Oct 24, 2001 The Gould Belt and other large star forming complexes 1
Structure and kinematics of the Gould Belt from Hipparcos Data
Structure and kinematics of the Gould Belt from Hipparcos Data
Francesca Figueras, Jordi Torra, David Fernández
Universitat de Barcelona
Oct 24, 2001 The Gould Belt and other large star forming complexes 2
Structure and kinematics of the Gould Belt from Hipparcos Data
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)
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Structure and kinematics of the Gould Belt from Hipparcos Data
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 stellar component of the Gould’s belt
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Structure and kinematics of the Gould Belt from Hipparcos Data
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
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Structure and kinematics of the Gould Belt from Hipparcos Data
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
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Structure and kinematics of the Gould Belt from Hipparcos Data
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”
Higher degree of completeness for distant stars
The fraction is not a flat function
kinematic bias present in our sample Needs for evaluation through numerical simulations
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Structure and kinematics of the Gould Belt from Hipparcos Data
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
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Structure and kinematics of the Gould Belt from Hipparcos Data
Initial Sample 6922 Hipparcos stars. Vlim ~ 8
Stroemgren Photometry 3031 stars
Radial Velocity 3397 stars
Sample 13915 stars r, Completeness Vlim ~ 6.5 < > = 0.60 mas< /> = 0.16< cos> = 0.83 mas /yr < >= 0.70 mas/yr2468 if ages are considered
Sample 22272 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
Working samples
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Structure and kinematics of the Gould Belt from Hipparcos Data
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.
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Structure and kinematics of the Gould Belt from Hipparcos Data
Simulations for the structure analysis
> 30 Myr
30 – 60 Myr
> 60 Myr
Zo= 40 pc Zo= 80 pc
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)
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Structure and kinematics of the Gould Belt from Hipparcos Data
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)
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Structure and kinematics of the Gould Belt from Hipparcos Data
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 + 2
i, 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
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Structure and kinematics of the Gould Belt from Hipparcos Data
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
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Structure and kinematics of the Gould Belt from Hipparcos Data
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
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Structure and kinematics of the Gould Belt from Hipparcos Data
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 simulationsthis 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)
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Structure and kinematics of the Gould Belt from Hipparcos Data
Results
Discrepances in Vo ( 4 km/s between Vr & proper motion solution::
•It remains when eliminating some particular regions
•From our simulationsNo 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 cos determination.
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Structure and kinematics of the Gould Belt from Hipparcos Data
Kinematics of young stars: the global behaviour
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
Bias expected from simulations:
•(U,V,W) underestimated in 0.4 km/s
•B underestimated in 0.8 km/s/kpc
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
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Structure and kinematics of the Gould Belt from Hipparcos Data
Variations of Oort parameters with age
•A increase of A,B with age
•A decrease of C,K with age
GB age from kinematic behaviour
=
GB age from spatial distribution
GB age = (30-60) Myr
100 < R < 600 pc
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Structure and kinematics of the Gould Belt from Hipparcos Data
Oort constants when excluding Sco-Cen and Ori OB1
•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
Excluded: A B C K
None 5.7(1.4) -20.7(1.4) 5.2(1.4) 7.1(1.4)
Sco-Cen 6.9(1.6) -19.7(1.6) 4.7(1.6) 5.8(1.6)
Ori OB1 6.1(1.6) -20.7(1.6) 5.3(1.6) 6.3(1.6)
Both Complexes 7.2(1.8) -19.7(1.8) 4.9(1.9) 6.0(1.9)
The two main complexes in the GB
Stars selected from Brown et al. (1994; Ori OB1), de Zeeuw et al. (1999; Sco-Cen)
100 < R < 600 pc, age < 30 Myr
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Structure and kinematics of the Gould Belt from Hipparcos Data
Expansion of the system as a function of distance
Variation of KR as a function of heliocentric distance (stars with < 60 Myr)
•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
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Structure and kinematics of the Gould Belt from Hipparcos Data
Residuals, local irregularities
OB stars < 60 Myr: residual space velocity vectors
< 150 pc 150 < < 300 pcOlano’s ring (t=0)
Breitschwerdt et al. (2000)
Loop I + LB
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
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Structure and kinematics of the Gould Belt from Hipparcos Data
Residuals, local irregularities
OB stars < 60 Myr: residual space velocity vectors
< 150 pc 150 < < 300 pcOlano’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
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Structure and kinematics of the Gould Belt from Hipparcos Data
Residuals, local irregularities
Distribution of OB stars in 225o<l<285o in (U,V)
(kernel estimator for isocontours)
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 Pleiades MG (Asiain et al., 1999)
Related to Puppis MG ( Roser & Bastian, 1994)
Origen of these streams in the context of GB, LB, Loop I (interaction) ?
Related to IC2391