arXiv subjects

Paul McMillan

Publications and source records attributed to Paul McMillan.

12 recordsLinked to original sources

arXiv2026

Impact of selection criteria on the structural parameters of the Galactic thin and thick discs

Context: The Milky Way contains a thick and a thin disc that differ in chemical, kinematic, structural, and spatial properties. There is significant overlap in the distributions of these properties, especially so at higher metallicities. Distinguishing between these major structural components is crucial for understanding the formation and evolution of the Galaxy. Multiple selection methods exist to classify stars as thin or thick disc stars, each with its own advantages and limitations. Aims: We investigate how different classification methods for categorising stars into the thick and thin disc populations influence the determination of structural properties of the two discs. Methods: We apply five different selection methods. Two methods use cuts in the [$α$/Fe]-[Fe/H] and [Mg/Mn]-[Al/Fe] planes; one uses a dynamical separation in $J_ϕ$ -$J_Z$ space; one uses an age-based cut; and the last one uses a kinematic likelihood method. For each method, we derive relative density profiles of each component as functions of height above the Galactic plane and Galactocentric radius, and fit these to a simple two-exponential disc model. We use red giant stars from APOGEE DR17 and stellar ages from astroNN. Results: Methods based on abundance or age data produce the cleanest separations, while kinematic and dynamical methods suffer higher contamination due to difficulties in separating well-mixed populations. The thin disc scale heights show a clear flaring as they increase with radius, while the thick disc stays approximately constant at around 1 kpc over most radii for all methods. All methods find the thin disc to have a longer scale length than the thick disc, with the difference being greatest for the chemical selection methods. A scale length of the thick disc of 2.0 kpc leads to one of between 2.3 and 3.0 kpc for the thin disc.

astro-ph.GA↗

arXiv2024

Limitations and rotation of the two-armed phase spiral in the Milky Way stellar disc

Context: The Milky Way's history of recent disturbances is vividly demonstrated by a structure in the vertical phase-space distribution known as the Gaia phase spiral. A one-armed phase spiral has been seen widely across the Milky Way disc, while a two-armed one has only been observed in the solar neighbourhood. Aims: This study aims to determine the properties of the two-armed phase spiral and to put it in a Galactic context, with the ultimate goal of understanding the structure and history of the Milky Way disc. Methods: The Gaia DR3 data is used to trace and characterise the two-armed phase spiral. Special focus is put on the phase spiral's spatial distribution, rotational behaviour, and chemical characteristics. To quantify the properties of the phase spiral we use a model that fits a spiral pattern to the phase space distribution of the stars. Results: We find that the two-armed phase spiral is detectable only within a narrow range of galactocentric distances and angular momenta in the solar neighbourhood, $R = 8 \pm 0.5 $ kpc, $L_Z = 1450 \pm 50$ kpc km s$^{-1}$. Outside this region, the phase spiral is one-armed. The two-armed phase spiral rotates with the phase angle, like the one-armed phase spiral, and changes axis ratio with phase angle. Additionally, stars within the phase-space overdensity caused by the two-armed phase spiral pattern have slightly higher mean metallicity than stars in the underdense regions of the pattern at equivalent galactocentric distances, angular momenta, and vertical orbit extents. Conclusions: The two-armed phase spiral rotates with phase angle and its effect can be seen in metallicity, like the one-armed phase spiral. However, the limited range over which it can be found, and its variation in shape are quite different from the one-armed version, suggesting it is a much more localised phenomenon in the Galactic disc.

astro-ph.GA↗

arXiv2023

Finding the dispersing siblings of young open clusters. Dynamical traceback simulations using Gaia DR3

Context. Stars tend to form in clusters, but many escape their birth clusters very early. Identifying the escaped members of clusters can inform us about the dissolution of star clusters, but also about the stellar dynamics in the galaxy. Methods capable of finding escaped stars from many clusters are required to fully exploit the large amounts of data in the Gaia era. Aims. We present a new method of identifying escaped members of nearby clusters and apply it to ten young clusters. Methods. We assumed the escaped stars were close to the cluster in the past and performed traceback computations based on the Gaia DR3 radial velocity subsample. For each individual star, our method produces a probability estimate that it is an escaped member of a cluster, and for each cluster it also estimates the field star contamination rate of the identified fugitives. Results. Our method is capable of finding fugitives that have escaped from their cluster in the last few ten million years. In many cases the fugitives form an elongated structure that covers a large volume. Conclusions. The results presented here show that traceback computations using Gaia DR3 data can identify stars that have recently escaped their cluster. Our method will be even more useful when applied to future Gaia data releases that contain more radial velocity measurements.

astro-ph.GA↗

arXiv2020

GaiaNIR: Combining optical and Near-Infra-Red (NIR) capabilities with Time-Delay-Integration (TDI) sensors for a future Gaia-like mission

ESA recently called for new "Science Ideas" to be investigated in terms of feasibility and technological developments -- for technologies not yet sufficiently mature. These ideas may in the future become candidates for M or L class missions within the ESA Science Program. With the launch of Gaia in December 2013, Europe entered a new era of space astrometry following in the footsteps of the very successful Hipparcos mission from the early 1990s. Gaia is the successor to Hipparcos, both of which operated in optical wavelengths, and Gaia is two orders of magnitude more accurate in the five astrometric parameters and is surveying four orders of magnitude more stars in a vast volume of the Milky Way. The combination of the Hipparcos/Tycho-2 catalogues with the first early Gaia data release will give improved proper motions over a long ~25 year baseline. The final Gaia solution will also establish a new optical reference frame by means of quasars, by linking the optical counterparts of radio (VLBI) sources defining the orientation of the reference frame, and by using the zero proper motion of quasars to determine a non-rotating frame. A weakness of Gaia is that it only operates at optical wavelengths. However, much of the Galactic centre and the spiral arm regions, important for certain studies, are obscured by interstellar extinction and this makes it difficult for Gaia to deeply probe. Traditionally, this problem is overcome by switching to the infra-red but this was not possible with Gaia's CCDs. Additionally, to scan the entire sky and make global absolute parallax measurements the spacecraft must have a constant rotation and this requires that the CCDs operate in TDI mode, increasing their complexity.

astro-ph.IM↗

arXiv2019

Distances and parallax bias in Gaia DR2

We derive Bayesian distances for all stars in the RV sample of Gaia DR2, and use the statistical method of Schoenrich, Binney & Asplund(2012) to validate the distances and test the Gaia parallaxes. In contrast to other methods, which rely on special sources, our method directly tests the distances to all stars in our sample. We find clear evidence for a near-linear trend of distance bias f with distance s, proving a parallax offset delta p. On average, we find delta p = -0.054 mas (parallaxes in Gaia DR2 need to be increased) when accounting for the parallax uncertainty under-estimate in the Gaia set (delta p = -0.048 mas on the raw parallax errors) with negligible formal error and a systematic uncertainty of about 0.006 mas. The value is in concordance with results from asteroseismic measurements, but differs from the much lower bias found on quasar samples. We further use our method to compile a comprehensive set of quality cuts in colour, apparent magnitude, and astrometric parameters. Last, we find that for this sample delta p appears to strongly depend on the parallax uncertainty sigma_p (when including the additional 0.043 mas) with a statistical confidence far in excess of 10σand a proportionality factor close to 1, though the dependence varies somewhat with sigma_p. Correcting for the sigma_p dependence also resolves otherwise unexplained correlations of the offset with the number of observation periods n_{vis} and ecliptic latitude. Every study using Gaia DR2 parallaxes/distances should investigate the sensitivity of their results on the parallax biases described here and - for fainter samples - in the DR2 astrometry paper.

astro-ph.GA↗

arXiv2018

Spiral arm crossings inferred from ridges in Gaia stellar velocity distributions

The solar neighborhood contains disc stars that have recently crossed spiral arms in the Galaxy. We propose that boundaries in local velocity distributions separate stars that have recently crossed or been perturbed by a particular arm from those that haven't. Ridges in the stellar velocity distributions constructed from the second Gaia data release trace orbits that could have touched nearby spiral arms at apocentre or pericentre. The multiple ridges and arcs seen in local velocity distributions are consistent with the presence of multiple spiral features and different pattern speeds and imply that the outer Galaxy is flocculent rather than grand design.

astro-ph.GA↗

arXiv2018

Correlations between age, kinematics, and chemistry as seen by the RAVE survey

We explore the connections between stellar age, chemistry, and kinematics across a Galactocentric distance of $7.5 < R\,(\mathrm{kpc}) < 9.0$, using a sample of $\sim 12\,000$ intermediate-mass (FGK) turnoff stars observed with the RAdial Velocity Experiment (RAVE) survey. The kinematics of this sample are determined using radial velocity measurements from RAVE, and parallax and proper motion measurements from the Tycho-Gaia Astrometric Solution (TGAS). In addition, ages for RAVE stars are determined using a Bayesian method, taking TGAS parallaxes as a prior. We divide our sample into young ($0 < τ< 3$ Gyr) and old ($8 < τ< 13$ Gyr) populations, and then consider different metallicity bins for each of these age groups. We find significant differences in kinematic trends of young and old, metal-poor and metal-rich, stellar populations. In particular, we find a strong metallicity dependence in the mean Galactocentric radial velocity as a function of radius ($\partial {V_{\rm R}}/\partial R$) for young stars, with metal-rich stars having a much steeper gradient than metal-poor stars. For $\partial {V_ϕ}/\partial R$, young, metal-rich stars significantly lag the LSR with a slightly positive gradient, while metal-poor stars show a negative gradient above the LSR. We interpret these findings as correlations between metallicity and the relative contributions of the non-axisymmetries in the Galactic gravitational potential (the spiral arms and the bar) to perturb stellar orbits.

astro-ph.GA↗

arXiv2017

Understanding inverse metallicity gradients in galactic discs as a consequence of inside-out formation

The early stages of a galaxy's evolution leave an imprint on its metallicity distribution. We discuss the origins and evolution of radial metallicity gradients in discs of spiral galaxies using an analytical chemical evolution model. We explain how radial metallicity gradients in stellar populations are determined by three factors: the radial metallicity profile of the star-forming ISM, radial changes in the star-formation history (in particular inside-out formation), and radial mixing of stars. Under reasonable assumptions, inside-out formation steepens the negative ISM metallicity gradient, but contributes positively to the stellar metallicity gradient, up to inverting the metallicity profile to a positive d[Fe/H]/dR. This reconciles steep negative d[Fe/H]/dR in some high redshift galaxies to generally flatter gradients in local observations. We discuss the evidence for inverse radial metallicity gradients (positive d[X/H]/dR) at high redshifts and the inverse relationship between azimuthal velocity and the metallicity (positive dV_ϕ/d[Fe/H]) of stars for the Milky Way's thick disc. The former can be achieved by high central gas-loss rates and re-distribution processes, e.g. re-accretion of enriched material in conjunction with inside-out formation, and near-disc galactic fountaining. For the Milky Way thick disc, we show that the positive dV_ϕ/d[Fe/H] correlation points to comparable timescales for inside-out formation, initial metal enrichment and SNIa enrichment. We argue that the original ISM metallicity gradient could be inferred with better data from the high-metallicity tail of the alpha enhanced population. Including inside-out formation in our models changes the local vertical metallicity gradient by about -0.2dex/kpc, in line with local measurements.

astro-ph.GA↗

arXiv2017

The selection function of the RAVE survey

We characterize the selection function of RAVE using 2MASS as our underlying population, which we assume represents all stars which could have potentially been observed. We evaluate the completeness fraction as a function of position, magnitude, and color in two ways: first, on a field-by-field basis, and second, in equal-size areas on the sky. Then, we consider the effect of the RAVE stellar parameter pipeline on the final resulting catalogue, which in principle limits the parameter space over which our selection function is valid. Our final selection function is the product of the completeness fraction and the selection function of the pipeline. We then test if the application of the selection function introduces biases in the derived parameters. To do this, we compare a parent mock catalogue generated using Galaxia with a mock-RAVE catalogue where the selection function of RAVE has been applied. We conclude that for stars brighter than I = 12, between $4000 \rm K < T_{\rm eff} < 8000 \rm K$ and $0.5 < \rm{log}\,g < 5.0$, RAVE is kinematically and chemically unbiased with respect to expectations from Galaxia.

astro-ph.GA↗

arXiv2016

The Radial Velocity Experiment (RAVE): Fifth Data Release

Data Release 5 (DR5) of the Radial Velocity Experiment (RAVE) is the fifth data release from a magnitude-limited (9< I < 12) survey of stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra ($R\sim7500$) covering the Ca-triplet region (8410-8795Å) span the complete time frame from the start of RAVE observations in 2003 to their completion in 2013. Radial velocities from 520,781 spectra of 457,588 unique stars are presented, of which 255,922 stellar observations have parallaxes and proper motions from the Tycho-Gaia astrometric solution (TGAS) in Gaia DR1. For our main DR5 catalog, stellar parameters (effective temperature, surface gravity, and overall metallicity) are computed using the RAVE DR4 stellar pipeline, but calibrated using recent K2 Campaign 1 seismic gravities and Gaia benchmark stars, as well as results obtained from high-resolution studies. Also included are temperatures from the Infrared Flux Method, and we provide a catalogue of red giant stars in the dereddened color $(J-Ks)_0$ interval (0.50,0.85) for which the gravities were calibrated based only on seismology. Further data products for sub-samples of the RAVE stars include individual abundances for Mg, Al, Si, Ca, Ti, Fe, and Ni, and distances found using isochrones. Each RAVE spectrum is complemented by an error spectrum, which has been used to determine uncertainties on the parameters. The data can be accessed via the RAVE Web site or the Vizier database.

astro-ph.SR↗

arXiv2015

Comment on "Evidence for dark matter in the inner Milky Way"

This is a brief rebuttal to arXiv:1502.03821, which claims to provide the first observational proof of dark matter interior to the solar circle. We point out that this result is not new, and can be traced back at least a quarter century.

astro-ph.GA↗

arXiv2011

Models of our Galaxy II

Stars near the Sun oscillate both horizontally and vertically. In Paper I the coupling between these motions was modelled by determining the horizontal motion without reference to the vertical motion, and recovering the coupling by assuming that the vertical action is adiabatically conserved as the star oscillates horizontally. Here we show that, although the assumption of adiabatic invariance works well, more accurate results can be obtained by taking the vertical action into account when calculating the horizontal motion. We use orbital tori to present a simple but fairly realistic model of the Galaxy's discs in which the motion of stars is handled rigorously, without decomposing it into horizontal and vertical components. We examine the ability of the adiabatic approximation to calculate the model's observables, and find that it performs perfectly in the plane, but errs slightly away from the plane. When the new correction to the adiabatic approximation is used, the density, streaming velocity and velocity dispersions are in error by less than 10 per cent for distances up to $2.5\kpc$ from the Sun. The torus-based model reveals that at locations above the plane the long axis of the velocity ellipsoid points almost to the Galactic centre, even though the model potential is significantly flattened. This result contradicts the widespread belief that the shape of the Galaxy's potential can be strongly constrained by the orientation of velocity ellipsoid near the Sun. An analysis of orbits reveals that in a general potential the orientation of the velocity ellipsoid depends on the structure of the model's distribution function as much as on its gravitational potential, contrary to what is the case for Staeckel potentials. We argue that the adiabatic approximation will provide a valuable complement to torus-based models in the interpretation of current surveys of the Galaxy.

astro-ph.GA↗