Resolved Gas Interior to the Dust Rings of the HD 141569 Disk

0.9 MB·

by  Kevin M. Flaherty

Draftversion January13,2016
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
RESOLVED CO GAS INTERIOR TO THE DUST RINGS OF THE HD 141569 DISK
Kevin M. Flaherty1, A. Meredith Hughes1, Sean M. Andrews2, Chunhua Qi2, David J. Wilner2, Aaron C.
Boley3, Jacob A. White3, Will Harney4, Julia Zachary1
Draft version January 13, 2016
ABSTRACT
ThediskaroundHD141569isoneofahandfulofsystemswhoseweakinfraredemissionisconsistent
6
with a debris disk, but still has a significant reservoir of gas. Here we report spatially resolved mm
1
observationsof the CO(3-2)and CO(1-0)emission as seen with the SMA and CARMA. We find that
0
the excitation temperature for CO is lower than expected from cospatial blackbody grains, similar
2
to previous observations of analogous systems, and derive a gas mass that lies between that of gas-
n
rich primordial disks and gas-poor debris disks. The data also indicate a large inner hole in the
a
CO gas distribution and an outer radius that lies interior to the outer scattered light rings. This
J
spatial distribution, with the dust rings just outside the gaseousdisk, is consistent with the expected
1 interactions between gas and dust in an optically thin disk. This indicates that gas can have a
1
significant effect on the location of the dust within debris disks.

]
R
1. INTRODUCTION byHerschelaroundHD172555(Riviere-Marichalaret al.
S 2012) and HD 32297 (Donaldson et al. 2013). The gas
Debris disks are characterized by weak scattered light
h. and thermal emission from small dust grains encircling revealedby these observationsmay be left overfrom the
p the central star. Most observations focus on this dust primordialdisk,whichisexpectedtodissipateafterafew
- emission (Wyatt 2008; Krivov 2010; Matthews et al. million years (Fedele et al. 2010), or created in a second
o generation process, such as the evaporation of infalling
2014, and references therein) but the presence of gas
r comets (Beust & Valiron 2007), or the collisionalrelease
t can substantially influence the observed dust structures
s ofgasthroughsublimation(Zuckerman & Song2012)or
a (Takeuchi & Artymowicz 2001; Lyra & Kuchner 2013).
vaporization (Czechowski & Mann 2007).
[ While surveys have found that most debris disks have
a negligible gas mass (Pascucci et al. 2006), some sys- HD 141569, a B9.5, 5Myr old star, located 108pc
1 away (Mer´ın et al. 2004), hosts one of the first de-
tems possess significant amounts of gas in addition
v bris disks discovered to have a significant gas reservoir
to their debris-like dust distributions. Radio surveys
2 (Zuckerman et al. 1995). While still fairly young, this
revealed that HD 21997, a 30 Myr old A star, is
4 systemdoes notexhibit astronginfraredexcess;instead
surrounded by a broad disk of CO with a mass of
26 MalsCoOh∼a5s×a10la−r2geMri⊕ng(Kof´osCpOa´leemt iasls.io2n0,1w3)ithwhailme a4s9s eCsetti-i itthsindudsetbreimsidssisioknwiisthmLorIeR/cLh∗ara8cte1r0is−t3ic(oSfylavnestoeprtiectalally.
1.0 ombasteerdvaattioMnsCOof∼β10P−i3cMfi⊕nd(HCuOghgeassetcoanl.fin2e0d08t)o. AaLbMelAt, 1fi9n9e6d).toStcwatotenraerdrolwighrtinigmsaagtes24fi∼0nda×nsdm4a0ll0dauust(Bgrilaleinrsectoanl-.
0 with clear asymmetries, and a mass of only 2 10−5 2015),withagapbetweenthem(Weinberger et al.1999;
6 M⊕ (Dent et al. 2014). Recently HD 131835 w×as dis- Augereau et al. 1999) and evidence for spiral arms in
1 the outermost ring (Clampin et al. 2003). Interior to
covered to have CO emission consistent with M
v: 4 10−4 M⊕ (Mo´or et al. 2015). In addition to tChOes∼e these rings there is very little scattered light emission
× from small dust grains, although mid-infrared thermal
i radio observations, optical surveys have revealed gas in
X dust emission has been detected down to tens of au
thethedebrisdisksystemsHR10(Lagrange-Henriet al.
(Fisher et al. 2000; Marsh et al. 2002; Moerchen et al.
r 1990), HD 32297 (Redfield 2007), 49 Ceti and ι Cyg
a 2010). The weak dust emission and ring features are
(Montgomery & Welsh 2012), HD 2160, HD 110411,
typical of a debris disk, but the strong CO emission, in-
HD 145964, HD 183324 (Welsh & Montgomery 2013)
dicatingasubstantialgasmass,isnot(Zuckerman et al.
and HD 17255 (Kiefer et al. 2014). β Pic has
1995). The presence of a gas reservoir has implications
been extensively studied using FUV/optical gas trac-
for the origin and evolution of the dust within this sys-
ers (e.g. Petterson & Tobin 1999; Roberge et al. 2000;
tem. Cospatialgasanddustcanleadtosharpringswith-
Brandeker et al. 2004; Roberge et al. 2006) revealing a
out the need for shepherding planets (Lyra & Kuchner
variablegascomponentduetotheevaporationofcomet-
2013), while collisions with gas as dust is pushed out-
like bodies as they approach the central star. Far-
wards by radiation pressure can lead to a ring at the
infrared gas emission lines have also been detected
outer edge of the gaseous disk (Takeuchi & Artymowicz
2001).
1Van Vleck Observatory, Astronomy Department, Wesleyan
Here we present radio interferometric observations of
University,96FossHillDrive,Middletown,CT06459
2Harvard-Smithsonian Center for Astrophysics, 60 Garden the CO(3-2)andCO(1-0)emissionatspatialresolutions
Street, Cambridge,MA02138 of 1′.′5 and 4′.′5 respectively. The high spatial reso-
3DepartmentofPhysicsandAstronomy,UniversityofBritish ∼ ∼
lution of these observations allow us to measure the lo-
Columbia,Vancouver BC,Canada
4DepartmentofPhysicsandAstronomy,UnionCollege,Sch- cation of the gas, as well as its overall temperature and
enectady, NY density structure. In Section 2 we present the observa-
2
tions, and in Section 3 we present our model and fitting April track and used to calibrate the bandpass; since no
procedure. InSection4wepresentourresults,inpartic- standardpassbandcalibratorwasobservedduringthe19
ularthefindingthatthegashasalowexcitationtemper- April track, we used the quasar J1512-090 as the pass-
ature and is confined to within the scattered light rings, band calibrator and obtained satisfactory solutions.
while in Section 5 we discuss the implications of these Forbothnights,thefirstlocaloscillatorfrequencywas
results for debris disk structure and evolution. set to 108.0314GHz. Two spectral windows were tuned
respectively to CO(1-0) and 13CO(1-0) rest frequencies
2. OBSERVATIONSANDRESULTS of115.27GHzand110.201GHz. Thesespectralwindows
We observed HD 141569 with the Submillimeter Ar- wereobservedataspectralresolutionof0.488MHz,with
ray (SMA) over the course of two nights in 2012. On 28 thelineplacedintheuppersideband. Theremainingsix
April2012weutilizedthearray’scompactconfiguration, spectral windows were each set the maximum available
including six of the array’s eight antennas spread across bandwidth of 0.468GHz to maximize continuum sensi-
projected baseline lengths ranging from 12 to 86m. The tivity, resulting in an aggregate bandwidth of 5.6GHz
weather was excellent, with the 225GHz atmospheric across the lower and upper sidebands.
opacity holding steady between 0.03 and 0.05 through- Both lines are clearly detected at high signal-to-noise
out the night. Both Titan and Neptune were used as in our data (Table 1). The moment maps for CO(3-
flux calibrators, allowing us to derive a flux density of 2)and CO(1-0),derivedusing the MIRIAD (Sault et al.
1.74Jy for the quasar J1512-090,which was used as the 1995) task moment and shown in Figure 1, demonstrate
primarygaincalibrator. Observationsofthe sourcewere that the spatially resolved CO(3-2) emission is confined
interleavedwithbriefobservationsofbothJ1512-090and to within the scattered light rings. A uniform disk fit
J1549+026, which was included to test the atmospheric to the CO(3-2) emission, using the MIRIAD task imfit,
gain transfer to the source, over a series of 13.5-minute derivessanouterradiusof290aufortheCOgas,similar
loops. Passbandcalibrationusingthe quasar3C279(de- to the locationofthe inner dust ring(Biller et al.2015).
rived flux 13.7Jy) occured for one hour at the start of This is similar to the outer radius derived from the CO
the track. On 10 Feb 2012 we observedHD 141569with lineprofileinsingle-dishobservations(Dent et al.2005).
the SMA in its extended configuration, including seven Our integrated flux measurements are similar to those
ofthearray’seightantennasspanningprojectedbaseline of previous CO(3-2) observations of the HD 141569disk
lengths of 22 to 252m. The weather was good, with the (Zuckerman et al.1995;Dent et al.2005). Theimproved
225GHz opacity steady at0.05for most ofthe night, al- spatial resolution of uniform weighting of the CO(3-2)
though puncutated by a brief period with values as high data (1′.′2 0′.′6 vs 1′.′5 1′.′0 for natural weighting) re-
× ×
as 0.12for approximatelytwo hoursin the middle of the veals a double-peaked structure indicative of an inner
10-hourtrack. Weusedthesamecalibratorsandobserv- holeintheCOgas(Figure1). The twopeaksinthemo-
ing strategy as in April and derived a flux of 1.47Jy for ment0mapareseparatedbyroughlyonebeamFWHM,
the quasar J1512-090 using only Titan as the flux cal- whichimplies a radiusfor the inner hole of<50 au, con-
ibrator. All SMA and CARMA flux measurements are sistent with the 11 au inner hole inferred using spectro-
subjecttoa20%systematicfluxuncertaintythatresults astrometric measurements of mid-infrared CO emission.
fromuncertainties inthe flux models of the solarsystem Withcleardetectionsoftwospectrallines,wehavead-
objects used to derive the flux scale. ditionalinformationonthetemperatureofthegas. Their
For both nights the receivers were tuned to the CO(3- flux ratio of 10 1 implies an excitation temperature of
±
2) rest frequency of 345.79899GHz. The CO(3-2) line 17 2 K, assuming both are optically thin, which indi-
±
was observed in the center of the lower 2GHz of the 4 catesthatthe gasis verycold. TheCO(1-0)flux implies
GHz-wide upper sideband, with a spectral resolution of a CO mass of MCO = 1.05 0.06 10−8 M⊙, assuming
256channelsacrossthe104MHzcorrelator“chunk”: the a temperature derived from±the lin×e ratio, which is simi-
remainderoftheeffective4GHzbandwidthwasobserved lar to 49 Ceti, HD 21997 and HD 131835 (Hughes et al.
atalowerspectralresolutionof64channelsper104MHz 2008; K´ospa´l et al. 2013; Mo´or et al. 2015). This corre-
chunkandsubsequentlyaveragedtogethertoformasin- spondsto Mgas=1.05 0.06 10−4M⊙=0.11 0.01Mjup,
gle continuum channel with an effective bandwidth of assumingCO/H =10±−4, wh×ichis notenough±to createa
2
approximately 8GHz.
Jupiter-sizedgasgiant,butislargerthantypicallyfound
Observations of the HD 141569 system with CARMA
among debris disks (Pascucci et al. 2006). This mass
took place over the course of two nights in April 2012.
fallswithintherangeinferredbyZuckerman et al.(1995)
On 17 April 2012 we observed the source for four hours
from single-dish CO observations (Table 2). These sim-
using CARMA’s D configuration with maximum base-
ple estimates paint a picture of a radially extended cold
line length 146m. The weather was very good for 3mm
gas disk whose mass is still significant, although it is di-
observations at the Cedar Flat site, with the 230GHz
minished relative to younger systems.
opacity averaging 0.19 over the course of the night and
Inaddition to the gas,we alsodetect continuumemis-
the sky RMS averaging 219µm. We continued D con-
sion from the dust at 870µm, with peak S/N 6, and
figuration observations for an additional four hours on ∼
marginallydetect the dust at2.8mm, with peak S/N 4.
the night of 19 April 2012. The weather remained sta- Ourobservedradioemissionisunresolvedinour1′.′5 ∼1′.′0
ble,withthe230GHzopacityaveraging0.23andthesky ×
beam,suggestingthatthemm-sizeddustisconcentrated
RMS averaging 146µm. Mars was used as the primary
in the inner disk. Using the MIRIAD task uvfit we fit
fluxcalibratorforbothnights. Wederivedafluxdensity
a Gaussian to the short baseline 870µm visibilities and
fortheprimarygaincalibrator,thequasarJ1549+026,of measure a total flux of 8.2 10−3 Jy (Table 1). Previ-
1.48Jy on 17 April and 1.42Jy on 19 April. The quasar ×
ous single-dish measurements of the continuum measure
3C279wasobservedfor15minutes atthe startofthe 17
3
Table 1
EmissionProperties
CO(3-2) CO(1-0) 870µm 2.8mm
Channelwidth(kms−1) 0.35 1.3 - -
Beamsize(FWHM)a 1′.′51×1′.′02 4′.′8×4′.′0 1′.′66×1′.′16 5′.′1×4′.′2
P.A. -83.0◦ 28.6◦ -82.2◦ 29.1◦
rms(mJybeam−1) 81 31 0.7 0.3
Integrated intensity 15.8±0.6b Jykms−1 1.6±0.2b Jykms−1 8.2±2.4c mJy 0.78±0.3d mJy
PeakFlux(Jybeam−1) 1.39 0.31 4.1×10−3 1.1×10−3
a Derivedassumingnaturalweighting
b Derived by integrating the moment 0 map, within the 3σ contour. Uncertainty is the 1σ noise on the
integratedintensity.
c Derived byfitting an elliptical gaussian to the short-baseline visibilities(uv<50kλ) usingthe MIRIAD task
uvfit. Uncertaintyisthe1σ noiseontheintegrated intensity.
d DerivedbyfittingapointsourcetothevisibilitiesusingtheMIRIADtaskuvfit. Uncertaintyisthe1σ noise
ontheintegratedintensity.
4 4.5 4 4.5 4 3.9
3.0 3.0 2.6
2 2 2
Dec (")∆−02 -011..05.5Velocity (km/sec) Dec (")∆−02 -011..05.5Velocity (km/sec) Dec (")∆−02 -011..03.3Velocity (km/sec)
-3.0 -3.0 -2.6
−4 -4.5 −4 -4.5 −4 -3.9
−6
4 2 0 −2 −4 4 2 0 −2 −4 4 2 0 −2 −4 −6
∆RA (") ∆RA (") ∆RA (")
Figure 1. (Left) CO(3-2) moments zero(total intensity, black contours) andone (intensity-weighted velocity, filledcontours). Intensity
contours are in units of 3σ, 5σ, 7σ,... (σ=0.24Jy/beam). The two dashed circles mark the locations of the two rings seen in scattered
light(Billeretal.2015). TheCOgasisnotcospatialwiththesmalldustgrains,butisinsteadlocatedwithintheinnerdisk. (Middle)CO(3-
2)momentzeroandonesmapswithuniformweighting. Whilethenoiseishigher(σ=0.32Jy/beam)thehigherspatialresolutionhighlights
thedouble-peakedstructure,whichisindicativeofaninnerholeintheCOemission. (Right)CO(1-0)momentszero(totalintensity,black
contours) and one (intensity-weighted velocity, filledcontours). Intensity contours are in units of 3σ, 5σ, 7σ,... (σ=0.12Jy/beam). In all
panelstheblackhorizontalbarmarksalengthscaleof100auatadistanceof108pc,whilethecircleinthelowerrightindicatesthebeam
size.
fluxes of 10.9 1.3 mJy at 850µm (Sandell et al. 2011) systematic uncertainties due to the assumptions about
±
and 12.6 4.6mJy at 870µm (Nilsson et al. 2010). Our dust/gas temperature, the dust opacity, the gas to dust
±
870µmcontinnumfluxof8.2 2.4mJyisconsistentwith, mass ratio, the CO/H conversion factor, and the abil-
2
±
althoughslightlysmallerthan,thesemeasurementslikely ityofcontinuumemissionto tracethe fullrangeofgrain
due to missing short-baseline flux. Assuming the dust is sizes. Our dust-based disk mass estimate is larger than
optically thin, our measured flux can be converted to a derived previously (Table 2), Sandell et al. (2011) find
mass of mm-sized dust by Mgas=9 10−5M⊙usingsingle-dish850µmfluxmeasure-
ment w×hile Li & Lunine (2003) derive M =1.1 10−5
gas
M = Fνd2 , (1) M⊙ from SED modeling. The discrepancy cou×ld be
dust κ B (T ) due to differences in the dust temperature, SED mod-
ν ν d
elingofinfraredemissionfindstemperaturesof70-200K
where F is the flux density, d is the distance, κ is the (Li & Lunine 2003) while the single dish measurements
ν ν
massopacityandB (T )istheplanckfunctionevaluated assume 40 K (Sandell et al. 2011), or differences in the
ν d
at a dust temperature of T (e.g. Andrews et al. 2013). assumeddustopacity. Despitethe differencesinthe var-
d
This flux is only sensitive to the small dust grains; in ious studies, and the limited ability of sub-mm emission
a system with collisionally generated dust grains a sub- to trace the totaldust mass,it is clear the HD 141569is
stantial amount of mass is locked in meter to km-sized surrounded a low mass disk of gas and dust.
planetesimals that do not emit efficiently at these wave-
lengths and any mass inferred from the radio emission 3. MODELINGTHEGASEMISSION
underestimatesthetotalmass. Withthiscaveatinmind, While a preliminary analysis of the CO emission indi-
wederiveamassinsmalldustgrainsofM =1.8 10−6 cates a significant mass distributed inside the scattered
dust
M⊙=0.6M⊕ assuminganopacity(1.5cm2 g−1 mo×reap- light rings, detailed modeling is needed to determine
propriate for mm-sized dust grains in protoplanetary the uncertainties on this structure. To characterize the
∼
disks, and the temperature (51 K) of a blackbody dust emission in greater detail we employ a simple paramet-
grain at 150 AU from the central star. Assuming a gas ric model and derive the posterior distribution functions
to dust mass ratio of 100, this implies M =1.8 10−4 (PDFs)foreachmodelparameter,usingaMarkov-Chain
gas
×
M⊙, which is similar to the gas mass derived from the Monte-Carlo technique. Our model of the temperature
CO(1-0) emission, although both are subject to large structure is a radialpower law with an isothermal verti-
4
Table 2
DiskMassMeasurements
Mdisk(M⊙) Method Reference
6-140×10−5 Single-dishCOa Zuckermanetal.(1995)
1.1×10−5 SEDmodelingb Li&Lunine(2003)
2.4×10−4 Radiativetransfermodellingc Jonkheidetal.(2007)
9×10−5 dustcontinuumd Sandelletal.(2011)
2.5-5×10−4 Radiativetransfermodellingc Thietal.(2014)
1.8×10−4 dustcontinuumd thispaper
1.05×10−4 CO(1-0)a thispaper
3.8+−125.8×10−5 MCMCmodeling thispaper
Note. —EstimatesofthegasmassofthediskaroundHD141569A
aGasmassisderivedassumingtheobservedlineisopticallythinandtracers
theentireH2 masswithaconstant CO/H2 abundance.
b Gas mass is derived from fitting the full SED with a dust model, and
extrapolatingtothegasmass
c Gas massisderivedusingradiativetransfermodel fittingtomultiplegas
anddustemissiondiagnostics
d Gas mass is derived from the mm dust continuum emission, assuming a
gastodustmassratio
cal profile. coefficient and S (s) is the source function. The line is
ν
r q assumed to be a Gaussian with a width given by ther-
T (r)=T . (2)
gas 0(cid:16)150 au(cid:17) malmotion,withzeroturbulentmotion. Wealsoassume
the line is in Local Thermodynamic Equilibrium (LTE),
We assume that the surface density is a power law,
an assumption that is consistent with the observed disk
with index γ, between R and R .
in out properties, as we confirm below. The disk structure and
M (2 γ) radiative transfer calculation is carried out using code
Σgas(r)= 2π(Rg2a−sγ −R2−γ)r−γ. (3) originallydevelopedin Rosenfeld et al. (2012, 2013) and
out − in Flaherty et al. (2015) with modifications for our simpli-
fied disk structure. The central star is assumed to lie
Inaverticallyisothermaldiskthe densityprofileisspec-
ified by ρ(r,z) = ρ(r,z = 0)exp( (z/H)2) where H is at a distance of 108 pc (Mer´ın et al. 2004) with a posi-
− tion angle of 356◦ (Weinberger et al. 2000). Unless oth-
the pressure scale height (H = √2c /Ω) and ρ(r,z =
s erwise specified, we also assume a stellar mass of 2 M⊙
0) = Σ (r)/(√πH). The isothermal sound speed is
gas (Mer´ın et al. 2004). The disk emission in the SMA data
given by c2 = k T /µm , and Ω is the Keplerian ro-
s B gas h is offset by [-0.2”,-0.2”]fromthe phase center due to the
tation frequency. The disk is assumed to be made of
proper motion of the system between the J2000 epoch
80% molecular hydrogen by mass with µ = 2.37. We
coordinates and when the data were taken; this offset is
spread CO throughout the disk, with a spatially con-
includedinourmodel. Nospatialoffsetis appliedto the
stant CO to H2 abundance of 10−4. While we assume CARMA data.
that CO traces the entire gas disk, this may not be the
Weusetheaffine-invariantMarkov-ChainMonte-Carlo
case. Freeze-outonto dustgrains andphoto-dissociation
routine EMCEE (Foreman-Mackey et al. 2013) to sam-
by high energy photons can remove CO from the sys- ple the posterior probability distribution. Our initial
tem. While the dust temperature is high enough that
sampling of the posterior distribution functions involves
freeze-out is unlikely (Li & Lunine 2003), photodissoca-
a smaller number of walkers run for a substantial length
tionatthe innerandouter edgesofthe disk maystill be of time (e.g. 80 walkers over 1000 steps), followed by a
a factor. Jonkheid et al. (2007) find that the gas mass
more detailed sampling of the PDFs using more walk-
of the disk around HD 141569 is in a regime where self-
ers, whose starting positions are spread throughout the
shielding begins to set in, resulting in large changes in 3σ ranges defined from the initial run, for fewer steps
CO structure in result to small variations in CO mass.
(e.g. 200 walkers over 300 steps). Initial steps corre-
As discussed below there is evidence of gas within our
sponding to burn-in are removed from the chains. The
derived inner radius based on measurements of ongoing goodness of fit between the model and observed visibili-
accretion, while the morphology of the CO relative to
tiesiscalculatedusingthechi-squaredstatistic,account-
the scattered light rings suggests that little gas extends
ing for the complex visibility weights. Model visibilities
beyond the outermost CO edge. arecalculatedfromthemodelimagesusingtheMIRIAD
Thefluxiscalculatedalongsightlinesthroughthedisk task uvmodel. Our fitting uses both the CO(3-2) and
according to: CO(1-0) lines simultaneously by summing the χ2 from
∞ each line. The starting positions of the walkers during
Iν =Z Sν(s)exp[−τν(s)]Kν(s)ds (4) the initial sampling are chosen to be evenly distributed
0 throughoutparameterspaceoveranareathatcoversthe
where s is the linear coordinate along the line of sight expected best fit (Table 3). Priors are placed on the pa-
increasingoutwardfromtheobserver. Theopticaldepth rameters to ensure physically realistic models, including
is τν(s) = 0sKν(s′)ds′ where Kν(s) is the absorption 0 < Mdisk < M∗, Rin > 0, Rout > Rin, T0 > 0, and
R
5
Interestingly we find that the CO gas does not over-
Table 3 lap with the scattered light rings. These scattered
InitialParameterSpace
light rings are at a radius of roughly 245 au and
400 au (Weinberger et al. 1999; Augereau et al. 1999;
Parameter
Weinberger et al. 2000; Clampin et al. 2003; Biller et al.
-5<log(Mgas(M⊙))<-3 2015), while the 3σ upper limit on the CO gas outer
2<log(Rout(au)) <2.6 radiusis 245au. Forcingthe models to include gas coin-
5au<Rin <60au cident with the scattered light rings, by extending R
10K<T0 <40K out
0.5<γ <2 to 405 au, and re-running the MCMC modeling routine,
50◦ <incl<70◦ produces a significantly worse fit as the gas is forced to
extend further out in the disk (Figure 6). The walkers
are driven towards very steep γ, eventually running into
our arbitrary upper limit of 3 as these models try to ac-
Table 4 commodatethelargeextentofthediskwithaverysteep
ModelFit density gradient that minimizes the flux from the outer
disk. Despite this, they still predict a surface density of
Parameter Result Σ 4 10−5 g cm−2 between the two rings, which is
log(Mgas(M⊙)) -4.4+−00..76 lar∼ge en×ough to produce detectable emission, although
log(Rout(au)) 2.31±0.04 none is seen in the data. The poor quality of these large
Rin 29+−1240 au radius models confirms that there is negligible extended
T0 27+−141 K CO gas at the location of the outer rings.
γ 1.7+−11..01 The marginalized PDF of Rin indicates that we can
incl(◦) 60±3 constrain the inner radius of the CO. Our result is con-
χ2red 1.91a sistent, within the errors,with spectro-astrometric mea-
surements of the CO v=2-1 line which find R =11 2
in
Note. — Median of the au (Goto et al. 2006). Our ability to constrainthe inn±er
marginalized PDF for each
radius comes from the spatial and spectral resolution as
parameter, with 3σ uncer-
tainties that are defined by well as model dependent factors. At uniform weighting,
theareaunder thePDFthat our beam has a FWHM of 0′.′6 along the major axis of
contains 99.7% of the sam- the disk, equivalent to a diameter of 50 au, or a radius
ples. ∼
of 30 au at the center of the disk, which corresponds to
a Reduced chi-squared, sum-
the median of the marginalized PDF. Similarly the line
ming the contribution from
each line, for the model de- wings probe high velocity gas arising from the center of
fined by the medians of the the disk; a projected velocity of 5 km sec−1 corresponds
marginalizedPDFs tomaterialat50au. Partofourconstraintisalsomodel
3 < γ < 3. With only 2-3 resolution elements across
− dependent. In the context of the continuous power law
the radiusofthe disk wedo nothavea strongconstraint
surfacedensityandtemperaturestructuresthat wehave
on the radial profile of the emission. For this reason we
chosen, the flux in the central beam can be reduced by
fix q = 0.5 while allowing γ to vary. Our final list of
− increase the inner radius. The consistency between our
variable model parameters is [M ,R ,R ,T ,i,γ].
gas out in 0 resultsandtheprevioushighresolutioninfraredobserva-
4. THERESULTS tions suggests that our result is not strongly influenced
by these model dependent effects.
The PDFs for each parameter are shown in Figure 2, We derive a disk mass of M =3.8+15 10−5
withmedianvaluesand3σuncertaintieslistedinTable4. gas −2.8 ×
We find that the data are well fit with CO gas that ex- M⊙=13+−590 M⊕ indicating a small, but still substan-
tendsfromR =29+14autoR =224+21auwithatotal tial, amount of gas within the disk, consistent with
in −20 out −20 our simple estimates earlier (Table 2). Our result is
gasmasslog(Mgas (M⊙))=-4.4+−00..76 anda surfacedensity smaller than derived from modeling of unresolved emis-
powerlawindexofγ =1.7+1.0,alongwithatemperature sion (Thi et al. 2014; Jonkheid et al. 2007) although
−1.1
profile that has T =27+11 K. We can place strong con- these models place a substantial amount of mass in the
0 −4
straintsonthe disk mass,temperature,outer radiusand cold outer disk, which is not present in our resolved ob-
inclination, while there are substantial uncertainties on servations. Thesurfacedensityfallsoffwithapowerlaw
the inner radius and γ. While some constraint on Rin is exponent of 1.7+−11..01, similar to the minimum mass solar
providedbythehigh-velocitychannelsofthespectra,the nebula (Weidenschilling 1977) and protoplanetary disks
limitedspatialresolutionpreventstightlimitsfrombeing (Andrews et al. 2009; Isella et al. 2009; Andrews et al.
placed on both R and γ. Figure 3 shows the CO(3-2) 2010), although our result is poorly constrained due to
in
andCO(1-0)spectraalongwiththebestfitmodeldrawn the lack of spatial resolution.
from the posterior distributions, which is able to match Our inclination (60 3) is slightly higher than
±
the strength and shape of both spectra. Residuals, de- that derived from scattered light observations (51 3)
±
rivedby subtractingthe modelanddatavisibilities from (Weinberger et al.1999). Thismaybeduetoanunderes-
each other and creating a cleaned image of these differ- timate ofthe stellar mass;White et al. (in prep)find, in
ences, are shown in Figures 4 and 5. their modeling of ALMA CO(3-2)observations,that the
velocity profile can be fit with an inclination consistent
4.1. Surface Density with the scattered light observations when including a
6
4.0 45 0.08
3.5 40 0.07
3.0 35 0.06
30
2.5 0.05
25
2.0 0.04
20
1.5 0.03
15
1.0 10 0.02
0.5 5 0.01
0.0 0 0.00
−5.2 −5.0 −4.8 −4.6 −4.4 −4.2 −4.0 −3.8 −3.6 2.30 2.32 2.34 2.36 2.38 2.40 2.42 5 10 15 20 25 30 35 40 45 50
R (AU)
log(Mgas (M )) log(Rout (AU)) in
⊙
0.35 1.6 0.6
0.30 1.4 0.5
1.2
0.25
0.4
1.0
0.20
0.8 0.3
0.15
0.6
0.2
0.10
0.4
0.05 0.2 0.1
0.00 0.0 0.0
22 24 26 28 30 32 34 36 38 40 0.5 1.0 1.5 2.0 2.5 3.0 56 57 58 59 60 61 62 63
T0(K) γ incl
Figure 2. MarginalizedPDFs for the parameters inour model. Thevertical dashed lineinthe PDFof log(Rout) marksthe location of
theinnermostscatteredlightdustring.
3.0 2001) and indeed the ratio of the line fluxes, assuming
2.5 both are optically thin, implies T = 17 2 K. While
) ±
y2.0 CO(3-2) this simple estimate is a lower limit, since CO(3-2) is
x (J1.5 not entirely optically thin, our more detailed MCMC
Flu01..50 modeling finds T0=27+−141 K, which is still significantly
lower than the blackbody dust temperature. Fitting the
0.0
−6 −4 −2 0 2 4 6 data with models restricted to have T0=51 K finds so-
Velocity (km/sec) lutions that underproduce the CO(1-0) flux, consistent
0.30 with the need to lower the excitation temperature to
0.25 more heavily populate the J=1 level compared to the
)
x (Jy00..2105 CO(1-0) Jer=b3atleedveilf.wSeuucshedamdiosrcereapcacnucryatweoeustldimbaetefsuortfhtehreedxuasct-
Flu0.10 temperature from SED modeling, which find that the
0.05
dust is much warmer than our simple blackbody esti-
0.00
−6 −4 −2 0 2 4 6 mate(Li & Lunine2003). Thelowgastemperatureruns
Velocity (km/sec) contrary to models of gas heating and cooling, which
predict T >T (Kamp & van Zadelhoff 2001), al-
Figure 3. Spectraderivedfromimageswithina10”boxforCO(3- gas dust
thoughsimilarbehaviorisseeninthe debrisdiskaround
2)(toppanel)andCO(1-0)(bottom panel). Dataareshownwith
thesolidblacklines,whilemodelspectraareshownwithreddashed HD 21997 (K´ospa´l et al. 2013).
lines.
4.3. Residual Features
largerstellarmass. Wefindthatfixingtheinclinationat
51◦andallowingstellarmasstovaryresultsinasatisfac- Within the CO(1-0) data, the v=2.5kms−1 channel is
tory fit to the emission profiles with M∗=2.15−+00..1038 M⊙, sreusbisdtuaanltfliaullxyobfri0g.h14teJryt/hbaenatmheamndodaeSl/(FNigoufr4e.73.,5I)twisitohffa-
largerthantheM∗ =2.00+−00..1185M⊙ derivedfromanalysis setfromthephasecenterof∆RA=0.2′′ and∆Dec=1.4′′,
of the optical spectrum (Mer´ın et al. 2004). While fur- is unresolved in the 4′′ beam and no comparable fea-
ther workis needed to understandthe sourceof this dis- ∼
ture is seen in the red-shifted side of the line, leading to
crepancy,it does notsubstantially affect ourconclusions
alargeasymmetryinthespectrum. Thisunresolvedfea-
about the structure of the disk. We find that the disk
turemaybe suggestiveofanadditionalemissioncompo-
parameters (M , R , etc.) fall within the 3σ ranges
gas in nent,suchasaplanetorcloudofgas,withinthedisk. Its
derived from the fiducial model.
spatialoffsetcorrespondstoadistanceof160aufromthe
star,anditsvelocityisconsistentwithKeplerianmotion
4.2. Excitation Temperature
at this distance, consistent with a body in orbit around
Anopticallythindiskofblackbodydustgrainsaround the central star. Assuming a temperature of 25 K, as
HD 141569 would have T = 51 K. In the low density expected from our derived temperature structure at the
0
environment of a debris disk the gas temperature will observed distance of this feature, its flux implies a CO
not be equal to that of the dust (Kamp & van Zadelhoff mass of 10−9 M⊙, or a gas mass of 10−5 M⊙ assuming
7
6.51 6.16 5.81 5.46 5.11 4.75 4.40 4.05 3.70 3.35 2.99 2.64 2.29
1.34
1.22
1.10
0.98
00..7835Jy/beam
0.61
3
Dec (")∆−−02112 00..3479
−3 0.24
−4
3210−1−2−3−4
∆RA (")
1.94 1.58 1.23 0.88 0.53 0.18 -0.18 -0.53 -0.88 -1.23 -1.58 -1.94 -2.29
1.34
1.22
1.10
0.98
00..7835Jy/beam
0.61
3
Dec (")∆−−02112 00..3479
−3 0.24
−4
3210−1−2−3−4
∆RA (")
-2.64 -2.99 -3.35 -3.70 -4.05 -4.40 -4.75 -5.11 -5.46 -5.81 -6.16 -6.51
1.34
1.22
1.10
0.98
00..7835Jy/beam
0.61
3
Dec (")∆−−02112 00..3479
−3 0.24
−4
3 2 10−1−2−3−4
∆RA (")
Figure 4. Data (top row), model (middle row) and residuals (bottom row) for the CO(3-2) data. Data and model start at 3σ (σ =
0.08Jy/beam),whilefortheimagedresidualsthecontoursareat3σ,5σ,7σ,... withblacksolidcontourswheredata>modelandreddashed
contours where model>data. Beam size and a 100 au scale bar are marked in the bottom and top row, respectively, of the first column.
The first two and last two channels were not included in the fitting, but are shown here to illustrate the noise properties in signal-free
channels.
7.62 6.35 5.08 3.81 2.54 1.27 0.00 -1.27 -2.54 -3.81 -5.08 -6.35 -7.62 0.31
0.28
0.25
00..1292Jy/beam
0.16
Dec (") 05 0.13
∆−5 0.09
5 0 −5
∆RA (")
Figure 5. SimilartoFigure4,butwithCO(1-0). AswithCO(3-2),thefirstandlasttwochannels werenotusedinthefitting. Outside
ofthe2.54kmsec−1 channel,thereareonlyminorresiduals.
8
disks to gain insight as to the origin of the gas and dust
3.5 in the system, as well as their influence on each other
Data
Rout = 224 AU (e.g. Wyatt et al. 2015).
3.0 Rout = 405 AU Around HD 141569, we observe CO gas that ex-
tends out to R =224+21 au with a surface density
2.5 out −20
power law index of γ =1.7+1.0. Typical protoplanetary
−1.1
y)2.0 disks have outer radii that cover a wide range from 15-
x (J 200 au (Hughes et al. 2008; Andrews et al. 2009, 2010;
Flu1.5 Isella et al. 2009) 5. Within these tapered power law
profiles, the power law portion has an average index of
1.0 1, although with very large dispersion, and much of the
power law portion is unresolved (Andrews et al. 2009,
0.5
2010; Isella et al. 2009). These protoplanetary disk pro-
files are similar to that of the HD 141569 system.
0.0
−6 −4 −2 0 2 4 6 The large inner radius is also similar to those seen
Velocity (km/sec)
in transition disks (Espaillat et al. 2014) although in
Figure 6. CO(3-2)spectracomparingthedata(blacksolidline) HD 141569 we find that the CO inner radius is simi-
to models with various outer radii. When allowed to vary, the lar to the inner radius of the mid-infrared dust emis-
outer radius settles at 224+21 au (red dashed line). If fixed at
−20 sion (Fisher et al. 2000; Marsh et al. 2002), while in
the location of the outermost scattered light ring (406 au, blue
transition disks CO has been found to extend inside
dashedline)thefitissignificantlyworse. Thisconfirmsthelackof
asubstantialgasmassextending totheoutermostdustring. the dust gaps (Salyk et al. 2009; van der Marel et al.
CO/H2=10−4. While this feature is prominent in the 2015). Active accretion, at a rate of ∼6×10−9 M⊙ yr−1
CO(1-0) data, there are only hints of residual emission (Garcia Lopez et al. 2006; Salyk et al. 2013), indicates
at a similar position in the CO(3-2) data, with a total that gas exists within the CO inner radius although its
flux of 0.11 Jy km s−1 integrated over the four CO(3-2) exact spatial distribution is unknown. Gas that is de-
channels that overlap with the single CO(1-0) channel. void of CO may extend continuously from the CO inner
The low CO(3-2)/CO(1-0) flux ratio implies an excita- radius toward the star; photodissociation of CO should
tiontemperatureofonly5K.Thiscold,compactfeature occurintheareaclosetothestarthatisdirectlyexposed
maybearesultofasymmetriesinthediskstructure,sim- to high energy radiation from HD 141569.
ilar to what has been seen in the CO gas of the debris While the spatial distribution is similar to proto-
disk around β Pic (Dent et al. 2014). Further data are planetary disks, the mass of the HD 141569 disk
needed to confirm the nature of this residual feature. is much smaller than in younger systems. Typi-
In the CO(3-2) data there are some 3-5σ residuals, cal protoplanetary disks have gas masses of 10−4-
corresponding to 10-30% of the peak flux, that may 10−2 M⊙ (Andrews et al. 2013; Carpenter et al∼. 2014;
reflectdeviationsf∼romoursimplemodelduetoe.g. non- Williams et al. 2013; Mann et al. 2015) while we infer
Keplerianmotionorasymmetriesinthedisk. Higherspa- Mgas=3.8+−125.8×10−5M⊙ intheHD141569system. Con-
tial resolution ALMA observations of CO(3-2) also find versely, Pascucci et al. (2006), using mid-infrared spec-
evidenceofasymmetricemissioninthedisk(Whiteetal. troscopyandsinglediskmmCOspectroscopy,putupper
in prep). While we assumed LTE in our modeling, the limitsonthegasintheouterradiiofdebrisdisksofafew
densities inourbestfitting diskarestartingtoapproach earth masses, below what we derive in the HD 141569
the critical densities of these lines (n 103 104 system. Our gas mass is similar to that derived using
crit
cm−3), which may contribute to the impe∼rfect m−odel. CO emission from cold gas in the disks around 49 Ceti,
To test our assumption of LTE at such low densities with Mgas=3.9 0.9 10−5 M⊙ (Hughes et al. 2008),
we input our density andtemperature structure into the HD 21997, wit±h Mga×s=8-20 10−5 M⊙ (K´ospa´l et al.
lineradiativetransfercodeLIME(Brinch & Hogerheijde 2013), and HD 131835 with×Mgas=1.3 0.6 10−5 M⊙
2010), whichis capableofperforming the full NLTE cal- (Mo´or et al. 2015), all examples of ma±ssive×CO disks.
culation. NLTE and LTE spectra are indistinguishable Ourdustmass,estimatedfromthe 870µmcontinuum,is
indicating that NLTE effects are minimal and justifying 1.8 10−4 M⊙=0.6 M⊕ similar to the 0.1 M⊕ dust
our use of the LTE assumption. Since the residuals do ma×sses seen in debris disks at ages o∼f 10-100 Myr
not lead to large discrepancies between the model and (Wyatt2008)andimpliesanaveragegas/d∼ustmassratio
data spectra outside of the bright CO(1-0) feature, and of 10.
wecanruleoutstrongNLTEeffects,ourresultsarelikely T∼he low gas mass may be a result of the multiple pro-
representative of the true disk structure. cesses that contribute to the removal of CO, and gas in
general,fromthesystem. Atthecurrentrateofaccretion
5. DISCUSSION
the observedgaswouldbe removedinonly 2500years.
TheHD141569systemhasbeenreferredtointheliter- Even if CO-poor gas extended down to 0.1∼au, with the
atureasbothatransitiondisk(Espaillat et al.2014)and same surface density profile as inferred from the outer
adebrisdisk(Zuckerman et al.1995). Itsyoungage,gas
disk and active accretionare reminiscentofa protoplan-
etary disk, although the strength of the infrared excess 5 Protoplanetary disks are often fit with an exponentially ta-
peredpowerlawinsteadofastraightpowerlaw. Inthesetapered
is much more typical of a debris disk. With constraints
models the characteristic radius, Rc is not the same as the outer
on the temperature and density structure of the gas we edgeofatruncatedpowerlaw,withtypicallyRout∼2Rc forγ∼1
can compare to typical protoplanetary disks and debris (Hughesetal.2008)
9
disk, accretion would still be able to deplete the disk in smalldustgrainsascloseas10-30au(Fisher et al.2000;
5000 years. A photoevaporative flow can also remove Marsh et al. 2002). While our 870µm continuum ob-
∼
gasfromthe disk onsimilar timescales(Alexander et al. servations are unresolved, at a spatial resolution corre-
2014, and references therein). The CO itself can be se- sponding to 150 au, higher resolution ALMA observa-
∼
lectively removed through its photodissociation by high tions of the dust continuum find that the dust disk is
energy photons. The observed CO mass would be de- compact and consistent with the mid-infrared observa-
stroyed in 200 years, using the photodissociation rates tions (White et al. in prep). Collisions between plan-
∼
of Visser et al. (2009) and only considering the inter- etesimals, either due to self-stirring or through secular
stellar radiation field. This CO depletion could be par- perturbations by a planet, could generate the inner dust
tiallyoffsetbyasourceofreplenishmentswithinthedisk, seenin the mid-infraredwhichis then rapidly blownout
suchas the collisions ofcomets andsubsequent sublima- ofthe system,populating the outerscatteredlightrings.
tionandreleaseoftrappedvolatiles(Zuckerman & Song Within the 5 Myr lifetime ofthe HD 141569system, the
2012). A total of 1.6 1019 kg of CO would need to be Pluto-size planetesimals needed for self-stirring could be
released each year, wh×ich corresponds to roughly 12000 created as far out as 70 au (Kenyon & Bromley 2008,
∼
kHgalaen-Bdoapp10si%zeCcoOmceotsn,teansstu.mJionngkMheHidalee−tBaolp.p(=2010.37×)1fi0n1d6 2n0er10e)d,gwehoiflethaemCiOlddlyisekcccoeuntldricpeprltaunrebtm(ea∼te0r.i1a)laotutthaesfianr-
that the mass of the HD 141569system is on the border as 60 au (Mustill & Wyatt 2009). The relative youth of
ofregimewhereself-shieldingsignificantlydiminishesthe thissystemmakesin-situgenerationofsmalldustgrains
depletion of CO, although their disk mass is an order of in the outer rings unlikley, unless by a planet embedded
magnitude larger than what we have derived here. The within the rings, but is consistent with their creation
accretionandphotoevaporativeflows,aswellasthepho- close to the star.
todissociationofCO,makeitunlikely thatthe systemis Generatingsmalldustgrainsintheinnerdiskandhav-
in steady state but instead it is likely in the process of ingthemgetblownoutwardbyradiationpressureiscon-
transitioning from a gas-rich protoplanetary disk to a sistentwiththerelativelocationofthegasanddust,but
gas-poor debris disk (Wyatt et al. 2015). itdoesnotfullyexplainthemorphologyoftheouterdisk.
Our finding of CO gas confined to radii smaller than Takeuchi & Artymowicz (2001) predict a single ring,
that of the scattered light rings has important implica- while two distinct rings are observed (Weinberger et al.
tions for the origin of the small dust grains. In a pro- 1999; Clampin et al. 2003; Biller et al. 2015). A Saturn-
toplanetary disk, much of which is shielded from the mass planet embedded in the outer disk could carve out
stellar radiation by the dust itself, the motion of dust a gap and stir up the grains enough to create two rings
grains depends on how well-coupled they are to the gas (Wyatt 2005), and Biller et al. (2015) find tentative evi-
(Weidenschilling 1977). Oncethe disk becomes optically dence ofastellocentricoffsetthat isconsistentwithper-
thin, the grainsexperience anoutwardforce due toradi- turbations from a planet. Another possibility relies on
ation pressure, which can often overwhelm the gravita- perturbations from one of the two distant companions
tional pull of the central star. The ratio of radiation (Weinberger et al. 2000). A close passage between one
to gravitational force is characterized by the parame- ofthe companions andthe disk wouldtruncate the disk,
ter β, which scales with the stellar luminosity and in- and excite dust grains just inside this truncation radius
versely with the grain size (Burns et al. 1979). Grains leadingtothe creationofsmalldustgrains(Ardila et al.
withβ >0.5( sub-micronsizes)arequicklyblownoutof 2005; Reche et al. 2009). Such a passage has been in-
the system on∼dynamical timescales, while larger grains voked to explain the large, low-surface brightness spiral
settleintoorbitsatslightlysub-Keplerianvelocities. For features in the outer reaches of the disk (Clampin et al.
the larger grains the presence of gas can further affect 2003).
these orbits. If the radiation pressure influenced orbit is Within the gas disk we derive an excitation temper-
such that the grains move slowerthan the gas then they ature of T =27+11 K. Spitzer observations find typ-
0 −4
will experience a tail-wind which increases their angular ical dust temperatures of 50-200 K in debris disks
momentum. These grains will move outward until they (Morales et al. 2011; Ballering et al. 2013), with sim-
reach the outer edge of the gas disk. For HD 141569 ilar results found by Herschel (Matthews et al. 2010;
this can occur for small-ish grains (tens of microns), Eiroa et al.2013;Thureau et al.2014),althoughthisde-
while larger grains ( hundreds of microns) settle into pends strongly on the location and composition of the
∼
stable orbits within the gaseuous disk where radiation dust grains. In debris disks the dust temperature at a
pressure and gas drag balance (Takeuchi & Artymowicz given radius is often found to be higher than that for
2001). Poynting-Robertson drag will drive grains with blackbodygrains,orconverselytheobservedradialloca-
sizes oftens to hundreds ofmicrons inwardstowardsthe tionofthe dustgrainsis largerthanpredictedfor black-
star (Li & Lunine 2003). The combined effect of these bodygrains(Booth et al.2013). Previousmodelsofdust
processes is a disk with a ring of dust at the outer edge emissionfromtheHD141569system(Li & Lunine2003;
of the gaseous disk, similar to what is seen in the HD Nilsson et al.2010)findtypicaldusttemperaturesof50-
141569 system. 200Kinthe outerdisk,higherthanourestimatedblack-
In this scenario the small dust grains are generated in body temperature of 51 K, consistent with the presence
the inner disk and blown out on dynamical timescales. of small dust grains.
Biller et al. (2015) do not find any scattered light in- While in the HD 141569system we find a gas temper-
side of 175 au, although they are limited by residuals ature that is lower than that of the dust, it is important
from subtration of the central star within 0.5” (=54 to keep in mind that we directly measure a gas excita-
au). Resolved imaging in the mid-infrared has found tion temperature that, under certain conditions, can be
10
equatedtothegaskinetictemperatureandthedusttem- scattered light dust rings. This configuration is consis-
perature. K´ospa´l et al.(2013)findthatinHD21997the tent with the generation of small grains within the in-
gas excitation temperature implied by the CO lines is ner disk, followed by interactions with the gas and the
much lowerthan the expected dust temperature, similar stellar radiation field that pushes them outward until
to what is seen here. The deviation of the gas tempera- they collect just beyond the outer edge of the gaseous
turefromanopticallythindiskofblackbodygrainsmay disk. The dust may then be further stirred by a recent
be a property of a low density disk in which the col- passage of one of the companions creating the observed
lisions of the gas and dust are not frequent enough to multi-ring structure. While the CO gas mass is much
equilibrate their temperatures. While the midplane gas lower than seen in protoplanetary disks, the active ac-
densities reach105-106 cm−3, withoutdetailedmodeling cretion and wide radial extent are consistent with a pri-
of the dust emission we cannot know if the gas is well- mordial protoplanetary disk origin, rather than the re-
mixed with the dust or if it is subject to a much differ- lease of CO through collisions of gas-rich comets. We
ent radial distribution. The low density environment in also find that the gas excitation temperature is colder
debris disks can lead to substantial differences between than the equivalent blackbody dust temperature. This
the gas and dust temperature (Kamp & van Zadelhoff isconsistentwiththebehaviorseeninthegaseousdebris
2001), although these models predict the gas tempera- diskHD21997(K´ospa´l et al.2013)butinconsistentwith
ture to be much higher than the dust temperature. The predictions from models (Kamp & van Zadelhoff 2001;
HD141569diskismoremassivethantheβ PicandVega Jonkheid et al.2007;Thi et al.2014)possiblyduetoen-
type disks considered by Kamp & van Zadelhoff (2001), hancedcoolingfromCOitself(e.g.Hollenbach & Tielens
whichcouldleadtoadditionalprocessesloweringthedisk 1999), or NLTE effects (e.g. Matr`a et al. 2015). Overall
temperature. Kamp & van Zadelhoff(2001) predict[OI] the data indicate a circumstellar disk in a later stage of
and[CII]playalargeroleingascooling,whileThi et al. its protoplanetaryevolution, with a diminishing gaseous
(2014), in their modeling of the HD 141569 system, find disk that may not be large enough to create a gas gi-
that CO itself can play a majorrole. Strong far-infrared ant planet, but can still influence the distribution of the
emission from [OI] and [CII], at levels more comparable small dust grains.
to protoplanetary disks than to debris disks, has been
measuredfromthediskaroundHD141569(Meeus et al.
We thank the referee for helpful comments regard-
2012) although these lines may only contribute signifi-
ing the context and interpretation of our results. We
cantlytotheupperlowdensityregionsofthediskrather
gratefully acknowledge support from NSF grant AST-
thanthe midplane being tracedbyourCO observations.
1412647. The Submillimeter Array is a joint project be-
Two-dimensional photodissociation region models (e.g.
tween the Smithsonian Astrophysical Observatory and
Hollenbach & Tielens 1999) predictthat at the densities
the Academia Sinica Institute of Astronomy and Astro-
whereCObecomesthedominantcoolant,butbeforecol-
physics and is funded by the Smithsonian Institution
lisional equilibrium with the dust sets in, the gas tem-
and the Academia Sinica. The authors wish to recog-
perature can drop below that of the dust. With dust
nize and acknowledge the very significant cultural role
temperatures of &50K,CO will not freeze-out onto dust
and reverance that the summit of Maunakea has always
grainsbut insteadwill continue to exist inthe gas phase
hadwithin the indigenous Hawaiiancommunity. We are
where it cancontinue to efficiently radiateawaythermal
most fortunate to have the opportunity to conduct ob-
energy.
servations from this mountain. This research made use
Comparingourderivedgastemperaturetothesemod-
of Astropy, a community-developed core Python pack-
elsassumesthattheexcitationtemperatureisequivalent
age for Astronomy (Astropy Collaborationet al. 2013).
to the gas kinetic temperature, while NLTE effects can
Support for CARMA construction was derived from the
cause these two temperatures to diverge. Our NLTE vs
states of California, Illinois, and Maryland, the James
LTEtestrulesoutsignificantNLTEeffects,butitdidso
undertheassumptionofCO/H =10−4. AdiminishedH S. McDonnell Foundation, the Gordonand Betty Moore
2 2 Foundation, the Kenneth T. and Eileen L. Norris Foun-
abundance will lead to fewer collisions and a deviation
dation, the University of Chicago, the Associates of the
betweenthe excitationtemperature inferredby our data
CaliforniaInstitute of Technology,and the NationalSci-
and the kinetic temperature predicted by the models.
ence Foundation. CARMA development and operations
Matr`a et al. (2015) find that, in the case of the debris
were supported by the National Science Foundation un-
disk around Fomalhaut, the density and nature of the
der a cooperative agreement, and by the CARMA part-
collision partners can induce NLTE effects that strongly
ner universities.
affect the gas mass derived from CO upper limits. Fur-
ther modeling is needed to fully explore the possibility
of a low-density NLTE dominated disk as a fit to our
observed CO emission.
6. CONCLUSION
The disk around HD 141569 likely represents a step-
pingstonebetweenafullyopticallythickprimordialdisk
and a more evolved debris disk (Wyatt et al. 2015). Us-
ing CO(3-2) and CO(1-0) we have spatially resolved the
cold gas emission from the disk around HD 141569. We
have found that the gas has a large inner hole, and
extends out to, but does not overlap with, the outer