arXiv subjects

Sofia Feltzing

Publications and source records attributed to Sofia Feltzing.

At least 37 records · Page 2Linked to original sources

arXiv2019

The COMBS survey I: Chemical Origins of Metal-Poor Stars in the Galactic Bulge

Chemistry and kinematic studies can determine the origins of stellar population across the Milky Way. The metallicity distribution function of the bulge indicates that it comprises multiple populations, the more metal-poor end of which is particularly poorly understood. It is currently unknown if metal-poor bulge stars ([Fe/H] $<$ -1 dex) are part of the stellar halo in the inner most region, or a distinct bulge population or a combination of these. Cosmological simulations also indicate that the metal-poor bulge stars may be the oldest stars in the Galaxy. In this study, we successfully target metal-poor bulge stars selected using SkyMapper photometry. We determine the stellar parameters of 26 stars and their elemental abundances for 22 elements using R$\sim$ 47,000 VLT/UVES spectra and contrast their elemental properties with that of other Galactic stellar populations. We find that the elemental abundances we derive for our metal-poor bulge stars have much lower overall scatter than typically found in the halo. This indicates that these stars may be a distinct population confined to the bulge. If these stars are, alternatively, part of the inner-most distribution of the halo, this indicates that the halo is more chemically homogeneous at small Galactic radii than at large radii. We also find two stars whose chemistry is consistent with second-generation globular cluster stars. This paper is the first part of the Chemical Origins of Metal-poor Bulge Stars (COMBS) survey that will chemo-dynamically characterize the metal-poor bulge population.

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arXiv2018

Estimating stellar ages and metallicities from parallaxes and broadband photometry - successes and shortcomings

A deep understanding of the Milky Way galaxy, its formation and evolution requires observations of huge numbers of stars. Stellar photometry, therefore, provides an economical method to obtain intrinsic stellar parameters. With the addition of distance information - deriving reliable ages from photometry is a possibility. We have developed a Bayesian method that generates 2D probability maps of a star's age and metallicity from photometry and parallax using isochrones. Our synthetic tests show that including a near-UV passband enables us to break the degeneracy between a star's age and metallicity for certain evolutionary stages. It is possible to find well-constrained ages and metallicities for turn-off and sub-giant stars with colours including a U band and a parallax with uncertainty less than ~20%. Metallicities alone are possible for the main sequence and giant branch. We find good agreement with the literature when we apply our method to the Gaia benchmark stars, particularly for turn-off and young stars. Further tests on the old open cluster NGC 188, however, reveal significant limitations in the stellar isochrones. The ages derived for the cluster stars vary with evolutionary stage, such that turn-off ages disagree with those on the sub-giant branch, and metallicities vary significantly throughout. Furthermore, the parameters vary appreciably depending on which colour combinations are used in the derivation. We identify the causes of these mismatches and show that improvements are needed in the modelling of giant branch stars and in the creation and calibration of synthetic near-UV photometry. Our results warn against applying isochrone fitting indiscriminately. In particular, the uncertainty on the stellar models should be quantitatively taken into account. Further efforts to improve the models will result in significant advancements in our ability to study the Galaxy.

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arXiv2018

Benchmark ages for the Gaia benchmark stars

In the era of large-scale surveys of stars in the Milky Way, stellar ages are crucial for studying the evolution of the Galaxy. But determining ages of field stars is notoriously difficult; therefore, we attempt to determine benchmark ages for the extensively studied Gaia benchmark stars which can be used for validation purposes. By searching the literature for age estimates from different methods and deriving new ages based on Bayesian isochrone fitting, we are able to put reliable limits on the ages of 16 out of the 33 benchmark stars. The giants with well-defined ages are all young, and an expansion of the sample to include older giants with asteroseismic ages would be beneficial. Some of the stars have surface parameters inconsistent with isochrones younger than 16 Gyr. Including $α$-enhancement in the models when relevant resolves some of these cases, but others clearly highlight discrepancies between the models and observations. We test the impact of atomic diffusion on the age estimates by fitting to the actual surface metallicity of the models instead of the initial value and find that the effect is negligible except for a single turn-off star. Finally, we show that our ability to determine isochrone-based ages for large spectroscopic surveys largely mirrors our ability to determine ages for these benchmark stars, except for stars with $\log g \gtrsim 4.4$ dex since their location in the HR diagram is almost age insensitive. Hence, isochrone fitting does not constrain their ages given the typical uncertainties of spectroscopic stellar parameters.

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arXiv2018

The ELT-MOS (MOSAIC): towards the construction phase

When combined with the huge collecting area of the ELT, MOSAIC will be the most effective and flexible Multi-Object Spectrograph (MOS) facility in the world, having both a high multiplex and a multi-Integral Field Unit (Multi-IFU) capability. It will be the fastest way to spectroscopically follow-up the faintest sources, probing the reionisation epoch, as well as evaluating the evolution of the dwarf mass function over most of the age of the Universe. MOSAIC will be world-leading in generating an inventory of both the dark matter (from realistic rotation curves with MOAO fed NIR IFUs) and the cool to warm-hot gas phases in z=3.5 galactic haloes (with visible wavelenth IFUs). Galactic archaeology and the first massive black holes are additional targets for which MOSAIC will also be revolutionary. MOAO and accurate sky subtraction with fibres have now been demonstrated on sky, removing all low Technical Readiness Level (TRL) items from the instrument. A prompt implementation of MOSAIC is feasible, and indeed could increase the robustness and reduce risk on the ELT, since it does not require diffraction limited adaptive optics performance. Science programmes and survey strategies are currently being investigated by the Consortium, which is also hoping to welcome a few new partners in the next two years.

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arXiv2018

Stellar multiplicity in high-resolution spectroscopic surveys. I. Application to APOGEE subgiants and giants

Many field stars reside in binaries, and the analysis and interpretation of photometric and spectroscopic surveys must take this into account. We have developed a model to predict how binaries influence the scientific results inferred from large spectroscopic surveys. Based on the rapid binary evolution code BSE, it allows us to model a representative population of binaries and generate synthetic survey observations. We describe this model in detail, and apply it to the radial velocity variation of subgiant and giant stars in the Galactic disc, as observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey III. APOGEE provides an excellent data set for testing our binary models since a large fraction of the stars have been observed multiple times. By comparing our model to the APOGEE observations we constrain the initial binary fraction of solar-metallicity stars in the sample to be $f_{\rm b,0}=0.35\pm0.01$, in line with the solar neighbourhood. We find that the binary fraction is higher at lower metallicities, consistent with other observational studies. Our model matches the shape of the high-velocity scatter in APOGEE, which suggests that most velocity variability above 0.5 km/s comes from binaries. Our exploration of binary initial properties shows that APOGEE is mostly sensitive to binaries with periods between 3 and 3000 years, and is largely insensitive to the detailed properties of the population. We can, however, rule out a population where the mass of the lower-mass star is drawn from the IMF independently of its companion.

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arXiv2017

Stellar multiplicity in the Milky Way Galaxy

We present our models of the effect of binaries on high-resolution spectroscopic surveys. We want to determine how many binary stars will be observed, whether unresolved binaries will contaminate measurements of chemical abundances, and how we can use spectroscopic surveys to better constrain the population of binary stars in the Galaxy. Using a rapid binary-evolution algorithm that enables modelling of the most complex binary systems we generate a series of large binary populations in the Galactic disc and evaluate the results. As a first application we use our model to study the binary fraction in APOGEE giants. We find tentative evidence for a change in binary fraction with metallicity.

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arXiv2017

First evidence of multiple populations along the AGB from Strömgren photometry

Spectroscopic studies have demonstrated that nearly all Galactic globular clusters (GCs) harbour multiple stellar populations with different chemical compositions. Moreover, colour-magnitude diagrams based exclusively on Strömgrem photometry have allowed us to identify and characterise multiple populations along the RGB of a large number of clusters. In this paper we show for the first time that Strömgren photometry is also very effcient at identifying multiple populations along the AGB, and demonstrate that the AGB of M3, M92, NGC362, NGC1851, and NGC6752 are not consistent with a single stellar population. We also provide a catalogue of RGB and AGB stars photometrically identified in these clusters for further spectroscopic follow-up studies.We combined photometry and elemental abundances from the literature for RGB and AGB stars in NGC6752 where the presence of multiple populations along the AGB has been widely debated. We find that, while the MS, SGB, and RGB host three stellar populations with different helium and light element abundances, only two populations of AGB stars are present in the cluster. These results are consistent with standard evolutionary theory.

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arXiv2015

Strömgren uvby photometry of the peculiar globular cluster NGC 2419

NGC 2419 is a peculiar Galactic globular cluster in terms of size/luminosity, and chemical abundance anomalies. Here, we present Strömgren $uvby$ photometry of the cluster. Using the gravity- and metallicity-sensitive $c_1$ and $m_1$ indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius with an estimated contamination by non-members of only 1%. We derive photometric [Fe/H] of red giants, and depending on which literature metallicity relation we use, find reasonable to excellent agreement with spectroscopic [Fe/H]. We demonstrate explicitly that the photometric errors are not Gaussian, and using a realistic model for the photometric uncertainties, find a formal internal [Fe/H] spread of $σ=0.11^{+0.02}_{-0.01}$ dex. This is an upper limit to the cluster's true [Fe/H] spread and may partially/entirely reflect the limited precision of the photometric metallicity estimation and systematic effects. The lack of correlation between spectroscopic and photometric [Fe/H] of individual stars is further evidence against a [Fe/H] spread on the 0.1 dex level. Finally, the CN-sensitive $δ_4$ anti-correlates strongly with Mg abundance, indicating that the 2nd generation stars are N-enriched. Absence of similar correlations in some other CN-sensitive indices supports the second generation being He-rich, which in these indices approximately compensates the shift due to CN. Compared to a single continuous distribution with finite dispersion, the observed $δ_4$ distribution is slightly better fit by two discrete populations, with the N-enhanced stars accounting for 53$\pm$5%. NGC 2419 appears to be very similar to other metal-poor Galactic globular clusters with a similarly N-enhanced second generation and little or no variation in [Fe/H], which sets it apart from other suspected accreted nuclei such as ωCen. (abridged)

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arXiv2015

Galactic Archeology - requirements on survey spectrographs

Galactic Archeology is about exploring the Milky Way as a galaxy by, mainly, using its (old) stars as tracers of past events and thus figure out the formation and evolution of our Galaxy. I will briefly outline some of the key scientific aspects of Galactic Archeology and then discuss the associated instrumentations. Gaia will forever change the way we approach this subject. However, Gaia on its own is not enough. Ground-based complementary spectroscopy is necessary to obtain full phase-space information and elemental abundances for stars fainter than the top few percent of the bright part of the Gaia catalogue. I will review the requirement on instrumentation for Gaia follow-up that Galactic Archeology sets. In particular, I will discuss the requirements on radial velocity and elemental abundance determination, including a brief look at potential pit-falls in the abundance analysis (e.g., NLTE, atomic diffusion). This contribution also provides a non-exhaustive comparison of the various current and future spectrographs for Galactic Archeology. Finally, I will discuss the needs for astrophysical calibrations for the surveys and inter-survey calibrations.

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arXiv2015

The Science Case for Multi-Object Spectroscopy on the European ELT

This White Paper presents the scientific motivations for a multi-object spectrograph (MOS) on the European Extremely Large Telescope (E-ELT). The MOS case draws on all fields of contemporary astronomy, from extra-solar planets, to the study of the halo of the Milky Way and its satellites, and from resolved stellar populations in nearby galaxies out to observations of the earliest 'first-light' structures in the partially-reionised Universe. The material presented here results from thorough discussions within the community over the past four years, building on the past competitive studies to agree a common strategy toward realising a MOS capability on the E-ELT. The cases have been distilled to a set of common requirements which will be used to define the MOSAIC instrument, entailing two observational modes ('high multiplex' and 'high definition'). When combined with the unprecedented sensitivity of the E-ELT, MOSAIC will be the world's leading MOS facility. In analysing the requirements we also identify a high-multiplex MOS for the longer-term plans for the E-ELT, with an even greater multiplex (>1000 targets) to enable studies of large-scale structures in the high-redshift Universe. Following the green light for the construction of the E-ELT the MOS community, structured through the MOSAIC consortium, is eager to realise a MOS on the E-ELT as soon as possible. We argue that several of the most compelling cases for ELT science, in highly competitive areas of modern astronomy, demand such a capability. For example, MOS observations in the early stages of E-ELT operations will be essential for follow-up of sources identified by the James Webb Space Telescope (JWST). In particular, multi-object adaptive optics and accurate sky subtraction with fibres have both recently been demonstrated on sky, making fast-track development of MOSAIC feasible.

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arXiv2015

An inefficient dwarf: Chemical abundances and the evolution of the Ursa Minor dwarf spheroidal galaxy

We present detailed chemical element abundance ratios of 17 elements in three metal poor stars in the Ursa Minor dwarf spheroidal galaxy, which we combine with extant data from the literature to assess the predictions of a novel suite of galaxy chemical evolution models. The spectroscopic data were obtained with the Keck/HIRES instrument and revealed low metallicities of [Fe/H]=-2.12, -2.13 and -2.67 dex. While the most metal poor star in our sample shows an overabundance of [Mn/Fe] and other Fe-peak elements, our overall findings are in agreement with previous studies of this galaxy: elevated values of the [alpha/Fe] ratios that are similar to, or only slightly lower than, the halo values but with SN Ia enrichment at very low metallicity, as well as an enhancement of the ratio of first to second peak neutron capture elements [Y/Ba] with decreasing metallicity. The chemical evolution models which were tailored to reproduce the metallicity distribution function of the dSph, indicate that UMi had an extended star formation which lasted nearly 5 Gyr with low efficiency and are able to explain the [Y/Ba] enhancement at low metallicity for the first time. In particular, we show that the present day lack of gas is probably due to continuous loss of gas from the system, which we model as winds.

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arXiv2014

The SAGA so far: reading the history of the Galaxy with asteroseismology

Asteroseismology has the capability of delivering stellar properties which would otherwise be inaccessible, such as radii, masses and thus ages of stars. When this information is coupled with classical determinations of stellar parameters, such as metallicities, effective temperatures and angular diameters, powerful new diagnostics for stellar and Galactic studies can be obtained. The ongoing Stroemgren survey for Asteroseismology and Galactic Archaeology (SAGA) is pursuing such a goal, by determining photometric stellar parameters for stars with seismic oscillations measured by the Kepler satellite. As the survey continues and expands in sample size, SAGA will provide an unprecedented opportunity to constrain theories of the evolution of the Milky Way disc.

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arXiv2014

The hunt for the Milky Way's accreted disc

The Milky Way is expected to host an accreted disc of stars and dark matter. This forms as massive >1:10 mergers are preferentially dragged towards the disc plane by dynamical friction and then tidally shredded. The accreted disc likely contributes only a tiny fraction of the Milky Way's thin and thick stellar disc. However, it is interesting because: (i) its associated `dark disc' has important implications for experiments hoping to detect a dark matter particle in the laboratory; and (ii) the presence or absence of such a disc constrains the merger history of our Galaxy. In this work, we develop a chemo-dynamical template to hunt for the accreted disc. We apply our template to the high-resolution spectroscopic sample from Ruchti et al. (2011), finding at present no evidence for accreted stars. Our results are consistent with a quiescent Milky Way with no >1:10 mergers since the disc formed and a correspondingly light `dark disc'. However, we caution that while our method can robustly identify accreted stars, our incomplete stellar sample makes it more challenging to definitively rule them out. Larger unbiased stellar samples will be required for this.

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arXiv2013

A spectroscopic binary in the Hercules dwarf spheroidal galaxy

We present the radial velocity curve of a single-lined spectroscopic binary in the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individual spectra covering more than two years of observations. This is the first time that orbital elements could be derived for a binary in a dSph. The system consists of a metal-poor red giant and a low-mass companion, possibly a white dwarf, with a 135-days period in a moderately eccentric ($e=0.18$) orbit. Its period and eccentricity are fully consistent with metal-poor binaries in the Galactic halo, while the projected semimajor axis is small, at $a_p$ sin$i$ = 38 R$_{sun}$. In fact, a very close orbit could inhibit the production of heavier elements through $s$-process nucleosynthesis, leading to the very low abundances of neutron-capture elements that are found in this star. We discuss the further implications for the chemical enrichment history of the Hercules dSph, but find no compelling binary scenario that could reasonably explain the full, peculiar abundance pattern of the Hercules dSph galaxy.

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arXiv2013

The case for high precision in elemental abundances of stars in the era of large spectroscopic surveys

A number of large spectroscopic surveys of stars in the Milky Way are under way or are being planned. In this context it is important to discuss the extent to which elemental abundances can be used as discriminators between different (known and unknown) stellar populations in the Milky Way. We aim to establish the requirements in terms of precision in elemental abundances, as derived from spectroscopic surveys of the Milky Way's stellar populations, in order to detect interesting substructures in elemental abundance space. We present a simple relation between the minimum number of stars needed to detect a given substructure and the precision of the measurements. The results are in agreement with recent small- and large-scale studies, with high and low precision, respectively. Large-number statistics cannot fully compensate for low precision in the abundance measurements and each survey should carefully evaluate what the main science drivers are for the survey and ensure that the chosen observational strategy will result in the precision necessary to answer the questions posed.

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arXiv2013

Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 AA absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log epsilon (Ba) < 0.7 dex (3 sigma). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 AA-line, but most of those would only lead to even lower abundances. A better match with metal-poor halo and dSph stars can only be reached by including a large uncertainty in the continuum placement. This contrasts with the high dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the latter spreads are typical of the very low luminosity, dark-matter dominated dSphs, a high level of depletion in heavy elements suggests that chemical enrichment in Hercules was governed by very massive stars, coupled with a very low star formation efficiency. While very low abundances of some heavy elements are also found in individual stars of other dwarf galaxies, this is the first time that a very low Ba abundance is found within an entire dSph over a broad metallicity range.

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arXiv2013

An intermediate-band photometric study of the "Globular Cluster" NGC 2419

NGC 2419 is one of the remotest star clusters in the Milky Way halo and its exact nature is yet unclear: While it has traits reminiscent of a globular cluster (GC), its large radius and suggestions of an abundance spread have fueled the discussion about its origin in an extragalactic environment, possibly the remnants of the accretion of a dwarf galaxy. Here, we present first results from deep intermediate-band photometry of NGC 2419, which enables us to search for chemical (light element) abundance variations, metallicity spreads, and thus multiple stellar populations through well calibrated Stroemgren indices.

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arXiv2012

Principal Component Abundance Analysis of Microlensed Bulge Dwarf and Subgiant Stars

Elemental abundance patterns can provide vital clues to the formation and enrichment history of a stellar population. Here we present an investigation of the Galactic bulge, where we apply principal component abundance analysis (PCAA)---a principal component decomposition of relative abundances [X/Fe]---to a sample of 35 microlensed bulge dwarf and subgiant stars, characterizing their distribution in the 12-dimensional space defined by their measured elemental abundances. The first principal component PC1, which suffices to describe the abundance patterns of most stars in the sample, shows a strong contribution from alpha-elements, reflecting the relative contributions of Type II and Type Ia supernovae. The second principal component PC2 is characterized by a Na--Ni correlation, the likely product of metallicity-dependent Type II supernova yields. The distribution in PC1 is bimodal, showing that the bimodality previously found in the [Fe/H] values of these stars is robustly and independently recovered by looking at only their relative abundance patterns. The two metal-rich stars that are alpha-enhanced have outlier values of PC2 and PC3, respectively, further evidence that they have distinctive enrichment histories. Applying PCAA to a sample of local thin and thick disk dwarfs yields a nearly identical PC1; in PC1, the metal-rich and metal-poor bulge dwarfs track kinematically selected thin and thick disk dwarfs, respectively, suggesting broadly similar alpha-enrichment histories. However, the disk PC2 is dominated by a Y--Ba correlation, likely indicating a greater contribution of s-process enrichment from long-lived asymptotic giant branch stars that is absent from the bulge PC2 because of its rapid formation.

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