arXiv subjects

Steven J. Desch

Publications and source records attributed to Steven J. Desch.

41 records · Page 3Linked to original sources

arXiv2012

Mixing of Clumpy Supernova Ejecta into Molecular Clouds

Several lines of evidence, from isotopic analyses of meteorites to studies of the Sun's elemental and isotopic composition, indicate that the solar system was contaminated early in its evolution by ejecta from a nearby supernova (SN). Previous models have invoked SN 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 SN ejecta, as observed in the Cas A SN remnant, into the molecular gas at the periphery of an HII region created by the SN's progenitor star. To track these interactions we have conducted a suite of high-resolution (1500^3 effective) 3D 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, but, if cooling is properly accounted for, clumpy ejecta penetrate to distances ~10^18 cm and mix effectively with star-forming molecular gas. The ~2 M_\odot high-metallicity ejecta from a core-collapse SN is likely to mix with ~2 \times 10^4 M_\odot of molecular gas. Thus all stars forming late (~5 Myr) in the evolution of an HII region may be contaminated by SN ejecta at a 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. SN contamination of forming planetary systems may be a common, universal process.

astro-ph.SR↗

arXiv2012

Chondrule Formation in Bow Shocks around Eccentric Planetary Embryos

Recent isotopic studies of Martian meteorites by Dauphas & Pourmond (2011) have established that large (~ 3000 km radius) planetary embryos existed in the solar nebula at the same time that chondrules - millimeter-sized igneous inclusions found in meteorites - were forming. We model the formation of chondrules by passage through bow shocks around such a planetary embryo on an eccentric orbit. We numerically model the hydrodynamics of the flow, and find that such large bodies retain an atmosphere, with Kelvin-Helmholtz instabilities allowing mixing of this atmosphere with the gas and particles flowing past the embryo. We calculate the trajectories of chondrules flowing past the body, and find that they are not accreted by the protoplanet, but may instead flow through volatiles outgassed from the planet's magma ocean. In contrast, chondrules are accreted onto smaller planetesimals. We calculate the thermal histories of chondrules passing through the bow shock. We find that peak temperatures and cooling rates are consistent with the formation of the dominant, porphyritic texture of most chondrules, assuming a modest enhancement above the likely solar nebula average value of chondrule densities (by a factor of 10), attributable to settling of chondrule precursors to the midplane of the disk or turbulent concentration. We calculate the rate at which a planetary embryo's eccentricity is damped and conclude that a single planetary embryo scattered into an eccentric orbit can, over ~ 10e5 years, produce ~ 10e24 g of chondrules. In principle, a small number (1-10) of eccentric planetary embryos can melt the observed mass of chondrules in a manner consistent with all known constraints.

astro-ph.EP↗

arXiv2009

Spitzer Observations of the HII 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 HII region affects the process of star formation. Our observations comprise IRAC (3.6, 4.5, 5.8, and 8.0 um) and MIPS (24 um) maps of the region, covering approximately 400 square arcminutes. 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 HII 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 HII region. At the current rate of star formation, we estimate at least 25-50% of the total population of YSOs formed by this process.

astro-ph↗

arXiv2005

Understanding our Origins: Star Formation in H II Region Environments

Recent analysis of the decay products of short-lived radiounclides (SLRs) in meteorites, in particular the confirmation of the presence of live 60Fe in the early Solar System, provides unambiguous evidence that the Sun and Solar System formed near a massive star. We consider the question of the formation of low-mass stars in environments near massive stars, presenting a scenario for the evolution of a star and its disk around the periphery of an expanding H II region. The stages in this scenario begin with compression of molecular gas around the edge of an H II region, continue as forming stars are overrun by the advancing ionization front, and culminate when ejecta from one or more nearby supernova explosions sweeps over YSO disks located in the low density interior of the H II region, injecting SLRs including 26Al and 60Fe. We review the evidence that this mode of star formation is more characteristic of formation of low-mass stars than is the mode of star formation seen in regions such as the Taurus-Auriga molecular cloud. We discuss the implications of this scenario for our understanding of star formation, as well as the effects of the young Sun's astrophysical environment on the formation and evolution of the Solar System. We conclude that low-mass stars and their accompanying disks form and evolve very differently near massive stars than they do in regions like Taurus-Auriga, and that these differences have profound implications for our understanding of our origins.

astro-ph↗

arXiv2001

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 > 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 protoSun, 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, 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 {\it in situ} thermal annealing of submicron and micron-sized silicate dust grains can occur, obviating the need for large-scale radial transport.

astro-ph↗