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THE DIFFUSE CLOUD THAT
SURROUNDS THE SOLAR SYSTEM
PECULIARITIES OF AN INTERSTELLAR CLOUD
EXPLORED FROM WITHIN
Cecile GRY (Laboratoire d’Astrophysique de Marseille)
Edward B. JENKINS (Princeton University)
From results from GHRS and STIS data
The local Interstellar
Medium (~100 pc)
is characterized by
its very low density
Existence of a cavity found
from the absence of
reddening of the nearby stars
and weakness of visible
interstellar absorption lines
Sun
the Local Bubble
The bubble interior
contains
hot, low density gas
T~106 K nHI ~0.005 cm-3
(soft X-ray background)
Existence of neutral gas
in the Local Bubble
N
Capella
12.6 pc
α Cen
1.3 pc
Sirius
2.7 pc
ε CMa
124 pc
Diffuse, partially ionized, warm gas
(7 000 K, 0.2 cm-3) detected in
strong UV absorption lines
Previous pictures:
Complex of distinct clouds
N
α Cen
1.3 pc
Sirius
2.7 pc
ε CMa
124 pc
G (Lallement & Bertin, 1992)
LIC (local
interstellar cloud)
Common bulk motion (e.g.
Frisch, Grodnicki & Welty, 2002)
but in previous pictures various
components are distributed in a
complex of several distinct
clouds (CLIC) with slightly
different velocities
If the term ‘‘clouds’’ refers to
contiguous parcels of interstellar gas
moving as rigid bodies with
homogeneous kinematical and
physical properties
then several of these clouds are
needed to account for the velocities
measured around the sun.
Cloud model of
Redfield & Linsky (2008)
with 15 clouds within 15 pc
- In this picture, the sun is at the edge of the LIC
- We note:
- No line of sight with no absorption
- Main clouds seem to complement each other
- Main clouds have similar velocity vectors
‘G’ cloud had been introduced to explain a velocity discrepancy of a few km/s
towards α Cen but LIC and G predict the same velocity in several directions
Question:
N
To what extent can we
identify a single,
continuous, cloud which
envelopes the Sun in all
directions ?
Aim:
Offer a simpler
picture of the LISM
α Cen
1.7 pc
We re-examine with this new focus
the UV-absorption database for the
LISM from Redfield & Linsky (2002)
Only MgII and FeII,
observed with HST GHRS & STIS
R=100 000 (ΔV~3km/s)
V (km/s), N (at cm-2), b-value (km/s)
total of 59 lines of sight
length 1.3 pc to 100 pc
111 velocity components
Step 1: select in every line of sight the component that is most
likely to originate in the cloud surrounding the sun
In each line of sight (li,bi) we select
the absorption component closest in
velocity to the projection of the
velocity vector of the ISM flow
interacting with the heliosphere as
measured by IBEX or Ulysses
Component 1
We call it «Component 1».
By definition, there is one
“Component 1” per line of sight.
(no filtering)
Interstellar flow into the heliosphere (McComas et al, 2012)
Step 2: Derive the mean velocity vector that best fit all Components 1.
(least squares fitting)
 lm= 185.84° ±0.83 bm=−12.79° ±0.67
Vm= 25.53 ±0.26 km/s
Quite close to the Ulysses or IBEX velocity vectors
Components 1
stand out as
forming a
conspicuous
Vm cosθi,m curve
They also appear
to be
the dominant
component
(large ☐s)
in most sight
lines
θm
Vm
θm: angular distance to the
apex of the mean vector
Step 3: Examine the velocity deviations of Components 1 from the best-fit
velocity vector
Ο=MgII
Δ=FeII
θm
Vm
The velocity
deviations show
a clear trend
with θm
We think that the
simple pattern
indicates a
fundamental, secondorder dynamical
effect associated with
an otherwise
coherently moving
cloud.
Conclusion:
Our Components 1 sample can indeed be interpreted as the
projected velocities of a unique interstellar cloud,
detected in all directions,
provided we relax the assumption that the cloud is rigid
• This cloud accounts for 70 % of the
column density in the first 50 parsecs
• It accounts for more than half of the
velocity components
• It includes most components of the
LIC, G, NGP, Blue, Leo, Aur, Cet
clouds and half of the Eri and Gem
clouds in the Redfield & Linsky
(2008) model
A single cloud surrounds the sun
Why does it matter ?
* Simpler and more stable conditions outside the heliosphere
(the sun had been predicted to be immersed in the hot gas in about 4000 yrs if G
was a separate cloud, with possible induced climate changes. Should not happen
before 500 000 years)
*This gives a unique opportunity to study a diffuse
cloud from within
*This reveals a novel picture of an interstellar cloud that
differs from the usual view of rigid, homogeneous and
detached bodies
dynamical deformations
its inhomogeneity
its interaction with the surrounding hot gas
V
1- Dynamics of a diffuse cloud
compression in the direction of motion,
expansion in perpendicular directions
indicate a differential deceleration
which may result from the
interaction of the cloud with a magnetic field
or an external medium.
Then, the minor axis for the velocity
deformation indicates a natural, fundamental
direction for the cloud’s internal movement,
independent of the chosen reference frame.
Simple, idealized model: a spherical
volume is differentially decelerated
To maintain its volume –and internal
pressure it is deformed into an oblate
ellipsoid ∆R(θd)
hot gas
θd
ld,bd
deformation
axis
a
b
The velocity perturbations ∆V(θd) from a solid-body motion
are proportional to ∆R(θd), and are expressed in function of θd, the angle
away from the minor axis of the ellipsoid
This yields
And for the velocity perturbations:
Free variables : e, ld and bd
Minimizing the quadratic differences of
the observed velocity deviations with
respect to ∆V(θd).
minor axis direction (origin of the angle θd)
ld = 174.17 ± 5.25°
bd = −12.10 ± 4.21°
 We adopt (ld, bd) as the natural reference
direction for the cloud
Residuals not much higher than velocity uncertainties
This yields
And for the velocity perturbations:
Free variables : e, ld and bd
Minimizing the quadratic differences of
the observed velocity deviations with
respect to ∆V(θd).
minor axis direction (origin of the angle θd)
ld = 174.17 ± 5.25°
bd = −12.10 ± 4.21°
 We adopt (ld, bd) as the reference
direction for the cloud
34 new sight-lines (Malamut et al 2014)
2- Measuring the HI density of a diffuse cloud
We need to know length of the cloud
ISM
Astrospheres = result of
the interaction of a stellar
wind with neutral IS atoms.
Lyman α
1- detection of an
astrosphere => the star is
embedded in neutral gas.
2- detection of an
astrosphere in a singlecomponent l.o.s ==> the
Local Cloud fills up the
sightline.
astrosphere
d
=> N(HI)/d = n(HI)
Wood et al. 2005: compilation of N(HI) from HST
7 targets verify both criteria in our cloud
- Densities vary between 0.03 and 0.1 cm-3
heliosphere
2-b- Cloud boundaries
From the 7 targets in the HI compilation and results by Wood et al. 2005 that
verify both criteria
Mean neutral density for the Local Cloud: <nHI> = 0.05 cm−3
In agreement with solar system measurements of interstellar pick-up ions and Anomalous Cosmic Rays n(H I) =
0.055 ± 0.029 cm−3 (Gloeckler et al. 2009)
Boundary of the cloud is located <N(HI)/nHI > = 9.3 pc away
with an rms dispersion of 7.3 pc  very irregular boundary
No directions where the Sun lies near the cloud edge.
Minimum column density NHI=4 1017 cm-2
 Minimum distance to the boundary: 1.3 - 4.7 pc
 the sun has entered the cloud and will remain in the cloud for more
than 500 000 years
 no reason to believe that the heliosphere has been or will be
immersed in the hot gas in historical times (Climate changes could
result due to changes in e.g. magnetic field or Cosmic Ray (interesting
study cases for paleo-astronomy)
2- Physical conditions in a diffuse cloud
2-b- electron density and temperature
Temperature and electron density
are derived from the combination of
Mg II/Mg I and C II*/C II
ε CMa
Gry & Jenkins (2001):
ε CMa : T= 7000 +/- 1200 K
ne = 0.12 +/- 0.05 cm-3
Redfield & Falcon (2008):
7 lines of sight through the LIC
< ne> =0.12 +/- 0.04 cm-3
New ASTRAL data (PI T. Ayres)
α Leo : T= 7200 +/- 1300 K
ne =0.12 +0.12/-0.04 cm-3
α Leo
3- Metal abundance gradient in a diffuse cloud
HI columns from Wood et al. (2005)
MgII, FeII from Redfield & Linsky (2002)
A correlation exists
between Mg II and Fe II
abundances and θd
indicating an abundance
gradient within the cloud
• No evidence that ionization influences the apparent abundances.
• Depletion is gradually decreasing from front to rear of the cloud:
Cloud zone
Front (apex)
Middle
Rear (anti-apex)
Minimum ISM depletion
[Mg/H]0 = - 0.27
[Fe/H]0 = - 0.95
(Jenkins 2009)
( [Mg/H]0-[Fe/H]0 = 0.68)
Significant depletion in the cloud head  Grains are not totally destroyed
Lower depletion in the rear this part has probably undergone a grain-destruction process
Possibly related to the origin of the cloud
(Kimura et al 2003)
If e.g the LIC is one of the cloudlets
expelled from the interaction zone
between the Local Bubble and the Loop I
superbubble (Breitschwerdt et al. 2000).
Breitschwerdt D.et al , 2000, A&A 361,303
4- Interaction of the cloud with its surroundings
4-a- Nature of other velocity components
Out of 59 sight lines,
34 (58%) show at least one other component
13 (22%) show at least two other components,
1 shows three and 1 shows five other components
Velocity shifts of the “other components” relative to “Component 1”
☐: 2nd component
Δ: 3rd component
: 4th component
+ : 5th & 6th components
Δ V = −7.2 +/-1.5 km s−1
The gas responsible for most
of the other velocity
components is very nearby
Consistency in the velocity shifts:
Half of the secondary components have
Δ V = −7.2 km s−1 +/- 1.5 km s−1 rms
relative to the Local Cloud component
• Half of the secondary components have
Δ V = −7.2 km s−1 +/- 1.5 km s−1 rms
relative to the Local Cloud component
• half of all sight lines include such a
component
• These components cover half of the sky
We call the ☐’s
the « Cetus Ripple » components
Possible signature of a
shock progressing towards the cloud interior
Pre-shock gas (Local Cloud)
electron density n1 (e) = 0.10 cm−3
neutral hydrogen n1 (H I) = 0.05 cm−3
Temperature T1=6680 K
Pressure p1/k = 1770 cm−3 K
Magnetic field B1 = [2.2, 3.8]μG
field direction //shock front
☐ Positive shift
☐ shift of −7.2 ±1.5 km/s
☐ Other negative shift
+ origin of the velocity vector
fitting the ☐’s
Vs ~ 20 km/s
p2/k ~3000 cm−3K
x2~1.4 compression T2 ~ 8500 K
Post-shock (Cetus Ripple)
Velocity vobs= − 7.2 km/s
relative to the main Local Cloud.
Column density: log N(H I)=17.8
b-values: 2 to 4 km/s
hot
gas
106 K
If the shock is driven by an enhancement of the external thermal pressure on one
side of the Local Cloud (e.g. Local Cloud suddenly overtaken by a SN blast wave),
with T~106 K and X-ray emission =1.1 Snowden pc-1, p/k ~3700 cm−3 K
4- Interaction of a warm cloud with its hot environment
4-b Interface with the hot bubble gas
We plan to study the outer boundary of
the Local Cloud and how it interacts with
the surrounding hot gas in different
locations.
• Conduction fronts ?
• Evaporation ?
• Turbulent Mixing
Layer?
By observing
Si III, C IV, Si IV, N V
with STIS and relate
them to OVI observed
by FUSE
Growth of a Turbulent Mixing Layer
(Kwak & Shelton, 2010)
 Understand mass transfer processes
in hot bubbles
 Understand the role of warm cloud
interfaces in enhancing the cooling
of SN remnants.
 How such processes can modify the
evolution of SNRs and galaxies
Summary :
We do identify a single cloud surrounding the sun
*
*
The local cloud and its Cetus Ripple account for 85 % of the total
column density in the first 50 pc.
The new picture differs significantly from the usual interpretation of
interstellar absorption components regarded as rigid, homogeneous and
detached bodies.
1-The cloud extent is about 9 pc,
it is irregular (+/- 7 pc)
2- The cloud undergoes a deformation, due to a
deceleration in the direction of motion and an
expansion in directions perpendicular to the flow
3- The cloud shows an abundance gradient :
metal depletion increases steadily from the rear
of the cloud to the apex
4- The cloud is in interaction with its surrounding:
* evidence for a shock progressing toward the
cloud interior, covering half of the cloud
* Future STIS observations to confirm the
existence of thermal interfaces with the hot gas
and characterize them. Determine their role in
enhancing the cooling of bubbles and SNRs.
Details in:
Gry C. & Jenkins E.B., 2014, A&A 567, A58