2,451 research outputs found
Average motion of emerging solar active region polarities I: Two phases of emergence
Our goal is to constrain models of active region formation by tracking the
average motion of active region polarity pairs as they emerge onto the surface.
We measured the motion of the two main opposite polarities in 153 emerging
active regions (EARs) using line-of-sight magnetic field observations from the
Solar Dynamics Observatory Helioseismic Emerging Active Region (SDO/HEAR)
survey (Schunker et al. 2016). We first measured the position of each of the
polarities eight hours after emergence and tracked their location forwards and
backwards in time. We find that, on average, the polarities emerge with an
east-west orientation and the separation speed between the polarities
increases. At about 0.1 days after emergence, the average separation speed
reaches a peak value of 229 +/- 11 m/s, and then starts to decrease, and about
2.5 days after emergence the polarities stop separating. We also find that the
separation and the separation speed in the east-west direction are
systematically larger for active regions with higher flux. Our results reveal
two phases of the emergence process defined by the rate of change of the
separation speed as the polarities move apart. Phase 1 begins when the opposite
polarity pairs first appear at the surface, with an east-west alignment and an
increasing separation speed. We define Phase 2 to begin when the separation
speed starts to decrease, and ends when the polarities have stopped separating.
This is consistent with the picture of Chen, Rempel, & Fan (2017): the peak of
a flux tube breaks through the surface during Phase 1. During Phase 2 the
magnetic field lines are straightened by magnetic tension, so that the
polarities continue to move apart, until they eventually lie directly above
their anchored subsurface footpoints.Comment: accepted A&
Sexual selection on plumage and behavior in an avian hybrid zone: Experimental tests of male-male interactions
In western Panama, an unusual hybrid zone exists between white-collared manakins, Manacus candei, and golden-collared manakins, M. vitellinus. Unidirectional introgression of plumage traits from vitellinus into candei has created a region in which all definitively plumaged males have a collar that is lemon-colored. These males are nearly indistinguishable from white-collared candei genetically and morphometrically, but strongly resemble golden-collared vitellinus due to the introgression of secondary sexual plumage traits, particularly the lemon-colored collar. The introgression could be explained by sexual selection for golden-collared traits or by a series of mechanisms that do not invoke sexual selection (e.g., neutral diffusion, dominant allele). Sexual selection on male-male interactions implies behavioral differences among the plumage forms - specifically that golden- and lemon-collared males should be more aggressive than white-collared males, In contrast, the nonsexual hypotheses predict behavioral similarity between lemon- and white-collared males, based on their nearly identical genetics. We tested the sexual selection hypothesis experimentally, by presenting males with taxidermic mounts of the three forms. As response variables, we monitored vocalizations and attacks on the mounts by replicate subject males. Both golden-collared and lemon-collared males were more likely to attack than were white-collared males, as predicted under sexual selection but not by the nonsexual hypotheses. Lemon-collared males were more vocally reactive than either parental form, contrary to the prediction of the nonsexual hypotheses. Our study demonstrates that sexual selection on male-male interactions may play an important role in the dynamics of character evolution and hybrid zones
Identification of intramembrane hydrogen bonding between 131 keto group of bacteriochlorophyll and serine residue α27 in the LH2 light-harvesting complex
AbstractIntramembrane hydrogen bonding and its effect on the structural integrity of purple bacterial light-harvesting complex 2, LH2, have been assessed in the native membrane environment. A novel hydrogen bond has been identified by Raman resonance spectroscopy between a serine residue of the membrane-spanning region of LH2 α-subunit, and the C-131 keto carbonyl of bacteriochlorophyll (BChl) B850 bound to the ÎČ-subunit. Replacement of the serine by alanine disrupts this strong hydrogen bond, but this neither alters the strongly red-shifted absorption nor the structural arrangement of the BChls, as judged from circular dichroism. It also decreases only slightly the thermal stability of the mutated LH2 in the native membrane environment. The possibility is discussed that weak H-bonding between the C-131 keto carbonyl and a methyl hydrogen of the alanine replacing serine(â4) or the imidazole group of the nearby histidine maintains structural integrity in this very stable bacterial light-harvesting complex. A more widespread occurrence of H-bonding to C-131 not only in BChl, but also in chlorophyll proteins, is indicated by a theoretical analysis of chlorophyll/polypeptide contacts at <3.5 Ă
in the high-resolution structure of Photosystem I. Nearly half of the 96 chlorophylls have aa residues suitable as hydrogen bond donors to their keto groups
Supernova Remnants in the Fossil Starburst in M82
We report the discovery of ten compact H-alpha-bright sources in the
post-starburst region northeast of the center of M82, ``M82 B.'' These objects
have H alpha luminosities and sizes consistent with Type II supernova remnants
(SNRs). They fall on the same H alpha surface brightness-diameter (Sigma-D)
relation defined by SNRs in other nearby star-forming galaxies, with the M82
candidates lying preferentially at the small diameter end. These are the first
candidates for optically-visible SNRs in M82 outside the heavily obscured
central starburst within ~250 pc from the galactic center. If these sources are
SNRs, they set an upper limit to the end of the starburst in region ``B2,''
about 500 pc from the galaxy's core, of ~50 Myr. Region ``B1,'' about 1000 pc
from the core, lacks good SNR candidates and is evidently somewhat older. This
suggests star formation in the galaxy has propagated inward toward the
present-day intense starburst core.Comment: Re-submitted to AJ, referee's comments taken into account, 15 pages
LaTeX preprint style, 4 postscript figures; full-resolution figures available
from http://www.astro.virginia.edu/~rd7a/snrs/ Changes: minor textual changes
and orientation/axes of Fig.
Spectrophotometry of HII Regions, Diffuse Ionized Gas and Supernova Remnants in M31: The Transition from Photo- to Shock-Ionization
We present results of KPNO 4-m optical spectroscopy of discrete emission-line
nebulae and regions of diffuse ionized gas (DIG) in M31. Long-slit spectra of
16 positions in the NE half of M31 were obtained over a 5-15 kpc range in
radial distance from the center of the galaxy. The spectra have been used to
confirm 16 supernova remnant candidates from the Braun & Walterbos (1993)
catalog. The slits also covered 46 HII regions which show significant
differences among the various morphological types (center-brightened, diffuse,
rings). Radial gradients in emission-line ratios such as [OIII]/H and
[OII]/[OIII] are observed most prominently in the center-brightened HII
regions. These line ratio trends are either much weaker or completely absent in
the diffuse and ring nebulae. The line ratio gradients previously seen in M31
SNRs (Blair, Kirshner, & Chevalier 1981; 1982) are well reproduced by our new
data. The spectra of center-brightened HII regions and SNRs confirm previous
determinations of the radial abundance gradient in M31. We use diagnostic
diagrams which separate photoionized gas from shock-ionized gas to compare the
spectral properties of HII regions, SNRs and DIG. This analysis strengthens
earlier claims (Greenawalt, Walterbos, & Braun 1997) that the DIG in the disk
of M31 is photoionized by a dilute radiation field.Comment: 45 pages, 9 figures, 7 tables, to appear in the Astronomical Journal
(December 1999
Numerical Hermitian Yang-Mills Connections and Vector Bundle Stability in Heterotic Theories
A numerical algorithm is presented for explicitly computing the gauge
connection on slope-stable holomorphic vector bundles on Calabi-Yau manifolds.
To illustrate this algorithm, we calculate the connections on stable monad
bundles defined on the K3 twofold and Quintic threefold. An error measure is
introduced to determine how closely our algorithmic connection approximates a
solution to the Hermitian Yang-Mills equations. We then extend our results by
investigating the behavior of non slope-stable bundles. In a variety of
examples, it is shown that the failure of these bundles to satisfy the
Hermitian Yang-Mills equations, including field-strength singularities, can be
accurately reproduced numerically. These results make it possible to
numerically determine whether or not a vector bundle is slope-stable, thus
providing an important new tool in the exploration of heterotic vacua.Comment: 52 pages, 15 figures. LaTex formatting of figures corrected in
version 2
Opening the door to greater phylogeographic inference in Southeast Asia: Comparative genomic study of five codistributed rainforest bird species using target capture and historical DNA
Indochina and Sundaland are biologically diverse, interconnected regions of Southeast Asia with complex geographic histories. Few studies have examined phylogeography of bird species that span the two regions because of inadequate population sampling. To determine how geographic barriers/events and disparate dispersal potential have influenced the population structure, gene flow, and demographics of species that occupy the entire area, we studied five largely codistributed rainforest bird species: Arachnothera longirostra, Irena puella, Brachypodius atriceps, Niltava grandis, and Stachyris nigriceps. We accomplished relatively thorough sampling and data collection by sequencing ultraconserved elements (UCEs) using DNA extracted from modern and older (historical) specimens. We obtained a genome-wide set of 753â4,501 variable loci and 3,919â18,472 single nucleotide polymorphisms. The formation of major within-species lineages occurred within a similar span of time (0.5â1.5 mya). Major patterns in population genetic structure are largely consistent with the dispersal potential and habitat requirements of the study species. A population break across the Isthmus of Kra was shared only by the two hill/submontane insectivores (N. grandis and S. nigriceps). Across Sundaland, there is little structure in B. atriceps, which is a eurytopic and partially frugivorous species that often utilizes forest edges. Two other eurytopic species, A. longirostra and I. puella, possess highly divergent populations in peripheral Sunda Islands (Java and/or Palawan) and India. These species probably possess intermediate dispersal abilities that allowed them to colonize new areas, and then remained largely isolated subsequently. We also observed an eastâwest break in Indochina that was shared by B. atriceps and S. nigriceps, species with very different habitat requirements and dispersal potential. By analyzing high-throughput DNA data, our study provides an unprecedented comparative perspective on the process of avian population divergence across Southeast Asia, a process that is determined by geography, species characteristics, and the stochastic nature of dispersal and vicariance events
Crossover Scaling in Dendritic Evolution at Low Undercooling
We examine scaling in two-dimensional simulations of dendritic growth at low
undercooling, as well as in three-dimensional pivalic acid dendrites grown on
NASA's USMP-4 Isothermal Dendritic Growth Experiment. We report new results on
self-similar evolution in both the experiments and simulations. We find that
the time dependent scaling of our low undercooling simulations displays a
cross-over scaling from a regime different than that characterizing Laplacian
growth to steady-state growth
- âŠ