Star formation is a multi-scale, multi-physics problem ranging from the size scale of molecular clouds (
∼
10
s pc) down to the size scales of dense prestellar cores (
∼
0.1
pc) that are the birth ...sites of stars. Several physical processes like turbulence, magnetic fields and stellar feedback, such as radiation pressure and outflows, are more or less important for different stellar masses and size scales. During the last decade a variety of technological and computing advances have transformed our understanding of star formation through the use of multi-wavelength observations, large scale observational surveys, and multi-physics multi-dimensional numerical simulations. Additionally, the use of synthetic observations of simulations have provided a useful tool to interpret observational data and evaluate the importance of various physical processes on different scales in star formation. Here, we review these recent advancements in both high- (
M
≳
8
M
⊙
) and low-mass star formation.
Most, if not all, young stars are initially surrounded by protoplanetary disks. Owing to the preferential formation of stars in stellar clusters, the protoplanetary disks around these stars may ...potentially be affected by the cluster environment. Various works have investigated the influence of stellar fly-bys on disks, although many of them consider only the effects due to parabolic, coplanar encounters often for equal-mass stars, which is only a very special case. We perform numerical simulations to study the fate of protoplanetary disks after the impact of parabolic star-disk encounter for the less investigated case of inclined up to coplanar, retrograde encounters, which is a much more common case. Here, we concentrate on the disk size after such encounters because this limits the size of the potentially forming planetary systems. In addition, with the possibilities that ALMA offers, now a direct comparison to observations is possible. Covering a wide range of periastron distances and mass ratios between the mass of the perturber and central star, we find that despite the prograde, coplanar encounters having the strongest effect on the disk size, inclined and even the least destructive retrograde encounters mostly also have a considerable effect, especially for close periastron passages. Interestingly, we find a nearly linear dependence of the disk size on the orbital inclination for the prograde encounters, but not for the retrograde case. We also determine the final orbital parameters of the particles in the disk such as eccentricities, inclinations, and semi-major axes. Using this information the presented study can be used to describe the fate of disks and also that of planetary systems after inclined encounters.
Recent observations have suggested that circumstellar disks may commonly form around young stellar objects. Although the formation of circumstellar disks can be a natural result of the conservation ...of angular momentum in the parent cloud, theoretical studies instead show disk formation to be difficult from dense molecular cores magnetized to a realistic level, owing to efficient magnetic braking that transports a large fraction of the angular momentum away from the circumstellar region. We review recent progress in the formation and early evolution of disks around young stellar objects of both low-mass and high-mass, with an emphasis on mechanisms that may bridge the gap between observation and theory, including non-ideal MHD effects and asymmetric perturbations in the collapsing core (e.g., magnetic field misalignment and turbulence). We also address the associated processes of outflow launching and the formation of multiple systems, and discuss possible implications in properties of protoplanetary disks.
Aims. In this study, the main goal is to understand the molecular cloud core collapse through the stages of first and second hydrostatic core formation. We investigate the properties of Larsons first ...and second cores following the evolution of the molecular cloud core until the formation of Larson’s cores. We expand these collapse studies for the first time to span a wide range of initial cloud masses from 0.5 to 100 M⊙. Methods. Understanding the complexity of the numerous physical processes involved in the very early stages of star formation requires detailed thermodynamical modelling in terms of radiation transport and phase transitions. For this we used a realistic gas equation of state via a density- and temperature-dependent adiabatic index and mean molecular weight to model the phase transitions. We used a grey treatment of radiative transfer coupled with hydrodynamics to simulate Larsons collapse in spherical symmetry. Results. We reveal a dependence of a variety of first core properties on the initial cloud mass. The first core radius and mass increase from the low-mass to intermediate-mass regime and decrease from the intermediate-mass to high-mass regime. The lifetime of first cores strongly decreases towards the intermediate- and high-mass regimes. Conclusions. Our studies show the presence of a transition region in the intermediate-mass regime. Low-mass protostars tend to evolve through two distinct stages of formation that are related to the first and second hydrostatic cores. In contrast, in the high-mass star formation regime, collapsing cloud cores rapidly evolve through the first collapse phase and essentially immediately form Larson’s second cores.
We reveal properties of global modes of linear buoyancy instability in stars, characterized by the celebrated Schwarzschild criterion, using non-Hermitian topology. We identify a ring of exceptional ...points of order 4 that originates from the pseudo-Hermitian and pseudochiral symmetries of the system. The ring results from the merging of a dipole of degeneracy points in the Hermitian stably-stratified counterpart of the problem. Its existence is related to spherically symmetric unstable modes. We obtain the conditions for which convection grows over such radial modes. Those are met at early stages of low-mass stars formation. We finally show that a topological wave is robust to the presence of convective regions by reporting the presence of a mode transiting between the wavebands in the non-Hermitian problem, strengthening their relevance for asteroseismology.
Aims. Out of the ~3000 exoplanets detected so far, only 14 planets are members of open clusters: one of them is the exoplanet system around Pr 0211 in the cluster M44. The system consists of at least ...2 planets, and the outer planet moves on a highly eccentric orbit at 5.5 AU. One hypothesis is that a close fly-by of a neighbouring star was responsible for the eccentric orbit. We test this hypothesis. Methods. First we determined the type of fly-by that would lead to the observed parameters, and then we used this result to determine the history of such fly-bys in simulations of the early dynamics in an M44-like environment. Results. We find that although very close fly-bys are required to obtain the observed properties of Pr 0211c, such fly-bys are relatively common as a result of the high stellar density and longevity of the cluster. Such close fly-bys are most frequent during the first 1−2 Myr after cluster formation, corresponding to a cluster age ≤3 Myr. During the first 2 to 3 Myr, about 6.5% of stars experience a fly-by that would lead to such a small system-size as observed for Pr 0211 or even smaller. It is unclear whether planets generally form on such short timescales. However, after this time, the close fly-by rate is still 0.2−0.5 Myr−1, which means that 12−20% of stars would experience such close fly-bys over this time span when we extrapolate the situation to the age of M44. Conclusions. Our simulations show that the fly-by scenario is a realistic option for the formation of eccentricity orbits of the planets in M44 (Wang et al. 2015). The occurrence of such events is relatively high, leading to the expectation that similar systems are likely common in open clusters in general.
Context.
Within just two years, two interstellar objects (ISOs) – 1I/‘Oumuamua and 2I/Borisov – have been discovered, the first of their kind. Large quantities of planetesimals form as a by-product ...of planet formation. Therefore, it seems likely that ISOs are former planetesimals that became somehow unbound from their parent star. The discoveries raise the question of the dominant ISO formation process.
Aims.
Here, we concentrate on planetesimals released during another star’s close flybys. Such close flybys happen most frequently during the first 10 Myr of a star’s life. Here, we quantify the amount of planetesimals released during close stellar flybys, their ejection velocity and likely composition.
Methods.
We numerically study the dependence of the effect of parabolic flybys on the mass ratio between the perturber and parent star, the periastron distance, inclination, and angle of periastron.
Results.
As expected, close prograde flybys of high-mass stars produce the most considerable amount of ISOs. Especially flybys of stars with
M
> 5
M
⊙
on trajectories closer than 250 AU can lead to more planetesimals turning into ISOs than remaining bound to the parent star. Even strongly inclined orbits do not significantly reduce the ISO production; only retrograde flybys lead to a significantly lower ISO production. For perturbers slightly more massive than the parent star, there is a competition between ISO production and planetesimals being captured by the perturber. Whenever ISOs are produced, they leave their parent system typically with velocities in the range of 0.5–2 km s
−1
. This ejection velocity is distinctly different to that of ISOs produced by planet scattering (~4–8 km s
−1
) and those shed during the stellar post-main-sequence phase (~0.1–0.2 km s
−1
). Using the typical disc truncation radius in various cluster environments, we find that clusters like the Orion nebula cluster are likely to produce the equivalent of 0.85 Earth-masses of ISOs per star. In contrast, compact clusters like NGC 3603 could produce up to 50 Earth-masses of ISOs per star. Our solar-system probably produced the equivalent of 2–3 Earth masses of ISOs, which left our solar system at a mean ejection velocity of 0.7 km s
−1
.
Conclusions.
Most ISOs produced by flybys should be comet-like, similar to Borisov and unlike ‘Oumuamua. ISOs originating from compact long-lived clusters would often show a deficiency in CO. As soon as a statistically significant sample of ISOs is discovered, the combined information of their observed velocities and composition might help in constraining the dominant production process.
Context . Signposts of early planet formation are ubiquitous in substructured young discs. Dense, hot, and high-pressure regions that formed during the gravitational collapse process, integral to ...star formation, facilitate dynamical mixing of dust within the protostellar disc. This provides an incentive to constrain the role of gas and dust interaction and resolve potential zones of dust concentration during star and disc formation stages. Aims . We explore whether the thermal and dynamical conditions that developed during protostellar disc formation can generate gas flows that efficiently mix and transport the well-coupled gas and dust components. Methods . We simulated the collapse of dusty molecular cloud cores with the hydrodynamics code PLUTO augmented with radiation transport and self-gravity. We used a two-dimensional axisymmetric geometry and followed the azimuthal component of the velocity. The dust was treated as Lagrangian particles that are subject to drag from the gas, whose motion is computed on a Eulerian grid. We considered 1, 10, and 100 µm-sized neutral, spherical dust grains. Importantly, the equation of state accurately includes molecular hydrogen dissociation. We focus on molecular cloud core masses of 1 and 3 M ⊙ and explore the effects of different initial rotation rates and cloud core sizes. Results . Our study underlines mechanisms for the early transport of dust from the inner hot disc regions via the occurrence of two transient gas motions, namely meridional flow and outflow. The vortical flow fosters dynamical mixing and retention of dust, while the thermal pressure driven outflow replenishes dust in the outer disc. Notably, these phenomena occur regardless of the initial cloud core mass, size, and rotation rate. Conclusions . Young dynamical precursors to planet-forming discs exhibit regions with complex hydrodynamical gas features and high-temperature structures. These can play a crucial role in concentrating dust for subsequent growth into protoplanets. Dust transport, especially, from sub-au scales surrounding the protostar to the outer relatively cooler parts, offers an efficient pathway for thermal reprocessing during pre-stellar core collapse.
Most stars form as part of a stellar group. These young stars are mostly surrounded by a disk from which potentially a planetary system might form. Both, the disk and later on the planetary system, ...may be affected by the cluster environment due to close fly-bys. The here presented database can be used to determine the gravitational effect of such fly-bys on non-viscous disks and planetary systems. The database contains data for fly-by scenarios spanning mass ratios between the perturber and host star from 0.3 to 50.0, periastron distances from 30 au to 1000 au, orbital inclination from 0
∘
to 180
∘
and angle of periastron of 0
∘
, 45
∘
and 90
∘
. Thus covering a wide parameter space relevant for fly-bys in stellar clusters. The data can either be downloaded to perform one’s own diagnostics like for e.g. determining disk size, disk mass, etc. after specific encounters, obtain parameter dependencies or the different particle properties can be visualized interactively. Currently the database is restricted to fly-bys on parabolic orbits, but it will be extended to hyperbolic orbits in the future. All of the data from this extensive parameter study is now publicly available as DESTINY.
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