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

Abstract

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...

Herschel/PACS observations of the 69μm band of crystalline olivine around evolved stars

“…In the case of circumstellar disks, it is possible that the grains reside in the inner part of the disk close to the central star where the temperature is warm enough for annealing. It is also possible to anneal amorphous grains in-situ in colder parts of the disk by shocks or other processes (Desch & Cuzzi 2000;Ábrahám et al 2009;Edgar et al 2008). The composition of crystalline olivine formed by these processes is difficult to predict, since it is unlikely that the in-situ formation of crystals happens under equilibrium conditions.…”

“…Crystalline silicates are, however, also known to exist in disks that are too cold for annealing (Molster et al 1999a). This suggests that it should also be possible to anneal amorphous grains in-situ in colder parts of the disk by shocks or other processes (Desch & Cuzzi 2000;Ábrahám et al 2009;Edgar et al 2008).…”

Location and sizes of forsterite grains in protoplanetary disks

“…This may reflect the lack of large-scale mixing (Shu et al 1994;Bockelée-Morvan et al 2002;Gail 2004;Jacquet 2014) and indicates the importance of crystal formation (i.e. local heating events) associated with the inner and outer disk region (Urey 1967;Huss et al 2001;Harker & Desch 2002;Desch et al 2005;Ábrahám et al 2009;Morlok et al 2010;Tanaka et al 2010).…”

Mid-infrared interferometric variability of DG Tauri: Implications for the inner-disk structure

“…Additionally, the presence of the jet is also a sign for the existence of outward material transport. The fact that we do not detect crystalline dust in the inner disk may suggest that the crystal production is not continuous, but linked to episodes of higher activity, after which the fresh crystals are transported outwards, as in the case of EX Lupi (Ábrahám et al 2009;Juhász et al 2012).…”