E. Seperuelo Duarte

Heavy ion irradiation of condensed CO2: sputtering and molecule formation

Context : Ices present in different astrophysical environments are exposed to ion irradiation from cosmic rays (H to heavier than Fe) in the keV to GeV energy range. Aims : The objective of this work is to study the effects produced in astrophysical ices by heavy ions at relatively high energies (MeV) in the electronic energy loss regime and compare them with those produced by protons. Methods : C18O2 was condensed on a CsI substrate at 13 K and it was irradiated by 46 MeV 58Ni11+ up to a final fluence of 1.5 × 1013 cm−2 at a flux of 2 × 109 cm−2 s−1. The ice was analyzed in situ by infrared spectroscopy (FTIR) in the 5000−600 cm−1 range. Results : The CO2 destruction was observed, as well as the formation of other species such as CO, CO3, O3, and C3. The destruction cross section of CO2 is found to be 1.7×10−13 cm2, while those for the formation of CO, CO3, and O3 molecules are 1.6 × 10−13 cm2, 4.5 × 10−14 cm2, and 1.5 × 10−14 cm2, respectively. The sputtering yield of the CO2 ice is 4.0 × 104 molecules/impact, four orders of magnitude higher than for H projectiles at the same velocity. This allows us to estimate the contribution of the sputtering by heavy ions as compared to protons in the solar winds and in cosmic rays. Conclusions : The present results show that heavy ions play an important role in the sputtering of astrophysical ices. Furthermore, this work confirms the quadratic stopping power dependence of sputtering yields.

Laboratory simulation of heavy-ion cosmic-ray interaction with condensed CO

Context. Within dense interstellar clouds, from their periphery to regions deep inside, ice mantles on dust grains are exposed to cosmic-ray irradiation. Various swift ions contribute from protons to iron in the keV to TeV energy range. Observations show that in some lines of sight condensed CO molecules are an important component of the ice. Aims. We irradiate CO ices with Ni ions of relatively high energy (50 and 537 MeV) to simulate the effects produced by fast heavy cosmic-ray ions in interstellar grain mantles. Methods. CO gas is condensed on a CsI substrate at 13 K and irradiated by 50 MeV 58Ni13+ and 537 MeV 64Ni24+ ions up to a final fluence of ≈1 × 1013 cm−2, at a flux of 1 × 109 cm−2 s−1. The sputtering yields, the destruction rate of CO, and the rate of formation of new molecular species are measured in situ by Fourier transform infrared spectroscopy (FTIR). Results. The measured CO destruction cross-sections and sputtering yields induced by Ni ions are, respectively, (i) for 50 MeV, σd = 1.0×10−13 cm2 and Y = 7×104 molecules/impact; (ii) for 537 MeV, σd = 3.0×10−14 cm2 and Y = 5.85×104 molecules/impact. Based on the present and previous results, the desorption rates induced by H, Ni, and Fe ions are estimated for a wide range of energies. The contribution of the heavy ions is found to dominate over that of protons in the interstellar medium.

Cosmic ray impact on astrophysical ices: laboratory studies on heavy ion irradiation of methane

Laboratory data of CH4 ice radiolysis promoted by fast heavy ions are obtained by infrared spectroscopy (FTIR). CH4 molecules are condensed on a CsI substrate at 15 K, and the ice layer is bombarded by 220 MeV 16 O 7+ ion beam. The ice thickness is thin enough to be traversed by projectiles at constant velocity close to the equilibrium charge state. The induced CH4 dissociation gives rise to the formation of molecular species CH3 ,C 2H2 ,C 2H4 ,C 2H6 ,a nd C 3H8. Their formation and dissociation cross sections are determined. C2H6 represent the most abundant daughter molecules. The carbon budget analysis of CH4 and its radiolysis products shows that the column density of carbon atoms contained in the methane destroyed during ion irradiation is 30−50% greater than the sum for the column densities of the newly formed species. As an astrophysical implication, the current results allow estimation of chemical reaction rates in ices covering interstellar grains.

Spectrophotometric analysis of the T Tauri star GQ Lupi A

Context. GQ Lup A is a classical T Tauri star that shows clear signs of accretion through the presence of inverse P Cygni profiles in its main emission lines. Recently, Neuhauser et al. (2005, A&A, 435, L13) found a co-moving sub-stellar companion of GQ Lup A, raising the importance of determining precise stellar parameters, system age, and distance to GQ Lup. Aims. Our main objective is to use spectrophotometric time series to determine GQ Lup A stellar parameters and predict its photospheric spectral distribution. The excess spectral luminosity can then be measured and employed to test predictions of magnetospheric accretion models of classical T Tauri stars. Methods. We present the analysis of 18 spectrophotometric observations of the T Tauri star GQ Lup A (K7) obtained with the Boller & Chivens spectrograph at the 1.52 m ESO telescope in La Silla. We also revisited archival photometric data of this star, comparing previous light curve variability with our more recent data. Results. We determined the photospheric flux of GQ Lup A on each observing night and obtained the stellar radius (R� = 1.8 ± 0.3 R� ), adopting a mean distance of 150 ± 20 pc to the Lupus 1 cloud. Assuming a K7 V temperature of 4060 K, the lu