jump to navigation

The M-Dwarf

Some background on M-dwarfs and how the make good harbingers of terrestrial ESPs (extrasolar planets):

So what is a Red, or M-dwarf Star about which GEMSS seeks to locate terrestrial planets? Located at the lower end of the Main Sequence of the H-R diagram, they are known to be small and relatively cool stars, and are in the M spectral class. Being small, the ratio of say, a terrestrial planet (in mass and occulting diameter) to its red-dwarf parent is much greater than for planets-to-sun-like stars. That’s a plus for GEMSS. Also this is by far the most common class of star, with about 80% of all stars being red dwarfs — which also makes them attractive targets (some really nearby, like Proxima Centauri, or the celebrated Gl 876). As well, the spectrum of an M star shows lines belonging to molecules and all neutral metals, but hydrogen is often absent; and — titanium oxide can be found. Metallicity seems to be a key to the likelihood of a star harboring a planet (makes common sense too). Debra Fischer at SFSU works N2K here, and Sally Robinson at UCSC talks about metallicity here, and in fact Greg Laughlin’s related Core Accretion talk his here, but that’s another story…. These searches are going after Jovian sized planets; our search-survey is a new approach (although Mike Endl is working on the red dwarf exoplanet challenge, just as we have been, but spectroscopically slanted). He comments on the m-dwarf exoplanet low-metallicity challenge HERE. In fact Ken Croswell is “aiming” low on the metallicity target, too…

I digress. Back to the host star. These stars are pretty small – with a mass & radius below a third of our Sun (to 0.08 solar masses, which are then classified as brown dwarfs) and a surface temperature below 3,500K. M dwarfs incorporate a slow P-P fusion (due to the temperature) to turn hydrogen into helium. This slow burn is likely to last billions to trillions of years (!) – and so they also burn dimly (1/10,000th the sun). This is a detractor to photometric work by GEMSS… But c’est-la vie. Also the m-dwarf moves its energy from the core to its surface by means of convection, which makes all its hydrogen burnable — and lengthens the lifetime even more. By the way, the triple Alpha fusion process is not possible so red dwarfs don’t move beyond the red giant phase (being more long-lived than the Universe itself, notwithstanding!) – and so are all on the main sequence. Also strange is the lack of no-metal (ie “Big Bang’ed”) red dwarf stars — which should have just hydrogen, helium, and lithium – none of these have been seen – another twist to metallicities in m dwarfs. Generally, their dimness keeps our understanding to the population within about 20 parsecs of Earth, perhaps another “systematic” skew to our understanding of them.

M Dwarfs are also considered to be flar stars (some include brown dwarfs, too). These are a-periodic variable stars which can undergo unpredictable dramatic increases in brightness from several minutes to several hours. The flares are thought to be analogs to solar flares. Their brightness increase is pan-spectral, and this too, is a detractor to good, clean light curves which can be teased for transit dips…

That’s the exoplanet’s host in a nutshell…. and here are some links to systems in general, and the background on their formation, evolution and their dust. These are Wyatt’s papers on ESP’s, Dust, Disks, and related info (we have moved to millimeter studies of Red Dwarfs as well); Graduate lecture course on “Planetary systems” given at IoA Jan-Mar 2007, updated from course given in 2006:

Lecture 1: Solar system
Lecture 2: Planetary system dynamics
Lecture 3: Extrasolar planets
Lecture 4: Planet formation
Lecture 5: Proto-planetary disks
Lecture 6: Debris disks
Lecture 7a: Debris Disk Structure from Perturbations
Lecture 7b: Debris Disk Evolution

Also:  Debris disks: dynamics of small par

%d bloggers like this: