Analyzing the Galactic plane CO survey with the Nobeyama 45-m telescope, we compared the spectral column density (SCD) of H$_2$ calculated for $^{12}$CO line using the current conversion factor $X_{\rm CO}$ to that for $^{13}$CO line under LTE in M16 and W43 regions. Here, SCD is defined by $dN_{\rm H_2}/dv$ with $N_{\rm H_2}$ and $v$ being the column density and radial velocity, respectively. It is found that the $X_{\rm CO}$ method significantly under-estimates the H$_2$ density in a cloud or region, where SCD exceeds a critical value ($ \sim 3\times 10^{21}\ [{\rm H_2 \ cm^{-2} \ (km \ s^{-1})^{-1}}]$), but over-estimates in lower SCD regions. We point out that the actual CO-to-H$_2$ conversion factor varies with the H$_2$ column density or with the CO-line intensity: It increases in the inner and opaque parts of molecular clouds, whereas it decreases in the low-density envelopes. However, in so far as the current $X_{\rm CO}$ is used combined with the integrated $^{12}$CO intensity averaged over an entire cloud, it yields a consistent value with that calculated using the $^{13}$CO intensity by LTE. Based on the analysis, we propose a new CO-to-\Htwo conversion relation, $N_{\rm H_2}^* = \int X_{\rm CO}^*(T_{\rm B}) T_{\rm B} dv$, where $X_{\rm CO}^*=(T_{\rm B}/T_{\rm B}^*)^βX_{\rm CO}$ is the modified spectral conversion factor as a function of the brightness temperature, $T_{\rm B}$, of the ${12}$CO ($J=1-0$) line, and $β\sim 1-2$ and $T_{\rm B}^*=12-16$ K are empirical constants obtained by fitting to the observed data. The formula corrects for the over/under estimation of the column density at low/high-CO line intensities, and is applicable to molecular clouds with $T_{\rm B} \ge 1$ K (rms noise in the data) from envelope to cores at sub-parsec scales (resolution). (Full resolution copy available at http://www.ioa.s.u-tokyo.ac.jp/~sofue/news/2020_mn_Xco12co13_fugin.pdf)
Column-density maps of molecular clouds are one of the most important observables in the context of molecular cloud- and star-formation (SF) studies. With the Herschel satellite it is now possible to determine the column density from dust emission. We use observations and simulations to demonstrate how LOS contamination affects the column density probability distribution function (PDF). We apply a first-order approximation (removing a constant level) to the molecular clouds of Auriga, Maddalena, Carina and NGC3603. In perfect agreement with the simulations, we find that the PDFs become broader…
The structure of molecular clouds can be characterized with the probability distribution function (PDF) of the mass surface density. In particular, the properties of the distribution can reveal the nature of the turbulence and star formation present inside the molecular cloud. In this paper, we explore how these structural characteristics evolve with time and also how they relate to various cloud properties as measured from a sample of synthetic column density maps of molecular clouds. We find that, as a cloud evolves, the peak of its column density PDF will shift to surface densities below th…
The probability distribution function of column density (PDF) has become the tool of choice for cloud structure analysis and star formation studies. Its simplicity is attractive, and the PDF could offer access to cloud physical parameters otherwise difficult to measure, but there has been some confusion in the literature on the definition of its completeness limit and shape at the low column density end. In this Letter we use the natural definition of the completeness limit of a column density PDF, the last closed column-density contour inside a surveyed region, and apply it to a set of large-…
Recent observations of column densities in molecular clouds find lognormal distributions with power-law high-density tails. These results are often interpreted as indications that supersonic turbulence dominates the dynamics of the observed clouds. We calculate and present the column-density distributions of three clouds, modeled with very different techniques, none of which is dominated by supersonic turbulence. The first star-forming cloud is simulated using smoothed particle hydrodynamics (SPH); in this case gravity, opposed only by thermal-pressure forces, drives the evolution. The second …
We have obtained column density maps for an unbiased sample of 120 molecular clouds in the third quadrant of the Milky Way mid-plane (b$\le |5|^{\circ}$) within the galactic longitude range from 195$^{\circ}$ to 225$^{\circ}$, using the high sensitivity $^{12}$CO and $^{13}$CO ($J=1-0$) 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 log-normal (LN) function and log-normal plus power-law (LN+PL) function. The molecular clouds are classified into three categori…
Larson (1981) first noted a scaling relation between masses and sizes in molecular clouds that implies that these objects have approximately constant column densities. This original claim, based upon millimeter observations of carbon monoxide lines, has been challenged by many theorists, arguing that the apparent constant column density observed is merely the result of the limited dynamic range of observations, and that in reality clouds have column density variations over two orders of magnitudes. In this letter we investigate a set of nearby molecular clouds with near-infrared excess methods…
Recent submillimeter and far-infrared wavelength observations of absorption in the rotational ground-state lines of various simple molecules against distant Galactic continuum sources have opened the possibility of studying the chemistry of diffuse molecular clouds throughout the Milky Way. In order to calculate abundances, the column densities of molecular and atomic hydrogen, HI, must be known. We aim at determining the atomic hydrogen column densities for diffuse clouds located on the sight lines toward a sample of prominent high-mass star-forming regions that were intensely studied with th…
It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function (Npdf). Instead, clouds without star formation seem to possess a lognormal distribution, while clouds with active star formation develope a power-law tail at high column densities. The lognormal behavior of the Npdf has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power-law behavior occurs when the cloud is dominated by gravity. In the present contribution we use thermally bi-stable numerical simulations of cl…
The formation of stars is inextricably linked to the structure of their parental molecular clouds. Here we take a number of nearby giant molecular clouds (GMCs) and analyse their column density and mass distributions. This investigation is based on four new all-sky median colour excess extinction maps determined from 2MASS. The four maps span a range of spatial resolution of a factor of eight. This allows us to determine cloud properties at a common spatial scale of 0.1pc, as well as to study the scale dependence of the cloud properties. We find that the low column density and turbulence domin…
Both observational and theoretical research over the past decade has demonstrated that the probability distribution function (PDF) of the gas density in turbulent molecular clouds is a key ingredient for understanding star formation. It has recently been argued that the PDF of molecular clouds is a pure power-law distribution. It has been claimed that the log-normal part is ruled out when using only the part of the PDF up/down to which it is complete, that is where the column density contours are still closed. By using the results from high-resolution magnetohydrodynamical simulations of molec…
A new method to determine the CO-to-H$_2$ conversion factor $X_{\rm CO}$ using absorption of diffuse X-ray emission by local molecular clouds was developed. It was applied to the Ophiucus (G353+17) and Corona Australis (G359-18) clouds using CO-line and soft X-ray archival data. We obtained a value $X_{\rm CO} =1.85 \pm 0.45 \times 10^{20} {\rm H_2 cm^{-2}/(K~ km~ s^{-1})}$ as the average of least-$chi^2$ fitting results for R4 (0.7 keV) and R5 (0.8 keV) bands. Full resolution pdf available at http://www.ioa.s.u-tokyo.ac.jp/~sofue/htdocs/2016Xco/
We present N-PDFs of 29 Galactic regions obtained from Herschel imaging at high angular resolution, covering diffuse and quiescent clouds, and those showing low-, intermediate-, and high-mass star formation (SF), and characterize the cloud structure using the Delta-variance tool. The N-PDFs are double-log-normal at low column densities, and display one or two power law tails (PLTs) at higher column densities. For diffuse, quiescent, and low-mass SF clouds, we propose that the two log-normals arise from the atomic and molecular phase, respectively. For massive clouds, we suggest that the first …
$Context.$ The traditional approach to characterize the structure of molecular clouds is to map their line emission. $Aims.$ We aim to test and apply a stratified random sampling technique that can characterize the line emission from molecular clouds more efficiently than mapping. $Methods.$ We sampled the molecular emission from the Perseus cloud using the H2 column density as a proxy. We divided the cloud into ten logarithmically spaced column density bins, and we randomly selected ten positions from each bin. The resulting 100 cloud positions were observed with the IRAM 30m telescope, cover…
Theoretical and observational investigations have indicated that the abundance of carbon monoxide (CO) is very sensitive to intrinsic properties of the gaseous medium, such as density, metallicity, and the background UV field. In order to accurately interpret CO observations, it is thus important to understand how well CO traces the gas, which in molecular clouds (MCs) is predominantly molecular hydrogen (H2). Recent hydrodynamic simulations by Glover & Mac Low have explicitly followed the formation and destruction of molecules in model MCs under varying conditions, confirming that CO formatio…
The Galactic Cold Cores (GCC) project has made Herschel observations of interstellar clouds where Planck detected compact sources of cold dust emission. Our aim is to characterise the structure of the clumps and their parent clouds. We also examine the accuracy to which the structure of dense clumps can be determined from submillimetre data. We use standard statistical methods to characterise the GCC fields. Clumps are extracted using column density thresholding and we construct for each field a three-dimensional radiative transfer (RT) model. These are used to estimate the relative radiation …
We demonstrate relations between column densities of simple molecules: CH, CH$^{+}$, H$_{2}$ and OH. The H$_{2}$, CH and OH molecules seem to occupy the same environments because of tight relations between their column densities. In contrary to this CH$^{+}$ column density does not correlate with those of other simple molecules.
We made CO(1-0) observations of 103 lines of sight in the core and envelope of the high-latitude cloud MBM 40 to determine how the CO-H_2 conversion factor (X_CO) varies throughout the cloud. Calibrating X_CO with CH data at similar resolution (1' for CO, 1.5' for CH) yields values of X_CO ranging from 0.6 10^20 to 3.3 10^20 cm^-2 [K km s^-1]^-1 with an average of 1.3 10^20 cm^-2 [K km s^-1]^-1. Given that the cloud has a peak reddening of 0.24 mag, it should be classed as a diffuse rather than a translucent molecular cloud. The mass obtained from the CO data and our values of X_CO is 9.6 M(so…
Power-law tails are often seen in probability density functions (PDFs) of molecular cloud column densities, and have been attributed to the effect of gravity. We show that extinction PDFs of a sample of five molecular clouds obtained at a few tenths of a parsec resolution, probing extinctions up to A$_{\mathrm{V}}$ $\sim$ 10 magnitudes, are very well described by lognormal functions provided that the field selection is tightly constrained to the cold, molecular zone and that noise and foreground contamination are appropriately accounted for. In general, field selections that incorporate warm, …
Absorption line studies are essential to understanding the origin, nature, and impact of starburst-driven galactic outflows. Such studies have revealed a multiphase medium with a number of poorly-understood features leading to a need to study the ionization mechanism of this gas. To better interpret these observations, we make use of a suite of adaptive mesh refinement hydrodynamic simulations of cold, atomic clouds driven by hot, supersonic outflows, including the effect of radiative cooling, thermal conduction, and an ionizing background characteristic of a starbursting galaxy. Using a new a…
We analyse column density and temperature maps derived from Herschel dust continuum observations of a sample of massive infrared dark clouds (G11.11-0.12, G18.82-0.28, G28.37+0.07, G28.53-0.25). We disentangle the velocity structure of the clouds using 13CO 1-0 and 12CO 3-2 data, showing that these IRDCs are the densest regions in massive giant molecular clouds and not isolated features. The probability distribution function (PDF) of column densities for all clouds have a power-law distribution over all (high) column densities, regardless of the evolutionary stage of the cloud: G11.11-0.12, G1…
Effects of low resolution on the column density PDF of molecular clouds
Shanghai Normal University（上海师范大学）；University of Massachusetts Amherst（马萨诸塞大学阿默斯特分校）；Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；Max Planck Institute for Astronomy（马克斯·普朗克天文学研究所）；Yunnan Observatories, Chinese Academy of Sciences（中国科学院云南天文台）；Universidad de Chile（智利大学）
The onset of massive star formation is not well understood because of observational and theoretical difficulties. To find the dense and cold clumps where massive star formation can take place, we compiled a sample of high infrared extinction clouds, which were observed previously by us in the 1.2 mm continuum emission and ammonia. We try to understand the star-formation stages of the clumps in these high extinction clouds by studying the infall and outflow properties, the presence of a young stellar object (YSO), and the level of the CO depletion through a molecular line survey with the IRAM 3…
We have used \textit{Far Ultraviolet Spectroscopic Explorer (FUSE)} observations of the Large Magellanic Cloud (LMC) to determine the O {\small VI} column densities in the Milky Way (MW) towards 6 LMC lines of sight. The mean column density of O {\small VI} in the MW is found to be log N(O {\small VI})=14.257$_{-0.084}^{+0.096}$. The results confirm the patchiness of O {\small VI} absorption in the MW and the column densities are higher or comparable to the LMC.
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 pro…
Measuring the mass distribution of infrared dark clouds (IRDCs) over the wide dynamic range of their column densities is a fundamental obstacle in determining the initial conditions of high-mass star formation and star cluster formation. We present a new technique to derive high-dynamic-range, arcsecond-scale resolution column density data for IRDCs and demonstrate the potential of such data in measuring the density variance - sonic Mach number relation in molecular clouds. We combine near-infrared data from the UKIDSS/Galactic Plane Survey with mid-infrared data from the Spitzer/GLIMPSE surve…
Deuterium fractionation, i.e. the enhancement of deuterated species with respect to the non-deuterated ones, is considered to be a reliable chemical clock of star-forming regions. This process is strongly affected by the ortho-to-para (o-p) H$_2$ ratio. In this letter we explore the effect of the o-p H$_2$ conversion on grains on the deuteration timescale in fully depleted dense cores, including the most relevant uncertainties that affect this complex process. We show that (i) the o-p H$_2$ conversion on grains is not strongly influenced by the uncertainties on the conversion time and the stic…
Simulations generally show that non-self-gravitating clouds have a lognormal column density ($Σ$) probability distribution function (PDF), while self-gravitating clouds with active star formation develop a distinct power-law tail at high column density. Although the growth of the power law can be attributed to gravitational contraction leading to the formation of condensed cores, it is often debated if an observed lognormal shape is a direct consequence of supersonic turbulence alone, or even if it is really observed in molecular clouds. In this paper we run three-dimensional magnetohydrodynam…
We present analytic theory for the total column density of singly ionized carbon (C+) in the optically thick photon dominated regions (PDRs) of far-UV irradiated (star-forming) molecular clouds. We derive a simple formula for the C+ column as a function of the cloud (hydrogen) density, the far-UV field intensity, and metallicity, encompassing the wide range of galaxy conditions. When assuming the typical relation between UV and density in the cold neutral medium, the C+ column becomes a function of the metallicity alone. We verify our analysis with detailed numerical PDR models. For optically …
We have modified the iterative procedure introduced by Lin et al. (2016), to systematically combine the submm images taken from ground based (e.g., CSO, JCMT, APEX) and space (e.g., Herschel, Planck) telescopes. We applied the updated procedure to observations of three well studied Infrared Dark Clouds (IRDCs): G11.11-0.12, G14.225-0.506 and G28.34+0.06, and then performed single-component, modified black-body fits to derive $\sim$10$"$ resolution dust temperature and column density maps. The derived column density maps show that these three IRDCs exhibit complex filamentary structures embeddi…
Infrared dark clouds (IRDCs) are cold and dense reservoirs of gas potentially available to form stars. Many of these clouds are likely to be pristine structures representing the initial conditions for star formation. The study presented here aims to construct and analyze accurate column density and dust temperature maps of IRDCs by using the first Herschel data from the Hi-GAL galactic plane survey. These fundamental quantities, are essential for understanding processes such as fragmentation in the early stages of the formation of stars in molecular clouds. We have developed a simple pixel-by-…
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 ro…
We present the results of a numerical study designed to address the question of whether there is a column density threshold for star formation within molecular clouds. We have simulated a large number of different clouds, with volume and column densities spanning a wide range of different values, using a state-of-the-art model for the coupled chemical, thermal and dynamical evolution of the gas. We show that star formation is only possible in regions where the mean (area-averaged) column density exceeds $10^{21} \: {\rm cm^{-2}}$. Within the clouds, we also show that there is a good correlatio…
Describing how the properties of the interstellar medium combine across size-scales is crucial for understanding star formation scaling laws and connecting Galactic and extragalactic data of molecular clouds. We describe how the statistical structure of clouds, and its connection to star formation, changes from sub-parsec to kiloparsec scales in a complete region within the Milky Way disk. We build a census of molecular clouds within 2 kpc from the Sun using literature. We examine the dust-based column density probability distributions (N-PDFs) of the clouds and their relation to star formatio…
The shape of the probability distribution function (PDF) of molecular clouds is an important ingredient for modern theories of star formation and turbulence. Recently, several studies have pointed out observational difficulties with constraining the low column density (i.e. Av <1) PDF using dust tracers. In order to constrain the shape and properties of the low column density probability distribution function, we investigate the PDF of multiphase atomic gas in the Perseus molecular cloud using opacity-corrected GALFA-HI data and compare the PDF shape and properties to the total gas PDF and the…
Sub-millimetre dust emission is often used to derive the column density N of dense interstellar clouds. The observations consist of data at several wavelengths but of variable resolution. We examine two procedures that been proposed for the estimation of high resolution N maps. Method A uses a low-resolution temperature map combined with higher resolution intensity data while Method B combines N estimates from different wavelength ranges. Our aim is to determine the accuracy of the methods relative to the true column densities and the estimates obtainable with radiative transfer modelling. We …
Simulations of molecular clouds often begin from highly idealised initial conditions, such as a uniform-density sphere with an artificially imposed turbulent velocity field. While the resulting structures may appear qualitatively similar to those detected in continuum and line observations, it is unclear whether they are genuinely representative of real molecular clouds. Recent observational work has discovered a tight, often close-to-linear relationship between the integrated intensity of molecular lines and the total column density of the cloud material. We combine magnetohydrodynamical simu…
We present the first study of the relationship between the column density distribution of molecular clouds within nearby Galactic spiral arms and their evolutionary status as measured from their stellar content. We analyze a sample of 195 molecular clouds located at distances below 5.5 kpc, identified from the ATLASGAL 870 micron data. We define three evolutionary classes within this sample: starless clumps, star-forming clouds with associated young stellar objects, and clouds associated with HII regions. We find that the N(H2) probability density functions (N-PDFs) of these three classes of o…
We show that the XCO factor, which converts the CO luminosity into the column density of molecular hydrogen has similar values for dense, fully molecular gas and for diffuse, partially molecular gas. We discuss the reasons of this coincidence and the consequences for the understanding of the interstellar medium.
Based on the analysis of available published data and archival data along 24 sightlines (5 of which are new) we derive more accurate estimates of the column densities of OH and CH towards diffuse/translucent clouds and revisit the typically observed correlation between the abundances of these species. The increase in the sample size was possible because of the equivalence of the column densities of CH derived from a combination of the transitions at 3137 & 3143 Angstrom, and a combination of transitions at 3886 & 3890 Angstrom, which we have demonstrated here. We find that with the exception o…
We present Very Large Array observations of NH3 (1,1) and (2,2) in a sample of six infrared-dark clouds (IRDCs) with distances from 2 to 5 kpc. We find that ammonia serves as an excellent tracer of dense gas in IRDCs, showing no evidence of depletion, and the average abundance in these clouds is 8.1 x 10^-7 relative to H2. Our sample consists of four IRDCs with 24 micron embedded protostars and two that appear starless. We calculate the kinetic temperature of the gas in IRDCs and find no significant difference between starless and star-forming IRDCs. We find that the bulk of the gas is between…
We investigate the impact of high optical depth on the HI saturation observed in the Perseus molecular cloud by using Arecibo HI emission and absorption measurements toward 26 radio continuum sources. The spin temperature and optical depth of individual HI components are derived along each line-of-sight, enabling us to estimate the correction for high optical depth. We examine two different methods for the correction, Gaussian decomposition and isothermal methods, and find that they are consistent (maximum correction factor ~ 1.2) likely due to the relatively low optical depth and insignifican…
The $^{13}$CO molecule is often used as a column density tracer in regions where the $^{12}$CO emission saturates. The $^{13}$CO column density is then related to that of $^{12}$CO by a uniform isotopic ratio. A similar approximation is frequently used when deriving $^{13}$CO emission maps from numerical simulations of molecular clouds. To test this assumption we calculate the $^{12}$CO/$^{13}$CO ratio self-consistently, taking the isotope selective photodissociation and the chemical fractionation of CO into account. We model the coupled chemical, thermal and dynamical evolution and the emerge…
We present newly processed archival Herschel images of molecular cloud MCLD 123.5+24.9 in the Polaris Flare. This cloud contains five starless cores. Using the spectral synthesis code Cloudy, we explore uncertainties in the derivation of column densities, hence, masses of molecular cores from Herschel data. We first consider several detailed grain models that predict far-IR grain opacities. Opacities predicted by the models differ by more than a factor of two, leading to uncertainties in derived column densities by the same factor. Then we consider uncertainties associated with the modified bl…
We derive an analytic expression for the transitional column density value ($s_t$) between the lognormal and power-law form of the probability distribution function (PDF) in star-forming molecular clouds. Our expression for $s_t$ depends on the mean column density, the variance of the lognormal portion of the PDF, and the slope of the power-law portion of the PDF. We show that $s_t$ can be related to physical quantities such as the sonic Mach number of the flow and the power-law index for a self-gravitating isothermal sphere. This implies that the transition point between the lognormal and pow…
Context: Observations of scattered light in the 1-5 $μ$m range have revealed dust grains in molecular cores with sizes larger than commonly inferred for the diffuse interstellar medium. It is currently unclear whether these grains are grown within the molecular cores or are an ubiquitous component of the interstellar medium. Aims: We investigate whether the large grains necessary for efficient scattering at 1-5 mum are associated with the abundance of water-ice within molecular clouds and cores. Methods: We combined water-ice abundance measurements for sight lines through the Lupus IV molecula…
Observed HI accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion happens in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To contrain the presence and properties of such flows, we present deep HI observations obtained with the NRAO Green Bank Telescope of an area measuring 4 by 4 degrees around NGC 2403. These observations, with a 5 sigma detection limit of 2.4 x 10^18 cm^-2 over a 20 km/s linewidth, reveal…
(abridged) We report a study of the relation between dust and gas over a 100deg^2 area in the Taurus molecular cloud. We compare the H2 column density derived from dust extinction with the CO column density derived from the 12CO and 13CO J= 1-0 lines. We derive the visual extinction from reddening determined from 2MASS data. The comparison is done at an angular size of 200", corresponding to 0.14pc at a distance of 140pc. We find that the relation between visual extinction Av and N(CO) is linear between Av~3 and 10 mag in the region associated with the B213--L1495 filament. In other regions th…
The amount of cold, molecular gas in high-redshift galaxies is typically inferred from proxies of molecular hydrogen (H$_2$), such as carbon monoxide (CO) or neutral atomic carbon ([CI]) and molecular gas mass conversion factors. The use of these proxies, however, relies on modeling and observations that have not been directly measured outside the local universe. Here, we use recent samples of high-redshift gamma-ray burst (GRB) and quasar molecular gas absorbers to determine this conversion factor $α_{\rm [CI]}$ from the column density of H$_2$, which gives us the mass per unit column, and th…
Chemical diversity of the gas in low-mass protostellar cores is widely recognized. In order to explore its origin, a survey of chemical composition toward 36 Class 0/I protostars in the Perseus molecular cloud complex, which are selected in an unbiased way under certain physical conditions, has been conducted with IRAM 30 m and NRO 45 m telescope. Multiple lines of C2H, c-C3H2 and CH3OH have been observed to characterize the chemical composition averaged over a 1000 au scale around the protostar. The derived beam-averaged column densities show significant chemical diversity among the sources, …
We present Herschel/HIFI observations of nine transitions of \hho and \hheo towards six high-mass star-forming regions, obtained as part of the PRISMAS Key Program. Water vapor in translucent clouds is detected in absorption along every sightline. We derive the column density of \hho or \hheo for the lower energy level of each transition observed. The total water column density is about a few $10^{13} \rm{cm^{-2}}$. We find that the abundance of water relative to hydrogen nuclei is $1\times10^{-8}$ in agreement with models for oxygen chemistry with high cosmic ray ionization rates. Relative to…
We searched for X-ray shadowing toward two infrared dark clouds (IRDCs) using the MOS detectors on XMM-Newton to learn about the Galactic distribution of X-ray emitting plasma. IRDCs make ideal X-ray shadowing targets of 3/4 kev photons due to their high column densities, relatively large angular sizes, and known kinematic distances. Here we focus on two clouds near 30 deg. Galactic longitude at distances of 2 and 5 kpc from the Sun. We derive the foreground and background column densities of molecular and atomic gas in the direction of the clouds. We find that the 3/4 kev emission must be dis…
High-velocity clouds (HVCs) are neutral or ionised gas clouds in the vicinity of the Milky Way that are characterised by high radial velocities inconsistent with participation in the regular rotation of the Galactic disc. Previous attempts to create a homogeneous all-sky HI map of HVCs have been hampered by a combination of poor angular resolution, limited surface brightness sensitivity and suboptimal sampling. Here, a new and improved HI map of Galactic HVCs based on the all-sky HI4PI survey is presented. The new map is fully sampled and provides significantly better angular resolution (16.2 …
We present a series of numerical simulations that explore how the `X-factor', $X_{CO}$ -- the conversion factor between the observed integrated CO emission and the column density of molecular hydrogen -- varies with the environmental conditions in which a molecular cloud is placed. Our investigation is centred around two environmental conditions in particular: the cosmic ray ionisation rate (CRIR) and the strength of the interstellar radiation field (ISRF). Since both these properties of the interstellar medium have their origins in massive stars, we make the assumption in this paper that both…
We characterize the column density probability distributions functions (PDFs) of the atomic hydrogen gas, HI, associated with seven Galactic molecular clouds (MCs). We use 21 cm observations from the Leiden/Argentine/Bonn Galactic HI Survey to derive column density maps and PDFs. We find that the peaks of the HI PDFs occur at column densities ranging from ~1-2$\times 10^{21}$ cm$^2$ (equivalently, ~0.5-1 mag). The PDFs are uniformly narrow, with a mean dispersion of $σ_{HI}\approx 10^{20}$ cm$^2$ (~0.1 mag). We also investigate the HI-to-H$_2$ transition towards the cloud complexes and estimat…
We have used the Taeduk Radio Astronomy Observatory to observe the Orion A and Ophiuchus clouds in the $J=$1$-$0 lines of $^{13}$CO, C$^{18}$O, HCN, HCO$^+$, and N$_2$H$^+$ and the $J=$2$-$1 line of CS. The fully sampled maps with uniform noise levels are used to create moment maps. The variations of the line intensity and velocity dispersion with total column density, derived from dust emission maps, are presented and compared to previous work. The CS line traces dust column density over more than one order of magnitude, and the N$_2$H$^+$ line best traces the highest column density regime ($…
