CHEMICAL COMPOSITIONS OF
FIELD DISK STARS
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A unique drawing of the Milky Way:
This was produced in 1955 at Lund Observatory by Knut Lundmark and
colleagues
(copyright:
Lund Observatory).
Click on picture to see larger image
Red Horizontal-Branch Stars in the Galactic Disk
Horizontal-branch (HB) stars are relatively low-mass objects that are fusing
helium quiescently in their cores. They have arrived on HB usually
after experiencing the He-flash at the end of their first ascent of
the red-giant branch. Metal-rich, Population I HB stars occupy a
small domain in HR Diagram (Teff ~ 4900K, MV ~ 1) called
the red clump. More metal-poor lower-mass thick disk and halo stars
lie on the HB at roughly the same absolute magnitude but with temperatures
than can range from T~5000K to T>15000K, depending on their masses and
metallicities. Such stars are labeled red HB (RHB), variable RR Lyr,
blue HB (BHB) and extreme blue HB (EBHB) as their temperatures.
Recently Melike Afşar
and her group at Ege University's
Department of Astronomy and Space Sciences
have been collaborating with me in a spectroscopic investigation of
purported RHB stars in the field. We wanted to know why there appear
to be so many of them that are relatively bright. If they are all thick
disk stars they should be few in number. In our first study, we identified
18 true field RHB stars. Surprisingly however, 13 of these
stars are members of the high-metallicity thin-disk population,
contrary to our expectations that all would be thick-disk,
modereately metal-poor stars.
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- Evolutionary states of our program stars are shown on
the spectroscopic log g-Teff diagram.
Stars with 12C/13C ≤ 20,
20 ≤ 12C/13C ≤30 and
12C/13C > 30 (no detection) are shown
by filled circles, (blue)
crosses and open circles, respectively.
(Fig 13 from
Afşar et al. 2012).
Click on picture to see larger image
|
Our contributions to this area of research concentrate on the chemical
compositions of the RHB candidate stars.
We derive metallicities, abundance ratios for the light α and n-capture
elemnets, but focus especially on the evolutionary-sensitive CNO group. The
most obvious chemical signature of an evolved star that has undergone interior
hydrogen fusion and convective envelope mixing of its products to the
stellar surface is a low 12C/13C. An example
of this is given in this figure:
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- Comparison of synthetic and observed spectrum of an
RHB star for 13CN features around 8004A. The
fit is illustrated by a red solid line for
12C/13C = 9.
(Fig 9 from
Afşar et al. 2012).
Click on picture to see larger image
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We are expanding our survey to obvserve hundreds of RHB candidates, hoping
to discriminate better between stellar population effects and interior
evolutionary effects in RHB stars.
Acknowledgments
All of the research described here has resulted from the collective efforts
of large numbers of people, including my current and former students, and
investigators at many astronomical institutions around the world.
Their contributions are gratefully acknowledged.
For many years I have benefited from grants by the U.S. National Science
Foundation and the National Aeronautics and Space Administration.
Their support has been crucial to my research.
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