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Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

Hot and Cold Gas in Extragalactic Superbubbles---Probing the Interstellar Medium of the Superbubble LMC2 in the Large Magellanic Cloud

Adeline Caulet
ST-ECF, ESO, Karl--Schwarzschild--Str.2, Garching bei München, D--85748 Germany
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Affiliated to the Astrophysics Division, Space Science Department of the European Space Agency



Superbubbles are gigantic bubbles blown in galaxies by the action of very powerful phenomenae (stellar winds and multiple supernovae explosions in stellar associations, collisions of clouds with galactic disks). They represent the largest space laboratory to study the physics of a dynamic interstellar medium. We present UV and optical absorption line measurements of gas clouds detected towards 7 OB supergiants in the direction of the superbubble LMC2 in the Large Magellanic Cloud. The UV observations, obtained with the GHRS spectrograph onboard HST, have revealed absorption lines from the ions S II, Si II, C IV that are tracers of warm and hot gas from Galactic to LMC velocities. Using the optical data (Ti II and Ca II ions), we summarize the kinematics and origins of the absorption clouds between the Galaxy and the LMC in correlation with the neutral (H I) and ionized (H ) 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.

Ti II and Ca II Absorption Lines Towards LMC2

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.

Ultraviolet Absorption Lines Towards LMC2

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


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Next: Gas and Dust Up: The Interstellar Medium Previous: Observations of Interstellar