7,472 research outputs found
Insight Into the Formation of the Milky Way Through Cold Halo Substructure. III. Statistical Chemical Tagging in the Smooth Halo
Full text linkWe find that the relative contribution of satellite galaxies accreted at high
redshift to the stellar population of the Milky Way's smooth halo increases
with distance, becoming observable relative to the classical smooth halo about
15 kpc from the Galactic center. In particular, we determine
line-of-sight-averaged [Fe/H] and [alpha/Fe] in the metal-poor main-sequence
turnoff (MPMSTO) population along every Sloan Extension for Galactic
Understanding and Exploration (SEGUE) spectroscopic line of sight. Restricting
our sample to those lines of sight along which we do not detect elements of
cold halo substructure (ECHOS), we compile the largest spectroscopic sample of
stars in the smooth component of the halo ever observed in situ beyond 10 kpc.
We find significant spatial autocorrelation in [Fe/H] in the MPMSTO population
in the distant half of our sample beyond about 15 kpc from the Galactic center.
Inside of 15 kpc however, we find no significant spatial autocorrelation in
[Fe/H]. At the same time, we perform SEGUE-like observations of N-body
simulations of Milky Way analog formation. While we find that halos formed
entirely by accreted satellite galaxies provide a poor match to our
observations of the halo within 15 kpc of the Galactic center, we do observe
spatial autocorrelation in [Fe/H] in the simulations at larger distances. This
observation is an example of statistical chemical tagging and indicates that
spatial autocorrelation in metallicity is a generic feature of stellar halos
formed from accreted satellite galaxies.Comment: 27 pages, 8 figures, and 7 tables in emulateapj format; accepted for
publication in ApJ. Full tables can be extracted from LaTeX sourc
Detection of Diatomic Carbon in 2I/Borisov
Get PDF2I/Borisov is the first-ever observed interstellar comet (and the second detected interstellar object (ISO)). It was discovered on 2019 August 30 and has a heliocentric orbital eccentricity of ~3.35, corresponding to a hyperbolic orbit that is unbound to the Sun. Given that it is an ISO, it is of interest to compare its properties—such as composition and activity—with the comets in our solar system. This study reports low-resolution optical spectra of 2I/Borisov. The spectra were obtained by the MDM Observatory Hiltner 2.4 m telescope/Ohio State Multi-Object Spectrograph (on 2019 October 31.5 and November 4.5, UT). The wavelength coverage spanned from 3700 to 9200 Å. The dust continuum reflectance spectra of 2I/Borisov show that the spectral slope is steeper in the blue end of the spectrum (compared to the red). The spectra of 2I/Borisov clearly show CN emission at 3880 Å, as well as C2 emission at both 4750 and 5150 Å. Using a Haser model to covert the observed fluxes into estimates for the molecular production rates, we find Q(CN) = 2.4 ± 0.2 × 10²⁴ s⁻¹, and Q(C₂) = (5.5 ± 0.4) × 10²³ s⁻¹ at the heliocentric distance of 2.145 au. Our Q(CN) estimate is consistent with contemporaneous observations, and the Q(C₂) estimate is generally below the upper limits of previous studies. We derived the ratio Q(C₂)/Q(CN) = 0.2 ± 0.1, which indicates that 2I/Borisov is depleted in carbon-chain species, but is not empty. This feature is not rare for the comets in our solar system, especially in the class of Jupiter-family comets
Future of computer-assisted reporting courses in university journalism programs: A Delphi study
Get PDFBiosynthesis of plant-specific stilbene polyketides in metabolically engineered Escherichia coli
Get PDFBACKGROUND: Phenylpropanoids are the precursors to a range of important plant metabolites such as the cell wall constituent lignin and the secondary metabolites belonging to the flavonoid/stilbene class of compounds. The latter class of plant natural products has been shown to function in a wide range of biological activities. During the last few years an increasing number of health benefits have been associated with these compounds. In particular, they demonstrate potent antioxidant activity and the ability to selectively inhibit certain tyrosine kinases. Biosynthesis of many medicinally important plant secondary metabolites, including stilbenes, is frequently not very well understood and under tight spatial and temporal control, limiting their availability from plant sources. As an alternative, we sought to develop an approach for the biosynthesis of diverse stilbenes by engineered recombinant microbial cells. RESULTS: A pathway for stilbene biosynthesis was constructed in Escherichia coli with 4-coumaroyl CoA ligase 1 4CL1) from Arabidopsis thaliana and stilbene synthase (STS) cloned from Arachis hypogaea. E. coli cultures expressing these enzymes together converted the phenylpropionic acid precursor 4-coumaric acid, added to the growth medium, to the stilbene resveratrol (>100 mg/L). Caffeic acid, added in the same way, resulted in the production of the expected dihydroxylated stilbene, piceatannol (>10 mg/L). Ferulic acid, however, was not converted to the expected stilbene product, isorhapontigenin. Substitution of 4CL1 with a homologous enzyme, 4CL4, with a preference for ferulic acid over 4-coumaric acid, had no effect on the conversion of ferulic acid. Accumulation of tri- and tetraketide lactones from ferulic acid, regardless of the CoA-ligase expressed in E. coli, suggests that STS cannot properly accommodate and fold the tetraketide intermediate to the corresponding stilbene structure. CONCLUSION: Phenylpropionic acids, such as 4-coumaric acid and caffeic acid, can be efficiently converted to stilbene compounds by recombinant E. coli cells expressing plant biosynthetic genes. Optimization of precursor conversion and cyclization of the bulky ferulic acid precursor by host metabolic engineering and protein engineering may afford the synthesis of even more structurally diverse stilbene compounds
Navigating the Return Trip from the Moon Using Earth-Based Ground Tracking and GPS
Get PDFNASA s Constellation Program is planning a human return to the Moon late in the next decade. From a navigation perspective, one of the most critical phases of a lunar mission is the series of burns performed to leave lunar orbit, insert onto a trans-Earth trajectory, and target a precise re-entry corridor in the Earth s atmosphere. A study was conducted to examine sensitivity of the navigation performance during this phase of the mission to the type and availability of tracking data from Earth-based ground stations, and the sensitivity to key error sources. This study also investigated whether GPS measurements could be used to augment Earth-based tracking data, and how far from the Earth GPS measurements would be useful. The ability to track and utilize weak GPS signals transmitted across the limb of the Earth is highly dependent on the configuration and sensitivity of the GPS receiver being used. For this study three GPS configurations were considered: a "standard" GPS receiver with zero dB antenna gain, a "weak signal" GPS receiver with zero dB antenna gain, and a "weak signal" GPS receiver with an Earth-pointing direction antenna (providing 10 dB additional gain). The analysis indicates that with proper selection and configuration of the GPS receiver on the Orion spacecraft, GPS can potentially improve navigation performance during the critical final phases of flight prior to Earth atmospheric entry interface, and may reduce reliance on two-way range tracking from Earth-based ground stations
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