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Kinematic distance determinations are complicated by a kinematic distance ambiguity (KDA) within the Solar orbit. For an axisymmetric Galactic rotation model, two distances, a "near" and "far" distance, have the same radial velocity. Formaldehyde (H2CO) absorption measurements have been used to resolve the KDA toward Galactic HII regions. This method relies on the detection of H2CO absorption against the broadband radio continuum emission from HII regions. H2CO absorption at velocities between the HII region velocity and the maximum velocity along the line of sight (the tangent point velocity) implies that the HII region lies at the far kinematic distance whereas a lack of absorption implies that it lies at the near kinematic distance. The reliability of KDA resolutions using H2CO is unclear, however, as disagreements between distances derived using H2CO absorption and those derived using other methods are common. Here we use new H2CO and radio recombination line data from the Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) to test whether H2CO absorption measurements can accurately resolve the KDA for 44 Galactic HII regions that have known distances from maser parallax measurements. For each of the 44 HII regions we determine whether the parallax distance is consistent with either the near or the far kinematic distance. We find that the Galactic distribution of H2CO is too sparse to reliably determine whether an HII region is at its near kinematic distance. The H2CO method also incorrectly resolves the KDA for 80% of HII regions that it places at the far kinematic distance; in such cases H2CO absorption may be caused by other sources of radio continuum emission (possibly the CMB, diffuse free-free, or synchrotron). Our results indicate that the H2CO method is unsuitable to resolve the KDA toward Galactic HII regions. 

Plasma of temperature 10,000K is created by OB stars and takes the form of discrete HII regions and diffuse gas. It is the key to determining the impact of massive stars on the interstellar medium (ISM) and the lifecycle of ISM gas. We review research this plasma, highlight outstanding questions, and provide recommendations for future facilities. 

ABSTRACT Citizen science has helped astronomers comb through large data sets to identify patterns and objects that are not easily found through automated processes. The Milky Way Project (MWP), a citizen science initiative on the Zooniverse platform, presents internet users with infrared (IR) images from Spitzer Space Telescope Galactic plane surveys. MWP volunteers make classification drawings on the images to identify targeted classes of astronomical objects. We present the MWP second data release (DR2) and an updated data reduction pipeline written in python. We aggregate ∼3 million classifications made by MWP volunteers during the years 2012–2017 to produce the DR2 catalogue, which contains 2600 IR bubbles and 599 candidate bow shock driving stars. The reliability of bubble identifications, as assessed by comparison to visual identifications by trained experts and scoring by a machine-learning algorithm, is found to be a significant improvement over DR1. We assess the reliability of IR bow shocks via comparison to expert identifications and the colours of candidate bow shock driving stars in the 2MASS point-source catalogue. We hence identify highly reliable subsets of 1394 DR2 bubbles and 453 bow shock driving stars. Uncertainties on object coordinates and bubble size/shape parameters are included in the DR2 catalogue. Compared with DR1, the DR2 bubbles catalogue provides more accurate shapes and sizes. The DR2 catalogue identifies 311 new bow shock driving star candidates, including three associated with the giant H ii regions NGC 3603 and RCW 49.