The main efforts were directed towards determining the frequency of disk occurrence and the timescales for disk evolution for solar-type and intermediate mass stars. The results of the investigation showed that optically thick disks are accretion disks. The projected accomplishments are also discussed

Edwards, Suzan

Strom, Karen M.

Strom, Stephen E.

English

NASA Technical Reports Server

N92-12847
The Evolution of Young Stellar Object Disks and Their Environment "
Five Colleges Astronomy Department
University of Massachusetts
LGR TWR B
Amherst, MA 01003
Stephen E. Strom (P.I.)
Suzan Edwards (Co-El.)
Karen M. Strom (Co-P.I.)
Progress and Accomplishments
Our main efforts have been directed toward determining the frequency of disk occurrence and
the timescales for disk evolution for solar-type and intermediate mass stars. Results to date
indicate that:
• Optically thick disks (x > 1 in regions sampled by excess emission _. < 100m) are
found around 30% to 50% of young (x < 3 Myr) stars of all masses M < 3 M®. Optically
thick disks are found around PMS stars of higher mass, but with unknown frequency. For the
relatively limited sample of objects for which sub-ram and ram-continuum measurements are
available, nearly all stars with x > 1 show disk masses M_ > 0.01 M e (-Mmm). Disk sizes
range from ~10 AU to ~1000 AU. Hence disks of solar s),stem sizes, and of masses
comparable to or greater than the minimum mass solar nebula appear to form around a large
.h'action of stars with M < 3 M®.
• Nearly all optically thick disks show evidence of (1) boundary layer emission
diagnosed by "spectral veiling" at all optical wavelengths, and indicative of accretion of
material onto the stellar surface; and (2) strong, broad Hot emission and forbidden line
emission arising in energetic winds whose mechanical luminosity is proportional to the disk
accretion luminosity. We conclude that optically thick disks are accretion disks with typical
accretion rates M,_, > 10 _ MJyr.
• Disks survive as massive, optically thick structures extending nearly to the stellar
surface for times ranging from t << 3 Myr to t~10 Myr around solar-type PMS stars. No
optically thick disks have been identified around solar-type stars with ages t > 30 Myr. The
survival time for disks around more massive stars are not well established, particularly for
stars M > 3M,,. However, the limited data available suggests that the disk survival times for
such massive stars may be considerably shorter (t < 1 Myr) than those for solar-type stars.
• The descendants of massive, optically thick disks are disks containing masses of
distributed micron-size dust grains M < 104 M.; such disks are optically thin, and perhaps
analogous to the structures observed to surround the intermediate mass stars Vega and [_ Pic.
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Evidenceof infrared excess emission consistent with emission arising in optically thin disks is
found for stars of all masses. Stars surrounded by optically thin disks show none of the
signatures associated with disk accretion (strong boundary layer emission; emission from
spectral features arising in energetic winds). Hence, when disks become optically thin, disk
accretion ceases.
• The presence of micron-size grains in optically thin circumstellar disks at distances r <
1 AU from the surface of solar-type stars, requires that such grains be continuously supplied
to these disk regions; Poynting-Robertson or aerodynamic drag would otherwise deplete the
grains on timescales short compared to the age of stars surrounded by optically thin disks.
Small grains can be supplied either by collisions among larger grains or planetesimals; by
evaporation of cometesimals gravitationally scattered into the inner disk regions; or by grains
spiraling in from cold, outer disk regions (r >> I AU). Millimeter continuum observations
currently underway can place stringent limits on the amount of distributed cold dust.
• Disks with apparent "inner holes" have been discovered. These are most likely
structures in transition between disks that extend inward to the stellar surface and are
optically thick throughout, and those that are optically thin. The presence of an "inner hole"
may require that the inner regions of a disk be isolated from the exterior, optically thick disk
regions. If so, the formation of a giant planet provides an attractive mechanism for tidally
isolating the outer and inner disk regions. In this picture, the formation of a giant planet
creates a gapinthe disk. Gas and small grains located inward of the gap accrete onto the
parent star; small grains, and possibly gas exterior to the gap survive lk_r times t > 0.1 Myr.
• Our assessment of disk properties to date rests entirely on observations of emission
arising from dust contained within the disk. lnterferometric observations reveal CO mm-line
emission arising in disk gas surrounding two extremely young solar-type PMS stars (HL Tau
and L1551, IRS 5). The estimated ,,'_eas mass for these systems lies in the range 0.1 to 1M,_
(consistent with the masses determined from mm-continuum emission, "standard" dust
emissivities, and an interstellar gas/dust ratio). Systematic searches for disk gas associated
with stars of differing ages and masses are critical to determining characteristic times for the
evolution of the gas component. However, no such searches are yet available, primarily
because the sensitivity required demands extensive time on large mm-wave telescopes. We
have searched for CO emission associated with G and K main sequence stars in the nearby
Ursa Major cluster (d=25 pc; t~200-300 Myr). We find no evidence of CO emission
associated with these stars, despite the fact that our measurements are sensitive enough to
permit detection of M - 10.4 M,D of gaseous material with a CO/H__ ratio comparable to
interstellar values. If CO survives in the circumstellar environment, then these observations
,,* ' survival time t < 200-300 Myr.suggest a eas
Projected A ccomplishments
We plan to complete program of precise near- and mid-infrared measurements aimed at
probing the distribution of dust located within the inner regions (r < 1 AU) of circumstellar
disks surrounding solar-type stars. Our g¢_al is to establish the magnitude of excess (above
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photosphericlevels)near-infraredemissionfor a large(150) sampleof starswith agesranging
from 30 Myr to 300 Myr, and to determinetherebythe timescale(s)over which dustemission
arising from inner disk regionsbecomesundetectable.Becausedust in the inner regionsof
optically thin disksmust be replenishedcontinuously,we believethat this programwill
providea strongastronomicalconstrainton thetimescale(s)for assemblingearth-massbodies
from planetesimalsby monitoringtherateat which small grainsareproducedvia collisions
betweenplanetesimalsasa functionof stellarage.
We also plan to carry out a programof sensitivemm-continuumobservationsaimedat:
• Determining for solar-type stars with ages t > I0 Myr, the fraction, as a function of
parent star age, that are still surrounded by circumstellar disks containing a significant (M >
0.001 M,,) amount of material in the Giant Planet Region (3 < r < 30 AU). These
observations will provide an important astronomical constraint on the timescale available for
building giant planets, and will complement recent infrared and mm-continuum surveys of
much younger (t<<10 Myr) solar-type stars.
• Identifying solar-type stars in which planet-building may already have taken place at r
< 1 AU. Such stars should be surrounded by disks which produce strong mm-continuum
excess, but no near- and mid-IR excesses. This spectral signature is indicative of a star
surrounded by a significant mass of cold dust located at distances of r>>l AU, but little or no
warm, micron-size dust located within -1AU of the stellar surface. The development of such
"inner holes" may signify that agglomeration of dust into planetesimals has begun in the
terrestrial planet region.
Finally, we will pursue efforts to detect C() er0ission from disks surrounding young (t < 100
Myr) solar-type stars using the IRAM antenna.
Publications
Evolutionary Timescales for Circumstellar Disks Associated with Solar-Type Pre-Main
Sequence Stars, 1989, Sixth Conference on Cool Stars, Stellar Systems, and the Sun,
Astronomical Society of the Pacific Conference Series Volume 9, ed. G. Wallerstein -- with
S. Edwards and M. Skrutskie.
Forbidden Lines in T Tauri Star Spectra: A Clue to the Origin of T Tauri Winds? July 1989,
ESO Workshop on Low Mass Star Formation and Pre-Main Sequence Objects, in Garching,
F.R.G. -- with S. Edwards, S. Cabrit, and L.O. Ghandour.
Lithium Abundances Among Solar-Type Pre-Main Sequence Stars, 1989, A.J. 98, 1444 --
with K.M. Stnm_, F.P. Wilkin, and R.L. Seaman.
Further Evidence for Differential Rotation in VI057 Cygni, 1990, Ap. J. 349, 328 -- with
A.D. Welty, K.M. Strom, L.W. Hartmann, S.J. Kenyon, G.L. Grasdalcn, and J.R. Stauffer.
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An IRAS Surveyof theTaurus-AurigaMolecularCloud, 1990,A.J. 99, 869 -- with S.
Kenyon, L. Hartmann, and K.M. Strom.
Evolutionary Timescales for Circumstellar Disks Associated with Solar-Type Pre-Main
Sequence Stars, 1990, in Protostars and Planets II1, (ed. E. H. Levy and J. Lunine, University
of Arizona Press: Tucson) -- with S. Edwards and M. Skrutskie.
A Sensitive 101a Search for Emission Arising from Circumstellar Dust Associated with
Solar-Type Pre-Main Sequence Stars, 1990, A.J. 99, 1187 -- with M.F. Skrutskie, D.
Dutkevitch, S. Edwards, K.M. Strom, and M.A. Shure.
Correlation of Optical and Infrared Excesses in T Tauri Stars, 1990, Ap.J.Lett. 354, L25 --
with P. Hartigan, L. Hartmann, S.J. Kenyon, and M.F. Skrutskie.
Forbidden Line Emission and IR Exces_s in T Tauri Stars: Evidence for Accretion-Driven
Mass-Loss? 1990, Ap.J. 354, 687 -- with S. Cabrit, S. Edwards, and K.M. Strom.
Disks Associated with Intermediate Mass Stars, 1990, in NATO Advanced Research
Workshop: Angular Momentum Evolution of Young Stars 17-21 Sept. 1990, NOTO, Italy (to
appear in Conference proceedingsi Kiuwer Academic Publishers; Dordrecht, The Netherlands)
-- with J. Keene, S. Edwards, L.A. Hillenbrand, K.M. Strom, L. Gauvin, and G. Condon.
Z
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The evolution of young stellar object disks and their environment

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