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OCR for page 1
The Properties and Environment of Primitive
Solar Nebulae as Deduced from Observations of
Solar-Type Pre-main Sequence Starsi
STEPHEN E. STROM
SUZAN EDWARDS2
KAREN M. STROM
University of Massachusetts, Amherst
ABSTRACT
This contribution reviews a) current observational evidence for the
presence of circumstellar disks associated with solar type pre-main sequence
(PMS) stars, b) the properties of such disks, and c) the disk environment.
The best evidence suggests that at least 60% of stars with ages t < 3 x 106
years are surrounded by disks of sizes ~ 10 to 100 AU and masses ~ 0.01 to
0.1 Me. Because there are no known main sequence stars surrounded by
this much distributed matter, disks surrounding newborn stars must evolve
to a more tenuous state. The time scales for disk survival as massive (M
~ 0.01 to 0.1 Me), optically thick structures appear to lie in the range t
<< 3 x 106 years to t ~ 107 years. At present, this represents the best
astrophysical constraint on the time scale for assembling planetary systems
from distributed material in circumstellar disks. The infrared spectra of
some solar-type EMS stars seem to provide evidence of inner disk clearing,
perhaps indicating the onset of planet-building.
The material in disks may be bombarded by energetic (~ 1 key)
particles from stellar winds driven by pre-main sequence stars. However,
it is not known at present whether, or for how long such winds leave the
stars a) as highly collimated polar streams which do not interact with disk
matenal, or b) as more isotropic outflows.
1 Based in part on a review presented at the Space Telescope Science Institute Workshop:
The Formation and Evolution of Planetary Systems, Cambridge University Press (in press).
2Also at Smith College, Northampton, Massachusetts.
1
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PLANETARY SCIENCES
INTRODUCTION
Recent theoretical and observational work suggests that the process of
star formation for single stars of low and intermediate mass (0.1 < M/M
< 5) results naturally in the formation of a circumstellar disk, which may
then evolve to form a planetary system. This process appears to involve
the following steps (Shu et al. 1987~:
· The formation of opaque, cold, rotating protostellar "cores" within
larger molecular cloud complexes;
· The collapse of a core when self-gravity exceeds internal pressure
support;
· The formation within the core of a central star surrounded by
a massive (0.01 to 0.1 Met circumstellar disk (embedded infrared source
stage). At this stage, the star/disk system is surrounded by an optically
opaque, infalling protostellar envelope. Gas and dust in the envelope rains
in upon both the central stellar core and the surrounding disk, thus in-
creasing both the disk and stellar mass. Infall of low angular momentum
material directly onto the central star and accretion of high angular mo-
mentum material through the disk provide the dominant contributions to
the young stellar object's (YSO) luminosity, far exceeding the luminosity
produced by gravitational contraction of the stellar core. Because the in-
falling envelope is optically opaque, such YSOs can be observed only at
wavelengths ~ > 1~. They exhibit infrared spectral energy distributions,
AF>, vs. A, which are a) broad compared to a blackbody distribution and
b) flat, or rising toward long wavelengths (see Figure la). At some time
during the infall phase, the central PMS star begins to drive an energetic
wind (Lain`` ~ 0.1 Let; Veins ~ 200 kilometers per second). This is first
observable as a highly collimated "molecular outflow" as it transfers mo-
mentum to the surrounding protostellar and molecular cloud material and
later (in some cases) as a "stellar jet" (Edwards and Strom 1988~. The wind
momentum is sufficient to reverse infall from the protostellar envelope and
eventually dissipates this opaque cocoon, thus revealing the YSO at visible
wavelengths (Shu et al. 1987~.
.
The optical appearance of a young stellar object (YSO) whose
luminosity may be dominated in the infrared by accretion through the disk,
and in the ultraviolet and optical region, by emission from a hot "boundary
layer" (continuum ~ emission 11S phase). Accretion of material through
the still massive (0.01 to 0.1 Mail disk produces infrared continuum ra-
diation with a total luminosity ~ 0.5 times the stellar luminosity. At
this stage, the infrared spectrum is still much broader than a single, black-
body spectrum (Myers et al. 1987; Adams et al. 1986), and in most cases falls
OCR for page 3
AMERICAN AND SOVIET RESEARCH
i
9
-10
o
-11
-12
9
-10
o -11
-12
-13
3
1
! 1 1 1 1 1 1
Do_
-.5 0 .5 1
IRS5
L `3 = 43.06
1
~ 1 1 ~ !
log \(RM)
1.5 2
_ i , i i i 1 i ~ 1 1 1 1 i , 1 i 1 1 1 1 , 1 ~ _
_ , ~
_ ,~
HLTau
- L`, = 10.63 ~
1 ! ' ~ ~ i . . , ! 1 , . 1 . ! , ; _
5 0 ·5
1 1.5 2
log \(pM)
FIGURE 1 (a) A plot of the observed spectral energy distribution for the embedded
infrared source, LlSSl/IRS 5. This source drives a well-studied, highly~ollimated, bipolar
molecular outflow and a stellar jet. Note that the spectrum rises toward longer wavelength,
suggesting that IRS 5 is surrounded by a flattened distribution of circumstellar matter,
possible remnant material from the protostellar core from which this YSO was assembled.
(by A plot of the observed spectral energy distribution for the continuum + ~rnrnission T
Taun star, HL Tau. Near-infrared images of HL Tau suggest that this YSO is surrounded
by a flattened distribution of circumstellar dust. Its infrared spectrum is nearly flat, which
suggests that HL lbu is as well still surrounded by a remnant, partially opaque envelope.
OCR for page 4
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PLANETARY SCIENCES
toward longer wavelengths. In a few cases, the spectrum is flat or rises
toward long wavelengths, perhaps reflecting the presence of a partially
opaque, remnant infallin~ envelope (Figure lbl.
---o ~ r- ~~~o-
The accretion of material from the rapidly rotating (~ 200 kilometers
per second), inner regions of the disk to the slowly rotating (~ 20 kilometers
per second) stellar photosphere also produces a narrow, hot (T > 8000
K) emission region: the "boundary layer" (Kenyon and Hartmann 1987;
Bertout et al. 1988~. Radiation from the boundary layer overwhelms that
from the cool PMS star photosphere. Consequently, the photospheric
absorption spectrum cannot be seen against the boundary layer emission at
~ < 7000 ~ Strong permitted and forbidden emission lines (perhaps tracing
emission associated with energetic stellar winds and the boundary layer)
are also present during this phase; hence the classification "continuum +
emission" objects. All such objects drive energetic winds, some of which
are manifest as highly collimated stellar jets (Strom et al. 1988; Cabrit et
al. 1989).
· The first appearance of the stellar photosphere, as the contribution
of the disk to the YSO luminosity decreases (T Tauri or TTS (0.2 < M/M`3
Tauri < 1/5) and Herbig emission star or HES (1.5 < M/Mc3 < 5) phase).
Relatively massive disks are still present, but the accretion rate and mass
outflow rate both diminish. The infrared luminosity from accretion and the
boundary layer emission decrease as a consequence of the reduced accretion
rate through the disk "Reprocessing" of stellar radiation absorbed by dust
in the accretion disk and re-radiated in the infrared contributes up to 0.5
times the stellar luminosity (depending on the inclination of the star/disk
system with respect to the line of sight). The observed infrared spectra show
the combined contribution from a Rayleigh-Jeans (AFx ~ A-3) component
from the stellar photosphere and a broader, less rapidly falling (~-2/3 to
)-4/3) component arising from both disk accretion and passive reprocessing
by circumstellar dust (Figure 2~. Photospheric absorption spectra are visible,
though sometimes partially "veiled" by the hot boundary layer emission. H
emission is strong (typical equivalent widths ranging from 10 < W~ < 100
Ay. In ~, Ca II and selected forbidden and pace permitted metadic lines
are often prominent in emission. Energetic winds persist, although their
morphology and interaction with the circumstellar environment is unknown
at this stage. Highly collimated optical jets are not seen, but spatially
unresolved [O I] line profile structures require that the winds be at least
moderately collimated (on scales of ~ 100 AU).
· The settling of dust into the midplane of the disk followed by
the clearing of di*~ibuted material in the disk, as dust agglomerates into
planetesimals, first in the inner regions of the disk where terrestrial planets
form, and later in the outer disk where giant planets and comets are
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AMERICAN AND SOVIET RESEARCH
Is
1- ~ ' ' ' 1 ' ' 1 ~ 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1
9
an
-9.5
-10
a)
LL -1 0.5
_ q
_ I
--of
-1
-11 .
-11.5 ~ 1 1 1 1—
I
l
/'
,6
'_ ~ 0
/ A~ 0
FXTau
Av= 1.49
\\ 0
\
\
\
\
\
\
,,,, 1,,
,,1,,,,1,,,,1,,,,1
-4.5 -4 -3.5 -3 -2.5 -2
~ (cm)
FIGURE 2 A plot of the reddening corrected spectral energy distribution (0.35p < ~ <
100 ,u) for the T T=ri star, FX Mu (open circles). Also plotted is the spectral energy
distribution of a dwarf standard star (filled triangles) of spectral type M1 V corresponding
to that of FX Lou. The flux of the 11S and the standard star have been forced to agree
at R (0.65,u).
assembled. The disappearance of accretion signatures and energetic winds
("naked" T Tauri or NTTS phase?~.
The appearance of the star on the hydrogen-burning main se-
quence, accompanied by its planetary system and possibly by a tenuous
remnant or secondary dust disk (analogous to the edge-on disk surround-
ing B. Pictoris imaged recently by Smith and Terrile 1984~.
Is this picture correct even in broad outline for all single stars? Are
disks formed around members of binary and multiple star systems (which
constitute at least 50% of the stellar population in the solar neighborhood)?
For those stars that form disks, what is the range of disk sizes and masses?
What is the range in time scales for disk evolution in the inner and outer
disks, and how do these time scales compare with those inferred for our
solar system from meteoritic and primitive body studies, and theoretical
modeling of the early solar nebula? In what fraction of star/disk systems
can the gas in the outer disk regions survive removal by energetic winds
long enough to be assembled into analogs of the giant planets?
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PLANETARY SCIENCES
RECENT OBSERVATIONAL RESULTS
Current Observational Evidence for Disks
Associated with Pre-Main Sequence Stars
Observations carried out over the past five years provide strong evi-
dence for circumstellar disks associated with many young stars (ages < 3 x
106 years) of a wide range of masses. These disks appear to be massive (M
~ 0.01 to 0.1 Mob precursor structures to the highly evolved, low-mass (M
~ 10-7 Me)) disks discovered recently around B-Pictoris and its analogs
(Smith and Terrile 1984; Baclanan and Gillett 19883:
· The direct and speckle imaging at near-infrared wavelengths (Gras-
dalen et al. 1984; Beckwith et al. 1984; Strom et al. 1985) reveal flattened,
disk-like structures associated with three YSOs: HL Tau (a low-mass con-
tinuum + emission star), R Mon (an intermediate-mass continuum +
emission star), and L1551/IRS 5 (a low-mass embedded infrared source).
These structures are seen via light scattered in our direction by sub-micron
and micron-size dust grains. Associated structures are also seen in mm-
continuum and CO line images obtained with the Owens Valley interferom-
eter (Sargent and Beckwith 1987), although the relationship between the
optical and near-infrared and mm-region structures is not clear at this point.
Shu (1987, private communication) has argued that the flattened, scattered
light structures detected to date trace not disks, but rather remnants of
infalling, protostellar cores (see also Grasdalen et al. 1989~.
· The high-spectral resolution observations of [O I] and IS II] lines in
continuum + emission, T Tauri, and Herbig emission stars provide indirect
but compelling evidence of such disks. The forbidden line emission is
associated with the outer (r ~ 10 to 100 AU) regions of winds driven by
PMS stars. However, in nears all cases studied to date, only blue-shifted
emission is observed (see Figure 3) thus requiring the presence of structures
whose opacity and dimension is sufficient to obscure the receding part of
the outflowing gas diagnosed by the forbidden lines Awards et al 1987;
Appenzeller e! al. 1984~.
· The broad, far-infrared () > 10~) spectra characteristic of ad
continuum + emission, T Taun, and Herbig emission stars arise from
heated dust located over a wide range of distances (~ 0.1 to > 100 AU)
from a central pre-main sequence (PMS) star (Rucinsld 1985; Rydgren
and Zak 19863. In order to account for the fact that these IR-luminous
YSOs are visible at optical wavelengths, it is necessary to assume that the
observed far-IR radiation arises in an optically thick but phy~cabty dour
circumstellar envelope: a disk If the heated circumstellar dust responsible
for the observed far-infrared radiation intercepted a large solid angle, the
OCR for page 7
AMERICAN AND SOVIET RESEARCH
1.
1
c'
-
0}
o
.5
o
-500 - 00 -300 -200 -100 0
I I I I I I I I I I 1 1 1 1 1 1 1 1 1 1 1 I r I , I, I I ~ I ~ I I I 1 1 1 1 1 1 ~ 1 T I I I '
- CW Tau
_~ {~-~W ~ nvY
OCR for page 8
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PL-ANETARY SCIENCES
important kinematic confirmation of disk structures associated with PMS
stars.
· The mm-line and continuum observations of HL Tau and L1551/
IRS 5 made with the OVRO mm interferometer suggest that circumstellar
gas and dust is bound to the central PMS star and, in the case of HL Tau,
in Keplerian motion about the central object (Beckwith and Sargent 1987~.
Frequency of Disk Occurrence
What fraction of stars are surrounded by disks of distributed gas and
dust at birth? If excess infrared and mm-wave continuum emission is
produced by heated dust in disks, then all continuum + emission, T Tauri,
and Herbig emission stars must be surrounded by disks. The inferred
disk masses (0.01 < Ma,~k/M,3 < 0.1, comparable to the expected mass of
the primitive solar nebula) and optical depths (TV ~ 10003 for AS and
HES are relatively large (Beckwith et al. 1989 and Edwards e! al. 1987 for
estimates based on mm-continuum and far-IR measurements respectively).
However, the HR diagram presented by Walter e! al. (19853) suggests
that ~ 50% of low mass pre-main sequence stars with ages comparable to
those characterizing TTS (t < 3 x 106 years) are "naked" T Tauri stars
(NETS) which lack measureable infrared emission, and therefore massive,
optically thick disks. The observations of Warner e! al. (1977) suggest that
a comparable percentage (50~o to 70~) of young (t < 3 x 106 years)
intermediate mass stars (M ~ 1.5-2.0 Mel also lack infrared excesses.
Recently, Strom e! al. (1989) examined the frequency distribution of
near-IA (2.2~) excesses, AK——log {F2.2`, (PMS star)/ F2 2', (standard
star)}, associated with 47 NETS and 36 ITS in Taurus-Auriga (see Figure
4~. They find that 84% of the AS and 36% of the N"m have significant
excesses, AK > 0.10 dex. Thus, nearly 60% of solar-type PMS stars with
ages t < 3 x 106 years located in this nearby star-forming complex have
measurable3 infrared excesses; these excesses most plausibly arise in disks.
However, the sample includes only known PMS stars for which adequate
photometry is available; more NETS may yet be discovered when more
complete x-ray and proper motion searches of the ~urus-Auriga clouds
become available. It is also important to note that these disk frequent
statistics exclude several PMS stars which show small or undetectable near
IR excesses, but relatively strong mid- to far-IR excesses (see Figure 5~:
30bjects that lack measurable infrared excesses could be surrounded by low mass, tenuous
disks (M << 10-5 Me) disks For example, a disk of mass comparable to that surrounding ,6 Pic
(M ~ 10-7 M ~ could not be detected around a PMS star in the Taurus-Auriga clouds given
the current sensitivity of IR measurements.
OCR for page 9
AMERICAN AND SOVIET RESEARCH
20
15
10
5
o
20
15
5
-.2
9
~ ~ ~ I I I ~ I i l--rr~-i I I I ~ I I I I I I ~ ~ I
,
-.2
Distribution of OK for NTTS
-
0 .2 .4 .6 .8 1 1.2
l I I ~ ~ ~ ~ ~ r ~ ~ ~ ~ r I r I ~ l r To
Distnbu6on of AK for TTS
, .
l
_
0F ~ 1 1 1 1 1
0 .2
, 1, 1 1 1 1 1 1
1 ~ 1 1 1 1 1 1
1.2
.4 .6 .8 1
AK(dex)
FIGURE 4 The frequency distribution of the 2.2~1 excess, /\K _ log {F2.2p (PMS)IF2.2p
(standard)}, for NETS (top) and 11S (bottom). Note that a) 36% of the NITS show
excesses, /\K > 0.10 dex, and b) that while the distribution for the NETS peaks toward
smaller values of AK, there is significant overlap in the two distribution. It does not appear
as if NUTS as a class lack infrared excesses.
OCR for page 10
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PLANETARY SCIENCES
these objects may represent PMS stars surrounded by circumstellar disks
which are optically thin near the star (and therefore produce too little
near-IA radiation to be detected), but optically thick at distances r > 1 AU
(see below).
Do pre-main sequence stars with ages t < 3 x 106 years that lack
infrared excesses (40% of all solar-type PMS stars) represent a population
of stars born without disks? Or have their disks been destroyed by tides
raised by a companion star? Or have some fraction of these young PMS
stars already built planetary systems?
The Effect of Stellar Companions on Disk Survival
Do disks form around members of binary and multiple star systems?
If so, are these disks perturbed by tidal forces and perhaps disrupted when
the disk size is comparable to the separation between stellar components?
date, the overwhelming majority of binaries discovered among low- and
intermediate-mass PMS stars in nearby star-forming complexes have been
wide (~8 > 1"; r > 150 AU) doubles with separations well in excess of the
radius of our solar system (and thus possibly of lesser interest to addressing
the above questions). In the last few years, ground-based observations
have uncovered a few examples of a) spectroscopic binaries with velocity
amplitudes, Av > 10 km/s (separations < 3 AU; Hartmann e! al. 1986~; b)
binaries with separations in the range 0.1 to 50 AU detected from lunar
occultation observations of YSOs in Taurus-Auriga and Ophiuchus (Simon,
private communication); and c) binaries discovered in the course of optical
and infrared speckle interferometric observations (Chelli et al. 1988~. Of
the known binaries in Taurus-Auriga with observed or inferred separations
/~8 < 0.5" (r < 70 AU; DF Mu, FF Tau, HV Mu, HO Tau, T Bud, all
but FF Mu appear to have IR excesses similar to those of TTS thought
to be surrounded by disks. This somewhat surprising result implies that
circumstellar envelopes of mass > 10-4 Me and of dimension 10 to 100
AU are present even in close binary systems.
Disk Evolutionary Time Scales
On what time scales do disks evolve from massive "primitive" to low
mass, perhaps "post planet-building" disks? Current observations suggest
that more than 50~O of low- to intermediate-mass PMS stars with ages t <
3 x 106 years are surrounded by disks with masses in excess of 0.01 M of
distributed material (see above). There are no Blown main sequence stars
surrounded by this much distributed matter, although a few stars such as
Vega and ,6 Pictoris (Smith and Terrile 1984; Backman and Gillett 1988)
OCR for page 11
-
0
~ -1(
-9
-
~n
ru
E -10
-11
._9
u,
cat
E -10
-1 1
'1 ~
- .5
\ O
4 -3.5 -3 -2.5 -2
~ (cm)
~8 I , , I I I I I T ~ ~ ~ I T T T T I r ~ I T I
_0 _ I 'A : _
HE -9 ~ ~ o o - E
a, \\ (D
-10 _ HDE283447 '
- AV=2.00 \`
-4 -3.5 -3 -2.5 -2
(cm)
' ' ' ' I ' ' ' ' 1 ' ' ' ' I ' ' ' ' 1 ' ' ' ' I
,' ~0
:/ \' :
1 '\
DlTau `\ O °
1— Av = 1.06 \ O _
,,,, I,,,, I I I
- .5 -4 -3.5 -3 -2.5 -2
(cm)
~'''1~11~ ~~ ~ ~ ,
- ~ s,'~~ °~°
// it\
·
~ -1 \;
J \`
V826Tau \`
.— Av = 0~53 \~ r
\
~ O
.5 -4 -3.5 -3 -2.5 -2
~ (cm)
_9
-
as -1<
-
~ -11
_ -12 .
.5 -4 -3.5 -3 -2.5 -2
~ (cm)
-10
-1 1
V81 9Tau
An = 1.25
.
\
\
SA07641 1A ``
A,, = 0.00
FIGURE 5 A plot of the reddening corrected spectral energy distributions (Q35p < ~ <
100~) for 7 N1= and the T Tauri star, FX Tau (see Figure 23; the observed points
for the N1= and TTS are plotted as open circles. Also plotted are the spectral energy
distributions of dwarf standard stars (filled triangles) of spectral types corresponding to
those of the NITS and 11~. The fluxes of the NITS and those of the standard stars have
been forced to agree at R (0.65~. Note the small near-infrared excess and relatively large
mid- to far-infrared excesses for several of the NTIS. A spectral energy distribution of this
character might be produced by a circumstellar disk in which the optical depth of emitting
material in the inner disk is small, while that in the outer disk is large.
OCR for page 12
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P1~ETARY SCIENCES
are surrounded by disks with masses ~ 10-7 Me (~ 0.1 Earth masses).
Thus, disks surrounding newborn stars must evolve to a more tenuous state.
Recent work by Strom et al. (1989) suggests that nearly 60% of PMS
stars in Taurus-Auriga with ages younger than 3 x 106 years, and only 40%
of older stars, show evidence of significant near-infrared excesses /`K >
0.1 dex (see above). These results are illustrated in Figure 6. Fewer than
10% of PMS stars older than 107 years show AK > al. If excess near-IA
emission arises in the warm, inner regions of circumstellar disks, then we
can use these statistics to discuss the range of time scales for disks to evolve
from massive, optically thick structures (with large K values) to low-mass,
tenuous entities (with small 1\K values).
If all solar-type stars form massive (0.01 to 0.1 Me disks, then by t =
3 x 106 years, 40% of PMS stars (the fraction of young PMS stars with
AK ~ 0.103 are surrounded by remnant disks too tenuous to detect. The
evolutionary time scale for such rapidly evolving disks must be t << 3 x 106
years. Because fewer than 10% of PMS stars older than 107 years show AK
> 0.1 (Strom et al. 1989), the disks surrounding all but 10% of PMS stars
must have completed their evolution by this time. The majority of disks
must have evolutionary time scales in the range 3 x 106 to 107 years. This
range represents the best astrophysical constraint on the likely time scale
for planet building available at present.
As noted earlier, the above statistics obtain for all known IS and
NETS in Taurus-Auriga for which adequate photometry is available. A1-
though the exact fraction of PMS stars surrounded by disks may change
somewhat as more complete surveys for PMS stars become available, our
qualitative conclusions regarding the approximate time scale range for disk
evolution are unlikely to be vitiated.
Disk Sizes and Morphologies
In our solar system, all known planets lie within 40 AU of the Sun.
Yet the circumstellar disk imaged around ,0 Pic extends to a distance
approximately several thousand AU from the central star. Do primitive
solar nebulae typically extend to radii considerably larger than our own
planetary system? If so, how far, and how much material do they contain?
How do properties such as size and surface mass distribution change with
time? Do such changes reflect the consequences of angular momentum
transport within the disk? Of planet building episodes?
Current estimates of disk sizes are indirect, and are based on: (1) the
size of the YSO wind region required to account for the observed blue-
shifted [O I] and [S II] forbidden line emission fluxes; disks must be large
enough to occult the receding portion of the stellar wind (Appenzeller e! al.
1984; Edwards et al. 1987) and (2) the projected radiating area required to
OCR for page 13
AMERICAN AND SOVIET RESEARCH
20 ;, 1 i,; I,
_ l
_
o -1 1 ~ 1 1 ~ 1
-2 n
Z 10
5
· j i;; i, 1 1 j i i i 1
Distnbubon of AK for NTTS
=
.2 .4 .6 .8 1
I ~ ~ 1 ~ I !
o
20~ ~ ' ' 1 ' '
~ 1 1 1 ~ 1 1 1 1 _
15
o
; _
Z 10 _
r
1, 1 1 ~ 1 1 .
-.2 0
Distnbubon of AK for TTS
- 1
1,,
. ~ ! ~1 ~ ~ 1
=
1.2
.2 .4 .6 .8 1
~K(dex)
13
FIGURE 6 The frequent distribution of the near-infrared excess AK (see text) for stem
with ages t ~ 3 X 106 yea m and t < 3 X 106 years. Note that a) nearly 60% of young
PMS stars have measurable (AK > 0.10) near-infrared excesses and b) that the fraction of
PMS stem with such excesses decreases for ages t A> 3 X 106 years. If IR excesses derive
from emitting dust embedded in massive, optically thick circumstellar disks surrounding
PMS stars, then the fraction of stars surrounded by such disks must decrease with time.
Our data suggests that the time scales for evolving from a massive, optically thick disk to
a low mass, tenuous disk must range from ~ 3 X 106 to 107 years. This range represents
the best available astrophysical constraint on the time scale for planet building.
explain the observed far-infrared radiation emanating from optically thick,
cool dust in the outer disk regions (Myers et al. 1987; Adams et al. 1987~.
In both cases, these estimates provide lower limits to the true extent of the
disk. Edwards et al. (1987) have shown that these independent methods
predict comparable lower limit disk size estimates: r~i.sk > 10 to 100 AU,
for a sample of continuum ~ emission, T Mauri, and Herbig emission stars.
At the distance of the nearest star-forming regions, such disks intercept
an angular radius, r > 0.07 to 0.7 arc seconds. Thus far, it has proven
difficult to image disks of this size from the ground. Decisive information
regarding disk isophotal sizes and surface brightnesses must await sensitive
OCR for page 14
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PLANETARY SCIENCES
imaging with HST, whose stable point spread function and high angular
resolution will permit imaging of low-surface brightness circumstellar disks
around bright PMS stars. When available, HST measurements of disk
sizes will prove an invaluable complement to ground-based sub-mm and
mm-continuum measurements which provide strong constraints on the disk
mass, but which lack the spatial resolution to determine size. Together, these
data will yield average surface mass densities and estimates of midplane
optical depths—critical parameters for modeling the evolution of primitive
solar nebulae.
Prior to HST, ground-based observations of the ratio of near- to far-
IR excess radiation may provide a qualitative indication of the distribution
of material in circumstellar disks. For example, Strom et al. (1989) discuss
several solar-type PMS stars which show small near-IA excesses compared
to far-IR excesses (Figure 5~. They suggest that the disks surrounding
these stars have developed central holes as a first step in their evolution
from massive, optically thick structures (such as those surrounding ITS)
to tenuous structures (such as those surrounding ,8 Pic and Vega). Such
central holes may provide the first observational evidence of planet-building
around young stars.
The Disk Environment
High resolution ground-based spectra suggest that energetic winds
(L~osn~ > 0.01 L*; v ~ 200 kilometers per second) characterize all 11S
and HES (Edwards and Strom 1988~. However, mass-loss rates are not
well determined: estimates range from 10-6 to 10-9 Me per year and are
greatly hampered by uncertainties in our knowledge of the wind geometry.
The broad, blueshifted, often double-peaked forbidden line profiles of [O
I] and IS II] (see Figure 3), suggest that typical 11S and HES winds
are not spherically symmetric and may be at least moderately collimated.
The models proposed to account for the forbidden line profiles include
a) latitude-dependent winds characterized by higher velocities and lower
densities in the polar regions, b) sub-arc second, highly collimated polar
jets; and c) largely equatorial mass outflows obliquely shocking gas located
at the raised surface of a slightly flaring disk (Hartmann and Raymond
1988~.
Knowledge of the wind geometry is necessary if we are to derive
more accurate estimates of PMS star mass loss rates from [O I] profiles.
Depending on their mass loss rate and geometry, PMS star winds may have
a profound effect on the survival of gas in circumstellar disks and on the
physical and chemical characteristics of the grains:
OCR for page 15
AMERICAN AND sorer RESEARCH
15
· Energetic winds can remove gas from the disk during the epoch
of planet building, thereby eliminating an essential "raw material" for
assembling massive giant planets in the outer disk.
· Exposure of interstellar grains to ~ 1 kev wind particles carried
by a wind with M ~ 10-9 Me per year for times of 106 to 107 years,
can alter the chemical composition of grain mantles. For a grain with a
water-ammonia-methane-ice mantle, energetic particle irradiation can in
principle: (1) create large quantities of complex organic compounds on
grain surface and (2) drastically reduce the grain albedo (Greenberg 1982;
Lanzerotti et al. 1985; Strazzula 1985~. Recent observations of the dust
released from the nucleus of comet Halley show these grains to be "black"
(albedos of ~ 0.02 to 0.05) and probably rich in organic material (Chyba
and Sagan 1987~. Does this cometary dust owe its origin to irradiation of
grains by the Sun's T Tauri wind during the early lifetime of the primitive
solar nebula?
HST will allow us to image low-density, outflowing ionized gas in the
light of [O I] )6300 ~ and thus allow us to trace YSO wind morphologies
directly. HST observations will determine whether energetic outflows in-
teract with the disk (as opposed to leaving the system in highly collimated
polar jets). In combination with improved estimates of mass outflow rates,
we can then determine the integrated flux of energetic particles through the
disk for a sample of PMS objects of differing age and thus assess the role
of winds in the evolution of disks.
ACKNOWLEDGMENTS
The authors acknowledge support from the National Science Founda-
tion, the NASA Astrophysics Data Program (IRAS and Einstein), and the
NASA Planetary Program. Many of the arguments developed here have
been sharpened and improved following consultation with Steven Beckwith,
Robert ~ Brown, Belva Campbell, Luis Carrasco, H. Melvin Dyck, Galy
Grasdalen, Lee Hartmann, S. Eric Persson, Frank Shu, T. Sunon, M. Simon,
R. Stachnik, John Stauffer, and Fred Vrba. Comments from Sylvie Cabnt,
Scott Kenyon and Michael Skrutskie have also been valuable.
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Representative terms from entire chapter:
circumstellar disks
