Found 25 talks archived in ISM and nebulae
More than 200 species have been detected in the interstellar medium (ISM), among them many molecules, radicals and ions, containing the −C≡N functional group. Both linear and branched isomers of propyl cyanide (PrCN; C 3 H 7 CN) are ubiquitous in interstellar space. To date, PrCN is one of the most complex molecules found in the interstellar medium. Furthermore, it is the only one observed species to share the branched atomic backbone of amino acids, some of the building blocks of life. Radical-radical chemical reactions in gas phase and on an ice model are examined in detail using density functional theory M062X/6-311++g(d,p) and ab initio methods CCSD(T)-F12//MP2. The reaction mechanism involves the following radicals association: CH 3 CHCH 3 +CN, CH 3 +CH3CHCN for iso-PrCN and CH 3 CH 2 +CH 2 CN, CH 3 +CH 2 CH 2 CN, CN+CH 3 CH 2 CH 2 for n-PrCN formation. Rate constants (see Figure 1) are also reported for gas phase association reactions. All reaction paths are exoergic and barrier-less in the gas phase and on the ice-model, suggesting that the formation of iso-PrCN and n-PrCN is efficient on the water-ice model adopted.
Another molecule : acetaldehyde (CH 3 CHO) is ubiquitous in interstellar space and is important for astrochemistry as it can contribute to the formation of amino acids through reaction with nitrogen containing chemical species. Quantum chemical and reaction kinetics studies are reported for acetaldehyde formation from the chemical reaction of C(3 P) with a methanol molecule adsorbed at the eighth position of a cubic water cluster. We present extensive quantum chemical calculations by means of CCSD(T)//wB97XD/6-311++G(2d,p) for total spin S=1 and S=0. The rate limiting step for forming acetaldehyde is the C–O bond breaking in CH 3 OCH to form adsorbed
CH 3 and HCO. We find two positions on the reaction path where spin crossing may be possible such that acetaldehyde can form in its singlet spin state.
1. I. BenChouikha, B. Kerkeni, et al. “Quantum chemical study of the reaction paths and kinetics of acetaldehyde formation on a methanol-water ice model”, ACS Adv., 12,18994 (2022).
2. B. Kerkeni, V Gámez, G. Ouerfelli, M-L. Senent, and N. Feautrier “Understanding Propyl-cyanide and its isomers Formation: Ab initio Study of the Spectroscopy and Reaction Mechanisms.”, Mon. Not. Roy. Astron. Soc. https://doi.org/10.48550/arXiv.2301.12297 (2023).
Artificial intelligence techniques are increasingly used in our daily lives. They also play an important role in science, including astrophysics. I am particularly interested in the use of machine learning regressors. I will present an overview of the current situation and some recent uses of these methods in the study of planetary nebulae or HII regions.
We present some results on a project based on high-resolution UVES@VLT spectroscopy and HST imaging of photoionized Herbig-Haro (HH) objects in the Orion Nebula. We study physical conditions, chemical abundances and other properties such as proper motions and the origin of the driving jets. Our study will include at least 9 HH objets, of which we will focus on HH529II, HH529III and HH204 in this talk. Our data allow us to separate the spectrum of the outflows from the main nebular emission, studying each object with an unprecedented detail. The HHs are located at different distances from the main ionization source of the Orion Nebula, with different ionization/physical conditions and flow velocities. In all objects, the electronic density (ne) is substantially higher than in the surrounding Orion Nebula, while the electronic temperature (Te) is maintained under photoionization equilibrium for the most abundant ion stages. In HH204 we observe a Te([OIII]) gradient due to the contribution of [OIII] emission from the cooling layer behind the bow shock, which is also detected in the HST imaging. The ionization degree of the gas in the different HH objects is very different, allowing us to determine the chemical composition of the Orion Nebula under both ionization conditions, avoiding the use of ionization correction factors (ICFs) for many elements. HH204 shows an abundance discrepancy -the difference between abundances derived from recombination and collisionally excited lines- thas is actually zero. We find direct evidence of dust destruction in the bow shock in all objects. This increases the gaseous abundances of Fe, Ni and Cr with respect to the Orion Nebula ones. We show that a failure to resolve the different kinematic components -as in a low spectral resolution spectrum- can lead to significant error in the determination of chemical abundances -40% underestimate of O in the case of HH204-, mainly due to incorrect estimation of the electron density.
APOGEE contains more than hundred thousands new giant stars. This enabled
us to collected an unprecedented and homogeneous sample of giant stars with
light-element abundance variations similar to observed in “
*second-generation*” globular cluster stars. If they are really former
members of dissolved globular clusters, stars in these groups should show
some of the basic SG-like chemical patterns known for stars currently
belonging to the Milky Way globular clusters, such as depletion in C and O
together with N and Al enrichments. Here, I will present the results of an
updated census of *SG-like* stars from a near-infrared manual analysis
using the Brussels Automatic Stellar Parameter (BACCHUS) code to provide
the abundances of C, N, O, Mg, Si, Al, Fe, Ce and Nd for every line of
possible cluster member stars, which they migrate to the disk, halo and
bulge as unbound stars, and become part of the general stellar population
of the Milky Way. By combining wide-field time-series photometry with
APOGEE-2S spectroscopy data, we are in a good position to put the big
picture together. The VVV survey have produced a large variability dataset
towards the Milky Way bulge and disk, including data in the near-IR (J and
Ks). These data will allow us to place constraints on the “polluters" that
are responsible for the chemical peculiarities, with candidates including
TP-AGB stars, binary mass transfer, accretion of material from the winds of
AGB stars, etc. A cross match between VVV sources and APOGEE targets is
Molecular hydrogen (H2) is a fundamental component of galaxies, being the most abundant element in molecular clouds, where stars form, and an important source of radiative cooling at low temperature. With the advent of the ALMA telescope, a large amount of data about the distribution of H2 in galaxies has become available. However, the large majority of numerical simulations on galactic and cosmological scales still lacks the ability to directly follow the formation and dissociation of H2, and must rely on pre-calibrated sub-grid models to compare the results with observations. I will present a new model to self-consistently track the evolution of H2, including gas and dust shielding, H2 self-shielding, star formation (SF), supernova feedback, and extragalactic and local stellar radiation. I will discuss the results of a suite of hydrodynamic simulations of an isolated gas-rich galaxy at z=3, showing that the model can naturally reproduce the observed correlation between SF and H2 surface densities, without assuming any a priori dependence of SF on the H2 abundance. I will also present a study of the kinematics and dynamics of molecular gas in high-redshift quasars (z=6), where we investigate whether a central accreting black hole (BH) can significantly affect the H2 distribution in the host galaxy and generate molecular outflows.
Planetary nebulae (PNe) are excellent tracers to study the chemistry, kinematics, and stellar populations of galaxies. They can be used to
constrain the properties of galactic substructures and peer into the past tidal interactions. In this talk, I present our successful GTC observations of PNe in the Northern Spur and the Giant Stream, two
most prominent substructures of M31. The deep spectroscopy enabled detection of the weak [O III] 4363 temperature-diagnostic line in all target PNe and as a consequence, reliable determination of elemental abundances. Our PN sample have homogeneous oxygen abundances, although
slight difference between the two substructures are marginally noticed. The study of abundances and the spatial and kinematical properties of our sample leads to the tempting conclusions: 1) their progenitors might
belong to the same stellar population, and 2) the Northern Spur and the Giant Stream may have the same origin and may be explained by the stellar orbit proposed by Merrett et al.
The dwarf satellite M32 might be responsible for the two substructures. Deep spectroscopy of PNe in M32 will help to assess this hypothesis.
From the structure of PHL 293B and the physical properties of its ionizing cluster and based on results of hydrodynamic models, we point at the various events required to explain in detail the emission and absorption components seen in its optical spectrum. We ascribe the narrow and well centered emission lines, showing the low metallicity of the galaxy, to an HII region that spans through the main body of the galaxy. The broad emission line components are due to two off-centered supernova remnants evolving within the ionizing cluster volume and the absorption line profiles are due to a stationary cluster wind able to recombine at a close distance from the cluster surface as originally suggested by Silich et al. 2004. Our numerical models and analytical estimates confirm the ionized and neutral column density values and the inferred X-ray emission derived from the observations.
There is an increasing multiplicity of proposed methodologies to derive chemical abundances in HII regions from the measurement of the relative fluxes of their optical emission lines. Particularly there is a known discrepancy between the prediction of some widely used grids of photoionization models and the results of the direct analysis of the spectra from their integrated physical properties (i.e. density, temperature). In this seminar, I will introduce HII-CHI-mistry, a Chi square approximation to compare observations with results of a large grid of models calculated using CLOUDY and varying the oxygen abundance, the nitrogen-to-oxygen ratio and the ionization parameter, covereing all possible conditions observed so far in massive complexes of star-formation. Including N/O as an additional variable allows the correct interpretation of the [NII] 6584 emission lines, widely used to derive abundances both in the Local and the Early Universe in the infra-red part of the spectrum.
The use of this method leads to a derivation of both Z and N/O totally consistent with the results from the direct method when emission line ratios sensitive to the temperature are available (e.g. [OIII] 50007/4363). On the contrary, when these ratios are not available, what is the most common situation in metal-rich/distant objects, it is necessary to assume empirical constraints to the space of parameters covered by the model-grid to arrive to solutions in the whole range of metallicity. Among the applications of this methods it is a consistent study of the metallicity in a wide range of potential variations (e.g. gradients of Z in disc galaxies, mass-metallicity relation, etc ...)
Evolved stars are factories of cosmic dust. This dust is made of tiny grains that are injected into the interstellar medium and plays a key role in the evolution of astronomical objects from galaxies to the embryos of planets. However, the processes involved in dust formation and evolution are still a mystery. The increased angular resolution of the new generation of telescopes will provide for the first time a detailed view of the conditions in the dust formation zone of evolved stars, as shown by our first observations with ALMA. The aim of the NANOCOSMOS project is to take advantage of these new observational capabilities to change our view on the origin and evolution of dust. We will combine astronomical observations, modelling, and top-level experiments to produce stardust analogues in the laboratory and identify the key species and steps that govern the formation of these nanoparticles. We will build two innovative setups: the Stardust chamber to simulate dust formation in the atmosphere of evolved stars, and the gas evolution chamber to identify novel molecules in the dust formation zone. We will also improve existing laboratory setups and combine different techniques to achieve original studies on individual nanoparticles, their processing to produce complex polycyclic aromatic hydrocarbons, the chemical evolution of their precursors and their reactivity with abundant astronomical molecules. Our simulation chambers will be equipped with state-of-the-art in situ and ex situ diagnostics. Our astrophysical models, improved by the interplay between observations and laboratory studies, will provide powerful tools for the analysis of the wealth of data provided by the new generation of telescopes.
The synergy in NANOCOSMOS between astronomers, vacuum and microwave engineers, molecular and plasma physicists, surface scientists, including both experimentalists and theoreticians is the key to provide a cutting-edge view of cosmic dust.
Following the observational and theoretical evidence that points at core collapse supernovae as major producers of dust, we calculate the hydrodynamics of the matter reinserted within young and massive super stellar clusters under the assumption of gas and dust radiative cooling. The large supernova rate expected in massive clusters allows for a continuous replenishment of dust immersed in the high temperature thermalized reinserted matter and warrants a stationary presence of dust within the cluster volume during the type II supernova era (~ 3 Myr - 40 Myr). Such a balance determines the range of dust to gas mass ratio and this the dust cooling law. We then search for the critical line in the cluster mechanical luminosity (or cluster mass) vs cluster size, that separates quasi- adiabatic and strongly radiative cluster wind solutions from the bimodal cases. In the latter, strong radiative cooling reduces considerably the cluster wind mechanical energy output and affects particularly the cluster central regions, leading to frequent thermal instabilities that diminish the pressure and inhibit the exit of the reinserted matter. Instead matter accumulates there and is expected to eventually lead to gravitational instabilities and to further stellar formation with the matter reinserted by former massive stars. The main outcome of the calculations is that the critical line is almost two orders of magnitude or more, depending on the assumed value of V\infty, lower than when only gas radiative cooling is applied. And thus, massive clusters (M_sc > 10^5 Msun) are predicted to enter the bimodal regime.
- Ultra-Diffuse Galaxies (UDGs) and the Stellar Mass – Halo Mass Relationship Dr. Jonah GannonTuesday June 6, 2023 - 12:30 GMT+1 (Aula)
- The complex Milky Way historyDr. Cristina ChiappiniThursday June 8, 2023 - 10:30 GMT+1 (Aula)