50 research outputs found
Dissociation of the benzene molecule by UV and soft X-rays in circumstellar environment
Benzene molecules, present in the proto-planetary nebula CRL 618, are ionized
and dissociated by UV and X-ray photons originated from the hot central star
and by its fast wind. Ionic species and free radicals produced by these
processes can lead to the formation of new organic molecules. The aim of this
work is to study the photoionization and photodissociation processes of the
benzene molecule, using synchrotron radiation and time of flight mass
spectrometry. Mass spectra were recorded at different energies corresponding to
the vacuum ultraviolet (21.21 eV) and soft X-ray (282-310 eV) spectral regions.
The production of ions from the benzene dissociative photoionization is here
quantified, indicating that C6H6 is more efficiently fragmented by soft X-ray
than UV radiation, where 50% of the ionized benzene molecules survive to UV
dissociation while only about 4% resist to X-rays. Partial ion yields of H+ and
small hydrocarbons such as C2H2+, C3H3+ and C4H2+ are determined as a function
of photon energy. Absolute photoionization and dissociative photoionization
cross sections have also been determined. From these values, half-life of
benzene molecule due to UV and X-ray photon fluxes in CRL 618 were obtained.Comment: The paper contains 8 pages, 9 figures and 4 tables. Accepted to be
published on MNRAS on 2008 November 2
Destruction and multiple ionization of PAHs by X-rays in circumnuclear regions of AGNs
The infrared signatures of polycyclic aromatic hydrocarbons (PAHs) are
observed in a variety of astrophysical objects, including the circumnuclear
medium of active galactic nuclei (AGNs). These are sources of highly energetic
photons (0.2 to 10 keV), exposing the PAHs to a harsh environment. In this
work, we examined experimentally the photoionization and photostability of
naphthalene (CH), anthracene (CH),
2-methyl-anthracene (CHCH) and pyrene (CH)
upon interaction with photons of 275, 310 and 2500 eV. The measurements were
performed at the Brazilian Synchrotron Light Laboratory using time-of-flight
mass-spectrometry (TOF-MS). We determined the absolute photoionization and
photodissociation cross sections as a function of the incident photon energy;
the production rates of singly, doubly and triply charged ions; and the
molecular half-lives in regions surrounding AGNs. Even considering moderate
X-ray optical depth values () due to attenuation by the dusty
torus, the half-lives are not long enough to account for PAH detection. Our
results suggest that a more sophisticated interplay between PAHs and dust
grains should be present in order to circumvent molecular destruction. We could
not see any significant difference in the half-life values by increasing the
size of the PAH carbon backbone, N, from 10 to 16. However, we show that
the multiple photoionization rates are significantly greater than the single
ones, irrespective of the AGN source. We suggest that an enrichment of multiply
charged ions caused by X-rays can occur in AGNs.Comment: 20 pages (appendix: 3 pages), 10 figures, 4 tables. Accepted for
publication in the Montly Notices of the Royal Astronomical Society (MNRAS).
Accepted 2019 April
Ionisation and dissociation of cometary gaseous organic molecules by solar wind particles I: Formic Acid
In order to simulate the effects of energetic charged particles present in
the solar wind colliding with the cometary gaseous formic acid molecule
(HCOOH), laboratory experiments have been performed. The absolute ionisation
and dissociation cross sections for this molecule interacting with solar wind
particles were measured employing fast electrons in the energy range of 0.5 to
2 keV and energetic protons with energies varying from 0.128 to 2 MeV. Despite
the fact that both projectiles lead to a very similar fragmentation pattern,
differences in the relative intensities of the fragments were observed. Formic
acid survives about 4-5 times more to the proton beam than to the energetic
electron collision.The minimum momentum transfer in the electron impact case
was estimated to be 3-38% larger than the minimum momentum transfer observed
with the equivelocity protons. The UV photodissociation rates and half-lives
for HCOOH are roughly closer to the values obtained with energetic electrons.
It is consequently important to take electron impact data into account when
developing chemical models to simulate the interplanetary conditions.Comment: 11 pages, 7 figures, 5 tables, Accepted to be published in MNRA
Photostability of gas- and solid-phase biomolecules within dense molecular clouds due to soft X-rays
An experimental photochemistry study involving gas- and solid-phase amino
acids (glycine, DL-valine, DL-proline) and nucleobases (adenine and uracil)
under soft X-rays was performed. The aim was to test the molecular stabilities
of essential biomolecules against ionizing photon fields inside dense molecular
clouds and protostellar disks analogs. In these environments, the main energy
sources are the cosmic rays and soft X-rays. The measurements were taken at the
Brazilian Synchrotron Light Laboratory (LNLS), employing 150 eV photons.
In-situ sample analysis was performed by Time-of-flight mass spectrometer
(TOF-MS) and Fourier transform infrared (FTIR) spectrometer, for gas- and
solid- phase analysis, respectively. The half-life of solid phase amino acids,
assumed to be present at grain mantles, is at least 3E5 years and 3E8 years
inside dense molecular clouds and protoplanetary disks, respectively. We
estimate that for gas-phase compounds these values increase one order of
magnitude since the dissociation cross section of glycine is lower at gas-phase
than at solid phase for the same photon energy. The half-life of solid phase
nucleobases is about 2-3 orders of magnitude higher than found for amino acids.
The results indicate that nucleobases are much more resistant to ionizing
radiation than amino acids. We consider these implications for the survival and
transfer of biomolecules in space environments.Comment: 10 pages, 5 figures, 2 tables. Accepted to be published in MNRA
Crocodiles in the Sahara Desert: An Update of Distribution, Habitats and Population Status for Conservation Planning in Mauritania
A taxonomic backbone for the global synthesis of species diversity in the angiosperm order Caryophyllales
The Caryophyllales constitute a major lineage of flowering plants with approximately 12500 species in 39 families. A taxonomic backbone at the genus level is provided that reflects the current state of knowledge and accepts 749 genera for the order. A detailed review of the literature of the past two decades shows that enormous progress has been made in understanding overall phylogenetic relationships in Caryophyllales. The process of re-circumscribing families in order to be monophyletic appears to be largely complete and has led to the recognition of eight new families (Anacampserotaceae, Kewaceae, Limeaceae, Lophiocarpaceae, Macarthuriaceae, Microteaceae, Montiaceae and Talinaceae), while the phylogenetic evaluation of generic concepts is still well underway. As a result of this, the number of genera has increased by more than ten percent in comparison to the last complete treatments in the Families and genera of vascular plants” series. A checklist with all currently accepted genus names in Caryophyllales, as well as nomenclatural references, type names and synonymy is presented. Notes indicate how extensively the respective genera have been studied in a phylogenetic context. The most diverse families at the generic level are Cactaceae and Aizoaceae, but 28 families comprise only one to six genera. This synopsis represents a first step towards the aim of creating a global synthesis of the species diversity in the angiosperm order Caryophyllales integrating the work of numerous specialists around the world