Thor - The Hi, Oh, Recombination Line Survey Of The Milky Way - The Pilot Study: Hi Observations Of The Giant Molecular Cloud W43

To study the atomic, molecular and ionized emission of Giant Molecular Clouds (GMCs), we have initiated a Large Program with the VLA: \u27THOR - The HI, OH, Recombination Line survey of the Milky Way\u27. We map the 21cm HI line, 4 OH lines, 19 H_alpha recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way (l=15-67deg, |b|\u3c1deg) at ~20 resolution. In this paper, we focus on the HI emission from the W43 star-formation complex. Classically, the HI 21cm line is treated as optically thin with column densities calculated under this assumption. This might give reasonable results for regions of low-mass star-formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth, and thus correct the HI 21cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the HI mass of W43 more accurately and our analysis reveals a lower limit of M=6.6x10^6 M_sun, which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The HI column densities are as high as N(HI)~150 M_sun/pc^2 ~ 1.9x10^22 cm^-2, which is an order of magnitude higher than for low mass star formation regions. This result challenges theoretical models that predict a threshold for the HI column density of ~10 M_sun/pc^2, at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profiles. While at the cloud edge atomic and molecular hydrogen are well mixed, the center of the cloud is strongly dominated by molecular hydrogen. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results are an important characterization of the atomic to molecular hydrogen transition in an extreme environment and challenges current theoretical models

Hierarchical fragmentation and collapse signatures in a high-mass starless region

Aims: Understanding the fragmentation and collapse properties of the dense gas during the onset of high-mass star formation. Methods: We observed the massive (~800M_sun) starless gas clump IRDC18310-4 with the Plateau de Bure Interferometer (PdBI) at sub-arcsecond resolution in the 1.07mm continuum andN2H+(3-2) line emission. Results: Zooming from a single-dish low-resolution map to previous 3mm PdBI data, and now the new 1.07mm continuum observations, the sub-structures hierarchically fragment on the increasingly smaller spatial scales. While the fragment separations may still be roughly consistent with pure thermal Jeans fragmentation, the derived core masses are almost two orders of magnitude larger than the typical Jeans mass at the given densities and temperatures. However, the data can be reconciled with models using non-homogeneous initial density structures, turbulence and/or magnetic fields. While most sub-cores remain (far-)infrared dark even at 70mum, we identify weak 70mum emission toward one core with a comparably low luminosity of ~16L_sun, re-enforcing the general youth of the region. The spectral line data always exhibit multiple spectral components toward each core with comparably small line widths for the individual components (in the 0.3 to 1.0km/s regime). Based on single-dish C18O(2-1) data we estimate a low virial-to-gas-mass ratio <=0.25. We discuss that the likely origin of these spectral properties may be the global collapse of the original gas clump that results in multiple spectral components along each line of sight. Even within this dynamic picture the individual collapsing gas cores appear to have very low levels of internal turbulence.Comment: 8 pages, 4 figures, A&A in pres

Experimental mechanics study of a dam Tainter gate

Abstract only availableThe primary objective of this instrumentation project is to determine the performance of the Tainter gate's trunnion bearings at the Carlyle Lake Dam in Illinois. The Tainter gate is a type of radial arm floodgate used in dams and canal locks to control water flow. The trunnion bearings are the critical component of the structure. Friction in the bearing can cause excessive forces in the structural arms of the Tainter gate and can result in a structural failure of the gate Experimental determination of the stresses in the structural arms facilitates computation of friction encountered in the trunnion bearing when raising or lowering the gate. Specially designed transducers which could be readily attached to the structural arm were fabricated and installed to measure the strains present in the gate during routine operation. The transducer consisted of four strain gauges mounted on the flexural transducer and wired in a Wheatstone bridge configuration. The change in resistance of the Wheatstone bridge is linearly related to the strain imposed on the transducer. By measuring the voltage history of the transducer it is possible to determine strain in the structural arms during operations of the Tainter gate. The strains were recorded by using a LabView program custom written for this purpose. Simple mechanics based analysis of the resulting strains is expected to facilitate analysis of forces on the bearing. Preliminary results from the field investigation will be presented and discussed.US Army Corps of Engineers Engineer Research and Development Center, College of Engineering Undergraduate Research Optio

Kinematic and Thermal Structure at the onset of high-mass star formation

We want to understand the kinematic and thermal properties of young massive gas clumps prior to and at the earliest evolutionary stages of high-mass star formation. Do we find signatures of gravitational collapse? Do we find temperature gradients in the vicinity or absence of infrared emission sources? Do we find coherent velocity structures toward the center of the dense and cold gas clumps? To determine kinematics and gas temperatures, we used ammonia, because it is known to be a good tracer and thermometer of dense gas. We observed the NH$_3$(1,1) and (2,2) lines within seven very young high-mass star-forming regions with the VLA and the Effelsberg 100m telescope. This allows us to study velocity structures, linewidths, and gas temperatures at high spatial resolution of 3-5$"$, corresponding to $\sim$0.05 pc. We find on average cold gas clumps with temperatures in the range between 10 K and 30 K. The observations do not reveal a clear correlation between infrared emission peaks and ammonia temperature peaks. We report an upper limit for the linewidth of $\sim$1.3 km s$^{-1}$, at the spectral resolution limit of our VLA observation. This indicates a relatively low level of turbulence on the scale of the observations. Velocity gradients are present in almost all regions with typical velocity differences of 1 to 2 km s$^{-1}$ and gradients of 5 to 10 km s$^{-1}$ pc$^{-1}$. These velocity gradients are smooth in most cases, but there is one exceptional source (ISOSS23053), for which we find several velocity components with a steep velocity gradient toward the clump centers that is larger than 30 km s$^{-1}$ pc$^{-1}$. This steep velocity gradient is consistent with recent models of cloud collapse. Furthermore, we report a spatial correlation of ammonia and cold dust, but we also find decreasing ammonia emission close to infrared emission sources.Comment: 20 pages, 10 figure

Chemical evolution in the early phases of massive star formation. I

Understanding the chemical evolution of young (high-mass) star-forming regions is a central topic in star formation research. Chemistry is employed as a unique tool 1) to investigate the underlying physical processes and 2) to characterize the evolution of the chemical composition. We observed a sample of 59 high-mass star-forming regions at different evolutionary stages varying from the early starless phase of infrared dark clouds to high-mass protostellar objects to hot molecular cores and, finally, ultra-compact HII regions at 1mm and 3mm with the IRAM 30m telescope. We determined their large-scale chemical abundances and found that the chemical composition evolves along with the evolutionary stages. On average, the molecular abundances increase with time. We modeled the chemical evolution, using a 1D physical model where density and temperature vary from stage to stage coupled with an advanced gas-grain chemical model and derived the best-fit chi^2 values of all relevant parameters. A satisfying overall agreement between observed and modeled column densities for most of the molecules was obtained. With the best-fit model we also derived a chemical age for each stage, which gives the timescales for the transformation between two consecutive stages. The best-fit chemical ages are ~10,000 years for the IRDC stage, ~60,000 years for the HMPO stage, ~40,000 years for the HMC stage, and ~10,000 years for the UCHII stage. The total chemical timescale for the entire evolutionary sequence of the high-mass star formation process is on the order of 10^5 years, which is consistent with theoretical estimates. Furthermore, based on the approach of a multiple-line survey of unresolved data, we were able to constrain an intuitive and reasonable physical and chemical model. The results of this study can be used as chemical templates for the different evolutionary stages in high-mass star formation.Comment: 31 pages, 11 figures, 21 tables, accepted by A&A; typos adde

Galactic Supernova Remnant Candidates Discovered By Thor

There is a considerable deficiency in the number of known supernova remnants (SNRs) in the Galaxy compared to that expected. This deficiency is thought to be caused by a lack of sensitive radio continuum data. Searches for extended low-surface brightness radio sources may find new Galactic SNRs, but confusion with the much larger population of H II regions makes identifying such features challenging. SNRs can, however, be separated from H II regions using their significantly lower mid-infrared (MIR) to radio continuum intensity ratios. Aims: Our goal is to find missing SNR candidates in the Galactic disk by locating extended radio continuum sources that lack MIR counterparts. Methods: We use the combination of high-resolution 1-2 GHz continuum data from The HI, OH, Recombination line survey of the Milky Way (THOR) and lower-resolution VLA 1.4 GHz Galactic Plane Survey (VGPS) continuum data, together with MIR data from the Spitzer GLIMPSE, Spitzer MIPSGAL, and WISE surveys to identify SNR candidates. To ensure that the candidates are not being confused with H II regions, we exclude radio continuum sources from the WISE Catalog of Galactic H II Regions, which contains all known and candidate H II regions in the Galaxy. Results: We locate 76 new Galactic SNR candidates in the THOR and VGPS combined survey area of 67.4° \u3e ℓ \u3e 17.5°, | b | ≤ 1.25° and measure the radio flux density for 52 previously-known SNRs. The candidate SNRs have a similar spatial distribution to the known SNRs, although we note a large number of new candidates near ℓ ≃ 30°, the tangent point of the Scutum spiral arm. The candidates are on average smaller in angle compared to the known regions, 6.4\u27 ± 4.7\u27 versus 11.0\u27 ± 7.8\u27, and have lower integrated flux densities. Conclusions: The THOR survey shows that sensitive radio continuum data can discover a large number of SNR candidates, and that these candidates can be efficiently identified using the combination of radio and MIR data. If the 76 candidates are confirmed as true SNRs, for example using radio polarization measurements or by deriving radio spectral indices, this would more than double the number of known Galactic SNRs in the survey area. This large increase would still, however, leave a discrepancy between the known and expected SNR populations of about a factor of two

Coexistence of dilute and densely packed domains of ligand-receptor bonds in membrane adhesion

We analyze the stability of micro-domains of ligand-receptor bonds that mediate the adhesion of biological model membranes. After evaluating the effects of membrane fluctuations on the binding affinity of a single bond, we characterize the organization of bonds within the domains by theoretical means. In a large range of parameters, we find the commonly suggested dense packing to be separated by a free energy barrier from a regime in which bonds are sparsely distributed. If bonds are mobile, a coexistence of the two regimes should emerge, which agrees with recent experimental observations.Comment: 6 pages, 6 figures, accepted by EP

The Hi/Oh/Recombination Line Survey Of The Inner Milky Way (Thor) - Survey Overview And Data Release 1

The past decade has witnessed a large number of Galactic plane surveys at angular resolutions below 20 `\u27. However, no comparable high-resolution survey exists at long radio wavelengths around 21 cm in line and continuum emission. Aims. We remedy this situation by studying the northern Galactic plane at similar to 20 `\u27 resolution in emission of atomic, molecular, and ionized gas. Methods. Employing the Karl G. Jansky Very Large Array (VLA) in the C-array configuration and a large program, we observe the HI 21 cm line, four OH lines, nineteen Hn alpha radio recombination lines as well as the continuum emission from 1 to 2 GHz in full polarization over a large part of the first Galactic quadrant. Results. Covering Galactic longitudes from 14.5 to 67.4 deg and latitudes between +/- 1.25 deg, we image all of these lines and the continuum at similar to 20 `\u27 resolution. These data allow us to study the various components of the interstellar medium (ISM): from the atomic phase, traced by the HI line, to the molecular phase, observed by the OH transitions, to the ionized medium, revealed by the cm continuum and the Hn alpha radio recombination lines. Furthermore, the polarized continuum emission enables magnetic field studies. In this overview paper, we discuss the survey outline and present the first data release as well as early results from the different datasets. We now release the first half of the survey; the second half will follow later after the ongoing data processing has been completed. The data in fits format (continuum images and line data cubes) can be accessed through the project web-page. Conclusions. The HI/OH/Recombination line survey of the Milky Way (THOR) opens a new window to the different parts of the ISM. It enables detailed studies of molecular cloud formation, conversion of atomic to molecular gas, and feedback from HII regions as well as the magnetic field in the Milky Way. It is highly complementary to other surveys of our Galaxy, and comparing the different datasets will allow us to address many open questions

Fragmentation and dynamical collapse of the starless high-mass star-forming region IRDC18310-4

Aims: We study the fragmentation and dynamical properties of a massive starless gas clump at the onset of high-mass star formation. Methods: Based on Herschel continuum data we identify a massive gas clump that remains far-infrared dark up to 100mum wavelengths. The fragmentation and dynamical properties are investigated by means of Plateau de Bure Interferometer and Nobeyama 45m single-dish spectral line and continuum observations. Results: The massive gas reservoir fragments at spatial scales of ~18000AU in four cores. Comparing the spatial extent of this high-mass region with intermediate- to low-mass starless cores from the literature, we find that linear sizes do not vary significantly over the whole mass regime. However, the high-mass regions squeeze much more gas into these similar volumes and hence have orders of magnitude larger densities. The fragmentation properties of the presented low-to high-mass regions are consistent with gravitational instable Jeans fragmentation. Furthermore, we find multiple velocity components associated with the resolved cores. Recent radiative transfer hydrodynamic simulations of the dynamic collapse of massive gas clumps also result in multiple velocity components along the line of sight because of the clumpy structure of the regions. This result is supported by a ratio between viral and total gas mass for the whole region <1. Conclusions: This apparently still starless high-mass gas clump exhibits clear signatures of early fragmentation and dynamic collapse prior to the formation of an embedded heating source. A comparison with regions of lower mass reveals that the linear size of star-forming regions does not necessarily have to vary much for different masses, however, the mass reservoirs and gas densities are orders of magnitude enhanced for high-mass regions compared to their lower-mass siblings.Comment: 11 pages, 10 figures, accepted to Astronomy and Astrophysics, high-resolution version with all figures included can be found at http://www.mpia.de/homes/beuther/papers.htm

Continuum Sources From The Thor Survey Between 1 And 2 Ghz

We carried out a large program with the Karl G. Jansky Very Large Array (VLA): THOR: The H1, OH, Recombination line survey of the Milky Way . We observed a significant portion (~100 deg2) of the Galactic plane in the first quadrant of the Milky Way in the 21 cm H I line, 4 OH transitions, 19 radio recombination lines, and continuum from 1 to 2 GHz. In this paper we present a catalog of the continuum sources in the first half of the survey (l = 14.0-37.9° and l = 47.1-51.2°, | b | = 1.1°) at a spatial resolution of 10.25 , depending on the frequency and sky position with a spatially varying noise level of ~0.3.1 mJy beam-1. The catalog contains ~4400 sources. Around 1200 of these are spatially resolved, and ~1000 are possible artifacts, given their low signaltonoise ratios. Since the spatial distribution of the unresolved objects is evenly distributed and not confined to the Galactic plane, most of them are extragalactic. Thanks to the broad bandwidth of the observations from 1 to 2 GHz, we are able to determine a reliable spectral index for ~1800 sources. The spectral index distribution reveals a doublepeaked profile with maxima at spectral indices of a ˜-1 and a ˜ 0, corresponding to steep declining and flat spectra, respectively. This allows us to distinguish between thermal and nonthermal emission, which can be used to determine the nature of each source. We examine the spectral index of ~300 known HII regions, for which we find thermal emission with spectral indices around a˜ 0. In contrast, supernova remnants (SNR) show nonthermal emission with a ˜-0.5 and extragalactic objects generally have a steeper spectral index of a ˜-1. Using the spectral index information of the THOR survey, we investigate potential SNR candidates. We classify the radiation of four SNR candidates as nonthermal, and for the first time, we provide strong evidence for the SNR origin of these candidates