We report the discovery of the rotational spectrum of CH+ in the Infrared Space Observatory Long Wavelength Spectrometer (LWS) spectrum of the planetary nebula NGC 7027. The identification relies on a 1996 reanalysis of the LWS spectrum by Liu et al. and on new LWS data. The strong line at 179.62 μm (coinciding with the 212-101 transition of water vapor) and the lines at 119.90 and 90.03 μm (reported as unidentified by Liu et al.), whose frequencies are in the harmonic relation 2 : 3:4, are shown to arise from the J = 2-1, 3-2, and 4-3 rotational transitions of CH+. This identification is strengthened by the new LWS spectra of NGC 7027, which clearly show the next two rotational lines of CH+ at 72.140 and 60.247 μm. This is the first time that the pure rotational spectrum of CH+ has been observed. This discovery opens the possibility of probing the densest and warmest zones of photodissociation regions. We derive a rotational temperature for the CH+ lines of 150 ± 20 K and a CH+/CO abundance ratio of 2-6 × 10-4

Barlow, MJ

Cernicharo, J

Cox, P

GonzalezAlfonso, E

Lim, T

Liu, XW

Swinyard, BM

English

UCL Discovery

DISCOVERY OF FAR-INFRARED PURE ROTATIONAL TRANSITIONS OF CH1 IN NGC 70271J. CERNICHAROConsejo Superior de Investigaciones Cientı́ficas, Instituto de Estructura de la Materia, Departamento Fı́sica Molecular, Serrano 123, E-28006 Madrid, SpainX.-W. LIUDepartment of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, England, UKE. GONZÁLEZ-ALFONSOUniversidad de Alcalá de Henares, Departamento Fı́sica, Campus Universitario, E-28871 Alcalá de Henares, Spain; and Observatorio Astronómico Nacional,E-28800, Alcalá de Henares, Madrid, SpainP. COXInstitut d’Astrophysique Spatiale, Bâtiment 121, Université de Paris XI, F-91405 Orsay Cedex, FranceM. J. BARLOWDepartment of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, England, UKT. LIMThe LWS Instrument—Dedicated Team, ISO Science Operations Centre, P.O. Box 50727, E-28080 Madrid, SpainANDB. M. SWINYARDRutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, England, UKReceived 1997 March 4; accepted 1997 April 17ABSTRACTWe report the discovery of the rotational spectrum of CH1 in the Infrared Space Observatory Long WavelengthSpectrometer (LWS) spectrum of the planetary nebula NGC 7027. The identification relies on a 1996 reanalysisof the LWS spectrum by Liu et al. and on new LWS data. The strong line at 179.62 mm (coinciding with the 212–101transition of water vapor) and the lines at 119.90 and 90.03 mm (reported as unidentified by Liu et al.), whosefrequencies are in the harmonic relation 2;3:4, are shown to arise from the J 5 2–1, 3–2, and 4–3 rotationaltransitions of CH1. This identification is strengthened by the new LWS spectra of NGC 7027, which clearly showthe next two rotational lines of CH1 at 72.140 and 60.247 mm. This is the first time that the pure rotationalspectrum of CH1 has been observed. This discovery opens the possibility of probing the densest and warmestzones of photodissociation regions. We derive a rotational temperature for the CH1 lines of 150 H 20 K and aCH1yCO abundance ratio of 2–6 3 1024.Subject headings: planetary nebulae: individual (NGC 7027)— infrared: ISM: lines and bands1. INTRODUCTIONThe spectrometers on board the Infrared Space Observatory(ISO) provide a unique opportunity to study important molec-ular species through transitions that are inaccessible from theground or from airborne platforms. This has been illustratedby the recent ISO results on the thermal lines of water vapor.Water vapor has been measured in high abundance [x(H2O) 31025] in star-forming regions and evolved stars using both theShort Wavelength Spectrometer (SWS) and the Long Wave-length Spectrometer (LWS) (e.g., van Dishoeck & Helmich1996; Barlow et al. 1996; Cernicharo et al. 1997; Cox et al.1997). The detection of water vapor (together with OH) wasalso reported by Liu et al. (1996, hereafter L96) in the LWSspectrum of the planetary nebula NGC 7027. This was anunexpected result because NGC 7027 is a carbon-rich source.In this Letter, we rediscuss the previous identification of thewater vapor emission lines in the LWS spectrum of NGC 7027.The 179.53 mm line together with two lines at 119.89 mm (nearthe OH 119.3 mm doublet) and 90.07 mm, both unidentified inL96, are now shown to arise from the pure rotational spectrumof CH1. We also present new ISO LWS data that show twofurther rotational lines of CH1 and confirm the previousdetection of OH and probably also of H2O.2. IDENTIFICATION OF CH1The identification of water vapor in NGC 7027 by L96 reliesmainly on the observation of a strong line at 179.56 mmcoinciding with the 179.53 mm H2O rotational 212–101 transi-tion. Two other features quoted as possible water lines in L96were flagged as “identification questionable.” In addition tothe H2O line, L96 also reported the detection of OH throughthe 119.3 mm fundamental transition. This line is blended withan equally strong feature at 119.89 mm, which remainedunidentified. Two other OH lines (at 79.2 and 84.5 mm) weretentatively reported.L96 also reported a strong unidentified line at 90.07 mm.Wenote that the wavelengths of the lines at 179.56, 119.89, and90.07 mm are nearly in the harmonic relation 2;3:4. Assumingthat the 179.56 mm line does not arise from water vapor butinstead that the three lines arise from another molecularcarrier, these lines could correspond to the rotational transi-tions J 5 2–1, 3–2, and 4–3 of a linear molecule. From the1 Based on observations with ISO, an ESA project with instruments fundedby ESAMember States (especially the Principal Investigator countries: France,Germany, the Netherlands, and the United Kingdom) and with the participa-tion of ISAS and NASA.THE ASTROPHYSICAL JOURNAL, 483 :L65–L68, 1997 July 1q 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.L65three lines we derive the following rotational constants for thecarrier: B 5 13.95 H 0.03 cm21 and D 5 0.0017 H 0.0005 cm21(1 s errors). The observed frequencies and inferred rotationalconstants indicate that the carrier must be diatomic and shouldcontain a hydrogen atom. The rotational constants are com-parable to those of CH (B 5 14.19 cm21, D 5 0.0014 cm21)and show an excellent agreement with those of the CH1 ion(B 5 13.9302 cm21 and D 5 0.0014 cm21; Carrington &Ramsay 1982).Prompted by the observation of three lines in harmonicrelation that are consistent with CH1, we have processed allthe ISO LWS01 grating spectra of NGC 7027 so far obtainedup to ISO revolution 377, in an effort to detect the J5 5–4 and6–5 transitions of CH1, which are predicted at wavelengths of72.15 and 60.26 mm and were not detected by L96. In total,there are nine separate observations, each covering a wave-length range from 43 to 195 mm, obtained during revolutions27, 39, 342, 349, 356, 363, and 377. All the data were calibratedusing Version 6 of the Off-Line Processing package (OLP).The observation from revolution 27 consists of three scans,with two 0.5 s integration ramps at each grating position,sampled at 1y4.5 of a spectral resolution element, the latterbeing 0.3 mm for the SW1–SW5 detectors (l , 93 mm) and0.6 mm for the LW1–LW5 detectors (l . 84 mm). Each of thefive observations obtained between revolutions 342 and 377consists of six scans with one 0.5 s ramp at each gratingposition, sampled at 1y9 of a resolution element. Threeobservations were obtained in revolution 39. Each consists offive scans with two 0.5 s ramps at each grating position,sampled at 1y15 of a resolution element. The total on-targetintegration time was 23,930 s. The LWS has a circular apertureof diameter 850, significantly larger than the ionized or neutralregions of NGC 7027. The analysis presented in L96 was basedon the three observations carried out in revolution 39, pro-cessed with Version 5 of the OLP. Although the current datahave a total on-target time that is only 65% larger than thatanalyzed by L96, the final co-added spectrum has a muchbetter SyN, by more than a factor of 2 in some wavelengthregions, thanks to the much-improved instrumental filterprofiles derived from additional observations of Uranus andthe implementation of an algorithm that corrects for detectorsensitivity drifts. The fluxes derived from the individual inde-pendent observations are in good agreement, and we estimatean accuracy of better than 20% for the absolute flux calibra-tion.Figure 1 shows the five CH1 lines detected in our newspectra, including the two lines at 72.15 and 60.26 mm, whichare clearly detected in the present data. These five linesdefinitely establish the presence of CH1 in NGC 7027. This isthe first time that CH1 is seen through its pure rotationalspectrum. CH1, a common molecule in the diffuse interstellarmedium, has been detected in the optical spectrum of only twoevolved carbon-rich stars: in emission in the Red Rectangle,and in absorption in HD 213985 (Balm & Jura 1992; Hall et al.1992; Waelkens et al. 1992; Bakker et al. 1997). Given thecarbon-rich nature of NGC 7027 and the strong radiation fieldFIG. 1.—CH1 lines observed in NGC 7027 (the continuum has been subtracted from the data). The solid lines are Gaussian line profile fits to the observed features(for blends, contributions from individual components are shown as dashed lines). The J 5 2–1 line at 179.62 mm was previously assigned to H2O. For the J 5 4–3line, spectra from two different detectors—LW1 (bottom panel; the sharp rise at short wavelengths is due to the strong [O III] 88.36 mm line) and SW5 (upperpanel)—are shown. The J 5 4–3 line is blended with the J 5 29–28 line of CO at 90.16 mm, which could contribute with an intensity less than 10219 W cm22—theJ 5 27–26 and J 5 26–25 lines of CO can be seen in the bottom left-hand panel of Fig. 2 with intensities of 110219 W cm22. The lines at 59.5 and 60.7 mm in thetop left-hand panel are unknown.L66 CERNICHARO ET AL. Vol. 483in its inner regions, it is thus not too surprising to find CH1 inthis prototypical planetary nebula. In fact, soon after thedetection of H2 in NGC 7027 (Treffers et al. 1978), Black(1978) predicted that CH1, OH, and HeH1 might havedetectable abundances in this nebula.In addition, five OH lines are present (Fig. 2), confirmingthe previous detection of OH by L96. The CH1 and OH linefluxes are given in Table 1. Apart from the 179.6 mm line, twomarginal features at 174.6 and 180.5 mm were tentativelyassigned in L96 as possible water vapor lines. The 180.5 mmline is now shown to arise from CH (see Liu et al. 1997).However, the new data confirm the 174.6 mm line [F5 (1.44 H0.17) 3 10220 W cm22] and, in addition, show two lines at108.05 H 0.1 mm [F 5 (5.9 H 0.6) 3 10220 W cm22] and75.39 H 0.03 mm [F 5 (6.6 H 2.5) 3 10220 W cm22], coinci-dent in wavelength with the 221–110 (108.08 mm) and the 321–212(75.38 mm) lines of ortho-H2O. Taking into account thedensity of lines and the lack of emission in the lines ofpara-water, the definitive assignment of these lines to watervapor will require high spectral resolution observations. No13CH1 lines were detected in current spectrum. Finally, Figure1 shows that the CH rotational line at 180.6 mm is muchweaker than the nearby CH1 J5 2–1 line. A comparison of theobserved intensities yields a CH1yCH abundance ratio of35–10 (see also Liu et al. 1997).3. DISCUSSIONThe line fluxes of Table 1 have been used to derive therotational temperature, Trot, of CH1. From a standard rota-tional analysis, in which we assume a uniform rotationaltemperature for all the rotational levels of CH1, we obtainTrot 3 150 H 20 K (see Fig. 3) and a CH1 total column densityof 2.5 3 1047. It is worth noting that similar CH1 rotationaltemperatures were derived from optical observations in theRed Rectangle and in HD 213985 by Bakker et al. (1997), i.e.,202 and 155 K, respectively. A similar rotational analysis forthe CO line fluxes given by L96 yields a rotational temperaturefor CO of 240 K and a total number of CO molecules 31051.Assuming that the molecular emission of both species arisesfrom the same region, we obtain a CH1yCO abundance ratioof 2 3 1024. Assuming that the size of the photodissociationregion (PDR) in NGC 7027 is about 60, we derive a columndensity for CH1 of 8 3 1013 cm22.In order to estimate the physical conditions needed toFIG. 2.—OH lines observed in NGC 7027 (after subtraction of the contin-uum). The OH 119.23, 119.44 mm lines, blending with the CH1 119.86 mm line,are shown in the lower middle panel of Fig. 1. Due to the L-doubling, each ofthe OH transitions consists of two components, and two Gaussians of equal linewidth and intensity, having a central wavelength difference given by theirlaboratory values, were used to fit the line profiles, except for the 98.73 mmtransition, which has a negligible wavelength splitting. Two lines in the bottomleft-hand panel correspond to the J 5 27–26 and J 5 26–25 transitions of CO(at 96.77 and 100.46 mm, respectively), while the line at 97.6 mm in the samepanel is unknown.TABLE 1CH1 AND OH LINE FLUXES FROM NGC 7027lobs(mm) Fluxalvac(mm) TransitionCH1179.60 H 0.04 . . . . . . . . . . 1.51 H 0.05 179.61 J 5 2–1119.88 H 0.06 . . . . . . . . . . 2.18 H 0.17 119.86 J 5 3–290.09 H 0.06 . . . . . . . . . . 2.64 H 0.28 90.02 J 5 4–3b90.02 H 0.03 . . . . . . . . . . 2.00 H 0.22 90.02 J 5 4–3c72.17 H 0.03 . . . . . . . . . . 2.50 H 0.41 72.15 J 5 5–460.27 H 0.03 . . . . . . . . . . 2.41 H 0.33 60.26 J 5 6–5OH119.36 H 0.06 . . . . . . . . . . 1.53 H 0.11 119.34 2P3y2 J 5 5y2–3y2d84.50 H 0.02 . . . . . . . . . . 1.98 H 0.15 84.51 2P3y2 J 5 7y2–5y2,2.4 (3 s) 65.20 2P3y2 J 5 9y2–7y2163.23 H 0.08 . . . . . . . . . . 0.62 H 0.07 163.26 2P1y2 J 5 3y2–1y2e98.85 H 0.10 . . . . . . . . . . 0.69 H 0.12 98.73 2P1y2 J 5 5y2–3y279.12 H 0.04 . . . . . . . . . . 1.76 H 0.23 79.15 2P1y223y2 J 5 1y2–3y2a In 10219 W cm22.b LW1 detector.c SW5 detector.d Blended with CH1 J 5 3–2.e Blended with CO J 5 16–15.FIG. 3.—Rotational temperature analysis for CH1 in NGC 7027. Theordinates represent the natural logarithm of the observed fluxes (in W cm22)divided by xi [ (16 3 1027p3n4m2Jup)y(3c3), where the symbols have the usualmeaning and are in cgs units. The abscissa corresponds to the energy of theupper level of each rotational transition. The solid line represents the fit to thedata assuming an uniform rotational temperature, while the dashed linerepresents the fluxes derived from the LVG models quoted in the text.No. 1, 1997 FAR-IR TRANSITIONS OF CH1 IN NGC 7027 L67produce the observed CH1 fluxes, we have made large velocitygradient (LVG) calculations that allow us to estimate the H2volume density and the CH1 column density. Collisional crosssections for CH1 with H2 are not available, so we have usedthose of N2H1 (Green 1975). The values of the collisional crosssections for high kinetic temperatures were obtained from theN2H1 data available for 10–40 K by extrapolating the ratecoefficients as the root square of the kinetic temperature.Typical estimates for the excitation rate coefficients are110210cm3 s21. With a dipole moment of 1.68 D (e.g., Follmeg,Rosmus, & Werner 1987), the relevant critical H2 densities forthe CH1 J5 1 level are on the order of1107 cm23. Even largerdensities could be necessary to pump the high J levels (the J 56–5 rotational transition has an Einstein coefficient for spon-taneous emission of 1.9 s21) by collisions. For a distance of 700pc (Hajian, Terzian, & Bignell 1993) and adopting a line widthof 30 km s21, the observed CH1 fluxes can be roughly explainedwith the following parameters: n(H2) 5 2–5 3 107 cm23, TK 5300–500 K, and a CH1 column density of 2.5 3 1014 cm22,which implies a CH1yCO abundance ratio of 36 3 1024. Thedashed line in Figure 3 shows the computed CH1 line intensityfluxes for n(H2) 5 5 3 107 cm23 and TK 5 300 K. An im-proved fit to the data could be obtained with a model wherethe low J transitions are excited in a region of moderate kinetictemperature and the high J levels are excited in a region withtemperatures around 1000 K. But in any case, the density mustbe very high in order to explain the population of the J 5 4, 5,and 6 rotational levels of CH1. The rotational lines of CH1could also be collisionally excited by impact of H and e2. Forcollisions with e2, g[J 1 13 J] is approximately a few 3 1027cm3 s21 for Te between 100 and 1000 K (Dickinson & Flower1981). Hence, electrons may compete with H2 in populating theCH1 rotational levels if neyn(H)1 1023, a condition reached onlyin the warm PDR, where all the carbon is in the form of C1. Inthis region, the H2 densities quoted above could be lowered by afactor of 2–3 if collisions with e2 are taken into account. Inaddition, we must consider the possibility of excitation of theCH1 rotational levels through absorption of photons in the 1P–1Stransition, followed by decay back to the ground state (in the RedRectangle, the CH1 1P–1S lines are observed in emission).Observations at optical wavelengths of CH1 in NGC 7027 mightprovide important insights into the excitationmechanism of CH1.If such a pumping mechanism plays a role in the excitation of therotational levels of CH1, then the densities necessary to explainthe emission would be considerably lower.The discrepancy between the CH1yCO abundance ratioderived from the LVG calculations and that derived from therotational analysis of the CH1 lines arises from two facts: first,the assumptions in the rotational analysis that the CH1 linesare optically thin (from the LVG calculations, the opacities areestimated to be 31–5) and have a uniform rotational temper-ature and, second, from the uncertainties in the collisionalcross sections used in the LVG models.The formation of CH1 via the reaction C1 1 H2 3 CH1 1H, which has an activation energy of 14000 K, requires a gasat high kinetic temperature (31000 K). However, the presentobservations suggest that, in NGC 7027, the temperature ofthe region where CH1 is emitted is much lower. In theinterface region between the cold neutral gas and the fullyionized zone, a significant fraction of the H2 population will bevibrationally excited, carrying an important part of the energyneeded to activate the above reaction (Sternberg & Dalgarno1995). A similar situation has been proposed by Jura, Turner,& Balm (1997) to explain the CH1 emission in the RedRectangle. For densities 106 , n(H2) , 107 cm23, the forma-tion rate of CH1 could be high enough to produce columndensities similar to those derived in this work, i.e., 0.8–2 31014 cm22, as predicted in the models of Sternberg & Dalgarno(1995). However, if the density becomes too high, CH1 mightnot survive because it will be rapidly destroyed by the reactionCH1 1 H23 CH12 1 H. As in the diffuse interstellar medium,CH1 could be formed efficiently in a region of moderatetemperature in the presence of shocks with velocities of a fewkm s21. Low-velocity shocks have been observed in NGC 7027(Jaminet et al. 1991) and should thus be taken into consider-ation in any detailed study of the formation and excitation ofCH1 in this planetary nebula.The CH1 J 5 2–1 line can contaminate the LWS observa-tions of water vapor at 179.5 mm. The water line is seen inabsorption toward several molecular clouds (Cernicharo et al.1997; Cox et al. 1997) and arises from the ground level, whilethe possible absorption by CH1 will arise from the J 5 1 level,which is 40 K above the ground. Contamination of the H2O179.5 mm absorption line by CH1 absorption is thus unlikely.However, the interpretation of the 179.6 mm line in emissionin PDRs will require high spectral resolution data or the studyof other transitions to disentangle the contributions of theH2O and CH1 lines.REFERENCESBakker, E. J., van Dishoeck, E. F., Waters, L. B. F. M., & Schoenmaker, T.1997, A&A, in pressBalm, S. P., & Jura, M. 1992, A&A, 261, L25Barlow, M. J., et al. 1996, A&A, 315, L241Black, J. H. 1978, ApJ, 222, 125Carrington, A., & Ramsay, D. A. 1982, Physica Scripta, 25, 272Cernicharo, J., et al. 1997, A&A, in pressCox, P., et al. 1997, A&A, in pressDickinson, A. S., & Flower, D. R. 1981, MNRAS, 196, 297Follmeg, B., Rosmus, P., & Werner, H.-J. 1987, Chem. Phys. Lett., 136, 562Green, S. 1975, ApJ, 201, 366Hajian, A. R., Terzian, Y., & Bignell, C. 1993, AJ, 106, 1965Hall, D. I., Miles, J. R., Sarre, P. J., & Fossey, S. J. 1992, Nature, 358, 629Jaminet, P. A., Danchi, W. C., Sutton, E. C., Russell, A. P. G., Sandell, G.,Bieging, J. H., & Wilner, D. 1991, ApJ, 380, 461Jura, M., Turner, J., & Balm, S. P. 1997, ApJ, 474, 741Liu, X.-W., et al. 1996, A&A, 315, L257 (L96)———. 1997, ApJ, submittedSternberg, A., & Dalgarno, A. 1995, ApJS, 99, 565Treffers, R. R., Fink, U., Larson, H. P., & Gautier, T. N., III. 1976, ApJ, 209,793van Dishoeck, E. F., & Helmich, F. P. 1996, A&A, 315, L177Waelkens, C., van Winckel, H., Trams, N. R., & Waters, L. B. F. M. 1992,A&A, 256, L15L68 CERNICHARO ET AL.

Discovery of far-infrared pure rotational transitions of CH+ in NGC 7027

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Discovery of far-infrared pure rotational transitions of CH+ in NGC 7027

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