707 research outputs found
Numerical Study of Two-fluid Flowing Equilibria of Helicity-driven Spherical Torus Plasmas
Two-fluid flowing equilibrium configurations of a helicity-driven spherical
torus (HD-ST) are numerically determined by using the combination of the finite
difference and the boundary element methods. It is found from the numerical
results that electron fluids near the central conductor are tied to an external
toroidal field and ion fluids are not. The magnetic configurations change from
the high- HD-ST (>1) with paramagnetic toroidal field and low-
(volume average value, ( 2 %) through the
helicity-driven spheromak and RFP to the ultra low- HD-ST (0<q<1) with
diagmagnetic toroidal field and high- ($\beta\approx 18\%) as the
external toroidal field at the inner edge region decreases and reverses the
sign. The two-fluid effecs are more significant in this equilibrium transition
when the ion diagmagnetic drift is dominant in the flowing two-fluid.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Star-Crossed Lovers DI Tau A and B: Orbit Characterization and Physical Properties Determination
The stellar companion to the weak-line T Tauri star DI Tau A was first
discovered by the lunar occultation technique in 1989 and was subsequently
confirmed by a speckle imaging observation in 1991. It has not been detected
since, despite being targeted by five different studies that used a variety of
methods and spanned more than 20 years. Here, we report the serendipitous
rediscovery of DI Tau B during our Young Exoplanets Spectroscopic Survey
(YESS). Using radial velocity data from YESS spanning 17 years, new adaptive
optics observations from Keck II, and a variety of other data from the
literature, we derive a preliminary orbital solution for the system that
effectively explains the detection and (almost all of the) non-detection
history of DI Tau B. We estimate the dynamical masses of both components,
finding that the large mass difference (q 0.17) and long orbital period
(35 years) make DI Tau system a noteworthy and valuable addition to
studies of stellar evolution and pre-main-sequence models. With a long orbital
period and a small flux ratio (f2/f1) between DI Tau A and B, additional
measurements are needed for a better comparison between these observational
results and pre-main-sequence models. Finally, we report an average surface
magnetic field strength () for DI Tau A, of 0.55 kG, which is
unusually low in the context of young active stars.Comment: 21 pages, 10 figures. Accepted to Ap
Orbital characterization of GJ1108A system, and comparison of dynamical mass with model-derived mass for resolved binaries
We report an orbital characterization of GJ1108Aab that is a low-mass binary
system in pre-main-sequence phase. Via the combination of astrometry using
adaptive optics and radial velocity measurements, an eccentric orbital solution
of =0.63 is obtained, which might be induced by the Kozai-Lidov mechanism
with a widely separated GJ1108B system. Combined with several observed
properties, we confirm the system is indeed young. Columba is the most probable
moving group, to which the GJ1108A system belongs, although its membership to
the group has not been established. If the age of Columba is assumed for
GJ1108A, the dynamical masses of both GJ1108Aa and GJ1108Ab ( and ) are more massive than what an
evolutionary model predicts based on the age and luminosities. We consider the
discrepancy in mass comparison can attribute to an age uncertainty; the system
is likely older than stars in Columba, and effects that are not implemented in
classical models such as accretion history and magnetic activity are not
preferred to explain the mass discrepancy. We also discuss the performance of
the evolutionary model by compiling similar low-mass objects in evolutionary
state based on the literature. Consequently, it is suggested that the current
model on average reproduces the mass of resolved low-mass binaries without any
significant offsets.Comment: Accepted in Ap
The shadow knows: using shadows to investigate the structure of the pretransitional disk of HD 100453
This is the final version of the article. Available from American Astronomical Society via the DOI in this record.We present GPI polarized intensity imagery of HD 100453 in Y-, J-, and K1 bands which reveals an inner gap ( au), an outer disk ( au) with two prominent spiral arms, and two azimuthally-localized dark features also present in SPHERE total intensity images (Wagner 2015). SED fitting further suggests the radial gap extends to au. The narrow, wedge-like shape of the dark features appears similar to predictions of shadows cast by a inner disk which is misaligned with respect to the outer disk. Using the Monte Carlo radiative transfer code HOCHUNCK3D (Whitney 2013), we construct a model of the disk which allows us to determine its physical properties in more detail. From the angular separation of the features we measure the difference in inclination between the disks 45, and their major axes, PA = 140 east of north for the outer disk and 100for the inner disk. We find an outer disk inclination of from face-on in broad agreement with the Wagner 2015 measurement of 34. SPHERE data in J- and H-bands indicate a reddish disk which points to HD 100453 evolving into a young debris disk.Based in part on data obtained at the Gemini Observatory via the time exchange program between Gemini and the Subaru Telescope (GS-2015A-C-1). The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, TecnologÃa e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). M.T. is partly supported by JSPS KAKENHI 2680016. C.A.G. is supported under NASA Origins of Solar Systems Funding via NNG16PX39P. Y.H. is supported by Jet Propulsion Laboratory, California Institute of Technology under a contract from NASA. M.S. is supported by NASA Exoplanet Research Program NNX16AJ75G. J.K. acknowledges support from Philip Leverhulme Prize (PLP-2013-110, PI: Stefan Kraus). S.K. acknowledges support from an ERC Starting Grant (Grant Agreement No. 639889). We also thank the referee for their comments and suggestions which added clarity to this paper
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