This talk was part of the UH Centre for Astrophysics Research (CAR) 2023-2024 seminar series. For the ongoing programme, please see:
Date: 11 Oct 2023
Speaker: Prof Konstantin Getman – Pennsylvania State University
Title: X-ray Flaring and Magnetic Activity Evolution in Young Suns
Abstract: Our recent multiwavelength studies quantify relations between magnetic activity, stellar mass, rotation and age in stars younger than 25 Myr, together with new insights into the physics of super-flares and changes in the magnetic dynamo mechanisms during the pre-main sequence phase. Utilizing X-ray data obtained from the NASA Chandra X-ray telescope, along with complementary optical and infrared data from the ZTF, Gaia, UKIDSS, 2MASS, and Spitzer surveys, we have analyzed flaring activity levels for tens of thousands members of Galactic star-forming regions and young open clusters. Our analyses consider both quasi-continuous flaring and discrete super-flare events, accounting for the presence of X-ray non-detections and partially captured flares. Additionally, we have derived rotational periods and spot sizes for several hundred stellar members of open clusters.
Based on the analysis of time-averaged X-ray luminosities, we find that the X-ray luminosities of young stars remain high and constant during the first few mega years, after which they begin to decay. This decay is more rapid for more massive stars. We interpret this change as the highly efficient turbulent convective alpha-squared magnetic dynamo is replaced by the less efficient solar-type alpha-omega tachoclinal dynamo as radiative cores grow. In the case of solar-mass stars, both X-ray luminosity and starspot area decrease linearly with age. This implies that the spot magnetic fields remain roughly constant, and young stars adhere to the universal solar-stellar scaling law between X-ray luminosity (Lx) and surface magnetic flux (Phi). This Lx-Phi relationship holds true across an extraordinary range of 13 orders of magnitude in X-ray luminosity and ages from Myr to Gyr.
Regarding X-ray flare activity, we have found that the mass-stratified flare energy distributions for young stars, dN/dEx ~ Ex^{-2}, exhibit a slope consistent with that of optical/X-ray flaring observed in older stars and the Sun. The X-ray flare properties of young stars are similar in both disk-bearing and diskless stars, indicating that star-disk magnetic fields do not play a significant role. Compared to the contemporary Sun, the convective dynamo observed in young stars supports significantly larger surface spot areas, magnetic fluxes, X-ray coronal extents, and higher levels of both flare and baseline X-ray activity. Notably, this convective dynamo mechanism enables the frequent occurrence of X-ray super-flares with energies greater than 10^34 erg. Super-flares in early pre-main sequence stars are a million times more frequent than those observed in solar-type main sequence stars. Even X-ray mega-flares with energies greater than 10^36 erg are seen, far surpassing the capabilities of the contemporary Sun. These powerful flares are expected to have a substantial impact on the photoevaporation and dispersal processes affecting surrounding protoplanetary disks and, when the disks have disappeared, on the primordial atmospheres of inner young planets. The effects of late-pre main sequence mega-flares (and possibly coronal mass ejections) on planetary atmospheres are only beginning to be studied.
[ad_2]
source
