Spitzer observations of the Hyades: Circumstellar debris disks at 625 Myr of age

We use the Spitzer Space Telescope to search for infrared excess at 24, 70, and 160 micron due to debris disks around a sample of 45 FGK-type members of the Hyades cluster. We supplement our observations with archival 24 and 70 micron Spitzer data of an additional 22 FGK-type and 11 A-type Hyades members in order to provide robust statistics on the incidence of debris disks at 625 Myr of age an era corresponding to the late heavy bombardment in the Solar System. We find that none of the 67 FGK-type stars in our sample show evidence for a debris disk, while 2 out of the 11 A-type stars do so. This difference in debris disk detection rate is likely to be due to a sensitivity bias in favor of early-type stars. The fractional disk luminosity, L_dust/L*, of the disks around the two A-type stars is ~4.0E-5, a level that is below the sensitivity of our observations toward the FGK-type stars. However, our sensitivity limits for FGK-type stars are able to exclude, at the 2-sigma level, frequencies higher than 12% and 5% of disks with L_dust/L* > 1.0E-4 and L_dust/L* > 5.0E-4, respectively. We also use our sensitivity limits and debris disk models to constrain the maximum mass of dust, as a function of distance from the stars, that could remain undetected around our targets.Comment: 33 pages, 11 figures, accepted by Ap

Nonharmonic phonons in MgB_2 at elevated temperatures

Inelastic neutron scattering was used to measure phonon spectra in MgB_2 and Mg_(0.75)Al_(0.25)B_2 from 7 to 750 K to investigate anharmonicity and adiabatic electron-phonon coupling. First-principles calculations of phonons with a linear response method were performed at multiple unit cell volumes, and the Helmholtz free energy was minimized to obtain the lattice parameters and phonon dynamics at elevated temperature in the quasiharmonic approximation. Most of the temperature dependence of the phonon density of states could be understood with the quasiharmonic approximation, although there was also significant thermal broadening of the phonon spectra. In comparison to Mg_(0.75)Al_(0.25)B_2, in the energy range of 60 to 80 meV the experimental phonon spectra from MgB_2 showed a nonmonotonic change with temperature around 500 K. This may originate from a change with temperature of the adiabatic electron-phonon coupling

Study of an automatic trajectory following control system

It is shown that the estimator part of the Modified Partitioned Adaptive Controller, (MPAC) developed for nonlinear aircraft dynamics of a small jet transport can adapt to sensor failures. In addition, an investigation is made into the potential usefulness of the configuration detection technique used in the MPAC and the failure detection filter is developed that determines how a noise plant output is associated with a line or plane characteristic of a failure. It is shown by computer simulation that the estimator part and the configuration detection part of the MPAC can readily adapt to actuator and sensor failures and that the failure detection filter technique cannot detect actuator or sensor failures accurately for this type of system because of the plant modeling errors. In addition, it is shown that the decision technique, developed for the failure detection filter, can accurately determine that the plant output is related to the characteristic line or plane in the presence of sensor noise

Arkansas Landlord Selection of Land-Leasing Contract Type and Terms

Land leasing is a major source of the land input to production agriculture. Responses from a survey of landlords leasing crop land in Arkansas are analyzed to better understand those factors motivating landlords in the type of lease they select and the terms of those leases. Probit models are estimated to determine the relative importance of variables representing credit constraint, agency problem, and risk aversion factors. Regression models then estimate the impact of site, landlord, and tenant characteristics on contract terms – the percentage of crop and cost sharing arrangements between landlord and tenant. Probit results suggest credit constraint factors influence lease-type selection. Risk aversion, managerial ability, and social capital factors are also supported. Regression models show that land and crop characteristics are significant determinants of contract terms.Land leasing, Probit, Contract, Production agriculture, Land Economics/Use,

Long-term Stellar Variability in the Galactic Centre Region

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.We report the detection of variable stars within a 11.5' x 11.5' region near the Galactic centre (GC) that includes the Arches and Quintuplet clusters, as revealed by the VISTA Variables in the Via Lactea (VVV) survey. There are 353 sources that show Ks-band variability, of which the large majority (81%) correspond to red giant stars, mostly in the asymptotic giant branch (AGB) phase. We analyze a population of 52 red giants with long-term trends that cannot be classified into the typical pulsating star categories. Distances and extinctions are calculated for 9 Mira variables, and we discuss the impact of the chosen extinction law on the derived distances. We also report the presence of 48 new identified young stellar object (YSO) candidates in the region.Peer reviewe

CONTRACT CHOICE SELECTION WITH LAND-LEASING AGREEMENTS

This study tests cropland contract hypotheses utilizing a landlord data set. Ordered probit and classical regression models are estimated and presented identifying factors that affect the contract type selection and terms. Results suggest credit constraints are a viable land-leasing hypothesis. Risk aversion, managerial ability, and social capital are also supported.Land Economics/Use,

A Candidate Protoplanet in the Taurus Star Forming Region

HST/NICMOS images of the class I protostar TMR-1 (IRAS04361+2547) reveal a faint companion with 10.0" = 1400 AU projected separation. The central protostar is itself resolved as a close binary with 0.31" = 42 AU separation, surrounded by circumstellar reflection nebulosity. A long narrow filament seems to connect the protobinary to the faint companion TMR-1C, suggesting a physical association. If the sources are physically related then we hypothesize that TMR-1C has been ejected by the protobinary. If TMR-1C has the same age and distance as the protobinary then current models indicate its flux is consistent with a young giant planet of several Jovian masses.Comment: 16 pages, 1 figure, Accepted by Astrophysical Journal Letters, Related information is available at http://www.extrasolar.co

A Cluster of Class I/f/II YSOs Discovered Near the Cepheid SU Cas

Preliminary constraints are placed on a cluster of YSOs (J2000 02:54:31.4 +69:20:32.5) discovered in the field of the classical Cepheid SU Cas. WISE 3.4, 4.6, 12, and 22 um images reveal that the cluster deviates from spherical symmetry and exhibits an apparent diameter of 3x6'. SEDs constructed using 2MASS Ks (2.2 um) and WISE photometry indicate that 19 (36%) class I, 21 (40%) class f, and 13 (25%) class II objects lie r<3' from the cluster center. Conversely, 11 (18%) class I, 13 (21%) class f, and 37 (61%) class II objects were detected for r>3'. Approximately 50% of the class I sources within r<3' were classified solely using WISE photometry owing to the absence of detections by 2MASS.Comment: Accepted for Publication (MNRAS