A total of image tube spectrograms of HM Sagittae were obtained. More than 70 emission lines, including several broad emission features, were identified. An analysis of the spectra indicates that HM Sagittae is a planetary nebula excited by a Wolf-Rayet star. The most conspicuous Wolf-Rayet feature is that attributed to a blend of C III at 4650 A and He II at 4686 A

Brown, L. W.

Feibelman, W. A.

Hobbs, R. W.

Mccracken, C. W.

English

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r
In A
THE SPECTRUM OF HM SAGITTAE:
A PLANETARY NEBULA EXCITED BY
A WOLF-kAYET STAR
(NASA-TM-X- 1 1415)	 rAh Si'ECTHJM OF NM	 N78-10989
SA3ITTAE:	 ^ P1 ANF.TAAY NEBULA EXCITED © V A
WOLF - PAYET SM , (NASA)	 15 N NC A02/MF a01
C';CL 03B	 1711clas
G3/90 52044
L. W. BROWN
W. A. FEIBELMAN
R. W. HOGBS
C. W. McCRACKEN
OCTOBER 1977
	 1
4D GODDARD SPACE FLIGHT CENTER ' =iGREENBELT, MARYLAND
ol
iThe Spectrum of HM Sagittae: A Planetary Nebula
Excited by a Wolf-Rayet Star
 
}7k
4f9
L.W. Brown
Laboratory for Extraterrestrial Physics
i
W.A. Feibelman, R.W. Hobbs and C.W. McCracken
Laboratory for Astronomy and Solar Physics
NASA-Goddard Space Flight Center
Greenbelt, Maryland 20771
ABSTRACT
Eight image tube spectrograms of HM Sagittae were obtained
during July and August, 1977. More than 70 emission lines,
including several broad emission features, have been identified. 	 I
An analysis of the spectra indicates that HM Sge is a planetary
nebula excited by a Wolf-Rayet star. Tll, most conspicuous
Wolf-Rayet feature is that attributed
	
a blend of C III at
0	 0
4650 A and He II at 4686 A.
^N^F
INTRODUCTION
A new emission line object with continuous and emission
line spectral features was reported by Dokuchaeva (1976) , based
on an objective prism plate. According to that report, the
magnitude of this stellar object rose from 16m to 11m between
April and September, 1976. Examination of the Palomar Sky
Survey prints for 1950 showed an object of about 17m (0-chart)
with a remarkably red color. The magnitude varied between
11m
 and 12m during the interval September 1975 to July 1976.
A spectrum published by Stover and Sivertsen (1977) showed
the object to have emission lines characteristic of a planetary
nebula in the region XX4470-9500. A spectrum similar to this
was reported by Bopp (1977) and by McCracken (1977). The
1950.0 coordinates for this object, IIM Sagittae, are:
h• m	 s	 0	 It
a=19 39 41.40,6=+16 37 1 33.1 (Shao, 1977). A radio source
was detected at this position by Feldman (1977), who suggested
that HY Sge may be an embryonic planetary nebula.
OBSERVATIONS
HM Sge was observed on four nights with the Goddard 91-cm
reflector and Cassegrain spectrograph equipped with an RCA
C33011 image intensifier having a S-20 photocathode. The
0
dispersion was approximately 82A/mm in the first-order red
and 41A/mm in the second-order blue, with a resolution of
o
approximately 2 to 3 A ^n the second-order blue. The dates of
observation, exposure times, plates used, and wavelength ranges
for the spectrograms are listed in Table 1. The positions of the
3
k
1	 -e
1
3
`	
c,
l2
emission lines were measured on a small Gaertner measuring
machine; their identifications and qualitative estimates of
strength are listed in Tables 2(a) and 2(b). All spectro-
grams listed in Table 1 were traced on a Joyce-Loebl
micro dens itometer to obtain measurements of photographic
density as a function of position along the direction of
dispersion. Tracings of the spectrograms for Aug. 18-19 are
shown in Figs. 1(a) and 1(b). Conversion from density to
intensity (incident on the plate) has not yet been accomplished,
although the plate for each night's observations was sensito-
metrically calibrated by imaging a Kodak 21-step wedge onto
the same plate that contained the spectrograms.
The first spectrograms were exposed through a Schott
GG-13 (2 mm) filter so as to block out the second-order blue
spectrum. Examination of the spectro gram show ed an emission
line spectrum superimposed on a continuum completely free of
absorption lines. This suggested that subsequent observations
should be made at the highest dispersion possible with the
available gratings and camera, and that two different grating
tilts should be used to obtain full wavelength coverage in the
visible region of the spectrum. one of these tilts involved
mixed first-order red and second-order blue. At least one
exposure at each grating tilt was made during each of the
subsequent observing sessions. The spectrograms were not
spectrophotometrically calibrated by taking spectra of faint
standard stars because of the variability of extinction and
lack of observing time.
a
i
3DISCUSSION
HD+ Sge had a stellar appearance each night on which it
I
i
was observed, but this undoubtedly was influenced by the 3
'	 I
to 5 arc-sec seeing prevalent on all four nights. The guided
(unwiilened) spectra consist of two components: (a) an emission 	 t
line spectrum characteristic of planetary nebulae and (b) an
underlying continuum without any absorption lines but with	 a
several wide emission features characteristic of a Wolf-Rayet
star of type WC. Approximately 100 separate emission features
have been measured; 7G have been definitely identified (six
of them due to Hg night sky contamination), 3 have been
observed in other high excitation planetaries but not identified,
and the remainder are weak and either unidentified or doubtful.
a
n
The Balmer lines of hydrogen from Il a to H2O are present.
The TN Ii] 6548, 6584A lines are well resolved from He on the i
short exposures. Prominent lines of He I, IO I], IO II],
TO III], INe III], and TAr III] are obvious. Weaker lines of
t
He II, Si II, TS II], [S III], [Cl III], [Ar IV], and several
ti
others are detectable. The TO II] 3726, 3729 ^ doublet is present
and is appropriately exposed and adequately resolved on one of
the spectrograms to show that the ratio of intensities I(3729)/
I(3726)41. This ratio eualitatively indicates a very high
electron density (cf. Eissner, et al., 1969) similar to that
of IC 4997. The [0 II] 7320, 7330 A doublet is clearly present
on the appropriate spectrograms for each of the four nights.
r	 fl
I'
4	 )
The IS 111 6717, G731 A doublet (which could have been used	
4
to derive an electron density) is conspicuously absent, but
i
the IS II] 4068, 4076 A doublet is present. This anamoly
also suqqests a high electron density (Aller and Liller,
1968, pp 528-9) in a greement with that indicated by the IO II]
1(3729)/2(3726) ratio.
s
An electron temperature, T e ?.10 4 K, is indicated by the
presence of the IN II] 5755 Aand [0 I1I] 4363 A lines.
Collisional de-excitation in a very high density nebula will,
however, affect the electron temperatures derived from the
IN II] 5755/(6548+6584) and 10 111] 4363/(4959+5007) intensity
ratios (Osterbrock, 1974). The electron temperature cannot
be much in excess of 10 4K because the He II 4686 A line is
completely undetectable in the nebular spectrun. Evidence
for a moderately high excitation nebula is provided by the
nearly equal intensity of. the H 5 and TO III] 5007 A lines
and by the presence of [Ne III], Ire II'II, and [Am IV] lines.
Contamination of the IO 2121 4363 Aline by the IIg I 4358 A
line prevents a direct comparison with H., but visual
0
comparison shows that the intensity of the 10 III] 4363 A
line is somewhat lower than that of HY.
Several broad emission features are present which
identify the exciting star as a Wolf-Rayet of type WC. The
strongest Wolf-Rayet feature is that of C III 4650 A and He II
04686 A which stands out clearly in Figure 1(a). Weaker WR
0	 0
features are present for He II 5411 A - 0 V 5471 A and
possibly for the 0 Ili and O IV feature at 3400 A. There
r^
I
_	 I	
r
5
are broad emission features (pedestals) under the hydrogen
lines from H a to H E . The pedestals under these hydrogen lines
and the tie II 5411, 4542, and 4200 A lines can be attributed
to the Pickering series of lie II in the spectrum of the
Wolf-Rayet star (Aller, 1968, p. 344). A Wolf-Rayet feature
of CII at 6580 A may also contibute to the apparent pedestal
0
under the H a
 and [N II] lines. The tie II 4686 A line of the
Fowler series together with C III 4650 A constitutes the rather
broad, flat-topped emission feature shown in Figure 1(a) and is
characteristic of a Wolf-Rayet star, but with the IFe III]
0	 0
4658 A and 4701 A lines of the nebula superimposed. The tie II
0
4686 A emission line is not present in the spectrum of the
nebula, either by visual inspection of the plates or on the
microdensitometer tracings. The intensity of 1114 is enhanced
by IS III] and tI 16 is enhanced by He I. The spectrum of Hbi Sge
resembles most closely that of either NGC 7026 or IC 1747,
both of which are excited by W if-Rayet central stars (Aller,
1968, p. 352). A high contras'k print of the Aug. 18-19, 1977
observations which enhances th0. Wolf-Rayet features is reproduced
i
in p late 1.
Short term variability in the spectrum of HNI Sge is
0
exhibited by the behavior of th
i
e INe III] 3868 A line as shown
in Figures 2(a) and 2(b). The Ne III] 36681 line is fainter
than the neighboring H8 3889 A line-an Aug. 5-6, 1977 but
more intense than H8 on Aug. 18-19.	 L
0111GV pAG )
OF IP?	 QUA3^ZL`X
S
i
1
6CONCLUSIONS
The emission lines and spectral features identified in
the observed spectrum of FIM Sge are characteristic of a high
density and moderately high excitation planetary nebula
having a type WC Wolf-Rayet central star. Short term
0
variability is shown by the behavior of the [No III1 38G8 A line
between observations separated by two weeks. HM Sagittae
therefore manifests itself as a Wolf-Rayet star of type WC
exciting a gaseous envelope of material ejected during an
earlier stage of evolution. The observations reported here
also support earlier suggestions that HM Sge may be a
proto-planetary nebula or a planetary nebula in its early
stages of observability.
ACKNOWLEDGEMENTS
A Polaroid print and microdensitometer tracing of a
spectrogram of HM Sge obtained June 26-27, 1977 at CTIO by
Liller (1977) was kindly made available by him to us immed-
iately after the results of our spectrogram of July 22-23,
1977 were announced on IAU Circular 3099. The information
contained in Liller's spectrogram is essentially the same as
that contained in our spectrograms, minus the Hg contamination.
The intensity of the INe 111] 3868 A line relative to H8 and
H9 is comparable to that on our spectrogram of August 5-6,
but supports Liller's suggestion that a collaborative effort 	 3
may be advantageous, especially when spectrograms covering
a longer interval of time have accumulated.
REFERENCES
Allen,	 L.Ft., 1968,	 I.A.U.	 Symposium No.	 34,	 "Planetary
Nebulae",	 D.C.	 Osterbrock	 and C.R.	 O'Dell,	 ed.,
Reidel Publ., Dordrecht, The Netherlands.
a
Aller,	 L.F1. and Liller, W.,	 1968,	 "Stars	 and Stellar Systems",
Vol.	 'VII,	 B.M.	 Middlehurst and L.H.	 Aller,	 ed.
'University of Chicago Press.
Bopp,	 B.W., 1977,	 TAU	 Circular 3088, July 11.
e
Dokuchaeva, O.D.,	 1976,	 IAU	 Inform.	 Bull.	 Var.	 Stars,
No,	 1189,	 Oct.	 14.
Eissner,	 W., Martins,	 P.	 deA.	 P.,	 Nussbaumer,	 H., Saraph,
H.P.,	 and Seaton,	 M.J.,	 1969, Monthly Notices
Roy.	 Ast.	 Soc.,	 146,	 63.
Feldman, I.A.,	 1977,
	
TAU	 Circular 3081,	 June	 B.
Liller,
	
W., 1977, private communication.
McCracken,	 C.W.,	 1977,	 IAU Circular 3094,	 Aug.	 9.
Osterbrock, D.B.,	 1974,	 "Astrophysics	 of	 Gaseous Nebulae",
W.H.	 Freeman Co.,	 San Francisco.
Shao,	 C.Y., 1977,	 IAU Circular 3081,	 June 8.
Stover,	 R.J. and Sivertsen,	 S.,	 1977,	 Ap.J.	 Lett.,	 214,	 L33. 4
ORIGINAL PAGE IS
OF POOR QUALITY
I'
)
	 n
'table 1
Summary of Obser+aa:ionn
Plato	 Spectral Range
Pirst Order Second Order
0
rra-0, unbaked	 46+00-7350 A	 ---
0
Iia-0, unbakod	 4600-735n A	 ---
0	 0
	
Iia-0, baked 26 hr. at 50 C 4000-6700 A 	 ---
Ira-0, baked 26 hr. at 5 0	0100-7350 A 3400-4500
IIa-n, Naked 50 hr, at 50 C G100-7350 	 3400-4500
0	 0
	IIa-0, baked 50 hr. at 50 C 4000-6700 A 	 ---
0	 0
II a•• 0, baked 50 hr. at 50 	 4000-6700 A	 ---
IIa-0, baked 50 hr. at 5 C 6100-7350 A 3400-4500 A
Date
	 Expos-
(1977)	 uro
July 22-23	 5 min.
July 22-23
	
30 min
Aug. 4-5
	
2 hrs.
Aug. 4-5	 2 hrs.
Aug. 5-6
	
2 hrs.
Aug. 5-6
	
2 hrs.
Aug. 18-19
	
V, hrs
Aug. 18-19
	 3 hrs.
I
J
I(
i
i
^i	 I
3 4 00'A w 0 III , o IV 5411 w	 He II
46150 s C III 5471 w	 0 V
4686 5 Ile 11 G580 C II
* Notes:
*= unidentified in NGC 7027
11= unidentified in Ic 4997
a- strong
M.
 medium
w- weak
vw- very weak
d= doubtful
NS= night sky
arlroo^i aurs^^
r--, [
Table 2(a)
Pmlooion Linea in IIM Sge.
A Strength* Ion A Strength Ion A Strength Ion
3683 vw 1120 4340 s BY 5676 a lie I
3657 Vw 1119 4350 w 0 II 5905 w 7
3092 vw I118 4358 NS Ila 5911 w lie II
3697 vw 1117 4363 m IO	 II1] 6228 w IK VI]
3703 w 1116+1101 4471 m Ile	 I 6 30 0 w [0 11
3711 vw 1115 4550 w Si	 111 6312 w IS II1)
3721 w 1114+	 SIII 4658 vw C IV , Irollll G337 vw ?
3727 w Its	 111 4669 vw Ir	 I11 G347 w Si II
3734 w 1113 4701 w Ire	 III] 6354 w 7
3744 vw * 4733 vw Ire	 ITT] 6364 w to I]
3750 w 1112 4754 w Ire	 111] 6507 w 7
3762 vw [0	 II1] 48611 a 118 6548 m IN I1)
3770 m 1I11 4921 w lie	 1 6563 a Ito
3785 vw lie.	 1 4959 a IO	 111] 6584 m IN II]
3791 vw 0 TIT 4972 vw Ire	 III] 6678 a lie I
3797 m 1110 5007 a 10	 III] 6854 w ?
3807 Vw lie	 1 5039 w Si	 II, [Fe 111 6863 vw ?
3819 vw tie	 I 5151 w Ire ITT] 6881 vw He IIC?)
3835 5 119 5161 NS Hq 6693 vw Ile II M
3856 vW Si	 11 5159 vw IF0	 111] 7003 vw [Ar IV]
3858 vw lie	 TI 5262 w it 7032 vw 7
3868 a [NO	 1111 5271 m Ire	 III] 7065 a Ile I
3877 vw 7 5411 w tie	 II 7087 vw 7
3889 w 118+110I 5461 NS Jig 7108 vw 7
3926 d tie	 1 5486 w Ire	 V11 7135 s [Ar III]
3933 d ? 5537 w [Cl	 III] 7155 w 7
3944 d 0 IV 5562 w 7 7244 vw 7
3970 s 117 5577 NS+Nab 10	 11 7256 w ?
4026 d 1101 5702 w * 7279 w ?
4046 NS Ila 5718 vw [re VII] 7283 m lie I
4067 w IS	 111 5755 5 IN	 1I 1 7319 a to I11
4077 NS liq 5770 NS Ila 7330 a IO 121
4101 a H.6 5790 NS Hg
4326 w C 1I1 5820 w Cr III
Table 2(b)
i
Broad Emission Features in Hfi	 Sqe.
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i
	
1
^	 tom,• ^.
cR,
Figure Captions
Figure 1 Microdensitometer tracings of the two spectrograms
of HM Sqe for Aug. 18-19, 1977; the ordinate is the
microdensitometer signal (proportional to density)
while the abscissa is position along the direction
of dispersion (proportional to wavelength).
a(a) T6acing covering the spectral range 4000 A to
6700 A in first order. Ha, fig, and the conspicuous
Wolf-Rayet feature at ),X4650-4686 are labeled.
(b) Tracinq ocoveri$q (in mixed orders) the s ectral
range 6100 A-7350 A in first order and 34040-4500 R
in second order. H and the [Ar IIII 7135 P. line in
first order are labeled; several higher members of
the Balmer series in second order are also labeled.
The I0 IIl 7320, 7380 A doublet in first ardor
straddles the 3663 A line of the Iig 1360 R triplet
in second order. The [0 I11 3726, 3729 A doublet
(in second order) is not resolved on this long
exposure spectrogram.
Figure 2 Microdensi tome tax tracings showing variability in
intensity of the INe III] 3868 A line relati^,e to
118 and H9 on:
(a) Aug. 5-6, 1977, and
(b) Aug. 18-19, 1977.
Plate 1. Iligh contrast pri g t of the spectrum of HM Sge. Some
of the weaker emission lines are lost on this print
in order to enhance the Wolf-Rayet features. 	 1
3
1 A'A
Plate 1
Spectrum of HM Sge
First order spectrum
WR Ii a	WR	 Ha
First order
	 - ---	 Second order
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The spectrum of HM Sagittae: A planetary nebula excited by a Wolf-Rayet star

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