Bayesian Cluster and Moving Group identification with Orbital Traceback

Michael Ireland Collaborators include: Tim Crundall, Jonah Hansen, Chris Tinney, Sarah Martell, Michael Bessell, Eric Mamajek, Aaron Rizzuto

Outline •  Motivation: what Gaia+RVs will give us for nearby

star formation. •  Degeneracies in the kinematic solution and

Bayesian moving group selection.. •  RVs, abundances and youth indicators from

FunnelWeb.

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Moving Groups •  Star formation is kinematically cold at typical densities,

producing nearby young moving groups and associations. •  Young asociations are critical for:

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1.  Dating young stars to test and tweak stellar evolutionary models (esp. PMS, e.g. inflated M dwarfs due to magnetic fields, Feiden 2016)

2.  Dating directly imaged exoplanets, to determine (model-dependent) masses.

3.  Studying stellar and planetary properties as a function of environment.

4.  …

LCC ULC Upper Sco

Input Kinematic Data (12 months, DR2) •  Ground-based ~1km/s RVs (complementing ~1km/s Gaia

RVs) •  ~0.15 mas/yr proper motions = 150m/s @ 200pc. •  1% distances at 200pc (2pc error box) •  For all G dwarfs within 200pc… Tracing back orbits gives: •  15 x 15 x 100pc error ellipse @ 100Myr age •  5 x 5 x 30pc error ellipses @ 30 Myr age

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Input Kinematic Data (Now) •  Ground-based ~1km/s RVs for a select ~60,000 stars (RAVE

+ GALAH + Gontcharov + various) •  ~0.5 mas/yr proper motions = 150m/s @ 200pc. (0.07 to 1.5

mas/yr depending on details) •  ~10% distances at 200pc (20pc error box) •  Only ~10% of G dwarfs within 200pc… Tracing back orbits gives: •  50 x 50 x 100pc error ellipse @ 100Myr age •  20 x 20 x 30pc error ellipses @ 30 Myr age

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Traceback Process •  Use galpy (Bovy 2015) to compute:

for a orbital traceback ftb and coordinate transform T •  Also compute the Jacobian, to produce an

observed (highly non-diagonal) covariance matrix in [X,Y,Z,U,V,W] space:

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“Big Picture”, top down traceback view

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Gaussian PDFs integrated over all stars. Integrated through the thin disk, and with only good Southern RVs (RAVE), only a few clusters or moving groups are obvious. Coma Ber is so prominent because proper motions correspond to ~0.2 km/s velocity errors.

Zooming in – Previous work on β Pic •  Mamajek and Bell (2014) : no evidence for the

12 Myr age for classical members. •  ~20 Myr preferred from isochrones.

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Problem: Filamentary Star Formation

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Luhman et al. (2009) Taurus

A single moving group may have multiple sites of star formation. e.g. Taurus-Auriga is spread over ~30pc with relatively small clusters. Rho Oph is often listed as an example of nearby compact star formation – but it it may not look compact in time to a future observer as a small component of Upper Sco.

Solution: Bayesian Group-Fitting •  The data D for a star comprises 6D kinematic parameters, their

covaraiance matrix, and auxilliary data. For models M for groups gi:

•  If Mg is a 6D Gaussian in x~[X,Y,X,U,V,W] for each component of a moving group, we compute P(D|M) via an overlap integral between two Gaussians for every star/group pair:.

•  Group parameters to describe P(x|Mg) can then be fitted using Monte-Carlo Markov Chain, either with traceback time as a parameter with a single value or as another variable to integrate over.

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β Pic : 20 Myr traceback in XY plane

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β Pic : 20 Myr traceback in XY,YZ planes

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•  Although trajectories still diverge, there is an overlap with most stars.

•  11 out of 23 “classical” β Pic member have <50% probability of being part of this core.

•  The age of the β Pic is an a posteriori result from this process: 19.5 +/- 2.0 Myr.

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β Pic : Best fit age

•  It is not completely model dependent yet, due to the assumption of one star forming core.

•  We really want to marginalise over multi-core association models.

Funnel-Web… 1.  Uses the UKST, one of the

widest-field >1m telescopes in the world.

2.  Uses the Starbugs fiber positioners, repositioning multiple objects faster than other technologies.

3.  Uses the TAIPAN spectrograph.

4.  Is a Stellar Survey that shares these resources with the TAIPAN galaxy survey

5.  Will begin April 2017 Ireland (ANU) , Tinney, Martell (UNSW), Hopkins (AAO) Executive Committee

The UK Schmidt Telescope •  “Modern” wide-field telescopes began with

the 48” Samuel Oschin Schmidt on Mt Palomar (1949)

•  Still used for very competitive science (PTF: 2016 supernovae, M. Brown: Sedna, Haumea, Makemake).

•  The UKST is based on this telescope, which has done some great science with RAVE but could do more…

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Starbugs – A Prototype for the GMT

TAIPAN Spectrograph •  R~2000, 370-870nm. •  150 fibres fed at f/2.5 •  Refractive design •  Max 100 mm beam •  5 lens collimator, 5 lens cameras •  Total of 8 aspheric surfaces in

design with 4 distinct lenses •  2 arms using 2kx2k E2V 42-40 BI

NIMO CCDs •  COTS cooler/controller

Slit

Beamsplitter

Blue Arm

Red Arm

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http://funnel-web.wikispaces.com

FunnelWeb Survey

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•  G<10 for all-sky •  G<12 for an off-plane, all Southern sky survey. •  G<14 for targeted subsamples (M dwarfs, kinematically

selected young stars, metal poor stars). •  RV uncertainties <1km/s for BAF stars (better than Gaia) •  Lithium equivalent width (youth) •  H-α,Ca H/K and Triplet •  10,000 stars/night

Stellar Parameters with Low-Resolution Spectra

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•  Funnelweb significantly augments Gaia spectra, e.g. log(g) for unresolved binary stars, metalicity/gravity/temperature degeneracies (age e.g. Diane Feuillet’s talk)

•  Elemental abundances, especially [α/Fe] will be available (e.g. Anna Ho et al)– the core survey will maintain SNR>100 per resolution element.

Funnelweb Team Membership

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Management document is in preparation, but it is expected to say: •  Membership is open to anyone who can contribute 0.1 FTE or more to

core survey operations, subject to exec approval (checking for overlaps/conflicts etc)

•  Due to project funding sources, membership of people at Australian institutions will be prioritised.

•  Team members will have access to pre-release data, and the ability to submit high priority targets and prioritise fields or regions of the southern sky.

•  We will have at least 2 public data releases, with the first release prior to Gaia DR3 (tight timeline, with <6 months proprietary period on data collected in April 2018).

•  During 2017, we’ll start looking for funding for 2019 until completion (which may include external partners).

Conclusions and Outlook

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Gaia Orbital traceback + RV, youth indicators and parameters from FunnelWeb will enable the majority of young stars within 200pc to be placed in their birth clusters/associations. This has wide-reaching stellar and exoplanetary implications. FunnelWeb membership is open to you! AND – we’ll have a 2.5y+ postdoc advertised ASAP