Bright Core Research Articles

A GLOBAL 86 GHZ VLBI SURVEY OF COMPACT RADIO SOURCES

We present results from a large 86 GHz global very long baseline interferometry (VLBI) survey of compact radio sources. The main goal of the survey is to increase by factors of 3-5 the total number of objects accessible for future 3 mm VLBI imaging. The survey observations reach a baseline sensitivity of 0.1 Jy and an image sensitivity of better than 10 mJy beam–1. A total of 127 compact radio sources have been observed. The observations have yielded images for 109 sources, extending the database of the sources imaged at 86 GHz with VLBI observation by a factor of 5, and only six sources have not been detected. The remaining 12 objects have been detected but could not be imaged due to insufficient closure phase information. Radio galaxies are less compact than quasars and BL Lac objects on the sub-milliarcsecond scale. The flux densities and sizes of the core and jet components of all imaged sources have been estimated using Gaussian model fitting. From these measurements, brightness temperatures have been calculated, taking into account the resolution limits of the data. The cores of 70% of the imaged sources are resolved. The core brightness temperatures of the sources peak at ~1011 K and only 1% have brightness temperatures higher than 1012 K. The cores of intraday variable (IDV) sources are smaller in angular size than those of non-IDV sources, and so yield higher brightness temperatures.

The determination of the structure of Saturn’s F ring by nearby moonlets

Saturn's narrow F ring exhibits several unusual features that vary on timescales of hours to years. These include transient clumps, a central core surrounded by a multistranded structure and a regular series of longitudinal channels associated with Prometheus, one of the ring's two 'shepherding' satellites. Several smaller moonlets and clumps have been detected in the ring's immediate vicinity, and a population of embedded objects has been inferred. Here we report direct evidence of moonlets embedded in the ring's bright core, and show that most of the F ring's morphology results from the continual gravitational and collisional effects of small satellites, often combined with the perturbing effect of Prometheus. The F-ring region is perhaps the only location in the Solar System where large-scale collisional processes are occurring on an almost daily basis.

High-mass star formation in the IRAS 17233-3606 region: a new nearby and bright hot core in the southern sky

Aims. We present molecular line observations of the massive star-forming region IRAS 17233-3606 aimed at studying the molecular core associated with the source. Methods. The observations were made using the Atacama Pathfinder Experiment telescope in the CO (3-2) and HCO + (4–3) transitions and in the CH 3 OH (6$_K{-}5_K$), (7$_K{-}6_K$), and CH 3 CN (16$_K{-}15_K$) bands. For the CO(3–2) and HCO + (4–3) transitions, we obtained maps with a size of $70''\times 70''$. The typical angular resolution of the data is ∼ $18\arcsec$. Results. Our observations reveal an exceptionally rich molecular spectrum, one signpost of hot core activity. Comparisons with two other prominent southern hot cores were made through observations in the same frequency setups. We also detected a bipolar outflow in CO (3–2) and HCO + (4–3) lines. Modelling reveals a hot core of size ∼ $3\arcsec$ and a temperature of 150 K in the IRAS 17233-3606 region. The parameters of the molecular outflow are derived through the analysis of the CO (3–2) emission, and are typical of outflows driven by high-mass young stellar objects.

Near-infrared imaging polarimetry of young stellar objects in ρ Ophiuchi

The results of a near-infrared (J H K LP) imaging linear polarimetry survey of 20 young stellar objects in ρ Ophiuchi are presented. The majority of the sources are unresolved, with K band polarizations, PK 20% are seen over their envelopes. Correlations are observed between the degree of core polarization and the evolutionary status inferred from the spectral energy distribution. K band core polarizations >6% are only observed in Class I YSOs. A 3-dimensional Monte Carlo model with oblate grains aligned with a magnetic field is used to investigate the flux distributions and polarization structures of three of the ρ Oph young stellar objects with extended nebulae. A ρ / r 1.5 power law for the density is applied throughout the envelopes. The large scale centrosymmetric polarisation structures are due to scattering. However, the polarization structure in the bright core of the nebula appears to require dichroic extinction by aligned nonspherical dust grains. The position angle indicates a toroidal magnetic field in the inner part of the envelope. Since the measured polarizations attributed to dichroic extinction are usually 610%, the grains must either be nearly spherical or very weakly aligned. The higher polarizations observed in the outer parts of the reflection nebulae require that the dust grains responsible for scattering have maximum grain sizes 61.05 � m.

MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. IV. The Parent Luminosity Function of Radio‐Loud Blazars

We use a complete sample of active galactic nuclei (AGN) selected on the basis of relativistically beamed 15 GHz radio flux density (MOJAVE: Monitoring of Jets in AGN with VLBA Experiments) to derive the parent radio luminosity function (RLF) of bright radio-selected blazar cores. We use a maximum likelihood method to fit a beamed RLF to the observed data and thereby recover the parameters of the intrinsic (unbeamed) RLF. We analyze two subsamples of the MOJAVE sample: the first contains only objects of known FR II class, with a total of 103 sources, and the second subsample adds 24 objects of uncertain FR class for a total of 127 sources. Both subsamples exclude four known FR I radio galaxies and two gigahertz-peaked spectrum sources. We obtain good fits to both subsamples using a single power law intrinsic RLF and a pure density evolution function of the form zmexp{−1/2[(z − z0)/σ]2}. We find that a previously reported break in the observed MOJAVE RLF actually arises from using incomplete bins (because of the luminosity cutoff) across a steep and strongly evolving RLF, and does not reflect a break in the intrinsic RLF. The derived space density of the parent population of the FR II sources from the MOJAVE sample (with L15 GHz ⩾ 1.3 × 1025 W Hz−1) is approximately 1.6 × 103 Gpc −3.

Optical line profiles of the Helix planetary nebula (NGC 7293) to large radii

New, very long (25 arcmin), cuts of spatially resolved profiles of the Hα and [N II] optical emission lines have been obtained over the face of the Helix planetary nebula, NGC 7293. These directions were chosen to supplement previous similar, though shorter, cuts as well as crossing interesting phenomena in this nebular envelope. In particular, one new cut crosses the extremes of the proposed CO J = 2‐1 emitting outer ‘torus’ shown by Huggins and his coworkers to be nearly orthogonal to its inner counterpart. The second new cut crosses the extensive outer filamentary arcs on either side of the bright nebular core. It is shown that NGC 7293 is composed of multiple bipolar outflows along different axes. Hubble-type outflows over a dynamical time-scale of 11 000 yr are shown to be occurring for all of the phenomena from the smallest He II emitting core out to the largest outer filamentary structure. All must then have been ejected over a short time-scale but with a range of ejection velocities.

The comet 17P/Holmes 2007 outburst: the early motion of the outburst material

Context. On October 24, 2007 the periodic comet 17P/Holmes underwent an astonishing outburst that increased its apparent total brightness from magnitude V ∼ 17 up to V ∼ 2.5 in roughly two days. In this contribution we report on Wendelstein 0.8 m telescope (WST) photometric observations of the early evolution stages of the outburst. Aims. We studied the evolution of the structure morphology and its kinematic and provide an estimate of the ejected dust mass. Methods. We analyzed 126 images of the comet in the BVRI photometric bands spread between October 26, 2007 and November 20, 2007. The bright comet core appeared to be separated from a quickly expanding dust cloud in all the data, and the bulk of the cloud was contained in the field of view of our instrument during the days soon after the outburst, allowing precise estimates both of the separation velocities of the two luminous baricenters and of the expansion velocity of the dust cloud. The ejected dust mass was derived on the basis of differential photometry on background stars occulted by the moving cloud. Results. The two cores were moving apart from each other at a relative, projected constant velocity of (9.87 ± 0.07) arcsec/day (0.135 ± 0.001 km s −1 ). In the inner regions of the dust cloud we observed a linear increase in size at a mean constant velocity of (14.6 ± 0.3) arcsec/day (0.200 ± 0.004 km s −1 ). Evidence of a radial velocity gradient in the expanding cloud was also found. Our estimate for the expanding coma’s mass was approximately 10 −2 −1 comet’s mass, implying a significant disintegration event. Conclusions. We interpret our observations in the context of an explosive scenario that was more probably triggered by some internal instability processes rather than by an impact with an asteroidal body. Due to the peculiar characteristics of this event, further observations and investigations are necessary to bring the nature of the physical processes that determined it to light.

Stochastic Acceleration in the Western Hot Spot of Pictor A

Chandra's high-resolution observations of radio galaxies require a revisit of the relevant electron acceleration processes. Although the diffusive shock particle acceleration model may explain spectra of spatially unresolved sources, it encounters difficulties in explaining the structure and spectral properties of recently discovered Chandra X-ray features in several low-power radio sources. We argue that these observations strongly suggest stochastic electron acceleration by magnetized turbulence, and show that the simplest stochastic particle acceleration model with energy-independent acceleration and escape timescales can overcome most of these difficulties. We use the bright core of the western hot spot of Pictor A as an example to demonstrate the model characteristics, which may be tested with high-energy observations.

MR manifestations of solitary necrotic nodule of the liver

目的 探讨MRI对各型肝脏孤立性坏死结节(SNN)的诊断价值.方法 分析经手术病理证实的SNN 9例,并结合文献资料探讨其分型、各型的MR表现及其病理基础.结果 (1)单纯凝固性坏死型(5例),MRI的T1WI及T2WI上分别表现为低信号或低-等信号,增强扫描病灶内部无强化,周围包膜呈轻-中度强化.病理显示为单纯凝固性坏死,外围有薄层纤维包膜.(2)伴液化性坏死型(1例),T1WI上表现为低信号,较大病灶内可见点片状更低信号;T2WI上病灶呈低-等信号,其内可见点片状高信号;增强后病灶内部无强化,周围包膜呈轻-中度强化.病理上以凝固性坏死为主,中心可见裂隙状液化性坏死,外围有薄层纤维包膜.(3)多结节融合型(3例),T1WI为低信号,其内可见低-等信号分隔;T2WI病灶以低信号为主,其内可见等或略高信号分隔;增强扫描病灶内部无强化,其内分隔及周围包膜呈轻-中度强化.病理呈多个坏死结节融合,病灶以凝固性坏死为主,部分较大结节内可见小片状液化性坏死,结节间可见纤维分隔及外围薄层纤维包膜.结论 MRI能很好反映SNN的分型及各型的病理特征。

APEX and ATCA observations of the southern hot core G327.3-0.6 and its environs

There is no generally accepted evolutionary scheme for high mass star formation yet. A simple approach to address this problem is to cover several of the known stages during the formation of massive stars in the same cloud and then investigate their properties trying to construct an evolutionary sequence. Here we present such a project conducted with complementary APEX and ATCA observations. These observations show a compact and bright single hot core in the G327.3-0.6 region on a 0.03 pc scale with a mass of 500 M⊙ and 0.5–1.5 105 L⊙. Additionally a clumpy filament is seen in N2H+. Together with cm continuum observations, the data reveal like pearls on a string several stages of massive star formation, with likely the youngest stages hiding in the cold N2H+ cores analysed with a multilevel study of the APEX and ATCA observations.

Circumbinary molecular rings around young stars in Orion

We present high angular resolution 1.3 mm continuum, methyl cyanide molecular line, and 7 mm continuum observations made with the Submillimeter Array and the Very Large Array, toward the most highly obscured and southern part of the massive star forming region OMC1S located behind the Orion Nebula. We find two flattened and rotating molecular structures with sizes of a few hundred astronomical units suggestive of circumbinary molecular rings produced by the presence of two stars with very compact circumstellar disks with sizes and separations of about 50 AU, associated with the young stellar objects 139-409 and 134-411. Furthermore, these two circumbinary rotating rings are related to two compact and bright hot molecular cores. The dynamic mass of the binary systems

GRB 050822: detailed analysis of an XRF observed bySwift

We report on the temporal and spectral characteristics of the early X-ray emission from the GRB 050822 as observed by Swift. This burst is likely to be an XRF showing major X-ray flares in its XRT light-curve. The quality of the data allows a detailed spectral analysis of the early afterglow in the X-ray band. During the X-ray flares, a positive correlation between the count rate and the spectral hardness (i.e. the higher the count rate, the harder the spectrum) is clearly seen for the X-ray flares. This behaviour, similar to that seen for Gamma-ray pulses, indicates that the energy peak of the spectrum is in the XRT energy band and it moves towards lower energies with time. We show evidence for the possible detection of the emergence of the forward-shock emission, produced at a radius larger than 4 1016 cm in the case of a CBM afterglow model (a formation region clearly different from that producing the prompt emission). Finally, we show that the null detection of a jet break up to T0+4 106 s in the X-ray light curve of this XRF can be understood: i) if the jet seen on-axis is uniform with a large opening angle (); or ii) if the jet is a structured Gaussian-like jet with the line-of-sight outside the bright Gaussian core.

WC 첨가 방법에 따른 Ti(CN)-Co/Ni 계 서멧트의 미세조직 및 특성변화

In the Ti(CN)-Co/Ni cermet, WC is an effective additive for increasing sinterability and mechanical properties such as toughness and hardness. In this work, WC, (WTi)C and (WTi)(CN) were used as the source of WC and their effects were investigated in the respect of microstructural evolution and mechanical properties. Regardless of the kinds of WC sources, the hard phase with dark core and bright rim structure was observed in the Ti(CN)-Co/Ni cermet under the incorporation of relatively small amount of WC. However, hard phases with bright core began to appear and their frequency increased with the increase of all kinds of WC source addition. The ratio of bright core to dark one in the (TiW)(CN)-Co/Ni cermet was greatest under the incorporation of (WTi)C compared at the same equivalent amount of WC. The mechanical properties were improved with the addition of WC irrespective of the kinds of sources, but the addition of (WTi)(CN) was less effective for the increase of fracture toughness.

A Novel ARID/Bright-like Protein Involved in Transcriptional Activation of Cyst Wall Protein 1 Gene in Giardia lamblia

The capability of protozoan parasite Giardia lamblia to encyst is critical for survival outside the host and its transmission. AT-rich interaction domain (ARID) or Bright homologs constitute a large family of transcription factors in higher eukaryotes that regulate cell proliferation, development, and differentiation. We asked whether Giardia has ARID-like genes and whether they influence gene expression during Giardia encystation. Blast searches of the Giardia genome data base identified two genes with putative ARID/Bright domains (gARID1 and 2). Epitope-tagged gARID1 was found to localize to nuclei. Recombinant gARID1 specifically bound to the encystation-induced cyst wall protein (cwp) gene promoters. Mutation analysis revealed that AT-rich initiators were required for binding of gARID1 to the cwp promoters. gARID1 contains several key residues for DNA binding, and its binding sequences are similar to those of the known ARID family proteins. The gARID1 binding sequences were positive cis-acting elements of the cwp1 promoter during both vegetative growth and encystation. We also found that gARID1 transactivated the cwp1 promoter through its binding sequences in vivo. Our results suggest that the ARID family has been conserved during evolution and that gARID1 is an important transactivator in regulation of the Giardia cwp1 gene, which is key to Giardia differentiation into cysts.

Long-lived hot coronal structures observed with CORONAS-F/SPIRIT in the Mg XII line

Large-scale hot features were detected and observed several times high in the solar corona in the high-temperature Mg XII line (T = 5–20 MK, T max = 10 MK) with the soft X-ray telescope of the SPIRIT instrumentation complex onboard the CORONAS-F spacecraft. These features look like a spider up to 300000 km in size and live up to a few days. Their bright cores observed at heights were from 0.1 to 0.3 solar radii are connected with active regions by darker legs, giant loops. These features are disposed above arcades, which are simultaneously observed in cooler emission lines sensitive to temperatures of 1 to 2 MK. For the core of such a feature observed December 28–29, 2001, Zhitnik et al. (2003a) estimated an electron temperature of 10 MK and a number density of n e ≈ 1010 cm−3. A high activity and an association with eruptive phenomena were found for such features in continuous (up to 20-day) observations with a cadence of 0.6–1.7 min. In the present paper, we discuss the relation of such features to coronal structures, which are known from previous studies. We identify such off-limb features observed with SPIRIT on October 22, November 12, and December 28–29, 2001, with hot upper parts of post-eruptive arcades. The results of multifrequency analysis of these features based on the data obtained in various spectral ranges by different instruments (Yohkoh/SXT, SOHO/EIT, SOHO/LASCO, Nobeyama and SSRT radioheliographs) are briefly discussed. We address the physical conditions of the long-term existence of giant hot coronal structures. It is demonstrated that the post-eruptive energy release must be prolonged and the condition β ≪ 1 is not satisfied in these structures. It is argued that the so-called “standard flare model” should be better considered as a “standard post-eruptive energy release model.”

Lyman Break Galaxies under a Microscope: The Small‐Scale Dynamics and Mass of an Arc in the Cluster 1E 0657−56

Using the near-infrared integral field spectrograph SPIFFI on the VLT, we have studied the spatially resolved dynamics in the z = 3.2 strongly lensed galaxy 1E 0657-56 arc+core by observing the rest-frame optical emission lines [O III] λ5007 and Hβ. The lensing configuration suggests that the high surface brightness core is the ~ 20 magnified central ~1 kpc of the galaxy, whereas the fainter arc is the more strongly magnified peripheral region of the same galaxy at about a half-light radius, which otherwise appears to be a typical z ~ 3 Lyman break galaxy. The overall shape of the position-velocity diagram resembles the rotation curves of the inner few kpc of nearby ~ spiral galaxies. For = 20, our data have a spatial resolution of ~200 pc in the source plane. The projected velocities vrot rise rapidly to ~75 km s-1 within radii ~0.5 kpc from the center and asymptotically reach a velocity of ~190 km s-1 within the arc, at a projected radius of a few kpc radius. The rotation curve implies a dynamical mass of log Mdyn/M☉ ~ 9.3 within the central kpc and suggests that in this system the equivalent of the mass of a present-day ~ bulge at the same radius was already in place by z 3. Approximating the circular velocity of the halo by the measured asymptotic velocity of the rotation curve, we estimate a dark matter halo mass of log Mhalo/M☉ ~ 11.7 ± 0.3, in good agreement with large-scale clustering studies of Lyman break galaxies. The baryonic collapse fraction is low compared to z ~ 2 actively star-forming BX and low-redshift galaxies, perhaps implying comparatively less gas infall to small radii or efficient feedback. Even more speculatively, the high central mass density might indicate highly dissipative gas collapse in very early stages of galaxy evolution, in approximate agreement with what is expected for inside-out galaxy formation models.

Milliarcsecond-Scale Structure in the Gamma-Ray Loud Quasar PKS 1622−297

Abstract We made a high-resolution VLBI observation of the gamma-ray loud quasar PKS 1622$-$297 with the HALCA spacecraft and ground radio telescopes at 5 GHz in 1998 February, almost 3 yr after the source exhibited a spectacular GeV gamma-ray flare. The source shows an elongated structure toward the west on the parsec scale. The visibility data are well modeled by three distinct components: a bright core and two weaker jet components. Comparison with previous observations confirms that the jet components have an apparent superluminal motion up to $12.1 \,h^{-1} \, c$, with the inner jet components having lower superluminal speeds. We applied the inverse Compton catastrophe model and derived a Doppler factor, $\delta$, of 2.45, which is rather lower than those of other gamma-ray loud active galactic nuclei (AGNs), suggesting that the source was in a more quiescent phase at the epoch of our observation. As an alternative probe of the subparsec-scale structure, we also present the results from multiepoch ATCA total flux monitoring, which indicate the presence of persistent intraday variability consistent with refractive interstellar scintillation. We examined the gamma-ray emission mechanism in light of these observations.

Photometric survey of the polar ring galaxy NGC 6822

We have previously established, from a carbon star survey, that the Local Group dwarf irregular galaxy NGC 6822 is much larger than its central bright core. Four MegaCam fields are acquired to survey a 2$^\circ\times$ 2$^\circ$ area centred on NGC 6822 to fully determine its extent and map its stellar populations. Photometry of over one million stars is obtained in the SDSS g$'$, r$'$, i$'$ to three magnitudes below the TRGB. RGB stars, selected from their magnitudes and colours, are used to map the NGC 6822 stellar distribution up to a distance of 60 arcmin. We map the reddening over the whole area. We establish that the stellar outer structure of NGC 6822 is elliptical in shape, with $\epsilon=0.36$ and a major-axis PA = 65$^\circ$, contrasting with the orientation of the HI disk. The density enhancement can be seen up to a semi-major axis of 36$'$ making NGC 6822 as big as the Small Magellanic Cloud. We fit two exponentials to the surface density profile of the spheroid, and identify a bulge with a scale length of 3.85$'$ and an outer spheroid with a scale length of 10.0$'$. We find intermediate-age C stars up to $\sim$ 40$'$ while demonstrating that the SDSS filters are unsuitable to identify extragalactic C stars. NGC 6822 is a unique Local Group galaxy with shape and structure suggesting a polar ring configuration. Radial velocities of carbon stars have indeed demonstrated that there are two kinematical systems in NGC 6822.

The Calm before the Storm: The Link between Quiescent Cavities and Coronal Mass Ejections

Determining the state of the corona prior to CMEs is crucial to understanding and ultimately predicting solar eruptions. A common and compelling feature of CMEs is their three-part morphology, as seen in white-light observations of a bright expanding loop, followed by a relatively dark cavity, and finally a bright core associated with an erupting prominence/filament. This morphology is an important constraint on CME models. It is also quite common for a three-part structure of loop, cavity, and prominence core to exist quiescently in the corona, and this is equivalently an important constraint on models of CME-precursor magnetic structure. These quiescent structures exist in the low corona, primarily below approximately 1.6R� , and so are currently observable in white light duringsolar eclipses, or else by the Mauna Loa Solar Observatory Mk4 coronameter. We present the first comprehensive, quantitative analysis of white-light quiescent cavities as observed by the Mk4 coronameter. We find that such cavities are ubiquitous, as they are the coronal limb counterparts to filament channels observed on the solar disk. We consider examples that range from extremely long-lived, longitudinally extended polar-crown-filament-related cavities to smallercavitiesassociated withfilamentsnearorwithinactiveregions.Theformerareoftenvisiblefordaysandeven weeks at a time and can be identified as long-lived cavities that survive for months. We quantify cavity morphology and intensity contrast properties and consider correlations between these properties. We find multiple cases in which quiescentcavitiesdirectlyeruptintoCMEsandconsiderhowmorphologicalandintensitycontrastpropertiesofthese cases differ from the general population of cavities. Finally, we discuss the implications that these observations may have for the state of the corona just prior to a CME, and more generally for the nature of coronal MHD equilibria.

On the Nature of the X-Ray Bright Core in a Stable Filament Channel

In a search for the cause of the intense heating revealed by X-ray emission in filament channels, we have simulated the evolution of a twisted toroidal flux rope emerging quasi-statically into the corona. Initially, the simulated flux rope remains confined in equilibrium as the stored magnetic energy increases. With enough twist buildup, there is a sudden catastrophic loss of equilibrium and total expulsion of the flux rope. We focused on the quasi-static phase in which a current sheet forms within the flux rope cavity, along the so-called bald-patch separatrix surface (BPSS). This comprises an envelope of field lines that graze the anchoring lower boundary, enclosing the detached helical field that supports the prominence. Significant magnetic energy dissipation and heating are expected to center around such current sheets. The heating that should result provides a plausible explanation for the hot X-ray sources, although they appear to be colocated with cool material. If our physical picture is correct, then the development of X-ray bright cores or sigmoids in a filament channel suggests the presence of a BPSS separating the helical field of a twisted flux rope in stable confinement from the surrounding untwisted fields.