Primordial Disk Research Articles

Structure Formation Through Magnetohydrodynamical Instabilities in Primordial Disks

The shear flow instabilities under the presence of magnetic fields in the primordial disk can greatly facilitate the formation of density structures that serve as seeds prior to the onset of the gravitational Jeans instability. We evaluate the effects of the Parker, magnetorotational and kinematic dynamo instabilities by comparing the properties of these instabilities. We calculate the mass spectra of coagulated density structures by the above mechanism in the radial direction for an axisymmetric magnetohydrodynamic (MHD) torus equilibrium and power density profile models. Our local three-dimensional MHD simulation indicates that the coupling of the Parker and magnetorotational instability creates spiral arms and gas blobs in an accretion disk, reinforcing the theory and model. Such a mechanism for the early structure formation may be tested in a laboratory. The recent progress in experiments involving shear flows in rotating tokamak, field reversed configuration (FRC) and laser plasmas may become a key element to advance in nonlinear studies.

Evidence for Primordial Alignment: Insights from Stellar Obliquity Measurements for Compact Sub-Saturn Systems

Despite decades of effort, the mechanisms by which the spin axis of a star and the orbital axes of its planets become misaligned remain elusive. In particular, it is of great interest whether the large spin–orbit misalignments observed are driven primarily by high-eccentricity migration—expected to have occurred for short-period, isolated planets—or reflect a more universal process that operates across systems with a variety of present-day architectures. Compact multiplanet systems offer a unique opportunity to differentiate between these competing hypotheses, as their tightly packed configurations preclude violent dynamical histories, including high-eccentricity migration, allowing them to trace the primordial disk plane. In this context, we report measurements of the sky-projected stellar obliquity (λ) via the Rossiter–McLaughlin effect for two sub-Saturns in multiple-transiting systems: TOI-5126 b (λ = 1 ± 48°) and TOI-5398 b ( λ=−8.1−6.3+5.3° ). Both are spin–orbit aligned, joining a fast-growing group of just three other compact sub-Saturn systems, all of which exhibit spin–orbit alignment. In aggregate with archival data, our results strongly suggest that sub-Saturn systems are primordially aligned and become misaligned largely in the postdisk phase, as appears to be the case increasingly for other exoplanet populations.

More realistic planetesimal masses alter Kuiper belt formation models and add stochasticity

Much of the modern Kuiper belt is thought to be the result of Neptune’s migration through a primordial belt of planetesimals, possibly interrupted by an instability amongst the giant planets. While most prior work has employed massless test particles to study this Kuiper belt formation process, we perform simulations here that include the gravitational effects of the primordial planetesimal belt consisting of ∼105 massive bodies. In our simulations, Neptune unlocks from resonance with the other giant planets and begins to migrate outward due to interactions with planetesimals before a planetary orbital instability is triggered, and afterward, residual Neptunian migration completes the formation of the modern Kuiper belt. Compared to past simulations using massless test particles, our present work exhibits a number of notable differences. First, Neptune’s planetary resonance unlocking requires the Neptunian 3:2 mean motion resonance to sweep much of the primordial disk interior to 30 au prior to the giant planet instability. This generates a pre-instability population of planetesimals that is significantly lower in semimajor axis, eccentricity, and inclination than predicted by many prior models, and this effect persists after the instability. Second, direct scattering between Pluto-mass bodies and other small bodies removes material from Neptunian resonances significantly more efficiently than the resonant dropout resulting from small changes in Neptune’s semimajor axis during scattering between Pluto-mass bodies and Neptune. Consequently, the primordial population of Pluto-mass bodies may be as few as ∼200 objects. Finally, our simulation end states display a wide variety of orbital distributions, and straightforward relationships between final bulk Kuiper belt properties and Neptune’s migration or initial planetesimal properties largely elude us. In particular, we find that the rapid, stochastic planetary orbital evolution occurring during the giant planet instability can significantly alter final Kuiper belt properties such as its inclination dispersion and the prominence of resonant populations. This complicates efforts to use modern Kuiper belt properties to confidently constrain early solar system events and conditions, including planetary orbital migration and the primordial Kuiper belt’s characteristics.

Stellar populations and the origin of thick disks in AURIGA simulations

Context. Recent integral-field spectroscopy observations of edge-on galaxies have led to significant progress in our knowledge of the ages and chemical compositions of thick disks. However, the origin of thick disks and their evolutionary connection with thin disks is still a matter of debate. Aims. We provide new insights into this topic by connecting the stellar populations of thick disks at redshift z = 0 with their past formation and growth in 24 Milky Way-mass galaxies from the AURIGA zoom-in cosmological simulations. We assess the role played by mergers of satellite galaxies in the mass assembly of geometrically defined thick disks. Methods. We projected each galaxy edge on and decomposed it morphologically into two disk components in order to geometrically define the thin and thick disks, as is usually done in observations of external galaxies. We produced age, metallicity, and [Mg/Fe] edge-on maps of the 24 galaxies. We quantified the impact of satellite mergers by mapping the distribution of ex situ stars. Results. Thick disks are on average ∼3 Gyr older, ∼0.25 dex more metal poor, and ∼0.06 dex more [Mg/Fe]-enhanced than thin disks. Their average ages range from ∼6 to ∼9 Gyr, metallicities from ∼ − 0.15 to ∼0.1 dex, and [Mg/Fe] from ∼0.12 to ∼0.16 dex. These properties are the result of an early initial in situ formation, followed by a later growth driven by the combination of direct accretion of stars, some in situ star formation fueled by mergers, and dynamical heating of stars. The balance between these processes varies from galaxy to galaxy and impacts thick-disk ages and metallicities. The oldest thick disks (older than 8 Gyr) are hosted by galaxies with a low mass fraction of accreted stars (below 8%), while the youngest thick disks (younger than 7 Gyr) are found in galaxies with higher accreted fractions (larger than 25%). Mergers play a key role in the mass assembly of thick disks, contributing an average accreted mass fraction of ∼22% in the analyzed thick-disk-dominated regions. In two galaxies, about half of the geometric thick-disk mass was directly accreted. The mass fraction of accreted stars is lower than 10% only in four thick disks. While primordial thick disks form at high redshifts in all galaxies, young metal-rich thin disks, with much lower [Mg/Fe] abundances, start to form later but at different times (at higher or lower redshifts) depending on the galaxy. Conclusions. We conclude that thick disks, although mostly formed in situ, grow thanks to the significant contribution of satellite mergers, especially through the direct accretion of stars. They result from the interplay of external processes with the internal evolution of the galaxy.

The evolution of collision debris near the ν6 secular resonance and its role in the origin of terrestrial water

ABSTRACT This work presents novel findings that broaden our understanding of the amount of water that can be transported to the Earth. The key innovation lies in the combined usage of smoothed particle hydrodynamics (SPH) and N-body codes to assess the role of collision fragments in water delivery. We also present a method for generating initial conditions that enables the projectile to impact at the designated location on the target’s surface with the specified velocity. The primary objective of this study is to simulate giant collisions between two Ceres-sized bodies by SPH near the ν6 secular resonance and follow the evolution of the ejected debris by numerical N-body code. With our method 6 different initial conditions for the collision were determined, and the corresponding impacts were simulated by SPH. Examining the orbital evolution of the debris ejected after collisions, we measured the amount of water delivered to the Earth, which is broadly 0.001 ocean equivalents of water, except in one case where one large body transported 7 per cent oceans of water to the planet. Based on this and taking into account the frequency of collisions, the amount of delivered water varies between 1.2 and 8.3 ocean’s worth of water, depending on the primordial disc mass. According to our results, the prevailing external pollution model effectively accounts for the assumed water content on the Earth, whether it’s estimated at 1 or 10 ocean’s worth of water.

Revisiting the membership, multiplicity, and age of the Beta Pictoris Moving Group in the Gaia era

ABSTRACT Determining the precise ages of young (tens to a few hundred Myr) kinematic (‘moving’) groups is important for placing star, protoplanetary disc, and planet observations on an evolutionary timeline. The nearby ∼25 Myr-old β Pictoris Moving Group (BPMG) is an important benchmark for studying stars and planetary systems at the end of the primordial disc phase. Gaia DR3 astrometry and photometry, combined with ground-based observations and more sophisticated stellar models, permit a systematic re-evaluation of BPMG membership and age. We combined Gaia astrometry with previously published radial velocities to evaluate moving group membership in a Bayesian framework. To minimize the effect of unresolved stellar multiplicity on age estimates, we identified and excluded multistar systems using Gaia astrometry, ground-based adaptive optics imaging, and multi-epoch radial velocities, as well as literature identifications. We estimated age using isochrone and lithium-depletion-boundary fitting with models that account for the effect of magnetic activity and spots on young, rapidly rotating stars. We find that age estimates are highly model-dependent; Dartmouth magnetic models with ages of 23 ± 8 and 33$^{+9}_{-11}$ Myr provide best fits to the lithium depletion boundary and Gaia MG versus BP–RP colour–magnitude diagram, respectively, whereas a Dartmouth standard model with an age of 11$^{+4}_{-3}$ Myr provides a best fit to the 2-Micron All-Sky Survey-Gaia$M_{K_S}$ versus BP–RP colour–magnitude diagram.

Chaotic Capture of a Retrograde Moon by Venus and the Reversal of Its Spin

Planets are surrounded by fractal surfaces (traditionally called Hill spheres), separating the inner zones of long-term stable orbital motion of their satellites from the outer space where the gravitational pull from the Sun takes over. Through this surface, external minor bodies in trajectories loosely co-orbital to a planet can be stochastically captured by the planet without any assistance from external perturbative forces, and can become moons chaotically orbiting the planet for extended periods of time. Using state-of-the-art orbital integrators, we simulate such capture events for Venus, resulting in long-term attachment phases by reversing the forward integration of a moon initially attached to the planet and escaping it after an extended period of time. Chaotic capture of a retrograde moon from a prograde heliocentric orbit appears to be more probable because the Hill sphere is almost four times larger in area for a retrograde orbit than for a prograde orbit. Simulated capture trajectories include cases with attachment phases up to 860,000 years for prograde moons and up to 370,000 years for retrograde moons. Although the probability of a long-term chaotic capture from a single encounter is generally low, the high density of co-orbital bodies in the primordial protoplanetary disk makes this outcome possible, if not probable. The early Venus was surrounded by a dusty gaseous disk of its own, which, coupled with the tidal dissipation of the kinetic energy in the moon and the planet, could shrink the initial orbit and stabilize the captured body within the Hill surface. The tidal torque from the moon, for which we use the historical name Neith, gradually brakes the prograde rotation of Venus, and then reverses it, while the orbit continues to decay. Neith eventually reaches the Roche radius and disintegrates, probably depositing most of its material on Venus’ surface. Our calculations show that surface density values of about 0.06 kg m−2 for the debris disk may be sufficient to stabilize the initial chaotic orbit of Neith and to bring it down within several radii of Venus, where tidal dissipation becomes more efficient.

A Primordial Origin for the Gas-rich Debris Disks around Intermediate-mass Stars

While most debris disks consist of dust with little or no gas, a fraction have significant amounts of gas detected via emission lines of CO, ionized carbon, and/or atomic oxygen. Almost all such gaseous debris disks known are around A-type stars with ages up to 50 Myr. We show, using semianalytic disk evolution modeling, that this can be understood if the gaseous debris disks are remnant protoplanetary disks that have become depleted of small grains compared to the interstellar medium. Photoelectric heating by the A stars’ far-UV (FUV) radiation is then inefficient, while the stars’ extreme-UV (EUV) and X-ray emissions are weak owing to a lack of surface convective zones capable of driving magnetic activity. In this picture, it is relatively difficult for stars outside the range of spectral types from A through early B to have such long-lived gas disks. Less-massive stars have stronger magnetic activity in the chromosphere, transition region, and corona with resulting EUV and X-ray emission, while more-massive stars have photospheres hot enough to produce strong EUV radiation. In both cases, primordial disk gas is likely to photoevaporate well before 50 Myr. These results come from 0D disk evolution models where we incorporate internal accretion stresses, MHD winds, and photoevaporation by EUV and X-ray photons with luminosities that are functions of the stellar mass and age. A key issue this work leaves open is how some disks become depleted in small dust so that FUV photoevaporation slows. Candidates include the grains’ growth, settling, radial drift, radiation force, and incorporation into planetary systems.

The GAPS programme at TNG

Context. Observational data from space- and ground-based campaigns have revealed that the 10-30 Ma old V1298Tau star hosts a compact and massive system of four planets. Mass estimates are available for the two outer giant planets and point to unexpectedly high densities for their young ages. Aims. We investigate the formation of these two outermost giant planets, V1298 Tau b and e, and the present dynamical state of V1298 Tau’s global architecture in order to shed light on the history of this young and peculiar extrasolar system. Methods. We performed detailed N-body simulations to explore the link between the densities of V1298 Tau b and e and their migration and accretion of planetesimals within the native circumstellar disk. We combined N-body simulations and the normalized angular momentum deficit (NAMD) analysis of the architecture to characterize V1298 Tau’s dynamical state and connect it to the formation history of the system. We searched for outer planetary companions to constrain V1298 Tau’s planetary architecture and the extension of its primordial circumstellar disk. Results. The high densities of V1298 Tau b and e suggest they formed at quite a distance from their host star, likely beyond the CO2 snowline. The higher nominal density of V1298 Tau e suggests it formed farther out than V1298 Tau b. The current architecture of V1298 Tau is not characterized by resonant chains. Planet-planet scattering with an outer giant planet is the most likely cause for the lack of a resonant chain between V1298 Tau’s planets, but currently our search for outer companions using SPHERE and Gaia observations can exclude only the presence of planets more massive than 2 MJ. Conclusions. The most plausible scenario for V1298 Tau’s formation is that the system formed by convergent migration and resonant trapping of planets born in a compact and plausibly massive disk. In the wake of their migration, V1298 Tau b and e would have left a dynamically excited protoplanetary disk, naturally creating the conditions for the later breaking of the resonant chain by planet-planet scattering.

Chemical footprints of giant planet formation

Context. Protoplanetary disks, the birthplaces of planets, commonly feature bright rings and dark gaps in both continuum and line emission maps. Accreting planets interact with the disk, not only through gravity, but also by changing the local irradiation and elemental abundances, which are essential ingredients for disk chemistry. Aims. We propose that giant planet accretion can leave chemical footprints in the gas local to the planet, which potentially leads to the spatial coincidence of molecular emissions with the planet in the ALMA observations. Methods. Through 2D multi-fluid hydrodynamical simulations in Athena++ with built-in sublimation, we simulated the process of an accreting planet locally heating up its vicinity, opening a gas gap in the disk, and creating the conditions for C-photochemistry. Results. An accreting planet located outside the methane snowline can render the surrounding gas hot enough to sublimate the C-rich organics off pebbles before they are accreted by the planet. This locally elevates the disk gas-phase C/O ratio, providing a potential explanation for the C2H line-emission rings observed with ALMA. In particular, our findings provide an explanation for the MWC 480 disk, where previous work identified a statistically significant spatial coincidence of line-emission rings inside a continuum gap. Conclusions. Our findings present a novel view of linking the gas accretion of giant planets and their natal disks through the chemistry signals. This model demonstrates that giant planets can actively shape their forming chemical environment, moving beyond the traditional understanding of the direct mapping of primordial disk chemistry onto planets.

Long-term Evolution of Warps in Debris Disks—Application to the Gyr-old System HD 202628

We present the results of N-body simulations meant to reproduce the long-term effects of mutually inclined exoplanets on debris disks, using the HD 202628 system as a proxy. HD 202628 is a Gyr-old solar-type star that possesses a directly observable, narrow debris ring with a clearly defined inner edge and nonzero eccentricity, hinting at the existence of a sculpting exoplanet. The eccentric nature of the disk leads us to examine the effect on it over Gyr timescales from an eccentric and inclined planet, placed on its orbit through scattering processes. We find that, in systems with dynamical timescales akin to that of HD 202628, a planetary companion is capable of completely tilting the debris disk. This tilt is preserved over the Gyr age of the system. Simulated observations of our models show that an exoplanet around HD 202628 with an inclination misalignment ≳10° would cause the disk to be observably diffuse and broad, which is inconsistent with Atacama Large Millimeter Array (ALMA) observations. With these observations, we conclude that, if there is an exoplanet shaping this disk, it likely had a mutual inclination of less than 5° with the primordial disk. The conclusions of this work can be applied either to debris disks appearing as narrow rings (e.g., Fomalhaut and HR 4796) or to disks that are vertically thick at ALMA wavelengths (e.g., HD 110058).

On the likelihoods of finding very metal-poor (and old) stars in the Milky Way’s disc, bulge, and halo

ABSTRACT Recent observational studies have uncovered a small number of very metal-poor (VMP) stars with cold kinematics in the Galactic disc and bulge. However, their origins remain enigmatic. We select a total of 138 Milky Way (MW) analogues from the TNG50 cosmological simulation based on their z = 0 properties: discy morphology, stellar mass, and local environment. In order to make more predictive statements for the MW, we further limit the spatial volume coverage of stellar populations in galaxies to that targeted by the upcoming 4MOST high-resolution survey of the Galactic disc and bulge. We find that across all galaxies, ∼20 per cent of VMP ([Fe/H] < −2) stars belong to the disc, with some analogues reaching 30 per cent. About 50 ± 10 per cent of the VMP disc stars are, on average, older than 12.5 Gyr and ∼70 ± 10 per cent come from accreted satellites. A large fraction of the VMP stars belong to the halo (∼70) and have a median age of 12 Gyr. Our results with the TNG50 cosmological simulation confirm earlier findings with simulations of fewer individual galaxies, and suggest that the stellar disc of the MW is very likely to host significant amounts of very- and extremely-metal-poor stars that, although mostly of ex situ origin, can also form in situ, reinforcing the idea of the existence of a primordial Galactic disc.

Accretion of aerodynamically large pebbles

ABSTRACT Due to their aerodynamical coupling with gas, pebbles in protoplanetary discs can drift over large distances to support planet growth in the inner disc. In the past decade, this pebble accretion has been studied extensively for aerodynamically small pebbles (Stokes number St < 1). However, accretion can also operate in the St > 1 mode, e.g. when planetesimals collisionally fragment to smaller bodies or when the primordial gas disc disperses. This work aims to extend the study of pebble accretion to these aerodynamically loosely coupled particles. We integrate the pebble’s equation of motion, accounting for gas drag, stellar and planetary gravity, in the mid-plane of a laminar disc. The accretion probability (ϵ) is calculated as function of Stokes number, disc pressure gradient index, planet mass, and eccentricity. We find that for Stokes number above unity ϵ(St) first rises, due to lower drift and aided by a large atmospheric capture radius, until it reaches a plateau where the efficiency approaches 100 per cent. At high St, the plateau region terminates as particles become trapped in resonance. These results are well described by a semi-analytical ‘kick-and-drift’ model and we also provide fully analytical prescriptions for ϵ. We apply our model to the accretion of ${\sim }30\, \mu \mathrm{m}$ dust particles in a dispersing protoplanetary and secondary (CO-rich) debris disc. It shows that such physically small particles are mainly accreted as aerodynamically large Stokes number pebbles during the debris disc phase. Earth-mass planets may obtain ∼25 per cent of their heavy elements through this late accretion phase.

Collisional heating of icy planetesimals – I. Catastrophic collisions

ABSTRACTPlanetesimals in the primordial disc may have experienced a collisional cascade. If so, the comet nuclei later placed in the Kuiper belt, scattered disc, and Oort Cloud would primarily be fragments and collisional rubble piles from that cascade. However, the heating associated with the collisions cannot have been strong enough to remove the hypervolatiles that are trapped within more durable ices, because comet nuclei are rich in hypervolatiles. This places constraints on the diameter of the largest bodies allowed to participate in collisional cascades, and limits the primordial disc lifetime or population size. In this paper, the thermophysical code nimbus is used to study the thermal evolution of planetesimals before, during, and after catastrophic collisions. The loss of CO during segregation of CO2:CO mixtures and during crystallization of amorphous H2O is calculated, as well as mobilization and internal relocation of CO2. If an amorphous H2O host existed, and was protected by a CO2:CO heat sink, only diameter $D\lt 20\, \mathrm{km}$ (inner disc) and $D\lt 64\, \mathrm{km}$ (outer disc) bodies could have been involved in a collisional cascade. If CO2 was the only CO host, the critical diameters drop to D < 20–$32\, \mathrm{km}$. Avoiding disruption of larger bodies requires a primordial disc lifetime of <9 Myr at 15 au and <50–70 Myr at 30 au. Alternatively, if a 450 Myr disc lifetime is required to associate the primordial disc disruption with the Late Heavy Bombardment, the disc population size must have been 6–60 times below current estimates.

The chemical abundance pattern of the extremely metal-poor thin disc star 2MASS J1808−5104 and its origins

ABSTRACT We present a high-resolution (R ∼ 35 000), high signal-to-noise (S/N = 350) Magellan/MIKE spectrum of the bright extremely metal-poor star 2MASS J1808−5104. We find [Fe/H] = −4.01 (spectroscopic LTE stellar parameters), [Fe/H] = −3.8 (photometric stellar parameters), and [Fe/H] = −3.7 (spectroscopic NLTE stellar parameters). We measured a carbon-to-iron ratio of [C/Fe] = 0.38 from the CH G-band. J1808−5104 is thus not carbon-enhanced, contrary to many other stars with similarly low-iron abundances. We also determine, for the first time, a barium abundance ([Ba/Fe] = −0.78), and obtain a significantly reduced upper limit for the nitrogen abundance ([N/Fe] < −0.2). For its [Ba/Fe] abundance, J1808−5104 has a lower [Sr/Ba] ratio compared to other stars, consistent with behaviour of stars in ultra-faint dwarf galaxies. We also fit the abundance pattern of J1808−5104 with nucleosynthesis yields from a grid of Population III supernova models. There is a good fit to the abundance pattern that suggests J1808−5104 originated from gas enriched by a single massive supernova with a high explosion energy of E = 10 × 1051 erg and a progenitor stellar mass of M = 29.5 M⊙. Interestingly, J1808−5104 is a member of the Galactic thin disc, as confirmed by our detailed kinematic analysis and calculated stellar actions and velocities. Finally, we also established the orbital history of J1808−5104 using our time-dependent Galactic potential the ORIENT. J1808−5104 appears to have a stable quasi-circular orbit and been largely confined to the thin disc. This unique orbital history, the star’s very old age (∼13.5 Gyr), and the low [C/Fe] and [Sr/Ba] ratios suggest that J1808−5104 may have formed at the earliest epoch of the hierarchical assembly of the Milky Way, and it is most likely associated with the primordial thin disc.

On the Unusual Variability of 2MASS J06195260–2903592: A Long-lived Disk around a Young Ultracool Dwarf

We present the characterization of the low-gravity M6 dwarf 2MASS J06195260-2903592, previously identified as an unusual field object based on its strong IR excess and variable near-IR spectrum. Multiple epochs of low-resolution (R ≈ 150) near-IR spectra show large-amplitude (≈0.1–0.5 mag) continuum variations on timescales of days to 12 yr, unlike the small-amplitude variability typical for field ultracool dwarfs. The variations between epochs are well-modeled as changes in the relative extinction (ΔA V ≈ 2 mag). Similarly, Panoramic Survey Telescope and Rapid Response System 1 optical photometry varies on timescales as long as 11 yr (and possibly as short as an hour) and implies comparable A V changes. Near Earth Object Wide-field Infrared Survey Explorer mid-IR light curves also suggest changes on 6 month timescales, with amplitudes consistent with the optical/near-IR extinction variations. However, near-IR spectra, near-IR photometry, and optical photometry obtained in the past year indicate that the source can also be stable on hourly and monthly timescales. From comparison to objects of similar spectral type, the total extinction of 2MASS J0619-2903 seems to be A V ≈ 4–6 mag, with perhaps epochs of lower extinction. Gaia Early Data Release 3 (EDR3) finds that 2MASS J0619-2903 has a wide-separation (1.′2 = 10,450 au) stellar companion, with an isochronal age of Myr and a mass of M ☉. Adopting this companion’s age and EDR3 distance (145.2 ± 0.6 pc), we estimate a mass of 0.11–0.17 M ☉ for 2MASS J0619-2903. Altogether, 2MASS J0619-2903 appears to possess an unusually long-lived primordial circumstellar disk, perhaps making it a more obscured analog to the “Peter Pan” disks found around a few M dwarfs in nearby young moving groups.

Impact of magnetic fields on Population III star formation

ABSTRACT The theory of the formation of the first stars in the Universe, the so-called Population III (Pop III), has until now largely neglected the impact of magnetic fields. Complementing a series of recent studies of the magnetohydrodynamic (MHD) aspects of Pop III star formation, we here carry out a suite of idealized numerical experiments where we ascertain how the fragmentation properties of primordial protostellar discs are modified if MHD effects are present. Specifically, starting from cosmological initial conditions, we focus on the central region in a select minihalo at redshift z ∼ 25, inserting a magnetic field at an intermediate evolutionary stage, normalized to a fraction of the equipartition value. To explore parameter space, we consider different field geometries, including uniform, radial, toroidal, and poloidal field configurations, with the toroidal configuration being the most realistic. The collapse of the gas is followed for ∼8 orders of magnitude in density after the field was inserted, until a maximum of $10^{15} {\rm \, cm}^{-3}$ is reached. We find that the magnetic field leads to a delay in the collapse of the gas. Moreover, the toroidal field has the strongest effect on the collapse as it inhibits the fragmentation of the emerging disc surrounding the central core and leads to the formation of a more massive core. The full understanding of the formation of Pop III stars and their mass distribution thus needs to take into account the effect of magnetic fields. We further conclude that ideal MHD is only a first step in this endeavour, to be followed up with a comprehensive treatment of dissipative effects, such as ambipolar diffusion and Ohmic dissipation.

Chandra Observations of Six Peter Pan Disks: Diversity of X-Ray-driven Internal Photoevaporation Rates Does Not Explain Their Rare Longevity

We present Chandra X-ray observations of six previously identified Peter Pan objects, rare ∼40 Myr systems with evidence of primordial disk retention. We observe X-ray luminosities (0.8–3.0 keV) ranging from log L x ∼ 27.7–29.1. We find that our Peter Pan sample exhibits X-ray properties similar to that of weak-lined T Tauri stars and do not exhibit evidence of stellar accretion induced X-ray suppression. Our observed Peter Pan X-ray luminosities are consistent with that measured for field dM stars of similar spectral type and age, implying their long primordial disk lifetimes are likely not a consequence of unusually faint X-ray host stars. Our derived X-ray photoevaporative mass-loss rates predict our systems have passed the point of rapid gas dispersal and call into question the impact of this internal mechanism for primordial disk dispersal around dM stars. Our qualitative assessment of the surrounding Peter Pan environments also does not predict unusually low levels of external photoevaporation relative to other respective moving group members. Overall, our results suggest Peter Pan disks may be a consequence of the low far-UV flux incident on the disk in low-mass dM stars given their relatively lower levels of accretion over the course of their pre-main-sequence evolution.

Forming short period sub-stellar companions in 47 Tucanae – II. Analytical expressions for the orbital evolution of planets in dense environments

ABSTRACT Short period, massive planets, known as hot Jupiters (HJs), have been discovered around ∼1 per cent of local field stars. The inward migration necessary to produce HJs may be ‘low eccentricity’, due to torques in the primordial disc, or ‘high eccentricity’ (HEM). The latter involves exciting high orbital eccentricity, allowing sufficiently close passages with the host star to raise circularizing tides in the planet. We present an analytical framework for quantifying the role of dynamical encounters in high density environments during HEM. We show that encounters can enhance or suppress HEM, depending on the local stellar density and the initial semimajor axis a0. For moderate densities, external perturbations can excite large eccentricities that allow a planet to circularize over the stellar lifetime. At extremely high densities, these perturbations can instead result in tidal disruption of the planet, thus yielding no HJ. This may explain the apparent excess of HJs in M67 compared with their local field star abundance versus their apparent deficit in 47 Tuc. Applying our analytical framework, we demonstrate that for an initial massive planet population similar to the field, the expected HJ occurrence rate in 47 Tuc is fHJ = 2.2 × 10−3, which remains consistent with present constraints. Future large (sample sizes ≳105) or sensitive transit surveys of stars in globular clusters are required to refute the hypothesis that the initial planet population is similar to the solar neighbourhood average. Non-detection in such surveys would have broad consequences for planet formation theory, implying planet formation rates in globular clusters must be suppressed across a wide range of a0.

Is the orbital distribution of multiplanet systems influenced by pure three-planet resonances?

ABSTRACT We analyse the distribution of known multiplanet systems (N ≥ 3) in the plane of mean-motion ratios, and compare it with the resonance web generated by two-planet mean-motion resonances (2P-MMR) and pure three-planet commensurabilities (pure 3P-MMR). We find intriguing evidence of a statistically significant correlation between the observed distribution of compact low-mass systems and the resonance structure, indicating a possible causal relation. While resonance chains, such as Kepler-60, Kepler-80, and TRAPPIST-1, are strong contributors, most of the correlation appears to be caused by systems not identified as resonance chains. Finally, we discuss their possible origin through planetary migration during the last stages of the primordial disc and/or an eccentricity damping process.