In the Star Formation Update, I comment on papers which I personally find interesting, in case my time allows it. This newsletter is not designed to provide a comprehensive overview of star formation research. This series forms part of the Professional Astronomy Blog.
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Star Formation Meetings in 2011
- IAU S280: The Molecular Universe. Toledo, Spain. May 30-June 3
- Star Formation Summer School. Taipei, Taiwan. June 15-18
- Star Formation through Spectroimaging at High Angular Resolution. Taipei, Taiwan. June 20-24
- Recent Advances in Star Formation: Observations and Theory. Bangalore, India. June 28-July 1
- Four Decades of Research on Massive Stars: A Scientific Meeting in the Honour of Anthony F.J. Moffat. Saint-Michel-des-Saints, Lanaudière, Québec, Canada. July 11-15
- Origins of Solar Systems. South Hadley, MA, USA. July 17-22
- FIR2011: Star Formation and Feedback in Galaxies as Revealed by Far Infrared and Submillimeter Wavelengths. London, UK. September 14-16
- The Milky Way in the Herschel Era: Towards a Galaxy-scale view of the Star Formation Life-cycle. Rome, Italy. September 19-23
- Formation and Early Evolution of Very Low Mass Stars and Brown Dwarfs. Garching (near Munich), Germany. October 11-14
- The emerging, multi-wavelength view of the Galactic Centre Environment. Heidelberg, Germany. October 17-20
Articles in Depth
Clumps in the non-lognormal Part of Column Density PDFs
Kainulainen et al. (2009) investigated the probability density functions (PDFs) of extinction-based column density maps of molecular clouds. They found that the PDFs are log-normal at low column density. However, above some threshold value, they take a power-law shape. Investigation shows that the presence of this “tail” correlates with the presence or absence of star formation activity in clouds.
This threshold was found to be of order 2–4 mag in visual extinction. In the present paper, the authors explore the physical properties of cloud fragments bound by continuous contours of this threshold value. The results are as follows.
- Objects bound by contours of 2–4 mag have very different physical properties. Their size ranges from 0.1–3 pc, their mass from 0.1–1000 Msun. However, all fragments have similar densities ~103 cm-3.
- These fragments are typically gravitationally unbound. However, when taking the pressure of the external parental cloud into account, objects bound by contours ~2–4 mag might exactly reside in the column density domain where external pressure is sufficient to confine clouds.
The argument about the external pressure confinement is mostly based on simple analytical calculations. It is not clear that they indeed describe all relevant processes. Still, the idea of the onset of pressure confinement at a visual extinction of 2–4 mag is an interesting concept that deserves further consideration.
J. Kainulainen, H. Beuther, R. Banerjee, C. Federrath, T. Henning; Probing the evolution of molecular cloud structure II: From chaos to confinement
Column Density, Outflows, and Massive Star Formation
A few years ago, Krumholz & McKee (2008) proposed that clumps forming massive stars have to have large column density (>1 g cm-2), since otherwise heating is not trapped in the clump, leading to Jeans-fragmentation of the cold gas into stars of low mass. That study did not include outflows. In the present paper, such flows are included. The effect is that radiation can easily escape through the outflow cavities. Consequently, less heating is available to suppress fragmentation. Thus, the inclusion of outflows into these models means that the original Krumholz & McKee (2008) column density limit for massive star formation (>1 g cm-2) has to be increased to a larger number.
Andrew J. Cunningham, Richard I. Klein, Mark R. Krumholz and Christopher F. McKee; Radiation-Hydrodynamic Simulations of Massive Star Formation with Protostellar Outflows
Radiation-Hydrodynamic Simulations give too top-heavy IMFs
As mentioned above, Krumholz & McKee (2008) proposed that fragmentation is stopped in regions of high column density. Because of heating by embedded stars, the gas temperature increases, and only fragments of relatively high mass can turn into stars. The present paper executes a first such simulation on a clump forming several 102 stars. This permits to study the IMF produced in the simulation. As it turns out, the heating strongly suppresses the formation of low-mass stars. This results in a model IMF inconsistent with observations. One simple solution would be to slow down the star formation process, which would yield lower accretion luminosity and reduce heating.
Mark R. Krumholz, Richard I. Klein and Christopher F. McKee; Radiation-Hydrodynamic Simulations of the Formation of Massive Star Clusters I. Implications for the Origin of the Initial Mass Function
Another Very Low Mass Star with an Outflow
SMA observations of the Taurus Very Low Mass (VLM; ~90 MJup) star MHO5 reveal a CO outflow. This supports the view that VLM stars and brown dwarfs can form in isolation, just like stars of higher mass. Otherwise, if MHO5 was ejected from a multiple stellar system, it is not clear that the disk needed to drive the outflow would survive the ejection.
Ngoc Phan-Bao, Chin-Fei Lee, Paul T.P. Ho, Ya-Wen Tang; Molecular Outflows in the Substellar Domain: Millimeter Observations of Young Very Low Mass Objects in Taurus and rho Ophiuchi
A WISE Study of the Western Circinius Cloud
Data from the Wide Field Infrared Survey Explorer became publicly available in April of 2011. This paper, focusing on a cloud at ~700 pc distance from earth, gives a good idea about the data quality to be expected from this mission.
Wilson M. Liu, Deborah L. Padgett, David Leisawitz, Sergio Fajardo-Acosta, Xavier P. Koenig; Wide-field Infrared Survey Explorer Observations of Young Stellar Objects in the Western Circinus Molecular Cloud
Further Articles
S. Leurini, C. Codella, L. Zapata, M.T. Beltran, P. Schilke and R. Cesaroni;On the kinematics of massive star forming regions: the case of IRAS 17233-3606
Dawn E. Peterson et al.; The Spitzer Survey of interstellar Clouds in the Gould Belt. III. A Multi-Wavelength View of Corona Australis
Mohsen Nejad-Asghar; Matter Infall in Collapsing Molecular Cloud Cores with an Axial Magnetic Field
Richard Allison and Simon Goodwin; On the formation of trapezium-like systems
D. Falceta-Goncalves and A. Lazarian; Evolution and lifetime of transient clumps in the turbulent interstellar medium
D. Ward-Thompson, J.M. Kirk, J.S. Greaves, P. Andre; The immediate environment of the Class 0 protostar VLA1623, on scales of ~50-100 AU, observed at millimetre and centimetre wavelengths
H. Beuther, H. Linz, Th. Henning, A. Bik, F. Wyrowski, F. Schuller, P. Schilke, S. Thorwirth, K.-T. Kim; High-mass star formation at high luminosities: W31 at >106 Lsun
Charles Bonatto and Eduardo Bica; Constraining the star formation rate in the Solar neighbourhood with star clusters
Kazushi Sakamoto, Rui-Qing Mao, Satoki Matsushita, Alison B. Peck, Tsuyoshi Sawada, and Martina C. Wiedner; Star-forming Cloud Complexes in the Central Molecular Zone of NGC253
Elly M. Berkhuijsen and Andrew Fletcher; Density PDFs of diffuse gas in the Milky Way
Kohji Tomisaka and Kengo Tomida; Observational Identification of First Cores: Non-LTE Radiative Transfer Simulation
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