High Galactic Latitude B Stars
The Origins and Evolutionary Status of B Stars Found Far From The Galactic Plane
by
John C. Martin
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy
At the Case Western Reserve Univesity, Warner & Swasey Observatory
On April 24th, 2003
Dissertation Adviser: Dr. R. Earle Luck
Associated Papers
John C. Martin, 2006, AJ, 131, 2006
"The Origins and Evolutionary Status of B Stars Found Far From the Galactic Plane I: Composition and Spectral Features"
John C. Martin, 2004, AJ, 128, 2474
"The Masses Of The B-Stars In The High Galactic Latitude Eclipsing Binary IT Lib"
John C. Martin, 2003, PASP, 115, 49
Here it is at the PASP web site.
Pre-print available on the LANL preprint server.
Here is the ADS entry.
Executive Summary
Since the discovery of faint blue stars at high galactic latitudes, which spectroscopically appear to be similar to Population I B stars, a debate has raged about their origin and characteristics. Their faint apparent magnitudes have been interpreted to place many of them several kiloparsecs above the galactic plane in the halo. Some have been thought to be too far from the galactic plane and moving too slowly to be young B stars which were ejected from the star forming regions of the disk. A sample of these stars, taken from the Hipparcos catalog, was observed at high spectroscopic dispersion (R=60000), analyzed for chemical composition, traced back in time along their trajectory in the galactic potential using their full computed space velocities (determined from radial velocity and proper motions), and investigated for other oddities including infrared flux excesses.
The results and conclusions drawn from this comprehensive survey include:
There are no massive Pop I stars in this sample that could have formed in situ in the galactic halo. Additionally there is no need to invoke this scenario as an explanation for B stars found far from the galactic plane since massive Population I runaway stars, BHB stars, and PAGB stars are able to account for all high galactic latitude B stars.
There is remarkable consensus between this study and others that about 30% of the apparently “normal” B stars found far from the galactic plane are old low mass evolved stars. The remaining fraction (70%) are high mass Population I B stars which have been ejected from the disk.
Combined, relative abundance analysis and analysis of full space velocities in a galactic potential model are very effective tools for determining the natures and origins of high galactic latitude B stars.
Infrared excesses, which are normally associated with older evolved stars, are a relatively reliable way to quickly identify the most likely low mass evolved star candidates. The final release of the 2MASS data makes this tool more widely available for B stars with V magnitudes as faint as 13 or 14. Down the road, the data from SIRTF might be mined for this purpose as well.
A majority of high galactic B stars with vsin(i) less than 50 km/s are older evolved stars. In fact, the proportion of stars with small projected rotational velocities is significantly smaller among runaway Pop I B Stars than for B stars in the field of the galactic disk. The reason for this is uncertain. The most likely explanation is that the ejection mechanism(s) which produce runaways somehow discriminate against slowly rotating B stars.
Even though conventional wisdom dictates that all evolved stars should be slow rotators, there are PAGB stars which rotate much more rapidly than horizontal branch stars. Since almost nothing is known about the rotational properties of PAGB stars a survey is need to determine how many and what types of PAGB stars are rapid rotators. Better understanding of PAGB star rotation could lead to better understanding of the late stages of stellar evolution and the formation of planetary nebula.
Since most runaways are observed near the apex of their trajectory and the distance of that point from the galactic plane correlates with ejection velocity, the volume occupied by a sample of runaway B stars has a strong influence on the distribution of ejection velocities observed in that sample . Future work with the aim of examining the distribution of ejection velocities for runaway stars should be designed to avoid the inherent biases that are present in a sample limited by volume.
Runaway B stars ejected from the disk cannot presently be used to constrain the galactic potential in any meaningful way. There is too much uncertainty in the main sequence lifetimes and only the flight times of the stars closest to the galactic plane are sensitive to changes in the potential.
The Dissertation
This dissertation is being made available by the author (who holds non-exclusive rights to this document) to anyone who wishes to use it for educational or research purposes as long as proper citations and footnotes are made crediting the author. This document may not be reproduced or distributed for profit under penalty of US copyright law.
Copies of this dissertation can be found in the collections of the Case Western Reserve University Library (Cleveland, OH), the Library of the United States Naval Observatory (Washington, DC), and the UMI Dissertation Archives.
I strongly suggest that you use the papers above as reference for scientific material and conclusions since they have been through peer review. I am providing PDFs of the chapters themselves only for the supplementary material and explanation which some people may find useful.
Contents:
Errata!
This is a list of known errata in my diseration. They have not been corrected in the PDF files above or the archived copies.
The oscillator strengths listed in the tables of atomic data in Chapter 5 are give as the natural log (base e) rather than the base 10 logarithm. This was corrected in the paper published for that data in the AJ.
Citing this Work
This represents a lot of hard work on my part. If you use my dissertation itself, please cite it. I'd suggest:
Martin, J.C. 2003, PhD Thesis, Case Western Reserve University
Or in LaTeX:
\bibitem[Martin(2003)]{2003PhDT.........4M} Martin, J.~C.\ 2003, Ph.D.~Thesis, Case Western Reserve University
Or you can cite one of the resulting peer-review publications (at top of the page).
Last Updated 9 March 2006