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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