arXiv subjects

Tim Naylor

Publications and source records attributed to Tim Naylor.

At least 37 records · Page 2Linked to original sources

arXiv2019

Spatial Statistics in Star Forming Regions: Testing the Limits of Randomness

Observational studies of star formation reveal spatial distributions of Young Stellar Objects (YSOs) that are `snapshots' of an ongoing star formation process. Using methods from spatial statistics it is possible to test the likelihood that a given distribution process could produce the observed patterns of YSOs. The aim of this paper is to determine the usefulness of the spatial statistics tests Diggle's G function (G), the `free-space' function (F), Ripley's K and O-ring for application to astrophysical data. The spatial statistics tests were applied to simulated data containing 2D Gaussian clusters projected on random distributions of stars. The number of stars within the Gaussian cluster and number of background stars were varied to determine the tests' ability to reject complete spatial randomness (CSR) with changing signal-to-noise. The best performing test was O-ring optimised with overlapping logarithmic bins, closely followed by Ripley's K. The O-ring test is equivalent to the 2-point correlation function. Both F and G (and the minimum spanning tree, of which G is a subset) performed significantly less well, requiring a cluster with a factor of two higher signal-to-noise in order to reject CSR consistently. We demonstrate the tests on example astrophysical datasets drawn from the Spitzer catalogue.

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arXiv2018

Gaia 17bpi: An FU Ori Type Outburst

We report on the source Gaia 17bpi and identify it as a new, ongoing FU Ori type outburst, associated with a young stellar object. The optical lightcurve from Gaia exhibited a 3.5 mag rise with the source appearing to plateau in mid/late 2018. Mid-infrared observations from NEOWISE also show a $>$3 mag rise that occurred in two stages, with the second one coincident with the optical brightening, and the first one preceding the optical brightening by $\sim$1.5 years. We model the outburst as having started between October and December of 2014. This wavelength-dependent aspect of young star accretion-driven outbursts has never been documented before. Both the mid-infrared and the optical colors of the object become bluer as the outburst proceeds. Optical spectroscopic characteristics in the outburst phase include: a GK-type absorption spectrum, strong wind/outflow in e.g. Mgb, NaD, H$α$, KI, OI, and CaII profiles, and detection of LiI 6707 Å. The infrared spectrum in the outburst phase is similar to that of an M-type spectrum, notably exhibiting prominent $H_2O$ and $^{12}$CO (2-0) bandhead absorption in the K-band, and likely HeI wind in the Y-band. The new FU Ori source Gaia 17bpi is associated with a little-studied dark cloud in the galactic plane, located at a distance of 1.27 kpc.

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arXiv2018

Improving Catalogue Matching By Supplementing Astrometry with Additional Photometric Information

The matching of sources between photometric catalogues can lead to cases where objects of differing brightness are incorrectly assumed to be detections of the same source. The rejection of unphysical matches can be achieved through the inclusion of information about the sources' magnitudes. The method described here uses the additional photometric information from both catalogues in the process of accepting or rejecting counterparts, providing approximately a factor 10 improvement in Bayes' factor with its inclusion. When folding in the photometric information we avoid using prior astrophysical knowledge. Additionally, the method allows for the possibility of no counterparts to sources as well as the possibility that sources overlap multiple potential counterparts. We formally describe the probability of two sources being the same astrometric object, allowing systematic effects of astrometric perturbation (by, e.g., contaminant objects) to be accounted for. We apply the method to two cases. First, we test IPHAS-Gaia matches to compare the resulting matches in two catalogues of similar wavelength coverage but differing dynamical ranges. Second, we apply the method to matches between IPHAS and 2MASS and show that the method holds when considering two catalogues with approximately equal astrometric precision. We discuss the importance of including the magnitude information in each case. Additionally, we discuss extending the method to multiple catalogue matches through an iterative matching process. The method allows for the selection of high-quality matches by providing an overall probability for each pairing, giving the flexibility to choose stars known to be good matches.

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arXiv2018

A Contaminant-Free Catalogue of Gaia DR2-WISE Galactic Plane Matches: Including the Effects of Crowding in the Cross-Matching of Photometric Catalogues

Faint, hidden contaminants in the point-spread functions (PSFs) of stars cause shifts to their measured positions. Wilson & Naylor (2017) showed failing to account for these shifts can lead to a drastic decrease in the number of returned catalogue matches in crowded fields. Here we highlight the effect these perturbations have on cross-matching, for matches between Gaia DR2 and WISE stars in a crowded Galactic plane region. Applying the uncertainties as quoted to Gaussian-based astrometric uncertainty functions (AUFs) can lead, in dense Galactic fields, to only matching 55% of the counterparts. We describe the construction of empirical descriptions for AUFs, building on the cross-matching method of Wilson & Naylor (2018), utilising the magnitudes of both catalogues to discriminate between true and false counterparts. We apply the improved cross-matching method to the Galactic plane |b| < 10. We provide the most likely counterpart matches and their respective probabilities. We also analyse several cases to verify the robustness of the results, highlighting some important caveats and considerations. Finally, we discuss the effect PSF resolution has by comparing the intra-catalogue nearest neighbour separation distributions of a sample of likely contaminated WISE objects and their corresponding Spitzer counterpart. We show that some WISE contaminants are resolved in Spitzer, with smaller intra-catalogue separations. We have highlighted the effect contaminant stars have on WISE, but it is important for all photometric catalogues, playing an important role in the next generation of surveys, such as LSST.

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arXiv2018

The Massive Star-Forming Regions Omnibus X-Ray Catalog, Second Installment

We present the second installment of the Massive Star-forming Regions (MSFRs) Omnibus X-ray Catalog (MOXC2), a compilation of X-ray point sources detected in Chandra/ACIS observations of 16 Galactic MSFRs and surrounding fields. MOXC2 includes 13 ACIS mosaics, three containing a pair of unrelated MSFRs at different distances, with a total catalog of 18,396 point sources. The MSFRs sampled range over distances of 1.3 kpc to 6 kpc and populations varying from single massive protostars to the most massive Young Massive Cluster known in the Galaxy. By carefully detecting and removing X-ray point sources down to the faintest statistically-significant limit, we facilitate the study of the remaining unresolved X-ray emission. Through comparison with mid-infrared images that trace photon-dominated regions and ionization fronts, we see that the unresolved X-ray emission is due primarily to hot plasmas threading these MSFRs, the result of feedback from the winds and supernovae of massive stars. The 16 MSFRs studied in MOXC2 more than double the MOXC1 sample, broadening the parameter space of ACIS MSFR explorations and expanding Chandra's substantial contribution to contemporary star formation science.

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arXiv2017

The Effect of Unresolved Contaminant Stars on the Cross-Matching of Photometric Catalogues

A fundamental process in astrophysics is the matching of two photometric catalogues. It is crucial that the correct objects be paired, and that their photometry does not suffer from any spurious additional flux. We compare the positions of sources in WISE, IPHAS, 2MASS, and APASS with Gaia DR1 astrometric positions. We find that the separations are described by a combination of a Gaussian distribution, wider than naively assumed based on their quoted uncertainties, and a large wing, which some authors ascribe to proper motions. We show that this is caused by flux contamination from blended stars not treated separately. We provide linear fits between the quoted Gaussian uncertainty and the core fit to the separation distributions. We show that at least one in three of the stars in the faint half of a given catalogue will suffer from flux contamination above the 1% level when the density of catalogue objects per PSF area is above approximately 0.005. This has important implications for the creation of composite catalogues. It is important for any closest neighbour matches as there will be a given fraction of matches that are flux contaminated, while some matches will be missed due to significant astrometric perturbation by faint contaminants. In the case of probability-based matching, this contamination affects the probability density function of matches as a function of distance. This effect results in up to 50% fewer counterparts being returned as matches, assuming Gaussian astrometric uncertainties for WISE-Gaia matching in crowded Galactic plane regions, compared with a closest neighbour match.

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arXiv2016

HARPS3 for a Roboticized Isaac Newton Telescope

We present a description of a new instrument development, HARPS3, planned to be installed on an upgraded and roboticized Isaac Newton Telescope by end-2018. HARPS3 will be a high resolution (R = 115,000) echelle spectrograph with a wavelength range from 380-690 nm. It is being built as part of the Terra Hunting Experiment - a future 10 year radial velocity measurement programme to discover Earth-like exoplanets. The instrument design is based on the successful HARPS spectrograph on the 3.6m ESO telescope and HARPS-N on the TNG telescope. The main changes to the design in HARPS3 will be: a customised fibre adapter at the Cassegrain focus providing a stabilised beam feed and on-sky fibre diameter ~ 1.4 arcsec, the implementation of a new continuous flow cryostat to keep the CCD temperature very stable, detailed characterisation of the HARPS3 CCD to map the effective pixel positions and thus provide an improved accuracy wavelength solution, an optimised integrated polarimeter and the instrument integrated into a robotic operation. The robotic operation will optimise our programme which requires our target stars to be measured on a nightly basis. We present an overview of the entire project, including a description of our anticipated robotic operation.

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arXiv2016

V346 Normae: First post-outburst observations of an FU Orionis star

During their formation phase stars gain most of their mass in violent episodic accretion events, such as observed in FU Orionis (FUor) and EXor stars. V346 Normae is a well-studied FUor that underwent a strong outburst beginning in $\sim1980$. Here, we report photometric and spectroscopic observations which show that the visual/near-infrared brightness has decreased dramatically between the 1990s and 2010 ($ΔR\approx10.9^{\rm m}$, $ΔJ\approx7.8^{\rm m}$, $ΔK\approx5.8^{\rm m}$). The spectral properties of this fading event cannot be explained with variable extinction alone, but indicate a drop in accretion rate by 2-3 orders of magnitude, marking the first time that a member of the FUor class has been observed to switch to a very low accretion phase. Remarkably, in the last few years (2011-2015) V346 Nor has brightened again at all near-infrared wavelengths, indicating the onset of a new outburst event. The observed behaviour might be consistent with the clustered luminosity bursts that have been predicted by recent gravitational instability and fragmentation models for the early stages of protostellar evolution. Given V346 Nor's unique characteristics (concerning outburst duration, repetition frequency, and spectroscopic diagnostics), our results also highlight the need for revisiting the FUor/EXor classification scheme.

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arXiv2016

Rapid Circumstellar Disk Evolution and an Accelerating Star Formation Rate in the Infrared Dark Cloud M17 SWex

We present a catalog of 840 X-ray sources and first results from a 100 ks Chandra X-ray Observatory imaging study of the filamentary infrared dark cloud G014.225$-$00.506, which forms the central regions of a larger cloud complex known as the M17 southwest extension (M17 SWex). In addition to the rich population of protostars and young stellar objects with dusty circumstellar disks revealed by Spitzer Space Telescope archival data, we discover a population of X-ray-emitting, intermediate-mass pre--main-sequence stars (IMPS) that lack infrared excess emission from circumstellar disks. We model the infrared spectral energy distributions of this source population to measure its mass function and place new constraints on the inner dust disk destruction timescales for 2-8 $M_{\odot}$ stars. We also place a lower limit on the star formation rate (SFR) and find that it is quite high ($\dot{M}\ge 0.007~M_{\odot}$ yr$^{-1}$), equivalent to several Orion Nebula Clusters in G14.225$-$0.506 alone, and likely accelerating. The cloud complex has not produced a population of massive, O-type stars commensurate with its SFR. This absence of very massive (${\ge}20~M_{\odot}$) stars suggests that either (1) M17 SWex is an example of a distributed mode of star formation that will produce a large OB association dominated by intermediate-mass stars but relatively few massive clusters, or (2) the massive cores are still in the process of accreting sufficient mass to form massive clusters hosting O stars.

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arXiv2015

The age of Taurus - environmental effects on disc lifetimes

Using semi-empirical isochrones, we find the age of the Taurus star-forming region to be 3-4 Myr. Comparing the disc fraction in Taurus to young massive clusters suggests discs survive longer in this low density environment. We also present a method of photometrically de-reddening young stars using $iZJH$ data.

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arXiv2015

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 β 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 η 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/.

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arXiv2015

Isochronal age scale of 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, which is consistent with recent lithium depletion boundary ages for both the Beta Pic and Tucana-Horologium moving groups. This age scale was derived using a set of semi-empirical pre-main-sequence model isochrones that incorporate an empirical colour-Teff relation and bolometric corrections based on the observed colours of Pleiades members, with theoretical corrections for the dependence on log g. Absolute ages for young, nearby groups are vital as these regions play a crucial role in our understanding of the early evolution of low- and intermediate-mass stars, as well as providing ideal targets for direct imaging and other measurements of dusty debris discs, substellar objects and, of course, extrasolar planets.

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arXiv2014

Pre-main-sequence isochrones -- III. The Cluster Collaboration isochrone server

We present an isochrone server for semi-empirical pre-main-sequence model isochrones in the following systems: Johnson-Cousins, Sloan Digital Sky Survey, Two-Micron All-Sky Survey, Isaac Newton Telescope (INT) Wide-Field Camera, and INT Photometric H$α$ Survey (IPHAS)/UV-Excess Survey (UVEX). The server can be accessed via the Cluster Collaboration webpage {http://www.astro.ex.ac.uk/people/timn/isochrones/}. To achieve this we have used the observed colours of member stars in young clusters with well-established age, distance and reddening to create fiducial loci in the colour-magnitude diagram. These empirical sequences have been used to quantify the discrepancy between the models and data arising from uncertainties in both the interior and atmospheric models, resulting in tables of semi-empirical bolometric corrections (BCs) in the various photometric systems. The model isochrones made available through the server are based on existing stellar interior models coupled with our newly derived semi-empirical BCs. As part of this analysis we also present new cluster parameters for both the Pleiades and Praesepe, yielding ages of $135^{+20}_{-11}$ and $665^{+14}_{-7}\,\rm{Myr}$ as well as distances of $132 \pm 2$ and $184 \pm 2\,\rm{pc}$ respectively (statistical uncertainty only).

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arXiv2014

The Spatial Structure of Young Stellar Clusters. I. Subclusters

The clusters of young stars in massive star-forming regions show a wide range of sizes, morphologies, and numbers of stars. Their highly subclustered structures are revealed by the MYStIX project's sample of 31,754 young stars in nearby sites of star formation (regions at distances <3.6 kpc that contain at least one O-type star.) In 17 of the regions surveyed by MYStIX, we identify subclusters of young stars using finite mixture models -- collections of isothermal ellipsoids that model individual subclusters. Maximum likelihood estimation is used to estimate the model parameters, and the Akaike Information Criterion is used to determine the number of subclusters. This procedure often successfully finds famous subclusters, such as the BN/KL complex behind the Orion Nebula Cluster and the KW-object complex in M17. A catalog of 142 subclusters is presented, with 1 to 20 subclusters per region. The subcluster core radius distribution for this sample is peaked at 0.17 pc with a standard deviation of 0.43 dex, and subcluster core radius is negatively correlated with gas/dust absorption of the stars -- a possible age effect. Based on the morphological arrangements of subclusters, we identify four classes of spatial structure: long chains of subclusters, clumpy structures, isolated clusters with a core-halo structure, and isolated clusters well fit by a single isothermal ellipsoid.

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arXiv2014

Age Gradients in the Stellar Populations of Massive Star Forming Regions Based on a New Stellar Chronometer

A major impediment to understanding star formation in massive star forming regions (MSFRs) is the absence of a reliable stellar chronometer to unravel their complex star formation histories. We present a new estimation of stellar ages using a new method that employs near-infrared (NIR) and X-ray photometry, AgeJX. Stellar masses are derived from X-ray luminosities using the Lx - Mass relation from the Taurus cloud. J-band luminosities are compared to mass-dependent pre-main-sequence evolutionary models to estimate ages. AgeJX is sensitive to a wide range of evolutionary stages, from disk-bearing stars embedded in a cloud to widely dispersed older pre-main sequence stars. The MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray) project characterizes 20 OB-dominated MSFRs using X-ray, mid-infrared, and NIR catalogs. The AgeJX method has been applied to 5525 out of 31,784 MYStIX Probable Complex Members. We provide a homogeneous set of median ages for over a hundred subclusters in 15 MSFRs; median subcluster ages range between 0.5 Myr and 5 Myr. The important science result is the discovery of age gradients across MYStIX regions. The wide MSFR age distribution appears as spatially segregated structures with different ages. The AgeJX ages are youngest in obscured locations in molecular clouds, intermediate in revealed stellar clusters, and oldest in distributed populations. The NIR color index J-H, a surrogate measure of extinction, can serve as an approximate age predictor for young embedded clusters.

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arXiv2014

An Anomalous Extinction Law in the Cep OB3b Young Cluster: Evidence for dust processing during gas dispersal

We determine the extinction law through Cep OB3b, a young cluster of 3000 stars undergoing gas dispersal. The extinction is measured toward 76 background K giants identified with MMT/Hectospec spectra. Color excess ratios were determined toward each of the giants using $V$ and $R$ photometry from the literature, $g$,$r$,$i$ and $z$ photometry from SDSS and $J$, $H$, and $K_{s}$ photometry from 2MASS. These color excess ratios were the used to construct the extinction law through the dusty material associated with Cep OB3b. The extinction law through Cep OB3b is intermediate between the $R_{V} = 3.1$ and $R_{V} = 5$ laws commonly used for the diffuse atomic ISM and dense molecular clouds, respectively. The dependence of the extinction law on line-of-sight $A_{V}$ is investigated and we find the extinction law becomes shallower for regions with $A_{V} > 2.5$ magnitudes. We speculate that the intermediate dust law results from dust processing during the dispersal of the molecular cloud by the cluster.

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arXiv2013

Ages of young stars

Determining the sequence of events in the formation of stars and planetary systems and their time-scales is essential for understanding those processes, yet establishing ages is fundamentally difficult because we lack direct indicators. In this review we discuss the age challenge for young stars, specifically those less than ~100 Myr old. Most age determination methods that we discuss are primarily applicable to groups of stars but can be used to estimate the age of individual objects. A reliable age scale is established above 20 Myr from measurement of the Lithium Depletion Boundary (LDB) in young clusters, and consistency is shown between these ages and those from the upper main sequence and the main sequence turn-off -- if modest core convection and rotation is included in the models of higher-mass stars. Other available methods for age estimation include the kinematics of young groups, placing stars in Hertzsprung-Russell diagrams, pulsations and seismology, surface gravity measurement, rotation and activity, and lithium abundance. We review each of these methods and present known strengths and weaknesses. Below ~20 Myr, both model-dependent and observational uncertainties grow, the situation is confused by the possibility of age spreads, and no reliable absolute ages yet exist. The lack of absolute age calibration below 20 Myr should be borne in mind when considering the lifetimes of protostellar phases and circumstellar material.

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arXiv2013

Overview of the Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) Project

MYStIX (Massive Young Star-Forming Complex Study in Infrared and Xray) 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 (MPCMs) with several hundred OB stars and > 30, 000 low mass premain sequence 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 HII regions, the survivability of protoplanetary disks in HII regions. This paper give 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.

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