ALBERT

Beschreibung: We examine the performance of four different methods which are used to measure mass segregation in star-forming regions: the radial variation of the mass function $\mathcal {M}_{\rm MF}$ ; the minimum spanning tree-based MSR method; the local surface density LDR method; and the GSR technique, which isolates groups of stars and determines whether the most massive star in each group is more centrally concentrated than the average star. All four methods have been proposed in the literature as techniques for quantifying mass segregation, yet they routinely produce contradictory results as they do not all measure the same thing. We apply each method to synthetic star-forming regions to determine when and why they have shortcomings. When a star-forming region is smooth and centrally concentrated, all four methods correctly identify mass segregation when it is present. However, if the region is spatially substructured, the GSR method fails because it arbitrarily defines groups in the hierarchical distribution, and usually discards positional information for many of the most massive stars in the region. We also show that the MSR and LDR methods can sometimes produce apparently contradictory results, because they use different definitions of mass segregation. We conclude that only MSR measures mass segregation in the classical sense (without the need for defining the centre of the region), although LDR does place limits on the amount of previous dynamical evolution in a star-forming region.

Beschreibung: The presence and abundance of the short-lived radioisotopes (SLRs) 26 Al and 60 Fe during the formation of the Solar system is difficult to explain unless the Sun formed in the vicinity of one or more massive star(s) that exploded as supernovae. Two different scenarios have been proposed to explain the delivery of SLRs to the protosolar nebula: (i) direct pollution of the protosolar disc by supernova ejecta, and (ii) the formation of the Sun in a sequential star formation event in which supernovae shockwaves trigger further star formation which is enriched in SLRs. The sequentially triggered model has been suggested as being more astrophysically likely than the direct pollution scenario. In this paper, we investigate this claim by analysing a combination of N -body and smoothed particle hydrodynamics simulations of star formation. We find that sequential star formation would result in large age spreads (or even bi-modal age distributions for spatially coincident events) due to the dynamical relaxation of the first star formation event(s). Secondly, we discuss the probability of triggering spatially and temporally discrete populations of stars and find this to be only possible in very contrived situations. Taken together, these results suggest that the formation of the Solar system in a triggered star formation event is as improbable, if not more so, than the direct pollution of the protosolar disc by a supernova.

Beschreibung: Heating by short-lived radioisotopes (SLRs) such as 26 Al and 60 Fe fundamentally shaped the thermal history and interior structure of Solar system planetesimals during the early stages of planetary formation. The subsequent thermo-mechanical evolution, such as internal differentiation or rapid volatile degassing, yields important implications for the final structure, composition and evolution of terrestrial planets. SLR-driven heating in the Solar system is sensitive to the absolute abundance and homogeneity of SLRs within the protoplanetary disc present during the condensation of the first solids. In order to explain the diverse compositions found for extrasolar planets, it is important to understand the distribution of SLRs in active planet formation regions (star clusters) during their first few Myr of evolution. By constraining the range of possible effects, we show how the imprint of SLRs can be extrapolated to exoplanetary systems and derive statistical predictions for the distribution of 26 Al and 60 Fe based on N -body simulations of typical to large clusters (10 3 –10 4 stars) with a range of initial conditions. We quantify the pollution of protoplanetary discs by supernova ejecta and show that the likelihood of enrichment levels similar to or higher than the Solar system can vary considerably, depending on the cluster morphology. Furthermore, many enriched systems show an excess in radiogenic heating compared to Solar system levels, which implies that the formation and evolution of planetesimals could vary significantly depending on the birth environment of their host stars.

Beschreibung: We present a large-scale, volume-limited companion survey of 245 late-K to mid-M (K7-M6) dwarfs within 15 pc. Infrared adaptive optics (AO) data were analysed from the Very Large Telescope, Subaru Telescope, Canada–France–Hawaii Telescope, and MMT Observatory to detect close companions to the sample from ~ 1 to 100 au, while digitized wide-field archival plates were searched for wide companions from ~ 100 to 10 000 au. With sensitivity to the bottom of the main sequence over a separation range of 3 to 10 000 au, multiple AO and wide-field epochs allow us to confirm candidates with common proper motions, minimize background contamination, and enable a measurement of comprehensive binary statistics. We detected 65 comoving stellar companions and find a companion star fraction of 23.5 ± 3.2 per cent over the 3 au to 10 000 au separation range. The companion separation distribution is observed to rise to a higher frequency at smaller separations, peaking at closer separations than measured for more massive primaries. The mass ratio distribution across the q  = 0.2–1.0 range is flat, similar to that of multiple systems with solar-type primaries. The characterization of binary and multiple star frequency for low-mass field stars can provide crucial comparisons with star-forming environments and hold implications for the frequency and evolutionary histories of their associated discs and planets.

Beschreibung: We follow the dynamical evolution of young star-forming regions with a wide range of initial conditions and examine how the radial velocity dispersion, , evolves over time. We compare this velocity dispersion to the theoretically expected value for the velocity dispersion if a region were in virial equilibrium, vir and thus assess the virial state (/ vir ) of these systems. We find that in regions that are initially subvirial, or in global virial equilibrium but subvirial on local scales, the system relaxes to virial equilibrium within several million years, or roughly 25–50 crossing times, according to the measured virial ratio. However, the measured velocity dispersion, , appears to be a bad diagnostic of the current virial state of these systems as it suggests that they become supervirial when compared to the velocity dispersion estimated from the virial mass, vir . We suggest that this discrepancy is caused by the fact that the regions are never fully relaxed, and that the early non-equilibrium evolution is imprinted in the one-dimensional velocity dispersion at these early epochs. If measured early enough (〈2 Myr in our simulations, or ~20 crossing times), the velocity dispersion can be used to determine whether a region was highly supervirial at birth without the risk of degeneracy. We show that combining , or the ratio of to the interquartile range (IQR) dispersion, with measures of spatial structure, places stronger constraints on the dynamical history of a region than using the velocity dispersion in isolation.

Beschreibung: Mass segregation in star clusters is often thought to indicate the onset of energy equipartition, where the most massive stars impart kinetic energy to the lower-mass stars and brown dwarfs/free-floating planets. The predicted net result of this is that the centrally concentrated massive stars should have significantly lower velocities than fast-moving low-mass objects on the periphery of the cluster. We search for energy equipartition in initially spatially and kinematically substructured N -body simulations of star clusters with N = 1500 stars, evolved for 100 Myr. In clusters that show significant mass segregation we find no differences in the proper motions or radial velocities as a function of mass. The kinetic energies of all stars decrease as the clusters relax, but the kinetic energies of the most massive stars do not decrease faster than those of lower-mass stars. These results suggest that dynamical mass segregation – which is observed in many star clusters – is not a signature of energy equipartition from two-body relaxation.

Beschreibung: The presence and abundance of short-lived radioisotopes 26 Al and 60 Fe in chondritic meteorites implies that the Sun formed in the vicinity of one or more massive stars that exploded as supernovae (SNe). Massive stars are more likely to form in massive star clusters (〉1000 M ) than lower mass clusters. However, photoevaporation of protoplanetary discs from massive stars and dynamical interactions with passing stars can inhibit planet formation in clusters with radii of ~1 pc. We investigate whether low-mass (50–200 M ) star clusters containing one or two massive stars are a more likely avenue for early Solar system enrichment as they are more dynamically quiescent. We analyse N -body simulations of the evolution of these low-mass clusters and find that a similar fraction of stars experience SN enrichment than in high-mass clusters, despite their lower densities. This is due to two-body relaxation, which causes a significant expansion before the first SN even in clusters with relatively low (100 stars pc –3 ) initial densities. However, because of the high number of low-mass clusters containing one or two massive stars, the absolute number of enriched stars is the same, if not higher than for more populous clusters. Our results show that direct enrichment of protoplanetary discs from SNe occurs as frequently in low-mass clusters containing one or two massive stars (〉20 M ) as in more populous star clusters (1000 M ). This relaxes the constraints on the direct enrichment scenario and therefore the birth environment of the Solar system.

Beschreibung: We present 3.7 arcsec (~0.05 pc) resolution 3.2 mm dust continuum observations from the Institut de Radioastronomie Millimétrique Plateau de Bure Interferometer, with the aim of studying the structure and fragmentation of the filamentary infrared dark cloud (IRDC) G035.39–00.33. The continuum emission is segmented into a series of 13 quasi-regularly spaced ( obs ~ 0.18 pc) cores, following the major axis of the IRDC. We compare the spatial distribution of the cores with that predicted by theoretical work describing the fragmentation of hydrodynamic fluid cylinders, finding a significant (a factor of 8) discrepancy between the two. Our observations are consistent with the picture emerging from kinematic studies of molecular clouds suggesting that the cores are harboured within a complex network of independent sub-filaments. This result emphasizes the importance of considering the underlying physical structure, and potentially, dynamically important magnetic fields, in any fragmentation analysis. The identified cores exhibit a range in (peak) beam-averaged column density (3.6 x 10 23 cm –2 〈 N H, c 〈 8.0 x 10 23 cm –2 ), mass (8.1 M 〈 M c 〈 26.1 M ), and number density (6.1 x 10 5 cm –3 〈 n H, c, eq 〈 14.7 x 10 5 cm –3 ). Two of these cores, dark in the mid-infrared, centrally concentrated, monolithic (with no traceable substructure at our PdBI resolution), and with estimated masses of the order ~20–25 M , are good candidates for the progenitors of intermediate-to-high-mass stars. Virial parameters span a range 0.2 〈 α vir 〈 1.3. Without additional support, possibly from dynamically important magnetic fields with strengths of the order of 230 μG 〈 B 〈 670 μG, the cores are susceptible to gravitational collapse. These results may imply a multilayered fragmentation process, which incorporates the formation of sub-filaments, embedded cores, and the possibility of further fragmentation.

Beschreibung: We compare the spatial distribution of stars which form in hydrodynamical simulations to the spatial distribution of the gas, using the $\mathcal {Q}$ -parameter. The $\mathcal {Q}$ -parameter enables a self-consistent comparison between the stars and gas because it uses a pixelated image of the gas as a distribution of points, in the same way that the stars (sink particles in the simulations) are a distribution of points. We find that, whereas the stars have a substructured, or hierarchical spatial distribution ( $\mathcal {Q} \sim 0.4 {-}0.7$ ), the gas is dominated by a smooth, concentrated component and typically has $\mathcal {Q} \sim 0.9$ . We also find no statistical difference between the structure of the gas in simulations that form with feedback, and those that form without, despite these two processes producing visually different distributions. These results suggest that the link between the spatial distributions of gas, and the stars which form from them, is non-trivial.