Sequential formation of subgroups in OB associations

Journal Article · Wed Jun 15 04:00:00 EDT 1977 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/155302· OSTI ID:7259622

Elmegreen, B G; Lada, C J

We reconsider the structure and formation of OB association in view of recent radio and infrared observations of the adjacent molecular clouds. As a result of this reexamination, we propose that OB subgroups are formed in a step-by-step process which involves the propagation of ionization (I) and shock (S) fronts through a molecular cloud complex. OB stars formed at the edge of a molecular cloud drive these I-S fronts into the cloud. A layer of dense neutral material accumulates between the I and S fronts and eventually becomes gravitationally unstable. This process is analyzed in detail. Several arguments concerning the temperature and mass of this layer suggest that a new OB subgroup will form. After approximately one-half million years, these stars will emerge from and disrupt the star-forming layer. A new shock will be driven into the remaining molecular cloud and will initiate another cycle of star formation.Several observed properties of OB associations are shown to follow from a sequential star-forming mechanism. These include the spatial separation and systematic differences in age of OB subgroups in a given association, the regularity of subgroup masses, the alignment of subgroups along the galactic plane, and their physical expansion. Detailed observations of ionization fronts, masers, IR sources, and molecular clouds are also in agreement with this model. Finally, this mechanism provides a means of dissipating a molecular cloud and exposing less massive stars (e.g., T Tauri stars) which may have formed ahead of the shock as part of the original cloud collapsed and fragmented.

• Research Organization: Center for Astrophysics, Harvard College Observatory and Smithsonian Astrophysical Observatory
• OSTI ID: 7259622
• Journal Information: Astrophys. J.; (United States), Journal Name: Astrophys. J.; (United States) Vol. 214:3; ISSN ASJOA
• Country of Publication: United States
• Language: English

Related Subjects

640102* -- Astrophysics & Cosmology-- Stars & Quasi-Stellar
Radio & X-Ray Sources
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
COSMIC GASES
ELECTROMAGNETIC RADIATION
FLUIDS
GASES
GRAVITATIONAL COLLAPSE
INFRARED RADIATION
IONIZATION
NEBULAE
PROTOSTARS
RADIATIONS
RADIOWAVE RADIATION
SHOCK WAVES
STAR ACCRETION
STAR CLUSTERS
STAR EVOLUTION