A Discontinuity in the Low‐Mass Initial Mass Function

Thies, Ingo; Kroupa, Pavel

doi:10.1086/522512

Cited by 100 publications

(178 citation statements)

References 70 publications

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“…This reflects the empirically determined value for Taurus-Auriga and the Pleiades (Thies & Kroupa 2007), i.e. for every three star-like bodies there is one object forming through peripheral fragmentation.…”

Section: The Imfsupporting

confidence: 71%

“…the shape of the IMF around the HBML (Thies & Kroupa 2007, 2008Thies et al 2015), the binary statistics of BDs which are different from higher mass stars (Thies & Kroupa 2008;Dieterich et al 2012, Sec. 5.3.4), the generation of free-floating planetary mass objects and BDs (Stamatellos & Whitworth 2009;Li et al 2015;Forgan et al 2015;Vorobyov 2016) and the lack of BD companions to solar-type stars at low separations known as the BD desert (Marcy & Butler 2000;Grether & Lineweaver 2006;Dieterich et al 2012;Evans et al 2012;Wilson et al 2016).…”

Section: Discussionmentioning

confidence: 99%

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Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

Marks¹,

Martín

Béjar

et al. 2017

A&A

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Context. Whether brown dwarfs (BDs) form just as stars directly from the gravitational collapse of a molecular cloud core ("star-like") or whether BDs and some very low-mass stars (VLMSs) constitute a separate population which form alongside stars comparable to the population of planets, e.g. through circumstellar disk ("peripheral") fragmentation, is one of the key questions of the star-formation problem. Aims. For young stars in Taurus-Auriga the binary fraction has been shown to be large with little dependence on primary mass above ≈ 0.2M⊙, while for BDs it is < 10%. Here we investigate a case in which BDs in Taurus formed dominantly, but not exclusively, through peripheral fragmentation, which naturally results in low binary fractions. The decline of the binary frequency in the transition region between star-like formation and peripheral formation is modelled. Methods. A dynamical population synthesis model is employed in which stellar binary formation is universal with a large binary fraction close to unity. Peripheral objects form separately in circumstellar disks with a distinctive initial mass function (IMF), own orbital parameter distributions for binaries and a low binary fraction according to observations and expectations from smoothed particle hydrodynamics (SPH) and grid-based computations. A small amount of dynamical processing of the stellar component is accounted for as appropriate for the low-density Taurus-Auriga embedded clusters. Results. The binary fraction declines strongly in the transition region between star-like and peripheral formation, exhibiting characteristic features. The location of these features and the steepness of this trend depend on the masslimits for star-like and peripheral formation. Such a trend might be unique to low density regions like Taurus which host dynamically largely unprocessed binary populations in which the binary fraction is large for stars down to M-dwarfs and low for BDs. Conclusions. The existence of a strong decline in the binary fraction -primary mass diagram will become verifiable in future surveys on BD and VLMS binarity in the Taurus-Auriga star forming region. It is a test of the (non-)continuity of star formation along the mass-scale, the separateness of the stellar and BD populations and the dominant formation channel for BDs and BD binaries in regions of low stellar density hosting dynamically unprocessed populations.

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Section: The Imfsupporting

confidence: 71%

Section: Discussionmentioning

confidence: 99%

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

Marks¹,

Martín

Béjar

et al. 2017

A&A

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“…This common presence of disks around stellar and substellar objects has been used to argue for a common formation mechanism (Scholz & Jayawardhana 2008;Haisch et al 2010), while other observations hint at possible differences in the dynamical history of low mass stars/brown dwarfs compared to higher mass stars (Thies & Kroupa 2007). Formation scenarios for brown dwarfs also have implications for the properties of brown dwarf disks, and a number of possible mechanisms have been proposed, including ejection of stellar embryos (e.g., Bate et al 2003;Reipurth & Clarke 2001), erosion of star-forming clouds by radiation from massive stars (Whitworth & Zinnecker 2004), gravitational instabilities in the early stages of disks (e.g., Stamatellos & Whitworth 2011;Basu & Vorobyov 2012), and gravitational collapse of clouds analogous to the formation of stars (e.g., Padoan et al 2005;Hennebelle & Chabrier 2008).…”

Section: Introductionmentioning

confidence: 99%

The Taurus Boundary of Stellar/Substellar (TBOSS) Survey

Bulger

Patience

Ward-Duong

et al. 2014

A&A

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With the PACS instrument on Herschel, 134 low mass members of the Taurus star-forming region spanning the M4-L0 spectral type range and covering the transition from low mass stars to brown dwarfs were observed. Combining the new Herschel results with other Herschel programs, a total of 150 of the 154 M4-L0 Taurus members have observations, and we have added an additional 3 targets from Spitzer to form the 153-object TBOSS (Taurus Boundary of Stellar/Substellar) sample, a 99% complete study. Among the 150 targets, 70 µm flux densities were measured for 7 of the 7 Class I objects, 48 of the 67 Class II objects, and 3 of the 76 Class III objects. For the detected Class II objects, the median 70 µm flux density level declines with spectral type; however, the distribution of excess relative to central object flux density does not change across the stellar/substellar boundary in the M4-L0 range. Connecting the 70 µm TBOSS values with the results from K0-M3 Class II members results in the first comprehensive census of far-IR emission across the full mass spectrum of the stellar and substellar population of a star-forming region, and the median flux density declines with spectral type in a trend analogous to the flux density decline expected for the central objects. Spectral energy distributions (SEDs) were constructed for all TBOSS targets covering the optical to far-IR range and extending to the submm/mm for a subset of sources with longer wavelength data. Based on an initial exploration of the impact of different physical parameters on the Herschel flux densities, geometrical factors such as inclination and structural parameters such as scale height and flaring have the largest influence on the flux densities in the PACS bands. From the 24 µm to 70 µm spectral index of the SEDs, 5 new candidate transition disks were identified. Considering the previously known and new candidate transition disks, the spectral indices over longer wavelengths (≥70 µm) are not distinct from those of the full Class II population, suggesting that the outer regions of the transition disks are similar to Class II disks. The steep 24 µm to 70 µm slope for a subset of 8 TBOSS targets may be an indication of truncated disks in these systems, however additional measurements are required to establish the outer radii of these disks conclusively. From existing high angular resolution companion search observations, two examples of mixed pair systems that include secondaries with disks were measured in the Herschel data. Finally, comparing the TBOSS results with a Herschel study of Ophiuchus brown dwarfs reveals a lower fraction of disks around the Taurus substellar population with flux densities comparable to the Ophiuchus disks.

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“…In a first set of models, we assume a universal log-normal IMF (Chabrier 2005) before considering a possible discontinuity (Thies & Kroupa 2007) around the substellar limit, and a truncated IMF with no system below 0.1 M . The simulations still start after the gas has been expelled but virial equilibrium is not required.…”

Section: Introductionmentioning

confidence: 99%

Reverse dynamical evolution ofηChamaeleontis

Becker

Moraux

Duchêne

et al. 2013

A&A

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Context. In the scope of the star formation process, it is unclear how the environment shapes the initial mass function (IMF). While observations of open clusters propose a universal picture for the IMF from the substellar domain up to a few solar masses, the young association η Chamaeleontis presents an apparent lack of low mass objects (m < 0.1 M ). Another unusual feature of this cluster is the absence of wide binaries with a separation >50 AU. Aims. We aim to test whether dynamical evolution alone can reproduce the peculiar properties of the association under the assumption of a universal IMF. Methods. We use a pure N-body code to simulate the dynamical evolution of the cluster for 10 Myr, and compare the results with observations. A wide range of values for the initial parameters are tested (number of systems, typical radius of the density distribution and virial ratio) in order to identify the initial state that would most likely lead to observations. In this context we also investigate the influence of the initial binary population on the dynamics and the possibility of having a discontinuous single IMF near the transition to the brown dwarf regime. We consider as an extreme case an IMF with no low mass systems (m < 0.1 M ). Results. The initial configurations cover a wide range of initial density, from 10 2 to 10 8 stars/pc 3 , in virialized, hot and cold dynamical state. We do not find any initial state that would evolve from a universal single IMF to fit the observations. Only when starting with a truncated IMF without any very low mass systems and no wide binaries, can we reproduce the cluster core properties with a success rate of 10% at best. Conclusions. Pure dynamical evolution alone cannot explain the observed properties of η Chamaeleontis from universal initial conditions. The lack of brown dwarfs and very low mass stars, and the peculiar binary properties (low binary fraction and lack of wide binaries), are probably the result of the star formation process in this association.

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A Discontinuity in the Low‐Mass Initial Mass Function

Cited by 100 publications

References 70 publications

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

The Taurus Boundary of Stellar/Substellar (TBOSS) Survey

Reverse dynamical evolution ofηChamaeleontis

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