Symbiotic Variables in the Local Group and Beyond

WDS'19 Proceedings of Contributed Papers — Physics, 165–170, 2019. ISBN 978-80-7378-409-6 © MATFYZPRESS

J. Merc Astronomical Institute, Faculty of Mathematics and Physics, Charles University, Czechia, Institute of Physics, Faculty of Science, P. J. SafárikUniversityˇ in Koˇsice,Slovakia. R. Gális Institute of Physics, Faculty of Science, P. J. SafárikUniversityˇ in Koˇsice,Slovakia.

M. Wolf Astronomical Institute, Faculty of Mathematics and Physics, Charles University, Czechia.

Abstract. Symbiotic stars belong to an interesting class of interacting binaries. However, many of these systems are still relatively unexplored, especially ones outside the Milky Way. In this short review, we focus especially on the objects in the Magellanic Clouds which are the targets of our ongoing research. We emphasize the advantages in studying of this symbiotic population.

Introduction Symbiotic variables are the widest interacting binaries consisting of a cool giant (or a su- pergiant) of a spectral type K or M (rarely G) as the donor and a compact star, the most commonly a hot white dwarf1 (≈ 105 K), as the accretor [Mikolajewska , 2007]. The mass transfer between the components most likely takes place via the cool giant’s wind [e.g., Allen, 1984a; Kenyon and Webbink, 1984; Mikolajewska , 2007], which is also the source of a dense circumbinary environment. The spectra of these objects are therefore the superposition of three components of radiation — two stellar and one nebular [e.g., Skopal, 2005]. Usually, the cool giant is dominating the spectrum at longer wavelengths (in IR) and the hot component mainly radiates in UV and blue part of optical region. Optical spectra are often rich in emission lines (Balmer lines of H, neutral and ionized He, Fe lines). Emission lines of a special interest are Raman-scattered lines of O vi which are exclusive feature of symbiotic binaries [Akras et al., 2019a] and provide a strong criterion for spectroscopic identification of new symbiotics. The investigation of these systems requires long-term monitoring as the orbital periods can be from hundreds to thousands of days [e.g., Mürsetand Schmid, 1999] and the activity of symbiotic variables can last for years. Moreover, several other processes contribute to the complexity of the light curves, such as eclipses, ellipsoidal effects, reflection effects, intrinsic variation of the both components or flickering. All of these occur on the various timescales from minutes to years. Recently, a systematic search for symbiotic binaries has begun, and not only in the Milky Way [e.g., Miszalski et al., 2013; Miszalski and Miko lajewska, 2014], but especially beyond of it, in nearby galaxies. Some surveys resulted in the identification of new extragalactic symbiotic variables or at least promising candidates [e.g., Gon¸calveset al., 2008, 2012, 2015; Kniazev et al., 2009; Mikolajewska et al., 2014, 2017]. Thanks to these surveys, the number of known symbiotic systems is growing rapidly.

New Online Database of Symbiotic Variables The ﬁrst comprehensive catalog of symbiotic variables [Allen, 1984b] contained 129 con- ﬁrmed and 15 suspected objects of which 6 were extragalactic. Another one, published by

1It is worth noting that several symbiotic binaries with accreting neutron stars were also identiﬁed [e.g., Masetti et al., 2007; Corbet et al., 2008; Enoto et al., 2014, and references therein].

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Kenyon [1986] was the compilation of 133 known symbiotic stars and 20 stars suspected of being symbiotic. The latest published catalog [Belczyńskiet al., 2000] contained 188 confirmed symbiotic systems as well as 30 objects suspected of being symbiotic, including 17 extragalactic objects. Over the past two decades, the number of confirmed systems has more than doubled, and many dozen candidates for symbiotic variables have emerged. At the same time, there had been a growing demand for the new catalog of symbiotic objects allowing their systematic studies. The census of symbiotic stars based on the 2MASS, WISE and Gaia surveys was published by Akras et al. [2019a]. It contained the list of 323 known and 87 candidate symbiotic stars. They focused mainly on the temperatures and IR types of symbiotic systems that were obtained by modeling of their IR spectra. Recently, we presented the New Online Database of Symbiotic Variables [Merc et al., 2019]. The database should serve not only as a catalog of data for all known symbiotic systems with consistent references, but we prepared also a web-portal for easy access to this information. The database is available online, allowing the addition of new objects as soon as they are discovered, and adding or updating the data when new information become available. In this way, the up-to-date lists of symbiotic variables and information about particular objects can be available to the community at any time. The database contains data about the position of the objects, their brightness in different spectral regions and other observational properties (e.g., the presence of outbursts, flickering, detectable X-ray or radio emission, symbiotic type), orbital properties (the orbital period, orbital ephemeris, presence of eclipses, etc.) and parameters of the binary components (their spectral types, temperatures, masses, radii, luminosities, presence of pulsations, etc.). For the cataloged symbiotic variables, we have prepared their own object pages covering all available information included in the database. The information and values provided are accompanied by appropriate references that are directly linked to the ADS database, which makes it easy and quick to find the quoted articles. The database is divided into two main parts according to the location of symbiotic variables. The first part consists of extragalactic symbiotic systems which are located in 14 galaxies (Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), Draco Dwarf, IC 10, M31, M33, M81, M87, NGC 55, NGC 185, NGC 205, NGC 300, NGC 2403, NGC 6822). The latest version of the New Online Database of Symbiotic Variables and the web-portal, including the up-to-date lists of all known symbiotic systems and candidates, is available at the internet address: http://astronomy.science.upjs.sk/symbiotics/.

Extragalactic symbiotic variables The number of extragalactic symbiotic stars and candidates have increased by the factor of 9 since the beginning of this century. In the recent version of our database, there are 74 conﬁrmed and 88 suspected extragalactic symbiotic systems. For the proper description of the extragalactic symbiotic population, systematic search for these objects is required. Such surveys have started in the recent years. For example, surveys focusing on M31 and M33 [Mikolajewska et al., 2014, 2017] resulted in discovery of 31 and 13 new symbiotic systems, respectively. The discovery of almost 200 other objects in M31 should be announced this year (J. Mikolajewska, private communication). Other galaxies are still waiting for systematic surveys. Several objects suspected of being symbiotic stars were detected thanks to their prominent X-ray emission [e.g., 7 objects in M31 or 3 in Draco Dwarf; Saeedi et al., 2019]. Other were suspected due to their photometric properties [e.g., long lasting outbursts of 9 symbiotic nova candidates in M87; Shara et al., 2016]. The greatest advantage of studying extragalactic symbiotic systems is that their distances are usually known with suﬃcient precision because they correspond to the distances of their parent galaxies. If the distances of symbiotic systems are known, the luminosities of their

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Figure 1. The position of symbiotic stars in the Magellanic Clouds (left: LMC, right: SMC). Conﬁrmed and suspected systems are denoted by blue pluses and red squares, respectively. components and other distance-related parameters could be determined precisely, which is very useful for comparing observational features with theoretical models [Mikolajewska et al., 2015].

Symbiotic variables in the Magellanic Clouds Our present research focuses mainly on those systems which are located in the Magellanic Clouds, because symbiotic variables in these galaxies are bright enough to be photometrically studied using meter-class telescopes. These objects evolved in the environment with a lower average metallicity than the symbiotic systems in the Milky Way, therefore the comparison of these two populations can be useful for understanding galactic objects containing low metallicity giants (e.g., AG Dra). The positions of confirmed and suspected symbiotic stars in the Magellanic Clouds are shown in Figure 1. Several of these objects have been observed during their outbursts. The carbon symbiotic star LMC S63 experienced the outburst in 1930 and the system remained active for 14 years [Ilkiewicz et al., 2015]. LMC S154 showed the nova-like outbursts in 1940s and 1980s and was recently classified as a symbiotic recurrent nova [Ilkiewicz et al., 2019]. Sanduleak’s star probably also experienced the outburst and the large stellar jet is associated with this object [Angeloni et al., 2011]. Actually, it is the largest known stellar jet from a star (14 parsecs long). LIN 9 was the first symbiotic system with the proven Z And-type outburst in SMC and Miszalski et al. [2014] observed several maxima in 2006–2010. SMC3 experienced the outburst in 1980–1981 [Morgan, 1992] and at least two outbursts of SMC2 detected in 1973 and 1985 are also reported in the same article. Moreover, our preliminary analysis of the OGLE-III light curves of SMC2 showed, that the outburst activity of this system has recently been repeated. However, the number of known symbiotic variables in the Magellanic Clouds is much lower than any estimate of the size of their population in these two galaxies. Therefore a systematic search for symbiotic stars in these galaxies is inevitable in the following years [the first results from the survey in the Small Magellanic Cloud were presented by Ilkiewicz et al., 2018]. More- over, several systems are very poorly studied. For some symbiotic candidates, spectroscopic information is completely missing. Brighter (mainly galactic) objects could be spectroscopically observed by amateur observers [see, e.g., the review of results of ARAS group presented by Teyssier, 2019], but spectra for extragalactic systems may be obtained only using the 10 meter- class telescopes. For example, in the most up-to-date version of our New Online Database of Symbiotic Variables, there are 21 objects in the LMC without derived spectroscopic parameters.

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Figure 2. The ﬁelds around the symbiotic star SMP LMC 94 obtained by 1.54-m Danish telescope at La Silla, Chile (left) and TESS satellite (right).

Spectroscopic observations should unambiguously determine the nature of these objects and can be used to determine the parameters of both their components (e.g., spectral types, effective temperatures) and symbiotic nebula (presence of emission lines, line ratios, maximal ionization potential). Proper characterization of these objects then would be useful for statistical studies of symbiotic stars and results can be also used as an input for machine-learning algorithms focused on discovering new ones [Akras et al., 2019b]. We plan to obtain spectroscopic observations of these objects using XSHOOTER/VLT and to supplement these data with photometric measurements which should be obtained at 1.54-m Danish telescope at La Silla in Chile (in B, V , R, I and Hα filters). Moreover, certain objects have been observed by ASAS-SN, OGLE or MACHO surveys and several symbiotic variables from the Magellanic Clouds have been observed by the TESS satellite while observing the south sectors (1–13) during the first year of its operation. It is worth to note that due to the TESS pixel size of ≈ 21”/px, the angular resolution is rather small and one should be very careful while using these data. Definitely, it is not possible to obtain light curves of all symbiotic stars located in the Magellanic Clouds from the TESS. For a comparison, Figure 2 shows the field around the symbiotic star SMP LMC 94 as observed by TESS and by 1.54-m Danish telescope.

Conclusion Symbiotic stars can serve as unique astrophysical laboratories to study accretion processes, winds or jets. They are important also for evolutionary models of binaries and other statistical research. Recently, we presented the new catalog of symbiotic stars which is available online to the whole community. The data are presented in the format of tables (for online and oﬄine use) and every system in the database has its own object page with information, references, notes and links. In the recent years, many interesting objects have been discovered in the Local Group and in other neighbouring galaxies, which need to be studied in more details. Several systems experienced outburst in the previous decades and some of them are proven to be active also recently. Our research focuses on the symbiotic systems in the Magellanic Clouds, as these are observable (at least photometrically) by the meter-class telescopes.

Acknowledgments. This research was supported by the Slovak Research and Development Agency grant No. APVV-15-0458.

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