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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Articles in Depth
A common Star–Formation Mechanism in Galaxies and the Milky Way
Lada et al. (2010) showed that the star formation rate in nearby galactic clouds is linearly correlated with the mass these clouds contain above a threshold AK ~ 0.8 mag (i.e., AV ~ 7 mag). In other words, the star formation rate per unit dense gas seems to be constant in these clouds. In contrast, the total gas mass of a cloud, e.g. integrated down to low extinction levels, is not tightly correlated with the star formation rate. One could also say that the star formation rate per unit total gas mass is only high in clouds where the fraction of dense gas, fd, is high. Solar neighborhood clouds have fd in the range 0.01 to 0.1.
The present paper explores whether this relation also holds in other galaxies. The authors assume that the mass at AK > 0.8 mag can be replaced with the dense gas mass obtained from HCN emission (including a further boost by a factor 2.7), while the total gas mass can be measured with CO emission. Using these assumptions, extragalactic star formation is typically characterized by fd ~ 0.1, with increases in the dense gas fraction in some galaxies.
So, what are the new results? We already knew (or suspected) from previous work that the star formation rate scales linearly with the mass of dense gas. And we already knew that the dense gas only makes up a fraction of the total gas in clouds. But there was always the problem that the galactic observations of star formation rates exceed those predicted from surface densities on the basis of extragalactic star formation laws (e.g., Kennicut–Schmidt). This can for example be seen in Fig. 4 of Evans et al. (2009). In the present study, surface densities are replaced by total masses and star formation rates. This seems to lead to a better use of properties and terminologies: e.g., in extragalactic research surface densities are measured on scales of kpc, much larger than used in local clouds.
I would thus say that this paper clarifies terminologies, and collects more data than used before. For example, Fig. 2 of the present paper is very similar to Fig. 4 of Evans et al. (2009) — which however uses surface densities of mass and star formation rate.
Charles J. Lada, Jan Forbrich, Marco Lombardi, and Joao F. Alves; Star Formation Rates in Molecular Clouds and the Nature of the Extragalactic Scaling Relations
A very young Outflow in Orion MMS–6
In maps of dust continuum emission, as probed by bolometers, MMS–6 is the brightest core in the region to the very north of the Orion Nebula (OMC–3). Still, previous searches have failed to reveal any YSOs in this very dense and massive core, which was a bit surprising.
Using SMA data of sub–arcsecond resolution, the present paper now reveals a very compact, but clearly detected, outflow in this core. This is consistent with the expected evolutionary future of this region. Previous observations apparently did not have sufficient spatial resolution to detect this structure.
Most importantly, the small size of 1000 to 2000 AU, and the high outflow velocities of 10 to 40 km s-1, yield very small dynamical outflow ages of only 30 to 115 yr. Given the small IR luminosity of the region, these properties are consistent with a very recent formation of a YSO (“second core”) in this region. To me, this is the most believable claim of a very young YSO (i.e., first or second core) being detected inside a molecular cloud.
Satoko Takahashi and Paul Ho; Discovery of the Youngest Molecular Outflow associated with an Intermediate-mass protostellar Core, MMS-6/OMC-3
Solving the Luminosity Problem in Star Formation
As first noticed by Kenyon et al. (1990), the observed luminosities of YSOs are much lower than those expected if they were to accrete with the standard rate predicted for the collapse of 10 K singular isothermal spheres, i.e., 2×10-6 Msun yr-1. This is the so–called “luminosity problem” in star formation research. Kenyon et al. proposed that the actual accretion is thus lower on average, or that accretion occurs in bursts.
The present paper continues previous papers by Dunham and collaborators in which this problem is investigated using recent models and observations. This paper combines 23 hydrodynamical simulations of collapsing cores — predicting accretion rates and luminosities, as well as the physical disk–core structure — with radiative transfer calculations — predicting observational parameters like SED shapes and bolometric luminosities. After weighting with the IMF, and averaging of viewing directions, these model data can be used to construct, e.g., a theoretical bolometric temperature–luminosity diagram that can be compared to observations.
As also shown in previous work, the assumption of sporadic accretion does indeed solve the luminosity problem. And the model produces a reasonable match to the observed distribution of YSOs in the bolometric temperature–luminosity diagram. However, the observational sample against which models can be matched is still small (112 YSOs), and it is not clear whether the hydrodynamical simulations do indeed provide the best
Michael M. Dunham and Eduard I. Vorobyov; Resolving the Luminosity Problem in Low-Mass Star Formation
Three Types of Dense Cores in the Pipe Nebula
Based on observations of dense cores in the Pipe Nebula, this paper proposed the existence of three distinct types of cores:
- “diffuse” cores, at AV < 15 mag, that show poor chemistry with mainly simple species (e.g. CS and C2H);
- “oxo-sulfurated” cores, at AV = 15–22 mag, that appear to be abundant in species like SO and SO2, but also in HCO, which seem to disappear at higher densities; and
- “deuterated” cores, at AV > 22 mag, that show typical evolved chemistry prior to the onset of the star formation process, with nitrogenated and deuterated species, as well as carbon chain molecules.
This is all not too surprising, but a helpful concept.
What is also interesting is that these data are from a line survey covering the 80.0–89.5 and 97.5–105.0 GHz windows at 0.6 km s-1 velocity resolution. This was obtained using the IRAM 30m–telescope. To my understanding, these observations were obtained in a single shot per spectral window! This opens up entirely new avenues for emission line science.
P. Frau, J.M. Girart, M. T. Beltran; Chemical Differentiation toward the Pipe Nebula Starless Cores
Introduction to Bayesian Analysis in Astronomy
This article describes a few examples of Bayesian probability analysis in astronomy. This presents a rather practical approach, including problem implementation in actual code.
S. Andreon; Understanding better (some) astronomical data using Bayesian methods
Non–Ideal MHD prevents Catastrophic Breaking of Cicumstellar Disks
Magnetic fields can slow down the rotation of circumstellar disks. In simulations of ideal MHD, where the field is frozen into the gas, the breaking action is actually so strong that motions are too slow to provide for centrifugal support of disks. In this case, disks cannot form. However, the present paper shows that Ohmic dissipation and ambipolar diffusion reduce the field strength sufficiently for centrifugal motion to be possible.
Wolf B. Dapp, Shantanu Basu, Matthew W. Kunz; Bridging the gap: disk formation in the Class 0 phase with ambipolar diffusion and Ohmic dissipation
Interstellar Dust of Biologic Origin?
I was not aware that it has been argued that dust in space, or compounds in it, may be a product of biologic processes. (Of course, this idea has been introduced by Hoyle and his collaborators.) Interesting review on this slightly controversial subject.
N. Chandra Wickramasinghe; Interstellar Grains: 50 Years On
Large Radii of Population III Protostars: Accretion in Cluster Environments
Simulations of the stellar evolution of population III stars during their formation suggests that they had large radii of up to 1 AU and more. These large radii can influence their gas accretion. Also, large stellar radii may increase the probability of mergers, and mass transfer between stars could become more frequent. These issues are investigated in the present SPH simulation of a cloud forming stars, which is paired with a stellar evolution code to model the nature of the sink particles (i.e., stars). These calculations suggest that a few 10% of the stars may be influenced by close encounters of stars.
Rowan J. Smith, Takashi Hosokawa, Kazuyuki Omukai, Simon C. O. Glover and Ralf S. Klessen; Variable Accretion Rates and Fluffy First Stars
Westerlund I is bound
The cluster Westerlund I, located at a distance of 4 kpc somewhere in the Norma or Scutum–Centaurus arm, has a mass of order 5×104 Msun. The velocity dispersion of the stars residing in it has not been measured well in the past, the problem being that binary motions and too small samples influenced such measurements. The present paper infers a dispersion ~2 km s-1, though the uncertainties are significant. Still, with a confidence of 90%, these internal motions will not disrupt the cluster, i.e., the motions are subvirial. This suggests that Westerlund I may evolve into a globular cluster.
Michiel Cottaar, Michael R. Meyer, Morten Andersen and Pablo Espinoza; Is the massive young cluster Westerlund I bound?
Further Articles
Y. Aikawa, D. Kamuro, I. Sakon, Y. Itoh, H. Terada, J.A. Noble, K.M. Pontoppidan, H.J. Fraser, M. Tamura, R. Kandori, A. Kawamura, M. Ueno; AKARI observations of ice absorption bands towards edge-on YSOs
F.F.S. van der Tak, V. Ossenkopf, Z. Nagy, A. Faure, M. Roellig, E.A. Bergin; Detection of HF emission from the Orion Bar
Matthew J. Turk, Jeffrey S. Oishi, Tom Abel and Greg Bryan; Magnetic Fields in Population III Star Formation
H. Beuther et al.; Galactic structure based on the ATLASGAL 870mum survey
Oskari Miettinen, Jorma Harju, Lauri K. Haikala, Mika Juvela; A (sub)millimetre study of dense cores in Orion B9
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