Detection of complex nitrogen-bearing molecule ethyl cyanide towards the hot molecular core G10.47 + 0.03

The studies of the complex organic molecular lines towards the hot molecular cores at millimeter and submillimeter wavelengths provide instructive knowledge about the chemical complexity in the interstellar medium (ISM). We present the detection of the rotational emission lines of the complex nitrogen-bearing molecule ethyl cyanide (C2H5CN) towards the chemically rich hot molecular core G10.47 + 0.03 using the Atacama Large Millimeter/Submillimeter Array (ALMA) band 4 observations. The estimated column density of C2H5CN towards the G10.47 + 0.03 is (7.7 ± 0.5) × 1016 cm−2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$10^{16}\text{ cm}^{-2}$\end{document} with the high rotational temperature of 352.9 ± 66.8 K. The estimated fractional abundance of C2H5CN with respect to H2 towards the G10.47 + 0.03 is 5.70 × 10−9\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$10^{-9}$\end{document}. We observe that the estimated fractional abundance of C2H5CN is similar to the existing three-phase warm-up chemical modelling abundance of C2H5CN. We also discuss the possible formation mechanism of C2H5CN towards the hot molecular cores, and we claim the barrierless and exothermic radical-radical reaction between CH2 and CH2CN is responsible for the production of low abundance of C2H5CN (∼10−9\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${\sim} 10^{-9}$\end{document}) in the grain surface of G10.47 + 0.03.


Introduction
In the ISM, or circumstellar shells, more than 270 interstellar complex and prebiotic molecules are identified at mil-

Arijit Manna
Sabyasachi Pal 1 Department of Physics and Astronomy, Midnapore City College, Paschim Medinipur, West Bengal, India 721129 email: arijitmanna@mcconline.org.inlimeter and submillimeter wavelengths 1 .The identification of the molecular lines from the ISM is a key step in the understanding of the chemical evolution from simple molecular species to molecules of biological relevance (Herbst & van Dishoeck 2009).The hot molecular cores are known as the early processes of the high-mass star-formation region, which play a key role in increasing the chemical complexity in the ISM (Shimonishi et al. 2021).At this stage, the complex organic molecules are ejected from the icy surfaces of dust grains (Herbst & van Dishoeck 2009).The hot molecular cores are characterised by their warm temperature (≳100 K), small source size (≤0.1 pc), and high gas density (≳10 6 cm −3 ) (van Dishoeck & Blake 1998; Kurtz et al. 2000).The time scale of the hot molecular cores lies between ∼10 5 years and ∼10 6 years (van Dishoeck & Blake 1998; Garrod & Herbst 2006;Garrod 2013).The hot molecular cores contain the high-velocity water (H 2 O) maser emission, which is located near the ultra-compact (UC) H II regions (Mehringer et al. 2004;Manna & Pal 2022a).The hot core phase is identified by chemically rich molecular emission spectra with many complex organic molecules, including methyl cyanide (CH 3 CN) and methanol (CH 3 OH) (Allen et al. 2017).These molecules may be created in the hot molecular cores at a high temperature (≳150 K) via endothermic chemical reactions (Allen et al. 2017).Using high spectral and spatial resolution data like ALMA, VLA, etc., we can identify the different complex organic molecules and the spatial distribution of those complex molecules in the hot molecular cores.The identification of the disk candidates in the hot molecular cores is very rare, which suggests a correlation between disks and hot molecular core chemistry (Allen et al. 2017).The study of the molecular lines in the disk candidate hot molecular cores can help us understand the highmass star-formation process and chemical evaluation on the small physical scales (≤0.05-0.1 pc) (Allen et al. 2017).The complex organic molecules that contain -C≡N functional group play a key role in prebiotic chemistry because this functional group is responsible for the formation of peptides, nucleic acids, amino acids, and nucleobases of DNA and RNA in the universe (Balucani 2009).The complex nitrogen-bearing molecule ethyl cyanide (C 2 H 5 CN) is also known as propanenitrile, and this is a well-known interstellar molecule which is found in the hot molecular cores and the massive star-formation regions (Mehringer et al. 2004).The dipole moments of the a-and b-conformers of the asymmetric top molecule C 2 H 5 CN are µ a = 3.85 D and µ b = 1.23 D, respectively (Heise et al. 1974).The high dipole moment of C 2 H 5 CN indicates that it exhibits a high intensity and dense rotational spectrum.The C 2 H 5 CN molecules form on the dust grains of high gas density, the warm inner region of the hot molecular cores (Mehringer et al. 2004).The emission lines of C 2 H 5 CN were first detected towards the Orion molecular cloud (OMC-1) and Sgr B2, with estimated column densities of 1.8×10 14 cm −2 and 1.6×10 14 cm −2 , respectively (Johnson et al. 1977).Later, hundreds of the rotational emission lines of C 2 H 5 CN were detected towards the high-mass star-formation regions Sgr B2, Orion, and W51 (Miao & Snyder 1997;Liu et al. 2001).The column density of C 2 H 5 CN towards the Sgr B2 (N) large molecule heimat source or Sgr B2 (N-LMH) hot core reaches 1×10 17 cm −2 (Miao & Snyder 1997).The higher-excited transitional lines of C 2 H 5 CN were also detected in the hot molecular cores Orion KL and Sgr B2 (N) (Daly et al. 2013;Belloche et al. 2013).The vibrationally excited transition lines of C 2 H 5 CN were also detected in the Sgr B2 and in W51 e2 (Mehringer et al. 2004;Demyk et al. 2008).Earlier, three 13 C isotopologues of ethyl cyanide, like 13 CH 3 CH 2 CN, CH 3 13 CH 2 CN, and CH 3 CH 2 13 CN were identified in the Orion hot molecular cloud in the frequency ranges of 80-40 GHz and 160-360 GHz (Demyk et al. 2007).The evidence of the C 2 H 5 CN was also found in the low-mass protostars NGC 1333-IRAS 4A, NGC 1333-IRAS 2A, and IRAS 16293-2422 (Cazaux et al. 2003;Taquet et al. 2015).Recently, evidence of the emission lines of C 2 H 5 CN was also found in the atmosphere of Titan using the ALMA (Cordiner et al. 2014;Manna & Pal 2022b).

Observations and data reductions
We have used the cycle 4 archival data of the hot molecular core G10.47+0.03,which was observed using the highresolution Atacama Large Millimeter/Submillimeter Array (ALMA) (#2016.1.00929.S., PI: Ohishi, Masatoshi).The G10.47+0.03 was located at the observed phase center of (α, δ) J2000 = 18:08:38.232,-19:51:50.400.The observation of G10.47+0.03 was carried out on 28 January 2017, 5 March 2017, 6 March 2017, and 7 March 2017, using the thirty-nine, forty, forty-one, and thirty-nine antennas, respectively.During the observations, the minimum baseline of the antennas was 15 m, and the maximum baseline of the antennas was 331 m.The observations were made with ALMA-band 4 receivers with spectral ranges of 129.50 GHz-160.43GHz and a corresponding angular resolution of 1.67 ′′ -1.76 ′′ .The corresponding spectral resolutions of the observed data were 1128.91 kHz and 488.28 kHz.The flux calibrator was taken as J1733-1304, the bandpass calibrator was taken as J1924-2914, and the phase calibrator was taken as J1832-2039.The observation details are shown in Table 1.
We use the Common Astronomy Software Application (CASA 5.4.1) for the reduction of the ALMA data (Mc-Mullin et al. 2007).We use the Perley-Butler 2017 flux calibrator model to scale the continuum flux density of the flux calibrator using the CASA task SETJY with 5% accuracy (Perley & Butler 2017).We use the task MSTRANSFORM to split the target data with all rest frequencies after the initial data reduction.After the splitting of the target data, we create the continuum images of G10.47+0.03 in all rest frequencies using CASA task TCLEAN with hogbom deconvolar.Now we perform the continuum subtraction procedure from the UV plane using the task UVCONTSUB.After the continuum subtraction, we use the TCLEAN task with a Briggs weighting robust parameter of 0.5 to make the spectral data cubes of G10.47+0.03.Finally, we apply the IMPBCOR task in CASA for the primary beam pattern correction in the synthesised image.Our data analysis procedure is similar to Manna & Pal (2022c) and Manna & Pal (2022d).147.692GHz,147.992 GHz,148.237 GHz,149.044 GHz,153.486 GHz,153.597 GHz,154.038 GHz,154.482 GHz,158.942 GHz,159.414 GHz,159.928 GHz,and 160.155GHz in Figure 1, where the surface brightness colour scale has the unit of Jy beam −1 .We estimate the integrated flux density, peak flux density, synthesised beam size, position angle, and RMS of the G10.47+0.03 to fit a 2D Gaussian over the continuum images using CASA task IMFIT.The estimated continuum image properties of the G10.47+0.03 are shown in Table .2. After fitting the 2D Gaussian, we observe that the synthesised beam size of the continuum image of G10.47+0.03 is not sufficient to resolve the continua.Previously, Rolffs et al. (2011) detected the submillimeter continuum emission from the G10.47+0.03 in the frequency range of 201-691 GHz with the variation of flux density as 6-95 Jy, corresponding to the spectral index 2.8.

Identification of C 2 H 5 CN towards the G10.47+003
We generate the millimeter-wavelength chemically rich spectra from the continuum-subtracted spectral data cubes of G10.47+0.03 to create a circular region with a diameter of 2.5 ′′ centred at RA (J2000) = 18 h 08 m 38 s .232,Dec (J2000) = -19 • 51 ′ 50 ′′ .440.The systematic velocity (V LS R ) of the G10.47+0.03 is 68.50 km s −1 (Rolffs et al. 2011).For identification of the rotational emission lines of C 2 H 5 CN towards the G10.47+0.03,we use the local thermodynamic equilibrium (LTE) model with the Cologne Database for Molecular Spectroscopy (CDMS) (Müller et al. 2005) spectroscopic molecular database.We have used the LTE-RADEX module in CASSIS for LTE computing (Vastel et al. 2015).The LTE assumption is valid in the warm inner region of G10.47+0.03 because the gas density of the hot core region of G10.47+0.03 is 7×10 7 cm −3 (Rolffs et al. 2011).After the LTE analysis, we detect a total of thirty-six transition lines of C 2 H 5 CN within the observable frequency ranges.There are no missing transitions of C 2 H 5 CN between the frequency ranges of 130.  GHz.At the full-beam offset position, the best-fit column density of C 2 H 5 CN is 8.02×10 16 cm −2 with excitation temperature 300 K and source size 2.5 ′′ (beam size of the data cubes).The FWHM of the LTE model spectra of C 2 H 5 CN is 10.2 km s −1 .The LTE-fitted rotational emission lines of C 2 H 5 CN are shown in Figure 2.
Recently, Mondal et al. (2023) also analysed the molecular spectra of G10.47+0.03using the ALMA, and they estimated the column density of C 2 H 5 CN to be (1.7±0.10)×10 17 cm −2 with an excitation temperature of 150 K. Mondal et al. (2023) fit above the second-order polynomial to reduce the noise level of the spectra, which contributed to the measurement of a higher column density of C 2 H 5 CN.Mondal et al.  2023) is confusing.Earlier, Rolffs et al. (2011) showed that the temperature of C 2 H 5 CN emiting region is above 200 K.

Spatial distribution of C 2 H 5 CN
We create the integrated emission maps of non-blended transitions of C 2 H 5 CN towards the G10.47+0.03.To create the integrated emission maps, we use the task IMMOMENTS in CASA.During the run of the task IMMOMENTS, we define the channel ranges of the data cubes where the emission lines of C 2 H 5 CN are detected.The integrated emission maps of C 2 H 5 CN are shown in Figure 3, which are overlaid on the 2.29 mm continuum emission map of G10.47+0.03.After overlaying the continuum emission map over the integrated emission maps of C 2 H 5 CN, we found that the emission maps of C 2 H 5 CN have a peak at the position of the continuum.The resultant non-blended integrated emission maps indicate that the different transitions of the C 2 H 5 CN molecule arise from the highly dense warm inner hot core region of G10.47+0.03.To estimate the emitting regions of C 2 H 5 CN, we apply the task IMFIT for fitting the 2D Gaussian over the integrated emission maps of C 2 H 5 CN.The deconvolved synthesised beam size of the C 2 H 5 CN emitting regions is estimated by the following equation: where θ beam is the half-power width of the synthesised beam of the C 2 H 5 CN integrated emission maps and is the diameter of the circle whose area (A) is surrounding the 50% line peak of C 2 H 5 CN (Manna & Pal 2022c,d).
The estimated emitting regions of C 2 H 5 CN are shown in Table 4.The emitting regions of C 2 H 5 CN are observed in the range of 1.36 ′′ -1.54 ′′ .We notice that the estimated emitting regions of C 2 H 5 CN are comparable to or slightly greater than the synthesised beam size of the integrated emission maps, which indicates the observed transitions of C 2 H 5 CN are not well spatially resolved or, at best, marginally resolved.As a result, no conclusions can be drawn about the morphology of the spatial distribution of C 2 H 5 CN towards the G10.47+0.03.

Rotational diagram analysis of C 2 H 5 CN
We use the rotational diagram model for the estimation of the column density and rotational temperature of the detected non-blended rotational emission lines of C 2 H 5 CN.Initially, we consider that the detected molecular spectra of C 2 H 5 CN are optically thin and obey the local thermodynamic equilibrium (LTE) conditions.The LTE approximation is appropriate in the G10.47+0.03environment because the gas density of the warm inner region of G10.47+0.03 is 7×10 7 cm −3 (Rolffs et al. 2011).The column density of the optically thin molecular emission lines can be written as (Goldsmith & Langer 1999), where g u indicates the degeneracy of the upper energy (E u ), k B represents the Boltzmann constant, T mb dV indicates the integrated intensity, µ is the electric dipole moment, S denote the strength of the detected molecular emission lines, and ν is the rest frequency of the identified emission lines of C 2 H 5 CN.The equation for total column density under LTE conditions is as follows,  After fitting the straight line, the rotational temperature is estimated from the inverse of the slope, and the column density is estimated from the intercept of the slope.For the rotational diagram, we extract the spectral line parameters of the six non-blended rotational emission lines of C 2 H 5 CN using the Gaussian model.We have used the Levenberg-Marquardt (LM)2 algorithm in CASSIS for fitting the Gaussian model over the detected emission lines of C 2 H 5 CN.We observe that the E up of the J = 15(1,15)-14(1,14) and J = 15(0,15)-14(0,14) transition lines of C 2 H 5 CN are nearly similar.So, we use only the J = 15(0,15)-14(0,14) transition line for the rotational diagram because the line intensity of J = 15(0,15)-14(0,14) is higher than J = 15(1,15)-14 (1,14).The best fit non-blended rotational emission lines of C 2 H 5 CN with the Gaussian model is shown in Figure 4 and spectral line fitting parameters are shown in Table 5.
To draw the rotational diagram of non-blended transitions of C 2 H 5 CN, we use the ROTATIONAL DIAGRAM module in CASSIS.The resultant rotational diagram is shown in Figure 5.In the rotational diagram, the blue error bars represent the error bar of ln(N u /g u ), which is calculated from the estimated error of T mb dV by fitting a Gaussian model over the observed emission lines of C 2 H 5 CN.The estimated total column density of C 2 H 5 CN towards the G10.47+0.03 is (7.7±0.5)×10 16cm −2 with the high rotational temperature of 352.9±66.8K. Our estimated rotational temperature indicate that the emission lines of C 2 H 5 CN originated from the hot core region of G10.47+0.03 because Rolffs et al. (2011) claimed the temperature of the hot core of G10.47+0.03 is above 100 K. Our estimated total column density and rotational temperature of C 2 H 5 CN are nearly similar to the LTEfitted column density and excitation temperature of C 2 H 5 CN towards the G10.47+0.03.The fractional abundance of C 2 H 5 CN with respect to H 2 is calculated to be 5.70×10 −9 , and the hydrogen column density towards G10.47+0.03 is calculated to be 1.35×10 25 cm −2 (Gorai et al. 2020).2013) assumed there would be an isothermal collapse phase after a static warm-up phase.In the first phase, the gas density increase from 3×10 3 to 10 7 cm −3 under the free-fall collapse, and the dust temperature decreases from 16 K to 8 K.In the second phase, the gas density remains constant at ∼10 7 cm −3 but the dust temperature fluctuates from 8 K to 400 K.The gas density (n H ) of G10.47+0.03 is 7×10 7 cm −3 (Rolffs et al. 2011) and the temperature of the warm region is ∼150 K (Rolffs et al. 2011).So, the three-phase warm-up chemical modelling of Garrod (2013), which is applied towards hot molecular cores, is suitable for understanding the chemical evolution of C 2 H 5 CN towards G10.47+0.03.In the three-phase warm-up chemical modelling, Garrod (2013) used the fast (5×10 4 -7.12×10 4 years), medium (2×10 5 -2.85×10 5 years), and slow (1×10 6 -1.43×10 6 years) warm-up models based on the time scales.Garrod (2013)  The complex nitrogen-bearing molecule C 2 H 5 CN is produced on the grain surface of the hot molecular cores (Mehringer et al. 2004;Garrod 2013;Garrod et al. 2017Garrod et al. , 2022)).In the first stage (the free-fall collapse phase), the peak abundance of C 2 H 5 CN is ∼10 −9 (Garrod 2013).In the free-fall collapse stage, the addition of radical CH 2 with the radical CH 2 CN produces the radical CH 2 CH 2 CN, and again hydrogenation of the radical CH 2 CH 2 CN produces the low abundance of C 2 H 5 CN (∼10 −9 ) on the grain surface of hot molecular cores (Garrod 2013).The chemical reactions are as follows: CH 2 +CH 2 CN→CH 2 CH 2 CN (1) CH 2 CH 2 CN+H→C 2 H 5 CN (2) Reaction 1 indicates that the addition of radical CH 2 and radical CH 2 CN (alternatively, radical-radical reactions) is barrierless and exothermic (Singh et al. 2021).Our estimated abundance of C 2 H 5 CN towards the G10.47+0.03 is 5.70×10 −9 , which indicates that reactions 1 and 2 are responsible for the production of the C 2 H 5 CN in the grain surface of the G10.47+0.03.Garrod et al. (2017) and Garrod et al. (2022) also used this reaction to simulate the abundance of C 2 H 5 CN in the environment of the other hot molecular cores in the free-fall collapse and warm-up phases.

Conclusion
We analyse the ALMA band 4 data of the hot molecular core G10.47+0.03 and extract the millimeter wavelength rotational molecular lines.The main conclusions of this work are as follows: 1. We detect a total of thirty-six rotational emission lines of C 2 H 5 CN towards the G10.47+0.03using the ALMA band 4 observation.2. The estimated column density of C 2 H 5 CN towards the G10.47+0.03 is (7.7±0.5)×10 16cm −2 with a rotational temperature of 352.9±66.8K.The derived abundance of C 2 H 5 CN with respect to H 2 towards G10.47+0.03 is 5.70×10 −9 .
3. We create the integrated emission maps of non-blended transitions of C 2 H 5 CN.From the emission maps, we observe that the non-blended transitions of C 2 H 5 CN is arising from the warm-inner region of the G10.47+0.03.
4. We also compare our estimated abundance of C 2 H 5 CN with the existing three-phase warm-up chemical modelling abundance of C 2 H 5 CN, which is applied towards particularly hot molecular cores.After the comparison, we found that the derived abundance of C 2 H 5 CN is nearly similar to the modelled abundance of C 2 H 5 CN under the fast warm-up conditions.
5. We also discuss the possible formation mechanism of C 2 H 5 CN towards the G10.47+0.03 and we claim the barrierless and exothermic radical-radical reaction between CH 2 and CH 2 CN is responsible for the production of the C 2 H 5 CN in the grain surface of G10.47+0.03.The identification of the emission lines of C 2 H 5 CN towards the G10.47+0.03indicates that more complex nitrogen-bearing molecules like propyl cyanide (C 3 H 7 CN) are also detectable towards the G10.47+0.03using the ALMA.

Fig. 1
Fig. 1 Millimeter-wavelength continuum emission images of the hot molecular core G10.47+0.03obtained with ALMA band 4 in the frequency range 130.234 GHz-160.155GHz.The contour levels start at 3σ, where σ is the RMS of each continuum image, and the contour levels increase by a factor of √ 2. The red circles indicate the synthesised beam of the continuum images.The corresponding synthesised beam size and RMS of all continuum images are presented in Table.2.
Fig. 1 Millimeter-wavelength continuum emission images of the hot molecular core G10.47+0.03obtained with ALMA band 4 in the frequency range 130.234 GHz-160.155GHz.The contour levels start at 3σ, where σ is the RMS of each continuum image, and the contour levels increase by a factor of √ 2. The red circles indicate the synthesised beam of the continuum images.The corresponding synthesised beam size and RMS of all continuum images are presented in Table.2.
Fig. 2 Rotational emission lines of C 2 H 5 CN with different molecular transitions towards the G10.47+0.03 in the frequency ranges of 130.234-130.949GHz, 147.692-149.044GHz, 153.486-154.482GHz, and 158.942-160.155GHz.The blue spectra indicate the observed millimeter-wavelength molecular spectra of G10.47+0.03,while the red synthetic spectra indicate the best-fitting LTE model over the observed spectra of C 2 H 5 CN.The black lines indicate the rest frequency positions of the C 2 H 5 CN transitions.

Figure
Figure 2 continued.

Fig. 3
Fig. 3 Integrated emission maps of non-blended transitions of C 2 H 5 CN towards the G10.47+0.03.The integrated emission maps of C 2 H 5 CN are overlaid with the 130.773 GHz (2.29 mm) continuum emission map (black contour).The contour levels start at 3σ, where σ is the RMS of each emission map, and the contour levels increase by a factor of √ 2. The cyan circle indicate the synthesised beam of the C 2 H 5 CN integrated emission maps.

Fig. 4
Fig. 4 Non-blended rotational emission lines of C 2 H 5 CN with different molecular transitions towards the G10.47+0.03.The blue spectra indicate the observed millimeter-wavelength molecular spectra of G10.47+0.03,while the red synthetic spectra indicate the best-fitting Gaussian model, which was fitted over the non-blended observed spectra of C 2 H 5 CN.The black lines indicate the rest frequency positions of the C 2 H 5 CN transitions.

Fig. 5
Fig. 5 Rotational diagram of C 2 H 5 CN towards the G10.47+0.03.The blue-filled squares indicate the optically thin approximation data points and blue vertical lines represent the error bars.The best-fit column density and rotational temperature are mentioned inside the image.

Table 2
Summary of the continuum image properties of G10.47+0.03.

Table 3
Summary of the LTE fitting line properties of the C 2 H 5 CN towards the G10.47+0.03
rot ) denotes the partition function as a function of rotational temperature (T rot ) and E u denote the identified molecule's upper energy.The partition functions of C 2 H 5 CN at 75 K, 150 K, and 300 K are 4667.9361,13209.5867,and 37424.5763,respectively.Equation 3 can be rewritten as, /g u ) and E u of the detected emission lines of C 2 H 5 CN.The values ln(N u /g u ) are calculated from equation 2. The column density and rotational temperature of C 2 H 5 CN can be estimated by fitting a straight line over the values of ln(N u /g u ), which are plotted as a function of E u .

Table 5
Summary of the Gaussian fitting line properties of the non-blended emission lines of C 2 H 5 CN towards the G10.47+0.03