This cooperative agreement was established with the scientific goal of understanding the conditions of early solar-type planetary systems. We investigated two facets of young solar systems: The effects on planetary bodies of young solar-type stellar mass loss, and photo-production of various organic materials due to radiation under comet-like conditions

Bernstein, Max

Doyle, Laurance R.

English

NASA Technical Reports Server

NASA-CR-200209 -*
FINAL TECHNICAL REPORT
NCC 2-758
"Origins of Planetary Systems: Observations and Analysis"
Laurance R. Doyle, Principal Investigator
Max Bernstein, Co-Investigator
(NASA-CR-200209) ORIGINS OF N96-1904A
PLANETARY SYSTEMS: OBSERVATIONS AND
ANALYSIS Final Report, 1 Mar. 1992
- 31 Dec. 1995 (Search for Unclas
Extraterrestrial Intelligence
Inst.) 6 p
63/90 0100679
March 1,1992 to December 31,1995
JM 1 7 1996
SETI Institute ^-
2035 Landings Drive \ °
Mountain View, CA 94043
https://ntrs.nasa.gov/search.jsp?R=19960013894 2020-06-16T04:44:15+00:00Z
Abstract
This cooperative agreement was established with the scientific goal of understanding the
conditions of early solar-type planetary systems. We investigated two facets of young
solar systems: The effects on planetary bodies of young solar-type stellar mass loss, and
photo-production of various organic materials due to radiation under comet-like
conditions.
Stellar Mass Loss and the Young Solar System
Background
An understanding of the astrophysical and planetary conditions under which biological
organisms originated on Earth is essential to an understanding and estimate of the
prevalence of such organisms around other stars. Modeling of the early solar system
ecoshell (region where liquid water can exist—roughly in between Venus and Mars in our
solar system) however, has run into problems. The standard solar model indicates that
when the solar system was born the Sun was only 70% as bright as it is today. If one
examines early Mars (the outer edge of the ecoshell) there is widespread evidence for
significant amounts of liquid water having existed on the surface (Carr 1987, McKay and
Stoker 1989). The ancient (4.6 Gyr ago) thick ^2 "greenhouse" atmosphere (and other
gases) that have been invoked to explain the presence of liquid water on Mars (Pollack et
al. 1987) have, however, become questionable since recent models including condensation
(Kasting 1991) have shown that the ^2 would have condensed out of the atmosphere at
any solar luminosity less than about 84%, a brightness the Sun did not reach until long
after widespread liquid water disappeared from the Martian surface. In addition, it has
recently been shown that, for a few bar atmosphere under the conditions of the standard
solar model, the Earth should have glaciated over (Caldeira and Kasting 1992) and would
not have been able to recover once frozen.
Consequently, Doyle et al. (1993) and Whitmire et al. (1993) have suggested that the early
Sun actually had 3-14% more mass than the present Sun. These mass-loss limits are
constrained by both planetary as well as astrophysical conditions (the luminosity of a
solar-type star scales with the mass as L oc M
 ) making the luminosity very sensitive to
changes in mass; Iben 1967). Planetary lower limits on early solar mass-loss require liquid
water to have existed on early Mars and the avoidance of complete glaciation on Earth
while allowing a runaway greenhouse to have taken place on Venus. Upper limits require
the avoidance of a runaway greenhouse on Earth. The Sun is also known to be depleted
in lithium by a factor of about 100 which can be accounted for by mass loss of about 10%
in the first Gyr after the Sun's formation (Boothroyd et al. 1991), or a mass-loss rate
10~'° K/f
about j/year (solar masses per year, about 10,000 times the present solar wind
rate). Magnetic braking to the present solar spin rate (Bohigas et al. 1986) would also be
expected to produce a mass-loss rate of about J/yr for the first Gyr of the solar
system. Solar-type stars in the Hyades star cluster (age about 0.5 Gyrs) indicate the same
degree of lithium depletion while solar-type stars in the Pleiades star cluster (age about
0.05 Gyrs) do not (Soderblom et al. 1990), indicating that the lithium depletion
mechanism takes place after complete star formation but within the first Gyr or so for
solar-type stars. Other preliminary evidence points to an early, more massive Sun, as well.
Meteorites and lunar samples indicate a significantly more flare active (Caffee et al. 1987)
young Sun with some models indicating a solar wind proton flux over four orders of
magnitude greater than that of today (Caffee et al. 1991). Thus, there are numerous
indications that the solar system ecoshell may have been significantly effected by early
mass loss from the Sun. But does this process really take place in young solar-type stars?
This is what we wish to measure directly.
We selected four young, nearby solar-type stars to observe with the VLA for stellar mass
loss (we obtained 29 hours and took observations at 3.6, 6.0, and 20 cm). Their young
ages were determined using several techniques such as membership in a young cluster of
stars, high starspot activity, and/or rapid rotation periods (for example, Soderblom et al.
o
1991, Gray 1992). All ages are taken to be less than 0.5 Gyrs. The expected flux, °" (in
milliJanskys), from these stars can be calculated using (after Wright and Barlow 1975,
Leitherer and Robert 1991, and Mullen et al. 1992):
where A^» is the stellar mass-loss rate (in solar masses per year), Z is the rms ionic charge
of the wind particles, Y is the number of electrons per ion, %v is the free-free Gaunt
factor, v is the frequency (in Hertz), v» is the stellar wind velocity (in km/sec; we use a
solar wind distribution), ^ is the mean molecular weight of wind particles, and, finally, d
is the distance to the star (in parsecs). For a typical solar-type stellar wind Z = y = /* = 1
may be approximated. An upper limit on distance for detectability within reasonable
observing times for our stars will also be about 15 parsecs. The free-free Gaunt factor,
gv = 10.6 + 1.901ogr-L261og v-1.261ogZ at radio frequencies (Allen 1973; where Ti*
the stellar wind temperature in degrees K). From these considerations we determined that
the measurement could be made, that is, an excess stellar wind could be detected and
identified by its unique 2/3 spectral index (flux depends on the frequency to the 2/3'
power).
Results
An upper limit on the mass loss rate of young solar-type stars, will have substantial impact
on our understanding of the early evolution of the solar system. We have reached down
to the level of 7% mass loss rate with the star HD72509, but more observations are
needed to reduce the error bars. We have also reached this approximate level with the star
HD39587, but have since discovered that its flare activity from a very close M-star
companion will also require further observations to determine if mass loss is taking place
(flare will not generally have a 2/3 spectral index). The stars HD115383 and HD206860
have also been observed but not sufficiently to have detected radio emissions indicative of
mass loss at the 7% rate at present. A more detailed report of the result of these
preliminary observations to detect young solar-type stellar mass loss can be found in
Doyle et al. (1995). Subsequent support for this effort has not, however, been
forthcoming. Nevertheless, either positive or negative results of these observations have
significant impact on our understanding of the conditions under which biology originated
on the early Earth as well as applicability to the evolution of young habitable zones in
general.
For example, the importance of non-detection of mass loss from young solar-type stars
would imply that the Sun did not lose substantial mass in the first billion years after the
origin of the solar system. Completely new theories would have to be developed to
explain the presence of liquid water on early Mars and the avoidance of permanent
glaciation on the Earth (along with modifications of solar evolution, interpretations of
meteoritic data and the cratering records, etc.). A detection of small but significant stellar
wind from young solar-type stars would strongly imply that this process took place in our
young solar system and takes place, in general, for very young solar-mass stars. It would
provide a straightforward explanation for the presence of liquid water on early Mars and
modify the timescale by which the runaway greenhouse effect took place on early Venus.
Meteoritic data from this epoch could be interpreted in terms of an increased early solar
wind and a mechanism could be suggested for the cause of the late heavy bombardment in
the cratering record. Finally, it could provide a mechanism by which the young Earth
avoided glaciation. Although it represents a small measurement on the stellar scale, such a
study would have major repercussions for our understanding on the planetary scale,
including the environment in which biological organisms first originated.
Organics In Comets
This part of the project was a study of the organic photochemistry of comets and
interstellar ice grains. We developed an improved technique for the generation of organic
residues of the type believed to reside on the surface of comets and interstellar ice grains,
and modified a Gas Chromatograph-Mass Spectrometer (GC-MS) to allow for analysis of
volatiles. The Infrared Spectra of the residues are an order of magnitude better those seen
before and, combined with GC-MS data, have allowed us to identify Hexamethylene-
tetramine (HMT) as the major component of the organic residue, in addition to CO,
formaldehyde and ethanol, which are lost as volatiles during annealing. We then studied
the spectroscopy and UV photolysis of HMT in Ar and "-fl at 12 K and found that UV
photolysis of HMT in the lab caused the formation of a cyano (CN) species, with the same
IR signature that has been seen in the direction of the object W33A.
Boothroyd, A.I., I. Sackmann, and F.A. Fowler, (1991), " Our Sun JJ. Early Mass Loss of
0.1 Solar Mass and the Case of the Missing Lithium", Ap.J. 377, 318-329.
Caffee, M.W., C.M. Hohenberg, T.D. Swindle, and J.N. Goswami, (1987), "Evidence in
Meteorites for an Active Early Sun", Ap.J. Lett. 313, L31-L35.
Caffee, M.W., C.M. Hohenberg, R.H. Nichols Jr., C.T. Olinger, R. Wieler, A. Pedroni, P.
Signer, T.D. Swindle, and J.N. Goswami, (1991), "Do Meteorites Contain Irradiation
Records From Exposure to an Enhanced-Activity Sun?" in The Sun in Time, C.P. Sonett,
M.S. Giampapa, and M.S. Matthews (Eds.), U. of Arizona Press, Tucson, AZ.
Caldeira, K. and J. Kasting, (1992), " Susceptibility of the Early Earth to Irreversible
Glaciation Caused by Carbon Dioxide Clouds", Nature 359, 226-228.
Carr, M., (1987), "Water on Mars", Nature 326, 30-35.
L.R. Doyle, C.P. McKay, D.P. Whitmire, J.J. Matese, R.T. Reynolds, and W.L. Davis,
(1993), " Astrophysical Constraints on Exobiological Habitats", in Third Decennial US-
USSR Conference on SETI, A.S.P. Conference Series, S. Shostak (Ed.), 47, 198-217.
L.R. Doyle, N.C. Heather, R. Vikramsingh, and D.P. Whitmire, 1995, "Early Solar Mass
Loss and the Solar System Habitable Zone", in Circumstellar Habitable Zones:
Proceedings of the First International Conference, (ed) L.R. Doyle, Travis House
Publications, Menlo Park, CA, in press.
Gray, D.F., (1992), The Observation and Analysis of Stellar Photospheres, Cambridge
University Press, N. Y.
Iben, I., (1967), " Stellar Evolution VI. Evolution of Low Mass Population I Stars", Ap.J.
147, 624.
Kasting, J.T., (1991), " Carbon Dioxide Condensation and the Climate of Early Mars",
Icarus 94, 1.
Leitherer, C. and C. Robert, (1991), " Observations of Stellar Winds From Hot Stars at
1.3 Millimeters", Ap.J. 377, 629-638.
»
McKay, C.P. and C.R. Stoker, (1991), " The Early Environment and Its Evolution On
Mars: Implications For Life", Rev. Geophys. Space Phys. 27, 189-214.
Mullen, D.J., J.G. Doyle, R.O. Redman, andM. Mathioudakis, (1992), "Limits on
Detectability of Mass Loss from Cool Dwarfs", Ap.J. 397, 225-231.
Pollack, J.B., J.F. Kasting, S.M. Richardson, and K. Poliakoff, (1987), " The Case For a
Wet, Warm Climate on Early Mars", Icarus 71, 203-224.
Soderblom, D.R., M.S. Oey, D.R.H. Johnson, andR.P.S. Stone, (1990), " The Evolution
of the Lithium Abundances of Solar-Type stars. I. The Hyades and Coma Berenices
Clusters", A.J. 99, 595-607.
Soderblom, D.K. Duncan, D.R.H. Johnson, (1991), "The Chromospheric Emission-Age
Relation for Stars of the Lower Main Sequence and its Implications for the Star Formation
Rate" ,Ap.J. 375,722-739.
Wright, A.E. and MJ7. Barlow, (1975), "The Radio and Infrared Spectrum of Early-type
Stars Undergoing Mass Los", Man. Not. R. Astr. Soc. 170, 41-51.


Origins of planetary systems: Observations and analysis

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