bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nonlinear, Nonradial Pulsation in Rapidly Oscillating Ap Stars
Authors: Kurtz D. W. 
Published: July 2016

AbstractThe rapidly oscillating Ap stars pulsate in high-overtone, low degree p-modes with their pulsation axes aligned with their oblique magnetic axes. They show non-linearity in their pulsation in three ways:
1)The harmonics of the basic pulsation frequency are detectable.2)The pulsation phase seems to vary stochastically on a time scale of days to years depending on the star.3)The form of the nonradial surface distortion is not constant with time.These three effects are illustrated with HR 3831, the best studied of the roAp stars. HR 3831 pulsates in distorted dipole mode which can be modelled as a linear sum of axisymmetric l = 0, 1, 2, and 3 spherical harmonics aligned with the magnetic axis. This gives rise to a 7-frequency multiplet split by exactly the rotation frequency. The form of the distortion shows small changes on a time-scale of years. HR 3831 shows a 5-frequency rotationally split first harmonic multiplet, a 3-frequency rotationally split second harmonic multiplet, and a single third harmonic frequency has probably been detected at an amplitude of 0.065 mmag. The first harmonic has changed its form significantly over the last 10 years. A technique for decomposing the fundamental frequency septuplet into its component spherical harmonics is used to fit the pulsation phase as a function of rotation phase. This allows a unique O-C to be defined for any length of light curve. The long term behaviour of the O-C diagram cannot be modelled adequately with a combination of periodic (Doppler shift) and quadratic (evolution) terms; there seems to be a significant stochastic component. The direction of the pulsation phase reversal at rotational phase 0.747 is indeterminate; sometimes it is a positive-going reversal, sometimes negative-going. At present it is not known whether this is a numerical artifact, or a physical effect in the star. If it is a physical effect, it means that small non-periodic differences in pulsation amplitude between the bipolar hemispheres have been detected.
Double-Mode Stellar Pulsation
Authors: Kovács G. 
Published: July 2016

AbstractThe current observational and theoretical status of the double-mode variables is reviewed. Focusing mostly on the RR Lyrae stars, we address the question of the observational evidence of modal stability. The problem of stationarity is a crucial issue in the modelling of these stars.We mention past efforts in hydrodynamical and analytical modelling together with a detailed discussion of some very recent results. It is suggested that stochastic forcing due to turbulent convection may play a crucial role in exciting some marginally stable modes in the limiting pulsation. The latest hydrodynamical results first demonstrate that purely radiative models are able to show permanent double-mode behavior in the relevant period regime of RRd stars. The reason for the previous lack of double-mode behavior is attributed to the large dissipation, i.e. artificial viscosity, generally used in the codes to ensure numerical stability and to obtain amplitudes comparable to the observed ones.We think that better models should include some physical dissipation, most probably turbulent convection, and a more accurate numerical treatment of the radiative hydrodynamics.
An Oscillator Model for Stellar Variability
Authors: Nakahara T. , Tanaka Y. 
Published: July 2016

AbstractThe dependence of coupling constants in a coupled oscillator model is examined with simplified methods. The Lyapunov exponents are preliminary introduced for the model. The behaviors of oscillator model are examined in a parameter plane. So-called the Arnold’s tongues for phase-locking states are observed in fractal patterns.
A Unified Model of Dwarf Nova Outbursts Based on the Disk Instability
Authors: Osaki Y. , Hirose M. , Ichikawa S. 
Published: July 2016

AbstractA unified model for outbursts of dwarf novae is proposed based on the disk instability model in cataclysmic variable stars. In this model, two different intrinsic instabilities (i.e., the thermal instability and the tidal instability) within accretion disks are considered in non-magnetic cataclysmic variable stars. It is suggested that all of three sub-classes of dwarf novae (i.e., U Gem-type, Z Cam-type and SU UMa-type dwarf novae) may be explained in terms of two model parameters of the orbital period of the binary and of the mass transfer rate within the framework of the disk instability model.
Numerical Simulations of Pulsationally Unstable Accretion Disks around Supermassive Black Holes
Authors: Honma F. , Matsumoto R. , Kato S. 
Published: July 2016

AbstractThe innermost region of slim accretion disks with standard α viscosity is unstable against axisymmetric radial inertial acoustic perturbations under certain conditions. Numerical simulations are performed in order to demonstrate behaviors of such unstable disks. It is shown that oscillations with the period of ~ 10−3(MBH/M⊙) s can be excited near the inner edge of the disks, where MBH is the mass of the central object. This kind of unstable disks is a possible origin of the periodic X-ray time variabilities with period of ~ 104s observed in a Seyfert galaxy NGC 6814.
Interstellar Molecules of Prebiotic Interest
Authors: Winnewisser Gisbert 
Published: July 2016

AbstractThe field of interstellar molecules is reviewed with special consideration of molecules which are of potential biological interest. At present more than 110 interstellar molecules have been identified in interstellar clouds and circumstellar envelopes. The more complex molecules are found in the dense cores which are often the sites of active star formation. These locations represent prime targets for the search of larger molecules such as glycine and possibly other amino acids. However, in the list of detected interstellar molecules still many simple hydrides are missing, e.g. SH, PH, PH2, etc., which constitute the building blocks for larger molecules and biomolecules. With the technological opening of the terahertz region to both laboratory and interstellar spectroscopy, great scientific advances are to expected such as the direct detection of the low energy bending vibrations of larger (linear) molecules or the ring-puckering motion of larger ring molecules like the polycyclic (multiring) aromatic hydrocarbons, which might shed new light on the assignment of the «unidentified infrared» features or even on the diffuse visible interstellar bands (DIBs).
The Diffuse Interstellar Bands And Organic Molecules In Space
Authors: Snow Theodore P. 
Published: July 2016

AbstractThe diffuse interstellar bands have been recognized for over 75 years, and remain unidentified today. This family of broad interstellar absorption features, now numbering well over 100, has been the subject of a great deal of speculation, intensive observations, and lately has stimulated the interest of chemists as well as astronomers. In this review I briefly summarize the history of the problem and then provide an overview of the current state of affairs. I emphasize recent evidence that the carriers are large molecular species, most likely organic in nature, which are abundant in the diffuse interstellar medium. If so, then the ultimate identification of the responsible species will provide a new window into the detailed chemical and physical properties of the interstellar gas. I also discuss constraints imposed on the carriers by a recent reassessment of the interstellar carbon abundance.
Organics In Comets
Authors: Mumma Michael J. 
Published: July 2016

AbstractThe birth-endowed organic fraction of the newly formed (hot) Earth was destroyed by thermal decomposition during the cooling epoch. After Earth cooled sufficiently, an early organic inventory was likely replenished by the impact of comets and asteroids — a process which continues even today. The present organic composition of comets and asteroids can provide information relevant to this secondary organic seeding of the planets, for comparison with scenarios leading to self-replicating organic entities. Although impacts no longer deliver organics in significant quantities, compared with the existing terrestrial inventory, small bodies can deliver their organics intact to Earth‘s surface while giant impacts may cause punctuated extinction of living species (and create opportunities for renewed speciation). Hence, the exogenous organic flux has great importance for life’s origins and terminations. Current knowledge of the organic composition of comets is reviewed, and recent progress in our understanding of the chemical evolution of organic material from its formation to its incorporation into comets and later into planets is outlined. The need for detailed quantitative chemical analysis of material obtained directly from the cometary nucleus is indicated.
On the Nature of the Newly Discovered Extrasolar Planets
Authors: Guillot Tristan , Saumon Didier , Burrows Adam , Hubbard William B. , Lunine Jonathan I. , Marley Mark S. , Freedman Richard S. 
Published: July 2016

AbstractThe discovery of planets around nearby stars gives us hope that life may be able to develop close to our solar system. It should also shed light on the problem of the formation of planets in general. In the past few years, we have developed theoretical evolutionary models of solar and extrasolar giant planets. With the help of these models, we discuss the nature and characteristics of these «new planets», in aid of both NASA’s and ESA’s recent plans to directly detect them.
Infrared Interferometers for Observing Extra-Solar Planets
Authors: Fisher Philip C. , Houston Joseph B. , Stuhlinger Tilman W. 
Published: July 2016

AbstractConcepts are outlined for ground - and space-based interferometers for observing nearby extra-solar earth-like planets (in the 8-20 μ range). The collecting elements are special coronagraphs, two of which constitute part of a Michelson interferometer. Performance characteristics are compared to those inferred for several competing systems. One conclusion is that one could construct a groundbased instrument that seems superior to the Keck interferometer system («superior» meaning up to a tenfold increase in angular resolution in each of two orthogonal directions, up to a tenfold increase in sensitivity, and a ten-fold lower cost). Space-based instruments of comparable/superior (?) performance to the Oases-1 and-2 systems might also be constructed.
The Tempo and mode(S) of Prebiotic Evolution
Authors: Lazcano Antonio 
Published: July 2016

AbstractDifferent current ideas on the origin of life are critically examined. Comparison of the now fashionable FeS/H2S pyrite-based autotrophic theory of the origin of life with the heterotrophic viewpoint suggest that the later is still the most fertile explanation for the emergence of life. However, the theory of chemical evolution and heterotrophic origins of life requires major updating, which should include the abandonment of the idea that the appearance of life was a slow process involving billions of years. Stability of organic compounds and the genetics of bacteria suggest that the origin and early diversification of life took place in a time period of the order of 10 million years. Current evidence suggest that the abiotic synthesis of organic compounds may be a widespread phenomenon in the Galaxy and may have a deterministic nature. However, the history of the biosphere does not exhibits any obvious trend towards greater complexity or «higher» forms of life. Therefore, the role of contingency in biological evolution should not be understimated in the discussions of the possibilities of life in the Universe.
Self-Replication and Evolution in Primordial Mutually Catalytic Sets
Authors: Segré Daniel , Pilpel Yitzhak , Glusman Gustavo , Lancet Doron 
Published: July 2016

AbstractA kinetic model for random mutually catalytic chemical sets is used here to estimate the probability for the spontaneous origin of molecular self-replication. Simplifying assumptions on the chemistry in such systems enable us to explore the possible mechanisms for chemical evolution of simple metabolic networks, and the emergence of well organized structures. The model affords an estimate of the probability that a given rate of growth or replication will appear by chance in a pre-biotic milieu, thus assisting in understanding the potential precursors of life.
Optical and Infrared Seti
Authors: Townes Charles H. 
Published: July 2016

When in 1959 Cocconi and Morrison first suggested communication with extraterrestrial intelligence and showed it could be done in the microwave region, there were no lasers. In addition, since the hydrogen line at 21 centimeters was the only astronomical line known at that time in the microwave region, it seemed unique and the obvious place to look. Only one year later, however, the laser came along and shortly after that the use of lasers for SETI could also be suggested (Schwartzet al., 1961). And still later, a number of important microwave lines at various frequencies were detected from interstellar clouds, making a choice of frequency less obvious.The first laser was built in 1960. But the laser perhaps came along unnecessarily late; there is no principle of science or technology involved in lasers that was not understood as early as the 1920s. One can find an early discussion by Tolman (1924), for example, about so-called «negative absorption,» produced by molecules in excited states. At that time, the coherence of stimulated radiation was not completely clear, but Tolman said it was probably coherent, even though that it had not yet been mathematically proven. In a few years, it was proven and so all the principles were known. Lasers could indeed have been invented at that time, even though their variety would have been limited because solid state physics and infrared spectroscopy were not very far along.
Interstellar Communication Channel Search of 49 Stars Closer than 11.5 PC
Authors: Zadnik Marjan G. , Winterflood John , Williams Andrew J. , Wellington Kelvin J. , Vaile Roberta , Blair David G. , Tarter Jill C. , Backus Peter R. , Norris Ray P. , Heiligman Gary 
Published: July 2016

AbstractWe report on a search for narrow band emissions from the vicinity of 36 solar type (F6 to K4) target stars and 13 other non-solar type stars within 11.5 parsecs at 5 possible interstellar beacon frequencies of π*fH, 2π*fH, e*fOH, e*(fOH+fH) and3He (4.462, 8.925, 4.532, 8.393 and 8.666 GHz respectively). The frequencies were selected by scaling important naturally occurring lines by fundamental constants. The stars were selected for a distance of less than 11.5 parsecs to give sufficient time for a nearby civilisation to have detected early Earth radio leakage signals and to have replied. The search was carried using the 64-m Parkes telescope with the Phoenix instrumentation and the 22-m Mopra telescope for candidate verification. The instrument combination allowed for a low interference, high resolution, high sensitivity search. We did not detect any signals from any of the target stars. If a civilisation at a distance of 10 pc had beamed 10 kW signals at us using a 100-m dish, we would have detected the transmission at the 10 sigma level. Thus we can exclude signals above this level.
The Serendip IV Interference Rejection and Signal Detection System
Authors: Cobb Jeff , Donnelly Charles , Bowyer Stuart , Werthimer Dan , Lampton Michael 
Published: July 2016

AbstractThe SERENDIP Interference Rejection and ETI Notification system (SIREN) is a software package developed at UC Berkeley for use in the search for extraterrestrial intelligence (SETI). Originally designed for and used by the SERENDIP III project, it is being substantially rewritten for use with SERENDIP IV. This paper will present the SIREN system and discuss the algorithms employed. SIREN performs both interference rejection and signal detection. The end result of SIREN processing is a database of candidate ETI signals. The most interesting candidates will be the subject of a targeted search.
A One Square Kilometer Radiotelescope: Design Challenges
Authors: Wellington Kelvin J. 
Published: July 2016

AbstractSETI searches, as well as radioastronomy, require large radiotelescopes for high sensitivity. In recent years a strong scientific case has been made for the construction of a radiotelescope operating at decimeter wavelengths with an aperture of about one square kilometer. Such an instrument would have a sensitivity about two orders of magnitude greater than is currently available. However, current projections indicate that this will require about an order of magnitude reduction in cost per unit area compared to existing structures in order to make it financially feasible. This reduction will demand new and innovative approaches in design. Even with this reduction in capital cost such a facility is expected to be several hundred million dollars. Such a major facility will need to be developed as an international project. The International Radio Science Union (URSI) and the International Astronomical Union (IAU) have recognized this and set up a joint Working Group to facilitate the development. Several countries and institutes have become actively involved and are working together in an international framework. Several different design approaches are currently being studied. These vary from a small number of large spherical radiotelescopes, through a moderate number of medium-sized parabolic antennae, to a phased array of a large number of small nonmoving «tiles». Each approach has advantages, disadvantages and particular challenges.
No known Energy Source can Power Manned Interstellar Flight
Authors: Ksanfomality Leonid V. 
Published: July 2016

AbstractWe consider a possible future energy source: deuterium-helium thermonuclear reaction. When this source is used even with no losses the closest star (4.4 l.y.) cannot be reached during the human lifetime. Calculations prove that the averaged speed of such an interstellar probe consisting only of deuterium fuel (with no mass for any construction!) and the module for astronauts with the mass of about 0.01 of the total mass of the mission, cannot exceed 17,600 km/s, and the two-way flight duration is 150 years. Even with the totally unrealistic mass of the module of 0.0001 of the total mass, it is still 105 years. Unless some as yet undiscovered physics leads to new sources of energy, interstellar flights piloted by a single generation of humans will remain a dream forever.
Detecting Interstellar Migrations
Authors: Matloff Gregory L. , Pazmino John 
Published: July 2016

AbstractInterstellar Migrations may occur when a civilization’s star enters the Red Giant phase, thereby dooming the life-bearing planet(s). Ecologically self-contained «world ships», massing billions of kilograms and propelled by hyperthin, space-manufactured solar sails thousands of kilometers in diameter unfurled near the home star are possible vehicles to transfer a threatened civilization to a neighboring star. Consideration of the nearest Red Giants reveals that Pollux is the nearest formerly solar-type Red Giant. Known stellar neighbors of Pollux are surveyed to determine likely directions for an interstellar migration departing Pollux. Such migrations might consist of many world ships launched over millenia on voyages of about 1000 terrestrial-year duration; discovery of such events will be serendipitous. The difficulties of observing solar-sail star ships near Pollux are considered. A facility dedicated to imaging extrasolar planets within 10 parsecs might be capable of detecting these large spacecraft.
The Biophysical Cosmology: The Place of Bioastronomy in the History of Science
Authors: Dick Steven J. 
Published: July 2016

AbstractThe recent discoveries of planets around Sun-like stars, possible primitive Martian fossil life, and conditions on Europa conducive to microbial life, render more urgent the question of the place of bioastronomy in the history of science. This paper argues that the tenets of bioastronomy constitute a «biophysical cosmology», a scientific world view that holds that life is common throughout the universe. Many of the activities of the field of bioastronomy are tests of this cosmology. Like other cosmologies, the biophysical cosmology bears strongly on humanity’s place in the universe. Cosmological status may also be useful in discussing die implications of contact, when one considers the response to other cosmologies as partial, if imperfect, analogues.
Resonance Effect of Binary Orbital Motion upon δ Scuti Type Stars
Authors: Paparó M. , Shibahashi H. , Sterken C. 
Published: July 2016

AbstractThe colour behaviour of θ Tucanae with orbital phase is shown. A break in the residuals is checked as the criterion for stopping the period search. A resonance of the dominant pulsational mode and the period of ellipticity is reported, with a Resonance number of 72.
RR Lyrae Variables in Local Group Galaxies: LMC, Leo I. New Results and Distances to these Galaxies
Authors: Clementini G. , Bragaglia A. , Fabrizio L. Di , Maio M. , Carretta E. , Gratton R.G. , Held E.V. , Momany Y. , Rizzi L. , Saviane I. 
Published: July 2016

AbstractWe present new results on a sample of RR Lyrae variables in the bar of the Large Magellanic Cloud (LMC), and report on the first detection of RR Lyrae in the dwarf spheroidal galaxy Leo I. Emphasis is given to the discussion of the distances to these galaxies as derived from their RR Lyrae populations, and of the impact on the “short” and “long” distance scale dichotomy.
Empirical Luminosities and Radii of Stars in the β Cep, SPB and δ Sct Instability Domains
Authors: Jerzykiewicz M. 
Published: July 2016

AbstractFor 26 stars with effective temperatures known from directly measured angular diameters and total absolute fluxes (Code et al., 1976), HIPPARCOS parallaxes (ESA, 1997) can be used to derive empirical luminosities and radii. The empirical effective temperatures and luminosities allow a comparison of the position of these stars in the fundamental H-R diagram with evolutionary tracks. Note that no calibration, photometric or any other, is involved in the process. As noted by Jerzykiewicz & Molenda-Żakowicz (2000), the comparison shows an overall coincidence with the phases of slow nuclear burning of the solar composition evolutionary tracks, computed with OPAL opacities and moderate amount of overshooting from the convective core.Several of the stars fall in the theoretical β Cep, SPB or δ Sct instability domains. Unfortunately, none of them is known to be an SPB variable, while the two that have been classified as δ Sct stars lack convincing evidence of variability. For the four β Cep stars that occur in the sample, the empirical radii are used to identify the radial order of pulsation.
Interferometric-Doppler Imaging of Nonradial Stellar Pulsations
Authors: Jankov S. , Vakili P. , Domiciano de Souza A. , Janot-Pacheco E. 
Published: July 2016

AbstractWe present the imaging potential of a tomographic technique which combines time-resolved spectroscopy and long baseline interferometry, providing information that cannot be obtained otherwise with each of these techniques separately.