N O T I C E
THIS DOCUMENT HAS BEEN REPRODUCED FROM MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED IN THE INTEREST OF MAKING AVAILABLE AS MUCH INFORMATION AS POSSIBLE
5
DISTRIBU."ION OF HOT STARS AND HYDROGEN IN THE LARGE MAGELLANIC CLOUD
THORNTON PACE NASA Johnson Space tenter Houston, Texas 77058 ..•' V I ^^^sMMM V 4 .^j J GEORGE R. CARRUTHERS E. 0. Hul,burt Center for Space Research Navin. Research Laboratory JUL1081 Washington, D.C. 20375 RECEIVED NR$A 3TI FACILITY 4 ACCESS DEPT. ABSTRACT
Imagery of the Large Magellanic Cloud, in the wavelength ranges 1050-
1600 A and 1250-1600 A, was obtained by the S201 Far Ultraviolet Camera
during the Apollo 16 mission in April 1972. These images have been
reduced to absolute far-UV intensity distributions over the area of the
LMC, wit-h 3-5 arc min angular resolution.
Comparison of our far-UV measurements in the LMC with Ha and 21-cm
surveys reveals that interstellar hydrogen in the LMC is often concentrated
in 100-pc clouds within the 500-pc clouds detected by McGee and Milton.
Furthermore, at least 2.5 associations of 0- B stars in the LMC are outside
the interstellar hydrogen clouds; four of them appear to be on the far side.
Far-UV and . mid-*UV spectra were obtained of stars in 12 of these
associations, using the International Ultraviolet Explorer. Equivalent
widths of La and six other lines, and relative intensities of the
continuum at seven wavelenths from 1300 A to 2900 A, have been measured
and are discussed.
(NASA-TM-62378) DISTRIBUTION OF NOT STAGS N61-26996 AND nYDRCGEN IN THE LARG E MAGELLANTC CLCUD (NASA) 52 p BC A04/11f A01 CSCL 03A Unc',d G3/89 26653 Key words: Large Magellanic Cloud, interstellar hydrogen, 0-B stars, far-UV
(Accepted by the Astrophysical Journal for 1 5 Sep 1981.)
I I I. INTRODUCTION
Far-ultraviolet Imagery of the Large Magellanic Cloud was obtained with an electrographic Schmidt camera (Experiment 5201) during the Apollo-
16 mission, 21-23 April 1972. This imagery covered two wavelength ranges,
1050-1600 A and 1250-1600 AR with a limiting resolution of about 3 arc- min (Carruthers and Page, 1572). Figure 1 (Plate X) shows prints of the
3 min and 30 min exposures of the WC in the 1250-1600 A band. Analysis of these images was briefly discussed (together with three spectra) by
Page and Carruthers (1977) 0 and in much more detail in our 5201 Far-UV
Atlas of the LMC (1978) which includes absolute far-UV fluxes, in the two wavelength ranges for all measurable objects,; in the IMC images.
Previously, Henize (1956) and Doherty, Henize, and Allex (1956) had
surveyed the LMC with an objective-prism camera to obtain Ila emission
intensities for all identifiable emission nebulae and emission-line
stars, and McCee and Milton (1966) had surveyed the IMC in the 21-cm
emission of atomic hydrogen. More recently, Davies, Elliott, and Heaburn
(1976), hereafter DIN, conducted a more sensitive Ha survey and compared
their observations with 21-cm and radio continuum measurements.
In this paper, we compare the results of these four surveys and
discuss their significance in studies of hydrogen distributions and of
far- and extreme--ultraviolet stellar flux distributions in the LMC. We
also discuss recent observations of selected INC stars made with the
International Ultraviolet Explorer, and their relevance both to determina-
tions of IMC hydrogen distributions and to the absolute and relative UV
bri,ghtnesses of INC objects. r
.z c
11. DATA AND ANALYSIS
The for-ultraviolet images shown in Figure l (Plate X) are qualitatively useful for determining the spatial distributions of early-type store in
this UIC without confusion by images of the fnr more nunaruus cooler stars
(almost all stars detected in the 5201 imagery are of spectral type
carltwr than A7; i.e., with effective: temperatures above 9000 K). It will
be noted that the ,distribution of hot stars differs considerably from the
,Seneral stellar pipulation distribution as revealed by visual imagery;
the short exposure shows the previously known 08 associations and clusters,
whereas the longer exposure shows the general distribution of hot stars,
most of which are less luminous than those in the associations. Some
structural features of interest are noted in the figure. Comparison of
the UV,imasery with the F{a and blue imagery of DEM (their Plates I and
XXI) indicates that, for the most part, the extended nebulosities in the
LMC (many of which are considerably larger than the S201 resolution
limit) are not conspicuous in the far ultraviolet. This is also indicated
by IUE observations of the 30 Doradus nebula (Koornneef and Mathis, 1.980)
and of local galactic 11 It regions. Thus, we presume in the following
that the observed far-UV is either direct star1ight or starlight scattered
by oust in close proximity to the stars. As discussed in more detail
latex, virtually all of the Henize and DEM Hu emission regions appear to
be associated with hot stars apparev,t, in the far-OV imagery, but the
converse is not true.
Quantitative analysis of the imagery is, to some extent, complicated
by the effects of interstellar extinction, correction for which is
particularly uncertain in the 11C because of incomplete knowledge of
i 3 k(H-V), and of the extinction vs. wavelength in the [MC. It iL known from ANS and IOC: observations that the IMC extinction law is considerably different from that applicable in the local, regions of our galaxy and 0 shows large variation with position in the 1MC (see, for exampla, Nandy y et al., 1980).
The procedures used for the reduction and processing of the 5201 cicctrographic imagery have been presented in detail in our Par Ultra- violet Atlas of the large Magellanic Cloud (Page and Carruthers, 1978) and in the Revised 5201 Catalog- of Par Ul traviolet Obliects (Page, Carruthers, and Neckathorn, in preparatlon). In summary, for any identifiable image, the integrated intensity is proportional to tho,% denntty volume
V - X(dL-bL), where 41, and bL are the optical densities (as measured by the PDq microdensi.teiaeter used to scan the films, times 100) of each pixel In the image, and in background areas near (but outside) the image, respectively; the sum is over all pixels delectably above the adopted background. The subscript 1, indicates that the densities have been corrected for nonlinearities of the emulsion and mi.crodensitometer. The density volume can then be related to ultraviolet brightness by reference to preflight calibrations of the instrument and/or comparison of observations of objects in common w i th other photometrically calibrated observations, such as those of OAn-2 (Code and Meade, 1979; Code, Holm, and Bortemil.ler j
1980). We have determined, through comparison of our preflight calibration predictions with OAO-2 measurements by Code et al. (1930) that the absolute sensitivity of the S201 camera was probably a factor of 1.5 (0.45 stellar magnitudes) less, at the time of the observations, than :predicted by our
preflight calibrations.
J
4 I
Inspection of the far-UV images gives the qualitative impression
that the surface brightness of the UIC in the far -ultraviolet, relative
to the visible, is very high; particularly in comparison to the local A region of our galaxy and to the Andromeda Calaxy (Carrt,thers, Neckathorn,
and Opal., 1978). Figure 2 gives a more quantitative presentation of the
UV surface brightness of the IMC; shown are isodensity contours from the
10-minute 1250-.1600 A exposure. The density values have been smoothed
And corrected for nonlinearity. Based on our preflight calibrations, a
density above background of 0.1 corresponds to an intensity of 1.89 x 106
photons/cm2sec sterad at the effective wavelength (1400 A) of the camera.
For a flat continuum extending over the camera effective passband of
250 A, this corresponds to 7.56 It 103 photons/cm2 sec A sterad (1.07 x 10-7
erg/cm2sec A sterad).
in the IM C, determination of the 1N brightnesses of individual
objects is difficult, because of the Limited resolution of our imagery
and because of the multitude offield stars against which an individual
object must be observed. This makes determination of the true background
which should be subtracted from the measured density, in determinations
of the density volumes, very uncertain. -However, contour plots such as
that in Fig. 2 give useful measurements of the ultraviolet brightness
distribution over the face of the UIC, which are significant to studies
of the interstellar medium in the IMC (photoionizati.on and photodissociation
equilibria of many interstellar species are largely controlled by the
stellar ultraviolet radiation field longward of 912 A) and which, in
conjzinction with other determinations of stellar spectral type or effective
temperature, provide indications of the distribution of dust extinction
5 — over the 'UIC. Our measurements of the UV brightneooes of solected objects or areaf, are of poct4cal ut,il.ity in guiding observations with more sensitive nand/or higher resolution instrtrnonts, such as the International Ultraviolet Ex plorer and tha Space Telescope. We obtained a measure of the total UV brightness of the UIC in the
1050-1600 A and 1250-1600 A ranges oy skimming the donaitias of all pixels In the 1110 region of cacti frame, using an a background reference the uniform background donsities outside, but around the borders of.• , the DIC
Image. The contrMst,ions of seven SAO stars were also subtracted. The total brightness of the MC (based on our preflight calibrations) i►y the 1250-1600 A wavelengtl► range ( Aef f - 1400 A) is 220 }photons/am2 see A or
F1400 - 3.12 x 10-9 ergs/em 2 see A. This corresponds to a UV magnitude, in the system of Code at al. (1980), of m1400 - 0.23. Xn the 1050-1600 A range (lef f - 1300 A) the corresponding UV nfagnitude is 1111300 0.13. .+ .Averaged over the APPArent angular sic of the UIC on our image (about V diameter, or 9 x 10"3 sterad) the mean surface brightness is S1400
2.4 x 104 photons /M' 2 sec A sterad (3.4 x 10-7 ergs/cm2 see A sterad), and 51300 ** 2.5 x 1011 photons/cm 2 sec A sterad (3.8 x 10"7 ergs/cm 2 A sterad).
These measurements include both di.rc°ct and dust:-scattered starlight (we assume that nebular emission l.iw7s make a negligible contribution to the total UV brightness). As mentioned earlier, use of the OAO 2 photometry as a reference standard will increase the above intensity by a factor of I.S. Except for a minor correction due to interstellar extinction within our galaxy in the line I of sight: to the IlfC, this give ., an indication of the local stellar radiation field, on. the average, within tl►e UIC. The average surface brightness at 1400 A 4n corresponds to a radiation density of U1400 ^ c 81400 4 1.4 x ]O^lf' ergs/cm3 A• This may be compared with estimates of the radiation field within our own galaxy of about 10`16 ergs/cm3 A at 1400 A (Witt and Johnson, 1973) and about half this value predicted by Henry (1977).
i %o our Wa s (1978) we derIved a "hydrogen Index" (hereafter 11 Ind) as
Che ratio of fla flux, ►1A, to far-VV flux $ OF (corrected for dust extinctlon), at over 100 places in the UIC. This Index wan first presented no a rough on measure of the hydrogen near hot stars or star groups detected our far-UV 0 That Images. is, if the ionizing, extreme-UV (A < 912 A) flux is assuilled rovighly proportional to the far-11V flux, then the intensity of Ila emission to related to the local hydrogen density. It e re, ^•.e present a revised determina- tion of 11 Ind andits vari -.cion over the TAC, using n more recent determination % I of the 1110 extinction law t allowing for extinction at Zia as well as in the uV, the in and utilizing additional data oil Hot brip,litnene distribution the 1111.
Figure 3 is a contour plot of 11 Ind (times 100)0 the derivation of which is discussed in the following. The for-UV flux values are proportional to the measured densfty volume, V (corrected for nonlinearities and background) divided by the In in exposure title, E'o minutes. An shown our 8201 Catalog of rar-UV
ObJectis (1978) t a density-volume
V - 0.03 7 n (1) where n is the number of photoelectrons forming G a far-UV image, Thus,
V/E, - 6.17 x 10 -4 n per see (2) in where F, is the exposure title min, and n/sec is related to the photons arriving each see from the object. Tito, detection efficiency (photo- c1cct-rons, par p1hoto-a, boackli on flight caliLvatio,th) of Lite 5201 01flieril in the itinging mode averages 0.05 over the range 1050-1000 A with the U1,4 corrector, and 0.04 over the range 1250-1600 A with the CnF2 corrector.
Hence, the photon flux in these wavelengths is 3 "L/0-05(30.0) - 1,08 x 10 O /O Photons/sec C111 2 for 1300 A + 250 A, (3) Ni. - L and
N M n0/0.04(30.0) - 1.35 x 10 3 C /F:) photons/sec CM 2 for U00 A j- 150 A, (4) O (V
7 where 30.0 cm 2 is the aperture area of this 5201 camerae. Since these each photons carry 1.52 x 10-11 erg and 1.42 x 10 -11 erg respectively, the far-UV' flux in f -8 -1 -2 FL 0 1.64 x 10 (%/E) erg sac cm and (5) VC PW 1.92 x 10-8 (VC/ ) erg sac`l cm-2 . (6) These were corrected for interstellar extinction, based on previous estimates (Lucke 1.974) of the visual reddening (RE .. E(D-V)). In order to estimate reddening for all our measurements of V/13, for which specific values of RE were not available, we plotted Lucke's (1974) RE valuas and sketched In contour lines ( see Fig. 4). Although Lucke's 81 measured values are good
to + 0.05 0 corresponding to k 16 to + 176 in corrected ultraviolet flux ► Utz, there is inevitably some uncertainty in the interpolated values of 'RE, duce to small scale variations in the extinction at a given distance, and the uncertainty in distance to an object along the line of sight. '11w stellar associations for which Lucke determined RE may lie in front of or behind far--UV objects with nearly the same celestiatl. coordinates. However., it Is highly likely that an 1.11 cluster and an associated 11anixe nebula are
in close 3-dimensional proximity.
Tr. the Far-UV Atlas, we used the "average" galactic inter g tel.lar ex- tinction curve of Bless and Savage (1172). However, measurements with the ANS satellite (Bo'rgman and Danks, 1977; Koornncef, 1978) in the 30 Doradus region, and with 1UU (handy et al. , 1080) there and elsewhere in the i.J`iC indicate a higher ratio of far-UV extinction to L(B-V) in the 1,MC Haan is typical in the local region of our galaxy (see Figure 5). Using the extinction curve of Nandy et al. (1980) with Al - 3 E(B-V) + E(A-V), we have, for effective wavelengths of 1300 h (LiF corrector) and 1400 A (Ca F2. corrector), rX1300-V)/E(B--V) - 8.97 and 1.(1400--V)/E(B-V) - 7.09.
8
4-
Therefore, the ultraviolet fluxes corrected for reddening area
104.8 I1g UI'L W F (7)
iO4.0 HP UPC . F'C (8) An expected, UFL values for an object are generally larger than the i UFC values because of the wider bandpass and letrger extinction correction
at the effective wavelength of 1300 A. The scatter in the JAC extinction
curve of A'andy at al. (1980) in about 0.2 mng. The extinction correction r at Ila is asnuned to be A6563- 2.5 RE; branch tbp corrected Fla flux is
1111A w IIA-1011 ', approximately, where FIA is titer Ila flux as measured by
Hanire et al. (1956) in unite: of 10"4 erg/cmzsec sterad. The I1A
values given here are often summed for severed close 11 11 regions that
could not be separately resolved on our 5201 photos. For instance,
I N18OA-C means the summed flux from ` .A,OI 1+1£3013, and N180C. 'ter order to
gent a single hydrogen Index represent;Ing all measurements or a given
object, we nvernged tire, values for two 111 frames with 1/2 the values
for two ICa frames:
If Ink ; UIIA/UF'L .(9) If IndC R UIIA/UFC (10)
If Ind .. (if IndL1 + 11 IndL2 + 1/2 H IndCl + 1/2 it IndC2)/4 (1].)
The major errors in V/F, UF, and H Ind are due to uncertainty in
background, b. As Fig. 2 shows, many of the objects measured are in
regions where the background density is changing. The local background
was estimated on mosaics of d, taking{ the first minimum in d in each of
four directions from the peak density, along +x, +y, -x, and -y, and
averaging these to let b. The background is high and posed the most
} difficulties on the 3-min ILi exposure, frame Al25. I
9 k+
a m__ JM .i.a riB 1 Tito IIA values Ara probably good to ± 10 %. although values near zero Ara subject w lar11ar percentage arrors. In fact, DER, in a careful
survey of a 5-hour exposure with the SRC arm-inch Schmidt camera using an r Interference filter with 100 A bandpasa centered on }la and [N11), found
the faint lionize 11 11 regions much larger, and detected 100 more, most of
them fainter than Flenixe's limit. They give no quantitative moanurements
of brightness, but,uOe tho steps of (very faint), f (faint), fb (fairly
bright), Is (bright), and vb (very bright). We calibrated this scale
against lIA by assigning the numbers of - 1 0 f .. 2, 1b - 3, b - 5, vb - 10,
and ►nulti.plyin,g by the dimensions given in are-.t ►in. For instances, n
faint (f)nebula of sire 3.5' x 2' has a brightness (arc-mi,n) 2 of 2 x 3.5
x 2 - 14. Fig. 6 is n plot of these values against IIA for 64 cases where
the 1)EM dimenslons are roughly the same as Ilenize's. To a fairly good
approximation,
D M brightness (arc-min) 2 w 3 NA. (12)
0 Using thin calibration, we cool.d fill in 227 11 11 regions at
positions in the 111C where we had measured far-UV flux, leaving out only
19 DEM objects of the total of 356. (These positions were all searched
on our mosaics.)
Ilse surface; brightness of a pure hydrogen emission nebula at fla is
proportional to the volumetric recombination rate (which in turn is
proportional to the square of the hydrogen density) and to the diameter
of the Str8mgren sphere. For a given local hydrogen density, the diameter
of the Strlimgren sphere varies as the cube root of the stellar extreme
ultraviolet (MV) Lyman-conti tits um, photon flux Ng UV ; for a gives'r M stellar HUV flux, it is proportional to nH-2/3 . Tr►js the Ila surface brightness is, in total, proportional to'N'EUVi/3 nil ' /3 . lie assume that•
t I°- 10 the diameter of the Str5mgran sphore in larger than the limiting resolution (about S are nee) of the Ilenire survey, which at the 52-kpc distance of than DIC amounts to About 1:. 1¢ pc. lining the data for typiciil, theoretical StrNgron aphaares Opitrar, 1978 0 p. 110), thin will he truce for all stare 110 or earlier at a ll C 60/cm30
The effects of interstell ar extinetiton, both Within and near the 11 XX regions, can be very marked, especially ':or regions with Urga till and correopoodi.ngly high dusL densities. Although the Ho extinction is arauc:h lass than the far-UV extinction, the average extinction over the projected area of tho It IT region is not necessarily The saamt an that in :front of the enclosed hot stars. Monsurrments of radio continuuat and recombination lines should help determine, the extinction corrections to the IN E nieasurementoo lleve ,aer, since the far-UV exti.rction corrections for most objects we observed are amnll., our atioumpti.on that the nebular lia extinction is equal to the stellar extinction at Ila should have little effect on our results. It is apparent that determinations of local hydrogen densities from
Ila intensity measurements are very sensitive to the Inferred or predicted BIJV ionizing flux. Ground-biased methods includti determinntion of the stellar spectral class which, in combination with model-atmosphere pro-- di,ctions, can be used to infer the EUV flux. Another method is measuring directly the ditarieter as well as the surface brightness of the Str8mgren sphere. Both of these methods have many well-known difficulties. The measurement of the stellar far-UV (1050-,1600 A) fluxes provides additional independent data which, in combination with the groa-nd-based mica surementa, hell, to specify more accurately the stellar effective tempertature, and
a
f^ 11 a
thereby better Infer tho NV flux. This, in turn, can yield more accurate estimates of local hydrogen density. Mansuremants of the exciting in the far ultraviolet, although not directly indicative of the Lyman continuum, are much more useful than measurements In visible because wavelengt1is they are much closer to the Lyman continuum, Also, by being near the peak continuum of eatty-typa stars, far-UV Is much more sensitive to small differences In effective temperature. Figure 7 shown model ntmosp;lera stellar flux distributions computed by Xurucz, Paytremann, and Avratt (1974), normalized to 5500 A. In addition, Ar-11V measurements, in combination with ground-based measurements, can be used to estimate the affecto of interstellar. extinction with much better accuracy than call measurements in the F,round--accessible in wavelengths alone, because extinction (patticulorly the 1110) 113 so much larger below 1600 A. rdeallyp it would be batter still to also have the and measurements in "extinction bump" near 220011 (see Bless Savage
(1972)p Savage (1975)o Nandy et al. (1980)) * Assuming that interstellar extinction can be determined from a combination of ground-based and UV data Figure 7 shows that measurements of the far-UV/visible ratio car, be Used to infer the effective temperature, and hence the ETV/visible ratio.
This ratio is, however, increasing considerably faster with effective temperature than is the far-UV/visible ratio in the temperature range of interest, and hence accurate measurements are nacessary. Figure 8 shows ratios of integrated EUV/visible, and far-UV (W spectral ran8e)/visible, where the visible photon flux Is integrated over the range 5000-6000 A.
Also shown are the ratios of EUV to far-UV photon flux, and of photon fluxes In than ILI and No spectral ranges. (EUV is labelled LyC for
Lyman continuum. )
12 an The Hydrogen Inden, derived from o-jr mansuromanta t Sivas only
qualitative indications of stellar effective te .mporoturd And local hydrogen
donalti000 Thin is because the limited angular resolution of the $201 camera
prevonte p in most cases, the Attribution of a given UP value to n single stag, the and Nance compArition of 1JV flux with ground-based measurements of the same
star. In a rich cluster or association, therefore, A given UP could be
produced by a single 0 star with effective tomparature of 40 0 000 K 0 or by it 19 I I Cluster Stars Of It with effective temparaturao near 20,000 K 6 but the Lyman
continuum flux would be much larger in the former can*# 11owaver l the Hydrogen is index a clunntitatively useful criterion for analysis of higher•renolutlon
measurements, in which single stars can be isolated with the WE
satellite), and in which flux distributions and effective temperatures can - be determined individually for all of the UV ' bright stars associ a ted with i n given 11 11 rrg^,on, In the following, we present a qualitative c omparison
of our Hydrogen Tn4ex mensurements with 21•cin observations of atomic hydrogon,
and ditwuss the potential of TUE observations for refinements of both
interstellar hydro ijen and effective temperature mansurementso
III * COMPARISON WITH 21 ••CH OBSERVATIONS
More direct methods of estimating interstellar hydrogen cont. ,en trat ions
include at-masurirg the ljyman-a interstellar absorption Aloe; in the spectra
of hot UIC stars, and mapping the 21-cm radio emission across the 91C.
t Ilie 'La measurements are to be preferred over 21-cm measurements for
several reasons (Carruthers, 1970, and Jenkins, 1970) 0 such as better
spatial resolution, discrimination against hydrogen beyond the star of
4
13
I - . interest, and freedom from the effects of spin temperature, concentration, etc. Nevertheless, since 21-cm measurements were .available and La measurements (until recently, with the advent of the WE satellite) were r not, we decided to compare our hydrogen Index values first with radio measurements of hydrogen in the IMC.
We compared Fig. 3 with the 21-cm survey by McGee and Milton (1966).
'11ieir measurements of brightness temperature, Tb , are presented I.n three different contour plots, showing values for radial velocities near 3()0,
273, and 243 km/sec. We combined these., taking the largest Tb at each location, and this combined 21^-cm flux is presented in Fig. 9, where contours of 20, 30 0 40, and 50 flux units are shown. (1 flux unit
1.76 K in Tb.) Although there are some similarities between Figs. 3 and
9, therc are some notable differences, where peaks in if lnd occur at low values of the 21-cm flux. McGee and Milton noted one of these in comparing their hydrogen clouds with Karl llenize' q (1956) 11 II regions; the nebula ► N55 at 5h32m3 - 660:28 1 has no strong 21.-cm flux near it.
There are at least 30 otbet, similar cases in Tables 2 and 2. Moreover, we find about 50 regions of high U1• in the 11 x clouds with little or no ila emission, and therefore zero or low 11 Ind, as shown In Table 3. Tables 1,
2, and 3 have 17 columns, the first 16 being the some in all three tables.
Col. 1 is the Davies- El l.i.ott-Meaburn (1976) or Heni:ze (1956) number,
mostly blank in 'fable 3. For the. Henive (N) numbers, N77A-E means
N77A + N7711 + N77C + N77D + N77F N79CC mans N79C + N79C; Nb,A means N8 4• N8A. A blank means no measured Ha flux.
Col. 2 lists Che 1950 coordinates of the area measured on S201 frames
Al24, Al25, Al29, and A130.
14 Col.. 3 is the Lucke-1lodge (1970) number of a stellar Association in the
IMC.
Col.. G is the NCC number of a star cluster in the 11G.
Col. 5 lists the north-south and east-vast dimensions Qf the measured
area in arc-min.
Col.. 6 is the 110 flux in units of 10-4 erg sec`, cm-2 sterad-1 summed for
all the n*o'-wins entered undir N No. Col. 7 is the unreddened flux (Ul+) measured on W frtsmes Al24 and Al25r
avernged.
Col. 8 is 100 times the 114 Hydrogen Index calculated from cols. 6 and 7.
Col. 9 is the unrectdeneJ flux (UP) measured on ICa frames Al29 and A130,
averaged. a
Col,. 10, is 100 times the ICa Hydrogen, Index calculated from cols. , 6 and 9.
Col.. 11 is 100 times the mean of 1>,i 11 Ind and 1/2 ICa 11 Ind, our best
estimate of the Hydrogen Index for the 11 11 region(s) listed in
col. 1.
Col.. 12 is the McCee--Milton (1966) H I-cloud number.
Col.. 13 lists the north-south and east--west diameters of the cloud in arc-
min.
Col. l ei is the 21-cm flux at the location of the measured area giv>,tn in
col. 2, in units of 1.76 K in Tb.
Col. 15 lists the 1950 cw dinates of the H I-cloud center.
Col. 16 lists the distance in arc-min and the approximate direction from
cloud center to the measurer, area given in col. 2.
Col. 17 in Tables 1 and 2 gives the MC catalog (McCee et al. 1972) number
of a radio source coincident with the H 11 region, and the letters
15
J SNR qtr supernova remnant identified by its non-thermal radio spectrum.
Col. 17 in Table 3 is the reddening (RE), or color excess, interpolnted from
Lucke's (1974) measurements.
In five regions of the LMC, Table 1 shows both high hydrogen Index and high 21 -Cm flux. These regions are centered at:
4 517.5-69p 20' involving N77,79,81, mean 11 Ind - 69, and 11 I cloud L34, 21-cm flux 38
4:58.6•-66:18 N11012,130 64 L2 42 5:14.3-69:25 N1120114 127? 139940043 30 5: 29.1-71:15 N1999200,206 47 L46j47 30
5:34.0-67:39 N56157159 100 113914 35
, These regions are evidently in the VT clouds and well populated with clusters of 0-1i stars, from 1112 and NCC 1727 in L34 to 11176 and NGC 2014 i in L13 and 114. The mean 11 Ind x 100 is about 2.3 times the 21-em flux.
Five other 11 Ind maxima in Table 1, and all those in Table, 2, total
25 11 11 regions outside of the 11 I clouds, where the unreddened far-UV flux, UC, is strong enough to ionize the hydrogen where the 21-cm -flux is only 10 to 20. This indicates small 11 I concentrations along the line of sight. The mean H Ind x 100 is about 4.75 times the 21-cm flux. The ratio of the 11 I1-region area to the H I-cloud area io Table 1 ranges from less than 1% to 52% for 114 and 59% for L2, with some indication of a correlation with the 21-cm flux, which ranges from 15 to 50 units. Eleven 11 1 clouds have more than 15% of their area covered with 11 1I regions, and the average for all listed in Table 1 is 127..
16
• In Table 3 there are 38 regions in the It I clouds where there is high OF from clusters of 0-H stars and little or no Ito flux, leading to
low 11 . zero or Ind. These clusters of f0 11 stars must therefore be in front of or behind the N 1 clouds. From the RE values -- E(8 -V) -- in Table 3, it seems likely that four clusters are behind the H I clouds:
It m o NCC 1734 (and 1)l4,16) tit 4:53.3-68:56' , behind L23
L1185 0 89 and NCC 2042 at 5:36.2-68:55, behind L32
and at 5:36.5-68:57, behind L32
and NCC 2100 at 5:42.4-69:13, behind 1,32.
Most of the others are probably in front of the 11 1 clouds. The sizes of these strong OF areas are smaller, than the H 11 regions in Tabl 1.
They range from less than 17. to 207, of the 11 I--cloud area in L25, and
28% in L10; the average of alA listed in 'fable 3 is 6%. It is unlikely that these are foreground stars, since SAO stars have been omitted from
the list.
We conclude that the local hydrogen density near the objects listed
in Table 3 is too low (below a 2/cm3) to produce a measurable Ha nebula, although the total column densities are large. This indicates that the
Hydrogen-Index method may provide useful measurements of local hydrogen
density, which can be compared with the column densities observed by
other methods, such as 21-cm emission and La absorption.
IV. ME OBSERVATIONS
Extensive measurements have been made of the column densities of
interstellar atomic hydrogen in the lines of sight to relatively nearby i galactic stars using the OAO-2 fax ultraviolet; spectrometer (Savage and
17 Jenkins, 1972; Jenkins and Savage, 1974) and with the much higher-resolution
Instrument on the OAO-3 (Copernicus) spacecraft (1lohlin, 1975; Roblin, Savage, and Drake, 1975). However, troth of these instruments were limited to relatively bright stars, and so were unable to obtain observations in the Large Magellanic Cloud.
The International Ultravioict Explorer (IUC) satellite, however, can observe much fwinter objects than its predecessors, allowing observations w of interstellar 11 in the directions of 0 and early B stars in the UIC at low dispersion (7 A resolution); at longer wavelengths, a few objects have been observed at high dispersion (0.1 A resolution). de Boer,
Koornneef, and Savage (1980) observed 111] 38252 (8144) and 11n 38268 (8136), obtaining 11 cols mr. densities of 1.9 x 1021 /cm2 and 7 x 1021/cm2,
respectively. St:,traction of an cstimated local. galactic column density f of 7 x 1020/cm2 yielded 1.2 and 6.3 x 1021 /cm2 , respectively, for the LMC contribution to the observed column densities.
In early 1979 (27 ,January - 3 February) one of its (TP) obtained WE observations of a number of LMC stars, in low dispersion mode, which were associated with bright objects in the S201 UV imagery. These spectra were taken for the purpose of measuring the hydrogen column densities
(from the Ira absorption features) and for obtaining measures of the
absolute flux distributions and spectral types for correlation with the
direct imagery. The procedure used was to select the brightest star (as
camera) associated with a selected 1.N association seen by the WE slit ,jaw f
And/or ilenize nebula. In some cases , this star turned out to be of late
spectral type,and yielded an underexposed spectrum. (The coordinates
I8 we had available were not sufficiently accurate to allow us to select individual stars for which ground-based photometry and spectral classification were available.)
Table 4 lists 30 WE spectra of 14 different stars in 12 of the associations, including one from fable 1 and three each from Tables 2 and 3. In one set of two exposures, two stars were in the slit (large aperture, 23.2 x 10.4 aresec), and two separate spectra of LH 74 NGC 2015 stars were obtained, somewhat underexposed. Column 1 of Table 4 lists the ltenize (1956) N number or the Davies-Elliott-Meaburn (1976) D number, and the the Lucke-Hodge (1970) Association or NCC number. In some cases, there is no Ila nebula. Column 2 lists our Hydrogen Index (x 100); Column 3 the McGee-itilton (1966) 21-cm flux; Column 4 is IUC far -UV (SWP) image , number; Column 5 the exposure in minutes; Columns 6 to 8 are the continuum fluxes at 1300, 1400, and 1500 A relative to that at 1925 A; Columns 9 to 14 are rough equivalent widths of La, Si 111 (1300 A), C it (1335 A), Si IV
(1394 4 1403 A), C IV (1550 A), and the feature at 1720 A. Column 15 i> the reddening (RE) - C(B V), interpolated from the values of Lucke (1974);
Column 16 is the mid-UV (LWR) image number; Column 17 the exposure in minutes; Columns 18 to 20 are the yontinuum fluxes at 2325, 2675, and
2900 A, relative to that at 1925 A; Column 21 is the equivalent width of Mg II (2804 A); and Column 22 is the spectral type estimated by Karl
Nenixe (private communication, 1980) from the Si IV to C IV line ratios,,
or by us from a comparison with Copernicus U2 spectra of standard stars
degraded to 6.2 A resolution comparable to that of the WE low-dispersion
spectrograph.
19 All of the measurements were made from the WE Calcomp plots of the net spectrum flux number vs wavelength after correction for distortion, nonlinearity, and initial IM, calibration error. The equivalent widthu are products of the line width at half depth and the central depth as percentage of continuum. La has teen corrected for geocoronal emission by using the La emission in the small-aperture spectrum, as shown in
Fig. 10. The ratio of Area of the large aperture to' that of the small aperture (3.2-arc-sec circle) was determined (Penston, private communi- cation) to be 31.1 ± 1.9, and the ratio of widths parallel to dispersion is 4.0. The small-aperture profile on the Calcomp plot is always a triangle of base w and height P, This was scaled up to a triangle of base 4w and height 31.1 P/4 - 7.75 P centered at 1215 A (the apparent wavelength of La at the WIC radial velocity), and subtracted from the large.-aperture plot. (This scaling factor was confirmed by ME spectra where: only pa:ocoronal La was present in both apertures.) The remaining
La, the stellar absorption line (or no line, or emission line) was then measured for equivalent width in the same way as the other stellar lines.
In two cases, the stellar contribution came out as an emission line i (LB 88 and LH 89 in Table 4).
The errors in measuring these line profiles are rather large, but the potential for improvement is limited by the low spectral resolution and photometric accuracy of the raw data. For stars of spectral type later than B2-133, the width of the ztellar La absorption is comparable to or greater than that of the interstellar line, preventing determinations of the interstellar H column density by this method. However, for the hotter stars in which the stellar contribution is neg-l igibla ► the hydrogen column density can be estimated from the relationship
N (11 I) - 1.37 x 1019(W1216)2 (13) where W1216 - 1.476 MIN (York, 1976)= These column densities, also listed in Table t+ ► are subject to large uncertainties clue to errors in correcting for geocoronal Ira emission (as mentioned above) and for nearby stellar features, such as blueshifted N V absorption and Si 111 (1200 h) absorption. Two examples in Table 4 where the La equivalent widths are clearly in excess of the stellar component (based on they spectral type derived from other lines), are 1.11 111 and 1,11 6/4.
Spectral types were estimated, and flux distributions measured, For the observed stars for comparison wi.fih the 5201 imagery ► and for •refining our Hydrogen Indox measures. The spectral typrv.: in col. 22 of Table 4 are entlmates by K. Henize based on the equivalent widths of Si III (1300 A), Si IV (1394, 1403 A), and C IV (1550 A) and by one of us (T1') using Si. 1I1 0 C IT (1335 A) , and Si IV for types later than 133.
Surprisingly, there is only one supergiant in this sample of bright early-type stars in the INC. The deviations are about two spectral classes In the 1, types. As indicated in Table Vii, the luminosity classes are also approximate.
The continuum fluxes ( relative to 1925 A) were determined using the
latest ME calibration (Bohlin et al., 1980). These are listed in Table 11
(and plotted in Fig. ll.). The Continuum intensity generally decreases
from 1300 tot 2900 A, as expected for early-type stars. For comparison with the model atmosphere predictions of l urucz et al. (19711), Table 5 was
21. generated using the reddening law of Nandy et a1. (1980), Fig. 5, and the RE values of pucka (1974), see fig. A. Table 6 gives the bast matches of the observed flux distributions to ,the reddened model predictions. It
Is seen that, in several cases, the observed flux distributions Indicate considerably higher effective temperatures than do the ultraviolet line ratios. This could be due to (a) hotter but less luminous Tatars which contribute to the continuum more than to the spectral line absorptions,
(b) overestimation of the local reddening, and (c) measurement errors in the line and/or continuum determinations. All in all, these IUE measurements tend to confirm our conclusions from the S201 far-UV measurements, including spectrographic results (Carruthers and Page, 1977) but do not add very much. Because of an unexpected early assignment of IUE observing time, we could not select individual stars classified by Lucke and llodge (1970), Ardeberg et al. (1972), and Walborn (1977). Also, we were not able to systematically observe all of the UV-bright stars in specific associations associated with specific Ilse emission regions. Ilo«ever, further analysis of our present observations, and follow-on observations with IUE, will allow better correlation of our results with previous ground-based observations as well as with the S201 measurements.
V. CONCLUSIONS
The far-ultraviolet brightness distribution over the face of the
Large Magellanic Cloud has been determined from calibrated electrographic imagery in the 1050-1600 and 1250-1600 A ranges. Far-UV fluxes for individual hot-star groupings in the 11-IC have been compared with lla measurements (llenize, 1956; Doherty et al. , 1956; navies et al. , 1976) z 22 and with the McGee and Milton (1966) 21-cm survey. These comparisons indicate that large clouds of interstellar hydrogen contain smaller concentrations revealed by Ilax emioolon, and other clear regions where hot
Q-1 stairs excite no 11 TI regions (undetectable Ila), Four of thecae associations probably lie behind the large interatellar clouds.
Alternntively, these clouds oay be very diffuse and extended in the line of sight. Six other peaks in far-1V flux not previously catalogued are also indicated. Initial exploratory investigations of It 14 absorption using the JUL satellite tend to confirm these results.
t►'e recommend ,further IU[ measurements of la absorption and Burt extinction in the spectra of hot stars observed by Lucke and Hodge (1970) in the DIC asaoci.ations listed in Tables 1, 20 and 3, as well as radio continuum and recombination-line measurements, and higher-rosolts ion 21- em measurements in these regions. More detailed HE measurements of flux distributions, and spoetral typa/effecti.ve temperaa:ure deternainaations, for all stars associated with particular ila emission regions would allow for our Hydrogen Indiex measure to be placed on a more quantitative basis. Ili.gh-resol.uti,on observations at La would be highly desirable; although marginal with TUE these should be readily possible with Space Telescope. Cround-based photome4ry, with angulnr resolution equal to that of the
S201 Camera (3 arc-min) in areas around the OF peaks we observed, would
be pnrticularly useful, as well as more detailed photometric measures of
the individual stars. The photometers should utilize narrow-band inter-
Terence filters, so as to isolate emission-line--free segments of continuum
for stellar photometry." In addition, measurements of nebular emissions such as Fla, but with higher photometric accuracy than in previous worn are
d
23 needed. Higher angular resolution 21-cm mansuremcenta, if possible equal to or batter than the 3 are min resolution of $201, would be very useful. Part of this research wnn oupported by NASA Grant NASW-3023. We thank Dr. Karl Ionize for useful, discussions, and tho IUK observatory Staff for their support of our observations. The IUB observations and data reduction were oupported by NASA Grant NAS5-254$1.
K
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C- pr 93 itl o O in 00 00 0 O O in O in M cn t0 U) F+ 'y • • • • i e • • • ° O in in O M N. •-+ rz .-i .-, '-^ v H in v o°'o 14 H 00 (n 00 ^D N C4 cn t•. d .M O N N O ttS `c1 •v 2 M M• • • • • • • • • • • • • • • G N iJ O .4 -to W N N 1; r-4 14 M N H N .4 ,C UJ 'd p ;o .7 rn N O co in O 0) %D M 00 00 d •r^ L:^ w-i .-1 N .-1 14 w 4 O 4 4 M N N /ed) QId1 3 u I N N en ^ O T1 00 M O f\ co O; 00 %O O t-% 00 O +"1 t`'1 0. 'rr tvi M • • • • • • • • • • • • • • • 14 .0 ,? i al N N .7 M N u1 4 in .? .7 in • ►-1 H M M M • 0 eC +- W in 1- a! in in r* a^ a r. O 00 W O -4 .^-1 •N-1 "1 .M-1 cei 1` .-1 N M W ra Ch tto N . ^ O M kD -*.7 ^D in Cl %O U) in r-1 C\ ON N O ^ •ai O cn + eT .7 m 1-. 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H 1-i .. eU ,'a N ad p^ ODa) O^ OQ P^ ^D 00 M .-1 N C,% C^ .7 O 00 +t 9S t^ 1 Cl tan M N M in N N %D in in N M v1 00 QI N S p0 W .--i O O O .-1 r-1 .-1 -4 v C+ --1 O O .-1 •• •-^ p .7 IT O O 10 ppj .7 .7' .1 .7 -7 .7 T .7 .7 .71 O 94 N ro d -+ 01 M o p o n G in 00 00 m O. in o rn M t; in N a ► ►O N .-1 M N N .-1 r1 r 1 . 1 f . 1 1-1 N N Cn O .-1 u x WO qq .7 in .7 Ol Ch d O p (D C• C) O in O O N .-4 to ^{ rl -.0 ^O ^O O l en O N N 'ri 1a p x u/; o u U x' in D. r, N o r-1 al U U W S in x 00 nh +.. -4 0O C% • Cq In N * CO •-1 .-1 r--1 * .Y N ^-r N .'17 i` ON C) ►'1 •r) in 1 i 4 U N .-1 in in i` 00O r-1 O .-1 ;; 00 -.D ^.c M P- M P- fl- 00 i^ 00 r'1 10 M .-1 O 0 C) '.4 x ^ -I 1,4 ra ^ a'^ a ►° zaa A ^ A aa^ *4 a a %1. Aa Table S. Continuum Flux Relative to 1925A, Reddened KPA Hode1r. To RE 1300A 1400A 1500A 1925A 2325A 2675A 2900A 14000K .00 1.805 1.705 1.543 1.000 0.667 0.531 0.460 14000K .05 1.57 1.59 1.51 1.000 0.701 0.600 0.535 14000K .10 1,37 1.49 1.48 1.000 0,738 0.678 0.624 14000K .15 1.195 1.39 1,445 1.000 0.776 0.765 0.727 14000K .36 0.670 1.053 1.33 1.000 0.963 1.275 1.385 16000K .00 2.06 1.86 1.65 1.000 0.649 0.489 0.412 16000K .05 1.79 1.73 1.615 1.000 0.682 0.552 0.478 16000K .10 1.565 1.625 1.58 1.000 0.717 0.623 0.559 16000K .15 1.363 1.52 1.545 1.000 0.755 0.705 0.650 16000K .36 0.765 1.15 1.42 1.000 0.935 1.173 1.24 f 18000K .00 2.30 2.00 1.757 1.000 0.630 0.454 0.374 ` 16000K .05 2.00 1.87 1.72 1.000 0.662 0.512 0.435 18000K .10 1.745 1.75 1.68 1.000 0.696 0.578 0.507 18000K .15 1.52 1.633 1.655 1.000 0.733 0.655 0.591 18000K .36 0.853 1.235 1.51 1.000 0.909 1.09 1.125 20000K .00 2.455 2.125 1.853 1:000 0.612 0.427 0.346 ► 200001. .05 2.14 1.98 1.81 1.000 0.643 0.482 0.402 20000K .10 1.86 1.86 1.78 1.000 0.676 0.545 0.470 20000K .15 1.625 1.735 1.735 1.000 0.713 0.615 0.547 20000K .36 0.910 1.315 1.595 1.000 0.893 1.025 1.04 25000K .00 2.96 2.34 2.025 1.000 0.575 0.376 0.294 25000K .05 2.58 2.18 1.98 1.000 0.604 0.424 0.342 25000K .1.0 2.245 2.04 1.94 1.000 0.635 0.479 0.399 25000K .15 1.96 1.91 1.90 1.000 0.670 0.542 0.465 25000K .36 1.10 1.445 1.745 1.000 0.830 0.903 0.885 r iI Table S. (Cont.) 7o RC 1300A 11#0011 T15 00A 1925A 2325A 267511 2900A 30000K .00 3.135 2.411 2.077 1.000 0.555 0.350 0.267 30000K .05 2.72 2.26 2.03 16000 0.583 0.395 0.310 30000K .10 2.38 2.13 1.99 1.000 0.614 0.446 0.363 30000K .15 2.07 1.99 1.94 1.000 0.6115 0.504 0.422 30000K .36 1.16 1.51 1.79 1.000 0.800 0.840 0.805 35000K .00 3.08 2.48 2.037 1.000 0.565 0.355 0.271 35000K .05 2.68 2.31 1.99 1.000 0.593 0.400 0.314 35000K .10 2.34 2.17 1.95 1.000 0.625 0.452 0.368 35000K x15 2.04 2.025 1.905 1.000 0.657 0.511 0.428 35000K .36 1.14 1.535 1.75 1.000 0.815 0.851 0.115 40000K .00 3.32 2.66 2.135 1.000 0.558 0.346 0.262 40000K 105 2.88 2.48 2.085 1.000 0.585 0.390 0.304 40000K .10 2.52 2.32 2.045 10000 0.617 0.441 0.356 40000K .15 2.20 2.17 2.00 1.000 0.649 0.498 0.414 40000K .36 1.23 1.645 1.84 1.000 0.805 0.830 0.790 45000K .00 3.42 2.735 2.175 10000 0.550 0.339 0.255 45000K .05 2.97 2.55 2.125 1.000 0.578 0.382 0.296 45000K .10 2.59 2.38 2.08. 1.000 0.608 0.1'32 0.346 45000K .15 2.26 2.23 2.035 1.000 0.640 0.488 0.403 45000K .36 1.27 1.69 1.87 1.000 0.7 0.4 0.81E 0.768 i 50000K 1100 3.48 2.80 2.21 1.000 0.542 0.332 0.249 50000K .05 3.025 2.61 2.16 1.000 0.569 0.374 0.289 50000K .10 2.64 2.1+45 2.16 1.000 0.600 0.423 0.338 50000K .15 2.30 2.29 2.07 1.000 0.630 0.478 0.394 50000K .36 1.29 1.73 1.92 1.000 0.782 0.795 0.750 i 1 Table 6. ttatch of Specrrral. Type, KPA Continuum nux , and Color Excess Continuum Match „1 tl Re reds Expected 1300-192: 19?.5=2900A No. RE Typo To To RE Te RH "30000 .05 72 .05* 138- 9? 13000K ^ ,40000 .10 >50000 .00? 64 .09* N1 24000 50000 .05 50000 .10 79 .11* 111-3 22000 50000 .05 20000 .10 35000 .05 50000 .05 114 .12 04-81 32000 40000 .10 40000 .05 88 .11 Ve 14500 30000 .15 30000 .15 20000 .20 N1818 .17 R2 22000 , 20000 .05 { 14000 .10 77 ' .11 131 24000 20000 .05 14000 .10 Jj 1 16000 .15 89 .42* l31-2e 23000 { 18000 .36 18000 .20 Underexp. 15 .09* 132-3 20000 18000 .15 20000 .05? 14000 .10? 14000 .17? 111 .31* 131 24000 { 20000 .36? 20000 .20 .36? 55 .11 Obi-07 38000 1x000 ^.20? >50000 .00? i 74#1 .14* 135? 16000 20000 .10? >50000 .00? Underexp. 7412 .14* B9? 12000 20000 .107 >50000 .05? Underexp. a 0 r-- REFERENCES ArdeberB, A., Brunet, J. P., Maur ca, IK., and Prevot, L. 19720 & e: a 1. , 6, 249. Astron. Astro_.. eh a. S M Bless, R. C. and Savaged Be D. , 1972, Ap. J. 0 171 0 2913. t BoRin, R. C. 0 1975 0 Al>• Jam, 200, 1102. Bohlin, R. C. , Savage, B. D. , and Drake, J, E. , 197 8p Ap. J. , 224, 132. Bohlin, It. C. , Holm, A. V. , Savage, 13. D. , Snijders, M. A. J. , and Sparks, W. M., 1980, Astron. & Astropb s. , 85, 1. Borgittan, J. and Danks, A. C. , 1977, Astron. & Astrophxs. , 54, 0. Carruthers, C. R., 1970, Spnee Sci. Revs. , 10, 11 59. Ca rru the rs, G. It. , llecha tho rn, H. H. , it nd Opal , C. B. , 1978, Ap. , J. 225 0 346. b Carruthers, G. R. and Page, T. , 1972, Chap. 13 in Apollo 16 Pre. im " Sci. I+`.eport, NASA SP-315. Carruthers, G. R. and Page, T. , :977, A . J., 211, 723. Carruthers, C. R., 1973, Appl. Optics, 12, 2501. Code, A. U. and Moade, M. R. , 1979 0 Ap. J. Supp. , 39, 195. Code, A. 1?. , Holm, A. V. , and Bottemiller, R. L. , 1980, Ap. J. Supp. , LO O 501. Davies, R. D. , Elliott, K. 11. , and Meaburn, J. , 1976, Mem. R. Astr. Soc. , 81 , 89. de Baer, K. S., KoornneeC, J., and Savage, B. D., 1980, An.! J., 236, 769. Doherty, L. , Ilenize, K. G. , and Aller, L. H. , 1956 0 Ap. J. Std gyp. , 2, 345. Heni.ze, K. G. , 1956, Ap. J. Supp. , t, 315. Henry, R. C. , 1977 0 p. J. Supp. , 33, 451. Jenkins, E. B., 1970, in Ultraviolet Stellar Spectra and Cround-Rased Observa- tion;;, (Houziaux, L. and Butler, 11. E. , E ds., Reidel, D. , Dotit;recht), p. 2.81. Jenkins, E. B. and Savage, B. D., 1974 0 Ap. J., 187 0 243. Koornneei', J., 1978, Astron.,& Astropllys. 6^i, 179. Koornneef, J. and Mathis, J. S. , 1980, preprint. W ^ Kurucz, R. Le , Peytremann, k;. , And Avrett, V. It. , 1974, Illanket:ed Model Atmospheres for tart -T r Stars, Smith soil On Inati.tuta, Wnshing,ton, A. C. Lucke, P. Be and !lodge, P. We, 1970, Astron. Jour., 75, 171. f^, l,uckr, P. Be , 1974 0 A?. J. Sup 28, 73. MaGoe, R. X. and Milton, J. A. 0 1966, Australian Josar. Pby ., 19, 343• HcCce , R. X. , Broolts, J. 14. , and 1 itchel.or, R. A. , 1972, Australian Jo,ar. Ph e. , 25, 581. Nandy, R. , Morgan, U. He , Willis, A. J. , Wilson t R. , Gondhaletthar, P. M., and Houziaux, L. , 1980, Nnturr, 283, 725. Page, T. and Carruthers, C. R. , 1977, COSPAR S nee, Research, Vol. (Perf;ampn, New York) , p. 74. Page, T. , Carruthers, G. It. , and Hill, R. 1 .,, 1978, 5201 Caralo8 of Isar-UV Oh' acts, NRL ''Report 81,73. Page, T. and Carruthers, C. R. , 1978, 5201 F,,tr-UV Atlns of the LMC, NRL Report 8206. Pottasch, S. R. , 1965, Vistns in Astronon4 , Vol. 6 (Bo er, A. , Ed. , Porgamonp New York) p. 149. Savage, Be U. , 1975 s h2!_ J. , 199, 92. Savage, Be D. and Jenkins, E. U., 1972, Ape J. , 172, 491. Spitzer, L., Jr., 1978, Phys_ic al. Pr.occsses in the Interstel lar. Medium (Wiley/Inter science, New York). Walborn, N. R. , 1977 0 An- J., 215, 53. Witt, A. N. and Johnson, It. 14. ,. 1'973 0 A pe J. , 181, 363. York, D. G. 1976, Mem. Soc. Astr. Italiann, 47, 493. a THORNTON PAGE, Code SN, NASA Johnson Spnee Center, Houston, Texas 77058 GEORGE 11. CARRUTHERS, Coale 4143, Naval. Research Unboratory, Wtashin t:on; U. C. 20375 FICURE CAPTIONS 1. Far--ultraviolet (1230-1600 A) Imnges of the Large Magcallanic Cloud obtained with the S201 Far Ultraviolet Camera: (Top) 3 min exposu *eo (Hottom) 30 min exposure. The shorter exposure ohows the prominent OB associations and individual UV-bright~ staru. We longer expaaure reveals the general distribution of less luminous OB stars. Note the apparent sharp outer boundary of the UV star distribution (arrows). North is up. 2. `lsodensity contour plot generated for the 10 min 12501600 A e xposure on the DIC. Contour interval is 0.10 D. The vertical. and horizontal axes are x and y scan coordinates, in Lusters. Superimposed on the plot is an approximate RA-P C (1950) grid, with nor .h to the right. a ► Posh Lions of 1,11 associations, 1lenire nebulne (N numbs ers), and foreg,round SAO stars Are indicated. 3. Contours of the Hydrop r, Index ( times 100) in the Large, Magellanie Cloud. Contour lines are for 100 11 Ind - 10, 20, 50, and 100. The vertical and horizontal taxes are as for Fig. 2. 4. Contour plot of E(B-V) in the IMC, based on values liven by Lucke (1974). These were used for correcting the far UV and Fla bri.gbItnesses for interstellar extinction using the curve of Nandy at al. (1950) in Fig. 5. Axes, orientation, and scale are as for Fig. 3. 5. Inter^. tellar extinction curves typical of the Local regions of our galaxy (Bless and Savage 1972) and for the 30 Doradus region of the DIC (Mandy et nl. 1980). C and L indicate the efrc:„tive wavelengths of the 5201 imagery with CaF 2 corrector (1400 A) and with L iF• corrector (1300 h). 1 estimates 6. Plot of our of IN brightness x (are m1.n)2 for emissioli nobulne ob&crved by Davies of al. (1976) vs. Ila brichtneases of Ilenize in (1956) for objects common. 7. Photon flux vs. wavelongtho ., -rmalizod to 5500 Ao for untroddoned stars of various of fective tompairaturas based on the model atmosphere calculations of Kur ►ez, Ppytromann, and Avrett (1974). on B. Intogrnted flux ration vs. effective temperature, based the inodol atmosphere flux distributions of Yurue-z et al. (1974). The ratios plottod are: IIA/Via - (1050-1600 A)/(5000-6000 A)p ILi/ICn (1050-1600 A)/(1250-1600 A) * LyC/Vis o (X < 912 A)/(5000-6000 A) O and LYC/11'i " (X < 912 A)/(1050-1600 A). on the 9. Contours of neutral hydrogen 21-cm emission ifi the 11111C O Lased meaf;urements of Mccee and 1111ton (1966). Contour lines ave for 20, 30, 40 0 and 50 flux units, where I flux unit - 1.76 K brightness tompern- tore at 21 cm. Errors are about + 10%, rind the niigulnr resolution Is about 14.5 arc min. Coordinates, orientation, and scale are as for Figs. 3 and 6. I 10. JUE cpeara, corrected for nonlinearity and distortion but not for instrumental spectral response, in the wisociation LIT 114. The large in aperture was used to obtain a spectrum of a star the nssociation (Top) and the foreground goocoronal La emission near the star was observed I simultaneously using the small aperture (Bottom). The omall aperture the La intensity %ns scaled up to value appr--printe for the large aperture and subtracted from the large aperture spectrum. on DIC otnra observod vith IUFO. Those, and other data the observed store, are lintod in Table 4. ~ ^ ^ ^ ". ^ 0 i a • , k F .. k ^.. Asa * x xm ^{^11^^/' I . SFr ,l,xXr.'xYl'Y ^.i x ^ p k , f „ r Y^ x a Y ` ! + r s r r *s^ ^`' *A,' ♦ ^ *^/ M *'s tW sa.+ I's ► : ► r I, .t ve^^r+ *..y "^ s ^ + ^ * r r t a M w►;'riGrr Ri „ !r tai } ^ 1 Yh^^ ^R ^ ^lerw*^ ^+",^ r'rra r^ `tr^♦ x x srj + .rtIt /./ xt""ltti"aRlf t ry ;^^r,i ^; + r .L;^IF^'^ir ``^ ^* y r ^/ , ^F^} ;♦e' .. 7'r+ i"° r , ^X r'r*1.'tts se ! yS y/,^"r^xr y^ a':r *ar,ia* 147I • + `. E ^" .,Pit t ^. ^pk;. ,A r'!A. r.x r; l Yxi T x f'; a* n.., * t. ` y,t y a ;`l1 t t. ^R•ilf•. t A ay + ^ • 1 q a ^ ^ y^FilJl I}7 y lx^h 1'xa^I^L 4^»i a I kMt. K 5 ^' a^ +IIF a ! . T • '" _,J .i, •: + vt"^vtt,r^^ s, Y to r k 'F ,•/ +1. t I I^t^ i 'r r s ` 'i r n F. ar."tll%kIIFxF "° I^'1r / F n• ♦ 1 s ^rr^; w, ^y Y +_`; ^ iatg#'i Is' ^l" ,♦^t^t t^1,^^'AC as tr r ^ • i t t1V ^•^^xl r tt ,t^a eft ,^^:Y"`' k Rat '^'^^•, r 17 §" '`i ^^l c•.^^F• ° ^^j It r+. j * r ,^ u r ^'* I f.» .. xar ^ ^A...^ +tA ^,'l ^"s. µ , 4 * n , ;' E,r A I^ :! t ♦ 5^ !. ^ ,^,.: A x sA!'xAr`w*rc.. o.' t + r ' I a ''^+ t{ ' ♦ ^pv `^ r .y +t..Y /'^r + sw R v 1 t }^ ` y1,/ F ' ^y1' * r • t F# ' ^, twy. r`. 1 ^ t; t'' r .. ►d •i^ ^yy4^ „"► , e It * x Ilk r • s x ^, ' * ^?. ^••* .Mir ^, ^ # ( A u - ^' s _ r r s r.° ^' d i kit Af } :, t • • r ^+ wr d * i^l r L t R # t • ,t +• c , A ' ♦ tt y 1 . g rA ^^fr• a 1 ?._ A• t P. x t ` • I .r + a ^, Ij ^S MiNwir- 0 I s $boom cj ^4 a^ ^gegop Cv. • oludINAL PAGE IS OF POOR QUAIM a` NORTH SCAN X -660 -65• -720 -71* .,70* -690 '68• '67• 950 600 650 700 750 800 850 900 6130 6110 WEST a, 6,20 400 -- 6100 6+10 450 ° 6000 a 5150 t ° ° ^. 2 O 5150 o tt ^ ^'`'H , Esc ` 16 o 500 ~ 11^^^ .+ ► 7 5 9 3t ) 1r O ^----' 5+40 ^ ^,..^'' e, 6 0V o oaa o 0 ..- O }- r 554 -- 7 e13^fr 0 9 ,x z a° oc, 1 6 °" Q 5.30 6 " ^+1a 5tgo -go 1sb O cn ' 15 ^(d as 016'7" 600 •- t ea X27 13 a O ^^14 5120 -^ 2 6161 8 4S 0^ 4 a 0 19 3 0 11 650 n ° 3e 1 19 5120 17 00 16 1 510 a o' W 1 st 0 O O r 96 0 0 81 2 4 $ 0 a o 0 Z 0 5110 5100 ^s o_z 700 "'--.•9-- ^._„ 70 00 R 30 O O 0 0 ^ O 51 6`'; 1 11^i41 SC.. Z 0° ° ° is ° 27 1s0 ayas r.., di 3 8 0 ° ^ t 'es :,3 5100 750 — t 13 5 1 O o 440 ° ° 800 j -71 150 •4( -70• 1 .690 1 -680 1 -670 1 -660 1 -.65° J r NORTH SCAN X -720 -710 -700 •100 .660 .6.7 0 .860 .650 6:10 WES 6:00 6:50 6:40 Z Q u N 5.30 5:20 6:10 5:00 r 1 V uL^e ♦ ♦ m > Q co J -u ti w / / / R i^ i V s ^e N 'd _"t: tt N D m (A N r` co W \LJ .-t 9_ (14 2z au x V) V) z X c m W O 10 20 30 40 50 60 HENIZE ,H Q 100 H+ II L He++ He+ -r--- I Ca 50000 K 45, 000 K 25,000 40, 000 K 20,000 I< 10 35,000 K __ J 1. NA 11 1 1 N 5500 1.0 30,000 K 25,000K .092 .0; 0.1 200 400 600 800 1000 1200 1400 0 A(A) J this 100 A 10 IU/1 Ca A 0 Q 1 V\ ac v,. 0. 1 0.0 1 20 25 30 35 40 45 T" ( I f- NAL PAGE iy i OF POOR QUALM a e 14 LARGE APERTURE SPECTRUM 12 10 8 i SATURATED X 104 RESEAU,^ i 6 0 ►--La SUBTRACTED SPECTRUM 4 i ^d l GEOCORONAL 2 Lo( SCALED TO LARGE APEPTURC J LL. w 0 1050 1100 1150 1200 1250 1300 WAVELENGTH (4) 8 SMALL APERTURE SPECTRUM RONAL Lo( X 103 4 0 L i P 1.5 x = Ul 89 1,0 + = LH 15 X 0.5 ,,\x J X - U1111 LL- 1,0 • LH55 w X---X---X—X d 0.6 J X Cr 1.5 X= LH74#1 I ,0 + = LH74"'2 a , 4 ----+-I 1300 1700 ' - 2100 2500 2900 WAVELENGTH (A) R d 310 2.5 3,01- 2.0 X ^ 2.5 1.5 w 2,0 1,0 QJ 1.5 ^- 1.01- 2,0 0.5 1,5 0 0,5 1300 1700 2100 2500 2900 O WAVELENGTH (A)rlta`t' t, °t•. •s = F )' a s ,{ n y / g +fir;. s * g"tat *i t *{ t*. t*•^t^, s ,sl . * * F ,rarer• ^ 1s f a r^ A .r rt rr' q t t+e•"1 „^f ^ r mx . . t - a. * ",. ^j` Ott ,^ e ^^•19l""jjjors r ,tx• . ^ A , \;, I • + tr [I tt I ?• + r` • x i* A`f • xr ^1 . t s ► / i c ^ • 9 ♦ f " ^' a } `1t ` rtrs ^" 4 iF * ' JC" ek„_ " x rS.sl,r^ ibd ,t ♦ R.a , ^. . •'jryd e , f ! 5r rs A^ to 4 . s ' '^`,,'l ^«r a ^ y ( x^ti ` ^r ► '1, earl. I r+" ^ f . 9 t +s t ^ A * + t; - ^, , xl *q k I^ r` 9` ,,. . yy, ,SuJ^. X +^^•Yt ..'. ^. Aa t ^r.} 4 4l y+riYf t^Er^rh'tx'r^ r ir 1r ^^'5;^2^;^Y}x, ! .^xrs s xt{ lP ka ,M^i^j,^* a♦ ji ^ f taY . r# I AV ! i^ y''bx g t ♦ r^ ;.. a •' a ' S t ,f r °Iq x. r * ; ..:/ ^. .y . ,•-• to 3 • ' ► * rl x A V seY ! ► ♦^t^:. wtf ytr+^^n^r t,r+^rrt w*^'i',, ^ 'r'; ^w i rx ^ ".. is rr'. *li.,^► , t . '°; 1^ i* kt xlq o,^ f ^U►,,,^. t,. .,.,, ♦ • r r• w' M{^+*r{-•^ M, a ,r 1•^ t .'^i rr. r _ w ,s' : "A •s ^i t • 1 ^i y'"'"^.'9Y (" ^.NF + t 1 ( a ,x//t x^/' a, - ^ t