High-velocity Molecular Flows Research Articles

A distance-limited sample of massive molecular outflows

We have observed 99 mid-infrared-bright, massive young stellar objects and compact HII regions drawn from the Red MSX source (RMS) survey in the J=3$-$2 transition of $^{12}$CO and $^{13}$CO, using the James Clerk Maxwell Telescope. 89 targets are within 6 kpc of the Sun, covering a representative range of luminosities and core masses. These constitute a relatively unbiased sample of bipolar molecular outflows associated with massive star formation. Of these, 59, 17 and 13 sources (66, 19 and 15 percent) are found to have outflows, show some evidence of outflow, and have no evidence of outflow, respectively. The time-dependent parameters of the high-velocity molecular flows are calculated using a spatially variable dynamic timescale. The canonical correlations between the outflow parameters and source luminosity are recovered and shown to scale with those of low-mass sources. For coeval star formation we find the scaling is consistent with all the protostars in an embedded cluster providing the outflow force, with massive stars up to $\sim$30 M$_{\odot}$ generating outflows. Taken at face value, the results support the model of a scaled-up version of the accretion-related outflow-generation mechanism associated with discs and jets in low-mass objects with time-averaged accretion rates of $\sim$10$^{-3}$ M$_{\odot}$ yr$^{-1}$ onto the cores. However, we also suggest an alternative model, in which the molecular outflow dynamics are dominated by the entrained mass and are unrelated to the details of the acceleration mechanism. We find no evidence that outflows contribute significantly to the turbulent kinetic energy of the surrounding dense cores.

A large-scale CO survey of the Rosette Molecular Cloud: assessing the effects of O stars on surrounding molecular gas

We present a new large-scale survey of the J= 3–2 12CO emission covering 4.8 deg2 around the Rosette Nebula. The results reveal the complex dynamics of the molecular gas in this region. We identify about 2000 compact gas clumps having a mass distribution given by dN/dM∼M−1.8, with no dependence of the power-law index on distance from the central O stars. A detailed study of a number of the clumps in the inner region shows that most exhibit velocity gradients in the range 1–3 km s−1 pc−1, generally directed away from the exciting nebula. The magnitude of the velocity gradient decreases with distance from the central O stars, and we compare the apparent clump acceleration with a photoionized gas acceleration model. For most clumps outside the central nebula, the model predicts lifetimes of a few 105 yr. In one of the most extended of these clumps, however, a near-constant velocity gradient can be measured over 1.7 pc, which is difficult to explain with radiatively driven models of clump acceleration. As well as the individual accelerated clumps, an unresolved limb-brightened rim lies at the interface between the central nebular cavity and the Rosette Molecular Cloud. Extending over 4 pc along the edge of the nebula, this region is thought to be at an earlier phase of disruption than the accelerating compact globules. Blueshifted gas clumps around the nebula are in all cases associated with dark absorbing optical globules, indicating that this material lies in front of the nebula and has been accelerated towards us. Redshifted gas shows little evidence of associated line-of-sight dark clouds, indicating that the dominant bulk molecular gas motion throughout the region is expansion away from the O stars. In addition, we find evidence that many of the clumps lie in a molecular ring, having an expansion velocity of 30 km s−1 and radius 11 pc. The dynamical time-scale derived for this structure (∼106 yr) is similar to the age of the nebula as a whole (2 × 106 yr). The J= 3–2/1–0 12CO line ratio in the clumps decreases with radial distance from the exciting O stars, from 1.6 at ∼8 pc distance to 0.8 at 20 pc. This can be explained by a gradient in the surface temperature of the clumps with distance, and we compare the results with a simple model of surface heating by the central luminous stars. We identify seven high-velocity molecular flows in the region, with a close correspondence between these flows and embedded young clusters or known young luminous stars. These flows are sufficiently energetic to drive gas turbulence within each cluster, but fall short of the turbulent energy of the Rosette giant molecular cloud by two orders of magnitude. We find 14 clear examples of association between an embedded young star (as seen by Spitzer at 24 μm) and a CO clump in the molecular cloud facing the nebular. The CO morphology indicates that these are photoevaporating circumstellar envelopes. CO clumps without evidence of embedded stars tend to have lower gas velocity gradients. It is suggested that the presence of the young star may extend the lifespan of the externally photoevaporating envelope.

Molecular line observations of the extremely high velocity molecular flow near HH 7-11

view Abstract Citations (18) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecular Line Observations of the Extremely High Velocity Molecular Flow near HH 7--11 Koo, Bon-Chul Abstract The characteristics of CO J = 2-1 lines of extremely high velocity (EHV) molecular gas near HH 7-11 are described. The CO J = 2-1 to J = 1-0 line ratio in the EHV wing implies a large gas density of 1000 or 10,000/cu cm, depending on whether CO molecules are excited by collisions with H2 or H. While the results are not inconsistent with the EHV CO wing being the emission from CO molecules formed in the neutral stellar wind, the implied large gas density suggests that the wind is not homogeneous but rather blows in either lumps, filaments, or sheets. An upper limit of 0.01 is obtained for the abundance ratio SiO/CO in the EHV gas, which is consistent with a chemical model for neutral stellar winds. Publication: The Astrophysical Journal Pub Date: September 1990 DOI: 10.1086/169176 Bibcode: 1990ApJ...361..145K Keywords: Herbig-Haro Objects; Line Spectra; Molecular Spectra; Stellar Winds; Carbon Compounds; Flow Velocity; Infrared Sources (Astronomy); Interstellar Gas; Magnetohydrodynamic Flow; Oxygen Compounds; Astrophysics; INTERSTELLAR: MOLECULES; LINE PROFILES; STARS: PRE--MAIN-SEQUENCE; STARS: WINDS full text sources ADS | data products SIMBAD (2)

Extremely high-velocity molecular flows in young stellar objects

view Abstract Citations (62) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Extremely High-Velocity Molecular Flows in Young Stellar Objects Koo, Bon-Chul Abstract Very sensitive, wideband CO J = 1-0, J = 2-1, and HCO+ J = 1-0 spectra of several young stellar objects are presented. Extremely weak and very broad extremely high velocity (EHV) wings are found in GL 4909, HH 7-11, S140, and MWC 1080. The line profiles of EHV wings are smooth or flat, quite different from steep HV wings. In L551 and HL Tau, EHV wings as broad as other sources are not found, which would be due to the inclination of the bipolar flows to the line of sight. From the excitation temperature of about 10 K derived for the EHV flows, the estimated column density of CO yields a number density too small to explain the observed ratio of CO J = 2-1 to J = 1-0 intensity. It is concluded that either EHV flows are composed of small dense clumps or most of the carbon in the flow is atomic carbon, unless the kinetic temperature of the flow is much higher than 100 K. Publication: The Astrophysical Journal Pub Date: February 1989 DOI: 10.1086/167105 Bibcode: 1989ApJ...337..318K Keywords: Molecular Flow; Pre-Main Sequence Stars; Stellar Spectra; Stellar Winds; Carbon Monoxide; Formyl Ions; Line Spectra; Molecular Spectra; Astrophysics; INTERSTELLAR: MOLECULES; LINE PROFILES; STARS: PRE--MAIN-SEQUENCE; STARS: WINDS full text sources ADS | data products SIMBAD (11)

Are the high-velocity molecular flows really clumpy?

view Abstract Citations (10) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Are the High-Velocity Molecular Flows Really Clumpy? Canto, J. ; Rodriguez, L. F. ; Anglada, G. Abstract The paper examines previous findings that the derived filling factors in the calculations of high-velocity molecular flow parameters are less than unity, thus indicating that these flows are clumpy. Here, the relative line intensities of the low rotational transitions of (C-12)O and (C-13)O molecules were calculated for regions with temperature gradients and were found to be very different from those expected from an isothermal region at any temperature. The standard procedure used in the analysis of CO data under assumption of uniform temperature was then applied to the results of the calculations with a temperature gradient. It was found that the deduced filling factors were less (or greater) than unity if the temperature increased (decreased) toward the center of the molecular cloud. It is concluded that the deduced filling factors (and, thus, the clumpiness) are an artifact of the assumption of uniform temperature adopted in the data analysis. Publication: The Astrophysical Journal Pub Date: October 1987 DOI: 10.1086/165679 Bibcode: 1987ApJ...321..877C Keywords: Carbon Monoxide; Interstellar Matter; Molecular Flow; Radiative Transfer; Radio Sources (Astronomy); Stellar Evolution; Computational Astrophysics; Magnetohydrodynamic Waves; Stellar Winds; Temperature Dependence; Temperature Gradients; Astrophysics; INTERSTELLAR: MOLECULES; RADIATIVE TRANSFER; RADIO SOURCES: LINES full text sources ADS |

Rotation measures of extragalactic radio sources viewed through the molecular clouds Cepheus A and L1551

VLA data on polarized sources in 11 high-velocity molecular flow regions at C-and L-band frequencies with 50 MHz resolution are analyzed to derive the Faraday rotation measures (RM) of the magnetic fields associated with the flows. The survey revealed the L1551 field, which has a strong source behind the molecular flow, and Cep A, where numerous polarized sources are located near the flow. The calculations of the RM were based on least squares analyses of the polarized intensity, position angle and total intensity maps. Differences were calculated in the RMs for different lines of sight in order to determine the causes of variations in the electron densities and magnetic fields within the clouds. The magnetic field within the L1551 flow was estimated to be 0.1 mG, indicating the presence of significant shock ionization at relative velocities exceeding 50 km/sec. Further observational techniques that may be employed to increase the understanding of the physical characteristics of bipolar flows are discussed. 31 references

On the energetics of high-velocity molecular flows

On the energetics of high-velocity molecular flows

High Velocity Gas Flows Associated with h2 Emission Regions - how are they Related and what Powers Them

view Abstract Citations (126) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-velocity gas flows associated with H2 emission regions : how are they related and what powers them ? Fischer, J. ; Sanders, D. B. ; Simon, M. ; Solomon, P. M. Abstract We report on an investigation of high-velocity molecular material near the vibrationally excited H2 emission regions in the objects DR 21, W75 N, NGC 7538, OMC-2, NGC 6334/Peak V, and the Herbig-Haro Object 2. The 12CO(J = 1-0) and 13CO(J = 1-0) transitions were used as tracers of the high-velocity molecular flows. All the H2 sources are associated with bipolar CO high-velocity flows. We also present a new highspatial-resolution map (11") of the DR 21 H2 region. The spatial relationship of the high-velocity gas and the vibrationally excited gas, together with the similarity of the energies involved in the two molecular components, is convincing evidence to support the scenario in which the H2 emission traces the cooling gas that has been shocked by outflowing gas. Although radiation pressure could be responsible for driving the flows in OMC-2 and NGC 7538, it is unlikely to be the driving mechanism in DR 21 and W75 N. Similarly, H II region expansion does not appear to be the driving mechanism for any of the outflows. Publication: The Astrophysical Journal Pub Date: June 1985 DOI: 10.1086/163256 Bibcode: 1985ApJ...293..508F full text sources ADS | data products SIMBAD (7)

The nature of AFGL 2591 and its associated molecular outflow Infrared and millimeter-wave observations

The results of infrared photometry from 2 to 160 microns of AFGL and CO(12) observations of its associated molecular cloud and high velocity molecular outflow are presented and discussed. The observed solar luminosity is 6.7 x 10(4) at a distance of 2 kpc. The spectrum of AFGL 2591 is interpreted in the context of a model in which a single embedded object is the dominant source of the infrared luminosity. This object is determined to be surrounded by a compact, optically thick dust shell with a temperature in excess of several hundred degrees kelvin. The extinction to this source is estimated to be between 26 and 50 visual magnitudes. The absolute position of the infrared sources at 10 microns was determined to an accuracy of + or - 1 in. This indicates for the first time that the IR source and H2O source are not coincident. The CO(12) observations show the high-velocity molecular flow near AFGL 2591 to be extended, bipolar and roughly centered on the infrared emission. The observations suggest that the red-shifted flow component extends beyond the boundary of the ambient cloud within which AFGL 2591 is embedded. The CO(12) observations also show that AFGL 2591 is embedded in a molecular cloud with an LSR velocity of -5 km/s.

Discovery of a family of Herbig–Haro objects in M42: implications for the geometry of the high velocity molecular flow?

Resultats d'une observation electronographique [OI] λ 6300 des objets de Herbig Haro dans la nebuleuse d'Orion. On a decouvert six objets proches de M42 HH1. Les donnees spectroscopiques de haute dispersion revelent la presence d'ailes bleues a grand vitesse dans les raies de faible excitation. Les vitesses maximales vont de −100 a −380 kms −1 par rapport au gaz [OI] ambiant. Les objets semblent etre des condensations stationnaires incluses dans un vent stellaire a grande vitesse

Source of the high-velocity molecular flow in Orion

We present 6'' resolution aperture synthesis maps of the 86.8 GHz SiO (v = 0, J = 2-1) emission from Orion-KL. The maps show that the ''high-velocity'' molecular flow in this region is bipolar and originates within a few arc seconds of the infrared source IRc2. The blueshifted SiO line wing arises from a well-defined lobe which extends W from IRc2, while the redshifted line wing is strongest to the E of IRc2. The axis defined by the high-velocity SiO emission is not well aligned with the lobe of vibrationally excited H/sub 2/ emission to the NW of IRc2.

The high-velocity molecular flows near young stellar objects

The results of a search for high-velocity molecular gas associated with star forming molecular clouds are reported. Faint, high-velocity (..delta..V> or approx. =30 km s/sup -1/) /sup 12/CO emission is freqently found near buried infrared sources when sufficiently deep (T/sub rms/ <100 mK) integrations are obtained. In most clouds, the high-velocity gas is observed over an extended area having a typical radius around 10/sup 18/ cm and exhibits bipolar structure with the redshifted emission formed in a different region from the corresponding blueshifted emission. The ratio of the flow major and minor diameters varies from about 6 to less than 2, with most sources in the range between 2 and 3, indicating that most flows are not highly collimated. In many sources, the red portion of the line wings is stronger and/or covers a larger angular region of the sky than the corresponding blue wing. The opacity of the /sup 12/CO line wings appears to be greater than unity. Correlations between the central source luminosity and both the flow mechanical luminosity and momentum flux are found. A correlation between the mass of a flow and its apparent size suggests that the flows may be located in molecular clouds with steeplymore » decreasing radial density gradients. The presence of high-velocity outflow in molecular clouds indicates an unanticipated phase of energetic mass loss associated with the birth and early evolution of stars. Even though the flows have large energies, momenta, and a high birthrate, they may not be capable of supporting molecular clouds against free-fall gravitational collapse.« less

Observations of 2 micron molecular hydrogen emission from NGC 2071, Cepheus A, and GL 961

The infrared emission line at lambda2.12 ..mu..m from the v = 1--0 S(1) transition of molecular hydrogen has been detected and partially mapped in the vicinity of the high-velocity molecular flows associated with NGC 2071, Cepheus A, and GL 961. The H/sub 2/ emission regions are extended on a scale of at least 0.2 to 0.3 pc, comparable to or somewhat greater than the size of the Orion H/sub 2/ source. The H/sub 2/ surface brightness, uncorrected for possible extinction, is, however, at least an order of magnitude lower in NGC 2071, Cep A, and GL 961 than in Orion. For sources with CO emission showing spatial displacement of redshifted and blueshifted peaks, there is a tendency for the strongest H/sub 2/ emission to occur in regions where blueshifted CO wings are the most prominent component. Emission at the positions of redshifted CO wings is weaker or absent. The source GL 490 was mapped over a region subtending several arcminutes and does not exhibit S(1) line emission above a flux level of 1 x 10/sup -20/ W cm/sup -2/.