Arm-like Structures Research Articles

Multiple Misaligned Outflows and Warped Accretion Flows in the Proto-multiple System Per-emb-8 and 55

To investigate the formation process of multiple systems, we have analyzed the Atacama Large Millimeter/submillimeter Array archival data of the 1.3 mm continuum, 12CO (2-1) and C18O (2-1) emission in a proto-multiple system consisting of a Class 0 protostar Per-emb-8 and a Class I protobinary Per-emb-55 A and B. The 1.3 mm continuum emission is likely to primarily trace their protostellar disks, and the Keplerian disk rotation is observed in Per-emb-8 and Per-emb-55 A in the emission lines. In Per-emb-8, we identify two arm-like structures with a length of ∼1000 au connecting the eastern and western of its disk in the continuum and C18O emission. Our analysis suggests that these arm-like structures are most likely infalling flows. In the 12CO emission, we discover a second bipolar outflow associated with Per-emb-8. The two bipolar outflows in Per-emb-8 are possibly launched along the normal axes of the misaligned inner and outer parts of its warped protostellar disk. In Per-emb-55, we find that the red- and blueshifted lobes of its bipolar outflow are misaligned by 90°. The presence of the warped disk, multiple misaligned outflows, and asymmetric infalling flows suggest complex dynamics in proto-multiple systems, and these could be related to the tidal interactions between the companions and/or the turbulent environments forming this proto-multiple system.

Plocamiomonas psychrophila gen. et sp. nov. (Pelagophyceae, Heterokontophyta), an Arctic marine nanoflagellate characterized by microscopy, pigments and molecular phylogeny

ABSTRACT During a field campaign in Baffin Bay (June, 1998), a sample dominated by small yellow brown biflagellates was collected from a small pocket of liquid water on the sea ice and established into a unialgal culture. Later it was given strain number CCMP2097 and deposited at the NCMA collection. Growth experiments over a range of temperatures and salinities indicated adaptation to variable Arctic conditions. This strain was studied here using light and transmission electron microscopy, HPLC for characterization of photosynthetic pigments and the relationship to related taxa was elucidated from molecular phylogeny. This integrative approach resulted in suggesting Plocamiomonas psychrophila gen. et sp. nov. the first systematically named Arctic species of the Pelagophyceae. Plocamiomonas had two unequally long flagella and the spherical cells measured 7.5 µm in diameter. Novel morphological characters included (1) a swelling on the mature flagellum, (2) a bi-layered theca, thin elongated arm-like structures emanating from the cell surface and (3) a thin filament above the flagellar transition region. This set of morphological characters represented a novel combination for the Pelagophyceae and together with evidence from the phylogenetic position of Plocamiomonas and eight uncultured cold-water pelagophytes supported the proposal of a new order Plocamiomonadales and a new family Plocamiomonadaceae.

ALMA Spectral Survey of an Eruptive Young Star, V883 Ori (ASSAY). I. What Triggered the Current Episode of Eruption?

An unbiased spectral survey of V883 Ori, an eruptive young star, was carried out with the Atacama Large Millimeter/submillimeter Array in Band 6. The detected line emission from various molecules reveals morphological/kinematical features in both the Keplerian disk and the infalling envelope. A direct infall signature, redshifted absorption against the continuum, has been detected in CO, HCO+, HCN, HNC, and H2CO. HCO+ and SO show large armlike structures that probably connect the infalling envelope to the disk. HCN and H2CO reveal a distinct boundary between the inner and outer disks and tentative spiral structures connecting the outer disk to the inner disk. HNC shows a large central emission hole (r ∼ 0.″3) due to its chemical conversion to HCN at high temperatures. HDO emission, a direct tracer of the water sublimation region, has been detected in the disk. Molecular emission from complex organic molecules is confined within the HDO emission boundary, and HCO+ has an emission hole in its distribution due to its destruction by water. Together, these features suggest that the current episode of eruption in V883 Ori may be triggered by the infall from the envelope to the outer disk, generating a spiral wave that propagates inward and greatly enhances the accretion onto the central star.

A new species of the iconic triradial Ediacaran genus Tribrachidium from Nilpena Ediacara National Park, Flinders Ranges (South Australia)

AbstractTribrachidium heraldicum Glaessner in Glaessner and Daily, 1959 is a triradial Ediacaran organism found in abundance within the Ediacara Member of the Flinders Ranges, South Australia. Here we report and describe a new species within the genus Tribrachidium Glaessner in Glaessner and Daily, 1959: Tribrachidium gehlingi new species from Nilpena Ediacara National Park (NENP), South Australia. Tribrachidium gehlingi n. sp. has low relief and three slightly curved, main arm-like structures that leave a conspicuous gap between the end of the arm-like structures and rim. In place of the ‘bulla’ found on T. herladicum, there are three secondary arm-like structures approximately half of the length of the main arm-like structures. Key morphological differences between the two species are statistically significantly different. Additionally, the species occur together within the same fossiliferous event horizons, indicating that the observed morphological differences are unlikely a result of taphonomy.UUID: http://zoobank.org/196bef68-54da-42a9-9327-dcb55411a457

The Magellanic Edges Survey – III. Kinematics of the disturbed LMC outskirts

ABSTRACT We explore the structural and kinematic properties of the outskirts of the Large Magellanic Cloud (LMC) using data from the Magellanic Edges Survey (MagES) and Gaia EDR3. Even at large galactocentric radii (8° < R < 11°), we find the north-eastern LMC disc is relatively unperturbed: its kinematics are consistent with a disc of inclination ∼36.5° and line-of-nodes position angle ∼145° east of north. In contrast, fields at similar radii in the southern and western disc are significantly perturbed from equilibrium, with non-zero radial and vertical velocities, and distances significantly in front of the disc plane implied by our north-eastern fields. We compare our observations to simple dynamical models of the Magellanic or Milky Way system which describe the LMC as a collection of tracer particles within a rigid potential, and the Small Magellanic Cloud (SMC) as a rigid Hernquist potential. A possible SMC crossing of the LMC disc plane ∼400 Myr ago, in combination with the LMC’s infall to the Milky Way potential, can qualitatively explain many of the perturbations in the outer disc. Additionally, we find the claw-like and arm-like structures south of the LMC have similar metallicities to the outer LMC disc ([Fe/H] ∼ −1), and are likely comprised of perturbed LMC disc material. The claw-like substructure is particularly disturbed, with out-of-plane velocities >60 km s−1 and apparent counter-rotation relative to the LMC’s disc motion. More detailed N-body models are necessary to elucidate the origin of these southern features, potentially requiring repeated interactions with the SMC prior to ∼1 Gyr ago.

Comparison of Two 3D-Printed Indirect Bonding (IDB) Tray Design Versions and Their Influence on the Transfer Accuracy.

Objective: This study aims to investigate the transfer accuracy of two different design versions for 3D-printed indirect bonding (IDB) trays. Materials and Methods: Digital plaster models of 27 patients virtually received vestibular attachments on every tooth using OnyxCeph³™ (Image Instruments, Chemnitz, Germany). Based on these simulated bracket and tube positions, two versions of transfer trays were designed for each dental arch and patient, which differed in the mechanism of bracket retention: Variant one (V1) had arm-like structures protruding from the tray base and reaching into the horizontal and vertical bracket slots, and variant two (V2) had a pocket-shaped design enclosing the brackets from three sides. Both tray designs were 3D-printed with the same digital light processing (DLP) printer using a flexible resin-based material (IMPRIMO® LC IBT/Asiga MAX™, SCHEU-DENTAL, Iserlohn, Germany). Brackets and tubes (discovery® smart/pearl, Ortho-Cast M-Series, Dentaurum, Ispringen, Germany) were inserted into the respective retention mechanism of the trays and IDB was performed on corresponding plaster models. An intraoral scan (TRIOS® 3W, 3Shape, Copenhagen, Denmark) was performed to capture the actual attachment positions and compared to the virtually planned positions with Geomagic© Control (3D Systems Inc., Rock Hill, SC, USA) using a scripted calculation tool, which superimposed the respective tooth surfaces. The resulting attachment deviations were determined in three linear (mesiodistal, vertical and orovestibular) and three angular (torque, rotation and tip) directions and analyzed with a descriptive statistical analysis. A comparison between the two IDB tray designs was conducted using a mixed model analysis (IBM, SPSS® Statistics 27, Armonk, NY, USA). Results: Both design versions of the 3D-printed IDB trays did not differ significantly in their transfer accuracy (p > 0.05). In total, 98% (V1) and 98.5% (V2) of the linear deviations were within the clinically acceptable range of ±0.2 mm. For the angular deviations, 84.9% (V1) and 86.8% (V2) were within the range of ±1°. With V1, most deviations occurred in the mesiodistal direction (3.3%) and in rotation (18%). With V2, most deviations occurred in the vertical direction (3.8%) and in palatinal and lingual crown torque (16.3%). Conclusions: The transfer accuracies of the investigated design versions for 3D-printed IDB trays show good and comparable results albeit their different retention mechanisms for the attachments and are, therefore, both suitable for clinical practice.

Morphological and kinematical analysis of the double-barred galaxy NGC 3504 using ALMA CO (2–1) data

ABSTRACT We present results obtained from ALMA CO (2–1) data of the double-barred galaxy NGC 3504. With three times higher angular resolution (∼0${_{.}^{\prime\prime}}$8) than previous studies, our observations reveal an inner molecular gas bar, a nuclear ring, and four inner spiral arm-like structures in the central 1-kpc region. Furthermore, the CO emission is clearly aligned with the two dust lanes in the outer bar region, with differences in shape and intensity between them. The total molecular gas mass in the observed region (50 arcsec × 57 arcsec) is estimated to be ${\sim}3. 1\times 10^9 \, {\rm M}_{\odot }$, which is 17 per cent of the stellar mass. We used the Kinemetry package to fit the velocity field and found that circular motion strongly dominates at R = 0.3–0.8 kpc, but radial motion becomes important at R < 0.3 kpc and R = 1.0–2.5 kpc, which is expected due to the presence of the inner and outer bars. Finally, assuming that the gas moves along the dust lanes in the bar rotating frame, we derived the pattern speed of the outer bar to be 18 ± 5 km s−1 kpc−1, the average streaming velocities on each of the two dust lanes to be 165 and 221 km s−1, and the total mass inflow rate along the dust lanes to be $12\, {\rm M}_{\odot }$ yr−1. Our results give a new example of an inner gas bar within a gas-rich double-barred galaxy and suggest that the formation of double-barred galaxies could be associated with the existence of such gas structures.

Untangling the Galaxy. I. Local Structure and Star Formation History of the Milky Way

Abstract Gaia DR2 provides unprecedented precision in measurements of the distance and kinematics of stars in the solar neighborhood. Through applying unsupervised machine learning on DR2's 5D data set (3D position + 2D velocity), we identify a number of clusters, associations, and comoving groups within 1 kpc and (many of which have not been previously known). We estimate their ages with the precision of ∼0.15 dex. Many of these groups appear to be filamentary or string-like, oriented in parallel to the Galactic plane, and some span hundreds of parsec in length. Most of these string lack a central cluster, indicating that their filamentary structure is primordial, rather than the result of tidal stripping or dynamical processing. The youngest strings (<100 Myr) are orthogonal to the Local Arm. The older ones appear to be remnants of several other arm-like structures that cannot be presently traced by dust and gas. The velocity dispersion measured from the ensemble of groups and strings increase with age, suggesting a timescale for dynamical heating of ∼300 Myr. This timescale is also consistent with the age at which the population of strings begins to decline, while the population in more compact groups continues to increase, suggesting that dynamical processes are disrupting the weakly bound string populations, leaving only individual clusters to be identified at the oldest ages. These data shed a new light on the local galactic structure and a large-scale cloud collapse.

ALMA High Angular Resolution Polarization Study: An Extremely Young Class 0 Source, OMC-3/MMS 6

Abstract Using the ≈16 km long baseline data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA), we imaged the Stokes I emission and linearly polarized intensity (PI) in the 1.1 mm continuum band of a very young intermediate-mass protostellar source, MMS 6, in the Orion Molecular Cloud-3. The achieved angular resolution, 0.″02 × 0.″03 (≈10 au), shows for the first time a wealth of data on the dust emission polarization in the central 200 au of a protostar. The PI peak is offset to the southeast (SE) by ≈20 au with respect to the Stokes I peak. Its polarization degree is 11% with its E-vector orientation of the position angle ≈135°. A partial ringlike structure with a radius of ≈80 au is detected in PI but not in the Stokes I. Northwest (NW) and SE parts of the ring are bright, with a high polarization degree of ≳10%, and their E-vector orientations are roughly orthogonal to those observed near the center. We also detected an armlike polarized structure, extending to 1000 au scale to the north, with the E-vectors aligned along the minor axis of the structure. We explored possible origins of the polarized emission by comparing them with magnetohydrodynamical simulations of the toroidal wrapping of the magnetic field. The simulations are consistent with the PI emission in the ringlike and the extended armlike structures observed with ALMA. However, the current simulations do not completely reproduce observed polarization characteristics in the central 50 au. Although the self-scattering model can explain the polarization pattern and positional offset between the Stokes I and PI in the central 50 au, this model is not able to reproduce the observed high degree of polarization.

Interpreting observations of edge-on gravitationally unstable accretion flows

Gravitational collapse of molecular cloud or cloud core/clump may lead to the formation of geometrically flattened, rotating accretion flow surrounding the new born star or star cluster. Gravitational instability may occur in such accretion flow when the gas to stellar mass ratio is high (e.g. over $\sim$10\%). This paper takes the OB cluster-forming region G10.6-0.4 as an example. We introduce the enclosed gas mass around its central ultra compact (UC) H\textsc{ii} region, addresses the gravitational stability of the accreting gas, and outline the observed potential signatures of gravitational instability. The position-velocity (PV) diagrams of various molecular gas tracers on G10.6-0.4 consistently show asymmetry in the spatial and the velocity domain. We deduce the morphology of the dense gas accretion flow by modeling velocity distribution of the azimuthally asymmetric gas structures, and by directly de-projecting the PV diagrams. We found that within the 0.3 pc radius, an infall velocity of 1-2 km\,s$^{-1}$ may be required to explain the observed PV diagrams. In addition, the velocity distribution traced in the PV diagrams can be interpreted by spiral arm-like structures, which may be connected with exterior infalling gas filaments. The morphology of dense gas structures we propose appears very similar to the spatially resolved gas structures around the OB cluster-forming region G33.92+0.11 with similar gas mass and size, which however is likely to be approximately in a face-on projection. The dense gas accretion flow around G10.6-0.4 appears to be Toomre unstable, which is consistent with the existence of large-scale spiral arm-like structures, and the formation of localize gas condensations.

The SPHERE view of the planet-forming disk around HD 100546

We image with unprecedented spatial resolution and sensitivity disk features that could be potential signs of planet-disk interaction. Two companion candidates have been claimed in the disk around the young Herbig Ae/Be star HD100546. Thus, this object serves as an excellent target for our investigation of the natal environment of giant planets. We exploit the power of extreme adaptive optics operating in conjunction with the new high-contrast imager SPHERE to image HD100546 in scattered light. We obtain the first polarized light observations of this source in the visible (with resolution as fine as 2 AU) and new H and K band total intensity images that we analyze with the Pynpoint package. The disk shows a complex azimuthal morphology, where multiple scattering of photons most likely plays an important role. High brightness contrasts and arm-like structures are ubiquitous in the disk. A double-wing structure (partly due to ADI processing) resembles a morphology newly observed in inclined disks. Given the cavity size in the visible (11 AU), the CO emission associated to the planet candidate 'c' might arise from within the circumstellar disk. We find an extended emission in the K band at the expected location of 'b'. The surrounding large-scale region is the brightest in scattered light. There is no sign of any disk gap associated to 'b'.

High-Resolution Optical and Near-Infrared Images of the FS Tauri Circumbinary Disk

Abstract We present an $H$-band image of FS Tauri, a 0$''\!\!\!.$2-separated classical T Tauri binary system, taken with the Coronagraphic Imager with Adaptive Optics (CIAO) on the Subaru Telescope. This new image, combined with Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) F606W-band polarimetric images, shows that the binary has complicated circumbinary features, including a circumbinary disk, western and eastern arm-like structures, and two cavities at the northeast and southwest. The circumbinary disk is 630 AU in radius and the southeast side of the disk is bright in the $H$-band. The brightness ratio (southeast$/$northwest) is 1.6$\ \pm\ $0.4. A single Rayleigh-like scattering model indicates that the disk is inclined by 30$^{\circ}$ to 40$^{\circ}$ and that the southeast side corresponds to the near side along our line of sight. The $H$-band surface brightness of the southeast side decreases as $r^{-1.9\pm0.1}$ from 15.2 mag arcsec$^{-2}$ to 16.8 mag arcsec$^{-2}$. The outer portion of the disk is possibly more flared than its inner portion. The weak centro-symmetric polarization pattern and redder F606W $-$$H$ color (4.2 $\ \pm\ $ 0.2 mag) of the southeast side are probably caused by multiple-scattering events from dust grains associated with the binary. The F606W-band image shows the bright northwest side of the disk in contrast with the $H$-band image. The F606W $-$$H$ color of the northwest side is between 1.7 mag and 3.0 mag. We consider that Haro 6-5 B (FS Tauri B), 20$''$ away, produces neutral scattered light from the northwest side. This idea is supported by the polarization pattern of the northwest side, which is centro-symmetric with respect to Haro 6-5 B. The arms appear to encompass the western and eastern cavities, suggesting that the arms $+$ cavity systems are created by a bipolar outflow from the binary. However, the direction of this outflow is inconsistent with that of outflows inferred from the circumbinary disk model. These differences may arise from a misalignment between the circumbinary disk and the circumstellar disks. Another mechanism forming the arms $+$ cavity systems is considered to be the inhomogeneous density distribution of materials in the circumbinary disk.

Signatures of recent star formation in ring S0 galaxies

We present a study of the stellar populations of ring and/or arm-like structures in a sample of S0 galaxies using GALEX far- and near-ultraviolet imaging and SDSS optical data. Such structures are prominent in the UV and reveal recent star formation. We quantitatively characterize these rejuvenation events, estimating the average age and stellar mass of the ring structures, as well as of the entire galaxy. The mass fraction of the UV-bright rings is a few percent of the total galaxy mass, although the UV ring luminosity reaches 70% of the galaxy luminosity. The integrated colors of these S0s locates them in the red sequence (NGC 2962) and in the so-called green valley. We suggest that the star formation episodes may be induced by different triggering mechanisms, such as the inner secular evolution driven by bars, and interaction episodes.

Nearby early-type galaxies with ionized gas: the UV emission from GALEX observations★

We present GALEX far-ultraviolet (FUV, $\lambda_{eff}$=1538 \AA) and near-ultraviolet (NUV, $\lambda_{eff}$=2316 \AA) surface photometry of 40 early-type galaxies (ETGs) selected from a wider sample of 65 nearby ETGs showing emission lines in their optical spectra. We derive FUV and NUV surface brightness profiles, (FUV-NUV) colour profiles and D$_{25}$ integrated magnitudes. We extend the photometric study to the optical {\it r} band from SDSS imaging for 14 of these ETGs. In general, the (FUV-NUV) radial colour profiles become redder with galactocentric distance in both rejuvenated ($\leq 4$ Gyr) and old ETGs. Colour profiles of NGC 1533, NGC 2962, NGC 2974, NGC 3489, and IC 5063 show rings and/or arm-like structures, bluer than the body of the galaxy, suggesting the presence of recent star formation. Although seven of our ETGs show shell systems in their optical image, only NGC 7135 displays shells in the UV bands. We characterize the UV and optical surface brightness profiles, along the major axis, using a Sersic law. The Sersic law exponent, $n$, varies from 1 to 16 in the UV bands. S0 galaxies tend to have lower values of $n$ ($\leq5$). The Sersic law exponent $n=4$ seems to be a watershed: ETGs with $n>4$ tend to have [$\alpha$/Fe] greater than 0.15, implying a short star-formation time scale. We find a significant correlation between the FUV$-$NUV colour and central velocity dispersions $\sigma$, with the UV colours getting bluer at larger $\sigma$. This trend is likely driven by a combined effect of `downsizing' and of the mass-metallicity relation.

Versatility of Trigger Factor Interactions with Ribosome-Nascent Chain Complexes

Trigger factor (TF) is the first molecular chaperone that interacts with nascent chains emerging from bacterial ribosomes. TF is a modular protein, consisting of an N-terminal ribosome binding domain, a PPIase domain, and a C-terminal domain, all of which participate in polypeptide binding. To directly monitor the interactions of TF with nascent polypeptide chains, TF variants were site-specifically labeled with an environmentally sensitive NBD fluorophore. We found a marked increase in TF-NBD fluorescence during translation of firefly luciferase (Luc) chains, which expose substantial regions of hydrophobicity, but not with nascent chains lacking extensive hydrophobic segments. TF remained associated with Luc nascent chains for 111 +/- 7 s, much longer than it remained bound to the ribosomes (t((1/2)) approximately 10-14 s). Thus, multiple TF molecules can bind per nascent chain during translation. The Escherichia coli cytosolic proteome was classified into predicted weak and strong interactors for TF, based on the occurrence of continuous hydrophobic segments in the primary sequence. The residence time of TF on the nascent chain generally correlated with the presence of hydrophobic regions and the capacity of nascent chains to bury hydrophobicity. Interestingly, TF bound the signal sequence of a secretory protein, pOmpA, but not the hydrophobic signal anchor sequence of the inner membrane protein FtsQ. On the other hand, proteins lacking linear hydrophobic segments also recruited TF, suggesting that TF can recognize hydrophobic surface features discontinuous in sequence. Moreover, TF retained significant affinity for the folded domain of the positively charged, ribosomal protein S7, indicative of an alternative mode of TF action. Thus, unlike other chaperones, TF appears to employ multiple mechanisms to interact with a wide range of substrate proteins.

Chemical enrichment in the cluster of galaxies Hydra A

We analyzed global properties, radial profiles, and 2D maps of the metal abundances and temperature in the cool core cluster of galaxies Hydra A using a deep ~120 ks XMM-Newton exposure. The best fit among the available spectral models is provided by a Gaussian distribution of the emission measure (gdem). We can accurately determine abundances for 7 elements in the cluster core with EPIC (O, Si, S, Ar, Ca, Fe, Ni) and 3 elements (O, Ne, Fe) with RGS. The gdem model gives lower Fe abundances than a single-temperature model. Based on this, we explain why simulations show that the best-fit Fe abundance in clusters with intermediate temperatures is overestimated. The abundance profiles for Fe, Si, S, but also O are centrally peaked. Combining the Hydra A results with 5 other clusters for which detailed chemical abundance studies are available, we find a significant decrease in O with radius, while the increase in the O/Fe ratio with radius is small within 0.1 r200, where the O abundances can be accurately determined, with d. We compare the observed abundance ratios with the mixing of various supernova type Ia and core-collapse yield models in different relative amounts. Producing the estimated O, Si, and S peaks in Hydra A requires either the amount of metals ejected by stellar winds to be 3–8 times higher than predicted by available models or the initial enrichment by core-collapse supernovae in the protocluster phase not to be as well mixed on large scales as previously thought. The temperature map shows cooler gas extending in arm-like structures towards the north and south. These structures, and especially the northern one, appear to be richer in metals than the ambient medium and spatially correlated with the large-scale radio lobes. With different sets of assumptions, we estimate the mass of cool gas, which was probably uplifted by buoyant bubbles of relativistic plasma produced by the AGN, to , and the energy associated with this uplift to erg. The best estimate of the mass of Fe uplifted together with the cool gas is , 15% of the total mass of Fe in the central 0.5′ region.

Ram-pressure stripped molecular gas in the Virgo spiral galaxy NGC 4522

IRAM 30 m 12CO(1–0) and 12CO(2–1) HERA observations are presented for the ram-pressure stripped Virgo spiral galaxy NGC 4522. The CO emission is detected in the galactic disk and the extraplanar gas. The extraplanar CO emission follows the morphology of the atomic gas closely but is less extended. The CO maxima do not appear to correspond to regions where there is peak massive star formation as probed by Hα emission. The presence of molecular gas is a necessary but not sufficient condition for star formation. Compared to the disk gas, the molecular fraction of the extraplanar gas is 30% lower and the star formation efficiency of the extraplanar gas is about 3 times lower. The comparison with an existing dynamical model extended by a recipe for distinguishing between atomic and molecular gas shows that a significant part of the gas is stripped in the form of overdense arm-like structures. It is argued that the molecular fraction depends on the square root of the total large-scale density. Based on the combination of the CO/Hα and an analytical model, the total gas density is estimated to be about 4 times lower than that of the galactic disk. Molecules and stars form within this dense gas according to the same laws as in the galactic disk, i.e. they mainly depend on the total large-scale gas density. Star formation proceeds where the local large-scale gas density is highest. Given the complex 3D morphology this does not correspond to the peaks in the surface density. In the absence of a confining gravitational potential, the stripped gas arms will most probably disperse; i.e. the density of the gas will decrease and star formation will cease.

Molecular Line Observations of IRAM 04191+1522

We have mapped the CO, HCO+, CS, and N2H+ emission around a low-luminosity Class 0 source IRAM 04191+1522 in the Taurus molecular cloud using the Berkeley-Illinois-Maryland Association array. A bipolar molecular outflow is seen in CO, HCO+, and CS emission originating from around the IRAM source, while a flattened envelope is seen in N2H+ emission surrounding the waist of the molecular outflow around the IRAM source. Outflow, rotation, and probably infall are seen around the Class 0 source, indicating a complicated context for star formation in the earliest stages. The N2H+ envelope is a thick clumpy torus with a tenuous outer part and a ringlike dense inner edge. The dense inner edge has a mean radius of 10'', or 1400 AU. The outer part of the torus exhibits differential rotation, and the infalling material appears to carry angular momentum inward toward the central source. The region surrounded by the inner edge of the torus may have a solid-body rotation. The envelope may result from the collapse of a rotating, magnetized toroid toward the central source. Outflow motion is seen in the outer part of the inner edge of the torus, probably due to an interaction with the molecular outflow. Two armlike structures are seen extending out from the inner edge in the torus. One of them is clearly seen with the velocity increasing roughly linearly with the distance. The HCO+ emission around the source may trace the central core around the source, showing a velocity structure connecting to that of the outer part of the torus. The molecular outflow is best seen in CO. It is bipolar with both a southwest and a northeast lobe. Two internal structures are seen along the main outflow axis within the lobes: (1) a strong CO emission at ~20'' to the northeast of the IRAM source, likely tracing an internal bow shock driven by a collimated, episodic jet and (2) a linear HCO+ structure of ~50'' length at low blueshifted velocity to the southwest of the IRAM source, requiring some interaction with a jet to be produced. In addition, a dense condensation is also seen in HCO+ and CS to the south of the IRAM source, likely representing a recent strong outflow interaction.

The modular structure of Escherichia coli threonyl-tRNA synthetase as both an enzyme and a regulator of gene expression.

In addition to its role in tRNA aminoacylation, Escherichia coli threonyl-tRNA synthetase is a regulatory protein which binds a site, called the operator, located in the leader of its own mRNA and inhibits translational initiation by competing with ribosome binding. This work shows that the two essential steps of regulation, operator recognition and inhibition of ribosome binding, are performed by different domains of the protein. The catalytic and the C-terminal domain of the protein are involved in binding the two anticodon arm-like structures in the operator whereas the N-terminal domain of the enzyme is responsible for the competition with the ribosome. This is the first demonstration of a modular structure for a translational repressor and is reminiscent of that of transcriptional regulators. The mimicry between the operator and tRNA, suspected on the basis of previous experiments, is further supported by the fact that identical regions of the synthetase recognize both the operator and the tRNA anticodon arm. Based on these results, and recent structural data, we have constructed a computer-derived molecular model for the operator-threonyl-tRNA synthetase complex, which sheds light on several essential aspects of the regulatory mechanism.

Chandra Observations of NGC 4636--an Elliptical Galaxy in Turmoil

Chandra images show symmetric, 8 kpc long, arm-like features in the X-ray halo surrounding NGC 4636. The leading edges of these features are sharp and are accompanied by temperature increases of ~30%. These properties, along with their scale and symmetry, suggest that the arm-like structures are produced by nuclear outburst driven shocks. We interpret these observations as part of a cycle in which cooling gas originally fueled a nuclear outburst about 3 X 10^{6} years ago leading to shocks that reheat the cooling gas, thus preventing the accumulation of significant amounts of cooled gas in the galaxy center and temporarily starving the central AGN.