Neutrino production in matter with time-dependent density or velocity

We show that neutrinos can be produced through standard electroweak interactions in matter with time-dependent density.Comment: 4 pages, revte

Dusty, Self-obscured Transients from Stellar Coalescence

We discuss the central role that dust condensation plays in shaping the observational appearance of outflows from coalescing binary systems. As binaries begin to coalesce, they shock-heat and expel material into their surroundings. Depending on the properties of the merging system, this material can expand to the point where molecules and dust form, dramatically increasing the gas opacity. We use the existing population of Luminous Red Novae (LRNe) to constrain the thermodynamics of these ejecta, then apply our findings to the progressive obscuration of merging systems in the lead in to their coalescence. Compact progenitor stars near the main sequence or in the Hertzsprung gap along with massive progenitor stars have sufficiently hot circumstellar material to remain unobscured by dust. By contrast, more extended, low-mass giants should become completely optically obscured by dust formation in the circumbinary environment. We predict that 30--50\% of stellar coalescence transients for solar-mass stars will be dusty, infrared-luminous sources. Of these, the optical transients may selectively trace complete merger outcomes while the infrared transients trace common envelope ejection outcomes.Comment: v2, published in AAS Journal

Water Masers in the Andromeda Galaxy: The First Step Toward Proper Motion

We have detected and confirmed five water maser complexes in the Andromeda Galaxy (M31) using the Green Bank Telescope. These masers will provide the high brightness temperature point sources needed for proper motion studies of M31, enabling measurement of its full three-dimensional velocity vector and its geometric distance via proper rotation. The motion of M31 is the keystone of Local Group dynamics and a gateway to the dark matter profiles of galaxies in general. Our survey for water masers selected 206 luminous compact 24 micron-emitting regions in M31 and was sensitive enough to detect any maser useful for ~10 microarcsecond per year astrometry. The newly discovered masers span the isotropic luminosity range 0.3-1.9 x 10^-3 L(Sun) in single spectral components and are analogous to luminous Galactic masers. The masers are distributed around the molecular ring, including locations close to the major and minor axes, which is nearly ideal for proper motion studies. We find no correlation between 24 micron luminosity and water maser luminosity, suggesting that while water masers arise in star-forming regions, the nonlinear amplification pathways and beamed nature of the water masers means that they are not predictable based on IR luminosity alone. This suggests that there are additional bright masers to be found in M31. We predict that the geometric distance and systemic proper motion of M31 can be measured in 2-3 years with current facilities. A "moving cluster" observation of diverging masers as M31 approaches the Galaxy may be possible in the long term.Comment: 6 pages, 3 figures, 2 tables, accepted by ApJ Letter

Redescription of the freshwater anchovy Anchoviella vaillanti (Steindachner, 1908) (Clupeiformes: Engraulidae) with notes on the distribution of estuarine congeners in the Rio São Francisco basin, Brazil

Anchoviella vaillanti (Steindachner, 1908) was described based on few specimens from the middle Rio São Francisco; however, several specimens of the species have been collected in recent decades. The range of morphological variation of A. vaillanti could thus be reassessed based on a larger number of specimens currently available in fish collections, and the species redescribed. Anchoviella vaillanti can be recognized among freshwater congeners by the relative position of the pelvic, dorsal and anal fins. Records of the species in ichthyological collections are restricted to the upper and middle portions of the Rio São Francisco basin, but the species might also occur in the lower Rio São Francisco. Comments on the distribution of the marine species of Anchoviella from the lower Rio São Francisco basin and an identification key including those species and A. vaillanti are provided

Future Evolution of the Intergalactic Medium in a Universe Dominated by a Cosmological Constant

We simulate the evolution of the intergalactic medium (IGM) in a universe dominated by a cosmological constant. We find that within a few Hubble times from the present epoch, the baryons will have two primary phases: one phase composed of low-density, low-temperature, diffuse, ionized gas which cools exponentially with cosmic time due to adiabatic expansion, and a second phase of high-density, high-temperature gas in virialized dark matter halos which cools much more slowly by atomic processes. The mass fraction of gas in halos converges to ~40% at late times, about twice its calculated value at the present epoch. We find that in a few Hubble times, the large scale filaments in the present-day IGM will rarefy and fade away into the low-temperature IGM, and only islands of virialized gas will maintain their physical structure. We do not find evidence for fragmentation of the diffuse IGM at later times. More than 99% of the gas mass will maintain a steady ionization fraction above 80% within a few Hubble times. The diffuse IGM will get extremely cold and dilute but remain highly ionized, as its recombination time will dramatically exceed the age of the universe.Comment: 22 pages, 10 figures. Accepted to New Astronomy. Movies and a higher resolution version of the paper are available at http://cfa-www.harvard.edu/~knagamine/FutureIG

Destruction of Molecular Hydrogen During Cosmological Reionization

We investigate the ability of primordial gas clouds to retain molecular hydrogen (H_2) during the initial phase of the reionization epoch. We find that before the Stromgren spheres of the individual ionizing sources overlap, the UV background below the ionization threshold is able to penetrate large clouds and suppress their H_2 abundance. The consequent lack of H_2 cooling could prevent the collapse and fragmentation of clouds with virial temperatures T_vir < 10^4 K (or masses 10^8 Msun [(1+z_vir)/10]^{-3/2}). This negative feedback on structure-formation arises from the very first ionizing sources, and precedes the feedback due to the photoionization heating.Comment: 14 pages, uuencoded compressed Postscript, 4 figures included. To appear in Ap

Muscle coordination is habitual rather than optimal

When sharing load among multiple muscles, humans appear to select an optimal pattern of activation that minimizes costs such as the effort or variability of movement. How the nervous system achieves this behavior, however, is unknown. Here we show that contrary to predictions from optimal control theory, habitual muscle activation patterns are surprisingly robust to changes in limb biomechanics. We first developed a method to simulate joint forces in real time from electromyographic recordings of the wrist muscles. When the model was altered to simulate the effects of paralyzing a muscle, the subjects simply increased the recruitment of all muscles to accomplish the task, rather than recruiting only the useful muscles. When the model was altered to make the force output of one muscle unusually noisy, the subjects again persisted in recruiting all muscles rather than eliminating the noisy one. Such habitual coordination patterns were also unaffected by real modifications of biomechanics produced by selectively damaging a muscle without affecting sensory feedback. Subjects naturally use different patterns of muscle contraction to produce the same forces in different pronation-supination postures, but when the simulation was based on a posture different from the actual posture, the recruitment patterns tended to agree with the actual rather than the simulated posture. The results appear inconsistent with computation of motor programs by an optimal controller in the brain. Rather, the brain may learn and recall command programs that result in muscle coordination patterns generated by lower sensorimotor circuitry that are functionally "good-enough.

The Adventures of the Rocketeer: Accelerated Motion Under the Influence of Expanding Space

It is well known that interstellar travel is bounded by the finite speed of light, but on very large scales any rocketeer would also need to consider the influence of cosmological expansion on their journey. This paper examines accelerated journeys within the framework of Friedmann- Lemaitre-Robertson-Walker universes, illustrating how the duration of a fixed acceleration sharply divides exploration over interstellar and intergalactic distances. Furthermore, we show how the universal expansion increases the difficulty of intergalactic navigation, with small uncertainties in cosmological parameters resulting in significantly large deviations. This paper also shows that, contrary to simplistic ideas, the motion of any rocketeer is indistinguishable from Newtonian gravity if the acceleration is kept small.Comment: 9 pages, 7 figures, accepted for publication in PAS

Signatures of Relativistic Helical Motion in the Rotation Measures of AGN Jets

Polarization has proved an invaluable tool for probing magnetic fields in relativistic jets. Maps of the intrinsic polarization vectors have provided the best evidence to date for uniform, toroidally dominated magnetic fields within jets. More recently, maps of the rotation measure (RM) in jets have for the first time probed the field geometry of the cool, moderately relativistic surrounding material. In most cases, clear signatures of toroidal magnetic field are detected, corresponding to gradients in RM profiles transverse to the jet. However, in many objects these profiles also display marked asymmetries which are difficult to explain in simple helical jet models. Furthermore, in some cases the RM profiles are strongly frequency and/or time dependent. Here we show that these features may be naturally accounted for by including relativistic helical motion in the jet model. In particular, we are able to reproduce bent RM profiles observed in a variety of jets, frequency dependent RM profile morphologies and even the time dependence of the RM profiles of knots in 3C 273. Finally, we predict that some sources may show reversals in their RM profiles at sufficiently high frequencies, depending upon the the ratio of the components of jet sheath velocity transverse and parallel to the jet. Thus, multi-frequency RM maps promise a novel way in which to probe the velocity structure of relativistic outflows.Comment: 5 pages, 4 figures, submitted to ApJ