298 research outputs found
Evidence of a large scale positive rotation-metallicity correlation in the Galactic thick disk
This study is based on high quality astrometric and spectroscopic data from
the most recent releases by Gaia and APOGEE. We select thin and thick
disk red giants, in the Galactocentric (cylindrical) distance range ~kpc and within ~kpc, for which full chemo-kinematical information
is available. Radial chemical gradients, , and rotational velocity-metallicity correlations, , are re-derived firmly uncovering that the thick disk
velocity-metallicity correlation maintains its positiveness over the ~kpc
range explored. This observational result is important as it sets experimental
constraints on recent theoretical studies on the formation and evolution of the
Milky Way disk and on cosmological models of Galaxy formation.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
New Signatures of the Milky Way Formation in the Local Halo and Inner Halo Streamers in the Era of Gaia
We explore the vicinity of the Milky Way through the use of
spectro-photometric data from the Sloan Digital Sky Survey and high-quality
proper motions derived from multi-epoch positions extracted from the Guide Star
Catalogue II database. In order to identify and characterise streams as relics
of the Milky Way formation, we start with classifying, select, and study
subdwarfs with up to kpc away from the Sun as tracers
of the local halo system. Then, through phase-space analysis, we find
statistical evidence of five discrete kinematic overdensities among of the
fastest-moving stars, and compare them to high-resolution N-body simulations of
the interaction between a Milky-Way like galaxy and orbiting dwarf galaxies
with four representative cases of merging histories. The observed overdensities
can be interpreted as fossil substructures consisting of streamers torn from
their progenitors, such progenitors appear to be satellites on prograde and
retrograde orbits on different inclinations. In particular, of the five
detected overdensities, two appear to be associated, yelding twenty-one
additional main-sequence members, with the stream of Helmi et al. (1999) that
our analysis confirms on a high inclination prograde orbit. The three newly
identified kinematic groups could be associated with the retrograde streams
detected by Dinescu (2002) and Kepley et al. (2007), whatever their origin, the
progenitor(s) would be on retrograde orbit(s) and inclination(s) within the
range . Finally, we use our simulations to
investigate the impact of observational errors and compare the current picture
to the promising prospect of highly improved data expected from the Gaia
mission.Comment: 16 pages, 10 figures, 6 Tables. Accepted for publication in The
Astronomical Journa
The radial metallicity gradients in the Milky Way thick disk as fossil signatures of a primordial chemical distribution
In this letter we examine the evolution of the radial metallicity gradient
induced by secular processes, in the disk of an -body Milky Way-like galaxy.
We assign a [Fe/H] value to each particle of the simulation according to an
initial, cosmologically motivated, radial chemical distribution and let the
disk dynamically evolve for 6 Gyr. This direct approach allows us to take into
account only the effects of dynamical evolution and to gauge how and to what
extent they affect the initial chemical conditions. The initial [Fe/H]
distribution increases with R in the inner disk up to R ~ 10 kpc and decreases
for larger R. We find that the initial chemical profile does not undergo major
transformations after 6 Gyr of dynamical evolution. The final radial chemical
gradients predicted by the model in the solar neighborhood are positive and of
the same order of those recently observed in the Milky Way thick disk.
We conclude that: 1) the spatial chemical imprint at the time of disk
formation is not washed out by secular dynamical processes, and 2) the observed
radial gradient may be the dynamical relic of a thick disk originated from a
stellar population showing a positive chemical radial gradient in the inner
regions.Comment: 10 pages, 5 figures, Accepted for publication on Astrophysical
Journal Letter
Growth-survival trade-offs and the restoration of non-forested open ecosystems
The growth-survival trade-off has been extensively documented for phanerophyte species, whereas there is little evidence for non-phanerophyte species. However, information on the growth-survival trade-offs in non-phanerophyte species could be of great use in non-forested open ecosystem restoration by providing insights for plant production and transplantation, thus impacting the planning of cost-effective restoration actions. In this study, we explored the relationship between growth and survival of individual plants of non-phanerophyte species used in a coastal dune restoration project, and we investigated whether plant functional traits explained patterns of trade-off between growth and survival. We monitored 355 individual plants of 13 perennial non-phanerophyte species belonging to foredune and transition dune communities every 30 days after planting and calculated relative growth and survival rates. In addition, we regressed the relationship between growth and survival on values of leaf and floral traits. We found that, besides being a widely recognised axis of life history variation in phanerophyte species, the growth-survival trade-off can also be observed in perennial non-phanerophyte species. Species of different coastal dune communities (i.e., foredune vs. transition dune communities) differed with respect to the growth-survival trade-off, with plant species of foredune communities exhibiting higher growth but lower survival rates than plant species of transition dune communities. Leaf dry matter content and mean number of floral displays explained species position on the growth-survival trade-off axis; species with relatively high growth and low survival rates exhibited an acquisitive strategy, with low values of leaf dry matter content, but also a low sexual reproductive effort, as revealed by low number of floral displays. In contrast, plant species with relatively low growth and high survival rates exhibited a conservative strategy but also high sexual reproductive effort, suggesting that trade-offs occur in resource allocation among vegetative and reproductive plant structures. The trade-off we found between growth and survival in perennial non-phanerophyte species provides useful insights for planning cost-effective ecosystem restoration actions of non-forested open ecosystems, especially when the actions are nature-based and involve planting individual plants. The results of this study suggest that individual plant production for coastal dune restoration should disproportionately target plant species of foredune communities because they have low survival rates associated with low sexual reproductive effort. Planning plant production based on ecological knowledge of plant species’ growth and survival after planting in the field could help achieve restoration goals while meeting project cost-effectiveness requirements
Point Defects in Two-Dimensional Indium Selenide as Tunable Single-Photon Sources
In the past few years remarkable interest has been kindled by the development of nonclassical light sources and, in particular, of single-photon emitters (SPE), which represent fundamental building blocks for optical quantum technology. In this Letter, we analyze the stability and electronic properties of an InSe monolayer with point defects with the aim of demonstrating its applicability as an SPE. The presence of deep defect states within the InSe band gap is verified when considering substitutional defects with atoms belonging to group IV, V, and VI. In particular, the optical properties of Ge as substitution impurity of Se predicted by solving the Bethe-Salpeter equation on top of the GW corrected electronic states show that transitions between the valence band maximum and the defect state are responsible for the absorption and spontaneous emission processes, so that the latter results in a strongly peaked spectrum in the near-infrared. These properties, together with a high localization of the involved electronic states, appear encouraging in the quest for novel SPE materials
First-Principles Calculations of Exciton Radiative Lifetimes in Monolayer Graphitic Carbon Nitride Nanosheets: Implications for Photocatalysis
In this work, we report on the exciton radiative lifetimes of graphitic carbon nitride monolayers in the triazine-based (gC3N4-t) and heptazine-based (gC3N4-h) forms, as obtained by means of ground-state plus excited-state ab initio calculations. By analyzing the exciton fine structure, we highlight the presence of dark states and show that the photogenerated electron-hole (e-h) pairs in gC3N4-h are remarkably long-lived, with an effective radiative lifetime of 260 ns. This fosters the employment of gC3N4-h in photocatalysis and makes it attractive for the emerging field of exciton devices. Although very long intrinsic radiative lifetimes are an important prerequisite for several applications, pristine carbon nitride nanosheets show very low quantum photoconversion efficiency, mainly due to the lack of an efficient e-h separation mechanism. We then focus on a vertical heterostructure made of gC3N4-t and gC3N4-h layers, which shows a type-II band alignment and looks promising for achieving net charge separation
Icarus: A Flat and Fast Prograde Stellar Stream in the Milky Way Disk
We explore the local volume of the Milky Way via chemical and kinematical measurements from high-quality astrometric and spectroscopic data recently released by the Gaia, APOGEE, and GALAH programs. We chemically select 1137 stars up to 2.5 kpc of the Sun and [Fe/H] ≤ −1.0 dex, and find evidence of statistically significant substructures. Clustering analysis in velocity space classifies 163 objects into eight kinematical groups, whose origin is further investigated with high-resolution N-body numerical simulations of single merging events. The two retrograde groups appear associated with Gaia-Sausage-Enceladus (GSE), while the slightly prograde group could be connected to GSE or possibly Wukong. We find evidence of a new 44-member-strong prograde stream that we name Icarus; to our knowledge, Icarus is the fast-rotating stream closest to the Galactic disk to date (, 〈V + V LSR〉 sime 231 km s−1). Its peculiar chemical (〈[Fe/H]〉 sime −1.45, 〈[Mg/Fe]〉 sime −0.02) and dynamical (mean eccentricity sime 0.11) properties are consistent with the accretion of debris from a dwarf galaxy progenitor with a stellar mass of ~109 M ☉ on an initial prograde low-inclination orbit, ~10°. The remaining prograde groups are either streams previously released by the same progenitor of Icarus (or Nyx), or remnants from different satellites accreted on initial orbits at higher inclination
Active Surface Structure of SnO2 Catalysts for CO2 Reduction Revealed by Ab Initio Simulations
Tin oxide (SnO2) is an efficient catalyst for the CO2 reduction reaction (CO2RR) to formic acid; however, the understanding of the SnO2 surface structure under working electrocatalytic conditions and the nature of catalytically active sites is a current matter of debate. Here, we employ ab initio density functional theory calculations to investigate how the selectivity and reactivity of SnO2 surfaces toward the CO2RR change at varying surface stoichiometry (i.e., reduction degree). Our results show that SnO2(110) surfaces are not catalytically active for the CO2RR or hydrogen evolution reaction, but rather they reduce under an applied external bias, originating surface structures exposing few metal tin layers, which are responsible for formic acid selectivity
The formation history of the Milky Way disc with high-resolution cosmological simulations
We analyse from an observational perspective the formation history and kinematics of a Milky Way-like galaxy from a high- resolution zoom-in cosmological simulation that we compare to those of our Galaxy as seen by Gaia DR2 to better understand the origin and evolution of the Galactic thin and thick discs. The cosmological simulation was carried out with the GADGET-3 TreePM+SPH code using the MUlti-Phase Particle Integrator (MUPPI) model. We disentangle the complex overlapping of stellar generations that rises from the top-down and inside-out formation of the galactic disc. We investigate cosmological signatures in the phase-space of mono-age populations and highlight features stemming from past and recent dynamical perturbations. In the simulation, we identify a satellite with a stellar mass of 1.2 × 109 M⊙, i.e. stellar mass ratio ∼ 5.5 per cent at the time, accreted at z ∼ 1.6, which resembles the major merger Gaia–Sausage–Enceladus that produced the Galactic thick disc, i.e. ∼ 6 per cent. We found at z ∼ 0.5–0.4 two merging satellites with a stellar mass of 8.8 × 108 M⊙ and 5.1 × 108 M⊙ that are associated to a strong starburst in the star formation history, which appears fairly similar to that recently found in the solar neighbourhood. Our findings highlight that detailed studies of coeval stellar populations kinematics, which are made available by current and future Gaia data releases and in synergy with simulations, are fundamental to unravel the formation and evolution of the Milky Way discs
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