arXiv subjects

M. Shao

Publications and source records attributed to M. Shao.

At least 55 records · Page 3Linked to original sources

arXiv2007

The orbits of the quadruple star system 88 Tau A from PHASES differential astrometry and radial velocity

We have used high precision differential astrometry from the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) project and radial velocity measurements covering a time-span of 20 years to determine the orbital parameters of the 88 Tau A system. 88 Tau is a complex hierarchical multiple system comprising a total of six stars; we have studied the brightest 4, consisting of two short-period pairs orbiting each other with an 18-year period. We present the first orbital solution for one of the short-period pairs, and determine the masses of the components and distance to the system to the level of a few percent. In addition, our astrometric measurements allow us to make the first determination of the mutual inclinations of the orbits. We find that the sub-systems are not coplanar.

astro-ph↗

arXiv2007

Masses, Luminosities, and Orbital Coplanarities of the mu Orionis Quadruple Star System from PHASES Differential Astrometry

mu Orionis was identified by spectroscopic studies as a quadruple star system. Seventeen high precision differential astrometry measurements of mu Ori have been collected by the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES). These show both the motion of the long period binary orbit and short period perturbations superimposed on that caused by each of the components in the long period system being themselves binaries. The new measurements enable the orientations of the long period binary and short period subsystems to be determined. Recent theoretical work predicts the distribution of relative inclinations between inner and outer orbits of hierarchical systems to peak near 40 and 140 degrees. The degree of coplanarity of this complex system is determined, and the angle between the planes of the A-B and Aa-Ab orbits is found to be 136.7 +/- 8.3 degrees, near the predicted distribution peak at 140 degrees; this result is discussed in the context of the handful of systems with established mutual inclinations. The system distance and masses for each component are obtained from a combined fit of the PHASES astrometry and archival radial velocity observations. The component masses have relative precisions of 5% (component Aa), 15% (Ab), and 1.4% (each of Ba and Bb). The median size of the minor axes of the uncertainty ellipses for the new measurements is 20 micro-arcseconds. Updated orbits for delta Equulei, kappa Pegasi, and V819 Herculis are also presented.

astro-ph↗

arXiv2007

A Planetary Mass Companion to the K0 Giant HD 17092

We report the discovery of a substellar-mass companion to the K0-giant HD 17092 with the Hobby-Eberly Telescope. In the absence of any correlation of the observed 360-day periodicity with the standard indicators of stellar activity, the observed radial velocity variations are most plausibly explained in terms of a Keplerian motion of a planetary-mass body around the star. With the estimated stellar mass of 2.3Msun, the minimum mass of the planet is 4.6MJ. The planet's orbit is characterized by a mild eccentricity of e=0.17 and a semi-major axis of 1.3 AU. This is the tenth published detection of a planetary companion around a red giant star. Such discoveries add to our understanding of planet formation around intermediate-mass stars and they provide dynamical information on the evolution of planetary systems around post-main sequence stars.

astro-ph↗

arXiv2007

SIM PlanetQuest: The Most Promising Near-Term Technique to Detect, Find Masses, and Determine Three-Dimensional Orbits of Nearby Habitable Planets

The past two Decadal Surveys in Astronomy and Astrophysics recommended the completion of a space-based interferometry mission, known today as SIM PlanetQuest, for its unique ability to detect and characterize nearby rocky planets (Bahcall 1991, McKee & Taylor 2001), as well as contributions to a broad range of problems in astrophysics. Numerous committees of the National Research Council as well as NASA Roadmaps have similarly highlighted SIM as the one technology that offers detection and characterization of rocky planets around nearby stars and which is technically ready. To date, SIM remains the only program with the capability of detecting and confirming rocky planets in the habitable zones of nearby solar-type stars. Moreover, SIM measures masses and three-dimensional orbits of habitable planets around nearby stars (within 25 pc); these are the only stars for which follow-up by other techniques is feasible, such as space-based spectroscopy, ground-based interferometry, and of course TPF.

astro-ph↗

arXiv2007

Size-dependent properties of dithallium selenide

We report on size-dependent properties of dithallium selenide, Tl2Se. We have carried out a comparative nuclear magnetic resonance (NMR) study of Tl2Se nanorods and bulk samples, measuring NMR spectra and spin-lattice relaxation rate of 203Tl and 205Tl isotopes. Though bulk Tl2Se was reported to be a metal, the Korringa-like spin-lattice relaxation behavior is observed only at low temperatures and is transformed to an activation regime above ~200 K. This finding is interpreted assuming a two-band model in the semimetallic compound. Our measurements show significant difference in the Knight shift and indirect nuclear exchange coupling for the bulk and nanorod Tl2Se samples, reflecting noticeable distinction in their electronic structure. At that, Tl2Se nanorods are semiconductors and exhibit a characteristic activation behavior in the spin-lattice relaxation rate due to the thermal excitation of carriers to the conduction band. The obtained size dependence of the Tl2Se properties is interpreted in terms of the semimetal-semiconductor transformation due to the quantum confinement.

cond-mat.other↗

arXiv2006

Limits to Tertiary Astrometric Companions in Binary Systems

The Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) has monitored 37 sub-arcsecond binary systems to determine whether their Keplerian orbits are perturbed by faint astrometric companions to either star. Software has been developed to evaluate the regions in a companion mass-period phase space in which the PHASES observations can exclude the possibility of face-on orbit perturbations. We present results for 8 systems for which astrometric companions with masses as small as those of giant planets can be excluded.

astro-ph↗

arXiv2006

Scientific Results from High-precision Astrometry at the Palomar Testbed Interferometer

A new observing mode for the Palomar Testbed Interferometer was developed in2002-2003 which enables differential astrometry at the level of 20 micro-arcseconds for binary systems with separations of several hundred milli-arcseconds (mas). This phase-referenced mode is the basis of the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES), a search for giant planets orbiting either the primary or secondary star in fifty binary systems. We present the first science results from the PHASES search. The properties of the stars comprising binary systems are determined to high precision. The mutual inclinations of several hierarchical triple star systems have been determined. We will present upper limits constraining the the existence of giant planets in a few of the target systems.

astro-ph↗

arXiv2005

Keck Interferometer Observations of FU Orionis Objects

We present new K-band long baseline interferometer observations of three young stellar objects of the FU Orionis class, V1057 Cyg, V1515 Cyg and Z CMa-SE, obtained at the Keck Interferometer during its commissioning science period. The interferometer clearly resolves the source of near-infrared emission in all three objects. Using simple geometrical models we derive size scales (0.5-4.5 AU) for this emission. All three objects appear significantly more resolved than expected from simple models of accretion disks tuned to fit the broadband optical and infrared spectro-photometry. We explore variations in the key parameters that are able to lower the predicted visibility amplitudes to the measured levels, and conclude that accretion disks alone do not reproduce the spectral energy distributions and K-band visibilities simultaneously. We conclude that either disk models are inadequate to describe the near-infrared emission, or additional source components are needed. We hypothesize that large scale emission (10s of AU) in the interferometer field of view is responsible for the surprisingly low visibilities. This emission may arise in scattering by large envelopes believed to surround these objects.

astro-ph↗

arXiv2005

PHASES Differential Astrometry and the Mutual Inclination of the V819 Herculis Triple Star System

V819 Herculis is a well-studied triple star system consisting of a ``wide'' pair with 5.5 year period, one component of which is a 2.2-day period eclipsing single-line spectroscopic binary. Differential astrometry measurements from the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) are presented and used to determine a relative inclination between the short- and long-period orbits of 23.6 +- 4.9 degrees. This represents only the sixth unambiguous determination of the mutual inclination of orbits in a hierarchical triple system. This result is combined with those for the other five systems for analysis of the observed distribution of mutual inclinations in nearby triple systems. It is found that this distribution is different than that which one would expect from random orientations with statistical significance at the 94% level; implications for studying the spatial distribution of angular momentum in star forming regions is discussed.

astro-ph↗

arXiv2005

PHASES Differential Astrometry and Iodine Cell Radial Velocities of the kappa Pegasi Triple Star System

kappa Pegasi is a well-known, nearby triple star system. It consists of a ``wide'' pair with semi-major axis 235 milli-arcseconds, one component of which is a single-line spectroscopic binary (semi-major axis 2.5 milli-arcseconds). Using high-precision differential astrometry and radial velocity observations, the masses for all three components are determined and the relative inclinations between the wide and narrow pairs' orbits is found to be 43.8 +/- 3.0 degrees, just over the threshold for the three body Kozai resonance. The system distance is determined to 34.60 +/- 0.21 parsec, and is consistent with trigonometric parallax measurements.

astro-ph↗

arXiv2005

PHASES High Precision Differential Astrometry of delta Equulei

delta Equulei is among the most well-studied nearby binary star systems. Results of its observation have been applied to a wide range of fundamental studies of binary systems and stellar astrophysics. It is widely used to calibrate and constrain theoretical models of the physics of stars. We report 27 high precision differential astrometry measurements of delta Equulei from the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES). The median size of the minor axes of the uncertainty ellipses for these measurements is 26 micro-arcseconds. These data are combined with previously published radial velocity data and other previously published differential astrometry measurements using other techniques to produce a combined model for the system orbit. The distance to the system is determined to within a twentieth of a parsec and the component masses are determined at the level of a percent. The constraints on masses and distance are limited by the precisions of the radial velocity data; we outline plans improve this deficiency and discuss the outlook for further study of this binary.

astro-ph↗

arXiv2005

Fundamental Physics with the Laser Astrometric Test Of Relativity

The Laser Astrometric Test Of Relativity (LATOR) is a joint European-U.S. Michelson-Morley-type experiment designed to test the pure tensor metric nature of gravitation - a fundamental postulate of Einstein's theory of general relativity. By using a combination of independent time-series of highly accurate gravitational deflection of light in the immediate proximity to the Sun, along with measurements of the Shapiro time delay on interplanetary scales (to a precision respectively better than 0.1 picoradians and 1 cm), LATOR will significantly improve our knowledge of relativistic gravity. The primary mission objective is to i) measure the key post-Newtonian Eddington parameter γwith accuracy of a part in 10^9. (1-γ) is a direct measure for presence of a new interaction in gravitational theory, and, in its search, LATOR goes a factor 30,000 beyond the present best result, Cassini's 2003 test. The mission will also provide: ii) first measurement of gravity's non-linear effects on light to ~0.01% accuracy; including both the Eddington βparameter and also the spatial metric's 2nd order potential contribution (never measured before); iii) direct measurement of the solar quadrupole moment J2 (currently unavailable) to accuracy of a part in 200 of its expected size; iv) direct measurement of the "frame-dragging" effect on light by the Sun's gravitomagnetic field, to 1% accuracy. LATOR's primary measurement pushes to unprecedented accuracy the search for cosmologically relevant scalar-tensor theories of gravity by looking for a remnant scalar field in today's solar system. We discuss the mission design of this proposed experiment.

gr-qc↗

arXiv2005

Extensive Particle Identification with TPC and TOF at the STAR Experiment

Particle identification (PID) capabilities are studied by using the Time Projection Chamber (TPC) and a Time-Of-Flight (TOF) detector together at STAR. The identification capability of charged hadrons is greatly extended compared with that achieved by TPC and TOF separately. Particle spectra from p+p, d+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV and Au+Au collisions at $\sqrt{s_{_{NN}}}=62.4$ GeV are used to develop the methods. The transverse momentum ($p_T$) ranges of $π$, and $p(\bar{p})$ identification are from $\sim0.3$ GeV/$c$ to $\sim10$ GeV/$c$. The high $p_T$ reach is limited by statistics in current data sets. An important conceptual advance was developed to identify electrons by using a combination of dE/dx in TPC and velocity information from the TOF detectors, which is important for future low-mass dilepton program at STAR.

nucl-ex↗

arXiv2005

The near-infrared size-luminosity relations for Herbig Ae/Be disks

We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes using the 85-m baseline Keck Interferometer. Targets were chosen to span the maximum range of stellar properties to probe the disk size dependence on luminosity and effective temperature. For most targets, the measured near-infrared sizes (ranging from 0.2 to 4 AU) support a simple disk model possessing a central optically-thin (dust-free) cavity, ringed by hot dust emitting at the expected sublimation temperatures (T_sub~1000-1500K). Furthermore, we find a tight correlation of disk size with source luminosity R propto L^(1/2) for Ae and late Be systems (valid over more than 2 decades in luminosity), confirming earlier suggestions based on lower-quality data. Interestingly, the inferred dust-free inner cavities of the highest luminosity sources (Herbig B0-B3 stars) are under-sized compared to predictions of the optically-thin cavity model, likely due to optically-thick gas within the inner AU.

astro-ph↗

arXiv2003

Interferometer Observations of Subparsec-scale Infrared Emission in the Nucleus of NGC 4151

We report novel, high-angular resolution interferometric measurements that imply the near-infrared nuclear emission in NGC 4151 is unexpectedly compact. We have observed the nucleus of NGC 4151 at 2.2 microns using the two 10-meter Keck telescopes as an interferometer and find a marginally resolved source ~0.1 pc in diameter. Our measurements rule out models in which a majority of the K band nuclear emission is produced on scales larger than this size. The interpretation of our measurement most consistent with other observations is that the emission mainly originates directly in the central accretion disk. This implies that AGN unification models invoking hot, optically thick dust may not be applicable to NGC 4151.

astro-ph↗

arXiv2003

Observations of DG Tauri with the Keck Interferometer

We present the first science results from the Keck Interferometer, a direct-detection infrared interferometer utilizing the two 10-meter Keck telescopes. The instrument and system components are briefly described. We then present observations of the T Tauri object DG Tau, which is resolved by the interferometer. The resolved component has a radius of 0.12 to 0.24 AU, depending on the assumed stellar and extended component fluxes and the model geometry used. Possible origins and implications of the resolved emission are discussed.

astro-ph↗

arXiv2002

Probing the Invisible Universe: The Case for Far-IR/Submillimeter Interferometry

The question "How did we get here and what will the future bring?" captures the human imagination and the attention of the National Academy of Science's Astronomy and Astrophysics Survey Commitee (AASC). Fulfillment of this "fundamental goal" requires astronomers to have sensitive, high angular and spectral resolution observations in the far-infrared/submillimeter (far-IR/sub-mm) spectral region. With half the luminosity of the universe and vital information about galaxy, star and planet formation, observations in this spectral region require capabilities similar to those currently available or planned at shorter wavelengths. In this paper we summarize the scientific motivation, some mission concepts and technology requirements for far-IR/sub-mm space interferometers that can be developed in the 2010-2020 timeframe.

astro-ph↗

arXiv1998

Astrometric Observation of MACHO Gravitational Microlensing

Following previous suggestions of other researchers, this paper discusses the prospects for astrometric observation of MACHO gravitational microlensing events. We derive the expected astrometric observables for a simple microlensing event with either a dark or self-luminous lens, and demonstrate that accurate astrometry can determine the lens mass, distance, and proper motion in a very general fashion. In particular we argue that in limited circumstances ground-based, narrow-angle differential astrometric techniques are sufficient to measure the lens mass directly, and other lens properties (distance, transverse motion) by applying an independent model for the source distance and motion. We investigate the sensitivity of differential astrometry in determining lens parameters by Monte Carlo methods, and derive a quasi-empirical relationship between astrometric accuracy and mass uncertainty.

astro-ph↗