H2 Column Density Research Articles

Study of the 13CO/C18O abundance ratio towards the filamentary infrared dark cloud IRDC 34.43 + 0.24

Abstract Nowadays, there are several observational studies about the 13CO/C18O abundance ratio ( $X^{13/18}$ ) towards nearby molecular clouds. These works give observational support to the C18O selective photodissociation due to the interaction between the far ultraviolet (FUV) radiation and the molecular gas. It is necessary to increase the sample of molecular clouds located at different distances and affected in different ways by nearby or embedded H ii regions and OB associations to study the selective photodissociation. Using 12CO, 13CO, and C18O J = 1–0 data obtained from the FOREST unbiased Galactic plane imaging survey performed with the Nobeyama 45-m telescope, we analyse the filamentary infrared dark cloud IRDC $34.43+0.24$ located at the distance of about 3.9 kpc. This infrared dark cloud (IRDC) is related to several H ii regions and young stellar objects. Assuming local thermodynamic equilibrium, we obtain: $0.8 \times 10^{16} <$ N(13CO) $<4 \times 10^{17}$ cm–2 (average value $= 4.2 \times 10^{16}$ cm–2), $0.6 \times 10^{15} <$ N(C18O) $<4.4 \times 10^{16}$ cm–2 (average value $= 5.0 \times 10^{15}$ cm–2), and 3 $<$ $X^{13/18}$ $<$ 30 (average $= 8$ ) across the whole IRDC. Larger values of $X^{13/18}$ were found towards portions of the cloud related to the H ii regions associated with the N61 and N62 bubbles and with the photodissociation regions, precisely the regions in which FUV photons are strongly interacting with the molecular gas. Our result represents an observational support to the C18O selectively photodissociation phenomenon occurring in a quite distant filamentary IRDC. Additionally, based on IR data from the Hi-GAL survey, the FUV radiation field was estimated in Habing units, and the dust temperature (T $_{dust}$ ) and H2 column density (N(H2)) distribution were studied. Using the average of N(H2), values in close agreement with the ‘canonical’ abundance ratios [H2]/[13CO] and [H2]/[C18O] were derived. However, the obtained ranges in the abundance ratios show that if an accurate analysis of the molecular gas is required, the use of the ‘canonical’ values may introduce some bias. Thus, it is important to consider how the gas is irradiated by the FUV photons across the molecular cloud. The analysis of $X^{13/18}$ is a good tool to perform that. Effects of beam dilution and clumpiness were studied.

HD/H2 absorption systems at high redshifts

We present new measurements of HD/H2 in absorption systems at redshift z ∼ 2 in quasar spectrum J 0812+3208 and J 2340−0053. In J 0812+3208 we detect and measure column density of H2 at z ∼ 2.07 for the first time and place upper limit on HD column density. In J 2340−0043 we identify HD in already known multicomponent H2 absorption system at z ∼ 2.05. In this system we obtain relative abundance of HD/H2 molecules close to isotopic ratio, suggesting the similar molecular fraction of deuterium and hydrogen. We discuss possible explanation of measured HD/H2 abundance.

The Serpens filament at the onset of slightly supercritical collapse

The Serpens filament, as one of the nearest infrared dark clouds, is regarded as a pristine filament at a very early evolutionary stage of star formation. In order to study its molecular content and dynamical state, we mapped this filament in seven species: C18O, HCO+, HNC, HCN, N2H+, CS, and CH3OH. Among them, HCO+, HNC, HCN, and CS show self-absorption, while C18O is most sensitive to the filamentary structure. A kinematic analysis demonstrates that this filament forms a velocity-coherent (trans)sonic structure, a large part of which is one of the most quiescent regions in the Serpens cloud. Widespread C18O depletion is found throughout the Serpens filament. Based on the Herschel dust-derived H2 column density map, the line mass of the filament is 36–41 M⊙ pc−1, and its full width at half maximum is 0.17 ± 0.01 pc, while its length is ≈1.6 pc. The inner radial column density profile of this filament can be well fitted with a Plummer profile with an exponent of 2.2 ± 0.1, a scale radius of 0.018 ± 0.003 pc, and a central density of (4.0 ± 0.8) × 104 cm−3. The Serpens filament appears to be slightly supercritical. The widespread blue-skewed HNC and CS line profiles and HCN hyperfine line anomalies across this filament indicate radial infall in parts of the Serpens filament. C18O velocity gradients also indicate accretion flows along the filament. The velocity and density structures suggest that such accretion flows are likely due to a longitudinal collapse parallel to the filament’s long axis. Both the radial infall rate (~72 M⊙ Myr−1, inferred from HNC and CS blue-skewed profiles) and the longitudinal accretion rate (~10 M⊙ Myr−1, inferred from C18O velocity gradients) along the Serpens filament are lower than all previously reported values in other filaments. This indicates that the Serpens filament lies at an early evolutionary stage when collapse has just begun, or that thermal and nonthermal support are effective in providing support against gravity.

Circumnuclear Multiphase Gas in the Circinus Galaxy. II. The Molecular and Atomic Obscuring Structures Revealed with ALMA

Abstract We used the Atacama Large Millimeter/Submillimeter Array (ALMA) to map the CO(3–2) and [C i](1–0) lines, as well as their underlying continuum emission, from the central ∼200 pc region of the Circinus galaxy that hosts the nearest type 2 Seyfert-class active galactic nucleus (AGN), with a spatial resolution of ∼6–15 pc. The lines and continuum-emitting regions consist of a circumnuclear disk (CND; 74 pc × 34 pc) and spiral arms. The distribution of the continuum emission revealed a temperature-dependent dust geometry and possibly polar dust elongation in the torus region. The molecular mass of the CND is , with a beam-averaged H2 column density of ∼5 × 1023 cm−2 toward the AGN position, which contributes significantly to the nuclear obscuration. The [C i](1–0)/CO(3–2) ratio at the AGN position is unusually high, suggesting an X-ray-dominated region-type chemistry. We decomposed the observed velocity fields into rotational and dispersion components, and revealed a multiphase dynamic nature in the r ≲ 10 pc torus region, i.e., the diffuse atomic gas is more spatially extended along the vertical direction of the disk than the dense molecular gas. Through comparisons with our model predictions based on the radiation-driven fountain scheme, we indicate that atomic outflows are the driver of the geometrical thickness of the atomic disk. This supports the validity of the radiation-driven fountain scheme in the vicinity of this AGN, which would explain the longstanding mystery of the physical origin of the AGN torus.

Opening PANDORA’s box: APEX observations of CO in PNe

Context. Observations of molecular gas have played a key role in developing the current understanding of the late stages of stellar evolution. Aims. The survey Planetary nebulae AND their cO Reservoir with APEX (PANDORA) was designed to study the circumstellar shells of evolved stars with the aim to estimate their physical parameters. Methods. Millimetre carbon monoxide (CO) emission is the most useful probe of the warm molecular component ejected by low- to intermediate-mass stars. CO is the second-most abundant molecule in the Universe, and the millimetre transitions are easily excited, thus making it particularly useful to study the mass, structure, and kinematics of the molecular gas. We present a large survey of the CO (J = 3−2) line using the Atacama Pathfinder EXperiment (APEX) telescope in a sample of 93 proto-planetary nebulae and planetary nebulae. Results. CO (J = 3−2) was detected in 21 of the 93 objects. Only two objects (IRC+10216 and PN M2-9) had previous CO (J = 3−2) detections, therefore we present the first detection of CO (J = 3−2) in the following 19 objects: Frosty Leo, HD 101584, IRAS 19475+3119, PN M1-11, V* V852 Cen, IC 4406, Hen 2-113, Hen 2-133, PN Fg 3, PN Cn 3-1, PN M2-43, PN M1-63, PN M1-65, BD+30 3639, Hen 2-447, Hen 2-459, PN M3-35, NGC 3132, and NGC 6326. Conclusions. CO (J = 3−2) was detected in all 4 observed pPNe (100%), 15 of the 75 PNe (20%), one of the 4 wide binaries (25%), and in 1 of the 10 close binaries (10%). Using the CO (J = 3−2) line, we estimated the column density and mass of each source. The H2 column density ranges from 1.7 × 1018 to 4.2 × 1021 cm−2 and the molecular mass ranges from 2.7 × 10−4 to 1.7 × 10−1 M⊙.

Chemical Evolution of N2H+ in Six Massive Star-forming Regions

Abstract To investigate how the abundance of N2H+ varies as massive clumps evolve, here we present a multiwavelength study toward six molecular clouds. All of these clouds contain several massive clumps in different evolutionary stages of star formation. Using archival data of the Herschel infrared Galactic Plane Survey (Hi-GAL), we made H2 column density and dust temperature maps of these regions by the spectral energy distribution method. We found that all of the six clouds show distinct dust temperature gradients, ranging from ∼20 to ∼30 K. This makes them good candidates to study chemical evolution of molecules (such as N2H+) in different evolutionary stages of star formation. Our molecular line data comes from the Millimeter Astronomy Legacy Team Survey at 90 GHz (MALT90). We made column density and then abundance maps of N2H+. We found that when the dust temperature is above 27 K, the abundance of N2H+ begins to decrease or reaches a plateau. We regard that this is because in the photodissociation regions around classical H ii regions, N2H+ is heavily destroyed by free electrons. However, when the dust temperature is below 27 K, the abundance of N2H+ increases with the dust temperature. This seems to be inconsistent with previous chemical models made in low-mass star-forming regions. In order to investigate whether this inconsistency is caused by a different chemistry in high-mass star-forming clumps, higher angular resolution observations are necessary.

Molecular gas and star formation in an absorption-selected galaxy: Hitting the bull’s eye atz≃ 2.46

We present the detection and detailed analysis of a diffuse molecular cloud atzabs= 2.4636 towards the quasar SDSS J 1513+0352 (zem≃ 2.68) observed with the X-shooter spectrograph at the Very Large Telescope. We measured very high column densities of atomic and molecular hydrogen with logN(H I, H2) ≃ 21.8, 21.3. This is the highest H2column density ever measured in an intervening damped Lyman-αsystem but we did not detect CO, implying logN(CO)/N(H2) < −7.8, which could be due to a low metallicity of the cloud. From the metal absorption lines, we derived the metallicity to beZ≃ 0.15Z⊙and determined the amount of dust by measuring the induced extinction of the background quasar light,AV≃ 0.4. We simultaneously detected Lyman-αemission at the same redshift with a centroid located at a most probable impact parameter of onlyρ≃ 1.4 kpc. We argue that the line of sight is therefore likely passing through the interstellar medium (ISM), as opposed to the circumgalactic medium (CGM), of a galaxy. The relation between the surface density of gas and that of star formation seems to follow the global empirical relation derived in the nearby Universe although our constraints on the star formation rate (SFR) and the galaxy extent remain too loose to be conclusive. We study the transition from atomic to molecular hydrogen using a theoretical description based on the microphysics of molecular hydrogen. We use the derived chemical properties of the cloud and physical conditions (Tk≃ 90 K andn≃ 250 cm−3) derived through the excitation of H2rotational levels and neutral carbon fine structure transitions to constrain the fundamental parameters that govern this transition. By comparing the theoretical and observed H Icolumn densities, we are able to bring an independent constraint on the incident ultra-violet (UV) flux, which we find to be in agreement with that estimated from the observed SFR.

Molecular Gas toward Supernova Remnant Cassiopeia A

Abstract We mapped 12CO J = 1–0, 12CO J = 2–1, 13CO J = 1–0, and 13CO J = 2–1 lines toward supernova remnant (SNR) Cassiopeia A with the IRAM 30 m telescope. The molecular clouds (MCs) along the line of sight of Cas A do not show optically thin, shock-broadened 12CO lines ( toward Cas A), or high-temperature features from shock heating ( toward Cas A). Therefore, we suggest that there is no physical evidence to support that the SNR is impacting the molecular gas. All the detected MCs are likely in front of Cas A, as implied by the HCO+ absorption line detected in the same velocity ranges. These MCs contribute H2 column densities of , , and in the west, south, and center of the SNR, respectively. The 20 K warm gas at is distributed along a large-scale molecular ridge in the south of Cas A. Part of the gas is projected onto Cas A, providing a foreground H2 mass of , consistent with the mass of cold dust (15–20 K; 2–4 ) found in front of the SNR. We suggest that the 20 K warm gas is heated by background cosmic rays with an ionization rate of . The cosmic rays and X-ray emission from Cas A are excluded as the heating sources of the clouds.

Physical Properties of Large-scale Galactic Filaments

Abstract The characterization of our Galaxy’s longest filamentary gas features has been the subject of several studies in recent years, producing not only a sizable sample of large-scale filaments, but also confusion as to whether all these features (e.g., “Bones,” “Giant Molecular Filaments”) are the same. They are not. We undertake the first standardized analysis of the physical properties (H2 column densities, dust temperatures, morphologies, radial column density profiles) and kinematics of large-scale filaments in the literature. We expand and improve upon prior analyses by using the same data sets, techniques, and spiral arm models to disentangle the filaments’ inherent properties from selection criteria and methodology. Our results suggest that the myriad filament-finding techniques are uncovering different physical structures, with length (11–269 pc), width (1–40 pc), mass ( ), aspect ratio (3:1–117:1), and high column density fraction (0.2%–100%) varying by over an order of magnitude across the sample of 45 filaments. We develop a radial profile-fitting code, RadFil, which is publicly available. We also perform a position–position–velocity (p–p–v) analysis on a subsample and find that while 60%–70% lie spatially in the plane of the Galaxy, only 30%–45% concurrently exhibit spatial and kinematic proximity to spiral arms. In a parameter space defined by aspect ratio, dust temperature, and column density, we broadly distinguish three filament categories, which could indicate different formation mechanisms or histories. Highly elongated “Bone-like” filaments show the most potential for tracing gross spiral structure (e.g., arms, spurs), while other categories could be large concentrations of molecular gas (giant molecular clouds, core complexes).

Optical and UV surface brightness of translucent dark nebulae

Context. Dark nebulae display a surface brightness because dust grains scatter light of the general interstellar radiation field (ISRF). High-galactic-latitudes dark nebulae are seen as bright nebulae when surrounded by transparent areas which have less scattered light from the general galactic dust layer. Aims. Photometry of the bright dark nebulae LDN 1780, LDN 1642, and LBN 406 shall be used to derive scattering properties of dust and to investigate the presence of UV fluorescence emission by molecular hydrogen and the extended red emission (ERE). Methods. We used multi-wavelength optical photometry and imaging at ground-based telescopes and archival imaging and spectroscopic UV data from the spaceborn GALEX and SPEAR/FIMS instruments. In the analysis we used Monte Carlo RT and both observational data and synthetic models for the ISRF in the solar neighbourhood. The line-of-sight extinctions through the clouds have been determined using near infrared excesses of background stars and the 200/250 μm far infrared emission by dust as measured using the ISO and Herschel space observatories. Results. The optical surface brightness of the three target clouds can be explained in terms of scattered light. The dust albedo ranges from ~0.58 at 3500 Å to ~0.72 at 7500 Å. The spectral energy distribution of LDN 1780 is explained in terms of optical depth and background scattered light effects instead of the original published suggestion in terms of ERE. The far-ultraviolet surface brightness of LDN 1780 cannot be explained by scattered light only. In LDN 1780, H2 fluorescent emission in the wavelength range 1400–1700 Å has been detected and analysed. Conclusions. Our albedo values are in good agreement with the predictions of the dust model of Weingartner and Draine and with the THEMIS CMM model for evolved core-mantle grains. The distribution of H2 fluorescent emission in LDN 1780 shows a pronounced dichotomy with a strong preference for its southern side where enhanced illumination is impinging from the Sco OB2 association and the O star ζ Oph. A good correlation is found between the H2 fluorescence and a previously mapped 21-cm excess emission. The H2 fluorescence emission in LDN 1780 has been modelled using a PDR code; the resulting values for H2 column density and the total gas density are consistent with the estimates derived from CO observations and optical extinction along the line of sight.

Molecular gas and dark neutral medium in the outskirts of Chamaeleon

Context. More gas is inferred to be present in molecular cloud complexes than can be accounted for by H I and CO emission, a phenomenon known as dark neutral medium (DNM) or CO-dark gas for the molecular part. Aims. We aim to investigate whether molecular gas can be detected in Chamaeleon where gas column densities in the DNM were inferred and CO emission was not detected. Methods. We took λ3 mm absorption profiles of HCO+ and other molecules toward 13 background quasars across the Chamaeleon complex, only one of which had detectable CO emission. We derived the H2 column density assuming N(HCO+)/N(H2) = 3 × 10−9 as before. Results. With the possible exception of one weak continuum target, HCO+ absorption was detected in all directions, C2H in eight directions and HCN in four directions. The sightlines divide into two groups according to their DNM content, with one group of eight directions having N(DNM) ≳ 2 × 1020 cm−2 and another group of five directions having N(DNM) < 0.5 × 1020 cm−2. The groups have comparable mean N(H I) associated with Chamaeleon 6−7 × 1020 cm−2 and total hydrogen column density per unit reddening 6−7 × 1021 cm−2 mag−1. They differ, however, in having quite different mean reddening 0.33 vs. 0.18 mag, mean N(DNM) 3.3 vs. 0.14 × 1020 cm−2 and mean molecular column density 2N(H2) = 5.6 vs. 0.8 × 1020 cm−2. The gas at more positive velocities is enriched in molecules and DNM. Conclusions. Overall the quantity of H2 inferred from HCO+ can fully account for the previously inferred DNM along the sightlines studied here. H2 is concentrated in the high-DNM group, where the molecular fraction is 46% vs. 13% otherwise and 38% overall. Thus, neutral gas in the outskirts of the complex is mostly atomic but the DNM is mostly molecular. Saturation of the H I emission line profile may occur along three of the four sightlines having the largest DNM column densities, but there is no substantial reservoir of “dark” atomic or molecular gas that remains undetected as part of the inventory of dark neutral medium.

A Study of the c-C3HD/c-C3H2 Ratio in Low-mass Star-forming Regions*

Abstract We use the deuteration of c-C3H2 to probe the physical parameters of starless and protostellar cores, related to their evolutionary states, and compare it to the N2H+-deuteration in order to study possible differences between the deuteration of C- and N-bearing species. We observed the main species c-C3H2, the singly and doubly deuterated species c-C3HD and c-C3D2, as well as the isotopologue c-H13CC2H toward 10 starless cores and five protostars in the Taurus and Perseus complexes. We examined the correlation between the N(c-C3HD)/N(c-C3H2) ratio and the dust temperature along with the H2 column density and the CO depletion factor. The resulting N(c-C3HD)/N(c-C3H2) ratio is, within error bars, consistent with 10% in all starless cores with detected c-C3HD. This also accounts for the protostars except for the source HH211, where we measure a high deuteration level of 23%. The deuteration of N2H+ follows the same trend but is considerably higher in the dynamically evolved core L1544. We find no significant correlation between the deuteration of c-C3H2 and the CO depletion factor among the starless and protostellar cores. Toward the latter the coolest objects show the largest deuterium fraction in c-C3H2. We show that the deuteration of c-C3H2 can trace the early phases of star formation and is comparable to that of N2H+. However, the largest c-C3H2 deuteration level is found toward protostellar cores, suggesting that while c-C3H2 is mainly frozen onto dust grains in the central regions of starless cores, active deuteration is taking place on ice.

SMA observations of polarized dust emission in solar-type Class 0 protostars: Magnetic field properties at envelope scales

Aims. Although from a theoretical point of view magnetic fields are believed to play a significant role during the early stages of star formation, especially during the main accretion phase, the magnetic field strength and topology is poorly constrained in the youngest accreting Class 0 protostars that lead to the formation of solar-type stars.Methods. We carried out observations of the polarized dust continuum emission with the SMA interferometer at 0.87 mm to probe the structure of the magnetic field in a sample of 12 low-mass Class 0 envelopes in nearby clouds, including both single protostars and multiple systems. Our SMA observations probed the envelope emission at scales ~600 − 5000 au with a spatial resolution ranging from 600 to 1500 au depending on the source distance.Results. We report the detection of linearly polarized dust continuum emission in all of our targets with average polarization fractions ranging from 2% to 10% in these protostellar envelopes. The polarization fraction decreases with the continuum flux density, which translates into a decrease with the H2column density within an individual envelope. Our analysis show that the envelope-scale magnetic field is preferentially observed either aligned or perpendicular to the outflow direction. Interestingly, our results suggest for the first time a relation between the orientation of the magnetic field and the rotational energy of envelopes, with a larger occurrence of misalignment in sources in which strong rotational motions are detected at hundreds to thousands of au scales. We also show that the best agreement between the magnetic field and outflow orientation is found in sources showing no small-scale multiplicity and no large disks at ~100 au scales.

Herschel spectral-line mapping of the HH211 protostellar system

Context. Mid- and far-infrared observations of the environment around embedded protostars reveal a plethora of high-excitation molecular and atomic emission lines. A number of different mechanisms for the origin of these lines have been proposed, including shocks induced by protostellar jets and radiation by the embedded protostar interacting with its immediate surroundings. Aims. We employ extended spectral-line maps that spatially resolve regions where diverse excitation processes appear to dominate. Studying the morphology and excitation of the most important molecular and atomic coolants, we aim to constrain the physical conditions around the embedded protostellar system HH 211-mm. Methods. Spectro-imaging observations with Herschel/PACS provide emission from major molecular (CO, H2O and OH) and atomic coolants (e.g. [O I]). Emission line maps reveal the morphology of the observed emissions and allow associations between the different species. Comparisons are also made with mid-infrared line maps from Spitzer and sub-mm interferometers. The excitation conditions of the detected molecular species along with the ortho-to-para ratio of water are assessed through Boltzmann diagrams. Further investigations focus on constraining the CO/H2 ratio in shocks and the mass flux of the atomic jet as traced by [O I]. Results. Molecular lines are exited mainly at the terminal bowshocks of the outflow and around the position of the protostar. All lines show maxima at the SE bowshock with the exception of water emission that peaks around the central source. Excitation analysis in all positions shows that CO and H2O are mainly thermally excited at Tex ~ 350 and 90 K respectively, with the CO showing a second temperature component at 750 K towards the SE peak. Excitation analysis breaks down in the case of OH at the blue-shifted bowshock, indicating that the molecule is non-thermally excited. Comparisons between the CO and H2 column densities suggest that the X[CO] value in shocks can be up to an order of magnitude lower than the canonical value of 10−4. The water ortho-to-para ratio around the protostar is only 0.65, indicating low-temperature water ice formation followed by non-distructive photodesorption from the dust grains. The two-sided total atomic mass flux estimated from the [O I] jet sums to 1.65 × 10−6 M⊙ yr−1, a value that is very close to the mass flux previously estimated for the SiO jet and the H2Outflow. Conclusions. The bulk of the cooling from CO, OH and [O I] is associated with gas excited in outflow shocks, with the blue-shifted (SE) outflow showing evidence of a shock-induced UV field responsible for the OH emission. Water lines around the protostar reveal a very low ortho-to-para ratio that can be interpreted in terms of formation from a primordial gas reservoir in the envelope. Finally comparison of the [O I] jet mass flux with the mass fluxes derived for SiO and H2 renders HH 211 the first embedded system where an atomic jet is demonstrably shown to possess enough momentum to drive the observed molecular jets and large-scale outflows.

Resolving the Nuclear Obscuring Disk in the Compton-thick Seyfert Galaxy NGC 5643 with ALMA

Abstract We present ALMA Band 6 12CO(2–1) line and rest-frame 232 GHz continuum observations of the nearby Compton-thick Seyfert galaxy NGC 5643 with angular resolutions 0.″11–0.″26 (9–21 pc). The CO(2–1) integrated line map reveals emission from the nuclear and circumnuclear region with a two-arm nuclear spiral extending ∼10″ on each side. The circumnuclear CO(2–1) kinematics can be fitted with a rotating disk, although there are regions with large residual velocities and/or velocity dispersions. The CO(2–1) line profiles of these regions show two different velocity components. One is ascribed to the circular component and the other to the interaction of the AGN outflow, as traced by the [O iii]λ5007 Å emission, with molecular gas in the disk a few hundred parsecs from the AGN. On nuclear scales, we detected an inclined CO(2–1) disk (diameter 26 pc, FWHM) oriented almost in a north–south direction. The CO(2–1) nuclear kinematics can be fitted with a rotating disk that appears to be tilted with respect to the large-scale disk. There are strong non-circular motions in the central 0.″2–0.″3 with velocities of up to 110 km s−1. In the absence of a nuclear bar, these motions could be explained as radial outflows in the nuclear disk. We estimate a total molecular gas mass for the nuclear disk of M(H2) = 1.1 × 107 M ⊙ and an H2 column density toward the location of the AGN of N(H2) ∼ 5 × 1023 cm−2, for a standard CO-to-H2 conversion factor. We interpret this nuclear molecular gas disk as the obscuring torus of NGC 5643 as well as the collimating structure of the ionization cone.

Gravity drives the evolution of infrared dark hubs: JVLA observations of SDC13

Context. Converging networks of interstellar filaments, that is hubs, have been recently linked to the formation of stellar clusters and massive stars. Understanding the relationship between the evolution of these systems and the formation of cores and stars inside them is at the heart of current star formation research. Aims. The goal is to study the kinematic and density structure of the SDC13 prototypical hub at high angular resolution to determine what drives its evolution and fragmentation. Methods. We have mapped SDC13, a ~1000 M⊙ infrared dark hub, in NH3(1,1) and NH3(2,2) emission lines, with both the Jansky Very Large Array and Green Bank Telescope. The high angular resolution achieved in the combined dataset allowed us to probe scales down to 0.07 pc. After fitting the ammonia lines, we computed the integrated intensities, centroid velocities and line widths, along with gas temperatures and H2 column densities. Results. The mass-per-unit-lengths of all four hub filaments are thermally super-critical, consistent with the presence of tens of gravitationally bound cores identified along them. These cores exhibit a regular separation of ~0.37 ± 0.16 pc suggesting gravitational instabilities running along these super-critical filaments are responsible for their fragmentation. The observed local increase of the dense gas velocity dispersion towards starless cores is believed to be a consequence of such fragmentation process. Using energy conservation arguments, we estimate that the gravitational to kinetic energy conversion efficiency in the SDC13 cores is ~35%. We see velocity gradient peaks towards ~63% of cores as expected during the early stages of filament fragmentation. Another clear observational signature is the presence of the most massive cores at the filaments’ junction, where the velocity dispersion is largest. We interpret this as the result of the hub morphology generating the largest acceleration gradients near the hub centre. Conclusions. We propose a scenario for the evolution of the SDC13 hub in which filaments first form as post-shock structures in a supersonic turbulent flow. As a result of the turbulent energy dissipation in the shock, the dense gas within the filaments is initially mostly sub-sonic. Then gravity takes over and starts shaping the evolution of the hub, both fragmenting filaments and pulling the gas towards the centre of the gravitational well. By doing so, gravitational energy is converted into kinetic energy in both local (cores) and global (hub centre) potential well minima. Furthermore, the generation of larger gravitational acceleration gradients at the filament junctions promotes the formation of more massive cores.

Constraining the H2 column density distribution at z ∼ 3 from composite DLA spectra

Abstract We present the detection of the average H2 absorption signal in the overall population of neutral gas absorption systems at z∼ 3 using composite absorption spectra built from the Sloan Digital Sky Survey-III damped Lyman α catalogue. We present a new technique to directly measure the H2 column density distribution function $f_{\rm H_2}(N)$ from the average H2 absorption signal. Assuming a power-law column density distribution, we obtain a slope $\beta = -1.29 \pm 0.06(\rm stat) \pm 0.10 (\rm sys)$ and an incidence rate of strong H2 absorptions [with N(H2) ≳ 1018 cm−2] to be $4.0 \pm 0.5(\rm stat) \pm 1.0 (\rm sys)\, \hbox{ per cent}$ in H i absorption systems with N(H i) ≥1020 cm−2. Assuming the same inflexion point where $f_{\rm H_2}(N)$ steepens as at z = 0, we estimate that the cosmological density of H2 in the column density range $\log N(\rm H_2) ({\rm cm}^{-2})= 18{\text{--}}22$ is ${\sim } 15\hbox{ per cent}$ of the total. We find one order of magnitude higher H2 incident rate in a sub-sample of extremely strong damped Lyman α absorption systems (DLAs) [$\log N(\rm{H\,\small {I}}) ({\rm cm}^{-2}) \ge 21.7$], which, together with the derived shape of $f_{\rm H_2}(N)$, suggests that the typical H i–H2 transition column density in DLAs is log N(H)(cm−2) ≳ 22.3 in agreement with theoretical expectations for the average (low) metallicity of DLAs at high-z.

The Orion Fingers: H2 Temperatures and Excitation in an Explosive Outflow

Abstract We measure H2 temperatures and column densities across the Orion Becklin-Neugebauer/Kleinmann-Low (BN/KL) explosive outflow from a set of 13 near-infrared (IR) H2 rovibrational emission lines observed with the TripleSpec spectrograph on Apache Point Observatory’s 3.5 m telescope. We find that most of the region is well characterized by a single temperature (∼2000–2500 K), which may be influenced by the limited range of upper-energy levels (6000–20,000 K) probed by our data set. The H2 column density maps indicate that warm H2 comprises 10−5–10−3 of the total H2 column density near the center of the outflow. Combining column density measurements for co-spatial H2 and CO at T = 2500 K, we measure a CO/H2 fractional abundance of 2 × 10−3 and discuss possible reasons why this value is in excess of the canonical 10−4 value, including dust attenuation, incorrect assumptions on co-spatiality of the H2 and CO emission, and chemical processing in an extreme environment. We model the radiative transfer of H2 in this region with ultraviolet (UV) pumping models to look for signatures of H2 fluorescence from H i Lyα pumping. Dissociative (J-type) shocks and nebular emission from the foreground Orion H ii region are considered as possible Lyα sources. From our radiative transfer models, we predict that signatures of Lyα pumping should be detectable in near-IR line ratios given a sufficiently strong source, but such a source is not present in the BN/KL outflow. The data are consistent with shocks as the H2 heating source.

The Properties of Planck Galactic Cold Clumps in the L1495 Dark Cloud

Abstract Planck Galactic Cold Clumps (PGCCs) possibly represent the early stages of star formation. To understand better the properties of PGCCs, we studied 16 PGCCs in the L1495 cloud with molecular lines and continuum data from Herschel, JCMT/SCUBA-2, and the PMO 13.7 m telescope. Thirty dense cores were identified in 16 PGCCs from 2D Gaussian fitting. The dense cores have dust temperatures of T d = 11–14 K, and H2 column densities of N H 2 = (0.36–2.5) × 1022 cm−2. We found that not all PGCCs contain prestellar objects. In general, the dense cores in PGCCs are usually at their earliest evolutionary stages. All the dense cores have non-thermal velocity dispersions larger than the thermal velocity dispersions from molecular line data, suggesting that the dense cores may be turbulence-dominated. We have calculated the virial parameter α and found that 14 of the dense cores have α <2, while 16 of the dense cores have α >2. This suggests that some of the dense cores are not bound in the absence of external pressure and magnetic fields. The column density profiles of dense cores were fitted. The sizes of the flat regions and core radii decrease with the evolution of dense cores. CO depletion was found to occur in all the dense cores, but is more significant in prestellar core candidates than in protostellar or starless cores. The protostellar cores inside the PGCCs are still at a very early evolutionary stage, sharing similar physical and chemical properties with the prestellar core candidates.

Cosmic-rays, gas, and dust in nearby anticentre clouds

Aim. H I 21-cm and 12CO 2.6-mm line emissions trace the atomic and molecular gas phases, respectively, but they miss most of the opaque H I and diffuse H2 present in the dark neutral medium (DNM) at the transition between the H I-bright and CO-bright regions. Jointly probing H I, CO, and DNM gas, we aim to constrain the threshold of the H I–H2 transition in visual extinction, AV, and in total hydrogen column densities, NHtot. We also aim to measure gas mass fractions in the different phases and to test their relation to cloud properties. Methods. We have used dust optical depth measurements at 353 GHz, γ-ray maps at GeV energies, and H I and CO line data to trace the gas column densities and map the DNM in nearby clouds toward the Galactic anticentre and Chamaeleon regions. We have selected a subset of 15 individual clouds, from diffuse to star-forming structures, in order to study the different phases across each cloud and to probe changes from cloud to cloud. Results. The atomic fraction of the total hydrogen column density is observed to decrease in the (0.6–1) × 1021 cm−2 range in NHtot (AV ≈ 0.4 mag) because of the formation of H2 molecules. The onset of detectable CO intensities varies by only a factor of 4 from cloud to cloud, between 0.6 × 1021 cm−2 and 2.5 × 1021 cm−2 in total gas column density. We observe larger H2 column densities than linearly inferred from the CO intensities at AV > 3 mag because of the large CO optical thickness; the additional H2 mass in this regime represents on average 20% of the CO-inferred molecular mass. In the DNM envelopes, we find that the fraction of diffuse CO-dark H2 in the molecular column densities decreases with increasing AV in a cloud. For a half molecular DNM, the fraction decreases from more than 80% at 0.4 mag to less than 20% beyond 2 mag. In mass, the DNM fraction varies with the cloud properties. Clouds with low peak CO intensities exhibit large CO-dark H2 fractions in molecular mass, in particular the diffuse clouds lying at high altitude above the Galactic plane. The mass present in the DNM envelopes appears to scale with the molecular mass seen in CO as MHDNM = 62 ± 7 MH2CO0.51 ± 0.02 across two decades in mass. Conclusions. The phase transitions in these clouds show both common trends and environmental differences. These findings will help support the theoretical modelling of H2 formation and the precise tracing of H2 in the interstellar medium.