Molecules in star forming regions: our astrochemical origins

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The formation of a Sun-like star and its plane-tary system

Stars like our Sun form in dark clouds invisible to optical observations. Only the development of infrared and (sub-)millimeter telescopes enables the direct studies of this complex process. Dense giant molecular clouds originated by the cooling of the di↵use medium and their extent is on scales of ⇠100 pc. The in-ternal structure is complex because of the interplay of several phenomena such as gravitational collapse, magnetic fields and turbulence (Klessen & Glover 2016).
Observations performed by the ESA Herschel space telescope2 revealed that molec-ular clouds have a filamentary structure (Andre´ et al. 2014). In the densest part of the filaments fragmentation due to gravitation instabilities can occur forming dense (nH2 > 104 cm 3) prestellar cores (⇠ 0.1–0.01 pc). These objects are grav-itationally bound clouds with low temperatures (around 10 K). During a process lasting fews Myr prestellar cores collapse under gravity forming one or more pro-tostars (Shu 1977). It is in these cold objects that the initial conditions of a com-plex process ending with a star and its planetary system are set. Young Stellar Ob-jects (YSOs) have been empirically classified by Lada (1987) in a scheme based on the slope of their infrared spectral energy distribution (SED), which reflects their evolutionary stage. Other criteria have been introduced to classify objects too deeply embedded to be detected in the near-infrared as for example the ratio
of submillimeter to bolometric luminosity (Lsubmm/Lbol; Andre´ et al. 1993) and the bolometric temperature (Myers & Ladd 1993), that is the temperature of a black body with the same mean frequency as the observed SED. Figure 1.2 ex-tends the Lada’s classification scheme to the current known evolutionary stages from the prestellar cores phase to the formation of a planetary system. On the right panels observations in di↵erent wavelengths of objects in the di↵erent stages are reported. The first stage is the prestellar core, represented by the Hubble im-age of the Barnard object B68 which appears at visual wavelengths like a dark object on the stellar background because of the large visual extinction. Once the collapse occurs, the material starts to accrete into a central object forming a pro-tostar which enters in the Class 0 phase. In this stage the central object is still deeply embedded in a large scale envelope. The infalling material has to lose an-gular momentum in order to be accreted from the protostar. The infalling material tends to conserve angular momentum, producing the so-called centrifugal bar-rier (see Fig. 1.3) which is this transition zone (a ring) from an infalling-rotating envelope to a rotationally (Keplerian) supported disc (Sakai et al. 2014b, 2017, Oya et al. 2016). At the centrifugal barrier, not only the gas motion is discon-tinuous, but also the chemical composition of the gas drastically changes, due to slow accretion shocks which drives the sputtering of the dust mantles and trig-gers warm gas chemistry. This prevents the material from falling directly onto the protostar surface and generate a disk-like structure. The angular momentum is removed thanks to the ejection of gas through low-velocity and poorly colli-mated molecular outflows and high-velocity and highly collimated protostellar jets. While the outflows are very massive and made of swept up material, jets are denser and generated very close to the protostellar surface. Jets from young accreting stars remain one of the most spectacular phenomena in astrophysics. Although their exact launch zone is still debated (in the inner 0.05 AU, according recent measurements by Lee et al. 2017b), it is currently accepted that they are powered by the rotation and accretion energy of the system, and are accelerated or collimated via a magneto-hydrodynamical (MHD) process (see e.g. Ferreira et al. 2006; Shang 2007; Frank et al. 2014, and references therein). The ejected material drives bow-shocks through the surrounding high-density medium and traced by H2 ro-vibrational lines at excitation temperatures above 1000 K. Slower and cold (10–20 K) molecular outflows are in turn formed by swept-up material, usually traced by CO. Shocks heat the gas and trigger several processes such as endothermic chemical reactions and ice grain mantles sublimation or sputtering. A large number of molecular species undergo a dramatic enhancement in their abundances (see e.g. van Dishoeck & Blake 1998), as observed by observations at millimeter wavelengths towards a number of outflows (e.g. Bachiller et al. 2001). The Class 0 object reported in Fig. 1.2 is the HH212 protostar in the Orion star forming region, recently observed in the submillimeter (Lee et al. 2017b) where a bipolar jet and a forming dusty disk have been identified. After about 105 yr, the protostar reaches the Class I phase in which the large scale envelope is still present but the luminosity start to be dominated by the central object. The SED is characterized by an infrared excess due to the protostellar light absorbed and scattered by the disk and the dust envelope. The object reported in Fig. 1.2 is HH30 in Taurus as observed with the Hubble space telescope. The image shows a jet and an edge-on disk illuminated by the protostellar light. After about 106 yr the protostar enter in the Class II phase when most of the envelope has dissipated and the young star is surrounded by an accretion disk. Fig. 1.2 reports the spec-tacular ALMA image of the protoplanetary disk around the Class II protostar HL Tauri. Within the disk dust coagulation occurs ending up with the formation of a debris disk during the Class III phase and a planetary system. In Fig. 1.2 the direct imaging of three planets orbiting around the protostellar objects HH 8799 are shown.

Molecules in star forming regions: our astro-chemical origins

During the low-mass star forming process, the interstellar medium evolves to-ward denser condensations and molecules can therefore be formed, destroyed or incorporated at the varius stages so that the chemical composition of the gas be-comes increasingly more complex (see e.g. Caselli & Ceccarelli 2012, Ceccarelli et al. 2014, Yamamoto 2017). Molecular complexity builds up at each step of the Sun–like star formation process, starting from simple molecules and ending up in large polyatomic species (see Figure 1.4). While matter slowly accumulates toward the center of a molecular cloud, the central density increases and the tem-perature decrease. Atoms and molecules in the gas phase freeze-out onto the cold surfaces of the dust grains, forming the grain mantles. Hydrogenation of atoms and molecules takes place, forming molecules such as water (H2O), and formalde-hyde (H2CO). In these regions the formation of new molecules in icy mantles is also caused by the e↵ects of UV photons and low-energy cosmic rays. At this point the collapse starts, the gravitational energy is converted into radiation and the envelope around the central object warms up. The molecules frozen on the mantles acquire mobility and form new, more complex species. When the tem-perature reaches about 100 K the mantle sublimates, given origin to the so-called hot-corino phase, which has a very small size (< 0.01 pc). Molecules in the man-tle are injected in the gas forming the so-called ”first generation” species. Succes-sively they react in the gas-phase to form the ”second generation” of more com-plex molecules. The abundance of iCOMs (such as methyl formate, HCOOCH3 or dimethyl ether, CH3OCH3) dramatically increases. However recent works sug-gest that gas-phase chemistry could play an important role also in the formation of the first generation species (Balucani et al. 2015, Codella et al. 2017). A clas-sical example of a chemically rich hot-corino is provided by IRAS16293–2422 (e.g. Ceccarelli et al. 2007), where recently also glycolaldehyde (HCOCH2OH), a crucial molecule for the formation of metabolic molecules, has been detected (Jørgensen et al. 2012). Simultaneously to the collapse, a newborn protostar gen-erates a fast and well collimated jet, possibly surrounded by a wider angle wind. In turn, the ejected material drives shocks travelling through the surrounding high-density medium. Shocks heat the gas up to thousands of K and trigger several processes such as endothermic chemical reactions and ice grain mantle sublima-tion or sputtering (e.g. van Dishoeck & Blake 1998). Several molecular species undergo significant enhancements in their abundances. The envelope dissipates with time and eventually only a circumstellar disk remains, which is also called protoplanetary disk. In the hot regions, close to the central forming star, new complex molecules are synthesized by reactions between the species formed in the protostellar phase. In the cold regions of the disk, where the vast majority of matter resides, the molecules formed in the protostellar phase freeze-out onto the grain mantles again. Dust grains then coagulate into larger planetesimals, the bricks of future planets, comets, and asteroids.
Figure 1.4: Star formation and chemical complexity. The formation of a star and a planetary system, like the Solar System, passes through fundamental phases, marked in the sketch (Caselli & Ceccarelli 2012) iCOMs such as methyl formate, HCOOCH3, dimethyl ether, CH3OCH3, or glycolaldehyde, HCOCH2OH) have been found in all the components of the star formation recipe (prestellar cores, hot-corinos, shocks induced by fast jets). These species are thought to be either formed in solid state chemistry of grain mantles and then released in the gas-phase due to ice grain mantle sublimation or sput-tering (see Fig. 1.5), or produced in the gas phase using simpler species released by mantles (such as H2CO or CH3OH). This question is still hotly debated. In both the proposed mechanism for iCOMs formation (in the gas or onto mantles surface), the chemical complexity is related to the initial ice composition (Oberg¨ et al. 2009, Sakai et al. 2010), confirming the importance of recovering the phys-ical conditions of the natal protostellar cloud. In particular, one of the key bricks for the iCOMs grain surface chemistry is CH3OH, formed through CO hydrogena-tion (Sakai & Yamamoto 2013).
In the last years it has been discovered a new class of YSOs characterised by a composition as rich as that of hot-corinos but in di↵erent species. These objects are deficient in saturated iCOMs but rich in unsaturated species like carbon-chain molecules. The particular chemistry associated with these sources has been de-fined warm carbon-chain chemistry (WCCC; see Sakai et al. 2008, 2009). Even if the origin of this chemical diversity is yet unclear, it looks not related to a di↵er-ent evolutionary stage or to a particular star forming region. It has been proposed that the chemical di↵erences between hot-corinos and WCCC sources is related to the di↵erent duration of the prestellar core phase which is reflected in a dif-ferent chemical composition of the dust mantles (Sakai & Yamamoto 2011). For WCCC objects, the short prestellar stage ensures that carbon atoms deplete onto dust grains before being converted to CO. Indeed a recent paper by Oya et al. (2017) is proposing L483 as a WCCC source harbouring hot-corino activity. Nev-ertheless only the analysis of a larger number of sources, both WCCC objects and hot-corinos, will allow us to build a unified model of low-mass star-formation. But it is clear that the key to understand the origin of the chemical di↵erentiation is in the early prestellar and protostellar stages.
The last developments of astrochemistry demonstrates that complex species represent a powerful diagnostic tool to study the present and the past conditions of an astronomical source. However, it is challenging to extract the rich infor-mation provided by the observed spectra given it is required the synergy between di↵erent research fields. The starting point is the line identification process in
Figure 1.5: Schematic showing the main routes of interstellar ice processing that takes place in astrophysical environments (Burke & Brown 2010). The temperatures in interstellar dense clouds are low enough that most gas phase molecules freeze into ice mantles on dust grains. Organic material may be created in dense molecular clouds on the surface of icy dust grains by UV-driven chemistry. Heat generated by new born stars or shocks stimulates further surface chemistry prior to evaporation or sputtering of the icy mantles during cloud collapse. Once the molecules have been released into the gas phase, they drive a rich chemistry leading to the formation of larger organics.
which the observed lines are compared with the spectroscopic data collected in spectral databases. These contain the predicted frequency and spectroscopic pa-rameters for rotational transitions of a particular species. The most used are the Jet Propulsor Laboratory (JPL3, Pickett et al. 1998) and Cologne Database for Molecular Spectroscopy (CDMS4; Muller¨ et al. 2001, Muller¨ et al. 2005) molecu-lar databases. Once a molecular species has been identified, the observed lines in-tensities need to be converted to molecular abundances. This is done with the help of radiative transfer codes, once the collisional coefficients are known. Finally, to derive the physical and chemical source structure the observed abundances have to be compared with abundances predicted by an astrochemical model which take into account the chemical reactions pathways and their rate coefficients. Cur-rent astrochemical models ar e far to reproduce the large abundances of iCOMsobserved so far in the protostellar objects. This demonstrates the need of explor-ing new chemical pathways and of obtaining more observational evidence to put stronger constraints on the models.

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Deuterium fractionation

The deuterium fractionation is the process that enriches the amount of deu-terium with respect to hydrogen in molecules. While the D/H elementary abun-dance ratio is ⇠ 1.6 ⇥ 10 5 (Linsky 2007), for molecules this ratio can be definitely higher and can be a precious tool to understand the chemical evolution of inter-stellar gas (see e.g. Ceccarelli et al. 2014, and references therein). In particular, during the process leading to the formation of a Sun-like star, large deuteration is observed in cold and dense prestellar cores (e.g. Bacmann et al. 2003, Caselli & Ceccarelli 2012, and references therein). The process start when cosmic rays ionise H2 and H, forming the ion H+3. Reactions with HD, which is the major deuterium reservoir in cold environments, produce H2D+ through the exothermic reaction H+3 + HD ! H2D+ + H2 + 230 K (e.g. Caselli & Ceccarelli 2012, Cecca-relli et al. 2014 and references therein). In a similar way the ions D2H+ and D+3 are formed. In cold gas (where T < 20 K) the inverse reaction cannot proceed. The formation of all the other deuterated molecules take place both in gas-phase or on the gain surfaces via reactions of the other species with H2D+, D2H+ and D+3. The enhancement of H2D+ which trigger all the deuteration process is due to the low temperature. Indeed in these conditions abundant neutral species as O and CO which are destruction partner of H+3 are depleted from the gas-phase and freeze-out onto grain mantles. Moreover the relative abundance of the ortho and para modifications of H2 can have an e↵ect on the deuterium fractionation. Normally, the statistical ratio of the ortho/para is 3 but at low temperatures it is predicted to drop below 10 3 (Sipila¨ et al. 2013) enhancing the H2D+ abundance. In prestellar cores, deuterated and double deuterated formaldehyde can be formed through gas phase chemistry and then the formaldehyde D/H ratio could reflect the gas phase chemistry (Roberts & Millar 2000). The picture is di↵erent for formaldehyde as well as for methanol around protostars which are mostly formed via active grain surface chemistry (e.g. Tielens 1983).

Table of contents :

1 Introduction 
1.1 The formation of a Sun-like star and its planetary system
1.2 Molecules in star forming regions: our astrochemical origins
1.2.1 Deuterium fractionation
1.3 Previous observations and related limits
2 Methods 
2.1 Radiative transfer
2.2 Spectral line fundamentals
2.3 Rotation diagram analysis
2.3.1 Multiplets
2.3.2 Large Velocity Gradient (LVG) approach
2.4 Molecular lines
2.4.1 Spin statistic
3 Observations 
3.1 Single-dish antennas: basic concepts
3.2 The IRAM-30m large program ASAI
3.3 Interferometers: basic concepts
3.4 ALMA Cycle 1 and Cycle 4 observations
4 Chemical complexity around Class 0 protostars 
4.1 The pristine jet-disk system in HH212
4.2 Deuteration on a solar system scale around HH212
4.2.1 Line spectra and maps
4.2.2 Methanol deuteration
4.2.3 Emitting region of deuterated methanol
4.2.4 Single deuterated methanol CH3OD
4.2.5 Conclusions on HH212 deuteration
4.3 iCOMs and water in HH212: accretion shocks and disk winds
4.3.1 Line spectra and maps
4.3.2 Physical properties
4.3.3 Kinematics
4.3.4 Conclusions on iCOMs
5 Chemical complexity in a later Class I protostellar stage 
5.1 The SVS13-A Class I laboratory
5.2 Decrease of deuteration in Class I objects
5.2.1 Line identification
5.2.2 Formaldehyde isotopologues
5.2.3 Methanol isotopologues
5.2.4 Conclusions on the line identification
5.2.5 LVG analysis
5.2.6 Rotation Diagram analysis
5.2.7 Methanol and formaldehyde deuteration
5.2.8 The [CH2DOH]/[CH3OD] ratio
5.2.9 D/H of organics: from Class 0 to Class I
5.2.10 Conclusions on deuteration
5.3 From Class 0 to Class I: chemical heritage
5.3.1 Ketene (H2CCO)
5.3.2 Acetaldehyde (CH3CHO)
5.3.3 Methyl Formate (HCOOCH3)
5.3.4 Dimethyl ether (CH3OCH3)
5.3.5 Formamide (NH2CHO)
5.3.6 Constraints on other iCOMs
5.3.7 Conclusions on iCOMs
6 Conclusions 
6.1 Deuterium fractionation to shed light on the past
6.2 On the nature of a hot-corino
6.3 How chemistry evolves from the Class 0 to the Class I phase
6.4 iCOMs abundances: new constraints and related limits
7 Perspectives: the NOEMA-SOLIS revolution


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