X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

8 CHAPTER 2. REVIEW OF LOW-MASS YOUNG STELLAR OBJECTS2.3 X-ray Observations of Low-mass Stars2.3.1 X-rays from low-mass main-sequence starsLow-mass MSs emit significant X-rays. The most famous sample of this class is, of course, our sun.There are two origins of the solar X-rays as follows.FlareA “solar flare” means a phenomenon in which a large amount of energy (10 29 –10 32 ergs) suddenlyarises in a surface magnetic loop with timescales from several minutes to hours. First, particles areaccelerated at the loop top, then move down along the loop and penetrate into a cold chromosphere.During this phase, the non-thermal particles emit radio (synchrotron) and hard X-ray – γ-ray(bremsstrahlung and nuclear line emission) photons. Cold gas at the chromosphere is abruptlyheated to high temperature, then expands and flows up into the loop, which is called “chromosphericevaporation”. High temperature (0.1–1 keV) and density (∼10 10 cm −3 ) plasma fills the loop, thenemits thermal X-rays. The plasma gradually cools down due to radiative and conductive energylosses, hence X-ray emission disappears about several hours later (Figure 2.5).It is obvious that the flare activity has a tight correlation with magnetic field. However, themechanism to release such large magnetic energy with a short timescale had been a long-standingissue. In 1990’s, the Japanese solar X-ray satellite Yohkoh brought us key observational evidence.Figure 2.6 (left) shows a Yohkoh SXT (soft X-ray telescope) image of the solar flare occurred onFebruary 21, 1992 (Tsuneta et al., 1992). This shows a beautiful cusp-like shape, which is in goodagreement with the magnetic reconnection models (Sweet, 1958; Parker, 1957; Petschek, 1964, seealso Figure 2.6 right); when a pair of anti-parallel lines of magnetic field crosses each other at theloop top, a reconfiguration of the magnetic field occurs and a large amount of magnetic energy issuddenly released. Now it is widely accepted that magnetic reconnections trigger the solar flares.Another important issue is the origin of surface magnetic field. The main mechanism for thesun is the standard α-ω dynamo (Parker, 1955, 1970, see also Figure 2.7); poloidal magnetic fieldmakes toroidal field with the effect of differential rotation in a surface convection zone (ω term),and vice versa with the effect of turbulent motion (α term). Theories predict that the boundarybetween the surface convection zone and the radiative core acts as the anchor point of magneticfield, hence the α-ω dynamo mechanism may not work for sources with fully convective structuresuch as protostars and brown dwarfs, although a small-scale turbulent dynamo (α 2 dynamo) wouldbe conceivable (Durney et al., 1993).