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Comprehensive maritime domain awareness includes detection, tracking, and identification of vessels, and we describe work concerned with the detection and tracking elements of this problem. Millions of maritime vessels operate without the means for their positions to be independently tracked, presenting a problem for safety and security of life at sea. To date, visible wavelength imaging from space has been of limited use for vessel detection and tracking due to poor area coverage provided by individual satellites. This situation is now changing. We present a survey of currently operating spacecraft carrying electro-optical imagers offering adequate imaging resolution and commercial data availability. We model the coverage provided by 54 satellites and 85 sensors over a target area to assess the value of these assets for space-based maritime domain awareness. The results show that useful levels of coverage can now be obtained through the collective observations of existing spacecraft, although on-board resources will limit the amount of imagery that can be acquired. The launch of large numbers of high-resolution imaging nanosatellites produced by private companies will improve this coverage, and the increasing capability of small satellite platforms offers the possibility of a dedicated Electro-Optical Space-Based Maritime Domain Awareness constellation that can realise the full benefit of the concept. We propose a cooperative approach, based on cloud computing and crowdsourcing philosophies, to the operation of the ground segment and the sharing and analysis of image data, to create an effective constellation of satellites and associated data handling infrastructure that can be used to enhance maritime security, and improve the safety of lives at sea. [ABSTRACT FROM PUBLISHER]

We present spectra of the DAB white dwarf PG 1115+166. Radial velocity measurements of the Balmer lines and the He i 6678 Å line show that this is a binary white dwarf with a period of 30.09 d in which the Balmer lines move in anti-phase to the He i line, i.e. PG 1115+166 is a DA+DB binary. The minimum masses of the stars are M[sub DA]= 0.43 ± 0.15 M[sub ⊙] and M[sub DB]= 0.52 ± 0.12 M[sub ⊙]. The separation of the stars is about 45 R[sub ⊙], which is much smaller than a typical asymptotic giant branch (AGB) progenitor of a white dwarf, implying that there has been at least one common envelope phase in this binary. Indeed, it is possible that this binary may have suffered up to three mass transfer episodes – two associated with the red giant phase prior to the formation of each white dwarf, and a third associated with the ‘born-again’ red giant phase of the DB white dwarf. PG 1115+166 has the longest orbital period of any post common envelope white dwarf–white dwarf binary found to date. Published models for the formation of white dwarf–white dwarf binaries do not predict any white dwarfs with the combination of a long orbital period and high mass found in PG 1115+166. We conclude that PG 1115+166 is a key object for testing models of binary star evolution, and it may also be a key object for our understanding of the formation of DB white dwarfs. We outline the observational tests which can be applied to scenarios for the formation of PG 1115+166 and apply them to the simplest case of a single common-envelope phase. This suggests that some part of the internal energy stored in the envelope of the AGB star, e.g. as ionized hydrogen, may have contributed to the ejection of the common envelope, but there are several unanswered questions concerning this simple scenario. [ABSTRACT FROM AUTHOR]

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the very hot (T[sub eff]≈60 000 K) DA white dwarf PG1342+444, describing our data reduction and analysis techniques. The spectrum reveals a number of photospheric absorption lines from high ionization species along with numerous interstellar features. The photospheric detections include the 1031.9- and 1037.0-Å O vi lines which are seen for the first time in a hot DA atmosphere and are usually associated with the much hotter PG1159 stars and so-called O vi central stars of planetary nebulae. Estimates of the stellar effective temperature made independently using both the Balmer and Lyman series lines are in disagreement (T[sub eff]≈67 000 and ≈54 000 K respectively), when taking into account just the statistical uncertainties in the analyses. However, the presence of weak absorption from the C iii multiplet near 1176 Å, which is predicted to be much stronger if the star were as cool as the Lyman measurement suggests, leads us to favour the higher temperature. PG1342+444 appears to have enhanced C, Fe and Ni abundances in its atmosphere compared with all the other G191-B2B-like DA white dwarfs, which might affect the temperature structure of the atmosphere if not homogeneously distributed, as assumed in this study. [ABSTRACT FROM AUTHOR]

We report a study of the photospheric composition of the hot DA white dwarf WD 2218+706, which is also the central star of the old planetary nebula DeHt5. Helium is detected in the far-UV spectrum. In addition, the star clearly contains significant quantities of elements heavier than He at abundances generally a factor of 2 to 10 higher than those found in the archetypal heavy element-rich DA G191-B2B. This is the first detection of trace He using the He iiλ1640 line in an isolated DA white dwarf, but the low surface gravity is more indicative of a binary evolution route from the red giant branch rather than a path along the asymptotic giant branch (AGB) as a single star. However, the absence of any evidence for a companion star and the uncertainty in the measured mass for WD 2218+706 still allow the possibility of an origin along an AGB evolutionary track. We reanalyse the existing optical spectra of WD 2218+706 using our latest pure H and heavy element-rich model atmospheres, obtaining a good match between the observed and synthetic spectra with either set of models. We find little evidence of any inconsistency in the temperature required to fit individual Balmer lines, as reported elsewhere for this star. Any discrepancies we see are confined to the Hα line and the core of Hβ but they do not compromise our analysis. [ABSTRACT FROM AUTHOR]