Secular Evolution Research Articles

Relativistic Phase Space Diffusion of Compact Object Binaries in Stellar Clusters and Hierarchical Triples

The LIGO/Virgo detections of compact object mergers have posed a challenge for theories of binary evolution and coalescence. One promising avenue for producing mergers dynamically is through secular eccentricity oscillations driven by an external perturber, be it a tertiary companion (as in the Lidov–Kozai, LK, mechanism) or the tidal field of the stellar cluster in which the binary orbits. The simplest theoretical models of these oscillations use a “doubly averaged” (DA) approximation, averaging both over the binary’s internal Keplerian orbit and its “outer” barycentric orbit relative to the perturber. However, DA theories do not account for fluctuations of the perturbing torque on the outer orbital timescale, which are known to increase a binary’s eccentricity beyond the maximum DA value, potentially accelerating mergers. Here we reconsider the impact of these short-timescale fluctuations in the test-particle quadrupolar limit for binaries perturbed by arbitrary spherical cluster potentials (including LK as a special case), in particular including 1pN general relativistic (GR) apsidal precession of the internal orbit. Focusing on the behavior of the binary orbital elements around peak eccentricity, we discover a new effect, relativistic phase space diffusion (RPSD), in which a binary can jump to a completely new dynamical trajectory on an outer orbital timescale, violating the approximate conservation of DA integrals of motion. RPSD arises from an interplay between secular behavior at extremely high eccentricity, short-timescale fluctuations, and rapid GR precession, and can change the subsequent secular evolution dramatically. This effect occurs even in hierarchical triples, but has not been uncovered until now.

A laser–laser method for carbonate C and O isotope measurement, metrology assessment, and stratigraphic applications

The stable isotopic compositions of carbon and oxygen (δ13Ccarb and δ18Ocarb) measured from carbonates are widely used in geology, notably to reconstruct paleotemperatures and the secular evolution of the biogeochemical carbon cycle, to characterize limestone sediment diagenesis, and to provide chemostratigraphy records. The standard technique used since the mid-20th century to measure C and O isotopic ratios is based on a wet chemical acid digestion protocol in order to evolve CO2 from carbonates—the latter being analyzed by mass spectrometry and, more recently, infrared spectroscopy. A newly developed laser-based method aims to circumvent this chemical preparation step by producing CO2 via an instant and spatially resolved calcination reaction. We describe an evolution of the laser calcination benchtop system previously described and used as a proof of concept toward a portable system, and we present the efficiency of this tool for performing carbon and oxygen isotope measurements from carbonate matrixes following standard evaluation metrology protocol. This metrological study explores the following: i) the use of internal standards; ii) inter-calibration with the traditional acid chemical preparation method; iii) analysis of the uncertainties of using GUM and ANOVA. Using 15 different types of carbonate minerals encompassing a range of isotopic VPDB compositions between −18.6‰ and +16.06‰ and between −14.80‰ and −1.72‰ for δ13Ccarb and δ18Ocarb, we show that isotopic cross-calibration is verified for both carbon and oxygen, respectively, and we demonstrate that the uncertainties (1σ) of the δ13Ccarb and δ18Ocarb measurements of laser–laser isotopic analysis are within 0.41‰ and 0.68‰, respectively. The advantages of this method in saving time and spatially resolved and automated analysis in situ are demonstrated by high-resolution chemostratigraphic analysis of a laminated lacustrine travertine sample.

The distribution of stellar orbits in eagle galaxies – the effect of mergers, gas accretion, and secular evolution

ABSTRACT The merger history of a galaxy is thought to be one of the major factors determining its internal dynamics, with galaxies having undergone different types or mergers (e.g. dry, minor, or major mergers) predicted to show different dynamical properties. We study the instantaneous orbital distribution of galaxies in the eagle simulation, colouring the orbits of the stellar particles by their stellar age, in order to understand whether stars form in particular orbits (e.g. in a thin or thick disc). We first show that eagle reproduces well the observed stellar mass fractions in different stellar orbital families as a function of stellar mass and spin parameter at z = 0. We find that the youngest stars reside in a thin disc component that can extend to the very inner regions of galaxies, and that older stars have warmer orbits, with the oldest ones showing orbits consistent with both hot and counter-rotating classifications, which is consistent with the trend found in the Milky Way and other disc galaxies. We also show that counter-rotating orbits trace galaxy mergers – in particular dry mergers, and that in the absence of mergers, counter-rotating orbits can also be born from highly misaligned gas accretion that leads to star formation.

Africa's Crustal Architecture Inferred From Probabilistic and Perturbational Inversion of Ambient Noise: ADAMA

AbstractAfrica's continental crust hosts a variety of geologic terrains and is crucial for understanding the evolution of its longest‐lived cratons. However, few of its seismological models are yet to incorporate the largest continent‐wide noise dispersion data sets. Here, we report on new insights into Africa's crustal architecture obtained using a new data set and model assessment product, ADAMA, which comprises a large ensemble of short‐period surface wave dispersion measurements: 5–40 s. We construct a continent‐wide model of Africa's Crust Evaluated with ADAMA's Rayleigh Phase maps (ACE‐ADAMA‐RP). Dispersion maps, and uncertainties, are obtained with a probabilistic approach. This model update, and a crustal taxonomy derived from unsupervised machine learning, reveals that the architecture of Africa's crust can be classified into two main types: primitive (C1: faster velocities with little gradients) and modified (C2–C4: slower velocities in the shallow crust with more pronounced gradients). The Archean shields are “primitive,” showing little variation or secular evolution. The basins, orogens, and continental margins are “modified” and retain imprints of surface deformation. The crustal taxonomy is obtained without a‐priori geological information and differs from previous classification schemes. While most of our reported features are robust, probabilistic modeling suggests caution in the quantitative interpretations where illumination is compromised by low‐quality measurements, sparse coverage or both. Future extension of our approach to other complementary seismological and geophysical data sets—for example, multimode earthquake dispersion, receiver functions, gravity, and mineral physics, will enable continent‐wide lithospheric modeling that extends resolution to the upper mantle.

Orbital pacing and secular evolution of lake-level changes reconstructed by sedimentary noise modeling during the Early Jurassic icehouses-(super)greenhouses

Orbital pacing and secular evolution of lake-level changes reconstructed by sedimentary noise modeling during the Early Jurassic icehouses-(super)greenhouses

The star formation histories of galaxies in different stages of pre-processing in the Fornax A group

ABSTRACT We study the recent star formation histories of 10 galaxies in the Fornax A galaxy group, on the outskirts of the Fornax cluster. The group galaxies are gas-rich, and their neutral atomic hydrogen (H i) was studied in detail with observations from the MeerKAT telescope. This allowed them to be classified into different stages of pre-processing (early, ongoing, advanced). We use long-slit spectra obtained with the South African Large Telescope (SALT) to analyse stellar population indicators to constrain quenching time-scales and to compare these to the H i gas content of the galaxies. The H α equivalent width, EW(H α), suggest that the pre-processing stage is closely related to the recent (<10 Myr) specific Star Formation Rate (sSFR). The early-stage galaxy (NGC 1326B) is not yet quenched in its outer parts, while the ongoing-stage galaxies mostly have a distributed population of very young stars, though less so in their outer parts. The galaxies in the advanced stage of pre-processing show very low recent sSFR in the outer parts. Our results suggest that NGC 1326B, FCC 35, and FCC 46 underwent significantly different histories from secular evolution during the last Gyr. The fact that most galaxies are on the secular evolution sequence implies that pre-processing has a negligible effect on these galaxies compared to secular evolution. We find EW(H α) to be a useful tool for classifying the stage of pre-processing in group galaxies. The recent sSFR and H i morphology show that galaxies in the Fornax A vicinity are pre-processing from the outside in.

Light curves for 10 R Coronae Borealis stars for longer than a century: secular evolution, dip statistics, and a general model for the shape of isolated light-curve dips

ABSTRACT R Coronae Borealis stars (RCBs) are cool supergiants that display non-periodic deep dips in brightness. Recently, a group of ‘Hot RCB stars’ has been discovered to be fast evolving across the HR (Hertzsprung-Russell) diagram, as these stars leave the RCB region, with brightness changes at the rate of ∼1 mag per century. Perhaps cool RCB stars can also be seen evolving, either increasing in temperature as they evolve to become Hot RCB stars, or perhaps increasing in luminosity as the stars arrive at the RCB region. To seek these changes, the only possible method is to extract archival data going back more than a century, looking for the brightness changes associated with the evolution. I have measured and extracted 323 464 mag (mostly from the Harvard plates and from the American Association of Variable Star Observers) for 10 cool RCB stars, all with over a century for the light curves, all consistently calibrated to a modern magnitude system. For times away from any dips, these light curves are flat to within the typical uncertainty of ±0.10 mag per century. That is, I see no significant evolution. I also have collected a large data base of light-curve dips and their properties. From this, the light curves for all the well-observed isolated dips have the same shape, featuring a flat slope for the few days immediately after the minima. Further, I derive a general model for the shape of the light curve for all isolated RCB dips, with a simple equation accurately describing the observed recovery to maximum light.

Warp and flare of the old Galactic disc as traced by the red clump stars

ABSTRACT Our study aims to investigate the outer disc structure of the Milky Way Galaxy using the red clump (RC) stars. We analysed the distribution of the largest sample of RC stars to date, homogeneously covering the entire Galactic plane in the range of 40° ≤ ℓ ≤ 340° and −10° ≤ b ≤ +10°. This sample allows us to model the RC star distribution in the Galactic disc to better constrain the properties of the flare and warp of the Galaxy. Our results show that the scale length of the old stellar disc weakly depends on azimuth, with an average value of 1.95 ± 0.26 kpc. On the other hand, a significant disc flaring is detected, where the scale height of the disc increases from 0.38 kpc in the solar neighbourhood to ∼2.2 kpc at R ≈15 kpc. The flare exhibits a slight asymmetry, with ∼1 kpc more scale height below the Galactic plane as compared to the Northern flare. We also confirm the warping of the outer disc, which can be modelled with Zw = (0.0057 ± 0.0050) [R − (7358 ± 368)(pc)]1.40 ± 0.09sin (ϕ − (− 2${_{.}^{\circ}}$03 ± 0${_{.}^{\circ}}$18)). Our analysis reveals a noticeable north–south asymmetry in the warp, with a greater amplitude observed in the southern direction compared to the northern. Comparing our findings with younger tracers from the literature, we observe an age dependency of both the flare and warp. An increase in flare strength with age suggests the secular evolution of the disc as the preferred mechanism for forming the flare. The increase of the maximum warp amplitude with age indicates that the warp dynamics could be the possible cause of the variation in the warp properties with age.

Geochemical variations of anatectic melts in response to changes of P–T–H2O conditions: Implication for the relationship between dehydration and hydration melting in the Himalayan orogen

The anatectic mechanism is the most fundamental criterion in determining the petrogenetic relationship between migmatite, granulite and granite at convergent plate margins. Although two main anatectic mechanisms, namely water-absent (dehydration) and water-fluxed (hydration) melting, have been identified in many collisional orogens, their spatiotemporal relationship and genetic connection remain obscure. To address this issue, we conduct an integrated study of forward phase equilibrium and trace element modelling for partial melting of metapelite in the Himalayan orogen. The results are used to decipher changes of phase relation and melt composition in response to changes of temperature, pressure and water content. Pressure and water content mainly affect the solidus and evolving residual mineral assemblages, whereas temperature controls the anatectic reaction. Hydration melting at lower pressure and temperature would produce anatectic melts with relatively high SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios, but low Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios. In contrast, dehydration melting at higher pressure and temperature would generate anatectic melts with relatively high Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios, but low SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios. A comparison of geochemical compositions between the modelled melts and the two groups of well-characterized leucogranites in the Himalayan orogen indicates that the coeval dehydration and hydration melting would likely occur at the deeper and shallower crustal levels, respectively. In combination with the secular evolution of metamorphic facies series in the Himalayan orogen, we propose a spatiotemporally coupled dehydration-hydration melting mechanism. The early metamorphism at lower geothermal gradients would result in metamorphic dehydration in the lower crust and hydration in the upper crust during the syn-collisional period in the Early Cenozoic. As a result, the later metamorphism at higher geothermal gradients would lead to dehydration melting in the lower crust and hydration melting in the upper crust during the post-collisional period in the Late Cenozoic. This study highlights the applicability of phase equilibrium and trace element modelling to reveal the geochemical variations of anatectic melts in response to changes of P–T–H2O conditions. This provides new insights into the relationship between dehydration and hydration melting in collisional orogens.

Dynamical aspects of Galactic habitability in N-body simulations

Abstract Recent studies of Galactic evolution revealed that the dynamics of the stellar component might be one of the key factors when considering galactic habitability. We run an N-body simulation model of the Milky Way, which we evolve for 10 Gyr, to study the secular evolution of stellar orbits and the resulting galactic habitability-related properties, i.e., the density of the stellar component and close stellar encounters. The results indicate that radial migrations are not negligible, even in a simple axisymmetric model with mild levels of dynamical heating, and that the net outward diffusion of the stellar component can populate galactic outskirts with habitable systems. Habitable environment is also likely even at sub-Solar galactocentric radii, because the rate of close encounters should not significantly degrade habitability. Stars that evolve from non-circular to stable nearly-circular orbits typically migrate outwards, settling down in a broad Solar neighborhood. The region between R ≈ 3 kpc and R ≈ 12 kpc represents the zone of radial mixing, which can blur the boundaries of the Galactic Habitable Zone, as it has been conventionally understood. The present-day stable population of the stars in the Solar neighborhood originates from this radial mixing zone, with most of the stars coming from the inner regions. The Solar system can be considered as a typical Milky Way habitable system because it migrated outwards from the metal-rich inner regions of the Disk and has a circular orbit in the present epoch. We conclude that the boundaries of the Galactic Habitable Zone cannot be sharply confined for a given epoch because of the mixing caused by the stellar migrations and secular evolution of stellar orbits.

The MAGPI survey: effects of spiral arms on different tracers of the interstellar medium and stellar populations at z ∼ 0.3

ABSTRACT Spiral structures are important drivers of the secular evolution of disc galaxies, however, the origin of spiral arms and their effects on the development of galaxies remain mysterious. In this work, we present two three-armed spiral galaxies at z ∼ 0.3 in the Middle Age Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey. Taking advantage of the high spatial resolution (∼0.6 arcsec) of the Multi-Unit Spectroscopic Explorer (MUSE), we investigate the two-dimensional (2D) distributions of different spectral parameters: Hα, gas-phase metallicity, and D4000. We notice significant offsets in Hα (∼0.2 dex) and gas-phase metallicities (∼0.05 dex) among the spiral arms, downstream and upstream of MAGPI 1202197197 (SG 1202). This observational signature suggests the spiral structure in SG 1202 is consistent with arising from density wave theory. No azimuthal variation in Hα or gas-phase metallicities is observed in MAGPI 1204198199 (SG 1204), which can be attributed to the tighter spiral arms in SG 1204 than SG 1202, coming with stronger mixing effects in the disc. The absence of azimuthal D4000 variation in both galaxies suggests the stars at different ages are well mixed between the spiral arms and distributed around the disc regions. The different azimuthal distributions in Hα and D4000 highlight the importance of time-scales traced by various spectral parameters when studying 2D distributions in spiral galaxies. This work demonstrates the feasibility of constraining spiral structures by tracing interstellar medium (ISM) and stellar population at z ∼ 0.3, with a plan to expand the study to the full MAGPI survey.

Magnetic support, wind-driven accretion, coronal heating, and fast outflows in a thin magnetically arrested disc

ABSTRACT Accretion discs properties should deviate from standard theory when magnetic pressure exceeds the thermal pressure. To quantify these deviations, we present a systematic study of the dynamical properties of magnetically arrested discs (MADs), the most magnetized type of accretion disc. Using an artificial cooling function to regulate the gas temperature, we study MADs of three different thermal thicknesses, hth/r = 0.3, 0.1, and 0.03. We find that the radial structure of the disc is never mostly supported by the magnetic field. In fact, thin MADs are very near Keplerian. However, as discs gets colder, they become more magnetized and the largest deviations from standard theory appear in our thinnest disc with hth/r = 0.03. In this case, the disc is much more extended vertically and much less dense than in standard theory because of vertical support from the turbulent magnetic pressure and wind-driven angular momentum transport that enhances the inflow speed. The thin disc also dissipates a lot of thermal energy outside of z/r = ±0.03 and a significant fraction of this dissipation happens in mildly relativistic winds. The enhanced dissipation in low-density regions could possibly feed coronae in X-ray binaries (XRBs) and active galactic nuclei (AGNs). Wind-driven accretion will also impact the dynamical evolution of accretion discs and could provide a mechanism to explain the rapid evolution of changing-look AGN and the secular evolution of XRBs. Finally, our MAD winds have terminal velocities and mass-loss rates in good agreement with the properties of ultrafast outflows observed in AGN.

Evolution of Planetary Obliquity: The Eccentric Kozai–Lidov Mechanism Coupled with Tide

Planetary obliquity plays a significant role in determining the physical properties of planetary surfaces and climate. As direct detection is constrained due to the present observation accuracy, kinetic theories are helpful for predicting the evolution of planetary obliquity. Here the coupling effect between the eccentric Kozai–Lidov effect and the equilibrium tide is extensively investigated; the planetary obliquity is observed to follow two kinds of secular evolution paths, based on the conservation of total angular momentum. The equilibrium timescale of the planetary obliquity t eq varies along with r t , which is defined as the initial timescale ratio of the tidal dissipation and secular perturbation. We numerically derive the linear relationship between t eq and r t with the maximum likelihood method. The spin-axis orientation of S-type terrestrials orbiting M-dwarfs reverses over 90° when r t > 100, then enters the quasi-equilibrium state between 40° and 60°, while the maximum obliquity can reach 130° when r t > 104. Numerical simulations show that the maximum obliquity increases with the semimajor axis ratio a 1/a 2, but is not so sensitive to the eccentricity e 2. The likelihood of an obliquity flip for S-type terrestrials in general systems with a 2 < 45 au is closely related to m 1. The observed potentially oblique S-type planets HD 42936 b, GJ 86 Ab, and τ Boo Ab are explored and found to have a great possibility of rotating head-down over the secular evolution of spin.

The JWST view of the barred galaxy population in the SMACS0723 galaxy cluster

Context. The cosmic evolution of the barred galaxy population provides key information about the secular evolution of galaxies and the settling of rotationally dominated discs. Aims. We study the bar fraction in the SMACSJ0723.37323 (SMACS0723) cluster of galaxies at z = 0.39 using the Early Release Observations obtained with the NIRCam instrument mounted on the JWST telescope. Methods. We visually inspected all cluster member galaxies using the images from the NIRCam F200W filter. We classified the galaxies into ellipticals and discs and determine the presence of a bar. The cluster member selection was based on a combined method using both the available spectroscopy and the color-magnitude relation. Results. As has previously been found in nearby galaxy samples, we find that the bar fraction distribution of SMACS0723 is a strong function of the galaxy stellar luminosity (or stellar mass). The analogy with local clusters, such as Virgo and Coma, reveals a similar distribution among the three clusters for low-mass galaxies (log(M⋆/M⊙)≤9.5). The comparison with a sample of local galaxies in a field environment shows a remarkable lack of bars in this low-mass regime for the SMACS0723 cluster (and, therefore, in Virgo and Coma) with respect to the field. At high masses (log(M⋆/M⊙)≥10.25), galaxies in SMACS0723 show a slightly lower bar fraction than those in Coma. At these high masses, we find a much larger bar fraction in SMACS0723 than previous works on field galaxies at z ∼ 0.4. Nevertheless, the difference is only marginal when we compare with a sample of well-resolved local field galaxies. Thus, we suggest that the improved capabilities of JWST with respect to HST in terms of both spatial resolution and image depth are responsible for the higher bar fraction we obtained. Conclusions. Our results support a scenario where cluster environment affects the formation of bars in a mass-dependent way. At high masses, the mild increase in the bar fraction of local clusters (Coma) with respect to both SMACS0723 and local field galaxies suggests a weak effect coming from the cluster environment possibly triggering bar formation. On the other hand, low-mass galaxies show the same bar fraction in the three clusters (different redshifts) and a significant drop with respect to field galaxies at z = 0, thus suggesting that: (i) the bar fraction of low-mass galaxies in clusters is not evolving during the last 4 Gyr; and (ii) bar formation is severely inhibited in low-mass galaxies residing in clusters.

Fitting the Light Curve of 1I/‘Oumuamua with a Nonprincipal Axis Rotational Model and Outgassing Torques

In this paper, we investigate the nonprincipal axis (NPA) rotational state of 1I/‘Oumuamua—the first interstellar object discovered traversing the inner solar system—from its photometric light curve. Building upon Mashchenko, we develop a model which incorporates NPA rotation and Sun-induced, time-varying outgassing torques to generate synthetic light curves of the object. The model neglects tidal forces, which are negligible compared to outgassing torques over the distances at which ‘Oumuamua was observed. We implement an optimization scheme that incorporates the NPA rotation model to calculate the initial rotation state of the object. We find that an NPA rotation state with an average period of 〈P〉 ≃ 7.34 hr best reproduces the photometric data. The discrepancy between this period and previous estimates is due to continuous period modulation induced by outgassing torques in the rotational model, as well as different periods being used. The best fit to the 2017 October data does not reproduce the 2017 November data (although the later measurements are too sparse to fit). The light curve is consistent with there being no secular evolution of the angular momentum, which is somewhat in tension with the empirical correlations between nuclear spin-up and cometary outgassing. The complex rotation of ‘Oumuamua may be the result of primordial rotation about the smallest principal axis if (i) the object experienced hypervolatile outgassing and (ii) our idealized outgassing model is accurate.

The main evolutionary pathways of massive hierarchical triple stars

Context. So far, stellar population studies have mainly focused on the evolution of single and binary stars. Recent observations show that triple and higher order multiple star systems are ubiquitous in the local population, especially among massive stars. Introducing three-body dynamical effects can influence the evolution of an individual stellar system and can therefore affect the predicted rates of astrophysical sources that are a product of stellar evolution. Therefore, predictions of triple star evolution are necessary for a more complete understanding of the evolutionary behaviour of stellar populations and their end products. Aims. We aim to constrain the main evolutionary pathways of massive hierarchical triple star systems and to quantify the effect of the third star on the evolution of the system. Methods. We model the massive triple star population by performing simulations of triple star evolution with the TRES code, which combines stellar evolution with secular evolution of triple systems, and explore how robust the predictions of these simulations are under variations of uncertain initial conditions. We focus on coeval, hierarchical stellar triples in pre-mass-transfer phases. Results. Interactions are common among massive triple stars. The majority of systems (65%–77%) experience a phase of mass transfer in the inner binary, often with a main sequence donor star. This differs significantly from isolated binary evolution, where mass transfer is less frequent (52.3% instead of 67% for our fiducial model) and the donors are typically post-main sequence stars. Initial constraints for dynamical stability as well as eccentricity oscillations driven by the third body facilitate the occurrence of interactions, such as mass transfer. The requirement of dynamical stability at formation places quite stringent constraints on allowed orbital properties, reducing uncertainties in triple evolution that resort from these initial conditions. Ignoring three-body dynamics during evolution of non-interacting triples leads to triple compact-object systems with stronger eccentricity oscillations and thereby likely over-predicts the merger rate of compact objects in such systems.

The dynamical evolution of protoplanetary discs and planets in dense star clusters

ABSTRACT Most stars are born in dense stellar environments, where the formation and early evolution of planetary systems may be significantly perturbed by encounters with neighbouring stars. To investigate the fate of circumstellar gas discs and planets around young stars dense stellar environments, we numerically evolve star–disc–planet systems. We use the N-body codes NBODY6++ GPU and SnIPES for the dynamical evolution of the stellar population, and the SPH-based code GaSPH for the dynamical evolution of protoplanetary discs. The secular evolution of a planetary system in a cluster differs from that of a field star. Most stellar encounters are tidal, adiabatic, and nearly-parabolic. The parameters that characterize the impact of an encounter include the orientation of the protoplanetary disc and planet relative to the orbit of the encountering star, and the orbital phase, and the semimajor axis of the planet. We investigate this dependence for close encounters (rp/a ≤ 100, where rp is the periastron distance of the encountering star and a is the semimajor axis of the planet). We also investigate distant perturbers (rp/a ≫ 100), which have a moderate effect on the dynamical evolution of the planet and the protoplanetary disc. We find that the evolution of protoplanetary discs in star clusters differs significantly from that of isolated systems. When interpreting the outcome of the planet formation process, it is thus important to consider their birth environments.

GEOCHEMISTRY AND U-Pb GEOCHRONOLOGY OF THE NEOARCHEAN GNEISSES AND PALEOPROTEROZOIC GRANITES FROM JANUÁRIA HIGH: RECORDS OF JUVENILE AND YOUTHFUL EARTH IN THE SÃO FRANCISCO CRATON NUCLEUS (BRAZIL)

Within the proposal of secular evolution of the Earth system divided into seven phases: “Proto-Earth” (4.57–4.45 Ga); “Primordial Earth” (4.45–3.80 Ga); “Primitive Earth” (3.8–3.2 Ga); “Juvenile Earth” (3.2–2.5 Ga); “Youthful Earth” (2.5–1.8 Ga); “Middle Earth” (1.8–0.8 Ga); and “Contemporary Earth” (since 0.8 Ga), the 2.61-2.64 Ga High-K, meta- to peraluminous Januária High orthogneisses are inserted in the context of almost rigid behavior of the lithosphere, by the end of the Juvenile Earth. With protolith origin suggestive of intracrustal partial melting of local older TTG (tonalite-trondhjemite-granodiorite), these orthogneisses are the representatives in the São Francisco craton nucleus of the 2.8 Ga to 2.6 Ga potassic rocks found in archean cratons worldwide, and the occurrence of these rocks is a defining characteristic of the Juvenile Earth. Supported on evidence of the plate tectonic activity in Paleoproterozoic, the 2.14-2.19 Ga Januária High biotite granites are inserted in the context of Youthful Earth and they are the record of accretionary orogeny delineated by the edification of magmatic arc at that time. They are calc-alkaline, I-type rocks typical of subduction-related continental magmatic arcs. These biotite granites are probably derived from a High-K mafic source associated with tonalites.

The linear response of stellar systems does not diverge at marginal stability

ABSTRACT The linear response of a stellar system’s gravitational potential to a perturbing mass comprises two distinct contributions. Most famously, the system will respond by forming a polarization ‘wake’ around the perturber. At the same time, the perturber may also excite one or more ‘Landau modes’, i.e. coherent oscillations of the entire stellar system which are either stable or unstable depending on the system parameters. The amplitude of the first (wake) contribution is known to diverge as a system approaches marginal stability. In this paper, we consider the linear response of a homogeneous stellar system to a point mass moving on a straight line orbit. We prove analytically that the divergence of the wake response is in fact cancelled by a corresponding divergence in the Landau mode response, rendering the total response finite. We demonstrate this cancellation explicitly for a box of stars with Maxwellian velocity distribution. Our results imply that polarization wakes may be much less efficient drivers of secular evolution than previously thought. More generally, any prior calculation that accounted for wakes but ignored modes – such as those based on the Balescu-Lenard equation – may need to be revised.

Galactic Bar Resonances with Diffusion: An Analytic Model with Implications for Bar–Dark Matter Halo Dynamical Friction

The secular evolution of disk galaxies is largely driven by resonances between the orbits of “particles” (stars or dark matter) and the rotation of non-axisymmetric features (spiral arms or a bar). Such resonances may also explain kinematic and photometric features observed in the Milky Way and external galaxies. In simplified cases, these resonant interactions are well understood: for instance, the dynamics of a test particle trapped near a resonance of a steadily rotating bar is easily analyzed using the angle-action tools pioneered by Binney, Monari, and others. However, such treatments do not address the stochasticity and messiness inherent to real galaxies—effects that have, with few exceptions, been previously explored only with complex N-body simulations. In this paper, we propose a simple kinetic equation describing the distribution function of particles near an orbital resonance with a rigidly rotating bar, allowing for diffusion of the particles’ slow actions. We solve this equation for various values of the dimensionless diffusion strength Δ, and then apply our theory to the calculation of bar–halo dynamical friction. For Δ = 0, we recover the classic result of Tremaine and Weinberg that friction ultimately vanishes, owing to the phase mixing of resonant orbits. However, for Δ > 0, we find that diffusion suppresses phase mixing, leading to a finite torque. Our results suggest that stochasticity—be it physical or numerical—tends to increase bar–halo friction, and that bars in cosmological simulations might experience significant artificial slowdown, even if the numerical two-body relaxation time is much longer than a Hubble time.