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Register a new userMassive Star Forming Regions: from diagnostic tools to derived properties
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Daniel Schaerer
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The current state-of-the-art of multi-wavelength diagnostic tools (evolutionary synthesis, photoionisation models) for massive star forming regions (HII regions, starbursts, etc.) and some of their input physics (especially model atmospheres) is reviewed. Analysis of stellar populations based on integrated spectra from both stellar features and nebular emission lines from the UV to IR are summarised. We stress the importance of template studies at various scales (from individual stars to well studied galaxies) and various wavelengths, to understand the processes operating in massive star forming regions and to reliably derive their properties.
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We have conducted a search for ionized gas at 3.6 cm, using the Very Large Array, towards 31 Galactic intermediate- and high-mass clumps detected in previous millimeter continuum observations. In the 10 observed fields, 35 HII regions are identified, of which 20 are newly discovered. Many of the HII regions are multiply peaked indicating the presence of a cluster of massive stars. We find that the ionized gas tends to be associated towards the millimeter clumps; of the 31 millimeter clumps observed, 9 of these appear to be physically related to ionized gas, and a further 6 have ionized gas emission within 1. For clumps with associated ionized gas, the combined mass of the ionizing massive stars is compared to the clump masses to provide an estimate of the instantaneous star formation efficiency. These values range from a few percent to 25%, and have an average of 7 +/- 8%. We also find a correlation between the clump mass and the mass of the ionizing massive stars within it, which is consistent with a power law. This result is comparable to the prediction of star formation by competitive accretion that a power law relationship exists between the mass of the most massive star in a cluster and the total mass of the remaining stars.
The determination of accurate distances to star-forming regions are discussed in the broader historical context of astronomical distance measurements. We summarize recent results for regions within 1 kpc and present perspectives for the near and more distance future.
In order to distinguish between the various components of massive star forming regions (i.e. infalling, outflowing and rotating gas structures) within our own Galaxy, we require high angular resolution observations which are sensitive to structures on all size scales. To this end, we present observations of the molecular and ionized gas towards massive star forming regions at 230 GHz from the SMA (with zero spacing from the JCMT) and at 22 and 23 GHz from the VLA at arcsecond or better resolution. These observations (of sources such as NGC7538, W51e2 and K3-50A) form an integral part of a multi-resolution study of the molecular and ionized gas dynamics of massive star forming regions (i.e. Klaassen & Wilson 2007). Through comparison of these observations with 3D radiative transfer models, we hope to be able to distinguish between various modes of massive star formation, such as ionized or halted accretion (i.e Keto 2003 or Klaassen et al. 2006 respectively).
Imaging polarimetry is a useful tool to reveal the 3D structure of dust distributions and to localize embedded young stellar objects. We present maps of the linear polarization at 2.2 micron for three ultra-compact HII regions (G192.16-3.82, G331.28-0.19, G339.88-1.26) and the methanol maser source G305.21+0.21. From the polarization maps, we draw conclusions on the morphology of these objects and the presence of luminous illuminating sources.
We present results of a survey of 14 star-forming regions from the Perseus spiral arm in CS(2-1) and 13CO(1-0) lines with the Onsala Space Observatory 20 m telescope. Maps of 10 sources in both lines were obtained. For the remaining sources a map in just one line or a single-point spectrum were obtained. On the basis of newly obtained and published observational data we consider the relation between velocities of the quasi-thermal CS(2-1) line and 6.7 GHz methanol maser line in 24 high-mass star-forming regions in the Perseus arm. We show that, surprisingly, velocity ranges of 6.7 GHz methanol maser emission are predominantly red-shifted with respect to corresponding CS(2-1) line velocity ranges in the Perseus arm. We suggest that the predominance of the red-shifted masers in the Perseus arm could be related to the alignment of gas flows caused by the large-scale motions in the Galaxy. Large-scale galactic shock related to the spiral structure is supposed to affect the local kinematics of the star-forming regions. Part of the Perseus arm, between galactic longitudes from 85deg to 124deg, does not contain blue-shifted masers at all. Radial velocities of the sources are the greatest in this particular part of the arm, so the velocity difference is clearly pronounced. 13CO(1-0) and CS(2-1) velocity maps of G183.35-0.58 show gas velocity difference between the center and the periphery of the molecular clump up to 1.2 km/s. Similar situation is likely to occur in G85.40-0.00. This can correspond to the case when the large-scale shock wave entrains the outer parts of a molecular clump in motion while the dense central clump is less affected by the shock.
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