On the origin and nature of brown dwarfs and massive planets

In this thesis the apparent peculiarities of brown dwarfs (BDs) are pointed out which force us to treat them as a population apart from, but yet still related to the stellar population. In particular, the properties that make BDs so special are deduced from the observational evidence in Chapter 1: 1. The brown dwarf desert, i.e. the apparent dearth of substellar, non-planetary companions to stars, 2. the truncated distribution of the semi-major axes of BD-BD binaries, 3. the unusually top-heavy mass ratio distribution of BD-BD binaries, compared to star-star binaries, and 4. the theoretical difficulties to explain a star-like, i.e. isolated formation scenario for BDs (Adams & Fatuzzo 1996; Goodwin & Whitworth 2007). These combined facts effectively rule out star-like formation as the predominant mechanism of BD formation. In addition, some fraction of very-low-mass stars may also have a non-star-like origin. As an immediate consequence, a separate population, here called BD-like, is introduced. Although the present observational data do not yet allow a precise determination of the mass range of this population, a steep descent of the BD-like initial mass function between 0.1 and 0.2 Msun as well as a steep onset of the stellar IMF slightly below the hydrogen-burning mass (0.075 Msun) appears to be in good agreement with the lower end of the observational mass function if it has been corrected for unresolved binaries. This two-populations model results necessarily in a discontinuity in the overall IMF of BDs and stars together. This discontinuity may be hidden in the observed mass function unless the mass function is corrected for unseen binaries. In Chapter 2 the issue is revised for the case of a high abundance of BD-BD binaries. It is shown that the discontinuity persists even if a star-like BD-BD binary fraction of up to 60% is assumed. This further supports the two-populations model. In Chapter 3 a probable formation scenario of BDs (as well as for massive gas planets) is presented. Earlier work by Stamatellos et al. (2007) and subsequent studies had shown that fragmentation of massive circumstellar disc beyond 100~AU is a valid mechanism to form substellar companions. My research now shows that less massive (and thus more frequent) discs can also fragment upon a tidal perturbation. While direct hydrodynamic perturbation in disc-disc collisions (Shen et al. 2010) requires two colliding massive discs, rendering such a scenario highly improbable, a tidal perturbation by a low-mass star with no disc or only a negligibly small disc (i.e. a typical protoplanetary disc in contrast to a massive extended one) appears to be reasonably probable for encounter distances around 500~AU, as estimated by Thies et al. (2005) and briefly revisited in Chapter 3

Induced planet formation in stellar clusters - a parameter study of star-disk encounters

We present a parameter study of the possibility of tidally triggered disk instability. Using a restricted N-body model which allows for a survey of an extended parameter space, we show that a passing dwarf star with a mass between 0.1 and 1 M_sun can probably induce gravitational instabilities in the pre-planetary solar disk for prograde passages with minimum separations below 80-170 AU for isothermal or adiabatic disks. Inclined and retrograde encounters lead to similar results but require slightly closer passages. Such encounter distances are quite likely in young moderately massive star clusters (Scally & Clarke 2001; Bonnell et al. 2001). The induced gravitational instabilities may lead to enhanced planetesimal formation in the outer regions of the protoplanetary disk, and could therefore be relevant for the existence of Uranus and Neptune, whose formation timescale of about 100 Myr (Wuchterl, Guillot & Lissauer 2000) is inconsistent with the disk lifetimes of about a few Myr according to observational data by Haisch, Lada & Lada (2001). The relatively small gas/solid ratio in Uranus and Neptune can be matched if the perturbing fly-by occurred after early gas depletion of the solar system, i.e. when the solar system was older than about 5 Myr. We also confirm earlier results by Heller (1993) that the observed 7 degree tilt of the solar equatorial plane relative to the ecliptic plane could be the consequence of such a close encounter.Comment: 11 pages, 9 figures, using aas_macros.sty. MNRAS, accepte

Tidally induced brown dwarf and planet formation in circumstellar discs

Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar discs and therefore the formation of planets and brown dwarfs. Recent findings suggest that tidal perturbations of typical circumstellar discs due to close encounters may inhibit rather than trigger disc fragmentation and so would seem to rule out planet formation by external tidal stimuli. However, the disc models in these calculations were restricted to disc radii of 40 AU and disc masses below 0.1 M_sun. Here we show that even modest encounters can trigger fragmentation around 100 AU in the sorts of massive (~0.5 M_sun), extended (>=100 AU) discs that are observed around young stars. Tidal perturbation alone can do this, no disc-disc collision is required. We also show that very-low-mass binary systems can form through the interaction of objects in the disc. In our computations, otherwise non-fragmenting massive discs, once perturbed, fragment into several objects between about 0.01 and 0.1 M_sun, i.e., over the whole brown dwarf mass range. Typically these orbit on highly eccentric orbits or are even ejected. While probably not suitable for the formation of Jupiter- or Neptune-type planets, our scenario provides a possible formation mechanism for brown dwarfs and very massive planets which, interestingly, leads to a mass distribution consistent with the canonical substellar IMF. As a minor outcome, a possible explanation for the origin of misaligned extrasolar planetary systems is discussed.Comment: 9 pages, 5 figures, uses emulateapj. Published in ApJ. Minor changes to match published version. For associated media files see http://www.astro.uni-bonn.de/~webaiub/english/downloads.ph

A discontinuity in the low-mass initial mass function

The origin of brown dwarfs (BDs) is still an unsolved mystery. While the standard model describes the formation of BDs and stars in a similar way recent data on the multiplicity properties of stars and BDs show them to have different binary distribution functions. Here we show that proper treatment of these uncovers a discontinuity of the multiplicity-corrected mass distribution in the very-low-mass star (VLMS) and BD mass regime. A continuous IMF can be discarded with extremely high confidence. This suggests that VLMSs and BDs on the one hand, and stars on the other, are two correlated but disjoint populations with different dynamical histories. The analysis presented here suggests that about one BD forms per five stars and that the BD-star binary fraction is about 2%-3% among stellar systems.Comment: 14 pages, 11 figures, uses emulateapj.cls. Minor corrections and 1 reference added after being accepted by the Ap

Barred spiral galaxies in modified gravity theories

Funding Information: IB is supported by an Alexander von Humboldt Foundation postdoctoral research fellowship. BF acknowledges funding from the Agence Nationale de la Recherche (ANR projects ANR-18-CE31-0006 and ANR-19-CE31-0017), and from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement number 834148). EA is supported by a stipend from the Stellar Populations and Dynamics Research Group at the University of Bonn. For NLG, MOG, RHH, and LPH simulations, this work made use of the Sci-HPC centre of the Ferdowsi University of Mashhad.When bars form within galaxy formation simulations in the standard cosmological context, dynamical friction with dark matter (DM) causes them to rotate rather slowly. However, almost all observed galactic bars are fast in terms of the ratio between corotation radius and bar length. Here, we explicitly display an 8σ tension between the observed distribution of this ratio and that in the EAGLE simulation at redshift 0. We also compare the evolution of Newtonian galactic discs embedded in DM haloes to their evolution in three extended gravity theories: Milgromian Dynamics (MOND), a model of non-local gravity, and a scalar-tensor-vector gravity theory (MOG). Although our models start with the same initial baryonic distribution and rotation curve, the long-term evolution is different. The bar instability happens more violently in MOND compared to the other models. There are some common features between the extended gravity models, in particular the negligible role played by dynamical friction − which plays a key role in the DM model. Partly for this reason, all extended gravity models predict weaker bars and faster bar pattern speeds compared to the DM case. Although the absence of strong bars in our idealized, isolated extended gravity simulations is in tension with observations, they reproduce the strong observational preference for 'fast' bar pattern speeds, which we could not do with DM. We confirm previous findings that apparently 'ultrafast' bars can be due to bar-spiral arm alignment leading to an overestimated bar length, especially in extended gravity scenarios where the bar is already fast.Publisher PDFPeer reviewe

3D hydrodynamic simulations for the formation of the local group satellite planes

Funding: IB is supported by Science and Technology Facilities Council grant ST/V000861/1. He acknowledges support from a ‘Pathways to Research’ fellowship from the University of Bonn in 2021 after an Alexander von Humboldt Foundation postdoctoral research fellowship (2018–2020). IT acknowledges support through the Stellar Populations and Dynamics research group at the University of Bonn. GC acknowledges support from Chile’s National Fund for Scientific and Technological Development (FONDECYT) Regular No. 1181708. BF and RI acknowledge funding from the Agence Nationale de la Recherche (ANR project ANR-18-CE31-0006 and ANR-19-CE31-0017) and from the European Research Council (ERC) under the European Union’s Horizon 2020 Framework Programme (grant agreement number 834148). MSP was supported by the Leibniz Junior Research Group grant J94/2020 via the Leibniz Competition, and a Klaus Tschira Boost Fund provided by the Klaus Tschira Stiftung and the German Scholars Organization.The existence of mutually correlated thin and rotating planes of satellite galaxies around both the Milky Way (MW) and Andromeda (M31) calls for an explanation. Previous work in Milgromian dynamics (MOND) indicated that a past MW–M31 encounter might have led to the formation of these satellite planes. We perform the first-ever hydrodynamical MOND simulation of the Local Group using PHANTOM OF RAMSES. We show that an MW–M31 encounter at z ≈ 1, with a perigalactic distance of about 80 kpc, can yield two disc galaxies at z = 0 oriented similarly to the observed galactic discs and separated similarly to the observed M31 distance. Importantly, the tidal debris are distributed in phase space similarly to the observed MW and M31 satellite planes, with the correct preferred orbital pole for both. The MW–M31 orbital geometry is consistent with the presently observed M31 proper motion despite this not being considered as a constraint when exploring the parameter space. The mass of the tidal debris around the MW and M31 at z = 0 compare well with the mass observed in their satellite systems. The remnant discs of the two galaxies have realistic radial scale lengths and velocity dispersions, and the simulation naturally produces a much hotter stellar disc in M31 than in the MW. However, reconciling this scenario with the ages of stellar populations in satellite galaxies would require that a higher fraction of stars previously formed in the outskirts of the progenitors ended up within the tidal debris, or that the MW–M31 interaction occurred at z > 1.Publisher PDFPeer reviewe

A discontinuity in the low-mass IMF - the case of high multiplicity

The empirical binary properties of brown dwarfs (BDs) differ from those of normal stars suggesting BDs form a separate population. Recent work by Thies & Kroupa revealed a discontinuity of the initial mass function (IMF) in the very-low-mass star regime under the assumption of a low multiplicity of BDs of about 15 per cent. However, previous observations had suggested that the multiplicity of BDs may be significantly higher, up to 45 per cent. This contribution investigates the implication of a high BD multiplicity on the appearance of the IMF for the Orion Nebula Cluster, Taurus-Auriga, IC 348 and the Pleiades. We show that the discontinuity remains pronounced even if the observed MF appears to be continuous, even for a BD binary fraction as high as 60%. We find no evidence for a variation of the BD IMF with star-forming conditions. The BD IMF has a power-law index alpha = +0.3 and about two BDs form per 10 low-mass stars assuming equal-mass pairing of BDs.Comment: 7 pages, 5 figures. Updated to match published versio