Sagittarius B2 Molecular Cloud Research Articles

Sulfur isotopes toward the extended envelope of Sagittarius B2 in the Galactic Center

Abstract Isotopic ratios are good tools for probing stellar nucleosynthesis and chemical evolution. We perform high-sensitivity mapping observations of the J = 7–6 rotational transitions of OCS, OC34S, O13CS, and OC33S toward the Galactic Center giant molecular cloud Sagittarius B2 with the IRAM 30 m telescope. Positions with optically thin and uncontaminated lines are chosen to determine the sulfur isotope ratios. A 32S/34S ratio of 17.1 ± 0.9 was derived with OCS and OC34S lines, while a 34S/33S ratio of 6.8 ± 1.9 was derived directly from the integrated intensity ratio of OC34S and OC33S. With independent and accurate measurements of the 32S/34S ratio, our results confirm the termination of the decreasing trend of 32S/34S ratios toward the Galactic Center, suggesting a drop in the production of massive stars at the Galactic Center.

Discovery of non-metastable ammonia masers in Sagittarius B2

We report the discovery of widespread maser emission in non-metastable inversion transitions of NH3 toward various parts of the Sagittarius B2 molecular cloud and star-forming region complex. We detect masers in the J, K = (6, 3), (7,4), (8,5), (9,6), and (10,7) transitions toward Sgr B2(M) and Sgr B2(N), an NH3 (6,3) maser in Sgr B2(NS), and NH3 (7,4), (9,6), and (10,7) masers in Sgr B2(S). With the high angular resolution data of the Karl G. Jansky Very Large Array (JVLA) in the A-configuration, we identify 18 maser spots. Nine maser spots arise from Sgr B2(N), one from Sgr B2(NS), five from Sgr B2(M), and three in Sgr B2(S). Compared to our Effelsberg single-dish data, the JVLA data indicate no missing flux. The detected maser spots are not resolved by our JVLA observations. Lower limits to the brightness temperature are > 3000 K and reach up to several 105 K, manifesting the lines’ maser nature. In view of the masers’ velocity differences with respect to adjacent hot molecular cores and/or UCH II regions, it is argued that all the measured ammonia maser lines may be associated with shocks caused either by outflows or by the expansion of UCH II regions. Overall, Sgr B2 is unique in that it allows us to measure many NH3 masers simultaneously, which may be essential in order to elucidate their thus far poorly understood origin and excitation.

The physical and chemical structure of Sagittarius B2

Context. The giant molecular cloud Sagittarius B2 (hereafter SgrB2) is the most massive region with ongoing high-mass star formation in the Galaxy. Two ultra-compact Hii (UCHii ) regions were identified in SgrB2’s central hot cores, SgrB2(M) and SgrB2(N). Aims. Our aim is to characterize the properties of the Hii regions in the entire SgrB2 cloud. Comparing the Hii regions and the dust cores, we aim to depict the evolutionary stages of different parts of SgrB2. Methods. We use the Very Large Array in its A, CnB, and D configurations, and in the frequency band C (~6GHz) to observe the whole SgrB2 complex. Using ancillary VLA data at 22.4 GHz and ALMA data at 96 GHz, we calculated the physical parameters of the UCHii regions and their dense gas environment. Results. We identify 54 UCHii regions in the 6 GHz image, 39 of which are also detected at 22.4 GHz. Eight of the 54 UCHii regions are newly discovered. The UCHii regions have radii between 0.006 pc and 0.04 pc, and have emission measure between 106 pc cm6 and 109 pc cm6. The UCHii regions are ionized by stars of types from B0.5 to O6. We found a typical gas density of ~106–109 cm3 around the UCHii regions. The pressure of the UCHii regions and the dense gas surrounding them are comparable. The expansion timescale of these UCHii regions is determined to be ~104–105 yr. The percentage of the dust cores that are associated with Hii regions are 33%, 73%, 4%, and 1% for SgrB2(N), SgrB2(M), SgrB2(S), and SgrB2(DS), respectively. Two-thirds of the dust cores in SgrB2(DS) are associated with outflows. Conclusions. The electron densities of the UCHii regions we identified are in agreement with that of typical UCHii regions, while the radii are smaller than those of the typical UCHii regions. The dust cores in SgrB2(M) are more evolved than in SgrB2(N). The dust cores in SgrB2(DS) are younger than in SgrB2(M) or SgrB2(N).

New Constraints on Cosmic Particle Populations at the Galactic Center Using X-Ray Observations of the Molecular Cloud Sagittarius B2

The Sagittarius B2 (Sgr B2) molecular cloud complex is an X-ray reflection nebula whose nonthermal X-ray emissions have continued to decrease since 2001 as it reprocesses one or more past energetic outbursts from the supermassive black hole Sagittarius A* at the Galactic Center. The X-ray reflection model explains the observed time variability of Sgr B2 and provides a window into the luminous evolutionary history of our nearest supermassive black hole. In light of evidence of elevated cosmic particle populations in the Galactic Center, X-rays from Sgr B2 are also of interest as a probe of low-energy (sub-GeV) cosmic rays, which may be responsible for an increasing relative fraction of the nonthermal emission as the contribution from X-ray reflection decreases. Here, we present the most recent NuSTAR and XMM-Newton observations of Sgr B2, from 2018, and we emphasize the Kα fluorescence line of neutral Fe. These 2018 observations reveal small-scale variations within lower-density portions of the complex, including brightening features, yet still enable upper limits on X-rays from low-energy cosmic-ray interactions in Sgr B2. We present Fe Kα line fluxes from cloud regions of different densities, facilitating comparison with models of ambient low-energy cosmic-ray interactions throughout the cloud.

Rotational spectroscopy of isotopic cyclopropenone, c-H2C3O, and determination of its equilibrium structure

Context. Cyclopropenone was first detected in the cold and less dense envelope of the giant molecular cloud Sagittarius B2(N). It was found later in several cold dark clouds and it may be possible to detect its minor isotopic species in these environments. In addition, the main species may well be identified in warmer environments. Aims. We aim to extend existing line lists of isotopologs of c-H2C3O from the microwave to the millimeter region and create one for the singly deuterated isotopolog to facilitate their detections in space. Furthermore, we aim to extend the line list of the main isotopic species to the submillimeter region and to evaluate an equilibrium structure of the molecule. Methods. We employed a cyclopropenone sample in natural isotopic composition to investigate the rotational spectra of the main and 18O-containing isotopologs as well as the two isotopomers containing one 13C atom. Spectral recordings of the singly and doubly deuterated isotopic species were obtained using a cyclopropenone sample highly enriched in deuterium. We recorded rotational transitions in the 70−126 and 160−245 GHz regions for all isotopologs and also in the 342−505 GHz range for the main species. Quantum-chemical calculations were carried out to evaluate initial spectroscopic parameters and the differences between ground-state and equilibrium rotational parameters in order to derive semi-empirical equilibrium structural parameters. Results. We determined new or improved spectroscopic parameters for six isotopologs and structural parameters according to different structure models. Conclusions. The spectroscopic parameters are accurate enough to identify minor isotopic species at centimeter and millimeter wavelengths while those of the main species are deemed to be reliable up to 1 THz. Our structural parameters differ from earlier ones. The deviations are attributed to misassignments in the earlier spectrum of one isotopic species.

Ketene and Ammonia Forming Acetamide in the Interstellar Medium

Background: Peptide bonds are among the fundamental building blocks of life, polymerizing amino acids to form proteins that make up the structural components of living cells and regulate biochemical processes. The detection of glycine by NASA in comet Wild 2 in 2009 suggests the possibility of the formation of biomolecules in extraterrestrial environments through the interstellar medium. Detected in the dense molecular cloud Sagittarius B2, acetamide is the largest molecule containing a peptide bond and is hypothesized to be the precursor to all amino acids; as such, viability of its formation is of important biological relevance.
 Methods: Under a proposed mechanism of ammonia and ketene reactants, which have also been detected in dense molecular clouds in the ISM, the reaction pathway for the formation of acetamide was modelled using quantum chemical calculations in Gaussian16, using Austin-Frisch-Petersson functional with dispersion density functional theory at a 6-31G(d) basis set level of theory to optimize geometries and determine the thermodynamic properties for the reaction. Stability of the reactants, transition states, and products were examined to establish a reasonable mechanism.
 Conclusion: Product formation of acetamide was found to be highly exergonic and exothermic with a low energy barrier, suggesting a mechanism that is viable in the extreme density and temperature conditions found in ISM.

The physical and chemical structure of Sagittarius B2

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.

The complex chemistry of hot cores in Sgr B2(N): influence of cosmic-ray ionization and thermal history

Context. As the number of complex organic molecules (COMs) detected in the interstellar medium increases, it becomes even more important to place meaningful constraints on the origins and formation pathways of such chemical species. The molecular cloud Sagittarius B2(N) is host to several hot molecular cores in the early stage of star formation, where a great variety of COMs are detected in the gas phase. Given its exposure to the extreme conditions of the Galactic center (GC) region, Sgr B2(N) is one of the best targets to study the impact of environmental conditions on the production of COMs. Aims. Our main goal is to characterize the physico-chemical evolution of Sgr B2(N)’s sources in order to explain their chemical differences and constrain their environmental conditions. Methods. The chemical composition of Sgr B2(N)’s hot cores, N2, N3, N4, and N5 is derived by modeling their 3 mm emission spectra extracted from the Exploring Molecular Complexity with ALMA (EMoCA) imaging spectral line survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA). We derived the density distribution in the envelope of the sources based on the masses computed from the ALMA dust continuum emission maps. We used the radiative transfer code RADMC-3D to compute temperature profiles and inferred the current luminosity of the sources based on the COM rotational temperatures derived from population diagrams. We used published results of 3D radiation-magnetohydrodynamical (RMHD) simulations of high-mass star formation to estimate the time evolution of the source properties. We employed the astrochemical code MAGICKAL to compute time-dependent chemical abundances in the sources and to investigate how physical properties and environmental conditions influence the production of COMs. Results. The analysis of the abundances of 11 COMs detected toward Sgr B2(N2-N5) reveals that N3 and N5 share a similar chemical composition while N2 differs significantly from the other sources. We estimate the current luminosities of N2, N3, N4, and N5 to be 2.6 × 105 L⊙, 4.5 × 104 L⊙, 3.9 × 105 L⊙, and 2.8 × 105 L⊙, respectively. We find that astrochemical models with a cosmic-ray ionization rate of 7 × 10−16 s−1 best reproduce the abundances with respect to methanol of ten COMs observed toward Sgr B2(N2-N5). We also show that COMs still form efficiently on dust grains with minimum dust temperatures in the prestellar phase as high as 15 K, but that minimum temperatures higher than 25 K are excluded. Conclusions. The chemical evolution of Sgr B2(N2-N5) strongly depends on their physical history. A more realistic description of the hot cores’ physical evolution requires a more rigorous treatment with RMHD simulations tailored to each hot core.

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.

A quantum chemical study on the formation of ethanimine (CH3CHNH) in the interstellar ice

Ethanimine (CH3CHNH) is an important prebiotic molecule since it is a precursor of amino acid $\alpha $ -alanine in Strecker synthesis. Two isomers (E and Z) of ethanimine were detected in the molecular cloud Sagittarius B2 north during GBT-PRIMOS survey. A possible radical-molecule reaction pathway has been proposed for the formation of ethanimine in the interstellar medium (ISM) from some previously detected interstellar molecules like methylene (both triplet CH2 (3B1) and singlet CH2 (1A1)) and methyenimine (CH2NH). The mechanism has been studied in the gas phase and in water ice with the help of density functional theory at B2PLYPD/6-311++G (2d, p) level of theory. It is observed that E-ethanimine forms efficiently in gas phase but ice reactions are favorable only in the hot core of molecular clouds. Same is true for the formation of Z-ethanimine which forms only at the surface of water cluster as the height of entrance barrier for formation of Z-ethanimine is similar to that of E-ethanimine. Isomerization from E to Z form is also studied and found to be forbidden due to large entrance barrier. Out of the two reaction system CH2 (3B1) + CH2NH and CH2 (1A1) + CH2NH, later is more favorable then the former one due to the small entrance barrier. Still, much of the detected abundance of ethanimine comes from the reaction of CH2 (3B1) with CH2NH as since CH2 (1A1) has very low abundance compared to the CH2 (3B1) in ISM. The proposed pathway seems to be a promising candidate for the ethanimine formation in ISM.

Widespread Presence of Glycolaldehyde and Ethylene Glycol around Sagittarius B2

Abstract We report the detection of widespread CH2OHCHO and HOCH2CH2OH emission in Galactic center giant molecular cloud Sagittarius B2 using the Shanghai Tianma 65 m Radio Telescope. Our observations show for the first time that the spatial distribution of these two important prebiotic molecules extends over 15 arcmin, corresponding to a linear size of approximately 36 pc. These two molecules are not just distributed in or near the hot cores. The abundance of these two molecules seems to decrease from the cold outer region to the central region associated with star formation activity. Results presented here suggest that these two molecules are likely to form through a low temperature process. Recent theoretical and experimental studies demonstrated that prebiotic molecules can be efficiently formed in icy grain mantles through several pathways. However, these complex ice features cannot be directly observed, and most constraints on the ice compositions come from millimeter observations of desorbed ice chemistry products. These results, combined with laboratory studies, strongly support the existence of abundant prebiotic molecules in ices.

A 3-mm molecular line study of the Central Molecular Zone of the Galaxy

AbstractWe are studying the Central Molecular Zone (CMZ) in the inner few degrees around the Galactic Centre, by mapping multiple 3-mm molecular lines, with the 22-m Mopra telescope. During 2006, we covered a 5 × 5 arcmin2 area of the Sagittarius B2 molecular cloud complex (Jones et al. 2008). We find substantial differences in chemical and physical conditions within the complex. We show some results here of Principal Component Analysis (PCA) of line features in this Sgr B2 area. During 2007 we covered the larger region of longitude −0.2 to 0.9 deg. and latitude -0.20 to 0.12 deg., including Sgr A and Sgr B2, in the frequency range 85.3 to 91.3 GHz. This includes lines of C3H2, CH3CCH, HOCO+, SO, H13CN, H13CO+, SO, H13NC, C2H, HNCO, HCN, HCO+, HNC, HC3N, 13CS and N2H+.

A 3-mm molecular line study of the Central Molecular Zone

We are studying the Central Molecular Zone (CMZ) in the inner few degrees around the Galactic Centre, by mapping a large number of 3-mm molecular lines. We have used the 22-m Mopra telescope, with the new capabilities of the 8-GHz bandwidth MOPS digital filter bank. During 2006, we covered a 5 × 5 arcmin2 area of the Sagittarius B2 molecular cloud complex, with spectral coverage of nearly all of the range 81.7 to 113.5 GHz with 4 tunings of the broad band mode. We imaged over 50 spectral lines with strong extended emission and around 110 more lines which were weak, or concentrated at the Sgr B2(N) and Sgr B2(M) cores (Jones et al. 2008). We find substantial differences in chemical and physical conditions within the complex. During 2007 we covered the larger region of longitude -0.2 to 0.9 deg. and latitude -0.20 to 0.12 deg., including Sgr A and Sgr B2, in the frequency range 85.3 to 91.3 GHz. This includes lines of C3 H2 , CH3 CCH, HOCO+ , SO, H13 CN, H13 CO+ , SO, HN13 C, C2 H, HNCO, HCN, HCO+ , HNC, HC3 N, 13 CS and N2 H+ .

X-ray studies of massive star birth regions

I report X-ray features of young stage of intermediate and high mass stars. The giant molecular cloud Sagittarius B2 (Sgr B2) exhibits more than dozen X-ray sources, two are associated with the ultra compact (UC) HII complex, Sgr B2 Main. The sources show large absorption of $\gg$ 10 23 Hcm −2 , the largest among any known stellar X-ray sources. The Arches cluster exhibits 3 extremely bright (a few $\times10^{33}$ ergs s −1 ) X-ray sources associated with the infrared (IR) massive stars. The X-ray spectra have 1-3 keV temperature in thin thermal model. Elongated diffuse 6.4 keV line emission is found. The Monoceros R2 cloud exhibits half dozen X-ray sources associated with young high-mass IR stars. They show rapid time variability and a thin thermal spectrum of ${\sim }2$ keV temperature. Among 28 ASC A pointing on intermediate-mass pre-main-sequence stars, or Herbig Ae/Be stars (HAeBes), eleven are found to be plausible X-ray sources. The general X-ray properties of these HAeBes are; (1) the plasma temperature and time variability are higher than those of high mass main-sequence stars, and more similar to low mass stars; (2) the X-ray luminosities come to the upper end of low mass pre-main sequence stars, or in some cases exceed that; (3) the X-ray activity of HAeBe decreases at the age of about a few×10 6 years. Using above observational facts, I propose a unified picture of X-ray activity of young stars in the wide mass range.

Far-infrared detection of $^{\mathsf{17}}$OH towards Sagittarius B2

The frequencies and line strengths of the 2 Π3/2 J = 5/2-3/2 rotational transition of 17 OH have been calculated from an analysis of its far-infrared laser magnetic resonance spectrum. These results have been used to make the first detection of a pure rotational transition of 17 OH in the ISM. Two resolved components of this transition appear in absorption towards the giant molecular cloud Sagittarius B2, which was observed at a spectral resolution of 33 km s −1 with the Fabry-Perot mode of the ISO Long Wavelength Spectrometer. The corresponding transition of 18 OH was also observed and its line shape was modelled using H  measurements. The 18 O/ 17 O ratio of 3.5 was then used to compare this with the observed 17 OH line shape.

X‐Ray Sources and Star Formation Activity in the Sagittarius B2 Cloud Observed withChandra

We report the X-ray population study in the giant molecular cloud Sagittarius B2 (Sgr B2). More than a dozen X-ray cloud members (and candidates) are discovered with Chandra. Two bright X-ray sources are located near Sgr B2 Main, the most copious complex of the ultracompact H II sources. The X-ray spectra are fitted with a thin thermal plasma model of 5-10 keV temperature. The intrinsic luminosity, after correcting the absorption of ~5 × 1023 H cm-2, is ~1033 ergs s-1. Although these two X-ray sources are attributable to young stellar objects (YSOs) in the same H II complex, they are in sharp contrast; the one at the center of the H II complex exhibits strong K-shell transition lines of iron, while the other near the east has only weak lines. The other H II complexes, Sgr B2 North and South, also show hard and highly absorbed X-ray emissions due possibly to the star formation activity. The composite X-ray spectrum of the other cloud member X-ray sources is fitted with a thin thermal plasma of ~10 keV temperature with hydrogen column density (NH) of 1.3 × 1023 H cm-2 and individual X-ray luminosity of a few times 1031-1032 ergs s-1. These are likely to be a single YSO or cluster of YSOs, but neither radio nor infrared counterpart is found. An alternative scenario of isolated white dwarfs powered by Bondi-Hoyle accretion from the dense cloud gas is also discussed. The X-ray spectra exhibit an additional 6.4 keV line of neutral or low-ionization irons, which indicates that the environment gas is concentrated near the sources.

Detection of HD in emission towards Sagittarius B2

The giant molecular cloud Sagittarius B2, located near the Galactic Centre, has been observed in the far-infrared by the ISO Long Wavelength Spectrometer. Wavelengths in the range 47-196 microns were covered with the high resolution Fabry-Perot spectrometer, giving a spectral resolution of 30-40 km/s. The J=1-0 and J=2-1 rotational transitions of HD fall within this range at 112 microns and 56 microns. A probable detection was made of the ground state J=1-0 line in emission but the J=2-1 line was not detected above the noise. This allowed us to calculate an upper limit on the temperature in the emitting region of approximately 80 K and a value for the deuterium abundance in the Sgr B2 envelope of D/H=(0.2-11)x10^-6.

ChandraObservations of Diffuse X‐Rays from the Sagittarius B2 Cloud

We present the first Chandra results of the giant molecular cloud Sagittarius B2 (Sgr B2), located about 100 pc away from the Galactic center. Diffuse X-rays are clearly separated from one-and-a-half-dozen resolved point sources. The X-ray spectrum exhibits pronounced iron K-shell transition lines at 6.40 keV (Kα) and 7.06 keV (Kβ), deep iron K-edge at 7.11 keV, and large photoelectric absorption at low energy. The absorption-corrected X-ray luminosity is ~1 × 1035 ergs s-1, 2 orders of magnitude larger than the integrated luminosity of all the resolved point sources. The diffuse X-rays come mainly from the southwest half of the cloud with a concave shape pointing to the Galactic center direction. These results strongly support the ASCA model that Sgr B2 is irradiated by an X-ray source at the Galactic center side.

Far-Infrared Detection of H[TINF]3[/TINF]O[TSUP]+[/TSUP] in Sagittarius B2

We report the first far-infrared detection of the hydronium ion (H3O+) toward the Sagittarius B2 molecular cloud, near the Galactic center. Using the Infrared Space Observatory Long-Wavelength Spectrometer, we have detected three lines arising from the ν2 ground-state inversion mode (0+ → 0-) at 55.3 cm-1. All transitions are observed in absorption against the optically thick infrared continuum emission of the dust. Two different absorption regions can be distinguished, the H3O+ associated with the Sgr B2 extended envelope and that of foreground gas along the line of sight. The derived abundances for both components are rather similar, 1 × 10-9, and may explain the high water abundance found in the low-density gas toward Sgr B2 as its dissociative recombination leads to the formation of important molecules OH and H2O. Contribution of the Q(1, 1) line of para-H3O+ to the 212-101 line of H218O is also analyzed. This allows a better determination of water abundance from far-infrared observations. The corrected H2O abundances are ≤10-6 for the diffuse clouds along the line of sight and a few times 10-6 for the gas associated with the Sgr B2 envelope, in good agreement with water vapor observations from the Submillimeter Wave Astronomy Satellite.

Hot Expanding Shells in the Envelope of the Sagittarius B2 Molecular Cloud

We present high-resolution (3'') maps of the (3, 3) and (4, 4) lines of NH3 toward the southern part of the molecular envelope of the Sagittarius B2 star-forming region. These maps reveal, for the first time, that the morphology of the hot gas in the Sgr B2 envelope is dominated by at least six rings, two arcs, and a filament. The sizes of the rings are between 1 and 2.6 pc and their thicknesses between 0.2 and 0.4 pc. Most of the gas in the rings is warm, with kinetic temperatures, Tk, of 40-70 K, although some parts of the low-velocity rings reach temperatures larger than 100 K. These hot rings and arcs represent regions of enhanced H2 density and/or enhanced NH3 abundance in the Sgr B2 envelope. Some of the hot rings show radial velocity gradients, which suggests that the rings and arcs correspond to three-dimensional shells expanding at velocities of ~6-10 km s-1. The walls of the hot shells are highly clumped and contain unresolved condensations (1'') in the line maps. There are two kinds of unresolved condensations: those appearing in the (3, 3) line maps with only weak emission in the (4, 4) line and those with rather strong emission in the (4, 4) line compared with that of the (3, 3) line. For the first kind, we identify six condensations in the NH3 (3, 3) line maps that have brightness temperatures larger than the kinetic temperatures. It is likely that these are newly found NH3 masers. We also find that other masers in the region such as class II CH3OH and H2CO masers are very well correlated in velocity and position with the hot shells. The large number of masers observed in the hot shells can be explained as a result of the combination of high abundance of volatile molecules like NH3, CH3OH, and H2CO (produced by hot temperature and/or shock chemistry) and sufficient velocity coherence at the edges of the expanding shells. For the second kind of condensations, we have identified three compact sources in the walls of the shells that are hot (Tk 300 K) and have high H2 densities (106-107 cm-3). These exhibit characteristics similar to those of the hot cores associated with newly formed massive stars. The inferred dust luminosity for the hot cores is at least 105 L☉, similar to the Orion A hot core. The high luminosity of the hot cores and the lack of associated radio continuum emission indicates that these are internally heated and very likely associated with the dense circumstellar material surrounding newly formed stars. The detection of hot cores suggests that massive star formation is not restricted to only Sgr B2N and Sgr B2M but is also taking place in the envelope of Sgr B2. Very likely, this star formation has been triggered by the expanding bubbles that produce the shells seen in the NH3 inversion lines. We discuss the possible origin of these hot shells. We find that a wind-blown bubble driven by typical Galactic Wolf-Rayet stars could account for the kinetic energies and the momenta observed in the hot NH3 shells. A cluster containing massive evolved stars such as the Quintuplet could easily explain the large concentration of hot shells, the heating of the warm envelope, and the peculiar chemistry observed in the envelope of Sgr B2. The implication of these findings for the heating of Galactic center molecular clouds is briefly discussed.