Bright Knots Research Articles

Twisted Jets from L1551 IRS5

We have carried out high-resolution near-infrared imaging observations of a protostar L1551 IRS 5 with the Subaru Telescope. The jet structure of IRS5 is resolved into two independent jets for the first time from the ground. Successive near-infrared spectroscopy has revealed that the jet emission is dominated by [Fe II] lines in the J and H-bands. While the visual-extinction reaches more than 20 mag in the close vicinity of IRS 5, it decreases rapidly at ∼1″ from IRS 5 and remains constant around 7 mag at larger distances. The twisted structure and bright emission knots are intrinsic to the jets, not due to a spatial variation of the extinction.

Smoke Signals from IRC +10216. I. Milliarcsecond Proper Motions of the Dust

The results of a seven epoch interferometric imaging study, at wavelengths in the near-infrared K band, of the carbon star IRC+10216 are presented. The use of non- and partially redundant aperture masking techniques on the 10 m Keck I telescope has allowed us to produce images of the innermost regions of the circumstellar dust envelope with unprecedented detail. With roughly twice the resolving power of previous work, the complex asymmetric structures reported within the central 05 (~20 R*) have been imaged at the size scale of the stellar disk itself (~50 mas). A prominent dark lane at a position angle of approximately 120° is suggested to be an optically thick disk or torus of dust which could help to explain IRC +10216's well-known bipolarity at a position angle of ~20°. Observations spanning more than a pulsational cycle (~638 days) have revealed significant temporal evolution of the nebula, including the outward motion of bright knots and clumps. Registering these displacements against the compact bright core, which we tentatively identify as marking the location of the star, has allowed us to determine the apparent angular velocity at a number of points. The magnitudes of the proper motions were found to be in agreement with current estimates of the stellar distance and radial velocity. Higher outflow speeds were found for features with greater separation from the core. This is consistent with acceleration taking place over the region sampled by the measurements; however, alternate interpretations are also presented. Although a number of changes of morphology were found, none were clearly interpreted as the condensation of new dust over the pulsation cycle. Unfortunately, ambiguities associated with the true three-dimensional nature of the nebula weaken a number of our quantitative and qualitative conclusions.

ITAL]ISO[/ITAL] Mid-Infrared Observations of Normal Star-Forming Galaxies: The Key Project Sample

We present mid-infrared maps and preliminary analysis for 61 galaxies observed with the ISOCAM instrument aboard the Infrared Space Observatory. Many of the general features of galaxies observed at optical wavelengths?spiral arms, disks, rings, and bright knots of emission?are also seen in the mid-infrared, except the prominent optical bulges are absent at 6.75 and 15 ?m. In addition, the maps are quite similar at 6.75 and 15 ?m, except for a few cases where a central starburst leads to lower I?(6.75 ?m)/I?(15 ?m) ratios in the inner region. We also present infrared flux densities and mid-infrared sizes for these galaxies. The mid-infrared color I?(6.75 ?m)/I?(15 ?m) shows a distinct trend with the far-infrared color I?(60 ?m)/I?(100 ?m). The quiescent galaxies in our sample [I?(60 ?m)/I?(100 ?m) 0.6] show I?(6.75 ?m)/I?(15 ?m) near unity, whereas this ratio drops significantly for galaxies with higher global heating intensity levels. Azimuthally averaged surface brightness profiles indicate the extent to which the mid-infrared flux is centrally concentrated, and provide information on the radial dependence of mid-infrared colors. The galaxies are mostly well resolved in these maps: almost half of them have <10% of their flux in the central resolution element. A comparison of optical and mid-infrared isophotal profiles indicates that the flux at 4400 ? near the optical outskirts of the galaxies is approximately 8 (7) times that at 6.75 ?m (15 ?m), comparable to observations of the diffuse quiescent regions of the Milky Way.

Kinematics and Morphology of Ionized Gas in Hickson Compact Group 18

We present new observations of Hα emission in the Hickson compact group 18 (HCG 18), obtained with a scanning Fabry-Perot interferometer. The velocity field does not show motions of individual group members but instead a complex common velocity field for the whole group. The gas distribution is very asymmetric, with clumps of maximum intensity coinciding with the optically brightest knots. Comparing Hα and H I data, we conclude that HCG 18 is not a compact group but instead a large irregular galaxy with several star-forming clumps.

Bars and Warps Traced by the Molecular Gas in the Seyfert 2 Galaxy NGC 1068

We present new interferometer observations of the 12CO (1-0) and 12CO (2-1) line emission of NGC 1068 with a resolution of 07. The molecular gas in the inner 5'' is resolved into a ring with two bright knots east and west of the nuclear continuum emission. For the first time in NGC 1068, we can trace molecular gas at ≈018 (13 pc) from the nucleus. The high velocities in this region imply an enclosed mass of ~108 M☉. This value is consistent with a black hole mass of 1.7 × 107 M☉, as estimated from nuclear H2O maser emission, plus a contribution from a compact nuclear stellar cluster. Perpendicular to the kinematic major axis, optical images of NGC 1068 show a bright, stellar, oval structure of eccentricity 0.8 and a deprojected length of 17 kpc. Analysis of the rotation curve shows the CO spiral arms are at the inner Lindblad resonance of this barlike structure. Inside the molecular spiral arms, 10'' from the nucleus, the CO kinematic axis changes direction probably in response to the 2.5 kpc (deprojected) long stellar bar seen in the near-infrared (NIR). The low velocity dispersion indicates the molecular gas is in a disk with a thickness of 10 pc in the nuclear region and 100 pc in the spiral arms.

Kinematics of Herbig-Haro Objects in the Protostellar Outflow L1551 as Mapped by Fabry-Perot Spectroscopy

We present new velocity-resolved Fabry-Perot images in [S II] λ6731 and Hα of the blueshifted portion of the protostellar outflow L1551. These new data isolate the line emission from the reflected continuum and make it possible to visualize the kinematics of the shock waves in the outflow clearly for the first time on large scales with subarcsecond spatial resolution. Velocity images of the L1551 jet confirm that a fainter, slower jet lies a few arcseconds below the main jet. Emission from the main jet decreases sharply in radial velocity and the emission-line width increases suddenly as the jet encounters bright knot 3, in agreement with bow shock models. This knot must move into previously ejected material to account for the observed radial velocities, supporting the idea that shocks in Herbig-Haro (HH) flows form as the result of variable velocity ejections from the embedded protostar. However, a velocity gradient observed along the entire edge of the bow shock is spatially larger than expected if the bow shock alone were responsible for all the line emission. Deviations from the simple model are most easily explained if a Mach disk alters the emission and kinematics within the bow shock region, though a precursor to the bow shock is an alternate possibility. The spatial distribution of radial velocities and emission-line widths across HH 29 implies that this object is a slower portion of the outflow currently being overtaken by faster material. New proper-motion images of HH 29 independently confirm this result. The large-scale velocity structure of the L1551 outflow is complex but can generally be understood if faster material drives shock waves into slower material around the edge of a cavity. A striking circular feature whose center lies near the intersection of the axis of the bright jet and the cavity resembles similar structures in the Orion Nebula and could define a hole through which a fast jet has penetrated. The velocity structure along a string of HH objects to the southwest of HH 29 is consistent with the recent result of Devine et al. that L1551 NE drives this portion of the outflow. Linear features that cross the L1551 flow may be associated with separate, unrelated jets from the HH 30 region or elsewhere within L1551.

Complex Extended Line Emission in the [CLC]c[/CLC]D Galaxy in Abell 2390

This paper reports maps of the cD galaxy in the rich z = 0.23 cluster Abell 2390 at UV, [O II], Lyα, and Hα wavelengths. Spatially resolved UV and optical spectra were obtained with Space Telescope Imaging Spectrograph on the Hubble Space Telescope; the 2'' wide slit was aligned close to the long axis and blue lane of the galaxy and includes all the inner bright features seen in Wide Field Planetary Camera 2 images. The Lyα is seen in emission from several bright knots and over an extended region from 4'' northwest to 2'' southeast of the nucleus. Three of these knots have detected UV continuum as well. Hα images were obtained with OSIS at the Canada-France-Hawaii Telescope; both Hα and [O II] show extended emission that roughly trace Lyα, primarily to the northwest. Notable differences between the spatial distributions of Hα, Lyα, and [O II] emission and the blue knots in the galaxy may be the result of nonhomogeneous dust extinction or variations in ionization. The Lyα emission velocities depend on the uncertain identification with features in the undispersed images; there is strong evidence for resolved emission knots to have high velocities, 1000 km s-1, indicative of infall. The Lyα spectrum shows a sharp cutoff which may be due to absorption; we entertain the possibility that this edge is due to a shell of absorbing gas outflowing at about 5000 km s-1.

A Pair of Twisted Jets of Ionized Iron from L 1551 IRS 5

High-resolution near-infrared J-band imaging with the Subaru Telescope has demonstrated, for the first time from the ground, two independent jets emanating from the protostar L 1551 IRS 5. Successive near-infrared spectroscopy has revealed that the jet emission is dominated by [FeII] lines in the J- and H-bands. While the visual-extinction estimated from the [FeII] line ratios reaches more than 20 mag in the close vicinity of IRS 5, it decreases rapidly at ∼ 1″ from IRS 5 and remains constant around 7 mag at larger distances. The electron density in the jets is estimated to be 106 cm−3 near IRS 5 and is 104 to 105 cm−3 in their outer section. The twisted appearance of the jets, with bright knots especially prominent in the northern jet, is similar to the R-band jets observed with the Hubble Space Telescope. These results suggest that the twisted structure and bright emission knots are intrinsic to the jets and are not due to a spatial variation of the extinction.

Dynamics in restructuring active regions observed during SOHO/Yohkoh/GBO campaigns

JOP17 and JOP 33 are SOHO Joint Observing Programs in collaboration with Yohkoh/SXT and ground based observatories (GBO's), dedicated to observe dynamical events through the atmosphere. During runs of these programs we observed in restructuring active regions (ARs), surges, subflares, bright knots, but not large flares and jets. From these observations we have been able to derive some of the responses of the coronal and chromospheric plasma to the evolution of the photospheric magnetic field. Emerging flux in an AR led to the formation of Arch Filament Systems in the chromosphere, hot loops and knots in the transition region, and X-ray loops. Frequent surges have been observed in relation to parasitic or mixed polarities, but coronal jets have not yet been found. We discuss the possible mechanisms acting during the restructuring of the active regions (reconnection or “sea-serpent” geometries).

Emergence of sheared magnetic flux tubes in an active region observed with the SVST and TRACE

The active region NOAA AR 8331 was a target of an international ground-based observational campaign in the Canaries and coordinated with space instruments (TRACE and Yohkoh). We focus our study on observations obtained with the SVST at LaPalma, and with TRACE. On 10 September 1998, arch-filament systems were observed with high spatial and temporal resolution, from the lower to the upper atmosphere of the Sun, during five hours. Flux tubes emerged with increasing shear, which apparently led to energy release and heating in the overlying corona. A model for filament formation by the emergence of U-shaped loops from the subphotosphere, as proposed by Rust and Kumar (1994), is supported by the present observations. The coronal response to these events is visualized by rising, medium-scale loop brightenings. The low-lying X-ray loops show short-lived, bright knots which are thought to result from interaction between different loop systems.

Observations of magnetic field evolution in a solar flare

I±V and I±Q profiles of nine spectral lines of Fe i, Fe ii, and H i in the 2B flare of 16 June 1989 have been analyzed. Two bright flare knots outside and inside of a spot were investigated. To measure the true magnetic field strength in the flare, two different methods were applied. In addition to these data, the magnetic field and thermodynamic conditions were determined using the non-LTE program for line profile synthesis. According to the measurements, the magnetic field in both flare knots changed in synchronism and non-monotonically, and reached its peak (nearly 1.6 kG for non-spot areas and approximately 4.0 kG for sunspot locations) at the time of flare peak. For the flare knot outside the spot, a background field component was also detected; the magnetic field in this component was found to have mixed polarity and remained practically unchanged during the flare. The non-LTE calculations show that the unique local magnetic field peak existed near the temperature minimum zone in the flare peak too. The observed perturbations do not exclude such phenomena as a magnetic field transient in flare.

Hubble Space TelescopeObservations of the Optical Jets of PKS 0521−365, 3C 371, and PKS 2201+044

Hubble Space Telescope (HST) observations have led to the discovery of the optical counterpart of the radio jet of PKS 2201+044 and to a detailed analysis of the optical jets of PKS 0521-365 and 3C 371. At HST spatial resolution these jets are well resolved, displaying knotty morphologies. When compared with radio maps of appropriate resolution, a clear one-to-one correspondence between optical and radio structures is found, showing that all detected optical structures are indeed related to the radio synchrotron emission. Photometry of the brightest knots shows that the radio-to-optical spectral index and the derived intensity of the equipartition magnetic field are approximately constant along the jet. Thus, present observations suggest that the electron energy distribution does not change significantly all along the jet.

The Excitation Mechanism of Emission‐Line Regions in Seyfert Galaxies

We present Hubble Space Telescope UV/optical spectrophotometry and ground-based optical spectrophotometry of the brightest knots in the emission-line regions of the Seyfert galaxies NGC 5643 and NGC 5728. These objects have ionization cones, radio and soft X-ray emission, and evidence of nuclear outflows and winds, suggesting that shocks may be associated with the emission-line gas. Comparison of the UV/optical data with grids of ionization-bounded and mixed ionization- and matter-bounded photoionization models, and shock models with and without ionized precursors, are used to determine whether radiative shocks that photoionize the shock precursor material (autoionizing shocks) are a significant excitation mechanism for the emission-line gas in these objects. Although the UV/optical spectra studied here are generally compatible with both photoionization and shock + precursor models, an unambiguous shock + precursor signature is not present. This suggests that mechanical energy input may not play the same dominant role in exciting these objects with weak radio jets as appears to be the case in strong radio jet sources such as M87.

SBSG 0335−052W: An Extremely Low Metallicity Dwarf Galaxy

We present Multiple Mirror Telescope (MMT) and Keck II telescope spectrophotometry and 3.5m Calar Alto telescope R, I photometry of the western component of the extremely low-metallicity blue compact galaxy SBS 0335-052. The components, separated by 22 kpc, appear to be members of a unique, physically connected system. It is shown that SBS 0335-052W consists of at least three stellar clusters and has the same redshift as SBS 0335-052. The oxygen abundance in its two brightest knots is extremely low, 12+log(O/H)= 7.22+/-0.03 and 7.13+/-0.08, respectively. These values are lower than in SBS 0335-052 and are nearly the same as those in I Zw 18. The (R-I) color profiles are very blue in both galaxies due to the combined effects of ionized gas and a young stellar population emission. We argue that SBS 0335-052W is likely to be a nearby, young dwarf galaxy.

Detection and Velocities of Hα Emission Regions in Stephan's Quintet

We present observations of Hα emission of Stephan's Quintet obtained with a scanning Fabry-Perot interferometer and samplings of ~11 km s-1 and ~1''. Our observations show an intragroup arclike feature formed of bright Hα knots and diffuse gas distributed along tidal tails associated with NGC 7318B and possibly also NGC 7318A. New velocities for 23 emitting regions detected along the Hα structure are presented. South of NGC 7318B, along a path covering ~12 h−175 kpc, H II regions with velocities ranging from 5540 to 6700 km s-1 are detected, suggesting that two or more overlapping tidal tails and/or a bow-shock region may have been formed through several interaction events within the group.

Optical and Radio Polarimetry of the M87 Jet at 0[farcs]2 Resolution

We discuss optical (HST/WFPC2 F555W) and radio (15 GHz VLA) polarimetry observations of the M87 jet taken during 1994-1995. Many knot regions are very highly polarized ($\sim 40-50%$, approaching the theoretical maximum for optically thin synchrotron radiation), suggesting highly ordered magnetic fields. High degrees of polarization are also observed in interknot regions. While the optical and radio polarization maps share many similarities, we observe significant differences between the radio and optical polarized structures, particularly for bright knots in the inner jet, giving us important insight into the jet's radial structure. Unlike in the radio, the optical magnetic field position angle becomes perpendicular to the jet at the upstream ends of knots HST-1, D, E and F. Moreover, the optical polarization decreases markedly at the position of the flux maxima in these knots. In contrast, the magnetic field position angle observed in the radio remains parallel to the jet in most of these regions, and the decreases in radio polarization are smaller. More minor differences are seen in other jet regions. Many of the differences between optical and radio polarimetry results can be explained in terms of a model whereby shocks occur in the jet interior, where higher-energy electrons are concentrated and dominate both polarized and unpolarized emissions in the optical, while the radio maps show strong contributions from lower-energy electrons in regions with {\bf B} parallel, near the jet surface.

Magnetic Field Effects on the Structure and Evolution of Overdense Radiatively Cooling Jets

We investigate the effect of magnetic fields on the propagation dynamics and morphology of overdense, radiatively cooling, supermagnetosonic jets, with the help of fully three-dimensional SPMHD simulations. Evaluated for a set of parameters which are mainly suitable for protostellar jets (with density ratios between the jet and the ambient medium 3-10, and ambient Mach number ~ 24), these simulations are also compared with baseline non-magnetic and adiabatic calculations. We find that, after amplification by compression and re-orientation in nonparallel shocks at the working surface, the magnetic field that is carried backward with the shocked gas into the cocoon improves the jet collimation relative to the purely hydrodynamic (HD) systems. Low-amplitude, approximately equally spaced internal shocks (which are absent in the HD systems) are produced by MHD K-H reflection pinch modes. The longitudinal field geometry also excites non-axisymmetric helical modes which cause some beam wiggling. The strength and amount of these modes are, however, reduced (by ~ twice) in the presence of radiative cooling relative to the adiabatic cases. Besides, a large density ratio between the jet and the ambient medium also reduces, in general, the number of the internal shocks. As a consequence, the weakness of the induced internal shocks makes it doubtful that the magnetic pinches could produce by themselves the bright knots observed in the overdense, radiatively cooling protostellar jets.

Bidimensional Spectroscopic Mapping and Chemical Abundances of the Star‐forming Dwarf Galaxy I Zw 18

We present bidimensional, long-slit spectroscopic mapping of the prototypical low-metallicity dwarf galaxy I Zw 18. A total of eight spaced slit positions were observed, covering most of the body of the galaxy and part of the extended envelope of the diffuse ionized halo seen in deep Hα images of I Zw 18. Spectroscopic maps of the galaxy have been produced for Hα, Hβ equivalent width, [O III], [O II], reddening, excitation, and line-free continuum. Chemical abundances have been derived for 16 positions in I Zw 18, including zones sampling part of the extended ionized envelope of the galaxy. Our results confirm and extend previous findings that I Zw 18 presents a substantially homogeneous chemical composition over the whole galaxy. The chemical abundances of I Zw 18 as derived from a σ-weighted average of all the observed positions give 12 + log O/H = 7.19 ± 0.07 and 12 + log N/H = 5.64 ± 0.12 for oxygen and nitrogen, respectively, and He/H = 0.073 ± 0.004 for helium, a value consistent with some previous determinations and one that would lead to a helium mass fraction of Y = 0.228 ± 0.009. However, we notice that when the bright central knots are excluded from the mean, a slightly larger helium abundance is derived, yielding Y = 0.242 ± 0.010. We argue that underlying stellar absorption in the helium lines is present in the spectra of the central bright knots of I Zw 18 and will be affecting the derivation of the helium abundance for these central knots. The implications of our results for the chemical evolution models and for the overall enrichment history of the I Zw 18 galaxy are discussed.

Radially aligned clumps and tails in planetary nebulae

ABSTRA C T We study the formation of radially aligned condensations and tails through the compression of material inside ionization shadows at early ionization phases of planetary nebulae. A dense clump, formed before ionization starts, forms an ionization shadow behind it. The surroundings, which are ionized before the shadow, have a higher temperature, and as a result compress the material in the shadow, forming a compressed tail. If the compressed tail crosses a dense shell, a dense condensation (clump) is formed there. At later stages this condensation is ionized and observed as a bright knot, radially aligned with the inner clump. We find that for the shadow to be effective, the clump should be already present as the ionization by the central star starts, and its density enhancement should be by a factor of * 5. We propose this mechanism as an explanation for the radially aligned condensations recently found in the planetary nebula IC 4593.

ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]Telescope[/ITAL] Resolved Image and Spectra of the [CLC][ITAL]z[/ITAL][/CLC] ≃ 2 QSO 1345+584

QSO 1345+584 has been spatially resolved by direct images and in spectral images, and has asymmetrically extended flux to the west, where its inner radio structure is seen. The brightest knots in the resolved flux correspond closely with knots in the curved radio jet, and the brightest knot has a velocity of approach of some 3000 km s-1 with respect to the nucleus. Other parts of the line-emitting material appear to follow a systematic velocity field with values up to 1000 km s-1 with respect to the nucleus. The signal from the resolved continuum is not detected spectroscopically, but accounts for two-thirds of the (rest-UV) flux, so that it is likely to originate in hot stars. The QSO lies in or behind a compact group of galaxies of comparable brightness and irregular and knotty morphology, which probably form a dense physical group with very young stellar populations.