Search

We present an analysis of the outer Galaxy giant molecular filament (GMF) G214.5-1.8 (G214.5) using IRAM 30m data of 12CO, 13CO, and C18O. We find that the 12CO (1-0) and (2-1) derived excitation temperatures are near identical and are very low, with a median of 8.2 K, showing that the gas is extremely cold across the whole cloud. Investigating the abundance of 13CO across G214.5, we find that there is a significantly lower abundance along the entire 13 pc spine of the filament, extending out to a radius of ∼0.8 pc, corresponding to Av ≳ 2 mag and Tdust ≲ 13.5 K. Due to this, we attribute the decrease in abundance to CO freeze-out, making G214.5 the largest scale example of freeze-out yet. We construct an axisymmetric model of G214.5's 13CO volume density considering freeze-out and find that to reproduce the observed profile significant depletion is required beginning at low volume densities, n ≳ 2000 cm−3. Freeze-out at this low number density is possible only if the cosmic-ray ionization rate is ∼1.9 × 10−18 s−1, an order of magnitude below the typical value. Using time scale arguments, we posit that such a low ionization rate may lead to ambipolar diffusion being an important physical process along G214.5's entire spine. We suggest that if low cosmic-ray ionization rates are more common in the outer Galaxy, and other quiescent regions, cloud-scale CO freeze-out occurring at low column and number densities may also be more prevalent, having consequences for CO observations and their interpretation. [ABSTRACT FROM AUTHOR]

To what extent magnetic fields affect how molecular clouds (MCs) fragment and create dense structures is an open question. We present a numerical study of cloud fragmentation using the SILCC-Zoom simulations. These simulations follow the self-consistent formation of MCs in a few hundred parsec-sized region of a stratified galactic disc; and include magnetic fields, self-gravity, supernova-driven turbulence, as well as a non-equilibrium chemical network. To discern the role of magnetic fields in the evolution of MCs, we study seven simulated clouds, five with magnetic fields, and two without, with a maximum resolution of 0.1 parsec. Using a dendrogram, we identify hierarchical structures, which form within the clouds. Overall, the magnetized clouds have more mass in a diffuse envelope with a number density between 1 and 100 cm−3. We find that six out of seven clouds are sheet-like on the largest scales, as also found in recent observations, and with filamentary structures embedded within, consistent with the bubble-driven MC formation mechanism. Hydrodynamic simulations tend to produce more sheet-like structures also on smaller scales, while the presence of magnetic fields promotes filament formation. Analysing cloud energetics, we find that magnetic fields are dynamically important for less dense, mostly but not exclusively atomic structures (typically up to ∼100−1000 cm−3), while the denser, potentially star-forming structures are energetically dominated by self-gravity and turbulence. In addition, we compute the magnetic surface term and demonstrate that it is generally confining, and some atomic structures are even magnetically held together. In general, magnetic fields delay the cloud evolution and fragmentation by ∼ 1 Myr. [ABSTRACT FROM AUTHOR]

We present an analysis of the outer Galaxy giant molecular filament (GMF) G214.5−1.8 (G214.5) using Herschel data. We find that G214.5 has a mass of ∼16 000 M⊙, yet hosts only 15 potentially protostellar 70- |$\mu$| m sources, making it highly quiescent compared to equally massive clouds such as Serpens and Mon R2. We show that G214.5 has a unique morphology, consisting of a narrow 'Main filament' running north–south and a perpendicular 'Head' structure running east–west. We identify 33 distinct massive clumps from the column density maps, 8 of which are protostellar. However, the star formation activity is not evenly spread across G214.5 but rather predominantly located in the Main filament. Studying the Main filament in a manner similar to previous works, we find that G214.5 is most like a 'Bone' candidate GMF, highly elongated and massive, but it is colder and narrower than any such GMF. It also differs significantly due to its low fraction of high column density gas. Studying the radial profile, we discover that G214.5 is highly asymmetric and resembles filaments which are known to be compressed externally. Considering its environment, we find that G214.5 is co-incident, spatially and kinematically, with a H  i superbubble. We discuss how a potential interaction between G214.5 and the superbubble may explain G214.5's morphology, asymmetry and, paucity of dense gas and star formation activity, highlighting the intersection of a bubble-driven interstellar medium paradigm with that of a filament paradigm for star formation. [ABSTRACT FROM AUTHOR]

The interstellar medium is highly structured, presenting a range of morphologies across spatial scales. The large data sets resulting from observational surveys and state-of-the-art simulations studying these hierarchical structures means that identification and classification must be done in an automated fashion to be efficient. Here we present RJ-plots, an improved version of the automated morphological classification technique J -plots developed by Jaffa et al. This method allows clear distinctions between quasi-circular/elongated structures and centrally over/underdense structures. We use the recent morphological SEDIGISM catalogue of Neralwar et al. to show the improvement in classification resulting from RJ-plots, especially for ring-like and concentrated cloud types. We also find a strong correlation between the central concentration of a structure and its star formation efficiency and dense gas fraction, as well as a lack of correlation with elongation. Furthermore, we use the accreting filament simulations of Clarke, Williams & Walch to highlight a multiscale application of RJ-plots, finding that while spherical structures become more common at smaller scales they are never the dominant structure down to r ∼ 0.03 pc. [ABSTRACT FROM AUTHOR]

Simulations are very useful for testing our theoretical understanding of star formation by varying the initial conditions or treatment of various physical mechanisms. However, large well-resolved simulations including complex physics are computationally costly and therefore are normally only performed once. This leads to a crisis in modelling, because star formation is a chaotic system, where a small variation in initial conditions can be magnified to a large change in results. We create a suite of cluster-scale simulations with 30 different random realizations of the turbulent velocity field applied to the same initial conditions of an isolated spherical molecular cloud. We test commonly used metrics of star formation activity and cloud structure and measure the variance cause by this random variation in initial conditions to quantify the error that should be applied to single-realization simulations. We find that after 5 Myr the number of stars varies by 58 per cent of the mean, the star formation efficiency by 60 per cent of the mean, and the shape of the column density PDF by 7 per cent of the mean. We provide a standard deviation at different times that should be applied as an error margin on all single realization simulations to enable robust statistical comparison. [ABSTRACT FROM AUTHOR]

During the early phases of low-mass star formation, episodic accretion causes the ejection of high-velocity outflow bullets, which carry a fossil record of the driving protostar's accretion history. We present 44 SPH simulations of |$1\, {{\mathrm{M}}}_{\odot }$| cores, covering a wide range of initial conditions, and follow the cores for five free-fall times. Individual protostars are represented by sink particles, and the sink particles launch episodic outflows using a sub-grid model. The Optics algorithm is used to identify individual episodic bullets within the outflows. The parameters of the overall outflow and the individual bullets are then used to estimate the age and energetics of the outflow, and the accretion events that triggered it, and to evaluate how reliable these estimates are, if observational uncertainties and selection effects (like inclination) are neglected. Of the commonly used methods for estimating outflow ages, it appears that those based on the length and speed of advance of the lobe are the most reliable in the early phases of evolution, and those based on the width of the outflow cavity and the speed of advance are most reliable during the later phases. We describe a new method that is almost as accurate as these methods, and reliable throughout the evolution. In addition, we show how the accretion history of the protostar can be accurately reconstructed from the dynamics of the bullets if each lobe contains at least two bullets. The outflows entrain about 10 times more mass than originally ejected by the protostar. [ABSTRACT FROM AUTHOR]

The properties of synthetic CO emission from 3D simulations of forming molecular clouds are studied within the SILCC-Zoom project. Since the time-scales of cloud evolution and molecule formation are comparable, the simulations include a live chemical network. Two sets of simulations with an increasing spatial resolution (d x  = 3.9 pc to d x  = 0.06 pc) are used to investigate the convergence of the synthetic CO emission, which is computed by post-processing the simulation data with the radmc-3d radiative transfer code. To determine the excitation conditions, it is necessary to include atomic hydrogen and helium alongside H2, which increases the resulting CO emission by ∼7–26 per cent. Combining the brightness temperature of 12CO and 13CO, we compare different methods to estimate the excitation temperature, the optical depth of the CO line and hence, the CO column density. An intensity-weighted average excitation temperature results in the most accurate estimate of the total CO mass. When the pixel-based excitation temperature is used to calculate the CO mass, it is over-/underestimated at low/high CO column densities where the assumption that 12CO is optically thick while 13CO is optically thin is not valid. Further, in order to obtain a converged total CO luminosity and hence 〈 X CO〉 factor, the 3D simulation must have d x  ≲ 0.1 pc. The 〈 X CO〉 evolves over time and differs for the two clouds; yet pronounced differences with numerical resolution are found. Since high column density regions with a visual extinction larger than 3 mag are not resolved for d x  ≳ 1 pc, in this case the H2 mass and CO luminosity both differ significantly from the higher resolution results and the local X CO is subject to strong noise. Our calculations suggest that synthetic CO emission maps are only converged for simulations with d x  ≲ 0.1 pc. [ABSTRACT FROM AUTHOR]

Recent observations have revealed the presence of small fibres or sub-filaments within larger filaments. We present a numerical fragmentation study of fibrous filaments investigating the link between cores and sub-filaments using hydrodynamical simulations performed with the moving-mesh code arepo. Our study suggests that cores form in two environments: (i) as isolated cores, or small chains of cores, on a single sub-filament, or (ii) as an ensemble of cores located at the junction of sub-filaments. We term these isolated and hub cores, respectively. We show that these core populations are statistically different from each other. Hub cores have a greater mean mass than isolated cores, and the mass distribution of hub cores is significantly wider than isolated cores. This fragmentation is reminiscent of parsec-scale hub-filament systems, showing that the combination of turbulence and gravity leads to similar fragmentation signatures on multiple scales, even within filaments. Moreover, the fact that fragmentation proceeds through sub-filaments suggests that there exists no characteristic fragmentation length-scale between cores. This is in opposition to earlier theoretical works studying fibre-less filaments which suggest a strong tendency towards the formation of quasi-periodically spaced cores, but in better agreement with observations. We also show tentative signs that global collapse of filaments preferentially form cores at both filament ends, which are more massive and dense than other cores. [ABSTRACT FROM AUTHOR]

Observations with the Herschel Space Telescope have established that most star forming gas is organised in filaments, a finding that is supported by numerical simulations of the supersonic interstellar medium (ISM) where dense filamentary structures are ubiquitous. We aim to understand the formation of these dense structures by performing observations covering the 12CO(4→3), 12CO(3→2), and various CO(2–1) isotopologue lines of the Musca filament, using the APEX telescope. The observed CO intensities and line ratios cannot be explained by PDR (photodissociation region) emission because of the low ambient far-UV field that is strongly constrained by the non-detections of the [C II] line at 158 μm and the [O I] line at 63 μm, observed with the upGREAT receiver on SOFIA, as well as a weak [C I] 609 μm line detected with APEX. We propose that the observations are consistent with a scenario in which shock excitation gives rise to warm and dense gas close to the highest column density regions in the Musca filament. Using shock models, we find that the CO observations can be consistent with excitation by J-type low-velocity shocks. A qualitative comparison of the observed CO spectra with synthetic observations of dynamic filament formation simulations shows a good agreement with the signature of a filament accretion shock that forms a cold and dense filament from a converging flow. The Musca filament is thus found to be dense molecular post-shock gas. Filament accretion shocks that dissipate the supersonic kinetic energy of converging flows in the ISM may thus play a prominent role in the evolution of cold and dense filamentary structures. [ABSTRACT FROM AUTHOR]

We have analysed the Herschel and SCUBA-2 dust continuum observations of the main filament in the Taurus L1495 star-forming region, using the Bayesian fitting procedure ppmap. (i) If we construct an average profile along the whole length of the filament, it has FWHM |$\simeq 0.087\pm 0.003\, {\rm pc};\,\,$| but the closeness to previous estimates is coincidental. (ii) If we analyse small local sections of the filament, the column-density profile approximates well to the form predicted for hydrostatic equilibrium of an isothermal cylinder. (iii) The ability of ppmap to distinguish dust emitting at different temperatures, and thereby to discriminate between the warm outer layers of the filament and the cold inner layers near the spine, leads to a significant reduction in the surface-density, |$\varSigma$|⁠ , and hence in the line-density, μ. If we adopt the canonical value for the critical line-density at a gas-kinetic temperature of |$10\, {\rm K}$|⁠ , |$\mu _{{\rm CRIT}}\simeq 16\, {\rm M_{\odot }\, pc^{-1}}$|⁠ , the filament is on average trans-critical, with |${\bar{\mu }}\sim \mu _{{\rm CRIT}};\,\,$| local sections where μ > μCRIT tend to lie close to prestellar cores. (iv) The ability of ppmap to distinguish different types of dust, i.e. dust characterized by different values of the emissivity index, β, reveals that the dust in the filament has a lower emissivity index, β ≲ 1.5, than the dust outside the filament, β ≳ 1.7, implying that the physical conditions in the filament have effected a change in the properties of the dust. [ABSTRACT FROM AUTHOR]

The C+ion is an important coolant of interstellar gas, and so the [C ii ] fine structure line is frequently observed in the interstellar medium. However, the physical and chemical properties of the [C ii ]-emitting gas are still unclear. We carry out non-LTE (local thermal equilibrium) radiative transfer simulations with radmc -3 d to study the [C ii ] line emission from a young, turbulent molecular cloud before the onset of star formation, using data from the SILCC-Zoom project. The [C ii ] emission is optically thick over 40 per cent of the observable area with I [C ii ]> 0.5 K km s−1. To determine the physical properties of the [C ii ] emitting gas, we treat the [C ii ] emission as optically thin. We find that the [C ii ] emission originates primarily from cold, moderate density gas (40 ≲ T ≲ 65 K and 50 ≲ n ≲ 440 cm−3), composed mainly of atomic hydrogen and with an effective visual extinction between ∼0.50 and ∼0.91. Gas dominated by molecular hydrogen contributes only ≲20 per cent of the total [C ii ] line emission. Thus, [C ii ] is not a good tracer for CO–dark H2at this early phase in the cloud's lifetime. We also find that the total gas, H and C+column densities are all correlated with the integrated [C ii ] line emission, with power law slopes ranging from 0.5 to 0.7. Further, the median ratio between the total column density and the [C ii ] line emission is Y C ii ≈ 1.1 × 1021cm−2(K km s−1)−1, and |$Y_{{\rm C\, {\small II}}}$| scales with |$I_{[\rm{C\,{{\small II}}}]}^{-0.3}$|⁠. We expect |$Y_{{\rm C\, {\small II}}}$| to change in environments with a lower or higher radiation field than simulated here. [ABSTRACT FROM AUTHOR]

Molecular-line observations of filaments in star-forming regions have revealed the existence of elongated coherent features within the filaments; these features are termed fibres. Here we caution that, since fibres are traced in PPV space, there is no guarantee that they represent coherent features in PPP space. We illustrate this contention using simulations of the growth of a filament from a turbulent medium. Synthetic C18O observations of the simulated filaments reveal the existence of fibres very similar to the observed ones, i.e. elongated coherent features in the resulting PPV data-cubes. Analysis of the PPP data-cubes (i.e. 3D density fields) also reveals elongated coherent features, which we term sub-filaments. Unfortunately there is very poor correspondence between the fibres and the sub-filaments in the simulations. Both fibres and sub-filaments derive from inhomogeneities in the turbulent accretion flow onto the main filament. As a consequence, fibres are often affected by line-of-sight confusion. Similarly, sub-filaments are often affected by large velocity gradients, and even velocity discontinuities. These results suggest that extreme care should be taken when using velocity coherent features to constrain the underlying substructure within a filament. [ABSTRACT FROM AUTHOR]

Large-scale surveys have brought about a revolution in astronomy. To analyse the resulting wealth of data, we need automated tools to identify, classify, and quantify the important underlying structures. We present here a method for classifying and quantifying a pixelated structure, based on its principal moments of inertia. The method enables us to automatically detect, and objectively compare, centrally condensed cores, elongated filaments, and hollow rings. We illustrate the method by applying it to (i) observations of surface density from Hi-GAL, and (ii) simulations of filament growth in a turbulent medium. We limit the discussion here to 2D data; in a future paper, we will extend the method to 3D data. [ABSTRACT FROM AUTHOR]

We use smoothed particle hydrodynamic simulations of cold, uniform density, self-gravitating filaments, to investigate their longitudinal collapse time-scales; these time-scales are important because they determine the time available for a filament to fragment into cores. A filament is initially characterized by its line-mass, µO, its radius, RO (or equivalently its density ?O=µO/pR²O), and its aspect ratio, AO?(=ZO/RO, where ZO is its half-length). The gas is only allowed to contract longitudinally, i.e. parallel to the symmetry axis of the filament (the z-axis). Pon et al. (2012) have considered the global dynamics of such filaments analytically. They conclude that short filaments (AO ? 5) collapse along the z-axis more-or-less homologously, on a time-scale tHOM ~ 0.44 AO (G?O)-1/2; in contrast, longer filaments (AO ? 5) undergo end-dominated collapse, i.e. two dense clumps form at the ends of the filament and converge on the centre sweeping up mass as they go, on a time-scale tEND ≳0.98A1/2O(G?O)-1/2. Our simulations do not corroborate these predictions. First, for all AO ~2, the collapse time satisfies a single equation tCOL ~(0.49+0.26AO)(G?O)-1/2, which for large AO is much longer than the Pon et al. prediction. Secondly, for all AO ≲2, the collapse is end-dominated. Thirdly, before being swept up, the gas immediately ahead of an end-clump is actually accelerated outwards by the gravitational attraction of the approaching clump, resulting in a significant ram pressure. For high aspect ratio filaments, the end-clumps approach an asymptotic inward speed, due to the fact that they are doing work both accelerating and compressing the gas they sweep up. Pon et al. appear to have neglected the outward acceleration and its consequences. [ABSTRACT FROM AUTHOR]