[en] Clouds of high infrared extinction are promising sites of massive star/cluster formation. A large number of cloud cores discovered in recent years allow for the investigation of a possible evolutionary sequence among cores in early phases. We have conducted a survey of deuterium fractionation toward 15 dense cores in various evolutionary stages, from high-mass starless cores to ultracompact H II regions, in the massive star-forming clouds of high extinction, G34.43+0.24, IRAS 18151–1208, and IRAS 18223–1243, with the Submillimeter Telescope. Spectra of N₂H⁺ (3-2), N₂D⁺ (3-2), and C¹⁸O (2-1) were observed to derive the deuterium fractionation of N₂H⁺, D_frac ≡ N(N₂D⁺)/N(N₂H⁺), as well as the CO depletion factor for every selected core. Our results show a decreasing trend in D_frac with both gas temperature and line width. Since colder and quiescent gas is likely to be associated with less evolved cores, larger D_frac appears to correlate with early phases of core evolution. Such decreasing trend resembles the behavior of D_frac in the low-mass protostellar cores and is consistent with several earlier studies in high-mass protostellar cores. We also find a moderate increasing trend of D_frac with the CO depletion factor, suggesting that sublimation of ice mantles alters the competition in the chemical reactions and reduces D_frac. Our findings suggest a general chemical behavior of deuterated species in both low- and high-mass protostellar candidates at early stages. In addition, upper limits to the ionization degree are estimated to be within 2 × 10^–7 and 5 × 10^–6. The four quiescent cores have marginal field-neutral coupling and perhaps favor turbulent cooling flows.

[en] We have imaged the extremely high velocity outflowing gas in CO (2-1) and (3-2) associated with the shell-like ultracompact H II region G5.89–0.39 at a resolution of ∼3'' (corresponding to ∼4000 AU) with the Submillimeter Array. The integrated high-velocity (∼>45 km s^–1) CO emission reveals at least three blueshifted lobes and two redshifted lobes. These lobes belong to two outflows, one oriented N-S, the other NW-SE. The NW-SE outflow is likely identical to the previously detected Brγ outflow. Furthermore, these outflow lobes all clearly show a Hubble-like kinematic structure. For the first time, we estimate the temperature of the outflowing gas as a function of velocity with large velocity gradient calculations. Our results reveal a clear increasing trend of temperature with gas velocity. The observational features of the extremely high velocity gas associated with G5.89–0.39 qualitatively favor the jet-driven bow shock model.

[en] We present the first interferometric polarization map of the W3(OH) massive star-forming region observed with the Submillimeter Array (SMA) at 878 μm with an angular resolution of 1.''5 (about 3 × 10³ AU). Polarization is detected in the W3(H₂O) hot core, an extended emission structure in the northwest of W3(H₂O), and part of the W3(OH) ultracompact H II region. The W3(H₂O) hot core is known to be associated with a synchrotron jet along the east-west direction. In this core, the inferred magnetic field orientation is well aligned with the synchrotron jet and close to the plane of sky. Using the Chandrasekhar-Fermi method with the observed dispersion in polarization angle, we estimate a plane-of-sky magnetic field strength of 17.0 mG. Combined with water maser Zeeman measurements, the total magnetic field strength is estimated to be 17.1 mG, comparable to the field strength estimated from the synchrotron model. The magnetic field energy dominates over turbulence in this core. In addition, the depolarization effect is discerned in both SMA and James Clerk Maxwell Telescope measurements. Despite the great difference in angular resolutions and map extents, the polarization percentage shows a similar power-law dependence with the beam averaged column density. We suggest that the column density may be an important factor to consider when interpreting the depolarization effect.

[en] We present the results of a high-resolution study with the Submillimeter Array toward the massive star-forming complex G9.62+0.19. Three submillimeter cores are detected in this region. The masses are 13, 30, and 165 M_sun for the northern, middle, and southern dust cores, respectively. Infall motions are found with HCN (4-3) and CS (7-6) lines at the middle core (G9.62+0.19 E). The infall rate is 4.3 x 10^-3 M_sun yr^-1. In the southern core, a bipolar outflow with a total mass about 26 M_sun and a mass-loss rate of 3.6 x 10^-5 M_sun yr^-1 is revealed in SO (8₇-7₇) line wing emission. CS (7-6) and HCN (4-3) lines trace higher velocity gas than SO (8₇-7₇). G9.62+0.19 F is confirmed to be the driving source of the outflow. We also analyze the abundances of CS, SO, and HCN along the redshifted outflow lobes. The mass-velocity diagrams of the outflow lobes can be well fitted by a single power law. The evolutionary sequence of the centimeter/millimeter cores in this region is also analyzed. The results support that ultracompact H II regions have a higher blue excess than their precursors.

[en] Sensitive and high angular resolution (∼0.''7) (sub)millimeter line and continuum observations of the massive star-forming region W3(OH) made with the Submillimeter Array are presented. We report the first detection of two bipolar outflows emanating from the young and massive 'Turner-Welch' (TW) protobinary system detected through the emission of carbon monoxide. The outflows are massive (10 M_sun), highly collimated (10⁰), and seem to be the extended molecular component of the strong radio jets and a 22 GHz maser water outflow energized also by the stars in the W3(OH)TW system. Observations of the 890 μm continuum emission and the thermal emission of CH₃OH might suggest the presence of two rotating circumstellar-disk-like structures associated with the binary system. The disk-like structures have sizes of about 1500 AU, masses of a few M_sun, and appear to energize the molecular outflows and radio jets. We estimate that the young stars feeding the outflows and surrounded by the massive disk-like structures may be of B-type.

[en] We present new spectral line observations of the CH₃CN molecule in the accretion disk around the massive protostar IRAS 20126+4104 with the Submillimeter Array, which, for the first time, measure the disk density, temperature, and rotational velocity with sufficient resolution (0.″37, equivalent to ∼600 au) to assess the gravitational stability of the disk through the Toomre- Q parameter. Our observations resolve the central 2000 au region that shows steeper velocity gradients with increasing upper state energy, indicating an increase in the rotational velocity of the hotter gas nearer the star. Such spin-up motions are characteristics of an accretion flow in a rotationally supported disk. We compare the observed data with synthetic image cubes produced by three-dimensional radiative transfer models describing a thin flared disk in Keplerian motion enveloped within the centrifugal radius of an angular-momentum-conserving accretion flow. Given a luminosity of 1.3 × 10⁴ L _⊙, the optimized model gives a disk mass of 1.5 M _⊙ and a radius of 858 au rotating about a 12.0 M _⊙ protostar with a disk mass accretion rate of 3.9 × 10⁻⁵ M _⊙ yr⁻¹. Our study finds that, in contrast to some theoretical expectations, the disk is hot and stable to fragmentation with Q > 2.8 at all radii which permits a smooth accretion flow. These results put forward the first constraints on gravitational instabilities in massive protostellar disks, which are closely connected to the formation of companion stars and planetary systems by fragmentation.

[en] How rapidly collapsing parsec-scale massive molecular clumps feed high-mass stars and how they fragment to form OB clusters have been outstanding questions in the field of star formation. In this work, we report the resolved structures and kinematics of the approximately face-on, rotating massive molecular clump, G33.92+0.11. Our high-resolution Atacama Large Millimeter/submillimeter Array images show that the spiral arm-like gas overdensities form in the eccentric gas accretion streams. First, we resolved that the dominant part of the ∼0.6 pc scale massive molecular clump (${3.0}_{-<!--\; ???\; -->1.4}^{+2.8}$·10³ $M_{\odot <!--\; ???\; -->}$) G33.92+0.11 A is tangled with several 0.5–1 pc size molecular arms spiraling around it, which may be connected further to exterior gas accretion streams. Within G33.92+0.11 A, we resolved the ∼0.1 pc width gas mini-arms connecting to the two central massive (100–300 $M_{\odot <!--\; ???\; -->}$) molecular cores. The kinematics of arms and cores elucidate a coherent accretion flow continuing from large to small scales. We demonstrate that the large molecular arms are indeed the cradles of dense cores, which are likely current or future sites of high-mass star formation. Since these deeply embedded massive molecular clumps preferentially form the highest-mass stars in the clusters, we argue that dense cores fed by or formed within molecular arms play a key role in making the upper end of the stellar and core mass functions.

[en] We have performed a dense core survey toward the Infrared Dark Cloud G14.225-0.506 at 3 mm continuum emission with the Atacama Large Millimeter/Submillimeter Array (ALMA). This survey covers the two hub-filament systems with an angular resolution of $\sim <!--\; ???\; -->3^{\mathrm{\prime <!--\; ???\; -->}\mathrm{\prime <!--\; ???\; -->}}$ (∼0.03 pc). We identified 48 dense cores. 20 out of the 48 cores are protostellar due to their association with young stellar objects (YSOs) and/or X-ray point-sources, while the other 28 cores are likely prestellar and unrelated with known IR or X-ray emission. Using APEX 870 μm continuum emission, we also identified the 18 clumps hosting these cores. Through virial analysis using the ALMA N₂H⁺ and VLA/Effelsberg NH₃ molecular line data, we found a decreasing trend in the virial parameter with decreasing scales from filaments to clumps, and then to cores. The virial parameters of 0.1–1.3 in cores indicate that cores are likely undergoing dynamical collapse. The cumulative core mass function for the prestellar core candidates has a power law index of $\mathrm{\alpha <!--\; ??\; -->}=1.6$, with masses ranging from 1.5 to 22 $M_{\odot <!--\; ???\; -->}$. We find no massive prestellar or protostellar cores. Previous studies suggest that massive O-type stars have not been produced yet in this region. Therefore, high-mass stars should be formed in the prestellar cores by accreting a significant amount of gas from the surrounding medium. Another possibility is that low-mass YSOs become massive by accreting from their parent cores that are fed by filaments. These two possibilities might be consistent with the scenario of global hierarchical collapse.

[en] We observed the high-mass star-forming region G335.579–0.292 with the Atacama Large Millimeter/submillimeter Array (ALMA) at 226 GHz with an angular resolution of 0.″3 (∼1000 au resolution at the source distance). G335.579–0.292 hosts one of the most massive cores in the Galaxy (G335–MM1). The continuum emission shows that G335–MM1 fragments into at least five sources, while molecular line emission is detected in two of the continuum sources (ALMA1 and ALMA3). We found evidence of large- and small-scale infall in ALMA1 revealed by an inverse P-Cygni profile and the presence of a blueshifted spot at the center of the first moment map of the CH₃CN emission. In addition, hot gas expansion in the innermost region is unveiled by a redshifted spot in the first moment map of HDCO and (CH₃)₂CO (both with E _u > 1100 K). Our modeling reveals that this expansion motion originates close to the central source, likely due to reversal of the accretion flow induced by the expansion of the H ii region, while infall and rotation motions originate in the outer regions. ALMA3 shows clear signs of rotation, with a rotation axis inclination with respect to the line of sight close to 90°, and a system mass (disk + star) in the range of 10–30 M _☉.

[en] We have modified the iterative procedure introduced by Lin et al., to systematically combine the submillimeter images taken from ground-based (e.g., CSO, JCMT, APEX) and space (e.g., Herschel, Planck) telescopes. We applied the updated procedure to observations of three well-studied Infrared Dark Clouds (IRDCs): G11.11−0.12, G14.225−0.506, and G28.34+0.06, and then performed single-component, modified blackbody fits to each pixel to derive ∼10″ resolution dust temperature and column density maps. The derived column density maps show that these three IRDCs exhibit complex filamentary structures embedded with rich clumps/cores. We compared the column density probability distribution functions (N-PDFs) and two-point correlation (2PT) functions of the column density field between these IRDCs with several OB-cluster-forming regions. Based on the observed correlation between the luminosity-to-mass ratio and the power-law index of the N-PDF, and complementary hydrodynamical simulations for a 10⁴ $M_{\odot <!--\; ???\; -->}$ molecular cloud, we hypothesize that cloud evolution can be better characterized by the evolution of the (column) density distribution function and the relative power of dense structures as a function of spatial scales, rather than merely based on the presence of star-forming activity. An important component of our approach is to provide a model-independent quantification of cloud evolution. Based on the small analyzed sample, we propose four evolutionary stages, namely, cloud integration, stellar assembly, cloud pre-dispersal, and dispersed cloud. The initial cloud integration stage and the final dispersed cloud stage may be distinguished from the two intermediate stages by a steeper than −4 power-law index of the N-PDF. The cloud integration stage and the subsequent stellar assembly stage are further distinguished from each other by the larger luminosity-to-mass ratio (>40 $L_{\odot <!--\; ???\; -->}/M_{\odot <!--\; ???\; -->}$) of the latter. A future large survey of molecular clouds with high angular resolution may establish more precise evolutionary tracks in the parameter space of N-PDF, 2PT function, and luminosity-to-mass ratio.