Non-thermal Velocity Dispersion Research Articles

The Initial Conditions of Clustered Core Collapse: Multiwavelength Analysis of Oph A SM1N and N6 at 100 au Resolution

We present new Atacama Large Millimeter/submillimeter Array (ALMA) continuum and NH2D, N2D+, and H2D+ line emission at matched, ∼100 au resolution toward the dense star-forming cores SM1N and N6 within the Ophiuchus molecular cloud. We determine the density and temperature structure of SM1N based on radiative transfer modeling and simulated observations of the multiwavelength continuum emission at 0.8, 2, and 3 mm. We show that SM1N is best fit by either a broken power-law or Plummer-like density profile with high central densities (n ∼ 108 cm−3), and an inner transition radius of only ∼80–300 au. The free-fall time of the inner region is only a few ×103 yr. The continuum modeling rules out the presence of an embedded first hydrostatic core (FHSC) or protostar. SM1N is therefore a dynamically unstable but still starless core. We find that NH2D is likely depleted at high densities within SM1N. The nonthermal velocity dispersions increase from NH2D to N2H+ and H2D+, possibly tracing increasing (but still subsonic) infall speeds at higher densities as predicted by some models of starless core contraction. Toward N6, we confirm the previous ALMA detection of a faint, embedded point source (N6-mm) in 0.8 mm continuum emission. NH2D and N2D+ avoid N6-mm within ∼100 au, while H2D+ is not strongly detected toward N6. The distribution of these tracers is consistent with heating by a young, warm object. N6-mm thus remains one of the best candidate FHSCs detected so far, although its observed (sub)millimeter luminosity remains below predictions for FHSCs.

Surveys of clumps, cores, and condensations in Cygnus X. Temperature and nonthermal velocity dispersion revealed by VLA NH3 observations

The physical properties, evolution, and fragmentation of massive dense cores (MDCs 0.1 pc) are fundamental pieces in our understanding of high-mass star formation. We aim to characterize the temperature, velocity dispersion, and fragmentation of the MDCs in the giant molecular cloud and to investigate the stability and dynamics of these cores. We present the Karl G. Jansky Very Large Array (VLA) observations of the ($J,K$) = and inversion lines towards 35 MDCs in from which we calculated the temperature and velocity dispersion. We extracted 202 fragments ( 0.02 pc) from the moment-0 maps with the GAUSSCLUMPS algorithm. We analyzed the stability of the MDCs and their fragments through evaluating the corresponding kinetic, gravitational potential, and magnetic energies and the virial parameters. The MDCs in Cygnus X have a typical mean kinetic temperature of 20 K. Our virial analysis shows that many MDCs are in subvirialized states, indicating that the kinetic energy is insufficient to support these MDCs against their gravity. The calculated nonthermal velocity dispersions of most MDCs are at transonic to mildly supersonic levels, and the bulk motions make only a minor contribution to the velocity dispersion. Regarding the fragments, with 19 K, their nonthermal velocity dispersions are mostly trans-sonic to subsonic. Unless there is a strong magnetic field, most fragments are probably not in virialized states. We also find that most of the fragments are dynamically quiescent, while only a few are active due to star formation activity.

Ammonia observations of Planck cold cores

Single-pointing observations of NH$_ $\,(1,1) and (2,2) were conducted toward 672 Planck Early Cold Cores (ECCs) using the Nanshan 26-m radio telescope. Out of these sources, a detection rate of 37<!PCT!> (249 cores) was achieved, with a $\,(1,1) hyperfine structure detected in 187 cores and $\,(2,2) emission lines detected in 76 of them. The detection rate of $ is positively correlated with the continuum emission fluxes at a frequency of 857\,GHz. Among the observed 672 cores, sim 22<!PCT!> have associated stellar and infrared objects within the beam size (sim 2arcmin ). This suggests that most of the cores in our sample may be starless. The kinetic temperatures of the cores range from 8.9 to 20.7\,K, with an average of 12.3\,K, indicating a coupling between gas and dust temperatures. The ammonia column densities range from 3.6\,times \,1014 to 6.07\,times $, with a median value of 2.04\,times $. The fractional abundances of ammonia range from 0.3 to 9.7\,times $, with an average of 2.7\,times $, which is one order of magnitude larger than that of massive star-forming (MSF) regions and infrared dark clouds (IRDCs). The correlation between thermal and nonthermal velocity dispersion of the NH$_ $\,(1,1) inversion transition indicates the dominance of supersonic nonthermal motions in the dense gas traced by $, and the relationship between these two parameters in Planck cold cores is weaker, with lower values observed for both parameters relative to other samples under our examination. The cumulative distribution shapes of line widths in the Planck cold cores closely resemble those of the dense cores found in regions of Cepheus, in addition to Orion L1630 and L1641, with higher values compared to Ophiuchus.

Fragmentation and dynamics of dense gas structures in the proximity of massive young stellar object W42-MME

ABSTRACT We present an analysis of the dense gas structures in the immediate surroundings of the massive young stellar object (MYSO) W42-MME, using the high-resolution (0″.31 × 0″.25) Atacama Large Millimetre/submillimetre Array dust continuum and molecular line data. We performed a dendrogram analysis of H13CO+ (4–3) line data to study multiscale structures and their spatio–kinematic properties, and analysed the fragmentation and dynamics of dense structures down to ∼2000 au scale. Our results reveal 19 dense gas structures, out of which 12 are leaves and 7 are branches in dendrogram terminology. These structures exhibit transonic–supersonic gas motions (1$\lt \mathcal {M}\lt 5$) with overvirial states (αvir ≥ 2). The non-thermal velocity dispersion–size relation (σnt–L) of dendrogram structures shows a weak negative correlation, while the velocity dispersion across the sky ($\delta \mathit {V_{\rm lsr}}$) correlates positively with structure size (L). Velocity structure function (S2(l)1/2) analysis of H13CO+ data reveals strong power-law dependences with lag (l) up to a scale length of ≲6000 au. The mass–size (M–R) relation of dendrogram structures shows a positive correlation with power-law index of 1.73 ± 0.23, and the leaf L17 hosting W42-MME meets the mass–size conditions for massive star formation. Blue asymmetry is observed in the H12CO+ (4–3) line profiles of most of the leaves, indicating infall. Overall, our results observationally support the hierarchical and chaotic collapse scenario in the proximity of the MYSO W42-MME.

The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). VIII. Dynamics of Embedded Dense Cores

We present dynamical properties of 294 cores embedded in twelve IRDCs observed as part of the ASHES Survey. Protostellar cores have higher gas masses, surface densities, column densities, and volume densities than prestellar cores, indicating core mass growth from the prestellar to the protostellar phase. We find that ∼80% of cores with virial parameter (α) measurements are gravitationally bound (α < 2). We also find an anticorrelation between the mass and the virial parameter of cores, with massive cores having on average lower virial parameters. Protostellar cores are more gravitationally bound than prestellar cores, with an average virial parameter of 1.2 and 1.5, respectively. The observed nonthermal velocity dispersion (from N2D+ or DCO+) is consistent with simulations in which turbulence is continuously injected, whereas the core-to-core velocity dispersion is neither in agreement with driven nor decaying turbulence simulations. We find a not significant increment in the line velocity dispersion from prestellar to protostellar cores, suggesting that the dense gas within the core traced by these deuterated molecules is not yet severely affected by turbulence injected from outflow activity at the early evolutionary stages traced in ASHES. The most massive cores are strongly self-gravitating and have greater surface density, Mach number, and velocity dispersion than cores with lower masses. Dense cores do not have significant velocity shifts relative to their low-density envelopes, suggesting that dense cores are comoving with their envelopes. We conclude that the observed core properties are more in line with the predictions of clump-fed scenarios rather than with those of core-fed scenarios.

Turbulence measurements in the neutral ISM from Hi-21 cm emission–absorption spectra

Abstract We study the correlation between the non-thermal velocity dispersion ( $\sigma_{nth}$ ) and the length scale (L) in the neutral interstellar medium (ISM) using a large number of Hi gas components taken from various published Hi surveys and previous Hi studies. We notice that above the length-scale (L) of 0.40 pc, there is a power-law relationship between $\sigma_{nth}$ and L. However, below 0.40 pc, there is a break in the power law, where $\sigma_{nth}$ is not significantly correlated with L. It has been observed from the Markov chain Monte Carlo (MCMC) method that for the dataset of L $\gt$ 0.40 pc, the most probable values of intensity (A) and power-law index (p) are 1.14 and 0.55, respectively. Result of p suggests that the power law is steeper than the standard Kolmogorov law of turbulence. This is due to the dominance of clouds in the cold neutral medium. This is even more clear when we separate the clouds into two categories: one for L is $\gt$ 0.40 pc and the kinetic temperature ( $T_{k}$ ) is $\lt$ 250 K, which are in the cold neutral medium (CNM) and for other one where L is $\gt$ 0.40 pc and $T_{k}$ is between 250 and 5 000 K, which are in the thermally unstable phase (UNM). Most probable values of A and p are 1.14 and 0.67, respectively, in the CNM phase and 1.01 and 0.52, respectively, in the UNM phase. A greater number of data points is effective for the UNM phase in constructing a more accurate estimate of A and p, since most of the clouds in the UNM phase lie below 500 K. However, from the value of p in the CNM phase, it appears that there is a significant difference from the Kolmogorov scaling, which can be attributed to a shock-dominated medium.

The Role of Filamentary Structures in the Formation of Two Dense Cores, L1544 and L694-2

We present mapping results of two prestellar cores, L1544 and L694-2, embedded in filamentary clouds in C18O (3–2), 13CO (3–2), 12CO (3–2), HCO+ (4–3), and H13CO+ (4–3) lines with the James Clerk Maxwell Telescope to examine the role of the filamentary structures in the formation of dense cores in the clouds, with new distance estimates for L1544 ( pc) and L694-2 ( pc). From these observations, we found that the nonthermal velocity dispersion of two prestellar cores and their surrounding clouds is smaller than or comparable to the sound speed. This may indicate that the turbulence has already been dissipated for both filaments and cores during their formation time. We also found a λ/4 shift between the periodic oscillations in the velocity and the column density distributions, implying the possible presence of gravitational core-forming flow motion along the axis of the filament. The mass accretion rates due to these flow motions are estimated to be 2–3 M ⊙ Myr−1, being comparable to that for Serpens cloud but much smaller than those for the Hub filaments, cluster, or high-mass forming filaments by 1 or 2 orders of magnitude. From this study, we suggest that the filaments in our targets might be formed from the shock compression of colliding clouds, and then the cores are formed by gravitational fragmentation of the filaments to evolve to the prestellar stage. We conclude that the filamentary structures in the clouds play an important role in the entire process of formation of dense cores and their evolution.

On the accuracy of H i observations in molecular clouds – More cold H i than thought?

ABSTRACT We present a study of the cold atomic hydrogen (H i) content of molecular clouds simulated within the SILCC-Zoom project for solar neighbourhood conditions. We produce synthetic observations of H i at 21 cm, including H i self-absorption (HISA) and observational effects. We find that H i column densities, $N_{\rm H\, \small {\rm I}}$, of ≳1022 cm−2 are frequently reached in molecular clouds with H i temperatures as low as ∼10 K. Hence, HISA observations assuming a fixed H i temperature tend to underestimate the amount of cold H i in molecular clouds by a factor of 3–10 and produce an artificial upper limit of $N_{\rm H\, \small {\rm I}}$ around 1021 cm−2. We thus argue that the cold H i mass in molecular clouds could be a factor of a few higher than previously estimated. Also, $N_{\rm H\, \small {\rm I}}$ PDFs obtained from HISA observations might be subject to observational biases and should be considered with caution. The underestimation of cold H i in HISA observations is due to both the large H i temperature variations and the effect of noise in regions of high optical depth. We find optical depths of cold H i around 1–10, making optical depth corrections essential. We show that the high H i column densities (≳1022 cm−2) can in parts be attributed to the occurrence of up to 10 individual H i–H2 transitions along the line of sight. This is also reflected in the spectra, necessitating Gaussian decomposition algorithms for their in-depth analysis. However, also for a single H i–H2 transition, $N_{\rm H\, \small {\rm I}}$ frequently exceeds 1021 cm−2, challenging one-dimensional, semi-analytical models. This is due to non-equilibrium chemistry effects and the fact that H i–H2 transition regions usually do not possess a one-dimensional geometry. Finally, we show that the H i gas is moderately supersonic with Mach numbers of a few. The corresponding non-thermal velocity dispersion can be determined via HISA observations within a factor of ∼2.

A FAST survey of H I narrow-line self-absorptions in Planck Galactic cold clumps guided by HC3N

Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we search for H I narrow-line self-absorption (HINSA) features in twelve Planck Galactic cold clumps (PGCCs), the starless core L1521B, and four star forming sources. Eight of the 12 PGCCs have detected emission of J = 2–1 of cyanoacetylene (HC3N). With an improved HINSA extraction method more robust for weaker and blended features with high velocity resolution, the detection rates of HINSA in PGCCs are high, at 92% overall (11/12) and 87% (7/8) among sources with HC3N J = 2–1 emissions. Combining the data of molecular spectra and Planck continuum maps, we studied the morphologies, dynamics, abundances and excitations of H I, CO and HC3N in PGCCs. The spatial distribution of HINSA is similar to that of CO, implying that HINSA features are confined to regions within and around CO emission kernels. HINSA tends to be not detected in regions associated with warm dust and background ionizing radiation, as well as regions associated with stellar objects. The L-band continnum and average background H I emission may be non-ignorable for the excitation of HINSA. The abundances of cold H I in PGCCs are approximately 3 × 10−4, and vary within a factor of ~3. The non-thermal velocity dispersions traced by C18O J = 1–0 and HINSA are consistent with each other (0.1–0.4 km s−1), larger than the typical value of HC3N (~0.1 km s−1). Carbon chain molecule (CCM) abundant PGCCs provide a good sample to study HINSA.

Planck Galactic Cold Clumps at High Galactic Latitude—a Study with CO Lines

Abstract Gas at high Galactic latitude is a relatively little noticed component of the interstellar medium. In an effort to address this, 41 Planck Galactic Cold Clumps at high Galactic latitude (HGal; ∣b∣ &gt; 25°) were observed in 12CO, 13CO, and C18O J = 1−0 lines, using the Purple Mountain Observatory 13.7 m telescope. 12CO (1−0) and 13CO (1−0) emission was detected in all clumps, while C18O (1−0) emission was only seen in 16 clumps. The highest and average latitudes are 71.°4 and 37.°8, respectively. Fifty-one velocity components were obtained, and then each was identified as a single clump. Thirty-three clumps were further mapped at 1′ resolution, and 54 dense cores were extracted. Among dense cores, the average excitation temperature T ex of 12CO is 10.3 K. The average line widths of thermal and nonthermal velocity dispersions are 0.19 and 0.46 km s−1, respectively, suggesting that these cores are dominated by turbulence. Distances of the HGal clumps given by Gaia dust reddening are about 120–360 pc. The ratio of X 13/X 18 is significantly higher than that in the solar neighborhood, implying that HGal gas has a different star formation history compared to the gas in the Galactic disk. HGal cores with sizes from 0.01 to 0.1 pc show no notable Larson’s relation, and the turbulence remains supersonic down to a scale of slightly below 0.1 pc. None of the HGal cores that bear masses from 0.01 to 1 M ⊙ are gravitationally bound, and all appear to be confined by outer pressure.

Molecular Cloud Cores with High Deuterium Fractions: Nobeyama Mapping Survey

Abstract We present the results of on-the-fly mapping observations of 44 fields containing 107 SCUBA-2 cores in the emission lines of molecules N2H+, HC3N, and CCS at 82–94 GHz using the Nobeyama 45 m telescope. This study aimed at investigating the physical properties of cores that show high deuterium fractions and might be close to the onset of star formation. We found that the distributions of the N2H+ and HC3N line emissions are approximately similar to the distribution of the 850 μm dust continuum emission, whereas the CCS line emission is often undetected or is distributed in a clumpy structure surrounding the peak position of the 850 μm dust continuum emission. Occasionally (12%), we observe CCS emission, which is an early-type gas tracer toward the young stellar object, probably due to local high excitation. Evolution toward star formation does not immediately affect the nonthermal velocity dispersion.

Neutral versus Ion Line Widths in Barnard 5: Evidence for Penetration by Magnetohydrodynamic Waves

Abstract Dense cores are the final place where turbulence is dissipated. It has been proposed from theoretical arguments that the nonthermal velocity dispersion should be narrower both for molecular ions (compared to neutrals) and for transitions with higher critical densities. To test these hypotheses, we compare the velocity dispersion of (1–0) ( = 6 × 104 ) and ( = 2 × 103 ), in the dense core Barnard 5. We analyze well-resolved and high signal-to-noise observations of (1,1) and (2,2) obtained with combining Robert C. Byrd Green Bank Telescope (GBT) and Very Large Array (VLA) data, and (1–0) obtained with GBT Argus, which present a similar morphology. Surprisingly, the nonthermal velocity dispersion of the ion is systematically higher than that of the neutral by 20%. The derived sonic Mach number, , has peak values and for and , respectively. This observed difference may indicate that the magnetic field even deep within the dense core is still oscillating, as it is in the turbulent region outside the core. The ions should be more strongly dynamically coupled to this oscillating field than the neutrals, thus accounting for their broader line width. If corroborated by further observations, this finding would shed additional light on the transition to quiescence in dense cores.

OH Evolution in Molecular Clouds

We have conducted a OH 18 cm survey toward 141 molecular clouds in various environments, including 33 optical dark clouds, 98 Planck Galactic cold clumps (PGCCs), and 10 Spitzer dark clouds with the Arecibo telescope. The deviations from local thermal equilibrium are common for intensity ratios of both OH main lines and satellite lines. Line intensity of OH 1667 MHz is found to correlate linearly with visual extinction when is less than 3 mag. It was converted into OH column density by adopting the excitation temperature derived from Monte Carlo simulations with 1σ uncertainty. The relationship between OH abundance X(OH) relative to H2 and is found to follow an empirical formula, Linear correlation is found between OH and 13CO intensity. Besides, nonthermal velocity dispersions of OH and 13CO are closely correlated. These results imply tight chemical evolution and spatial occupation between OH and 13CO. No obvious correlation is found between column density and nonthermal velocity dispersion of OH and H i narrow self-absorption (HINSA), indicating different chemical evolution and spatial volume occupation between OH and HINSA. Using the age information of HINSA analysis, OH abundance X(OH) is found to increase linearly with cloud age, which is consistent with previous simulations. Fourteen OH components without corresponding CO emission were detected, implying the effectiveness of OH in tracing the "CO-dark" molecular gas.

NH3 (1,1) hyperfine intensity anomalies in the Orion A molecular cloud

Ammonia (NH3) inversion lines, with their numerous hyperfine components, are a common tracer used in studies of molecular clouds (MCs). In local thermodynamical equilibrium, the two inner satellite lines (ISLs) and the two outer satellite lines (OSLs) of the NH3(J, K) = (1,1) transition are each predicted to have equal intensities. However, hyperfine intensity anomalies (HIAs) are observed to be omnipresent in star formation regions, a characteristic which is still not fully understood. In addressing this issue, we find that the computation method of the HIA by the ratio of the peak intensities may have defects, especially when used to process the spectra with low-velocity dispersions. Therefore, we defined the integrated HIAs of the ISLs (HIAIS) and OSLs (HIAOS) by the ratio of their redshifted to blueshifted integrated intensities (unity implies no anomaly) and developed a procedure to calculate them. Based on this procedure, we present a systematic study of the integrated HIAs in the northern part of the Orion A MC. We find that integrated HIAIS and HIAOS are commonly present in the Orion A MC and no clear distinction is found at different locations of the MC. The medians of the integrated HIAIS and HIAOS are 0.921 ± 0.003 and 1.422 ± 0.009, respectively, which is consistent with the HIA core model and inconsistent with the collapse or expansion (CE) model. In the selection of those 170 positions, where both integrated HIAs deviate by more than 3σ from unity, most (166) are characterized by HIAIS &lt; 1 and HIAOS &gt; 1, which suggests that the HIA core model plays a more significant role than the CE model. The remaining four positions are consistent with the CE model. We compare the integrated HIAs with the para-NH3 column density (N(para-NH3)), kinetic temperature (TK), total velocity dispersion (σv), non-thermal velocity dispersion (σNT), and the total opacity of the NH3(J, K) = (1,1) line (τ0). The integrated HIAIS and HIAOS are almost independent of N(para-NH3). The integrated HIAIS decreases slightly from unity (no anomaly) to about 0.7 with increasing TK, σv, and σNT. The integrated HIAOS is independent of TK and reaches values close to unity with increasing σv and σNT. The integrated HIAIS is almost independent of τ0, while the integrated HIAOS rises with τ0, thus showing higher anomalies. These correlations cannot be fully explained by either the HIA core nor the CE model.

Ubiquitous NH3 supersonic component in L1688 coherent cores

Context. Star formation takes place in cold dense cores in molecular clouds. Earlier observations have found that dense cores exhibit subsonic non-thermal velocity dispersions. In contrast, CO observations show that the ambient large-scale cloud is warmer and has supersonic velocity dispersions. Aims. We aim to study the ammonia (NH3) molecular line profiles with exquisite sensitivity towards the coherent cores in L1688 in order to study their kinematical properties in unprecedented detail. Methods. We used NH3 (1,1) and (2,2) data from the first data release (DR1) in the Green Bank Ammonia Survey (GAS). We first smoothed the data to a larger beam of 1′ to obtain substantially more extended maps of velocity dispersion and kinetic temperature, compared to the DR1 maps. We then identified the coherent cores in the cloud and analysed the averaged line profiles towards the cores. Results. For the first time, we detected a faint (mean NH3(1,1) peak brightness &lt; 0.25 K in TMB), supersonic component towards all the coherent cores in L1688. We fitted two components, one broad and one narrow, and derived the kinetic temperature and velocity dispersion of each component. The broad components towards all cores have supersonic linewidths (ℳS ≥ 1). This component biases the estimate of the narrow dense core component’s velocity dispersion by ≈28% and the kinetic temperature by ≈10%, on average, as compared to the results from single-component fits. Conclusions. Neglecting this ubiquitous presence of a broad component towards all coherent cores causes the typical single-component fit to overestimate the temperature and velocity dispersion. This affects the derived detailed physical structure and stability of the cores estimated from NH3 observations.

ALMA Observations of NGC 6334S. I. Forming Massive Stars and Clusters in Subsonic and Transonic Filamentary Clouds

Abstract We present Atacama Large Millimeter/submillimeter Array (ALMA) and Karl G. Jansky Very Large Array (JVLA) observations of the massive infrared dark cloud NGC 6334S (also known as IRDC G350.56+0.44), located at the southwestern end of the NGC 6334 molecular cloud complex. The H13CO+ and NH2D lines covered by the ALMA observations at a ∼3″ angular resolution (∼0.02 pc) reveal that the spatially unresolved nonthermal motions are predominantly subsonic and transonic, a condition analogous to that found in low-mass star-forming molecular clouds. The observed supersonic nonthermal velocity dispersions in massive star-forming regions, often reported in the literature, might be significantly biased by poor spatial resolutions that broaden the observed line widths owing to unresolved motions within the telescope beam. Our 3 mm continuum image resolves 49 dense cores, whose masses range from 0.17 to 14 M ⊙. The majority of them are resolved with multiple velocity components. Our analyses of these gas velocity components find an anticorrelation between the gas mass and the virial parameter. This implies that the more massive structures tend to be more gravitationally unstable. Finally, we find that the external pressure in the NGC 6334S cloud is important in confining these dense structures and may play a role in the formation of dense cores and, subsequently, the embedded young stars.

TRAO Survey of Nearby Filamentary Molecular Clouds, the Universal Nursery of Stars (TRAO FUNS). I. Dynamics and Chemistry of L1478 in the California Molecular Cloud

Abstract “TRAO FUNS” is a project to survey the Gould Belt’s clouds in molecular lines. This paper presents its first results on the central region of the California molecular cloud, L1478. We performed on-the-fly mapping observations using the Taeduk Radio Astronomy Observatory 14 m single-dish telescope equipped with a 16 multibeam array covering a ∼1.0 square degree area of this region using C 18 O ( 1 – 0 ) , mainly tracing low-density clouds, and a ∼460 square arcminute area using N 2 H + ( 1 – 0 ) , mainly tracing dense cores. CS ( 2 – 1 ) and SO ( 3 2 – 2 1 ) were also used simultaneously to map a ∼440 square arcminute area of this region. We identified 10 filaments by applying the dendrogram technique to the C 18 O data cube and 8 dense N 2 H + cores using FellWalker. Basic physical properties of filaments such as mass, length, width, velocity field, and velocity dispersion are derived. It is found that L1478 consists of several filaments with slightly different velocities. Particularly, the filaments that are supercritical are found to contain dense cores detected in N 2 H + . A comparison of nonthermal velocity dispersions derived from C 18 O and N 2 H + for the filaments and dense cores indicates that some of the dense cores share kinematics similar to those of the surrounding filaments, while several dense cores have different kinematics from those of their filaments. This suggests that the formation mechanism of dense cores and filaments can be different in individual filaments depending on their morphologies and environments.

Magnetic Field Structure of Dense Cores Using Spectroscopic Methods

We develop a new ''core field structure'' (CFS) model to predict the magnetic field strength and magnetic field fluctuation profile of dense cores using gas kinematics. We use spatially resolved observations of the nonthermal velocity dispersion from the Green Bank Ammonia survey along with column density maps from SCUBA-2 to estimate the magnetic field strength across seven dense cores located in the L1688 region of Ophiuchus. The CFS model predicts the profile of the relative field fluctuation, which is related to the observable dispersion in direction of the polarization vectors. Within the context of our model we find that all the cores have a transcritical mass-to-flux ratio.

Thermal balance and comparison of gas and dust properties of dense clumps in the Hi-GAL survey

We present a comparative study of physical properties derived from gas and dust emission in a sample of 1068 dense Galactic clumps. The sources are selected from the crossmatch of the Hi-GAL survey with 16 catalogues of NH$_3$ line emission in its lowest inversion (1,1) and (2,2) transitions. The sample covers a large range in masses and bolometric luminosities, with surface densities above $\Sigma=0.1$ g cm$^{-2}$ and with low virial parameters $\alpha<1$. The comparison between dust and gas properties shows an overall agreement between $T_{\textit{kin}}$ and $T_{\textit{dust}}$ at volumetric densities $n\gtrsim1.2\times10^{4}$ cm$^{-3}$, and a median fractional abundance $\chi$(NH$_3$)$=1.46\times10^{-8}$. While the protostellar clumps in the sample have small differences between $T_{\textit{kin}}$ and $T_{\textit{dust}}$, prestellar clumps have a median ratio $T_{\textit{kin}}/T_{\textit{dust}}=1.24$, suggesting that these sources are thermally decoupled. A correlation is found between the evolutionary tracer $L/M$ and the parameters $T_{\textit{kin}}/T_{\textit{dust}}$ and $\chi$(NH$_3$) in prestellar sources and protostellar clumps with $L/M<1$ L$_\odot$ M$_\odot^{-1}$. In addition, a weak correlation is found between non-thermal velocity dispersion and the $L/M$ parameter, possibly indicating an increase of turbulence with protostellar evolution in the interior of clumps. Finally, different processes are discussed to explain the differences between gas and dust temperatures in prestellar candidates, and the origin of non-thermal motions observed in the clumps.

Recovering the Physical Properties of Molecular Gas in Galaxies from CO SLED Modeling

Abstract Modeling of the spectral line energy distribution (SLED) of the CO molecule can reveal the physical conditions (temperature and density) of molecular gas in Galactic clouds and other galaxies. Recently, the Herschel Space Observatory and ALMA have offered, for the first time, a comprehensive view of the rotational J = 4−3 through J = 13−12 lines, which arise from a complex, diverse range of physical conditions that must be simplified to one, two, or three components when modeled. Here we investigate the recoverability of physical conditions from SLEDs produced by galaxy evolution simulations containing a large dynamical range in physical properties. These simulated SLEDs were generally fit well by one component of gas whose properties largely resemble or slightly underestimate the luminosity-weighted properties of the simulations when clumping due to nonthermal velocity dispersion is taken into account. If only modeling the first three rotational lines, the median values of the marginalized parameter distributions better represent the luminosity-weighted properties of the simulations, but the uncertainties in the fitted parameters are nearly an order of magnitude, compared to approximately 0.2 dex in the “best-case” scenario of a fully sampled SLED through J = 10−9. This study demonstrates that while common CO SLED modeling techniques cannot reveal the underlying complexities of the molecular gas, they can distinguish bulk luminosity-weighted properties that vary with star formation surface densities and galaxy evolution, if a sufficient number of lines are detected and modeled.