Habitable zones around L and T dwarfs: detecting Earthlike planets with Nahual

1
Habitable zones around L (and T?) dwarfs
detecting Earth-like planets with Nahual?

• José Antonio Caballero
• Instituto de Astrofísica de Canarias

2
HZs around L dwarfs L and T dwarfs

• Two new spectral types have been defined in the
  last decade in order to classify objects cooler
  than M dwarfs L and T
• Oh, Be A Fine Girl, Kiss My Lips Tonight
• L dwarfs objects with effective temperatures
  between 2200 and 1350 K. Evolved L dwarfs have
  typical masses in the range 0.080 to 0.065 solar
  masses (Msol)
• T dwarfs objects with effective temperatures
  between 1350 and 700? K. Typical of masses
  below 0.065 Msol
• L and T dwarfs could outnumber M dwarfs in our
  Galaxy
• They seem to form as 0.5-0.1-Msol stars do ? Many
  of them have accretion discs ? planetary
  formation around them?

3
HZs around L dwarfs evolution of an ultracool
dwarf

• Substellar objects (with masses below the
  hydrogen burning limit, around 0.072 Msol solar
  metallicity-) are never able to stabilize their
  temperature and continuously dim and cool with
  time
• In contrast, a very low mass star, just above the
  hydrogen burning mass limit, will stay in the
  Main Sequence for, perhaps, terayears
• Only about one third of the early-L dwarfs are
  substellar objects
• Evolution of luminosity, radius, lithium
  abundance... of objects with masses below 0.100
  Msol has been modeled by several authors (in this
  talk I am using Chabrier et al. (2000) models
  DUSTY models are more suitable than COND ones to
  describe the photospheres of early- and mid-L
  dwarfs)

4
HZs around L dwarfs evolution of an ultracool
dwarf
5
HZs around L dwarfs effective temperature of a
planet

• The habitable zone (HZ) around a star is defined
  as the range of orbital distances where a planet
  can support liquid water (273 to 373 K). Also
  continuously habitable zone (Kasting et al. 1993)
  (Earth 7 Myr?)
• The effective temperature (Teff) is the
  temperature that the outermost raditive surface
  of a planet (cloud or solid) would have if the
  starlight absorbed by the planet were distributed
  uniformly around the surface and radiated to
  space (Caldwell 1992)
• The usual radiative energy balance equation is
• (1-A) p R²planet S 4p R²planet s Teff4planet,
• where A is the planetary albedo (the ratio of
  all incoming radiation to the immeditely
  reflected to space) and S is the radiation power
  per surface unit reaching the planet (S is equal
  to Lstar/4p a²)

6
HZs around L dwarfs surface temperature of a
planet

• The effective temperature of a planet is a
  function of the planetary albedo (A), the
  luminosity of the star (Lstar) and the distance
  between the star and the planet (semimajor axis,
  a)
• However, the real surface temperature of a planet
  is not so easy to estimate. It strongly depends
  on
• the surface pressure
• the tropospheric adiabatic lapse rate, G
• the presence of greenhouse gases (especially CO2
  and H2O)
• the wind and oceanic circulation pattern and the
  global heat exchange (both affected by orbital
  locking, distribution and extension of oceans and
  continents...)
• deviations in the radiative energy balance
  equation (internal energy source of the planet
  tidal?-, emissivity of the planetary top
  atmosphere different from unity, flux factor for
  a slowly rotating planet with a thin atmosphere,
  non-zero orbital eccentricity...)

7
HZs around L dwarfs greenhouse effect

• The use of an optical thickness is a first order
  approximation that takes into account all the
  parameters Tsurf4Teff4 (1 3t/4)
• All the terrestrial planets in the Solar System
  have appreciable greenhouse effect (from Houghton
  1986)

8
HZs around L dwarfs more details to take into
account...

• Orbital locking atmospheric models indicate that
  relatively moderate climates could exist on
  Earth-sized planets in synchronous rotation
  around M dwarf stars (Joshi et al.1997).
  Circularized orbits?
• Mass constrains The planet must be able to
  retain an appreciable atmosphere around its rocky
  core (? Mplanet gt 0.8 M?), and the planet must
  not bring deep gaseous envelopes (? Mplanet lt 10
  M?)
• The cool central object emits most of the energy
  in the near infrared, where the CO2 and H2O
  absorption telluric bands are stronger ? lower
  albedo?, larger optical thickness? (Tsurf gt Teff)
• The planet must have an ocean where most of the
  CO2 is dissolved to avoid runaway greenhouse (as
  in Venus). The planet must have plate tectonics
  to avoid runaway glaciation (as in Mars, related
  to Mplanet) soft-Gaia theory?
• Bioastronomy lack of UV emission (no source of
  mutations) and nIR photosynthesis
  (bacteryclorophyle a 750-800 nm)

9
HZs around L dwarfs A simple model

• Terrestrial planets in HZ around 0.100-0.060
  Msol-dwarfs there are many solutions at
  different age-semimajor axis-albedo-optical
  thickness values
• For example, a realistic case A0.10, t1.0.
  Luminosities from Chabrier et al. (2000). Orbital
  distances 0.2 (gtRoche radius) to 20 Rsol
• Continuously habitable zones are getting narrower
  and closer to the dwarf when the masses get down
  (from 9-16 Rsol for 0.100 Msol to 1-2 Rsol for
  0.060 Msol)

10
HZs around L dwarfs Radial velocity results

• Continuously HZs around 0.080-0.065 Mso objects
  terrestrial planets with periods from ¼ to 8 days
  (being the most probable from 8 to 72 h)
• Accounting for the ltsin igt?2/3 factor, only
  massive teluric planets (i.e. 3 to 10 M?) give
  radial velocity amplitudes K1 from 2 to 18 m s-1
  (the most probable from 3 to 10 m s-1)
• Do 3-10-M? bodies exist at 2-10 Rsol of L dwarfs?
  (a/RL? satellites in the Solar System around
  jovian planets, RL/Rplanet? jovian exoplanets
  around main sequence stars)

11
HZs around L dwarfs conclusions

• Why in the near infrared? Because of the low flux
  in the optical. Disadvantages (and questions to
  the audience) Does the technology to build a
  HARPS-like spectograph (R?105) in the nIR exist
  now?. Are there enough lines in the nIR to reach
  the sensitivities attained in the optical?. Do
  rotational broadening affect?
• Massive teluric planets in broad continuously HZs
  around L dwarfs could be detected in a few nights
  if RV precision of 1-2 m s-1 can be achieved
• Earth-like planets can also be detected with
  larger RV amplitudes around T dwarfs in only one
  night, but the CHZ is narrower and shorter in
  time
• Althought detecting Earth-like planets in HZs
  around L-T dwarfs seem to be difficult, it is
  very easy to detect hot planets around brown
  dwarfs at distances similar to those found in the
  Jupiter system!

12
HZs around L dwarfs appendix

• RV searches capabilities (following Desidera
  1999)
• RV precision dv ? c w/ l d(Npix Ic)½ (c speed
  of light, w width of spectral line, Npix number
  of pixels in a line, d line depth as a fraction
  of the continuum intensity, l wavelength, Ic
  continuum intensity)
• Assuming v sin i 10 km s-1 (? d ? 0.4), J band
  (? l 1.25 mm), R 150 000 dv ? 10 180 / (S/N)
  ms-1
• Considering simultaneous coverage of several
  spectral lines dvN ? dv / (Nlines)½. gt100-200
  lines in the J band?
• If Nlines 500, S/N 300, then dvN 1.5 ms-1
  (of the order of the systematic errors). 1 h in a
  10-m class telescope with Adaptive Opticcs?
• Wanted spectral coverage, resolution and
  telescope size