The long-period Galactic Cepheid RS Puppis - III. A geometric distance from HST polarimetric imaging of its light echoes

As one of the most luminous Cepheids in the Milky Way, the 41.5-day RS Puppis is an analog of the long-period Cepheids used to measure extragalactic distances. An accurate distance to this star would therefore help anchor the zero-point of the bright end of the period-luminosity relation. But, at a distance of about 2 kpc, RS Pup is too far away for measuring a direct trigonometric parallax with a precision of a few percent with existing instrumentation. RS Pup is unique in being surrounded by a reflection nebula, whose brightness varies as pulses of light from the Cepheid propagate outwards. We present new polarimetric imaging of the nebula obtained with HST/ACS. The derived map of the degree of linear polarization pL allows us to reconstruct the three-dimensional structure of the dust distribution. To retrieve the scattering angle from the pL value, we consider two different polarization models, one based on a Milky Way dust mixture and one assuming Rayleigh scattering. Considering the derived dust distribution in the nebula, we adjust a model of the phase lag of the photometric variations over selected nebular features to retrieve the distance of RS Pup. We obtain a distance of 1910 +/- 80 pc (4.2%), corresponding to a parallax of 0.524 +/- 0.022 mas. The agreement between the two polarization models we considered is good, but the final uncertainty is dominated by systematics in the adopted model parameters. The distance we obtain is consistent with existing measurements from the literature, but light echoes provide a distance estimate that is not subject to the same systematic uncertainties as other estimators (e.g. the Baade-Wesselink technique). RS Pup therefore provides an important fiducial for the calibration of systematic uncertainties of the long-period Cepheid distance scale.Comment: 14 pages, 14 figures, accepted for publication in Astronomy & Astrophysic

Adzebills of New Zealand.

69 pages : illustrations (some color) ; 26 cm.Relationships among Neoaves, a group comprising approximately 95% of all extant birds, are difficult to resolve because of multiple short internodes presumably created by a rapid evolutionary radiation around the K/Pg boundary. This difficulty has plagued both morphological and molecular studies. Compared with molecular studies with extensive taxon and character sampling, morphological datasets have largely failed to provide insight into the phenotypic evolutionary transitions of the neoavian radiation. Extinct neoavian taxa remain an understudied but critical key to resolving relationships among these problematic stem lineages and understanding evolutionary changes in structure and function. Adzebills (Aptornis), some of the most phylogenetically controversial fossil neoavians, are extinct terrestrial birds endemic to New Zealand since at least the early Miocene. Past morphological studies have placed adzebills as a sister taxon to the flightless Kagu of New Caledonia (Rhynochetos jubatus) or to the land- and waterfowl group Galloanseres. Recent molecular studies reveal the Kagu and Sunbittern (Eurypyga helias) to be sister taxa, whereas adzebills have been postulated to be within Rallidae (rails, gallinules, and coots) or the sister taxon of Sarothruridae (flufftails) or Ralloidea (finfoots, flufftails, and rails). To better resolve the position of adzebills and begin constructing a fine-scale total evidence phylogenetic dataset for the base of Neoaves, we constructed a new and more comprehensive morphological dataset of 368 discrete osteological characters for 38 extant and two extinct taxa that includes extensive sampling of nearly all neoavian stem lineages. We then combined this dataset with 32 DNA sequences of the slowly evolving nuclear RAG1 and RAG2 genes. Morphological results place adzebills as the sister taxon of trumpeters (Psophia) within core Gruiformes and confirm strong support for a Kagu+Sunbittern sister group (99% bootstrap value). Results for analyses of the combined data were identical, and the adzebill+trumpeter clade was supported by a 99% Bayesian clade credibility value. Although the Kagu+Sunbittern sister group is consistent with recent molecular hypotheses, the adzebill+trumpeter group is novel

Flash Mixing of the White Dwarf Cooling Curve Spectroscopic Confirmation in NGC 2808

We present new HST far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of "blue-hook" stars. The blue-hook stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; the hotter temperatures and associated decrease in the hydrogen opacity shortward of the Lyman limit makes the stars brighter in the extreme UV but appear sub luminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the blue-hook stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the blue-hook stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the blue-hook and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in C, Si, and the Fe-peak elements indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 K to 50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our blue-hook sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen

New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve

Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters omega Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf cooling curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear subluminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).Comment: To appear in the Proceedings of the Fifth Meeting on Hot Subdwarf Stars and Related Objects (sdOB5), 25 - 29 July 2011, Stellenbosch, South Africa. ASP Conference Series, 10 pages, 5 figures, LaTe

The Mid-Infrared Instrument for the James Webb Space Telescope, VIII: The MIRI Focal Plane System

We describe the layout and unique features of the focal plane system for MIRI. We begin with the detector array and its readout integrated circuit (combining the amplifier unit cells and the multiplexer), the electronics, and the steps by which the data collection is controlled and the output signals are digitized and delivered to the JWST spacecraft electronics system. We then discuss the operation of this MIRI data system, including detector readout patterns, operation of subarrays, and data formats. Finally, we summarize the performance of the system, including remaining anomalies that need to be corrected in the data pipeline

Biogeography and diversification of Rhegmatorhina (Aves: Thamnophilidae): Implications for the evolution of Amazonian landscapes during the Quaternary

© 2018 John Wiley & Sons Ltd Aim: To test the importance of alternative diversification drivers and biogeographical processes for the evolution of Amazonian upland forest birds through a densely sampled analysis of diversification of the endemic Amazonian genus Rhegmatorhina at multiple taxonomic and temporal scales. Location: Amazonia. Taxon: Antbirds (Thamnophilidae). Methods: We sequenced four mtDNA and nuclear gene regions of 120 individuals from 50 localities representing all recognized species and subspecies of the genus. We performed molecular phylogenetic analyses using both gene tree and species tree methods, molecular dating analysis and estimated population demographic history and gene flow. Results: Dense sampling throughout the distribution of Rhegmatorhina revealed that the main Amazonian rivers delimit the geographic distribution of taxa as inferred from mtDNA lineages. Molecular phylogenetic analyses resulted in a strongly supported phylogenetic hypothesis for the genus, with two main clades currently separated by the Madeira River. Molecular dating analysis indicated diversification during the Quaternary. Reconstruction of recent demographic history of populations revealed a trend for population expansion in eastern Amazonia and stability in the west. Estimates of gene flow corroborate the possibility that migration after divergence had some influence on the current patterns of diversity. Main Conclusions: Based on broad-scale sampling, a clarification of taxonomic boundaries, and strongly supported phylogenetic relationships, we confirm that, first, mitochondrial lineages within this upland forest Amazonian bird genus agree with spatial patterns known for decades based on phenotypes, and second, that most lineages are geographically delimited by the large Amazonian rivers. The association between past demographic changes related to palaeoclimatic cycles and the historically varying strength and size of rivers as barriers to dispersal may be the path to the answer to the long-standing question of identifying the main drivers of Amazonian diversification

Multiple Species And Deep Genomic Divergences Despite Little Phenotypic Differentiation In An Ancient Neotropical Songbird, Tunchiornis Ochraceiceps (Sclater, 1860) (Aves: Vireonidae)

Several bird taxa have been recently described or elevated to full species and almost twice as many bird species than are currently recognized may exist. Defining species is one of the most basic and important issues in biological science because unknown or poorly defined species hamper subsequent studies. Here, we evaluate the species limits and evolutionary history of Tunchiornis ochraceiceps-a widespread forest songbird that occurs in the lowlands of Central America, Choco and Amazonia-using an integrative approach that includes plumage coloration, morphometrics, vocalization and genomic data. The species has a relatively old crown age (similar to 9 Ma) and comprises several lineages with little, if any, evidence of gene flow among them. We propose a taxonomic arrangement composed of four species, three with a plumage coloration diagnosis and one deeply divergent cryptic species. Most of the remaining lineages have variable but unfixed phenotypic characters despite their relatively old origin. This decoupling of genomic and phenotypic differentiation reveals a remarkable case of phenotypic conservatism, possibly due to strict habitat association. Lineages are geographically delimited by the main Amazonian rivers and the Andes, a pattern observed in studies of other understory upland forest Neotropical birds, although phylogenetic relationships and divergence times among populations are idiosyncratic