The ALMA view of W33A: A Spiral Filament Feeding the Candidate Disc in MM1-Main

0.34 MB·

by  L. T. Maud

Mon.Not.R.Astron.Soc.000,1–??(2017) Printed26January2017 (MNLATEXstylefilev2.2)
The ALMA view of W33A: A Spiral Filament Feeding the
Candidate Disc in MM1-Main
L. T. Maud1⋆, M. G. Hoare2, R. Galva´n-Madrid3, Q. Zhang4,
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1 E. Keto4, K. G. Johnston2, J. E. Pineda5
0
2 1Leiden Observatory,Leiden University,PO Box 9513, 2300 RA Leiden, The Netherlands
2School of Physics and Astronomy, Universityof Leeds, Leeds, LS2 9JT, UK

n 3Universidad Nacional Auto´noma de M´exico, Instituto de Radioastronom´ıa y Astrof´ısica, Morelia, Michoac´an, 58089, Mexico
a 4Harvard-Smithsonian Centerfor Astrophysics, 160 Garden St, Cambridge, MA 02420, USA
J 5Max-Planck-Institut fu¨rextraterrestrische Physik, PO Box 1312, D-85741 Garching, Germany

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Accepted 2017January19.Received2017January9;inoriginalform2016October20
]
R
S
ABSTRACT
.
h
p We targeted the massive star forming region W33A using the Atacama Large
- Sub/Millimeter Array (ALMA) in band 6 (230 GHz) and 7 (345 GHz) to search for
o
a sub−1000au disc around the central O-type massive young stellar object (MYSO)
r
t W33A MM1-Main. Our data achieves a resolution of ∼0.2′′ (∼500au) and resolves
s
a thecentralcore,MM1,intomultiplecomponentsandrevealscomplexandfilamentary
[ structures. There is strong molecular line emission covering the entire MM1 region.

The kinematic signatures are inconsistent with only Keplerian rotation although we
1
propose that the shift in the emission line centroids within ∼1000au of MM1-Main
v
couldhintatanunderlyingcompactdiscwithKeplerianrotation.Wecannothowever
8
5 ruleoutthe possibilityofanunresolvedbinaryormultiplesystem.Aputativesmaller
9 disc couldbe fed by the largescale spiral‘feeding filament’ we detect in both gasand
6 dust emission. We also discuss the nature of the now-resolvedcontinuum sources.
0
Key words: stars:formation - stars:protostars - stars:massive - tech-
.
1 niques:interferometric - techniques:high angular resolution - submillimetre: stars
0
7
1
:
v 1 INTRODUCTION sive B-type protostars (Cesaroni et al. 2014; Beltr´an et al.
i
X 2014; S´anchez-Monge et al. 2014).
There are a growing number of ALMA observations in
r the millimetre regime to support the existence of discs In the context of star formation, discs are a key re-
a quirementtoallowsuchmassivesourcestoaccretematerial
aroundOB-typeprotostars(e.g.S´anchez-Monge et al.2014;
in the face of strong radiation pressures so that they ex-
Johnston et al. 2015). These agree with a scaled up
paradigm of low-mass star formation beyond ∼10-15M⊙. ceed>10−15M⊙ (Krumholzet al.2007;Kuiperet al.2010,
2011).Therefore,naively,discsshouldbeseentowardsmany
Generally, high-resolution and high-sensitivity observations
more sources than have been found. Studies with ‘scaled-
at such wavelengths have been lacking as previous inter-
up’ models of low mass star formation applied to higher
ferometric arrays could not achieve either; or have only
masses suggest that discs are required (e.g. Maud et al.
achieved enough resolution for the most nearby sources
2013; Keto & Zhang 2010; Johnston et al. 2011). Observa-
(e.g.Beutheret al.2013;Maud & Hoare2013;Hunteret al.
tions at other wavelengths also provide evidence for discs,
2014;Carrasco-Gonz´alez et al.2012).Todate,themostcon-
typicallythroughtheanalysisofemissionlineprofilesinthe
vincingobservationsofanO-typeprotostellardiscarethose
IR modelled as arising from the hot inner regions of Ke-
of AFGL 4176 (Johnston et al. 2015) where a clear Keple-
plarian discs (e.g. Ilee et al. 2013). Some multi-baseline IR
rian signature is found in the CH3CN molecular line emis-
sionon∼1000auscalesaroundthe∼25M⊙source.Recently interferometric studies have also suggested small <100au
hot discs (Krauset al. 2010; Boley et al. 2013).
Ilee et al. (2016) also report Keplerian like rotation around
a putative OB-type protostar, although it is marginally re- TherecentreviewbyBeltr´an & deWit(2016)provides
solved.TypicallyKepleriansignaturesarefoundinlessmas- a comprehensive summary of discs around luminous YSOs.
In particular, they note that B-type protostars appear to
have traits of scaled-up low-mass protostars, whereas, the
⋆ E-mail:[email protected](LTM) mostmassiveearly-Otypeprotostars(L>105L⊙)arefound
(cid:13)c 2017RAS
2 L. T. Maud et al.
to have toroidal structures that may never become stable modelling is beyond thescope of this work and will be pre-
Kepleriandiscs.Cesaronietal.(2016-inprep.)usedALMA sented in a forthcoming article.
(∼0.2′′ resolution) to spatially resolved six of these most
luminous sources. However they find a very heterogeneous
3.1 The Continuum Emission
sample with little evidenceof Keplerian discs.
Here we detail our ALMA observations zooming into Figure 1a shows the band 7 continuum emission from
W33A(G12.91-0.26),arelativelynearbyarchetypalmassive the W33A MM1 region with a power-law scaling. With
star formation site (2.4kpc, Immer et al. 2013). We focus these high-resolution and high-sensitivity ALMA observa-
on one of two previously detected strong continuum emis- tionswefurtherresolvetheregioncomparedtoourprevious
sion regions, MM1 (Galv´an-Madrid et al. 2010) in an effort work(Galv´an-Madrid et al.2010).Specifically,theemission
to search for a Keplerian disc around the dominating high- around the central brightest source MM1-Main is well re-
mass protostar designated MM1-Main (L∼3.2×104L⊙1). A solved (peak flux of 0.96mJybeam−1 at Band 7 without
plethoraofhigh-resolution,sub-arcsecond,multi-wavelength free-freecorrection)astheNWandSEcomponentsarenow
observations support such a scenario. Our previous SMA separatedfromMM1-Main.Wealsodefinetheextraregions
data indicated a change in position of the blue- and red- MM1-E and MM1-Ridge (only resolved at Band 7).
shifted emission close to the peak of the continuum emis- Masses are estimated from the Band 7 data, assum-
sion, hinting at rotation, although it is marginally resolved ing optically thin emission, using the calculated dust opac-
(Galv´an-Madrid et al. 2010). Other work includes: VLTI ity index in the region (β - see below) and the tempera-
MIDI observations (deWit et al. 2010) where IR emis- turesasdiscussedinSection3.2.WhereCH3CNistooweak
sion is attributed to the hot dust from the outflow cav- for our temperature estimation technique we fix it to 80K
ity walls; VLT CRIRES data (Ilee et al. 2013) which at- (the lowest temperature in MM1). We use Equation 1 of
tribute the 2.3µm CO bandhead emission to a close 1-2au Maud et al. (2013), a dust to gas ratio of 100 and a dust
disc; and NIR adaptive optics integral field spectroscopy opacityκν =κ0(ν/ν0)β whereκ0 =0.5cm2g−1 at230GHz
(Davies et al. 2010) in which a 300kms−1 Brγ jet along is (Galv´an-Madrid et al. 2010). Table 1 lists details the posi-
found ∼perpendiculartoan assumed discseen in CO emis- tions, fluxes and masses of the various sources. The masses
sion and absorption. are consistent with our SMA data (Galv´an-Madrid et al.
2010)consideringwerecovernearlyallthefluxatthehigher
resolution but accounting for a higher temperature and
2 OBSERVATIONS closer source distance.
We calculate β using the band 6 and band 7 emission
Our ALMA Cycle 1 band 6 and 7 observations
with matched u,v coverage (40to 610 Klambda) and beam
(2012.1.00784.S − PI: M. G. Hoare) were observed in Cy-
sizes (c.f. Galv´an-Madrid et al. 2010; Maud et al. 2013) af-
cle 2 taken on June 24, 25 and 26, 2015. The PWV was
ter the free-free emission is removed under the Rayleigh-
<0.7mm and there was good phase stability. Both bands Jeans approximation (hν ≪ kBT,) where the dust opacity
had 4 spectral windows, each with a 937.5MHz bandwidth
index β is related to the spectral index by α = β+2. We
and 488kHz frequency resolution (∼0.65kms−1 at band 6
modelthefree-freeemissionbypointsourcesforMM1-Main
and ∼0.42kms−1 at band 7). The spectral settings covered
andMM1-SE, afterextrapolating from theradio detections
theCH3CN(J=12-11andJ=13-12)K-laddersfromK=0to at 8.4, 15.0 and 43.3GHz (van derTak & Menten 2005;
9 in band 6, CH3CN (J=19-18) K=0 to 8 and SiO (8-7) in Rengarajan & Ho 1996) with a spectral index of the free-
band7,andhadenough‘line-free’channelstodeterminethe
free emission, αff=1.03±0.08 (Galv´an-Madrid et al. 2010).
continuum emission. The array configurations had baseline
The corresponding free-free fluxes at band 6 range from 25
lengths from 30.8m to 1300m in band 6 and out to 1600m
to 39mJy and at band 7 from 37 to 61mJy when consider-
in band 7. A minimum of 37 antennas were used. The re-
sulting images have a beam size of 0.32′′×0.26′′ in band 6 ing ∆αff.van der Tak & Menten (2005) resolve MM1-Main
(Q1)andSE(Q2)andsowedividethefree-freefluxbetween
and 0.21′′×0.14′′ at band 7 using a robust=0.5 weighting
thetwo sources according to the their43GHzflux ratio.
(∼700au and ∼500au respectively). The data were man-
The spectral index valuesα for MM1-Main range from
ually reduced using casa (McMullin et al. 2007) version
2.0 to 2.5 and the dust opacity index β from 0.0 to 0.5 de-
4.5.1. Self calibration was performed, although a minimal pending on the free-free contribution. α of ∼2 could point
timescale of ∼30s was used as the signal-to-noise was no
to the continuum emission becoming optically thick (τ >1)
longer sufficient to improve the phase residuals on shorter within the central ∼500au. This is discrepant with the low
times. peak brightness temperature of MM1-Main (Tmb ∼27K)
within the 0.21′′×0.14′′ beam. The column density at the
peak of MM1-Main reaches ∼1024cm−2, although the cal-
3 RESULTS AND DISCUSSION culatedmaximalopticaldepthonlyreachesτ ∼0.06(follow-
ing equations 2 and 3 from Schulleret al. 2009). Assuming
HerewefocusonthecontinuumandCH3CNemission from
a coupling of the gas and dust temperatures, we estimate
W33AMM1whichharboursthehigh-massprotostar,aswell
thatonlytheverycentral∼30mas(milli-arcseconds) diam-
as briefly discussing the SiO emission. The W33A region
eter would be optically thick, hence we are dominated by
is chemically rich with over 20 other species detected (c.f.
optically thin emission. Accounting for the uncertainties in
Galv´an-Madrid et al.2010),althoughthefulldiscussionand
the free-free spectral index and consistency with optically
thin emission we emphasise that an estimated β value of
1 http://rms.leeds.ac.uk/cgi-bin/public/RMS DATABASE.cgi ∼0.5providesgoodevidenceforlargerdustgrainsasacon-
(cid:13)c 2017RAS,MNRAS000,1–??
A Spiral Filament Feeding the Candidate Disc 3
Table1.SourceparametersestablishedfromthecontinuummapsattheresolutionsnotedinSection2priortofree-freecorrection.We
use apertures measuring down to the 5σ level (σ = 0.107 and 0.221mJybeam−1 forband 6 and band 7) while dividingthe sources at
the intersection between them. The flux was summedinan aperture at the 15percent level forMM1-Mainthat was ‘circular’(∼0.42′′
diameter)beforetheemissionblendsintotheothersources.MM1-SandMM1-NWarenotresolvedat1.3mm.Uncertaintiesinthepeak
valuecanbeconsideredastheσlevelandoftheorder∼5−10percentfortheintegratedfluxesandmasses.Themassiscalculatedusing
theband7dataandshowsarangeforsomesourcesafter removaloftheextremesofthefree-freeemission.Notethetotalislargerthan
thesumofthesourcesasitalsoincludesdiffuseemission.Therangeoffree-freefluxesforMainandSEareshown.
Source RA. DEC. 0.87mm 0.87mmPeak 0.87mm 1.3mm 1.3mmPeak 1.3mm Mass
Name (J2000) (J2000) Flux(mJy) (mJy/bm) ff(mJy) Flux(mJy) (mJy/bm) ff(mJy) (M⊙)
Main 18:14:39.512 −17:52:00.13 126.1 100.3 23.7−45.1 73.4 56.8 18.5−28.9 0.15−0.21
S 18:14:39.511 −17:52:00.40 40.2 16.1 ... ... ... ... 0.10
SE 18:14:39.552 −17:52:00.48 23.9 7.7 8.2−15.9 10.1 5.6 6.5−10.0 0.00−0.07
E 18:14:39.566 −17:52:00.01 49.0 8.3 ... 18.1 6.1 ... 0.71
NW 18:14:39.491 −17:51:59.87 21.1 6.9 ... ... ... ... 0.32
Ridge 18:14:39.466 −17:51:59.47 59.1 9.4 ... 18.2 5.0 ... 0.58
Total ... ... 410.7 100.3 31.9−61.0 170.3 56.8 25.0−38.9 2.22−2.45
sequence of grain growth (Draine 2006; Williams & Cieza 3.3 SiO Outflow
2011). Future modelling will bring both the density and
Strong SiO emission is detected towards MM1-Main. Clip-
temperature distribution to bear on the dust opacity index
ping at ∼3σ (σ ∼1.71mJybeam−1 per 0.42kms−1) we es-
calculation.
tablish the velocity ranges from ∼29 to 36.7kms−1 in the
blue shifted and ∼39.6 to 42.8kms−1 in the red shifted
regimes (limited at blue velocities by line contamination).
The blue shifted emission highlights a linear jet-like struc-
ture although the red-shifted emission is more complex
3.2 CH CN Emission
3 in shape. The PA of the central flow based on the lin-
ear blue-shifted emission is ∼120◦ and is coincident with
Figure 1b presents the temperature map of the MM1 re-
that of the larger scale CO outflow (Maud et al. 2015;
gion using the higher spatial and spectral resolution, and
signal-to-noise of the band 7 CH3CN J=19−18 K-ladder. Galv´an-Madrid et al. 2010) and the near-IR reflection neb-
ula (Davieset al. 2010).
We follow the rotation temperature diagram (RTD) anal-
ysis outlined in Arayaet al. (2005) on a pixel-by-pixel ba-
sis for a single optically thin component in LTE (see Hollis
3.4 Nature of the other continuum sources
1982;Loren & Mundy1984;Turner1991;Zhanget al.1998
fordiscussion).ThisbreaksdownforMM1-Maininthecen- It is unclear whether the components S, NW, E, SE and
tral region as the column density divided by the statisti- ridge,areembeddedsources,filamentsfeedingthemaincore
cal weight versus the energy level no longer follows a linear (e.g. Peretto et al. 2013), or over-densities at the outflow
trend, and so we cap the temperature at 624K conforming cavity wall (e.g. Fuenteet al. 2009). Considering the direc-
to the upper energy level of the J=19−18 K=8 line. With tionofthejet/outflow(SE−NW)thenoneplausibledescrip-
radex (van derTak et al. 2007) we can reproduce individ- tion is that the extensions E and S (curved line through
ual K-ladder line fluxes with temperatures >600K, column Main)couldformpartofablue-shiftedcavitywall,whereas
densities>1017−18cm−2 anddensities>107cm−3,butfora NWwouldformpartofthered-shiftedcavitywall.Certainly
fixed set of these parameters we cannot reproduce the ob- the moment 1 map Figure 1c indicates the blue and red-
servedratiosbetweenthem.Thissuggestsatleast2temper- shiftedemissionisnotconfinedonlytotheNEandSWbut
aturecomponentsmayberequired.Afullradiativetransfer isco-locatedwiththeoutflowdirectionSE-NW(Maud et al.
model is requiredbut is beyondthescope of thisletter and 2015; deWit et al. 2010). The moment 2 map also high-
will be presented in the future. lightsthelargervelocitydispersionsintheregionoftheblue-
Figure 1c shows the moment 1 (intensity weighted ve- shiftedoutflowdirectiontracingthecavitywallworkingsur-
locity) and 1d the moment 2 (velocity dispersion) maps fo- faces. Suchadescription corroborates thefaint <10percent
cussing on MM1 over the full velocity range of the CH3CN level dust emission in the region of MM1-E and -S as from
19−18emission from∼28kms−1 to∼42kms−1 oftheK=4 an outflow cavity. However Figure 1b indicates that S has
line. Thisis representativeof theK=3,5& 6line emission, additional dust emission and elevated temperatures consis-
but not K>7 which traces hotter gas closer to the central tentwithcentralheatingandalsohasbroaderemissionlines
source in addition to decreasing in flux. Compared to our comparabletothoseinMM1-Main.Ahighlyprobablyalter-
previous SMA observations we resolve the gas and find the native scenario is that S is an embedded intermediate-high
kinematics are much more complex than the lower resolu- mass source providing the heating and velocity dispersion
tion observations might have indicated. We do not find a although it is not separated in thecontinuum emission.
Keplerian disc (c.f. Johnston et al. 2015) in MM1-Main as The mm emission from SE appears in the path of the
suggested by the position shift of the blue- and red-shifted SiOjetandcanattributedtothatfromathermaljet‘knot’,
emission in Galv´an-Madrid et al. 2010, Fig. 6. as the mm emission is accounted for by the extrapolated
(cid:13)c 2017RAS,MNRAS000,1–??
4 L. T. Maud et al.
Figure 1. a) 0.87mm continuum image of the W33A MM1region. The mm sources are marked by crosses (+), the radio sources are
indicatedbydiamondsandtheCOoutflowdirectionbythesolidlinewitha±5◦range(dashedlines).Thegreycontoursareat15,2550,
75and100σ(σ=0.22mJybeam−1).Thesourcesarelabelledandascalebarandbeamareshownatthebottomoftheimage.Theblue-
andred-shiftedSiOemissionintegratedfrom29to36.5and39.5to43kms−1 respectively, arerepresentedbyblueandredcontours at
20to90percentin10percentsteps.b)TemperaturemapestablishedfromthefittingofCH3CN.Thecontoursaretheband7continuum
as(a). c)Moment1mapofCH3CNJ=19-18K=4integratingvelocities between 28and42kms−1 above 15σ (σ= 2.6mJybeam−1per
0.42kms−1 channel).Therangeofthecolourbarislimitedtodisplayfrom35kms−1 asonlyblueremissionislocatedclosetothepeak
of MM1-Main.Thegrey contours represent the moment 0map of CH3CN J=19-18 K=4from 10to 90 percent in10percent steps. d)
Themoment2velocitydispersionmapofCH3CNJ=19-18K=4.Notethespiralstructureinbothfigures(b), (c)and(d)
free-free emission. Such an interaction is supported by the south of ridge, near VLSR velocities flowing through ridge,
elevatedtemperaturesatthe‘knot’,althoughthelineprofile and finally onto the north of MM1-Main at red-shifted ve-
do not show outflow like wings and there is no SiO contri- locities ∼41kms−1.Based on thesekinematics and thespi-
bution at this location. Considering the lower limit of the ral structure we strongly suggest that it is a ‘feeding’ fil-
free-freecontributiontoSEatband7leavessomemmemis- ament drawing material from the reservoir to the South-
sion as from dust and assuming the free-free emission as a West(joinedtoMM2NEandMM2−Galv´an-Madrid et al.
tracer of star-formation activity itself, we believe SE could 2010 and in our continuum emission at the 5−10percent
also be explained by an embedded self heating source. SE flux levels). Interestingly similar spiral-like structures were
isveryrichincomplexchemicalspecieswhichmaybemore seeninthesimulatedimages ofKrumholzet al.(2007)who
closely related to protostellar heating although these trac- modelled the formation of a massive stars via discs. The
ers could still be generated by grain sputtering due to out- spirals in their work feed and join the <500−1000au size
flowactivity(e.g.Arceet al.2007,2008;Codella et al.2009; discs surrounding the compact, multiple cores formed in
Drozdovskayaet al. 2015). thedynamicenvironment.Observationsof thelow-mass bi-
nary L1551 and the high-mass proto-cluster G33.92 indi-
MM1-Ridge could potentially harbour a protostellar
cate such accretion/infall spiral features maybe common-
source, or simply be an enhancement associated with the
place (Takakuwaet al. 2014; Liu et al. 2015).
red-shifted outflow lobe. However, the whole structure has
a clear velocity gradient (Figure 1c) from a blue-shifted ve-
locity of ∼37kms−1 at the tail of the spiral shape to the
(cid:13)c 2017RAS,MNRAS000,1–??
A Spiral Filament Feeding the Candidate Disc 5
3.5 W33A MM1-Main as a disc candidate? IA101715.JEPacknowledgethefinancialsupportoftheEu-
ropean Research Council (ERC; project PALs320620).
We do not find a Keplerian rotation signature at ∼500au
resolution. The spectra of all the unblended lines of the
CH3CN ladders in the region of MM1-Main are not Gaus-
sianbutareverybroad.Theprofileaveragedoverthecentral
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ACKNOWLEDGEMENTS
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