We review recent determinations of the present-day mass function (PDMF) and initial mass function (IMF) in various components of the Galaxy—disk, spheroid, young, and globular clusters—and in conditions characteristic of early star formation. As a general feature, the IMF is found to depend weakly on the environment and to be well described by a power-law form forM , and a lognormal form below, except possibly for m!1 early star formation conditions. The disk IMF for single objects has a characteristic mass around M , m!0.08 c and a variance in logarithmic mass , whereas the IMF for multiple systems hasM , and . j!0.7 m!0.2 j!0.6 c The extension of the single MF into the brown dwarf regime is in good agreement with present estimates of L- and T-dwarf densities and yields a disk brown dwarf number density comparable to the stellar one, n!n! BD " pc !3 .T he IMF of young clusters is found to be consistent with the disk fi eld IMF, providing the same correction 0.1 for unresolved binaries, confirming the fact that young star clusters and disk field stars represent the same stellar population. Dynamical effects, yielding depletion of the lowest mass objects, are found to become consequential for ages!130 Myr. The spheroid IMF relies on much less robust grounds. The large metallicity spread in the local subdwarf photometric sample, in particular, remains puzzling. Recent observations suggest that there is a continuous kinematic shear between the thick-disk population, present in local samples, and the genuine spheroid one. This enables us to derive only an upper limit for the spheroid mass density and IMF. Within all the uncertainties, the latter is found to be similar to the one derived for globular clusters and is well represented also by a lognormal form with a characteristic mass slightly larger than for the disk, M , ,e xcluding as ignif icant population of m!0.2-0.3 c brown dwarfs in globular clusters and in the spheroid. The IMF characteristic of early star formation at large redshift remains undetermined, but different observational constraints suggest that it does not extend below!1M , .T hese results suggest a characteristic mass for star formation that decreases with time, from conditions prevailing at large redshift to conditions characteristic of the spheroid (or thick disk) to present-day conditions.Theseconclusions,however, remain speculative, given the large uncertainties in the spheroid and early star IMF determinations. These IMFs allow a reasonably robust determination of the Galactic present-day and initial stellar and brown dwarf contents. They also have important galactic implications beyond the Milky Way in yielding more accurate mass-to-light ratio determinations. The mass-to-light ratios obtained with the disk and the spheroid IMF yield values 1.8-1.4 times smaller than for a Salpeter IMF, respectively, in agreement with various recent dynamical determinations. This general IMF determination is examined in the context of star formation theory. None of the theories based on a Jeans-type mechanism, where fragmentation is due only to gravity, can fulfill all the observational constraints on star formation and predict a large number of substellar objects. On the other hand, recent numerical simulations of compressible turbulence, in particular in super-Alfvenic conditions, seem to reproduce both qualitatively and quantitatively the stellar and substellar IMF and thus provide an appealing theoretical foundation. In this picture, star formation is induced by the dissipation of large-scale turbulence to smaller scales through radiative MHD shocks, producing filamentary structures. These shocks produce local nonequilibrium structures with large density contrasts, which collapse eventually in gravitationally bound objects under the combined influence of turbulence and gravity. The concept of a single Jeans mass is replaced by a distribution of local Jeans masses, representative of the lognormal probability density function of the turbulent gas. Objects below the mean thermal Jeans mass still have a possibility to collapse, although with a decreasing probability.

/pdf/galactic-stellar-and-substellar-initial-mass-function-1fntlgtk9q.pdf

Galactic stellar and substellar initial mass function

US Department of Energy; US National Science Foundation; Ministry of Science and Education of Spain; Science and Technology Facilities Council of the United Kingdom; Higher Education Funding Council for England; National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign; Kavli Institute of Cosmological Physics at the University of Chicago; Center for Cosmology and Astro-Particle Physics at the Ohio State University; Mitchell Institute for Fundamental Physics and Astronomy at Texas AM University; Financiadora de Estudos e Projetos; Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro; Conselho Nacional de Desenvolvimento Cientifico e Tecnologico and the Ministerio da Ciencia; Tecnologia e Inovacao; Deutsche Forschungsgemeinschaft; Collaborating Institutions in the Dark Energy Survey; National Science Foundation [AST-1138766]; University of California at Santa Cruz; University of Cambridge, Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid; University of Chicago, University College London; DES-Brazil Consortium; University of Edinburgh; Eidgenossische Technische Hochschule (ETH) Zurich, Fermi National Accelerator Laboratory; University of Illinois at Urbana-Champaign; Institut de Ciencies de l'Espai (IEEC/CSIC); Institut de Fisica d'Altes Energies, Lawrence Berkeley National Laboratory; Ludwig-Maximilians Universitat Munchen; European Research Council [FP7/291329]; MINECO [AYA2012-39559, ESP2013-48274, FPA2013-47986]; Centro de Excelencia Severo Ochoa [SEV-2012-0234]; European Research Council under the European Union [240672, 291329, 306478]

/pdf/the-dark-energy-survey-more-than-dark-energy-an-overview-1g8hpx4t8y.pdf

The Dark Energy Survey: more than dark energy - an overview

We conjecture that brown dwarfs are substellar objects because they have been ejected from small newborn multiple systems that have decayed in dynamical interactions. In this view, brown dwarfs are stellar embryos for which the star formation process was aborted before the hydrostatic cores could build up enough mass to eventually start hydrogen burning. The disintegration of a small multiple system is a stochastic process, which can be described only in terms of the half-life of the decay. A stellar embryo competes with its siblings in order to accrete infalling matter, and the one that grows slowest is most likely to be ejected. With better luck, a brown dwarf would therefore have become a normal star. This interpretation of brown dwarfs readily explains the rarity of brown dwarfs as close companions to normal stars, the absence of wide brown dwarf binaries, and the flattening of the low-mass end of the initial mass function. Possible observational tests of this scenario include statistics of brown dwarfs near Class 0 sources and the kinematics of brown dwarfs in star-forming regions, while they still retain a kinematic signature of their expulsion. Because the ejection process limits the amount of gas brought along in a disk, it is predicted that substellar equivalents to the classical T Tauri stars should be rather short-lived.

https://iopscience.iop.org/article/10.1086/321121/pdf

The Formation of Brown Dwarfs as Ejected Stellar Embryos

Particle Stirring in Turbulent Gas Disks: Including Orbital Oscillations

1. Introduction 2. Radial velocities 3. Astrometry 4. Timing 5. Microlensing 6. Transits 7. Imaging 8. Host stars 9. Brown dwarfs and free-floating planets 10. Formation and evolution 11. Interiors and atmospheres 12. The Solar System Appendixes References Index.

/pdf/the-exoplanet-handbook-3g3nzlvc20.pdf

The Exoplanet Handbook

The existence of an outer planet beyond Pluto has been a matter of debate for decades and the recent discovery of 2012 VP113 has just revived the interest for this controversial topic. This Sedna-like object has the most distant perihelion of any known minor planet and the value of its argument of perihelion is close to 0 degrees. This property appears to be shared by almost all known asteroids with semimajor axis greater than 150 au and perihelion greater than 30 au (the extreme trans-Neptunian objects or ETNOs), and this fact has been interpreted as evidence for the existence of a super-Earth at 250 au. In this scenario, a population of stable asteroids may be shepherded by a distant, undiscovered planet larger than the Earth that keeps the value of their argument of perihelion librating around 0 degrees as a result of the Kozai mechanism. Here, we study the visibility of these ETNOs and confirm that the observed excess of objects reaching perihelion near the ascending node cannot be explained in terms of any observational biases. This excess must be a true feature of this population and its possible origin is explored in the framework of the Kozai effect. The analysis of several possible scenarios strongly suggest that at least two trans-Plutonian planets must exist.

Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets.

A number of Earth co-orbital asteroids experience repeated transitions between the quasi-satellite and horseshoe dynamical states. Asteroids 2001 GO2, 2002 AA29, 2003 YN107 and 2015 SO2 are well-documented cases of such a dynamical behaviour. These transitions depend on the gravitational influence of other planets, owing to the overlapping of a multiplicity of secular resonances. Here, we show that the recently discovered asteroid (469219) 2016 HO3 is a quasi-satellite of our planet -the fifth one, joining the ranks of (164207) 2004 GU9, (277810) 2006 FV35, 2013 LX28 and 2014 OL339. This new Earth co-orbital also switches repeatedly between the quasi-satellite and horseshoe configurations. Its current quasi-satellite episode started nearly 100 yr ago and it will end in about 300 yr from now. The orbital solution currently available for this object is very robust and our full N-body calculations show that it may be a long-term companion (time-scale of Myr) to our planet. Among the known Earth quasi-satellites, it is the closest to our planet and as such, a potentially accessible target for future in situ study. Due to its presumably lengthy dynamical relationship with the Earth and given the fact that at present and for many decades this transient object remains well positioned with respect to our planet, the results of spectroscopic studies of this small body, 26-115 m, may be particularly useful to improve our understanding of the origins -local or captured- of Earth's co-orbital asteroid population. The non-negligible effect of the uncertainty in the value of the mass of Jupiter on the stability of this type of co-orbitals is also briefly explored.

Asteroid (469219) 2016 HO3, the smallest and closest Earth quasi-satellite

https://arxiv.org/pdf/astro-ph/0401360.pdf

On the recent star formation history of the Milky Way disk

Minor bodies trapped in 1:1 co-orbital resonances with a host planet could be relevant to explain the origin of captured satellites. Among the giant planets, Uranus has one of the smallest known populations of co-orbitals, three objects, and all of them are short-lived. Asteroid 2015 DB216 has an orbital period that matches well that of Uranus, and here we investigate its dynamical state. Direct N-body calculations are used to assess the current status of this object, reconstruct its immediate dynamical past, and explore its future orbital evolution. A covariance matrix-based Monte Carlo scheme is presented and applied to study its short-term stability. We find that 2015 DB216 is trapped in a temporary co-orbital resonance with Uranus, the fourth known minor body to do so. A detailed analysis of its dynamical evolution shows that it is an unstable but recurring co-orbital companion to Uranus. It currently follows an asymmetric horseshoe trajectory that will last for at least 10 kyr, but it may remain inside Uranus' co-orbital zone for millions of years. As in the case of other transient Uranian co-orbitals, complex multibody ephemeral mean motion resonances trigger the switching between the various resonant co-orbital states. The new Uranian co-orbital exhibits a secular behaviour markedly different from that of the other known Uranian co-orbitals because of its higher inclination, nearly 38 degrees. Given its rather unusual discovery circumstances, the presence of 2015 DB216 hints at the existence of a relatively large population of objects moving in similar orbits.

Asteroid 2015 DB216: a recurring co-orbital companion to Uranus

Venus has three known co-orbitals: (322756) 2001 CK32, 2002 VE68 and 2012 XE133. The first two have absolute magnitudes 18 < H < 21. The third one, significantly smaller at H = 23.4 mag, is a recent discovery that signals the probable presence of many other similar objects: small transient companions to Venus that are also potentially hazardous asteroids (PHAs). Here, we study the dynamical evolution of the recently discovered asteroid 2013 ND15. At H = 24.1 mag, this minor body is yet another small Venus co-orbital and PHA, currently close to the Lagrangian point L4 and following the most eccentric path found so far for objects in this group. Due to its high eccentricity (0.6), its dynamics is different from that of the other three known Venus co-orbitals even if they all are near-Earth objects (NEOs). A Monte Carlo simulation that uses the orbital data and discovery circumstances of the four objects as proxies to estimate the current size of this population, indicates that the number of high-eccentricity, low-inclination Venus co-orbital NEOs may have been greatly underestimated by current models. Three out of four known objects were discovered with solar elongation at perigee greater than 135 degrees even if visibility estimates show that less than 4 per cent of these objects are expected to reach perigee at such large elongations. Nearly 70 per cent of the objects discussed here have elongation at perigee < 90 degrees and 65 per cent are prospective PHAs.

/pdf/asteroid-2013-nd15-trojan-companion-to-venus-pha-to-the-352uv0ejt7.pdf

C. de la Fuente Marcos

Papers

Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets.

Asteroid (469219) 2016 HO3, the smallest and closest Earth quasi-satellite

On the recent star formation history of the Milky Way disk

Asteroid 2015 DB216: a recurring co-orbital companion to Uranus

Asteroid 2013 ND15: Trojan companion to Venus, PHA to the Earth