Ammonia in infrared dark clouds

Sometimes, early star formation can be found in cold and dense molecular clouds, such as an infrared dark cloud. Considering that star formation often occurs in clusters, H II regions may be triggering a new generation of star formation, so we can search for the initial stage of massive star formation around H II regions. Based on the above, this work introduces one method to search for the initial stage of massive star formation around H II regions. Towards one section of the H II region G18.2–0.3, multiwavelength observations are carried out to investigate its physical properties. Through analysis, we find three potential initial stages of massive star formation, suggesting that it is feasible to use in searching for the initial stage of massive star formation around H II regions.

Using the H13CN and HN13C J = 1–0 line observations, the abundance ratio of HCN/HNC has been estimated for different evolutionary stages of massive star formation: infrared dark clouds (IRDCs), high-mass protostellar objects (HMPOs), and ultracompact H ii regions (UCH iis). IRDCs were divided into “quiescent IRDC cores (qIRDCc)” and “active IRDC cores (aIRDCc),” depending on star formation activity. The HCN/HNC ratio is known to be higher at active and high temperature regions related to ongoing star formation, compared to cold and quiescent regions. Our observations toward 8 qIRDCc, 16 aIRDCc, 23 HMPOs, and 31 UCH iis show consistent results; the ratio is 0.97 (±0.10), 2.65 (±0.88), 4.17 (±1.03), and 8.96 (±3.32) in these respective evolutionary stages, increasing from qIRDCc to UCH iis. The change of the HCN/HNC abundance ratio, therefore, seems directly associated with the evolutionary stages of star formation, which have different temperatures. One suggested explanation for this trend is the conversion of HNC to HCN, which occurs effectively at higher temperatures. To test the explanation, we performed a simple chemical model calculation. In order to fit the observed results, the energy barrier of the conversion must be much lower than the value provided by theoretical calculations.

We analyze both HCN J = 1–0 and HNC J = 1–0 line profiles to study the inflow motions in different evolutionary stages of massive star formation: 54 infrared dark clouds (IRDCs), 69 high-mass protostellar objects (HMPOs), and 54 ultra-compact H ii regions (UCHIIs). Inflow asymmetry in the HCN spectra seems to be prevalent throughout all the three evolutionary phases, with IRDCs showing the largest excess in the blue profile. In the case of the HNC spectra, the prevalence of blue sources does not appear, apart from for IRDCs. We suggest that this line is not appropriate to trace the inflow motion in the evolved stages of massive star formation, because the abundance of HNC decreases at high temperatures. This result highlights the importance of considering chemistry in dynamics studies of massive star-forming regions. The fact that the IRDCs show the highest blue excess in both transitions indicates that the most active inflow occurs in the early phase of star formation, i.e., in the IRDC phase rather than in the later phases. However, mass is still inflowing onto some UCHIIs. We also find that the absorption dips of the HNC spectra in six out of seven blue sources are redshifted relative to their systemic velocities. These redshifted absorption dips may indicate global collapse candidates, although mapping observations with better resolution are needed to examine this feature in more detail.

A comparison of star formation within the galactic centre and galactic disc

<p>The formation of massive stars and stellar clusters is important in understanding the light we receive from other galaxies, the life cycle of matter in the Galaxy, and the global process of star formation. However, this problem has remained elusive as the relative rarity, large distances, confusion, and obscured nature of massive star forming regions has made a global and high-resolution understanding of their formation intractable for decades. The advent of large Galactic Plane Surveys and high-resolution observing facilities have allowed us to make large strides in this  field by constraining the physical properties at the onset of massive star formation in clustered environments, identifying the stages of massive star formation, and estimating the lifetimes of these phases.</p> <p>We present a detailed analysis of two young massive star forming regions in different evolutionary stages embedded within a single Infrared Dark Cloud using NH<sub>3</sub> on the Karl G. Jansky Very Large Array. In this analysis, we characterize the physical structure (column density, temperature, and virial parameter) just prior to the onset of massive star formation and infer evolution in this structure by measuring it at different evolutionary stages. We expand this analysis to a global scale using Herschel and Spitzer surveys of the Galactic Plane from mid-to far-IR, devising a method to identify precursors to stellar clusters throughout the Galaxy for the first time. By separating the diffuse Galactic cirrus emission from the dense molecular clumps, we derive the dust temperatures and column densities characteristic of cluster-forming clumps. We compare these physical properties with star formation tracers in a systematic way to distinguish and characterize their evolutionary phases. We compare the physical properties derived from gas with those derived using dust. We estimate lifetimes for these evolutionary phases and speculate on the large-scale dynamics in the formation of stellar clusters.</p> <p>We constrain the conditions at the onset of massive star formation, measure how these conditions change with evolutionary phase, and estimate the duration of each phase. This thesis places global and high-resolution constraints on the physical properties, evolution, and lifetimes of massive star and cluster forming regions.</p>

It is commonly assumed that cold and dense Infrared Dark Clouds (IRDCs)\nlikely represent the birth sites massive stars. Therefore, this class of\nobjects gets increasing attention. To enlarge the sample of well-characterised\nIRDCs in the southern hemisphere, we have set up a program to study the gas and\ndust of southern IRDCs. The present paper aims at characterizing the continuuum\nproperties of this sample of objects. We cross-correlated 1.2 mm continuum data\nfrom SIMBA@SEST with Spitzer/GLIMPSE images to establish the connection between\nemission sources at millimeter wavelengths and the IRDCs we see at 8 $\\mu$m in\nabsorption against the bright PAH background. Analysing the dust emission and\nextinction leads to a determination of masses and column densities, which are\nimportant quantities in characterizing the initial conditions of massive star\nformation. The total masses of the IRDCs were found to range from 150 to 1150\n$\\rm M_\\odot$ (emission data) and from 300 to 1750 $\\rm M_\\odot$ (extinction\ndata). We derived peak column densities between 0.9 and 4.6 $\\times 10^{22}$\ncm$^{-2}$ (emission data) and 2.1 and 5.4 $\\times 10^{22}$ cm$^{-2}$\n(extinction data). We demonstrate that the extinction method fails for very\nhigh extinction values (and column densities) beyond A$_{\\rm V}$ values of\nroughly 75 mag according to the Weingartner & Draine (2001) extinction relation\n$R_{\\rm V} = 5.5$ model B. The derived column densities, taking into account\nthe spatial resolution effects, are beyond the column density threshold of 3.0\n$\\times 10^{23}$ cm$^{-2}$ required by theoretical considerations for massive\nstar formation. We conclude that the values for column densities derived for\nthe selected IRDC sample make these objects excellent candidates for objects in\nthe earliest stages of massive star formation.\n

It is assumed that objects which will eventually evolve into high-mass main sequence stars can initially appear as low- to intermediate-mass young stellar objects that are still accreting mass. By initiating a Spitzer mid-infrared spectroscopic survey of intermediate mass young stellar objects, selected from massive star-forming clumps and infrared dark clouds (IRDCs), an evolutionary sequence was established. A group of particularly young stellar objects (YSOs) was found showing several ice and silicate absorption features. A second group was identified: these objects represent more evolved YSOs that already emit a significant amount of UV radiation and feature shock-excited
\ngas driven by proto-stellar outflows. Importantly, the observed extended emission of cationic polycyclic aromatic hydrocarbons (PAHs) and the analysis of molecular hydrogen indicates the creation of a very young photon dissociation region while the expected radio emission from the associated HII region is still undetectable.
\n
\nAn automated photometric pipeline was developed to detect cold dense cores and extract their far-infrared fluxes in Herschel bolometer maps while taking into account the complicated background and additional instrumental effects. By studying the fragmentation in the high-mass part of the Herschel EPoS program a typical point source separation of ~ 0.5 pc was found throughout the sample. The detected sources are in an evolutionary stage where they are embedded in the dust clumps associated with the parental cloud. This typical source separation is not retained in later stages, such as in young stellar clusters.
\n
\nFurthermore, a comprehensive case study is presented in which an isolated IRDC region is used to analyze the conditions of massive star formation in the absence of strong external effects. Two point sources found in this region are candidates for evolving into high-mass main-sequence stars. The filamentary star-forming cloud is in free collapse while the embedded molecular clumps are gravitationally bound. The observed dust temperature structure shows that the dark cloud is not isothermal. The locations of temperature minima are in good spatial agreement with the column density peaks. Together with the structure of molecular clumps identified by radio line observations these results confirm that the dust temperatures are lowest in the densest parts of the dark cloud.

Massive stars govern the evolution of galaxies by providing ionizing photons and energy as well as enriching heavy elements into interstellar medium; however, their formation is still poorly understood. Infrared dark clouds (IRDCs) are cold (< 25 K) and very dense (> 105 cm−3) interstellar clouds which are seen silhouette against the bright Galactic background in mid-IR. With very high column densities (∼ 1023–1025cm−2), IRDCs are believed to be the precursors to massive stars and star clusters (Simon et al. 2006).We report a remarkable IRDC at (l, b) ∼ (53°.2, 0°.0) which shows a number of bright mid-IR stellar sources along the cloud that are likely young stellar objects (YSOs). There are also several H2 (at 2.122 μm) outflow features in the cloud revealed by UWISH2 (Ukirt Widefield Infrared Survey for H2, Froebrich et al. 2011), in particular where earlier evolutionary stage of YSOs are located. The IRDC was previously partly identified as three separate IRDCs in the MSXDC catalog (Simon et al. 2006), whereas we have found that a long, filamentary cloud extending ∼ 30 pc including these three IRDCs is very well coincident with a CO cloud at v ∼ 23.5 km/s (or at d ∼ 2 kpc) which is clearly distinct from the other velocity components. Therefore, in this study, we investigate the overall star formation activity in this IRDC (IRDC G53.2, hereafter).We perform the PRF photometry of Spitzer MIPSGAL 24 μm data using MOPEX and build a catalog of YSOs by matching the detected 24 μm sources with published catalogs. The limiting magnitude in 24 μm is ∼ 7.8 mag, and YSO candidates which have counterparts in GLIMPSE I catalog are 354. The YSO candidates are classified using spectral index derived between 2 and 24 μm, following Greene et al. (1994). We also remove the field-star contamination using reference fields where there is no CO cloud; the fraction of each class after reference field analysis is 18, 22, 45, 10, and 5% for Class I, Flat, Class II, Class III, and sources which cannot be classified due to the lack of data. The spatial distribution that earlier classes (i.e., Class I and Flat) are concentrated where far-IR or millimeter emission is strong and larger fraction of Flat objects compared to other low-mass star forming regions (e.g., Evans et al. 2009 and Billot et al. 2010) may imply that the IRDC G53.2 is indeed an active star-forming region in rather early evolutionary stage. Further investigation of each YSO such as SED modeling will reveal detailed information on star formation activity in this intriguing IRDC.

Context.In the inner parts of the Galaxy the Infrared Dark Clouds (IRDCs) are presently believed to be the progenitors of massive stars and star clusters. Many of them are predominantly devoid of active star formation and for now they represent the earliest observed stages of massive star formation. Their Outer Galaxy counterparts, if present, are not easily identified because of a low or absent mid-IR background.

Infrared dark clouds (IRDCs) are extinction features against the Galactic infrared background, mainly in the mid-infrared band. Recently they were proposed to be potential sites of massive star formation. In this work we have made a12CO,13CO, and C18O (J = 1→ 0) survey of 61 IRDCs, 52 of which are in the first Galactic quadrant, selected from a catalog given by Simon and coworkers, while the others are in the outer Galaxy, selected by visually inspecting the MSX images. Detection rates in the three CO lines are 90%, 71%, and 62%, respectively. The distribution of IRDCs in the first Galactic quadrant is consistent with the 5 kpc molecular ring picture, while a slight trace of a spiral pattern is also noticeable, and needs to be further examined. The IRDCs have a typical excitation temperature of 10 K and typical column density of several 1022 cm−2. Their typical physical size is estimated to be several parsecs using angular sizes from the Simon catalog. Typical volume density and typical LTE mass are ~5000 cm−3 and ~5000 M☉, respectively. The IRDCs are in or near virial equilibrium. The properties of IRDCs are similar to those of star-forming molecular clumps, and they seem to be intermediate between giant molecular clouds and Bok globules; thus they may represent early stages of massive star formation.

Infrared dark clouds (IRDCs) are believed to host the earliest stages of high-mass star and cluster formation. Because O stars typically travel short distances over their lifetimes, if IRDCs host the earliest stages of high-mass star formation then these cold, dense molecular clouds should be located in or near the spiral arms in the Galaxy. The Galactic distribution of a large sample of IRDCs should therefore provide information on Galactic structure. Moreover, determination of distances enables mass and luminosity calculations. We have observed a large sample of IRDC candidates in the first Galactic quadrant in the dense gas tracer CS (2-1) using the Mopra telescope in order to determine kinematic distances from the molecular line velocities. We find that the IRDCs are concentrated around a Galactocentric distance of ~4.5 kpc, agreeing with the results of Simon et al. This distribution is consistent with the location of the Scutum-Centaurus spiral arm. The group of IRDCs near the Sun in the first quadrant detected in 13CO (1-0) in Simon et al. is not detected in the CS data. This discrepancy arises from the differences in the critical densities between the 13CO (1-0) and CS (2-1) lines. We determine that the Midcourse Space Experiment selected IRDCs are not a homogeneous population, and 13CO (1-0) traces a population of IRDCs with lower column densities and lower 1.1 mm flux densities in addition to more dense IRDCs detected in CS. Masses of the first quadrant IRDCs are calculated from 13CO (1-0) maps. We find a strong peak in the Galactocentric IRDC mass surface density distribution at R Gal ~ 4.5 kpc.

With a mass of ∼1000 M ⊙ and a surface density of ∼0.5 g cm−2, G023.477+0.114, also known as IRDC 18310-4, is an infrared dark cloud (IRDC) that has the potential to form high-mass stars and has been recognized as a promising prestellar clump candidate. To characterize the early stages of high-mass star formation, we have observed G023.477+0.114 as part of the Atacama Large Millimeter/submillimeter Array (ALMA) Survey of 70 μm Dark High-mass Clumps in Early Stages. We have conducted ∼1.″2 resolution observations with ALMA at 1.3 mm in dust continuum and molecular line emission. We have identified 11 cores, whose masses range from 1.1 to 19.0 M ⊙. Ignoring magnetic fields, the virial parameters of the cores are below unity, implying that the cores are gravitationally bound. However, when magnetic fields are included, the prestellar cores are close to virial equilibrium, while the protostellar cores remain sub-virialized. Star formation activity has already started in this clump. Four collimated outflows are detected in CO and SiO. H2CO and CH3OH emission coincide with the high-velocity components seen in the CO and SiO emission. The outflows are randomly oriented for the natal filament and the magnetic field. The position-velocity diagrams suggest that episodic mass ejection has already begun even in this very early phase of protostellar formation. The masses of the identified cores are comparable to the expected maximum stellar mass that this IRDC could form (8–19 M ⊙). We explore two possibilities on how IRDC G023.477+0.114 could eventually form high-mass stars in the context of theoretical scenarios.

Context. Filamentary infrared dark clouds (IRDCs) are a useful class of interstellar clouds for studying the cloud fragmentation mechanisms on different spatial scales. Determination of the physical properties of the substructures in IRDCs can also provide useful constraints on the initial conditions and early stages of star formation, including those of high-mass stars. Aims. We aim to determine the physical characteristics of two filamentary IRDCs, G1.75-0.08 and G11.36+0.80, and their clumps. We also attempt to understand how the IRDCs are fragmented into clumps. Methods. We imaged the target IRDCs at 350 and 450 µm using the bolometer called Architectures de bolomètres pour des Télescopes à grand champ de vue dans le domaine sub-Millimétrique au Sol (ArTéMiS). These data were used in conjunction with our previous 870 µm observations with the Large APEX BOlometer CAmera (LABOCA) and archival Spitzer and Berschel data. The LABOCA clump positions in G11.36+0.80 were also observed in the N2H+(1–0) transition with the Institut de Radioastronomie Millimétrique (IRAM) 30-metre telescope. Results. On the basis of their far-IR to submillimetre spectral energy distributions (SEDs), G1.75-0.08 was found to be composed of two cold (~14.5 K), massive (several ~103 M⊙) clumps that are projectively separated by ~3.7 pc. Both clumps are 70 µm dark, but they do not appear to be bounded by self-gravity. The G1.75-0.08 filament was found to be subcritical by a factor of ~14 with respect to its critical line mass, but the result is subject to uncertain gas velocity dispersion. The IRDC G11.36+0.80 was found to be moderately (by a factor of ~2) supercritical and composed of four clumps that are detected at all wavelengths observed with the ground-based bolometers. The SED-based dust temperatures of the clumps are ~13–15 K, and their masses are in the range ~232–633 M⊙. All the clumps are gravitationally bound and they appear to be in somewhat different stages of evolution on the basis of their luminosity-to-mass ratio. The projected, average separation of the clumps is ~1 pc. At least three clumps in our sample show hints of fragmentation into smaller objects in the ArTéMiS images. Conclusions. A configuration that is observed in G1.75-0.08, namely two clumps at the ends of the filament, could be the result of gravitational focussing acting along the cloud. The two clumps fulfil the mass-radius threshold for high-mass star formation, but if their single-dish-based high velocity dispersion is confirmed, their gravitational potential energy would be strongly overcome by the internal kinetic energy, and the clumps would have to be confined by external pressure to survive. Owing to the location of G1.75-0.08 near the Galactic centre (~270 pc), environmental effects such as a high level of turbulence, tidal forces, and shearing motions could affect the cloud dynamics. The observed clump separation in G11.36+0.80 can be understood in terms of a sausage instability, which conforms to the findings in some other IRDC filaments. The G11.36+0.80 clumps do not lie above the mass-radius threshold where high-mass star formation is expected to be possible, and hence lower-mass star formation seems more likely. The substructure observed in one of the clumps in G11.36+0.80 suggests that the IRDC has fragmented in a hierarchical fashion with a scale-dependent physical mechanism. This conforms to the filamentary paradigm for Galactic star formation.

Ever since their discovery, infrared dark clouds (IRDCs) are generally considered to be the sites just at the onset of high-mass (HM) star formation. In recent years, it has been realized that not all IRDCs harbor HM young stellar objects (YSOs). Only those IRDCs satisfying a certain mass–size criterion, or equivalently above a certain threshold density, are found to contain HMYSOs. In all cases, IRDCs provide ideal conditions for the formation of stellar clusters. In this paper, we study the massive stellar content of IRDCs to readdress the relation between IRDCs and HM star formation. For this purpose, we have identified all IRDCs associated with a sample of 12 Galactic molecular clouds (MCs). The selected MCs have been the target of a systematic search for YSOs in an earlier study. The cataloged positions of YSOs have been used to search all YSOs embedded in each identified IRDC. In total, we have found 834 YSOs in 128 IRDCs. The sample of IRDCs have mean surface densities of 319 M ⊙ pc − 2 , mean mass of 1062 M ⊙ , and a mass function power-law slope −1.8, which are similar to the corresponding properties for the full sample of IRDCs and resulting physical properties in previous studies. We find that all those IRDCs containing at least one intermediate to HM young star satisfy the often-used mass–size criterion for forming HM stars. However, not all IRDCs satisfying the mass–size criterion contain HM stars. We find that the often-used mass–size criterion corresponds to 35% probability of an IRDC forming a massive star. Twenty-five (20%) of the IRDCs are potential sites of stellar clusters of mass more than 100 M ⊙ .

ABSTRACTTo unravel the star formation process, we present a multi-scale and multi-wavelength study of the filamentary infrared dark cloud (IRDC) G333.73 + 0.37, which hosts previously known two H ii regions located at its center. Each H ii region is associated with a mid-infrared source, and is excited by a massive OB star. Two filamentary structures and a hub-filament system (HFS) associated with one H ii region are investigated in absorption using the Spitzer 8.0 μm image. The 13CO(J = 2–1) and C18O(J = 2–1) line data reveal two velocity components (around −35.5 and −33.5 km s−1) toward the IRDC, favouring the presence of two filamentary clouds at different velocities. Non-thermal (or turbulent) motions are depicted in the IRDC using the C18O line data. The spatial distribution of young stellar objects (YSOs) identified using the VVV near-infrared data traces star formation activities in the IRDC. Low-mass cores are identified toward both the H ii regions using the ALMA 1.38 mm continuum map. The VLT/NACO adaptive-optics L′-band images show the presence of at least three point-like sources and the absence of small-scale features in the inner 4000 AU around YSOs NIR31 and MIR 16 located toward the H ii regions. The H ii regions and groups of YSO are observed toward the central part of the IRDC, where the two filamentary clouds intersect. A scenario of cloud–cloud collision or converging flows in the IRDC seems to be applicable, which may explain star formation activities including HFS and massive stars.