archiv~1.txt: SETI SCIENCE-WEEK EXPRESS March 19, 1999


Larry Klaes ( )
Wed, 17 Mar 1999 08:12:37 -0500

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>An Abridged Version of SCIENCE-WEEK
>A Weekly Email Digest of the News of Science
>A journal devoted to the improvement of communication
>between the scientific disciplines, and between scientists,
>science educators, and science policy makers.
>At the last dim horizon, we search among ghostly
>errors of observations for landmarks that are
>scarcely more substantial. The search will continue.
>The urge is older than history. It is not satisfied
>and it will not be oppressed.
>-- Edwin Hubble (1889-1953)
>Contents of this Issue of SCIENCE-WEEK EXPRESS:
>March 19, 1999
>-- On Brown Dwarf Stars
>-- Optical Rotation and Molecular Chirality
>(see below for contents of current Full-Text SCIENCE-WEEK)
>FYI: A new book page has been introduced at the SW website, the
>page an annotated list of selected titles of interest in science
>and technology. The titles on this frequently updated list are
>selected by the Editors of SW. The URL of the page at the SW
>website is: []. -- The Editors.
>Brown dwarf stars are formed by the contraction of a lump of gas
>with a mass too small for nuclear reactions to begin in the core
>[*Note #1]. Such a star has a relatively short-lived luminosity
>(with estimates ranging from approximately 100 million years to
>several billion years) as the result of conversion of
>gravitational energy to radiation. The surface temperature of a
>brown dwarf is estimated to range from below 2500 degrees kelvin
>to less than 1000 degrees kelvin. As recently as 1994, brown
>dwarf stars were "theoretical" stars, with no brown dwarf stars
>considered to be unambiguously observed, but in recent years a
>number of brown dwarf stars have been identified.
>... ... C.G. Tinney (Anglo-Australian Observatory Epping, AU)
>presents a review of recent observations of brown dwarf stars,
>the author making the following points: 1) Most stars spend most
>of their lives in a state of pressure balance maintained between
>gravitational contraction and the energy generated by nuclear
>reactions. In 1963, Kumar suggested there may exist a class of
>star-like bodies with masses too low to create the central
>temperature and densities required to ignite nuclear fusion
>reactions. These "failed stars" became known to astronomers as
>"brown dwarfs". 2) The lowest-mass ordinary stars can
>theoretically maintain a quasi-equilibrium luminosity for almost
>6000 billion years. Brown dwarf stars, in contrast, are expected
>to fade throughout their lifetime, cooling to temperatures below
>1000 degrees kelvin and becoming undetectable by direct
>observation after just a few billion years. This has engendered
>considerable interest in brown dwarf stars as possible candidates
>for the *dark matter which apparently composes more than 90
>percent of the mass our Galaxy. 3) The past 4 years have seen
>success finally achieved in the hunt for brown dwarf stars. These
>detections have confirmed predictions that both methane and dust
>play an important role in determining the spectral behavior of
>these objects. But the detection of brown dwarf stars in
>significant numbers, when combined with results for the space
>density of low-mass stars and *gravitational microlensing
>results, allows us to conclude that brown dwarf stars do not make
>a significant contribution to the dark matter of our Galaxy. The
>author concludes: "No matter how nicely brown dwarfs would solve
>the *baryonic dark matter problem, it appears we must look
>elsewhere for a solution to this long-standing astronomical
>Editor's note: In addition to the background material below, see
>SW Focus Reports "The Birth of Galaxies and Stars" and
>"Cosmology: Dark Matter", both available at URL
>C.G. Tinney: Brown Dwarfs: The stars that failed.
>(Nature 7 Jan 99 397:37)
>QY: C.G. Tinney, Anglo-Australian Observatory, PO Box 296, Epping
>NSW 1710, AU.
>Text Notes:
>... ... *Note #1: Present theoretical models predict a lower
>mass-limit for fusion burning stars with the same element mix as
>the Sun of 0.07 solar-mass, equivalent to 74 times the mass of
>... ... *dark matter: In general, in this context, the term "dark
>matter" refers to material whose presence can be inferred from
>its effects on the motions of stars and galaxies, but which
>cannot be seen directly because it emits little or no radiation.
>It is believed that at least 90 percent of the mass in the
>Universe exists as some form or dark matter.
>... ... *gravitational microlensing: Gravitational lensing is the
>bending of light and other radiation by a massive gravitational
>entity such as a star, a black hole, a galaxy, or a cluster of
>galaxies. The effect is predicted by Einstein's theory of
>relativity and was first detected during a total solar eclipse by
>Eddington in 1919. Large-scale gravitational lensing causes
>multiple images of an object, the type and arrangement of the
>images determined by the specifics of the lensing entity.
>Gravitational "Microlensing" is a small-scale lensing effect, the
>gravitational field of the lensing object not strong enough to
>form distinct images of the background source, but instead
>causing an apparent brightening of the source. Stars are expected
>to vary in brightness in a characteristic manner if low-mass
>stars pass in front of them.
>... ... *baryonic dark matter: Ordinary matter too dim to be
>observed. A baryon is a nuclear particle, e.g., a proton, built
>from 3 quarks (fundamental particles that combine to make up
>protons, neutrons, and mesons).
>Summary & Notes by SCIENCE-WEEK [] 19Mar99
>Related Background:
>... C.A. Griffith et al (3 authors at 3 installations, US) now
>report observations of the brown dwarf star Gliese 229B, which
>exhibits certain unique characteristics. At 900 degrees kelvin,
>the atmosphere of this object is too warm to contain ice clouds
>like those on Jupiter and too cool to contain silicate clouds
>like those on low-mass stars. These unique conditions (high
>gravity and the lack of high clouds) permit spectroscopic
>visibility of the atmosphere down to higher pressures (i.e.,
>closer to the surface) than possible in cool stars or planets.
>The authors investigated the structure of the atmosphere of
>Gliese 229B by analyzing its optical spectrum in the interval
>0.85 to 1.0 micron, the spectrum obtained at the *Keck 1
>telescope. The authors report that the spectrum of Gliese 229B
>indicates deep-atmosphere particulate matter with the optical
>properties of neither ice nor silicates. The authors suggest the
>reddish color of the particles indicates an organic composition
>characteristic of aerosols in planetary stratospheres, and that
>the *mass fraction of the particles agrees with a photochemical
>origin involving incident radiation from its companion primary
>star (Gliese 229A).
>C.A. Griffith et al: The dusty atmosphere of the brown dwarf
>Gliese 229B.
>(Science 11 Dec 98 282:2063)
>QY: Caitlin A. Griffith, Northern Arizona University 520-523-5511
>Text Notes:
>... ... *Keck 1 telescope: The Keck telescopes are a pair of twin
>telescopes at the W. M. Keck Observatory on Mauna Kea, HI US,
>each with a 10 meter mirror, the pair constructed 1992-1996. The
>installation is managed by the University of California (US) and
>the California Institute of Technology (US).
>... ... *mass fraction: The mass fraction of aerosols is related
>to the *eddy diffusion coefficient k, the mass density of the
>atmosphere d, the net mass flux f, and the scale height of the
>atmosphere h according to F = fh/kd.
>... ... *eddy diffusion coefficient: (turbulent diffusion
>coefficient) The exchange coefficient for the diffusion of a
>conserved property by eddies in a turbulent flow. In general, an
>"eddy" is a vortex-like motion of fluid running contrary to the
>main current.
>Summary & Notes by SCIENCE-WEEK [] 12Feb99
>Related Background:
>Filipe D. Santos (Centro de Fisica da Universidade de Lisboa, PT)
>presents a short review of current ideas concerning giant
>extrasolar planets and *brown dwarf stars. The author makes the
>following points: 1) The recent discoveries of planets orbiting
>nearby Sun-like stars have revealed that planetary systems can be
>surprisingly diverse. The initial discovery in 1995 of the planet
>around the star 51 Pegasi was a surprise because it is apparently
>a planet with mass about that of Jupiter (at least 0.44 Jupiter-
>mass) and an orbital period of only 4.2 days, which implies that
>the planet is 20 times closer to its star than Earth is to the
>sun. 2) Seven additional planets around solar-type stars have
>since been discovered, with Jupiter-mass values ranging from 0.44
>to 6.84. 3) Two critical questions are, a) Where should we set
>the dividing line that distinguishes massive planets from brown
>dwarfs? and, b) What are the mechanisms leading to the formation
>of massive planets and brown dwarfs? 4) Brown dwarfs are expected
>to have masses smaller than the hydrogen-burning limit of
>approximately 0.075 solar-mass (approximately 75 Jupiter-mass),
>but probably larger than the deuterium-burning limit of 0.013
>solar-mass (approximately 13 Jupiter-mass). 5) Like the companion
>massive planets mentioned, several companion brown dwarfs to
>solar-type stars have also been identified. One method of
>investigating brown dwarfs involves *astrometric measurements,
>and in all cases of brown dwarfs investigated by the astrometric
>method, the masses are above or very close to the hydrogen-
>burning limit. The extant data thus suggest that the distribution
>of mass of brown dwarfs does not extend to masses as small as
>giant planets. Also, the new measurements indicate that brown
>dwarfs orbiting solar-type stars are very rare. 6) The discovery
>of Jupiter-mass planets with orbits very close to their stars
>poses a considerable problem, because it is difficult to
>understand how such planets could form in place. (Five known
>Jupiter-mass planets have orbital radii smaller than the distance
>from Mercury to the Sun.) The suggestion has been made that these
>planets formed at larger distances and migrated inward, but the
>proposed migration mechanisms are not yet empirically
>distinguishable. The author concludes: "Clearly the discovery of
>planetary systems outside our solar system has opened a Pandora's
>box of startling phenomena and new questions."
>QY: Filipe D. Santos []
>(Science 17 Jul 98 281:359) (Science-Week 31 Jul 98)
>Text Notes:
>... ... *brown dwarf stars: See previous report.
>... ... *astrometric measurements: This method of detection
>infers the presence of a companion to a star by measuring the
>position of the star as it orbits the center of mass of the
>entire system. From the orbital inclination, the real mass of the
>companion can be derived.
>Summary & Notes by SCIENCE-WEEK [] 31Jul99
>Related Background:
>Joel R. Primack (University of California Santa Cruz, US)
>presents a commentary on a paper by E. Gawiser and J. Silk
>(University of California Berkeley, US) ((Science 29 May 98
>280:1405), Primack making the following points: 1) One of the
>fundamental issues facing cosmologists concerns the evidence that
>observable matter in the universe makes up only a fraction of
>what is needed to explain the properties of the universe. A large
>portion of matter in the universe must therefore be unobserved,
>or "dark matter". 2) In current cosmology, "hot" dark matter is
>defined as particles that were still moving at nearly the speed
>of light at about a year after the big bang. "Cold" dark matter
>is defined as particles that were moving sluggishly at that time.
>Neutrinos are the standard example of hot dark matter, although
>other more exotic possibilities have been discussed. 3) Gawiser
>and Silk (ref. cited above) conclude that of all the currently
>popular cosmological models, the only one whose predictions agree
>with the data on the cosmic microwave background anisotropies and
>the large-scale distribution of galaxies is the cold + hot dark
>matter model, with 70% of the matter cold dark, 20% hot dark, and
>10% ordinary matter (baryonic). 3) There are 3 species of
>neutrinos, and there are mounting astrophysical and laboratory
>data suggesting that neutrinos oscillate from one species to
>another, which can only happen if they have nonzero mass. As
>dark-matter candidates, neutrinos are entities with masses that
>may be 10^(-5) of the mass of the electron, but with an expected
>density more than 8 orders of magnitude greater than the density
>of electrons and protons in the universe. Neutrinos, therefore,
>can provide a substantial fraction of dark matter. 4) The success
>of the cold + hot dark matter model in fitting the cosmic
>microwave background and galaxy distribution data indicates that
>this type of model should be investigated in more detail.
>QY: Joel R. Primack []
>(Science 29 May 98 280:1398) (Science-Week 19 Jun 98)
>In chemistry, chirality is a property of certain asymmetric
>molecules, the property being that the mirror images of the
>molecules cannot be superimposed one on the other while facing in
>the same direction. Chiral molecules are characterized by a
>specific rotation angle, the angle through which *plane-polarized
>light is rotated on passing through an *enantiomerically enriched
>solution. Recent developments in methodology allow computation of
>both the sign and magnitude of these rotation angles, but what
>remains elusive is a general strategy for assigning the
>individual contributions that atoms and functional groups make to
>the optical rotation angle and to the molecular chirality in
>general. Linking the rotation angle to the molecular structure is
>a challenge of fundamental as well as practical importance. For
>example, the chemical bonding pattern is often known for natural
>and synthetic products of potential use in medicine, but
>determination of the absolute handedness of every chiral center
>can be a formidable challenge. When N *stereocenters are present,
>there are 2^(N) possible *stereoisomers for a structure.
>... ... R.K. Kondru et al (University of Pittsburgh, US) present
>a quantitative method to dissect the optical rotation angle into
>its individual atomic contributions. The authors suggest this
>atomic mapping protocol provides a foundation for establishing
>fundamental relations between chemical structure and optical
>rotation angles in molecules and links modern quantitatively
>reliable computation to numerous empirical models developed over
>the past 100 years. The authors also suggest that atomic analyses
>of optical rotation may help to establish new quantitative
>definitions of molecular asymmetry and the nature of its
>propagation through bonds and through space.
>R.K. Kondru et al: Atomic contributions to the optical rotation
>angle as a quantitative probe of molecular chirality.
>(Science 18 Dec 98 282:2247)
>QY: Rama K. Kondru, Department of Chemistry, University of
>Pittsburgh, Pittsburgh, PA 15260 US.
>Text Notes:
>... ... *plane-polarized light: Electromagnetic radiation
>involves the propagation of both electric and magnetic forces,
>and at each point in a light beam, there is a component electric
>field and a component magnetic field, both of which oscillate in
>all directions perpendicular to each other and to the direction
>in which the beam is propagated. In plane-polarized light, the
>component electric field oscillates as in ordinary light except
>that the direction of oscillation is contained within a plane.
>Likewise, in plane-polarized light, the component magnetic field
>oscillates within a plane, the planes in question being
>perpendicular. Circularly polarized light has a component
>electric field that varies in direction but not in magnitude, so
>that the field traverses a spiral path in either a clockwise or
>counterclockwise direction.
>... ... *enantiomerically enriched solution: In chemistry, an
>enantiomer is a compound whose structure is not superimposable on
>its mirror image, the compound being one of a pair of optical
>isomers, each of which interacts differently with polarized light
>(i.e., shows optical activity). A mixture of two optical isomers
>in equal amounts is called a racemic mixture, and racemic
>mixtures do not show optical activity. A reactant or process that
>produces or selects an enantiomeric excess is simply a reactant
>or process that produces or selects one enantiomer in excess over
>the other enantiomer. The phrase "enantiomerically enriched
>solution" thus refers to a solution with an excess of one
>enantiomer over another.
>... ... *stereocenters: (chiral center) Any atom (in a molecule)
>attached to 4 different groups.
>... ... *stereoisomers: In general, stereoisomers are compounds
>whose molecules have the same number and kinds of atoms and the
>same atomic arrangement, but whose molecules differ in their
>spatial arrangements.
>Summary & Notes by SCIENCE-WEEK [] 19Mar99
>Related Background:
>Chirality is a property of certain asymmetric molecules such that
>their mirror images cannot be superimposed. Such molecules
>usually exist in two mirror image forms (enantiomorphs), and in
>processes involving the binding of such molecules with receptors,
>it is usually only one of the forms that binds. For this reason,
>and other reasons, laboratory separation methods for enantio-
>morphs are of great interest. This week Brinda B. Lakshmi and
>Charles R. Martin (Colorado State University, US) report a method
>of chiral separation involving the embedding of apoenzymes in
>porous polymer membranes. An apoenzyme is an enzyme with its co-
>factor removed (and therefore its catalytic action obliterated).
>The essential idea is that the apoenzyme still binds to one
>enantiomorph, but now without altering it, and by proper
>arrangement of the apoenzyme in the pores of the polymer
>membrane, the binding enantiomorph diffuses through the membrane,
>and the separation of the enantiomorphs can be effected.
>QY: Charles R. Martin <>
>(Nature 21 August) (Science-Report 29 Aug 97)
>Related Background:
>In chemistry, chirality is a property of certain asymmetric
>molecules, the property being that the mirror images of the
>molecules cannot be superimposed one on the other while facing in
>the same direction. A pair of gloves, for example, has this
>property. The two forms of the molecule are called enantiomorphs,
>and beginning chemistry students are often taught that
>enantiomorphs are chemically identical. This may be true in
>ordinary solutions, but it is definitely not true when the
>reaction of significance involves, for example, the binding of
>one enantiomorph to a receptor which itself exhibits chirality.
>In that case, one enantiomorph binds and the other does not, a
>situation quite prevalent in biological systems, where any carbon
>atom with four different bonds (and there are many) exhibits
>chirality. One of the interesting properties of molecules that
>possess chirality is that the enantiomorphs are optically active,
>rotating the plane of polarized light equally but in opposite
>directions, and this optical activity is often used as a means of
>analysis. Recently, Takuzo Aida et al (University of Tokyo, JP)
>reported the synthesis of organic molecules with what they call
>"chirality memory". The group of compounds are derivatives of
>porphyrin, an interesting molecule having a fully conjugated
>cyclic structure of four pyrrole rings. Although porphyrin itself
>does not exist in nature, the porphyrin ring system is found in
>several quite important natural products such as hemoglobin and
>chlorophyll. What Aida and his group have done is design a fully
>substituted porphyrin that adopts nonplanar saddle shapes that
>alternately tilt up and down. On its own, this compound exists as
>a mixture of the two possible conformations, as many tilted one
>way as the other way. The essence of this little story is that
>when an optically active reactant is introduced, all of this new
>porphyrin entity flips into one form or the other, depending on
>the reactant (not surprising, so far), but when the optically
>active reactant is replaced by one not optically active, the
>porphyrin molecules retain their flipped configuration,
>remembering the chirality of the previous reactant, and that
>memory can be erased by visible light, and then caused to
>reappear when the light is switched off. The experts in porphyrin
>chemistry are calling this work unique. The idea is that such
>organic molecules might one day form the basis for nanoscale
>optical and electronic devices. Meanwhile, they will be used as
>chirality sensors.
>(J. Amer. Chem. Soc. 119:5267 1997) (Science-Week 25 July 97)
>Contents of the Current Full-Text Issue of SCIENCE-WEEK:
>March 19, 1999 -- Vol. 3 Number 12
>1. On Brown Dwarf Stars
>2. Optical Rotation and Molecular Chirality
>3. Observation of Transient Structures in Chemical Reactions
>4. Electroluminescence in Conjugated Polymers
>5. Extreme Climate Warmth in the Late Cretaceous Period
>6. On Apoptosis Inhibitor Proteins
>-- In Focus: On the Brains of Mice and Humans
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