Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “molecular clouds.”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

A measurement of the turbulence-driven density distribution in a non-star-forming molecular cloud

Molecular clouds are supersonically turbulent. This turbulence governs the initial mass function and the star formation rate. In order to understand the details of star formation, it is therefore essential to understand the properties of turbulence, in particular the probability distribution of density in turbulent clouds. We present H{sub 2}CO volume density measurements of a non-star-forming cloud along the line of sight toward W49A. We use these measurements in conjunction with total mass estimates from {sup 13}CO to infer the shape of the density probability distribution function. This method is complementary to measurements of turbulence via the column density distribution and should be applicable to any molecular cloud with detected CO. We show that turbulence in this cloud is probably compressively driven, with a compressive-to-total Mach number ratio b=M{sub C}/M>0.4. We measure the standard deviation of the density distribution, constraining it to the range 1.5 < σ {sub s} < 1.9, assuming that the density is lognormally distributed. This measurement represents an essential input into star formation laws. The method of averaging over different excitation conditions to produce a model of emission from a turbulent cloud is generally applicable to optically thin line observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

GRAVITATIONAL CONTRACTION VERSUS SUPERNOVA DRIVING AND THE ORIGIN OF THE VELOCITY DISPERSION–SIZE RELATION IN MOLECULAR CLOUDS

Molecular cloud (MC) observations show that clouds have non-thermal velocity dispersions that scale with the cloud size as σ ∝ R {sup 1/2} at a constant surface density, and for varying surface density scale with both the cloud’s size and surface density, σ {sup 2} ∝ R Σ. The energy source driving these chaotic motions remains poorly understood. We describe the velocity dispersions observed in a cloud population formed in a numerical simulation of a magnetized, stratified, supernova (SN)-driven, interstellar medium, including diffuse heating and radiative cooling, before and after we include the effects of the self-gravity of the gas. We compare the relationships between velocity dispersion, size, and surface density measured in the simulated cloud population to those found in observations of Galactic MCs. Our simulations prior to the onset of self-gravity suggest that external SN explosions alone do not drive turbulent motions of the observed magnitudes within dense clouds. On the other hand, self-gravity induces non-thermal motions as gravitationally bound clouds begin to collapse in our model, approaching the observed relations between velocity dispersion, size, and surface density. Energy conservation suggests that the observed behavior is consistent with the kinetic energy being proportional to the gravitational energy. However, the clouds in our model show no sign of reaching a stable equilibrium state at any time, even for strongly magnetized clouds. We conclude that gravitationally bound MCs are always in a state of gravitational contraction and their properties are a natural result of this chaotic collapse. In order to agree with observed star formation efficiencies, this process must be terminated by the early destruction of the clouds, presumably from internal stellar feedback.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Wide-field {sup 12}CO (J=2--1) and {sup 13}CO (J=2--1) Observations toward the Aquila Rift and Serpens Molecular Cloud Complexes. I. Molecular Clouds and Their Physical Properties

We present the results of wide-field {sup 12}CO (J=2--1) and {sup 13}CO (J=2--1) observations toward the Aquila Rift and Serpens molecular cloud complexes (25{sup ∘}<l<33{sup ∘} and 1{sup ∘}<b<6{sup ∘}) at an angular resolution of 3.′4 (≈0.25 pc) and at a velocity resolution of 0.079 km s{sup −1} with velocity coverage of −5 km s{sup −1}<V{sub LSR}<35 km s{sup −1}. We found that the {sup 13}CO emission better traces the structures seen in the extinction map, and derived the X{sub {sup 1}{sup 3}CO}-factor of this region. Applying SCIMES to the {sup 13}CO data cube, we identified 61 clouds and derived their mass, radii, and line widths. The line width–radius relation of the identified clouds basically follows those of nearby molecular clouds. The majority of the identified clouds are close to virial equilibrium, although the dispersion is large. By inspecting the {sup 12}CO channel maps by eye, we found several arcs that are spatially extended to 0.°2–3° in length. In the longitude–velocity diagrams of {sup 12}CO, we also found two spatially extended components that appear to converge toward Serpens South and the W40 region. The existence of two components with different velocities and arcs suggests that large-scale expanding bubbles and/or flows play a role in the formation and evolution of the Serpens South and W40 cloud.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A deep near-infrared survey toward the aquila molecular cloud. I. Molecular hydrogen outflows

We have performed an unbiased, deep near-infrared survey toward the Aquila molecular cloud with a sky coverage of ∼1 deg{sup 2}. We identified 45 molecular hydrogen emission-line objects (MHOs), of which only 11 were previously known. Using the Spitzer archival data, we also identified 802 young stellar objects (YSOs) in this region. Based on the morphology and the location of MHOs and YSO candidates, we associate 43 MHOs with 40 YSO candidates. The distribution of jet length shows an exponential decrease in the number of outflows with increasing length, and the molecular hydrogen outflows seem to be oriented randomly. Moreover, there is no obvious correlation between jet lengths, jet opening angles, or jet H{sub 2}1−0S(1) luminosities and the spectral indices of the possible driving sources in this region. We also suggest that molecular hydrogen outflows in the Aquila molecular cloud are rather weak sources of turbulence, unlikely to generate the observed velocity dispersion in the region of survey.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

TURBULENCE DECAY AND CLOUD CORE RELAXATION IN MOLECULAR CLOUDS

The turbulent motion within molecular clouds is a key factor controlling star formation. Turbulence supports molecular cloud cores from evolving to gravitational collapse and hence sets a lower bound on the size of molecular cloud cores in which star formation can occur. On the other hand, without a continuous external energy source maintaining the turbulence, such as in molecular clouds, the turbulence decays with an energy dissipation time comparable to the dynamic timescale of clouds, which could change the size limits obtained from Jean's criterion by assuming constant turbulence intensities. Here we adopt scaling relations of physical variables in decaying turbulence to analyze its specific effects on the formation of stars. We find that the decay of turbulence provides an additional approach for Jeans' criterion to be achieved, after which gravitational infall governs the motion of the cloud core. This epoch of turbulence decay is defined as cloud core relaxation. The existence of cloud core relaxation provides a more complete understanding of the effect of the competition between turbulence and gravity on the dynamics of molecular cloud cores and star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Magnetized interstellar molecular clouds – II. The large-scale structure and dynamics of filamentary molecular clouds

Abstract We perform ideal magnetohydrodynamics high-resolution adaptive mesh refinement simulations with driven turbulence and self-gravity and find that long filamentary molecular clouds are formed at the converging locations of large-scale turbulence flows and the filaments are bounded by gravity. The magnetic field helps shape and reinforce the long filamentary structures. The main filamentary cloud has a length of ∼4.4 pc. Instead of a monolithic cylindrical structure, the main cloud is shown to be a collection of fibre/web-like substructures similar to filamentary clouds such as L1495. Unless the line-of-sight is close to the mean field direction, the large-scale magnetic field and striations in the simulation are found roughly perpendicular to the long axis of the main cloud, similar to L1495. This provides strong support for a large-scale moderately strong magnetic field surrounding L1495. We find that the projection effect from observations can lead to incorrect interpretations of the true three-dimensional physical shape, size, and velocity structure of the clouds. Helical magnetic field structures found around filamentary clouds that are interpreted from Zeeman observations can be explained by a simple bending of the magnetic field that pierces through the cloud. We demonstrate that two dark clouds form a T-shaped configuration that is strikingly similar to the infrared dark cloud SDC13, leading to the interpretation that SDC13 results from a collision of two long filamentary clouds. We show that a moderately strong magnetic field (${{\cal M}_{\rm A}}\sim 1$) is crucial for maintaining a long and slender filamentary cloud for a long period of time ∼0.5 Myr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Quantifying the interstellar medium and cosmic rays in the MBM 53, 54, and 55 molecular clouds and the pegasus loop using Fermi-LAT gamma-ray observations

A study of the interstellar medium (ISM) and cosmic rays (CRs) using Fermi Large Area Telescope (LAT) data, in a region encompassing the nearby molecular clouds MBM 53, 54, and 55 and a far-infrared loop-like structure in Pegasus, is reported. By comparing the Planck dust thermal emission model with Fermi-LAT γ-ray data, it was found that neither the dust radiance (R) nor the dust opacity at 353 GHz (τ {sub 353}) was proportional to the total gas column density N(H{sub tot}) primarily because N(H{sub tot})/R and N(H{sub tot})/τ {sub 353} depend on the dust temperature (T {sub d}). The N(H{sub tot}) distribution was evaluated using γ-ray data by assuming the regions of high T {sub d} to be dominated by optically thin atomic hydrogen (H I) and by employing an empirical linear relation of N(H{sub tot})/R to T {sub d}. It was determined that the mass of the gas not traced by the 21 cm or 2.6 mm surveys is ∼25% of the mass of H I in the optically thin case and is larger than the mass of the molecular gas traced by carbon monoxide by a factor of up to 5. The measured γ-ray emissivity spectrum is consistent with a model based on CR spectra measured at the Earth and a nuclear enhancement factor of ≤1.5. It is, however, lower than local H I emissivities reported by previous Fermi-LAT studies employing different analysis methods and assumptions on ISM properties by 15%–20% in energies below a few GeV, even if we take account of the statistical and systematic uncertainties. The origin of the discrepancy is also discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

FORMATION OF MASSIVE MOLECULAR CLOUD CORES BY CLOUD-CLOUD COLLISION

Recent observations of molecular clouds around rich massive star clusters including NGC 3603, Westerlund 2, and M20 revealed that the formation of massive stars could be triggered by a cloud-cloud collision. By using three-dimensional, isothermal, magnetohydrodynamics simulations with the effect of self-gravity, we demonstrate that massive, gravitationally unstable, molecular cloud cores are formed behind the strong shock waves induced by cloud-cloud collision. We find that the massive molecular cloud cores have large effective Jeans mass owing to the enhancement of the magnetic field strength by shock compression and turbulence in the compressed layer. Our results predict that massive molecular cloud cores formed by the cloud-cloud collision are filamentary and threaded by magnetic fields perpendicular to the filament.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Magnetohydrodynamic simulations of a jet drilling an H I cloud: Shock induced formation of molecular clouds and jet breakup

The formation mechanism of the jet-aligned CO clouds found by NANTEN CO observations is studied by magnetohydrodynamical (MHD) simulations taking into account the cooling of the interstellar medium. Motivated by the association of the CO clouds with the enhancement of H I gas density, we carried out MHD simulations of the propagation of a supersonic jet injected into the dense H I gas. We found that the H I gas compressed by the bow shock ahead of the jet is cooled down by growth of the cooling instability triggered by the density enhancement. As a result, a cold dense sheath is formed around the interface between the jet and the H I gas. The radial speed of the cold, dense gas in the sheath is a few km s{sup –1} almost independent of the jet speed. Molecular clouds can be formed in this region. Since the dense sheath wrapping the jet reflects waves generated in the cocoon, the jet is strongly perturbed by the vortices of the warm gas in the cocoon, which breaks up the jet and forms a secondary shock in the H I-cavity drilled by the jet. The particle acceleration at the shock can be the origin of radio and X-ray filaments observed near the eastern edge of the W50 nebula surrounding the galactic jet source SS433.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The Spitzer survey of interstellar clouds in the gould belt. VI. The Auriga-California molecular cloud observed with IRAC and MIPS

We present observations of the Auriga-California Molecular Cloud (AMC) at 3.6, 4.5, 5.8, 8.0, 24, 70, and 160 μm observed with the IRAC and MIPS detectors as part of the Spitzer Gould Belt Legacy Survey. The total mapped areas are 2.5 deg{sup 2} with IRAC and 10.47 deg{sup 2} with MIPS. This giant molecular cloud is one of two in the nearby Gould Belt of star-forming regions, the other being the Orion A Molecular Cloud (OMC). We compare source counts, colors, and magnitudes in our observed region to a subset of the SWIRE data that was processed through our pipeline. Using color-magnitude and color-color diagrams, we find evidence for a substantial population of 166 young stellar objects (YSOs) in the cloud, many of which were previously unknown. Most of this population is concentrated around the LkHα 101 cluster and the filament extending from it. We present a quantitative description of the degree of clustering and discuss the relative fraction of YSOs in earlier (Class I and F) and later (Class II) classes compared to other clouds. We perform simple SED modeling of the YSOs with disks to compare the mid-IR properties to disks in other clouds and identify 14 classical transition disk candidates. Although the AMC is similar in mass, size, and distance to the OMC, it is forming about 15-20 times fewer stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

QUANTIFYING OBSERVATIONAL PROJECTION EFFECTS USING MOLECULAR CLOUD SIMULATIONS

The physical properties of molecular clouds are often measured using spectral-line observations, which provide the only probes of the clouds' velocity structure. It is hard, though, to assess whether and to what extent intensity features in position-position-velocity (PPV) space correspond to 'real' density structures in position-position-position (PPP) space. In this paper, we create synthetic molecular cloud spectral-line maps of simulated molecular clouds, and present a new technique for measuring the reality of individual PPV structures. Using a dendrogram algorithm, we identify hierarchical structures in both PPP and PPV space. Our procedure projects density structures identified in PPP space into corresponding intensity structures in PPV space and then measures the geometric overlap of the projected structures with structures identified from the synthetic observation. The fractional overlap between a PPP and PPV structure quantifies how well the synthetic observation recovers information about the three-dimensional structure. Applying this machinery to a set of synthetic observations of CO isotopes, we measure how well spectral-line measurements recover mass, size, velocity dispersion, and virial parameter for a simulated star-forming region. By disabling various steps of our analysis, we investigate how much opacity, chemistry, and gravity affect measurements of physical properties extracted from PPV cubes. For the simulations used here, which offer a decent, but not perfect, match to the properties of a star-forming region like Perseus, our results suggest that superposition induces a ∼40% uncertainty in masses, sizes, and velocity dispersions derived from {sup 13}CO (J = 1-0). As would be expected, superposition and confusion is worst in regions where the filling factor of emitting material is large. The virial parameter is most affected by superposition, such that estimates of the virial parameter derived from PPV and PPP information typically disagree by a factor of ∼2. This uncertainty makes it particularly difficult to judge whether gravitational or kinetic energy dominate a given region, since the majority of virial parameter measurements fall within a factor of two of the equipartition level α ∼ 2.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SPECTRAL LINE SURVEY TOWARD MOLECULAR CLOUDS IN THE LARGE MAGELLANIC CLOUD

Spectral line survey observations of seven molecular clouds in the Large Magellanic Cloud (LMC) have been conducted in the 3 mm band with the Mopra 22 m telescope to reveal chemical compositions in low metallicity conditions. Spectral lines of fundamental species such as CS, SO, CCH, HCN, HCO{sup +}, and HNC are detected in addition to those of CO and {sup 13}CO, while CH{sub 3}OH is not detected in any source and N{sub 2}H{sup +} is marginally detected in two sources. The molecular-cloud scale (10 pc scale) chemical composition is found to be similar among the seven sources regardless of different star formation activities, and hence, it represents the chemical composition characteristic of the LMC without influences by star formation activities. In comparison with chemical compositions of Galactic sources, the characteristic features are (1) deficient N-bearing molecules, (2) abundant CCH, and (3) deficient CH{sub 3}OH. Feature (1) is due to a lower elemental abundance of nitrogen in the LMC, whereas features (2) and (3) seem to originate from extended photodissociation regions and warmer temperature in cloud peripheries due to a lower abundance of dust grains in the low metallicity condition. In spite of general resemblance of chemical abundances among the seven sources, the CS/HCO{sup +} and SO/HCO{sup +} ratios are found to be slightly higher in a quiescent molecular cloud. An origin of this trend is discussed in relation to possible depletion of sulfur along the molecular cloud formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

H{sub 2}, CO, and Dust Absorption through Cold Molecular Clouds

The abundance of H{sub 2} in molecular clouds, relative to the commonly used tracer CO, has only been measured toward a few embedded stars, which may be surrounded by atypical gas. We present observations of near-infrared absorption by H{sub 2}, CO, and dust toward stars behind molecular clouds, providing a representative sample of these molecules in cold molecular gas, primarily in the Taurus Molecular Cloud. We find N{sub H{sub 2}}/A{sub V} ≈ 1.0 × 10{sup 21} cm{sup −2}, N{sub CO}/A{sub V} ≈ 1.5 × 10{sup 17} cm{sup −2} (1.8 × 10{sup 17} including solid CO), and N{sub H{sub 2}}/N{sub CO} ≈ 6000. The measured N{sub H{sub 2}}/N{sub CO} ratio is consistent with that toward embedded stars in various molecular clouds, but both are less than that derived from millimeter-wave observations of CO and star counts. The difference apparently results from the higher directly measured N{sub CO}/A{sub V} ratio.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A large catalog of accurate distances to molecular clouds from PS1 photometry

Distance measurements to molecular clouds are important but are often made separately for each cloud of interest, employing very different data and techniques. We present a large, homogeneous catalog of distances to molecular clouds, most of which are of unprecedented accuracy. We determine distances using optical photometry of stars along lines of sight toward these clouds, obtained from PanSTARRS-1. We simultaneously infer the reddenings and distances to these stars, tracking the full probability distribution function using a technique presented in Green et al. We fit these star-by-star measurements using a simple dust screen model to find the distance to each cloud. We thus estimate the distances to almost all of the clouds in the Magnani et al. catalog, as well as many other well-studied clouds, including Orion, Perseus, Taurus, Cepheus, Polaris, California, and Monoceros R2, avoiding only the inner Galaxy. Typical statistical uncertainties in the distances are 5%, though the systematic uncertainty stemming from the quality of our stellar models is about 10%. The resulting catalog is the largest catalog of accurate, directly measured distances to molecular clouds. Our distance estimates are generally consistent with available distance estimates from the literature, though in some cases the literature estimates are off by a factor of more than two.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SUBMILLIMETER POLARIZATION SPECTRUM IN THE VELA C MOLECULAR CLOUD

Polarization maps of the Vela C molecular cloud were obtained at 250, 350, and 500 μ m during the 2012 flight of the balloon-borne telescope BLASTPol. These measurements are used in conjunction with 850 μ m data from Planck to study the submillimeter spectrum of the polarization fraction for this cloud. The spectrum is relatively flat and does not exhibit a pronounced minimum at λ ∼ 350 μ m as suggested by previous measurements of other molecular clouds. The shape of the spectrum does not depend strongly on the radiative environment of the dust, as quantified by the column density or the dust temperature obtained from Herschel data. The polarization ratios observed in Vela C are consistent with a model of a porous clumpy molecular cloud being uniformly heated by the interstellar radiation field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Planck intermediate results: XXXV. Probing the role of the magnetic field in the formation of structure in molecular clouds

Within ten nearby (d < 450 pc) Gould belt molecular clouds we evaluate in this paper statistically the relative orientation between the magnetic field projected on the plane of sky, inferred from the polarized thermal emission of Galactic dust observed by Planck at 353 GHz, and the gas column density structures, quantified by the gradient of the column density, N H . The selected regions, covering several degrees in size, are analysed at an effective angular resolution of 10' FWHM, thus sampling physical scales from 0.4 to 40 pc in the nearest cloud. The column densities in the selected regions range from N H ≈ 10 21 to10 23 cm -2 , and hence they correspond to the bulk of the molecular clouds. The relative orientation is evaluated pixel by pixel and analysed in bins of column density using the novel statistical tool called “histogram of relative orientations”. Throughout this study, we assume that the polarized emission observed by Planck at 353 GHz is representative of the projected morphology of the magnetic field in each region, i.e., we assume a constant dust grain alignment efficiency, independent of the local environment. Within most clouds we find that the relative orientation changes progressively with increasing N H , from mostly parallel or having no preferred orientation to mostly perpendicular. In simulations of magnetohydrodynamic turbulence in molecular clouds this trend in relative orientation is a signature of Alfvénic or sub-Alfvénic turbulence, implying that the magnetic field is significant for the gas dynamics at the scales probed by Planck. Finally, we compare the deduced magnetic field strength with estimates we obtain from other methods and discuss the implications of the Planck observations for the general picture of molecular cloud formation and evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The giant molecular cloud environments of infrared dark clouds

We study giant molecular cloud (GMC) environments surrounding 10 infrared dark clouds (IRDCs), using {sup 13}CO(1−0) emission from the Galactic Ring Survey. We measure physical properties of these IRDCs/GMCs on a range of scales extending to radii, R, of 30 pc. By comparing different methods for defining cloud boundaries and for deriving mass surface densities and velocity dispersions, we settle on a preferred “CE,τ,G” method of “Connected Extraction” in position–velocity space plus Gaussian fitting to opacity-corrected line profiles for velocity dispersion and mass estimation. We examine how cloud definition affects measurements of the magnitude and direction of line-of-sight velocity gradients and velocity dispersions, including associated dependencies on size scale. CE,τ,G-defined GMCs show velocity dispersion versus size relations σ ∝ s{sup 1/2}, which are consistent with the large-scale gradients being caused by turbulence. However, IRDCs have velocity dispersions that are moderately enhanced above those predicted by this scaling relation. We examine the dynamical state of the clouds, finding mean virial parameters α-bar {sub vir}≃1.0 for GMCs and 1.6 for IRDCs, broadly consistent with models of magnetized virialized pressure-confined polytropic clouds, but potentially indicating that IRDCs have more disturbed kinematics. CE,τ, G-defined clouds exhibit a tight correlation of σ/R{sup 1/2}∝Σ{sup n}, with n ≃ 0.7 for GMCs and 1.3 for IRDCs (cf. a value of 0.5 expected for a population of virialized clouds). We conclude that while GMCs show evidence for virialization over a range of scales, IRDCs may be moderately supervirial. Alternatively, IRDCs could be virialized but have systematically different {sup 13}CO gas-phase abundances, i.e., owing to freeze-out, affecting mass estimations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Colliding filaments and a massive dense core in the Cygnus OB 7 molecular cloud

We report the results of molecular line observations carried out toward a massive dense core in the Cyg OB 7 molecular cloud. The core has an extraordinarily large mass (∼1.1 × 10{sup 4} M {sub ☉}) and size (∼2 × 5 pc{sup 2}), but there is no massive young star forming therein. We observed this core in various molecular lines such as C{sup 18}O(J = 1-0) using the 45 m telescope at Nobeyama Radio Observatory. We find that the core has an elongated morphology consisting of several filaments and core-like structures. The filaments are massive (10{sup 2}-10{sup 3} M {sub ☉}), and they are apparently colliding with one another. Some candidates for young stellar objects are distributed around their intersection, suggesting that the collisions of the filaments may have influenced their formation. To understand the formation and evolution of such colliding filaments, we performed numerical simulations using the adaptive mesh refinement technique, adopting the observed core parameters (the mass and size) as the initial conditions. The results indicate that the filaments are formed as seen in other earlier simulations for small cores in the literature, but we could not reproduce the collisions of the filaments simply by assuming a large initial mass and size. We find that collisions of the filaments occur only when there is a large velocity gradient in the initial core, in a sense compressing it. We suggest that the observed core was actually compressed by an external effect, e.g., shocks from nearby supernova remnants, including HB 21 which has been suggested to be interacting with the Cyg OB 7 molecular cloud.

79 ASTRONOMY AND ASTROPHYSICS↗