We have measured Faraday Rotation Measures (RM) at Arecibo Observatory for 36 pulsars, 17 of them new. We combine these and earlier measurements to study the galactic magnetic field and its possible temporal variations. Many RM values have changed significantly on several-year timescales, but these variations probably do not reflect interstellar magnetic field changes. By studying the distribution of pulsar RMs near the plane in conjunction with the new NE2001 electron density model, we note the following structures in the first galactic longitude quadrant: (1) The local field reversal can be traced as a null in RM in a 0.5-kpc wide strip interior to the Solar Circle, extending ∼ 7 kpc around the Galaxy.(2) Steadily increasing RMs in a 1-kpc wide strip interior to the local field reversal, and also in the wedge bounded by 42 < l < 52 • , indicate that the large-scale field is approximately steady from the local reversal in to the Sagittarius arm. (3) The RMs in the 1-kpc wide strip interior to the Sagittarius arm indicate another field reversal in this strip. (4) The RMs in a final 1-kpc wide interior strip, straddling the Scutum arm, also support a second field reversal interior to the Sun, between the Sagittarius and Scutum arms. (5) Exterior to the nearby reversal, RMs from 60 < l < 78 • show evidence for two reversals, on the near and far side of the Perseus arm. (6) In general, the maxima in the large-scale fields tend to lie along the spiral arms, while the field minima tend to be found between them.We have also determined polarized profiles of 48 pulsars at 430 MHz. We present morphological pulse profile classifications (Rankin 1983) of the pulsars, based on our new measurements and previously published data.

The luminous, massive star formation region RCW 49, located in the southern Galactic plane, was imaged with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope as part of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) program. The IRAC bands contain polycyclic aromatic hydrocarbon (PAH) features at 3.3, 6.2, 7.7, and 8.6 m, as well as the Br line. These features are the major contributors to the diffuse emission from RCW 49 in the IRAC bands. The Spitzer IRAC images show that the dust in RCW 49 is distributed in a network of fine filaments, pillars, knots, sharply defined boundaries, bubbles, and bow shocks. The regions immediately surrounding the ionizing star cluster and W-R stars are evacuated of dust by stellar winds and radiation. The IRAC images of RCW 49 suggest that the dust in RCW 49 has been sculpted by the winds and radiation from the embedded luminous stars in the inner 5 0 (inner $6 pc) of the nebula. At projected angular radii > 5 0 from the central ionizing cluster, the azimuthally averaged infrared intensity falls off as $ À3 . Both high-resolution radio and mid-IR images suggest that the nebula is density bounded along its western boundary. The filamentary structure of the dust in RCW 49 suggests that the nebula has a small dust filling factor and, as a consequence, the entire nebula may be slightly density bounded to H-ionizing photons. Subject headingg s: astrochemistry -dust, extinction -H ii regions -infrared: ISM -ISM: lines and bands

GLIMPSE imaging using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope indicates that star formation is ongoing in the RCW 9 giant H ii region. A photometric comparison of the sources in RCW 49 to a similar area to its north finds that at least 300 stars brighter than 13th magnitude in band [3.6] have infrared excesses inconsistent with reddening due to foreground extinction. These are likely young stellar objects (YSOs) more massive than 2.5 M , suggesting that thousands more low-mass stars are forming in this cloud. Some of the YSOs are massive (B stars) and therefore very young, suggesting that a new generation of star formation is occurring, possibly triggered by stellar winds and shocks generated by the older (2-3 Myr) central massive cluster. The Spitzer IRAC camera has proven to be ideally suited for distinguishing young stars from field stars, and the GLIMPSE survey of the Galactic plane will likely find thousands of new star formation regions.

Infrared Dark Clouds (IRDCs) harbor the earliest phases of massive star formation, and many of the compact cores in IRDCs, traced by millimeter continuum or by molecular emission in high critical density lines, host massive young stellar objects (YSOs). We used the Robert C. Byrd Green Bank Telescope (GBT) and the Karl G. Jansky Very Large Array (VLA) to map NH 3 and CCS in nine IRDCs to reveal the temperature, density, and velocity structures and explore chemical evolution in the dense (> 10 22 cm −2 ) gas. Ammonia is an excellent molecular tracer for these cold, dense environments. The internal structure -2and kinematics of the IRDCs include velocity gradients, filaments, and possibly colliding clumps that elucidate the formation process of these structures and their YSOs. We find a wide variety of substructure including filaments and globules at distinct velocities, sometimes overlapping at sites of ongoing star formation. It appears that these IRDCs are still being assembled from molecular gas clumps even as star formation has already begun, and at least three of them appear consistent with the morphology of "hub-filament structures" discussed in the literature. Furthermore, we find that these clumps are typically near equipartition between gravitational and kinetic energies, so these structures may survive for multiple free-fall times.University Galactic Ring Survey (BU-GRS) , and thus determined velocities, kinematic distances, and physical properties of the population of clouds. The darkest clouds have very high column densities, as high as approximately 10 24 -10 25 cm −2 .The terms "core" and "clump" are frequently used in the literature, but the meanings are not standardized. We use the term "core" to refer to an unresolved or marginally resolved overdense structure <0.1 pc across and tens of solar masses. We then use the term "clump" to refer to a resolved structure (e.g. projected area greater than three times the area of the beam) within an IRDC and is assigned by a clump deconvolution algorithm (see §4.1 for description of algorithms used). "Average" results of spectral line fitting presented in this paper are averaged over clumps. We also occasionally refer to the velocity components within IRDCs when the IRDC's emission primarily occurs at two or more distinct velocities with little emission at the intermediate velocities; a velocity component may have one or more clump associated with it.

We have searched for OH absorption against seven pulsars using the Arecibo telescope. In both OH mainlines (at 1665 and 1667 MHz), deep and narrow absorption features were detected toward PSR B1849+00. In addition, we have detected several absorption and emission features against B33.6+0.1, a nearby supernova remnant (SNR). The most interesting result of this study is that a pencil-sharp absorption sample against the PSR differs greatly from the large-angle absorption sample observed against the SNR. If both the PSR and the SNR probe the same molecular cloud then this finding has important implications for absorption studies of the molecular medium, as it shows that the statistics of absorbing OH depends on the size of the background source. We also show that the OH absorption against the PSR most likely originates from a small (< 30 arcsec) and dense (> 10 5 cm −3 ) molecular clump.