C2D Spitzer-IRS spectra of disks around T Tauri stars

Abstract

Context. Dust particles evolve in size and lattice structure in protoplanetary disks, due to coagulation, fragmentation and crystallization, and are radially and vertically mixed in disks due to turbulent diffusion and wind/radiation pressure forces. Aims. This paper aims at determining the mineralogical composition and size distribution of the dust grains in planet forming regions of disks around a statistical sample of 58 T Tauri stars observed with Spitzer/IRS as part of the Cores to Disks (c2d) Legacy Program. Methods. We present a spectral decomposition model, named "B2C", that reproduces the IRS spectra over the full spectral range (5-35 μm). The model assumes two dust populations: a warm component responsible for the 10 μm emission arising from the disk inner regions ( 1 AU) and a colder component responsible for the 20-30 μm emission, arising from more distant regions ( 10 AU). The fitting strategy relies on a random exploration of parameter space coupled with a Bayesian inference method. Results. We show evidence for a significant size distribution flattening in the atmospheres of disks compared to the typical MRN distribution, providing an explanation for the usual flat, boxy 10 μm feature profile generally observed in T Tauri star spectra. We reexamine the crystallinity paradox, observationally identified by Olofsson et al. (2009, A&A, 507, 327), and we find a simultaneous enrichment of the crystallinity in both the warm and cold regions, while grain sizes in both components are uncorrelated. We show that flat disks tend to have larger grains than flared disk. Finally our modeling results do not show evidence for any correlations between the crystallinity and either the star spectral type, or the X-ray luminosity (for a subset of the sample). Conclusions. The size distribution flattening may suggests that grain coagulation is a slightly more effective process than fragmentation (helped by turbulent diffusion) in disk atmospheres, and that this imbalance may last over most of the T Tauri phase. This result may also point toward small grain depletion via strong stellar winds or radiation pressure in the upper layers of disk. The non negligible cold crystallinity fractions suggests efficient radial mixing processes in order to distribute crystalline grains at large distances from the central object, along with possible nebular shocks in outer regions of disks that can thermally anneal amorphous grains.

Transient dust in warm debris disks

“…Min et al 2008). Additionally, optical properties of large crystalline grains can become degenerate with optical properties of other dust species, since the recognizable emission features of small crystals fade away when increasing s. Limiting s max to 1 μm is in line with previous studies of dust in protoplanetary disks (e.g., Bouwman et al 2008;Juhász et al 2010;Olofsson et al 2010) where crystalline grains are found to be much smaller than amorphous grains. Concerning the maximum grain size for amorphous grains, we tested several values for s max and discuss the outcomes of this exercise in Sect.…”

“…sion features, have been observed numerous times in disks around young Class II objects (e.g., Bouwman et al 2001Bouwman et al , 2008Juhász et al 2010;Oliveira et al 2011;Olofsson et al 2009Olofsson et al , 2010Sargent et al 2009b), and disks around evolved Class III objects (e.g., Beichman et al 2011;Lisse et al 2007Lisse et al , 2008Lisse et al , 2009Weinberger et al 2011). Amorphous grains of olivine (Mg 2x Fe 2(1−x) SiO 4 ) and pyroxene (Mg 2x Fe 2(1−x) Si 2 O 6 ) stoichiometries are also the dominant species in the interstellar medium (∼98%, Kemper et al 2005;Min et al 2007).…”

“…To assess if the grains trace a second generation or are the remnants of the primordial, gas-rich disks one has to search for possible differences between the mineralogy of dust in Class II disks compared to the dust we observe in debris disks. For several stars, Juhász et al (2010) found pyroxene crystalline grains with a Fe fraction of about ∼10%; however, a result that appears to be quite general for the petrology of crystalline olivine grains in protoplanetary disks is that they are Mg-rich (and hence Fe-poor, see Bouwman et al 2008;Olofsson et al 2010). The overall lack of Fe-bearing crystalline grains has been studied by several authors and can be explained by crystallization processes that take place in massive disks (gas-phase condensation, Gail 2010;or thermal annealing, Nuth & Johnson 2006).…”

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

“…Thus that a transition disk geometry does not necessarily go hand in hand Fig. 13. a) Comparison of the mass-averaged sizes of amorphous dust grains as derived from the shorter and longer wavelength part of the IRS spectra (indicated with "W" and "C" superscripts for "warm" and "cold" disk regions) of sources in Cha, MBM 12, η Cha, and the Coronet cluster, and the targets from Olofsson et al (2010). The circles are for the targets in MBM 12, η Cha, and the Coronet cluster.…”

“…The circles are for the targets in MBM 12, η Cha, and the Coronet cluster. The diamonds show the targets from Olofsson et al (2010). The pentagram marks object ID#10 and 11 in Cha.…”