Transcript Why there are apparently so few debris disks among post

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 830Myr)
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.
OBSERVATIONS METODOLOGY
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)
A CORONOGRAPHIC SURVEY
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.
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)
Beta Pic Moving Group (BPMG)
GAYA1 (Tuc/Hor A)
GAYA2
8 Myr
12 Myr
30 Myr
20 Myr
(de la Reza et al.2004)
(Zuckerman et al.2001;
Ortega et al. 2002,2004)
(Torres et al. 2003,2004
'' ''
The Hg Coronograph
(Bourget et al. 2002)
TWA (8 Myr)
TWA3
TWA7
TWA8
TWA11A
TWA19
BPMG (12 Myr)
BetaPic
PZ Tel
V343nor
HD164249
HD 6749
HR 6070
HD172555
Eps Cha (7 Myr)
89863110 eps cha
HD105923 eps cha
PDS66
V4046 Sgr
GAYA 1 = Tuc/HorA (30 Myr)
85842682
94120059
92431332
92520033
92151181
GAYA 2 (20 Myr)
92450535
90340968
Other
CD29125883
V877
68041870
CD2711535
CD547336
HD176269
CD691055
CD337795
872802262
930000891
946700543
952301575
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
Adonis (AO) ESO 3.6m Several
Hawaii 2.2m & Keck 10m --------
Weingerber et al.2000
Schultz et al. 2004
Kalas et al. 2004
Liu (2004)
No new disks
No new disks
AU Mic
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 few
3) 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
HR 4796 A star
Au Mic M star
(belong to BPMG)
(belong to TWA)
12 Myr
8 Myr
(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)
2) For Eta-Epsilon Chamaleontis (Jilinski et al.2005)
2) For TWA
(de la Reza et al.2005)
Age 11.5 Myr
Age 7.0 Myr
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!
A GREAT COLLECTION OF OBJECTS IN TWA
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
HR 4796A
Au Mic
--> CTTS disk
A type --> Ring type debris disk
M type --> Beta Pic type debris disk
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.