The Formation of the Double Gaussian Line Profiles Of
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Journal of the Korean Astronomical Society https://doi.org/10.5303/JKAS.2020.53.2.35 53: 35 ∼ 42, 2020 April pISSN: 1225-4614 · eISSN: 2288-890X Published under Creative Commons license CC BY-SA 4.0 http://jkas.kas.org THE FORMATION OF THE DOUBLE GAUSSIAN LINE PROFILES OF THE SYMBIOTIC STAR AG PEGASI Siek Hyung and Seong-Jae Lee Department of Earth Science Education (Astronomy), Chungbuk National University, Chungbuk 28644, Korea [email protected], [email protected] Received December 12, 2019; accepted February 8, 2020 Abstract: We analyze high dispersion emission lines of the symbiotic nova AG Pegasi, observed in 1998, 2001, and 2002. The Hα and Hβ lines show three components, two narrow and one underlying broad line components, but most other lines, such as H i, He i, and He ii lines, show two blue- and red-shifted components only. A recent study by Lee & Hyung(2018) suggested that the double Gaussian lines emitted from a bipolar conical shell are likely to form Raman scattering lines observed in 1998. In this study, we show that the bipolar cone with an opening angle of 74◦, which expands at a velocity of 70 km s−1 along the polar axis of the white dwarf, can accommodate the observed double line profiles in 1998, 2001, and 2002. We conclude that the emission zone of the bipolar conical shell, which formed along the bipolar axis of the white dwarf due to the collimation by the accretion disk, is responsible for the double Gaussian profiles. Key words: stars: binaries: symbiotic | stars: individual: AG Peg { ISM: dynamics and kinematics | ISM: lines and bands 1.I NTRODUCTION a GS transit cannot form the observed Gaussian line Symbiotic stars are identified as binary systems by their profiles. Using spectroscopic data obtained at phases peculiar spectra, which show contributions by emission φ ∼ 0:0 or 0.5 with the Lick observatory, LH18 investi- of both relatively low and very high temperature. How- gated whether the GS blocks the emission zone around ever, it is difficult to confirm the presence of two stars the hot WD, causing the observed Gaussian line profiles. in imaging studies. AG Pegasi was first reported as a They provided some kinematic inference that the GS Be star by Fleming(1907), after which Merrill(1916, passages were not the source of the double Gaussian line profiles. Instead, LH18 demonstrated that a bipolar 1942) observed He and Ca ii absorption lines, numerous conical shell formed double Gaussian lines and that a ionized or neutral emission lines, and TiO2 absorption bands. Considered a Cyg-Be type star at the time of its broad line wing was formed by Raman scattering in the discovery, AG Peg is now known as a symbiotic system neutral region of the shell. composed of a Wolf-Rayet type white dwarf (WD) and Raman scattering processes in symbiotic stars were a type M3 giant star (GS) (Merrill 1916; Hutchings et al. proposed first by Schmid(1989), and Raman wings were 1975). Spectroscopic monitoring of AG Peg has shown first discussed by Nussbaumer et al.(1989) and further variations of its brightness and spectral intensities on studied by Lee(2002). Lee & Hyung(2000) also showed time scales from months to decades. The orbital period that Raman scattering can occur in a dense planetary is about 800 days. The temperature of the WD is about nebula such as IC 4997. Symbiotic stars have been 100 000 K, while its cold component is a GS (Allen 1980; considered as candidates for type Ia supernovae. The Viotti 1988; Nussbaumer 1992). Since the beginning of kinematic structure, including the physical conditions astronomical observations, two major outbursts have of the ionized hydrogen region and the symbiotic binary occurred: a first one in 1850, a second, shorter one in system, is of great interest. The mass inflow from the 2015. GS into the hot component might form an accretion disk. In a symbiotic binary system, variations of emission The fast stellar wind from the hot star is likely to develop line profiles might be caused by shielding effects partly bipolar conical shells that are responsible for both the due to the relative orientation of observer and binary sys- double and broad emission lines. As the orbital phase, tem. Some earlier studies noted the presence of double and thus the relative orientation of binary components Gaussian profiles or an absorption line at the center that and observer, changes, the shape of the line profiles might be associated with the GS passage at a specific changes accordingly. LH18 suggested that the bipolar phase (Contini 1997). However, the double Gaussian conical shell is the most likely source of double Gaussian profiles seemed to exist at all phases and the GS cannot line profiles, but they did not compute theoretical line always be in front of the emission zone surrounding the profiles for multiple phases. In this study, we fit the Hα WD. Lee & Hyung(2018), hereafter LH18, showed that or Hβ double Gaussian line profiles observed in arbitrary phases to a bipolar conical model. Corresponding author: S.-J. Lee Section2 describes the observed spectra for three 35 36 Hyung & Lee Table 1 matic subcomponents responsible for the full line profile, AG Peg observation log which involves a trial and error process that requires choosing the number of line components the observer can Observation date (UT) Julian Date Phase (φ) resolve. We used StarLink/Dipso provided by the Inter- 1998-09-17 2451073.70 10.24 active Data Language (IDL) software and the European 2001-08-30 2452151.60 11.56 Southern Observatory (ESO) for this deconvolution. 2002-08-11 2452498.92 11.98 Figure1 shows the H α lines observed in three epochs. To avoid saturation in the spectral line pro- files, we used the 3 min and 5 min exposures. LH18 epochs acquired by the Hamilton Echelle Spectrograph concluded that the Hα and Hβ hydrogen lines observed at Lick Observatory. Section3 presents the predicted in 1998 are composed of three parts, a double line plus line profiles and the best parameters of a bipolar coni- a broad wing, while the previous studies assumed four cal shell suitable for three epochs. We investigate the to five separate components with different kinematics, geometric structure of the system, responsible for the with suggested radial velocities being 60, 120, 400, and double Gaussian line profiles, observed in 1998, 2001, 1000 km s−1 (Kenyon et al. 1993; Eriksson et al. 2004). and 2002. Theoretical line profiles corresponding to dif- At first glance, only two line components are obvious. ferent phases are provided to model the observed double However, our detailed analysis (using IDL) separates the line profiles. We conclude in Section4. observed line profiles into three components. A detailed discussion of the three components is given in LH18 2.L ICK OBSERVATORY SPECTRAL DATA based on the 1998 data. The narrow double lines consist Spectroscopic data were obtained by Lawrence H. Aller of blue- and red-shifted Gaussian components. and Siek Hyung in 1998, 2001, and 2002 at the Lick The Hα line observed in 1998 shows a red compo- observatory in the USA using the high-resolution Hamil- nent that is substantially stronger than the blue one, ton Echelle Spectrograph (HES) attached to the 3-meter while in 2001 the blue component is stronger than the Shane telescope. The spectral resolution of the HES red one. All observed line profiles in Figure1 show is R ∼ 50 000. The slit width of the HES is 640 µm the two narrow red-shifted and blue-shifted line com- which corresponds to 1.200 on the sky, the wavelength ponents and one broad component. The full width at resolution is 0.1{0.2 A/pixel.˚ The observed spectral half maximum (FWHM) of the narrow lines is 60{120 range is 3470{9775 A,˚ and we selected only the H and km s−1 each, while the width of the broad line is 400{ He lines from the various emission lines. Long exposure 500 km s−1. We adopted the kinematic center from the times of 1800 sec and 3600 sec were chosen for weak result of LH18 who analyzed the strong lines observed lines. Additional short-exposure spectra of 300 sec and in 1998. One expects to see He ii 6560 as well. Due to 180 sec were obtained to avoid saturation of the strong the relatively strong Hα emission, however, the He ii Hα line. The IRAF standard star HR 7596 served as 6560 line component(s) to the left of the Hα line cannot flux calibrator. be resolved. Table1 shows the observation dates of AG Peg, The Hβ line spectra shown in Figure2 display two Greenwich universal time (UT), Julian day, and phase. lines to the left of the Hβ components, corresponding The phase of AG Peg is defined as zero when the WD is to N iii 4858 and He ii 4559. Although the FWHMs of in front of the GS. The 2002 data (phase φ = 11:98, 0.98 the Hβ line components are slightly different from those hereafter), close to φ = 0:0, were secured when the WD of the Hα spectral line profiles, both consist of three was in front of the GS. The 2001 data (phase φ = 11:56, components. 0.56 hereafter) is close to φ = 0:5 when the GS was in We corrected the observed Hα and Hβ fluxes for front of the WD, i.e. when the GS could block the view interstellar extinction using a value of the extinction onto the emission zone near the WD.