The Cygnus X region XXIII. Is 18P87 galactic or extragalactic?

The radio source 18P87, previously thought to be a point source, has been serendipitously found to be resolved into a core-jet geometry in VLA maps. HI absorption of continuum emission (in data from the Canadian Galactic Plane Survey) appears in gas with radial velocities > +2 km/s but not in brightly emitting gas at lower radial velocity. Examination of further archival observations at radio, infrared and optical wavelengths suggests that the "obvious" interpretation as a radio galaxy requires a rather unusual object of this kind and a highly unusual local line of sight. We argue that 18P87 may be a Galactic object, a local astrophysical jet. If this is correct it could have arisen from outbursts of a microquasar.Comment: Accepted for publication in Astronomische Nachrichte

Floquet engineering of interlayer couplings: Tuning the magic angle of twisted bilayer graphene at the exit of a waveguide

We introduce a new approach that allows one complete control over the modulation of the effective twist angle change in few-layer van der Waals heterostructures by irradiating them with longitudinal waves of light at the end of a waveguide. As a specific application, we consider twisted bilayer graphene and show that one can tune the magic angles to be either larger or smaller, allowing in-situ experimental control of the phase diagram of this and other related materials. A waveguide allows one to circumvent the free-space constraints on the absence of longitudinal electric field components of light. We propose to place twisted bilayer graphene at a specific location at the exit of a waveguide, such that it is subjected to purely longitudinal components of a transverse magnetic modes (TM) wave

Effective Floquet Hamiltonians for periodically-driven twisted bilayer graphene

We derive effective Floquet Hamiltonians for twisted bilayer graphene driven by circularly polarized light in two different regimes beyond the weak-drive, high frequency regime. First, we consider a driving protocol relevant for experiments with frequencies smaller than the bandwidth and weak amplitudes and derive an effective Hamiltonian, which through a symmetry analysis, provides analytical insight into the rich effects of the drive. We find that circularly polarized light at low frequencies can selectively decrease the strength of AA-type interlayer hopping while leaving the AB-type unaffected. Then, we consider the intermediate frequency, and intermediate-strength drive regime. We provide a compact and accurate effective Hamiltonian which we compare with the Van Vleck expansion and demonstrate that it provides a significantly improved representation of the exact quasienergies. Finally, we discuss the effect of the drive on the symmetries, Fermi velocity and the gap of the Floquet flat bands

Floquet engineering of twisted double bilayer graphene

Motivated by the recent experimental realization of twisted double bilayer graphene (TDBG) samples we study, both analytically and numerically, the effects of circularly polarized light propagating in free space and confined into a waveguide on the band structure and topological properties of these systems. These two complementary Floquet protocols allow us to selectively tune different parameters of the system by varying the intensity and the frequency of the light. For the drive protocol in free space, in the high-frequency regime, we find that in TDBG with AB/BA stacking, we can selectively close the zone-center quasienergy gaps around one valley while increasing the gaps near the opposite valley by tuning the parameters of the drive. In TDBG with AB/AB stacking, a similar effect can be obtained upon the application of a perpendicular static electric field. Furthermore, we study the topological properties of the driven system in different settings, provide accurate effective Floquet Hamiltonians, and show that relatively strong drives can generate flat bands. On the other hand, longitudinal light confined into a waveguide couples to the components of the interlayer hopping that are perpendicular to the TDBG sheet, allowing for selective engineering of the bandwidth of Floquet zone center quasienergy bands without breaking the symmetries of the static system.Comment: 12 pages, 12 figure

Drivers of ecosystem vulnerability to Corbicula invasions in southeastern North America

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGInvasive species introduction is one of the major ongoing ecological global crises. Identifying factors responsible for the success of invasive species is key for the implementation of effective management actions. The invasive filter-feeding bivalve, Corbicula, is of particular interest because it has become ubiquitous in many river basins across North America and elsewhere. Here we sampled bivalve assemblages, environmental indicators, and land cover parameters in the Ouachita highlands in southeastern Oklahoma and southwestern Arkansas, and in the Gulf Coastal Plain of Alabama to test three working models (using structural equation modeling, SEM) based on a priori scientific knowledge regarding Corbicula invasions. Our models tested three competing hypotheses: (1) Native mussel declines are related to land use changes at the watershed level and subsequent Corbicula colonization is a result of an empty niche; (2) Corbicula abundance is one of the factors responsible for native mussel declines and has an interactive effect with land use change at the watershed level; (3) Native mussel declines and Corbicula success are both related to land use changes at the watershed level. We found no evidence for the first two hypotheses. However, we found that environmental indicators and land cover parameters at the watershed scale were robust predictors of Corbicula abundance. In particular, agricultural land cover was positively related with Corbicula density. These results suggest that further improvement of conventional agricultural practices including the optimization of fertilizer delivery systems may represent an opportunity to manage this species by limiting nutrient inputs to stream ecosystems. Preservation of extensive floodplain habitats may help buffer these inputs by providing key ecosystem services including sediment and nutrient retention.National Science Foundation | Ref. DEB-1831512Xunta de Galicia | Ref. I2C 2017-202

Radio Spectral Index Analysis and Classes of Ejection in LS I +61 303

LS I +61303 is a gamma-ray binary with periodic radio outbursts coincident with the orbital period of P=26.5 d. The origin of the radio emission is unclear,it could be due either to a jet, as in microquasars, or to the shock boundary between the Be star and a possible pulsar wind. We here analyze the radio spectral index over 6.7 yr from Green Bank Interferometer data at 2.2 GHz and 8.3 GHz. We find two new characteristics in the radio emission. The first characteristic is that the periodic outbursts indeed consist of two consecutive outbursts; the first outburst is optically thick, whereas the second outburst is optically thin. The spectrum of LS I +61 303 is well reproduced by the shock-in-jet model commonly used in the context of microquasars and AGNs: the optically thin spectrum is due to shocks caused by relativistic plasma ("transient jet") traveling through a pre-existing much slower steady flow ("steady jet"). This steady flow is responsible for the preceding optically thick spectrum. The second characteristic we find is that the observed spectral evolution, from optically thick to optically thin emission, occurs twice during the orbital period. We observed this occurrence at the orbital phase of the main 26.5 d outburst and also at an earlier phase, shifted by $\Delta \Phi \sim$ 0.3 (i.e almost 8 days before). We show that this result qualitatively and quantitatively agrees with the two-peak accretion/ejection model proposed in the past for LS I +61303. We conclude that the radio emission in LS I +61303 originates from a jet and suggest that the variable TeV emission comes from the usual Compton losses expected as an important by-product in the shock-in-jet theory.Comment: 27 pages, 7 figures, accepted for publication in Ap

Estimation of Direct and Maternal Breed Effects for Prediction of Expected Progeny Differences for Birth and Weaning Weights in Three Multibreed Populations

Direct and maternal breed effects on birth and 200-d weights were estimated for nine parental breeds (Hereford [H], Angus [A], Braunvieh [B], Limousin [L], Charolais [C], Simmental [S], Gelbvieh [G], Red Poll [R], and Pinzgauer [P]) that contributed to three composite populations (MARC I = ¼B, ¼C, ¼L, ⅛H, ⅛A; MARC II = ¼G, ¼S, ¼H, ¼A; and MARC III = ¼R, ¼P, ¼H, ¼A). Records from each population, the composite plus pure breeds and crosses used to create each composite, were analyzed separately. The animal model included fixed effects of contemporary group (birth year-sex-dam age), proportions of individual and maternal heterosis and breed inheritance as covariates, and random effects of additive direct genetic ( a ) and additive maternal genetic (m) with covariance (a,m), permanent environment, and residual. Sampling correlations among estimates of genetic fixed effects were large, especially between direct and maternal heterosis and between direct and maternal breed genetic effects for the same breed, which were close to &#;1. This resulted in some large estimates with opposite sign and large standard errors for direct and maternal breed genetic effects. Data from a diallel experiment with H, A, B, and R breeds, from grading up and from a top cross experiment were required to separate breed effects satisfactorily into direct and maternal genetic effects. Results indicate that estimation of direct and maternal breed effects needed to predict hybrid EPD for multibreed populations from field data may not be possible. Information from designed crossbreeding experiments will need to be incorporated in some way

Group Leaders’ Perceptions of Interventions with Grandparent Caregivers: Content and Process

Nineteen (Mage = 45, SD = 12.8) professionally trained group leaders were surveyed regarding their experiences in leading a 10-week program with one of three RCT conditions (cognitive behavior training, parenting skills training, information only support). While a high percentage indicated that the intervention led by them was beneficial, leaders nevertheless felt that some participants benefited more so than others. Perceived program benefits were seen as being linked to regular attendance and the completion of weekly homework. The major benefits to participants were gaining personal insight, receiving and providing support to others, successfully applying learned skills and knowledge to everyday life, and feeling empowered and hopeful about the future. Peer leaders were viewed positively as was the provision of food and childcare. Group leaders faced numerous practical challenging in conducting group interventions: ensuring regular attendance, keeping participants focused and on track, and dealing with participants who dominated discussion. These unprecedented findings not only allow us insight into the dynamics of leading group interventions with grandmother caregivers, but they may also have implications as influences on the measured efficacy of such programs