Infrared Spectroscopy of the Merger Candidate KIC 9832227

arXiv:1804.01333v1 [astro-ph.SR] 4 Apr 2018 hr a eetycm olgtanwcaso eruptive maximum of merger at class stellar luminosity ( a 2003) new high al., to display et a (Bond attributed generally light be events can to These outbursts come whose recently events has There Introduction 1. aigbe ugse)te“etbt cnro–bsdo th on based is – – Sco scenario V1309 “best-bet” of behaviour the (w suggested) unclear been initially therma having late was very eruptions stars, planet-swallowing these eruptions, 20 nova of al., et al., nature Smith et the Williams 2012; (Kasliwal, 2003; While galaxies al., other et in (Bond and M31 2015) in LRVs of occurrence 01.Teeutv vn htmre h egrbgni 200 luminosity in began peak a merger reaching et the (Tylenda marked March, peri eruption that 2008 orbital event the eruptive an to The up had 2011). decreased progenitor that m The day 1.4 binary. stellar of contact a to, a point in are conclusively event eruption but subsequent with, consistent the only and behaviour pre-eruptive its oigaselrmre sa da niomn o utform that dust showed for fol- they environment significant ejecta growth; ideal a an the and of is that tion merger suggests presence stellar Sco the a V1309 lowing that around b dust note of the (2013) amount obscured al. eventually et which Zhu nary. point, Pejc merger. Lagrangian throug the loss outer before mass year experienced the Sco a V1309 least that at shown has excess (2014) IR s an 2014) had al., it et that (McCollum observations (IR) Infrared 2011). n rnds a rdcdi h merger. the in produced was dust iron and pi ,2018 5, April ⋆ Astrophysics & Astronomy E-mail:[email protected] 10 c ste“oet tn”o tla egr,i that in mergers, stellar of Stone” “Rosetta the is Sco V1309 e words. Key origin. n oa ealct,cnitn ihtefc httesys the that fact the with consistent metallicity, solar and eevd accepted Received; 4 xie ycrnlatvt nacruselrevlp,su envelope, circumstellar a in activity coronal by excited 3 2 1 Conclusions. Results. Methods. Aims. Context. h erifae pcrmo hsojc sa meu oobt to impetus an as object this of spectrum infrared near the niiulsas euetebnr peei odtriet determine to ephemeris binary the use We stars. individual h rsneo hoopee smgtb xetdfo rap from expected be might as chromosphere, a of presence the ota h egr–i n hni cus–cnb nepee i interpreted be can – occurs it when and if – merger the that so eateto hsc,FoiaSaeUiest,7 Chieft 77 University, State Florida Physics, of Department hsclRsac aoaoy arnpr,Amdbd Gu Ahmedabad, Navrangpura, Laboratory, Research Physical srpyisGop el nvriy el,Sta Keele, University, Keele Group, Astrophysics anAtooia bevtr,Aaeyo cecso h U the of Sciences of Academy Observatory, Astronomical Main nrrdsetocp ftemre addt I 9832227 KIC candidate merger the of spectroscopy Infrared eamt hrceie()tentr fteidvda compo individual the of nature the (i) characterise to aim We Pavlenko V. Ya. efidta h nrrdsetu sbs te yasnl com single a by fitted best is spectrum infrared the that find We thsbe rdce htteojc I 822 otc b contact a – 9832227 KIC object the that predicted been has It eueifae pcrsoyi h aeeghrne0.7 range wavelength the in spectroscopy infrared use We icmtla atr—bnre:coe—bnre:eclipsi binaries: — close binaries: — matter circumstellar h tegho h nrrdHlnsi ossetwt high a with consistent is lines H infrared the of strength The M bol − ∼ ⋆ h egro w stars two of merger the aucitn.KIC˙accepted no. manuscript 10 1 .Evans A. , , M ∼ v ∼ ∼ < 3 5 × − . 2 )a vdne ythe by evidenced as 9) 10 × 2 4 .P .Banerjee K. P. D. , 10 L ⊙ − 4 Tlnae al., et (Tylenda M ⊙ ff . rsie T B,UK 5BG, ST5 ordshire, fsilicate of pulses l e a bevda conjunction. at observed was tem gsigta h ekeso h Ca the of weakness the that ggesting erger eobtlpaea h ieo observation. of time the at phase orbital he ABSTRACT how 16). i r-egrdata. pre-merger ain i a,Tlaase L33645,USA 32306-4350, FL Tallahassee, Way, ain not ith od ha l., a- i- nifre manner. informed an n 8 h e dselrrtto.TeHe The rotation. stellar id aa 809 India 380009, jarat . 3 rie ooivWos yv17 38 Ukraine 03680 Kyiv-127, Woods, Golosiiv kraine, .Southworth J. , et ftebnr n i)telkl icmiayenviron circumbinary likely the (ii) and binary the of nents ad h aawr eue n airtduigteSpextoo the using calibrated and stan- reduced telluric were the as data used The was dard. HD194354 star A0V The seconds. power ltrsligi pcrlcvrg 0 coverage spectral a in resulting slit pXwsue ntecosdsesdmd sn 0 a using Telescope mode spectrograph cross-dispersed Infra-Red the NASA in 57941.54) SpeX (MJD used m UT was 3 7.54 SpeX the July the 2017 on Hawaii, on with (IRTF), Facility 2003) observed al., et (Rayner was 9832227 KIC observation IRTF 3.1. an amplitude with binary, and contact days a 0.458 and period eclipsing orbital an both is 9832227 KIC 9832227 KIC 2. spect object. IR merger near candidate a this here of present we and candidate, merger tential .Observations 3. M17. by given is 9832227 KIC of h rmr scnay trhsms . M 1.4 mass has star (secondary) primary the 41) hr sas oeeiec o h rsneo thir a 590 period of orbital presence with the system, for the evidence in some component also is There (4.10). aetetm fmre ob 2022 es be they to merger merger; of impending time an the of mate ra signature was derivative sure period a the that decreasing, and years, previous in than . R 2.6 ia.Mla ta.(07 eefe 1)fudta h orb the that found from M17) determined hereafter as (2017, period al. et Molnar tical. M µ m–2.5 g—Sas niiul(I 822)—Ifae:stars Infrared: — 9832227) (KIC individual Stars: — ng sin nr tr–wl neg egri 2022 in merger a undergo will – star inary I 822 a dnie yMla ta.(05 sapo- a as (2015) al. et Molnar by identified was 9832227 KIC oethvn e having ponent ⊙ au flog of value i R 07 R (0.78 = µ ,t hc efi oe topee orpeetthe represent to atmospheres model fit we which to m, 0 = , 1M 11 λ/δλ ⊙ 2 ), .Shahbandeh M. , g ⊙ n h tegho h Ca the of strength the and , T ealddsuso ftemre potential merger the of discussion detailed A . = ff i e ff cietmeaue590K o [ log K, 920 5 temperature ective bopinw bev ieyaie nHe in arises likely observe we absorption 0.Tettlitgaintm a 717 was time integration total The 200. 1 = ii rpe sas ieycrmshrcin chromospheric likely also is triplet 0 590K n log and K) 920 (5 K 800 5 Kepler aawssgicnl shorter significantly was data 4 . 2 n .Davis S. and , . 77 ± 0 . 2 − . .M7dtriethat determine M17 7. ± ∼ ii 2 0 . rpe indicates triplet . ⊙ 50 0 .W describe We 7. . a nteop- the in mag 2 03 M (0.32 µ ,a resolving at m,

c g ± 4 ] S 2018 ESO asand days 8 ment, = . g ⊙ 5 4 ,radius ), ′′ . = 1 × pidly 4 rum 15 ital . 19 ti- ′′ d 1 l . Ya. V. Pavlenko et al.: Infrared spectroscopy of the merger candidate KIC 9832227

0.95 Observed 0.9 b=0 b=1 0.9 0.8

0.7 0.85

0.6 0.8

0.5 S 0.75 0.4 Normalised Flux 0.3 0.7

0.2 0.65 0.1 0.6 8000 10000 12000 14000 16000 18000 20000 22000 24000 0 0.2 0.4 0.6 0.8 1 Wavelength(A) b Fig. 1. Left: dereddened IRTF spectrum of KIC9832227, together with the ﬁt with b = 1; the case b = 0 is also shown for comparison. See text for details. Right: minimisation parameter S as the function of b.

Table 1. Infrared fluxes from 2MASS and WISE. 3.3. Orbital phase of KIC9832227 at the time of the IRTF observation Survey Band λ (µm) Flux (mJy) 2MASS J 1.25 48.23 ± 0.92 KIC9832227 is an eclipsing binary: in order to interpret the IR H 1.65 39.55 ± 0.62 spectrum we need to know the orbital phase and stellar veloc- Ks 2.2 26.59 ± 0.39 ity separation at the time of the IRTF observation. M17 do not WISE W1 3.3 13.13 ± 0.28 give the full information necessary to deduce these quantities, W2 4.6 7.14 ± 0.14 so this process was non-trivial. We neglected the effect of the W3 12 1.31 ± 0.09 third body, which affects the orbital timings by only ±0.002 cy- cles (Figure 10 in M17), and the proposed three-body solutions suggested by M17 because this body was not detected in the spectrum. We adopted the parameters of the exponential fit given in Figure 12 of M17 and the linear orbital ephemeris from the 10 Kepler Eclipsing Binary Catalogue1 (see Kirk et al., 2016). We further imposed a normalisation which forces the first point in (mJy) ν

F the year 2004 to be at zero O − C. The resulting orbital phases 1 correctly reproduce the O − C, instantaneous orbital period, and dP/dt as plotted in Figures 12, 13 and 14 in M17. The UTC time of mid-observation of our spectrum (2457942.04199) was converted into the BJD(TDB) timescale 0.1 1 2 4 8 16 using the IDL routines from Eastman et al. (2010), giving a λ (µm) time of 2457942.04135. This corresponds to an orbital phase of Fig. 2. Infrared spectrum spectral energy distribution of 0.010, so our spectrum was obtained around a time of inferior KIC9832227 from 2MASS (red) and WISE (blue) surveys. conjunction. The velocity separation of the two stars at this time Black curve is a 5930 K blackbody, corresponding to the stellar was negligible compared to their spectral line broadening, so the components of KIC 9832227. lines of the two stars are superposed in the spectrum.

4. Theoretical spectra and SEDs software (Cushing et al., 2005), and corrections for telluric ab- Synthetic spectra and SEDs were computed using the sorption were performed using the IDL tool Xtellcor (Vacca et WITA6 program for the SAM12 model 1D atmospheres al. 2003). The observed spectrum, which was dereddened by (Pavlenko, 2003) with Te(in K)/log g/[Fe/H]= 5920/4.10/0.0 E(B − V) = 0.03 (M17), is shown in Fig. 1. and 5800/4.19/0.0 for the hotter (A) and cooler (B) components, respectively, as determined by M17. Significant molec- 3.2. Complementary observations ular features are not seen in the observed spectrum, therefore we accounted for only atomic lines from the VALD3 database KIC9832227 is included in the 2MASS (Skrutskie et al., 2006) (Ryabchikova et al., 2015). Our procedure for producing the the- and WISE (Wright et al., 2010) IR surveys; the source was not oretical spectra is described elsewhere (see Pavlenko, 2003; detected in other surveys, such as IRAS, AKARI and Herschel Pavlenko et al., 2008). The synthetic spectra were convolved PACS. The 2MASS and WISE fluxes are given in Table 1 and with a gaussian profile having the instrumental broadening cor- shown in Fig. 2; KIC9832227 was not detected in WISE Band 4 responding to the spectral resolution R, and rotational profiles − − (22 µm). The IR spectral energy distribution (SED) is photo- corresponding to v sin i = 85kms 1 (A) and 150kms 1 (B; spheric out to at least 12 µm, indicating that dust – if present in significant quantity as in V1309 Sco – has temperature ∼< 425 K. 1 http://keplerebs.villanova.edu/overview/?k=9832227

2 Ya. V. Pavlenko et al.: Infrared spectroscopy of the merger candidate KIC 9832227

0.9 0.48 Observed b=0 b=1 0.8 H I lines, log g =2.0 0.46

0.7 0.44

0.6 0.42

Normalised Flux 0.5 Normalised Flux 0.4 ??

0.4 0.38 Observed A He I B 0.3 0.36 8500 9000 9500 10000 10500 11000 11500 10500 10600 10700 10800 10900 11000 Wavelength(A) Wavelength(A) Fig. 3. Left: best the ﬁt to the observed spectrum with b = 1 in the regionof the Ca ii triplet and He i; the case of b = 0 is also shown, as is the hydrogen line spectrum computed for a model atmosphere with log g = 2.0. Right: ﬁt to the spectrum containing Si I lines, marked by vertical arrows; the He i line is also shown. Note the broad feature at 10700Å, marked by ’??’; see text.

M17). To account for rotational broadening of the spectra we 10 694.17Å; these lines have relatively high excitation potentials used formulae given by Gray (1976). (∼ 7.5 eV).

5. Results 5.3. The Ca ii triplet: evidence of a chromosphere

5.1. The overall spectrum Absorption in the Ca ii triplet (8498.023, 8542.091, 8662.14Å) We computed theoretical spectral energy distributions for the – by comparison with other metallic lines – is a notable feature A and B components, and created a grid of combined spectra of KIC9832227’s spectrum (see Fig. 3). This Figure shows rel- t F = b × FA + (1 − b) × FB, where 0 < b < 1. We com- evant portion of Fig.1 on a larger scale, to compare the observed puted the best fits to the dereddened spectrum for the cases Ca ii lines with those computed for the case of a classical model b = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0; the procedure to determine the atmosphere. The observed lines are notably weaker by compar- best fit used a χ2 minimising algorithm, details of which are ison with the computed. The discrepancies between computed given elsewhere (see Pavlenko et al., 2008). The minimisation and observed Ca ii lines strength may be explained by several t obs 2 factors, such as NLTE effects or a Ca deficit. In the former case, parameter S = P(Fλ − Fλ ) indicates a minimum S at b = 1 (see Fig. 1). This is consistent with the fact that the observed NLTE effects in other lines should also be apparent, but this is spectrum was obtained at conjuction, when the hotter component not the case. Also H i and metallic lines are well fitted for the dominates the combined spectrum (although a more compelling case of solar abundances. reason for not seeing the second component is the very similar The components of the KIC9832227 system consist of Teff). Furthermore, as the eclipses are very shallow, component rapidly rotating, tightly bound stars in a contact binary system A dominates the spectrum at all orbital phases. and we would not expect the atmospheres of such stars so be adequtely described by the classical model. In particular rapid 5.2. Absorption lines in KIC9832227’s spectrum stellar rotation is likely to lead to strong chromospheric (and re- lated) activity, which could certainly account for the weakness Generally speaking, both the intermediate resolution of the ob- of the Ca triplet. = served spectra (R 1200) and the rapid rotation of both com- We have simulated a chromosphere by imposing a temper- ponents complicate the use of atomic lines for abundance analy- ature minimum of 5000 K, and a temperature gradient G = sis. However, we see that practically all notable lines in the near dT/d(log m)(m being the column density) in the model atmo- IR spectrum are well fitted with close-to-solar abundances; the sphere. We fit the Ca ii line profiles by choosing the value of G likely errors do not exceed 0.3 dex. In the right panel of Fig. 3 i that minimises the fitting parameter S (details of our approach we show a few well-fitted Si lines in the spectral range 10500- to this calculation is given in Pavlenko, 1998). A model with a 11 000 Å. We find fits of the same quality to metallic lines in weak chromosphere (G = −200, cf. the solar value G = −800, other spectral regions. with T in K, m in g cm−2) provides a good fit to the data in the We draw particular attention to the modelling of the moder- region of the Ca ii triplet (see Fig. 4). ately strong hydrogenlines, to which our model provides a satisfactory fit. In the left panel of Fig. 3 we show the hydrogenlines We also note here an alternative interpretion: our classical g = . model atmospheres were computed for Rosseland mean optical computedfor the case of lower log 2 0. In the observedspec- −5 trum the upper members of the Bracket series are much weaker; depth in the range τRoss = 10 – 100 but such models lead to ii this can be taken as independent evidence of the higher log g, Ca lines that are too strong. Interestingly, a “stripped” model atmosphere for the A component that lacks atmospheric layers as determined by M17. Indeed, our fits of the theoretical spec- −5 −2 tra computed for log g = 4.19 and log g = 4.10 to the observed in the τRoss range [10 –3 × 10 ] provides an equally good fit ii spectra are far superior. to the Ca triplet (see Fig. 4). We also note the presence of a broad feature at 10700Å However, while both approaches provide equally good fits to (identified by “??” in Fig. 3). Given the width of the feature, the data, for reasons to be discussed below we prefer the chro- a possible identification is C i (3P−2S) at 10686.01, 10688.27, mospheric interpretation.

3 Ya. V. Pavlenko et al.: Infrared spectroscopy of the merger candidate KIC 9832227

11000 6. Conclusions

10000 We have carried out an analysis of the near IR spectrum of 9000 KIC9832227. We computed synthetic spectra for two classi- 8000 cal 1D model atmospheres, and ﬁtted the dereddened spec-

7000 trum. While we acknowledge that the computed model atmo-

Temperature (K) Classical model atmosphere spheres may be overly simplistic for the case of the complex 6000

Chromosphere KIC9832227 system, we do nonetheless obtain a satisfactory 5000 Stripped model reproduction of the observed SEDs. This may be taken as evi- 4000 + 100 1000 10000 100000 dence that, at the time of observation, the 1D LTE approach is Pressure (dyne/cm2) valid.

0.75 Within the constraints of our simpliﬁed model we conclude Observed Classical MA that: MA with chromosphere 0.7 Stripped MA 1. our ephemeris analysis and spectral synthesis show that, at 0.65 the time of observation, the contribution of the hotter component prevails in the combined spectrum; 0.6 i

Normalised Flux 2. thestrengthoftheH lines is well reproducedby our models, providing independent conﬁrmation of the high log g (4.10, 0.55 component A; 4.19, component B; M17) at least in the atmosphere of component A. Model atmospheres with lower log g 0.5 8500 8550 8600 8650 result in Bracket and Paschen lines that are too srong; Wavelength(A)

0.74 3. a comparatively narrow feature at 10830 Å is identified as 3 3 0.72 the He i ( S− P) line, and is formed in the outer part of the 0.7 common envelope; 0.68 4. a satisfactory fit to the Ca ii IR triplet is obtained by intro- 0.66 ducing a chromosphere into the model atmosphere. This is 0.64 0.62 likely a consequence of rapid stellar rotation; 0.6 5. despite the intermediate resolution of observed spectra and Normalised Flux 0.58 the fast rotation of both components, we confirm that com- 0.56 Observed Classical MA ponent A has near-solar abundances. 0.54 MA with chromosphere Stripped MA 0.52 8525 8530 8535 8540 8545 8550 8555 8560 8565 8570 Acknowledgements. We warmly thank David Sand for making possible the Wavelength(A) IRTF observations, and for help with the data reduction. The Infrared Telescope Facility is operated by the University of Hawaii under contract NNH14CK55B Fig. 4. Top panel: the structure of the “classical” and “chromo- with the National Aeronautics and Space administration. We thank the referee spheric” model atmosphere of component A. Middle panel: best for their helpful comments. the fit to the observed Ca ii triplet, with classical and “chromospheric” model atmospheres. Bottom panel: fit to strongest component of the Ca ii triplet shown on a larger scale. In all three References panels the “Stripped model atmosphere” refers to the model Avrett, E. H., Fontenla, J. M., Loeser, R., 1994, in IAU Symposium, 154, Infrared briefly discussed at the end of Section 5.3. Solar Physics, eds D. M. Rabin, J. T. Jefferies, C. Lindsey, Kluwer Academic Publishers; Dordrecht, p. 35 Bond, H. E., Henden, A., Levay, Z. G., Panagia, N., Sparks, W. B., Starrfield, S., Wagner, R. M., Corradi, R. L. M., Munari, U. 2003, Nature, 422, 405 Cushing M. C., Rayner J. T., Vacca W. D., 2005, ApJ, 623, 1115 Eastman, J., Siverd, R., Gaudi, B. S., 2010, PASP, 122, 935 Gray, D. F., 1976, The observation and analysis of stellar photospheres, Wiley 5.4. He i at 10830Å Interscience, New York Kasliwal, M. M. 2012, PASA, 29, 482 Kirk, B., et al., 2016, AJ, 151, 68 A strong absorption feature is present at 10830Å (see Fig. 3); McCollum, B., Laine, S., Väisänen, P., Bruhweiler, F. C., Rottler, L., Ryder, S., we attribute this to He i 10830Å (3S−3P0), which is also ob- Wahlgren, G. M., Barway, S., Nagayama, T., Ramphul, R. 2014, AJ, 147, 11 Molnar, L. A., Van Noord, D. M., Steenwyk, S. D., Spedden, C. J., Kinemuchi, served in absorption in the spectra of metal poor subdwarfs K. 2015, BAAS, 225, 415.05 (Smith et al., 2012) and red giants (Smith et al., 2014). This line Molnar, L. A., Van Noord, D. M., Kinemuchi, K., Smolinski, J. P., Alexander, C. is noticeably narrower than the rotationally broadened photo- E., Cook, E. M., Jang, B., Kobulnicky, H. A., Spedden, C. J., Steenwyk, S. D. spheric lines, and most likely forms outside the stellar photo- 2017, ApJ, 840, 1 (M17) spheres, possibly in the common envelope. Nicholls, C. P., et al., 2013, MNRAS, 431, L33 Pavlenko, Ya. V. 1998, Astronomy Reports, 42, 501 In the case of the Sun the line is also observed in absorp- Pavlenko, Ya. V. 2003, Astronomy Reports, 47, 59 tion; Avrett et al. (1994) suggested that coronal radiation pene- Pavlenko, Ya. V., Evans, A., Kerr, T., et al. 2008, A&A, 485, 541 Pejcha, O. 2014, ApJ, 788, 22 trates into the upper solar chromosphere, causing sufficient he- Rayner J. T., Toomey D. W., Onaka P. M., Denault A. J., Vacca W. D., Cushing lium ionisation to populate the lower level of the He i 10830Å M. C., Rayner J. T., 2003, PASP, 115, 389 transition, thus resulting in optically-thin absorption against the Ryabchikova, T., Piskunov, N., Kurucz, R. L., et al. 2015, Phys. Scr, 90, 054005 photospheric continuum. This, together with the presence of the Strutskie, M. F., et al., 2006, AJ, 131, 1163 Smith, G. H., Dupree, A. K., Strader, J., 2012, PASP, 124, 1252 C i lines at ∼ 10690Å, lends support to the chromospheric ex- Smith, G. H., Dupree, A. K., Strader, J., 2014, ArXiv e-prints [arXiv:1410.3487] planation offered in Section 5.3. Smith et al. 2016, MNRAS, 458, 950

4 Ya. V. Pavlenko et al.: Infrared spectroscopy of the merger candidate KIC 9832227

Tylenda, R., Hajduk, M., Kamiński, T., Udalski, A., Soszyński, I., Szymaáski, M. K., Kubiak, M., PietrzyÅski, G., Poleski, R., Wyrzykowski, Ł., Ulaczyk, K. 2011, A&A, 528, A114 Vacca, W. D., Cushing, M. C., Rayner, J. T., 2003, PASP, 115, 389 Williams, S. C., Darnley, M. J., Bode, M. F., Steele, I. A. 2015, ApJ, 805, L18 Wright, E. L., et al., 2010, AJ, 140, 1868 Zhu, C., Lü, G., Wang, Z., 2013, ApJ, 777, 23