Interference filter photographs were used to identify an arc of nebulosity that is coincident with the radio contours of the galactic supernova remnant G 126.2 + 1.6. Spectrophotometry of the filament shows that the emission line spectrum matches the spectra of other galactic supernova remnants. In particular, the arc shows the usual strong SII, and NII emission lines seen in other remnants and unusually strong OII emission as seen in a few remnants. The spectrum can be adequately matched by a shock of velocity near 100 km/s in an interstellar cloud of density 3. If the SNR is at a distance of 4.5 kpc as indicated by the radio signal-D relation, then the observed pressure in the filament requires an initial energy near 4 x 10 to the 51st power (d/4.5 kpc) to the 3rd power ergs

Blair, W. P.

Gull, T. R.

Kirshner, R. P.

Parker, R. A. R.

Sawyer, D. L.

English

NASA Technical Reports Server

IASA
Technical Mernorandum 80708
The Discovery of Optical emission
from the SNR G 126.2 +11-6
(NASA-TM-80708)
	 Ttih UrSCUVLRY OF OPTICAL
	 iVi ;3- Zb2Zti
EMISSION FROM THE SNR G 12b.2 + 1.6 (NASA)
18 p HC A02/MF L01	 CSCL 03A
U ii C1cia
G3/89 22941
W. P. Blair, R. P. Kirshner, T. R. Gull,
D. L. Sawyer and R. A. R. Parker
MAY 1980
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771	 r	 3^.
https://ntrs.nasa.gov/search.jsp?R=19800017729 2020-03-21T18:41:10+00:00Z
THE DISCOVERY OF OPTICAI. FMISSION
FROM
THE SNR G 126.2 + 1.6
William P. Blair l 2
Robert P. Kirshner '
Department of Astronomy, University of Michigan
T. R. Gull 
Laboratory for Astronomy and Solar Physics
NASA/Goddard Space Flight Center
D. L. Sawyer
Wesleyan. University
and
R. A. R. Parkerl
NASA/Johnson Space Center
Received
1Visiting Astronomers at Kitt Peak National Observatory which is operated
by the Association of Universities for Research in Astronomy, Inc. under
contract with the National Science Foundation
2Alfred P. Sloan Foundation Research Fellow
ABSTRACT
We have used interference filter Photographs to indentify an arc of
nebulosity that is coincident with the radio contours of the galactic
supernova remnant G 126.2 r 1.6. Spectrophotometry of the filament shows
that the emission line spectrum matches the spectra of other galactic
supernova remnants. In particular, the arc shows the usual strong [S III,
and N II emission lines seen in other remnants and unusually strong
[0 III] emission as seen in a fcw remnants. The spectrum can be adequately
matched by a shock of velocity near 100 km s -1 in an interstellar cloud of
density 3. If the SNR is at a distance: of 4.5 kpc as indicated by the radio
E-D relation, then the observed pressure in the filament requires an initial
energy near 4 x 1051d5 kpc'3 ergs'
Subject headings
nebulae: Supernova remnants
Interstellar: matter
Shock waves
I.	 INTRODUCTION
We present the first optical observations of the galactic supernova
remnant (SNR) G 126.2 + 1.6 that was recently discovered with the Efflesburg
100 m radio telescope by Reich, Kallas, and Stube (1979; hereafter RKS).
In the radio, the remnant appears as a large (68 are minute diameter)
shell of emission with an exceptionally low surface brightness (E (408 Mliz)
12..2 x 10 -22 watts m-2 Hz -I sterad 1 ) and a non-thermal spectrum (a = 0.85).
We find that optical filaments in An Emission Line Survey of the Milky Way
by Parker., Gull, and Kirshner (1979) are coincident with the radio contours
mapped by RKS at 1400 MHz and 2700 MHz. Further images and spectrophotometric
measurements made at McGraw-Hill Observatory confirm the identification of
the optical emission with the radio source, show that the optical spectrum of
the filaments is that of a supernova remnant, and provide estimates of the
physical conditions in the remnant.
H. OBSERVATIONS
a.	 Imagery
The region of the galactic plane near G 126.2 + 1.6 is included in
An Emission Line Survey of the Milky Way by Parker, Gull and Kirshner(19791.
That work presents monochromatic images obtained through interference filters
a
at [S II], Ha + [N II], [0 III], M and a blue continuum band at 4215 A. The
plates were obtained with a 300 mm focal length f/2.' Nikkor lens coupled
with a Carnegie Image Tube. Each IIIa-,T plate from the survey covers a
seven degree field with about 30 arc second resolution.
1
The survey was done on a systematic grid with considerable overlap
between fields. the region (if G 120.2 + 1.6 is recorded on three separate
sets of plates centered at 123 + 0, 128 + 0 and 125.5 + 5. An arc of
emission nebulosity is easily visible on all three plates which recorded
[0 II11 emission through a filter centered at 5010 R, with a full width at
half transmissicn of 28 A. Figure 1 shows the bust image of the [0 III]
emission, from the 128 + 0 field. In Figure 2 wt- she ►w a sketch of the
[0 III] arc superposed on a portion of the 1410 ` 111-, map of' RKS. Thir, arc,
which extends about 12 arc minutes in the north-south direction has an
apparent width of 2 are minutes on these small-scale plates. The optical
arc iies just outside the western radio rim of the remnant and is curved
in the same way as the radio contours. This close correspondence between
the optical and radio features; is tyl,iCal of the detailed connection seen
at the peripher y of most supernova remn- ► nts.
We also detect this arc or nebulosity in the 1 i,;h* c)f Ha + [h' II] as
00imaged through a filter centered at 6570 A with	 bandriss of 70 A. Through
our [S II] filter (at 6730 R, tea - 5 1) AS ) the aic i at the very l imit of
detestability.
To impro^re the angular resolution of our hv., ges we employed the same
[0 III] filter with a 114 no ;ingle - stake I`IT i mage '.ube at the f/7.5 focus
of the McGraw-hill 1.3 m telescope. The tntc • fol• -ice filter has 130 mm of
clear aperture so that nearl y thL' whole rielG	 the image tube has
a fiber optic output which was rjcordca on baltd Iia-ii	 Figure 3
shows that the arc of jo 11 1 1 enii: inn	 s';r : ► tee ► i tb C 1 2(;. 2 + 1 .6 is
resolved into two filaments when photographed at an original plate scale of
22" mm-1.
b.	 Spectroscopy
We employed the 2000-channel Intensified Reticon Spectrometer
attached to the McGraw-Hill 1.3 m telescope to obtain spectrophotometry of
the arc. The entrance aperture of the spectrometer measured 2.8 11 ,x 40" and
was placed across the filaments as shown in Figure 3. Sky subraction was
achieved by moving the telescope to a region of sky about 3 m west every
10 minutes. The spectrum was placed on a flux scale through measurements
of standard stars, and corrected for atmospheric extinction based on a mean
extinction table. The reduced spectrum shown in Figure 4 is the result of
60 minutes of integration on the filament.
In Table 1, we list the measured fluxes relative to Ha, F(X) a: ►d the
fluxes corrected for interstellar extinction I(X), with A  - 1.4 as described
below. For reference, we have also tabulated Miller's (1974) measurements
of Position 3 in the Cygnus Loop and Fesen and Kirshner's (1980) measurements
of G 65.5 + 0.5.
III. DISCUSSION
The spectroscopic data leave no doubt that the observed emission nebulo-
sity is a supernova remnant. In particular, the line ratios of [N II]
(16548 + X6584)/Ha z 1 and [S II) (X6717 + X6731)/Ha Z 1 correspond well to
the ratios found for SNR's by Daltabuit, D'Odorico and Sabbadin (1976) and are
distinctly different from the values usually measured in H II regions.
3
L.
Despite the poor statistics at W, the rat!,) [() 1119 (5007 + 	 is
at least 10. This is a relatively larg( value but not unprccedented in the
spectra of remnants, as shown by filler's (197 .4) ohservations of the Cygnus
Loop.
A large [0 II11/11S ratio has been measured by Fesen and Kirshner (1980)
in G 65.5 + 6.5, which is the other SNR which ha:. been identified by the
emission line survey described hr-, , ((,ull, kirshner and Parker, 1977). ine
emission line survey is much more sensitivc to I f ) 1111 emi--;ion than the
0-emulsions used for the blue plate of the Palomar Sk y `purvey, which may
account, at least in part, for the strong 10 III1 /11- ratio in the remnants we
have found.	 In particular, G 116.2 + 1.6 is easil y visible in our 10 III]
plate, but not found by RKS on the blue Sky Survey print.
The uncertainties in all the line_; in Table 1 ire at least 200, and the
weak lines suc h as Ilff are even less precise17 mf,a,,urod.
Nevertheless we have made a rough estimate of the interstellar reddening o
G 126.2 + 1.6 from the observed ratio of IV4 to !I'. 	 If we assnine that the
emitted ratio is about 3 based on model ealculat ions (Rayriond 1979) then the
observed value of 5.5 requires ;rl >out l . d M,11'11 i tudes Of ext i l!ct ion at V if we
employ the Whitford (11150 relation i1i the forlA ,l iver y hV % 1 1(1 ;uld Mathews
(1972).
We can estimate -ijr& of tl,e rhy ,,ic : ii propk , rt1	 of t 11 rrmnant by Coln
paring the observations	 Meta ; l cd	 the cr1 i	 i'^^n f r-o1n shock I cat cd
gas.	 In particular, b y LO,upal'ill;' t ri(' JO :I;)I''I" =tr'(-ngth with Paymend's
shock models we f..nd t}i;ii	 it v	 1 i (• -h '0, ' q tr "i *h : Fi,' , a q we ob.;erve
must be more than about. 70 I,m s 1 .	 ro. , 	Al, cl	 • tic o„ •t — hock t ,rmera-
tune is too low to produci , g tr ^:,i; (r' 1 I f emi	 ill,,_
4
As an example, we have listed Raymond's model X along with the observa-
tions. This model has a shock velocity of 100 km s -1 in a fully ionized
medium with a very small magnetic field and initial density, n c , of 10 cm-3.
The assumed abundances are those observed in the interstellar gas, in par-
ticular carbon and silicon are depleted relative to oxygen. The match is
quite good for the limited selection of lines that we observe.
We can use the observed [8 III line ratio to infer the electron density at the
temperature where sulfur is once ionized (about 8000 - 10,000 K). The
observed ratio of [S II] X6716/X6731 is about 1.3, which corresponds
to an electron density of n(S II) - 125 cm-3 , using Pradhan's (1978) cross
sections! Roughly speaking, the pressure in the shock is constant so that
the immediate post-shock pressure, n 
s 
T 
s 
is about equal to the pressure in
the zone emitting the sulfur lines where we know the temperature and density.
The post-shock pressure is directly related to the density of the cloud, nc,
and the shock velocity which we take to be V7 in units of 107 cm s -l . Model
calculations by Dopita (1977) and by Raymond show that
n z n(S II) V -2 cm-3
C	 45	 7
For our case, V7 - 1, n(S II) -125, so the derived density is n  = 3.
This seems like a perfectly reasonable density for an interstellar cloud,
and is consistent with the parameters for the shock model that we used for
comparison.
(1)
5
h­._ A.	 - r
We expect that the interstePa r medium is clumpy so that the shock
veloc i ty in the intercloud medium can be r•ubstantiall y higher titan 70 km ^'I
(McKee and Cowie 1975). For example, if the average density for the inter-
cloud medium is about 0.3 cm -3 , the shock velocity in that gas could be of
order 200 km s -1 , and G 126 + 1.0 could he a galarttr soft X-ray source.
Inspection of 11EAO -A data by Garmire (1980) did not reveal a source, but
the absorption of soft X-rays by gas in the galaxy could he significant in
this case. We can use our ostimatc of the pre-ssurc in the gas we see to
estimate the initial energy of the supernova explosion. Following McKee
and Cowie (1975) and Chevalier (1974) we wr;te
E :Y 2 x 104o	 (r,' ) -I n	 1', It ( pc ) 3 erg.	 (2)
Here h' describes the coupling of the blast wave pressure to the cloud
	
pressure. Using n_ V 7 2 z 3 from (1) and adopting f3'	 I.S. the angular dia-
meter of 68 are min from RKS, implies an initial energy
L - 4 x I0 I t I) k")c 	 erg.	 (3)
From the radio surface bri t;htne5s . RKS estimate D -- 4.5 l pc which requires
an initial of 3.6 x 10 51 erg. 'this numi^er i ,; large, but not excessive.
For comparison, the game computation appl icd to tho Cv ,vnus hout r data in Tal,lc
1 gives (for r " 20 pc), 1 --7 ^ 10 50 er,.
The principal ,-incertaint y in tilis tnnergy	 i= `he distance.	 A
crude id,ra of the dist.inLc +niijit be dorivc . l fror, the inter— tollar absorption
a l oni; tho l ine of ,, !;ht ,	 1. !	 !11	 t	 1 , :1 	 tor aV.
17
1.____
distance of 4.5 kpc, we find from integrating the 21 cm M I map by Westerhout
(1973) that the total coluwi density out to a velocity of -40 km/s which
corresponds to a distance of 4.5 koc. is n.. = 3.4 x 1021 cm-2 . Using the gas-
to-dust ratio of A.axleltoms cm-2
 mag-1 (Bohlin, Savage, and Drake 1978), this
implies Av - 1.8, which is consistent with the spectrophotometry.
In addition, Westerhout's survey shows a hole in the neutral hydrogen
which corresponds to the angular size of the SNR in the direction of
G 126.2 + 1.6. This local minimum in the hydrogen density is in the velocity
range -40 to -55 km s -1 which is consistent with a distance of 4.5 kpc in a
Schmidt gzlactic rotation model. If we imagine that the remnant has ionized
the volume it contains then it seems plausible to associate this region with
the remnant. On the other hand, this may be a mere coincidence.
We regard the distance tQ G 126.2 + :.6 as uncertain - the radio E-D
relation has very few SNR at the low-radio surface brightness measured by
RKS, and even fewer for which an independent distance measurement is avail-
able. It will be extremely interesting to see if a 21 cm 2bsorption line
profile can be obtained for this source to improve the distance estimate.
IV. G 127.3 + 0.7
The SNR G 127.3 + 0.7 is also within the region mapped by RKS and photo-
graphed by us in our 128 + 0 field. Careful examination of our original
plates shows no evidence for optical emission coincident with the radio
contours of this remnant. We note that this remnant surrounds the point
radio source G 127.1 + 0.5 which Caswell (1977) suggested might be radio
7
emission from a collapsed stellar remnant . Optical tO , :,ervat iuns by Kirshner-
and Chevalier (1978) show that the point radio : ")W	 i'. associated with .1
distant radio galaxy,
 rather than a galactic %ource. laswelI I % cab%^rVations
of the extended SNR are consistent with a distance for this remnant of 3.0
or 5.7 kpc. This is in contrast to the view cif Pauls (1377) who suggested
that the 5NR was physically associated with thr op:•1 ► Aoster Nt.t: S9, which
is at a distance of about 1.3 kpc(Lindhoff, l y (,j). since tiu• interstellar
absorption to the open cluster is about 
A 
	 1.3S, one might expert to "et,
optical emission if the remnant is realiy at that ilist:ins •.
	
l'he fact that
no optical emission is preient fovo.-. the larger dirt.-ace prupo.,:Cd by
Caswell.
RI I K's research un supernovac is : til)l>urtvd b^ .V,;I' t,r.xnt AST ; 7 -17ooli
and by the Alfrel I'. Sloan 1`oundati , m.	 '1110 I'll 11111 1-1:1 . ;e III!'(- i:^ on joall
from the Fitt I'eak National Ubsurvatnny.
8
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Figure 3. (O II1] emission imaged by the McGraw-Hill 1.3 m telescope showing the slit location of
the spectrum illustrated in figure 4.
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12
TABLE: 1
Emission Lines in G 126.2 + 1.6
ION a I 1 Mode 1 I CY}MUS 1.001 G 65.S3(H6 =	 100) (AV	=	 1.4) x it
Iit3 4861 100 100 100 100 100
[0 111] 4959 + 5007 1095 1031 881 1276 2447
[0 11 6300 + 6363 <100 <60 50 29 -
(N I11 6548 + 6583 539 291 272 343 369
Ha 6562 554 300 308 306 250
[S I11 6716 339 177 155 130 250
Is 111 6731 254 133 126 114 240
1. Raymond (1979), 2. Miller (1974), 3. Fesen and Kirshner (1980)
13
RI:FIiRENCES
Bohl in, R. C., Savaqe, B. D., and Drake, J. F. 1978, AL). J., 224, 132.
Caswell, J. L. 1977, M.N.R.A.S., 18:.
Chevalier, R. A. 1974, Ak. J., 188, S..
Daltabuit, L., D'Odorico, S., and Sabbadin, F. 1976, Astr. Ap., 52, 93.
Dopita, M. A. 1977, Ap._J_ Suppl., 33, 437.
Fesen, R. A., and Kirshner, R. 1 1 . 1980, in preparation.
Garmire, G. 1980, private communication.
Gull, 1. R., Kirshner, R. P., and Parker, R. A. R. 1977, Ap. J. (Letters),
215, 1.69.
Kirshner, R. P., and Chevalier, R. A. 1978, Nature, 216, 480.
Lindhoff, U. 1969, Arkiv. Astron., 5, 221.
McKee, C. F., and Cowie, L. L. 1975, AE .J., 195, 715.
Parker, R. A. R., Gull, T. R., anti Kirshner, R. P. 1979, An [:mission Line
Survey of the Milky Wes, (NASA SP-434).
Miller, J. S. 1974, Ap. J., 189, 239.
Miller, J. S., and Mathews, W. G. 1972, Ap. J., 172, 593.
Paul s, 't. 1977, Astr. Ap., 59, 1.13.
Pradhan, A. K. 1978, M.N.R.A.S., 183, 89P.
Raymond, J. C. 1979, ^L)_ J. Su 1., 39, 1.
Reich, W., Kallas, F., and Steube, R. 1979, Astr. Ap., 78, 1.13.
Whitford, A. E. 1958, A. J., 6z, 201.
Westerhout, G. 1973, Mar}land-Green Bank Galactic 21-cm Line Survey,
(3d ed.; Baltimore: Univ. Maryland).
14
BIBLIOGRAPHIC DATA SHEET
1	 Repor t No. Accession NoT777rnment 3.	 Recipi	 tit's Catalog No
TM 80708
4. Title and Subtitle 5. Report Date
The Discovery of Optical Frnission fiam the
6. Performing Organization CodeSNR G 126.2 + 1.6
7. Authorls) W .
 p. Blair, R. N. hirshnet, T. R. Gull, 8. Perfor-rung Organization Report No
1). L. Sa wyer, and R. A. R. Pa rker
9. Performing Organization Name and Address 10. Work Unit No.
Goddard Space Flight Center 11. Contrac' or Grant No.Greenbelt, Maryland 20771
13. Type of Report and Period Covered
12. Sponsoring A(ymcy Name and Address
1'echnical Memorandum
National Aeronautics and Space Administration
Washington, D.C. 20546 14	 Sponsoring Agency Code
15. Supplementary Notes
16. Abstract
We have used interference filter photographs to identify an arc of tichulosltV th:it Is
coincident with the radio contours of the galactic supernova remnant G 120.2 t 1.6.
Spectrophotonletry of the filament shows that the emission line spectrum matches the
spectra of other galactic supernova remnants. 	 In particular, the arc shows the usual
strong I S ll ) , and N If emission lines sc.tl in other remnants and unusually strong
10 1111 emission as seen in a few remnants. vic spectrum can be adequately matched
by a shock of velocity near 100 kill s - 1 in all 	 cloud of density 3.	 If the SNR
is at a distance of 4.5 kpc as indicated by the radio 1-1) relation, then the observed
z
Pressure in the filament requires an initial energy near 4 r 1051 	 ergs.
\4
 S`1'pc/
17. Key Words (Selected by Author(s))	 18. Distribution Statement
nehulae:	 Supernova remnants
Interstellar:	 matter
Shock waves
19. Security OdSSIf. (of this report)	 20. Security Classif. (of this page)	 21. No	 of Pages	 22. Price"
Unclassified	 Unclassified
'For sah , by the Nationa; T.v,nical Information Service, Springfield, Virginia 22151	 GSFC 25 44 (10177)


The discovery of optical emission from the SNR G 126.2 + 1.6

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