A 10 $\mathsf{\mu}$m spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization

Abstract

Abstract. 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.

Young stars inϵChamaleontis and their disks: disk evolution in sparse associations

“…Figure 13c shows that the dust in the warm inner regions of protoplanetary disks generally contains a higher fraction of crystalline material than the dust in cooler regions. This is consistent with earlier findings (e.g., van Boekel et al 2004;Meeus et al 2009). …”

“…The shape and the strength of the 10 μm silicate feature were found to be correlated and this was interpreted as evidence for grain growth in disks (van Boekel et al 2005;Kessler-Silacci et al 2006;Bouwman et al 2008). The Spitzer/IRS has provided spectra of many young objects, covering the wavelength range from 5.3 to 38 μm.…”

“…This may be a contrast effect: because of the very low luminosity of ID#10, its 10 μm silicate feature arises mainly in the inner ∼0.5 AU of the disk. The central regions of disks can be highly crystalline (van Boekel et al 2004), possibly leading to very high apparent crystallinities if only the very central disk regions contribute to the part of the spectrum used in the mineralogical analysis (see also Apai et al 2005). In Fig.…”

“…The most convincing evidence that silicates are present in the protoplanetary disks is the strong "10 μm feature" in midinfrared spectra of Herbig Ae/Be and T Tauri stars (Cohen & Witteborn 1985;Natta et al 2000;Bouwman et al 2001Bouwman et al , 2003van Boekel et al 2004;van Boekel et al 2005). This feature is emitted by silicate grains with sizes of up to several microns that reside in the optically thin surface layers of the disk (Men'shchikov & Henning 1997;Chiang & Goldreich 1997).…”

Photoprocesses in protoplanetary disks

“…Third, the grain sizes and properties may be very different from those found in clouds. In particular, there is strong evidence from infrared observations that grains in the upper disk layers have grown from ∼ 0.1 µm to a few µm in size regardless of the spectral type of the star (e.g., [71,32]). Figure 2 illustrates this growth using recent Spitzer Space Telescope spectra of the 10 and 20 µm silicate feature.…”