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Register a new userMagnetic field in Cepheus A as deduced from OH maser polarimetric observations
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A.Bartkiewicz
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We present the results of MERLIN polarization mapping of OH masers at 1665 and 1667 MHz towards the Cepheus A star-forming region. The maser emission is spread over a region of 6 arcsec by 10 arcsec, twice the extent previously detected. In contrast to the 22 GHz water masers, the OH masers associated with H II regions show neither clear velocity gradients nor regular structures. We identified ten Zeeman pairs which imply a magnetic field strength along the line-of-sight from -17.3 to +12.7 mG. The magnetic field is organised on the arcsecond scale, pointing towards us in the west and away from us in the east side. The linearly polarized components, detected for the first time, show regularities in the polarization position angles depending on their position. The electric vectors of OH masers observed towards the outer parts of H II regions are consistent with the interstellar magnetic field orientation, while those seen towards the centres of H II regions are parallel to the radio-jets. A Zeeman quartet inside a southern H II region has now been monitored for 25 years; we confirm that the magnetic field decays monotonically over that period.
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We present linear and circular polarization observations of the water masers in 4 distinct regions spread over 1x2 arcseconds around the HW2 high-mass young stellar object in the Cepheus A star-forming region. We find magnetic fields between 100-500 mG in the central maser region, which has been argued to trace a circumstellar disk. The masers further from HW2 have field strengths between 30-100 mG. In all cases the magnetic field pressure is found to be similar to the dynamic pressure, indicating that the magnetic field is capable of controlling the outflow dynamics around HW2. In addition to several water maser complexes observed before, we also detect a new maser filament, ~1 arcsec (~690 AU) East of HW2, which we interpret as a shocked region between the HW2 outflow and the surrounding medium. We detect a linear polarization gradient along the filament as well as a reversal of the magnetic field direction. This is thought to mark the transition between the magnetic field associated with the outflow and that found in the surrounding molecular cloud. In addition to the magnetic field we determine several other physical properties of the maser region, including density and temperatures as well as the maser beaming angles.
The southern maser site OH 300.969+1.147 has been studied using the Long Baseline Array of the Australia Telescope National Facility. The 1665- and 1667-MHz hydroxyl ground-state transitions were observed simultaneously. A series of maps with tenth-arcsec spatial resolution, at velocity spacing 0.09 km/s, and in both senses of circular polarization, reveal 59 small diameter maser spots. The spots are scattered over 2-arcsec, coincident with a strong ultracompact HII region, at a distance of 4.3 kpc. 17 Zeeman pairs of oppositely polarized spots were found, all yielding magnetic field estimates towards us (negative), ranging from -1.1 to -4.7 mG, with a median value of -3.5 mG. Excited state masers of OH at 6035 MHz and 6030 MHz at this site also display Zeeman pairs revealing a magnetic field of -5.0 mG. Weak methanol maser emission is intermingled with the OH masers, but there is no detectable closely related water maser. The consistent magnetic field direction found within this site is a striking feature of several other maser sites associated with strong HII regions studied in comparable detail. We interpret the site as a mature region nearing the end of the brief evolutionary stage that can support maser emission.
We present a (sub)millimeter line survey of the methanol maser outflow located in the massive star-forming region DR21(OH) carried out with the Submillimeter Array (SMA) at 217/227 GHz and 337/347 GHz. We find transitions from several molecules towards the maser outflow such as CH$_3$OH, H$_2$CS, C$^{17}$O, H$^{13}$CO$^+$ and C$^{34}$S. However, with the present observations, we cannot discard the possibility that some of the observed species such as C$^{17}$O, C$^{34}$S, and H$_2$CS, might be instead associated with the compact and dusty continuum sources located in the MM2 region. Given that most of transitions correspond to methanol lines, we have computed a rotational diagram with CASSIS and a LTE synthetic spectra with XCLASS for the detected methanol lines in order to estimate the rotational temperature and column density in small solid angle of the outflow where enough lines are present. We obtain a rotational temperature of $28pm 2.5$K and a column density of $6.0pm 0.9 times 10^{15}$ cm$^{-2}$. These values are comparable to those column densities/rotational temperatures reported in outflows emanating from low-mass stars. Extreme and moderate physical conditions to excite the maser and thermal emission coexist within the CH$_3$OH flow. Finally, we do not detect any complex molecules associated with the flow, e.g., CH3OCHO, (CH3)2CO, and CH$_3$CH$_2$CN.
We have observed the OH 1612-MHz maser emission towards the proto-planetary nebula candidate OH17.7-2.0 that underwent a very strong and unusual outburst in 2003. Phase-referencing data were obtained with the EVN in order to localize the outburst and to examine its possible causes. The majority of the emission comes from an incomplete spherical shell with inner and outer radii of 220 and 850 mas, respectively. There is a strong evidence for maser components that arise due to the interaction of a jet-like post-AGB outflow with the remnant outer AGB shell. The most prominent signature of such an interaction is the strongly bursting polarized emission near 73.3km/s coming from two unresolved components of brightness temperature up to 10^11K located at the edge of the biconal region 2500 AU from the central star. It is remarkable that this OH biconal region is well-aligned with the polar outflow inferred from the near-infrared image.
We present the results of daily monitoring of 6.7 GHz methanol maser in Cepheus A (Cep A) using Yamaguchi 32-m radio telescope as well as the results of imaging observations conducted with the JVN (Japanese VLBI Network). We indentified five spectral features, which are grouped into red-shifted (-1.9 and -2.7 km/s) and blue-shifted (-3.8, -4.2, and -4.9 km/s), and we detected rapid variabilities of these maser features within a monitoring period of 81 days. The red-shifted features decreased in flux density to 50% of its initial value, while the flux density of the blue-shifted features rapidly increased within a 30 days. The time variation of these maser features showed two remarkable properties; synchronization and anti-correlation between the red-shifted and the blue-shifted. The spatial distribution of the maser spots obtained by the JVN observation showed an arclike structure with a scale of $sim$1400 AU, and separations of the five maser features were found to be larger than 100 AU. The absolute position of the methanol maser was also obtained based on the phase-referencing observations, and the arclike structure were found to be associated with the Cep A-HW2 object, with the elongation of the arclike structure nearly perpendicularly to the radio continuum jet from the Cep A-HW2 object. These properties of the masers, namely, the synchronization of flux variation, and the spectral and spatial isolation of features, suggest that the collisional excitation by shock wave from a common exciting source is unlikely. Instead, the synchronized time variation of the masers can be explained if all the maser features are excited by infrared radiation from dust which is heated by a common exciting source with a rapid variability.
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