Mass models of the Milky Way

A parametrized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scalelength R _d* of the stellar disc to R ₀. The disc and bulge dominate v _c( R ) at R ≲ R ₀ only for R _d,*/ R ₀≲0.3. Since the only knowledge we have of the halo derives from studies like the present one, we allow it to contribute to the density at all radii. When allowed this freedom, however, the halo causes changes in assumptions relating to R  ≪  R ₀ to affect profoundly the structure of the best-fitting model at R  ≫  R ₀. For example, changing the disc slightly from an exponential surface-density profile significantly changes the form of v _c( R ) at R  ≫  R ₀, where the disc makes a negligible contribution to v _c. Moreover, minor changes in the constraints can cause the halo to develop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct haloes rather than outwards-increasing mass-to-light ratios.   The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z ≳ R and at R  >  R ₀ are especially important.     

The radio properties of radio-quiet quasars

Although radio-quiet quasars (RQQs) constitute ≳ 90 per cent of optically identified quasar samples, their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (≤ 0.24 arcsec). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (α ∼ 0.7, where S  ∝ ν^−α), and high brightness temperatures ( T _B ≥ 10⁵ K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity–absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample.     

Magnetospheric accretion and pre-main-sequence stellar masses

We present a method of determining lower limits on the masses of pre-main-sequence (PMS) stars and so constraining the PMS evolutionary tracks. This method uses the redshifted absorption feature observed in some emission-line profiles of T Tauri stars, indicative of infall. The maximum velocity of the accreting material measures the potential energy at the stellar surface, which, combined with an observational determination of the stellar radius, yields the stellar mass. This estimate is a lower limit owing to uncertainties in the geometry and projection effects. Using available data, we show that the computed lower limits can be larger than the masses derived from PMS evolutionary tracks for M   0.5 M. Our analysis also supports the notion that accretion streams do not impact near the stellar poles but probably hit the stellar surface at moderate latitudes.     

Meteorological noise in crustal gas emission and relevance to geochemical exploration

The emission of gas from the earth's crust is a complex process influenced by meteorological and seasonal processes which must be understood for effective application of gas emission to geochemical exploration. Free mercury vapor emission and radon emanation are being measured in a shallow instrument vault at a single nonmineralized site in order to evaluate these influences on gas emission.Mercury concentrations in the instrument vault average 9.5 ng/m³ and range from < 1 ng/m³ to 53 ng/m³ with a strong seasonal effect. Mercury has a direct relationship to vault temperature, air temperature, soil temperature, barometric pressure, water table, and the frozen or thawed state of the soil. Air and soil temperature, barometric pressure, and relative humidity are most important in influencing mercury emission while soil moisture is also important in radon emanation. Diurnal cycles are common but do not occur on all days. A heavy precipitation event on a dry soil seals the soil resulting in a rise in mercury concentration. Precipitation on a soil that is already wet does not increase mercury emission because of the compensation caused by lowering of the soil temperature by the precipitation event. Freezing of the soil changes the physical state of the vault-soil-soil gas-atmosphere system and emits the lowest concentrations of mercury. Phase lag effects are likely important. Stepwise multiple regression of mercury as dependent variable with meteorological and seasonal parameters as independent variables gives a cumulative R value of 0.563 and R² of 0.317. The short-term noise coupled with phase lags are an important factor.The radon measurements integrated over weekly intervals smooth out much of the short-term noise. Stepwise multiple regression of radon as dependent variable with meteorological and seasonal parameters as independent variables gives a cumulative R value of 0.967 and R² of 0.934. In this portion of the study the variation in the radon emanation is adequately predicted by meteorological and seasonal parameters.     

Seawater sulfate reduction and sulfur isotope fractionation in basaltic systems: Interaction of seawater with fayalite and magnetite at 200–350°C

Sulfate reduction during seawater reaction with fayalite and with magnetite was rapid at 350°C, producing equilibrium assemblages of talc-pyrite-hematite-magnetite at low water/rock ratios and talc-pyrite-hematite-anhydrite at higher water/rock ratios. At 250°C, seawater reacting with fayalite produced detectable amounts of dissolved H₂S, but extent of reaction of solid phases was minor after 150 days. At 200°C, dissolved H₂S was not detected, even after 219 days, but mass balance calculations suggest a small amount of pyrite may have formed. Reaction stoichiometry indicates that sulfate reduction requires large amounts of H⁺, which, in subseafloor hydrothermal systems is provided by Mg metasomatism. Seawater contains sufficient Mg to supply all the H⁺ necessary for quantitative reduction of seawater sulfate.Systematics of sulfur isotopes in the 250 and 350°C experiments indicate that isotopic equilibrium is reached, and can be modeled as a Rayleigh distillation process. Isotopic composition of hydrothermally produced H₂S in natural systems is strongly dependent upon the seawater/basalt ratio in the geothermal system, which controls the relative sulfide contributions from the two important sulfur sources, seawater sulfate and sulfide phases in basalt. Anhydrite precipitation during geothermal heating severely limits sulfate ingress into high temperature interaction zones. Quantitative sulfate reduction can thus be accomplished without producing strongly oxidized rocks and resultant sulfide sulfur isotope values represent a mixture of seawater and basaltic sulfur.     

13C12C exchange between calcite and graphite: A possible thermometer in Grenville marbles

The fractionation of ¹³C between calcite and graphite, Δ(Cc-Gr). is consistently small (2.6–4.8 permil) in 34 assemblages from upper amphibolite- and granulite-facies marbles of the Grenville Province. In 25 samples from the Adirondack Mountains, New York, it decreases regularly with increasing metamorphic temperature. The fractionations are independent of absolute δ¹³C values of calcite (?2.9 to +5.0). For T = 600–800°C, the Adirondack data are described by Δ(Cc-Gr) = ?0.00748T (°C) + 8.68. This good correlation between Δ and T suggests that carbon isotope equilibrium was attained in these high-grade marbles and that the theoretical calculations of this fractionation by Bottinga are approximately 2 permil too large in this temperature range. Because of the relatively high temperature sensitivity suggested by these results and by Bottinga's calculations, and the pressure independence of isotope fractionation, Δ(Cc-Gr) may provide a very good thermometer for high-grade marbles.Comparison of this field calibration for Δ(Cc-Gr) vs temperature with results from other terranes supports the utility of Δ(Cc-Gr) for geothermometry and suggests that graphite is much more sluggish to exchange than calcite, that exchange between calcite and graphite occurs at temperatures as low as 300°C, and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500–600°C.Because ¹³C exchange is an unavoidable metamorphic process at temperatures above 300°C, high values of δ¹³C(Gr) in moderate- to high-grade carbonate-bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite.     

Oxygen and hydrogen in the primitive atmosphere

From time to time there appears in the literature the assertion that photolysis of water vapor could have maintained an appreciable concentration of oxygen in the primitive (prebiological) atmosphere. The implausibility of this assertion is argued in this paper.By itself, photolysis does not provide a source of oxygen because it is usually followed by recombination of the products of photolysis. Only the escape to space (at a much smaller rate) of the hydrogen produced by photolysis of water results in a net source of oxygen. The oxidation state of the primitive atmosphere depended on the relative magnitudes of this net source of oxygen and a volcanic source of hydrogen and other reduced gases. Today the volcanic source of reduced gases is approximately equal to the oxygen source provided by photolysis followed by escape. The oxygen source depends on the mixing ratio of water vapor in the stratosphere, which ultimately determines the rate of escape of hydrogen produced from water vapor. Its magnitude may not have been very different in the past. The volcanic source of hydrogen, on the other hand, is likely to have been much larger when the earth was tectonically young. Hydrogen was therefore released to the primitive atmosphere more rapidly than oxygen, probably. Photochemical reactions with the excess hydrogen maintained oxygen mixing ratios at negligibly small levels. The hydrogen mixing ratio was determined by a balance between the volcanic source (reduced by recombination with oxygen) and escape to space.In time, either because of decline of the volcanic source of hydrogen or because of addition of a biological source of oxygen, the input of oxygen to the atmosphere rose above the input of hydrogen. The oxidation state of the atmosphere changed rapidly. Volcanic hydrogen was now consumed by photochemical reactions with excess oxygen, while the oxygen mixing ratio was determined by a balance between the source (reduced by recombination with volcanic hydrogen) and consumption in reactions with reduced material at the surface.     

Ontogeny ofHybocrinus punctatus (Miller and Gurley), an ordovician crinoid

Relative growth of the Ordovician crinoid Hybocrinus punctatus (Miller and Gurley)was studied using regression, correlation, and R-mode factor analysis. Systematic growth gradients of both initial intercepts and slopes existed throughout the calyx. Development of a single plate was best correlated with growth of the directly adjacent plates and was less well correlated with that of more distant plates. A total of 86 percent of the R-mode correlation matrix variance is associated with overall size increase of all dimensions and size-correlated shape changes. Most of the remaining variance is linked with divergent growth of small groups of specimens with respect to various shape parameters.     

Magnesium removal in reducing marine sediments by cation exchange

Cation exchange capacity determinations on sediments of the California Borderland indicate that exchange capacities are increased under highly reducing conditions, and result in magnesium removal from the pore water. Calculations indicate the removal process may account for 5–10 per cent of the magnesium brought to the sea by rivers.