Motions Of Knots Research Articles

Proper motions in the sub-kiloparsec jet of 3C 78: novel constraints on the physical nature of relativistic jets

ABSTRACT Jets from active galactic nuclei are thought to play a role in the evolution of their host and local environments, but a detailed prescription is limited by the understanding of the jets themselves. Proper motion studies of compact bright components in radio jets can be used to produce model-independent constraints on their Lorentz factor, necessary to understand the quantity of energy deposited in the intergalactic medium. We present our initial work on the jet of radio–galaxy 3C 78, as part of Catalogue of proper motions in active galactic nuclei using Very Large Array Studies (CAgNVAS), with a goal of constraining nature of jet plasma on larger (>100 parsec) scales. In 3C 78, we find three prominent knots (A, B, and C), where knot B undergoes sub-luminal longitudinal motion (∼0.6c at ∼ 200 pc), while knot C undergoes extreme (apparent) backward motion and eventual forward motion (∼−2.6c, 0.5c, at ∼ 300 pc). Assuming knots are shocks, we infer the bulk speeds from the pattern motion of Knots B and C. We model the spectral energy distribution of the large-scale jet and observe that a physically motivated two-zone model can explain most of the observed emission. We also find that the jet profile remains approximately conical from parsec to kiloparsec scales. Using the parsec-scale speed from very long baseline interferometry studies (∼0.1c) and the derived bulk speeds, we find that the jet undergoes bulk acceleration between the parsec and the kiloparsec scales providing the first direct evidence of jet acceleration in a conical and matter-dominated jet.

Collision of protostellar jets in the star-forming region IC 1396N

Context. The bright-rimmed cloud IC 1396N is believed to host one of the few known cases where two bipolar CO outflows driven by young stellar objects collide. The CO outflows are traced by chains of knots of H2 emission, with enhanced emission at the position of the possible collision. Aims. The aim of this work is to use the proper motions of the H2 knots to confirm the collision scenario. Methods. A second-epoch H2 image was obtained, and the proper motions of the knots were determined with a time baseline of ~11 yr. We also performed differential photometry on the images to check the flux variability of the knots. Results. For each outflow (N and S), we classified the knots as pre-collision or post-collision. The axes of the pre-collision knots, the position of the possible collision point, and the axes of the post-collision knots were estimated. The difference between the proper motion direction of the post-collision knots and the position angle from the collision point was also calculated. For some of the knots, we obtained the 3D velocity using the radial velocity derived from H2 spectra. Conclusions. The velocity pattern of the H2 knots in the area of interaction (post-collision knots) shows a deviation from that of the pre-collision knots, consistent with being a consequence of the interaction between the two outflows. This favours the interpretation of the IC 1396N outflows as a true collision between two protostellar jets instead of a projection effect.

Observations of metastable states of the free swelling knots and directional motion of tensioned knots in vibrated bead chains.

The free swelling of knots and the directional motion of knots under tension were studied in vertically vibrated bead chains. A metastable state of swelling was observed in the strongly vibrated two-end-free bead chains, as predicted by Grosberg and Rabin (Phys. Rev. Lett. 99, 217801 (2007)). Knots in the two-end-fixed chains were found to move directionally. The direction of motion could be changed by flipping the knot over. The velocity of motion depended on the tension in the bead chain. The effects of tension on the motion of knots were studied in one-end-fixed chains. The directional reptation might have been influenced by the random motion of the leading arc of the knot. The knots might move in a forced-reptation manner under the interaction with a simulated translocase.

Motion of Knots in DNA Stretched by Elongational Fields.

Knots in DNA occur in biological systems, serve as a model system for polymer entanglement, and affect the efficacy of modern genomics technologies. We study the motion of complex knots in DNA by stretching molecules with a divergent electric field that provides an elongational force. We demonstrate that the motion of knots is nonisotropic and driven towards the closest end of the molecule. We show for the first time experimentally that knots can go from a mobile to a jammed state by varying an applied strain rate, and that this jamming is reversible. We measure the mobility of knots as a function of strain rate, demonstrating the conditions under which knots can be driven towards the ends of the molecule and untied.

Emission Knots and Polarization Swings of Swinging Jets

Knots (emission features in jets of active galactic nuclei) often show non-ballistic dynamics and variable emission/polarization properties. We model these features as emission pattern propagating in a jet that carries a helical magnetic field and is launched along a changing direction. The model can reproduce a wide range of phenomena observed in the motion of knots: non-ballistic motion (both smooth and occasional sudden change of direction, and/or oscillatory behavior), variable brightness, and confinement of knots’ motion within an overlaying envelope. The model also reproduces smooth large polarization angle swings, and at the same time allows for the seemingly random behavior of synchrotron fluxes, polarization fraction, and occasional π / 2 polarization jumps.

Proper motions of embedded protostellar jets in Serpens

Context. To investigate the dynamical properties of protostellar jets. Aims. Determine the proper motion of protostellar jets around Class 0 and Class I sources in an active star forming region in Serpens. Methods. Multi-epoch deep images in the 2.122 $\mu$m line of molecular hydrogen, v=1-0 S(1), obtained with the near-infrared instrument NOTCam over a time-scale of 10 years, are used to determine proper motion of knots and jets. K-band spectroscopy of the brighter knots is used to supply radial velocities, estimate extinction, excitation temperature, and H$_2$ column densities towards these knots. Results. We measure the proper motion of 31 knots over different time scales (2, 4, 6, and 10 years). The typical tangential velocity is around 50 km/s for the 10 year base-line, but for shorter time-scales a maximum tangential velocity up to 300 km/s is found for a few knots. Based on morphology, velocity information and the locations of known protostars, we argue for the existence of at least three partly overlapping and deeply embedded flows, one Class 0 flow and two Class I flows. The multi-epoch proper motion results indicate time-variable velocities of the knots, for the first time directly measured for a Class 0 jet. We find in general higher velocities for the Class 0 jet than for the two Class I jets. While the bolometric luminosites of the three driving sources are about equal, the derived mass flow rate (dM/dt)$_{\rm out}$ is two orders of magnitude higher in the Class 0 flow than in the two Class I flows.

ASTROMETRY AND ASTROPHYSICS WITH THE SPACE TELESCOPE RADIOASTRON

It is well-known that gravitational lensing is a powerful tool in the investigation of the distribution of matter, including that of dark matter (DM). Typical angular distances between images and typical time scales depend on the gravitational lens masses. For the case of microlensing, angular distances between images or typical astrometric shifts are about 10-5 - 10-6 arcsec. Such an angular resolution will be reached with the space–ground VLBI interferometer, Radioastron. The basic targets for microlensing searches should be bright point-like radio sources at cosmological distances. In this case, an analysis of their variability and a solid determination of microlensing could lead to an estimation of their cosmological mass density. Moreover, one could not exclude the possibility that non-baryonic dark matter could also form microlenses if the corresponding optical depth were high enough. It is known that in gravitationally lensed systems the probability (the optical depth) of observing microlensing is relatively high. Therefore, for example, gravitationally lensed objects, like the CLASS gravitational lens B1600+434, appear to be most suitable to detect astrometric microlensing, since features of photometric microlensing have been detected in these objects. However, to directly resolve these images and to directly detect the apparent motion of the knots, the Radioastron sensitivity would have to be improved, since the estimated flux density is below the sensitivity threshold. Alternatively, they may be observed by increasing an integration time, assuming that a radio source has a typical core–jet structure and microlensing phenomena are caused by the superluminal apparent motions of knots. In the case of a confirmation (or a negation) of claims about microlensing in gravitational lens systems, one can speculate about the microlens contribution to the gravitational lens mass. Astrometric microlensing due Galactic MACHOs is not very important because of low optical depths and long typical time scales. Therefore, the launch of the space interferometer Radioastron will enable the investigation of microlensing in the radio band, giving rise to the possibility of not only resolving microimages but also of observing astrometric microlensing.

The nearest X-ray emitting protostellar jet (HH 154) observed with Hubble

Context. The jet coming from the YSO binary L1551 IRS5 is the closest astrophysical jet known. It is therefore a unique laboratory for studies of outflow mechanisms and of the shocks occurring when expanding material hits the ambient medium as well as of how the related processes influence the star- (and planet-) forming process. Aims. The optical data are related to other data covering the spectrum from the optical band to X-rays with goal of understanding the energetics of low-mass star jets, in general, and of this jet in particular. We study the time evolution of the jet, by measuring the proper motions of knots as they progress outwards from the originating source. Methods. The nebulosities associated with the jet(s) from the protostellar binary L1551 IRS5 were imaged in a number of spectral bands using the Hubble Space Telescope. This allows the proper motion to be measured and permits a simple characterization of the physical conditions in different structures. To this end we developed a reproducible method of data analysis, which allows us to define the position and shape of each substructure observed within the protostellar jet. Using this approach, we derive the proper motion of the knots in the jet, as well as their flux variability and shock emission. Results. The time base over which HST observations were carried out is now about ten years. The sub-structures within the jet undergo significant morphological variations: some knots seem to disappear in a few years and collision between different knots, ejected at different epochs and maybe with different speed, may occur. The velocities along the jet vary between ~100 km s -1 and over 400 km s -1 , with the highest speed corresponding to the knots at the base of the jet. Conclusions. There are indications that the HH 154 jet has been active relatively recently. Our results suggest the presence of a new shock front at the base of the jet identified with an internal working surface. From the analysis of the terminal and internal working surfaces within the jet, we find that the more likely scenario for the HH 154 jet is that of a jet traveling through a denser ambient medium (a “light jet”). These results are consistent with the Bonito et al. (2007) model predictions. Furthermore, there is strong evidence that the knots at the base of the northern jet correspond to the location where the highest velocity and the highest excitation component are measured along the jet. More important, this is the location where the X-ray source has been discovered.

Where Centaurus A Gets Its X-Ray Knottiness

We report an X-ray spectral study of the transverse structure of the Centaurus A jet using new data from the Chandra Cen A Very Large Project. We find that the spectrum steepens with increasing distance from the jet axis, and that this steepening can be attributed to a change in the average spectrum of the knotty emission. Such a trend is unexpected if the knots are predominantly a surface feature residing in a shear layer between faster and slower flows. We suggest that the spectral steepening of the knot emission as a function of distance from the jet axis is due to knot migration, implying a component of transverse motion of knots within the flow.

Collimation, Proper Motions, and Physical Conditions in the HH 30 Jet fromHubble Space TelescopeSlitless Spectroscopy

We present STIS spectral images of the HH 30 stellar jet taken through a wide slit over two epochs. The jet is unresolved spectrally, so the observations produce emission-line images for each line in the spectrum. This rich data set shows how physical conditions in the jet vary with distance and time, produces precise proper motions of knots within the jet, resolves the jet width close to the star, and gives a spectrum of the reflected light from the disk over a large wavelength range at several positions. We introduce a new method for analyzing a set of line ratios based on minimizing a quadratic form between models and data. The method generates images of the density, temperature, and ionization fraction computed using all the possible line ratios appropriately weighted. In HH 30, the density declines with distance from the source in a manner consistent with an expanding flow and is larger by a factor of 2 along the axis of the jet than it is at the periphery. Ionization in the jet ranges from ~5% to 40%, and high-ionization/excitation knots form at about 100 AU from the star and propagate outward with the flow. These high-excitation knots are not accompanied by corresponding increases in the density, so if formed by velocity variations the knots must have a strong internal magnetic pressure to smooth out density increases while lengthening recombination times.

Studies of relativistic effects with radioastron space mission

In the review we discuss possible studies of GR phenomena such as gravitational microlensing and shadow analysis with the forthcoming RadioAstron space mission. It is well-known that gravitational lensing is a powerful tool in the investigation of the distribution of matter, including that of dark matter (DM). Typical angular distances between images and typical time scales depend on the gravitational lens masses. For the microlensing, angular distances between images or typical astrometric shifts are about 10-5 ? 10-6 as1. Such an angular resolution will be reached with the space-ground VLBI interferometer, Radioastron. The basic targets for microlensing searches should be bright point-like radio sources at cosmological distances. In this case, an analysis of their variability and a reliable determination of microlensing could lead to an estimation of their cosmological mass density. Moreover, one could not exclude the possibility that non-baryonic dark matter could also form microlenses if the corresponding optical depth were high enough. It is known that in gravitationally lensed systems, the probability (the optical depth) to observe microlensing is relatively high; therefore, for example, such gravitationally lensed objects, like CLASS gravitational lens B1600+434, appear the most suitable to detect astrometric microlensing, since features of photometric microlensing have been detected in these objects. However, to directly resolve these images and to directly detect the apparent motion of the knots, the Radioastron sensitivity would have to be improved, since the estimated flux density is below the sensitivity threshold, alternatively, they may be observed by increasing the integration time, assuming that a radio source has a typical core - jet structure and microlensing phenomena are caused by the superluminal apparent motions of knots. In the case of a confirmation (or a disproval) of claims about microlensing in grav?itational lens systems, one can speculate about the microlens contribution to the gravitational lens mass. Astrometric microlensing due to Galactic Macho's action is not very important because of low optical depths and long typical time scales. Therefore, the launch of the space interferometer Radioastron will give excellent new facilities to investigate microlensing in the radio band, allowing the possibility not only to resolve microimages but also to observe astrometric microlensing. Shadows around supermassive black holes can be detected with the RadioAstron space interferometer. .

Proper Motions of the HH 47 Jet Observed with theHubble Space Telescope

We present a proper-motion study of the shock waves within the classic stellar jet HH 47 based on Hubble Space Telescope (HST) Hα and [S II] images of the region taken over two epochs. Individual knots within the jet and in the bow shock/Mach disk working surface of HH 47A move significantly in the 5 yr that separate the images, and the excellent spatial resolution of HST makes it possible to measure the proper motions with enough precision to easily observe differential motions throughout the flow. The bright portion of the jet emerges at 375 ± 25 from the plane of the sky with an average velocity of 300 km s-1. Dynamical ages of the shock waves in the jet range from a few decades for knots recently ejected by the source to ~1300 yr for the faint extended bow shock HH 47D. The jet curves, but motions of knots in the jet are directed radially away from the exciting source, and velocity variability in the flow drives the shock waves that heat the jet internally. The jet orientation angle varies with time by about 15° and currently points to the northwestern portion of a cavity outlined by a reflection nebula, where a quasi-stationary shock deflects the jet. The major working surface HH 47A is more complex than a simple bow shock/Mach disk and contains numerous clumps that move relative to one another with velocities of ~±40 km s-1. Small clumps or instabilities affect the Mach disk, and dense clumps may move all the way through the working surface to cause the bumpy morphology seen at the bow shock. A localized area between the bow shock and Mach disk varies significantly between the two sets of images.

Further observations of IRAS 04302+2247

We present near-infrared broad-band and H2 images of the quadrupolar source IRAS 04302+2247. High-resolution data at 3.8 μm show that the circumstellar envelope has a high degree of axisymmetry: the asymmetry at shorter wavelengths is almost certainly the result of extinction by dust in the cavity. Photometry of the flux peaks confirms that the source is variable but little change in appearance is observed between 1995 November and 1997 September. We report the first clear detection of the molecular outflow in H2 at 2.1 μm, revealing discrete streams which are inclined to the axis of the cavity. The motion of knots in the outflow remains the most likely cause of the variability.

Physics of the M87 Jet

Motivated by the recent determination of proper motions of knots in the M87 jet by Biretta, Zhou, & Owen (1995), we have investigated the relationship between the dynamics of the jet and the kpc-scale radio and optical structure. We interpret the knots as internal shocks in the jet, and find that relativistic effects play an important role in their appearance. In particular, knot A, which appears to be almost transverse to the flow and seems to be viewed nearly edge-on, is in fact a highly oblique shock. The direction of its normal with respect to the flow lies within about 10° of the Mach angle, i.e., the angle of obliquity beyond which no shock is possible. In a frame comoving with the shock front, the incident streamlines make an angle of about 40° – 60° with respect to the shock normal. Knot A has a proper motion corresponding to only 0.51c, and the relativistic effects are modest. Its edge-on appearance is largely fortuitous. We have found, however, that relativistic effects would create a strong bias for shocks with apparent superluminal speeds to appear nearly edge-on.

On the relationship between BL Lacertae objects and Fanaroff-Riley I radio galaxies

The idea that Fanaroff-Riley Class~I Radio Galaxies contain core jets with Lorentz factors of the order of a few and are the parent population for BL~Lac Objects is examined, with particular reference to the data on two FRI Radio Galaxies, NGC~315 and NGC~6251. Conservation laws for an entraining relativistic jet are derived and are used to determine a relationship between Mach number and velocity for initially relativistic jets. One immediate consequence of this relationship is that, if an initially relativistic jet is decelerated to a transonic Mach number between, say 0.5 and 1.5, then its velocity at this point is between 0.3 and 0.7 times the speed of light. Analysis of the milliarcsecond and arcsecond data on NGC~315 and NGC~6251 shows that the Lorentz factors of the core jets in these galaxies can be as high as 2-4 provided that the jet pressures are not much more than an order of magnitude higher than the minimum pressures. Such Lorentz factors are more plausible if the jets are inclined at $\sim 30$ degrees to the line of sight, although larger angles would also be possible. For core Lorentz factors in the range 2-4, the jets are mildly relativistic on the large scale and their velocities are consistent with the jet brightness asymmetries being caused by Doppler beaming. The Lorentz factors required for significant beaming of core jets may be reconciled with subluminal motions of knots in the core if these are reverse shocks advected by the jet. Moderate jet Lorentz factors are possible if the shocks are normal and higher Lorentz factors are possible if the shocks are oblique. This paper provides support for the idea that the transition from two-sided to one-sided jets across the FRI/II break is due to the transition from

A possible superluminal motion of the VLBI core in 3C345

The recent VLBI observations of the QSO 3C345 have shown that the apparent motions of knots c 6 and C 7 are northwestward. This is approximately parallel to the apparent motion of knot C 4 during the period 1981–1984. The observed apparent motions of knots C 6 and C 7 are consistent with the prediction of the “core motion model” proposed. In this paper we use all the VLBI data available for 3C345 and a more stringent method to determine the apparent motion of the VLBI core of 3C345 in the N-S direction and the common apparent trajectory along which the knots move. The results show that the apparent N-S motion of the VLBI core in 3C345 is oscillatory. The derived maximal speed of the apparent motion is 0.10–0.15 mas/yr (i.e. 2 c or 3 c, h0 = 100 km −1 sec −1 Mpc −1). The VLBI monitoring observations of the apparent trajectories for knots C 6 and C 7 before 1993 is decisive to the test of the model. Direct measurements of the possible apparent motion of the VLBI core by VLBI phasereferencing technique would be most desirable.

Motion of knots in the superluminal source 3C 345

In the typical superluminal source 3C 345, there are two radio knots C 4 and C 5 which move along two different curved orbits and which show acceleration near the core. An explanation of these observations is presented here. We assume that the knots follow spiral-shaped magnetic lines inside the jet. From simple geometrical consideration, with an appropriate form of the spiral and a suitable size of the velocity in space, the observed paths and velocities of these two knots can be well explained.

A General Study of a Prominence Field

This study is an attempt to find some order or pattern in the motions of knots in a prominence field and to view the prominence against the background of the general solar neighbourhood. It is based on the nine moving knots of the prominence observed on 1947 September 16 which traversed similar, though not identical, trajectories, from a region high above a sunspot area to a point of attraction on the disk. Velocities of the knots increased from zero to more than 100 km./sec. under accelerations which became as large as one-third of solar gravity. Time-distance diagrams for the fast-moving knots are brought into coincidence for a large portion of the trajectory through arbitrary changes in the zero points from which the distances are measured. The similarity of these curves suggests that the velocity of the knots is a function of the distance, or time, from their starting-points, and that they form a system subject to the same force or forces.

Electromagnetic Forces in Solar Prominences

The paper considers the hypothesis that the forms of solar prominences and the motion of knots in them is to be accounted for as motion of ions in a magnetic field. In Section I the dispersion of systems of ions under their mutual electrostatic repulsion is considered, and the computed velocities compared with observations by McMath and his co-workers. Agreement with observation is obtained by assuming the existence of a background cloud of 10 −11 or 10 −12 elementary charges per c.c. In Section II the possible existence of a general electrostatic field above the solar surface is discussed. It is suggested that this may account for the support of prominences and of highly ionized atoms in the corona. Space tracks of moving knots are computed and found to be consistent with the observed forms of streamers. The forms of tornado and coronal prominences are also considered.