Adeline Caulet
ST-ECF, ESO, Karl--Schwarzschild--Str.2, Garching bei München,
D--85748 Germany
Electronic mail: acaulet
eso.org
Affiliated to the Astrophysics Division, Space Science Department of the European Space Agency
) gas detected along and near the star
lines of sight.
Keywords: superbubbles, multi--phase interstellar medium
We have undertaken detailed absorption line studies of the gas of the
superbubble LMC2 located eastward of the 30 Dor Nebula in the nearby Large
Magellanic Cloud. The previous work relevant to this paper can be found in
the following published and in press papers: Caulet (1980), Caulet et al.
(1982), Caulet (1995), Caulet & Newell (1996, Paper I). The early
publications include the LMC2 kinematics from H 
line emission, the
discussion of H I 21 cm emission line, stellar content and ionization of
the optical filaments. Several mechanisms have been investigated for the
origin of the expanding H 
supershell LMC2. The recent papers show
that probing the interstellar medium of superbubbles can be done
efficiently via absorption line studies at high spectroscopic resolution.
Paper I is the completed study of the Ti II and Ca II optical absorption
lines towards LMC2 and should be consulted because only a brief summary is
given below. The questions being investigated now and in future work are
the dynamics of optically invisible IS gas layers towards LMC2, the
connection of the superbubble phenomenon with galactic superwinds, the
turbulent ISM, the multi-phase ISM, the formation and history of gaseous
halos. We are aiming at a satisfactory model of LMC2 that takes into
account the physical characteristics of the superbubble and the effects of
supershell expansion and break-out into the LMC halo.
Figure 1 shows the location of 7 OB supergiants towards LMC2 used to probe the ISM in absorption. The background stars were observed with the spectrograph CASPEC on the ESO 3.6 meter telescope at La Silla.
Figure: Location of 7 target stars superimposed on the superbubble LMC2
(reproduction of a Schmidt H
plate, Davies et al. 1976).
The spectral resolutions are 0.11 and 0.16Å for 
3384 Ti II
and 
3934 Ca II, respectively. The signal--to--noise ratios vary
between 16 and 84 for Ti II, and between 58 and 143 for Ca II. These
observations are the first time detections of extragalactic Ti II and Ca II
absorption throughout an extragalactic superbubble. The physical
parameters derived from line profile fitting are the velocity, ionic column
density N and Doppler-width b of all clouds: for Ti II, N(Ti II) ranges
from 2 to 38 
10
cm
, b(Ti II) from 2 to
18
; for Ca II, N(Ca II) ranges from 1 to 84 
10
cm
, b(Ca II) from 3 to 28
. The absorption velocity V
ranges from -30 to 360
. The clouds with V 
100
belong to local gas and the Galactic halo. The clouds with V
between 120 and 140
are either in the Galactic halo or tidally
torn Magellanic gas. Clouds with V between 150 and 360
are
cold and warm clouds detected in the multi-phase disk and halo ISM of the
LMC with reduced depletion. Remarkable absorption profiles are observed
with broad asymmetric wings bluewards of the main disk component between
150 and 220
, perhaps the signature of thermalization (shock
heating) in high speed collisions between material ejected in the shocked
wind and multiple supernova explosions.
The absorption clouds attest of downward and upward motions of gas within
the superbubble. High velocity components around 300
are
located in front of the LMC main velocity layers at 
245 and
280
which are also the large scale bodies of neutral hydrogen in this
direction of the LMC. Therefore, the 300
clouds may be falling
towards the LMC plane. The gas clouds in the velocity range
150--220
may be moving upwards, as pushed by the expansion of the
superbubble above the disk (towards the observer). In Paper I, the
1980--82 interpretation of an ionized filamentary half-supershell expanding
at 30
into a LMC surrounding disk layer at 245
has
been revised to incorporate the existence of the absorption line components
not detected in H 
and recent theoretical ideas on superbubble
expansion (Tomisaka & Ikeuchi 1986, Li & Ikeuchi 1992). LMC2 extends
probably to a very large scale height above the LMC plane in the direction
of the observer. It may have originated in a collision between a low
velocity H I component, so--called L component at 245
(Luks &
Rohlfs 1992) and the main H I disk, i.e., D component at 280
.
In Paper I, we suggested that some of the absorption line velocity
components throughout LMC2 can be interpreted as falling high velocity
clouds, and clouds pushed by galactic fountains and superwinds.
The velocity field seen in the many Ca II and Ti II absorption components across LMC2 bears some similarity in its complexity and the shapes of the absorption line profiles with the velocity field observed in optical emission and absorption across the 30 Dor nebula on the western boundary of LMC2 (Chu & Kennicutt 1994, Blades & Meaburn 1980). This infers that energetic stellar winds could be important to explain the multiple velocity absorption IS components.
Twelve GHRS spectra of six stars shown in Figure 1 (SK -69282 not observed)
were obtained over wavelength intervals chosen to search for 1239--1243 N V
and 1548--1551 C IV doublets from hot gas. The grating G160M and the small
science aperture were chosen to give a spectral resolution of 0.075Å.
Signal--to--noise ratios are 
10 in about 100 minutes exposure time.
We give only preliminary results as further detailed analysis is required
(Caulet & Smith 1996). The clouds detected in Ca II are also detected in
1250, 1253 and 1259 S II, and 1260 Si II. The ions S II and Si II exist in
photoionised gas, and in gas compressed by shocks. High temperatures above
10
K are expected inside the superbubble cavity, associated with
conduction fronts and coronal gas. C IV absorption is indeed detected, but
not N V which would be the signature of shocks in collisionally ionized gas
at 
2 
10
K (equivalent width limit at a 3 
level
of 23mÅ). The absence of N V is somewhat surprising if the interstellar
medium of LMC2 is representative of a multi-phase ISM, since the diffuse
hot IS gas at 
5 
10
K from soft X-ray emission has been
observed over this region and even beyond the optical filaments (Wang et
al. 1991, Trümper et al. 1991). Because gas must cool, temperature in the
intermediate range corresponding to N V should be observed at the
interfaces of cloud--hot gas in the LMC2 cavity. N V has been observed in
the vicinity of LMC2, along the line of sight to 30 Dor nebula (W(N V)
50mÅ, de Boer & Savage 1980). Figure 2 shows the comparison of
S II 1250 and 1253 absorption and Ca II K towards the star HD 38448.
Figure 3 shows the absence of N V absorption along the same line of sight.
N V profiles at 
1239 were calculated for six absorbing clouds at
velocities near those obtained for Ca II. No corresponding absorption at
1243 was found to match the transition at 
1239, thus
invalidating the possibility of N V detection.
Figure: Heliocentric velocity profiles of 1250 and 1253 S II and Ca II K
IS absorption lines, and the H I emission line towards the star
HD 38448. The H I profile y-axis is the 21 cm brightness temperature divided by 50
from the data of Rohlfs et al (1984). The spectra have been normalized to
their stellar continua. Also shown, the range of H 
emission
velocity near the star position from Caulet (1980). The multiple
component fits to the absorption spectra are plotted as thin solid lines.
Figure: Heliocentric velocity profiles of 1239 and 1243 N V and Ca II K
IS absorption lines to show the non--detection of N V. Otherwise same caption
as in Figure 2
Blades, J. C. & Meaburn, J. 1980, MNRAS, 190, 59p
Caulet, A. 1980, these de 3eme cycle, U. de Paris, Bulle geante de gas ionise dans le Grand Nuage de Magellan
Caulet, A., Deharveng, L., Georgelin, Y. M., & Georgelin, Y. P. 1982, A&A, 110, 185
Caulet, A. 1995, in Cold Gas at High Redshift, ed. H. Rottgering, C. Carilli et al., Hoogeven: Kluwer, in press
Caulet, A. & Newell, R. 1996, ApJ, in press (paper I)
Caulet, A. & Smith, A. M. 1996, ApJ, in preparation
Chu, Y.-H. & Kennicutt, R. C. 1994, ApJ, 425, 720
Davies, R. D., Elliot, K. H., & Meaburn, J. 1976, MNRAS, 81, 89
de Boer, K. S., & Savage, B. D. 1980, ApJ, 238, 86
Li, F. & Ikeuchi, S. 1992, ApJ, 390, 405
Luks, Th. & Rohlfs, K. 1992, A&A, 263, 41
Rohlfs, K., Kreitschmann, J., Siegman, B. C., & Feitzinger, J. V. 1984, A&A, 137, 343
Tomisaka, K., & Ikeuchi, S. 1986, PASJ, 38, 697
Trümper, J., Hasinger, G., Aschenbach, B., Braeuninger, H., & Briel, U. G. 1991, Nature, 349, 579
Wang, Q., Hamilton, T., Helfand, D. J., & Wu, X. 1991, ApJ, 374, 475