© The Authors 2025

1 Introduction

Life on Earth appeared about 700 million years after planetary formation (~3.8 billion years ago; Pearce et al. 2018), yet the mechanisms driving its origin remain unknown. What is clear is that life depends on recurring chemical motifs. Among them, the amide functional group (−NH−C(O)−) is central to peptide bonds and essential for biochemistry (Pauling et al. 1951).

Understanding how amides appeared on early Earth is vital for tracing the chemical origin of life. While they may have formed through prebiotic processes on Earth (Patel et al. 2015), an alternative scenario involves their formation in space and subsequent delivery by meteorites or comets (Chyba et al. 1990; Chyba & Sagan 1992). Such molecules could have originated in the interstellar medium (ISM), within the parental molecular cloud of the Solar System. This raises the possibility that analogous prebiotic pathways operate in other planetary systems across the Milky Way.

To assess the prebiotic chemistry relevance of amides, it is crucial to determine their presence in star- and planet-forming regions. Formamide (NH₂CHO), the simplest amide and a key prebiotic precursor, has been widely detected across various astrophysical environments (see, e.g., Zheng et al. 2024; Zeng et al. 2023; Liu et al. 2022; Ligterink et al. 2020, and the review by López-Sepulcre et al. 2019 for a comprehensive list of observations). Recent observations further confirmed its prevalence in star-forming regions (Xu et al. 2025). In contrast, acetamide (CH₃CONH₂), a methylated derivative of formamide that represents a potential step toward more complex biomolecules, has been detected in only a handful of star-forming regions, including Sgr B2 (Hollis et al. 2006; Halfen et al. 2011; Belloche et al. 2017; Zheng et al. 2024), NGC 6334I (Ligterink et al. 2020), G31.41+0.31 (Colzi et al. 2021), and Orion KL (Cernicharo et al. 2016), as well as the intermediate-mass protostar Serpens SMM1-a (Ligterink et al. 2022) and the low-mass protostar IRAS 16293-2422B (Ligterink et al. 2017). Notably, no large-sample survey has yet confirmed widespread acetamide emission.

Expanding the number of amide detections is essential in order to evaluate its role in prebiotic chemistry. Although astrochemical models have incorporated potential reaction networks for formamide and acetamide (Belloche et al. 2017, 2019; Garrod et al. 2022), observational constraints – especially for larger amides – remain sparse. To address this gap, we present the first systematic search for acetamide in 52 hot molecular cores (HMCs) using Band 6 observations from the Atacama Large Millimeter/submillimeter Array (ALMA). Our detections substantially increase the number of known acetamide sources, enabling a comparative analysis with formamide. These findings offer new insights into interstellar amide chemistry and its potential connection to prebiotic molecular evolution.

This paper is structured as follows. Section 2 outlines the source sample and the observational setup. Section 3 presents detection results and the derivation of column densities and excitation temperatures. Section 4 discusses chemical correlations and formation pathways. Section 5 summarizes our conclusions.

2 Observations

This study is based on a sample of HMCs summarized in Table 1. The targets were selected from the Querying Underlying mechanisms of massive star formation with ALMA-Resolved gas Kinematics and Structures (QUARKS) survey (Liu et al. 2024) based on HMC candidates previously identified by Qin et al. (2022). QUARKS is a 1.3 mm follow-up to the ALMA Three-millimeter Observations of Massive Star-forming Regions (ATOMS; Liu et al. 2020) and its goal is to resolve substructures within 3 mm core clusters in massive star-forming clumps (Liu et al. 2020).

The QUARKS survey was conducted with the Atacama Compact 7 m Array (ACA) and 12 m arrays at Band 6 toward 139 protoclusters from October 2021 to June 2024. Further details on the observational setup and data reduction are provided in Liu et al. (2024). Briefly, the combined ACA and 12 m array C-2 configuration dataset of this work yields an angular resolution of ~1.3″ and a typical rms noise level of ~5 mJy beam⁻¹. The flux calibration uncertainty is estimated to be ~10%. Four spectral windows (SPWs 1-4), each with a bandwidth of 1.875 GHz, were centered at 217.918, 220.319, 231.370, and 233.520 GHz. The uniform spectral resolution of 976.56 kHz corresponds to a velocity resolution (δV) of 1.2 km s⁻¹, sufficient to spectrally resolve broad molecular lines.

3 Results

3.1 Molecular line identifications

Spectroscopic data for NH₂CHO and its ¹³C isotopolog (NH₂¹³CHO) were obtained from the Cologne Database for Molecular Spectroscopy¹ (CDMS; Müller et al. 2001, 2005), while data for CH₃CONH₂ were adopted from Ilyushin et al. (2004). The spectral modeling was performed using GILDAS² (Grenoble Image and Line Data Analysis Software) to simulate the observed emission, with five parameters: source size, line width, velocity offset, rotational temperature, and column density. Of these, only the rotational temperature and column density were treated as free parameters. Source sizes were based on deconvolved continuum angular sizes; line widths were derived via Gaussian fitting; velocity offsets were calibrated using the CH₃OH line at 218440.063 MHz. Transitions heavily blended with intense lines from other species were excluded from the fitting. For partially blended transitions, multicomponent Gaussian fitting was applied, and the contribution from overlapping species was estimated and subtracted using the best-fit synthetic spectra based on local thermodynamic equilibrium (LTE) modeling.

Although line intensities peak at the continuum center, strong absorption and line blending complicate the analysis. A detailed line-by-line inspection revealed that most CH₃CONH₂ transitions are blended with other species near the continuum peak, while NH₂CHO transitions suffer from significant absorption against bright continuum emission. To improve line identification in such cases, spectra were extracted from offset positions (as shown in Fig. A.1), where most of the lines exhibit Gaussian profiles and are relatively bright compared to other positions.

CH₃CONH₂ was considered confidently detected in ten HMCs (see Table 2), each showing at least five unblended transitions with signal-to-noise ratios ≥3σ that match the modeled rest frequencies and intensities. This criterion minimizes contamination from noise or line blending and ensures robust molecular identification. This constitutes the largest known sample of CH₃CONH₂ detections in HMCs to date and forms the basis for our subsequent analysis. Figure 1 presents representative spectra of CH₃CONH₂, NH₂CHO, and NH₂¹³CHO in I17175 MM2, highlighting clearly resolved or slightly blended transitions. Spectra for the other nine sources with CH₃CONH₂ detections are provided in Fig. B.1. Tables C.1–C.10 list all detected lines for each amide in ten HMCs.

Although several other HMCs also exhibit emission features potentially attributable to CH₃CONH₂, it is insufficient to claim a secure detection of this species in these sources, as two or fewer unblended transitions are observed. Thus, the focus of this study is solely on the secure detection of CH₃CONH₂. Sources that lack conclusive evidence of the existence of this species are discussed in Appendix D.

3.2 Molecular parameter calculations

Column densities and excitation temperatures were derived using rotational diagrams for molecules with at least three unblended transitions spanning a broad energy range (Fig. 2). This method was applied to nine sources with CH₃CONH₂ detections (excluding I18089) and to one source (I18089) with sufficient NH₂¹³CHO transitions.

For CH₃CONH₂ in I18089, the rotational diagram was not employed due to the limited energy range (117.7–136.6 K) of five detected transitions, which precludes a reliable fit. Instead, its column density was estimated from a single unblended line, assuming the excitation temperature derived from NH₂¹³CHO in the same source. Similarly, for sources with fewer than three clean NH₂¹³CHO transitions, column densities were estimated using individual lines and the excitation temperature obtained from CH₃CONH₂. Column densities (N) were calculated under the assumptions of LTE and optically thin emission, following the standard rotational diagram formalism (Li et al. 2022): $$N_{\mathrm{t}}=\frac{8\pi k{\nu }^2}{h{c}^3{A}_{\mathrm{u}\mathrm{l}}}\frac{Q}{g_{\mathrm{u}}}{e}^{E_{\mathrm{u}}/k{T}_{\mathrm{e}\mathrm{x}}}\int T_{\mathrm{m}\mathrm{b}}dv,$$(1)

where k is the Boltzmann constant, ν the transition frequency, h the Planck constant, c the speed of light, A_ul the Einstein emission coefficient, g_u the upper level degeneracy, and E_u the upper-level energy. Due to optical thickness of NH₂CHO lines, their column densities were instead derived from the optically thin NH₂¹³CHO isotopolog, scaled by the appropriate ¹²C/¹³C ratio for each source. These ratios were calculated following the equation described by Yan et al. (2023) and are tabulated in Column 8 of Table 1. While this method effectively avoids opacity-related biases, several factors can introduce uncertainties into the observed ¹²C/¹³C ratios. As noted by Yan et al. (2023), these include distance effects, beam size variations, excitation temperature mismatches, isotope-selective photodissociation, and chemical fractionation. Although the first three effects are likely minor, the last two can be significant in regions with strong UV fields or nonequilibrium chemistry. For example, isotope-selective photodissociation can enhance the ¹²C/¹³C ratio in photon-dominated regions, whereas chemical fractionation can either increase or decrease the abundance of ¹³C-bearing species depending on formation pathways.

Table 2 summarizes the derived column densities and excitation temperatures, along with abundances relative to CH₃OH. The CH₃OH column densities were adopted from Qin et al. (2022). Abundances relative to H₂ are not reported due to uncertainties in dust opacity. CH₃CONH₂ column densities range from (2.5 ± 0.9) × 10¹⁴ to (1.5 ± 0.6) × 10¹⁶ cm⁻², with excitation temperatures between 110 and 178 K. NH₂CHO exhibits consistently higher column densities, ranging from (1.1 ± 0.1) × 10¹⁵ to (6.9 ± 0.4) × 10¹⁶ cm⁻², typically 3–9 times those of CH₃CONH₂.

Fig. 1 Observed (black) and modeled (red) spectra of CH3CONH2, NH2CHO, and NH213CHO in I17175 MM2. Clearly resolved or slightly blended transitions are shown. Dashed green lines mark the 3σ noise level. No modeled spectrum is shown for NH2CHO due to optical thickness. Dashed red lines indicate the rest frequencies of NH2CHO transitions. Spectra for other sources are shown in Fig. B.1.

Fig. 2 Rotational diagrams of CH3CONH2 and NH213CHO. Black points represent the observed data, while red lines indicate the best-fit results. Shaded red regions denote the associated fitting uncertainties. Each panel includes the source name and molecule in the top-right corner, with the derived rotational temperature (Trot) and column density (N) shown in the bottom-left.

Fig. 3 Integrated emission maps of NH2CHO (black) and CH3CONH2 (blue), overlaid with 1.3 mm continuum emission. Contour levels are 10%, 20%, 40%, 60%, and 80% of the peak values. The solid ellipse in the bottom-left corner indicates the synthesized beam for the continuum.

3.3 Spatial distribution

The spatial distributions of NH₂CHO and CH₃CONH₂ were analyzed using the Cube Analysis and Rendering Tool for Astronomy (CARTA; Comrie et al. 2021) to investigate their morphology and potential chemical link. Figure 3 presents the integrated intensity (moment-0) maps of both molecules overlaid on the 1.3 mm continuum emission. All selected transitions were individually checked to avoid contamination from line blending. Overall, amide emissions appear compact and generally coincide with the continuum peaks across all sources, except for I16164, where spatial offsets are observed. This deviation is consistent with the presence of an ultracompact H_II region reported by (Zhang et al. 2023) and similar offsets for other complex organic molecules (COMs) reported in Qin et al. (2022). In all HMCs, CH₃CONH₂ and NH₂CHO show similar spatial distributions, supporting their chemical linkage.

4 Discussion

4.1 Detection and distribution of CH₃CONH₂

CH₃CONH₂, a key interstellar amide following NH₂CHO, has previously been detected in only a few individual sources. Based on unblended emission features, we report its detection in ten HMCs from the QUARKS sample, significantly increasing the number of known CH₃CONH₂ emitters in the ISM (Fig. 4). This findings suggests that complex amides are more widespread in star-forming regions than previously recognized, providing new observational constraints on their potential role in prebiotic chemistry.

4.2 Chemical links between NH₂CHO and CH₃CONH₂

To explore potential chemical connections between CH₃CONH₂ and NH₂CHO, their abundances were normalized to CH₃OH, a standard reference for COMs in the ISM. For our sources, CH₃OH column densities were adopted from Qin et al. (2022). For literature sources, we adopted values from Cernicharo et al. (2016), Bøgelund et al. (2018), Jørgensen et al. (2018), Bonfand et al. (2019), Ligterink et al. (2022), and Mininni et al. (2023), assuming a 20% uncertainty where not specified.

For comparison, we incorporated observational data from both high- and low-mass sources including Sgr B2(N2) (Belloche et al. 2017), G31.41+0.31 (Colzi et al. 2021), Orion KL (Cernicharo et al. 2016), IRAS 16293-2422B (Ligterink et al. 2018), NGC6334I MM1/MM2 (Ligterink et al. 2020), and Serpens SMM1-a (Ligterink et al. 2022), along with relevant chemical models (Garrod et al. 2022) and laboratory experimental result (Ligterink et al. 2018). While NGC6334I MM1/MM2 (Ligterink et al. 2020) correspond to the same physical sources as I17175 MM1/MM2 in our sample, the CH₃CONH₂ column densities derived in this work are lower, likely due to more offset spectral extraction positions. Despite this, the NH₂CHO/CH₃CONH₂ abundance ratios agree well between the two studies (4.5 for MM1 and 3.0 for MM2 in our work vs. 12.3 for MM1 and 3.1 for MM2 in Ligterink et al. 2020), supporting the reliability of our detections. Therefore, values from NGC 6334I were excluded from further statistical analysis to avoid duplication.

As shown in Fig. 5a, CH₃CONH₂ and NH₂CHO abundances exhibit a strong positive correlation, with a best-fit power law: $$X_{{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}}\left({\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{O}\mathrm{N}\mathrm{H}}_2\right)=(0.08\pm 0.07)\times {\mathrm{X}}_{{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}}{\left({\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{H}\mathrm{O}\right)}^{(0.86\pm 0.15)}.$$(2)

This trend indicates a potential chemically linked for the two species across diverse astrophysical environments.

The NH₂CHO/CH₃CONH₂ ratios in our sample range from 2.9 to 8.9, with an average of ~5.6 and a standard deviation of ±1.8, and vary relatively little across sources (Fig. 4b). These values are broadly consistent with those reported for Orion KL and IRAS 16293-2422B, although they lie near the lower end of our observed range. G31.41+0.31, with a ratio of ~2, is significantly lower than our sample average (~5.6), while higher ratios have been reported for Sgr B2(N2) (~25; Belloche et al. 2017) and SMM1-a (~18; Ligterink et al. 2022), exceeding the average of this work by factors of ~4.5, and ~3.2, respectively. The observed discrepancies may arise from multiple factors. While uncertainties in excitation temperatures significantly affect the derived column densities, other contributors include optical depth, line blending, spatial structure, and assumptions of LTE. In this work, potential limitations involve the use of ¹³C isotopologs of NH₂CHO to correct for optical depth effects in its emission lines. Additionally, the adopted assumption of uniform source-filling emission for CH₃CONH₂ may lead to systematic uncertainties. Nevertheless, it is noteworthy that even with these discrepancies, the reported ratios from these works still fall within the same order of magnitude as our study. Given the sensitivity of COM abundances to both temperature and evolutionary stage (Garrod 2013; Garrod et al. 2022), the observed stability suggests that NH₂CHO and CH₃CONH₂ likely form under similar physical and chemical conditions (Quénard et al. 2018; Belloche et al. 2020).

Fig. 4 Spatial distribution of CH3CONH2-detected sources projected onto a schematic top-down view of the Milky Way. Cyan symbols represent sources with CH3CONH2 detections in this work, red symbols sources where CH3CONH2 was not detected, and orange symbols previously reported detections from other studies. Notably, IRAS 17175 MM1/MM2 (this work) and NGC 6334I MM1/MM2 (Ligterink et al. 2020) are actually the same HMCs and are therefore shown exclusively with cyan symbols.

Fig. 5 (a) XCH3OH(CH3CONH2) as a function of XCH3OH(NH2CHO). The cyan line and shaded region denote the power-law fit and its uncertainty derived in this work; the red line and shaded region represent the fit and its uncertainty obtained by combining our sources with previous studies; and the gray line shows the fit from Colzi et al. (2021). Symbols and colors correspond to different sources, as shown in the legend. (b) NH2CHO/CH3CONH2 ratios. Ratios from this work are indicated in cyan, while those from previous studies are shown in different colors. The cyan line and shaded region represent the mean ratio and its uncertainty for our data, and the solid red line and shaded region represent the combined sample ratio.

4.3 Formation pathways of CH₃CONH₂

Several formation pathways have been proposed for CH₃CONH₂. Hollis et al. (2006) suggested a gas-phase route involving the addition of a CH₂ radical to NH₂CHO. However, theoretical studies by Quan & Herbst (2007) indicate that this reaction requires a spin-flip transition of CH₂ from the triplet to singlet state, with an energy barrier exceeding 1000 K – rendering it unlikely under typical interstellar conditions. Other proposed gas-phase reactions, such as radiative association between NH₂CHO and ${\mathrm{C}\mathrm{H}}_3^{+}$ (Quan & Herbst 2007), predict CH₃CONH₂ abundances (~10⁻¹⁵) far below observed levels.

In contrast, grain-surface chemistry offers a more viable formation mechanism. Agarwal et al. (1985) proposed that CH₃CONH₂ can form via the reaction between the CH₃ radical and the NH₂CO intermediate – a key species linking NH₂CHO and CH₃CONH₂. The NH₂CO radical can arise through hydrogen abstraction from NH₂CHO, radical addition of NH₂ and CO, or hydrogenation of HNCO (Belloche et al. 2017, 2019; Garrod et al. 2022). Belloche et al. (2017) predicted an NH₂CHO/CH₃CONH₂ ratio of ~15 assuming only the hydrogen abstraction route. In contrast, Garrod et al. (2022) considered multiple grain-surface formation pathways for CH₃CONH₂, including: (i) CH₃ + NH₂CO → CH₃CONH₂, (ii) H-abstraction from NH₂CHO forming NH₂CO, followed by CH₃ addition, and (iii) hydrogenation of HNCO leading to NH₂CO intermediates. These mechanisms all involve NH₂CO as a key radical intermediate, thereby establishing a direct chemical connection between NH₂CHO and CH₃CONH₂. Our observed abundance ratios (2.9–8.9) align with Garrod et al.’s predicted range (5–13), providing strong support for a linked formation pathway through grain-surface chemistry.

The strong abundance correlation between NH₂CHO and CH₃CONH₂ may also reflect their co-formation on dust grains, followed by thermal desorption under similar physical conditions – paralleling trends observed for HNCO and NH₂CHO (Quénard et al. 2018). Laboratory data further support this view: Ligterink et al. (2018) measured similar desorption temperatures for CH₃CONH₂ (219 K) and NH₂CHO (210 K), while ice mantle simulations yielded a desorption ratio of NH₂CHO/CH₃CONH₂ ~${2.5}_{-1.2}^{+1.3}$, comparable to our average observed value of 5.6.

Taken together, these findings strongly favor a formation scenario in which CH₃CONH₂ is synthesized via grain-surface chemistry closely linked to NH₂CHO, followed by co-desorption in star-forming regions. Nonetheless, additional laboratory, observational, and modeling efforts are required to fully constrain the dominant formation routes.

5 Conclusions

We present the first systematic survey of acetamide (CH₃CONH₂), a key interstellar amide, in HMCs using ALMA-QUARKS data. The joint detections of formamide (NH₂CHO) enable a comparative analysis of their abundance correlation and formation chemistry. Our main findings are as follows:

1. Both NH₂CHO and CH₃CONH₂ were detected in ten HMCs, increasing the number of known CH₃CONH₂ sources to 17 and providing the most extensive sample to date for this molecule;

2. CH₃CONH₂ column densities, derived via rotational diagram analysis, range from (2.5 ± 0.9) × 10¹⁴ to (1.5 ± 0.6) × 10¹⁶ cm⁻². NH₂CHO column densities, estimated from optically thin ¹³C isotopologs, range from (1.1 ± 0.1) × 10¹⁵ to (6.9 ± 0.4) × 10¹⁶ cm⁻² – typically 3–9 times those of CH₃CONH₂;

3. The NH₂CHO/CH₃CONH₂ ratio remains nearly constant (~5.6 on average) across all detected sources. A strong correlation between the two species follows a power-law relation, supporting a chemically linked formation pathway, likely dominated by grain-surface reactions.

These results provide compelling evidence for the presence of complex amide molecules in star-forming regions. The inferred chemical association between NH₂CHO and CH₃CONH₂ highlights the potential role of surface chemistry in assembling peptide-relevant structures prior to planetary formation. The eventual incorporation of such molecules into nascent planetary systems—via cometary or meteoritic delivery – may have contributed essential building blocks for prebiotic chemistry on early Earth and potentially elsewhere. Continued laboratory, observational, and modeling efforts are required to further constrain the dominant formation routes and the broader astrochemical role of interstellar amides.

Data availability

The derived data underlying this article are available in the article and in its online supplementary material on Zenodo.

Acknowledgements

This work makes use of the following ALMA data: ADS/JAO.ALMA#2021.1.00095.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan, China), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. We sincerely thank Prof. Shengli Qin and Dongting Yang (Yunnan University) for their assistance with the data processing. This work was supported by the Fundamental Research Funds for the Central Universities (Grant Nos. 2024CDJGF-025 and 2023CDJXY-045), Chongqing Municipal Natural Science Foundation General Program (Grant No. cstc2021jcyj-msxmX0867), National Natural Science Foundation of China (Grant No. 12103010), and Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB0800303).

Appendix A Spectral extraction positions

To minimize the effects of absorption and line blending, we extracted spectra at offset positions from the continuum peaks for further analysis. The extraction positions for each source are shown in Fig. A.1, where they are marked by white points.

Fig. A.1 1.3 mm continuum image of I15254, I16065, I16164, I16272, I16318, I17008, I17175, I18089, and I18411 obtained with ALMA Band 6. The background shows the continuum emission of each source. Contour levels are drawn at (3, 5, 10, 20, 30, 50, 100, 200) ×σ, where σ is the rms noise level. The 1σ values of I15254, I16065, I16164, I16272, I16318, I17008, I17175, I18089, and I18411 are 1.92, 2.75, 4.13, 1.75, 1.51, 2.80, 6.90, 2.60, and 2.80 mJy beam−1, respectively. The synthetic beam for continuum is indicated in the bottom-left corner by the ellipse. The spectral extraction positions are indicated with a white point.

Appendix B The identified spectral lines of amide molecules

The emissions of CH₃CONH₂, NH₂CHO, and NH₂¹³CHO are detected in ten sources. Figure B.1 show the detected transitions of these amide molecules, which are not covered in the Fig. 1, for all hot cores.

Fig. B.1 Observed (black) and modeled (red) spectra of CH3CONH2, NH2CHO, and NH213CHO in nine HMCs. Clearly resolved or slightly blended transitions are shown. Dashed green lines mark the 3σ noise level. No modeled spectrum is shown for NH2CHO due to optical thickness. Dashed red lines indicate the rest frequencies of NH2CHO transitions. The y-axis shows the line intensity in K, and the x-axis shows the frequency in MHz.

Appendix C Summary of detected molecular lines

This appendix presents the statistics of detected lines in our observations. Tables C.1–C.10, only available on Zenodo, compile all detected line information for each amide species across the ten HMCs. These data provide essential constraints for the derivation of column densities and excitation temperatures.

Appendix D The upper limits of column density for CH₃CONH₂ in undetected sources

For the 42 sources where CH₃CONH₂ was not detected, 3σ upper limits for its column density has been derived. Fig D.1(a) compares these upper limits with the derived column densities in the 10 detections. The upper limits generally fall within an order of magnitude of the detected values, suggesting that sensitivity limitations, rather than a true absence of CH₃CONH₂, can account for many of the non-detections. To further explore this, the relationship between CH₃CONH₂/CH₃OH ratios and CH₃OH column densities have been examined (Fig. D.1b). All ten CH₃CONH₂-detected sources exhibit higher CH₃OH column densities, supporting the idea that CH₃CONH₂ is more likely to be detected in chemically rich environments. Nonetheless, 6 non-detected sources with CH₃OH column densities > 4 × 10¹⁸ cm⁻², show no CH₃CONH₂ detection, suggesting that CH₃OH abundance alone is not a sufficient predictor. These results emphasize the need for higher-sensitivity observations to better constrain the prevalence and formation conditions of interstellar acetamide.

Fig. D.1 (a) Derived column densities (10 sources with CH3CONH2 detections, indicated in red) or upper limits (42 sources without CH3CONH2 detections, indicated in cyan) for all sources. (b) Relationship between the CH3CONH2/CH3OH and CH3OH column densities.

References

All Tables

All Figures

	Fig. 1 Observed (black) and modeled (red) spectra of CH3CONH2, NH2CHO, and NH213CHO in I17175 MM2. Clearly resolved or slightly blended transitions are shown. Dashed green lines mark the 3σ noise level. No modeled spectrum is shown for NH2CHO due to optical thickness. Dashed red lines indicate the rest frequencies of NH2CHO transitions. Spectra for other sources are shown in Fig. B.1.
In the text

	Fig. 2 Rotational diagrams of CH3CONH2 and NH213CHO. Black points represent the observed data, while red lines indicate the best-fit results. Shaded red regions denote the associated fitting uncertainties. Each panel includes the source name and molecule in the top-right corner, with the derived rotational temperature (Trot) and column density (N) shown in the bottom-left.
In the text

	Fig. 3 Integrated emission maps of NH2CHO (black) and CH3CONH2 (blue), overlaid with 1.3 mm continuum emission. Contour levels are 10%, 20%, 40%, 60%, and 80% of the peak values. The solid ellipse in the bottom-left corner indicates the synthesized beam for the continuum.
In the text

Fig. 4 Spatial distribution of CH3CONH2-detected sources projected onto a schematic top-down view of the Milky Way. Cyan symbols represent sources with CH3CONH2 detections in this work, red symbols sources where CH3CONH2 was not detected, and orange symbols previously reported detections from other studies. Notably, IRAS 17175 MM1/MM2 (this work) and NGC 6334I MM1/MM2 (Ligterink et al. 2020) are actually the same HMCs and are therefore shown exclusively with cyan symbols.

In the text

Fig. 5 (a) XCH3OH(CH3CONH2) as a function of XCH3OH(NH2CHO). The cyan line and shaded region denote the power-law fit and its uncertainty derived in this work; the red line and shaded region represent the fit and its uncertainty obtained by combining our sources with previous studies; and the gray line shows the fit from Colzi et al. (2021). Symbols and colors correspond to different sources, as shown in the legend. (b) NH2CHO/CH3CONH2 ratios. Ratios from this work are indicated in cyan, while those from previous studies are shown in different colors. The cyan line and shaded region represent the mean ratio and its uncertainty for our data, and the solid red line and shaded region represent the combined sample ratio.

In the text

Fig. A.1 1.3 mm continuum image of I15254, I16065, I16164, I16272, I16318, I17008, I17175, I18089, and I18411 obtained with ALMA Band 6. The background shows the continuum emission of each source. Contour levels are drawn at (3, 5, 10, 20, 30, 50, 100, 200) ×σ, where σ is the rms noise level. The 1σ values of I15254, I16065, I16164, I16272, I16318, I17008, I17175, I18089, and I18411 are 1.92, 2.75, 4.13, 1.75, 1.51, 2.80, 6.90, 2.60, and 2.80 mJy beam−1, respectively. The synthetic beam for continuum is indicated in the bottom-left corner by the ellipse. The spectral extraction positions are indicated with a white point.

In the text

Fig. B.1 Observed (black) and modeled (red) spectra of CH3CONH2, NH2CHO, and NH213CHO in nine HMCs. Clearly resolved or slightly blended transitions are shown. Dashed green lines mark the 3σ noise level. No modeled spectrum is shown for NH2CHO due to optical thickness. Dashed red lines indicate the rest frequencies of NH2CHO transitions. The y-axis shows the line intensity in K, and the x-axis shows the frequency in MHz.

In the text

	Fig. D.1 (a) Derived column densities (10 sources with CH3CONH2 detections, indicated in red) or upper limits (42 sources without CH3CONH2 detections, indicated in cyan) for all sources. (b) Relationship between the CH3CONH2/CH3OH and CH3OH column densities.
In the text