Ammonia clumps in the Orion and Cepheus clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900199292 ◽

1991 ◽

Vol 147 ◽

pp. 436-437

Author(s):

J. Harju ◽

C.M. Walmsley ◽

J.G.A. Wouterloot

Keyword(s):

Density Distribution ◽

Column Density ◽

Velocity Dispersion ◽

Line Emission ◽

Main Interest ◽

Molecular Line ◽

Dynamical State

We present statistics of clump properties in the Orion and Cepheus cloud complexes based on ammonia mapping observations. Surroundings of about 50 IRAS sources earlier found to have associated molecular line emission (Wouterloot, Walmsley and Henkel, 1988) were mapped in NH3(1,1) and (2,2) with the Effelsberg 100-m telescope. Our main interest has been in determining the clump sizes and masses on the basis of the ammonia column density distribution, which together with the observed velocity dispersion lead to a rough estimate of the dynamical state. We also have studied the star-clump separations which should give us estimates of the source ages. Special attention has been paid to comparison of our Orion data with the Benson and Myers (1989, hereafter BM89) results in Taurus because the linear resolutions in the two studies are similar.

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Understanding biases in measurements of molecular cloud kinematics using line emission

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2432 ◽

2020 ◽

Vol 498 (2) ◽

pp. 2440-2455

Author(s):

Yuxuan (宇轩) Yuan (原) ◽

Mark R Krumholz ◽

Blakesley Burkhart

Keyword(s):

Magnetic Field ◽

Molecular Cloud ◽

Molecular Clouds ◽

Velocity Dispersion ◽

Signal To Noise Ratio ◽

Line Emission ◽

Molecular Line ◽

Velocity Dispersions ◽

Cloud Kinematics ◽

Measurement Biases

ABSTRACT Molecular line observations using a variety of tracers are often used to investigate the kinematic structure of molecular clouds. However, measurements of cloud velocity dispersions with different lines, even in the same region, often yield inconsistent results. The reasons for this disagreement are not entirely clear, since molecular line observations are subject to a number of biases. In this paper, we untangle and investigate various factors that drive linewidth measurement biases by constructing synthetic position–position–velocity cubes for a variety of tracers from a suite of self-gravitating magnetohydrodynamic simulations of molecular clouds. We compare linewidths derived from synthetic observations of these data cubes to the true values in the simulations. We find that differences in linewidth as measured by different tracers are driven by a combination of density-dependent excitation, whereby tracers that are sensitive to higher densities sample smaller regions with smaller velocity dispersions, opacity broadening, especially for highly optically thick tracers such as CO, and finite resolution and sensitivity, which suppress the wings of emission lines. We find that, at fixed signal-to-noise ratio, three commonly used tracers, the J = 4 → 3 line of CO, the J = 1 → 0 line of C18O, and the (1,1) inversion transition of NH3, generally offer the best compromise between these competing biases, and produce estimates of the velocity dispersion that reflect the true kinematics of a molecular cloud to an accuracy of $\approx 10{{\ \rm per\ cent}}$ regardless of the cloud magnetic field strengths, evolutionary state, or orientations of the line of sight relative to the magnetic field. Tracers excited primarily in gas denser than that traced by NH3 tend to underestimate the true velocity dispersion by $\approx 20{{\ \rm per\ cent}}$ on average, while low-density tracers that are highly optically thick tend to have biases of comparable size in the opposite direction.

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Selected aspects of the analysis of molecular line observations of edge-on circumbinary disks

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935610 ◽

2020 ◽

Vol 642 ◽

pp. A127

Author(s):

R. Avramenko ◽

S. Wolf ◽

T. F. Illenseer ◽

S. Rehberg

Keyword(s):

Density Distribution ◽

Line Emission ◽

Density Waves ◽

Molecular Line ◽

Hydrodynamic Simulations ◽

Spatially Resolved ◽

Transition Area ◽

Multi Wavelength ◽

Inner Region ◽

Innermost Region

Context. Inner cavities, accretion arms, and density waves are characteristic structures in the density distribution of circumbinary disks. They are the result of the tidal interaction of the non-axisymmetric gravitational forces of the central binary with the surrounding disk and are most prominent in the inner region, where the asymmetry is most pronounced. Aims. The goal of this study is to test the feasibility of reconstructing the gas density distribution and quantifying properties of structures in the inner regions of edge-on circumbinary disks using multiple molecular line observations. Methods. The density distribution in circumbinary disks is calculated with 2D hydrodynamic simulations. Subsequently, molecular line emission maps are generated with 3D radiative transfer simulations. Based on these, we investigate the observability of characteristic circumbinary structures located in the innermost region for spatially resolved and unresolved disks. Results. We find that it is possible to reconstruct the inner cavity, accretion arms, and density waves from spatially resolved multi-wavelength molecular line observations of circumbinary disks seen edge-on. For the spatially unresolved observations only, an estimate can be derived for the density gradient in the transition area between the cavity and the disk’s inner rim.

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LEGO – II. A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way disc

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1814 ◽

2020 ◽

Vol 497 (2) ◽

pp. 1972-2001

Author(s):

A T Barnes ◽

J Kauffmann ◽

F Bigiel ◽

N Brinkmann ◽

D Colombo ◽

...

Keyword(s):

Column Density ◽

Milky Way ◽

Line Emission ◽

Chemical Properties ◽

Molecular Line ◽

Star Forming ◽

Emission Efficiency ◽

Single Region ◽

High Emission ◽

Molecular Lines

ABSTRACT The current generation of (sub)mm-telescopes has allowed molecular line emission to become a major tool for studying the physical, kinematic, and chemical properties of extragalactic systems, yet exploiting these observations requires a detailed understanding of where emission lines originate within the Milky Way. In this paper, we present 60 arcsec (∼3 pc) resolution observations of many 3 mm band molecular lines across a large map of the W49 massive star-forming region (∼100 pc × 100 pc at 11 kpc), which were taken as part of the ‘LEGO’ IRAM-30m large project. We find that the spatial extent or brightness of the molecular line transitions are not well correlated with their critical densities, highlighting abundance and optical depth must be considered when estimating line emission characteristics. We explore how the total emission and emission efficiency (i.e. line brightness per H2 column density) of the line emission vary as a function of molecular hydrogen column density and dust temperature. We find that there is not a single region of this parameter space responsible for the brightest and most efficiently emitting gas for all species. For example, we find that the HCN transition shows high emission efficiency at high column density (1022 cm−2) and moderate temperatures (35 K), whilst e.g. N2H+ emits most efficiently towards lower temperatures (1022 cm−2; <20 K). We determine $X_{\mathrm{CO} (1-0)} \sim 0.3 \times 10^{20} \, \mathrm{cm^{-2}\, (K\, km\, s^{-1})^{-1}}$, and $\alpha _{\mathrm{HCN} (1-0)} \sim 30\, \mathrm{M_\odot \, (K\, km\, s^{-1}\, pc^2)^{-1}}$, which both differ significantly from the commonly adopted values. In all, these results suggest caution should be taken when interpreting molecular line emission.

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Quantitative inference of the H2 column densities from 3 mm molecular emission: case study towards Orion B

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037871 ◽

2020 ◽

Vol 645 ◽

pp. A27 ◽

Cited By ~ 1

Author(s):

Pierre Gratier ◽

Jérôme Pety ◽

Emeric Bron ◽

Antoine Roueff ◽

Jan H. Orkisz ◽

...

Keyword(s):

Molecular Cloud ◽

Column Density ◽

Global Analysis ◽

Line Emission ◽

Dense Core ◽

Supervised Machine Learning ◽

Physical Parameters ◽

Wide Field ◽

Molecular Line ◽

Core Conditions

Context. Based on the finding that molecular hydrogen is unobservable in cold molecular clouds, the column density measurements of molecular gas currently rely either on dust emission observation in the far-infrared, which requires space telescopes, or on star counting, which is limited in angular resolution by the stellar density. The (sub)millimeter observations of numerous trace molecules can be effective using ground-based telescopes, but the relationship between the emission of one molecular line and the H2 column density is non-linear and sensitive to excitation conditions, optical depths, and abundance variations due to the underlying physico- chemistry. Aims. We aim to use multi-molecule line emission to infer the H2 molecular column density from radio observations. Methods. We propose a data-driven approach to determine the H2 gas column densities from radio molecular line observations. We use supervised machine-learning methods (random forest) on wide-field hyperspectral IRAM-30m observations of the Orion B molecular cloud to train a predictor of the H2 column density, using a limited set of molecular lines between 72 and 116 GHz as input, and the Herschel-based dust-derived column densities as “ground truth” output. Results. For conditions similar to those of the Orion B molecular cloud, we obtained predictions of the H2 column density within a typical factor of 1.2 from the Herschel-based column density estimates. A global analysis of the contributions of the different lines to the predictions show that the most important lines are 13CO(1–0), 12CO(1–0), C18O(1–0), and HCO+(1–0). A detailed analysis distinguishing between diffuse, translucent, filamentary, and dense core conditions show that the importance of these four lines depends on the regime, and that it is recommended that the N2H+(1–0) and CH3OH(20–10) lines be added for the prediction of the H2 column density in dense core conditions. Conclusions. This article opens a promising avenue for advancing direct inferencing of important physical parameters from the molecular line emission in the millimeter domain. The next step will be to attempt to infer several parameters simultaneously (e.g., the column density and far-UV illumination field) to further test the method.

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