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ScienceWeek
ASTROPHYSICS: ON STAR MASS
The following points are made by I. Neill Reid (Nature 2005 433:207):
1) Size matters for stars. Individually, a star's mass determines which fusion reactions are possible in the core, and hence its luminosity, surface temperature, and the length of its life. For example, objects less massive than approximately 0.075 times the mass of the Sun (about 75 Jupiter masses) never ignite hydrogen fusion, and therefore fade to obscurity in 100 million years or less; these are "brown dwarfs". Collectively, the "initial mass function", the number of stars and brown dwarfs forming as a function of mass, is critical in assessing the total amount of matter in the Milky Way. Until recently, this task loomed large, but within the past five years it has become clear that brown dwarfs are not viable candidates for "dark matter", the invisible material that constitutes some 90% of the mass of the Milky Way. The initial mass function is also important for testing theories of star formation, and remains vital for understanding how the material in molecular clouds is redistributed to form stars. In particular, we would like to know whether there is a low-mass cut-off to the formation process. Unfortunately, we cannot measure stellar mass directly, and although there are indirect techniques for accurately determining the mass of mature stars, these are unreliable for young stars.
2) To weigh a star, we have to determine the effects of its mass. Thus, we measure how mass regulates the orbits of objects in binary-star or planetary systems; how it affects the wavelength of light through gravitational redshift (about 0.5 km/s for the Sun); and its effects on the brightness, shape and/or location of distant objects in gravitational lensing. Each of these techniques can be applied only to a small number of objects, but luminosity and mass are correlated in mature hydrogen-burning stars, which follow a well-defined evolutionary sequence; as a result, we can map out a mass-luminosity relation using the relatively small number of stars in binary systems whose masses have been accurately determined. It is this relationship that provides the basis for the derivation of the mass function of stars in the solar neighborhood.
3) The mass-luminosity calibration, however, is not appropriate either for brown dwarfs, which never reach a stable configuration, or for young stars (those less than 100 million years old), which are still contracting onto the well-defined sequence followed by mature stars and therefore have larger radii and higher luminosities. This is unfortunate, because clusters of young stars offer the best hunting ground for low-mass stars and brown dwarfs exactly because those objects are more luminous and therefore easier to identify and characterize. Star-forming regions such as the Orion nebula (which is about 5 million years old) provide the best opportunity for identifying isolated brown dwarfs that have planet-size masses. However, the masses of individual objects can be derived only using theoretical models, matching the observed luminosities and temperatures against predicted mass-luminosity relations for the appropriate age. These analyses generally indicate that the mass function is turning over in the brown-dwarf regime, with perhaps 30% as many brown dwarfs as stars; moreover, a few studies have claimed to detect objects with masses as low as 1-2 Jupiter masses, suggesting that we have yet to reach a lower mass limit.
Nature http://www.nature.com/nature
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CLUSTERED STAR FORMATION AND THE ORIGIN OF STELLAR MASSES
The following points are made by Ralph E. Pudritz (Science 2002 295:63):
1) Stellar clusters are among nature's most beautiful and intriguing astronomical objects. They are associated with every type of galaxy and range from hundreds of stars, as is commonly observed for young star clusters in the disk of the Milky Way (1-3), to the millions of stars that populate the super star clusters (SSCs) in prototypical starburst galaxies such as M82 (4,5) and interacting galaxies such as the Antennae.
2) Stellar clusters were also among the first systems that formed as galaxies were assembled billions of years ago, as is evidenced by the ubiquitous presence of globular star clusters around galaxies of all kinds. Globular clusters are akin to SSCs in their mass and size but are the oldest objects yet discovered in the universe, ranging in age from 12 to 15 billion years, the oldest being found in a more spherical spatial distribution in the halos of galaxies.
3) There is also growing evidence that the stellar content of any star cluster, as measured by the mass spectrum of the stars that compose it [the initial mass function (IMF)], is fairly robust and independent of environment. Star clusters form at all epochs of galactic evolution, are associated with galaxies of all Hubble types, and have similar IMFs, which suggest a common and robust mechanism of star formation.
4) Stars in the Milky Way and other nearby galaxies form in cold [temperature (T) approximately 10 to 20 K], self-gravitating molecular clouds whose masses lie in the range from 10^(3) to 10^(6.5) solar masses [the Sun's mass (M-sol) = 2 × 1033 g]. Infrared (IR) observations of young embedded stars within clouds reveal that their formation is restricted to smaller regions of higher than average gas density called "clumps". One of the important recent advances in star formation research is the realization that most stars form as members of star clusters within such clumps and not in isolation from one another. Star clusters are therefore not exotic novelties in the universe but are the representative products of the process of star formation.
5) In summary: Star clusters are ubiquitous in galaxies of all types and at all stages of their evolution. We also observe them to be forming in a wide variety of environments, ranging from nearby giant molecular clouds to the supergiant molecular clouds found in starburst and merging galaxies. The typical star in our galaxy and probably in others formed as a member of a star cluster, so star formation is an intrinsically clustered and not an isolated phenomenon. The greatest challenge regarding clustered star formation is to understand why stars have a mass spectrum that appears to be universal.
References (abridged):
1. C. J. Clarke, I. A. Bonnell, L. A. Hillenbrand, in Protostars and Planets IV, V. Mannings, A. P. Boss, S. S. Russell, Eds. Univ. of Arizona Press, Tucson, AZ, 2000, pp. 151-177.
2. B. G. Elmegreen, Y. Efremov, R. E. Pudritz, H. Zinnecker, in Protostars and Planets IV, V. Mannings, A. P. Boss, S. S. Russell, Eds. Univ. of Arizona Press, Tucson, AZ, 2000, pp. 179-215.
3. C. J. Lada, in The Origins of Stars and Planetary Systems, C. J. Lada, N. D. Kylafis, Eds. Kluwer, Dordrecht, Netherlands, 2000.
4. R. W. O'Connell, J. S. Gallagher, D. A. Hunter, W. N. Colley, Astrophys. J. 446, L1 (1995)
5. J. S. Gallagher and L. J. Smith, Mon. Not. R. Astron. Soc. 304, 540 (1999)
Science http://www.sciencemag.org
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LOW-MASS RELICS OF EARLY STAR FORMATION
The following points are made by R. Schneider et al (Nature 2003 422:869):
1) The earliest stars to form in the Universe were the first sources of light, heat and metals after the Big Bang. The products of their evolution would have had a profound impact on subsequent generations of stars.
2) Recent studies(1-5) of primordial star formation have shown that in the absence of metals (elements heavier than helium), the formation of stars with masses 100 times that of the Sun would have been strongly favored, and that low-mass stars could not have formed before a minimum level of metal enrichment had been reached. The value of this minimum level is very uncertain, but is likely to be between 10^(-6) and 10^(-4) that of the Sun.
3) The authors demonstrate that the recent discovery of the most iron-poor star known indicates the presence of dust in extremely low-metallicity gas, and that this dust is crucial for the formation of lower-mass second-generation stars that could survive until today. The dust provides a pathway for cooling the gas that leads to fragmentation of the precursor molecular cloud into smaller clumps, which become the lower-mass stars.
References (abridged):
1. Omukai, K. & Nishi, R. Formation of primordial protostars. Astrophys. J. 508, 141-150 (1998)
2. Abel, T., Bryan, G. & Norman, M. The formation of the first star in the universe. Science 295, 93-98 (2002)
3. Bromm, V., Coppi, P. S. & Larson, R. B. The formation of the first stars. I. The primordial star-forming cloud. Astrophys. J. 564, 23-51 (2002)
4. Ripamonti, E., Haardt, F., Ferrara, A. & Colpi, M. Radiation from the first forming stars. Mon. Not. R. Astron. Soc. 334, 401-418 (2002)
5. Nakamura, F. & Umemura, M. The stellar initial mass function in primordial galaxies. Astrophys. J. 569, 549-557 (2002)
Nature http://www.nature.com/nature
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