We present the results of an extensive Arecibo observational survey of magnetic field strengths in the inter-core regions of molecular clouds to determine their role in the evolution and collapse of ...molecular clouds as a whole. Sensitive 18 cm OH Zeeman observations of absorption lines from Galactic molecular gas in the direction of extragalactic continuum sources yielded 38 independent measurements of magnetic field strengths. Zeeman detections were achieved at the 3 level toward nine clouds, while the others revealed sensitive upper limits to the magnetic field strength. Our results suggest that total field strengths in the inter-core regions of giant molecular clouds (GMCs) are about 15 G.
Magnetic fields have an important role in the evolution of interstellar medium and star formation
. As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse ...owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas
. Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the H I narrow self-absorption (HINSA)
towards L1544
-a well-studied prototypical prestellar core in an early transition between starless and protostellar phases
characterized by a high central number density
and a low central temperature
. A combined analysis of the Zeeman measurements of quasar H I absorption, H I emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the diffuse CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes
.
Abstract
Measurements of starlight polarized by aligned interstellar dust grains are used to probe the relation between the orientation of the ambient interstellar magnetic field (ISMF) and the ISMF ...traced by the ribbons of energetic neutral atoms discovered by the Interstellar Boundary Explorer spacecraft. We utilize polarization data, many acquired specifically for this study, to trace the configuration of the ISMF within 40 pc. A statistical analysis yields a best-fit ISMF orientation,
B
magpol
, aligned with Galactic coordinates
ℓ
= 42°,
b
= 49°. Further analysis shows the ISMF is more orderly for “downfield” stars located over 90° from
B
magpol
. The data subset of downfield stars yields an orientation for the nearby ISMF at ecliptic coordinates
λ
,
β
≈ 219° ± 15°, 43° ± 9° (Galactic coordinates
l
,
b
≈ 40°, 56°, ±17°). This best-fit ISMF orientation from polarization data is close to the field direction obtained from ribbon models. This agreement suggests that the ISMF shaping the heliosphere belongs to an extended ordered magnetic field. Extended filamentary structures are found throughout the sky. A previously discovered filament traversing the heliosphere nose region, “Filament A,” extends over 300° of the sky, and crosses the upwind direction of interstellar dust flowing into the heliosphere. Filament A overlaps the locations of the Voyager kilohertz emissions, three quasar intraday variables, cosmic microwave background (CMB) components, and the inflow direction of interstellar grains sampled by Ulysses and Galileo. These features are likely located in the upstream outer heliosheath where ISMF drapes over the heliosphere, suggesting Filament A coincides with a dusty magnetized plasma. A filament 55° long is aligned with a possible shock interface between local interstellar clouds. A dark spot in the CMB is seen within 5° of the filament and within 10° of the downfield ISMF direction. Two large magnetic arcs are centered on the directions of the heliotail. The overlap between CMB components and the aligned dust grains forming Filament A indicates the configuration of dust entrained in the ISMF interacting with the heliosphere provides a measurable foreground to the CMB.
The Disruption and Fueling of M33 Putman, M. E; Peek, J. E. G; Muratov, A ...
The Astrophysical journal,
10/2009, Letnik:
703, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The disruption of the M33 galaxy is evident from its extended gaseous structure. We present new data from the Galactic Arecibo L-Band Feed Array H I (GALFA-H I) Survey that show the full extent and ...detailed spatial and kinematic structure of M33's neutral hydrogen. Over 18% of the H I mass of M33 ( M ) is found beyond the star-forming disk as traced in the far-ultraviolet (FUV). The most distinct features are extended warps, an arc from the northern warp to the disk, diffuse gas surrounding the galaxy, and a southern cloud with a filament back to the galaxy. The features extend out to 22 kpc from the galaxy center (18 kpc from the edge of the FUV disk), and the gas is directly connected to M33's disk. Only five discrete clouds (i.e., gas not directly connected to M33 in position-velocity space) are cataloged in the vicinity of M33, and these clouds show similar properties to Galactic and M31 halo clouds. M33's gaseous features most likely originate from the tidal disruption of M33 by M31 1-3 Gyr ago as shown by an orbit analysis which results in a tidal radius <15 kpc in the majority of M33's possible orbits. M33 is now beyond the disruptive gravitational influence of M31, and the gas appears to be returning to M33's disk and redistributing its star formation fuel. M33's high mean velocity dispersion in the disk (~18.5 km s-1) may also be consistent with the previous interaction and high rate of star formation. M33 will either exhaust its star formation fuel in the next few Gyrs or eventually become star formation fuel for M31. The latter represents the accretion of a large gaseous satellite by a spiral galaxy, similar to the Magellanic Clouds'relationship to the Galaxy.
We search for fast-expanding H I shells associated with Galactic supernova remnants (SNRs) in the longitude range l approximately 32degrees to 77degrees using 21 cm line data from the Inner-Galaxy ...Arecibo L-band Feed Array (I-GALFA) H I survey. Among the 39 known Galactic SNRs in this region, we find such H I shells in 4 SNRs: W44, G54.4-0.3, W51C, and CTB 80. All four were previously identified in low-resolution surveys, and three of those (excluding G54.4-0.3) were previously studied with the Arecibo telescope. A remarkable new result, however, is the detection of H I emission at both very high positive and negative velocities in W44 from the receding and approaching parts of the H I expanding shell, respectively. This is the first detection of both sides of an expanding shell associated with an SNR in H I 21 cm emission. The high-resolution I-GALFA survey data also reveal a prominent expanding H I shell with high circular symmetry associated with G54.4-0.3. We explore the physical characteristics of four SNRs and discuss what differentiates them from other SNRs in the survey area. We conclude that these four SNRs are likely the remnants of core-collapse supernovae interacting with a relatively dense (gap1 cm super(-3)) ambient medium, and we discuss the visibility of SNRs in the H I 21 cm line.
The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting ...all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.
ABSTRACT The Local Leo Cold Cloud (LLCC, at a distance of 11-24 pc) was studied in its relation to the Local Hot Bubble (LHB) and the result suggested that much of the observed keV emission in that ...direction originates in front of the cloud. This placed a strong constraint on the distribution of X-ray emission within the LHB and called into question the assumption of a uniform distribution of X-ray emitting plasma within the Local Cavity. However, recent work has quantified the contribution of heliospheric solar wind charge exchange (SWCX) emission to the diffuse X-ray background measured by the ROSAT All-Sky Survey (RASS) at keV, and led to the consistency of pressure measurements between the LHB and the local cloud component of the complex of local interstellar clouds (CLICs) surrounding the Sun. In this paper we revisit the LLCC and improve the previous analysis by using higher resolution RASS data, a serendipitous ROSAT pointed observation, a rigorous treatment of the band-averaged X-ray absorption cross section, and models for the heliospheric and magnetospheric SWCX contributions. We find that the foreground emission to the cloud is in excess of the expected heliospheric (interplanetary plus near Earth) SWCX contribution but that it is marginally consistent with the range of possible LHB plasma path lengths between the LLCC and the CLICs given the currently understood plasma emissivity.
The total electron content (TEC) of the solar corona in June 2002 is calculated by three observational techniques and the results are compared. The first technique is solar rotational tomography ...(SRT) applied to a 14-day time series of LASCO-C2 polarized brightness images, and the other two techniques use the
Cassini
spacecraft radio beacon for Doppler tracking (phase delay) and ranging (group delay). While the Doppler-tracking technique has an arbitrary zero-point, it is otherwise found that the three methods produce consistent estimates of the TEC to within established uncertainties, providing an independent check on the calibrations. The verification of the accuracy of the Doppler-tracking technique enables a significant improvement to the use of spacecraft data sets in studying the heliosphere: the density component to Faraday rotation can be separated from the magnetic-field component as variable structures cross, such as coronal mass ejections and magnetohydrodynamic waves. Furthermore, we show that the unique frequency-time variable characteristics of the hydrodynamic components of waves can be studied. Based on this work, future Faraday rotation studies of variable solar phenomena will isolate the electron density changes from the magnetic-field contribution. This capability will enable advanced research into variable heliospheric magnetic fields.
The dark cloud Lynds 1622 is one of a few specific sites in the Galaxy where, relative to observed free-free and vibrational dust emission, there is a clear excess of microwave emission. In order to ...constrain models for this microwave emission, and to better establish the contribution which it might make to ongoing and near-future microwave background polarization experiments, we have used the Green Bank Telescope to search for linear polarization at 9.65 Ghz toward Lynds 1622. We place a 95.4% upper limit of 88 is a subset of K (123 is a subset of K at 99.7% confidence) on the total linear polarization of this source averaged over a 13 FWHM beam. Relative to the observed level of anomalous emission in Stokes I these limits correspond to fractional linear polarizations of 2.7% and 3.5%.
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