KIC10001893 and EPIC220641886 are V1093Her type pulsating subdwarf-B stars, which were observed by the Kepler spacecraft during nominal and K2 mission respectively. The amplitude spectrum of both stars show similar characteristics that the majority of the pulsation frequencies lay in the gravity g-mode domain. While the g-mode region contains 248 frequencies, the p-mode region contains just 15, altogether indicating the hybrid nature of both stars. We used one of the seismic tools, asymptotic period spacing effectively to identify the modal degree of the majority of the modes. We could not find rotational multiplets for both stars which is likely due to pole-on orientation. We assigned dipole and quadrupole modes for KIC10001893, while for EPIC220641886 we defined modal degrees ranging from l=1 to l=12, apart from l=3 and l=11 modes, which are not seen

Baran, A.

Ostensen, R. H.

SILVOTTI, Roberto

Uzundag, M.

Vuckovic, M.

OA@INAF - Istituto Nazionale di Astrofisica

2019Publication Year2021-02-22T16:06:34ZAcceptance in OA@INAFAsteroseismic analysis of subdwarf B variable stars of KIC 10001893 and EPIC 220641886TitleUzundag, M.; Vuckovic, M.; SILVOTTI, Roberto; Baran, A.; Ostensen, R. H.Authorshttp://hdl.handle.net/20.500.12386/30524HandleBOLETÍN DE LA ASOCIACIÓN ARGENTINA DE ASTRONOMÍAJournalBAAA, Vol. 61A, 2019 Asociación Argentina de Astronomı́aR. Gamen, N. Padilla, C. Parisi, F.A. Iglesias & M.A. Sgró, eds. Bolet́ın de art́ıculos cient́ıficosAsteroseismic analysis of subdwarf B variable stars ofKIC 10001893 and EPIC 220641886M. Uzundag1, M. Vučković1, R. Silvotti2, A. Baran3 & R.H. Østensen41 Instituto de F́ısica y Astronomı́a, Facultad de Ciencias, Universidad de Valparáıso, Valparáıso, Chile2 INAF-Osservatorio Astronomico di Torino, Pino Torinese, Italy3 Uniwersytet Pedagogiczny, Obserwatorium na Suhorze, Poland4 Department of Physics, Astronomy, and Materials Science, Missouri State University, USAContact / murat.uzundag@postgrado.uv.clResumen / KIC 10001893 y EPIC 220641886 son estrellas subenanas B pulsantes de tipo V1093 Her. Las mismashan sido observadas por Kepler en las misiones nominal y K2, respectivamente. A partir de la amplitud espectralde las estrellas, se puede decir que la mayoŕıa de las frecuencias de sus pulsos se encuentran en modo g. De lasfrecuencias detectadas, 248 pertenecen al dominio de modo g y 15 dentro del rango de modo p, mostrando lanaturaleza h́ıbrida de ambas estrellas. Para identificar el grado de los modos hemos utilizado la herramienta śısmicaasymptotic period spacing. En nuestro análisis, no hemos podido detectar multipletes rotacionales en ninguna delas dos estrellas, lo cual podŕıa indicar que se encuentran en dirección polar. A la estrella KIC 10001893 le hemosasignado modos dipolares y cuadrupolares, mientras que a la estrella EPIC 220641886 le hemos asignado modosque van desde l = 1 hasta l = 12, exceptuando los modos l = 3 y l = 11, que no han sido observados.Abstract / KIC 10001893 and EPIC 220641886 are V1093 Her type pulsating subdwarf-B stars, which wereobserved by the Kepler spacecraft during nominal and K2 mission respectively. The amplitude spectrum of bothstars show similar characteristics that the majority of the pulsation frequencies lay in the gravity g-mode domain.While the g-mode region contains 248 frequencies, the p-mode region contains just 15, altogether indicating thehybrid nature of both stars. We used one of the seismic tools, asymptotic period spacing effectively to identify themodal degree of the majority of the modes. We could not find rotational multiplets for both stars which is likelydue to pole-on orientation. We assigned dipole and quadrupole modes for KIC 10001893, while for EPIC 220641886we defined modal degrees ranging from l = 1 to l = 12, apart from l = 3 and l = 11 modes, which are not seen.Keywords / subdwarfs — stars: oscillations (including pulsations)1. IntroductionHot subdwarf stars of spectral class B (sdB) are corehelium-burning stars, found both in the disk and haloof our Galaxy. The observed properties locate themin the extreme horizontal branch (EHB) part of theHR diagram. Their effective temperatures range from20 000 to 40 000 K and surface gravities range from 5.0≤ log g ≤ 6.2 (in cgs units). They are compact ob-jects with radii typically on the order of 0.2 R andmasses confined to about 0.47 M. These stars haveexperienced extreme mass-loss at the end of the red gi-ant branch phase, in which nearly entire hydrogen en-velope is lost, leaving a helium burning core with a verythin inert hydrogen-rich envelope (Menv < 0.01 M)unable to ascend the asymptotic giant branch. Afterdepletion of helium in the core, they will become subd-warf O (sdO) stars burning helium in a shell surround-ing a C/O core and, eventually, they will end theirlives as DAO white dwarfs (Heber, 2016). SdB starsare found to pulsate with short period p-modes knownas V361 Hya stars and long period g-modes known asV1093 Her stars. Theoretically, Charpinet et al. (1996)explained excitation mechanism for V361 Hya stars aslow-order acoustic modes, while Fontaine et al. (2003)explained the excitation mechanism for V1093 Her starsas high-order g-modes. The short period pulsators havefrequencies higher than 2000µHz, while the long periodpulsators have frequencies usually below than 1000 µHz,which makes them quite hard to detect from the ground.The V361 Hya stars are found between 28 000 K and35 000 K with higher surface gravity log g and V1093 Herpulsators are somewhat cooler with temperatures rang-ing from 24 000 K to 30 000 K and lower logg. Thereexist also sdB stars that display both kinds of pulsa-tion modes and referred to as hybrid pulsators (Baranet al., 2005). Their nature as hybrid pulsators, givesan invaluable opportunity to understand both the coreand the envelope behavior of the stars. Understand-ing the interior structure of a pulsator heavily dependson the identification of the pulsation modes; which canbe achieved by the analyses of precise photometric datausing asteroseismological tools. High-precision Keplerphotometry allowed us to detect many frequencies thatwe had never obtained previously. Non-radial pulsa-tions are described by spherical harmonics as radialnodes, surface nodes and azimuthal orders n, l, m re-spectively. The detection of the frequencies with highresolution is a key point to apply asteroseismologicalmethods such as rotational multiplets and asymptoticperiod spacing to identify the spherical harmonics. InOral contribution - October 2018Asteroseismic analysis of subdwarf B pulsatorsthis paper, we present our analysis of two pulsation-ally rich long period hybrid sdB stars KIC 10001893 andEPIC 220641886. KIC 10001893 was studied extensivelyby Baran et al. (2011); Silvotti et al. (2014); Uzundaget al. (2017). The star was observed during the nominalmission of Kepler and it yields millions of data pointswhich allow us to characterize pulsation properties ofthe star. EPIC 220641886 (=HD 4539) is a well known,bright (V = 10.24 mag), sdB star for which g-mode pul-sations have been suspected but not confirmed (Schoe-naers & Lynas-Gray, 2007). From observations madeby the Kepler spacecraft during K2 mission, which af-ter failure of the second wheel of Kepler, the duty of themission changed slightly and this makes the data muchshorter in comparison with the normal mission of thespacecraft.Here, using the Kepler data, we provide a detailedanalysis of KIC 10001893 and EPIC 220641886. Firstly,we have calculated Fourier spectra of the stars. Then,using asteroseismological tools such as asymptotic pe-riod spacing and rotational multiplets, we have identi-fied the pulsational modes of both stars. After iden-tification of the pulsational modes, we constructed theéchelle diagrams.2. MethodsAfter we got all available data from the ”Barbara A.Mikulski Archive for Space Telescopes” (MAST)?, westarted to extract light curves of the objects. We usedshort-cadence (SC) data allowing for covering the fre-quency range up to 8495µHz, and assuring both p- andg-modes regions are covered. Then, in order to iden-tify pulsation frequencies of the targets, we used Fouriertechnique to calculate the amplitude spectrum of bothstars Fig. 1.2.1. Rotational multipletsIn the presence of stellar rotation, non-radial modes ofdegree l split into 2 l+ 1 components differing in az-imuthal (m) number. In the case of slow rotation, thefrequency splitting can be derived from the followingequation:νn,l,m = νn,l,0 + ∆νn,l,m = νn,l,0 +m1 − Cn,lProt(1)where ∆νn,l,m is a rotational splitting, Prot is a star’s ro-tation period and Cn,l is the Ledoux constant (Ledoux,1951). The method is quite useful both for identifica-tion of the pulsation modes and for determining the ro-tation period of the surface and the core. However, wecould not use this method for any of the two stars. Themethod is strongly dependent of both the inclinationof the star and on the frequency resolution. If the starsare pole-on oriented, there is no way to see azimuthal or-ders on surface of the star and therefore their amplitudespectra. Another dependence is frequency resolution toapply the method, in fact, the frequency resolution isenough for KIC 10001893, the star does not show any?archive.stsci.eduFigure 1: Amplitude spectra of KIC 10001893 andEPIC 220641886, the figures only cover gravity mode regionsof the targets.tendency amongst frequencies concerning splitting valueso it is quite possible that the star is either pole-on ori-ented or rather slow rotator. However, it is harder toevaluate the amplitude spectra of EPIC 220641886 dueto shorter length of the data and thus lower resolution.2.2. Asymptotic period spacingIn order to identify the pulsation modes in the g - moderegion, we used asymptotic period spacing. In theasymptotic limit (n >> l), consecutive radial overtonesare evenly spaced in period Charpinet et al. (2000). Forgiven n and l values, periods of consecutive overtonescan be derived from the following equation,Πl,n =Π0√l(l + 1)n+ ε, (2)where Πl,n is the period spacing, Π0 and ε are constants.3. ResultsWe found 110 oscillation frequencies for KIC 10001893and 153 frequencies for EPIC 220641886. We definedthe threshold as 5.4 and 5.6 sigma, respectively, to makesure the signals are produced by the stars intrinsically.The sigma is calculated based on median value of wholerange of Fourier spectra. Then, it has been multiply by5.4 for normal mission of Kepler (KIC 10001893) and5.6 for K2 mission (EPIC 220641886).BAAA 61A (2019)Uzundag et al.3.1. KIC 10001893The frequency resolution equals 0.0162µHz as definedby ∆f = 1.5/T, where T is the time coverage of the data.We identified 110 oscillation frequencies including p- andg-modes region. After identification of the frequencies,we have started to search for pulsational modes usingasymptotic period spacing. We identified 50 modal de-grees among the g-modes, which is more than 49% ofthe modes. Out of those 50 modes, 32 are dipole modesand 18 are quadrupole modes. Additionally, we calcu-lated échelle diagrams for l= 1 and 2 and the diagramfor dipole modes shows a common feature of a horizon-tal shift that called as a hook feature (for detail Baran& Winans, 2012; Uzundag et al., 2017).3.2. EPIC 220641886We have analysed K2 observations of one of the brightestknown sdBs, confirming the previous report by Schoe-naers & Lynas-Gray (2007) that EPIC 220641886 is aV1093 Her-type pulsator. We identify more than 150pulsation frequencies (144 g-modes, 9 p-modes). Al-though the frequency resolution is 11 times worse thannominal Kepler mission, the amplitude spectra of thestar displays quite rich content in terms of g-modes. Wehave identified 10 sequences between l= 1 and l= 12, ex-cluding l= 3 and l= 11 which are not seen, likely due tocancellation effects. Since the period spacing changeswith l (gets lower with increasing l), the identificationof the modes seems quite robust, at least up to l= 6.It is obvious that cancellation effects are important forhigh-l modes (Dziembowski, 1977), we are able to seethem in EPIC 220641886 is simply due to the fact thatthis star is particularly bright. In addition to that themodes are visible in the échelle diagrams for 1≤ l≤ 12(Silvotti, Uzundag, et al. 2019, in prep). Here, wepresent the échelle diagrams for dipole and quadrupolemodes Fig. 2.4. ConclusionWe have shown our analyses of two long period subd-warf B pulsating stars. Both objects show quite richcontents in terms of g-modes, which are sensitive to thecore properties of the stars. In total, we identified 248g-modes, 15 p-modes in both stars. Both stars are hy-brid which can be an important feature to constrainboth the core and the envelope parameters. Thanksto uninterrupted observations, the seismic tools are be-coming quite useful for pulsating sdB stars, specifically,asymptotic period spacing. Using asymptotic relations,we have identified sequences of near-consecutive modesfor degrees (1≤ l≤ 2) for KIC 10001893, (1≤ l≤ 12) forEPIC 220641886 which will be crucial to make futureasteroseismic modeling of both stars.Acknowledgements: MU gratefully acknowledge financial supportfrom organizing committee. The authors would like to thankMariana B. Sánchez for translation on the abstract. ASB grate-fully acknowledges financial support from the Polish National Sci-ence Center under projects No. UMO-2017/26/E/ST9/00703 andUMO-2017/25/B/ST9/02218.Figure 2: Échelle diagrams of EPIC 220641886 for dipole andquadrupole modes.ReferencesBaran A., et al., 2005, MNRAS, 360, 737Baran A.S., Winans A., 2012, AcA, 62, 343Baran A.S., et al., 2011, MNRAS, 414, 2871Charpinet S., et al., 1996, ApJL, 471, L103Charpinet S., et al., 2000, ApJS, 131, 223Dziembowski W., 1977, AcA, 27, 203Fontaine G., et al., 2003, ApJ, 597, 518Heber U., 2016, PASP, 128, 082001Ledoux P., 1951, ApJ, 114, 373Schoenaers C., Lynas-Gray A.E., 2007, Communications inAsteroseismology, 151, 67Silvotti R., et al., 2014, A&A, 570, A130Uzundag M., et al., 2017, MNRAS, 472, 700BAAA 61A (2019)

Asteroseismic analysis of subdwarf B variable stars of KIC 10001893 and EPIC 220641886

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Asteroseismic analysis of subdwarf B variable stars of KIC 10001893 and EPIC 220641886

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