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We report on the first uniform and systematic study of dust and molecular gas in nearby molecular clouds. We use surveys of dust extinction and emission to determine the opacity and map the distribution of the dust within a dozen local clouds in order to derive a uniform set of basic cloud properties. We find (1) the average dust opacity 〈 Îş d,353〉 = 0.8 cm2 gâˆ'1 with variations of a factor of ∼2 between clouds, (2) cloud probability density functions are exquisitely described by steeply falling power laws with a narrow range of slope, and (3) a tight M GMC ∼ R GMC 2 scaling relation for the cloud sample, indicative of a cloud population with an exactingly constant average surface density above a common fixed boundary. We compare these results to uniformly analyzed CO surveys. We measure the CO mass conversion factors and assess the efficacy of CO for tracing the physical properties of molecular clouds. We find 〈 α CO〉 = 4.31 ± 0.67 M ⊙ (K km sâˆ'1 pc2)âˆ'1 (corresponding to X CO = 1.97 Ă—1020 cmâˆ'2(K km sâˆ'1)âˆ'1). We demonstrate that CO observations are a poor tracer of column density and structure on sub-cloud spatial scales. On cloud scales, CO observations can provide measurements consistent with those of the dust, provided data are analyzed in a similar, self-consistent fashion. Measurements of average giant molecular cloud surface density are sensitive to choice of cloud boundary. Care must be exercised to adopt common fixed boundaries when comparing surface densities for cloud populations within and between galaxies. [ABSTRACT FROM AUTHOR]

We compare observations of H i from the Very Large Array (VLA) and the Arecibo Observatory and observations of HCO+ from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the diffuse (AV ≲ 1) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multiphase ISM simulations. Using a coarse grid of PDR models, we estimate the density, FUV radiation field, and cosmic-ray ionization rate (CRIR) for each structure identified in HCO+ and H i absorption. These structures fall into two categories. Structures with Ts < 40 K, mostly with N(HCO+) ≲ 1012 cm−2, are consistent with modest density, FUV radiation field, and CRIR models, typical of the diffuse molecular ISM. Structures with spin temperature Ts > 40 K, mostly with N(HCO+) ≳ 1012 cm−2, are consistent with high density, FUV radiation field, and CRIR models, characteristic of environments close to massive star formation. The latter are also found in directions with a significant fraction of thermally unstable H i. In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., nonequilibrium processes like turbulent dissipation and/or shocks) are required to explain the observed HCO+ in this direction. Similarly, while our observations and simulations of the turbulent, multiphase ISM agree that HCO+ formation occurs along sight lines with N(H I) ≳ 1021 cm−2, the simulated data fail to explain HCO+ column densities ≳ few × 1012 cm−2. Because a majority of our sight lines with HCO+ had such high column densities, this likely indicates that nonequilibrium chemistry is important for these lines of sight. [ABSTRACT FROM AUTHOR]

We use two existing molecular cloud catalogs derived from the same CO survey and two catalogs derived from local dust extinction surveys to investigate the nature of the giant molecular cloud (GMC) mass–size relation in the Galaxy. We find that the four surveys are well described by MGMC ∼ R2, implying a constant mean surface density, , for the cataloged clouds. However, the scaling coefficients and scatter differ significantly between the CO- and extinction-derived relations. We find that the additional scatter seen in the CO relations is due to a systematic variation in with Galactic radius that is unobservable in the local extinction data. We decompose this radial variation of into two components, a linear negative gradient with Galactic radius and a broad peak coincident with the molecular ring and superposed on the linear gradient. We show that the former may be due to a radial dependence of XCO on metallicity, while the latter likely results from a combination of increased surface densities of individual GMCs and a systematic upward bias in the measurements of due to cloud blending in the molecular ring. We attribute the difference in scaling coefficients between the CO and extinction data to an underestimate of XCO. We recalibrate the CO observations of nearby GMCs using extinction measurements to find that locally XCO = 3.6 ± 0.3 × 1020 cm−2 (K km s−1)−1. We conclude that outside the molecular ring, the GMC population of the Galaxy can be described to relatively good precision by a constant of 35 pc−2. [ABSTRACT FROM AUTHOR]

We report trigonometric parallax measurements of masers in the massive star-forming complex W43 from Very Long Baseline Array observations as part of the Bar and Spiral Structure Legacy Survey. Based on measurements of three 12 GHz methanol maser sources (G029.86–00.04, G029.95–00.01, and G031.28+00.06) and one 22 GHz water maser source (G031.58+00.07) toward W43, we derived a distance of kpc to W43. By associating the masers with CO molecular clouds, and associating the clouds kinematically with CO longitude-velocity spiral features, we assign W43 to the Scutum spiral arm, close to the near end of the Galactic bar. The peculiar motion of W43 is about 20 km s–1 toward the Galactic Center and is very likely induced by the gravitational attraction of the bar. [ABSTRACT FROM AUTHOR]

We report four trigonometric parallaxes for high-mass star-forming regions within 4 kpc of the Galactic center. These measurements were made with the Very Long Baseline Array as part of the BeSSeL Survey. By associating these sources kinematically with large-scale features in CO and H I longitude-velocity diagrams, we begin to outline some major features of the inner Milky Way: the Connecting arm, the near and far 3 kpc arms, and the Norma arm. The Connecting arm in the first Galactic quadrant lies closer to the Galactic center than the far 3 kpc arm and is offset by the long-bar's major axis near its leading edge, supporting the presence of an inner Lindblad resonance. Assuming the 3 kpc arms are a continuous physical structure, the relative Galactocentric distance of its near and far sides suggests highly elliptical streamlines of gas around the bar(s) and a bar corotation radius, rCR ≳ 3.6 kpc. At a Galactic longitude near 10° and a heliocentric distance of about 5 kpc, the near 3 kpc arm and the Norma arm intersect on a face-on view of our Galaxy, while passing at different Galactic latitudes. We provide an accurate distance measurement to the W 31 star-forming complex of kpc from the Sun, which associates it with a bright CO feature belonging to the near 3 kpc arm. [ABSTRACT FROM AUTHOR]

We have made new CO observations of two molecular clouds, which we call “jet” and “arc” clouds, toward the stellar cluster Westerlund 2 and the TeV γ-ray source HESS J1023–575. The jet cloud shows a linear structure from the position of Westerlund 2 on the east. In addition, we have found a new counter jet cloud on the west. The arc cloud shows a crescent shape in the west of HESS J1023–575. A sign of star formation is found at the edge of the jet cloud and gives a constraint on the age of the jet cloud to be ∼Myr. An analysis with the multi CO transitions gives temperature as high as 20 K in a few places of the jet cloud, suggesting that some additional heating may be operating locally. The new TeV γ-ray images by H.E.S.S. correspond to the jet and arc clouds spatially better than the giant molecular clouds associated with Westerlund 2. We suggest that the jet and arc clouds are not physically linked with Westerlund 2 but are located at a greater distance around 7.5 kpc. A microquasar with long-term activity may be able to offer a possible engine to form the jet and arc clouds and to produce the TeV γ-rays, although none of the known microquasars have a Myr age or steady TeV γ-rays. Alternatively, an anisotropic supernova explosion which occurred ∼Myr ago may be able to form the jet and arc clouds, whereas the TeV γ-ray emission requires a microquasar formed after the explosion. [ABSTRACT FROM AUTHOR]

Trigonometric parallax measurements of nine water masers associated with the Local Arm of the Milky Way were carried out as part of the BeSSeL Survey using the Very Long Baseline Array. When combined with 21 other parallax measurements from the literature, the data allow us to study the distribution and three-dimensional motions of star forming regions in the spiral arm over the entire northern sky. Our results suggest that the Local Arm does not have the large pitch angle characteristic of a short spur. Instead its active star formation, overall length (>5 kpc), and shallow pitch angle (∼10◦) suggest that it is more like the adjacent Perseus and Sagittarius Arms; perhaps it is a branch of one of these arms. Contrary to previous results, we find the Local Arm to be closer to the Perseus than to the Sagittarius Arm, suggesting that a branching from the former may be more likely. An average peculiar motion of near zero toward both the Galactic center and north Galactic pole, and counter rotation of ∼5 km s−1 were observed, indicating that the Local Arm has similar kinematic properties as found for other major spiral arms. [ABSTRACT FROM AUTHOR]

We report a trigonometric parallax measurement with the Very Long Baseline Array for the water maser in the distant high-mass star-forming region G75.30+1.32. This source has a heliocentric distance of 9.25 Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed kpc, which places it in the Outer arm in the first Galactic quadrant. It lies 200 pc above the Galactic plane and is associated with a substantial H I enhancement at the border of a large molecular cloud. At a Galactocentric radius of 10.7 kpc, G75.30+1.32 is in a region of the Galaxy where the disk is significantly warped toward the North Galactic Pole. While the star-forming region has an instantaneous Galactic orbit that is nearly circular, it displays a significant motion of 18 km s-1 toward the Galactic plane. The present results, when combined with two previous maser studies in the Outer arm, yield a pitch angle of about 12° for a large section of the arm extending from the first quadrant to the third. [ABSTRACT FROM AUTHOR]

We report parallaxes and proper motions of three water maser sources in high-mass star-forming regions in the Outer Spiral Arm of the Milky Way. The observations were conducted with the Very Long Baseline Array as part of Bar and Spiral Structure Legacy Survey and double the number of such measurements in the literature. The Outer Arm has a pitch angle of 14.°9 ± 2.°7 and a Galactocentric distance of 14.1 ± 0.6 kpc toward the Galactic anticenter. The average motion of these sources toward the Galactic center is 10.7 ± 2.1 km s–1 and we see no sign of a significant fall in the rotation curve out to 15 kpc from the Galactic center. The three-dimensional locations of these star-forming regions are consistent with a Galactic warp of several hundred parsecs from the plane. [ABSTRACT FROM AUTHOR]

We report measurements of trigonometric parallaxes for six high-mass star-forming regions in the Scutum spiral arm of the Milky Way as part of the BeSSeL Survey. Combining our measurements with 10 previous measurements from the BeSSeL Survey yields a total sample of 16 sources in the Scutum arm with trigonometric parallaxes in the Galactic longitude range from 5° to 32°. Assuming a logarithmic spiral model, we estimate a pitch angle of 19.°8 ± 3.°1 for the Scutum arm, which is larger than pitch angles reported for other spiral arms. The high pitch angle of the arm may be due to the arm's proximity to the Galactic bar. The Scutum arm sources show an average peculiar motion of 4 km s–1 slower than the Galactic rotation and 8 km s–1 toward the Galactic center. While the direction of this non-circular motion has the same sign as determined for sources in other spiral arms, the motion toward the Galactic center is greater for the Scutum arm sources. [ABSTRACT FROM AUTHOR]

We report trigonometric parallaxes and proper motions of water masers for 12 massive star forming regions in the Perseus spiral arm of the Milky Way as part of the Bar and Spiral Structure Legacy (BeSSel) Survey. Combining our results with 14 parallax measurements in the literature, we estimate a pitch angle of 9.°9 ± 1.°5 for a section of the Perseus arm. The three-dimensional Galactic peculiar motions of these sources indicate that on average they are moving toward the Galactic center and slower than the Galactic rotation. [ABSTRACT FROM AUTHOR]

Over 100 trigonometric parallaxes and proper motions for masers associated with young, high-mass stars have been measured with the Bar and Spiral Structure Legacy Survey, a Very Long Baseline Array key science project, the European VLBI Network, and the Japanese VLBI Exploration of Radio Astrometry project. These measurements provide strong evidence for the existence of spiral arms in the Milky Way, accurately locating many arm segments and yielding spiral pitch angles ranging from about 7° to 20°. The widths of spiral arms increase with distance from the Galactic center. Fitting axially symmetric models of the Milky Way with the three-dimensional position and velocity information and conservative priors for the solar and average source peculiar motions, we estimate the distance to the Galactic center, R0, to be 8.34 ± 0.16 kpc, a circular rotation speed at the Sun, Θ0, to be 240 ± 8 km s–1, and a rotation curve that is nearly flat (i.e., a slope of –0.2 ± 0.4 km s–1 kpc–1) between Galactocentric radii of ≈5 and 16 kpc. Assuming a “universal” spiral galaxy form for the rotation curve, we estimate the thin disk scale length to be 2.44 ± 0.16 kpc. With this large data set, the parameters R0 and Θ0 are no longer highly correlated and are relatively insensitive to different forms of the rotation curve. If one adopts a theoretically motivated prior that high-mass star forming regions are in nearly circular Galactic orbits, we estimate a global solar motion component in the direction of Galactic rotation, V☼ = 14.6 ± 5.0 km s–1. While Θ0 and V☼ are significantly correlated, the sum of these parameters is well constrained, Θ0 + V☼ = 255.2 ± 5.1 km s–1, as is the angular speed of the Sun in its orbit about the Galactic center, (Θ0 + V☼)/R0 = 30.57 ± 0.43 km s–1 kpc–1. These parameters improve the accuracy of estimates of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their Galactic orbits, significantly reducing the uncertainty in tests of gravitational radiation predicted by general relativity. [ABSTRACT FROM AUTHOR]