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Chapter 1 Introduction Cover Page The handle http://hdl.handle.net/1887/20669 holds various files of this Leiden University dissertation. Author: Nefs, Bas Title: The hunt for red dwarf binaries and hot planets in the WFCAM transit survey Issue Date: 2013-03-27 Chapter 1 Introduction The advent of infra-red-sensitive astronomical detectors and arrays has, over the last few decades, led to a revived interest into the fundamental properties of M-dwarf stars. This thesis presents the first results from the Wide Field Camera Transit Survey (WTS), a dedicated and ongoing photometric infra-red survey, that hunts for low-mass binaries and planetary companions around M-dwarfs. The goal of this work is, by investigating M-dwarfs in eclipsing binary systems, to gain a better understanding of how low-mass stars are formed and how they evolve. In this in- troduction we first describe some of the general characteristics of M-dwarf stars (Section 1.1), followed by a discussion in Section 1.2 on the importance of M-dwarf studies. Section 1.3 in- troduces possible low-mass binary formation scenarios and potential observational constraints on these theories. In Section 1.4 the importance of close (eclipsing) M-dwarf systems is empha- sized in relation to existing theories and simulations of binary formation and evolution. Current discrepancies of fundamental observed M-dwarf properties with evolution models are reviewed in Section 1.5, which pose a challenge to M-dwarf planet characterisation efforts, which are dis- cussed in Section 1.6. The observational data for this thesis, high quality infra-red light curves from the WTS, are detailed in Section 1.7. We end the introduction with a short outline of the various thesis chapters (Section 1.8) and a last section (Section 1.9) on possible future work. 1.1 M-dwarfs - general characteristics M-dwarfs are the smallest hydrogen burning stars that live on the stellar main sequence. They bridge the mass gap between cool deuterium-burning brown dwarfs and solar-like stars, and range in mass from 0.07-0.08M to 0.60-0.65M (Baraffe & Chabrier 1996). M-dwarfs are highly abundant throughout our Milky Way, e.g. Henry et al. (2007) find that M-dwarfs rep- resent >70% of all stars in number. Of the 77 known stars in 5 parsec (pc) around our Sun1, 62.3% are M-dwarfs, 5.2% brown dwarfs and only 2.6% are solar type (spectral class G) stars. The closest M-dwarf is Proxima Centauri, at a distance of only 1.3 pc. Yet, ironically, of all the ∼6000 stars accessible to the naked eye, none are M-dwarfs. The brightest observed M-dwarf is Lacaille 8760 (AX Microscopii; distance 12.9 pc) at V band magnitude 6.69. This is because M-dwarfs are intrinsically faint, with luminosities ranging from ∼7% to only 0.015% that of the Sun (Baraffe & Chabrier 1996). M-dwarfs are brightest at (infra)red wavelengths explaining why, historically, they were also difficult to access with telescopes. The observed atmospheric temperatures of M-dwarfs range from ∼2000 to ∼3900 K, which is low enough for simple molecules (e.g. Titanium Oxide, TiO and water, H2O) to be stable 1RECONS (REsarch Consortium On Nearby Stars) census of all known objects within 10 pc, 01 January 2012,http://www.chara.gsu.edu/RECONS/ 2 Chapter 1. Introduction and provide significant absorption in the optical and infra-red parts of their spectra. The M- dwarf spectral class (which ranges from M0 to M9) is actually defined by the presence of TiO absorption bands in the spectrum. An important difference between M-dwarfs and solar-type stars is in the structure of their atmospheres. Models indicate that stars with mass less then 35% of the Sun are fully convective, and higher mass M-dwarfs have a radiative core. This core increases in size from ∼50% of R∗ for a 0.4 M M-dwarf to ∼65% for 0.6M , and to ∼70% for a solar type star (Chabrier & Baraffe 1997). Convection occurs because the M-dwarf interior has a high density compared to the temperature and is consequently opaque to radiation. M- dwarfs are very slow hydrogen burners because their core temperature is relatively low (< 107 K) and the resulting helium is constantly remixed by the convection. This means that M-dwarfs have a nearly constant luminosity and spectral type while on the main sequence and that no M-dwarf has yet evolved from the main sequence since the Big Bang. Many M-dwarfs are chromospherically and magnetically active, and this activity manifests itself by flares, ejections of mass and periodic brightness variations caused by rotational mod- ulation of cool surface star spots. In Sun-like stars, with masses between about 0.35 and 1.3 M , the dynamo that gives rise to this activity (the aW − dynamo) is believed to be generated at the thin boundary between the convective envelope and radiative core, the tachocline (e.g. Parker 1993; Charbonneau & MacGregor 1997; Thompson et al. 2003). Here, magnetic fields are generated by the combined action of differential rotation (the W effect) and the twisting of field lines by cyclonic convection (the a effect) (e.g. Parker 1955; Steenbeck 1966; Leighton 1969). Both of these effects depend on the rotation - the W-effect because more rapidly ro- tating stars are expected to possess stronger internal angular velocity contrasts (Brown et al. 2008) and the a-effect because it depends on the helicity of the convection which itself senses the overall rotation rate. For stars with masses less than ∼0.35M (spectral types later than ∼M3.5), which are fully convective, the tachocline disappears. However, activity has been ob- served in such stars, suggesting a different dynamo mechanism (e.g. Rockenfeller, Bailer-Jones & Mundt 2006; Reiners & Basri 2008). Indeed, spectropolarimetric studies of fully convective M-dwarfs have shown that the magnetic field morphology appears to change with spectral type (e.g. Morin 2008; 2010). These findings have led to an alternative dynamo, the a2-dynamo, where turbulence and cyclonic convection play the main role (e.g. Chabrier & Küker 2006). West et al. (2008) find that magnetic activity is a function of subtype; earlier M-dwarf types are generally less active than late types (unless part of a close binary system). Also, they find that M-dwarf activity declines as a function of age, but extends with later subclass; activity life +0:5 +0:5 times in M0 dwarfs are 0.8−0:5Gyr, and increase to as much as 8.0−1:0Gyr for M7 type stars. 1.2 M-dwarfs - why study them? M-dwarfs are very interesting objects to study for several reasons: • M-dwarfs are an ideal stellar population for studying the structure and evolution of our Galaxy (e.g. Wielen 1977; Reid et al. 1995; Bochanski et al. 2007), and the star- formation history in the local Solar neighbourhood (e.g. Gizis et al. 2002), because of their ubiquity and very long main-sequence lifetimes. Chromospheric activity decays on time-scales of billions of years, which is a time-scale relevant for studies of Galactic Section 1.3. Low-mass binary formation 3 evolution. As emphasised by e.g. Reid et al. (1999), the local star formation history is one of the major requirements for modelling of the sub-stellar mass function. • M-dwarfs also encompass many important regions of parameter space of stellar structure, not only the onset of convection, but also of significant electron degeneracy in the core, and the formation of dust and subsequent depletion of metals onto dust grains in the stellar atmosphere. Note furthermore that the equation of state for M-dwarfs, which determines internal structure and forms an important ingredient for stellar atmosphere models (e.g. Chabrier & Baraffe 1996), may even need to be (slightly) revised (e.g. Torres & Ribas 2002; Lopez-Morales 2004). Such a revision may (partially) remedy the mismatch be- tween observed fundamental M-dwarf properties and models, but remains an interesting open question (e.g. Irwin et al. 2011). • M-dwarfs form important ingredients for dynamical stellar evolution simulations by con- necting solar-type stars and brown dwarfs, which are two mass regions that appear to have very different binary fractions. The change of these multiplicity characteristics through- out the M-dwarf regime is important to understand the evolution of both low-mass stars and brown dwarfs and their formation environment (e.g. Goodwin et al. 2007; Burgasser et al. 2007; Parker et al. 2009). Also, the observed distributions of orbital period and mass-ratios of M-dwarf binaries are constraints to models of star-formation and dynami- cal evolution (e.g. Bate et al. 2012). See also chapters 4 and 5. • Exoplanet detection techniques are significantly more sensitive to planets orbiting M- dwarfs than solar-type stars, making them sensitive to rocky planets in the habitable zone. In addition they occupy a different place in parameter space and are therefore important probes for planet formation theories (see also section 1.6). • There are still apparent discrepancies between theoretical stellar structure models for M- dwarfs and the observed fundamental M-dwarf properties (mass, luminosity, radius, ef- fective temperature), in addition to the lack of dynamical mass-radius measurements for mid-to-late type M-dwarfs (mass below 0.2 M ). See also chapters 3 and 5 and section 1.5. 1.3 Low-mass binary formation 1.3.1 Observational constraints Observations of both young clusters and the field show that a significant fraction of stars are formed as multiple systems (e.g.
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