X-Ray Super-Flares from Pre-Main Sequence Stars: Flare Energetics and Frequency
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Draft version May 12, 2021 Typeset using LATEX twocolumn style in AASTeX63 X-ray Super-Flares From Pre-Main Sequence Stars: Flare Energetics And Frequency Konstantin V. Getman1 and Eric D. Feigelson1, 2 1Department of Astronomy & Astrophysics Pennsylvania State University 525 Davey Laboratory University Park, PA 16802, USA 2Center for Exoplanetary and Habitable Worlds (Accepted for publication in ApJ, May 2021) ABSTRACT Solar-type stars exhibit their highest levels of magnetic activity during their early convective pre- main sequence (PMS) phase of evolution. The most powerful PMS flares, super-flares and mega-flares, −1 have peak X-ray luminosities of log(LX ) = 30:5−34:0 erg s and total energies log(EX ) = 34−38 erg. Among > 24; 000 X-ray selected young (t . 5 Myr) members of 40 nearby star-forming regions from our earlier Chandra MYStIX and SFiNCs surveys, we identify and analyze a well-defined sample of 1,086 X-ray super-flares and mega-flares, the largest sample ever studied. Most are considerably more powerful than optical/X-ray super-flares detected on main sequence stars. This study presents energy estimates of these X-ray flares and the properties of their host stars. These events are produced by young stars of all masses over evolutionary stages ranging from protostars to diskless stars, with the occurrence rate positively correlated with stellar mass. Flare properties are indistinguishable for disk- bearing and diskless stars indicating star-disk magnetic fields are not involved. A slope α ' 2 in the −α flare energy distributions dN=dEX / EX is consistent with those of optical/X-ray flaring from older stars and the Sun. Mega-flares (log(EX ) > 36:2 erg) from solar-mass stars have occurrence rate of +1:0 1:7−0:6 flares/star/year and contribute at least 10 − 20% to the total PMS X-ray energetics. These explosive events may have important astrophysical effects on protoplanetary disk photoevaporation, ionization of disk gas, production of spallogenic radionuclides in disk solids, and hydrodynamic escape of young planetary atmospheres. Our following paper details plasma and magnetic loop modeling of the > 50 brightest X-ray mega-flares. 1. INTRODUCTION (2017), Feigelson(2018), Sciortino et al.(2019) and Ar- 1.1. Pre-Main Sequence Super-flares and Mega-flares giroffi(2019). Though PMS X-ray flares were surprising at first, the X-ray imaging studies of nearby star forming regions, existence of strong magnetic dynamos in the interiors such as the Taurus clouds and Orion Nebula, typically of fully convective, rapidly rotating stars, followed by show that highly variable X-ray emission is a ubiquitous eruption of field lines and violent magnetic reconnection characteristic of pre-main sequence (PMS) stars (Feigel- above the stellar surface, is reasonable. The X-ray emis- son & Decampli 1981; Montmerle et al. 1983; Getman sion seems to be independent of the presence or absence arXiv:2105.04768v1 [astro-ph.SR] 11 May 2021 et al. 2005; G¨udelet al. 2007). The emission arises from of protoplanetary disks despite astrophysical calcula- magnetic reconnection events similar to, but much more tions that star-disk magnetic field lines may be involved powerful and frequent than, flares on the contemporary in X-ray emitting flares (Hayashi et al. 1996; Shu et al. Sun. Reviews relating to PMS X-ray emission are pro- 1997; Aarnio et al. 2010; L´opez-Santiago et al. 2016; vided by Feigelson & Montmerle(1999), G¨udel(2004), Colombo et al. 2019). A factor of two reduced X-ray Feigelson et al.(2007), Gregory et al.(2010), Stelzer activity level in accreting versus non-accreting systems (Flaccomio et al. 2003; Preibisch et al. 2005; Telleschi Corresponding author: Konstantin Getman et al. 2007a) could have several possible causes: cooling [email protected] of active regions by accreting material, attenuation of X-rays by accreting columns and/or inner disks, coronal 2 Getman & Feigelson stripping by disks, and distortion of magnetic topologies for heating the solar and stellar coronas (Vilangot by disks/accretion (Jardine et al. 2006; Gregory et al. Nhalil et al. 2020, and references therein). 2007; Flaccomio et al. 2003; Getman et al. 2008b; Flac- comio et al. 2012). Accretion shocks contribute a small 2. By taking advantage of the much increased super- fraction to the total X-ray emission from T-Tauri stars flare sample combined with a homogeneous set of in the form of soft X-ray excess emission (Telleschi et al. derived flare-host properties (such as stellar mass), 2007b). we evaluate flare occurrence rate as a function of Following previous researchers (Favata et al. 2005; flare energy and stellar mass, as well as contribu- Getman et al. 2008a; McCleary & Wolk 2011), we focus tion of powerful flares to the total X-ray fluence attention here on the most luminous PMS X-ray flares of PMS stars. Such unique estimates will pro- 30:5 with peak X-ray luminosities exceeding LX;pk = 10 vide better understanding the effects of PMS X- erg s−1 and/or total (time-integrated) energies exceed- ray emission on their molecular environs including 34 ing EX = 10 erg. In contrast, no solar flare has been their natal molecular cloud, the infalling envelope observed with total X-ray energy exceeding ∼ 1030 erg, of protostars, the protoplanetary disk around T- four orders of magnitude below our threshold (Schrijver Tauri stars, and the protoplanets revealed after 34 36:2 et al. 2012). We call events with 10 < EX < 10 erg the disk has dissipated. 36:2 'super-flares’ and events with EX > 10 erg 'mega- flares’1, recognizing that the super-flare designation is 3. Super- and mega-flares are important for both also used for less powerful optical flares seen with the their high fluency ionizing radiation and the pro- Kepler satellite in older stars. In both solar and PMS duction of hard X-rays that can penetrate deep flares, X-rays constitute only a minor fraction of the to- into molecular environments (Glassgold et al. tal radiated energy (Flaccomio et al. 2018), and the radi- 2000). Flare X-rays potentially can produce lay- ated energy may be dominated by the energy in ejected ers of ionization in otherwise neutral material, magnetic fields and energetic particles. induce non-equilibrium ion-molecular chemistry, In the present effort, we examine the largest sam- and sputter grain surfaces. Even low levels of ple of X-ray PMS super- and mega-flares ever collected ionization can couple molecular material to mag- to seek ensemble characteristics and relationships with netic fields resulting (in some circumstances) in other properties. The sample is drawn from observations turbulent motions and (in other circumstances) in of > 24; 000 PMS stars detected in 40 MYStIX (Mas- bulk outflows. There is some empirical evidence sive Young Star-forming complex study in Infrared and that PMS flares heat disks and diminish accre- X-rays; Feigelson et al. 2013) and SFiNCs (Star Forma- tion due to photoevaporation of disks (Drake et al. tion in Nearby Clouds; Getman et al. 2017) star forming 2009; Flaccomio et al. 2018; Flaischlen et al. 2021). regions with NASA's Chandra X-ray Observatory (§2). They may be accompanied by energetic particles that could produce spallogenic radionuclides and 1.2. Astrophysical Implications by coronal mass ejection shocks that could melt Our studies are aimed at partially addressing some of ices or solids. the important questions concerning flare related physics Flare radiation play an important role in photo- and phenomena: evaporative flows and dispersal of protoplanetary disks (reviewed by Williams & Cieza 2011; Alexan- 1. Previous studies using much smaller samples of der et al. 2014; Ercolano & Pascucci 2017) and X-ray flares from PMS stars (Wolk et al. 2005; young planetary atmospheres (Lammer et al. 2003; Stelzer et al. 2007; Caramazza et al. 2007; Al- Ribas et al. 2005; G¨udel 2007; Gronoff et al. 2020). bacete Colombo et al. 2007) report similar pow- There is particular concern that the effects of vi- erlaw slopes α ∼ 2 of flare energy distributions olent magnetic activity in young stars − extreme dN=dE / E−α consistent with those of flare flare ultraviolet emission and coronal mass ejections as older stars and the Sun. Such findings may have well as X-ray emission − can erode atmospheres of implications for understanding the relative impor- planets that otherwise might be habitable (Lam- tance of powerful flares and nano- or micro-flares mer et al. 2007; Gronoff et al. 2020; Atri & Car- berry Mogan 2020). The effects of energetic par- 1 This boundary represents our completeness limit: All mega-flares ticles and coronal mass ejections that may be as- in the observed stars have been confidently detected, while only sociated with super-flares is still uncertain (Drake an incomplete subset of super-flares are found (§ 4.2). et al. 2016; Atri 2020). Pre-Main Sequence X-ray Super-Flares 3 4. The geometry of PMS flare plasma seems remark- mega-flare occurrence rates as functions of flare energy able. Models of X-ray evolution of PMS super- and stellar mass, as well as their contribution to the flares are usually consistent with enormous loop total X-ray fluence of PMS stars are evaluated in §6. structures often larger than the star itself (Favata Comparison of X-ray and optical super-flares is given in et al. 2005; Getman et al. 2008b; Reale et al. 2018). §7. Effects of super- and mega-flares on the environment Magnetospheric calculations for classical T-Tauri of young stars are discussed in §8. Concluding remarks stars indicate that closed magnetic loops on this are presented in §9. scale can co-exist with open magnetic field lines accreting from a protoplanetary disk (Johnstone 2.