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

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GHRS Observations of Diffuse Clouds in the Local Bubble

Cécile Gry and Olivier Dupin
Laboratoire d'Astronomie Spatiale, B.P.8, F-13376 Marseille cedex 12, France

Abstract:

We analyze HST GHRS observations of the interstellar medium in the direction of two supergiant B stars, CMa and CMa, located 200 pc away in a region very deficient in neutral gas (N(HI) cm). The two lines of sight are very similar in direction, distance and neutral gas content but are very different in ionized gas content. We have found two strongly ionized components only visible towards CMa (ionization fraction at least 90). This important ionization could be produced by the two intense EUV radiation fields of the two stars.

Introduction

Many observations report that the Sun is embedded in a small interstellar cloud, with T=7000 K and n of the order of 0.1 cm, itself located in a region of low density (n5 ) and high temperature (T K) called the Local Bubble. This one extends in all directions to at least 50 pc and reaches the stars CMa and CMa respectively 188 and 206 pc away (Gry et al. 1985, Welsh et al. 1995).

We have obtained spectra of both CMa and CMa with the HST GHRS. The analysis of the interstellar medium towards CMa has been presented in Gry et al. (1995), as well as the data processing and line profile analysis methods used for both stars. We present here a new study of the CMa line of sight and its comparison to the CMa direction.

Structure of the CMa Line of Sight

Three main components, numbered 1 to 3, have been detected without ambiguity in the CMa FeII and MgII lines (see Gry et al. 1995). The first one has been identified to the Local Interstellar Cloud (LIC), in which the Sun is embedded, and the second one is identical to the second component seen in the line of sight to CMa, that lies only 3 pc away from the Sun (Lallement et al. 1994). Table 1 summarizes the temperatures and turbulent velocities of these three clouds derived from both FeII and MgII b-values.

The electron densities shown in Table 1 have been obtained from the observed MgII/MgI ratio. Despite of the high uncertainty on these values, it is clear that the ionization is important in these clouds. For the LIC, the ionization fraction ranges from 5 to 75% with a preferred value of 47% if T=7000 K and n(HI)=0.1 are adopted.

The LIC and the component 3 have been detected in highly ionized species (CIV and SiIII). The most plausible interpretation is a collisional ionization in a high temperature gas ( K) linked to these components. This gas could be the thermal conduction interface between the clouds and the very hot coronal gas ( K) which fills the Local Bubble and is responsible of the soft X-ray background.

Cassinelli et al. (1995a) and Vallerga et al. (1995) have established a total HI column density of 9 cm from the lyman continuum in the EUVE spectrum. We find an upper limit of 5 cm from the NI lines. The total H column density is slightly higher, indicating that about half of the gas is ionized : N(H) has been estimated to 1.5 cm for FeII and 1.4 cm for MgII using the warm cloud abundances determined by Jenkins et al. (1986).

The contribution of the clouds 1 and 2 to the total column density is located within the 3 first parsecs. Therefore, in the other 185 pc, the mean gas density is less than 4.5 cm. This makes the CMa line of sight the most devoid region of neutral gas known in the solar neighborhood.

 
Table 1: Physical conditions in the clouds:

Structure of the CMa Line of Sight

The line of sight towards CMa had already been studied by Gry et al. (1985) at lower resolution with Copernicus UV data. They reported an HI column density of 1--2 cm and a total H column density (derived from SII and SIII) of about 1.6 cm. The study of the EUV spectrum of CMa by Cassinelli et al. (1995b) has yield to a total column density of 2.00.2 10 cm for HI and at least 1.4 10 cm for HeI that implies a total H column density of at least 1.4 10 cm, both results in agreement with the previous Copernicus results. Our Ech-B GHRS data allow us for the first time to separate the interstellar absorption in at least 4 clouds labeled A, B, C and D (figure 1). All four are visible in both the FeII and MgII lines, but only the strongest C and D components in the MgI and SiIII lines. Physical parameters of the clouds are shown in Table 1.

The projection of the Local Cloud (LIC) velocity vector (v=26 towards the direction , (Lallement & Bertin 1992) on the CMa line of sight give us an heliocentric velocity of 20.3 for its absorption feature. So, the component due to the LIC is hidden in the blue wing of the component C; but we take it into account in our model adopting the column densities and b-values found in the CMa direction.

Given its velocity position, the cloud B can be identified to the component 2 of CMa. However, its location close to the component C has made the determination of both its column densities and b-values very uncertain.

Using the warm cloud abundances of Jenkins et al. (1986) and our column densities, we derived N(H)=1.50.2 10 cm and N(H)=1.70.3 10 cm from, respectively, FeII and MgII, that is in good agreement with the Copernicus and EUVE values. We, thus, confirm that at least 90% of the matter in the line of sight is ionized. The strength of our data is to show that the ionized gas is distributed in two distinct interstellar components (C and D) for which we measure the electron densities from the MgII/MgI ratio (the results are shown in table 1). This shows altogether that the ionized gas is not uniformly distributed in the line of sight and that it does not constitute either a usual HII region around the star CMa.

We have measured N(SiIII) for clouds C and D, but we failed in detecting CIV unlike in the CMa case. This lack of detection could induce inhomogeneities of the boundary layer.

 
Figure: GHRS Ech-B spectrum (R85000) of MgII, MgI and FeII lines. The solid points are the observations, the dashed line is the stellar continuum and the solid line the fit to the interstellar absorption profile.

Conclusion and Perspectives

Despite of their proximity and their similarities in HI content, the lines of sight of CMa and CMa are radically different. The CMa sigh-line intercepts mostly local gas and thereby allows the study of the physical conditions in local clouds. Its two main clouds have been detected in the CMa line of sight (3 pc away) and so, it is essentially empty after the 3 first parsecs.

The CMa line of sight is dominated by two ionized components, located after the 3 first parsecs (not detected in the CMa direction), where at least 90% of the gas responsible for the FeII and MgII absorption is not in an HI region. This ionization could be due to the strong EUV radiation fields produced by both CMa and CMa which are known to be the strongest EUV sources in the sky. Note that a collisional ionization induced by the particular location of the clouds in the void tunnel is also possible. The study of the ionization processes constitute the next step of our work.

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