Jane S. Greaves

Submillimetre images of dusty debris around nearby stars

Indirect detections of massive — presumably Jupiter-like — planets orbiting nearby Sun-like stars have recently been reported,. Rocky, Earth-like planets are much more difficult to detect, but clues to their possible existence can nevertheless be obtained from observations of the circumstellar debris disks of dust from which they form. The presence of such disks has been inferred from excess far-infrared emission but, with the exception of β Pictoris, it has proved difficult to image these structures directly as starlight dominates the faint light scattered by the dust. A more promising approach is to attempt to image the thermal emission from the dust grains at submillimetre wavelengths,. Here we present images of such emission around Fomalhaut, β Pictoris and Vega. For each star, dust emission is detected from regions comparable in size to the Suns Kuiper belt of comets. The total dust mass surrounding each star is only a few lunar masses, so any Earth-like planets present must already have formed. The presence of the central cavity, approximately the size of Neptunes orbit, that we detect in the emission from Fomalhaut may indeed be the signature of such planets.

Detection of Polarized Millimeter and Submillimeter Emission from Sagittarius A

We report the detection of linear polarization from Sgr A* at 750, 850, 1350, and 2000 µm which confirms the contribution of synchrotron radiation. From the lack of polarization at longer wavelengths, it appears to arise in the millimeter/submillimeter excess. There are large position angle changes between the millimeter and submillimeter results, and these are discussed in terms of a polarized dust contribution in the submillimeter and various synchrotron models. In the model that best explains the data, the synchrotron radiation from the excess is self-absorbed in the millimeter region and becomes optically thin in the submillimeter. This implies that the excess arises in an extremely compact source of approximately 2 Schwarzschild radii.

FIRST DETECTION OF SUBMILLIMETER POLARIZATION FROM T TAURI STARS

We report a first detection of polarization of the 850 μm continuum emission from two T Tauri stars (TTSs), GM Aur and DG Tau. These are both single and classical TTSs. The emission mostly comes from the compact (r < 100 AU) accretion disks. The polarization (at ~3% level at 3 σ) is interpreted in terms of thermal emission by magnetically aligned dust grains in the disk. Thus these submillimeter polarizations probe the magnetic field structure in the disk, while the previously reported millimeter/submillimeter polarizations of protostars and the transitional objects from the protostars to TTSs trace the magnetic fields in the larger envelope region. In both TTSs, the direction of the magnetic field inferred from our submillimeter polarizations is parallel to the plane of the compact dust disk measured by interferometric observations, suggesting the dominance of a toroidal magnetic field component in the disk. The magnetic evolution of the circumstellar environments is discussed, as well as the constraints on the MHD models of the jets and outflows from young stellar objects.

SUBMILLIMETER IMAGING POLARIMETRY OF THE NGC 7538 REGION

Imaging polarimetry of the 850 km continuum emission in the NGC 7538 region, obtained with the SCUBA Polarimeter, is presented. The polarization map is interpreted in terms of thermal radiation by magnetically aligned dust grains. Two prominent cores associated with IRS 1 and IRS 11, IRS 1(SMM), and IRS 11(SMM), are found in the surface brightness map. Although these cores look similar in surface brightness, their polarization shows striking diUerences. In IRS 11(SMM), the polarization vectors are extremely well-ordered, and the degrees of polarization are quite high with an average of D3.9%. In IRS 1(SMM), on the other hand, the directions of polarization vectors are locally disturbed, and the degrees of polarization are much lower than those of IRS 11(SMM). These diUerences suggest that small scale —uctuations of the magnetic —eld are more prominent in IRS 1(SMM). This can be interpreted in terms of the diUerence in evolutionary stage of the cores. Inside IRS 1(SMM), which seems to be at a later evolutionary stage than IRS 11(SMM), substructures such as subclumps or a cluster of infrared sources have already formed. Small scale —uctuations in the magnetic —eld could have developed during the formation of these substructures. The distribution of magnetic —eld directions derived from our polarization map agrees well with those of molecular out—ows associated with IRS 1(SMM) and IRS 11(SMM). Comparisons of energy densities between the magnetic —eld and the out—ows show that the magnetic —eld probably plays an important role in guiding the directions of the out—ows.

Magnetic field surrounding the starburst nucleus of the galaxy M82 from polarized dust emission

Magnetic fields may play an important role in the star-formation process, especially in the central regions of ‘starburst’ galaxies where star formation is vigorous. But the field directions are very difficult to determine in the dense molecular gas out of which the stars form, so it has hitherto been impossible to test this hypothesis. Dust grains in interstellar clouds tend to be magnetically aligned, and it is possible to determine the alignment direction based on the polarization of optical light due to preferential extinction along the long axes of the aligned grains. This technique works, however, only for diffuse gas, not for the dense molecular gas. Here we report observations of polarized thermal emission from the aligned dust grains in the central region of M82, which directly traces the magnetic field structure (as projected onto the plane of the sky). Organized field lines are seen around the brightest star-forming regions, while in the dusty halo the field lines form a giant magnetic bubble possibly blown out by the galaxys ‘superwind’.

Submillimeter Polarimetry of Class 0 Protostars: Constraints on Magnetized Outflow Models

Polarized 800 μm emission from magnetically aligned dust grains has been detected in five protostars. Previously, only two such detections had been made. All of these protostellar sources have associated bipolar outflows and presumably also circumstellar disks. We find that the angle between the deduced magnetic field direction and the outflow axis, Δθ, appears to be correlated with the angle between the line of sight to the observer and the outflow direction, θlos. If the outflow is in the plane of the sky, then the deduced magnetic field tends to be perpendicular to the outflow, and if the outflow is angled toward the observer, then the magnetic field tends to lie parallel to the outflow. Various magnetic field geometries could explain this trend. In addition, the 800 μm percentage polarization, p, appears to be inversely correlated with the ratio of Lbol/L1.3 mm, which is a measure of the evolutionary stage of the source, and with the bipolar outflow opening angle, which also increases as a source evolves, and it appears to be correlated with source distance. We interpret these results as indicating that the observed magnetic field is more ordered in younger sources. The most likely interpretation of the distance correlation appears to be that the field is more ordered in more massive sources. As yet, there are no theoretical models for the magnetic fields around protostars that can fully explain all of these correlations.

Measurement of the Magnetic Field Direction in the NGC 2024 FIR 5 Protostellar Outflow

Molecular outflows from young protostars are widely believed to be collimated by magnetic fields, but there has been little observational evidence to support this hypothesis. Using the new technique of millimeter-wavelength spectropolarimetry, we demonstrate the existence of a magnetic field in the NGC 2024 FIR 5 outflow lobe. The 1.3 mm J = 2-1 transition of carbon monoxide (CO) is polarized at a level of approximately 1%, in a direction within 10°-15° of the outflow axis. This agrees with theoretical models in which the magnetic field channels the outflowing gas, and it shows that the process can be effective as far as 0.1 pc from the protostar.

The 3.3 micron PAH feature in Vega-type stars

We present spectroscopic data of 3 isolated Vega-type systems which exhibit the 3.3 μm‘PAH’ feature ascribed to stretching of the C-H bonds in aromatic hydrocarbons. These are part of a sample of mainly hot stars. We discuss the implications of Vega-type stars having an organic component to their surroundings and compare them with the unique star, HD135344 (SAO 206462).