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The YETI Collaboration,
W. P. Chen,
S. C. -L. Hu,
R. Errmann,
Ch. Adam,
S. Baar,
A. Berndt,
L. Bukowiecki,
D. P. Dimitrov,
T. Eisenbeiß, [......],
C. Briceño,
R. Chini,
E. L. N. Jensen,
E. H. Nikogossian,
A. K. Pandey,
J. Sperauskas,
H. Takahashi,
F. M. Walter,
Z. -Y. Wu, X. Zhou
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ABSTRACT: GM Cep in the young (~4 Myr) open cluster Trumpler 37 has been known to be an
abrupt variable and to have a circumstellar disk with very active accretion.
Our monitoring observations in 2009-2011 revealed the star to show sporadic
flare events, each with brightening of < 0.5 mag lasting for days. These
brightening events, associated with a color change toward the blue, should
originate from an increased accretion activity. Moreover, the star also
underwent a brightness drop of ~1 mag lasting for about a month, during which
the star became bluer when fainter. Such brightness drops seem to have a
recurrence time scale of a year, as evidenced in our data and the photometric
behavior of GM Cep over a century. Between consecutive drops, the star
brightened gradually by about 1 mag and became blue at peak luminosity. We
propose that the drop is caused by obscuration of the central star by an
orbiting dust concentration. The UX Orionis type of activity in GM Cep
therefore exemplifies the disk inhomogeneity process in transition between
grain coagulation and planetesimal formation in a young circumstellar disk.
The Astrophysical Journal 03/2012; · 6.02 Impact Factor
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R. Neuhäuser,
R. Errmann,
A. Berndt,
G. Maciejewski,
H. Takahashi,
W.P. Chen,
D.P. Dimitrov,
T. Pribulla,
E.H. Nikogossian,
E.L.N. Jensen, [......],
E. Schmidt,
M.M. Hohle,
M. Kitze,
N. Chakrova,
C. Gräfe,
K. Schreyer,
V.V. Hambaryan,
C.H. Broeg,
J. Koppenhoefer,
A.K. Pandey
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ABSTRACT: We present the Young Exoplanet Transit Initiative (YETI), in which we use several 0.2 to 2.6-m telescopes around the world to monitor continuously young (≤100 Myr), nearby (≤1 kpc) stellar clusters mainly to detect young transiting planets (and to study other variability phenomena on time-scales from minutes to years). The telescope network enables us to observe the targets continuously for several days in order not to miss any transit. The runs are typically one to two weeks long, about three runs per year per cluster in two or three subsequent years for about ten clusters. There are thousands of stars detectable in each field with several hundred known cluster members, e.g. in the first cluster observed, Tr-37, a typical cluster for the YETI survey, there are at least 469 known young stars detected in YETI data down to R = 16.5 mag with sufficient precision of 50 millimag rms (5 mmag rms down to R = 14.5 mag) to detect transits, so that we can expect at least about one young transiting object in this cluster. If we observe ∼10 similar clusters, we can expect to detect ∼10 young transiting planets with radius determinations. The precision given above is for a typical telescope of the YETI network, namely the 60/90-cm Jena telescope (similar brightness limit, namely within ±1 mag, for the others) so that planetary transits can be detected. For targets with a periodic transit-like light curve, we obtain spectroscopy to ensure that the star is young and that the transiting object can be sub-stellar; then, we obtain Adaptive Optics infrared images and spectra, to exclude other bright eclipsing stars in the (larger) optical PSF; we carry out other observations as needed to rule out other false positive scenarios; finally, we also perform spectroscopy to determine the mass of the transiting companion. For planets with mass and radius determinations, we can calculate the mean density and probe the internal structure. We aim to constrain planet formation models and their time-scales by discovering planets younger than ∼100 Myr and determining not only their orbital parameters, but also measuring their true masses and radii, which is possible so far only by the transit method. Here, we present an overview and first results (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Astronomische Nachrichten 06/2011; 332(6):547 - 561. · 1.01 Impact Factor
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R. Neuhäuser,
R. Errmann,
A Berndt,
G. Maciejewski,
H Takahashi,
W P Chen,
D. P. Dimitrov,
T. Pribulla,
E. H. Nikogossian,
E. L. N. Jensen, [......],
E Schmidt,
M. M. Hohle,
M. Kitze,
N. Chakrova,
C. Gräfe,
K. Schreyer,
V. V. Hambaryan,
C. H. Broeg,
J. Koppenhoefer,
A. K. Pandey
[show abstract]
[hide abstract]
ABSTRACT: We present the Young Exoplanet Transit Initiative (YETI), in which we use
several 0.2 to 2.6m telescopes around the world to monitor continuously young
(< 100 Myr), nearby (< 1 kpc) stellar clusters mainly to detect young
transiting planets (and to study other variability phenomena on time-scales
from minutes to years). The telescope network enables us to observe the targets
continuously for several days in order not to miss any transit. The runs are
typically one to two weeks long, about three runs per year per cluster in two
or three subsequent years for about ten clusters. There are thousands of stars
detectable in each field with several hundred known cluster members, e.g. in
the first cluster observed, Tr-37, a typical cluster for the YETI survey, there
are at least 469 known young stars detected in YETI data down to R=16.5 mag
with sufficient precision of 50 milli-mag rms (5 mmag rms down to R=14.5 mag)
to detect transits, so that we can expect at least about one young transiting
object in this cluster. If we observe 10 similar clusters, we can expect to
detect approximately 10 young transiting planets with radius determinations.
The precision given above is for a typical telescope of the YETI network,
namely the 60/90-cm Jena telescope (similar brightness limit, namely within
+/-1 mag, for the others) so that planetary transits can be detected. For
planets with mass and radius determinations, we can calculate the mean density
and probe the internal structure. We aim to constrain planet formation models
and their time-scales by discovering planets younger than 100 Myr and
determining not only their orbital parameters, but also measuring their true
masses and radii, which is possible so far only by the transit method. Here, we
present an overview and first results. (Abstract shortened)
06/2011;
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ABSTRACT: Using a new design procedure termed as Algorithms by Design, which we have successfully introduced in our previous efforts for second-order systems, alternatively, we advance in this exposition, the design and development of a computational framework that permits order-preserving second-order time accurate, unconditionally stable, zero-order overshooting behavior, and features with controllable numerical dissipation and dispersion via a family of algorithms for effectively solving transient first-order systems. The key feature is the incorporation of a spurious root to introduce controllable numerical dissipation while preserving second-order accuracy (order-preserving feature) resulting in a two-root system, namely, the principal root (ρ1∞) and a spurious root (ρ2∞). In contrast to the classical Trapezoidal family of algorithms which are the most popular, the present framework has the same order of computational complexity, but a higher payoff that is a significant advance to the field for tackling a wide class of applications dealing with first-order transient systems. We also present the special case with selection of ρ1∞ = 1 and any ρ2∞ leading to the design of a family of generalized single-step single-solve [GS4-1] algorithms recovering the Crank–Nicolson method at one end (ρ2∞ = 1) and the Midpoint Rule at the other end (ρ2∞ = 0) and anything in between, all of which have spectral radius features resembling that of the Crank–Nicolson method. More interestingly, with the particular choice of ρ1∞ = ρ2∞ = 0, the developed framework additionally inherits L-stable features. We illustrate the successful design of the developed GS4-1 framework using two simple illustrative numerical examples. Copyright © 2011 John Wiley & Sons, Ltd.
International Journal for Numerical Methods in Engineering 06/2011; 88(13):1411 - 1448. · 2.01 Impact Factor
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ABSTRACT: A new approach termed as the hybrid displacement–strain-based normalized time-weighted residual approach is presented to accurately model non-linear structural dynamic problems. The focus here is restricted to all possible algorithmic designs within the class of linear multi-step methods that fall under the umbrella of the generalized single solve single step linear dynamic framework originally developed via the classical time-weighted residual approach involving a single solve within each time step as such designs are the most predominant in research and commercial software (Int. J. Numer. Meth. Engng 2004; 59:597–668; Int. J. Numer. Meth. Engng 2006; 66:1738–1790). However, traditional practices via classical time-weighted residual approaches fail to preserve the underlying physics and stability and do not serve the purposes of extensions to non-linear dynamic situations. In contrast, a new normalized time-weighted residual approach is proposed that naturally enables such extensions leading to the design of a family of conserving time operators for unconstrained conservative dynamic systems without resorting to enforcing energy constraints as in past practices. Consequently, the algorithmic stability (in the sense of energy stability) via this approach for non-linear dynamic problems is also preserved. For linear dynamic situations, it reverts to the classical paradigm. Only simple numerical illustrations are purposely presented to demonstrate the basic concepts. Copyright © 2009 John Wiley & Sons, Ltd.
International Journal for Numerical Methods in Engineering 03/2009; 79(9):1094 - 1146. · 2.01 Impact Factor
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International Journal for Computational Methods in Engineering Science and Mechanics 01/2009; 10(1):27-56.
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ABSTRACT: While in Part I (see [1]) and Part II (see [2]) of this three-part exposition we focused attention on non-dissipative symplectic-momentum conserving designs of time operators for applicability to nonlinear dynamics, here in Part III of this exposition we demonstrate how to further advance the theoretical developments and introduce controllable numerical dissipation via a novel time weighted residual approach we have previously described. The unique aspects of the algorithmic designs are such that when the numerical dissipative features are turned off, the resulting time operators readily recover the original designs of algorithms that are inherently symplectic-momentum conserving (we defer to those that are energy-momentum conserving elsewhere [3, 4]). The focus of the current work is on the theoretical developments of the new formulation termed the displacement based normalized time weighted residual approach for nonlinear dynamics applications. In particular, we provide extensions of the well—known Generalized Single Solve Single Step (GSSSS) framework, which comprises two distinct classifications, namely constrained U and V algorithmic architectures, and was originally developed for solving linear dynamic problems. Starting with the generalization of the classical time weighted residual approach, the GSSSS framework that was previously developed encompasses the general class of LMS methods represented by the single field form of the second order ordinary differential equations in time involving a single solve, and also covers most of the developments to date in the literature, including providing new avenues towards optimal designs of algorithms. However, the classical time weighted residual approach fails to adequately provide proper extensions to nonlinear dynamics applications. Consequently, the basic premise and argument that is herein advanced is that controllable numerical dissipative time operators designed for linear dynamic problems are very valuable, and are indeed the basis and can be readily employed as the basic parent algorithms, such that when implemented appropriately via the present new time weighted residual representation, they are now readily suitable for extensions to nonlinear dynamics applications. To demonstrate the basic concepts, we consider applications to the Saint Venant Kirchhoff material model simply for illustration, although the method can be readily extended to general material models. The numerical examples presented show that in contrast to the classical time weighted residual approach, which fails to recover the original designs of symplectic-momentum conservation when numerical dissipation is turned off, this new approach readily accomplishes this feature naturally and without enforcing any added constraints, and is the more appropriate way to design a particular class of controllable numerical dissipative schemes. It also leads to algorithm designs that yield fewer numerical oscillations in the energy and angular momentum in contrast to the classical approach, thus additionally confirming the improved effectiveness of the proposed approach. Further, we also show that amongst all the controllable numerical dissipative schemes considered in the sense of and under the framework of LMS methods in the single field form and involving a single solve and second-order time accuracy, the U0- V0optimal is the preferred choice of this particular class of symplectic-momentum conserving based controllable numerical dissipative schemes since: (i) it yields least amount of energy dissipation; and (ii) it is ideal for any given set of initial conditions in the sense that it possesses the highly desirable attributes involving zero order displacement and velocity overshooting behavior. Simple numerical examples are provided that illustrate the fundamental ideas for applications to nonlinear dynamics problems.
International Journal for Computational Methods in Engineering Science and Mechanics 12/2008; 10(1):57-90.
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ABSTRACT: Application of SAR images to detect the onset of breakup and dynamic decay process of coastal sea ice was investi-gated, using a time series of RADARSAT-1 C-band scanning SAR images of the areas around Seward Peninsula coastal region, Alaska, including Bering Strait and Norton Sound. Satellite image analyses mainly include a time series feature space images composed of ra-dar backscattering coefficient versus look angle derived from RADARSAT-1 amplitude images covering both coastal sea and land area. It was found that the feature space images have a single mode before the decay and breakup of the coastal sea ice, and have multiple modes when large-scale open water first appears until the coastal sea ice disappears completely. The mode patterns in the feature space images can then be determined by the water surface roughness caused by the wind field. The single-mode phenomenon before ice melting is thought to be due to a similarity of the backscattering of snow cover on sea ice and land surface due to the common coverage by snow and ice. The appearance of multiple modes is due to the differences in backscattering of sea ice surface at various decaying stages or open water with different wave roughness due to wind effect, and the land surface with snow and ice cover with different melting conditions once the melting or decay starts and proceeds. The feature space images reported here can be used as a useful tool in detection of decay of coastal sea ice and landfast ice in the field of sea ice dynamics as the imaging radar technology is weather independent, providing information of coastal ice on a regular basis.
Journal of Environmental Informatics 01/2007; 10:37-46. · 2.53 Impact Factor
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ABSTRACT: We report the results of a new time-series charge-coupled device (CCD) photometry survey of variable stars in the field of the old open cluster NGC 188. In a 1° field covering the cluster, a total number of 27 variable stars, including eight new variables, were detected during this survey. The periods, the classifications as well as the membership of the newly discovered variables are discussed. Among those, two are definite member stars and one is a probable member star of the cluster. They are all W UMa binaries. Thus the total number of W UMa stars belonging to the cluster is increased to 10. A peculiar variable (V26 = GSC 4619−450) is found from the field. It is very likely a new large-amplitude β Cephei-type variable with a pulsating period of about 0.1331 d. Revised basic data are given for the known variables in the programme field. The period of V8 is refined as 5.2096 d. The variability of V11 is confirmed by this observation. It is suggested to be a very probable FK Com-type variable with a period of about 1.2433 d. Finally, the relative frequency of the occurrences of W UMa stars and the distribution in the cluster are discussed. The result supports the hypothesis of mass segregation in NGC 188.
Monthly Notices of the Royal Astronomical Society 10/2004; 355(4):1369 - 1377. · 4.90 Impact Factor
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X Zhou,
R Dong,
S Li,
G Peng,
L Zhang,
J Hou,
J Xiao,
Xingxiang Zhou,
Renjie Dong,
Shujun Li,
Gaojun Peng,
Lanfang Zhang,
Jicong Hou,
Junhua Xiao,
Benhai Zhu
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ABSTRACT: China has a longest history of tools innovation and development for agriculture on the world but became backward between the Qing Dynasty and 1940s. Since the 1950s especially since the reform and opening policies started in 1978, agricultural engineering has made tremendous achievements in China. Agricultural engineering was referred to as agricultural mechanization before 1978. Agricultural engineering has been gradually understood and agricultural mechanization, agricultural water and soil engineering, agricultural bio-environment and energy engineering, and agricultural electrification and automation engineering have been developed and have contributed to the overall agricultural achievement.
CIGR Journal of Scientific Research and Development. Invited Overview Paper. August. 01/2003;
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ABSTRACT: This paper presents a physic-based reverse short channel effect (RSCE) model for threshold voltage (V th) modeling of deep submicron MOSFETs. Unlike those conventional empirically-based RSCE models, the proposed model is derived based on two Gaussian pile-up profiles located at the source and drain edges of a MOSFET. The model has a simple compact form that can be utilized to characterize the advanced halo-implant MOSFETs. A detailed comparison of the proposed RSCE model with the pre-viously proposed model is also presented. The analytical model has been applied to, and verified with, experimental data of a 0.25-µ m CMOS process for ten different gate lengths as well as various drain and substrate bias conditions.
Journal of Modeling and Simulation of Microsystems. 01/1999; 2:51-56.
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ABSTRACT: In computational mechanics to date, even after five decades of research dealing with integration of the linear and nonlinear dy-namic equations of motion, there exists for the general cases of algorithm designs still a clear lack of a fundamental understand-ing regarding evaluation of these equations of motion, and how, why, and what precise time levels the integrations need to take place. This has placed major limitations on commercial code de-velopers, and a lack of confidence in general, thereby restricting the implementation of LMS methods to only a select few that hap-pen to be correct, but without any rigorous proofs or underlying reasons explaining the ramifications. This is extremely critical for the general cases of integration of the equations of motion, in par-ticular for the class of Linear Multi-Step (LMS) that are implicit, involve a single solve, and are second-order accurate in time with unconditional stability (this pertains to linear dynamic situations only), since these serve as the backbone and drivers for most fi-nite element commercial and research software. In this regard, the present Part I of the three-part exposition puts the matter to rest and provides closure, while in Part II (see Ref [1]) and Part III (see Ref [2]) we describe extensions to nonlinear dynamics applications of the basic general framework for the classes of nondissipative and controllable numerical dissipative methods, respectively. Within the confines of these LMS methods, particular attention is paid to designing algorithms that are symplectic-momentum preserving, and with enhancements to include controllable dissipative features and optimal algorithm designs. Simply for illustration of the basic ideas, readily understandable numerical examples are presented that validate the overall concepts and developments. Other related support in the form of computer grants from the Min-nesota Supercomputer Institute (MSI), Minneapolis, Minnesota, is also gratefully acknowledged. Address correspondence to K.
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ABSTRACT: Of interest here are the class of static/dynamic finite deformation problems that arise in com-putational mechanics, and the question of the suitability in employing the total strain measure for this class of problems is raised. An attempt to resolve the problem by proposing a new arbitrary reference configuration (ARC) framework is described in this exposition. The ARC framework consists of the ARC elasticity which bridges the Truesdell stress rate hypo-elasticity and the St. Venant-Kirchhoff hyperelasticity, and the ARC Lagrangian formulation which bridges the updated Lagrangian formulation and the total Lagrangian formulation. The ARC framework serves as a generalized computational framework to handle both the computational infinitesimal and the finite deformation/strain deformation applications in a consistent and unified manner. In part II of the paper [1], we further extend the ARC framework to elasto-plasticity.
