Do accretion disks occur around massive protostars?. High-mass star formation.
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Do accretion disks occur around massive protostars?
High-mass star formation
We cannot form high-mass stars without involving a flattened structure unless they form through mergers of low-mass stars. High-mass stars may form the same way low-mass stars do, i.e. by forming an accretion disk and driving a molecular outflow. Both phenomena are needed to get rid of excess angular momentum and magnetic energy.
But massive stars form in dense molecular cores where the pressure from the surrounding cloud dominates over the radiation pressure from the newly formed star, allowing it to continue the accretion process much longer.
Disks are ubiquitous in low-mass (T Tauri) and intermediate-mass (HAEBE) stars. Disks are also seen in younger, i.e. Class I objects. It is therefore reasonable to expect that they might also be present at the protostellar stage.
There are some clear cases of disks around early B-stars, e.g. MWC349 & MWC297. We expect disks to have shorter lifetimes in high-mass stars, so they are likely to be rare. Disks around still-forming high-mass protostars will be even rarer. In this poster we present evidence for just such an object in NGC 7538S (Sandell, Wright & Forster 2003, ApJ, 590, L45).
J.R. Forster (UCB), Göran Sandell (SOFIA-USRA)
Mel Wright (UCB), W.M. Goss (NRAO)
The NGC 7538 Giant Molecular Complex
Spitzer view - IRAC 8 micron
Distance 2.8 kpc Diameter ~ 4 pc total mass > 4 104 Msun
Molecular cores are high-pressure, self-gravitating molecular clumps. They have a lifetime of ~ 106 yr, temperatures of 30 to 200 K, densities of 105 to 109 cm-3, sizes 0.1 to 1 pc, and mass 102 to 104 Msun.
Left: BIMA spectral index map of dust emission at 1.4, 2.7 and 3.4 mm l with VLA 6cm free-free image (insert).
Right: 1.4mm BIMA image of dust disk with VLA jet overlaid.
Below: BIMA CH3CN K=1 and K=3 transitions showing red and blue-shifted velocities offset in the direction of the VLA jet. The contours are integrated line intensity.
Currently the best evidence for the presence of a disk is extended dust emission at mm wavelengths oriented in a direction perpendicular to a free-free jet and bipolar outflow in CH3CN. Jets and outflows require a disk or torus surrounding a star to collimate the flow. Accretion onto the disk and rotation are also needed in order to maintain stability and power the outflows.
DCN and H13CN trace dense gas in the disk, and both show a clear velocity separation on opposite sides of the disk. The situation is complicated by the outflow, which is also seen in DCN. The white lines in the H13CN position-velocity diagram mark the location of the hyperfine components.
Properties and stability of the disk
The total disk mass is ~100 Msun, while the dynamical mass of the inner 5” rotating part of the disk is ~20 Msun. The large outer structure is probably unstable and is unlikely to survive against collapse and fragmentation.
BIMA 3mm spectra
BIMA HCO+ spectra
BIMA 1mm spectra
Michiel Hogerheijde & Floris van der TakA&A, 2001
Molecular line spectra toward NGC 7538S are complex. This indicates that a variety of kinematic components are present including rotation, outflow and possibly collapse. Emission from unrelated clouds along the line of sight, such as the one at a velocity of -49 km/s, further confuse the issue.
By comparing the shapes of optically thick and optically thin transitions of the same molecule, it is possible to distinguish between expansion and collapse of a molecular cloud core. In optically thick transitions radiation from the back side of the cloud is absorbed as it passes through the cloud toward the observer. This can produce asymmetries which are not present in the optically thin line.
Models which account for radiative transfer effects caused by radial temperature and density changes in a collapsing cloud predict strong self-absorption at the systemic velocity of the cloud, and reduced intensity of the red side relative to the blue. This is exactly what is observed in the optically thick HCO+ line. The optically thin H13CO+ line does not show this asymmetry.
NH2D (contours) on 450 mm image shows cold H2 filament running through the GMC.
Illustration of the protostar with rotating accreation disk embedded in the southern molecular clump NGC 7538S
The NGC 7538S molecular core has a total luminosity of ~104 Lsun, a size ~0.8 pc and total mass of ~5000 Msun. Embedded in this core is a high-mass protostar surrounded by a flattened dusty structure >20,000 AU in diameter and containing >100 Msun. Within ~6,500 AU of the protostar a 20 Msun accretion disk drives a highly collimated ionized jet and a molecular outflow. The disk is seen nearly edge-on and has a velocity gradient of 3.4 km/s across its diameter. Infalling molecular gas is observed out to at least 0.1 pc from the disk, and is accreting at an estimated rate of ~8 10-3 Msun/yr. The protostarcoincides with a VLA source, Spitzer 8 µm source and a cluster of H2O masers.
In recent years more rotating accretion disks have been found around massive protostars. The disks range in diameter from 1,520 to 26,800 AU (Cesaroni et al ’05 A&A 434, 1039; Schreyer et al ’06, ApJ 627, L129; Beltran et al ’05 A&A 435, 901). The larger disks are found in dense molecular cores and are coincident with OH/H2O masers, like NGC 7538S. The largest disks are probably also the youngest, with very high accretion rates and short lifetimes. These disks are not stable, except perhaps in the innermost regions nearest the protostar, and will probably not survive more than a few thousand years. With such short lifetimes it is remarkable that so many have been found. The high success rate is largely due to the availability of maser and molecular surveys, which pinpoint the locations of the dense molecular cores in which massive protostellar objects are most likely to form.