ALBERT

Description: In the context of the ASAI (Astrochemical Surveys At IRAM) project, we carried out an unbiased spectral survey in the millimetre window towards the well known low-mass Class I source SVS13-A. The high sensitivity reached (3–12 mK) allowed us to detect at least six HDO broad (full width at half-maximum ~4–5 km s –1 ) emission lines with upper level energies up to E u = 837 K. A non-local thermodynamic equilibrium Large Velocity Gradient (LVG) analysis implies the presence of very hot (150–260 K) and dense (≥3  x  10 7  cm –3 ) gas inside a small radius (~25 au) around the star, supporting, for the first time, the occurrence of a hot corino around a Class I protostar. The temperature is higher than expected for water molecules are sublimated from the icy dust mantles (~100 K). Although we cannot exclude we are observing the effects of shocks and/or winds at such small scales, this could imply that the observed HDO emission is tracing the water abundance jump expected at temperatures ~220–250 K, when the activation barrier of the gas phase reactions leading to the formation of water can be overcome. We derive X ( HDO ) ~ 3  x  10 –6 , and a H 2 O deuteration ≥1.5  x  10 –2 , suggesting that water deuteration does not decrease as the protostar evolves from the Class 0 to the Class I stage.

Description: As part of the Large Program Astrochemical Surveys At IRAM, we have used the IRAM 30 m telescope to lead a systematic search for the emission of rotational transitions of P-bearing species between 80 and 350 GHz towards L1157-B1, a shock position in the solar-type star-forming region L1157. We report the detection of several transitions of PN and, for the first time, of pre-biotic molecule PO. None of these species are detected towards the driving protostar of the outflow L1157-mm. Analysis of the line profiles shows that PN arises from the outflow cavity, where SiO, a strong shock tracer, is produced. Radiative transfer analysis yields an abundance of 2.5  x  10 –9 and 0.9  x  10 –9 for PO and PN, respectively. These results imply a strong depletion (100) of phosphorus in the quiescent cloud gas. Shock modelling shows that atomic N plays a major role in the chemistry of PO and PN. The relative abundance of PO and PN brings constraints both on the duration of the pre-shock phase, which has to be ~10 6 yr, and on the shock parameters. The maximum temperature in the shock has to be larger than 4000 K, which implies a shock velocity of 40 km s –1 .

Description: Formamide (NH 2 CHO) has been proposed as a pre-biotic precursor with a key role in the emergence of life on Earth. While this molecule has been observed in space, most of its detections correspond to high-mass star-forming regions. Motivated by this lack of investigation in the low-mass regime, we searched for formamide, as well as isocyanic acid (HNCO), in 10 low- and intermediate-mass pre-stellar and protostellar objects. The present work is part of the IRAM Large Programme ASAI (Astrochemical Surveys At IRAM), which makes use of unbiased broad-band spectral surveys at millimetre wavelengths. We detected HNCO in all the sources and NH 2 CHO in five of them. We derived their abundances and analysed them together with those reported in the literature for high-mass sources. For those sources with formamide detection, we found a tight and almost linear correlation between HNCO and NH 2 CHO abundances, with their ratio being roughly constant – between 3 and 10 – across 6 orders of magnitude in luminosity. This suggests the two species are chemically related. The sources without formamide detection, which are also the coldest and devoid of hot corinos, fall well off the correlation, displaying a much larger amount of HNCO relative to NH 2 CHO. Our results suggest that, while HNCO can be formed in the gas-phase during the cold stages of star formation, NH 2 CHO forms most efficiently on the mantles of dust grains at these temperatures, where it remains frozen until the temperature rises enough to sublimate the icy grain mantles. We propose hydrogenation of HNCO as a likely formation route leading to NH 2 CHO.

Description: Interstellar molecules with a peptide link (–NH–C(=O)–), like formamide (NH 2 CHO), acetamide (NH 2 COCH 3 ) and isocyanic acid (HNCO), are particularly interesting for their potential role in pre-biotic chemistry. We have studied their emission in the protostellar shock regions L1157-B1 and L1157-B2, with the IRAM 30 m telescope, as part of the ASAI Large Programme. Analysis of the line profiles shows that the emission arises from the outflow cavities associated with B1 and B2. Molecular abundances of (0.4–1.1) 10 –8 and (3.3–8.8) 10 –8 are derived for formamide and isocyanic acid, respectively, from a simple rotational diagram analysis. Conversely, NH 2 COCH 3 was not detected down to a relative abundance of a few ≤10 –10 . B1 and B2 appear to be among the richest Galactic sources of HNCO and NH 2 CHO molecules. A tight linear correlation between their abundances is observed, suggesting that the two species are chemically related. Comparison with astrochemical models favours molecule formation on icy grain mantles, with NH 2 CHO generated from hydrogenation of HNCO.

Description: We present a multiline CS survey towards the brightest bow-shock B1 in the prototypical chemically active protostellar outflow L1157. We made use of (sub-)mm data obtained in the framework of the Chemical HErschel Surveys of Star forming regions and Astrochemical Surveys at IRAM (ASAI) key science programs. We detected 12 C 32 S, 12 C 34 S, 13 C 32 S, and 12 C 33 S emissions, for a total of 18 transitions, with E u up to ~180 K. The unprecedented sensitivity of the survey allows us to carefully analyse the line profiles, revealing high-velocity emission, up to 20 km s –1 with respect to the systemic. The profiles can be well fitted by a combination of two exponential laws that are remarkably similar to what previously found using CO. These components have been related to the cavity walls produced by the ~2000 yr B1 shock and the older (~4000 yr) B2 shock, respectively. The combination of low- and high-excitation CS emission was used to properly sample the different physical components expected in a shocked region. Our CS observations show that this molecule is highlighting the dense, n H 2 = 1–5 10 5  cm –3 , cavity walls produced by the episodic outflow in L1157. In addition, the highest excitation ( E u  ≥ 130 K) CS lines provide us with the signature of denser (1–5 10 6  cm –3 ) gas, associated with a molecular reformation zone of a dissociative J -type shock, which is expected to arise where the precessing jet impacting the molecular cavities. The CS fractional abundance increases up to ~10 –7 in all the kinematical components. This value is consistent with what previously found for prototypical protostars and it is in agreement with the prediction of the abundances obtained via the chemical code Astrochem.

Description: We present the results of a line identification analysis using data from the IRAM Plateau de Bure Plateau de Bure Interferometer, focusing on six massive star-forming hot cores: G31.41+0.31, G29.96–0.02, G19.61–0.23, G10.62–0.38, G24.78+0.08A1 and G24.78+0.08A2. We identify several transitions of vibrationally excited methyl formate (HCOOCH 3 ) for the first time in these objects as well as transitions of other complex molecules, including ethyl cyanide (C 2 H 5 CN), and isocyanic acid (HNCO). We also postulate a detection of one transition of glycolaldehyde (CH 2 (OH)CHO) in two new hot cores. We find G29.96–0.02, G19.61–0.23, G24.78+0.08A1 and G24.78+0.08A2 to be chemically very similar. G31.41+0.31, however, is chemically different: it manifests a larger chemical inventory and has significantly larger column densities. We suggest that it may represent a different evolutionary stage to the other hot cores in the sample, or it may surround a star with a higher mass. We derive column densities for methyl formate in G31.41+0.31, using the rotation diagram method, of 4 10 17  cm –2 and a T rot of ~170 K. For G29.96–0.02, G24.78+0.08A1 and G24.78+0.08A2, glycolaldehyde, methyl formate and methyl cyanide, all seem to trace the same material and peak at roughly the same position towards the dust emission peak. For G31.41+0.31, however, glycolaldehyde shows a different distribution to methyl formate and methyl cyanide and seems to trace the densest, most compact inner part of hot cores.

Description: In a previous study of the L1157 B1 shocked cavity, a comparison between NH 3 (1 0 –0 0 ) and H 2 O(1 10 –1 01 ) transitions showed a striking difference in the profiles, with H 2 O emitting at definitely higher velocities. This behaviour was explained as a result of the high-temperature gas-phase chemistry occurring in the post-shock gas in the B1 cavity of this outflow. If the differences in behaviour between ammonia and water are indeed a consequence of the high gas temperatures reached during the passage of a shock, then one should find such differences to be ubiquitous among chemically rich outflows. In order to determine whether the difference in profiles observed between NH 3 and H 2 O is unique to L1157 or a common characteristic of chemically rich outflows, we have performed Herschel -HIFI observations of the NH 3 (1 0 –0 0 ) line at 572.5 GHz in a sample of eight bright low-mass outflow spots already observed in the H 2 O(1 10 –1 01 ) line within the Water In Star-forming regions with Herschel Key Programme. We detected the ammonia emission at high velocities at most of the outflows positions. In all cases, the water emission reaches higher velocities than NH 3 , proving that this behaviour is not exclusive of the L1157-B1 position. Comparisons with a gas–grain chemical and shock model confirms, for this larger sample, that the behaviour of ammonia is determined principally by the temperature of the gas.

Description: Sulphur-bearing molecules are highly reactive in the gas phase of the interstellar medium. However, the form in which most of the sulphur is locked on to interstellar dust grains is unknown. By taking advantage of the short time-scales of shocks in young molecular outflows, one could track back the main form of sulphur in the ices. In this paper, six transitions of H 2 S and its isotopologues in the L1157-B1 bow shock have been detected using data from the Herschel -Chemical HErschel Surveys of Star forming regions survey and the IRAM-30m Astrochemical Surveys At IRAM large programme. These detections are used to calculate the properties of H 2 S gas in L1157-B1 through use of a rotation diagram and to explore the possible carriers of sulphur on the grains. The isotopologue detections allow the first calculation of the H 2 S deuteration fraction in an outflow from a low-mass protostar. The fractional abundance of H 2 S in the region is found to be 6.0 x 10 –7 and the deuteration fraction is 2 x 10 –2 . In order to investigate the form of sulphur on the grains, a chemical model is run with four different networks, each with different branching ratios for the freeze out of sulphur-bearing species into molecules such as OCS and H 2 S. It is found that the model best fits the data when at least half of each sulphur-bearing species hydrogenates when freezing. We therefore conclude that a significant fraction of sulphur in L1157-B1 is likely to be locked in H 2 S on the grains.

Description: We present high spatial resolution (750 au at 250 pc) maps of the B1 shock in the blue lobe of the L1157 outflow in four lines: CS (3–2), CH 3 OH (3 K –2 K ), HC 3 N (16–15) and p-H 2 CO (2 02 –3 01 ). The combined analysis of the morphology and spectral profiles has shown that the highest velocity gas is confined in a few compact (5 arcsec) bullets, while the lowest velocity gas traces the wall of the gas cavity excavated by the shock expansion. A large velocity gradient model applied to the CS (3–2) and (2–1) lines provides an upper limit of 10 6 cm –3  to the averaged gas density in B1 and a range of $5 \times 10^3\leq n_{\rm H_2} \leq 5 \times 10^5$ cm –3  for the density of the high-velocity bullets. The origin of the bullets is still uncertain: they could be the result of local instabilities produced by the interaction of the jet with the ambient medium or could be clumps already present in the ambient medium that are excited and accelerated by the expanding outflows. The column densities of the observed species can be reproduced qualitatively by the presence in B1 of a C-type shock and only models where the gas reaches temperatures of at least 4000 K can reproduce the observed HC 3 N column density.