Progress Toward a National Serials Data System

published or submitted for publicatio

Hopf algebras up to homotopy and the Bockstein spectral sequence

Anick proved that every q-mild Hopf algebra up to homotopy is isomorphic to a primitively-generated chain Hopf algebra. We provide a new proof, that involves extensive use of the Bockstein spectral sequence.Comment: Published by Algebraic and Geometric Topology at http://www.maths.warwick.ac.uk/agt/AGTVol5/agt-5-7.abs.htm

Missing Red Supergiants and Carbon Burning

Recent studies on direct imaging of Type II core-collapse supernova progenitors indicate a possible threshold around $M_{\rm ZAMS}\sim 16-20$ M$_\odot$, where red supergiants with larger birth masses do not appear to result in supernova explosions and instead implode directly into a black hole. In this study we argue that it is not a coincidence that this threshold closely matches the critical transition of central Carbon burning in massive stars from the convective to radiative regime. In lighter stars, Carbon burns convectively in the center and result in compact final presupernova cores that are likely to result in explosions, while in heavier stars after the transition, it burns as a radiative flame and the stellar cores become significantly harder to explode. Using the KEPLER code we demonstrate the sensitivity of this transition to the rate of $^{12}$C$(\alpha,\gamma)^{16}$O reaction and the overshoot mixing efficiency, and we argue that the upper mass limit of exploding red supergiants could be employed to constrain uncertain input physics of massive stellar evolution calculations. The initial mass corresponding to the central Carbon burning transition range from 14 to 26 M$_\odot$ in recently published models from various groups and codes, and only a few are in agreement with the estimates inferred from direct imaging studies.Comment: submitted to MNRA

The Effects of Living Wage Laws: Evidence from Failed and Derailed Living Wage Campaigns

Living wage campaigns have succeeded in about 100 jurisdictions in the United States but have also been unsuccessful in numerous cities. These unsuccessful campaigns provide a better control group or counterfactual for estimating the effects of living wage laws than the broader set of all cities without a law, and also permit the separate estimation of the effects of living wage laws and living wage campaigns. We find that living wage laws raise wages of low-wage workers but reduce employment among the least-skilled, especially when the laws cover business assistance recipients or are accompanied by similar laws in nearby cities.

When Do Living Wages Bite?

Many features of living wage laws may influence the strength of their effects on wages and employment of low-skill individuals. Echoing past research, business assistance living wage laws generate stronger wage increases and employment reductions than contractor-only laws. But broader enforcement or implementation and geographic concentration of living wage laws also appear to strengthen their effects. Finally, geographic concentration may be more significant than the distinction between business assistance and contractor-only living wage laws.

The Economic Effects of Living Wage Laws: A Provisional Review

Nearly 100 cities and local governments in the United States passed living wage laws since the mid-1990s. The central goal of living wages is to reduce poverty, yet they may fail to do so because of disemployment effects. We summarize and critique the existing research on the effects of living wages on wages, employment, and family income, emphasizing common findings, points of disagreement, and important questions for future research. The evidence thus far points to wage increases as well as employment losses for the least-skilled although there is disagreement about the employment effects but on net some beneficial distributional effects. The evidence also points to efficiency wage-type effects of living wage laws that may offset some of the adverse impacts on employers.

Simple Closed Geodesics in Hyperbolic 3-Manifolds

This paper determines which orientable hyperbolic 3-manifolds contain simple closed geodesics. The Fuchsian group corresponding to the thrice-punctured sphere generates the only example of a complete non-elementary orientable hyperbolic 3-manifold that does not contain a simple closed geodesic. We do not assume that the manifold is geometrically finite or that it has finitely generated fundamental group.Comment: 7 pages, to appear in the Bulletin of the London Mathematical Societ

Stellar Mergers Are Common

The observed Galactic rate of stellar mergers or the initiation of common envelope phases brighter than M_V=-3 (M_I=-4) is of order 0.5 (0.3)/year with 90% confidence statistical uncertainties of 0.24-1.1 (0.14-0.65) and factor of 2 systematic uncertainties. The (peak) luminosity function is roughly dN/dL L^(-1.4+/-0.3), so the rates for events more luminous than V1309 Sco (M_V=-7 mag) or V838Mon (M_V=-10 mag) are lower at r~0.1/year and 0.03/year, respectively. The peak luminosity is a steep function of progenitor mass, L M^(2-3). This very roughly parallels the scaling of luminosity with mass on the main sequence, but the transients are ~2000-4000 times more luminous at peak. Combining these, the mass function of the progenitors, dN/dM M^(-2.0+/-0.8), is consistent with the initial mass function, albeit with broad uncertainties. These observational results are also broadly consistent with the estimates of binary population synthesis models. While extragalactic variability surveys can better define the rates and properties of the high luminosity events, systematic, moderate depth (I>16 mag) surveys of the Galactic plane are needed to characterize the low luminosity events. The existing Galactic samples are only ~20% complete and Galactic surveys are (at best) reaching a typical magnitude limit of <13 mag.Comment: Submitted to MNRAS (13 pages, 6 figures, 3 tables