Discovery of the Youngest Molecular Outflow associated with an Intermediate-mass protostellar Core, MMS-6/OMC-3
aa r X i v : . [ a s t r o - ph . GA ] D ec Accepted to ApJ (Dec. 15)
Discovery of the Youngest Molecular Outflow associated with anIntermediate-mass protostellar Core, MMS-6/OMC-3.
Satoko Takahashi, and Paul T.P. Ho , ABSTRACT
We present sub-arcsecond resolution HCN (4–3) and CO (3–2) observa-tions made with the Submillimeter Array (SMA ), toward an extremely youngintermediate-mass protostellar core, MMS 6-main, located in the Orion Molecu-lar Cloud 3 region (OMC-3). We have successfully imaged a compact molecularoutflow lobe ( ≈ ≤
100 yr. The line widthdramatically increases downstream at the end of the molecular outflow (∆ v ∼ − ), and clearly shows the bow-shock type velocity structure. The estimatedoutflow mass ( ≈ − M ⊙ ) and outflow size are approximately 2–4 orders and 1–3orders of magnitude smaller, while the outflow force ( ≈ − M ⊙ km s − yr − ) issimilar, as compared to the other molecular outflows studied in OMC-2/3. Theseresults show that MMS 6-main is a protostellar core at the earliest evolutionarystage, most likely shortly after the 2nd core formation. Subject headings:
ISM: jets and outflows — ISM: individual (OMC3-MMS 6)—ISM: molecules —stars: evolution Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 106, Taiwan;satoko [email protected]; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street Cambridge, MA 02138, U.S.A. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and theAcademia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution andthe Academia Sinica.
1. INTRODUCTION
Molecular outflows are ubiquitously observed in low- to high- mass star forming regions(e.g., Bachiller & Tafalla 1999; Arce et al. 2007). They have a variety of characteristics suchas size (0.01 pc–a few pc), mass (10 − –10 M ⊙ ), and outflow force (10 − –1 M ⊙ km s − yr − ) (e.g., Wu et al. 2004). These outflows are known to result from the entrainment ofcircumstellar gas, swept-up by the primary jet.It is well accepted that primary jets are launched by magnetohydrodynamic (MHD) pro-cesses in the accretion disks surrounding a protostar (e.g., Pudritz et al. 2007; Shang et al.2007). They play an important role to remove angular momentum from the circumstellardisk and thereby promote mass accretion onto the central star. Furthermore, recent 3D-MHDsimulations by Machida et al. (2008) focus on the outflows/jets at the earliest evolutionaryphase of star formation, between the prestellar phase and Class 0 phase such as outflowsfrom the first adiabatic core and right after 2nd core formation. Theoretical works are ableto resolve the jet launching points numerically. However, observational studies with currenttelescopes are not able to spatially resolve the region. The rarity of suitable targets (shortlife time at the earliest evolutionary phase) is a persistent problem.MMS 6 (414 pc; the distance to Orion; Menten et al. 2007), is located in the OMC-3 region (Chini et al. 1997; Lis et al. 1998; Johnstone & Bally 1999; Matthews et al. 2005;Takahashi et al. 2009, 2011, ; Hereafter, Takahashi et al. 2011a is denoted as Paper II).MMS 6-main is the brightest and the most compact submillimeter continuum source in theMMS 6 region (Takahashi et al. 2009) as well as in the OMC-3 region (Takahashi et al.2011). The sub-arcsecond 850 µ m continuum image (Paper II), spatially resolved a massiveenvelope (0.29 M ⊙ ) and hot gas ( ≥
50 K) in the central 120 AU. Comparisons with coremodels clearly showed the presence of a self-luminous source, which implies that MMS 6-main is an intermediate-mass protostellar core at the earliest evolutionary stage. However,neither molecular outflow, nor radio jet, nor infrared source has ever been detected at thesource center in the previous studies (Matthews et al. 2005; Takahashi et al. 2008, Hereafter,Takahashi et al. 2008 is denoted as Paper I).In order to search for molecular outflows and study their characteristics, we have per-formed sub-arcsecond angular resolution observations toward MMS 6-main in the CO (3–2)and HCN (4–3) lines with the SMA. Our observations have achieved a factor of seven im-provement in terms of beam size ( ∼
50 times better in terms of the beam surface area) thanthe previous molecular line studies by Takahashi et al. (2009). 3 –
2. OBSERVATIONS AND DATA REDUCTION
The observations of the CO (3–2; 345.796 GHz) and HCN (4–3; 354.505 GHz) emissionwere carried out with the SMA (Ho et al. 2004) in the extended configuration on September2, 2010. The phase center was set on MMS 6 (Chini et al. 1997, ; R.A. (J2000)=5 h m s ◦ ′ ′′ .20). The array configuration provided projected baselines rangingfrom 44–244 k λ for CO (3–2) and 45–249 k λ for HCN (4–3). Our observations were insensitiveto structures more extended than 3 ′′ .8 for CO (3–2) and 3 ′′ .7 for HCN (4–3) at the 10%level (Wilner & Welch 1994). The typical system noise temperatures in DSB mode werebetween 200–350 K at the observed elevations. The receivers have two sidebands with 4GHz bandwidth and the CO (3–2) and HCN (4–3) lines were simultaneously observed in thelower- and upper-sideband, respectively. We used the configuration that gave 128 channelsfor CO (3–2) and 64 channels for HCN (4–3). The corresponding velocity resolutions were0.7 km s − for CO (3–2) and 1.4 km s − for HCN (4–3). The phase and amplitude calibrator,0423-013 (1.5 Jy ), was observed every 18 minutes. Observations of Callisto provided theabsolute scale for the flux density calibrations. The overall flux uncertainty was estimated tobe ∼ ∼
30 minute integration.The raw data were calibrated using MIR (Scoville et al. 1993). After the calibration andthe flagging of bad data, final CLEANed images were made using the AIPS task “imager”with a natural weighting. The resulting synthesized beam sizes were 0 ′′ .82 × ′′ .64 with aposition angle of -74 ◦ for CO (3–2) and -79 ◦ for HCN (4–3). The achieved rms noise levelswere 0.14 Jy beam − km s − for CO (3–2) and 0.10 Jy beam − km s − for HCN (4–3), forthe 0.7 km s − bin and 1.4 km s − bin.
3. RESULTS
Figure 1 shows the zeroth-moment (total intensity), first-moment (velocity field de-scribed in the radial outflow velocity), and second-moment maps (velocity dispersion) pro-duced in CO (3–2) and HCN (4–3), superposed on the 850 µ m continuum emission (PaperII). With the SMA extended configuration observations, a very compact molecular outflowlobe ( ∼ ′′ ) associated with MMS 6-main has been successfully imaged and spatially resolvedfor the first time. The outflow has a collimated bipolar structure and the high-velocityemission is elongated along the north-south direction with a position angle of -6 ◦ . The firstmoment maps show that the redshifted and blueshifted components are clearly separated inthe north and south directions centered at the MMS 6-main continuum peak. The secondmoment maps show that the line width dramatically increases downstream at the ends of 4 –the molecular outflow particularly in the HCN (4–3) emission. This is further demonstratedby the position-velocity map presented in Figure 2 in HCN (4–3), where the radial outflowvelocity reaches its maximum only at ∼ ′′ from the exciting source.Note that the gas distribution and velocity structure as observed in CO (3–2) andHCN (4–3) are slightly different. In Figure 1 (first moment maps), the peak positions ofthe redshifted components are roughly the same in both CO (3–2) and HCN (4–3), whilethe peak positions of the blueshifted component are shifted approximately 1 ′′ with respect toeach other. Moreover, as clearly seen in Figure 2, the CO (3–2) emission traces only a partof gas observed in HCN (4–3) with the radial outflow velocity up to 15 km s − . No highervelocity emission is traced by the CO (3–2) emission. Note that the gas distribution does notdramatically change even if we smooth the CO (3–2) data to the same velocity resolutionas the HCN (4–3) data. It is difficult to explain that the CO (3–2) high-velocity outflowis fainter than in HCN (4–3), if the emissions are optically thin, since the CO molecule ismuch more abundant. It is likely that the CO (3–2) is optically thick, and may be sensi-tive to the less dense region, perhaps the outflow cavity. If the outflow walls are stratifiedin temperature but not in velocity, self-absorption by the cooler outer layers may suppressCO (3–2) relative to the HCN (4–3). In this context, we note that the CO emission regionis more extensive than in HCN. Multi-transition line observations are necessary to constraindetailed physical conditions of the high-velocity component.Paper I and Takahashi et al. (2009) did not detect molecular outflows associated withMMS 6-main in the CO (1–0) and CO (3–2) observations. This is likely due to the lim-ited sensitivity and the beam dilution. The 3 σ mass sensitivity limit was 10 − M ⊙ (withthe ASTE 26 ′′ beam size; Paper I) and 6.5 × − M ⊙ (with the NMA ∼ ′′ beam size;Takahashi et al. 2009). Here, with the sub-arcsec resolution (0 ′′ . × ′′ . ≈ ′′ ) with the mass sensitivity limit of 3.9 × − M ⊙ (3 σ mass sensi-tivity derived from HCN).
4. Outflow Parameters
Outflow parameters were calculated from the detected CO (3–2) and HCN (4–3) emis-sions. Note that, as we described in Section 2, the maximum detectable size with our SMAobservations is larger than the outflow lobe size ( ∼ ′′ ), so that our observations are notsignificantly affected by missing flux. 5 –The inclination angle of the outflow , i , is constrained from the position-velocity di-agram. We use a simple analytical outflow model as described in Lee et al. (2000). Thecomparison with these models shows that the inclination angle of 45 ◦ as denoted by theblack curves in Figure 2b, can delineate the MMS 6 outflow velocity structure. Therefore,the outflow inclination angle of 45 ◦ is adopted in the following discussion.In order to compare the MMS 6-main outflow properties with those derived from theother OMC-2/3 outflows reported in Paper I, the same derivations are adopted. Under theassumption of the local thermodynamical equilibrium (LTE) condition and optically thinmolecular line emissions, we estimate the outflow mass. The excitation temperature of themolecular line is adopted as 30K, which is the typical CO (3–2) brightness temperature mea-sured in the OMC-2/3 region (Paper I). Note that adopting the excitation temperature of 100K modifies the outflow mass estimation slightly (a factor of ≤ X [CO]=10 − (Frerking et al. 1982) and X [HCN]=5 × − (valuesderived in the L 1157 outflow by Bachiller & Perez Gutierrez 1997). The HCN (4–3) abun-dance can be enhanced by factors of 10-100 in the shock region (Bachiller & Perez Gutierrez1997; Jørgensen et al. 2004). In this paper, the maximum HCN (4–3) abundance derived forthe L 1157 outflow (Bachiller & Perez Gutierrez 1997), is adopted.The maximum radial outflow velocity and the outflow lobe size are derived as v flow = v max(obs) × [1 / sin i ] and R flow = R obs × [1 / cos i ], respectively. The dynamical time of the outflowis estimated to be t d = R flow /v flow yr with the maximum radial outflow velocity. The outflowlobe size, and outflow dynamical time, outflow momentum ( P = M CO v flow ), energy ( E = M CO v / F obs = P/t d ), mechanical luminosity ( L m = M CO × v / R ),and mass loss rate ( ˙ M out = M CO /t d ) are estimated. The estimated parameters using HCN(4–3) and CO (3–2) are listed in Table 1, with/without inclination angle corrections.
5. Nature of the Extremely Compact Outflow
In the MMS 6-main outflow case, HCN (4–3) is a better tracer of the morphology andvelocity structure of the collimated molecular outflow as compared with CO (3–2). Thismay be because the central part of the MMS 6-main envelope ( ≤ n ∼ cm − ; Paper II) and hence, the ejected gas is better traced by the highercritical density of HCN (4–3), and also because the CO may suffer from self-absorption Here, the plane of the sky is defined as i =0 ◦ . We correct all velocities by the systemic velocity (see Table 1). ◦ . The dynamical timescale of this outflow is estimated to be 33-44 yrwith the maximum radial outflow velocity of 35–39 km s − . (The dynamical time becomesa factor of 1.5–2.3 shorter when the intensity weighted mean outflow sizes and mean gasvelocities are adopted.) The derived outflow mass and outflow size are approximately 2–4orders and 1–3 orders of magnitude smaller than those derived for other molecular outflowspreviously detected in the OMC-2/3 region (Aso et al. 2000; Williams et al. 2003, , PaperI). The molecular outflow detected in MMS 6-main is the smallest and least massive bipolaroutflow that has ever been observed for the intermediate-mass protostars (c.f., Aso et al.2000; Williams et al. 2003; Zapata et al. 2006; Beltr´an et al. 2008, , Paper I). In contrast,outflow force, (9.1–13) × − M ⊙ km s − yr − , and the mass loss rate, 3.5 × − M ⊙ yr − ,have similar values as compared with the median values/mean value of those derived forthe other OMC-2/3 outflows ( F CO =5.4 × − M ⊙ km s − yr − / 2.7 × − M ⊙ km s − yr − ;Values are calculated from Paper I). These strongly imply that the outflow associated withMMS 6-main is much younger, but otherwise has similar outflow force as compared with theother molecular outflows in OMC-2/3.The projected outflow lobe size ( ≈ ∼
500 AU and outflow mass of < − M ⊙ . This source is a proto-brown dwarf candidate(Bourke et al. 2005). Detected molecular outflow in MMS 6-main is a factor of three largerthan this outflow. However, the estimated outflow force in MMS 6-main is more thantwo orders of magnitude larger than that estimated in L 1014L. Furthermore, similarlycompact molecular outflows are reported in young brown dwarf stars; ISO-Oph 102 andMHO5 (Phan-Bao et al. 2008, 2011). They have outflow lobe sizes of ∼ ∼ − M ⊙ , which are similar to the outflow in MMS 6-main. However, theirestimated outflow forces are on the order of 10 − M ⊙ km s − yr − , which is four ordersof magnitude smaller than those estimated in MMS 6-main. As summarized in Figure 3,the molecular outflow detected in MMS 6-main is similarly compact and less massive ascompared to those associated with young brown dwarfs, while the outflow force is muchhigher in MMS 6-main. If we consider that the outflow force is correlated with the mass 7 –accretion rate as suggested in Bontemps et al. (1996), this result indicates that MMS 6-mainhas a much higher accretion rate than for (proto) brown dwarf cases.The observed outflows are usually characterized by the jet-driven bow-shock models andwind-driven shell models (e.g., Lee et al. 2000). The jet-driven bow shock model shows thatmolecular outflows consist of the ambient gas interacting with the bow-shock of the jet head,while the wind-driven model shows that molecular outflows consist of swept-up gas entrainedby the wide-angle outflow. The velocity structures observed in HCN (4–3) emission showthe immediate velocity increment at the jet head up to 25 km s − . This clearly suggests thesignature of the jet-driven bow-shock type outflow. Similar velocity structures are reported inlow-mass protostellar outflows in the Class 0 phase (e.g., Lee et al. 2000, 2007; Hirano et al.2010).The primary jets from young stars are likely launched from accretion disks around pro-tostars (e.g., Pudritz et al. 2007; Shang et al. 2006). Magnetocentrifugal forces can producewinds with large opening-angles, while the magnetic pressure gradients (hoop stress inducedby the generated toroidal magnetic field) eventually dominate beyond the region around theAlfven radius, which provides the collimation (Pudritz et al. 2007). X-wind models can alsoexplain both wide and collimated winds as an effect due to the density contrast within thewinds (Shang et al. 2006).Furthermore, recent 3D-MHD simulations predict two distinct outflows associated withthe early evolutionary stages (Machida et al. 2008). First case is the outflow from the firstadiabatic core. These outflows have a strong magnetic field and are driven mainly by themagneto centrifugal mechanism and are guided by the hour glass-like field lines. Theseoutflows show a wide opening-angle low-velocity outflow ( ∼ − ). Second case is theoutflow from the protostar. These outflows have a weak magnetic field. They are drivenby the magnetic pressure gradient force and guided by straight field lines. They have acollimated high-velocity outflow ( ≥
30 km s − ).Our observations do not have enough angular resolution to resolve the outflow launchingpoint. Hence, this experiment cannot distinguish between the different launching mecha-nisms as proposed by theoretical models. Nevertheless, the nature of the observed outflow inMMS 6-main shares similarities with the outflows from protostellar cores, such as collimation,bipolarity, high-velocity gas, and bow-shocks. Negative detection of the wide opening-anglelow-velocity outflow as predicted in the first adiabatic core, is possibly due to the observa-tional limits on the mass sensitivity (Section 3) and the detectable size (Section 2).The extremely compact and less massive nature of MMS 6-main suggests that it maybe at the earliest evolutionary phase of the protostar, shortly after its formation. Paper II 8 –suggests that the observed density of the submillimeter source, MMS 6-main, is explained bythe combination of a central heating source and the surrounding warm envelope, supportingthe presence of a protostellar core.
6. Conclusion
An extremely compact molecular outflow was discovered toward an intermediate-massprotostellar core, MMS 6-main in the OMC-3 region. The velocity broadening at outflowtips, (∆ v ∼
25 km s − ), indicates the bow-shock type of velocity structure. The estimatedoutflow lobe size ( ≈ ≈ − M ⊙ ), and dynamical time ( ≤
100 yr), clearlysuggest that the outflow is compact, less massive, and young, while the estimated outflowforce ( ≈ − M ⊙ km s − yr − ) is similar to those derived in other OMC-2/3 outflows. Theseresults most likely suggest that the outflow is originated from a protostar (2nd core), shortlyafter its formation. These results are consistent with our recent SMA continuum observationsat 850 µ m, which imply that MMS 6-main is an intermediate mass protostellar core.We acknowledge the staff at the Submillimeter Array for assistance with observations.We thank Naomi Hirano, Masanori Nakamura, and Keiichi Asada for fruitful discussions,and our anonymous referee whose suggestions improved this manuscript. REFERENCES
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This preprint was prepared with the AAS L A TEX macros v5.2.
11 – F l u x ( Jy / bea m k m / s ) V e l o c i t y ( k m / s ) V e l o c i t y d i s pe r s i on ( k m / s ) F l u x ( Jy / bea m k m / s ) V e l o c i t y ( k m / s ) V e l o c i t y d i s pe r s i on ( k m / s ) CO (3-2) Moment 0 CO (3-2) Moment 1 CO (3-2) Moment 2HCN (4-3) Moment 0 HCN (4-3) Moment 1 HCN (4-3) Moment 2
Fig. 1.—
Color images of the zeroth, first and second moments maps of the HCN (4–3) andCO (3–2) emissions. The velocity labels in the first moment maps are described in the radialoutflow velocity ( v rad;obs = v LSR − v sys ). The moment maps were made using the AIPS task’momnt’ with 5 σ cut off level (i.e. 0.5 Jy beam − km s − for HCN and 7.0 Jy beam − kms − for CO). The black contours show the 850 µ m continuum image (Paper II). The contourlevels are 10 σ , 20 σ , 30 σ , 40 σ , 60 σ , 100 σ , 160 σ , 200 σ , and 240 σ with intervals of 20 σ (1 σ =2.7 mJy beam − ). Filled ellipses at the bottom-left corners show the synthesized beamsof the line data (Section 2) and continuum data ( ′′ . × ′′ . P.A.=32 ◦ ).
12 –
Resolution
NWSE (a) (b)
Fig. 2.— (a) HCN(4–3) and CO (3–2) position-velocity maps denoted by black and purplecontours, respectively. The velocity labels are described in the radial observed velocity ( v LSR ).These position-velocity maps were made along the major axis (P.A.=-6 ◦ ) of the molecularoutflow. Dashed lines along the horizontal and vertical directions show the position of the850 µ m continuum peak (Paper II) and the systemic velocity of MMS 6 as derived fromthe H CO + (1–0) emission (Takahashi et al. 2009), respectively. The resolution of eachmolecular line in the position-velocity diagrams are denoted in the bottom right corner. (b)HCN (4–3) position-velocity map as shown in figure (a) overlaid with the wide-angle windmodel curve produced by i =45 ◦ , C=0.88 arcsec − , v =7.0 km s − (refer Lee et al. 2001 fordefinitions of C and v ). This model is used in order to constrain the inclination angle ofthe molecular outflow.
13 –Fig. 3.— a: Outflow mass plotted as a function of outflow size. b: Outflow force plotted asa function of outflow size. c: Outflow force plotted as a function of bolometric luminosity.Connected lines of data points show the uncertainties of bolometric luminosities measuredby Paper I. Black and blue arrows show the lower and upper limits of the bolometric lu-minosities measure by Paper II and Chini et al. 1997, respectively. Blue filled diamonddenotes outflow from MMS 6-main. Open circles, filled circles, open squares, crosses, opentriangles, and hatched squares show outflow results reported towards intermediate-mass pro-tostars (Beltr´an et al. 2008, and their references), OMC-2/3 outflows (in the blueshiftedand redshifted components; Takahashi et al. 2008), Taurus outflows (in the blueshifted andredshifted components; Hogerheijde et al. 1998), and outflows associated with proto-browndwarf and first adiabatic core candidates (Bourke et al. 2005; Phan-Bao et al. 2008, 2011;Pineda et al. 2011), respectively. Data points from Taurus and OMC-2/3 surveys are sep-arately plotted in redshifted and blueshifted components. Adding both components shouldincrease the values by a factor of two as compared to the current plot.
14 –Table 1. Outflow Parameters Estimated from HCN (4–3) and CO (3–2)
Parameters Blue Lobe Red Lobe i =45 ◦ Uncorrected i =45 ◦ Uncorrected
HCN (4–3)
Mass ( M ⊙ ) 8.3e-05 8.3e-05 5.2e-05 5.2e-05Maximum velocity (km s − ) a
39 (20) b
27 (14) b
35 (18) b
25 (13) b Size (AU) 1700 (1100) c c c
830 (520) c Dynamical Time (yr) 44 (56) d
44 (56) d
33 (40) c
33 (40) d Momentum ( M ⊙ km s − ) 3.2e-04 2.3e-04 1.8e-04 1.3e-04Kinetic energy ( M ⊙ km s − ) 6.2e-03 3.1e-03 3.2e-03 1.6e-03Outflow force ( M ⊙ km s − yr − ) 7.3e-05 5.2e-05 5.5e-05 3.9e-05Mechanical luminosity ( L ⊙ ) 4.9e-03 2.4e-03 3.4e-03 1.7e-03Mass Loss rate ( M ⊙ yr − ) 1.9e-06 1.9e-06 1.6e-06 1.6e-06 CO (3–2)
Mass ( M ⊙ ) 1.9e-04 1.9e-04 1.4e-04 1.4e-04Maximum velocity (km s − ) a
21 (11) b
15 (7.5) b
11 (5.5) b b Size (AU) 1300 (570) c
950 (400) c c
770 (450) c Dynamical Time (yr) 64 (53) d
64 (53) d
100 (115) d
100 (115) d Momentum ( M ⊙ km s − ) 4.1e-04 2.9e-04 1.5e-04 1.1e-04Kinetic energy ( M ⊙ km s − ) 4.3e-03 2.1e-03 8.2e-04 4.1e-04Outflow force ( M ⊙ km s − yr − ) 6.3e-05 4.5e-05 1.5e-06 1.1e-05Mechanical luminosity ( L ⊙ ) 2.3e-03 1.2e-03 2.9e-04 1.4e-04Mass Loss rate ( M ⊙ yr − ) 3.0e-06 3.0e-06 1.4e-06 1.4e-06 a The maximum radial outflow velocities ( v obs = | v LSR − v sys | ) measured from the channel maps.Here, v sys =10.3 km s − is adopted (Takahshi et al. 2009) b Mean radial outflow velocities. c Distance to the peak intensity positions from the driving source measured from the zerothmoment maps dd