We present observations of ¹²CO J = 1-0, ¹³CO J = 1-0, H i, and all four ground-state transitions of the hydroxyl (OH) radical toward a sharp boundary region of the Taurus molecular cloud. Based on a photodissociation region (PDR) model that reproduces CO and [C i] emission from the same region, we modeled the three OH transitions, 1612, 1665, and 1667 MHz successfully through escape probability non-local thermal equilibrium radiative transfer model calculations. We could not reproduce the 1720 MHz observations, due to unmodeled pumping mechanisms, of which the most likely candidate is a C-shock. The abundance of OH and CO-dark molecular gas is well-constrained. The OH abundance [OH]/[H₂] decreases from to as A _v increases from 0.4 to 2.7 mag following an empirical law: which is higher than PDR model predictions for low-extinction regions by a factor of 80. The overabundance of OH at extinctions at or below 1 mag is likely the result of a C-shock. The dark gas fraction (DGF, defined as the fraction of molecular gas without detectable CO emission) decreases from 80% to 20% following a Gaussian profile: This trend of the DGF is consistent with our understanding that the DGF drops at low visual extinction due to photodissociation of H₂ and drops at high visual extinction due to CO formation. The DGF peaks in the extinction range where H₂ has already formed and achieved self-shielding but ¹²CO has not. Two narrow velocity components with a peak-to-peak spacing of ∼1 km s⁻¹ were clearly identified. Their relative intensity and variation in space and frequency suggest colliding streams or gas flows at the boundary region.

Hydroxyl (OH) is expected to be abundant in diffuse interstellar molecular gas because it forms along with H₂ under similar conditions and forms within a similar extinction range. We have analyzed absorption measurements of OH at 1665 MHz and 1667 MHz toward 44 extragalactic continuum sources, together with the J = 1–0 transitions of ¹²CO, ¹³CO, and C¹⁸O, and the J = 2–1 transition of ¹²CO. The excitation temperatures of OH were found to follow a modified lognormal distribution the peak of which is close to the temperature of the Galactic emission background (CMB+synchrotron). In fact, 90% of the OH has excitation temperatures within 2 K of the Galactic background at the same location, providing a plausible explanation for the apparent difficulty of mapping this abundant molecule in emission. The opacities of OH were found to be small and to peak around 0.01. For gas at intermediate extinctions (A _V ∼ 0.05–2 mag), the detection rate of OH with a detection limit N(OH) ≃ 10¹² cm⁻² is approximately independent of A _V . We conclude that OH is abundant in the diffuse molecular gas and OH absorption is a good tracer of “dark molecular gas (DMG).” The measured fraction of DMG depends on the assumed detection threshold of the CO data set. The next generation of highly sensitive low-frequency radio telescopes, such as FAST and SKA, will make feasible the systematic inventory of diffuse molecular gas through decomposing, in velocity, the molecular (e.g., OH and CH) absorption profiles toward background continuum sources with numbers exceeding what is currently available by orders of magnitude.

We have identified outflows and bubbles in the Taurus molecular cloud based on the ∼100 deg2 Five College Radio Astronomy Observatory 12CO(1-0) and 13CO(1-0) maps and the Spitzer young stellar object catalogs. In the main 44 deg2 area of Taurus, we found 55 outflows, of which 31 were previously unknown. We also found 37 bubbles in the entire 100 deg2 area of Taurus, none of which had been found previously. The total kinetic energy of the identified outflows is estimated to be erg, which is 1% of the cloud turbulent energy. The total kinetic energy of the detected bubbles is estimated to be erg, which is 29% of the turbulent energy of Taurus. The energy injection rate from the outflows is , which is 0.4–2 times the dissipation rate of the cloud turbulence. The energy injection rate from bubbles is erg s−1, which is 2–10 times the turbulent dissipation rate of the cloud. The gravitational binding energy of the cloud is erg, that is, 385 and 16 times the energy of outflows and bubbles, respectively. We conclude that neither outflows nor bubbles can provide sufficient energy to balance the overall gravitational binding energy and the turbulent energy of Taurus. However, in the current epoch, stellar feedback is sufficient to maintain the observed turbulence in Taurus.

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.