2,128 research outputs found
Photometry and polarimetry of the nucleus of comet 2P/Encke
Broadband imaging photometry, and broadband and narrowband linear polarimetry
was measured for the nucleus of 2P/Encke over the phase-angle range 4 - 28 deg.
An analysis of the point spread function of the comet reveals only weak coma
activity, corresponding to a dust production of the order of 0.05 kg/s. The
nucleus displays a color independent photometric phase function of almost
linear slope. The absolute R filter magnitude at zero phase angle is 15.05 +/-
0.05, and corresponds to an equivalent radius for the nucleus of 2.43 +/- 0.06
km (for an adopted albedo of 0.047). The nucleus color V - R is 0.47 +/- 0.07,
suggesting a spectral slope of 11 +/- 8 %/100nm. The phase function of linear
polarimetry in the V and R filters shows a widely color independent linear
increase with phase angle (0.12 +/- 0.02%/deg). We find discrepancies in the
photometric and polarimetric parameters between 2P/Encke and other minor bodies
in the solar system, which may indicate significant differences in the surface
material properties and light-scattering behavior of the bodies.
The linear polarimetric phase function of 2P/Encke presented here is the
first ever measured for a cometary nucleus, and its analysis encourages future
studies of cometary nuclei in order to characterize the light-scattering
behavior of comets on firm empirical grounds and provide suitable input to a
comprehensive modeling of the light scattering by cometary surfaces.Comment: Accepted by A&
Quantifying mixing using magnetic resonance imaging.
Mixing is a unit operation that combines two or more components into a homogeneous mixture. This work involves mixing two viscous liquid streams using an in-line static mixer. The mixer is a split-and-recombine design that employs shear and extensional flow to increase the interfacial contact between the components. A prototype split-and-recombine (SAR) mixer was constructed by aligning a series of thin laser-cut Poly (methyl methacrylate) (PMMA) plates held in place in a PVC pipe. Mixing in this device is illustrated in the photograph in Fig. 1. Red dye was added to a portion of the test fluid and used as the minor component being mixed into the major (undyed) component. At the inlet of the mixer, the injected layer of tracer fluid is split into two layers as it flows through the mixing section. On each subsequent mixing section, the number of horizontal layers is duplicated. Ultimately, the single stream of dye is uniformly dispersed throughout the cross section of the device. Using a non-Newtonian test fluid of 0.2% Carbopol and a doped tracer fluid of similar composition, mixing in the unit is visualized using magnetic resonance imaging (MRI). MRI is a very powerful experimental probe of molecular chemical and physical environment as well as sample structure on the length scales from microns to centimeters. This sensitivity has resulted in broad application of these techniques to characterize physical, chemical and/or biological properties of materials ranging from humans to foods to porous media (1, 2). The equipment and conditions used here are suitable for imaging liquids containing substantial amounts of NMR mobile (1)H such as ordinary water and organic liquids including oils. Traditionally MRI has utilized super conducting magnets which are not suitable for industrial environments and not portable within a laboratory (Fig. 2). Recent advances in magnet technology have permitted the construction of large volume industrially compatible magnets suitable for imaging process flows. Here, MRI provides spatially resolved component concentrations at different axial locations during the mixing process. This work documents real-time mixing of highly viscous fluids via distributive mixing with an application to personal care products
A simple and efficient method for predicting protein-protein binding sites.
In this work, we propose a strategy for predicting binding sites by exploiting the characteristic of core and rim regions of binding sites mentioned above, while using simple and well-know pattern recognition techniques.X-meeting 2007
A new photometric technique for the joint selection of star-forming and passive galaxies at 1.4<z<2.5
A simple two color selection based on B-, z-, and K- band photometry is
proposed for culling galaxies at 1.4<z<2.5 in K-selected samples and
classifying them as star-forming or passive systems. The method is calibrated
on the highly complete spectroscopic redshift database of the K20 survey,
verified with simulations and tested on other datasets. Requiring
BzK=(z-K)-(B-z)>-0.2 (AB) allows to select actively star-forming galaxies at
z>1.4, independently on their dust reddening. Instead, objects with BzK<-0.2
and (z-K)>2.5 (AB) colors include passively evolving galaxies at z>1.4, often
with spheroidal morphologies. Simple recipes to estimate the reddening, SFRs
and masses of BzK-selected galaxies are derived, and calibrated on K<20
galaxies. Based on their UV (reddening-corrected), X-ray and radio
luminosities, the BzK-selected star-forming galaxies with K<20 turn out to have
average SFR ~ 200 Msun yr^-1, and median reddening E(B-V)~0.4. Besides missing
the passively evolving galaxies, the UV selection appears to miss some relevant
fraction of the z~2 star-forming galaxies with K<20, and hence of the
(obscured) star-formation rate density at this redshift. The high SFRs and
masses add to other existing evidence that these z=2 star-forming galaxies may
be among the precursors of z=0 early-type galaxies. Theoretical models cannot
reproduce simultaneously the space density of both passively evolving and
highly star-forming galaxies at z=2. In view of Spitzer Space Telescope
observations, an analogous technique based on the RJL photometry is proposed to
complement the BzK selection and to identify massive galaxies at 2.5<z<4.0.
These color criteria should help in completing the census of the stellar mass
and of the star-formation rate density at high redshift (abridged).Comment: 19 pages, 17 figures, to appear on ApJ (20 December 2004 issue
The Fourth Positive System of Carbon Monoxide in the Hubble Space Telescope Spectra of Comets
The rich structure of the Fourth Positive System (A-X) of carbon monoxide
accounts for many of the spectral features seen in long slit HST-STIS
observations of comets 153P/Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM1
(LINEAR), as well as in the HST-GHRS spectrum of comet C/1996 B2 Hyakutake. A
detailed CO fluorescence model is developed to derive the CO abundances in
these comets by simultaneously fitting all of the observed A-X bands. The model
includes the latest values for the oscillator strengths and state parameters,
and accounts for optical depth effects due to line overlap and self-absorption.
The model fits yield radial profiles of CO column density that are consistent
with a predominantly native source for all the comets observed by STIS. The
derived CO abundances relative to water in these comets span a wide range, from
0.44% for C/2000 WM1 (LINEAR), 7.2% for 153P/Ikeya-Zhang, 8.8% for C/2001 Q4
(NEAT) to 20.9% for C/1996 B2 (Hyakutake). The subtraction of the CO spectral
features using this model leads to the first identification of a molecular
hydrogen line pumped by solar HI Lyman-beta longward of 1200A in the spectrum
of comet 153P/Ikeya-Zhang. (Abridged)Comment: 12 pages, 11 figures, ApJ accepte
Evolution of the Dust Coma in Comet 67P/Churyumov-Gerasimenko Before 2009 Perihelion
Comet 67P/Churyumov-Gerasimenko is the main target of ESA's Rosetta mission
and will be encountered in May 2014. As the spacecraft shall be in orbit the
comet nucleus before and after release of the lander {\it Philae}, it is
necessary necessary to know the conditions in the coma. Study the dust
environment, including the dust production rate and its variations along its
preperihelion orbit. The comet was observed during its approach to the Sun on
four epochs between early-June 2008 and mid-January 2009, over a large range of
heliocentric distances that will be covered by the mission in 2014. An
anomalous enhancement of the coma dust density was measured towards the comet
nucleus. The scalelength of this enhancement increased with decreasing
heliocentric distance of the comet. This is interpreted as a result of an
unusually slow expansion of the dust coma. Assuming a spherical symmetric coma,
the average amount of dust as well as its ejection velocity have been derived.
The latter increases exponentially with decreasing heliocentric distance (\rh),
ranging from about 1 m/s at 3 AU to about 25-35 m/s at 1.4 AU. Based on these
results we describe the dust environment at those nucleocentric distances at
which the spacecraft will presumably be in orbit.
Astronomy and Astrophysics, in pressComment: 5 pages, 4 figure
The contribution of geomagnetic observatories and magnetic models to the study of secular variation and jerks in Antarctica
Some of the most interesting features of the geomagnetic field and its time variations are displayed in polar
areas. Observatory monthly means usually provide an excellent opportunity to study the temporal changes of the
magnetic field at a given location. Unfortunately, on the Antarctic continent the distribution of the permanent ground-
based observatories does not permit a uniform coverage of the examined area. Furthermore, the magnetic records are
characterized by intense external disturbances and noise that make the analysis of the magnetic field difficult. To improve our knowledge of the secular variation and detect the presence of secular variation impulses (geomagnetic
jerks) in Antarctica, we use both observatory data and the CM4 quiet time magnetic field model. In particular CM4
improves our knowledge of geomagnetic jerks over Antarctica through the study of the sign changes of the secular
acceleration maps
Hubble Space Telescope Imaging in the Chandra Deep Field South: III. Quantitative Morphology of the 1Ms Chandra Counterparts and Comparison with the Field Population
We present quantitative morphological analyses of 37 HST/WFPC2 counterparts
of X-ray sources in the 1 Ms Chandra Deep Field-South (CDFS). We investigate:
1) 1-D surface brightness profiles via isophotal ellipse fitting; 2) 2-D, PSF-
convolved, bulge+disk+nucleus profile-fitting; 3) asymmetry and concentration
indices compared with all ~3000 sources in our three WFPC2 fields; and 4) near-
neighbor analyses comparing local environments of X-ray sources versus the
field control sample. Significant nuclear point-source optical components
appear in roughly half of the resolved HST/WFPC2 counterparts, showing a narrow
range of F_X/F_{opt,nuc} consistent with the several HST-unresolved X-ray
sources (putative type-1 AGN) in our fields. We infer roughly half of the
HST/WFPC2 counterparts host unobscured AGN, which suggests no steep decline in
the type-1/type-2 ratio out to the redshifts z~0.5-1 typical of our sources.
The concentration indices of the CDFS counterparts are clearly larger on
average than those of the field distribution, at 5-sigma, suggesting that the
strong correlation between central black hole mass and host galaxy properties
(including concentration index) observed in nearby galaxies is already evident
by z~0.5-1. By contrast, the asymmetry index distribution of the 21 resolved
CDFS sources at I<23 is indistinguishable from the I<23 field. Moreover, the
frequency of I<23 near neighbors around the CDFS counterparts is not
significantly different from the field sample. These results, combined with
previous similar findings for local samples, suggest that recent merger/
interaction history is not a good indicator of AGN activity over a substantial
range of look-back time.Comment: 30 pages, incl. 8 figures; accepted for publication in the
Astrophysical Journa
- âŠ