A L[CLC]y[/CLC]α Bright Jet from a Herbig A[CLC]e[/CLC] Star

Protostellar outflows are one of the most outstanding features of star formation. The observational studies over several decades successfully demonstrated that outflows are ubiquitously associated with low- and high-mass protostars in the solar-metallicity Galactic condition. However, the environmental dependence of protostellar outflow properties is still poorly understood, particularly in the low-metallicity regime. Here we report the first detection of a molecular outflow in the Small Magellanic Cloud with 0.2 $Z_{\odot}$, using ALMA observations at a spatial resolution of 0.1 pc toward the massive protostar Y 191. The bipolar outflow is nicely illustrated by high-velocity wings of CO(3-2) emission with $\gtrsim$15 km s$^{-1}$. The evaluated properties of the outflow (momentum, mechanical force, etc.) are consistent with those of the Galactic counterparts. Our results suggest that the molecular outflows, i.e., the guidepost of the disk accretion at the small scale, might be universally associated with protostars across the metallicity range of$\sim$0.2-1 $Z_{\odot}$.

Protostellar outflows are one of the most outstanding features of star formation. Observational studies over the last several decades have successfully demonstrated that outflows are ubiquitously associated with low- and high-mass protostars in solar-metallicity Galactic conditions. However, the environmental dependence of protostellar outflow properties is still poorly understood, particularly in the low-metallicity regime. Here we report the first detection of a molecular outflow in the Small Magellanic Cloud with 0.2 Z ⊙, using Atacama Large Millimeter/submillimeter Array observations at a spatial resolution of 0.1 pc toward the massive protostar Y246. The bipolar outflow is nicely illustrated by high-velocity wings of CO(3–2) emission at ≳15 km s−1. The evaluated properties of the outflow (momentum, mechanical force, etc.) are consistent with those of the Galactic counterparts. Our results suggest that the molecular outflows, i.e., the guidepost of the disk accretion at the small scale, might be universally associated with protostars across the metallicity range of ∼0.2–1 Z ⊙.

We review the theory of x-winds in young stellar objects (YSOs). In particular, we consider how a model where the central star does not coro-tate with the inner edge of the accretion disk may help to explain the enhanced emission of X-rays from embedded protostars. We argue, however, that the departure from corotation is not large, so a mathematical formulation that treats the long-term average state as steady and axisymmetric represents a useful approximation. Magnetocentrifugally driven x-winds of this description collimate into jets, and their interactions with the surrounding molecular cloud cores of YSOs yield bipolar molecular outflows.

HOPS 87 is a Class 0 protostellar core known to harbor an extremely young bipolar outflow and a hot corino. We report the discovery of localized, chemically rich regions near the bases of the two-lobe bipolar molecular outflow in HOPS 87 containing molecules such as H2CO, 13CS, H2S, OCS, and CH3OH, the simplest complex organic molecule (COM). The locations and kinematics suggest that these localized features are due to jet-driven shocks rather than being part of the hot-corino region encasing the protostar. The COM compositions of the molecular gas in these jet-localized regions are relatively simpler than those in the hot-corino zone. We speculate that this simplicity is due to either the liberation of ice with a less complex chemical history or the effects of shock chemistry. Our study highlights the dynamic interplay between the protostellar bipolar outflow, disk, inner-core environment, and the surrounding medium, contributing to our understanding of molecular complexity in solar-like young stellar objects.

Since their first discovery in 1980, nearly 200 bipolar molecular outflows have now been observed. These outflows appear to be a ubiquitous phenomenon in the star formation process, although their origin remains unclear. Here, we adopt the thin-shell model of Shu, Ruden, Lada, & Lizano (1991, ApJL, 370, 31) in which bipolar molecular outflows are the swept-up ambient material by powerful winds from central young stellar objects. Besides explaining many observed features of bipolar sources, such as shape, velocity, and the typical extent, this model also naturally accounts for the so-called “Hubble law” of velocity field. However, it was criticized by Masson & Chernin (1992, ApJ, 387, L47) on the ground that much less mass is observed at the high velocity end of some bipolar outflows than expected from this thin-shell model, unless the wind and/or the ambient medium are extremely anisotropic. We show that the extreme anisotropy is in fact inevitable both in the wind and in the ambient medium, because of the presence of dynamically important magnetic fields.The wind is anisotropic because it is magneto-centrifugally driven, and the frozen-in toroidal field tends to collimate the flow toward the rotation axis. This effect is studied using the well-known “method of characteristics” and we conclude that a MHD wind always has two components: a well-collimated jet and a wind-angle wind. It is the wide-angle wind component that will interact with the ambient medium and produce bipolar molecular outflows. The ambient core medium is anisotropic because it arises from lower-density, magnetically-supported molecular clouds through ambipolar diffusion. Using self-similarity technique, we model the “pivotal state” of star formation, which divides the core formation phase and the core collapse (accretion) phase. We find that pivotal states have toroid-like density contours in general, with lower-density regions around the magnetic axis (where matter drains more easily).When these anisotropics induced by magnetic fields are taken into account properly, the main argument against the wind-angle wind-driven, thin-shell model for bipolar molecular outflows disappears. Indeed, we show that the empirical mass-velocity relation found by Masson & Chernin and others for bipolar outflows provides the best quantitative supporting evidence for the thin-shell theory!

Protostellar feedback, both radiation and bipolar outflows, dramatically affects the fragmentation and mass accretion from star-forming cores. We use ORION, an adaptive mesh refinement gravito-radiation-hydrodynamics code, to simulate the formation of a cluster of low-mass stars, including both radiative transfer and protostellar outflows. We ran four simulations to isolate the individual effects of radiation feedback and outflow feedback as well as the combination of the two. Outflows reduce protostellar masses and accretion rates each by a factor of three and therefore reduce protostellar luminosities by an order of magnitude. Thus, while radiation feedback suppresses fragmentation, outflows render protostellar radiation largely irrelevant for low-mass star formation above a mass scale of 0.05 M_sun. We find initial fragmentation of our cloud at half the global Jeans length, ~ 0.1 pc. With insufficient protostellar radiation to stop it, these 0.1 pc cores fragment repeatedly, forming typically 10 stars each. The accretion rate in these stars scales with mass as predicted from core accretion models that include both thermal and turbulent motions. We find that protostellar outflows do not significantly affect the overall cloud dynamics, in the absence of magnetic fields, due to their small opening angles and poor coupling to the dense gas. The outflows reduce the mass from the cores by 2/3, giving a core to star efficiency ~ 1/3. The simulation with radiation and outflows reproduces the observed protostellar luminosity function. All of the simulations can reproduce observed core mass functions, though they are sensitive to telescope resolution. The simulation with both radiation and outflows reproduces the galactic IMF and the two-point correlation function of the cores observed in rho Oph.

ABSTRACTWe present a spectral line image in CO (J = 7–6) of the spatially compact molecular outflow from the massive protostar IRAS 20126+4014, observed using a novel superconductive hot‐electron bolometer (HEB) heterodyne receiver we recently installed at the 10 m Heinrich Hertz Telescope. The bipolar outflow is clearly detected and resolved with a center of symmetry situated at the position of the massive protostar, previously identified as an IRAS source and a millimeter continuum source. The peaks of the emission from the red‐ and blueshifted CO lobes are separated by ≈14″ (0.1 pc), and the velocity of the CO emission extends ±30 km s−1 from the ambient cloud velocity. The total outflowing gas mass is approximately 4M⊙, while the outflow rate is at least 6 × 10-4 M⊙ yr−1. Both high‐velocity CO emission maxima trail the NH3 (3, 3) and SiO clumps located 3″–4″ downstream (farther from the IR source) in the flow. The high‐excitation CO line and the NH3 are likely tracing the heated gas entrained in different parts of the bow shocks. By interpreting our observations in terms of a jet‐driven model, we estimate the density in the underlying jet to be at least 105 cm−3. The outflow appears much more compact and in a different orientation than the arcminute‐scale north/south flow seen in low‐J transitions of CO. These observations represent the first successful operation of a superconductive HEB receiver on a telescope and demonstrate the importance of high‐frequency submillimeter lines to the understanding of the protostellar environment.

CO J = 3–2 and 4–3 observations of V380 Ori-NE reveal a highly collimated bipolar molecular outflow associated with a jet traced here in H2 1–0 S(1) line emission. The source of the flow is also detected at 450 and 850 μm with SCUBA. The combined CO and near-IR observations offer compelling support for the prompt entrainment model of jet-driven molecular outflows. Not only are the H2 shock fronts spatially coincident with peaks in the CO outflow lobes, but the slope of the mass-velocity distribution in the flow, measured here at intervals along both flow lobes, also clearly decreases just behind the advancing shock fronts (and towards the ends of the flow lobes), as one would expect if the high-to-low velocity mass fraction was enhanced by the entraining shocks. We also find that both lobes of the CO outflow clearly deviate, by some 20°, from the H2 jet direction near the source. Both lobes may be being deflected at the locations of the observed H2 shock fronts, where they impact dense, ambient material. Alternatively, the almost point-symmetric CO flow pattern could be caused by precession at the source. The submillimetre (submm) data reveal the source of the outflow, V380 Ori-NE. The 450- and 850-μm maps show an elongated peak superimposed on to an extensive pedestal of weaker emission. The major axis of the source is oriented parallel with the inner flow axis. Indeed, weak 850-μm emission is detected along much of the bipolar outflow, particularly in the southern lobe and towards the southernmost CO intensity peak. The submm ‘continuum’ data therefore probably trace warm dust and CO associated with the outflow. These data also confirm the status of V380 Ori-NE as a Class I protostar. Overall, the orientation, simplicity and symmetry of this outflow, combined with the remarkable strength of the high-velocity line-wing emission in comparison to the ambient emission, make this system a perfect laboratory for future detailed studies of bipolar molecular outflows and their association with collimated jets from young, deeply embedded protostars.

Aims. We present the first simulations of the formation and feedback of massive stars which account for radiation forces as well as photoionization feedback (along with protostellar outflows). In two different accretion scenarios modeled, we determine the relative strength of these feedback components and derive the size of the reservoir from which the forming stars gained their masses. Methods. We performed direct hydrodynamics simulations of the gravitational collapse of high-density mass reservoirs toward the formation of massive stars including self-gravity, stellar evolution, protostellar outflows, continuum radiation transport, photoionization, and the potential impact of ram pressure from large-scale gravitational infall. For direct comparison, we executed these simulations with and without the individual feedback components. Results. Protostellar outflows alone limit the stellar mass growth only in an accretion scenario with a finite mass reservoir; when including accretion and ram pressure from large scales (>0.1 pc), protostellar outflows do not limit stellar mass growth at all. Photoionization and HII regions dominate the feedback ladder only at later times, after the star has already contracted down to the zero-age main sequence, and only on large scales. Specifically, photoionization yields a broadening of the bipolar outflow cavities and a reduction of the gravitational infall momentum by about 50%, but does not limit the stellar mass accretion. On the other hand, we find radiation forces restrain the gravitational infall toward the circumstellar disk, impact the gravito-centrifugal equilibrium at the outer edge of the disk, and eventually shut down stellar accretion completely. The most massive star formed in the simulations accreted 95 M⊙ before disk destruction; this mass was drawn-in from an accretion reservoir of ≈240 M⊙ and ≈0.24 pc in radius. Conclusions. In the regime of very massive stars, the final mass of these stars is controlled by their own radiation force feedback.

FS Tau B is one of the few T Tauri stars that possess a jet and a counterjet as well as an optically-visible cavity wall. We obtained images and spectra of its jet-cavity system in the near-infrared H and K bands using Subaru/IRCS and detected the jet and the counterjet in the [Fe II] 1.644 \mu m line for the first time. Within the inner 2" the blueshifted jet is brighter, whereas beyond ~ 5" the redshifted counterjet dominates the [Fe II] emission. The innermost blueshifted knot is spectrally resolved to have a large line width of ~ 110 km/s, while the innermost redshifted knot appears spectrally unresolved. The velocity ratio of the jet to the counterjet is ~ 1.34, which suggests that FS Tau B is driving an asymmetric jet, similar to those found in several T Tauri Stars. Combining with optical observations in the literature, we showed that the blueshifted jet has lower density and higher excitation than the redshifted counterjet. We suggest that the asymmetry in brightness and velocity is the manifestation of a bipolar outflow driving at different mass-loss rates, while maintaining balance of linear momentum. A full explanation to the asymmetry in the FS Tau B system awaits detail modeling and further investigation of the kinematic structure of the wind-associated cavity walls.

We present high spatial resolution mid-IR images of the ring of UCHII regions in W49A obtained at Gemini North, allowing us to identify the driving source of its powerful H2O maser outflow. These data also confirm our previous report that several radio sources in the ring are undetected in the mid-IR because they are embedded deep inside the cloud core. We locate the source of the water maser outflow at the position of the compact mid-IR peak of source G (source G:IRS1). This IR source is not coincident with any identified compact radio continuum source, but is coincident with a hot molecular core, so we propose that G:IRS1 is a hot core driving an outflow analogous to the wide-angle bipolar outflow in OMC-1. G:IRS1 is at the origin of a larger bipolar cavity and CO outflow. The water maser outflow is orthogonal to the bipolar CO cavity, so the masers probably reside near its waist in the cavity walls. Models of the IR emission require a massive protostar of 45Msun, 3e5Lsun, and an effective envelope accretion rate of 1e-3Msun/yr. Feedback from the central star could potentially drive the H2O maser outflow, but it has insufficient radiative momentum to have driven the large-scale CO outflow, requiring that this massive star had an active accretion disk over the past 10^4 yr. Combined with the spatialy resolved morphology in IR images, G:IRS1 in W49 provides compelling evidence for a massive protostar that formed by accreting from a disk, accompanied by a bipolar outflow.

We review the theory of x-winds in young stellar objects (YSOs). In particular, we consider how a model where the central star does not corotate with the inner edge of the accretion disk may help to explain the enhanced emission of X-rays from embedded protostars. We argue, however, that the departure from corotation is not large, so a mathematical formulation that treats the long-term average state as steady and axisymmetric represents a useful approximation. Magnetocentrifugally driven x-winds of this description collimate into jets, and their interactions with the surrounding molecular cloud cores of YSOs yield bipolar molecular outflows.

HST and ground-based [OII} and [NII] images obtained from 1996 to 1999 reveal the existence of a ionised optical nebula around the symbiotic binary CH Cyg extending out to 5000 A.U. from the central stars. The observed velocity range of the nebula, derived from long-slit echelle spectra, is of 130 km/s. In spite of its complex appearence, the velocity data show that the basic morphology of the inner regions of the optical nebula is that of a bipolar (or conical) outflow extending nearly along the plane of the sky out to some 2000 A.U. from the centre. Even if the extension of this bipolar outflow and its position angle are consistent with those of the radio jet produced in 1984 (extrapolated to the time of our optical imagery), no obvious counterpart is visible of the original, dense radio bullets ejected by the system. We speculate that the optical bipolar outflow might be the remannt of the interaction of the bullets with a relatively dense circumstellar medium.

The protostellar outflows have indispensable role in the formation of single stars, because they carry off the excess angular momentum from the centre of the shrinking gas cloud, and permits further collapse to form a star. On the other hand, a significant fraction of stars is supposed to be born as binaries with circumbinary disk that are frequently observed. Here, we investigate the evolution of a magnetized rotating cloud using three-dimensional resistive MHD nested-grid code, and show that the outflow is driven by the circumbinary disk and has an important role even in the binary formation. After the adiabatic core formation in the collapsing cloud core, the magnetic flux is significantly removed from the centre of the cloud by the Ohmic dissipation. Since this removal makes the magnetic braking ineffective, the adiabatic core continuously acquires the angular momentum to induce fragmentation and subsequent binary formation. The magnetic field accumulates in the circumbinary disk where the removal and accretion of magnetic field are balanced, and finally drives circumbinary outflow. This result explains the spectacular morphology of some specific young stellar objects such as L1551 IRS5. We can infer that most of the bipolar molecular outflows observed by low density tracers (i.e., CO) would correspond to circumbinary or circum-multiple outflows found in this report, since most of the young stellar objects are supposed to be binaries or multiples.

Context. The main accretion phase of protostars is characterized by the ejection of material in the form of bipolar jets and outflows. In addition, external UV irradiation can potentially have a significant impact on the excitation conditions within these outflows. High-resolution observations in the mid-infrared (mid-IR) allow us to investigate the details of those energetic processes through the emission of shock-excited H 2 . Aims. Our aim is to spatially resolve H 2 , ionic, and atomic emission within the outflows of low-mass protostars, and investigate its origin in connection to shocks influenced by external ultraviolet irradiation. Methods. We analyze spectral maps of 5 Class I protostars in the Ophiuchus molecular cloud from the James Webb Space Telescope (JWST) Medium Resolution Spectrometer (MIRI/MRS). The MIRI/MRS field of view covers an area between ∼3.2″ × 3.7″ at 6 μm and 6.6″ × 7.7″ at 25 μm and with a resolution of ∼0.3 to 1″, corresponding to spatial scales of a few hundred astronomical units. Results. Four out of five protostars in our sample show strong H 2 , [Ne II ], and [Fe II ] emission associated with outflows and jets. Pure rotational H 2 transitions from S(1) to S(8) are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ∼500–600 K and ∼1000–3000 K, respectively. Both C -type shocks propagating at high pre-shock densities ( n H ≥ 10 4 cm −3 ) and J -type shocks at low pre-shock densities ( n H ≤ 10 3 cm −3 ) reproduce the observed line ratios. However, only C -type shocks produce sufficiently high column densities of H 2 , whereas predictions from a single J -type shock reproduce the observed rotational temperatures of the gas better. A combination of various types of shocks could play a role in protostellar outflows as long as UV irradiation is included in the models. The origin of this radiation is likely internal, since no significant differences in the excitation conditions of outflows are seen at various locations in the cloud. Conclusions. Observations with MIRI offer an unprecedented view of protostellar outflows, allowing us to determine the properties of outflowing gas even at very close distances to the driving source. Further constraints on the physical conditions within outflows can be placed thanks to the possibility of direct comparisons of such observations with state-of-the-art shock models.