N. J. Mayne

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.

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.

CAN WE PREDICT THE GLOBAL MAGNETIC TOPOLOGY OF A PRE-MAIN-SEQUENCE STAR FROM ITS POSITION IN THE HERTZSPRUNG-RUSSELL DIAGRAM?

Zeeman-Doppler imaging studies have shown that the magnetic fields of T Tauri stars can be significantly more complex than a simple dipole and can vary markedly between sources. We collect and summarize the magnetic field topology information obtained to date and present Hertzsprung-Russell (H-R) diagrams for the stars in the sample. Intriguingly, the large-scale field topology of a given pre-main-sequence (PMS) star is strongly dependent upon the stellar internal structure, with the strength of the dipole component of its multipolar magnetic field decaying rapidly with the development of a radiative core. Using the observational data as a basis, we argue that the general characteristics of the global magnetic field of a PMS star can be determined from its position in the H-R diagram. Moving from hotter and more luminous to cooler and less luminous stars across the PMS of the H-R diagram, we present evidence for four distinct magnetic topology regimes. Stars with large radiative cores, empirically estimated to be those with a core mass in excess of ~40% of the stellar mass, host highly complex and dominantly non-axisymmetric magnetic fields, while those with smaller radiative cores host axisymmetric fields with field modes of higher order than the dipole dominant (typically, but not always, the octupole). Fully convective stars above ≳ 0.5 M_☉ appear to host dominantly axisymmetric fields with strong (kilo-Gauss) dipole components. Based on similarities between the magnetic properties of PMS stars and main-sequence M-dwarfs with similar internal structures, we speculate that a bistable dynamo process operates for lower mass stars (≾ 0.5 M_☉ at an age of a few Myr) and that they will be found to host a variety of magnetic field topologies. If the magnetic topology trends across the H-R diagram are confirmed, they may provide a new method of constraining PMS stellar evolution models.

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.

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.

Accuracy tests of radiation schemes used in hot Jupiter global circulation models

The treatment of radiation transport in global circulation models (GCMs) is crucial for correctly describing Earth and exoplanet atmospheric dynamics processes. The two-stream approximation and correlated-k method are currently state-of-the-art approximations applied in both Earth and hot Jupiter GCM radiation schemes to facilitate the rapid calculation of fluxes and heating rates. Their accuracy have been tested extensively for Earth-like conditions, but verification of the methods’ applicability to hot Jupiter-like conditions is lacking in the literature. We are adapting the UK Met O ce GCM, the Unified Model (UM), for the study of hot Jupiters, and present in this work the adaptation of the Edwards-Slingo radiation scheme based on the two-stream approximation and the correlated-k method. We discuss the calculation of absorption coe cients from high-temperature line lists and highlight the large uncertainty in the pressure-broadened line widths. We compare fluxes and heating rates obtained with our adapted scheme to more accurate discrete ordinate (DO) line-by-line (LbL) calculations ignoring scattering e ects. We find that, in most cases, errors stay below 10% for both heating rates and fluxes using 10 k-coe cients in each band and a di usivity factor D = 1:66. The two-stream approximation and the correlated-k method both contribute non-negligibly to the total error. We also find that using band-averaged absorption coe cients, which have previously been used in radiative-hydrodynamical simulations of a hot Jupiter, may yield errors of 100%, and should thus be used with caution.

The unified model, a fully-compressible, non-hydrostatic, deep atmosphere global circulation model, applied to hot Jupiters - ENDGame for a HD 209458b test case

We are adapting the global circulation model (GCM) of the UK Met Office, the so-called unified model (UM), for the study of hot Jupiters. In this work we demonstrate the successful adaptation of the most sophisticated dynamical core, the component of the GCM which solves the equations of motion for the atmosphere, available within the UM, ENDGame (Even Newer Dynamics for General atmospheric modelling of the environment). Within the same numerical scheme ENDGame supports solution to the dynamical equations under varying degrees of simplification. We present results from a simple, shallow (in atmospheric domain) hot Jupiter model (SHJ), and a more realistic (with a deeper atmosphere) HD 209458b test case. For both test cases we find that the large-scale, time-averaged (over the 1200 days prescribed test period), dynamical state of the atmosphere is relatively insensitive to the level of simplification of the dynamical equations. However, problems exist when attempting to reproduce the results for these test cases derived from other models. For the SHJ case the lower (and upper) boundary intersects the dominant dynamical features of the atmosphere meaning the results are heavily dependent on the boundary conditions. For the HD 209458b test case, when using the more complete dynamical models, the atmosphere is still clearly evolving after 1200 days, and in a transient state. Solving the complete (deep atmosphere and non-hydrostatic) dynamical equations allows exchange between the vertical and horizontal momentum of the atmosphere, via Coriolis and metric terms. Subsequently, interaction between the upper atmosphere and the deeper more slowly evolving (radiatively inactive) atmosphere significantly alters the results, and acts over timescales longer than 1200 days.

Pre-main-sequence isochrones -- I. The Pleiades benchmark

We present a critical assessment of commonly used pre-main-sequence isochrones by comparing their predictions to a set of well-calibrated colour-magnitude diagrams of the Pleiades in the wavelength range 0.4 to 2:5 m. Our analysis shows that for temperatures less than 4000 K the models systematically overestimate the ux by a factor two at 0:5 m, though this decreases with wavelength, becoming negligible at 2:2 m. In optical colours this will result in the ages for stars younger than 10 Myr being underestimated by factors between two and three. We show that using observations of standard stars to transform the data into a standard system can introduce signicant errors in the positioning of pre-mainsequences in colour-magnitude diagrams. Therefore we have compared the models to the data in the natural photometric system in which the observations were taken. Thus we have constructed and tested a model of the system responses for the Wide-Field Camera on the Isaac Newton Telescope. As a benchmark test for the development of pre-main-sequence models we provide both our system responses and the Pleiades sequence.

The Keele–Exeter young cluster survey – I. Low-mass pre-main-sequence stars in NGC 2169

We have used R c l c CCD photometry from the Isaac Newton telescope and intermediate-resolution spectroscopy from the Gemini North telescope to identify and characterize low-mass (0.15 < M /M ⊙ < 1.3) pre-main-sequence stars in the young open cluster NGC 2169. Isochrone fitting to the high- and low-mass populations yields an intrinsic distance modulus of 10.13 + 0.06 - 0.09 mag and a model-dependent age of 9 ± 2 Myr. Compared to the nearby, kinematically defined groups of a similar age, NGC 2169 has a large low-mass population which potentially offers a more precise statistical investigation of several aspects of star formation and early stellar evolution. By modelling the distribution of low-mass stars in the I c versus R c - I c diagram, we find that any age spread among cluster members has a Gaussian full width at half-maximum (FWHM) < 2.5 Myr. A young age and a small age spread (<10 Myr) are supported by the lack of significant lithium depletion in the vast majority of cluster members. There is no clear evidence for accretion or warm circumstellar dust in the low-mass members of NGC 2169, bolstering the idea that strong accretion has ceased and inner discs have dispersed in almost all low-mass stars by ages of 10 Myr.

Rotation of young stars in Cepheus OB3b

ABSTRACT We present a photometric study of I-band variability in the young associationCepheusOB3b. The study is sensitive to periodic variability on timescales of less than a day,to more than 20 days. After rejection of contaminating objects using V, I, R andnarrowband Hα photometry, we find 475 objects with measured rotation periods,which areverylikely pre-main-sequencemembers ofthe Cep OB3b starforming region.We revise the distance and age to Cep OB3b, putting it on the self-consistentage and distance ladder of Mayne & Naylor (2008). This yields a distance modulus of8.8±0.2 mags, corresponding to a distance of 580±60 pc, and an age of 4-5Myrs.The rotation period distribution confirms the general picture of rotational evo-lution in young stars, exhibiting both the correlation between accretion (determinedin this case through narrowband Hα photometry) and rotation expected from disclocking, and the dependence of rotation upon mass that is seen in other star formingregions. However, this mass dependence is much weaker in our data than found inother studies. Comparison to the similarly aged NGC 2362 shows that the low-massstars in Cep OB3b are rotating much more slowly. This points to a possible link be-tween star forming environment and rotation properties. Such a link would call intoquestion models of stellar angular momentum evolution, which assume that the rota-tional period distributions of young clusters and associations can be assembled intoan evolutionary sequence, thus ignoring environmental effects.Key words: accretion, accretion discs, stars:pre-main-sequence planetary systems:protoplanetary discs