bluuebunny/crossref_metadata_2025_split huggingface dataset:
Convective Overshooting and The Observed Width of The Main-Sequence Band
Authors: Napiwotzki R. , Rieschick A. , Blöcker T. , Schönberner D. , Wenske V. 
Published: July 2016

For many years we are witnessing a lively debate on the existence and extent of convective overshooting, mainly in the cores of main-sequence stars. This is an important issue, since even a small amount of overshooting increases considerably the mass of the finally hydrogen exhausted core and lenghthens the main-sequence lifetime correspondingly. The available evolutionary calculations assume either moderate overshooting, d/Hp = 0.25, (d = overshooting distance, Hp = pressure scale height; Maeder & Meynet 1988) or strong overshooting, d/Hp ≈ 0.50 (Bertelli et al. 1986). Presently theory is unable to quantify the exact amount of overshooting, and one has to resort to empirical determinations.Recently, Stothers (1991) collected all available information from the literature on stellar parameters and evolutionary calculations and concluded that, within the errors, d/Hp = 0 is an acceptable result, with a conservative upper limit of d/Hp < 0.2. However, such an approach is hampered by observational errors (like distance or temperature uncertainties, rotation) that are difficult to quantify and that may mask any definitive result. Detailed investigations of detached binaries may help in this matter (Andersen et al. 1990) but the number of suitable binary systems is probably not very large.
Excitation Mechanisms of Oscillations in Stars
Authors: Osaki Yoji 
Published: July 2016

AbstractExcitation mechanisms of oscillations in stars are discussed, in particular on the problems of the Beta Cephei pulsations and of the solar oscillations. A long-standing mystery about the excitation mechanism of the Beta Cephei pulsations seems to have finally been solved, which is due to availability of the new “OPAL” opacities. The very mechanism is the classical к-mechanism, however, due to enhanced heavy element opacities at temperature around 2 × 105K. By using the new opacities, three independent groups have found pulsational instabilities for the fundamental radial mode and a few low-degree non-radial modes in Beta Cephei models. Stochastic excitation of solar oscillations by turbulent convection is then discussed. It is shown that the noise generation by quadrupole radiation concentrated at the top of the convection zone may well explain the frequency dependence of the observed solar acoustic power pumped into individual p-modes.
The Subseismic Approximation For Low-Frequency Modes of the Earth Applied to Low-Degree, Low-Frequency G-Modes of Non-Rotating Stars
Authors: Boeck I.De , Hoolst T.Van , Smeyers P. 
Published: July 2016

AbstractThe subseismic approximation for low-frequency modes in the Earth is used in an asymptotic treatment of low-frequency g-modes belonging to low-degree spherical harmonics in a non-rotating star.
A Procedure For Two-Dimensional Asymptotic Rotational-Splitting Inversion
Authors: Sekii T. , Gough D.O. 
Published: July 2016

AbstractRotational splitting Δω(n,l,m) of the eigenfrequencies of a star rotating with angular velocity Ω(r, θ) about a unique axis can be represented as a weighted integral of Ω over r and θ, (r, θ, φ) being spherical polar coordinates about the axis of rotation. For high-frequency acoustic modes, Δω/m collapses essentially to a function of ω — ω /(l+1/2) and M = m/(l + 1/2) alone, and the weighting kernel K(r,θ) becomes asymptotically degenerate, each factor being of essentially Abel type. Therefore, formally, the splitting integral can be inverted, once a procedure has been found for extending Δω over the domain of (ω, M) such that the turning points (rt,ωt), given by (c(rt)/rt, sinωt) = (ω, M) where c is sound speed, span the star. Obtaining that representation is the most difficult stage of the inversion. We report on a procedure that treats the inverted two-dimensional Abel integral as a repeated double integral, representing the data successively along a set of parallel lines M =constant. The method is illustrated by an inversion of artificial data which is compared with the angular velocity from which those data were computed.
Stellar Dynamos
Authors: Krause F. 
Published: July 2016

AbstractEnergy is transported from the central regions of a star to its surface. Generally this transport is in certain layers carried on by convective motions. Because of the structure, which these motions have due to the influence of the overall rotation, the star becomes electromagnetically unstable, i.e. a large magnetic field grows from small seed fields as a result of the dynamo process. The internal structure, especially the symmetries of a star, will be, at least to some extend, reflected by the spatial structure and the time behaviour of the excited magnetic field. In this sense observations of the magnetic field on a surface of a star and the related activity phenomena can provide insight in the internal structure of the star, since characteristic parameters like thickness of the convection zone, mixing length, turnover time, profile of the differential rotation, etc. strongly influence the dynamo process.The actual magnetic field of a star is a product of a nonlinear process. Models elaborated on the kinematical (i.e. linear) level provide insight in the excitation conditions and the linear field modes. The marginal mode, i.e. the mode which is easiest to excite, reflects properties of the nonlinear solution in case the system operates not far from the margin to the dynamo unstable region. Here the solutions show symmetries with respect to the axis of rotation and the equatorial plane, properties which are, for example, to a large extend fulfilled for the solar average magnetic field. For systems operating far from this margin irregular or even chaotic behaviour has to be expected. From observations there is a strong indication that these theoretical possibilities find their realizations within the sample of late-type stars.
Evolution of Stellar Activity in Early Post-Main-Sequence Phases
Authors: Maggio A. , Sciortino S. , Bianchi L. , Harnden F.R. , Rosner R. 
Published: July 2016

We present preliminary observational evidences on the variation of the activity level in late type stars, during the evolutionary phases on the main sequence and beyond. We have selected a sample of 51 stars (Fig. 1), lying mostly along evolutionary tracks between 1.3 and 1.7 solar masses, which have been observed in soft X-rays with the Einstein Observatory, and in UV with IUE (Maggio et al. 1990; Haisch et al.1990). Two ROSAT targets, and four new IUE observations are also included.We find that the radiative emission from the outer atmospheres of stars with M > 1.6M⊙ seems to behave differently than for stars with lower mass.On the main sequence, the X-ray luminosity of most stars with B-V < 0.42 (spectral type F3) is relatively low, at Lx ~ 3 × 1028erg s-1 (Fig. 2). In the early evolutionary phases beyond the main sequence, the X-ray luminosity of the higher mass stars tend to increase sistematically up to ~ 1030erg s-1, while the lower mass stars show an initial moderate increase followed by a drop, at B-V ~ 0.6, below our sensitivity threshold.
Energy Losses by Stellar Activity Phenomena
Authors: Katsova Maria 
Published: July 2016

AbstractFor active late-type dwarfs, the total energy losses including both chromospheric and coronal radiation as well as the energy losses caused by stellar flares reach 3·10−3 L001. To maintain such high energetic losses requires a high efficiency of the dynamo action in the convection zone. Mass loss by active late-type stars can probably approach the rate of 10−11 M⊙/yr if we take into account the large-scale structure of their coronae and transient events. The phase of activity can continue for up to 1% of the lifetime of these stars on the main sequence due to braking by such a stellar wind.
Prisma the Instruments
Authors: Jones Andrew 
Published: July 2016

Our understanding of Solar structure has increased dramatically in the last couple of decades thanks mainly to the opening of new windows of observation providing high quality data to theoreticians with access to powerful computing facilities. Two of the new windows were UV and X-ray images of the Sun, allowing a detailed view of the upper solar atmosphere, and the development of very high resolution spectrometers allowing us to exploit the solar oscillations to probe the internal structure of the Sun. It is the goal of PRISMA to extend these techniques to other stars, which using the Sun as a calibration point will allow us to explore stellar structure and evolution in ways not possible now.In this poster I will present a possible selection of instruments able to achieve this goal, and explain some of the rationale in their design. A more general overview is presented by T. Appouchaux also in these proceedings. It must be stressed that these are not the definitive instruments to be flown on PRISMA, but rather result from a study to show the feasibility of such a mission. Should PRISMA be chosen as the next ESA medium sized mission, an ‘Announcement of Opportunity’ wiH be issued by ESA and the responses of all people interested in constructing the instrument will be considered.
Comparison of Stellar Oscillations and Activity Data to Infer Internal Rotation
Authors: Belvedere Gaetano 
Published: July 2016

AbstractRecently it has beeen suggested that the latitude distribution of the main surface features of solar activity is intimately related to the angular velocity profile inside the Sun through the working of a MHD dynamo in the boundary layer between the convective and the radiative zones (Belvedere et al. 1991).Although the present observational capabilities are not very encouraging, here we want to point out, in the framework of the analogy to the Sun (solar-stellar connection), that space observations of surface distribution and latitudinal migration of active regions on stellar surfaces, which could be carried out in this decade with more sophisticated techniques, may conversely allow us to infer the rotation profile, and consequently the angular momentum distribution, in stellar interiors. This methodology may in principle be considered alternate or complementary to the classical one based on observation of acoustic oscillations.
Radial Pulsations In 63 Herculis
Authors: Uyaniker Bülent 
Published: July 2016

AbstractConstructing a theoretical model of 1.88 M⊙ pulsation properties of 63 Her have been analyzed. For the model, In question, pulsation periods in the fundamental mode and first two overtones have been derived by solving the linearized pulsation equations. The model has been found to be vibrationally stable.
High- and Low-Degree Modes in Delta Scuti Stars: 1- and 2-D Fourier Analyses Of τ Peg
Authors: Kennelly Edward J. , Matthews Jaymie M. , Walker Gordon A.H. 
Published: July 2016

Stellar seismology of δ Scuti stars offers the best prospect for determining the interior properties of normal non-magnetic A-F stars on or near the main sequence. The discovery of rich eigenfrequency spectra in the light variations of several δ Scuti stars (e.g., θ2 Tau: Breger et al. 1989; GX Peg: Michel et al. 1990) should make this possible for low-degree modes (l ≤ 4). However, due to cancellation effects, modes of higher l cannot be studied through photometry or radial velocity measurements.The detection of rapid line-profile variations among rapidly rotating δ Scuti stars (e.g., Walker et al. 1987; Kennelly et al. 1992) has extended our sensitivity to much higher modes. These variations are consistent with moderate- to high-degree modes, where |m| ~ l. (In the simplest case, they could be pure sectorial modes with |m| = l and pulsation amplitudes strongly confined to the stellar equator.) To use these data effectively, we need an objective way to identify the modes and their time evolution.
Impact of Observations on Prejudice and Input Physics
Authors: Gough D.O. 
Published: July 2016

AbstractMy brief from the principal organizers of this colloquium is to draw your attention to some of the prejudices upon which the observations that have been discussed here have had or should have had some impact, and to remark on how that might revise our views about physics: it should not be a balanced summary of the meeting, but a prejudiced review.
Key Issues: A Round Table Discussion
Authors: Brown T.M. , Demarque P. , Noyes R. , Praderie F. , Roxburgh I.W. , Schatzman E. 
Published: July 2016

We have taken part to an exceptionally rich colloquium, characterized by a large amount of information in all fields of physics, and a remarkable collection of observational data. Our understanding of what is going on inside the stars has changed in a radical manner during the last years, on one side because the observations are bringing new kinds of information, and on the other side because theory in its development is taking into account a number of processes, some of them having been completely ignored only ten years ago.It is impossible to draw here a complete list of problems. There are many cases where some inconsistency could be found in the theory itself or some contradiction between observational data and theory. I recommended that such a list should be drawn carefully. The participants to the round table discussion will give their contribution, helping to open the way to new fields of research and to new discoveries.
Chemical Evolution of Algol-Type Stars
Authors: J.Sarna Marek 
Published: July 2016

AbstractThe evolution of the chemical abundances at the surface of both components of Algol-type binaries is examined. First, we have examined some mixing processes which can affect element distribution in components of Algol-type stars. Second, the C abundance determinations from theoretical models are compared with values known from observational analysis. We conclude that there is no significant difference in chemical composition in components between case AB and early case B mass transfer. We have found a real difference between systems which evolve from initial mass ratios of 10/4 and 10/9.
Microarcsecond VLBI: What will We See?
Authors: Valtaoja Esko 
Published: July 2016

With the exception of some pioneering mm-VLBI observations, the present-day VLBI cannot resolve the most compact components in active galaxies, nor measure their brightness temperatures. In addition, the observing frequencies are too low: the innermost components are still self-absorbed and often completely undetectable at standard VLBI frequencies. However, studies of continuum spectra and variations of AGN can give us estimates of what will eventually be seen when VLBI techniques reach sufficient resolutions and frequencies. These estimates are also useful in considering what capabilities will be required from future interferometry for studying AGN.We have used multifrequency continuum monitoring data together with standard shock-in-jet models to derive flux, size and brightness temperature distributions for the innermost core components in a complete 2 Jy sample of AGN. The millimeter spectra and its variations can be satisfactorily explained by just two types of high-frequency components, a relativistic inhomogeneous jet and evolving shocks in this jet (Valtaoja et al. 1988). Moreover, with extended multifrequency flux monitoring the contributions from these two components can be separated, which makes possible the extraction of limited amounts of structural data (average sizes, brightness distributions, etc.) just from continuum data.We have used two main methods. Cm-to-mm spectra can reveal the existence of persistent high frequency components and give estimates of their fluxes and turnover frequencies. Continuum monitoring gives fluxes and variability timescales of transient components, and further their sizes and brightness temperatures.
MM-VLBI Survey of Spectral Index on AGN
Authors: Matsumoto K. , Kawaguchi N. , Inoue M. , Takaba H. , Koyama Y. , Iwata T. , Kurihara N. 
Published: July 2016

AbstractThe preliminary results of an mm-VLBI survey of spectral index on active galactic nuclei (AGN) are presented which suggest that their activities are only in the central regions. The difference in central activities may correspond to their different stages of evolution. We found a strong concentration to α = 0 for quasar and a spread distribution for HPQ.
Wide-Band Tunerless Mixer Mounts for 100 GHz and 150 GHz SIS receivers
Authors: Sunada K. , Kawabe R. , Inatani J. 
Published: July 2016

AbstractThe performance of the new SIS receiver systems at the Nobeyama Millimeter Array are described. These receivers operate at 100 GHz and 150 GHz bands and tunerless mixer mounts have been adopted. These receivers show very low noise temperature (< 50 K) over a very wide frequency range because of the large embedding impedance range.
Combined-Array Imaging of Extra-Galactic Radio Sources
Authors: Akujor Chidi E. , Bååth Lars B. 
Published: July 2016

The resolving powers of existing telescope arrays are quite different. For example the extended new MERLIN at 5 GHz readily yields maps of ~ 50 mas resolution compared with the VLA ~ 350 mas, WSRT ~ 4000 mas, EVN ~ 5 mas, global VLBI ~ 1 mas, etc. This means that images obtained with these instruments yield information on different scales that sometimes appear unrelated. However, the need to have structural information on intermediate scales demands that data from different arrays be combined to make a single image. This is particularly important in extra-galactic radio sources where the relationship between core-jet features on different scales (parsec and kiloparsec) and the connection between small-scale and extended features need to be established.By combining uvdata from two or more arrays we can routinely make images that represent a balance between the capabilities of the combined arrays, (e.g. VLA + MERLIN, MERLIN+VLBI, VLA+WSRT etc) at cm wavelengths. In order to do this, one has to overcome a number of calibration and mapping difficulties (see Zhang et al. 1991; Akujor et al. 1992, in prep.).
Ionized and Neutral Gas in the Central 10 PC of the Galaxy
Authors: Lo K. Y. , Plante R. , Lacy J. , Wright M. , Killeen N. 
Published: July 2016

AbstractObservations of the 12.8 μm [NeII] line emission, the OH absorption against Sgr A and the HCN emission were compared to determine the relationship between the ionized and neutral gas in the central 10 pc of the Galaxy. The distribution of the neutral gas is too asymmetric to be a ring or disk. Along the western arc, the ionized gas velocity is very different from that of the neutral gas, suggesting it may not be the ionized inner edge of the circum-nuclear neutral gas.
Interstellar Chemistry in Orion KL
Authors: Yamamoto Satoshi , Mikami Hitomi , Saito Shuji 
Published: July 2016

AbstractA map of SiO (J = 2 − 1) in Ori KL has been obtained with 2.8” spatial resolution by using Nobeyama Millimeter Array. The distribution of SiO evidences an interaction between outflow and dense gas around IRc4 and IRc5; these infrared sources are thus considered to be shock-heated regions. Since distributions of (CH3)2O and HCOOCH3 have a peak at the IRc5 position, such shock heating seems to play an important role in production of these molecules. Detection of CH3OD around the interacting region most likely suggests that CH3OH is evaporating from dust grains. (CH3)2O and HCOOCH3 may be produced by gas-phase reactions starting from injected CH3OH, or they might be evaporated from dust grains.
Evolution of Disks in the Course of Star Formation
Authors: Hayashi Masahiko 
Published: July 2016

AbstractObservations of circumstellar disks with Nobeyama Millimeter Array (NMA) are presented for the following two topics. The first one is on the continued NMA survey for 13 complete samples of protostar candidates associated with Taurus molecular cloud. The observation confirmed the previous result that protostar candidates do not have detectable 3 mm continuum emission except for the two sources L1551-IRS5 and IRAS 04365+2535. This sets the upper limit to the circumstellar disk mass to be ~0.03 Mʘ for the protostar candidates. The disk mass for protostar candidates tends to be smaller than that around young T Tauri stars, suggesting that it may increase in the course of evolution from protostars into T Tauri stars. The second topic is on the detection of CO (J=1-0) emission toward GG Tau. The observations with the 45-m telescope and with NMA show strong evidence of the CO emission arising from a rotating disk with its size significantly extended with respect to the dust disk. Depletion of CO gas in the GG Tau disk is discussed.
Aperture Synthesis CS(J=1-0) Observations of the Molecular Outflow in NGC 2071: Subclumps in a Wind-Driven Shocked Shell and their Origin
Authors: Kitamura Yoshimi , Kawabe Ryohei , Ishiguro Masato 
Published: July 2016

AbstractAperture synthesis CS(J=l-0) observations were made of the expanding clumpy shocked shell that was found around the redshifted CO outflow in NGC 2071 during our previous study (Kitamura et al. 1990). It has been revealed that previously identified clumps in the shell comprise several small subclumps. The decelerating shock and Rayleigh-Taylor instabilities seem to be probable mechanisms by which the subclumps are formed in the shock-compressed shell.
High S/N HCO+ Maps of the Orion B Dense Core
Authors: Barnes Peter J. , Crutcher Richard M. 
Published: July 2016

AbstractBy combining new NRAO 12m data with previously published BIMA data, we have obtained very high signal-to-noise maps of HCO+ in the Orion B core, which contain information on both the large- and small-scale emission and put new constraints on the gas’ physical conditions.
Radiative Transfer Modeling of Molecular Clouds: HCO+ in the Star-Forming Region W 49 A North
Authors: Dickel Hélène R. , Auer Lawrence H. , Wilner David J. 
Published: July 2016

AbstractA multi-level, non-LTE, (non-LVG) radiative transfer code has been applied to HCO+ observations of W 49 A north. Profiles of both the J=1-0 and J=3-2 transitions are successfully reproduced with a variety of cloud models.
Millimeter Observations of G5.89-0.39
Authors: Wilner David J. , Welch W. J. , Forster J. R. , Murata Yasuhiro 
Published: July 2016

AbstractWe present multi-line millimeter interferometer maps HII single dish observations of the G5.89-0.39 ultracompact HII region. Radiative transfer modelling suggests an overall collapse.
CO Outflows in Low-Mass Protostar Candidates in Taurus
Authors: Tamura M. , Ohashi N. , Moriarty-Schieven G. , Hayashi M. , Hirano N. 
Published: July 2016

The Taurus dark cloud is one of the nearest active sites of low-mass star formation. The IRAS satellite has discovered numerous low-luminosity far-infrared sources in this cloud, some of which are completely invisible at optical wavelengths. There are several arguments suggesting that the invisible sources are low-mass protostars which are younger than T Tauri stars and powered by the accretion of infalling gas and dust (Adams, Lada, & Shu 1987; Beichman et al. 1986; Myers et al. 1987; Kenyon et al. 1990)Tamura et al. (1991) made a near-infrared imaging survey of a complete, flux-limited sample of cold IRAS sources in the Taurus dark cloud, of which 8 are optical T Tauri-like objects and 16 are optically invisible sources. They identified all 24 sources with near-infrared counterparts; one is entirely nebulous without a point-like source, and the others generally have an unresolved peak with or without an extended component at 2.2 µm 75 % of the 24 sample sources are associated with infrared/optical nebulosity about 1000 AU to 10000 AU in size. A number of the sources show a clear bipolar or monopolar morphology, suggestive of a close relation of the nebulosity with a bipolar mass outflow; the nebulosity is due to scattering of light from the central source by the dust associated with the mass outflow extending to the poles of a circumstellar dust disk.