Received 14th March 1998
Abstract.
Massive stars (∼25
Msun) are known to have strong
stellar winds and a high mass loss rate of about
10
-6 Msun yr
-1.
Ring nebulae around massive stars can therefore be formed by fast stellar winds
sweeping up the ambient medium.
For massive stars these bubbles can either be
interstellar or
circumstellar bubbles.
While the Large Magellanic Cloud reveals different large scale bubbles
formed by the combined actions of stellar winds and supernovae in OB
associations, like superbubbles and supergiant shells, there is a surprisingly
low number of bubbles around single OB stars.
The same paucity of OB star bubbles is observed in the Milky Way.
We present preliminary analysis of a survey for single bubbles around massive
stars in the Large Magellanic Cloud (LMC).
Statistics of the presence of bubbles around single, massive star will put
constraints on the predictions for the formation mechanism, early evolution
of bubbles and their coalesce to large-scale shells.
1. Introduction
All massive stars with ZAMS masses above ∼25 Msun
show a non-negligible mass loss through stellar wind during their lifetime.
With wind velocities of 1500-3000 km s-1 main-sequence O
stars remove already half of their mass into the interstellar medium in less
than 107 yr (depending on the initial mass and metallicities,
Z ↓ than d/dt M ↓).
While evolving, massive stars pass the red supergiant phase or if
MZAMS>50 Msun even experience a
short but highly unstable phase with high mass loss as Luminous Blue
Variables (LBVs).
In both cases a dense and slow wind will be shed into the ISM and, when
evolving further a fast wind in the evolved supergiant and Wolf-Rayet phase
can sweep up the older denser wind and form bubbles.
This scenario shows that massive stars influence the ISM in their whole
lifetime, deposing various winds, sweeping it up and forming bubbles and
nebulae.
For main-sequence massive stars, the ambient medium is interstellar, and
the ring nebulae are called interstellar bubbles.
For evolved stars, the ambient medium could be stellar material lost via
slow winds during the red supergiant phase or via outbursts during the LBV
phase, and the ring nebulae would be circumstellar bubbles or a
Luminous Blue Variable nebula, LBVN.
While several LBVNs and nebulae around Wolf-Rayet stars are known in
the LMC, only few examples are found for interstellar bubbles.
To analyze whether this is a physical effect or an observational bias we
try to improve the statistics by performing a dedicated search for small
bubbles in the LMC.
Parameters for a sample of bubbles and their energizing stars
will produce input for stellar evolution models.
In addition this leads to a better determination of connected parameters
to model the bubble formation of these stars.
2. The search for bubbles
We present here a status report of our search for bubbles around single stars
in the Magellanic Clouds.
The LMC is a perfect target for deriving a well controlled sample, because
of its low reddening, small inclination and line-of-sight depth, and its well
determined distance.
We used the CTIO 0.9 m telescope
to take images in Halpha and broad band B and R
filters.
Each field has a size of 13.5' × 13.5'.
We selected areas around star forming regions.
We avoided the core of 30 Dor and the interiors of supergiant shells, as
in these environments we expect that bubbles would be confused with effects
from groups of neighboring stars or would be unobservable due to low density,
hot gas environment.
We visually inspected a subset of our Halpha data.
The 28 fields used cover a total of 1.2° × 1.2°
(or 1 kpc × 1 kpc using a distance modulus of
18.5 mag).
As selection criteria we used a maximum bubble diameter of 15 pc,
to differentiate between bubbles created by a single star and larger
structures, which are more probably contaminated from neighboring
massive stars.
The seeing was better than 1.4" in all our images, therefore we can safely
detect structures around stars down to 0.5 pc.
Comparison with the scans of Hodge & Kennicutt (1986) showed that the
limiting surface brightness of our images is always below
1·10-17 erg cm-2 sec-1 arcsec-2.
We found 36 bubble candidates around single stars of which 8 candidates are
plotted in Fig. 1.
All candidates show clear bubble geometry or at least some edge-brightening,
the expected signature of bubbles.
Still, kinematics of some candidates is needed to decisively exclude classical
Stroemgren spheres from the sample.
A statistical analysis shows an increased frequency of bubbles with diameters
of 2-3 pc clearly indicating that we find structures intrinsic to single
stars.
With these sizes typical dynamical ages are of the order of
105 yr if they are interstellar bubble.
If they are circumstellar, ages cannot be estimated, since we cannot decide
from the current data if wind-wind interaction or a single eruption created
the bubble.
The second accumulation of diameters around 6-7 pc may indicate that
we pick up a different type of bubbles.
They might resemble bubbles around more evolved stars or older interstellar
bubbles.
[Click here to see Fig. 1!]
3. First Results
Our survey is able to pick up bubbles around reasonable isolated
massive stars in the LMC.
Bubbles around massive stars are not as rare as previously thought, and
they show the expected broad range of properties (size, surface brightness,
detailed morphology).
If they are interstellar bubbles we pick up very young effects of the star
on the surrounding ISM.
Our bubbles include a number of candidates for very massive stars if the
bubbles are circumstellar.
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| First version: | 08th | August, | 1998
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| Last update: | 30th | September, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)