... 

I 

FINAL TECHNICAL REPORT 

for 

NASA Grant No. NAG 5-710 

Entitled: "RESEARCH RELATIVE TO A MODEL OF THE ORION NEBULA" 

Submitted by 

THE REGENTS OF THE UNIVERSITY OF CALIFORNIA 

UNIVERSITY OF CALIFORNIA, LOS ANGELES 

DEPARTMENT OF ASTRONOMY 

405 HILGARD AVENUE 

LOS ANGELES, CALIFORNIA 90024 

Period Covered by Report: September I., 1985 - August 31, 1986 

PRINCIPAL INVESTIGATOR: 

Professor Benjamin M. Zuckerman 

Department of Astronomy 

Telephone No. (213) 825-9338 

1NASA-C6-179335) fiESEARCH BELATIVE TO A N67-14218 

HGCEL OF THE OliIcL NEBULA Firal Technical 

Eeport, 3 Sep. lSE5 - 31 Bug. 1466 [Columkia 

Univ.) 8 F CSCL 03A Unclas G3/89 43490 
Final Report 

This document comprises the final report for our IUE grant NAG 5-710 

entitled, llModel of the Orion Nebula Incorporating IUE Data". 

I. Introduction 

Our research basically has been directed along two avenues. First of 

all, we have a long-standing interest in modeling H I1 regions in order to 

understand the physical processes involved and to extract important 

astrophysical information. This includes knowledge of chemical elemental 

abundances and properties of the exciting stars, such as their far 

ultraviolet spectrum. The Orion Nebula is a prime candidate to study 

because it is nearby, bright, the extinction is not large, and its 

appearance is reasonably circular (e.g. Gordon et al. 1986). Such a shape 

is consistent with a geometrical structure that is spherically symmetric 

(1-D) or one that is axisymmetric (2-D) seen nearly face-on. Previously all 

detailed modeling of the ionization and thermal structure of H I1 regions 

has been confined to 1-D because of computational complexity. We now have 

the capability to treat the 2-D case with much of the level of physical 

sophistication as the 1-D case (Rubin 1984). The interpretation of the 

spectra of gaseous nebulae in terms of underlying physical processes 

requires measurements of line intensities at different points in the object 

and over as wide a set of excitation and ionization conditions as practical. 

Observations of nebulae have been made for many years in the optical and 

radio but only recently in the infrared and ultraviolet. These relatively 

new windows allow observations of lines of ionization states not available 

in the radio or optical--providing a much more complete set of known 

quantities to undertake a meaningful modeling effort. 

Whereas the first avenue of approach is a detailed study of the Orion 

Nebula, the second interest is to improve in general the determination of 

chemical elemental abundances in H I1 regions. In view of recent 

observational advances, in particular the availability of IR and UV lines, 

improved atomic data, and an improved knowledge of important physical 

processes, a reexamination of nebular abundances should be timely. Reliable 

values for quantities such as O/H, C/O, N/O in the interstellar medium are 

of utmost importance in understanding galactic chemical evolution. 

11. Studies of the Orion Nebula 

Since the last spherical modeling of the Orion Nebula (Simpson 1973), 

there have been numerous advances in virtually every aspect that contributes 

to solving the problem. 

measurements of lines of species that have no optical forbidden lines. 

As mentioned earlier IR and UV observations permit 
These include transitions in the IR of C I1 at 158 pm, N I11 at 57 pm, Ne I1 

at 12.8 pm, S IV at 10.5 pm, and Ar I1 at 6.99 pm and in the UV, C 111 1906, 

1909 A as well as C 11 2326 A. The last line is really the primary probe of 

C I1 in the H I1 region since the bulk of C I1 158 pm will generally occur 

outside the H+ region. 

using the Kuiper Airborne Observatory (KAO) to obtain new observations of 

several of these IR lines in Orion and other nebulae. We have attempted in 

our IUE proposal to obtain the UV line data from archival material. 

Our group (Erickson et al. 1986) has been active 

Before proceeding with the more difficult 2-D modeling described above, 

we fealt it was important to try to fit the available data for Orion with a 

spherical model. This paper will appear shortly (Simpson et al. 1986). 

However from dynamical considerations it is known that the Orion Nebula is 

probably a "blister" geometry object (Zuckerman 1973). The H I1 region is 

in the foreground of the large Orion Molecular Cloud (OMC1). 

geometry/density distribution is having more success at matching the 

intensities of lines of singly ionized species, which were in general 

significantly underestimated by the spherical models. Progress on this 

ongoing work is listed below. 

This 

1) Rubin (1985) gave a review paper at the Model Nebulae Workshop in 

Paris 8-19 July, 1985 that incorporated work on the Orion Nebula. 

2) Rubin et al. (1987) presented a paper at the IAU Symposium on Star 

Forming Regions in Tokyo 11-15 November, 1985 entitled Non-Spherical Models 

of the Orion Nebula and M17. Details of that meeting were in the trip 

report of 10 February, 1986. On 25 November, 1985 Rubin gave a paper at the 

IAU General Assembly in New Delhi on the N/O Abundance Ratio in Gaseous 

Nebulae (including Orion). 

observations directly but have been very much influenced by our concomitant 

IUE theoretical work. . 

These papers have not yet used the IUE 

3) Rubin, Simpson, and Haas (1987) will present a paper at the AAS 

meeting in January, 1987 entitled "A Two-Dimensional Model for the Ionized 

Gas in the Orion Nebula". 

Rubin spent 5 days in December, 1985 at the Goddard IUE data analysis 

facility beginning the archival work on the Orion spectra. He consulted 

with Dr. Barry Turnrose on the data processing since much of the useful 

original observations were made by Turnrose and Dr. Peter Perry. He 

systematically reprocessed about 40 spectra. 

and money to properly complete the reduction of the archival Orion Nebula 

data. 

We have not yet had the time 

111. Studies of the Elemental Abundance Ratios N/O, C/H, and C/O 

A major unexpected surprise of this work was that recombination 

processes may significantly enhance the intensities of some well known lines 

arising from low-lying metastable levels that have been treated as solely 

collisionally excited. As a result, under certain conditions which are 
'\ 

achievable in real nebulae, including Orion, recombinations can compete with 

collisional excitation in producing these lines (Rubin 1986). This work 

serves to bring closer agreement in general for H I1 regions between the 

optically determined average of - 0.07 (Shaver et al. 1983) and the lowest 

N/O values from IR lines - 0.2 (Lester et al. 1983). 

The largest effect is for the [O 111 lines, including the 2470 R line 

(blended with IUE), one of the strongest and most important emission lines 

observed by IUE in nebulae. There may also be a recombinational enhancement 

of C 1111 1908 R in nebulae that are higher excitation such as Planetary 

Nebulae. The physics has been added to the computer modeling code to 

predict the intensities of important C and 0 lines observed by IUE, which 

are relied upon heavily to determine C/O abundance ratios. In the treatment 

of [O I11 2470 R and C 1111 1908 A, recombination terms have been added to 

the statistical equilibrium calculation. For the C 111 2326 R lines, there 

is a considerable revision of the atomic constants used to compute the line 

intensities. When adding this to the code, we used the recent improvement 

of Lennon et al. (1985). , 

Since there have been in addition substantial changes in the relevant 

atomic data and UV extiction corrections, we rederived C/O and C/H for the 

Orion Nebula from previous IUE observations. The early low resolution IUE 

observations of Perinotto and Patriarchi (1980) and Torres-Peimbert 

et al. (1980) had been properly corrected for the SWP intensity transfer 

function problem. 

unlike the Perry and Turnrose observations, that we are reexamining which 

were made in the vicinity of the SE bar. The results of our study were 

presented at the London conference on New Insights in Astrophysics (Rubin 

and Zuckerman 1986, copy enclosed). We find that C/H is - 20-40% lower than 

the prior analyses. Our C/O results are presented in the paper and are not 

directly comparable with the earlier papers since the temperature used by 

Torres-Peimbert et al. (1980) were somewhat higher. The improved treatment 

does yield substantially more reliable values. Accurate abundance ratios 

are important for a wide variety of problems including, for example, the 

fundamental C/O ratio as well as the galactic chemical evolution of the 

heavy elements. 

They were also made in the region close to the Trapezium, 

REFERENCES 

Erickson, E.F., Haas, M.R., Hollenbach, D.J., Rubin, R.H., Simpson, J.P., 

and Wolf, J., 1986, "Far Infrared Spectroscopy from the Kuiper Airborne 

Observatory", Proposal for the Kuiper Airborne Observatory (funded). 

Ap.J., 308, 288. 

294, 200. 

R.L. 1983, Ap.J., 271, 618. 

Gordon, M.A., Jewell, P.R., Kaftan-Kassim, M.A., and Salter, C.J. 1986, 

Lennon, D.J., Dufton, P.L., Hibbert, A., and Kingston, A.E. 1985, Ap.J., 

Lester, D.F., Dinerstein, H.L., Werner, M.W., Watson, D.M., and Genzel, 

Perinotto, M., and Patriarchi, P. 1980, Ap.J.(Letters), 235, L13. 

Rubin, R.H. 1984, Ap.J., 287, 653. 
Rubin, R.H. 1985, Models of H I1 Regions in Proceedings of Model Nebulae 

Rubin, R.H. 1986, Ap.J., 309, 334. 

Rubin, R.H., Simpson, J.P., Erickson, E.F., and Haas, M.R. 1987, Proceedings 

of IAU Symposium No. 115, Star Forming Regions, Tokyo, eds. M. Peimbert 

and J. Jugaku. 

Workshop Meudon, ed. 0. PeqUignOt, p. 235. 

Rubin, R.H., Simpson, J.P., and Haas, M.R. 1987, AAS Meeting Pasadena. 

Rubin, R.H., and Zuckerman, B. 1986, Proceedings of New Insights in 

Astrophysics - 8 Years of UV Astronomy with IUE, London, 

ed. E.J. Rolfe, p. 581. 

S.R. 1983, M.N.R.A.S., 204, 53. 

Shaver, P.A., McGee, R.X., Newton, L.M., Danks, A.C., and Pottasch, 

Simpson, J.P. 1973, PASP, 85, 479. 

Simpson, J.P., Rubin, R.H., Erickson, E.F., and Haas, M.R. 1986, AP-J., 311, 

Torres-Peimbert S., Peimbert, M., and Daltabuit, E. 1980, Ap.J., 238, 133. 

Zuckerman, B. 1973, Ap.J., 183, 863. 

(in press). 

No inventions or patents were developed on this grant. 
. 

ELEMENTAL ABUNDANCE DETERMINATION IN GASEOUS NEBULAE FROM IUE LINES 

Robert H. Rubin B. Zuckerman 

NASA Ames Research Center 

Moffett Field, CA 94035 USA 

and Astronomy Dept. 

University of California, 

Los Angeles 

ABSTRACT 

A method for obtaining the C/O elemental 

abundance ratio for high excitation H I1 regions 

is discussed. 

C/O for the Orion Nebula from previous IUE 

observations using improved atomic data and 

extinction corrections. It is also found that 

C/H is - 20-401 lower than previously 

determined. When recombinations contribute 

importantly to EO 111 2470 and 7325 A emission, 

we caution agai:st use of these lines for 

assessing the 0 ionic abundance with the 

so-called empirical method. 

This is then applied to rederive 

Key uords: IUE, Abundances. Orion Nebula 

1. INTRODUCTION 

Several recent developments make it 

worthwhile to rederive the C/H and C/O ratio in 

Orion and other gaseous nebulae. Evaluation Of 

these ratios depends heavily on space 

ultraviolet transitions. For H I1 regions and 

low excitation planetary nebulae, carbon is 

essentially in two ionization stages -- C I1 and 

C I11 whose abundances are gleaned from the 

C I13 2326 A lines and C I111 1908 A lines. 

There have been substantial revisions in the 

atomic data since the early IUE observations of 

Orion (Perinotto and Patriarchi 1980 - PP; 

Torres-Peimbert, Peimbert, and Daltabuit 

1980 = TPPD). The most dramatic changes have 

occurred for the collision strengths and 

transition probabilities for the C 111 2326 A 

lines (Lennon et al. 1985). 

derived from [0 I13 2470 A, there is an improved 

A-value (Zeippen 1982). 

For O+ abundances 

In addition, Rubin (1986) has shown that 

there should be a substantial enhancement of the 

[O 111 2470 A line due to recombinations in the 

0 I11 region of the Orion Nebula and other 

Astronomy Department 

University of California 

Los Angeles, CA 90024 

USA 

nebulae as well. Previously this line (actually 

two lines) was treated as solely collisionally 

excited. 

model of Orion (Simpson et al. 1986), we predict 

I(2470 A) to increase by - 1501 over the purely 

collisionally excited value for a line of sight 

that corresponds to the brightest region (Ori 1 

of PP) and the IUE large aperture. Significant 

enhancements are also expected for other classes 

of gaseous nebulae. For instance in Rubin 

(1986) the PN NCC 7662 modeled by 

Harrington et al. (1982) was "retrofitted" for 

the 0 11 recombination terms utilizing their 

fractional ionic abundaices and average Te 

values appropriate for the relevant ionic 

species. 

integrated flux would increase - 50%. Figure 1 

illustrates the effect on I(7325)/1(3727). 

Throughout this paper, I(7325) and I(3727) mean 

I(7320,30j and i(3726.29). Since the 2470 A and 

7325 A multiplets both arise from a common upper 

*P level, a similar plot would result for 

I(2270 1 /I (3727 1. 

Working from the framework of our 

It was found that the 2470 A 

Therefore, when recombinations are contributing to the intensity of one or more of 

these 0 I1 lines, the empirical method (which 

assumes collision-only excitation) for obtaining 

N , T and 0 I1 abundance is inappropriate. In 

g8ner%, the direct effect of including 

recombination terms in the statistical 

equilibrium would be to lower the 0 abundance 

somewhat depending on how much 0 I11 was 

present. In addition, there is the possibility 

that the 0 11110 I1 ratio and values of T , N 

used for prior analyses might need revisign izi 

view of the recombination terms for [o 111 

affecting intensities of the 7325 A and even 

3727 A lines. 

Finally, there is a substantially improved 

Orion extinction curve derived from IUE 

observations by Bohlin and Savage (1981) that 

generally indicates less UV extinction than the 

earlier work of Bless and Savage (1972). In 

view of the above mentioned effects, we Shall 

reevaluate C/O and C/H for the Orion Nebula 

using the IUE observations of PP and TPPD. 

%ew insights in Asrroph.vsics, Proc. Joinr NASAIEIA/SERC Conference, University College London, 14-16 July 1986. €SA SP-263 (September 1986) 
SS2 R H RUBIS & B ZUCKERMAN 

Y 

0 

z 

\ 

0, 

t- i 

I 

1 

I 

I 

I 

1 

1 

t 

i 

j 

I I I 

13 

12 

11 

10 

- 

- 

- 

4.5 

- 

9- 

8- 

7- 

6- 

3.0 3.5 4.0 

log Ne (cm' 3 1 

FIGURE 1. We show curves that represent possible 

solutions for T and N for an assumed 

I(7325 A)/1(372? A) = 6.1, close to the value 

observed near the 'I'rapezium, corrected for 

absorption. The curves are ldbeled with 

different values of the ratio r = 

<N (O++ ) >/<N( O+ ) > representing different 

contributions to the statistical equilibrium due 

to recombinations. The notation, for example, 

for the average density, <N(O+)?, means INeN(O+)dV/INedV. For surface brightness 

corresponding to lines of sight near the 

Trapezium in the Orion Nebula, our models show 

the ratio r is -20. 

i 

2. DERIVATION OF C/O AND C/H 

Because of the significant contribution to 

the intensity of 2470 A in Orion due to 

recombinations in the 0 111 region, we take a 

somewhat different approach to the empirical 

evaluation of C/O. (With a detailed modeling 

analysis the contributions of both collisional 

excitation and recombinations could be properly 

accounted for so that Of lines could be used.) 

Basically, we assume that for Orion the volumes 

occupied by O++ and C++ are the same. 

N(c)/N(o) - N(c++)/N(o++), particularly 

along central lines of sight in resolved 

objects. For lower excitation objects 

V(C 111) > V(0 111) (see Rubin 1985) and the 

method will provide only an upper limit if the 

volumes are assumed coextensive. The procedure 

for obtaining C/O and C/H is as follows: 

Then, 

1) The original analyses of PP and TPPD 

both used the Orion Nebula extinction curve of 

Bless and Savage (1972) obtained from the 

Orbiting Astronomical Observatory 2. We employ 

the much improved average Orion Nebula 

extinction curve of Bohlin and Savage (1981) to 

correct the original line fluxes to intrinsic 

line intensities. In the derivation of f(X) 

(see PP or TPPD) from E(X-V)/E(B-V) (see Bohlin 

and Savage 1981), we use E(B-V) - 0.32, which is 

the average for 0' Ori A,C,D (Bohlin and Savage 

1981). We also use C(H8) values given in PP and 

TPPD. 

2) The observations of the 10 111 2470 A 

lines are utilized together with those of 7325 A 

to place the IUE observations on the same scale 

as the optical ones. This is done by using the 

expected multiplet intensity ratio 

1(2470)/1(7325) = 0.75, which is an updated 

value from that of Harrington et al. (1980). 

3) The C 1111 1908 A and the [O I111 5007 A 

lines are used to obtain the C++/O++ ratio. For 

objects where collisional deexcitation of the 

upper levels of these lines is negligible (valid 

for Orion), 

N(C++) 4.00 eV I(1908) 

N(O++) i kTo i I(5007) 

- 0.0658 exp - - . 

Since we are using this method when it is valid 

to assume that the C I11 and 0 I11 volumes are 

about the same, the value of T is the appropriate average for the C++,O++evolume. It is 

important to use the average temperature To 

given by 

I Ne N(O++,C++) Te dV 

I Ne N(O++.C++) dV 

To - 

4) Abundance ratios are rederived for 

region Ori 1 uv of TPPD and Ori 1 of PP. 

Observed areas were chosen to correspond closely 

with Ori la and 2b respectively of Peimbert and 

Tcrres-Peimbert (1977). (Because of the 

different aperture sizes, the optical and UV 

areas do not agree exactly.) In Table 1 we 

present the results for average temperatures of 

8000 and 8500 K, which correspond to the narrow 

range found to be self consistent with our 

detailed spherical modeling. (The temperatures 

derived in a non-spherical blister model may 

differ from these.) 

5) For the C/H derivation, C 111 2326 A and 

C 1111 1908 A provide ionic density information 

and are used in conjunction with H6 to obtain 

N(C)/N(H) - N(C+ +C++)/N(H+). Again 

calculations are made for two average 

temperatures of 8000 and 8500 K and utilizing 

the improved atomic data described earlier. The 

latter temperature is essentially what both PP 

and TPPD used for their original analyses. 

Results are summarized in Table 1. 

3. RESULTS AND DISCUSSION 

1. Our detailed computer models of the 

ionization and thermal structure of the Orion 

Nebula show that for the volumes sampled by the 

above mentioned observations, the 0 111 and 

C I11 emission is coming from the same region. 

From the models, the appropriate average 

temperature in this C 111. 0 111 region for the 

lines of sight in question are calculated to be 

- 8000 - 8500 K. using the range @f C and 0 

elemental abundances found. 
'Table 1 

Reg i on Ikf. Location N(C)/N(O) = N(C")/N(O'+) N(C)/N(H) 

relative to 

dori c 8000 K 8500 K 8000 K 8500 K 

Ori 1 uv TPPD 45" N 0.97 0.69 3.58( -4 ) 2.06(-4) 

previously 3.31 (-4)* 

previously 2.51 (-4) I 

Ori 1 PP 34" U,3" s 0.88 0.63 3.51(-4) 2.03(-4) 

*TPPD use <Te> = 8700 K for C 111 and 8500 K for C 11. 

UPP use <'Ye> = 8500 K. 

2. Recombinations play an important role 

in contributing to the [O 111 2470 and 7325 A 

lines in the Orion Nebula and other nebulae. 

When this is important, the derivation of 

properties of the plasma -- N , T , and 

elemental abundances -- from %bse$vations of 

affected lines will not be appropriate with a 

coilisional-excitation-only treatment. 

3. When recombinations are important in 

the production of the 10 IIj line emission, the 

2470 A line should not be used with the 

so-called empirical metnod for assessing 0' 

abundances. 

4. In light of 3, we find for the Orion 

?iebula that it is more reliable to determine the 

C/O ratio from lines of C 111 and 0 111. 

5. Even though there are some revisions in 

virtually every aspect of the abundance 

analysis, the effects are not all in the Same 

direction. The bottom line effect on the C/H 

ratio is to decrease the value -20-405 from the 

prior analyses. However, the improved treatment 

presently possible does yield Significantly more reliable values. 

obtained by TPPD was 0.79. This was obtained 

assuming an average temperature of 8700 K for 

the C 111 region and 9100 K for the 0 111 

region. 

The previous C/o ratio 

6. By using the temperature To in our analysis, it can be demonstrated that if there 

are temperature fluctuations, the effect would 

be to lower C/O and C/H. 

7. Reliable abundance ratios are important 

for a wide variety of problems including, for 

example, the fundamental C/O ratio in the 

interstellar medium as well as the evolution of 

heavy element abundances in the galaxy. Langer 

et al. (1984) have suggested that in the gas 

phase in molecular clouds, C/O is greater than 

unity. Since the Orion Nebula was recently a 

rr,olecular cloud, it is important to constrain 

accurately its C/O ratio. If C/O is greater 

than unity in the gas phase in molecular clouds, 

then our results indicate that relatively more 0 

than C, must be hidden (e.6. in dust grains) in 

these clouds compared to the Orion Nebula. In 

any event, since main-sequencqstars have 

C/O < 1, there must be more 0 than C in the 

interstellar medium as we deduce, at least for 

the gas phase in Orion. 

Support from NASA IUE grant NAG5-710 to UCLA is 

gratefully acknowledged. 

REFERENCES 

Bless R C & Savage B D 1972, Ap.J., 189, 253. 

Bohlin R C h Savage B D 1981, Ap.J., 3, 109. 

Harrington J P, Lutz J H. Seaton M J h Stickland 

D J i980, M.N.R.A.S., 191, 13. 

Harrington J P, Seaton M J, Adams S h Lutz J H 

1982, M.N.R.A.S., 199. 517. 

Langer W D, Graedel T E, Frerking M A & 

Armentrout P B 1984, Ap.J., 277, 581. 

Lennon D J, Dufton P L. Hibbert A h Kingston A E 

1985, Ap.J., 29'1, 200. 

Peimbert M h Torres-Peimbert S 1977, M.N.R.A.S., 

- 179, 217. 

Perinotto M & Patriarchi P 1980, Ap.J.(Letters), 

- 235, L13 (PP). 

Rubin R H 1985, Ap.J. Suppl., 51, 349. 

Rubin R H 1986, Ap.J., 309 in press. 

Simpson J P, Rubin R H, Erickson E F h Haas M R 

1986, Ap.J., in press. 

Torres-Peimbert S, Peimbert M h Daltabuit E 

1980, Ap.J., 238, 133 (TPPD). 

Zeippen C J 1982, M.N.R.A.S.. 198, 111.