arXiv subjects

Tim Naylor

Publications and source records attributed to Tim Naylor.

At least 55 records · Page 3Linked to original sources

arXiv2013

The Wide-Field Near Infrared Data: Optimal Photometry in Crowded Fields

We present JHK infrared data from the UK Infrared Telescope for a subset of the regions of the MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray) survey. Some of the data were obtained specifically for the MYStIX project, and some as part of the UKIRT Infrared Deep Sky Survey's Galactic Plane Survey. In most of these fields crowding is a significant issue for aperture photometry, and so we have re-extracted the photometry from the processed images using an optimal extraction technique, and we describe how we adapt the optimal technique to mitigate the effects of crowding.

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arXiv2013

Bayesian Matching for X-ray and Infrared Sources in the MYStIX Project

Identifying the infrared counterparts of X-ray sources in Galactic Plane fields such as those of the MYStIX project presents particular difficulties due to the high density of infrared sources. This high stellar density makes it inevitable that a large fraction of X-ray positions will have a faint field star close to them, which standard matching techniques may incorrectly take to be the counterpart. Instead we use the infrared data to create a model of both the field star and counterpart magnitude distributions, which we then combine with a Bayesian technique to yield a probability that any star is the counterpart of an X-ray source. In our more crowded fields, between 10 and 20% of counterparts that would be identified on the grounds of being the closest star to X-ray position within a 99% confidence error circle are instead identified by the Bayesian technique as field stars. These stars are preferentially concentrated at faint magnitudes. Equally importantly the technique also gives a probability that the true counterpart to the X-ray source falls beneath the magnitude limit of the infrared catalog. In deriving our method, we place it in the context of other procedures for matching astronomical catalogs.

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arXiv2013

The MYStIX InfraRed-Excess Source Catalog

The MYStIX project (Massive Young Star-Forming Complex Study in Infrared and X-rays) provides a comparative study of 20 Galactic massive star-forming complexes (d = 0.4 to 3.6 kpc). Probable stellar members in each target complex are identified using X-ray and/or infrared data via two pathways: (1) X-ray detections of young/massive stars with coronal activity/strong winds; or (2) infrared excess (IRE) selection of young stellar objects (YSOs) with circumstellar disks and/or protostellar envelopes. We present the methodology for the second pathway, using Spitzer/IRAC, 2MASS, and UKIRT imaging and photometry. Although IRE selection of YSOs is welltrodden territory, MYStIX presents unique challenges. We combine IR spectral energy distribution (SED) fitting with IR color cuts and spatial clustering analysis to identify IRE sources and isolate probable YSO members in each MYStIX target field from the myriad types of contaminating sources that can resemble YSOs: extragalactic sources, evolved stars, nebular knots, and even unassociated foreground/background YSOs. Applying our methodology consistently across 18 of the target complexes, we produce the MYStIX IRE Source (MIRES) Catalog comprising 20,719 sources, including 8686 probable stellar members of the MYStIX target complexes. We also classify the SEDs of 9365 IR counterparts to MYStIX X-ray sources to assist the first pathway, the identification of X-ray detected stellar members. The MIRES catalog provides a foundation for follow-up studies of diverse phenomena related to massive star cluster formation, including protostellar outflows, circumstellar disks, and sequential star formation triggered by massive star feedback processes.

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arXiv2013

Identifying Young Stars in Massive Star-Forming Regions for the MYStIX Project

The Massive Young star-forming Complex Study in Infrared and X-rays (MYStIX) project requires samples of young stars that are likely members of 20 nearby Galactic massive star-forming regions. Membership is inferred from statistical classification of X-ray sources, from detection of a robust infrared excess that is best explained by circumstellar dust in a disk or infalling envelope, and from published spectral types that are unlikely to be found among field stars. We present the MYStIX membership lists here, and describe in detail the statistical classification of X-ray sources via a \Naive Bayes Classi

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arXiv2013

A lithium depletion boundary age of 22 Myr for NGC 1960

We present a deep Cousins RI photometric survey of the open cluster NGC 1960, complete to R_C \simeq 22, I_C \simeq 21, that is used to select a sample of very low-mass cluster candidates. Gemini spectroscopy of a subset of these is used to confirm membership and locate the age-dependent "lithium depletion boundary" (LDB) --the luminosity at which lithium remains unburned in its low-mass stars. The LDB implies a cluster age of 22 +/-4 Myr and is quite insensitive to choice of evolutionary model. NGC 1960 is the youngest cluster for which a LDB age has been estimated and possesses a well populated upper main sequence and a rich low-mass pre-main sequence. The LDB age determined here agrees well with precise age estimates made for the same cluster based on isochrone fits to its high- and low-mass populations. The concordance between these three age estimation techniques, that rely on different facets of stellar astrophysics at very different masses, is an important step towards calibrating the absolute ages of young open clusters and lends confidence to ages determined using any one of them.

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arXiv2013

Pre-main-sequence isochrones -- II. Revising star and planet formation timescales

We have derived ages for 13 young (<30 Myr) star-forming regions and find they are up to a factor two older than the ages typically adopted in the literature. This result has wide-ranging implications, including that circumstellar discs survive longer (~10-12 Myr) and that the average Class I lifetime is greater (~1 Myr) than currently believed. For each star-forming region we derived two ages from colour-magnitude diagrams. First we fitted models of the evolution between the zero-age main-sequence and terminal-age main-sequence to derive a homogeneous set of main-sequence ages, distances and reddenings with statistically meaningful uncertainties. Our second age for each star-forming region was derived by fitting pre-main-sequence stars to new semi-empirical model isochrones. For the first time (for a set of clusters younger than 50 Myr) we find broad agreement between these two ages, and since these are derived from two distinct mass regimes that rely on different aspects of stellar physics, it gives us confidence in the new age scale. This agreement is largely due to our adoption of empirical colour-Teff relations and bolometric corrections for pre-main-sequence stars cooler than 4000 K. The revised ages for the star-forming regions in our sample are: ~2 Myr for NGC 6611 (Eagle Nebula; M 16), IC 5146 (Cocoon Nebula), NGC 6530 (Lagoon Nebula; M 8), and NGC 2244 (Rosette Nebula); ~6 Myr for σ Ori, Cep OB3b, and IC 348; ~10 Myr for λ Ori (Collinder 69); ~11 Myr for NGC 2169; ~12 Myr for NGC 2362; ~13 Myr for NGC 7160; ~14 Myr for χ Per (NGC 884); and ~20 Myr for NGC 1960 (M 36).

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arXiv2013

No evidence for intense, cold accretion onto YSOs from measurements of Li in T-Tauri stars

We have used medium resolution spectra to search for evidence that proto-stellar objects accrete at high rates during their early 'assembly phase'. Models predict that depleted lithium and reduced luminosity in T-Tauri stars are key signatures of 'cold' high-rate accretion occurring early in a star's evolution. We found no evidence in 168 stars in NGC 2264 and the Orion Nebula Cluster for strong lithium depletion through analysis of veiling corrected 6708 angstrom lithium spectral line strengths. This suggests that 'cold' accretion at high rates (M_dot > 5 x 10-4 M_sol yr-1) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 < M < 1.9 M_sol stars. We also find that the dispersion in the strength of the 6708 angstrom lithium line might imply an age spread that is similar in magnitude to the apparent age spread implied by the luminosity dispersion seen in colour magnitude diagrams. Evidence for weak lithium depletion (< 10 per cent in equivalent width) that is correlated with luminosity is also apparent, but we are unable to determine whether age spreads or accretion at rates less than 5 x 10-4 M_sol yr-1 are responsible.

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arXiv2012

MYStIX First Results: Spatial Structures of Massive Young Stellar Clusters

Observations of the spatial distributions of young stars in star-forming regions can be linked to the theory of clustered star formation using spatial statistical methods. The MYStIX project provides rich samples of young stars from the nearest high-mass star-forming regions. Maps of stellar surface density reveal diverse structure and subclustering. Young stellar clusters and subclusters are fit with isothermal spheres and ellipsoids using the Bayesian Information Criterion to estimate the number of subclusters. Clustering is also investigated using Cartwright and Whitworth's Q statistic and the inhomogeneous two-point correlation function. Mass segregation is detected in several cases, in both centrally concentrated and fractally structured star clusters, but a few clusters are not mass segregated.

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arXiv2012

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 microns. Our analysis shows that for temperatures less than 4000 K the models systematically overestimate the flux by a factor two at 0.5 microns, though this decreases with wavelength, becoming negligible at 2.2 microns. 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 significant errors in the positioning of pre-main-sequences 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.

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arXiv2012

Spitzer Imaging of the Nearby Rich Young Cluster, Cep OB3b

We map the full extent of a rich massive young cluster in the Cep OB3b association with the IRAC and MIPS instruments aboard the {\it Spitzer} Space Telescope and the ACIS instrument aboard the $\it{Chandra}$ X-Ray Observatory. At 700 pc, it is revealed to be the second nearest large ($>1000$ member), young ($< 5$ Myr) cluster known. In contrast to the nearest large cluster, the Orion Nebula Cluster, Cep OB3b is only lightly obscured and is mostly located in a large cavity carved out of the surrounding molecular cloud. Our infrared and X-ray datasets, as well as visible photometry from the literature, are used to take a census of the young stars in Cep OB3b. We find that the young stars within the cluster are concentrated in two sub-clusters; an eastern sub-cluster, near the Cep B molecular clump, and a western sub-cluster, near the Cep F molecular clump. Using our census of young stars, we examine the fraction of young stars with infrared excesses indicative of circumstellar disks. We create a map of the disk fraction throughout the cluster and find that it is spatially variable. Due to these spatial variations, the two sub-clusters exhibit substantially different average disk fractions from each other: $32% \pm 4%$ and $50% \pm 6%$. We discuss whether the discrepant disk fractions are due to the photodestruction of disks by the high mass members of the cluster or whether they result from differences in the ages of the sub-clusters. We conclude that the discrepant disk fractions are most likely due to differences in the ages.

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arXiv2011

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 H-R 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 timescales of only a few Myr, then 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 log-normal age distribution, there is an upper limit of <0.14 dex (at 99% confidence) to any real age dispersion, compared to the ~=0.4 dex implied by the H-R diagram. If the mean age of the ONC is 2.5 Myr, this would mean at least 95% of its low-mass stellar population has 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 H-R 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.

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arXiv2011

Accretion-induced luminosity spreads in young clusters: evidence from stellar rotation

We present an analysis of the rotation of young stars in the associations Cepheus OB3b, NGC 2264, NGC 2362 and the Orion Nebula Cluster (ONC). We discover a correlation between rotation rate and position in a colour-magnitude diagram (CMD) such that stars which lie above an empirically determined median pre-main sequence rotate more rapidly than stars which lie below this sequence. The same correlation is seen, with a high degree of statistical significance, in each association studied here. If position within the CMD is interpreted as being due to genuine age spreads within a cluster, then the stars above the median pre-main sequence would be the youngest stars. This would in turn imply that the most rapidly rotating stars in an association are the youngest, and hence those with the largest moments of inertia and highest likelihood of ongoing accretion. Such a result does not fit naturally into the existing picture of angular momentum evolution in young stars, where the stars are braked effectively by their accretion discs until the disc disperses. Instead, we argue that, for a given association of young stars, position within the CMD is not primarily a function of age, but of accretion history. We show that this hypothesis could explain the correlation we observe between rotation rate and position within the CMD.

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arXiv2009

Rotation of young stars in Cepheus OB3b

We present a photometric study of I-band variability in the young association Cepheus OB3b. 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 and narrowband H-alpha photometry, we find 475 objects with measured rotation periods, which are very likely pre-main-sequence members of the Cep OB3b star forming region. We revise the distance and age to Cep OB3b, putting it on the self-consistent age and distance ladder of Mayne & Naylor (2008). This yields a distance modulus of 8.8 +/- 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 evolution in young stars, exhibiting both the correlation between accretion (determined in this case through narrowband H-alpha photometry) and rotation expected from disc locking, and the dependence of rotation upon mass that is seen in other star forming regions. However, this mass dependence is much weaker in our data than found in other studies. Comparison to the similarly aged NGC 2362 shows that the low-mass stars in Cep OB3b are rotating much more slowly. This points to a possible link between star forming environment and rotation properties. Such a link would call into question models of stellar angular momentum evolution, which assume that the rotational period distributions of young clusters and associations can be assembled into an evolutionary sequence, thus ignoring environmental effects.

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arXiv2009

Are pre-MS stars older than we thought?

We present a consistent age ordering for young clusters and groups determined using the contraction of stars through their pre-main-sequence phase. We compare these with ages derived from the evolution of the upper main-sequence stars, and find the upper MS ages are older by a factor 1.5 to 2. We show that increasing the binary fraction and number of equal-mass binaries amongst the O-stars compared to the rest of the MS cannot remove this discrepancy.

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arXiv2009

New Methods for Determining the Ages of PMS Stars

We present three new methods for determining the age of groups of pre-main-sequence stars. The first, creating empirical isochrones allows us to create a robust age ordering, but not to derive actual ages. The second, using the width of the gap in colour-magnitude space between the pre-main-sequence and main-sequence (the radiative convective gap) has promise as a distance and extinction independent measure of age, but is as yet uncalibrated. Finally we discuss tau-squared fitting of the main sequence as the stars approach the terminus of the main sequence. This method suggests that there is a factor two difference between these "nuclear" ages, and more conventional pre-main-sequence contraction ages.

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arXiv2009

Are pre-main-sequence stars older than we thought?

We fit the colour-magnitude diagrams of stars between the zero-age main-sequence and terminal-age main sequence in young clusters and associations. The ages we derive are a factor 1.5 to 2 longer than the commonly used ages for these regions, which are derived from the positions of pre-main-sequence stars in colour-magnitude diagrams. From an examination of the uncertainties in the main-sequence and pre-main-sequence models, we conclude that the longer age scale is probably the correct one, which implies we must revise upwards the commonly used ages for young clusters and associations. Such a revision would explain the discrepancy between the observational lifetimes of proto-planetary discs and theoretical calculations of the time to form planets. It would also explain the absence of clusters with ages between 5 and 30Myr. We use the $τ^2$ statistic to fit the main-sequence data, but find that we must make significant modifications if we are to fit sequences which have vertical segments in the colour-magnitude diagram. We present this modification along with improvements to methods of calculating the goodness-of-fit statistic and parameter uncertainties. Software implementing the methods described in this paper is available from http://www.astro.ex.ac.uk/people/timn/tau-squared/

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arXiv2009

Pre-main-sequence variability across the radiative-convective gap

We use I band imaging to perform a variability survey of the 13 Myr-old cluster h Per. We find a significant fraction of the cluster members to be variable. Most importantly, we find that variable members lie almost entirely on the convective side of the gap in the cluster sequence between fully convective stars and those which have a radiative core. This result is consistent with a scenario in which the magnetic field changes topology when the star changes from being fully convective, to one containing a radiative core. When the star is convective the magnetic field appears dominated by large-scale structures, resulting in global-size spots that drive the observed variability. For those stars with radiative cores we observe a marked absence of variability due to spots, which suggests a switch to a magnetic field dominated by smaller-scale structures, resulting in many smaller spots and thus less apparent variability. This implies that wide field variability surveys may only be sensitive to fully convective stars. On the one hand this reduces the chances of picking out young groups (since the convective stars are the lower mass and therefore fainter objects), but conversely the absolute magnitude of the head of the convective sequence provides a straightforward measure of age for those groups which are discovered.

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arXiv2008

The stellar association around Gamma Velorum and its relationship with Vela OB2

We present the results of a photometric BVI survey of 0.9 square degrees around the Wolf-Rayet binary gamma^2 Vel and its early-type companion gamma^1 Vel. Several hundred PMS stars are identified and the youth of a subset of these is confirmed by the presence of lithium, H-alpha emission and X-ray activity. We show that the PMS stars are kinematically coherent and spatially concentrated around gamma Vel. The PMS stars have similar proper motions to gamma Vel, to main-sequence stars around gammaVel and to early-type stars of the wider Vela OB2 association of which gamma^2 Vel is the brightest member. The ratio of main-sequence stars to low-mass (0.1-0.6 Msun) PMS stars is consistent with a Kroupa mass function. Main-sequence fitting to stars around gamma Vel gives a distance modulus of 7.76+/-0.07 mag, consistent with a similarly-determined distance for Vela OB2 and with interferometric distances to gamma^2 Vel. High-mass stellar models indicate an age of 3-4 Myr for gamma^2 Vel, but the low-mass PMS stars have ages of ~10 Myr according to low-mass evolutionary models and 5-10 Myr by empirically placing them in an age sequence with other clusters based on colour-magnitude diagrams and lithium depletion. We conclude that the low-mass PMS stars form a genuine association with gamma Vel and that this is a subcluster within the larger Vela OB2 association. We speculate that gamma^2 Vel formed after the low-mass stars, expelling gas, terminating star formation and unbinding the association. The velocity dispersion of the PMS stars is too low for this star forming event to have produced all the stars in Vela OB2. Instead, star formation must have started at several sites within a molecular cloud, either sequentially or, simultaneously after some triggering event [abridged].

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