Infrared polarimetry of the southern massive star-forming region G333.6−0.2

We present     spectropolarimetry, and 12- and 2-μm imaging polarimetry of the southern massive star-forming region G333.6−0.2. Spectro-polarimetry measurements show that the polarization observed towards the nebula contains a mixture of both absorptive and emissive polarizations. Model fitting to the spectra indicates that the temperature of the mid-infrared emitting dust grains is generally ∼200 K and the optical depth of the absorbing dust at 9.7 μm is ∼1.5. Fits are also made to the polarimetry spectra, which show a reasonably constant peak absorptive polarization (∼3.4 per cent at 43°) across the face of the H  ii region. This absorptive polarization position angle is consistent with that found by the 2-μm imaging polarimetry     and is most likely due to the Galactic magnetic field local to G333.6−0.2. When the absorptive polarization is subtracted from the 12-μm polarization image, the emissive polarization pattern that is intrinsic to the star-forming region is revealed. A probable magnetic field configuration implied by the intrinsic polarization suggests star formation initially influenced by the Galactic magnetic field which is eventually perturbed by the star formation process.     

Ice in the Southern Coalsack

To better understand the conditions under which ice mantles form on grains in molecular clouds, three globules in the Southern Coalsack have been searched for the presence of H₂O ice. Given the total lack of star formation in the Coalsack, it is an ideal site for studying unprocessed icy molecular mantles. In our sample of eight field stars lying behind the Coalsack we detect strong H₂O ice absorption in the lines of sight to two stars and possible weak absorption in four others. We estimate H₂O ice column densities or upper limits for these lines of sight. Compared to dark clouds such as Taurus, the Coalsack H₂O ice column densities are lower than expected given the quiescent nature of the Coalsack region. It is possible that the chemical evolution of the Coalsack may simply be at too early a stage for significant ice mantles to appear on the grains, except perhaps in the densest parts of some of the globules. Alternatively, the presence or absence of ice absorption may be related to the distribution of dust along each line of sight, specifically, the relative contributions of dense globules and a more extended diffuse component. For example, our observations are consistent with an ice threshold extinction similar to that observed in the Taurus dark cloud if extinction amounting to   A _V∼5  towards Globules 2 and 3 arises in the extended component. Globule 1 appears to have no extended component.     

Mid-infrared polarization studies of SgrA: a three-dimensional study of the central parsec

Thermal emission from magnetically aligned dust grains produces the observed mid-infrared polarization in the northern arm and east–west bar of SgrA West; recent arcsecond-resolution imaging polarimetry at 12.5 μm of these ionized filaments is presented, which confirms and extends previous studies. A lower limit ∼2 mG is found for the magnetic field in the northern arm and the IRS16 complex appears to be displaced from the northern arm by ∼ 0.15 pc along the line of sight. It is shown that the physical conditions in the ionized filaments of the central parsec lead to a very uniform grain alignment that is directed along the local magnetic field. The position angle of polarized emission will then be at right angles to the projection of the field direction on the plane of the sky and its amplitude a measure of the component of field along the line of sight; this makes possible a partial reconstruction of the field in three dimensions. We present the first application of the use of polarimetry in this way. This partial reconstruction is compared with the H92α observations of Roberts et al. and the implications are that the northern arm and east–west bar do not define either an orbital path or a spiral arm but rather represent a tidally stretched structure in free fall about SgrA^★ with significant deviations from a single plane, and most likely represent the inner ionized rim of a more extended neutral cloud.     

Grain alignment in dense interstellar environments: spectropolarimetry of the 4.67-μm CO-ice feature in the field star Elias 16 (Taurus dark cloud)

We present spectropolarimetry of the solid CO feature at 4.67 μm along the line of sight to Elias 16, a field star background to the Taurus dark cloud. A clear increase in polarization is observed across the feature with the peak of polarization shifted in wavelength relative to the peak of absorption. This shows that dust grains in dense, cold environments (temperatures ∼20 K or less) can align and produce polarization by dichroic absorption. For a grain model, consisting of a core with a single mantle, the polarization feature is best modelled by a thick CO mantle, possibly including 10 per cent water-ice, with the volume ratio of mantle to bare grain of ∼5. Radiative torques could be responsible for the grain alignment provided the grain radius is at least 0.5 μm. This would require the grain cores to have a radius of at least 0.3 μm, much larger than grain sizes in the diffuse interstellar medium. Sizes of this order seem reasonable on the basis of independent evidence for grain growth by coagulation, as well as mantle formation, inside dense clouds.     

Polarization models of young stellar objects – II. Linear and circular polarimetry of R Coronae Australis

Near-infrared linear imaging polarimetry of the young stellar objects R CrA and T CrA in the J , H and K n bands, and circular imaging polarimetry in the H band, is presented. The data are modelled with the Clark and McCall scattering model. The R CrA and T CrA system is shown to be a particularly complex scattering environment. In the case of R CrA there is evidence that the wavelength dependence of polarization changes across the nebula. MRN dust grain models do not explain this behaviour. Depolarization by line emission is considered as an alternative explanation. The dust grain properties could also be changing across the nebula.
Although surrounded by reflection nebulosity, there is a region of particularly low polarization surrounding R CrA that is best modelled by the canonical bipolar outflow being truncated by an evacuated spherical cavity surrounding the star. The symmetry axis of the nebula appears inclined by 50° to the plane of the sky.
The H -band circular polarimetry of R CrA clearly shows a quadrupolar structure of positive and negative degrees of circular polarization that reach peak magnitudes of ∼5 per cent within our limited map. It is shown that spherical MRN grains are incapable of producing this circular polarization given the observed linear polarization of the R CrA system. Instead, scattering from aligned non-spherical grains is proposed as the operating mechanism.
T CrA is a more archetypical bipolar reflection nebula, and this object is modelled as a canonical parabolic reflection nebula that lies in the plane of the sky. The wavelength independence of linear polarization in the T CrA reflection nebula suggests that the scattering particles are Rayleigh sized. This is modelled with the MRN interstellar grain size distribution.     

The magnetic field and geometry of the oblique shock in the jet of 3C 346

We investigate the brightest regions of the kpc-scale jet in the powerful radio galaxy 3C 346, using new optical Hubble Space Telescope ( HST ) ACS/F606W polarimetry together with Chandra X-ray data and 14.9 and 22.5 GHz Very Large Array (VLA) radio polarimetry. The jet shows a close correspondence between optical and radio morphology, while the X-ray emission shows a  0.80 ± 0.17 kpc  offset from the optical and radio peak positions. Optical and radio polarimetry show the same apparent magnetic field position angle and fractional polarization at the brightest knot, where the jet undergoes a large kink of almost 70° in the optical and radio images. The apparent field direction here is well aligned with the new jet direction, as predicted by earlier work that suggested the kink was the result of an oblique shock. We have explored models of the polarization from oblique shocks to understand the geometry of the 3C 346 jet, and find that the upstream flow is likely to be highly relativistic  (β_u= 0.91^+0.05_−0.07)  , where the plane of the shock front is inclined at an angle of  η= 51°± 11°  to the upstream flow which is at an angle  θ= 14⁺⁸₋₇  deg to our line of sight. The actual deflection angle of the jet in this case is only 22°.     

Detection of the old stellar component of the major Galactic bar

We present near-infrared colour–magnitude diagrams and star counts for a number of regions along the Galactic plane. It is shown that along the l =27°, b =0° line of sight there is a feature at 5.7±0.7 kpc with a density of stars at least a factor of 2 and probably more than a factor of 5 times that of the disc at the same position. This feature forms a distinct clump on an H versus J − H diagram and is seen at all longitudes from the bulge to about l =28°, but at no longitude greater than this. The distance to the feature at l =20° is about 0.5 kpc further than at l =27°, and by l =10° it has merged with, or has become, the bulge. Given that at l =27° and l =21° there is also a clustering of very young stars, the only component that can reasonably explain what is seen is a bar with half-length of around 4 kpc and a position angle of about 43°±7°.     

Polarization in the young cluster NGC 6611: circumstellar, interstellar, or ... both?

We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south–east to the north–west. The cloud is threaded by a very uniform magnetic field.     

A high sampling-density polarization study of the Southern Coalsack

We present a densely sampled map of visual polarimetry of stars in the direction of the Southern Coalsack dark cloud. Our sample consists of new polarimetric observations of 225 stars drawn from the spectrophotometric survey of Seidensticker, and an additional 173 stars, covering the surrounding areas of the cloud, taken from the literature. Because all the target stars have at least spectroscopic parallaxes, we can reliably investigate the spatial origins of the polarization, in three dimensions. We decompose the polarization into three components, due to (i) the wall of the local hot bubble, (ii) the Coalsack cloud and (iii) material in the Carina spiral arm. The polarization due to the Coalsack varies, both in alignment efficiency  ( p / A_V )  and in the dispersion in polarization angle, across the cloud. Using a simplified radiative transfer treatment we show that the measured polarization in background gas is significantly affected by foreground polarization, and specifically that the analysis of the Coalsack polarization must take the effects of the local hot bubble wall into consideration. Correcting for this effect as well as for the internal line-of-sight averaging in the Coalsack, we find, based on a Chandrasekhar–Fermi analysis, a plane-of-the-sky magnetic field for the Coalsack cloud of  〈 B _⊥〉= 93 ± 23 μG  . A systematic error, best described by a multiplicative factor between 0.5 and 1.5, additionally arises from radiative transfer effect uncertainties. We propose that this high value for the magnetic field in the cloud envelope is due to the fact that the Coalsack cloud is embedded in the hot interior of the Upper Centaurus–Lupus superbubble.     

Near-infrared imaging polarimetry of young stellar objects in ρ Ophiuchi

The results of a near-infrared ( J H K L _P) imaging linear polarimetry survey of 20 young stellar objects (YSOs) in ρ Ophiuchi are presented. The majority of the sources are unresolved, with K -band polarizations, P _K < 6 per cent. Several objects are associated with extended reflection nebulae. These objects have centrosymmetric vector patterns with polarization discs over their cores; maximum polarizations of P _K > 20 per cent are seen over their envelopes. Correlations are observed between the degree of core polarization and the evolutionary status inferred from the spectral energy distribution. K -band core polarizations >6 per cent are only observed in Class I YSOs.
A 3D Monte Carlo model with oblate grains aligned with a magnetic field is used to investigate the flux distributions and polarization structures of three of the ρ Oph YSOs with extended nebulae. A ρ∝ r ^−1.5 power law for the density is applied throughout the envelopes. The large-scale centrosymmetric polarization structures are due to scattering. However, the polarization structure in the bright core of the nebula appears to require dichroic extinction by aligned non-spherical dust grains. The position angle indicates a toroidal magnetic field in the inner part of the envelope. Since the measured polarizations attributed to dichroic extinction are usually ≤10 per cent, the grains must either be nearly spherical or very weakly aligned. The higher polarizations observed in the outer parts of the reflection nebulae require that the dust grains responsible for scattering have maximum grain sizes 1.05 μm.     

Near-infrared imaging polarimetry of Cygnus A

We present a 2.2-μm polarization image of the nuclear regions of Cygnus A. The degree of polarization in the central 1 arcsec is (4.1±0.50) per cent, at a position angle of (23.6±3.6)°, approximately perpendicular to the axis of the radio jet.
Modelling of the results suggests that at this wavelength the polarization along the line of sight to the central source is most likely produced by dichroism, through an A _v∼40 mag, with the polarization in surrounding regions produced by scattering. For this model, the K -band luminosity of the central source is calculated to be ∼2×10⁴⁴ erg s⁻¹.     

Chemically driven desorption of CO from icy grains in dark clouds

The chemical desorption of an adsorbed CO molecule in the vicinity of H₂-forming sites on cosmic dust grains in cold dense clouds is investigated theoretically, mainly using a model based on a classical molecular dynamics computational simulation. As a model surface for icy mantles of dust grains, an amorphous water ice slab is generated at 10 K, and the first and the second H atoms are thrown on to the model surface to reproduce the recombination process of the two H atoms, H+H→H₂. Then, the time and space dependence of the local temperature increase of icy mantles caused by the release of H₂ formation energy in the vicinity of H₂-forming sites is examined. It is found that icy mantles are heated locally up to about 30 K in the surface region at R 4 Å and about 20 K at 4 R 6 Å, where R is the distance from the H₂-forming site. The critical temperature of CO desorption is estimated to be about 20–30 K under conditions in typical dense clouds, which might be seen to be comparable to the above result. However, the lifetime of local heating of icy mantles is found to be too short, compared with the time-scale of CO desorption (10¹³ s) and that for H₂ forming in the vicinity of an adsorbed CO molecule (more than 2×10¹³ s). Thus, it is found that the efficiency of chemical desorption of CO on a large dust grain is negligible. On the other hand, chemical desorption can occur on a small dust grain with size less than 20 Å.     

Mid-infrared, spatially resolved spectroscopy of the nucleus of the Circinus galaxy

High spatial resolution spectroscopy at 8–13 μm with T-ReCS on Gemini-S has revealed striking variations in the mid-infrared emission and absorption in the nucleus of the Circinus galaxy (hereafter Circinus) on subarcsecond scales. The core of Circinus is compact and obscured by a substantial column of cool silicate dust. Weak extended emission to the east and west coincides with the coronal line region and arises from featureless dust grains which are probably heated by line emission in the coronal emission zone. The extended emission on the east side of the nucleus displays a much deeper silicate absorption than that on the west, indicating significant columns of cool material along the line of sight and corresponding to an additional extinction of   A_V ∼ 25 mag  . Emission bands from aromatic hydrocarbons are not subject to this additional extinction, are relatively weak in the core and in the coronal line region, and are much more spatially extended than the continuum dust emission; they presumably arise in the circumnuclear star-forming regions. These data are interpreted in terms of an inclined disc-like structure around the nucleus extending over tens of parsecs and possibly related to the inner disc found from observations of water masers by Greenhill et al..     

Imaging and integral-field spectroscopy of shocked H2 around G25.65+1.05

The results of B -band CCD imaging linear polarimetry obtained for stars from the Hipparcos catalogue are used to re-examine the distribution of the local interstellar medium towards the IRAS 100-μm emission void in the Lupus dark clouds. The analysis of the obtained parallax–polarization diagram assigns to the dark cloud Lupus 1 a distance between 130 and 150 pc and assures the existence of a low column density region coincident with the observed infrared void. Moreover, there are clear indications of the existence of absorbing material at distances closer than 60–100 pc, which may be associated with the interface boundary between the Local Bubble and its neighbourhood Loop I superbubble.     

Linear and Circular Polarization Properties of Jets

I discuss the transfer of polarized synchrotron radiation in relativistic jets. I argue that the main mechanism responsible for the circular polarization properties of compact synchrotron sources is likely to be Faraday conversion and that, contrary to common expectation, a significant rate of Faraday rotation does not necessarily imply strong depolarization. The long-term persistence of the sign of circular polarization, observed in some sources, is most likely due to a small net magnetic flux generated in the central engine, carried along the jet axis and superimposed on a highly turbulent magnetic field. I show that the mean levels of circular and linear polarizations depend on the number of field reversals along the line of sight and that the gradient in Faraday rotation across turbulent regions can lead to`correlation depolarization'. The model is potentially applicable to a wide range of synchrotron sources. In particular, I demonstrate how the model can naturally explain the excess of circular over linear polarization in the Galactic Center (SgrA^*) and the low-luminosity AGN M81^*.     

Discovery of an Optical Jet in the Rosette Nebula： Rosette HH2

We report on the discovery of an optical jet-Rosette HH2-in the Rosette Nebula. The jet system bears unique features for residing at the center of a giant HII region, and its energy source is visible with apparently very low extinction along the line of sight. Unlike most other Herbig-Haro jets, this jet indicates a high-excitation origin, and its extended portion shows a seemingly intact structure, instead of normally a shocked working surface, which is attributed to photoablation.     

The 10-μm profile of molecular-cloud and diffuse ISM silicate dust

High signal-to-noise ratio spectra are presented of the 10-μm silicate absorption feature in lines of sight towards Elias 16 and 18 in the Taurus dark cloud, and towards the heavily reddened supergiant Cyg OB2 no. 12. The observations are fitted with laboratory and astronomical spectra to produce intrinsic absorption profiles. These features, which represent molecular-cloud and diffuse ISM dust respectively, are better fitted with emissivity spectra of the Trapezium and μ Cephei than they are with those of laboratory, terrestrial, or other observations of circumstellar silicates. The difference in width between the silicate band in the two environments can be almost entirely ascribed to a broad excess absorption in the long-wavelength wing of the profiles, which is much stronger in the molecular-cloud lines of sight, and possibly reflects grain growth in the denser environment. Limits are placed on the strength of fine spectral structure; if there is a crystalline silicate component in these spectra, it is most likely to be serpentine. Column-density upper limits for methanol and the photolysis product hexamethylenetetramine (HMT) are less than a few per cent of those of water ice and silicates.     

Polarimetry in the visible and infrared: application to CMB polarimetry

Interstellar polarization from aligned dust grains can be measured both in transmission at visible and near-infrared wavelengths and in emission at far-infrared and sub-mm wavelengths. These observations can help predict the behavior of foreground contamination of CMB polarimetry by dust in the Milky Way. Fractional polarization in emission from aligned dust grains will be at the higher range of currently observed values of 4–10%. Away from the galactic plane, fluctuations in Q and U will be dominated by fluctuations in intensity, and less influenced by fluctuations in fractional polarization and position angle.     

High resolution observations of linear polarization of the 6818.8 Å feature of methane for Uranus and Saturn

Observations of the linear polarization in the fully resolved 6818.8 Å feature of CH₄ in Saturn and Uranus show dramatic changes of linear polarization across the line profile. The change in position angle of polarization across the line core, especially for Saturn, indicates a likely Rayleigh scattering origin for the observed polarization.     

The CO Cameron band emission in the Red Rectangle

The origin of the strong CO Cameron band emission in the Red Rectangle is investigated. From a comparison of laboratory spectroscopic data and astronomical observations in combination with chemical modelling, it is concluded that the Cameron band emission of CO originates from the dissociative recombination of HCO⁺, and its intensity provides a measure of the product of the density of HCO⁺ ions and the electron density, integrated along the line of sight.