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
Committee Chair: Stephen E. Strom
- Committee Members:
- Suzan Edwards
Michael F. Skrutskie
Ronald L. Snell
Frederick W. Byron, Jr.
Department Chair: John F. Dubach
We have carried out a molecular mapping, optical and infrared imaging,
and stellar-classification spectroscopic survey of star-forming sites
young ( <
intermediate- and high-mass (M > 3 M )
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 ( 1000 stars pc as an
expansion to the developing theory of protostellar collapse
for individual high-mass stars.
The Postscript files are also available.
- 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
- 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