Search Publications
Spiral Structure as a Recurrent Instability
Sellwood, J. A.
A long-standing controversy in studies of spiral structure has concerned the lifetimes of individual spiral patterns. Much theoretical work has sought quasi-stationary spiral modes while N-body simulations have consistently displayed recurrent, short-lived patterns. The simulations manifest a recurrent cycle of true instabilities related to small-…
Pick-up Ion Measurements in the Heliosphere – A Review
Geiss, J.; Gloeckler, G.; von Steiger, R. +1 more
Measurements of the composition and spatial distribution of pick-up ions inside the heliosphere are reviewed. The first interstellar 4He+pick-up ions were detected with the SULEICA instrument on the AMPTE spacecraft near Earth's orbit. Most data on pick-up ions were taken in the solar-wind and suprathermal energy range of SWI…
Local Kinematics of Classical Cepheids
Zhu, Zi
Combining Hipparcos proper motions and the radial velocity data, we have studied the Cepheid kinematics on the basis of the three-dimensional Ogorodnikov-Milne model. The results seem to show a slight contracting motion of the Galaxy in the solar neighbourhood, ∂ Vθ / ∂θ / R = ‑2.60 ± 1.07 km s-1 kpc-1, which is al…
Gas Dynamics in Planetary Nebulae: From Macro-structures to FLIERs
Perinotto, Mario
Purpose of this paper is to clarify how Planetary Nebulae (PNe) are very interesting laboratories to study cosmic gas dynamics. I first recall the history of PNe which are generated from low and intermediate mass stars through successive mass loss processes starting in the Reg Giant phase of evolution and continuing also after the termination of t…
The Velocity Field of Young Stars in the Solar Neighbourhood
Comerón, F.; Figueras, F.; Torra, J. +1 more
A sample of O- and B-type stars with Hipparcos astrometric data, ages computed from Strömgren photometry and radial velocities, has been used to characterize the structure, age and kinematics of the Gould Belt system. The local spiral structure of our galaxy is determined from this sample, and also from a sample of Hipparcos Cepheid stars. The Gou…
The Influence of Fluctuations of the Solar Emission Line Profileon the Doppler Shift of Interplanetary HLyα Lines Observed by the Hubble–Space–Telescope
Scherer, H.; Fahr, H. J.; Bzowski, M. +1 more
In an earlier research the employment of a radiation transport model with angle-dependent partial frequency redistribution, self-absorption by interplanetary hydrogen, realistic solar HLyαemission profile, and a time dependent `hot' hydrogen model to analyze 5 interplanetary HLyα glow spectra obtained with theHubble–Space–Tel…
Faint field galaxies: an explanation of the faint blue excess using number evolution models
Zhang, Yuan-Zhong; He, Ping; Zou, Zhen-Long
Pure luminosity evolution models for galaxies provide an unacceptable fit to the redshifts and colors of faint galaxies. In this paper we demonstrate, using HST morphological number counts derived both from the I814-band of WFPC2 in the Medium Deep Survey (MDS) and the Hubble Deep Field (HDF) and from the H1.6-band of NICMOS,…
A Search for Warm Neutral Gas Associated with the Giant HII Region NGC 604
Beckman, J.; Gómez de Castro, A. I.; Sanz, L.
The characteristics of the halo gas and its interaction with the galactic disk in spiral galaxies are poorly known; this is particularly true for the warm neutral gas associated with HVCs and galactic chimneys. The detection of absorption features such as the NaI D or the CaII (H,K)lines is instrumental to study its detailed physical properties bu…
Separation between Gould Belt and Disk Stars in the Solar Neighbourhood
Alfaro, E. J.; Cabrera-Caño, J.; Elias, F.
The adoption of a parametric model for the spatial distribution of Gould Belt and Galactic Disk stars allows the classification by their membership probabilities of stars from the Hipparcos catalogue in the Solar vicinity and the estimation of structural parameters for both stellar systems for several age groups. The observed kinematics of Gould B…
High Angular Resolution Studies of Coronal Structures with SOHO
Karovska, Margarita; Wood, Brian; Cook, John W. +2 more
Since January 1996, the Solar and Heliospheric Observatory (SOHO)has been providing unprecedented views of the extended solar coronato heliocentric distances of up to 32 solar radii. During the past threeyears we carried out studies of the morphology and dynamical evolutionof various structures in the solar corona using two instruments on boardSOH…