Using the DR12 public release of APOGEE data, we show that the thin and thick discs separate very well in the space defined by α/Fe, Fe/H and C/N. Thick disc giants have both higher C/N and higher ...α/Fe than do thin disc stars with similar Fe/H. We deduce that the thick disc is composed of lower mass stars than the thin disc. Considering the fact that at a given metallicity there is a one-to-one relation between stellar mass and age, we are then able to infer the chronology of disc formation. Both the thick and the thin discs – defined by α/Fe – converge in their dependence on C/N and C+N/Fe at Fe/H≈−0.7. We conclude that (1) the majority of thick disc stars formed earlier than did the thin disc stars, (2) the formation histories of the thin and thick discs diverged early on, even when the Fe/H abundances are similar and (3) that the star formation rate in the thin disc has been lower than in the thick disc, at all metallicities. Although these general conclusions remain robust, we also show that current stellar evolution models cannot reproduce the observed C/N ratios for thick disc stars. Unexpectedly, reduced or inhibited canonical extra mixing is very common in field stars. While subject to abundance calibration zero-point uncertainties, this implies a strong dependence of non-canonical extra mixing along the red giant branch on the initial composition of the star and in particular on the α-elemental abundance.
We present a chemical abundance distribution study in 14 α, odd-Z, even-Z, light, and Fe-peak elements of approximately 3200 intermediate-metallicity giant stars from the Apache Point Observatory ...Galactic Evolution Experiment (APOGEE) survey. The main aim of our analysis is to explore the Galactic disc–halo transition region within −1.20 < Fe/H < −0.55 as a means to study chemical difference (and similarities) between these components. In this paper, we show that there is an α-poor and α-rich sequence within both the metal-poor and intermediate-metallicity regions. Using the Galactic rest-frame radial velocity and spatial positions, we further separate our sample into the canonical Galactic components. We then studied the abundances ratios of Mg, Ti, Si, Ca, O, S, Al, C+N, Na, Ni, Mn, V, and K for each of the components and found the following: (1) the α-poor halo subgroup is chemically distinct in the α-elements, particularly O, Mg, S, Al, C+N, and Ni, from the α-rich halo, consistent with the literature confirming the existence of an α-poor accreted halo population; (2) the canonical thick disc and halo are not chemically distinct in all elements indicating a smooth transition between the thick disc and halo; (3) a subsample of the α-poor stars at metallicities as low as Fe/H ∼ −0.85 dex are chemically and dynamically consistent with the thin disc indicating that the thin disc may extend to lower metallicities than previously thought; and (4) the locations of the most metal-poor thin disc stars are consistent with a negative radial metallicity gradient. Finally, we used our analysis to suggest a new set of chemical abundance planes (α/Fe, C+N/Fe, Al/Fe, and Mg/Mn) that may be able to chemically label the Galactic components in a clean and efficient way independent of kinematics.
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 V
φ and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, V
R, finding that the gradient is marked below the plane (δ〈V
R〉/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, V
Z
, also shows clear, large-amplitude (|V
Z
| = 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 V
R and V
Z
along the line of sight, with the V
R 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.
Context. The APOGEE survey has obtained high-resolution infrared spectra of more than 100 000 stars. Deriving chemical abundances patterns of these stars is paramount to piecing together the ...structure of the Milky Way. While the derived chemical abundances have been shown to be precise for most stars, some calibration problems have been reported, in particular for more metal-poor stars. Aims. In this paper, we aim to (1) re-determine the chemical abundances of the APOGEE+Kepler stellar sample (APOKASC) with an independent procedure, line list and line selection, and high-quality surface gravity information from asteroseismology; and (2) extend the abundance catalogue by including abundances that are not currently reported in the most recent APOGEE release (DR12). Methods. We fixed the Teff and log g to those determined using spectrophotometric and asteroseismic techniques, respectively. We made use of the Brussels Automatic Stellar Parameter (BACCHUS) code to derive the metallicity and broadening parameters for the APOKASC sample. In addition, we derived differential abundances with respect to Arcturus. Results. We have validated the BACCHUS code on APOGEE data using several well-known stars, and stars from open and globular clusters. We also provide the abundances of C, N, O, Mg, Ca, Si, Ti, S, Al, Na, Ni, Mn, Fe, K, and V for every star and line, and show the impact of line selection on the final abundances. Improvements have been made for some elements (e.g. Ti, Si, V). Additionally, we measure new abundance ratios not found in the current APOGEE release including P, Cu, Rb, and Yb, which are only upper limits at this time, as well as Co and Cr which are promising. Conclusions. In this paper, we present an independent analysis of the APOKASC sample and provide abundances of up to 21 elements. This catalogue can be used not only to study chemical abundance patterns of the Galaxy but also to train data driven spectral approaches which can improve the abundance precision in a restricted dataset, but also full APOGEE sample.
The development of cell printing is vital for establishing biofabrication approaches as clinically relevant tools. Achieving this requires bio-inks which must not only be easily printable, but also ...allow controllable and reproducible printing of cells. This review outlines the general principles and current progress and compares the advantages and challenges for the most widely used biofabrication techniques for printing cells: extrusion, laser, microvalve, inkjet and tissue fragment printing. It is expected that significant advances in cell printing will result from synergistic combinations of these techniques and lead to optimised resolution, throughput and the overall complexity of printed constructs.
The mass distribution of Population III stars Fraser, M; Casey, A. R; Gilmore, G ...
Monthly notices of the Royal Astronomical Society,
06/2017, Letnik:
468, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
Extremely metal-poor (EMP) stars are uniquely informative on the nature of massive Population III stars. Modulo a few elements that vary with stellar evolution, the present-day photospheric ...abundances observed in EMP stars are representative of their natal gas cloud composition. For this reason, the chemistry of EMP stars closely reflects the nucleosynthetic yields of supernovae from massive Population III stars. Here we collate detailed abundances of 53 EMP stars from the literature and infer the masses of their Population III progenitors. We fit a simple initial mass function (IMF) to a subset of 29 of the inferred Population III star masses, and find that the mass distribution is well represented by a power-law IMF with exponent
$\alpha = 2.35^{+0.29}_{-0.24}$
. The inferred maximum progenitor mass for supernovae from massive Population III stars is
$M_{\rm {max}} = 87^{+13}_{-33}$
M⊙, and we find no evidence in our sample for a contribution from stars with masses above ∼120 M⊙. The minimum mass is strongly consistent with the theoretical lower mass limit for Population III supernovae. We conclude that the IMF for massive Population III stars is consistent with the IMF of present-day massive stars and there may well have formed stars much below the supernova mass limit that could have survived to the present day.
We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia astrometric solution (TGAS) catalogue to compute the velocity fields yielded by the radial (V-R), azimuthal (V-phi), and ...vertical (V-z) components of associated Galactocentric velocity. We search in particular for variation in all three velocity components with distance above and below the disc midplane, as well as how each component of V-z (line-of-sight and tangential velocity projections) modifies the obtained vertical structure. To study the dependence of velocity on proper motion and distance, we use two main samples: a RAVE sample including proper motions from the Tycho-2, PPMXL, and UCAC4 catalogues, and a RAVE-TGAS sample with inferred distances and proper motions from the TGAS and UCAC5 catalogues. In both samples, we identify asymmetries in V-R and V-z. Below the plane, we find the largest radial gradient to be partial derivative V-R/partial derivative R = -7.01 +/- 0.61 km s(-1) kpc(-1), in agreement with recent studies. Above the plane, we find a similar gradient with partial derivative V-R/partial derivative R = -9.42 +/- 1.77 km s(-1) kpc(-1). By comparing our results with previous studies, we find that the structure in V-z is strongly dependent on the adopted proper motions. Using the Galaxia Milky Way model, we demonstrate that distance uncertainties can create artificial wave-like patterns. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms.
Dwarf spheroidal galaxies have shallow central dark matter density profiles, low angular momentum and approximately exponential surface brightness distributions. Through N-body simulations and ...analytic calculations we investigate the extent to which these properties can be generated from ‘typical’ΛCDM galaxies, which differ in all of these properties, by the dynamical consequences of feedback. We find that, for a wide range of initial conditions, one impulsive mass-loss event will naturally produce a surface brightness profile in the remaining stellar component of a dwarf spheroidal galaxy (dSph) which is well-fitted over many scalelengths by an exponential, in good qualitative agreement with observations of Local Group dSphs. Furthermore, two impulsive mass-loss phases, punctuated by significant gas re-accretion, are found to be sufficient to transform a central density cusp in the dark matter profile into a near-constant density core. This may then provide the missing link between current cosmological simulations, which predict a central cusp in the dark matter density profile, and current observations, which find much shallower central density profiles. We also look at the angular momentum history of dSphs and demonstrate that if these galaxies have spent most of their lifetime in tidal isolation from massive galaxies then they cannot have formed from high angular momentum gas discs.
We combine simultaneous constraints on stellar evolutionary status from asteroseismology, and on nitrogen abundances derived from large spectroscopic surveys, to follow nitrogen surface abundances ...all along the evolution of a low-mass star, comparing model expectations with data. After testing and calibrating the observed yields from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey, we first show that nitrogen surface abundances follow the expected trend after the first dredge-up occurred, i.e. that the more massive is the star, the more nitrogen is enhanced. Moreover, the behaviour of nitrogen data along the evolution confirms the existence of non-canonical extramixing on the red giant branch (RGB) for all low-mass stars in the field. But more surprisingly, the data indicate that nitrogen has been depleted between the RGB tip and the red clump. This may suggest that some nitrogen has been burnt near or at the He flash episode.
We investigate the hypothesis that some fraction of the globular clusters presently observed in the Galactic halo formed in external dwarf galaxies. This is done by means of a detailed comparison ...between the ‘old halo’, ‘young halo’ and ‘bulge/disc’ subsystems defined by Zinn and the globular clusters in the Large Magellanic Cloud, Small Magellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies. We first use high-quality photometry from Hubble Space Telescope images to derive a complete set of uniform measurements of horizontal branch (HB) morphology in the external clusters. We also compile structural and metallicity measurements for these objects and update the data base of such measurements for the Galactic globular clusters, including new calculations of HB morphology for 11 objects. Using these data together with recent measurements of globular cluster kinematics and ages we examine the characteristics of the three Galactic cluster subsystems. Each is quite distinct in terms of their spatial and age distributions, age–metallicity relationships, and typical orbital parameters, although we observe some old halo clusters with ages and orbits more similar to those of young halo objects. In addition, almost all of the Galactic globular clusters with large core radii fall into the young halo subsystem, while the old halo and bulge/disc ensembles are characterized by compact clusters. We demonstrate that the majority of the external globular clusters are essentially indistinguishable from the Galactic young halo objects in terms of HB morphology, but ∼20–30 per cent of external clusters have HB morphologies most similar to the Galactic old halo clusters. We further show that the external clusters have a distribution of core radii which very closely matches that for the young halo objects. The old halo distribution of core radii can be very well represented by a composite distribution formed from ∼83–85 per cent of objects with structures typical of bulge/disc clusters, and ∼15–17 per cent of objects with structures typical of external clusters. Taken together our results fully support the accretion hypothesis. We conclude that all 30 young halo clusters and 15–17 per cent of the old halo clusters (10–12 objects) are of external origin. Based on cluster number counts, we estimate that the Galaxy may have experienced approximately seven merger events with cluster-bearing dwarf-spheroidal-type galaxies during its lifetime, building up ∼45–50 per cent of the mass of the Galactic stellar halo. Finally, we identify a number of old halo objects which have properties characteristic of accreted clusters. Several of the clusters associated with the recently proposed dwarf galaxy in Canis Major fall into this category.
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