High-mass Star-forming Region NGC 6334I Research Articles

Physical Environments of the Luminosity Outburst Source NGC 6334I Traced by Thermal and Maser Lines of Multiple Molecules

We have conducted a systematic line survey, primarily focused on transitions of the methanol and ammonia molecules, and monitoring observations of masers toward the high-mass star-forming region NGC 6334I. These observations were undertaken between 2019 and 2022 in the C, K, Ka, and Q bands with the Tianma Radio Telescope. In total, 63 CH3OH (including 11 class I and nine class II maser or maser candidate), 18 13CH3OH, and 34 NH3 (including seven maser or maser candidate) transitions were detected. The emission is likely associated with the luminosity outburst source MM1. Rotation diagram analysis of multiple ammonia transitions shows that the gas temperature in the molecular core was a factor of 2 higher than that measured in previous observations in the pre-burst stage. This suggests that the molecular core has likely been heated by radiation originating from the luminosity outburst. Maser variability in the methanol and excited-state OH masers shows a general trend that the maser components associated with the luminosity outburst have decreased in their intensity since 2020. The decay in the maser luminosity indicates that the outburst is possibly declining, and as a result, the duration of the MM1 luminosity outburst may be shorter than the predicted 40 yr duration. Compared to the masers detected toward another luminosity outburst source, G358.93-0.03, abundant class I methanol masers and strong water maser flares were also detected toward NGC 633I, but masers from rare class II methanol transitions and new molecules were absent toward NGC 6334I. The large number of detections of maser transitions toward the two burst sources provided a database for further maser modeling to explore the physical environments associated with accretion burst events.

Magnetic Fields in Massive Star-forming Regions (MagMaR). II. Tomography through Dust and Molecular Line Polarization in NGC 6334I(N)

Abstract Here, we report ALMA detections of polarized emission from dust, CS(J = 5 → 4), and C33S(J = 5 → 4) toward the high-mass star-forming region NGC 6334I(N). A clear “hourglass” magnetic field morphology was inferred from the polarized dust emission, which is also directly seen from the polarized CS emission across velocity, where the polarization appears to be parallel to the field. By considering previous findings, the field retains a pinched shape that can be traced to clump length scales from the envelope scales traced by ALMA, suggesting that the field is dynamically important across multiple length scales in this region. The CS total intensity emission is found to be optically thick (τ CS = 32 ± 12) while the C33S emission appears to be optically thin ( τ C 33 S = 0.1 ± 0.01 ). This suggests that sources of anisotropy other than large velocity gradients, i.e., anisotropies in the radiation field, are required to explain the polarized emission from CS seen by ALMA. By using four variants of the Davis–Chandrasekhar–Fermi technique and the angle dispersion function methods (ADF), we obtain an average of the estimates for the magnetic field strength on the plane of the sky of B pos = 16 mG from the dust and B pos ∼ 2 mG from the CS emission, where each emission traces different molecular hydrogen number densities. This effectively enables a tomographic view of the magnetic field within a single ALMA observation.

The Family of Amide Molecules toward NGC 6334I

Abstract Amide molecules produced in space could play a key role in the formation of biomolecules on a young planetary object. However, the formation and chemical network of amide molecules in space is not well understood. In this work, Atacama Large Millimeter/submillimeter Array observations are used to study a number of amide(-like) molecules toward the high-mass star-forming region NGC 6334I. The first detections of cyanamide (NH2CN), acetamide (CH3C(O)NH2), and N-methylformamide (CH3NHCHO) are presented for this source. These are combined with analyses of isocyanic acid (HNCO) and formamide (NH2CHO), and a tentative detection of urea (carbamide; NH2C(O)NH2). Abundance correlations show that most amides are likely formed in related reactions occurring in ices on interstellar dust grains in NGC 6334I. However, in an expanded sample of sources, large abundance variations are seen for NH2CN that seem to depend on the source type, which suggests that the physical conditions within the source heavily influence the production of this species. The rich amide inventory of NGC 6334I strengthens the case that interstellar molecules can contribute to the emergence of biomolecules on planets.

The Submillimeter Rotational Spectrum of Ethylene Glycol up to 890 GHz and Application to ALMA Band 10 Spectral Line Data of NGC 6334I.

The rotational spectrum of the most stable conformer of ethylene glycol (HO(CH2)2OH) has been recorded between 360-890 GHz using a frequency-modulation submillimeter spectrometer. The refinement and extension of the spectroscopic parameters over previous efforts provide predicted catalog frequencies for ethylene glycol with sufficient accuracy for comparison to high-frequency astronomical data. The improvement in the cataloged line positions, and the need for improved accuracy enabled by high-frequency laboratory work, is demonstrated by an analysis of ethylene glycol emission at 890 GHz in the high-mass star-forming region NGC 6334I in ALMA Band 10 observations. The need for accurate rotational spectra at submillimeter wavelengths/THz frequencies is discussed.

ALMA Detection of Vibrationally Excited (v t = 1, 2) Acetic Acid toward NGC 6334I

Vibrationally excited states of detected interstellar molecules have been shown to account for a large portion of unidentified spectral lines in observed interstellar spectra toward chemically rich sources. Here, we present the first interstellar detection of the first and second vibrationally excited torsional states of acetic acid ($v_\mathrm{t} = 1, 2$) toward the high-mass star-forming region NGC 6334I. The observations presented were taken with the Atacama Large Millimeter/submillimeter Array in bands 4, 6, and 7 covering a frequency range of 130 - 352 GHz. By comparing a single excitation temperature model to the observations, the best-fit excitation temperature and column density are obtained to be 142(25) K and $1.12(7) \times 10^{17} \mathrm{cm^{-2}}$ respectively. Based on the intensity maps of the vibrationally excited CH$_3$COOH transitions, we found that the CH$_3$COOH emissions are compact and concentrated toward the MM1 and MM2 regions with a source size smaller than 2 arcsec. After locating the emission from different CH$_3$COOH transitions, which cover a large range of excitation energies, we are able to explain the variation of the CH$_3$COOH emission peak within the MM2 core by invoking continuum absorption or outflows.

Untangling the Formation of Methoxymethanol (CH3OCH2OH) and Dimethyl Peroxide (CH3OOCH3) in Star-forming Regions

Abstract Methoxymethanol (CH3OCH2OH) was recently detected toward the MM1 core in the high-mass star-forming region NGC 6334I. However, the underlying formation mechanisms of this complex organic molecule (COM) as well as its structural isomers ethylene glycol (HOCH2CH2OH) and the hitherto unobserved dimethyl peroxide (CH3OOCH3) are still elusive. Here, we report the very first confirmed synthesis of dimethyl peroxide—at various deuteration levels within interstellar analogous ices of D3-methanol (CD3OH) exposed to ionizing radiation at ultralow temperatures of 5 K. The discrimination of specific isomers is achieved by exploiting reflectron time-of-flight mass spectrometry coupled with isomer-selective photoionization of the subliming molecules in the temperature programmed desorption phase of the experiment. Based on the distribution of the identified species at distinct mass-to-charge ratios, we reveal primary and secondary reaction pathways to methoxymethanol, ethylene glycol, and dimethyl peroxide involving radical–radical recombination of methoxy (CH3O) and hydroxymethyl (CH2OH). Our findings help to constrain the formation mechanism of COMs detected within star-forming regions (methoxymethanol, ethylene glycol) and propose that the hitherto elusive dimethyl peroxide isomer represents an excellent candidate for future astronomical searches.

Methylamine and other simple N-bearing species in the hot cores NGC 6334I MM1–3

Context. In the search for the building blocks of life, nitrogen-bearing molecules are of particular interest since nitrogen-containing bonds are essential for the linking of amino acids and ultimately the formation of larger biological structures. The elusive molecule methylamine (CH3NH2) is thought to be a key pre-biotic species but has so far only been securely detected in the giant molecular cloud Sagittarius B2. Aims. We identify CH3NH2 and other simple nitrogen-bearing species involved in the synthesis of biologically relevant molecules towards three hot cores associated with the high-mass star-forming region NGC 6334I, located at a distance of 1.3 kpc. Column density ratios are derived in order to investigate the relevance of the individual species as precursors of biotic molecules. Methods. High sensitivity, high angular and spectral resolution observations obtained with the Atacama Large Millimeter/ submillimeter Array were used to study transitions of CH3NH2, CH2NH, NH2CHO, and the 13C- and 15N-methyl cyanide (CH3CN) isotopologues, detected towards NGC 6334I. Column densities are derived for each species assuming local thermodynamic equilibrium and excitation temperatures in the range 220–340 K for CH3NH2, 70–110 K for the CH3CN isotopologues and 120–215 K for NH2CHO and CH2NH. Results. We report the first detections of CH3NH2 towards NGC 6334I with column density ratios with respect to CH3OH of 5.9 × 10−3, 1.5 × 10−3 and 5.4 × 10−4 for the three hot cores MM1, MM2, and MM3, respectively. These values are slightly lower than the values derived for Sagittarius B2 but higher by more than an order of magnitude as compared with the values derived for the low-mass protostar IRAS 16293–2422B. The column density ratios of NH2CHO, 13CH3CN, and CH3C15N with respect to CH3OH are (1.5 – 1.9) × 10−4, (1.0 – 4.6) × 10−3 and (1.7 – 3.0) × 10−3 respectively. Lower limits of 5.2, 1.2, and 3.0 are reported for the CH3NH2 to CH2NH column density ratio for MM1, MM2, and MM3 respectively. These limits are largely consistent with the values derived for Sagittarius B2 and higher than those for IRAS 16293–2422B. Conclusions. The detections of CH3NH2 in the hot cores of NGC 6334I hint that CH3NH2 is generally common in the interstellar medium, albeit that high-sensitivity observations are essential forthe detection of the species. The good agreement between model predictions of CH3NH2 ratios and the observations towards NGC 6334I indicate a main formation pathway via radical recombination on grain surfaces. This process may be stimulated further by high grain temperatures allowing a lager degree of radical mobility. Further observations with ALMA will help evaluate the degree to which CH3NH2 chemistry depends on the temperature of the grains in high- and low-mass star-forming regions respectively.

First Results of an ALMA Band 10 Spectral Line Survey of NGC 6334I: Detections of Glycolaldehyde (HC(O)CH2OH) and a New Compact Bipolar Outflow in HDO and CS

Abstract We present the first results of a pilot program to conduct an Atacama Large Millimeter Array (ALMA) band 10 spectral line survey of the high-mass star-forming region NGC 6334I. The observations were taken in exceptional weather conditions (0.19 mm precipitable water) with typical system temperatures T sys < 950 K at ∼890 GHz. A bright, bipolar north–south outflow is seen in HDO and CS emission, driven by the embedded massive protostar MM1B. This has allowed, for the first time, a direct comparison of the thermal water in this outflow to the location of water maser emission from prior 22 GHz Very Large Array observations. The maser locations are shown to correspond to the sites along the outflow cavity walls, where high-velocity gas impacts the surrounding material. We also compare our new observations to prior Herschel Heterodyne Instrument for the Far-infrared (HIFI) spectral line survey data of this field, detecting an order of magnitude more spectral lines (695 versus 65) in the Atacama Large Millimeter/submillimeter Array (ALMA) data. We focus on the strong detections of the complex organic molecule glycolaldehyde (HC(O)CH2OH) in the ALMA data that is not detected in the heavily beam-diluted HIFI spectra. Finally, we stress the need for dedicated THz laboratory spectroscopy to support and exploit future high-frequency molecular line observations with ALMA.

Low levels of methanol deuteration in the high-mass star-forming region NGC 6334I

Context. The abundance of deuterated molecules in a star-forming region is sensitive to the environment in which they are formed. Deuteration fractions, in other words the ratio of a species containing D to its hydrogenated counterpart, therefore provide a powerful tool for studying the physical and chemical evolution of a star-forming system. While local low-mass star-forming regions show very high deuteration ratios, much lower fractions are observed towards Orion and the Galactic centre. Astration of deuterium has been suggested as a possible cause for low deuteration in the Galactic centre. Aims. We derive methanol deuteration fractions at a number of locations towards the high-mass star-forming region NGC 6334I, located at a mean distance of 1.3 kpc, and discuss how these can shed light on the conditions prevailing during its formation. Methods. We use high sensitivity, high spatial and spectral resolution observations obtained with the Atacama Large Millimeter/ submillimeter Array to study transitions of the less abundant, optically thin, methanol-isotopologues: 13CH3OH, CH318OH, CH2DOH and CH3OD, detected towards NGC 6334I. Assuming local thermodynamic equilibrium (LTE) and excitation temperatures of ~120–330 K, we derive column densities for each of the species and use these to infer CH2DOH/CH3OH and CH3OD/CH3OH fractions. Results. We derive column densities in a range of (0.8–8.3) × 1017 cm−2 for 13CH3OH, (0.13–3.4) × 1017 cm−2 for CH318OH, (0.03–1.63) × 1017 cm−2 for CH2DOH and (0.15–5.5) × 1017 cm−2 for CH3OD in a ~1″ beam. Interestingly, the column densities of CH3OD are consistently higher than those of CH2DOH throughout the region by factors of 2–15. We calculate the CH2DOH to CH3OH and CH3OD to CH3OH ratios for each of the sampled locations in NGC 6334I. These values range from 0.03% to 0.34% for CH2DOH and from 0.27% to 1.07% for CH3OD if we use the 13C isotope of methanol as a standard; using the 18 O-methanol as a standard, decreases the ratios by factors of between two and three. Conclusions. All regions studied in this work show CH2DOH/CH3OH as well as CH2DOH/CH3OD values that are considerably lower than those derived towards low-mass star-forming regions and slightly lower than those derived for the high-mass star-forming regions in Orion and the Galactic centre. The low ratios indicate a grain surface temperature during formation ~30 K, for which the efficiency of the formation of deuterated species is significantly reduced. Therefore, astration of deuterium in the Galactic centre cannot be the explanation for its low deuteration ratio but rather the high temperatures characterising the region.

ALMA Detection of Interstellar Methoxymethanol (CH3OCH2OH)

Abstract We report the detection of interstellar methoxymethanol (CH3OCH2OH) in Atacama Large Millimeter/submillimeter Array (ALMA) Bands 6 and 7 toward the MM1 core in the high-mass star-forming region NGC 6334I at ∼0.″1–1″ spatial resolution. A column density of 4(2) × 1018 cm−2 at T ex = 200 K is derived toward MM1, ∼34 times less abundant than methanol (CH3OH), and significantly higher than predicted by astrochemical models. Probable formation and destruction pathways are discussed, primarily through the reaction of the CH3OH photodissociation products, the methoxy (CH3O) and hydroxymethyl (CH2OH) radicals. Finally, we comment on the implications of these mechanisms on gas-phase versus grain-surface routes operative in the region, and the possibility of electron-induced dissociation of CH3OH rather than photodissociation.

Molecular line survey of the high-mass star-forming region NGC 6334I withHerschel/HIFI and the Submillimeter Array

We aim at deriving the molecular abundances and temperatures of the hot molecular cores in the high-mass star-forming region NGC 6334I and consequently deriving their physical and astrochemical conditions. In the framework of the Herschel guaranteed time key program CHESS, NGC 6334I is investigated by using HIFI aboard the Herschel Space Observatory. A spectral line survey is carried out in the frequency range 480-1907 GHz, and auxiliary interferometric data from the SMA in the 230 GHz band provide spatial information for disentangling the different physical components contributing to the HIFI spectrum. The spectral lines are identified with the aid of former surveys and spectral line catalogs. The observed spectrum is then compared to a simulated synthetic spectrum with XCLASS, assuming local thermal equilibrium, and best fit parameters are derived using the model optimization package MAGIX. A total of 46 molecules are identified, with 31 isotopologues, resulting in about 4300 emission and absorption lines. High- energy levels of the dominant emitter methanol and vibrationally excited HCN are detected. The number of unidentified lines remains low with 75, or less than 2 percent of the lines detected. The modeling suggests that several spectral features need two or more components to be fitted properly. Other components could be assigned to cold foreground clouds or to outflows, most visible in the SiO emission. A chemical variation between the two embedded hot cores is found, with more N-bearing molecules identified in SMA1 and O-bearing molecules in SMA2. Spectral line surveys give powerful insights into the study of the interstellar medium. Different molecules trace different physical conditions like the inner hot core, the envelope, the outflows or the cold foreground clouds. The derived molecular abundances provide further constraints for astrochemical models.

Herschel/HIFI observations of spectrally resolved methylidyne signatures toward the high-mass star-forming core NGC 6334I

<i>Context. <i/>In contrast to the more extensively studied dense star-forming cores, little is known about diffuse gas surrounding star-forming regions. <i>Aims. <i/>We study the molecular gas in the Galactic high-mass star-forming region NGC 6334I, which contains diffuse, quiescent components that are inconspicuous in widely used molecular tracers such as CO. <i>Methods. <i/>We present <i>Herschel<i/>/HIFI observations of methylidyne (CH) toward NGC 6334I observed as part of the “Chemical <i>HErschel<i/> Survey of Star forming regions” (CHESS) key program. HIFI resolves each of the six hyperfine components of the lowest rotational transition (<i>J<i/> = –) of CH, observed in both emission and absorption.<i>Results. <i/>The CH emission features appear close to the systemic velocity of NGC 6334I, while its measured <i>FWHM<i/> linewidth of 3 km s<sup>-1<sup/> is smaller than previously observed in dense gas tracers such as NH<sub>3<sub/> and SiO. The CH abundance in the hot core is ~7 × 10<sup>-11<sup/>, two to three orders of magnitude lower than in diffuse clouds. While other studies find distinct outflows in, e.g., CO and H<sub>2<sub/>O toward NGC 6334I, we do not detect any outflow signatures in CH. At least two redshifted components of cold absorbing material must be present at –3.0 and +6.5 km s<sup>-1<sup/> to explain the absorption signatures. We derive a CH column density (<i>N<i/><sub>CH<sub/>) of 7 × 10<sup>13<sup/> and 3 × 10<sup>13<sup/> cm<sup>-2<sup/> for these two absorbing clouds. We find evidence of two additional absorbing clouds at +8.0 and 0.0 km s<sup>-1<sup/>, both with <i>N<i/><sub>CH<sub/> <i>≈<i/> 2 × 10<sup>13<sup/> cm<sup>-2<sup/>. Turbulent linewidths for the four absorption components vary between 1.5 and 5.0 km s<sup>-1<sup/> in <i>FWHM<i/>. We constrain the physical properties and locations of the clouds by matching our CH absorbers with the absorption signatures seen in other molecular tracers.<i>Conclusions. <i/>In the hot core, molecules such as H<sub>2<sub/>O and CO trace gas that is heated and dynamically influenced by outflow activity, whereas the CH molecule traces more quiescent material. The four CH absorbing clouds have column densities and turbulent properties that are consistent with those of diffuse clouds: two are located in the direct surroundings of NGC 6334, and two are unrelated foreground clouds. Local density and dynamical effects influence the chemical composition of the physical components of NGC 6334, which causes some components to be seen in CH but not in other tracers, and vice versa.