Bieging, John H.; Patel, Saahil; Peters, William L.; Toth, L. Viktor; Marton, Gábor; Zahorecz, Sarolta; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2016-09-26)
      We present the results of a program to map the Sh2-235 molecular cloud complex in the CO and (CO)-C-13 J = 2 - 1 transitions using the Heinrich Hertz Submillimeter Telescope. The map resolution is 38 '' (FWHM), with an rms noise of 0.12K brightness temperature, for a velocity resolution of 0.34 km s(-1). With the same telescope, we also mapped the CO J = 3 - 2 line at a frequency of 345 GHz, using a 64 beam focal plane array of heterodyne mixers, achieving a typical rms noise of 0.5 K brightness temperature with a velocity resolution of 0.23 km s(-1). The three spectral line data cubes are available for download. Much of the cloud appears to be slightly sub-thermally excited in the J = 3 level, except for in the vicinity of the warmest and highest column density areas, which are currently forming stars. Using the CO and (CO)-C-13. J = 2 - 1 lines, we employ an LTE model to derive the gas column density over the entire mapped region. Examining a 125 pc(2). region centered on the most active star formation in the vicinity of Sh2-235, we find that the young stellar object surface density scales as approximately the 1.6-power of the gas column density. The area distribution function of the gas is a steeply declining exponential function of gas column density. Comparison of the morphology of ionized and molecular gas suggests that the cloud is being substantially disrupted by expansion of the H II regions, which may be triggering current star formation.
    • The Arizona Radio Observatory CO Mapping Survey of Galactic Molecular Clouds. VI. The Cep OB3 Cloud (Cepheus B and C) in CO J=2-1, (CO)-C-13 J=2-1, and CO J=3-2

      Bieging, John H.; Patel, Saahil; Hofmann, Ryan; Peters, William L.; Kainulainen, Jouni; Zhang, Miaomiao; Stutz, Amelia M.; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2018-10)
      We present (1) new fully sampled maps of CO and (CO)-C-13 J = 2-1 emission and CO J = 3-2 emission toward the molecular clouds Cep B and C, associated with the Cep OB3 association; (2) a map of extinction, A(V), derived from IR colors of background stars; and (3) the distribution of young stellar objects (YSOs) over the same field as the molecular maps. An LTE analysis of the CO and (CO)-C-13. maps yields the distribution of molecular column densities and temperatures. Substantial variations are evident across the clouds; smaller subregions show correlations between molecular properties and dust extinction, consistent with a picture of outer photodissociation regions with a layer of CO-dark molecular gas, a CO self-shielded interior, and an inner cold dense region where CO is largely depleted onto grains. Comparing the distribution of YSOs with molecular gas surface density shows a power-law relation very similar in slope to that for the giant molecular cloud associated with the H II region Sh2-235 from a previous paper in this series that employed the same methodology. We note the presence of several compact, isolated CO emission sources in the J = 3-2 maps. The gas temperature and (CO)-C-13. velocity dispersion yield a map of the sonic Mach number, which varies across the cloud but always exceeds unity, confirming the pervasiveness of supersonic turbulence over length scales greater than or similar to 0.1 pc (the map resolution). We also compute a J = 2-1 CO X-factor that varies with position but is, on average, within. 20% of the Galactic average derived from CO J = 1-0 observations.
    • 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.
    • Inflow Motions Associated with High-mass Protostellar Objects

      Yoo, Hyunju; Kim, Kee-Tae; Cho, Jungyeon; Choi, Minho; Wu, Jingwen; Evans, Neal J., II; Ziurys, L. M.; Univ Arizona, Dept Astron; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2018-04-02)
      We performed a molecular line survey of 82 high-mass protostellar objects in a search for inflow signatures associated with high-mass star formation. Using the (HCO+)-C-13 (1-0) line as an optically thin tracer, we detected a statistically significant excess of blue asymmetric line profiles in the HCO+ (1-0) transition, but nonsignificant excesses in the HCO+ (3-2) and H2CO (2(12)-1(11)) transitions. The negative blue excess for the HCN (3-2) transition suggests that the line profiles are affected by dynamics other than inflow motion. The HCO+ (1-0) transition thus seems to be the suitable tracer of inflow motions in high-mass star-forming regions, as previously suggested. We found 27 inflow candidates that have at least 1 blue asymmetric profile and no red asymmetric profile, and derived the inflow velocities to be 0.23-2.00 km s(-1) for 20 of them using a simple two-layer radiative transfer model. Our sample is divided into two groups in different evolutionary stages. The blue excess of the group in relatively earlier evolutionary stages was estimated to be slightly higher than that of the other in the HCO+ (1-0) transition.
    • The Lifetimes of Phases in High-mass Star-forming Regions

      Battersby, Cara; Bally, John; Svoboda, Brian; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2017-02-01)
      High-mass stars form within star clusters from dense, molecular regions (DMRs), but is the process of cluster formation slow and hydrostatic or quick and dynamic? We link the physical properties of high-mass star-forming regions with their evolutionary stage in a systematic way, using Herschel and Spitzer data. In order to produce a robust estimate of the relative lifetimes of these regions, we compare the fraction of DMRs above a column density associated with high-mass star formation, N(H-2) > 0.4-2.5 x 10(22) cm(-2), in the "starless" (no signature of stars >= 10 M circle dot forming) and star-forming phases in a 2 degrees x 2(degrees) region of the Galactic Plane centered at l = 30 degrees. Of regions capable of forming high-mass stars on similar to 1 pc scales, the starless (or embedded beyond detection) phase occupies about 60%-70% of the DMR lifetime, and the star-forming phase occupies about 30%-40%. These relative lifetimes are robust over a wide range of thresholds. We outline a method by which relative lifetimes can be anchored to absolute lifetimes from large-scale surveys of methanol masers and UCHII regions. A simplistic application of this method estimates the absolute lifetime of the starless phase to be 0.2-1.7 Myr (about 0.6-4.1 fiducial cloud free-fall times) and the star-forming phase to be 0.1-0.7 Myr (about 0.4-2.4 free-fall times), but these are highly uncertain. This work uniquely investigates the star-forming nature of high column density gas pixel by pixel, and our results demonstrate that the majority of high column density gas is in a starless or embedded phase.
    • An Ordered Envelope–Disk Transition in the Massive Protostellar Source G339.88-1.26

      Zhang, Yichen; Tan, Jonathan C.; Sakai, Nami; Tanaka, Kei E. I.; De Buizer, James M.; Liu, Mengyao; Beltrán, Maria T.; Kratter, Kaitlin; Mardones, Diego; Garay, Guido; et al. (IOP PUBLISHING LTD, 2019-03-01)
      We report molecular line observations of the massive protostellar source G339.88-1.26 with the Atacama Large Millimeter/Submillimeter Array. The observations reveal a highly collimated SiO jet extending from the 1.3. mm continuum source, which connects to a slightly wider but still highly collimated CO outflow. Rotational features perpendicular to the outflow axis are detected in various molecular emissions, including SiO, SO2, H2S, CH3OH, and H2CO emissions. Based on their spatial distributions and kinematics, we find that they trace different parts of the envelope-disk system. The SiO emission traces the disk and inner envelope in addition to the jet. The CH3OH and H2CO emissions mostly trace the infalling-rotating envelope and are enhanced around the transition region between envelope and disk, i.e., the centrifugal barrier. The SO2 and H2S emissions are enhanced around the centrifugal barrier and also trace the outer part of the disk. Envelope kinematics are consistent with rotating-infalling motion, while those of the disk are consistent with Keplerian rotation. The radius and velocity of the centrifugal barrier are estimated to be about 530 au and 6 km s(-1), respectively, leading to a central mass of about 11 M-circle dot, consistent with estimates based on spectral energy distribution fitting. These results indicate that an ordered transition from an infalling-rotating envelope to a Keplerian disk through a centrifugal barrier, accompanied by changes of types of molecular line emissions, is a valid description of this massive protostellar source. This implies that at least some massive stars form in a similar way to low-mass stars via core accretion.
    • The PDR structure and kinematics around the compact H ii regions S235 A and S235 C with [C ii], [13C ii], [O i], and HCO+ line profiles

      Kirsanova, M S; Ossenkopf-Okada, V; Anderson, L D; Boley, P A; Bieging, J H; Pavlyuchenkov, Ya N; Luisi, M; Schneider, N; Andersen, M; Samal, M R; et al. (OXFORD UNIV PRESS, 2020-07-30)
      The aim of this work is to study structure and gas kinematics in the photodissociation regions (PDRs) around the compact H II regions S235 A and S235 C. We observe the [C II], [C-13 II], and [O I] line emission, using SOFIA/upGREAT, and complement them by data of HCO+ and CO. We use the [C-13 II] line to measure the optical depth of the [C II] emission, and find that the [C II] line profiles are influenced by self-absorption, while the [C-13 II] line remains unaffected by these effects. Hence, for dense PDRs, [C-13 II] emission is a better tracer of gas kinematics. The optical depth of the [C II] line is up to 10 in S235 A. We find an expanding motion of the [C II]-emitting layer of the PDRs into the front molecular layer in both regions. Comparison of the gas and dust columns shows that gas components visible neither in the [C II] nor in low-J CO lines may contribute to the total column across S235 A. We test whether the observed properties of the PDRs match the predictions of spherical models of expanding H II region + PDR + molecular cloud. Integrated intensities of the [C-13 II], [C II], and [O I] lines are well represented by the model, but the models do not reproduce the double-peaked [C II] line profiles due to an insufficient column density of C+. The model predicts that the [O I] line could be a more reliable tracer of gas kinematics, but the foreground self-absorbing material does not allow using it in the considered regions.
    • 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.
    • Proper motions of collimated jets from intermediate-mass protostars in the Carina Nebula

      Reiter, Megan; Kiminki, Megan M.; Smith, Nathan; Bally, John; Univ Arizona, Steward Observ (OXFORD UNIV PRESS, 2017-10)
      We present proper motion measurements of 37 jets and HH objects in the Carina Nebula measured in two epochs of H alpha images obtained similar to 10 yr apart with Hubble Space Telescope/Advanced Camera for Surveys (ACS). Transverse velocities in all but one jet are faster than greater than or similar to 25 km s(-1), confirming that the jet-like H alpha features identified in the first epoch images trace outflowing gas. Proper motions constrain the location of the jet-driving source and provide kinematic confirmation of the intermediate-mass protostars that we identify for 20/37 jets. Jet velocities do not correlate with the estimated protostar mass and embedded driving sources do not have slower jets. Instead, transverse velocities (median similar to 75 km s(-1)) are similar to those in jets from low-mass stars. Assuming a constant velocity since launch, we compute jet dynamical ages (median similar to 10(4) yr). If continuous emission from inner jets traces the duration of the most recent accretion bursts, then these episodes are sustained longer (median similar to 700 yr) than the typical decay time of an FU Orionis outburst. These jets can carry appreciable momentum that may be injected into the surrounding environment. The resulting outflow force, dp/dt, lies between that measured in low- and high-mass sources, despite the very different observational tracers used. Smooth scaling of the outflow force argues for a common physical process underlying outflows from protostars of all masses. This latest kinematic result adds to a growing body of evidence that intermediate-mass star formation proceeds like a scaled-up version of the formation of low-mass stars.
    • The role of non-ionizing radiation pressure in star formation: the stability of cores and filaments

      Seo, Young Min; Youdin, Andrew N.; Univ Arizona, Dept Astron; Univ Arizona, Steward Observ (OXFORD UNIV PRESS, 2016-09-01)
      Stars form when filaments and dense cores in molecular clouds fragment and collapse due to self-gravity. In the most basic analyses of gravitational stability, the competition between self-gravity and thermal pressure sets the critical (i.e. maximum stable) mass of spheres and the critical line density of cylinders. Previous work has considered additional support from magnetic fields and turbulence. Here, we consider the effects of non-ionizing radiation, specifically the inward radiation pressure force that acts on dense structures embedded in an isotropic radiation field. Using hydrostatic, isothermal models, we find that irradiation lowers the critical mass and line density for gravitational collapse, and can thus act as a trigger for star formation. For structures with moderate central densities, similar to 10(3) cm(-3), the interstellar radiation field in the Solar vicinity has an order unity effect on stability thresholds. For more evolved objects with higher central densities, a significant lowering of stability thresholds requires stronger irradiation, as can be found closer to the Galactic centre or near stellar associations. Even when strong sources of ionizing radiation are absent or extincted, our study shows that interstellar irradiation can significantly influence the star formation process.
    • Transition from coherent cores to surrounding cloud in L1688

      Choudhury, S.; Pineda, J.E.; Caselli, P.; Offner, S.S.R.; Rosolowsky, E.; Friesen, R.K.; Redaelli, E.; Chacón-Tanarro, A.; Shirley, Y.; Punanova, A.; et al. (EDP Sciences, 2021)
      Context. Stars form in cold dense cores showing subsonic velocity dispersions. The parental molecular clouds display higher temperatures and supersonic velocity dispersions. The transition from core to cloud has been observed in velocity dispersion, but temperature and abundance variations are unknown. Aims. We aim to measure the temperature and velocity dispersion across cores and ambient cloud in a single tracer to study the transition between the two regions. Methods. We use NH3 (1,1) and (2,2) maps in L1688 from the Green Bank Ammonia Survey, smoothed to 1′, and determine the physical properties by fitting the spectra. We identify the coherent cores and study the changes in temperature and velocity dispersion from the cores to the surrounding cloud. Results. We obtain a kinetic temperature map extending beyond dense cores and tracing the cloud, improving from previous maps tracing mostly the cores. The cloud is 4-6 K warmer than the cores, and shows a larger velocity dispersion (Δσv = 0.15-0.25 km s-1). Comparing to Herschel-based dust temperatures, we find that cores show kinetic temperatures that are ≈1.8 K lower than the dust temperature, while the gas temperature is higher than the dust temperature in the cloud. We find an average p-NH3 fractional abundance (with respect to H2) of (4.2 ± 0.2) × 10-9 towards the coherent cores, and (1.4 ± 0.1) × 10-9 outside the core boundaries. Using stacked spectra, we detect two components, one narrow and one broad, towards cores and their neighbourhoods. We find the turbulence in the narrow component to be correlated with the size of the structure (Pearson-r = 0.54). With these unresolved regional measurements, we obtain a turbulence-size relation of σv,NT ∝ r0.5, which is similar to previous findings using multiple tracers. Conclusions. We discover that the subsonic component extends up to 0.15 pc beyond the typical coherent boundaries, unveiling larger extents of the coherent cores and showing gradual transition to coherence over ∼0.2 pc. © S. Choudhury et al. 2021.
    • The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Perseus Protostars. VI. Characterizing the Formation Mechanism for Close Multiple Systems

      Tobin, John; Looney, Leslie W.; Li, Zhi-Yun; Sadavoy, Sarah I.; Dunham, Michael M.; Segura-Cox, Dominique; Kratter, Kaitlin M.; Chandler, Claire J.; Melis, Carl; Harris, Robert J.; et al. (IOP PUBLISHING LTD, 2018-11-01)
      We present Atacama Large Millimeter/submillimeter Array observations of multiple protostar systems in the Perseus molecular cloud, previously detected by the Karl G. Jansky Very Large Array. We observe 17 close (<600 au separation) multiple systems at 1.3 mm in continuum and five molecular lines (i.e., (CO)-C-12, (CO)-O-18, (CO)-C-13, H2CO, SO) to characterize the circum-multiple environments in which these systems are forming. We detect at least one component in the continuum for the 17 multiple systems. In three systems one companion is not detected, and for two systems the companions are unresolved at our observed resolution. We also detect circum-multiple dust emission toward eight out of nine Class 0 multiples. Circum-multiple dust emission is not detected toward any of the eight Class I multiples. Twelve systems are detected in the dense gas tracers toward their disks/inner envelopes. For these 12 systems, we use the dense gas observations to characterize their formation mechanism. The velocity gradients in the circum-multiple gas are clearly orthogonal to the outflow directions in eight out of the 12 systems, consistent with disk fragmentation. Moreover, only two systems with separations <200 au are inconsistent with disk fragmentation, in addition to the two widest systems (>500 au). Our results suggest that disk fragmentation via gravitational instability is an important formation mechanism for close multiple systems, but further statistics are needed to better determine the relative fraction formed via this method.
    • 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.