Jet-like Outflows Research Articles

Massive Molecular Outflows at High Spatial Resolution

We present high-spatial resolution Plateau de Bure Interferometer CO(2-1) and SiO(2-1) observations of one intermediate-mass and one high-mass star-forming region. The intermediate-mass region IRAS20293+3952 exhibits four molecular outflows, one being as collimated as the highly collimated jet-like outflows observed in low-mass star formation sources. Furthermore, comparing the data with additional infrared H2 and cm observations we see indications that the nearby ultracompact HII region triggers a shock wave interacting with the outflow. The high-mass region IRAS19217+1651 exhibits a bipolar outflow as well and the region is dominated by the central driving source. Adding two more sources from the literature, we compare position-velocity diagrams of the intermediate- to high-mass sources with previous studies in the low-mass regime. We find similar kinematic signatures, some sources can be explained by jet-driven outflows whereas other are better constrained by wind-driven models. The data also allow to estimate accretion rates varying from a few times 10^{-5}Msun/yr for the intermediate-mass sources to a few times 10^{-4}Msun/yr for the high-mass source, consistent with models explaining star formation of all masses via accretion processes.

General Relativistic Magnetohydrodynamic Simulations of Collapsars

We have performed 2.5-dimensional general relativistic magnetohydrodynamic (MHD) simulations of the gravitational collapse of a magnetized rotating massive star as a model of gamma-ray bursts (GRBs). The current calculation focuses on general relativistic MHD with simplified microphysics (we ignore neutrino cooling, physical equation of state, and photodisintegration). Initially, we assume that the core collapse has failed in this star. A few M☉ black hole is inserted by hand into the calculation. The simulations presented in the paper follow the accretion of gas into a black hole that is assumed to have formed before the calculation begins. The simulation results show the formation of a disklike structure and the generation of a jetlike outflow inside the shock wave launched at the core bounce. We have found that the jet is accelerated by the magnetic pressure and the centrifugal force and is collimated by the pinching force of the toroidal magnetic field amplified by the rotation and the effect of geometry of the poloidal magnetic field. The maximum velocity of the jet is mildly relativistic (~0.3c). The velocity of the jet becomes larger as the initial rotational velocity of stellar matter gets faster. On the other hand, the dependence on the initial magnetic field strength is a bit more complicated: the velocity of the jet increases with the initial field strength in the weak field regime, then is saturated at some intermediate field strength, and decreases beyond the critical field strength. These results are related to the stored magnetic energy determined by the balance between the propagation time of the Alfven wave and the rotation time of the disk (or twisting time).

Evidence for Evolution of the Outflow Collimation in Very Young Stellar Objects

We present Very Long Baseline Array proper-motion measurements of water masers toward two young stellar objects (YSOs) of the W75 N star-forming region. We find that these two objects are remarkable for having a similar spectral type, being separated by 07 (corresponding to 1400 AU), and sharing the same environment, but with a strikingly different outflow ejection geometry. One source has a collimated, jetlike outflow at a 2000 AU scale, while the other has a shell outflow at a 160 AU scale expanding in multiple directions with respect to a central compact radio continuum source. This result reveals that outflow collimation is not only a consequence of ambient conditions but is something intrinsic to the individual evolution of stars and brings to light the possibility of noncollimated outflows in the earliest stages of YSOs.

A collimated, high-speed outflow from the dying star V Hydrae

Stars with masses in the range 1-8 solar masses (M(\circ)) live ordinary lives for approximately 10(9)-10(10) years, but die extraordinary deaths. First, during their death throes as asymptotic giant branch (AGB) stars they eject, over 10(4)-10(5) years, half or more of their mass in slowly expanding, spherical winds, and then, in a short (a few 100-1,000 years) and poorly understood phase, they are transformed into aspherical planetary nebula. Recent studies support the idea that high-speed, jet-like flows play a crucial role in this transformation. Evidence for such outflows is indirect, however; this phase is so short that few nearby stars are likely to be caught in the act. Here we report the discovery of a newly launched, high-speed jet-like outflow in the nearby AGB star, V Hydrae. We have detected both proper motions and ongoing evolution in the jet. These results support a model in which the jet is driven by an accretion disk around an unseen, compact companion. We also find a central, dense equatorial disk-like structure which may enable and/or enhance the formation of the accretion disk.

Millimeter Observations of GRB 030329: Continued Evidence for a Two-Component Jet

We present the results of a dedicated campaign on the afterglow of GRB 030329 with the millimeter interferometers of the Owens Valley Radio Observatory (OVRO), the Berkeley-Illinois-Maryland Association (BIMA), and with the MAMBO-2 bolometer array on the IRAM 30-m telescope. These observations allow us to trace the full evolution of the afterglow of GRB 030329 at frequencies of 100 GHz and 250 GHz for the first time. The millimeter light curves exhibit two main features: a bright, constant flux density portion and a steep power-law decline. The absence of bright, short-lived millimeter emission is used to show that the GRB central engine was not actively injecting energy well after the burst. The millimeter data support a model, advocated by Berger et al., of a two-component jet-like outflow in which a narrow angle jet is responsible for the high energy emission and early optical afterglow, and a wide-angle jet carrying most of the energy is powering the radio and late optical afterglow emission

A Late‐Time Flattening of Afterglow Light Curves

We present a sample of radio afterglow light curves with measured decay slopes which show evidence for a flattening at late times compared to optical and X-ray decay indices. The simplest origin for this behavior is that the change in slope is due to a jet-like outflow making a transition to sub-relativistic expansion. This can explain the late-time radio light curves for many but not all of the bursts in the sample. We investigate several possible modifications to the standard fireball model which can flatten late-time light curves. Changes to the shock microphysics which govern particle acceleration, or energy injection to the shock (either radially or azimuthally) can reproduce the observed behavior. Distinguishing between these different possibilities will require simultaneous optical/radio monitoring of afterglows at late times.

The magnetic field of the $\boldmath\beta$ Lyrae system: Orbital and longer time-scale variability

The presence of a magnetic field in β Lyrae was firstly suggested by Babcock in 1958 and then confirmed by Skulsky in 1982. This kG-order large-scale organized magnetic field has been neglected in interpreting and modeling the large variety of phenomena presented by β Lyrae. Here, we present circular spectropolarimetry of β Lyrae showing that its magnetic field is variable with the orbital period and that it has changed in sign and strength between 1980 and 2000. Unfortunately, there are not enough data to conclude if a longer-time variability is super-imposed on the orbital period variability or if the field changes abruptly. This magnetic field, to our knowledge, is unique. Since we measured the magnetic field in metal lines of the brightest star of the system, we can conclude that this is the first magnetic B-type giant star. In this case, the magnetic field is significantly different from that of Magnetic Chemically Peculiar stars and the long-time-scale variability suggests the presence of a dynamo. However, we cannot rule out that the magnetic field measured on the brightest star is generated by the accretion disk, or that the magnetic field of the embedded star is so elongated in the orbital plane by the disk that it still has a significant strength even at the companion distance. In any case, the accretion disk is certainly related to the magnetic field of the β Lyrae system. We found that variations of the field in sign and strength corresponded to variations in the disk structure, as it is inferred from photometry and spectroscopy. Also, a magnetized disk explains the observed jet-like outflow from the β Lyrae system.

GRB afterglow light curves from uniform and non-uniform jets

It is widely believed that gamma-ray bursts are produced by a jet-like outflows directed towards the observer, and the jet opening angle (theta(j)) is often inferred from the time at which there is a break in the afterglow light curves. Here we calculate the GRB afterglow light curves from a relativistic jet as seen by observers at a wide range of viewing angles (theta(v)) from the jet axis, and the jet is uniform or non-uniform (the energy per unit solid angle decreases smoothly away from the axis epsilon(theta) proportional to (theta/theta(c))(-k)). We find that, for uniform jet (k = 0), the afterglow light curves for different viewing angles are somewhat different: in general, there are two breaks in the light curve, the first one corresponds to the time at which gamma similar to (theta(j) - theta(v))(-1), and the second one corresponds to the time when gamma similar to (theta(j) + theta(v))(-1). However, for non-uniform jet, the things become more complicated. For the case theta(v) = 0, we can obtain the analytical results, for k 8/(p + 4) there should be only one break corresponds to gamma similar to theta(c)(-1), and this provides a possible explanation for some rapidly fading afterglows whose light curves have no breaks since the time at which gamma similar to theta(c)(-1) is much earlier than our first observation time. For the case theta(v) not equal 0, our numerical results show that, the afterglow light curves are strongly affected by the values of theta(v), theta(c) and k. If theta(v) is close to theta(c) and k is small, then the light curve is similar to the case of k = 0, except the flux is somewhat lower. However, if the values of theta(v)/theta(c) and k are larger, there will be a prominent flattening in the afterglow light curve, which is quite different from the uniform jet, and after the flattening a very sharp break will be occurred at the time gamma similar to (theta(v) + theta(c))(-1).

Collimated Outflows in Planetary Nebulae

The study of collimated outflows (COFs) in planetary nebulae (PNe) has experienced vigorous activity since the last IAU symposium on PNe, held in Groningen in 1996, for the understanding of the origin and evolution of these outflows is directly linked to the mechanisms of PN formation. Collimated outflows in planetary nebulae (PNe) have diverse characteristics and are observed as bipolar, multipolar, point-symmetric and jet-like outflows. The Hubble Space Telescope has been instrumental in revealing the complex structure and pervasive presence of COFs in a large number of PNe during the last lustrum. Significant advances have also been achieved in the study of the molecular component associated with these outflows. On the theoretical side, models that incorporate the influence of binary cores and magnetic fields have gained particular strength in explaining the origin of COFs. This review attempts to summarize some of the main progresses in the study of COFs during the last years.

Interferometric Observations of OH and H2O Masers in Protoplanetary Nebulae Imaged with HST - A Unique Diagnostic of their Spatio-Kinematic Structure

Recently, high-resolution imaging surveys of young planetary nebulae (PNe) and protoplanetary nebulae (PPNe) have revealed that the majority of these objects are characterised by multipolar bubbles distributed roughly point-symmetrically around the central star (e.g. Sahai & Trauger 1998, Sahai 2000). Sahai & Trauger (1998) have proposed that episodic high-speed jet-like outflows, operating during the protoplanetary or very late-AGB phase, are the primary agent for shaping PNe. OH and H2O masers provide a unique and crucial probe of the kinematics of the circumstellar material in PPNe, because of the general lack of other emission-line diagnostics. Here we present new results from our ongoing study of PPNe usingHSTimages with interferometric OH & H2O maser-line data to unravel their detailed spatio-kinematic structure (e.g. Sahai et al. 1999a, Sahai, Claussen, & Morris 2002).

Afterglow light curves, viewing angle and the jet structure of -ray bursts

Gamma-ray bursts are often modelled as jet-like outflows directed towards the observer; the cone angle of the jet is then commonly inferred from the time at which there is a steepening in the power-law decay of the afterglow. We consider an alternative model in which the jet has a beam pattern where the luminosity per unit solid angle (and perhaps also the initial Lorentz factor) decreases smoothly away from the axis, rather than having a well-defined cone angle within which the flow is uniform. We show that the break in the afterglow light curve then occurs at a time that depends on the viewing angle. Instead of implying a range of intrinsically different jets – some very narrow, and others with a similar power spread over a wider cone – the data on afterglow breaks could be consistent with a standardized jet, viewed from different angles. We discuss the implication of this model for the luminosity function.

Bow Shocks, Wiggling Jets, and Wide‐Angle Winds: A High‐Resolution Study of the Entrainment Mechanism of the PV Cephei Molecular (CO) Outflow

We present a new set of high-resolution molecular-line maps of the gas immediately surrounding various Herbig-Haro (HH) knots of the giant HH flow HH 315 from the young star PV Cephei. The observations, aimed at studying the entrainment mechanism of the 2.6 pc-long HH 315 flow, include IRAM 30 m maps of the 12CO (2-1), 12CO (1-0), and 13CO (1-0) lines, with beam sizes of 11'', 21'', and 22'', respectively. We compare the morphology and the kinematics of the outflow gas, as well as the temperature and momentum distribution of the molecular outflow, with those predicted by different entrainment models. With our detailed study we are able to conclude that jet bow shock entrainment by an episodic stellar wind, with a time-varying axis, produces most of the high-velocity molecular outflow observed far from the source. In addition, near PV Cep we find evidence for a poorly collimated, wide-angle molecular outflow and a collimated, wiggling jet-like molecular outflow. We propose that the poorly collimated component is entrained by a wide-angle wind and the collimated component is entrained by a variable jet with internal working surfaces. If this picture is true, then a stellar wind model that allows for the coexistence of a wide-angle component and a collimated (jetlike) stellar wind component is needed to explain the observed properties of the PV Cep outflow. The wiggling axis of the redshifted molecular outflow lobe indicates that the outflow ejection axis is changing over time. We find that the timescale of the axis variation shown by the molecular outflow lobe is about a factor of 10 less than that shown by the large-scale optical HH knots.

Interferometric observations of OH and H2O masers in protoplanetary nebulae imaged with HST - A unique diagnostic of their spatial-kinematic structure

One of the most exciting challenge facing theories of post-main sequence evolution today is to understand how Asymptotic Giant Branch (AGB) stars transform themselves into aspherical planetary nebulae (PNe). Recently, high-resolution imaging surveys of young planetary nebulae and protoplanetary nebulae (PPNe: objects in transition between the AGB and PN phases) have revealed that the majority of these objects are characterised by multipolar bubbles distributed roughly point-symmetrically around the central star. These data strongly suggest that the current model for the shaping of PNe is no longer adequate. High angular-resolution kinematic information is sorely needed to complement the imaging data in order to test new hypotheses, such as our proposal that episodic high-speed jet-like outflows, operating during the protoplanetary or very late-AGB phase, are the primary agent.We have therefore begun a program of using interferometric mapping of OH (and H2O, when feasible) maser emission in order to trace the kinematics of the structures discovered in protoplanetary nebulae with HST. These masers provide a unique and crucial probe of the kinematics of the circumstellar material in PPNe, because of the lack of other emission-line diagnostics. Although our work is still in its infancy (only two objects have been studied in detail), we find that the OH masers indicate the presence of multiple low-latitude outflows and an increase of outflow velocity with latitude. This paper summarises our progress so far, the state of current studies, and future prospects.

The role of tidal interactions in star formation

Nearly all of the initial angular momentum of the matter that goes into each forming star must somehow be removed or redistributed during the formation process. The possible transport mechanisms and the possible fates of the excess angular momentum are discussed, and it is argued that transport processes in discs are probably not sufficient by themselves to solve the angular momentum problem, while tidal interactions with other stars in forming binary or multiple systems are likely to be of very general importance in redistributing angular momentum during the star formation process. Most, if not all, stars probably form in binary or multiple systems, and tidal torques in these systems can transfer much of the angular momentum from the gas around each forming star to the orbital motions of the companion stars. Tidally generated waves in circumstellar discs may contribute to the overall redistribution of angular momentum. Stars may gain much of their mass by tidally triggered bursts of rapid accretion, and these bursts could account for some of the most energetic phenomena of the earliest stages of stellar evolution, such as jet-like outflows. If tidal interactions are indeed of general importance, planet-forming discs may often have a more chaotic and violent early evolution than in standard models, and shock heating events may be common. Interactions in a hierarchy of subgroups may play a role in building up massive stars in clusters and in determining the form of the upper initial mass function (IMF). Many of the processes discussed here have analogues on galactic scales, and there may be similarities between the formation of massive stars by interaction-driven accretion processes in clusters and the buildup of massive black holes in galactic nuclei.

GRB 000418: A Hidden Jet Revealed

We report on optical, near-infrared, and centimeter radio observations of GRB 000418 that allow us to follow the evolution of the afterglow from 2 to 200 days after the ?-ray burst (GRB). In modeling these broadband data, we find that an isotropic explosion in a constant-density medium is unable to simultaneously fit both the radio and optical data. However, a jetlike outflow into either a constant density or wind-stratified medium with an opening angle of 10?-20? provides a good description of the data. The evidence in favor of a jet interpretation is based on the behavior of the radio light curves, since the expected jet break is masked at optical wavelengths by the light of the host galaxy. We also find evidence for extinction, presumably arising from within the host galaxy, with A = 0.4 mag, as well as host flux densities of FR = 1.1 ?Jy and FK = 1.7 ?Jy. These values supersede previous work on this burst as a result of the availability of a broadband data set allowing a global fitting approach. A model in which the GRB explodes isotropically into a wind-stratified circumburst medium cannot be ruled out by these data. However, in examining a sample of other bursts (e.g., GRB 990510, GRB 000301C), we favor the jet interpretation for GRB 000418.

A possible explanation for the radio afterglow of GRB 980519: the dense medium effect

GRB 980519 is characterized by its rapidly declining optical and X-ray afterglows. Explanations of this behaviour include models invoking a dense medium environment, which makes the shock wave evolve quickly into the subrelativistic phase, a jet-like outflow, and a wind-shaped circumburst medium environment. Recently, Frail et al. found that the latter two cases are consistent with the radio afterglow of this burst. Here, by considering the transrelativistic shock hydrodynamics, we show that the dense medium model can also account for the radio light curve quite well. The potential virtue of the dense medium model for GRB 980519 is that it implies a smaller angular size of the afterglow, which is essential for interpreting the strong modulation of the radio light curve. Optical extinction arising from the dense medium is not important if the prompt optical-UV flash accompanying the gamma-ray emission can destroy dust by sublimation out to an appreciable distance. Comparisons with some other radio afterglows are also discussed.

High-resolution spectroscopy and broad-band imaging of the young planetary nebula K 3-35

We present high-resolution echelle and long-slit spectra and broad-band (R, I) images of the very young planetary nebula K 3-35. Several emission lines are identified, including the He ii 4686 line and strong [N ii]6548, 6583 and [O iii]4959, 5007 emissions [I([N ii])/I(Hα)≃5.5, I([O iii])/I(Hβ)≃30]. A systemic velocity VLSR≃10±2 km s−1 for K 3-35 is obtained from the optical emission lines. Two different kinematic components are identified in the nebula. One of them is probably related to the elliptical envelope previously observed. The second component exhibits systematic changes of the radial velocity with position, and a relatively small velocity width. This component may be attributed to the precessing jet-like outflows previously identified. The R and I images and the deduced R−I colour map strongly support the existence of a dense, partially neutral disc-like region in the equatorial plane of the nebula, which probably represents an equatorial density enhancement in a previously ejected slow wind. Diagnostic diagrams for line intensity ratios in K 3-35 and collimated components of other planetary nebulae suggest that the emission spectrum of this kind of structure is a combination of radiative and shock excitation, in agreement with recent models of shocks in a strongly photoionized medium.

Numerical simulations of astrophysical jets from Keplerian discs -- III. The effects of mass loading

We present 2.5D time-dependent simulations of the non-linear evolution of non-relativistic outflows from the surface of Keplerian accretion discs. The gas is accelerated from the surface of the disc (which is a fixed platform in these simulations) into a cold corona in stable hydrostatic equilibrium. We explore the dependence of the resulting jet characteristics upon the mass loading of the winds. Two initial configurations of the threading disc magnetic field are studied: a potential field and a uniform vertical field configuration. We show that the nature of the resulting highly collimated, jet-like outflows (steady or episodic) is determined by the mass load of the disc wind. The mass load controls the interplay between the collimating effects of the toroidal field and the kinetic energy density in the outflow. In this regard, we demonstrate that the onset of episodic behaviour of jets appears to be determined by the quantity which compares the speed for a toroidal Alfven wave to cross the diameter of the jet, with the flow speed vp along the jet. This quantity decreases with increasing load. For sufficiently large N (small mass loads), disturbances appear to grow leading to instabilities and shocks. Knots are then generated and the outflow becomes episodic. These effects are qualitatively independent of the initial magnetic configuration that we employed and are probably generic to a wide variety of magnetized accretion disc models.

Quenching of the Radio Jet during the X-Ray High State of GX 339−4

We have observed the black hole candidate X-ray binary GX 339-4 at radio wavelengths before, during, and after the 1998 high/soft X-ray state transition. We find that the radio emission from the system is strongly correlated with the hard X-ray emission and is reduced by a factor of ≥25 during the high/soft state compared with the more usual low/hard state. At the points of state transition, we note brief periods of unusually optically thin radio emission that may correspond to discrete ejection events. We propose that in the low/hard state, black hole X-ray binaries produce a quasi-continuous outflow, that in the high/soft state, this outflow is suppressed, and that state transitions often result in one or more discrete ejection events. Future models for low/hard states, such as advection-dominated solutions, need to take into account the strong outflow of relativistic electrons from the system. We propose that the inferred Comptonizing corona and the base of the jetlike outflow are the same thing, based on the strong correlation between radio and hard X-ray emission in GX 339-4 and other X-ray binaries and on the similarity in inferred location and composition of these two components.

Detection of Polarization in the Afterglow of GRB 990510 with the ESO Very Large Telescope

Following a BeppoSAX alert and the discovery of the OT at SAAO, we observed GRB 990510 with the FORS instrument on ESO's VLT Unit 1 (Antu). The burst is unremarkable in gamma rays, but in optical is the first one to show good evidence for jetlike outflow. We report the detection of significant linear polarization in the afterglow: it is 1.6 ± 0.2% 0.86 days after trigger, and after 1.81 days is consistent with that same value, but much more uncertain. The polarization angle is constant on a timescale of hours and may be constant over one day. We conclude that the polarization is intrinsic to the source and due to the synchrotron nature of the emission, and discuss the random and ordered field geometries that may be responsible for it.