Interferometric Astrometry of the Low-mass Binary Gl 791.2 (= HU Del) Using Hubble Space Telescope Fine Guidance Sensor 3: Parallax and Component Masses

With fourteen epochs of fringe tracking data spanning 1.7y from Fine Guidance Sensor 3 we have obtained a parallax (pi_abs=113.1 +- 0.3 mas) and perturbation orbit for Gl 791.2A. Contemporaneous fringe scanning observations yield only three clear detections of the secondary on both interferometer axes. They provide a mean component magnitude difference, Delta V = 3.27 +- 0.10. The period (P = 1.4731 yr) from the perturbation orbit and the semi-major axis (a = 0.963 +- 0.007 AU) from the measured component separations with our parallax provide a total system mass M_A + M_B = 0.412 +- 0.009 M_sun. Component masses are M_A=0.286 +- 0.006 M_sun and M_B = 0.126 +- 0.003 M_sun. Gl 791.2A and B are placed in a sparsely populated region of the lower main sequence mass-luminosity relation where they help define the relation because the masses have been determined to high accuracy, with errors of only 2%.Comment: 19 pages, 5 figures. The paper is to appear in August 2000 A

Competition dynamics in a chemical system of self-replicating macrocycles

Central to the origin of life is the question how a chemical system transitioned from interacting molecules to an entity with the capacity for self-replication, diversification and adaptive evolution. Here, we study a chemical system that is comprised of macrocycles that have been shown to spontaneously give rise to self-replicating entities. By combining experimental and theoretical approaches, we strive to understand the evolutionary potential of this system. In particular, we apply eco-evolutionary reasoning to investigate whether and when this system of chemical replicators can diversify. Here, we report first results of a simplified stochastic chemical reaction model that is parameterized on the basis of experimental data. The model considers the competition of two replicators that do not interact directly but need similar building blocks for their growth and reproduction. Interestingly, the replicator that emerges first is being overtaken by the later one. By means of stochastic simulations, we will explore how the competitive ability of a replicator is determined by its chemical characteristics, and under which conditions replicators can coexist. The results will subsequently inform the design of future experiments

Photometry of Proxima Centauri and Barnard's Star Using HST Fine Guidance Sensor 3: A Search for Periodic Variations

We have observed Proxima Centauri and Barnard's Star with Hubble Space Telescope Fine Guidance Sensor 3. Proxima Centauri exhibits small-amplitude, periodic photometric variations. Once several sources of systematic photometric error are corrected, we obtain 2 milli-magnitude internal photometric precision. We identify two distinct behavior modes over the past four years: higher amplitude, longer period; smaller amplitude, shorter period. Within the errors one period (P ~ 83d) is twice the other. Barnard's Star shows very weak evidence for periodicity on a timescale of approximately 130 days. If we interpret these periodic phenomena as rotational modulation of star spots, we identify three discrete spots on Proxima Cen and possibly one spot on Barnard's Star. We find that the disturbances change significantly on time scales as short as one rotation period.Comment: 39 pages, 17 figure

Interferometric Astrometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: Detection Limits for sub-Stellar Companions

We report on a sub-stellar companion search utilizing interferometric fringe-tracking astrometry acquired with Fine Guidance Sensor 3 (FGS 3) on the Hubble Space Telescope. Our targets were Proxima Centauri and Barnard's Star. We obtain absolute parallax values for Proxima Cen pi_{abs} = 0.7687 arcsecond and for Barnard's Star pi_{abs} = 0.5454 arcsecond. Once low-amplitude instrumental systematic errors are identified and removed, our companion detection sensitivity is less than or equal to one Jupiter mass for periods longer than 60 days for Proxima Cen. Between the astrometry and the radial velocity results we exclude all companions with M > 0.8M_{Jup} for the range of periods 1 < P < 1000 days. For Barnard's Star our companion detection sensitivity is less than or equal to one Jupiter mass for periods long er than 150 days. Our null results for Barnard's Star are consistent with those of Gatewood (1995).Comment: 35 pages, 13 figures, to appear in August 1999 A

Dynamical Masses for Low-Mass Pre-Main Sequence Stars: A Preliminary Physical Orbit for HD 98800 B

We report on Keck Interferometer observations of the double-lined binary (B) component of the quadruple pre-main sequence (PMS) system HD 98800. With these interferometric observations combined with astrometric measurements made by the Hubble Space Telescope Fine Guidance Sensors (FGS), and published radial velocity observations we have estimated preliminary visual and physical orbits of the HD 98800 B subsystem. Our orbit model calls for an inclination of 66.8 $\pm$ 3.2 deg, and allows us to infer the masses and luminosities of the individual components. In particular we find component masses of 0.699 $\pm$ 0.064 and 0.582 $\pm$ 0.051 M_{\sun} for the Ba (primary) and Bb (secondary) components respectively. Modeling of the component SEDs finds temperatures and luminosities in agreement with previous studies, and coupled with the component mass estimates allows for comparison with PMS models in the low-mass regime with few empirical constraints. Solar abundance models seem to under-predict the inferred component temperatures and luminosities, while assuming slightly sub-solar abundances bring the models and observations into better agreement. The present preliminary orbit does not yet place significant constraints on existing pre-main sequence stellar models, but prospects for additional observations improving the orbit model and component parameters are very good.Comment: 20 pages, 6 figures, ApJ in press; tables 2 and 3 to be included in ApJ versio

Photometry of Proxima Centauri and Barnard\u27s Star Using Hubble Space Telescope Fine Guidance Sensor 3: A Search for Periodic Variations

We have observed Proxima Centauri and Barnard\u27s star with the Hubble Space Telescope Fine Guidance Sensor 3. Proxima Cen exhibits small-amplitude, periodic photometric variations. Once several sources of systematic photometric error are corrected, we obtain 2 mmag internal photometric precision. We identify two distinct behavior modes over the past 4 years: higher amplitude, longer period and smaller amplitude, shorter period. Within the errors, one period (P ~ 83 days) is twice the other. Barnard\u27s star shows very weak evidence for periodicity on a timescale of approximately 130 days. If we interpret these periodic phenomena as rotational modulation of starspots, we identify three discrete spots on Proxima Cen and possibly one spot on Barnard\u27s star. We find that the disturbances change significantly on timescales as short as one rotation period

Interactions between irregular wave fields and sea ice: A physical model for wave attenuation and ice breakup in an ice tank

Irregular, unidirectional surface water waves incident on model ice in an ice tank are used as a physical model of ocean surface wave interactions with sea ice. Results are given for an experiment consisting of three tests, starting with a continuous ice cover and in which the incident wave steepness increases between tests. The incident waves range from causing no breakup of the ice cover to breakup of the full length of ice cover. Temporal evolution of the ice edge, breaking front, and mean floe sizes are reported. Floe size distributions in the different tests are analyzed. The evolution of the wave spectrum with distance into the ice-covered water is analyzed in terms of changes of energy content, mean wave period, and spectral bandwidth relative to their incident counterparts, and pronounced differences are found between the tests. Further, an empirical attenuation coefficient is derived from the measurements and shown to have a power-law dependence on frequency comparable to that found in field measurements. Links between wave properties and ice breakup are discussed

Advanced Data Chain Technologies for the Next Generation of Earth Observation Satellites Supporting On-Board Processing for Rapid Civil Alerts

The growing number of planned Earth Observation (EO) satellites, together with the increase in payload resolution and swath, brings to the fore the generation of unprecedented volumes of data that needs to be downloaded, processed and distributed with low latency. This creates a severe bottleneck problem, which overloads ground infrastructure, communications to ground, and hampers the provision of EO products to the End User with the required performances. The EO-ALERT project (http://eo-alert-h2020.eu/), an H2020 European Union research activity, proposes the definition of next-generation EO missions by developing an on-board high speed EO data processing chain, based on a novel flight segment architecture that moves optimised key EO data processing elements from the ground segment to on-board the satellite. EO-ALERT achieves, globally, latencies below five minutes for EO products delivery, reaching latencies below 1 minute in some scenarios. The proposed architecture solves the above challenges through a combination of innovations in the on-board elements of the data chain and the communications link. Namely, the architecture introduces innovative technological solutions, including on-board reconfigurable data handling, on-board image generation and processing for generation of alerts (EO products) using Artificial Intelligence (AI), high-speed on-board avionics, on-board data compression and encryption using AI and reconfigurable high data rate communication links to ground including a separate chain for alerts with minimum latency and global coverage. Those key technologies have been studied, developed, implemented in software/hardware (SW/HW) and verified against previously established technologies requirements to meet the identified user needs. The paper presents the development of the innovative solutions defined during the project for each of the above mentioned technological areas and the results of the testing campaign of the individual SW/HW implementations within the context of two operational scenarios: ship detection and extreme weather observation (nowcasting), both requiring a high responsiveness to events to reduce the response time to few hours, or even to minutes, after an emergency situation arises. The technologies have been experimentally evaluated during the project using relevant EO historical sensor data. The results demonstrate the maturity of the technologies, having now reached TRL 4-5. Generally, the results show that, when implemented using COTS components and available communication links, the proposed architecture can generate alerts with a latency lower than five minutes, which demonstrates the viability of the EO-ALERT concept. The paper also discusses the implementation on an Avionic Test Bench (ATB) for the validation of the integrated technologies chain