Noise driven translocation of short polymers in crowded solutions

In this work we study the noise induced effects on the dynamics of short polymers crossing a potential barrier, in the presence of a metastable state. An improved version of the Rouse model for a flexible polymer has been adopted to mimic the molecular dynamics by taking into account both the interactions between adjacent monomers and introducing a Lennard-Jones potential between all beads. A bending recoil torque has also been included in our model. The polymer dynamics is simulated in a two-dimensional domain by numerically solving the Langevin equations of motion with a Gaussian uncorrelated noise. We find a nonmonotonic behaviour of the mean first passage time and the most probable translocation time, of the polymer centre of inertia, as a function of the polymer length at low noise intensity. We show how thermal fluctuations influence the motion of short polymers, by inducing two different regimes of translocation in the molecule transport dynamics. In this context, the role played by the length of the molecule in the translocation time is investigated.Comment: 11 pages, 3 figures, to appear in J. Stat. Mechanics: Theory and Experiment, 200

'Auxiliary' Science with the WFIRST Microlensing Survey: Measurement of the Compact Object Mass Function over Ten Orders of Magnitude; Detection of ~10⁵ Transiting Planets; Astroseismology of ~10⁶ Bulge Giants; Detection of ~5x10³ Trans-Neptunian Objects; and Parallaxes and Proper Motions of ~6x10⁶ Bulge and Disk Stars

The Wide Field Infrared Survey Telescope (WFIRST) will monitor ∼2 deg² toward the Galactic bulge in a wide (∼1−2 μm) W149 filter at 15-minute cadence with exposure times of ∼50s for 6 seasons of 72 days each, for a total ∼41,000 exposures taken over ∼432 days, spread over the 5-year prime mission. This will be one of the deepest exposures of the sky ever taken, reaching a photon-noise photometric precision of 0.01 mag per exposure and collecting a total of ∼10⁹ photons over the course of the survey for a W149_(AB) ∼ 21 star. Of order 4×10⁷ stars will be monitored with W149_(AB) < 21, and 10⁸ stars with W145_(AB) < 23. The WFIRST microlensing survey will detect ∼54,000 microlensing events, of which roughly 1% (∼500) will be due to isolated black holes, and ∼3% (∼1600) will be due to isolated neutron stars. It will be sensitive to (effectively) isolated compact objects with masses as low as the mass of Pluto, thereby enabling a measurement of the compact object mass function over 10 orders of magnitude. Assuming photon-noise limited precision, it will detect ∼10⁵ transiting planets with sizes as small as ∼2 R⊕, perform asteroseismology of ∼10⁶ giant stars, measure the proper motions to ∼0.3% and parallaxes to ∼10% for the ∼6×10⁶ disk and bulge stars in the survey area, and directly detect ∼5×10³ Trans-Neptunian objects (TNOs) with diameters down to ∼10 km, as well as detect ∼10³ occulations of stars by TNOs during the survey. All of this science will completely serendipitous, i.e., it will not require modifications of the WFIRST optimal microlensing survey design. Allowing for some minor deviation from the optimal design, such as monitoring the Galactic center, would enable an even broader range of transformational science

The Scientific Context of WFIRST Microlensing in the 2020s

As discussed in Exoplanet Science Strategy (National Academies of Sciences Engineering and Medicine 2018), WFIRST (Akeson et al. 2019) is uniquely capable of finding planets with masses as small as Mars at separations comparable to Jupiter, i.e., beyond the current ice lines of main sequence stars. In semimajor axis, these planets fall between the close-in planets found by Kepler (Coughlin et al. 2016) and the wide separation gas giants seen by direct imaging (e.g. Lagrange et al. 2009) and ice giants inferred from ALMA observations (Zhang et al. 2018). Furthermore, the smallest planets WFIRST can detect are smaller than the planets probed by radial velocity (Mayor et al. 2011; Bonfils et al. 2013) and Gaia (Perryman et al. 2014) at comparable separations. Interpreting planet populations to infer the underlying formation and evolutionary processes requires combining results from multiple detection methods to measure the full variation of planets as a function of planet size, orbital separation, and host star mass. Microlensing is the only way to find planets from 0.5 to 5M⊕ at separations of 1 to 5 au. Fundamentally, the case for a microlensing survey from space has not changed in the past 20 years: going to space allows wide-field diffraction-limited observations that can resolve main-sequence stars in the bulge, which in turn allows the detection and characterization of the smallest microlensing signals including those from planets with masses at least as small as Mars (Bennett & Rhie 2002). What has changed is that ground-based microlensing is reaching its limits, which underscores the scientific necessity for a space-based microlensing survey to measure the population of the smallest planets. Ground-based microlensing has found a break in the mass-ratio distribution at about a Neptune mass-ratio (Suzuki et al. 2016; Jung et al. 2018), implying that Neptunes are the most common microlensing planet and that planets smaller than this are rare. However, ground-based microlensing reaches its detection limits at mass ratios only slightly below the observed break. The WFIRST microlensing survey will measure the shape of the mass-ratio function below the break by finding numerous smaller planets: ~ 500 Neptunes, a comparable number of large gas giants, and ~ 200 Earths (if they are as common as Neptunes), and it can detect planets as small as 0.1M⊕ (Penny et al. 2018). In addition, because it will also measure host star masses and distances, WFIRST will also track the behavior of the planet distribution as a function of separation and host star mass

New insights on the AU-scale circumstellar structure of FU Orionis

We report new near-infrared, long-baseline interferometric observations at the AU scale of the pre-main-sequence star FU Orionis with the PTI, IOTA and VLTI interferometers. This young stellar object has been observed on 42 nights over a period of 6 years from 1998 to 2003. We have obtained 287 independent measurements of the fringe visibility with 6 different baselines ranging from 20 to 110 meters in length, in the H and K bands. Our extensive (u,v)-plane coverage, coupled with the published spectral energy distribution data, allows us to test the accretion disk scenario. We find that the most probable explanation for these observations is that FU Ori hosts an active accretion disk whose temperature law is consistent with standard models. We are able to constrain the geometry of the disk, including an inclination of 55 deg and a position angle of 47 deg. In addition, a 10 percent peak-to-peak oscillation is detected in the data (at the two-sigma level) from the longest baselines, which we interpret as a possible disk hot-spot or companion. However, the oscillation in our best data set is best explained with an unresolved spot located at a projected distance of 10 AU at the 130 deg position angle and with a magnitude difference of DeltaK = 3.9 and DeltaH = 3.6 mag moving away from the center at a rate of 1.2 AU/yr. we propose to interpret this spot as the signature of a companion of the central FU Ori system on an extremely eccentric orbit. We speculate that the close encounter of this putative companion and the central star could be the explanation of the initial photometric rise of the luminosity of this object

Wide-Orbit Exoplanet Demographics

The Kepler, K2 and TESS transit surveys are revolutionizing our understanding of planets orbiting close to their host stars and our understanding of exoplanet systems in general, but there remains a gap in our understanding of wide-orbit planets. This gap in our understanding must be filled if we are to understand planet formation and how it affects exoplanet habitability. We summarize current and planned exoplanet detection programs using a variety of methods: microlensing (including WFIRST), radial velocities, Gaia astrometry, and direct imaging. Finally, we discuss the prospects for joint analyses using results from multiple methods and obstacles that could hinder such analyses. We endorse the findings and recommendations published in the 2018 National Academy report on Exoplanet Science Strategy. This white paper extends and complements the material presented therein

Self-energy limited ion transport in sub-nanometer channels

The current-voltage characteristics of the alpha-Hemolysin protein pore during the passage of single-stranded DNA under varying ionic strength, C, are studied experimentally. We observe strong blockage of the current, weak super-linear growth of the current as a function of voltage, and a minimum of the current as a function of C. These observations are interpreted as the result of the ion electrostatic self-energy barrier originating from the large difference in the dielectric constants of water and the lipid bilayer. The dependence of DNA capture rate on C also agrees with our model.Comment: more experimental material is added. 4 pages, 7 figure

Observations of T Tauri Disks at Sub-AU Radii: Implications for Magnetospheric Accretion and Planet Formation

We determine inner disk sizes and temperatures for four solar-type (1-2 M$_{\odot}$) classical T Tauri stars (AS 207A, V2508 Oph, AS 205A, and PX Vul) using 2.2 $\mu$m observations from the Keck Interferometer. Nearly contemporaneous near-IR adaptive optics imaging photometry, optical photometry, and high-dispersion optical spectroscopy are used to distinguish contributions from the inner disks and central stars in the interferometric observations. In addition, the spectroscopic and photometric data provide estimates of stellar properties, mass accretion rates, and disk co-rotation radii. We model our interferometric and photometric data in the context of geometrically flat accretion disk models with inner holes, and flared disks with puffed-up inner walls. Models incorporating puffed-up inner disk walls generally provide better fits to the data, similar to previous results for higher-mass Herbig Ae stars. Our measured inner disk sizes are larger than disk truncation radii predicted by magnetospheric accretion models, with larger discrepancies for sources with higher mass accretion rates. We suggest that our measured sizes correspond to dust sublimation radii, and that optically-thin gaseous material may extend further inward to the magnetospheric truncation radii. Finally, our inner disk measurements constrain the location of terrestrial planet formation as well as potential mechanisms for halting giant planet migration.Comment: Accepted for publication in ApJ (May 1, 2005 issue