The potential for Earth-mass planet formation around brown dwarfs

Recent observations point to the presence of structured dust grains in the discs surrounding young brown dwarfs, thus implying that the first stages of planet formation take place also in the substellar regime. Here, we investigate the potential for planet formation around brown dwarfs and very low-mass stars according to the sequential core accretion model of planet formation. We find that, for a brown dwarf mass 0.05 M_⊙, our models predict a maximum planetary mass of  ∼5   M_⊕  , orbiting with semimajor axis ∼ 1 au. However, we note that the predictions for the mass–semimajor axis distribution are strongly dependent upon the models chosen for the disc surface density profiles and the assumed distribution of disc masses. In particular, if brown dwarf disc masses are of the order of a few Jupiter masses, Earth-mass planets might be relatively frequent, while if typical disc masses are only a fraction of Jupiter mass, we predict that planet formation would be extremely rare in the substellar regime. As the observational constraints on disc profiles, mass dependencies and their distributions are poor in the brown dwarf regime, we advise caution in validating theoretical models only on stars similar to the Sun and emphasize the need for observational data on planetary systems around a wide range of stellar masses. We also find that, unlike the situation around solar-like stars, Type II migration is totally absent from the planet formation process around brown dwarfs, suggesting that any future observations of planets around brown dwarfs would provide a direct measure of the role of other types of migration.     

An optical spectroscopic HR diagram for low-mass stars and brown dwarfs in Orion

The masses and temperatures of young low-mass stars and brown dwarfs in star-forming regions are not yet well established because of uncertainties in the age of individual objects and the spectral type–temperature scale appropriate for objects with ages of only a few Myr. Using multi-object optical spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified and 44 of these confirmed as bona fide cluster members. The spectral types obtained have been converted to effective temperatures using a temperature scale intermediate between those of dwarfs and giants, which is suitable for young pre-main-sequence objects. The objects have been placed on a Hertzsprung–Russell (HR) diagram overlaid with theoretical isochrones. The low-mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while many of the lower mass substellar objects are located well above this isochrone. An average age of 1 Myr is found for the majority of the objects. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and range from  0.018 to 0.44 M_⊙  . The spectra also allow an investigation of the surface gravity of the objects by measurement of the sodium doublet equivalent width. With one possible exception, all objects have low gravities, in line with young ages, and the Na indices for the Trapezium objects lie systematically below those of young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index may provide a sensitive means of estimating ages in young clusters.     

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.     

Close binary systems among very low‐mass stars and brown dwarfs

Using Monte Carlo simulations and published radial velocity surveys we have constrained the frequency and separation (a ) distribution of very low‐mass star (VLM) and brown dwarf (BD) binary systems.We find that simple Gaussian extensions of the observed wide binary distribution, with a peak at 4AU and 0.6 < σ _{log(a /AU)} < 1.0, correctly reproduce the observed number of close binary systems, implying a close (a < 2.6 AU) binary frequency of 17–30% and overall frequency of 32–45%. N‐body models of the dynamical decay of unstable protostellar multiple systems are excluded with high confidence because they do not produce enough close binary VLMs/BDs. The large number of close binaries and high overall binary frequency are also completely inconsistent with published smoothed particle hydrodynamical modelling and argue against a dynamical origin for VLMs/BDs. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hotspots and a clumpy disc: variability of brown dwarfs and stars in the young σ Ori cluster

The properties of accretion discs around stars and brown dwarfs in the σ Ori cluster (age 3 Myr) are studied based on near-infrared (IR) time series photometry supported by mid-IR spectral energy distributions (SEDs). We monitor ∼30 young low-mass sources over eight nights in the J and K band using the duPont telescope at Las Campanas. We find three objects showing variability with J -band amplitudes  ≥0.5 mag  ; five additional objects exhibit low-level variations. All three highly variable sources have been previously identified as highly variable; thus, we establish the long-term nature of their flux changes. The light curves contain periodic components with time-scales of  ∼0.5–8 d  , but have additional irregular variations superimposed – the characteristic behaviour for classical T Tauri stars. Based on the colour variability, we conclude that hotspots are the dominant cause of variations in two objects (#19 and #33), including one likely brown dwarf, with spot temperatures in the range of 6000–7000 K. For the third one (#2), a brown dwarf or very low-mass star, inhomogeneities at the inner edge of the disc are the likely origin of variability. Based on mid-IR data from Spitzer , we confirm that the three highly variable sources are surrounded by circum-(sub)-stellar discs. They show typical SEDs for T Tauri-like objects. Using SED models, we infer an enhanced scaleheight in the disc for the object #2, which favours the detection of disc inhomogeneities in light curves and is thus consistent with the information from variability. In the σ Ori cluster, about every fifth accreting low-mass object shows persistent high-level photometric variability. We demonstrate that estimates for fundamental parameters in such objects can be significantly improved by determining the extent and origin of the variations.     

High-resolution spectra of very low-mass stars

We present the results of high-resolution (1–0.4 Å) optical spectroscopy of a sample of very low-mass stars. These data are used to examine the kinematics of the stars at the bottom of the hydrogen-burning main sequence. No evidence is found for a significant difference between the kinematics of the stars in our sample with I  −  K  > 3.5 ( M _bol ≳ 12.8) and those of more massive M dwarfs ( M _bol ≈ 7–10). A spectral atlas at high (0.4-Å) resolution for M8–M9+ stars is provided, and the equivalent widths of Cs  I , Rb  I and Hα lines present in our spectra are examined. We analyse our data to search for the presence of rapid rotation, and find that the brown dwarf LP 944-20 is a member of the class of 'inactive, rapid rotators'. Such objects seem to be common at and below the hydrogen-burning main sequence. It seems that in low-mass/low-temperature dwarf objects either the mechanism that heats the chromosphere, or the mechanism that generates magnetic fields, is greatly suppressed.     

Spatial distribution of stars and brown dwarfs in σ Orionis

I have re-visited the spatial distribution of stars and high-mass brown dwarfs in the σ Orionis (σ Ori) cluster (∼3 Ma, ∼360 pc). The input was a catalogue of 340 cluster members and candidates at separations less than 30 arcmin to σ Ori AB. Of them, 70 per cent have features of extreme youth. I fitted the normalized cumulative number of objects counting from the cluster centre to several power-law, exponential and King radial distributions. The cluster seems to have two components: a dense core that extends from the centre to   r ≈  20 arcmin and a rarified halo at larger separations. The radial distribution in the core follows a power law proportional to r ¹, which corresponds to a volume density proportional to   r ⁻²  . This is consistent with the collapse of an isothermal spherical molecular cloud. The stars more massive than  3.7 M_⊙  concentrate, however, towards the cluster centre, where there is also an apparent deficit of very low mass objects  ( M < 0.16 M_⊙)  . Last, I demonstrated through Monte Carlo simulations that the cluster is azimuthally asymmetric, with a filamentary overdensity of objects that runs from the cluster centre to the Horsehead Nebula.     

A population of very young brown dwarfs and free-floating planets in Orion

We describe the results of a very deep imaging survey of the Trapezium cluster in the IJH bands, using the UKIRT high-resolution camera UFTI. Approximately 32 per cent of the 515 point sources detected are brown dwarf candidates, including several free-floating objects with masses below the deuterium-burning (planetary) threshold at 0.013 M_⊙, which are detectable because of their extreme youth. We have confidence that almost all the sources detected are cluster members, since foreground contamination is minimal in the 33-arcmin² area surveyed, and the dense backdrop of OMC-1 obscures all background stars at these wavelengths. Extinction is calculated from the ( J − H ) colours, permitting accurate luminosity estimates, and temperatures are derived from the dereddened ( I − J ) colours. There is some evidence for a cut-off in the luminosity function below the level corresponding to several Jupiter masses, which may represent the bottom end of the initial mass function . Since star formation is complete in the Trapezium, this limit could have wide significance, if confirmed. However, it could well be an effect of the dispersal of the molecular cloud by the central O-type stars, a process for which the time-scale will vary between star formation regions.     

A near-infrared spectroscopic search for very-low-mass cool companions to notable DA white dwarfs

We have undertaken a detailed near-infrared spectroscopic analysis of eight notable white dwarfs, predominantly of southern declination. In each case the spectrum failed to reveal compelling evidence for the presence of a spatially unresolved, cool, late-type companion. Therefore, we have placed an approximate limit on the spectral type of a putative companion to each degenerate. From these limits we conclude that if GD659, GD50, GD71 or WD2359−434 possesses an unresolved companion then most probably it is substellar in nature  ( M < 0.072 M_⊙)  . Furthermore, any spatially unresolved late-type companion to RE J0457−280, RE J0623−374, RE J0723−274 or RE J2214−491 most likely has   M < 0.082 M_⊙  . These results imply that if weak accretion from a nearby late-type companion is the cause of the unusual photospheric composition observed in a number of these degenerates then the companions are of very low mass, beyond the detection thresholds of this study. Furthermore, these results do not contradict a previously noted deficit of very-low-mass stellar and brown dwarf companions to main sequence F, G, K and early-M type primaries ( a ≲ 1000 au).     

VISIR/VLT observations of brown dwarfs in Upper Scorpius

We present mid‐IR observations of a sample of very low‐mass stars and brown dwarfs in the Upper Scorpius association. The main goal of these observations is to derive the disk frequency and the disk properties of the sample in order to shed light on brown dwarf formation theories. The observations have been performed with VISIR at the Very Large Telescope (VLT, Paranal). A total of ten targets were observed and six were detected. In this paper we show preliminary results for two of them: DENIS‐PJ160334 and DENIS‐PJ161939. While the former shows mid‐IR fluxes consistent with pure photospheric emission, the latter displays strong mid‐IR excesses which are probably related with a circumstellar disk. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

IZ photometry of L dwarfs and the implications for brown dwarf surveys

The I − Z colour has recently been shown to be a good temperature indicator for M dwarfs. We present the first IZ photometry of a small sample of L dwarfs ranging in spectral type from L0.5V to L6.0V. We find that the I − Z colour is not a good temperature indicator for objects between L1V and L5V, such objects having colours that overlap with mid M dwarfs. We attribute this to the reduction in the strength of the TiO and VO bands in the L dwarfs, which are the dominant opacity source in the I band for late M dwarfs. Beyond L5V, I − Z appears to be a reasonable indicator. This has important implications for the planning of optical surveys for cool objects in clusters and the field. For example, I − Z will cease to be a good method of identifying brown dwarfs in the Pleiades below around 0.04 M_⊙, and at around 0.075 M_⊙ in the Hyades and Praesepe.     

A new population of brown dwarfs

In this paper we report the first results from a survey for low-mass stars and brown dwarfs, based on a photographic stack of around 100 Schmidt plates. This survey extends photographic searches by about 2 mag, and covers an area of 25 deg². Some 30 faint objects with large R − I colours were selected for further study, and were found to have very strong molecular absorption in their spectra, but only moderately red infrared colours. Five of these stars were selected for a parallax programme; three of these were found to be at a distance of around 45 pc, implying a very low luminosity. On the basis of their luminosity alone it is clear that these stars are field brown dwarfs, and we discuss their likely evolutionary status in the context of current models of low-mass stellar evolution.