A New Version of Reimers' law of Mass Loss Based on a Physical Approach

We present a new semi-empirical relation for the mass loss of cool stellar winds, which so far has frequently been described by "Reimers' law". Originally, this relation was based solely on dimensional scaling arguments without any physical interpretation. In our approach, the wind is assumed to result from the spill-over of the extended chromosphere, possibly associated with the action of waves, especially Alfven waves, which are used as guidance in the derivation of the new formula. We obtain a relation akin to the original Reimers law, but which includes two new factors. They reflect how the chromospheric height depends on gravity and how the mechanical energy flux depends, mainly, on effective temperature. The new relation is tested and sensitively calibrated by modelling the blue end of the Horizontal Branch of globular clusters. The most significant difference from mass loss rates predicted by the Reimers relation is an increase by up to a factor of 3 for luminous late-type (super-)giants, in good agreement with observations.Comment: 12 pages, 4 figures, accepted by ApJ Letter

Star-forming regions of the Aquila rift cloud complex. I. NH3 tracers of dense molecular cores

(Abridged) Aims. In the present part of our survey we search for ammonia emitters in the Aquila rift complex which trace the densest regions of molecular clouds. Methods. From a CO survey carried out with the Delingha 14-m telescope we selected ~150 targets for observations in other molecular lines. Here we describe the mapping observations in the NH3(1,1) and (2,2) inversion lines of the first 49 sources performed with the Effelsberg 100-m telescope. Results. The NH3(1,1) and (2,2) emission lines are detected in 12 and 7 sources, respectively. Among the newly discovered NH3 sources, our sample includes the following well-known clouds: the starless core L694-2, the Serpens cloud Cluster B, the Serpens dark cloud L572, the filamentary dark cloud L673, the isolated protostellar source B335, and the complex star-forming region Serpens South. Angular sizes between 40" and 80" (~0.04-0.08 pc) are observed for compact starless cores but as large as 9' (~0.5 pc) for filamentary dark clouds. The measured kinetic temperatures of the clouds lie between 9K and 18K. From NH3 excitation temperatures of 3-8K we determine H2 densities with typical values of ~(0.4-4) 10^4 cm^-3. The masses of the mapped cores range between ~0.05 and ~0.5M_solar. The relative ammonia abundance, X= [NH3]/[H2], varies from 10^-7 to 5 10^-7 with the mean = (2.7+/-0.6) 10^-7 (estimated from spatially resolved cores assuming the filling factor eta = 1). In two clouds, we observe kinematically split NH3 profiles separated by ~1 km/s. The splitting is most likely due to bipolar molecular outflows for one of which we determine an acceleration of <~ 0.03 km/s/yr. A starless core with significant rotational energy is found to have a higher kinetic temperature than the other ones which is probably caused by magnetic energy dissipation.Comment: 28 pages, 22 figures, 6 tables, accepted for publication in A&

Static Critical Behavior of the Spin-Freezing Transition in the Geometrically Frustrated Pyrochlore Antiferromagnet Y2Mo2O7

Some frustrated pyrochlore antiferromagnets, such as Y2Mo2O7, show a spin-freezing transition and magnetic irreversibilities below a temperature Tf similar to what is observed nonlinear magnetization measurements on Y2Mo2O7 that provide strong evidence that there is an underlying thermodynamic phase transition at Tf, which is characterized by critical exponents \gamma \approx 2.8 and \beta \approx 0.8. These values are typical of those found in random spin glasses, despite the fact that the level of random disorder in Y2Mo2O7 is immeasurably small.Comment: Latex file, calls for 4 encapsulated postscript figures (included). Submitted to Phys. Rev. Letters

A closer look at nitrification in pelagic sediments

Nutrient profiles in Southwest Pacific interstitial solutions suggest that in environments of oxic pelagic sedimentation microbially mediated nitrification is recognizable as a two-step process. During the first step partially oxidized nitrogenous intermediaries accumulate in distinctive ammonia and nitrite maxima along with nitrate. During the second step nitrification continues and all intermediate species are fully oxidized to nitrate. Both steps occur within a zone that corresponds in thickness to the biologically active surface layer. Similarly, experimental nitrogen regeneration from decomposition of plankton in seawater (VON BRAND and RAKESTRAW, 1941: VON BRAND et al., 1942) suggests that each step corresponds to a distinct reaction in the microbially mediated transformation of N-org → NH3 → NO2 → NO3. The resolution of distinct reaction zones in pore water nutrient profiles possibly depends on the nature and mode of supply of the organic matter undergoing nitrification or reflects the spatial succession downcore of microbial populations capable of deamination, ammonium oxidation and nitrite oxidation, respectively. Finally, stoichiometric ratios of nutrients in the free water column - here demonstrated on published data from Saanich Inlet - reflect the same two steps of nitrification as delineated by the dissolved pore water species. Future pore water studies should include dissolved oxygen measurements as well as accurate ∑CO2, PO4 and nitrogenous species profiles, to verify and better quantify these separate steps in nitrification mechanism of oxic pelagic sediments

Pyrochlore Antiferromagnet: A Three-Dimensional Quantum Spin Liquid

The quantum pyrochlore antiferromagnet is studied by perturbative expansions and exact diagonalization of small clusters. We find that the ground state is a spin-liquid state: The spin-spin correlation functions decay exponentially with distance and the correlation length never exceeds the interatomic distance. The calculated magnetic neutron diffraction cross section is in very good agreement with experiments performed on Y(Sc)Mn2. The low energy excitations are singlet-singlet ones, with a finite spin gap.Comment: 4 pages, 4 figure

Contribution of White Dwarfs to Cluster Masses

I present a literature search through 31 July 1997 of white dwarfs (WDs) in open and globular clusters. There are 36 single WDs and 5 WDs in binaries known among 13 open clusters, and 340 single WDs and 11 WDs in binaries known among 11 globular clusters. From these data I have calculated WD mass fractions for four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one globular cluster (NGC 6121). I develop a simple model of cluster evolution that incorporates stellar evolution but not dynamical evolution to interpret the WD mass fractions. I augment the results of my simple model with N-body simulations incorporating stellar evolution (Terlevich 1987; de la Feunte Marcos 1996; Vesperini & Heggie 1997). I find that even though these clusters undergo moderate to strong kinematical evolution the WD mass fraction is relatively insensitive to kinematical evolution. By comparing the cluster mass functions to that of the Galactic disk, and incorporating plausibility arguments for the mass function of the Galactic halo, I estimate the WD mass fraction in these two populations. I assume the Galactic disk is ~10 Gyrs old (Winget et al. 1987; Liebert, Dahn, & Monet 1988; Oswalt et al. 1996) and that the Galactic halo is ~12 Gyrs old (Reid 1997b; Gratton et al. 1997; Chaboyer et al. 1998), although the WD mass fraction is insensitive to age in this range. I find that the Galactic halo should contain 8 to 9% (alpha = -2.35) or perhaps as much as 15 to 17% (alpha = -2.0) of its stellar mass in the form of WDs. The Galactic disk WD mass fraction should be 6 to 7% (alpha = -2.35), consistent with the empirical estimates of 3 to 7% (Liebert, Dahn, & Monet 1988; Oswalt et al. 1996). (abridged)Comment: 20 pages, uuencoded gunzip'ed latex + 3 postscrip figures, to be published in AJ, April, 199

Ordering of the Antiferromagnetic Heisenberg Model on a Pyrochlore Slab

Ordering of the geometrically frustrated two-dimensional Heisenberg antiferromagnet on a pyrochlore slab is studied by Monte Carlo simulations. The model is expected to serve as a reference system of SrCrGaO compound studied extensively. In sharp contrast to the kagom\'e Heisenberg antiferromagnet, the model exhibits locally non-coplanar spin structures at low temperatures, bearing nontrivial chiral degrees of freedom. We find that under certain conditions the model exhibits a novel Kosterlitz-Thouless-type transition at a finite temperature associated with these chiral degrees of freedom. Implications to experiments are discussed.Comment: 26 figure

Red and Blue Shifted Broad Lines in Luminous Quasars

We have observed a sample of 22 luminous quasars, in the range 2.0<z<2.5, at 1.6 microns with the near-infrared (NIR) spectrograph FSPEC on the Multiple Mirror Telescope. Our sample contains 13 radio-loud and 9 radio-quiet objects. We have measured the systemic redshifts z_(sys) directly from the strong [O III]5007 line emitted from the narrow-line-region. From the same spectra, we have found that the non-resonance broad H$\beta$ lines have a systematic mean redward shift of 520+/-80 km/s with respect to systemic. Such a shift was not found in our identical analysis of the low-redshift sample of Boroson & Green. The amplitude of this redshift is comparable to half the expected gravitational redshift and transverse Doppler effects, and is consistent with a correlation between redshift differences and quasar luminosity. From data in the literature, we confirm that the high-ionization rest-frame ultraviolet broad lines are blueshifted ~550-1050 km/s from systemic, and that these velocity shifts systematically increase with ionization potential. Our results allow us to quantify the known bias in estimating the ionizing flux from the inter-galactic-medium J_(IGM) via the Proximity Effect. Using redshift measurements commonly determined from strong broad line species, like Ly\alpha or CIV1549, results in an over-estimation of J_(IGM) by factors of ~1.9-2.3. Similarly, corresponding lower limits on the density of baryon Omega_b will be over-estimated by factors of ~1.4-1.5. However, the low-ionization MgII2798 broad line is within ~50 km/s of systemic, and thus would be the line of choice for determining the true redshift of 1.0<z<2.2 quasars without NIR spectroscopy, and z>3.1 objects using NIR spectroscopy.Comment: 12 pages, Latex, 2 figures, 2 tables, Accepted for publication in ApJ Letter

Binaries discovered by the SPY project. IV, Five single-lined DA double white dwarfs

We present results from our ongoing follow-up observations of double white dwarf binaries detected in the ESO SN Ia Progenitor SurveY (SPY). We discuss our observing strategy and data analysis and present the orbital solutions of five close double white dwarf binaries: HE0320−1917, HE1511−0448, WD0326−273, WD1013−010 and WD1210+140. Their periods range from 0.44 to 3.22 days. In none of these systems we find any spectral lines originating from the companion. This rules out main sequence companions and indicates that the companion white dwarfs are significantly older and cooler than the bright component. Infrared photometry suggests the presence of a cool, helium-rich white dwarf companion in the binary WD 0326−273. We briefly discuss the consequences of our findings for our understanding of the formation and evolution of double white dwarfs

Local spin resonance and spin-Peierls-like phase transition in a geometrically frustrated antiferromagnet

Using inelastic magnetic neutron scattering we have discovered a localized spin resonance at 4.5 meV in the ordered phase of the geometrically frustrated cubic antiferromagnet $\rm ZnCr_2O_4$. The resonance develops abruptly from quantum critical fluctuations upon cooling through a first order transition to a co-planar antiferromagnet at $T_c=12.5(5)$ K. We argue that this transition is a three dimensional analogue of the spin-Peierls transition.Comment: 4 figures, revised and accepted in Phys. Rev. Let