Infall Model Research Articles

Introduction to the session

The generic label “high-velocity cloud” is a very unspecific term that tells us only that there are clouds of hydrogen gas moving with velocities which are anomalous for their position on the sky, if we assume that we live in a flat, uniformly rotating galaxy. If one attempts to explain them all away in one fell swoop we might draw an analogy with the situation extant in the 19th century, when optical astronomers were aware of patches of emission that were called nebulae. We now know that there are extragalactic as well as galactic nebulae, all quite different from one another. They were recognized to be so different as soon as the quality of data on these objects improved sufficiently. We may be in a similar, somewhat frustrating, position with regard to the high-velocity clouds. Soon after their discovery in 1963 it was believed that the clouds were mostly at negative velocities and that they were very local. Now we know of many more clouds at both positive and negative velocities. The models proposed for their existence cover a range of distances. We still have local (infall and explosive event) models, while other models place the clouds on relatively nearby spiral arms, in the high-z extension or warp areas of distant arms, in the outskirts and immediate neighborhood of the Galaxy, at the distance of the Magellanic Clouds, and even beyond. There may well be some truth in all of these models.

Abundances and Ages of Disk Clusters

Larson has emphasized at this meeting that one relation which is valuable in determining the correct model for the formation of the Galactic disk is the dependence of stellar abundances on the time of formation of the disk stars. While Mayor has demonstrated that correlations exist between metal abundance and both kinematics and positions of stars in the disk, Larson has suggested that it is possible in an extreme infall model to have such trends and still have no age dependence.

Neutral hydrogen in the normal elliptical galaxy NGC 4278.

view Abstract Citations (44) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Neutral hydrogen in the normal elliptical galaxy NGC 4278. Gallagher, J. S. ; Knapp, G. R. ; Faber, S. M. ; Balick, B. Abstract We report the first convincing detection of neutral hydrogen in a structurally normal elliptical galaxy, NOC 4278 (type El). The observed 21 cm emission-line profile is roughly Gaussian in shape, has a central velocity close to the optical velocity of the galaxy, and has a full width at half- maximum (FWHM) of 470 km s '. If D = 16 Mpc, the H i mass is 7 x 108 M0 and the H mass-to-photographic-luminosity ratio is 0.05 in solar units, a remarkably high value for an elliptical galaxy. The H I is unresolved with a FWHM 10' beam. NOC 4278 is a morphologically normal elliptical galaxy but has an unusually intense compact nuclear radio source as well as a relatively bright nuclear emission-line spectrum. It is not known whether the nuclear activity in NGC 4278 is related in some way to the unusually high H i content. Possible sources for the H i include mass loss from stars or infall from the intergalactic medium. Infall models seem more in accord with the data now available. The present situation is perhaps unstable, and possible evolutionary scenarios are briefly mentioned. Subject headings: galaxies: individual - galaxies: structure - interstellar: matter - radio sources: 21 cm radiation Publication: The Astrophysical Journal Pub Date: July 1977 DOI: 10.1086/155376 Bibcode: 1977ApJ...215..463G full text sources ADS | data products NED (1)

An infall model for the T Tauri phenomenon

view Abstract Citations (350) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An infall model for the T Tauri phenomenon. Ulrich, R. K. Abstract An infall model which produces P Cygni type II emission lines is presented. The blueshifted apparent absorption feature in these emission lines is interpreted in this model as the overlap region between a strong emission line centered near the rest velocity of the star and a weaker emission line at a velocity near -150 km/s. According to this model the energy source for the continuum and line-emission features observed in T Tauri stars is the kinetic energy of the infalling matter. A weak flux of soft X-rays is produced in the high-temperature postshock gas. These X-rays may be responsible for producing the nebulae observed near some T Tauri stars. Publication: The Astrophysical Journal Pub Date: December 1976 DOI: 10.1086/154840 Bibcode: 1976ApJ...210..377U Keywords: Emission Spectra; Gravitational Collapse; Stellar Evolution; Stellar Spectra; T Tauri Stars; Variable Stars; H Alpha Line; H Beta Line; Kinetic Energy; Line Spectra; Radiative Transfer; Shock Layers; Stellar Models; Astrophysics full text sources ADS |