Force dependent fragility in RNA hairpins

We apply Kramers theory to investigate the dissociation of multiple bonds under mechanical force and interpret experimental results for the unfolding/refolding force distributions of an RNA hairpin pulled at different loading rates using laser tweezers. We identify two different kinetic regimes depending on the range of forces explored during the unfolding and refolding process. The present approach extends the range of validity of the two-states approximation by providing a theoretical framework to reconstruct free-energy landscapes and identify force-induced structural changes in molecular transition states using single molecule pulling experiments. The method should be applicable to RNA hairpins with multiple kinetic barriers.Comment: Latex file, 4 pages+3 figure

The first detection of near-infrared CN bands in active galactic nuclei: signature of star formation

We present the first detection of the near-infrared CN absorption band in the nuclear spectra of active galactic nuclei (AGN). This feature is a recent star formation tracer, being particularly strong in carbon stars. The equivalent width of the CN line correlates with that of the CO at 2.3 microns, as expected in stellar populations (SP) with ages between ~ 0.2 and ~ 2 Gyr. The presence of the 1.1 microns CN band in the spectra of the sources is taken as an unambiguous evidence of the presence of young/intermediate SP close to the central source of the AGN. Near-infrared bands can be powerful age indicators for star formation connected to AGN, the understanding of which is crucial in the context of galaxy formation and AGN feedback.Comment: Accepted for publication in The Astrophysical Journal Letters. 4 pages, 3 figure

A Quantitative Evaluation of the Nighttime Visual Sign Inspection Method

A research project to determine the appropriate sign inspection and replacement procedure was conducted at North Carolina State University and sponsored by the North Carolina DOT. The purpose was to determine the optimum strategy for sign inspection and replacement under different conditions to respond to the pending retroreflectivity requirements. This paper reports on a spreadsheet tool developed to quantitatively evaluate the effectiveness of different sign inspection and replacement scenarios. The spreadsheet was designed for yellow and red engineer-grade sign sheetings, and takes into account sign vandalism and knock-downs as well as normal sign aging. The spreadsheet provides estimates of the number of signs in place that would not meet the minimum retroreflectivity standard and the cost of the sign inspection and replacement program. The results from a number of trials of the spreadsheet show that agencies that generally conform to the key assumptions made to build the spreadsheet should consider replacing all signs every seven years, as that insures that no aged signs are in place at a relatively low cost. If total replacement is not possible, an inspection program using retroreflectometers every three years appears very competitive in its effectiveness with a program using typical visual inspection rates each year. The retroreflectometers appear to allow fewer deficient signs, while the typical visual inspection program costs are lower for a given vandalism rate. More conservative visual sign replacement rates do not appear to offer distinct advantages, because typical replacement rates with visual inspections every two or three years allow relatively high numbers of deficient signs to remain on the roads

A Quantitative Evaluation of the Nighttime Visual Sign Inspection Method

A research project to determine the appropriate sign inspection and replacement procedure was conducted at North Carolina State University and sponsored by the North Carolina DOT. The purpose was to determine the optimum strategy for sign inspection and replacement under different conditions to respond to the pending retroreflectivity requirements. This paper reports on a spreadsheet tool developed to quantitatively evaluate the effectiveness of different sign inspection and replacement scenarios. The spreadsheet was designed for yellow and red engineer-grade sign sheetings, and takes into account sign vandalism and knock-downs as well as normal sign aging. The spreadsheet provides estimates of the number of signs in place that would not meet the minimum retroreflectivity standard and the cost of the sign inspection and replacement program. The results from a number of trials of the spreadsheet show that agencies that generally conform to the key assumptions made to build the spreadsheet should consider replacing all signs every seven years, as that insures that no aged signs are in place at a relatively low cost. If total replacement is not possible, an inspection program using retroreflectometers every three years appears very competitive in its effectiveness with a program using typical visual inspection rates each year. The retroreflectometers appear to allow fewer deficient signs, while the typical visual inspection program costs are lower for a given vandalism rate. More conservative visual sign replacement rates do not appear to offer distinct advantages, because typical replacement rates with visual inspections every two or three years allow relatively high numbers of deficient signs to remain on the roads

Near-Infrared Spectroscopy of McNeil's Nebula Object

We present 0.8-5.2 micron spectroscopy of the compact source at the base of a variable nebula (McNeil's Nebula Object) in the Lynds 1630 dark cloud that went into outburst in late 2003. The spectrum of this object reveals an extremely red continuum, CO bands at 2.3-2.5 microns in emission, a deep 3.0 micron ice absorption feature, and a solid state CO absorption feature at 4.7 microns. In addition, emission lines of H, Ca II, Mg I, and Na I are present. The Paschen lines exhibit P Cygni profiles, as do two lines of He I, although the emission features are very weak in the latter. The Brackett lines, however, are seen to be purely in emission. The P Cygni profiles clearly indicate that mass outflow is occurring in a wind with a velocity of ~400 km/s. The H line ratios do not yield consistent estimates of the reddening, nor do they agree with the extinction estimated from the ice feature (A_V ~ 11). We propose that these lines are optically thick and are produced in a dense, ionized wind. The near-infrared spectrum does not appear similar to any known FUor or EXor object. However, all evidence suggests that McNeil's Nebula Object is a heavily-embedded low-mass Class I protostar, surrounded by a disk, whose brightening is due to a recent accretion event.Comment: 11 pages, 2 ps figures, accepted for publication in ApJ Letter

Reionization Constraints on the Contribution of Primordial Compact Objects to Dark Matter

Many lines of evidence suggest that nonbaryonic dark matter constitutes roughly 30% of the critical closure density, but the composition of this dark matter is unknown. One class of candidates for the dark matter is compact objects formed in the early universe, with typical masses M between 0.1 and 1 solar masses to correspond to the mass scale of objects found with microlensing observing projects. Specific candidates of this type include black holes formed at the epoch of the QCD phase transition, quark stars, and boson stars. Here we show that accretion onto these objects produces substantial ionization in the early universe, with an optical depth to Thomson scattering out to z=1100 of approximately tau=2-4 [f_CO\epsilon_{-1}(M/Msun)]^{1/2} (H_0/65)^{-1}, where \epsilon_{-1} is the accretion efficiency \epsilon\equiv L/{\dot M}c^2 divided by 0.1 and f_CO is the fraction of matter in the compact objects. The current upper limit to the scattering optical depth, based on the anisotropy of the microwave background, is approximately 0.4. Therefore, if accretion onto these objects is relatively efficient, they cannot be the main component of nonbaryonic dark matter.Comment: 12 pages including one figure, uses aaspp4, submitted to Ap

K-band Spectroscopy of Clusters in NGC 4038/4039

Integral field spectroscopy in the K-band (1.9-2.4um) was performed on four IR-bright star clusters and the two nuclei in NGC 4038/4039 (``The Antennae''). Two of the clusters are located in the overlap region of the two galaxies, and together comprise ~25% of the total 15um and ~10% of the total 4.8 GHz emission from this pair of merging galaxies. The other two clusters, each of them spatially resolved into two components, are located in the northern galaxy, one in the western and one in the eastern loop of blue clusters. Comparing our analysis of Brgamma, CO band-heads, He I (2.058um), Halpha (from archival HST data), and V-K colors with stellar population synthesis models indicates that the clusters are extincted (A_V ~ 0.7 - 4.3 mags) and young, displaying a significant age spread (4-13 Myrs). The starbursts in the nuclei are much older (65 Myrs), with the nucleus of NGC 4038 displaying a region of recent star formation northward of its K-band peak. Using our derived age estimates and assuming the parameters of the IMF (Salpeter slope, upper mass cut-off of 100 M_sun, Miller-Scalo between 1 M_sun and 0.1 M_sun), we find that the clusters have masses between 0.5 and 5 * 10^6M_sun.Comment: 10 pages, 3 figures, ApJ accepte

Density fluctuations and the structure of a nonuniform hard sphere fluid

We derive an exact equation for density changes induced by a general external field that corrects the hydrostatic approximation where the local value of the field is adsorbed into a modified chemical potential. Using linear response theory to relate density changes self-consistently in different regions of space, we arrive at an integral equation for a hard sphere fluid that is exact in the limit of a slowly varying field or at low density and reduces to the accurate Percus-Yevick equation for a hard core field. This and related equations give accurate results for a wide variety of fields

Primordial Helium Abundance: A Reanalysis of the Izotov-Thuan Spectroscopic Sample

A reanalysis is made for the helium abundance determination for the Izotov-Thuan (2004) spectroscopic sample of extragalactic H II regions. We find that the effect of underlying stellar absorption of the He I lines, which is more important for metal poor systems, affects significantly the inferred primordial helium abundance $Y_p$ obtained in the zero metallicity limit and the slope of linear extrapolation, $dY/dZ$. This brings $Y_p$ from $0.234\pm0.004$ to $0.250\pm 0.004$ and $dY/dZ=4.7\pm 1.0$ to $1.1\pm 1.4$. Conservatively, this indicates the importance of the proper understanding of underlying stellar absorption for accurate determinations of the primordial helium abundance to the error of $\delta Y_p\simeq 0.002-0.004$.Comment: 12 pages, 3 figures, misplaced figures are corrected, typos in Table1 are correcte