Tristan Cantat-Gaudin

Transcription

1 Open Clusters in the Milky Way with Gaia ICCUB Winter Meeting 1-2 Feb 2018, Barcelona Tristan Cantat-Gaudin Carme Jordi, Antonella Vallenari, Laia Casamiquela, and Gaia people in Barcelona and around the world

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3 Messier 74

4 NGC 6814

5 Andromeda (Messier 31)

6 the Milky Way seen from inside only, unfortunately!

7 We are here. We can study the structure of the Milky Way, and even reconstruct its formation history!

8 Open Clusters and why study them Open clusters are groups of stars born together, from the same gas cloud. All stars within a cluster are the same age, and have the same chemical composition.

9 NGC 604 star formation

10 NGC million years

11 NGC 4755 («the Jewel Box») 16 million years

12 NGC million years!

13 Open Clusters and why study them Open clusters are groups of stars born together, from the same gas cloud. All stars within a cluster are the same age, and have the same chemical composition. [NGC 6705 from Cantat-Gaudin et al. 2014]

14 Open Clusters and why study them Open clusters are groups of stars born together, from the same gas cloud. All stars within a cluster are the same age, and have the same chemical composition. [NGC 6705 from Cantat-Gaudin et al. 2014]

15 The metallicity gradient in the disk traced by open clusters Stars in the inner regions of the Galaxy tend to have larger abundances of metals than stars on the outskirts. Since stars form from the gas enriched by the previous generations, the chemical composition traces the star formation history. [Cantat-Gaudin et al. 2016] inside outside

16 The metallicity gradient in the disk traced by open clusters Stars in the inner regions of the Galaxy tend to have larger abundances of metals than stars on the outskirts. Since stars form from the gas enriched by the previous generations, the chemical composition traces the star formation history. But the shape of the gradient also depends on internal dynamics, mixing of the gas, gas infall/outflow [Cantat-Gaudin et al. 2016] inside outside

17 The metallicity gradient in the disk traced by open clusters Stars in the inner regions of the Galaxy tend to have larger abundances of metals than stars on the outskirts. Since stars form from the gas enriched by the previous generations, the chemical composition traces the star formation history. But the shape of the gradient also depends on internal dynamics, mixing of the gas, gas infall/outflow [Cantat-Gaudin et al. 2016] We do not know the exact shape of the gradient: position of the break, does it flatten or smoothly decrease, etc.

18 The metallicity gradient in the disk traced by open clusters Stars in the inner regions of the Galaxy tend to have larger abundances of metals than stars on the outskirts. Since stars form from the gas enriched by the previous generations, the chemical composition traces the star formation history. But the shape of the gradient also depends on internal dynamics, mixing of the gas, gas infall/outflow [Cantat-Gaudin et al. 2016] We do not know the exact shape of the gradient: position of the break, does it flatten or smoothly decrease, etc.

19 The metallicity gradient in the disk traced by open clusters Stars in the inner regions of the Galaxy tend to have larger abundances of metals than stars on the outskirts. Since stars form from the gas enriched by the previous generations, the chemical composition traces the star formation history. But the shape of the gradient also depends on internal dynamics, mixing of the gas, gas infall/outflow [Cantat-Gaudin et al. 2016] We do not know the exact shape of the gradient: position of the break, does it flatten or smoothly decrease, etc.

20 The metallicity gradient in the disk traced by open clusters Open clusters are convenient tracers of the metallity gradient, because we can determine their ages and study the evolution of the gradient through time. The observed shape and time evolution of the gradient can then be compared with theoretical models. [Casamiquela 2017]

21 The metallicity gradient in the disk traced by open clusters Open clusters are convenient tracers of the metallity gradient, because we can determine their ages and study the evolution of the gradient through time. The observed shape and time evolution of the gradient can then be compared with theoretical models. Some studies find that old clusters trace a steeper gradient, some studies find the opposite. [Jacobson et al. 2016, model from Minchev et al, 2014]

22 The metallicity gradient in the disk traced by open clusters And you might find different gradients if you are using the current Galactocentric radius, or the birth radius of the cluster! [Casamiquela 2017]

23 The metallicity gradient in the disk traced by open clusters And you might find different gradients if you are using the current Galactocentric radius, or the birth radius of the cluster! [Casamiquela 2017]

24 The metallicity gradient in the disk traced by open clusters And you might find different gradients if you are using the current Galactocentric radius, or the birth radius of the cluster! Stars (and clusters) might be following elliptical orbits, and/or may have migrated from a different Galactocentric radius because of the non-axisymmetric features of the disk (bar, spiral arms). [Casamiquela 2017]

25 The metallicity gradient in the disk traced by open clusters And you might find different gradients if you are using the current Galactocentric radius, or the birth radius of the cluster! Stars (and clusters) might be following elliptical orbits, and/or may have migrated from a different Galactocentric radius because of the non-axisymmetric features of the disk (bar, spiral arms). We need better distance estimates. We need a better knowledge of the dynamical processes of mixing and migration. [Casamiquela 2017]

26 The Gaia mission Full-sky coverage, positions and parallaxes for more than a billion stars down to G~20: three-dimensional map of the Milky Way. Gaia DR1

27 The Gaia mission Full-sky coverage, positions and parallaxes for more than a billion stars down to G~20: three-dimensional map of the Milky Way. Proper motions for all stars, and radial velocities for stars brigher than G~15: three-dimensional velocities. In addition: mmag-level photometry in G/GBP/GRP bands Time series for variable stars, metallicities and stellar parameters for stars brighter than G~14.5

28 In Gaia Data Release 1 (GDR1), only the ~2 million brightest stars (G<12) have proper motions and parallaxes. This sample is called Tycho-Gaia Astrometric Solution (TGAS) The astrometry in GDR2 will be of much better quality than TGAS and extend to much deeper sources. GDR1 (September 2016) GDR2 (April 25th 2018) Positions, G magnitudes for ~1.5 billion stars Parallaxes, proper motions for ~2 million stars Positions, G/GBP/GRP magnitudes for ~1.5 billion stars Parallaxes, proper motions for ~1.1 billion stars Radial velocities for ~5 million stars

29 In Gaia Data Release 1 (GDR1), only the ~2 million brightest stars (G<12) have proper motions and parallaxes. This sample is called Tycho-Gaia Astrometric Solution (TGAS) The astrometry in GDR2 will be of much better quality than TGAS and extend to much deeper sources. GDR1 (September 2016) GDR2 (April 25th 2018) Positions, G magnitudes for ~1.5 billion stars Parallaxes, proper motions for ~2 million stars Positions, G/GBP/GRP magnitudes for ~1.5 billion stars Parallaxes, proper motions for ~1.1 billion stars Radial velocities for ~5 million stars This April 25th! Less than three months!

30 HIPPARCOS revolutionised our understanding of the solar neighbourhood. Gaia will be 100 times more precise than HIPPARCOS. It also reaches 10,000 times more sources, probing all components of the Milky Way. Gaia > 1 billion stars HIPPARCOS ~100,000 stars (Gaia actually sees bright sources as far as the Magellanic clouds)

31 Open clusters with Gaia DR1 The quality of astrometry in the TGAS subset of Gaia DR1 is unprecedented! Example: NGC 2527 The signature of the cluster can clearly be seen as a compact group of stars with the same proper motions, and we even have parallaxes to estimate its distance.

32 Open clusters with Gaia DR1 The quality of astrometry in the TGAS subset of Gaia DR1 is unprecedented! Example: NGC 2527 pre-gaia: errors 3-5 mas/yr Gaia DR1: errors ~1 mas/yr Gaia DR2 has parallax and proper motion errors under 0.1 mas

33 Open clusters with Gaia DR1 We identified ~130 open clusters from their proper motions and parallaxes in the TGAS dataset. Selecting cluster members also allows for a better determination of cluster ages from photometry. NGC 2567 age ~1 Gyr [Cantat-Gaudin et al. 2018]

34 If a cluster is on a perturbed orbit we can tell because of its excursion from the plane (z max ) eccentricity (e) We computed orbits using the positions, proper motions and distances from GDR1, and radial velocities from the literature

35 Open clusters with Gaia DR1 Ocs younger than 300 Myr stay within 150pc of the Galactic plane, while older clusters can reach as far as 400pc. This can be attributed to the orbits of the clusters being perturbed during their lifetime. We do not find a significant correlation of the eccentricity of orbits with age, suggesting that their Galactocentric radius is much more robust than their altitude with respect to perturbations: the timescale for radial heating is longer than vertical heating. [Cantat-Gaudin et al. 2018]

36 Prospects for cluster science with Gaia DR2 Identify cluster members and estimate distances for more distant objects. Make use of Gaia radial velocities in order to compute 3D orbits for more objects. Make use of Gaia photometry to derive ages. Expected discovery of unknown objects from Gaia data.

37 [from C. Jordi] Prospects for cluster science with Gaia DR2 Identify cluster members and estimate distances for more distant objects. Make use of Gaia radial velocities in order to compute 3D orbits for more objects. Make use of Gaia photometry to derive ages. Expected discovery of unknown objects from Gaia data. Currently known ~2000 OCs:

38 [from C. Jordi] Prospects for cluster science with Gaia DR2 Identify cluster members and estimate distances for more distant objects. Make use of Gaia radial velocities in order to compute 3D orbits for more objects. Make use of Gaia photometry to derive ages. Expected discovery of unknown objects from Gaia data. Currently known ~2000 OCs: Open questions Is there a cluster mass function? Is it universal? What are the factors for a group of stars to remain bound as a cluster? What are the factors for a cluster to survive?

39 [Cantat-Gaudin et al. 2016] Prospects for cluster science with Gaia DR2 Clusters of extragalactic origin? Some clusters on the edge of the Milky Way are located far from the Galactic plane. Are they extragalactic clusters falling into the disk? Are they Galactic clusters perturbed by a merger event? The currently available data is insufficient to understand what is going on.

40 Concluding remarks Gaia DR1 contains data of unprecedented quality, and DR2 will be better by one order of magnitude, and much deeper. And it is coming this April 25th. Observing 3D positions and 3D velocities (a three-dimensional moving picture!) will reveal the processes taking place in the Milky Way, as well as traces of past events. Gaia will revolutionise our knowledge of both the current structure of the Milky Way and its history.

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