Infrared Dark Clouds Research Articles

Near-infrared imaging polarimetry toward M 17 SWex

AbstractWe conducted near-infrared ($\mathit {JHK}_{\rm s}$) imaging polarimetry toward the infrared dark cloud (IRDC) M 17 SWex, including almost all of the IRDC filaments as well as its outskirts, with the polarimeter SIRPOL on the IRSF 1.4 m telescope. We revealed the magnetic fields of M 17 SWex with our polarization-detected sources that were selected by some criteria based on their near-IR colors and the column densities toward them, which were derived from the Herschel data. The selected sources indicate not only that the ordered magnetic field is perpendicular to the cloud elongation as a whole, but also that at both ends of the elongated cloud the magnetic field appears to be bent toward its central part, i.e., a large-scale hourglass-shaped magnetic field perpendicular to the cloud elongation. In addition to this general trend, the elongations of the filamentary subregions within the dense parts of the cloud appear to be mostly perpendicular to their local magnetic fields, while the magnetic fields of the outskirts appear to follow the thin filaments that protrude from the dense parts. The magnetic strengths were estimated to be ∼70–$300\, \mu$G in the subregions, of which the lengths and average number densities are ∼3–9 pc and ∼2–7 × 103 cm−3, respectively, by the Davis–Chandrasekhar–Fermi method with the angular dispersion of our polarization data and the velocity dispersion derived from the C18O (J = 1–0) data obtained by the Nobeyama 45 m telescope. These field configurations and our magnetic stability analysis of the subregions imply that the magnetic field has controlled the formation/evolution of the M 17 SWex cloud.

A survey of molecular cores in M 17 SWex

Abstract A survey of molecular cores covering the infrared dark cloud known as the M 17 southwest extension (M 17 SWex) has been carried out with the 45 m Nobeyama Radio Telescope. Based on the N2H+ (J = 1–0) data obtained, we have identified 46 individual cores whose masses are in the range from 43 to $3026\, {M}_{\odot }$. We examined the relationship between the physical parameters of the cores and those of young stellar objects (YSOs) associated with the cores found in the literature. The comparison of the virial mass and the core mass indicates that most of the cores can be gravitationally stable if we assume a large external pressure. Among the 46 cores, we found four massive cores with YSOs. They have large masses of $\gtrsim 1000\, M_{\odot }$ and line widths of $\gtrsim 2.5\:$km s−1 which are similar to those of clumps forming high-mass stars. However, previous studies have shown that there is no active massive star formation in this region. Recent measurements of near-infrared polarization imply that the magnetic field around M 17 SWex is likely to be strong enough to support the cores against self-gravity. We therefore suggest that the magnetic field may prevent the cores from collapsing, causing the low level of massive star formation in M 17 SWex.

A SiO J = 5 → 4 Survey Toward Massive Star Formation Regions

Abstract We performed a survey in the SiO J = 5 → 4 line toward a sample of 199 Galactic massive star-forming regions at different evolutionary stages with the Submillimeter Telescope (SMT) 10 m and Caltech Submillimeter Observatory (CSO) 10.4 m telescopes. The sample consists of 44 infrared dark clouds (IRDCs), 86 protostellar candidates, and 69 young H ii regions. We detected SiO J = 5 → 4 line emission in 102 sources, with a detection rate of 57%, 37%, and 65% for IRDCs, protostellar candidates, and young H ii regions, respectively. We find both broad line emissions with full widths at zero power >20 km s−1 and narrow line emissions of SiO in objects at various evolutionary stages, likely associated with high-velocity shocks and low-velocity shocks, respectively. The SiO luminosities do not show apparent differences among various evolutionary stages in our sample. We find no correlation between the SiO abundance and the luminosity-to-mass ratio, indicating that the SiO abundance does not vary significantly in regions at different evolutionary stages of star formation.

Gravity, Magnetic Field, and Turbulence: Relative Importance and Impact on Fragmentation in the Infrared Dark Cloud G34.43+00.24

Abstract We investigate the interplay between magnetic (B) field, gravity, and turbulence in the fragmentation process of cores within the filamentary infrared dark cloud G34.43+00.24. We observe the magnetic field morphology across G34.43, traced with thermal dust polarization at 350 μm with an angular resolution of 10″ (0.18 pc), and compare with the kinematics obtained from N2H+ across the filament. We derive local velocity gradients from N2H+, tracing motion in the plane of sky, and compare with the observed local B field orientations in the plane of sky. The B field orientations are found to be perpendicular to the long axis of the filament toward the MM1 and MM2 ridge, suggesting that the B field can guide material toward the filament. Toward MM3, the B field orientations appear more parallel to the filament and aligned with the elongated core of MM3, indicating a different role of the B field. In addition to a large-scale east–west velocity gradient, we find a close alignment between local B field orientations and local velocity gradients toward the MM1/MM2 ridge. This local correlation in alignment suggests that gas motions are influenced by the B field morphology or vice versa. Additionally, this alignment seems to become even closer with increasing integrated emission in N2H+, possibly indicating that a growing gravitational pull alignes the B field and gas motion more and more. We analyze and quantify B field, gravity, turbulence, and their relative importance toward the MM1, MM2, and MM3 regions with various techniques over two scales, a larger clump area at 2 pc scale and the smaller core area at 0.6 pc scale. While gravitational energy, B field, and turbulent pressure all grow systematically from large to small scale, the ratios among the three constituents clearly develop differently over scale. We propose that this varying relative importance between B field, gravity, and turbulence over scale drives and explains the different fragmentation types seen at subparsec scale (no fragmentation in MM1; aligned fragmentation in MM2; clustered fragmentation in MM3). We discuss uncertainties, subtleties, and the robustness of our conclusion, and we stress that a multiscale joint analysis is required to understand the dynamics in these systems.

Molecular environs and triggered star formation around the large Galactic infrared bubble N 24

Abstract A multi-wavelength analysis of the large Galactic infrared bubble N 24 is presented in this paper in order to investigate the molecular and star-formation environment around expanding H ii regions. Using archival data from Herschel and ATLASGAL, the distribution and physical properties of the dust over the entire bubble are studied. Using the Clumpfind2d algorithm, 23 dense clumps are identified, with sizes and masses in the range 0.65–1.73 pc and 600–16 300 M⊙, respectively. To analyse the molecular environment in N 24, observations of NH3 (1,1) and (2,2) were carried out using the Nanshan 26-m radio telescope. Analysis of the kinetic temperature and gravitational stability of these clumps suggests gravitational collapse in several of them. The mass–size distributions of the clumps and the presence of massive young protostars indicate that the shell of N 24 is a region of ongoing massive-star formation. The compatibility of the dynamical and fragmentation timescales and the overabundance of young stellar objects and clumps on the rim suggest that the ‘collect-and-collapse’ mechanism is in play at the boundary of the bubble, but the existence of the infrared dark cloud at the edge of bubble indicates that a ‘radiation-driven implosion’ mechanism may also have played a role there.

Massive Young Stellar Objects and Outflow in the Infrared Dark Cloud G79.3+0.3

Abstract G79.3+0.3 is an infrared dark cloud in the Cygnus-X complex that is home to massive deeply embedded young stellar objects (YSOs). We have produced a Submillimeter Array (SMA) 1.3 mm continuum image and 12CO line maps of the eastern section of G79.3+0.3 in which we detect five separate YSOs. We have estimated physical parameters for these five YSOs and others in the vicinity of G79.3+0.3 by fitting existing photometry from Spitzer, Herschel, and ground-based telescopes to spectral energy distribution models. Through these model fits we find that the most massive YSOs seen in the SMA 1.3 mm continuum emission have masses in the 5–6 M ☉ range. One of the SMA sources was observed to power a massive collimated 12CO outflow extending at least 0.94 pc in both directions from the protostar, with a total mass of 0.83 M ☉ and a dynamical timescale of 23 kyr.

Filamentary Accretion Flows in the Infrared Dark Cloud G14.225–0.506 Revealed by ALMA

Abstract Filaments are ubiquitous structures in molecular clouds and play an important role in the mass assembly of stars. We present results of dynamical stability analyses for filaments in the infrared dark cloud G14.225−0.506, where a delayed onset of massive star formation was reported in the two hubs at the convergence of multiple filaments of parsec length. Full-synthesis imaging is performed with the Atacama Large Millimeter/submillimeter Array to map the emission in two hub-filament systems with a spatial resolution of ∼0.034 pc. Kinematics are derived from a sophisticated spectral fitting algorithm that accounts for line blending, large optical depth, and multiple velocity components. We identify five velocity coherent filaments and derive their velocity gradients with principal component analysis. The mass accretion rates along the filaments are up to and are significant enough to affect the hub dynamics within one freefall time (∼105 yr). The filaments are in equilibrium with virial parameter α vir ∼ 1.2. We compare α vir measured in the filaments, filaments, 870 μm dense clumps, and 3 mm dense cores. The decreasing trend in α vir with decreasing spatial scales persists, suggesting an increasingly important role of gravity at small scales. Meanwhile, α vir also decreases with decreasing nonthermal motions. In combination with the absence of high-mass protostars and massive cores, our results are consistent with the global hierarchical collapse scenario.

Magnetized interstellar molecular clouds – II. The large-scale structure and dynamics of filamentary molecular clouds

Abstract We perform ideal magnetohydrodynamics high-resolution adaptive mesh refinement simulations with driven turbulence and self-gravity and find that long filamentary molecular clouds are formed at the converging locations of large-scale turbulence flows and the filaments are bounded by gravity. The magnetic field helps shape and reinforce the long filamentary structures. The main filamentary cloud has a length of ∼4.4 pc. Instead of a monolithic cylindrical structure, the main cloud is shown to be a collection of fibre/web-like substructures similar to filamentary clouds such as L1495. Unless the line-of-sight is close to the mean field direction, the large-scale magnetic field and striations in the simulation are found roughly perpendicular to the long axis of the main cloud, similar to L1495. This provides strong support for a large-scale moderately strong magnetic field surrounding L1495. We find that the projection effect from observations can lead to incorrect interpretations of the true three-dimensional physical shape, size, and velocity structure of the clouds. Helical magnetic field structures found around filamentary clouds that are interpreted from Zeeman observations can be explained by a simple bending of the magnetic field that pierces through the cloud. We demonstrate that two dark clouds form a T-shaped configuration that is strikingly similar to the infrared dark cloud SDC13, leading to the interpretation that SDC13 results from a collision of two long filamentary clouds. We show that a moderately strong magnetic field (${{\cal M}_{\rm A}}\sim 1$) is crucial for maintaining a long and slender filamentary cloud for a long period of time ∼0.5 Myr.

Widespread Molecular Outflows in the Infrared Dark Cloud G28.37+0.07: Indications of Orthogonal Outflow-filament Alignment

Abstract We present ALMA CO(2-1) observations toward a massive infrared dark cloud G28.37+0.07. The ALMA data reveal numerous molecular (CO) outflows with a wide range of sizes throughout the cloud. Sixty-two 1.3 mm continuum cores were identified to be driving molecular outflows. We have determined the position angle in the plane-of-sky of 120 CO outflow lobes and studied their distribution. We find that the distribution of the plane-of-sky outflow position angles peaks at about 100°, corresponding to a concentration of outflows with an approximately east–west direction. For most outflows, we have been able to estimate the plane-of-sky angle between the outflow axis and the filament that harbors the protostar that powers the outflow. Statistical tests strongly indicate that the distribution of outflow-filament orientations is consistent with most outflow axes being mostly orthogonal to their parent filament in three dimensions. Such alignment may result from filament fragmentation or continuous mass transportation from the filament to the embedded protostellar core. The latter is suggested by recent numerical studies with moderately strong magnetic fields.

The Core Mass Function in the Infrared Dark Cloud G28.37+0.07

Abstract In this paper, we analyze the 1.3 mm continuum ALMA data that cover the majority of the infrared dark cloud (IRDC) G28.37+0.07. With a spatial resolution of 0.″5 (2500 au at 5 kpc), the continuum image reveals five groups of dense cores. Each core group has a projected physical scale of about 1 pc, with core masses spanning a dynamic range of about 100. We use the dendrogram method (astrodendro) and a newly developed graph method (astrograph) to identify individual cores. The core masses are estimated through the millimeter continuum flux, assuming constant temperature and using an NH3-based gas temperature. We construct core mass functions (CMFs) based on the two methods and fit a power-law relation dN/d log M ∝ M −α to the CMFs for M > 0.79 M ⊙. In the constant-temperature scenario, astrograph gives α = 0.80 ± 0.10, while astrodendro gives α = 0.71 ± 0.11, both significantly shallower than the Salpeter-type initial mass function with α = 1.35. In the scenario where the NH3 gas temperature is applied to cores, astrograph gives α = 1.37 ± 0.06, while astrodendro gives α = 0.87 ± 0.07. Regional CMF slope variation is seen between the core groups. We also compare CMFs in three different environments, including IRDC G28.37+0.07, IRDC clumps, and G286.21+0.17, using the identical dendrogram method. Results show that IRDCs have smaller α than the cluster-forming cloud G286.21+0.17.

Multicomponent Kinematics in a Massive Filamentary Infrared Dark Cloud

Abstract To probe the initial conditions for high-mass star and cluster formation, we investigate the properties of dense filaments within the infrared dark cloud (IRDC) G035.39–00.33 (G035.39) in a combined Very Large Array and Green Bank Telescope mosaic tracing the NH3 (1, 1) and (2, 2) emission down to 0.08 pc scales. Using agglomerative hierarchical clustering on multiple line-of-sight velocity component fitting results, we identify seven extended velocity-coherent components in our data, likely representing spatially coherent physical structures, some exhibiting complex gas motions. The velocity gradient magnitude distribution peaks at its mode of 0.35 and has a long tail extending into higher values of 1.5–2 , and it is generally consistent with those found toward the same cloud in other molecular tracers and with the values found toward nearby low-mass dense cloud cores at the same scales. Contrary to observational and theoretical expectations, we find the nonthermal ammonia line widths to be systematically narrower (by about 20%) than those of N2H+ (1–0) line transition observed with similar resolution. If the observed ordered velocity gradients represent the core envelope solid-body rotation, we estimate the specific angular momentum to be about 2 × 1021 cm2 s−1, similar to the low-mass star-forming cores. Together with the previous finding of subsonic motions in G035.39, our results demonstrate high levels of similarity between kinematics of a high-mass star-forming IRDC and the low-mass star formation regime.

Massive and low-mass protostars in massive “starless” cores

The infrared dark clouds (IRDCs) G11.11−0.12 and G28.34+0.06 are two of the best-studied IRDCs in our Galaxy. These two clouds host clumps at different stages of evolution, including a massive dense clump in both clouds that is dark even at 70 and 100 μm. Such seemingly quiescent massive dense clumps have been speculated to harbor cores that are precursors of high-mass stars and clusters. We observed these two “prestellar” regions at 1 mm with the Submillimeter Array (SMA) with the aim of characterizing the nature of such cores. We show that the clumps fragment into several low- to high-mass cores within the filamentary structure of the enveloping cloud. However, while the overall physical properties of the clump may indicate a starless phase, we find that both regions host multiple outflows. The most massive core though 70 μm dark in both clumps is clearly associated with compact outflows. Such low-luminosity, massive cores are potentially the earliest stage in the evolution of a massive protostar. We also identify several outflow features distributed in the large environment around the most massive core. We infer that these outflows are being powered by young, low-mass protostars whose core mass is below our detection limit. These findings suggest that low-mass protostars have already formed or are coevally formed at the earliest phase of high-mass star formation.

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.

Extreme fragmentation and complex kinematics at the center of the L1287 cloud

Aims. The filamentary ~10-pc-scale infrared dark cloud L1287 located at a parallax distance of ~929 pc is actively forming a dense cluster of low-mass young stellar objects (YSOs) at its inner ~0.1 pc region. To help understand the origin of this low-mass YSO cluster, the present work aims at resolving the gas structures and kinematics with high angular resolution. Methods. We performed ~1′′ angular resolution (~930 AU) observations at ~1.3 mm wavelengths using the Submillimeter Array (SMA), which simultaneously cover the dust continuum emission and various molecular line tracers for dense gas, warm gas, shocks, and outflows. Results. From a 1.3-mm continuum image with a resolution of ~2′′ we identified six dense cores, namely SMA1-6. Their gas masses are in the range of ~0.4–4 M⊙. From a 1.3-mm continuum image with a resolution of ~1′′, we find a high fragmentation level, with 14 compact millimeter sources within 0.1 pc: SMA3 contains at least nine internal condensations; SMA5 and SMA6 are also resolved with two internal condensations. Intriguingly, one condensation in SMA3 and another in SMA5 appear associated with the known accretion outburst YSOs RNO 1C and RNO 1B. The dense gas tracer DCN (3–2) well traces the dust continuum emission and shows a clear velocity gradient along the NW-SE direction centered at SMA3. There is another velocity gradient with opposite direction around the most luminous YSO, IRAS 00338 + 6312. Conclusions. The fragmentation within 0.1 pc in L1287 is very high compared to other regions at the same spatial scales. The incoherent motions of dense gas flows are sometimes interpreted by being influenced by (proto)stellar feedback (e.g., outflows), which is not yet ruled out in this particular target source. On the other hand, the velocities (with respect to the systemic velocity) traced by DCN are small, and the directions of the velocity gradients traced by DCN are approximately perpendicular to those of the dominant CO outflow(s). Therefore, we alternatively hypothesize that the velocity gradients revealed by DCN trace the convergence from the ≳0.1 pc scales infalling motion towards the rotational motions around the more compact (~0.02 pc) sources. This global molecular gas converging flow may feed the formation of the dense low-mass YSO cluster. Finally, we also found that IRAS 00338 + 6312 is the most likely powering source of the dominant CO outflow. A compact blue-shifted outflow from RNO 1C is also identified.

Cluster formation in the W 40 and Serpens South complex triggered by the expanding H ii region

Abstract We present the results of mapping observations covering a large area of 1 square degree around W 40 and Serpens South carried out in the 12CO (J = 1–0), 13CO (J = 1–0), C18O (J = 1–0), CCS (JN = 87–76), and N2H+ (J = 1–0) emission lines with the 45 m Nobeyama Radio Telescope. W 40 is a blistered H ii region, and Serpens South is an infrared dark cloud accompanied by a young cluster. The relation between these two regions, which are separated by ∼20′ on the sky, has not been recognizable so far. We found the C18O emission is distributed smoothly throughout the W 40 and Serpens South regions, and that the two regions seem to be physically connected. We divided the C18O emission into four groups in terms of the spatial distributions around the H ii region which we call 5, 6, 7, and 8 km s−1 components according to their typical LSR velocity, and propose a three-dimensional model of the W 40 and Serpens South complex. We found two elliptical structures in the position–velocity diagrams, which can be explained as part of two expanding shells. One of the shells is small inner shell just around the H ii region, and the other is a large outer shell corresponding to the boundary of the H ii region. Dense gas associated with the young cluster of Serpens South is likely to be located at the surface of the outer shell, indicating that the natal clump of the young cluster is interacting with the outer shell being compressed by the expansion of the shell. We suggest that the expansion of the shell induced the formation of the young cluster.

Zooming in to Massive Star Birth

Abstract We present high-resolution (0.″2, 1000 au) 1.3 mm ALMA observations of the massive infrared dark cloud clump, G028.37+00.07-C1, thought to harbor the early stages of massive star formation. Using (3–2), we resolve the previously identified C1-S core, separating the bulk of its emission from two nearby protostellar sources. C1-S is thus identified as a massive (∼50 M ⊙), compact (∼0.1 pc diameter) starless core, e.g., with no signs of outflow activity. Being highly deuterated, this is a promising candidate for a pre-stellar core on the verge of collapse. An analysis of its dynamical state indicates a sub-virial velocity dispersion compared to a trans-Alfvénic turbulent core model. However, virial equilibrium could be achieved with sub-Alfvénic conditions involving magnetic field strengths of ∼2 mG.

A Parsec-scale Bipolar H2 Outflow in the Massive Star-forming Infrared Dark Cloud Core MSXDC G053.11+00.05 MM1* †

Abstract We present a parsec-scale molecular hydrogen (H2 1–0 S(1) at 2.12 μm) outflow discovered from the UKIRT Widefield Infrared Survey for H2. The outflow is located in the infrared dark cloud core MSXDC G053.11+00.05 MM1 at 1.7 kpc and is likely associated with two young stellar objects (YSOs) at the center. Although the overall morphology of the outflow is bipolar along the NE–SW direction with a brighter lobe to the southwest, the detailed structure consists of several flows and knots. With a total length of ∼1 pc, the outflow luminosity is fairly high with , implying a massive outflow-driving YSO if the entire outflow is driven by a single source. The two putative driving sources that are located at the outflow center show photometric variability of ≳1 mag in H- and K-bands. Together with their early evolutionary stage from spectral energy distribution (SED) fitting, this indicates that both are capable of ejecting outflows and may be eruptive variable YSOs. The YSO masses inferred from SED fitting are ∼10 M ⊙ and ∼5 M ⊙, which suggests the association of the outflow with massive YSOs. The geometrical morphology of the outflow is well-explained by the lower-mass YSO by assuming a single-source origin; however, without kinematic information, the contribution from the higher mass YSO cannot be ruled out. Considering star formation process by fragmentation of a high-mass core into several lower-mass stars, we also suggest the possible presence of another, yet-undetected driving source that is deeply embedded in the core.

Study of the filamentary infrared dark cloud G192.76+00.10 in the S254–S258 OB complex

We present results of a high resolution study of the filamentary infrared dark cloud G192.76+00.10 in the S254–S258 OB complex in several molecular species tracing different physical conditions. These include three isotopologues of carbon monoxide (CO), ammonia (NH3) and carbon monosulfide (CS). The aim of this work is to study the general structure and kinematics of the filamentary cloud, and its fragmentation and physical parameters. The gas temperature is derived from the NH3 (J, K) = (1, 1), (2, 2) and 12CO(2–1) lines, and the 13CO(1–0), 13CO(2–1) emission is used to investigate the overall gas distribution and kinematics. Several dense clumps are identified from the CS(2–1) data. Values of the gas temperature lie in the range 10 – 35K, and column density N(H2) reaches the value 5.1 × 1022 cm−2. The width of the filament is of order 1 pc. The masses of the dense clumps range from ∼ 30 M⊙ to ∼ 160 M⊙. They appear to be gravitationally unstable. Themolecular emission shows a gas dynamical coherence along the filament. The velocity pattern may indicate longitudinal collapse.

Dust and polarization of cold clumps

AbstractThe Planck catalogue of Galactic cold clumps, PGCC, contains sources of ongoing and future star formation. The data show clear variations also in their dust properties.We use Planck polarization measurements to investigate the polarization fraction in PGCC clumps and the relative orientation of filamentary structures and magnetic fields (Alina et al. 2017). The decrease of polarization fraction as a function of column density can be related to the field geometry but also suggest some loss of grain alignment.PGCCs have been studied with ground-based observations (Liu et al. 2018). The first SCUBA-2/POL-2 polarization studies have targeted the infrared dark cloud G35.39-0.33. The magnetic field is found to be mostly perpendicular to the main filament. The plane-of-the-sky field strength is $\sim 50\mu \,{\rm{G}}$ , a noticeable support against gravity. The polarization fraction decreases with increasing column density. This matches predictions of RAT grain alignment models but the relative contribution of the field morphology is hard to quantify (Juvela et al. 2018).We continue to use MHD simulations to study the same phenomena, with synthetic observations of clumps and filaments.

The Core Mass Function across Galactic Environments. II. Infrared Dark Cloud Clumps

Abstract We study the core mass function (CMF) within 32 dense clumps in seven infrared dark clouds (IRDCs) with the Atacama Large Millimeter/submillimeter Array via 1.3 mm continuum emission at a resolution of ∼1″. We have identified 107 cores with the dendrogram algorithm, with a median radius of about 0.02 pc. Their masses range from 0.261 to 178 M ⊙. After applying completeness corrections, we fit the combined IRDC CMF with a power law of the form and derive an index of α ≃ 0.86 ± 0.11 for M ≥ 0.79 M ⊙ and α ≃ 0.70 ± 0.13 for M ≥ 1.26 M ⊙, which is a significantly more top-heavy distribution than the Salpeter stellar initial mass function index of 1.35. We also make a direct comparison of these IRDC clump CMF results to those measured in the more evolved protocluster G286 derived with similar methods, which have α ≃ 1.29 ± 0.19 and 1.08 ± 0.27 in these mass ranges, respectively. These results provide a hint that, especially for the M ≥ 1.26 M ⊙ range where completeness corrections are modest, the CMF in high pressure, early-stage environments of IRDC clumps may be top-heavy compared to that in the more evolved, global environment of the G286 protoclusters. However, larger samples of cores probing these different environments are needed to better establish the robustness of this potential CMF variation.