X-ray photodesorption of complex organic molecules in protoplanetary disks

II. Formic acid, HCOOH

¹ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 75005 Paris, France
² Université Paris-Saclay, CNRS, ISMO, 91405 Orsay, France

^★ Corresponding author: romain.basalgete@sorbonne-universite.fr

Received: 7 May 2025
Accepted: 17 July 2025

Abstract

Context. Nonthermal desorption from interstellar ices induced by X-rays, known as X-ray photodesorption, is a possible route to explain gas phase molecules detected in protoplanetary disks. However, the efficiency of such a process remains to be quantified for organic molecules, such as formic acid, HCOOH.

Aims. We experimentally estimate the X-ray photodesorption yields of intact HCOOH and its photoproducts from its pure ice and when it is mixed in CO-dominated and H₂O-dominated ices.

Methods. Ices were grown in an ultrahigh vacuum setup and soft X-rays (530–560 eV) from the SEXTANTS beamline of the SOLEIL synchrotron facility were used to irradiate the ices at 15 K. X-ray photodesorption from the ices was probed in the gas phase of the chamber via quadrupole mass spectrometry. X-ray photodesorption yields were derived from the mass signals; they are expressed in molecules desorbed per incident photon, denoted as molecules photon⁻¹ in the manuscript. They were extrapolated and averaged in the 0.53–10 keV range in order to provide yields that can be easily implemented in astrochemical models, referred to as astrophysical yields in the following.

Results. X-ray photodesorption of intact HCOOH at 560 eV is found to be close to our detection limit of 10⁻³ molecules photon⁻¹ for pure and mixed-CO ices. When mixed in H₂O ices, no clear desorption signal of intact HCOOH is observed. X-ray photodesorption yields of the HCOOH photoproducts, which we attribute to H₂O, CO, CO₂, HCO, H₂CO, and O₂, are derived as a function of the ice composition. Possible factors that may explain the relative intensities of the yields are discussed. The astrophysical yields of intact HCOOH from CO-dominated ices vary from ~10⁻³ to ~5 × 10⁻⁶ molecules photon⁻¹ depending on the region of the disk considered. Only upper limits, of the same order of magnitude, can be provided for the desorption of intact HCOOH from H₂O-dominated ices.

Conclusions. X-ray photodesorption should enrich the gas phase of the cold regions of protoplanetary disks with organic molecules such as HCOOH. However, as has been suggested in previous studies on CH₃OH and CH₃CN, X-ray photodesorption from H₂O-rich ice surfaces should be less efficient than from CO-rich ice surfaces by a factor that remains to be quantified for HCOOH.