Crystalline Silicates and Dust Processing in the Protoplanetary Disks of the Taurus Young Cluster

Watson, D. M.; Leisenring, Jarron; Furlan, Elise; Bohac, C. J.; Sargent, B.; Forrest, W. J.; Calvet, Nuria; Hartmann, L.; Nordhaus, Jason; Green, Joel D.; Kim, K. H.; Sloan, G. C.; Chen, C. H.; Keller, L. D.; D’Alessio, Paola; Najita, J.; Uchida, K. I.; Houck, J. R.

doi:10.1088/0067-0049/180/1/84

Cited by 134 publications

(194 citation statements)

References 95 publications

(84 reference statements)

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“…Moreover, this scenario may work even in disks with large inner holes (like EX Lup), where the mid-plane temperature will never be high enough for crystallization. Our proposal on episodic crystallization is in line with recent observations which imply that there must be another grain crystallizing mechanism uncorrelated with the steady mass accretion rate, stellar luminosity, disk mass, or disk/star mass ratio 26 . Although in a single outburst only the thin surface layer of the disk is crystallized, in EX Lupi we saw that subsequent major outbursts transform always a new layer of amorphous grains to crystals, potentially enriching the disk interior via vertical mixing.…”

supporting

confidence: 89%

Episodic formation of cometary material in the outburst of a young Sun-like star

Ábrahám

Juhász

Dullemond

et al. 2009

Nature

133

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Our Solar System originated in interstellar gas and dust; the latter is in the form of amorphous silicate particles 1,2 and carbonaceous dust. The composition of cometary material shows that a significant fraction of the amorphous silicates was transformed into crystalline form during the early evolution of the protosolar nebula 3 . How and when this transformation happened has been controversial, with the main options being heating by the young Sun 4,5 or shock heating 6 . Here we report mid-infrared features in the outburst spectrum of the young solar-like star EX Lupi that were not present in quiescence. We attribute them to crystalline forsterite; the crystals were produced via thermal annealing in the surface layer of the inner disk by heat from the outburst, a process that has hitherto not been considered. The observed lack of cold crystals excludes shock heating at larger radii.The year 2008 brought a rare opportunity to study high temperature dust processing on human timescale in a cosmic laboratory. The experiment took place in the circumstellar disk of EX Lupi, an M0 star, that is the prototype of a class of young eruptive stars named EXors 7 . These objects are defined by their large repetitive outbursts, attributed to temporarily increased mass accretion from the circumstellar disk onto the star 8 . Such outbursts represent the most intense accretion episodes in assembling the final stellar mass. In January 2008 EX Lupi entered one of its largest outbursts, brightening by approximately a factor of 100 and reaching a maximum brightness of 8 magnitude in visual light 9,10 . We observed EX Lupi in the 5.2-37 µm wavelength range with the InfraRed Spectrograph on-board the Spitzer Space Telescope, on 2008 April 21. EX Lupi was already slowly fading after its peak brightness in 2008 February, but was still a factor of 30 brighter in visual light than in quiescence.Comparing our EX Lupi spectrum with a pre-outburst measurement from the Spitzer archive obtained in 2005, we observed a remarkable change. In the 8 -12 µm spectral range (Figure 1), the silicate profile in quiescence exhibited a triangular shape, similar to that of the amorphous interstellar grains 1,11 (panels a, b). In contrast, in outburst (panel c), we clearly detected several narrower spectral features, which we identified as crystalline silicates, on top of the broad peak of amorphous silicates. The sharp peak at 10 µm and a shoulder at 11.3 µm suggest that forsterite, the Mg-rich form of olivine, dominates the observed crystal population 12,13 . The appearance of a weaker peak at 16 µm, shown in the Supplementary Information, supports this conclusion. Longward of this peak no other crystalline features are present in the spectrum. The observed crystalline features are similar to those present in comet spectra 3,14 (panel d) and in a number of protoplanetary disks 15,16 . The remaining differences between EX Lupi and 2 the cometary spectra can be related to different temperatures and different relative abundances of dust components. . Aft...

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supporting

confidence: 89%

Episodic formation of cometary material in the outburst of a young Sun-like star

Ábrahám

Juhász

Dullemond

et al. 2009

Nature

133

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“…3). In Watson et al (2009), any departure from the 10 μm pristine reference spectrum is considered as being evidence for crystalline features, while we argue that this frequent departure that we also observe has more to do with a grain size effect (see Sects. 4.1 and 5.3 for further (2006) model to investigate the impact of dust sedimentation on the observed dispersion in equivalent width for the 10 μm amorphous feature.…”

Section: The Prevalence Of Crystalline Featuresmentioning

confidence: 52%

“…As the accretion rate becomes smaller, the crystalline fraction drops accordingly: ∼10% and ∼1% forṀ = 10 −7 M /yr and 10 −8 M /yr, respectively. However, Watson et al (2009) find only a weak correlation between the accretion rate and the crystalline features arising at around 10 μm, and no correlation with the 33 μm feature, leading the authors to suggest that some mechanisms may be erasing such trends. Alternatively, other crystallisation processes at colder temperatures (e.g.…”

Section: On Radial Mixingmentioning

confidence: 98%

C2D Spitzer-IRS spectra of disks around T Tauri stars

Olofsson¹,

Augereau²,

Dishoeck

et al. 2009

A&A

103

148

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Aims. Dust grains in the planet-forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation, and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks for a statistically significant number of TTauri stars (96). Methods. As part of the cores to disks (c2d) legacy program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars, over a spectral range of 5−35 μm where many silicate amorphous and crystalline solid-state features are present. At these wavelengths, observations probe the upper layers of accretion disks up to distances of a dozen AU from the central object.Results. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. The Fe-rich crystalline silicates are largely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at λ > 20 μm correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 μm feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (≤1 AU) emitting at λ ∼ 10 μm and a much colder region emitting at λ > 20 μm (≤10 AU). We identify a crystallinity paradox, as the long-wavelength (λ > 20 μm) crystalline silicate features are detected 3.5 times more frequently (∼55% vs. ∼15%) than the crystalline features arising from much warmer disk regions (λ ∼ 10 μm). This suggests that the disk has an inhomogeneous dust composition within ∼10 AU. The analysis of the shape and strength of both the amorphous 10 μm feature and the crystalline feature around 23 μm provides evidence for the prevalence of μm-sized (amorphous and crystalline) grains in upper layers of disks. Conclusions. The abundant crystalline silicates found far from their presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of μm-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests efficient (turbulent) vertical diffusion, probably accompanied by grain-grain fragmentation to balance the expected efficient growth. In this scenario, the depletion of submicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure.

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“…Bouwman et al 2003;Meeus et al 2001) for intermediate-mass Herbig Ae/Be stars and later with much improved sensitivity provided by Spitzer for disks around brown dwarfs/low-mass stars (e.g. Pascucci et al 2009), T Tauri stars (Bouwman et al 2008;Kessler-Silacci et al 2006;Olofsson et al 2009;Furlan et al 2009;Watson et al 2009) and Herbig Ae/Be stars (Juhász et al 2010). Regions closer to the mid-plane can only be probed by observations at (sub)-millimetre wavelengths where the disks become optically thin.…”

Section: Introductionmentioning

confidence: 99%

The 69μm forsterite band in spectra of protoplanetary disks. Results from theHerschelDIGIT programme

Sturm

Bouwman

Henning

et al. 2013

A&A

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Context. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The spectra were taken with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory. In this paper we focus on the detection and analysis of the 69 μm emission band of the crystalline silicate forsterite. Aims. This work aims at providing an overview of the 69 μm forsterite bands present in the DIGIT sample. We use characteristics of the emission band (peak position and FWHM) to derive the dust temperature and to constrain the iron content of the crystalline silicates. With this information, constraints can be placed on the spatial distribution of the forsterite in the disk and the formation history of the crystalline grains. Methods. The 69 μm forsterite emission feature is analysed in terms of position and shape to derive the temperature and composition of the dust by comparison to laboratory spectra of that band. The PACS spectra are combined with existing Spitzer IRS spectra and we compare the presence and strength of the 69 μm band to the forsterite bands at shorter wavelengths. Results. A total of 32 disk sources have been observed. Out of these 32, 8 sources show a 69 μm emission feature that can be attributed to forsterite. With the exception of the T Tauri star AS 205, all of the detections are for disks associated with Herbig Ae/Be stars. Most of the forsterite grains that give rise to the 69 μm bands are found to be warm (∼100-200 K) and iron-poor (less than ∼2% iron). AB Aur is the only source where the emission cannot be fitted with iron-free forsterite requiring approximately 3-4% of iron. Conclusions. Our findings support the hypothesis that the forsterite grains form through an equilibrium condensation process at high temperatures. The large width of the emission band in some sources may indicate the presence of forsterite reservoirs at different temperatures. The connection between the strength of the 69 and 33 μm bands shows that at least part of the emission in these two bands originates fom the same dust grains. We further find that any model that can explain the PACS and the Spitzer IRS observations must take the effects of a wavelength dependent optical depth into account. We find weak indications of a correlation of the detection rate of the 69 μm band with the spectral type of the host stars in our sample. However, the sample size is too small to obtain a definitive result.

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Crystalline Silicates and Dust Processing in the Protoplanetary Disks of the Taurus Young Cluster

Abstract: We characterize the crystalline silicate content and spatial distribution of small dust grains in a large sample of protoplanetary disks in the Taurus-Auriga young cluster,

Cited by 134 publications

References 95 publications

Episodic formation of cometary material in the outburst of a young Sun-like star

Episodic formation of cometary material in the outburst of a young Sun-like star

C2D Spitzer-IRS spectra of disks around T Tauri stars

The 69μm forsterite band in spectra of protoplanetary disks. Results from theHerschelDIGIT programme

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