X-Ray Emission and Disk Irradiation of HL Tau and HD 100546

Abstract

We present new X-ray observations of the optically-obscured protostar HL Tau and the intermediate mass Herbig Be star HD 100546. Both objects are surrounded by spectacular disks showing complex morphology including rings and gaps that may have been sculpted by protoplanets. HL Tau was detected as a variable hard X-ray source by Chandra, typical of late-type magnetically-active coronal sources. No extended X-ray emission was seen along the HL Tau jet, or along the jet of the T Tauri binary system XZ Tau located 23 ′′ to its east. In contrast, HD 100546 was detected by XMM-Newton as a soft X-ray source (kT < ∼ 1 keV) with no short-term (<1 day) variability. Its X-ray properties are remarkably similar to the Herbig stars AB Aur and HD 163296, strongly suggesting that their X-ray emission arises from the same mechanism and is intrinsic to the Herbig stars themselves, not due to unseen late-type companions. We consider several possible emission mechanisms and conclude that the X-ray properties of HD 100546 are consistent with an accretion shock origin, but higher resolution grating spectra capable of providing information on individual emission lines are needed to more reliably distinguish between accretion shocks and alternatives. We show that X-ray ionization and heating are mainly confined to the upper disk layers in both HL Tau and HD 100546, and any exoplanets near the midplane at distances ≥1 au are well-shielded from X-rays produced by the central star. 2The X-ray luminosity of low and intermediate mass PMS stars is typically in the range L x ∼ 10 28 -10 31 ergs s −1 , about 10 1 -10 4 times that of the quiet Sun. As young solar-like stars age and spin down, their X-ray luminosity declines (Güdel, Guinan, & Skinner 1997). Stellar X-rays heat and ionize disk gas and the gaseous atmospheres of any close-in protoplanets, and must be taken into account in realistic models of planet formation. X-ray ionization of disk gas also promotes the magnetorotational instability (Balbus & Hawley 1991) and can thereby affect accretion onto the central star.In magnetically-active class I protostars obscured by surrounding envelopes and optically-revealed T Tauri stars (TTS), very high plasma temperatures (T ∼ 100 MK) can be reached during powerful short-duration (minutes to hours) magnetic reconnection flares. In addition to flash-heating, such flares can produce high energy particles that bombard the inner disk and any close-in protoplanets (reviewed by Feigelson & Montmerle 1999; Feigelson 2010).The photoelectric cross section for absorption of X-rays by gas scales inversely with photon energy as σ ∝ E −p , where p ≈ 2.5 for solar-abundance gas. Soft X-rays (E < ∼ 1 keV) are thus heavily absorbed in outer disk layers, whereas harder X-rays with energies of several keV penetrate deeper and can potentially reach the disk midplane, as can cosmic rays. As a young star evolves and its inner disk gas disperses, close-in protoplanets become more heavily exposed to stellar X-rays. Detailed discussions of X-ray heating and ionization ...

Stable Coronal X-Ray Emission over 20 yr of XZ Tau

“…Similarly, with carefully chosen initial parameters for fitting our data, we can produce a fit to our faint XMM-Newton data with similar (though somewhat worse) goodness of fit with temperatures similar to the temperatures found in this work. We thus conclude that the two temperatures found by Skinner & Güdel (2020) reproduce our cool component. Güdel et al (2007) present multiple fit options to all sources in the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST), including a two-temperature plasma model similar to the parameterization we adopt.…”

“…We also observe clear brightening events of durations that would suggest multiple EXOr-like outbursts, including ongoing outbursts during the 2018 Chandra and 2020 XMM-Newton observations. The 2017 Chandra observations (Skinner & Güdel 2020) appear to take place during a local minimum of optical brightness, but at an elevated optical brightness compared to the baseline set in 2004.…”

Exoplanet X-ray irradiation and evaporation rates with eROSITA

“…The median hardness ratio, corresponding to a coronal temperature of about 0.3 keV, is depicted by the dashed line. The quite blue star to the left is the Herbig Be star HD 100546, a known X-ray emitter(Skinner & Güdel 2020).…”

Nearby Young Stars and Young Moving Groups

“…The massive exoplanet hosts β Pic and HR 8799, as well as HD 100453 -all of which are orbited by debris disks -all fall into the former (weak coronal emission) category [26,59,123]. The stars HD 100546 [134], HD 163296 [147], and HD 169142 [54] are examples of the latter, i.e., stars for which some or most of the X-ray emission may be derived from accretion shocks, with the accretion streams fed by material from their gas-rich circumstellar disks [although HD 169142 appears to be a borderline case ; 54]. Conclusive evidence that accretion is the origin of the X-ray emission from these stars might be provided by future X-ray observations at high spectral resolution (see next).…”