Romanian Academy

About: Romanian Academy is a archive organization based out in Bucharest, Romania. It is known for research contribution in the topics: Population & Nonlinear system. The organization has 3662 authors who have published 10491 publications receiving 146447 citations. The organization is also known as: Academia Română & Societatea Literară Română.

Obstructions to the existence of Kähler structures on compact complex manifolds

Abstract: We prove that a manifold in the Fujiki class C which supports a i∂∂̄-closed metric is Kähler. This result implies that on a compact complex manifold in the Fujiki class C which is not Kähler there exists a nonzero i∂∂̄exact, positive current of bidimension (1, 1). Introduction In [HaLa], Harvey and Lawson proved that the obstruction to the existence of a Kähler metric on a given compact complex manifold X of dimension n is a positive, nonzero current of bidegree (n− 1, n− 1) which is the (n− 1, n− 1) component of a d-exact current on X. In general, such currents are not d-closed, therefore the theory of closed positive currents cannot be used to study them (although some results extending this theory to the case of i∂∂̄-closed, positive currents do exist). The main result of this paper is that, in the case when X is a manifold in the Fujiki class C, the obstruction current can be chosen to be d-closed: Theorem 0.1. Let X be a compact complex manifold of dimension n in the Fujiki class C and which is not Kähler. Then there exists a positive, nonzero current T of bidegree (n− 1, n− 1) which is i∂∂̄-exact. Theorem 0.1 follows immediately from Theorem 0.2. Let X be a compact complex manifold of dimension n in the Fujiki class C and suppose there exists ω a strictly positive (1, 1) form on X such that i∂∂̄ω = 0. Then X is a Kähler manifold. The two theorems are generalizations to the analytic case of the algebraic case which was proved by Peternell [Pe]. Theorem 0.2 is similar to Moishezon’s theorem which states that a Moishezon manifold which is Kähler is in fact projective. A (1, 1) form ω as in Theorem 0.2 (i.e., positive defined, and i∂∂̄-closed) is called a strong Kähler with torsion (SKT ) metric. See for instance [FiTo] for an introduction to SKT metrics. Therefore, Theorem 0.2 states that a manifold in Fujiki class C which supports an SKT metric is in fact Kähler. On surfaces, Theorems 0.1 and 0.2 are vacuous since any surface in the Fujiki class C is Kähler. But on 3-folds, Theorem 0.1 implies that any closed obstruction contains a nonzero curve: Received by the editors November 6, 2012. 2010 Mathematics Subject Classification. Primary 32J27; Secondary 32Q15. The author was supported by a Marie Curie International Reintegration Grant within the 7th European Community Framework Programme and the CNCS grant PN-II-ID-PCE-2011-3-0269. c ©2014 American Mathematical Society 3561 License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use

Linking Remote-Sensing and In Situ Observations of Coronal Mass Ejections Using STEREO

Abstract: The twin STEREO spacecraft have been observing the Sun since 2006. Even though STEREO has only been active during solar minimum conditions so far, an important number of coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs) have been observed. Many of the ICMEs can be linked back to the corresponding CMEs on the Sun through the combination of remote-sensing and in situ observations. This paper aims to answer the question whether a CME observed by a coronagraph will be detected in situ by a spacecraft in a specific location in the heliosphere. We use a flux-rope-like model fit to the STEREO SECCHI/COR2 data to obtain the direction of CME propagation and its geometrical configuration in three dimensions. Based on model parameters, we then calculate their angular widths and determine whether they should have been detected by STEREO-A, STEREO-B, Wind or ACE. We compare the results with corresponding in situ observations of ICMEs. We find that predictions of ICME detections on the base of COR2 data generally match well the actual in situ observations.

Comparative seismology of pre- and main sequence stars in the instability strip

Abstract: Pulsational properties of 1.8 stellar models covering the latest stages of contraction toward the main sequence up to early hydrogen burning phases are investigated by means of linear nonadiabatic analyses. Results confirm that pre-main sequence stars (pms) which cross the classical instability strip on their way toward the main sequence are pulsationally unstable with respect to the classical opacity mechanisms. For both pms and main sequence types of models in the lower part of the instability strip, the unstable frequency range is found to be roughly the same. Some non-radial unstable modes are very sensitive to the deep internal structure of the star. It is shown that discrimination between pms and main sequence stages is possible using differences in their oscillation frequency distributions in the low frequency range.

Hybrid fullerene conjugates as vectors for DNA cell-delivery

Abstract: The present study reports fullerene conjugates that act as efficient binders of double stranded DNA (dsDNA) into cytofriendly polyplexes. The conjugates are designed to generate dendrimeric structures, having C60 as the core and bearing linear or branched PEI and polyethyleneglycol (PEG) arms (∼2 kDa). Simple and reproducible synthesis pathways provided C60-PEI and C60-PEG-PEI conjugates. They were able to bind linear and plasmidic dsDNA and they form particulate polyplexes of 50 to 200 nm in diameter. The resulted polyplexes toggle between the anionic and cationic state at nitrogen to phosphorous ratios (N/P) of about 5, as revealed by their zeta potential and became colloidally stable at N/P ratios above 10, as determined by atomic force microscopy (AFM). They are electrophoretically unbreakable starting with N/P ratios of 3 and of 5 when salmon sperm DNA and pEYFP-C1 plasmid, respectively are loaded. Both C60-PEI·pEYFP and C60-PEG-PEI·pEYFP polyplexes are non-cytotoxic against HEK 293T cells in culture and exhibit transfection efficiency better than 25% (N/P ratios above 20) and 6% (N/P ratios above 60) respectively, measured by flow cytometry. For comparison, the commercial SuperFect® from Qiagen (positive control) was able to provide an efficiency of 15–20%, under similar conditions. Moreover, the C60-PEG-PEI conjugate is as performant as the positive control in terms of expression of EYFP reporter gene in cultured cells and exhibited high cytocompatibility, determining cell proliferation up to 200%. Our study proved that C60-PEG-PEI is effective vector for DNA delivery being, in addition, easily synthesizable, practically non-cytotoxic and as efficient the commercially available transfection tools.