Trigonometric distance and proper motions of H2O maser bowshocks in AFGL 5142

0.63 MB·

by  Ross A. Burns

Mon.Not.R.Astron.Soc.000,1–??(2002) Printed24January2017 (MNLATEXstylefilev2.2)
Trigonometric distance and proper motions of H O maser
2
bowshocks in AFGL 5142
R. A. Burns1,2⋆, T. Handa2, H. Imai 2, T. Nagayama 3, and T. Omodaka2
7
1 T. Hirota4,5, K. Motogi6, H. J. van Langevelde1,7, W. A. Baan,2,8
0
2 1Joint Institute for VLBI ERIC (JIVE), Postbus 2, 7990 AA Dwingeloo, the Netherlands
2Graduate School of Science and Engineering, Kagoshima University,1-21-35 Kˆorimoto, Kagoshima 890-0065, Japan
n 3Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka-cho, Mizusawa-ku, Oshu, Iwate, Japan 023-0861
a 4Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo181-8588, Japan
J 5Department of Astronomical Sciences, SOKENDAI, Osawa 2-21-1, Mitaka, Tokyo181-8588, Japan

0 6Graduate School of Sciencesand Technology for Innovation, Yamaguchi University.Yoshida1677-1, Yamaguchi 753-8512, Japan
2 7Sterrewacht Leiden, Leiden University,Postbus 9513, 2300 RA Leiden, the Netherlands
8ASTRON, PO Box 2, NL-7990 AA Dwingeloo, the Netherlands

]
R
S
Accepted 1988December 15.Received1988December14;inoriginalform1988October11
.
h
p
- ABSTRACT
o
r
t Wepresenttheresultsofmulti-epochVLBIobservationsofwatermasersintheAGFL
s 5142massivestarformingregion.Wemeasureanannualparallaxofπ =0.467±0.010
[a mas, corresponding to a source distance of D = 2.14+−00..005419 kpc. Proper motion and
lineofsightvelocitiesrevealthe3Dkinematicsofmasersinthisregion,mostofwhich

1 associatewithmillimetersourcesfromthe literature.Inparticularwefindremarkable
v
bipolarbowshocksexpandingfromthemostmassivemember,AFGL5142MM1,which
2
areusedtoinvestigatethephysicalpropertiesofitsprotostellarjet.Weattempttolink
1
the known outflows in this region to possible progenitors by considering a precessing
9
jet scenario and we discuss the episodic nature of ejections in AFGL 5142.
5
0
Key words: Masers - Massive Star Formation - Stars; individual (AFGL 5142)
.
1
0
7
1
: 1 INTRODUCTION (Burnset al. 2015) are now becoming common targets of
v
investigation in MYSOs.
i Massivestarformationisachallengingfieldofobservational
X In disk-jet systems, ejection rates correlate with accre-
astronomy,with massiveyoungstellarobjects(MYSOs)re-
r sidingindeeplyembeddedenvironments,oftenatgreatdis- tion rates (Corcoran & Ray 1998; Caratti o Garatti et al.
a 2015) - thus the ejection history of an MYSO allows in-
tances from Earth. From a theoretical perspective, high-
ferenceofitsaccretion history-whichisitself unobservable
andlow-massstarformationwereonceconsidereddistinctly
inapracticalsenseduetothetimescalesinvolved.Accretion
different, with MYSOs initiating nuclear fusion while still
history, based on ejection history, can therefore be used to
deeply embedded in their parent cores, accretion persisting
compare low- and high-mass star formation.
intothemain sequence,in addition toproducing strongra-
In addition to accretion mechanisms, outflows serve as
diation capable of limiting accretion onto the central star
another point of comparison - with the outflows of massive
(see reviews byZinnecker& Yorke2007; Tan et al. 2014).
starsconsideredtobelesscollimatedthanthoseoflowmass
However, with recent advances in instrument sensitiv-
stars (Wu et al. 2004). However, such conclusions are often
ity, resolution and data reduction techniques used in as-
based on surveys of large scale outflows which could have
tronomy there has emerged a gradual convergence in the
formed via entrainment by a collimated, precessing jet as
observed features present in low- and high-mass star for-
was observed in IRAS 20126+4104 (Shepherdet al. 2000).
mation. MYSOs are increasingly found to harbour disk-
Evidently,allphysicalscalesmustbeconsideredwhencom-
jet systems - a feature ubiquitous to low mass star forma-
paring theoutflows of low- and high-mass YSOs.
tion. Phenomena such as circumstellar disks (Beltr´an et al.
2004; Hirota et al. 2014; Chen et al. 2016; Ilee et al. 2016), AFGL 5142 is a massive star forming region which
episodic ejection (Burns et al. 2016) and even jet rotation has multiple outflows on multiple scales. The region con-
tains ninemillimeter cores, by far themost predominant of
which being MM1 and MM2 which exhibit hot core chem-
⋆ E-mail:[email protected] istry(Zhang et al.2007;Palau et al. 2011, 2013).MM1 has
(cid:13)c 2002RAS
2 R. A. Burns
amassofabout6.5M⊙ (Liu et al.2016)andhousesanem- Table 1.Summaryofobservations
bedded massive star, indicated by the presence of 6.7 GHz
methanolmasers which tracean infalling disk (Goddi et al.
Epoch Observation Modified Numberof
2011).Anionized bipolarjet extendstotheNW-SE-atan number date Juliandate features
angle near perpendicular to the disk. Water masers tracing
1 21Apr2014 56768 12
the leading tip of the ionised jet reveal expanding motions 2 20May2014 56797 9
inthelineofsightandskyplane(Goddi & Moscadelli2006; 3 2Oct2014 56932 17
4 25Nov2014 56986 22
Goddi et al.2011),thusgivingAFGL5142MM1theimage 5 31Jan2015 57053 24
ofaprototypicaldisk-jetsysteminanMYSO.Furthermore, 6 29Mar2015 57110 25
7 29May2015 57171 19
the combination of features associated with both low- and
high-mass star formation make AFGL 5142 MM1 a good
target for a comparative investigation.
About1′′tothesouthisMM2,withamassofabout6.2
M⊙ (Liu et al.2016)butnocentimeteremissionor6.7GHz
methanol masers. Both MM1 and MM2 exhibit similar hot
molecular core chemical compositions (Zhang et al. 2007;
Palau et al. 2011), suggesting similar evolutionary stages.
Water masers have also been detected near AFGL 5142
MM2 (Hunteret al. 1995; Goddi & Moscadelli 2006).
InterferometricobservationsoftheAFGL5142regionin
CO(2−1)(Zhang et al.2007;Palau et al.2011),SiO(2−1)
and HCO+ (1−0) (Hunteret al. 1999) reveal at least four
distinct collimated molecular outflows on 10′′ scales; out-
flowsA,B,C(Zhang et al.2007)andoutflowD(Palau et al.
2011). Several of these outflows intersect both MM1 and
MM2, barring a simple assignment of progenitors. Single
dish observations reveal the presence of larger, arc-minute
scale outflowstraced in CO (2−1) which also intersect the
Figure1.Scalaraveragedcross-powerspectraofthemaseremis-
MM1andMM2region(Hunteret al.1995)thesepropagate
sioninAFGL5142asafunctionoftime,colouredarbitrarily.
primarily in the skyplane. Recently Liu et al. (2016) also
find a ∼ 9′′ length, extremely wide-angle bipolar outflow
(EWBO), with an opening angle of ∼180◦, driven from the 2000) with beams centered on the maser target, AFGL
MM1 - MM2 region, leading the authors to interpret its 5142,andareferencequasar,J0533+3451. Forbothbeams,
formation as being driven by a precessing jet rather than left-hand circular polarisation signals were recorded at
entrainment at wide angles from a narrow jet. each of the 4 VERA station. Data were collected and
In this work we investigate the outflows seen in AFGL correlated using the Mitaka FX correlator (Chikada et al.
5142 at the smallest scales by performing new multi-epoch 1991), adopting a rest frequency of 22.235080 GHz. For
very long baseline interferometry (VLBI) observations of AFGL 5142 data we used a phase tracking center of
water masers at 22 GHz. VLBI is an ideal observational (α,δ)J2000.0 = (05h30m48s.01733742, +33◦47’54”.56750)
approach to investigating protostellar ejections as multi- and for J0533+3451 data we set a phase tracking center at
epoch observations provide sky-plane and line of sight in- (α,δ)J2000.0 = (05h33m12s.76510600, +34◦51’30”.336995).
formation, thus revealing the 3D kinematics of gas in the Further phase corrections, including more accurate atmo-
vicinity of MYSOs. Our investigation aims to provide ob- spheric models and antennae positions than those used at
servationalcharacterisationofoutflowbehaviour(launching correlation, were madeand applied post-calibration.
mechanismsandepisodicejection)foraprototypicalMYSO, Thetotalcorrelatorbandwidthof240MHzwasshared
to provide comparison between low- and high-mass forma- into16intermediatefrequencies(IFs).OneIFwasallocated
tionmechanisms.Ourreportconcentratesonthekinematics to the maser data with a bandwidth of 8 MHz and 15.63
andenergeticsofthejetsassociatedwithAFLG5142MM1. kHzchannelspacing,providinga0.12kms−1 velocityspac-
Weuse ourfindingsto offer areinterpretation of the recent ing.Theremaining 15 IFswereallocated tothedataof the
history of jets and outflows in this region and attempt to quasar reference source - one IF had similar properties to
assign progenitors to each of the known outflows from the the maser data while the other 14 IFs had 16 MHz band-
literature.Viaannualparallaxwealsoprovidethefirstpre- widthandspacingsof125kHz.The15quasardataIFswere
cisemeasurementofthedistancetoAFGL5142;aquantity manipulatedintocommonformandmerged-resultinginal-
essential for converting observations to physical quantities most continuousfrequency coverage spanning232 MHz.
such as ejection velocities, ages and momentum rates. DatareductionwasperformedusingAIPS(Astronomi-
calImageProcessingSystem),developedbyNationalRadio
Astronomy Observatory. VLBI phase referencing data re-
ductionmadeuseoftheinversephasereferencing technique,
2 OBSERVATIONS AND DATA REDUCTION
customisedforthedual-beamdataofVERA.Thetechnique
VLBI observations of AFGL 5142 were carried out with was first introduced in Imai et al. (2012) and furtherdevel-
VERA(VLBIexplorationofradioastrometry).Allobserva- opment,inadditiontoaguidetoitsimplementation,isgiven
tionswere conductedin dual-beammode (Kawaguchi et al. in Burns et al. (2015).
(cid:13)c 2002RAS,MNRAS000,1–??
H O maser astrometry: AFGL 5142 3
2
Table 2.ThegeneralpropertiesofH2OmasersinAFGL5142,detected withVERA.
Maser VLSR Detected ∆αcosδ ∆δ µαcosδ µδ Fint π
ID (kms−1) epochs (mas) (mas) (masyr−1) (masyr−1) (Jy) (mas)
N.W.1 2.47 1234567 -35.216 35.133 −0.55±0.03 0.89±0.26 23.67 0.460±0.014
N.W.2 0.42 1234567 -39.637 29.447 −1.03±0.02 0.23±0.11 11.09 0.479±0.021
N.W.3 3.49 ****567 -28.365 38.437 0.07±0.27 0.16±0.05 1.27
N.W.4 3.07 1*34*67 -37.371 32.979 −0.58±0.08 1.07±0.08 2.99
N.W.5 1.81 **34567 -38.121 31.934 −0.19±0.48 1.51±0.80 4.13
N.W.6 1.24 ****567 -38.838 30.803 −1.28±0.06 −0.21±0.11 4.14
N.W.7 0.86 **3*567 -41.625 24.954 −0.77±0.12 0.87±0.31 2.22
N.W.8 −0.02 ****567 -45.198 19.198 −0.73±0.42 1.98±0.21 2.46
S.E.1 −6.69 123456* 162.415 -232.163 1.13±0.14 −1.59±0.04 4.83
S.E.2 −6.20 1234567 156.283 -237.916 1.93±0.03 −1.16±0.03 12.87 0.479±0.022
S.E.3 −6.71 1**4567 147.805 -222.823 1.38±0.19 −0.83±0.11 1.77
S.E.4 −4.93 **34567 134.663 -228.13 0.84±0.04 −1.19±0.08 5.31
S.E.5 −5.36 ***4567 117.507 -230.753 0.36±0.04 −1.08±0.02 2.72
S.E.6 −6.91 ****567 160.06 -235.217 1.97±0.70 −1.67±0.26 2.52
S.E.7 −4.08 **3**** 121.616 -227.994 − − 1.41
S.E.8 −7.05 *****6* 131.305 -229.803 − − 5.80
S.E.9 −7.05 *****6* 159.065 -236.213 − − 0.88
S.E.10 −5.77 **3**** 54.856 -143.514 − − 2.15
F.S.1 −5.78 1234567 11.783 -1241.213 1.42±0.03 −0.43±0.07 13.98 0.450±0.029
F.S.2 −4.41 ***4567 12.405 -1227.991 1.64±0.08 0.05±0.21 17.80
F.S.3 −5.15 123456* 13.319 -1236.244 1.71±0.04 0.71±0.18 5.74
F.S.4 −4.43 **34567 13.227 -1230.149 1.35±0.08 1.00±0.11 4.67
F.S.5 −5.54 1234567 11.416 -1225.99 1.13±0.09 1.44±0.23 11.02
B 0.77 1*3456* 639.914 148.019 1.89±0.06 −0.61±0.04 3.06
F.S.W. −3.04 **3456* -3457.993 -1553.682 −2.56±−0.46 0.01±0.18 3.29
A −3.18 123456* 6.68 -40.77 − − 1.27
Simultaneous fit 0.467±0.010
Column(2): Lineofsightvelocities,withrespecttothelocalstandardofrest,arequotedasthosemeasuredforthefirstdetection.
Column(3): Numbersindicatedetectioninthecorrespondingepoch,whileasteriskrepresentsnon-detection.
Column(8): Integrated fluxesaregivenastheaverageofmeasurementsfromallepochs.
In short, data reduction involves solving frequency de- featureandcollocatedwithinaradiusof1mas.Theassocia-
pendant phase terms (group delay) using bright calibrators tionsofspotstofeatures,andthefluxweightedastrometric
in the wider 15 IF data set, while time dependant phase positions of maser features, were determined using a basic
terms are solved using the narrow but bright line emission FORTRAN code which also removes the offset introduced
of the maser. Both sets of solutions are then applied to the bythedifferencein coordinates between thephasetracking
reference source, J0533+3451, inversely giving the angular center and the reference maser used in each epoch, in ad-
separation of the maser from the phase center. Assuming dition to shifting the maser maps to the absolute reference
thatthereferencesourceisfixedatitsICRFcoordinateswe frame.
then obtain the astrometric position of the maser from the
derivedseparations.Sincephasesolutionsdeterminedusing
themaserareappliedtoitself wealsoproducehighquality,
astrometricallyaccuratemasermaps.Typically,masermaps
3 RESULTS
achieved rms noise values of 100−200 mJy beam−1 while
thewiderbandcontinuumdataachievedtypicalrmsvalues 3.1 Maser distribution and l.o.s. velocities
of ∼1 mJy.
Atotalof27independentmaserfeaturesweredetectedwith
Maser maps were produced using the AIPS task VERA during the observing calendar. Their coordinates,
CLEANinautomaticmode(DOTV=0),withasynthesised lineofsight(l.o.s)velocities,detectionfrequenciesandother
beam of dimensions 1.0×1.2 mas. Masers were identified propertiesarerecoredinTable2.Withregardstotemporal
from the maps using an automated SAD routine in AIPS, variability,somemaserfeaturespersistedthroughallepochs,
employing a detection signal to noise cutoff of 7. Follow- whileothersexhibitedmoresporadicbehaviour.Noperiodic
ing common nomenclature, a ‘spot’ refers to an individual behaviour was noted. Figure 1 shows the temporal changes
maser emission peak, imaged in a single spectral channel - in the overall emission spectrum, and the variability of in-
whileamaser‘feature’referstoacollection ofspotsconsid- dividual maser features can be read from the detection fre-
eredtoemanatefromthesamephysicalmasercloud.Maser quencies in column 3 of Table 2. The distributions of all
spots were grouped into a feature if they shared a spectral maser features detected by VERAare shown in Figure 2.
(cid:13)c 2002RAS,MNRAS000,1–??
4 R. A. Burns
Figure2.InternalmotionsofwatermasersinAFGL5142.VectormagnitudesindicatepropermotionwhilecoloursindicateLSRvelocity.
AnnotationsarethesameasthoseinTable2,withS.E.iandoreferringtothe‘inner’and‘outer’SouthWestgroups,respectively.The
maporiginisatthephasetracingcenter ofthemaserdata.
system center, at map position (α,δ) = (7,−41) mas. This
maser exhibits some remarkable properties which will be
discussed in a later publication.
Our observations did not detect all of the masers re-
portedbyGoddi & Moscadelli(2006).IntheirVLBAobser-
vations they find more water masers associated with MM1
which are located further to the N.W. We include their re-
sultsinourinvestigationofthemaserdistributioninAFGL
5142 MM1 (see Section 3.4).
Water masers associated with MM2 are arranged in a
single arc (see Figure A2). Due to their location ‘far south’
of thephasetracking centerwe refer to thesemasers as the
F.S. group in Table 2. These masers likely correspond to
maser features ‘IIb’ of Goddi & Moscadelli (2006). All F.S.
masers have slightly blueshifted l.o.s. velocities.
We found one maser to the North East (‘B’ in Ta-
ble2)ofthephasetrackingcenter-possiblyassociatedwith
MM1 and probably corresponding to maser feature ‘IIr’ of
Goddi & Moscadelli(2006).Onemaserfeaturewasfoundin
the far South West (‘F.S.W.’), associated with AFGL 5142
Figure 3.SameasFigure2fortheMM1coreregion. MM6.
Figure 3 shows a magnified view of the MM1 region.
3.2 Fitting parallax and proper motion
Masers trace three arc structures contained in a region of
about 300 ×300 mas. There exists one maser arc to the The astrometry of water masers observed with VERA was
North West of the system center, and two proximate and used to measure the annual parallax and resulting trigono-
similarly oriented arcs lying to the South East, one closer metricdistanceoftheAFGL5142region.Toassurethebest
and one further from the system center. We refer to these possible precision for the distance estimate we performed
as the ‘N.W.’, the ‘S.E. inner’, and ‘S.E. outer’ maser arcs, parallax fitting only using maser features which were de-
respectively - and are hence categorised accordingly in col- tected in all 7 epochs - spanning just over one year. Four
umn1ofTable2.TheN.W.masersarcsareredshifted-and maser features passed this criterion, two of which were in
theS.E.masersblueshiftedwithrespecttothel.o.svelocity the N.W. bowshock, one in the S.E. bowshock and one in
of the region (−1.1 km s−1 Zhang et al. 2007). theF.S.bowshock.Parallaxfittingwasdonesimultaneously
Maser feature A (see Table 2) is also associated with for the four maser features, assuming a common distance,
MM1. This maser does not associate with the aforemen- toaverageand smooth outnoise-like andstructureinduced
tioned arcs, and rather, stands in the midway - near the errors. Additionalerror floors were addedand reducediter-
(cid:13)c 2002RAS,MNRAS000,1–??
H O maser astrometry: AFGL 5142 5
2
Figure 5. Ellipses fit to the VERA data, assuming a common
originandindipendantejectionsforN.W.,S.E.iandS.E.omasers.
Figure 4. Annual parallax motions of four maser features in VLBAdatafrom2004(Goddi &Moscadelli2006)arealsoshown.
AFGL 5142 in (above) R.A. and (below) Dec. directions. Maser
features are coloured arbitrarilyand error floors are represented
byverticalbars.
quadrature sum of the average errors of all maser motions
in each bowshock.
ativelyuntilaχ2 valueofunitywasreached.Requirederror Subtracting the systemic proper motion from all abso-
floorswere0.04and0.27masinR.AandDec.,respectively. lutepropermotionsrevealstheinternalmotionsintheframe
We estimate the annual parallax of AFGL 5124 to be of thedriving source, theseare shown in Figures 2 and 3.
π=0.467±0.010mas,correspondingtoatrigonometricdis- Masers in MM1 trace arc shaped shocks which are ex-
tanceofD=2.14+−00..005419 kpc,firmlyplacingitinthePerseus panding from the center of the system at an average veloc-
Arm.FitsareshowninFigure4.Toconfirmthereliabilityof ity of 15 km s−1. We interpret these as bow shocks tracing
oursimultaneousfittingtheparallaxes(see Table2:column theprotostellar jetreportedbyGoddi & Moscadelli (2006);
7) and proper motions of each of the four maser features Goddi et al.(2011)anddiscusstheseinfurtherdetaillater.
werethenfitindependentlyandwerefoundtobeconsistent The masers associated with MM2 also appear to trace an
witheachotherandwiththesimultaneousfit.Notethatour expandingarc.HoweverthedetectionofonlyEasterlyprop-
consistentdistancestoMM1andMM2confirmthattheyare agatingcomponentsi.e.withoutasymmetriccounterpartin
in close physical proximity. theWest, precludesdetailed analysis of its kinematics.
UsingFeatureN.W.1asreferenceweobtainedtherela- MaserfeatureAcouldnotbeassigned apropermotion
tivepropermotionsofallothermaserfeaturesthatwerede- since its motion in thesky-planewas strongly non-linear.
tectedinatleastthreeepochs,ofwhichtherewere21.These
relativemotionswerethenconvertedtoabsolutepropermo-
3.4 Single vs Episodic ejection
tions (see Table 2: columns 6,7) using the absolute proper
motionofFeatureN.W.1fromtheastrometricfittingproce- To investigate the physical nature of ejections from MM1
duredescribedearlier.Propermotionerrorswerecalculated we performed least squares fitting of elliptical jet shells to
as the quadraturesum of thestandard deviation in therel- the maser distributions to evaluate their morphology and
ative proper motion, and the proper motion uncertainty of to determine the center position of the system. Since our
Feature N.W.1. maser data reveal two independent bowshocks in the S.E.
lobe we must consider that ejections may occur in multi-
pleepisodesratherthanasingleejectionevent,thusellipses
3.3 Systemic motion and internal motions
were fit under the condition that the three ejections, each
Absolute (observed) proper motions are a composition of tracedbyabowshock,originate from acommon center.We
the internal motions of masers in the frame of the driv- findthat the dataare best fit to concentricellipses with an
ing source, and the apparent systemic motion of the source origin at (α,δ) = (33,−54) mas, shown as a star in Fig-
acrosstheskyasgovernedbyGalacticdynamics.Inthecase ure5.Thecenterposition coincides with themethanoldisk
where a source’s internal motions are expected to be sym- reportedbyGoddi et al.(2007)andisneartomaserfeature
metricthesumof all motion vectorsshould balance tozero Awhichexhibitsnon-linearmotions(see Section4.1.5).The
- the systemic proper motion of the region is derived from physicaldistances of thebowshocks from thesystem center
the residual of this summation. In AFGL 5124 we used the were 240, 437 and 475 AU for the N.W., S.E. inner and
propermotionsofmasersassociatedwiththeN.W.andS.E. S.E. outer masers, respectively, assuming that these bow-
maser bowshocks in MM1 (8 and 6 maser features, respec- shocks are close the plane of the sky. The shocked surfaces
tively) since the masers in this region are known to trace extend about ∼ 50 AU in a direction perpendicular to the
a symmetric bipolar ejection (Goddi et al. 2007). The re- outflow direction, while their separations from the system
sultingsystemic motion was (µαcosδ,µδ)= (+0.32±0.27, centerrangefrom240−475 AU,indicatingahighdegreeof
−0.22±0.47) masyr−1,whereerrorswerecalculatedasthe collimation.
(cid:13)c 2002RAS,MNRAS000,1–??
6 R. A. Burns
ThedifferenceintheejectionradiioftheN.W.andS.E. 4.1.2 Jet momentum rate
bowshocks groups in our model can be interpreted either
In addition to investigating the driving mechanism, we can
as separate asynchronus ejections expanding from the cen-
calculate the momentum rate (force) of the jet via the ex-
ter at constant velocity, or as a quasi-single ejection event
pression used byGoddi et al. (2011):
with non-equal propagation velocities - possibly due to a
density gradient in the vicinity of the YSO. The former
interpretation appears favoured considering the results of P˙outflow =1.5×10−3 V120 R1200 (Ω/4π) n8 [M⊙ kms−1 yr−1]. (1)
Goddi & Moscadelli (2006) who find masers further in the
N.W.directionthanthoseobservedwithVERA(Figure5). In which V is the maser velocity in units of 10 km
10
These masers align well with the ellipses fit to the VERA s−1,R istheradialdistanceofthemasers fromthedriv-
100
data(eventhoughtheVLBAdatawerenotused forfitting) ing source in units of 100 AU, Ω is the opening solid angle
and may thusbecounterparts to theS.E. bow shocks. of the jet and n is the volume density of ambient gas in
8
Assuming expansion at constant velocity the ages of units of 108 cm−3. The following parameters used in the
ejection events- i.e. thetimeit would havetakenfor ejecta calculation were derived from our observations of the N.W.
to reach the positions of the observed bowshocks - can be maser bowshock; v = 14 km s−1, R = 240 AU, and S.E.
estimatedfromtheirdistancefromthestardividedbytheir bowshock; v = 14 km s−1, R = 475 AU. Furthermore we
outward motion. We measured average internal motions of take Ω = 1.1 sr from the jet continuum observations of
1.4 mas yr−1 for each of the lobes, in opposite directions. Goddi et al. (2011). Regarding the ambient density, n , we
8
Fortheejections observedinAFGL5142MM1weestimate considervaluesintherange107−109 cm−3 whicharesimu-
ejection ages of 80, 145 and 159 years for the N.W., S.E. latedinitialambientdensitiessuitableforproducingmasers
innerandS.E.outerbowshocks,respectively.Inthisscenario in shocks (Kaufman & Neufeld 1996). We arrive at a range
the S.E. bow shocks would be roughly twice the age of the of momentum rates between P˙ =10−4 to 10−2 M⊙ km s−1
N.W.bowshock,with14yearsintervalbetweentheejections yr−1 for both bowshocks. These ranges are consistent with
of the inner- and outer-S.E. ejections. Goddi et al. (2011) and theextended molecular outflows in
AFGL 5142 MM1 (see Table 3).
Ifweinstead derivethejet openingangle directlyfrom
the maser data we arrive at Ω = 0.07 sr and momentum
4 DISCUSSION
rate ranges of P˙ = 10−6 to 10−4 M⊙ km s−1 yr−1. The
4.1 Physical properties of the jet fundamental property that maser emission arises in regions
of special physical conditions, and that they are observed
4.1.1 Outflow driving mechanism
preferentially along surfaces tangental to the observer, can
Masers in AFGL 5142 MM1 trace arcs of shocked gas at complicatemaser-derivedestimatesofthejetopeningangle
theleading edge of a protostellar jet. Using the propermo- -exceptforcaseswherethejetheadandbodyaremorefully
tionsof masersmeasured with VLBI wecaninvestigate the traced.Assuch,wearguethatthejetopeninganglederived
physicalproperties of thejet bycomparison with published from the continuum observations of Goddi et al. (2011) is
outflowmodels.VLBImaserdataarewellsuitedforcompar- preferred.
isonwiththemodelsofLee et al.(2001)andOstriker et al.
(2001)whodescribethemotionsofgasintheshockedregion
betweenaprotostellaroutflowandtheambientgas.Intheir
workstheycomparetwomodels;ajetdrivenbowshock and
a stellar wind. Generally, the prevalence of each model can
4.1.3 Primary jet velocity
bedeterminedbytheamountoftransversemotionobserved
near the head of the jet; in a jet driven outflow the trans- When the ram pressure of the jet is sufficient to balance
verse velocity is large as the jet material disperses at the that of the swept up material the relationship between the
bowshock while the transverse velocities in a stellar wind velocitiesoftheprimaryjet,andthatofthesweptupgas,is
tend to zero at the tip since in a momentum driven stellar givenasvj ≈vws(1+pn/nj),adaptedfrom Chernin et al.
wind the gas propagates radially outward from the YSO, (1994). Where vj and vws are the velocities of the primary
and thermal pressure is negligible. jet and working surface (traced by the bowshock velocity),
Using a jet position angle of −35◦ we measure trans- and nand nj are theambient and jet numberdensities, re-
verse proper motions in a velocity range of 0−13 km s−1, spectively. For the ambient density we use n = 108 cm−1,
withameanvalueof4kms−1.Thelargerangeoftransverse adopted from shock models Hollenbach et al. (2013). Re-
motions indicate that the outflow in AFGL 5142 MM1 is garding the jet density, nj, Caratti o Garatti et al. (2015)
dominatedbyjet-drivenbowshockkinematics.Sincemasers find primary jet electron densities of ne ∼103 to 104 cm−3
in AFGL 5142 MM1 exclusively trace the leading surface in their survey of MYSOs. Employing an ionisation frac-
of the outflow it is difficult to quantify any contribution tion of xe=ne/nH =0.1 (Hartigan et al. 1994) theMYSO
from a stellar wind - whose influence is more easily seen primary jet densities fall in the range of n = 104 to 105
alongthebodyoftheoutflow.Othersimilar workscompar- cm−3. Finally, we estimate primary jet velocities in the
ing VLBI determined maser proper motions with the mod- range of vj ∼ 100 to 1000 km s−1. The jet velocity de-
els of Lee et al. (2001) and Ostrikeret al. (2001) also find rived here for AFGL 5142 MM1 is consistent with other
MYSO jets to be dominated by jet-driven bowshocks with values for MYSOs from the literature (Marti et al. 1995;
some contribution from a lower velocity wind (Sannaet al. Curiel et al. 2006; Sannaet al. 2016; Guzma´n et al. 2016;
2012;Burns et al. 2016). Rodr´ıguez-Kamenetzky et al. 2016).
(cid:13)c 2002RAS,MNRAS000,1–??
H O maser astrometry: AFGL 5142 7
2
Figure 6. Combined view of 22 GHz water masers (filled circles) observed with VERA in 2010 (this work), 22 GHz water masers
(asterisk)observedwiththeVLBAin2004(Goddi &Moscadelli2006)and6.7GHzmethanolmasers(triangles)observedwiththeEVN
in2004(Goddi etal.2007).TheinsetshowsthetrajectoryofmaserfeatureA,movinginaclockwisefashion.Thetrajectoryoffeature
Aand propermotions of other masersareallconverted to theYSO frame.Theblack asterisksymbol indicates the approximate origin
oftheepisodicejections,estimatedfromleast-squaresfittingofellipsestotheVERAmaserdata.
4.1.4 bowshock velocity gradient ture ubiquitous in low mass star formation and becom-
ing increasingly encountered in MYSOs with well studied
As a closing remark on the observationally determined jet
cases including Orion Source I (Hirota et al. 2014), IRAS
physical properties in AFGL 5142 MM1, we note a shallow
20126+4104 (Moscadelli et al.2011;Chen et al. 2016).Wa-
yet well structured velocity gradient across the N.W. bow-
termaserstraceanarrowepisodicjetwhichemanatesatan
shock masers (see Figure A1). This could be interpreted as
angle near perpendicular to the protostellar disk traced by
a flattened and expandingejection or as rotation of theen-
6.7GHzmethanolmasers-atransitionexclusivetoMYSOs.
trained material about the jet axis - a phenomenon widely
sought in the context of massive star formation as it would Maser feature A, detected in this work only, exhibits
suggestthatoutflowsaredrivenbymagneto-centrifugaljets a curved trajectory and lies close to the likely position of
(seealsoBurnset al.2015).OurdatashowthatAFGL5142 the star as was inferred independently by the center of
MM1 haspotential as a target for futurerotating jet inves- kinematics of the 6.7 GHz methanol masers (Goddi et al.
tigations. 2007), the center of kinematics of the VLBA water masers
(Goddi & Moscadelli 2006)andtheconcentricellipsesfitto
VERA water masers - presented in Section 3.4. Non-linear
motion implies the application of force - whether that be
4.1.5 Combined VERA, VLBA and EVN view of AFGL
gravitationally or magnetically dominated we expect that
5142 MM1
the maser must be in close proximity to the central object.
AFGL 5142 has been observed by three different VLBI Thetrajectory offeature Aisshownin aninset in Figure6
arrays; the VLBA (Goddi & Moscadelli 2006), the EVN where motion progresses from the upper right of the inset
(Goddi et al. 2007), and VERA (thiswork). These are pre- in a clockwise direction. The trajectory has been shifted to
sented together in Figure 6. theframeofthestar.Wepursueafulldecomposition ofthe
The combined view of VLBI maser observations of trajectoryofthismaserinafuturepublicationtargetingthe
AFGL 5142 depicts a prototypical disk-jet system; a struc- central <100 AUregion.
(cid:13)c 2002RAS,MNRAS000,1–??
8 R. A. Burns
Overall, the features highlighted by the combination
of VLBA, EVN and VERA observations reveal a system
comprising a circumstellar disk from within which a nar- -40
rowandcollimated jet islaunched.Thesefeatures resemble
those representative of low mass YSOs. As we showed in
-20
Section 4.1.1, the outflow launching mechanism in AFGL
g]
5142MM1alsoresemblesthosecommontolow-massYSOs, e
whiletheoutflowmomentumrateofAFGL5142MM1(Sec- A. [d 0
tion 4.1.2) is larger than those typically seen in low-mass P.
stars(Beutheret al.2002).Episodicejection,whichinturn 20
impliesthataccretionalsooccursepisodically,isanotherfea-
tureshared with low-mass disk-aided starformation. These
40
points, taken together with the morphology and 3D kine-
matics of structures near the MYSO compiled from VLBI
40 35 30 25 20 15 10 5 0
observations,revealsthatAFGL5142MM1depictsa‘scaled
up’picture of low-mass star formation. extent [asec]
Figure7.TheangularextentofoutflowsfromTable3asafunc-
4.2 Molecular outflows in the AFGL 5142 region tion of position angle. A sinusoid represents expected values for
aslowlyprecessingoutflow,fittothedata.
Previousattemptsatlocatingtheprogenitorsofthemultiple
outflows seen in AFGL 5142 struggled on account of there
Table3.Outflowparametersfromthisworkandtheliterature.
beingmoreoutflows(atleast4)thanmajormillimetercores
(onlytwo),andbecausemostoutflowsintersectoneorboth
ofMM1orMM2.Weproposethatseveraloftheoutflowsin Tracer Length P.A. Momentumrate Notes
this region could be explained by episodic ejections from a ′′ [◦] [M⊙kms−1yr−1]
slowlyprecessingoutflowsystem.InarecentpaperLiu et al. CO(2-1) 40 34 1.11×10−4 1
(2016) consider that a precessing jet may be the best way HCO+(1-0) - - 4.52×10−3 2
SiO(2-1) 30 10 - 2
to produce the extremely wide-angle bipolar outflow which CO(2-1) 30 5 2.85×10−3 ‘OutflowA’,3
forms the subject of their investigation. Weshare this view CO(2-1) 10 -40 2.38×10−3 ‘OutflowC’,3,4
and extend on justifications described as follows. H2O(616−523) 0.1 -35 10−4 to10−2 thiswork
Outflows A and C from Zhanget al. (2007), the ‘com-
Column (5): 1 - Hunteretal. (1995); 2 - Hunteretal. (1999); 3 -
pact CO outflow’ of Hunteret al. (1995) and the SiO Zhangetal.(2007);4-Palauetal.(2011)
and HCO+ outflows of Hunteret al. (1999) share a trend
whereby the redshifted lobe of the outflow extends to the
ures 8,A2), and also the proper motions of the F.S. and
North, while the blueshifted lobes extend to the south. We
F.S.Emasers(Figure8).Notethatbothmillimeter coresof
evaluate this trend in Figure 7 which shows theP.A. of the
MM1 and MM2 are elongated in the direction perpendicu-
aforementioned outflows as a function of angular extent,
lar to the assigned outflows, a known attribute of disk-jet
which we take as analogous to the outflow age (more ex-
systems (Reid et al. 2007).
tended outflows are older). Given that the age of the more
extended outflows are a few 104 years old (Hunteret al. ThevelocityorientationandpositionangleofoutflowB
isoppositetothoseofoutflowsAandC,rulingoutanassoci-
1995, 1999), the precession has an approximate period of
onerevolutionper104−105 years.Forthisanalysisweonly ation with MM1. Outflow B may be driven by MM3 which
could be a low mass YSO (Zhanget al. 2007). MM3 sits
use the Northern, redshifted outflows since the southernly
between the blue- and redshifted lobes of outflow B where
directed blueshifted outflows often appear to truncate near
Zhanget al. (2007) detect weak 8.4 GHz continuum emis-
MM2, which is known to coincide with the densest part of
sionatthelocationofthemillimetercore.Theauthorsthus
the molecular envelope (see Figure 4 of Zhang et al. 2007).
arguethatMM3maybeofstellarnature,whiletheremain-
Outflow details used in the analysis and references to the
ing millimeter cores, MM6, MM7, MM8 and MM9 can be
original works are given in Table 3.
explainedasdustcondensationssweptupbymolecularout-
Figure7demonstratestheplausibilitythattheoutflows
flows, D, A/C, A and C, respectively.
listedaboveemanatedfromasingle,precessingjetsystem-
whichwecanconfidentlyattributetoMM1thankstoworks
atthehighestangularresolution(Goddi & Moscadelli2006;
4.3 Episodic ejection in AFGL 5142
Goddi et al. 2011 and this work). To further test this hy-
pothesis we consider the momentum rates of each outflow, Bowshockstracetheleadingedgesofnewejectionevents.As
which we calculate or re-evaluate from the literature using such, multiple bowshocks imply a history of multiple ejec-
ourtrigonometricdistanceof2.14kpcandrecordincolumn tions.InSection3.4wepresentevidenceofepisodicejection
4 of Table 3. All outflows under consideration have compa- operating on timescales of 101 years, based on maser data.
rablemomentumrates,supportingthehypothesisthatthey In Section 4.2 we argue that several of the molecular out-
were driven by thesame progenitor. flowsinAFGL5142appeartostemfromAFGL5142MM1.
Regardingtheremainingoutflows,outflowDwaslikely Regardingthedynamicaltimescales ofthesemolecular out-
driven by MM2 since its East-West orientation matches flows, the older, more extended outflows from MM1 are a
the cental axis of the bowshock in the F.S. masers (Fig- few104 yearsold(Hunteret al.1995,1999),whilethereare
(cid:13)c 2002RAS,MNRAS000,1–??
H O maser astrometry: AFGL 5142 9
2
Figure 8.Overlayshowing water maser activity inthe context ofthe millimetercoreand molecular outflows. Small arrowvectors are
thesameasinFigure2.Greyscaleshowsthe1.3mmcontinuumemissionintherangeof0-0.07mJywithcontoursat-4,4,8,12,16,20
and24timesthermsnoiseof2.8mJybeam−1,fromPalauetal.(2013),withthemillimetercoreslabeled.Theredcontourindicatesthe
6σdetectionadoptedbyPalauetal.(2013).Largerarrowswithlettersannotatedindicatethedirectionsofmolecularoutflowsdescribed
inpreviousworks(Zhang etal.2007;Palauetal.2013).
outflowsassociatedwithMM1on4differentscales(Table3) massivestarformingregionofπ=0.467±0.010 mas,corre-
-correspondingtoejectiontimescalesofeveryfew104years. sponding toa trigonometric distance of D=2.14+0.051 kpc.
−0.049
Since outflow activity is expected to correlate with
accretion activity (Caratti o Garatti et al. 2015) the 104 Using the latest VLBI H2O maser data we analyse a
year timescale of ejections could indicate alternating pe- recentejectionfromtheAFLG5142MM1regionwhichpro-
riods of accretion bursts and quiescence in AFGL 5142 duced prototypical bi-polar bowshocks that expand from a
MM1, consistent with timescales predicted for episodic ac- commoncenterinacircumstellardisk.Weusethe3Dkine-
cretioninlow-(Stamatellos et al.2011)andhigh-massstars matics of the maser bowshocks to investigate the physical
(Meyer et al.2017).Theshortepisodicejectionseenona10 properties of theprotostellar jet.
yeartimescales(Section3.4)wouldlikelyoriginateinthein-
nerdisk. Oneofthemasersnearthelikelypositionofthestarex-
The concept of episodic accretion is only recently be- hibitsclearnon-linearmotion.Theimmediateregionaround
ing considered in the framework of massive star formation, the star requires further deep observations to provide vital
context for this very unusualfinding.
thoughitsabilitytodealwiththe”luminosityproblem”and
suppress the accretion inhibiting radiation field show great
CombiningourdatawiththoseofotherpublishedVLBI
potential in explaining how massive stars accumulate their
observations and through proper motion analysis we find
mass.AFGL5142presentsapromisingobservationaltarget
that AFGL 5142 MM1 is forming as a disk-jet system with
for studies of episodic ejection and accretion in MYSOs.
evidence of episodic ejection, giving it the appearance of
‘scaled up’low-mass star formation.
Finally, we reinterpret the outflow history of AFGL
5 CONCLUSIONS
5142MM1asformation viaanepisodic,precessingoutflow.
UsingVERA,adual-beamVLBIarraydedicatedtoastrom- We were able to suggest progenitor allocations for all out-
etry, we measured an annual parallax for the AFGL 5142 flows in AFLG5142 known from thecurrent literature.
(cid:13)c 2002RAS,MNRAS000,1–??
10 R. A. Burns
ACKNOWLEDGMENTS LiuT.,ZhangQ.,KimK.-T.,WuY.,LeeC.-W.,Goldsmith
P.F., Li D., Liu S.-Y.,Chen H.-R.,Tatematsu K.,Wang
R.B. would like to acknowledge the Ministry of Education,
K., Lee J.-E., Qin S.-L., Mardones D., Cho S.-H., 2016,
Culture,Sports,ScienceandTechnology(MEXT)Japanfor
ApJ,824, 31
support as part of theMonbukagakushoscholarship.
Marti J., Rodriguez L. F., Reipurth B., 1995, ApJ, 449,
We kindly thank the referee for providing helpful feed-
184
back and guidance during thereview of this work.
MeyerD.M.-A.,VorobyovE.I.,KuiperR.,KleyW.,2017,
MNRAS,464, L90
Moscadelli L., Cesaroni R., Rioja M. J., Dodson R., Reid
M. J., 2011, A&A,526, A66
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