Photoelectric Cross-Sections of Gas and Dust in Protoplanetary Disks

Bethell, Thomas J.; Bergin, Edwin A.

doi:10.1088/0004-637x/740/1/7

Cited by 51 publications

(42 citation statements)

References 41 publications

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“…7 and 8 show the X-ray spectra used to calculate the values given in table 2 of the text. They correspond to the X-ray spectrum of two different sources, the star TW Hya [26] and the emission from the ULIRG MKN231 [27], attenuated by different column densities of gas (using the crosssections of ref [46]), which displaces the peak of the spectrum towards higher and higher energies. The spectra are all normalized to their respective area for readability.…”

Section: Extrapolated Photodesorption Yieldsmentioning

confidence: 99%

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

et al. 2018

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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...

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Section: Extrapolated Photodesorption Yieldsmentioning

confidence: 99%

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

et al. 2018

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“…We consider this possible deviation as not significant for the results presented here. We further neglect the dust in the X-ray radiative transfer, since with the assumed gas to dust ratio of 100 and dust settling X-ray photons mainly interact with the gas (Bethell & Bergin 2011;Glassgold et al 2012). …”

Section: Appendix A: X-ray Radiative Transfermentioning

confidence: 99%

Stellar energetic particle ionization in protoplanetary disks around T Tauri stars

Rab

Güdel

Padovani

et al. 2017

A&A

102

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Context. Anomalies in the abundance measurements of short lived radionuclides in meteorites indicate that the protosolar nebulae was irradiated by a high amount of energetic particles (E 10 MeV). The particle flux of the contemporary Sun cannot explain these anomalies. However, similar to T Tauri stars the young Sun was more active and probably produced enough high energy particles to explain those anomalies. Aims. We want to study the interaction of stellar energetic particles with the gas component of the disk (i.e. ionization of molecular hydrogen) and identify possible observational tracers of this interaction. Methods. We use a 2D radiation thermo-chemical protoplanetary disk code to model a disk representative for T Tauri stars. We use a particle energy distribution derived from solar flare observations and an enhanced stellar particle flux proposed for T Tauri stars. For this particle spectrum we calculate the stellar particle ionization rate throughout the disk with an accurate particle transport model. We study the impact of stellar particles for models with varying X-ray and cosmic-ray ionization rates. Results. We find that stellar particle ionization has a significant impact on the abundances of the common disk ionization tracers HCO + and N 2 H + , especially in models with low cosmic-ray ionization rates (e.g. 10 −19 s −1 for molecular hydrogen). In contrast to cosmic rays and X-rays, stellar particles cannot reach the midplane of the disk. Therefore molecular ions residing in the disk surface layers are more affected by stellar particle ionization than molecular ions tracing the cold layers/midplane of the disk. Conclusions. Spatially resolved observations of molecular ions tracing different vertical layers of the disk allow to disentangle the contribution of stellar particle ionization from other competing ionization sources. Modeling such observations with a model like the one presented here allows to constrain the stellar particle flux in disks around T Tauri stars.

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“…The UV field, including the propagation of Ly α photons, and the X-ray radiation field within the disk were generated using a Monte Carlo radiative transfer code as described by Bethell & Bergin (2011a, 2011b. The input UV spectrum is taken to be that observed for TW Hya (Herczeg et al 2002(Herczeg et al , 2004.…”

Section: Physical Conditionsmentioning

confidence: 99%

Destruction of Refractory Carbon in Protoplanetary Disks

Anderson

Bergin

Blake

et al. 2017

ApJ

Self Cite

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The Earth and other rocky bodies in the inner solar system contain significantly less carbon than the primordial materials that seeded their formation. These carbon-poor objects include the parent bodies of primitive meteorites, suggesting that at least one process responsible for solid-phase carbon depletion was active prior to the early stages of planet formation. Potential mechanisms include the erosion of carbonaceous materials by photons or atomic oxygen in the surface layers of the protoplanetary disk. Under photochemically generated favorable conditions, these reactions can deplete the near-surface abundance of carbon grains and polycyclic aromatic hydrocarbons by several orders of magnitude on short timescales relative to the lifetime of the disk out to radii of ∼20-100+au from the central star depending on the form of refractory carbon present. Due to the reliance of destruction mechanisms on a high influx of photons, the extent of refractory carbon depletion is quite sensitive to the disk's internal radiation field. Dust transport within the disk is required to affect the composition of the midplane. In our current model of a passive, constant-α disk, where α=0.01, carbon grains can be turbulently lofted into the destructive surface layers and depleted out to radii of ∼3-10au for 0.1-1 μm grains. Smaller grains can be cleared out of the planet-forming region completely. Destruction may be more effective in an actively accreting disk or when considering individual grain trajectories in non-idealized disks.

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Photoelectric Cross-Sections of Gas and Dust in Protoplanetary Disks

Cited by 51 publications

References 41 publications

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

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

Stellar energetic particle ionization in protoplanetary disks around T Tauri stars

Destruction of Refractory Carbon in Protoplanetary Disks

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