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High-energy Irradiance Evolution of Low-mass
Stars: Effects on Planetary Atmospheres
I. Ribas1, E. F. Guinan2, F. Selsis3, H. Lammer4
11 Institut d’Estudis Espacials de Catalunya/CSIC, Campus UAB, Facultat de Ciències, Torre C-5 – parell – 2ª planta, E-08193 Bellaterra, SPAIN
22 Department of Astronomy & Astrophysics, Villanova University, 800 Lancaster Ave, Villanova, PA 19085, USA
33 Centro de Astrobiologia (INTA-CSIC), Carretera de Ajalvir, km 4, E-28850, Torrejón de Ardoz, Madrid, SPAIN
44 Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, AUSTRIA
Abstract:
The results from the “Sun in Time” program suggest that the coronal X-ray-EUV emissions of the young main-sequence Sun were ~100-1000 times stronger than those of the present Sun. Similarly,
the transition region and chromospheric FUV-UV emissions of the young Sun are expected to be 20-60 and 10-20 times stronger, respectively, than presently. In the entire XUV interval from 1 to 1200 Å, we find
that the solar high-energy flux was about 3 times the present value 2.5 Gyr ago and about 6 times the present value about 3.5 Gyr ago (when life arose on Earth). In summary, compelling observational evidence
indicates that the Sun underwent a much more active phase in the past. The enhanced activity revealed itself in the form of strong high-energy emissions, frequent flares, and a powerful stellar wind. Such energy
and particle environment certainly had an impact on the genesis and evolution of Solar System planets and planetary atmospheres. Because lower mass stars are especially common and hence may host habitable
planets, progress in this direction has started by expanding the ``Sun in Time'' program to time sequences of the high-energy emissions, wind, and flare activity of low-mass K and M stars. Because of the low
luminosities, their “habitable zones” can be quite close to the host stars. Low-mass stars have deeper outer convective zones than Sun-like stars and thus possess very efficient magnetic dynamos. The initial
studies of K and M main-sequence stars have revealed that they have very strong coronal/chromospheric XUV emission fluxes compared to solar-type with similar rotation periods or ages. This enhanced XUV
radiation environment is expected to play a major role in the development of the atmospheres and ultimately of life on planets located in their habitable zones. This research is of great importance to characterize
the targets of future space missions that will search for (terrestrial) planets in the habitable zone such as COROT, Eddington, and Darwin/TPF.
Stellar samples – age is the issue
Thermal escape in planetary atmospheres
A key aspect of the investigation is the compilation of a sample of single, nearby stars to serve as proxies
to study the evolution of spectral irradiances. Stars within a narrow interval in masses must be selected
to avoid variations of their intrinsic magnetic activity levels because of different convective zone depths.
Characterization includes accurate determinations of Teff, R, L, [m/H], Prot, and, most notably, their age.
The stellar radiation with λ<1700 Å is absorbed (and thus deposits its energy) in the upper layers
of the planetary atmospheres. XUV radiation affects the vertical temperature profile of the
atmosphere and raises the temperature of the most external layer (exosphere). For high radiation
inputs, thermal (Jeans) escape of the atmospheric constituents (most notably H, which drags away
also heavier atoms), although usually neglected in the Solar System, becomes important.
Stars around solar-type stars (e.g., “Hot Jupiters”) may lose significant fractions of their hydrogen
envelopes under intense XUV radiation and eventually evaporate down to their core sizes (Lammer
et al. 2003). The calculations are in excellent agreement with direct observations of HD209458 by
Vidal-Madjar et al. (2003). This effect may partly explain the paucity of exoplanets detected at
close orbital distances (<0.05 AU).
Stellar ages are determined using several methods:
• Cluster or moving group membership (young)
• Calibrated relationships with Prot and LX (intermediate)
• Theoretical isochrones (old)
Above is the sample of solar-type stars. Similar compilations
have been put together for K- and M-type stars
Rotation-period age relationship
for solar-type stars
High-energy irradiance evolution
High-energy emissions are associated with magnetic activity in cool stars. Chromospheric, transition
region and coronal plasmas are heated through dynamo processes to temperatures in the range 104-106
K and emit from the X-rays to the UV (XUV). The resulting high-energy fluxes are variable in timescales
of hours to Gyr. In this work we focus on the evolution of these irradiances during the longest timescale,
i.e, the main sequence lifetime, of low-mass (G-K-M) stars.
Solar-type stars (G0-G5)
In the case of K and M stars the effects are enhanced
X = 1.5→ blow off
because the XUV radiation levels can be significantly
stronger (in the habitable zone; HZ) than those of
solar-type stars for extended periods of time. Even
planets with heavier atmospheric constituents may
suffer intense evaporation. Our calculations indicate
that only Earth-like planets with a dense CO2
atmosphere will be able to keep its constituents
because of efficient IR cooling.
Tidal locking is also
Very high
loss rates
an issue for planets
for O, N,
C atoms
in the HZ around M
stars because of the
lack of a protecting
magnetic field and
Vertical temperature profile of an Earthline planetary atmosphere under different
condensation on the
XUV radiation inputs
dark side.
Habitable zone around main sequence stars
H
Observations of the “Sun in Time”
targets have been acquired over the
course of 20 yr with a variety of
high-energy spacecrafts/instruments
including: ASCA & ROSAT (Xrays), EUVE (EUV), FUSE (FUV),
IUE (UV), HST (UV). The measured
fluxes, which cover a wavelength
interval 1-3300 Å, were carefully
calibrated and scaled to match the
properties of a 1 M~ star at a
distance of 1 AU (Ribas et al. 2004).
X-ray (ASCA & ROSAT) and FUV (FUSE; detail) irradiances
of solar-type stars. Note the obvious trend of decreasing flux
with increasing age
¾The stellar fluxes are well approximated by power laws of
different slopes
¾The slope decreases monotonically from X-rays to the UV
(hotter plasmas diminish more rapidly as stars spin down)
¾The emissions of young G stars are stronger than the
Sun’s:
• 100-1000× in X-rays • 20-60× in the FUV • 10-20× in the UV
Normalized fluxes of the Sun in Time targets
with the corresponding power law fits
Temperatures of H-based planetary exospheres Time evolution of the mass of two “Hot Jupiters” as a
as a function of the orbital distance for different consequence of thermal escape from XUV radiation.
stellar ages (solar type) or XUV fluxes. The
The thick line indicates the maximum mass for which
shaded area indicates blow off conditions.
complete evaporation occurs in 1 Gyr.
¾In the interval 1-1200 Å we find:
• About 3× and 6× the present flux 2.5 and 3.5 Gyr ago
• Up to 100× stronger than today for ZAMS stars!
K and M stars
The initial stages of the extension of the project to lowermass stars have been completed. A sample of K-M stars
covering 100 Myr to 10 Gyr has been put together. The ages
have been determined in an analogous way to G stars.
We use the X-ray flux as an activity proxy. The normalized
X-ray flux is useful to scale the irradiances.
¾The irradiances stay at saturated levels (LX/Lbol≈10-3)
for longer (up to 1 Gyr in the case of M stars!)
¾The slope of the decreasing power laws are similar for
G-K-M stars
¾If the plasma emissions scale similarly to G stars:
• K stars XUV B 3-4× XUV of G stars at same age
• M stars XUV B 10-100× XUV of G stars
X-ray fluxes normalized to the bolometric stellar
luminosity for stars of spectral type G-K-M
Stellar wind and flares
One of the major loss processes affecting planetary atmospheres is erosion from charged
particles (stellar wind). In this case, atmospheric constituents are eroded away via two
mechanisms known as sputtering and ion pickup. These are most effective when the planet is
not protected by a magnetic field.
Direct detections of low-mass star winds
are challenging but recent results (Wood
et al. 2002) indicate that active stars may
have winds up to 1000× stronger than the
Sun today. Work is underway to measure
directly the winds of K-M stars. Flares
and coronal mass ejections are other
phenomena present in active stars that
have an impact on planetary atmospheres.
For example, active K and M stars have
strong flares up to once per hour. The
particle and XUV fluxes may be sufficient
to erode away a significant fraction of the
Relationship between X-ray surface flux and stellar
atmospheric constituents of terrestrial
wind (Wood et al. 2002). The relationship seems to
planets in orbits < 0.5 AU.
hold for G-K stars but winds of M stars may be weaker.
¾ Our preliminary results suggest that rocky
planets in the HZ of M stars up to 0.2 AU
may never evolve into habitable worlds!
¾ For K stars it remains to be seen…
References
• Lammer, H., Selsis, F., Ribas, I., et al. 2003, ApJ, 598, L121
• Ribas, I., Guinan, E.F., Güdel, M. 2004, ApJ, submitted
• Vidal-Madjar, A., Lecavelier des Etangs, A., Désert, J.-M., et al. 2003,
Nature, 422, 143
• Wood, B.E., Müller, H.-R., Zank, G.P., Linsky, J.L. 2002, ApJ, 574, 412