G. Kordopatis

Observatoire de la Côte d’Azur

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Julio F. Navarro

University of Victoria

Else Starkenburg

University of Groningen

Nicolas F. Martin

Max Planck Institute for Astronomy

Federico Sestito

University of Victoria

G. M. Seabroke

University College London

T. Zwitter

University of Ljubljana

V. Hill

University of Pittsburgh

Joss Bland‐Hawthorn

University of Sydney

Anke Arentsen

University of Cambridge

Matthias Steinmetz

Leibniz Institute for Astrophysics Potsdam

Cooperative Institutions

Max Planck Society

113

Centre National de la Recherche Scientifique

112

National Institute for Astrophysics

Max Planck Institute for Astronomy

Leibniz Institute for Astrophysics Potsdam

University of Cambridge

Instituto de Astrofísica de Canarias

University of Groningen

Universidad de La Laguna

Galaxies, Etoiles, Physique et Instrumentation

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The disturbed outer Milky Way disc

Monthly Notices of the Royal Astronomical Society (2022)

P. J. McMillan Jonathan Petersson Thor Tepper-García Joss Bland‐Hawthorn T. Antoja

ABSTRACT 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 $14\, \mathrm{kpc}$ 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.

10.1093/mnras/stac2571

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Citations (18)

Gaia Focused Product Release: Spatial distribution of two diffuse interstellar bands

arXiv (Cornell University) (2023)

M. Schultheis H. Zhao T. Zwitter C. Ordénovic F. Pailler

Diffuse interstellar bands (DIBs) are absorption features seen in optical and infrared spectra of stars that are probably caused by large and complex molecules in the ISM. Here we investigate the Galactic distribution and properties of two DIBs identified in almost six million stellar spectra collected by the Gaia Radial Velocity Spectrometer. These measurements constitute a part of the Gaia Focused Product Release to be made public between the Gaia DR3 and DR4 data releases. In order to isolate the DIB signal from the stellar features in each individual spectrum, we identified a set of 160 000 spectra at high Galactic latitudes which we consider to be the DIB-free reference sample. Matching each target spectrum to its closest reference spectra in stellar parameter space allowed us to remove the stellar spectrum empirically, without reference to stellar models, leaving a set of six million ISM spectra. Identifying the two DIBs at 862.1 nm and 864.8 nm in the stacked spectra, we modelled their shapes and report the depth, central wavelength, width, and equivalent width (EW) for each, along with confidence bounds on these measurements. Our main results are as follows: (1) the strength and spatial distribution of the DIB $\lambda$862.1 are very consistent with what was found in Gaia DR3, but for this work we attained a higher signal-to-noise ratio in the stacked spectra to larger distances, which allowed us to trace DIBs in the outer spiral arm and beyond the Scutum--Centaurus spiral arm; (2) we produced an all-sky map below ${\pm}65^{\circ}$ of Galactic latitude to $\sim$4000 pc of both DIB features and their correlations; (3) we detected the signals of DIB\,$\lambda$862.1 inside the Local Bubble; and (4) there is a reasonable correlation with the dust reddening found from stellar absorption and EWs of both DIBs.

10.48550/arxiv.2310.06175

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The wobbly Galaxy: kinematics north and south with RAVE red-clump giants

Monthly Notices of the Royal Astronomical Society (2013)

M. Williams Matthias Steinmetz James Binney A. Siebert H. Enke

The RAdial Velocity Experiment survey, combined with proper motions and distance estimates, can be used to study in detail stellar kinematics in the extended solar neighbourhood (solar suburb). Using 72 365 red-clump stars, we examine the mean velocity components in 3D between 6 < R < 10 kpc and −2 < Z < 2 kpc, concentrating on north–south differences. Simple parametric fits to the (R, Z) trends for Vϕ and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, VR, finding that the gradient is marked below the plane (δ〈VR〉/δR = −8 km s−1 kpc−1 for Z < 0, vanishing to zero above the plane), with a Z gradient thus also present. The vertical velocity, VZ, also shows clear, large-amplitude (|VZ| = 17 km s−1) structure, with indications of a rarefaction–compression pattern, suggestive of wave-like behaviour. We perform a rigorous error analysis, tracing sources of both systematic and random errors. We confirm the north–south differences in VR and VZ along the line of sight, with the VR estimated independent of the proper motions. The complex three-dimensional structure of velocity space presents challenges for future modelling of the Galactic disc, with the Galactic bar, spiral arms and excitation of wave-like structures all probably playing a role.

Red clump

Galactic plane

Radial velocity

10.1093/mnras/stt1522

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The Pristine survey – XIX. Cu and Zn abundances in metal-poor giants

Monthly Notices of the Royal Astronomical Society (2022)

E. Caffau Linda Lombardo L. Mashonkina T. Sitnova P. Bonifacio

ABSTRACT Metal-poor stars formed from a gas enriched by the ejecta of the explosion of one/few generations of first massive stars. With the Pristine photometry combined with the Gaia data, we selected a sample of bright giants metal-poor candidates to be observed at high resolution. Of the 43 stars observed, 36 were confirmed to be metal-poor, supporting the high success-rate of Pristine in selecting metal-poor stars. We centred the investigation on Cu and Zn, which are elements usually neglected, also because they are the ‘killing elements’ for identifying the Pair Instability Supernovae (PISN) descendants, the explosion of very massive first stars. We derived detailed chemical abundances of 22 species of 18 elements for 36 giant stars. The study of the kinematics of the stars in the sample revealed the presence of metal-poor stars on disc-like orbits (17 per cent of the sample) and even on thin disc orbits (8 per cent of the sample). Four stars are members of the Gaia–Sausage–Enceladus, one star is also likely a member of it. Three stars in the sample (TYC 1118−595−1, TYC 2207−992−1, and TYC 1194−507−1) show a chemical pattern compatible with the one theoretically derived for the PISN descendants, i.e. stars formed out from a gaseous environment enriched by PISN (>50 per cent level) and subsequent generations of normal stars evolving as core-collapse SNe.

10.1093/mnras/stac3372

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TheGaia-ESO Survey: the selection function of the Milky Way field stars

Monthly Notices of the Royal Astronomical Society (2016)

E. Stonkutė S. E. Koposov L. M. Howes S. Feltzing C. C. Worley

The Gaia-ESO Survey was designed to target all major Galactic components (i.e. bulge, thin and thick discs, halo and clusters), with the goal of constraining the chemical and dynamical evolution of the Milky Way. This paper presents the methodology and considerations that drive the selection of the targeted, allocated and successfully observed Milky Way field stars. The detailed understanding of the survey construction, specifically the influence of target selection criteria on observed Milky Way field stars is required in order to analyse and interpret the survey data correctly. We present the target selection process for the Milky Way field stars observed with Very Large Telescope/Fibre Large Array Multi Element Spectrograph and provide the weights that characterize the survey target selection. The weights can be used to account for the selection effects in the Gaia-ESO Survey data for scientific studies. We provide a couple of simple examples to highlight the necessity of including such information in studies of the stellar populations in the Milky Way.

Stellar kinematics

10.1093/mnras/stw1011

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Citations (35)

An artificial neural network to discover hypervelocity stars: candidates in Gaia DR1/TGAS

Monthly Notices of the Royal Astronomical Society (2017)

Tommaso Marchetti Elena M. Rossi G. Kordopatis A. G. A. Brown Alex Rimoldi

The paucity of hypervelocity stars (HVSs) known to date has severely hampered their potential to investigate the stellar population of the Galactic Centre and the Galactic potential. The first Gaia data release (DR1, 2016 September 14) gives an opportunity to increase the current sample. The challenge is the disparity between the expected number of HVSs and that of bound background stars. We have applied a novel data mining algorithm based on machine learning techniques, an artificial neural network, to the Tycho–Gaia astrometric solution catalogue. With no pre-selection of data, we could exclude immediately ∼99 per cent of the stars in the catalogue and find 80 candidates with more than 90 per cent predicted probability to be HVSs, based only on their position, proper motions and parallax. We have cross-checked our findings with other spectroscopic surveys, determining radial velocities for 30 and spectroscopic distances for five candidates. In addition, follow-up observations have been carried out at the Isaac Newton Telescope for 22 stars, for which we obtained radial velocities and distance estimates. We discover 14 stars with a total velocity in the Galactic rest frame >400 km s−1, and five of these have a probability of >50 per cent of being unbound from the Milky Way. Tracing back their orbits in different Galactic potential models, we find one possible unbound HVS with v ∼ 520 km s−1, five bound HVSs and, notably, five runaway stars with median velocity between 400 and 780 km s−1. At the moment, uncertainties in the distance estimates and ages are too large to confirm the nature of our candidates by narrowing down their ejection location, and we wait for future Gaia releases to validate the quality of our sample. This test successfully demonstrates the feasibility of our new data-mining routine.

Parallax

Astrometry

Hypervelocity

10.1093/mnras/stx1304

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Citations (26)

The Pristine Inner Galaxy Survey (PIGS) VI: Different vertical distributions between two DIBs at 442.8 nm and 862.1 nm

arXiv (Cornell University) (2022)

H. Zhao M. Schultheis Anke Arentsen G. Kordopatis M. Fouesneau

Although diffuse interstellar bands (DIBs) were discovered over 100 years ago, for most of them, their origins are still unknown. Investigation on the correlations between different DIBs is an important way to study the behavior and distributions of their carriers. Based on stacking thousands of spectra from the Pristine Inner Galaxy Survey, we study the correlations between two DIBs at 442.8 nm ($λ$442.8) and 862.1 nm ($λ$862.1), as well as the dust grains, in a range of latitude spanning ${\sim}22^{\circ}$ ($4^{\circ}\,{<}\,|b|\,{<}\,15^{\circ}$) toward the Galactic center ($|\ell|\,{<}\,11^{\circ}$). Tight linear intensity correlations can be found between $λ$442.8, $λ$862.1, and dust grains for $|b|\,{<}\,12^{\circ}$ or $E(B-V)\,{>}\,0.3$ mag. For $|b|\,{>}\,12^{\circ}$, $λ$442.8 and $λ$862.1 present larger relative strength with respect to the dust grains. A systematic variation of the relative strength between $λ$442.8 and $λ$862.1 with $|b|$ and $E(B-V)$ concludes that the two DIBs do not share a common carrier. Furthermore, the carrier of $λ$862.1 is more abundant at high latitudes than that of $λ$442.8. This work can be treated as an example showing the significance and potentials to the DIB research covering a large latitude range.

Variation (astronomy)

10.48550/arxiv.2211.15224

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The Pristine survey XXV: The Galactic underdogs -- Dynamic tales of a Milky Way metal-poor population

arXiv (Cornell University) (2024)

Isaure González Rivera de La Vernhe V. Hill G. Kordopatis F. Gran Emma Fernández-Alvar

Through the chemodynamical characterisation of metal-poor stars, one can efficiently probe the early history of the Milky Way. We aim at decontaminating a sample of $\sim$ 3M giant stars with Gaia DR3 XP-based \textit{Pristine-Gaia} metallicities, to investigate a subset of very metal-poor stars ([Fe/H] < -1.7) with disc-like orbits. We construct a statistically robust sample of $\sim$ 36 000 very metal-poor giants, using APOGEE and LAMOST to estimate and remove contamination from high $V_\phi$ stars. We investigate the spatial and kinematic properties of the decontaminated sample, using $V_\phi$ and the action space, both powerful to disentangle stellar populations. As in previous works, we find a pronounced asymmetry in $L_z$ and $V_\phi$ in favour of prograde stars. This excess is mostly made of prograde-planar stars (10% of the very metal-poor population), and contains stars with $V_\phi$ > 180 km s$^{-1}$ and $Z_{\text{max}}$ < 1.5 kpc, down to [Fe/H] = -2.9 at a 2$\sigma$ confidence level. While the overall orbital distributions of our sample match that of a halo, the highly prograde and planar subset (2% of the very metal-poor population) also bears characteristics classically associated with a thick disc, i.e., a spatial distribution compatible with a short-scaled thick disc, and a thick disc-like $Z_{\text{max}}$ - $R_{\text{max}}$ distribution. Additionally, assuming a stationary or prograde halo with $\overline{V_\phi}$ $\sim$ 30-40 km.s$^{-1}$ is not sufficient to suppress the kinematic signature of the highly prograde and planar subset. These results rule out any link with a thin disc, and instead, support a contribution from a \textit{metal-weak thick disc}.

10.48550/arxiv.2406.05728

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Characterizing the high-velocity stars of RAVE: the discovery of a metal-rich halo star born in the Galactic disc

Monthly Notices of the Royal Astronomical Society (2015)

Keith Hawkins G. Kordopatis G. Gilmore T. Masseron R. F. G. Wyse

We aim to characterize high-velocity (HiVel) stars in the solar vicinity both chemically and kinematically using the fourth data release of the RAdial Velocity Experiment (RAVE). We used a sample of 57 HiVel stars with Galactic rest-frame velocities larger than 275 km s$^{-1}$. With 6D position and velocity information, we integrated the orbits of the HiVel stars and found that, on average, they reach out to 13 kpc from the Galactic plane and have relatively eccentric orbits consistent with the Galactic halo. Using the stellar parameters and [$\alpha$/Fe] estimates from RAVE, we found the metallicity distribution of the HiVel stars peak at [M/H] = -1.2 dex and is chemically consistent with the inner halo. There are a few notable exceptions that include a hypervelocity star (HVS) candidate, an extremely high-velocity bound halo star, and one star that is kinematically consistent with the halo but chemically consistent with the disk. High-resolution spectra were obtained for the metal-rich HiVel star candidate and the second highest velocity star in the sample. Using these high-resolution data, we report the discovery of a metal-rich halo star that has likely been dynamically ejected into the halo from the Galactic thick disk. This discovery could aid in explaining the assembly of the most metal-rich component of the Galactic halo.

Thick disk

Galactic plane

Galactic corona

Radial velocity

10.1093/mnras/stu2574

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Citations (54)

The rich are different: evidence from the RAVE survey for stellar radial migration

Monthly Notices of the Royal Astronomical Society (2015)

G. Kordopatis James Binney G. Gilmore Rosemary F. Ġ. Wyse Vasily Belokurov

Using the RAdial Velocity Experiment fourth data release (RAVE DR4), and a new metallicity calibration that will be also taken into account in the future RAVE DR5, we investigate the existence and the properties of super-solar metallicity stars ([M/H] > +0.1 dex) in the sample, and in particular in the Solar neighbourhood. We find that RAVE is rich in super-solar metallicity stars, and that the local metallicity distribution function declines remarkably slowly up to +0.4 dex. Our results show that the kinematics and height distributions of the super-solar metallicity stars are identical to those of the [M/H] < 0 thin-disc giants that we presume were locally manufactured. The eccentricities of the super-solar metallicity stars indicate that half of them are on a roughly circular orbit (e < 0.15), so under the assumption that the metallicity of the interstellar medium at a given radius never decreases with time, they must have increased their angular momenta by scattering at corotation resonances of spiral arms from regions far inside the Solar annulus. The likelihood that a star will migrate radially does not seem to decrease significantly with increasing amplitude of vertical oscillations within range of oscillation amplitudes encountered in the disc.

Solar radius

10.1093/mnras/stu2726

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Citations (78)