A&A 390, 299-306 (2002)
DOI: 10.1051/0004-6361:20020627
T. W. Berghöfer 1 - D. Breitschwerdt2
1 - DESY Hamburg, Notkestraße 85, 22607 Hamburg,
Germany
2 -
Max-Planck-Institut für Extraterrestrische Physik,
Giessenbachstr. 1, 85740 Garching, Germany
Received 1 May 2001 / Accepted 19 April 2002
Abstract
We have analyzed the trajectories of moving stellar groups in the
solar neighborhood in an attempt to estimate the number of supernova
explosions in our local environment during the past 20 million years.
Using Hipparcos stellar distances and the results of kinematical
analyses by Asiain et al. (1999a) on the Pleiades moving groups, we
are able to show that subgroup B1, consisting of early type B stars
up to
,
but lacking more massive objects,
has passed through the local interstellar medium within less than 100 pc.
Comparing the stellar content of B1 with the initial mass function
derived from the analysis of galactic OB associations, we estimate the
number of supernova explosions and find that about 20 supernovae must have
occurred during the past
10 - 20 million years, which is suggested
to be the age of the Local Bubble; the age of the star cluster is about
20 - 30 million years. For the first time, this provides
strong evidence that the Local Bubble must have been created and shaped by
multi-supernova explosions and presumably been reheated more than 1 million years ago, consistent with recent findings of an excess of
in a deep ocean ferromanganese crust.
Calculating similarity solutions of an expanding
superbubble for time-dependent energy input, we show that the number of
explosions is sufficient to explain the size of the Local Bubble.
The present energy input rate is about
erg/s, in good agreement with the
estimated local soft X-ray photon output rate.
It seems plausible that the origin of the Local
Bubble is also linked to the formation of the Gould Belt, which originated
about 30-60 Myrs ago.
Key words: stars: early-type - ISM: bubbles - ISM: general - ISM: kinematics and dynamics - ISM: structure - Galaxy: solar neighborhood
Our solar system is embedded in a medium of low H I column density (e.g. Frisch & York 1983) largely filled with hot plasma radiating in the soft X-rays (McCammon et al. 1983). The obvious anticorrelation between neutral gas and X-ray emitting plasma on large angular scales has been interpreted in terms of an elongated local hot cavity (Tanaka & Bleeker 1977; Sanders et al. 1977; Snowden et al. 1990) with an extension between 80 and 200 pc, now commonly known as the Local Bubble (LB). Recently, based on the spatial distribution of stellar EUV sources detected with EUVE and subsequent Na I absorption line studies (Sfeir et al. 1999), Welsh et al. (1999) suggested that the LB shows a "chimney like'' structure with no definite border towards high Galactic latitude in the northern direction rather than a full bubble. Despite the detailed mappings of the LB and our knowledge about its morphology, its origin is still a mystery.
Although it has often been suggested that the LB is the result of one or more
supernova (SN) explosions (Cox & Anderson 1982; Innes & Hartquist 1984),
there is no direct evidence for this
hypothesis. Unlike in the neighboring Loop I superbubble,
with still ongoing star formation and
about 40 SN explosions to occur in the future (Egger 1998), no
obvious active cluster of early type stars resides inside the LB.
Moreover, the analysis of the line of sight in the direction of the star
CMa, which extends over 200 pc, seems to indicate that the cavity is
not homogeneously filled with X-ray emitting plasma (Gry et al. 1985;
Welsh et al. 1998) and a non-negligible part of the LB material must exist in
a warm phase with a temperature of several 104 K.
This rules out a LB formation by one single SN explosion since an expanding
pressure driven remnant would
always fill the cavity inside a swept-up shell.
On the other hand, there is also support for an isotropic nature of the LB emission, coming from EUVE observations of cloud shadows in the local ISM. Based on these observations, Berghöfer et al. (1998) found evidence that the pressure of the hot interstellar medium is the same in three different directions near the Galactic plane. Therefore, it seems appropriate to distinguish between the local cavity as the region largely devoid of H I gas and the present day LB as the X-ray emitting region. These ambiguous observational results have consequently led to a variety of different ideas concerning the origin of the LB.
Some time ago the mysterious -ray source Geminga seemed to be the
most promising candidate for a recent nearby SN explosion, after having
been identified with an X-ray millisecond pulsar (Gehrels & Chen 1993).
However, HST observations have provided a parallactic distance of
160 pc (Caraveo et al. 1996). Therefore, due to its high proper
motion, Geminga most probably originated
yr ago in the Orion
region and is thus not related to the present day EUV and soft X-ray emission
of the LB.
In another scenario, the superbubble picture of the LB is disputed, and the LB is conceived as being related to a local interarm region between the Sagittarius and the Perseus spiral arms of the Galaxy (Bochkarev 1987; Frisch 1995, 1996). It is assumed that during different epochs of star formation in the Sco-Cen association hot gas was generated and subsequently swept into the surrounding inhomogeneous medium. While some morphological features of the local ISM can be explained by such a model, it cannot account for the distinct X-ray shadow observed towards the Loop I bubble in the ROSAT PSPC (Position Sensitive Proportional Counter) data (Egger & Aschenbach 1995), which is most naturally explained by an interaction ring between Loop I and the LB. The existence of a "wall'' between these two bubbles has also been demonstrated by stellar absorption line measurements of stars with known Hipparcos distances. The formation of such a wall is rather difficult to explain once hot plasma is flowing out from Loop I directly into the LB. For a detailed discussion we refer the reader to Breitschwerdt et al. (2000).
Recently, Smith & Cox (2001) have performed 1-D hydrodynamic simulations to explore whether a low energy input rate in combination with a high ambient pressure can result in the formation of a bubble like the LB. They find that 2-3 SN explosions within a warm ionized medium are sufficient to explain most observed characteristics of the LB. It is argued that due to the absence of a parent star cluster, these should be due to random explosions, e.g. due to runaway OB stars, in the Galactic disk. As a result of the low energy input rate and high ambient pressure all models predict a collapse of interstellar bubbles once the evolution time scale is larger than about 6 Myr. Although an interesting idea, there is to our knowledge as yet no observational evidence of a bubble in contraction.
Given these widely different concepts for the origin of the LB, it seems appropriate to investigate the evolution of the stellar content of the local ISM back in time for the last tens of million years. Thus we are looking for a "smoking gun'' of a stellar cluster consisting of massive stars that may have passed through the LB; its most massive members should have already ended as SNe, but the later-type stars should still be present. Applying an initial mass function (IMF) appropriate for OB stars we are able to infer the number of members that may have exploded within a region that now forms the local cavity. However, these stars may have dispersed and are not easily identifiable as a cluster any more. As pointed out by Eggen (1996), small perturbations in the space motion of stellar clusters can lead to a significant spread of cluster members on a relatively short time scale. In Sect. 2 we take a detailed look at the population of stars in the solar neighborhood. In Sect. 3, we provide results of our investigations on the youngest stars known to exist, and in Sect. 4 we derive the time-dependent energy input due to successive SN explosions and calculate the evolution of the LB. Finally, in Sect. 5, we discuss our findings and present our conclusions on the formation of the LB.
Investigations of the stellar content in the solar neighborhood have shown that young stars do exist. Obviously, these stars cannot have the same origin as the much older stellar population including our Sun. Ongoing star formation in combination with phase-mixing and relaxation must have led to young and old stars now co-existing in the solar neighborhood.
Detailed studies of the Hipparcos data have provided the existence of a number of stellar moving groups. Such moving groups are star streams identified by a similar velocity component in the direction of Galactic rotation. A complete review of the concept of such star streams and the Galactic structure can be found in Eggen (1996). It is suggested that some, if not all, of these moving groups belong to so-called superclusters. According to Eggen (1996), internal and external stresses (e.g., disk heating and Galactic differential rotation) may cause perturbations in the space velocities of the individual stars in a cluster, which led the original cluster to expand in a relatively short time and form a supercluster.
Recently, Asiain et al. (1999a) studied the kinematical properties of 1924
B, A and F main sequence stars extracted from the Hipparcos catalog.
They included radial velocity measurements to derive galactic space velocity
components (U, V, W) and Strömgren photometry to determine stellar ages for
their sample stars. Using a method based on non-parametric density estimators
they find evidence for moving groups in the four-dimensional (U, V, W, age) space.
In addition to already known moving groups, e.g., the Sirius and the
Pleiades moving groups, they detect finer substructures with kinematical
properties apparently consistent with those of nearby open clusters or
associations. In particular, they find that the Pleiades moving group splits
into 4 significantly different subgroups
of vastly different ages ranging from 20 to 300 Myrs. According to Asiain et al. (1999a), the youngest (B1 in their notation) is also the
most distinct subgroup and the only one still concentrated in space. Most of
the 27 member B stars lie in the nearest direction of the Gould Belt,
.
The center of mass of these stars is located
pc away from the Sun, whereas
the distance of the Gould Belt in this direction is
160 pc. Obviously,
this Pleiades moving group is located in the foreground of the Gould Belt.
According to Asiain et al. (1999b) the average age of these stars is
Myrs. Based on simulations of the evolution of a stellar complex
including the influence of the Galactic gravitational potential and the effect
of disk heating with a constant diffusion coefficient, Asiain et al. (1999b)
are also able to explain the gross properties of the kinematics of the
Pleiades moving group substructures.
The stars associated with Pleiades subgroup B1 constitute the youngest stars known to exist in the solar neighborhood. In order to explore whether more massive stars existed in this association that already exploded as SNe while moving through the LB region, we assume coeval star formation and investigate the mass spectrum of the stars in the Pleiades subgroup B1.
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Figure 1: Mass histogram of the stars in the Pleiades B1 moving group (dark shaded area). The thick solid line shows the best fit initial mass function. Since the number of stars known so far to be associated with subgroup B1 is actually a lower limit, we also plot the uncertainties as a hatched region (see Sect. 3). |
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Employing evolutionary tracks by Schaller et al. (1992) for the 27 stars in
subgroup B1 we derived individual stellar masses from their spectral types
and luminosities. In Fig. 1 we show the mass spectrum of the
stars in this
subgroup. As can be seen, subgroup B1 is dominated by early B stars of spectral
types B3-B1.5, with masses in the range 6-9 .
It is
worthwhile to mention that 11 of these stars have already evolved from the
main-sequence. Note that the sample of stars investigated by Asiain et al.
(1999a) is not a complete sample.
The apparent deficit of stars with masses less than
in
Fig. 1 can be attributed to this incompleteness. In
fact, the number of stars known so far to be associated with subgroup
B1 is a lower limit.
In order to investigate how many stars more
massive than 9
may have existed in the Pleiades subgroup B1 we
assume that all stars in the mass range 8-9
are detected.
Next, an initial mass function (IMF) is fit to the
distribution of stars in the mass range 8-9
.
We adopted a
standard power law IMF with a power law index
of the form
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We next compare location and space motion of the Pleiades subgroup B1 with
respect to the LB. Figure 2 provides a sketch of the solar
neighborhood in a projection seen from the top of the galactic plane; the
direction towards the galactic center is to the right (
).
The positions of a number of nearby open stellar clusters (open circles in
Fig. 2) are shown together with the center of mass position of
the Pleiades subgroup B1 (filled circle in Fig. 2).
We point out that the Pleiades
subgroup B1 is localized in a small volume in space. The evolution of the space
distribution of the single member stars is plotted in detail in Asiain et al.
(1999b, cf. Fig. 4).
The LB is idealized as a sphere; radius and
center have been chosen to be consistent with the EUVE measurements presented
by Berghöfer et al. (1998, cf. Fig. 13). Additionally, we plot the border
of the local cavity as derived by Sfeir et al. (1999) from Na I absorption
line studies.
The solid line, ending at the actual position of B1, provides the
space motion of the moving group during the past 30 Myrs
in the epicyclic approximation; tickmarks indicate the center of mass position
30, 20, and 13 Myrs ago. The path of B1 has been computed by employing the
equation of motion provided by Asiain et al. (1999; Eq. (1)). The IAU
standard for the solar motion has been adopted to transform the B1 velocity
components in the local standard of rest (LSR) system, following the assumption
that the system of the LB is bound to the LSR. The time of -13 Myrs is
estimated to reflect the time when the most massive star(s) in B1
(
)
must have exploded. At this time the
center of mass of B1 is located inside the volume of the LB.
We note in passing that there exists a controversy in the interpretation of
Hipparcos data, resulting in different values for the solar
motion. While there is good agreement on ,
Dehnen & Binney (1998)
obtain a value
km s-1, by arguing that
because there is a linear relationship between V and the velocity
dispersion
(also known as asymmetric drift), which
is fulfilled for all stars but the youngest, that the latter ones should be
excluded from the sample of stars. On the other hand,
Torra et al. (2000) have analyzed the kinematics of young
stars, using the Hipparcos data plus photometric distances, and by
taking into account a distortion of the velocity field by the expansion of the
Gould Belt, obtain
km s-1, which is close to
the IAU standard value of
km s-1.
A more detailed discussion of these problems is clearly beyond the scope of
this paper.
Assuming the smaller value for
by Dehnen & Binney (1998) stretches
the B1 trajectory to the upper direction in Fig. 2 by a factor of
2; however, the trajectory still passes through the LB as outlined
by the Na I observations.
Taking into account the spread in the actual positions of individual B1 member
stars and the deviations in space velocities
(,
4.7,3.3) km s-1, a significant fraction of
former more massive members must have crossed the LB region even closer to the
center, while a few might have exploded outside. From the present positions
of the individual stars in B1 and their
space velocities we estimate that an individual star position may deviate from
the center of mass 13 Myrs ago by up to
100 pc. Even if the
path of B1 did not run through the very center of the LB it
should be emphasized (and will also be discussed in the next section) that
the shape or boundary of the bubble are mainly determined by the density and
pressure of the ambient
medium rather than by the location of individual SNe with respect to the
center of explosion. The reason is that a shock wave is weak and propagates
fast in a rarefied hot medium, but moves rather slowly when it encounters
high density material, which it has to compress and set into motion.
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Figure 2:
Sketch of the solar neighborhood seen from above the galactic
plane. The center of mass position of Pleiades subgroup B1 is labeled with
"B1''. The solid line, ending at the actual position of B1, provides the
trajectory of the moving group during the past 30 Myrs in the epicyclic
approximation (see Sect. 3); center of mass positions 13, 20, and 30 Myrs
ago are labeled with -13, -20, and -30. Approximately 13 Myrs ago the most
massive B1 star(s) (
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While the details of the dynamics and evolution of the LB have to be deferred to a future paper, which concentrates on numerical simulations, we will discuss in the following the time-dependent energy input rate and its consequences for the evolution of the LB.
Obviously,
the contribution by stellar winds is only significant at early times and thus
is relevant only if massive O stars exist in the cluster, which is not the
case here.
We will therefore only consider the input rate by SN events,
which is by far the dominant at later times (Mac Low & McCray 1988).
The main sequence life times,
,
of stars within
the mass range
can empirically be
approximated by
yr
(Stothers 1972), with
.
Since this defines m as a function of time
,
implicitly
assuming that the energy input can be described as a continuous process,
we obtain
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The assumption of a continuous energy input into the ISM at the center of the first explosion is much better than thought of at first glance. The reason is that every off-center blast wave in a hot rarefied medium with a high speed of sound rapidly degenerates into a weak shock with a low factor of compressibility of the downstream medium. Therefore, hardly any material is swept up and the bubble remains roughly uniform with negligible radiative cooling due to its high initial temperature. The net effect is simply pressurization of the bubble. The time for the interior weak shock to reach the bubble boundary is of the order of a sound crossing time and thus during most of the evolution much smaller than the dynamical time scale; hence the eccentricity of subsequent explosions has only a minor influence on the evolution of the bubble.
Similarity solutions of superbubbles (e.g. McCray & Kafatos 1987) and
their classical analogue, stellar wind bubbles (e.g.
Pikelner 1968; Dyson & deVries 1972; Castor et al. 1975;
Weaver et al. 1977), can be calculated, provided that the dependent
variables and boundary conditions do not explicitly depend on time
or length scales that may enter the problem. This is still true for
the case of a time-dependent energy input rate,
and a density gradient of the ambient medium,
.
Similarity conditions hold during the adiabatic expansion phase as long
as the bubble pressure is much larger than the ambient medium pressure,
i.e.
.
The mass of the bubble is dominated by the mass
swept into a shell,
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According to ROSAT PSPC broad band spectral fitting the Loop I bubble
temperature
is about
K (Egger 1998). This gives support to the
idea that we live in an environment of ongoing star formation, although
the LB itself is probably extinct.
There are several reasons why we may have overestimated the size of the LB
in the similarity solutions above. Firstly, the mass inside the bubble
is significantly higher than the pure ejecta mass, as can be inferred
from the ROSAT X-ray emission measures; when assuming bubble parameters
of
pc and
(e.g.
Snowden et al. 1990) a mass of at least 600
is derived whereas
non-equilibrium ionization plasma models (Breitschwerdt & Schmutzler 1994)
result in a more than a factor of five higher mass.
The contribution of ejecta is of the order of 100 ,
and the
bulk of the bubble mass is due to hydrodynamic mixing of shell material,
heat conduction between shell and bubble and evaporation of entrained
clouds; therefore the flow is mass-loaded.
The net effect is to reduce the amount of specific energy per unit
mass, because the material mixed in is essentially cold, and thereby also
increases the rate of radiative cooling. Secondly, the
stellar association has probably been surrounded by a molecular cloud
with a density in excess of
with subsequent
break-out of the bubble and dispersal of the parent cloud
(Breitschwerdt et al. 1996). Thirdly, the number of SN explosions could
be less; here we have assumed that all 20 SNe have
occurred inside the LB. This need not be the case as is also suggested
by Fig. 2. ROSAT PSPC observations have revealed an annular
shadow centered toward the direction (
,
), which has been interpreted as an interaction
between the LB and the neighboring Loop I superbubble (Egger & Aschenbach
1995). The trajectory of the cluster B1 may have partly crossed the Loop I
region. Alternatively and more likely, part of the thermal energy might
have been liberated into the Galactic halo, since there is some evidence
(see Sect. 5) that the LB is open toward the North Galactic
Pole. It should also be mentioned that due to small number statistics
the true number of SNe can vary by a factor of 2.
Finally, although there is no stringent evidence, it would be very
unusual, if the LB would not be bounded by a magnetic field, whose
tension and pressure forces would decrease the size of the LB.
Given these uncertainties and the fact that the simple analytic model
discussed above can be considered as an upper limit, the agreement with
observations is quite good. The bubble radius and shell velocity are
rather insensitive to the energy input rate and the ambient density
(due to the power of 1/5) and therefore not well constrained, but
depend more sensitively on the expansion time scale. Thus we can assert
with some confidence that the age of the LB should be between
yr.
The radiative cooling time scale of the LB can be estimated using
Kahn's (1976) cooling law (in the temperature range
K),
,
with
being the adiabatic parameter and
.
Using
and
K, which have been derived from
X-ray broad band spectral fitting of a plasma in collisional
ionization equilibrium (e.g. Snowden et al. 1990), we obtain
yr, comparable to the dynamical time scale.
If the X-ray emission is mainly due to the delayed recombination of highly
ionized species of a plasma not in in ionization equilibrium, then the cooling
time can be as short as
yr (Breitschwerdt et al. 1996).
In either case, the energy input of previous SNe has largely been used
up for expansion and radiative cooling of the bubble.
We thus conclude that the emergent X-ray emission must be largely due
to the energy input of the last supernova.
We can express the number of stars (and potential SNe) in the mass interval
,
N(m), by a number of stars ending their lives in the time
interval
,
,
using Eq. (3) and
(
yr), which results in
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Analysis of ferromanganese crust samples in deep ocean layers have
shown an increased flux rate of
,
which is indicative of
at least one SN which exploded inside the LB
at about 5 Myr ago (Knie et al. 1999). Thus, using t7 = 0.8, we obtain
for the associated energy input rate,
erg/s.
This is a moderate value, but still comfortably larger by a factor of 5
than the total present day soft X-ray photon energy output rate of the LB.
The fact that the Gould Belt stars show receding velocities, and that the stellar associations are also closely related to molecular clouds and local H I (see Pöppel 1997) argues in favor of a common history of gas and stars in the local environment. The nature of the Gould Belt is yet unclear; current ideas favor either an explosive origin (Blaauw 1965; Olano 1982) or a collision between a high velocity cloud and the Galactic disk (e.g. Tenorio-Tagle et al. 1986; Comerón & Torra 1992). It is noteworthy that the LB extends perpendicular to the Gould Belt rather than to the Galactic disk (Sfeir et al. 1999), hinting at an origin of the LB connected to the Gould Belt stars and gas.
Regardless
whether the expansion was caused by multiple SN events or by the
oblique impact of a high velocity cloud,
the implications are similar: the shock fronts that
have propagated into the ISM, presumably at some angle with respect
to the disk, will overrun ambient clouds. If the clouds
have low masses, they could be disrupted hydrodynamically and/or
evaporated, thereby mass loading and decelerating the flow.
As a matter of fact, the spectral properties of the soft X-ray emission
combined with the H I distribution in the local cavity show that the hot
LB plasma must consist of a total mass of the order of
.
This cannot
be provided solely by ejecta material, but must be due to hydrodynamic
mixing and/or evaporation of ambient gas clouds.
In the case of more massive clouds, the
shock propagating into the gas will compress it
without destroying the cloud, so that
a growing number of Jeans unstable clumps will eventually collapse
into protostellar cores. A considerable number of nearby stars with
ages substantially lower than the Gould Belt's kinematical age and the
existence of active superbubbles (e.g. Loop I with an estimated dynamical age
of 107 years, see Egger 1998) seem to support this hypothesis.
Note that the age of the Gould Belt is rather uncertain, Comerón (1999)
provides an age estimate of
Myrs whereas Torra et al. (2000) derive
an age in the range of 30-60 Myrs.
In the above picture, the origin of the LB is connected to the evolution of
the Gould Belt itself as the disturbed medium that formed the LB.
We favor a scenario in which the LB was created by about 10 - 20 SNe
about
yr ago, with a preferred value of
yr; the star cluster itself is about
yr.
It is conceivable that the stars have also been embedded in a
molecular cloud for some time (Breitschwerdt et al. 1996).
The present day total LB X-ray luminosity
is fairly moderate,
at most
(Cox & Reynolds 1987), and can easily be supplied by the last SN
reheating the cavity about a million years ago. This could also explain
why only part of the local cavity is illuminated now by X-rays. Note
that the sound crossing time out to a distance of 200 pc (in direction of
CMa) in a 106 K plasma is roughly 2 Myrs. Moreover, the
recombination time scale of major coolants, such as O VII and
C VI can be estimated from analytical approximations to the rate
coefficients (Shull & Van Steenberg 1982) to be 4.7 and 12.2 Myrs,
respectively, for commonly used LB fit parameters of
and
K (Breitschwerdt & Freyberg 2002).
Therefore the emergent X-ray spectrum could be a fairly complex superposition
of freshly ionized and recombining plasma (from previous explosions) and
definitely be out ionization equilibrium.
We have shown that the subgroup B1 of the Pleiades moving group
has actually passed through the LB. If the IMF of this
cluster of young stars is similar to the OB associations we observe in
our local neighborhood, then obviously some of
these stars must have exploded as SNe on their way. Since the kinematic age
of B1 can be fairly well determined (assuming they are not bound
gravitationally) to be
Myrs,
it can certainly not be responsible for the origin of the Gould Belt
itself. Note that all age determinations for the Gould Belt point to ages
older than 30 Myrs. However, the SN events in B1 should have left their
imprint on the LB. One important question is when the
last one injected mass and energy. According to Eq. (3)
and Fig. 1 about 4-5 stars of
should have been members of B1.
Since two of them are
still with us and the average life time of such stars is about 25 Myrs,
while stars with
live 30 Myrs
the last SN is expected to have exploded about 1 - 2 Myrs ago, or 3 SNe
occurred within the last 5 Myrs. This is roughly consistent with the
measurements of radioactive
(Knie et al. 1999), where
the high flux rate is interpreted by one SN, but within a radius of 30 pc
from the Sun and a progenitor mass of about
.
The existence of a "Local Chimney'' as claimed by Sfeir et al. (1999) and Welsh et al. (1999) may support the hypothesis that the LB is created and shaped by as many as 20 SNe, because it allows a significant fraction of the large superbubble thermal energy to be vented into the Galactic halo. Consequently, after the last explosion, the LB is left in a state of long-term recombination. According to non-equilibrium models (Breitschwerdt & Schmutzler 1994; Breitschwerdt 2002) the observed modest amount of soft X-ray emission can still be generated, even if the last SN occurred more than a million years ago.
B1 still provides another 10 SN candidate members. Since it is about to
leave the volume of the local cavity, the LB will remain in a state of
recombination, which is what we probably observe at present. Reheating will
become progressively less important and the future of
the LB should be fairly quiescent.
:
We realize that after submission of our manuscript
to the editorial office of Astronomy & Astrophysics an other publication on this
subject was submitted to the Astrophysical Journal.
In this publication by J. Maiz-Apellaniz (2001, ApJ, 560, L83) the author
comes to a conclusion about the formation of the Local Bubble that is similar
to one of the conclusions provided here.
Acknowledgements
TWB acknowledges support from the Alexander-von-Humboldt-Stiftung (AvH) by a Feodor-Lynen Fellowship. The research of DB has been funded by the Deutsche Forschungsgemeinschaft (DFG) with a Heisenberg Fellowship and by the Max-Planck-Gesellschaft (MPG). DB thanks G. Hasinger, J. Trümper and the Max-Planck-Institut für extraterrestrische Physik as well as the Department of Astrophysical Sciences at Princeton University, where part of this work was done, for their hospitality.We are indebted to F. Figueras and R. Asiain of the Departament d'Astronomia i Meteorologia, Universitat de Barcelona, who kindly provided us with the listings of stars in the Pleiades moving groups, and to D. Fernandez for helping us in calculating the trajectories of B1. We kindly acknowledge valuable discussions with F. Comerón (ESO), B. Fuchs (ARI Heidelberg) and C. de Vegt (Hamburger Sternwarte).