Region Of NGC Research Articles

Probing fragmentation and velocity sub-structure in the massive NGC 6334 filament with ALMA

Context.Herschelimaging surveys of galactic interstellar clouds support a paradigm for low-mass star formation in which dense molecular filaments play a crucial role. The detailed fragmentation properties of star-forming filaments remain poorly understood, however, and the validity of the filament paradigm in the intermediate- to high-mass regime is still unclear.Aims.Here, following up on an earlier 350μm dust continuum study with the ArTéMiS camera on the APEX telescope, we investigate the detailed density and velocity structure of the main filament in the high-mass star-forming region NGC 6334.Methods.We conducted ALMA Band 3 observations in the 3.1 mm continuum and of the N2H+(1–0), HC5N(36–35), HNC(1–0), HC3N(10–9), CH3CCH(6–5), and H2CS(3–2) lines at an angular resolution of ~3′′, corresponding to 0.025 pc at a distance of 1.7 kpc.Results.The NGC 6334 filament was detected in both the 3.1 mm continuum and the N2H+, HC3N, HC5N, CH3CCH, and H2CS lines with ALMA. We identified twenty-six compact (<0.03 pc) dense cores at 3.1 mm and five velocity-coherent fiber-like features in N2H+within the main filament. The typical length (~0.5 pc) of, and velocity difference (~0.8 km s−1) between, the fiber-like features of the NGC 6334 filament are reminiscent of the properties for the fibers of the low-mass star-forming filament B211/B213 in the Taurus cloud. Only two or three of the five velocity-coherent features are well aligned with the NGC 6334 filament and may represent genuine, fiber sub-structures; the other two features may trace accretion flows onto the main filament. The mass distribution of the ALMA 3.1 mm continuum cores has a peak at ~10M⊙, which is an order of magnitude higher than the peak of the prestellar core mass function in nearby, low-mass star-forming clouds. The cores can be divided into seven groups, closely associated with dense clumps seen in the ArTéMiS 350μm data. The projected separation between ALMA dense cores (0.03–0.1 pc) and the projected spacing between ArTéMiS clumps (0.2–0.3 pc) are roughly consistent with the effective Jeans length (0.08 ± 0.03 pc) in the filament and a physical scale of about four times the filament width, respectively, if the inclination angle of the filament to line of sight is ~30°. These two distinct separation scales are suggestive of a bimodal fragmentation process in the filament.Conclusions.Despite being one order of magnitude denser and more massive than the Taurus B211/B213 filament, the NGC 6334 filament has a density and velocity structure that is qualitatively very similar. The main difference is that the dense cores embedded in the NGC 6334 filament appear to be an order of magnitude denser and more massive than the cores in the Taurus filament. This suggests that dense molecular filaments may evolve and fragment in a similar manner in low- and high-mass star-forming regions, and that the filament paradigm may hold in the intermediate-mass (if not high-mass) star formation regime.

Massive Stars in the Tarantula Nebula: A Rosetta Stone for Extragalactic Supergiant HII Regions

A review of the properties of the Tarantula Nebula (30 Doradus) in the Large Magellanic Cloud is presented, primarily from the perspective of its massive star content. The proximity of the Tarantula and its accessibility to X-ray through radio observations permit it to serve as a Rosetta Stone amongst extragalactic supergiant HII regions since one can consider both its integrated characteristics and the individual properties of individual massive stars. Recent surveys of its high mass stellar content, notably the VLT FLAMES Tarantula Survey (VFTS), are reviewed, together with VLT/MUSE observations of the central ionizing region NGC 2070 and HST/STIS spectroscopy of the young dense cluster R136, provide a near complete Hertzsprung-Russell diagram of the region, and cumulative ionizing output. Several high mass binaries are highlighted, some of which have been identified from a recent X-ray survey. Brief comparisons with the stellar content of giant HII regions in the Milky Way (NGC 3372) and Small Magellanic Cloud (NGC 346) are also made, together with Green Pea galaxies and star forming knots in high-z galaxies. Finally, the prospect of studying massive stars in metal poor galaxies is evaluated.

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.

ALMA view of the12C/13C isotopic ratio in starburst galaxies

We derive molecular-gas-phase12C/13C isotope ratios for the central few hundred parsecs of the three nearby starburst galaxies NGC 253, NGC 1068, and NGC 4945 making use of theλ ∼ 3 mm12CN and13CNN= 1–0 lines in the ALMA Band 3. The12C/13C isotopic ratios derived from the ratios of these lines range from 30 to 67 with an average of 41.6 ± 0.2 in NGC 253, from 24 to 62 with an average of 38.3 ± 0.4 in NGC 1068, and from 6 to 44 with an average of 16.9 ± 0.3 in NGC 4945. The highest12C/13C isotopic ratios are determined in some of the outskirts of the nuclear regions of the three starburst galaxies. The lowest ratios are associated with the northeastern and southwestern molecular peaks of NGC 253, the northeastern and southwestern edge of the mapped region in NGC 1068, and the very center of NGC 4945. In the case of NGC 1068, the measured ratios suggest inflow from the outer part of NGC 1068 into the circum-nuclear disk through both the halo and the bar. Low12C/13C isotopic ratios in the central regions of these starburst galaxies indicate the presence of highly processed material.

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.

The First Candidate Colliding-wind Binary in M33

Abstract We present the detection of the first candidate colliding-wind binary (CWB) in M33, located in the giant H ii region NGC 604. The source was first identified in archival Chandra imaging as a relatively soft X-ray point source, with the likely primary star determined from precise astrometric alignment between archival Hubble Space Telescope and Chandra imaging. The candidate primary star in the CWB is classified for the first time in this work as a carbon-rich Wolf–Rayet star with a likely O-star companion based on spectroscopy obtained from Gemini-North. We model the X-ray spectrum using Chandra and XMM-Newton observations, and find the CWB to be well fit as a ∼1 keV thermal plasma with a median unabsorbed luminosity in the 0.5–2.0 keV band of L X ∼ 3 × 1035 erg s−1, making this source among the brightest of CWBs observed to date. We present a long-term light curve for the candidate CWB from archival Chandra and XMM-Newton observations, and discuss the constraints placed on the binary by this light curve, as well as the X-ray luminosity at maximum. Finally, we compare this candidate CWB in M33 to other well-studied, bright CWBs in the Galaxy and Magellanic Clouds, such as η Car.

Deuterated methanol toward NGC 7538-IRS1

We investigate the deuteration of methanol towards the high-mass star-forming region NGC 7538-IRS1. We carried out a multi-transition study of CH3OH, 13CH3OH, and the deuterated flavors, CH2DOH and CH3OD, between 1.0 and 1.4 mm with the IRAM-30 m antenna. In total, 34 13CH3OH, 13 CH2DOH lines and 20 CH3OD lines spanning a wide range of upper-state energies (Eup) were detected. From the detected transitions, we estimate that the measured D/H does not exceed 1%, with a measured CH2DOH/CH3OH and CH3OD/CH3OH of about (32 ± 8) × 10−4 and (10 ± 4) × 10−4, respectively. This finding is consistent with the hypothesis of a short-timescale formation during the pre-stellar phase. We find a relative abundance CH2DOH/CH3OD ratio of 3.2 ± 1.5. This result is consistent with a statistical deuteration. We cannot exclude H/D exchanges between water and methanol if water deuteration is of the order 0.1%, as suggested by recent Herschel observations.

The Milky Way Project second data release: bubbles and bow shocks

ABSTRACT Citizen science has helped astronomers comb through large data sets to identify patterns and objects that are not easily found through automated processes. The Milky Way Project (MWP), a citizen science initiative on the Zooniverse platform, presents internet users with infrared (IR) images from Spitzer Space Telescope Galactic plane surveys. MWP volunteers make classification drawings on the images to identify targeted classes of astronomical objects. We present the MWP second data release (DR2) and an updated data reduction pipeline written in python. We aggregate ∼3 million classifications made by MWP volunteers during the years 2012–2017 to produce the DR2 catalogue, which contains 2600 IR bubbles and 599 candidate bow shock driving stars. The reliability of bubble identifications, as assessed by comparison to visual identifications by trained experts and scoring by a machine-learning algorithm, is found to be a significant improvement over DR1. We assess the reliability of IR bow shocks via comparison to expert identifications and the colours of candidate bow shock driving stars in the 2MASS point-source catalogue. We hence identify highly reliable subsets of 1394 DR2 bubbles and 453 bow shock driving stars. Uncertainties on object coordinates and bubble size/shape parameters are included in the DR2 catalogue. Compared with DR1, the DR2 bubbles catalogue provides more accurate shapes and sizes. The DR2 catalogue identifies 311 new bow shock driving star candidates, including three associated with the giant H ii regions NGC 3603 and RCW 49.

Determination of the Primordial Helium Abundance Based on NGC 346, an H ii Region of the Small Magellanic Cloud

Abstract To place meaningful constraints on Big Bang Nucleosynthesis models, the primordial helium abundance determination is crucial. Low-metallicity H ii regions have been used to estimate it because their statistical uncertainties are relatively small. We present a new determination of the primordial helium abundance, based on long-slit spectra of the H ii region NGC 346 in the small Magellanic cloud. We obtained spectra using three 409″ × 0.51″ slits divided into 97 subsets. They cover the range λλ3600–7400 of the electromagnetic spectrum. We used PyNeb and standard reduction procedures to determine the physical conditions and chemical composition. We found that for NGC 346 X = 0.7465, Y = 0.2505, and Z = 0.0030. By assuming ΔY/ΔO = 3.3 ± 0.7 we found that the primordial helium abundance is Y P = 0.2451 ± 0.0026 (1σ). Our Y P value is in agreement with the value of neutrino families, N ν , and with the neutron half-life time, τ n , obtained in the laboratory.

Modeling of CoRoT and Spitzer lightcurves in NGC 2264 caused by an optically thick warp

Aims. We present an analysis of simultaneously observed CoRoT and Spitzer lightcurves for four systems in the stellar forming region NGC 2264: Mon-660, Mon-811, Mon-1140, and Mon-1308. These objects share in common a strong resemblance between the optical and infrared lightcurves, such that the mechanism responsible for producing them is the same. The aim of this paper is to explain both lightcurves simultaneously with only one mechanism. Methods. We modeled the infrared emission as coming from a warp composed of an optically thick wall and an optically thick asymmetric disk beyond this location. We modeled the optical emission mainly by partial stellar occultation by the warp. Results. The magnitude amplitude of the CoRoT and Spitzer observations for all the objects can be described with the emission coming from the system components. The difference between them is the value of the disk flux compared with the wall flux and the azimuthal variations of the former. This result points out the importance of the hydrodynamical interaction between the stellar magnetic field and the disk. Conclusions. CoRoT and Spitzer lightcurves for the stellar systems Mon-660, Mon-811, Mon-1140, and Mon-1308 can be simultaneously explained using the emission coming from an asymmetric disk and emission with stellar occultation by an optically thick wall.

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.

Relationship Between the Ionization Energy and the Radial Velocity of the Ionized Gas in Gaseous Nebulae

We study the relationship between the radial velocity of different ionic emission lines as a function of their ionization potential for the two largest giant extragalactic HII regions in NGC 6822. Our aim is to study the spatial velocity stratification along the line of vision within the nebula.

X-Ray Structure between the Innermost Disk and Optical Broad-line Region in NGC 4151

Abstract We present an analysis of the narrow Fe Kα line in Chandra/HETGS observations of the Seyfert active galactic nucleus (AGN) NGC 4151. The sensitivity and resolution afforded by the gratings reveal asymmetry in this line. Models including weak Doppler boosting, gravitational redshifts, and scattering are generally preferred over Gaussians at the 5σ level of confidence, and generally measure radii consistent with . Separate fits to “high/unobscured” and “low/obscured” phases reveal that the line originates at smaller radii in high-flux states; model-independent tests indicate that this effect is significant at the 4–5σ level. Some models and Δt ≃ 2 × 104 s variations in line flux suggest that the narrow Fe Kα line may originate at radii as small as in high-flux states. These results indicate that the narrow Fe Kα line in NGC 4151 is primarily excited in the innermost part of the optical broad line region (BLR), or X-ray BLR. Alternatively, a warp could provide the solid angle needed to enhance Fe Kα line emission from intermediate radii, and might resolve an apparent discrepancy in the inclination of the innermost and outer disk in NGC 4151. Both warps and the BLR may originate through radiation pressure, so these explanations may be linked. We discuss our results in detail, and consider the potential for future observations with Chandra, XARM, and ATHENA to measure black hole masses and to study the intermediate disk in AGNs using narrow Fe Kα emission lines.

A High-quality Velocity-delay Map of the Broad-line Region in NGC 5548

Abstract NGC 5548 has been well spectroscopically monitored for reverberation mapping of the central kinematics by 19 campaigns. Using the maximum entropy method in this Letter, we build up a high-quality velocity-delay map of the Hβ emission line in the light curves of the continuum and the line variations observed between 2015 and 2016. The map shows the response strength and lags of the velocity fields of the Hβ-emitting regions. The velocity-delay structure of the map is generally symmetric, with strong red and blue wings at time lag τ ≲ 15 days, a narrower velocity distribution at τ ≳ 15 days, and a deficit of response in the core. This is suggestive of a disk geometry of the broad-line region (BLR). The relatively weaker Hβ response at the longer lags in the red side indicates anisotropic emission from the outer part of the BLR. We also recover the velocity-delay maps of NGC 5548 from the historical data of 13 years to investigate the long-term variability of its BLR. In general, the BLR of NGC 5548 was switching between the inflow and virialized phases in the past years. The resultant maps of seven years reveal inflow signatures and show decreasing lags, indicating that the changes in the BLR size are related to the infalling BLR gas. The other four maps show potential disk signatures that are similar to our map.

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.

Twinkle little stars: Massive stars are quenched in strong magnetic fields

AbstractThe role of the magnetic fields in the formation and quenching of stars with different mass is unknown. We studied the energy balance and the star formation efficiency in a sample of molecular clouds in the central kpc region of NGC 1097, known to be highly magnetized. Combining the full polarization VLA/radio continuum observations with the HST/Hα, Paα and the SMA/CO lines observations, we separated the thermal and non-thermal synchrotron emission and compared the magnetic, turbulent, and thermal pressures. Most of the molecular clouds are magnetically supported against gravitational collapse needed to form cores of massive stars. The massive star formation efficiency of the clouds also drops with the magnetic field strength, while it is uncorrelated with turbulence (Tabatabaei et al. 2018). The inefficiency of the massive star formation and the low-mass stellar population in the center of NGC 1097 can be explained in the following steps: I) Magnetic fields supporting the molecular clouds prevent the collapse of gas to densities needed to form massive stars. II) These clouds can then be fragmented into smaller pieces due to e.g., stellar feedback, non-linear perturbations and instabilities leading to local, small-scale diffusion of the magnetic fields. III) Self-gravity overcomes and the smaller clouds seed the cores of the low-mass stars.

A New Detection of Extragalactic Anomalous Microwave Emission in a Compact, Optically Faint Region of NGC 4725

Abstract We discuss the nature of a discrete, compact radio source (NGC 4725 B) located ≈1.9 kpc from the nucleus in the nearby star-forming galaxy NGC 4725, which we believe to be a new detection of extragalactic anomalous microwave emission (AME). Based on detections at 3, 15, 22, 33, and 44 GHz, NGC 4725 B is a microjansky radio source peaking at ≈33 GHz. While the source is not identified in optical (BVRI) photometry, we detect counterparts in the midinfrared Spitzer/IRAC bands (3.6, 4.5, 5.8, 8.0 μm) that appear to be associated with dust emission in the central region of NGC 4725. Consequently, we conclude that NGC 4725 B is a new detection of AME and is very similar to a recent detection of AME in an outer-disk star-forming region in NGC 6946. We find that models of electric dipole emission from rapidly rotating ultra-small grains are able to reproduce the radio spectrum for reasonable interstellar medium conditions. Given the lack of an optical counterpart and the shape of the radio spectrum, NGC 4725 B appears consistent with a nascent star-forming region in which young (≲3 Myr) massive stars are still highly enshrouded by their natal cocoons of gas and dust with insufficient supernovae occurring to produce a measurable amount of synchrotron emission.

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.

The Charge State of Polycyclic Aromatic Hydrocarbons across a Reflection Nebula, an H ii Region, and a Planetary Nebula

Abstract Low-resolution Spitzer-IRS spectral map data of a reflection nebula (NGC 7023), H ii region (M17), and planetary nebula (NGC 40), totaling 1417 spectra, are analyzed using the data and tools available through the NASA Ames PAH IR Spectroscopic Database. The polycyclic aromatic hydrocarbon (PAH) emission is broken down into PAH charge and size subclass contributions using a database-fitting approach. The resulting charge breakdown results are combined with those derived using the traditional PAH band strength ratio approach, which interprets particular PAH band strength ratios as proxies for PAH charge. Here the 6.2/11.2 μm PAH band strength ratio is successfully calibrated against its database equivalent: the ratio. In turn, this ratio is converted into the PAH ionization parameter, which relates it to the strength of the radiation field, gas temperature, and electron density. Population diagrams are used to derive the density and temperature. The bifurcated plot of the 8.6 versus 11.2 μm PAH band strength for the northwest photo dissociation region in NGC 7023 is shown to be a robust diagnostic template for the ratio in all three objects. Template spectra for the PAH charge and size subclasses are determined for each object and shown to favorably compare. Using the determined template spectra from NGC 7023 to fit the emission in all three objects yields, upon inspection of the Structure SIMilarity maps, satisfactory results. The choice of extinction curve proves to be critical. Concluding, the distinctly different astronomical environments of a reflection nebula, H ii region, and planetary nebula are reflected in their PAH emission spectra.

Triggering the formation of the supergiant H ii region NGC 604 in M 33

Abstract Formation mechanism of a supergiant H ii region NGC 604 is discussed in terms of collision of H i clouds in M 33. An analysis of the archival H i data obtained with the Very Large Array (VLA) reveals complex velocity distributions around NGC 604. The H i clouds are composed of two velocity components separated by ∼20 km s−1 for an extent of ∼700 pc, beyond the size of the the H ii region. Although the H i clouds are not easily separated in velocity with some mixed component represented by merged line profiles, the atomic gas mass amounts to $6 \times 10^{6}\,M_{\odot }$ and $9 \times 10^{6}\,M_{\odot }$ for each component. These characteristics of H i gas and the distributions of dense molecular gas in the overlapping regions of the two velocity components suggest that the formation of giant molecular clouds and the following massive cluster formation have been induced by the collision of H i clouds with different velocities. Referring to the existence of a gas bridging feature connecting M 33 with M 31 reported by large-scale H i surveys, the disturbed atomic gas possibly represents the result of past tidal interaction between the two galaxies, which is analogous to the formation of the R 136 cluster in the LMC.