The 870 μm dust continuum and its linearly polarized emission is detected and resolved in the Orion BN/KL region. With an angular resolution of 1, the polarized emission from the dust continuum at submillimeter is resolved at 2 milli-pc (480 AU)
for the first time. As compared to dust polarization previously detected, the revealed polarization in the south of IRc2 changes smoothly by 90◦ instead of abrupt change previously observed at poorer resolution. The continuum emission is resolved into three clumps, and the strongest clump is associated with source I and SMA 1. The associated masses are 2 to 12 M. There is no continuum peak detected at source BN, n and the BN-I center. The upper limit of the associated mass is 0.01 M. We interpret the detected dust continuum as part of a remnant disk. The center of the proposed remnant disk is suggested to be at the common center of source BN and I about 500 years ago, as proposed by Gomez et al. (2005). Our proposed scenario of remnant disk is supported by the kinematics inferred from the OH masers by Cohen et al. (2006), where a clear signature of rotation is seen the positional-velocity plot.
Furthermore, our proposed orientation and the center of the disk is consistent with the disk proposed by the same authors. In this scenario, the detected B field inferred from the dust continuum is toroidal in the south of source I, and is poloidal in the east of the main dust ridge and in the compact ridge. The poloidal B field is consistent with the field direction in the OMC-1 dust ridge. In the main dust ridge, source I and SMA 1 have been suggested to be active in star formation, indicating that further fragmentation is ongoing within the remnant disk. We note that the possibility of mechanical alignment can not be ruled out.
Table 4.1. Observational parameters
Date Configuration gain calib. flux calib. BP/Pol. calib.
’08.Jan.6 Compact 0528+134 Uranus 3c273
’09.Jan.25 Subcompact 0423-013/0510+180 Titan 3c273
’09.Feb.15 Extended 0423-013 Titan 3c273
Note. — BP/Pol. refers to bandpass/polarization. Calib. refers to the calibra-tor. In all of the observations, the local oscillator frequencies are tuned at 341.482 GHz. With a bandwidth of 2 GHz, the frequency covered is from 345.5 to 347.5 GHz and from 335.5 to 337.5 GHz in the upper and lower sidebands, respectively.
Maps of continuum emission at wavelengths of 870μm is generated by averaging over the line-free channels.
Figure 4.1 Left-panel: SMA 870 μm continuum map in Orion BN/KL with natural weighting. Solid contours are the 870 μm continuum strength at 3, 6, 9, ..., 33, 36, 39 × 70 mJy beam−1. The dashed contours are -3 and -6 × 70 mJy beam−1, which are due to the missing of short spacing visibilities. The three resolved dust clumps are called main dust ridge, compact ridge and north-west clump anti-clockwise from the strongest clump. Asterisk marks the position of BN. Stars mark the radio sources identified by Menten & Reid (1995). Cross marks the submillimeter continuum source SMA1 identified by Beuther et al. (2004). Red pluses are the mid-infrared sources, where the adjacent numbers refer to the IRc source names in Shuping, Morris &
Bally (2004), where the position uncertainties of these sources are typically 0.1 to 0.3. The cyan square marks the common center proposed by Gomez et al. (2005).
The synthesized beam is 1.2×1.1 with a P.A. of −16◦, shown as solid black ellipse at lower-left corner. Right Panel: Polarization map in Orion BN/KL with natural weighting. The plotted red vectors are above 3σIP, and black vectors are between 2 to 3 σIP. Polarized intensity map is shown in color scale with strength in wedge in units of mJy beam−1.
Figure 4.1 –continue
Figure 4.1 –continue
Figure 4.2 Polarization maps at 3 mm and 1 mm obtained with BIMA and at 870 μm obtained with the SMA. The sizes of the synthesized bemas are plotted in the lower-left corner in the corresponding color as indicated in the upper-right corner. The length of the indicated segments in these three wavelengths represents the polarization percentage of 8%. The black contours are the 3 mm continuum emission strength at 3, 6, 9, ..., 33, 36, 39 × 0.01 Jy beam−1. The large red circle marks the field of view of 30 of the SMA at wavelength of 0.87 mm. All the other symbols are the same as in Figure 1.
Figure 4.2 Polarization map in Orion BN/KL inferred from 0.87 mm with compact and subcompact array data. With robust weighting of 0.5, the synthesized beam is 2.8×1.8. All the symbols are the same as in Figure 1. The continuum emission at 0.87 mm is shown in contours plotted at 3, 6, 9, 12, 15, 20, 25, 30, 35, 40 × 0.16 Jy beam−1. The polarized emission is shown in color scale. The cyan ellipse marks the proposed inclined disk, where the BN-I center is at the center of the ellipse with size equivalent to the positional uncertainty given in Zapata et al. (2009). The black circle marks the primary beam of the SMA at this wavelength. The red and black segments mark the inferred B field direction with S/N ratio above 3 and in between 2 to 3σIP, respectively.
Figure 4.3 B field map in Orion BN/KL inferred from 3 mm, 1 mm, and 0.87 mm.
All the symbols are the same as in Figure 1. The continuum emission at 0.87 mm is shown in contours plotted at 3, 6, 9, 12, 15, 20, 25, 30, 35, 40× 0.07 Jy beam−1, and at 1 mm in grey scale with strength indicated in the wedge in units of Jy beam−1. The cyan ellipse marks the proposed inclined disk, where the BN-I center is at the center of the ellipse with size equivalent to the positional uncertainty given in Zapata et al. (2009).
Figure 4.3 B field map in Orion BN/KL inferred from 0.87 mm with compact and sub-compact array data. With robust weighting of 0.5, the synthesized beam is 2.8×1.8.
All the symbols are the same as in Figure 1. The continuum emission at 0.87 mm is shown in contours plotted at 3, 6, 9, 12, 15, 20, 25, 30, 35, 40× 0.16 Jy beam−1. The cyan ellipse marks the proposed inclined disk, where the BN-I center is at the center of the ellipse with size equivalent to the positional uncertainty given in Zapata et al.
(2009). The black circle marks the primary beam of the SMA at this wavelength.
The red and black segments mark the inferred B field direction with S/N ratio above 3 and in between 2 to 3σIP, respectively.
Figure 4.4 Left-panel: Map of 870μm dust continuum emission with highest angular resolution. This map is constructed with extended array data only with uniform weighting. The size of the synthesized beam is 0.8×0.6 with a P.A. of −74◦. The triangles mark the positions of clumps identified in NH3 by Migenes et al. (1989) with VLSR labelled in units of km s−1. Note that near SMA 1, the VLSRs of these clumps vary significantly, ranging from 1.6 to 9.3 km s−1.
Figure 4.4 Middle-panel: B field map of the extended array track with uniform weight-ing overlayed on the maps shown in the left panel. The polarized intensity is presented in color scale in units of mJy beam−1. The presented vectors are B field direction above 3σIp and between 2 to 3σIp in red and black segments, respectively.
Figure 4.4 Right-panel: NH3maps by Wilson et al. (2000). Note that the distribution of the NH3 gas is similar to the 870 μm continuum emission, i.e. have a cavity near the center. Source I and n are also at the edge of the NH3 gas.
Figure 4.5 Left-panel: B field map in Orion BN/KL of the combined three tracks with uniform weighting. Contours are plotted from and step in 3σ, where 1 σ = 53 mJy beam−1. All the symbols are the same as in Figure 1. The B field vectors are derived by rotating the polarization by 90◦ with identical length. The synthesized beam is 1.1×0.8 with a P.A. of −72◦.
Figure 4.5 Right-panel: Zoomed-in B field map in Orion BN/KL of the combined tracks with uniform weighting. The polarized intensity map is shown in color scale in units of mJy beam−1 shown in color wedge. In both panels, the vectors are gridded with 0.45, ∼ half of the synthesize beam in order to show the variation of B field direction across each independent data points. Note that the B field direction vary smoothly by more than 90◦ across the main dust ridge.
Figure 4.6 870μm dust continuum (grey scale) of the combined tracks with natural weighting and CO outflows in blue and red contours by Zapata et al. (2009).
Submillimeter Array Dust Polarization Image of the Ultracompact H II Region G5.89-0.39
Ya-Wen Tang1,2, Paul T. P. Ho2,3, Josep Miquel Girart4, Ramprasad Rao2, Patrick Koch2, and Shih-Ping Lai5
1Department of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617
2Academia Sinica Institute of Astronomy and Astrophysics, P. B. Box 23-141, Taipei 10617
3Harvard-Smithsonian Center for Astrophysics,60 Garden Street Cambridge, MA 02138, U.S.A.
4Institut de Ci`encies de l’Espai (CSICIEEC), Campus UAB, Facultat de Ciencies, Torre C5 -parell 2, 08193 Bellaterra, Catalunya, Spain
5Institute of Astronomy and Department of Physics, National Tsing Hua University, 101, Section 2, Kuang Fu Road, Hsinchu, Taiwan 300, R. O. C.
2009 ApJ, 695, 1399
85
Abstract
We report high angular resolution (3) Submillimeter Array (SMA) observations of the molecular cloud associated with the Ultracompact H II region G5.89-0.39. Imaged dust continuum emission at 870μm reveals significant linear polarization. The position angles (P.A.s) of the polarization vary enormously but smoothly in a region of 2×104 AU.
Based on the distribution of the P.A.s and the associated structures, the polarized emission can be separated roughly into two components. The component ”x” is associated with a well-defined dust ridge at 870 μm, and is likely tracing a compressed B field. The component
”o” is located at the periphery of the dust ridge and is probably from the original B field associated with a pre-existing extended structure. The global B field morphology in G5.89, as inferred from the P.A.s, is clearly disturbed by the expansion of the HII region and the molecular outflows. Using the Chandrasekhar−Fermi method, we estimate from the smoothness of the field structures that the B field strength in the plane of sky can be no more than 2−3 mG. We then compare the energy densities in the radiation, the B field, and the mechanical motions as deduced from the C17O 3-2 line emission. We conclude that the B field structures are already overwhelmed and dominated by the radiation, outflows, and turbulence from the newly formed massive stars.
Keywords: ISM: individual (G5.89-0.39) – ISM: magnetic fields – polarization – stars:
formation
5.1 Introduction
One of the main puzzles in the study of star formation is the low star formation ef-ficiency in molecular clouds. Since molecular clouds are known to be cold, the thermal pressure is small. Hence, if there are no other supporting forces against gravity, the free-fall timescale will be short and the star formation rate will be much higher than what is observed. Magnetic (B) fields have been suggested to play the primary role in providing a supporting force to slow down the collapsing process (see the reviews by Shu et al. 1999 and Mouschovias & Ciolek 1999). In these models, the B field is strong enough and has an orderly structure in the molecular cloud. The B field lines, which are anchored to the ionized particles, will then be dragged in along the direction of accretion, only when the ambipolar diffusion process allows the neutral component to slip pass the ionized component. In the
standard low-mass star formation model (Galli & Shu 1993; Fiedler & Mouschovias 1993), an hourglass-like B field morphology is expected with an accreting disk near the center of the pinched field. Alternatively, turbulence has also been suggested as a viable source of support against contraction (see reviews by Mac Low & Klessen 2004 and Elmegreen &
Scalo 2004). The relative importance of B field and turbulence continues to be a hot topic as the two methods of support will lead to different scenarios for the star formation process.
Compared with the low-mass stars, the formation process of high-mass stars is relatively poorly understood. High mass star-forming regions, because of their rarity, are usually at larger distances and are always located in dense and massive regions because they are typ-ically formed in a group. Hence, both poor resolution and complexity have hampered past observational studies. Furthermore, the environments of high-mass star-forming regions are very different from the low-mass case because of higher radiation intensity, higher temper-ature, and stronger gravitational fields. Will the B fields in massive star-forming sites have a similar morphology to the low-mass cases?
Polarized emission from dust grains can be used to study the B field in dense regions, because the dust grains are not spherical in shape. They are thought to be aligned with their minor axes parallel to the B field in most of the cases, even if the alignment is not magnetic (Lazarian 2007). Due to the differences in the emitted light perpendicular and parallel to the direction of alignment, the observed thermal dust emission will be polarized, the direction of polarization is then perpendicular to the B field. Although the alignment mechanism of the dust grains has been a difficult topic for decades (see review by Lazarian 2007), the radiation torques seem to be a promising mechanism to align the dust grains with the B field (e.g., Draine & Weingartner 1996; Lazarian & Hoang 2007). However, other processes such as mechanical alignments by outflows can also be important.
Polarized dust emission has been detected successfully at arcsecond scales. The best example might be the low-mass star-forming region NGC 1333 IRAS 4A (Girart, Rao &
Marrone 2006), which reveals the classic predicted hourglass B field morphology. Results on the massive star-forming regions, such as W51 e1/e2 cores (Lai et al. 2001), NGC2024 FIR5 (Lai et al. 2002), DR21 (OH) (Lai et al. 2003), G30.79 FIR 10 (Cortes et al. 2006), and G34.4+0.23 MM (Cortes et al. 2008), typically show an organized and smooth B field morphology. However, this could be due to the lack of spatial resolution. Indeed, for the nearby high-mass cases such as Orion KL (Rao et al. 1998) and NGC 2071IR (Cortes,
Crutcher, & Matthews 2006), abrupt changes of the polarization direction on small physical scales have been seen, which may suggest mechanical alignments by outflows as proposed by these authors. Whether high-mass star-forming regions will all show complicated B field structures on small scales remains to be examined.
In this study, we report on one of the first SMA measurements of dust polarization for a high-mass star-forming region, G5.89-0.39 (hereafter, G5.89). The linearly polarized thermal dust emission is used to map the B field at∼3resolution, and the C17O 3-2 line is used to study the structure and kinematics of the dense molecular cloud. The description of the source, the observations and the data analysis, the results, and the discussion are in Sec. 2, 3, 4, and 5, respectively. The conclusions and summary are in Sec. 6.