Barlow, MJ

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ISO LWS OBSERVATIONS OF EVOLVED STARS AND NEBULAEM. J. BARLOWDept. of Physics & Astronomy, University College London,Gower Street, London WC1E 6BT, UK1. IntroductionThis review reports, on behalf of the LWS Consortium1, some of the spec-troscopic results on evolved stars and nebulae that have been obtained atfar-infrared wavelengths by the Infrared Space Observatory (ISO) since itslaunch in November 1995. The results reported have been obtained mainlyusing ISO’s Long Wavelength Spectrometer (LWS; Clegg et al. 1996), butfor some objects complementary spectra have also been obtained with theShort Wavelength Spectrometer (SWS; de Graauw et al. 1996). In thisreview I present some of the results that have been obtained (a) for oxygen-rich cool stars and (b) for carbon stars and carbon-rich post-AGB objectsand planetary nebulae. Highlights from the first area include the first de-tection of the far-infrared rotational lines of water vapour, the dominantcoolant in their outflows, and the detailed confirmation of the proposedradiative pump mechanism for circumstellar OH masers. Highlights fromthe second area include the detection of a rich HCN and CO far-infraredline spectrum from the carbon star IRC+10o216, with HCN found to be thedominant coolant, the detection of very strong shock-excited CO rotationallines from the post-AGB object AFGL 2688, and the first detection of thefar-infared rotational lines of CH+ (from the the PDR of the carbon-richplanetary nebula NGC 7027), as well as the first detection in emission ofthe far-IR rotational lines of CH (also from NGC 7027).1presented on behalf of members of the ISO LWS Post Main Sequence SpecialistAstronomy Group: J.-P. Baluteau, M.J. Barlow, J. Cernicharo, M. Cohen, R.J. Cohen, P.Cox, R.J. Emery, E. Gonzalez-Alfonso, C.Gry, T.Lim, X-W. Liu, A.Omont, D.Péquignot,N-Q.Rieu, B. Swinyard, R.J. Sylvester and Truong-Bach2 M. J. BARLOWFigure 1. The best-fitting H2O line spectrum model is plotted below the continuum–subtracted LWS spectrum of W Hya, with the dominant rotational transitions labelled.From Barlow et al. (1996).2. Oxygen-rich cool stars2.1. WATER VAPOUR LINES FROM COOL STARSISO has provided the first extra-solar system detection of the important far-infrared rotational transitions of water molecules. These lines had not beendetected previously, even from airborne altitudes, because of the enormousopacity of the Earth’s atmosphere in the same lines. Their detection by ISOhas confirmed predictions (e.g. Deguchi & Rieu 1990; Chen & Neufeld 1995)of the importance of water vapour cooling lines in a variety of astrophysicalenvironments. One of the earliest LWS targets, the oxygen-rich AGB starW Hya, was found to exhibit more than 40 detectable water vapour lines inthe 43–180 µm range (Barlow et al. 1996), even at the relatively low LWSgrating-mode resolution (0.6 µm FWHM in first order, longwards of 90 µm;and 0.3 µm in second order, longwards of 43 µm). Fig. 1 shows portionsof the continuum-subtracted LWS spectrum of W Hya, along with theirbest-fitting model spectrum. Large Velocity Gradient (LVG) Sobolev escapeprobability models were constructed for the wind of this M8e-M9e semi-ISO OBSERVATIONS OF EVOLVED STARS AND NEBULAE 3Figure 2. The rectified SWS02 spectrum of IRC+10o420’s OH 34.6-µm line, which isresolved into its doublet components (rest wavelengths 34.6035 and 34.6294 µm). FromSylvester et al. (1997).regular variable AGB star in order to try to reproduce the observed waterline fluxes. The excitation of H2O is dominated by collisions, primarily withH2. The calculated spectrum is found to be sensitive to the adopted massloss rate and to the H2O/H2 abundance ratio, and only weakly dependenton the wind velocity field and temperature structure. All of the observedwater lines are found to be very optically thick, explaining the ease withwhich transitions from the higher pure rotational levels were detected. Afirst fit with a constant H2O/H2 ratio, while matching the fluxes of the low-J transitions (those with excitation energies less than 200 cm−1), tended tosystematically underestimate the the fluxes of the higher-J H2O lines. Thesehigher-J transitions arise mainly from the innermost regions of the wind,and an improved fit was obtained by adopting a higher H2O/H2 numberratio of 8×10−4 within 4 stellar radii and a lower ratio (3×10−4) beyondthat point. The spectrum predicted by this model is shown in Fig. 1. Itcorresponds to a mass loss rate of 6×10−7 M⊙ yr−1 for a distance of 130 pc,and has a Goldreich & Scoville (1976)-type wind temperature distribution(a constant 1300 K inside 3 R∗; but decreasing as r−0.82 beyond there).2.2. THE OH MASER PUMP MECHANISM FOR IRC+10O420The derived decrease in the abundance of water in the outer parts ofthe envelope of W Hya is consistent with photochemical models, whichpredict that H2O should be photodissociated to OH at larger radii by the4 M. J. BARLOWFigure 3. The four detected OH emission lines in the LWS01 spectrum of IRC+10o420.In each plot the underlying continuum has been subtracted. From Sylvester et al. (1997).interstellar radiation field (e.g. Huggins & Glassgold 1982). The populationinversions which lead to circumstellar hydroxyl (OH) maser emission fromcool stars have long been thought to be radiatively pumped (Elitzur, Gol-dreich & Scoville 1976; hereafter EGS). However, the proposed pumpingmechanism had not been directly observed before now, because the OHrotational lines involved in this pumping scheme all occur at far-infraredwavelengths, between 30 and 200 µm, unobservable from the ground butaccessible to ISO’s spectrometers.OH maser sources exhibit emission from transitions within the J = 3/22Π3/2 ground state. EGS proposed that the upper sublevel of the 1612 MHztransition is inverted by 34.6-µm and 53.3-µm photons, emitted by dust inthe wind, which are absorbed from the ground state up to excited 2Π1/2rotational levels, followed by a cascade back to the ground state, involvingthe emission of 98.7-, 163.2- and 79.2-µm photons. If the transitions to andfrom the ground state (34.6, 53.3 and 79.2 µm) are optically thick, whilethose between the excited levels are optically thin, this scheme causes aninversion of the F=2/F=1 levels of the ground state, leading to 1612 MHzmaser action. The scheme also requires that there be sufficient 34.6- and/or53.3-µm flux available: this is provided by continuum emission from dustin the outflow.ISO OBSERVATIONS OF EVOLVED STARS AND NEBULAE 5We have observed the infrared OH pump lines from IRC+10o420. Thisstar is a massive F8Ia hypergiant and a Type II OH maser source, with the1612 MHz line stronger than the main lines. It is one of the warmest knownOH maser stars. Nedoluha & Bowers (1992) modelled the shell of maser-emitting regions as an oblate spheroid with a semi-major axis of 8700 AU,with the near side of the equatorial plane tilted 25o above the line of sightand the outflow velocity increasing from 28 km s−1 in the equatorial plane to63 km s−1 along the polar axis. We have obtained SWS grating observationsof the 34.6 µm pumping line, together with an LWS grating spectrum of thefull 43–197 µm region containing the predicted cascade lines that shouldresult from absorption of the dust continuum flux by the 34.6 µm line.The OH 34.6-µm absorption doublet (Fig. 2) and the emission lines inthe resulting cascade (98.7, 163.2 and 79.2 µm; see Fig. 3) are all clearlydetected in the SWS and LWS spectra (Sylvester et al. 1997), confirmingthat the EGS radiative pumping cycle does indeed operate. Additionally,the 119.3 µm OH doublet is seen in emission (Fig. 3). This ground-statetransition may well be collisionally excited: it has the largest cross-sectionamongst the far-IR OH transitions for excitation by H2 collisions (Offer &van Dishoeck 1992).The observed photon absorption rate in the 34.6-µm transition is 83×104m−2 s−1, while the integrated photon flux in the 1612-MHz maser line mea-sured by the Nancay radiotelescope at the time of the ISO observationswas 6.2 photons m−2 s−1. The corresponding pumping efficiency is there-fore 7.5%. Since the 1612 MHz maser can also be pumped by radiativeabsorption by the 53.3 µm transition, as well as by radiative absorptionand collisional excitation in the 79.2 µm line (see Fig. 1 of EGS), a moreappropriate estimate for the pumping efficiency comes from ratioing theemission rate in the 1612 MHz maser transition to that in the final down-ward pumping cascade line at 79.2 µm. The latter rate is measured to be(200±30)×104 m−2 s−1, corresponding to an overall pumping efficiency forthe 1612 MHz transition of 3%, in excellent agreement with the predictionsof EGS.3. Carbon-rich evolved stars and nebulae3.1. IRC+10O216 AND GL 2688The archetypal IR-bright carbon star IRC+10o216 was only visible to ISOfor a relatively brief period, in the first half of 1996, and was observed withthe LWS right at the end of this period. Its far-IR spectrum turned out tobe extremely rich in emission lines and these were identified by Cernicharoet al. (1996) with lines of CO and HCN (see the upper spectrum in Fig. 4).The CO pure rotational line spectrum is strongly present, as are the pure6 M. J. BARLOWFigure 4. The continuum-subtracted 90–190-µm LWS01 spectra of IRC+10o216 (upper)and AFGL 2688 (lower). The upward-pointing arrows at the bottom of the plot indicatethe positions of pure rotational transitions of 12CO (longer arrows) and 13CO (shorterarrows). The uppermost row of downward-pointing arrows indicates the wavelengths ofpure rotational transitions of H12CN and H13CN. The second and third rows of arrowsfrom the top indicate the wavelengths of HCN rotational transitions from the ν2=1,2 andν1,3=1 bending vibrational modes.rotational lines of HCN, but in addition a forest of strong, blended emissionfeatures is present. Cernicharo et al. (1996) identified these new featureswith rotational transitions from bending vibrational excited modes of HCN.From modelling of the wind they derived a HCN/CO number ratio of 0.1.Although HCN was derived to be the less abundant of the two molecules,its larger dipole moment leads to it being the dominant coolant in the wind(0.44 L⊙ in the HCN lines versus 0.28 L⊙ in the CO lines).Deep LWS grating spectra (16000 secs) of the carbon-rich post-AGB ob-ject AFGL 2688 (the ‘Cygnus Egg’; F5I central star) were acquired duringthe early Performance Verification phase of ISO (Cox et al. 1996). The 90–190 µm continuum-subtracted spectrum of AFGL 2688 is shown in Fig. 4(lower spectrum). It is dominated by the pure rotational line spectrum ofCO (three high-J CO lines had earlier been detected with the KAO by Just-tanont et al. 1997). At the longest wavelengths, several high-J rotationalISO OBSERVATIONS OF EVOLVED STARS AND NEBULAE 7lines of HCN were detected, but they are significantly weaker than the COlines (by an order of magnitude compared to the case of IRC+10o216; up-per plot in Fig. 4). The [O i] 63 and 146-µm lines and the [C ii] 158-µmline were not detected, although these lines become the dominant lines fromthe neutral zone once photodissociation of the molecular envelope beginsduring the subsequent PN phase. A number of other, relatively strong andbroad, emission features are apparent in the spectrum of AFGL 2688 plot-ted in Fig. 4 – these features do not appear to be due to HCN and remainto be identified.Cox et al. considered two models for the excitation of the CO lines. Onewas a standard AGB-wind type model, with a mass loss rate of 1.7×10−4M⊙ yr−1, a CO/H2 ratio of 6×10−4, and an r−0.65 temperature distributionwith a starting temperature of 235 K at a radius r of 2×1016 cm, whichcorresponds to the dust temperature at that radius previously derived frommid-infrared data. Although capable of matching the relative intensities ofthe CO lines, this model was unable to match the observed CO line fluxlevels. Since mm-wave and near-IR observations have provided evidence forhigh-velocity shock-heated molecular gas (CO and H2) in AFGL 2688, Coxet al. therefore constructed a shock-excitation model for the CO emission.The LWS data were found to be well-fit by 400 K molecular gas with atotal mass of 0.005 M⊙.3.2. THE CARBON-RICH PLANETARY NEBULA NGC 7027The carbon-rich planetary nebula NGC 7027 lived up to its reputation ofalways producing suprises when it is observed with new infrared instru-mentation. Liu et al. (1996) presented a deep (14500 sec) LWS gratingspectrum of NGC 7027 acquired during ISO’s PV-phase. In addition tothe expected ionic fine structure emission lines from the ionized zone, thespectrum showed strong fine structure lines of [O i] and [C ii], arising fromabout 0.15 M⊙ of dissociated molecular gas in the photodissociation region(PDR) outside the ionized region; and a rich CO rotational line spectrumthat could be well fitted by emission from 1000 K (PDR) gas with a H2 den-sity of 105 cm−3. The major suprise though was the detection of emissionin the ground-state 119.3-µm OH doublet, with the tentative detection ofemission in the ground-state OH 79.2 and 84.5-µm lines, and the apparentdetection of strong emission in the ground-state rotational line of o-H2Oat 179.53 µm. The OH and H2O molecules were interpreted by Liu et al.(1996) as having formed in the PDR of this carbon-rich nebula followingthe dissociation of CO into atomic C and O and the reaction of the lat-ter with H2 molecules to yield (succesively) OH and H2O, with the 1000 Ktemperature of the PDR enabling the activation energies for these reactions8 M. J. BARLOWFigure 5. CH+ rotational lines observed in NGC7027. The solid lines are gaussianline profile fits to the observed features (for the case of blends, the contributions fromindividual components are shown as dashed lines). The J=2-1 line of CH+ at 179.62 µmhad been previously assigned to H2O. For the J=4-3 line CH+ line, spectra from twodifferent detectors - LW1 (bottom panel) – where the sharp rise at short wavelengths isdue to the strong [O iii] 88.36-µm line – and SW5 (upper panel) - are shown. The J=4-3line is blended with the J=29-28 line of CO at 90.16 µm. From Cernicharo et al. (1997).to be overcome.The identification of the line at 179.6 µm with o-H2O was overturnedby Cernicharo et al. (1997), who showed that it was due to the J=2–1rotational line of CH+ at 179.62 µm. This identification was supportedby the simultaneous identification of the J=3–2, 4–3, 5–4 and 6–5 lines ofCH+ in the LWS spectrum, whose S/N had been significantly increasedby the availability of a new calibration and the addition of several moreroutine-phase observations, yielding a total integration time of 24000 secs.The newly identified CH+ lines are illustrated in Fig. 5. The J=3–2 line ofCH+ at 119.9 µm (middle-bottom panel of Fig. 5) is clearly blended withthe 119.3-µm doublet of OH. In addition, the new spectrum provided cleardetections of the OH 79.2 and 84.5-µm lines. The presence of H2O mustbe considered doubtful at the moment, although three lines of o-H2O, at174.6, 108.05 and 75.4-µm are tentatively identified. The formation of CH+,ISO OBSERVATIONS OF EVOLVED STARS AND NEBULAE 9via the reaction C+ + H2 → CH+ + H, requires an activation energy of4000 K, which can be supplied by the warm PDR.The formation of CH+ and OH in the PDR around NGC 7027 hadbeen predicted by Black (1978), who also predicted that HeH+ might beformed in sufficient quantities to be detectable. The HeH+ molecule is ofinterest as it is one of the few molecules predicted to be capable of formingin the early post-recombination phase of the Universe. It has never beendetected astrophysically, but NGC 7027 has been considered to be one ofthe most promising sites for its detection, due to the high density andhigh degree of ionization of the nebula (HeH+ is, unusually, predicted to beformed within the ionized zone, via the reaction He+ + H → HeH+ + γ; seeCecchi-Pestellini & Dalgarno 1993, hereafter CD93). HeH+ has its J=1–0fundamental pure rotational line at 149.14 µm. Liu et al. (1997) analysedthe deep LWS spectrum of NGC 7027 in a search for this line. They detectedan emission feature at about the correct wavelength and with a flux inagreement with that predicted by CD93. However, the CH radical has itsJ=3/2–1/2 fundamental pure rotational doublet at 149.09 and 149.39µm.The presence in the NGC 7027 spectrum of the CH J=5/2–3/2 doubletat 180.48 and 180.93µm, with a flux consistent with the observed featureat 149.18 µm being entirely due to CH, led Liu et al. (1997) to concludethat the HeH+ J=2–1 line was not detected and to assign an upper limitto its flux that was three times lower than predicted by CD93. The CH lineidentifications represent the first detection of the pure rotational emissionline spectrum of CH from an astrophysical object. Liu et al. (1997) deriveda CH/CO abundance ratio of ∼ 0.06 and a CH/CH+ ratio of ∼ 0.2.ReferencesBarlow M. J., et al., 1996, A&A, 315, L241Black J. 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ISO LWS observations of evolved stars and nebulae

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ISO LWS observations of evolved stars and nebulae

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