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ScienceWeek
SCIENCEWEEK
ScienceWeek
August 22, 2003
Vol. 7 Number 34A
An Online Digest of Research in the Sciences
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And the end of all our exploring
Will be to arrive where we started
And know the place for the first time.
-- T.S. Eliot (1888-1965)
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Section 1
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Part A - Symposium: Star Formation
1. Introduction
2. The Formation of the First Star in the Universe
3. An Early Stellar Nursery
4. History of Element Creation and Star Formation
5. Massive Star Formation in 100,000 Years from Turbulent and
Pressurized Molecular Clouds
6. Low-Mass Relics of Early Star Formation
7. A Stellar Relic from the Early Milky Way
8. The Initial Mass Function of Stars: Evidence for Uniformity in
Variable Systems
9. Clustered Star Formation and The Origin Of Stellar Masses
10. Isolated Star Formation: From Cloud Formation to Core
Collapse
11. A Molecular Einstein Ring: Imaging a Starburst Disk
Surrounding a Quasi-Stellar Object
12. Black Holes at The Cosmic Dawn
Notices and Subscription Information
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Section 2
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1. INTRODUCTION
ON THE BIRTH OF STARS
One of the great achievements of 20th century science was the
detailed quantitative understanding of star formation. In
general, a star is born when a massive clump of gas contracts
under its own gravity, an idea first introduced by the
mathematician and physicist Pierre Simon de Laplace (1749-1827).
Laplace proposed that a rotating cloud of gas, as it pulled
itself together, would flatten into a disk, and the central
portion of the disk would gather itself into a ball to form a
star, while the outlying regions of the disk coalesced into
planets. Although physics and mathematics were not advanced
enough during the time of Laplace to develop the details of this
model, the general idea is still a good qualitative description
of the process that creates new stars.
Today we know that the most likely stellar nurseries are gigantic
molecular clouds, huge aggregations of cold gas, these clouds
containing a significant number of molecules, rather than merely
solitary atoms. As each would-be star collapses due to its self-
gravity, the gas retains its spin (angular momentum), and forms a
disk similar to that imagined by Laplace. The collapse compresses
the gas, causing the gas to increase in temperature. Some of the
rotational energy of the gas is carried away, possibly by
magnetic fields threading the cloud, allowing further collapse
and compression at the center. Eventually, most of the matter
accumulates at the center, while the rest remains in an
encircling disk. The central sphere of condensed gas, now a
"protostar", continues to contract and heat. As its temperature
rises, more and more of its hydrogen ionizes. Free electrons
scatter and absorb photons very effectively, so the more
electrons that are liberated, the more opaque the protostar
becomes. If photons cannot escape from the gas, their energy is
trapped within the protostar, causing the temperature to rise
even further. Finally, the temperature within the core of the
protostar rises to a sufficient level to ignite nuclear fusion,
and the energy generated from this process provides the newborn
star with the pressure required to prevent further collapse.
Thus, a star is born. The entire gravitational condensation
followed by nuclear fusion ignition may take 100 million years;
if the star is of "ordinary" mass (like our Sun), it will then
survive approximately 10 billion years before its nuclear fuel is
exhausted and it begins the phase of star death.
ON NEW STARS AND THE INTERSTELLAR MEDIUM
The very first stars were made only of hydrogen and helium; then
some of them exploded and enriched the interstellar medium (in
fact, we have never identified any of these primordial stars,
which must have all died before even the oldest stars that we see
were born). The next generation of stars was made out of slightly
enriched material, and the process repeated time and again until
today we have stars like the Sun, formed from interstellar
material about 4.5 billion years ago, when it had already been
enriched by many generations of exploding stars, over a
comparable span of time (the Milky Way is a little more than 10
billion years old).
The process is remarkably slow, by human standards. It is
estimated that the amount of interstellar material being reworked
into new stars each year in our Milky Way Galaxy at the present
time is less than about ten times the mass of our Sun. Since most
stars are smaller than the Sun, in round numbers you can say that
between ten and twenty new stars light up in our Galaxy each
year. But in 10 billion years, that means that 100 billion solar
masses of material, perhaps a third of the mass of all the stars
in our Galaxy today, and ten times the mass of the present
interstellar medium, has been recycled in this way. All this
requires is the ejection of about ten solar masses of recycled
material from stars over the Galaxy as a whole each year --
either in the form of stellar winds from red giants or in rare
supernova explosions -- to replace the material turning into new
stars.
There must also have been a much more intensive burst of
activity, in which tens of millions, possibly hundreds of
millions, of stars formed at the same time when the Universe was
young. We can see such spectacular activity going on in systems
known as "starburst" galaxies, sometimes as a result of tidal
interactions that occur when two galaxies pass close by each
other.
One implication of this continuing process of star formation and
recycling of interstellar material is that the interstellar
medium today is already richer in heavy elements than it was when
the Sun formed, so that stars forming today will have a different
concentration of chemical ingredients than the Sun. But they are
still the same ingredients -- stars that formed when the Galaxy
was young started out with fewer atoms of heavy elements overall
and with, for example, a higher proportion of oxygen, compared
with iron, than stars forming today. But they each started out
with traces of the same elements.
Adapted from: Stardust: Supernovae and Life - The Cosmic
Connection. John Gribbin. Yale University Press 2000, p.188.
More information at:
http://www.amazon.com/exec/obidos/ASIN/0300084196/scienceweek
ON PROTOSTARS
The following points are made by Thomas P. Greene (American
Scientist 2001 89:316):
1) Theoretical models of how a star forms -- its embryonic
development from a cold cloud of interstellar gas to a blazing
furnace of nuclear fusion -- had until recently outpaced the
astronomer's ability to observe the process. This peculiar state
of affairs in astrophysics is largely due to the reclusive nature
of embryonic stars. A stellar embryo grows in a cloudy womb of
molecular gas, which is so choked with dust that visible light
has little hope of passing through. A view of these molecular
clouds in even the largest optical telescope would reveal little
more than a dark patch of sky. Observations are further hindered
by the forbidding distances to the stars. The nearest region of
stellar birth is more than 400 light-years away. At such
distances, a prenatal star the size of our Sun is exceedingly
dim.
2) Despite such impediments, astrophysicists have been able to
piece together the broad outlines of how a low-mass (Sun-like)
star forms. It's a surprisingly complex process. Although it may
involve the simplest of elements and molecules, the making of a
star is directed by a maelstrom of competing forces -- including
gravitational collapse, magnetic fields, nuclear processes,
thermal pressures and fierce stellar winds -- all of which wish
to have their way with the unformed star. Because the interaction
of these forces is not fully understood, there is much that
remains mysterious about the birth of a star. How exactly does a
growing star accrete matter from its surroundings, and how does
the process stop? Why do stars form in the numbers and range of
sizes that they do? And why do some stars form planetary systems?
These are fundamental questions that cannot be answered without
actually observing the process of star formation.
3) Fortunately, the field of star formation has recently improved
as an observational science. Although visible light is unable to
penetrate the dusty cloud that swaddles a prenatal star, the
longer wavelengths of infrared radiation can easily slip through
the dust and so escape the inner confines of the cloud. Such
radiation has been detected for decades, but until recently
infrared telescopes have lacked the sensitivity and resolution to
provide detailed information about the youngest prenatal stars,
the protostars. With the development of giant telescopes and
extremely sensitive infrared detectors in the past decade,
astronomers have now been able to observe these secluded stellar
embryos.
TERMINOLOGY AND NOTES
The term "supernova" refers to a class of violently exploding
stars whose luminosity after eruption suddenly increases millions
or billions of times its normal level, the supernova explosion a
cataclysmic event associated with the essential end of the active
(energy-generating) life of the star.
Black hole: When a star exhausts its fuel and dies, it blows off
a considerable amount of material in a massive explosion. If the
terminal stages of star death leave a remnant star mass greater
than 3 solar masses, the ultimate gravitational collapse will
produce a black hole, a relativistic singularity. A black hole is
a localized region of space from which neither matter nor
radiation can escape. The "trapping" occurs because the requisite
escape velocity, which can be calculated from the relevant
equations, exceeds the velocity of light and is therefore
unattainable. Another view of a black hole is that it is a mass
that has collapsed to such a small volume that its gravity
prevents the escape of all radiation. Space and time essentially
have no meaning in a black hole. The boundary of the black hole
is called the "event horizon", because any event within the
boundary is invisible outside, the invisibility resulting from
the fact that no radiation can escape to be detected. The radius
of the black hole depends upon how much matter has fallen into
the region; it is called the "Schwarzchild radius", and it is
usually a few kilometers. However, massive black holes are
possible and are thought to be the source of quasars (quasi-
stellar objects), which are extremely luminous sources radiating
energy over the entire spectrum from x-rays to radio waves, and
which are apparently the oldest and most distant objects in the
universe. If quasars indeed involve black holes, the radiation is
from material just outside the black hole, and not from anything
within it. Nothing inside a black hole can get out of it.
Gravitational lens: An effect in which light rays are bent by the
gravitational field of a massive object such as a galaxy or black
hole. On cosmological scales, the effect is seen as the formation
of double or multiple images of a distant galaxy or quasar by a
foreground object.
Einstein ring: A circular image of a distant source produced by a
gravitational lens. Theoretically, such an image can be produced
when a point mass lies exactly on the line of sight to a distant
galaxy or quasar.
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2. THE FORMATION OF THE FIRST STAR IN THE UNIVERSE
The following points are made by Tom Abel et al (Science 2002
295:93):
1) Chemical elements heavier than lithium are synthesized in
stars. Such "metals" are observed, at times when the Universe was
only approximately less than 10% of its current age, in the
intergalactic medium (IGM) as absorption lines in quasar
spectra.(1). Hence, these heavy elements not only had to be
synthesized but also released and distributed in the IGM within
the first billion years. Only supernovae of sufficiently short-
lived massive stars are known to provide such an enrichment
mechanism. This leads to the prediction that the first generation
of cosmic structures formed massive stars (although not
necessarily only massive stars).
2) In the past 30 years, it has been argued that the first
cosmological objects formed globular clusters (2), supermassive
black holes (3), or even low-mass stars (4). This disagreement of
theoretical studies might at first seem surprising. However, the
first objects formed via the gravitational collapse of a
thermally unstable reactive medium, which inhibits conclusive
analytical calculations. The problem is particularly acute
because the evolution of all other cosmological objects (and in
particular the larger galaxies that follow) depends on the
evolution of the first stars.
3) Nevertheless, in comparison to present-day star formation, the
physics of the formation of the first star in the Universe are
rather simple: (i) The chemical and radiative processes in the
primordial gas are readily understood. (ii) Strong magnetic
fields are not expected to exist at early times. (iii) By
definition, no other stars exist to influence the environment
through radiation, winds, supernovae, etc. (iv) The emerging
standard model for structure formation provides appropriate
initial conditions.
4) In summary: The authors describe results from a fully self-
consistent three-dimensional hydrodynamical simulation of the
formation of one of the first stars in the Universe. In current
models of structure formation, dark matter initially dominates,
and pregalactic objects form because of gravitational instability
from small initial density perturbations. As they assemble via
hierarchical merging, primordial gas cools through ro-vibrational
lines of hydrogen molecules and sinks to the center of the dark
matter potential well. The high-redshift analog of a molecular
cloud is formed. As the dense, central parts of the cold gas
cloud become self-gravitating, a dense core of ~100 M (where M is
the mass of the Sun) undergoes rapid contraction. At particle
number densities greater than 10^(9) per cubic centimeter, a 1 M
protostellar core becomes fully molecular as a result of three-
body H2 formation. Contrary to analytical expectations, this
process does not lead to renewed fragmentation and only one star
is formed. The calculation is stopped when optical depth effects
become important, leaving the final mass of the fully formed star
somewhat uncertain. At this stage the protostar is accreting
material very rapidly (~10^(2) M per year). Radiative feedback
from the star will not only halt its growth but also inhibit the
formation of other stars in the same pregalactic object (at least
until the first star ends its life, presumably as a supernova).
The authors conclude that at most one massive (M >> 1 solar mass)
metal-free star forms per pregalactic halo, consistent with
recent abundance measurements of metal-poor galactic halo stars.
References (abridged):
1. S. L. Ellison, A. Songaila, J. Schaye, M. Pettini, Astron. J.
120, 1175 (2000)
2. P. J. E. Peebles and R. H. Dicke, Astrophys. J. 154, 891
(1968)
3. T. Hirasawa, Prog. Theor. Phys. 42, 523 (1969)
4. F. Palla, E. E. Salpeter, S. W. Stahler, Astrophys. J. 271,
632 (1983)
5. T. Abel, thesis, University of Regensburg, Germany (1995)
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3. AN EARLY STELLAR NURSERY
The following points are made by Philip Solomon (Nature 2003
424:382):
1) In the astronomical hunt for ever more distant objects, and
hence a window on the early Universe, the current leader(1) is
J1148 + 5251 at a redshift of 6.4. The light we now see from this
particularly luminous object was emitted only 800 million years
after the Big Bang, making it the youngest object known. J1148 +
5251 is a quasar (from "quasi-stellar" object). Whereas stars are
powered by nuclear fusion, a quasar's power is derived from
gravitation: quasars are powered by matter from rotating
accretion disks spiraling into massive black holes at the centers
of galaxies.
2) Observations of an object as young as J1148 + 5251 could
reveal much about the evolution of galaxies early in the history
of the Universe, and about the relation between the formation of
stars and massive black holes. New data are reported by Walter et
al.(2), and by Bertoldi et al.(3). These authors have detected
radiation at millimeter wavelengths from molecules of carbon
monoxide, indicating the presence of a large mass of interstellar
molecular gas, and from which the presence of molecular hydrogen
(the dominant component in molecular clouds) can be inferred.
This is the raw material from which stars form. These
measurements, combined with the observation of strong emission at
far-infrared wavelengths from interstellar dust(4) in this very
young galaxy, point to an ongoing burst of star formation that
began only a short time after the Big Bang.
3) The combination of high luminosity at far-infrared wavelengths
and a large mass of molecular gas and dust is an accepted
signature of star formation in galaxies. Young stars embedded in
the molecular clouds heat the interstellar dust, which then
radiates at infrared wavelengths. In the local Universe, many
spiral galaxies show significant infrared luminosity from star
formation, and the most powerful galaxies -- ultraluminous
infrared galaxies(5) -- all have large masses of molecular gas
and CO emission similar to that seen in the distant J1148 + 5251.
Most ultraluminous galaxies seem to have formed from collisions
between separate galaxies(5), and their molecular gas is found
concentrated in rotating disks or rings a few thousand light
years across -= a central region much smaller than the galaxies
but much larger than a quasar accretion disk. An analysis of the
CO emission from a quasar at lower redshift than J1148 + 5251
showed that the molecular gas is also distributed in a star-
forming ring with a size of about 6000 light years.
References (abridged):
1. Fan, X. et al. Astron. J. 125, 1649-1659 (2003)
2. Walter, F. et al. Nature 424, 406-408 (2003)
3. Bertoldi, F. et al. Astron. Astrophys. (in press)
4. Bertoldi, F. et al. Astron. Astrophys. (in press); preprint at
http://arxiv.org/astro-ph/0305116 (2003)
5. Sanders, D. B. et al. Astrophys. J. 325, 74-91 (1988)
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4. HISTORY OF ELEMENT CREATION AND STAR FORMATION
The following points are made by John Cowan (Nature 2003 423:29):
1) In the Big Bang at the beginning of the Universe, the lightest
elements, hydrogen, helium and lithium, were created. Two other
light elements (beryllium and boron) are produced in interstellar
space in interactions between cosmic-ray particles and gas
atoms(1). But all of the other elements that exist in nature have
been synthesized in nuclear reactions -- "nucleosynthesis" --
inside stars, from where they are ejected into interstellar space
and eventually find their way into new stars and planets.
2) Astronomers have made detailed studies of the synthesis of
elements in the Milky Way(2) and in some relatively nearby
galaxies(3). But little was known about the production of
elements, and the associated history of star formation, in the
most distant galaxies that formed early in the history of the
Universe. Prochaska et al.(4) report observations of the
abundance of elements in a galaxy far away and less than 2.5
billion years old. Their work opens a new window on the early
formation of elements and stars in the Universe.
3) A number of studies have explored "damped Lyman alpha" (DLA)
systems, in which clouds of hydrogen gas are detected through the
radiation they absorb from even more distant quasars. These
studies probe the earliest chemical history of gas in the
Universe, the gas that would form the first stars in the first
galaxies. Prochaska et al(4) were able to identify a DLA galaxy
along the line of sight to a more distant quasar (known as
FJ081240.6+320808). The distance of an object is usually
indicated in terms of its "redshift" -- how much the wavelength
of its emitted light has increased on its way to Earth, due to
the expansion of the Universe. The DLA galaxy has a redshift (Z)
of 2.626 and the quasar of 2.701. Such large shifts towards the
red end of the spectrum indicate that these objects are at great
distances: in this case, it took almost 12 billion years for the
light from this galaxy to reach Earth.(5)
References (abridged):
1. Fields, B. D. & Olive, K. A. Astrophys. J. 516, 797-810 (1999)
2. Sneden, C. & Cowan, J. J. Science 299, 70-75 (2003)
3. Shetrone, M. et al. Astron. J. 125, 684-706 (2003)
4. Prochaska, J., Howk, J. C. & Wolfe, A. M. Nature 423, 57-59
(2003)
5. Woosley, S. E. & Weaver, T. A. Astrophys. J. Suppl. Ser. 101,
181-235 (1995)
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5. MASSIVE STAR FORMATION IN 100,000 YEARS FROM TURBULENT AND
PRESSURIZED MOLECULAR CLOUDS
The following points are made by C.F. Mckee and J.C. Tan (Nature
2002 416:59):
1) Massive stars (with mass > 8 solar masses (M-sol) are
fundamental to the evolution of galaxies, because they produce
heavy elements, inject energy into the interstellar medium, and
possibly regulate the star formation rate. The individual star
formation time, t*f, determines the accretion rate of the star;
the value of the former quantity is currently uncertain by many
orders of magnitude(1-5), leading to other astrophysical
questions. For example, the variation of t*f with stellar mass
dictates whether massive stars can form simultaneously with low-
mass stars in clusters.
2) The authors demonstrate that t*f is determined by the
conditions in the star's natal cloud, and is typically 10^(5)
years. The corresponding mass accretion rate depends on the
pressure within the cloud -- which the authors relate to the gas
surface density -- and on both the instantaneous and final
stellar masses.
3) Characteristic accretion rates are sufficient to overcome
radiation pressure from 100(M-sol) protostars, while
simultaneously driving intense bipolar gas outflows. The weak
dependence of t*f on the final mass of the star allows high- and
low-mass star formation to occur nearly simultaneously in
clusters.
References (abridged):
1. Bernasconi, P. A. & Maeder, A. About the absence of a proper
zero age main sequence for massive stars. Astron. Astrophys. 307,
829-839 (1996)
2. McLaughlin, D. E. & Pudritz, R. E. Gravitational collapse and
star formation in logotropic and nonisothermal spheres.
Astrophys. J. 476, 750-765 (1997)
3. Stahler, S. W., Palla, F. & Ho, P. T. P. in Protostars &
Planets IV (eds Mannings, V., Boss, A. P. & Russell, S. S.) 327-
351 (Univ. Arizona Press, Tucson, 2000).
4. Behrend, R. & Maeder, A. Formation of massive stars by growing
accretion rate. Astron. Astrophys. 373, 190-198 (2001)
5. Osorio, M., Lizano, S. & D'Alessio, P. Hot molecular cores and
the formation of massive stars. Astrophys. J. 525, 808-820 (1999)
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6. 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)
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7. A STELLAR RELIC FROM THE EARLY MILKY WAY
The following points are made by N. Christlieb et al (Nature 2002
419:904):
1) The chemical composition of the most metal-deficient stars
largely reflects the composition of the gas from which they
formed. These old stars provide crucial clues to the star
formation history and the synthesis of chemical elements in the
early Universe. They are the local relics of epochs otherwise
observable only at very high redshifts(1,2).
2) If totally metal-free ("population III") stars could be found,
this would allow the direct study of the pristine gas from the
Big Bang. Earlier searches for such stars found none with an iron
abundance less than 1/10,000 that of the Sun(3,4), leading to the
suggestion(5) that low-mass stars could form from clouds above a
critical iron abundance.
3) The authors report the discovery of a low-mass star with an
iron abundance as low as 1/200,000 of the solar value. This
discovery suggests that population III stars could still exist --
that is, that the first generation of stars also contained long-
lived low-mass objects. The previous failure to find them may be
an observational selection effect.
References (abridged):
1. Norris, J. E., Ryan, S. G. & Beers, T. C. Extremely metal-poor
stars. VIII. High-resolution, high signal-to-noise ratio analysis
of five stars with [Fe/H] < -3.5. Astrophys. J. 561, 1034-1059
(2001)
2. Cohen, J. G., Christlieb, N., Beers, T. C., Gratton, R. &
Carretta, E. Stellar archaeology: A Keck pilot program on
extremely metal-poor stars from the Hamburg/ESO survey. I.
Stellar parameters. Astron. J. 124, 470-480 (2002)
3. Bond, H. E. Where is population III? Astrophys. J. 248, 606-
611 (1981)
4. Beers, T. C. in The Third Stromlo Symposium: The Galactic Halo
(eds Gibson, B. K., Axelrod, T. S. & Putman, M. E.) Astron. Soc.
Pacif. Conf. Ser. 165, 202-212 (1999)
5. Bromm, V., Ferrara, A., Coppi, P. S. & Larson, R. B. The
fragmentation of pre-enriched primordial objects. Mon. Not. R.
Astron. Soc. 328, 969-976 (2001)
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8. THE INITIAL MASS FUNCTION OF STARS: EVIDENCE FOR UNIFORMITY IN
VARIABLE SYSTEMS
The following points are made by P. Kroupa (Science 2002 295:63):
1) The physics of star formation determines the conversion of gas
to stars. The outcome of star formation are stars with a range of
masses. Astrophysicists refer to the distribution of stellar
masses as the stellar "initial mass function" (IMF). Together
with the time-modulation of the star-formation rate, the IMF
dictates the evolution and fate of galaxies and star clusters.
2) The evolution of a stellar system is driven by the relative
initial numbers of brown dwarfs [BDs, < 0.072 times the mass of
the Sun (M-sol)] that do not fuse H to He, very-low-mass stars
(0.072 to 0.5 M-sol), low-mass stars (0.5 to 1 M-sol),
intermediate-mass stars (1 to 8 M-sol), and massive stars (m > 8
M-sol). Nonluminous BDs through to dim low-mass stars remove gas
from the interstellar medium (ISM), locking-up an increasing
amount of the mass of galaxies over cosmological time scales.
Intermediate and luminous but short-lived massive stars expel a
large fraction of their mass when they die and thereby enrich the
ISM with elements heavier than H and He. They heat the ISM
through radiation, outflows, winds, and supernovae (1,2).
3) It is therefore of much importance to quantify the relative
numbers of stars in different mass ranges and to find systematic
variations of the IMF with different star-forming conditions.
Identifying systematic variations of star formation would allow
us to understand the physics involved in assembling each of the
mass ranges, and thus to probe early cosmological events.
4) Determining the IMF of a stellar population with mixed ages is
a difficult problem. Stellar masses cannot be weighed directly in
most instances (3), so the mass has to be deduced indirectly by
measuring the star's luminosity and evolutionary state. The
history of the subject began in 1955 at the Australian National
University, when Edwin E. Salpeter published the first estimate
(4) of the IMF for stars in the solar-neighborhood (5).
5) In summary: The distribution of stellar masses that form in
one star formation event in a given volume of space is called the
initial mass function (IMF). The IMF has been estimated from low-
mass brown dwarfs to very massive stars. Combining IMF estimates
for different populations in which the stars can be observed
individually unveils an extraordinary uniformity of the IMF. This
general insight appears to hold for populations including
present-day star formation in small molecular clouds, rich and
dense massive star-clusters forming in giant clouds, through to
ancient and metal-poor exotic stellar populations that may be
dominated by dark matter. This apparent universality of the IMF
is a challenge for star formation theory, because elementary
considerations suggest that the IMF ought to systematically vary
with star-forming conditions.
References (abridged):
1. D. Chappell and J. Scalo, Mon. Not. R. Astron. Soc. 325, 1
(2001)
2. G. Hensler, Astrophys. Space Sci. 265, 397 (1999)
3. Stellar masses can be measured directly in binary systems.
Unfortunately, the Kepler orbits are available only for very few
well-studied cases. These do not constitute a volume-limited
unbiased sample.
4. E. E. Salpeter, Astrophys. J. 121, 161 (1955)
5. The solar neighborhood is the region of the Milky Way close to
the Sun. There is no definition of the exact radius of this
region, and it is admissible to refer to the immediate solar
neighborhood (within 5 pc), the solar neighborhood (within about
25 pc) and the extended solar neighborhood (within a few hundred
pc). On the northern hemisphere virtually all very-low-mass stars
in the immediate neighborhood (93) and all low-mass stars in the
neighborhood are known and have accurate distance measurements
using trigonometric parallax. Their properties are cataloged in
the Jahreiss-Gliese Catalogue of Nearby Stars (7) (available
online at http://www.ari.uni-heidelberg.de/aricns/).
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9. 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)
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10. ISOLATED STAR FORMATION: FROM CLOUD FORMATION TO CORE
COLLAPSE
xThe following points are made by Derek Ward-Thompson (Science
2002 295:63):
1) Stars are among the most fundamental building blocks of the
universe, and yet the processes by which they are formed are not
understood. Models can give different predictions for the masses,
densities, and temperatures of the objects formed, even if they
assume only slightly different initial conditions. The variations
of the initial density, temperature, velocity, and magnetic field
are crucial to the gravitational collapse of molecular clouds
(the chief sites of star formation), but one of the main problems
is that the initial conditions that pertain in the clouds from
which stars form are still not known sufficiently accurately.
This gap is currently one of the major limiting factors in the
understanding of the star formation process, at least for
relatively low-mass stars [~0.2 to 3 times the mass of the Sun
(M-sol)]. It is believed that different physical mechanisms
dominate in isolated star-forming regions (which are more quasi-
static) and cluster-forming regions (which are more dynamic).(1-
3).
2) In the quasi-static picture of isolated star formation, there
are a number of distinct stages that can be identified, starting
from the diffuse matter occupying the space between the stars,
known as the interstellar medium (ISM), with a volume number
density of H atoms (n) approximately 1 H atom/cm^(3) (in the
solar neighborhood) through to regions known as diffuse clouds,
with a volume number density of H atoms (n) approximately 10 to
100 H atoms/cm3 and temperature approximately 30 to 50 K. The
more dense parts of the ISM are known as "molecular clouds",
because the gas within them is primarily molecular and of higher
density and lower temperature [(n) approximately 10^(3)
H2/cm^(3), temperature (T) approximately 20 to 30 K]. The gas is
molecular for two reasons: (i) the higher density provides a
shorter mean free path for collisions between the atomic gas and
dust grains (the chief molecule formation mechanism is via
surface interactions on dust grains) and hence a higher formation
rate of molecules; and (ii) the molecules are not dissociated by
the ultraviolet (UV) component of the interstellar radiation
field, because the embedded dust extinguishes the UV radiation
and shields the molecules (4); approximately 1% of a molecular
cloud's mass is in the form of silicate (with some carbonaceous)
dust grains approximately 0.1 microns in size.
3) In summary: The formation of stars is one of the most
fundamental problems in astrophysics, as it underlies many other
questions, on scales from the formation of galaxies to the
formation of the Solar System. The physical processes involve the
turbulent behavior of a partially ionized medium containing a
non-uniform magnetic field. Current debate centers around the
time taken for turbulence to decay and the relative importance of
the roles played by magnetic fields and turbulence. Technological
advances such as millimeter-wave cameras have made possible
observations of the temperature and density profiles, and
statistical calculations of the lifetimes, of objects collapsing
under their own self-gravity and those on the verge of collapse.
Increased computing power allows more complex models to be made
that include magnetic and turbulent effects. No current model can
reproduce all of the observations.
References (abridged):
1. The exact definitions of the words "clustered" and "isolated"
are open to interpretation. The author defines "isolated star
formation" as referring to stars that form without substantial
interference from the winds and outflows of neighboring stars.
2. R. Pudritz, Science 295, 68 (2002)
3. For reviews of binary star formation, see the proceedings of
IAU Symposium 200, The Formation of Binary Stars, B. Reipurth, H.
Zinnecker, Eds. Reidel, Dordrecht, Netherlands, 2001.
4. For a general review of the structure of molecular clouds, see
J. P. Williams, L. Blitz, C. F. McKee, in Protostars and Planets
IV, V. Mannings, A. P. Boss, S. S. Russell, Eds. Univ. of Arizona
Press, Tucson, AZ, 2000, pp. 97-120.
5. E. N. Parker, Astrophys. J. 145, 811 (1966); E. N. Parker,
Cosmical Fields: Their Origin and Their Activity Clarendon Press,
Oxford, 1979.
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11. A MOLECULAR EINSTEIN RING: IMAGING A STARBURST DISK
SURROUNDING A QUASI-STELLAR OBJECT
The following points are made by C.L. Carilli et al (Science 2003
300:773):
1) Establishing a link between galaxy formation and massive black
hole formation has become of paramount importance to
observational astronomy since the correlation between black hole
mass and stellar bulge mass in nearby [redshift (z) < 0.1]
galaxies was discovered (1). This correlation suggests a "causal
connection between the formation and evolution of the black hole
and the bulge" (2) and has led to the hypothesis of coeval
formation at high redshift (z > 2) of massive black holes and
spheroidal galaxies (3–5). Supermassive black holes [10^(9) solar
masses (M-sol)] at high redshift manifest themselves as optically
luminous quasi-stellar objects (QSOs), powered by mass accretion
onto the hole.
2) Observations of high-redshift QSOs have shown that 30% are
luminous infrared (IR) sources, with far infrared (FIR)
luminosities 10^(13) times the luminosity of the Sun (L-sol),
corresponding to thermal emission from warm dust. The key
question for these FIR luminous QSOs is: What is the dominant
dust-heating mechanism: star formation or the active galactic
nucleus (AGN)?
3) If the dust is heated by star formation, the star formation
rates must be on the order of 10^(3) M-sol/year, supporting the
idea of coeval formation of the stars and black holes in these
systems. Molecular (CO) line emission has been detected from a
number of these FIR-luminous high-redshift QSOs, which implies
that there are large reservoirs of molecular gas (>10^(10) M-sol)
and suggests that star formation may be inevitable. However, in
most cases the FIR luminosity corresponds to only 10% of the
bolometric luminosity of the system, and hence the case for
coeval star formation and mass accretion onto a supermassive
black hole at high redshift remains circumstantial.
4) In summary: Images of the molecular CO 2-1 line emission and
the radio continuum emission from the redshift 4.12
gravitationally lensed quasi-stellar object (QSO) PSS J2322+1944
reveal an Einstein ring with a diameter of 1.5". These
observations are modeled as a star-forming disk surrounding the
QSO nucleus with a radius of 2 kiloparsecs. The implied massive
star formation rate is 900 solar masses per year. At this rate, a
substantial fraction of the stars in a large elliptical galaxy
could form on a dynamical time scale of 10^(8) years. The
observation of active star formation in the host galaxy of a
high-redshift QSO supports the hypothesis of coeval formation of
supermassive black holes and stars in spheroidal galaxies.
References (abridged):
1. L. Ferrarese, D. Merritt, Astrophys. J. 539, L9 (2000)
2. K. Gebhardt et al., Astrophys. J. 539, L13 (2000)
3. A. W. Blain et al., Mon. Not. R. Astron. Soc. 309, 715 (1999)
4. G. Kauffmann, M. Haehnelt, Mon. Not. R. Astron. Soc. 311, 576
(2000)
5. M. J. Page, J. A. Stevens, J. Mittaz, F. J. Carrera, Science
294, 2516 (2001)
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12. BLACK HOLES AT THE COSMIC DAWN
The following points are made by Xiaohui Fan (Science 2003
300:752):
1) When did the first generation of galaxies and quasars form?
How did these first sources of light end the cosmic "dark ages"?
And what is the relation between the star formation in the first
galaxies and the initial growth of supermassive black holes in
the first quasars?
2) The most distant galaxies and quasars with confirmed redshifts
are at approximately 6.6 (2) and 6.4 (3), respectively. At these
high redshifts, the Universe was less than 1 billion years old
and the first generations of galaxies and black holes were
forming. At the other end of cosmic history, Hubble Space
Telescope observations have shown that most, if not all, galaxies
contain supermassive black holes. The masses of the black holes
are tightly correlated with the velocity dispersions and masses
of their host galaxies (4,5). This result suggests that the
evolution of the black holes and the galaxies are connected, such
that the process responsible for the assembly of the galaxy also
feeds the growth of the black holes.
3) Optically bright quasars represent the critical phase of black
hole evolution when it is acquiring most of its mass. The
luminous quasars at redshifts of > 6 likely represent black holes
with several billion solar masses (M-sol), residing in a halo of
~10^(13) M-sol -- an amazing feat of early structure formation. A
growing body of evidence suggest that high-redshift quasars are
accompanied by intense star-formation activities on a galaxy
scale.
4) First, the chemical composition of quasars hints at early
enrichments, indicative of star formation. Emission lines in the
quasar spectrum can be used to measure their abundance of heavy
elements, or "metallicity." Luminous, high-redshift quasars have
roughly solar or higher metallicity, even at redshifts > 6,
indicating that they existed in a metal-rich environment similar
to that found in the centers of massive galaxies. The existence
of large amounts of heavy elements in quasars, only a few hundred
million years after the Big Bang, has important implications for
the initial starburst and initial growth of the black hole: These
events must have happened at a redshift much higher than 6, and
must have coevolved to the observed epoch.
5) Second, quasars can serve as markers of concentrated galaxy
formation. The luminous high-redshift quasars represent the
rarest and highest mass density peaks in the early universe. The
surroundings of luminous high-redshift quasars are among the most
active galaxy-forming environments, with a high concentration of
young galaxies.
6) Third, dust and molecular emission from quasars can be closely
linked to star formation. A substantial fraction of radio-quiet
quasars at redshifts of ~4 show strong thermal dust emission.
Extensive surveys of high-redshift quasars have been carried out
with the IRAM 30-m telescope on Pico Veleta, Spain, and at the
James Clerk Maxwell Telescope on Mauna Kea, Hawaii. The detected
quasars have far-infrared luminosities of ~10^(12) to 10^(13)
solar luminosities (L-sol), and implied dust masses of ~10^(8) M-
sol. Some of them exhibit molecular gas emission from CO,
indicating molecular masses of ~10^(11) M-sol. The detection of
large reservoirs of molecular gas provides strong circumstantial
evidence of possible active star formation in the quasar host
galaxy.
References (abridged):
1. C. L. Carilli et al. Science 300, 773 (2003)
2. E. M. Hu et al., Astrophys. J. Lett. 568, L75 (2002)
3. X. Fan et al., Astron. J. 125, 1649 (2003)
4. K. Gerbhardt et al., Astrophys. J. Lett. 539, L13 (2001)
5. L. Ferrarese, D. Merritt, Astrophys. J. Lett. 539, L9 (2001)
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