydrodynamics ◽

Smoothed Particle ◽

Grid Codes ◽

Viscous Transport ◽

High Column

Abstract Self-gravitating disks are believed to play an important role in astrophysics in particular regarding the star and planet formation process. In this context, disks subject to an idealized cooling process, characterized by a cooling timescale β expressed in unit of orbital timescale, have been extensively studied. We take advantage of the Riemann solver and the 3D Godunov scheme implemented in the code Ramses to perform high resolution simulations, complementing previous studies that have used Smoothed Particle Hydrodynamics (SPH) or 2D grid codes. We observe that the critical value of β for which the disk fragments is consistent with most previous results, and is not well converged with resolution. By studying the probability density function of the fluctuations of the column density (∑-PDF), we argue that there is no strict separation between the fragmented and the unfragmented regimes but rather a smooth transition with the probability of apparition of fragments steadily diminishing as the cooling becames less effective. We find that the high column density part of the ∑-PDF follows a simple power law whose slope turns out to be proportional to β and we propose an explanation based on the balance between cooling and heating through gravitational stress. Our explanation suggests that a more efficient cooling requires more heating implying a larger fraction of dense material which, in the absence of characteristic scales, results in a shallower scale-free power law. We propose that the gravitational cascade proceeds in two steps, first the formation of a dense filamentary spiral pattern through a sequence of quasi-static equilibrium triggered by the viscous transport of angular momentum, and second the collapse alongside these filaments that eventually results in the formation of bounded fragments.

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TWO-FLUID MAGNETOHYDRODYNAMICS SIMULATIONS OF CONVERGING H I FLOWS IN THE INTERSTELLAR MEDIUM. II. ARE MOLECULAR CLOUDS GENERATED DIRECTLY FROM A WARM NEUTRAL MEDIUM?

The Astrophysical Journal ◽

10.1088/0004-637x/704/1/161 ◽

2009 ◽

Vol 704 (1) ◽

pp. 161-169 ◽

Cited By ~ 63

Author(s):

Tsuyoshi Inoue ◽

Shu-ichiro Inutsuka

Keyword(s):

Interstellar Medium ◽

Molecular Clouds ◽

Neutral Medium ◽

Two Fluid

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

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2628 ◽

2019 ◽

Vol 490 (2) ◽

pp. 2760-2778 ◽

Cited By ~ 2

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.

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Gravity, turbulence and the scaling “laws” in molecular clouds

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316006499 ◽

2015 ◽

Vol 11 (A29B) ◽

pp. 716-716

Author(s):

Javier Ballesteros-Paredes

Keyword(s):

Molecular Clouds ◽

Column Density ◽

Scaling Laws ◽

Cloud Formation ◽

List Type ◽

Present Contribution ◽

Scale Free ◽

Dense Cores ◽

Star Formation Histories ◽

Size Relation

AbstractThe so-called Larson (1981) scaling laws found empirically in molecular clouds have been generally interpreted as evidence that the clouds are turbulent and fractal. In the present contribution we discussed how recent observations and models of cloud formation suggest that: (a)these relations are the result of strong observational biases due to the cloud definition itself: since the filling factor of the dense structures is small, by thresholding the column density the computed mean density between clouds is nearly constant, and nearly the same as the threshold (Ballesteros-Paredes et al. 2012).(b)When accounting for column density variations, the velocity dispersion-size relation does not appears anymore. Instead, dense cores populate the upper-left corner of the δ v-R diagram (Ballesteros-Paredes et al. 2011a).(c)Instead of a δ v-R relation, a more appropriate relation seems to be δ v2 / R = 2 GMΣ, which suggest that clouds are in collapse, rather than supported by turbulence (Ballesteros-Paredes et al. 2011a).(d)These results, along with the shapes of the star formation histories (Hartmann, Ballesteros-Paredes & Heitsch 2012), line profiles of collapsing clouds in numerical simulations (Heitsch, Ballesteros-Paredes & Hartmann 2009), core-to-core velocity dispersions (Heitsch, Ballesteros-Paredes & Hartmann 2009), time-evolution of the column density PDFs (Ballesteros-Paredes et al. 2011b), etc., strongly suggest that the actual source of the non-thermal motions is gravitational collapse of the clouds, so that the turbulent, chaotic component of the motions is only a by-product of the collapse, with no significant “support" role for the clouds. This result calls into question if the scale-free nature of the motions has a turbulent, origin (Ballesteros-Paredes et al. 2011a; Ballesteros-Paredes et al. 2011b, Ballesteros-Paredes et al. 2012).

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Formation of the super star cluster RCW 38 triggered by cloud-cloud collision

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316000478 ◽

2015 ◽

Vol 12 (S316) ◽

pp. 208-213

Author(s):

Yasuo Fukui

Keyword(s):

Molecular Clouds ◽

Column Density ◽

Cluster Formation ◽

Star Cluster ◽

Cluster Center ◽

Molecular Gas ◽

Velocity Difference ◽

The Galaxy ◽

Massive Cluster ◽

High Column

AbstractRCW 38 is the youngest super star cluster in the Galaxy and is located at a distance of 1.7 kpc. Molecular observations revealed that the cluster is associated with two molecular clouds having velocity difference of 12 km s−1. We interpret that the two clouds are colliding with each other and the collision triggered the cluster formation. The natal molecular gas still survives within ~ 0.5 pc of the central O stars which have an age of 0.1 Myrs as inferred from the collision morphology. We suggest that the high column density of one of the clouds 1023 cm−2 enabled formation of ~ 20 O stars in the cluster center and discuss the implications on massive cluster formation.

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