arXiv subjects

Steven J. Desch

Publications and source records attributed to Steven J. Desch.

At least 37 records · Page 2Linked to original sources

arXiv2020

Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds

The search for life on exoplanets is one of the grand scientific challenges of our time. The strategy to date has been to find (e.g., through transit surveys like Kepler) Earth-like exoplanets in their stars habitable zone, then use transmission spectroscopy to measure biosignature gases, especially oxygen, in the planets atmospheres (e.g., using JWST, the James Webb Space Telescope). Already there are more such planets than can be observed by JWST, and missions like the Transiting Exoplanet Survey Satellite and others will find more. A better understanding of the geochemical cycles relevant to biosignature gases is needed, to prioritize targets for costly follow-up observations and to help design future missions. We define a Detectability Index to quantify the likelihood that a biosignature gas could be assigned a biological vs. non-biological origin. We apply this index to the case of oxygen gas, O2, on Earth-like planets with varying water contents. We demonstrate that on Earth-like exoplanets with 0.2 weight percent (wt%) water (i.e., no exposed continents) a reduced flux of bioessential phosphorus limits the export of photosynthetically produced atmospheric O2 to levels indistinguishable from geophysical production by photolysis of water plus hydrogen escape. Higher water contents >1wt% that lead to high-pressure ice mantles further slow phosphorus cycling. Paradoxically, the maximum water content allowing use of O2 as a biosignature, 0.2wt%, is consistent with no water based on mass and radius. Thus, the utility of an O2 biosignature likely requires the direct detection of both water and land on a planet.

astro-ph.EP↗

arXiv2019

Haumea's Shape and Composition

We have calculated the figure of equilibrium of a rapidly rotating, differentiated body to determine the shape, structure, and composition of the dwarf planet Haumea. Previous studies of Haumea's light curve have suggested Haumea is a uniform triaxial ellipsoid consistent with a Jacobi ellipsoid with axes $\approx 960 \times 774 \times 513$ km, and bulk density $\approx 2600 \, {\rm kg} \, {\rm m}^{-3}$. In contrast, observations of a recent stellar occultation by Haumea indicate its axes are $\approx 1161 \times 852 \times 523$ km and its bulk density $\approx 1885 \, {\rm kg} \, {\rm m}^{-3}$; these results suggest that Haumea cannot be a fluid in hydrostatic equilibrium and must be partially supported by interparticle forces. We have written a code to reconcile these contradictory results and to determine if Haumea is in fact a fluid in hydrostatic equilibrium. The code calculates the equilibrium shape, density, and ice crust thickness of a differentiated Haumea after imposing (semi-) axes lengths $a$ and $b$. We find Haumea is consistent with a differentiated triaxial ellipsoid fluid in hydrostatic equilibrium with axes of best fit $a$ = 1050 km, $b$ = 840 km, and $c$ = 537 km. This solution for Haumea has $ρ_{\rm avg} = 2018 \, {\rm kg} \, {\rm m}^{-3}$, $ρ_{\rm core} = 2680 \, {\rm kg} \, {\rm m}^{-3}$, and core axes $a_{\rm c} = 883$ km, $b_{\rm c} = 723$ km, and $c_{\rm c}=470$ km, which equates to an ice mantle comprising $\sim 17\%$ of Haumea's volume and ranging from 67 to 167 km in thickness. The thick ice crust we infer allows for Haumea's collisional family to represent only a small fraction of Haumea's pre-collisional ice crust. For a wide range of parameters, the core density we calculate for Haumea suggests that today the core is composed of hydrated silicates and likely underwent serpentinization in the past.

astro-ph.EP↗

arXiv2019

Effect of Different Angular Momentum Transport mechanisms on the Distribution of Water in Protoplanetary Disks

The snow line in a protoplanetary disk demarcates regions with H$_2$O ice from regions with H$_2$O vapor. Where a planet forms relative to this location determines how much water and other volatiles it forms with. Giant planet formation may be triggered at the water snow line if vapor diffuses outward and is cold-trapped beyond the snow line faster than icy particles can drift inward. In this study we investigate the distribution of water across the snow line, considering three different radial profiles of the turbulence parameter $α(r)$, corresponding to three different angular momentum transport mechanisms. We consider the radial transport of water vapor and icy particles by diffusion, advection, and drift. We show that even for similar values of $α$, the gradient of $α$(r) across the snow line significantly changes the snow line location, the sharpness of the volatile gradient across the snow line, and the final water/rock ratio in planetary bodies. A profile of radially decreasing $α$, consistent with transport by hydrodynamic instabilities plus magnetic disk winds, appears consistent with the distribution of water in the solar nebula, with monotonically-increasing radial water content and a diverse population of asteroids with different water content. We argue that $Σ(r)$ and water abundance $N_{\rm H_2O}(r)/N_{\rm H_2}(r)$ are likely diagnostic of $α(r)$ and thus the mechanism for angular momentum transport in inner disks.

astro-ph.EP↗

arXiv2018

Inferring the Composition of Disintegrating Planet Interiors from Dust Tails with Future James Webb Space Telescope Observations

Disintegrating planets allow for the unique opportunity to study the composition of the interiors of small, hot, rocky exoplanets because the interior is evaporating and that material is condensing into dust, which is being blown away and then transiting the star. Their transit signal is dominated by dusty effluents forming a comet-like tail trailing the host planet (or leading it, in the case of K2-22b), making these good candidates for transmission spectroscopy. To assess the ability of such observations to diagnose the dust composition, we simulate the transmission spectra from 5-14 $μ$m for the planet tail assuming an optically-thin dust cloud comprising a single dust species with a constant column density scaled to yield a chosen visible transit depth. We find that silicate resonant features near 10 $μ$m can produce transit depths that are at least as large as those in the visible. For the average transit depth of 0.55% in the Kepler band for K2-22b, the features in the transmission spectra can be as large as 1%, which is detectable with the JWST MIRI low-resolution spectrograph in a single transit. The detectability of compositional features is easier with an average grain size of 1 $μ$m despite features being more prominent with smaller grain sizes. We find most features are still detectable for transit depths of ~0.3% in the visible range. If more disintegrating planets are found with future missions such as the space telescope TESS, follow-up observations with JWST can explore the range of planetary compositions.

astro-ph.EP↗

arXiv2018

The Effect of Jupiter's Formation on the Distribution of Refractory Elements and Inclusions in Meteorites

We present a comprehensive evolutionary model of the Sun's protoplanetary disk, constructed to resolve the "CAI Storage" problem of meteoritics. We predict the abundances of calcium-rich, aluminum-rich inclusions (CAIs) and refractory lithophile elements under the central assumption that Jupiter's $\sim 30 \, M_{\oplus}$ core forms at about 3 AU at around 0.6 Myr and opened a gap CAIs are trapped in the pressure maximum beyond Jupiter; carbonaceous chondrites formed there. Inside Jupiter's orbit, CAIs were depleted by aerodynamic drag; ordinary and enstatite chondrites formed there. For 16 chondrites and achondrites, we review meteoritic data on their CAI and refractory abundances and their times of formation, constrained by radiometric dating and thermal models. We predict their formation locations, finding excellent consistency with other location information (water content, asteroid spectra and parent bodies). We predict the size of particle concentrated by turbulence for each chondrite, finding excellent matches to each chondrites's mean chondrule diameter. These consistencies imply meteorite parent bodies assembled quickly from local materials concentrated by turbulence, and usually did not migrate far. We predict CI chondrites are depleted in refractory lithophile elements relative to the Sun, by about 12% (0.06 dex). We constrain the variation of turbulence parameter $α$ in the disk, and find a limited role for magnetorotational instability, favoring hydrodynamical instabilities in the outer disk, plus magnetic disk winds in the inner disk. Between 3 and 4 Myr at least, gas persisted outside Jupiter but was depleted inside it, and the solar nebula was a transition disk.

astro-ph.EP↗

arXiv2018

Updated Compositional Models of the TRAPPIST-1 Planets

After publication of our initial mass-radius-composition models for the TRAPPIST-1 system in Unterborn et al. (2018), the planet masses were updated in Grimm et al. (2018). We had originally adopted the data set of Wang et al., 2017 who reported different densities than the updated values. The differences in observed density change the inferred volatile content of the planets. Grimm et al. (2018) report TRAPPIST-1 b, d, f, g, and h as being consistent with <5 wt% water and TRAPPIST-1 c and e has having largely rocky interiors. Here, we present updated results recalculating water fractions and potential alternative compositions using the Grimm et al., 2018 masses. Overall, we can only reproduce the results of Grimm et al., 2018 of planets b, d and g having small water contents if the cores of these planets are small (<23 wt%). We show that, if the cores for these planets are roughly Earth-sized (33 wt%), significant water fractions up to 40 wt% are possible. We show planets c, e, f, and h can have volatile envelopes between 0-35 wt% that are also consistent with being totally oxidized and lacking an Fe-core entirely. We note here that a pure MgSiO$_3$ planet (Fe/Mg = 0) is not the true lowest density end-member mass-radius curve for determining the probability of a planet containing volatiles. All planets that are rocky likely contain some Fe, either within the core or oxidized in the mantle. We argue the true low density end-member for oxidizing systems is instead a planet with the lowest reasonable Fe/Mg and completely core-less. Using this logic, we assert that planets b, d and g likely must have significant volatile layers because the end-member planet models produce masses too high even when uncertainties in both mass and radius are taken into account.

astro-ph.EP↗

arXiv2018

Magnetic Fields Recorded by Chondrules Formed in Nebular Shocks

Recent laboratory efforts (Fu et al., 2014) have constrained the remanent magnetizations of chondrules and the magnetic field strengths at which the chondrules were exposed to as they cooled below their Curie points. An outstanding question is whether the inferred paleofields represent the background magnetic field of the solar nebula or were unique to the chondrule-forming environment. We investigate the amplification of the magnetic field above background values for two proposed chondrule formation mechanisms, large-scale nebular shocks and planetary bow shocks. Behind large-scale shocks, the magnetic field parallel to the shock front is amplified by factors $\sim 10-30$, regardless of the magnetic diffusivity. Therefore, chondrules melted in these shocks probably recorded an amplified magnetic field. Behind planetary bow shocks, the field amplification is sensitive to the magnetic diffusivity. We compute the gas properties behind a bow shock around a 3000 km-radius planetary embryo, with and without atmospheres, using hydrodynamics models. We calculate the ionization state of the hot, shocked gas, including thermionic emission from dust, and thermal ionization of gas-phase potassium atoms, and the magnetic diffusivity due to Ohmic dissipation and ambipolar diffusion. We find that the diffusivity is sufficiently large that magnetic fields have already relaxed to background values in the shock downstream where chondrules acquire magnetizations, and that these locations are sufficiently far from the planetary embryos that chondrules should not have recorded a significant putative dynamo field generated on these bodies. We conclude that, if melted in planetary bow shocks, chondrules probably recorded the background nebular field.

astro-ph.EP↗

arXiv2018

Inward Migration of the TRAPPIST-1 Planets as Inferred From Their Water-Rich Compositions

Multiple planet systems provide an ideal laboratory for probing exoplanet composition, formation history and potential habitability. For the TRAPPIST-1 planets, the planetary radii are well established from transits (Gillon et al., 2016, Gillon et al., 2017), with reasonable mass estimates coming from transit timing variations (Gillon et al., 2017, Wang et al., 2017) and dynamical modeling (Quarles et al., 2017). The low bulk densities of the TRAPPIST-1 planets demand significant volatile content. Here we show using mass-radius-composition models, that TRAPPIST-1f and g likely contain substantial ($\geq50$ wt\%) water/ice, with b and c being significantly drier ($\leq15$ wt\%). We propose this gradient of water mass fractions implies planets f and g formed outside the primordial snow line whereas b and c formed inside. We find that compared to planets in our solar system that also formed within the snow line, TRAPPIST-1b and c contain hundreds more oceans worth of water. We demonstrate the extent and timescale of migration in the TRAPPIST-1 system depends on how rapidly the planets formed and the relative location of the primordial snow line. This work provides a framework for understanding the differences between the protoplanetary disks of our solar system versus M dwarfs. Our results provide key insights into the volatile budgets, timescales of planet formation, and migration history of likely the most common planetary host in the Galaxy.

astro-ph.EP↗

arXiv2018

A procedure for observing rocky exoplanets to maximize the likelihood that atmospheric oxygen will be a biosignature

Here we advocate an observational strategy to help prioritize exoplanet observations. It starts with more easily obtained observational data, and ranks exoplanets for more difficult follow-up observations based on the likelihood of avoiding planets for which oxygen is a false positives or even an inconclusive signature of life. We find that for oxygen to be a robust biosignature, both land and surface water must be present. Landless exoplanets have much slower biogeochemical cycles, so while oxygenic photosynthesizing life could exist on such planets, it could not produce oxygen at a rate competitive with abiotic rates such as photolysis. These habitable planets, whose life would not be detectable, should be avoided.

astro-ph.EP↗

arXiv2017

A Catalog of Stellar Unified Properties (CATSUP) for 951 FGK-Stars Within 30 pc

Almost every star in our Galaxy is likely to harbor a terrestrial planet, but accurate measurements of an exoplanet's mass and radius demands accurate knowledge of the properties of its host star. The imminent TESS and CHEOPS missions are slated to discover thousands of new exoplanets. Along with WFIRST, which will directly image nearby planets, these surveys make urgent the need to better characterize stars in the nearby solar neighborhood (< 30 pc). We have compiled the CATalog of Stellar Unified Properties (CATSUP) for 951 stars, including such data as: Gaia astrometry; multiplicity within stellar systems; stellar elemental abundance measurements; standardized spectral types; Ca II H and K stellar activity indices; GALEX NUV and FUV photometry; and X-ray fluxes and luminosities from ROSAT, XMM, and Chandra. We use this data-rich catalog to find correlations, especially between stellar emission indices, colors, and galactic velocity. Additionally, we demonstrate that thick-disk stars in the sample are generally older, have lower activity, and have higher velocities normal to the galactic plane. We anticipate CATSUP will be useful in discerning other trends among stars within the nearby solar neighborhood, for comparing thin-disk vs. thick-disk stars, for comparing stars with and without planets, and for finding correlations between chemical and kinematic properties.

astro-ph.SR↗

arXiv2017

Formulas for Radial Transport in Protoplanetary Disks

Quantification of the radial transport of gaseous species and solid particles is important to many applications in protoplanetary disk evolution. An especially important example is determining the location of the water snow lines in a disk, which requires computing the rates of outward radial diffusion of water vapor and the inward radial drift of icy particles; however, the application is generalized to evaporation fronts of all volatiles. We review the relevant formulas using a uniform formalism. This uniform treatment is necessary because the literature currently contains at least six mutually exclusive treatments of radial diffusion of gas, only one of which is correct. We derive the radial diffusion equations from first principles, using Fick's law. For completeness, we also present the equations for radial transport of particles. These equations may be applied to studies of diffusion of gases and particles in protoplanetary and other accretion disks.

astro-ph.EP↗

arXiv2016

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.

astro-ph.SR↗

arXiv2016

The effect of multiple particle sizes on cooling rates of chondrules produced in large-scale shocks in the solar nebula

Chondrules represent one of the best probes of the physical conditions and processes acting in the early solar nebula. Proposed chondrule formation models are assessed based on their ability to match the meteoritic evidence, especially experimental constraints on their thermal histories. The model most consistent with chondrule thermal histories is passage through shock waves in the solar nebula. Existing models of heating by shocks generally yield a good first-order approximation to inferred chondrule cooling rates. However, they predict prolonged heating in the pre-shock region, which would cause volatile loss and isotopic fractionation, which are not observed. These models have typically included particles of a single (large) size, i.e., chondrule precursors, or at most, large particles accompanied by micron-sized grains. The size distribution of solids present during chondrule formation controls the opacity of the affected region, and significantly affects the thermal histories of chondrules. Micron-sized grains evaporate too quickly to prevent excessive heating of chondrule precursors. However, isolated grains in chondrule-forming regions would rapidly coagulate into fractal aggregates. Pre-shock heating by infrared radiation from the shock front would cause these aggregates to melt and collapse into intermediate-sized (tens of microns) particles. We show that inclusion of such particles yields chondrule cooling rates consistent with petrologic and isotopic constraints.

astro-ph.EP↗

arXiv2015

Magneto-rotational instability in the protolunar disk

(Abridged) We perform the first study of magnetohydrodynamic processes in the protolunar disk (PLD). With the use of published data on the chemical composition of the PLD, along with existing analytical models of the disk structure, we show that the high temperatures that were prevalent in the disk would have led to ionization of Na, K, SiO, Zn and, to a lesser extent, O$_2$. We assume that the disk has a vapor structure. The resulting ionization fractions, together with a relatively weak magnetic field, would have been sufficient to trigger the magneto-rotational instability, or MRI. We calculate the intensity of the resulting magnetohydrodynamic turbulence, as parameterized by the dimensionless ratio $α$ of turbulent stresses to gas pressure, and obtain maximum values $α\sim 10^{-2}$ along most of the vertical extent of the disk, and at different orbital radii. Under these conditions, turbulent mixing within the PLD due to the MRI was likely capable of transporting isotopic and chemical species efficiently. To test these results in a conservative manner, we carry out a numerical magnetohydrodynamic simulation of a small, rectangular patch of the PLD, located at 4 Earth radii ($r_{\rm E}$) from the center of the Earth, and assuming once again that the disk is completely gaseous. We obtain turbulence with an average intensity $α\sim 7\times 10^{-6}$ over the course of 280 orbital periods (133 days at 4$r_{\rm{E}}$). Despite this relatively low value of $α$, the effective turbulent diffusivity of a passive tracer introduced in the flow is large enough to allow the tracer to spread across a radial distance of $10r_{\rm{E}}$ in $\sim13-129$ yr, less than the estimated cooling time of the PLD of $\sim250$ yr. Further improvements to our model will need to incorporate the energy balance in the disk, a complete two-phase structure, and a more realistic equation of state.

astro-ph.EP↗

arXiv2015

High-Temperature Ionization in Protoplanetary Disks

We calculate the abundances of electrons and ions in the hot (> 500 K), dusty parts of protoplanetary disks, treating for the first time the effects of thermionic and ion emission from the dust grains. High-temperature ionization modeling has involved simply assuming that alkali elements such as potassium occur as gas-phase atoms and are collisionally ionized following the Saha equation. We show that the Saha equation often does not hold, because free charges are produced by thermionic and ion emission and destroyed when they stick to grain surfaces. This means the ionization state depends not on the first ionization potential of the alkali atoms, but rather on the grains' work functions. The charged species' abundances typically rise abruptly above about 800 K, with little qualitative dependence on the work function, gas density, or dust-to-gas mass ratio. Applying our results, we find that protoplanetary disks' dead zone, where high diffusivities stifle magnetorotational turbulence, has its inner edge located where the temperature exceeds a threshold value ~1000 K. The threshold is set by ambipolar diffusion except at the highest densities, where it is set by Ohmic resistivity. We find that the disk gas can be diffusively loaded onto the stellar magnetosphere at temperatures below a similar threshold. We investigate whether the "short-circuit" instability of current sheets can operate in disks and find that it cannot, or works only in a narrow range of conditions; it appears not to be the chondrule formation mechanism. We also suggest that thermionic emission is important for determining the rate of Ohmic heating in hot Jupiters.

astro-ph.EP↗

arXiv2014

The carbon atom in intense magnetic fields

The energy levels of the first few low-lying states of carbon in intense magnetic fields upwards of $\approx 10^7$ T are calculated in this study. We extend our previously employed pseudospectral approach for calculating the eigenstates of the carbon atom. We report data for the ground state and a low-lying state that are in good agreement with findings elsewhere, as well as new data for ten other states of the carbon atom that have not been investigated until now. It is seen that these hitherto uninvestigated states also become strongly bound with increasing magnetic field strengths. The data presented in this study are relevant for astrophysical applications, such as magnetized white dwarf and neutron star spectral analysis as well as opacity calculations and absorption features, including in the context of material accreting onto the surfaces of these compact objects.

physics.atom-ph↗

arXiv2013

Short GRB 130603B: Discovery of a jet break in the optical and radio afterglows, and a mysterious late-time X-ray excess

We present radio, optical/NIR, and X-ray observations of the afterglow of the short-duration 130603B, and uncover a break in the radio and optical bands at 0.5 d after the burst, best explained as a jet break with an inferred jet opening angle of 4-8 deg. GRB 130603B is only the third short GRB with a radio afterglow detection to date, and the first time that a jet break is evident in the radio band. We model the temporal evolution of the spectral energy distribution to determine the burst explosion properties and find an isotropic-equivalent kinetic energy of (0.6-1.7) x 10^51 erg and a circumburst density of 5 x 10^-3-30 cm^-3. From the inferred opening angle of GRB 130603B, we calculate beaming-corrected energies of Egamma (0.5-2) x 10^49 erg and EK (0.1-1.6) x 10^49 erg. Along with previous measurements and lower limits we find a median short GRB opening angle of 10 deg. Using the all-sky observed rate of 10 Gpc^-3 yr^-1, this implies a true short GRB rate of 20 yr^-1 within 200 Mpc, the Advanced LIGO/VIRGO sensitivity range for neutron star binary mergers. Finally, we uncover evidence for significant excess emission in the X-ray afterglow of GRB 130603B at >1 d and conclude that the additional energy component could be due to fall-back accretion or spin-down energy from a magnetar formed following the merger.

astro-ph.HE↗

arXiv2013

Phyllosilicate Emission from Protoplanetary Disks: Is the Indirect Detection of Extrasolar Water Possible?

Phyllosilicates are hydrous minerals formed by interaction between rock and liquid water and are commonly found in meteorites originating in the asteroid belt. Collisions between asteroids contribute to zodiacal dust, which therefore reasonably could include phyllosilicates. Collisions between planetesimals in protoplanetary disks may also produce dust containing phyllosilicates. These minerals possess characteristic emission features in the mid-infrared and could be detectable in extrasolar protoplanetary disks. Here we determine whether phyllosilicates in protoplanetary disks are detectable in the infrared using instruments such as those on board the Spitzer Space Telescope and SOFIA (Stratospheric Observatory for Infrared Astronomy). We calculate opacities for the phyllosilicates most common in meteorites and compute the emission of radiation from a protoplanetary disk using a 2-layer radiative transfer model. We find that phyllosilicates present at the 3% level lead to observationally significant differences in disk spectra, and should therefore be detectable using infrared observations and spectral modeling. Detection of phyllosilicates in a protoplanetary disk would be diagnostic of liquid water in planetesimals in that disk and would demonstrate similarity to our own Solar System. We also discuss use of phyllosilicate emission to test the "waterworlds" hypothesis, which proposes that liquid water in planetesimals should correlate with the inventory of short-lived radionuclides in planetary systems, especially 26Al.

astro-ph.SR↗