Giovanni Picogna

Giovanni Picogna

I am a post-doc in the Star and Planet Formation group led by Barbara Ercolano

Dust entrainment in photoevaporative winds: The impact of X-rays

Published in Astronomy & Astrophysics, 2020

Recommended citation: Franz, Picogna et al. (2020). "Dust entrainment in photoevaporative winds: The impact of X-rays." Astronomy & Astrophysics. 635, A53. https://www.aanda.org/articles/aa/full_html/2020/03/aa36615-19/aa36615-19.html

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Abstract

Context. X-ray- and extreme ultraviolet (XEUV) driven photoevaporative winds acting on protoplanetary disks around young T Tauri stars may crucially impact disk evolution, affecting both gas and dust distributions.

Aims. We investigate the dust entrainment in XEUV-driven photoevaporative winds and compare our results to existing magnetohydrodynamic and EUV-only models.

Methods. We used a 2D hydrodynamical gas model of a protoplanetary disk irradiated by both X-ray and EUV spectra from a central T Tauri star to trace the motion of passive Lagrangian dust grains of various sizes. The trajectories were modelled starting at the disk surface in order to investigate dust entrainment in the wind.

Results. For an X-ray luminosity of $L_X = 2\times10^{30}$ erg s$^{-1}$ emitted by a $M_\star = 0.7 M_\odot$ star, corresponding to a wind mass-loss rate of $\dot{M)w \approx 2.6\times 10^{−8}$ $M\odot$ yr$^{−1}$, we find dust entrainment for sizes $a_0 \lesssim 11$ $\mu$m (9 $\mu$m) from the inner 25 AU (120 AU). This is an enhancement over dust entrainment in less vigorous EUV-driven winds with $\dot{M)w = 10^{−10}$ $M\odot$ yr$^{−1}$. Our numerical model also shows deviations of dust grain trajectories from the gas streamlines even for $\mu$m-sized particles. In addition, we find a correlation between the size of the entrained grains and the maximum height they reach in the outflow.

Conclusions. X-ray-driven photoevaporative winds are expected to be dust-rich if small grains are present in the disk atmosphere.

grain sizes entrained
Size of the largest grains entrained from a point R along the base of the wind (blue, peak at about 20 AU); the saw-tooth appearance of the curve at larger R is caused by the finite resolution of the underlying gas grid. When comparing to Hutchison et al. (2016a, their Fig. 7 for M* = 0.75 M⊙) (orange, peak at around 40 AU), we can see the size enhancement – especially at smaller R – caused by the inclusion of X-rays in our photoevaporative wind model. The blue dashed line represents our results scaled down by a factor of 3, to compensate for the differing internal grain densities of Hutchison et al. (2016a) and this work; yet, Ṁw still differsbetween the models, making a direct comparison difficult. The MHD wind model investigated by Miyake et al. (2016, their Fig. 4) (green) shows a distinctly different entrainment curve, starting off at very high a0 in the jet region but dropping towards max (a0) = 0 very quickly.Around R ≃ 140 AU, our max (a0) plummets to 0.