PROTOPLANETARY DISK EVOLUTION AT THE AGES OF PLANET
FORMATION
Aurora Sicilia-Aguilar1, Lee Hartmann1, Jesús Hernández1,2, César Briceño2, Nuria Calvet1,2, James Muzerolle3, Bruno Merín4
1. Smithsonian Astrophysical Observatory, Cambridge USA 2. Centro de Investigaciones de Astronomía, Venezuela 3. University of Arizona, USA 4. Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF), Spain
Email: [email protected] - Smithsonian Astrophysical Observatory; 60 Garden St. MS-42, Cambridge MA02138, USA
ABSTRACT
We present the first identification of low mass stars and their disks in the young clusters Tr37 and NGC7160, members of the
CepOB2 association. This is part of a program to follow the evolution of protoplanetary accretion disks through the ages
thought to be crucial to understanding disk dissipation and planet formation (~3-10 Myr). Combining optical photometry
and optical spectroscopy, we have identified 135 low mass members in Tr37, and 45 in NGC7160, confirming the age estimates of 3-5 Myr for Tr37 and 10 Myr for NGC7160. Using extinction, we have identified the high and intermediate mass
stars in both clusters, finding ~59 and ~69 B-F stars in Tr37 and NGC7160, respectively.
We find active accretion in Tr37, with average accretion rates of ~10-8MSUN yr-1, and ~40% disk fraction, whereas only 1 of
the stars in NGC7160 in our sample is accreting. These results are consistent with those from other populations and are a
clear sign of disk evolution within the CepOB2 region. Using IRAC GTO (3.6, 4.5, 5.8 and 8.0 µm) data and MIPS (24µm), we
investigate the presence of accretion disks and debris disks in the clusters. We find evidences of inner disk removal, suggesting either dust settling or inner holes in the disks of low mass stars, and some very low accretion rates in Tr37, which
could be indicative of transition objects. We find 2 Herbig AeBe stars and 9 debris disks in Tr37, and 4 in NGC7160, suggesting disk evolution for the higher mass stars, and evidences of debris disks already at the age of 3-5 Myr.
Spitzer 3 color (3.6, 4.5 and 8.0µm ) image of a dust bubble inTr37, heated by HD206267.
This work is supported by NASA grants NAG5-9670 and NAG5-13210.
IDENTIFYING STARS IN CEP OB2
ACCRETION IN PROTOPLANETARY
DISKS
The CepOB2 region is a bubble-shaped structure (Garrison &
Kormendy, 1976, Simonson & van Someren Greve, 1976) of 3
degrees diameter, located at 900pc (Contreras et al. 2001). It
contains two clusters: NGC7160 near the center; and Tr37 (with
the O6 star HD206267) in the rim (Garrison & Kormendy 1976).
We use optical photometry (VRI, 4Shooter/1.2m FLWO) and
variability to identify the potential members. Optical low resolution spectra (Hectospec/6.5m FLWO, FAST/1.5m FLWO, Hydra/
3.5m Kitt Peak) are used to confirm the youth of the low mass
stars via Li 6707 Å absorption ( Hartmann 2003) and Hα emission. We also obtain spectral types, and determine the extinction and its variation over the cluster (AV=1.57 for Tr37, AV=1.27
for NGC7160, standard deviation ~0.5). The combination of criteria ensures a low contamination in our sample (Sicilia-Aguilar
et al. 2004). Color-magnitude diagrams and theoretical isochrones (Siess et al. 2000) give age estimates of ~3-5 Myr for Tr37
and ~10 Myr for NGC7160.
Accretion is detected spectroscopically using the EW of the Hα
emission line. Lines formed in the accretion columns have
EW>10 Å for M2-K6 stars or EW>3 Å for K5-G (White & Basri 2003).
The accretion shock produces an excess hot emission, which is
strongly correlated to the accretion luminosity and to the mass
accretion rate (Gullbring et al. 1998). We have used U band
photometry from the 4Shooter/1.2m FLWO telescope to obtain
accretion rates ranging 10-7 to few 10-10 (average ~10-8) for ~60
stars in Tr37.
V vs. V-I diagrams for low and intermediate (G-M4) mass stars in Tr 37 (left) and
NGC7160 (right). Objects are corrected using their individual reddening. The Siess
et al. 2000 isochrones for 1, 10 and 100 Myr are displayed for comparison. Ages are
consistent with 3-5 and 10 Myr respectively. Some younger objects in Tr37 seem
associated with dust and gas structures in the cluster (see Spitzer image).
High and intermediate mass members (which do not show Li
absorption nor Hα emission) are identified using their optical
spectra, spectral types and extinction.
3-5 MYR OLD DISKS IN TR37: DUST
SETTLING AND DISK EVOLUTION
The low mass members of Tr37 are consistent with ages around
3-5 Myr (there could be some 1Myr population mostly associated to dust structures). A considerable fraction (~40%) of the
stars have accretion disks, but about half of these accretion
disks show no excess in JHK, which suggest that dust settling
and/or clearing of the inner disk has occurred.
We find accretion rates about ~10-8MSUN yr-1, but some of the
stars have accretion rates lower than ~10-9MSUN yr-1. We also
find 2 transition objects, with no signs of accretion but evidences of a disk detectable only at 5.8 and/or 8.0µm. Since we
do not find any transition objects in NGC7160, this may suggest
a rapid transition phase, finished by the age of 10 Myr.
IRAC color-color diagrams for the
Tr37 members.
Ch1 - 3.6 µm
Ch2 - 4.5µm
Ch3 - 5.8 µm
Ch4 - 8.0 µm
Dashed lines delimit the color-color
areas predicted by the models
(D’Alessio et al. 2001) for CTTS and
protostars (Allen 2004).
Protoplanetary disks produce an excess in IR emission (Meyer,
Calvet & Hillenbrand 1997). The near-IR excess traces the inner
disk (~0.1-few AU), and it is consistent with a model of a heated
wall located at the dust sublimation radius (Muzerolle et al.
2003). This excess decreases with time, suggesting dust settling
and/or opening of a gap. We have studied JHK data from the
2MASS survey (Cutri et al. 2003), finding a substantial lack of
near-IR emission in about half of the CTTS in Tr37 (see SEDs).
IRAC and MIPS observations (3.6 to 24µm) reveal the disk emission at ~10-20 AU, allowing us to detect disks with dust settling
and/or inner holes (maybe indicative of planet formation). The
stars in Tr37 show IRAC colors that are very consistent with the
colors predicted by the accretion disk models (D’Alessio et al.
2001, Allen et al. 2004), and we can confirm the presence of
disks in stars with very low (or even zero) accretion rates. IRAC
and MIPS reveal the presence of debris disks around high and
intermediate stars as well.
The lack of excess of some CTTS (red dots in the figure) at shorter wavelengths is
indicative of dust settling or holes in the inner disk. Excesses in stars defined as WTTS
(green triangles in the CTTS box) may correspond to transition objects (WTTS with
disks at ~10 AU) or stars with very low accretion rates ~few times 10-10MSUN yr-1.
DISK EVOLUTION IN CEP OB2
The comparative study of low mass stars in Tr37 and
NGC7160 reveals striking differences between the disks
around 3-5 and 10 Myr low mass stars. The disk fraction is
~40% at 3-5 Myr, but only 1/45 stars have disks at 10 Myr.
3-5 Myr disks have lower IR excesses than disks around
younger stars (i.e., Taurus), and about half of the CTTS do
not show any near-IR excess; this could be an indication of
disk removal or dust settling. The detection of 2 transition
objects (WTTS with disks) and some potential stars with
very low accretion rates aged 3-5 Myr and none aged 10
Myr suggest that the termination of accretion processes is
happening very rapidly at the age of 3-5 Myr.
We have detected debris disks around ~15% of the high
and intermediate mass stars in Tr37, and around~6% in
NGC7160. Moreover, we have detected two potential
AeBe stars with disks in Tr37, but none of them in NGC7160.
This seems to indicate that there is important disk evolution
in the age range of 3 to 10 Myr for the higher mass stars.
SEDs for low (left) and high-intermediate mass stars (right) in Tr37, compared to the photospheric emission for each spectral type (dotted
line) and to the brightest debris disk, HR 4796. Spectral types are indicated, as well as CTTS (c) or WTTS (w) class for the low mass stars.
Note the remarkable lack of near IR excess in almost half of the CTTS, (i.e. 21-33, red arrow), and the presence of outer disks around
some stars for which no accretion is detected using U band nor Hα; these could correspond to objects with very low or zero accretion
rates, intermediate stage between CTTS and WTTS (i.e., 13-1250, green arrow). Only one active accretor is found in our NGC7160 sample.
We find several debris disks around high and intermediate mass stars in Tr37 (i.e., KUN-314S, black arrow), as well as a couple of potential
Herbig AeBe stars (i.e., MVA-426, blue arrow). The number of debris disks in NGC7160 is lower, and no Herbig AeBe are found in this older
cluster.
Mass accretion rates versus age for the accreting stars in Tr37, compared
to the results obtained in several clusters (Muzerolle et al. 2000) and the viscous disk evolutionary model (green line, Hartmann et al. 1998). Ages for
individual stars are obtained by comparison with theoretical isochrones
(Siess et al. 2000). Error estimation in the lower left corner. Higher mass stars
(G) tend to have more massive disks and to look older due to birth line
effects. The detection limit is about few times 10-10 MSUNyr-1.
Combining our knowledge of spectral types and extinction with U band
photometry, we can calculate mass accretion rates using the relation
between U excess luminosity and accretion luminosity in Gullbring et al.
1998:
Lacc
Lu
GMstarṀ
Rstar
log  ------------ = 1.09 log  -------- + 0.98 ⇒ Lacc = --------------------------  1 – --------------------------------
 LΘ 
 LΘ
Rstar 
Rtruncation
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