A Swift view of X-ray and UV radiation in the planet-forming T Tauri system PDS 70

Abstract

PDS 70 is a ∼5 Myr old star with a gas and dust disc in which several proto-planets have been discovered. We present the first UV detection of the system along with Xray observations taken with the Neil Gehrels Swift Observatory satellite. PDS 70 has an X-ray flux of 3.4×10 −13 erg cm −2 s −1 in the 0.3-10.0 keV range, and UV flux (U band) of 3.5×10 −13 erg cm −2 s −1 . At the distance of 113.4 pc determined from Gaia DR2 this gives luminosities of 5.2×10 29 erg s −1 and 5.4×10 29 erg s −1 respectively. The X-ray luminosity is consistent with coronal emission from a rapidly rotating star close to the log L X L bol ∼ −3 saturation limit. We find the UV luminosity is much lower than would be expected if the star were still accreting disc material and suggest that the observed UV emission is coronal in origin.

Variable Accretion onto Protoplanet Host Star PDS 70

“…The solid lines show the total emission for different mass accretion rates and the dashed line of the same color showing only the shock emission. Joyce et al (2019) obtained an accretion rateṀ ∼ 6×10 −12 M yr −1 using the correlation of the flux excess in the U band and the accretion luminosity from Venuti et al (2014). As can be seen in Figure 7, the flux at the U band is dominated by stellar photospheric and chromospheric emission, as expected for low accretors Ingleby et al (2011), so we argue that this band is not optimal to obtain accretion shock emission in this star.…”

“…Indicators that the planets are still forming include the submillimeter emission detected around them, interpreted as arising in circumplanetary disks (CPDs, Isella et al 2019), and by the Hα emission coincident with the location of the protoplanets in NIR images (Wagner cently, Long et al (2018) classified the star as a non accretor, based on the lack of emission in Paβ. Similarly, Joyce et al (2019) found essentially no excess over the photosphere in the Swift U band flux of PDS 70, from which they infer that the star is accreting at a extremely low level, Ṁ ∼ 6 × 10 −12 M yr −1 , consistent with no accretion. On the other hand, Haffert et al (2019) reported a redshifted absorption component in the Hα profile of the star and concluded that the star is accreting.…”

“…As shown in Figure 7, the observed Swift fluxes are consistent with the accretion rates estimated from the magnetospheric modeling of Hα (Table 3). Joyce et al (2019). The gray line is the spectrum of the weak T Tauri star HBC 427 with the same spectral type as that of PDS 70; we added the expected emission from the accretion shock to this spectrum.…”

“…We estimate the parameter η by calculating a typical fraction between the number density of the lower level of He I λ10830 to the total hydrogen number density. We use the C17.01 release of the software Cloudy (Ferland et al 2017) to calculate the level populations in a slab of gas assuming that the ionization radiation is an X-ray with a blackbody temperature of 5 × 10 6 K and L X = 5 × 10 29 erg s −1 consistent with the luminosity measured by Swift (Joyce et al 2019). 3 Assuming that the wind is ∼ 10 R from the star with a thickness of 1 R , we find 3 Joyce et al (2019) model the X-ray spectra with two temperatures.…”

“…We show in Figure 7 optical fluxes for PDS 70 from Gregorio-Hetem & Hetem (2002) and fluxes in the Swift/UVOT U, uvw1, and uvw2 bands (Joyce et al 2019). Except for one case, observations at uvw1 and uvw2 were obtained at different epochs.…”