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A spectroscopic hunt for binary RR Lyrae pulsators
Elisabeth GuggenbergerMax Planck Institute for Solar System Research, Göttingen
In collaboration with Thomas G. Barnes and Katrien Kolenberg
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Mag
nitu
de
Time (d)
Some Background on RR Lyraes • Pulsating stars (κ-mechanism in
He II, predominantly radial mode)• Large amplitudes: 0.3 – 1.5 mag • Periods: approx. 0.5 d• Evolved stars (He core burning) • 6000 K < T < 7500 K• 0.5 - 0.9 M
• Some are modulated
Time (d)
D m
ag
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Many RR Lyraes known, for example from the OGLE experiment:
• 38257 in the Galactic Bulge (Soszynski et al. 2014)• 7612 in the LMC • 2475 in the SMC (Soszynski 2010)
Credit: J. Hartman (Harcard CfA) & K. Stanek (Ohio State Univ.)
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Any Binaries?• Very few known!• Would be good to get mass (independently from pulsation/evolution models)• OGLE-BLG-RRLYR-02792:
+ TU Uma and some of other candidates that still await confirmation
Question: Are they really rare or just hard to find?
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How to find them?
• O-C technique:Use the pulsation as a clockLight time effectAssume constant period
Mag
nitu
de
Time (d)
Sharp maximum
timeObs
erve
d - e
xpec
ted Wrong period. Too short.
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How to find them?
timeObs
erve
d - e
xpec
ted Wrong period. Too long.
• O-C technique:Use the pulsation as a clockLight time effectAssume constant period
Mag
nitu
de
Time (d)
Sharp maximum
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How to find them?
timeObs
erve
d - e
xpec
ted Nothing going on. Correct period.
• O-C technique:Use the pulsation as a clockLight time effectAssume constant period
Mag
nitu
de
Time (d)
Sharp maximum
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How to find them?
timeObs
erve
d - e
xpec
ted Changing period. Evolution?
• O-C technique:Use the pulsation as a clockLight time effectAssume constant period
Mag
nitu
de
Time (d)
Sharp maximum
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How to find them?
timeObs
erve
d - e
xpec
ted Changing period. Evolution?
• O-C technique:Use the pulsation as a clockLight time effectAssume constant period
Mag
nitu
de
Time (d)
Sharp maximum
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How to find them?
timeObs
erve
d - e
xpec
ted
Binary or…
• O-C technique:Use the pulsation as a clockLight time effectAssume constant period
Mag
nitu
de
Time (d)
Sharp maximum
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…Modulation?br
ight
ness
• We do not fully understand the Blazhko effect• What shapes can the curve have? • Cannot differentiate it from a light time effect without additional
information..
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…other intrinsic variations• sudden jumps, irregular changes,…
(e.g. LeBorgne et al. 2007)
Studies systematically searching for O-C variations:Li & Qian (2014)Guggenberger & Steixner (2014)Hajdu et al. (2015): 1952 analyzed –> initially 29 candidates -> reduced to 12Liska et al (2016)
Radial velocities are more reliable indicators
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Target selection
• TU Uma:
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Target selection
• Other stars:– Follow up on candidates that have either:
• discordant velocities in the past• Suspicious O-C variations
– Cover RV curve well to prevent phasing errors– Create individual templates if possible– Lay a solid foundation for future studies (we
expect long orbits!)
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Target selection III
Solano et al. 1997
✔✔✔✔
Too south
✔
Fernley and Barnes 1997
✔✔
✔✔
✔
✔✔
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Challenge: High RV amplitude of pulsation!Challenge: High RV amplitude of pulsation!
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Challenges
• Incomplete phase coverage is pernicious• Often templates are used (e.g. Liu 1991, Sesar
et al. 2012), but:– Amplitude needs to be known
(from spectra or photometry)– Phases need to be correct – Template does not fit every
star equally well
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Observations @ McDonald Observatory
• 2.1m Otto Struve telescope• 22 nights awarded (15 useful with good weather)• Sandiford Echelle Spectrometer• R=60000, λ=4250-4750 Å• Spring 2014 - ? (new proposal sent in)
Image credit: Tim Jones/McDonald observatory
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Techniques
• Integration times < 5% of P• Several radial velocity standard stars/night• ThAr spectra after each exposure• Cross correlation with iraf/fxcor• Metal lines only!• Orders analyzed separately for consistency
check
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TU UMa
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TU UMa
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Finding the center of mass velocity
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Individual template
RVc,2014=92.6km/sRVc,2014=92.6km/s
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Comparison to orbit model
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CN Lyr
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CN Lyr
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Z CVn
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BX Leo
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Targets observed so farTarget # of velocity
measurements
TU Uma 47+5
CN Lyr 11
DM Cyg 11
Z CVn 9
BK Dra 9
AO Peg 8
BX Leo 7
SZ Leo 7
AV Vir 7
Target # of velocity measurements
ST Leo 7
BK And 6
XX Hya 5
RR Gem 4
RV UMa 4
CI And 3
U Tri 2
TT Lyn 1
SS Leo 1
Hopefully more to come…Hopefully more to come…
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Conclusions
• Campaign ongoing…• Extraction of systemic RVs from spectra in
progress• Detailed comparison with literature values and
photometry upcoming• New individual TU Uma template future
measurements easier and more accurate • Individual as well as center of mass velocities will
be published for further use