Peremennye Zvezdy (Variable Stars) 34, No. 3, 2014 Received 3 October; accepted 14 November.

Article in PDF

New Double-Mode RR Lyrae Variables

Institute of Astronomy, Russian Academy of Sciences, Moscow, Russia; e-mail: khruslov@bk.ru

1. Introduction

According to the International Variable Star Index (VSX, AAVSO), about 600 double-mode RR Lyrae variable stars are currently known in our Galaxy, usually pulsating in the fundamental and first overtone radial modes (most of them, outside globular clusters). During the recent years, automated data surveys resulted in a considerable progress in the identification of stars of this type. Wils (2010) reported seven new RR(B) stars from WASP data. About 90 RR(B) stars in the bulge of our Galaxy were found in the OGLE project (Soszynski et al. 2011). In the Catalina Surveys Periodic Variable Star Catalog (Drake et al. 2014), results are presented for 502 RR(B) stars, at least 300 of them previously unknown. Then, a discovery of 59 new Galactic double-mode RR Lyrae stars found in the LINEAR survey data (a small part of them were found earlier by other authors) was reported by Poleski (2014). The author of the present paper, with his co-authors, earlier discovered 28 double-mode RR Lyrae variable stars, pulsating in the fundamental and first overtone radial modes ( Khruslov 2007, 2010, 2011, 2012a, 2012b; Khruslov, Huemmerich, and Bernhard 2013; Antipin and Khruslov 2013; Huemmerich and Khruslov 2014).

In this article, I present the discovery of 28 more double-mode RR Lyrae variable stars, pulsating in the first-overtone and fundamental modes.

2. Results

I studied 28 known RR Lyrae variable stars, analyzing all observations available for them in the Catalina Surveys (Drake et al. 2009), LINEAR (Sesar et al. 2011), Northern Sky Variability Survey (NSVS , Wozniak et al. 2004), and SuperWASP (Butters et al. 2010) online public archives using the period-search software developed by Dr. V.P. Goranskij for Windows environment. According to these data, the variables are double-mode RR Lyrae variables, pulsating in the first-overtone and fundamental modes.

Light curves, finding charts, and data (CSS, SSS, MLS, LINEAR, NSVS, SWASP) are available online in the html version of this paper as a zip-archive. The light curves are given in the format displayed in Fig. 1. Top panels present data folded with the fundamental-mode and first overtone periods. Bottom panels show the same curves after prewhitening the other oscillation (if the frequencies and were excluded, it is also noted). These light curves are given for all data series. In three cases (No. 16, 19, 20), the data from LINEAR and Catalina surveys could not be represented with a unified system of light elements, probably indicating that both periods, and , vary.

Along with the light curves, we present power spectra of the RR Lyrae variables, for the raw data and after subtraction of the first-overtone (or fundamental mode) oscillations, as is shown in Fig. 2. The structure of the power spectra shows that the secondary periods are real.

Their period ratios, , are typical of radially pulsating double-mode RR Lyrae stars. The Petersen diagram for double-mode RR Lyrae variables of this article is displayed in Fig. 3.

Information on the studied stars is presented in Tables 1-4. Table 1 contains numbers from the GSC and USNO-B1.0 catalogs; equatorial coordinates (J2000); type and period previously known for the star, according to references from the last column, explained below the table. The tabulated coordinates of the variables were drawn from either the GSC2.3 (Lasker et al. 2008; star No. 1 in this paper) or 2MASS catalogs (Skrutskie et al. 2006).

References in Table 1:

1. Drake et al. (2013)

2. Palaversa et al. (2013)

3. Hadon, VS-COMPAS team, 2012

( http://www.vs-compas.belastro.net/stars/by/type/RRC )

4. Otero, International Variable Star Index, 2014

5. Keller et al. (2008)

Table 2 presents identification numbers for the LINEAR database, other data used in the analysis, magnitude in the maximum and minimum in the Catalina surveys and LINEAR photometric systems.

Light elements of all oscillations: the first-overtone period , the fundamental period , the first-overtone and fundamental mode epoch of maxima, period ratios , periods of the frequencies () and () are collected in Table 3.

Table 4 presents semi-amplitudes of all the oscillations, separately for Catalina surveys and LINEAR data: semi-amplitudes of first-overtone () and fundamental mode () oscillations, for the () and () frequencies. Asterisks mark the amplitudes based on other data, as explained in Comments, also containing additional information about some of the stars.

Comments:

No. 1. A rare case when the amplitude of the first overtone mode is much lower than that of the fundamental mode.

No. 6. The variability of GSC 5512-01099 was discovered by Siarhey Hadon, project VS-COMPAS (2012), from ROTSE-I/NSVS data (NSVS 15876164). Combined brightness of several stars was measured in the NSVS, the corresponding amplitudes are somewhat too low. The ROTSE data with photometric correction flags (usually rejected) were kept for the analysis. Asterisks in Table 4 mark the semi-amplitudes in the NSVS photometric system.

No. 16. Two systems of light elements were derived. The light elements for the CSS data are given in Table 3. The light elements for the Linear data are:

JD(max) = 2453600.273 + 0.421227 (the first-overtone period);

JD(max) = 2453600.363 + 0.564883 (the fundamental mode period).

No. 19. Two systems of light elements. The light elements for the CSS data are given in Table 3. The light elements for the Linear data are:

JD(max) = 2453600.125 + 0.413947 (the first-overtone period);

JD(max) = 2453600.225 + 0.554846 (the fundamental mode period).

No. 20. Two systems of light elements. The light elements for the CSS data are given in Table 3. The light elements for the Linear data are:

JD(max) = 2453600.300 + 0.444207 (the first-overtone period);

JD(max) = 2453600.065 + 0.596291 (the fundamental mode period).

No. 26. The variability of USNO-B1.0 0667-0901820 was discovered by Luyten (1938), who did not provide accurate coordinates. In the NSV catalog (Samus et al. 2007-2012), the star is numbered NSV 12483, the catalog position of NSV 12483 marked with a crosshair in our finding chart. Its type or period are not contained in the NSV catalog. The variability of USNO-B1.0 0667-0901820 was also reported by Keller et al. (2008; Id. 109453.71).The International Variable Star Index (VSX, AAVSO), based on Catalina survey data, gives the type RRC: (a possible RRC star) for the variable; initially, two periods were given, 0.201067 days and 0.290046 days. Later, in 2014, S. Otero gave it the period days, with the following remark: "It might be a double-mode pulsator (RRD) with a possible secondary period of 0.45397 d. (period ratio 0.7415) but more data are needed". I confirm the double-mode nature of this RR Lyrae variable, based on SWASP data. Combined brightness of two stars, USNO-B1.0 0667-0901820=var and the brighter neighbor GSC 6891-00928, was measured in the SWASP data, so the corresponding amplitudes are considerably underestimated. When plotting the phased light curves for the identified frequencies, and , I also subtracted variations of the mean brightness in the 1SWASP data, probably of instrumental origin. I considered the JD2453860-2454388 time range in the 1SWASP data. According to SSS data, it is USNO-B1.0 0667-0901820 that varies, and GSC 6891-00928 is a constant star. I was not able to reliably detect the double-mode behavior in the SSS data. Table 4 gives the combined-brightness semi-amplitudes in the SWASP photometric system. The SuperWASP observations are available as FITS tables, which were converted into ASCII tables using the OMC2ASCII program as described by Sokolovsky (2007).

Acknowledgments: Thanks are due to Dr. K.V. Sokolovsky for his advice concerning data retrieving. The author wishes to thank Dr. V.P. Goranskij for providing his software. This study was supported by the Russian Foundation for Basic Research (grant 13- 02-00664) and by the Programme "Non-stationary Phenomena of Objects in the Universe" of the Presidium of Russian Academy of Sciences.

References:

Antipin, S. V., Khruslov, A. V., 2013, Perem. Zvezdy Prilozh., 13, 15

Butters, O.W., West, R.G., Anderson, D.R., et al., 2010, Astron. and Astrophys., 520, L10

Drake, A.J., Djorgovski, S.G., Mahabal, A., et al., 2009, Astrophys. J., 696, 870

Drake, A. J., Catelan, M., Djorgovski, S. G., et al., 2013, Astrophys. J., 763, 32

Drake, A. J., Graham, M. J., Djorgovski, S. G., et al., 2014, Astrophys. J. Suppl., 213, 9

Huemmerich, S., Khruslov, A. V., 2014, Perem. Zvezdy Prilozh., 14, 2

Keller, S.C., Murphy, S., Prior, S., et al., 2008, Astrophys. J., 678, 851

Khruslov, A. V., 2007, Perem. Zvezdy Prilozh., 7, 7

Khruslov, A. V., 2010, Perem. Zvezdy Prilozh., 10, 11

Khruslov, A. V., 2011, Perem. Zvezdy Prilozh., 11, 25

Khruslov, A. V., 2012a, Perem. Zvezdy Prilozh., 12, 9

Khruslov, A. V., 2012b, Perem. Zvezdy Prilozh., 12, 25

Khruslov, A. V., Huemmerich, S., Bernhard, K., 2013, Perem. Zvezdy Prilozh., 13, 11

Lasker, B., Lattanzi, M.G., McLean, B.G., et al., 2008, Astron. J., 136, 735

Luyten, W.J., 1938, Publ. Astr. Obs. University Minnesota, 2, No.6, 114

Palaversa, L., Ivezic, Z., Eyer, L., et al., 2013, Astron. Journal, 146, 101

Poleski, R., 2014, PASP, 126, 509

Samus, N.N., Durlevich, O.V., Kazarovets, E.V., et al., 2007 - 2012, General Catalogue of Variable Stars, Centre de Donnees Astronomiques de Strasbourg, B/gcvs

Sesar, B., Stuart, J.S., Ivezic, Z., et al., 2011, Astron. J., 142, 190

Skrutskie, M.F., Cutri, R.M., Stiening, R., et al., 2006, Astron. J., 131, 1163

Sokolovsky, K.V., 2007, Perem. Zvezdy Prilozh., 7, No. 30

Soszynski, I., Dziembowski, W. A., Udalski, A., et al., 2011, Acta Astronomica, 61, 1

Wils, P., 2010, Inform. Bull. Var. Stars, No. 5955

Wozniak, P.R., Vestrand, W.T., Akerlof, C.W., et al., 2004, Astron. J., 127, 2436

---