The Araucaria Project. Binary Classical Cepheids in the LMC
D. Graczyk, B. Pilecki, G. Pietrzyński, W. Gieren, P. Konorski, I. Soszyński, A. Udalski, A. Gallenne
aa r X i v : . [ a s t r o - ph . S R ] J u l Title : will be set by the publisher
Editors : will be set by the publisherEAS Publications Series, Vol. ?, 2019
THE ARAUCARIA PROJECT. BINARY CLASSICALCEPHEIDS IN THE LMC
Dariusz Graczyk , Bogumi l Pilecki , , Grzegorz Pietrzy´nski , , WolfgangGieren , Piotr Konorski , Igor Soszy´nski , Andrzej Udalski andAlexandre Gallenne Abstract.
The status of our work on binary classical cepheid systems inthe Large Magellanic Cloud is presented. We report on results from ourfollow up of two eclipsing binary cepheids OGLE-LMC-CEP-0227 andOGLE-LMC-CEP-1812. Here we presented for the first time confir-mation that a third cepheid OGLE-LMC-CEP-2532 is a true eclipsingbinary cepheid with a period of 800 days. Two other very good candi-dates for eclipsing binaries detected during OGLE-IV survey are alsodiscussed.
The eclipsing binary systems containing classical cepheids are very rare. Althoughwe know tens of binary cepheids in our Galaxy (e.g. Evans et al. et al. et al. etal. et al. Departamento de Astronom´ıa, Universidad de Concepci´on, Chile Astronomical Observatory of Warsaw University, Poland c (cid:13)
EDP Sciences 2019DOI: (will be inserted later)
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Fig. 1.
Position of eclipsing binary classical cepheids in the LMC monitored by ourproject and discovered from OGLE III survey (diamonds) and OGLE-IV survey (stars).Additionally we plotted position of late type eclipsing binary stars used to determinedistance to the LMC (circles). Background image is from ASAS-3 survey by Po-jma´nski (2002). verification of the mean cepheid radius - pulsation period relation; 3) developing ofa consistent scheme for solving light and radial velocity curves of radially pulsatingstar in an eclipsing binary system; 4) determination of distance independent theprojection factor for cepheids; 5) analysis of the limb darkening dependence on apulsation phase.Below we presented some results and details of our cepheids research. Thesections are devoted to particular eclipsing binary systems. The Figure 1 presentsposition of our target eclipsing binary cepheids within the LMC body.
This relatively bright star (V = 15.2 mag) was discovered to be a classical funda-mental mode cepheid by Soszy´nski et al. (2008). In the same paper the cepheidwas mentioned as possible eclipsing binary candidate. We confirmed it to be aneccentric double lined binary with orbital period of 309 days and deep eclipsesclipsing binary cepheids 3
Fig. 2.
A part of OGLE-III V-band optical light curve of eclipsing binary cepheid OGLE-LMC-ECL-0227 (points) covering two primary and secondary eclipses. The cepheid iseclipsed during deeper minimum. The continues line is our model synthetic light curve. (Pietrzy´nski et al. et al. et al. (2010) but by a factor of two more precise. We detected anoma-lously large limb darkening in the pulsating component in disagreement with pre-dictions from static atmosphere models. For more we calculated the projectionfactor of this cepheid which is an important parameter in Baade-Wesselink typemethods of distance determination to radially pulsating stars. This direct determi-nation is a distance and a limb darkening independent what differs our result fromthose obtained for galactic cepheids from interferometry (e.g. M´erand et al. et al.
Fig. 3.
Left : the Broadening Function of the MIKE spectrum of CEP-1812 taken closeto the first quadrature. The high peak corresponds to the cepheid component and theoptical blend produces a small peak at radial velocity of 265 km/s.
Right : OGLE-IIII-band image and close-up done with HST of a region centred on CEP-1812 (below). Aclump of stars in vicinity of the cepheid system is clearly visible in R-band HST image.
The star was confirmed to be an eccentric double lined eclipsing binary with anorbital period of 552 days by Pietrzy´nski et al. (2011). The pulsating component isa fundamental mode classical cepheid having shorter pulsation period than CEP-0227 - see Table 1. However during analysis of this system we detected non-negligible third light. First hint of it came from spectroscopy as we observed anadditional peak in the Broadening Function power distribution - Fig. 3. The peakis not observed on all spectra suggesting that the third light comes from an opticalblend. In our analysis we assumed that the light contribution from this blend isabout 10% of the total flux in I-band. The presence of the optical blend(s) wasconfirmed later through HST multicolour imaging. As the orbital inclination ofthe system is very close to 90 degrees the presence of the third light does notaffect significantly masses reported by Pietrzy´nski et al. (2011). Thus the cepheidcomponent is only the second one with mass known with accuracy better than3%. However radii and luminosities of both component are a subject of someuncertainty up to 5-10%. This system is somewhat puzzling because the cepheidhas much larger mass (and luminosity) than the companion star, however we meetboth these stars in a short evolutionary phase of giant evolution. Currently wework on improving our previous solution and deriving the p-factor for this cepheid.clipsing binary cepheids 5
Fig. 4.
The radial velocities and a model orbital motion of components of the systemCEP-2532 after subtracting cepheid’s pulsation velocities - points along continuous line.The points along dashed line signify radial velocities of red giant companion star.
This star contains a first overtone cepheid in a eccentric double lined eclipsingsystem having an orbital period of 800 days. We confirm here that both starsare gravitationally bounded by detecting their mutual orbital motion - see Fig 4.Because of the eccentricity and a position of the orbit we observe only one min-imum in this system when a red giant companion is eclipsing the cepheid star.Our photometric campaign to detect secondary minimum around expected orbitalphase 0.55 gave null result. It is interesting that the companion star is in factmore massive and more luminous than the cepheid.
Star LMC562.05.9009 is reported by Soszy´nski et al.
Fig. 5.
Left : the Broadening Function of the HARPS spectrum of LMC562.05. The twohigh peaks correspond to the cepheid component and its probable physical companion.
Right : OGLE-IV I-band light curve of LMC506.14 phased with pulsation period (the leftsubplot) and with supposed orbital period.
Table 1.
Physical parameters of the LMC classical Cepheids in eclipsing binaries
Parameter CEP-0227 CEP-1812 CEP-2532 LMC562 LMC506Orb. period (d) 309 552 800 >
820 242 (?)Pul. period (d) 3.98 1.31 2.04 2.99 4.51Pul. mode F F 1O F FMass (M ⊙ ) 4 . ± .
03 3 . ± . ∼ . a ? b ?Mass ratio 0 . ± .
01 0 . ± .
02 1 . ± .
10 ? ?Radius (R ⊙ ) 34 . ± . . ± . . ± . ∼ . Notes: a Assuming the orbital inclination of 90 deg. b ? - not directly determined yet.a possible secondary eclipse at orbital phase 0.53 - see Fig. 5. At the moment wehave no information about a presence of a companion star in spectra. During a few last years we were able to significantly improve the knowledge aboutphysical parameters of classical cepheids by observing and analysing two eclipsingbinary cepheids in the LMC. At least one more eclipsing binary system OGLE-2532 is confirmed. Our future plans consist of 1) performing similar analysisfor CEP-1812 like we did in the case of CEP-0227, 2) follow up of two newcandidate eclipsing binaries, 3) confirmation of binary nature of double eclips-ing cepheid OGLE-LMC-CEP-1718 and 4) improving our understanding of theBaade-Wesselink method applied for cepheids.clipsing binary cepheids 7
Acknowledgements:
We gratefully acknowledge financial support for this work from the Polish NationalScience Center grant MAESTRO DEC-2012/06/A/ST9/00269.
References
Alcock, C., et al. (MACHO) 1995, AJ, 109, 1653Evans, R. N., Bond, H. E., Schaefer, G. H., Mason, B. D., Karovska, M. and Tingle, E.2013, AJ, 146, 93M´erand, A., Kervella, P., Coud´e de Foresto, V. et al. et al. et al. et al.et al.