arXiv subjects

A. P. Sarma

Publications and source records attributed to A. P. Sarma.

15 recordsLinked to original sources

arXiv2026

Measuring Magnetic Field Strengths in Galactic Star-forming Regions via the Zeeman Effect with the SKA

Magnetic fields thread the interstellar medium from the largest to the smallest scales and play an important role in molecular cloud evolution and star formation. Quantifying this requires measurements of the field strengths, and the most direct way to measure them is via the Zeeman effect in spectral lines. The effect is subtle for the typical field strengths expected from theory, from a few $μ$G in diffuse molecular clouds to a few 10s of mG in dense star-forming regions, and detections are scarce. Existing measurements of magnetic field strength suggest dense clouds and cores are marginally supercritical (cannot prevent collapse, but can inhibit it), but may be biased due to small sample sizes. Zeeman effect measurements tracing different scales and densities within molecular clouds can reveal the variation of field strengths, providing critical measurements to address the question of whether star formation is primarily regulated by magnetic fields or turbulence on different scales. Observations with SKA precursors such MeerKAT and FAST are beginning to increase the number of Zeeman effect detections in nearby star-forming regions. The SKA will extend their reach to many regions within our Galaxy that are best representative of where most stars form, while zooming in on the densest star-forming regions, providing a statistical basis for the role of magnetic fields in molecular cloud evolution and star formation. We present predictions and plans for Zeeman effect observations with the SKA telescopes, demonstrating the significant advances they will provide for studies of magnetic fields in molecular clouds.

astro-ph.GA↗

arXiv2023

The Discovery of the Zeeman Effect in 38 GHz Class II Methanol Masers

Magnetic fields likely play an important role in star formation, but the number of directly measured magnetic field strengths remains scarce. We observed the 38.3 and 38.5 GHz Class II methanol (CH$_3$OH) maser lines toward the high mass star forming region NGC 6334F for the Zeeman effect. The observed spectral profiles have two prominent velocity features which can be further decomposed through Gaussian component fitting. In several of these fitted Gaussian components we find significant Zeeman detections, with $zB_{\rm los}$ in the range from 8 to 46 Hz. If the Zeeman splitting factor $z$ for the 38 GHz transitions is of the order of $\sim$1 Hz mG$^{-1}$, similar to that for several other CH$_3$OH maser lines, then magnetic fields in the regions traced by these masers would be in the range of 8-46 mG. Such magnetic field values in high mass star forming regions agree with those detected in the better-known 6.7 GHz Class II CH$_3$OH maser line. Since Class II CH$_3$OH masers are radiatively pumped close to the protostar and likely occur in the accretion disk or the interface between the disk and outflow regions, such fields likely have significant impact on the dynamics of these disks.

astro-ph.GA↗

arXiv2022

Long-term variability of Class I methanol masers in the high mass star forming region DR21(OH)

High mass stars play an important role in the Interstellar Medium, but much remains to be known about their formation. Class I methanol masers may be unique tracers of an early stage of high mass star formation, and a better understanding of such masers will allow them to be used as more effective probes of the high mass star forming process. We present an investigation of the long-term variability of Class I methanol masers at 44 GHz toward the high mass star forming region DR21(OH). We compare observations taken in 2017 to 2012 and also to 2001 data from the literature. A total of 57 maser spots were found in the 2017 data, with center velocities ranging between -8.65 km/s to +2.56 km/s. The masers are arranged in a western and an eastern lobe with two arcs in each lobe that look like bowshocks, consistent with previous observations. The general trend is an increase in intensity from 2001 to 2012, and a decrease from 2012 to 2017. Variability appears to be more prevalent in the inner arc of the western lobe than in the outer arc. We speculate that this may be a consequence of episodic accretion, in which a later accretion event has resulted in ejection of material whose shock reached the inner arc at some point in time after 2001. We conclude that class I methanol masers are variable on long timescales (of the order of 5-10 years).

astro-ph.GA↗

arXiv2019

A Curious Case of Circular Polarization in the 25 GHz Methanol Maser Line toward OMC-1

We report the detection of a circular polarization signature in the Stokes $V$ profile of a 25 GHz Class I CH$_3$OH maser toward the high mass star forming region OMC-1. Such a feature usually constitutes a detection of the Zeeman effect. If due to a magnetic field in OMC-1, this would represent the first detection and discovery of the Zeeman effect in the 25 GHz Class I CH$_3$OH maser. The feature in Stokes $V$ is detected in two observations with different angular resolutions taken eight years apart with the Very Large Array (VLA); for our 2009 D-configuration observations, the fitted value for $z B_\text{los}$ is $152 \pm 12$ Hz, where $z$ is the Zeeman splitting factor and $B_\text{los}$ is the line-of-sight magnetic field. For our 2017 C-configuration observations, the fitted value for $zB_\text{los} = 149 \pm 19~$Hz, likely for the same maser spot. These correspond to $B_\text{los}$ in the range 171-214 mG, depending on which hyperfine transition is responsible for the maser line. While these $B_\text{los}$ values are high, they are not implausible. If the magnetic field increases in proportion to the molecular hydrogen density in shocked regions, then our detected fields predict values for the pre-shock magnetic field that are in agreement with observations. With $B_\text{los}$ =171-214 mG, the magnetic energy in the post-shocked regions where these 25 GHz Class I CH$_3$OH masers occur would dominate over the kinetic energy density and be at least of the order of the pressure in the shock, implying that the magnetic field would exert significant influence over the dynamics of these regions.

astro-ph.GA↗

arXiv2016

The Zeeman Effect in the 44 GHz Class I Methanol Maser Line toward DR21(OH)

We report the detection of the Zeeman effect in the 44 GHz Class I methanol maser line toward the star forming region DR21(OH). In a 219 Jy/beam maser centered at an LSR velocity of 0.83 km s$^{-1}$, we find a 20-$σ$ detection of $zB_{\text{los}} = 53.5 \pm 2.7$ Hz. If 44 GHz methanol masers are excited at $n \sim 10^{7-8}$ cm$^{-3}$, then the $B~vs.~n^{1/2}$ relation would imply from comparison with Zeeman effect detections in the CN($1-0$) line toward DR21(OH) that magnetic fields traced by 44 GHz methanol masers in DR21(OH) should be $\sim$10 mG. Together with our detected $zB_{\text{los}} = 53.5$ Hz, this would imply that the value of the 44 GHz methanol Zeeman splitting factor $z$ is $\sim$5 Hz mG$^{-1}$. Such small values of $z$ would not be a surprise, as the methanol molecule is non paramagnetic, like H$_2$O. Empirical attempts to determine $z$, as demonstrated, are important because currently there are no laboratory measurements or theoretically calculated values of $z$ for the 44 GHz methanol transition. Data from observations of a larger number of sources are needed to make such empirical determinations robust.

astro-ph.GA↗

arXiv2013

VLA OH Zeeman Observations of the star forming region S88B

We present observations of the Zeeman effect in OH thermal absorption main lines at 1665 and 1667 MHz taken with the Very Large Array (VLA) toward the star forming region S88B. The OH absorption profiles toward this source are complicated, and contain several blended components toward a number of positions. Almost all of the OH absorbing gas is located in the eastern parts of S88B, toward the compact continuum source S88B-2 and the eastern parts of the extended continuum source S88B-1. The ratio of 1665/1667 MHz OH line intensities indicates the gas is likely highly clumped, in agreement with other molecular emission line observations in the literature. S88-B appears to present a similar geometry to the well-known star forming region M17, in that there is an edge-on eastward progression from ionized to molecular gas. The detected magnetic fields appear to mirror this eastward transition; we detected line-of-sight magnetic fields ranging from 90-400 μG, with the lowest values of the field to the southwest of the S88B-1 continuum peak, and the highest values to its northeast. We used the detected fields to assess the importance of the magnetic field in S88B by a number of methods; we calculated the ratio of thermal to magnetic pressures, we calculated the critical field necessary to completely support the cloud against self-gravity and compared it to the observed field, and we calculated the ratio of mass to magnetic flux in terms of the critical value of this parameter. All these methods indicated that the magnetic field in S88B is dynamically significant, and should provide an important source of support against gravity. Moreover, the magnetic energy density is in approximate equipartition with the turbulent energy density, again pointing to the importance of the magnetic field in this region.

astro-ph.GA↗

arXiv2012

Comparison of Two Epochs of the Zeeman Effect in the 44 GHz Class I methanol (CH3OH) maser line in OMC-2

We present a second epoch of observations of the 44 GHz Class I methanol maser line toward the star forming region OMC-2. The observations were carried out with the Very Large Array, and constitute one of the first successful Zeeman effect detections with the new WIDAR correlator. Comparing to the result of our earlier epoch of data for this region, we find that the intensity of the maser increased by 50%, but the magnetic field value has stayed the same, within the errors. This suggests that the methanol maser may be tracing the large-scale magnetic field that is not affected by the bulk gas motions or turbulence on smaller scales that is causing the change in maser intensity.

astro-ph.GA↗

arXiv2011

Discovery of the Zeeman Effect in the 44 GHz Class I methanol maser line

We report the discovery of the Zeeman effect in the 44 GHz Class I methanol maser line. The observations were carried out with 22 antennas of the EVLA toward a star forming region in OMC-2. Based on our adopted Zeeman splitting factor of z = 1.0 Hz/mG, we detect a line of sight magnetic field of 18.4 +/- 1.1 mG toward this source. Since such 44 GHz methanol 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 GHz 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, methanol 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.

astro-ph.GA↗

arXiv2011

The structure of the magnetic field in the massive star-forming region W75N

A debated topic in star formation theory is the role of magnetic fields during the protostellar phase of high-mass stars. It is still unclear how magnetic fields influence the formation and dynamics of massive disks and outflows. Most current information on magnetic fields close to high-mass protostars comes from polarized maser emissions, which allows us to investigate the magnetic field on small scales by using very long-baseline interferometry. The massive star-forming region W75N contains three radio continuum sources (VLA1, VLA2, and VLA3), at three different evolutionary stages, and associated masers, while a large-scale molecular bipolar outflow is also present. Very recently, polarization observations of the 6.7 GHz methanol masers at milliarsecond resolution have been able to probe the strength and structure of the magnetic field over more than 2000 AU around VLA1. The magnetic field is parallel to the outflow, suggesting that VLA1 is its powering source. The observations of water masers at 22 GHz can give more information about the gas dynamics and the magnetic fields around VLA1 and VLA2. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman-splitting of the 22 GHz water masers in the star-forming region W75N. We detected 124 water masers, 36 around VLA1 and 88 around VLA2 of W75N, which indicate two different physical environments around the two sources, where VLA1 is in a more evolved state. The linear polarization of the masers confirms the tightly ordered magnetic field around VLA1, which is aligned with the large-scale molecular outflow, and also reveals an ordered magnetic field around VLA2, which is not parallel to the outflow. [abridged]

astro-ph.SR↗

arXiv2009

First Detection of the Zeeman Effect in the 36 GHz Class I methanol 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 $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.

astro-ph.GA↗

arXiv2008

Conference Summary: The Cosmic Agitator - Magnetic Fields in the Galaxy

We present a summary of the conference "The Cosmic Agitator: Magnetic Fields in the Galaxy" held in Lexington KY in 2008 Mar 26-29. The presentation draws primarily from material in the slides prepared for the Conference Summary by one of us (Carl Heiles). Interested readers may navigate to the conference web site given in the paper to view the posted presentations in detail.

astro-ph↗

arXiv2007

Arcsecond Scale Kinematic and Chemical Complexity in Cepheus A-East

We present results from SMA observations of the star forming region Cepheus A-East at ~340 GHz (875 micron) with 0.7'' - 2'' resolution. At least four compact submm continuum sources have been detected, as well as a rich forest of hot core line emission. Two kinematically, chemically, and thermally distinct regions of molecular emission are present in the vicinity of the HW2 thermal jet, both spatially distinct from the submm counterpart to HW2. We propose that this emission is indicative of multiple protostars rather than a massive disk as reported by Patel et al. (2005).

astro-ph↗

arXiv2007

Optically Thick [O I] and [C II] Emission Towards NGC 6334 A

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.

astro-ph↗

arXiv2002

Gaussian Spectral Line Profiles of Astrophysical Masers

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 (VLBA) 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} ster 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.

astro-ph↗

arXiv1999

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 (del-v_FWHM ~ 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 microG 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.

astro-ph↗