Neutral hydrogen in galactic clusters.
Existing theories of stellar evolution predict that, in a given galactic cluster, all stars of mass less than a certain limiting value will be in the protostar state at present. If these protostars consist of a not too compact structure of neutral hydrogen, they should emit observable 21- centimeter radiation. A successful search for such radiation has been made with the Harvard 6o-foot radio telescope. Neutral hydrogen in excess of that present in the general interstellar medium, and coinciding in position and radial velocity with the cluster position and radial velocity, has been found in h and x Persei, the Pleiades (previously detected by Heeschen and Drake), Coma Berenices, and Praesepe. No neutral hydrogen has been detected in M67. A strong age effect is shown in the cluster neutral-hydrogen content, the youngest clusters having about as much HI as stellar mass, with the ratio Hi/stellar mass decreasing with increased age until the ratio is zero in M67. The total mass of hydrogen found in each cluster is consistent with the total mass of protostars predicted by theory, the agreement becoming very good in the older clusters. The absence of interstellar lines connected with the cluster hydrogen, the low cluster escape velocity, and the probable existence of interstellar winds militate strongly against the cluster neutral hydrogen being an extended residue of cluster formation, or extended material ejected from cluster stars, and demand that the cluster neutral hydrogen occur in compact bodies. Both theory and observation, then, support the thesis that the neutral hydrogen observed is in the form of cluster protostars. Accurate frequency profiles obtained for the Pleiades and Praesepe are almost identical in form, and resemble typical line profiles broadened by collisional damping. If the line broadening is interpreted as pressure broadening, densities of the order of i~'~ atoms/cc are indicated, requiring the radiating bodies to be highly flattened if high optical depth and very large deduced hydrogen masses are considered unreasonable. Harvard College Observatory, Cambridge, Mass.
The Astronomical Journal
- Pub Date: