114 research outputs found
Modulation of LISA free-fall orbits due to the Earth-Moon system
We calculate the effect of the Earth-Moon (EM) system on the free-fall motion
of LISA test masses. We show that the periodic gravitational pulling of the EM
system induces a resonance with fundamental frequency 1 yr^-1 and a series of
periodic perturbations with frequencies equal to integer harmonics of the
synodic month (9.92 10^-7 Hz). We then evaluate the effects of these
perturbations (up to the 6th harmonics) on the relative motions between each
test masses couple, finding that they range between 3mm and 10pm for the 2nd
and 6th harmonic, respectively. If we take the LISA sensitivity curve, as
extrapolated down to 10^-6 Hz, we obtain that a few harmonics of the EM system
can be detected in the Doppler data collected by the LISA space mission. This
suggests that the EM system gravitational near field could provide an absolute
calibration for the LISA sensitivity at very low frequencies.Comment: 15 pages, 5 figure
Optical nanofibers and spectroscopy
We review our recent progress in the production and characterization of
tapered optical fibers with a sub-wavelength diameter waist. Such fibers
exhibit a pronounced evanescent field and are therefore a useful tool for
highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist
or of the medium surrounding. We use a carefully designed flame pulling process
that allows us to realize preset fiber diameter profiles. In order to determine
the waist diameter and to verify the fiber profile, we employ scanning electron
microscope measurements and a novel accurate in situ optical method based on
harmonic generation. We use our fibers for linear and non-linear absorption and
fluorescence spectroscopy of surface-adsorbed organic molecules and investigate
their agglomeration dynamics. Furthermore, we apply our spectroscopic method to
quantum dots on the surface of the fiber waist and to caesium vapor surrounding
the fiber. Finally, towards dispersive measurements, we present our first
results on building and testing a single-fiber bi-modal interferometer.Comment: 13 pages, 18 figures. Accepted for publication in Applied Physics B.
Changes according to referee suggestions: changed title, clarification of
some points in the text, added references, replacement of Figure 13
Fundamentals of the LISA Stable Flight Formation
The joint NASA-ESA mission LISA relies crucially on the stability of the
three spacecraft constellation. Each of the spacecraft is in heliocentric
orbits forming a stable triangle. The principles of such a formation flight
have been formulated long ago and analysis performed, but seldom presented if
ever, even to LISA scientists. We nevertheless need these details in order to
carry out theoretical studies on the optical links, simulators etc. In this
article, we present in brief, a model of the LISA constellation, which we
believe will be useful for the LISA community.Comment: 9 Pages, 2 Figure Submitted to Classical and Quantum Gravit
Numerical solution of a pursuit-evasion differential game involving two spacecraft in low earth orbit
This paper considers a spacecraft pursuit-evasion problem taking place in low earth orbit. The problem is formulated as a zero-sum differential game in which there are two players, a pursuing spacecraft that attempts to minimize a payoff, and an evading spacecraft that attempts to maximize the same payoff. We introduce two associated optimal control problems and show that a saddle point for the differential game exists if and only if the two optimal control problems have the same optimal value. Then, on the basis of this result, we propose two computational methods for determining a saddle point solution: a semi-direct control parameterization method (SDCP method), which is based on a piecewise-constant control approximation scheme, and a hybrid method, which combines the new SDCP method with the multiple shooting method. Simulation results show that the proposed SDCP and hybrid methodsare superior to the semi-direct collocation nonlinear programming method (SDCNLP method), which is widely used to solve pursuit-evasion problems in the aerospace field
Suppression of Mcl-1 via RNA interference sensitizes human hepatocellular carcinoma cells towards apoptosis induction
BACKGROUND: Hepatocelluar carcinoma (HCC) is one of the most common cancers worldwide and a major cause of cancer-related mortality. HCC is highly resistant to currently available chemotherapeutic drugs. Defects in apoptosis signaling contribute to this resistance. Myeloid cell leukemia-1 (Mcl-1) is an anti-apoptotic member of the Bcl-2 protein family which interferes with mitochondrial activation. In a previous study we have shown that Mcl-1 is highly expressed in tissues of human HCC. In this study, we manipulated expression of the Mcl-1 protein in HCC cells by RNA interference and analyzed its impact on apoptosis sensitivity of HCC cells in vitro. METHODS: RNA interference was performed by transfecting siRNA to specifically knock down Mcl-1 expression in HCC cells. Mcl-1 expression was measured by quantitative real-time PCR and Western blot. Induction of apoptosis and caspase activity after treatment with chemotherapeutic drugs and different targeted therapies were measured by flow cytometry and fluorometric analysis, respectively. RESULTS: Here we demonstrate that Mcl-1 expressing HCC cell lines show low sensitivity towards treatment with a panel of chemotherapeutic drugs. However, treatment with the anthracycline derivative epirubicin resulted in comparatively high apoptosis rates in HCC cells. Inhibition of the kinase PI3K significantly increased apoptosis induction by chemotherapy. RNA interference efficiently downregulated Mcl-1 expression in HCC cells. Mcl-1 downregulation sensitized HCC cells to different chemotherapeutic agents. Sensitization was accompanied by profound activation of caspase-3 and -9. In addition, Mcl-1 downregulation also increased apoptosis rates after treatment with PI3K inhibitors and, to a lower extent, after treatment with mTOR, Raf I and VEGF/PDGF kinase inhibitors. TRAIL-induced apoptosis did not markedly respond to Mcl-1 knockdown. Additionally, knockdown of Mcl-1 efficiently enhanced apoptosis sensitivity towards combined treatment modalities: Mcl-1 knockdown significantly augmented apoptosis sensitivity of HCC cells towards chemotherapy combined with PI3K inhibition. CONCLUSION: Our data suggest that specific downregulation of Mcl-1 by RNA interference is a promising approach to sensitize HCC cells towards chemotherapy and molecularly targeted therapies
The Role of Rumination and Reduced Concreteness in the Maintenance of Posttraumatic Stress Disorder and Depression Following Trauma
Rumination has been linked to posttraumatic stress disorder (PTSD) and depression following trauma. A cross-sectional (N = 101) and a prospective longitudinal study (N = 147) of road traffic accident survivors assessed rumination, PTSD and depression with self-report measures and structured interviews. We tested the hypotheses that (1) rumination predicts the maintenance of PTSD and depression and (2) reduced concreteness of ruminative thinking may be a maintaining factor. Rumination significantly predicted PTSD and depression at 6 months over and above what could be predicted from initial symptom levels. In contrast to the second hypothesis, reduced concreteness in an iterative rumination task was not significantly correlated with self-reported rumination frequency, and did not consistently correlate with symptom severity measures. However, multiple regression analyses showed that the combination of reduced concreteness and self-reported frequency of rumination predicted subsequent PTSD better than rumination frequency alone. The results support the view that rumination is an important maintaining factor of trauma-related emotional disorders
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