Abstracts
The ASAI view on the evolution of molecular complexity along the formation of sun-like stars
Bertrand Lefloch (IPAG)
The Large Program ASAI carried out at the IRAM 30m telescope joins the efforts of several groups in Astrochemistry, in Spain and France, to address the question of our “chemical origins”. Its goal is to obtain a complete census of the gas chemical composition, its evolution along the main stages of the star formation process, from prestellar cores and protostars to protoplanetary disks, in order to understand the processes which governs the emergence of molecular complexity, and the formation of pre-biotic molecules. This is achieved through highly sensitive and systematic spectral line surveys of a sample of sources illustrative of the various stages of protostellar evolution. The resulting data set is aimed to serve as a reference database for the astrochemical community: astronomers, chemists, and theoreticians. I will present the main results obtained in the framework of ASAI, with some special attention to the importance of shocks. I will discuss their implication for the formation route of this molecule and pre-biotic species in solar-type protostellar environments.
Hot water disk around Orion Source I
Tomoya Hirota (NAOJ)
We report ALMA cycle 0 and 1 observations of submillimeter H2O lines at bands 6, 7, and 9 toward a massive protostar candidate Source I in the Orion KL. We detect the 321 GHz (v=0, 10,2,9-9,3,6) and 658 GHz (v=1, 1,1,0-1,0,1) H2O lines, which have lower state energies of 1846 K and 2329 K, respectively, toward Source I. They are elongated along the NE-SW bipolar outflow traced by the thermal SiO lines and the 22 GHz H2O masers (v=0, 6,1,6-5,2,3). In addition, we also detect compact emissions of the higher excitation lines at 232 GHz (v=1, 5,5,0-6,4,3) and 336 GHz (v=1, 5,2,3-6,1,6) with the lower state energies of 3451 K and 2939 K, respectively. All of the submillimeter lines show clear velocity gradients perpendicular to the bipolar outflow. We interpret that the 321 GHz and 658 GHz lines trace the base of the bipolar outflow similar to the vibrationally excited SiO masers. In contrast, the 232 GHz and 336 GHz lines are most likely emitted from a midplane of the edge-on hot molecular gas disk rotating around Source I.
The physical and chemical structure of low-velocity MHD shocks
Izaskun Jimenez-Serra (UCL)
In the past decade, the wealth of data acquired at (sub-/)millimetre and far-IR wavelengths has provided key information about the physical and chemical properties of supersonic shock waves in molecular outflows. In this talk I will summarise our results on the chemical signatures of the early interaction of low-velocity MHD shocks (occurring via a magnetic precursor) and I will present our recent work on the characterisation of the chemistry of H2O, NH3 and sulphur-bearing species in young molecular outflows. I will also mention our current plans to enhance our shock and dust grain/chemical codes at UCL, and I will discuss a few observing projects that could benefit from the high sensitivity and image fidelity of NOEMA and ALMA.
Carbon chemistry in evolved stars: The Nanocosmos project
Jose Cernicharo (ICMM)
Dust from which planetary systems are formed is produced in the envelopes of red giants. In, and around, the photosphere of these objects, small molecules are formed from which molecular clusters of refractory materials growth. Once these nanoparticles are ejected and escape the stellar gravity, they will grow through their interaction with the gas of the circumstellar envelope created by the star. Depending on the atomic abundance ratio C/O, a different chemistry is produced, with SiC, SiC2 and Si2C as main refractory species in Carbon-rich objects, and silicates in Oxygen rich stars. I will present a brief summary of the physical and chemical processes leading to the formation of dust in red giants and present the main goals of the NANOCOSMOS Project which aims to produce dust analogues in our laboratories by bulding a set of experimental devices that will work near the physical conditions prevailing in the atmosphere of cool red giant stars.
Isotopic fractionation of carbon, deuterium and nitrogen: a full chemical approach
Evelyne Roueff (Paris)
The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an expanding number of isotopically substituted molecules in the pre-stellar environments. I will discuss the basic principles sustaining the possibility of isotopic exchange gas phase reactions and present an updated summary of the various reactions at the light of recent experimental and theoretical studies (Roueff, Loison & Hickson 2015, Grozdanov, Mc Carroll & Roueff 2016). Model results obtained for two different physical conditions that correspond to a moderately dense cloud in an early evolutionary stage and a dense, depleted prestellar core tend to show that ammonia and its singly deuterated form are somewhat enriched in 15N, which agrees with observations. The 14N/15N ratio in N2H+ is found to be close to the elemental value, in contrast to observations showing much larger values. Finally, we find that nitriles and isonitriles may be significantly depleted in 13C, thereby challenging previous interpretations of observed 15N/14N ratios in C15N, HC15N, and H15NC abundances from 13C containing isotopologues. References Roueff, Loison, Hickson, 2015, A&A 576, A99 Grozdanov, McCarroll & Roueff, 2016, submitted
Experimental approach to ortho-to-para ratio of hydrogen and water molecules desorbed from ice at around 10 K
Naoki Watanabe (Hokkaido U.)
Since the radiative transformation of molecular nuclear spins is forbidden in the gas phase, the ortho-to-para abundance ratios (OPRs) of hydrogen and water molecules observed toward various targets have been often considered as tracers of chemical history of those molecules. Especially, the OPR of H2 is crucial for chemical evolution and deuterium fraction of molecules in molecular clouds because H2 in the ortho-ground state (J=1) is more energetic and thus reactive than that in the para-ground state (J=0) by approximately 14.6 meV corresponding to 170 K. In contrast to the gas phase, it was not obvious how the nuclear spins behave on ice dust. It was often assumed without the experimental evidences that the OPR of H2 formed on the dust surface is statistical value of 3 and that the OPRs of H2O molecule astronomically observed in the gas phase indicate the temperatures of icy grains where H2O was formed. Recently, our group has tackled these issues experimentally. Using experimental techniques of molecular beam, photostimulated-desorption, and resonance-enhanced multiphoton ionization, we measured the OPRs of H2 and H2O photodesorbed from ice at around 10 K. We obtained the clear evidence that the OPR of H2 easily varies on ice sensitively depending on the surface temperature and the OPR of H2O formed on ice always shows the statistical value of 3. I will review our recent experiments regarding this.
Cooling dynamics of negative carbon cluster ions studied by an ion storage ring
Toshiyuki Azuma (RIKEN)
The composition of interstellar clouds is heavily influenced by the fate of the molecules after excitation by light absorption or after the collision. In the dilute interstellar space, emission of IR radiation is the only means by which the molecules can stabilize after excitation above the decomposition threshold. From such a context, our new findings of recurrent fluorescence (Poincar.ANi Fluorescence) using an ion storage ring will play a significant role of efficient cooling of the hot molecules in addition to IR radiation. For polyatomic molecules, internal conversion (IC) is a common phenomenon, by which electronic excitation energy is dissipated into the vibrational modes in the time scale on picosecond timescales, and vibrational radiative cooling by IR photon emission follows. What we found is the reversed process of IC, i.e. the inverse internal conversion (IIC) followed by visible photon emission from low-lying electronic excited states. We also introduce a cryogenic electrostatic ion storage ring newly built at RIKEN dedicated to the laboratory astrochemistry. This device enables to keep molecular ions at the temperature of around 5 K, and to observe the de-excitation process and the collision process of them.
New insights on the earliest phase of low-mass star forming regions
Charlotte Vastel (IRAP)
A tremendous advance in instrumentation for spectroscopy of the interstellar medium took place during the last decade. Major facilities such as ALMA, SOFIA and Herschel have been constructed and commissioned, so that science opportunities in the field of astrochemistry have increased by a huge factor. Major discoveries have occurred because of the greater sensitivities of existing telescopes such as IRAM (30 meters and Plateau de Bure), and also because new spectral ranges, so far hidden by the Earth's atmosphere, were finally revealed. The high sensitivity as well as the spectral resolution of the instruments led to the discovery of many species, and the spatial resolution was the key point to uncover the spatial distribution of these species. I will present the recent advances made in the earliest phase of star formation in particular, and will acknowledge the links made between molecular physics and chemistry and the beautiful observations performed with nowadays instruments. As a title I propose "New insights on the earliest phase of low-mass star forming regions"
Chemistry in the disk formation
Y. Aikawa (Univ. Tsukuba), H. Yoneda (Kobe U.), Y. Tsukamoto (RIKEN), K. Furuya (Leiden Obs.)
We investigate the chemistry in a forming disk using the radiation-hydrodynamics model of Tsukamoto et al. (2015). The core evolves from a cold (~10 K) prestellar core to the main accretion phase in ~105 yr. A rotationally supported disk with gravitational instability is formed at the center. We extract the temporal variation of physical parameters of 103 SPH particles which end up near the disk midplane, and calculate a network model of 3-phase gas-grain chemistry as a post process. While stable abundant molecules in the prestellar phase, such as H2O, CH4, NH3 and CH3OH, tend to simply sublimate as the fluid parcel migrates warmer regions, various molecules such as carbon chains and complex organic species are formed in the infalling envelope and rotating disk. Their radial distribution varies depending on the formation path (gas phase or grain-surface reactions) and destruction timescale in the gas phase after the sublimation.
A drastic change in the disk forming regions
Yoko Oya (U. Tokyo)
One of the important frontiers in star-formation studies is to understand when and how rotationally-supported disks are formed around young low-mass protostars. We are conducting molecular observations with ALMA to answer this question. L1527 (d = 137 pc) is a low-mass Class 0-I source, which is known as a prototypical warm-carbon-chain chemistry (WCCC) source characterized by rich carbon-chain molecules. In this source, an infalling-rotating envelope (IRE) is selectively traced by CCH, c-C3H2, and CS, which is beautifully reproduced by a simple ballistic model. The centrifugal barrier of the IRE, which is defined as the perihelion radius in the ballistic model (a half of the centrifugal radius), is clearly identified. On the other hand, SO is found to be distributed in a ring structure around the centrifugal barrier. A similar physical and chemical structure is also seen in the evolved WCCC source TMC-1A (Class I). In addition, we analyzed the ALMA archival data toward IRAS 16293-2422 Source A (d = 120 pc), which is known as a prototypical hot corino source characterized by rich complex organic molecules. In this source, OCS is found to essentially trace the IRE, which can also be explained by the simple ballistic model. On the other hand, CH3OH and HCOOCH3 tend to be distributed around the centrifugal barrier. Moreover, the H2CO lines toward L1527 and IRAS 15398-3359 as well as the H2CS lines toward IRAS 16293-2422 Source A have high-velocity components concentrated to the protostar, suggesting the existence of the rotationally-supported disks. Thus the infalling matter is significantly processed around the centrifugal barrier before being delivered into the rotationally-supported disk. These results provide us an important clue to detailed understandings of the physical and chemical evolution from the IRE to the disk.
Sulfur chemistry in Class 0 disks and jets with ALMA
Linda Podio (Arcetri)
While there have been numerous studies of the disk and jet chemistry in evolved T Tauri stars, observations of the jet-disk system in protostars are very difficult due to their embedded nature and to the occurrence of numerous other kinematical components in the circumstellar region (cavities of swept-up material, infalling envelope, static ambient cloud). I will show how to exploit the combination of the ALMA capabilities and the emission of the sulfur-bearing molecules to simultaneously probe for the first time the forming disk and the jet base in the HH 212 Sun-like protostellar system. SO and SO2 lines turn out to be effective tracers of the collimated and fast molecular jet in the inner few hundreds AU from the protostar. Their abundances indicate that from 1% to 40% of elemental sulfur is converted into SO and SO2 due to shocks in the jet and/or to ambipolar diffusion at the wind base. On the other hand, the SO abundance in the disk is found to be 3 - 4 orders of magnitude larger than in evolved protoplanetary disks around Class II sources. The observed SO enhancement may be produced in the accretion shock at the envelope-disk interface or in the shocks occuring in the disk spirals caused by gravitational instabilities if the disk is partly gravitationally unstable.
From cores to disk: Just follow the gas
Jaime Pineda (MPI)
In our current understanding of low-mass star formation disks, outflows and multiplicity are natural consequences of the collapse of rotating dense cores. However, some of the most important questions still remain unanswered, how much angular momentum needs to be removed to form a disk? When do disks gather most of their mass? How important is the role of outflow feedback? How are multiple systems formed? I will discuss some of the efforts we are carrying out to answer these questions, and I will also discuss how the selection of the "right" tracer can determine the success for some of these projects.
ALMA observations of CO gas depletion in the protoplanetary disk around TW Hya
Hideko Nomura (TITEC)
Protoplanetary disks are the natal place of planets and ALMA observations are now revealing the physical and chemical structure of planet forming regions in the disks. Understanding chemical components of gas, dust and ice in the disks is essential to investigate the origins of materials in the plants. In the talk, I shall report our recent ALMA Band 7 observations of CO isotopologue lines from the protoplanetary disk around TW Hya. The result shows a significant decrement in CO gas throughout the disk even inside the CO snowline, indicating freeze-out of gas-phase CO onto grain surfaces and possible subsequent surface reactions to form larger molecules. Complex organic molecules could be efficiently produced in the observed CO gas depleted regions.
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