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
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
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.)
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?
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.
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
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
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
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
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
…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..
…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
Target selection
• TU Uma:
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!)
Target selection III
Solano et al. 1997
✔✔✔✔
Too south
✔
Fernley and Barnes 1997
✔✔
✔✔
✔
✔✔
Challenge: High RV amplitude of pulsation!Challenge: High RV amplitude of pulsation!
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
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
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
TU UMa
TU UMa
Finding the center of mass velocity
Individual template
RVc,2014=92.6km/sRVc,2014=92.6km/s
Comparison to orbit model
CN Lyr
CN Lyr
Z CVn
BX Leo
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…
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
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