Introduction
At millimetre and submillimetre wavelengths, approximately 290 prebiotic and complex organic molecules have been discovered in the interstellar medium (ISM) or circumstellar shellsFootnote 1. The identification of complex prebiotic molecules in the ISM is important to understand the chemical evolution of biologically relevant prebiotic molecules from fundamental molecular species (Herbst and van Dishoeck, Reference Herbst and van Dishoeck2009). Hot molecular cores are one of the early stages of high-mass star-formation regions (van Dishoeck and Blake, Reference van Dishoeck and Blake1998; Herbst and van Dishoeck, Reference Herbst and van Dishoeck2009; Shimonishi et al., Reference Shimonishi, Izumi, Furuya and Yasui2021; Manna et al., Reference Manna, Pal, Viti and Sinha2023; Manna and Pal, Reference Manna and Pal2024a). The early stages of the high-mass star-formation regions are known as the chemically rich phase, which plays an essential role in understanding the formation of chemical complexity in the ISM (Tan et al., Reference Tan, Beltrán, Caselli, Fontani, Fuente, Krumholz, McKee and Stolte2014; Shimonishi et al., Reference Shimonishi, Izumi, Furuya and Yasui2021). In the hot molecular cores, the complex organic molecules escape from the icy surfaces of dust grains, or the complex organic molecules are created in the hot circumstellar gas (Herbst and van Dishoeck, Reference Herbst and van Dishoeck2009). Hot molecular cores are identified by their high gas density (>106 cm−3), small source size (<0.1 pc) and warm temperature (>100 K) (van Dishoeck and Blake, Reference van Dishoeck and Blake1998; Kurtz et al., Reference Kurtz, Cesaroni, Churchwell, Hofner and Walmsley2000). The warm-up time scale for hot molecular cores ranges from ~104 to ~106 years (van Dishoeck and Blake, Reference van Dishoeck and Blake1998; Garrod and Herbst, Reference Garrod and Herbst2006; Garrod, Reference Garrod2013). The hot molecular phase is characterized by the rich molecular spectra of several complex organic molecules such as methanol (CH3OH) and methyl cyanide (CH3CN) (Allen et al., Reference Allen, van der Tak, Sánchez-Monge, Cesaroni and Beltrán2017). These complex molecules can form on the surface of dust grains on a cooler surface and are then released when the grains are heated owing to the formation of stars (Allen et al., Reference Allen, van der Tak, Sánchez-Monge, Cesaroni and Beltrán2017; Manna et al., Reference Manna, Pal, Viti and Sinha2023). Alternatively, these complex molecules may be created in massive young objects when the high temperature (>100 K) allows for endothermic reactions (Allen et al., Reference Allen, van der Tak, Sánchez-Monge, Cesaroni and Beltrán2017). Therefore, both formation pathways of complex organic molecules are important for acquiring molecular abundance around the hot molecular cores. Higher spectral and spatial resolution observations are required to identify the different complex organic molecules and the spatial distribution of these molecules in hot molecular cores. The detection of disc candidates in hot molecular cores is extremely rare, implying a link between the hot molecular core chemistry and discs (Allen et al., Reference Allen, van der Tak, Sánchez-Monge, Cesaroni and Beltrán2017). Studying the chemistry of the hot molecular cores of disc candidates can help us to understand the chemical evolution of high-mass star formations on small physical scales (Allen et al., Reference Allen, van der Tak, Sánchez-Monge, Cesaroni and Beltrán2017).
The disc-like hot molecular core G10.47+0.03 is known as the ultra-compact (UC) H II region, which is located at a distance of 8.6 kpc with a luminosity of 5×105 L$_{\odot }$
(Cesaroni et al., Reference Cesaroni, Hofner, Araya and Kurtz2010; Sanna et al., Reference Sanna, Reid and Menten2014). G10.47+0.03 is a disc-like candidate because, in this source, the hot core is embedded in the disc (Sanna et al., Reference Sanna, Reid and Menten2014; Manna and Pal, Reference Manna and Pal2022a). Earlier, Rolffs et al. (Reference Rolffs, Schilke, Zhang and Zapata2011) conducted a molecular spectral line survey of G10.47+0.03, using the Submillimeter Array (SMA) telescope in the frequency range of 199.9–692.2 GHz. Using the local thermodynamic equilibrium (LTE) modelling, Rolffs et al. (Reference Rolffs, Schilke, Zhang and Zapata2011) detected the rotational emission lines of several simple and complex organic molecules such as sulphur monoxide (SO), sulphur dioxide (SO2), cyanide (CN), hydrogen cyanide (HCN), hydrogen isocyanide (HNC), formamide (NH2CHO), cyanoacetylene (HC3N), vinyl cyanide (C2H3CN), formaldehyde (H2CO), ethynol (H2C2O), ethanol (C2H5OH), dimethyl ether (CH3OCH3), methyl formate (CH3OCHO), methanol (CH3OH) and acetone (CH3COH3) towards the G10.47+0.03. The rotational emission lines of methylamine (CH3NH2) and amino acetonitrile (NH2CH2CN) are also detected towards the G10.47+0.03 (Ohishi et al., Reference Ohishi, Suzuki, Hirota, Saito and Kaifu2019; Manna and Pal, Reference Manna and Pal2022a). The CH3NH2 and NH2CH2CN molecules are known to be other possible precursors of the simplest amino acid, NH2CH2COOH, towards hot molecular cores. The emission lines of cyanamide (NH2CN) and ethyl cyanide (C2H5CN) are detected from the hot molecular core G10.47+0.03 using ALMA (Manna and Pal, Reference Manna and Pal2022b, Reference Manna and Pal2023a). Recently, the rotational emission line of phosphorus nitride (PN) is detected towards the G10.47+0.03 (Manna and Pal, Reference Manna and Pal2024b).
The asymmetric top-molecule methylenimine (CH2NH) is known to be a possible precursor of NH2CH2COOH in the ISM. The CH2NH molecule was created by the hydrogenation of HCN on the dust surface of hot molecular cores (Woon, Reference Woon2002; Theule et al., Reference Theule, Borget, Mispelaer, Danger, Duvernay, Guillemin and Chiavassa2011). When CH2NH and HCN both react with each other via the Strecker synthesis reaction, the complex amino and nitrile-bearing molecule amino acetonitrile (NH2CH2CN) is produced (Danger, Reference Danger, Borget, Chomat, Duvernay, Theulé, Guillemin, Le Sergeant D'Hendecourt and Chiavassa2011). The hydrolysis of NH2CH2CN on the grain surface creates glycinamide (O)NH2) (Alonso et al., Reference Alonso, Kolesniková, Białkowska-Jaworska, Kisiel, León, Guillemin and Alonso2018). Finally, NH2CH2COOH can be created via hydrolysis of NH2CH2C(O)NH2 on the grain surface of hot molecular cores (Alonso et al., Reference Alonso, Kolesniková, Białkowska-Jaworska, Kisiel, León, Guillemin and Alonso2018). The proposed formation processes for CH2NH and NH2CH2COOH are shown in Fig. 1. Earlier, Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) claimed that the CH2NH molecule is created in the gas phase between the reactions of CH3 and NH (CH3 + NH → CH2NH). Quantum chemical studies have shown that $\rm {CH_{3}NH_{2}}$
is produced via the sequential hydrogenation of $\rm {CH_{2}NH}$
(Joshi and Lee, Reference Joshi and Lee2022). This indicates that both $\rm {CH_{2}NH}$
and $\rm {CH_{3}NH_{2}}$
are chemically linked in ISM (Joshi and Lee, Reference Joshi and Lee2022). Subsequently, Garrod et al. (Reference Garrod, Jin, Matis, Jones, Willis and Herbst2022) confirmed that both $\rm {CH_{2}NH}$
and $\rm {CH_{3}NH_{2}}$
are chemically linked towards hot molecular cores by using three-phase warm-up chemical models. Both Joshi and Lee (Reference Joshi and Lee2022) and Garrod et al. (Reference Garrod, Jin, Matis, Jones, Willis and Herbst2022) showed that $\rm {CH_{3}NH_{2}}$
and $\rm {CH_{2}NH}$
are chemically connected with $\rm {NH_{2}CH_{2}COOH}$
. Previously, many authors claimed that CH2NH was detected in the high-mass star-formation region Sgr B2. Evidence of CH2NH was found near Sgr B2 (OH) (Godfrey et al., Reference Godfrey, Brown, Robinson and Sinclair1973; Turner, Reference Turner1989), Sgr B2 (N) (Halfen et al., Reference Halfen, Ilyushin and Ziurys2013) and Sgr B2 (M) (Sutton et al., Reference Sutton, Jaminet, Danchi and Blake1991). Previously, Jones et al. (Reference Jones, Burton, Cunningham, Menten, Schilke and Belloche2008, Reference Jones, Burton, Tothill and Cunningham2011) created the spatial distribution of CH2NH from Sgr B2 (N) to Sgr B2 (S) at wavelengths of 3 and 7 mm using the MOPRA telescope. Belloche et al. (Reference Belloche, Müller, Menten, Schilke and Comito2013) demonstrated a detailed analysis of CH2NH towards the Sgr B2 (N) and Sgr B2 (M) using the IRAM 30 m telescope. The rotational emission lines of CH2NH were also detected for W51 e1/e2, Orion KL, G34.3+0.15, G19.61-0.23, IRAS 16293–2422 B and NGC 6334I (Dickens et al., Reference Dickens, Irvine, Devries and Ohishi1997; White et al., Reference White, Araki, Greaves, Ohishi and Higginbottom2003; Qin et al., Reference Qin, Wu, Huang, Zhao, Li, Wang and Chen2010; Ligterink et al., Reference Ligterink, Calcutt, Coutens, Kristensen, Bourke, Drozdovskaya, Müller, Wampfler, van der Wiel, van Dishoeck and Jørgensen2018; Bøgelund et al., Reference Bøgelund, McGuire, Hogerheijde, van Dishoeck and Ligterink2019). Recently, CH2NH megamaserFootnote 2 lines were detected in six compact obscured nuclei using the Very Large Array (VLA) (Gorski et al., Reference Gorski, Aalto, Mangum, Momjian, Black, Falstad, Gullberg, KÖnig, Onishi, Sato and Stanley2021).
Figure 1. Proposed possible formation mechanism of CH2NH and NH2CH2COOH. In the chemical diagram, the black/grey dumbbells indicate the carbon (C) atom, the white dumbbells indicate the hydrogen (H) atom, the blue dumbbells indicate the nitrogen (N) atom and the red dumbbells indicate the oxygen (O) atom. In the chemical reaction, ‘H2O’ represents the hydrolysis process. References: (A) Woon (Reference Woon2002); Theule et al. (Reference Theule, Borget, Mispelaer, Danger, Duvernay, Guillemin and Chiavassa2011); (B) Danger (Reference Danger, Borget, Chomat, Duvernay, Theulé, Guillemin, Le Sergeant D'Hendecourt and Chiavassa2011); (C) Alonso et al. (Reference Alonso, Kolesniková, Białkowska-Jaworska, Kisiel, León, Guillemin and Alonso2018).
In this article, we present the identification of the possible NH2CH2COOH precursor molecule CH2NH towards the G10.47+0.03, using ACA. To estimate the column density and rotational temperature of CH2NH, we used a rotational diagram model. ACA observations and data reductions are presented in section ‘Observation and data reductions’. The results of the detection of the emission lines of CH2NH are presented in section ‘Result’. The discussion and conclusion of the detection of CH2NH are presented in sections ‘Discussion’ and ‘Conclusion’.
Observation and data reductions
We used the archival data of G10.47+0.03 in cycle 4, which was observed using the Atacama Compact Array (ACA) with a 7 m array (PI: Rivilla, Victor; ID: 2016.2.00005.S). The ACA is the heart of the Atacama Large Millimeter/submillimeter Array (ALMA). The observed phase centre of the hot molecular core G10.47+0.03 is (α, δ)J2000 = 18 : 08 : 38.232, –19:51:50.400. The observation was carried out on 16 September 2017, using 11 antennas. The observations were made with ACA band 4 with spectral ranges of 127.47–128.47, 129.74–130.74, 139.07–140.07 and 140.44–141.44 GHz and a corresponding spectral resolution of 488 kHz. During the observation, the flux calibrator and bandpass calibrator were J1924–2914, and the phase calibrator was J1833–210B.
For data reduction and imaging, we used the Common Astronomy Software Application (CASA 5.4.1) with an ALMA data reduction pipeline (McMullin et al., Reference McMullin, Waters, Schiebel, Young and Golap2007). The data analysis flow chart is shown in Manna and Pal (Reference Manna and Pal2024). For flux calibration using the flux calibrator, we used the Perley-Butler 2017 flux calibrator model for each baseline to scale the continuum flux density of the flux calibrator using the CASA task SETJY (Perley and Butler, Reference Perley and Butler2017). We constructed the flux and bandpass calibration after flagging bad antenna data and channels using the CASA pipeline with tasks hifa_bandpassflag and hifa_flagdata. After the initial data reduction, we used the task MSTRANSFORM with all available rest frequencies to separate the target data of G10.47+0.03. For continuum and background subtraction, we used task UVCONTSUB in the UV plane of the separated calibrated data. We used the CASA task TCLEAN with a Briggs weighting robust value of 0.5, to create continuum and spectral images of the G10.47+0.03. To produce spectral images, we used the SPECMODE = CUBE parameter in the TCLEAN task. The final spatial resolutions of the spectral data cubes were 10.48$^{\prime \prime }\times$
6.28$^{\prime \prime }$
, 10.82$^{\prime \prime }\times$
6.39$^{\prime \prime }$
, 12.08$^{\prime \prime }\times$
6.90$^{\prime \prime }$
and 12.08$^{\prime \prime }\times$
6.79$^{\prime \prime }$
between the frequency ranges of 127.47–128.47, 129.74–130.74, 139.07–140.07 and 140.44–141.44 GHz with a spectral resolution of 488.28 kHz. Finally, we used the CASA task IMPBCOR to correct the primary beam pattern in continuum images and spectral data cubes.
Result
Continuum emission towards the G10.47+0.03
We presented the continuum emission images of the hot molecular core G10.47+0.03 at frequencies of 127.97, 130.25, 139.56 and 140.92 GHz. The continuum images are shown in Fig. 2, where the surface brightness colour scale has units of the Jy beam−1. After the creation of the continuum emission images, we fitted the 2D Gaussian over the continuum emission images using the CASA task IMFIT and estimated the integrated flux density in Jy, peak flux density in Jy beam−1, synthesized beam size in arcsec ($^{\prime \prime }$
), deconvolved beam size in arcsec ($^{\prime \prime }$
), position angle in degrees (°) and RMS in mJy of the hot core G10.47+0.03. The estimated continuum image properties of hot core G10.47+0.03 are shown in Table 1. We noticed that the continuum emission region of G10.47+0.03 is smaller than the synthesized beam size, which was estimated after fitting the 2D Gaussian over the continuum emission region. This indicates that the continuum emission image of G10.47+0.03 was not resolved between the frequency range of 127.97 and 140.92 GHz. Recently, Manna and Pal (Reference Manna and Pal2023a) reported the detection of continuum emission from the G10.47+0.03 in the frequency range of 130.23–160.15 GHz with a flux density variation of 1.36–2.71 Jy.
Figure 2. Millimetre-wavelength continuum emission images of the hot molecular core G10.47+0.03. Continuum emission images are obtained with ACA band 4 at frequencies of 127.97, 130.25, 139.56 and 140.92 GHz. The contour levels start at 3σ, where σ is the RMS of each continuum image. The contour levels increase by a factor of $\surd$
2. The red circles indicate the synthesized beams of the continuum images. The corresponding synthesized beam sizes and RMS values of all continuum images are presented in Table 1.
Table 1. Summary of the millimetre wavelength continuum images of G10.47+0.03
Identification of the CH2NH in the G10.47+0.03
First, we extracted the millimetre-wavelength molecular spectra from the spectral data cubes to create a 23.75$^{\prime \prime }$
diameter circular region over the G10.47+0.03. The synthesized beam sizes of the spectral data cubes of hot core G10.47+0.03 are 10.48$^{\prime \prime }\times$
6.28$^{\prime \prime }$
, 10.82$^{\prime \prime }\times$
6.39$^{\prime \prime }$
, 12.08$^{\prime \prime }\times$
6.90$^{\prime \prime }$
and 12.08$^{\prime \prime }\times$
6.79$^{\prime \prime }$
. Hot core G10.47+0.03 is located at a distance of 8.6 kpc and at that distance, a ~10$^{\prime \prime }$
resolution refers to a spatial scale of 0.4 pc. This implies that the extracted spectrum mostly represents the outer envelope. The systematic velocity (V LSR) of G10.47+0.03 is 68.50 km s−1 (Rolffs et al., Reference Rolffs, Schilke, Zhang and Zapata2011). We used the second-order polynomial to subtract the baseline of the entire spectra. To identify the rotational emission lines of CH2NH, we used the LTE model with the Cologne Database for Molecular Spectroscopy (CDMS) database (Müller et al., Reference Müller, Schlmöder, Stutzki and Winnewisser2005). For LTE modelling, we used CASSIS (Vastel et al., Reference Vastel, Bottinelli, Caux, Glorian and Boiziot2015). The LTE assumptions are valid in the inner region of G10.47+0.03 because the gas density of the warm inner region of the hot core is 7×107 cm−3 (Rolffs et al., Reference Rolffs, Schilke, Zhang and Zapata2011). To fit the LTE model spectra of CH2NH over the millimetre wavelength spectra of G10.47+0.03, we applied the Markov Chain Monte Carlo (MCMC) algorithm in CASSIS. After the LTE analysis, we have detected a total of three transitions of CH2NH, i.e. J = 2(0, 2) − 1(0, 1), J = 6(2, 4) − 7(1, 7) and J = 10(3, 7) − 11(2, 10). The three detected transitions of CH2NH had hyperfine lines. We do not discuss the hyperfine lines regarding the identified transitions of CH2NH because the current spectral resolution is insufficient to resolve the hyperfine lines. The CH2NH is a simple asymmetric top with all atoms being on the simple plane, and the transitions are described using labels of $J^{\prime }$
, $K_{p}^{\prime }$
, $K_{o}^{\prime }$
, and $J^{\prime \prime }$
, $K_{p}^{\prime \prime }$
, $K_{o}^{\prime \prime }$
. In the transition of CH2NH, J indicated the total rotational angular momentum quantum number, K p indicated the projection of J on the symmetry axis in the limiting prolate symmetric top, K o indicated the projection of J on the symmetry axis in the limiting oblate symmetric top and F indicated the total angular momentum quantum number, which includes the nuclear spin for the nucleus with the largest χ or eQq where χ or eQq denoted the nuclear electric quadrupole coupling constant along the indicated principal axis (Kirchhoff et al., Reference Kirchhoff, Johnson and Lovas1973). There were no missing transitions of CH2NH in the observable frequency ranges. As per the CDMS and online molecular database Splatalogue, we find that all the detected transitions of $\rm {CH_{2}NH}$
are not blended with other nearby molecular transitions. Using the LTE model, the best-fit column density of CH2NH is (3.21±1.5)×1015 cm−2 with an excitation temperature of 210.50 ± 32.82 K and a source size of 10.78$^{\prime \prime }$
. The full-width half maximum (FWHM) of the LTE-fitted rotational emission spectra of CH2NH is 9.5 km s−1. The LTE-fitted rotational emission spectra of CH2NH are shown in Fig. 3. After identifying the rotational emission lines of CH2NH using the LTE model, we obtained the molecular transitions, upper-state energy (E u) in K, Einstein coefficients (A ij) in s−1, line intensity (Sμ 2) in Debye2 and optical depth (τ). We also verified the detected transitions of CH2NH from Kirchhoff et al. (Reference Kirchhoff, Johnson and Lovas1973). To estimate the proper FWHM and integrated intensity ($\rm {\int T_{mb}dV}$
) of the detected emission lines of CH2NH, we fitted a Gaussian model to the observed spectra of CH2NH. A summary of the detected transitions and spectral line properties of CH2NH are presented in Table 2.
Figure 3. Identified rotational emission lines of CH2NH towards the G10.47+0.03 in the frequency ranges of 127.47–128.47, 129.74–130.74 and 140.44–141.44 GHz. The green spectra indicate the millimetre-wavelength molecular spectra of G10.47+0.03. The black spectra present the best-fit LTE model spectra of CH2NH, and the red spectra indicate the Gaussian model. The radial velocity of the spectra is 68.50 km s−1.
Table 2. Summary of the molecular line parameters of the CH2NH towards the G10.47+0.03
aAll detected transitions of CH2NH are also verified from Table 2 in Kirchhoff et al. (Reference Kirchhoff, Johnson and Lovas1973) and CDMS molecular database.
bThe values of Eup, Aij and gup are taken from the CDMS database.
cThe line intensity Sμ 2 is defined by the product of the transition line strength S and square of the dipole moment μ 2 of CH2NH. The values of Sμ 2 of detected transitions of CH2NH are taken from online molecular database splatalogue.
dFWHM and $\rm {\int T_{mb}dV}$
are estimated from the fitting of the Gaussian model over the observed spectra of CH2NH.
Rotational diagram analysis of CH2NH
In this work, we used the rotational diagram method to estimate the total column density (N) in cm−2 and the rotational temperature (T rot) in K of CH2NH because we detected multiple transition lines of CH2NH towards the G10.47+0.03. Initially, we assumed that the detected CH2NH emission lines were optically thin and populated under the LTE conditions. The equation of column density for optically thin molecular emission lines can be expressed as (Goldsmith and Langer, Reference Goldsmith and Langer1999),
where g u is the degeneracy of the upper state, μ is the electric dipole moment, S indicates the strength of the transition lines, ν is the rest frequency, k B is Boltzmann's constant and $\int T_{mb}dV$
indicates the integrated intensity. The total column density of CH2NH under LTE conditions can be written as,
where E u is the upper-state energy of CH2NH, T rot is the rotational temperature of CH2NH and Q(T rot) is the partition function at the extracted rotational temperature. The rotational partition function of CH2NH at 75 K is 740.457, that at 150 K is 2084.970, and that at 300 K is 5892.504 (Müller et al., Reference Müller, Schlmöder, Stutzki and Winnewisser2005). Equation (2) can be rearranged as,
Equation (3) indicates a linear relationship between the upper state energy (E u) and ln(N u/g u) of CH2NH. The value ln(N u/g u) was estimated using Equation (1). Equation (3) indicates that the spectral parameters with respect to the different transition lines of CH2NH should be fitted with a straight line, whose slope is inversely proportional to the rotational temperature (T rot), with its intercept yielding ln(N/Q), which will help estimate the molecular column density of CH2NH. For the rotational diagram analysis, we estimated the spectral line parameters of CH2NH after fitting the Gaussian model over the observed spectra of CH2NH using the Levenberg–Marquardt algorithm in CASSIS (see section ‘Identification of the CH2NH in the G10.47+0.03’ for details on spectral fitting). For the rotational diagram analysis, we used all the detected transitions of CH2NH to estimate the accurate column density and rotational temperature because all the detected transitions of CH2NH are non-blended. The resultant rotational diagram of CH2NH is shown in Fig. 4, which was created using the ROTATIONAL DIAGRAM module in CASSIS. In the rotational diagram, the vertical red error bars indicate the absolute uncertainty of ln(N u/g u), which was determined from the estimated error of $\int T_{mb}dV$
. From the rotational diagram, we estimated the column density of CH2NH to be (3.40 ± 0.2) × 1015 cm−2 with a rotational temperature of 218.70 ± 20 K. From the LTE spectral modelling, we found that the column density and excitation temperature of $\rm {CH_{2}NH}$
are (3.21 ± 1.5) × 1015 cm−2 and 210.50±32.82 K, which are nearly similar to the estimated column density and rotational temperature of $\rm {CH_{2}NH}$
using the rotational diagram model. Our derived rotational temperature of CH2NH indicates that the detected transitions of CH2NH arise from the warm-inner region of G10.47+0.03 because the temperature of the hot molecular core is above ≥100 K (van Dishoeck and Blake, Reference van Dishoeck and Blake1998). To determine the fractional abundance of $\rm {CH_{2}NH}$
, we use the column density of $\rm {CH_{2}NH}$
inside the 12.08$^{\prime \prime }$
beam and divide it by the column density of $\rm {H_{2}}$
. The estimated fractional abundance of CH2NH towards G10.47+0.03 with respect to H2 is 2.61×10−8, where the column density of H2 towards the G10.47+0.03 is 1.30 × 1023 cm−2 (Suzuki et al., Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016).
Figure 4. Rotational diagram of CH2NH towards G10.47+0.03. In the rotational diagram, the red blocks represent the statistical data points of all detected transitions, and the solid black line indicates the fitted straight line, which helps estimate the column density and rotational temperature of CH2NH.
Spatial distribution of CH2NH in the G10.47+0.03
We created integrated emission maps of CH2NH towards the G10.47+0.03, using the CASA task IMMOMENTS. Integrated emission maps of CH2NH were created by integrating the spectral data cubes in the velocity ranges of 61.06–74.11, 63.80–75.28 and 64.40–70.93 km s−1, where the emission lines of CH2NH were detected. We created integrated emission maps for the three non-blended transitions of CH2NH towards the G10.47+0.03. The integrated emission maps of CH2NH with different transitions towards the G10.47+0.03 are shown in Fig. 5. The resultant integrated emission maps of CH2NH were overlaid with the 2.34 mm continuum emission map of G10.47+0.03. The integrated emission maps of CH2NH exhibit a peak at the continuum position. From the integrated emission maps, it is evident that the transitions of the CH2NH molecule arise from the warm inner part of the hot core region of G10.47+0.03. This indicates that the temperature of the detected transition lines of $\rm {CH_{2}NH}$
is above 100 K because the temperature of the hot core ≥100 K (van Dishoeck and Blake, Reference van Dishoeck and Blake1998). From the rotational diagram, we estimate the temperature of $\rm {CH_{2}NH}$
is 218.7 ± 20 K, which indicates the emission lines of $\rm {CH_{2}NH}$
arise from the warm-inner region of G10.47+0.03. After the generation of the integrated emission maps of all identified lines of CH2NH, we estimated the emitting regions of CH2NH towards the G10.47+0.03 by fitting the 2D Gaussian over the integrated emission maps of CH2NH using the CASA task IMFIT. The deconvolved beam size of the emitting region of CH2NH was estimated by the following equation,
where $\theta _{50} = 2\sqrt {A/\pi }$
indicates the diameter of the circle whose area (A) corresponds to the 50% line peak of CH2NH and θ beam is the half-power width of the synthesized beam (Rivilla et al., Reference Rivilla, Beltrán, Cesaroni, Fontani, Codella and Zhang2017; Manna and Pal, Reference Manna and Pal2022c, Reference Manna and Pal2023b). The estimated emitting regions of the J = 2(0, 2) − 1(0, 1), J = 6(2, 4) − 7(1, 7), and J = 10(3, 7) − 11(2, 10) transitions of CH2NH were 10.520$^{\prime \prime }$
(0.44 pc), 10.275$^{\prime \prime }$
(0.430 pc), and 10.781$^{\prime \prime }$
(0.451 pc). The emitting region of CH2NH varied between 10.520$^{\prime \prime }$
and 10.781$^{\prime \prime }$
. We observed that the emitting regions of CH2NH were similar or small with respect to the synthesized beam size of the integrated emission maps, which means that the transition lines of CH2NH were not spatially resolved or, at best, marginally resolved. Therefore, we cannot draw any conclusions about the morphology of the integrated emission maps of CH2NH towards the G10.47+0.03. Higher spectral resolution observations were needed using the ALMA 12 m array to solve the spatial distribution morphology of CH2NH towards the hot molecular core G10.47+0.03.
Figure 5. Integrated emission maps of detected transitions of CH2NH towards the G10.47+0.03, which are overlaid with the 2.34 mm continuum emission map. The contour levels are at 20, 40, 60 and 80% of peak flux. The cyan circle represents the synthesized beam of the integrated emission maps.
Discussion
CH2NH towards the G10.47+0.03
We presented the first interferometric detection of possible $\rm {NH_{2}CH_{2}COOH}$
precursor molecule CH2NH towards the G10.47+0.03 using the ACA band 4. We identified a total of three transition lines of CH2NH towards the G10.47+0.03, and after spectral analysis using the LTE model, we observed that all identified transitions of CH2NH are non-blended. Subsequently, these non-blended transition lines were used for rotational diagram analysis to estimate the total column density and rotational temperature of CH2NH. Earlier, Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) first attempted to search the rotational emission lines of CH2NH from G10.47+0.03 and other hot molecular core objects. Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) identified three transition lines of CH2NH i.e., J = 4(0, 4) − 3(1, 3), J = 4(1, 4) − 3(1, 3), and J = 4(2, 3) − 3(2, 2) towards the G10.47+0.03 using the Nobeyama Radio Observatory (NRO) 45 m single dish telescope. Many questions arise in the detection of CH2NH towards the G10.47+0.03 by Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016). First, all detected spectral lines of CH2NH towards the G10.47+0.03 were below 2.5σ statistical significance (for details, see Fig. 3 in Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016)), and the authors did not use any radiative transfer model for spectral characterization of CH2NH. Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) also did not discuss the blending effect of CH2NH with nearby molecular transitions in the molecular spectra of G10.47+0.03. The single-dish observation of CH2NH by Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) could not study the spatial distribution of CH2NH towards G10.47+0.03. Thus, Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) did not estimate any information regarding the source size or emitting region of CH2NH, which restricted the accuracy of their measurements of the proper column density of the detected molecules. We also observed that the upper state energy (E u) of the transition lines of CH2NH detected by Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) varies between 10 and 30 K. In the rotational diagram, lower energy levels will not enable accurate determination of the column density. In the rotational diagram, transitions at significantly higher energy levels can determine a more accurate column density. Our interferometric detection of CH2NH using ACA gives us confidence in the more accurate column density of CH2NH because the upper state energies of the detected transitions vary between 9 K and 240 K. From the spatial distribution analysis, we estimated that the emission regions of CH2NH vary between 10.520$^{\prime \prime }$
–10.781$^{\prime \prime }$
. The estimated molecular column density of CH2NH towards the G10.47+0.03 using the ACA was (3.40 ± 0.2) × 1015 cm−2 with a rotational temperature of 218.70 ± 20 K. Earlier, Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) estimated that the column density of CH2NH towards the G10.47+0.03 using the NRO telescope is (4.70 ± 1.6) × 1015 cm−2 with a rotational temperature of 84 ± 57 K. We find that our estimated column density of CH2NH is similar to that reported by Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016), but the temperature is different. The estimated rotational temperature of $\rm {CH_{2}NH}$
by Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) indicates that the detected transition lines of $\rm {CH_{2}NH}$
arise from the cold region of G10.47+0.03. Our estimated temperature indicates that the emission lines of $\rm {CH_{2}NH}$
arise from the warm-inner region of G10.47+0.03. Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) found the lower rotational temperature due to the lower spatial and spectral resolution of the NRO telescope. Our estimated higher excitation temperature of $\rm {CH_{2}NH}$
using ACA is accurate because the temperature of the hot molecular cores is above 100 K (van Dishoeck and Blake, Reference van Dishoeck and Blake1998). The other two precursors of $\rm {NH_{2}CH_{2}COOH}$
, such as $\rm {CH_{3}NH_{2}}$
(Ohishi et al., Reference Ohishi, Suzuki, Hirota, Saito and Kaifu2019) and $\rm {NH_{2}CH_{2}CN}$
(Manna and Pal, Reference Manna and Pal2022a) were also detected towards G10.47+0.03 using the NRO and ALMA telescopes. The detection of $\rm {CH_{2}NH}$
towards G10.47+0.03 indicates that three possible precursors of $\rm {NH_{2}CH_{2}COOH}$
are present in G10.47+0.03. That means the hot molecular core G10.47+0.03 is an ideal candidate for searching the emission lines of $\rm {NH_{2}CH_{2}COOH}$
.
Comparison with modelled and observed abundance of CH2NH
After estimating the fractional abundance of CH2NH towards the G10.47+0.03, we compared the estimated abundance of CH2NH with the modelled abundance of CH2NH, which was estimated from the two-phase warm-up chemical model (Suzuki et al., Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016). For chemical modelling, Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) used the gas-grain chemical kinetics code NAUTILUS in an environment with hot molecular cores. In chemical modelling, Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) assumed an isothermal collapse phase after a static warm-up phase. In the first phase, the gas density rapidly increased from 3 × 103 to 1 × 107 cm−3, and under free-fall collapse, the dust temperature decreased from 16 to 8 K. In the second phase, the gas density remained constant at 1 × 107 cm−3 and the gas temperature fluctuated rapidly from 8 to 400 K (Suzuki et al., Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016). In chemical modelling, Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) used the neutral–neutral reaction between $\rm {CH_{3}}$
and NH radicals in the gas phase and the neutral–neutral reaction between $\rm {CH_{2}}$
and NH radicals on the grain surface to create CH2NH under the condition of hot molecular cores. The gas temperature of G10.47+0.03 was ~150 K (Rolffs et al., Reference Rolffs, Schilke, Zhang and Zapata2011) and the gas density was 7 × 107 cm−3 (Rolffs et al., Reference Rolffs, Schilke, Zhang and Zapata2011). Therefore, the two-phase warm-up chemical model of Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016), which is based on the time scale, is appropriate for explaining the chemical abundance and evolution of CH2NH towards the G10.47+0.03. After the simulation, Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016) observed that the modelled abundance of CH2NH varied between ~10−9–10−8 in the gas phase. Similarly, the abundance of CH2NH on the grain surface is ≤10−12. We found that the abundance of CH2NH towards the G10.47+0.03 is 2.61 × 10−8, which is nearly similar to the modelled abundance of CH2NH in the gas phase derived by Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016). This result indicates that CH2NH is created towards G10.47+0.03, via the gas-phase neutral–neutral reaction between $\rm {CH_{3}}$
and NH radicals.
Previously, Manna and Pal (Reference Manna and Pal2022a) claimed that $\rm {NH_{2}CH_{2}CN}$
was the daughter molecule of $\rm {CH_{2}NH}$
(see Fig. 1). The identification of both $\rm {CH_{2}NH}$
and $\rm {NH_{2}CH_{2}CN}$
indicates that G10.47+0.03 is an ideal candidate in the ISM, where $\rm {NH_{2}CH_{2}COOH}$
may exist. In ISM, G10.47+0.03 is the only source where the maximum number of possible $\rm {NH_{2}CH_{2}COOH}$
precursors (such as $\rm {NH_{2}CN}$
, $\rm {H_{2}CO}$
, $\rm {CH_{3}NH_{2}}$
, $\rm {CH_{2}NH}$
and $\rm {NH_{2}CH_{2}CN}$
) is detected, and several prebiotic chemistries have been proposed to understand the possible formation mechanism of these molecules and their possible connection with $\rm {NH_{2}CH_{2}COOH}$
. After detecting the maximum number of $\rm {NH_{2}CH_{2}COOH}$
precursors towards the G10.47+0.03, we created a possible chemical network to understand the prebiotic chemistry of $\rm {NH_{2}CH_{2}COOH}$
towards the G10.47+0.03. The chemical network is shown in Fig. 6. In the chemical network, all reactions were obtained from Woon (Reference Woon2002); Theule et al. (Reference Theule, Borget, Mispelaer, Danger, Duvernay, Guillemin and Chiavassa2011); Danger (Reference Danger, Borget, Chomat, Duvernay, Theulé, Guillemin, Le Sergeant D'Hendecourt and Chiavassa2011); Garrod (Reference Garrod2013); Alonso et al. (Reference Alonso, Kolesniková, Białkowska-Jaworska, Kisiel, León, Guillemin and Alonso2018); Ohishi et al. (Reference Ohishi, Suzuki, Hirota, Saito and Kaifu2019); Manna and Pal (Reference Manna and Pal2022a) and UMIST 2012 astrochemistry molecular reaction databases. The chemical network clearly indicates the maximum number of parent molecules detected towards the G10.47+0.03, which gives us an idea about the chemical complexity towards hot molecular cores.
Figure 6. Proposed chemical network for the formation of $\rm {NH_{2}CH_{2}COOH}$
from other molecules. In the network, the red box molecule is the final daughter molecule ($\rm {NH_{2}CH_{2}COOH}$
) and the black box molecules are the parent molecules that are detected towards the G10.47+0.03.
Searching of NH2CH2COOH towards the G10.47+0.03 using the ACA
After the identification of three possible $\rm {NH_{2}CH_{2}COOH}$
precursor molecules like $\rm {CH_{2}NH}$
(present paper), $\rm {CH_{3}NH_{2}}$
(Ohishi et al., Reference Ohishi, Suzuki, Hirota, Saito and Kaifu2019) and $\rm {NH_{2}CH_{2}CN}$
(Manna and Pal, Reference Manna and Pal2022a) towards the G10.47+0.03, we searched the emission lines of $\rm {NH_{2}CH_{2}COOH}$
conformers I and II towards the G10.47+0.03. After the careful spectral analysis using the LTE model, we did not detect any evidence of $\rm {NH_{2}CH_{2}COOH}$
conformers I and II towards the G10.47+0.03 within the limits of our LTE modelling. The estimated upper limit column density of $\rm {NH_{2}CH_{2}COOH}$
conformers I and II towards the G10.47+0.03 was ≤1.02 × 1015 cm−2 and ≤2.36 × 1013 cm−2 respectively. The energy of $\rm {NH_{2}CH_{2}COOH}$
conformer I is 705 cm−1 (1012 K) lower than that of $\rm {NH_{2}CH_{2}COOH}$
conformer II (Lovas et al., Reference Lovas, Kawashima, Grabow, Suenram, Fraser and Hirota1995). The dipole moments of $\rm {NH_{2}CH_{2}COOH}$
conformer I are μ a = 0.911 D (a-type) and μ b = 0.607 D (b-type), whereas $\rm {NH_{2}CH_{2}COOH}$
conformer II has dipole moments of μ a = 5.372 D (a-type) and μ b = 0.93 D (b-type) (Lovas et al., Reference Lovas, Kawashima, Grabow, Suenram, Fraser and Hirota1995). In ISM, the detection of a-typeFootnote 3 transitions of $\rm {NH_{2}CH_{2}COOH}$
are expected compared to b-type transitions because the line intensity of the molecule is proportional to the square of the dipole moments (Lovas et al., Reference Lovas, Kawashima, Grabow, Suenram, Fraser and Hirota1995). The detection of three possible $\rm {NH_{2}CH_{2}COOH}$
precursor molecules towards the G10.47+0.03 gives more confidence about the presence of $\rm {NH_{2}CH_{2}COOH}$
towards the G10.47+0.03.
Conclusion
In this article, we present the identification of the possible $\rm {NH_{2}CH_{2}COOH}$
precursor molecule $\rm {CH_{2}NH}$
towards the G10.47+0.03, using the ACA band 4. The main conclusions of this study are as follows:
1. We successfully identified three non-blended transition lines of $\rm {CH_{2}NH}$
towards the G10.47+0.03 using the ACA observation.2. The estimated column density of $\rm {CH_{2}NH}$
towards the G10.47+0.03 is (3.40 ± 0.2) × 1015 cm−2 with a rotational temperature of 218.70 ± 20 K. The estimated fractional abundance of $\rm {CH_{2}NH}$ towards the G10.47+0.03 with respect to $\rm {H_{2}}$ is 2.61×10−8.3. We compare the estimated abundance of $\rm {CH_{2}NH}$
with the two-phase warm-up chemical model abundance of $\rm {CH_{2}NH}$ proposed by Suzuki et al. (Reference Suzuki, Ohishi, Hirota, Saito, Majumdar and Wakelam2016). We noticed that the modelled abundance of $\rm {CH_{2}NH}$ is nearly similar to the observed abundance of $\rm {CH_{2}NH}$ towards the G10.47+0.03. This comparison indicates that $\rm {CH_{2}NH}$ is created via a gas-phase neutral–neutral reaction between $\rm {CH_{3}}$ and NH radicals towards the G10.47+0.03.4. After the successful detection of $\rm {CH_{2}NH}$
towards the G10.47+0.03, we also search the emission lines of the simplest amino acid $\rm {NH_{2}CH_{2}COOH}$ conformers I and II towards the G10.47+0.03. We do not detect the emission lines of $\rm {NH_{2}CH_{2}COOH}$ conformers I and II within the limits of LTE modelling. The estimated upper-limit column densities of $\rm {NH_{2}CH_{2}COOH}$ conformers I and II are ≤1.02 × 1015 cm−2 and ≤2.36 × 1013 cm−2 respectively.5. The unsuccessful detection of $\rm {NH_{2}CH_{2}COOH}$
towards G10.47+0.03 using ACA indicate that the emission lines of $\rm {NH_{2}CH_{2}COOH}$ may be below the confusion limit in G10.47+0.03.
Acknowledgements
We thank the anonymous referees for their helpful comments, which improved the manuscript. A. M. acknowledges the Swami Vivekananda merit cum means scholarship (SVMCM), Government of West Bengal, India, for financial support for this research. The plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. This paper makes use of the following ALMA data: ADS /JAO.ALMA#2016.2.00005.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in co-operation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.
Competing interest
None.
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