Structure and kinematics of the Gould Belt from Hipparcos Data

1. Structure and kinematics of the Gould Belt from Hipparcos Data Francesca Figueras, Jordi Torra, David Fernández Universitat de Barcelona The Gould Belt and other large star forming complexes

2. Barcelona work Praga (97)  Garching (01) • Kinematics of young stars. I. Local Irregularities (GB structure and kinematics) • Torra, J., Fernández, D., Figueras, F., A&A 359, 82 (2000) • On the evolution of moving groups: an aplication to Pleiades moving group • Asiain, R., Figueras, F., Torra, J., A&A 350, 434 (1999) • Kinematics of young stars. II. Galactic Spiral Structure • Fernández, D., Figueras, F., Torra, J., A&A 372, 833 (2001) • Young stars in the nearest solar neighbourhood • Fernández, D., Figueras, F., Torra, J. (Garching, tomorrow) The Gould Belt and other large star forming complexes

3. The stellar component of the Gould’s belt • Before Hipparcos data • Poppel review 1997 • The stellar component of the Gould’s belt from Hipparcos data: • GB: Venice’s 97, Palous (98), Torra et al. (99), Lindblad (00), Alfaro et al. (00) • OC and Assocciations: De Zeeuw et al (99), Robichon et al. (99) , Brown (01) • X-ray & RASS-Tycho data: • Guillout, Sterzik, Neuhauser,.. • High energy sources • Gehrels, Grenier The Gould Belt and other large star forming complexes

4. Working sample : 6922 O and B stars • Data compilation: • Astrometric Data (Hipparcos) • photometric data (H&M,98) • radial velocities (Grenier, 1997 + Barbier-Brossat, 2000) • Careful treatement of: • Stellar distances • Radial velocities • Stellar ages The Gould Belt and other large star forming complexes

5. Stellar distances Trigonometric and/or Photometric (individual error evaluation)  the one with smallest error Trigonometric distances accepted only if / < 25 %: Photometric distances (Crawford ,75) : nos systematic trends for / < 15 % “For distances estimated as R = 1/ , a symmetric error law for parrallaxes results in a non-symmetric, biased distribution for distances” • The bias is: • always less than 5.5 % • Smaller than 3 % for 88 % of the stars • Distance bias smaller than 5 pc for 82 % of the stars The Gould Belt and other large star forming complexes

6. Higher degree of completeness for distant stars The fraction is not a flat function Stellar radial velocities (for 3397 stars) Possible kinematic bias: Binney & Merrifield (1998): “Due to observational programmes: radial velocity availability is higher for high proper motion stars” kinematic bias present in our sample Needs for evaluation through numerical simulations The Gould Belt and other large star forming complexes

7. Stellar ages (for 2864 stars) Individual ages from photometry, using evolutionary models of Bressan et al (1993) Bias (F&B,98): over-estimation on 30-50 % due to stellar rotation (not taken into account in the models) Careful treatement with the aim of retaining as many as possible of the very young stars To be considered when deriving GB age The Gould Belt and other large star forming complexes

8. Working samples Initial Sample 6922 Hipparcos stars. Vlim ~ 8 Stroemgren Photometry 3031 stars Radial Velocity 3397 stars Sample 2 2272 stars r, , vr Completeness Vlim ~ 6.5 < > = 0.57 mas < /> = 0.16 <  cos > = 0.81 mas/yr <  > = 0.67 mas/yr < vr > = 3.44 km/s 1789 if ages are considered Sample 1 3915 stars r,  Completeness Vlim ~ 6.5 < > = 0.60 mas < /> = 0.16 <  cos> = 0.83 mas /yr <  >= 0.70mas/yr 2468 if ages are considered The Gould Belt and other large star forming complexes

9. Structural parameters of the Gould Belt • Method: • Comerón et al. (1994) + iteration until convergence • Requirement: homogeneous completeness of the sample over the celestial sphere • Numerical simulations to evaluate biases and to estimate errors on parameters • Critical questions to answer: • Older stars have a small limiting distance: Can our method be able to detect an inclined structure if present? • For which scale heigh of the belts our method looses its statistical capability? • Are the available number of stars enough to undertake this study? • Realistic error estimation from simulations. The Gould Belt and other large star forming complexes

10. Simulations for the structure analysis • Conclusions: • The angular halfwidths correctly reflect the growth of the scale heigh of the simulated belts (2-5o) • q is well recovered though with 0.13-0.17 • There is a presence of a small systematic trend in (iG, G) when increasing Zo (always smaller that the errors) • For old stars, when forcing and inclination of 20o, there is a probability less than 5 % to obtain a null inclination ( iG,  4o, as in the real sample) Zo= 40 pc Zo= 80 pc > 30 Myr 30 – 60 Myr > 60 Myr The Gould Belt and other large star forming complexes

11. Structural parameters of the Gould Belt GB extended up to 600 pc GB orientation iG = 16-22o, p= 275-295o , depending on age GB is narrow than the Galactic Belt For R < 600pc : 60 % of stars younger than 60 Myr belong to the GB The inclination i = 27.5o +/- 1o (Guillot et al. , 1998, RASS-Tycho) is not compatible with our results (very nearby sample possible influenced by the Sco-Cen) The Gould Belt and other large star forming complexes

12. Kinematic model & resolution procedure • Model: • First order development of the systematic velocity field (A,B,C,K) • Palous (98): “the second-order terms have low significance” • No systematic motion perpendicular to the galactic plane • Solutions for V , (l + b) , (V + l + b) Resolution procedure: Weighted least squares Weights: (2i, obs + 2i, cos)-1: individual observational errors (considering correlations) cosmic residual velocity dispersion ellipsoid Iterative process: simultaneous determination of model parameters and cosmic dispersion obtained  cos increasing with age, close to Wielen (77) values  cos effects in solar motion and Oort Constants are 0.5 km/s/kpc The Gould Belt and other large star forming complexes

13. Detailed treatement • Possible biases in the fitting parameters induced by: • observational constraints • - irregular spatial distribution of the stars • - incompleteness effects • - biases in the availability of radial velocities • the presence of observational errors in the right hand side of equations, not considered in a WLS fit) • Correlation among variables Numerical experiments to globally evaluate all these effects The Gould Belt and other large star forming complexes

14. Simulations for kinematic analysis • Case 1: Pseudostars with real spatial and Vr distribution + errors in Vr and  • Case 2: Case 1 + error in distances • Case 3: Case 2 + rejection criteria • Expected biases in the combined solution: • 100 < R < 600 pc: biases of A  + 0.5 on B,  - 0.8 on B. C & K negligible • Solar motion 0.3, 0.4 km/s • 600 < R < 2000 pc : A,C,K negligible, B + 0.9 • Solar motion + 0.3, 0.4 km/s • No bias from the rejection criteria • Lack of radial velocity data: bias  0.2 km/s/kpc The Gould Belt and other large star forming complexes

15. Results • A long standing problem: • Discrepances in the A Oort contant between solutions from radial velocities and from proper motions: • Real Data: A 2-3 km/s/kpc • Crézé(1970): error in distance underestimation in A from radial velocity equations • Our simulationsthis effect is less important than the distance cut • Feast et al. (1998): no bias was present with the new distance scale • From our simulations: • A bias of 1,1.5 km/s/kpc is present in the opposite sense  even enlarge the difference • An overestimation in our photometric distances by 20 % (rotation effects) •  account only for a difference of 1-2 km/s/kpc • The discrepance is not due to the irregular spatial distribution of the stars • Atributted to the departure of some stellar groups from the adopted linear model • (removing stars 200 < l<250o the discrepance in A vanishes, 2 statistics improve) The Gould Belt and other large star forming complexes

16. Results • Discrepances in Vo (  4 km/s between Vr & proper motion solution:: • It remains when eliminating some particular regions • From our simulationsNo effect from irregular distribution of stars • Again, Atributted to local departure from the linear model (MG?) • Correlations: • Are small in all cases (vr,  + , vr +  + ) • Not the cause of differences in vr,  +  • The combined solution presents the smallest correlations • The largest 2 value comes from radial velocity (not in the simulations), due to underestimation of errors in vr or to cosdetermination. The Gould Belt and other large star forming complexes

17. Kinematics of young stars: the global behaviour • Bias expected from simulations: • (U,V,W) underestimated in 0.4 km/s • B underestimated in  0.8 km/s/kpc 600 < R < 2000 pc Uo = 9.0 +/- 0.8 Vo = 13.4 +/- 0.7 Wo = 8.3 +/- 0.5 A = 13.0 +/- 0.7 B = -12.1 +/- 0.7 C = 0.5 +/- 0.8 K = -2.9 +/- 0.6 • Our resuts indicate a tendency to obtain lower values of A when the distance horizon of the sample is approached. The same for: • Feast et al. (1998), Cepheids: A = 15.1 +/- 0.3 • Hanson (1987), nearby stars: A = 11.3 +/- 1.1 • Explanation: Oling & Merrifield (1998): variation of Oort constants as a function of galactocentric distance Our resuts indicate near null values for C & K: pure differential rotation, in good agreeement with Lindblad et al. (1997): C = 0.8 +/- 1.1 , K = -1.1 +/- 0.8 The Gould Belt and other large star forming complexes

18. GB age from kinematic behaviour GB age from spatial distribution = Variations of Oort parameters with age 100 < R < 600 pc • A increase of A,B with age • A decrease of C,K with age GB age = (30-60) Myr The Gould Belt and other large star forming complexes

19. The two main complexes in the GB Oort constants when excluding Sco-Cen and Ori OB1 100 < R < 600 pc, age < 30 Myr Stars selected from Brown et al. (1994; Ori OB1), de Zeeuw et al. (1999; Sco-Cen) • These associations are not the only responsible for the peculiar kinematics observed for the youngest stars and attributed to the Gould Belt • The GB is not a casual arrangement of these two associations The Gould Belt and other large star forming complexes

20. Expansion of the system as a function of distance • The expansion diminish rapidly with increasing distance for R < 250 pc • At R > 300 pc only Per OB2 has a mean residual motion away from the Sun • Ori OB1 has an almost null radial residual motion (U,V,W)res= (-1.2, -2.8, 2.1) km/s Variation of KR as a function of heliocentric distance (stars with  < 60 Myr) The Gould Belt and other large star forming complexes

21. Residuals, local irregularities OB stars  < 60 Myr: residual space velocity vectors  < 150 pc 150 <  < 300 pc Olano’s ring (t=0) • A well defined concentration of OB stars in 225o<l<285o: • 3 different residual motions • Mainly 100< <300 pc • Ages 30 <  < 60 Myr • Only 7 stars identified as members of OC or Ass. =>a large number of field OB spread in a large area Breitschwerdt et al. (2000) Loop I + LB The Gould Belt and other large star forming complexes

22. Residuals, local irregularities OB stars  < 60 Myr: residual space velocity vectors  < 150 pc 150 <  < 300 pc Olano’s ring (t=0) • A well defined concentration of OB stars in 225o<l<285o: • 3 different residual motions • Mainly 100< <300 pc • Ages 30 <  < 60 Myr • Only 7 stars identified as members of OC or Ass. =>a large number of field OB spread in a large area The Gould Belt and other large star forming complexes

23. Residuals, local irregularities • Other structures in this region: • Open clusters from Hipparcos data (Robichon et al., 1999) • IC2602,NGC2232, NGC2516, IC2391, NGC 2451, Tr10 • Kinematic structures identified by Platais et al. (1998) • A Car, HR 3661 Related to IC2391 Related to Pleiades MG (Asiain et al., 1999) Related to Puppis MG ( Roser & Bastian, 1994) Distribution of OB stars in 225o<l<285o in (U,V) (kernel estimator for isocontours) Origen of these streams in the context of GB, LB, Loop I (interaction) ? The Gould Belt and other large star forming complexes