Yellow symbiotic star AG Draconis in the scope of the New Online Database of Symbiotic Variables
YYellow symbiotic star AG Draconis in the scope ofthe New Online Database of Symbiotic Variables
Jaroslav Merc ∗ Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, CzechRepublicFaculty of Science, P. J. Šafárik University, Park Angelinum 9, 040 01 Košice, Slovak RepublicE-mail: [email protected]
Rudolf Gális
Faculty of Science, P. J. Šafárik University, Park Angelinum 9, 040 01 Košice, Slovak RepublicE-mail: [email protected]
Laurits Leedjärv
Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere, 61602 Tartumaa, EstoniaE-mail: [email protected]
Marek Wolf
Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, CzechRepublicE-mail: [email protected]
Symbiotic stars are strongly interacting binaries, consisting of a white dwarf and a cool giant,mainly of spectral type M. AG Draconis belongs to a less numerous group of the yellow symbioticsystems, as the cool component in this binary is of a spectral type earlier than K4. Recently,after seven years of quiescence, this symbiotic star exhibited a very unusual active stage with thefour minor outbursts observed. Thanks to the excellent involvement of amateur astronomers andprofessional observatories, we can study the activity of AG Draconis in unprecedented details. Inthe present work, we discuss the activity and peculiarities of this interacting system within theentire group of symbiotic stars whose properties have recently been presented in our New OnlineDatabase of Symbiotic Variables.
The Golden Age of Cataclysmic Variables and Related Objects V (GOLDEN2019)2-7 September 2019Palermo, Italy ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ a s t r o - ph . S R ] J un G Dra in the New Online Database of Symbiotic Variables
Jaroslav Merc
1. AG Draconis
Symbiotic variables are the widest interacting binaries consisting of a cool giant (or a super-giant) of a spectral type K or M (rarely G) as a donor and a compact star, most commonly a hotwhite dwarf ( ≈ K), as the accretor of transferred matter (Mikołajewska, 2007).AG Dra is one of the best studied symbiotic systems, but as discussed below, there are someunique features of this star. AG Dra belongs to a less common group of the yellow symbiotic stars- the cool component is a red giant of an early spectral type (K0 - K4), with a low metallicity anda higher luminosity than that of a standard class III (giant). Its hot component is considered to be awhite dwarf (WD) sustaining a high luminosity and temperature (Mikołajewska et al., 1995; Sionet al., 2012). The orbital period of this binary system is 551 days (Hric et al., 2014).The system undergoes characteristic symbiotic activity with alternating quiescent and activestages. The active stages occur in intervals of 9 - 15 years and consist of several outbursts repeatingat about one-year interval (Gális et al., 2017). The activity of AG Dra usually begins with themajor, prominent outburst followed by several minor ones.As previously indicated by UV observations (González-Riestra et al., 1999), the characteristicsof the optical emission lines clearly confirmed the presence of two types of the outbursts of AG Dra:major ones are usually cool and smaller-scale outbursts are of the hot type (Leedjärv et al., 2016).During the hot outbursts, the brightness of AG Dra is more or less linearly correlated with variationsof emission lines characteristics (e.g. EWs) and the He II Zanstra temperature increases or remainsunchanged. On the other hand, during the cool outbursts, the He II Zanstra temperature drops asthe pseudo-atmosphere of the WD expands.
Prodigious amount of observations of the symbiotic system have revealed that AG Dra under-went at least six active stages since 1889. Previous, particularly long active stage (E+F) started in1993 and continued until 2008 (Hric et al., 2014). This active stage was followed by seven years ofquiescence (Q6) and AG Dra begun rising again in brightness in the late spring of 2015 (see the toppanel of Fig. 1). We have observed series of four outbursts during the ongoing active stage (Gáliset al., 2019).According to the brightness of AG Dra, all four recent outbursts can be classified as minorones. The increase of EWs observed during these outbursts is indicating their hot character (Mercet al., 2018). However, outbursts in the beginning of active phases of AG Dra are usually major, cool ones which makes the ongoing active stage very unusual. A detailed discussion of the featuresof this activity will be the subject of the forthcoming article.
2. New Online Database of Symbiotic Variables
AG Dra is a variable star whose symbiotic character has been known for years (e.g. Kenyon,1986). It was discovered accidentally like most symbiotic binaries in the previous century. How-ever, systematic search for such objects has begun in recent decades and this effort has alreadybrought a lot of new results. Surveys have led to the discovery of many new objects and dozensof candidates in the Milky Way (e.g. Miszalski et al., 2013; Miszalski & Mikołajewska, 2014)1
G Dra in the New Online Database of Symbiotic Variables
Jaroslav Merc
JD - 2 400 000 [days] B [ m a g ] AG Dra
JD - 2 400 000 [days] B [ m a g ] AG Peg
JD - 2 400 000 [days] B [ m a g ] Z And
JD - 2 400 000 [days] B [ m a g ] AX Per
Figure 1: The light curves in B filter over the period 1997 - 2019 of four galactic symbiotic starswhich experienced outbursts in recent years. Data are obtained from AAVSO database (Kafka,2019), Skopal et al. (2002, 2004, 2007, 2012) and Sekeráš et al. (2019).2 G Dra in the New Online Database of Symbiotic Variables
Jaroslav Merc and the Local Group (e.g. Gonçalves et al., 2008, 2012, 2015; Kniazev et al., 2009; Mikołajewskaet al., 2014, 2017; Shara et al., 2016; Roth et al. 2018). Subsequently, as the number of knownsystems grew, the catalog of symbiotic stars which was published by Belczy´nski et al. (2000) be-came rapidly outdated. For this reason, we have prepared a new online database of the galactic andextragalactic symbiotic systems (Merc et al., 2019a).The database is divided into two main parts according to the location of symbiotic variables.The first part consists of 74 confirmed and 88 suspected extragalactic symbiotic systems whichare located in 14 galaxies (LMC, SMC, Draco Dwarf, IC 10, M31, M33, M81, M87, NGC 55,NGC 185, NGC 205, NGC 300, NGC 2403, NGC 6822). The second part of the database consistsof more than 480 galactic objects. The data of symbiotic variables are presented in the form oftables, which can be explored directly through the dedicated web-portal or can be downloadedand used offline in different formats (csv, xlsx, txt and pdf). Moreover, for all symbiotic bina-ries included in the database, we have prepared their object pages covering all available infor-mation, references, notes, and useful links. The database is accessible through the web-page:http://astronomy.science.upjs.sk/symbiotics/.
With the increasing number of known symbiotic stars, the vast amount of observations ob-tained particularly thanks to amateur observers and the availability of the New Online Database ofSymbiotic Variables, symbiotic binaries can be studied as a whole population of interesting vari-able stars. In addition, individual stars can be studied in relation to other objects from the knownpopulation, allowing comparison of parameters and behavior.Several symbiotic stars have been studied very extensively during previous decades. Amongthem, AG Dra is an exceptional case with hundreds of articles available on the Astrophysics DataSystem mentioning this object. However, there are still many open questions regarding the com-ponents of this interacting system or the outburst mechanisms as well. Moreover, it is not only thenumber of observations and studies, which is unique about the star.AG Dra is a classical symbiotic star of the infrared type S (Friedjung et al., 1998). Majorityof the objects in the present version of the New Online Database of Symbiotic Variables ( ≈
83 %)belongs to this type (Fig. 2). The radius of the giant was estimated to be 33 ±
11 R (cid:12) (Skopal, 2005)and if we assume the volume radius of its corresponding Roche lobe to be 170 R (cid:12) (Ogley et al.,2002), the giant is under-filling it, which is typical for most symbiotic binaries.On the other hand, AG Dra belongs to a small group ( ≈
10 %) of so-called yellow symbioticstars characterized by an early spectral type of the giant (K or even G) which is different frommore typical spectral type M. While the exact spectral type of the giant in the AG Dra system isnot known, it is definitely of an early spectral type (K0 - K4; Fig. 3). The metallicity of the giantis very low (Fe/H ≈ − .
3; Smith et al., 1996). Together with the radial velocity of the systemof -147 km s − and its position on the sky (Fig. 4a), these suggest that AG Dra belongs to the oldpopulation of the galactic halo. Although the location of AG Dra on the sky favors convenient long-term observations, at the same time it is not common for variable stars from this group, as nearlyall galactic symbiotic stars are located around the Milky Way equator ( | b | < ◦ ). For the objectsin the current version of the database, 89 % of the confirmed and 83 % of candidate symbiotic starsare located in this sky region (Merc et al., 2020; Fig. 4b and 4c).3 G Dra in the New Online Database of Symbiotic Variables
Jaroslav Merc
Figure 2: The position of symbiotic stars of various infrared types (S, S+IR, D and D’) as wellas other objects in the near IR color-color diagram based on 2MASS observations. The Mainsequence (MS) and Red giant branch (RGB) loci are taken from Straižys & Lazauskait˙e (2009),Be stars from Zhang et al. (2005), planetary nebulae from Ramos-Larios & Phillips (2005) andT Tauri stars from Dahm & Simon (2005).The hot component of AG Dra is a WD. Although some symbiotic binaries with a neutron staras an accretor have been discovered (e.g. Enoto et al., 2014), majority of them contain accretingWDs. Quasi-steady shell burning of the hydrogen-rich material on the surface of WD is probablyalso the source of super-soft X-ray emission of AG Dra. There are 58 other symbiotic stars whichhave been detected in X-rays (Merc et al., 2019c). Nine sources, AG Dra among them, showthe super-soft emission which have been classified as the α type in the scheme introduced byMuerset et al. (1997) and extended by Luna et al. (2013). In addition to nine α sources, 18 havebeen classified as the β type in which the X-ray emission is interpreted as due to the shock-heatedplasma emerging in the collision of winds from the components, nine are neutron star accretors (the γ type), and twelve have been classified as the δ type in which X-ray emission originates from theboundary layer between the accretion disk and the WD. Eleven systems are showing characteristicsof both β and δ types and are therefore classified as the β / δ type.Very prominent emission lines are observed in optical spectra of the α and β types (Akras etal., 2019; Fig. 2 in Merc et al., 2019c). In the case of AG Dra, the emission lines of H I , He I ,He II and Raman-scattered O VI are observed almost all the time, with the emission lines of Fe II appearing during the outbursts and the O VI lines vanishing during the cool outbursts. The Raman-4 G Dra in the New Online Database of Symbiotic Variables
Jaroslav Merc
Figure 3: Optical spectra of AG Dra (the giant’s spectral type K0 - K4) and AG Peg (the giant’sspectral type M3) during their quiescence (blue) and outburst (red) stages. Data are obtained fromthe ARAS database (Teyssier, 2019).scattered O VI lines, which are typically considered as an evidence of the symbiotic nature of anobject are observed in ≈
55 % of symbiotic stars (Akras et al., 2019).The spectral characteristics of symbiotic systems are changing dramatically throughout theiroutbursts (comparison of the quiescent and active optical spectra of AG Dra is shown in Fig. 3).The AG Dra binary star manifests the Z And-type activity, characterized by recurrent outburstswith amplitudes of ≈ − U filter, and are not so pronounced in B or V filters. In symbiotic stars comprising Mgiants, these variations are easily observable at least in B filter (Fig. 1), because the giant dominatesat longer wavelengths. In case of AG Dra, the light curves in the B or V filters are dominated bymodulations with a period of around 355 days, explained by the pulsations of the giant as suggestedby Gális et al. (1999).Comparing of the light curves of outbursting symbiotic stars is interesting for several reasons.The similarities may indicate the same nature of the outburst mechanisms responsible for theirobserved activity. For example, Sokoloski et al. (2006) proposed the combination nova model as an5 G Dra in the New Online Database of Symbiotic Variables
Jaroslav Merc(a) The distribution of galactic symbiotic stars overlaid on the 2MASS infrared image of the sky (in thegalactic coordinates). Confirmed and suspected symbiotic stars are denoted by blue dots and red squares,respectively. The position of AG Dra is shown by the green cross.(b) Histogram showing the distribution of symbi-otic stars over the galactic longitude. The positionof AG Dra is shown by the green line. (c) Histogram showing the distribution of symbi-otic stars over the galactic latitude. The position ofAG Dra is shown by the green line.
Figure 4: The position of AG Dra on the sky.6
G Dra in the New Online Database of Symbiotic Variables
Jaroslav Merc
Table 1: Comparison of the parameters of AG Dra and LT Del.
AG Dra LT DelTemperature of the giant [K]
Metallicity [Fe/H] -1.3 -1.1
Radial velocity [km/s] -147 -107
IR type
S S
Activity
Z And Z And
Orbital period [d]
551 476
Dominant spectral lines
H I, He I, He II H I, He I, He II
Fe II lines
During some outbursts No
O VI lines
Yes No
X-rays
Yes Noexplanation of the outburst activity of Z And, in which smaller-scale hot outbursts are explained bythe accretion disc instability model, as in the dwarf novae (Warner 1995) and major cool outburstsare due to the thermonuclear runaway as in the classical nova outbursts. This model may also beapplicable in the case of AG Dra (Leedjärv et al., 2016) and other symbiotic stars.On the other hand, it was very exciting to observe the symbiotic system AG Peg to show theZ And-type outburst in 2015 (Skopal et al., 2017, Merc et al., 2019b), 165 years after its nova-likeflare-up. In the case of AG Dra, very long period of quiescence had also been observed since thebeginning of observations in 1889 until 1927 (or maybe even 1932). It is therefore possible, thatAG Dra (and other classical symbiotic stars as well) experienced the symbiotic nova stage in pastand then started to show Z And-type activity. Comparing the current behavior of these symbioticsystems can help to solve this issue.
3. Comparison of AG Dra and LT Del
As already mentioned, the cool component of AG Dra is the giant of a spectral type K. Most ofthe symbiotic stars mentioned in the previous section (e.g. AG Peg, Z And) are comprising M gi-ants. When looking for an object markedly similar to AG Dra, besides an early spectral type of thegiant (a yellow symbiotic star), it should meet several other criteria: low giant metallicity, infraredtype S, Z And-type activity, and it should be regularly observed to some extent, allowing compari-son of both objects. The New Online Database of Symbiotic Variables can be used effectively forsuch a search.One of the relatively appropriate candidates is the yellow, infrared type S symbiotic starLT Del. The binary system with an orbital period of 476 days (Arkhipova et al., 2011) is con-sisting of a giant of a spectral type K3 with a low metallicity (Fe/H ≈ -1.1; Pereira et al., 1998)and a hot WD with a temperature of ≈ K. The symbiotic system LT Del, similarly to AG Dra,belongs to the halo population which is suggested by the low metallicity and high radial velocity(-107 km s − ). Comparison of the parameters of AG Dra and LT Del is listed in Tab. 1.During the observation period, LT Del experienced two outbursts (in 1994 and 2017). Thelatter one has been studied by Ikonnikova et al. (2019). They reported that the outburst was7 G Dra in the New Online Database of Symbiotic Variables
Jaroslav Merc of the hot type, in contrast with the one observed in 1994 during which the temperature did notchange compared to its quiescent values. As mentioned before, such behavior is typical for thesymbiotic system AG Dra. Similarly to AG Dra, the quiescent light curves of LT Del are dominatedby sinusoidal variations caused by the changing visibility of emitting regions during the orbitalrevolution of the binary system. Ikonnikova et al. (2019) reported similar variability observed alsoin the case of the EWs of low excitation emission lines.The main difference of the two symbiotic stars is in the presence of Raman-scattered O VI emission lines, which have not been detected in spectra of LT Del. This symbiotic star has not beendetected in the X-rays as well which points to a difference in the hot components of these systemsor in geometry and optical thickness of the symbiotic nebulae.
4. Conclusions
Symbiotic stars are important astrophysical laboratories in studying a great variety of interest-ing phenomena. Yellow symbiotic stars are of particular interest because their absorption spectraare not influenced by TiO molecule absorption features which allows more precise measurementsof atomic lines. The symbiotic system AG Dra is the prototype of this small but growing group.Recent efforts have resulted in several new discoveries that allow for more systematic studies ofthe symbiotic population.The purpose of this work was not only to present the New Online Database of SymbioticVariables but also to show how it can be utilized for the study of specific symbiotic system, in ourcase of AG Dra. The binary aroused the interest recently when it showed very unusual activitystage and therefore it is more than ever interesting to compare the behavior of AG Dra to othersymbiotic stars, e.g. those showing Z And-type activity, those containing K giants or those with thesuper-soft X-ray emission. As we have demonstrated, there are several options to select samples forcomparison based on similarities or differences of AG Dra to other known symbiotic stars. Basedon this example, the New Online Database of Symbiotic Variables can be used for studies of otherinteresting symbiotic systems.
Acknowledgements
This research was supported by the Slovak Research and Development Agency grant No.APVV-15-0458, by the Faculty of Science, P. J. Šafárik University in Košice under the internalgrant VVGS-PF-2019-1047, by the Charles University, project GA UK No. 890120, and by theEstonian Ministry of Education and Research institutional research funding IUT 40-1.
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DISCUSSIONALESSANDRO EDEROCLITE:
Is the database VO-compatible?