Astrobiology

1
Very Low Mass Stars as Optimum Sites of Habitable
Planets and Their Detection
Andrey Andreeshchev John Scalo, Univ. Texas at
Austin David Smith, Univ. Texas and Harvard
INTRODUCTION Properties of Very Low Mass Stars
(VLMS) Masses limit (0.075 Msun) and below. Nearby examples
Proxima Centauri, Barnards star, YY Gem, UV
Ceti, VB8, CN Leo,many still being discovered
(DENIS, 2MASS). Here for simplicity we include
brown dwarfs with VLMS. Temperatures Red, cool
(Teff ? 3500K), spectra dominated by molecules
(TiO, VO, H2O,). Spectral types roughly K7 to
M9 to L, T brown dwarfs. Flares Mostly
convective interiors. Intense surface activity
and fast rotation during youth. First few Gyr
(e.g. Stauffer Hartmann 1986, Gizis et al.
2002) spent as dMe/flare stars, with large flares
(relative amplitude order of magnitude) roughly
once per hour (see Gershberg Shakhovskaya
1983). Strongest in U, UV and x-rays.
Interesting radiation environment! Faint Lbol
from about 10-1 to 10-4 Lsun and smaller ?
conventional habitable planets (liquid water)
will be very close to parent star, leading to
synchronization of spin and orbit by tidal
forces. Fig. 1 shows habitable zone radii for M
realistic atmospheric boundary conditions, along
with tidal locking and Roche radii. Standard
view poor bets for habitability because
synchronous rotation leads to rapid atmospheric
collapse by circulation to the dark side.
Joshii et al. (1997) 3D hydro/climate
simulations show that circulation on
synchronously rotating planets allows retention
of atmosphere for pressures greater than a few
percent of Earths ? main objection to
habitability now removed. Now consider the
viability of VLMS habitable planets in fresh
light. (See also Heath et al. 1999)

• Top 10 reasons why very low mass stars (VLMS)
  may be optimum sites for habitable planets
• Common VLMS are the most numerous stars in the
  Galaxy (75 to 85 see Chabrier 2000, 2001).
  Even given uncertainly in IMF at low masses, this
  is a robust result.
• Formation Disks and planets should be able to
  form around VLMS (simulations of planetesimal
  growth down to 0.5 Msun, Wetherill 1996) and are
  observed (Delfosse et al. 1998, Marcy et al. 1998
  for M star Gl 876 planet Muench et al. 2001,
  Haisch et al. 2001 for disks, or at least IR
  excesses).
• Get climate stability without a large moon
  Tidal effects on habitable planets around VLMS
  damp rapid ( 107 yr) chaotic obliquity
  variations (Touma Wisdom 1993, Laskar Robutel
  1993), without need for a massive moon like ours
  (very unlikely event if collisional origin see
  Lissauer 2001).
• Huge lifetimes VLMS lifetimes ? age of Galaxy ?
  virtually no brightening, no faint young Sun
  paradox. Once in HZ, always in HZ! (unless
  youre orbiting a brown dwarf--even then duration
  of habitability can be many Gyr Andreeschchev
  Scalo 2002, in preparation Desidera 1999 ).
• Safety from ejection HZ planets around VLMS are
  tightly bound (hard orbits), so risk of
  ejection by stellar encounters (free-floating
  planets) during cluster phase (cf. Davies
  Sigurdsson 2001, Harley Shara 2002) is small.
  Easy to verify that orbital speeds much larger
  than typical cluster velocity dispersions for
  VLMS HZ planets.
• Safety from disk photoevaporation Frustration of
  planet formation by photoevaporation of disks by
  massive stars in clusters (Throop et al. 2001) is
  less probable for smaller-mass stars because of
  primordial mass segregation observed in young
  clusters (Meyer et al. 2000 and references
  there).
• Tidepools One of most serious origin of life
  problems It is difficult if not impossible to
  polymerize amino acids and nucleotides into
  proteins and RNA/DNA (or even lengths 30-100
  required for self-replication) in aqueous
  solution (see extensive refs. In Lahav 1999,
  Lahav 2000).
• Many suggest adsorption on minerals followed by
  washing (e.g. Orgel 1998). On Earth, this
  requires large moon (improbable). On HZ planets
  around VLMS tides are strong without a moon,
  although tides are stationary without
  eccentricity or resonances, since synchronous
  rotation. (5 bucks to anyone with better
  solution!)
• Flares Extremely strong and frequent flares
  usually regarded as threat to life on HZ planets
  around VLMS. Average U-band fluxes are weak, but
  U amplitude of flares (e.g. Lacey et al. 1976,
  Gershberg Shakhovskaya 1983, Gershberg et al.
  1999 and references therein) can easily exceed
  100 (Fig. 2). Flare x-ray fluxes and fluences
  of order 106 times larger than on Earth due to
  Sun (Fig. 3) once per 100 hr (or so). Coronal
  x-rays 102 to 103 stronger relative to Lbol
  than Sun.
• But not so clear whether the enhanced and
  stochastic mutation rate would have
  prevented/sterilized life, or instead accelerated
  evolution.
• In vitro and Artificial Life evolution genome
  lengths become large only in information-rich
  environment (e.g. Adami et al. 2000 and refs).
  Mutations that increase fitness can be regarded
  as random measurements on the environment.
  Flares provide a wide information channel on
  which natural selection can operate.
• Contrast with Ward Brownlee (2000) assumption
  that development of complex organisms primarily
  requires self-regulated stability of environment
  (which may only be true for considerations of
  survival, but not evolution).
• Production of biological precursor molecules
• Fig. 4 shows radiative transfer calculation
  (Smith et al., this meeting for details and more
  cases) of energy deposition by coronal or flare
  soft x-rays for two column densities and incident
  peak energies, for a Koren-Tucker model stellar
  x-ray spectrum. Through x-ray redistribution
  mediated by secondary electron ionization and
  excitation (similar to auroral emission), about a
  percent of the flare and coronal radiation
  reaching the surface is below about 100nm (for a
  column density like the Earths). Important!
• Examples
• a. Well-known that in weakly reducing
  atmospheres important precursor H2CO only easily
  formed from photolysis of H2O and CO2 (Kasting
  1993).
• b. HCN (important precursor to amino acid and
  nucleotide synthesis) is difficult to form in
  weakly reducing atmospheres because need to break
  strong bonds of N2 and CO flare and coronal
  radiation may remove this dilemma.
• c. Production of complex prebiotic organic
  compounds by irradiation of icy surfaces should
  also be much more important on flare star planets
  than on solar analogues.

log FX/Fbol
log FX/Fbol

• Why this might be completely wrong
• Intense flares from very young VLMS could
  sublimate grains and planetesimals out to the
  future HZ radius, suppressing planet formation
  there.
• Atmospheric energy deposition from VLMS ionizing
  radiation could lead to atmospheric loss.
• Much future work to do!

This work was supported by NSF grant 9907582.