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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

On the minimum core mass for giant planet formation at wide separations

In the core accretion hypothesis, giant planets form by gas accretion onto solid protoplanetary cores. The minimum (or critical) core mass to form a gas giant is typically quoted as 10 M {sub ⊕}. The actual value depends on several factors: the location in the protoplanetary disk, atmospheric opacity, and the accretion rate of solids. Motivated by ongoing direct imaging searches for giant planets, this study investigates core mass requirements in the outer disk. To determine the fastest allowed rates of gas accretion, we consider solid cores that no longer accrete planetesimals, as this would heat the gaseous envelope. Our spherical, two-layer atmospheric cooling model includes an inner convective region and an outer radiative zone that matches onto the disk. We determine the minimum core mass for a giant planet to form within a typical disk lifetime of 3 Myr. The minimum core mass declines with disk radius, from ∼8.5 M {sub ⊕} at 5 AU to ∼3.5 M {sub ⊕} at 100 AU, with standard interstellar grain opacities. Lower temperatures in the outer disk explain this trend, while variations in disk density are less influential. At all distances, a lower dust opacity or higher mean molecular weight reduces the critical core mass. Our non-self-gravitating, analytic cooling model reveals that self-gravity significantly affects early atmospheric evolution, starting when the atmosphere is only ∼10% as massive as the core.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Thermal starless ammonia core surrounded by CCS in the Orion a cloud

We imaged two starless molecular cloud cores, TUKH083 and TUKH122, in the Orion A giant molecular cloud in the CCS and NH{sub 3} emission with the Very Large Array. TUKH122 contains one NH{sub 3} core 'TUKH122-n', which is elongated and has a smooth oval boundary. Where observed, the CCS emission surrounds the NH{sub 3} core. This configuration resembles that of the N{sub 2}H{sup +} and CCS distribution in the Taurus starless core L1544, a well-studied example of a dense prestellar core exhibiting infall motions. The linewidth of TUKH122-n is narrow (0.20 km s{sup –1}) in the NH{sub 3} emission line and therefore dominated by thermal motions. The smooth oval shape of the core boundary and narrow linewidth in N{sub 2}H{sup +} seem to imply that TUKH122-n is dynamically relaxed and quiescent. TUKH122-n is similar to L1544 in the kinetic temperature (10 K), linear size (0.03 pc), and virial mass (∼2 M {sub ☉}). Our results strongly suggest that TUKH122-n is on the verge of star formation. TUKH122-n is embedded in the 0.2 pc massive (virial mass ∼30 M {sub ☉}) turbulent parent core, while the L1544 NH{sub 3} core is embedded in the 0.2 pc less-massive (virial mass ∼10 M {sub ☉}) thermal parent core. TUKH083 shows complicated distribution in NH{sub 3}, but was not detected in CCS. The CCS emission toward TUKH083 appears to be extended, and is resolved out in our interferometric observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Colliding filaments and a massive dense core in the Cygnus OB 7 molecular cloud

We report the results of molecular line observations carried out toward a massive dense core in the Cyg OB 7 molecular cloud. The core has an extraordinarily large mass (∼1.1 × 10{sup 4} M {sub ☉}) and size (∼2 × 5 pc{sup 2}), but there is no massive young star forming therein. We observed this core in various molecular lines such as C{sup 18}O(J = 1-0) using the 45 m telescope at Nobeyama Radio Observatory. We find that the core has an elongated morphology consisting of several filaments and core-like structures. The filaments are massive (10{sup 2}-10{sup 3} M {sub ☉}), and they are apparently colliding with one another. Some candidates for young stellar objects are distributed around their intersection, suggesting that the collisions of the filaments may have influenced their formation. To understand the formation and evolution of such colliding filaments, we performed numerical simulations using the adaptive mesh refinement technique, adopting the observed core parameters (the mass and size) as the initial conditions. The results indicate that the filaments are formed as seen in other earlier simulations for small cores in the literature, but we could not reproduce the collisions of the filaments simply by assuming a large initial mass and size. We find that collisions of the filaments occur only when there is a large velocity gradient in the initial core, in a sense compressing it. We suggest that the observed core was actually compressed by an external effect, e.g., shocks from nearby supernova remnants, including HB 21 which has been suggested to be interacting with the Cyg OB 7 molecular cloud.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

G305.136+0.068: A MASSIVE AND DENSE COLD CORE IN AN EARLY STAGE OF EVOLUTION

We report molecular line observations, made with ASTE and SEST, and dust continuum observations at 0.87 mm, made with APEX, toward the cold dust core G305.136+0.068. The molecular observations show that the core is isolated and roughly circularly symmetric and imply that it has a mass of 1.1 × 10{sup 3} M {sub ☉}. A simultaneous model fitting of the spectra observed in four transitions of CS, using a non-LTE radiative transfer code, indicates that the core is centrally condensed, with the density decreasing with radius as r {sup –1.8}, and that the turbulent velocity increases toward the center. The dust observations also indicate that the core is highly centrally condensed and that the average column density is 1.1 g cm{sup –2}, a value slightly above the theoretical threshold required for the formation of high-mass stars. A fit to the spectral energy distribution of the emission from the core indicates a dust temperature of 17 ± 2 K, confirming that the core is cold. Spitzer images show that the core is seen in silhouette from 3.6 to 24.0 μm and that it is surrounded by an envelope of emission, presumably tracing an externally excited photo-dissociated region. We found two embedded sources within a region of 20'' centered at the peak of the core, one of which is young, has a luminosity of 66 L {sub ☉}, and is accreting mass with a high accretion rate of ∼1 × 10{sup –4} M {sub ☉} yr{sup –1}. We suggest that this object corresponds to the seed of a high-mass protostar still in the process of formation. The present observations support the hypothesis that G305.136+0.068 is a massive and dense cold core in an early stage of evolution, in which the formation of a high-mass star has just started.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The structure and dark halo core properties of dwarf spheroidal galaxies

The structure and dark matter halo core properties of dwarf spheroidal galaxies (dSphs) are investigated. A double-isothermal (DIS) model of an isothermal, non-self-gravitating stellar system embedded in an isothermal dark halo core provides an excellent fit to the various observed stellar surface density distributions. The stellar core scale length a{sub *} is sensitive to the central dark matter density ρ{sub 0,d}. The maximum stellar radius traces the dark halo core radius r{sub c,d}. The concentration c{sub *} of the stellar system, determined by a King profile fit, depends on the ratio of the stellar-to-dark-matter velocity dispersion σ{sub ∗}/σ{sub d}. Simple empirical relationships are derived that allow us to calculate the dark halo core parameters ρ{sub 0,d}, r{sub c,d}, and σ{sub d} given the observable stellar quantities σ{sub *}, a{sub *}, and c{sub *}. The DIS model is applied to the Milky Way’s dSphs. All dSphs closely follow the same universal dark halo scaling relations ρ{sub 0,d}×r{sub c,d}=75{sub −45}{sup +85} M{sub ⊙} pc{sup −2} that characterize the cores of more massive galaxies over a large range in masses. The dark halo core mass is a strong function of core radius, M{sub c,d}∼r{sub c,d}{sup 2}. Inside a fixed radius of ∼400 pc the total dark matter mass is, however, roughly constant with M{sub d}=2.6±1.4×10{sup 7} M{sub ⊙}, although outliers are expected. The dark halo core densities of the Galaxy’s dSphs are very high, with ρ{sub 0,d} ≈ 0.2 M{sub ⊙} pc{sup −3}. dSphs should therefore be tidally undisturbed. Evidence for tidal effects might then provide a serious challenge for the CDM scenario.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The spin rate of pre-collapse stellar cores: Wave-driven angular momentum transport in massive stars

The core rotation rates of massive stars have a substantial impact on the nature of core-collapse (CC) supernovae and their compact remnants. We demonstrate that internal gravity waves (IGWs), excited via envelope convection during a red supergiant phase or during vigorous late time burning phases, can have a significant impact on the rotation rate of the pre-SN core. In typical (10 M{sub ⊙}≲M≲20 M{sub ⊙}) supernova progenitors, IGWs may substantially spin down the core, leading to iron core rotation periods P{sub min,Fe}≳30 s. Angular momentum (AM) conservation during the supernova would entail minimum NS rotation periods of P{sub min,NS}≳3 ms. In most cases, the combined effects of magnetic torques and IGW AM transport likely lead to substantially longer rotation periods. However, the stochastic influx of AM delivered by IGWs during shell burning phases inevitably spin up a slowly rotating stellar core, leading to a maximum possible core rotation period. We estimate maximum iron core rotation periods of P{sub max,Fe}≲5×10{sup 3} s in typical CC supernova progenitors, and a corresponding spin period of P{sub max,NS}≲500 ms for newborn neutron stars (NSs). This is comparable to the typical birth spin periods of most radio pulsars. Stochastic spin-up via IGWs during shell O/Si burning may thus determine the initial rotation rate of most NSs. For a given progenitor, this theory predicts a Maxwellian distribution in pre-collapse core rotation frequency that is uncorrelated with the spin of the overlying envelope.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

STAR FORMATION: STATISTICAL MEASURE OF THE CORRELATION BETWEEN THE PRESTELLAR CORE MASS FUNCTION AND THE STELLAR INITIAL MASS FUNCTION

We present a simple statistical analysis of recent numerical simulations exploring the correlation between the core mass function (CMF) obtained from the fragmentation of a molecular cloud and the stellar mass function which forms from these collapsing cores. Our analysis shows that the distributions of bound cores and sink particles obtained in the simulations are consistent with the sinks being formed predominantly from their parent core mass reservoir, with a statistical dispersion of the order of one-third of the core mass. Such a characteristic dispersion suggests that the stellar initial mass function (IMF) is relatively tightly correlated to the parent CMF, leading to two similar distributions, as observed. This in turn argues in favor of the IMF being essentially determined at the early stages of core formation and being only weakly affected by the various environmental factors beyond the initial core mass reservoir, at least in the mass range explored in the present study. Accordingly, the final IMF of a star-forming region should be determined reasonably accurately, statistically speaking, from the initial CMF, provided some uniform efficiency factor. The calculations also show that these statistical fluctuations, due to, e.g., variations among the core properties, broaden the low-mass tail of the IMF compared with the parent CMF, providing an explanation for the fact that the latter appears to underestimate the number of 'pre brown dwarf' cores compared with the observationally derived brown dwarf IMF.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

MASSIVE QUIESCENT CORES IN ORION: DYNAMICAL STATE REVEALED BY HIGH-RESOLUTION AMMONIA MAPS

We present combined Very Large Array and Green Bank Telescope images of NH{sub 3} inversion transitions (1, 1) and (2, 2) toward OMC2 and OMC3. We focus on the relatively quiescent Orion cores, which are away from the Trapezium cluster and have no sign of massive protostars or evolved star formation. The 5'' angular resolution and 0.6 km s{sup -1} velocity resolution enable us to study the thermal and dynamic state of these cores at {approx}0.02 pc scales, comparable to or smaller than those of the current dust continuum surveys. We measure temperatures for a total of 30 cores, with average masses of 11 M{sub Sun }, radii of 0.039 pc, virial mass ratio R{sub vir}-bar = 3.9, and critical mass ratio R{sub C}-bar = 1.5. Twelve sources contain Spitzer protostars. The thus defined starless and protostellar subsamples have similar temperature, line width, but different masses, with an average of 7.3 M{sub Sun} for the former and 16 M{sub Sun} for the latter. Compared to other Gould Belt dense cores, more Orion cores have a high gravitational-to-kinetic energy ratio and more cores have a larger than unity critical mass ratio. Orion dense cores have velocity dispersions similar to those of cores in low-mass star-forming regions but larger masses for given size. Some cores appear to have truly supercritical gravitational-to-kinetic energy ratios, even when considering significant observational uncertainties: thermal and non-thermal gas motions alone cannot prevent collapse.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Dynamics in Star-forming Cores (DiSCo): project overview and the first look towards the B1 and NGC 1333 regions in Perseus

The internal velocity structure within dense gaseous cores plays a crucial role in providing the initial conditions for star formation in molecular clouds. However, the kinematic properties of dense gas at core scales (~0.01–0.1 pc) has not been extensively characterized because of instrument limitations until the unique capabilities of GBT-Argus became available. The ongoing GBT-Argus Large Program, Dynamics in Star-forming Cores (DiSCo) thus aims to investigate the origin and distribution of angular momentum of star-forming cores. DiSCo will survey all starless cores and Class 0 protostellar cores in the Perseus molecular complex down to ~0.01 pc scales with <0.05 km s –1 velocity resolution using the dense gas tracer N 2 H + . Here, we present the first data sets from DiSCo towards the B1 and NGC 1333 regions in Perseus. Our results suggest that a dense core’s internal velocity structure has little correlation with other core-scale properties, indicating these gas motions may be originated externally from cloud-scale turbulence. These first data sets also reaffirm the ability of GBT-Argus for studying dense core velocity structure and provided an empirical basis for future studies that address the angular momentum problem with a statistically broad sample.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Active Field Cancellation to Prevent Saturation in Ferromagnetic-Core Loop Antennas

Ferromagnetic-core loop antennas with high-permeability core materials are typically used in size-constrained applications requiring the detection and reception of weak very low frequency and extremely low frequency signals. However, high-permeability materials are susceptible to saturation in the presence of strong magnetic fields such as those generated by power lines, in research laboratories, or through natural phenomena. Here, a system is presented that actively controls the magnetic flux in the core of the antenna by applying a control signal to an independent set of windings on the same core. This control signal is phase locked to the offending signal and the two signals destructively combine in the core. Our results show that it is possible to actively cancel the fundamental component of a strong interferer in the core of a high sensitivity ferromagnetic-core loop antenna and prevent core saturation. This approach eliminates signal distortion caused by magnetic saturation that passive signal cancellation or signal processing in the receiver do not. Furthermore, high-sensitivity ferromagnetic-core antennas can be used in applications where they were previously thought unsuitable.

42 ENGINEERING↗
DOE OSTI2022

Power distribution estimation method for SMR using ex-core detectors: experimental demonstration by plural control rod patterns at KUCA

The power distribution estimation method based on the ex-core detectors, or PHOEBE, was demonstrated at Kyoto University Critical Assembly, KUCA. Generally, core monitoring systems use in-core neutron detectors. Since inside the core is a harsh environment, the maintainability and reliability of the detectors are deteriorated. On the other hand, the environment outside the core is milder: core monitoring by the ex-core detectors improves the maintainability and reliability especially for small modular reactors and micro-reactors. However, neutron information from the inner region of the core is lost at the ex-core detectors. To recover the information, the authors proposed to utilize the power correlation between the fuel regions. PHOEBE concept was demonstrated at KUCA with distorted power distributions simulated by control rod patterns. The relative power distribution estimated by PHOEBE agreed with that calculated by Monte Carlo simulation code MVP. PHOEBE approximately reproduced the trend of the distorted power distributions calculated by MVP code. By contrast, the case of without power correlation between fuel regions produced significant different power distributions. Therefore, the advantage of considering the power correlation between fuel regions was also demonstrated. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2023

Applying the core-satellite species concept: Characteristics of rare and common riverine dissolved organic matter

Introduction: Dissolved organic matter (DOM) composition varies over space and time, with a multitude of factors driving the presence or absence of each compound found in the complex DOM mixture. Compounds ubiquitously present across a wide range of river systems (hereafter termed core compounds) may differ in chemical composition and reactivity from compounds present in only a few settings (hereafter termed satellite compounds). Here, we investigated the spatial patterns in DOM molecular formulae presence (occupancy) in surface water and sediments across 97 river corridors at a continental scale using the “Worldwide Hydrobiogeochemical Observation Network for Dynamic River Systems—WHONDRS” research consortium. Methods: We used a novel data-driven approach to identify core and satellite compounds and compared their molecular properties identified with Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Results: In this work, we found that core compounds clustered around intermediate hydrogen/carbon and oxygen/carbon ratios across both sediment and surface water samples, whereas the satellite compounds varied widely in their elemental composition. Within surface water samples, core compounds were dominated by lignin-like formulae, whereas protein-like formulae dominated the core pool in sediment samples. In contrast, satellite molecular formulae were more evenly distributed between compound classes in both sediment and water molecules. Core compounds found in both sediment and water exhibited lower molecular mass, lower oxidation state, and a higher degree of aromaticity, and were inferred to be more persistent than global satellite compounds. Higher putative biochemical transformations were found in core than satellite compounds, suggesting that the core pool was more processed. Discussion: The observed differences in chemical properties of core and satellite compounds point to potential differences in their sources and contribution to DOM processing in river corridors. Overall, our work points to the potential of data-driven approaches separating rare and common compounds to reduce some of the complexity inherent in studying riverine DOM.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

THE JCMT GOULD BELT SURVEY: A FIRST LOOK AT DENSE CORES IN ORION B

We present a first look at the SCUBA-2 observations of three sub-regions of the Orion B molecular cloud: LDN 1622, NGC 2023/2024, and NGC 2068/2071, from the JCMT Gould Belt Legacy Survey. We identify 29, 564, and 322 dense cores in L1622, NGC 2023/2024, and NGC 2068/2071 respectively, using the SCUBA-2 850 μm map, and present their basic properties, including their peak fluxes, total fluxes, and sizes, and an estimate of the corresponding 450 μm peak fluxes and total fluxes, using the FellWalker source extraction algorithm. Assuming a constant temperature of 20 K, the starless dense cores have a mass function similar to that found in previous dense core analyses, with a Salpeter-like slope at the high-mass end. The majority of cores appear stable to gravitational collapse when considering only thermal pressure; indeed, most of the cores which have masses above the thermal Jeans mass are already associated with at least one protostar. At higher cloud column densities, above 1–2 × 10{sup 23} cm{sup −2}, most of the mass is found within dense cores, while at lower cloud column densities, below 1 × 10{sup 23} cm{sup −2}, this fraction drops to 10% or lower. Overall, the fraction of dense cores associated with a protostar is quite small (<8%), but becomes larger for the densest and most centrally concentrated cores. NGC 2023/2024 and NGC 2068/2071 appear to be on the path to forming a significant number of stars in the future, while L1622 has little additional mass in dense cores to form many new stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

CHEMICAL AND PHYSICAL CHARACTERIZATION OF COLLAPSING LOW-MASS PRESTELLAR DENSE CORES

The first hydrostatic core, also called the first Larson core, is one of the first steps in low-mass star formation as predicted by theory. With recent and future high-performance telescopes, the details of these first phases are becoming accessible, and observations may confirm theory and even present new challenges for theoreticians. In this context, from a theoretical point of view, we study the chemical and physical evolution of the collapse of prestellar cores until the formation of the first Larson core, in order to better characterize this early phase in the star formation process. We couple a state-of-the-art hydrodynamical model with full gas-grain chemistry, using different assumptions for the magnetic field strength and orientation. We extract the different components of each collapsing core (i.e., the central core, the outflow, the disk, the pseudodisk, and the envelope) to highlight their specific physical and chemical characteristics. Each component often presents a specific physical history, as well as a specific chemical evolution. From some species, the components can clearly be differentiated. The different core models can also be chemically differentiated. Our simulation suggests that some chemical species act as tracers of the different components of a collapsing prestellar dense core, and as tracers of the magnetic field characteristics of the core. From this result, we pinpoint promising key chemical species to be observed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The quest for the largest depleted galaxy core: supermassive black hole binaries and stalled infalling satellites

Partially depleted cores are practically ubiquitous in luminous early-type galaxies (M {sub B} ≲ −20.5 mag) and are typically smaller than 1 kpc. In one popular scenario, supermassive black hole (SMBH) binaries—established during dry (i.e., gas-poor) galaxy mergers—kick out the stars from a galaxy’s central region via three-body interactions. Here, this “binary black hole scouring scenario” is probed at its extremes by investigating the two galaxies reported to have the largest partially depleted cores found to date: 2MASX J09194427+5622012 and 2MASX J17222717+3207571 (the brightest galaxy in Abell 2261). We have fit these galaxy’s two-dimensional light distribution using the core-Sérsic model and found that the former galaxy has a core-Sérsic break radius R{sub b,cS}=0.55 kpc, which is three times smaller than the published value. We use this galaxy to caution that other reportedly large break radii may too have been overestimated if they were derived using the “sharp-transition” (inner core)-to-(outer Sérsic) model. In the case of 2MASX J17222717+3207571, we obtain R {sub b,cS} = 3.6 kpc. While we confirm that this is the biggest known partially depleted core of any galaxy, we stress that it is larger than expected from the evolution of SMBH binaries—unless one invokes substantial gravitational-wave-induced (black hole-)recoil events. Given the presence of multiple nuclei located (in projection) within the core radius of this galaxy, we explored and found support for the alternative “stalled infalling perturber” core-formation scenario, in which this galaxy’s core could have been excavated by the action of an infalling massive perturber.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Distortion of Magnetic Fields in a Starless Core: Near-infrared Polarimetry of FeSt 1–457

Magnetic fields are believed to play an important role in controlling the stability and contraction of the dense condensations of gas and dust that lead to the formation of stars and planetary systems. In the present study, the magnetic field of FeSt 1–457, a cold starless molecular cloud core, was mapped on the basis of the polarized near-infrared light from 185 background stars after being dichroically absorbed by dust aligned with the magnetic field in the core. A distinct “hourglass-shaped” magnetic field was identified in the region of the core, and was interpreted as the first evidence of a magnetic field structure distorted by mass condensation in a starless core. The steep curvature of the magnetic field lines obtained in the present study indicates that the distortion was mainly created during the formation phase of the dense core. The derived mass-to-magnetic flux ratio indicates that the core is in a magnetically supercritical state. However, the stability of the core can be considered to be in a nearly critical state if the additional contributions from the thermal and turbulent support are included. Further diffusion of the magnetic field and/or turbulent dissipation would cause the onset of the dynamical collapse of the core. The geometrical relationship between the direction of the magnetic field lines and the elongation of the core was found to be in good agreement with theoretical predictions for the formation of Sun-like stars under the influence of a magnetic field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Temperature and precipitation signal in two Alpine ice cores over the period 1961&ndash;2001

Water stable isotope ratios and net snow accumulation in ice cores are commonly interpreted as temperature or precipitation proxies. However, only in a few cases has a direct calibration with instrumental data been attempted. In this study we took advantage of the dense network of observations in the European Alpine region to rigorously test the relationship of the annual and seasonal resolved proxy data from two highly resolved ice cores with local temperature and precipitation. We focused on the time period 1961–2001 with the highest amount and quality of meteorological data and the minimal uncertainty in ice core dating (±1 year). The two ice cores were retrieved from the Fiescherhorn glacier (northern Alps, 3900 m a.s.l.), and Grenzgletscher (southern Alps, 4200 m a.s.l.). A parallel core from the Fiescherhorn glacier allowed assessing the reproducibility of the ice core proxy data. Due to the orographic barrier, the two flanks of the Alpine chain are affected by distinct patterns of precipitation. The different location of the two glaciers therefore offers a unique opportunity to test whether such a specific setting is reflected in the proxy data. On a seasonal scale a high fraction of δ 18 O variability was explained by the seasonal cycle of temperature (~60% for the ice cores, ~70% for the nearby stations of the Global Network of Isotopes in Precipitation – GNIP). When the seasonality is removed, the correlations decrease for all sites, indicating that factors other than temperature such as changing moisture sources and/or precipitation regimes affect the isotopic signal on this timescale. Post-depositional phenomena may additionally modify the ice core data. On an annual scale, the δ 18 O/temperature relationship was significant at the Fiescherhorn, whereas for Grenzgletscher this was the case only when weighting the temperature with precipitation. In both cases the fraction of interannual temperature variability explained was ~20%, comparable to the values obtained from the GNIP stations data. Consistently with previous studies, we found an altitude effect for the δ 18 O of -0.17‰/100 m for an extended elevation range combining data of the two ice core sites and four GNIP stations. Significant correlations between net accumulation and precipitation were observed for Grenzgletscher during the entire period of investigation, whereas for Fiescherhorn this was the case only for the less recent period (1961–1977). Local phenomena, probably related to wind, seem to partly disturb the Fiescherhorn accumulation record. Spatial correlation analysis shows the two glaciers to be influenced by different precipitation regimes, with the Grenzgletscher reflecting the characteristic precipitation regime south of the Alps and the Fiescherhorn accumulation showing a pattern more closely linked to northern Alpine stations.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1976

Some aspects of core formation in Mercury

Some questions dealing with the nature and history of a large metallic core within Mercury are considered. These include the existence of a core, its size, whether it is fluid or solid, the timescale for core formation, the geological consequences of core formation, and whether such consequences are consistent with the surface geology. Several indirect lines of evidence are discussed which suggest the presence of a large iron-rich core. A core-formation model is examined in which core infall is accompanied by an increase of 17 km in planetary radius, an increase of 700 K in mean internal temperature, and substantial melting of the mantle. It is argued that if the core differentiated from an originally homogeneous planet, that event must have predated the oldest geological units comprising most of the planetary surface. A convective dynamo model for the source of Mercury's magnetic field is shown to conflict with cosmochemical models that do not predict a substantial radiogenic heat source in the core.

Solomon, S. C.↗