The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. V. R Coronae Borealis Stars in the Large Magellanic Cloud
I. Soszynski, A. Udalski, M.K. Szymanski, M. Kubiak, G. Pietrzynski, L. Wyrzykowski, O. Szewczyk, K. Ulaczyk, R. Poleski
aa r X i v : . [ a s t r o - ph . S R ] F e b ACTA ASTRONOMICA
Vol. (2009) pp. 335–347 The Optical Gravitational Lensing Experiment.The OGLE-III Catalog of Variable Stars.V. R Coronae Borealis Stars in the Large Magellanic Cloud ∗ I. S o s z y ´n s k i , A. U d a l s k i , M. K. S z y m a ´n s k i ,M. K u b i a k , G. P i e t r z y ´n s k i , , Ł. W y r z y k o w s k i ,O. S z e w c z y k , K. U l a c z y k and R. P o l e s k i Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Polande-mail: (soszynsk,udalski,msz,mk,pietrzyn,wyrzykow,kulaczyk,rpoleski)@astrouw.edu.pl Universidad de Concepción, Departamento de Fisica, Casilla 160–C, Concepción, Chilee-mail: [email protected] Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB30HA, UKe-mail: [email protected]
Received November 20, 2009
ABSTRACTThe fifth part of the OGLE-III Catalog of Variable Stars presents 23 R CrB (RCB) stars inthe Large Magellanic Cloud (LMC). 17 of these objects have been spectroscopically confirmed byprevious studies, while 6 stars are new candidates for RCB variables. We publish the VI multi-epochOGLE photometry for all objects.We use the sample of carbon-rich long-period variables released in the previous part of this cat-alog to select objects with severe drops in luminosity, i.e. , with the DY-Per-like light curves. DY Perstars are often related to R CrB variables. We detect at least 600 candidates for DY Per stars, mostlyamong dust enshrouded giants. We notice that our candidate DY Per stars form a continuity withother carbon-rich long-period variables, so it seems that DY Per stars do not constitute a separategroup of variable stars. Key words:
Stars: AGB and post-AGB – Stars: carbon – Magellanic Clouds
1. Introduction
R CrB (RCB) stars are hydrogen-deficient, carbon-rich supergiants which un-dergo sudden and severe declines in brightness due to the formation of carbon dustat irregular intervals. This is a very rare type of variable stars, with only about ∗ Based on observations obtained with the 1.3-m Warsaw telescope at the Las Campanas Observa-tory of the Carnegie Institution of Washington. A. A.
50 known representatives in the Galaxy (Clayton 1996, Zaniewski et al. et al. e.g. , Iben et al. ≈
20 000 K), warm( ≈ ≈ ≈ et al. et al. et al.
2. Observational Data
All observations provided in this catalog were obtained with the 1.3-meter War-saw telescope located at Las Campanas Observatory in Chile. The observatory isoperated by the Carnegie Institution of Washington. The telescope was equippedwith the CCD mosaic camera consisting of eight detectors with 2048 × × . ol. 59 I photometric band and 40–60 measurements in the V -band. For thecentral 4.5 square degrees of the LMC the OGLE-III photometry has been supple-mented by the OGLE-II data collected between 1997 and 2000, which gives thetotal time span of observations of over 12 years. For more information on the in-strumentation setup and the photometric reduction techniques see Udalski (2003),Udalski et al. (2008) and the previous papers of this series.The near- and middle-infrared single-epoch photometric measurements used inthis study were originated in the 2MASS point source catalog (Cutri et al. et al. ′′ wasused to match the visual and infrared data.
3. Identification of Variable Stars
The majority of RCB stars presented in this catalog were identified by visualinspection of light curves during the selection of LPVs in the LMC (Soszy´nski etal. et al. (2009). From 19 spectroscopicallyconfirmed RCB stars in the LMC two objects (HV 12842 and EROS2-LMC-RCB-6) are located outside the OGLE-III fields, so there are no OGLE light curves forthese stars. Further two RCB stars (HV 5637, EROS2-LMC-RCB-3) do not showany significant magnitude declines during the time span covered by the OGLE sur-vey. The same behavior for HV 5637 was noticed by Alcock et al. (2001) andTisserand et al. (2009). Even so, both stars are included in our catalog, becauseit is well-known that some RCB stars may go for long intervals without minima( e.g. , XX Cam, Diethelm 1994).Thus, this catalog includes 17 of the 19 confirmed RCB stars in the LMC. Their2MASS and SAGE infrared magnitudes were used to plot the color–color diagramspresented in Fig. 1. We also show here the C-rich Miras and semiregular variables(SRVs) in the LMC from the previous part of the OIII-CVS (Soszy´nski et al. e.g. , Morgan et al. I -bandlight curves are shown in Fig. 2. As can be seen, these stars exhibit irregular fadingepisodes, however usually not so deep and not so sudden as for typical RCB stars.38 A. A.
Fig. 1. Color–color diagrams for the confirmed (blue dots) and candidate (cyan crosses) RCB starsin the LMC. Small black dots mark C-rich AGB stars from Soszy´nski et al. (2009). ol. 59
Fig. 2. The OGLE-III I -band light curves of candidates for RCB stars in the LMC. The only exception is OGLE-LMC-RCB-21 with the light decline up to 5.7 mag.It looks that virtually all bona fide
RCB stars in the region covered by the OGLE-III fields in the LMC have been already cataloged by the MACHO and EROS-2projects. Our six new candidates are likely low-active RCB stars, but obviously,their status has to be confirmed spectroscopically.
From ten confirmed DY Per stars (Alcock et al. et al.
A. A.
Fig. 3. Color–color diagrams for the confirmed (red dots) and candidate (yellow crosses) DY Perstars in the LMC. Small black dots mark C-rich AGB stars from Soszy´nski et al. (2009). ol. 59
Fig. 4. The OGLE-II and OGLE-III I -band light curves of six selected candidates for DY Per stars inthe LMC. infrared color–color diagrams (Fig. 3). As noticed by Alcock et al. (2001) andTisserand et al. (2009), DY Per stars occupy the same region in the color–colordiagrams as other C-rich AGB stars. Actually, the light curves of DY Per stars aretypical for C-rich LPVs and all of them were included in our catalog of LPVs in theLMC (Soszy´nski et al. A. A.
III catalog and detected 600 objects with similar features as possessed by the con-firmed DY Per stars, i.e. , with significant irregular declines of brightness ( > . i.e. , ( J − K ) ? ( K − [ ]) ? .
4. The Catalog of RCB Stars
In Table 1 we present the full list of confirmed and candidate RCB stars iden-tified in the OGLE-III fields in the LMC. The stars are sorted by increasing rightascension and designated with symbols OGLE-LMC-RCB-NN, where NN is a two-digit consecutive number. Table 1 provides information about the equinox J2000.0RA/DEC coordinates of stars and their identifications in the OGLE-III, OGLE-II,MACHO and EROS-2 databases, as well as in the General Catalogue of VariableStars (Artyukhina et al. O P HOT profile photom-etry for this object.Table 2 contains the basic parameters of the objects in the catalog: I- and V -band magnitudes at maximum light, pulsation periods, amplitudes of pulsation, andthe maximum drop amplitudes in the I -band. Several very active RCB stars havenot fully recovered at all during the OGLE observations and for these objects weprovide just the maximum of measured brightness. The periods and amplitudesof pulsation are given only for stars that have spent outside the declines enoughtime to determine periods. In some cases we used the MACHO light curves toimprove the periods. Note that RCB stars usually show semiregular and multi-periodic variations, but we provide only one, the primary period.The content of Tables 1 and 2 can be downloaded in the electronic form throughthe WWW interface or via the anonymous FTP site: http://ogle.astrouw.edu.pl/ftp://ftp.astrouw.edu.pl/ogle/ogle3/OIII-CVS/lmc/rcb/ The multi-epoch VI photometry and finding charts for all stars are also available inthe OGLE Internet Archive. o l . T a b l e 1
The identifications of RCB stars in the LMC
Star name OGLE-III ID Status RA DEC OGLE-II ID MACHO ID EROS-2 ID GCVS ID OtherField No [J2000.0] [J2000.0] LMC... designationOGLE-LMC-RCB-01 LMC126.5 51348 conf. 04 h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ A. A.
T a b l e 2
Parameters of the RCB stars in the LMCStar name I max V max P puls A puls ( I ) A drop ( I ) [mag] [mag] [days] [mag] [mag]OGLE-LMC-RCB-01 13.143 14.333 39.216 0.067 0.18OGLE-LMC-RCB-02 13.324 14.580 84.679 0.117 7.36OGLE-LMC-RCB-03 15.627 16.566 18.839 0.004 0.32OGLE-LMC-RCB-04 13.265 14.483 42.723 0.056 2.43OGLE-LMC-RCB-05 13.528 14.798 39.377 0.051 0.27OGLE-LMC-RCB-06 13.570 15.239 53.426 0.188 6.16OGLE-LMC-RCB-07 13.427 15.200 45.472 0.089 7.63OGLE-LMC-RCB-08 13.583 15.390 53.840 0.136 9.14OGLE-LMC-RCB-09 15.102 15.563 20.770 0.011 0.42OGLE-LMC-RCB-10 14.943 17.912 – – 6.63OGLE-LMC-RCB-11 13.520 14.490 57.690 0.156 7.09OGLE-LMC-RCB-12 14.223 16.140 – – 6.75OGLE-LMC-RCB-13 13.323 13.797 66.957 0.033 2.98OGLE-LMC-RCB-14 13.667 15.690 58.116 0.106 5.64OGLE-LMC-RCB-15 15.656 16.645 – – 0.34OGLE-LMC-RCB-16 14.179 14.437 – – 0.81OGLE-LMC-RCB-17 14.440 16.580 128.139 0.114 6.82OGLE-LMC-RCB-18 15.391 15.758 46.272 0.038 5.80OGLE-LMC-RCB-19 13.378 15.142 54.993 0.151 5.47OGLE-LMC-RCB-20 13.976 15.173 18.857 0.041 0.52OGLE-LMC-RCB-21 15.793 – – – 5.72OGLE-LMC-RCB-22 13.410 15.087 46.435 0.142 7.44OGLE-LMC-RCB-23 13.249 14.380 84.717 0.124 1.80 All the confirmed and candidate DY Per stars are included in the OGLE-IIIcatalog of LPVs (Soszy´nski et al.
5. Discussion
We performed an independent search for RCB in the LMC and found no un-doubted new objects of that type. It suggests that the catalogs of RCB stars in theLMC prepared on the basis of the MACHO and EROS-2 data are close to beingcomplete. The six new candidates for RCB stars identified among the OGLE lightcurves have infrared colors similar to the confirmed RCB stars, and their I -bandlight curves exhibit declines, however not so deep as for typical RCB stars. Wepropose to consider these new candidates as low-active RCB stars, similar to the ol. 59 Fig. 5. Spatial distribution of RCB and DY Per stars in the LMC. Color symbols represent the sametypes of stars as in Figs. 1 and 3. The background image of the LMC is originated from the ASASwide field sky survey (Pojma´nski 1997). confirmed HV 5637 and EROS2-LMC-RCB-3. Only one candidate RCB variable– OGLE-LMC-RCB-21 – undergoes large brightness variations ( > et al. (2003) in the list of the suspected RCBstars on the basis of its spectral characteristics. We also checked two candidateRCB stars selected by Tisserand et al. (2009). EROS2-LMC-RCB-7 seems to bea highly-obscured star with the I -band luminosity varying between 19 mag and20 mag. A sign of periodicity of about 700 days and the infrared colors of this ob-ject suggest that this is a C-rich Mira. The other EROS candidate for RCB stars –EROS-LMC-RCB-8 – lies outside the OGLE-III fields. The object MSX050825.4–685359, considered by Wood and Cohen (2001) as a possible RCB star, seems tobe a DY Per variable.Eight of ten DY Per stars identified and spectroscopically confirmed by Alcock et al. (2001) and Tisserand et al. (2009) can be detected in the LMC OGLE-IIIfields. All these stars are included in the OGLE-III catalog of LPVs in the LMC46 A. A. (Soszy´nski et al. ≈ . ( J − K ) > i.e. , they arethought to be AGB stars with thick circumstellar envelopes. We stress that theselected objects do not constitute any separate group of variable stars, but theyform a continuity with other C-rich AGB stars. Thus, it should be considered thatDY Per stars are just the extreme cases of dust-enshrouded AGB stars, and they arenot separate type of variables.The vast majority of our DY Per candidates are Miras or SRVs, but a numberof objects were categorized as OGLE Small Amplitude Red Giants (OSARGs).Some of these stars exhibit Long Secondary Periods (LSPs), just like OGLE-LMC-LPV-70666 presented in Fig. 4 with the LSP equal to ≈ Acknowledgements.
We are grateful to P. Tisserand for pointing out errors inFigs. 1 and 3. We thank G. Pojma´nski and J. Skowron for providing software anddata which enabled us to prepare this study.This work has been supported by the Foundation for Polish Science throughthe Homing (Powroty) Program and by MNiSW grants: NN203293533 to IS andN20303032/4275 to AU. REFERENCES
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