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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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

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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

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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