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Report of the Star Formation and Stellar Evolution Working Group
F. Adams (Univ. of Michigan),
J. Bieging (Univ. of Arizona), Chair,
G. Blake (Caltech),
L. Bronfman (Univ. de Chile),
C. Chandler (NRAO),
E. Churchwell (Univ. of Wisconsin),
R. Crutcher (Univ. of Illinois),
N. Evans (Univ. of Texas),
G. Knapp (Princeton),
J. Mangum (Univ. of Arizona/SMTO),
R. Mathieu (Univ. of Wisconsin),
M. Meixner (Univ. of Illinois),
M. Morris (UCLA),
L. Mundy (Univ. of Maryland),
P. Myers (CfA),
W. Peters (Univ. of Arizona/SMTO),
A. Sargent (Caltech),
C. K. Walker (Univ. of Arizona),
D. Wilner (CfA),
A. Wootten (NRAO)
The Millimeter Array will provide astronomers with unprecedented
capabilities for observational studies of stars, from the phenomena which lead
to their formation to the events which surround their final stages of evolution.
Our working group looked in some detail at what we believe to be the most
important topics in the fields of both star formation and the final stages of
stellar evolution, and considered how the MMA will be able to address these
problems. We present here the highlights of our discussions. In each area,
the MMA will make major contributions to our state of knowledge. We also
expect that the unprecedented flexibility and sensitivity of the proposed
instrument will open up previously unanticipated areas of research, and so
ensure additional dramatic advances in our understanding of star formation
and evolution.
One of the most fundamental goals in the field of star formation is the
measurement of the motions associated with gravitational infall leading to star
formation. Infall velocities and spatial structure are key to our understanding
of how stars form. The MMA will greatly improve our knowledge of this
fundamental process, by extending inward to the scales of the solar system
(< 100 AU), and by allowing us to make sensitive images of hundreds of infalling
objects. The wide frequency coverage of the array and the broad spectral
bandwidth will allow complementary imaging in spectral lines which trace infall
motions (e.g., H
CO 2
-1
at 2 mm wavelength, and C
S
J = 5-4 at 1.2 mm),
outflow motions (e.g. CO 2-1 at 1.3 mm), and the motions of the more extended
ambient gas (e.g. N
H
J=1-0 and C
S J=2-1 at 3 mm). Comparison of such maps
is essential to determine accurately the structure of each type of motion.
The array will be sensitive enough to image infall regions with a
velocity resolution of 0.02 km s
, good enough to resolve details in the line
structure which reveal information about the velocity patterns and opacity.
To estimate the rms brightness
temperature in a practical observation, we assume a synthesized beam of diameter
2
, or 300 AU at the distance (150 pc) of the star-forming complexes in
Taurus, Ophiuchus, Lupus, Chamaeleon, and Corona Australis. The standard array
sensitivity formula gives rms brightness temperatures of 0.2, 0.03, and 0.024
K, for wavelengths of 3, 1.1, and 0.45 mm, respectively for an eight-hour
integration. The line brightness in
single-dish spectra is typically 
3K, so the MMA will be capable of achieving
high signal-to-noise ratios, especially at the shorter wavelengths.
A wealth of indirect evidence shows that disks of gas and dust exist around
recently formed solar-type stars, and that the properties of these disks
resemble those of the early Solar System. These disks play a central role in
the formation of stars. They are the conduit through which material from dense
molecular cores flows onto protostars. Disks are probably the origin of
collimated molecular outflows, and they regulate the angular momentum evolution
of young stars. Disks are also the birthplaces of planets. The MMA will permit
the first exploration of the structure of nearby protostellar disks on Solar
System size scales. The MMA should also be able to detect a proto-Jupiter in
nearby star-forming regions.
As a proto-Jovian planet evolves, it contracts and cools (Bodenheimer and
Pollack, 1986, Icarus, 67, 391). The present Jupiter at 1 pc distance would
emit only 10 
Jy at 0.85 mm, where the sensitivity of the MMA is greatest.
However, in the first few million years of its formation, a proto-Jovian planet
may reach a luminosity of 10
that of the Sun; at 160 pc, its signal would be
about 0.1 mJy at 0.85 mm. Such an object would be detected at a 3 
level
in one hour with the nominal MMA. (The detection limits for proto-Jupiters at
the distance of several star-forming clouds are illustrated in Figure 1.)
The resolution of the nominal array is 9 AU at 0.85 mm; at least 3 AU
resolution is needed to resolve a proto-planet at the separation of our Jupiter.
This could be achieved if three antennas were placed at 10 km baselines. The
sensitivity on these long baselines would only be a factor of 3 lower than the
full array. Hence a proto-Jupiter at Jupiter's distance from a young star could
be detected and resolved by an MMA with a few longer baselines in one day's
integration, provided that continuum dynamic ranges currently achievable with
the VLA can also be realized with the MMA. The orbital motion of such a
proto-Jupiter could be detected in a few years. With a portion of the orbit
curve the mass ratio of the star/planet system could be determined, providing
definitive evidence of the detection of an extra-solar planet.
Jovian companions are expected to clear gaps in disks; indeed it has been
argued that the formation of the inner solar system required the formation of
Jupiter in order to terminate the accretion flow of the protosolar disk onto
the Sun. At the same time, the coagulation of dust into planetesimals and
ultimately planets will clear the planet-forming domains of disks. The presence
of gaps or inner holes in disks may be one of the most readily accessible
signatures of the formation of a planetary system.
Theories for the formation of the Jovian and terrestrial planets depend
critically on the dissipation timescale of the gas in the disk, and the
coagulation timescale of the dust grains. The capability of the MMA to image
such systems at high resolution in optically thin molecular lines of many
different species and isotopomers will allow us to determine the effects of
molecular depletion onto dust grains, and so to measure accurately the amount
and distribution of gas in protoplanetary disks.
More than 50% of the stars in the solar neighborhood are binary, and recent
surveys suggest that the frequency of binaries among young stars may be even
higher. Roughly half of these binaries have separations comparable to or less
than typical disk radii of 100 AU. Binary companions should therefore play a
dominant role in protostellar disk evolution. These stellar companions will
clear large gaps in associated disks. In addition, the companions will
terminate the flow of material from the outer circumbinary disks to the inner
circumstellar disks. In this case, if accretion onto the stars continues, the
circumstellar disks will become depleted, ending any possibility of planet
formation. Interferometric observations of young binaries have already
demonstrated that systems with companion separations of less than about 100 AU
emit much less mm-wavelength continuum emission than do single stars. The MMA
will have sensitivity to measure disk emission from large samples of such stars
to test whether the diminution of flux is due to clearing of material by binary
companions.
The MMA will be able to resolve the structure of disks in binary
environments. The size and morphology of gaps will test the dynamical theory,
which is also fundamental to theories of planet formation. In addition, the MMA
will measure directly the masses of the circumstellar disks in young binary
systems, constraining disk accretion physics and reflecting on the planet
formation rate in binary systems. Finally, the circumbinary disk masses are a
primary discriminant of binary formation theories--for example, capture
scenarios predict dispersal of circumbinary material.
Molecular outflows are a ubiquitous feature of the star formation process,
yet neither the mechanism nor the evolution of outflows is understood. The
competition between infall and outflow plays a key role in determining the
initial stellar mass function and the structure of any planet-forming disk.
The key to understanding the small scale structure of outflows is high
resolution images of the acceleration and entrainment region. With a resolution
of 0
1, a region of 15 AU can be resolved at the distance of the nearest star
forming clouds and a brightness temperature of 1.3 K can be detected in eight
hours. The combination of CO and hydrogen recombination lines at the above
resolution will permit the entrainment process to be observed very near the
origin of the flow.
On the intermediate scale, outflows interact with the surrounding medium
to produce shocked interfaces. The physics and chemistry of those shocks can be
studied with unprecedented resolution and sensitivity with the MMA using probes
which cover all components from pre-shock to post-shock regions. Although the
shocks themselves cannot be resolved, the changes in physical properties
(density, temperature, ionization, velocity structure, etc.) can be used to
dissect shock interactions. High angular resolution is essential to provide
information on the re-formation of molecules and dust in post-shocked regions.
On the largest scales, outflows associated with massive star formation
regions provide a major injection of kinetic energy into the interstellar
medium. With the MMA it will be possible to map essentially every massive
outflow in the Galaxy, thus providing a global estimate of the kinetic energy
input. It would be possible to do the same in the LMC if the MMA were located
in Chile.
Magnetic fields play a crucial role in star formation. Fields support
clouds against collapse, ambipolar diffusion provides a mechanism for
fragmentation and core formation, and magnetic braking solves the angular
momentum problem. The MMA will provide a unique capability to map magnetic
fields on the relevant scales with the required sensitivity. Linear
polarization maps of warm dust emission at wavelengths near 1 mm will
provide both the overall field morphology and its small-scale structure,
and should provide information on MHD waves and/or turbulence.
The MMA will also provide the best opportunity for the detection and study
of circular polarization in emission lines. Mapping the circular polarization
of molecular lines caused by the Zeeman effect, such as the J = 3-2 transition
of CCS at 33 GHz and the N = 1-0 transitions of CN at 113 GHz, will probe
densities of 
and 
cm
respectively with typical sensitivities
(1
) of 0.06 milliGauss for an eight-hour synthesis.
(The Zeeman effect has
recently been detected in the N=1-0 transition of CN with single dish
observations, with measured fields of 0.5 to 1.0 mG.) The MMA will allow
routine mapping of the magnetic fields in star formation regions and will
provide, for the first time, full information on all physical parameters
important for cloud evolution and star formation.
Mass loss from cool evolved stars is a key process in the cycling of matter
from the interstellar medium (ISM) into stars and back again to the ISM. In
their late stages of evolution, stars shed copious amounts of gas and dust in
the form of cool molecular winds. These winds are well-suited to studies with
the MMA. By such studies, we will learn just how the mass loss process occurs,
how much material is ejected globally in the galaxy, what chemical and physical
processes occur in the circumstellar envelope produced by mass loss, and how the
process leads to the formation of visible planetary nebulae.
The MMA will be capable of detecting mass-losing evolved stars in the whole
Milky Way galaxy. At the distance of the galactic center (8.5 kpc), it can
detect the CO rotational lines (J=1-0 and 2-1) of a typical carbon star
envelope like CIT 6 with a signal-to-noise ratio of 25 in 100 minutes of
integration, with 1 MHz velocity resolution. The MMA will therefore be able
to measure the mass loss rates of a large sample of stars from the outer galaxy
to the galactic center, and examine correlations of mass loss rate with
galacto-centric radius, stellar type, evolutionary state, etc. High-resolution images
of CO emission in several transitions will reveal the temperature and density
structure of stellar winds from a wide variety of cool evolved stars, for which
the history of mass loss (e.g., episodic or continuous) can be constructed.
Such data will constrain mass loss mechanisms and the global rate of mass return
to the interstellar medium.
Imaging capability in multiple spectral lines of other chemical species
will permit studies of the distribution of molecules containing refractory
elements like Si, Al, and Mg. These images can be related to the process of
grain formation and growth, discussed in the next section. At greater distances
from the star, the distribution of organic molecules, radicals, and ions is
determined by the photo-chemical processes which occur in outer envelopes of
cool evolved stars. Currently available instruments have been limited to
studies of only a handful of nearby stars, such as the carbon star IRC+10216
(see Fig.2). The sensitivity and imaging capability of the MMA will permit
similar molecular line studies for evolved stars with a wide range of physical
and chemical properties.
Evolved stars form dust, seen by large IR excesses. Since the photospheric
temperatures are 2000 -- 3500 K, dust-formation models involve levitating the
atmosphere to several R
(by pulsations?) where the gas is cool enough for dust
to form.
The dust formation process is important: red giant stars put 
M
of dust per year into the Galaxy, and dust is very likely to be the agent that
causes mass loss. How does dust formation work? The MMA will let us measure
the transition zone from the stellar radius to the inner edge of the dust shell
in both dust continuum and line emission.
Consider the nearest and brightest carbon star, IRC+10216. Its distance is
200 pc and photospheric radius 
cm.
(The MMA will be able to measure
a large number of stars like IRC+10216.) The photospheric angular diameter is
0
035. The gas and dust temperature at the inner edge of the dust shell is
about 1000 K and the dust optical depth is 
0.1 at 1 mm wavelength. The
brightness temperature is therefore 
100 K, which is detectable at the
12
level in one minute for an angular resolution of 0
02.
Likewise, the line
brightness temperatures should be several hundred Kelvins, detectable at the
10
level in 2 hours.
We can thus map the inner 10
to 20
of the IRC+10216 envelope at a
resolution of 0
02 at several continuum wavelengths and in many spectral
lines. We can compare molecular and dust abundances over 10 resolution
elements within the inner dust shell. High resolution images in the continuum
and in many molecular species will help answer a host of questions: What is the
dust made of? How does the size distribution change with distance from the
stars? How big is the star, and how does this relate to the dust formation
zone? What is the density and temperature distribution of the envelope at
large scales? Which molecules disappear into dust? Can asymmetries in the
gas and dust distributions be related to those in the star? Does the dust
formation zone change with stellar pulsation period?
How many other stars can be studied besides IRC+10216? There are between
10 and 50 stars within 200 pc with bright molecular emission, covering a range
of spectral types, chemistries and mass loss rates. None of these is as bright
as IRC+10216 so we have to work a bit harder, but each can be mapped in
approximately a day's observing time, certainly in the range of feasibility.
The transition from red giant to planetary nebula is rapid and dramatic,
but poorly understood. Observations of post-red giant stars show that they
develop very fast (> 100 km s
) molecular outflows with large mass loss rates,
which may strip off the cool outer layer of the former giant and reveal the hot
central core which will become a white dwarf. This ``super wind" may be bipolar,
not spherical, and must overtake the slower red giant wind. The interaction of
fast and slow winds should produce shocks and morphological effects which shape
the resulting planetary nebula. The hot central star photoionzes the former
red giant wind, gradually eroding it from the inside.
All these complex processes--fast winds, bipolar mass ejection, shocks, and
ionization/dissociation fronts--occur in regions of temperature and density, and
on angular scales, which require high resolution and sensitivity images to
understand the details. Studies with existing interferometers have shown that
fast molecular winds can be detected and imaged with few-arcsecond resolution.
To probe deeper, near the source of the outflow, will require the molecular line
imaging sensitivity and resolution of which the MMA will be capable. Likewise,
the interaction region where the precursor cool molecular envelope is being
photo-ionized offers a host of physical and chemical processes which MMA imaging
will be able to reveal. The proposed instrument should lead to major progress
in understanding the final stages of evolution of solar-like stars from red
giant to planetary nebula and white dwarf.
The scientific goals described above imply several requirements for the design
of the MMA, which are summarized here.
Spectral Resolution:
To resolve the spectral structure of line profiles which are diagnostic of
infall in protostellar cores, it is necessary to have at least 5 resolution
elements across the thermal width of the tracer line. This implies a velocity
resolution of 
0.02 km s
, which corresponds to 7 kHz at 3 mm wavelength or 2
kHz at the longest array wavelength of 1 cm. A resolution of 2 kHz is the
finest available in the current correlator design, and should be retained in
the design specifications.
Spectrometer Configuration Flexibility:
With the relatively large instantaneous IF bandwidth now specified (8 GHz,
possibly 16 GHz), and the separation of mixer sidebands by phase-switching
techniques, it should often be possible to obtain images in several different
spectral lines from multiple molecular species simultaneously. Especially if
dual-polarization is available at all bands, it will be very desirable to allow
more than just 4 spectral windows in the correlator configuration. We recommend
that correlator designs with 8 spectral windows be considered.
Angular Resolution and Brightness Temperature Sensitivity:
The MMA will be the premier instrument sensitive to protoplanets and
protoplanetary disk emission from 1 AU to 40 AU, the domain of Jovian planet
formation. A primary design goal must be the direct study of planet formation
in the nearest star-forming regions (150 pc distance). Even there, 0
1
resolution (14 AU) cannot resolve structure on the scale of Jupiter's orbit. It
is essential that the MMA resolution be pushed to the technical limits,
preferably to 0
01 at 1 mm wavelength.
The same criteria apply to studies of proto-binary systems. Only binaries
with separations less than disk radii (roughly 100 AU) will significantly impact
on disk structure. Theory predicts that gaps will extend from 0.5 to 2 times the
stellar separations in circular orbits, and will be larger in eccentric orbits.
Consequently, the circumstellar disks of binaries with separations of roughly 30
AU or greater can be resolved by the MMA with 3 km baselines. A decrease in
resolution would significantly reduce the sample of interesting binaries.
Binary studies would greatly benefit from the higher resolution driven by the
planet formation science.
Likewise, studies of dust formation and of the mass loss process in evolved
stars will require resolutions of the order of 0
02 at wavelengths near 1 mm,
also implying the need for baselines longer than 3 km if site considerations
permit.
Excellent sensitivity is essential, but the present goals are appropriate.
At a resolution of 0
02 (2.8 AU), the anticipated rms noise in a 6-hour
integration is 0.3 K. At the separation of Jupiter, disks are expected to have
temperatures between 30 K -- 50 K and to have 1 mm optical depths of order unity.
Thus, gaps and holes on size scales of one beam are straightforwardly detectable
with the present sensitivity specification. Structure in the innermost zones of
circumstellar envelopes of evolved stars will be resolvable as well, which is
essential to studies of the (intimately related) dust formation and wind
acceleration processes.
Polarization Properties:
Technical requirements for magnetic field measurements include instrumental
polarizations in the primary beam below 0.1% and polarization capabilities in
the 33 GHz, 113 GHz, and 300 GHz bands.
Submillimeter Capabilities:
Almost all of the scientific problems described in this section would
benefit from observations at the shortest wavelengths that can be observed
with a ground-based instrument. We therefore recommend that at a minimum, the
MMA design not preclude the possibility of adding receivers for short submm
wavelengths. We also urge that receivers for at least one short submm band be
included in the initial construction of the array.
kweather@aoc.nrao.edu