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EROS results towards the galactic spiral arms

Explore EROS2 Workshop results towards Milky Way spiral arms, microlensing challenges, field data analysis, event detection efficiency, and implications for galactic structure. Spanish.

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EROS results towards the galactic spiral arms

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  1. EROSresults towardsthe galactic spiral arms M. Moniez (on behalf of the EROS2 Coll.) Daejon microlensing workshop 2007-01-15

  2. We are here Our position in the Milky-way

  3. Our position in the Milky-way

  4. Why search for microlensing towards the spiral arms?-EROS exclusivity- • Because there is a significant signal • To calibrate the LMC/SMC/GC measurements through « easy-to-control » directions • To improve Milky-way structure knowledge • Bar size and inclination • Constrain thick disk

  5. Specific difficulties compared with LMC/SMC searches

  6. 1: Lenses along the line of sight belong to several structures • Bulge • Thin disk • Thick disk (?) • Bar (extension?) • Halo • Spiral arms Different Distances/Masses/Velocities

  7. Galactic center Gamma Normae 2: The source distances are poorly known and widely distributed! • Also strong and very variable interstellar absorption. For example: red giant clump not well defined in magnitude-color diagrams of spiral arms

  8. The Milky-way

  9. 29 fields in 4 zones away from center (spiral arms) (88 fields towards the Galactic center)

  10. Photometric precision 7 years of data Spiral arms • 12.9 million stars • 7 seasons -all data- • Improved catalog (+70%)since 3 year paper A&A373, 126, 2001

  11. From EROS <-> HST comparisons (contributor) (1rst contributor) IC Our GSA catalogue

  12. EROS vs HST

  13. Event selection • After “standard”pre-filtering • 2nd fluctuation probability : (lp2/lp1)B, R < 0.5 • B and R fitted peak overlap : xrec > 40% • Sampling • Tpeak = T(u<2) < Tobs - 600 days • duration between max and closest measurement < 40% x Tpeak • goodness of (simple) ML fit : (c2monochromatic ML /NDDL)B, R < 1.8 • Stability out of the peak : c2Base/NDDL < 8 • improvement vs constant fit : Dc2B, R > 60 ⇒26 candidates (+ 1 uncertain -very long duration) • With respect to previous analysis (3years, 7 evts) : • 4 lowest S/N events now rejected (2 genuine background ?) • events in all 4 directions

  14. « exotic » events Exoticity vs quality • fit measures the diffe-rence in fit quality in the base and during the peak • large >0 for «exotics» • large <0 for «variable» • 2/tE measures the significance per tE

  15. Microlensing detection efficiency

  16. 26 candidates found low S/N • 1 found 2 times, 1 found 3 times (overlaps…) • 2 candidates with parallax • 4 with blending • 1 Xallarap(see A&A 351, 87-96, 1999) • Maybe one binary lens high S/N

  17. Xallarap eventwith extremely red source

  18. From 3 year to 7 year analysis • 3 ---> 7 years • 9 ---> 13 million stars • 7 ---> 26 (≠ 7 +19) events • From 7 events in 3yr • 3 killed (2nd fluctuation) • 1 with u0=1-e ---> 1+e

  19. <t> vs u0 Stability of <t> measurement

  20. Microlensed stars are redder (?) • An effect of the non-uniformity of source distance • increases with distance • I increases with distance BUT faint stars do not enter the catalog => <I> is ~ stable • Absorption increases with distance => (V-I) increases I vs (V-I)

  21. Spatial distributionof the 26 events gNor qMus gScu bScu

  22. Results

  23. Interpretation of the optical depths • The Milky-way

  24. Interpretation of the optical depths • The Milky-way • And the model…

  25. t vs galactic longitude @7kpc and b=-2.5° Interpretation of the optical depths • Compare with simulations • Bulge & thin disk: known contributions • No halo • No thick disk

  26. Interpretation of the optical depths No thick disk b +/- 1° t vs galactic longitude @7kpc and b=-2.5°

  27. Guidelines for further interpretation • t measurement is specific of • Our stellar catalog (+/- unbiased for I<18.5) • => concept of (catalog) instead of (distance) • Events shorter than 800 days • Why these precisions? • Distribution of source distances poorly known • Strong spatial variations of the absorption

  28. Conclusions perspectives • <tGSA>=0.49±0.09 (stat.)x10-6 on the 4 directions • Statistics x4 since 3 year results • Preliminary indications of a long bar not confirmed, no need for thick disk • Sensitivity to the bar inclination • Complementary measurements (source distance distribution study) would make easier further interpretation • Complete interpretation under study (CM diagram+) • Future: VISTA survey (IR) will be able to search for microlensing through dust.

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