The CII158um line is one of the dominant cooling lines in star-forming active regions. The commonly assumed clumpy UV-penetrated cloud models predict a CII line profile similar to that of CO. ...However, recent spectral-resolved observations show that they are often very different, indicating a more complex origin of the line emission including the dynamics of the source region. The aim of our study is to investigate the physical properties of the star-forming ISM in the Large Magellanic Cloud (LMC) by separating the origin of the emission lines spatially and spectrally. In this paper, we focus on the spectral characteristics and the origin of the emission lines, and the phases of carbon-bearing species in the N159 star-forming region in the LMC. We mapped a 4'x(3-4)' region in N159 in CII158um and NII205um with the GREAT on board SOFIA, and in CO(3-2), (4-3), (6-5), 13CO(3-2), and CI3P1-3P0 and 3P2-3P1 with APEX. The emission of all transitions observed shows a large variation in the line profiles across the map and between the different species. At most positions the CII emission line profile is substantially wider than that of CO and CI. We estimated the fraction of the CII integrated line emission that cannot be fitted by the CO line profile to be 20%-50%. We derived the relative contribution from C+, C, and CO to the column density in each velocity bin. The contribution from C+ dominates the velocity range far from the velocities traced by the dense molecular gas, and the region located between the CO cores of N159 W and E. We estimate the contribution of the ionized gas to the CII emission using the ratio to the NII emission to be < 19% to the CII emission at its peak position, and <15% over the whole observed region. Using the integrated line intensities, we present the spatial distribution of I(CII)/I(FIR). (abridged for arXiv)
Carbon monoxide is an excellent tracer of the physical conditions of gas in molecular outflows from young stars. To understand the outflow mechanism we need to investigate the origin of the molecular ...emission and the structure and interaction of the outflowing molecular gas. Deriving the physical parameters of the gas will help us to trace and understand the various gas components in the flow. We observed CO(12-11) line emission at various positions along the L1157 bipolar outflow with GREAT aboard SOFIA. Comparing these new data with CO(2-1), we find basically constant line ratios along the outflow and even at the position of the source. These line ratios lead us to estimates of 10^5 to 10^6 cm^-3 for the gas density and 60 to 100 K for the gas temperature of the outflowing gas. The constrained density and temperature values indicate that we are mostly tracing a low-velocity gas component everywhere along the outflow, which is intermediate between the already known cold gas component, which gets entrained into the flow, and the hot gas, which gets shocked in the outflow.
Imaging polarimetry is an important tool for the study of cosmic magnetic fields. In our Galaxy, polarization levels of a few up to \(\sim\)10\% are measured in the submillimeter dust emission from ...molecular clouds and in the synchrotron emission from supernova remnants. Only few techniques exist to image the distribution of polarization angles, as a means of tracing the plane-of-sky projection of the magnetic field orientation. At submillimeter wavelengths, polarization is either measured as the differential total power of polarization-sensitive bolometer elements, or by modulating the polarization of the signal. Bolometer arrays such as LABOCA at the APEX telescope are used to observe the continuum emission from fields as large as \(\sim0\fdg2\) in diameter. %Here we present the results from the commissioning of PolKa, a polarimeter for Here we present PolKa, a polarimeter for LABOCA with a reflection-type waveplate of at least 90\% efficiency. The modulation efficiency depends mainly on the sampling and on the angular velocity of the waveplate. For the data analysis the concept of generalized synchronous demodulation is introduced. The instrumental polarization towards a point source is at the level of \(\sim0.1\)\%, increasing to a few percent at the \(-10\)db contour of the main beam. A method to correct for its effect in observations of extended sources is presented. Our map of the polarized synchrotron emission from the Crab nebula is in agreement with structures observed at radio and optical wavelengths. The linear polarization measured in OMC1 agrees with results from previous studies, while the high sensitivity of LABOCA enables us to also map the polarized emission of the Orion Bar, a prototypical photon-dominated region.
We aim to investigate the polarization properties of a starless core in a very early evolutionary stage. Linear polarization data reveal the properties of the dust grains in the distinct phases of ...the interstellar medium. Our goal is to investigate how the polarization degree and angle correlate with the cloud and core gas. We use optical, near infrared and submillimeter polarization observations toward the starless object Pipe-109 in the Pipe nebula. Our data cover a physical scale range of 0.08 to 0.4 pc, comprising the dense gas, envelope and the surrounding cloud. The cloud polarization is well traced by the optical data. The near infrared polarization is produced by a mixed population of grains from the core border and the cloud gas. The optical and near infrared polarization toward the cloud reach the maximum possible value and saturate with respect to the visual extinction. The core polarization is predominantly traced by the submillimeter data and have a steep decrease with respect to the visual extinction. Modeling of the submillimeter polarization indicates a magnetic field main direction projected onto the plane-of-sky and loss of grain alignment for densities higher than \(6\times10^4\) cm\(^{-3}\) (or \(A_V > 30\) mag). Pipe-109 is immersed in a magnetized medium, with a very ordered magnetic field. The absence of internal source of radiation significantly affects the polarization efficiencies in the core, creating a polarization hole at the center of the starless core. This result supports the theory of dust grain alignment via radiative torques.
The methylidyne radical CH is commonly used as a proxy for H\(_2\) in the cold, neutral phase of the interstellar medium. The optical spectroscopy of CH is limited by interstellar extinction, whereas ...far-infrared observations provide an integral view through the Galaxy. While the HF ground state absorption, another H\(_2\) proxy in diffuse gas, frequently suffers from saturation, CH remains transparent both in spiral-arm crossings and high-mass star forming regions, turning this light hydride into a universal surrogate for H\(_2\). However, in slow shocks and in regions dissipating turbulence its abundance is expected to be enhanced by an endothermic production path, and the idea of a "canonical" CH abundance needs to be addressed. The \(N=2\leftarrow 1\) ground state transition of CH at \(\lambda 149\,\mu\)m has become accessible to high-resolution spectroscopy thanks to GREAT aboard SOFIA. Its unsaturated absorption and the absence of emission makes it an ideal candidate for the determination of column densities with a minimum of assumptions. Here we present an analysis of four sightlines towards distant, far-infrared bright Galactic star forming regions. If combined with the sub-millimeter line of CH at \(\lambda 560\,\mu\)m, environments forming massive stars can be analyzed. For this we present a case study on the "proto-Trapezium" cluster W3 IRS5, and demonstrate that the sub-millimeter/far-infrared lines of CH reliably trace not only diffuse but also dense, molecular gas. While we confirm the global correlation between the column densities of HF and those of CH, clear signposts of an over-abundance of CH are observed towards lower densities. A quiescent ion-neutral chemistry alone cannot account for this over-abundance. Vortices forming in turbulent, diffuse gas may be the setting for an enhanced production path.
(Context.) A number of millimeter and submillimeter facilities with linear polarization observing capabilities have started operating during last years. These facilities, as well as other previous ...millimeter telescopes and interferometers, require bright and stable linear polarization calibrators to calibrate new instruments and to monitor their instrumental polarization. The current limited number of adequate calibrators implies difficulties in the acquisition of these calibration observations. (Aims.) Looking for additional linear polarization calibrators in the millimeter spectral range, in mid-2006 we started monitoring 3C 286, a standard and highly stable polarization calibrator for radio observations. (Methods.) Here we present the 3 and 1 mm monitoring observations obtained between September 2006 and January 2012 with the XPOL polarimeter on the IRAM 30 m Millimeter Telescope. (Results.) Our observations show that 3C 286 is a bright source of constant total flux with 3 mm flux density S_3mm = (0.91 \pm 0.02) Jy. The 3mm linear polarization degree (p_3mm =13.5\pm0.3%) and polarization angle (chi_3mm =37.3\pm0.8deg.,expressed in the equatorial coordinate system) are also constant during the time span of our observations. Although with poorer time sampling and signal-to-noise ratio, our 1 mm observations of 3C 286 are also reproduced by a constant source of 1 mm flux density (S_1mm = 0.30 \pm 0.03 Jy), polarization fraction (p_1mm = 14.4 \pm 1.8 %), and polarization angle (chi_1mm = 33.1 \pm 5.7deg.). (Conclusions.) This, together with the previously known compact structure of 3C 286 -extended by ~3.5" in the sky- allow us to propose 3C 286 as a new calibrator for both single dish and interferometric polarization observations at 3 mm, and possibly at shorter wavelengths.
The origin of magnetism around AGB stars is uncertain. If these stars drive a dynamo, it cannot be sufficient to generate a strong global field, otherwise the observed X-ray luminosities would be ...higher. Other explanations for the circular polarization of SiO masers in the atmospheres are needed. The interaction of the AGB wind with previously ejected matter and with planets bears complex magneto-hydrodynamic phenomena on a short time scale, such that strong magnetic fields can be maintained locally. Here we provide observational evidence for the corresponding magnetic fluctuations. We trace magnetic activity with the circular polarization of the v=1, J=2-1 SiO masers, using a correlation polarimeter. In V Cam and R Leo, we find evidence of pseudo-periodic fluctuations of the circular polarization on a timescale of a few hours, from which we infer magnetic fluctuations of ~1G. The phenomenon is rare and restricted to a narrow range of velocities. It seems to be associated with planetary wake flows suggested by VLBI maps. While scenarios involving magnetic activity in the extended stellar atmosphere have problems to explain all observed features, precessing Jovian magnetospheres predict all of them. For the case of R Leo, we constrain the orbit of the planet (estimated period 5.2 years) and estimate a stellar mass of 0.7 solar masses.
The chemical composition of the interstellar medium is determined by gas phase chemistry, assisted by grain surface reactions, and by shock chemistry. The aim of this study is to measure the ...abundance of the hydroxyl radical (OH) in diffuse spiral arm clouds as a contribution to our understanding of the underlying network of chemical reactions. Owing to their high critical density, the ground states of light hydrides provide a tool to directly estimate column densities by means of absorption spectroscopy against bright background sources. We observed onboard the SOFIA observatory the 2Pi3/2, J = 5/2 3/2 2.5 THz line of ground-state OH in the diffuse clouds of the Carina-Sagittarius spiral arm. OH column densities in the spiral arm clouds along the sightlines to W49N, W51 and G34.26+0.15 were found to be of the order of 10^14 cm^-2, which corresponds to a fractional abundance of 10^-7 to 10^-8, which is comparable to that of H_2O. The absorption spectra of both species have similar velocity components, and the ratio of the derived H_2O to OH column densities ranges from 0.3 to 1.0. In W49N we also detected the corresponding line of ^18OH.
The BD+40\degree4124 region was observed with high angular and spectral resolution with the German heterodyne instrument GREAT in CO J = 13 \rightarrow 12 and CII on SOFIA. These observations show ...that the CII emission is very strong in the reflection nebula surrounding the young Herbig Ae/Be star BD+40\degree4124. A strip map over the nebula shows that the CII emission approximately coincides with the optical nebulosity. The strongest CII emission is centered on the B2 star and a deep spectrum shows that it has faint wings, which suggests that the ionized gas is expanding. We also see faint CO J = 13 \rightarrow 12 at the position of BD+40\degree4124, which suggests that the star may still be surrounded by an accretion disk.We also detected CII emission and strong CO J = 13 \rightarrow 12 toward V1318 Cyg. Here the CII emission is fainter than in BD+40\degree4124 and appears to come from the outflow, since it shows red and blue wings with very little emission at the systemic velocity, where the CO emission is quite strong. It therefore appears that in the broad ISO beam the CII emission was dominated by the reflection nebula surrounding BD+40\degree4124, while the high J CO lines originated from the adjacent younger and more deeply embedded binary system V1318 Cyg.
XPOL, the first correlation polarimeter at a large millimeter telescope, uses a flexible digital correlator to measure all four Stokes parameters simultaneously, i.e. the total power I, the linear ...polarization components Q and U, and the circular polarization V. The versatility of the backend provides adequate bandwidth for efficient continuum observations as well as sufficient spectral resolution (40 kHz) for observations of narrow lines. We demonstrate that the polarimetry specific calibrations are handled with sufficient precision, in particular the relative phase between the Observatory's two orthogonally linearly polarized receivers. The many facets of instrumental polarization are studied at 3mm wavelength in all Stokes parameters: on-axis with point sources and off-axis with beam maps. Stokes Q which is measured as the power difference between the receivers is affected by instrumental polarization at the 1.5% level. Stokes U and V which are measured as cross correlations are very little affected (maximum sidelobes 0.6% (U) and 0.3% (V)). These levels critically depend on the precision of the receiver alignment. They reach these minimum levels set by small ellipticities of the feed horns when alignment is optimum (<~ 0.3"). A second critical prerequisite for low polarization sidelobes turned out to be the correct orientation of the polarization splitter grid. Its cross polarization properties are modeled in detail. XPOL observations are therefore limited only by receiver noise in Stokes U and V even for extended sources. Systematic effects set in at the 1.5% level in observations of Stokes Q. With proper precautions, this limitation can be overcome for point sources. Stokes Q observations of extended sources are the most difficult with XPOL.
