arXiv subjects

Tim Naylor

Publications and source records attributed to Tim Naylor.

At least 73 records · Page 4Linked to original sources

arXiv2008

Autonomous software: Myth or magic?

We discuss work by the eSTAR project which demonstrates a fully closed loop autonomous system for the follow up of possible micro-lensing anomalies. Not only are the initial micro-lensing detections followed up in real time, but ongoing events are prioritised and continually monitored, with the returned data being analysed automatically. If the ``smart software'' running the observing campaign detects a planet-like anomaly, further follow-up will be scheduled autonomously and other telescopes and telescope networks alerted to the possible planetary detection. We further discuss the implications of this, and how such projects can be used to build more general autonomous observing and control systems.

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arXiv2008

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 (2007). The new (approximate) nominal ages are: 2 Myrs for NGC6530 and the ONC, 3 Myrs for Lambda Orionis, NGC2264 and Sigma Orionis, 4-5 Myrs for NGC2362, 13 Myrs for h and chi Per, 20 Myrs for NGC1960 and 40 Myrs for NGC2547. In cases of significantly variable extinction we have derived individual extinctions using a revised Q-method (Johnson and Morgan, 1953). 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 R-C 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.

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arXiv2008

An Autonomous Adaptive Scheduling Agent for Period Searching

We describe the design and implementation of an autonomous adaptive software agent that addresses the practical problem of observing undersampled, periodic, time-varying phenomena using a network of HTN-compliant robotic telescopes. The algorithm governing the behaviour of the agent uses an optimal geometric sampling technique to cover the period range of interest, but additionally implements proactive behaviour that maximises the optimality of the dataset in the face of an uncertain and changing operating environment.

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arXiv2006

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

We have selected pre-main-sequence stars in 12 groups of notional ages ranging from 1 Myr to 35 Myrs, using heterogeneous membership criteria. Using these members we have constructed empirical isochrones in V, V-I colour magnitude diagrams (CMDs). This allows us to identify clearly the gap between the radiative main sequence and the convective pre-main-sequence (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 Myrs 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, NGC6530 and IC5146 (nominally 1 Myrs); next Cep OB3b, NGC2362, Lambda Ori and NGC2264 (nominally 3 Myrs); and finally Sigma Ori and IC348 (nominally 4-5 Myrs). This suggests Cep OB3b is younger than previously thought, and IC348 older. For IC348 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 ionising radiation may be an important factor in the removal of discs in other clusters.

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arXiv2006

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

We have used R_C I_C CCD photometry from the Isaac Newton telescope and intermediate resolution spectroscopy from the Gemini North telescope to identify and characterise low-mass (0.15<M/M_sun<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 with 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 <=2.5 Myr. A young age and 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.

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arXiv2006

A maximum likelihood method for fitting colour-magnitude diagrams

We present a maximum likelihood method for fitting two-dimensional model distributions to stellar data in colour-magnitude space. This allows one to include (for example) binary stars in an isochronal population. The method also allows one to derive formal uncertainties for fitted parameters, and assess the likelihood that a good fit has been found. We use the method to derive an age of 38.5 +3.5/-6.5 Myrs and a true distance modulus of 7.79 +0.11/-0.05 mags from the V vs V-I diagram of NGC2547 (the uncertainties are 67 percent confidence limits, and the parameters are insensitive to the assumed binary fraction). These values are consistent with those previously determined from low-mass isochronal fitting, and are the first measurements to have statistically meaningful uncertainties. The age is also consistent with the lithium depletion age of NGC2547, and the HIPPARCOS distance to the cluster is consistent with our value. The method appears to be quite general and could be applied to any N-dimensional dataset, with uncertainties in each dimension. However, it is particularly useful when the data are sparse, in the sense that both the typical uncertainties for a datapoint and the size of structure in the function being fitted are small compared with the typical distance between datapoints. In this case binning the data will lose resolution, whilst the method presented here preserves it. 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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arXiv2006

Kinematic structure in the young Sigma Orionis association

We have used precise radial velocity measurements for a large number of candidate low-mass stars and brown dwarfs, to show that the young Sigma Ori ``cluster'' consists of two spatially superimposed components which are kinematically separated by 7 km/s in radial velocity, and which have different mean ages. We examine the relationship of these two kinematic groups to other populations in the Orion OB1 association and briefly discuss the consequence of mixed age samples for ongoing investigations of the formation and evolution of low-mass objects in this much-observed region.

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arXiv2006

Optimal placement of a limited number of observations for period searches

Robotic telescopes present the opportunity for the sparse temporal placement of observations when period searching. We address the best way to place a limited number of observations to cover the dynamic range of frequencies required by an observer. We show that an observation distribution geometrically spaced in time can minimise aliasing effects arising from sparse sampling, substantially improving signal detection quality. The base of the geometric series is however a critical factor in the overall success of this strategy. Further, we show that for such an optimal distribution observations may be reordered, as long as the distribution of spacings is preserved, with almost no loss of quality. This implies that optimal observing strategies can retain significant flexibility in the face of scheduling constraints, by providing scope for on-the-fly adaptation. Finally, we present optimal geometric samplings for a wide range of common observing scenarios, with an emphasis on practical application by the observer at the telescope. Such a sampling represents the best practical empirical solution to the undersampling problem that we are aware of. The technique has applications to robotic telescope and satellite observing strategies, where target acquisition overheads mean that a greater total target exposure time (and hence signal-to-noise) can often in practice be achieved by limiting the number of observations.

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arXiv2005

Can variability account for apparent age spreads in OB association colour-magnitude diagrams?

We have investigated the role of photometric variability in causing the apparent age spreads observed in the colour-magnitude diagrams of OB associations. We have found that the combination of binarity, photometric uncertainty and variability on timescales of a few years is not sufficient to explain the observed spread in either of the OB associations we have studied. Such effects can account for about half the observed spread in the sigma Orionis subgroup and about 1/20 of the observed spread in Cep OB3b. This rules out variability caused by stellar rotation and rotation of structures within inner accretion discs as the source of the majority of the the apparent age spreads. We also find that the variability tends to move objects parallel to isochrones in V/V-i' CMDs, and thus has little influence on apparent age spreads. We conclude that the remaining unexplained spread either reflects a true spread in the ages of the PMS objects or arises as a result of longer term variability associated with changes in accretion flow.

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arXiv2005

T Tauri stellar magnetic fields: He I measurements

We present measurements of the longitudinal magnetic field in the circumstellar environment of seven classical T Tauri stars. The measurements are based on high-resolution circular spectropolarimetry of the He I 5876 emission line, which is thought to form in accretion streams controlled by a stellar magnetosphere. We detect magnetic fields in BP Tau, DF Tau and DN Tau, and detect statistically significant fields in GM Aur and RW Aur A at one epoch but not at others. We detect no field for DG Tau and GG Tau, with the caveat that these objects were observed at one epoch only. Our measurements for BP Tau and DF Tau are consistent, both in terms of sign and magnitude, with previous studies, suggesting that the characteristics of T Tauri magnetospheres are persistent over several years. We observed the magnetic field of BP Tau to decline monotonically over three nights, and have detected a peak field of 4kG in this object, the highest magnetic field yet observed in a T Tauri star. We combine our observations with results from the literature in order to perform a statistical analysis of the magnetospheric fields in BP Tau and DF Tau. Assuming a dipolar field, we determine a polar field of ~3kG and a dipole offset of 40deg for BP Tau, while DF Tau's field is consistent with a polar field of ~-4.5kG and a dipole offset of 10deg. We conclude that many classical T Tauri stars have circumstellar magnetic fields that are both strong enough and sufficiently globally-ordered to sustain large-scale magnetospheric accretion flows.

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arXiv2005

Do accretion discs regulate the rotation of young stars?

We present a photometric study of I-band variability in the young cluster IC 348. The main purpose of the study was to identify periodic stars. In all we find 50 periodic stars, of which 32 were previously unknown. For the first time in IC 348, we discover periods in significant numbers of lower-mass stars (M < 0.25 Msun) and classical T-Tauri stars. This increased sensitivity to periodicities is due to the enhanced depth and temporal density of our observations, compared with previous studies. The period distribution is at first glance similar to that seen in the Orion Nebula Cluster, with the higher-mass stars (M > 0.25 Msun) showing a bi-modal period distribution concentrated around periods of 2 and 8 days, and the lower-mass stars showing a uni-modal distribution, heavily biassed towards fast rotators. Closer inspection of the period distribution shows that the higher mass stars show a significant dearth of fast rotators, compared to the Orion Nebula Cluster, whilst the low mass stars are rotating significantly faster than those in Orion. We find no correlation between rotation period and K-L colour or H-alpha equivalent width. We also present a discussion of our own IC 348 data in the context of previously published period distributions for the Orion Nebula Cluster, the Orion Flanking Fields and NGC 2264. We find that the previously claimed correlation between infrared excess and rotation period in the ONC might in fact result from a correlation between infrared excess and mass. We also find a marked difference in period distributions between NGC 2264 and IC 348, which presents a serious challenge to the disc locking paradigm, given the similarity in ages and disc fractions between the two clusters.

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arXiv2004

Contamination and exclusion in the sigma Orionis young group

We present radial velocities for 38 low-mass candidate members of the sigma Orionis young group. We have measured their radial velocities by cross-correlation of high resolution (R~6000) AF2/WYFFOS spectra of the gravity sensitive NaI doublet at 8183, 8195Angstroms. The total sample contained 117 objects of which 54 have sufficient signal-to-noise to detect NaI at an equivalent width of 3Angstroms, however we only detect NaI in 38 of these. This implies that very low-mass members of this young group display weaker NaI absorption than similarly aged objects in the Upper Scorpius OB association. We develop a technique to assess membership using radial velocities with a range of uncertainties that does not bias the selection when large uncertainties are present. The resulting membership probabilities are used to assess the issue of exclusion in photometric selections, and we find that very few members are likely to be excluded by such techniques. We also assess the level of contamination in the expected pre-main sequence region of colour-magnitude space brighter than I = 17. We find that contamination by non-members in the expected PMS region of the colour-magnitude diagram is small. We conclude that although radial velocity alone is insufficient to confirm membership, high signal-to-noise observations of the NaI doublet provide the opportunity to use the strength of NaI absorption in concert with radial velocities to asses membership down to the lowest masses, where Lithium absorption no longer distinguishes youth.

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arXiv2004

Membership, binarity and accretion among very low-mass stars and brown dwarfs of the Sigma Orionis cluster

Intermediate resolution (R=7000) spectroscopy is presented for 76 photometrically selected very low mass (0.04<M<0.3M_{sun}) candidate members of the Sigma Orionis cluster. More than two thirds appear to be genuine cluster members on the basis of Li I absorption, weak Na I features and radial velocities. Photometric selection alone therefore appears to be very effective in identifying cluster members in this mass range. Only 6 objects appear to be certain non-members, however a substantial subset of 13 have ambiguous or contradictory indications of membership and lack Li absorption. Four candidate binary cluster members are identified. Consideration of sampling and precision leads us to conclude that either the fraction of very low mass stars and brown dwarfs in small separation (a<1au) binary systems is larger than in field M-dwarfs, or the distribution of separations is much less skewed towards large separations. This conclusion hinges critically on the correct identification of the small number of binary candidates, but is significant even when only Li-rich candidate members are considered. Broadened H alpha emission, indicative of circum(sub)stellar accretion discs is found in 5 or 6 of the candidate cluster members, 3 of which probably have substellar masses. The fraction of accretors (10+/-5 per cent) is similar to that found in stars of higher mass in the cluster using H alpha emission as a diagnostic, but much lower than found for very low mass stars and brown dwarfs of younger clusters. The timescale for accretion rates to drop to less than 10^{-11} M_sun/yr is hence less than the age of the Sigma Ori cluster (3 to 7 Myr) for most low-mass objects (abridged).

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arXiv2004

Low mass stars, brown dwarf candidates and the mass function of the young open cluster NGC 2547

We present a catalogue of RIZ photometry over an area of 0.855 square degrees, centred on NGC 2547. The survey is substantially complete to limits of R_c=21.5, I_c=19.5, Z=19.5. We use the catalogue to define NGC 2547 candidates with masses of about 0.05-1.0 M_sun. After correcting for incompleteness and contamination by foreground field dwarfs, we investigate the mass function of the cluster, its binary content, and search for evidence of mass segregation among lower mass stars. There is evidence for mass segregation between high (>3 M_sun) and lower mass stars, but over the range 0.1<M<0.7 M_sun, the data are consistent with no further mass segregation. By fitting King profiles we conclude that at least 60 percent of the low-mass stellar population are contained in our survey. The cluster mass function is remarkably similar to the Pleiades for 0.075<M<0.7 M_sun. Because of its age (~30 Myr), we demonstrate that this mass function is robust to several systematic uncertainties likely to affect older and younger clusters and is therefore one of the best available estimates for the initial mass function in young disc populations. For 0.05<M<0.075 M_sun there is some evidence for a deficit of brown dwarfs in NGC 2547 compared with other clusters. This deficit may extend to lower masses or may only be a dip, perhaps caused by an imperfect understanding of the mass-magnitude relationship at temperatures of 2800K.Incompleteness in our survey, and the luminosity functions from which we estimate contamination, leave this question open. The binary fraction for systems with mass ratios greater than about 0.5 is 20-35 per cent for M-dwarfs in NGC 2547, quite consistent with that found in the field (abridged).

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arXiv2003

Evidence for high accretion-rates in Weak-Line T Tauri stars?

We have discovered T Tauri stars which show startling spectral variability between observations seperated by 20 years. In spectra published by Bouvier & Appenzeller (1992) these objects showed very weak H-alpha emission, broad CaII absorption and so called ``composite spectra'', where the spectral type inferred from the blue region is earlier than that inferred from the red. We present here new spectroscopy which shows that all four stars now exhibit strong H-alpha emission, narrow CaII emission and a spectral type which is consistent at all wavelengths. We propose a scheme to understand these changes whereby the composite spectra of these stars can be explained by a period of active accretion onto the central, young star. In this scheme the composite spectrum consists of a contribution from the stellar photosphere and a contribution from a hot, optically thick, accretion component. The optically thick nature of the accretion flow explains the weakness of the H-alpha emission during this phase. Within this scheme, the change to a single spectral type at all wavelengths and emergence of strong H-alpha emission are consistent with the accretion columns becoming optically thin, as the accretion rate drops. There is a strong analogy here with the dwarf novae class of interacting binaries, which show similar behaviour during the decline from outbursts of high mass-transfer rate. The most important consequence of this interpretation is that these objects bring into question the association of Weak-Line T Tauri stars (WTTs) with non-accreting or discless objects. In light of this result we consider the justification for this paradigm.

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arXiv2003

On the Nature of Collinder 121: Insights from the Low-Mass Pre-Main Sequence

We present a VI photometric catalogue towards the open cluster Cr 121. XMM-Newton and ROSAT data are used to discover a low-mass pre-main sequence (PMS) along this sight-line. de Zeeuw et al (1999) identified Cr 121 as a moving group, using HIPPARCOS data, at a distance of 592 pc. We reject the scenario that these low-mass PMS stars are associated with that association. By considering the higher mass main sequence stellar membership of the groups along this sight-line, the density of low-mass PMS stars and their age spread we argue that the low-mass PMS stars are associated with a young, compact cluster at a distance of 1050 pc. This is consistent with the original description of Cr 121 (Collinder, 1931), and we argue that this distant compact cluster should retain its original designation. The moving group detected by de Zeeuw et al (1999) resembles a foreground association and we agree with Eggen (1981) that this should be called CMa OB2. This study demonstrates that although the de Zeeuw et al (1999) census of OB associations is an invaluable resource for studying local star formation, it must be interpreted in the context other data when considering structure over distances of the same order as the limits of the Hipparcos parallaxes.

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arXiv2003

Mass segregation in the young open cluster NGC 2547

We present a study of mass segregation of the young (20-35 Myr isochronal age), open cluster NGC 2547. We find good evidence that mass segregation exists in NGC 2547 down to 3 Msun, and weak evidence for mass segregation down to 1 Msun. Theoretical models of an initially unsegregated model of NGC 2547 using the NBODY2 code show weaker mass segregation, implying that at least some of the observed mass segregation has a primordial origin. We also report the discovery of three possible escaped cluster members, which share the proper motion and colours of the cluster, but lie nearly a degree from the cluster centre.

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arXiv2002

Optimal photometry for colour-magnitude diagrams and its application to NGC 2547

We have developed the techniques required to use the optimal photometry algorithm of Naylor (1998) to create colour-magnitude diagrams with well defined completeness functions. To achieve this we first demonstrate that the optimal extraction is insensitive to uncertainties in the star's measured position. We then show how to correct the optimally extracted fluxes such that they correspond to those measured in a large aperture, so aperture photometry of standard stars can be used to place the measurements on a standard system. The technique simultaneously removes the effects of a position dependent point-spread function. Finally we develop a method called "ghosting", which calculates the completeness corrections in the absence of an accurate description of the point spread function. We apply these techniques to the young cluster NGC 2547 (=C0809-491), and use an X-ray selected sample to find an age of 20-35Myr and an intrinsic distance modulus of 8.00-8.15 magnitudes. We use these isochrones to select members from our photometric surveys. Our derived luminosity function shows a well defined Wielen dip, making NGC 2547 the youngest cluster in which such a feature has been observed. Our derived mass function spans the range 0.1-6Mo and is similar to that for the field and the older, more massive clusters M35 and the Pleiades, supporting the idea of a universal initial mass function.

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