The Late Stages of Protoplanetary Disk Evolution: A Millimeter Survey of Upper Scorpius
ABSTRACT We present deep 1.2 millimeter photometry of 37 stars in the young (5 Myr)
Upper Scorpius OB association, sensitive to ~4 x 10^-3 Mjup of cool millimeter
dust. Disks around four low- and solar-mass stars are detected, as well as one
debris disk around an intermediate mass star, with dust masses ranging from 3.6
x 10^-3 -- 1.0 x 10^-1 Mjup. The source with the most massive disk exhibits a
transition-disk spectral energy distribution. Combining our results with
previous studies, we find the millimeter-detection fraction of Class II sources
has significantly decreased from younger ages, and comparison with
near-infrared and Halpha measurements indicates the present disks have
undergone significant evolution in composition or structure at all radii. The
disks of Upper Scorpius represent the tail-end of the depletion of primordial
disks; while a few near-solar mass stars may still sustain giant planet
formation, this process has finished around higher mass stars
arXiv:1111.0101v2 [astro-ph.SR] 3 Nov 2011
Preprint typeset using LATEX style emulateapj v. 11/10/09
THE LATE STAGES OF PROTOPLANETARY DISK EVOLUTION:
A MILLIMETER SURVEY OF UPPER SCORPIUS
Geoffrey S. Mathews1, Jonathan P. Williams1, Francois M´ enard2, Neil Phillips3, Gaspard Duchˆ ene4,2, and
Institute for Astronomy, University of Hawaii, Honolulu, HI 96826
We present deep 1.2 millimeter photometry of 37 stars in the young (5 Myr) Upper Scorpius OB
association, sensitive to ∼ 4 × 10−3MJupof cool millimeter dust. Disks around four low- and solar-
mass stars are detected, as well as one debris disk around an intermediate mass star, with dust
masses ranging from 3.6 × 10−3– 1.0 × 10−1MJup. The source with the most massive disk exhibits
a transition-disk spectral energy distribution. Combining our results with previous studies, we find
the millimeter-detection fraction of Class II sources has significantly decreased from younger ages,
and comparison with near-infrared and Hα measurements indicates the present disks have undergone
significant evolution in composition or structure at all radii. The disks of Upper Scorpius represent
the tail-end of the depletion of primordial disks; while a few near-solar mass stars may still sustain
giant planet formation, this process has finished around higher mass stars.
Subject headings: circumstellar matter — open clusters and associations: individual (Upper Scorpius
OB1) — planetary systems: protoplanetary disks — stars: pre-main-sequence
Our understanding of circumstellar disks, the birth-
place of planets, has been driven in the last decade largely
by the Spitzer space telescope, which has allowed for
the identification of young stars with warm, optically
thick inner dust disks. Spitzer has observed all known
nearby sites of recent star formation (e.g. c2d, FEPS,
Gould’s Belt Survey), revealing hundreds of new disk
bearing systems with ages from 1 – 100 Myr.
disks have shown a remarkable variety of spectral energy
distributions (SED), indicating the presence of large-
excess primordial disks, weaker excess “anemic” disks,
transition disks showing an emission gap at short wave-
lengths, and debris disks showing weak emission charac-
teristic of small amounts of second generation dust (for
an overview, see Williams & Cieza 2011). These many
surveys have revealed a picture of circumstellar disk evo-
lution in which inner disk dust dissipates on timescales
of a few Myr (e.g. Hillenbrand 2006), with inner disks
around higher mass stars evolving more quickly (e.g.
Carpenter et al. 2006).
Optical and near-infrared spectroscopic surveys for ac-
cretion indicators (e.g. Hα line equivalent width) have
revealed a similar variety in accretion rates among young
stars (e.g. Dahm 2008). The presence of accretion indi-
cates the existence of gas in the inner disk.
On the other hand, probing the outer disk, the main
reservoir of dust, requires sensitive (sub-)millimeter pho-
1Institute for Astronomy, University of Hawaii, 2680 Wood-
lawn Dr., Honolulu HI 96826
2CRNS-INSU / UJF-Grenoble 1, Institut de Plan´ etologie et
d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-
3Institute for Astronomy, University of Edinburgh, Royal Ob-
servatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
4Astronomy Department, University of California, Berkeley
CA 94720-3411 USA
thin, allowing for the direct measurement of dust mass.
Past large mm-surveys in young (<10 Myr) regions have
reached dust mass sensitivities of a few 10−2MJup for
standard assumptions (e.g. Andrews & Williams 2005;
Roccatagliata et al. 2009). In this work, we present re-
sults of a sensitive search for millimeter continuum in
a sample of nearby stars in the Upper Scorpius region
which have signs of significant evolution in the inner disk
dust and gas.
At a distance of 145 pc (de Zeeuw et al. 1999), Up-
per Scorpius (Upper Sco) is one of the closest sites
of recent star formation.
(Preibisch & Zinnecker 1999), it has largely exhausted
its interstellar gas and dust, reducing contamination
from field emission. Spectroscopic studies indicate that
few stars are still accreting (e.g. Walter et al. 1994;
Dahm & Carpenter 2009); this suggests that most disks
have depleted their inner disk gas content. Spitzer stud-
ies using IRAC, IRS, and MIPS (Carpenter et al. 2006,
2009) directly revealed that the disks present in Up-
per Sco are more evolved than disks in younger star
forming regions. The majority of Upper Sco members
(∼80%), at all spectral types, exhibit no infrared ex-
cess.SEDs for the infrared detections in Upper Sco
indicate anemic or transition disks around K and M
stars, and debris disks around stars across the entire
sample (B through M spectral types). Similar results
have been found in other 5 Myr associations (e.g.
Ori and NGC 2362, Barrado y Navascu´ es et al. 2007;
Dahm & Hillenbrand 2007).
The variety of disk types around low mass stars in Up-
per Sco suggest that an age of 5 Myr may be a time
at which disk properties rapidly change, providing an
ideal laboratory for examining the evolution of disks.
Of all mechanisms affecting disk evolution, perhaps the
one that has stirred the greatest interest in recent years
At these wavelengths, the dust is optically
At an age of 5 Myr
2 Mathews et al.
has been the possibility of disruption by giant planets.
Disk mass measurements will provide direct constraints
on the potential for on-going giant planet formation, in
turn constraining planet formation models.
We have carried out the most sensitive survey to date
for millimeter emission from 5 Myr stars, reaching a dust
mass sensitivity of ∼4×10−3MJup. In section 2, we de-
scribe our target sample and our observations. In section
3, we discuss the inferred properties of our sources. In
§4, we discuss the properties of disks in Upper Sco, and
in §5, we compare to disks at younger ages and discuss
the implications for planet formation. We summarize our
findings in section 6.
2. SAMPLE, OBSERVATIONS, AND VALUES FROM THE
We selected our sample from the 218 Upper Sco mem-
bers observed in the near-infrared (NIR) continuum sur-
vey of Carpenter et al. (2006). They selected survey tar-
gets that were identified as group members on the basis
of stellar parameters so as to not bias towards disk bear-
ing sources. Moreover, their targets were chosen so as to
uniformly sample the stellar mass range in Upper Sco.
We thus believe their sample to be the best representa-
tion of the Upper Sco population. Our sample focused
on sources identified by Carpenter et al. as having a NIR
excess at 8 or 16 µm; we observed 8 of the 9 B and A
(henceforth BA) excess sources, 20 out of 24 K and M
stars (henceforth KM) with an 8 or 16 µm excess, and 3
BA and 6 KM stars with no excess.
Our targets were observed for continuum emission
at 1.2 mm using the MAMBO2 bolometer array
(Kreysa et al. 1998) on the IRAM 30m telescope at Pico
Veleta, Spain. Observations were conducted during the
bolometer pools in 2008 (Nov.
(February 24-25, March 8-9, and Nov. 8). Zenith opacity
for our observations was typically ∼0.2 – 0.3. Observa-
tions were carried out to a target 1σ sensitivity of 1 mJy,
typically 20 minutes on source, in an ON-OFF pattern
of 1 minute on target followed by 1 minute on sky, with a
throw of 32′′. Observations during November 2009 were
limited to 10 minutes on source, resulting in poorer sensi-
tivity. In addition, deeper observations of objects show-
ing 2σ detections at the target RMS were carried out,
resulting in the discovery of millimeter emission from a
pair of sources (HIP 76310, ScoPMS 31) at a higher sen-
sitivity than the median of the sample. Flux calibration
was carried out using Mars, Saturn, or Jupiter (depend-
ing on availability), and local pointing and secondary flux
calibration was carried out using IRAS 16293-2422B.The
data were reduced using the facility reduction software,
We list the physical properties of the sources in this
study in Table 1. We include values for three objects
from other studies: [PBB2002] USco J160823.2-193001
and [PBB2002] USco J160900.7-190852 (observed at 1.3
mm in Cieza et al. 2008), and [PZ99] J161411.0-230536
(observed at 1.2 mm in Roccatagliata et al. 2009). These
are members of the parent sample from Carpenter et al.
(2006), and would have been observed as part of this
work if photometry did not already exist in the literature.
13-18, 23) and 2009
2.1. Spectral type and stellar mass
of BA stars
Hern´ andez et al.
Hern´ andez et al. (2005), who used the stellar isochrones
of Palla & Stahler (2000). For KM stars, we use masses
from Kraus & Hillenbrand (2007), who used the models
ofD’Antona & Mazzitelli
(1998) for solar-mass and low mass stars, respectively.
Two stars lack a mass estimate in the literature,
[PBB2002] USco J160823.2-193001 and [PBB2002] USco
J160900.0-190836; we assigned these stars masses ac-
cording to their spectral type, as in Kraus & Hillenbrand
(2007).Comparison of the KM star masses to other
estimates in the literature (e.g. Preibisch & Zinnecker
1999) suggests a typical stellar mass uncertainty of
10–20%. Both spectral type and estimated stellar mass
are listed in Table 1.
from Preibisch et al.
For BA stars, we adopt stellar masses from
(1997)and Baraffe et al.
The Hα equivalent width, Wλ(Hα), was taken from
Hern´ andez et al. (2005), Preibisch et al. (1998, 2002),
Dahm & Carpenter (2009), and Riaz et al. (2006). This
is then used to determine the accretion state of the
stars. Traditionally, KM stars with Wλ(Hα) of 10˚ A or
greater were considered to be accreting, and referred
to as Classical T-Tauri stars (CTTS). Those with
lower or undetected emission were considered to be
non-accreting, and referred to as Weak-line T-Tauri
stars (WTTS). Recent work has refined this classi-
fication system (Barrado y Navascu´ es & Mart´ ın 2003;
White & Basri 2003), identifying spectral type depen-
dent Wλ(Hα) that unambiguously indicate accretion.
Here, we adopt the system of White & Basri — Wλ(Hα)
in emission of 3, 10, 20, and 40˚ A indicate accretion for
K0–K5, K7–M2.5, M3–M5.5, and M6 and later spectral
types, respectively. While there are a few accreting sys-
tems among the KM stars of Upper Sco, none of the BA
stars in Upper Sco show Hα in emission, suggesting that
none of these stars still experience accretion.
We note that the use of an accretion indicator to
demonstrate the presence of gas in the disk is subject
to two main problems: accretion and photospheric Hα
emission can be difficult to distinguish at low equiva-
lent widths, and accretion may vary by large amounts
on timescales as short as days (Bouvier et al. 2007;
Nguyen et al. 2009). Among the 20 KM stars in our sam-
ple with multiple epochs of Wλ(Hα) measurements, two
show values which would produce different classifications
as CTTS or WTTS over a decade timescale. In these
two cases ([PBB2002] J160545.4-202308 and [PBB2002]
J160702.1-201938), we classify the sources as CTTS. We
list the Wλ(Hα) and accretion state of our sources in
Our observations include 8 of the 10 CTTS in the orig-
inal sample of Carpenter et al. (2006), and 17 of the 95
WTTS. There are an additional 22 KM stars with no
Hα measurement in the literature, of which we include
1. The millimeter-detected sources we include from the
literature are split between accreting and non-accreting
systems; 1 is a CTTS, and 2 are WTTS.
Millimeter Survey of Upper Sco3
2.3. Infrared disk classification
Table 1 also shows the disk classification based on
SEDs. SEDs were constructed using 2MASS J, H, and
K band photometry (Skrutskie et al. 2006), 4.5, 8.0,
and 16 µm photometry from Carpenter et al. (2006),
and 24 and 70 µm photometry from Carpenter et al.
(2009). We used extinction estimates from the litera-
ture (Hern´ andez et al. 2005; Preibisch et al. 1998, 2002)
and the R=3.1 reddening law of Fitzpatrick (1999) to
deredden NIR photometry.
For the BA stars in Upper Sco, we use the SED clas-
sifications of Carpenter et al. (2009). Among BA stars,
they found only debris disks. Carpenter et al. classified
stars in Upper Sco as hosting debris or primordial disks
on the basis of the strength of the NIR excess and com-
parison of NIR colors with those of disks in the younger
For KM stars, the classification is based upon the slope
of the SED at NIR wavelengths, as in (Greene et al.
1994). Assuming λFλ ∝ λn, sources exhibiting a near
infrared power law index greater than 0.3 are considered
Class I, −0.3 < n < 0.3 are flat-spectrum, −1.6 < n <
−0.3 are Class II, and n < −1.6 are Class III. The classes
roughly trace the expected evolution of inner disk dust.
We use the broadest baseline applicable to our entire
sample, calculating the index from K-band and 24 µm
We classify the K2 star [PZ99] J160421.7-213028 as
hosting a transition disk, based on its lack of NIR excess
at wavelengths less than 16 µm and steeply rising excess
at longer wavelengths (e.g. Cieza et al. 2007).
The Upper Sco sample of Carpenter et al. (2006) con-
tains 19 Class II sources, comprising 15% of the KM
stars, and 107 Class III sources. We observed 16 Class II
and 9 Class III sources. The three additional detections
included from the literature are Class II sources.
Due to the variety of disk types among KM stars, we
summarize their disk classifications in Table 2, where we
show the number of each type of object in the parent
sample of Carpenter et al. (2006) (applying our Class II
/ III and CTTS / WTTS criteria as described above),
the number included in this work, and the number of
3. RESULTS AND ANALYSIS
We detect 1 out of 11 BA stars, and 4 out of 26
KM stars. Our median 3σ sensitivity is 2.8 mJy. We
report observation times, fluxes, and 3σ upper limits
in Table 3. We also include fluxes from the literature
for [PBB2002] USco J160823.2-193001, [PBB2002] USco
J160900.7-190852, and [PZ99] J161411.0-230536.
Overall, millimeter observations exist for 9 of the 10
CTTS in the original sample, of which 4 are detected.
18 of the 95 WTTS have been observed for millimeter
emission, with 3 detections, and the single millimeter-
observed M star lacking Hα information is not detected.
Examining disks by NIR classification, 6 Class II
sources are detected in millimeter emission. The addi-
tional two millimeter-detections are the debris disk of
HIP 76310, and the transition disk of [PZ99] J160421.7-
We show the SEDs of the 8 millimeter detected Up-
per Sco sources in Figure 1. We also display the upper
quartile, median, and lower quartile SEDs of disk bearing
sources in the younger Taurus association (Furlan et al.
2006) for comparison. Most sources lie in the lower quar-
tile of disk emission in Taurus, implying extensive evo-
lution or settling of the dust. For the object [PBB2002]
USco J161420.3-190648, the photospheric model appears
to be a poor fit at short wavelengths. This source is likely
being viewed close to edge on.
3.1. Mass estimates
Dust masses were estimated from the 1.2 mm flux
density assuming the dust to be optically thin, and are
listed in Table 3. Under this assumption, the dust mass
(Mdust) will be directly proportional to the flux (Fν), as
in Hildebrand (1983):
Assuming a dust mass opacity (κν) of 10 cm2/g at 1000
GHz and opacity power law index β=1 (Beckwith et al.
1990), as well as a characteristic dust temperature (Tc)
of 20 K (the median temperature of Taurus disks;
Andrews & Williams 2005), we can then estimate dust
masses as Mdust= 1.5 × 10−3F1.2mm(mJy) MJupat the
145 pc distance of Upper Sco. For the two objects ob-
served at 1.3 mm, [PBB2002] USco J160823.2-193001
and [PBB2002] USco J160900.7-190852, we must adopt
a different constant: Mdust = 1.9 × 10−3F1.3mm(mJy)
MJup. While there is considerable theoretical uncer-
tainty in these terms, the values are consistent with pre-
vious disk studies. The addition of far-infrared photome-
try will allow for detailed modeling of the dust structure
of the disk and an improved dust mass estimate (e.g.
Pinte et al. 2006), while multi-line spectroscopy is neces-
sary to constrain the gas mass (e.g. Kamp et al. 2011).
These are the subject of a future paper.
Our median 3σ sensitivity of 2.8 mJy corresponds to
an estimated dust mass sensitivity of 4.2×10−3MJup.
In order to estimate an upper limit to the average dust
mass for non-detected BA and KM stars, we stack our
non-detections. If many sources have mm-emission just
below our 3σ limits, then we would expect stacking to
result in a measurable flux. However, this procedure re-
sults in a non-detection for both the stacked BA and KM
observations. The 3σ upper limits of the stacked obser-
vations, 0.89 mJy for BA stars, and 0.73 mJy for KM
stars, do not differ from what would be expected from
the noise in a single observation of the combined observa-
tion times. These upper limits suggest that on average,
BA and KM stars have dust masses less than 1.3×10−3
and 1.1×10−3MJup, respectively.
4.1. Comparison of detected and non-detected sources
The millimeter-detected KM stars exhibit Hα emission
similar to the non-detected KM stars. For each star,
we calculate the mean Hα equivalent width from values
in the literature, and show these in comparison to the
dust masses in Figure 2. We show sources with Hα in
absorption as having an upper limit to emission of 0.1
˚ A. We calculate the Kendell’s rank statistic, generalized
for a censored data set (Isobe et al. 1986), for the KM
4 Mathews et al.
stars and find a Z-value of 0.028, indicating a 98% chance
there is no correlation between the Hα equivalent widths
of Upper Sco KM stars and their disk dust masses. Con-
sidering just the mm-detected sources, there is ambigu-
ous evidence for an anti-correlation (Z-value of 1.05, 29%
chance of no correlation).
To carry out detailed comparison of the SED shapes,
we plot the SED indices from 2.2 to 8 microns and 8 to
24 microns in Fig. 3, defining the SED index n1−2 =
λ1) as in Furlan et al. (2006). The cen-
tral cluster of points correspond to Class II sources, while
points along the left hand side show no excess at 8 µm,
indicative of a void inner region. However, some of these
sources exhibit 24 µm excesses indicative of some amount
of cooler dust which may either represent transition or
debris disks. The majority of millimeter-detected sources
in Upper Sco exhibit NIR SEDs typical of other Upper
Sco Class II sources.
While the NIR and Hα properties of the majority of
millimeter detected sources do not stand out among Up-
per Sco source, disk dust mass does appear as if it could
be correlated with stellar mass. In Figure 4, we show the
dust mass as a function of stellar mass. The highest dust
masses appear clustered around stars of 0.7–1.2 M⊙, sug-
gesting a stellar mass dependence in the evolution of the
mass of millimeter-sized grains in disks.
We calculate the Kendall’s rank correlation statistic
and find that the comparison is ambiguous when consid-
ering the entire sample. With a Z-value of 0.87, there is a
39% chance of rejecting the null-hypothesis that the disk
mass and stellar mass are unrelated. Noting that the BA
stars only host debris disks, we examine the stellar mass
- dust mass relation for KM stars. In this case, the rela-
tionship is clear; with a Z-value of 3.17, there is only a
0.2% chance of disk dust mass not being related to the
stellar mass. In Upper Sco, K stars tend to have disks
with higher dust mass than do M stars. This could re-
flect numerous factors; these higher mass stars may have
emerged from their natal clouds with higher mass disks
or some mechanism may be active in this mass range to
The observation that millimeter detected sources are
not outliers in their NIR and Hα emission suggests that
the process leading to low millimeter emission is not nec-
essarily tied to the loss of disk gas nor small warm grains.
However, the concentration of high dust masses around
near-solar mass stars suggests a stellar mass dependent
mechanism in dust evolution. This expands upon the
results of Carpenter et al. (2006) and Carpenter et al.
(2009), who found evidence that small dust grains de-
plete more rapidly around stars of greater than solar
mass than around stars of near-solar mass or less; our
results suggest millimeter-emitting grains are retained
longer around near-solar mass stars.
4.2. Effects of detectability
The NIR and Hα classification systems we use above
serve as indicators for the presence of dust and gas com-
ponents of disks. While in younger groups the two prop-
erties are strongly correlated, in Upper Sco the major-
ity of Class II systems show no evidence for accretion.
The statistics of our observations and data collected from
the literature indicate that we are seeing the tail-end of
the primordial disk stage. All disks hosted by BA stars
appear to be debris disks. Only 22 out of 127 (17%)
KM stars show any evidence for remaining primordial
disk material (either NIR excess, Hα emission, or mm-
emission), and only 7 (6%) have disks detected in mil-
These remaining primordial disks appear to be in the
last stages of dissipation — their SEDs lie below the Tau-
rus median, indicating significant inner disk dust evolu-
tion or flattening of the disk; they generally have low
Hα equivalent widths, suggesting depleted gas reservoirs
for sustained accretion; and their low mm fluxes indicate
that the mass in millimeter-sized and smaller grains has,
for the most part, decreased to the edge of detectability
with current facilities.
One must keep in mind, however, that each of these
measures reaches a different mass sensitivity. Because
of this, one should exercise caution in interpreting the
statistics regarding Class II / Class III sources and CTTS
/ WTTS sources.
Our millimeter observations reach a dust mass sensi-
tivity of ∼ 4×10−3MJup, while near-IR observations are
sensitive to dust masses of ∼ 10−5MJup(e.g. Cieza et al.
2007). These correspond to disk mass sensitivities of 0.4
MJupand 10−3MJup, assuming the ISM gas-to-dust ra-
tio of 100 (e.g. Beckwith et al. 1990).
To estimate an equivalent disk mass sensitivity for Hα
observations, we use the empirical relation of accretion
rate as a function of time of Hartmann et al. (1998).
Modeling the accretion rate (˙M) as a power law in time
(˙M ∝ t−η, with stellar age t and η ≈ 1.5 − 2.8), one
can estimate the remaining disk gas mass as Mgas ≈
(0.5 − 2) ×˙M(t) × t. The Wλ(Hα) cutoffs for classi-
fying systems as accreting roughly correspond to accre-
tion rates of 10−10M⊙/yr (e.g. Dahm 2008). Thus, for
our Upper Sco sources with t = 5 Myr, Wλ(Hα) ob-
servations provide an estimated disk mass sensitivity of
∼ (0.3−1)×10−3M⊙. While this estimate spans a large
range of values, and neglects many of the details of ac-
cretion (e.g. accretion variability, observation geometry),
it illustrates that Hα observations are likely sensitive to
disks of similar mass as those detected by our millimeter
photometry. Both Wλ(Hα) and millimeter photometry
based estimates of the disk mass are at least 2 orders of
magnitude less sensitive than near-IR observations.
Therefore, one should expect the presence of numerous
systems exhibiting infrared excesses, but lacking one or
both of detectable Hα emission and detectable millimeter
emission. This is indeed the case in Upper Sco, with
only 4 sources exhibiting all three disk tracers, while 80%
of accreting sources and all millimeter detected sources
exhibit an excess at 24 µm and shorter wavelengths.
4.3. Notes on selected sources
HIP 76310: The sole mm-detected A star in our
sample, HIP 76310, has the brightest infrared excess
of the BA stars in Upper Sco.
to stellar luminosity ratio (LIR/L∗
Dahm & Carpenter 2009) falls within the observational
definition of a debris disk (LIR/L∗ < 10−3). Our as-
sumption of 20K for the average dust temperature of
primordial disks is based on KM stars in Taurus and
does not apply to the case of a debris disk around an
However, its infrared
= 2.1 × 10−4,
Millimeter Survey of Upper Sco5
A star. The SED fit by Dahm & Carpenter implies a
much higher dust temperature of 122K and thus a cor-
respondingly lower dust mass (a factor of ∼8). Both our
initial mass estimate (3.6×10−3MJup) and revised dust
mass of ∼ 5 × 10−4MJup are several orders of magni-
tude higher than the lower limit of 2.6×10−7MJupfound
by Dahm & Carpenter, highlighting the high NIR opti-
cal depth achieved by small quantities of dust, and the
importance of (sub-)millimeter observations to directly
constrain the dust mass. This dust mass also fits the
evolutionary trend of debris disk dust masses as a func-
tion of time as shown in Williams & Andrews (2006).
J160421.7-213028 stands out not only for having a
uniquely steep infrared SED in Upper Sco, but also for
having the largest millimeter emission. The SED shape
for this object is typical of a transition disk, where dust
grains have been cleared from the inner disk but are
still abundant at larger radii.
emission implies a large dust mass (0.10 MJup). How-
ever, this source is not clearly accreting; the Hα equiv-
alent width of -0.57˚ A suggests that some process has
greatly reduced or cut off accretion to the stellar surface.
By current disk evolution models, the two most likely
candidate processes are the onset of photoevaporation
(Alexander et al. 2006) or the formation of a planet large
enough to reduce stellar accretion below our detection
limits (Alexander & Armitage 2007). Detailed examina-
tion of this disk requires millimeter imaging, which we
defer to a later paper.
Binaries: ScoPMS 31 and [PZ99] J161411.0-230536
are detected in millimeter continuum and exhibit pri-
mordial type infrared excesses, yet have binary compan-
ions at projected separations of 84 and 32 AU, respec-
tively (K¨ ohler et al. 2000; Metchev & Hillenbrand 2009).
There is ambiguity between observing a circumstellar or
circumbinary disk; both cases allow for the presence of
cold grains at large radii. While the question of the effect
of binarity on disk mass and lifetime is worth consider-
ing, our numbers here are too small to provide a useful
The K2 star [PZ99]
The strong millimeter
5. DISK EVOLUTION
We can investigate the evolution of observable disk
properties from 1 to 5 Myr by comparing the properties
of KM sources in Upper Sco with those in Taurus (mean
age 1 Myr, Andrews & Williams 2005, AW05), and in
IC348 (mean age ∼2.5 Myr; Lee et al. 2011, LWC11).
These studies are among the largest millimeter surveys at
younger ages, for populations that have served as bench-
marks in many disk studies. The Upper Sco and Tau-
rus samples are readily comparable, as our study has
similar dust mass limits to AW05 (4 × 10−3MJup ver-
sus 6×10−3MJup). Comparisons with IC348 (dust mass
limit of 2×10−2MJup) will necessarily focus on the high
mass end of the dust mass distributions. We examine
trends in disk mass as a function of disk accretion and
NIR excess, and the implications for planet formation.
5.1. Accretion status
At younger ages accreting systems dominate the pop-
ulation of sources with millimeter continuum detections.
In Taurus, 67 out of 76 (88±11%) millimeter detected
sources with Hα measurements in AW05 are accreting,
while in IC348, all 9 mm-detected sources are accreting
(LWC11). There is no such clear distinction in Upper
Sco where, of the 7 millimeter detections, 4 are accreting
and 3 are not.
Furthermore, as noted in Section 4.1, the Hα distri-
butions of millimeter detected and non-detected sources
in Upper Sco are similar. This is a marked contrast to
the younger IC348, where millimeter-detected disks are
biased towards larger Hα equivalent widths than non-
Examining the detection statistics in greater detail, we
find that the disk detection rate among CTTS in Upper
Sco is 44±22%, significantly lower than that in Taurus.
The detection rate for WTTS is similar. In Table 4, we
show contingency tables constructed for detections and
non-detections of CTTS and WTTS in the two regions.
Using the Fisher exact test, we find for a two-tailed distri-
bution the mm-detection rate among CTTS has ∼ 0.25%
chance of being independent of region; the apparent drop
in mm-detection rate for CTTS we see in Upper Sco is
significant. For WTTS, the two-tailed Fisher’s exact test
indicates that the mm-detection rate for WTTS remains
the same between Taurus and Upper Sco.
mm-detection rate for WTTS has changed little from 1
to 5 Myr, the detection rate for CTTS has decreased
5.2. Dust mass distribution
The millimeter-detection rate for Class II sources drops
significantly from 1 to 5 Myr. In Taurus, 86% of Class
II sources have millimeter detections (AW05), while in
Upper Sco that fraction has dropped to 32±13% (6/19).
Furthermore, the median disk mass decreases. Taurus
Class II disks have a median dust mass of 3×10−2MJup,
while in Upper Sco the median dust mass of detected
disks is 1.3×10−2MJup. The true median is likely to be
lower, due to the high censoring rate in our data.
The Class III sources in Upper Sco are unlikely to host
additional massive disks. We did not detect any of the
9 Class III disks in our survey, and the inferred upper
limit to the detection rate of <11% is consistent with
the detection rate for Class III sources in AW05, 7±4%.
As with the accretion state, we construct contingency
tables of the detection statistics of Taurus and Upper Sco
Class II and Class III sources. We show the number of
detections of each type of object in Taurus and Upper
Sco in Table 4. As with CTTS, we find that the Class II
sources are detected at a lower rate in Upper Sco than in
Taurus. The detection rate for Class III sources, on the
other hand, is not clearly distinguishable.
Due to the variation in exposure times in our observa-
tions, and hence sensitivities, we use the Kaplan-Meier
product limit estimator for randomly censored data sets
(Feigelson & Nelson 1985) to generate the cumulative
dust mass distribution functions, F(≥MDust), for Class II
sources in these regions (Fig. 5). The dust mass distribu-
tion of Upper Sco Class II sources is clearly different from
that of Taurus Class II sources, with the upper quartile
of Upper Sco sources overlapping the lowest quartile of
Taurus sources; 27±10% of Upper Sco sources have disks
with a dust mass greater than or equal to 4×10−3MJup,
compared to 89 ± 4% of those in Taurus. We make use
of the Gehan, Peto-Prentice, and log-rank statistics to
6 Mathews et al.
compare the highly censored dust mass distributions of
Upper Sco and IC 348; the distributions are different,
with mild statistical significance (there is only a 2.5 –7%
chance that the distributions are drawn from the same
5.3. Disk dissipation timescale
Having established that the dust mass distribution
has changed from 1 to 5 Myr, we examine the rate at
which the highest dust masses change. Comparing sim-
ilarly constructed mass distributions, LWC11 find that
the fraction of IC348 disks with Mdust greater than
2 × 10−2MJupis approximately equal to the fraction of
Taurus disks with Mdust greater than 40 × 10−2MJup,
suggesting a systematic factor of 20 decrease in the dust
mass of Class II sources from 1 Myr to 2.5 Myr. If this
trend were to continue, one would not expect to find any
disks with millimeter continuum emission in Upper Sco.
However, our results indicate dust masses do not decline
so greatly from 2.5 to 5 Myr; the fraction of disks with
Mdust > 2 × 10−2MJup is similar between IC348 and
Upper Sco, and the highest masses for Class II disks in
Upper Sco are lower than masses of IC348 disks by only
a factor of∼< 2.
These results indicate that for Class II sources, the
observed dust mass rapidly declines from 1 to 2.5 Myr,
but may only slowly change from then until 5 Myr. In
contrast, the fraction of disks exhibiting IR-excess con-
sistent with primordial dust continues its marked de-
cline. In Taurus, 50% of KM stars are Class I or Class
II (Luhman et al. 2010). We include Class I sources here
as their disks consist of primordial material. In IC348,
only 39% of KM stars are Class II, with 1% of sources
being Class I (Spitzer photometry of Lada et al. 2006;
Muench et al. 2007). In Upper Sco the Class II fraction
is 15%, with no Class I sources.
While there is no clear evidence for a large age spread
among Upper Sco stars, Preibisch et al. (2002) could not
rule out a 1–2 Myr age spread. This allows for the possi-
bility of some overlap in stellar ages with the stars of IC
348 (where there may be an age spread of several Myr).
It is possible that the mm-detected sources in Upper Sco
are also the youngest sources. If these sources were only
marginally older than the sources observed in IC 348, the
small decrease in the maximum mm-emission for Class
II disks would be consistent with a continuation of the
steep drop in mm-emission seen in comparing Taurus to
However, this argument could be equally applied to
IC 348 and Taurus, with the corresponding claim that
only the youngest disks are detected in the survey of
LWC11, and the most massive disks in Taurus are also
the youngest. Assuming similar age spreads in these re-
gions, this scenario would not change the general picture
we find of rapid change in the greatest disk masses over
several Myr, followed by a more gradual decline.
There have been recent attempts to trace the evolu-
tion of disk properties as a function of stellar age (e.g.
Isella et al. 2009; Guilloteau et al. 2011). The size of
star forming regions, however, suggest distance uncer-
tainties to individual stars large enough to cause 30% or
greater uncertainties in luminosity estimates; other fac-
tors (e.g. photometric errors) can make this uncertainty
even higher. In turn, 30% luminosity uncertainties are
large enough to cause age uncertainties of several Myr
(Hillenbrand et al. 2008). Until the age of individual
stars can be accurately determined, interpretation may
need to be restricted to consideration of average ages.
Comparison of HR diagrams for these regions indicates
that the median age in Upper Sco is older than Taurus
and IC 348 (see Figure 1 of Hillenbrand et al. 2008). The
comparison between Taurus and IC 348 is more ambigu-
ous; across all temperatures, the median sequences could
be equivalent within the luminosity errors. However, at
temperatures of logTeff less than 3.625 (corresponding
to a spectral type of about K6), the median Taurus se-
quence has luminosity equal to or higher than the IC 348
sequence, suggesting that the median Taurus age for KM
stars is likely younger than IC 348. These interpretations
of the HR diagrams are consistent with the decrease in
Class II and CTTS fraction seen from Taurus to IC 348,
and then further to Upper Sco. While the uncertainty in
absolute ages of star forming regions, age spread within
those regions, and ages of individual stars complicate in-
terpretation, the overall trend is clear.
Significant progress has been made in recent years
in modeling the dust evolution in disks. The amount
of millimeter sized grains will decrease over time as
dust aggregates to form centimeter and larger bod-
ies, to which millimeter photometry is insensitive
(Dullemond & Dominik 2005).
modeled mm-slopes and fluxes in disks where millimeter-
size grains remained present due to an equilibrium state
between coagulation and fragmentation. Their models
are consistent with observations of young disks at 0.5–
2 Myr, and later work (Birnstiel et al. 2011) shows the
same mechanisms can maintain populations of micron-
sized grains that produce NIR excess emission. Observa-
tions of millimeter continuum emission from older disks,
such as this work, provide important constraints on the
later time evolution of dust, and perhaps constrain the
mechanisms leading to loss of observable dust (e.g. radial
drift, agglomeration into centimeter and larger grains).
In summary, there appears to be a rapid evolution of
disks from 1 to 2.5 Myr, an interval during which both
millimeter-dust masses and disk fractions rapidly drop.
While the infrared disk fraction continues to decrease
from 2.5 to 5 Myr, the dust masses of the remaining
disks do not rapidly change. In addition, disks appear
to reach a low baseline of dust mass where there is no
longer a correlation between dust and inner disk gas.
Birnstiel et al. (2010b)
5.4. Implications for planet formation
In addition to providing a point of comparison for the
evolution of circumstellar disk mass as a function of age
and inner disk properties, our disk dust mass measure-
ments provide new insights on giant planet formation.
Under the assumption of a primordial 100:1 gas-to-
dust mass ratio (e.g. Beckwith et al. 1990) as if planet
formation has yet to begin, Upper Sco retains only a sin-
gle known Minimum Mass Solar Nebula disk (MMSN, 10
MJup, Weidenschilling 1977), the 10 MJup disk around
[PZ99] J160421.7-213028. If planet formation were to
begin now in Upper Sco, then the region could not form
Jupiter mass planets at the frequency observed in the
field (∼10%, Johnson 2009). However, the disks in Upper
Sco are clearly evolved, and the dust has grown substan-
Millimeter Survey of Upper Sco7
tially. Jupiter mass planets must have already formed, or
be well on their way to formation, as seen in the younger
sources of IC348 (LWC11). The discovery of planetary
companions to the Upper Sco members 1RXS J160929.1-
210524 and GSC 06214-00210 (Lafreni` ere et al. 2008;
Ireland et al. 2011) serve as confirmation to this scenario.
Due to our greater sensitivity, however, we can push this
argument further to examine the formation of Neptune
At the 5 Myr age of Upper Sco, large rock and ice
protoplanets may have formed, or may be in the pro-
cess of formation. There are still uncertainties in the
growth of grains from centimeter to planetesimal (105
m) sizes, but simulations indicate growth from micron to
centimeter size grains occurs in as little as 2 × 105years
(Birnstiel et al. 2010a), and planetesimals can aggregate
to form protoplanets with enough mass to accrete gas
(∼0.03 MJup) in less than 1 Myr (Pollack et al. 1996).
Under core accretion models, the formation of a gas gi-
ant planet requires that gas be retained in the disk until
the accreting protoplanet reaches the critical mass (∼0.1
MJup) at which runaway accretion begins; this process
could take from less than 1 Myr (Alibert et al. 2005) to
several tens of Myr (Pollack et al. 1996), depending on
several factors such as disk surface density, formation ra-
dius, and the effects of planetary migration.
Debris disks are expected to be seen in systems in the
final stages of giant planet formation, as well as after
the conclusion of giant planet formation. Our results
verify that BA stars in Upper Sco have no residual cold
millimeter-size dust above debris levels; these disks are
unlikely to retain substantial gas mass, signaling the end
of giant planet formation.
However, some lower mass KM stars have NIR or mm
continuum emission and / or Hα emission indicating the
presence of gas and dust. The median inferred mass for
the handful of mm-detected disks, under the assumption
of the primordial 100:1 gas-to-dust ratio, is ∼0.1 MMSN.
By linear scaling of the MMSN, such a disk could pro-
duce a planet approximately twice the mass of Neptune
(MN = 0.05 MJup) if planet formation were to begin
now. Only 4 sources have disks with inferred masses of
0.1 MMSN or greater, implying ∼2% of stars in the 5
Myr Upper Sco group retain disks able to form a 2MN
planet. In comparison, results of the first 4.5 months
of the Kepler mission suggest that up to 19% of field
stars have Neptune-size planet candidates within 0.5 AU
(Borucki et al. 2011). The formation of Neptune mass
planets, in addition to the formation of Jupiter mass
planets, must be at an advanced stage by 5 Myr.
We have carried out the largest survey to date for mil-
limeter continuum emission in a group of 5 Myr stars,
an intermediate age in the evolution from ubiquitous gas
and dust rich primordial disks at 1 Myr to, largely, gas
and dust poor debris disks by 10 Myr. The overallpicture
of Upper Sco is of a region where a handful of primordial
disks remain, with small disk masses, weak NIR excesses,
and low Hα emission. We have found:
1. 5 new 1.2 millimeter detections of 2.4 mJy and
brighter, with estimated dust masses typically an
order of magnitude lower than Taurus disks.
2. The sole Minimum-Mass Solar Nebula equivalent
mass disk is the transition disk of [PZ99] J160421.7-
3. The only BA star exhibiting mm-dust emission is
the debris disk system with the highest 24 µm ex-
cess, HIP 76310.
4. Other than [PZ99] J160421.7-213028 and HIP
76310, only Class II KM stars are detected in mm-
emission. The Class II millimeter-fraction is 32%,
a significant drop from 86% in Taurus.
5. The mm-detection rate for CTTS is 44%, signif-
icantly lower than the detection rate for CTTS
in Taurus. Millimeter detections are roughly split
among accreting and non-accreting sources, sug-
gesting the action of mechanisms that can halt ac-
cretion while preserving mm-sized grains.
6. Dust mass in millimeter-emitting grains appears to
decrease more slowly from 2.5 to 5 Myr than from
1 to 2.5 Myr.
7. We verify that BA stars with debris disk SEDs do
not have large amounts of remnant cold dust, and
find that with only four exceptions, KM stars with
primordial disks lack sufficient mass to form new
Neptune mass or larger planets. If the stars of Up-
per Scorpius are to host giant planets at the same
rate as field stars, then planet formation must be
at an advanced stage.
The authors would like to thank Jagadheep Pandian
for thoughtful feedback on this paper.
J.P.W. acknowledge NASA/JPL and NSF for funding
support through grants RSA-1369686 and AST08-08144
respectively. F.M., G.D., and C.P. acknowledge PNPS,
CNES and ANR (contract ANR-07-BLAN-0221 and
ANR-2010-JCJC-0504-01) for financial support.
acknowledges the funding from the EC 7th Framework
Program as a Marie Curie Intra-European Fellow (PIEF-
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Millimeter Survey of Upper Sco9
Figure 1. SEDs of the millimeter detected sources discussed here, shown with Kurucz photospheric models (Kurucz 1979) and overlaid
with the upper quartile, median, and lower quartile SEDs of Taurus KM stars (Furlan et al. 2006) normalized to the H band flux. Red
circles are dereddened fluxes from the literature (R=3.1, Fitzpatrick 1999), with open squares indicating original measurements. Our
measurements are indicated as filled red squares. Inverted triangles indicate upper limits. 1σ errors are smaller than symbols.
10 Mathews et al.
Upper Sco. The displayed errors on Mdustonly include flux errors; we neglect uncertainties due to adopted opacity and average disk
temperature. There is no overall correlation between disk mass and Wλ(Hα).
Estimated disk mass versus mean Hα equivalent widths for millimeter detected (red) and non-detected KM stars (gray) in
Millimeter Survey of Upper Sco 11
Figure 3. Near– and mid–infrared SED indices, following the example of Furlan et al. (2006). Squares indicate BA sources, circles indicate
KM sources, and plus signs indicate millimeter detected sources. The symbol color indicates accretion status, and “×” marks in the center
and bottom left indicate the expected indices for a uniform flat disk and a stellar photosphere, respectively. The majority of detections
have similar SED shapes to other Upper Sco Class II sources.
12 Mathews et al.
Figure 4. Estimated disk dust mass vs. stellar mass in Upper Sco (5 Myr). Solid circles indicate Class II or transition disks, solid squares
indicate sources with debris disk-like IR excesses, and inverted triangles indicate non-excess sources. Millimeter detected sources are red
symbols, with 1σ error bars.
Millimeter Survey of Upper Sco 13
Figure 5. Cumulative dust mass distribution for Class II disks in Upper Sco, IC348, and Taurus, as well as Class III Taurus disks. The
sample distributions are calculated using the Kaplan-Meier estimator to include 3σ upper limits. The long vertical lines at the left indicate
the lowest mass or upper limit in the sample.
14 Mathews et al.
Name RADec. Sp.Typea
[PBB2002] USco J155624.8-222555
[PBB2002] USco J155706.4-220606
[PBB2002] USco J155729.9-225843
[PBB2002] USco J155829.8-231007
[PBB2002] USco J160210.9-200749
[PBB2002] USco J160245.4-193037
[PBB2002] USco J160357.9-194210
[PBB2002] USco J160525.5-203539
[PBB2002] USco J160532.1-193315
[PBB2002] USco J160545.4-202308
[PBB2002] USco J160600.6-195711
[PBB2002] USco J160622.8-201124
[PBB2002] USco J160702.1-201938
[PBB2002] USco J160827.5-194904
[PBB2002] USco J160900.0-190836
[PBB2002] USco J160953.6-175446
[PBB2002] USco J160959.4-180009
[PBB2002] USco J161115.3-175721
[PBB2002] USco J161420.3-190648
[PBB2002] USco J160823.2-193001
[PBB2002] USco J160900.7-190852
0.96, 0.80, 0.38
aSpectral types from Hern´ andez et al. (2005); Preibisch et al. (1998, 2002)
bWλ(Hα) from Hern´ andez et al. (2005); Preibisch et al. (1998, 2002); Dahm & Carpenter (2009); Riaz et al. (2006)
KM classification summary
Disk classCTTS WTTSUnknown Hα
Note. — Summary of KM star classifications, showing the number of each type in the parent
sample (Carpenter et al. 2006), the number in our study, and the number detected in millimeter
Millimeter Survey of Upper Sco 15
Time on source, measured flux, and 3σ Limits
[PBB2002] USco J160823.2-193001
[PBB2002] USco J160900.7-190852
a1.3 mm flux from Cieza et al. (2008).
b1.2 mm flux from Roccatagliata et al. (2009).
Comparisons with Taurus disks
CTTSWTTS Class II Class III
Note. — Contingency tables for comparing detection statistics of CTTS / WTTS
and Class II / Class III sources in Upper Sco and Taurus. P(fTau= fUS) shows the
result of the two-tailed Fisher exact test for each table, showing the probability for
the mm-detection rate in Taurus being equal to the mm-detection rate in Upper Sco
for each type of object.