Structure and kinematics of the Gould Belt from Hipparcos Data

Oct 24, 2001 The Gould Belt and other large star forming complexes 1

Structure and kinematics of the Gould Belt from Hipparcos Data


Francesca Figueras, Jordi Torra, David Fernández

Universitat de Barcelona



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)



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



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



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



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



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



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



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.



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)



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)



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



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



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



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)



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.



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



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



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



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



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




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




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

Date post:	10-Jan-2016
Category:	Documents
Upload:	glynn
View:	40 times
Download:	0 times

Structure and kinematics of the Gould Belt from Hipparcos Data

Documents