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Atomic and molecular gas in the environs
of the ring nebula RCW 78
C. Cappa1,2, M. Rubio3,
M.C. Martin1 and N. McClure-Griffiths4
1 Instituto Argentino de Radioastronomía, Argentina
2 Facultad de Ciencias Astronómicas y Geofísicas, UNLP, La Plata, Argentina
3 Departamento de Astronomía, Universidad de Chile, Chile
4 Australia Telescope Compact Array, Australia
Valparaiso, Chile, October 2004
Introduction
Wolf-Rayet stars: Mdot ~ 10-5 Mo/yr (van der Hucht 2001)
Terminal velocities: Vw = 1000-3000 km/s (Prinja et al. 1997)
 contribute to the shaping and the chemical enrichment of the ISM
 create interstellar bubbles (IB)
RCW58 (E)
Interstellar bubbles are detected as:
 Optical ring nebulae (e.g. Chu et al. 1982,
Lozinskaya 1982; Heckathorn et al. 1982;
Marston et al. 1994a,b)
 Different formation mechanisms (Chu
1991): mass lost by the star (E type) or
swept-up material (UV radiation [Rstype],
Stellar winds [W type])
 25-30 % of the 227 Galactic WR stars
are surrounded by ring nebulae
NGC6888 (W+E)
Introduction
Interstellar bubbles are
detected as:
 Thermal radio continuum shells
(e.g. Goss & Lozinskaya 1995,
Cappa et al. 2002)
 Cavities and expanding shells in
the HI 21 cm line emission
distribution (Cappa et al. 2003
and references therein)
 Infrared shells (Marston 1991;
Mathis et al. 1992)
 X-rays sources (Bochkarev et al.
1987; Chu et al. 2004)
 Molecular lines
Anon(WR101), VLA, 1465 MHz, 38 arcsec
Anon(WR23), HI emission, 2.5 arcmin
Molecular observations towards ring nebulae
 Molecular material was found to be related to
some WR ring nebulae:
- Anon (WR16) (Marston et al. 1999)
-
NGC2359
NGC3199 (Marston 2001)
NGC2359 (Rizzo et al 2001, Cappa, Rubio and Goss 2001)
Anon (WR134) and NGC 6888 (Rizzo et al. 2001)
Anon(WR16), nebula around WR 103 (Duronea and Arnal 2004)
 Origin of the molecular gas:
- NGC 3199: stellar wind material (Marston 2001)
- All the other nebulae: interstellar matter
- NGC2359: interstellar matter, PDR (Cappa et al. 2001),
shock fronts are also present (Rizzo et al. 2001)
 Other molecules detected in NGC2359:
- H2 lines (St-Louis et al. 1999)
- CS, HCO+, CN, HCN (Rizzo et al. 2001b)
 Studies of molecular material associated with
ring nebulae are needed to investigate the
physical state of the gas, their kinematics and
energetics.
CO observations (black
contours): SEST, 22 arcsec)
Velocity: 54 km/s
HI data (white contours):
VLA, 45 arcsec
The ring nebula RCW 78 around WR 55
 Here, we present CO and HI
observations in the environs of the ring
nebula RCW 78 associated with the WR
star HD 117688.
WR 55 = HD 117688 = MR 49
 (l,b) = (307.8°,+0.16°),
(a,d)2000 = (13h 33m 30.1S, -62° 19’ 1.2’’)
 Spectral type: WN7 (van der Hucht
2001)
 Distance: 5.5 kpc (Conti & Vacca 1990)
6.0 kpc (van der Hucht 2001)
 Terminal wind velocity: 1100 km/s
(Hamann et al. 1995)
WR 55
Ha image (Treffers et al. 1983)
The ring nebula RCW 78 around WR 55
RCW 78
 Optical appearance: Brightest region
to the W and fainter regions to the E
 RS-type ring nebula, no shell structure
(Chu & Treffers 1981).
 Size: 35 arcmin in diameter
 Brightest region to the west: 10 arcmin
in size
 Kinematics from Ha line: from –44
km/s near the star to –53 km/s 7
arcmin north of the star.
 Kinematical distance: 5 kpc (Brand &
Blitz 1993)
-53 km/s
-44 km/s
HD92206
WR 55
Ha image (Treffers et al. 1983)
Database
 CO data: SEST telescope at La Silla, January
2002, March 2003
 12CO(1-0) at 115 GHz:
- HPBW = 44 arcsec
- velocity resolution = 0.43 km/s after smoothing
- rms noise = 0.20 K (Tmb)

12CO(2-1)
at 230 GHz:
- HPBW = 22 arcsec
- velocity resolution = 0.32 km/s after smoothing
- rms noise = 0.15 K (Tmb)
 Both lines acquired simultaneously in the
position-switching mode on a grid spacing 45
arcsec
 HI data: Southern Galactic Plane Survey (SGPS) (McClure-Griffiths et al.
2000), obtained with the ATCA and Parkes radiotelescopes
–
Synthesized beam = 2.4 x 2.1 arc min, Velocity resolution = 1.6 km/s, RMS noise: 1.5 K_Tb
 Radio continuum at 4.9 GHz from the Parkes-MIT-NRAO Survey (PMN)
(Griffith et al. 1993)
–
Angular resolution: 5 arcmin, rms noise: 8 mJy/b
RCW 78 in CO emission
 Molecular components with velocities in the range –65 to -8 km/s are
detected.
 Sample of CO profiles:
13h33m36.0s, -62o16’01”
CO(1-0)
CO(2-1)
13h33m21.0s, -62o22’01”
13h33m15.0s, -62o17’30”
RCW 78 in CO emission
 Mean Tmb
[-54.4,-53.1]
km/s
 Mean Tmb
[-53.1,-49.1]
km/s
 Mean Tmb
[-48.7,-44.7]
km/s
 Mean Tmb
[-42.5,-39.4]
km/s
CO contour
lines: 1 to 7
K in steps of
1K
RCW 78 in CO emission
 Mean Tmb
[-38.5,-36.7]
km/s
Some results
 CO emission within the range –54
to –33 km/s is associated with the
ring nebula.
 CO velocities agree with Ha
velocities.
 Mean Tmb
[-35.8,-32.7]
km/s
CO contour
lines: 1 to 7 K
in steps of 1 K
 The slight velocity gradient within
the range –54 to –40 km/s is similar
to the one observed for the ionized
gas.
 Molecular mass = about 3 x 104 Mo
RCW 78 in the HI line
emission distribution
Some results:
HI cavity and shell detected within the
velocity range -54 to –36 km/s
HI emission borders the NE, E and SE
sections of RCW 78, the agreement is
not so clear to the W
HI velocties agrees with CO and Ha
velocities
 Systemic velocity: -47 km/s
 Expansion velocity: about 10 km/s
 HI shell: neutral atomic counterpart of
the optical nebula
 Radius of the HI bubble: 15.5 arcmin or
26 pc (at d = 6 kpc)
 Dynamical age: 1.4 x 106 yr
 Neutral swept-up mass: 1800 Mo
Colour
scale:
82 to
110 K
HI emission [-49,-40] km/s
WR 55
RCW 78 in the radio continuum
 The image at 4.9 GHz shows a radio
source coincident in position with the
brightest part of RCW 78
 Flux density: S
4.9
= 0.5 Jy
 Physical parameters of the ionized gas
in the brightest part of the nebula :





EM = (1.0  0.1)x103 pc cm-6
Angular diameter = 8 arcmin
Adopted distance = 6.0 kpc
Mion = 900 Mo
rms Ne = 7 cm-3
RCW 78 at 4.9 GHz
RCW 78: energetics
Kinetic energy Ek (10 48 erg)
3-10
Stellar wind luminosity (1036 erg/s)
4
Mechanical energy of the wind Ew (1050 erg)
1.7
Energy conversion eficiency  = Ek / Ew
<0.04
Some results:
 The value of  indicates that the stellar wind of WR 55 is
strong enough to blow the interstellar bubble
 RCW 78 is in the momentum conserving phase or in an
intermediate stage between energy and momentum conserving
stages.
 This value is typical for interstellar bubbles around WolfRayet stars.
Valparaiso, Chile, 2004 October
Conclusions
 WR 55 is the only massive star related to RCW 78 and the main responsible for
the ionization of the gas.
 The HI gas emission distribution reveals an HI shell associated with RCW 78,
which can be interpreted as an HI bubble linked to the ionized ring nebula.
 CO observations show the presence of molecular gas related to the nebula with
velocities similar to those of the HI and HII material. The same velocity
gradient is observed in CO and Ha lines. The surface of the molecular cloud has
probably been photodisociated and ionized by the stellar UV photons.
 The stellar winds from the WR star are strong enough to create the interstellar
bubble. Since the dynamical age is larger than the duration of the WR phase of
the star, the progenitor of the current WR star has also contributed in shaping
the nebula.
 The nebula is in the momentum conserving stage or in an intermediate stage
between energy and momentum conservation.
Valparaiso, Chile, 2004 October
Future prospects
 To perform molecular observations towards other ring nebulae to
investigate the presence of PDRs and shock fronts.
 To investigate if star formation occurs in the surrounding shells.
 To investigate the energetics of these nebulae taking into account all
the gas components linked to the nebulae.
Valparaiso, Chile, 2004 October