FU Orionis outbursts, preferential recondensation of water ice, and the formation of giant planets

FU Orionis outbursts, preferential recondensation of water ice, and the formation of giant planets

Author:

Hubbard

Abstract:

Ices, including water ice, prefer to recondense onto pre-existing nuclei rather than spontaneously forming grains from a cloud of vapor. Interestingly, different potential recondensation nuclei have very different propensities to actually nucleate water ice at the temperatures associated with freeze-out in protoplanetary discs. Therefore, if a region in a disc is warmed and then recooled, water vapor should not be expected to refreeze evenly onto all available grains. Instead it will preferentially recondense onto the most favorable grains. When the recooling is slow enough, only the most favorable grains will nucleate ice, allowing them to recondense thick ice mantles. We quantify the conditions for preferential recondensation to rapidly create pebble-sized grains in protoplanetary discs and show that FU Orionis type outbursts have the appropriate cooling rates to drive pebble creation in a band about 5 astronomical units wide outside of the quiescent frost line from approximately Jupiter's orbit to Saturn's (about 4 to 10 au). Those pebbles could be of the appropriate size to proceed to planetesimal formation via the Streaming Instability, or to contribute to the growth of planetesimals through pebble accretion. We suggest that this phenomenon contributed to the formation of the gas giants in our own Solar System.

The Mid-Infrared Evolution of the FU Orionis Disk

The Mid-Infrared Evolution of the FU Orionis Disk

Authors:

Green et al

Abstract:

We present new SOFIA-FORCAST observations obtained in Feburary 2016 of the archetypal outbursting low mass young stellar object FU Orionis, and compare the continuum, solid state, and gas properties with mid-IR data obtained at the same wavelengths in 2004 with Spitzer-IRS. In this study, we conduct the first mid-IR spectroscopic comparison of an FUor over a long time period. Over a 12 year period, UBVR monitoring indicates that FU Orionis has continued its steady decrease in overall brightness by ~ 14%. We find that this decrease in luminosity occurs only at wavelengths < 20 microns. In particular, the continuum short ward of the silicate emission complex at 10 microns exhibits a ~ 12% (~ 3 sigma) drop in flux density, but no apparent change in slope; both the Spitzer and SOFIA spectra are consistent with a 7200 K blackbody. Additionally, the detection of water absorption is consistent with the Spitzer spectrum. The silicate emission feature at 10 microns continues to be consistent with unprocessed grains, unchanged over 12 years. We conclude that either the accretion rate in FU Orionis has decreased by ~ 12-14% over this time baseline, or that the inner disk has cooled, but the accretion disk remains in a superheated state outside of the innermost region.

Inner Disk in Mass Accretion into Host Star Early in Formation

The Role for the Inner Disk in Mass Accretion to the Star in the Early Phase of Star Formation

Authors:

Ohtani et al

Abstract:

A physical mechanism that drives FU Orionis-type outbursts is reconsidered. We study the effect of inner part of a circumstellar disk covering a region from near the central star to the radius of approximately 5 AU (hereafter, the inner disk). Using the fluctuated mass accretion rate onto the inner disk M˙out, we consider the viscous evolution of the inner disk and the time variability of the mass accretion rate onto the central star M˙in by means of numerical calculation of an unsteady viscous accretion disk in a one-dimensional axisymmetric model. First, we calculate the evolution of the inner disk assuming an oscillating M˙out. It is shown that the time variability of M˙in does not coincide with M˙out due to viscous diffusion. Second, we investigate the properties of spontaneous outbursts with temporally constant M˙out. Outburst occur only in a limited range of mass accretion rates onto the inner disk 10−10 greater than M˙out greater than 3×10−6 M⊙yr−1 due to gravo-magneto limit cycle (GML). Finally, we discuss the case with a combination of episodic M˙out and accretion outbursts cause by the GML in the inner disk. The GML can drive accretion outbursts onto the star even for the case of fluctuating M˙out, although fluctuations of M˙ decay during transmitting the inner disk inwards. We newly identified two modes of outburst which are spontaneous one and stimulated one. In a stimulated mode of outburst, M˙out does appear directly in M˙in (the latter defining the stellar accretion luminosity). In a spontaneous mode of outburst, M˙out appears as the interval between outbursts.