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.     

Rotation and activity in the solar-type stars of NGC 2547

We present high-resolution spectroscopy of a sample of 24 solar-type stars in the young (15–40 Myr), open cluster NGC 2547. We use our spectra to confirm cluster membership in 23 of these stars, to determine projected equatorial velocities and chromospheric activity, and to search for the presence of accretion discs. We find examples of both fast ( v _e sin  i >50 km s⁻¹) and slow ( v _e sin  i <10 km s⁻¹) rotators, but no evidence for active accretion in any of the sample. The distribution of projected rotation velocities is indistinguishable from the slightly older IC 2391 and IC 2602 clusters, implying similar initial angular momentum distributions and circumstellar disc lifetimes. The presence of very slow rotators indicates either that long (10–40 Myr) disc lifetimes or internal differential rotation are needed, or that NGC 2547 (and IC 2391/2602) were born with more slowly rotating stars than are presently seen in even younger clusters and associations. The solar-type stars in NGC 2547 follow a similar rotation–activity relationship to that seen in older clusters. X-ray activity increases until a saturation level is reached for v _e sin  i >15–20 km s⁻¹. We are unable to explain why this saturation level, of log( L _x L _bol)≃−3.3, is a factor of 2 lower than in other clusters, but rule out anomalously slow rotation rates or uncertainties in X-ray flux calculations.     

Episodic accretion in magnetically layered protoplanetary discs

We study protoplanetary disc evolution assuming that angular momentum transport is driven by gravitational instability at large radii, and magnetohydrodynamic (MHD) turbulence in the hot inner regions. At radii of the order of 1 au such discs develop a magnetically layered structure, with accretion occurring in an ionized surface layer overlying quiescent gas that is too cool to sustain MHD turbulence. We show that layered discs are subject to a limit cycle instability, in which accretion on to the protostar occurs in ∼10⁴-yr bursts with ̇ ∼10⁻⁵ M_⊙ yr⁻¹, separated by quiescent intervals lasting ∼10⁵ yr where ̇ ≈10⁻⁸ M_⊙ yr⁻¹. Such bursts could lead to repeated episodes of strong mass outflow in young stellar objects. The transition to this episodic mode of accretion occurs at an early epoch ( t ≪1 Myr), and the model therefore predicts that many young pre-main-sequence stars should have low rates of accretion through the inner disc. At ages of a few Myr, the discs are up to an order of magnitude more massive than the minimum-mass solar nebula, with most of the mass locked up in the quiescent layer of the disc at r ∼1 au. The predicted rate of low-mass planetary migration is reduced at the outer edge of the layered disc, which could lead to an enhanced probability of giant planet formation at radii of 1–3 au.     

Radiatively heated, protoplanetary discs with dead zones – I. Dust settling and thermal structure of discs around M stars

The irradiation of protoplanetary discs by central stars is the main heating mechanism for discs, resulting in their flared geometric structure. In a series of papers, we investigate the deep links between two-dimensional self-consistent disc structure and planetary migration in irradiated discs, focusing particularly on those around M stars. In this first paper, we analyse the thermal structure of discs that are irradiated by an M star by solving the radiative transfer equation by means of a Monte Carlo code. Our simulations of irradiated hydrostatic discs are realistic and self-consistent in that they include dust settling with multiple grain sizes  ( N = 15)  , the gravitational force of an embedded planet on the disc and the presence of a dead zone (a region with very low levels of turbulence) within it. We show that dust settling drives the temperature of the mid-plane from an   r ^−3/5  distribution (well mixed dust models) towards an   r ^−3/4  . The dead zone, meanwhile, leaves a dusty wall at its outer edge because dust settling in this region is enhanced compared to the active turbulent disc at larger disc radii. The disc heating produced by this irradiated wall provides a positive gradient region of the temperature in the dead zone in front of the wall. This is crucially important for slowing planetary migration because Lindblad torques are inversely proportional to the disc temperature. Furthermore, we show that low turbulence of the dead zone is self-consistently induced by dust settling, resulting in the Kelvin–Helmholtz instability (KHI). We show that the strength of turbulence arising from the KHI in the dead zone is  α= 10⁻⁵  .     

Substellar companions and isolated planetary-mass objects from protostellar disc fragmentation

Self-gravitating protostellar discs are unstable to fragmentation if the gas can cool on a time-scale that is short compared with the orbital period. We use a combination of hydrodynamic simulations and N -body orbit integrations to study the long-term evolution of a fragmenting disc with an initial mass ratio to the star of   M _disc/ M _*= 0.1  . For a disc that is initially unstable across a range of radii, a combination of collapse and subsequent accretion yields substellar objects with a spectrum of masses extending (for a Solar-mass star) up to  ≈0.01 M_⊙  . Subsequent gravitational evolution ejects most of the lower mass objects within a few million years, leaving a small number of very massive planets or brown dwarfs in eccentric orbits at moderately small radii. Based on these results, systems such as HD 168443 – in which the companions are close to or beyond the deuterium burning limit – appear to be the best candidates to have formed via gravitational instability. If massive substellar companions originate from disc fragmentation, while lower-mass planetary companions originate from core accretion, the metallicity distribution of stars which host massive substellar companions at radii of ∼1 au should differ from that of stars with lower mass planetary companions.     

Circumstellar discs in the young σ Orionis cluster

We present new K - and L '-band imaging observations for members of the young (3–5 Myr) σ Orionis cluster, obtained at the United Kingdom Infrared Telescope (UKIRT) with the UKIRT 1–5 micron Imager Spectrometer (UIST). We determine ( K − L ') colour excesses with respect to the photospheres, finding evidence for warm circumstellar dust around 27 out of 83 cluster members that have masses between 0.04 and  1.0 M_⊙  . This indicates a circumstellar disc frequency of at least (33 ± 6) per cent for this cluster, consistent with previous determinations from smaller samples and also consistent with the 3-Myr disc half-life suggested by Haisch et al. There is marginal evidence that the disc frequency declines towards lower masses, but the data are also consistent with no mass dependence at all. There is no evidence for spatial segregation of objects with and without circumstellar discs.     

Non-emission-line young stars of intermediate mass

We present optical spectra of four intermediate-mass candidate young stellar objects that have often been classified as Herbig Ae/Be stars. Typical Herbig Ae/Be emission features are not present in the spectra of these stars. Three of them, HD 36917, HD 36982 and HD 37062, are members of the young Orion nebula cluster (ONC). This association constrains their ages to be ≲1 Myr. The lack of appreciable near-infrared excess in them suggests the absence of hot dust close to the central star. However, they do possess significant amounts of cold and extended dust as revealed by the large excess emission observed at far-infrared wavelengths. The fractional infrared luminosities  ( L _ir/ L _★)  and the dust masses computed from IRAS fluxes are systematically lower than those found for Herbig Ae/Be stars but higher than those for Vega-like stars. These stars may thus represent the youngest examples of the Vega phenomenon known so far. In contrast, the other star in our sample, HD 58647, is more likely to be a classical Be star, as is evident from the low   L _ir/ L _★  , the scarcity of circumstellar dust, the low polarization, the presence of H α emission and near-infrared excess, and the far-infrared spectral energy distribution consistent with free–free emission similar to other well-known classical Be stars.     

On the origin of high-velocity runaway stars

We explore the hypothesis that some high-velocity runaway stars attain their peculiar velocities in the course of exchange encounters between hard massive binaries and a very massive star (either an ordinary  50–100 M_⊙  star or a more massive one, formed through runaway mergers of ordinary stars in the core of a young massive star cluster). In this process, one of the binary components becomes gravitationally bound to the very massive star, while the second one is ejected, sometimes with a high speed. We performed three-body scattering experiments and found that early B-type stars (the progenitors of the majority of neutron stars) can be ejected with velocities of  ≳200–400 km s⁻¹  (typical of pulsars), while  3–4 M_⊙  stars can attain velocities of  ≳300–400 km s⁻¹  (typical of the bound population of halo late B-type stars). We also found that the ejected stars can occasionally attain velocities exceeding the Milky Ways's escape velocity.     

Gravitational interaction between a planet and an optically thin disc

We investigate the gravitational interaction between a planet and an optically thin protoplanetary disc, performing local three-dimensional hydrodynamical simulations. In the present study, we take account of radiative energy transfer in optically thin discs. Before the stage of planetary accretion, dust opacity is expected to decrease significantly because of grain growth and planetesimal formation. Thus, it would be reasonable to consider optically thin discs in the disc–planet interaction. Furthermore, we focus on small planets that can neither capture disc gas nor open a disc gap. The one-sided torque exerted on a planet by an optically thin disc is examined for various values of the disc optical thickness (<1). In optically thin discs, the temperature behind the density waves is lower than the unperturbed value because of radiative cooling. Heating due to shock dissipation is less effective than radiative cooling. Because of radiative cooling, the density distribution around the planet is not axisymmetric, which exerts an additional torque on the planet. The torque enhancement becomes maximum when the cooling time is comparable with the Keplerian period. The enhancement is significant for low-mass planets. For planets with  3 M_⊕  , the additional one-sided torque can be 40 per cent of the torque in the isothermal case. The radiative cooling is expected to change the differential torque and the migration speed of planets, too.     

Sakurai's Object: characterizing the near-infrared CO ejecta between 2003 and 2007

We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around  4.7 μm  , we determine the excitation conditions in the line-forming region. We find  ¹²C/¹³C = 3.5^+2.0_−1.5  , consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of  2.2 × 10⁻⁶≤ M _CO≤ 2.7 × 10⁻⁶ M_⊙  of CO ejecta outside the dust, forming a high-velocity wind of  500 ± 80 km s⁻¹  . We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor.     

Possible detection of V4334 Sgr (Sakurai's Object) at 450 and 850 μm

We report the possible detection of V4334 Sgr (Sakurai's Object) at 450 and 850 μm with SCUBA on the James Clerk Maxwell Telescope. The submillimetre photometry, combined with a  1–5 μm  spectrum and  8–10 μm  photometry obtained nearly contemporaneously, suggests that the submillimetre emission originates in material ejected during the 1995 event. The dust mass is a  few×10^-7 M_⊙  , the average mass-loss in the form of dust is  few×10^-8 M_⊙ yr^-1  , and the integrated luminosity is  log( L /L_⊙)=3.66  for a distance of 2 kpc. The ejected shell had angular diameter ∼55 mas in 2001 August, and should by now be resolvable in the mid-infrared by  8–10 m  class telescopes.     

HST and ground-based eclipse observations of V2051 Ophiuchi: binary parameters

We report on high-speed eclipse photometry of the dwarf nova V2051 Oph while it was in a low brightness state, at B  ≃ 16.2 mag. In comparison with the average IUE spectra, the ultraviolet continuum and emission lines appear reduced by factors of, respectively, ≃ 4 and ≃ 5. Flickering activity is mostly suppressed and the light curve shows the eclipse of a compact white dwarf at the disc centre which contributes ≃ 60 per cent of the total light at 3900–4300 Å. We use measurements of contact phases in the eclipse light curve to derive the binary geometry and to estimate masses and relevant dimensions. We find a mass ratio of q  = 0.19 ± 0.03 and an inclination of i  = 83 ± 2°. The masses of the component stars are M ₁ = 0.78 ± 0.06 M⊙ and M ₂ = 0.15 ± 0.03 M⊙. Our photometric model predicts K ₁ = 83 ± 12 km s⁻¹ and K ₂ = 436 ± 11 km s⁻¹. The predicted value of K₁ is in accordance with the velocity amplitude obtained from the emission lines after a correction for asymmetric line emission in the disc is made. The secondary of V2051 Oph is significantly more massive than the secondaries of the other ultrashort period dwarf novae. V2051 Oph is probably a relatively young system, with a secondary star that has not had enough time to evolve out of thermal equilibrium.     

IRAS 22036+5306: an Al2O3 oxide-dominated post-AGB star

Radiative transfer modelling of the Infrared Space Observatory ( ISO ) spectrum of IRAS 22036+5306 has shown that its unusual 11-μm band can be suitably modelled with an alumina-olivine mixture substantially dominated by the former. The results of this work add further credence to recent findings that significant amounts of Al  ₂O₃  dust grains are present in the dust shells of stars near or beyond the tip of the asymptotic giant branch. Indeed, in the case of IRAS 22036+5306, Al  ₂O₃  dominates the dust composition to the extent that it shifts the 9.8-μm band due to amorphous silicates to 11 μm. IRAS 22036+5306 may be an unusual case in that the inner dust torus is maintained at a sufficiently high temperature for Al  ₂O₃  condensation, but not silicate.     

Spitzer observations of acetylene bands in carbon-rich asymptotic giant branch stars in the Large Magellanic Cloud

We investigate the molecular bands in carbon-rich asymptotic giant branch (AGB) stars in the Large Magellanic Cloud (LMC), using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope ( SST ) over the 5–38 μm range. All 26 low-resolution spectra show acetylene (C₂H₂) bands at 7 and 14 μm. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 μm C₂H₂  band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 μm C₂H₂  band by circumstellar dust emission. This 14-μm band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C₂H₂ column density, derived from the 13.7 μm band, shows a gas mass-loss rate in the range 3 × 10⁻⁶ to 5 × 10⁻⁵ M_⊙ yr⁻¹. This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 μm C₂H₂  band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C₂H₂  abundance among LMC and Galactic stars. This reflects the effect of the third dredge-up bringing self-produced carbon to the surface, leading to high carbon-to-oxygen ratio at low metallicity.     

The photoevaporation of discs around young stars in massive clusters

We present models in which the photoevaporation of discs around young stars by an external ultraviolet source (as computed by Adams et al.) is coupled with the internal viscous evolution of the discs. These models are applied to the case of the Orion Nebula Cluster (ONC), where the presence of a strong ultraviolet field from the central OB stars, together with a detailed census of circumstellar discs and photoevaporative flows, is well established. In particular we investigate the constraints that are placed on the initial disc properties in the ONC by the twin requirement that most stars possess a disc on a scale of a few astronomical unit (au), but that only a minority (<20 per cent) are resolved by Hubble Space Telescope ( HST ) at a scale of 50 au. We find that these requirements place very weak constraints on the initial radius distribution of circumstellar discs: the resulting size distribution readily forgets the initial radius distribution, owing to the strong positive dependence of the photoevaporation rate on disc radius. Instead, the scarcity of large discs reflects the relative scarcity of initially massive discs (with mass  >0.1 M_⊙  ). The ubiquity of discs on a small scale, on the other hand, mainly constrains the time-span over which the discs have been exposed to the ultraviolet field (<2 Myr). We argue that the discs that are resolved by HST represent a population of discs in which self-gravity was important at the time that the dominant central OB star switched on, but that, according to our models, self-gravity is unlikely to be important in these discs at the present time. We discuss the implications of our results for the so-called proplyd lifetime problem.     

Impact of cosmic rays on Population III star formation

We explore the implications of a possible cosmic-ray (CR) background generated during the first supernova explosions that end the brief lives of massive Population III stars. We show that such a CR background could have significantly influenced the cooling and collapse of primordial gas clouds in minihaloes around redshifts of   z ∼ 15–20  , provided the CR flux was sufficient to yield an ionization rate greater than about 10⁻¹⁹ s⁻¹ near the centre of the minihalo. The presence of CRs with energies  ≲10⁷  eV would indirectly enhance the molecular cooling in these regions, and we estimate that the resulting lower temperatures in these minihaloes would yield a characteristic stellar mass as low as  ∼10 M_⊙  . CRs have a less-pronounced effect on the cooling and collapse of primordial gas clouds inside more massive dark matter haloes with virial masses  ≳10⁸ M_⊙  at the later stages of cosmological structure formation around   z ∼ 10–15  . In these clouds, even without CR flux the molecular abundance is already sufficient to allow cooling to the floor set by the temperature of the cosmic microwave background.     

The cooling of shock-compressed primordial gas

We find that at redshifts   z ≳ 10, HD  line cooling allows strongly shocked primordial gas to cool to the temperature of the cosmic microwave background (CMB). This temperature is the minimum value attainable via radiative cooling. Provided that the abundance of HD, normalized to the total number density, exceeds a critical level of  ∼10⁻⁸  , the CMB temperature floor is reached in a time which is short in comparison to the Hubble time. We estimate the characteristic masses of stars formed out of shocked primordial gas in the wake of the first supernovae, and resulting from the virialization of dark matter haloes during hierarchical structure formation to be  ∼10 M_⊙  . In addition, we show that cooling by HD enables the primordial gas in relic H  ii regions to cool to temperatures considerably lower than those reached via H₂ cooling alone. We confirm that HD cooling is unimportant in cases where the primordial gas does not go through an ionized phase, as in the formation process of the very first stars in   z ≳ 20  minihaloes of mass  ∼10⁶ M_⊙  .     

Metal-rich carbon stars in the Sagittarius dwarf spheroidal galaxy

We present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C₂H₂ and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range  1.0–3.3 × 10⁻⁸ M_⊙ yr⁻¹  . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample.     

Contribution to the search for binaries among Am stars – VIII. New spectroscopic orbits of eight systems and statistical study of a sample of 91 Am stars

Red clump giant (RCG) stars can be used as distance indicators to trace the mass distribution of the Galactic bar. We use RCG stars from 44 bulge fields from the OGLE-II microlensing collaboration data base to constrain analytic triaxial models for the Galactic bar. We find the bar major-axis is oriented at an angle of 24°–27° to the Sun–Galactic Centre line-of-sight. The ratio of semimajor and semiminor bar axis scalelengths in the Galactic plane   x ₀, y ₀  , and vertical bar scalelength z ₀, is   x ₀ :  y ₀ :  z ₀= 10 : 3.5 : 2.6  , suggesting a slightly more prolate bar structure than the working model of Gerhard which gives the scalelength ratios as   x ₀ :  y ₀ :  z ₀= 10 : 4 : 3  .