SiO Observations Research Articles

Polarization of thermal molecular lines in the envelope of IK Tauri

Molecular line polarization is a unique source of information about the magnetic fields and anisotropies in the circumstellar envelopes of evolved stars. Here we present the first detection of thermal CO(J=2-1) and SiO(J=5-4, v=0) polarization, in the envelope of the asymptotic giant branch star IK Tau. The observed polarization direction does not match predictions for circumstellar envelope polarization induced only by an anisotropic radiation field. Assuming that the polarization is purely due to the Goldreich-Kylafis effect, the linear polarization direction is defined by the magnetic field as even the small Zeeman splitting of CO and SiO dominates the molecular collisional and spontaneous emission rates. The polarization was mapped using the Submillimeter Array (SMA) and is predominantly north-south. There is close agreement between the CO and SiO observations, even though the CO polarization arises in the circumstellar envelope at ~800 AU and the SiO polarization at <250 AU. If the polarization indeed traces the magnetic field, we can thus conclude that it maintains a large-scale structure throughout the circumstellar envelope. We propose that the magnetic field, oriented either east-west or north-south is responsible for the east-west elongation of the CO distribution and asymmetries in the dust envelope. In the future, the Atacama Large Millimeter/submillimeter Array will be able to map the magnetic field using CO polarization for a large number of evolved stars.

Surviving the hole

The interstellar region within the few central parsecs around the super-massive black hole, Sgr A* at the very Galactic center is composed by a number of overlapping molecular structures which are subject to one of the most hostile physical environments in the Galaxy. We present high resolution (4"x3"~0.16x0.11 pc) interferometric observations of CN, 13CN, H2CO, SiO, c-C3H2 and HC3N emission at 1.3 mm towards the central ~4 pc of the Galactic center region. Strong differences are observed in the distribution of the different molecules. The UV resistant species CN, the only species tracing all previously identified circumnuclear disk (CND) structures, is mostly concentrated in optically thick clumps in the rotating filaments around Sgr A*. H2CO emission traces a shell-like structure that we interpret as the expansion of Sgr A East against the 50 km/s and 20 km/s giant molecular clouds (GMCs). We derive isotopic ratios 12C/13C~15-45 across most of the CND region. The densest molecular material, traced by SiO and HC3N, is located in the southern CND. The observed c-C3H2/HC3N ratio observed in the region is more than an order of magnitude lower than in Galactic PDRs. Toward the central region only CN was detected in absorption. Apart from the known narrow line-of-sight absorptions, a 90 km/s wide optically thick spectral feature is observed. We find evidences of an even wider (>100 km/s) absorption feature. Around 70-75% of the gas mass, concentrated in just the 27% densest molecular clumps, is associated with rotating structures and show evidences of association with each of the arcs of ionized gas in the mini-spiral structure. Chemical differentiation has been proven to be a powerful tool to disentangle the many overlapping molecular components in this crowded and heavily obscured region.

Simultaneous observations of SiO and H2O masers toward known stellar SiO and/or H2O maser sources

AbstractWe present the results of simultaneous observations of SiO v = 1, 2, 29SiO v = 0, J = 1−0 and H2O 616-523 maser lines toward 318 known stellar SiO and/or H2O maser sources using the Yonsei 21-m radio telescope of the Korean VLBI Network. Toward 166 known SiO and H2O maser sources, both SiO and H2O maser emissions were detected from 112 sources giving a detection rate of 67.5 %. On the other hand, toward 152 known H2O-only maser sources, both SiO and H2O maser emissions were detected from 62 sources, giving a detection rate of 40.8 %. Characteristics of all observed sources in the IRAS two-color diagram is investigated including their evolutionary sequence and mutual relations between SiO and H2O maser properties.

Revisiting the shocks in BHR71: new observational constraints and H2O predictions forHerschel

During the formation of a star, material is ejected along powerful jets that impact the ambient material. This outflow phenomenon plays an important role in the regulation of star formation. Understanding the associated shocks and their energetic effects is therefore essential to the study of star formation. We present comparisons of shock models with observations of H$_2$ and SiO emission in the bipolar outflow BHR71, and predict water emission, under the basic assumption that the emission regions of the considered species coincide, at the resolution of currently available observations. New SiO observations from APEX are presented, and combined with \textit{Spitzer} and ground-based observations of H$_2$ to constrain shock models. The shock regions associated with targeted positions in the molecular outflow are studied by means of a 1D code that generates models of the propagation of stationary shock waves, and approximations to non-stationary ones. The SiO emission in the inner part of the outflow is concentrated near the apex of the corresponding bow-shock that is also seen in the pure rotational transitions of H$_2$. Simultaneous modelling is possible for H$_2$ and SiO and leads to constraints on the silicon pre-shock distribution on the grain mantles and/or cores. The best-fitting models are found to be of the non-stationary type, but the degeneracy of the solutions is still large. Pre-shock densities of 10$^4$ and 10$^5$ cm$^{-3}$ are investigated, and the associated best-model candidates have rather low velocity (respectively, 20-30 and 10-15 km s$^{-1}$) and are not older than 1000 years. We provide emission predictions for water, focusing on the brightest transitions, to be observed with the PACS and HIFI instruments of the \textit{Herschel} Telescope.

Oxidative Reactions of Silicon Atoms and Clusters at Ultralow Temperature in Helium Droplets

The reaction between Si and O(2) was studied in liquid He droplets at low temperature (T = 0.37 K) by monitoring the energy release during the reaction. Additionally, the reactions of Si atoms and clusters with the oxidation agents H(2)O and O(2) have been studied by mass spectrometry. It was found that Si atoms react fast with O(2) molecules. On the other hand, Si atoms and clusters do not react with H(2)O molecules. The energy released during the chemical reaction leads to the ejection of the products from small He droplets. In contrast, large He droplets (N(He) > 20000) are capable of keeping part of the reaction products in their interior. The observation of SiO(2) products with the mass spectrometer reveals that the He droplet can stabilize intermediate products in the exit channel.

SPITZERSPECTRAL LINE MAPPING OF PROTOSTELLAR OUTFLOWS. II. H2EMISSION IN L1157

We present an analysis of Spitzer-IRS spectroscopic maps of the L1157 protostellar outflow in the H2 pure-rotational lines from S(0) to S(7). The aim of this work is to derive the physical conditions pertaining to the warm molecular gas and study their variations within the flow. The mid-IR H2 emission follows the morphology of the precessing flow, with peaks correlated with individual CO clumps and H2 2.12{\mu}m ro-vibrational emission. More diffuse emission delineating the CO cavities is detected only in the low-laying transitions, with J(lower) less or equal to 2. The H2 line images have been used to construct 2D maps of N(H2), H2 ortho-to-para ratio and temperature spectral index beta, in the assumption of a gas temperature stratification where the H2 column density varies as T^(beta). Variations of these parameters are observed along the flow. In particular, the ortho-to-para ratio ranges from 0.6 to 2.8, highlighting the presence of regions subject to recent shocks where the ortho-to-para ratio has not had time yet to reach the equilibrium value. Near-IR spectroscopic data on ro-vibrational H2 emission have been combined with the mid-IR data and used to derive additional shock parameters in the brightest blue- and red-shifted emission knots. A high abundance of atomic hydrogen (H/H2 about 0.1-0.3) is implied by the observed H2 column densities, assuming n(H2) values as derived by independent SiO observations. The presence of a high fraction of atomic hydrogen, indicates that a partially-dissociative shock component should be considered for the H2 excitation in these localized regions. However, planar shock models, either of C- or J-type, are not able to consistently reproduce all the physical parameters derived from our analysis of the H2 emission. Globally, H2 emission contributes to about 50% of the total shock radiated energy in the L1157 outflow.

SIMULTANEOUS OBSERVATIONS OF SiO AND H2O MASERS TOWARD SYMBIOTIC STARS

We present the results of simultaneous observations of SiO v = 1, 2, J = 1–0, 29SiO v = 0, J = 1–0, and H2O 616–523 maser lines performed with the KVN Yonsei 21 m radio telescope from 2009 November to 2010 January. We searched for these masers in 47 symbiotic stars and detected maser emission from 21 stars, giving the first time detection from 19 stars. Both SiO and H2O masers were detected from seven stars of which six were D-type symbiotic stars and one was an S-type star, WRAY 15-1470. In the SiO maser emission, the 28SiO v = 1 maser was detected from 10 stars, while the v = 2 maser was detected from 15 stars. In particular, the 28SiO v = 2 maser emission without the v = 1 maser detection was detected from nine stars with a detection rate of 60%, which is much higher than that of isolated Miras/red giants. The 29SiO v = 0 maser emission was also detected from two stars, H 2-38 and BF Cyg, together with the 28SiO v = 2 maser. We conclude that these different observational results between isolated Miras/red giants and symbiotic stars may be related with the presence of hot companions in a symbiotic binary system.

SIMULTANEOUS OBSERVATIONS OF SiO AND H 2 O MASERS TOWARD KNOWN STELLAR SiO AND H 2 O MASER SOURCES. I.

We present the results of simultaneous observations of both SiO and H2O masers toward 166 known SiO and H2O maser sources using the Korean VLBI Network Yonsei 21 m radio telescope during 2009 June. Both SiO and H2O maser emission were detected from 112 sources giving a detection rate of 67% at one epoch observation. SiO-only maser emission was detected from 42 sources, while H2O-only maser emission was detected from four sources. Most of the SiO masers appear around the stellar velocity, while H2O masers show different characteristics compared with SiO masers. There are more than 20 sources that show a one-way peak or double peaks with respect to the stellar velocity and SiO maser peak velocity. The H2O maser peak and integrated intensity ratios with respect to those of SiO (v = 1) show increasingly larger values from Mira variables, to OH/IR stars, to semi-regular variables. In addition, the IRAS two-color diagram of SiO and H2O maser observational results is discussed.

Japanese VLBI Network Mapping of SiO v = 3 J = 1–0 Maser Emission in W Hydrae

Abstract We report first on two-epoch mapping observations of SiO $v =$ 3 $J =$ 1–0 maser emission in the semiregular variable W Hydrae using the Japanese VLBI Network. The flux density of the $v =$ 3 $J =$ 1–0 emission detected on 2009 February 28 was two orders of magnitude smaller than those of the $v =$ 1 and $v =$ 2 emissions, while a month and half later the $v =$ 3 flux density suddenly increased by a factor of $\sim $25. In contrast, the $v =$ 1 and $v =$ 2 flux densities decreased during this period, as expected from the optical light curve. At the first epoch, the $v =$ 3 maser features were located inside of a ring composed of the $v =$ 1 and $v =$ 2 features by 6 mas ($\gtrsim $0.5 AU) toward the central star. These offsets are meaningfully larger than the error of the fitted ring radius and the difference in the ring sizes of $v =$ 1 and $v =$ 2 masers ($\lesssim $0.2 AU). The present result at the first epoch suggests that $v =$ 3 $J =$ 1–0 SiO masers are predominantly excited in a pumping mechanism (e.g., collisional pumping) different from that recently proposed on the basis of the line overlap with infrared H$_{2}$O lines. Interestingly, the second-epoch observation revealed that the $v =$ 3 features were located on almost the same ring as the $v =$ 2 ring, which is consistent with what line-overlap theory suggests.

The magnetic precursor of L1448-mm: excitation differences between ion and neutral fluids

Shock modelling predicts an electron density enhancement within the magnetic precursor of C-shocks. Previous observations of SiO, H13CO+, HN13C and H13CN toward the young L1448-mm outflow showed an over-excitation of the ion fluid that was attributed to an electron density enhancement in the precursor. We re-visit this interpretation and test if it still holds when we consider different source morphologies and kinetic temperatures for the observed molecules, and also give some insight on the spatial extent of the electron density enhancement around L1448-mm. We estimate the opacities of H13CO+ and HN13C by observing the J=3\to2 lines of rarer isotopologues to confirm that the emission is optically thin. To model the excitation of the molecules, we use the large velocity gradient (LVG) approximation with updated collisional coefficients to i) re- analyse the observations toward the positions where the over-excitation of H13CO+ has previously been observed [i.e. toward L1448- mm at offsets (0,0) and (0,-10)], and ii) to investigate if the electron density enhancement is still required for the cases of extended and compact emission, and for kinetic temperatures of up to 400 K. We also report several lines of SiO, HN13C and H13CO+ toward new positions around this outflow, to investigate the spatial extent of the over-excitation of the ions in L1448-mm. From the isotopologue observations, we find that the emission of H13CO+ and HN13C from the precursor is optically thin if this emission is extended. Using the new collisional coefficients, an electron density enhancement is still needed to explain the excitation of H13CO+ for extended emission and for gas temperatures of\le 400 K toward L1448-mm (0,-10), and possibly also toward L1448-mm (0,0). For compact emission the data cannot be fitted. We do not find any evidence for the over-excitation of the ion fluid toward the newly observed positions around L1448-mm. The observed line emission of SiO, H13CO+ and HN13C toward L1448-mm (0,0) and (0,-10) is consistent with an electron density enhancement in the precursor component, if this emission is spatially extended. This is also true for the case of high gas temperatures (\le400 K) toward the (0,-10) offset. The electron density enhancement seems to be restricted to the southern, redshifted lobe of the L1448-mm outflow. Interferometric images of the line emission of these molecules are needed to confirm the spatial extent of the over-excitation of the ions and thus, of the electron density enhancement in the magnetic precursor of L1448-mm.

OBSERVATIONS OF SiO J = 3-2 AND J = 2-1 EMISSION IN LATE-TYPE STARS. II.

Second observations of SiO v = 1, 2, J = 3-2 transitions, together with SiO v = 1, J = 2-1 transitions, have been performed toward 42 program late-type stars, using the 14 m radio telescope at Taeduk Radio Astronomy Observatory, from 1997 February to 1998 March. The v = 1, J = 3-2 maser was detected from 24 stars giving a detection rate of 57%, which is slightly higher than that of the first observations of 1995-1996. Four of these are new detections. For the v = 2, J = 3-2 maser, the line was detected from six stars including two new detections. The peak velocities and the velocity patterns of the v = 1, J = 3-2 masers are different from those of the v = 1, J = 2-1 indicating a difference in the traced regions. The average total photon flux of the v = 1, J = 3-2 maser is estimated to be 10{sup 42.7}, that of the v = 2, J = 3-2 maser 10{sup 42.2}, and that of the v = 1, J = 2-1 maser 10{sup 43.2} photons s{sup -1}. In addition, we confirm that the v = 2, J = 4-3 masermore » is stronger than the v = 1, J = 4-3.« less

NUMERICAL SIMULATIONS OF A PROTOSTELLAR OUTFLOW COLLIDING WITH A DENSE MOLECULAR CLOUD

High-resolution SiO observations of the NGC 1333 IRAS 4A star-forming region showed a highly collimated outflow with a substantial deflection. The deflection was suggested to be caused by the interactions of the outflow and a dense cloud core. To investigate the deflection process of protostellar outflows, we have carried out three-dimensional hydrodynamic simulations of the collision of an outflow with a dense cloud. Assuming a power-law type density distribution of the obstructing cloud, the numerical experiments show that the deflection angle is mainly determined by the impact parameter and the density contrast between the outflow and the cloud. The deflection angle is, however, relatively insensitive to the velocity of the outflow. Using a numerical model with physical conditions that are particularly suitable for the IRAS 4A system, we produce a column-density image and a position-velocity diagram along the outflow, and they are consistent with the observations. Based on our numerical simulations, if we assume that the initial density and the velocity of the outflow are $\sim 10 \cm3$ and $\sim 100 \kms$, the densities of the dense core and ambient medium in the IRAS 4A system are most likely to be $\sim 10^5 \cm3$ and $\sim 10^2 \cm3$, respectively. We therefore demonstrate through numerical simulations that the directional variability of the IRAS 4A outflow can be explained reasonably well using the collision model.

SiO outflow signatures toward massive young stellar objects with linearly distributed methanol masers

Methanol masers are often found in linear distributions, and it has been hypothesized that these masers are tracing circumstellar accretion disks around young massive stars. However, recent observations in H2 emission have shown what appear to be outflows at similar angles to the maser distribution angles, not perpendicular as expected in the maser-disk scenario. The main motivation behind the observations presented here is to determine from the presence and morphology of an independent outflow tracer, namely SiO, if there are indeed outflows present in these regions and if they are consistent or inconsistent with the maser-disk hypothesis. For ten sources with H2 emission we obtained JCMT single dish SiO (6-5) observations to search for the presence of this outflow indicator. We followed up those observations with ATCA interferometric mapping of the SiO emission in the (2-1) line in six sources. The JCMT observations yielded a detection in the SiO (6-5) line in nine of the ten sources. All of the sources with bright SiO lines display broad line wings indicative of outflow. A subset of the sources observed with the JCMT have methanol maser velocities significantly offset from their parent cloud velocities, supporting the idea that the masers in these sources are likely not associated with circumstellar disks. The ATCA maps of the SiO emission show five of the six sources do indeed have SiO outflows. The spatial orientations of the outflows are not consistent with the methanol masers delineating disk orientations. Overall, the observations presented here seem to provide further evidence against the hypothesis that linearly distributed methanol masers generally trace the orientations of circumstellar disks around massive young stars.

SiO line emission from interstellar jets and outflows: silicon-containing mantles and non-stationary shock waves

Context. We study the production and emission of SiO and H 2 in the gas phase of molecular outflows, extending previous work in which we considered steady-state C-type shock waves and assumed the silicon to be present only in the cores of silicate grains. Aims. We place constraints on the physical parameters of the pre-shock region, using recent observations of SiO and observations of molecular hydrogen. We show the effects of introducing SiO-containing mantles and of varying the age of the shock wave. We consider simultaneously the emission of SiO and H 2 from the young L1157 outflow. Methods. The molecular outflows are studied by means of a code that can generate stationary C- and J-type shock models and approximate non-stationary solutions, which combine these two types of shock wave. The emission of molecular hydrogen is computed by this code, whereas the SiO emission is computed by means of a separate LVG model, which uses the calculated physical and chemical profiles. A grid of models has been computed, with shock speeds in the range 10 ≤ u s < 35 km s -1 and pre-shock gas densities 10 4 ≤ n H ≤ 10 6 cm -3 . A wide range of magnetic field strengths has been investigated, from 45 pG to about 600 pG. Results. We illustrate our results by means of observational data obtained on the blue lobe of the L1157 outflow. Given the combinations of pre-shock densities and shock velocities necessary to fit the H 2 observations, we find that the erosion only of the silicate material in the grains cores cannot account for the observed SiO line intensities. We investigate the possiblity that a fraction of the SiO is present initially in the grain mantles, and we succeed in constraining this fraction. Introducing even a few percent of the silicon (as SiO) into the mantles is sufficient to increase the SiO line widths and fluxes by an order of magnitude. With this assumption, it is possible to find a non-stationary shock model that provides a reasonable fit of the observations of both H 2 and SiO. Conclusions. With a few percent of the silicon present initially in the grain mantles, good agreement is obtained with recent observations of SiO line integrated line intensities for a pre-shock density n H = 10 4 cm - and a shock speed v s = 20 km s -1 . The magnetic field strength and the shock age are not well constrained by the observations of either H 2 or SiO. We show that CO observations (in particular, with the Herschel satellite) could provide further discrimination between the models.

Outflow and Infall in a Sample of Massive Star Forming Regions. II. Large‐Scale Kinematics

We present maps of seven sources selected from Klaassen & Wilson (2007a) in SiO (J=8-7) and HCO$^+$ and H$^{13}$CO$^+$ (J=4-3) which were obtained using HARP-B on the James Clerk Maxwell Telescope. We find that four out of our seven sources have infall signatures based on their HCO$^+$ emission profiles. From our maps, we have determined the extent of both the outflowing and infalling regions towards these sources, and constrained the amount of infalling and outflowing mass as well as the mass infall rate for each massive star forming region. From our SiO observations, we estimate the source luminosity required to shock the surroundings of these massive star forming regions and find luminosities similar to those of the HII regions themselves. We find that the ratio between our infall and outflow masses is less than one, suggesting high mass entrainment rates in the molecular outflows. We also find that the large scale molecular infall rate towards G10.6-0.4 is comparable to the small scale molecular infall rate found in previous studies.

Outflow and Infall in a Sample of Massive Star‐forming Regions

We present single pointing observations of SiO, HCO$^+$ and H$^{13}$CO$^+$ from the James Clerk Maxwell Telescope towards 23 massive star forming regions previously known to contain molecular outflows and ultracompact HII regions. We detected SiO towards 14 sources and suggest that the non-detections in the other nine sources could be due to those outflows being older and without ongoing shocks to replenish the SiO. We serendipitously detected SO$_2$ towards 17 sources in the same tuning as HCO$^+$. We detected HCO$^+$ towards all sources, and suggest that it is tracing infall in nine cases. For seven infall candidates, we estimate mass infall rates between 1$\times10^{-2}$ and 2$\times10^{-5}$ M$_{\odot}$ yr$^{-1}$. Seven sources show both SiO detections (young outflows) and HCO$^+$ infall signatures. We also find that the abundance of H$^{13}$CO$^+$ tends to increase along with the abundance of SiO in sources for which we could determine abundances. We discuss these results with respect to current theories of massive star formation via accretion. From this survey, we suggest that perhaps both models of ionized accretion and halted accretion may be important in describing the evolution of a massive protostar (or protostars) beyond the formation of an HII region.

Outflow and Dense Gas Emission from Massive Infrared Dark Clouds

Infrared Dark Clouds are expected to harbor sources in different, very young evolutionary stages. To better characterize these differences, we observed a sample of 43 massive Infrared Dark Clouds, originally selected as candidate high-mass starless cores, with the IRAM 30m telescope covering spectral line tracers of low-density gas, high-density gas, molecular outflows/jets and temperature effects. The SiO(2-1) observations reveal detections toward 18 sources. Assuming that SiO is exclusively produced by sputtering from dust grains, this implies that at least in 40% of this sample star formation is on-going. A broad range of SiO line-widths is observed (between 2.2 and 65km/s), and we discuss potential origins for this velocity spread. While the low-density tracers 12CO(2-1) and 13CO(1-0) are detected in several velocity components, the high-density tracer H13CO+(1--0) generally shows only a single velocity component and is hence well suited for kinematic distance estimates of IRDCs. Furthermore, the H13CO+ line-width is on average 1.5 times larger than that of previously observed NH3(1,1). This is indicative of more motion at the denser core centers, either due to turbulence or beginning star formation activity. In addition, we detect CH3CN toward only six sources whereas CH3OH is observed toward approximately 40% of the sample. Estimates of the CH3CN and CH3OH abundances are low with average values of 1.2x10^{-10} and 4.3x10^{-10}, respectively. These results are consistent with chemical models at the earliest evolutionary stages of high-mass star formation. Furthermore, the CH3OH abundances compare well to recently reported values for low-mass starless cores.

Observations of SiO J=2–1 and J=3–2 masers towards evolved stars with the TRAO 14 m telescope

AbstractWe present the results of survey and time-monitoring observations of SiO J=2–1 and J=3–2 masers towards evolved stars with the 14 m radio telescope at Taeduk Radio Astronomy Observatory (TRAO) from 1995 February to 2001 February. The first detection of SiO v=3, J=2–1 maser emission toward S-type Mira variable χ Cyg is also presented.

First Detection of the SiO (v= 3,J= 2–1) Maser Emission from χ Cygni

From the simultaneous observations of SiO v = 3, J = 2-1 and J = 3-2 masers, SiO v = 3, J = 2-1 maser emission was detected for the first time toward the S-type Mira variable χ Cyg. The line was a single spike and was redshifted with respect to the stellar velocity. The SiO v = 3, J = 3-2 maser emission was not detected from this star. However, for the O-rich Mira variable TX Cam, SiO v = 3, J = 3-2 maser emission was detected, while v = 3, J = 2-1 maser was not detected. Two possible line overlaps were adopted as an explanation of these observational results.

High‐Resolution Imaging of Molecular Outflows in Massive Young Stars

We present high angular resolution observations toward two massive star-forming regions, IRAS 18264-1152 and IRAS 23151+5912, with the Plateau de Bure Interferometer (PdBI) in the SiO(J = 2-1) and H13CO+(J = 1-0) lines and at 1.3 and 3.4 mm continuum, and with the Very Large Array (VLA) in the NH3(J,K) = (1,1), (2, 2) lines. The NH3(1, 1) and (2, 2) emission is detected toward IRAS 18264-1152 only. For IRAS 18264-1152, the SiO observations reveal at least two quasi-perpendicular outflows with high collimation factors, and the most dominant feature is a redshifted jetlike outflow with very high velocities up to about Δv = 60 km s-1 with respect to the systemic velocity. The very high velocity component (Δv = 22-60 km s-1) of this outflow is spatially offset from its high-velocity (Δv = 3-21 km s-1) component. The SiO line profiles and position-velocity characteristics of these two components suggest that this outflow could be driven by an underlying precessing jet. For IRAS 23151+5912, the bipolar but mainly blueshifted SiO outflow coincides with the inner parts of the single-dish CO outflow. In particular, the quasi-parabolic shape of the blueshifted outflow coincides with the near-infrared nebulosity and is consistent with entrainment of the gas by an underlying wide-angle wind. The analysis of the molecular outflow data of the two luminous sources further supports high-mass stars forming via a disk-mediated accretion process similar to that in low-mass stars.