WHY THERE ARE APPARENTLY SO FEW DEBRIS DISKS AMONG

POST- T TAURI STARS ?

R. de la Reza (1), R. Almeida (1), I. Oliveira (1), P. Bourget (1),

C.A.O. Torres (2), G. Quast (2) & G. Pinzón (1)

(1) Observatório Nacional, Rio de Janeiro-RJ, Brazil(2) Laboratório Nacional de Astrofísica, Itajuba-MG, Brazil

CONTENTS

1) Protoplanetary disk evolution and planetary formation

2) A coronographic search for disks around post-T Tauri stars (ages 8-30Myr)

3) Debris disks: Ages and present situation

4) Conclusions

WHEN PLANETS AROUND LOW MASS STARS ARE FORMED ?

Following the popular coagulation of dust theory of planetary formation, planets are formed inside disks around young stars.

Initial disks contain gas and dust where dust is formed by micrometric sized particles radiating mainly in the far-IR. This is at least the case of Classical T Tauri stars (CTTS) with ages below 5 Myr old.

As evolution continues a complex scenario appears. This evolution is an inspiring source of a large number of recent theoretical models. These models anticipate detailed features as protoplanets connected to rings, gaps and spiral tubes inside the disks etc.

In the future, very high resolution observations as is the case of the project ALMA will permit a confrontation of models (people) and observation (facts) which are the ingredients of our understanding of how the worlds are made.

Some recent models treat for instance gas and dust as separate fluids with different properties. Both fluids interact in order to form aggregates of dust by means of sticky slow collisions in these initial phases of the evolution.

As a result it is expected to form kilometric sized particles called "planetesimals".

Gas is lost from the disk mainly by means of accretion into the central stellar surface inside magnetic tubes connecting the disk to the star.

AND THE EVOLUTION CONTINUES

Before gas is lost from the disk it is expected that Earth type planets can be form inside the dust hidden to an external observer.

When a sufficient large mass of ice and rock is formed, producing a core of the order of ~15 Earth masses, the surrounding gas is captured quiet rapidly to form a giant planet.

The giant planet and disk interaction produces an inward migration of the planet.

WHEN ALL THESE PROCESSES BEGAN TO OCCUR ?

They occur mainly in the first ~10 Myr when disks are still alive.

They depend on the total mass of the disk and maybe on metallicity.

Different critical disks masses produced different planetary systems ?

Debris Disks

In this complex evolution there is an epoch when gas is at its mininum mass, planets have been already formed or be in formation.Strong collisions between remanescent planetesimals produced a new generation of dust (debris), even a few quantity, but continually renewed.

Disk are asymmetric and deformed, produced probably by hidden planets.

IMPORTANCE OF THE AGES

Determination of the ages, of the scarce known debris disks is then the key of our understanding of planetary formation. This is the main subject of the work presented here.

DIRECT 1) Optical and Near-IR coronography 2) Sub-millimetric interferometry

INDIRECT 1) Far – IR photometry – IRAS (cool dust) 2) Near IR photometry (warm dust)

OBSERVATIONS METODOLOGY

THIS WORK: A coronography based survey has been made among selected nearby post – T Tauri stars with ages between 8 and 30 Myr in order to try to discover new debris disks.

A CORONOGRAPHIC SURVEY

OUR OBSERVATIONS

During three consecutive nights (datas) we observed 37 PTTS at the Pico dos Dias Observatory (MG - Brasil) using a Hg-mask coronograph constructed at the Observatorio Nacional (Bourget et al. 2002, 2004) attached to a 1.62m telescope. For comparison purposes, two stars with known disks have been observed (Beta Pic and HR4796A).

TARGETS

TW Hya Association (TWA) 8 Myr (de la Reza et al.2004)Beta Pic Moving Group (BPMG) 12 Myr (Zuckerman et al.2001; Ortega et al. 2002,2004) GAYA1 (Tuc/Hor A) 30 Myr (Torres et al. 2003,2004

GAYA2 20 Myr '' ''

The Hg Coronograph

(Bourget et al. 2002)

TWA (8 Myr)

TWA3TWA7TWA8

TWA11ATWA19

BPMG (12 Myr)

BetaPicPZ Tel

V343norHD164249HD 6749HR 6070

HD172555

Eps Cha (7 Myr)

89863110 eps chaHD105923 eps cha

PDS66V4046 Sgr

GAYA 1 = Tuc/HorA (30 Myr)

8584268294120059924313329252003392151181

GAYA 2 (20 Myr)9245053590340968

OtherCD29125883

V87768041870

CD2711535CD547336HD176269CD691055CD337795872802262930000891946700543952301575

MAIN RESULTS

Positive detection of the Beta Pictoris disk.

No detection of the disk of HR4796A because the occulting disk was larger than the ring type disk of this star.

Apparently no new disks were detected among the other 35 targets.

OTHER RECENT CORONOGRAPHIC SURVEYS

Coronograph Targets Authors Results==============================================================

Hubble (NICMOS) TWA Weingerber et al.2000 No new disksAdonis (AO) ESO 3.6m Several Schultz et al. 2004 No new disksHawaii 2.2m & Keck 10m -------- Kalas et al. 2004 AU Mic

Liu (2004)

CONSEQUENCES OF THE NEGATIVE RESULTS

We have search for disks for a collection of ages between 8 and 30 Myr. Our apparently negative results can have a deeper meaning suggesting a rapid consumation of the disk material. If planets formed, they formed relatively early and rapidly.

These results are compatible with the surveys of the presence of warm dust. (near - IR)

1) In TWA (Weinberger et al.2004) --> Positive only in CTTS of TWA 2) In BPMG (Song et al.2003) --> Very few3) In Tuc/Hor (Mamajek et al.2004) --> Absence

YOUNG DEBRIS DISKS

Among the known debris disks around low mass stars in the interval of 8 and 12 Myrs, only three cases are known

Beta Pic A star (belong to BPMG) 12 Myr HR 4796 A star (belong to TWA) 8 Myr Au Mic M star

(AU Mic is a recent important discovery Kalas et al. 2004, Liu 2004)

Beta Pic versus AU Mic

Because AU Mic was considered to be a member of BPMG (Zuckerman et al. 2001), Kalas et al. and Liu 2004 considered that at that age (12 Myr) these two debris disk pointed to follow a synchronous evolution.

But because to the different radiative blowout of an A star (Beta Pic) to that of a M star AU Mic, the dust in the last star was considered to be a primordial one, whereas that of Beta Pic is a second generation dust.

We elucidate even more this question of two kinds of dust at the same age using a completely different strategy based in a new way to determine ages and members of post – T Tauri associations.

IMPORTANT NEW AGE DETERMINATIONS OF POST-T TAURI STARS

Our group in the Observatorio Nacional in Rio is introducing a new age indicator based on retracing 3D galactic orbits of the members of a post-T Tauri association, using a general Galactic potential.

The high precision of this method is now being used in the literature as a calibrated of ages found by means of an HR diagram.

Applications:

1) For BPMG (Ortega et al.2002,2004) Age 11.5 Myr2) For Eta-Epsilon Chamaleontis (Jilinski et al.2005) Age 7.0 Myr2) For TWA (de la Reza et al.2005) Age 8.0 Myr

THE AGE OF AU Mic

The dynamical study of TWA showed that in reality AU Mic is not a member of BPMG but of TWA.

AU Mic then has an age of 8 Myr !!

This result justifies even more that the observed dust in AU Mic is a primordial one.

This result is also important because shows that together with HR4796 A (another member of TWA) that planetary formation is then very probably, already present at 8 Myr or before!

TWA is really a quite exciting group, because is the only young group of PTTS which present at the same age of 8 Myr the whole collection of different types of disks morphologies:

TW Hya K type star --> Classical T tauri star disk

Hen 600 A M type --> CTTS disk

HR 4796A A type --> Ring type debris disk

Au Mic M type --> Beta Pic type debris disk

A GREAT COLLECTION OF OBJECTS IN TWA

CONCLUSIONS

1) No new disks or debris disks have been found up to an age of 30 Myr suggesting that the disk material, if present before, have been consumed quite rapidly, suggesting an eventual rapid planetary formation.

2) The important phase of the disk evolution characterized by the presence of a debris disk appears to be present observationally between 8 and 12 Myr. This short time interval can help to explain why we observe so few initial debris disks.

CONSEQUENCES FOR COROT RESEARCH

A large group of Brazilian researchers are devoted to try to detect by means of CoRot observed indications of the presence of protoplanetary and planetary formation.

We are conscious that this is a difficult task because the evolutionary interval of these processes is short.

Nevertheless, the results presented here indicate that planetary formation must to be searched already in the T Tauri stages and not only in the post – T Tauri stage.

Main consequences of this, an increasing number of targets.