Melissa Ness

Flatiron Health (United States)

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K. C. Freeman

Australian National University

M. Asplund

Australian Academy of Science

Sven Buder

ARC Centre of Excellence for All-sky Astrophysics

Joss Bland‐Hawthorn

University of Sydney

Geraint F. Lewis

University of Sydney

Andrew R. Casey

Monash University

Sarah L. Martell

ARC Centre of Excellence for All-sky Astrophysics

K. Lind

Medical University of Graz

David M. Nataf

Johns Hopkins University

D. B. Zucker

Macquarie University

Cooperative Institutions

Max Planck Society

108

Max Planck Institute for Astronomy

101

Australian National University

ARC Centre of Excellence for All-sky Astrophysics

Centre National de la Recherche Scientifique

ASTRO-3D

The Ohio State University

Instituto de Astrofísica de Canarias

Mount Stromlo Observatory

University of Virginia

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Red giant masses and ages derived from carbon and nitrogen abundances

Monthly Notices of the Royal Astronomical Society (2016)

Marie Martig M. Fouesneau Hans‐Walter Rix Melissa Ness Szabolcs Mészáros

We show that the masses of red giant stars can be well predicted from their photospheric carbon and nitrogen abundances, in conjunction with their spectroscopic stellar labels log g, Teff, and [Fe/H]. This is qualitatively expected from mass-dependent post-main-sequence evolution. We here establish an empirical relation between these quantities by drawing on 1475 red giants with asteroseismic mass estimates from Kepler that also have spectroscopic labels from Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR12. We assess the accuracy of our model, and find that it predicts stellar masses with fractional rms errors of about 14 per cent (typically 0.2 M⊙). From these masses, we derive ages with rms errors of 40 per cent. This empirical model allows us for the first time to make age determinations (in the range 1–13 Gyr) for vast numbers of giant stars across the Galaxy. We apply our model to ∼52 000 stars in APOGEE DR12, for which no direct mass and age information was previously available. We find that these estimates highlight the vertical age structure of the Milky Way disc, and that the relation of age with [α/M] and metallicity is broadly consistent with established expectations based on detailed studies of the solar neighbourhood.

Red-giant branch

Red giant

Giant star

Stellar mass

10.1093/mnras/stv2830

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Citations (201)

The X-shaped Milky Way bulge in OGLE-III★ photometry and in N-body models

Monthly Notices of the Royal Astronomical Society (2014)

David M. Nataf A. Udalski J. Skowron M. K. Szymański M. Kubiak

We model the split red clump of the Galactic bulge in OGLE-III photometry, and compare the results to predictions from two N-body models. Our analysis yields precise maps of the brightness of the two red clumps, the fraction of stars in the more distant peak, and their combined surface density. We compare the observations to predictions from two N-body models previously used in the literature. Both models correctly predict several features as long as one assumes an angle αBar ≈ 30° between the Galactic bar's major axis and the line of sight to the Galactic Centre. In particular that the fraction of stars in the faint red clump should decrease with increasing longitude. The biggest discrepancies between models and data are in the rate of decline of the combined surface density of red clump stars towards negative longitudes and of the brightness difference between the two red clumps towards positive longitudes, with neither discrepancy exceeding ∼25 per cent in amplitude. Our analysis of the red giant luminosity function also yields an estimate of the red giant branch bump parameters towards these high-latitude fields, and evidence for a high rate (∼25 per cent) of disc contamination in the bulge at the colour and magnitude of the red clump, with the disc contamination rate increasing towards sightlines further distant from the plane.

Red clump

Red giant

Star count

Red-giant branch

Galactic plane

Giant star

10.1093/mnras/stu2497

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Citations (44)

The GALAH survey: chemical tagging of star clusters and new members in the Pleiades

Monthly Notices of the Royal Astronomical Society (2017)

Janez Kos Joss Bland‐Hawthorn K. C. Freeman Sven Buder G. Traven

The technique of chemical tagging uses the elemental abundances of stellar atmospheres to 'reconstruct' chemically homogeneous star clusters that have long since dispersed. The GALAH spectroscopic survey – which aims to observe one million stars using the Anglo-Australian Telescope – allows us to measure up to 30 elements or dimensions in the stellar chemical abundance space, many of which are not independent. How to find clustering reliably in a noisy high-dimensional space is a difficult problem that remains largely unsolved. Here, we explore t-distributed stochastic neighbour embedding (t-SNE) – which identifies an optimal mapping of a high-dimensional space into fewer dimensions – whilst conserving the original clustering information. Typically, the projection is made to a 2D space to aid recognition of clusters by eye. We show that this method is a reliable tool for chemical tagging because it can: (i) resolve clustering in chemical space alone, (ii) recover known open and globular clusters with high efficiency and low contamination, and (iii) relate field stars to known clusters. t-SNE also provides a useful visualization of a high-dimensional space. We demonstrate the method on a data set of 13 abundances measured in the spectra of 187 000 stars by the GALAH survey. We recover seven of the nine observed clusters (six globular and three open clusters) in chemical space with minimal contamination from field stars and low numbers of outliers. With chemical tagging, we also identify two Pleiades supercluster members (which we confirm kinematically), one as far as 6° – one tidal radius away from the cluster centre.

Pleiades

Supercluster (genetic)

10.1093/mnras/stx2637

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Citations (43)

Soviet Material on Hydroacoustics

Georgina Gordon Melissa Ness Stuart G. Hibben Lee Boylan

Abstract : This is a collection of abstracts from recent Soviet publications in the field of hydroacoustics. The bulk of the material is from the proceedings of the Fifth All-Union Training Seminar in Statistical Hydroacoustics, held in Sukhumi on the Black Sea in 1973. An extended review is also include of a recent Soviet article describing in detail the organization and operation of the US/ NATO submarine surveillance network. Finally, a reference list of other articles and monographs appearing in the past two years and dealing with aspects of hydroacoustics is appended.

Black Sea

10.21236/ada019647

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Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way’s quiescent merger history and in-situ bulge

Monthly Notices of the Royal Astronomical Society (2020)

Francesca Fragkoudi Robert J. J. Grand Rüdiger Pakmor Guillermo Blázquez-Calero Ignacio D. Gargiulo

ABSTRACT We explore the chemodynamical properties of a sample of barred galaxies in the Auriga magnetohydrodynamical cosmological zoom-in simulations, which form boxy/peanut (b/p) bulges, and compare these to the Milky Way (MW). We show that the Auriga galaxies which best reproduce the chemodynamical properties of stellar populations in the MW bulge have quiescent merger histories since redshift z ∼ 3.5: their last major merger occurs at $t_{\rm lookback}\gt 12\, \rm Gyr$, while subsequent mergers have a stellar mass ratio of ≤1:20, suggesting an upper limit of a few per cent for the mass ratio of the recently proposed Gaia Sausage/Enceladus merger. These Auriga MW-analogues have a negligible fraction of ex-situ stars in the b/p region ($\lt 1{{\ \rm per\ cent}}$), with flattened, thick disc-like metal-poor stellar populations. The average fraction of ex-situ stars in the central regions of all Auriga galaxies with b/p’s is 3 per cent – significantly lower than in those which do not host a b/p or a bar. While the central regions of these barred galaxies contain the oldest populations, they also have stars younger than 5 Gyr (>30 per cent) and exhibit X-shaped age and abundance distributions. Examining the discs in our sample, we find that in some cases a star-forming ring forms around the bar, which alters the metallicity of the inner regions of the galaxy. Further out in the disc, bar-induced resonances lead to metal-rich ridges in the Vϕ − r plane – the longest of which is due to the Outer Lindblad Resonance. Our results suggest the Milky Way has an uncommonly quiet merger history, which leads to an essentially in-situ bulge, and highlight the significant effects the bar can have on the surrounding disc.

10.1093/mnras/staa1104

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Citations (104)

Monthly Notices of the Royal Astronomical Society (2022)

Yuxi Lu Melissa Ness Tobias Buck Joel Zinn Kathryn V. Johnston

ABSTRACT The detailed age-chemical abundance relations of stars measure time-dependent chemical evolution. These trends offer strong empirical constraints on nucleosynthetic processes, as well as the homogeneity of star-forming gas. Characterizing chemical abundances of stars across the Milky Way over time has been made possible very recently, thanks to surveys like Gaia, APOGEE, and Kepler. Studies of the low-α disc have shown that individual elements have unique age–abundance trends and the intrinsic dispersion around these relations is small. In this study, we examine and compare the age distribution of stars across both the high and low-α disc and quantify the intrinsic dispersion of 16 elements around their age–abundance relations at [Fe/H] = 0 using APOGEE DR16. We examine the age–metallicity relation and visualize the temporal and spatial distribution of disc stars in small chemical cells. We find: (1) the high-α disc has shallower age–abundance relations compared to the low-α disc, but similar median intrinsic dispersions of ∼0.03 dex; (2) turnover points in the age-[Fe/H] relations across radius for both the high- and low-α disc. The former constrains the mechanisms that set similar intrinsic dispersions, regardless of differences in the enrichment history, for stars in both disc, and the latter indicates the presence of radial migration in both disc. Our study is accompanied by an age catalogue for 64 317 stars in APOGEE derived using the cannon with a median uncertainty of 1.5 Gyr (26 per cent; APO-CAN stars), and a red clump catalogue of 22 031 stars with a contamination rate of 2.7 per cent.

10.1093/mnras/stac610

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Citations (11)

Neutron-capture elements record the ordered chemical evolution of the disc over time

Monthly Notices of the Royal Astronomical Society (2022)

Danny Horta Melissa Ness J. Rybizki Ricardo P. Schiavon Sven Buder

An ensemble of chemical abundances probing different nucleosynthetic channels can be leveraged to build a comprehensive understanding of the chemical and structural evolution of the Galaxy. Using GALAH DR3 data, we seek to trace the enrichment by the supernovae Ia, supernovae II, asymptotic giant branch stars, and neutron-star mergers and/or collapsars nucleosynthetic sources by studying the [Fe/H], [$\alpha$/Fe], [Ba/Fe], and [Eu/Fe] chemical compositions of $\sim$50,000 red giant stars, respectively. Employing small [Fe/H]-[$\alpha$/Fe] cells, which serve as an effective reference-frame of supernovae contributions, we characterise the abundance-age profiles for [Ba/Fe] and [Eu/Fe]. Our results disclose that these age-abundance relations vary across the [Fe/H]-[$\alpha$/Fe] plane. Within cells, we find negative age-[Ba/Fe] relations and flat age-[Eu/Fe] relations. Across cells, we see the slope of the age-[Ba/Fe] relations evolve smoothly and the [Eu/Fe] relations vary in amplitude. We subsequently model our empirical findings in a theoretical setting using the flexible Chempy Galactic chemical evolution (GCE) code, using the mean [Fe/H], [Mg/Fe], [Ba/Fe], and age values for stellar populations binned in [Fe/H], [Mg/Fe], and age space. We find that within a one-zone framework, an ensemble of GCE model parameters vary to explain the data. Using present day orbits from \textit{Gaia} EDR3 measurements we infer that the GCE model parameters, which set the observed chemical abundance distributions, vary systematically across mean orbital radii. Under our modelling assumptions, the observed chemical abundances are consistent with a small gradient in the high mass end of the initial mass function (IMF) across the disc, where the IMF is more top heavy towards the inner disc and more bottom heavy in the outer disc.

Chemical evolution

10.1093/mnras/stac953

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The imprint of clump formation at high redshift – I. A disc α-abundance dichotomy

Monthly Notices of the Royal Astronomical Society (2019)

Adam J. Clarke Victor P. Debattista David L. Nidever Sarah Loebman Raymond C. Simons

The disc structure of the Milky Way is marked by a chemical dichotomy, with high-alpha and low-alpha abundance sequences, traditionally identified with the geometric thick and thin discs. This identification is aided by the old ages of the high-alpha stars, and lower average ages of the low-alpha ones. Recent large scale surveys such as APOGEE have provided a wealth of data on this chemical structure, including showing that an identification of chemical and geometric thick discs is not exact, but the origin of the chemical dichotomy has remained unclear. Here we demonstrate that a dichotomy arises naturally if the early gas-rich disc fragments, leading to some fraction of the star formation occurring in clumps of the type observed in high-redshift galaxies. These clumps have high star formation rate density. They, therefore, enrich rapidly, moving from the low-alpha to the high-alpha sequence, while more distributed star formation produces the low-alpha sequence. We demonstrate that this model produces a chemically-defined thick disc that has many of the properties of the Milky Way's thick disc. Because clump formation is common in high redshift galaxies, we predict that chemical bimodalities are common in massive galaxies.

10.1093/mnras/stz104

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Citations (91)

Chemical tagging with APOGEE: discovery of a large population of N-rich stars in the inner Galaxy

Monthly Notices of the Royal Astronomical Society (2016)

Ricardo P. Schiavon O. Zamora R. Carrera S. Lucatello A. C. Robin

Formation of globular clusters (GCs), the Galactic bulge, or galaxy bulges in general, are important unsolved problems in Galactic astronomy. Homogeneous infrared observations of large samples of stars belonging to GCs and the Galactic bulge field are one of the best ways to study these problems. We report the discovery by APOGEE of a population of field stars in the inner Galaxy with abundances of N, C, and Al that are typically found in GC stars. The newly discovered stars have high [N/Fe], which is correlated with [Al/Fe] and anti-correlated with [C/Fe]. They are homogeneously distributed across, and kinematically indistinguishable from, other field stars in the same volume. Their metallicity distribution is seemingly unimodal, peaking at [Fe/H]~-1, thus being in disagreement with that of the Galactic GC system. Our results can be understood in terms of different scenarios. N-rich stars could be former members of dissolved GCs, in which case the mass in destroyed GCs exceeds that of the surviving GC system by a factor of ~8. In that scenario, the total mass contained in so-called "first-generation" stars cannot be larger than that in "second-generation" stars by more than a factor of ~9 and was certainly smaller. Conversely, our results may imply the absence of a mandatory genetic link between "second generation" stars and GCs. Last, but not least, N-rich stars could be the oldest stars in the Galaxy, the by-products of chemical enrichment by the first stellar generations formed in the heart of the Galaxy.

10.1093/mnras/stw2162

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Citations (175)

The GALAH survey: observational overview andGaiaDR1 companion

Monthly Notices of the Royal Astronomical Society (2016)

Sarah L. Martell Sanjib Sharma Sven Buder Ly Duong Katharine J. Schlesinger

The Galactic Archaeology with HERMES (GALAH) Survey is a massive observational project to trace the Milky Way's history of star formation, chemical enrichment, stellar migration and minor mergers. Using high-resolution (R$\simeq$28,000) spectra taken with the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) instrument at the Anglo-Australian Telescope (AAT), GALAH will determine stellar parameters and abundances of up to 29 elements for up to one million stars. Selecting targets from a colour-unbiased catalogue built from 2MASS, APASS and UCAC4 data, we expect to observe dwarfs at 0.3 to 3 kpc and giants at 1 to 10 kpc. This enables a thorough local chemical inventory of the Galactic thin and thick disks, and also captures smaller samples of the bulge and halo. In this paper we present the plan, process and progress as of early 2016 for GALAH survey observations. In our first two years of survey observing we have accumulated the largest high-quality spectroscopic data set at this resolution, over 200,000 stars. We also present the first public GALAH data catalogue: stellar parameters (Teff, log(g), [Fe/H], [alpha/Fe]), radial velocity, distance modulus and reddening for 10680 observations of 9860 Tycho-2 stars that may be included in the first Gaia data release.

10.1093/mnras/stw2835

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Citations (179)