arXiv subjects

Ken C. Freeman

Publications and source records attributed to Ken C. Freeman.

At least 37 records · Page 2Linked to original sources

arXiv2020

The GALAH Survey: Chemical Clocks

Previous studies have found that the elemental abundances of a star correlate directly with its age and metallicity. Using this knowledge, we derive ages for a sample of 250,000 stars taken from GALAH DR3 using only their overall metallicity and chemical abundances. Stellar ages are estimated via the machine learning algorithm $XGBoost$, using main sequence turnoff stars with precise ages as our input training set. We find that the stellar ages for the bulk of the GALAH DR3 sample are accurate to 1-2 Gyr using this method. With these ages, we replicate many recent results on the age-kinematic trends of the nearby disk, including the age-velocity dispersion relationship of the solar neighborhood and the larger global velocity dispersion relations of the disk found using $Gaia$ and GALAH. The fact that chemical abundances alone can be used to determine a reliable age for a star have profound implications for the future study of the Galaxy as well as upcoming spectroscopic surveys. These results show that the chemical abundance variation at a given birth radius is quite small, and imply that strong chemical tagging of stars directly to birth clusters may prove difficult with our current elemental abundance precision. Our results highlight the need of spectroscopic surveys to deliver precision abundances for as many nucleosynthetic production sites as possible in order to estimate reliable ages for stars directly from their chemical abundances. Applying the methods outlined in this paper opens a new door into studies of the kinematic structure and evolution of the disk, as ages may potentially be estimated for a large fraction of stars in existing spectroscopic surveys. This would yield a sample of millions of stars with reliable age determinations, and allow precise constraints to be put on various kinematic processes in the disk, such as the efficiency and timescales of radial migration.

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arXiv2020

The GALAH Survey: A new constraint on cosmological lithium and Galactic lithium evolution from warm dwarf stars

Lithium depletion and enrichment in the cosmos is not yet well understood. To help tighten constraints on stellar and Galactic evolution models, we present the largest high-resolution analysis of Li abundances A(Li) to date, with results for over 100 000 GALAH field stars spanning effective temperatures $5900\,\mathrm{K} \lesssim \rm{T_{eff}} \lesssim7000\,\mathrm{K}$ and metallicities $-3 \lesssim \rm[Fe/H] \lesssim +0.5$. We separated these stars into two groups, on the warm and cool side of the so-called Li-dip, a localised region of the Kiel diagram wherein lithium is severely depleted. We discovered that stars in these two groups show similar trends in the A(Li)-[Fe/H] plane, but with a roughly constant offset in A(Li) of 0.4 dex, the warm group having higher Li abundances. At $\rm[Fe/H]\gtrsim-0.5$, a significant increasing in Li abundance with increasing metallicity is evident in both groups, signalling the onset of significant Galactic production. At lower metallicity, stars in the cool group sit on the Spite plateau, showing a reduced lithium of around 0.4 dex relative to the primordial value predicted from Big Bang nucleosynthesis (BBN). However, stars in the warm group between [Fe/H] = -1.0 and -0.5, form an elevated plateau that is largely consistent with the BBN prediction. This may indicate that these stars in fact preserve the primordial Li produced in the early Universe.

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arXiv2019

The GALAH Survey: Temporal Chemical Enrichment of the Galactic Disk

We present isochrone ages and initial bulk metallicities ($\rm [Fe/H]_{bulk}$, by accounting for diffusion) of 163,722 stars from the GALAH Data Release 2, mainly composed of main sequence turn-off stars and subgiants ($\rm 7000 K>T_{eff}>4000 K$ and $\rm log g>3$ dex). The local age-metallicity relationship (AMR) is nearly flat but with significant scatter at all ages; the scatter is even higher when considering the observed surface abundances. After correcting for selection effects, the AMR appear to have intrinsic structures indicative of two star formation events, which we speculate are connected to the thin and thick disks in the solar neighborhood. We also present abundance ratio trends for 16 elements as a function of age, across different $\rm [Fe/H]_{bulk}$ bins. In general, we find the trends in terms of [X/Fe] vs age from our far larger sample to be compatible with studies based on small ($\sim$ 100 stars) samples of solar twins but we now extend it to both sub- and super-solar metallicities. The $α$-elements show differing behaviour: the hydrostatic $α$-elements O and Mg show a steady decline with time for all metallicities while the explosive $α$-elements Si, Ca and Ti are nearly constant during the thin disk epoch (ages $\lessapprox $ 12 Gyr). The s-process elements Y and Ba show increasing [X/Fe] with time while the r-process element Eu have the opposite trend, thus favouring a primary production from sources with a short time-delay such as core-collapse supernovae over long-delay events such as neutron star mergers.

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arXiv2019

The GALAH Survey: Chemically tagging the Fimbulthul stream to the globular cluster $ω$ Centauri

Using kinematics from Gaia and the large elemental abundance space of the second data release of the GALAH survey, we identify two new members of the Fimbulthul stellar stream, and chemically tag them to massive, multi-metallic globular cluster $ω$ Centauri. Recent analysis of the second data release of Gaia had revealed the Fimbulthul stellar stream in the halo of the Milky Way. It had been proposed that the stream is associated with the $ω$ Centauri, but this proposition relied exclusively upon the kinematics and metallicities of the stars to make the association. In this work, we find our two new members of the stream to be metal-poor stars that are enhanced in sodium and aluminium, typical of second population globular cluster stars, but not otherwise seen in field stars. Furthermore, the stars share the s-process abundance pattern seen in $ω$ Centauri, which is rare in field stars. Apart from one star within 1.5 deg of $ω$ Centauri, we find no other stars observed by GALAH spatially near $ω$ Centauri or the Fimbulthul stream that could be kinematically and chemically linked to the cluster. Chemically tagging stars in the Fimbulthul stream to $ω$ Centauri confirms the earlier work, and further links this tidal feature in the Milky Way halo to $ω$ Centauri.

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arXiv2019

The GALAH survey and Gaia DR2: Linking ridges, arches and vertical waves in the kinematics of the Milky Way

Gaia DR2 has revealed new small-scale and large-scale patterns in the phase-space distribution of stars in the Milky Way. In cylindrical Galactic coordinates $(R,ϕ,z)$, ridge-like structures can be seen in the \vphiR{} plane and asymmetric arch-like structures in the \vphivR{} plane. We show that the ridges are also clearly present when the third dimension of the \vphiR{} plane is represented by $\langle z \rangle$, $\langle V_z \rangle$, $\langle V_R \rangle$, $\langle$[Fe/H]$\rangle$ and $\langle[α/{\rm Fe}]\rangle$. The maps suggest that stars along the ridges lie preferentially close to the Galactic midplane ($|z|<0.2$ kpc), and have metallicity and $α$ elemental abundance similar to that of the Sun. We show that phase mixing of disrupting spiral arms can generate both the ridges and the arches. It also generates discrete groupings in orbital energy $-$ the ridges and arches are simply surfaces of constant energy. We identify 8 distinct ridges in the \gaia{} data: six of them have constant energy while two have constant angular momentum. Given that the signature is strongest for stars close to the plane, the presence of ridges in $\langle z \rangle$ and $\langle V_z \rangle$ suggests a coupling between planar and vertical directions. We demonstrate, using N-body simulations that such coupling can be generated both in isolated discs and in discs perturbed by an orbiting satellite like the Sagittarius dwarf galaxy.

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arXiv2019

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

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.

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arXiv2018

The GALAH Survey: Verifying abundance trends in the open cluster M67 using non-LTE spectroscopy

Open cluster members are coeval and share the same initial bulk chemical compositions. Consequently, differences in surface abundances between members of a cluster that are at different evolutionary stages can be used to study the effects of mixing and internal chemical processing. We carry out an abundance analysis of seven elements (Li, O, Na, Mg, Al, Si, Fe) in 66 stars belonging to the open cluster M67, based on high resolution GALAH spectra, 1D MARCS model atmospheres, and, for the first time for a large sample of M67 stars, non-local thermodynamic equilibrium (non-LTE) radiative transfer. From the non-LTE analysis, we find a typical star-to-star scatter in the abundance ratios of around 0.05 dex; this scatter is slightly but systematically larger when LTE is assumed instead. We find trends in the abundance ratios with effective temperature, indicating systematic differences in the surface abundances between turn-off and giant stars; these trends are more pronounced when LTE is assumed. However, in the non-LTE analysis, most of the element trends have been flattened. Two species are exceptions to this behaviour, namely Al and Si, which both clearly display remaining trends in the non-LTE analysis. We comment on the possible origin of these trends, by comparing them with recent stellar models that include atomic diffusion.

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arXiv2018

The GALAH survey: Co-orbiting stars and chemical tagging

We present a study using the second data release of the GALAH survey of stellar parameters and elemental abundances of 15 pairs of stars identified by Oh et al 2017. They identified these pairs as potentially co-moving pairs using proper motions and parallaxes from Gaia DR1. We find that 11 very wide (>1.7 pc) pairs of stars do in fact have similar Galactic orbits, while a further four claimed co-moving pairs are not truly co-orbiting. Eight of the 11 co-orbiting pairs have reliable stellar parameters and abundances, and we find that three of those are quite similar in their abundance patterns, while five have significant [Fe/H] differences. For the latter, this indicates that they could be co-orbiting because of the general dynamical coldness of the thin disc, or perhaps resonances induced by the Galaxy, rather than a shared formation site. Stars such as these, wide binaries, debris of past star formation episodes, and coincidental co-orbiters, are crucial for exploring the limits of chemical tagging in the Milky Way.

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arXiv2018

The Origin of the Milky Way's Halo Age Distribution

We present an analysis of the radial age gradients for the stellar halos of five Milky Way mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately -7 to -19 Myr/kpc , shallower than those determined by recent observational studies of the Milky Way's stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values ranging from $-$8 to $-$32~Myr/kpc, closer to those observed in the Galaxy. Two of the accretion-dominated simulated halos show a large concentration of old stars in the center, in agreement with the Ancient Chronographic Sphere reported observationally. The stellar halo that best reproduces the current observed characteristics of the age distributions of the Galaxy is that formed principally by the accretion of small satellite galaxies. Our findings suggest that the hierarchical clustering scenario can reproduce the MW's halo age distribution if the stellar halo was assembled from accretion and disruption of satellite galaxies with dynamical masses less than ~10^9.5M_sun, and a minimal in situ contribution.

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arXiv2018

The GALAH and TESS-HERMES surveys: high-resolution spectroscopy of luminous supergiants in the Magellanic Clouds and Bridge

We report the serendipitous observations of 571 luminous supergiants in the Magellanic Clouds by the spectroscopic GALAH and TESS-HERMES surveys: 434 stars in the Large Magellanic Cloud and 137 in the Small Magellanic Cloud. We also find one star that appears associated with structured star formation in the Magellanic Bridge. Both of these surveys are aimed at the local volume of the Galaxy but have simple, magnitude-limited selection functions that mean they include some observations of luminous extra-Galactic stars. The surveys determine stellar parameter and abundances using The Cannon, a data-driven generative modelling approach. In this work, we explore the results from The Cannon when it is fed the spectra of these intrinsically luminous supergiants in the Magellanic Clouds, which are well outside the normal bounds of The Cannon's training set. We find that, although the parameters are astrophysically incorrect, the $v\sin i$ and the abundances of lithium, barium, and magnesium are excellent discriminants of these stars. It shows that in the future, with an expanded training set, it should be possible to determine accurate values for these types of stars.

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arXiv2016

The GALAH survey: The data reduction pipeline

We present the data reduction procedures being used by the GALAH survey, carried out with the HERMES fibre-fed, multi-object spectrograph on the 3.9~m Anglo-Australian Telescope. GALAH is a unique survey, targeting 1 million stars brighter than magnitude V=14 at a resolution of 28,000 with a goal to measure the abundances of 29 elements. Such a large number of high resolution spectra necessitates the development of a reduction pipeline optimized for speed, accuracy, and consistency. We outline the design and structure of the Iraf-based reduction pipeline that we developed, specifically for GALAH, to produce fully calibrated spectra aimed for subsequent stellar atmospheric parameter estimation. The pipeline takes advantage of existing Iraf routines and other readily available software so as to be simple to maintain, testable and reliable. A radial velocity and stellar atmospheric parameter estimator code is also presented, which is used for further data analysis and yields a useful verification of the reduction quality. We have used this estimator to quantify the data quality of GALAH for fibre cross-talk level ($\lesssim0.5$%) and scattered light ($\sim5$ counts in a typical 20 minutes exposure), resolution across the field, sky spectrum properties, wavelength solution reliability (better than $1$ $\mathrm{km\ s^{-1}}$ accuracy) and radial velocity precision.

astro-ph.IM↗

arXiv2014

Beyond the Brim of the Hat: Kinematics of Globular Clusters out to Large Radius in the Sombrero Galaxy

We have obtained radial velocity measurements for 51 new globular clusters around the Sombrero galaxy. These measurements were obtained using spectroscopic observations from the AAOmega spectrograph on the Anglo-Australian Telescope and the Hydra spectrograph at WIYN. Combined with our own past measurements and velocity measurements obtained from the literature we have constructed a large database of radial velocities that contains a total of 360 confirmed globular clusters. Previous studies' analyses of the kinematics and mass profile of the Sombrero globular cluster system have been constrained to the inner ~9' (~24 kpc or ~5 effective radii), but our new measurements have increased the radial coverage of the data, allowing us to determine the kinematic properties of M104 out to ~15' (~41 kpc or ~9 effective radii). We use our set of radial velocities to study the GC system kinematics and to determine the mass profile and V-band mass-to-light profile of the galaxy. We find that the V-band mass-to-light ratio increases from 4.5 at the center to a value of 20.9 at 41 kpc (~9 effective radii or 15'), which implies that the dark matter halo extends to the edge of our available data set. We compare our mass profile at 20 kpc (~4 effective radii or ~7.4') to the mass computed from x-ray data and find good agreement. We also use our data to look for rotation in the globular cluster system as a whole, as well as in the red and blue subpopulations. We find no evidence for significant rotation in any of these samples.

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arXiv2011

Metal-Poor Lithium-Rich Giants in the Radial Velocity Experiment Survey

We report the discovery of eight lithium-rich field giants found in a high resolution spectroscopic sample of over 700 metal-poor stars ([Fe/H]<-0.5) selected from the RAVE survey. The majority of the Li-rich giants in our sample are very metal-poor ([Fe/H]<-1.9), and have a Li abundance (in the form of 7Li), A(Li)=log(n(Li)/n(H))+12, between 2.30 and 3.63, well above the typical upper red giant branch limit, A(Li)<0.5, while two stars, with A(Li)~1.7-1.8, show similar lithium abundances to normal giants at the same gravity. We further included two metal-poor, Li-rich globular cluster giants in our sample, namely the previously discovered M3-IV101 and newly discovered (in this work) M68-A96. This comprises the largest sample of metal-poor Li-rich giants to date. We performed a detailed abundance analysis of all stars, finding that the majority our sample stars have elemental abundances similar to that of Li-normal halo giants. Although the evolutionary phase of each Li-rich giant cannot be definitively determined, the Li-rich phase is likely connected to extra mixing at the red giant branch bump or early asymptotic giant branch that triggers cool bottom processing in which the bottom of the outer convective envelope is connected to the H-burning shell in the star. The surface of a star becomes Li-enhanced as 7Be (which burns to 7Li) is transported to the stellar surface via the Cameron-Fowler mechanism. We discuss and discriminate among several models for the extra mixing that can cause Li-production, given the detailed abundances of the Li-rich giants in our sample.

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arXiv2011

The Case for the Dual Halo of the Milky Way

Carollo et al. have recently resolved the stellar population of the Milky Way halo into at least two distinct components, an inner halo and an outer halo. This result has been criticized by Schoenrich et al., who claim that the retrograde signature associated with the outer halo is due to the adoption of faulty distances. We refute this claim, and demonstrate that the Schoenrich et al. photometric distances are themselves flawed because they adopted an incorrect main-sequence absolute magnitude relationship from the work of Ivezić et al. When compared to the recommended relation from Ivezić et al., which is tied to a Milky Way globular cluster distance scale and accounts for age and metallicity effects, the relation adopted by Schoenrich et al. yields up to 18% shorter distances for stars near the main-sequence turnoff (TO). Use of the correct relationship yields agreement between the distances assigned by Carollo et al. and Ivezić et al. for low-metallicity dwarfs to within 6-10%. Schoenrich et al. also point out that intermediate-gravity stars (3.5 <= log g <= 4.0) with colors redder than the TO region are likely misclassified, with which we concur. We implement a new procedure to reassign luminosity classifications for the TO stars that require it. New derivations of the rotational behavior demonstrate that the retrograde signature and high velocity dispersion of the outer-halo population remains. We summarize additional lines of evidence for a dual halo, including a test of the retrograde signature based on proper motions alone, and conclude that the preponderance of evidence strongly rejects the single-halo interpretation.

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arXiv2010

Structure and Kinematics of the Stellar Halos and Thick Disks of the Milky Way Based on Calibration Stars from SDSS DR7

The structure and kinematics of the recognized stellar components of the Milky Way are explored, based on well-determined atmospheric parameters and kinematic quantities for 32360 calibration stars from the Sloan Digital Sky Survey (SDSS) and its first extension, (SDSS-II), which included the sub-survey SEGUE: Sloan Extension for Galactic Understanding and Exploration. Full space motions for a sub-sample of 16920 stars, exploring a local volume within 4 kpc of the Sun, are used to derive velocity ellipsoids for the inner- and outer-halo components of the Galaxy, as well as for the canonical thick-disk and proposed metal-weak thick-disk populations. We first examine the question of whether the data require the presence of at least a two-component halo in order to account for the rotational behavior of likely halo stars in the local volume, and whether more than two components are needed. We also address the question of whether the proposed metal-weak thick disk is kinematically and chemically distinct from the canonical thick disk. In addition, we consider the fractions of each component required to understand the nature of the observed kinematic behavior of the stellar populations of the Galaxy as a function of distance from the plane. Scale lengths and scale heights for the thick-disk and metal-weak thick-disk components are determined. Spatial density profiles for the inner- and outer-halo populations are inferred from a Jeans Theorem analysis. The full set of calibration stars (including those outside the local volume) is used to test for the expected changes in the observed stellar metallicity distribution function with distance above the Galactic plane in-situ, due to the changing contributions from the underlying stellar populations. [abridged]

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arXiv2009

The Abundance Gradient in the Extremely Faint Outer Disk of NGC 300

In earlier work, we showed for the first time that the resolved stellar disk of NGC 300 is very extended with no evidence for truncation, a phenomenon that has since been observed in other disk galaxies. We revisit the outer disk of NGC 300 in order to determine the metallicity of the faint stellar population. With the GMOS camera at Gemini South, we reach 50% completeness at (g', i')=(26.8-27.4,26.1-27.0) in photometric conditions and 0.7" seeing. At these faint depths, careful consideration must be given to the background galaxy population. The mean colors of the outer disk stars fall within the spread of colors for the background galaxies, but the stellar density dominates the background galaxies by ~2:1. The predominantly old stellar population in the outer disk exhibits a negative abundance gradient - as expected from models of galaxy evolution - out to about 10 kpc where the abundance trend changes sign. We present two scenarios to explain the flattening, or upturn, in the metallicity gradient of NGC 300 and discuss the implication this has for the broader picture of galaxy formation.

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arXiv2008

The Arcturus Moving Group: Its Place in the Galaxy

The Arcturus moving group is a well-populated example of phase space substructure within the thick disk of our Galaxy. Traditionally regarded as the remains of a dissolved open cluster, it has recently been suggested to be a remnant of a satellite accreted by our Galaxy. To investigate the origin of the group we undertook a high-resolution spectroscopic abundance study of Arcturus group members and candidates. Examining abundance of Fe, Mg, Ca, Ti, Cr, Ni, Zn, Ce, Nd, Sm and Gd for 134 stars we found that the group is chemically similar to disk stars and does not exhibit a clear chemical homogeneity. Furthermore, we confirm the existence of the Arcturus group using the Nordstroem et al. (2004), Schuster et al. (2006) and RAdial Velocity Experiment (RAVE) surveys (Steinmetz et al. 2006). The origin of the group still remains unresolved: the chemical results are consistent with a dynamical origin but do not entirely rule out a merger one. Certainly, the Arcturus group provides a challenge to our understanding of the nature and origin of the Galaxy's thick disk.

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arXiv2006

Virgo's Intracluster Globular Clusters as Seen by the Advanced Camera for Surveys

We report the discovery of 4 candidate intracluster globular clusters (IGCs) in a single deep HST ACS field of the Virgo Cluster. We show that each cluster is roughly spherical, has a magnitude near the peak of the Virgo globular cluster luminosity function, has a radial profile that is best-fit by a King model, and is surrounded by an excess of point sources which have the colors and magnitudes of cluster red giant stars. Despite the fact that two of our IGC candidates have integrated colors redder than the mean of the M87 globular cluster system, we propose that all of the objects are metal-poor with [M/H] < -1. We show that the tidal radii of our intracluster globulars are all larger than the mean for Milky Way clusters, and suggest that the clusters have undergone less tidal stress than their Galactic counterparts. Finally, we normalize our globular cluster observations to the luminosity of intracluster stars, and derive a value of S_N ~ 6 for the specific frequency of Virgo intracluster globular clusters. We use these data to constrain the origins of Virgo's intracluster population, and suggest that globular clusters in our intracluster field have a different origin than globular clusters in the vicinity of M87. In particular, we argue that dwarf elliptical galaxies may be an important source of intracluster stars.

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