Detection of antifreeze molecule ethylene glycol in the hot molecular core G358.93–0.03 MM1

The identification of complex prebiotic molecules using millimeter and submillimeter telescopes allows us to understand how the basic building blocks of life are formed in the universe. In the interstellar medium (ISM), ethylene glycol ((CH 2 OH) 2 ) is the simplest sugar alcohol molecule, and it is the reduced alcohol of the simplest sugar-like molecule, glycolaldehyde (CH 2 OHCHO). We present the first detection of the rotational emission lines of 𝑎𝐺𝑔 ′ conformer of ethylene glycol ((CH 2 OH) 2 ) towards the hot molecular core G358.93–0.03 MM1 using the Atacama Large Millimeter/Submillimeter Array (ALMA). The estimated column density of 𝑎𝐺𝑔 ′ -(CH 2 OH) 2 towards the G358.93–0.03 MM1 is (4.5 ± 0.1) × 10 16 cm − 2 with an excitation temperature of 155 ± 35 K. The abundance of 𝑎𝐺𝑔 ′ -(CH 2 OH) 2 with respect to H 2 is (1.4 ± 0.5) × 10 − 8 . Similarly, the abundances of 𝑎𝐺𝑔 ′ - (CH 2 OH) 2 with respect to CH 2 OHCHO and CH 3 OH are 3.1 ± 0.5 and (6.1 ± 0.3) × 10 − 3 . We compare the estimated abundance of 𝑎𝐺𝑔 ′ -(CH 2 OH) 2 with the existing three-phase warm-up chemical model abundance of (CH 2 OH) 2 , and we notice the observed abundance and modelled abundance are nearly similar. We discuss the possible formation pathways of 𝑎𝐺𝑔 ′ -(CH 2 OH) 2 towards the hot molecular cores, and we find that 𝑎𝐺𝑔 ′ -(CH 2 OH) 2 is probably created via the recombination of two CH 2 OH radicals on the grain surface of G358.93–0.03 MM1.


INTRODUCTION
In the past few years, the higher sensitivity of radio and (sub)millimeter telescopes have made it possible to identify simple and complex molecules with an increasing number of atoms in the interstellar medium (ISM).Interstellar molecules, which contain more than six atoms, including carbon, are known as complex organic molecules (hereafter COMs) (van Dishoeck & Blake 1998;Herbst & van Dishoeck 2009).COMs are mainly found in dense collapsing cloud cores (Bacmann et al. 2012;Taquet et al. 2017), protoplanetary disks (Walsh et al. 2016;Favre et al. 2018), hot molecular cores (hereafter HMCs) (Rivilla et al. 2017;Jørgensen et al. 2020;Mondal et al. 2021), high-mass protostars (Isokoski et al. 2013;Manna et al. 2024), and hot corinos (Blake et al. 1987;Fayolle et al. 2015;Bergner et al. 2017;Ceccarelli et al. 2017;Ospina-Zamudio et al. 2018).The presence of complex prebiotic molecules in the ISM suggests that those molecules are created during the initial stage of star formation and that they are preserved until the development of small bodies.The formation mechanisms of COMs in the star-forming regions are being intensively debated in astrochemistry.Two possible routes have been proposed to create COMs: gas phase and grain surface chemical reactions (Viti et al. 2004;Garrod & Herbst 2006;Garrod et al. 2008;Garrod 2013;Balucani et al. 2015; Skouteris et ★ E-mail:amanna.astro@gmail.com† E-mail:sabya.pal@gmail.comal. 2018;Garrod et al. 2022;Enrique-Romero et al. 2022;Puzzarini 2022;Ceccarelli et al. 2023).Only the identification of COMs and their relative abundances and two-or three-phase warm-up chemical models in a large number of star-forming regions and HMCs will help us understand the chemical formation pathways of the COMs (Viti et al. 2004;Garrod 2013;Coutens et al. 2018;Garrod et al. 2022).
Ethylene glycol (hereafter EG) is known as a di-alcohol molecule that is commonly used in prebiotic sugar synthesis.The EG molecule is the sugar alcohol of the aldehyde sugar molecule glycolaldehyde (hereafter GA).EG is an asymmetric top molecule with coupled rotation around its two C-O bonds and one C-C bond, which generates different conformers (Christen et al. 1995(Christen et al. , 2001)).There are six conformers with respect to the C-C bond with gauche (G) arrangement of the two H 2 C = CH 2 groups and four low-stable conformers with anti-arrangement of the CH 2 groups.In the G conformer, the two OH groups pick the gauche orientation with respect to each other.The 'a' and 'g' notations are used for the anti (a) and gauche (g) conformers of the OH groups with respect to the two C-O bonds.When the OH group provides the hydrogen for the intramolecular bonding, the gauche orientation is noted as  ′ .The EG molecule has only two conformers,  ′ and  ′ .The 3D molecular structure of  ′ -EG and  ′ -EG is shown in Fig. 1.The  ′ conformer of EG is 200 cm −1 or 290 K higher in energy than  ′ (Müller & Christen 2004).For  ′ and  ′ conformers of EG, the tunneling is observed between two equivalent equilibrium configurations, and aGg' -EG gGg' -EG it splits each rotational level into two different states,  = 0 and  = 1 (Christen et al. 2001).For the  ′ and  ′ conformers, the  = 1 state is around 7 and 1.4 GHz higher than the  = 0 (Müller & Christen 2004).The emission lines of EG were first detected towards the galactic center along the line of sight to Sgr B2, but the several detected lines of EG were blended (Hollis et al. 2002;Requena-Torres et al. 2008;Belloche et al. 2013).Evidence of EG was also found towards the HMCs Orion KL, W51 e2, G34.3+0.2,G31.41+0.31,G10.47+0.03(Favre et al. 2011;Fuente et al. 2014;Brouillet et al. 2015;Lykke et al. 2015;Rivilla et al. 2017;Mondal et al. 2021), and hot corinos IRAS 16293-2422 B, NGC 1333 IRAS 2A (Jørgensen et al. 2016;Coutens et al. 2015).Additionally, the emission lines of EG were also found towards the comets Hale-Bopp, C/2012 F6 (Lemmon), C/2013 R1 (Lovejoy), 67P/Churyumov-Gerasimenko, and meteorites Murchinson and Murray (Cooper et al. 2001;Crovisier et al. 2004;Biver et al. 2014Biver et al. , 2015;;Goesmann et al. 2015).
In the ISM, HMCs are one of the earliest stages of the highmass star-formation regions, and most of the complex and prebiotic molecules, including possible glycine (NH 2 CH 2 COOH) precursors, are found in these objects (van Dishoeck & Blake 1998;Herbst & van Dishoeck 2009;Garrod 2013).COMs in HMCs play a crucial role in increasing the chemical complexity in the ISM (Shimonishi et al. 2021).HMCs are small, compact objects (⩽0.1 pc) with a warm temperature (⩾100 K) and a high gas density (⩾10 6 cm −3 ) (van Dishoeck & Blake 1998;Williams & Viti 2014).In the ISM, these objects are short-lived (10 5 yr to 10 6 yr) (van Dishoeck & Blake 1998; Viti et al. 2004;Garrod & Herbst 2006;Garrod et al. 2008;Garrod 2013).Most of the COMs are observed in the warm-inner regions of the envelope of HMCs, where the temperature exceeds the water ice desorption temperature of ∼100 K (Shimonishi et al. 2021).The fractional abundances of the COMs in the HMCs are found to be between 10 −7 and 10 −11 with respect to molecular H 2 (Herbst & van Dishoeck 2009).In the HMCs, the gas temperature starts to increase due to the collapse of the pre-stellar core.This is called the "warm-up" phase (Viti et al. 2004;Garrod 2013).In this phase, several COMs are created in the ice, then sublimated and destroyed in the gas phase (Viti et al. 2004).So the resultant abundance of the complex molecules in the gas depends on the initial abundances of the precursors (Garrod 2013).Studies at the beginning of the 2000s showed that known (at the time) gas phase reactions were not efficient enough to explain the high abundances of COMs observed in HMCs.However, more recent quantum chemical studies have brought attention back to the gas phase chemistry in forming COMs (Balucani et al. 2015;Skouteris et al. 2018).
In this article, we present the first detection of the simplest sugar alcohol molecule, EG, towards the HMC object G358.93-0.03MM1 using the ALMA.In Section 2, we discuss the ALMA data and their reduction.The result of the identification and spatial distribution of EG is presented in Section 3. The discussion and conclusions of the identification of EG are shown in Section 4 and 5, respectively.

OBSERVATIONS AND DATA REDUCTION
The high-mass star-formation region G358.93-0.03 was observed using ALMA band 7 receivers to study the massive protostellar accretion outburst (ID: 2019.1.00768.S., PI: Crystal Brogan).The observations were performed on October 11th, 2019 with on-source integration times of 756 s.The observed phase center of G358.93-0.03 was (, ) J2000 = 17:43:10.000,-29:51:46.000.A total of 47 antennas were used to observe the G358.93-0.03 with a minimum baseline of 14 m and a maximum baseline of 2517 m.During the observation, J1550+0527 was used as a flux calibrator and bandpass calibrator, and J1744-3116 was used as a phase calibrator.The observed frequency ranges of G358.93-0.03were 290.GHz, with a spectral resolution of 977 kHz.
For data reduction and imaging, we used the Common Astronomy Software Application (CASA 5.4.1) with the ALMA data analysis pipeline (McMullin et al. 2007).In the pipeline, we used the task SETJY with the Perley-Butler 2017 flux calibrator model for flux calibration (Perley & Butler 2017).We also applied the pipeline tasks hifa_bandpassflag and hifa_flagdata for bandpass calibration and flagging the bad antenna data.After the initial data reduction, we used the task MSTRANSFORM to separate the target object G358.93-0.03.We created the four continuum images of G358.93-0.03 by using line-free channels at frequency ranges of 290.GHz using the CASA task TCLEAN with the HOGBOM deconvolver.Recently, Manna et al. (2023) analyzed this data, and they showed the continuum emission image of G358.93-0.03 at frequency 303.39 GHz (988 m) (Fig. 1. of Manna et al. (2023)).Manna et al. (2023) clearly detected the eight sub-millimeter wavelength continuum sources, G358.93-0.03MM1 to G358.93-0.03MM8.Manna et al. (2023) also detected other two continuum sources associated with G358.93-0.03MM1 and G358.93-0.03MM2, which are defined as G358.93-0.03 MM1A and G358.93-0.03MM2A.Before making the spectral images, we use the task UVCONTSUB to subtract the background continuum emission from the UV plane of the calibrated data.We also made self-calibration multiple times using the tasks GAINCAL and APPLYCAL for better RMS of the final images.We create the spectral images of G358.93-0.03 at frequency ranges of 290.GHz, and 304.14-306.01GHz using the task TCLEAN with the SPECMODE = CUBE parameter.Finally, we apply the task IMPBCOR for the correction of the primary beam pattern in the spectral images of G358.93-0.03.

Detection of EG towards the G358.93-0.03 MM1
We assume local thermodynamic equilibrium (LTE) and used the Cologne Database for Molecular Spectroscopy (CDMS) (Müller et al. 2005) to identify the rotational emission lines of EG from the molecular spectra of G358.93-0.03MM1.We use CASSIS for the LTE modelling (Vastel et al. 2015).The LTE assumption is reasonable in the inner region of the G358.93-0.03MM1 because the maximum gas density of the warm inner region of the hot core is 1×10 8 cm −3 (Stecklum et al. 2021).We used the Markov Chain Monte Carlo     2023) estimated the FWHM of CH 2 OHCHO in the spectra of G358.93-0.03MM1 using this data was 3.2 km s −1 .The LTE-fitted rotational emission spectra of  ′ -EG towards the G358.93-0.03MM1 are shown in Fig. 2. The LTE-fitted spectral line parameters of  ′ -EG are shown in Tab. 1.After the detection of the emission lines  ′ -EG from the spectra of G358.93-0.03MM1, we also search the emission lines of  ′ -EG using the LTE modelling spectra.After the spectral analysis, we observe that all detected lines of  ′ -EG are blended with other nearby molecular transitions.Thus, all detected lines of  ′ -EG are blended with other molecules, so we estimate the upper limit column density of  ′ -EG towards the G358.93-0.03MM1 is ⩽(2.9±0.6)×10 16cm −2 .Manna et al. (2023) estimated that the temperature of GA is 300 K, and we estimate that the temperature of EG towards the G358.93-0.03MM1 is 155 K. Previously, Shimonishi et al. (2021) showed that the temperatures of complex molecules vary between 100 K to 300 K in the inner shell of hot molecular core (see Fig 8 in Shimonishi et al. (2021)).So, both molecules EG and GA exist in slightly different regions of the inner shell of G358.93-0.03MM1.

Spatial distribution of 𝑎𝐺𝑔 ′ -EG towards the G358.93-0.03 MM1
We produce the integrated emission maps (moment zero maps) of some selected highly intense non-blended emission lines of  ′ -EG towards the G358.93-0.03MM1 using the task IMMOMENTS.
During the run of the task IMMOMENTS, we use the channel ranges of the spectral images where the emission lines of  ′ -EG are detected.We create the integrated emission maps of  ′ -EG for non-blended transitions towards the G358.9-0.03MM1 at Fig. 3.We overlaid the 988 m continuum emission map of G358.93-0.03over the integrated emission maps of  ′ -EG.The continuum emission map of G358.93-0.03 is taken from Manna et al. (2023).We found that the integrated emission maps of  ′ -EG exhibit a peak at the position of the continuum.The integrated emission maps indicate that the emission lines of  ′ -EG originate from the high density, the warm inner region of G358.93-0.03MM1.After the extraction, we apply the CASA task IMFIT to fit the 2D Gaussian over the integrated emission maps of  ′ -EG to estimate the size of the emitting regions.The following equation is used  where  beam indicates the half-power width of the synthesised beam and  50 = 2 √︁ / denotes the diameter of the circle whose area surrounded the 50% line peak of  ′ -EG (Rivilla et al. 2017).The derived emitting regions of  ′ at several frequencies are shown in Tab. 2. The synthesized beam sizes of the integrated emission maps vary between 0.41 arcsec × 0.35 arcsec and 0.42 arcsec × 0.36 arcsec.The derived emitting regions of  ′ -EG at different frequencies vary between 0.40 arcsec and 0.42 arcsec.After fitting a 2D Gaussian, we observe that the size of the emitting regions of  ′ -EG is comparable to or slightly greater than the synthesized beam sizes of the integrated emission maps.This result indicates that the detected transition lines of  ′ -EG are not spatially resolved or are only marginally resolved towards the G358.93-0.03MM1.As a result, drawing any conclusions about the morphology of the spatial distributions of  ′ -EG towards the G358.93-0.03MM1 is impossible.Higher spatial and angular resolution observations are required to understand the spatial distribution of  ′ -EG towards the G358.93-0.03MM1.

Comparison of the EG/GA ratio in different star-forming regions
We compared the EG/GA ratio in G358.93-0.03MM1 with other regions such as the massive star-forming regions Orion KL (Brouillet et al. 2015), W51e2 (Lykke et al. 2015), G34.3+0.2 (Lykke et al. 2015), the intermediate-mass hot core NGC 7129 FIRS2 (Fuente et al. 2014), the HMC candidate G31.41+0.31(Rivilla et al. 2017), the low-mass protostars IRAS 16293 B (Jørgensen et al. 2016), and NGC 1333 IRAS 2A (Coutens et al. 2015).The EG/GA ratios of the above-mentioned sources are shown in Tab. 3. The uncertainty of the EG/GA ratio in G358.93-0.03MM1 can be deduced by considering the propagation of the uncertainty of the column densities of EG and GA.The behaviour of the EG/GA with respect to the luminosity of the different star-forming regions is shown in Fig. 4. As per Fig. 4, the EG/GA ratios increased with luminosities, with a lower value of 1 for the low-mass protostar IRAS 16293 B and a higher value of 10 for the HMC object G31.41+0.31.The lower limit of the hot cores in Orion KL, G34.3+0.2, and W51e2 is as high as 15.Earlier Rivilla et al. (2017) also see the same nature from the variation of the EG/GA ratios with luminosities in the star-forming regions, but they do not describe why these ratios increase with luminosities.We observe different EG/GA ratios because different initial compositions of the ices could produce very different values of the EG/GA ratios (Öberg et al. 2009).Pure CH 3 OH ices create a ratio of >10, but ices with a composition of CH 3 OH:CO 1:10 produce a ratio of <0.25.This implies that the initial composition of the grains is significantly differ- We see that the modelled EG/GA ratios in Coutens et al. (2018) cannot match any sources because the modelled EG/GA ratios with respect to luminosities are larger than the observed EG/GA ratios for different high-and low-mass sources.

Possible formation pathways of EG in HMCs
In HMCs, grain surface chemistry is crucial for the formation of COMs, including EG and GA.Previously, several grain surface chemical routes have been proposed, in which some of the reactions involved thermal and energetic processes (Garrod et al. 2008;Beltrán et al. 2009;Woods et al. 2012;Garrod 2013;Butscher et al. 2015).In HMCs and hot corinos, the warm-up phase is thought to be important for the production of COMs.The warm-up phase allows the more strongly bound radicals to become mobile at the grain surface (Garrod et al. 2008;Garrod 2013).Earlier, several experimental studies showed that the photochemistry in CH 3 OH-CO ice mixtures is sufficient to explain the observed abundance of EG in the star-forming regions (Öberg et al. 2009) (5) Reaction 1 indicates that the reaction between radical HCO and radical CH 2 OH on the gain surfaces creates GA.Similarly, reactions 2 and 3 indicate that the subsequential hydrogenation of GA on the grain surface produces the EG (Fedoseev et al. 2015;Chuang et al. 2016;Garrod 2013;Coutens et al. 2018).Earlier, Rivilla et al. (2017) raises many questions about the formation of EG using reactions 2 and 3.If EG is created via GA, a relatively constant EG/GA ratio would be expected (Rivilla et al. 2017).In Fig. 4, we observe that the EG/GA ratio varies by more than an order of magnitude (1 to >15) with increasing luminosity, which suggests that there is no direct link between EG and GA.The large variation in the EG/GA ratio indicates that GA is not a direct precursor of EG, as well as that they do not share a common precursor (Rivilla et al. 2017).We observe that EG is more abundant than GA in  (Garrod et al. 2008;Garrod 2013;Butscher et al. 2015;Coutens et al. 2015Coutens et al. , 2018;;Garrod et al. 2022).Mondal et al. (2021) also used reaction 5 in the three-phase warm-up chemical models and showed that reaction 5 is the most efficient for the formation of EG in the HMC candidate G10.47+0.03.Previously, Rivilla et al. (2017) claimed that reaction 5 is the most efficient pathway to the formation of EG but Coutens et al. (2018) showed that reaction 3 is the most efficient for the formation of EG towards the G31.41+0.31using the two-phase warm-up chemical modelling.Recently, Mininni et al. (2023) supports the three-phase warm-up chemical model of Garrod et al. (2022) and they show that reaction 5 is the most efficient route to the formation of EG towards the G31.41+0.31.

Comparison between the modelled and observed abundance of EG
Recently, Garrod et al. ( 2022) developed a three-phase (gas + grain + ice mantle) warm-up chemical model using the three-phase astrochemical model code MAGICKAL (Garrod 2013) to understand the formation mechanisms and abundance of different COMs, including EG, towards the HMCs.To model a HMC, they first assumed a free-fall collapse of a cloud (phase I), followed by a warm-up phase (phase II).In the first phase, the gas density increases from   = 3×10 3 cm −3 to 2×10 8 cm −3 , and Garrod et al. (2022) assume a constant temperature of 10 K.In the second phase (warm-up phase), the gas density remains constant at 2×10 8 cm −3 and the temperature increases with time from 8 to 400 K.  2022)).Similarly, the modelled abundances of EG with respect to CH 3 OH was 5.5 × 10 −3 , 1.0 × 10 −3 , and 2.7 × 10 −7 corresponding to the fast, medium, and slow warm-up models (see Tab. 18 of Garrod et al. ( 2022)).After chemical modelling, Garrod et al. (2022) showed that the EG is formed via the recombination of two CH 2 OH radicals (reaction 5) on the grain surface of HMCs.
To understand the formation pathways of EG towards the G358.93-0.03MM1, we compare our estimated abundance of EG with the modelled abundance by Garrod et al. (2022).This comparison is physically reasonable because the maximum gas density and dust temperature of G358.93-0.03MM1 are 1×10 8 cm −3 (Stecklum et al. 2021) and ∼150 K (Chen et al. 2020), respectively.Hence, the threephase warm-up chemical model of Garrod et al. (2022) is appropriate for explaining the chemical evolution of EG towards the G358.93-0.03MM1.We estimate the abundance of EG with respect to H 2 towards the G358.93-0.03MM1 to be (1.4±0.5)×10−8 , which is very close to the medium warm-up model abundance of EG.Similarly, the abundance of EG with respect to CH 3 OH towards the G358.93-0.03MM1 is (6.1±0.3)×10−3 , which is very close to the fast warm-up model abundance of EG with respect to CH 3 OH.This comparison indicates that the simplest sugar alcohol molecule, EG, may form due to the recombination of two CH 2 OH radicals (reaction 5) on the grain surface of G358.93-0.03MM1.For G31.41+0.31,Mininni et al. (2023) estimated that the abundance of EG with respect to H 2 is (1.5±0.4)×10−8 , which is very close to the medium warm-up model abundance of EG.This means that reaction 5 may be responsible for the production of EG towards the G31.41+0.31.Earlier, Enrique-Romero et al. (2022) showed that reaction 5 has an activation barrier, and Garrod et al. (2022) did not include an activation barrier during the chemical modelling.Therefore, it is necessary to verify if the main formation pathway of EG is via the recombination of two CH 2 OH radicals by using a three-phase warm-up model with the activation barrier.

CONCLUSION
We present the first confirmed detection of the rotational emission lines of the simplest sugar alcohol molecule,  ′ -EG, towards the HMC candidate G358.93-0.03MM1 using ALMA.The derived abundance of  ′ -EG with respect to H 2 towards the G358.93-0.03MM1 is (1.4±0.5)×10−8 .The EG/GA ratio towards the G358.93-0.03MM1 is 3.1±0.5.Similarly, the abundance of  ′ -EG with respect to CH 3 OH towards the G358.93-0.03MM1 is (6.1±0.3)×10−3 .We observe that the fractional abundance of EG with respect to H 2 towards the G358.93-0.03MM1 is nearly similar to the abundance of EG towards the other two HMCs, G10.47+0.03 and G31.41+0.31(Mondal et al. 2021;Mininni et al. 2023).We discuss the possible formation mechanisms of EG in HMCs.We compare our estimated abundance of EG with respect to H 2 towards the G358.93-0.03MM1 with the three-phase warm-up chemical model abundances of EG from Garrod et al. (2022), and we find that the derived and medium warm-up modelled abundances of EG are close.Similarly, we also compare the abundance of EG with respect to CH 3 OH with the modelled abundance values of EG from Garrod et al. (2022), and we find that the derived and fast warm-up modelled abundances of EG with respect to CH 3 OH are close.We also notice that the modelled abundance of EG by Garrod et al. (2022) is similar to the observed abundance of EG towards other HMC candidate G31.41+0.31.So, EG may formed via the recombination of two CH 2 OH radicals on the grain surface of G358.93-0.03 MM1 and G31.41+0.31. Earlier, Enrique-Romero et al. (2022) showed that the grain surface reaction between two CH 2 OH radicals has an activation barrier, and Garrod et al. (2022) did not include the activation barrier during the chemical modelling.As a result, it is essential to confirm whether the main formation mechanism of EG is via the recombination of two CH 2 OH radicals, using a three-phase warm-up model that incorporates the activation barrier.The identification of high abundances of EG in G358.93-0.03MM1 suggests that grain surface chemistry is efficient for the formation of other COMs, which should therefore be detectable.A combined spectral line study with an LTE model and three-phase warm-up chemical modelling is required to understand the hot core chemistry towards the G358.93-0.03MM1, which will be carried out in our follow-up study.

Figure 1 .
Figure 1.Three-dimensional molecular structure of  ′ -EG and  ′ -EG.The grey atoms are carbon (C), the red atoms are oxygen (O), and the white atoms are hydrogen (H).

Figure 2 .
Figure 2. Rotational emission lines of  ′ -EG towards the G358.93-0.03MM1.The green spectra indicate the observed molecular spectra of the G358.93-0.03MM1.The red spectra present the LTE model spectrum of  ′ -EG and the black spectra indicate the model spectra for all species identified in the spectrum, including those published in Manna et al. (2023), Manna & Pal (2024a), and Manna & Pal (2024b).

Figure 3 .
Figure 3. Integrated emission maps (moment zero) of  ′ -EG towards the G358.93-0.03MM1.The integrated emission maps are overlaid with the 988 m continuum emission map of G358.93-0.03(black contour).The contour levels are at 20%, 40%, 60%, and 80% of the peak flux.The yellow circles represent the synthesized beams of the integrated emission maps.In the first panel image, the red circle indicates the 0.91 arcsec diameter circular region.

Figure 4 .
Figure 4. Variation of the EG/GA ratios with luminosities in different highand low-mass star-forming regions.The red circle is the EG/GA ratio of G358.93-0.03MM1, and the blue circles are lower limits of EG/GA ratios towards Orion KL, G34.3+0.2, and W51e2.
*-The transition of  ′ -EG contain double with frequency difference ⩽100 kHz.The second transition is not shown.

Table 2 .
Gorai et al. (2020)  -EG towards the G358.93-0.03MM1.-Thetransition of  ′ -EG contain double with frequency difference ⩽100 kHz.The second transition is not shown.(MCMC)algorithm in CASSIS for fitting the LTE model spectra of EG over the observed molecular spectra of G358.93-0.03MM1.The details of the MCMC fitting method using the CASSIS are described inGorai et al. (2020).After the LTE analysis, we have detected a total of 106 transitions of the  ′ conformer of EG with ⩾2.5 statistical significance in the spectra of G358.93-0.03MM1 in the observable frequency ranges.After the spectral analysis using the LTE model, we observe that 35 transitions of Manna & Pal (2024b)ed, and all of those non-blended transitions are detected above 5.To understand the non-blended transitions of  ′ , we have used more than 200 different molecular transitions in the LTE modelling, which are taken from the CDMS database, including those molecules detected byManna & Pal (2023),Manna et al. (2023),Manna & Pal (2024a), andManna & Pal (2024b).We find that the 71 transition lines of  ′ -EG are blended with the other nearby molecular transitions.The upper-level energies of the detected 106 transitions of  ′ -EG vary from 76.49 K to 597.41 K.The upper-level energies of the non-blended transitions of  ′ -EG vary between 206.05 K and 416.16K.There are no missing high-intensity transitions of  ′ -EG in the observable frequency ranges.After the LTE modelling, the best-fit column density of  ′ -EG is (4.5±0.1)×1016cm −2 with an excitation temperature of 155±35 K and a source size of 0.50 arcsec.During the LTE fitting, we used the FWHM of the LTE spectra of  ′ -EG is 3.21 km s −1 .We have taken this FWHM value for LTE modelling of  ′ -EG because recentlyManna et  al. (

Table 3 .
Summary of the EG/GA ratios in the star-forming region.
Rivilla et al. (2017)h means the unsaturated molecule GA is broken towards the G358.93-0.03MM1.For that reason,Rivilla et al. (2017)claims that reactions 2 and 3 are not sufficient to produce EG from GA. Earlier, Requena-Torres et al. (2008) claimed that the double bond C = O in the unsaturated molecule GA can be easily broken in the gas phase of star-forming regions which indicates there is a possibility that GA can survive on the grain surface and EG can be produced by reactions 2 and 3. Similarly, reactions 4 and 5 indicate that the EG is formed by the recombination of two CH 2 OH radicals in the grain surface, where radical CH 2 OH is created via the subsequential hydrogenation of HCO Garrod et al. (2022)ed abundances of EG with respect to H 2 inGarrod et al. (2022)was 6.0 × 10 −8 , 1.1 × 10 −8 , and 2.2 × 10 −12 corresponding to the fast, medium, and slow warm-up models (see Tab. 17 ofGarrod et al. ( Garrod et al. (2022)used three different warm-up stages during the chemical model corresponding to a fast, medium, or slow warm-up from 8 to 400 K.The warm-up time scales of this chemical model were 5 × 10 4 yr (fast), 2 × 10 5 yr (medium), and 1 × 10 6 yr (slow).During chemical modelling,Garrod et al. (2022)used different types of chemical reactions for the formation of EG on the grain surface of HMCs (see Tab. 3 ofGar-  rod et al. (2022)).