Forming chondrules in impact splashes. I. Radiative cooling model

The formation of chondrules is one of the oldest unsolved mysteries in meteoritics and planet formation. Recently an old idea has been revived: the idea that chondrules form as a result of collisions between planetesimals in which the ejected molten material forms small droplets which solidify to become chondrules. Pre-melting of the planetesimals by radioactive decay of 26Al would help producing sprays of melt even at relatively low impact velocity. In this paper we study the radiative cooling of a ballistically expanding spherical cloud of chondrule droplets ejected from the impact site. We present results from a numerical radiative transfer models as well as analytic approximate solutions. We find that the temperature after the start of the expansion of the cloud remains constant for a time t_cool and then drops with time t approximately as T ~ T_0[(3/5)t/t_cool+ 2/5]^(-5/3) for t>t_cool. The time at which this temperature drop starts t_cool depends via an analytical formula on the mass of the cloud, the expansion velocity and the size of the chondrule. During the early isothermal expansion phase the density is still so high that we expect the vapor of volatile elements to saturate so that no large volatile losses are expected

Model atmospheres of irradiated exoplanets: The influence of stellar parameters, metallicity, and the C/O ratio

Many parameters constraining the spectral appearance of exoplanets are still poorly understood. We therefore study the properties of irradiated exoplanet atmospheres over a wide parameter range including metallicity, C/O ratio and host spectral type. We calculate a grid of 1-d radiative-convective atmospheres and emission spectra. We perform the calculations with our new Pressure-Temperature Iterator and Spectral Emission Calculator for Planetary Atmospheres (PETIT) code, assuming chemical equilibrium. The atmospheric structures and spectra are made available online. We find that atmospheres of planets with C/O ratios $\sim$ 1 and $T_{\rm eff}$ $\gtrsim$ 1500 K can exhibit inversions due to heating by the alkalis because the main coolants CH$_4$, H$_2$O and HCN are depleted. Therefore, temperature inversions possibly occur without the presence of additional absorbers like TiO and VO. At low temperatures we find that the pressure level of the photosphere strongly influences whether the atmospheric opacity is dominated by either water (for low C/O) or methane (for high C/O), or both (regardless of the C/O). For hot, carbon-rich objects this pressure level governs whether the atmosphere is dominated by methane or HCN. Further we find that host stars of late spectral type lead to planetary atmospheres which have shallower, more isothermal temperature profiles. In agreement with prior work we find that for planets with $T_{\rm eff}$ $<$ 1750 K the transition between water or methane dominated spectra occurs at C/O $\sim$ 0.7, instead of $\sim$ 1, because condensation preferentially removes oxygen.Comment: 30 pages, 20 figures. Accepted for publication in Ap

Cloudlet capture by Transitional Disk and FU Orionis stars

After its formation, a young star spends some time traversing the molecular cloud complex in which it was born. It is therefore not unlikely that, well after the initial cloud collapse event which produced the star, it will encounter one or more low mass cloud fragments, which we call "cloudlets" to distinguish them from full-fledged molecular clouds. Some of this cloudlet material may accrete onto the star+disk system, while other material may fly by in a hyperbolic orbit. In contrast to the original cloud collapse event, this process will be a "cloudlet flyby" and/or "cloudlet capture" event: A Bondi-Hoyle-Lyttleton type accretion event, driven by the relative velocity between the star and the cloudlet. As we will show in this paper, if the cloudlet is small enough and has an impact parameter similar or less than $GM_{*}/v^2_\infty$ (with $v_\infty$ being the approach velocity), such a flyby and/or capture event would lead to arc-shaped or tail-shaped reflection nebulosity near the star. Those shapes of reflection nebulosity can be seen around several transitional disks and FU Orionis stars. Although the masses in the those arcs appears to be much less than the disk masses in these sources, we speculate that higher-mass cloudlet capture events may also happen occasionally. If so, they may lead to the tilting of the outer disk, because the newly infalling matter will have an angular momentum orientation entirely unrelated to that of the disk. This may be one possible explanation for the highly warped/tilted inner/outer disk geometries found in several transitional disks. We also speculate that such events, if massive enough, may lead to FU Orionis outbursts

Forming chondrules in impact splashes-II Volatile retention

Solving the mystery of the origin of chondrules is one of the most elusive goals in the field of meteoritics. Recently, the idea of planet(esimal) collisions releasing splashes of lava droplets, long considered out of favor, has been reconsidered as a possible origin of chondrules by several papers. One of the main problems with this idea is the lack of quantitative and simple models that can be used to test this scenario by directly comparing to the many known observables of chondrules. In Paper I of this series, we presented a simple thermal evolution model of a spherically symmetric expanding cloud of molten lava droplets that is assumed to emerge from a collision between two planetesimals. The production of lava could be either because the two planetesimals were already in a largely molten (or almost molten) state due to heating by 26Al, or due to impact jetting at higher impact velocities. In the present paper, number II of this series, we use this model to calculate whether or not volatile elements such as Na and K will remain abundant in these droplets or whether they will get depleted due to evaporation. The high density of the droplet cloud (e.g., small distance between adjacent droplets) causes the vapor to quickly reach saturation pressure and thus shuts down further evaporation. We show to what extent, and under which conditions, this keeps the abundances of these elements high, as is seen in chondrules. We find that for most parameters of our model (cloud mass, expansion velocity, initial temperature) the volatile elements Mg, Si, and Fe remain entirely in the chondrules. The Na and K abundances inside the droplets will initially stay mostly at their initial values due to the saturation of the vapor pressure, but at some point start to drop due to the cloud expansion. However, as soon as the temperature starts to decrease, most or all of the vapor recondenses again. At the end, the Na and K elements retain most of their initial abundances, albeit occasionally somewhat reduced, depending on the parameters of the expanding cloud model. These findings appear to be qualitatively consistent with the analysis of Semarkona Type II chondrules by Hewins et al. who found evidence for sodium evaporation followed by recondensation

Redistribution of CO at the location of the CO ice line in evolving gas and dust disks

Context. Ice lines are suggested to play a significant role in grain growth and planetesimal formation in protoplanetary disks. Evaporation fronts directly influence the gas and ice abundances of volatile species in the disk and therefore the coagulation physics and efficiency and the chemical composition of the resulting planetesimals. Aims. In this work, we investigate the influence of the existence of the CO ice line on particle growth and on the distribution of CO in the disk. Methods. We include the possibility of tracking the CO content and/or other volatiles in particles and in the gas in our existing dust coagulation and disk evolution model and present a method for studying evaporation and condensation of CO using the Hertz-Knudsen equation. Our model does not yet include fragmentation, which will be part of further investigations. Results. We find no enhanced grain growth immediately outside the ice line where the particle size is limited by radial drift. Instead, we find a depletion of solid material inside the ice line, which is solely due to evaporation of the CO. Such a depression inside the ice line may be observable and may help to quantify the processes described in this work. Furthermore, we find that the viscosity and diffusivity of the gas heavily influence the re-distribution of vaporized CO at the ice line and can lead to an increase in the CO abundance by up to a factor of a few in the region just inside the ice line. Depending on the strength of the gaseous transport mechanisms, the position of the ice line in our model can change by up to ~ 10 AU and consequently, the temperature at that location can range from 21 to 23 K