We report systematic variations in the emission line ratio of the CO J = 2 − 1 and J = 1 − 0 transitions (R 2−1/1−0 ) in the grand-design spiral galaxy M51. The R 2−1/1−0 ratio shows clear evidence for the evolution of molecular gas from the upstream interarm regions, passage into the spiral arms and back into the downstream interarm regions. In the interarm regions, R 2−1/1−0 is typically < 0.7 (and often 0.4-0.6); this is similar to the ratios observed in Galactic giant molecular clouds (GMCs) with low far infrared luminosities. However, the ratio rises to > 0.7 (often 0.8-1.0) in the spiral arms, particularly at the leading (downstream) edge of the molecular arms. These trends are similar to those seen in Galactic GMCs with OB star formation (presumably in the Galactic spiral arms). R 2−1/1−0 is also high, ∼ 0.8 − 1.0, in the central region of M51. Analysis of the molecular excitation using a Large Velocity Gradient radiative transfer calculation provides insight into the changes in the physical conditions of molecular gas between the arm and interarm regions: cold and low density gas ( 10 K, 300 cm −3 ) is required for the interarm GMCs but this gas must become warmer and/or denser in the more active star forming spiral arms. The ratio R 2−1/1−0 is higher in areas of high 24µm dust surface brightness (which is an approximate tracer of star formation rate surface density) and high CO(1-0) integrated intensity (i.e., a well-calibrated tracer of total molecular gas surface density). The systematic enhancement of the CO(2-1) line relative to CO(1-0) in luminous star forming regions suggests that some caution is needed when using CO(2-1) as a tracer of bulk molecular gas mass, especially when galactic structures are resolved.

We report measurements of geometric offsets between gas spiral arms and associated star forming regions in the grand-design spiral galaxy M51. These offsets are a suggested measure of the star formation timescale after the compression of gas at spiral arm entry. A surprising discrepancy, by an order of magnitude, has been reported in recent offset measurements in nearby spiral galaxies. Measurements using CO and Hα emission find large and ordered offsets in M51. On the contrary, small or non-ordered offsets have been found using the HI 21cm and 24µm emissions, possible evidence against gas flow through spiral arms, and thus against the conventional density-wave theory with a stationary spiral pattern. The goal of this paper is to understand the cause of this discrepancy. We investigate potential causes by repeating those previous measurements using equivalent data, methods, and parameters. We find offsets consistent with the previous measurements and conclude that the difference of gas tracers, i.e., HI versus CO, is the primary cause. The HI emission is contaminated significantly by the gas photo-dissociated by recently-formed stars and does not necessarily trace the compressed gas, the precursor of star formation. The HI gas and star forming regions coincide spatially and tend to show small offsets. We find mostly positive offsets with substantial scatter between CO and Hα, suggesting gas flow through spiral arms (i.e., density-wave) though the spiral pattern may not necessarily be stationary.

A growing number of close binary stars are being discovered among central stars of planetary nebulae. Recent and ongoing surveys are finding new systems and contributing to our knowledge of the evolution of close binary systems. The push to find more systems was largely based on early discoveries which suggested that 10%-15% of all central stars are close binaries. One goal of this series of papers is confirmation and classification of these systems as close binaries and determination of binary system parameters. Here we provide time-resolved multiwavelength photometry of the central star of Abell 65 as well as further analysis of the nebula and discussion of possible binary-nebula connections. Our results for Abell 65 confirm recent work showing that it has a close, cool binary companion, though several of our model parameters disagree with the recently published values. With our longer time baseline of photometric observations from 1989 to 2009 we also provide a more precise orbital period of 1.0037577 days.

The digital fluorescent imaging for documentation and analysis of gel electrophoretic separations of nucleic acids and proteins is widely used in quantitative biology. Most fluorescent stains used in postelectrophoretic analysis of proteins and nucleic acids have significant excitation peaks with UV light (300-365 nm), making midrange UV (UV-B) as the excitation source of choice. However, coupling quantitative CCD imaging with UV is difficult due to lack of uniformity found in typical UV transilluminators. The apparent amount of those macromolecules depends on the position of the gel band on the imaging surface of the transilluminator. Here, we report the development and validation of a highly uniform UV transillumination system. Using a novel high density lighting system containing a single lamp formed into a high density grid, an electronic ballast, a phosphor coating, and a bandpass filter to convert 254 nm light produced to 300-340 nm, uniformity of 80% CV observed in typical UV transilluminators. This system has been used for the quantitative analysis of electrophoretically separated nucleic acids and proteins (CV