arXiv subjects

H. S. P. Müller

Publications and source records attributed to H. S. P. Müller.

At least 55 records · Page 3Linked to original sources

arXiv2019

Rotational spectra of isotopic species of methyl cyanide, CH$_3$CN, in their $v_8 = 1$ excited vibrational states

Methyl cyanide is an important trace molecule in space, especially in star-forming regions where it is one of the more common molecules used to derive kinetic temperatures. We want to obtain accurate spectroscopic parameters of minor isotopologs of methyl cyanide in their lowest excited $v_8 = 1$ vibrational states to support astronomical observations, in particular, with interferometers such as ALMA. The laboratory rotational spectrum of methyl cyanide in natural isotopic composition has been recorded from the millimeter to the terahertz regions. Transitions with good signal-to-noise ratios could be identified for the three isotopic species CH$_3^{13}$CN, $^{13}$CH$_3$CN, and CH$_3$C(15)N up to about 1.2 THz ($J'' \le 66$). Accurate spectroscopic parameters were obtained for all three species. The present data were already instrumental in identifying $v_8 = 1$ lines of methyl cyanide with one $^{13}$C in IRAM 30 m and ALMA data toward Sagittarius B2(N).

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arXiv2019

Spectroscopic parameters for silacyclopropynylidene, SiC$_2$, from extensive astronomical observations toward CW Leo (IRC +10216) with the Herschel satellite

A molecular line survey has been carried out toward the carbon-rich asymptotic giant branch star CW Leo employing the HIFI instrument on board of the Herschel satellite. Numerous features from 480 GHz to beyond 1100 GHz could be assigned unambiguously to the fairly floppy SiC$_2$ molecule. However, predictions from laboratory data exhibited large deviations from the observed frequencies even after some lower frequency data from this survey were incorporated into a fit. Therefore, we present a combined fit of all available laboratory data together with data from radio-astronomical observations.

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arXiv2019

Laboratory spectroscopic study of isotopic thioformaldehyde, H$_{2}$CS, and determination of its equilibrium structure

Thioformaldehyde is an abundant molecule in various regions of the interstellar medium. However, available laboratory data limit the accuracies of calculated transition frequencies in the submillimeter region, in particular for minor isotopic species. We aim to determine spectroscopic parameters of isotopologs of H2CS that are accurate enough for predictions well into the submillimeter region. We investigated the laboratory rotational spectra of numerous isotopic species in natural isotopic composition almost continuously between 110 and 377 GHz. Individual lines were studied for most species in two frequency regions between 566 and 930 GHz. Further data were obtained for the three most abundant species in the 1290-1390 GHz region. New or improved spectroscopic parameters were determined for seven isotopic species. Quantum-chemical calculations were carried out to evaluate the differences between ground state and equilibrium rotational parameters to derive semi-empirical equilibrium structural parameters. The spectroscopic parameters are accurate enough for predictions well above 1 THz with the exception of H2(13)C(34)S where the predictions should be reliable to around 700 GHz.

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arXiv2019

Laboratory spectroscopic study of the $^{15}$N isotopomers of cyanamide, H$_2$NCN, and a search for them toward IRAS 16293$-$2422 B

Cyanamide is one of the few interstellar molecules containing two chemically different N atoms. It was detected recently toward the solar-type protostar IRAS 16293-2422 B together with H$_2$N$^{13}$CN and HDNCN in the course of the Atacama Large Millemeter/submillimeter Array (ALMA) Protostellar Interferometric Line Survey (PILS). The detection of the 15N isotopomers or the determination of upper limits to their column densities was hampered by the lack of accurate laboratory data at the frequencies of the survey. We wanted to determine spectroscopic parameters of the $^{15}$N isotopomers of cyanamide that are accurate enough for predictions well into the submillimeter region and to search for them in the PILS data. We investigated the laboratory rotational spectra of H$_2^{15}$NCN and H$_2$NC$^{15}$N in the selected region between 192 and 507~GHz employing a cyanamide sample in natural isotopic composition. Additionally, we recorded transitions of H$_2$N$^{13}$CN. We obtained new or improved spectroscopic parameters for the three isotopic species. Neither of the $^{15}$N isotopomers of cyanamide were detected unambiguously in the PILS data. Two relatively clean lines can be tentatively assigned to H$_2^{15}$NCN. If confirmed, their column densities would imply a low $^{14}$N/$^{15}$N ratio for cyanamide toward this source. The resulting line lists should be accurate enough for observations up to about 1 THz. More sensitive observations, potentially at different frequencies, may eventually lead to the astronomical detection of these isotopic species.

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arXiv2019

The ALMA-PILS survey: First detection of nitrous acid (HONO) in the interstellar medium

Nitrogen oxides are thought to play a significant role as a nitrogen reservoir and to potentially participate in the formation of more complex species. Until now, only NO, N$_2$O and HNO have been detected in the interstellar medium. We report the first interstellar detection of nitrous acid (HONO). Twelve lines were identified towards component B of the low-mass protostellar binary IRAS~16293--2422 with the Atacama Large Millimeter/submillimeter Array, at the position where NO and N$_2$O have previously been seen. A local thermodynamic equilibrium model was used to derive the column density ($\sim$ 9 $\times$ 10$^{14}$ cm$^{-2}$ in a 0.5'' beam) and excitation temperature ($\sim$ 100 K) of this molecule. HNO, NO$_2$, NO$^+$, and HNO$_3$ were also searched for in the data, but not detected. We simulated the HONO formation using an updated version of the chemical code Nautilus and compared the results with the observations. The chemical model is able to reproduce satisfactorily the HONO, N$_2$O, and NO$_2$ abundances, but not the NO, HNO, and NH$_2$OH abundances. This could be due to some thermal desorption mechanisms being destructive and therefore limiting the amount of HNO and NH$_2$OH present in the gas phase. Other options are UV photodestruction of these species in ices or missing reactions potentially relevant at protostellar temperatures.

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arXiv2019

Small-scale physical and chemical structure of diffuse and translucent molecular clouds along the line of sight to Sgr B2

The diffuse and translucent molecular clouds traced in absorption along the line of sight to strong background sources have so far been investigated mainly in the spectral domain because of limited angular resolution or small sizes of the background sources. We aim to resolve and investigate the spatial structure of molecular clouds traced by several molecules detected in absorption along the line of sight to SgrB2(N). We have used spectral line data from the EMoCA survey performed with ALMA, taking advantage of the high sensitivity and angular resolution. We identify, on the basis of the spectral analysis of c-C3H2 across the field of view, 15 main velocity components along the line of sight to SgrB2(N) and several components in the envelope of SgrB2. The c-C3H2 column densities reveal two categories of clouds. Clouds in Category I (3 kpc arm, 4 kpc arm, and some GC clouds) have smaller c-C3H2 column densities, smaller linewidths, and smaller widths of their column density PDFs than clouds in Category II (Scutum arm, Sgr arm, and other GC clouds). To investigate the spatial structure we derive opacity maps for the following molecules: c-C3H2, H13CO+, 13CO, HNC, HN13C, HC15N, CS, C34S, 13CS, SiO, SO, and CH3OH. These maps reveal that most molecules trace relatively homogeneous structures that are more extended than the field of view defined by the background continuum emission (about 15", that is 0.08-0.6pc depending on the distance). SO and SiO show more complex structures with smaller clumps of size ~5-8". Our analysis suggests that the driving of the turbulence is mainly solenoidal in the investigated clouds. On the basis of HCO+, we conclude that most line-of-sight clouds towards SgrB2 are translucent, including all clouds where complex organic molecules were recently detected. We also conclude that CCH and CH are good probes of H2 in both diffuse and translucent clouds.

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arXiv2018

The ALMA-PILS survey: The first detection of doubly-deuterated methyl formate (CHD2OCHO) in the ISM

Studies of deuterated isotopologues of complex organic molecules can provide important constraints on their origin in regions of star formation. In particular, the abundances of deuterated species are very sensitive to the physical conditions in the environment where they form. Due to the low temperatures in regions of star formation, these isotopologues are enhanced to significant levels, making detections of multiply-deuterated species possible. However, for complex organic species, only the multiply-deuterated variants of methanol and methyl cyanide have been reported so far. The aim of this paper is to initiate the characterisation of multiply-deuterated variants of complex organic species with the first detection of doubly-deuterated methyl formate, CHD2OCHO. We use ALMA observations from the Protostellar Interferometric Line Survey (PILS) of the protostellar binary IRAS 16293-2422, in the spectral range of 329.1 GHz to 362.9 GHz. We report the first detection of doubly-deuterated methyl formate CHD2OCHO in the ISM. The D/H ratio of CHD2OCHO is found to be 2-3 times higher than the D/H ratio of CH2DOCHO for both sources, similar to the results for formaldehyde from the same dataset. The observations are compared to a gas-grain chemical network coupled to a dynamical physical model, tracing the evolution of a molecular cloud until the end of the Class 0 protostellar stage. The overall D/H ratio enhancements found in the observations are of the same order of magnitude as the predictions from the model for the early stages of Class 0 protostars. However, the higher D/H ratio of CHD2OCHO compared to the D/H ratio of CH2DOCHO is still not predicted by the model. This suggests that a mechanism is enhancing the D/H ratio of singly- and doubly-deuterated methyl formate that is not in the model, e.g. mechanisms for H-D substitutions.

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arXiv2018

The ALMA-PILS survey: Isotopic composition of oxygen-containing complex organic molecules toward IRAS 16293-2422B

This paper presents a systematic survey of the deuterated and 13C isotopologues of a variety of oxygen-bearing complex organic molecules on Solar System scales toward the protostar IRAS 16293-2422B. We use the data from an unbiased molecular line survey between 329 and 363 GHz from the Atacama Large Millimeter/submillimeter Array (ALMA). The observations probe scales of 60 AU where most of the organic molecules have sublimated off dust grains and are present in the gas-phase. The complex organic molecules can be divided into two groups with one group, the simpler species, showing a D/H ratio of approximately 2% and the other, the more complex species, D/H ratios of 4-8%. This division may reflect the formation time of each species in the ices before or during warm-up/infall of material through the protostellar envelope. No significant differences are seen in the deuteration of different functional groups for individual species, possibly a result of the short time-scale for infall through the innermost warm regions where exchange reactions between different species may be taking place. The species show differences in excitation temperatures between 125 K and 300 K. This likely reflects the binding energies/sublimation temperatures of the individual species, in good agreement to what has previously been found for high-mass sources. For dimethyl ether, the 12C/13C ratio is found to be lower by up to a factor of 2 compared to typical ISM values similar to what has previously been inferred for glycolaldehyde. The results point to the importance of ice surface chemistry for the formation of these complex organic molecules at different stages in the evolution of embedded protostars and demonstrate the use of accurate isotope measurements for understanding the history of individual species.

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arXiv2018

The ALMA-PILS survey: Stringent limits on small amines and nitrogen-oxides towards IRAS 16293-2422B

Hydroxylamine (NH$_{2}$OH) and methylamine (CH$_{3}$NH$_{2}$) have both been suggested as precursors to the formation of amino acids and are therefore of interest to prebiotic chemistry. Their presence in interstellar space and formation mechanisms, however, are not well established. We aim to detect both amines and their potential precursor molecules NO, N$_{2}$O and CH$_{2}$NH towards the low-mass protostellar binary IRAS 16293--2422, in order to investigate their presence and constrain their interstellar formation mechanisms around a young Sun-like protostar. ALMA observations from the unbiased, high angular resolution and sensitivity Protostellar Interferometric Line Survey (PILS) are used. Spectral transitions of the molecules under investigation are searched for with the CASSIS line analysis software. CH$_2$NH and N$_{2}$O are detected for the first time towards a low-mass source, the latter molecule through confirmation with the single-dish TIMASSS survey. NO is also detected. CH$_{3}$NH$_{2}$ and NH$_{2}$OH are not detected and stringent upper limit column densities are determined. The non-detection of CH$_{3}$NH$_{2}$ and NH$_{2}$OH limits the importance of formation routes to amino acids involving these species. The detection of CH$_{2}$NH makes amino acid formation routes starting from this molecule plausible. The low abundances of CH$_2$NH and CH$_{3}$NH$_{2}$ compared to Sgr B2 indicate that different physical conditions influence their formation in low- and high-mass sources.

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arXiv2018

The ALMA-PILS survey: first detection of methyl isocyanide (CH$_3$NC) in a solar-type protostar

Methyl isocyanide (CH$_3$NC) is the isocyanide with the largest number of atoms confirmed in the interstellar medium (ISM), but it is not an abundant molecule, having only been detected towards a handful of objects. Conversely, its isomer, methyl cyanide (CH$_3$CN), is one of the most abundant complex organic molecules detected in the ISM, with detections in a variety of low- and high-mass sources. We use ALMA observations from the Protostellar Interferometric Line Survey (PILS) to search for methyl isocyanide and compare its abundance with that of its isomer methyl cyanide. We use a new line catalogue from the Cologne Database for Molecular Spectroscopy (CDMS) to identify methyl isocyanide lines. We also model the chemistry with an updated version of the three-phase chemical kinetics model {\em MAGICKAL}, presenting the first chemical modelling of methyl isocyanide to date. We detect methyl isocyanide for the first time in a solar-type protostar, IRAS 16293$-$2422 B, and present upper limits for its companion protostar, IRAS 16293$-$2422 A. Methyl isocyanide is found to be at least 20 times more abundant in source B compared to source A, with a CH$_3$CN/CH$_3$NC abundance ratio of 200 in IRAS 16293--2422 B and >5517 in IRAS 16293$-$2422 A. We also present the results of a chemical model of methyl isocyanide chemistry in both sources, and discuss the implications on methyl isocyanide formation mechanisms and the relative evolutionary stages of both sources.

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arXiv2018

The ALMA-PILS survey: Complex nitriles towards IRAS 16293--2422

Complex organic molecules are readily detected in the inner regions of the gaseous envelopes of forming protostars. In particular, molecules that contain nitrogen are interesting due to the role nitrogen plays in the development of life and the compact scales such molecules have been found to trace around forming protostars. The goal of this work is to determine the inventory of one family of nitrogen-bearing organic molecules, complex nitriles (molecules with a $-$CN functional group) towards two hot corino sources in the low-mass protostellar binary IRAS 16293$-$2422. This work explores the abundance differences between the two sources, the isotopic ratios, and the spatial extent derived from molecules containing the nitrile functional group. Using data from the Protostellar Interferometric Line Survey (PILS) obtained with ALMA we determine abundances and excitation temperatures for the detected nitriles. We also present a new method for determining the spatial structure of sources with high line density and large velocity gradients $-$ Velocity-corrected INtegrated emission (VINE) maps. We detect methyl cyanide (CH$_3$CN) as well as 5 of its isotopologues, including the detection of CHD$_2$CN which is the first detection in the ISM. We also detect ethyl cyanide (C$_2$H$_5$CN), vinyl cyanide (C$_2$H$_3$CN), and cyanoacetylene (HC$_3$N). We find that abundances are similar between IRAS 16293A and IRAS 16293B on small scales except for vinyl cyanide which is only detected towards the latter source. This suggests an important difference between the sources either in their evolutionary stage or warm-up timescales. We also detect a spatially double-peaked emission for the first time in molecular emission in the A source, suggesting that this source is showing structure related to a rotating toroid of material.

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arXiv2018

Outflowing OH$^+$ in Markarian 231: the ionization rate of the molecular gas

The oxygen-bearing molecular ions OH+, H2O+, and H3O+ are key species that probe the ionization rate of (partially) molecular gas that is ionized by X-rays and cosmic rays permeating the interstellar medium. We report Herschel far-infrared and submillimeter spectroscopic observations of OH+ in Mrk 231, showing both ground-state P-Cygni profiles, and excited line profiles with blueshifted absorption wings extending up to ~1000 km s^{-1}. In addition, OH+ probes an excited component peaking at central velocities, likely arising from the torus probed by the OH centimeter-wave megamaser. Four lines of H2O+ are also detected at systemic velocities, but H3O+ is undetected. Based on our earlier OH studies, we estimate an abundance ratio of OH/OH+~5-10 for the outflowing components and ~20 for the torus, and an OH+ abundance relative to H nuclei of ~>10^{-7}. We also find high OH+/H2O+ and OH+/H3O+ ratios, both are ~>4 in the torus and ~>10-20 in the outflowing gas components. Chemical models indicate that these high OH+ abundances relative to OH, H2O+, and H3O+ are characteristic of gas with a high ionization rate per unit density, ζ/n_H~(1-5)x10^{-17} cm^3 s^{-1} and ~(1-2)x10^{-16} cm^3 s^{-1} for the above components, respectively, and an ionization rate of ζ~(0.5-2)x10^{-12} s^{-1}. X-rays appear to be unable to explain the inferred ionization rate, and thus we suggest that low-energy (10-400 MeV) cosmic-rays are primarily responsible for the ionization with \dot{M}_{CR}~0.01 M_{sun} yr^{-1} and \dot{E}_{CR}~10^{44} erg s^{-1}, the latter corresponding to 1% of the AGN luminosity and similar to the energetics of the molecular outflow. We suggest that cosmic-rays accelerated in the forward shock associated with the molecular outflow are responsible for the ionization, as they diffuse through the outflowing molecular phase downstream.

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arXiv2017

First detection of cyanamide (NH$_2$CN) towards solar-type protostars

Searches for the prebiotically-relevant cyanamide (NH$_2$CN) towards solar-type protostars have not been reported in the literature. We here present the first detection of this species in the warm gas surrounding two solar-type protostars, using data from the Atacama Large Millimeter/Submillimeter Array Protostellar Interferometric Line Survey (PILS) of IRAS 16293-2422 B and observations from the IRAM Plateau de Bure Interferometer of NGC1333 IRAS2A. We furthermore detect the deuterated and $^{13}$C isotopologues of NH$_2$CN towards IRAS 16293-2422 B. This is the first detection of NHDCN in the interstellar medium. Based on a local thermodynamic equilibrium analysis, we find that the deuteration of cyanamide ($\sim$ 1.7%) is similar to that of formamide (NH$_2$CHO), which may suggest that these two molecules share NH$_2$ as a common precursor. The NH$_2$CN/NH$_2$CHO abundance ratio is about 0.2 for IRAS 16293-2422 B and 0.02 for IRAS2A, which is comparable to the range of values found for Sgr B2. We explored the possible formation of NH$_2$CN on grains through the NH$_2$ + CN reaction using the chemical model MAGICKAL. Grain-surface chemistry appears capable of reproducing the gas-phase abundance of NH$_2$CN with the correct choice of physical parameters.

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arXiv2017

The ALMA-PILS survey: 3D modeling of the envelope, disks and dust filament of IRAS 16293-2422

Context. The Class 0 protostellar binary IRAS 16293-2422 is an interesting target for (sub)millimeter observations due to, both, the rich chemistry toward the two main components of the binary and its complex morphology. Its proximity to Earth allows the study of its physical and chemical structure on solar system scales using high angular resolution observations. Such data reveal a complex morphology that cannot be accounted for in traditional, spherical 1D models of the envelope. Aims. The purpose of this paper is to study the environment of the two components of the binary through 3D radiative transfer modeling and to compare with data from the Atacama Large Millimeter/submillimeter Array. Such comparisons can be used to constrain the protoplanetary disk structures, the luminosities of the two components of the binary and the chemistry of simple species. Methods. We present 13CO, C17O and C18O J=3-2 observations from the ALMA Protostellar Interferometric Line Survey (PILS), together with a qualitative study of the dust and gas density distribution of IRAS 16293-2422. A 3D dust and gas model including disks and a dust filament between the two protostars is constructed which qualitatively reproduces the dust continuum and gas line emission. Results and conclusions. Radiative transfer modeling of source A and B, with the density solution of an infalling, rotating collapse or a protoplanetary disk model, can match the constraints for the disk-like emission around source A and B from the observed dust continuum and CO isotopologue gas emission. If a protoplanetary disk model is used around source B, it has to have an unusually high scale-height in order to reach the dust continuum peak emission value, while fulfilling the other observational constraints. Our 3D model requires source A to be much more luminous than source B; LA ~ 18 $L_\odot$ and LB ~ 3 $L_\odot$.

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arXiv2017

The ALMA-PILS survey: Formaldehyde deuteration in warm gas on small scales toward IRAS 16293-2422 B

[abridged] The enhanced degrees of deuterium fractionation observed in envelopes around protostars demonstrate the importance of chemistry at low temperatures, relevant in pre- and protostellar cores. formaldehyde is an important species in the formation of methanol and more complex molecules. Here, we present the first study of formaldehyde deuteration on small scales around the prototypical low-mass protostar IRAS 16293-2422 using high spatial and spectral resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations. Numerous isotopologues of formaldehyde are detected, among them H$_2$C$^{17}$O, and D$_2^{13}$CO for the first time in the ISM. The large range of upper energy levels covered by the HDCO lines help constrain the excitation temperature to 106$\pm$13 K. Using the derived column densities, formaldehyde shows a deuterium fractionation of HDCO/H$_2$CO=6.5$\pm$1%, D$_2$CO/HDCO=12.8$^{+3.3}_{-4.1}$%, and D$_2$CO/H$_2$CO=0.6(4)$\pm$0.1%. The isotopic ratios derived are $^{16}$O/$^{18}$O=805, $^{18}$O/$^{17}$O=3.2 and $^{12}$C/$^{13}$C=56. The HDCO/H$_2$CO ratio is lower than found in previous studies, highlighting the uncertainties involved in interpreting single dish observations of the inner warm regions. The D$_2$CO/HDCO ratio is only slightly larger than the HDCO/H$_2$CO ratio. This is consistent with formaldehyde forming in the ice as soon as CO has frozen onto the grains, with most of the deuteration happening towards the end of the prestellar core phase. A comparison with available time-dependent chemical models indicates that the source is in the early Class 0 stage.

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arXiv2017

Complex organic molecules in diffuse clouds along the line of sight to Sgr B2

Up to now, mostly relatively simple molecules have been detected in interstellar diffuse molecular clouds in our galaxy, but more complex species have been reported in the diffuse/translucent medium of a z = 0.89 spiral galaxy. We aim at searching for complex organic molecules (COMs) in diffuse molecular clouds along the line of sight to Sgr B2(N), taking advantage of the high sensitivity and angular resolution of the Atacama Large Millimeter/submillimeter Array (ALMA). We use data acquired as part of the EMoCA survey performed with ALMA. To analyse the absorption features of the molecules detected towards the ultracompact HII region K4 in Sgr B2(N), we calculate synthetic spectra for these molecules and fit their column densities, line widths, centroid velocities, and excitation temperatures. We report the detection of CH3OH, CH3CN, CH3CHO, HC3N, and NH2CHO in Galactic center (GC) diffuse clouds and CH3OH and CH3CN in a diffuse cloud in the Scutum arm. The chemical composition of one of the diffuse GC clouds is found to be similar to the one of the diffuse/translucent medium of the z=0.89 spiral galaxy. The chemical processes leading to chemical complexity in the diffuse molecular ISM appear to have remained similar since z=0.89. As proposed in previous studies, the presence of COMs in diffuse molecular clouds may result from a cyclical interstellar process of cloud contraction and expansion between diffuse and dense states.

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arXiv2017

The ALMA-PILS survey: Detection of CH$_{3}$NCO toward the low-mass protostar IRAS 16293-2422 and laboratory constraints on its formation

Methyl isocyanate (CH$_{3}$NCO) belongs to a select group of interstellar molecules considered to be relevant precursors in the formation of larger organic compounds, including those with peptide bonds. The molecule has only been detected in a couple of high-mass protostars and potentially on comets. A formation route on icy grains has been postulated for this molecule but experimental evidence is lacking. Here we extend the range of environments where methyl isocyanate is found, and unambiguously identify CH$_{3}$NCO through the detection of 43 unblended transitions in the ALMA Protostellar Interferometric Line Survey (PILS) of the low mass solar-type protostellar binary IRAS 16293-2422. The molecule is detected toward both components of the binary with a ratio HNCO/CH$_3$NCO $\sim$4--12. The isomers CH$_{3}$CNO and CH$_3$OCN are not identified, resulting in upper abundance ratios of CH$_{3}$NCO/CH$_{3}$CNO > 100 and CH$_{3}$NCO/CH$_3$OCN > 10. The resulting abundance ratios compare well with those found for related N-containing species toward high-mass protostars. To constrain its formation, a set of cryogenic UHV experiments is performed. VUV irradiation of CH$_{4}$:HNCO mixtures at 20 K strongly indicate that methyl isocyanate can be formed in the solid-state through CH$_{3}$ and (H)NCO recombinations. Combined with gas-grain models that include this reaction, the solid-state route is found to be a plausible scenario to explain the methyl isocyanate abundances found in IRAS 16293-2422.

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arXiv2017

Rotational spectroscopy, tentative interstellar detection, and chemical modelling of N-methylformamide

N-methylformamide, CH3NHCHO, may be an important molecule for interstellar pre-biotic chemistry because it contains a peptide bond. The rotational spectrum of the most stable trans conformer of CH3NHCHO is complicated by strong torsion-rotation interaction due to the low barrier of the methyl torsion. We use two absorption spectrometers in Kharkiv and Lille to measure the rotational spectra over 45--630 GHz. The analysis is carried out using the Rho-axis method and the RAM36 code. We search for N-methylformamide toward the hot molecular core Sgr B2(N2) using a spectral line survey carried out with ALMA. The astronomical results are put into a broader astrochemical context with the help of a gas-grain chemical kinetics model. The laboratory data set for the trans conformer of CH3NHCHO consists of 9469 line frequencies with J <= 62, including the first assignment of the rotational spectra of the first and second excited torsional states. All these lines are fitted within experimental accuracy. We report the tentative detection of CH3NHCHO towards Sgr B2(N2). We find CH3NHCHO to be more than one order of magnitude less abundant than NH2CHO, a factor of two less abundant than CH3NCO, but only slightly less abundant than CH3CONH2. The chemical models indicate that the efficient formation of HNCO via NH + CO on grains is a necessary step in the achievement of the observed gas-phase abundance of CH3NCO. Production of CH3NHCHO may plausibly occur on grains either through the direct addition of functional-group radicals or through the hydrogenation of CH3NCO. Provided the detection of CH3NHCHO is confirmed, the only slight underabundance of this molecule compared to its more stable structural isomer acetamide and the sensitivity of the model abundances to the chemical kinetics parameters suggest that the formation of these two molecules is controlled by kinetics rather than thermal equilibrium.

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