122781581
The Tycho-2 Spectral Type Catalog
The Tycho-2 Catalogue contains positions, proper motions, and BT and VT magnitudes for approximately 2.5 million stars. We have obtained spectral types for 351,864 of the Tycho-2 stars by cross-referencing Tycho-2 to several catalogs containing spectral types, using the VizieR astronomical database. The positional matching requirements were stringent (97.5% of the matches were within 1''), and the validity of the positional matches is further supported by the distribution of magnitude differences for matched pairs, which is symmetric about the median value (dV)med = 0.043 and has a small standard deviation, σdV = 0.40 mag.
118163991
Anisotropic Galaxy-Galaxy Lensing
We investigate the weak lensing shear due to dark matter galaxy halos whose mass distributions, as projected on the sky, are nearly elliptical. The shear pattern due to these halos is anisotropic about the lens centers and we quantify the level of anisotropy by comparing the mean shear experienced by sources located closest to the major axes of the lenses $ _{major}$, to that experienced by sources located closest to the minor axes, $ _{minor}$. We also compute the degree to which the anisotropy in the lensing signal is degraded due to a noisy determination of the position angles of the lens galaxies and find that provided the typical error on the orientation of the lenses is less than $15^\circ$, more than 90% of the true lensing signal is recovered in the mean. We discuss our our results in the context of detecting anisotropic galaxy-galaxy lensing in large, ground-based data sets and conclude that a modest net flattening of dark matter halos should be detectable at a statistically significant level. The forthcoming Sloan Digital Sky Survey (SDSS) data will necessarily provide a very useful data set for this analysis, but a detection of anisotropic galaxy-galaxy lensing is not dependent upon the very large sky coverage of the SDSS. Rather, we argue that a significant detection of this effect can also be obtained from an imaging survey that is of order two magnitudes fainter than SDSS and which covers only a relatively small area of sky, of order one square degree. [Abridged]
7233194
Gravitational Lensing by NFW Halos
We investigate the gravitational lensing properties of dark matter halos with Navarro, Frenk & White (NFW) density profiles and derive an analytic expression for the radial dependence of the shear, γ(x), due to these objects. In addition, we derive an expression for the mean shear interior to a given radius, (x), and use this to quantify systematic errors that will arise in weak-lensing mass estimates of astronomical objects in cases where the mass estimate is based on an a priori assumption that the underlying potential is that of a singular isothermal sphere when in fact the potential is that of an NFW-type object. On mass scales of 1011 M☉ M200 1015 M☉, the assumption of an isothermal sphere potential results in an overestimate of the halo mass, and the amount by which the mass is overestimated increases linearly with the value of the NFW concentration parameter. Considerable overestimates of the mass (~60%) can occur for galaxy-sized halos, but for rich clusters the mass overestimate is small. The degree to which the mass is systematically in error depends on the cosmology adopted, with a COBE-normalized standard cold dark matter (CDM) model yielding the largest systematic errors for a given true value of the halo mass.
16842883
A Comparison of Simple Mass Estimators for Galaxy Clusters
High-resolution N-body simulations are used to investigate systematic trends in the mass profiles and total masses of clusters as derived from three simple estimators: (1) the weak gravitational lensing shear field, under the assumption of an isothermal cluster potential; (2) the dynamical mass obtained from the measured velocity dispersion, under the assumption of an isothermal cluster potential; and (3) the classical virial estimator. The clusters used for the analysis were obtained from simulations of a standard cold dark matter universe at z = 0.5 and consist of order ? 105 particles of mass mp 1010 M?. The clusters are not smooth and spherically symmetric but, rather, the mass distribution is triaxial, and significant substructure exists within the clusters. Not surprisingly, the level of agreement between the cluster mass profiles obtained from the various estimators and the actual mass profiles is found to be scale-dependent. We define the total cluster mass to be the mass contained within a three-dimensional radius r200 of the cluster centers, where r200 is the radius inside which the mean interior overdensity is equal to 200. Under this definition the classical virial estimator yields a good measurement of the total cluster mass, although it is systematically underestimated by ~10%. This result suggests that, at least in the limit of ideal data, the virial estimator is quite robust to deviations from pure spherical symmetry and the presence of substructure within a cluster. The dynamical mass estimate based upon a measurement of the cluster velocity dispersion and an assumption of an isothermal potential yields a poor measurement of the total cluster mass, underestimating it by ~40% for the case in which ?v is computed from an average over the entire cluster. The weak lensing estimate yields a very good measurement of the total cluster mass, provided that the mean shear used to determine the equivalent cluster velocity dispersion is computed from an average of the lensing signal over the entire cluster (i.e., the mean shear is computed interior to a projected cluster radius of R200).
15605593
Multiple weak deflections and the detection of flattened halos with galaxy-galaxy lensing
We investigate the occurrence of multiple weak deflections in deep data sets which are used to detect galaxy-galaxy lensing. Using the galaxies in the HDF (North) for which both redshifts and rest-frame blue luminosities are known, we show that the probability for a given source galaxy to be lensed by two or more foreground galaxies exceeds 50% for z_s > 1, and for which the separate, individual deflections yield gamma > 0.005. Neglecting multiple deflections when obtaining best-fitting halo parameters for the lens galaxies can lead to an overestimate by a factor of order 2 for the halo mass interior to a radius of 100 kpc/h. We also show that multiple weak deflections create systematic effects which may hinder observational efforts to use weak lensing to constrain the projected shapes of the halos of field galaxies. For a data set in which the galaxies have magnitudes in the rage 19 < I < 23, multiple deflections result in strong correlations between the post-lensing image shapes of most foreground-background pairs of galaxies. Imposing a simple redshift cut during the data anaysis is sufficient to reduce the correlation between the final images of lenses and sources to the point that the expected anisotropy in the weak lensing signal can be detected via a straightforward average. We conclude that previous theoretical calculations of weak lensing due to flattened halos have considerably underestimated the sizes of the observational data sets which would be required to detect this effect. [Abridged]
117044525
Gas-phase H2O and CO2 toward massive protostars
We present a study of gas-phase H2O and CO2 toward a sample of 14 massive protostars with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). Modeling of the H2O spectra using a homogeneous model with a constant excitation temperature T_ex shows that the H2O abundances increase with temperature, up to a few times 10^-5 with respect to H2 for the hottest sources (T_ex ~500 K). This is still a factor of 10 lower than the H2O ice abundances observed toward cold sources in which evaporation is not significant (Keane et al. 2001). Gas-phase CO2 is not abundant in our sources. The abundances are nearly constant for T_ex>~100 K at a value of a few times 10^-7, much lower than the solid-state abundances of ~1--3 times 10^-6 (Gerakines et al. 1999). For both H2O and CO2 the gas/solid ratio increases with temperature, but the increase is much stronger for H2O than for CO2, suggesting a different type of chemistry. In addition to the homogeneous models, a power law model has been developed for one of our sources, based on the physical structure of this region as determined from submillimeter data by van der Tak et al. (1999). The resulting H2O model spectrum gives a good fit to the data.
118866213
ISO-SWS observations of pure rotational water absorption lines toward Orion-IRc2
First detections of thermal water vapor absorption lines have been made toward Orion IRc2 using the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). Grating spectra covering wavelengths 25-45 micron yield 19 pure rotational lines, originating from energy levels 200-750 K above ground. Fabry-Perot spectra of 5 transitions resolve the line profiles and reveal the water gas kinematics. The fact that all lines are seen in absorption is in striking contrast with data from the ISO Long Wavelength Spectrometer (LWS), where the water lines appear in emission. At least one line displays a P-Cygni type profile, which suggests that the water is located in an expanding shell centered on or near IRc2. The expansion velocity is 18 km per second, in agreement with the value inferred from water maser observations by Genzel et al. (1981). Because the continuum is intense and likely formed in or near the water-containing gas, the excitation of the observed transitions is dominated by radiative processes. A simple, generalised curve-of-growth method is presented and used to analyze the data. A mean excitation temperature of 72 K and a total water column density of 1.5e18 molecules per cm squared are inferred, each with an estimated maximum uncertainty of 20%. Combined with the molecular hydrogen column density derived from ISO observations of the pure rotational lines, and an assumed temperature of 200-350 K, the inferred water abundance is 2e-4 to 5e-4 in the warm shocked gas. This abundance is similar to that found recently by Harwit et al. (1998) toward Orion using data from the LWS, but higher than that found for most other shocked regions by, for example, Liseau et al. (1996).
17282332
Disks around the Young Stars TW Hydrae and HD 100546 Imaged at 3.4 Millimeters with the Australia Telescope Compact Array
We present observations of the young stars TW Hya and HD 100546 made with the Australia Telescope Compact Array at 89 GHz with ~2'' resolution and ~3 mJy continuum sensitivity. Compact thermal dust continuum emission is detected from disks surrounding both stars. HD 100546 also shows hints of extended emission, presumably a residual protostellar envelope, which is also visible in scattered light at optical wavelengths. For TW Hya, HCO+ J = 1-0 line emission from the circumstellar disk is detected and spatially resolved. The observed size and intensity are in good agreement with model calculations based on an irradiated disk with substantial depletions derived previously from single-dish observations of higher J HCO+ transitions.
14480836
ISOCAM spectro-imaging of the H2 rotational lines in the supernova remnant IC 443
We report spectro-imaging observations of the bright western ridge of the supernova remnant IC 443 obtained with the ISOCAM circular variable filter (CVF) on board the In- frared Space Observatory (ISO) . This ridge corresponds to a location where the interaction between the blast wave of the supernova and ambient molecular gas is amongst the strongest. The CVF data show that the 5 to 14 m spectrum is domi- nated by the pure rotational lines of molecular hydrogen (v = 0-0, S(2) to S(8) transitions). At all positions along the ridge, the H2 rotational lines are very strong with typical line fluxes of 10 4 to10 3 erg s 1 cm 2 sr 1 . We compare the data to a new time-dependent shock model; the rotational line fluxes in IC 443 are reproduced within factors of 2 for evolutionary times between 1,000 and 2,000 years with a shock velocity of 30kms 1 and a pre-shock density of 10 4 cm 3 .
94022512
SOLVING DYNAMIC EQUATIONS FOR THE COLLECTION AND EVAPORATION OF AN AEROSOL
We investigate the coalescence of drops undergoing random Brownian motion and evaporation. The evolution of the drop size spectrum is determined by solving a partial integro-differential equation with a kernel that describes the pairwise coalescence of drops. Numerical results for fuel injection with n-octane suggest that the size distribution of drops is determined by two competing processes, namely coalescence and evaporation. 
 
 
 
Nous etudions ici la coalescence de gouttes dans des conditions de deplacement brownien aleatoire et d'evaporation. L'evolution du spectre de la taille des gouttes est determinee par la resolution d'une equation integro-differentielle partielle avec un noyau qui decrit la coalescence par paire des gouttes. Les resultats numeriques pour l'injection du fuel avec du n-octane suggkrent que la distribution de taille des gouttes est determinee par deux processus se faisant concurrence, a savoir la coalescence et l'evaporation.