119032296
Modelling collision products of triple‐star mergers
In dense stellar clusters, binary-single and binary-binary encounters can ultimately lead to collisions involving two or more stars during a resonant interaction. A comprehensive survey of multistar collisions would needto explore an enormous amount of parameter space, but here we focus on a number of representative cases involving low-mass (0.4-, 0.6- and 0.8-M O .) main-sequence stars. Using both smoothed particle hydrodynamics (SPH) calculations and a much faster fluid sorting software package (MMAS), we study scenarios in which a newly formed product from an initial collision collides with a third parent star. By varying the order in which the parent stars collide, as well as the orbital parameters of the collision trajectories, we investigate how factors such as shock heating affect the chemical composition and structure profiles of the collision product. Our simulations and models indicate that the distribution of most chemical elements within the final product is not significantly affected by the order in which the stars collide, the direction of approach of the third parent star, or the periastron separations of the collisions. Although the exact surface abundances of beryllium and lithium in the product do depend on the details of the dynamics, these elements are always severely depleted due to mass loss during the collisions. We find that the sizes of the products, and hence their collisional cross-sections for subsequent encounters, can be sensitive to the order and geometry of the collisions. For the cases that we consider, the radius of the product formed in the first (single-single star) collision ranges anywhere from roughly 2-30 times the sum of the radii of its parent stars. The size of the final product formed in our triple-star collisions is more difficult to determine, but it can easily be as large or larger than a typical red giant. Although the vast majority of the volume in such a product contains diffuse gas that could be readily stripped in subsequent interactions, we nevertheless expect the collisional cross-section of a newly formed product to be greatly enhanced over that of a thermally relaxed star of the same mass. Our results also help establish that the algorithms of MMAS can quickly reproduce the important features of our SPH models for these collisions, even when one of the parent stars is itself a former product.
118446053
The Metallicity of the CM Draconis System
The CM Draconis system comprises two eclipsing mid-M dwarfs of nearly equal mass in a 1.27?day orbit. This well-studied eclipsing binary has often been used for benchmark tests of stellar models, since its components are among the lowest mass stars with well-measured masses and radii ( 1% relative precision). However, as with many other low-mass stars, non-magnetic models have been unable to match the observed radii and effective temperatures for CM Dra at the 5%-10% level. To date, the uncertain metallicity of the system has complicated comparison of theoretical isochrones with observations. In this Letter, we use data from the SpeX instrument on the NASA Infrared Telescope Facility to measure the metallicity of the system during primary and secondary eclipses, as well as out of eclipse, based on an empirical metallicity calibration in the H and K near-infrared (NIR) bands. We derive an [Fe/H] = -0.30 ? 0.12 that is consistent across all orbital phases. The determination of [Fe/H] for this system constrains a key dimension of parameter space when attempting to reconcile model isochrone predictions and observations.
51201355
Discovery of a Low-mass Companion to a Metal-rich F Star with the MARVELS Pilot Project
We report the discovery of a low-mass companion orbiting the metal-rich, main sequence F star TYC 2949-00557-1 during the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) pilot project. The host star has an effective temperature T_(eff) = 6135 ± 40 K, logg = 4.4 ± 0.1, and [Fe/H] = 0.32 ± 0.01, indicating a mass of M_⊙ = 1.25 ± 0.09 M_⊙ and R = 1.15 ± 0.15 R_⊙. The companion has an orbital period of 5.69449 ± 0.00023 days and straddles the hydrogen burning limit with a minimum mass of 64 M_J , and thus may be an example of the rare class of brown dwarfs orbiting at distances comparable to those of "Hot Jupiters." We present relative photometry that demonstrates that the host star is photometrically stable at the few millimagnitude level on time scales of hours to years, and rules out transits for a companion of radius ≳ 0.8 R_J at the 95% confidence level. Tidal analysis of the system suggests that the star and companion are likely in a double synchronous state where both rotational and orbital synchronization have been achieved. This is the first low-mass companion detected with a multi-object, dispersed, fixed-delay interferometer.
8100425
A TRANSITING PLANET OF A SUN-LIKE STAR
A planet transits an 11th magnitude, G1 V star in the constellation Corona Borealis. We designate the planet XO-1b and the star XO-1, also known as GSC 02041-01657. XO-1 lacks a trigonometric distance; we estimate it to be 200 ± 20 pc. Of the 10 stars currently known to host extrasolar transiting planets, the star XO-1 is the most similar to the Sun in its physical characteristics: its radius is 1.0 ± 0.08 R☉, its mass is 1.0 ± 0.03 M☉, its V sin i < 3 km s-1, and its metallicity [Fe/H] is 0.015 ± 0.04. The orbital period of the planet XO-1b is 3.941534 ± 0.000027 days, one of the longer ones known. The planetary mass is 0.90 ± 0.07MJ, which is marginally larger than that of other transiting planets with periods between 3 and 4 days. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. If the stellar radius is 1.0 ± 0.08 R☉, then the planetary radius is 1.30 ± 0.11RJ and the inclination of the orbit is 877 ± 12. We have demonstrated a productive international collaboration between professional and amateur astronomers that was important to distinguishing this planet from many other similar candidates.
14122353
Ground-Based Multisite Observations of Two Transits of HD 80606b
We present ground-based optical observations of the 2009 September and 2010 January transits of HD 80606b. Based on three partial light curves of the 2009 September event, we derive a midtransit time of T_c [HJD] = 2455099.196 ± 0.026, which is about 1σ away from the previously predicted time. We observed the 2010 January event from nine different locations, with most phases of the transit being observed by at least three different teams. We determine a midtransit time of Tc [HJD] = 2455210.6502 ± 0.0064, which is within 1.3σ of the time derived from a Spitzer observation of the same event.
17485541
MEASURING STELLAR RADIAL VELOCITIES WITH A DISPERSED FIXED-DELAY INTERFEROMETER
We demonstrate the ability to measure precise stellar barycentric radial velocities with the dispersed fixed-delay interferometer technique using the Exoplanet Tracker (ET), an instrument primarily designed for precision differential Doppler velocity measurements using this technique. Our barycentric radial velocities, derived from observations taken at the KPNO 2.1 m telescope, differ from those of Nidever et al. by 0.047 km s−1 (rms) when simultaneous iodine calibration is used, and by 0.120 km s−1 (rms) without simultaneous iodine calibration. Our results effectively show that a Michelson interferometer coupled to a spectrograph allows precise measurements of barycentric radial velocities even at a modest spectral resolution of R ~ 5100. A multiobject version of the ET instrument capable of observing ~500 stars per night is being used at the Sloan 2.5 m telescope at Apache Point Observatory for the Multiobject APO Radial Velocity Exoplanet Large-area Survey (MARVELS), a wide-field radial velocity survey for extrasolar planets around TYCHO-2 stars in the magnitude range 7.6 < V < 12. In addition to precise differential velocities, this survey will also yield precise barycentric radial velocities for many thousands of stars using the data analysis techniques reported here. Such a large kinematic survey at high velocity precision will be useful in identifying the signature of accretion events in the Milky Way and understanding local stellar kinematics, in addition to discovering exoplanets, brown dwarfs, and spectroscopic binaries.
14771068
Soft Collinear Effective Theory: An Overview
In this talk I give an overview of soft collinear effective theory (SCET), including a discussion of some recent advances. First, I briefly cover the foundation upon which SCET is built, namely QCD factorization, and review the theoretical framework of SCET. Next, I cover some recent calculations involving SCET. Since there are more interesting results than I can cover, I have picked four that that I find particularly exciting. The first two topics are dynamical threshold enhancements in Drell-Yan and resummation of higgs production at the LHC. Both topics center on the resummation of large Sudakov logarithms, however, for each there is a twist. In the case of threshold enhancements in Drell-Yan, it is partonic, not hadronic logarithms that are summed. In the case of higgs production at the LHC, the twist is that the logarithms are of a time-like scale. The third topic highlighted is electroweak corrections at high energy. In particular the electroweak Sudakov form factor. Finally, I end by discussing generalized event shapes in e+e− annihilation.
119508366
Flavor-singlet light-cone amplitudes and radiative Upsilon decays in SCET
We study the evolution of flavor-singlet, light-cone amplitudes in the soft-collinear effective theory (SCET), and reproduce results previously obtained by a different approach. We apply our calculation to the color-singlet contribution to the photon endpoint in radiative Upsilon decay. In a previous paper, we studied the color-singlet contributions to the endpoint, but neglected operator mixing, arguing that it should be a numerically small effect. Nevertheless the mixing needs to be included in a consistent calculation, and we do just that in this work. We find that the effects of mixing are indeed numerically small. This result combined with previous work on the color-octet contribution and the photon fragmentation contribution provides a consistent theoretical treatment of the photon spectrum in radiative Upsilon decay.
119091462
Photoproduction of J / psi in the forward region
We study the phenomenology of fixed-target elastic J/{psi} photoproduction in the nonrelativistic QCD (NRQCD) factorization formalism. Our goal is to test an essential feature of this formalism{emdash}the color-octet mechanism. We obtain an order-of-magnitude estimate for a certain linear combination of NRQCD color-octet matrix elements. Our estimate is consistent with other empirical determinations and with the v-scaling rules of NRQCD. {copyright} {ital 1997} {ital The American Physical Society}
2000018
Systematics of quarkonium production at the LHC and double parton fragmentation
In this paper we discuss the systematics of quarkonium production at the LHC. In particular, we focus on the necessity to sum logs of the form log(Q/p⊥) and log(p⊥/mQ). We show that the former contributions are power suppressed, while the latter, whose contribution in fragmentation is well known, also arise in the short distance (i.e., non-fragmentation) production mechanisms. Though these contributions are suppressed by powers of mQ/p⊥, they can be enhanced by inverse powers of v, the relative velocity between heavy quarks in the quarkonium. In the limit p⊥ ≫ mQ short distance production can be thought of as the fragmentation of a pair of partons (i.e., the heavy quark and anti-quark) into the final state quarkonium. We derive an all order factorization theorem for this process in terms of double parton fragmentation functions (DPFF) and calculate the one-loop anomalous dimension matrix for the DPFF.
15677547
The Large Energy Expansion for B Decays: Soft Collinear Effective Theory
I this talk I give an introduction to the soft collinear effective theory by considering in detail the decay rate for $B \to X_s \gamma$ near the endpoint.