Effect of temperature on the gas-phase reaction of CH3CN with OH radicals: experimental (T = 11.7-177.5 K) and computational (T = 10-400 K) kinetic study.

Acetonitrile (CH3CN) is present in the interstellar medium (ISM) in a variety of environments. However, at the ultracold temperatures of the ISM, radical-molecule reactions are not widely investigated because of the experimental handicap of getting organic molecules in the gas phase by conventional techniques. The CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique solves this problem. For this reason, we present in this work the kinetic study of the gas-phase reaction of CH3CN with one of the most ubiquitous radicals, the hydroxyl (OH) radical, as a function of temperature (11.7-177.5 K). The kinetic technique employed to investigate the CH3CN + OH reaction was the pulsed laser photolysis-laser induced fluorescence. The rate coefficient for this reaction k(T) has been observed to drastically increase from 177.5 K to 107.0 K (about 2 orders of magnitude), while the increase in k(T) from 107.0 K to 11.7 K was milder (around 4 times). The temperature dependent expressions for k(T) are provided in the two distinct T-ranges, excluding the upper limit obtained for k(177.5 K): In addition, the rate coefficients estimated by the canonical competitive unified statistical (CCUS) theory show a similar behaviour to the experimental results, when evaluated within the high-pressure limit. This is consistent with the experimentally observed independence of k(T) with total gas density at selected temperatures. Astrochemical networks, such as the KIDA database or UMIST, do not include the CH3CN + OH reaction as a potential depletion process for acetonitrile in the ISM because the current studies predict very low rate coefficients at IS temperatures. According to the model (T = 10 K), the impact of the titled reaction on the abundances of CH3CN appears to be negligible in dark molecular clouds of the ISM (∼1% of the total depletion reactions included in UMIST network). With respect to the potential formation of the CH2CN radical in those environments, even in the most favourable scenario, where this radical could be formed in a 100% yield from the CH3CN + OH reaction, this route would only contribute around 2% to the current assumed formation routes by the UMIST network.

Searches for bridged bicyclic molecules in space-norbornadiene and its cyano derivatives.

The norbornadiene (NBD) molecule, C7H8, owes its fame to its remarkable photoswitching properties that are promising for molecular solar-thermal energy storage systems. Besides this photochemical interest, NBD is a rather unreactive species within astrophysical conditions and it should exhibit high photostability, properties that might also position this molecule as an important constituent of the interstellar medium (ISM)-especially in environments that are well shielded from short-wavelength radiation, such as dense molecular clouds. It is thus conceivable that, once formed, NBD can survive in dense molecular clouds and act as a carbon sink. Following the recent interstellar detections of large hydrocarbons, including several cyano-containing ones, in the dense molecular cloud TMC-1, it is thus logical to consider searching for NBD-which presents a shallow but non-zero permanent electric dipole moment (0.06 D)-as well as for its mono- and dicyano-substituted compounds, referred to as CN-NBD and DCN-NBD, respectively. The pure rotational spectra of NBD, CN-NBD, and DCN-NBD have been measured at 300 K in the 75-110 GHz range using a chirped-pulse Fourier-transform millimetre-wave spectrometer. Of the three species, only NBD was previously studied at high resolution in the microwave domain. From the present measurements, the derived spectroscopic constants enable prediction of the spectra of all three species at various rotational temperatures (up to 300 K) in the spectral range mapped at high resolution by current radio observatories. Unsuccessful searches for these molecules were conducted toward TMC-1 using the QUIJOTE survey, carried out at the Yebes telescope, allowing derivation of the upper limits to the column densities of 1.6 × 1014 cm-2, 4.9 × 1010 cm-2, and 2.9 × 1010 cm-2 for NBD, CN-NBD, and DCN-NBD, respectively. Using CN-NBD and cyano-indene as proxies for the corresponding bare hydrocarbons, this indicates that-if present in TMC-1-NBD would be at least four times less abundant than indene.

Formation of the methyl cation by photochemistry in a protoplanetary disk.

Forty years ago, it was proposed that gas-phase organic chemistry in the interstellar medium can be initiated by the methyl cation CH3+ (refs. 1-3), but so far it has not been observed outside the Solar System4,5. Alternative routes involving processes on grain surfaces have been invoked6,7. Here we report James Webb Space Telescope observations of CH3+ in a protoplanetary disk in the Orion star-forming region. We find that gas-phase organic chemistry is activated by ultraviolet irradiation.

The conformational behavior of N­ethylformamide as observed by rotational spectroscopy and quantum chemistry.

A peptide linkage (CO)NH containing molecule, N-ethylformamide, was investigated by rotational spectroscopy in order to determine the molecular constants of its highest-energy conformer, cis-ac. Its rotational spectrum was observed in two different frequency ranges, in the 4-26 GHz frequency region using a Fourier transform microwave spectrometer and at millimeter wave frequencies between 75 and 116 GHz, employing a broadband high-resolution rotational spectrometer. The measurements at low frequencies allowed to resolve the hyperfine structure components due to nitrogen nuclear quadrupole coupling while the data at higher frequencies provided spectroscopic information about high order centrifugal effects. From a merged fit using all the observational data we have determined a total of thirteen molecular constants that provide a more accurate spectral modelling of the cis-ac conformer and serves a basis for their astronomical search. We have also observed spectra of five singly substituted isotopologues for the cis-ac conformer, three 13C and one for each of 15N and the deuterated species on the N-D position, from which we derived a partial r0 structure, in fair agreement with an ab initio result. In addition, the rotational transitions of the deuterated species of the most stable trans-sc conformer were observed and assigned and three rotational, five centrifugal distortion constants and nuclear quadrupole coupling constants of the nitrogen and deuterium nuclei were determined.

Rotational spectroscopy of the large saturated dinitriles hexanedinitrile and heptanedinitrile.

Dinitriles with a saturated hydrocarbon skeleton and a -C≡N group at each end can have large electric dipole moments. Their formation can be related to highly reactive radicals such as CH2CN, C2N, or CN. Thus, these saturated dinitriles are potential candidates to be observed in the interstellar medium. In this work, two members of this family, hexanedinitrile and heptanedinitrile, have been investigated through their rotational spectra. The jet-cooled broadband chirped-pulse Fourier transform microwave spectra of both molecules were measured in the 2-8 GHz frequency region. Three and six conformers of hexanedinitrile and heptanedinitrile, respectively, were detected and assigned based on the rotational and quadrupole coupling constants.

Metal-catalyst-free gas-phase synthesis of long-chain hydrocarbons.

Development of sustainable processes for hydrocarbons synthesis is a fundamental challenge in chemistry since these are of unquestionable importance for the production of many essential synthetic chemicals, materials and carbon-based fuels. Current industrial processes rely on non-abundant metal catalysts, temperatures of hundreds of Celsius and pressures of tens of bars. We propose an alternative gas phase process under mild reaction conditions using only atomic carbon, molecular hydrogen and an inert carrier gas. We demonstrate that the presence of CH2 and H radicals leads to efficient C-C chain growth, producing micron-length fibres of unbranched alkanes with an average length distribution between C23-C33. Ab-initio calculations uncover a thermodynamically favourable methylene coupling process on the surface of carbonaceous nanoparticles, which is kinematically facilitated by a trap-and-release mechanism of the reactants and nanoparticles that is confirmed by a steady incompressible flow simulation. This work could lead to future alternative sustainable synthetic routes to critical alkane-based chemicals or fuels.

Laboratory microwave spectroscopy of the doubly deuterated cyanomethyl radical, D2CCN.

The microwave spectrum of the doubly deuterated cyanomethyl radical (D2CCN) in its ground electronic state (2 B 1) has been observed for the lowest four rotational transitions (NKa,Kc = 10,1-00,0, 20,2-10,1, 21,2-10,1 and 21,1-11,0) using a Fourier transform microwave spectrometer in combination with a pulsed discharge nozzle. A total of 394 hyperfine components were measured and subjected to a least squares analysis which allowed determining twelve hyperfine constants for nitrogen and deuterium nuclei. With this new set of molecular constants we obtained accurate predictions for low N rotational transitions with hyperfine structure, and searched for this species in TMC-1.

Silicon and Hydrogen Chemistry under Laboratory Conditions Mimicking the Atmosphere of Evolved Stars.

Silicon is present in interstellar dust grains, meteorites and asteroids, and to date thirteen silicon-bearing molecules have been detected in the gas-phase towards late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those at the atmosphere of evolved stars. We have found that the chemistry of Si, H and H2 efficiently produces silane (SiH4), disilane (Si2H6) and amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely detected towards the carbon-rich star IRC+10216, while disilane has not been detected in space yet. Thus, based on our results, we propose that gas-phase reactions of atomic Si with H and H2 are a plausible source of silane in C-rich AGBs, although its contribution to the total SiH4 abundance may be low in comparison with the suggested formation route by catalytic reactions on the surface of dust grains. In addition, the produced a-Si:H dust analogs decompose into SiH4 and Si2H6 at temperatures above 500 K, suggesting an additional mechanism of formation of these species in envelopes around evolved stars. We have also found that the exposure of these dust analogs to water vapor leads to the incorporation of oxygen into Si-O-Si and Si-OH groups at the expense of SiH moieties, which implies that, if this type of grains are present in the interstellar medium, they will be probably processed into silicates through the interaction with water ices covering the surface of dust grains.

Laboratory Observation of, Astrochemical Search for, and Structure of Elusive Erythrulose in the Interstellar Medium.

Rotational spectroscopy provides the most powerful means of identifying molecules of biological interest in the interstellar medium (ISM), but despite their importance, the detection of carbohydrates has remained rather elusive. Here, we present a comprehensive Fourier transform rotational spectroscopic study of elusive erythrulose, a sugar building block likely to be present in the ISM, employing a novel method of transferring the hygroscopic oily carbohydrate into the gas phase. The high sensitivity of the experiment allowed the rotational spectra of all monosubstituted isotopologue species of 13C-12C3H8O4 to be recorded, which, together with quantum chemical calculations, enabled us to determine their equilibrium geometries (reSE) with great precision. Searches employing the new experimental data for erythrulose have been undertaken in different ISM regions, so far including the cold areas Barnard 1, the pre-stellar core TMC-1, Sagittarius B2. Although no lines of erythrulose were found, this data will serve to enable future searches and possible detections in other ISM regions.

INFRA-ICE: An ultra-high vacuum experimental station for laboratory astrochemistry.

Laboratory astrochemistry aims at simulating, in the laboratory, some of the chemical and physical processes that operate in different regions of the universe. Amongst the diverse astrochemical problems that can be addressed in the laboratory, the evolution of cosmic dust grains in different regions of the interstellar medium (ISM) and its role in the formation of new chemical species through catalytic processes present significant interest. In particular, the dark clouds of the ISM dust grains are coated by icy mantles and it is thought that the ice-dust interaction plays a crucial role in the development of the chemical complexity observed in space. Here, we present a new ultra-high vacuum experimental station devoted to simulating the complex conditions of the coldest regions of the ISM. The INFRA-ICE machine can be operated as a standing alone setup or incorporated in a larger experimental station called Stardust, which is dedicated to simulate the formation of cosmic dust in evolved stars. As such, INFRA-ICE expands the capabilities of Stardust allowing the simulation of the complete journey of cosmic dust in space, from its formation in asymptotic giant branch stars to its processing and interaction with icy mantles in molecular clouds. To demonstrate some of the capabilities of INFRA-ICE, we present selected results on the ultraviolet photochemistry of undecane (C11H24) at 14 K. Aliphatics are part of the carbonaceous cosmic dust, and recently, aliphatics and short n-alkanes have been detected in situ in the comet 67P/Churyumov-Gerasimenko.

The Chemistry of Cosmic Dust Analogues from C, C2, and C2H2 in C-Rich Circumstellar Envelopes.

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H2 and CO. In a previous study, we addressed the chemistry of carbon (C and C2) with H2 showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C2) with C2H2. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp2 and sp3 hydrocarbons with amorphous morphology.

Gas-phase kinetics of CH3CHO with OH radicals between 11.7 and 177.5 K.

Gas-phase reactions in the interstellar medium (ISM) are a source of molecules in this environment. The knowledge of the rate coefficient for neutral-neutral reactions as a function of temperature, k(T), is essential to improve astrochemical models. In this work, we have experimentally measured k(T) for the reaction between the OH radical and acetaldehyde, both present in many sources of the ISM. Laser techniques coupled to a CRESU system were used to perform the kinetic measurements. The obtained modified Arrhenius equation is k(T = 11.7-177.5 K) = (1.2 ± 0.2) × 10-11 (T/300 K)-(1.8±0.1) exp-{(28.7 ± 2.5)/T} cm3 molecule-1 s-1. The k(T) value of the title reaction has been measured for the first time below 60 K. No pressure dependence of k(T) was observed at ca. 21, 50, 64 and 106 K. Finally, a pure gas-phase model indicates that the title reaction could become the main CH3CO formation pathway in dark molecular clouds, assuming that CH3CO is the main reaction product at 10 K.

Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes.

Evolved stars are a foundry of chemical complexity, gas and dust that provides the building blocks of planets and life, and dust nucleation first occurs in their photosphere. Despite their importance, the circumstellar regions enveloping these stars remain hidden to many observations, thus dust formation processes are still poorly understood. Laboratory astrophysics provides complementary routes to unveil these chemical processes, but most experiments rely on combustion or plasma decomposition of molecular precursors under physical conditions far removed from those in space. We have built an ultra-high vacuum machine combining atomic gas aggregation with advanced in-situ characterization techniques to reproduce and characterize the bottom-up dust formation process. We show that carbonaceous dust analogues formed from low-pressure gas-phase condensation of C atoms in a hydrogen atmosphere, in a C/H2 ratio similar to that reported for evolved stars, leads to the formation of amorphous C nanograins and aliphatic C-clusters. Aromatic species or fullerenes do not form effectively under these conditions, raising implications for the revision of the chemical mechanisms taking place in circumstellar envelopes.

Alkaline and Alkaline-earth Cyanoacetylides: A Combined Theoretical and Rotational Spectroscopic Investigation.

The metallic cyanoacetylides LiC3N, NaC3N, MgC3N and CaC3N have been investigated by combined spectroscopy measurements and theoretical calculations. The theoretical calculations predict for the four species that the linear isomer with formula MCCCN (M= Li, Na, Mg and Ca) is the most stable one. We used the laser ablation molecular beam Fourier transform microwave spectroscopy to synthesize these species by the reaction of metal vapors, produced by laser ablation, and the 3-bromo-2-propynenitrile (BrCCCN). The pure rotational spectra were observed by Fourier transform microwave spectroscopy in the 2-18 GHz frequency region only for LiCCCN and NaCCCN, while no spectral signatures for MgCCCN and CaCCCN could be detected. Finally, we have searched for LiCCCN and NaCCCN species towards the carbon-rich evolved star IRC + 10216 but only upper limits to their abundances have been obtained.

Oxygen fractionation in dense molecular clouds.

We have developed the first gas-grain chemical model for oxygen fractionation (also including sulphur fractionation) in dense molecular clouds, demonstrating that gas-phase chemistry generates variable oxygen fractionation levels, with a particularly strong effect for NO, SO, O2, and SO2. This large effect is due to the efficiency of the neutral 18O + NO, 18O + SO, and 18O + O2 exchange reactions. The modeling results were compared to new and existing observed isotopic ratios in a selection of cold cores. The good agreement between model and observations requires that the gas-phase abundance of neutral oxygen atoms is large in the observed regions. The S16O/S18O ratio is predicted to vary substantially over time showing that it can be used as a sensitive chemical proxy for matter evolution in dense molecular clouds.

Hints of the existence of C-rich massive evolved stars.

We aim at studying the properties of a particular type of evolved stars, the C-rich evolved stars with high expansion velocities. For this purpose we have focused on the two best studied objects within this group, IRC +10401 and AFGL 2233. We focused on determining their luminosity by studying their spectral energy distribution. Also we have obtained single-dish line profiles and interferometric maps of the CO J=1-0 and J=2-1emission lines for both objects. We have modeled this emission using a LVG radiative transfer code to determine the kinetic temperature and density profiles of the gas ejected by these stars. We have found that the luminosities obtained for these objects (log(L/L ⊙) = 4.1 & 5.4) locate them in the domain of the massive Asymptotic Giant Branch stars (AGBs) and the Red Supergiant stars (RSGs). In addition, the mass-loss rates obtained (1.5 × 10-5 - 6 × 10-3 M ⊙ yr-1) suggest that while IRC+10401 might be an AGB star, AFGL 2233 could be a RSG star. All these results, together with those from previous works suggest that both objects are massive objects, IRC +10401 a massive evolved star with M init ~ 5 - 9M ⊙ which could correspond to an AGB or a RSG and AFGL 2233 a RSG with M init ~ 20M ⊙, which would confirm the existence of massive C-rich evolved stars. Two scenarios are proposed to form these types of objects. The first one capable of producing high mass AGB stars up to ~ 8M ⊙ and a second one capable of forming C-rich RSGs as AFGL 2233.

Building Blocks of Dust: A Coordinated Laboratory and Astronomical Study of AGB Stars.

This article provides an overview of recent astronomical studies and a closely coordinated laboratory program devoted to the study of the physics and chemistry of carbon rich Asymptotic Giant Branch (AGB) stars. The increased sensitivity and angular resolution of high altitude ground-based millimeter-wave interferometers in the past few years has enabled molecular astronomers to determine the excitation and spatial distribution of molecules within a few stellar radii of the central star where the molecular seeds of dust are formed, and to critically assess the physicochemical mechanisms of dust formation and growth. However the astronomical studies are crucially dependent on precise laboratory measurements of the rotational spectra - both in the ground and vibrationally excited states of the normal and rare isotopic species - of the principal molecules in the inner region which appear to contain only two or three heavy atoms Much remains to be done by laboratory spectroscopists as evidenced by the large number of unassigned millimeters-wave rotational lines that are observed in the inner envelope of carbon rich AGB stars. As an illustration we refer to the example of an initial laboratory approach for establishing whether vibrationally excited SiC2 and HCN are the carriers of some of the unassigned features observed in the prototypical carbon rich AGB star IRC+10216 with ALMA. Also highlighted are ongoing laboratory studies of the silicon carbides SiC2 and SiCSi in their ground and excited vibrational states, and SiC3 in the ground vibrational state. Following the initial detection of SiC3 and SiCSi in the outer molecular envelope of IRC+10216, the laboratory spectroscopy was extended to higher frequency in support of the recent interferometric measurements. Thirty-two new millimeter-wave rotational transitions of SiCSi with J ≤ 48, Ka ≤ 3 and upper level energies Eu ≤ 484 K in the range from 178 - 391 GHz, and 35 new transitions of SiC3 with J ≤ 38, Ka ≤ 20 and Eu ≤ 875 K between 315 and 440 GHz were measured in the laboratory. In addition five to six rotational transitions in one quanta of each of the three fundamental vibrational modes of SiCSi, and the two lowest rotational transitions in the previously unexplored C-C stretching mode (ν 1) of SiCC were measured in the normal and doubly substituted 13C isotopic species.

Molecular tracers of radiative feedback in Orion (OMC-1) Widespread CH+ (J = 1-0), CO (10-9), HCN (6-5), and HCO+ (6-5) emission.

Young massive stars regulate the physical conditions, ionization, and fate of their natal molecular cloud and surroundings. It is important to find tracers that help quantifying the stellar feedback processes that take place at different spatial scales. We present ~85 arcmin2 (~1.3 pc2) velocity-resolved maps of several submillimeter molecular lines, taken with Herschel/HIFI, toward the closest high-mass star-forming region, the Orion molecular cloud 1 core (OMC-1). The observed rotational lines include probes of warm and dense molecular gas that are difficult, if not impossible, to detect from ground-based telescopes: CH+ (J = 1-0), CO (J = 10-9), HCO+ (J = 6-5) and HCN (J = 6-5), and CH (N, J =1, 3/2-1, 1/2). These lines trace an extended but thin layer (A V ≃3-6 mag or ~1016 cm) of molecular gas at high thermal pressure, P th = n H · T k ≈ 107 - 109 cm-3 K, associated with the far ultraviolet (FUV) irradiated surface of OMC-1. The intense FUV radiation field, emerging from massive stars in the Trapezium cluster, heats, compresses and photoevaporates the cloud edge. It also triggers the formation of specific reactive molecules such as CH+. We find that the CH+ (J = 1-0) emission spatially correlates with the flux of FUV photons impinging the cloud: G 0 from ~103 to ~105. This correlation is supported by constant-pressure photodissociation region (PDR) models in the parameter space P th/G 0 ≈ [5 · 103 - 8 · 104] cm-3 K where many observed PDRs seem to lie. The CH+ (J = 1-0) emission spatially correlates with the extended infrared emission from vibrationally excited H2 (v ≥ 1), and with that of [C ii] 158 µm and CO J = 10-9, all emerging from FUV-irradiated gas. These correlations link the presence of CH+ to the availability of C+ ions and of FUV-pumped H2 (v ≥ 1) molecules. We conclude that the parsec-scale CH+ emission and narrow-line (Δv ≃ 3 km s-1) mid-J CO emission arises from extended PDR gas and not from fast shocks. PDR line tracers are the smoking gun of the stellar feedback from young massive stars. The PDR cloud surface component in OMC-1, with a mass density of 120-240 M ⊙ pc-2, represents ~5% to ~10% of the total gas mass, however, it dominates the emitted line luminosity; the average CO J = 10-9 surface luminosity in the mapped region being ~35 times brighter than that of CO J = 2-1. These results provide insights into the source of submillimeter CH+ and mid-J CO emission from distant star-forming galaxies.

High-speed molecular cloudlets around the Galactic center's supermassive black hole.

We present 1″-resolution ALMA observations of the circumnuclear disk (CND) and the interstellar environment around Sgr A*. The images unveil the presence of small spatial scale 12CO (J=3-2) molecular "cloudlets" (≲20,000 AU size) within the central parsec of the Milky Way, in other words, inside the cavity of the CND, and moving at high speeds, up to 300 km s-1 along the line-of-sight. The 12CO-emitting structures show intricate morphologies: extended and filamentary at high negative-velocities (vLSR ≲-150 km s-1), more localized and clumpy at extreme positive-velocities (vLSR ≳+200 km s-1). Based on the pencil-beam 12CO absorption spectrum toward Sgr A* synchrotron emission, we also present evidence for a diffuse molecular gas component producing absorption features at more extreme negative-velocities (vLSR <-200 km s-1). The CND shows a clumpy spatial distribution traced by the optically thin H13CN (J=4-3) emission. Its motion requires a bundle of non-uniformly rotating streams of slightly different inclinations. The inferred gas density peaks, molecular cores of a few 105 cm-3, are lower than the local Roche limit. This supports that CND cores are transient. We apply the two standard orbit models, spirals vs. ellipses, invoked to explain the kinematics of the ionized gas streamers around Sgr A*. The location and velocities of the 12CO cloudlets inside the cavity are inconsistent with the spiral model, and only two of them are consistent with the Keplerian ellipse model. Most cloudlets, however, show similar velocities that are incompatible with the motions of the ionized streamers or with gas bounded to the central gravity. We speculate that they are leftovers of more massive molecular clouds that fall into the cavity and are tidally disrupted, or that they originate from instabilities in the inner rim of the CND that lead to fragmentation and infall from there. In either case, we show that molecular cloudlets, all together with a mass of several 10 M ⊙, exist around Sgr A*. Most of them must be short-lived, ≲104 yr: photoevaporated by the intense stellar radiation field, G 0≃105.3 to 104.3, blown away by winds from massive stars in the central cluster, or disrupted by strong gravitational shears.