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Molecular hydrogen clouds are a key component of the interstellar medium because they are the birthplaces for stars. They are embedded in atomic gas that pervades the interstellar space. However, the details of how molecular clouds assemble from and interact with the atomic gas are still largely unknown. As a result of new observations of the 158~$\mu$m line of ionized carbon CII in the Cygnus region within the FEEDBACK program on SOFIA (Stratospheric Observatory for Infrared Astronomy), we present compelling evidence that CII unveils dynamic interactions between cloud ensembles. This process is neither a head-on collision of fully molecular clouds nor a gentle merging ofonly atomic clouds. Moreover, we demonstrate that the dense molecular clouds associated with the DR21 and W75N star-forming regions and a cloud at higher velocity are embedded in atomic gas and all components interact over a large range of velocities (20 km/s). The atomic gas has a density of 100 cm$^{-3}$ and a temperature of 100 K. We conclude that the CII 158 $\mu$m line is an excellent tracer to witness the processes involved in cloud interactions and anticipate further detections of this phenomenon in other regions, Comment: Nature Astronomy in press

Molecular hydrogen clouds are a key component of the interstellar medium because they are the birthplaces for stars. However, the details of how molecular clouds assemble and interact are still largely unknown. Thanks to new observations of the 158 micron line of ionized carbon [CII] in the Cygnus region within the FEEDBACK program on SOFIA, we present for the first time compelling evidence that [CII] unveils dynamic interactions between cloud ensembles. This process is neither a head-on collision of fully molecular clouds nor a gentle merging of only atomic clouds. Moreover, we demonstrate that the dense molecular clouds associated with the DR21 and W75N star-forming regions and a cloud at higher velocity are embedded in atomic gas and all components interact over a large range of velocities (~20 km/s). The atomic gas has a density of ~100 cm-3 and a temperature of ~100 K. We promote that this type of interaction, solely traced in [CII], is the most common process of molecular cloud formation and anticipate further detections of this phenomenon in other regions.

The aim of the present work is to study structure and gas kinematics in the photodissociation regions (PDRs) around the compact HII regions S235A and S235C. We observe the [CII], [13CII] and [OI] line emission, using SOFIA/upGREAT and complement them by data of HCO+ and CO. We use the [13CII] line to measure the optical depth of the [CII] emission, and find that the [CII] line profiles are influenced by self-absorption, while the [13CII] line remains unaffected by these effects. Hence, for dense PDRs, [13CII] emission is a better tracer of gas kinematics. The optical depth of the [CII] line is up to 10 in S235A. We find an expanding motion of the [CII]-emitting layer of the PDRs into the front molecular layer in both regions. Comparison of the gas and dust columns shows that gas components visible neither in the [CII] nor in low-J CO lines may contribute to the total column across S235A. We test whether the observed properties of the PDRs match the predictions of spherical models of expanding HII region + PDR + molecular cloud. Integrated intensities of the [13CII], [CII] and [OI] lines are well-represented by the model, but the models do not reproduce the double-peaked [CII] line profiles due to an insufficient column density of C+. The model predicts that the [OI] line could be a more reliable tracer of gas kinematics, but the foreground self-absorbing material does not allow using it in the considered regions., Comment: accepted to MNRAS

The mass distribution of dense cores is a potential key to understand the process of star formation. Applying dendrogram analysis to the CARMA-NRO Orion C$^{18}$O ($J$=1--0) data, we identify 2342 dense cores, about 22 \% of which have virial ratios smaller than 2, and can be classified as gravitationally bound cores. The derived core mass function (CMF) for bound starless cores which are not associate with protostars has a slope similar to Salpeter's initial mass function (IMF) for the mass range above 1 $M_\odot$, with a peak at $\sim$ 0.1 $M_\odot$. We divide the cloud into four parts based on the declination, OMC-1/2/3, OMC-4/5, L1641N/V380 Ori, and L1641C, and derive the CMFs in these regions. We find that starless cores with masses greater than 10 $M_\odot$ exist only in OMC-1/2/3, whereas the CMFs in OMC-4/5, L1641N, and L1641C are truncated at around 5--10 $M_\odot$. From the number ratio of bound starless cores and Class II objects in each subregion, the lifetime of bound starless cores is estimated to be 5--30 free-fall times, consistent with previous studies for other regions. In addition, we discuss core growth by mass accretion from the surrounding cloud material to explain the coincidence of peak masses between IMFs and CMFs. The mass accretion rate required for doubling the core mass within a core lifetime is larger than that of Bondi-Hoyle accretion by a factor of order 2. This implies that more dynamical accretion processes are required to grow cores., Comment: 58 pages, 33 figures, 21 tables, accepted by ApJS

We present the first results from a CARMA high-resolution $^{12}$CO(1-0), $^{13}$CO(1-0), and C$^{18}$O(1-0) molecular line survey of the North America and Pelican (NAP) Nebulae. CARMA observations have been combined with single-dish data from the Purple Mountain 13.7m telescope to add short spacings and produce high-dynamic-range images. We find that the molecular gas is predominantly shaped by the W80 HII bubble that is driven by an O star. Several bright rims are probably remnant molecular clouds heated and stripped by the massive star. Matching these rims in molecular lines and optical images, we construct a model of the three-dimensional structure of the NAP complex. Two groups of molecular clumps/filaments are on the near side of the bubble, one being pushed toward us, whereas the other is moving toward the bubble. Another group is on the far side of the bubble and moving away. The young stellar objects in the Gulf region reside in three different clusters, each hosted by a cloud from one of the three molecular clump groups. Although all gas content in the NAP is impacted by feedback from the central O star, some regions show no signs of star formation, while other areas clearly exhibit star formation activity. Other molecular gas being carved by feedback includes the cometary structures in the Pelican Head region and the boomerang features at the boundary of the Gulf region. The results show that the NAP complex is an ideal place for the study of feedback effects on star formation., accepted by AJ

The paper aims to study relation between the distributions of the young stellar objects (YSOs) of different ages and the gas-dust constituents of the S254-S258 star-formation complex. This is necessary to study the time evolution of the YSO distribution with respect to the gas and dust compounds which are responsible for the birth of the young stars. For this purpose we use correlation analysis between different gas, dust and YSOs tracers. We compared the large-scale CO, HCO$^+$, near-IR extinction, and far-IR {\it Herschel} maps with the density of YSOs of the different evolutionary Classes. The direct correlation analysis between these maps was used together with the wavelet-based spatial correlation analysis. This analysis reveals a much tighter correlation of the gas-dust tracers with the distribution of Class I YSOs than with that of Class II YSOs. We argue that Class I YSOs which were initially born in the central bright cluster S255-IR (both N and S parts) during their evolution to Class II stage ($\sim$2 Myr) had enough time to travel through the whole S254-S258 star-formation region. Given that the region contains several isolated YSO clusters, the evolutionary link between these clusters and the bright central S255-IR (N and S) cluster can be considered. Despite the complexity of the YSO cluster formation in the non-uniform medium, the clusters of Class II YSOs in the S254-258 star-formation region can contain objects born in the different locations of the complex., Comment: Accepted for publication in MNRAS

26 pages, 26 figures, 1 table, 4 appendixes, Observations of the interstellar medium (ISM) show a complex density and velocity structure, which is in part attributed to turbulence. Consequently, the multifractal formalism should be applied to observation maps of the ISM in order to characterize its turbulent and multiplicative cascade properties. However, the multifractal formalism, even in its more advanced and recent canonical versions, requires a large number of realizations of the system, which usually cannot be obtained in astronomy. We present a self-contained introduction to the multifractal formalism in a “microcanonical” version, which allows us, for the first time, to compute precise turbulence characteristic parameters from a single observational map without the need for averages in a grand ensemble of statistical observables (e.g., a temporal sequence of images). We compute the singularity exponents and the singularity spectrum for both observations and magnetohydrodynamic simulations, which include key parameters to describe turbulence in the ISM. For the observations we focus on the 250 μm Herschel map of the Musca filament. Scaling properties are investigated using spatial 2D structure functions, and we apply a two-point log-correlation magnitude analysis over various lines of the spatial observation, which is known to be directly related to the existence of a multiplicative cascade under precise conditions. It reveals a clear signature of a multiplicative cascade in Musca with an inertial range from 0.05–0.65 pc. We show that the proposed microcanonical approach provides singularity spectra that are truly scale invariant, as required to validate any method used to analyze multifractality. The obtained singularity spectrum of Musca, which is sufficiently precise for the first time, is clearly not as symmetric as usually observed in log-normal behavior. We claim that the singularity spectrum of the ISM toward Musca features a more log-Poisson shape. Since log-Poisson behavior is claimed to exist when dissipation is stronger for rare events in turbulent flows, in contrast to more homogeneous (in volume and time) dissipation events, we suggest that this deviation from log-normality could trace enhanced dissipation in rare events at small scales, which may explain, or is at least consistent with, the dominant filamentary structure in Musca. Moreover, we find that subregions in Musca tend to show different multifractal properties: While a few regions can be described by a log-normal model, other regions have singularity spectra better fitted by a log-Poisson model. This strongly suggests that different types of dynamics exist inside the Musca cloud. We note that this deviation from log-normality and these differences between subregions appear only after reducing noise features, using a sparse edge-aware algorithm, which have the tendency to “log-normalize” an observational map. Implications for the star formation process are discussed. Our study establishes fundamental tools that will be applied to other galactic clouds and simulations in forthcoming studies, This work is supported by the GENESIS project (GENeration and Evolution of Structure in the ISm), via the french ANR and the german DFG through grant numbers ANR-16-CE92-0035-01 and DFG1591/2-1. N. Schneider acknowledges support by the German Deutsche Forschungsgemeinschaft, DFG project number SFB 956. L. Bonne acknowledges support by the Région Nouvelle-Aquitaine, France., With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

Takemura, H., et al., Applying dendrogram analysis to the CARMA-NRO C18O (J = 1-0) data having an angular resolution of ∼8″, we identified 692 dense cores in the Orion Nebula Cluster region. Using this core sample, we compare the core and initial stellar mass functions in the same area to quantify the step from cores to stars. About 22% of the identified cores are gravitationally bound. The derived core mass function (CMF) for starless cores has a slope similar to Salpeter's stellar initial mass function (IMF) for the mass range above 1 M o˙, consistent with previous studies. Our CMF has a peak at a subsolar mass of ∼0.1 M o˙, which is comparable to the peak mass of the IMF derived in the same area. We also find that the current star formation rate is consistent with the picture in which stars are born only from self-gravitating starless cores. However, the cores must gain additional gas from the surroundings to reproduce the current IMF (e.g., its slope and peak mass), because the core mass cannot be accreted onto the star with 100% efficiency. Thus, the mass accretion from the surroundings may play a crucial role in determining the final stellar masses of stars., P.S. was partially supported by a Grant-in-Aid for Scientific Research (KAKENHI Number 18H01259) of Japan Society for the Promotion of Science (JSPS). R.S.K. acknowledges financial support from the DFG via the collaborative research center (SFB 881, Project-ID 138713538) “The Milky Way System” (subprojects A1, B1, B2, and B8). He is also thankful for subsidies from the Heidelberg Cluster of Excellence STRUCTURES in the framework of Germanyʼs Excellence Strategy (grant EXC-2181/1-390900948) and for funding from the European Research Council (ERC) via the ERC Synergy Grant ECOGAL (grant 855130). P.P. acknowledges support by the Spanish MINECO under project AYA2017- 88754-P, and financial support from the State Agency for Research of the Spanish Ministry of Science and Innovation through the “Unit of Excellence María de Maeztu 2020-2023” award to the Institute of Cosmos Sciences (CEX2019-000918-M). V.O., A.S.M., and P.S. were supported by the Collaborative Research Centre 956, subprojects C1, A6, and C3, funded by the Deutsche Forschungsgemeinschaft (DFG), project ID 184018867. T.G.S.P. gratefully acknowledges support by the National Science Foundation under grant No. AST-2009842. J.E.P. acknowledges the support by the Max Planck Society. We thank the anonymous referee for many useful comments that have improved the presentation

FEEDBACK is a SOFIA (Stratospheric Observatory for Infrared Astronomy) legacy program dedicated to study the interaction of massive stars with their environment. It performs a survey of 11 galactic high mass star-forming regions in the 158 μm (1.9 THz) line of [C II] and the 63 μm (4.7 THz) line of [O I]. We employ the 14 pixel Low Frequency Array and 7 pixel High Frequency Array upGREAT heterodyne instrument to spectrally resolve (0.24 MHz) these far-infrared fine structure lines. With a total observing time of 96h, we will cover ∼6700 arcmin2 at 14 1) angular resolution for the [C II] line and 6 3 for the [O I] line. The observations started in spring 2019 (Cycle 7). Our aim is to understand the dynamics in regions dominated by different feedback processes from massive stars such as stellar winds, thermal expansion, and radiation pressure, and to quantify the mechanical energy injection and radiative heating efficiency. This is an important science topic because feedback of massive stars on their environment regulates the physical conditions and sets the emission characteristics in the interstellar medium (ISM), influences the star formation activity through molecular cloud dissolution and compression processes, and drives the evolution of the ISM in galaxies. The [C II] line provides the kinematics of the gas and is one of the dominant cooling lines of gas for low to moderate densities and UV fields. The [O I] line traces warm and high-density gas, excited in photodissociations regions with a strong UV field or by shocks. The source sample spans a broad range in stellar characteristics from single OB stars, to small groups of O stars, to rich young stellar clusters, to ministarburst complexes. It contains well-known targets such as Aquila, the Cygnus X region, M16, M17, NGC7538, NGC6334, Vela, and W43 as well as a selection of H II region bubbles, namely RCW49, RCW79, and RCW120. These [C II] maps, together with the less explored [O I] 63 μm line, provide an outstanding database for the community. They will be made publically available and will trigger further studies and follow-up observations.

37 pags., 12 figs., 1 tab., The far-infrared (FIR) regime is one of the wavelength ranges where no astronomical data with sub-arcsecond spatial resolution exist. None of the medium-term satellite projects like SPICA, Millimetron, or the Origins Space Telescope will resolve this malady. For many research areas, however, information at high spatial and spectral resolution in the FIR, taken from atomic fine-structure lines, from highly excited carbon monoxide (CO), light hydrides, and especially from water lines would open the door for transformative science. A main theme will be to trace the role of water in proto-planetary discs, to observationally advance our understanding of the planet formation process and, intimately related to that, the pathways to habitable planets and the emergence of life. Furthermore, key observations will zoom into the physics and chemistry of the star-formation process in our own Galaxy, as well as in external galaxies. The FIR provides unique tools to investigate in particular the energetics of heating, cooling, and shocks. The velocity-resolved data in these tracers will reveal the detailed dynamics engrained in these processes in a spatially resolved fashion, and will deliver the perfect synergy with ground-based molecular line data for the colder dense gas., Open Access funding enabled and organized by Projekt DEAL. The investigations by H.L., D.B., L.B., R.F. and M.L. have been funded by the German Space Agency (DLR) under FKZ 50NA1816– 19. H.B. acknowledges support from the European Research Council under the Horizon 2020 Framework Program via the ERC Consolidator Grant CSF-648505. H.B. also acknowledges support from the Deutsche Forschungsgemeinschaft in the Collaborative Research Center (SFB 881) “The Milky Way System” (subproject B1). J.R.G. thanks the Spanish MICIU for funding support under grant AYA2017-85111-P. V.O.-O. and U.G. were supported by the Collaborative Research Centre 956, sub-projects C1 and D2, funded by the Deutsche Forschungsgemeinschaft (DFG), project ID 184018867

We identify 45 protostellar outflows in CO maps of the Orion A giant molecular cloud from the CARMA-NRO Orion survey. Our sample includes 11 newly detected outflows. We measure the mass and energetics of the outflows, including material at low-velocities by correcting for cloud contributions. The total momentum and kinetic energy injection rates of outflows is comparable to the turbulent dissipation rate of the cloud. We also compare the outflow position angles to the orientation of C$^{18}$O filaments. We find that the full sample of outflows is consistent with being randomly oriented with respect to the filaments. A subsample of the most reliable measurements shows a moderately perpendicular outflow-filament alignment which may reflect accretion of mass across filaments and onto the protostellar cores., 22 pages, 11 figures in main text, 47 additional figures in appendices. Accepted to ApJ

A unique filament is identified in the {\it Herschel} maps of the Orion A giant molecular cloud. The filament, which, we name the Stick, is ruler-straight and at an early evolutionary stage. Transverse position-velocity diagrams show two velocity components closing in on the Stick. The filament shows consecutive rings/forks in C$^{18}$O(1-0) channel maps, which is reminiscent of structures generated by magnetic reconnection. We propose that the Stick formed via collision-induced magnetic reconnection (CMR). We use the magnetohydrodynamics (MHD) code Athena++ to simulate the collision between two diffuse molecular clumps, each carrying an anti-parallel magnetic field. The clump collision produces a narrow, straight, dense filament with a factor of $>$200 increase in density. The production of the dense gas is seven times faster than free-fall collapse. The dense filament shows ring/fork-like structures in radiative transfer maps. Cores in the filament are confined by surface magnetic pressure. CMR can be an important dense-gas-producing mechanism in the Galaxy and beyond., 39 pages, 31 figures, 2 tables, accepted by ApJ

Although the 2P3/2-2P1/2 transition of [CII] at 158um is known to be an excellent tracer of active star formation, we still do not have a complete understanding of where within star formation regions the emission originates. Here, we use SOFIA upGREAT observations of [CII] emission toward the HII region complex Sh2-235 (S235) to better understand in detail the origin of [CII] emission. We complement these data with a fully-sampled Green Bank Telescope radio recombination line map tracing the ionized hydrogen gas. About half of the total [CII] emission associated with S235 is spatially coincident with ionized hydrogen gas, although spectroscopic analysis shows little evidence that this emission is coming from the ionized hydrogen volume. Velocity-integrated [CII] intensity is strongly correlated with WISE 12um intensity across the entire complex, indicating that both trace ultra-violet radiation fields. The 22um and radio continuum intensities are only correlated with [CII] intensity in the ionized hydrogen portion of the S235 region and the correlations between the [CII] and molecular gas tracers are poor across the region. We find similar results for emission averaged over a sample of external galaxies, although the strength of the correlations is weaker. Therefore, although many tracers are correlated with the strength of [CII] emission, only WISE 12um emission is correlated on small-scales of the individual HII region S235 and also has a decent correlation at the scale of entire galaxies. Future studies of a larger sample of Galactic HII regions would help to determine whether these results are truly representative., Comment: Accepted to ApJ

We investigate the relationship between turbulence and feedback in the Orion A molecular cloud using maps of $^{12}$CO(1-0), $^{13}$CO(1-0) and C$^{18}$O(1-0) from the CARMA-NRO Orion survey. We compare gas statistics with the impact of feedback in different parts of the cloud to test whether feedback changes the structure and kinematics of molecular gas. We use principal component analysis, the spectral correlation function, and the spatial power spectrum to characterize the cloud. We quantify the impact of feedback with momentum injection rates of protostellar outflows and wind-blown shells as well as the surface density of young stars. We find no correlation between shells or outflows and any of the gas statistics. However, we find a significant anti-correlation between young star surface density and the slope of the $^{12}$CO spectral correlation function, suggesting that feedback may influence this statistic. While calculating the principal components, we find peaks in the covariance matrix of our molecular line maps offset by 1-3 km s$^{-1}$ toward several regions of the cloud which may be produced by feedback. We compare these results to predictions from molecular cloud simulations., 25 pages, 17 figures, accepted to ApJ

We present an initial overview of the filamentary structure in the Orion A molecular cloud utilizing a high angular and velocity resolution C$^{18}$O(1-0) emission map that was recently produced as part of the CARMA-NRO Orion Survey. The main goal of this study is to build a credible method to study varying widths of filaments which has previously been linked to star formation in molecular clouds. Due to the diverse star forming activities taking place throughout its $\sim$20 pc length, together with its proximity of 388 pc, the Orion A molecular cloud provides an excellent laboratory for such an experiment to be carried out with high resolution and high sensitivity. Using the widely-known structure identification algorithm, DisPerSE, on a 3-dimensional (PPV) C$^{18}$O cube, we identified 625 relatively short (the longest being 1.74 pc) filaments over the entire cloud. We study the distribution of filament widths using FilChaP, a python package that we have developed and made publicly available. We find that the filaments identified in a 2 square degree PPV cube do not overlap spatially, except for the complex OMC-4 region that shows distinct velocity components along the line of sight. The filament widths vary between 0.02 and 0.3 pc depending on the amount of substructure that a filament possesses. The more substructure a filament has, the larger is its width. We also find that despite this variation, the filament width shows no anticorrelation with the central column density which is in agreement with previous Herschel observations., Comment: 19 pages, 20 figures. Accepted for publication in Astronomy and Astrophysics

With this research note, we are releasing the CARMA-NRO Orion Survey data first presented in Kong et al., enhanced with additional coverage of the L1641-C region to the south of the integral-shaped filament. We are including position–position–velocity cubes for the molecular lines 12CO(1–0), 13CO(1–0), and C18O(1–0). The original paper includes details of the data reduction and final sensitivity. The mapped region now spans about 2.°5 along the Orion A cloud in the north–south direction, providing an unprecedented overview of the extended molecular gas. The associated data cubes are publicly available on the Harvard Dataverse 10.7910/DVN/6Q26PN (Version 3). Kong et al. and this research note should be cited when using these data.

The structure of molecular clouds (MCs) holds important clues on the physical processes that lead to their formation and subsequent evolution. While it is well established that turbulence imprints a self-similar structure to the clouds, other processes, such as gravity and stellar feedback, can break their scale-free nature. The break of self-similarity can manifest itself in the existence of characteristic scales that stand out from the underlying structure generated by turbulent motions. We investigate the structure of the Cygnus-X North and the Polaris MCs which represent two extremes in terms of their star formation activity. We characterize the structure of the clouds using the delta-variance ($\Delta$-variance) spectrum. In Polaris, the structure of the cloud is self-similar over more than one order of magnitude in spatial scales. In contrast, the $\Delta$-variance spectrum of Cygnus-X exhibits an excess and a plateau on physical scales of ~0.5-1.2 pc. In order to explain the observations for Cygnus-X, we use synthetic maps in which we overlay populations of discrete structures on top of a fractal Brownian motion (fBm) image. The properties of these structures such as their major axis sizes, aspect ratios, and column density contrasts are randomly drawn from parameterized distribution functions. We show that it is possible to reproduce a $\Delta$-variance spectrum that resembles the one of the Cygnus-X cloud. We also use a "reverse engineering" approach in which we extract the compact structures in the Cygnus-X cloud and re-inject them on an fBm map. The calculated $\Delta$-variance using this approach deviates from the observations and is an indication that the range of characteristic scales observed in Cygnus-X is not only due to the existence of compact sources, but is a signature of the whole population of structures, including more extended and elongated structures, Comment: Accepted to A&A. Version after language and other minor corrections

Context. The [CII] 158 {\mu}m far-infrared (FIR) fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). High spectral resolution observations have shown complex structures in the line profiles of the [CII] emission. Aims. Our aim is to determine whether the complex profiles observed in [^{12}CII] are due to individual velocity components along the line-of-sight or to self-absorption based on a comparison of the [^{12}CII] and isotopic [^{13}CII] line profiles. Methods. Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in M43, Horsehead~PDR, Monoceros~R2, and M17~SW allow for the detection of optically thin [^{13}CII] emission lines, along with the [^{12}CII] emission lines, with a high signal-to-noise ratio. We first derived the [^{12}CII] optical depth and the [CII] column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multi-component velocity fit allows us to derive the physical conditions of the [CII] gas: column density and excitation temperature. Results. We find moderate to high [^{12}CII] optical depths in all four sources and self-absorption of [^{12}CII] in Mon R2 and M17 SW. The high column density of the warm background emission corresponds to an equivalent Av of up to 41 mag. The foreground absorption requires substantial column densities of cold and dense [CII] gas, with an equivalent Av ranging up to about 13 mag. Conclusions. The column density of the warm background material requires multiple photon-dominated region (PDR) surfaces stacked along the line of sight and in velocity. The substantial column density of dense and cold foreground [CII] gas detected in absorption cannot be explained with any known scenario and we can only speculate on its origins, Comment: 40 pages, 49 figures

The ubiquitous presence of filamentary structures in the interstellar medium asks for an unbiased characterization of their properties including a stability analysis. We propose a novel technique to measure the spectrum of filaments in any two-dimensional data set. Using anisotropic wavelets we can quantify and distinguish local and global anisotropies and measure the size distribution of filaments. The wavelet analysis does not need any assumptions on the alignment or shape of filaments in the maps, but directly measures their typical spatial dimensions. In a rigorous test program, we calibrate the scale-dependence of the method and test the angular and spatial sensitivity. We apply the method to molecular line maps from magneto-hydrodynamic (MHD) simulations and observed column density maps from Herschel observations. When applying the anisotropic wavelet analysis to the MHD data, we find that the observed filament sizes depend on the combination of magnetic-field dominated density-velocity correlations with radiative transfer effects. This can be exploited by observing tracers with different optical depth to measure the transition from a globally ordered large-scale structure to small-scale filaments with entangled field lines. The unbiased view to Herschel column density maps does not confirm a universal characteristic filament width. The map of the Polaris Flare shows an almost scale-free filamentary spectrum up to the size of the dominating filament of about 0.4pc. For the Aquila molecular cloud the range of filament widths is limited to 0.05-0.2pc. The filaments in Polaris show no preferential direction in contrast to the global alignment that we trace in Aquila. By comparing the power in isotropic and anisotropic structures we can measure the relative importance of spherical and cylindrical collapse modes and their spatial distribution., Accepted for publication by A&A

We present the first results from a new, high resolution, $^{12}$CO(1-0), $^{13}$CO(1-0), and C$^{18}$O(1-0) molecular line survey of the Orion A cloud, hereafter referred to as the CARMA-NRO Orion Survey. CARMA observations have been combined with single-dish data from the Nobeyama 45m telescope to provide extended images at about 0.01 pc resolution, with a dynamic range of approximately 1200 in spatial scale. Here we describe the practical details of the data combination in uv space, including flux scale matching, the conversion of single dish data to visibilities, and joint deconvolution of single dish and interferometric data. A $\Delta$-variance analysis indicates that no artifacts are caused by combining data from the two instruments. Initial analysis of the data cubes, including moment maps, average spectra, channel maps, position-velocity diagrams, excitation temperature, column density, and line ratio maps provides evidence of complex and interesting structures such as filaments, bipolar outflows, shells, bubbles, and photo-eroded pillars. The implications for star formation processes are profound and follow-up scientific studies by the CARMA-NRO Orion team are now underway. We plan to make all the data products described here generally accessible; some are already available at https://dataverse.harvard.edu/dataverse/CARMA-NRO-Orion, Comment: 46 pages, 28 figures, 2 tables, accepted by ApJS

Photon Dominated Regions (PDRs) are interfaces between the mainly ionized and mainly molecular material around young massive stars. Analysis of the physical and chemical structure of such regions traces the impact of far-ultraviolet radiation of young massive stars on their environment. We present results on the physical and chemical structure of the prototypical high UV-illumination edge-on Orion Bar PDR from an unbiased spectral line survey with a wide spectral coverage. A spectral scan from 480-1250 GHz and 1410-1910 GHz at 1.1 MHz resolution was obtained by the HIFI instrument onboard the Herschel Space Observatory. For molecules with multiple transitions we used rotational diagrams to obtain excitation temperatures and column densities. For species with a single detected transition we used an optically thin LTE approximation. In case of species with available collisional rates, we also performed a non-LTE analysis to obtain kinetic temperatures, H2 volume densities, and column densities. About 120 lines corresponding to 29 molecules (including isotopologues) have been detected in the Herschel/HIFI line survey, including 11 transitions of CO, 7 transitions of 13CO, 6 transitions of C18O, 10 transitions of H2CO, and 6 transitions of H2O. Most species trace kinetic temperatures in the range between 100 and 150 K and H2 volume densities in the range between 10^5 and 10^6 cm^-3. The species with temperatures and / or densities outside of this range include the H2CO transitions tracing a very high temperature (315 K) and density (1.4x10^6 cm^-3) component and SO corresponding to the lowest temperature (56 K) measured as a part of this line survey. The observed lines/species reveal a range of physical conditions (gas density /temperature) involving structures at high density / high pressure, obsoleting the traditional 'clump / interclump' picture of the Orion Bar., Comment: Accepted for publication in A&A, abstract abridged

Spatial association of clumps from different tracers turns out to be a valuable tool to determine the physical properties of molecular clouds. It provides a reliable estimate for the $X$-factors, serves to trace the density of clumps seen in column densities only and allows to measure the velocity dispersion of clumps identified in dust emission. We study the spatial association between clump populations, extracted by use of the GAUSSCLUMPS technique from $^{12}$CO (1-0), $^{13}$CO (1-0) line maps and Herschel dust-emission maps of the star-forming region Rosette, and analyse their physical properties. All CO clumps that overlap with another CO or dust counterpart are found to be gravitationally bound and located in the massive star-forming filaments of the molecular cloud. They obey a single mass-size relation $M_{\rm cl}\propto R_{\rm cl}^\gamma$ with $\gamma\simeq3$ (implying constant mean density) and display virtually no velocity-size relation. We interpret their population as low-density structures formed through compression by converging flows and still not evolved under the influence of self-gravity. The high-mass parts of their clump mass functions are fitted by a power law ${\rm d}N_{\rm cl}/{\rm d}\,\log M_{\rm cl}\propto M_{\rm cl}^{\Gamma}$ and display a nearly Salpeter slope $\Gamma\sim-1.3$. On the other hand, clumps extracted from the dust-emission map exhibit a shallower mass-size relation with $\gamma=2.5$ and mass functions with very steep slopes $\Gamma\sim-2.3$ even if associated with CO clumps. They trace density peaks of the associated CO clumps at scales of a few tenths of pc where no single density scaling law should be expected., Comment: 23 pages, 13 figures, 8 tables; accepted for publication in MNRAS

[13CII] observations in several Galactic sources show that the fine-structure [12CII] emission is often optically thick (the optical depths around 1 to a few). The aim of our study is to test whether this also affects the [12CII] emission from nearby galaxies like the Large Magellanic Cloud (LMC). We observed three star-forming regions in the LMC with upGREAT on board SOFIA at the frequency of the [CII] line. The 4GHz band width covers all three hyperfine lines of [13CII] simultaneously. For the analysis, we combined the [13CII] F=1-0 and F=1-1 hyperfine components, as they do not overlap with the [12CII] line in velocity. Three positions in N159 and N160 show an enhancement of [13CII] compared to the abundance-ratio-scaled [12CII] profile. This is likely due to the [12CII] line being optically thick, supported by the fact that the [13CII] line profile is narrower than [12CII], the enhancement varies with velocity, and the peak velocity of [13CII] matches the [OI] 63um self-absorption. The [12CII] line profile is broader than expected from a simple optical depth broadening of the [13CII] line, supporting the scenario of several PDR components in one beam having varying [12CII] optical depths. The derived [12CII] optical depth at three positions (beam size of 14arcsec, corresponding to 3.4pc) is 1--3, which is similar to values observed in several Galactic sources shown in previous studies. If this also applies to distant galaxies, the [CII] intensity will be underestimated by a factor of approximately 2., 5 pages with 3 figures plus one page appendix with 4 figures, accepted for publication in A&A Letter

We present results from a comprehensive submillimeter spectral survey toward the source Orion South, based on data obtained with the Heterodyne Instrument for the Far-Infrared instrument on board the Herschel Space Observatory, covering the frequency range of 480 to 1900 GHz. We detect 685 spectral lines with signal-to-noise ratios (S/Ns) > 3 sigma, originating from 52 different molecular and atomic species. We model each of the detected species assuming conditions of Local Thermodynamic Equilibrium. This analysis provides an estimate of the physical conditions of Orion South (column density, temperature, source size, and VLSR). We find evidence for three different cloud components: a cool (T-ex similar to 20-40 K), spatially extended (> 60 ''), and quiescent (Delta V-FWHM similar to 4 km s(-1)) component; a warmer (T-ex similar to 80-100 K), less spatially extended (similar to 30 ''), and dynamic (Delta V-FWHM similar to 8 km s(-1)) component, which is likely affected by embedded outflows; and a kinematically distinct region (T-ex > 100 K; V-LSR similar to 8 km s(-1)), dominated by emission from species that trace ultraviolet irradiation, likely at the surface of the cloud. We find little evidence for the existence of a chemically distinct "hot-core" component, likely due to the small filling factor of the hot core or hot cores within the Herschel beam. We find that the chemical composition of the gas in the cooler, quiescent component of Orion South more closely resembles that of the quiescent ridge in Orion-KL. The gas in the warmer, dynamic component, however, more closely resembles that of the Compact Ridge and Plateau regions of Orion-KL, suggesting that higher temperatures and shocks also have an influence on the overall chemistry of Orion South.

The Far Infrared Spectroscopic Explorer (FIRSPEX) is a novel European-led astronomy mission concept developed to enable large area ultra high spectroscopic resolution surveys in the THz regime. FIRSPEX opens up a relatively unexplored spectral and spatial parameter space that will produce an enormously significant scientific legacy by focusing on the properties of the multi-phase ISM, the assembly of molecular clouds in our Galaxy and the onset of star formation; topics which are fundamental to our understanding of galaxy evolution. The mission uses a heterodyne instrument and a ~1.2 m primary antenna to scan large areas of the sky in a number of discreet spectroscopic channels from L2. The FIRSPEX bands centered at [CI] 809 GHz, [NII]1460 GHz, [CII]1900 GHz and [OI]4700 GHz have been carefully selected to target key atomic and ionic fine structure transitions difficult or impossible to access from the ground but fundamental to the study of the multi-phase ISM in the Universe. The need for state-of-the-art sensitivity dictates the use of superconducting mixers configured either as tunnel junctions or hot electron bolometers. This technology requires cooling to low temperatures, approaching 4K, in order to operate. The receivers will operate in double sideband configuration providing a total of 7 pixels on the sky. FIRSPEX will operate from L2 in both survey and pointed mode enabling velocity resolved spectroscopy of large areas of sky as well as targeted observations.

The CO+ reactive ion is thought to be a tracer of the boundary between a HII region and the hot molecular gas. In this study, we present the spatial distribution of the CO+ rotational emission toward the Mon R2 star-forming region. The CO+ emission presents a clumpy ring-like morphology, arising from a narrow dense layer around the HII region. We compare the CO+ distribution with other species present in photon-dominated regions (PDR), such as [CII] 158 mm, H2 S(3) rotational line at 9.3 mm, polycyclic aromatic hydrocarbons (PAHs) and HCO+. We find that the CO+ emission is spatially coincident with the PAHs and [CII] emission. This confirms that the CO+ emission arises from a narrow dense layer of the HI/H2 interface. We have determined the CO+ fractional abundance, relative to C+ toward three positions. The abundances range from 0.1 to 1.9x10^(-10) and are in good agreement with previous chemical model, which predicts that the production of CO+ in PDRs only occurs in dense regions with high UV fields. The CO+ linewidth is larger than those found in molecular gas tracers, and their central velocity are blue-shifted with respect to the molecular gas velocity. We interpret this as a hint that the CO+ is probing photo-evaporating clump surfaces., Comment: The main text has 4 pages, 2 pages of Appendix, 4 figures, 1 table. Accepted for publication in Astronomy and Astrophysics letters

Probability distribution functions (PDFs) of column densities are an established tool to characterize the evolutionary state of interstellar clouds. Using simulations, we show to what degree their determination is affected by noise, line-of-sight contamination, field selection, and the incomplete sampling in interferometric measurements. We solve the integrals that describe the convolution of a cloud PDF with contaminating sources and study the impact of missing information on the measured column density PDF. The effect of observational noise can be easily estimated and corrected for if the root mean square (rms) of the noise is known. For $\sigma_{noise}$ values below 40% of the typical cloud column density, $N_{peak}$, this involves almost no degradation of the accuracy of the PDF parameters. For higher noise levels and narrow cloud PDFs the width of the PDF becomes increasingly uncertain. A contamination by turbulent foreground or background clouds can be removed as a constant shield if the PDF of the contamination peaks at a lower column or is narrower than that of the observed cloud. Uncertainties in the definition of the cloud boundaries mainly affect the low-column density part of the PDF and the mean density. As long as more than 50% of a cloud are covered, the impact on the PDF parameters is negligible. In contrast, the incomplete sampling of the uv plane in interferometric observations leads to uncorrectable distortions of the PDF of the produced maps. An extension of ALMA's capabilities would allow us to recover the high-column density tail of the PDF but we found no way to measure the intermediate and low column density part of the underlying cloud PDF in interferometric observations., Comment: Accepted for publication in A&A

Context. In bright photodissociation regions (PDR) associated with massive star formation, the presence of dense “clumps” that are immersed in a less dense interclump medium is often proposed to explain the difficulty of models to account for the observed gas emission in high-excitation lines. Aims. We aim to present a comprehensive view of the modelling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces. Methods. We observed the 12CO and 13CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel. We also considered line emission from key species in the gas cooling of PDRs (C+, O, and H2) and other tracers of PDR edges such as OH and CH+. All the intensities are collected from Herschel observations, the literature and the Spitzer archive and were analysed using the Meudon PDR code. Results. A grid of models was run to explore the parameter space of only two parameters: thermal gas pressure and a global scaling factor that corrects for approximations in the assumed geometry. We conclude that the emission in the high-J CO lines, which were observed up to Jup = 23 in the Orion Bar (Jup = 19 in NGC 7023), can only originate from small structures with typical thicknesses of a few 10−3 pc and at high thermal pressures (Pth ~ 108 K cm−3). Conclusions. Compiling data from the literature, we find that the gas thermal pressure increases with the intensity of the UV radiation field given by G0, following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help to rationalise the analysis of high-J CO emission in massive star formation and provides an observational constraint for models which study stellar feedback on molecular clouds.

Context. Models of photon-dominated regions (PDRs) still fail to fully reproduce some of the observed properties. In particular they do not reproduce the combination of the intensities of different PDR cooling lines together with the chemical stratification, as observed for example for the Orion Bar PDR.Aims. We aim to construct a numerical PDR model, KOSMA-τ 3D, to simulate full spectral cubes of line emission from arbitrary PDRs in three dimensions (3D). The model will reproduce the intensity of the main cooling lines from the Orion Bar PDR and the observed layered structure of the different transitions.Methods. We built up a 3D compound, made of voxels (3D pixels) that contain a discrete mass distribution of spherical “clumpy” structures, approximating the fractal ISM. To analyse each individual clump the new code was combined with the KOSMA-τ PDR model. Probabilistic algorithms were used to calculate the local FUV flux for each voxel as well as the voxel-averaged line emissivities and optical depths, based on the properties of the individual clumps. Finally, the computation of the radiative transfer through the compound provided full spectral cubes. To test the new model we tried to simulate the structure of the Orion Bar PDR and compared the results to observations from HIFI/Herschel and from the Caltech Submillimetre Observatory (CSO). In this context new Herschel data from the HEXOS guaranteed-time key program is presented. Results. Our model is able to reproduce the line-integrated intensities within a factor of 2.5 and the observed stratification pattern within 0.016 pc for the [Cii] 158 μ m and different 12/13 CO and HCO+ transitions, based on the representation of the Orion Bar PDR by a clumpy edge-on cavity wall. In the cavity wall, a large fraction of the total mass needs to be contained in clumps. The mass of the interclump medium is constrained by the FUV penetration. Furthermore, the stratification profile cannot be reproduced by a model that has the same amount of clump and interclump mass in each voxel; dense clumps need to be removed from the PDR surface.

The CH$^+$ ion is a key species in the initial steps of interstellar carbon chemistry. Its formation in diverse environments where it is observed is not well understood, however, because the main production pathway is so endothermic (4280 K) that it is unlikely to proceed at the typical temperatures of molecular clouds. We investigation CH$^+$ formation with the first velocity-resolved spectral mapping of the CH$^+$ $J=1-0, 2-1$ rotational transitions, three sets of CH $\Lambda$-doubled triplet lines, $^{12}$C$^+$ and $^{13}$C$^+$, and CH$_3$OH 835~GHz E-symmetry Q branch transitions, obtained with Herschel/HIFI over $\approx$12 arcmin$^2$ centered on the Orion BN/KL source. We present the spatial morphologies and kinematics, cloud boundary conditions, excitation temperatures, column densities, and $^{12}$C$^+$ optical depths. Emission from C$^+$, CH$^+$, and CH is indicated to arise in the diluted gas, outside of the explosive, dense BN/KL outflow. Our models show that UV-irradiation provides favorable conditions for steady-state production of CH$^+$ in this environment. Surprisingly, no spatial or kinematic correspondences of these species are found with H$_2$ S(1) emission tracing shocked gas in the outflow. We propose that C$^+$ is being consumed by rapid production of CO to explain the lack of C$^+$ and CH$^+$ in the outflow, and that fluorescence provides the reservoir of H$_2$ excited to higher ro-vibrational and rotational levels. Hence, in star-forming environments containing sources of shocks and strong UV radiation, a description of CH$^+$ formation and excitation conditions is incomplete without including the important --- possibly dominant --- role of UV irradiation., Comment: Accepted for publication in The Astrophysical Journal

The C$^{+}$ ion is an important coolant of interstellar gas, and so the [CII] fine structure line is frequently observed in the interstellar medium. However, the physical and chemical properties of the [CII]-emitting gas are still unclear. We carry out non-LTE radiative transfer simulations with RADMC-3D to study the [CII] line emission from a young, turbulent molecular cloud before the onset of star formation, using data from the SILCC-Zoom project. The [CII] emission is optically thick over 40% of the observable area with $I_{[\textrm{CII}]} > 0.5$ K km s$^{-1}$. To determine the physical properties of the [CII] emitting gas, we treat the [CII] emission as optically thin. We find that the [CII] emission originates primarily from cold, moderate density gas ($40 \lesssim T \lesssim 65$ K and $50 \lesssim n \lesssim 440$ cm$^{-3}$), composed mainly of atomic hydrogen and with an effective visual extinction between $\sim 0.50$ and $\sim 0.91$. Gas dominated by molecular hydrogen contributes only $\lesssim$20% of the total [CII] line emission. Thus, [CII] is not a good tracer for CO-dark H$_2$ at this early phase in the cloud's lifetime. We also find that the total gas, H and C$^+$ column densities are all correlated with the integrated [CII] line emission, with power law slopes ranging from 0.5 to 0.7. Further, the median ratio between the total column density and the [CII] line emission is $Y_{{\rm CII}}\approx 1.1 \times 10^{21}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$, and $Y_{{\rm CII}}$ scales with $I_{[\textrm{CII}]}^{-0.3}$. We expect $Y_{{\rm CII}}$ to change in environments with a lower or higher radiation field than simulated here., Comment: 27 pages, 23 figures, accepted for publication in MNRAS