TYC 8606-2025-1: a mild barium star surrounded by the ejecta of a very late thermal pulse
V.V. Gvaramadze, Yu.V. Pakhomov, A.Y. Kniazev, T.A. Ryabchikova, N. Langer, L. Fossati, E.K.Grebel
aa r X i v : . [ a s t r o - ph . S R ] S e p Mon. Not. R. Astron. Soc. , 000–000 (0000) Printed 5 September 2019 (MN L A TEX style file v2.2)
TYC 8606-2025-1: a mild barium star surrounded by theejecta of a very late thermal pulse
V. V. Gvaramadze, , ⋆ Yu. V. Pakhomov, A. Y. Kniazev, , , T. A. Ryabchikova, N. Langer, L. Fossati and E. K. Grebel Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetskij Pr. 13, Moscow 119992, Russia Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 119017, Moscow, Russia South African Astronomical Observatory, PO Box 9, 7935 Observatory, Cape Town, South Africa Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory, Cape Town, South Africa Argelander-Institut f¨ur Astronomie, Auf dem H¨ugel 71, 53121 Bonn, Germany Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria Astronomisches Rechen-Institut, Zentrum f¨ur Astronomie der Universit¨at Heidelberg, M¨onchhofstr. 12-14, D-69120 Germany
Accepted 2019 September 01. Received 2019 August 30; in original form 2019 August 15
ABSTRACT
We report the discovery of a spiral-like nebula with the
Wide-field Infrared Survey Ex-plorer ( WISE ) and the results of optical spectroscopy of its associated star TYC 8606-2025-1 with the Southern African Large Telescope (SALT). We find that TYC 8606-2025-1 is a G8 III star of ≈ ⊙ , showing a carbon depletion by a factor of two anda nitrogen enhancement by a factor of three. We also derived an excess of s-processelements, most strongly for barium, which is a factor of three overabundant, indicatingthat TYC 8606-2025-1 is a mild barium star. We thereby add a new member to thesmall group of barium stars with circumstellar nebulae. Our radial velocity measure-ments indicate that TYC 8606-2025-1 has an unseen binary companion. The advancedevolutionary stage of TYC 8606-2025-1, together with the presence of a circumstellarnebula, implies an initial mass of the companion of also about 3 M ⊙ . We conclude thatthe infrared nebula, due to its spiral shape, and because it has no optical counterpart,was ejected by the companion as a consequence of a very late thermal pulse, duringabout one orbital rotation. Key words: stars: abundances – circumstellar matter – stars: individual: TYC 8606-2025-1 – stars: low-mass
In the search for rare types of massive stars through the de-tection of their infrared (IR) circumstellar shells (e.g. Gvara-madze, Kniazev & Fabrika 2010; Gvaramadze et al. 2012),we discovered numerous compact nebulae of various shapesusing data from the
Spitzer Space Telescope (Werner et al.2004) and the
Wide-field Infrared Survey Explorer ( WISE ;Wright et al. 2010). Follow-up spectroscopy of their cen-tral stars showed that most of them are produced by lumi-nous blue variables, blue supergiants, and Wolf-Rayet stars(e.g. Kniazev & Gvaramadze 2015; Gvaramadze & Kniazev2017 and references therein). As a by-product, we found thatseveral of the detected nebulae were created in the course ⋆ E-mail: [email protected] of evolution of low-mass stars (e.g. Gvaramadze & Kniazev2017; Gvaramadze et al. 2019). One of these nebulae and itsassociated star TYC 8606-2025-1 are the topic of this paper.In Section 2, we present multiwavelength images ofthe nebula and review the existing data on TYC 8606-2025-1. Spectroscopic observations and spectral analysis ofTYC 8606-2025-1 are described, respectively, in Sections 3and 4. In Section 5, we discuss the obtained results and out-line further work. Finally, Section 6 provides a summary.
The nebula around TYC 8606-2025-1 was discovered usingdata from the
WISE mid-IR all sky survey (Wright et al.2010). This survey provides images at four wavelengths: 22, c (cid:13) V. V. Gvaramadze et al. NE Figure 1.
From left to right and from top to bottom:
Herschel
160 and 70 µ m, WISE
22 and 12 µ m, Spitzer µ m, DSS-IIred-band, SHS H α +[N ii ] and VPHAS+ H α images of TYC 8606-2025-1 (marked by a circle) and its associated circumstellar nebula. Awhite stripe in the VPHAS+ image is a gap between the CCDs in the camera mosaic. A white stripe in the VPHAS+ image is a gapbetween the CCDs in the camera mosaic. The orientation and the scale of the images are the same. At the distance to TYC 8606-2025-1of 0.97 kpc, 1 arcmin corresponds to ≈ .
28 pc.
12, 4.6 and 3.4 µ m, with an angular resolution of 6.1, 6.4,6.5 and 12.0 arcsec, respectively. The nebula is clearly seenin the 22 and 12 µ m images (see Fig. 1), and at both wave-lengths its angular size is ≈ indicates the Infrared Astronomical Satellite ( IRAS )source IRAS 09546 − Spitzer and
WISE , most of which have circular or bipo-lar appearance (Gvaramadze et al. 2010), the nebula pro-duced by TYC 8606-2025-1 appears as a spiral. The spiral-like shape of the nebula is more obvious in the
WISE µ mimage showing that the spiral starts from TYC 8606-2025-1and then bends around this star on the east side. The bright-ness distribution over the nebula is inhomogeneous, with itsmaximum close to the star and dropping by a factor of sev-eral at the periphery.The nebula was also observed with the Infrared ArrayCamera (IRAC; Fazio et al. 2004) on board the Spitzer SpaceTelescope at 8 and 4.5 µ m, and with the Photodetector Array http://simbad.harvard.edu/simbad/ Camera and Spectrometer (PACS) instrument on board the
Herschel
Space Observatory (Pilbratt et al. 2010) at 160and 70 µ m. Fig. 1 shows that the nebula is visible (at leastpartially) at all these wavelengths. In the 160 µ m image, itappears that the nebula is blended with back- or foregroundemission.We also searched for an optical counterpart to the neb-ula using the Digitized Sky Survey II (DSS-II; McLean et al.2000), the SuperCOSMOS H-alpha Survey (SHS; Parker etal. 2005) and the VST Photometric H α Survey of the South-ern Galactic Plane and Bulge (VPHAS+; Drew et al. 2014),but did not find it (see, however, Section 5.2). Moreover, wedid not find any signature of the nebula in the optical long-slit spectrum of TYC 8606-2025-1 (see Section 3).In Table 1, we summarize some of the properties ofTYC 8606-2025-1. The spectral type is based on our spectro-scopic observations and spectral analysis. The B and V mag-nitudes are from the Tycho-2 catalogue (Høg et al. 2000).The coordinates and the JHK s photometry are taken fromthe Two-Micron All Sky Survey (2MASS; Skrutskie et al.2006) All-Sky Catalog of Point Sources (Cutri et al. 2003).The WISE photometry is from the AllWISE Data Release(Cutri et al. 2014). The distance is based on the
Gaia second c (cid:13) , 000–000 YC 8606-2025-1: a mild barium star Table 1.
Properties of TYC 8606-2025-1.Spectral type G8 III Ba0RA(J2000) 09 h m . s − ◦ ′ . ′′ l . ◦ b − . ◦ B (mag) 11 . ± . V (mag) 10 . ± . J (mag) 8 . ± . H (mag) 7 . ± . K s (mag) 7 . ± . .
4] (mag) 7 . ± . .
6] (mag) 7 . ± . . ± . . ± . d (kpc) d = 0 . +0 . − . data release (DR2; Gaia Collaboration et al. 2018) parallaxof TYC 8606-2025-1 of 1 . ± . d = 0 . +0 . − . kpc. At this distance, thelinear size of the nebula is ≈ .
28 pc.
To get an idea on the nature of TYC 8606-2025-1, we ob-tained its long-slit spectrum on 2014 April 11 with theRobert Stobie Spectrograph (RSS; Burgh et al. 2003; Kob-ulnicky et al. 2003) at the Southern African Large Telescope(SALT; Buckley, Swart & Meiring 2006; O’Donoghue et al.2006). The volume phase holographic (VPH) grating GR900was used to cover the spectral range from 4200 to 7200 ˚A,with a slit width of 1.25 arcsec and the spectral resolutionof FWHM ≈ R ≈ ◦ (measured from north to east), i.e. insuch a way to cover the brightest part of the nebula.We obtained two short (30 s) and two long (300 s) ex-posures with an average seeing of 1.3 arcsec. A referencespectrum of a Xe arc lamp was obtained immediately af-ter each observation. The spectrophotometric standard starCD − ◦ ≈
100 and 150.The HRS is a dual beam, fibre-fed ´echelle spectrograph.In the MR mode it has 2.23 arcsec diameter for both the ob-ject and sky fibres providing a spectrum in the blue and red arms over the spectral range of ≈ R ≈
36 500 −
39 000 (Kniazev et al. 2019). In ourobservations, both the blue and red arm CCDs were readout by a single amplifier with a 1 × R = 48 000) ´echelle spectrograph —mounted on the 2.2-m Max Planck Gesellschaft telescopeat La Silla. Four spectra covering the spectral range of ≈ − ≈ The RSS spectrum of TYC 8606-2025-1 does not show emis-sion lines and is typical of cool stars of the G–K spectraltype. The H γ /Fe i λ < We used the SME (Spectroscopy Made Easy) package(Valenti & Piskunov 1996; Piskunov & Valenti 2017) for theatmospheric parameter determination and abundance esti-mates. This package was designed for the analysis of stellarspectra using spectral fitting techniques. After correction forheliocentric velocity and radial velocity variations (see Sec-tion 4.2.2), the HRS spectra were co-added to increase theSNR. Four spectral regions 5100–5250, 5600–5705, 6100–6250, and 6450–6630 ˚A were chosen for the fitting. Thischoice was based on the presence of spectral lines sensitiveto different atmospheric parameters (Mg i triplet at 5167-5183 ˚A, H α line wings, etc), as well as on the presence ofmolecular lines of C and CN used for carbon and nitrogenabundance estimates.The fundamental atmospheric parameters, effectivetemperature T eff , surface gravity log g , metallicity [Fe/H],micro- and macroturbulent velocities V mic and V mac , and c (cid:13) , 000–000 V. V. Gvaramadze et al.
Table 2.
Atmospheric parameters of TYC 8606-2025-1 based onthe HRS spectra.Parameter Value T eff (K) 4900 ± g . ± . − . ± . v sin i ( km s − ) 0 . ± . V mic ( km s − ) 1 . ± . V mac ( km s − ) 3 . ± . projected rotational velocity v sin i , were derived duringthe first step of the SME analysis by varying the above-mentioned parameters. As a first approximation, we usedthe line masks defined in Ryabchikova et al. (2016). Thesemasks were further corrected for defects (e.g., cosmics) inthe observed spectrum of TYC 8606-2025-1. The model at-mosphere parameters were searched within a grid of theMARCS spherical atmosphere models (Gustafsson et al.2008). The derived parameters together with the error esti-mates are given in Table 2. The error estimates are basedon fit residuals, partial derivatives, and data uncertainties,as described in detail in Ryabchikova et al. (2016) andPiskunov & Valenti (2017).In the second step of the SME analysis, we fixed all pa-rameters derived during the first step and varied the abun-dances of the individual elements using the same masks. Fora few elements we extracted the lines of the particular el-ement from the common masks and ran SME with theseindividual masks. The abundances of O, Sr, La, and Euwere derived from the spectral synthesis of individual linesnot falling into the chosen SME spectral regions: the O i IR triplet λλ i λλ ii λλ ii λλ and CN molecular lines distributedin all regions covered by the SME analysis.The atomic parameters of the lines were extracted fromthe Vienna Atomic Line Database ( vald ; Kupka et al. 1999)using its 3rd release ( vald3 ; Ryabchikova et al. 2015). Theatomic parameters for the C and CN molecular lines weretaken from Brooke et al. (2013) and Kurucz (2010), re-spectively. The isotopic structure (IS) of Cu ii , Ba ii , andEu ii was taken into account. For elements with odd iso-topes the synthetic calculations included hyperfine splitting(hfs). Most of the data for isotopic and hyperfine splittingwere obtained from the website of R. L. Kurucz , who col-lected data and a bibliography for individual atomic species.In particular, the hyperfine structure constants for Na i weretaken from Griffith et al. (1977), Tsekeris et al. (1976), andYei et al. (1993), for Sc i and ii from Ertmer & Hofer (1976)and Mansour et al. (1989), for V i from Childs et al. (1979),Palmeri et al. (1995), and Unkel et al. (1989), for Mn i , ii from Blackwell-Whitehead et al. (2005) and Holt et al.(1999), for Co i from Pickering (1996), for Cu i from Fis-cher et al. (1967) and Ney (1966), for Y i from Dinneen etal. (1991) and Villemoes et al. (1992), and for Ba i from http://kurucz.harvard.edu/atoms.html Table 3.
Surface element (X) abundances of TYC 8606-2025-1and the Sun in log(X/H)+12 units (second and third columns,respectively). The fourth column lists abundances relative to theSun, [X/H]=log(X/H) − log(X ⊙ /H ⊙ ). The last column providesinformation about different effects taken into account in derivingabundances.X Star Sun [X/H] RemarkC 8 . ± .
04 8 . − .
32 C N 8 . ± .
06 7 .
83 0.54 CNO** 8 . ± .
09 8 .
69 0.02 NLTENa* 6 . ± .
06 6 .
21 0.32 NLTE, hfsAl 6 . ± .
02 6 . − . . ± .
10 7 . − . . ± .
07 7 .
51 0.01Ca* 6 . ± .
05 6 .
32 0.07Sc* 3 . ± .
04 3 . − .
13 hfsTi 4 . ± .
06 4 . − . . ± .
04 3 . − .
10 hfsCr 5 . ± .
06 5 . − . . ± .
08 5 . − .
23 hfsFe 7 . ± .
07 7 . − . . ± .
02 4 . − .
24 hfsNi 6 . ± .
06 6 . − . . ± .
21 2 .
83 0.31 NLTEY 2 . ± .
08 2 .
21 0.20 hfsZr* 2 . ± .
04 2 .
59 0.18Ba 2 . ± .
06 2 .
25 0.55 hfs, ISLa** 1 . ± .
03 1 .
11 0.22Ce* 1 . ± .
05 1 .
58 0.25Nd* 1 . ± .
06 1 .
42 0.12Eu** 0 . ± .
13 0 .
52 0.02 hfs, IS
Note : One asterisk marks the elements for which abun-dances were derived using individual line masks inSME. Two asterisks mark the elements whose abun-dances were derived by spectral synthesis of individuallines. In the latter case, the uncertainty corresponds tothe standard deviation of the single line abundances.
C O Mg Si Sc V Mn Co Sr Zr La Nd -0.500.5 [ X / H ] N Na Al Ca Ti Cr Fe Ni Y Ba Ce Eu
Figure 2.
Elemental abundances of TYC 8606-2025-1 relative tothe Sun. The (black) solid line corresponds to solar abundancevalues. The metallicity [Fe/H] level is marked by a (blue) dashedline.
Villemoes et al. (1993). All hfs data were collected in a newdatabase (Pakhomov et al., in preparation) and used by theinternal vald programs for the hfs calculations.The average abundances for the final atmospheric modelof TYC 8606-2025-1 are given in log(X/H)+12 units in thesecond column of Table 3. The third column of this ta-ble presents the current solar photospheric abundances (As-plund et al. 2009; Scott et al. 2015a,b; Grevesse et al. 2015),while the fourth columns lists the elemental abundances de-rived for TYC 8606-2025-1 relative to the Sun. The last col- c (cid:13) , 000–000 YC 8606-2025-1: a mild barium star o N o r m a li ze d f l ux o N o r m a li ze d f l ux o N o r m a li ze d f l ux o N o r m a li ze d f l ux Figure 3.
Portions of the HRS spectrum of TYC 8606-2025-1 with Ba ii lines used to determine the abundance of this element. Theobserved spectra (black dots) are compared with synthetic NLTE spectra calculated from the derived atmospheric parameters for thesolar abundances (dashed red line) and the final estimated abundances (solid blue line). One can see that the solar Ba abundance is nota good match to the observed spectrum, indicating that Ba is overabundant. CN 1 Ti 1 CN 1 CN 1 CN 1 o N o r m a li ze d f l ux C N Figure 4.
Profiles of the CN molecular lines in the HRS spectrumof TYC 8606-2025-1 (black dots). The observed profiles (blackdots) are compared with models (blue lines), of which the solidone corresponds to C/ C=14, while the dashed ones corre-spond to this ratio changed by ± umn of Table 3 provides information about different effectstaken into account in the SME or spectral synthesis anal-yses. For clarity, the elemental abundances of TYC 8606-2025-1 relative to the Sun are shown in Fig. 2, highlightingthe overabundance of s-process elements, with the Ba abun-dance increased by a factor of about three with respect tosolar.The current analysis was performed in the LTE approxi-mation. In principle, SME allows one to include NLTE calcu-lations in the spectral fitting (Piskunov et al. 2017), but thisrequires to provide NLTE departure coefficients for the gridcalculations. At present, this is done for the MARCS gridof plane-parallel models. However, for a few elements NLTEcorrections were applied to the LTE abundances. An aver-age NLTE correction for the IR O i triplet of − .
27 dex was calculated for us by T.M. Sitnova following a model atomfrom Sitnova & Mashonkina (2018). For Na i , a NLTE cor-rection of − . i i line at7070.071 ˚A for which no NLTE correction was estimated byBergemann et al. (2012). The resulting Sr abundance givenin Table 3 is the mean of the two estimates. The NLTE cor-rection for Ba does not exceed − . i resonancelines at 4238.19, 4262.27, and 4297.06 ˚A. All of them arestrongly blended with other absorption lines, which couldsuccessfully be fitted without adding the Tc i lines, indicat-ing that there is no sign of the presence of this elementin the spectrum. The non-detection of technetium impliesthat the enhanced abundances of the s-process elementsin TYC 8606-2025-1 are likely due to mass transfer from a(more evolved) companion star (see Section 5.1).We also estimated the ratio C/ C, which is an indi-cator of the stellar evolutionary status (Iben 1966). For thiswe used the spectral region around λ C N. We used the molecular data fromSneden et al. (2014) to create a list of C N lines. The c (cid:13) , 000–000 V. V. Gvaramadze et al.
Table 4.
Changes in the heliocentric radial velocity of TYC 8606-2025-1. Date JD V r , hel ( km s − )2015 May 6 2457148.0 +0 . ± . − . ± . − . ± . − . ± . − . ± . − . ± . observed spectrum was fitted with a synthetic one as shownin Fig. 4. We found that C/ C=14 ±
4, implying thatTYC 8606-2025-1 is a giant star (e.g. Dearborn, Eggleton &Schramm 1976).
We measured the heliocentric radial velocity, V r , hel , ofTYC 8606-2025-1 from each of the five available HRS spec-tra. The obtained results are given in Table 4, to which wealso added the radial velocity derived from the FEROS spec-trum. From Table 4 it follows that V r , hel has changed byabout 2 km s − during the last four years, suggesting thatTYC 8606-2025-1 is a binary system. Note also that the pos-itive sign of V r , hel in 2015 is consistent with that of the Gaia
DR2 median (barycentric) radial velocity of TYC 8606-2025-1 of +1 . ± .
20 km s − , based on six measurements carriedout during a period of 22 months between 25 July 2014 and23 May 2016. Unfortunately, the individual Gaia measure-ments of the radial velocity are not available in DR2 (Sar-toretti et al. 2018), which did not allow us to attempt con-straining the orbital parameters of TYC 8606-2025-1 (see,however, Section 5.2).
Using photometric data from Table 1, we fit the observedspectral energy distribution (SED) of TYC 8606-2025-1 witha model SED (see Fig. 6), calculated by interpolation ofgrids of ATLAS9 model atmospheres by Castelli & Ku-rucz (2003) and scaled to d = 0 .
97 kpc according to theparameters given in Table 2. We adopted the extinction lawof Mathis (1990) with a total-to-selective absorption ratioof R V = 3 .
1. The best fit was achieved with the colour ex-cess E ( B − V ) = 0 . ± .
01 mag. The excess flux seen at12 and 22 µ m is obviously due to nebular emission, sincethe point spread function of the WISE instrument at thesewavelengths of ≈ − .
28 magfrom Bessell, Castelli & Plez (1998), we derive the abso-lute visual magnitude and luminosity of the star of M V ≈− . ± .
09 mag and log( L/ L ⊙ ) ≈ . ± .
04, respectively,which correspond to an initial mass of the star of ≈ ⊙ (e.g. Girardi et al. 2000; Ekstr¨om et al. 2012; see also Fig. 5). T eff , K l og L / L PSfrag replacements ⊙ ⊙⊙⊙⊙⊙
Figure 5.
Position of TYC 8606-2025-1 in the Hertzsprung-Russell diagram (circle with error bars). Also shown are the evo-lutionary tracks of Girardi et al. (2000). Wavelength, A o -17 -16 -15 -14 -13 F l ux , e r g s - c m - A o - Figure 6.
Observed flux distribution of TYC 8606-2025-1 (filledcolour circles) based on the photometric data compiled in Table 1,compared to the model continuum (blue line).
Inspection of Table 3 shows (see also Fig. 2) that the sur-face carbon and nitrogen abundances in TYC 8606-2025-1are, respectively, depleted and enhanced by factors of twoand three, while the oxygen abundance is equal to the so-lar one within 1 σ . These abundances are typical of normalG and K giants and could be understood as the result offirst dredge-up occurring on the red-giant branch (Lambert& Ries 1981). Indeed, according to the evolutionary tracksby Ekstr¨om et al. (2012), a 3 M ⊙ star reaches T eff = 4900 Kthe first time evolving redward, after core hydrogen exhaus-tion. At this time the surface abundances remain unchanged.Then, during the core helium-burning, the star undergoes ablue loop and goes back to T eff = 4900 K. At this stage, c (cid:13) , 000–000 YC 8606-2025-1: a mild barium star the CN-processed material is dredged-up to the surface ofthe star, with the C and N abundances, correspondingly, re-duced and increased by factors of 1.7 and 3, and O remainingunchanged.Table 3 and Fig. 2 also show an excess of elements pro-duced in the slow neutron-capture nucleosynthesis (s-processelements), such as strontium, yttrium, zirconium, barium,lanthanum and cerium, of which the most overabundant (bya factor of ≈
3) is Ba. The enhanced Ba abundance is typicalof the so-called barium stars (first recognized as a spectro-scopic class by Bidelman & Keenan 1951). These (classical)barium stars, however, exhibit an increased carbon abun-dance and much stronger excess of the s-process elementswith Ba enhanced by up to a factor of 10–20.Observations show that all classical barium stars are bi-nary systems (McClure, Fletcher & Nemec 1980; McClure& Woodsworth 1990; Jorissen et al. 2019), meaning that bi-narity is a prerequisite for their formation. It is generallyaccepted that barium stars are post mass-transfer binariescomposed of a giant (or dwarf) G–K star and a white dwarf(McClure et al. 1980; Boffin & Jorissen 1988). Detection ofwhite dwarf companions to some barium stars lends weightysupport to this view (B¨ohm-Vitense 1980; Schindler et al.1982; Gray et al. 2011). Barium and other s-process elementsare synthesized during the thermally pulsing asymptotic gi-ant branch (TP-AGB) evolutionary phase of the primarystar (now white dwarf) and are transferred by stellar wind tothe companion star, thereby polluting its surface with heavyelements. In most cases, however, the primary star has fadedin the visual and IR wavelengths below detectability, and itspresence can only be revealed by UV (B¨ohm-Vitense 1980;Schindler et al. 1982) and/or X-ray observations (Schindleret al. 1982; Jorissen et al. 1996), or through indirect signs,such as a radial velocity variability in the companion (bar-ium) star (e.g. McClure et al. 1980) or detection of an opti-cally visible planetary nebula (PN) around a cool chemicallypeculiar star (e.g. Miszalski et al. 2013).The moderate enhancement of barium and other s-process elements and the depleted carbon abundance inTYC 8606-2025-1 imply that this star is a mild (or marginal)barium star (MacConnell, Frye & Upgren 1972; Pilachowski1977; Boyarchuk et al. 2002). Following the spectral classifi-cation of Morgan & Keenan (1973), we assign to TYC 8606-2025-1 the barium index (a characteristic of barium enhance-ment) of Ba0, so that it is a G8 III Ba0 star. Althoughthe origin of mild barium stars is still open to debate, itis possible that the relatively low excess of s-process ele-ments in these stars is at least partly a consequence of alarger separation between companion stars, reducing the ef-ficiency of wind accretion (B¨ohm-Vitense, Nemec & Proffitt1984). This possibility is consistent with the observed trendin barium stars towards a decrease of the s-process elementexcess with the increase of the binary period (see table 8 inJorissen et al. 2019). Interestingly, stars with large bariumindices form a population of binary systems with nearly cir-cular, short period ( < ∼
100 yr) orbital periods (Jorissenet al. 2019). The combination of large orbital periods andeccentricities is still unclear.The detection of radial velocity variations in TYC 8606-2025-1 (indicating that it is a binary system) and the lumi- nosity of this star (not high enough for any dredge up ofthe s-process elements from the stellar interior to occur)suggest that, like in the classical barium stars, the surfaceof TYC 8606-2025-1 was polluted with heavy elements viawind mass transfer from the companion star during its TP-AGB phase. Moreover, from the fact that TYC 8606-2025-1is in the core He-burning stage and because the post-AGBphase is very short ( ∼
10 000 yr), it is likely that the initialmass ratio of the binary system was close to unity (withinabout 10 per cent), meaning that the initial mass of theprimary star was ≈ ⊙ .Currently only few classical and mild barium stars areknown to be associated with PNe (Th´evenin & Jasniewicz1997; Bond, Pollacco & Webbink 2003; Miszalski et al. 2012,2013; Tyndall et al. 2013; L¨obling, Boffin & Jones2019), andfor some of them a hot companion star (white dwarf) hasbeen detected in the UV. Moreover, for one of these stars theorbital period of ≈ T eff <
30 000 K) to ionize itssurroundings. In this case, however, the primary star wouldbe visually 2–4 mag brighter than TYC 8606-2025-1 becauseon the horizontal part of the post-AGB track its luminosityof log( L/ L ⊙ ) ≈ ∼
100 000 K) core of ≈ . ⊙ (e.g. Bl¨ocker& Sch¨onberner 1997) and that it is currently on the whitedwarf cooling track. In this case, the star would contributemostly to the UV part of the SED of TYC 8606-2025-1, whileat optical wavelengths it would be several magnitudes fainterthan the G star. This, however, raises a question: Why doesnot the spiral nebula have an optical counterpart and did notexhibit evidence of nebular emissions in the 2D RSS spec-trum in spite of the low reddening of its associated star? The inference that TYC 8606-2025-1 is a binary system com-posed of a G8 giant star and a hot primary star (whitedwarf) implies that the origin of the spiral nebula shouldbe somehow related to the mass lost from the latter stareither during its AGB or post-AGB evolution.If the nebula was formed during the AGB and/or earlypost-AGB phase, then it should shine at optical wavelengthsas a PN because the wind material ejected at these phasesis hydrogen-rich. The lack of optical emission suggests thatthe nebula is composed of hydrogen-poor material and thatthe oxygen, nitrogen and neon atoms (responsible for thestrongest optical forbidden emission lines in spectra of PNe)are at least triply ionized and therefore did not form nebu-lar lines at optical wavelengths (e.g. Gurzadyan 1970). Thiswould imply that the observed IR emission from the nebulain the
WISE
22 and 12 µ m bands is due to [O iv ] 25.9 µ m c (cid:13) , 000–000 V. V. Gvaramadze et al. and [Ne v ] 14.3 and 24.3 µ m line emission, while at longerwavelengths it is due to dust emission (Chu et al. 2009;Flagey et al. 2011). Also, since the nebula is visible (in part)in both 8 and 4.5 µ m IRAC bands, and the 4.5 µ m banddoes not include PAH (polycyclic aromatic hydrocarbon)features, it is likely that at these wavelengths the nebularemission is also due to forbidden lines from highly ionizedspecies (Chu et al. 2009).For this explanation to work, the nebula should haveformed after the primary star has experienced a very latethermal pulse (VLTP), i.e. a thermal pulse that occurredwhen the star was on the white dwarf cooling track andhydrogen burning was already off. In this case, the pulse-driven convection can led to a mixing and total burning ofthe remaining hydrogen shell in the star (Fujimoto 1977;Sch¨onberner 1979; Iben 1984; Herwig et al. 1999). As aresult, the star expands and cools, and rapidly returns tothe AGB. During this “born-again” episode the star devel-ops a hydrogen-poor stellar wind, which could produce anew circumstellar nebula embedded in the more extendedhydrogen-rich material shed during the AGB phase (e.g.Wesson et al. 2008). Such nebulae were detected around sev-eral hydrogen-poor central stars of PNe (e.g. Zijlstra 2002and references therein).The post-VLTP evolution is very fast and proceeds ona time scale of ∼
100 yr. After the brief AGB phase thestar undergoes a final blue loop and finds itself again on thewhite dwarf cooling track as a very hot bare stellar nucleus.The UV photons emitted by this nucleus are hard enoughto triply ionize metals in the newly formed hydrogen-poornebula.The shape of the IR nebula around TYC 8606-2025-1also suggests that this star has recently gone through a briefepisode of mass loss. Like in the case of spiral circumstellarnebulae produced by AGB stars (e.g. Mauron & Huggins2006; Dinh-V.-Trung & Lim 2009), it could be due to theorbital motion of the mass-losing star (Soker 1994; Mas-trodemos & Morris 1999). The stellar wind from such a starorbiting a windless companion star interacts with itself andinduces a one arm spiral shock (Cant´o et al. 1999; Wilkin& Hausner 2017), which for circular orbits appears as anArchimedean spiral if the line of sight crosses the orbitalplane at the right angle. The distance between successiveturns of the spiral depends on the binary orbital period ( P ),and the orbital ( v orb ) and wind ( v ∞ ) velocities (Kim & Taam2002; Wilkin & Hausner 2017). In the case of a fast wind( v ∞ >> v orb ) this separation is simply equal to the productof the stellar wind velocity and the orbital period (Cant´o etal. 1999; Wilkin & Hausner 2017).The IR nebula associated with TYC 8606-2025-1 ap-pears as a spiral that can be traced only about one turnaround the star. This indicates that the nebula was formedduring one orbital period. Since the G companion star couldsafely be considered windless and assuming that during theblue loop phase the wind velocity of the post-VLTP starwas v ∞ ∼ − , one finds that in order to explainthe separation of the spiral arm from the star of ∼ . ∼
100 yr. This valueis at the upper end of the range of orbital periods derivedfor barium stars (Jorissen et al. 2019), and of the same orderof magnitude as the time it takes the star after a VLTP toreturn in the white dwarf cooling track. NE Figure 7.
SHS H α +[N ii ] image of the 5 arcmin × ≈ .
28 pc.
Although the existing data did not allow us to derive theorbital parameters of TYC 8606-2025-1, we note that thesedata are in agreement with our suggestion of a binary orbitalperiod as long as ∼
100 yr. The spiral shape of the nebulaimplies that the orbital plane of the binary system makes amoderate angle, θ , with the plane of the sky. Assuming that θ = 30 ◦ and P = 100 yr, one finds that the observed values of V r , hel could be fit with an eccentric orbit with an eccentricityof > .
7, which agrees with the observed tendency in bariumstars to have more eccentric orbits with the increase of theorbital period (Jorissen et al. 2019; see their fig. 7).If our suggestions on the origin of the IR nebula arecorrect, then one can expect that the hydrogen-rich materialejected by the primary star during the AGB and early post-AGB phases should be somewhere around TYC 8606-2025-1. This material should be ionized by UV photons from thewhite dwarf and therefore be visible at optical wavelengths.The typical expansion velocity of PNe of ≈ −
30 km s − (Gesicki & Zijlstra 2000) and the duration of the post-AGBphase of ∼
10 000 yr imply that a hydrogen-rich nebula couldbe found at ≈ . ≈
45 arcsec long) filament at about 90 arcsec (or ≈ .
42 pc) to the northeast of TYC 8606-2025-1 and an arcu-ate region of diffuse emission of angular radius of ≈
75 arcsec(or ≈ .
35 pc) to the southwest of TYC 8606-2025-1, whichapparently encircles the IR nebula (see Fig. 7). We speculatethat these features are what remains of the slow AGB windmaterial swept-up by the fast post-AGB wind during thefirst post-AGB track. Unfortunately, the position angle ofthe RSS slit was chosen in such a way (see Section 4.1) thatit did not cross the optical features, which precludes us from c (cid:13) , 000–000 YC 8606-2025-1: a mild barium star making any decisive conclusion on their nature. Note thatthese features are not visible in the VPHAS+ H α image.This could be due to the larger FWHM of the VPHAS+H α filter (107 ˚A) as compared to that of SHS (70 ˚A).Finally, we note that the Gaia
DR2 provides high-precision proper motion measurements for TYC 8606-2025-1: µ α = − . ± .
06 mas yr − and µ δ cos b = 2 . ± . − . After correction for the Galactic differential ro-tation and solar motion, one finds the peculiar transversevelocity of TYC 8606-2025-1 to be v tr = ( v l + v b ) / =54 . ± . − , where v l = 46 . ± . − and v b =27 . ± . − are, respectively, the peculiar velocity com-ponents along the Galactic longitude and latitude. To cal-culate v tr , we used the Galactic constants R = 8 . = 240 km s − (Reid et al. 2009) and the solar peculiarmotion ( U ⊙ , V ⊙ , W ⊙ ) = (11 . , . , .
3) km s − (Sch¨onrich,Binney & Dehnen 2010). For the error calculation, only theerrors of the proper motion measurements were considered.The derived values of v l and v b indicate that TYC 8606-2025-1 is a runaway star moving along a position angle of ≈ ◦ , i.e. almost from west to east. Surprisingly, there isno obvious correlation between the direction of the stellarmotion and the shape of the nebula. This could be under-stood if TYC 8606-2025-1 is still embedded in the hydrogenenvelope lost by the primary, which shields the nebula fromspace motion (cf. Gvaramadze et al. 2009). Further work would be needed to check our conclusionsabout the nature of TYC 8606-2025-1 and its associated cir-cumstellar nebula(e). It includes: • Search for UV and X-ray emission from TYC 8606-2025-1 to confirm the presence of the white dwarf companion. • Narrowband imaging and spectroscopy of the opticalfeatures around TYC 8606-2025-1 to establish their possiblerelationship with the star, and, potentially, to derive theirchemical abundances. • Constraining the orbital parameters of TYC 8606-2025-1 using forthcoming
Gaia
DR3 data and new radial velocitymeasurements. •
3D hydrodynamic modelling of the spiral nebula.
We have discovered an IR spiral nebula using data fromthe
WISE all-sky survey. Follow-up optical spectroscopywith SALT showed that the star associated with the neb-ula, TYC 8606-2025-1, is a G8 III mild barium star. Spec-tral analysis of TYC 8606-2025-1 along with the
Gaia
DR2data allowed us to derive the luminosity of this star oflog( L/ L ⊙ ) ≈ ≈ ⊙ . An analysisof our own and archival ´echelle spectroscopic data has re-vealed radial velocity variability in TYC 8606-2025-1, mean-ing that this star is in a binary system and that its surfaceis polluted with barium (and other s-process elements) asa result of wind accretion from the more evolved compan-ion star (now a white dwarf). We did not find an opticalcounterpart to the spiral nebula in the available sky surveysin spite of the low distance and reddening of the star, nor did we detect nebular lines in the long slit spectrum of thestar. We have interpreted this non-detection as an indica-tion that the nebula is composed of hydrogen-poor materialshed by the companion (primary) star after the very latethermal pulse (VLTP), and that metals in this material areat least triply ionized and therefore do not form nebularlines at optical wavelengths. The very likely binary statusof TYC 8606-2025-1 implies that the formation of the spiralnebula around this star was induced by the orbital motionof the mass-losing post-VLTP star around the windless Gstar. From the shape and linear size of the nebula, we in-ferred that it was formed during approximately one orbitalperiod of ∼
100 yr, which is of the same order of magni-tude as the duration of the post-VLTP phase. Our sugges-tions on the origin of the IR nebula imply that it shouldbe surrounded by a more extended optically visible nebulaproduced by a hydrogen-rich wind from the primary starduring the AGB and early post-AGB phases. We have de-tected several optical filaments in the SHS H α +[N ii ] imageof the region around the IR nebula. Unfortunately, our longslit spectroscopy did not cover these filaments, so that theirpossible connection to TYC 8606-2025-1 remains unclear. This work is based on observations obtained with theSouthern African Large Telescope (SALT), programmes2013-2-RSA OTH-003 and 2018-1-MLT-008, and collectedat the European Southern Observatory under ESO pro-gramme 095.A-9011(A). V.V.G. acknowledges support fromthe Russian Science Foundation under grant 19-12-00383(analysis and interpretation of observational data) andfrom the Russian Foundation for Basic Research (RFBR)under grant 19-02-00779 (search for infrared nebulae with
Spitzer and
WISE ). A.Y.K. acknowledges support fromRFBR under grant 19-02-00779 (spectroscopic observationsand data reduction) and from the National ResearchFoundation (NRF) of South Africa. E.K.G. gratefullyacknowledges funding by the Sonderforschungsbereich“The Milky Way System” (SFB 881, especially subprojectA5) of the German Research Foundation (DFG). Thisresearch has made use of the NASA/IPAC Infrared ScienceArchive, which is operated by the Jet Propulsion Labo-ratory, California Institute of Technology, under contractwith the National Aeronautics and Space Administration,the SIMBAD database and the VizieR catalogue accesstool, both operated at CDS, Strasbourg, France, anddata from the European Space Agency (ESA) mission
Gaia
Gaia
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