Each session will begin with a round of introductions, then one or two 15-20min talks. After the talks we’ll open the floor for questions and discussion of the session topic. The topics listed below are only suggestions, feel free to discuss anything! If more time is needed to plan future projects, two time-slots (but unlimited breakout rooms!) are available on Friday for you to use as you wish.
All pre-recorded sessions (6am and 8pm time-slots) will only include the talk portion of the previous session, so you can have your own discussions. Each session will also have it’s own Slack space, so you can join the conversation there.
After the sessions the recordings will be uploaded to YouTube and linked below in the session titles, and the slides will be linked in the talk titles.

Sessions in detail:

S1: Observing diffuse OH and how to get time on telescopes
Allison Smith, University of Puerto Rico Mayagüez – A search for OH 18cm emission from intermediate-velocity gas at high galactic latitudes.
Michael Busch, University of California San Diego – Observing thermal OH in M31 and the Outer Galaxy with the Green Bank Telescope.
Discussion/collaboration topics: How to apply for time on the Green Bank Telescope (with Michael Busch), Murriyang and ATCA (with Anita Hafner).

S2: RFI challenges and solutions
Nobuyuki Sakai, NARIT and NAOJ – The Thai National Radio Telescope: OH 18cm cometary database project, and the challenges of developing an automatic RFI mitigation script.
Tommy Marshman, Macquarie University – RFI mitigation: methods you may be interested in.
Discussion/collaboration topics: How do you deal with RFI?

S3: HI/H₂ transition and OH/H₂ abundances
Sergei Balashev, Ioffe Institute St Petersburg – Modelling OH/H₂ abundances in the diffuse ISM.
Recent molecular hydrogen (H2) absorption line surveys start to provide statistical studies on the cold diffuse interstellar medium. The abundance of OH molecules in the diffuse ISM is tightly linked with H2 abundance. I will present the results of the semianalytical chemical model that calculates OH/H2 abundances in the diffuse ISM as a function of the physical parameters such as number density, metallicity, UV flux and cosmic ray ionization rate. This is especially interesting to interpret future blind OH absorption surveys such as MALS and compare its results with statistics from H2 absorption line surveys.
Discussion/collaboration topics: The HI to H₂ transition (with Van Hiep Nguyen)

S4: Pre-recorded session: S1: Observing diffuse OH and how to get time on telescopes

S5: Pre-recorded session: S3: HI/H₂ transition and OH/H₂ abundances

S6: Data Analysis: Bayesian spectral line models
Trey Wenger, University of Wisconsin Madison – Bayesian spectral line models: applications to OH.
Discussion/collaboration topics: simultaneous Gaussian decomposition of OH 18cm transitions (with Anita Hafner)

S7: OH absorption surveys in the Milky Way and beyond
Michael Rugel, NRAO – Faint absorption of the ground state hyperfine-splitting transitions of hydroxyl at 18 cm in the Galactic Disk.
The interstellar hydride hydroxyl (OH) is a potential tracer of CO-dark molecular gas. We present new high-sensitivity absorption line observations of the four ground state hyperfine-splitting transitions of OH at 18-cm wavelength towards four Galactic and extra-galactic continuum sources as follow-up to the THOR survey. We compare these to deep observations of the [CII] line at 1.9 THz obtained with the upGREAT instrument on SOFIA, observations of the neutral atomic hydrogen (H I) 21 cm line with the VLA, and CO (𝐽 = 2–1) lines obtained with the APEX PI230 receiver at the APEX 12 m sub-mm telescope. Our results confirm that OH absorption traces molecular gas across diffuse and dense environments of the interstellar medium. At the sensitivity limits of the present observations our detection of only one CO-dark gas feature appears in agreement with previous studies. We conclude that if OH absorption was to be used as a CO-dark gas tracer, deeper observations or stronger background targets are necessary to unveil its full potential as a CO-dark gas tracer, and yet it will never be an exclusive tracer of CO-dark gas.
Neeraj Gupta, IUCAA – The MeerKAT Absorption Line Survey (MALS).
The MeerKAT Absorption Line Survey (MALS) view of cold gas in and around galaxies The MeerKAT Absorption Line Survey (MALS; https://mals.iucaa.in/) is a large project (1655 hrs) to blindly search for HI and OH absorption lines with the primary goal to better characterize the cold atomic and molecular gas in and around normal and active galaxies at 0≤z≤1.8. In this talk, I will present an update on HI and OH absorption line search using MALS, with focus on Galactic ISM and z<0.1.
Discussion/collaboration topics: What can we do with the data from these (and other) large surveys?

S8: Evolution from diffuse to dense OH, clumping and HI/OH mixing
Ningyu Tang, Anhui Normal University – OH evolution in diffuse and dense clouds.
Discussion/collaboration topics: OH abundance over time, spatial distribution of OH, OH-HI coexistence hypothesis.

S9: Pre-recorded session: S6: Data Analysis: Bayesian spectral line models

S10: Pre-recorded session: S7: OH absorption surveys in the Milky Way and beyond

S11: Molecular excitation modelling
Michael Busch, University of California San Diego – Using MOLPOP.
Malcolm Gray, NARIT – 3D polarization code.
I introduce the pol3D code, originally written to study the effects of source geometry on maser polarization in the narrow-splittiing case. I briefly present results for narrow tube domains that are close analogues of 1D models, noting the important 3D effects. I demonstrate flexibility in the Zeeman splitting, where the model Lande factor can be arbitrarily varied from the narrow to the wide (e.g. OH) splitting regime. I continue with a preliminary study of the effects of source shape on synthetic spectra and images, and what these suggest for the shape of actual clouds, and the orientation of the magnetic field within them.
Discussion/collaboration topics: How can we make better use of existing codes?

S12: Physical environments as traced by OH excitation
Esteban Araya and Wei Siang Tan, Western Illinois University – Physical Conditions of a Molecular Jet Traced by Excited OH Emission and Absorption in a High-Mass Star Forming Region.
We present preliminary results of VLA (A configuration) observations of multiple OH transitions toward the cometary HII region in G34.26+0.15. Previous high sensitivity but low angular resolution observations by our group resulted in the detection of broad OH absorption from 6 GHz transitions and emission from 4 GHz transitions. The new high angular resolution observations reveal that the OH is tracing a collimated molecular jet, which seems to originate from a hot molecular core at the vertex of a cometary HII region. A grid of models were generated to investigate the physical conditions traced by OH; our observations are consistent with models characterized by OH column densities of the order of 10^15 cm^-2 and molecular densities of ~10^7 cm^-3.
Anita Hafner, University of Sydney – The OH satellite-line ‘flip’ as a tracer of expanding HII regions.
The OH satellite-line ‘flip’ is a unique profile pattern in which the satellite lines at 1612 and 1720 MHz flip – one from emission to absorption and the other the reverse – across a closely blended double feature. From a study of flips in the literature we found that the vast majority have the same velocity orientation and have close association (in position and velocity) with HII regions. We propose that the flip traces pre- and post-shock gas on the near side of an expanding HII region. We justify this through reference to HII region modelling and our own OH molecular excitation modelling.
Beth Cappellazzo, Macquarie University – Tracing Interactions Between HII Regions and Their Parent Molecular Clouds.
Recent work by Hafner, Dawson and Wardle (2020) proposes a hypothesis that hydroxyl (OH) may trace HII region-molecular cloud interactions and identify gas pre- and post-feedback through the OH satellite-line flip spectral feature. The OH flip may trace a shock front driven by HII region expansion moving through the surrounding molecular cloud. We assess this hypothesis using dedicated VLA observations of three Galactic HII regions including hydrogen radio recombination lines and the four 18-cm OH lines. A Gaussian decomposition of the molecular gas data reveals complex OH emission and absorption across our targets. The OH flip was identified towards two of our sources, G049.205-0.343 and G034.256+0.145. We assess predictions of the OH flip hypothesis against the observed multi-wavelength spectra, kinematics and morphology of the sources, finding an agreement between the sources with the OH flip and the hypothesis. We find that the hypothesis does not hold for G024.471+0.492 (without an OH flip), although we do see evidence of interacting molecular gas traced by OH. These results show support for the ability of OH to trace HII region-molecular cloud interactions. Future investigations with excitation modelling and a wider range of sources will provide more definitive evidence for or against the OH flip hypothesis.
Discussion/collaboration topics: Available molecular excitation modelling codes and how we can use them.

S13: OH masers in star-forming regions
Dmitry Ladeyschikov, Ural Federal University – MaserDB.net update: OH maser database in star forming regions and its applications.
We present a new online database compiling multi-transition hydroxyl (OH) maser observations in Galactic star-forming regions, creating a unified catalog covering > 90% of all published detections. This includes the main ground-state transitions at 18 cm and key excited-state lines like those at 4.7 and 6.035 GHz. Each maser is linked to its host dust clump from ATLASGAL and Hi-GAL surveys, enabling analysis with physical properties like luminosity, mass, and dust temperature. Using this cross-matched catalog, we perform statistical analyses: we develop predictive models for OH maser occurrence using generalized linear models and a neural network; we analyze angular offsets between precise maser positions and clump centers; we quantify maser variability and its correlations; and we compare evolutionary trends with water and methanol masers. Our models reliably predict OH maser presence with > 70% accuracy, suggesting dozens of undetected masers in known clumps. We find OH masers are tightly linked to the most evolved massive clumps, exhibiting high luminosity-to-mass ratios and frequent association with radio continuum emission, indicating a later stage in high-mass star formation. Their positions are typically very close to clump peaks, similar to water and class II methanol masers. While some OH masers vary significantly, overall variability is lower and less tied to clump properties than for water masers. Crucially, clumps hosting OH masers show the highest luminosity-to-mass ratios compared to those with water or methanol masers, reinforcing OH masers as tracers of a later evolutionary phase. This database serves as a community resource for maser studies in star formation and is accessible online via the MaserDB.net platform. This research was supported by the RSF grant 23-12-00258.
Jayender Kumar, CSIRO – VLBI astrometry with OH maser in the Milky Way
Most of the spiral structure in the third and fourth quadrants of the Galaxy remains poorly defined in current Milky Way models, largely due to the lack of distance measurements to prominent objects such as high-mass star-forming regions (HMSFRs) in these regions. Increasing the number of parallax measurements here will significantly improve the accuracy of existing Galactic models, especially in the outer Galaxy. The lower metallicity of the outer Galaxy leads to a reduced abundance of complex molecules like methanol and water. However, OH masers remain relatively abundant, making them excellent targets for VLBI astrometry. Precise VLBI astrometry with OH masers have been more challenging due to strong ionospheric effects at low frequencies, but the multiview VLBI phase-referencing technique can overcome these limitations. Multiview is key to achieving the highest levels of astrometric precision. I plan to use the Southern Hemisphere VLBI network to conduct parallax measurements of hydroxyl, methanol, and water masers associated with HMSFRs in the third and fourth quadrants of the Milky Way. This effort will be highly complementary to the BeSSeL and VERA surveys, which focus primarily on the inner and mid-Galactic regions from the Northern Hemisphere at higher frequencies. By improving constraints on the Milky Way’s structure from the Southern Hemisphere and determining how far spiral arms extend from the Galactic center, this work will provide critical insights into the global morphology of our Galaxy.
Discussion/collaboration topics: Correlations between OH and other maser species

S14: Pre-recorded session: S11: Molecular excitation modelling

S15: Pre-recorded session: S13: OH masers in star-forming regions

S16: OH masers and magnetic fields 1
Gordon MacLeod, XAO and HartRAO – Long-term OH monitoring and magnetic fields: possible magnetic field variations.
Hydroxyl (OH) is a paramagnetic molecule. The implication of this is that, under the influence of a magnetic field, OH transitions experience hyper-fine splitting or Zeeman splitting. The preferred method of identifying Zeeman (right- and left-circularly-polarised – R and LCP) pairs is via proximity in interferometric maser spot maps. However, possible line-of-sight coincidences leave uncertainty in pair identification in these spot maps. Single-dish monitoring to the rescue! Zeeman pairs reside in the same maser cloudlet. Our premise is simple, if this cloudlet is moving, then velocity drifts for both the R and LCP components must be similar. We present long-term monitoring of OH masers associated with NGC6334F using the 26-m Hartebeesthoek Radio Astronomy Observatory telescope (HartRAO). We confirm Zeeman pairs by measuring the velocity drifts of the R and LCP velocity features. Finally, variations in the magnetic field are estimated through the slight differences in these velocity drifts. The more we look, the more we find!
Anita Hafner, University of Sydney – Maser monitoring Parkes project data description and preliminary results.
The Maser Monitoring Parkes Project (M2P2) is an ongoing project in its 5th year to regularly (~1-2 week cadence) monitor OH masers in high-mass star forming regions with the UWL receiver on the 64m Murriyang telescope in Parkes. Our observations include full polarisation products of all 4 18cm OH lines along with 7 CH lines. This is a rich data set in need of a talented PhD student!
Discussion/collaboration topics: Techniques needed to extract insights from maser monitoring data.

S17: OH masers and magnetic fields 2
Lucero Uscanga, University of Guanajuato – OH maser polarimetric observations of planetary nebulae.
I would like to present full-polarization observations of the four ground-state transitions of OH, taken with the Australia Telescope Compact Array towards two very young planetary nebulae (PNe). We detect Zeeman splitting in the 1720 MHz transition in IRAS 16333-4807, with magnetic field strengths of ~2 to 11 mG. In IRAS 17393-2727 we derive a magnetic field magnitude of ~3 to 20 mG at 1612 MHz. These values are consistent with those estimated previously, but it seems that there are changes in the configuration of the magnetic field in both PNe.
Discussion/collaboration topics: OH Zeeman doublets and triplets (with Malcolm Gray and Busaba Kramer)

S18: Observing masers with various telescopes
Nobuyuki Sakai, NARIT and NAOJ – First detection of OH maser emission in the 71-year periodic comet 12P/Pons–Brooks using the 40-m
Thai National Radio Telescope (TNRT).
I will introduce the scientific results on OH masers obtained with the 40m TNRT. In collaboration with the Max Planck Institute for Radio Astronomy (MPIfR), we have installed an L-band (1 – 1.8 GHz) receiver at the 40 m TNRT for H I, OH maser, and continuum observations. I hope to extend the TNRT scientific case for the OH molecule through discussion with the workshop audience.
Max Voronkov, CSIRO – Observing with the ATCA with the new BIGCAT.
The Australia Telescope Compact Array (ATCA) is a workhorse of Australian spectral line astronomy. It is undergoing a major upgrade of its backend system at the moment. The new backend called BIGCAT (the Broadband Integrated GPU Correlator for ATCA) is replacing the outdated CABB (Compact Array Broadband Backend) system. The new backend is in general much more flexible in its operating modes and in the near future will be extended to double the instantaneous bandwidth (from 4 to 8 GHz). The upgrade involves a notable change in software including the parts the telescope users normally interact with. I will review the changes brought by the BIGCAT from the point of view of a spectral line observer.
Discussion/collaboration topics: How to get time on these telescopes, ideas for future projects

S19: Pre-recorded session: S16: OH masers and magnetic fields 1

S20: Pre-recorded session: S17: OH masers and magnetic fields 2

S21: Existing, ongoing and future surveys and projects
Jo Dawson, Macquarie University – SPLASH, GASKAP-OH
Discussion/collaboration topics: Ideas for projects based on existing and ongoing datasets, ideas for future surveys and projects.

S22: New Projects with the Thai National Radio Telescope
TNRT team, NARIT – New projects with the 40m Thai National Radio Telescope.
Discussion/collaboration topics: Ideas for projects with the TNRT

S23: How to make molecular excitation codes more user-friendly
Topics will be announced here.