Raman Scattered HeII 6545 in the Young and Compact Planetary Nebula NGC 6790

📝 Abstract

We present the high resolution spectra of the youn and compact planerary nebula NGC 6790 obtained with the echelle spectrograph at Bohyunsan Optical Astronomy Observatory and report the discovery of Raman scattered He II 6545 in this object. This line feature is formed in a thick neutral region surrounding the hot central star, where He II 1025 line photons are scattered inelastically by hydrogen atoms. A Monte Carlo technique is adopted to compute the line profile with a simple geometric model, in which the neutral region is in the form of a cylindrical shell that is expanding from the central star. From our line profile analysis, the expansion velocity of the HI region lies in the range V_exp = 15- 19 Km/s. Less stringent constraints are put on the HI column density N_HI and covering factor C, where the total flux of Raman He II 6545 is consistent with the product of N_HI and C being CN_HI ~ 0.5 X 10^20 cm^-2. The Monte Carlo profiles from stationary emission models exhibit deficit in the wing parts. A much better fit is obtained when the He II emission region is assumed to take the form of a ring that slowly rotates with a rotation speed ~ 18Km/s. Brief discussions are presented regarding the mass loss processes and future observations.

💡 Analysis

We present the high resolution spectra of the youn and compact planerary nebula NGC 6790 obtained with the echelle spectrograph at Bohyunsan Optical Astronomy Observatory and report the discovery of Raman scattered He II 6545 in this object. This line feature is formed in a thick neutral region surrounding the hot central star, where He II 1025 line photons are scattered inelastically by hydrogen atoms. A Monte Carlo technique is adopted to compute the line profile with a simple geometric model, in which the neutral region is in the form of a cylindrical shell that is expanding from the central star. From our line profile analysis, the expansion velocity of the HI region lies in the range V_exp = 15- 19 Km/s. Less stringent constraints are put on the HI column density N_HI and covering factor C, where the total flux of Raman He II 6545 is consistent with the product of N_HI and C being CN_HI ~ 0.5 X 10^20 cm^-2. The Monte Carlo profiles from stationary emission models exhibit deficit in the wing parts. A much better fit is obtained when the He II emission region is assumed to take the form of a ring that slowly rotates with a rotation speed ~ 18Km/s. Brief discussions are presented regarding the mass loss processes and future observations.

📄 Content

arXiv:0901.2189v1 [astro-ph.SR] 15 Jan 2009 Raman Scattered He II λ 6545 in the Young and Compact Planetary Nebula NGC 6790 Eun-Ha Kang1, Byeong-Cheol Lee2 & Hee-Won Lee1 1 Department of Astronomy and Space Science, Astrophysical Research Center for the Structure and Evolution of the Cosmos, Sejong University, Seoul, 143-747, Korea 2 Department of Astronomy and Atmospheric Sciences, Kyungpook National University hwlee@sejong.ac.kr Received ; accepted Submitted to ApJ – 2 – ABSTRACT We present the high resolution spectra of the young and compact planetary nebula NGC 6790 obtained with the echelle spectrograph at Bohyunsan Optical Astronomy Observatory and report the discovery of Raman scattered He II λ 6545 in this object. This line feature is formed in a thick neutral region surrounding the hot central star, where He IIλ 1025 line photons are scattered inelastically by hydrogen atoms. A Monte Carlo technique is adopted to compute the line profiles with a simple geometric model, in which the neutral region is in the form of a cylindrical shell that is expanding from the central star. From our line profile analysis, the expansion velocity of the H I region lies in the range vexp = 15 −19 km s−1. Less stringent constraints are put on the H I column density NHI and covering factor C, where the total flux of Raman He IIλ6545 is consistent with their product CNHI ∼0.5×1020 cm−2. The Monte Carlo profiles from stationary emission models exhibit deficit in the wing parts. A much better fit is obtained when the He II emission region is assumed to take the form of a ring that slowly rotates with a rotation speed ∼18 km s−1. Brief discussions are presented regarding the mass loss processes and future observations. Subject headings: planetary nebulae — planetary nebulae: individual NGC 6790 — radiative transfer — scattering — mass loss – 3 – 1. Introduction Mass loss is an important process that mainly occurs in the late stage of stellar evolution. A star with a mass less than 8 M⊙loses a significant amount of mass in the giant stage before becoming a planetary nebula with a hot white dwarf at its center. Considering the Chandrasekhar limit of 1.4 M⊙, the mass loss process in the giant stage with enriched heavy elements should be important in the chemical evolution of the interstellar medium. In this regard, with a recent history of mass loss, young planetary nebulae are interesting objects to study the mass loss process. It is expected that around a young planetary nebula there may be a significant amount of neutral material that was lost in the previous stage of stellar evolution. In this case, the neutral region is exposed to the strong UV emission line source in the vicinity of the hot central star of the planetary nebula. Therefore, important information related with the mass loss process can be gathered from investigations of the scattering processes of the UV radiation originating from the center region. Taylor, Gussie & Pottasch (1990) performed H I 21 cm radio observations for a number of compact planetary nebulae (see also Altschuler et al. 1986, Gussie & Taylor 1995, Schneider et al. 1987). Their target selection was made on the basis of high radio brightness temperature, which is indicative of the nebular compactness. They searched an absorption trough that may be formed at the radial velocity of a compact planetary nebula when the neutral region blocks the background H I radio emission from our Galaxy. A number of compact young planetary nebulae including IC 5117 and NGC 6790 have been detected. Adopting an excitation temperature THI = 100 K, the typical H I column density was determined to be of order NHI ∼1020 cm−2 in these objects. Astrophysical Raman spectroscopy involving atomic hydrogen was initiated by Schmid (1989), who identified the mysterious broad emission bands occurring at 6825 ˚A and 7088 – 4 – ˚A in many symbiotic stars (see also Nussbaumer, Schmid & Vogel 1989). He proposed that a hydrogen atom in the ground state is excited with the absorption of an incident far UV O VIλ 1032 photon and de-excites into the 2s level with the re-emission of an optical photon at 6825 ˚A. An analogous process for far UV O VIλ1038 yields optical photons at 7088 ˚A. The large line width and prominent linear polarization exhibited by these scattered features strongly support his proposal (e.g. Harries & Howarth 1996). Observations made simultaneous in the UV and optical regions also confirm the Raman scattering nature (Espey et al. 1995). In the spectrum of the symbiotic star RR Telescopii, Van Groningen (1993) discovered Raman scattered He II features that are formed blueward of hydrogen Balmer emission lines. He II emission lines arising from transitions between n = 2k and n = 2 levels have wavelengths that are slightly shorter than hydrogen Lyman lines owing to the fact that He II ions are single electron atoms with a slightly larger two body reduced mass. The proximity to resonance is responsible for a large scatteri

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