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Register a new userDiscovery of water vapor around IRC+10216 as evidence for comets orbiting another star
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Authors
Gary J. Melnick
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Since 1995, astronomers have discovered planets with masses comparable to that of Jupiter (318 times Earths mass) in orbit around approximately 60 stars. Although unseen directly, the presence of these planets is inferred by the small reflex motions that they gravitationally induce on the star they orbit; these result in small periodic wavelength shifts in the stellar spectrum. Since this method favors the detection of massive objects orbiting in close proximity to the star, the question of whether these systems also contain analogs of the smaller constituents of our Solar System has remained unanswered. Using an alternative approach, we report here observations of an aging carbon-star, IRC+10216, that reveal the presence of circumstellar water vapor, a molecule not expected in measurable abundances around such a star and thus a distinctive signature of an orbiting cometary system. The only plausible explanation for this water vapor is that the recent evolution of IRC+10216 - which is accompanied by a prodigious increase in its luminosity - is now causing the vaporization of a collection of orbiting icy bodies, a process first considered in a previous theoretical study.
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We have modeled the emission of H2O rotational lines from the extreme C-rich star IRC+10216. Our treatment of the excitation of H2O emissions takes into account the excitation of H2O both through collisions, and through the pumping of the nu2 and nu3 vibrational states by dust emission and subsequent decay to the ground state. Regardless of the spatial distribution of the water molecules, the H2O 1_{10}-1_{01} line at 557 GHz observed by the Submillimeter Wave Astronomy Satellite (SWAS) is found to be pumped primarily through the absorption of dust-emitted photons at 6 $mu$m in the nu2 band. As noted by previous authors, the inclusion of radiative pumping lowers the ortho-H2O abundance required to account for the 557 GHz emission, which is found to be (0.5-1)x10^{-7} if the presence of H2O is a consequence of vaporization of orbiting comets or Fischer-Tropsch catalysis. Predictions for other submillimeter H2O lines that can be observed by the Herschel Space Observatory (HSO) are reported. Multitransition HSO observations promise to reveal the spatial distribution of the circumstellar water vapor, discriminating among the several hypotheses that have been proposed for the origin of the H2O vapor in the envelope of IRC+10216. We also show that, for observations with HSO, the H2O 1_{10}-1_{01} 557 GHz line affords the greatest sensitivity in searching for H2O in other C-rich AGB stars.
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.
H13CN J=8-7 sub-millimetre line emission produced in the circumstellar envelope around the extreme carbon star IRC+10216 has been imaged at sub-arcsecond angular resolution using the SMA. Supplemented by a detailed excitation analysis the average fractional abundance of H13CN in the inner wind (< 5E15 cm) is estimated to be about 4E-7, translating into a total HCN fractional abundance of 2E-5 using the isotopic ratio 12C/13C=50. Multi-transitional single-dish observations further requires the H13CN fractional abundance to remain more or less constant in the envelope out to a radius of about 4E16 cm, where the HCN molecules are effectively destroyed, most probably, by photodissociation. The large amount of HCN present in the inner wind provides effective line cooling that can dominate over that generated from CO line emission. It is also shown that great care needs to be taken in the radiative transfer modelling where non-local, and non-LTE, effects are important and where the radiation field from thermal dust grains plays a major role in exciting the HCN molecules. The amount of HCN present in the circumstellar envelope around IRC+10216 is consistent with predicted photospheric values based on equilibrium chemical models and indicates that any non-equilibrium chemistry occurring in the extended pulsating atmosphere has no drastic net effect on the fractional abundance of HCN molecules that enters the outer envelope. It further suggests that few HCN molecules are incorporated into dust grains.
We describe Very Large Array observations of the extreme carbon star IRC+10216 at 8.4, 14.9, and 22.5 GHz made over a two year period. We find possible variability correlated with the infrared phase and a cm- to sub-millimeter wavelength spectral index very close to 2. The variability, observed flux densities, and upper limit on the size are consistent with the emission arising from the stellar photosphere or a slightly larger radio photosphere.
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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