We have compiled a significantly updated and comprehensive census of massive stars in the nearby Cygnus OB2 association by gathering and homogenising data from across the literature. The census contains 169 primary OB stars, including 52 O-type stars and 3 Wolf-Rayet stars. Spectral types and photometry are used to place the stars in a Hertzprung-Russell diagram, which is compared to both non-rotating and rotating stellar evolution models, from which stellar masses and ages are calculated. The star formation history and mass function of the association are assessed, and both are found to be heavily influenced by the evolution of the most massive stars to their end states. We find that the mass function of the most massive stars is consistent with a 'universal' power-law slope of Γ = 1.3. The age distribution inferred from stellar evolutionary models with rotation and the mass function suggest the majority of star formation occurred more or less continuously between 1 and 7 Myr ago, in agreement with studies of low-and intermediate mass stars in the association. We identify a nearby young pulsar and runaway O-type star that may have originated in Cyg OB2 and suggest that the association has already seen its first supernova. Finally we use the census and mass function to calculate the total mass of the association of 16500 +3800 −2800 M ⊙ , at the low end, but consistent with, previous estimates of the total mass of Cyg OB2. Despite this Cyg OB2 is still one of the most massive groups of young stars known in our Galaxy making it a prime target for studies of star formation on the largest scales.

The VST Photometric Hα Survey of the Southern Galactic Plane and Bulge (VPHAS+) is surveying the southern Milky Way in u, g, r, i and Hα at ∼1 arcsec angular resolution. Its footprint spans the Galactic latitude range −5 o < b < +5 o at all longitudes south of the celestial equator. Extensions around the Galactic Centre to Galactic latitudes ±10 • bring in much of the Galactic Bulge. This ESO public survey, begun on 28th December 2011, reaches down to ∼20th magnitude (10σ) and will provide single-epoch digital optical photometry for ∼300 million stars. The observing strategy and data pipelining is described, and an appraisal of the segmented narrowband Hα filter in use is presented. Using model atmospheres and library spectra, we compute main-sequence (u − g), (g − r), (r − i) and (r − Hα) stellar colours in the Vega system. We report on a preliminary validation of the photometry using test data obtained from two pointings overlapping the Sloan Digital Sky Survey. An example of the (u − g, g − r) and (r − Hα, r − i) diagrams for a full VPHAS+ survey field is given. Attention is drawn to the opportunities for studies of compact nebulae and nebular morphologies that arise from the image quality being achieved. The value of the u band as the means to identify planetary-nebula central stars is demonstrated by the discovery of the central star of NGC 2899 in survey data. Thanks to its excellent imaging performance, the VST/OmegaCam combination used by this survey is a perfect vehicle for automated searches for reddened early-type stars, and will allow the discovery and analysis of compact binaries, white dwarfs and transient sources.

The INT/WFC Photometric Hα Survey of the Northern Galactic Plane (IPHAS) is a 1800 deg 2 imaging survey covering Galactic latitudes |b| < 5 • and longitudes = 30 • to 215 • in the r, i and Hα filters using the Wide Field Camera (WFC) on the 2.5-metre Isaac Newton Telescope (INT) in La Palma. We present the first quality-controlled and globally-calibrated source catalogue derived from the survey, providing single-epoch photometry for 219 million unique sources across 92 per cent of the footprint. The observations were carried out between 2003 and 2012 at a median seeing of 1.1 arcsec (sampled at 0.33 arcsec/pixel) and to a mean 5σ-depth of 21.2 (r), 20.0 (i) and 20.3 (Hα) in the Vega magnitude system. We explain the data reduction and quality control procedures, describe and test the global re-calibration, and detail the construction of the new catalogue. We show that the new calibration is accurate to 0.03 mag (rms) and recommend a series of quality criteria to select accurate data from the catalogue. Finally, we demonstrate the ability of the catalogue's unique (r − Hα, r − i) diagram to (i) characterise stellar populations and extinction regimes towards different Galactic sightlines and (ii) select and quantify Hα emission-line objects. IPHAS is the first survey to offer comprehensive CCD photometry of point sources across the Galactic Plane at visible wavelengths, providing the much-needed counterpart to recent infrared surveys.

An unsettled question concerning the formation and distribution of massive stars is whether they must be born in massive clusters and, if found in less dense environments, whether they must have migrated there. With the advent of wide-area digital photometric surveys, it is now possible to identify massive stars away from prominent Galactic clusters without bias. In this study we consider 40 candidate OB stars found in the field around the young massive cluster, Westerlund 2, by Mohr-Smith et al (2017): these are located inside a box of 1.5×1.5 square degrees and are selected on the basis of their extinctions and K magnitudes. We present VLT/X-shooter spectra of two of the hottest O stars, respectively 11 and 22 arcmin from the centre of Westerlund 2. They are confirmed as O4V stars, with stellar masses likely to be in excess of 40 M . Their radial velocities relative to the non-binary reference object, MSP 182, in Westerlund 2 are −29.4±1.7 and −14.4±2.2 km s −1 , respectively. Using Gaia DR2 proper motions we find that between 8 and 11 early O/WR stars in the studied region (including the two VLT targets, plus WR 20c and WR 20aa) could have been ejected from Westerlund 2 in the last one million years. This represents an efficiency of massive-star ejection of up to ∼ 25%. On sky, the positions of these stars and their proper motions show a near N-S alignment. We discuss the possibility that these results are a consequence of prior sub-cluster merging combining with dynamical ejection.

Massive OB stars are critical to the ecology of galaxies, and yet our knowledge of OB stars in the Milky Way, fainter than V ∼ 12, remains patchy. Data from the VST Photometric Hα Survey (VPHAS+) permit the construction of the first deep catalogues of blue excess-selected OB stars, without neglecting the stellar field. A total of 14900 candidates with 2MASS cross-matches are blue-selected from a 42 squaredegree region in the Galactic Plane, capturing the Carina Arm over the Galactic longitude range 282 • 293 • . Spectral energy distribution fitting is performed on these candidates' combined VPHAS+ u, g, r, i and 2MASS J, H, K magnitudes. This delivers: effective temperature constraints, statistically separating O from early-B stars; high-quality extinction parameters, A 0 and R V (random errors typically < 0.1). The high-confidence O-B2 candidates number 5915 and a further 5170 fit to later B spectral type. Spectroscopy of 276 of the former confirms 97% of them. The fraction of emission line stars among all candidate B stars is 7-8% . Greyer (R V > 3.5) extinction laws are ubiquitous in the region, over the distance range 2.5-3 kpc to ∼10 kpc. Near prominent massive clusters, R V tends to rise, with particularly large and chaotic excursions to R V ∼ 5 seen in the Carina Nebula. The data reveal a hitherto unnoticed association of 108 O-B2 stars around the O5If+ star LSS 2063 ( = 289.77 • , b = −1.22 • ). Treating the OB star scale-height as a constant within the thin disk, we find an orderly mean relation between extinction (A 0 ) and distance in the Galactic longitude range, 287.6 • < < 293.5 • , and infer the subtle onset of thin-disk warping. A halo around NGC 3603, roughly a degree in diameter, of ∼500 O-B2 stars with 4 < A 0 (mag) < 7 is noted.