The motion after-effect: local and global contributions to contrast sensitivity

Motion adaptation is a widespread phenomenon analogous to peripheral sensory adaptation, presumed to play a role in matching responses to prevailing current stimulus parameters and thus to maximize efficiency of motion coding. While several components of motion adaptation (contrast gain reduction, output range reduction and motion after-effect) have been described, previous work is inconclusive as to whether these are separable phenomena and whether they are locally generated. We used intracellular recordings from single horizontal system neurons in the fly to test the effect of local adaptation on the full contrast-response function for stimuli at an unadapted location. We show that contrast gain and output range reductions are primarily local phenomena and are probably associated with spatially distinct synaptic changes, while the antagonistic after-potential operates globally by transferring to previously unadapted locations. Using noise analysis and signal processing techniques to remove ‘spikelets’, we also characterize a previously undescribed alternating current component of adaptation that can explain several phenomena observed in earlier studies

Particle Physics Probes Of Extra Spacetime Dimensions

The possibility that spacetime is extended beyond the familiar 3+1-dimensions has intrigued physicists for a century. Indeed, the consequences of a dimensionally richer spacetime would be profound. Recently, new theories with higher dimensional spacetimes have been developed to resolve the hierarchy problem in particle physics. These scenarios make distinct predictions which allow for experiment to probe the existence of extra dimensions in new ways. We review the conceptual framework of these scenarios, their implications in collider and short-range gravity experiments, their astrophysical and cosmological effects, as well as the constraints placed on these models from present data.Comment: Submitted to Annual Review of Nuclear and Particle Science, 29 page

See a Black Hole on a Shoestring

The modes of vibration of hanging and partially supported strings provide useful analogies to scalar fields travelling through spacetimes that admit conformally flat spatial sections. This wide class of spacetimes includes static, spherically symmetric spacetimes. The modes of a spacetime where the scale factor depends as a power-law on one of the coordinates provide a useful starting point and yield a new classification of these spacetimes on the basis of the shape of the string analogue. The family of corresponding strings follow a family of curves related to the cycloid, denoted here as hypercycloids (for reasons that will become apparent). Like the spacetimes that they emulate these strings exhibit horizons, typically at their bottommost points where the string tension vanishes; therefore, hanging strings may provide a new avenue for the exploration of the quantum mechanics of horizons.Comment: 5 pages, 1 figure, extensive changes to refect version accepted to PR

The role of binaries in the enrichment of the early Galactic halo. II. Carbon-Enhanced Metal-Poor Stars - CEMP-no stars

The detailed composition of most metal-poor halo stars has been found to be very uniform. However, a fraction of 20-70% (increasing with decreasing metallicity) exhibit dramatic enhancements in their abundances of carbon - the so-called carbon-enhanced metal-poor (CEMP) stars. A key question for Galactic chemical evolution models is whether this non-standard composition reflects that of the stellar natal clouds, or is due to local, post-birth mass transfer of chemically processed material from a binary companion; CEMP stars should then all be members of binary systems. Our aim is to determine the frequency and orbital parameters of binaries among CEMP stars with and without over-abundances of neutron-capture elements - CEMP-s and CEMP-no stars, respectively - as a test of this local mass-transfer scenario. This paper discusses a sample of 24 CEMP-no stars, while a subsequent paper will consider a similar sample of CEMP-s stars. Most programme stars exhibit no statistically significant radial-velocit variation over this period and appear to be single, while four are found to be binaries with orbital periods of 300-2,000 days and normal eccentricity; the binary frequency for the sample is 17+-9%. The single stars mostly belong to the recently-identified ``low-C band'', while the binaries have higher absolute carbon abundances. We conclude that the nucleosynthetic process responsible for the strong carbon excess in these ancient stars is unrelated to their binary status; the carbon was imprinted on their natal molecular clouds in the early Galactic ISM by an even earlier, external source, strongly indicating that the CEMP-no stars are likely bona fide second-generation stars. We discuss potential production sites for carbon and its transfer across interstellar distances in the early ISM, and implications for the composition of high-redshift DLA systems. Abridged.Comment: 16 pages, 5 figures, accepted for publication in Astronomy and Astrophysic

Current driven magnetization dynamics in helical spin density waves

A mechanism is proposed for manipulating the magnetic state of a helical spin density wave using a current. In this paper, we show that a current through a bulk system with a helical spin density wave induces a spin transfer torque, giving rise to a rotation of the order parameter.The use of spin transfer torque to manipulate the magnetization in bulk systems does not suffer from the obstacles seen for magnetization reversal using interface spin transfer torque in multilayered systems. We demonstrate the effect by a quantitative calculation of the current induced magnetization dynamics of Erbium. Finally we propose a setup for experimental verification.Comment: In the previous version of this paper was a small numerical mistake made when evaluating equation 3 and 9. The number of digits given in the calculation of the torque current tensor is reduced to better represent the accuracy of the calculation. A slightly modified paper have been published in Phys. Rev. Lett. 96, 256601 (2006) 4 pages 3 figure

Abundances and kinematics of carbon-enhanced metal-poor stars in the Galactic halo*; A new classification scheme based on Sr and Ba

Carbon-enhanced metal-poor (CEMP) stars span a wide range of stellar populations, from bona fide second-generation stars to later forming stars that provide excellent probes of, e.g., binary mass transfer. Here we analyse 11 metal-poor stars of which 10 are CEMP stars. Based on high signal-to-noise (SNR) X-Shooter spectra, we derive abundances of 20 elements (C, N, O, Na, Mg, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Sr, Y, Ba, La, Ce, Pr, Nd, Eu). From the high SNR spectra, we trace the chemical contribution of the rare earth elements (REE) from various production sites, finding a preference for metal-poor low-mass AGB stars of 1.5Mo in CEMP-s stars, while CEMP-r/s stars may indicate a more massive AGB contribution (2-5Mo). A contribution from the r-process - possibly from neutron star mergers (NSM), is also detectable in the REE abundances, especially in the CEMP-r/s. Combining spectra with Gaia DR2 astrometric data indicates that all but one star in our sample (and most literature stars) belong to the Galactic halo. They exhibit a median orbital eccentricity of 0.7, and are found on both pro- and retrograde orbits. The orbital parameters of CEMP-no and CEMP4s stars are remarkably similar in the 98 stars we study. A special CEMP-no star, with very low Sr and Ba content, possesses the most eccentric orbit among the stars in our sample, passing close to the Galactic centre. Finally, we propose an improved scheme to sub-classify the CEMP stars, making use of the Sr$/$Ba ratio, which can also be used to separate very metal-poor stars from CEMP stars in 93 stars in the metallicity range $-4.2<$[Fe/H]$<-2$. The Sr/Ba ratio can also be used for distinguishing CEMP-s,-r/s and -no stars. The Sr/Ba ratio is also a powerful astro-nuclear indicator, as AGB stars exhibit very different Sr/Ba ratios, compared to fast rotating massive stars and NSM, and it is fairly unbiased by NLTE and 3D corrections.(abridged)Comment: 15 pages, 4 pages appendix, 11 figures, accepted for publication in A&

Electrically charged fluids with pressure in Newtonian gravitation and general relativity in d spacetime dimensions: theorems and results for Weyl type systems

Previous theorems concerning Weyl type systems, including Majumdar-Papapetrou systems, are generalized in two ways, namely, we take these theorems into d spacetime dimensions (${\rm d}\geq4$), and we also consider the very interesting Weyl-Guilfoyle systems, i.e., general relativistic charged fluids with nonzero pressure. In particular within Newton-Coulomb theory of charged gravitating fluids, a theorem by Bonnor (1980) in three-dimensional space is generalized to arbitrary $({\rm d}-1)>3$ space dimensions. Then, we prove a new theorem for charged gravitating fluid systems in which we find the condition that the charge density and the matter density should obey. Within general relativity coupled to charged dust fluids, a theorem by De and Raychaudhuri (1968) in four-dimensional spacetimes in rendered into arbitrary ${\rm d}>4$ dimensions. Then a theorem, new in ${\rm d}=4$ and ${\rm d}>4$ dimensions, for Weyl-Guilfoyle systems, is stated and proved, in which we find the condition that the charge density, the matter density, the pressure, and the electromagnetic energy density should obey. This theorem comprises, as particular cases, a theorem by Gautreau and Hoffman (1973) and results in four dimensions by Guilfoyle (1999). Upon connection of an interior charged solution to an exterior Tangherlini solution (i.e., a Reissner-Nordstr\"om solution in d-dimensions), one is able to give a general definition for gravitational mass for this kind of relativistic systems and find a mass relation with the several quantities of the interior solution. It is also shown that for sources of finite extent the mass is identical to the Tolman mass.Comment: 27 page

Role of hyperpolarization-activated cyclic nucleotide-gated channel HCN2 in embryonic neural stem cell proliferation and differentiation

Funding Information: Supported by grants from the K. Albin Johansson foundation, the Magnus Ehrnrooth foundation, The Finnish Society of Sciences and Letters and the Medical Society of Finland. We are grateful for the laboratory assistance provided by Jarno Hörhä. Publisher Copyright: © 2022 The AuthorsHyperpolarization-activated cyclic nucleotide-gated channels (HCN channels) are involved in spontaneous activity in many electrically active cell types such as cardiomyocytes and neurons. In this study, the role of HCN channels in proliferation and migration of Nestin and Sox2 expressing embryonic neural progenitor cells (NPC) originating from the subventricular zone (SVZ) was examined. Immunostaining and PCR data showed that the HCN2 subtype was highly expressed in these cells. Patch clamp recordings revealed a hyperpolarization-activated current, which was sensitive to inhibitors of HCN channels. Using the fluorescence dye bis-(1,3-dibutylbarbituric acid)-trimethineoxonol (DiBAC(4)(3)) we found that a prompt reduction of the extracellular K+ concentration, or exposing the cells to acute hypoxia, induced an instant hyperpolarization in the whole cell population. Recovery from low K+ induced hyperpolarization after extracellular calcium removal, or by re-oxygenation of hypoxic cells, was sensitive to ZD7288, a HCN channel inhibitor. Treatment of neurosphere cultures from the SVZ with ZD7288 caused a significant and reversible inhibition of neurosphere formation from single cells indicating that proliferation of progenitor cells was reduced. Furthermore, the migration of neuronal cells from neurospheres was considerably retarded in the presence of ZD7288. The results suggest that HCN2 channels are involved in controlling the proliferation of NPC and that HCN2 channel-induced spontaneous electrical activity may trigger the motility response of neurosphere-derived neurons in concert with other ion channels. Furthermore, the response to hypoxia suggests that HCN2 channels may trigger the chemotactic response of NPC to ischemic brain regions seen in many studies.Peer reviewe

Asymptotically cylindrical 7-manifolds of holonomy G_2 with applications to compact irreducible G_2-manifolds

We construct examples of exponentially asymptotically cylindrical Riemannian 7-manifolds with holonomy group equal to G_2. To our knowledge, these are the first such examples. We also obtain exponentially asymptotically cylindrical coassociative calibrated submanifolds. Finally, we apply our results to show that one of the compact G_2-manifolds constructed by Joyce by desingularisation of a flat orbifold T^7/\Gamma can be deformed to one of the compact G_2-manifolds obtainable as a generalized connected sum of two exponentially asymptotically cylindrical SU(3)-manifolds via the method given by the first author (math.DG/0012189).Comment: 36 pages; v2: corrected trivial typos; v3: some arguments corrected and improved; v4: a number of improvements on presentation, paritularly in sections 4 and 6, including an added picture

Lead abundance in the uranium star CS 31082-001

In a previous paper we were able to measure the abundance of uranium and thorium in the very-metal poor halo giant BPS CS 31082-001, but only obtained an upper limit for the abundance of lead (Pb). We have got from ESO 17 hours of additional exposure on this star in order to secure a detection of the minimum amount of lead expected to be present in CS 31082-001, the amount arising from the decay of the original content of Th and U in the star. We report here this successful detection. We find an LTE abundance log(Pb/H)+12=-0.55 \pm 0.15 dex, one dex below the upper limits given by other authors for the similar stars CS 22892-052 and BD +17d3248, also enhanced in r-process elements. From the observed present abundances of Th and U in the star, the expected amount of Pb produced by the decay of 232Th, and 238U alone, over 12-15 Gyr is -0.73\pm 0.17 dex. The decay of 235U is more difficult to estimate, but is probably slightly below the contribution of 238U, making the contribution of the 3 actinides only slightly below, or even equal to, the measured abundance. The contribution from the decay of 234U has was not included, for lack of published data. In this sense our determination is a lower limit to the contribution of actinides to lead production. We comment this result, and we note that if a NLTE analysis, not yet possible, doubles our observed abundance, the decay of the 3 actinides will still represent 50 per cent of the total lead, a proportion higher than the values considered so far in the literature.Comment: 4 pages, LateX, A&A Letters Accepte