Orion Revisited - I. The massive cluster in front of the Orion Nebula Cluster

The aim of this work is to characterize the stellar population between Earth and the Orion A molecular cloud where the well known star formation benchmark Orion Nebula Cluster (ONC) is embedded. We use the denser regions the Orion A cloud to block optical background light, effectively isolating the stellar population in front of it. We then use a multi-wavelength observational approach to characterize the cloud's foreground stellar population. We find that there is a rich stellar population in front of the Orion A cloud, from B-stars to M-stars, with a distinct 1) spatial distribution, 2) luminosity function, and 3) velocity dispersion from the reddened population inside the Orion A cloud. The spatial distribution of this population peaks strongly around NGC 1980 (iota Ori) and is, in all likelihood, the extended stellar content of this poorly studied cluster. We infer an age of ~4-5 Myr for NGC 1980 and estimate a cluster population of the order of 2000 stars, which makes it one of the most massive clusters in the entire Orion complex. What is currently taken in the literature as the ONC is then a mix of several intrinsically different populations, namely: 1) the youngest population, including the Trapezium cluster and ongoing star formation in the dense gas inside the nebula, 2) the foreground population, dominated by the NGC 1980 cluster, and 3) the poorly constrained population of foreground and background Galactic field stars. Our results support a scenario where the ONC and L1641N are not directly associated with NGC 1980, i.e., they are not the same population emerging from its parental cloud, but are instead distinct overlapping populations. This result calls for a revision of most of the observables in the benchmark ONC region (e.g., ages, age spread, cluster size, mass function, disk frequency, etc.). (abridged)Comment: Version 2 includes comments and clarifications from John Tobin, Nicola Da Rio, and Lynne Hillenbrand (minor clarifying changes were made to Figures 1, 8, and 10). A&A accepted (15 pages, 10 figures). Higher resolution figures available upon reques

Yang-Lee zeros and the helix-coil transition in a continuum model of polyalanine

We calculate the Yang-Lee zeros for characteristic temperatures of the helix-coil transition in a continuum model of polyalanine. The distribution of these zeros differs from predictions of the Zimm-Bragg theory and supports recent claims that polyalanine exhibits a true phase transition. New estimates for critical exponents are presented and the relation of our results to the Lee-Yang theorem is discussed.Comment: 15 pages and 5 figure

Ab initio calculation of the dynamical properties of PPP and PPV

In this work, we have calculated the vibrational modes and frequencies of the crystalline PPP (in both the Pbam and Pnnm symmetries) and PPV (in the P21/c symmetry). Our results are in good agreement with the available experimental data. Also, we have calculated the temperature dependence of their specific heats at constant volume, and of their vibrational entropies. Based on our results, at high temperatures, the PPP is more stable in the Pnnm structure than in the Pbam one.Comment: 5 pages, 7 figures, accepted for publication in Braz. J. Phys., special number, Proceedings of BWSP-12, 12th Brazilian Workshop on Semiconductor Physic

On special limits of the Mixed Painlev\'e PIII−V_{\mathbf{III-V}} Model

The paper discusses P$_{III-V}$ equation for special values of its parameters for which this equation reduces to P$_{III}$, I$_{12}$, as well as, to some special cases of I$_{38}$ and I$_{49}$ equations from the Ince's list of $50$ second order differential equations possessing Painlev\'e property. These reductions also yield symmetries governing the reduced models obtained from the P$_{III-V}$ equation. We point out that the solvable equations on Ince's list emerge in this reduction scheme when the underlying reflections of the Weyl symmetry group no longer include an affine reflection through the hyperplane orthogonal to the highest root and therefore do not give rise to an affine Weyl group. We hypothesize that on the level of the underlying algebra and geometry this might be a fundamental feature that distinguishes the six Painlev\'e equations from the remaining $44$ solvable equations on the Ince's list.Comment: 10 pages, Proc. of the 32nd International Colloquium on Group Theoretical Methods in Physics (Group32), 2018, Prague, Czech Republi

Optimized cross-slot flow geometry for microfluidic extension rheometry

A precision-machined cross-slot flow geometry with a shape that has been optimized by numerical simulation of the fluid kinematics is fabricated and used to measure the extensional viscosity of a dilute polymer solution. Full-field birefringence microscopy is used to monitor the evolution and growth of macromolecular anisotropy along the stagnation point streamline, and we observe the formation of a strong and uniform birefringent strand when the dimensionless flow strength exceeds a critical Weissenberg number Wicrit 0:5. Birefringence and bulk pressure drop measurements provide self consistent estimates of the planar extensional viscosity of the fluid over a wide range of deformation rates (26 s1 "_ 435 s1) and are also in close agreement with numerical simulations performed by using a finitely extensible nonlinear elastic dumbbell model

Structural transitions in biomolecules - a numerical comparison of two approaches for the study of phase transitions in small systems

We compare two recently proposed methods for the characterization of phase transitions in small systems. The usefulness of these techniques is evaluated for the case of structural transition in alanine-based peptides.Comment: Accepted for publication in Int. J. Mol. Sci., to appear in a special issue devoted to R.S. Berr

Helix Formation and Folding in an Artificial Peptide

We study the relation between $\alpha$-helix formation and folding for a simple artificial peptide, Ala$_{10}$-Gly$_5$-Ala$_{10}$. Our data rely on multicanonical Monte Carlo simulations where the interactions among all atoms are taken into account. The free-energy landscape of the peptide is evaluated for various temperatures. Our data indicate that folding of this peptide is a two-step process: in a first step two $\alpha$-helices are formed which afterwards re-arrange themselves into a U-like structure.Comment: 15 pages, with 9 eps figure