The formation of brown dwarfs

We review four mechanisms for forming brown dwarfs: (i) turbulent fragmentation (producing very low‐mass prestellar cores); (ii) gravitational instabilities in discs; (iii) dynamical ejection of stellar embryos from their placental cores; and (iv) photo‐erosion of pre‐existing cores in HII regions. We argue (a) that these are simply the mechanisms of low‐mass star formation , and (b) that they are not mutually exclusive. If, as seems possible, all four mechanisms operate in nature, their relative importance may eventually be constrained by their ability to reproduce the binary statistics of brown dwarfs, but this will require fully 3‐D radiative magneto‐hydrodynamic simulations. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Convective radiation fluid‐dynamics: formation and early evolution of ultra low‐mass objects

The formation process of ultra low‐mass objects is some kind of extension of the star formation process. The physical changes towards lower mass are discussed by investigating the collapse of cloud cores that are modelled as Bonnor‐Ebert spheres. Their collapse is followed by solving the equations of fluid dynamics with radiation and a model of time‐dependent convection that has been calibrated to the Sun. For a sequence of cloud‐cores with 1 to 0.01 solar masses, evolutionary tracks and isochrones are shown in the mass‐radius diagram, the Hertzsprung‐Russel diagram and the effective temperaturesurface gravity or Kiel diagram. The collapse and the early hydrostatic evolution to ages of few Ma are briefly discussed and compared to observations of objects in Upper Scorpius and the low‐mass components of GG Tau. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spitzer discovery of very low luminosity objects

The Spitzer Space Telescope allows for the .rst time to search systematically for very low luminosity (≲0.1 L_⊙) objects (VeLLOs) associated with dense molecular cores. They may be the .rst candidate Class 0 sources with sub‐stellar masses. We describe such a source in the dense molecular core L1148. VeLLO natal cores show properties that are unusual for star‐forming cores. The low luminosity and in some cases the lack of prominent out.ow could be the result of the small gas supply near the VeLLO. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rotation and variability of young very low‐mass objects

Variability studies are an important tool to investigate key properties of stars and brown dwarfs. From photometric monitoring we are able to obtain information about rotation and magnetic activity, which are expected to change in the mass range below 0.3 solar masses, since these fully convective objects cannot host a solar‐type dynamo. On the other hand, spectroscopic variability information can be used to obtain a detailed view on the accretion process in very young objects. In this paper, we report about our observational efforts to analyse the variability and rotational evolution of young brown dwarfs and very low‐mass stars. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Brown dwarf candidates in the Serpens cloud core

The Serpens cloud core is populated by a young, low‐mass stellar cluster with members in many different evolutionary stages. We revisit the near‐IR properties of the cluster in this contribution. A total of 188 near‐IR sources are identified, including new members suggested in this work. Near‐IR colour‐magnitude and colour‐colour diagrams allow us to suggest 42 objects among the cluster members as brown dwarf candidates. A considerable fraction of them present near‐IR excesses which indicates that many of these very low mass objects are still surrounded by prominent dust accretion disks. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Accretion in brown dwarfs down to nearly planetary masses

We show that in accreting ultra low‐mass stars and brown dwarfs, the CaII λ 8662 emission line flux correlates remarkably well with the mass accretion rate ( ), just as it does in higher mass classical T Tauri stars (CTTs). A straightforward measurement of the CaII flux thus provides an easier determination technique than detailed modeling of the Hα emission line profile (except at the very lowest accretion rates, where CaII does not appear to be in emission for ultra low‐mass objects, and Hα modeling is required). Using optical high‐resolution spectra, we infer from CaII emission for young ultra low‐mass objects down to nearly the deuterium‐burning (planetary‐mass) limit. Our results, in combination with previous determinations of in CTTs, illustrate that the accretion rate declines steeply with mass, roughly as ∝ M_*² (albeit with considerable scatter). A similar relationship has been suggested by previous studies; we extend it down to nearly the planetary regime. The physical reason for this phenomenon is not yet clear; we discuss various possible mechanisms. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Star-like activity from a very young 'isolated planet'

The discovery of isolated bodies of planetary mass has challenged the paradigm that planets form only as companions to stars. To determine whether 'isolated planets', brown dwarfs and stars can have a common origin, we have made deep submillimetre observations of part of the ρ Oph B star formation region. Spectroscopy of the 9-Jupiter-mass core Oph B-11 has revealed carbon monoxide line wings such as those of a protostar. Moreover, the estimated mass of outflowing gas lies on the force versus core-mass relation for protostars and protobrown dwarfs. This is evidence for a common process that can form any object between planetary and stellar masses in a molecular cloud. In a submillimetre continuum map, six compact cores in ρ Oph B were found to have masses presently below the deuterium-burning limit, extending the core mass function down to  0.01 M_⊙  with the approximate form  d N /d M ∝ M ^−3/2  . If these lowest-mass cores are not transient and can collapse under gravity, then isolated planets should be very common in ρ Oph in the future, as is the case in the Orion star formation region. In fact, the isolated planetary objects that may form from these cores would outnumber the massive planets that have been found as companions to stars.     

Calibrating models of ultralow‐mass stars

Evolutionary and atmospheric models have become available for young ultralow‐mass objects. These models are being used to determine fundamental parameters from observational properties. TiO bands are used to determine effective temperatures in ultralow‐mass objects, and together with Na‐ and K‐lines to derive gravities at the substellar boundary. Unfortunately, model calibrations in (young) ultralow‐mass objects are rare. As a first step towards a calibration of synthetic spectral features, I show molecular bands of TiO, which is a main opacity source in late M‐dwarfs. The TiO ε ‐band at 8450 Å is systematically too weak. This implies that temperatures determined from that band are underestimated, and I discuss implications for determining fundamental parameters from high resolution spectra. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mid-infrared source multiplicity within hot molecular cores traced by methanol masers

We present high resolution, mid-infrared (MIR) images towards three hot molecular cores signposted by methanol maser emission: G173.49+2.42 (S231, S233IR), G188.95+0.89 (S252, AFGL-5180) and G192.60−0.05 (S255IR). Each of the cores was targeted with Michelle on Gemini North using five filters from 7.9 to 18.5 μm. We find each contains both large regions of extended emission and multiple, luminous point sources which, from their extremely red colours ( F _18.5/ F _7.9≥ 3), appear to be embedded young stellar objects. The closest angular separations of the point sources in the three regions are 0.79, 1.00 and 3.33 arcsec corresponding to linear separations of 1700, 1800 and 6000 au, respectively. The methanol maser emission is found closest to the brightest MIR point source (within the assumed 1-arcsec pointing accuracy). Mass and luminosity estimates for the sources range from 3 to  22 M_⊙  and from 50 to 40 000  L_⊙  , respectively. Assuming the MIR sources are embedded objects and the observed gas mass provides the bulk of the reservoir from which the stars formed, it is difficult to generate the observed distributions for the most massive cluster members from the gas in the cores using a standard form of the initial mass function.     

An X‐shooter survey of star forming regions: Low‐mass stars and sub‐stellar objects

We present preliminary results of our X‐shooter survey in star forming regions. In this contribution we focus on subsamples of young stellar and sub‐stellar objects (YSOs) in the Lupus star forming region and in the TW Hya association. We show that the X‐shooter spectra are suitable for conducting several parallel studies such as YSO + disk fundamental parameters, accretion and outflow activity in the very low‐mass (VLM) and sub‐stellar regimes, as well as magnetic activity in young VLM YSOs, and Li abundance determinations. The capabilities of X‐shooter in terms of wide spectral coverage, resolution and limiting magnitudes, allow us to assess simultaneously the accretion/outflow, magnetic activity, and disk diagnostics, from the UV and optical to the near‐IR, avoiding ambiguities due to possible YSO variability (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A near‐infrared/optical/X‐ray survey in the centre of σ Orionis

Because of the intense brightness of the OB‐type multiple star system σ Ori, the low‐mass stellar and substellar populations close to the centre of the very young σ Orionis cluster is poorly know. I present an IJHK_s survey in the cluster centre, able to detect from the massive early‐type stars down to cluster members below the deuterium burning mass limit. The near‐infrared and optical data have been complemented with X‐ray imaging. Ten objects have been found for the first time to display high‐energy emission. Previously known stars with clear spectroscopic youth indicators and/or X‐ray emission define a clear sequence in the I vs. I – K_s diagram. I have found six new candidate cluster members that follow this sequence. One of them, in the magnitude interval of the brown dwarfs in the cluster, displays X‐ray emission and a very red J – K_s colour, indicative of a disc. Other three low‐mass stars have excesses in the K_s band as well. The frequency of X‐ray emitters in the area is 80±20 %. The spatial density of stars is very high, of up to 1.6±0.1 arcmin^–2. There is no indication of lower abundance of substellar objects in the cluster centre. Finally, I also report two cluster stars with X‐ray emission located at only 8000–11000 AU to σ Ori AB, two sources with peculiar colours and an object with X‐ray emission and near‐infrared magnitudes similar to those of previously‐known substellar objects in the cluster. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observational characteristics of dense cores with deeply embedded young protostars

Class 0 objects, which are thought to be the youngest protostars, are identified in terms of NIR or radio emission and/or the presence of molecular outflows. We present combined hydrodynamic and radiative transfer simulations of the collapse of a star‐forming molecular core, which suggest two criteria for identifying dense cores with deeply embedded very young protostars that may not be observable in the NIR or radio with current telescopes. We find that cores with protostars are relatively warm (T > 15 K) with their SEDs peaking at wavelengths <170 µm, and they tend to appear circular. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Searching for proto‐brown dwarfs: Extending near IR spectroscopy of protostars below the hydrogen burning limit

Recent observations of nearby star forming regions have offered evidence that young brown dwarfs undergo a period of mass accretion analogous to the T Tauri phase observed in young stars. Brown dwarf analogs to stellar protostars, however, have yet to be definitively observed. These young, accreting objects would shed light on the nature of the dominant brown dwarf formation process, as well as provide ideal laboratories to investigate the dependence of the accretion mechanism on protostellar mass. Recent near infrared surveys have identified candidate proto‐brown dwarfs and characterized low mass protostars in nearby star forming regions. These techniques allow near infrared spectra to diagnose the effective temperature, accretion luminosity, magnetic field strength and rotation velocity of young low mass stars across the stellar/substellar boundary. The lowest mass proto‐brown dwarfs (M < 40 M_Jup), however, will prove challenging to observe given current near IR observational capabilities. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X‐shooting Herbig Ae/Be stars: Accretion probed by near‐infrared He I emission

The Herbig Ae/Be stars are intermediate mass pre‐main sequence stars that bridge the gap between the low mass T Tauri stars and the Massive Young Stellar Objects. In this mass range, the acting star forming mechanism switches from magnetically controlled accretion to an as yet unknown mechanism, but which is likely to be direct disk accretion onto the star. We observed a large sample of Herbig Ae/Be stars with X‐shooter to address this issue from a multi‐wavelength perspective. It is the largest such study to date, not only because of the number of objects involved, but also because of the large wavelength coverage from the blue to the near‐infrared. This allows many accretion diagnostics to be studied simultaneously. By correlating the various properties with mass, temperature and age, we aim to determine where and whether the magnetically controlled mass accretion mechanism halts and the proposed direct disk accretion takes over. Here, we will give an overview of the background, present some observations and discuss our initial results. We will introduce a new accretion diagnostic for the research of Herbig Ae/Be stars, the HeI 1.083 μm line (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time-dependent models of the structure and stability of self-gravitating protoplanetary discs

Angular momentum transport within young massive protoplanetary discs may be dominated by self-gravity at radii where the disc is too weakly ionized to allow the development of the magneto-rotational instability. We use time-dependent one-dimensional disc models, based on a local cooling time calculation of the efficiency of transport, to study the radial structure and stability (against fragmentation) of protoplanetary discs in which self-gravity is the sole transport mechanism. We find that self-gravitating discs rapidly attain a quasi-steady state in which the surface density in the inner disc is high and the strength of turbulence very low (  α∼ 10⁻³  or less inside 5 au). Temperatures high enough to form crystalline silicates may extend out to several astronomical units at early times within these discs. None of our discs spontaneously develop regions that would be unambiguously unstable to fragmentation into substellar objects, though the outer regions (beyond 20 au) of the most massive discs are close enough to the threshold that fragmentation cannot be ruled out. We discuss how the mass accretion rates through such discs may vary with disc mass and with mass of the central star, and note that a determination of the     relation for very young systems may allow a test of the model.     

Very low mass stars in binaries: a theoretical look

We have undertaken a series of hydrodynamic + N ‐body simulations in order to explore the binary properties of young stars. We find that multiple stars are a natural outcome of collapsing turbulent flows, with a high incidence of N > 2 multiples, specially among the higher mass objects. We find a positive correlation of multiplicity with primary mass and a companion frequency that decreases with age, during the first few Myr after formation. Binary brown dwarfs are rarely formed, in conflict with observations. Brown dwarfs as companions are predominantly found orbiting binaries or triples at large separations. The paucity of ultra low mass and low mass ratio binaries has been investigated further, and we tentatively conclude that their formation is intricately related to an appropriate selection of initial conditions and an accurate modelling of disc accretion and evolution. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic field measurements of CP stars from hydrogen line cores

We present the results of measurements of magnetic fields of chemically peculiar (CP) stars, performed from the shifts between the circularly polarized components of metal and hydrogen lines (the Babcock method). The observations are carried out with an analyzer of circular polarization at the 6‐m telescope of the SAO RAS. We found that for the absolute majority of the objects studied (in 22 CP stars out of 23), the magnetic fields, determined from the Zeeman shifts in the hydrogen line cores, are significantly lower than those obtained from metal lines in the same spectra. This disparity varies between the stars. We show that instrumental effects can not produce the above features, and discuss the possible causes of the observed effect. The discovered condition reveals a more complicated structure of magnetic fields of CP stars than a simple dipole, in particular, a reduction of the field strength in the upper atmosphere with the vertical gradient, significantly higher than the dipole (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations of Stephenson's Hα stars

Hitherto unstudied objects from Stephenson's list of Hα emission line objects at high galactic latitude were observed spectroscopically to prove their nature. 9 out of 11 objects show Hα in emission. Spectroscopy combined with photometric information indicates most of them being classical Be stars, while one object is a Post‐AGB star and one a T‐Tauri star. The classification of two objects, which are showing Hα in emission, is unclear. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

>“Resolving” neutron stars physics and geometry

The unprecedented harvest of X‐ray photons detected from dozens of isolated neutron stars has made it possible to glimpse at their emission mechanisms as well as at their emission geometry. Rotating hot spot(s), superimposed to the global thermal emission from the neutron star surface, are seen from several objects, allowing to probe the stars' external heating sources. Non‐thermal emission is also seen to vary as the stars rotate. Moreover, absorption features have been detected in the spectra of several objects, allowing to probe (tentatively) the stars' magnetic fields. Spectacular tails, trailing the stars' supersonic motion, trace the boundaries of the relativist winds streaming from the star's magnetosphere. Apart from classical radio pulsar and certified radio‐quiet neutron stars, XMM‐Newton has devoted significant observation time to the enigmatic central compact objects, presumably isolated neutron stars shining at the center of their supernova remnants. Far from showing a unifying behaviour, XMM‐Newton data have unveiled a surprising diversity. Understanding the reason(s) behind such diversity is the challenge for the next decade of X‐ray observations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variability in the Lambda Orionis cluster substellar domain

We present the first results on variability of very low mass stars and brown dwarfs belonging to the ∼5 Myr Lambda Orionis cluster (Collinder 69). We have monitored almost continuously in the J filter a small area of the cluster which includes 12 possible members of the cluster during one night. Some members have turned to be short‐term variables. One of them, LOri167, has a mass close to the planetary mass limit and its variability might be due to instabilities produced by the deuterium burning, although other mechanism cannot be ruled out. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)