• ALMA Observations of Fragmentation, Substructure, and Protostars in High-mass Starless Clump Candidates

      Svoboda, Brian E.; Shirley, Yancy L.; Traficante, Alessio; Battersby, Cara; Fuller, Gary A.; Zhang, Qizhou; Beuther, Henrik; Peretto, Nicolas; Brogan, Crystal; Hunter, Todd; et al. (IOP PUBLISHING LTD, 2019-11-15)
      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 x 10(2) M-circle dot less than or similar to Mcl less than or similar to 4 x 10(3) M-circle dot) within d(circle dot) < 5 kpc. The joint 12 + 7 m array maps have a high spatial resolution of.3000 au (0.015 pc,.syn 0.8) and have high point-source mass-completeness down to M >> 0.3 M. at 6srms (or 1srms column density sensitivity of N = 1.1 x 10(22) cm(-2)). We discover previously undetected signposts of low-luminosity star formation from CO J = 2 -> 1 and SiO J = 5 -> 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-bol less than or similar to 50 L-circle dot), 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(circle dot) when integrated over size scales of R < 2 x 10(4) 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(j,th); i.e., separations equal to 0.4-1.6 x lambda(j,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 (approximate to 3-4 x lambda(j, 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.
    • CO (J = 1–0) Observations toward Filamentary Molecular Clouds in the Galactic Region with l = [169.°75, 174.°75], b = [−0.°75, 0.°5]

      Xiong, Fang; Chen, Xuepeng; Zhang, Qizhou; Yang, Ji; Fang, Min; Zhang, Miaomiao; Guo, Weihua; Sun, Li; Univ Arizona, Dept Astron (IOP PUBLISHING LTD, 2019-07-30)
      We present observations of the CO isotopologues (12CO, 13CO, and C18O) toward the Galactic region with 169fdg75 ≤ l ≤ 174fdg75 and −0fdg75 ≤ b ≤ 0fdg5 using the Purple Mountain Observatory 13.7 m millimeter-wavelength telescope. Based on the 13CO (J = 1 − 0) data, we find five molecular clouds within the velocity range between −25 and 8 km s−1 that are all characterized by conspicuous filamentary structures. We have identified eight filaments with a length of 6.38–28.45 pc, a mean H2 column density of 0.70 × 1021–6.53 × 1021 cm−2, and a line mass of 20.24–161.91 M ☉ pc−1, assuming a distance of ~1.7 kpc. Gaussian fittings to the inner parts of the radial density profiles lead to a mean FWHM width of 1.13 ± 0.01 pc. The velocity structures of most filaments present continuous distributions with slight velocity gradients. We find that turbulence is the dominant internal pressure to support the fragmentation of filaments instead of thermal pressure. Most filaments have virial parameters smaller than 2; thus, they are gravitationally bound. Four filaments have an LTE line mass close to the virial line mass. We further extract dense clumps using the 13CO data and find that 64% of the clumps are associated with the filaments. According to the complementary IR data, most filaments have associated Class II young stellar objects. Class I objects are mainly found to be located in the filaments with a virial parameter close to 1. Within two virialized filaments, 12CO outflows have been detected, indicating ongoing star-forming activity therein.
    • HALOGAS: the properties of extraplanar HI in disc galaxies

      Marasco, A.; Fraternali, F.; Heald, G.; de Blok, W. J. G.; Oosterloo, T.; Kamphuis, P.; Józsa, G. I. G.; Vargas, C. J.; Winkel, B.; Walterbos, R. A. M.; et al. (EDP SCIENCES S A, 2019-10-17)
      We present a systematic study of the extraplanar gas (EPG) in a sample of 15 nearby late-type galaxies at intermediate inclinations using publicly available, deep interferometric HI data from the Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. For each system we masked the HI emission coming from the regularly rotating disc and used synthetic datacubes to model the leftover "anomalous" HI flux. Our model consists of a smooth, axisymmetric thick component described by three structural and four kinematical parameters, which are fit to the data via a Markov chain Monte Carlo (MCMC) based Bayesian method. We find that extraplanar HI is nearly ubiquitous in disc galaxies as we fail to detect it in only two of the systems with the poorest spatial resolution. The EPG component encloses similar to 5-25% of the total HI mass with a mean value of 14%, and has a typical thickness of a few kpc which is incompatible with expectations based on hydrostatic equilibrium models. The EPG kinematics is remarkably similar throughout the sample, and consists of a lagging rotation with typical vertical gradients of similar to-10 km s(-1) kpc(-1), a velocity dispersion of 15-30 km s(-1), and, for most galaxies, a global inflow in both the vertical and radial directions with speeds of 20-30 km s(-1). The EPG HI masses are in excellent agreement with predictions from simple models of the galactic fountain that are powered by stellar feedback. The combined effect of photo-ionisation and interaction of the fountain material with the circumgalactic medium can qualitatively explain the kinematics of the EPG, but dynamical models of the galactic fountain are required to fully test this framework.
    • Probing ISM Structure in Trumpler 14 and Carina I Using the Stratospheric Terahertz Observatory 2

      Seo, Young Min; Goldsmith, Paul F.; Walker, Christopher K.; Hollenbach, David J.; Wolfire, Mark G.; Kulesa, Craig A.; Tolls, Volker; Bernasconi, Pietro N.; Kavak, Ümit; van der Tak, Floris F. S.; et al. (IOP PUBLISHING LTD, 2019-06-20)
      We present observations of the Trumpler 14/Carina I region carried out using the Stratospheric Terahertz Observatory 2. The Trumpler 14/Carina I region is in the western part of the Carina Nebula Complex (CNC), which is one of the most extreme star-forming regions in the Milky Way. We observed Trumpler 14/Carina I in the 58 mu m transition of [C II] with a spatial resolution of 48 '' and a velocity resolution of 0.17 km s(-1). The observations cover a 0 degrees.25 by 0 degrees.28 area with central position l = 297 degrees.34, b = -0 degrees.60. The kinematics show that bright [C II] structures are spatially and spectrally correlated with the surfaces of CO clouds, tracing the photodissociation region (PDR) and ionization front of each molecular cloud. Along seven lines of sight (LOSs) that traverse Tr 14 into the dark ridge to the southwest, we find that the [C II] luminosity from the H II region is 3.7 times that from the PDR. In the same LOS, we find in the PDRs an average ratio of 1 : 4.1 : 5.6 for the mass in atomic gas : dark CO gas : molecular gas traced by CO. Comparing multiple gas tracers, including H I 21 cm, [C II], CO, and radio recombination lines, we find that the H II regions of the CNC are well described as H II regions with one side freely expanding toward us, consistent with the Champagne model of ionized gas evolution. The dispersal of the GMC in this region is dominated by EUV photoevaporation; the dispersal timescale is 20-30 Myr.
    • Relative alignment between dense molecular cores and ambient magnetic field: the synergy of numerical models and observations

      Chen, Che-Yu; Behrens, Erica A; Washington, Jasmin E; Fissel, Laura M; Friesen, Rachel K; Li, Zhi-Yun; Pineda, Jaime E; Ginsburg, Adam; Kirk, Helen; Scibelli, Samantha; et al. (OXFORD UNIV PRESS, 2020-03-28)
      The role played by magnetic field during star formation is an important topic in astrophysics. We investigate the correlation between the orientation of star-forming cores (as defined by the core major axes) and ambient magnetic field directions in (i) a 3D magnetohydrodynamic simulation, (ii) synthetic observations generated from the simulation at different viewing angles, and (iii) observations of nearby molecular clouds. We find that the results on relative alignment between cores and background magnetic field in synthetic observations slightly disagree with those measured in fully 3D simulation data, which is partly because cores identified in projected 2D maps tend to coexist within filamentary structures, while 3D cores are generally more rounded. In addition, we examine the progression of magnetic field from pc to core scale in the simulation, which is consistent with the anisotropic core formation model that gas preferably flows along the magnetic field towards dense cores. When comparing the observed cores identified from the Green Bank Ammonia Survey and Planck polarizationinferred magnetic field orientations, we find that the relative core-field alignment has a regional dependence among different clouds. More specifically, we find that dense cores in the Taurus molecular cloud tend to align perpendicular to the background magnetic field, while those in Perseus and Ophiuchus tend to have random (Perseus) or slightly parallel (Ophiuchus) orientations with respect to the field. We argue that this feature of relative core-field orientation could be used to probe the relative significance of the magnetic field within the cloud.
    • Searching for Inflow toward Massive Starless Clump Candidates Identified in the Bolocam Galactic Plane Survey

      Calahan, Jenny K.; Shirley, Yancy L.; Svoboda, Brian E.; Ivanov, Elizabeth A.; Schmid, Jonathan R.; Pulley, Anna; Lautenbach, Jennifer; Zawadzki, Nicole; Bullivant, Christopher; Cook, Claire W.; et al. (IOP PUBLISHING LTD, 2018-07-20)
      Recent Galactic plane surveys of dust continuum emission at long wavelengths have identified a population of dense, massive clumps with no evidence for ongoing star formation. These massive starless clump candidates are excellent sites to search for the initial phases of massive star formation before the feedback from massive star formation affects the clump. In this study, we search for the spectroscopic signature of inflowing gas toward starless clumps, some of which are massive enough to form a massive star. We observed 101 starless clump candidates identified in the Bolocam Galactic Plane Survey (BGPS) in HCO+ J = 1-0 using the 12 m Arizona Radio Observatory telescope. We find a small blue excess of E = (N-blue - N-red)/N-total = 0.03 for the complete survey. We identified six clumps that are good candidates for inflow motion and used a radiative transfer model to calculate mass inflow rates that range from 500 to 2000 M-circle dot Myr(-1). If the observed line profiles are indeed due to large-scale inflow motions, then these clumps will typically double their mass on a freefall time. Our survey finds that massive BGPS starless clump candidates with inflow signatures in HCO+ J = 1-0 are rare throughout our Galaxy.
    • Star-forming content of the giant molecular filaments in the Milky Way

      Zhang, M.; Kainulainen, J.; Mattern, M.; Fang, M.; Henning, Th.; Univ Arizona, Dept Astron (EDP SCIENCES S A, 2019-02-04)
      Through observations numerous giant molecular filaments (GMFs) have been discovered in the MilkyWay. Their role in the Galactic star formation and Galaxy-scale evolution of dense gas is unknown. Aims. We investigate systematically the star-forming content of all currently known GMFs. This allows us to estimate the star formation rates (SFRs) of the GMFs and to establish relationships between the SFRs and the GMF properties. Methods. We identified and classified the young stellar object (YSO) population of each GMF using multiwavelength photometry from near-to far-infrared. We estimated the total SFRs assuming a universal and fully sampled initial mass function and luminosity function. Results. We uniformly estimate the physical properties of 57 GMFs. The GMFs show correlations between the (CO)-C-13 line width, mass, and size, similar to Larson's relations. We identify 36 394 infrared excess sources in 57 GMFs and obtain SFRs for 46 GMFs. The median SFR surface density (Sigma(SFR)) and star formation efficiency (SFE) of GMFs are 0.62 M-circle dot Myr(-1) pc(-2) and 1%, similar to the nearby star-forming clouds. The star formation rate per free-fall time of GMFs is between 0.002-0.05 with the median value of 0.02. We also find a strong correlation between SFR and dense gas mass that is defined as gas mass above a visual extinction of 7 mag, which suggests that the SFRs of the GMFs scale similarly with dense gas as those of nearby molecular clouds. We also find a strong correlation between the mean SFR per unit length and dense gas mass per unit length. The origin of this scaling remains unknown, calling for further studies that can link the structure of GMFs to their SF activity and explore the differences between GMFs and other molecular clouds.
    • The dependence of the hierarchical distribution of star clusters on galactic environment

      Menon, S.H.; Grasha, K.; Elmegreen, B.G.; Federrath, C.; Krumholz, M.R.; Calzetti, D.; Sánchez, N.; Linden, S.T.; Adamo, A.; Messa, M.; et al. (Oxford University Press, 2021)
      We use the angular two-point correlation function (TPCF) to investigate the hierarchical distribution of young star clusters in 12 local (3-18 Mpc) star-forming galaxies using star cluster catalogs obtained with the Hubble Space Telescope (HST) as part of the Treasury Program Legacy ExtraGalactic UV Survey. The sample spans a range of different morphological types, allowing us to infer how the physical properties of the galaxy affect the spatial distribution of the clusters. We also prepare a range of physically motivated toy models to compare with and interpret the observed features in the TPCFs. We find that, conforming to earlier studies, young clusters ($T \lesssim 10\, \mathrm{Myr}$) have power-law TPCFs that are characteristic of fractal distributions with a fractal dimension D2, and this scale-free nature extends out to a maximum scale lcorr beyond which the distribution becomes Poissonian. However, lcorr, and D2 vary significantly across the sample, and are correlated with a number of host galaxy physical properties, suggesting that there are physical differences in the underlying star cluster distributions. We also find that hierarchical structuring weakens with age, evidenced by flatter TPCFs for older clusters ($T \gtrsim 10\, \mathrm{Myr}$), that eventually converges to the residual correlation expected from a completely random large-scale radial distribution of clusters in the galaxy in $\sim 100 \, \mathrm{Myr}$. Our study demonstrates that the hierarchical distribution of star clusters evolves with age, and is strongly dependent on the properties of the host galaxy environment. © 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.
    • X Marks the Spot: Nexus of Filaments, Cores, and Outflows in a Young Star-forming Region

      Imara, Nia; Lada, Charles; Lewis, John; Bieging, John H.; Kong, Shuo; Lombardi, Marco; Alves, Joao; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2017-05-15)
      We present a multiwavelength investigation of a region of a nearby giant molecular cloud that is distinguished by a minimal level of star formation activity. With our new (CO)-C-12(J = 2-1) and (CO)-C-13(J = 2-1) observations of a remote region within the middle of the California molecular cloud, we aim to investigate the relationship between filaments, cores, and a molecular outflow in a relatively pristine environment. An extinction map of the region from Herschel Space Observatory observations reveals the presence of two 2 pc long filaments radiating from a highextinction clump. Using the (CO)-C-13 observations, we show that the filaments have coherent velocity gradients and that their mass-per-unit-lengths may exceed the critical value above which filaments are gravitationally unstable. The region exhibits structure with eight cores, at least one of which is a starless, prestellar core. We identify a low-velocity, low-mass molecular outflow that may be driven by a flat spectrum protostar. The outflow does not appear to be responsible for driving the turbulence in the core with which it is associated, nor does it provide significant support against gravitational collapse.