Anuj Pratim Sarma

Very Long Baseline Array Observations of the Zeeman Effect in H2O Masers in W3 IRS 5

We present Very Long Baseline Array (VLBA) observations of the Zeeman effect in H2O masers in W3 IRS 5. These observations have yielded the first ultrahigh spatial resolution maps of the magnetic field in a star-forming region measured via the Zeeman effect—masers for which we can independently measure the magnetic field are separated by ≤25 mas (60 AU). This is the first VLBA detection of the Zeeman effect in H2O masers in a star-forming region. These observations also remove the confusion in observed magnetic fields from closely spaced masers. We shall explicitly demonstrate that observations at lower resolution can yield lower magnetic fields than with the VLBA, even though there are no field reversals within the larger beam.

Magnetic Fields in Shocked Regions: Very Large Array Observations of H2O Masers

We present VLA observations of the Zeeman effect in 22 GHz H2O masers in several high-mass star-forming regions. These masers are believed to arise from collisional pumping in postshock environments. Therefore, the Zeeman effect data provide the most direct measurements of magnetic field strengths in high-density (n 108 cm-3) postshock gas, where the field energy determines other physical conditions. Our observations yield significant magnetic field detections in W3 IRS 5, W3(OH), W49 N, and OH 43.8-0.1. In these sources, we detect line-of-sight field strengths ranging from 13 to 49 mG. For some regions, the detected fields provide a 2-3 point sampling of the magnetic field, indicating the nature of field variations on arcsecond scales. These field strengths are consistent with a shock-driven maser model having relatively low speed (20 km s-1), C-type shocks. We examine the balance between magnetic field energy and turbulent kinetic energy in the masing regions. These energies appear close to equilibrium.

Australia Telescope Observations of the CTB 33 Complex

We present high-resolution radio continuum, H I absorption, and H90α recombination line observations of the CTB 33 complex and nearby sources (l = 337°, b = 0°) taken with the Australia Telescope Compact Array. We estimate distances to several discrete sources on the basis of H I absorption spectra. Radio recombination lines are detected for two sources in the field. The compact source G337.0-0.1 in the CTB 33 complex, whose nature was previously in doubt, is identified as a supernova remnant with a diameter of ~5 pc. This classification is based on several contributing features: a nonthermal spectral index (-0.6), a shell structure, and the absence of detectable recombination lines. In addition, a 1720 MHz OH maser is probably associated with G337.0-0.1. We conclude that the CTB 33 region is a supernova remnant-H II complex at a distance of ~11 kpc.

VLA OH and H I Zeeman Observations of the NGC 6334 Complex

We present OH and H I Zeeman observations of the NGC 6334 complex taken with the Very Large Array. The OH absorption profiles associated with the complex are relatively narrow (ΔvFWHM ~ 3 km s-1) and single-peaked over most of the sources. The H I absorption profiles contain several blended velocity components. One of the compact continuum sources in the complex (source A) has a bipolar morphology. The OH absorption profiles toward this source display a gradient in velocity from the northern continuum lobe to the southern continuum lobe; this velocity gradient likely indicates a bipolar outflow of molecular gas from the central regions to the northern and southern lobes. Magnetic fields of the order of 200 μG have been detected toward three discrete continuum sources in the complex. Virial estimates suggest that the detected magnetic fields in these sources are of the same order as the critical magnetic fields required to support the molecular clouds associated with the sources against gravitational collapse.

VLA H I Zeeman Observations of Centaurus A

We present H I absorption observations toward the nucleus and neighboring radio features of Centaurus A, with the aim of studying the magnetic field using the Zeeman effect. The H I profiles toward the nucleus reveal for the first time a broad absorption component that extends up to 635 km s-1 and is centered near 583 km s-1. This broad component may be a blend of absorption in several clouds located in the ~100 pc radius circumnuclear ring of Cen A. We set upper limits on the line-of-sight magnetic field (Blos) in Cen A in the regions traced by the H I absorption components. The best value (fit ± 1 σ error) for Blos in the dust lane is -3 ± 7 μG. For the redshifted narrow line width components observed toward the nucleus, Blos is ~10 ± 20 μG. If we assume the spin temperature Ts ≤ 50 K in these redshifted clouds, our analysis shows that the magnetic fields will be energetically significant if Blos ≥ 5 μG for cloud sizes around a few parsecs or larger.

DETECTION OF THE ZEEMAN EFFECT IN THE 36 GHz CLASS I CH3OH MASER LINE WITH THE EVLA

We report the first detection of the Zeeman effect in the 36 GHz Class I methanol maser line. The observations were carried out with 13 antennas of the EVLA toward the high mass star forming region M8E. Based on our adopted Zeeman splitting factor of

Gaussian Spectral Line Profiles of Astrophysical Masers

z = 1.7 Hz/mG, we detect a line of sight magnetic field of -31.3 +/- 3.5 mG and 20.2 +/- 3.5 mG to the northwest and southeast of the maser line peak respectively. This change in sign over a 1300 AU size scale may indicate that the masers are tracing two regions with different fields, or that the same field curves across the regions where the masers are being excited. The detected fields are not significantly different from the magnetic fields detected in the 6.7 GHz Class II methanol maser line, indicating that methanol masers may trace the large scale magnetic field, or that the magnetic field remains unchanged during the early evolution of star forming regions. Given what is known about the densities at which 36 GHz methanol masers are excited, we find that the magnetic field is dynamically significant in the star forming region.

Discovery of the Zeeman Effect in the 44 GHz Class I Methanol (CH3OH) Maser Line

Calculations are performed to demonstrate the deviations from Gaussian that occur in the spectral line profiles of a linear maser as a result of the amplification process. Near-Gaussian profiles are presented for bright, interstellar 22 GHz water masers obtained from high-resolution Very Long Baseline Array observations of W3 IRS 5. For the profiles to be so close to Gaussian, the calculations indicate that these masers must originate in quite hot gas with temperatures greater than 1200 K—a conclusion that is supportive of C-type shocks as the origin of these masers. In addition, the degree of saturation of these masers must be less than approximately one-third, from which it follows that the beaming angles are less than about 10-4 sr and the actual luminosities are modest. If spectral profiles that are as close to Gaussian as the profiles presented in this initial investigation are found to occur widely, they can be valuable diagnostics for the environments of astrophysical masers.

Optically thick [O I] and [C II] emission toward NGC 6334A

We report the discovery of the Zeeman effect in the 44?GHz Class I methanol (CH3OH) maser line. The observations were carried out with 22 antennas of the Expanded Very Large Array toward a star-forming region in OMC-2. Based on our adopted Zeeman splitting factor of z = 1.0 Hz mG-1, we detect a line-of-sight magnetic field of 18.4 ? 1.1 mG toward this source. Since such 44?GHz CH3OH masers arise from shocks in the outflows of star-forming regions, we can relate our measurement of the post-shock magnetic field to field strengths indicated by species tracing pre-shock regions, and thus characterize the large-scale magnetic field. Moreover, since Class I masers trace regions more remote from the star-forming core than Class II masers, and possibly earlier phases, magnetic fields detected in 6.7?GHz Class II and 36 and 44?GHz Class I methanol maser lines together offer the potential of providing a more complete picture of the magnetic field. This motivates further observations at high angular resolution to find the positional relationships between Class I and Class II masers, and masers at various frequencies within each category. In particular, CH3OH masers are widespread in high- as well as intermediate-mass star-forming regions, and our discovery provides a new method of studying the magnetic field in such regions, by observing small physical scales that are not accessible by any other lines.

VLBA Observations of the Zeeman Effect in H2O Masers in OH 43.8-0.1

This work focuses on [O I] and [C II] emission toward NGC 6334A, an embedded H+ region/PDR only observable at infrared or longer wavelengths. A geometry in which nearly all the emission escapes out the side of the cloud facing the stars, such as Orion, is not applicable to this region. Instead, we find the geometry to be one in which the H+ region and associated PDR is embedded in the molecular cloud. Constant-density PDR calculations are presented which predict line intensities as a function of AV [or N(H)], hydrogen density (nH), and incident UV radiation field (G0). We find that a single-component model with AV ~ 650 mag, nH = 5 × 105 cm-3, and G0 = 7 × 104 reproduces the observed [O I] and [C II] intensities, and that the low [O I] 63 to 146 μm ratio is due to line optical depth effects in the [O I] lines, produced by a large column density of atomic/molecular gas. We find that the effects of a density law would increase our derived AV, while the effects of an asymmetric geometry would decrease AV, with the two effects largely canceling. We conclude that optically selected H+ regions adjacent to PDRs, such as Orion, likely have a different viewing angle or geometry than similar regions detected through IR observations. Overall, the theoretical calculations presented in this work have utility for any PDR embedded in a molecular cloud.This work focuses on [O I] and [C II] emission towards NGC 6334 A, an embedded H+ region/PDR only observable at infrared or longer wavelengths. A geometry where nearly all the emission escapes out the side of the cloud facing the stars, such as Orion, is not applicable to this region. Instead, we find the geometry to be one where the H+ region and associated PDR is embedded in the molecular cloud. Constant density PDR calculations are presented which predict line intensities as a function of AV (or N(H)), hydrogen density (nH), and incident UV radiation field (G0). We find that a single component model with AV ~650 mag, nH = 5x10^5 cm-3, and G0 = 7x10^4 reproduces the observed [O I] and [C II] intensities, and that the low [O I] 63 to 146 micron ratio is due to line optical depth effects in the [O I] lines, produced by a large column density of atomic/molecular gas. We find that the effects of a density-law would increase our derived AV, while the effects of an asymmetric geometry would decrease AV, with the two effects largely canceling. We conclude that optically selected H+ regions adjacent to PDRs, such as Orion, likely have a different viewing angle or geometry than similar regions detected through IR observations. Overall, the theoretical calculations presented in this work have utility for any PDR embedded in a molecular cloud.