A Profile Analysis of Raman-scattered OVI Bands at 6825 Å and 7082 Å in Sanduleak's Star (Heo et al. 2016, ApJ, 833, 286)

Jeong-Eun Heo1,2, Rodolfo Angeloni2, Francesco Di Mille3, Tali Palma5 and Hee-Won Lee1 1Department of Physics and Astronomy, Sejong University, Korea, 2Gemini Observatory, Chile, 3Las Campanas Observatory, Chile 5Universidad Andrés Bello, Chile

Sanduleak's star is a suspected symbiotic binary in the Large Magellanic Cloud. In 2011 it was also discovered to power a highly-collimated bipolar jet extending over almost 15 pc. We present a detailed modelling of two broad bands at 6825 and 7082 Å in Sanduleak's star. These bands are the results of Raman-scattering of O VI 𝜆 𝜆 1032 Å and 1038 Å photons by atomic hydrogen and are only observed in symbiotic stars. Our high-resolution spectrum obtained with MIKE at the Magellan-Clay telescope allowed us to reveal that the profiles of the two bands look quite different: while the Raman 6825 Å band shows a single broad profile, the Raman 7082 Å band exhibits a distinct triple-peak profile. In our analysis we propose that the O VI emission region can be decomposed into three components: an accretion disk, a bipolar outflow and a further optically thick compact component. We also perform Monte Carlo simulations with the aim of fitting the observed flux ratio F(6825)/F(7082) ~ 4.5, which indicates that the H I scattering region in Sanduleak’s star is characterized by a column density NHI ∼ 1 × 1023 cm−2

ABSTRACT

- Spectropolarimetry would have the power of shedding much more light on the detailed structure of the O VI emission region in symbiotic stars. Spectropolarimetric observations performed by Harries & Howarth (1996) showed for example that Raman-scattered O VI bands are strongly polarized, and that in many cases the red wing is polarized in the direction perpendicular to the polarization direction of the main part.

DISCUSSION

Sanduleak’s Star- Sanduleak’s star was discovered by Sanduleak in 1977 as a

variable emission-line object in the Large Magellanic Cloud.- Despite the absence of any late-type stellar signatures, it is

tentatively classified as a D-type symbiotic stars (SSs) on the based of the optical emission-line spectrum and the presence of the Raman-scattered OVI features, the latter only observed in bona fide SSs (Allen 1980).

- The discovery of a giant, highly-collimated bipolar jet extending over almost 15 pc has shed new light on the nature of this object.

Raman-scattering of OVI 𝜆 𝜆 1032 and 1038 in H I- Raman-scattering is an inelastic scattering process which results

in the re-emission of a photon with a significantly longer wavelength than the incident photon.

- For the Raman-scattering of OVI photons to be efficient, a strong far-UV emission region and a very optically thick H I scattering region are required. This astrophysical conditions are ideally met in SSs.

- A number of SSs exhibit broad emission feature at around 6825 Å and 7082 Å which are formed through Raman-scattering of OVI 𝜆 𝜆 1032 and 1038 Å by atomic hydrogen (Schmid 1989).

- The Raman profiles reflect the relative kinematics between the H I scattering region and the far-UV O VI emission region and are almost independent of the observer’s line of sight.

INTRODUCTION

Figure 1.Hα+[N II] emission-line image of Sanduleak's star (Angeloni et al., 2011)

Figure 2. Schematic energy level diagram showing Raman-scattering of

O VI process involving H I

F(1032)/F(1038)

MIKE Spectroscopy - The Magellan Inamori Kyocera Echelle (MIKE)- 6.5m Clay Telescope, Las Campanas Observatory, Chile - Observing Date: Nov. 21, 2010 - Spectral Coverage: (Red) 4,900~9,500 Å- Resolving Power: ~32,000 - Exposure Time: 3×900 sec

Raman Bands in Sanduleak’s Star - Raman O VI profiles in SSs are known to exhibit multiple-peak profiles including double-peak and

triple-peak structures (Harris & Howarth 1996). - In the Sanduleak’s star spectrum (Fig.3), the two Raman profiles are quite different: while the Raman

6825 Å band shows a single broad profile, the 7082 Å one exhibits a distinct triple-peak profile.- We transformed the observed spectrum into the Doppler factor (∆V) space (upper axis of Fig.3), which

is measured by the atomic centers of OVI 𝜆 𝜆 1032 and 1038 Å doublet.

Figure 3. The Magellan Telescopes at LCO, Chile

Figure 4. The Raman-scattered OVI bands at 6825 Å (left) and 7082 Å (right) in Sanduleak’s star.

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OBSERVATIONS

OVI EMISSION REGION MODEL

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① Blue Emission Part (BEP) & Red Emission Part (REP)- BEP is a part of the accretion disk approaching the scattering region with ∆VOVI ~ -34 km/s, whereas REP is receding from the giant with ∆VO VI ~ +34 km/s.- The accretion disk must be quite asymmetric based on their flux ratio F(1032)/F(1038) : BEP is an optically thin component with F(1032)/F(1038)=2, REP is an emitting region with high optical depth F(1032)/F(1038)=1.- Both components are characterized by same velocity width ∆v ~ 28 km/s.

② Central Emission Part (CEP)- An optically thin gas surrounding the white dwarf characterized by ∆VOVI ~ + 6 km/s and F(1032)/F(1038)=2.- Considerable emission volume leads to a wide occupancy in velocity space (∆v ~ 42 km/s).

③ Bipolar Outflow- The bipolar outflow moving away from the scattering region has a representative speed of ∆VOVI ~ +60 km/s.- This region is characterized by F(1032)/F(1038)=2 and ∆v ~ 67 km/s.

④ Optically Thick Compact Component - An optically thick nebula with ∆VOVI ~ +5 km/s and F(1032)/F(1038)=1.- This emitting component might be localized (∆v ~ 11 km/s).- It might be interpreted as a local density enhancement in the accretion stream, e.g., in the form of spiral structure, which is also suggested by the numerical work of Walder et al. (2008).

Monte Carlo Simulation- We normalize the simulated spectra with the observed Raman 6825 Å band

and look for the best fitting profile of the observed Raman 7082Å band. - A good fit is obtained for NHI ∼ 1 × 1023 cm−2. - Our Monte Carlo result shows that F(6825)/F(7082) > 5 appropriate for

D-type symbiotic stars are obtained for NHI < 1022 cm−2 . On the other hand, for NHI > 3×1023 cm−2 the flux ratio becomes lower than 4, resulting in poor fit to the observed data.

- In order to characterize the emission regions of Sanduleak’s star, we decomposed the far-UV O VI emission lines into five Gaussian components based on the observed Raman-scattered bands.- We propose that the O VI emission region can be decomposed into blue, red and central emission part of an accretion disk, a bipolar outflow and a further compact, optically thick region.- Since our interest is the kinematics of the O VI emission region with respect to the H I scattering region, we introduce ∆VOVI , and set its zero-point at the average value of the blue and red peaks (shown by dotted line in Fig. 5).

Figure 6. Monte-Carlo results depend on the column density of the H I scattering region

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Figure 5. Profile synthesis of the far-UV OVI lines

Figure 7. Schematic model of Sanduleak’s star

- Allen, D. A. 1980, ApL, 20, 131- Angeloni, R., Di Mille, F., Bland-Hawthorn, J., & Osip, D. J. 2011, ApJ,

743, L8- Harries, T. J., & Howarth, I. D. 1996, A&AS, 119, 61- Sanduleak, N., 1997, IBVS, 1304, 1- Schmid, H. M. 1989, A&A, 211, L31- Walder, R., Folini, D., & Shore, S. N. 2008, A&A, 484, L9

REFERENCES

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KOREA-CHILE Collaboration in Stellar Astophysics

2017 KAS Spring Meeting