Bipolar Outflow Source Research Articles

Sources of Bipolar Outflows and Model Challenges for the Egg Nebula

Abstract The striking features of the Egg Nebula—searchlight beams that illuminate a pair of bipolar lobes, quadrupolar outflows, and arcs imprinted on an expanding spherical envelope—have long defied a coherent explanation. Here we present images at high angular resolutions of emission from dust and dense molecular gas in the nebula. Away from the systemic velocity, the molecular gas primarily traces the walls of the bipolar lobes, where bipolar outflows collide with the envelope ejected by the progenitor AGB star. At and close to the systemic velocity, both the molecular gas and dust trace the walls of a cylindrical channel swept aside by these bipolar outflows; in addition, the dust also traces a channel orthogonal to the bipolar lobes, swept aside by the orthogonal set of bipolar outflows. Close to the center, the dense gas traces an expanding spherical shell, corresponding to a very dense final puff from the progenitor AGB star. The center of this shell lies within the channel aligned with the optical lobes but is displaced to the south of the channel orthogonal to these lobes, and it closely coincides with the illumination center of the nebula (location of post-AGB star), as well as the expansion center of the bipolar lobes. We highlight difficulties faced by models whereby a binary companion both induces arcs in the wind of the progenitor AGB star and gives rise to the quadrupolar outflows. Instead, we suggest that the addition of a second companion, thus forming a triple-star system, can naturally produce all the features seen.

New bow-shock source with bipolar morphology in the vicinity of Sgr A*

Context. We find an extended source in the direct vicinity of Sgr A* with an approximate projected mean distance of 425 ± 26 mas. Its sky-projected elongated shape can be described by an averaged spatial extension ofx = 110 ± 20 mas andy = 180 ± 20 mas. With this, the observed object points in the analyzed SINFONI data sets between 2006 and 2016 directly toward the supermassive black hole. We discuss different possible scenarios that could explain the detected blueshifted line emission source.Aims. Here we present a detailed and extensive analysis of the adaptive optics corrected SINFONI data between 2006 and 2016 with a spatial pixel scale of 0.″025 and a corresponding field of view of 0.″8 × 0.″8 per single data cube with the focus on the newly discovered source. We spectroscopically identify the source, which we name X8, in the blueshifted Brγline maps. Additionally, an upper limit for the continuum magnitude can be derived from the close-by S-star S41.Methods. We applied the standard reduction procedure with the SINFONI/EsoRex pipeline for the analysis. We applied pre- and post-data correction in order to establish various calibration procedures. For the sharpened images, we used the Lucy–Richardson algorithm with a low iteration number. For the high-pass filtered images, we used the smooth-subtracting process in order to increase the signal-to-noise ratio.Results. We are able to detect the elongated line emission source in quantified data sets between 2006 and 2016. We find a lower limit for the infrared continuum magnitude ofKs ≳ 17.0 ± 0.1. The alignment of X8 toward Sgr A* can be detected in data sets that fulfill a sufficient number of observations with a defined quality level. A more detailed analysis of the results shows indications of a bipolar outflow source that might be associated with either a young stellar object, or with a post-AGB star or young planetary nebula.Conclusions. The near-infrared excess source X8 close to S24, S25, and S41 can be detected between 2006 and 2016. In addition to an apparent bow-shock morphology, the source shows clear signatures of a bipolar outflow that is consistent with both a young stellar object and a post-AGB star. If confirmed, this would be the closest ever detected bipolar outflow source to the supermassive black hole. Similar to the case of the DSO/G2 source and other dusty sources, it further supports the in situ star formation in the direct vicinity of Sgr A*. If X8 were a bow-shock source, it would be the third object of this type that can be found in projection in the mini-cavity. This scenario would support the idea that the cavity is created by a wind from Sgr A*.

Near-Infrared Imaging Polarization Study of M 17

Abstract We conducted wide-field ($ \sim$ 8$ '$$ \times$ 8$ '$ ) $ \textit{JHK}_{\rm s}$ imaging polarimetric observations toward the massive star-formation region M 17. The southern H II bar is identified as large-scale infrared reflection nebula (IRN) illuminated by OB stars in the cluster center, while the northern bar shows polarization originating from dichroic extinction. Three small-scale bipolar IRN with centro-symmetric polarization patterns were identified, and their illuminating sources, embedded young stellar objects (YSOs), are most likely the driving sources of bipolar outflows. $ \textit{JHK}_{\rm s}$ polarizations of point sources in M 17 show a wavelength dependence of the polarization of dichroic extinction in the general interstellar medium, indicating $ \alpha$$ \sim$ 1.8 for the empirical relation $ P$$ \propto$$ \lambda^{-\alpha}$ of interstellar polarization. The distribution of the polarization angle shows two peaks at 170$ ^{\circ}$ and 110$ ^{\circ}$ , indicating a non-uniform magnetic field structure in M 17. The direction of the magnetic field in the cluster region is south-north. However, the magnetic field in the H II region is roughly between 100$ ^{\circ}$ –140$ ^{\circ}$ , roughly perpendicular to the Galactic disk. Such a configuration indicates that the magnetic field in the cluster region is twisted from the primordial orientation in the cluster formation. The magnetic field in the southern bar has a total strength of $ \sim$ 230$ \ \mu$ G, and is 40$ ^{\circ}$ inclined to the plane of the sky. The outflows driven by YSOs in M 17 show a poor relation with the surrounding magnetic fields. This fact implies that the magnetic field in M 17 does not dominate the orientations of protostars formed inside.

HCO+emission excess in bipolar outflows

A plausible model is proposed for the enhancement of the abundance of molecular species in bipolar outflow sources. In this model, levels of HCO+ enhancement are considered based on previous chemical calculations, that are assumed to result from shock-induced desorption and photoprocessing of dust grain ice mantles in the boundary layer between the outflow jet and the surrounding envelope. A radiative transfer simulation that incorporates chemical variations within the flow shows that the proposed abundance enhancements in the boundary layer are capable of reproducing the observed characteristics of the outflow seen in HCO+ emission in the star forming core L1527. The radiative transfer simulation also shows that the emission lines from the enhanced molecular species that trace the boundary layer of the outflow exhibit complex line profiles indicating that detailed spatial maps of the line profiles are essential in any attempt to identify the kinematics of potential infall/outflow sources. This study is one of the first applications of a full three dimensional radiative transfer code which incorporates chemical variations within the source.

Evidence for an Ionized Rotating Structure Around RW Aur

Profiles of the UV semiforbidden lines of C III](1908) and Si III](1892) of RW Aur have been obtained with the HST/STIS. The C III](1908) profile shows two high velocity components at v = +/- 170 km s(-1) and a central one. The Si III](1892) profile is very broad (FWHM = 293 km s(-1)) and the high velocity components are unresolved. It is shown that the high velocity components are most probably produced in a rotating belt alike the detected in other sources of bipolar outflows. A radius between 2.7 R-* and the corotation radius (6.1 R-*) is derived and a log T-e ( K) similar or equal to 4.7 and log n(e)(cm(-3)) = 11.6 are estimated. The belt is clumpy and the most likely source of heating is local X-rays radiation, probably associated with the release of magnetic energy.

Hubble Space TelescopeSTIS Spectrum of RW Aurigae A: Evidence for an Ionized Beltlike Structure and Mass Ejection in Timescales of a Few Hours

Profiles of the UV semiforbidden lines of C III](1908) and Si III](1892) of RW Aur have been obtained with the Hubble Space Telescope (HST) STIS. The C III](1908) profile shows two high-velocity components at upsilon = +/-170 km s(-1) and a central one. The Si III](1892) profile is very broad (FWHM = 293 km s(-1)), and the high-velocity components are unresolved. The high-velocity components are most probably produced in a rotating belt similar to that detected in other sources of bipolar outflows. A radius between 2.7 R-* and the corotation radius (6.1 R-*) is derived, and a log T-e(K) similar or equal to 4.7 and log n(e)(cm(-3)) = 11.6 are estimated. The belt is clumpy, and the most likely source of heating is local X-ray radiation, probably associated with the release of magnetic energy. In addition, profiles of the optical lines of He I, Fe II, and Halpha retrieved from the HST archive have been analyzed. The spectra were obtained by shifting the STIS slit between exposures in the transverse direction to the flow. Two features vary from one exposure to another: a blueshifted emission feature (detected in the Fe II and He I lines) and a redshifted absorption feature (detected in Halpha), which are observed at the velocity of the blueshifted and redshifted components of the jet, respectively. There is a clear-cut correlation between the equivalent widths of these two components. These components are not associated with the flaring activity of the source, since they remain stable timescales of similar or equal to40 minutes. They are most likely associated with variations in the stellar/circumstellar environment in timescales of a few (similar or equal to5) hours, consistent with reports by other authors for variations of the veiling and the appearance of shell signatures in the optical spectrum.

OH masers and magnetic fields in the bipolar outflow source W75N

MERLIN polarization measurements of 1665-,1667- and 1720-MHz OH masers in the bipolar outflow source W75N are presented. The OH masers are distributed over a region 5 arcsec across (10 000 au at 2 kpc), which is elongated in a north-west‐ south-east (NW ‐SE) direction. The distribution is interpreted in terms of a rotating disc orthogonal to the bipolar outflow that is seen in CO and H2. A compact cluster of OH masers with a large velocity spread appears to mark the source of the outflow. Most of the OH masers detected have strong circular polarization up to 100 per cent. Seven Zeeman groups are identified, including the first example of a complete Zeeman triplet (two s-components and one p-component). The field strength measured from Zeeman pairs of opposite circular polarization ranges from 2 8 toa 8 mG, with a field reversal on opposite sides of the disc. This is interpreted in terms of a toroidal component of the magnetic field in the disc. Linear polarization is also detected in many of the OH masers with some 1665-MHz masers showing up to 100 per cent linear polarization. The polarization position angles are preferentially along the direction of the bipolar outflow or perpendicular to it. The polarization data lend support to the twisted magnetic field model originally proposed by Uchida & Shibata.

Milliarcsecond structure of methanol masers in L1206 and GL2789

We report results of EVN interferometric study of two star-forming regions L1206 and GL2789 in the brightest methanol maser line at 6.7 GHz. Using measured absolute positions both methanol masers were identified with protostars which are sources of bipolar outflows. Both masers consist of several maser spots, with some of them being aligned in a linear structure with velocity gradient probably delineating edge-on circumstellar disks. We estimated the radii of such disks to be 140 AU and 280 AU (or 700 AU for the whole structure treated as a disk) for L1206 and GL2789 respectively. The brightness temperatures of the most intense features in L1206 and GL2789 are at least 1.1 × 1010 K and 1.4 × 109K respectively.

Evidence for a solar system-size accretion disk around the massive protostar G192.16-3.82.

Seven-millimeter continuum observations of a massive bipolar outflow source, G192.16-3.82, were made at a milli-arc-second resolution with a capability that links the National Radio Astronomy Observatory's Very Large Array radio interferometer with the Very Long Baseline Array antenna, located in Pie Town, New Mexico. The observations provide evidence for a true accretion disk that is about the size of our solar system and located around a massive star. A model of the radio emission suggests the presence of a binary protostellar system. The primary protostar, G192 S1, at the center of the outflow, with a protostar mass of about 8 to 10 times the solar mass, is surrounded by an accretion disk with a diameter of 130 astronomical units (AU). The mass of the disk is on the order of the protostar mass. The outflow is poorly collimated with a full opening angle of about 40 degrees; there is no indication of a more highly collimated jetlike component. The companion source, G192 S2, is located 80 AU north of the primary source.

Discovery of a dense bipolar outflow from a new class 0 protostar in NGC 2068/LBS 17

We report the discovery of high-velocity dense gas from a bipolar outflow source near NGC 2068 in the L1630 giant molecular cloud. CO and HCO+J=3→2 line wings have a bipolar distribution in the vicinity of LBS 17-H with the flow orientated roughly east-west and perpendicular to the elongation of the submillimetre dust continuum emission. The flow is compact (total extent ∼0.2 pc) and contains of the order of 0.1 M⊙ of swept-up gas. The high-velocity HCO+ emission is distributed over a somewhat smaller area <0.1 pc in extent. A map of C18O J=2→1 emission traces the LBS 17 core and follows the ambient HCO+ emission reasonably well, with the exception of the direction towards LBS 17-H where there is a significant anticorrelation between the C18O and HCO+. A comparison of beam-matched C18O and dust-derived H2 column densities suggests that CO is depleted by up to a factor of ∼50 at this position if the temperature is as low as 9 K, although the difference is substantially reduced if the temperature is as high as 20 K. Chemical models of collapsing clouds can account for this discrepancy in terms of different rates of depletion on to dust grains for CO and HCO+. LBS 17-H has a previously known water maser coincident with it but there are no known near-infrared, IRAS or radio continuum sources associated with this object, leading to the conclusion that it is probably very young. A greybody fit to the continuum data gives a luminosity of only 1.7 L⊙ and a submillimetre-to-bolometric luminosity ratio of 0.1, comfortably satisfying the criteria for classification as a class 0 protostar candidate.

Magnetic field structure in the bipolar outflow source G 35.2-0.74N: MERLIN spectral line results

MERLIN polarization measurements of the OH 1665-MHz maser emission from the bipolar outflow source G 35.2-0.74N are presented. The masers have an elongated distribution, and a velocity gradient which is interpreted in terms of a disc surrounding the source of the outflow. The radius of the maser disc is 1.3 arcsec (2600 au at 2 kpc). Analysis of the OH maser polarization reveals four Zeeman pairs, with field strengths ranging from -2.5 to +5.3 mG. The direction of the field reverses on opposite sides of the disc. Linear polarization is found in five of the masers, and is used to infer the three-dimensional magnetic field direction at these points. The data lend support to the hydromagnetic model proposed by Uchida & Shibata.

The Density, Extinction, and Excitation Characteristics of NGC 6445

We have acquired [O III] λ5007, H I λλ6563 and 4861, and [S II] λλ6717 and 6731 imaging of the bipolar outflow source NGC 6445. As a result, we find that density is preferentially enhanced within an interior optical/radio emission annulus and takes a maximum value ~1.1 × 103 cm-3. Visual extinction appears also to be enhanced within this regime, taking values ~0.8 mag greater than for the enveloping shell, a result that is shown to imply ratios Av/N(H2) comparable to that of NGC 7027 and at least 4 times greater than for the interstellar medium. Finally, we investigate the variation of excitation through the projected nebular envelope. While the nucleus possesses excitation properties consistent with normal (radiatively excited) sources, it appears that line ratios toward the source periphery may indicate the presence of appreciable shock activity.

The density and shock characteristics of NGC 2818

We report the results of narrow band imaging of the bipolar outflow source NGC 2818 in transitions of [NII], [OIII], [SII], and HI. As a consequence, we are able to assess the overall excitation properties of the shell, and determine that line ratios towards the nebular periphery are anomalous, and characteristic of planar shocks with velocity km s-1 . Pre-shock densities are likely to be modest, and of order cm-3 . The [SII] imaging is further used to derive a fully-sampled map of electron density, whence it is clear that the density structure is rather complex and related to various features within the excitation maps. Peak values cm-3 occur close to the nucleus, although densities subsequently decline for major axis displacements arcsec. The minor axis distribution, on the other hand, appears to be somewhat more extended and bilobal. It is probable that complexities in the density mapping reflect the presence of condensations and filaments within the primary shell structure, and that these are also responsible for apparent variations in density as a function of forbidden and permitted line transition.

Large‐Scale Structure, Kinematics, and Heating of the Orion Ridge. I. VLA NH3(1, 1) and (2, 2) Multifield Mosaics

We present high-resolution VLA mosaics of 18 adjacent fields in OMC-1 covering an 8' ? 4' (1.2 ? 0.6 pc) region surrounding the Orion-KL core. The NH3 (J, K) = (1, 1) and (2, 2) inversion transitions were observed; the resulting maps were produced with both high spectral (0.3 km s-1) and high angular (8'') resolution. Both linear and nonlinear (maximum entropy method) techniques were employed to create the mosaics; we compare the challenges and results of each method. The complex effects of chemical excitation are discussed. We find extended clumpy filaments throughout the 0.5 pc region extending to the north and west from Orion-KL. The structure of filaments composed of chains of condensations appears to be hierarchical in OMC-1, as it is found on several scales. The filaments are separated into at least two major velocity components that appear to overlap in the central Orion-KL core region, suggesting interaction between cloud components as a possible triggering mechanism for the active high-mass star formation occurring there. The overlapping nature of the velocity components complicates simple global rotation models for OMC-1. The filaments appear to be fragmented into beadlike chains of dense clumps, which may show a continuing pattern of structural instabilities and star formation in the region. Some of these fragments may be sites, or future sites, of young stars. Some have very large velocity gradients and may be collapsing cores that have not yet shed their angular momentum. We also present a high-resolution NH3 (2, 2) to (1, 1) ratio map of the region, representing the temperature. Striking patterns of heating are apparent: around the central Orion-KL core, strong heating is found in a patchy ring surrounding the Orion bipolar outflow source, possibly delineating the paths by which the outflowing gas exits the central dense region and heats the gas along the way. Heating is evident along the edges of the filaments and clumps that face the path of the outflow, showing direct impact of outflows and radiation on the molecular environment. Other clumps along the filaments show slight temperature enhancements along their outer sheaths, which may be the result of radiation from external stars penetrating the clumpy medium and heating the core sheaths. The complex combination presented here of filaments, fragmentation, kinematical interaction, and heating along the OMC-1 ridge provides evidence for extensive and long-range interaction between a core of high-mass star formation and its surrounding cloud environment.

Bipolar outflow sources in the Serpens core: SVS 2 and SVS 20

Bipolar outflow sources in the Serpens core: SVS 2 and SVS 20

Mid-infrared Observations of Magnetic Fields in the Disks of Bi-Polar Outflow Sources

AbstractWe present the results from mid-infrared spectro-polarimetric observations of a number of bi-polar outflow sources. The specto-polarimetric data provides information on the polarization mechanism and the magnetic field direction. The field direction in the disks of the observed sources is most often normal to the ambient field direction and lies in the plane of the disk, indicating a toroidal rather than poloidal field configuration.

The physical structure of NGC 2440

We have acquired monochromatic imaging of the bipolar outflow source NGC 2440 in 12 ionic transitions using the 1.5 m Telescope at the Observatorio de San Pedro Mártir, Ensenada, Baja California, Mexico. As a result, we have been able to undertake detailed analysis of the shell ionisation structure. Then, we found that excitation is high close the center, extending towards the exterior at located radial positions. Edges and ansae are zones of low excitation indicating probably shock regions, in particular the outer shell.

BIPOLAR MOLECULAR OUTFLOWS FROM YOUNG STARS AND PROTOSTARS

▪ Abstract A violent outflow of high-velocity gas is one of the first manifestations of the formation of a new star. Such outflows emerge bipolarly from the young object and involve amounts of energy similar to those involved in accretion processes. The youngest (proto-)stellar low-mass objects known to date (the Class 0 protostars) present a particularly efficient outflow activity, indicating that outflow and infall motions happen simultaneously and are closely linked since the very first stages of the star formation processes. This article reviews the wealth of information being provided by large millimeter-wave telescopes and interferometers on the small-scale structure of molecular outflows, as well as the most recent theories about their origin. The observations of highly collimated CO outflows, extremely high velocity (EHV) flows, and molecular “bullets” are examined in detail, since they provide key information on the origin and propagation of outflows. The peculiar chemistry operating in the associated shocked molecular regions is discussed, highlighting the recent high-sensitivity observations of low-luminosity sources. The classification schemes and the properties of the driving sources of bipolar outflows are summarized with special attention devoted to the recently identified Class 0 protostars. All these issues are crucial for building a unified theory on the mass-loss phenomena in young stars.

The embedded young stars in the Taurus-Auriga molecular cloud. II - Models for scattered light images

We describe NIR imaging observations of embedded young stars in the Taurus-Auriga molecular cloud. We find a large range in J-K and H-K colors for these class I sources. The bluest objects have colors similar to the reddest T Tauri stars in the cloud; redder objects lie slightly above the reddening line for standard ISM dust and have apparent K extinctions of up to 5 mag. Most of these sources also show extended NIR emission on scales of 10-20 arcsec which corresponds to linear sizes of 1500-3000 AU. The NIR colors and nebular morphologies for this sample and the magnitude of linear polarization in several sources suggest scattered light produces most of the NIR emission in these objects. We present modeling results that suggest mass infall rates that agree with predictions for cold clouds and are generally consistent with rates estimated from radiative equilibrium models. For reasonable dust grain parameters, the range of colors and extinctions require flattened density distributions with polar cavities evacuated by bipolar outflows. These results support the idea that infall and outflow occur simultaneously in deeply embedded bipolar outflow sources. The data also indicate fairly large centrifugal radii and large inclinations to the rotational axis for a typical source.

Shock Modelling of the Bipolar Outflow Source NGC 6537

NGC 6537 is an unusual high excitation bipolar outflow source, with anomalous abundances indicative of a type I nebula. We have recently obtained a range of high resolution spectrophotometry for this source using the 2.5 m Isaac Newton Telescope (Observatorio del Roque de Los Muchachos, La Palma), together with narrow band optical imaging using the 2.6 m Nordic Optical Telescope (Observatorio del Roque de los Muchachos, La Palma). As a consequence, it is apparent that the source optical morphology is suggestive of the presence of an hyperbolic shock outflow surface, similar to those observed in other BPN (e.g. NGC 6302, Hb 5). Line ratio maps also indicate the presence of extremely strong [Nii] emission at the periphery of the outflow, whilst expansion velocities are of order ∼ 400 km sec−1. These large shell expansion velocities may in turn be driven by an extremely high velocity wind, which in this case appears to extend over a range ΔV ≥ 3000 km sec−1.