Herschel observations of extreme OH/IR stars The isotopic ratios of oxygen as a sign-post for the stellar mass

© ESO 2015. Aims. The late stages of stellar evolution are mainly governed by the mass of the stars. Low- and intermediate-mass stars lose copious amounts of mass during the asymptotic giant branch (AGB) which obscure the central star making it difficult to study the stellar spectra and determine the stellar mass. In this study, we present observational data that can be used to determine lower limits to the stellar mass. Methods. Spectra of nine heavily reddened AGB stars taken by the Herschel Space Observatory display numerous molecular emission lines. The strongest emission lines are due to H 2 O. We search for the presence of isotopologues of H 2 O in these objects. Results. We detected the 16O and 17O isotopologues of water in these stars, but lines due to H 2 18O are absent. The lack of 18O is predicted by a scenario where the star has undergone hot-bottom burning which preferentially destroys 18O relative to 16O and 17O. From stellar evolution calculations, this process is thought to occur when the stellar mass is above 5 M ⊙ for solar metallicity. Hence, observations of different isotopologues of H 2 O can be used to help determine the lower limit to the initial stellar mass. Conclusions. From our observations, we deduce that these extreme OH/IR stars are intermediate-mass stars with masses of ≥5 M ⊙ . Their high mass-loss rates of ∼10-4M ⊙ yr-1 may affect the enrichment of the interstellar medium and the overall chemical evolution of our Galaxy.

[1]  J. Lattanzio,et al.  The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars , 2014, Publications of the Astronomical Society of Australia.

[2]  D. A. García-Hernández,et al.  Hot bottom burning and s-process nucleosynthesis in massive AGB stars at the beginning of the thermally-pulsing phase , 2013, 1306.2134.

[3]  M. Barlow,et al.  OH/IR stars and their superwinds as observed by the Herschel Space Observatory ⋆ , 2013, 1306.1777.

[4]  D. A. Garc'ia-Hern'andez,et al.  HEAVY ELEMENT NUCLEOSYNTHESIS IN THE BRIGHTEST GALACTIC ASYMPTOTIC GIANT BRANCH STARS , 2012, 1203.2931.

[5]  K. Menten,et al.  Herschel/HIFI observations of O-rich AGB stars : molecular inventory ⋆ , 2011, 1111.5156.

[6]  O. Krause,et al.  MESS (Mass-loss of Evolved StarS), a Herschel key program , 2010, 1012.2701.

[7]  S. Ott,et al.  Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy , 2010, 1005.5331.

[8]  G. Davis,et al.  Galactic interstellar 18 O/{^17}O ratios - a radial gradient? , 2008, 0805.3399.

[9]  L. R. Nittler,et al.  On the asymptotic giant branch star origin of peculiar spinel grain OC2 , 2007 .

[10]  Lithium and zirconium abundances in massive Galactic O-rich AGB stars , 2006, astro-ph/0609106.

[11]  K. Justtanont,et al.  Near-infrared observations of water-ice in OH/IR stars , 2006, astro-ph/0601649.

[12]  F. Herwig.  Evolution of Asymptotic Giant Branch Stars , 2005 .

[13]  C. Kramer,et al.  The Herschel-Heterodyne Instrument for the Far-Infrared (HIFI) , 2005, Infrared and Millimeter Waves, Conference Digest of the 2004 Joint 29th International Conference on 2004 and 12th International Conference on Terahertz Electronics, 2004..

[14]  G. Wasserburg,et al.  Nucleosynthesis in asymptotic giant branch stars: Relevance for galactic enrichment and solar system formation , 1999 .

[15]  P. Cox,et al.  Detection by ISO of the far-infrared OH maser pumping lines in IRC+10420 , 1997 .

[16]  H. J. Habing,et al.  Circumstellar envelopes and Asymptotic Giant Branch stars , 1996 .

[17]  A. Tielens,et al.  Modeling of the Dust and Gas Outflows from OH 26.5+0.6: The Superwind , 1996 .

[18]  A. Boothroyd,et al.  The Creation of Superrich Lithium Giants , 1992 .

[19]  T. Wilson,et al.  Abundances in the interstellar medium , 1992 .

[20]  I. Iben,et al.  Asymptotic Giant Branch Evolution and Beyond , 1983 .

[21]  E. Salpeter.  The Luminosity function and stellar evolution , 1955 .