Young Solar-like Stars Research Articles

Chandra observation of an unusually long and intense X-ray flare from a young solar-like star in M 78

LkHα 312 has been observed serendipitously with the ACIS-I detector on board the Chandra X-ray Observatory with a 26 h continuous exposure. This Hα emission line star belongs to M 78 (NGC 2068), one of the star-forming regions of the Orion B giant molecular cloud at a distance of 400 pc. From the optical and the near-infrared (NIR) data, we show that LkHα 312 is a pre-main sequence (PMS) low-mass star with a weak NIR excess. This genuine T Tauri star displayed an X-ray flare with an unusually long rise phase (∼8 h). The X-ray emission was nearly constant during the first 18 h of the observation, and then increased by a factor of 13 during a fast rise phase (∼2 h), and reached a factor of 16 above the quiescent X-ray level at the end of a gradual phase (∼6 h) showing a slower rise. To our knowledge this flare, with ∼0.4-0.5 cts s −1 , has the highest count rate observed so far with Chandra from a PMS low-mass star. By chance, the source position, 8.2 � off-axis, protected this observation from pile-up. We make a spectral analysis of the X-ray emission versus time, showing that the plasma temperature of the quiescent phase and the flare peak reaches 29 MK and 88 MK, respectively. The quiescent and flare luminosities in the energy range 0.5-8 keV corrected from absorption (NH ≈ 1.7 × 10 21 cm −2 )a re 6× 10 30 erg s −1 and ∼10 32 erg s −1 , respectively. The ratio of the quiescent X-ray luminosity on the LkHα 312 bolometric luminosity is very high with log(LX/Lbol) = −2.9, implying that the corona of LkHα 312 reached the saturation level. The X-ray luminosity of the flare peak reaches ∼2% of the stellar bolometric luminosity. The different phases of this flare are finally discussed in the framework of solar flares, which leads to the magnetic loop height from 3.1 × 10 10 to 10 11 cm (0.2-0.5 R� , i.e., 0.5-1.3 R� ).

ASCAObservations of the T Tauri Star SU Aurigae and the Surrounding L1517 Dark Cloud

We present the results of a approximately equals 40 ks pointed Advanced Satellite for Cosmology and Astrophysics (ASCA) observation of the L1517 star-forming region, centered on the X-ray-bright T Tauri star SU Aurigae. This star has the highest X-ray luminosity of any classical T Tauri star in the Taurus-Auriga region, and its optical spectra show evidence for both mass inflow and outflow. Strong X-ray emission was detected from SU Aur (L(sub x) = 10(exp 30.9) ergs s(exp -1)) as well as weaker emission from five other pre-main-sequence stars. Although no large-amplitude flares were detected, the X-ray emission of SU Aur showed clear variability in the form of a slow decline in count rate during the 1.3 day observation. We provide the first direct comparison of the coronal differential emission measure (DEM) distribution of a classical T Tauri star with that of a young main-sequence star of similar spectral type. The DEM distributions of SU Aur (G2; age 3 Myr) and the young solar-like star EK Draconis (GO V; age 70 Myr) are qualitatively similar, with both showing a bimodal temperature distribution characterized by a cool plasma component peaking at approximately 8-9 MK and a hot component peaking at approximately 20-21 MK. However, there is a striking difference in the relative proportion of plasma at high temperatures in the two stars, with hot plasma (>20 MK) accounting for approximately equals 80% of the volume emission measure of SU Aur, compared to only approximately equals 40% in EK Dra. These results provide new insight into the changes that will occur in the corona of a T Tauri star as it descends onto the main sequence. A sharp decline in the fraction of coronal plasma at flarelike temperatures will occur during the late-T Tauri and post-T Tauri phases, and other recent X-ray studies have shown that this decline will continue after the young solar-like star reaches the main sequence.

The Infancy of Solar-Type Stars and its Relevance for the Origin of Life

AbstractFor life to develop, planets are a necessary condition. Likewise, for planets to form, stars must be surrounded by circumstellar disks, at least some time during their pre-main sequence evolution. Much progress has been made recently in the study of young solar-like stars. In the optical domain, these stars are known as «T Tauri stars». A significant number show IR excess, and other phenomena indirectly suggesting the presence of circumstellar disks. The current wisdom is that there is an evolutionary sequence from protostars to T Tauri stars. This sequence is characterized by the initial presence of disks, with lifetimes ~ 1-10 Myr after the intial collapse of a dense envelope having given birth to a star. While they are present, about 30% of the disks have masses larger than the minimum solar nebula. Their disappearance may correspond to the growth of dust grains, followed by planetesimal and planet formation, but this is not yet demonstrated.