by Staff Writers
Philadelphia PA (SPX) Jun 13, 2012
As a part of their "Living with a Red Dwarf" program, Villanova astronomers Scott Engle and Ed Guinan are trying to understand how the most common type of star known live its lives, and what kinds of environments they provide for possible life-bearing planets.
Dwarf M-stars (dM stars or M dwarfs) are by far the most numerous stars in the solar neighborhood (and likely the entire galaxy), comprising 70-80% of all stars. dM stars are cool, low luminosity stars with Teff = 2640-3850K and luminosities that range from L = 0.0008-0.06 Lsun (for dM8-dM0 stars, respectively).
These small, low-mass stars (~0.1-0.6 Rsun; ~0.1-0.6 Msun) have very slow nuclear fusion rates (compared to the more massive solar-type stars) and thus have very long lifetimes that range from ~40 Gyr for dM0 stars to longer than 100 Gyr for the lower mass, very low luminosity dM5-8 stars.
Given their large numbers and long lifetimes, determining the number of dM stars with planets and assessing planetary habitability is critically important because such studies would indicate how common life is in the universe.
Despite their abundance and importance, determining ages for these stars is very difficult. Because of their slow nuclear fusion rates, dM stars undergo almost negligible changes in temperature or luminosity over time, making traditional age determination methods (such as isochronal fits) essentially impossible.
There is, however, one property of dM stars that does noticeably change over time - the strength of their magnetic fields. As dM stars (along with K- and solar-type G-stars) age, they lose mass along stellar windows following magnetic field lines.
This causes the "spin-down" effect, where the rotation period lengthens over time. This is a quantity that can be directly measured and then calibrated as a "dating method" or "aging method."
The problem has been the need to calibrate a relationship between stars of known rotation periods and stars of known ages. The database of dM stars with reliably known ages has long been limited but, recently, two separate studies published by Garces et al. and Zhao et al. in 2011 furnished a nice list of dM stars with white dwarf companions. Recent work has allowed for much more reliable white dwarf ages to be determined, and that age can be assigned to the companion dM star through association.
These were the calibration stars the Villanova Astronomers had been waiting for. Observing as many stars as they can, and continuing to do so, they started to see rotation periods from the periodic brightening and dimming of the stars, as starspots come in and go out of view. This allowed for a much better-populated, and reliable, age-rotation relationship to be derived.
In addition to rotational effects, the magnetic activity also reveals itself in the levels of X-ray and UV activity the stars demonstrate. Engle was very grateful to receive an approved Chandra X-ray telescope program to observe some of his new calibrator stars, to define their X-ray activity levels. dM stars are believed to undergo magnetic activity cycles of several years, much like the Sun does, so the single X-ray exposures represent only the beginning of the path to understanding the true "average X-ray luminosity" of the dM stars.
But, as the character David says in the recently release Prometheus movie, "Big things have small beginnings." Engle and Guinan were also very excited to just recently be approved for a Hubble Space Telescope program to observe their targets, in order to derive an ultraviolet activity over time relationship, similar to the X-ray relationship that is under way.
The behaviors of this high-energy radiation as the dM stars age is of critical importance for possibly habitable planets that could be circling dM stars. A recent study by the European HARPS "planet-hunters" estimated that as many as 40% of all dM stars could host Earth-like planets in their habitable zones.
Engle points out, "With around 270 known dM stars in the local stellar neighborhood, that could mean there are as many as 100 dM stars with habitable, Earth-like worlds right in our own backyard."
The overall issue of dM star habitability is a tricky one, with pros and cons. dM stars have very long, stable lifetimes, which is always important to evolving life.
However, for a planet to be warm enough to have liquid water on its surface, it must be very close to the dM star, even closer than Mercury is to the Sun. This brings up the very important question of what the high energy radiation would do to the atmosphere and surface of the planet. Charged particles and high-energy radiation can strip away a planet's atmosphere, and also damage or destroy the DNA of any life forms on the planetary surface.
A habitable planet would need both an atmosphere and a good magnetic field to deflect or absorb the particles and radiation. Studies have shown that super-Earth planets (those with masses ~3-10x that of the Earth) are prevalent around dM stars, but recent studies have called into question the ability of the heavier super-Earths (~5-10x the Earth's mass) to generate sufficiently strong magnetic fields.
However, such studies are being revised continuously, and every specific planets can behave in slightly different way depending on its characteristics and history Studies agree, though, that planets closer to Earth's mass (~1-5x the mass) can still definitely generate a sufficient magnetic field and atmosphere to be habitable.
Given the sheer number of dM stars in our neighborhood, and the galaxy itself, understanding the lives they live and the environments they put their planets through has ever been more important.
Astronomy at Villanova University
Stellar Chemistry, The Universe And All Within It
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