arXiv subjects

Nicholas J. Wright

Publications and source records attributed to Nicholas J. Wright.

At least 37 records · Page 2Linked to original sources

arXiv2022

The Structure and 3D Kinematics of Vela OB2

The kinematics of stars in OB associations can provide insights into their formation, dynamical evolution, and eventual fate. The low-mass stellar content of OB associations are sufficiently numerous as to provide a detailed sampling of their kinematic properties, however spectroscopy is required to confirm the youth of individual stars and to get 3D kinematics. In this paper we present and analyse results from a large spectroscopic survey of Vela OB2 conducted using 2dF/HERMES on the AAT. This spectroscopy is used to confirm the youth of candidate young stars and determine radial velocities, which are combined with proper motions and parallaxes from Gaia to measure 3-dimensional positions and velocities. We identify multiple separate kinematic groups in the region, for which we measure velocity dispersions and infer their virial states. We measure expansion rates for all these groups and find strong evidence for anisotropic expansion in the Vela OB2 association of at least 11$σ$ significance in all three dimensions, as well as some evidence for expansion in the $γ$ Vel and P Puppis clusters. We trace back the motions of these groups into the past and find that the open cluster NGC 2547 is an interloper in the Vela OB2 region and actually formed $>$100 pc away from the association. We conclude that Vela OB2 must have formed with considerable spatial and kinematic substructure over a timescale of $\sim$10 Myr, with clear temporal substructure within the association, but no clear evidence for an age gradient.

astro-ph.SR↗

arXiv2022

Quantifying kinematic substructure in star-forming regions with statistical tests of spatial autocorrelation

We investigate whether spatial-kinematic substructure in young star-forming regions can be quantified using Moran's $I$ statistic. Its presence in young star clusters would provide an indication that the system formed from initially substructured conditions, as expected by the hierarchical model of star cluster formation, even if the cluster were spatially smooth and centrally concentrated. Its absence, on the other hand, would be evidence that star clusters form monolithically. The Moran's $I$ statistic is applied to $N$-body simulations of star clusters with different primordial spatial-velocity structures, and its evolution over time is studied. It is found that this statistic can be used to reliably quantify spatial-kinematic substructure, and can be used to provide evidence as to whether the spatial-kinematic structure of regions with ages $\lesssim$ 6 Myr is best reproduced by the hierarchical or monolithic models of star formation. Moran's $I$ statistic is also able to conclusively say whether the data are $not$ consistent with initial conditions that lack kinematic substructure, such as the monolithic model, in regions with ages up to, and potentially beyond, 10 Myrs. This can therefore provide a kinematic signature of the star cluster formation process that is observable for many Myr after any initial spatial structure has been erased.

astro-ph.GA↗

arXiv2022

Large-scale expansion of OB stars in Cygnus

The proper motions (PMs) of OB stars in Cygnus have recently been found to exhibit two large-scale kinematic patterns suggestive of expansion. We perform a 3D traceback on these OB stars, the newly-identified OB associations and related open clusters in the region. We find that there are two groups of stars, associations and clusters and that they were each more compact in the past, reaching their closest approach $7.9^{+3.0}_{-1.8}$ and $8.5^{+0.8}_{-2.8}$ Myr ago. We consider two main scenarios for the driver of these large-scale expansion patterns: feedback-driven expansion from a previous generation of massive stars, and expansion as a result of the turbulent velocity field in the primordial molecular cloud. While it is tempting to attribute such large-scale expansion patterns to feedback processes, we find that the observed kinematics are fully consistent with the turbulent origin, and therefore that the injection of further energy or momentum from feedback is not required. Similar conclusions may be drawn for other star forming regions with large-scale expansion patterns.

astro-ph.SR↗

arXiv2022

OB Associations

OB associations are low-density groups of young stars that are dispersing from their birth environment into the Galactic field. They are important for understanding the star formation process, early stellar evolution, the properties and distribution of young stars and the processes by which young stellar groups disperse. Recent observations, particularly from Gaia, have shown that associations are highly complex, with a high degree of spatial, kinematic and temporal substructure. The kinematics of associations have shown them to be globally unbound and expanding, with the majority of recent studies revealing evidence for clear expansion patterns in the association subgroups, suggesting the subgroups were more compact in the past. This expansion is often non-isotropic, arguing against a simple explosive expansion, as predicted by some models of residual gas expulsion. The star formation histories of associations are often complex, exhibit moderate age spreads and temporal substructure, but so far have failed to reveal simple patterns of star formation propagation (e.g., triggering). These results have challenged the historical paradigm of the origin of associations as the expanded remnants of dense star clusters and suggests instead that they originate as highly substructured systems without a linear star formation history, but with multiple clumps of stars that have since expanded and begun to overlap, producing the complex systems we observe today. This has wide-ranging consequences for the early formation environments of most stars and planetary systems, including our own Solar System.

astro-ph.SR↗

arXiv2021

Revisiting the Cygnus OB associations

OB associations play an important role in Galactic evolution, though their origins and dynamics remain poorly studied, with only a small number of systems analysed in detail. In this paper we revisit the existence and membership of the Cygnus OB associations. We find that of the historical OB associations only Cyg OB2 and OB3 stand out as real groups. We search for new OB stars using a combination of photometry, astrometry, evolutionary models and an SED fitting process, identifying 4680 probable OB stars with a reliability of $>$90\%. From this sample we search for OB associations using a new and flexible clustering technique, identifying 6 new OB associations. Two of these are similar to the associations Cyg OB2 and OB3, though the others bear no relationship to any existing systems. We characterize the properties of the new associations, including their velocity dispersions and total stellar masses, all of which are consistent with typical values for OB associations. We search for evidence of expansion and find that all are expanding, albeit anistropically, with stronger and more significant expansion in the direction of Galactic longitude. We also identify two large-scale (160 pc and 25 km s$^{-1}$) kinematic expansion patterns across the Cygnus region, each including three of our new associations, and attribute this to the effects of feedback from a previous generation of stars. This work highlights the need to revisit the existence and membership of the historical OB associations, if they are to be used to study their properties and dynamics.

astro-ph.SR↗

arXiv2020

Performance Assessment of OpenMP Compilers Targeting NVIDIA V100 GPUs

Heterogeneous systems are becoming increasingly prevalent. In order to exploit the rich compute resources of such systems, robust programming models are needed for application developers to seamlessly migrate legacy code from today's systems to tomorrow's. Over the past decade and more, directives have been established as one of the promising paths to tackle programmatic challenges on emerging systems. This work focuses on applying and demonstrating OpenMP offloading directives on five proxy applications. We observe that the performance varies widely from one compiler to the other; a crucial aspect of our work is reporting best practices to application developers who use OpenMP offloading compilers. While some issues can be worked around by the developer, there are other issues that must be reported to the compiler vendors. By restructuring OpenMP offloading directives, we gain an 18x speedup for the su3 proxy application on NERSC's Cori system when using the Clang compiler, and a 15.7x speedup by switching max reductions to add reductions in the laplace mini-app when using the Cray-llvm compiler on Cori.

cs.PF↗

arXiv2020

OB associations and their origins

OB associations are unbound groups of young stars made prominent by their bright OB members, and have long been thought to be the expanded remnants of dense star clusters. They have been important in astrophysics for over a century thanks to their luminous massive stars, though their low-mass members have not been well studied until the last couple of decades. This has changed thanks to data from X-ray observations, spectroscopic surveys and astrometry from Gaia that allows their full stellar content to be identified and their dynamics to be studied, which in turn is leading to changes in our understanding of these systems and their origins, with the old picture of Blaauw (1964) now being superseded. It is clear now that OB associations have considerably more substructure than once envisioned, both spatially, kinematically and temporally. These changes have implications for the star formation process, the formation and evolution of planetary systems, and the build-up of stellar populations across galaxies.

astro-ph.SR↗

arXiv2020

The dynamics of the γ Vel cluster and nearby Vela OB2 association

The kinematics of low-mass stars in nearby OB associations can provide clues about their origins and evolution. Combining the precise positions, proper motions and parallaxes given in the second Gaia Data Release with radial velocity measurements obtained with the Hermes spectrograph at the Anglo-Australian Telescope, we have an opportunity to study in detail the kinematics of low-mass stars belonging to the nearby $γ$ Vel cluster and the Vela OB2 association it is projected against. The presence of lithium is used to confirm the youth of our targets. We separate our sample into the cluster and association populations based on the membership probabilities of Jeffries et al. 2014, their parallaxes, and kinematics. We find strong evidence for expansion in the OB association population with at least 4$σ$ significance along all three axes, though the expansion is notably anisotropic. We discuss these results in the context of cluster and association dispersal theories.

astro-ph.SR↗

arXiv2019

Kinematic signatures of cluster formation from cool collapse in the Lagoon Nebula cluster NGC 6530

We examine the mass-dependence of the velocity dispersion of stars in the young cluster NGC 6530 to better understand how it formed. Using a large sample of members we find that the proper motion velocity dispersion increases with stellar mass. While this trend is the opposite to that predicted if the cluster were developing energy equipartition, it is in agreement with recent N-body simulations that find such a trend develops because of the Spitzer instability. In these simulations the massive stars sink to the centre of the cluster and form a self-gravitating system with a higher velocity dispersion. If the cluster has formed by the cool collapse of an initially substructured distribution then this occurs within 1-2 Myr, in agreement with our observations of NGC 6530. We therefore conclude that NGC 6530 formed from much more extended initial conditions and has since collapsed to form the cluster we see now. This cluster formation model is inconsistent with the idea that all stars form in dense, compact clusters and provides the first dynamical evidence that star clusters can form by hierarchical mergers between subclusters.

astro-ph.GA↗

arXiv2019

Dynamical evolution of star-forming regions: III. Unbound stars and predictions for Gaia

We use $N$-body simulations to probe the early phases of the dynamical evolution of star-forming regions and focus on mass and velocity distributions of unbound stars. In this parameter space study, we vary the initial virial ratio and degree of spatial and kinematic substructure and analyse the fraction of stars that become unbound in two different mass classes (above and below 8 M$_{\odot}$). We find that the fraction of unbound stars differs depending on the initial conditions. After 10 Myr, in initially highly subvirial, substructured simulations, the high-mass and lower-mass unbound fractions are similar at $\sim$23 per cent. In initially virialised, substructured simulations, we find only $\sim$16 per cent of all high-mass stars are unbound, whereas $\sim$37 per cent of all lower-mass stars are. The velocity distributions of unbound stars only show differences for extremely different initial conditions. The distributions are dominated by large numbers of lower-mass stars becoming unbound just above the escape velocity of $\sim$3 km s$^{-1}$ with unbound high-mass stars moving faster on average than lower-mass unbound stars. We see no high-mass runaway stars (velocity > 30 km s$^{-1}$) from any of our initial conditions and only an occasional lower-mass runaway star from initially subvirial/substructured simulations. In our simulations, we find a small number of lower-mass walkaway stars (with velocity 5-30 km s$^{-1}$) from all of our initial conditions. These walkaway stars should be observable around many nearby star-forming regions with Gaia.

astro-ph.SR↗

arXiv2019

The Gaia-ESO Survey: Asymmetric expansion of the Lagoon Nebula cluster NGC 6530 from GES and Gaia DR2

The combination of precise radial velocities from multi-object spectroscopy and highly accurate proper motions from Gaia DR2 opens up the possibility for detailed 3D kinematic studies of young star forming regions and clusters. Here, we perform such an analysis by combining Gaia-ESO Survey spectroscopy with Gaia astrometry for ~900 members of the Lagoon Nebula cluster, NGC 6530. We measure the 3D velocity dispersion of the region to be $5.35^{+0.39}_{-0.34}$~km~s$^{-1}$, which is large enough to suggest the region is gravitationally unbound. The velocity ellipsoid is anisotropic, implying that the region is not sufficiently dynamically evolved to achieve isotropy, though the central part of NGC 6530 does exhibit velocity isotropy that suggests sufficient mixing has occurred in this denser part. We find strong evidence that the stellar population is expanding, though this is preferentially occurring in the declination direction and there is very little evidence for expansion in the right ascension direction. This argues against a simple radial expansion pattern, as predicted by models of residual gas expulsion. We discuss these findings in the context of cluster formation, evolution and disruption theories.

astro-ph.SR↗

arXiv2019

Stellar Astrophysics and Exoplanet Science with the Maunakea Spectroscopic Explorer (MSE)

The Maunakea Spectroscopic Explorer (MSE) is a planned 11.25-m aperture facility with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. A rebirth of the 3.6m Canada-France-Hawaii Telescope on Maunakea, MSE will use 4332 fibers operating at three different resolving powers (R ~ 2500, 6000, 40000) across a wavelength range of 0.36-1.8mum, with dynamical fiber positioning that allows fibers to match the exposure times of individual objects. MSE will enable spectroscopic surveys with unprecedented scale and sensitivity by collecting millions of spectra per year down to limiting magnitudes of g ~ 20-24 mag, with a nominal velocity precision of ~100 m/s in high-resolution mode. This white paper describes science cases for stellar astrophysics and exoplanet science using MSE, including the discovery and atmospheric characterization of exoplanets and substellar objects, stellar physics with star clusters, asteroseismology of solar-like oscillators and opacity-driven pulsators, studies of stellar rotation, activity, and multiplicity, as well as the chemical characterization of AGB and extremely metal-poor stars.

astro-ph.SR↗

arXiv2019

White paper for Chandra cool attitude targets (CAT): Stellar activity with TESS and Chandra

All cool stars show magnetic activity, and X-ray emission is the hallmark of this activity. Gaining an understanding of activity aids us in answering fundamental questions about stellar astrophysics and in determining the impact of activity on the exoplanets that orbit these stars. Stellar activity is driven by magnetic fields, which are ultimately powered by convection and stellar rotation. However, the resulting dynamo properties heavily depend on the stellar interior structure and are far from being understood. X-ray radiation can evaporate exoplanet atmospheres and damage organic materials on the planetary surface, reducing the probability that life can form or be sustained, but also provides an important source of energy for prebiotic chemical reactions. Over the next two years, the TESS mission will deliver a catalog of the closest exoplanets, along with rotation periods and activity diagnostics for millions of stars, whether or not they have a planet. We propose to include all cool stars that are TESS targets and bright enough for Chandra observations, as determined by their detection and flux in the ROSAT all-sky survey (RASS), to the list of Chandra Cool Attitude Targets (CATs). For each target, the signal will be sufficient to fit the coronal plasma with at least two temperature components, and compare abundances of groups of elements with low, medium, or high first ionization potential. Similar to the known relation between X-ray luminosity $L_X$ and rotation period, we can correlate stellar properties with coronal temperatures and abundances to constrain models for stellar activity, coronal heating, and stellar dynamos. Detailed X-ray characterization for even a subset of planet-hosting systems would dramatically advance our knowledge of what impact these emissions have on orbiting planets.

astro-ph.SR↗

arXiv2019

Deep Chandra survey of the Small Magellanic Cloud. III. Formation efficiency of High-Mass X-ray binaries

We have compiled the most complete census of High-Mass X-ray Binaries (HMXBs) in the Small Magellanic Cloud with the aim to investigate the formation efficiency of young accreting binaries in its low metallicity environment. In total, we use 127 X-ray sources with detections in our \chandra X-ray Visionary Program (XVP), supplemented by 14 additional (likely and confirmed) HMXBs identified by \cite{2016A&A...586A..81H} that fall within the XVP area, but are not either detected in our survey (9 sources) or matched with any XVP source that has at least one OB counterpart in the OGLE-III catalog (5 sources). Specifically, we examine the number ratio of the HMXBs [N(HMXBs)] to {\it (a)} the number of OB stars, {\it (b)} the local star-formation rate (SFR), and {\it (c)} the stellar mass produced during the specific star-formation burst, all as a function of the age of their parent stellar populations. Each of these indicators serves a different role, but in all cases we find that the HMXB formation efficiency increases as a function of time (following a burst of star formation) up to $\sim$40--60\,Myr, and then gradually decreases. The peak formation efficiency N(HMXB)/SFR is (49 $\pm$ 14) $[10^{-5}~{\rm M_{\odot}/yr}]^{-1}$, in good agreement with previous estimates of the average formation efficiency in the broad $\sim$20--60\,Myr age range. The frequency of HMXBs is a factor of 8$\times$ higher than at $\sim$10\,Myr, and 4$\times$ higher than at $\sim$260\,Myr, i.e. at earlier and later epochs, respectively.

astro-ph.HE↗

arXiv2018

The Dynamics of OB Associations

The formation and evolution of young star clusters and OB associations is fundamental to our understanding of the star formation process, the conditions faced by young binary and planetary systems, and the formation of long-lived open and globular clusters. Despite this our understanding of the physical processes that drive this evolution has been limited by the static nature of most observations. This is all changing thanks to a revolution in kinematic data quality from large-scale radial velocity surveys and new astrometric facilities such as Gaia. Here I summarise recent studies of multiple OB associations from both {\it Gaia} and ground-based astrometric surveys. These observations show that OB associations have considerable kinematic substructure and no evidence for the radial expansion pattern predicted by theories such as residual gas expulsion. This means that, contrary to the standard view of OB associations as expanded star clusters, these systems could never have been dense star clusters in the past and were most likely born as extended and highly substructured groups of stars. This places strong constraints on the primordial clustering of young stars and the conditions faced by young planetary systems.

astro-ph.SR↗

arXiv2018

Evolution of spatio-kinematic structures in star-forming regions: are Friends of Friends worth knowing?

The Friends of Friends algorithm identifies groups of objects with similar spatial and kinematic properties, and has recently been used extensively to quantify the distributions of gas and stars in young star-forming regions. We apply the Friends of Friends algorithm to $N$-body simulations of the dynamical evolution of subvirial (collapsing) and supervirial (expanding) star-forming regions. We find that the algorithm picks out a wide range of groups (1 -- 25) for statistically identical initial conditions, and cannot distinguish between subvirial and supervirial regions in that we obtain similar mode and median values for the number of groups it identifies. We find no correlation between the number of groups identified initially and either the initial or subsequent spatial and kinematic tracers of the regions' evolution, such as the amount of spatial substructure, dynamical mass segregation, or velocity dispersion. We therefore urge caution in using the Friends of Friends algorithm to quantify the initial conditions of star formation.

astro-ph.GA↗

arXiv2018

The low mass population of the Vela OB2 association from Gaia

The first Gaia Data Release presents an opportunity to characterise the low-mass population of OB associations, providing larger statistical samples and better constraints on the formation and evolution of clusters and associations. Using previously known low mass members in a small region of Vela OB2 we have designed selection criteria that combine Gaia and 2MASS photometry, independently of any astrometric information, to identify low-mass pre-main-sequence (PMS) stars over the wider association area. Our method picks out the known clusters of young stars around $γ^2$ Velorum and NGC-2547, but also identifies other over-densities that may represent previously unknown clusters. There are clear differences in the spatial distributions of the low-mass and the high-mass OB populations, suggesting either that the structure and dynamics of these populations has evolved separately or that the initial mass function can vary considerably on small scales within a single association.

astro-ph.SR↗

arXiv2018

A sextet of clusters in the Vela OB2 region revealed by Gaia

Using Gaia DR2 data, combined with OmegaCAM ground-based optical photometry from the AD-HOC survey, and detailed Radial Velocity measurements from ESO-Gaia, we analyse in detail a 10x5 deg region around the Wolf-Rayet star $γ^2$ Vel, including the previously known clusters Gamma Vel and NGC2547. Using clustering analysis that considers positions, proper motions and parallax, we discover 6 clusters or associations -- 4 of which appear new. Analysis of the colour-magnitude diagram for these clusters show that 4 of them formed coevally from the same molecular clouds 10 Myr ago, while NGC 2547 formed together with a newly discovered cluster 30 Myr ago. This study shows the incredible wealth of data provided by Gaia for the study of young stellar clusters.

astro-ph.SR↗