T. Naylor

A γ-ray burst at a redshift of z ≈ 8.2

Long-duration gamma-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z > 20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-alpha emitting galaxy. Here we report that GRB 090423 lies at a redshift of z approximately 8.2, implying that massive stars were being produced and dying as GRBs approximately 630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.It is thought that the first generations of massive stars in the Universe were an important, and quite possibly dominant, source of the ultra-violet radiation that reionized the hydrogen gas in the intergalactic medium (IGM); a state in which it has remained to the present day. Measurements of cosmic microwave background anisotropies suggest that this phase-change largely took place in the redshift range z=10.8 +/- 1.4, while observations of quasars and Lyman-alpha galaxies have shown that the process was essentially completed by z=6. However, the detailed history of reionization, and characteristics of the stars and proto-galaxies that drove it, remain unknown. Further progress in understanding requires direct observations of the sources of ultra-violet radiation in the era of reionization, and mapping the evolution of the neutral hydrogen fraction through time. The detection of galaxies at such redshifts is highly challenging, due to their intrinsic faintness and high luminosity distance, whilst bright quasars appear to be rare beyond z~7. Here we report the discovery of a gamma-ray burst, GRB 090423, at redshift z=8.26 -0.08 +0.07. This is well beyond the redshift of the most distant spectroscopically confirmed galaxy (z=6.96) and quasar (z=6.43). It establishes that massive stars were being produced, and dying as GRBs, ~625 million years after the Big Bang. In addition, the accurate position of the burst pinpoints the location of the most distant galaxy known to date. Larger samples of GRBs beyond z~7 will constrain the evolving rate of star formation in the early universe, while rapid spectroscopy of their afterglows will allow direct exploration of the progress of reionization with cosmic time.Long-duration γ-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z > 20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-α emitting galaxy. Here we report that GRB 090423 lies at a redshift of z ≈ 8.2, implying that massive stars were being produced and dying as GRBs ∼630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.

Pre-main-sequence isochrones – II. Revising star and planet formation time-scales

CPMB is funded by a UK Science and Technology Facilities Council (STFC) studentship. SPL is supported by an RCUK fellowship. The authors would like to thank Charles D. H. Williams for maintaining the Xgrid facilities at the University of Exeter which were used to reduce the photometric data presented in this study. The authors thank Amelia Bayo for bringing to our attention the important work on the λ Ori region published in Bayo et al. (2011) and Bayo et al. (2012) which we overlooked in our original submission. The inclusion of these works does not change the results or conclusions of the paper. The authors also thank the referee for useful comments and constructive suggestions that have greatly improved this work. This research has made use of data obtained at the Isaac Newton Telescope which is operated on the island of La Palma by the Isaac Newton Group (ING) in the Spanish Observatorio del Roque de los Muchachos of the Institutio de Astrofisica de Canarias. This research has also made use of archival data products from the Two-Micron All-Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration (NASA) and the National Science Foundation.

A self-consistent, absolute isochronal age scale for young moving groups in the solar neighbourhood

We present a self-consistent, absolute isochronal age scale for young (< 200 Myr), nearby (< 100 pc) moving groups in the solar neighbourhood based on homogeneous fitting of semi-empirical pre-main-sequence model isochrones using the tau^2 maximum-likelihood fitting statistic of Naylor & Jeffries in the M_V, V-J colour-magnitude diagram. The final adopted ages for the groups are: 149+51-19 Myr for the AB Dor moving group, 24+/-3 Myr for the {\beta} Pic moving group (BPMG), 45+11-7 Myr for the Carina association, 42+6-4 Myr for the Columba association, 11+/-3 Myr for the {\eta} Cha cluster, 45+/-4 Myr for the Tucana-Horologium moving group (Tuc-Hor), 10+/-3 Myr for the TW Hya association, and 22+4-3 Myr for the 32 Ori group. At this stage we are uncomfortable assigning a final, unambiguous age to the Argus association as our membership list for the association appears to suffer from a high level of contamination, and therefore it remains unclear whether these stars represent a single population of coeval stars. Our isochronal ages for both the BPMG and Tuc-Hor are consistent with recent lithium depletion boundary (LDB) ages, which unlike isochronal ages, are relatively insensitive to the choice of low-mass evolutionary models. This consistency between the isochronal and LDB ages instills confidence that our self-consistent, absolute age scale for young, nearby moving groups is robust, and hence we suggest that these ages be adopted for future studies of these groups. Software implementing the methods described in this study is available from http: //www.astro.ex.ac.uk/people/timn/tau-squared/.

Fitting the young main-sequence: distances, ages and age spreads

We use several main-sequence models to derive distances (and extinctions), with statistically meaningful uncertainties for 11 star-forming regions and young clusters. The model dependency is shown to be small, allowing us to adopt the distances derived using one model. Using these distances, we have revised the age order for some of the clusters of Mayne et al. The new nominal ages are: ≈2 Myr for NGC 6530 and the ONC, ≈3 Myr for λ Orionis, NGC 2264 and σ Orionis, ≈4‐5 Myr for NGC 2362, ≈13 Myr for h and χ Per, ≈20 Myr for NGC 1960 and ≈40 Myr for NGC 2547. In cases of significantly variable extinction, we have derived individual extinctions using a revised Q-method. These new data show that the largest remaining uncertainty in deriving an age ordering (and necessarily ages) is metallicity. We also discuss the use of a feature we term the radiative‐convective gap overlap to provide a diagnostic of isochronal age spreads or varying accretion histories within a given star formation region. Finally, recent derivations of the distance to the ONC lie in two groups. Our new more precise distance of 391 +12 −9 pc allows us to decisively reject the further distance; we adopt 400 pc as a convenient value.

The VST Photometric Hα Survey of the Southern Galactic Plane and Bulge (VPHAS

The VST Photometric HSurvey of the Southern Galactic Plane and Bulge (VPHAS+) is surveying the southern Milky Way in u,g,r,i and Hat �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 sur- vey, begun on 28th December 2011, reaches down to �20th magnitude (10�) 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 narrow- band Hfilter in use is presented. Using model atmospheres and library spectra, we compute main-sequence (u g), (g r), (r i) and (r H�) 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 H�,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 per- fect vehicle for automated searches for reddened early-type stars, and will allow the discovery and analysis of compact binaries, white dwarfs and transient sources.

Empirical isochrones and relative ages for young stars, and the radiative–convective gap

We have selected pre-main-sequence (PMS) stars in 12 groups of notional ages ranging from 1 to 35 Myr, using heterogeneous membership criteria. Using these members we have constructed empirical isochrones in V, V − I colour‐magnitude diagrams. This allows us to identify clearly the gap between the radiative main sequence and the convective PMS (the R‐C gap). We follow the evolution of this gap with age and show that it can be a useful age indicator for groups less than � 15 Myr old. We also observe a reduction in absolute spreads about the sequences with age. Finally, the empirical isochrones allow us to place the groups in order of age, independently of theory. The youngest groups can be collated into three sets of similar ages. The youngest set is the ONC, NGC 6530 and IC 5146 (nominally 1 Myr); next Cep OB3b, NGC 2362, λ Ori and NGC 2264 (nominally 3 Myr); and finally σ Ori and IC 348 (nominally 4‐5 Myr). This suggests Cep OB3b is younger than previously thought, and IC 348 older. For IC 348 the stellar rotation rate distribution and fraction of stars with discs imply a younger age than we derive. We suggest this is because of the absence of O-stars in this cluster, whose winds and/or ionizing radiation may be an important factor in the removal of discs in other clusters.

Infrared spectroscopy of low-mass X-ray binaries – II

Using CGS4 on UKIRT, we have obtained 2.00–2.45 μ m infrared spectra of a number of low-mass X-ray binaries including Sco X-1, Sco X-2 and GX1 3+1. Sco X-1 shows emission lines only, supporting our previous conclusion that the spectral type of the evolved secondary must be earlier than G5. Emission lines are also seen in the spectrum of Sco X-2, confirming the identity of the IR counterpart. We report the detection of CO bands in GX1 3+1, and estimate the most likely spectral type of the secondary to be K5III. We also find P Cygni-type profiles in the Brackett γ lines of Sco X-1 and GX1 3+1, indicating the presence of high-velocity outflows in these systems. We present spectra of candidate IR counterparts to several other elusive X-ray binaries. Finally, implications for the nature and classification of these systems are discussed.

No wide spread of stellar ages in the Orion Nebula Cluster

The wide luminosity dispersion seen for stars at a given effective temperature in the Hertzsprung–Russell diagrams of young clusters and star-forming regions is often interpreted as due to significant (∼10 Myr) spreads in stellar contraction age. In the scenario where most stars are born with circumstellar discs, and that disc signatures decay monotonically (on average) over time-scales of only a few Myr, any such age spread should lead to clear differences in the age distributions of stars with and without discs. We have investigated large samples of stars in the Orion Nebula Cluster (ONC) using three methods to diagnose disc presence from infrared measurements. We find no significant difference in the mean ages or age distributions of stars with and without discs, consistent with expectations for a coeval population. Using a simple quantitative model, we show that any real age spread must be smaller than the median disc lifetime. For a lognormal age distribution, there is an upper limit of <0.14 dex (at 99 per cent confidence) to any real age dispersion, compared to the � 0.4 dex implied by the Hertzsprung–Russell diagram. If the mean age of the ONC is 2.5 Myr, this would mean at least 95 per cent of its low-mass stellar population have ages between 1.3–4.8 Myr. We suggest that the observed luminosity dispersion is caused by a combination of observational uncertainties and physical mechanisms that disorder the conventional relationship between luminosity and age for pre-main-sequence stars. This means that individual stellar ages from the Hertzsprung–Russell diagram are unreliable and cannot be used to directly infer a star formation history. Irrespective of what causes the wide luminosity dispersion, the finding that any real age dispersion is less than the median disc lifetime argues strongly against star formation scenarios for the ONC lasting longer than a few Myr.

The second data release of the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS DR2)

The INT/WFC Photometric Hα Survey of the Northern Galactic Plane (IPHAS) is a 1800 deg2 imaging survey covering Galactic latitudes |b| < 5° and longitudes l = 30°–215° in the r, i, and Hα filters using the Wide Field Camera (WFC) on the 2.5-m 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−1) 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 (root mean square) and recommend a series of quality criteria to select accurate data from the catalogue. Finally, we demonstrate the ability of the catalogues unique (r − Hα, r − i) diagram to (i) characterize 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.

OVERVIEW OF THE MASSIVE YOUNG STAR-FORMING COMPLEX STUDY IN INFRARED AND X-RAY (MYStIX) PROJECT

The Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) seeks to characterize 20 OB-dominated young clusters and their environs at distances d ≤ 4 kpc using imaging detectors on the Chandra X-ray Observatory, Spitzer Space Telescope, and the United Kingdom InfraRed Telescope. The observational goals are to construct catalogs of star-forming complex stellar members with well-defined criteria and maps of nebular gas (particularly of hot X-ray-emitting plasma) and dust. A catalog of MYStIX Probable Complex Members with several hundred OB stars and 31,784 low-mass pre-main sequence stars is assembled. This sample and related data products will be used to seek new empirical constraints on theoretical models of cluster formation and dynamics, mass segregation, OB star formation, star formation triggering on the periphery of H II regions, and the survivability of protoplanetary disks in H II regions. This paper gives an introduction and overview of the project, covering the data analysis methodology and application to two star-forming regions: NGC 2264 and the Trifid Nebula.