The Crab Nebula is marginally observable with the IUE. Observations of the optically brightest filamentary regions, made with IUE in August 1979, show the C IV x1549, He II x1640, and C III] x1909 emission lines. The intensities of these lines have been compared with visual-wavelength data. It appears that carbon is not overabundant in the Crab; carbon/- oxygen is approximately  normal   oxygen is slightly scarcer than  normal  as a fraction of the total mass.https://digitalcommons.chapman.edu/scs_books/1005/thumbnail.jp

Davidson, K.

Gull, T. R.

Kafatos, Menas

Maran, S. P.

Stecher, T. P.

Trimble, V. L.

English

Chapman University Digital Commons

Chapman UniversityChapman University Digital CommonsMathematics, Physics, and Computer ScienceFaculty Books and Book Chapters Mathematics, Physics, and Computer Science1981Preliminary Report on IUE Spectra of Crab NebulaK. DavidsonT. R. GullS. P. MaranT. P. StecherMenas KafatosChapman University, kafatos@chapman.eduSee next page for additional authorsFollow this and additional works at: http://digitalcommons.chapman.edu/scs_booksPart of the Instrumentation Commons, and the Stars, Interstellar Medium and the GalaxyCommonsThis Book is brought to you for free and open access by the Mathematics, Physics, and Computer Science at Chapman University Digital Commons. Ithas been accepted for inclusion in Mathematics, Physics, and Computer Science Faculty Books and Book Chapters by an authorized administrator ofChapman University Digital Commons. For more information, please contact laughtin@chapman.edu.Recommended CitationDavidson, K., Gull, T.R., Maran, S.P., Stecher, T.P., Kafatos, M., Trimble, V.L. (1981) Preliminary report on IUE Spectra of CrabNebula. In R.D. Chapman (Ed.), The Universe at Ultraviolet Wavelengths: the First Two Years of International Ultraviolet Explorer.Proceedings of a symposium held at NASA Goddard Space Flight Center, Greenbelt, MD, May 7-9, 1980 (pp. 693-700).AuthorsK. Davidson, T. R. Gull, S. P. Maran, T. P. Stecher, Menas Kafatos, and V. L. TrimbleThis book is available at Chapman University Digital Commons: http://digitalcommons.chapman.edu/scs_books/10NASA Conference Publication 21 71 NAsA-cp-2fl I 19810017357 The Universe ofe$nteixational I I I [ *  - i ? - 1  by f> t $  $ 4 "  A r : !  [ J : : , k  2 b & . L 2 r t  . I .  t f ' 3 ,  ,,,- jb!ili & '  , a T ?  \ h\fl;, 1,  1 5  8c l i  ( I l'l-r*: I +  I ' J ~ P .  ' i  i ' ' ~ , I ' < i ' I  w Proceedings of a symposium held at NASA Goddard Space Flight ~en'ter Greenbelt, Maryland May 7-9, 1980 PRELIMINARY REPORTON IUE SPECTRAOF THE CRAB NEBULAK. DavidsonDepartment of AstronomyUniversity of MinnesotaT. R. Gull, S. P. Maran, T. P. StecherLaboratory for Astronomy and Solar PhysicsNASA-Goddard Space Flight CenterM. KafatosDepartment of PhysicsGeorge Mason UniversityV. L. TrimbleDepartment of PhysicsUniversity of California, IrvineABSTRACTThe Crab Nebula is marginally observable with the IUE. Observationsof the optically brightest filamentary regions, made with IUE in August1979, show the C IV x1549, He II x1640, and C III] x1909 emisslon lines.The intensities of these lines have been compared with visual-wavelengthdata. It appears that carbon is not overabundant in the Crab; carbon/-oxygen is approximately "normal" _ oxygen is slightly scarcer than"normal" as a fraction of the total mass.INTRODUCTIONThe Crab Nebula is the only young supernova remnant (i.e., onecomposed of supernova ejecta rather--r_-FTan of swept-up interstellar matter)which can be detected with the IUE. In August 1979, several IUE shiftswere devoted to surveying a few spots in the nebula, using the large(IOX20 arc sec) apertures, at low dispersion. The brightest spot found inthis preliminary work will be further observed in 1980. The results ofthe 1979 work, described here, are preliminary. It appears that only nearthe brightest spot does the nebula produce ultraviolet emission linefluxes that are bright enough to be measured with the IUE. This is aregion of superimposed "filaments" southwest of the pulsar, near the"bright filament" of Davidson (ref. I) and "position 2" of Miller (ref. 2).OBSERVATIONSThe present results are based on two exposures with the short-wavespectrograph, with integration times of 260 and 420 minutes, respectively(Figures i and 2). At the bright region that we observed, the C IV x1549,He II x1640, and C III] x1909 lines appear to have been detected;radiation hits also affected the data shown in the Figures.693The He II _1640 brightness is essential, because it allows comparisonwith relative line intensities in the visual wavelength region, thanks tothe theoretical intrinsic relation (ref. 3) between the He IIrecombination lines,I(_1640)/I(_4686) _ 7.No observations reported in refs. 1 and 2 were taken at positionsthat coincide exactly with the location in the nebula that was observedwith IUE. However, for the purpose of the present preliminary analysis,we may average these previous results for the bright filamentary region.The lines that we take into account here are listed in Table I, where wehave assumed a reddening for the Crab,EB-V = 0.5 magfrom ref. 4. The adopted reddening was used to correct the visual lineratios, [0 III]/HB/He ll/He I, and separately to correct the ultravioletline ratios, C lll]/He II/C IV; then the previously-mentioned intrinsicratio of the He II _1640/_4686 recombination lines was used to combine allof the relative intensities on the common scale of Table I.The visual wavelength data of Table I were taken from refs. 1 and 2.Differences between these two references are lessthan 20 percent and'probably real, due to the slightly different locations that wereobserved. The ultraviolet data (present work) in Table I must be regardedas very rough estimates, with errors as large as a factor of 1.5 quitepossible.DISCUSSIONUltimately, appropriate photoionization calculations will be requiredto support a full analysis of the data in Table I and the furthermeasurements to be obtained this year. In the meantime, some roughestimates will be instructive.Theoretically, each blob or filament in the Crab Nebula is expectedto have a stratified structure in photoionization equilibrium (ref. 5).An illustrative model is shown in Figure 3. The scheme shown in thefigure is supported, to some extent, by temperature estimates from thelines of [S II] and [0 III] (refs. 2 and 6). Zones B and C in the modelare too cool to contribute significantly to the ultraviolet lines that weobserved. Therefore, in this analysis, we consider only zone A. The[0 III] _4363/_5007 temperature estimates generally give values around14,000 - 15,000 K for this zone. (Actually these are averages, sincethere must be a temperature gradient within the zone.)The intensity ratio of the He II _4686/He I _4471 recombination linesin the Crab Nebula shows that most of the helium in zone A is in the formof He+ , not He++ , in particular,694n(He++)In(He+) ~ 0.1.This probably means that 03+ and C4+ are negligible compared with0++ and C3+, respectively. Thus, in zone A we have C++, C3+,0+, and 0++. It is very unlikely that more than 50 percent of theoxygen in zone A is 0+. Most likely, in zone A, at least 60 percent ofthe oxygen is 0++ (see ref. 6). In the following discussion, we keepthis in mind and consider only the C Ill], C IV, and [0 Ill] lines.Using conventional values for collision strengths (ref. 8),(46,500 K) n(C++1I(C III] _1909) % 13 e -I([0 III] xx4959, 5007) T n(O++)and(64,400 K) n(C3+)I(C IV x1549) "_ 50 e -I([0 III] xx4959, 5007) "" T n(O++)The line intensities reported in Table I then imply ionic abundance ratiosfor various representative temperatures, as listed in Table II. Note thatthe most likely temperature is around 15,000 K.It seems clear that the carbon/oxygen ratio in the Crab is not verylarge. Also included in Table I are the observed relative lineintensities in the high-excitation planetary nebula NGC 7662 (ref. 7),normalized to HB. The intensities of the lines observed in the Crab,which are attributed to zone A, are remarkably similar to those found inthe planetary nebula. Since NGC 7662 was extensively modeled in ref. 7,an idea of the abundances in the Crab may also be obtained under theassumption that the physical conditions are similar. For NGC 7662, thecarbon abundance was found to be solar and the C/O ratio was equal tounity, with the oxygen abundance less than solar. The temperature of theinner zone of NGC 7662, where the C IV line is formed, was 14,000 K. Itis somewhat surprising that the C IV line is so strong in the Crab, sincethe C3+ ions are reduced by dielectronic recombination and by chargetransfer on neutral hydrogen, which is much more abundant in the Crab.The inferred carbon/oxygen ratio in the Crab does not confirm theprediction of Arnett's (ref. 9) Case B, in which the supernova ejected allmass above the helium-burning shell, nor does it confirm the suspicion ofDavidson (ref, 6). It is consistent with the Arnett Case A, in which allmass above the oxygen-burning shell is ejected. There are, however, avariety of models for the presupernova star and not all have beenaccompanied by explicit abundance predictions for the remnant nebula.Considering that some 0+ must be present, but has been omitted from thisanalysis, it is probable that695n(oxygen) : mn(°+) + n(°++)] zone A1 within a factor of twoIt is importantto recall that in the Crab Nebula,the abundanceof oxygenby mass is lower than solar (ref. 6). Incidentally,note that theCIV/C Ill] ratio is considerablyhigher (perhapsby 3 to 5 times) thananticipatedfrom the simplestphotoionizationcalculations. It is toosoon to say whether this indicatesa seriousdifficultyfor suchcalculations. It is unfortunatethat the 0 Ill] _1663 line cannot bemeasured as a check on this matter. The line is expectedto be faint andits positioncorrespondsto a reseaumark; a similarcomment appliestoN Ill near _1750.The interstellarabsorptionfeature at _2200 was observed in the•continuumand will providean independentdeterminationof the extinctionalong the line of sight to the Crab Nebula. The extinctionalso can beindependentlydeterminedfrom the ratio of the He II recombinationlines,which is known from theory,once the absolutefluxes are available.REFERENCESI. Davidson,K. 1978,Ap. J., 220, 177.2. Miller, J. S. 1978,Ap.-J., 220, 490.3. Seaton,M. J. 1978,_N._.A.S., 185, 5p.4. Miller, J. S. 1973, Ap. _. (Lett.),180, L83.5. Davidson,K. 1973,Ap. J., 16-_6_'-,223.6. Davidson,K. 1979,Ap. 7., 228, 179.7. Bohlin,R. C., Harringt_, J. P., and Stecher,T. P. 1978, Ap. _.,219, 575.8. Osterbrock,D. E. 1974,Astrophysicsof GaseousNebulae,San Francisco:W. H. Freemanand Co., p. 47.9. Arnett,W. D. 1975,Ap. _., 195, 727.696TABLE I. SOMERELATIVE LINE INTENSITIES IN THEBRIGHT FILAMENTARY REGION OF THE CRAB NEBULA(corrected for interstellar reddening)Identification Crab Nebula NGC 7662[0 III] 4959, 5007 16. 15.5HB 4861 1.0 1.0He II 4686 0.7 0.4He I 4471 0.3 0.03C III] 1909 7.5 5.5He II 1640 5. 3.0C IV 1549 5. 7.3Crab Nebula Data: Visual lines from refs. 1 and 2.NGC 7662 Data: From ref. 7.TABLE II. C/O IONIC ABUNDANCERATIOS IN THECRAB NEBULA ZONE AT n(C++) n(C 3+)(K) n(O++) n(O++)12,000 1.74 1.2914,000 1.00 0.6016,000 0.66 0.3418,000 0.48 0.226976000-4000-3000-Figure I. Results of 260-min exposure initiated at 02:51G.M.T. onAugust 15, 1980.698SNP 6 I_9RUEL = 1.3MU[..TIPLYFLU;';',BY I >",10%:_:-1770013 t t . t , i6000-5000.4000-30_-2000-1000-Figure 2. Results of 420-min exposure initiated at,02:55 G.M.T. onAugust 13, 1980.699STRATIFIED FILAMENT INPHOTOIONIZATION EQUILIBRIUMZONE AH+, He+, some He++.C++ , some C+++O++, some O+++ & O+T _ 12,000 - 18,000KZONE BH+, He, C ++, O+T = 6,000 - 10,000KZONE CH, He, ...Figure 3. Schematic model for stratified filament in photoionizationequilibrium in the Crab Nebula.700

Preliminary Report on IUE Spectra of Crab Nebula

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