X-ray photodesorption from water ice in protoplanetary disks and X-ray-dominated regions

Abstract

Water is the main constituent of interstellar ices, and it plays a key role in the evolution of many regions of the interstellar medium, from molecular clouds to planet-forming disks [1]. In cold regions of the ISM, water is expected to be completely frozen out onto the dust grains. Nonetheless, observations indicate the presence of cold water vapor, implying that non-thermal desorption mechanisms are at play. Photodesorption by UV photons has been proposed to explain these observations [2,3], with the support of extensive experimental and theoretical work on ice analogues [4,5,6]. In contrast, photodesorption by X-rays, another viable mechanism, has been little studied. The potential of this process to desorb key molecules, such as water, intact rather than fragmented or ionised, remains unexplored. We experimentally investigated X-ray photodesorption from water ice, monitoring all desorbing species. We find that desorption of neutral water is efficient, while ion desorption is minor. We derive for the first time yields that can be implemented in astrochemical models. These results open up the possibility of taking into account the X-ray photodesorption process in the modelling of protoplanetary disks or X-ray dominated re-Numerous studies have explored the effects of Xrays on the chemistry of various regions of the ISM [7,8,9,10], with a few taking into account solid phase processes [11,12]. In the example of protoplanetary disks, as sketched in figure 1, the X-ray dominated region corresponds to a thin layer. The extent of the X-ray layer will depend on the type of star, the degree of evolution of the disk (dust settling, etc) and the physical model of disk considered. In the T Tauri model of ref [11], the limit between the UV and X-ray dominated layers occurs around z/R ∼ 0.2. The location of ices is disk-dependent as well, with the horizontal onset varying between <1 and 10 AU. Beyond a few tens of AU dust temperatures are cold enough for water to freeze out throughout the whole disk, and the X-ray layer overlaps with the icy region (cf fig.1). The region where X-rays are dominant can extend to the whole midplane if the disk is shielded from cosmic rays, as suggested by [13].The goal of this study is to provide experimental constraints on X-ray photodesorption, which allows modellers to assess the relevance of this process to the physics and chemistry of disks and other regions. Previous experimental studies have mainly focused on ion desorption [15,16,17], and only a few have attempted to derive quantitative desorption yields [18]. We used synchrotron radiation from the SEXTANTS beamline (SOLEIL facility) to irradiate amorphous solid water at either 15 K or 90 K in an ultra-high vacuum chamber (see Methods). Our set-up allows us 1 arXiv:1807.03725v1 [astro-ph.GA] 10 Jul 2018 (External X-rays) Cosmic Rays (External UV) z R Figure 1: Schematic representation of a vertical slice of a protoplanetary disk showing the various sources of irradiation in the ice-containing regions.To the left is the central star, wi...

Desorption of neutrals, cations, and anions from core-excited amorphous solid water

“…We are confident with the values given for neutral species, within the uncertainty of ±40% that we estimated [4]. The absolute values for cations (and anions) rely on the validity of the measurements of Rosenberg et al The relative comparison of cations, however, is valid within the ±40% error of the apparatus function calibration.…”

“…In photodesorption, two volatile products of the chemistry, H 2 and O 2 are also observed. The desorption of neutral species is discussed in more details in section C and in a previous publication [4]. Other elements that play a role in water photochemistry and cannot be probed here are the important atomic H, the solvated electron, and the presence of defects in the ice.…”

“…Experiments were performed in the SPICES 2 set-up. Some aspects of the experiments have already been detailed elsewhere [4]. Briefly, the set-up is an ultra-high vacuum chamber equipped with a closed-cycle helium cryostat, reaching a base temperature of 15 K at the sample holder and a base pressure of ∼ 1 × 10 −10 mbar at 15 K. The substrate used was a technical copper surface (polycrystalline OFHC copper), electrically insulated from the holder by a kapton foil.…”

“…We estimated the absolute photodesorption yields of the different species we observed. The derivation of absolute yields for neutral species were presented in detail previously [4]. For deriving the yields of ions, several steps are necessary.…”

“…planet formation in protoplanetary disks [1,2]. The interaction of water ice with energetic radiation such as UV or X-rays interests different fields, such as planetary science [3] or astrochemistry: water frozen on interstellar dust grains can be released to the gas phase through a process called photodesorption, which has shown its importance in the field over the last decade [4,5,6,7]. Amorphous solid water is also often used as a model for liquid water to which a different variety of experimental techniques can be applied.…”