40 research outputs found
The Decay of Debris Disks around Solar-Type Stars
We present a Spitzer MIPS study of the decay of debris disk excesses at 24
and 70 m for 255 stars of types F4 - K2. We have used multiple tests,
including consistency between chromospheric and X-ray activity and placement on
the HR diagram, to assign accurate stellar ages. Within this spectral type
range, at 24 m, of the stars younger than 5 Gyr have
excesses at the 3 level or more, while none of the older stars do,
confirming previous work. At 70 m, of the younger stars
have excesses at 3 significance, while only
% of the older stars do. To characterize the far infrared
behavior of debris disks more robustly, we double the sample by including stars
from the DEBRIS and DUNES surveys. For the F4 - K4 stars in this combined
sample, there is only a weak (statistically not significant) trend in the
incidence of far infrared excess with spectral type (detected fractions of
21.9, late F; 16.5, G; and
16.9, early K). Taking this spectral type range together,
there is a significant decline between 3 and 4.5 Gyr in the incidence of
excesses with fractional luminosities just under . There is an
indication that the timescale for decay of infrared excesses varies roughly
inversely with the fractional luminosity. This behavior is consistent with
theoretical expectations for passive evolution. However, more excesses are
detected around the oldest stars than is expected from passive evolution,
suggesting that there is late-phase dynamical activity around these stars.Comment: 46 pages. 7 figures. Accepted to Ap
Debris Disks around Solar-Type Stars: Observations of the Pleiades with Spitzer Space Telescope
We present Spitzer MIPS observations at 24 um of 37 solar-type stars in the
Pleiades and combine them with previous observations to obtain a sample of 71
stars. We report that 23 stars, or 32 +/- 6.8%, have excesses at 24 um at least
10% above their photospheric emission. We compare our results with studies of
debris disks in other open clusters and with a study of A stars to show that
debris disks around solar-type stars at 115 Myr occur at nearly the same rate
as around A-type stars. We analyze the effects of binarity and X-ray activity
on the excess flux. Stars with warm excesses tend not to be in equal-mass
binary systems, possibly due to clearing of planetesimals by binary companions
in similar orbits. We find that the apparent anti-correlations in the incidence
of excess and both the rate of stellar rotation and also the level of activity
as judged by X-ray emission are statistically weak.Comment: 34 pages; accepted for publication in ApJ; new version included
corrections of typos, etc to match published versio
All Source Analysis System (ASAS): Migration from VAX to Alpha AXP computer systems
The Jet Propulsion Laboratory's (JPL's) experience migrating existing VAX applications to Digital Equipment Corporation's new Alpha AXP processor is covered. The rapid development approach used during the 10-month period required to migrate the All Source Analysis System (ASAS), 1.5 million lines of FORTRAN, C, and Ada code, is also covered. ASAS, an automated tactical intelligence system, was developed by the Jet Propulsion Laboratory for the U. S. Army. Other benefits achieved as a result of the significant performance improvements provided by Alpha AXP platform are also described
Comparison of velocimetry techniques for turbulent structures in gas-puff imaging data
Recent analysis of Gas Puff Imaging (GPI) data from Alcator C-Mod found blob velocities with a modified tracking time delay estimation (TDE). These results disagree with velocity analysis performed using direct Fourier methods. In this paper, the two analysis methods are compared. The implementations of these methods are explained, and direct comparisons using the same GPI data sets are presented to highlight the discrepancies in measured velocities. In order to understand the discrepancies, we present a code that generates synthetic sequences of images that mimic features of the experimental GPI images, with user-specified input values for structure (blob) size and velocity. This allows quantitative comparison of the TDE and Fourier analysis methods, which reveals their strengths and weaknesses. We found that the methods agree for structures of any size as long as all structures move at the same velocity and disagree when there is significant nonlinear dispersion or when structures appear to move in opposite directions. Direct Fourier methods used to extract poloidal velocities give incorrect results when there is a significant radial velocity component and are subject to the barber pole effect. Tracking TDE techniques give incorrect velocity measurements when there are features moving at significantly different speeds or in different directions within the same field of view. Finally, we discuss the limitations and appropriate use of each of methods and applications to the relationship between blob size and velocity.National Science Foundation (U.S.) (1122374
Debris discs in the 27 Myr old open cluster IC4665
We present Spitzer IRAC and MIPS 24um imaging of members of the 27+/-5Myr old
open cluster IC 4665. Models for the assembly of terrestrial planets through
planetesimal collisions and mergers predict episodic dust debris discs at this
epoch. We determine that 42(+18-13)% of the solar-type (F5-K5) cluster members
have excess emission at 24um indicative of these debris discs, the highest
frequency of the clusters studied with Spitzer to date. The majority of these
discs have intermediate levels of excess (F_24/F_phot < 2), and no source is
found to have extreme levels of excess indicative of a recent transient event
as opposed to steady-state collisional evolution. We find no evidence of a link
between multiplicity and 24um excess in this cluster sample. Only the
early-type star TYC424-473-1 (T_eff~8420K) has significant near-infrared excess
from 4.5um as measured with IRAC. Two solar-type targets have low significance
8um excess but no significant 24um excess. All other targets show no evidence
for near-infrared excess which could indicate the presence of an optically
thick primordial disc, demonstrating that the observed 24um excess arises from
a debris disc.Comment: 14 pages, 11 figures, accepted for publication in MNRA
Modeling the System Parameters of 2M1533+3759: A New Longer-Period Low-Mass Eclipsing sdB+dM Binary
We present new photometric and spectroscopic observations for 2M 1533+3759 (=
NSVS 07826147). It has an orbital period of 0.16177042 day, significantly
longer than the 2.3--3.0 hour periods of the other known eclipsing sdB+dM
systems. Spectroscopic analysis of the hot primary yields Teff = 29230 +/- 125
K, log g = 5.58 +/- 0.03 and log N(He)/N(H) = -2.37 +/- 0.05. The sdB velocity
amplitude is K1 = 71.1 +/- 1.0 km/s. The only detectable light contribution
from the secondary is due to the surprisingly strong reflection effect. Light
curve modeling produced several solutions corresponding to different values of
the system mass ratio, q(M2/M1), but only one is consistent with a core helium
burning star, q=0.301. The orbital inclination is 86.6 degree. The sdB primary
mass is M1 = 0.376 +/- 0.055 Msun and its radius is R1 = 0.166 +/- 0.007 Rsun.
2M1533+3759 joins PG0911+456 (and possibly also HS2333+3927) in having an
unusually low mass for an sdB star. SdB stars with masses significantly lower
than the canonical value of 0.48 Msun, down to as low as 0.30 Msun, were
theoretically predicted by Han et al. (2002, 2003), but observational evidence
has only recently begun to confirm the existence of such stars. The existence
of core helium burning stars with masses lower than 0.40--0.43 Msun implies
that at least some sdB progenitors have initial main sequence masses of
1.8--2.0 Msun or more, i.e. they are at least main sequence A stars. The
secondary is a main sequence M5 star.Comment: 47 pages, 7 figure
Nonlinear gyrokinetic simulations of the I-mode high confinement regime and comparisons with experimenta)
For the first time, nonlinear gyrokinetic simulations of I-mode plasmas are performed and compared with experiment. I-mode is a high confinement regime, featuring energy confinement similar to H-mode, but without enhanced particle and impurity particle confinement [D. G. Whyte et al., Nucl. Fusion 50, 105005 (2010)]. As a consequence of the separation between heat and particle transport, I-mode exhibits several favorable characteristics compared to H-mode. The nonlinear gyrokinetic code GYRO [J. Candy and R. E. Waltz, J Comput. Phys. 186, 545 (2003)] is used to explore the effects of E × B shear and profile stiffness in I-mode and compare with L-mode. The nonlinear GYRO simulations show that I-mode core ion temperature and electron temperature profiles are more stiff than L-mode core plasmas. Scans of the input E × B shear in GYRO simulations show that E × B shearing of turbulence is a stronger effect in the core of I-mode than L-mode. The nonlinear simulations match the observed reductions in long wavelength density fluctuation levels across the L-I transition but underestimate the reduction of long wavelength electron temperature fluctuation levels. The comparisons between experiment and gyrokinetic simulations for I-mode suggest that increased E × B shearing of turbulence combined with increased profile stiffness are responsible for the reductions in core turbulence observed in the experiment, and that I-mode resembles H-mode plasmas more than L-mode plasmas with regards to marginal stability and temperature profile stiffness.United States. Department of Energy (Contract No. DE-FC02-99ER54512-CMOD)United States. Department of Energy. Office of Science (Contract No. DE-AC02- 05CH11231
20 years of research on the Alcator C-Mod tokamak
The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994) and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only high-power radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing components—approaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated the critical role of cross-field transport in divertor operation, edge flows and the tokamak density limit. C-Mod developed the I-mode and the Enhanced Dα H-mode regimes, which have high performance without large edge localized modes and with pedestal transport self-regulated by short-wavelength electromagnetic waves. C-Mod has carried out pioneering studies of intrinsic rotation and demonstrated that self-generated flow shear can be strong enough in some cases to significantly modify transport. C-Mod made the first quantitative link between the pedestal temperature and the H-mode's performance, showing that the observed self-similar temperature profiles were consistent with critical-gradient-length theories and followed up with quantitative tests of nonlinear gyrokinetic models. RF research highlights include direct experimental observation of ion cyclotron range of frequency (ICRF) mode-conversion, ICRF flow drive, demonstration of lower-hybrid current drive at ITER-like densities and fields and, using a set of novel diagnostics, extensive validation of advanced RF codes. Disruption studies on C-Mod provided the first observation of non-axisymmetric halo currents and non-axisymmetric radiation in mitigated disruptions. A summary of important achievements and discoveries are included.United States. Dept. of Energy (Cooperative Agreement DE-FC02-99ER54512)United States. Dept. of Energy (Cooperative Agreement DE-FG03-94ER-54241)United States. Dept. of Energy (Cooperative Agreement DE-AC02-78ET- 51013)United States. Dept. of Energy (Cooperative Agreement DE-AC02-09CH11466)United States. Dept. of Energy (Cooperative Agreement DE-FG02-95ER54309)United States. Dept. of Energy (Cooperative Agreement DE-AC02-05CH11231)United States. Dept. of Energy (Cooperative Agreement DE-AC52-07NA27344)United States. Dept. of Energy (Cooperative Agreement DE-FG02- 97ER54392)United States. Dept. of Energy (Cooperative Agreement DE-SC00-02060
ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets
The affordable, robust, compact (ARC) reactor is the product of a conceptual design study aimed at reducing the size, cost, and complexity of a combined fusion nuclear science facility (FNSF) and demonstration fusion Pilot power plant. ARC is a ∼200–250 MWe tokamak reactor with a major radius of 3.3 m, a minor radius of 1.1 m, and an on-axis magnetic field of 9.2 T. ARC has rare earth barium copper oxide (REBCO) superconducting toroidal field coils, which have joints to enable disassembly. This allows the vacuum vessel to be replaced quickly, mitigating first wall survivability concerns, and permits a single device to test many vacuum vessel designs and divertor materials. The design point has a plasma fusion gain of Qp ≈ 13.6, yet is fully non-inductive, with a modest bootstrap fraction of only ∼63%. Thus ARC offers a high power gain with relatively large external control of the current profile. This highly attractive combination is enabled by the ∼23 T peak field on coil achievable with newly available REBCO superconductor technology. External current drive is provided by two innovative inboard RF launchers using 25 MW of lower hybrid and 13.6 MW of ion cyclotron fast wave power. The resulting efficient current drive provides a robust, steady state core plasma far from disruptive limits. ARC uses an all-liquid blanket, consisting of low pressure, slowly flowing fluorine lithium beryllium (FLiBe) molten salt. The liquid blanket is low-risk technology and provides effective neutron moderation and shielding, excellent heat removal, and a tritium breeding ratio ≥ 1.1. The large temperature range over which FLiBe is liquid permits an output blanket temperature of 900 K, single phase fluid cooling, and a high efficiency helium Brayton cycle, which allows for net electricity generation when operating ARC as a Pilot power plant