Discovery of non-metastable ammonia masers in Sagittarius B2

We have carried out polarization calibration for archival Jansky Very Large Array (JVLA) (∼9 mm) full polarization observations toward the Class 0 young stellar object (YSO) OMC-3/MMS 6 (also known as HOPS-87), and then compared the results with the archival Atacama Large Millimeter Array (ALMA) 1.2 mm observations. The resolved spectral indices show that the innermost ∼100 au region of OMC-3/MMS 6 is marginally optically thin (e.g., τ ≲ 1) at ∼9 mm wavelength, such that the JVLA observations can directly probe the linearly polarized emission from nonspherical dust. Assuming that the projected long axis of dust grains is aligned perpendicular to magnetic field (B-field) lines, we propose that the overall B-field topology resembles an hourglass shape. The geometry of this system is consistent with a magnetically regulated dense (pseudo)disk, although this “hourglass” appears to be ∼40° inclined with respect to the previously reported outflow axis. In contrast, the inner ∼100 au region of this YSO is likely very optically thick (e.g., τ ≫ 1) at ∼1 mm wavelength. The electric field position angles resolved by JVLA and ALMA present ∼90° offsets on this region, which indicate that the dominant polarization mechanism at 1 mm wavelength is dichroic extinction. This is the second case where the (sub)millimeter dichroic extinction is demonstrated by the direct comparison between the JVLA and ALMA polarization observations.

Context.The detection of a branched alkyl molecule in the high-mass star forming protocluster Sagittarius (Sgr) B2(N) permitted by the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) revealed a new dimension of interstellar chemistry. Astrochemical simulations subsequently predicted that beyond a certain degree of molecular complexity, branched molecules could even dominate over their straight-chain isomers.Aims.More generally, we aim to probe further the presence in the interstellar medium of complex organic molecules with the capacity to exhibit both a normal and iso form, via the attachment of a functional group to either a primary or secondary carbon atom. Methods. We used the imaging spectral line survey ReMoCA performed with ALMA at high angular resolution and the results of a recent spectroscopic study of propanol to search for the iso and normal isomers of this molecule in the hot molecular core Sgr B2(N2). We analyzed the interferometric spectra under the assumption of local thermodynamical equilibrium. We expanded the network of the astrochemical model MAGICKAL to explore the formation routes of propanol and put the observational results in a broader astrochemical context.Results.We report the first interstellar detection of iso-propanol, ¿-C3H7OH, toward a position of Sgr B2(N2) that shows narrow linewidths. We also report the first secure detection of the normal isomer of propanol, n-C3H7OH, in a hot core. Iso-propanol is found to be nearly as abundant as normal-propanol, with an abundance ratio of 0.6 which is similar to the ratio of 0.4 that we obtained previously for iso- and normal-propyl cyanide in Sgr B2(N2) at lower angular resolution with our previous ALMA survey, EMoCA. The observational results are in good agreement with the outcomes of our astrochemical models, which indicate that the OH-radical addition to propylene in dust-grain ice mantles, driven by water photodissociation, can produce appropriate quantities of normal- and iso-propanol. The normal-to-iso ratio in Sgr B2(N2) may be a direct inheritance of the branching ratio of this reaction process.Conclusions.The detection of normal- and iso-propanol and their ratio indicate that the modest preference for the normal form of propyl cyanide determined previously may be a more general feature among similarly sized interstellar molecules. Detecting other pairs of interstellar organic molecules with a functional group attached either to a primary or secondary carbon may help in pinning down the processes that dominate in setting their normal-to-iso ratios. Butanol and its isomers would be the next obvious candidates in the alcohol family, but their detection in hot cores will be challenging.

Context. Hot molecular cores correspond to the phase of star formation during which many molecules, in particular complex organic molecules (COMs), thermally desorb from the surface of dust grains. Sophisticated kinetic models of interstellar chemistry describe the processes that lead to the formation and subsequent evolution of COMs in star-forming regions. Aims. Our goal is to derive the chemical composition of hot cores in order to improve our understanding of interstellar chemistry. In particular, we want to test the models by comparing their predictions to the observed composition of the gas phase of hot cores. Methods. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to perform an imaging spectral line survey of the high mass star-forming region Sagittarius B2(N) at 3 mm, called Re-exploring Molecular Complexity with ALMA (ReMoCA). We modeled under the assumption of local thermodynamic equilibrium the spectra obtained with this survey toward the sources embedded in the secondary hot core Sgr B2(N2). We compared the chemical composition of these sources to that of sources from the literature and to predictions of the chemical kinetics model MAGICKAL. Results. We detected up to 58 molecules toward Sgr B2(N2)’s hot cores, including up to 24 COMs, as well as many less abundant isotopologs. The compositions of some pairs of sources are well correlated, but differences also exist, in particular for HNCO and NH2CHO. The abundances of series of homologous molecules drop by about one order of magnitude at each further step in complexity. The nondetection of radicals yields stringent constraints on the models. The comparison to the chemical models confirms previous evidence of a high cosmic-ray ionization rate in Sgr B2(N). The comparison to sources from the literature gives a new insight into chemical differentiation. The composition of most hot cores of Sgr B2(N2) is tightly correlated to that of the hot core G31.41+0.31 and the hot corino IRAS 16293–2422 B after normalizing the abundances by classes of molecules (O-bearing, N-bearing, O+N-bearing, and S-bearing). There is no overall correlation between Sgr B2(N2) and the shocked region G+0.693−0.027 also located in Sgr B2, and even less with the cold starless core TMC-1. The class of N-bearing species reveals the largest variance among the four classes of molecules. The S-bearing class shows in contrast the smallest variance. Conclusions. These results imply that the class of N-bearing molecules reacts more sensitively to shocks, low-temperature gas phase chemistry after nonthermal desorption, or density. The overall abundance shifts observed between the N-bearing and O-bearing molecules may indicate how violently and completely the ice mantles are desorbed.

The SgrB2 molecular cloud contains several sites forming high-mass stars. SgrB2(N) is one of its main centers of activity. It hosts several compact and UCHII regions, as well as two known hot molecular cores (SgrB2(N1) and SgrB2(N2)), where complex organic molecules are detected. Our goal is to use the high sensitivity of ALMA to characterize the hot core population in SgrB2(N) and shed a new light on the star formation process. We use a complete 3 mm spectral line survey conducted with ALMA to search for faint hot cores in SgrB2(N). We report the discovery of three new hot cores that we call SgrB2(N3), SgrB2(N4), and SgrB2(N5). The three sources are associated with class II methanol masers, well known tracers of high-mass star formation, and SgrB2(N5) also with a UCHII region. The chemical composition of the sources and the column densities are derived by modelling the whole spectra under the assumption of LTE. The H2 column densities are computed from ALMA and SMA continuum emission maps. The H2 column densities of these new hot cores are found to be 16 up to 36 times lower than the one of the main hot core Sgr B2(N1). Their spectra have spectral line densities of 11 up to 31 emission lines per GHz, assigned to 22-25 molecules. We derive rotational temperatures around 140-180 K for the three new hot cores and mean source sizes of 0.4 for SgrB2(N3) and 1.0 for SgrB2(N4) and SgrB2(N5). SgrB2(N3) and SgrB2(N5) show high velocity wing emission in typical outflow tracers, with a bipolar morphology in their integrated intensity maps suggesting the presence of an outflow, like in SgrB2(N1). The associations of the hot cores with class II methanol masers, outflows, and/or UCHII regions tentatively suggest the following age sequence: SgrB2(N4), SgrB2(N3), SgrB2(N5), SgrB2(N1). The status of SgrB2(N2) is unclear. It may contain two distinct sources, a UCHII region and a very young hot core.

The distribution of H2O masers in the Sgr B2 core was observed with a 2.5′×2.5′ wide field and with 540 km s−1 total velocity coverage by the Nobeyama Millimeter Array. Thirty-nine resolved maser spots were detected with a relative positional accuracy of 0.3″, which are clustered into four separate regions. In Sgr B2 north, the cluster lies at the edge of the continuum ridge. One of the maser spots shows strong and wide velocity-spread emission, suggesting it may correspond to a center of star forming activity. In Sgr B2 main, the strong maser spots are projected just on the face of a compact HII region, and are red-shifted relative to the central velocity of the HII region. There are two possibilities to interpret our results in Sgr B2 (M). One is that the H2O maser spots are distributed around the HII region and are infailing to the HII region. The other is that the H2O maser sources are associated with the cloud in the foreground of the HII region.

The distribution of H2O masers in the Sgr B2 core was observed with a 2.5′×2.5′ wide field and with 540 km s-1 total velocity coverage by the Nobeyama Millimeter Array. Thirty-nine resolved maser spots were detected with a relative positional accuracy of 0.3″, which are clustered into four separate regions. In Sgr B2 north, the cluster lies at the edge of the continuum ridge. One of the maser spots shows strong and wide velocity-spread emission, suggesting it may correspond to a center of star forming activity. In, Sgr B2 main, the strong maser spots are projected just on the face of a compact HII region, and are red-shifted relative to the central velocity of the HII region. There are two possibilities to interpret our results in Sgr B2 (M). One is that the H2O maser spots are distributed around the HII region and are infailing to the HII region. The other is that the H2O maser sources are associated with the cloud in the foreground of the HII region.

Context. The chemical differentiation of seven complex organic molecules (COMs) in the extended region around Sagittarius B2 (Sgr B2) has been previously observed: CH2OHCHO, CH3OCHO, t-HCOOH, C2H5OH, and CH3NH2 were detected both in the extended region and near the hot cores Sgr B2(N) and Sgr B2(M), while CH3OCH3 and C2H5CN were only detected near the hot cores. The density and temperature in the extended region are relatively low in comparison with Sgr B2(N) and Sgr B2(M). Different desorption mechanisms, including photodesorption, reactive desorption, and shock heating, and a few other mechanisms have been proposed to explain the observed COMs in the cold regions. However, they fail to explain the deficiency of CH3OCH3 and C2H5CN in the extended region around Sgr B2. Aims. Based on known physical properties of the extended region around Sgr B2, we explored under what physical conditions the chemical simulations can fit the observations and explain the different spatial distribution of these seven species in the extended region around Sgr B2. Methods. We used the macroscopic Monte Carlo method to perform a detailed parameter space study. A static physical model and an evolving physical model including a cold phase and a warm-up phase were used, respectively. The fiducial models adopt the observed physical parameters except for the local cosmic ray ionization rate ζCR. In addition to photodesorption that is included in all models, we investigated how chain reaction mechanism, shocks, an X-ray burst, enhanced reactive desorption and low diffusion barriers could affect the results of chemical modeling. Results. All gas-grain chemical models based on static physics cannot fit the observations, except for the high abundances of CH3NH2 and C2H5CN in some cases. The simulations based on evolving physical conditions can fit six COMs when T ~ 30−60 K in the warm-up phase, but the best-fit temperature is still higher than the observed dust temperature of 20 K. The best agreement between the simulations and all seven observed COMs at a lower temperature T ~ 27 K is achieved by considering a short-duration ≈102 yr X-ray burst with ζCR = 1.3 × 10−13 s−1 at the early stage of the warm-up phase when it still has a temperature of 20 K. The reactive desorption is the key mechanism for producing these COMs and inducing the low abundances of CH3OCH3 and C2H5CN. Conclusions. We conclude that the evolution of the extended region around Sgr B2 may have begun with a cold, T ≤ 10 K phase followed by a warm-up phase. When its temperature reached about T ~ 20 K, an X-ray flare from the Galactic black hole Sgr A* with a short duration of no more than 100 yr was acquired, affecting strongly the Sgr B2 chemistry. The observed COMs in Sgr B2 are able to retain their observed abundances only several hundred years after such a flare, which could imply that such short-term X-rays flares occur relatively often, likely associated with the accretion activity of the Sgr A* source.

Aims. A few years ago, we investigated the massive young stellar object (MYSO) G29.862–0.0044 (YSO-G29), an intriguing star-forming region at a distance of 6.2 kpc. Although the typical disc-jet scenario was proposed to explain the observations, it remained far from conclusive. We wonder if the puzzling observed near-IR features are produced by only one source or if it is due to confusion generated by an unresolved system of YSOs. Unveiling this issue is important for a better understanding of the star-forming processes. Methods. We analysed YSO-G29 using new observations in the near-IR from Gemini-NIFS, at the radio continuum (10 GHz) from the Jansky Very Large Array (JVLA) and new continuum (1.3 mm) and molecular line data from the Atacama Large Millimeter Array (ALMA). Results. The near-IR observations allowed us to detect emission of H2 1−0 S(1) and Brγ lines in YSO-G29, which are compatible with excitation and ionisation from UV radiation propagating in a highly perturbed ambient. In addition, we also found some evidence of H2 excitation by collisions. The ALMA data show the presence of a conspicuous and collimated molecular outflow propagating southwards, while to the north, an extended molecular feature perfectly surrounded by the Ks near-IR emission appears. The continuum emission at 1.3 mm allowed us to better resolve the molecular cores, one of which stands out due to its high temperatures and rich chemical composition. From the JVLA observations, we discovered a compact radio continuum source, a likely compact HII region or an ionised jet of a massive protostar, located at ~0.″7 (~0.02 pc) from the main millimetre core. In this way, we propose a YSO wide binary system. Conclusions. We can explain the nature of the intriguing near-IR features previously observed: Cone-like structures produced by jets or winds of one of the components of the binary system that cleared out the surroundings were disrupted by a molecular outflow probably from the other component. These results complete the picture of what is happening in YSO-G29 and reveal a phenomenon that should be considered when investigating massive star-forming regions.

view Abstract Citations (39) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS 6 Centimeter Formaldehyde Absorption toward the Sagittarius B Star-forming Complex Mehringer, David M. ; Palmer, Patrick ; Goss, W. M. Abstract Observations of the 6 cm transition of H2CO (formaldehyde) toward the Sgr B massive star-forming complex have been carried out using the Very Large Array (VLA). The purpose of this 15 sec resolution absorption study was to determine the parameters (e.g., velocities, optical depths, and line widths) of the molecular clouds that lie along the line of sight. The H2CO clouds studied have velocities in the range -130 km/s less than vLSR less than +130 km/s. The H2CO spectrum of Sgr B1 is very different from that of Sgr B2. The optical depths of the molecular clouds associated with Sgr B2 are greater than 1, while the opacities are less than 1 for the molecular clouds associated with Sgr B1. Thus, much of the molecular material in Sgr B1 seems to have dispersed, suggesting that Sgr B1 is more evolved than Sgr B2. This same conclusion was reached by Mehringer et al. on the basis of radio continuum observations. In Sgr B2, H2CO spectra vary dramatically from one position to another, which suggests that the H II regions lie at different depths within the clouds. Thus, several isolated events may have triggered star formation throughout Sgr B2, in contrast to many current models which suggest a single global event is responsible. The properties of a number of clouds which are in the central part of the Galaxy, but are not associated with the H II regions, are briefly discussed. Publication: The Astrophysical Journal Supplement Series Pub Date: April 1995 DOI: 10.1086/192148 Bibcode: 1995ApJS...97..497M Keywords: Absorption Spectra; Formaldehyde; H Ii Regions; Interstellar Matter; Molecular Clouds; Radio Astronomy; Radio Spectra; Star Formation; Centimeter Waves; Opacity; Optical Thickness; Superhigh Frequencies; Very Large Array (Vla); Astronomy; ISM: CLOUDS; ISM: H II REGIONS; ISM: INDIVIDUAL NAME: SAGITTARIUS B; ISM: KINEMATICS AND DYNAMICS; ISM: MOLECULES; RADIO LINES: ISM full text sources ADS | data products SIMBAD (31)

Context. The giant molecular cloud complex Sagittarius B2 (Sgr B2) in the central molecular zone of our Galaxy hosts several high-mass star formation sites, with Sgr B2(M) and Sgr B2(N) being the main centers of activity. This analysis aims to comprehensively model each core spectrum, considering molecular lines, dust attenuation, and free-free emission interactions. We describe the molecular content analysis of each hot core and identify the chemical composition of detected sources. Aims. Using ALMA’s high sensitivity, we aim to characterize the hot core population in Sgr B2(M) and N, gaining a better understanding of the different evolutionary phases of star formation processes in this complex. Methods. We conducted an unbiased ALMA spectral line survey of 47 sources in band 6 (211-275 GHz). Chemical composition and column densities were derived using XCLASS, assuming local thermodynamic equilibrium. Quantitative descriptions for each molecule were determined, considering all emission and absorption features across the spectral range. Temperature and velocity distributions were analyzed, and derived abundances were compared with other spectral line surveys. Results. We identified 65 isotopologs from 41 different molecules, ranging from light molecules to complex organic compounds, originating from various environments. Most sources in the Sgr B2 complex were assigned different evolutionary phases of high-mass star formation. Conclusions. Sgr B2(N) hot cores show more complex molecules such as CH3OH, CH3OCHO, and CH3OCH3, while M cores contain lighter molecules such as SO2, SO, and NO. Some sulfur-bearing molecules are more abundant in N than in M. The derived molecular abundances can be used for comparison and to constrain astrochemical models. Inner sources in both regions were generally more developed than outer sources, with some exceptions.

Context. Numerous complex organic molecules have been detected in the universe and among them are amides, which are considered as prime models for species containing a peptide linkage. In its backbone, acrylamide (CH2CHC(O)NH2) bears not only the peptide bond, but also the vinyl functional group that is a common structural feature in many interstellar compounds. This makes acrylamide an interesting candidate for searches in the interstellar medium. In addition, a tentative detection of the related molecule propionamide (C2H5C(O)NH2) has been recently claimed toward Sgr B2(N). Aims. The aim of this work is to extend the knowledge of the laboratory rotational spectrum of acrylamide to higher frequencies, which would make it possible to conduct a rigorous search for interstellar signatures of this amide using millimeter wave astronomy. Methods. We measured and analyzed the rotational spectrum of acrylamide between 75 and 480 GHz. We searched for emission of acrylamide in the imaging spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array toward Sgr B2(N). We also searched for propionamide in the same source. The astronomical spectra were analyzed under the assumption of local thermodynamic equilibrium. Results. We report accurate laboratory measurements and analyses of thousands of rotational transitions in the ground state and two excited vibrational states of the most stable syn form of acrylamide. In addition, we report an extensive set of rotational transitions for the less stable skew conformer. Tunneling through a low energy barrier between two symmetrically equivalent configurations has been revealed for this higher-energy species. Neither acrylamide nor propionamide were detected toward the two main hot molecular cores of Sgr B2(N). We did not detect propionamide either toward a position located to the east of the main hot core, thereby undermining the recent claim of its interstellar detection toward this position. We find that acrylamide and propionamide are at least 26 and 14 times less abundant, respectively, than acetamide toward the main hot core Sgr B2(N1S), and at least 6 and 3 times less abundant, respectively, than acetamide toward the secondary hot core Sgr B2(N2). Conclusions. A comparison with results of astrochemical kinetics model for related species suggests that acrylamide may be a few hundred times less abundant than acetamide, corresponding to a value that is at least an order of magnitude lower than the observational upper limits. Propionamide may be as little as only a factor of two less abundant than the upper limit derived toward Sgr B2(N1S). Lastly, the spectroscopic data presented in this work will aid future searches of acrylamide in space.

Context. Urea, NH2C(O)NH2, is a molecule of great importance in organic chemistry and biology. Two searches for urea in the interstellar medium have been reported in the past, but neither were conclusive. Aims. We want to take advantage of the increased sensitivity and angular resolution provided by the Atacama Large Millimeter/submillimeter Array (ALMA) to search for urea toward the hot molecular cores embedded in the high-mass-star-forming region Sgr B2(N). Methods. We used the new spectral line survey named ReMoCA (Re-exploring Molecular Complexity with ALMA) that was performed toward Sgr B2(N) with ALMA in its observing cycle 4 between 84 and 114 GHz. The spectra were analyzed under the local thermodynamic equilibrium approximation. We constructed a full synthetic spectrum that includes all the molecules identified so far. We used new spectroscopic predictions for urea in its vibrational ground state and first vibrationally excited state to search for this complex organic molecule in the ReMoCA data set. We employed the gas-grain chemical kinetics model MAGICKAL to interpret the astronomical observations. Results. We report the secure detection of urea toward the hot core Sgr B2(N1) at a position called N1S slightly offset from the continuum peak, which avoids obscuration by the dust. The identification of urea relies on nine clearly detected transitions. We derive a column density of 2.7 × 1016 cm−2 for urea, two orders of magnitude lower than the column density of formamide, and one order of magnitude below that of methyl isocyanate, acetamide, and N-methylformamide. The latter molecule is reliably identified toward N1S with 60 clearly detected lines, confirming an earlier claim of its tentative interstellar detection. We report the first interstellar detections of NH2CH18O and 15NH2CHO. We also report the nondetection of urea toward the secondary hot core Sgr B2(N2) with an abundance relative to the other four species at least one order of magnitude lower than toward the main hot core. Our chemical model roughly reproduces the relative abundances of formamide, methyl isocyanate, acetamide, and N-methylformamide, but it overproduces urea by at least one order of magnitude. Conclusions. Urea is clearly detected in one of the hot cores. Comparing the full chemical composition of Sgr B2(N1S) and Sgr B2(N2) may help understand why urea is at least one order of magnitude less abundant in the latter source.

We report the discovery of nine new hot molecular cores in the Deep South (DS) region of Sagittarius B2 using Atacama Large Millimeter/submillimeter Array Band 6 observations. We measure the rotational temperature of CH3OH and derive the physical conditions present within these cores and the hot core Sgr B2(S). The cores show heterogeneous temperature structure, with peak temperatures between 252 and 662 K. We find that the cores span a range of masses (203–4842 M ⊙) and radii (3587–9436 au). CH3OH abundances consistently increase with temperature across the sample. Our measurements show the DS hot cores are structurally similar to Galactic disk hot cores, with radii and temperature gradients that are comparable to sources in the disk. They also show shallower density gradients than disk hot cores, which may arise from the Central Molecular Zone’s higher density threshold for star formation. The hot cores have properties which are consistent with those of Sgr B2(N), with three associated with Class II CH3OH masers and one associated with an ultra-compact H ii region. Our sample nearly doubles the high-mass star-forming gas mass near Sgr B2(S) and suggests the region may be a younger, comparably massive counterpart to Sgr B2(N) and (M). The relationship between peak CH3OH abundance and rotational temperature traced by our sample and a selection of comparable hot cores is qualitatively consistent with predictions from chemical modeling. However, we observe constant peak abundances at higher temperatures (T ≳ 250 K), which may indicate mechanisms for methanol survival that are not yet accounted for in models.

view Abstract Citations (59) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Sagittarius B2 Star-forming Region. III. High-Resolution H52 alpha and H66 alpha Observations of Sagittarius B2 Main de Pree, C. G. ; Gaume, R. A. ; Goss, W. M. ; Claussen, M. J. Abstract The Galactic center star-forming region Sagittarius B2 Main [Sgr B2 (M)] has been observed with the Very Large Array in the continuum (7 mm and 1.3 cm) and in the radio recombination lines H52α, H66α, and He 66α. The H66α(∼0".25 resolution) and H52α(∼2".5 resolution) data provide high spatial resolution kinematics of the ionized gas associated with the massive stars. Both high spatial resolution and sensitivity to large-scale structures are essential to thoroughly sample the H II regions in Sgr B2 (M) where source sizes of the 36 H II regions range from unresolved (<0".3) to extended (∼10"). H66α recombination line emission has been detected toward 23 of the 36 continuum sources in Sgr B2 (M). The mean velocity of the detected sources is 62±2 km s-1. He 66α line emission is detected toward nine of the 36 continuum sources. The 1.3 cm helium and hydrogen data are used together as a high-resolution probe of Y+ values and variations within Sgr B2 (M). An average singly ionized helium abundance of <Y+> = 10.3%±1.2% is derived. Of the sources where a Y+ value is determined, only Sgr B2 F has an anomalously low value (Y+ ≤ 5%). The recombination line data are examined in detail toward several of the most unusual sources. A very broad radio recombination line (ΔVH66α ∼ 64 km s-1, ΔVH52α ∼ 59 km s-1) is measured, and high electron temperatures [Te(H66α)* = 26,400 K] are derived for Sgr B2 F. The broad lines and high temperatures may be due to a nonequilibrium environment at the edge of an expanding H II region. Assuming that the F and G H II regions have expanded in a dense molecular environment, they appear to be young, with an average age of <τ> = 0.7±0.3 x 104 yr. This lower limit on the age is consistent with the dynamical age of the nearby massive molecular outflow observed by Lis et al. A doubly peaked H66α line has been detected toward the ultracompact H II (UC HII) region Z10.24. This source may be either a very young (τ < 100 yr) shell source or the ionized base of a molecular outflow. The small number of morphological cometary H II regions in Sgr B2 (<5%) and the observed velocity gradients in those regions that do have a cometary morphology indicate that the moving star bow shock model cannot account for any of the H II regions in Sgr B2 (M). Publication: The Astrophysical Journal Pub Date: June 1996 DOI: 10.1086/177364 Bibcode: 1996ApJ...464..788D Keywords: ISM: KINEMATICS AND DYNAMICS; ISM: H II REGIONS; ISM: INDIVIDUAL NAME: SAGITTARIUS B2; GALAXY: CENTER; RADIO LINES: ISM full text sources ADS | data products SIMBAD (47) Related Materials (2) Part 1: 1995ApJ...449..663G Part 2: 1995ApJ...451..284D

Context. The giant molecular cloud Sagittarius B2 (hereafter Sgr B2) is the most massive region with ongoing high-mass star formation in the Galaxy. In the southern region of the 40-pc large envelope of Sgr B2, we encounter the Sgr B2(DS) region, which hosts more than 60 high-mass protostellar cores distributed in an arc shape around an extended H II region. Hints of non-thermal emission have been found in the H II region associated with Sgr B2(DS). Aims. We seek to characterize the spatial structure and the spectral energy distribution of the radio continuum emission in Sgr B2(DS). We aim to disentangle the contribution from the thermal and non-thermal radiation, as well as to study the origin of the non-thermal radiation. Methods. We used the Very Large Array in its CnB and D configurations, and in the frequency bands C (4–8 GHz) and X (8–12 GHz) to observe the whole Sgr B2 complex. Continuum and radio recombination line maps are obtained. Results. We detect radio continuum emission in Sgr B2(DS) in a bubble-shaped structure. From 4 to 12 GHz, we derive a spectral index between − 1.2 and − 0.4, indicating the presence of non-thermal emission. We decomposed the contribution from thermal and non-thermal emission, and find that the thermal component is clumpy and more concentrated, while the non-thermal component is more extended and diffuse. The radio recombination lines in the region are found to be not in local thermodynamic equilibrium but stimulated by the non-thermal emission. Conclusions. Sgr B2(DS) shows a mixture of thermal and non-thermal emission at radio wavelengths. The thermal free–free emission is likely tracing an H II region ionized by an O 7 star, while the non-thermal emission can be generated by relativistic electrons created through first-order Fermi acceleration. We have developed a simple model of the Sgr B2(DS) region and found that first-order Fermi acceleration can reproduce the observed flux density and spectral index.