Dust in brown dwarfs and extra-solar planets

Proceedings of The International Astronomical Union, 2007

Astronomy and Astrophysics

In this paper, first solutions of the dust moment equations developed in for the description of dust formation and precipitation in brown dwarf and giant gas planet atmospheres are presented. We consider the special case of a static brown dwarf atmosphere, where dust particles continuously nucleate from the gas phase, grow by the accretion of molecules, settle gravitationally and re-evaporate thermally. Mixing by convective overshoot is assumed to replenish the atmosphere with condensable elements, which is necessary to counterbalance the loss of condensable elements by dust formation and gravitational settling (no dust without mixing). Applying a kinetic description of the relevant microphysical and chemical processes for TiO 2 -grains, the model makes predictions about the large-scale stratification of dust in the atmosphere, the depletion of molecules from the gas phase, the supersaturation of the gas in the atmosphere as well as the mean size and the mass fraction of dust grains as function of depth. Our results suggest that the presence of relevant amounts of dust is restricted to a layer, where the upper boundary (cloud deck) is related to the requirement of a minimum mixing activity (mixing time-scale τ mix ≈ 10 6 s) and the lower boundary (cloud base) is determined by the thermodynamical stability of the grains. The nucleation occurs around the cloud deck where the gas is cool, strongly depleted, but nevertheless highly supersaturated (S 1). These particles settle gravitationally and populate the warmer layers below, where the in situ formation (nucleation) is ineffective or even not possible. During their descent, the particles grow and reach mean radii of ≈30 µm ... 400 µm at the cloud base, but the majority of the particles in the cloud layer remains much smaller. Finally, the dust grains sink into layers which are sufficiently hot to cause their thermal evaporation. Hence, an effective transport mechanism for condensable elements exists in brown dwarfs, which depletes the gas above and enriches the gas below the cloud base of a considered solid/liquid material. The dust-to-gas mass fraction in the cloud layer results to be approximately given by the mass fraction of condensable elements in the gas being mixed up. Only for artificially reduced mixing we find a self-regulation mechanism that approximately installs phase equilibrium (S ≈ 1) in a limited region around the cloud base.

Astronomy & Astrophysics, 2014

Context. Gravity modifies the spectral features of young brown dwarfs (BDs). A proper characterization of these objects is crucial for the identification of the least massive, and latest-type objects in star-forming regions, and to explain the origin(s) of the peculiar spectro-photometric properties of young directly imaged extrasolar planets and BD companions. Aims. We obtained medium-resolution (R∼1500-1700) near-infrared (1.1-2.5 µm) spectra of seven young M9.5-L3 dwarfs classified at optical wavelengths. We aim to empirically confirm the low surface gravity of the objects in the near-infrared. We also test whether self-consistent atmospheric models correctly represent the formation and the settling of dust clouds in the atmosphere of young late-M and L dwarfs. Methods. We used ISAAC (Infrared Spectrometer And Array Camera) at VLT (Very Large Telescope) to obtain the spectra of the targets. We compared them to those of mature and young BDs, and young late-type companions to nearby stars with known ages, in order to identify and study gravity-sensitive features. We computed spectral indices weakly sensitive to the surface gravity to derive near-infrared spectral types. Finally, we found the best fit between each spectrum and synthetic spectra from the BT-Settl 2010 and 2013 atmospheric models. Using the best fit, we derived the atmospheric parameters of the objects and identify which spectral characteristics the models do not reproduce. Results. We confirmed that our objects are young BDs and we found near-infrared spectral types in agreement with the ones determined at optical wavelengths. The spectrum of the L2γ dwarf 2MASSJ232252.99-615127.5 reproduces well the spectrum of the planetary mass companion 1RXS J160929.1-210524b. BT-Settl models fit the spectra and the 1-5 µm spectral energy distribution of the L0-L3 dwarfs for temperatures between 1600-2000 K. But the models fail to reproduce the shape of the H band, and the nearinfrared slope of some of our targets. This fact, and the best fit solutions found with super-solar metallicity are indicative of a lack of dust, in particular at high altitude, in the cloud models.

2018

Context. The cloud formation process starts with the formation of seed particles, after which, surface chemical reactions grow or erode the cloud particles. If seed particles do not form, or are not available by another means, an atmosphere is unable to form a cloud complex and will remain cloud free. Aims. We investigate which materials may form cloud condensation seeds in the gas temperature and pressure regimes (T gas = 100-2000 K, p gas = 10 -8 -100 bar) expected to occur in planetary and brown dwarf atmospheres. Methods. We apply modified classical nucleation theory which requires surface tensions and vapour pressure data for each solid species, which are taken from the literature. Input gas phase number densities are calculated assuming chemical equilibrium at solar metallicity. Results. We calculate the seed formation rates of TiO 2 [s] and SiO[s] and find that they efficiently nucleate at high temperatures of T gas = 1000-1750 K. Cr[s], KCl[s] and NaCl[s] are found to efficiently nucleate across an intermediate temperature range of T gas = 500-1000 K. We find CsCl[s] may serve as the seed particle for the water cloud layers in cool sub-stellar atmospheres. Four low temperature ice species, H 2 O[s/l], NH 3 [s], H 2 S[s/l] and CH 4 [s], nucleation rates (T gas = 100-250 K) are also investigated for the coolest sub-stellar/planetary atmospheres. Conclusions. Our results suggest a possibly, (T gas , p gas ) distributed hierarchy of seed particle formation regimes throughout the substellar and planetary atmospheric temperature-pressure space. With TiO 2 [s] providing seed particles for the most refractory cloud formation species (e.g. Al 2 O 3 [s], Fe[s], MgSiO 3 [s], Mg 2 SiO 4 [s]), Cr[s] providing the seed particles for MnS[s], Na 2 S[s] and ZnS[s] sulfides, and K/Na/Rb/Cs/NH 4 -Cl binding solid species providing the seed particles for H 2 O[s/l] and NH 4 -H 2 PO 4 /SH[s] clouds. A detached, high-altitude aerosol layer may form in some sub-stellar atmospheres from the nucleation process, dependent on the upper atmosphere temperature, pressure and availability of volatile elements. In order to improve the accuracy of the nucleation rate calculation, further research into the small cluster thermochemical data for each cloud species is warranted. The validity of these seed particle scenarios will be tested by applying it to more complete cloud models in the future.

Proceedings of The Life Cycle of Dust in the Universe: Observations, Theory, and Laboratory Experiments — PoS(LCDU2013), 2014

The Astrophysical …, 2000

We present evolutionary calculations for very-low-mass stars and brown dwarfs based on synthetic spectra and non-grey atmosphere models which include dust formation and opacity, i.e. objects with T eff < ∼ 2800 K. The interior of the most massive brown dwarfs is shown to develop a conductive core after ∼ 2 Gyr which slows down their cooling. Comparison is made in optical and infrared color-magnitude diagrams with recent late-M and L-dwarf observations. The saturation in optical colors and the very red near-infrared colors of these objects are well explained by the onset of dust formation in the atmosphere. Comparison of the faintest presently observed L-dwarfs with these dusty evolutionary models suggests that dynamical processes such as turbulent diffusion and gravitational settling are taking place near the photosphere. As the effective temperature decreases below T eff ≈ 1300 − 1400 K, the colors of these objects move to very blue near-infrared colors, a consequence of the ongoing methane absorption in the infrared. We suggest the possibility of a brown dwarf dearth in J, H, K color-magnitude diagrams around this temperature.

Astronomy and Astrophysics, 2008

Aims. The abundance evolution of interstellar dust species originating from stellar sources and from condensation in molecular clouds in the local interstellar medium of the Milky Way is studied and the input of dust material to the Solar System is determined. Methods. A one-zone chemical evolution model of the Milky Way for the elemental composition of the disk combined with an evolution model for its interstellar dust component similar to that of Dwek (1998) is developed. The dust model considers dust-mass return from AGB stars as calculated from synthetic AGB models combined with models for dust condensation in stellar outflows. Supernova dust formation is included in a simple parameterized form which is gauged by observed abundances of presolar dust grains with supernova origin. For dust growth in the ISM a simple method is developed for coupling this with disk and dust evolution models. Results. The time evolution of the abundance of the following dust species is followed in the model: silicate, carbon, silicon carbide, and iron dust from AGB stars and from SNe as well as silicate, carbon, and iron dust grown in molecular clouds. It is shown that the interstellar dust population is dominated by dust accreted in molecular clouds; most of the dust material entering the Solar System at its formation does not show isotopic abundance anomalies of the refractory elements, i.e., inconspicuous isotopic abundances do not point to a Solar System origin of dust grains. The observed abundance ratios of presolar dust grains formed in SN ejecta and in AGB star outflows requires that for the ejecta from SNe the fraction of refractory elements condensed into dust is 0.15 for carbon dust and is quite small (∼ 10 −4 ) for other dust species.

2003

Monthly Notices of the Royal Astronomical Society, 2020

We use Milky Way-like chemodynamical simulations with a new treatment for dust destruction and growth to investigate how these two processes affect the properties of the interstellar medium in galaxies. We focus on the role of two specific parameters, namely fdes (a new parameter that determines the fraction of dust destroyed in a single gas particle vicinity of a supernova) and Cs (the probability that a metal atom or ion sticks to the dust grain after colliding, i.e. the sticking coefficient), in regulating the amount and distribution of dust, cold gas and metals in galaxies. We find that simulated galaxies with low fdes and/or high Cs values not only produce more dust, but they also have a shallower correlation between the dust surface density and the total gas surface density, and a steeper correlation between the dust-to-gas ratio and the metallicity. Only for values of fdes between 0.01 and 0.02, and of Cs between 0.5 and 1 do our simulations produce an average slope of the du...

Monthly Notices of the Royal Astronomical Society, 2017

Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial metallicity in the range of 0 ≤ Z ini ≤ 10 -4 and initial mass 2 ≤ M ini /M ⊙ ≤ 5 are explored by the hydrodynamical calculations of dust-driven wind (DDW) along the AGB evolutionary tracks. We employ the MESA code to simulate the evolution of stars, assuming an empirical mass-loss rate in the post-main sequence phase, and considering the three types of low-temperature opacities (scaled-solar, CO-enhanced, and CNO-enhanced opacities) to elucidate the effect on the stellar evolution and the DDW. We find that the treatment of low-temperature opacity strongly affects the dust formation and resulting DDW; in the carbon-rich AGB phase, the maximum Ṁ of M ini ≥ 3 M ⊙ star with the CO-enhanced opacity is at least one order of magnitude smaller than that with the CNO-enhanced opacity. A wide range of stellar parameters being covered, a necessary condition for driving efficient DDW with Ṁ ≥ 10 -6 M ⊙ yr -1 is expressed as the effective temperature T eff < ∼ 3850 K and log(δ C L/κ R M) > ∼ 10.43 logT eff -32.33 with the carbon excess δ C defined as ε C -ε O and the Rosseland mean opacity κ R in units of cm 2 g -1 in the surface layer, and the stellar mass (lu- minosity) M (L) in solar units. The derived fitting formulae of gas and dust mass-loss rates in terms of input stellar parameters could be useful for investigating the dust yield from AGB stars in the early Universe being consistent with the stellar evolution calculations.

The Astrophysical Journal, 2006

We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRS low resolution) observations for 314 stars in the Formation and Evolution of Planetary Systems Legacy program. These data are used to investigate the properties and evolution of circumstellar dust around solar-type stars spanning ages from approximately 3 Myr-3 Gyr. We identify 46 sources that exhibit excess infrared emission above the stellar photosphere at 24 μm, and 21 sources with excesses at 70 μm. Five sources with an infrared excess have characteristics of optically thick primordial disks, while the remaining sources have properties akin to debris systems. The fraction of systems exhibiting a 24 μm excess greater than 10.2% above the photosphere is 15% for ages < 300 Myr and declines to 2.7% for older ages. The upper envelope to the 70 μm fractional luminosity appears to decline over a similar age range. The characteristic temperature of the debris inferred from the IRS spectra range between 60 and 180 K, with evidence for the presence of cooler dust to account for the strength of the 70 μm excess emission. No strong correlation is found between dust temperature and stellar age. Comparison of the observational data with disk models containing a power-law distribution of silicate grains suggests that the typical inner-disk radius is 10 AU. Although the interpretation is not unique, the lack of excess emission shortward of 16 μm and the relatively flat distribution of the 24 μm excess for ages 300 Myr is consistent with steady-state collisional models.

Astronomy and Astrophysics, 2004

In this paper, we propose a kinetic description for the growth and evaporation of oxygen-rich, dirty dust particles, which consist of numerous small islands of different solid materials like Mg 2 SiO 4 , SiO 2 , Al 2 O 3 , Fe and TiO 2 . We assume that the total surface of such a grain collects condensible molecules from the gas phase and that these molecules are rapidly transported by diffusive hopping on the surface to the respective solid islands, where finally the constructive surface chemical reactions take place which increase the size of the grain. Applied to a typical dust forming region in a brown dwarf atmosphere, turbulent temperature fluctuations enable the creation of first seed particles (nucleation) at high supersaturation ratios. These seeds are then quickly covered by different solid materials in a simultaneous way, which results in dirty grains. Our treatment by moment equations allows for the calculation of the time-dependent material composition of the dust grains and the elemental composition of the gas phase. We argue that the depletion of condensible elements from the gas phase by dust formation may be incomplete and occurs in a patchy, non-uniform way which possibly makes metallicity measurements highly uncertain.

The Astrophysical Journal, 2003

The average properties of interstellar dust have previously been inferred from remote astronomical observations of interstellar extinction and from gas depletion measurements. In addition to recent high-resolution observations of gas absorption spectra, in situ measurements of dust in the solar system provide an alternative approach to deducing the properties of interstellar dust, in particular, those of dust in the Local Interstellar Cloud (LIC), in which the Sun resides. We constrain the composition and structure of dust in the LIC by the dust-phase elemental abundances derived from gas absorption measurements and by the dynamical behavior inferred from dust impact measurements. The elemental abundances of the LIC dust are consistent with coremantle grains consisting of Mg-rich pyroxene and Mg-rich olivine with inclusions of troilite, Fe-rich kamacite, and corundum in the core and organic refractory compounds of C, N, and O in the mantle. The mass of the organic refractory mantle is comparable to the mass of the silicate core that is abundant in pyroxene compared to olivine. Taking into account these results, the dynamical behavior of the LIC dust in the solar system indicates that bare silicates and bare carbonaceous materials may be present as grains smaller than 10 À17 kg. The LIC grains with mass exceeding 10 À17 kg are most likely aggregates of submicron-sized silicate core, organic mantle grains. The mass distribution of dust in the LIC can be well explained by coagulation growth of core-mantle grains but is scarcely explained by severe destruction of grains in interstellar shocks.

Over the past decade a new generation of chemical models have included the dust in the treatment of the ISM. This major accomplishment has been spurred by the growing amounts of data on the highly obscured high-z Universe and the intriguing local properties of the Solar Neighbourhood (SoNE). We present here a new model able to simulate the formation and evolution of dust in the ISM. The model follows the evolution of 16 elemental species, with particular attention to those that are simultaneously present in form of gas and dust, e.g. C, N, O, Mg, Si, S, Ca and Fe. In this study we focus on the SoNe and the MW Disk as a whole which are considered as laboratories to test the physical ingredients governing the dust evolution. Infall of primordial gas, birth and death of stars, radial flows of matter between contiguous shells, presence of a central bar, star-dust emission by SNae and AGB stars, dust destruction and accretion are taken into account. The model reproduces the local depleti...

The Astrophysical Journal Supplement Series, 2009

We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRS low resolution) observations for 314 stars in the Formation and Evolution of Planetary Systems Legacy program. These data are used to investigate the properties and evolution of circumstellar dust around solar-type stars spanning ages from approximately 3 Myr-3 Gyr. We identify 46 sources that exhibit excess infrared emission above the stellar photosphere at 24 μm, and 21 sources with excesses at 70 μm. Five sources with an infrared excess have characteristics of optically thick primordial disks, while the remaining sources have properties akin to debris systems. The fraction of systems exhibiting a 24 μm excess greater than 10.2% above the photosphere is 15% for ages < 300 Myr and declines to 2.7% for older ages. The upper envelope to the 70 μm fractional luminosity appears to decline over a similar age range. The characteristic temperature of the debris inferred from the IRS spectra range between 60 and 180 K, with evidence for the presence of cooler dust to account for the strength of the 70 μm excess emission. No strong correlation is found between dust temperature and stellar age. Comparison of the observational data with disk models containing a power-law distribution of silicate grains suggests that the typical inner-disk radius is 10 AU. Although the interpretation is not unique, the lack of excess emission shortward of 16 μm and the relatively flat distribution of the 24 μm excess for ages 300 Myr is consistent with steady-state collisional models.