Steven Joseph Desch

Annealing of Silicate Dust by Nebular Shocks at 10 AU

Silicate dust grains in the interstellar medium are known to be mostly amorphous, yet crystalline silicate grains have been observed in many long-period comets and in protoplanetary disks. Annealing of amorphous silicate grains into crystalline grains requires temperatures of 1000 K, but exposure of dust grains in comets to such high temperatures is incompatible with the generally low temperatures experienced by comets. This has led to the proposal of models in which dust grains were thermally processed near the proto-Sun, then underwent considerable radial transport until they reached the gas giant planet region where the long-period comets originated. We hypothesize instead that silicate dust grains were annealed in situ by shock waves triggered by gravitational instabilities. We assume a shock speed of 5 km s-1, a plausible value for shocks driven by gravitational instabilities. We calculate the peak temperatures of micron and submicron amorphous pyroxene grains of chondritic composition under conditions typical in protoplanetary disks at 5-10 AU. Our results also apply to chondritic amorphous olivine grains. We show that in situ thermal annealing of submicron- and micron-sized silicate dust grains can occur, obviating the need for large-scale radial transport.

NEAR-INFRARED SPECTROSCOPY OF CHARON: POSSIBLE EVIDENCE FOR CRYOVOLCANISM ON KUIPER BELT OBJECTS

We present the first reported adaptive optics spectra of Charon in the H and K bands, which examine the anti-Pluto and sub-Pluto hemispheres. The ice temperature is estimated at 40-50 K, based on the 1.65 μm feature of crystalline water ice. We obtain the most accurate profiles of the 2.21 μm feature and confirm that the feature is due to hydrated ammonia. We attribute hemispheric differences in the features profile to different hydration states. We calculate the rate at which crystalline water ice is amorphized by solar UV/visible radiation, finding that at the depths probed by H and K observations (≈350 μm), the e-folding time to amorphize ice is (3-5) × 104 yr. This implies Charons ice crystallized from a melt, or has been heated to 90 K, during the last ~105 yr. The extent of the crystalline water ice and the short timescales involved argue that surface renewal is necessary, a conclusion reinforced by the presence of ammonia hydrates. We investigate possible mechanisms for surface renewal and conclude that cryovolcanism is the most likely.

Interaction of Supernova Ejecta with Nearby Protoplanetary Disks

The early solar system contained short-lived radionuclides such as 60Fe (t1/2 = 1.5 Myr) whose most likely source was a nearby supernova. Previous models of solar system formation considered a supernova shock that triggered the collapse of the Suns nascent molecular cloud. We advocate an alternative hypothesis, that the solar systems protoplanetary disk had already formed when a very close (<1 pc) supernova injected radioactive material directly into the disk. We conduct the first numerical simulations designed to answer two questions related to this hypothesis: Will the disk be destroyed by such a close supernova, and will any of the ejecta be mixed into the disk? Our simulations demonstrate that the disk does not absorb enough momentum from the shock to escape the protostar to which it is bound. Only low amounts (<1%) of mass loss occur, due to stripping by Kelvin-Helmholtz instabilities across the top of the disk, which also mix into the disk about 1% of the intercepted ejecta. These low efficiencies of destruction and injection are due to the fact that the high disk pressures prevent the ejecta from penetrating far into the disk before stalling. Injection of gas-phase ejecta is too inefficient to be consistent with the abundances of radionuclides inferred from meteorites. On the other hand, the radionuclides found in meteorites would have condensed into dust grains in the supernova ejecta, and we argue that such grains will be injected directly into the disk with nearly 100% efficiency. The meteoritic abundances of the short-lived radionuclides such as 60Fe therefore are consistent with injection of grains condensed from the ejecta of a nearby (<1 pc) supernova, into an already formed protoplanetary disk.

An interstellar origin for the beryllium 10 in calcium-rich, aluminum-rich inclusions

Beryllium 10 is a short-lived radionuclide (t1=2 ¼ 1:5 Myr) that was incorporated live into calcium-rich, aluminum-rich inclusions (CAIs) at the birth of our solar system. Beryllium 10 is unique among the short-lived radionuclides in that it is formed only by spallation reactions and not by nucleosynthesis, e.g., in a supernova. Recent work by McKeegan, Gounelle, and others has stated that the high initial abundance of 10 Be in CAIs ( 10 Be= 9 Be � 1 � 10 � 3 ) cannot be attributed to galactic cosmic rays (GCRs) and therefore concluded that the spallation reactions must have occurred within the solar nebula itself, because of energetic particles emitted by the early Sun. In this paper we reexamine this conclusion. We calculate the contributions of GCRs to the 10 Be abundance in a molecular cloud core as it collapses to form a protostar and protoplanetary disk. We constrain the flux of protons and 10 Be GCRs in the Sun’s molecular cloud core 4.5 Gyr ago. We use numerical magnetohydrodynamic simulations of star formation to model the time evolution of the magnetic field strength and column density of gas in a collapsing cloud core. We account for magnetic focusing and magnetic mirroring and the anisotropic distribution of GCR pitch angles in the cloud core. We calculate the rates at which GCR protons and � -particles induce spallation reactions producing 10 Be atoms, and the rates at which GCR 10 Be nuclei are trapped in the cloud core. Accounting also for the decay of 10 Be over the evolution of the cloud core, we calculate the time-varying 10 Be/ 9 Be ratio. We find that at the time of protostar formation 10 Be/ 9 Be � 1 � 10 � 3 , with an uncertainty of about a factor of 3. Spallation reactions account for 20% of the 10 Be in CAIs, while trapped GCR 10 Be nuclei account for the other 80%. The initial abundance of 10 Be in CAIs is therefore entirely attributable to cosmic rays. We discuss the implications of this finding for the origin of other short-lived radionuclides and for the use of 10 Be as a chronometer. Subject headings: cosmic rays — nuclear reactions, nucleosynthesis, abundances — solar system: formation — stars: formation

Linear Analysis of the Magnetorotational Instability, Including Ambipolar Diffusion, with Application to Protoplanetary Disks

We present a linear analysis of the magnetorotational instability (MRI) in differentially rotating disks and derive the most general instability criterion to date. Our analysis improves on earlier work on this topic in that it simultaneously accounts for arbitrary geometry and the full effects of magnetic diffusion. We allow the magnetic field to have arbitrary orientation and for linear modes to propagate at an angle to the rotation axis. We also include in our analysis all three forms of magnetic diffusion: ohmic dissipation, ambipolar diffusion, and Hall currents. Previous analyses have included either arbitrary geometry or ambipolar diffusion, but never both. The simultaneous inclusion of these effects allows us to identify a new unstable mode in which ambipolar diffusion and differential rotation can couple and amplify the magnetic field. We provide a physical explanation of this mode. Our linear analysis is aimed at determining which parts of protoplanetary disks may be unstable to the MRI. Accordingly, we outline the conditions that are likely to obtain in protoplanetary disks and make estimates of the coupling between the gas and the magnetic field. We derive a linear stability criterion that can be applied to protoplanetary disks.

Progress in planetary lightning

We review the progress in the last decade in the field of planetary lightning. We provide background covering terrestrial lightning and newly discovered associated phenomena such as sprites. We concentrate on the theory and observations regarding lightning at Jupiter, especially the discoveries made by the Galileo orbiter and entry probe. Recent observations of Titans atmosphere and a theory of possible lightning at Titan are reviewed. We discuss the potential for lightning detection by the Huygens probe that will enter Titans atmosphere in 2005. Directions for future progress in planetary lightning are outlined.

Nature of opaque components on Mercury: Insights into a Mercurian magma ocean

[1] Analysis of Mariner 10 and MESSENGER data sets reveal the importance of opaque components on Mercury’s surface. A global darkening agent, suggested to be ilmenite or other Fe-, Ti-bearing opaque mineral, has been invoked to explain the lower albedo of Mercury relative to the lunar highlands. Separately, a low-reflectance material (LRM) has been recognized as one of three dominant color terrains. We present laboratory reflectance spectra of ilmenite size separates and other candidate Fe-, Ti-bearing oxide minerals. These oxides cannot sufficiently darken Mercury without violating neutron spectrometer constraints on surface iron content. The spectra of all samples exhibit negative spectral slopes shortward of 500 nm, consistent with the LRM. We review models of crystallization of an FeO-poor Mercurian magma ocean and show that lack of a plagioclase flotation crust could lead to a thin quench crust with near surface layers of incompatible- and Ti-rich late stage cumulates, consistent with Mercury’s albedo and LRM. Citation: Riner, M. A., P. G. Lucey, S. J. Desch, and F. M. McCubbin (2009), Nature of opaque components on Mercury: Insights into a Mercurian magma ocean, Geophys. Res. Lett., 36, L02201, doi:10.1029/2008GL036128.

MIXING OF CLUMPY SUPERNOVA EJECTA INTO MOLECULAR CLOUDS

Several lines of evidence, from isotopic analyses of meteorites to studies of the Suns elemental and isotopic composition, indicate that the solar system was contaminated early in its evolution by ejecta from a nearby supernova. Previous models have invoked supernova material being injected into an extant protoplanetary disk, or isotropically expanding ejecta sweeping over a distant (>10 pc) cloud core, simultaneously enriching it and triggering its collapse. Here, we consider a new astrophysical setting: the injection of clumpy supernova ejecta, as observed in the Cassiopeia A supernova remnant, into the molecular gas at the periphery of an H II region created by the supernovas progenitor star. To track these interactions, we have conducted a suite of high-resolution (15003 effective) three-dimensional numerical hydrodynamic simulations that follow the evolution of individual clumps as they move into molecular gas. Even at these high resolutions, our simulations do not quite achieve numerical convergence, due to the challenge of properly resolving the small-scale mixing of ejecta and molecular gas, although they do allow some robust conclusions to be drawn. Isotropically exploding ejecta do not penetrate into the molecular cloud or mix with it, but, if cooling is properly accounted for, clumpy ejecta penetrate to distances ~1018 cm and mix effectively with large regions of star-forming molecular gas. In fact, the ~2 M ☉ of high-metallicity ejecta from a single core-collapse supernova is likely to mix with ~2 × 104 M ☉ of molecular gas material as it is collapsing. Thus, all stars forming late (5 Myr) in the evolution of an H II region may be contaminated by supernova ejecta at the level ~10–4. This level of contamination is consistent with the abundances of short-lived radionuclides and possibly some stable isotopic shifts in the early solar system and is potentially consistent with the observed variability in stellar elemental abundances. Supernova contamination of forming planetary systems may be a common, universal process.

SPITZER OBSERVATIONS OF THE H II REGION NGC 2467: AN ANALYSIS OF TRIGGERED STAR FORMATION*

We present new Spitzer Space Telescope observations of the region NGC 2467, and use these observations to determine how the environment of an H II region affects the process of star formation. Our observations comprise IRAC (3.6, 4.5, 5.8, and 8.0 μm) and MIPS (24 μm) maps of the region, covering approximately 400 arcmin2. The images show a region of ionized gas pushing out into the surrounding molecular cloud, powered by an O6V star and two clusters of massive stars in the region. We have identified as candidate young stellar objects (YSOs) 45 sources in NGC 2467 with infrared excesses in at least two mid-infrared colors. We have constructed color-color diagrams of these sources and have quantified their spatial distribution within the region. We find that the YSOs are not randomly distributed in NGC 2467; rather, over 75% of the sources are distributed at the edge of the H II region, along ionization fronts driven by the nearby massive stars. The high fraction of YSOs in NGC 2467 that are found in proximity to gas that has been compressed by ionization fronts supports the hypothesis that a significant fraction of the star formation in NGC 2467 is triggered by the massive stars and the expansion of the H II region. At the current rate of star formation, we estimate at least 25%-50% of the total population of YSOs formed by this process.

A COMPARISON of STELLAR ELEMENTAL ABUNDANCE TECHNIQUES and MEASUREMENTS

Stellar elemental abundances are important for understanding the fundamental properties of a star or stellar group, such as age and evolutionary history, as well as the composition of an orbiting planet. However, as abundance measurement techniques have progressed, there has been little standardization between individual methods and their comparisons. As a result, different stellar abundance procedures determine measurements that vary beyond quoted error for the same elements within the same stars (Hinkel et al. 2014). The purpose of this paper is to better understand the systematic variations between methods and offer recommendations for producing more accurate results in the future. We have invited a number of participants from around the world (Australia, Portugal, Sweden, Switzerland, and USA) to calculate ten element abundances (C, O, Na, Mg, Al, Si, Fe, Ni, Ba, and Eu) using the same stellar spectra for four stars (HD361, HD10700, HD121504, HD202206). Each group produced measurements for each of the stars using: 1) their own autonomous techniques, 2) standardized stellar parameters, 3) standardized line list, and 4) both standardized parameters and line list. We present the resulting stellar parameters, absolute abundances, and a metric of data similarity that quantifies homogeneity of the data. We conclude that standardization of some kind, particularly stellar parameters, improves the consistency between methods. However, because results did not converge as more free parameters were standardized, it is clear there are inherent issues within the techniques that need to be reconciled. Therefore, we encourage more conversation and transparency within the community such that stellar abundance determinations can be reproducible as well as accurate and precise.