1,562 research outputs found
H-alpha features with hot onsets. II. A contrail fibril
The solar chromosphere observed in H-alpha consists mostly of narrow fibrils.
The longest typically originate in network or plage and arch far over adjacent
internetwork. We use data from multiple telescopes to analyze one well-observed
example in a quiet area. It resulted from the earlier passage of an
accelerating disturbance in which the gas was heated to high temperature as in
the spicule-II phenomenon. After this passage a dark H-Halpha fibril appeared
as a contrail. We use Saha-Boltzmann extinction estimation to gauge the onset
and subsequent visibilities in various diagnostics and conclude that such
H-alpha fibrils can indeed be contrail phenomena, not indicative of the
thermodynamic and magnetic environment when they are observed but of more
dynamic happenings before. They do not connect across internetwork cells but
represent launch tracks of heating events and chart magnetic field during
launch, not at present.Comment: Accepted for Astronomy & Astrophysic
Germinant receptor diversity and germination responses of four strains of the Bacillus cereus group
Four strains of the Bacillus cereus group were compared for their germinant receptor composition and spore germination capacity. Phylogenetic analysis of the germinant receptor encoding operons of the enterotoxic strains B. cereus ATCC 14579 and ATCC 10987, the emetic strain AH187, and the psychrotolerant strain Bacillus weihenstephanensis KBAB4, indicated a core group of five germinant receptor operons to be present in the four strains, with each strain containing one to three additional receptors. Using quantitative PCR, induction of expression during sporulation was confirmed for all identified germinant receptor operons in these strains. Despite the large overlap in receptors, diversity in amino acid-induced germination capacity was observed, with six out of 20 amino acids, serving as germinants for spores of all four strains. Each strain showed unique features: efficient germination of strain KBAB4 spores required non-inducing amounts of inosine as the co-germinant, strain ATCC 10987 spores germinated only efficiently after heat activation. Furthermore, strain ATCC 14579 and AH187 spores germinated without heat activation or inosine, with strain ATCC 14579 spores being triggered by all amino acids except phenylalanine and strain AH187 spores being specifically triggered efficiently only by phenylalanine. Analysis of all germination data did not reveal strict linkages between specific germinants and germinant receptors. Finally, the diversity in nutrient-induced germination capacity was also reflected in the diverse germination responses of heat-activated spores of the four B. cereus strains in food matrices, such as milk, rice water and meat bouillon, indicating that amino acid composition and/or availability of inosine are important germination determinants in foods. Keywords: Ger operon; Food preservation; B. weihenstephanensis; Sporulatio
Dynamics of the solar magnetic bright points derived from their horizontal motions
The sub-arcsec bright points (BP) associated with the small scale magnetic
fields in the lower solar atmosphere are advected by the evolution of the
photospheric granules. We measure various quantities related to the horizontal
motions of the BPs observed in two wavelengths, including the velocity
auto-correlation function. A 1 hr time sequence of wideband H
observations conducted at the \textit{Swedish 1-m Solar Telescope}
(\textit{SST}), and a 4 hr \textit{Hinode} \textit{G}-band time sequence
observed with the Solar Optical telescope are used in this work. We follow 97
\textit{SST} and 212 \textit{Hinode} BPs with 3800 and 1950 individual velocity
measurements respectively. For its high cadence of 5 s as compared to 30 s for
\textit{Hinode} data, we emphasize more on the results from \textit{SST} data.
The BP positional uncertainty achieved by \textit{SST} is as low as 3 km. The
position errors contribute 0.75 km s to the variance of the observed
velocities. The \textit{raw} and \textit{corrected} velocity measurements in
both directions, i.e., , have Gaussian distributions with standard
deviations of and km s respectively. The BP
motions have correlation times of about s. We construct the power
spectrum of the horizontal motions as a function of frequency, a quantity that
is useful and relevant to the studies of generation of Alfv\'en waves.
Photospheric turbulent diffusion at time scales less than 200 s is found to
satisfy a power law with an index of 1.59.Comment: Accepted for publication in The Astrophysical Journal. 24 pages, 9
figures, and 1 movie (not included
Effect of the C-bridge length on the ultraviolet-resistance of oxycarbosilane low-k films
The ultra-violet (UV) and vacuum ultra-violet (VUV) resistance of bridging alkylene groups in organosilica films has been investigated. Similar to the Si-CH3 (methyl) bonds, the Si-CH2-Si (methylene) bonds are not affected by 5.6 eV irradiation. On the other hand, the concentration of the Si-CH2-CH2-Si (ethylene) groups decreases during such UV exposure. More significant difference in alkylene reduction is observed when the films are exposed to VUV (7.2 eV). The ethylene groups are depleted by more than 75% while only about 40% methylene and methyl groups loss is observed. The different sensitivity of bridging groups to VUV light should be taken into account during the development of curing and plasma etch processes of low-k materials based on periodic mesoporous organosilicas and oxycarbosilanes. The experimental results are qualitatively supported by ab-initio quantum-chemical calculations
Strongly Time-Variable Ultra-Violet Metal Line Emission from the Circum-Galactic Medium of High-Redshift Galaxies
We use cosmological simulations from the Feedback In Realistic Environments
(FIRE) project, which implement a comprehensive set of stellar feedback
processes, to study ultra-violet (UV) metal line emission from the
circum-galactic medium of high-redshift (z=2-4) galaxies. Our simulations cover
the halo mass range Mh ~ 2x10^11 - 8.5x10^12 Msun at z=2, representative of
Lyman break galaxies. Of the transitions we analyze, the low-ionization C III
(977 A) and Si III (1207 A) emission lines are the most luminous, with C IV
(1548 A) and Si IV (1394 A) also showing interesting spatially-extended
structures. The more massive halos are on average more UV-luminous. The UV
metal line emission from galactic halos in our simulations arises primarily
from collisionally ionized gas and is strongly time variable, with
peak-to-trough variations of up to ~2 dex. The peaks of UV metal line
luminosity correspond closely to massive and energetic mass outflow events,
which follow bursts of star formation and inject sufficient energy into
galactic halos to power the metal line emission. The strong time variability
implies that even some relatively low-mass halos may be detectable. Conversely,
flux-limited samples will be biased toward halos whose central galaxy has
recently experienced a strong burst of star formation. Spatially-extended UV
metal line emission around high-redshift galaxies should be detectable by
current and upcoming integral field spectrographs such as the Multi Unit
Spectroscopic Explorer (MUSE) on the Very Large Telescope and Keck Cosmic Web
Imager (KCWI).Comment: 16 pages, 8 figures, accepted for publication in MNRA
On the Moat-Penumbra Relation
Proper motions in a sunspot group with a delta-configuration and close to the
solar disc center have been studied by employing local correlation tracking
techniques. The analysis is based on more than one hour time series of G-band
images. Radial outflows with a mean speed of 0.67 km s^{-1} have been detected
around the spots, the well-known sunspots moats. However, these outflows are
not found in those umbral core sides without penumbra. Moreover, moat flows are
only found in those sides of penumbrae located in the direction marked by the
penumbral filaments. Penumbral sides perpendicular to them show no moat flow.
These results strongly suggest a relation between the moat flow and the
well-known, filament aligned, Evershed flow. The standard picture of a moat
flow originated from a blocking of the upward propagation of heat is commented
in some detail.Comment: 4 pages, 4 figures, To appear in ApJ Letter
The effect of magnetic fields on properties of the circumgalactic medium
We study the effect of magnetic fields on a simulated galaxy and its surrounding gaseous halo, or circumgalactic medium (CGM), within cosmological 'zoom-in' simulations of a Milky Way-mass galaxy as part of the Simulating the Universe with Refined Galaxy Environments (SURGE) project. We use three different galaxy formation models, each with and without magnetic fields, and include additional spatial refinement in the CGM to improve its resolution. The central galaxy's star formation rate and stellar mass are not strongly affected by the presence of magnetic fields, but the galaxy is more disc dominated and its central black hole is more massive when B > 0. The physical properties of the CGM change significantly. With magnetic fields, the circumgalactic gas flows are slower, the atomic hydrogen-dominated extended discs around the galaxy are more massive and the densities in the inner CGM are therefore higher, the temperatures in the outer CGM are higher, and the pressure in the halo is higher and smoother. The total gas fraction and metal mass fraction in the halo are also higher when magnetic fields are included, because less gas escapes the halo. Additionally, we find that the CGM properties depend on azimuthal angle and that magnetic fields reduce the scatter in radial velocity, whilst enhancing the scatter in metallicity at fixed azimuthal angle. The metals are thus less well-mixed throughout the halo, resulting in more metal-poor halo gas. These results together show that magnetic fields in the CGM change the flow of gas in galaxy haloes, making it more difficult for metal-rich outflows to mix with the metal-poor CGM and to escape the halo, and therefore should be included in simulations of galaxy formation
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