Northern Streamer Research Articles

Cloudlet capture model for the accretion streamer onto the disc of DG Tau

ABSTRACT DG Tau is a nearby T Tauri star associated with a collimated jet, a circumstellar disc, and a streamer a few hundred au long. The streamer connects to the disc at ∼50 au from DG Tau. At this location SO emission is observed, likely due to the release of sulphur from dust grains caused by the shock of the impact of the accretion streamer onto the disc. We investigate the possibility that the DG Tau streamer was produced via cloudlet capture on the basis of hydrodynamic simulations, considering a cloudlet initiating infall at 600 au from DG Tau with low angular momentum so that the centrifugal force is smaller than the gravitational force, even at 50 au. The elongation of the cloudlet into a streamer is caused by the tidal force when its initial velocity is much less than the free-fall velocity. The elongated cloudlet reaches the disc and forms a high-density gas clump. Our hydrodynamic model reproduces the morphology and line-of-sight velocity of CS (5 − 4) emission from the northern streamer observed with Atacama Large Millimeter/submillimeter Array. We discuss the conditions for forming a streamer based on the simulations. We also show that the streamer should perturb the disc after impact for several thousands of years.

The JCMT Gould Belt Survey: A First Look at IC 5146

Abstract We present 450 and 850 μm submillimeter continuum observations of the IC 5146 star-forming region taken as part of the James Clerk Maxwell Telescope Gould Belt Survey. We investigate the location of bright submillimeter (clumped) emission with the larger-scale molecular cloud through comparison with extinction maps, and find that these denser structures correlate with higher cloud column density. Ninety-six individual submillimeter clumps are identified using FellWalker, and their physical properties are examined. These clumps are found to be relatively massive, ranging from 0.5 to 116 with a mean mass of 8 and a median mass of 3.7 . A stability analysis for the clumps suggests that the majority are (thermally) Jeans stable, with . We further compare the locations of known protostars with the observed submillimeter emission, finding that younger protostars, i.e., Class 0 and I sources, are strongly correlated with submillimeter peaks and that the clumps with protostars are among the most Jeans unstable. Finally, we contrast the evolutionary conditions in the two major star-forming regions within IC 5146: the young cluster associated with the Cocoon Nebula and the more distributed star formation associated with the Northern Streamer filaments. The Cocoon Nebula appears to have converted a higher fraction of its mass into dense clumps and protostars, the clumps are more likely to be Jeans unstable, and a larger fraction of these remaining clumps contain embedded protostars. The Northern Streamer, however, has a larger number of clumps in total and a larger fraction of the known protostars are still embedded within these clumps.

DECONVOLUTION OF IMAGES FROM BLAST 2005: INSIGHT INTO THE K3-50 AND IC 5146 STAR-FORMING REGIONS

We present an implementation of the iterative flux-conserving Lucy-Richardson (L-R) deconvolution method of image restoration for maps produced by the Balloon-borne Large Aperture Submillimeter Telescope (BLAST). We have analyzed its performance and convergence extensively through simulations and cross-correlations of the deconvolved images with available highresolution maps. We present new science results from two BLAST surveys, in the Galactic regions K3-50 and IC 5146, further demonstrating the benefits of performing this deconvolution. We have resolved three clumps within a radius of 4.'5 inside the star-forming molecular cloud containing K3-50. Combining the well-resolved dust emission map with available multi-wavelength data, we have constrained the Spectral Energy Distributions (SEDs) of five clumps to obtain masses (M), bolometric luminosities (L), and dust temperatures (T). The L-M diagram has been used as a diagnostic tool to estimate the evolutionary stages of the clumps. There are close relationships between dust continuum emission and both 21-cm radio continuum and 12CO molecular line emission. The restored extended large scale structures in the Northern Streamer of IC 5146 have a strong spatial correlation with both SCUBA and high resolution extinction images. A dust temperature of 12 K has been obtained for the central filament. We report physical properties of ten compact sources, including six associated protostars, by fitting SEDs to multi-wavelength data. All of these compact sources are still quite cold (typical temperature below ~ 16 K) and are above the critical Bonner-Ebert mass. They have associated low-power Young Stellar Objects (YSOs). Further evidence for starless clumps has also been found in the IC 5146 region.

Molecular Excitation and Differential Gas‐Phase Depletions in the IC 5146 Dark Cloud

We present a combined near-infrared and molecular line study of a 25' × 8' area in the northern streamer of the IC 5146 cloud. Using the technique pioneered by Lada and coworkers, we construct a Gaussian-smoothed map of the infrared extinction with the same resolution as the molecular line observations in order to examine correlations of integrated intensities and molecular abundances with extinction for C17O, C34S, and N2H+. We find that over a visual extinction range of 0-40 mag, there is good evidence for the presence of differential gas-phase depletions in the densest portions of IC 5146. Both CO and CS exhibit a statistically significant (factor of ~3) abundance reduction near AV ~ 12 mag, while, in direct contrast, at the highest extinctions (AV > 10 mag), N2H+ appears relatively undepleted. Moreover, for AV < 4 mag, there exists little or no N2H+. This pattern of depletions is consistent with the predictions of chemical theory. Through the use of a time- and depth-dependent chemical model, we show that the near-uniform or rising N2H+ abundance with extinction is a direct result of a reduction in its destruction rate at high extinction because of the predicted and observed depletion of CO molecules. The observed abundance threshold for N2H+, A ~ 4 mag, is examined in the context of this same model, and we demonstrate how this technique can be used to test the predictions of depth-dependent chemical models. Finally, we find that cloud density gradients can have a significant effect on the excitation and detectability of high dipole moment molecules, which are typically far from local thermodynamic equilibrium. Density gradients also cause chemical changes since reaction rates and depletion timescales are density-dependent. Accounting for such density/excitation gradients is crucial to a correct determination and proper interpretation of molecular abundances.

Kuiper Widefield Infrared Camera Far‐Infrared Imaging of the Galactic Center: The Circumnuclear Disk Revealed

We present images of the Galactic center in the dust continuum at 31.5 and 37.7 μm obtained with the Kuiper Widefield Infrared Camera on the Kuiper Airborne Observatory. The raw images have a spatial resolution of ~85 and cover a region roughly 10' (R.A.) by 24' (decl.) in extent. Here we discuss the innermost 28 × 36 (6.9 × 9.1 pc) regions roughly centered on Sagittarius A*, where the high signal-to-noise ratio permits image restoration to ~36-57 spatial resolution at 31.5 and 37.7 μm, respectively. These images clearly delineate the warm dust in the but also show an elliptical structure centered on Sgr A* and intersecting the minispiral at the western arc that we identify as the photodissociated inner rim of the circumnuclear disk (CND) or ring. This is the first image of the complete disk and the first image of both the minispiral and CND in a single tracer, thereby permitting detailed studies of the intimate association between these two structures. The 31.5/37.7 μm color temperature map indicates that most of the far-UV flux required to heat the dust grains must come from centrally located sources. The cluster of He I emission-line stars recently identified near Sgr A* is sufficient to provide half the heating for the far-IR ring and minispiral structures, the balance likely being provided by O and B stars associated with the cluster. We also find ~16 color temperature peaks distributed within the CND that we suggest are the locations of early type (B4 to O9) main-sequence stars. To match the observed far-IR fluxes from the ring, we require an unusually high 30-40 μm dust grain emissivity and/or an usually high UV albedo. The far-IR morphology is reproduced by a simple model: an inclined (i ~ 65°), slightly elliptical (e ~ 0.06) torus with one focus at Sgr A* and two streamers on parabolic orbits with foci at Sgr A*. The torus has an inner radius of 1.58 pc, is ~0.4 pc thick, and consists of clumpy cloudlets with characteristic sizes less than 0.15 pc. The northern streamer is identified with the northern arm, and the east-west (EW) streamer is identified with the bar and eastern arm structures seen in both their [Ne II] fine-structure line emission and the radio continuum. The northern arm is traced in the far-IR continuum from regions ~1.4 pc outside of the far-IR ring to its apparent intersection with the EW streamer. The eastern CND is not detected in the radio continuum because of extinction by the intervening northern arm, which therefore must be nearly in the plane of the CND. Extensions of the EW streamer outside of the CND are apparent to the east and especially to the northwest. The EW streamer lies ~85° out of the plane of the CND and is on a parobolic orbit focused on Sgr A* at a distance of 0.33 pc, with its apex well in front of or behind Sgr A*.

Observation of sectored structure in the outer solar corona: Correlation with interplanetary magnetic field

Daily images of the white light corona between 3 and 10 R⊗ have been recorded by a coronagraph aboard the OSO-7 unmanned satellite since October 3, 1971. Images for the years 1972 and 1973 have been examined for persistent coronal forms. For most of 1972 there passed over the Sun's east limb a regular alternation of northern and southern streamers separated frequently by equatorial fans. The alternation suggested the rotation of a stable four-sectored coronal structure produced by two northern streamers, 180° apart in longitude and a similar pair of southern streamers shifted 90° in longitude. Toward the end of 1972 this structure evolved into a two-sectored structure produced by a single northern streamer and a single southern streamer separated by 180° in longitude. This structure remained stable during most of 1973. Transition from a northern to southern streamer, converted to Earth passage dates, correlated with the passage of a -/+ sector boundary in the interplanetary magnetic field. Conversely, the transition from a southern to northern streamer was associated with a +/-boundary passage. These correlations support the recent observations of Hansen et al. (1973).

The Aurora Borealis of Nov. 9 and 10

THERE have been two magnificent auroral displays on the nights of the 9th and 10th inst. That on the 9th commenced at 10 o'clock, and continued with little interruption until 12.45; and last night from 9.40 until 12 o'clock. Both displays were in the north and north-west, and at times the streamers reached the zenith, but I did not observe them to pass beyond that point. The colours were varied; at one time of a beautiful crimson, at another a greenish white. Last night's display was the most interesting, but not so brilliant as that of the previous night. The aurora made its first appearance by an undefined redness in the north; it then gradually developed into a crimson, and assumed the shape of a vertical pillar, the upper part tapering to a clearly defined point, within a few degrees of the zenith. It remained in this shape and position for two minutes, and then faded away. At 10.15 there appeared, at about 10 degrees above the horizon, a peculiar lightness, like the edge of a dark horizontal cloud illuminated by the hidden moon, but I could distinctly discern some stars below the illuminated stratum, which proves that the cloud was transparent; the stars could not, however, be seen through the lightness. At 10.40 there were three distinct streamers shooting up from this light, emanating from separate parts, but all in the north and north-west. They then assumed an easterly movement, the right hand streamer before disappearing being in the north-east. The centre one of these was of a very light colour, approaching a faint or whitish green; the others were crimson. At 11 o'clock I saw an exceedingly brilliant patch undefined in the north-east; by this time some clouds, stratified horizontally, rose from the northern horizon and passed into the light part of the heavens, which seemed to influence the display by intensifying the streamers, which were shooting up, at this time, to the zenith. At 11.30 I saw six beams start across east and west, of a whitish colour with dark spaces between, and the southern one in the zenith. The northern streamer now disappeared, but the auroral twilight was still visible, although gradually fading, and by 12 o'clock all was darkness. I did not continue my observation beyond this hour, the temperature not being conducive to personal comfort.