Young, high-velocity A stars
Creators: name: Lance, Catherine Margaret
Publisher: name: The Australian National University
Year: 1986
Subjects: subject: A stars Evolution
subject: A stars Spectra
subject: A stars Motion in line of sight

description: This thesis examines the group of high-velocity SGP A stars, discussed by Rodgers, Harding, and Sadler (1981, RHS), which had been observed at distances from the plane of more than 1 kpc. RHS found them to be main-sequence stars, with a velocity dispersion perpendicular to the plane of 66 km/s, and a range of abundances from -0.5 dex to 0.0 dex (around one third of Population I to normal Population I abundance). This combination of properties had not been previously observed in any population of stars, and after exploration and rejection of several alternative possibilities, RHS were led to suggest that the stars had been formed from the recent coalescence with the Galactic plane of metal-poor gas from a satellite galaxy undergoing a merger with the Milky Way. The rejection of alternative hypotheses was consequent upon the accuracy of all of the properties measured by RHS. If one or more of the parameters were to be incorrect, then it might be that the stars were actually misidentified members of other populations, such as blue stragglers, metal-rich horizontal branch stars, or normal disk stars accelerated by some unknown mechanism. In order to explore this possibility, in this thesis the parameters found by RHS have been re-derived and critically examined in the light of alternative hypotheses. A catalogue has been compiled of all early-type stars that have been identified (from many sources) in 217 square degrees at the SGP, to augment the sample of high-velocity stars. In addition a number of high-velocity stars in the Solar neighbourhood have also been studied. The results of this work are that: 1) The majority of the high-velocity stars are undoubtedly on the mainsequence. 2) Their rotational velocities (v sin i) are high, typical of young, Population I stars. They do not have the lower rotational velocities found for blue stragglers or horizontal branch stars. 3) They are all younger than around 0.6 billion years, in contrast to a control group of disk A stars with stochastic ages of formation of up to 2 billion years. They appear to be coeval, suggesting that they were formed as the result of a single event, rather than randomly over time. 4) They are relatively metal-poor, with [Ca/H] from around -0.4 dex to 0.0 dex, in comparison to young disk A stars which usually have abundances from about -0.1 dex to 0.2 dex. 5) Their W velocity dispersion was found to be 62 km/s, in very good agreement with RHS. 6) They are observed to be at distances of up to 6.5 kpc from the Galactic plane, and some stars may travel as far as 10 kpc from the plane in their orbits. The majority of stars seen are at less than 3 kpc distance because of the magnitude limit of the original survey: this is a selection effect. The stars would have to be observed to 16th magnitude to obtain a larger sample of the more distant, fainter stars. 7) The exponential scale height for the young stars at distances greater than 1 kpc is estimated to be around 700 pc. 8) The nearby young, high-velocity stars lag behind Solar rotation in the disk by around 40 km/s. 9) The nearby stars have large (30 to 40 km/s) and almost isotropic velocity dispersions in U, V and W. Their ratios of UVW velocity dispersions are unlike any other known group of stars. 10) The high-velocity A stars comprise around one percent of normal disk A stars. 11) The amount of gas required to form the observed density of high-velocity stars was estimated to be equivalent to the gas content of a galaxy at least midway in size between the Small and Large Magellanic Clouds. These results were discussed and compared to those that would be predicted by hypotheses proposed to account for the existence of the A stars.