School Days Fifty Years Ago

Not long ago I had a very interesting conversation with a friend of my grandfather. While discussing my graduation from high school and the coming year at Butler, r was\u27 amazed to discover that he had taught school in a rural district some fifty years ago. Naturally I wanted to know all about it because, although I\u27ve read much about the strict old schoolmasters who :\u27didn\u27t spare the rod, I had never seen or talked to one. As I listened to his tales, I felt almost as if I were reading the Hoosier Schoolmaster

Independent stratum formation on the avian sex chromosomes reveals inter-chromosomal gene conversion and predominance of purifying selection on the w chromosome

We used a comparative approach spanning three species and 90 million years to study the evolutionary history of the avian sex chromosomes. Using whole transcriptomes, we assembled the largest cross-species dataset of W-linked coding content to date. Our results show that recombination suppression in large portions of the avian sex chromosomes has evolved independently, and that long-term sex chromosome divergence is consistent with repeated and independent inversions spreading progressively to restrict recombination. In contrast, over short-term periods we observe heterogeneous and locus-specific divergence. We also uncover four instances of gene conversion between both highly diverged and recently evolved gametologs, suggesting a complex mosaic of recombination suppression across the sex chromosomes. Lastly, evidence from 16 gametologs reveal that the W chromosome is evolving with a significant contribution of purifying selection, consistent with previous findings that W-linked genes play an important role in encoding sex-specific fitness

A high power synchronously-pumped dye laser with diffraction-limited and transform-limited performance

The design and performance of a high-power synchronously pumped dye laser is described. Emphasis has been given to achieving bandwidth-limited and diffraction-limited performance so that efficient, reproducible nonlinear frequency conversion can be achieved. Pulses of 1 mJ from Rhodamine 6G and 250 µJ from DCM with tuning ranges of 557-584 nm and 609-676 nm, respectively, have been obtained. Second harmonic generation with 40 percent efficiency to ~280 nm and efficient multiple Stokes Raman shifting to cover the range 1.1-1.2 µm has been demonstrated. Blue dyes, pumped by third harmonic of Nd:YAG, have been operated. Typically, for Stilbene 3 (peak at 420 nm), pulses of 160µJ were obtained

Molecular ions in L1544. I. Kinematics

We have mapped the dense dark core L1544 in H13CO+(1-0), DCO+(2-1), DCO+(3-2), N2H+(1-0), NTH+(3-2), N2D+(2-1), N2D+(3-2), C18O(1-0), and C17O(1-0) using the IRAM 30-m telescope. We have obtained supplementary observations of HC18O+(1-0), HC17O+(1-0), and D13CO+(2-1). Many of the observed maps show a general correlation with the distribution of dust continuum emission in contrast to C18O(1-0) and C17O(1-0) which give clear evidence for depletion of CO at positions close to the continuum peak. In particular N2D+(2-1) and (3-2) and to a lesser extent N2H+(1-0) appear to be excellent tracers of the dust continuum. We find that the tracers of high density gas (in particular N2D+) show a velocity gradient along the minor axis of the L1544 core and that there is evidence for larger linewidths close to the dust emission peak. We interpret this using the model of the L1544 proposed by Ciolek & Basu (2000) and by comparing the observed velocities with those expected on the basis of their model. The results show reasonable agreement between observations and model in that the velocity gradient along the minor axis and the line broadening toward the center of L1544 are predicted by the model. This is evidence in favour of the idea that amipolar diffusion across field lines is one of the basic processes leading to gravitational collapse. However, line widths are significantly narrower than observed and are better reproduced by the Myers & Zweibel (2001) model which considers the quasistatic vertical contraction of a layer due to dissipation of its Alfvenic turbulence, indicating the importance of this process for cores in the verge of forming a star.Comment: 24 pages, 9 figures, to be published in Ap