1,222 research outputs found
RGS9, a GTPase Accelerator for Phototransduction
AbstractThe rod outer segment phototransduction GAP (GTPase-accelerating protein) has been identified as RGS9, a member of the RGS family of Gα GAPs. RGS9 mRNA expression is specific for photoreceptor cells, and RGS9 protein colocalizes with other phototransduction components to photoreceptor outer segment membranes. The RGS domain of RGS9 accelerates GTP hydrolysis by the visual G protein transducin (Gαt), and this acceleration is enhanced by the γ subunit of the phototransduction effector cGMP phosphodiesterase (PDEγ). These unique properties of RGS9 match those of the rod outer segment GAP and implicate it as a key element in the recovery phase of visual transduction
Near-UV Observations of CS29497-030: New Constraints on Neutron-Capture Nucleosynthesis Processes
Employing spectra obtained with the new Keck I HIRES near-UV sensitive
detector, we have performed a comprehensive chemical composition analysis of
the binary blue metal-poor star CS29497-030. Abundances for 29 elements and
upper limits for an additional seven have been derived, concentrating on
elements largely produced via neutron-capture nucleosynthesis. Included in our
analysis are the two elements that define the termination point of the slow
neutron-capture process, lead and bismuth. We determine an extremely high value
of [Pb/Fe] = +3.65 +/- 0.07 (sigma = 0.13) from three features, supporting the
single-feature result obtained in previous studies. We also detect Bi for the
first time in a metal-poor star. Our derived Bi/Pb ratio is in accord with
those predicted from the most recent FRANEC calculations of the slow
neutron-capture process in low-mass AGB stars. We find that the neutron-capture
elemental abundances of CS29497-030 are best explained by an AGB model that
also includes very significant amounts of pre-enrichment of rapid
neutron-capture process material in the protostellar cloud out of which the
CS29497-030 binary system formed. Thus, CS29497-030 is both an ``r+s'' and
``extrinsic AGB'' star. Furthermore, we find that the mass of the AGB model can
be further constrained by the abundance of the light odd-element [Na/Fe] which
is sensitive to the neutron excess.Comment: 7 pages = 4 + 2 colour encapsulated postscript figures + 1 table; to
appear in ApJ Letters; additional jpeg figure available at
ftp://www.astro.caltech.edu/users/iii/cs2949703
The Radio Recovery of SN 1970G: The Continuing Radio Evolution of SN 1970G
Using the Very Large Array, we have detected radio emission from the site of
SN 1970G in the Sc galaxy M101. These observations are 31 years after the
supernova event, making SN 1970G the longest monitored radio supernova. With
flux densities of 0.12 +/- 0.020 mJy at 6 cm and 0.16 +/- 0.015 mJy at 20 cm,
the spectral index of -0.24 +/- 0.20 appears to have flattened somewhat when
compared with the previously reported value of -0.56 +/- 0.11, taken in 1990.
The radio emission at 20 cm has decayed since the 1990 observations with a
power-law index of beta_20cm = -0.28 +/- 0.13. We discuss the radio properties
of this source and compare them to those of other Type II radio supernovae.Comment: 11 pages, 1 table and 2 figures; To appear in Astrophysical Journal
Letter
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The Abundances Of Neutron-Capture Species In The Very Metal-Poor Globular Cluster M15: A Uniform Analysis Of Red Giant Branch And Red Horizontal Branch Stars
The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of Delta(RHB-RGB) approximate to 0.1 dex in the iron abundance was found. The anti-correlative behavior of the light neutron-capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the s- and r-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the s-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.National Science Foundation AST 07-07447, AST 09-08978Astronom
Probing the Neutron-Capture Nucleosynthesis History of Galactic Matter
The heavy elements formed by neutron capture processes have an interesting
history from which we can extract useful clues to and constraints upon both the
characteristics of the processes themselves and the star formation and
nucleosynthesis history of Galactic matter. Of particular interest in this
regard are the heavy element compositions of extremely metal-deficient stars.
At metallicities [Fe/H] <= -2.5, the elements in the mass region past barium (A
>= 130-140 have been found (in non carbon-rich stars) to be pure r-process
products. The identification of an environment provided by massive stars and
associated Type II supernovae as an r-process site seems compelling. Increasing
levels of heavy s-process (e.g., barium) enrichment with increasing
metallicity, evident in the abundances of more metal-rich halo stars and disk
stars, reflect the delayed contributions from the low- and intermediate-mass (M
\~ 1-3 Msol) stars that provide the site for the main s-process nucleosynthesis
component during the AGB phase of their evolution. New abundance data in the
mass region 60 <~ A <~ 130 is providing insight into the identity of possible
alternative r-process sites. We review recent observational studies of heavy
element abundances both in low metallicity halo stars and in disk stars,
discuss the observed trends in light of nucleosynthesis theory, and explore
some implications of these results for Galactic chemical evolution,
nucleosynthesis, and nucleocosmochronology.Comment: 47 pages, 2 tables, 11 figures; To appear in PAS
The Nature of SN 1961V
The nature of SN 1961V has been uncertain. Its peculiar optical light curve
and slow expansion velocity are similar to those of super-outbursts of luminous
blue variables (LBVs), but its nonthermal radio spectral index and declining
radio luminosity are consistent with decades-old supernovae (SNe). We have
obtained Hubble Space Telescope STIS images and spectra of the stars in the
vicinity of SN 1961V, and find Object 7 identified by Filippenko et al. to be
the closest to the optical and radio positions of SN 1961V. Object 7 is the
only point source detected in our STIS spectra and only its H-alpha emission is
detected; it cannot be the SN or its remnant because of the absence of
forbidden lines. While the H-alpha line profile of Object 7 is remarkably
similar to that of eta Car, the blue color (similar to an A2Ib supergiant) and
lack of appreciable variability are unlike known post-outburst LBVs. We have
also obtained Very Long Baseline Array (VLBA) observations of SN 1961V at 18
cm. The non-detection of SN 1961V places a lower limit on the size of the
radio-emitting region, 7.6 mas or 0.34 pc, which implies an average expansion
velocity in excess of 4,400 km/s, much higher than the optical expansion
velocity measured in 1961. We conclude the following: (1) A SN occurred in the
vicinity of SN 1961V a few decades ago. (2) If the SN 1961V light maximum
originates from a giant eruption of a massive star, Object 7 is the most
probable candidate for the survivor, but its blue color and lack of significant
variability are different from a post-outburst eta Car. (3) The radio SN and
Object 7 could be physically associated with each other through a binary
system. (4) Object 7 needs to be monitored to determine its nature and
relationship to SN 1961V.Comment: 16 pages, 3 figures, accepted by the Astronomical Journal for the
2004 May issu
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A chemical genetic approach reveals distinct EphB signaling mechanisms during brain development.
EphB receptor tyrosine kinases control multiple steps in nervous system development. However, it remains unclear whether EphBs regulate these different developmental processes directly or indirectly. In addition, given that EphBs signal through multiple mechanisms, it has been challenging to define which signaling functions of EphBs regulate particular developmental events. To address these issues, we engineered triple knock-in mice in which the kinase activity of three neuronally expressed EphBs can be rapidly, reversibly and specifically blocked. We found that the tyrosine kinase activity of EphBs was required for axon guidance in vivo. In contrast, EphB-mediated synaptogenesis occurred normally when the kinase activity of EphBs was inhibited, suggesting that EphBs mediate synapse development by an EphB tyrosine kinase-independent mechanism. Taken together, our data indicate that EphBs control axon guidance and synaptogenesis by distinct mechanisms and provide a new mouse model for dissecting EphB function in development and disease
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