712 research outputs found
Mitigating Charge Transfer Inefficiency in the Chandra X-ray Observatory's ACIS Instrument
The ACIS front-illuminated CCDs onboard the Chandra X-ray Observatory were
damaged in the extreme environment of the Earth's radiation belts, resulting in
enhanced charge transfer inefficiency (CTI). This produces a row dependence in
gain, event grade, and energy resolution. We model the CTI as a function of
input photon energy, including the effects of de-trapping (charge trailing),
shielding within an event (charge in the leading pixels of the 3X3 event island
protect the rest of the island by filling traps), and non-uniform spatial
distribution of traps. This technique cannot fully recover the degraded energy
resolution, but it reduces the position dependence of gain and grade
distributions. By correcting the grade distributions as well as the event
amplitudes, we can improve the instrument's quantum efficiency. We outline our
model for CTI correction and discuss how the corrector can improve
astrophysical results derived from ACIS data.Comment: Accepted by ApJ Letters; see
http://www.astro.psu.edu/users/townsley/cti
Structure of the Cytoplasmic Loop between Putative Helices II and III of the Mannitol Permease of Escherichia coli: A Tryptophan and 5-Fluorotryptophan Spectroscopy Study
In this work, four single tryptophan (Trp) mutants of the dimeric mannitol transporter of Escherichia coli, EIImtl, are characterized using Trp and 5-fluoroTrp (5-FTrp) fluorescence spectroscopy. The four positions, 97, 114, 126, and 133, are located in a region shown by recent studies to be involved in the mannitol translocation process. To spectroscopically distinguish between the Trp positions in each subunit of dimeric EIImtl, 5-FTrp was biosynthetically incorporated because of its much simpler photophysics compared to those of Trp. The steady-state and time-resolved fluorescence methodologies used point out that all four positions are in structured environments, both in the absence and in the presence of a saturating concentration of mannitol. The fluorescence decay of all 5-FTrp-containing mutants was highly homogeneous, suggesting similar microenvironments for both probes per dimer. However, Stern-Volmer quenching experiments using potassium iodide indicate different solvent accessibilities for the two probes at positions 97 and 133. A 5 Ã… two-dimensional (2D) projection map of the membrane-embedded IICmtl dimer showing 2-fold symmetry is available. The results of this work are in better agreement with a 7 Ã… projection map from a single 2D crystal on which no symmetry was imposed.
Mary of Nemmegen: The ca. 1518 Translation and the Middle Dutch Analogue, Mariken van Nieumeghen
Mary of Nemmegen, a prose condensation in English of the Middle Dutch play Mariken van Nieumeghen, is an important example of the literature that was imported from Holland in the early part of the sixteenth century - literature that helped to establish an English taste for narrative prose fiction. It also may be compared to Everyman, described as a treatise in the manner of a moral play. Mary of Nemmegen is an analogue of the Faustus story, in which a person makes an agreement with the devil; hence the work deserves to be made available as background to Christopher Marlowe\u27s Tragical History of Doctor Faustus as well as to later redactions of the Faust story. As such, it is also a window on the obsession in its own time with the occult, including its relation to the issues of damnation and salvation. Further, because Mary is ultimately saved from damnation and becomes saintly, the story is related to the saint play tradition as exemplified by the Digby Mary Magdalene. The original Dutch drama itself has had a long history in various languages, appearing even in Arabic in the nineteenth century, and is regarded as the most important play of its type from the Dutch Renaissance. It remains a staple reading matter in Dutch education. Mary of Nemmegen: The ca. 1518 Translation and the Middle Dutch Analogue, Mariken van Nieumeghen presents in an old-spelling critical edition, with introduction and notes, along with the text and literal translation of the Dutch play.https://scholarworks.wmich.edu/mip_edam/1005/thumbnail.jp
A Chandra ACIS Study of the Young Star Cluster Trumpler 15 in Carina and Correlation with Near-infrared Sources
Using the highest-resolution X-ray observation of the Trumpler 15 star
cluster taken by the Chandra X-ray Observatory, we estimate the total size of
its stellar population by comparing the X-ray luminosity function of the
detected sources to a calibrator cluster, and identify for the first time a
significant fraction (~14%) of its individual members. The highest-resolution
near-IR observation of Trumpler 15 (taken by the HAWK-I instrument on the VLT)
was found to detect most of our X-ray selected sample of cluster members, with
a K-excess disk frequency of 3.8+-0.7%. The near-IR data, X-ray luminosity
function, and published spectral types of the brightest members support a
cluster age estimate (5-10 Myr) that is older than those for the nearby
Trumpler 14 and Trumpler 16 clusters, and suggest that high-mass members may
have already exploded as supernovae. The morphology of the inner ~0.7 pc core
of the cluster is found to be spherical. However, the outer regions (beyond 2
pc) are elongated, forming an `envelope' of stars that, in projection, appears
to connect Trumpler 15 to Trumpler 14; this morphology supports the view that
these clusters are physically associated. Clear evidence of mass segregation is
seen. This study appears in a Special Issue of the ApJS devoted to the Chandra
Carina Complex Project (CCCP), a 1.42 square degree Chandra X-ray survey of the
Great Nebula in Carina.Comment: Accepted for the ApJS Special Issue on the Chandra Carina Complex
Project (CCCP), scheduled for publication in May 2011. All 16 CCCP Special
Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
least. 30 pages; 8 figures; 3 table
- …