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Paul J. McMillan

Publications and source records attributed to Paul J. McMillan.

At least 19 recordsLinked to original sources
arXiv2024

The tangled warp of the Milky Way

We determine the influence of the Milky Way's warp on the kinematics of stars across the disc, and therefore measure its precession rate and line of nodes under different assumptions. We do this by applying Jeans' first equation to a model of a rigidly precessing warp. The predictions of these models are fit to the average vertical velocities of stars with measured line-of-sight velocities in Gaia DR3 data. We test models in which the warp's line of nodes and precession speed are fixed, and models in which they are allowed to vary linearly with radius. We also test models in which the velocity of stars radially in the disc is included in Jeans' equation. The kinematic data is best fit by models with a line of nodes that is 40 degrees offset from the Sun's Galactic azimuth, significantly leading the line of nodes found from the positions of stars. These models have a warp precession speed of around 13 km/s/kpc in the direction of Galactic rotation, close to other recent estimates. We find that including the velocity of stars radially in the disc in our kinematic model leads to a significantly worse fit to the data, and implausible warp parameters. We conclude that the Milky Way's warp appears to be rapidly precessing, but the structure and kinematics of the warped disc are not consistent within the approximation of a fixed, precessing, warp shape. This implies that the Milky Way's warp is dynamically evolving, which is a challenge to models of the warp's creation, and must be considered in the context of other known disturbances of the disc.

astro-ph.GA↗
arXiv2022

New stellar velocity substructures from Gaia DR3 proper motions

Local stellar motions are expected, and have been shown, to include signatures of the Galaxy's past dynamical evolution. These are typically divided into the disc, which shows the dynamical effects of spiral arms and the bar, and the stellar halo, with structures thought to be debris from past mergers. We use Gaia Data Release 3 to select large samples of these populations without limiting them to sources with radial velocities. We apply a penalised maximum likelihood method to these samples to determine the full 3D velocity distribution in Cartesian $(U, V, W)$ or spherical $(v_r, v_ϕ, v_θ)$ coordinates. We find that the disc population is dominated by four moving groups and also detect a new moving group at $(U, V) = (-10, -15)$ km s$^{-1}$ which we call MMH-0. For the stellar halo, we isolate the accreted component with cuts in transverse velocity and the colour-magnitude diagram. In this component we find several known structures believed to be caused by past mergers, particularly one around $(v_r, v_ϕ, v_θ) = (-150, -300, -100)$ km s$^{-1}$ appears more prominent than previously claimed. Furthermore we also identify two new structures near $(v_r, v_ϕ, v_θ) = (225, 25, 325)$ km s$^{-1}$ and $(0, 150, -125)$ km s$^{-1}$ which we refer to as MMH-1 and MMH-2 respectively. These results give new insights into local stellar motions and shows the potential of using samples that are not limited to stars with measured line-of-sight velocities, which is key to providing large samples of stars, necessary for future studies.

astro-ph.GA↗
arXiv2022

The disturbed outer Milky Way disc

The outer parts of the Milky Way's disc are significantly out of equilibrium. Using only distances and proper motions of stars from Gaia's Early Data Release 3, in the range |b|<10°, 130°<l<230°, we show that for stars in the disc between around 10 and 14 kpc from the Galactic centre, vertical velocity is strongly dependent on the angular momentum, azimuth, and position above or below the Galactic plane. We further show how this behaviour translates into a bimodality in the velocity distribution of stars in the outer Milky Way disc. We use an N-body model of an impulse-like interaction of the Milky Way disc with a perturber similar to the Sagittarius dwarf to demonstrate that this mechanism can generate a similar disturbance. It has already been shown that this interaction can produce a phase spiral similar to that seen in the Solar neighbourhood. We argue that the details of this substructure in the outer galaxy will be highly sensitive to the timing of the perturbation or the gravitational potential of the Galaxy, and therefore may be key to disentangling the history and structure of the Milky Way.

astro-ph.GA↗
arXiv2022

The velocity distribution of white dwarfs in Gaia EDR3

Using a penalised maximum likelihood we estimate, for the first time, the velocity distribution of white dwarfs in the Solar neighbourhood. Our sample consists of 129 675 white dwarfs within 500 pc in Gaia Early Data Release 3 The white dwarf velocity distributions reveal a similar structure to the rest of the Solar neighbourhood stars, reflecting that white dwarfs are subjected to the same dynamical processes. In the velocity distribution for three magnitude-binned subsamples we however find a novel structure at $(U, V) = (7, -19)$ km s$^{-1}$ in fainter samples, potentially related to the Coma Berenices stream. We also see a double-peaked feature in $U$-$W$ at $U \approx -30$ km s$^{-1}$ and in $V$-$W$ at $V \approx -20$ km s$^{-1}$ for fainter samples. We determine the velocity distribution and velocity moments as a function of absolute magnitude for two samples based on the bifurcation identified in Gaia Data Release 2 in the colour-magnitude diagram. The brighter, redder sequence has a larger velocity dispersion than the fainter, bluer sequence across all magnitudes. It is hard to reconcile this kinematic difference with a bifurcation caused purely by atmospheric composition, while it fits neatly with a significant age difference between the two sequences. Our results provide novel insights into the kinematic properties of white dwarfs and demonstrate the power of analytical techniques that work for the large fraction of stars that do not have measured radial velocities in the current era of large-scale astrometric surveys.

astro-ph.SR↗
arXiv2020

The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- II: Stellar Atmospheric Parameters, Chemical Abundances and Distances

We present part 2 of the 6th and final Data Release (DR6 or FDR) of the Radial Velocity Experiment (RAVE), a magnitude-limited (9<I<12) spectroscopic survey of Galactic stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra (R~7500) cover the Ca-triplet region (8410-8795A) and span the complete time frame from the start of RAVE observations on 12 April 2003 to their completion on 4 April 2013. In the second of two publications, we present the data products derived from 518387 observations of 451783 unique stars using a suite of advanced reduction pipelines focussing on stellar atmospheric parameters, in particular purely spectroscopically derived stellar atmospheric parameters (Teff, log(g), and the overall metallicity), enhanced stellar atmospheric parameters inferred via a Bayesian pipeline using Gaia DR2 astrometric priors, and asteroseismically calibrated stellar atmospheric parameters for giant stars based on asteroseismic observations for 699 K2 stars. In addition, we provide abundances of the elements Fe, Al, and Ni, as well as an overall [alpha/Fe] ratio obtained using a new pipeline based on the GAUGUIN optimization method that is able to deal with variable signal-to-noise ratios. The RAVE DR6 catalogs are cross matched with relevant astrometric and photometric catalogs, and are complemented by orbital parameters and effective temperatures based on the infrared flux method. The data can be accessed via the RAVE Web site (http://rave-survey.org) or the Vizier database.

astro-ph.SR↗
arXiv2020

The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- I: Survey Description, Spectra and Radial Velocities

The Radial Velocity Experiment (RAVE) is a magnitude-limited (9<I<12) spectroscopic survey of Galactic stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra (R~7500) cover the Ca-triplet region (8410-8795A). The 6th and final data release (DR6 or FDR) is based on 518387 observations of 451783 unique stars. RAVE observations were taken between 12 April 2003 and 4 April 2013. Here we present the genesis, setup and data reduction of RAVE as well as wavelength-calibrated and flux-normalized spectra and error spectra for all observations in RAVE DR6. Furthermore, we present derived spectral classification and radial velocities for the RAVE targets, complemented by cross matches with Gaia DR2 and other relevant catalogs. A comparison between internal error estimates, variances derived from stars with more than one observing epoch and a comparison with radial velocities of Gaia DR2 reveals consistently that 68% of the objects have a velocity accuracy better than 1.4 km/s, while 95% of the objects have radial velocities better than 4.0 km/s. Stellar atmospheric parameters, abundances and distances are presented in subsequent publication. The data can be accessed via the RAVE Web (http://rave-survey.org) or the Vizier database.

astro-ph.SR↗
arXiv2019

Voyage 2050 White Paper: All-Sky Visible and Near Infrared Space Astrometry

A new all-sky visible and Near-InfraRed (NIR) space astrometry mission with a wavelength cutoff in the K-band is not just focused on a single or small number of key science cases. Instead, it is extremely broad, answering key science questions in nearly every branch of astronomy while also providing a dense and accurate visible-NIR reference frame needed for future astronomy facilities. For almost 2 billion common stars the combination of Gaia and a new all-sky NIR astrometry mission would provide much improved proper motions, answering key science questions -- from the solar system and stellar systems, including exoplanet systems, to compact galaxies, quasars, neutron stars, binaries and dark matter substructures. The addition of NIR will result in up to 8 billion newly measured stars in some of the most obscured parts of our Galaxy, and crucially reveal the very heart of the Galactic bulge region. In this white paper we argue that rather than improving on the accuracy, a greater overall science return can be achieved by going deeper than Gaia and by expanding the wavelength range to the NIR.

astro-ph.IM↗
arXiv2018

Improved distances and ages for stars common to TGAS and RAVE

We combine parallaxes from the first Gaia data release with the spectrophotometric distance estimation framework for stars in the fifth RAVE survey data release. The combined distance estimates are more accurate than either determination in isolation - uncertainties are on average two times smaller than for RAVE-only distances (three times smaller for dwarfs), and 1.4 times smaller than TGAS parallax uncertainties (two times smaller for giants). We are also able to compare the estimates from spectrophotometry to those from Gaia, and use this to assess the reliability of both catalogues and improve our distance estimates. We find that the distances to the lowest log g stars are, on average, overestimated and caution that they may not be reliable. We also find that it is likely that the Gaia random uncertainties are smaller than the reported values. As a byproduct we derive ages for the RAVE stars, many with relative uncertainties less than 20 percent. These results for 219566 RAVE sources have been made publicly available, and we encourage their use for studies that combine the radial velocities provided by RAVE with the proper motions provided by Gaia. A sample that we believe to be reliable can be found by taking only the stars with the flag notification flag_any=0

astro-ph.GA↗
arXiv2016

The mass distribution and gravitational potential of the Milky Way

We present mass models of the Milky Way created to fit observational constraints and to be consistent with expectations from theoretical modelling. The method used to create these models is that demonstrated in McMillan (2011), and we improve on those models by adding gas discs to the potential, considering the effects of allowing the inner slope of the halo density profile to vary, and including new observations of maser sources in the Milky Way amongst the new constraints. We provide a best fitting model, as well as estimates of the properties of the Milky Way. Under the assumptions in our main model, we find that the Sun is $R_0 = (8.20\pm0.09)\,\mathrm{kpc}$ from the Galactic Centre, with the circular speed at the Sun being $v_0 = (232.8\pm3.0)\,\mathrm{km}\,\mathrm{s}^{-1}$; that the Galaxy has a total stellar mass of $(54.3\pm5.7)\times10^9\,{\rm M}_\odot$, a total virial mass of $(1.30 \pm 0.30)\times10^{12}\,{\rm M}_\odot$ and a local dark-matter density of $0.38\pm0.04\,\mathrm{GeV\,cm}^{-3}$, where the quoted uncertainties are statistical. These values are sensitive to our choice of priors and constraints. We investigate systematic uncertainties, which in some cases may be larger. For example, if we weaken our prior on $R_0$, we find it to be $(7.97\pm0.15)\,\mathrm{kpc}$ and that $v_0=(226.8\pm4.2)\,\mathrm{km}\,\mathrm{s}^{-1}$. We find that most of these properties, including the local dark-matter density, are remarkably insensitive to the assumed power-law density slope at the centre of the dark-matter halo. We find that it is unlikely that the local standard of rest differs significantly from that found under assumptions of axisymmetry. We have made code to compute the force from our potential, and to integrate orbits within it, publicly available.

astro-ph.GA↗
arXiv2016

Chemical separation of disc components using RAVE

We present evidence from the RAdial Velocity Experiment (RAVE) survey of chemically separated, kinematically distinct disc components in the solar neighbourhood. We apply probabilistic chemical selection criteria to separate our sample into $α$-low (`thin disc') and $α$-high (`thick disc') sequences. Using newly derived distances, which will be utilized in the upcoming RAVE DR5, we explore the kinematic trends as a function of metallicity for each of the disc components. For our $α$-low disc, we find a negative trend in the mean rotational velocity ($V_{\mathrmϕ}$) as a function of iron abundance ([Fe/H]). We measure a positive gradient $\partial V_{\mathrmϕ}$/$\partial$[Fe/H] for the $α$-high disc, consistent with results from high-resolution surveys. We also find differences between the $α$-low and $α$-high discs in all three components of velocity dispersion. We discuss the implications of an $α$-low, metal-rich population originating from the inner Galaxy, where the orbits of these stars have been significantly altered by radial mixing mechanisms in order to bring them into the solar neighbourhood. The probabilistic separation we propose can be extended to other data sets for which the accuracy in [$α$/Fe] is not sufficient to disentangle the chemical disc components a priori. For such datasets which will also have significant overlap with Gaia DR1, we can therefore make full use of the improved parallax and proper motion data as it becomes available to investigate kinematic trends in these chemical disc components.

astro-ph.GA↗
arXiv2015

Making action-angle disc models for Gaia

A brief review of recent work. I describe dynamical modelling of the Milky Way using action-angle coordinates. I explain what action-angle coordinates are, and what progress has been made in the past few years to ensuring they can be used in reasonably realistic Galactic potentials. I then describe recent modelling efforts, and progress they have made in constraining the potential of the Milky Way and the local dark matter density.

astro-ph.GA↗
arXiv2013

Analysing surveys of our Galaxy -- II. Determining the potential

We consider the problem of determining the Galaxy's gravitational potential from a star catalogue. We show that orbit-based approaches to this problem suffer from unacceptable numerical noise deriving from the use of only a finite number of orbits. An alternative approach, which requires an ability to determine the model's phase-space density at predetermined positions and velocities, has a level of numerical noise that lies well below the intrinsic uncertainty associated with the finite size of the catalogue analysed. A catalogue of 10000 stars brighter than V=17 and distributed over the sky at b>30 degrees enables us to determine the scaleheight of the disc that contributes to the potential with an uncertainty below 20pc if the catalogue gives proper motions, line-of-sight velocities and parallaxes with errors typical of the Gaia Catalogue, rising to 36pc if only proper motions are available. The uncertainty in the disc's scalelength is significantly smaller than 0.25kpc.

astro-ph.GA↗
arXiv2013

Extending the Hyades

We explore the implications of models of the Hyades moving group in which it has a resonant origin, for regions of the Galaxy beyond the Solar neighbourhood. We show that while models associated with different resonances can produce nearly identical substructure in the local velocity distribution, the velocity distribution away from the Solar neighbourhood has different properties for different models. In particular there is a variation between different models of where in Galactocentric radius the observed Hyades signal in velocity space is strongest, at a given Galactic azimuth. We note, however, that the uncertainties in currently available data, primarily due to uncertain distances to stars, hide these signatures rather effectively, meaning we are not yet able to determine which resonance is the cause of the Hyades.

astro-ph.GA↗
arXiv2011

Analysing surveys of our Galaxy I: basic astrometric data

We consider what is the best way to extract science from large surveys of the Milky Way galaxy. The diversity of data gathered in these surveys, together with our position within the Galaxy, imply that science must be extracted by fitting dynamical models to the data in the space of the observables. Models based on orbital tori promise to be superior for this task than traditional types of models, such as N-body models and Schwarzschild models. A formalism that allows such models to be fitted to data is developed and tested on pseudodata of varying richness.

astro-ph.GA↗
arXiv2011

Dynamical models of the Galaxy

I discuss the importance of dynamical models for exploiting survey data, focusing on the advantages of "torus" models. I summarize a number of applications of these models to the study of the Milky Way, including the determination of the peculiar Solar velocity and investigation of the Hyades moving group.

astro-ph.GA↗
arXiv2011

The Solar neighbourhood in angle coordinates: the Hyades moving group

I investigate the suggestion that the Hyades moving group in the Solar neighbourhood is the result of a recent inner Lindblad resonance. I use dynamical "torus" models of the Galaxy to understand the expected distribution of solar neighbourhood stars in angle coordinates for phase-mixed models and models which include a resonant component. I show that attempts to find the signatures of resonances in angle coordinates are strongly influenced by selection effects, including rather subtle effects associated with the relationship between action and angle for stars at a given point. These effects mean that one can not use simple tests to determine whether substructures seen in the Solar neighbourhood are associated with any given resonance.

astro-ph.GA↗