760 research outputs found
Inferring coronal structure from X-ray lightcurves and Doppler shifts: a Chandra study of AB Doradus
The Chandra X-ray observatory monitored the single cool star, AB Doradus,
continuously for a period lasting 88 ksec (1.98 Prot) in 2002 December with the
LETG/HRC-S. The X-ray lightcurve shows rotational modulation, with three peaks
that repeat in two consecutive rotation cycles. These peaks may indicate the
presence of compact emitting regions in the quiescent corona. Centroid shifts
as a function of phase in the strongest line profile, O VIII 18.97 A, indicate
Doppler rotational velocities with a semi-amplitude of 30 +/- 10 km/s. By
taking these diagnostics into account along with constraints on the rotational
broadening of line profiles (provided by archival Chandra HETG Fe XVII and FUSE
Fe XVIII profile) we can construct a simple model of the X-ray corona that
requires two components. One of these components is responsible for 80% of the
X-ray emission, and arises from the pole and/or a homogeneously distributed
corona. The second component consists of two or three compact active regions
that cause modulation in the lightcurve and contribute to the O VIII centroid
shifts. These compact regions account for 16% of the emission and are located
near the stellar surface with heights of less than 0.3R*. At least one of the
compact active regions is located in the partially obscured hemisphere of the
inclined star, while one of the other active regions may be located at 40
degrees. High quality X-ray data such as these can test the models of the
coronal magnetic field configuration as inferred from magnetic Zeeman Doppler
imaging.Comment: 28 pages, 11 figures, accepted by Ap
High Resolution X-ray Spectroscopy of the Post-T Tauri Star PZ Tel
We present an analysis of the Chandra High Energy Transmission Grating
Spectrometer observation of the rapidly rotating P_(rot)=0.94 d post T Tauri
(~20 Myr old) star PZ Telescopii, in the Tucana association. Using two
different methods we have derived the coronal emission measure distribution,
em(T), and chemical abundances. The em(T) peaks at log T = 6.9 and exhibits a
significant emission measure at temperatures log T > 7. The coronal abundances
are generally ~0.5 times the solar photospheric values that are presumed fairly
representative of the composition of the underlying star. A minimum in
abundance is seen at a first ionization potential (FIP) of 7-8 eV, with
evidence for higher abundances at both lower and higher FIP, similar to
patterns seen in other active stars. From an analysis of the He-like triplet of
Mg XI we have estimated electron densities of ~10^(12)-10^(13) cm^(-3). All the
coronal properties found for PZ Tel are much more similar to those of AB Dor,
which is slightly older than PZ Tel, than to those of the younger T Tauri star
TW Hya. These results support earlier conclusions that the soft X-ray emission
of TW Hya is likely dominated by accretion activity rather than by a
magnetically-heated corona. Our results also suggest that the coronae of
pre-main sequence stars rapidly become similar to those of older active
main-sequence stars soon after the accretion stage has ended.Comment: 15 pages, 8 Postscript figures, accepted for publication in
Astrophysical Journa
ESR, raman and conductivity studies on fractionated poly(2-methoxyaniline-5-sulfonic acid)
Synthesis methods used to produce poly(2-methoxyaniline-5-sulfonic acid) (PMAS), a water soluble, self-doped conducting polymer, have been shown to form two distinctly different polymer fractions with molecular weights of approximately 2 kDa and 8 -10 kDa. The low molecular weight (LMWT) PMAS fraction is redox inactive and non-conducting while the high molecular weight (HMWT) PMAS is electro-active with electrical conductivities of 0.94 0.05 S cm-1. Previous investigations have illustrated the different photochemical and electrochemical properties of these fractions, but have not correlated these properties with the structural and electronic interactions that drive them. Incomplete purification of the PMAS mixture, typically via bag dialysis, has been shown to result in a mixture of approximately 50:50 HMWT:LMWT PMAS with electrical conductivity significantly lower at approximately 0.10 to 0.26 S cm-1. The difference between the electrical conductivities of these fractions has been investigated by the controlled addition of the non-conducting LMWT PMAS fraction into the HMWT PMAS composite film with the subsequent electronic properties investigated by solid-state ESR and Raman spectroscopies. These studies illustrate strong electronic intereactions of the insulating LMWT PMAS with the emeraldine salt HMWT PMAS to substantially alter the population of the electronic charge carriers in the conducting polymer. ESR studies on these mixtures, when compared to HMWT PMAS, exhibited a lower level of electron spin in the presence of LMWT PMAS indicative of the the formation of low spin bipolarons without a change the oxidation state of the conducting HMWT fraction
Prepubertal exposure to arsenic(III) suppresses circulating insulin-like growth factor-1 (IGF-1) delaying sexual maturation in female rats
Arsenic (As) is a prevalent environmental toxin readily accessible for human consumption and has been identified as an endocrine disruptor. However, it is not known what impact As has on female sexual maturation. Therefore, in the present study, we investigated the effects of prepubertal exposure on mammary gland development and pubertal onset in female rats. Results showed that prepubertal exposure to 10 mg/kg of arsenite (As(III)) delayed vaginal opening (VO) and prepubertal mammary gland maturation. We determined that As accumulates in the liver, disrupts hepatocyte function and suppresses serum levels of the puberty related hormone insulin-like growth factor 1 (IGF-1) in prepubertal animals. Overall, this is the first study to show that prepubertal exposure to As(III) acts peripherally to suppress circulating levels of IGF-1 resulting in delayed sexual maturation. Furthermore, this study identifies a critical window of increased susceptibility to As(III) that may have a lasting impact on female reproductive function
Honesty mediates the relationship between serotonin and reaction to unfairness
How does one deal with unfair behaviors? This subject has long been investigated by various disciplines including philosophy, psychology, economics, and biology. However, our reactions to unfairness differ from one individual to another. Experimental economics studies using the ultimatum game (UG), in which players must decide whether to accept or reject fair or unfair offers, have also shown that there are substantial individual differences in reaction to unfairness. However, little is known about psychological as well as neurobiological mechanisms of this observation. We combined a molecular imaging technique, an economics game, and a personality inventory to elucidate the neurobiological mechanism of heterogeneous reactions to unfairness. Contrary to the common belief that aggressive personalities (impulsivity or hostility) are related to the high rejection rate of unfair offers in UG, we found that individuals with apparently peaceful personalities (straightforwardness and trust) rejected more often and were engaged in personally costly forms of retaliation. Furthermore, individuals with a low level of serotonin transporters in the dorsal raphe nucleus (DRN) are honest and trustful, and thus cannot tolerate unfairness, being candid in expressing their frustrations. In other words, higher central serotonin transmission might allow us to behave adroitly and opportunistically, being good at playing games while pursuing self-interest. We provide unique neurobiological evidence to account for individual differences of reaction to unfairness
High-Resolution Spectroscopy of Some Very Active Southern Stars
We have obtained high-resolution echelle spectra of 18 solar-type stars that
an earlier survey showed to have very high levels of Ca II H and K emission.
Most of these stars belong to close binary systems, but 5 remain as probable
single stars or well-separated binaries that are younger than the Pleiades on
the basis of their lithium abundances and H-alpha emission. Three of these
probable single stars also lie more than 1 magnitude above the main sequence in
a color-magnitude diagram, and appear to have ages of 10 to 15 Myr. Two of
them, HD 202917 and HD 222259, also appear to have a kinematical association
with the pre-main sequence multiple system HD 98800.Comment: 25 figures, 3 table
On the dynamics of the ABDoradus system
We present an astrometric analysis of the binary systems ABDorA /ABDorC and
ABDorBa / ABDorBb. These two systems of well-known late-type stars are
gravitationally associated and they constitute the quadruple ABDoradus system.
From the astrometric data available at different wavelengths, we report: (i)
a determination of the orbit of ABDorC, the very low mass companion to ABDorA,
which confirms the mass estimate of 0.090Msun reported in previous works; (ii)
a measurement of the parallax of ABDorBa, which unambiguously confirms the
long-suspected physical association between this star and ABDorA; and (iii)
evidence of orbital motion of ABDorBa around ABDorA, which places an upper
bound of 0.4Msun on the mass of the pair ABDorBa / ABDorBb (50% probability).
Further astrometric monitoring of the system at all possible wavelengths would
determine with extraordinary precision the dynamical mass of its four
components.Comment: 7 pages, 4 figure
Molecular evolution of HoxA13 and the multiple origins of limbless morphologies in amphibians and reptiles
Developmental processes and their results, morphological characters, are inherited through transmission of genes regulating development. While there is ample evidence that cis-regulatory elements tend to be modular, with sequence segments dedicated to different roles, the situation for proteins is less clear, being particularly complex for transcription factors with multiple functions. Some motifs mediating protein-protein interactions may be exclusive to particular developmental roles, but it is also possible that motifs are mostly shared among different processes. Here we focus on HoxA13, a protein essential for limb development. We asked whether the HoxA13 amino acid sequence evolved similarly in three limbless clades: Gymnophiona, Amphisbaenia and Serpentes. We explored variation in ω (dN/dS) using a maximum-likelihood framework and HoxA13sequences from 47 species. Comparisons of evolutionary models provided low ω global values and no evidence that HoxA13 experienced relaxed selection in limbless clades. Branch-site models failed to detect evidence for positive selection acting on any site along branches of Amphisbaena and Gymnophiona, while three sites were identified in Serpentes. Examination of alignments did not reveal consistent sequence differences between limbed and limbless species. We conclude that HoxA13 has no modules exclusive to limb development, which may be explained by its involvement in multiple developmental processes
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