arXiv subjects

F. Fontani

Publications and source records attributed to F. Fontani.

At least 91 records · Page 5Linked to original sources

arXiv2018

SOLIS IV. Hydrocarbons in the OMC-2 FIR 4 region, a probe of energetic particle irradiation of the region

We report new interferometric images of cyclopropenylidene, c-C$_3$H$_2$, towards the young protocluster OMC-2 FIR\,4. The observations were performed at 82 and 85 GHz with the NOrthern Extended Millimeter Array (NOEMA) as part of the project Seeds Of Life In Space (SOLIS). In addition, IRAM-30m data observations were used to investigate the physical structure of OMC-2 FIR\,4. We find that the c-C$_3$H$_2$ gas emits from the same region where previous SOLIS observations showed bright HC$_5$N emission. From a non-LTE analysis of the IRAM-30m data, the c-C$_3$H$_2$ gas has an average temperature of $\sim$40K, a H$_2$ density of $\sim$3$\times$10$^{5}$~cm$^{-3}$, and a c-C$_3$H$_2$ abundance relative to H$_2$ of ($7\pm1$)$\times$10$^{-12}$. In addition, the NOEMA observations provide no sign of significant c-C$_3$H$_2$ excitation temperature gradients across the region (about 3-4 beams), with T$_{ex}$ in the range 8$\pm$3 up to 16$\pm$7K. We thus infer that our observations are inconsistent with a physical interaction of the OMC-2 FIR\,4 envelope with the outflow arising from OMC-2 FIR\,3, as claimed by previous studies. The comparison of the measured c-C$_3$H$_2$ abundance with the predictions from an astrochemical PDR model indicates that OMC-2 FIR\,4 is irradiated by a FUV field $\sim$1000 times larger than the interstellar one, and by a flux of ionising particles $\sim$4000 times larger than the canonical value of $1\times10^{-17}$~s$^{-1}$ from the Galaxy cosmic rays, which is consistent with our previous HC$_5$N observations. This provides an important and independent confirmation of other studies that one or more sources inside the OMC-2 FIR\,4 region emit energetic ($\geq10$~MeV) particles.

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arXiv2018

Nitrogen fractionation in high-mass star-forming cores across the Galaxy

The fractionation of nitrogen (N) in star-forming regions is a poorly understood process. To put more stringent observational constraints on the N-fractionation, we have observed with the IRAM-30m telescope a large sample of 66 cores in massive star-forming regions. We targeted the (1-0) rotational transition of HN$^{13}$C, HC$^{15}$N, H$^{13}$CN and HC$^{15}$N, and derived the $^{14}$N/$^{15}$N ratio for both HCN and HNC. We have completed this sample with that already observed by Colzi et al. (2018), and thus analysed a total sample of 87 sources. The $^{14}$N/$^{15}$N ratios are distributed around the Proto-Solar Nebula value with a lower limit near the terrestrial atmosphere value ($\sim$272). We have also derived the $^{14}$N/$^{15}$N ratio as a function of the Galactocentric distance and deduced a linear trend based on unprecedented statistics. The Galactocentric dependences that we have found are consistent, in the slope, with past works but we have found a new local $^{14}$N/$^{15}$N value of $\sim$400, i.e. closer to the Prosolar Nebula value. A second analysis was done, and a parabolic Galactocentric trend was found. Comparison with Galactic chemical evolution models shows that the slope until 8 kpc is consistent with the linear analysis, while the flattening trend above 8 kpc is well reproduced by the parabolic analysis.

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arXiv2018

The fragmentation properties of massive protocluster gas clumps: an ALMA study

Fragmentation of massive dense molecular clouds is the starting point in the formation of rich clusters and massive stars. Theory and numerical simulations indicate that the population of the fragments (number, mass, diameter, separation) resulting from the gravitational collapse of such clumps is probably regulated by the balance between the magnetic field and the other competitors of self-gravity, in particular turbulence and protostellar feedback. We have observed 11 massive, dense and young star-forming clumps with the Atacama Large Millimeter Array (ALMA) in the thermal dust continuum emission at $\sim 1$~mm with an angular resolution of 0.25 arcseconds with the aim of determining their population of fragments. We find fragments on sub-arcsecond scales in 8 out of the 11 sources. The ALMA images indicate two different fragmentation modes: a dominant fragment surrounded by companions with much smaller mass and size, and many ($\geq 8$) fragments with a gradual change in masses and sizes. On average, the largest number of fragments is found towards the warmer and more massive clumps. Also, the warmer clumps tend to form fragments with larger mass and size. To understand the role of the different physical parameters to regulate the final population of the fragments, we have simulated the collapse of a massive clump of $100$ and $300$ M$_{\odot}$ having different magnetic support. The simulations indicate that: (1) fragmentation is inhibited when the initial turbulence is low, independent of the other physical parameters. (2) a filamentary distribution of the fragments is favoured in a highly magnetised clump. We conclude that the clumps that show many fragments distributed in a filamentary-like structure are likely characterised by a strong magnetic field, while the others are possible also in a weaker magnetic field.

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arXiv2018

On the origin of phosphorus nitride in star-forming regions

We present multi-transition observations of PN towards a sample of nine massive dense cores in different evolutionary stages. Using transitions with different excitation conditions, we have found for the first time that the excitation temperatures of PN are in the range 5-30 K. To investigate the main chemical route for the PN formation (surface-chemistry vs. gas-phase chemistry), and the dominant desorption mechanism (thermal vs. shock), we have compared our results with those obtained from molecules tracing different chemical and physical conditions (SiO, SO, CH3OH, and N2H+). We have found that the PN line profiles are very well correlated with those of SiO and SO in six out of the nine targets, which indicates that PN may be released by sputtering of dust grains due to shocks. This finding is corroborated by a faint but statistically significant positive trend between the PN abundance and those of SiO and SO. However, in three objects the PN lines have no hints of high velocity wings, which indicates an alternative origin of PN. Overall, our results indicate that the origin of PN is not unique, as it can be formed in protostellar shocks, but also in colder and more quiescent gas through alternative pathways.

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arXiv2018

Deuterium and $^{15}$N fractionation in N$_2$H$^+$ during the formation of a Sun-like star

Although chemical models predict that the deuterium fractionation in N$_2$H$^+$ is a good evolutionary tracer in the star formation process, the fractionation of nitrogen is still a poorly understood process. Recent models have questioned the similar evolutionary trend expected for the two fractionation mechanisms in N$_2$H$^+$, based on a classical scenario in which ion-neutral reactions occurring in cold gas should have caused an enhancement of the abundance of N$_2$D$^+$, $^{15}$NNH$^+$, and N$^{15}$NH$^+$. In the framework of the ASAI IRAM-30m large program, we have investigated the fractionation of deuterium and $^{15}$N in N$_2$H$^+$ in the best known representatives of the different evolutionary stages of the Sun-like star formation process. The goal is to ultimately confirm (or deny) the classical "ion-neutral reactions" scenario that predicts a similar trend for D and $^{15}$N fractionation. We do not find any evolutionary trend of the $^{14}$N/$^{15}$N ratio from both the $^{15}$NNH$^+$ and N$^{15}$NH$^+$ isotopologues. Therefore, our findings confirm that, during the formation of a Sun-like star, the core evolution is irrelevant in the fractionation of $^{15}$N. The independence of the $^{14}$N/$^{15}$N ratio with time, found also in high-mass star-forming cores, indicates that the enrichment in $^{15}$N revealed in comets and protoplanetary disks is unlikely to happen at core scales. Nevertheless, we have firmly confirmed the evolutionary trend expected for the H/D ratio, with the N$_2$H$^+$/N$_2$D$^+$ ratio decreasing before the pre-stellar core phase, and increasing monotonically during the protostellar phase. We have also confirmed clearly that the two fractionation mechanisms are not related.

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arXiv2018

Similar complex kinematics within two massive, filamentary infrared dark clouds

Infrared dark clouds (IRDCs) are thought to be potential hosts of the elusive early phases of high-mass star formation. Here we conduct an in-depth kinematic analysis of one such IRDC, G034.43+00.24 (Cloud F), using high sensitivity and high spectral resolution IRAM-30m N$_2$H$^+$ ($1-0$) and C$^{18}$O ($1-0$) observations. To disentangle the complex velocity structure within this cloud we use Gaussian decomposition and hierarchical clustering algorithms. We find that four distinct coherent velocity components are present within Cloud F. The properties of these components are compared to those found in a similar IRDC, G035.39-00.33 (Cloud H). We find that the components in both clouds have: high densities (inferred by their identification in N$_2$H$^+$), trans-to-supersonic non-thermal velocity dispersions with Mach numbers of $\sim$ $1.5-4$, a separation in velocity of $\sim$3 km s$^{-1}$, and a mean red-shift of $\sim$ 0.3 km s$^{-1}$ between the N$_2$H$^+$ (dense gas) and C$^{18}$O emission (envelope gas). The latter of these could suggest that these clouds share a common formation scenario. We investigate the kinematics of the larger-scale Cloud F structures, using lower-density-tracing $^{13}$CO ($1-0$) observations. A good correspondence is found between the components identified in the IRAM-30m observations and the most prominent component in the$^{13}$CO data. We find that the IRDC Cloud F is only a small part of a much larger structure, which appears to be an inter-arm filament of the Milky Way.

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arXiv2017

Phosphorus-bearing molecules in the Galactic Center

Phosphorus (P) is one of the essential elements for life due to its central role in biochemical processes. Recent searches have shown that P-bearing molecules (in particular PN and PO) are present in star-forming regions, although their formation routes remain poorly understood. In this Letter, we report observations of PN and PO towards seven molecular clouds located in the Galactic Center, which are characterized by different types of chemistry. PN is detected in five out of seven sources, whose chemistry is thought to be shock-dominated. The two sources with PN non-detections correspond to clouds exposed to intense UV/X-rays/cosmic-ray radiation. PO is detected only towards the cloud G+0.693$-$0.03, with a PO/PN abundance ratio of $\sim$1.5. We conclude that P-bearing molecules likely form in shocked gas as a result of dust grain sputtering, while are destroyed by intense UV/X-ray/cosmic ray radiation.

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arXiv2017

Widespread SiO and CH3OH emission in Filamentary Infrared Dark Clouds

Infrared-Dark Clouds (IRDCs) are cold, dense regions of high (optical and infrared) extinction, believed to be the birthplace of high-mass stars and stellar clusters. The physical mechanisms leading to the formation of these IRDCs are not completely understood and it is thus important to study their molecular gas kinematics and chemical content to search for any signature of the IRDCs formation process. Using the 30m-diameter antenna at the Instituto de Radioastronomía Milimétrica, we have obtained emission maps of dense gas tracers (H$^{13}$CO$^{+}$ and HN$^{13}$C) and typical shock tracers (SiO and CH$_3$OH) toward three IRDCs, G028.37+00.07, G034.43+00.24 and G034.77-00.55 (clouds C, F and G, respectively). We have studied the molecular gas kinematics in these clouds and, consistent with previous works toward other IRDCs, the clouds show complex gas kinematics with several velocity-coherent sub-structures separated in velocity space by a few km s$^{-1}$. Correlated with these complex kinematic structures, widespread (parsec-scale) emission of SiO and CH$_3$OH is present in all the three clouds. For clouds C and F, known to be actively forming stars, widespread SiO and CH$_3$OH is likely associated with on-going star formation activity. However, for cloud G, which lacks either 8 $μ$m or 24 $μ$m sources and 4.5 $μ$m H$_2$ shock-excited emission, the detected widespread SiO and CH$_3$OH emission may have originated in a large-scale shock interaction, although a scenario involving a population of low-mass stars driving molecular outflows cannot be fully ruled out.

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arXiv2017

Seeds Of Life In Space (SOLIS): The organic composition diversity at 300--1000 au scale in Solar-type star forming regions

Complex organic molecules have been observed for decades in the interstellar medium. Some of them might be considered as small bricks of the macromolecules at the base of terrestrial life. It is hence particularly important to understand organic chemistry in Solar-like star forming regions. In this article, we present a new observational project: SOLIS (Seeds Of Life In Space). This is a Large Project at the IRAM-NOEMA interferometer, and its scope is to image the emission of several crucial organic molecules in a sample of Solar-like star forming regions in different evolutionary stage and environments. Here, we report the first SOLIS results, obtained from analysing the spectra of different regions of the Class 0 source NGC1333-IRAS4A, the protocluster OMC-2 FIR4, and the shock site L1157-B1. The different regions were identified based on the images of formamide (NH2CHO) and cyanodiacetylene (HC5N) lines. We discuss the observed large diversity in the molecular and organic content, both on large (3000-10000 au) and relatively small (300-1000 au) scales. Finally, we derive upper limits to the methoxy fractional abundance in the three observed regions of the same order of magnitude of that measured in few cold prestellar objects, namely ~10^-12-10^-11 with respect to H2 molecules.

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arXiv2017

Nitrogen and hydrogen fractionation in high-mass star forming cores from observations of HCN and HNC

The ratio between the two stable isotopes of nitrogen, $^{14}$N and $^{15}$N, is well measured in the terrestrial atmosphere ($\sim 272$), and in the pre-Solar nebula ($\sim 441$). Interestingly, some pristine Solar System materials show enrichments in $^{15}$N with respect to the pre-Solar nebula value. However, it is not yet clear if, and how, these enrichments are linked to the past chemical history, due to the limited number of measurements in dense star-forming regions. In this respect, dense cores believed to be precursors of clusters containing also intermediate- and high-mass stars are important targets, as the Solar System was probably born within a rich stellar cluster. In this work, we show the results of IRAM-30m observations of the J=1-0 rotational transition of the molecules HCN and HNC, and their $^{15}$N-bearing counterparts, towards 27 intermediate/high-mass dense cores divided in three evolutionary categories: high-mass starless cores, high-mass protostellar objects, and ultra-compact HII regions. We have also observed the DNC(2-1) rotational transition, in order to search for a relation between the isotopic ratios D/H and $^{14}$N/$^{15}$N. We derive average $^{14}$N/$^{15}$N ratios of $359\pm16$ in HCN and of $438\pm21$ in HNC, with a dispersion of about 150-200. We find no trend of the $^{14}$N/$^{15}$N ratio with the evolutionary stage. This result agrees with what found from N$_{2}$H$^{+}$ and its isotopologues in the same sources, although the $^{14}$N/$^{15}$N ratios from N$_{2}$H$^{+}$ show a dispersion larger than that in HCN/HNC. Moreover, we have found no correlation between D/H and $^{14}$N/$^{15}$N in HNC. These findings indicate that: (1) the chemical evolution does not seem to play a role in the fractionation of nitrogen; (2) the fractionation of hydrogen and nitrogen in these objects are not related.

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arXiv2017

Seeds of Life in Space (SOLIS) III. Formamide in protostellar shocks: evidence for gas-phase formation

Context: Modern versions of the Miller-Urey experiment claim that formamide (NH$_2$CHO) could be the starting point for the formation of metabolic and genetic macromolecules. Intriguingly, formamide is indeed observed in regions forming Solar-type stars as well as in external galaxies. Aims: How NH$_2$CHO is formed has been a puzzle for decades: our goal is to contribute to the hotly debated question of whether formamide is mostly formed via gas-phase or grain surface chemistry. Methods: We used the NOEMA interferometer to image NH$_2$CHO towards the L1157-B1 blue-shifted shock, a well known interstellar laboratory, to study how the components of dust mantles and cores released into the gas phase triggers the formation of formamide. Results: We report the first spatially resolved image (size $\sim$ 9", $\sim$ 2300 AU) of formamide emission in a shocked region around a Sun-like protostar: the line profiles are blueshifted and have a FWHM $\simeq$ 5 km s$^{-1}$. A column density of $N_{\rm NH_2CHO}$ = 8 $\times$ 10$^{12}$ cm$^{-1}$, and an abundance (with respect to H-nuclei) of 4 $\times$ 10$^{-9}$ are derived. We show a spatial segregation of formamide with respect to other organic species. Our observations, coupled with a chemical modelling analysis, indicate that the formamide observed in L1157-B1 is formed by gas-phase chemical process, and not on grain surfaces as previously suggested. Conclusions: The SOLIS interferometric observations of formamide provide direct evidence that this potentially crucial brick of life is efficiently formed in the gas-phase around Sun-like protostars.

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arXiv2017

The L1157-B1 astrochemical laboratory: testing the origin of DCN

L1157-B1 is the brightest shocked region of the large-scale molecular outflow, considered the prototype of chemically rich outflows, being the ideal laboratory to study how shocks affect the molecular gas. Several deuterated molecules have been previously detected with the IRAM 30m, most of them formed on grain mantles and then released into the gas phase due to the shock. We aim to observationally investigate the role of the different chemical processes at work that lead to formation the of DCN and test the predictions of the chemical models for its formation. We performed high-angular resolution observations with NOEMA of the DCN(2-1) and H13CN(2-1) lines to compute the deuterated fraction, Dfrac(HCN). We detected emission of DCN(2-1) and H13CN(2-1) arising from L1157-B1 shock. Dfrac(HCN) is ~4x10$^{-3}$ and given the uncertainties, we did not find significant variations across the bow-shock. Contrary to HDCO, whose emission delineates the region of impact between the jet and the ambient material, DCN is more widespread and not limited to the impact region. This is consistent with the idea that gas-phase chemistry is playing a major role in the deuteration of HCN in the head of the bow-shock, where HDCO is undetected as it is a product of grain-surface chemistry. The spectra of DCN and H13CN match the spectral signature of the outflow cavity walls, suggesting that their emission result from shocked gas. The analysis of the time dependent gas-grain chemical model UCL-CHEM coupled with a C-type shock model shows that the observed Dfrac(HCN) is reached during the post-shock phase, matching the dynamical timescale of the shock. Our results indicate that the presence of DCN in L1157-B1 is a combination of gas-phase chemistry that produces the widespread DCN emission, dominating in the head of the bow-shock, and sputtering from grain mantles toward the jet impact region.

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arXiv2017

SOLIS II. Carbon-chain growth in the Solar-type protocluster OMC2-FIR4

The interstellar delivery of carbon atoms locked into molecules might be one of the key ingredients for the emergence of life. Cyanopolyynes are carbon chains delimited at their two extremities by an atom of hydrogen and a cyano group, so that they might be excellent reservoirs of carbon. The simplest member, HC3N, is ubiquitous in the galactic interstellar medium and found also in external galaxies. Thus, understanding the growth of cyanopolyynes in regions forming stars similar to our Sun, and what affects it, is particularly relevant. In the framework of the IRAM/NOEMA Large Program SOLIS (Seeds Of Life In Space), we have obtained a map of two cyanopolyynes, HC3N and HC5N, in the protocluster OMC2-FIR4. Because our Sun is thought to be born in a rich cluster, OMC2-FIR4 is one of the closest and best known representatives of the environment in which the Sun may have been born. We find a HC3N/HC5N abundance ratio across the source in the range ~ 1 - 30, with the smallest values (< 10) in FIR5 and in the Eastern region of FIR4. The ratios < 10 can be reproduced by chemical models only if: (1) the cosmic-ray ionisation rate $ζ$ is ~ $4 \times 10^{-14}$ s$^{-1}$; (2) the gaseous elemental ratio C/O is close to unity; (3) O and C are largely depleted. The large $ζ$ is comparable to that measured in FIR4 by previous works and was interpreted as due to a flux of energetic (> 10 MeV) particles from embedded sources. We suggest that these sources could lie East of FIR4 and FIR5. A temperature gradient across FIR4, with T decreasing by about 10 K, could also explain the observed change in the HC3N/HC5N line ratio, without the need of a cosmic ray ionisation rate gradient. However, even in this case, a high constant cosmic-ray ionisation rate (of the order of $10^{-14}$ s$^{-1}$) is necessary to reproduce the observations.

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arXiv2017

15N Fractionation in Infrared-Dark Cloud Cores

Nitrogen is one of the most abundant elements in the Universe and its 14N/15N isotopic ratio has the potential to provide information about the initial environment in which our Sun formed. Recent findings suggest that the Solar System may have formed in a massive cluster since the presence of short-lived radioisotopes in meteorites can only be explained by the influence of a supernova. The aim of this project is to determine the 14N/15N ratio towards a sample of cold, massive dense cores at the initial stages in their evolution. We have observed the J=1-0 transitions of HCN, H13CN, HC15N, HN13C and H15NC toward a sample of 22 cores in 4 Infrared-Dark Clouds (IRDCs). IRDCs are believed to be the precursors of high-mass stars and star clusters. Assuming LTE and a temperature of 15K, the column densities of HCN, H13CN, HC15N, HN13C and H15NC are calculated and their 14N/15N ratio is determined for each core. The 14N/15N ratio measured in our sample of IRDC cores range between ~70 and >763 in HCN and between ~161 and ~541 in HNC. They are consistent with the terrestrial atmosphere (TA) and protosolar nebula (PSN) values, and with the ratios measured in low-mass pre-stellar cores. However, the 14N/15N ratios measured in cores C1, C3, F1, F2 and G2 do not agree with the results from similar studies toward the same massive cores using nitrogen bearing molecules with nitrile functional group (-CN) and nitrogen hydrides (-NH) although the ratio spread covers a similar range. Amongst the 4 IRDCs we measured relatively low 14N/15N ratios towards IRDC G which are comparable to those measured in small cosmomaterials and protoplanetary disks. The low average gas density of this cloud suggests that the gas density, rather than the gas temperature, may be the dominant parameter influencing the initial nitrogen isotopic composition in young PSN.

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arXiv2017

Gas vs. solid phase deuterated chemistry: HDCO and D$_2$CO in massive star-forming regions

The formation of deuterated molecules is favoured at low temperatures and high densities. Therefore, the deuteration fraction D$_{frac}$ is expected to be enhanced in cold, dense prestellar cores and to decrease after protostellar birth. Previous studies have shown that the deuterated forms of species such as N2H+ (formed in the gas phase) and CH3OH (formed on grain surfaces) can be used as evolutionary indicators and to constrain their dominant formation processes and time-scales. Formaldehyde (H2CO) and its deuterated forms can be produced both in the gas phase and on grain surfaces. However, the relative importance of these two chemical pathways is unclear. Comparison of the deuteration fraction of H2CO with respect to that of N2H+, NH3 and CH3OH can help us to understand its formation processes and time-scales. With the new SEPIA Band 5 receiver on APEX, we have observed the J=3-2 rotational lines of HDCO and D2CO at 193 GHz and 175 GHz toward three massive star forming regions hosting objects at different evolutionary stages: two High-mass Starless Cores (HMSC), two High-mass Protostellar Objects (HMPOs), and one Ultracompact HII region (UCHII). By using previously obtained H2CO J=3-2 data, the deuteration fractions HDCO/H2CO and D2CO/HDCO are estimated. Our observations show that singly-deuterated H2CO is detected toward all sources and that the deuteration fraction of H2CO increases from the HMSC to the HMPO phase and then sharply decreases in the latest evolutionary stage (UCHII). The doubly-deuterated form of H2CO is detected only in the earlier evolutionary stages with D2CO/H2CO showing a pattern that is qualitatively consistent with that of HDCO/H2CO, within current uncertainties. Our initial results show that H2CO may display a similar D$_{frac}$ pattern as that of CH3OH in massive young stellar objects. This finding suggests that solid state reactions dominate its formation.

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arXiv2017

Silicon-bearing molecules in the shock L1157-B1: first detection of SiS around a Sun-like protostar

The shock L1157-B1 driven by the low-mass protostar L1157-mm is an unique environment to investigate the chemical enrichment due to molecules released from dust grains. IRAM-30m and Plateau de Bure Interferometer observations allow a census of Si-bearing molecules in L1157-B1. We detect SiO and its isotopologues and, for the first time in a shock, SiS. The strong gradient of the [SiO/SiS] abundance ratio across the shock (from >=180 to ~25) points to a different chemical origin of the two species. SiO peaks where the jet impacts the cavity walls ([SiO/H2] ~ 1e-6), indicating that SiO is directly released from grains or rapidly formed from released Si in the strong shock occurring at this location. In contrast, SiS is only detected at the head of the cavity opened by previous ejection events ([SiS/H2] ~ 2e-8). This suggests that SiS is not directly released from the grain cores but instead should be formed through slow gas-phase processes using part of the released silicon. This finding shows that Si-bearing molecules can be useful to distinguish regions where grains or gas-phase chemistry dominates.

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arXiv2017

Decrease of the organic deuteration during the evolution of Sun-like protostars: the case of SVS13-A

We present the results of formaldehyde and methanol deuteration measurements towards the Class I low-mass protostar SVS13-A, in the framework of the IRAM 30-m ASAI (Astrochemical Surveys At IRAM) project. We detected emission lines of formaldehyde, methanol, and their deuterated forms (HDCO, D2CO, CHD2OH, CH3OD) with Eup up to 276 K. The formaldehyde analysis indicates Tkin = 15 - 30 K, n (H2) >= 10^6 cm^-3, and a size of about 1200 AU suggesting an origin in the protostellar envelope. For methanol we find two components: (i) a high temperature (Tkin = 80 K) and very dense (> 10^8 cm^-3}) gas from a hot corino (radius about 35 AU), and (ii) a colder Tkin <= 70 K) and more extended (radius about 350 AU) region. The deuterium fractionation is 9 10^-2 for HDCO, 4 10^-3 for D2CO, and 2 - 7 10^-3 for CH2DOH, up to two orders of magnitude lower than the values measured in Class 0 sources. We derive also formaldehyde deuteration in the outflow: 4 10^-3, in agreement with what found in the L1157-B1 protostellar shock. Finally, we estimate [CH2DOH]/[CH3OD] about 2. The decrease of deuteration in the Class I source SVS13-A with respect to Class 0 sources can be explained by gas-phase processes. Alternatively, a lower deuteration could be the effect of a gradual collapse of less deuterated external shells of the protostellar evelope. The present measurements fill in the gap between prestellar cores and protoplanetary disks in the context of organics deuteration measurements.

astro-ph.SR↗

arXiv2016

On the chemical ladder of esters. Detection and formation of ethyl formate in the W51 e2 hot molecular core

The detection of organic molecules with increasing complexity and potential biological relevance is opening the possibility to understand the formation of the building blocks of life in the interstellar medium. One of the families of molecules with astrobiological interest are the esters, whose simplest member, methyl formate, is rather abundant in star-forming regions. The next step in the chemical complexity of esters is ethyl formate, C$_2$H$_5$OCHO. Only two detections of this species have been reported so far, which strongly limits our understanding of how complex molecules are formed in the interstellar medium. We have searched for ethyl formate towards the W51 e2 hot molecular core, one of the most chemically rich sources in the Galaxy and one of the most promising regions to study prebiotic chemistry, especially after the recent discovery of the P$-$O bond, key in the formation of DNA. We have analyzed a spectral line survey towards the W51 e2 hot molecular core, which covers 44 GHz in the 1, 2 and 3 mm bands, carried out with the IRAM 30m telescope. We report the detection of the trans and gauche conformers of ethyl formate. A Local Thermodynamic Equilibrium analysis indicates that the excitation temperature is 78$\pm$10 K and that the two conformers have similar source-averaged column densities of (2.0$\pm$0.3)$\times$10$^{16}$ cm$^{-2}$ and an abundance of $\sim$10$^{-8}$. We compare the observed molecular abundances of ethyl formate with different competing chemical models based on grain surface and gas-phase chemistry. We propose that grain-surface chemistry may have a dominant role in the formation of ethyl formate (and other complex organic molecules) in hot molecular cores, rather than reactions in the gas phase.

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