Keywords

This paper presents an overview of the QUARKS survey, which stands for "Querying Underlying mechanisms of massive star formation with ALMA-Resolved gas Kinematics and Structures." The QUARKS survey is observing 139 massive clumps covered by 156 pointings at Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 (λ ∼ 1.3 mm). In conjunction with data obtained from the ALMA-ATOMS survey at Band 3 (λ ∼ 3 mm), QUARKS aims to carry out an unbiased statistical investigation of massive star formation process within protoclusters down to a scale of 1000 au. This overview paper describes the observations and data reduction of the QUARKS survey, and gives a first look at an exemplar source, the mini-starburst Sgr B2(M). The wide-bandwidth (7.5 GHz) and high-angular-resolution (∼03) observations of the QUARKS survey allow for the resolution of much more compact cores than those could be done by the ATOMS survey, and to detect previously unrevealed fainter filamentary structures. The spectral windows cover transitions of species including CO, SO, N₂D⁺, SiO, H₃₀α, H₂CO, CH₃CN, and many other complex organic molecules, tracing gas components with different temperatures and spatial extents. QUARKS aims to deepen our understanding of several scientific topics of massive star formation, such as the mass transport within protoclusters by (hub-)filamentary structures, the existence of massive starless cores, the physical and chemical properties of dense cores within protoclusters, and the feedback from already formed high-mass young protostars.

Star-forming processes strongly influence the ISM chemistry. Nowadays, many high-quality databases are available at millimeter wavelengths. Using them, it is possible to carry out studies that review and deepen previous results. If these studies involve large samples of sources, it is preferred to use direct tools to study the molecular gas. With the aim of testing these tools such as the use of the HCN/HNC ratio as a thermometer, and the use of H¹³CO⁺, HC₃N, N₂H⁺ and C₂H as "chemical clocks," we present a molecular line study toward 55 sources representing massive young stellar objects at different evolutionary stages: infrared dark clouds (IRDCs), high-mass protostellar objects (HMPOs), hot molecular cores (HMCs) and ultracompact H ii regions. We found that the use of the HCN/HNC ratio as a universal thermometer in the ISM should be taken with care because the HCN optical depth is a big issue that can affect the method. Hence, this tool should be utilized only after a careful analysis of the HCN spectrum, checking that no line, neither the main nor the hyperfine ones, presents absorption features. We point out that the analysis of the emission of H¹³CO⁺, HC₃N, N₂H⁺ and C₂H could be useful to trace and distinguish regions among IRDCs, HMPOs and HMCs. The molecular line widths of these four species increase from the IRDC to the HMC stage, which can be a consequence of the gas dynamics related to the star-forming processes taking place in the molecular clumps. Our results not only contribute with more statistics, acting as a probe of such chemical tools, useful to obtain information in large samples of sources, but also complement previous works through the analysis of other types of sources.

The most extensive survey of carbon monoxide (CO) gas in the Taurus molecular cloud relied on ¹²CO and ¹³CO J = 1 → 0 emission only, distinguishing the region where ¹²CO is detected without ¹³CO (named mask 1 region) from the one where both are detected (mask 2 region) (Goldsmith et al. 2008; Pineda et al. 2010). We have taken advantage of recent ¹²CO J = 3 → 2 James Clerk Maxwell Telescope observations, where they include mask 1 regions to estimate density, temperature, and N(CO) with a large velocity gradient model. This represents 1395 pixels out of ∼1.2 million in the mark 1 region. Compared to Pineda et al. (2010) results and assuming a T_kin of 30 K, we find a higher volume density of molecular hydrogen of 3.3 × 10³ cm⁻³, compared to their 250–700 cm⁻³, and a CO column density of 5.7 × 10¹⁵ cm⁻², about a quarter of their value. The differences are important and show the necessity to observe several CO transitions to better describe the intermediate region between the dense cloud and the diffuse atomic medium. Future observations to extend the ¹²CO J = 3 → 2 mapping further away from the ¹³CO-detected region comprising mask 1 are needed to revisit our understanding of the diffuse portions of dark clouds.

We have started a systematic survey of molecular clumps with infall motions to study the very early phase of star formation. Our first step is to utilize the data products by MWISP to make an unbiased survey for blue asymmetric line profiles of CO isotopical molecules. Within a total area of ∼2400 square degrees nearby the Galactic plane, we have found 3533 candidates showing blue-profiles, in which 3329 are selected from the ¹²CO&¹³CO pair and 204 are from the ¹³CO&C¹⁸O pair. Exploration of the parametric spaces suggests our samples are in the cold phase with relatively high column densities ready for star formation. Analysis of the spatial distribution of our samples suggests that they exist virtually in all major components of the galaxy. The vertical distribution suggest that the sources are located mainly in the thick disk of ∼85 pc, but still a small part are located far beyond Galactic midplane. Our follow-up observation indicates that these candidates are a good sample to start a search for infall motions, and to study the condition of very early phase of star formation.

We present large-scale (2° × 2°) observations toward the molecular cloud M120.1+3.0, using ¹²CO, ¹³CO and C¹⁸O (J = 1 − 0) data from the Purple Mountain Observatory 13.7 m millimeter telescope. The distance of the cloud is measured to be ∼1.1 kpc. Using the ¹³CO data, we identify a main filament F1 and two sub-filaments F2 and F3 in the cloud, which together show a "hub-filament" structure. Filaments F1 and F2 are thermally supercritical. Furthermore, F1 displays clear localized systematic motions in the ¹³CO position–velocity diagram, which could be explained by accretion along the filament. The mean estimated accretion rate is ∼132 M_⊙ Myr⁻¹. Approximately 150 ¹³CO clumps are identified in the cloud, of which 39 are gravitationally bound. Most of these virialized clumps are well distributed along the supercritical filaments F1 and F2. Based on the complementary infrared and optical data, we identify ∼186 young stellar objects in the observed area and extract five clusters within the dense ridge of F1. The calculated star formation rate (SFR) surface densities (Σ_SFR) in the clusters range from 1.4 to 2.5 M_⊙ Myr⁻¹ pc⁻², with a mean value of ∼2.0 M_⊙ Myr⁻¹ pc⁻². We therefore regard them as mini-starburst cluster candidates. The comparison between Σ_SFR and column density N_gas along the skeleton of F1 suggests that star formation is closely related to the dense gas in the cloud. Along the main filament F1, five bipolar outflows are also found. All these results indicate intense star-forming activities in the M120.1+3.0 molecular cloud.

The distribution of ultraviolet (UV) radiation field provides critical constraints on the physical environments of molecular clouds. Within 1 kpc of our solar system and fostering protostars of different masses, the giant molecular clouds in the Gould Belt present an excellent opportunity to resolve the UV field structure in star-forming regions. We performed spectral energy distribution (SED) fitting of the archival data from the Herschel Gould Belt Survey (HGBS). Dust radiative transfer analysis with the DUSTY code was applied to 23 regions in 14 molecular complexes of the Gould Belt, resulting in the spatial distribution of the radiation field in these regions. For 10 of 15 regions with independent measurements of star formation rate, their star formation rate and UV radiation intensity largely conform to a linear correlation found in previous studies.

Star formation is governed by the interplay between gravity and turbulence in most of molecular clouds. Recent theoretical works assume that dense gas, whose column density is above a critical value in the column density probability distribution function (N-PDF), where gravity starts to overcome turbulence, becomes star-forming gas and will collapse to form stars. However, these high-density gases will include some very turbulent areas in the clouds. Will these dense but turbulent gases also form stars? We test this scenario in Ophiuchus molecular cloud using N-PDF analysis and find that at least in some regions, the turbulent, dense gas is not forming stars. We identified two isolated high-density structures in Ophiuchus, which are gravitationally unbound and show no sign of star formation. Their high densities may come from turbulence.

We revisit the mass–size relation of molecular cloud structures based on the column density map of the Cygnus-X molecular cloud complex. We extract 135 column density peaks in Cygnus-X and analyze the column density distributions around these peaks. The averaged column density profiles, N(R), around all the peaks can be well fitted with broken power-laws, which are described by an inner power-law index n, outer power-law index m, and the radius R_TP and column density N_TP at the transition point. We then explore the M–R relation with different samples of cloud structures by varying the N(R) parameters and the column density threshold, N₀, which determines the boundary of a cloud structure. We find that only when N₀ has a wide range of values, the M–R relation may largely probe the density distribution, and the fitted power-law index of the M–R relation is related to the power-law index of N(R). On the contrary, with a constant N₀, the M–R relation has no direct connection with the density distribution; in this case, the fitted power-law index of the M–R relation is equal to 2 (when N₀ ≥ N_TP and n has a narrow range of values), larger than 2 (when N₀ ≥ N_TP and n has a wide range of values), or slightly less than 2 (when N₀ < N_TP).

We present a large-scale simultaneous survey of the CO isotopologues (¹²CO, ¹³CO, and C¹⁸O) J = 1–0 line emission toward the Galactic plane region of l = 10665 to 10950 and b = −185 to 095 using the Purple Mountain Observatory 13.7 m millimeter-wavelength telescope. Except for the molecular gas in the solar neighborhood, the emission from the molecular gas in this region is concentrated in the velocity range of [−60, −35] km s⁻¹. The gas in the region can be divided into four clouds, with mass in the range of ∼10³–10⁴ M_☉. We have identified 25 filaments based on the ¹³CO data. The median excitation temperature, length, line mass, line width, and virial parameter of the filaments are 10.89 K, 8.49 pc, 146.11 M_⊙ pc⁻¹, 1.01 km s⁻¹, and 3.14, respectively. Among these filaments, eight have virial parameters of less than 2, suggesting that they are gravitationally bound and can lead to star formation. Nineteen H ii regions or candidates have previously been found in the region and we investigate the relationships between these H ii regions/candidates and surrounding molecular clouds in detail. Using morphology similarity and radial velocity consistency between H ii regions/candidates and molecular clouds as evidence for association, and raised temperature and velocity broadening as signatures of interaction, we propose that 12 H ii regions/candidates are associated with their surrounding molecular clouds. In the case of the H ii region of S142, the energy of the H ii region is sufficient to maintain the turbulence in the surrounding molecular gas.

The initial physical conditions of high-mass stars and protoclusters remain poorly characterized. To this end, we present the first targeted ALMA Band 6 1.3 mm continuum and spectral line survey toward high-mass starless clump candidates, selecting a sample of 12 of the most massive candidates ($4\times {10}^{2}\,{M}_{\odot }\lesssim {M}_{\mathrm{cl}}\lesssim 4\times {10}^{3}\,{M}_{\odot }$) within ${d}_{\odot }\lt 5\,\mathrm{kpc}$. The joint $12+7\,{\rm{m}}$ array maps have a high spatial resolution of $\lesssim 3000\,\mathrm{au}$ ($0.015\,\mathrm{pc}$, θ_syn ≈ 08) and have high point-source mass-completeness down to $M\approx 0.3\,{M}_{\odot }$ at $6{\sigma }_{\mathrm{rms}}$ (or $1{\sigma }_{\mathrm{rms}}$ column density sensitivity of $N=1.1\times {10}^{22}\,{\mathrm{cm}}^{-2}$). We discover previously undetected signposts of low-luminosity star formation from $\mathrm{CO}$ $J=2\to 1$ and $\mathrm{SiO}$ $J=5\to 4$ bipolar outflows and other signatures toward 11 out of 12 clumps, showing that current MIR/FIR Galactic plane surveys are incomplete to low- and intermediate-mass protostars (${L}_{\mathrm{bol}}\lesssim 50\,{L}_{\odot }$), and emphasizing the necessity of high-resolution follow-up. We compare a subset of the observed cores with a suite of radiative transfer models of starless cores. We find a high-mass starless core candidate with a model-derived mass consistent with ${29}_{15}^{52}\,{M}_{\odot }$ when integrated over size scales of $R\lt 2\times {10}^{4}\,\mathrm{au}$. Unresolved cores are poorly fit by radiative transfer models of externally heated Plummer density profiles, supporting the interpretation that they are protostellar even without detection of outflows. A high degree of fragmentation with rich substructure is observed toward 10 out of 12 clumps. We extract sources from the maps using a dendrogram to study the characteristic fragmentation length scale. Nearest neighbor separations, when corrected for projection with Monte Carlo random sampling, are consistent with being equal to the clump average thermal Jeans length (${\lambda }_{{\rm{j}},\mathrm{th}}$; i.e., separations equal to $0.4\mbox{--}1.6\times {\lambda }_{{\rm{j}},\mathrm{th}}$). In the context of previous observations that, on larger scales, see separations consistent with the turbulent Jeans length or the cylindrical thermal Jeans scale ($\approx 3\mbox{--}4\times {\lambda }_{{\rm{j}},\mathrm{th}}$), our findings support a hierarchical fragmentation process, where the highest-density regions are not strongly supported against thermal gravitational fragmentation by turbulence or magnetic fields.