Herschel/HIFI Observations of IRC+ 10216: Water Vapor in the Inner

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arXiv:1012.1854v1 [astro-ph.SR] 8 Dec 2010 nOrc SsaeC n H and CO molecules abundant are most CSEs the O-rich AGB while in oxygen-rich particular, qual- of In is typical that stars. that chemistry from circumstellar different a itatively to large photo- rise a giving The to and leading nu- K. ( – helium-burning core and ratio of the C/O F, from dredge-up P, products in the reaction enriched S, to clear significantly owing Si, C, been – Al, H, has circum- carbon Mg, IRC+10216 elements its of the Na, within sphere of F, detected up O, been made N, have (CSE), (e.g. envelope 2008) molecules stellar as- al. 60 carbon-rich et than a source He in more valuable chemistry environment; most of trophysical 2000), the study the Olofsson be for Schöier to & available proven and has 1997 IRC+10216 Menten & Crosas r O C,adC and HCN, CO, are oigevlp hthrosarc hmsr.Be- chemistry. ( rich Earth to a proximity its harbors out- of dense, that cause a envelope by surrounded flowing is Leonis) (CW IRC+10216 t ag asls ae( rate mass-loss large its ai .Neufeld A. David est,30 ot hre tet atmr,M 11,US 21218, MD Baltimore, Street, Charles North 3400 versity, NASA from participation important con with Investigator and Principal European-led by provided ments mtra,SinePr 0,N-08Asedm The Amsterdam, NL-1098 904, Park Netherlands Science Amsterdam, Madrid, Henares, de Alcalá E-28871 Spain Universitario, Campus Spain Onsala, den SE–43992 Technology, of University Chalmers ence, rpittpstuigL using 2018 typeset 15, Preprint June version Draft h abnrc G aypoi in rnh star branch) giant (asymptotic AGB carbon-rich The 1 * 7 6 5 4 3 2 9 8 eateto hsc n srnm,JhsHpisUni- Hopkins Johns Astronomy, and Physics instru- of science Department with observatory space ESA an is Herschel srnmclIsiue teh nvriy h Netherla The University, Utrecht Institute, of Astronomical University Pannekoek, Anton Belgium Leuven, Instituut U. K. Sterrenkundig Sterrenkunde, Toruń, voor Poland Center, Instituut Astronomical USA Copernicus 02138, N. MA Cambridge, CfA, Harvard-Smithsonian Henares Alcalá de de F´ısica, Universidad de Departamento A,IT-SC tad orjo jli,Madrid, Ajalvir, a Torrejón de Ctra INTA-CSIC, Sci- Space CAB, and Radio of Dept. Observatory, Space Onsala ihAB(smttcgatbac)sa R+01 C ens,c Leonis), (CW IRC+10216 star branch) giant (asymptotic AGB rich wthmd fHF.Temaue ierto ml htwtrvapor ob water that of imply means ratios ( line by distances measured detected small The at securely HIFI. of was mode transition switch Each instrument. ujc headings: Subject (3 edrv pe iiso h H IRC+1 the of envelope on vaporiz C-rich limits dense the the upper from within derive water results We of water Th origin The observed instruments. the orbits. SPIRE that and hypothesis PACS the Herschel’s with observations from erpr h eut fosrain ftnrttoa transitio rotational ten of observations of results the report We σ ∼ ,poiigadtoa osrit nmdl o h rgno h wa the of origin the for models on constraints additional providing ), 1 . ;Wnes oii,&Sdmy 1994) Sedlmayr & Dominik, Winters, 4; A E AO NTEINREVLP FACRO-IHABSTAR AGB CARBON-RICH A OF ENVELOPE INNER THE IN VAPOR WATER 1. enDecin Leen 1 2 A dad Gonz Eduardo , H INTRODUCTION T E 2 tl mltajv 11/10/09 v. emulateapj style X ro o20,teol oxygen- only the 2001, to Prior . ∼ icmtla atr–sas G n otAB–sas abundan stars: – post-AGB and AGB stars: – matter circumstellar ≤ 1 . 5 few HERSCHEL 2 − 5 ,toei IRC+10216 in those O, lxd Koter de Alex , D 3 × ∼ × 10 100 alez-Alfonso ´ 10 14 − 2 17 m rmtesa,cnrigrcn eut eotdb ei et Decin by reported results recent confirming star, the from cm) − 5 O/H M ∗ 7 c and pc) 170 HF BEVTOSO IRC+10216: OF OBSERVATIONS /HIFI ⊙ rf eso ue1,2018 15, June version Draft yr 2 16 − n H and O 6,7 1 sortia see ; 2 rdi Sch Fredrik , Swe- ABSTRACT ayJ Melnick J. Gary , nds A , 2 18 O/H ihmlclsrpre nteCEo R+01 were IRC+10216 of HCO CSE and the SiO, in CO, reported molecules rich 03,H 2003), fcro-ihsaspeitwtraudne fonly of abundances water predict photospheres ∼ the stars in carbon-rich equilibrium of thermochemical the for of els use Satellite the tronomy with GHz 557 atdcd orohrObaigmlclshv been have molecules O-bearing H other detected: four decade last o h pta itiuino h mtigwtrvapor. water emitting the of prediction distribution specific a spatial to the rise gave for hypotheses these of each enc 07 eefe N)promdi anticipation in the performed of GNM) hereafter 2007; Melnick between distinguish hypotheses. observa- readily place these single could to that a unable constraints was only it tional to vapor, access water of had transition SWAS Because (ISRF). field of nteotrevlp ymaso aitv association radiative of formed means by be H envelope might of outer water (2006; the that Cernicharo in proposed Agúndez and AC06) whilst hereafter grains, dust CO from lic formed be H could sug- water and observed subsequently – the un- (2004) that objects – gested Willacy icy star AGB orbiting the sublimation. which of derwent luminosity in enhanced vaporization analog, the the by belt water heated by Kuiper outflow the Ford a the whereby of vapor. into model released water a was observed proposed vapor the (2001) of Neufeld origin pro- & the been have for hypotheses posed Several observations. from SWAS the 2007) or Melnick and González-Alfonso, threshold Neufeld 2006; detection the below abundance magnitude of ders yosrain fte1 the of observations by 2006). al. et natertclsuy(ozae-los,Nuedand (González-Alfonso, Neufeld study theoretical a In h icvr fwtrvpri R+01,obtained IRC+10216, in vapor water of discovery The 10 13 2 16 2 Oepsdt h lrvoe neselrradiation interstellar ultraviolet the to exposed CO − oier ¨ stpcaudnerto of ratios abundance isotopic O 2 10 ihOaospoue ytephotodissociation the by produced atoms O with eshlSaeObservatory Space Herschel ymaso ice-rpc aayi nmetal- on catalysis Fischer-Tropsch of means by 2 eaiet H to relative 8 O(ode l 04,adC and 2004), al. et (Ford CO n Jos and , 3 yzr Szczerba Ryszard , so ae ao oadtecarbon- the toward vapor water of ns to fsaliyojcsi circular in objects icy small of ation ∼ 2 Mlike l 01,O Fr tal. et (Ford OH 2001), al. et (Melnick O 10 26rmispol understood. poorly remains 0216 evtosuigteda beam dual the using servations ridotwith out arried sfidn entvl ue out rules definitively finding is − speeti h ne outflow inner the in present is SA) a uzigbcuemod- because puzzling was (SWAS), 7 Cernicharo e ´ + e ao nIRC+10216. in vapor ter nerd(gudzadCernicharo and (Agúndez inferred 2 Lcs&G´ln19) u nthe in but Guélin 1999), & (Lucas 10 eg hrhe 06,mn or- many 2006), Cherchneff (e.g. − 1 01 oainltasto near transition rotational umliee aeAs- Wave Submillimeter 4 iolwSchmidt Miroslaw , Herschel ces 9 twsdsusdhow discussed was it , ∼ 5 3 sHIFI ’s × (Tenenbaum O 10 − al. 3 4 , 2 Neufeld et al.

GNM investigated how multitransition observations with Herschel might be used to determine that distribution, and showed, in particular, that the relative strength of lines of higher excitation than those accessible to SWAS was a decreasing function of the inner radius, Rin, of the region containing water vapor. The vaporization of a 15 Kuiper belt analog would lead to Rin ∼ 2 × 10 cm ∼ 13 30 R∗ (Model B in GNM; where R∗ ∼ 8 × 10 cm is the assumed stellar radius) because any icy object (at least of small size and on a circular orbit) within that radius would have been vaporized before IRC+10216 ascended the AGB. Water production by means of Fischer-Tropsch catalysis would result in a similar value of Rin, accord- ing to calculations of Willacy (2004). The formation of water following the photodissociation of CO in an outer shell, as suggested by AC06, by contrast, would lead to 16 a much larger Rin ∼ 4.3 × 10 cm ∼ 500 R∗ (Model C in GNM), and considerably smaller strengths for the higher-excitation water transitions. Conversely, were Rin significantly smaller than 2 × 1015 cm – a possibility that had not been anticipated by any specific model – then the high-excitation transitions would be relatively stronger (Model A in GNM). In this Letter, we report the results obtained from Her- Fig. 1.— Herschel/HIFI spectra of six rotational transitions of schel (Pilbratt et al. 2010) observations of 10 rotational H2O with upper state energies EU/k ≤ 160 K. The blue line super- transitions of water vapor, carried with the Heterodyne posed on each spectrum shows the profile of the 110 − 101 ground- state transition of ortho-water (top left panel). Doppler velocities Instrument for the Far Infrared (HIFI; de Graauw et are expressed relative to the LSR velocity of the source, taken as al. 2010). The observations and data reduction are de- −25.5kms−1. scribed in §2, and the spectral line profiles and line intensities are presented in §3. In §4, we discuss the spatial distribution inferred for the water vapor in the CSE of IRC+10216, in the context of various hypotheses for its origin. We compare our results with those reported recently in an entirely independent study performed by Decin et al. (2010; hereafter D10) with the use of the PACS and SPIRE instruments on Herschel.

2. OBSERVATIONS AND DATA REDUCTION Observations of IRC+10216 were carried out in May 2010 as part of the HIFISTARS Key Program. We used the HIFI instrument in dual beam switch (DBS) mode to target 10 rotational transitions of water vapor. The list of observed transitions, and the details of each observation, are given in Table 1. The tele- scope beam was centered on IRC+10216 at coordinates α = 9h47m57.38s, δ = +13016′43.7′′ (J2000), and the reference positions were located at offsets of 3′ on either side of the source. The data were processed using the standard HIFI pipeline to Level 2, providing fully calibrated spectra with the intensities expressed as antenna temperature and the frequencies in the frame of the Local Standard of Rest (LSR). For five of the ten observed transitions, we used a pair of local oscillator (LO) frequencies, separated by a small offset, to confirm the assignment of any observed spectral feature to either the upper or lower sideband of the (double side band) HIFI receivers. The resultant spectra were coadded so as to recover the signal-to-noise ratio that would have been obtained at a single LO setting. Spectra obtained for the horizontal and vertical polarizations were found to be very similar in their appearance and noise charac- Fig. 2.— same as Fig. 1, except for four rotational transitions of teristics and were likewise coadded. H2O with upper state energies EU/k ≥ 160 K, and one rotational transition each of CO, 13CO, SiO and SiS. Herschel/HIFIobservationsofIRC+10216 3

3. RESULTS TABLE 1 The integrated antenna temperatures and line fluxes Observations of IRC+10216 are given for each detected transition of water vapor in a b Table 2, together with the energy of the upper state. The Transition ν Mixer HPBW Date tobs HIFI instrument used for these observations provides a (GHz) band (′′) 2010 (s) spectral resolution (1.1 MHz for the Wide Band Spec- c d 110 − 101 (o) 556.936 1b 38 May 4, 11 3150 d trometer) that is much superior to that of the PACS or 211 − 202 (p) 752.033 2b 28 May 12 3236 d SPIRE instruments; this property of HIFI typically pro- 202 − 111 (p) 987.927 4a 22 May 16 3242 d vides for the unambiguous identification of strong lines, 312 − 303 (o) 1097.365 4b 19 May 11 10446 d and yields spectral line profiles that can be used to probe 111 − 000 (p) 1113.343 4b 19 May 11 3200 e 312 − 221 (o) 1153.127 5a 18 May 12 1539 the kinematics of the outflowing gas. In Figures 1 and 2, e 321 − 312 (o) 1162.912 5a 18 May 12 1539 we present the spectra obtained for each of the ten de- f 221 − 212 (o) 1661.008 6b 13 May 14 1615 tected water vapor transitions, each smoothed to a spec- f −1 212 − 101 (o) 1669.905 6b 13 May 14 1615 tral resolution of 1 km s to improve the signal-to-noise d 303 − 212 (o) 1716.770 7a 12 May 14,15 10412 ratio. The lower four panels in Figure 2 show compari- a Half power beam width son spectra for one transition each of the molecules CO, b 13 Total observing time, including overheads CO, SiO and SiS; taking advantage of the wide band- c The letters o and p indicate whether the transition is of ortho- width of the HIFI receivers (4 GHz for ν ≤ 1250 GHz or of para-water d and 2.4 GHz for ν ≥ 1410 GHz), we observed these lines Data obtained with two separate LO settings of duration tobs/2 simultaneously with nearby water transitions. In Fig- e,f Pairs of lines observed simultaneously ures 1 and 2, the blue line superposed on each spectrum shows the profile of the 110 − 101 ground-state transition adjusting the assumed water abundance to match the 557 of ortho-water. GHz line flux detected by SWAS. Clearly, the best fit to Although a full analysis of the spectral line profiles will the data is obtained for the smallest value of Rin con- be the subject of a future publication, several features are 14 sidered by GNM, 4.5 × 10 cm (∼ 5.6 R∗), but even for obvious from an inspection of the spectra. Most of the that value the model underpredicts the relative strengths water lines share a distinctive profile that is quite dissim- 12 13 of the transitions of highest excitation. Thus, our HIFI ilar from those of either the CO or CO transitions. In observations confirm the inference drawn by D10 – from particular, the blue sides of the water lines are very sim- the detection of high-lying water rotational lines with ilar to the nearly parabolic profile shown by 12CO, while 13 PACS and SPIRE – about the spatial distribution of wa- the red sides are more similar to that of the CO line, ter vapor; water is clearly present at distances smaller which shows the more rectangular profile expected of an 14 than 4.5 × 10 cm (∼ 5.6 R∗) from the star. The pres- optically-thin line. In these respects, the typical water ence of water that close to the star definitively falsifies line profiles are most similar to that of the silicon monox- the model proposed by Ford & Neufeld (2001), in which ide line shown in Figure 2. The observed asymmetry in the origin of the observed water vapor was the vaporiza- the line profiles likely reflects an asymmetry in the distri- tion of icy comets in a Kuiper belt analog, because any bution of the emitting H2O and SiO, perhaps suggesting icy object (at least of small size and on a circular orbit) a common origin11. The highest excitation water line to within 4.5 × 1014 cm would have been vaporized before have been detected in our study, the 1162.912 GHz tran- IRC+10216 ascended the AGB. The models proposed by sition, exhibits a profile that is rather different from the Willacy (2004; i.e. Fischer-Tropsch catalysis) and par- typical water line profile, with a narrow core that is sim- ticularly AC06 (production via radiative association in ilar to that present in the SiS 1102.029 GHz line profile; an outer shell) are similarly excluded. The fact that the here, emission from the acceleration zone may be impli- 1113 GHz / 557 GHz line ratio is in good agreement cated. In addition, the other water lines typically show a with the GNM model implies that the ortho-to-para ra- small narrow emission bump near the systemic velocity tio is close to 3, the value assumed in the model. The of the source, perhaps also representing emission from dashed black line in Figure 3 shows our best-fit model, material that has not yet been fully accelerated. 14 with Rin =1.0×10 cm (∼ 1.3 R∗), and a slightly larger 4. −8 DISCUSSION water abundance H2O/H2 = 8.1 × 10 than that as- In Figure 3, we compare the measured line fluxes tabu- sumed in GNM, now chosen to optimize the fit to all the lated in Table 2 with the predictions of the GNM models. transitions observed with HIFI. Here, the observed line fluxes are represented by black At present, the origin of water vapor in IRC+10216 crosses for each of the ten detected transitions, ordered remains poorly-understood. D10 have proposed an alter- from left to right in increasing energy of the upper state, native model – discussed in greater detail by Agúndez, and the GNM predictions are shown by the solid lines Cernicharo and Guélin (2010; hereafter ACG) – involving photochemistry in the inner envelope. As in the AC06 for various values of the inner radius, Rin, of the water emitting region. These predictions were “calibrated” by model, oxygen atoms are generated by photodissociation of 13CO and SiO by the ultraviolet ISRF, but – unlike 11 Cherchneff (2006) has proposed pulsation-driven shock waves the AC06 model – the UV radiation is assumed to pene- as the source of SiO in carbon-rich AGB stars, and – although the trate deeply into a clumpy CSE. In the D10/ACG model, models presented in that 2006 study failed to predict any enduring enhancement in the water abundance behind such shock waves – the oxygen atoms are liberated close to the star, where preliminary results from a more recent calculation do indeed sug- the temperature is sufficient to drive H2O production via gest (Cherchneff 2010) that the observed water vapor could be a sequence of two H-atom extraction reactions with ac- produced along with SiO by shocks in the inner envelope. tivation energy barriers: O(H2, H)OH(H2, H)H2O. Be- 4 Neufeld et al.

cause the mean visual extinction through the CSE is ∼ 100 mag, this scenario requires the existence of chan- nels of greatly reduced extinction through which the ultraviolet radiation can penetrate. The consequences of this model for the abundances and spatial distribution of the many other molecules detected in IRC+10216, many of which have been observed interferometrically, has yet to be fully investigated. One possible test of this model might involve a search 17 18 12 for H2 O or H2 O. Because the CO photodissociation rate is sharply reduced by self-shielding, D10 have em- phasized the importance of 13CO as a source of atomic O that can react to form H2O. The photodissociation rates for C17O andC18O would presumably be at least as large as that for 13CO. Thus, if 13CO, C17O and C18O were 16 18 the only suppliers of atomic oxygen, the H2 O/H2 O 16 17 13 18 and H2 O/H2 O ratios would approach the CO/C O and 13CO/C17O ratios respectively. Given the 13C/12C, 18O/16O and 17O/16O isotopic ratios determined by Ka- hane et al. (1992) and by Cernicharo, Guélin & Kahane (2000) for IRC+10216, the 13CO/C17O and 13CO/C18O ratios are respectively ∼ 18 and 28, each a factor of 45 (= 12C/13C) smaller than the elemental 16O/17O and 16O/18O ratios in the CSE. If SiO or 12CO are significant additional sources of atomic oxygen in the D10/ACG 16 17 picture, then a H2 O/H2 O ratio larger than 18 – or a 16 18 H2 O/H2 O ratio larger than 28 – might still be consistent with the model. Data obtained in a full HIFI spectral survey carried out toward IRC+10216 (Cernicharo et al. 2010) place upper 17 18 limits on the flux of the H2 O (552.021 GHz) and H2 O (547.676 GHz) 110 − 101 transitions. Comparing these 16 with the observed flux in the 556.936 GHz H2 O110 −101 transition, we determined that the spectral survey places Fig. 3.— Comparison of the measured water line fluxes (black crosses) with predictions of the GNM models for various values 3 σ lower limits on both the 556.936 GHz/552.021 GHz of the inner radius, Rin, of the water-emitting region. The ten and 556.936 GHz/547.676 GHz line ratios of ∼ 100. rotational transitions appear from left to right in order of increasing Taking account of optical depth effects, we find that energy for the upper state (except for the 1153 GHz and 1097 GHz these correspond to a lower limit of ∼ 200 on both the transitions which originate in the same upper state). 16 17 16 18 H2 O/H2 O and H2 O/H2 O abundance ratios. Given the elemental isotopic ratios 16O/18O ∼ 1260 and 16O/17O ∼ 840 (Kahane et al. 1992), these limits imply that the abundance of the minor isotopologues could only be enhanced (by means of isotope-selective photodissoci- 17 TABLE 2 ation of CO, for example) by at most a factor 4 (H2 O) Water line fluxes measured toward IRC+10216 18 or 6 (H2 O). Detailed modeling will be required to determine whether our non-detections of H17O and H18O a,b 2 2 Transition ν EU /k R TAdv Flux −1 −20 −2 are consistent with the water production mechanism pro- (GHz) (K) (K km s ) (10 W cm ) posed by D10/ACG. c 110 − 101 (o) 556.936 26.7 10.3 0.89 211 − 202 (p) 752.033 136.9 6.0 0.70 202 − 111 (p) 987.927 100.8 7.2 1.11 HIFI has been designed and built by a consortium of in- 312 − 303 (o) 1097.365 215.2 10.7 1.83 stitutes and university departments from across Europe, 111 − 000 (p) 1113.343 53.4 14.6 2.53 Canada and the United States under the leadership of 312 − 221 (o) 1153.127 215.2 3.4 0.62 321 − 312 (o) 1162.912 271.0 5.3 0.96 SRON Netherlands Institute for Space Research, Gronin- 221 − 212 (o) 1661.008 159.9 15.5 4.09 gen, The Netherlands and with major contributions 212 − 101 (o) 1669.905 80.1 35.1 9.56 from Germany, France and the US. Consortium mem- 303 − 212 (o) 1716.770 162.5 24.2 6.74 bers are: Canada: CSA, U. Waterloo; France: CESR, a For an unresolved source at the beam center LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, b We conservatively estimate the flux uncertainty to be less than MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, 15%. c The letters o and p indicate whether the transition is of ortho- Osservatorio Astrofisico di Arcetri- INAF; Netherlands: or of para-water SRON, TUD; Poland: CAMK, CBK; Spain: Observato- rio Astronómico Nacional (IGN), Centro de Astrobiologá (CSIC-INTA). Sweden: Chalmers University of Tech- Herschel/HIFIobservationsofIRC+10216 5 nology - MC2, RSS & GARD; Onsala Space Observa- JPL contract funded by the National Aeronautics and tory; Swedish National Space Board, Stockholm Univer- Space Administration. E.G-A is a Research Associate sity - Stockholm Observatory; Switzerland: ETH Zurich, at the Harvard-Smithsonian Center for Astrophysics. FHNW; USA: Caltech, JPL, NHSC. R. Sz. and M. Sch. acknowledge support from grant This research was performed, in part, through a N 203 581040.

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