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Herbig Ae/Be Stars:
An Investigation of Molecular Environments and
Associated Stellar Populations

Lynne A. Hillenbrand

Department of Physics and Astronomy
A.B., Princeton University
Ph.D., University of Massachusetts, Amherst

submited February 1995
© 1995

Committee Chair: Stephen E. Strom

Committee Members:
Suzan Edwards
Michael F. Skrutskie
Ronald L. Snell
Frederick W. Byron, Jr.
Department Chair: John F. Dubach

Abstract

We have carried out a molecular mapping, optical and infrared imaging, and stellar-classification spectroscopic survey of star-forming sites containing young (tau < 1 Myr) intermediate- and high-mass (M > 3 M_sun ) stars. Our target sample is drawn from the set of pre-main sequence Herbig Ae/Be stars with spectral types earlier than B7, and focusses on those which are isolated from large complexes of extensive star-formation. Our aim is to quantify the local environmental characteristics (molecular and stellar clustering) of regions in which high-mass stars are being produced, in order to compare these properties with those of regions in which only low-mass stars are being produced.

In the current analysis, we first investigate the general attributes of the small clouds within which our sample of massive Ae/Be stars is found. We then define an appropriate size scale (1/3 pc × 1/3 pc in area) over which to investigate stellar clustering properties, and identify small (a few to tens in number), dense (several hundred to several thousand stars per cubic parsec), partially - obscured stellar aggregates projected onto the same molecular cores as are our sample of Ae/Be stars. We find an apparent correlation between cluster density and the mass of the defining Ae/Be star. We then present evidence based on the observation of infrared and spectroscopic circumstellar activity signatures usually indicative of stellar youth, as well as on the actual location of the members of several of these groups in the HR diagram, that indeed the companions to young massive Ae/Be stars are also young pre-main sequence stars, of lower mass. Given the significant amount of ambient molecular material and the apparent youthfulness of the stellar populations, any and all objects found near the cores are likely not to have wandered far from their birthplaces. Thus, our survey enables us to carry out a complete census of stars formed relatively contemporaneously with the high-mass "signpost."

Our particular goals then, are to determine 1) the initial stellar mass spectrum in spatially and temporally coherent i. e., truly "initial") units which have given birth to at least one high-mass star, as well as 2) the time sequence of star formation as a function of stellar mass, that is, whether stars of all masses are formed simultaneously in a dense stellar environment, or whether high-mass stars might form significantly before or after the initiation of low-mass star-formation. With these goals, we explore techniques for investigating the distributions of stellar masses and stellar ages in young groups, as well as the observational and theoretical limits of these techniques.

Finally, in our discussion we address the issue of differences in local stellar environment between star-forming regions producing high-mass stars and those producing only low-mass stars. We pursue the hypothesis that if the Ae/Be stars studied herein are representative of a stellar evolutionary phase through which all young high-mass stars pass, and if their association with dense aggregates of stars is typical, then these young stellar systems may provide important clues to the physical processes associated with forming high-mass stars. We therefore urge the theoretical consideration of star formation in dense groups (approx 1000 stars pc^-3 as an expansion to the developing theory of protostellar collapse for individual high-mass stars.

Chapter 1. Introduction and Outline
Chapter 2. The Molecular Survey
Chapter 3. The Imaging Survey
Chapter 4. The Optical Classification-Resolution Spectroscopic Survey
Chapter 5. Clusters Associated with Ae/Be Stars
Chapter 6. Stars with Circumstellar Disks and Envelopes
Chapter 7. A First Look at Stellar Populations: C-M Diagrams
Chapter 8. Construction and Analysis of HR Diagrams
Chapter 9. Testing Universal Mass Functions
Chapter 10. Discussion
Chapter 11. Summary and Future Investigations
Appendices
Appendix A. Background and Stellar Properties of Herbig Ae/Be Stars
Appendix B. Nearby Companions to Ae/Be Stars
Appendix C. Ongoing Analysis in the Orion Nebula Cluster
Acknowledgements
References
The Postscript files are also available.

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