Disks and outflows in the massive protobinary system W3(OH)TW
Luis A. Zapata, Carolina Rodríguez-Garza, Luis F. Rodríguez, Josep M. Girart, Huei-Ru Chen
aa r X i v : . [ a s t r o - ph . GA ] A ug T o appear in the A p JL.
Preprint typeset using L A TEX style emulateapj v. 08 / / DISKS AND OUTFLOWS IN THE MASSIVE PROTOBINARY SYSTEM W3(OH)TW L uis A. Z apata , C arolina R odr ´ ıguez -G arza , L uis F. R odr ´ ıguez , J osep M. G irart , and H uei -R u , C hen To appear in the ApJL.
ABSTRACTSensitive and high angular resolution ( ∼ ′′
7) (sub)millimeter line and continuum observations of the massivestar forming region W3(OH) made with the Submillimeter Array are presented. We report the first detectionof two bipolar outflows emanating from the young and massive ”Turner-Welch” [TW] protobinary systemdetected by the emission of the carbon monoxide. The outflows are massive (10 M ⊙ ), highly-collimated (10 ◦ ),and seem to be the extended molecular component of the strong radio jets and a 22 GHz maser water outflowenergized also by the stars in the W3(OH)TW system. Observations of the 890 µ m continuum emission and thethermal emission of the CH OH might suggest the presence of two rotating circumstellar disk-like structuresassociated with the binary system. The disks-like structures have sizes of about 1500 AU, masses of a fewM ⊙ and appear to energize the molecular outflows and radio jets. We estimate that the young stars feeding theoutflows and that are surrounded by the massive disk-like structures maybe are B-type. Subject headings: stars: formation — ISM: jets and outflows — ISM: individual objects (W3(OH), W3(H O),W3(OH)TW) INTRODUCTION
There is recent observational evidence that the formationof massive stars (early B-type to late O-type) takes place ina way similar to that of solar-type objects, namely by accre-tion via a circumstellar disk: a handful of cases where a diskcould be present around a forming massive star have been pre-sented in the literature (e.g. Fern´andez-L´opez et al. 2011a,b;Galv´an-Madrid et al. 2010; Rodr´ıguez et al. 2007; Schreyeret al. 2006; Patel et al. 2005; Shepherd & Kurtz 1999). Thesedisks are larger than those found in forming solar stars andhave dimensions of less than 1000 AU and masses of a fewsolar masses. For more massive stars (i.e. early O-type), thedisks appear to be much more massive and larger (e.g. Zapataet al. 2010a; Qiu et al. 2009; Franco-Hern´andez et al. 2009)One of the nearest and best-studied sites of ongoing massivestar formation is the W3(OH) region. It is located at 2.04 kpc(Hachisuka et al. 2006). Within the W3(OH) region there areclearly two objects that host young massive stars, W3(OH)itself and the Turner-Welch” [TW] object, both bright at radioand millimeter wavelengths (Turner & Welch 1984). W3(OH)is a well-known ultracompact H II region ionized by youngOB stars, and rich in OH maser emission (Reid et al. 1995;Wilner et al. 1999; Fish & Sjouwerman 2007). W3(OH)TWor W3(H O), on the other hand, seems to be in a youngerstate, with no associated HII region and with strong dust andmolecular emission at (sub)millimeter wavelengths (Wilner etal. 1995; Wyrowski et al. 1997). Bally& Lada (1983) reportedfaint wings in the CO (J = ∼
26 km s − . However, no additional studies for highvelocity gas were reported in the literature.High angular radio observations resolved W3(OH)TW intoa binary system: W3(OH)TW-A and W3(OH)TW-C. Thespectrum of W3(OH)TW-A from 1.6 to 15 GHz exhibits apower law with spectral index − Centro de Radioastronom´ıa y Astrof´ısica, Universidad NacionalAut´onoma de Mexico, Morelia 58090, Mexico Institut de Ciencies de l’Espai (CSIC-IEEC), Campus UAB, Facultat deCiencies, Torre C5-parell 2, 08193 Bellaterra, Catalunya, Spain Department of Physics, National Tsing Hua University, Hsinchu, Taiwan that this object is a synchrotron jet (Reid et al. 1995; Wilneret al. 1999). The elongated continuum source is further coin-cident, within 0.1 arcsec, with the center of expansion of theH O masers and is aligned with the dominant H O outflowpattern (Alcolea et al. 1993). At millimeter wavelengths thisobject shows optically thin dust emission with a spectral in-dex of + + + OBSERVATIONS
Millimeter
The observations were made with 8 antennas of the SMA on 2007 August in its compact configuration. The phase refer-ence center for the observations was at R.A. = = + ◦ ′ ′′ (J2000.0). The frequency was centeredat 220.730 GHz in the Lower Sideband (LSB), while the Up-per Sideband (USB) was centered at 230.730 GHz. The pri-mary beam of the SMA at around 230 GHz has a FWHM ofabout 60 ′′ . The emission from the whole of the W3(OH) re-gion falls very well inside of our FWHM.The SMA digital correlator was configured in 24 spectralwindows (“chunks”) of 104 MHz each, with 128 channels dis-tributed over each spectral window, providing a resolution of0.812 MHz ( ∼ − ) per channel.The zenith opacity ( τ GHz ), measured with the NRAO tip-ping radiometer located at the Caltech Submillimeter Obser-vatory, was from 0.24 to 0.32, indicating reasonable weatherconditions during the observations. Observations of Uranusprovided the absolute scale for the flux density calibra- The Submillimeter Array (SMA) is a joint project between the Smithso-nian Astrophysical Observatory and the Academia Sinica Institute of Astron-omy and Astrophysics, and is funded by the Smithsonian Institution and theAcademia Sinica.
Zapata, et al.
W3(OH)
White: SubmillimeterBlue Red:Green: RadioCO(2−1)
DISKS+JETS
CW3(OH)TW A
W3(OH)TW F ig . 1.— LEFT: Integrated intensity color and contour maps of the CO(2-1) emission from the W3(OH) region overlaid in contours with the SMA 890 µ m continuum emission(white) and the VLA 3.6 cm continuum emission (green). The blue and red contours are from 30% to 90% with steps of 7% of the peak of the line emission; the peak the CO(2-1)emission is 100 Jy Beam − km s − . The white contours are from −
30% to 90% with steps of 7% of the peak of the emission; the peak of the 890 µ m map is 830 mJy Beam − . Thegreen contours are from 0.45% to 10% with steps of 0.1% of the peak of the emission; the peak at 3.6 cm is 23 mJy Beam − . The synthesized beams of the CO and 890 µ m continuumare described in the text and are shown in the left and right upper corners of the two images, respectively. RIGHT: A zoom into the W3(OH)TW region. The green and white contoursare the same as in the left Figure. The grey scale image shows the SMA 890 µ m continuum emission. The color-scale bar on the right indicates the flux scale in mJy. The blue and redarrows represent the position and orientation of the molecular outflows emanating from the massive protobinary. tion. The gain calibrators were the quasars 0102 +
584 and0359 + The calibrated data were imaged andanalyzed in the standard manner using the MIRIAD andKARMA softwares. We set the ROBUST parameter of thetask INVERT to − − for each velocity channelat an angular resolution of 2. ′′ × ′′
55 with a P.A. = . ◦ . Submillimeter
The observations were made with 7 antennas of the SMAon 2007 August in its extended configuration. The receiverswere tuned to a frequency of 346.383 GHz in the USB, whilethe LSB was centered on 336.383 GHz. The phase referencecenter for the observation was R.A. = =+ ◦ ′ ′′ (J2000.0).The zenith opacity ( τ GHz ) was from 0.06 to 0.08, indicat-ing excellent weather conditions. Observations of 3C273 withan adopted flux of 8.8 Jy, provided the absolute scale for theflux density calibration. The gain calibrators were the quasars3C84 and 3C111. For the continuum, we set the ROBUST pa-rameter to − =+ The MIR-IDL cookbook by C. Qi can be found at http: // / ∼ cqi / mircook.html images rms noises were around 50 mJy beam − and 150 mJybeam − , respectively at an angular resolution of 0. ′′ × ′′ = . ◦ for the continuum and 0.88 ′′ × ′′ witha P.A. of 80.4 ◦ for the line. RESULTS
Molecular outflows
In our 2 GHz upper side band of the millimeter observa-tions, we detected the line CO(2-1) at a rest frequency of230.53800 GHz. In Figure 1, we present a map of the inte-grated intensity over velocity (moment 0) of the line emissionobserved toward W3(OH). This map was additionally over-laid with the 890 µ m continuum emission obtained from theseobservations and the 3.6 cm radio emission from Reid et al.(1995), and Wilner et al. (1999). The velocity integration isover the velocity ranges: −
75 to −
55 km s − (blue) and − −
25 km s − (red). The emission at ambient velocities ( − −
46 km s − ) was clearly extended and poorly sampled withthe SMA, and was suppressed in this moment zero map. Thismap reveals two strong collimated and bipolar outflows em-anating from the massive protobinary system W3(OH)TW,both with similar orientations. One of the bipolar outflowsemanates from W3(OH)TW-A with its blueshifted side to-wards the southwest while its redshifted side is located tothe northeast. This outflow has a position angle of + ◦ .The second outflow emanates from W3(OH)TW-C with itsblueshifted side towards the northeast while its redshifted sideis located to the southwest. This outflow has a position angleof + ◦ . Both outflows show collimation factors of about 10 ◦ .The extension of the outflows is about 0.1 pc.We note that both red lobes appear to be significantly largerthan the blue lobes. We tentatively suggest that this e ff ectcould be created if very dense molecular material is presentisks and outflows in the massive protobinary system W3(OH)TW 3 Log F l u x D en s i t y F n ( m Jy ) Log Frequency n (GHz) W3(OH)TW−CBIMA + SMA n VLA n DiskThermal jet?
Log F l u x D en s i t y F n ( m Jy ) Log Frequency n (GHz) W3(OH)TW−ABIMA + SMA n VLA n -0.5 Synchrotron Jet
Disk F ig . 2.— Energy spectral distributions for W3(OH)TW-A (left) and W3(OH)TW-C (right) from radio to (sub)millimeter wavelengths. The radio and millimeter data were obtainedfrom Reid et al. (1995), Wilner et al. (1999), and Chen et al. (2006). The submillimeter data is presented here (Table 1). The line is a least-squares power-law fit (of the form S ν ∝ ν α )to the spectrum. The α -values of the fitting for the di ff erent components of the spectrum are shown in the panels. between us and the source and that this gas is stopping thedevelopment of the blue lobes.Assuming that we are in local thermodynamic equilibrium(LTE), the molecular emission is optically thin, an excitationtemperature equal 50 K, and an abundance ratio of CO / H equal to 1 × − , we can estimate the mass of the outflows forthe CO molecule in the transition ∆ J = ⊙ for each outflow.For a mechanical force of F M =
10 M ⊙
20 km s − / = ⊙ km s − yr − and from the correlation presented inWu et al. (2004) for the outflow mechanical force versus thebolometric luminosity of the exciting source, we very roughlyestimate a luminosity for the central powering source on theorder of 10 − L ⊙ , which corresponds to a massive B-type pro-tostar. This spectral type for the central star is in good agree-ment with that obtained from the dynamical considerations,as we will see in the next section.It is interesting to note that the molecular outflows are verylikely be the extended molecular component of the outflowstraced at smaller scales by the radio jets and 22 GHz maserwater outflow reported by Reid et al. (1995), Wilner et al.(1999), and Alcolea et al. (1993). However, for examplethe radio jet associated with the object W3(OH)TW-A hasan east-west orientation or a P.A. of + ◦ , while the molecu-lar outflow related with this source has a di ff erent orientationtoward the southwest-northeast or a P.A. of + ◦ . The 22GHz maser water outflow has a similar orientation to the ra-dio jet. This might be explained if the ejected material fromW3(OH)TW-A bends sometime after the ejection or maybethis source precesses as expected from a binary source. Thereare some cases where the molecular outflow changes orien-tation or even precesses, see for example Choi et al. (2006),Zapata et al. (2010b), and Cunningham et al. (2009). Maybethe outflows are arising from di ff erent very compact sourceswithin W3(OH)TW-A or perhaps this source could be enegiz-ing both outflows as this may precess.Argon et al. (2003) found several OH masers that are as-sociated with the bipolar outflow traced by the strong H O masers. These OH masers trace the outflow at distances of1-2” from the TW-A source, a factor of 2 larger that thedistances traced by the water masers. Interestingly, the OHmasers suggest that the outflow may be starting to bend in thedirection traced by the CO at scales or 5-10”.
Circumestellar disks?
Continuum emission
In Figure 1, we show the resulting submillimeter ( λ = µ m) continuum image obtained with the Submillimeter Ar-ray from the high-mass star forming region W3(OH). Wedetected continuum emission arising from the ultra-compactHII region W3(OH) itself and the massive protobinary sys-tem W3(OH)TW. At these wavelengths, the dominant sourceis W3(OH)TW. The flux density of W3(OH) is about 1.1Jy. With our present angular resolution ( ∼ ′′ σ =
200 mJy) associated with thesource W3(OH)TW-B reported by Wyrowski et al. (1997).W3(OH)TW-A and W3(OH)TW-C are well resolved atthese wavelengths and show sizes of about 1500 AU at a dis-tance of 2.04 kpc (Hachisuka et al. 2006). However, both ob-jects show di ff erent morphologies, the objects associated withW3(OH)TW-A is very elongated in the northeast-southwestdirection (with positional angle equal − ◦ ) and with a sizeof its mayor axis of about 2000 AU, while its minor axishas 1000 AU. W3(OH)TW-C, on the other hand, does notshow this marked elongation, this object instead shows a moreroundish morphology, with the size of its minor and mayoraxis quite similar (of about 1000 AU) and with a positionalangle equal − ◦ , see Table 1.In Figure 2, we present the Spectral Energy Distribu-tions (SEDs) for W3(OH)TW-A and -C. The spectrum ofboth objects shows a ”combined” two-regime spectrum, withone component observed at radio wavelengths with a nega- Zapata, et al. TABLE 1P hysical parameters of the circumstellar disk - like structures Position a Total Flux Disk Dyn. α (J2000) δ (J2000) Density (mJy) Deconvolved Angular Size b Spectral Mass MassW3(OH)TW 02 27 +
61 52 0.87 mm Index M ⊙ M ⊙ A 04.674 24.72 1500 ±
100 0. ′′ ± ′′ × ′′ ± ′′ − ◦ ± ◦ ±
150 0. ′′ ± ′′ × ′′ ± ′′ − ◦ ± ◦ a Units of right ascension are hours, minutes, and seconds and units of dec-lination are degrees, arcminutes, and arcseconds. b Major axis × minor axis; position angle of major axis. The values wereobtained using the task IMFIT of MIRIAD. A C p.a.=−90
W3(OH)TW CH3OH p.a.=−35 F ig . 3.— UPPER: Integrated intensity weighted velocity map of the CH OH[(3,0)-4(2,2)] emission from W3(OH)TW-A overlaid in contours with the 890 µ m continuumemission. The integrated velocity range is from −
54 to −
46 km s − . The systemicvelocity of W3(OH)TW-A is about − − . LOWER: Same map as in the upperpanel, but now for W3(OH)TW-B. The integrated velocity range is from −
42 to −
48 kms − . The systemic velocity of W3(OH)TW-B is about − − . The color-scalebars on the right indicates the LSR velocities in km s − . The synthesized beam of theline image is shown in the bottom left corner of each image. The dashed lines in eachpanel mark the orientation and position were the PV-diagrams presented in Figure 4were made. The synthesized beam of the images is 0.88 ′′ × ′′ with a P.A. of 80.4 ◦ and is shown in the left corner of the figures. tive / positive slowly rising spectrum (with α = − α = / thermal?jets (Reid et al. 1995; Wilner et al. 1999). The second com-ponent is observed at (sub)millimeter wavelengths with theemission that rises rapidly with frequency, and is associatedwith optically thin dust emission from a circumstellar disk ormaybe an envelope (Wyrowski et al. 1997, 1999; Chen et al.2006).We have obtained the spectral indices (S ν ∝ ν α ) forboth structures from the millimeter and submillimeter BIMA + SMA observations, see Table 1. The spectral indicesare very steep (2.6 for the component A and 2.9 for the com-ponent B) which are consistent with optically thin dust emis-sion and with a dust mass opacity coe ffi cient that varies withfrequency as κ ν ∝ ν . − . . This spectral index determinationis reliable since the 0.89, 1.4, and 2.8 mm observations havesimilar angular resolution ( ≤ ′′ ) and are in very good agree-ment with those values obtained at millimeter wavelengths byChen et al. (2006).With this information, we can estimate the masses of the890 µ m sources. Adopting a value of κ . mm = g − (the average of the values of 1.0 cm g − , valid for grains withthick dust mantles, and 2.0 cm g − , valid for grains withoutmantles). This implies κ µ m ∼ g − . Assuming opti-cally thin, isothermal dust emission and a gas-to-dust ratio of100, and a dust temperature of 100 K for the (sub)millimeterobjects (Chen et al. 2006), we derive masses of about a fewsolar masses (see Table 1). These values for the mass arein reasonable agreement with the values found by Chen et al.(2006) for W3(OH)TW-A equal to 5 M ⊙ and for W3(OH)TW-C equal to 4 M ⊙ . The di ff erences would be attributed to theobservations of Chen et al. (2006) probably are recoveringmore extended emission from the objects.The dimensions ( ∼ ⊙ ),and that from both sources emanate at large scales powerfulmolecular outflows and at small scales thermal / non-thermaljets (Figure 1) suggest that W3(OH)TW-A and -C containmassive circumstellar disks. Furthermore, the two submil-limeter objects have orientations consistent with the ejectionof the outflows (Table 1).The derived masses and sizes of the two hypothesized disks(Table 1) will make fairly large optical depths of 0.3 in thedust emission. In such case, the spectral index may be a ff ectedby the optical depth and the assumption of optically thin dustemission might be not correct at all.There is, however, a problem with the interpretation ofW3(OH)TW-A being a massive circumstellar disk. The ori-entation of the synchrotron jet and the H O maser outflow thatemanates from this source has an east-west orientation, and isnot consistent with the orientation of the disk-like structure.However, as mentioned before, maybe W3(OH)TW-A is pre-cessing or the putative disk is actually circumbinary, and onecompact source in the middle drives the radio jet, and the sec-ond one the molecular outflow.
Line emission
In our 2 GHz lower side band of the submillimeter obser-vations, we detected the line CH OH[(3, 0)-4(2, 2)] v t = µ m continuum emission obtained by averaging the line-free channels in our SMA observations. These maps revealisks and outflows in the massive protobinary system W3(OH)TW 5 W3TW−AW3TW−C
DISK OF W3TW−A F ig . 4.— Position velocity diagrams computed from Figure 3. UPPER: PV diagramfor W3(OH)TW-A. LOWER: PV diagram for W3(OH)TW-B. The units of the verticalaxis are in arcseconds. The synthesized beam of the images is 0.88 ′′ × ′′ with a P.A.of 80.4 ◦ . The spectral resolution is 0.704 km s − . The white lines in the panels mark thevelocity gradient found in every circumestellar disk. compact and strong submillimeter molecular emission arisingonly from the continuum sources and with clear velocity gra-dients of a few kilometers per second. W3(OH)TW-A showsa velocity gradient of about 8 km s − arcsec − at a positionangle of − ◦ and W3(OH)TW-C of about 6 km s − arcsec − at a position angle of − ◦ . In Figure 3, we only present thestrongest CH OH emission from both sources, however, in Figure 4 we show more clearly the full magnitude of the ve-locity gradients. The systemic velocities of each source are abit di ff erent, we find that W3(OH)TW-A is at − − ,while W3(OH)TW-C is at − − . Assuming that thetwo sources are separated by ∼ ′′ and in a bound circular or-bit, we estimate a lower limit of 30 M ⊙ for the whole system.We note that the orientation of the velocity gradient found inthese objects are in very good agreement with the position ofthe mayor axis of the circumstellar disk-like structures (Ta-ble 1) suggesting that we are likely seeing the rotation of themolecular gas in them.In Figure 4, we show the kinematics of the molecular gasof both possible circumstellar disks. In the position-velocitydiagrams it is much more clear to see the gradients and theirmagnitudes. The velocity gradients are clearly linear and as-sociated with the rotation of a rigid body. However, to detectthe Keplerian motions, typically observed in many low-masscircumstellar disks, we need much more angular and spectralresolution. Zapata et al. (2010a) noted that the Keplerian mo-tions of the extremely large circumstellar disk associated withthe massive young object W51 North are more pronouncedclose to the protostar while in the edges the molecular gasmoves more like a rotating ring. Assuming that the veloc-ity gradients are Keplerian, we find that the dynamical massfor W3(OH)TW-A is 16 M ⊙ and for W3(OH)TW-C is 6 M ⊙ .The sizes of the line emission are similar to those found inthe dust continuum emission. Those values are in good agree-ment with Chen et al. (2006) that reported a total dynamicalmass for the system of about 22 M ⊙ . However, we note thatthey could not separate the velocity gradient of each disk, theyinstead reported a single gradient across the system.Thus, the protostar associated with W3(OH)TW-A has amass of approximately 13 M ⊙ , while the associated withW3(OH)TW-C has a mass of 4 M ⊙ , which corresponds to B-type stars., which corresponds to B-type stars.