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Register a new userThe dynamical evolution of the dust shell of IRC+10216
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R. Osterbart
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We present high-resolution J-, H-, and K-band observations of the carbon star IRC+10216. The images were reconstructed from 6 m telescope speckle interferograms using the bispectrum speckle interferometry method. The H and K images consist of several compact components within a 0.2 radius and a fainter asymmetric nebula. The brightest four components are denoted with A to D in the order of decreasing brightness. A comparison of our images gives - almost like a movie of five frames - insight to the dynamical evolution of the inner nebula. For instance, the separation of the two brightest components A and B increased by almost 40% from 191 mas in 1995 to 265 mas in 1998. At the same time, component B is fading and the components C and D become brighter. The X-shaped bipolar structure of the nebula implies an asymmetric mass-loss suggesting that IRC+10216 is very advanced in its AGB evolution, shortly before turning into a protoplanetary nebula. The cometary shape of component A suggests that the core of A is not the central star, but the southern lobe of a bipolar structure. The position of the central star is probably at or near the position of component B.
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We present high-resolution J-, H-, and K-band observations and the first H-K color image of the carbon star IRC +10216. The images were reconstructed from 6m telescope speckle interferograms using the bispectrum speckle interferometry method. The H and K images with resolutions between 70mas and 92mas consist of several compact components within a 0.2 radius and a fainter asymmetric nebula. The brightest four components are denoted with A to D in the order of decreasing brightness in the 1996 image. A comparison of our images from 1995, 1996, 1997, and 1998 gives - almost like a movie of five frames - insight into the dynamical evolution of the inner nebula. For instance, the separation of the two brightest components A and B increased from 191 mas in 1995 to 265 mas in 1998. At the same time, component B is fading and the components C and D become brighter. The X-shaped bipolar structure of the nebula, most prominently present in the J-band image, implies an asymmetric mass loss. Such asymmetries are often present in protoplanetary nebulae but are unexpected for AGB stars. IRC +10216 is thus likely to be very advanced in its AGB evolution, shortly before turning into a protoplanetary nebula. The cometary shapes of A in the H and J images and in the 0.79 micron and 1.06 micron HST images suggest that the core of A is not the central star, but the southern lobe of a bipolar structure. The position of the central star is probably at or near the position of component B, where the H-K color has a value of 4.2. If the star is at or near B, then the components A, C, and D are likely to be located at the inner boundary of the dust shell.
We report the detection in IRC+10216 of lines of HNC $J$=3-2 pertaining to 9 excited vibrational states with energies up to $sim$5300 K. The spectrum, observed with ALMA, also shows a surprising large number of narrow, unidentified lines that arise in the vicinity of the star. The HNC data are interpreted through a 1D--spherical non--local radiative transfer model, coupled to a chemical model that includes chemistry at thermochemical equilibrium for the innermost regions and reaction kinetics for the external envelope. Although unresolved by the present early ALMA data, the radius inferred for the emitting region is $sim$0.06 (i.e., $simeq$ 3 stellar radii), similar to the size of the dusty clumps reported by IR studies of the innermost region ($r <$ 0.3). The derived abundance of HNC relative to H$_2$ is $10^{-8} <$ $chi$(HNC) $< 10^{-6}$, and drops quickly where the gas density decreases and the gas chemistry is dominated by reaction kinetics. Merging HNC data with that of molecular species present throughout the inner envelope, such as vibrationally excited HCN, SiS, CS, or SiO, should allow us to characterize the physical and chemical conditions in the dust formation zone.
We have used the Robert C. Byrd Green Bank Telescope to perform the most sensitive search to date for neutral atomic hydrogen (HI) in the circumstellar envelope (CSE) of the carbon star IRC+10216. Our observations have uncovered a low surface brightness HI shell of diameter ~1300 (~0.8 pc), centered on IRC+10216. The HI shell has an angular extent comparable to the far ultraviolet-emitting astrosphere of IRC+10216 previously detected with the GALEX satellite, and its kinematics are consistent with circumstellar matter that has been decelerated by the local interstellar medium. The shell appears to completely surround the star, but the highest HI column densities are measured along the leading edge of the shell, near the location of a previously identified bow shock. We estimate a total mass of atomic hydrogen associated with IRC+10216 CSE of M_HI~3x10e-3 M_sun. This is only a small fraction of the expected total mass of the CSE (<1%) and is consistent with the bulk of the stellar wind originating in molecular rather than atomic form, as expected for a cool star with an effective temperature T_eff<~2200 K. HI mapping of a 2 deg x 2 deg region surrounding IRC+10216 has also allowed us to characterize the line-of-sight interstellar emission in the region and has uncovered a link between diffuse FUV emission southwest of IRC+10216 and the Local Leo Cold Cloud.
Understanding the formation of planetary nebulae remains elusive because in the preceding asymtotic giant branch (AGB) phase these stars are heavily enshrouded in an optically thick dusty envelope. To further understand the morphology of the circumstellar environments of AGB stars we observe the closest carbon-rich AGB star IRC+10216 in scattered light. When imaged in scattered light at optical wavelengths, IRC+10216 surprisingly shows a narrow equatorial density enhancement, in contrast to the large-scale spherical rings that have been imaged much further out. We use radiative transfer models to interpret this structure in terms of two models: firstly, an equatorial density enhancement, commonly observed in the more evolved post-AGB stars, and secondly, in terms of a dust rings model, where a local enhancement of mass-loss creates a spiral ring as the star rotates. We conclude that both models can be used to reproduce the dark lane in the scattered light images, which is caused by an equatorially density enhancement formed by dense dust rather than a bipolar outflow as previously thought. We are unable to place constraints on the formation of the equatorial density enhancement by a binary system.
The J,K = 1,0-0,0 rotational transition of phosphine (PH3) at 267 GHz has been tentatively identified with a T_MB = 40 mK spectral line observed with the IRAM 30-m telescope in the C-star envelope IRC+10216. A radiative transfer model has been used to fit the observed line profile. The derived PH3 abundance relative to H2 is 6 x 10^(-9), although it may have a large uncertainty due to the lack of knowledge about the spatial distribution of this species. If our identification is correct, it implies that PH3 has a similar abundance to that reported for HCP in this source, and that these two molecules (HCP and PH3) together take up about 5 % of phosphorus in IRC+10216. The abundance of PH3, as that of other hydrides in this source, is not well explained by conventional gas phase LTE and non-LTE chemical models, and may imply formation on grain surfaces.
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