Aug 01, 2014

Natalie Gosnell, UW Madison

"Blue Stragglers and X-Ray Binaries: An Observational Study of Alternative Pathways in Stellar Evolution"

Membership studies of evolved open clusters reveal many alternative-pathway stellar products whose 
evolution cannot be explained using single-star evolutionary models. These stars are 
neither rare nor anomalous, and in fact are a common occurrence in cluster populations. 
The goal of this thesis is to investigate the origin and evolutionary pathways of such 
stars through the careful study of X-ray binaries in NGC 6819 and white dwarf (WD) companions of mass 
transfer-formed blue straggler stars (BSSs) in NGC 188.
 
I present the first X-ray study of the intermediate-age open cluster NGC 6819, using observations 
from XMM-Newton.  This study of NGC 6819 is part of a systematic survey  
to investigate the relationship between the number of X-ray sources and cluster 
dynamics in the regime of massive open clusters.  Of the 12 X-ray sources within the half-light 
radius of NGC 6819, four sources challenge single-star evolutionary models, 
including a candidate quiescent low-mass X-ray binary. 
 
Next, I present the first results from a Hubble Space Telescope (HST) far-ultraviolet (FUV) campaign 
to search for WD companions of BSSs as indicators of mass transfer formation. 
I find direct observational detections of young (< 250 Myr), 
hot WD companions in three BSS binaries. 
Their presence in a well-studied cluster environment allows for unparalleled constraints on the pre-mass 
transfer system. I outline potential formation timelines for these three BSSs, which all formed 
through recent mass transfer.
 
Finally, I use HST photometry of the complete NGC 188 BSS population to place 
limits on the mass transfer BSS formation frequency. Comparison of the observations 
with models for BSS FUV emission reveals seven WD companions with 
temperatures greater than 11,000 K. The location of the young BSSs on an optical color-magnitude 
diagram suggests that using single-star evolutionary models to age luminous 
BSSs may be problematic. Considering other formation scenarios, the total mass transfer formation 
frequency for the NGC 188 BSSs is 67%. These results indicate that the parameterization of stable 
versus unstable mass transfer in binary population synthesis models should be revised.

Event Details

Date:
Aug 01, 2014

Time:
1:15–2:15pm

Location:
4421 Sterling Hall

Notes:
Thesis Defense

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