X-ray photodesorption of complex organic molecules in protoplanetary disks. II. Formic acid, HCOOH

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. 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 H_2O-dominated ices. 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. 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 H_2O ices, no clear desorption signal of intact HCOOH is observed. X-ray photodesorption yields of the HCOOH photoproducts, which we attribute to H_2O, CO, CO_2, HCO, H_2CO, and O_2, 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 times 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 H_2O-dominated ices. 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 CH_3OH and CH_3CN, X-ray photodesorption from H_2O-rich ice surfaces should be less efficient than from CO-rich ice surfaces by a factor that remains to be quantified for HCOOH.