2MASS wide field extinction maps

We present an extinction map of a ~1700 deg sq region that encloses the Ophiuchus, the Lupus, and the Pipe dark complexes using 42 million stars from the Two Micron All Sky Survey (2MASS) point source catalog. The use of a robust and optimal near-infrared method to map dust column density (nicer, described in Lombardi & Alves 2001, A&A, 377, 1023) allow us to detect extinction as low as AK = 0.05 mag with a 2-σ significance, and still to have a resolution of 3 arcmin on our map. We also present a novel, statistically sound method to characterize the small-scale inhomogeneities in molecular clouds. Finally, we investigate the cloud structure function, and show that significant deviations from the results predicted by turbulent models are observed.

We present a 8 deg x 6 deg, high resolution extinction map of the Pipe nebula using 4.5 million stars from the Two Micron All Sky Survey (2MASS) point source catalog. The use of NICER, a robust and optimal technique to map the dust column density, allows us to detect a Av = 0.5 mag extinction at a 3-sigma level with a 1 arcmin resolution. We find for the Pipe nebula a normal reddening law, E(J-H) = (1.85 +/- 0.15) E(H-K). We measure the cloud distance using Hipparchos and Tycho parallaxes, and obtain ~130 pc. This, together with the total estimated mass, 10^4 Msun, makes the Pipe the closest massive cloud complex to Earth. We compare the NICER extinction map to the NANTEN 12CO observations and derive with unprecedented accuracy the relationship between the near-infrared extinction and the 12CO column density and hence (indirectly) the 12CO X-factor, that we estimate to be 2.91 10^20 cm^-2 K^-1 km^-1 s in the range Av <- [0.9, 5.4] mag. We identify ~1500 OH/IR stars located within the Galactic bulge in the direction of the Pipe field. This represents a significant increase of the known numbers of such stars in the Galaxy. Our analysis confirms the power and simplicity of the color excess technique to study molecular clouds. The comparison with the NANTEN 12CO data corroborates the insensitivity of CO observations to low column densities (up to approximately 2 mag in Av), and shows also an irreducible uncertainty in the dust-CO correlation of about 1 mag of visual extinction.

In the last years substantial progress has been made in modelling turbulent clouds and describing their structure by characteristic parameters. The missing link for a systematic comparison between models and observations is the lack of ecient radiative transfer algorithms to generate molecular line maps from the models comparable to the observed maps. A fully self- consistent solution of the radiative transfer problem is computationally very demanding and hardly suited to evaluate a large set of cloud models with regard to their agreement with observed molecular cloud structures. We introduce a new, computationally ecient code to calculate the line profiles based on two reasonable approximations. It is able to compute the molecular line maps in turbulent cloud models with an accuracy of about 20% fast enough to be run on large sets of model clouds. Applying the code to hydrodynamic, and magnetohydrodynamic cloud models we study how their structure would appear in molecular line observations. We show that no single molecular line provides a good measure for the density structure in the models. The X factor, translating the integrated line intensities into column densities, can be approximately constant within a density range covering up to a factor 100 in few transitions but for each line this behaviour breaks down outside of a limited range of densities. Optical depth eects and subthermal excitation result in a significant modification of the distribution of line intensities relative to the column density distribution. All lower transitions of CO isotopes only trace gas at low and intermediate densities which is distributed over all scales in molecular clouds. Turbulence models driven on the largest scales reproduce the observed scaling behaviour. Higher CO tran- sitions are only excited in dense cores resulting from shocks or gravitational collapse. The existence of massive dense cores resulting from collapse can only be inferred when comparing observations in dierent transitions taken with an excellent signal-to-noise ratio or from dedicated high-density tracers. The line profiles obtained from turbulence models driven on large scales break up into several fragments in contrast to obser- vations of molecular clouds without heavy star-formation which show typically smooth profiles with close-to-Gaussian shape. None of the turbulence simulations provides a good match of all observed properties for this type of clouds. The velocity scaling behaviour of all observations and turbulence models is consistent with the interpretation of a molecular cloud as shock- dominated medium. More observational data are needed to provide a reliable conclusion on the degree of intermittency. As molecular lines fail to reflect the density structure of an interstellar cloud line observations should be combined with dust continuum observations to deduce column densities. On the other hand we need the velocity information contained in line observations to discriminate between dierent turbulence models.

We use the COMPLETE Survey's observations of the Perseus star-forming region to assess and intercompare three methods for measuring column density in molecular clouds: extinction mapping (NIR); thermal emission mapping (FIR); and mapping the intensity of CO isotopologues. The structures shown by all three tracers are morphologically similar, but important differences exist. Dust-based measures give similar, log-normal, distributions for the full Perseus region, once careful calibration corrections are made. We also compare dust- and gas-based column density distributions for physically-meaningful sub-regions of Perseus, and we find significant variations in the distributions for those regions. Even though we have used 12CO data to estimate excitation temperatures, and we have corrected for opacity, the 13CO maps seem unable to give column distributions that consistently resemble those from dust measures. We have edited out the effects of the shell around the B-star HD 278942. In that shell's interior and in the parts where it overlaps the molecular cloud, there appears to be a dearth of 13CO, likely due either to 13CO not yet having had time to form in this young structure, and/or destruction of 13CO in the molecular cloud. We conclude that the use of either dust or gas measures of column density without extreme attention to calibration and artifacts is more perilous than even experts might normally admit. And, the use of 13CO to trace total column density in detail, even after proper calibration, is unavoidably limited in utility due to threshold, depletion, and opacity effects. If one's main aim is to map column density, then dust extinction seems the best probe. Linear fits amongst column density tracers are given, quantifying the inherent uncertainties in using one tracer (when compared with others). [abridged]

Aims: We derive the probability density functions (PDFs) of column density for a complete sample of prominent molecular cloud complexes closer than 200 pc. Methods: We derive near-infrared dust extinction maps for 23 molecular cloud complexes, using the "nicest" colour excess mapping technique and data from the 2MASS archive. The extinction maps are then used to examine the column density PDFs in the clouds. Results: The column density PDFs of most molecular clouds are well-fitted by log-normal functions at low column densities (0.5 mag < A_v < 3-5 mag). However, at higher column densities prominent, power-law-like wings are common. In particular, we identify a trend among the PDFs: active star-forming clouds always have prominent non-log-normal wings. In contrast, clouds without active star formation resemble log-normals over the whole observed column density range, or show only low excess of higher column densities. This trend is also reflected in the cumulative PDFs, showing that the fraction of high column density material is significantly larger in star-forming clouds. These observations are in agreement with an evolutionary trend where turbulent motions are the main cloud-shaping mechanism for quiescent clouds, but the density enhancements induced by them quickly become dominated by gravity (and other mechanisms) which is strongly reflected by the shape of the column density PDFs. The dominant role of the turbulence is restricted to the very early stages of molecular cloud evolution, comparable to the onset of active star formation in the clouds.

We present a near-infrared extinction map of a large region (approximately 2200 deg^2) covering the Orion, the Monoceros R2, the Rosette, and the Canis Major molecular clouds. We used robust and optimal methods to map the dust column density in the near-infrared (NICER and NICEST) towards ~19 million stars of the Two Micron All Sky Survey (2MASS) point source catalog. Over the relevant regions of the field, we reached a 1-sigma error of 0.03 mag in the K-band extinction with a resolution of 3 arcmin. We measured the cloud distances by comparing the observed density of foreground stars with the prediction of galactic models, thus obtaining d_{Orion A} = (371 +/- 10) pc, d_{Orion B} = (398 +/- 12) pc, $d_{Mon R2} = (905 +/- 37) pc, $d_{Rosette} = (1330 +/- 48) pc, and $d_{CMa} = (1150 +/- 64) pc, values that compare very well with independent estimates.

We have created new dust temperature and column density maps of Perseus, Ophiuchus, and Serpens using 60 and 100 μm data from the Improved Reprocessing of the IRAS Survey (IRIS) recalibration of Infrared Astronomical Satellite (IRAS) data. We describe an optimized method for finding the dust temperature, emissivity spectral index, and optical depth using optical and near-infrared extinction maps. The creation of these temperature and extinction maps (covering tens of square degrees of molecular clouds) is one of the first results from the ongoing Coordinated Molecular Probe Line Extinction Thermal Emission (COMPLETE) Survey of Star-Forming Regions. However, while the extinctions derived from the IRIS emission maps are globally accurate, we warn that far-infrared emission is not a good proxy for extinction on the scale of 1 pixel (~5'). In addition to describing the global dust properties of these clouds, we have found two particularly interesting features in the column density and temperature maps. In the Ophiuchus dark cloud complex, the new dust temperature map shows a little-known warm (25 K) dust ring with a 2 pc diameter. This shell is approximately centered on the B star ρ Ophiuchus, 1° north of the well-studied ρ Oph star-forming cluster. In Perseus, the column density map shows a 10 pc diameter ring, a feature not apparent in the filamentary chain of clouds seen in molecular gas. These rings are further discussed in detail in our companion papers.

We present an extinction map of the inner ∼15′ by 16′ of the Galactic center (GC) with map pixels measuring 5″ × 5″ using integrated light color measurements in the near- and mid-infrared. We use a variant of the Rayleigh–Jeans color excess (RJCE) method first described by Majewski et al. as the basis of our work, although we have approached our problem with a Bayesian mindset and dispensed with point-source photometry in favor of surface photometry, turning the challenge of the extremely crowded field at the GC into an advantage. Our results show that extinction at the GC is not inconsistent with a single power-law coefficient, β = 2.03 ± 0.06, and compare our results with those using the red clump (RC) point-source photometry method of extinction estimation. We find that our measurement of β and its apparent lack of spatial variation are in agreement with prior studies, despite the bimodal distribution of values in our extinction map at the GC with peaks at 5 and 7.5 mag. This bimodal nature of extinction is likely due to the infrared dark clouds that obscure portions of the inner GC field. We present our extinction law and map of the GC region using the point-source catalog of infrared sources compiled by DeWitt et al. The dereddening is limited by the error in the extinction measurement (typically 0.6 mag), which is affected by the size of our map pixels and is not fine-grained enough to separate out the multiple stellar populations present toward the GC.

We have used a numerical simulation of a turbulent cloud to synthesize maps of the thermal emission from dust at a variety of far-IR and submillimeter wavelengths. The average column density and external radiation field in the simulation is well matched to clouds such as Perseus and Ophiuchus. We use pairs of single-wavelength emission maps to derive the dust color temperature and column density, and we compare the derived column densities with the true column density. We demonstrate that longer wavelength emission maps yield less biased estimates of column density than maps made toward the peak of the dust emission spectrum. We compare the scatter in the derived column density with the observed scatter in Perseus and Ophiuchus. We find that while in Perseus all of the observed scatter in the emission-derived versus the extinction-derived column density can be attributed to the flawed assumption of isothermal dust along each line of sight, in Ophiuchus there is additional scatter above what can be explained by the isothermal assumption. Our results imply that variations in dust emission properties within a molecular cloud are not necessarily a major source of uncertainty in column density measurements.

Inlet boundary conditions at the supply opening play an important role in the accuracy and reliability of computational fluid dynamics (CFD) simulations for indoor airflow. A non-exhaustive overview of past CFD simulations of ventilation flow in generic enclosures indicates that uniform hypotheses of inlet boundary conditions are commonly used. However, due to thermal effects and a complex air supply system geometry, constant values of inlet airflow quantities can be insufficient for an accurate simulation of non-isothermal ventilation flow. In addition, this can lead to biased conclusions on the performance of turbulence models and other computational settings. To assess this issue, a well-documented experiment on non-isothermal mechanical ventilation with detailed information of inlet conditions from the literature is used in this study. Different turbulence models and inlet boundary conditions are validated, including seven steady Reynolds-averaged Navier-Stokes (RANS) models, six methods of imposing inlet air velocity, three methods of specifying air temperature, and a wide range of turbulence quantities. The parametric study demonstrates that in this particular case, i.e., cold air supply from a single round nozzle diffuser close to the ceiling, ω-based turbulence models perform better than the ε-based turbulence models. The impacts of different methods to specify inlet air temperature profile and turbulence quantities on the simulation results are not significant. However, if the inlet airflow direction is not taken into account appropriately, significant deviations are observed between CFD simulations and experimental data.

We studied the H2 column density probability distribution function (N-PDF) based on molecular emission lines using the Nobeyama 45-m Cygnus X CO survey data. Using the DENDROGRAM and SCIMES algorithms, we identified 124 molecular clouds in the 13CO data. From these identified molecular clouds, an N-PDF was constructed for 11 molecular clouds with an extent of more than 0.4 deg2. From the fitting of the N-PDF, we found that the N-PDF could be well fitted with one or two lognormal distributions. These fitting results provided an alternative density structure for molecular clouds from a conventional picture. We investigated the column density, dense molecular cloud cores, and radio continuum source distributions in each cloud and found that the N-PDF shape was less correlated with the star-forming activity over a whole cloud. Furthermore, we found that the lognormal N-PDF parameters obtained from the fitting showed two impressive features. First, the lognormal distribution at the low-density part had the same mean column density (∼1021.5 cm−2) for almost all the molecular clouds. Second, the width of the lognormal distribution tended to decrease with an increasing mean density of the structures. These correlations suggest that the shape of the N-PDF reflects the relationship between the density and turbulent structure of the whole molecular cloud but is less affected by star-forming activities.

The lognormal distribution of metal resources in mineral deposits

\n Context. Infrared dark clouds (IRDCs) harbor progenitors of high-mass stars. Little is known of the parental molecular clouds of IRDCs.\n Aims. We demonstrate the feasibility of the near-infrared (NIR) dust extinction mapping in tracing the parental molecular clouds of IRDCs at the distances of D ≈ 2.5−8 kpc. \n Methods. We derive NIR extinction maps for 10 prominent IRDC complexes using a color-excess mapping technique and NIR data from the UKIDSS/Galactic Plane Survey. We compare the resulting maps to the 13CO emission line data, to the 8 μm dust opacity data, and to the millimeter dust emission data. We derive distances for the clouds by comparing the observed NIR source densities to the Besançon stellar distribution model and compare them to the kinematic distance estimates.\n Results. The NIR extinction maps provide a view of the IRDC complexes over the dynamical range of AV ≈ 2 − 40 mag, in spatial resolution of ~30″. The NIR extinction data correlate well with the 13CO data and probe a similar gas component, but also extend to higher column densities. The NIR data reveal a wealth of extended structures surrounding the dense gas traced by the 8 μm shadowing features and sub-mm dust emission, showing that the clouds contain typically ≳ 10 times more mass than traced by those tracers. The IRDC complexes of our sample contain a relatively high amount of high-column density material, and their cumulative column density distributions resemble active nearby star-forming clouds like Orion rather than less active clouds like California. \n Conclusions. The NIR dust extinction data provide a new powerful tool to probe the mass distribution of the parental molecular clouds of IRDCs up to the distances of D ~ 8 kpc. This encourages deeper NIR observations of IRDCs, because the sensitivity and resolution of the data can be directly enhanced with dedicated observations. In addition to mass distributions, the NIR data provide relatively reliable distance estimates.\n

We have conducted a large-field simultaneous survey of 12CO, 13CO and C18O J = 1 – 0 emission toward the Orion A giant molecular cloud (GMC) with a sky coverage of ∼4.4 deg2 using the Purple Mountain Observatory (PMO)-13.7 m millimeter-wavelength telescope. We use the probability distribution function of the column density (N-PDF) to investigate the distribution of molecular hydrogen in the Orion A GMC. The H2 column density, derived from the 13CO emission, of the GMC is dominated by a log-normal distribution in the range from ∼ 4 × 1021 to ∼ 1.5 × 1023 cm−2 with excesses both at the low-density and high-density ends. The excess of the low-density end is possibly caused by an extended and low-temperature (∼10 K) component with velocities in the range of 5 – 8 km s−1. Compared with the northern sub-regions, the southern sub-regions of the Orion A GMC contain less gas with column density in NH2 > 1.25 × 1022 cm−2. The dispersions of the N-PDFs of the sub-regions are found to correlate with the evolutionary stages of the clouds across the Orion A GMC. The structure hierarchy of Orion A GMC is explored with the DENDROGRAM algorithm, and it is found that the GMC is composed of two branches. All structures except one in the tree have virial parameters less than 2, indicating self-gravity is important on the spatial scales from ∼0.3 to ∼4 pc. Although power-laws and departures from lognormal distributions are found at the high-density end of N-PDFs for active star-forming regions, the N-PDFs of structures in the Orion A GMC are predominantly lognormal on scales from R∼0.4 to 4 pc.

We have obtained column density maps for an unbiased sample of 120 molecular clouds in the third quadrant of the Milky Way midplane (b ≤ ∣5∣°) within the Galactic longitude range from 195° to 225°, using the high-sensitivity 12CO and 13CO (J = 1 − 0) data from the Milky Way Imaging Scroll Painting (MWISP) project. The probability density functions of the molecular hydrogen column density of the clouds, N-pdfs, are fitted with both a lognormal (LN) function and a lognormal plus power-law (LN+PL) function. The molecular clouds are classified into three categories according to their shapes of N-pdfs, i.e., LN, LN+PL, and UN (unclear). About 72% of the molecular clouds fall into the LN category, while 18% and 10% fall into the LN+PL and UN categories, respectively. A PL scaling relation, , exists between the width of the N-pdf, σ s , and the average column density, , of the molecular clouds. However, σ s shows no correlation with the mass of the clouds. A correlation is found between the dispersion of normalized column density, σ N/〈N〉, and the sonic Mach number, , of molecular clouds. Overall, as predicted by numerical simulations, the N-pdfs of the molecular clouds with active star formation activity tend to have N-pdfs with PL high-density tails.