R. van Boekel

The building blocks of planets within the `terrestrial' region of protoplanetary disks

Our Solar System was formed from a cloud of gas and dust. Most of the dust mass is contained in amorphous silicates, yet crystalline silicates are abundant throughout the Solar System, reflecting the thermal and chemical alteration of solids during planet formation. (Even primitive bodies such as comets contain crystalline silicates.) Little is known about the evolution of the dust that forms Earth-like planets. Here we report spatially resolved detections and compositional analyses of these building blocks in the innermost two astronomical units of three proto-planetary disks. We find the dust in these regions to be highly crystallized, more so than any other dust observed in young stars until now. In addition, the outer region of one star has equal amounts of pyroxene and olivine, whereas the inner regions are dominated by olivine. The spectral shape of the inner-disk spectra shows surprising similarity with Solar System comets. Radial-mixing models naturally explain this resemblance as well as the gradient in chemical composition. Our observations imply that silicates crystallize before any terrestrial planets are formed, consistent with the composition of meteorites in the Solar System.

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.

Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI

We present the first long baseline mid-infrared interferometric observations of the circumstellar disks surrounding Herbig Ae/Be stars. The observations were obtained using the mid-infrared interferometric instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI on Cerro Paranal. The 102 m baseline given by the telescopes UT1 and UT3 was employed, which provides a maximum full spatial resolution of 20 milli-arcsec (mas) at a wave- length of 10 µm. The interferometric signal was spectrally dispersed at a resolution of 30, giving spectrally resolved visibility information from 8 µm to 13.5 µm. We observed seven nearby Herbig Ae/Be stars and resolved all objects. The warm dust disk of HD 100546 could even be resolved in single-telescope imaging. Characteristic dimensions of the emitting regions at 10 µm are found to be from 1 AU to 10 AU. The 10 µm sizes of our sample stars correlate with the slope of the 10-25 µm infrared spectrum in the sense that the reddest objects are the largest ones. Such a correlation would be consistent with a different ge- ometry in terms of flaring or flat (self-shadowed) disks for sources with strong or moderate mid-infrared excess, respectively. We compare the observed spectrally resolved visibilities with predictions based on existing models of passive centrally irra- diated hydrostatic disks made to fit the SEDs of the observed stars. We find broad qualitative agreement of the spectral shape of visibilities corresponding to these models with our observations. Quantitatively, there are discrepancies that show the need for a next step in modelling of circumstellar disks, satisfying both the spatial constraints such as are now available from the MIDI observations and the flux constraints from the SEDs in a consistent way.

Grain growth in the inner regions of Herbig Ae/Be star disks

We present new mid-infrared spectroscopy of the emission from warm circumstellar dust grains in Herbig Ae/Be stars. Our survey significantly extends the sample that was studied by Bouwman et al. (2001). We find a correla- tion between the strength of the silicate feature and its shape. We interpret this as evidence for the removal of small (0.1m) grains from the disk surface while large (1-2m) grains persist. If the evolution of the grain size distribution is dominated by gravitational settling, large grains are expected to disappear first, on a timescale which is much shorter than the typical age of our programme stars. Our observations thus suggest a continuous replenishment of micron sized grains at the disk surface. If the grain replenishment is due to the dredge-up of dust from the disk interior, the mineralogy we observe is representative of the bulk composition of dust in these stars.

Evidence for grain growth in T Tauri disks

In this article we present the results from mid-infrared spectroscopy of a sample of 14 T Tauri stars with silicate emission. The qualitative analysis of the spectra reveals a correlation between the strength of the silicate feature and its shape similar to the one which was found recently for the more massive Herbig Ae/Be stars by van Boekel et al. (2003). The comparison with theoretical spectra of amorphous olivine ([Mg,Fe] 2 SiO 4 ) with different grain sizes suggests that this correlation is indicating grain growth in the disks of TTauri stars. Similar mechanisms of grain processing appear to be effective in both groups of young stars.

Direct measurement of the size and shape of the present-day stellar wind of eta Carinae

We present new high angular resolution observations at near-IR wavelengths of the core of the Luminous Blue Variable Carinae, using NAOS-CONICA at the VLT and VINCI at the VLT Interferometer (VLTI). The latter observations provide spatial information on a scale of 5 milli-arcsec or 11 AU at the distance of Carinae. The present-day stellar wind of Carinae is resolved on a scale of several stellar radii. Assuming spherical symmetry, we find a mass loss rate of 1:610 3 M/yr and a wind clumping factor of 0.26. The VLTI data taken at a baseline of 24 m show that the object is elongated with a de- projected axis ratio of approximately 1.5; the major axis is aligned with that of the large bi-polar nebula that was ejected in the 19th century. The most likely explanation for this observation is a counter-intuitive model in which stellar rotation near the critical velocity causes enhanced mass loss along the rotation axis. This results from the large temperature dierence between pole and equator in rapidly rotating stars. Carinae must rotate in excess of 90 percent of its critical velocity to account for the observed shape. The large outburst may have been shaped in a similar way. Our observations provide strong support for the existence of a theoretically predicted rotational instability, known as the limit.

Explaining UX Orionis star variability with self-shadowed disks

In this Letter we propose a new view on the phenomenon of Algol-type minima in the light curves of UX Orionis stars (UXORs). The idea is based on the earlier proposal by various authors that UXORs are nearly edge-on disks in which hydrodynamic fluctuations could cause clumps of dust and gas to cross the line of sight. However, early models of protoplanetary disks were based on the notion that these have a flaring geometry. If so, then it is mostly the outer regions of the disk that obscure the star. The timescales for such obscuration events would be too long to match the observed timescales of weeks to months. Recent two-dimensional self-consistent models of Herbig Ae/Be protoplanetary disks, however, have indicated that for Herbig Ae/Be star disks there exists, in addition to the usual flared disks, a new class of disks: disks that are fully self-shadowed. For these disks only their puffed-up inner rim (at the dust evaporation radius) is directly irradiated by the star, while the disk at larger radius resides in the shadow of the rim. For these disks there exist inclinations at which the line of sight toward the star skims the upper parts of the puffed-up inner rim, while passing high over the surface of outer-disk regions. These outer-disk regions therefore do not obscure the star nor the inner-disk regions, and small hydrodynamic fluctuations in the puffed-up inner rim could cause the extinction events seen in UXORs. If this idea is correct, it makes a prediction for the shape of the SEDs of these stars. It was shown by Dullemond and Dullemond, Dominik, & Natta that flared disks have a strong far-IR excess and can be classified as “Group I” (in the classification of Meeus et al.), while self-shadowed disks have a relatively weak far-IR excess and are classified as “Group II.” Our model therefore predicts that UXORs belong to the “Group II” sources. We show that this correlation is indeed found within a sample of 86 Herbig Ae/Be stars. Subject headings: circumstellar matter — planetary systems: protoplanetary disks — stars: pre–main-sequence — stars: variables: other

Correlation between grain growth and disk geometry in Herbig Ae/Be systems

We have calculated the (sub-)mm spectral indices of 26 Herbig Ae/Be stars, for which we can determine the infrared spectral energy distribution (SED). We find a clear correlation between the strength of the ratio of the near- to mid-infrared excess of these sources, and the slope of the (sub-)mm energy distribution. Based on earlier multi-dimensional modeling of disks around Herbig Ae stars, we interpret this as a correlation between the geometry of the disk (flared or self-shadowed) and the size of the grains: self-shadowed disks have, on average, larger grains than their flared counterparts. These data suggest that the geometry of a young stellar disk evolves from flared to self-shadowed.

Spatially and spectrally resolved 10 μm emission in Herbig Ae/Be stars

We present new mid-infrared spectroscopy of the emission from warm circumstellar dust grains in the Herbig Ae stars HD 100546, HD 97048 and HD 104237, with a spatial resolution of ≈0. �� 9. We find that the emission in the UIR bands at 8.6, 11.3 and (HD 97048 only) 12.7 µm is extended in the first two sources. The continuum emission is resolved in HD 97048 and possibly in HD 100546. HD 104237 is not spatially resolved in our observations. We find that the UIR emission in HD 100546 and HD 97048 is extended on a scale of (several) 100 AU, corresponding to the outer disk scale in flaring disk models. Small carbonaceous particles are the dominant source of opacity in the HD 97048 disk.

Flaring and self-shadowed disks around Herbig Ae stars: simulations for 10 μm interferometers

We present simulations of the interferometric visibilities of Herbig Ae star disks. We investigate whether interferometric measurements in the 10μm atmospheric window are sensitive to the presence of an increased scale height at the inner disk edge, predicted by recent models. Furthermore, we investigate whether such measurements can discriminate between disks with a flaring geometry and disks with a flat geometry. We show that both these questions can be addressed, using measurements at a small number of appropriately chosen baselines. The classification of Herbig Ae stars in two groups, based on the appearance of the spectral energy distribution (SED), has been attributed to a difference in disk geometry. Sources with a group I SED would have a flaring outer disk geometry, whereas the disk of group II sources is proposed to be flat (or self-shadowed). We show that this hypothesis can be tested using long-baseline interferometric measurements in the 10μm atmospheric window.