Nearly a century ago it was recognized that radiation absorption by stellar matter controls the internal temperature profiles within stars. Laboratory opacity measurements, however, have never been ...performed at stellar interior conditions, introducing uncertainties in stellar models. A particular problem arose when refined photosphere spectral analysis led to reductions of 30-50 per cent in the inferred amounts of carbon, nitrogen and oxygen in the Sun. Standard solar models using the revised element abundances disagree with helioseismic observations that determine the internal solar structure using acoustic oscillations. This could be resolved if the true mean opacity for the solar interior matter were roughly 15 per cent higher than predicted, because increased opacity compensates for the decreased element abundances. Iron accounts for a quarter of the total opacity at the solar radiation/convection zone boundary. Here we report measurements of wavelength-resolved iron opacity at electron temperatures of 1.9-2.3 million kelvin and electron densities of (0.7-4.0) × 10(22) per cubic centimetre, conditions very similar to those in the solar region that affects the discrepancy the most: the radiation/convection zone boundary. The measured wavelength-dependent opacity is 30-400 per cent higher than predicted. This represents roughly half the change in the mean opacity needed to resolve the solar discrepancy, even though iron is only one of many elements that contribute to opacity.
•An indirect bridge frequency estimation strategy utilizing two vehicles is proposed.•The estimation strategy improves the detectability of the bridge frequency.•Sensitivity of the strategy with ...respect to vehicle’s mass and stiffness variation is investigated.•The estimation strategy is proved to be feasible through a field experiment.
Bridge natural frequencies are among fundamental properties of bridges. However, natural frequencies of most bridges remain unknown. Natural frequency identification through acceleration measurement with sensors installed on bridges is not practical if a large number of bridges are investigated. Indirect methods to detect the frequencies from the acceleration responses of a vehicle driving over bridges, on the other hand, still have difficulties. Vehicle responses contains various components, making it difficult to distinguish the bridge frequency from other frequency components. This study proposes new frequency estimation strategy utilizing two vehicles. The key idea is that bridge vibration, a common vibration component among responses of multiple vehicles, is extracted through signal processing involving cross-spectral density function estimation. Numerical analyses employing a vehicle-bridge interaction (VBI) model are conducted to examine the algorithm performance under various conditions. The numerical simulations have confirmed the feasibility of such indirect frequency detection. An experimental study featuring synchronized-sensing of two vehicles is then performed. The first natural frequency of the bridge has been identified under various driving speed combinations, demonstrating the performance of the proposed approach.
The Serpens South embedded cluster, which is located in the constricted part of a long, filamentary, infrared dark cloud, is believed to be in a very early stage of cluster formation. We present ...results of near-infrared (JHKs) polarization observations of the filamentary cloud. Our polarization measurements of near-infrared point sources indicate a well-ordered global magnetic field that is perpendicular to the main filament, implying that the magnetic field is likely to have controlled the formation of the main filament. On the other hand, the sub-filaments, which converge on the central part of the cluster, tend to run along the magnetic field. The global magnetic field appears to be curved in the southern part of the main filament. Such morphology is consistent with the idea that the global magnetic field is distorted by gravitational contraction along the main filament toward the northern part, which contains larger mass. Applying the Chandrasekhar-Fermi method, the magnetic field strength is roughly estimated to be a few X100 Delta *mG, suggesting that the filamentary cloud is close to magnetically critical.
We present the discovery of two new X-ray transients in archival Chandra data. The first transient, XRT 110103, occurred in 2011 January and shows a sharp rise of at least three orders of magnitude ...in count rate in less than 10 s, a flat peak for about 20 s and decays by two orders of magnitude in the next 60 s. We find no optical or infrared counterpart to this event in pre-existing survey data or in an observation taken by the Simultaneous-3color InfraRed Imager for Unbiased Survey (SIRIUS) instrument at the Infrared Survey Facility ∼2.1 yr after the transient, providing limiting magnitudes of J > 18.1, H > 17.6 and K
s
> 16.3. This event shows similarities to the transient previously reported in Jonker et al. which was interpreted as the possible tidal disruption of a white dwarf by an intermediate-mass black hole. We discuss the possibility that these transients originate from the same type of event. If we assume these events are related a rough estimate of the rates gives 1.4 × 105 per year over the whole sky with a peak 0.3–7 keV X-ray flux greater than 2 × 10−10 erg cm−2 s−1. The second transient, XRT 120830, occurred in 2012 August and shows a rise of at least three orders of magnitude in count rate and a subsequent decay of around one order of magnitude all within 10 s, followed by a slower quasi-exponential decay over the remaining 30 ks of the observation. We detect a likely infrared counterpart with magnitudes J = 16.70 ± 0.06, H = 15.92 ± 0.04 and K
s
= 15.37 ± 0.06 which shows an average proper motion of 74 ± 19 mas yr−1 compared to archival 2MASS observations. The JHK
s
magnitudes, proper motion and X-ray flux of XRT 120830 are consistent with a bright flare from a nearby late M or early L dwarf.
The first systematic study of opacity dependence on atomic number at stellar interior temperatures is used to evaluate discrepancies between measured and modeled iron opacity J. E. Bailey et al., ...Nature (London) 517, 56 (2015)NATUAS0028-083610.1038/nature14048. High-temperature (>180 eV) chromium and nickel opacities are measured with ±6%-10% uncertainty, using the same methods employed in the previous iron experiments. The 10%-20% experiment reproducibility demonstrates experiment reliability. The overall model-data disagreements are smaller than for iron. However, the systematic study reveals shortcomings in models for density effects, excited states, and open L-shell configurations. The 30%-45% underestimate in the modeled quasicontinuum opacity at short wavelengths was observed only from iron and only at temperature above 180 eV. Thus, either opacity theories are missing physics that has nonmonotonic dependence on the number of bound electrons or there is an experimental flaw unique to the iron measurement at temperatures above 180 eV.
Context. The Ophiuchus cluster, at a redshift z = 0.0296, is known from X-rays to be one of the most massive nearby clusters, but its optical properties have not been investigated in detail because ...of its very low Galactic latitude. Aims. We discuss the optical properties of the galaxies in the Ophiuchus cluster, in particular, with the aim of understanding its dynamical properties better. Methods. We have obtained deep optical imaging in several bands with various telescopes, and applied a sophisticated method to model and subtract the contributions of stars to measure galaxy magnitudes as accurately as possible. The colour−magnitude relations obtained show that there are hardly any blue galaxies in Ophiuchus (at least brighter than r′ ≤ 19.5), and this is confirmed by the fact that we only detect two galaxies in Hα. We also obtained a number of spectra with ESO-FORS2, which we combined with previously available redshifts. Altogether, we have 152 galaxies with spectroscopic redshifts in the 0.02 ≤ z ≤ 0.04 range, and 89 galaxies with both a redshift within the cluster redshift range and a measured r′ band magnitude (limited to the Megacam 1 × 1 deg2 field). Results. A complete dynamical analysis based on the galaxy redshifts available shows that the overall cluster is relaxed and has a mass of 1.1 × 1015 M⊙. The Sernal-Gerbal method detects a main structure and a much smaller substructure, which are not separated in projection. Conclusions. From its dynamical properties derived from optical data, the Ophiuchus cluster seems overall to be a relaxed structure, or at most a minor merger, though in X-rays the central region (radius ~ 150 kpc) may show evidence for merging effects.
The spectroscopic method relies on hydrogen Balmer absorption lines to infer white dwarf (WD) masses. These masses depend on the choice of atmosphere model, hydrogen atomic line shape calculation, ...and which Balmer series members are included in the spectral fit. In addition to those variables, spectroscopic masses disagree with those derived using other methods. Here we present laboratory experiments aimed at investigating the main component of the spectroscopic method: hydrogen line shape calculations. These experiments use X-rays from Sandia National Laboratories' Z-machine to create a uniform ∼15 cm3 hydrogen plasma and a ∼4 eV backlighter that enables recording high-quality absorption spectra. The large plasma, volumetric X-ray heating that fosters plasma uniformity, and the ability to collect absorption spectra at WD photosphere conditions are improvements over past laboratory experiments. Analysis of the experimental absorption spectra reveals that electron density ( ) values derived from the Hγ line are ∼34% 7.3% lower than from Hβ. Two potential systematic errors that may contribute to this difference were investigated. A detailed evaluation of self-emission and plasma gradients shows that these phenomena are unlikely to produce any measurable Hβ-Hγ difference. WD masses inferred with the spectroscopic method are proportional to the photosphere density. Hence, the measured Hβ-Hγ difference is qualitatively consistent with the trend that WD masses inferred from their Hβ line are higher than that resulting from the analysis of Hβ and Hγ. This evidence may suggest that current hydrogen line shape calculations are not sufficiently accurate to capture the intricacies of the Balmer series.
The interpretation of x-ray spectra emerging from x-ray binaries and active galactic nuclei accreted plasmas relies on complex physical models for radiation generation and transport in photoionized ...plasmas. These models have not been sufficiently experimentally validated. We have developed a highly reproducible benchmark experiment to study spectrum formation from a photoionized silicon plasma in a regime comparable to astrophysical plasmas. Ionization predictions are higher than inferred from measured absorption spectra. Self-emission measured at adjustable column densities tests radiation transport effects, demonstrating that the resonant Auger destruction assumption used to interpret black hole accretion spectra is inaccurate.
A ‘water spout’ maser jet in S235AB-MIR Burns, R. A; Imai, H; Handa, T ...
Monthly Notices of the Royal Astronomical Society,
11/2015, Letnik:
453, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We report on annual parallax and proper motion observations of H2O masers in S235AB-MIR, which is a massive young stellar object in the Perseus Arm. Using multi-epoch VLBI (very long baseline ...interferometry) astrometry we measured a parallax of π = 0.63 ± 0.03 mas, corresponding to a trigonometric distance of
$D= 1.56^{+0.09}_{-0.08}$
kpc, and source proper motion of (μαcos δ, μδ) = (0.79 ± 0.12, −2.41 ± 0.14) mas yr−1. Water masers trace a jet of diameter 15 au which exhibits a definite radial velocity gradient perpendicular to its axis. 3D maser kinematics were well modelled by a rotating cylinder with physical parameters: v
out = 45 ± 2 km s−1, v
rot = 22 ± 3 km s−1, i = 12° ± 2°, which are the outflow velocity, tangential rotation velocity and line-of-sight inclination, respectively. One maser feature exhibited steady acceleration which may be related to the jet rotation. During our 15-month VLBI programme there were three ‘maser burst’ events caught ‘in the act’ which were caused by the overlapping of masers along the line of sight.
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