We analyze the present day structure and assembly history of a high-resolution hydrodynamic simulation of the formation of a Milky-Way-(MW)-like disk galaxy, from the "Eris" simulation suite, ...dissecting it into cohorts of stars formed at different epochs of cosmic history. The younger age cohorts populate disks of progressively longer radial scale lengths and shorter vertical scale heights. At a given radius, the vertical density profiles and velocity dispersions of stars vary smoothly as a function of age, and the superposition of old, vertically extended and young, vertically compact cohorts gives rise to a double-exponential profile like that observed in the MW. Turning to formation history, we find that the trends of spatial structure and kinematics with stellar age are largely imprinted at birth, or immediately thereafter. The predicted correlations of stellar age with spatial and kinematic structure are in good qualitative agreement with the correlations observed for mono-abundance stellar populations in the MW.
ABSTRACT Using a sample of 69,919 red giants from the SDSS-III/APOGEE Data Release 12, we measure the distribution of stars in the /Fe versus Fe/H plane and the metallicity distribution functions ...(MDFs) across an unprecedented volume of the Milky Way disk, with radius 3 < R < 15 kpc and height kpc. Stars in the inner disk (R < 5 kpc) lie along a single track in /Fe versus Fe/H, starting with -enhanced, metal-poor stars and ending at /Fe ∼ 0 and Fe/H ∼ +0.4. At larger radii we find two distinct sequences in /Fe versus Fe/H space, with a roughly solar- sequence that spans a decade in metallicity and a high- sequence that merges with the low- sequence at super-solar Fe/H. The location of the high- sequence is nearly constant across the disk; however, there are very few high- stars at R > 11 kpc. The peak of the midplane MDF shifts to lower metallicity at larger R, reflecting the Galactic metallicity gradient. Most strikingly, the shape of the midplane MDF changes systematically with radius, from a negatively skewed distribution at 3 < R < 7 kpc, to a roughly Gaussian distribution at the solar annulus, to a positively skewed shape in the outer Galaxy. For stars with kpc or /Fe > 0.18, the MDF shows little dependence on R. The positive skewness of the outer-disk MDF may be a signature of radial migration; we show that blurring of stellar populations by orbital eccentricities is not enough to explain the reversal of MDF shape, but a simple model of radial migration can do so.
We map the trends of elemental abundance ratios across the Galactic disk, spanning R = 3 - 15 kpc and midplane distance Z = 0 - 2 kpc , for 15 elements in a sample of 20,485 stars measured by the ...SDSS/APOGEE survey (O, Na, Mg, Al, Si, P, S, K, Ca, V, Cr, Mn, Fe, Co, Ni). Adopting Mg rather than Fe as our reference element, and separating stars into two populations based on Fe/Mg, we find that the median trends of X/Mg versus Mg/H in each population are nearly independent of location in the Galaxy. The full multi-element cartography can be summarized by combining these nearly universal median sequences with our measured metallicity distribution functions and the relative proportions of the low-Fe/Mg (high- ) and high-Fe/Mg (low- ) populations, which depend strongly on R and Z . We interpret the median sequences with a semi-empirical "two-process" model that describes both the ratio of core collapse and Type Ia supernova (SN Ia) contributions to each element and the metallicity dependence of the supernova yields. These observationally inferred trends can provide strong tests of supernova nucleosynthesis calculations. Our results lead to a relatively simple picture of abundance ratio variations in the Milky Way, in which the trends at any location can be described as the sum of two components with relative contributions that change systematically and smoothly across the Galaxy. Deviations from this picture and future extensions to other elements can provide further insights into the physics of stellar nucleosynthesis and unusual events in the Galaxy's history.
We use a suite of numerical simulations to investigate the mechanisms and effects of radial migration of stars in disc galaxies such as the Milky Way (MW). An isolated, collisionless stellar disc ...with an MW-like scaleheight shows only the radial 'blurring' expected of epicyclic orbits. Reducing the disc thickness or adding gas to the disc substantially increases the level of radial migration, induced by interaction with transient spiral arms and/or a central bar. We also examine collisionless discs subjected to gravitational perturbations from a cosmologically motivated satellite accretion history. In the perturbed disc that best reproduces the observed properties of MW, 20 per cent of stars that end up in the solar annulus 7 kpc < R < 9 kpc started at R < 6 kpc, and 7 per cent started at R > 10 kpc. This level of migration would add considerable dispersion to the age-metallicity relation of solar neighbourhood stars. In the isolated disc models, the probability of migration traces the disc's radial mass profile, but in perturbed discs migration occurs preferentially at large radii, where the disc is more weakly bound. The orbital dynamics of migrating particles are also different in isolated and perturbed discs: satellite perturbations drive particles to lower angular momentum for a given change in radius. Thus, satellite perturbations appear to be a distinct mechanism for inducing radial migration, which can operate in concert with migration induced by bars and spiral structures. We investigate correlations between changes in radius and changes in orbital circularity or vertical energy, identifying signatures that might be used to test models and distinguish radial migration mechanisms in chemodynamical surveys of the MW disc.
ABSTRACT
Kinematic studies of disc galaxies, using individual stars in the Milky Way or statistical studies of global disc kinematics over time, provide insight into how discs form and evolve. We use ...a high-resolution, cosmological zoom-simulation of a Milky Way-mass disc galaxy (h277) to tie together local disc kinematics and the evolution of the disc over time. The present-day stellar age–velocity relationship (AVR) of h277 is nearly identical to that of the analogous solar-neighbourhood measurement in the Milky Way. A crucial element of this success is the simulation’s dynamically cold multiphase ISM, which allows young stars to form with a low velocity dispersion (σbirth$\sim \!6 - 8 \ \mathrm{km\, s}^{-1}$) at late times. Older stars are born kinematically hotter (i.e. the disc settles over time in an ‘upside-down’ formation scenario), and are subsequently heated after birth. The disc also grows ‘inside-out’, and many of the older stars in the present-day solar neighbourhood are present because of radial mixing. We demonstrate that the evolution of σbirth in h277 can be explained by the same model used to describe the general decrease in velocity dispersion observed in disc galaxies from z ∼ 2–3 to the present-day, in which the disc evolves in quasi-stable equilibrium and the ISM velocity dispersion decreases over time due to a decreasing gas fraction. Thus, our results tie together local observations of the Milky Way’s AVR with observed kinematics of high z disc galaxies.
ABSTRACT
We derive empirical constraints on the nucleosynthetic yields of nitrogen by incorporating N enrichment into our previously developed and empirically tuned multizone galactic chemical ...evolution model. We adopt a metallicity-independent (‘primary’) N yield from massive stars and a metallicity-dependent (‘secondary’) N yield from AGB stars. In our model, galactic radial zones do not evolve along the observed N/O–O/H relation, but first increase in O/H at roughly constant N/O, then move upward in N/O via secondary N production. By t ≈ 5 Gyr, the model approaches an equilibrium N/O–O/H relation, which traces the radial oxygen gradient. Reproducing the N/O–O/H trend observed in extragalactic systems constrains the ratio of IMF-averaged N yields to the IMF-averaged O yield of core-collapse supernovae. We find good agreement if we adopt $y_\text{N}^\text{CC}/y_\text{O}^\text{CC}=0.024$ and $y_\text{N}^\text{AGB}/y_\text{O}^\text{CC} = 0.062(Z/Z_\odot)$. For the theoretical AGB yields we consider, simple stellar populations release half their N after only ∼250 Myr. Our model reproduces the N/O–O/H relation found for Milky Way stars in the APOGEE survey, and it reproduces (though imperfectly) the trends of stellar N/O with age and O/Fe. The metallicity-dependent yield plays the dominant role in shaping the gas-phase N/O–O/H relation, but the AGB time-delay is required to match the stellar age and O/Fe trends. If we add ∼40 per cent oscillations to the star formation rate, the model reproduces the scatter in the gas phase N/O–O/H relation observed in external galaxies by MaNGA. We discuss implications of our results for theoretical models of N production by massive stars and AGB stars.
ABSTRACT
We develop a hybrid model of galactic chemical evolution that combines a multiring computation of chemical enrichment with a prescription for stellar migration and the vertical distribution ...of stellar populations informed by a cosmological hydrodynamic disc galaxy simulation. Our fiducial model adopts empirically motivated forms of the star formation law and star formation history, with a gradient in outflow mass loading tuned to reproduce the observed metallicity gradient. With this approach, the model reproduces many of the striking qualitative features of the Milky Way disc’s abundance structure: (i) the dependence of the O/Fe–Fe/H distribution on radius Rgal and mid-plane distance |z|; (ii) the changing shapes of the O/H and Fe/H distributions with Rgal and |z|; (iii) a broad distribution of O/Fe at sub-solar metallicity and changes in the O/Fe distribution with Rgal, |z|, and Fe/H; (iv) a tight correlation between O/Fe and stellar age for O/Fe > 0.1; (v) a population of young and intermediate-age α-enhanced stars caused by migration-induced variability in the Type Ia supernova rate; (vi) non-monotonic age–O/H and age–Fe/H relations, with large scatter and a median age of ∼4 Gyr near solar metallicity. Observationally motivated models with an enhanced star formation rate ∼2 Gyr ago improve agreement with the observed age–Fe/H and age–O/H relations, but worsen agreement with the observed age–O/Fe relation. None of our models predict an O/Fe distribution with the distinct bimodality seen in the observations, suggesting that more dramatic evolutionary pathways are required. All code and tables used for our models are publicly available through the Versatile Integrator for Chemical Evolution (VICE; https://pypi.org/project/vice).
Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier ...generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)‐based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox‐potential‐driven. Remarkably, X‐ray scattering shows that despite their large 2‐nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.
The effect of dopant redox potential on the chemical doping of conjugated polymers is studied using dodecaborane dopants whose potential can be tuned at constant size and shape. The 2‐nm dodecaborane clusters preferentially intercalate into the polymer crystalline lamellae, and the doping efficiency increases with redox potential. The isolation of the counteranion from the hole leads to improved carrier mobility.
We study the occurrence of spectroscopic binaries in young star-forming regions using the INfrared Spectroscopy of Young Nebulous Clusters (IN-SYNC) survey, carried out in SDSS-III with the APOGEE ...spectrograph. Multi-epoch observations of thousands of low-mass stars in Orion A, NGC 2264, NGC 1333, IC 348, and the Pleiades have been carried out, yielding H-band spectra with a nominal resolution of R = 22,500 for sources with H < 12 mag. Radial velocity precisions of ∼0.3 were achieved, which we use to identify radial velocity variations indicative of undetected companions. We use Monte Carlo simulations to assess the types of spectroscopic binaries to which we are sensitive, finding sensitivity to binaries with orbital periods days, for stars with and < 100 . Using Bayesian inference, we find evidence for a decline in the spectroscopic binary fraction, by a factor of 3-4, from the age of our pre-main-sequence (PMS) sample to the Pleiades age . The significance of this decline is weakened if spot-induced radial-velocity jitter is strong in the sample, and is only marginally significant when comparing any one of the PMS clusters against the Pleiades. However, the same decline in both sense and magnitude is found for each of the five PMS clusters, and the decline reaches a statistical significance of greater than 95% confidence when considering the PMS clusters jointly. Our results suggest that dynamical processes disrupt the widest spectroscopic binaries ( days) as clusters age, indicating that this occurs early in the stars' evolution, while they still reside within their nascent clusters.
Identification of key molecules that drive angiogenesis is critical for the development of new modalities for the prevention of solid tumor progression. Using multiple models of colorectal cancer, we ...show that activity of the extracellular matrix-modifying enzyme lysyl oxidase (LOX) is essential for stimulating endothelial cells in vitro and angiogenesis in vivo. We show that LOX activates Akt through platelet-derived growth factor receptor β (PDGFRβ) stimulation, resulting in increased VEGF expression. LOX-driven angiogenesis can be abrogated through targeting LOX directly or using inhibitors of PDGFRβ, Akt, and VEGF signaling. Furthermore, we show that LOX is clinically correlated with VEGF expression and blood vessel formation in 515 colorectal cancer patient samples. Finally, we validate our findings in a breast cancer model, showing the universality of these observations. Taken together, our findings have broad clinical and therapeutic implications for a wide variety of solid tumor types.
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