Red de Excelencia Polvo Cósmico
Polvo Cósmico

Cosmic Dust Excellence Network (funded by the Ministerio de Economía, Industría y Competitividad for a duration of two years, from 01/12/2015 to 30/11/2017)

Proposal abstract

The network includes the four national groups with a research career devoted to the study of cosmic dust. We intend to consolidate and reinforce our research activity creating a network dedicated to cosmic dust and ice characterization (dust and ice structure and composition, ice density and optical constants, spectroscopy) and the study of numerous physical and chemical processes that occur in the dust (formation of dust particles by agglomeration of smaller ones, ice accretion and desorption, dust ejection in comets, ultraviolet and ion irradiation, etc.).

To do this, we have labs that simulate these processes under conditions mimicking the interstellar medium (IEM-CSIC, UPV-Alcoy, CAB INTA-CSIC) or the scattering of light by dust (IAA of Granada). The experimental work is complemented by models to reproduce the dust shape/surface roughness and study their role in the scattering properties (IAA in Granada) and ice structure and spectroscopy (IEM-CSIC), which aid to interpret the experimental results and the observational data from comets and interstellar ice mantles.

The research lines of this proposal are:

  1. Rosetta cometary mission, determination of physical and chemical properties of dust
  2. Experimental and theoretical study of ice monolayers deposited on dust analogs (HAC, silicates)
  3. Measurement of optical constants applied to the dust scattering matrix
  4. optical constants of multi-component molecular ices, and e) study in the far infrared range of dust and ice for the future SPICA mission

The network proposes regular meetings among groups to undertake the research plan and interact with other researchers inside and outside our country. Specifically, we have the support of foreign institutions with whom we cooperate regularly: Dartois, IAS, Orsay; Ciaravella, O. Palermo; Chen, NCU, Taiwan; Linnartz, U. Leiden,; Watanabe, Sapporo; Hovenier, U. Amsterdam, etc.

Astrophysical context of cosmic dust

Dust is an ubiquitous component in space, it is mainly in the form of carbonaceous grains or silicate particles. It is present in the diffuse interstellar medium (ISM) with sizes below one micron, where it is exposed to a strong UV field. Dense molecular clouds are the birthplaces of young stars; the typical densities around 104-106 particles cm-3 and temperatures as low as 10 K allow accretion of ice mantles on dust grains. The ice is composed mainly of H2O, and other species like CO, CH3OH, CO2, and NH3 (e.g. Gibb et al. 2001, Dartois 2005). The physical properties (structure, density, optical constants) of these multi-component ice mantles, layered or mixed, different degrees of amorphicity, and their effect on the observed infrared bands is a topic of discussion (e.g. Herrero et al. 2010, Luna et al. 2012, Satorre et al. 2013, Escribano et al. 2013). Cosmic rays are able to penetrate into the cloud and produce UV photons by ionization of molecular hydrogen (known as the ¨secondary UV field¨). Icy grain mantles in dense clouds are thus processed energetically by cosmic rays (protons and heavy ions) and by the secondary UV field, about 103-104 photons cm-2 s-1.

The accretion and desorption processes of gas molecules on cold dust play an important role in the evolution of dense clouds and circumstellar regions of young stellar objects (YSOs) (e.g. Bisschop et al. 2006, Escribano et al. 2013; Luna et al. 2012, 2014). The onset of accretion is related to the so-called snowline in protoplanetary disks (the distance from the protostar where ices start to form). An adequate interpretation of the observations toward cold interstellar regions requires a good understanding of the processes occurring at the interface between the solid and the gas phase. Most observations are performed in the radio and detect molecules in the gas, the dust is often invoked as a sort of ¨black box¨ when gas phase reactions alone cannot explain the observed molecular abundances. Given the low temperatures of 10-20 K in dark clouds, thermal desorption is negligible and most molecules are expected to stick to dust grains leading to depletion in the gas phase. Carbon monoxide, CO, is expected to deplete at temperatures below 20 K. Nevertheless, CO gas is observed in cold clouds at densities below ≈ 3–8 × 104 cm−3 (e.g. Pagani et al. 2005). Other molecules like methanol and formaldehyde are also observed in cold regions (Guzman et al. 2013). This suggests that there must be a non-thermal desorption mechanism operating in dark clouds.

Organic molecules of considerable complexity have been found in dark interstellar clouds and in localized regions in hot cores, protoplanetary nebulae and disks, and circumstellar envelopes (Dalgarno 2006). Large partly hydrogen-saturated molecules were detected, many of them are of prebiotic interest (Snyder 2006). These species challenge the completeness of the standard ion-neutral scheme in interstellar chemistry, suggesting that reactions on dust grains are involved in their formation. Unfortunately, reaction pathways leading to the formation of such complex molecules are largely unknown. Secondary UV photons could chemically modify the pristine ices as condensed onto interstellar grains, producing a significant increase in the complexity of the photolyzed mixtures, as it has been shown in laboratory experiments (e.g. Muñoz Caro et al. 2002, Maté et al. 2014, Gálvez et al. 2010). After the ices are evaporated in the very dense and warm gas, in the vicinity of a newly forming star, these complex molecules are released to the gas phase. Thus, both the cold early stages of star formation and the following warm-up to the final hot-core phase are involved (Herbst & van Dishoeck 2009).

The envelopes around stars contain icy grain mantles similar in composition to those present in dense clouds (e.g., Thi et al. 2002, Pontoppidan et al. 2005) that will be exposed to photon and ion irradiation from the central star and the surrounding diffuse ISM, thus providing a new scenario for energetic ice processing (e.g. Muñoz Caro & Schutte 2003). These envelopes often give rise to disks, which can lead to planetary systems. The evolution of the solar nebula led to the formation of comets, asteroids, and planetesimals.

Comets are presumably formed by agglomeration of icy dust particles during or prior to the formation of the Solar System, although their materials could be processed at later stages, as the presence of crystalline silicates and minerals formed at high temperatures indicates (see Muñoz Caro 2010 for a short review). Some are known to be rich in organics, as found by the Halley missions (Kissel & Krueger 1987) and more recently by Stardust (Kissel et al. 2004). The ESA-Rosetta mission is currently orbiting Comet 67P/Churyumov-Gerasimenko. The Philae lander was designed to analyze in situ the properties of the nucleus. The early Earth was bombarded by comets and asteroids (e.g. Schoenberg et al. 2002). This exogenous delivery provided a significant amount of organic matter that might have contributed to the origin of life (e.g. Chyba & Sagan 1992). It is therefore of high interest to study the processes leading to the formation of organic matter in space, including ice irradiation, and to characterize cometary organics (Muñoz Caro & Dartois 2013). Rosetta is being a great success since unprecedented data is measured by the orbiter instruments and Philae performed a series of measurements after the landing that may be continued in the coming weeks, probably depending on the current status of the communication antenna. In particular, this mission is providing valuable data on the physical parameters and the dynamics of the ejected dust due to the ice sublimation from the comet nucleus (Moreno et al. 2007, 2014; Muñoz et al. 2012) and the in situ chemical characterization of the dust (Goesmann et al. 2015).


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