Jeroen Bouwman

ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: Towardsan understanding of dust processing.

We present Infrared Space Observatory (ISO) spectra of fourteen isolated Herbig Ae/Be (HAEBE) stars, to study the characteristics of their circumstellar dust. These spectra show large star-to-star differences, in the emission features of both carbon-rich and oxygen-rich dust grains. The IR spectra were combined with photometric data ranging from the UV through the optical into the sub-mm region. We defined two key groups, based upon the spectral shape of the infrared region. The derived results can be summarized as follows: (1) the continuum of the IR to sub-mm region of all stars can be reconstructed by the sum of a power-law and a cool component, which can be represented by a black body. Possible locations for these components are an optically thick, geometrically thin disc (power-law component) and an optically thin flared region (black body); (2) all stars have a substantial amount of cold dust around them, independent of the amount of mid-IR excess they show; (3) also the near-IR excess is unrelated to the mid-IR excess, indicating different composition/location of the emitting material; (4) remarkably, some sources lack the silicate bands; (5) apart from amorphous silicates, we find evidence for crystalline silicates in several stars, some of which are new detections; (6) PAH bands are present in at least 50% of our sample, and their appearance is slightly different from PAHs in the ISM; (7) PAH bands are, with one exception, not present in sources which only show a power-law continuum in the IR; their presence is unrelated to the presence of the silicate bands; (8) the dust in HAEBE stars shows strong evidence for coagulation; this dust processing is unrelated to any of the central star properties (such as age, spectral type and activity).

Processing of silicate dust grains in Herbig Ae/Be systems

We have analysed the 10 m spectral region of a sample of Herbig Ae/Be (HAEBE) stars. The spectra are dominated by a broad emission feature caused by warm amorphous silicates, and by polycyclic aromatic hydrocarbons. In HD 163296 we nd aliphatic carbonaceous dust, the rst detection of this material in a HAEBE star. The silicate band shows a large variation in shape, due to variable contributions of three components: (i) a broad shoulder at 8.6 m; (ii) a broad maximum at 9.8 m; and (iii) a narrow feature with a broad underlying continuum at 11.3 m. From detailed modeling these features can be identied with silica (SiO2), sub-micrometer sized amorphous olivine grains and micrometer sized amorphous olivine grains in combination with forsterite (Mg2SiO4), respectively. Typical mass fractions are 5 to 10 per cent of crystalline over amorphous olivine, and a few per cent of silica compared to the olivines. The detection of silica in emission implies that this material is heated by thermal contact with other solids that have a high absorptivity at optical to near-IR wavelengths. The observed change in peak position of the silicate band in HAEBE stars from 9.7 m to 11.3 m is dominated by an increase in average grain size, while changes in composition play only a minor r^ ole. The HAEBE stars, Pic and the solar system comet Halley form a sequence of increasing crystallinity. We nd that the abundance of SiO2 tends to increase with increasing crystallinity. This is consistent with the compositional changes expected from thermal annealing of amorphous grains in the inner regions of the disk. We conrm earlier studies that the timescale for crystallisation of silicates in disks is longer than that of coagulation. Our results indicate that the processes that governed grain processing in the proto-solar nebula, are also at work in HAEBE stars.

A 10 μm spectroscopic survey of herbig Ae star disks : Grain growth and crystallization

We present spectroscopic observations of a large sample of Herbig Ae stars in the 10 µm spectral region. We perform compositional fits of the spectra based on properties of homogeneous as well as inhomogeneous spherical particles, and derive the mineralogy and typical grain sizes of the dust responsible for the 10 µm emission. Several trends are reported that can constrain theoretical models of dust processing in these systems: i) none of the sources consists of fully pristine dust comparable to that found in the interstellar medium; ii) all sources with a high fraction of crystalline silicates are dominated by large grains; iii) the disks around more massive stars (M > 2.5 M� , L > 60 L� ) have a higher fraction of crystalline silicates than those around lower mass stars, iv) in the subset of lower mass stars (M < 2.5 M� ) there is no correlation between stellar parameters and the derived crystallinity of the dust. The correlation between the shape and strength of the 10 micron silicate feature reported by van Boekel et al. (2003) is reconfirmed with this larger sample. The evidence presented in this paper is combined with that of other studies to present a likely scenario of dust processing in Herbig Ae systems. We conclude that the present data favour a scenario in which the crystalline silicates are produced in the innermost regions of the disk, close to the star, and transported outward to the regions where they can be detected by means of 10 micron spectroscopy. Additionally, we conclude that the final crystallinity of these disks is reached very soon after active accretion has stopped.

MOLECULAR SIGNATURES IN THE NEAR-INFRARED DAYSIDE SPECTRUM OF HD 189733b

We have measured the dayside spectrum of HD 189733b between 1.5 and 2.5 μm using the NICMOS instrument on the Hubble Space Telescope. The emergent spectrum contains significant modulation, which we attribute to the presence of molecular bands seen in absorption. We find that water (H2O), carbon monoxide (CO), and carbon dioxide (CO2) are needed to explain the observations, and we are able to estimate the mixing ratios for these molecules. We also find temperature decreases with altitude in the ~0.01 < P< ~1 bar region of the dayside near-infrared photosphere and set an upper limit to the dayside abundance of methane (CH4) at these pressures.

WATER, METHANE, AND CARBON DIOXIDE PRESENT IN THE DAYSIDE SPECTRUM OF THE EXOPLANET HD 209458b

Using the NICMOS instrument on the Hubble Space Telescope, we have measured the dayside spectrum of HD 209458b between 1.5 and 2.5 μm. The emergent spectrum is dominated by features due to the presence of methane (CH4) and water vapor (H2O), with smaller contributions from carbon dioxide (CO2). Combining this near-infrared spectrum with existing mid-infrared measurements shows the existence of a temperature inversion and confirms the interpretation of previous photometry measurements. We find a family of plausible solutions for the molecular abundance and detailed temperature profile. Observationally resolving the ambiguity between abundance and temperature requires either (1) improved wavelength coverage or spectral resolution of the dayside emission spectrum or (2) a transmission spectrum where abundance determinations are less sensitive to the temperature structure.

The Complete Census of 70 μm-Bright Debris Disks within “The Formation and Evolution of Planetary Systems” Spitzer Legacy Survey of Sun-like Stars

We report detection of cool dust surrounding solar-type stars from observations performed as part of the Spitzer Legacy Science Program FEPS. From a sample of 328 stars having ages ~0.003-3 Gyr we have selected sources with 70 μm flux densities indicating excess in their SEDs above expected photospheric emission. Six strong excess sources are likely primordial circumstellar disks, remnants of the star formation process. Another 25 sources having ≥3 σ excesses are associated with dusty debris disks, generated by collisions within planetesimal belts that are possibly stirred by existing planets. Six additional sources with ≥2 σ excesses require confirmation as debris disks. In our analysis, most (>80%) 70 μm excess sources have ≥3 σ excesses at 33 μm as well, while only a minority ( 1/3 of the debris sources we find that multiple temperature components are suggested, implying a dust distribution extending over many tens of AU. Because the disks are dominated by collisional processes, the parent body (planetesimal) belts may be extended as well. Preliminary assessment of the statistics of cold debris around Sun-like stars shows that ~10% of FEPS targets with masses between 0.6 and 1.8 M☉ and ages between 30 Myr and 3 Gyr exhibit excess 70 μm emission. We find that fractional excess amplitudes appear higher for younger stars and that there may be a trend in 70 μm excess frequency with stellar mass.

The Formation and Evolution of Planetary Systems: Grain Growth and Chemical Processing of Dust in T Tauri Systems

This paper is one in a series presenting results obtained within the Formation and Evolution of Planetary Systems (FEPS) Legacy Science Program on the Spitzer Space Telescope. Here we present a study of dust processing and growth in seven protoplanetary disks. Our spectra indicate that the circumstellar silicate dust grains have grown to sizes at least 10 times larger than observed in the interstellar medium and show evidence for a non-negligible (~5% in mass fractions) contribution from crystalline species. These results are similar to those of other studies of protoplanetary disks. In addition, we find a correlation between the strength of the amorphous silicate feature and the shape of the spectral energy distribution. This latter result is consistent with the growth and subsequent gravitational settling of dust grains toward the disk midplane. Furthermore, we find a change in the relative abundance of the different crystalline species: more enstatite than forsterite is observed in the inner warm dust population at ~1 AU, while forsterite dominates in the colder outer regions at ~5-15 AU. This change in the relative abundances argues for a localized crystallization process rather than a radial mixing scenario in which crystalline silicates are being transported outwards from a single formation region in the hot inner parts of the disk. Finally, we report the detection of emission from polycyclic aromatic hydrocarbon (PAH) molecules in five out of seven sources.We find a tentative PAH band at 8.2 µm that was previously undetected in the spectra of disks around low-mass pre-main-sequence stars.

A ground-based near-infrared emission spectrum of the exoplanet HD 189733b.

Detection of molecules using infrared spectroscopy probes the conditions and compositions of exoplanet atmospheres. Water (H2O), methane (CH4), carbon dioxide (CO2), and carbon monoxide (CO) have been detected in two hot Jupiters. These previous results relied on space-based telescopes that do not provide spectroscopic capability in the 2.4–5.2 μm spectral region. Here we report ground-based observations of the dayside emission spectrum for HD 189733b between 2.0–2.4 μm and 3.1–4.1 μm, where we find a bright emission feature. Where overlap with space-based instruments exists, our results are in excellent agreement with previous measurements. A feature at ∼3.25 μm is unexpected and difficult to explain with models that assume local thermodynamic equilibrium (LTE) conditions at the 1 bar to 1 × 10-6 bar pressures typically sampled by infrared measurements. The most likely explanation for this feature is that it arises from non-LTE emission from CH4, similar to what is seen in the atmospheres of planets in our own Solar System. These results suggest that non-LTE effects may need to be considered when interpreting measurements of strongly irradiated exoplanets.

9.7 μm Silicate Features in Active Galactic Nuclei: New Insights into Unification Models

We describe observations of 9.7 μm silicate features in 97 AGNs, exhibiting a wide range of AGN types and of X-ray extinction toward the central nuclei. We find that the strength of the silicate feature correlates with the H I column density estimated from fitting the X-ray data, such that low H I columns correspond to silicate emission, while high columns correspond to silicate absorption. The behavior is generally consistent with unification models in which the large diversity in AGN properties is caused by viewing-angle-dependent obscuration of the nucleus. Radio-loud AGNs and radio-quiet quasars follow roughly the correlation between H I columns and the strength of the silicate feature defined by Seyfert galaxies. The agreement among AGN types suggests a high-level unification with similar characteristics for the structure of the obscuring material. We demonstrate the implications for unification models qualitatively with a conceptual disk model. The model includes an inner accretion disk (<0.1 pc in radius), a middle disk (0.1-10 pc in radius) with a dense diffuse component and with embedded denser clouds, and an outer clumpy disk (10-300 pc in radius).

The Onset of Planet Formation in Brown Dwarf Disks

The onset of planet formation in protoplanetary disks is marked by the growth and crystallization of sub–micrometer-sized dust grains accompanied by dust settling toward the disk mid-plane. Here, we present infrared spectra of disks around brown dwarfs and brown dwarf candidates. We show that all three processes occur in such cool disks in a way similar or identical to that in disks around low- and intermediate-mass stars. These results indicate that the onset of planet formation extends to disks around brown dwarfs, suggesting that planet formation is a robust process occurring in most young circumstellar disks.