364 research outputs found
A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globular
clusters, obtained with the Blanco 4-m telescope and the R-C spectrograph at
the Cerro Tololo Interamerican Observatory. The spectra cover the range ~ 3350
-- 6430 A with ~ 3.1 A (FWHM) resolution. The spectroscopic observations and
data reduction were designed to integrate the full projected area within the
cluster core radii in order to properly sample the light from stars in all
relevant evolutionary stages. The S/N values of the flux-calibrated spectra
range from 50 to 240/A at 4000 A and from 125 to 500/A at 5000 A. The selected
targets span a wide range of cluster parameters, including metallicity,
horizontal-branch morphology, Galactic coordinates, Galactocentric distance,
and concentration. The total sample is thus fairly representative of the entire
Galactic globular cluster population and should be valuable for comparison with
similar integrated spectra of unresolved stellar populations in remote systems.
For most of the library clusters, our spectra can be coupled with deep
color-magnitude diagrams and reliable metal abundances from the literature to
enable the calibration of stellar population synthesis models. In this paper we
present a detailed account of the observations and data reduction. The spectral
library is publicly available in electronic format from the National Optical
Astronomical Observatory website.Comment: 39 Pages, including 2 tables and 15 Figures. To appear in the
Astrophysical Journal, Supplement Serie
Chandra X-Ray Spectroscopy Of The Very Early O Supergiant HD 93129A: Constraints On Wind Shocks And The Mass-Loss Rate
We present an analysis of both the resolved X-ray emission-line profiles and the broad-band X-ray spectrum of the O-2 If* star HD 93129A, measured with the Chandra High Energy Transmission Grating Spectrometer ( HETGS). This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind-shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10 per cent of the X-ray flux. We find results consistent with the predictions of numerical simulations of the line-driving instability, including line broadening indicating an onset radius of X-ray emission of several tenths of R-*. Helium-like forbidden-to-intercombination line ratios are consistent with this onset radius, and inconsistent with being formed in a wind-collision interface with the star\u27s closest visual companion at a distance of 100 au. The broad-band X-ray spectrum is fitted with a dominant emission temperature of just kT = 0.6 keV along with significant wind absorption. The broad-band wind absorption and the line profiles provide two independent measurements of the wind mass-loss rate:. M = 5.2(-1.5)(+1.8) x 10(-6) and 6.8(-2.2)(+2.8) x 10(-6) M-circle dot yr(-1), respectively. This is the first consistent modelling of the X-ray line-profile shapes and broad-band X-ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3-4 compared to the standard H alpha mass-loss rate that assumes a smooth wind
The Identification of Blue Horizontal Branch Stars in the Integrated Spectra of Globular Clusters
A major uncertainty in the spectroscopic dating of extragalactic globular
clusters concerns the degenerate effect that age and horizontal branch
morphology have on the strength of Balmer lines. In this Letter we show that
the ratio between the equivalent widths of Hdelta and Hbeta is far more
sensitive to horizontal branch morphology than to age, thus making it possible
to break the degeneracy. We show that it is possible to distinguish
intermediate-age globular clusters from those whose Balmer lines are
strengthened by the presence of blue horizontal branch stars, purely on the
basis of the clusters' integrated spectra. The degeneracy between age and
horizontal branch morphology can be lifted with Hbeta and Hdelta line strengths
from spectra with S/N >= 30 per Angstrom, which is typical of current studies
of integrated spectroscopy of extragalactic globular clusters.Comment: 4 pages. To appear in the Astrophysical Journal Letter
Modeling broadband X-ray absorption of massive star winds
We present a method for computing the net transmission of X-rays emitted by
shock-heated plasma distributed throughout a partially optically thick stellar
wind from a massive star. We find the transmission by an exact integration of
the formal solution, assuming that the emitting plasma and absorbing plasma are
mixed at a constant mass ratio above some minimum radius, below which there is
assumed to be no emission. This model is more realistic than either the slab
absorption associated with a corona at the base of the wind or the exospheric
approximation that assumes that all observed X-rays are emitted without
attenuation from above the radius of optical depth unity. Our model is
implemented in XSPEC as a pre-calculated table that can be coupled to a
user-defined table of the wavelength dependent wind opacity. We provide a
default wind opacity model that is more representative of real wind opacities
than the commonly used neutral interstellar medium (ISM) tabulation.
Preliminary modeling of \textit{Chandra} grating data indicates that the X-ray
hardness trend of OB stars with spectral subtype can largely be understood as a
wind absorption effect.Comment: 9 pages, 9 figures. Includes minor corrections made in proof
Timing of deployment does not affect the biodiversity outcomes of ecological enhancement of coastal flood defences in northern Europe
Timing of installation is an important factor when planning the deployment of ecological enhancements to intertidal coastal and marine infrastructure. Such nature-based solutions (NbS) are increasingly used worldwide, so understanding whether the timing of deployment affects colonisation success is crucial to enhance their success and identify any ecological sensitivities that must be taken into consideration during construction. To date, none of the previous marine eco-engineering studies globally have looked specifically at timing. An unexpected COVID19 interruption in retrofitting Ecotiles designed to improve urban marine biodiversity provided a unique window of opportunity to address this research gap. We examined if time of deployment affects the early colonisation (within 18 months) success of eco-engineering enhancements. Thirty concrete tiles (Ecotiles) cast with a novel multi-scale, multi-species textured formliner were deployed on rock armour in three sites along the coast in Edinburgh, Scotland, at two different time periods (early March and late May 2020). After two settlement seasons, the colonisation success of 85% of the studied species did not vary between the times of deployment. Early colonisation success of intertidal species equalised within two settlement seasons of deployment, along with an overall increase in species richness. Crucially, these results also show that summer construction periods designed to reduce impacts on overwintering birds, do not adversely impact intertidal species during their peak (spring-summer) recruitment period in northern Europe. This novel result provides further support for widespread use of eco-engineering to enhance large coastal infrastructure projects and achieve ecological goals in northern Europe. More widely, this work contributes to the understanding of the impact of deployment timing on the success of similar NbS worldwide
Titration-based screening for evaluation of natural product extracts: identification of an aspulvinone family of luciferase inhibitors
The chemical diversity of nature has tremendous potential for discovery of new molecular probes and medicinal agents. However, sensitivity of HTS assays to interfering components of crude extracts derived from plants, macro- and microorganisms has curtailed their use in lead discovery efforts. Here we describe a process for leveraging the concentration-response curves (CRCs) obtained from quantitative HTS to improve the initial selection of “actives” from a library of partially fractionated natural product extracts derived from marine actinomycetes and fungi. By using pharmacological activity, the first-pass CRC paradigm aims to improve the probability that labor-intensive subsequent steps of re-culturing, extraction and bioassay-guided isolation of active component(s) target the most promising strains and growth conditions. We illustrate how this process identified a family of fungal metabolites as potent inhibitors of firefly luciferase, subsequently resolved in molecular detail by x-ray crystallography
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