Density Of Molecular Clouds Research Articles

Distribution Properties of the 6.7 GHz Methanol Masers and Their Surrounding Gases in the Milky Way

An updated catalog consisting of 1092 6.7 GHz methanol maser sources is reported in this work. Additionally, the NH3 (1, 1), NH3 (2, 2), and NH3 (3, 3) transitions were observed toward 214 star-forming regions using the Shanghai Tianma radio telescope in order to examine the differences in physical environments, such as the excitation temperature and column density of molecular clouds associated with methanol masers on the Galactic scale. Statistical results reveal that the number of 6.7 GHz methanol masers in the Perseus arm is significantly lower than that in the other three main spiral arms. In addition, the Perseus arm also has the lowest gas column density among the main spiral arms traced by the NH3 observations. Both findings suggest that the Perseus arm has the lowest rate of high-mass star formation compared to the other three main spiral arms. We also observed a trend in which both the luminosity of the 6.7 GHz methanol masers and the ammonia gas column density decreased with the galactocentric distance. This finding indicates that the density of material in the inner Milky Way is generally higher than that in the outer Milky Way. This further suggests that high-mass stars are more easily formed at the head of the spiral arms. Furthermore, we found that the column density of ammonia gas is higher in the regions on the arms than in the inter-arm regions, supporting that the former is more likely to be the birthplace of high-mass stars.

Molecular Gas Structures Traced by 13CO Emission in the 18,190 12CO Molecular Clouds from the MWISP Survey

After morphological classification of 18,190 12CO molecular clouds, we further investigate the properties of their internal molecular gas structures traced by the 13CO (J = 1−0) line emissions. Using three different methods to extract the 13CO gas structures within each 12CO cloud, we find that ∼15% of the 12CO clouds (2851) have 13CO gas structures and these 12CO clouds contribute about 93% of the total integrated flux of 12CO emission. In each of the 2851 12CO clouds with 13CO gas structures, the 13CO emission area generally does not exceed 70% of the 12CO emission area, and the 13CO integrated flux does not exceed 20% of the 12CO integrated flux. We reveal a strong correlation between the velocity-integrated intensities of 12CO lines and those of 13CO lines in both 12CO and 13CO emission regions. This indicates the H2 column densities of molecular clouds are crucial for the 13CO line emission. After linking the 13CO structure detection rates of the 18,190 12CO molecular clouds to their morphologies, i.e., nonfilaments and filaments, we find that the 13CO gas structures are primarily detected in 12CO clouds with filamentary morphologies. Moreover, these filaments tend to harbor more than one 13CO structure. That demonstrates filaments not only have larger spatial scales, but also have more molecular gas structures traced by 13CO lines, i.e., local gas density enhancements. Our results favor the turbulent compression scenario for filament formation, in which dynamical compression of turbulent flows induces local density enhancements. The nonfilaments tend to be in the low-pressure and quiescent turbulent environments of the diffuse interstellar medium.

Molecular cloud catalogue from 13CO (1–0) data of the Forgotten Quadrant Survey

Context. New-generation spectroscopic surveys of the Milky Way plane have been revealing the structure of the interstellar medium, allowing the simultaneous study of dense structures from single star-forming objects or systems to entire spiral arms. Aims. The good sensitivity of the new surveys and the development of dedicated algorithms now enable building extensive catalogues of molecular clouds and deriving good estimates of their physical properties. This allows studying the behaviour of these properties across the Galaxy. Methods. We present the catalogue of molecular clouds extracted from the 13CO (1–0) data cubes of the Forgotten Quadrant Survey, which mapped the Galactic plane in the range 220° < l < 240° and −2.​​° 5 < b < 0° in 12CO (1–0) and 13CO (1–0). We compared the properties of the clouds of our catalogue with those of other catalogues. Results. The catalogue contains 87 molecular clouds for which the main physical parameters such as area, mass, distance, velocity dispersion, and virial parameter were derived. These structures are overall less extended and less massive than the molecular clouds identified in the 12CO (1–0) data-set because they trace the brightest and densest part of the 12CO (1–0) clouds. Conversely, the distribution of aspect ratio, equivalent spherical radius, velocity dispersion, and virial parameter in the two catalogues are similar. The mean value of the mass surface density of molecular clouds is 87 ± 55 M⊙ pc−2 and is almost constant across the galactocentric radius, indicating that this parameter, which is a proxy of star formation, is mostly affected by local conditions. Conclusions. In data of the Forgotten Quadrant Survey, we find a good agreement between the total mass and velocity dispersion of the clouds derived from 12CO (1–0) and 13CO (1–0). This is likely because in the surveyed portion of the Galactic plane, the H2 column density is not particularly high, leading to a CO emission with a not very high optical depth. This mitigates the effects of the different line opacities between the two tracers on the derived physical parameters. This is a common feature in the outer Galaxy, but our result cannot be readily generalised to the entire Milky Way because regions with higher particle density could show a different behaviour.

Structural organization of space polymers

Extraterrestrial polymers of glycine with iron have been characterized by mass spectrometry to have a core mass of 1494 Da with dominant rod-like variants at mass to charge ratios of 1567 and 1639 [McGeoch et al., “Meteoritic proteins with glycine, iron and lithium,” arXiv:2102.10700 (2021)]. Several principal macro-structural morphologies are observed in solvent extracts from a chondritic Vigarano class alteration type 3 meteoritic material. The first is an extended sheet of linked (three-legged) triskelia containing the 1494 Da core entity that encloses gas bubbles in the solvent. A second is of fiber-like crystals found here, via x-ray diffraction, to be multiple-walled nanotubes made from a square lattice of the 1494 Da polymer. A third is a dispersion of floating phantom-like short tubes of up to 100 μm length with characteristic angled bends that suggest the influence of a specific underlying protein structure. Here, it is proposed that the angled tubes are the observable result of a space-filling superpolymerization of 1638 Da polymer subunits guided by the tetragonal symmetry of linking silicon bonds. Distorted hexagonal sheets are linked by perpendicular subunits in a three-dimensional hexagonal diamond structure to fill the largest possible volume. This extended very low-density structure is conjectured to have dominated in a process of chemical selection because it captured a maximum amount of molecular raw material in the ultra-low density of molecular clouds or of the proto-solar nebula. This could have led ultimately to the accretion of the earliest planetary bodies.

Extended X-Ray Emission in Compton Thick AGN with Deep Chandra Observations

Abstract We present the spatial analysis of five Compton thick (CT) active galactic nuclei (AGNs), including MKN 573, NGC 1386, NGC 3393, NGC 5643, and NGC 7212, for which high-resolution Chandra observations are available. For each source, we find hard X-ray emission (>3 keV) extending to ∼kiloparsec scales along the ionization cone, and for some sources, in the cross-cone region. This collection represents the first, high-signal sample of CT AGN with extended hard X-ray emission for which we can begin to build a more complete picture of this new population of AGN. We investigate the energy dependence of the extended X-ray emission, including possible dependencies on host galaxy and AGN properties, and find a correlation between the excess emission and obscuration, suggesting a connection between the nuclear obscuring material and the galactic molecular clouds. Furthermore, we find that the soft X-ray emission extends farther than the hard X-rays along the ionization cone, which may be explained by a galactocentric radial dependence on the density of molecular clouds due to the orientation of the ionization cone with respect to the galactic disk. These results are consistent with other CT AGN with observed extended hard X-ray emission (e.g., ESO 428-G014 and the Ma et al. CT AGN sample), further demonstrating the ubiquity of extended hard X-ray emission in CT AGN.

Exploring the Galactic Anticenter Substructure with LAMOST and Gaia DR2

Abstract We characterize the kinematic and chemical properties of 589 Galactic anticenter substructure stars (GASS) with K/M giants in integrals-of-motion space. These stars likely include members of previously identified substructures such as Monoceros, A13, and the Triangulum-Andromeda cloud. We show that these stars are in nearly circular orbits on both sides of the Galactic plane. We can see a velocity (V Z ) gradient along Y-axis especially for the south GASS members. Our GASS members have similar energy and angular momentum distributions to thin-disk stars. Their location in [α/M] versus [M/H] space is more metal-poor than typical thin-disk stars, with [α/M] lower than that of the thick disk. We infer that our GASS members are part of the outer metal-poor disk stars and that the outer disk extends to 30 kpc. Considering the distance range and α-abundance features, GASS could be formed after the thick disk was formed due to the molecular cloud density decreasing in the outer disk where the star-formation rate might be less efficient compared to the inner disk.

CO-to-H2 conversion and spectral column density in molecular clouds: the variability of the XCO factor

ABSTRACT Analysing the Galactic plane CO survey with the Nobeyama 45-m telescope, we compared the spectral column density (SCD) of $N_{\rm H_2}$ calculated for the 12CO (J = 1–0) line using the current conversion factor $X_{\rm ^{12}CO}$ to that for the 13CO (J = 1–0) line under the LTE (local thermal equilibrium) assumption in the M16 and W43 regions. Here, SCD is defined by $\mathrm{d}N_{\rm H_2}/\mathrm{d}v$ with $N_{\rm H_2}$ and v being the column density and radial velocity, respectively. It is found that the $X_{\rm ^{12}CO}$ method significantly underestimates the H2 density in a cloud or region, where SCD exceeds a critical value (∼3 × 1021 [H2 cm−2 (km s−1)−1]), but overestimates in lower SCD regions. We point out that the actual CO-to-H2 conversion factor varies with the H2 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 $X_{^{12}{\rm CO}}$ is used combined with the integrated 12CO intensity averaged over an entire cloud, it yields a consistent value with that calculated using the 13CO intensity by LTE. Based on the analysis, we propose a new CO-to-H2 conversion relation, $N_{\rm H_2}^* = \int X^*_{\rm CO} (T_{\rm B}) T_{\rm B}\ \mathrm{d}v$, where $X^*_{\rm CO} (T_{\rm B})=(T_{\rm B}/T_{\rm B}^*)^\beta X_{\rm ^{12}CO}$ is the modified spectral conversion factor as a function of the brightness temperature, TB, of the 12CO (J = 1–0) line, and β ∼ 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/underestimation of the column density at low/high CO line intensities, and is applicable to molecular clouds with TB ≥ 1 K (12CO (J = 1–0) line rms noise in the data) from envelope to cores at sub-parsec scales (spatial resolution).

Protostellar birth with ambipolar and ohmic diffusion

Context. The transport of angular momentum is fundamental during the formation of low-mass stars; too little removal and rotation ensures stellar densities are never reached, too much and the absence of rotation means no protoplanetary disks can form. Magnetic diffusion is seen as a pathway to resolving this long-standing problem. Aims. We aim to investigate the impact of including resistive magnetohydrodynamics (MHD) in simulations of the gravitational collapse of a 1 M⊙ gas sphere, from molecular cloud densities to the formation of the protostellar seed; the second Larson core. Methods. We used the adaptive mesh refinement code RAMSES to perform two 3D simulations of collapsing magnetised gas spheres, including self-gravity, radiative transfer in the form of flux-limited diffusion, and a non-ideal gas equation of state to describe H2 dissociation which leads to the second collapse. The first run was carried out under the ideal MHD approximation, while ambipolar and ohmic diffusion was incorporated in the second calculation using resistivities computed from an equilibrium chemical network. Results. In the ideal MHD simulation, the magnetic field dominates the energy budget everywhere inside and around the first hydrostatic core, fueling interchange instabilities and driving a low-velocity outflow above and below the equatorial plane of the system. High magnetic braking removes essentially all angular momentum from the second core. On the other hand, ambipolar and ohmic diffusion create a barrier which prevents amplification of the magnetic field beyond 0.1 G in the first Larson core which is now fully thermally supported. A significant amount of rotation is preserved and a small Keplerian-like disk forms around the second core. The ambipolar and ohmic diffusions are effective at radii below 10 AU, indicating that a least ~1 AU is necessary to investigate the angular momentum transfer and the formation of rotationally supported disks. Finally, when studying the radiative efficiency of the first and second core accretion shocks, we found that it can vary by several orders of magnitude over the 3D surface of the cores. Conclusions. This proves that magnetic diffusion is a prerequisite to star formation. Not only does it enable the formation of protoplanetary disks in which planets will eventually form, it also plays a determinant role in the formation of the protostar itself.

Parallel Algorithms for Astrophysics Problems

The algorithm and the mathematical modeling package for 3-D gravitational gasdynamics problems with ultra-high resolution meshes are described. The modeling results of filamentary formations, i.e. concentrated areas with high gas density in molecular clouds (MC), and the nonisothermic compression calculation data are discussed. The spatial mesh resolution required for satisfying Jeans conditions in modeling is substantiated. The programming code developed uses dynamic gridding called local adaptive mesh refinement (AMR) at several (up to 10) resolution levels. To provide adequate resolution themeshes are added automatically and dynamically as well as destroyed as needed. The computation paralleling algorithm withOpenMPand CUDA is given. The programming language chosen to compute the problems of gravitational gas dynamics efficiently is justified and substantiated. The practice of applying algorithms for modeling the MC fragmentation after collisions, the filament and protostellar clouds formation, the star formation stages is analyzed.

The response of filamentary and spherical clouds to the turbulence and magnetic field

Recent observations have revealed that there is a power-law relation between magnetic field and density in molecular clouds. Furthermore, turbulence has been observed in some regions of molecular clouds and the velocity dispersion resulting from the turbulence is found to correlate with to the cloud density. Relating to these observations, in this study, we model filamentary and spherical clouds in magnetohydrostatic equilibrium in two quiescent and turbulent regions. The proposed equations are expected to represent the impact of magnetic field and turbulence on the cloud structure and the relation of cloud mass with shape. The Virial theorem is applied to consider the cloud evolution leading to important conditions for equilibrium of the cloud over its lifetime. The obtained results indicate that under the same conditions of the magnetic field and turbulence, each shape presents different responses. The possible ways for the formation of massive cores or coreless clouds in some regions as well as the formation of massive stars or low-mass stars can be discussed based on the results of this study. It should be mentioned that the shape of the clouds plays an important role in the formation of the protostellar clouds as well as their structure and evolution. This role is due to the effects of magnetic fields and turbulence.

The interstellar chemistry of C3H and C3H2 isomers.

We report the detection of linear and cyclic isomers of C3H and C3H2 towards various starless cores and review the corresponding chemical pathways involving neutral (C3Hx with x=1,2) and ionic (C3Hx+ with x = 1,2,3) isomers. We highlight the role of the branching ratio of electronic Dissociative Recombination (DR) reactions of C3H2+ and C3H3+ isomers showing that the statistical treatment of the relaxation of C3H* and C3H2* produced in these DR reactions may explain the relative c,l-C3H and c,l-C3H2 abundances. We have also introduced in the model the third isomer of C3H2 (HCCCH). The observed cyclic-to-linear C3H2 ratio vary from 110 ± 30 for molecular clouds with a total density around 1×104 molecules.cm-3 to 30 ± 10 for molecular clouds with a total density around 4×105 molecules.cm-3, a trend well reproduced with our updated model. The higher ratio for low molecular cloud densities is mainly determined by the importance of the H + l-C3H2 → H + c-C3H2 and H + t-C3H2 → H + c-C3H2 isomerization reactions.

Dynamical cooling of galactic discs by molecular cloud collisions – origin of giant clumps in gas-rich galaxy discs

Different from Milky Way-like galaxies, discs of gas-rich galaxies are clumpy. It is believed that the clumps form because of gravitational instability. However, a necessary condition for gravitational instability to develop is that the disc must dissipate its kinetic energy effectively, this energy dissipation (also called cooling) is not well understood. We propose that collisions (coagulation) between molecular clouds dissipate the kinetic energy of the discs, which leads to a dynamical cooling. The effectiveness of this dynamical cooling is quantified by the dissipation parameter D, which is the ratio between the free-fall time |$t_{\rm ff}\approx 1/ \sqrt{G \rho _{\rm {\rm disc}}}$| and the cooling time determined by the cloud collision process tcool. This ratio is related to the ratio between the mean surface density of the disc Σdisc and the mean surface density of molecular clouds in the disc Σcloud. When D < 1/3 (which roughly corresponds to |$\Sigma _{\rm {\rm disc}} < 1/3 \Sigma _{\rm cloud}$|⁠), cloud collision cooling is inefficient, and fragmentation is suppressed. When D > 1/3 (which roughly corresponds to Σdisc > 1/3Σcloud), cloud–cloud collisions lead to a rapid cooling through which clumps form. On smaller scales, cloud–cloud collisions can drive molecular cloud turbulence. This dynamical cooling process can be taken into account in numerical simulations as a sub-grid model to simulate the global evolution of disc galaxies.

The dust content of galaxies from z = 0 to z = 9

We study the dust content of galaxies from $z=0$ to $z=9$ in semi-analytic models of galaxy formation that include new recipes to track the production and destruction of dust. We include condensation of dust in stellar ejecta, the growth of dust in the interstellar medium (ISM), the destruction of dust by supernovae and in the hot halo, and dusty winds and inflows. The rate of dust growth in the ISM depends on the metallicity and density of molecular clouds. Our fiducial model reproduces the relation between dust mass and stellar mass from $z=0$ to $z=7$, the number density of galaxies with dust masses less than $10^{8.3}\,\rm{M}_\odot$, and the cosmic density of dust at $z=0$. The model accounts for the double power-law trend between dust-to-gas (DTG) ratio and gas-phase metallicity of local galaxies and the relation between DTG ratio and stellar mass. The dominant mode of dust formation is dust growth in the ISM, except for galaxies with $M_*<10^7\,\rm{M}_\odot$, where condensation of dust in supernova ejecta dominates. The dust-to-metal ratio of galaxies depends on the gas-phase metallicity, unlike what is typically assumed in cosmological simulations. Model variants including higher condensation efficiencies, a fixed timescale for dust growth in the ISM, or no growth at all reproduce some of the observed constraints, but fail to simultaneously reproduce the shape of dust scaling relations and the dust mass of high-redshift galaxies.

Electron Excitation of High Dipole Moment Molecules Re-examined

Abstract Emission from high-dipole moment molecules such as HCN allows determination of the density in molecular clouds, and is often considered to trace the “dense” gas available for star formation. We assess the importance of electron excitation in various environments. The ratio of the rate coefficients for electrons and H2 molecules, ≃105 for HCN, yields the requirements for electron excitation to be of practical importance if and , where the numerical factors reflect the critical values and . This indicates that in regions where a large fraction of carbon is ionized, will be large enough to make electron excitation significant. The situation is in general similar for other “high-density tracers,” including HCO+, CN, and CS. But there are significant differences in the critical electron fractional abundance, , defined by the value required for equal effect from collisions with H2 and e−. Electron excitation is, for example, unimportant for CO and C+. Electron excitation may be responsible for the surprisingly large spatial extent of emission from dense gas tracers in some molecular clouds. The enhanced estimates for HCN abundances and HCN/CO and HCN/HCO+ ratios observed in the nuclear regions of luminous galaxies may be in part a result of electron excitation of high dipole moment tracers. The importance of electron excitation will depend on detailed models of the chemistry, which may well be non-steady state and non-static.

Polarizing efficiency as a guide of grain growth and interstellar magnetic field properties

We interpret the relation between the polarizing efficiency $P_{\max}/E(B-V)$ and the wavelength of the maximum polarization $\lambda_{\max}$ observed for 17 objects (including 243 stars) separated into two groups: "dark clouds" and "open clusters". The objects are assigned to one of the groups according to the distribution of the parameter $\lambda_{\max}$. We use the model of homogeneous silicate and carbonaceous spheroidal particles with the imperfect alignment and a time-evolving size distribution. The polarization is assumed to be mainly produced by large silicate particles with the sizes $r_{V} \ga r_{V,\rm cut}$. The models with the initial size distribution reproducing the average curve of the interstellar extinction fail to explain the values of $\lambda_{\max} \ga 0.65\,\mkm$ observed for several dark clouds. We assume that the grain size distribution is modified due to accretion and coagulation, according to the model of Hirashita \& Voshchinnikov (2014). After including the evolutionary effects, $\lambda_{\max}$ shifts to longer wavelengths on time-scales $\sim 20 (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1}$ Myr where $n_\mathrm{H}$ is the hydrogen density in molecular clouds where dust processing occurs. The ratio $P_{\max}/E(B-V)$ goes down dramatically when the size of polarizing grains grows. The variations of the degree and direction of particle orientation influence this ratio only moderately. We have also found that the aspect ratio of prolate grains does not affect significantly the polarizing efficiency. For oblate particles, the shape effect is stronger but in most cases the polarization curves produced are too narrow in comparison with the observed ones.

The high-energy gamma-ray detection of G73.9+0.9, a supernova remnant interacting with a molecular cloud

We have analysed the Fermi LAT data on the SNR G73.9+0.9. We have confirmed a previous detection of high-energy gamma-rays from this source at a high significance of $\simeq 12\sigma$. The observed spectrum shows a significant curvature, peaking in $E F_E$ at $\sim$1 GeV. We have also calculated the flux upper limits in the mm-wavelength and X-ray ranges from Planck and XMM-Newton, respectively. We have inspected the intensity of the CO (1$\rightarrow $0) emission line and found a large peak at a velocity range corresponding to the previously estimated source distance of $\sim$4 kpc, which may indicate an association between a molecular cloud and the SNR. The gamma-ray emission appears due to interaction of accelerated particles within the SNR with the matter of the cloud. The most likely radiative process responsible for the gamma-ray emission is decay of neutral pions produced in ion-ion collisions. While a dominant leptonic origin of this emission can be ruled out, the relativistic electron population related to the observed radio flux will necessarily lead to a certain level of bremsstrahlung gamma-ray emission. Based on this broad-band modelling, we have developed a method to estimate the magnetic field, yielding $B\geq 80\,\mu$G at our best estimate of the molecular cloud density (or less at a lower density). G73.9+0.9 appears similar, though somewhat weaker, to other SNRs interacting with a local dense medium detected by the LAT.

SUPERMASSIVE BLACK HOLES IN A STAR-FORMING GASEOUS CIRCUMNUCLEAR DISK

Using N-body/SPH simulations we study the evolution of the separation of a pair of SMBHs embedded in a star forming circumnuclear disk (CND). This type of disk is expected to be formed in the central kilo parsec of the remnant of gas-rich galaxy mergers. Our simulations indicate that orbital decay of the SMBHs occurs more quickly when the mean density of the CND is higher, due to increased dynamical friction. However, in simulations where the CND is fragmented in high density gaseous clumps (clumpy CND), the orbits of the SMBHs are erratically perturbed by the gravitational interaction with these clumps, delaying, in some cases, the orbital decay of the SMBHs. The densities of these gaseous clumps in our simulations and in recent studies of clumpy CNDs are significantly higher than the observed density of molecular clouds in isolated galaxies or ULIRGs, thus, we expect that SMBH orbits are perturbed less in real CNDs than in the simulated CNDs of this study and other recent studies. We also find that the migration timescale has a weak dependence on the star formation rate of the CND. Furthermore, the migration timescale of a SMBH pair in a star-forming clumpy CND is at most a factor three longer than the migration timescale of a pair of SMBHs in a CND modeled with more simple gas physics. Therefore, we estimate that the migration timescale of the SMBHs in a clumpy CND is on the order of $10^7$ yrs.

On the probability distribution function of the mass surface density of molecular clouds. II.

The probability distribution function (PDF) of the mass surface density of molecular clouds provides essential information about the structure of molecular cloud gas and condensed structures out of which stars may form. In general, the PDF shows two basic components: a broad distribution around the maximum with resemblance to a log-normal function, and a tail at high mass surface densities attributed to turbulence and self-gravity. In a previous paper, the PDF of condensed structures has been analyzed and an analytical formula presented based on a truncated radial density profile, $\rho(r) = \rho_c/(1+(r/r_0)^2)^{n/2}$ with central density $\rho_c$ and inner radius $r_0$, widely used in astrophysics as a generalization of physical density profiles. In this paper, the results are applied to analyze the PDF of self-gravitating, isothermal, pressurized, spherical (Bonnor-Ebert spheres) and cylindrical condensed structures with emphasis on the dependence of the PDF on the external pressure $p_{ext}$ and on the overpressure $q^{-1} =p_c /p_{ext}$, where $p_c$ is the central pressure. Apart from individual clouds, we also consider ensembles of spheres or cylinders, where effects caused by a variation of pressure ratio, a distribution of condensed cores within a turbulent gas, and (in case of cylinders) a distribution of inclination angles on the mean PDF are analyzed. The probability distribution of pressure ratios $q^{-1}$ is assumed to be given by $P(q^{-1}) \propto q^{-k_1}/(1+(q_0/q)^{\gamma})^{(k_1+k_2)/{\gamma}}$, where $k_1$, ${\gamma}$, $k_2$, and $q_0$ are fixed parameters.

On the probability distribution function of the mass surface density of molecular clouds. I

The probability distribution function (PDF) of the mass surface density is an essential characteristic of the structure of molecular clouds or the interstellar medium in general. Observations of the PDF of molecular clouds indicate a composition of a broad distribution around the maximum and a decreasing tail at high mass surface densities. The first component is attributed to the random distribution of gas which is modeled using a log-normal function while the second component is attributed to condensed structures modeled using a simple power-law. The aim of this paper is to provide an analytical model of the PDF of condensed structures which can be used by observers to extract information about the condensations. The condensed structures are considered to be either spheres or cylinders with a truncated radial density profile at cloud radius r_cl. The assumed profile is of the form rho(r)=rho_c/(1+(r/r_0)^2)^{n/2} for arbitrary power n where rho_c and r_0 are the central density and the inner radius, respectively. An implicit function is obtained which either truncates (sphere) or has a pole (cylinder) at maximal mass surface density. The PDF of spherical condensations and the asymptotic PDF of cylinders in the limit of infinite overdensity rho_c/rho(r_cl) flattens for steeper density profiles and has a power law asymptote at low and high mass surface densities and a well defined maximum. The power index of the asymptote Sigma^(-gamma) of the logarithmic PDF (Sigma x P(Sigma)) in the limit of high mass surface densities is given by gamma = (n+1)/(n-1)-1 (spheres) or by gamma=n/(n-1)-1 (cylinders in the limit of infinite overdensity).

Stellar feedback by radiation pressure and photoionization

The relative impact of radiation pressure and photoionization feedback from young stars on surrounding gas is studied with hydrodynamic radiative transfer (RT) simulations. The calculations focus on the single-scattering (direct radiation pressure) and optically thick regime, and adopt a moment-based RT-method implemented in the moving-mesh code AREPO. The source luminosity, gas density profile and initial temperature are varied. At typical temperatures and densities of molecular clouds, radiation pressure drives velocities of order ~20 km/s over 1-5 Myr; enough to unbind the smaller clouds. However, these estimates ignore the effects of photoionization that naturally occur concurrently. When radiation pressure and photoionization act together, the latter is substantially more efficient, inducing velocities comparable to the sound speed of the hot ionized medium (10-15 km/s) on timescales far shorter than required for accumulating similar momentum with radiation pressure. This mismatch allows photoionization to dominate the feedback as the heating and expansion of gas lowers the central densities, further diminishing the impact of radiation pressure. Our results indicate that a proper treatment of the impact of young stars on the interstellar medium needs to primarily account for their ionization power whereas direct radiation pressure appears to be a secondary effect. This conclusion may change if extreme boosts of the radiation pressure by photon trapping are assumed.