An Extremely Large Excess of
                    <sup>18</sup>
                    O in the Hydrogen-deficient Carbon Star HD 137613

Clayton, Geoffrey C.; Herwig, Falk; Geballe, T. R.; Asplund, Martin; Tenenbaum, E. D.; Engelbracht, C. W.; Gordon, K. D.

doi:10.1086/430110

Cited by 52 publications

(62 citation statements)

References 32 publications

(42 reference statements)

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“…The remaining H-rich envelope is ingested by the He-shell and the ensuing nucleosynthesis includes large-scale conversion of H to He. Clayton et al (2005Clayton et al ( , 2007 made the remarkable discovery from medium-resolution infrared spectra that the oxygen in HdCs was primarily the isotope 18 O and not the usual dominant isotope 16 O. This work supports the origin of HdC and RCB stars via the DD scenario.…”

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confidence: 55%

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SpS1-Infrared spectroscopy of post-AGB objects

2009

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Post-AGB (pAGB) objects are low to intermediate initial mass ( 8 M ) objects that have terminated normal nuclear burning and as a result are undergoing rapid evolution toward the white dwarf sequence. In classical pAGB objects evolution is to hotter effective temperatures at roughly constant luminosity. However, there are also several classes of pAGB objects that have revived nuclear burning after approaching or being on the white dwarf sequence. These include objects with delayed final helium shell flashes (e.g. Sakurai's star) and white dwarf mergers (e.g. R CrB stars). Binary evolution plays a critical role in many of these systems. A group of pAGB supergiants with large infrared excesses are suspected to be binaries that have undergone common envelope evolution. Further details on many of these objects can be found in reviews, see for instance Van Winckel (2003) and Herwig (2005).The use of infrared (IR) tools has greatly increased our understanding of pAGB evolution. The strong infrared excess of these objects makes them very bright in the infrared and easily detectable in the mid-infrared. Many objects become embedded in their AGB mass loss and are optically obscured. The reduction of the obscuration and the brightness of pAGB objects in the IR makes them ideal targets for detailed analysis using IR spectroscopy. We report on three specific groups of pAGB objects.Post-common envelope binaries: Van Winckel et al. (1995) identified a class of pAGB singlelined, long-period, spectroscopic binaries. The primary star of these binaries is an early-type supergiant with very peculiar abundances. These objects typically have strong IR excesses, carbon-rich circumstellar dust, and a photospheric abundance pattern characterized by a severe deficiency of refractory elements and a near-solar abundance for volatile elements. The orbits of these objects suggest previous common envelope evolution when the primary was at the tip of the AGB. A circumbinary Keplerian rotating disk appears a common feature of these pAGB binaries (De Ruyter et al. 2006). Hinkle et al. (2007) report on high-resolution infrared spectroscopy in the 2.3-4.6 μm region. In the prototype HR 4049 the 4.6 μm spectrum shows a rich forest of CO and H2 O emission lines. All the spectral lines observed in the 2.3-4.6 μm spectrum are circumbinary in origin and originate in oxygen rich gas in contrast to the extended circumstellar carbon-rich gas. The emission and absorption line profiles show that the circumbinary gas is located in a thin, rotating layer near the dust disk. The velocities of the disk and the spectroscopic orbit yield masses for the individual stars, for HR 4049 MA I ∼0.58 M and MM V ∼0.34 M . Chemical processing in the disk is the likely method for producing the chemical peculiarities seen on the stellar photosphere.Final flash objects: Final helium shell flash evolution occurs in stars that have terminated their AGB stage and are either on the constant luminosity pAGB track or just entering the early white dwarf sequence. A final helium bu...

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supporting

confidence: 55%

“…This work supports the origin of HdC and RCB stars via the DD scenario. Exploratory calculations by Clayton et al (2007) Clayton et al(2005Clayton et al( , 2007.…”

mentioning

confidence: 99%

SpS1-Infrared spectroscopy of post-AGB objects

2009

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“…The resulting H-deficient star is produced either from the merger of a He white dwarf with a C-O white dwarf (DD scenario) 5 or from a final post-asymptotic giant branch helium flash in the central stars of planetary nebulae (PNe; FF scenario). Detailed abundance analysis of RCBs (Lambert & Rao 1994;Asplund et al 2000;Clayton et al 2005Clayton et al , 2007García-Hernández et al 2009, 2010aJeffery et al 2011;Pandey & Lambert 2011) suggests that the DD scenario may account for the great majority of the RCBs. Indeed, simulations by Longland et al (2011) show that a "hot" white dwarf merger may qualitatively explain the chemical abundances observed in RCB stars 6 (see also Staff et al 2012 for more details about merger simulations and RCBs).…”

Section: Introductionmentioning

confidence: 99%

Dust Around R Coronae Borealis Stars. Ii. Infrared Emission Features in an H-Poor Environment

García-Hernández

Rao

Lambert

2013

ApJ

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Residual Spitzer/Infrared Spectrograph spectra for a sample of 31 R Coronae Borealis (RCB) stars are presented and discussed in terms of narrow emission features superimposed on the quasi-blackbody continuous infrared emission. A broad ∼6-10 μm dust emission complex is seen in the RCBs showing an extreme H-deficiency. A secondary and much weaker ∼11.5-15 μm broad emission feature is detected in a few RCBs with the strongest ∼6-10 μm dust complex. The Spitzer infrared spectra reveal for the first time the structure within the ∼6-10 μm dust complex, showing the presence of strong C-C stretching modes at ∼6.3 and 8.1 μm as well as of other dust features at ∼5.9, 6.9, and 7.3 μm, which are attributable to amorphous carbonaceous solids with little or no hydrogen. The few RCBs with only moderate H-deficiencies display the classical "unidentified infrared bands (UIRs)" and mid-infrared features from fullerene-related molecules. In general, the characteristics of the RCB infrared emission features are not correlated with the stellar and circumstellar properties, suggesting that the RCB dust features may not be dependent on the present physical conditions around RCB stars. The only exception seems to be the central wavelength of the 6.3 μm feature, which is blueshifted in those RCBs showing also the UIRs, i.e., the RCBs with the smallest H deficiency.

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“…Recently, Clayton et al (2005) discovered that 16 O/ 18 O P 1 in the HdC star, HD 137613, from a spectrum of the first overtone bands of CO at 2.3-2.4 m. As the bands of 12 C 16 O are of typical strength for a cool star, the measured ratio implies a huge enhancement of 18 O rather than a depletion of 16 O. The isotopic ratio is orders of magnitude less than that measured in other stars .…”

Section: Introductionmentioning

confidence: 99%

Very Large Excesses of¹⁸O in Hydrogen‐deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers

Clayton

Geballe

Herwig

et al. 2007

ApJ

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157

232

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We have found that at least seven hydrogen-deficient carbon ( HdC ) and R Coronae Borealis ( RCB) stars, have 16 O/ 18 O ratios close to and in some cases less than unity, values that are orders of magnitude lower than measured in other stars (the solar value is 500). Greatly enhanced 18 O is evident in every HdC and RCB we have measured that is cool enough to have detectable CO bands. The three HdC stars measured have 16 O/ 18 O < 1, lower values than any of the RCB stars. These discoveries are important clues in determining the evolutionary pathways of HdC and RCB stars, for which two models have been proposed: the double degenerate (white dwarf [WD] merger) and the final helium-shell flash (FF ). No overproduction of 18 O is expected in the FF scenario. We have quantitatively explored the idea that HdC and RCB stars originate in the mergers of CO-and He-WDs. The merger process is estimated to take only a few days, with accretion rates of 150 M yr À1 producing temperatures at the base of the accreted envelope of 1:2 1:9 ð Þ; 10 8 K. Analysis of a simplified one-zone calculation shows that nucleosynthesis in the dynamically accreting material may provide a suitable environment for a significant production of 18 O, leading to very low values of 16 O/ 18 O, similar to those observed. We also find qualitative agreement with observed values of 12 C/ 13 C and with the CNO elemental ratios. H-admixture during the accretion process from the small H-rich C/O WD envelope may play an important role in producing the observed abundances. Overall, our analysis shows that WD mergers may very well be the progenitors of O 18 -rich RCB and HdC stars, and that more detailed simulations and modeling are justified.

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An Extremely Large Excess of ¹⁸ O in the Hydrogen-deficient Carbon Star HD 137613

Cited by 52 publications

References 32 publications

SpS1-Infrared spectroscopy of post-AGB objects

SpS1-Infrared spectroscopy of post-AGB objects

Dust Around R Coronae Borealis Stars. Ii. Infrared Emission Features in an H-Poor Environment

Very Large Excesses of¹⁸O in Hydrogen‐deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers

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An Extremely Large Excess of 18 O in the Hydrogen-deficient Carbon Star HD 137613

Cited by 52 publications

References 32 publications

SpS1-Infrared spectroscopy of post-AGB objects

SpS1-Infrared spectroscopy of post-AGB objects

Dust Around R Coronae Borealis Stars. Ii. Infrared Emission Features in an H-Poor Environment

Very Large Excesses of18O in Hydrogen‐deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers

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Product

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An Extremely Large Excess of ¹⁸ O in the Hydrogen-deficient Carbon Star HD 137613

Very Large Excesses of¹⁸O in Hydrogen‐deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers