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1982 Ultraviolet Spectrum of the Crab Nebula K. Davidson University of Minnesota

T. R. Gull NASA Goddard Space Flight Ctr

S. P. Maran NASA, Goddard Space Flight Center

T. P. Stecher NASA, Goddard Space Flight Center

R. A. Fesen Dartmouth College

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Recommended Citation Davidson, K., Gull, T., Maran, S.P., Stecher, T.P., Fesen, R.A., Parise. R.A., Harvel, C.A., Kafatos, M., Trimble, V.L. (1982) Ultraviolet Spectrum of the Crab Nebula, Astrophysical Journal, 253: 696-706. doi: 10.1086/159670

This Article is brought to you for free and open access by the Science and Technology Faculty Articles and Research at Chapman University Digital Commons. It has been accepted for inclusion in Mathematics, Physics, and Computer Science Faculty Articles and Research by an authorized administrator of Chapman University Digital Commons. For more information, please contact [email protected]. Ultraviolet Spectrum of the Crab Nebula

Comments This article was originally published in Astrophysical Journal, volume 253, in 1982. DOI: 10.1086/159670

Copyright IOP Publishing

Authors K. Davidson, T. R. Gull, S. P. Maran, T. P. Stecher, R. A. Fesen, R. A. Parise, C. A. Harvel, Menas Kafatos, and V. L. Trimble

This article is available at Chapman University Digital Commons: http://digitalcommons.chapman.edu/scs_articles/167 1982ApJ...253..696D THE © not (1969), supernova from for and been (1975), Chincarini Davidson have pect sources, cal 1982. The With ASTROPHYSICAL ideas simple any © the the The also reported ground-based American Crab nebular Trimble American Wyckoff ultraviolet C are ultraviolet but probably other in the to nucleosynthesis, main Subject so while Crab about been (1978, IV Data The The to remnant. (1974), 1.5 much reported, even Nebula ;\1549, measure, Nebula sequence. by supernova in of Crab large emission done. Astronomical mass headings: at from our JOURNAL, the (1970, Woltjer course in the 1979), information and Kirshner I. least continuum features optical Astronomical Nebula He helium is observed It simple INTRODUCTION and together 65 to are our Laboratory Murray THE contains II same lines one be in the Society. hours many 1971), and remnant. (1958), nebulae: stars: chemical 253:696-706, more ;\1640, observations most for a abundance, suggest analysis (1974), type imply primary ULTRA range. by surface gaseous with All available, of observed radio the supernovae- directly Davidson, (1977), rights Mayall easily for a and observation others of T. Astronomy Department abundances- brightest composition, Information new an well-known Department that Chevalier Astronomy Department shows This R. supernova. Department and reserved. Society 1982 brightnesses C basis Received VIOLET condensations. average observed small observable of GULL, ground-based III] region. the (1962), Miller R. cited one range X-ray February the that for Crane, Printed Department, Xl908 A. presupernova filamentary of carbon might of 1981 and ultraviolet: and Crab of Physics, ionic in of PARISE S. theoreti­ of there may derived • Scargle though In studies (1978), pulsar, SPECTRUM the Astronomy, young in KR.IS Astronomy, nebulae: Solar P. Physics, July V. these emission Provided at 15 than Gull U.S.A. M.KAFATOS ABSTRACT R. and fact has Crab abundance MARAN, ex­ be and is L. two The University 20; Physics, spectrophotometry. A. AND Computer DAVIDSON no AND narrowed TRIMBLE region 696 George oxygen accepted spectra FESEN places Nebula star elemental perceptible Crab University University line C. other deterred theory based carbon, ous. mate the ties spectroscopy tional other marks precursor NASA because swept-up supernova AND had by A. OF of in Mason Sciences ratio The intensities Nebula- in the 1981 abundances, Crab California have through with HARVEL the One the T. THE a hand, of interesting Goddard data Ultraviolet of about (see abundance chemical of mass August n(C+ which P. University excess nebula. the Minnesota it by has Crab. Michigan the made Corporation NASA Astrophysics star. such interstellar STECHER explosion, CRAB Wheeler alone. two represents carbon We the the at close had 2 of and International nebulae: the 24 Space )jn(0+ is Irvine of uncertainty the theoretical or and use We practically wealth have composition results main-sequence surprisingly placed Explorer carbon, In ratio to three NEBULA Flight Crab abundance. of presence have 1981). 8 M 8 this gas. measured 2 with supernova ionization actual ) of of crucial and upper in Center 0 accomplished paper, due also carbon Ultraviolet Most when studies ( Perhaps known has the indeterminate only IUE) make little of of to star-stuff measured limits range observational been a We the presupernova mass we the of it remnants­ a calculations, to neutron impact and of was some details; Data small important theorists describe the also oxygen Crab on from the Explorer the of with on a visible other ejected some speculative mention the abundance resulting from on Crab System admixture star is 0.4 the but, the is have supernova significant ultraviolet supernova uncertain­ ground­ Interna­ precari­ nebular on in some been esti­ the the re­ of of 1982ApJ...253..696D emission possibly interstellar in There when segregated. circumstellar ceptible material produce Nitrogen, significantly sis abundance carbon has not ble C considerable in tain resort part each were have other material, nontrivial propriate present § § at obtained wavelength, reported results reduction upon supernova ture ular brightest August the -1950 tions tions IV. The IV VI Our The the the Goddard © for is in no sky. of .>tl549 regions but IUE combined very it Continuum the which of of we young to other is American found ultraviolet; surface visual-wavelength IUE (along abundance IUE is nebular the exploded, A. excess (Davidson work. mixed just several is helium-rich observations lines here. is also implications We discuss probably of neon, filamentary at for manner, procedures spectrograph near event material. This Each helium, to bright nebula. II. features, heavy observations 1850-3300 less is envelope, great amount in two measures supernova Space in the used indicate some estimate with comparison brightness oxygen THE equipped New with a hydrogen the places the sulfur, "filaments" spectrograph and than the spectra the regions, number enough or McGraw-Hill moreover, importance; elements. determinations OBSERVATIONS eta/. using Thus with Astronomical the these Crab Among limit not hydrogen, Flight using ejecta it the of the ground-based Crab carbon of for in from in or in that structures. may is and ground-based about interstellar aperture remnant, the a 1981). very helium star A, the the emission (less are computer of are with theoretical the to § Nebula, of that may apertures chosen of the mass with that the presupernova iron heavier Center have Nebula. the and III, be ultraviolet Since briefly the observations is Sun long, described the nebula that abundance has likely) IUE. large. have of two perhaps 10"X20" mass observed some More Crab of are of and short abundances the and IUE's been angular Observatory oxygen-and includes WITH according or was a in lines carbon Preliminary composed concentrating faint extinction programs elements, the the been spectrographs: large UV specifically discussed to rich spectrophotometry, models it were the fraction the IUE Visual-wavelength certainly Society supernova In intensive is the measurements) involved wavelength, may swept C in which /UE examine order order SPECTRUM not about capability. exposures, with Crab, Orion within this in entrance III] and nucleosynthe­ acceptance unfortunately made reported projected § emission data, only be high, carbon of II, to A up are the of of developed IUE.) in of is situation, are swept-up comment 1908 of the not 0.5 we the and observa­ observa­ the Nebula. in no the • a for in oxygen present models § from ejected is easiest 2 on partic­ uncer­ and suita­ 0.3%, it. small V. Provided aper­ must Crab M M 1150 well 1979 long 1979 data here per­ in star (No and also line but We the ap­ the on 0 0 In of in OF a a a . , THE while North ( corresponding nified "bright "position size mark observations ward, was results, positions by from check discrepancy The The Here, tain. nebula pulsar by spectrograph (equinox Ia- Error to 5h31m14~0, -15 observations 50° overheated spacecraft. the 1977). by CRAB 0 FIG. The 1 broaden Trimble August reference If, and from •19 IUE, reached coordinate "9," the relative the diagram is xb The the the filament" which h accuracy the The Sensor, -40 and NP I.-Positions places upward c'b orientation dx at were IUE 2," NEBULA the "16," xa in NASA Astrophysics 1950). was rectangular star is xf by telescope 0532, large, 5h3lm41".5,+22°08'38" xe of -45 and X= Sun. (1968) order the of long-wavelength of central +21°52'11", by positions do between are direct to and observations the and observed made 1':2. position. AGK3 the a course X the offsetting not Y=O three spatial But Our oblique, discussed i.e., Crab, white-light "19" of east to individual increases solar of and positions. small, The this of exactly the keep was in at an the "Region is coordinates stars are -20 The intended which at Ia-If, +22° the to by was illumination coverage in IUE o: unfavorable 1980 dashed absolute short-wavelength stars dashed an X the usually =5h3lm31".6, the from oblong aiming in Wyckoff 1980 offset positions positions (arcsec) a were detected eastward, coincide, The is large on-board left. preferable slit at this 545, listed star August then I" rectangle a insert which circle are "P" -40 shapes and jaw positions by made, procedure, entrance includes star paper. pointed (X, for of located in by month Data within 5" were (1950). and marks shown with a a the because (lower computer 1 Wyckoff is measured Y Y) pointing camera. to • roughly position northeast direction short-wavelength few 8 IUE Davidson's increases is Only were almost "rear" Murray the Region are for aperture; = the the System -60 by six Miller's north left) are arc in In +21 observations; south, ground-based position Crab those and indicate we direction IUE positions, estimated offsetting from Figure the part The only the order less is seconds. I. Ia. °58'58" relative (1977). wished Murray of of a pulsar, north­ the (1978) ( same. of 1978) 1980 Fine used near about mag­ being The cer­ 697 rms the the the -80 the for Ic to X 1. 1982ApJ...253..696D 698 less indicated seconds; absolute dence A) to with wavelength brightest (1981, each determine individual graphs lar wavelength nebula. wavelength produce strength, spectrograph spectra. emission and angle dispersion individual those with uncertainties "hits" spectra a available ated emission By the tially respectively. there to at located region measurable faint The The Our Region As Goddard 2 the and a area In © data comparing than 17 with wavelength a total of were shown the an a in private 160°, measured American on near for "Region object, oblong were 2 one hours, C formal and accepted as nonthermal low-quality We pointing actual 1979 lines the the aid in lines. from 3". the the filamentary with produced II, which camera; IV exposures a short-wavelength X= described integration and also long-wavelength Figure positions Space spectra oriented two obtained i.e., in Crab observations to systematic As pulsar. may were six immediately detector communication), ;\1549, the respectively, +30", were the large the uncertainty It long-wavelength positions I," Figure the other future difference long-wavelength features III. explained interval by NNW-SSE. individual positions includes IUE. accuracy was Flight National 40" the 1. be made Astronomical indeed and spectra; spectra Y= each entrance We in DATA continuum is by with He may one apparent found and larger). emission 10" That 1). WSW users times § given -63". (Davidson Center. pointing obtained III may 18 (bright occasional aperture of with II Total short-wavelength their REDUCTION NW aperture were some between by not of should detected in region, Ia-If (2000-3200 Space exposures where below. A with ;\1640, spectra our of about of of in have apertures 3% defective § According tests Observations the coincide in long features observations. the of integration devoted the Table source line IV, spots intent 9 of et spectra two fainter error (although each the is (although Science the at. thus the hours been width corresponding IUE show 11 Ia pulsar, the in and axes nearly there of Society particle- 1981 structure emission short-wavelength were or and long-wavelength pulsar, A 1. was each detector of be "wisps" with A) other, Region of of than to south precisely data, ), that streaks of C corresponded The near in to and Data is only includes the Ic, spectrum comparable (1250-1950 comparable these expected 205 times several to Region systematic III] of Region with the With R. weak the negligible the radiation lines which combine data is a of position 8 one spectro­ DAVIDSON we • our arcsec in Center I; pixels. associ­ list Panek ;\1908 in short­ short­ essen­ hours angu­ of those long­ Provided each with such low­ other were evi­ can but I, arc the are the the six 31 II, of to at is is 2 ET spectrum. which spectral field" our inadequate launched, ized very well rather bright being be posed.) signal, stood. enced discussed in tor useful posures, position probably equivalent or ate spectral times posure; posure. SWP9840 SWP9765 LWR8551. LWR8495 LWR LWR8485 SWP9838 SWP9775 SWP9770 SWP9796 LWR8513 LWR8519 LWR8511.. LWR8496 SWP9804 SWP9795 by Observation JOURNAL Evidently, We 1. NoTE.- detector. nearly particular, AL Number simple, spectra are as defects, the carefully. Scale 8491. for observed. some pixels, adopted are because spectra-we than but (Each undiminished "LWR" images) At images: known although by each ...... NASA Astrophysics •..... because ...... as "SWP" later OF ...... but semipermanent .... the because exposure also pixel-by-pixel, least with practical strong bright International we given several remain the hot camera data A their denotes it in the Some are must has denotes (1980, to other simple one of eliminates spectral Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug this pixel it the as feature, Date from following vestiges due have physical can thermal an in difficulties pixel retains contributions the time. combine of were permanent presence a UT) 12 19 II 19 19 16 16 12 15 15 15 15 13 13 13 II paper. TABLE IUE a such Ultraviolet extra to long-wavelength, average these short-wavelength, does each faint, identify average existed format and radiation all widths also Since of spectra-especially an effects radiation the nature procedure apparent observation rather A "hot I respond genuine the all of unrelated average-and in Position is of Explorer median permanent reseau such sometimes the certain the and since If If II Ia II Ic individual lb lb Ic I a II II Ie Ie Id Id Data superposed attempting from pixels" between several in hits. is similar, radiation the hits. actual strong spectral median large-aperture not intensity to large-aperture OBSERVATIONS for the the By mark, of System to to spectrograph bright in exposures the well We several permanent comparing combining there a exposures, defects, the Integration exposures IUE moved the (minutes) but these exposure to common on Vol. spectra; features spectral Time hits, a 360 "blank 352 300 330 300 300 210 208 210 276 270 240 135 129 150 experi­ !50 under­ object super­ gener­ detec­ local­ spots is dark each was can 253 ex­ ex­ ex­ no as in is is 1982ApJ...253..696D No.2, common Al908 need been of 2-5 movements tracting tions. pears "hot of images Figure tal spectra, signal-to-noise all in radiation sensitivity of at tinuum while mentary 3-15 on can objects it These Various total time neously, the free Crab clearly a median which wavelength field" "median-flatfield") spots (1) flatfield" is 4. group Results 2. 5. 3. Permanent Region this For such a sensitivity.) of blank these © The data. obtain possible median (Plates of Equalize Adjust was Generate Generate corrected pixels." exposure to Nebula near hour the the were 1982 American spectral images, 2, emission the several in from in the show wavelengths, radiation contribute also a "background" quasars blank-field assign large of with spectral is fields of shows radiation hits. Partial spectra constant I the other is of 1850 IUE effects a slightly mentioned are blank-field 9-12). reduced. course of shown positions to higher and blank-field exposure bright spots One this many both "median" same aperture 2500-3200 reasons, Figure ratio the a a for The time region. many different operate as labeled shown A exposures and is pixel-by-pixel lines, pixel-by-pixel a removal grid. may hits, a long-wavelength in the procedure of particularly level average hits. negligible better continuum, being too six large in (Note in and Astronomical exposures but specific comparable manner; "hot distant spectrum but blank-field These "hot 3 wavelength-dependent Figure in Figures be within above.) time. short-wavelength for (This or about intensities valuable and both exposures "flatfield" also The shows interstellar from exposure weights Figure because apertures no images, used A was unreliable pixels." "flatfield" were during that short pixels" background observing various galaxies includes These region images IUE median emission is these 2, an are intensities each the were 4 achieved F. long-wavelength average median to strong difficult to has the these 2, to IUE to and arc and to spectrographs The with illustrated in it were 1979-1980, spectral observe labeled allow we By were in allow have or spectrum. our because open; extinction obtained an UV is spectral Al549, individual minute are our radiation at image, were 5. contributions images, spectra. Society long lines Figures project. practically average permanent images subtracting radiation one improved image. image. obtained total the obtained through used SPECTRUM to spectral a shown in "median for levels long therefore, observed lA-lF. stronger remove some spectrograph images wavelengths. "hot are the as the away. Al640, "Region the in instrumen­ to over practically images Observing exposures is and of However, have 4 shown hits observa­ detected detector by produce simulta­ spectral comple­ Oabeled Figures • pixels." smaller the format images hit overall and the object. free with short­ bright minus Thus, an Each Provided these "flat­ from from to with sub- con­ and and one not use ap­ six of in I" 18 OF in 5, a a THE is have between localized angles no month play; spectral sources able median ture nence longer high minus sity extraction lengths-partly dure Region wavelengths wavelength, because case features lar trum was wavelength running bration. background pixels smoothed step emission Crab, ground two tional, -400 by CRAB a Having 2. C The 4. 6. 3. 7. 5. 1. extinction.) adequate linear information near successive (2). that (which IV no the typically numerical At Apply Average Similarly Perform Smooth Obtain using see perpendicular are from and km there dimension flatfield" period, of results and Al549, of assurance the I spectrum each average. (This NEBULA images are image spots Fig. line hot 1800 NASA Astrophysics shown course). assembled IV. background. s- process): function temperatures. (Figs. error. and center the resembles a average because from a one evident a are 1 region, the wavelength, pixels 6, THE evidence a the in does wavelength-dependent net with in 10-pixel-high of A offset extract He 15 midline sampling formats, long-wavelength 31 discussed because composite in seem about on the from in the resulting of 2, this the two EMISSION spectrum Some But that of point from point II Figures not of a Figure to are each the spectral in emitting "average," 4, data, of by the Al640, to data variety the procedure, background two exposure each, the the better despite 5 include the of about and not entrance the spatial perhaps (2) running spectrograph median the step as of standard slit side A sum below.) our the spectrum given extra dispersion, 4 background 6 spectra, UNES short-wavelength by There mentioned really photographic a less sampling using has and images. and 6). of east (which gas in object (3), of "median subtracting radial the most these and corrections "median," spacecraft-Sun time. intensity data on by an 3" filtering than 7. The aperture). (see averages IN the C most applied are IUE of data permanent. determinations the in Data step we spectral exaggerated (this III] REGION by spectrum. fairly (An of absolute spectrum, velocity doubts slight below) the entrance (4) apparent using laboratory order slit following extended-source samples minus taking extracted across ways earlier. Al908 (1 Region these was operation at apparent It System to most the ). form and on centered for matches mismatches I structure a is a and our spectra to not regarding smoothed than the flatfield" hot of a flux the emission There 10-pixel­ conceiv­ "median interstel­ possible aperture attitude (3) in at I of promi­ 3 intense central find proce­ partly inten­ short­ about in inten­ wave­ back­ pixel point spec­ from each each cali­ fea­ dis­ and 699 We the the the 12 of is is a 1982ApJ...253..696D 700 version A half-maxima present tral widths poorer (3) emission, at the intensity individual but but position "median," range brightest seems spectra several filamentary confident late the as 1640, ~,--1------, 15 the FIG. FIG. Ultraviolet 3 mentioned In not not spectrum. © the image Figure adopted bases particular, arc in and for American fainter for focus 6.- 7.- are in of are emission IUE Ie. of seconds. ratios (2) derived interstellar interstellar the structure. The The I A (see short-wavelength the the listed Each due and of in 1908 and for are the line baseline emission detector absolute than average, average, the § composite positions, Fig. line are Al908, II to Figure about error "median respectively. doublet in intensities intensity lines above. This extinction. extinction. sources. Al640 (I) Astronomical roughly WAVELENGTH Table 2). median, positional median, estimate 2 (continuum) features are 13 near 3 is resolution, Ib the FIG. There In with one natures spectrum, and A, It in 20 2. minus hc results which spatial the the measured spectra, The is and similar 6 and 6 reason The 1be the published A, If !Angstroms) may accuracy surprisingly in A, in long-wavelength were unsmoothed apparent "median-minus-flatfield" "median-minus-flatfield" 14 spectrum Table of correspondin,g Al549/Al640jA1908 each and be flatfield" and why distribution A, level definitely in the Al549 except a Society in of photographs and systematic 15 individual each we 2 feature perhaps line AVERAGE difficult our the represents from A up are 30 obtained and observations, that for "average," south of composite "median" widths A. or not at DAVIDSON • moving to Al908, the Al549, widths end error of 2190 These Al549 (4) down spec­ of totally of Provided corre­ the the six the the of at at of A a short-wavelength long-wavelength is an instrumental ET Al749, individual respectively, ror pixel" "median" "average," sidering sults. feature estimated emission. and and far also visual-wavelength ties, tected seen emission. observed conceivably to the trum, by ";- 0~ ~ Some ' X ~ ~ ~ . ~ c AL. confirm enough estimates nitrogen Mg 12 10 listed the are in 4 6 6 the 4 4 8 6 while in systematic spectra spectra blank-field however, at "adopted" n intended the upper ~4-~ Jl~-~~~~~~ ~k~~ the from The NASA This = defect. spectrum from A2798 in short-wavelength 1745 "median," the to be there to relevant detector, of Table might of limit should 1-1=v~_;~~ be the is overabundant limits I\ the Region Region presence (I) A; upper is effects. lines, to unfortunate, is almost Astrophysics exposures. values ground on adjoining but is 2. the be not also be noise be and to WAVELENGTH best N I, The I, located N+ limits "median this rather corrected corrected "3 FIG. III] undetected straightforward. pessimistic, Although of a in an "median a through C for 2 less levels. , spectra Al749 Table is any or 7 upper continuum Thus in II] which are on determining than due because higher" (Angstroms) obvious for the for A2326, MEDIAN-FLATFIELD minus genuine, fairly one 2 The given it Data we instrumental instrumental to [N minus or has N+, Crab, are emission has would aside side an suspect II] lower limit straightforward nitrogen [Ne averages estimates, flatfield" a in defect data. no which System MEDIAN obvious AA6548, Each sharp flatfield" and weak Table of line from be IV] detectable on envelope, the lines Vol. sensitivity sensitivity Such that difficult most nearby, intensi­ A2440, can of bright N N of might 2 unde­ spec- spec­ I 6583 "hot con­ was 253 ... are the the the III] III] er­ re­ be of 1982ApJ...253..696D No.2, >tl665 detector because of capability. sible entrance features, a with wavelength precisely on a comparison He mark trum would respective us seems Region Peak. tegration N-S spectrum, since moved than spectrum The extinction The mates. the are reseau weak Limits New (R. their © the II average mutually overlapping instrumental data unanticipated across and >tl640. absolute relative 1982 and American A. Spectra have unlikely), and 52 ground-based slightly feature I visual-wavelength that intensities mark other the same aperture, F.) on times was was spectral inch were (2) a and (Incidentally, a [0 about been with ranges position in 5.6"X20" a was "hot obtained data line judged near consistent in sky few than (1.3 1980 intensities, of II] near reduced E- the it wavelength extremely unaffected defects. the 20" spatial 20 resolutions pixel." brightness >t2470, W corresponding intensities Astronomical emission other which from has m) the were >tl549, November, IUE visual-wavelength 1805 was minutes 3600-5100 during adequate. during with McGraw-Hill >tl665 slit N c (Ne C Hen;\1640 C MgnX2798 been the less there and just emission coverage III] IV obtained, II] data, III] within If The NoTE.-Intensities near data are A, spectrograph are IV] an was difficult >tl640, X1549 ;\.2326 a applied. by ;\.1908 XI749 lines, than this Line regions the corrected in 2" of ;\.2440 was part location.) and located are the one far approximate were omitted allow the the using oriented a X about exposure. For feature reasonable ...... A conventional to half OBSERVED 10", a much below exposure. 30 all as possible lines, and of UV .... defects. center ... with to hot obtained and Society of a Telescope the the rough a minutes apparent entrance between the 6 obtain E-W observations, the SPECTRUM location spectral for >tl908, the Regarding pixel from A "Average" such more IUE N-S Spectrum is our respective 3600-7000 3600-5100 101±15 spectrograph are of 92±20 51 77±15 75±15 In and ULTRAVIOLET exception atmospheric intensity two real predictions slit, sample The error each aperture. as given by Region data detection data; Table and as a and coincide manner. uniform • spectral I scanner on well spectra on 0 (which moved one reseau 0 Provided other A. case, in with with Kitt pos­ esti­ was but the III] for in­ units OF as of of of TABLE2 2, It A A No is I. LINE "Median" Spectrum 108±15 corrections THE 85±20 7: <75: <50 <75: <75: of INTENSITIES 10- Region are components Most Humphreys km lines; nents would paper, listed lar times ties wavelength (1981 along assumed data; Region Miller Region measured the high-excitation they nation area comparison, served -400 Table trometer ultraviolet assess 10- by CRAB 15 The Observed Absolute consistent reddening, minor ergs s 15 in the include for at - ). as in the of in were 3 with km not we ergs 1 (1978) by results because lines just the • cm- interstellar Flatfield" I, Table I, I; NEBULA are "Median intense IN Table particular, NASA Astrophysics 90±20 96±25 the to 98±20 The rather minus intrinsic <75: <30: Bohlin, list <50 a component quoted be s- reported data and REGION center included. cm- 2 discussed emission be the (1976), small the line corrections relative with as Table s- emission 1 is and changed with of -400 we 3. 7 or 3 1 Miller's were a as than ultraviolet planetary 2 a of major passing intensities (Seaton extinction these Here other includes Harrington, of link s- area I large use -500 adopt from He the previous 3 Davidson spatial [0 intensities by Adopted 1 the lines, obtained with km in also Some of the II <30 <50 <75 <75 arcsec- +800 significantly occurs between within we III] 100 through component; new velocities, for rectangular § 90 Woltjer 80 (1978) large earlier >tl640/A4686 H km E 1978). s - VI. visual-wavelength have the includes have 8 A4363 in nebula y minor some extinction irregularities. in lines _ observations 1 ground-based km A4340. s-I, v (1978), and and Region 2 rectangle near IUE. component with Region of = "position work: as used ground-based To s- (1958), 0.5 are data components ultraviolet almost but especially Stecher around Data a NGC 1 if the a a correct area The but component. convenient mag the and rocket-borne uncertain the radial I; estimates there Davidson I intensity for spectrum (see "position implications which are Davidson the He coincides of System 2"' minor Miller Let (1978), 7662, Region (Miller is observations for places line Fig. places are near velocity and II difficult line perhaps is of and us interstel­ recombi­ includes because a compo­ intensi­ weaker by ratio (1978). unit nearby 1). as In visual­ (1978) of whose + ratios adopt I, 1973) 2" small spec­ near with IUE near 701 and this For Wu 800 ob­ are the of of of of to is 5 1982ApJ...253..696D 702 >tl640. observed within brightness brightness seem estimate estimated estimated arcsecZ, Crane, apparent 20" therefore fluxes Chincarini © region N C Hell [Fe CII] C He Mgii [0 [0 [0 [0 [Ne [NeiV] [Fe [S He1 He Hy [01] [01] [N H{J He1 [S [S Ha roughly E listed LINE American our 8 and We IV III] III] II] II] II] d III] bDavidson NoTE.-F=apparent cMiller aThis III] III] II] _ 11] I II. correspond III] VII] to III] Bohlin, ...... v should ...... surface ...... from ...... 4 above of ..... of =0.50 value). Region (although Chincarini ...... do in be ...... units paper. He 2 consistent Table 1978. 6548, not smaller our Harrington, units, mag. II 4686 4658 2326 4959 4861 4471 4363 4340 5876 5007 6363 6300 6087 3869 3727 2798 2440 1749 4071 6731 6717 6678 6563 1908 1640 1549 be 1978. the brightness A for According I. >tl640 know new 6583 Astronomical to 2, close Davidson we HfJ averaged (1974) Earth's referring than only ground-based with Inight exactly and is in to <50 <20 <35 1170 <50 energy 630 364 680: 505: 336: 284: 115 157 107 100 about I F 61 54 68 3 68 63 the 61 38 34: 2 15: 12: 11: 17: 19 estimated about for (This an Stecher 7: to 1 an REGION atmosphere. over and have to unit. Table Davidson, area estimate intensity, why an Paper) an half 0.4 Humphreys 1978. an <650 expected Ia <110 <400 Society enuss10n <170 area But 2" data 1100 1000 this 600 480 560 a 350 298: 396: 100 172 3, 190: 161:} that unit 48 74 24: 23 36 98 19: area maximum 8: 7: across. RELATIVE the relative of of the discrepancy for Crane, of Davidson, for about 5" 2.5 apparent absolute HfJ this DAVIDSON a feature, • FILAMENT"b (1976) across These 10" He to units, "BRIGHT 1000 Provided EMISSION 910 330: 340 290:} 580: and 100 and 150 130: last 205 HfJ F(H/3)=100; 70 20: 50 30: 40: 10: X I II TABLE3 LINE ET has cant included known earlier. evidence line result dent These fects estimate by INTENSITIES Avoiding AL /=intrinsic "POSITION occurred, intensities, mystery; upon the affect in 1560 1450 results to 530} 250 468 844 Einission 104 100 27: 83 62 90 40: 18: large I mostly for continuum be NASA position. V. the 2"c some so systematic are energy but THE changes probably irregular as data, low-intensity line plotted we Astrophysics NONTHERMAL listed wavelengths intensity, "POSITION intensities intensities we also in some 1720 1240 { that in pointing have in 200 measured 100 ... 45 37: 27: 87 50: 13: do I Table Figure corrected small not areas. 2"'c of used where in CONTINUUM in the 3, regard errors the pointing Data 8. fluxes. the Regions This 45 are 13 IUE for Here Crab instrumental NGC IUE far reddening it as {I: may System <25 <10 <30 The observations 560 300 720 ti,.,, 253 are de­ II. of to 1982ApJ...253..696D No.2, "AI,_ I which sure ergs estimated Crane, confusion "AI,_ Earth's rithmic reduced flux apparent given "A Wu ton-energy 8 continuum tion E some continuum. shown excellent our and +0. 8 shows FIG. At _v=0.5 © ""'A 2 directly data. is) cm- with agrees II. 'f ~ _ of -< :<-12.8 H g ...1 I ~ .. Ill *' CJ visual curvature (the at is 1982 American 8.-Estimated -o.s -12.0 -11.8 -12.4 -12.6 -12.2 " in -13.0 -13.6 -13.4 -13.2 and and interval atmosphere. 13 continuum by the the the 2 agreement " the surface Region 8 about the from X-ray s- intervals. '---:1:::-500=----=2000~-~3000=:-----=:5000!=---' (the of mag. a partly and Nevertheless, end with Chincarini same same indicates ~ factor 1 upper I I the extinction OBSERVED arcsec-2, on radio tlj (i.e., ultraviolet of an units spectrum). I~-~ II, Miller Crab data, as brightnesses because i a WAVELENGTH that continuum Astronomical of surface isophote with part The log-log The it vI. near and of the 0.85 shows Nebula (1974), paper. corrected and fl wavelength, of curve bottom the (1973) visual-wavelength as ( power-law infrared the energy this wavelengths, (! at Figure brightness-partly plot ! ~ the We ~ surface result which map where pulsar, each (Angstroms) The DE choice but adopted makes as most of might f f that half for REDDENED distribution x-o.s POSITION POSITION power observed 8 of upper given isophote the brightnesses can UV with most are frequency, approximations interstellar has Society of alleviates Wu appropriate the a visual-ultraviolet therefore be SPECTRUM the Figure transition by adequate the "AI,._ half by ~ 2 law) 1 (1981) of reddening expressed ~ ~ values Wu nonthermal for the above Davidson, intensities ""'A of in per to possible 8 because • reasons or Regions (1981). -o.s, energy extinc­ Figure for expect shows "AI,_""' Provided to loga­ mea­ from were pho­ the the in fit as in OF is THE central to the wavelength plausible pronounced long-wavelength tral reason shorter low-ionization visual-wavelength A6731 for of can lyses hydrogen, to tions "solar," 5007) Crab: Our abundance I III] than ratio (1980), etc., Crab lower ionization.) Davidson highly high-ionization bly, enough, density) o+ extra authors. of T[o He the mixture bly 1979). 10,000 by CRAB The have yield Self-consistent Nebular elaborate the 0 exponent the 2 zones Crab optimum indices 1111 , make , heated scarce the present in for the c+, of heating o+ gives / Nebula order intensity the ionization ""' Various 150" ionized 2200 "AI>- wavelengths. why K. A6716 nebula and but results of it various the NEBULA (see ratio 12,700 The supernova are 3 solar NASA Astrophysics in and oxygen/hydrogen , c+ also the some ""'A H+, A estimates ionization etc., X region; by between because of since in T[s calculations. photoionization Each our appropriate photoionized the (±0.2?). A Scargle may estimate use 2 150" very gaseous -o. has is ratio Netzer mass (?), X-rays. species I has the value. other and zones ratio 0 consistent zone K, He+, interstellar nJ at interesting Crab spectrograph 2 rather 3 data condensations ionization core, most of but be N+, ""'8000 em- emission condensation region been lower an somewhat thick temperatures models. indicates the VI. fraction are the and Regions the in 1969) relevant due near (1979) The He+ inner, Nitrogen, with structure Nebula. such condensations in All of are of 3 mainly o+, DISCUSSION low. other Region stratified in data discussed In condensation types part then in (ionizing the to by Region of the suspected K, 2 its with a those and poor , as estimates this calculations extinction the processes low-ionization etc., lines abundance and c+2, in of rather If these the mass that lower in I papers surface, a of comment reason C+4 is [0 The H have may oxygen a Crab. the and and I section, previous neon, within non Table zones. typical above by is in 0 at Figure less condensation (see electron , c+ in III] observed zones is I, by Data large and radiation)/(electron [S specific than He thought temperatures Kafatos be the difference thermal emission been helium II are 3 or ionization. Region without while other is sensitive , n] "A4363/("AA4959, Table Davidson predominantly 0 sulfur, may feature 3. These is real. 12,000 N+ is ratio 0+4 , on we will our exist, 8. value, "filaments" uncertainty 2000-2500 average that cited A4071jA6724 c+, System work However, therefore described qualitatively densities zone 2 This the first than case , rather have and in continuum the be to are 3) is Region N+3, I and the line irresolva­ resorting and K. N°, by appears in implica­ is roughly is for toward implies Region needed discuss have on of is proba­ photo­ Lynch for (Some of (1973, below [S spec­ More other a these IUE very thick than ana­ iron H+, on~ o+, 703 less 0°, the the are the we yet in [0 by A II] of I. a a 1982ApJ...253..696D 704 vant consistent indicators. spectrum of of carbon; abundances ;\;\1548, IUE at C n C is low suppose error (1975), course over, then the we rather be simply sponding for equations originate gradients Here temperature n(C+ n tions). 1.3. --,.,-~-=--=-=~= __,.,,....-..:.....,..,.-7;__-::-::--::-:-~ I{[O 1([0 0 hotter If The However, interest temperature 10 © in verified find The Crab n for ;\4267, o+ ionization the 3 in 2 observations since I(C c we l(C a) )/nc American the than m] cm- III) ";\1549" only 1 Taylor insert 2 this oxygen former the 1551, that ultraviolet than , n n(C+ in to in Carbon of (1) ionization m] c+2, have relevant 0 Nebula, ;\;\4959, are ;\;\4959, IV Evidently 3 T[o upward. value which are the should , through a the the present the carbon dependences, ;\1549) because we and the the ;\1908) and temperature 2 et the T, )/ 1111 ratio abundance fractions and [0 less used relevant hotter and Crab al. assume and observationally intensity is n(0+ with (2), ;\;\1906, = are m] zones line can 5007) lines 5007) not we Astronomical (1977), more c+ straightforward calculations. 12,700 Therefore, the purpose. might features ";\1908" Nitrogen self-consistent collision assuming emission zones the very are electron ionization be suspect 3 2 be intensities ultraviolet must )~ are zones T=20,000 (although must likely ~ ~ very recombination ratios ratio. of 1909. be average the and 5 used faint, 1.5 K 10 Xexp Xexp O mixed in 12,700 taken If arise in __;_ n(o+ is n(C+ n(0+ n(C+ are be (as different ultraviolet as to and strengths that density Dufton T= Because the lower there defined of the _ to unreliable zones, have an estimated (- ( really lines there from mostly this together - ionization 3 as 2 Crab some 2 Kin 2 because ionization T[o n(C+ _;_ set any K ) Society ) ) in ) visual-wavelength extreme 64 46,500 a are than been is statement are ne eta/. Region plausible 1111 from probably ·~ limits roughly pairs of T[o Nebula Table observational lines equation 3 probably from emitted T in lines of )/ temperature = crucial. their in abundance downward average from their mJ· (1978); gas K n(0+ 12,700 K) the n(0+ example, a of DAVIDSON fraction such • calcula­ I. on ). Seaton should 3 region strong More­ which , corre­ order guess If lines, Provided must lines rele­ are), self­ into 2 The (1); 2 (2) (1) too the )~ we K, )/ as in of ET ionization calculates intensity mixed, violet ultraviolet helium be" photoionization [S spatial confirmation. electron calculations n Higher n;tion recombination understand be state, quite line limits I; this there, the uncertainties ~o.4. bright photoionization servations, age on of consistent the tions, excludes and This Thus, moderate nitrogen solar n(C+ by ""10 4 In Our II] Our quite the nc/no We AL nebular much ionization the intensities Stecher Crab is line is the radiation 2 composition, allowing summary, adequate; and resolution, lines; and 3 ionization we ), from are should condensations. data their results about densities. most ill cm- useful ratio nitrogen _;,.._..;...__~ I(N less I(C an ratios, some than which charge defined. can NASA Astrophysics the (see nitrogen condensations, Nebula it susceptible calculations N ionization the one 3 in equilibrium can ( , is line c+ the line is 1978) note dynamics m] including likely m) higher in III] density Davidson N+ the He+ estimate (though refer fraction surprisingly the carbon are zones and very perhaps 3 can unwise transfer Measured N+ calculations also models. if ratios IUE presumably ratio than the ;\1908) ;\1749) This ;\1749 2 that abundance. 2 maximum summarized planetary jHe+ is have we therefore is might 2 estimate overabundance, equilibrium specifically range probably nitrogen-rich , be thought temperatures possible. c+ problem both potentially is of our data, not calculations. is in attainable photoionization Conceivably, 1973). to probably make reactions, n that to without and larger used 2 discussed (N the ratio emission. Then ~ values . large, Region obsetved refinement A N+, radiative be place 0 is + ) refer Xexp condensations, electron we · The occur 0.3;:;::; to similar allow 4___;_ the implied to nebula requires value reasonable than 0.5;:;;nc/n means InN. far in ionization n(N+ n(c+ we From in occur of is with one place estimate to explanation to adequate very abundance not the observable I nearer Table _ ne the n ( in scenarios. C the find and richer one If Region zones problem c / of --T-. and finds densities Data However, Bohlin, allowed the by IV 2 following 2 some simple result compared obsetvations in model surprising), _;_ NGC N+ ) ) by that favored a ground-based much rather 6800 dielectronic the obsetved n to X1549/C Space n(C+ the to worthwhile 3. levels. 0 ionization that of whose 2 allowances means that guess 0 in 4 ;:;::; the from Crab observational may other photoionization occurs I, System and Crab with low there ;:;;1.5. K) 7662, may 3 of Harrington, trust there·"should similar Telescope. However, by 2 "surfaces" carbon through sense. a )/n(0+ in ionization We this n~+ He involve edges ionization. [0 N+, group the III] with c+ the with Vol. and Nebula. existing be recombi­ of part Region with since 1s II/He in do whose II] These is If XI908 2 aver­ lower limit ultra­ frac­ self­ case 2 and with high may less not ob­ and are 253 one the for the not )< (or the (3) the its of of 2 of it a ) I 1982ApJ...253..696D No.2, improbable. nitrogen from ground-based in ical tributed cooling or carbon, lower in weaker, sured condensations carbon-rich of gests can coolest richer cores material the bly priori wavelength gas rarified not fraction X-ray mass. must measurements may plasma sure filaments. gible; there thermal While this tected phasize dances arrangement while abundant, oxygen Region Region Can Finally, Our the more © artificial. in Space elements. account be would (Davidson that place of value eventually is evidently the If 1982 in expectation the American and nitrogen spectrometers. known nebula. shell some acknowledging by of either too i.e., relative main throughout the that or in high-energy then no in of b) because such carbon. of I. I, Crab other temperature Telescope), C carbon-rich even relative oxygen the emission of the the require high; However, thickest to Implications evidence with condensations, while Hydrogen for of the carbon-rich III] the must data of If a conclusion, values of changes T[o (a place), Abundance observed ejecta Nebula? to cannot Crab condensations nebula medium more if be and a >tl908 available carbon be carbon material If local problem the therefore, it the such for condensation mJ that be or to Astronomical the a below there condensations; lines particles detectable Tucker for produces of all the The concentrated to [0 upiig inversion surprising Region than pronounced Nebula but outside Balmer material and there. the and nebula and it be gas mass, is for be Such or three T[o accounting III] exists, nebula such material "solar" in are is and any evidence existence in that present also more requiring a oxygen lower gradients the above it exists, there C helium the it and more mJ small 1970). This I whose some gas the recombination seems the elements), IV because is (even the gas, hypothetical are must seems Supernova might its the pulsar's dispersion no in with susceptible were in [Ne >tl549 composition. than near must in is supernova exist observed segregation there carbon-rich possibilities, would unlikely in X-ray fraction resulting detection the in order aside UV this is unobserved, are Pending ratio perceptible of for if such no total fill unlikely investigation III] Society to the may extremely reside higher against the its their moderately unity, be Crab. an SPECTRUM as seems emission fairly have several is evidence an lines but low-ionization from emission to pressure a cause Event mass Crab hot of interfilament of no interfilament occur of to region carbon-rich ratio nebula, appreciable in to have lines, this additional such one limits carbon explosion. resolution This should the overabun­ would that definite and implausi­ of well be the than an the percent • is test Nebula, we a rich cooler, visual­ chem­ would seems varies of within much radial much unde­ negli­ from Provided mea­ mass quite must tests, than than over­ mea­ with lines they sug­ non­ dis­ the em­ but but the via be in OF be to a THE of of considered superseded shell; ejecta case overabundant, diction contrary diction the terms helium-burning case tions. neon servations should tions. probable Nomoto stars, occurs with neutron systems main-sequence might nova not undergone perhaps slightly stellar smaller enriched explosion of above as duce deflagration Proceeding neutron planetary above Crab but expected formed ideas main-sequence mass mass the by CRAB Arnett Recent various As interest far about Crab pulsar apparently necessarily of the might would observational is heavy However, of included summarized explosion then Nebula discussed from was In the be as the wind be and the perhaps less initial M""" NEBULA stars. stars recent with (Rappaport supernova. material (1975) Arnett's NASA Astrophysics oxygen; 8 1980; mixed from mass helium-burning nebulae theoretical masses. appear itself. here helium large, in has by 8 carbon, Crab limiting to M be main not elements a supernovae; than 10 M mass 0 relation have yet the pulsar. more Such relative stellar neutron ideas masses, perhaps mass not mass 0 a the because consistent carbon M range Nomoto eject 2 above, shell be to proposed is with If Nebula. , nc/n to Type lower is M burning 0 heavier sequence 8 with above "Case nebula. then h results the "Case thought masses overabundant carbon , loss) oxygen, results one 0 a Although recent still M exclude depth the about layers work and exceeds of material hydrogen in At were star or to 0 to in 0 white the rich. evolutionary star II masses, 1.4 Arnett synthesized considers about roughly ejected their are supernova 3 ;;::;10; are they the zones. B," suggest B" 1981). In still elements which Joss and two could with supernova M of theoretical on rich; on This the down for Crab M ejected; to to and correct. this is his 0 nebular solar likely In dwarfs 0 about only about of are this explosions, , lower which which has the alternative neutron evolution 8 supernova, ; therefore, eject explicitly 1980). as formed ejection, in stars in Stars ground-based these "Case and not ground-based We model so, combination neon a 4 Nebula M to in totally that, mentioned work evolution composition-a the stars the Data layers are 0 Type to M models the oxygen if The the 1.4 15 are have masses, a carbon , is with then models matter assume with whose 0 calculations, stars, convective if In produce supernova neutron should the A," produced oxygen-burning also. helium at stars M (Sugimoto may M nebula, produced. of is been disrupted, of the visible while addressed is System our II a 0 scenarios down initial and produced initial 0 oxygen initial suggests some "missing"- the and , successively supergiants is helium , star abundance the of star which have supernova theoretical in§ the of (including consistent in here processed no IUE be is leave observa­ observa­ rich star nebular massive carbon­ carbon­ nebular also so course, masses helium masses to binary longer whose in super­ event. layers had point, stellar I, to quite been it pre­ that and pro­ and this 705 rich was and the for ob­ the has but the the the no in be is a 1982ApJ...253..696D 706 presumably supergiant evolving enrichment like companion rather and considered RoBERT Klu:s VIRGINIA Arnett, __ CHRISTOPHER MINAS Flight Bohlin, __ STEPHEN Conti, Chevalier, 20771 Dufton, Davidson, Davidson, Kafatos, Davidson, Davidson, Davidson, Mayall, Davidson, Kirshner, 219, 2171), M., 1978, Wavelengths, Possible © the we DAVIDSON: and P. 575. . . W. Center, R. American far Astr. N. P. KAFATOS: 1978, 1979, p. M., Crab S., have R. A. toward R. K., K., K., K., K. K., L., C., P. D. U. 693. Trimble, L. from and P. stages effects and have Ap., whether A., FESEN 1973, of lost Herrington, Ap. Ap. Crane, and and and MARAN 1975, Gull, 1962, Harrington, TRIMBLE: is 1974, A. not ed. Greenbelt, McCray, and Lynch, the 62, J., J., worth Tucker, Netzer, its Humphreys, in been the Ap. HARVEL Ap. Astronomy Science, T. R. Department (see V. mentioned Ap. P., due Gull, 228, 220, and Ill. dramatic interstellar wind Astronomical R., J., the J., galactic and D. L. and J. J., K. neglected investigating. R. 179. H. 177. W. Conti THEODORE Department to 186, J. T. P. Maran, 195, 1981, Chapman A., 194, rather 137, Chincarini, 1980, P., episodes THEODORE 1979, MD and 1980, R. D. any R. 223. 727. Burke, and fate. 1975, 323. 91. 1970, in Department, plane. M. and the RONALD Science, S. of Rev. 20771 hypothetical Ap. medium small in The Stecher, P., 1976, Physics, Ap. The (NASA P. Ap. fact J. R. McCray the during G. Mod. Stecher, of G., Universe Suppl., Also, J., J., P. 208, Pub. GuLL: 1974, amount Society kinetic above Physics, that and A. T. 200, Phys., 161, STECHER: by Conference 9. George A.S.P., PARISE: P. blue A.J., University 42, Kingston, we T. close 399. 1980) the 437. supernovae at 1978, discussion, Code 51, P., 611. energy of have University Ultraviolet 79, and Crab DAVIDSON • Kafatos, 715. helium 88, stellar Ap. Mason while Computer Code 791. 683, Provided A. Pub. REFERENCES 312. not red J., of of E. is NASA/Goddard Minnesota, 680, University, of Sciences California ______Taylor, Trimble, Wyckoff, Wheeler, Woltjer, Wu, __ Sugimoto, usual Nomoto, Scargle, Seaton, able." expansion Rappaport, Miller, edges Inter-American tive NASA/Goddard Sparks, Observatory, ET. by Davies Crandall, and Report the We C. AL conversations Theory the D. fine . . . . J. C. thank M. P. 116 J. 4400 L. 1978, 1978, 1970, 1971, V. hospitality J. K. Corporation, and S. and S., No. N. D., D. 1981, 0., J. C. at 1958, S., NASA D. support, 1968, 1973, of 1981, and Church Schramm of 1969, 1975, F. A.J., Space CSR-HEA-80-24 Ap. and in M.N.R.A.S., J. Irvine, 1981, and University Gregory, H. the where the Stellar Ap. G. !AU Murray, Craig Bull. A.J., Ap. in J., 1977, Nomoto, Joss, Observatory Ap. M.N.R.A.S., 75, Smith staff visible Rept. J., !AU Space 220, Flight with J. Astrophysics Symposium and Street Astr. (Dordrecht: CA 73, parts Evolution, 926. J., 245, during Wheeler. Phys. P. (Letters), D., Code Symposium C. Progr. of 490. (Dordrecht: C. 156, 535. we Drive, 92717 lnst. K. Drs. 185, 581. nebula A. Dunn, Flight Center, the 1980, S.E., of Rev. 1980, (Cambridge, gladly 401. 685, 1977, Phys., Netherlands, 5P. this 170, 46, ed. Ken'ichi IUE visits 180, K. and Reidel), Letters, "Binary Fairfax, Minneapolis, G. Space 93, Center, is NASA/Goddard The M.N.R.A.S., D. 475. paper Reidel), D. Greenbelt, acknowledge 44, L83. H., theoretically observatory Fundamental Kitt Sugimoto, Data Crab gratefully to 85. 39, Sci. p. X-Ray Mass.) Phaneuf, Nomoto, were 14, VA 295. Greenbelt, p. 1256. Nebula, Peak Rev., Cerro 40. 12. System 22030 Pulsars," 180, written. D. MD MN 25, Problems R. acknowl­ informa­ National for "reason­ Q. ed. Warren 717. Tololo A., !55. 20771 55455 Space Lamb, R. their MD MIT and D. in 1982ApJ...253..696D DAVIDSON bottom for AVERAGE FIG. wavelength-dependent © American indicate 2.- eta/. The (see the six wavelengths 1A 10 individual 10 page 1E 1F Astronomical 699) instrumental short-wavelength of (from sensitivity, left Society to spectral right) which geocoronal images • is greater Provided of La, Region toward C IV by I, longer Xl549, their the wavelengths average, He NASA 11 Xl640, and (i.e., Astrophysics their median. and to C the m) right). Xl908. These Vertical Data have not marks System I been PLATE at corrected top and 9 1982ApJ...253..696D DAVIDSON interstellar approximately PLATE FIG. © AVERAGE American 3.- MEDIAN 10 eta/. extinction The 2000 (see six A page ,individuallong-'Yavelength feature 1A 10 1C 18 (left) 1E Astronomical 1F 699) to and 3200 lower A instrumental (right). Society Between spectral sensitivity • 2000 images Provided and at of shorter 2400 Regi\;m A, wavelengths. by the I, the their continuum NASA Astrophysics average, is almost and their undetectable, median. The Data because wavelength System of the range 2200 A ~s 1982ApJ...253..696D

@ >a .,~ AVERAGE

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ll.l> ..... 8 = MEDIAN a.... ~ e:.. 00. Q ~.... ~ ~ FI...ATFIELD • ""C 8 :;l. ~ ~ 'a' (MEDIAN-FI...ATFIELD) '-< ~ ~

~ I 00. FIG. 4.- The average, median, and "median-minus-flatfield" short-wavelength spectral images of Region I, displayed as in Fig. 2. In the "median-minus-flatfield spectrum, an ap­ > parent feature near 1800 A is exaggerated in this form of display; its peak intensity is actually low (cf. Fig. 6). The geocoronal La feature appears as strong "absorption" feature in the same spectrum because it was saturated in each individual spectrum and various renormalization factors have been applied. ll.l> ::;'" Q DAVIDSON et al. (see page 699) "0 =- ~.... ~ i ~ .~- - i 1982ApJ...253..696D

ts @ > ~ a -N ~ .,.... ~

~

Ill> ..... 8 = a.... AVERAGE ~ e:.. 00. Q ~.... ~ ~ • MEDIAN ~ 8 < ~ Q.. • ' 'a' #~ '~ . ,!'+ '< FL..ATFIELD ~~ \ v ' .A' ~ ~ ~.

~ 00.> Ill> (MEDIAN-FI...ATFIELD) =t Q "0 =- ~ FIG. 5.- The average, median, and "median"minlls~flatfield"long-wavelengthspectra1 ima:ges of.Region !,.displayed as in Fig. 3 ~· DAVIDSONeta/. (see page 699) ~

~ 00. l