High-order multiplicity of PMS stars: results from a VLT/NACO survey

1. High-order multiplicity of PMS stars: results from a VLT/NACO survey S. Correia (AIP), H. Zinnecker (AIP), Th. Ratzka (MPIA), M. Sterzik (ESO)

2. Why a high-order (N>2) multiplicity survey ? • Strong constraints for the models of star formation (cf Delgado-Donate talk) • Increasing number of individual triples/quadruples detected in the past few years, e.g. : A353 (Tokunaga et al. 2003), V 773 Tau (Duchene et al. 2002), VW Cha (Brandeker et al. 2001), etc. • Survey for young triples in Rho Oph (Koresko 2002 - 2um speckle holography at Keck) : among 14 widebinaries, 7 consistent with being triples (2 resolved) Motivation for our triple/quadruple survey with NACO : • Within 2 arcsec binaries (~300AU) existence of close (~0.1-0.2 arcsec) companions possible because in hierarchical configuration (Eggleton & Kisseleva 1995, Aarseth & Mardling 2001). • Probe separations down to 10 AU with the resolution provided by a 8m telescope (70mas at 140pc).

3. Sample • Observations • 58 wide (sep. > 1”) binaries • from Reipurth & Zinnecker 1993 (CTTS, based on H surveys) • Clouds : • At ~140-190 pc – 44 binaries • Mainly Taurus – Cha I/II – Oph - • Lupus I/III - L1642 – MBM 12 – B59 • At ~450 pc – 8 binaries • Gum Neb – Orion – L1622.

5. N E

6. N E apparent triple systems

7. N E apparent quadruple systems

8. chance projections PH 30 C and ESO H 283 C&D likely background stars

9. Multiplicity statistics Sample of 58 wide binaries -- 2 Herbig Ae/Be and one foreground star = 55 systems missed components : LkH 263 C (edge-on disk, Jayawardhana et al. 2002, Chauvin et al 2003) No incompletness correction (small sample statistical errors dominate) => 40 binaries, 8 triples, 7 quadruples Multiplicity frequency (MF)per (wide) binary : MF/B = (T+Q) / (B+T+Q) = 27.3 ± 7.0% Within the separation range 10/14 AU - 1700/2300 AU (140-190pc) : MF/B = 28.6 ± 8.3% In individual clouds : Taurus : 50 ± 22%(5/10) Cha I : 30 ± 17%(3/10) Oph : 20 ± 14%(2/10)

10. CTTS CTTS WTTS CTTS WTTS WTTS Multiplicity statisticscomparison with previous multiplicity surveys In the separation range ~ 14-1800 AU :  Frequencies agree with previous surveys within the (large) uncertainties. Are there reasons to suspect that CTTS statistics is different from WTTS statistics ?

11. Multiplicity statisticscomparaison with theory • Only apparent agreement since theoretical MF/B values include all binaries • and wider high-order companions, unlike the observations. • Unlike the observations, the simulations also include : • binary separations < 140 AU (1 arcsec), down to ~ 3-5 AU • higher-order companion separations > 2000 AU Probable overabundance of high-order multiples produced by the current simulations

12. Dynamical stability of the systems • Mostly systems in hierarchical configurations. • (Eggleton & Kisseleva 1995, Aarseth & Mardling 2001 criterions fulfilled) • Few non-hierarchical systems (3/15). • Dynamical decay should already have taken place (age of our systems ~ 10^6yr). • (cf Delgado-Donate talk). • Either these 3 systems are younger or hierarchical in projection.

13. Disk replenishment & accretion • 4/8 systems of mixed type • No common reservoir as invoked for binaries, or disrupted ? • Closest pairs (<50AU) are • mainly WTTS-WTTS types Exceptions : GG Tau AB UZ Tau BC

14. Conclusions - Perspectives • About 20-30% of multiple T Tau systems are high-order (N>2) multiple systems (MF/B =28.6 ± 8.3% in the sep. range 10/14 AU - 1700/2300 AU, mostly in Taurus) • Frequency seems lower than results from current numerical simulations of multiple star formation • We obtained high accuracy astrometric data useful for future orbit characterization (system stability) • CTTS-WTTS type statistics among component of multiple systems suggests no strong correlation as found for binaries. Half of the systems have mixed-types. • Closed pairs are predominantly WTTS pairs. PS : a suggestion for the theorists : please give details of the results of your simulations !