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A study of the η Carinae RegionBok, Bartholomeus Jan

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Publication date:1932

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INTIWDOCTIO:\" AND SUMMA l~Y.

The star i f Carinae or I, Arg lls ((.(HIOO 10"41111 .2 , bl900 - - 59"ro' ga l. long. = 254°) occurs in a part of the Ivlilky Way th a t has al'ways st ruck observers in the Southern hemisphere by it s ext reme beau ty a nd richness in stars. Sir J OH\" H EESCHEL made th e first extensive study of this star and the region around it during his visit to the Cape of Good Hope, th e resuHs 01' whi ch have been puhlished in a very lu cid form in his monumental work: " H.esults of astrono­mical observations at the Cape of Good Hope" . I wish to quot e as a n introd uc tion a few lin es from the chapter on "I) Argus and the grea t Nebula surrounding it " in the Cape Observations:

" There is perhaps no other sidereal object which unites more " points of interes t th an this. Its situa tion is very remarkable, "bein g in the mid st of those rich and brillia nt masses, a succession "of which curi ously contrasted with dark adjacent spaces (called " by the old navigators coal-sad s) constitute the milky way in " tha t portion of its course whir.h lies between the Centaur and " the main body of Argo,"

Sir J OH\" HERSCHEL'S words hold with undiminished strength at the present time and the developments of photography a ne! spec tros­copy h ave only served to emphasize the importance of this part of the south ern sky still more.

The s tar II Carinae itself has had a most va ried history, totally different from that of any other star. For some time it has been one of the brightest stars in the southern sky. Its brightness varied probably between the 2nd and 4th magnitude from th e end of the eventeenth to the beginning of th e nineteenth century 1). In December

1837 HE]{SCHEL noticed that th e s tar had increased considerably in brightness and it was only "after a momentary hesitat ion" that he hecame satisfied of its identity with his "old acquaintance 17 Argus". It increased in brightness until the spring of r 843, wh en according t o i'vIACLEAR it was even brighter than Canopus. The s tar remained

' ) For a complete record of the ea rly changes in brightness of 1j Carinae see:

h~ES , Cape Annals XI, 7.5 B, f903·

r

'

one of the bright es t in th e southern sky until rRS6 , but th e steady would h decrease in brightness had definit ely s tarted by 1858 and in r869 it only POS!

was as faint as the se venth magn itud e. Between [869 a od r894 it s sta r, as .

apparen t brightness flu ctuated between the seventh and eight h nebula h magnitude. \\ie In

}Iiss LE ·\\'JTT'S photographic light CllI've 1) shows that t lw lig-ht between \l'as practically constant (11'1 = 7.5) between r8RC) <Ind the alltumn o and B of 1Rc)4, wh en a further small decrease in brightness set in a nd in connect i lvIay 1R95 the star 's magnitude became equa l to lit = d,]. A cont in\lou s intricate se ries of Harva rd plates bringing the record up to elat e indicates no of the sp further changes of any appreciable amO\lnt. of th e d

The slight change in brightness of 1894-1895 must however have The re been accompanied by v iolent changes inside th e s tar and in its at mos­ of ga lact phere . A study of some H a rvard spec trum plates taken between the nebt IR92 and 1895 showed that the spect rum challged at the time that sca le (fo the change in appa rent brightness occ urred from a n absorption few regi(

spectrum of type cI'5 with a few bright lines superposed to a p ure tion COll<

emission spectrum with perhaps a very faint continuous background 2). limited Part of the emission lines appeared to be clue to hydroge n and ionized SO li thern iron a nd in 1928 M E RIULL succeeded in identifying practica lly all while to th e remainin g lines as being clue to trallsitions between forbidd ell rnagnitu sta t es of the atom of ionized iron :J). in addil

This remarkable s tar occurs nea r one of the brightest parts of fainter s the diffuse Il ebul a that bears its name. The fir st accurate drawing limit th, of the nebula, which is one of the brightest and largest in the sky, The J has been made by Sir J OH:\ HERSCHEL and was published in his Cape region 1I

Observations. The spectrum of at least the brightest part of the Similar nebula is of the em ission type 4) a nd the question of the relation the sky, between the diffuse nebula and th e sta r 1/ Carinae is of import ance. The pre: HUBBLE has shown that the light emission of diffuse nebulae is cllle cl istribu to the exc itation by nea rby stars of high temperature and has found these st2 in addition some striking relations between the brightnesses of the not esp exciting stars a nd the surface brightnesses of the nebula 5). 1/ Carinaf' been USI

has undergone tremendous changes in brightness and spectral type t ry to e (its color is described as "ruddy" in the Cape Observations). W e The res

dec isive ' ) Sec :\fiss C,,:-.;:-.;o:-.;'s paper in Pop..-\str. 28 , 324, 1<)20. lJreven t ') ]>op ..\ str. 38 . 3')<) , "!30. galactic") Ap. J. 67, 361, 1928.

' ) Li ck Pub!. 13, 121, 1<)18. 0) .\p . J. 56 , [(' 2, "00, 1<)22. ' ) ,-\n l

') •.l

but th e steady ane! in r869 it 9 and 1894 its 1h and eighth

that the light cl the autumn

se t in and in I. A continuou s e indicates no

however have cl in its atmos­'aken between the time that

an absorption )seel to a pure background 2) . rn anel ionized pract ica lly a ll een forbidd en

1test parts of urate drawing st in the sky, eel in his Cape t part of t he f the relat ion f importance. e bulae is clue ncl has fOll n d nesses of the 5). '/ Carin ae

;:;pectral type vations). We

would have a fin e opportunity to test HUBBLE'S theory, if it were only possible to prove definitely that i/ Carinae is act ually the exciting star, as there is no case on record, ",here a star that excites a diffuse nebula has changeel so mu ch in brigh tness as 1/ Carin ae.

Vile must however not be too rash in concludin g a connect ion bet'ween 1/ Carina e a nd the nebula as many stars of spectr(] 1 type~ o and B occur within the limits of th e nebula. Th e question of tile connect ion between particular sta rs in the region an d the ncl)lJ la is an intricate one and it can only be solved by a c1c>ta ilcc1 inves tigation oj' the space di stribution of the early type stars together with a study of the distribution of light throughout the nebula.

The region around 1; Carinae is of comiderable interest for s tudies of galac tic structure. The su rface brightness of the ;\Jilky Way outside the nebula is very high. P!\:\:\"E1WEK 1) estimates it as 6) on hi s scale (for ga l. long. = 2)7

C ) and its brightness is ollly surpassed for a

few regions het \\·een ga lactic longi tudes 300 0 and 3)o ~ . Our informa ­tion concerning the structure of the Southern l\Iilky \;Yay is still very limited as t he ohservational m ate rial available for studies of the S011 t hern hemispllere is natura lly small. It appeared therefon~ wort h while to determine apparent m agnitudes, color indices and absolut e magnitudes for the brightest stars in the '/ Carinae region and to obtain in addition some inform a tion concerning the distribution of th e fa inter stars from a series of starcounts clown to th e faintest magnitud e limi t t hat could he reached.

The Harvard Observatory has a large collec ti on of plates of the region under considera tion taken with man y different inst ruments. Simila r collec tions are now avail able at Harvard for rnan y regions of the sky, but few a rc so complete as the collection of .'/ Carinae plat e:-;. Th e present paper gives the results of an attempt to study the space distrihution of th e stars near '/ Carinae and the relation behveen th ese sta rs and the great nebu la, making use of existing Harvard plates, not especially taken for this purpose. The Harvard plates have been used a lready for a great variety of investigations a nd we only t ry to explore here some further possibilities for the plate collection. The results (Jf the st ructual analysis do not pretend to be final or dec isive in any respec ts; they will have served thf'ir purpose if they p revf'nt us in future from tackling the problem of the (] ll alysis of galact ic st ru ctu re in too inefficient a way. The sta rcou nts discussecl

' ) , \nn . BOSSCHA Sterrenll"acht II , ,st part, J<)2S.

.

4

in the first chapter may be taken as an illustration of th e point. The :1 nr! Sll\,~

Harvard Observatory plans to attack cer t ain ph ases of the problem error \\·a

of the surface distribution of the faint stars . Tlte invest igat ions of tude and the distance errors in the third section of chapter I have proved Each B a lreauy of value in select ing certa in in struments for starcoll nting coun ting problems and rej ecti ng others. W e had abo a splendid oppo rtulli ty VIll. A to test out diffe rent methods of count illg and \\e exprcss the hope sca le. that the expericnce ga ined by the present investigation will ll elp liS

§ 3·t o make a more direct and more effective attack on the larger problem. IR93 an BI~CC:E t

A short summary of the resu Its of eae h separa tc section follows: to 14 he

Chapter I. thro ugh

series all§ 1. Photographic and photov isual magnitudes are derived for !less rcg!III stars of kllown spectral type brighter than 1Jl = 9.0 in the Henry which nDraper Catalogue. The mea"m ernents were madc with the Schilt nebulosiphotometer of t he . H arvard Observatory on plates taken with the correspo 8 inch Bache telescope. :VIiss A:\GE[{ has determined spectroscopic SC H\L\Hparallaxes for most of these stars. Some difficulties were met in the scale of derivat ion of the reduction curves , The color illdices a re however by 111::\kreliable a nd have been obtain ed on the assumption of zero color diagramindex for the Ao sta rs (mcan distance = 300 parsecs). The probable

error of a final photovisual magnitucle is eq ual to =:.: 01".05 and to Clzapl~ 0''' .04 for a final photographic magnitude. The error depending

upon t he distance fro m the center of the plate has been investigatecl § I

and \vas found to be either sma ll or absent. The final magnitudes Special are in Table V. diffcren

Pho tographic magnitudes \\·ere determilled fo r 150 add itional .tars a nSlll g with known spectra and absolute magnitudes, all fainter th an m = g.o over th according to the H. D. C. The final magn itudes are in Table VI. Bo, BI

effect,§ 2, The starcounts have been made on plates taken with the nebula.I inch COOKE, 8 in ch BA CHE and 24 inch BRCCE telescopes. The amongadvantages of the use of a binocular microscope for the count· on

sm all scale plates are melltioned, The counts have been done in tely br

spectmsuch a way that completeness up to the limiting magnitude is guaran­~pac.e fteecl. These limi ting magnitudes ll a ve bel: n obta ined : a. by ~ompa ri. on in thewith the Harvard Standard I~egioIls (m < 14), b from the exposure red den. time (/11. > 14). The aCCl!racy and reliability of the counts have been

checked by repe8t ing th e COllnts of some platcs aft er an interval of § 2 .

a month or longe r. Th e di stance correct ions h a ve been investigaterl bu bon

'

: th e point . The

of the problem vestiga tions of

I have proved )r starcou n ti ng

lid opportuni ty :press th e hope on will h elp \\s

larger problem.

,ect ion follows:

lre ci eri ved for

o iII the Henry (ith the Schilt :aken wi th th e

~ spec troscopic ere met in the

s are however of zero color

. Thr p robabl e

: 0"'.05 and to rror cl epending

?n investiga ted la l magn itudes

,del i tional stars

r tha n 'lit 9.0 n Ta ble VI .

aken wi tb th e

elescopes. Th e the COUll ts on

been cl one in ucle is guma 11­

oy comparison

tb r. exposure nts have been

an interva l of i1 inves tigated

·1

ilnd small c()rrec tioll s 11 ,( v(; b re ll ;Ipplied, if thr c::'\ is tCll CC of s1l eh an

ern)r \\'as \\'cll esta hlishecl. F or th e HI\l'CE p la tes th e limiting m agni­

t ude a nd the di s t a n ce correc tion h a ve been inv es ti gated toge ther. Each BH l'C E pla te has a brge fi eld that lw s e:'\crll cnt qualities fo r

countin g problems. Th e com b in eci result s of th e counts a rc in T able \ ' III. All m agnitudes are photographi c a nd on th e in terna tional

sca le.

§ 3. 38 plates of t he ' / Cari nae region , taken la rge ly b etween

1893 an d 1 9 01 with th e 8 inch B.\CH E , q inch B OY DE " a nd 2 4 inch BH CCE te lescopes a nd ha ving e:'\posure tim es ra ngin g from 10 minut es to q homs, have been used for a s tlldy oJ th e d is tributi on of light th ro ugh th e nebula. A se t of normal pl a tes was form ed for each

seri es a nd the isoph otes corresponding t o th e limiting surface b ri ght­ness regis tered on each p la te were d rawn . T he e:'\pOo, ll1T tim es, for

which normal p lates of cI ifferen t se ri es showed t he sam e amoun t of nebulosity, h ave been int f'rc(l·n pa rect . The relat ions b etwee n these

correspondin g exposure ti mes have been d iscll ssed o n th e basi s of SCH\VA I<ZSClJILD'S law of b lacke ning for a ph otographic pla te. Th e sca le of surface brightn esses fo r t he isophotes h J.s been calibrat ed

by m a king li se o f SCH\Y.-\RZSC HILD 'S law and t J. kin g p :c-, 0. 85. The diagra ms of th e isopho tes will b e found at th e en d of thi s paper.

Chapter II.

§ 1. Th e color ill d ices for th e III bright es t st:1 rs a re di scussed .

Specia l attention has been p a id to th e p ossibili ty of errors due t o d iffe rences in distan ce from the center of th e plates and t o errors

J.r ising from th e vari a tions in density of th e n ebul a r backgro und over t he pla tes . A color excess is found fo r a fe\y sta rs classified as Bo, 131 and H, but evidence is given th a t thi s is entirely a locaJ

effec t , proba bly clu e to a sm a ll cloud of m a teri a l connec tecl with the

nebllla. There a re n o indi ca ti ons of th e ex is tence of a col or excess a m ong the numerous B2 to 139 st a rs. It is improbable tll a t the a b solu ­

tely brightest sta rs shou le! be e:'\ac tly so nl11 ch too &/'ue for the ir spec tra l cbss to make the tota l effect of absolute m agn it ude and

space red dening 7ero . If we rejec t t!l is possiiJility the color indices in the Ii Carin ae region indicate t he a bsence of a ny apprec ia b le space

redd ening over a d istance o f nea rl y 200 0 pa rsecs .

§ 2 . This sec tion con ta ins an in ves tiga ti on of t he space di s tri­bution of th e bright es t s t a r s alld the cl e termination of til e distance

'

·

" '

6

Chapieof the diffuse nebula. The stars with known absolute magnitudes have been divid ed into nine groups accord in g- to the values for th e S I. distan ce moclllii. \Ve find from a s tudy of the surfJce dis tributiOll is discu: for the different g roups th at the stars with j1 - in > -- rO.5 arc only pia: dist ributed uniformly over the regio n st udied , but th a t the stars of 1/ Carl with :\1 - m < - rO.5 show it disti nct connect ion with the nebula. has prot 1\ minimum va lue for the distance of the nebula is derived a nd the years. A probability for th e ex istence of a cloud of a bsorbing mat eri a l connected that the \\'ith the nebula is di scussed. Th e co nnect ion bebveen the sta rs and in spite c the neb ul a is investigated a little closer; the nearest 0, Band CA2 Jines to stars occur nea r th e brightest p;lrts of the nebula and the shape of The pos th e isophot es indicates that th ey are actua lly connec ted with it. The instance distance of the nebula is probably equ Cl l t o : our pho

IIaa parsecs. The C stars anIt is practically excluded that it is mu ch nearer by, unless we make diffuse L

th e a bsurd assumption that the nebula is se lf luminous . sit)' anc

The space dist ribution of the s tars within IrOO parsecs from our sun is studied next. The space density of the early type sta rs is

The I s lightly less than that near our sun at a distance of Goo parsecs , but

discussiit is a t least twice as high as th at in the so lar neigh borhoo<1 for a

emissiolpoint at Q di s tance of 250 p a rsecs in the direc t ion towards 'iI

necessa Carin;le.

cleratio § 3. Th e sta rcounts for fifteen regions A to 0 (see fi g. 3) are gaseou~

analyzed in thi s section . The cOlInts for som c neighboring reg ions § 2call be combined a nd six fin a l sets of starco unts are st udi ed . Th e

of a. spintercornpa rison of these si x represcn tat ive sets shows tha t: 1. a surplus the ionof nearby A sLa rs is always connected with a su rplus of s tars brigh te r to be ,than 111 = 12 . .5 ; 2. a clark nebula covers region H and perhaps pa rt clilutioof th e surruunding regions; 3. the numlJer of faint s ta rs is larges t ti on b)for the region S. \V. of the nebula. Th e counts have bee n com­makespared with tho~e for galactic se lected areas in the same part of the skv . butionThe space clensit ir:s have been computed by the m ethod of H . C. for th371, assll ming respective ly the vailles LI m = 0.00, Ll '111. = 0 .20 and $-->- 2,Ll m =.--co 0.40 for the coefficient of the ge neral absorption of light in

space per 1000 p arsecs. The sp acc d ensit ies for the direc ti on towards 1i Carinae exceed or a re at leas t eq llal to the d ensity in th e so lar

whereneighborhood over a large range of distance. The computed space ture 0densities a re nat urally uncertain, but they seem to confirm som e of of fretll e conclus ions reac hed in H. C. 371.

7

pte llldgni tudes values for the

~ce clistribu tion

~ ~ - 10.) arc that the stars

ith the nebula . lerived and the

erial connecteci 1 the stars and 0, Band cAz

d the shape of ed with it. The

nJ ess we make )us.

,"secs from our

r type stars is

10 parsecs , bu t lborhoocl for a

on towards 1{

,pe fig . .3) are

onng regions studiell . Th e

Elt: 1. a surplu s stars brigh tel'

perhaps part ta rs is largest

'e been com­~ rt of the sky. thod of 1-1. C. II = o.zo and jn of lig lI t in

:tion towards

in the solar

npu teel space finn so me of

Chapier III.

§ 1. The connection be1\\'ccn thc star Ii C:ninae aIld the nclJl1b is disc ll ssed and the conclusion is reached that Ii Carinae itself can only playa minor role in the excitation of the nebula. Th e brightness

of 11 Carinae has ciecreased nin e magnitudes since 1840 and the nebllla has prohably been at the limit of visibility throu gh a ll these ninety

years . A series of Harva rd plat es taken between 1890 a nd 1930 shows th at tIle part of the nebula near 1{ Carinae has not changed at a ll

in spite of tIle fact that a pure emission spectrum with forbidd en iron

lines took the place of the cF5 spectrum o f Ii C;nin ae in 1894--1895. The possibility of changes in the nebula is discussed and two instances of a marked difference between HE1{SC JlET.'S drawing and

our photographi c isophotes are mentioned. The 0 and l) stars excite the nebular em ission and a nu mber of

stars are probably responsible for what appears to us as one huge diffuse nebula.. The rel at ion between the brightest patch es of nebulo­sity and th e stars involved in them is d iscussed.

The last two sections of the thirrl cha.pter deal with a t heoretical

discussion of the physica l processes involved -in the excitation of the emission nebulae. The results apply to any emission n ebula and not necessar ily to the l) Carinae nebula alon e . These theoretical consi­derations may be of valu e for the computation of the density of

gaseous materia l in em ission n ehulae.

§ 2. The equations h ave heen \-vorked out for a simplified model

of a spherical nebula consisting of hydrogen only. Til e equation for the ionization is derived and it is shown how the dilution factor has to be computed, if we want to tak e int o account both geometrical dilution and the recJuctionfactor arising from the absorption of radi a­

tion by the nebula. A numerica.l method has been developed, which makes it a simple m a tter to so lve the equat ions, if the density distri ­bution of the hydrogen through the nebula is known. The formula for the amount of BALIIEH light, corresponding to th e transiti on s->- 2, emitted by I crn3 of the nebula in one second is:

1' 2 = Q.2 x 2n",

where Q".~ is a constant for each nebula depending upon the tempera­tu re of the free electrons, x 2 the number of protons a nd 1/.(. the number of free electrons pcr cm ~.

i

The influence of capt ures of electrons 111 the LY~IA:\ level, which arc ;]ccompanied by the em ission of quanta with I' > 1'0' is cliscmsecl.

§:). The formulae derived in the preceding secl ion arc applied to a typical case of a spherical nebula emitting 10000 times as much visual light per second as our sun , having a diameter of 5 parsecs. A central star with a surface temperature of 30000° and a radiu s equal to 15 times that of the sun is taken as the source of the nebular

xon,emlSS1On. The value of - "- ' can be founel from the equation for the

Xl

ionization and X 21t" can be determinecl from the observed Hi) emission for I cm:) of the ne bu la. The result is:

x2 1700

Xl

and the densit y of the hydrogen appears to be equa l to:

The optical depth of the nebula is fairly small and cloes not exceed the value T 2 for light just on the violet side of the LnIA:\ limit.=c

Most of the oxygen in the nebula is either once or twice ionized, the number of orr atoms being 2.5 times as large as the number of 0 III a.toms per cm:J.

The formula for ls2 is further discussed. the concl usion being that the densities in the emission nebula e fluctuate considerably . finally H c rnlu,'s relation between the spectra l type of the exciting star and the spect ru m of the nebula is discussed . The radiation with 1) > 1'0 cannot even penetrate over a few tenths of a parsec distance in th e nebula, if the central star has a temperature of 10000°

and the hydrogen in the nebula a density of 10 - 22 g/cm:J. A. star with T - 1 0000° call only cause sufficient ionization, if the density in the nebula is of the order of 10-24 g/cm 3 , but the ,lmount of R\UIEl'

light emitted by I cm3 of the nebula will then be too small to make the nehula visible.

§ 1. Apparen

The region aro type stars. The the sky the spect magnitudeancl it Harvard plates. scale of the ma approximate an apparent magn.i This was espeC!; measure the a bs by a method s on the detenni

The Harvard exposure gratin on plate I 111m center of the pl ~

a yellow filter determination do not show 5

appeared to bE for all stars \1

The main ~

accurate pho! with a photog Ca talogue. AI' and - 57°50 '

part of the P' region for \Vh

') I-I. C. 352