Region Of M82 Research Articles

Reassessing the proper motions of M31/M33 with Gaia DR3. Unravelling systematic uncertainties

We provide an updated inference of the proper motion of M31 using the Gaia DR3 proper motions of bright stars from the disc of M31. By refining the motion of the quasar reference frame, and statistically accounting for the variations in the inferred proper motions obtained across different regions of M31, we demonstrate that these inconsistencies most likely arise from systematic uncertainties. Our updated favoured values for the proper motion of M31 are $46.9 stat sys in the right ascension direction and $-29.1 stat sys in the declination direction, the systematics being determined at a 90<!PCT!> confidence level (the values for M33 are given in the paper). This clearly highlights that the systematics are the dominant source of uncertainty, their magnitudes being comparable to the proper motion of M31 itself. The analysis conducted using Gaia DR2 instead of DR3 revealed that a net reduction in these systematic uncertainties occurred between the two data releases. If similar progress is made with the upcoming DR4, the future Gaia -based estimates could match the uncertainty level of HST, and could be used to refine the dynamics and history of M31 and M33.

The spectroscopic binary fraction of the young stellar cluster M17

Context. Significant progress has been made toward understanding the formation of massive (M > 8 M⊙) binaries in close orbits (with periods of less than a month). Some of the observational studies leading to this progress are the detection of a very low velocity dispersion among the massive stars in the young region M17 and the measurement of a positive trend of velocity dispersion with age in Galactic clusters. The velocity dispersion observed in M17 could be explained either by the lack of binaries among the stars in this region, which implies the highly unlikely scenario of a different formation mechanism for M17 than for other Galactic regions, or by larger binary separations than typically observed, but with a binary fraction similar to other young Galactic clusters. The latter implies that, over time, the binary components migrate toward each other. This is in agreement with the finding that the radial velocity dispersion of young Galactic clusters correlates positively with their age. Aims. We aim to determine the origin of the strikingly low velocity dispersion by determining the observed and intrinsic binary fraction of massive stars in M17 through multi-epoch spectroscopy. Methods. We performed a multi-epoch spectroscopic survey consisting of three epochs separated by days and months, respectively. We complemented this survey with existing data covering timescales of years. We determined the radial velocity of each star at each epoch by fitting the stellar absorption profiles. The velocity shifts between epochs were used to determine whether a close companion is present. Results. We determined an observed binary fraction of 27% and an intrinsic binary fraction of 87%, consistent with that of other Galactic clusters. We conclude that the low velocity dispersion is due to a large separation among the young massive binaries in M17. Our result is in agreement with a migration scenario in which massive stars are born in binaries or higher-order systems at large separation and harden within the first million years of evolution. Such an inward migration may either be driven by interaction with a remnant accretion disk or with other young stellar objects present in the system, or by dynamical interactions within the cluster. Our results imply that possibly both dynamical interactions and binary evolution are key processes in the formation of gravitational wave sources.

Radio survey of the stellar population in the infrared dark cloud G14.225-0.506

Context. The infrared dark cloud (IRDC) G14.225-0.506 is part of the extended and massive molecular cloud located to the southwest of the H II region M17. The cloud is associated with a network of filaments, which result in two different dense hubs, as well as with several signposts of star formation activity and a rich population of protostars and young stellar objects (YSOs). Aims. The aim of this work is to study the centimeter continuum emission in order to characterize the stellar population in both regions, as well as to study the evolutionary sequence across the IRDC G14.225-0.506. Methods. We performed deep (~ 1.5–3 μJy) radio continuum observations at 6 and 3.6 cm toward the IRDC G14.225-0.506 using the Karl G. Jansky Very Large Array (VLA) in its most extended A configuration (~0.3″). Data at both C and X bands were imaged using the same (u,v) range in order to derive spectral indices. We have also made use of observations taken during different days to study the presence of variability at short timescales toward the detected sources. Results. We detected a total of 66 sources, 32 in the northern region G14.2-N and 34 in the southern region G14.2-S. Ten of the sources are found to be variable, with three located in G14.2-N and seven in G14.2-S. Based on their spectral index, the emission in G14.2-N is mainly dominated by nonthermal sources while G14.2-S contains more thermal emitters. Approximately 75% of the sources present a counterpart at other wavelengths. When considering the inner 0.4 pc region around the center of each hub, the number of infrared (IR) sources in G14.2-N is larger than in G14.2-S by a factor of 4. We also studied the relation between the radio luminosity and the bolometric luminosity, finding that the thermal emission of the studied sources is compatible with thermal radio jets. For our sources with X-ray counterparts, the nonthermal emitters follow a Güdel-Benz relation with κ = 0.03, as previously suggested for other similar regions. Conclusions. We found similar levels of fragmentation between G14.2-N and G14.2-S, suggesting that both regions are most likely twin hubs. The nonthermal emission found in the less evolved objects, mainly coming from G14.2-N, suggests that G14.2-N may be composed of more massive YSOs as well as being in a more advanced evolutionary stage, consistent with the “filament-halo” gradient in age and mass from previous works. Overall, our results confirm a wider evolutionary sequence from the southwest to northeast starting in G14.2-S as the youngest part, followed by G14.2-N, and ending with the most evolved region M17.

Spectroscopic variability of massive pre-main-sequence stars in M17

Context. It is a challenge to study the formation process of massive stars: their formation time is short, there are only few of them, they are often deeply embedded, and they lie at relatively large distances. Our strategy is to study the outcome of the star formation process and to search for signatures that remain of the formation. We have access to a unique sample of (massive) pre-main-sequence (PMS) stars in the giant H II region M17. These PMS stars can be placed on PMS tracks in the Hertzsprung–Russell diagram (HRD) as we can detect their photospheric spectrum, and they exhibit spectral features indicative of the presence of a circumstellar disk. These stars are most likely in the final stage of formation. Aims. The aim is to use spectroscopic variability as a diagnostic tool to learn about the physical nature of these massive PMS stars. More specifically, we wish to determine the variability properties of the hot gaseous disks to understand the physical origin of the emission lines, identify dominant physical processes in these disks, and to find out about the presence of an accretion flow and/or jet. Methods. We obtained multiple-epoch (four to five epochs) VLT/X-shooter spectra of six young stars in M17 covering about a decade. Four of these stars are intermediate to massive PMS stars with gaseous disks. Using stacked spectra, we updated the spectral classification and searched for the presence of circumstellar features. With the temporal variance method (TVS), we determined the extent and amplitude of the spectral line variations in velocity space. The double-peaked emission lines in the PMS stars with gaseous disks were used to determine peak-to-peak velocities, V/R ratios, and the radial velocity of the systems. Simultaneous photometric variations were studied using VLT acquisition images. Results. From detailed line identification in the PMS stars with gaseous disks, we identify many (double-peaked) disk features, including a new detection of CO bandhead and CI emission. In three of these stars, we detect significant spectral variability, mainly in lines originating in the circumstellar disk, in a velocity range up to 320 km s−1, which exceeds the rotational velocity of the central sources. The shortest variability timescale is about one day. We also detect long-term (months, years) variability. The ratio of the blue and red peaks in two PMS stars shows a correlation with the peak-to-peak velocity, which might be explained by a spiral-arm structure in the disk. Conclusions. The variable PMS stars lie at similar positions in the HRD, but show significant differences in disk lines and variability. The extent and timescale of the variability differs for each star and line (sets), showing the complexity of the region where the lines are formed. We find indications for an accretion flow, slow disk winds, and/or disk structures in the hot gaseous inner disks. We find no evidence for close companions or strong accretion bursts as the cause of the variability in these PMS stars.

Massive pre-main-sequence stars in M17

Context.The young massive-star-forming region M17 contains optically visible massive pre-main-sequence stars that are surrounded by circumstellar disks. Such disks are expected to disappear when these stars enter the main sequence. The physical and dynamical structure of these remnant disks are poorly constrained, especially the inner regions where accretion, photo-evaporation, and companion formation and migration may be ongoing.Aims.We aim to constrain the physical properties of the inner parts of the circumstellar disks of massive young stellar objects B243 (6M⊙) and B331 (12M⊙), two systems for which the central star has been detected and characterized previously despite strong dust extinction.Methods.Two-dimensional radiation thermo-chemical modelling with PRODIMOof double-peaked hydrogen lines of the Paschen and Brackett series observed with X-shooter was used to probe the properties of the inner disk of the target sources. The model was modified to treat these lines. Additionally, the dust structure was studied by fitting the optical and near-infrared spectral energy distribution.Results.B243 features a hot gaseous inner disk with dust at the sublimation radius at ~3 AU. The disk appears truncated at roughly 6.5 AU; a cool outer disk of gas and dust may be present, but it cannot be detected with our data. B331 also has a hot gaseous inner disk. A gap separates the inner disk from a colder dusty outer disk starting at up to ~100 AU. In both sources the inner disk extends to almost the stellar surface. Chemistry is essential for the ionization of hydrogen in these disks.Conclusions.The lack of a gap between the central objects and these disks suggests that they accrete through boundary-layer accretion. This would exclude the stars having a strong magnetic field. Their structures suggest that both disks are transitional in nature, that is to say they are in the process of being cleared, either through boundary-layer accretion, photo-evaporation, or through companion activity.

CHAOS. VII. A Large-scale Direct Abundance Study in M33

The dispersion in chemical abundances provides a very strong constraint on the processes that drive the chemical enrichment of galaxies. Due to its proximity, the spiral galaxy M33 has been the focus of numerous chemical abundance surveys to study the chemical enrichment and dispersion in abundances over large spatial scales. The CHemical Abundances Of Spirals project has observed ∼100 H ii regions in M33 with the Large Binocular Telescope (LBT), producing the largest homogeneous sample of electron temperatures (T e ) and direct abundances in this galaxy. Our LBT observations produce a robust oxygen abundance gradient of −0.037 ± 0.007 dex kpc−1 and indicate a relatively small (0.043 ± 0.015 dex) intrinsic dispersion in oxygen abundance relative to this gradient. The dispersions in N/H and N/O are similarly small, and the abundances of Ne, S, Cl, and Ar relative to O are consistent with the solar ratio as expected for α-process or α-process-dependent elements. Taken together, the ISM in M33 is chemically well-mixed and homogeneously enriched from inside out, with no evidence of significant abundance variations at a given radius in the galaxy. Our results are compared to those of the numerous studies in the literature, and we discuss possible contaminating sources that can inflate abundance dispersion measurements. Importantly, if abundances are derived from a single T e measurement and T e –T e relationships are relied on for inferring the temperature in the unmeasured ionization zone, this can lead to systematic biases that increase the measured dispersion up to 0.11 dex.

Revealing Efficient Dust Formation at Low Metallicity in Extragalactic Carbon-rich Wolf-Rayet Binaries

Abstract We present Spitzer/InfraRed Array Camera observations of dust formation from six extragalactic carbon-rich Wolf-Rayet (WC) binary candidates in low-metallicity (Z ≲ 0.65 Z ⊙) environments using multiepoch mid-infrared (IR) imaging data from the SPitzer InfraRed Intensive Transients Survey (SPIRITS). Optical follow-up spectroscopy of SPIRITS 16ln, 19q, 16df, 18hb, and 14apu reveals emission features from C iv λ5801−12 and/or the C iii–iv λ4650/He ii λ4686 blend that are consistent with early-type WC stars. We identify SPIRITS 16ln as the variable mid-IR counterpart of the recently discovered colliding-wind WC4 + O binary candidate, N604-WRXc, located in the subsolar metallicity NGC 604 H ii region in M33. We interpret the mid-IR variability from SPIRITS 16ln as a dust-formation episode in an eccentric colliding-wind WC binary. SPIRITS 19q, 16df, 14apu, and 18hb exhibit absolute [3.6] magnitudes exceeding that of one of the most IR-luminous dust-forming WC systems known, WR 104 (M [3.6] ≲ −12.3). An analysis of dust formation in the mid-IR outburst from SPIRITS 19q reveals a high dust production rate of M ⊙ yr−1, which may therefore exceed that of the most efficient dust-forming WC systems known. We demonstrate that efficient dust formation is feasible from early-type WC binaries in the theoretical framework of colliding-wind binary dust formation if the systems host an O-type companion with high mass-loss rates ( M ⊙ yr−1). This efficient dust formation from early-type WC binaries highlights their potential role as significant sources of dust in low-metallicity environments.

Physical Properties of H ii Regions in M51 from Spectroscopic Observations

M51 and NGC 5195 is an interacting system that can be explored in great details with ground-based telescopes. The H ii regions in M51 were observed using the 2.16 m telescope of the National Astronomical Observatories of the Chinese Academy of Sciences and the 6.5 m Multiple Mirror Telescope with spatial resolution of less than ∼100 pc. We obtain a total of 113 spectra across the galaxy and combine the literature data of Croxall et al. to derive a series of physical properties, including the gas-phase extinction, stellar population age, star formation rate (SFR) surface density, and oxygen abundance. The spatial distributions and radial profiles of these properties are investigated in order to study the characteristics of M51 and the clues to the formation and evolution of this galaxy. M51 presents a mild radial extinction gradient. The lower gas-phase extinction in the north spiral arms compared to the south arms are possibly caused by the past encounters with the companion galaxy of NGC 5195. A number of H ii regions have the stellar age between 50 and 500 Myr, consistent with the recent interaction history by simulations in the literatures. The SFR surface density presents a mild radial gradient, which is ubiquitous in spiral galaxies. There is a negative metallicity gradient of −0.08 dex in the disk region, which is also commonly found in many spiral galaxies. It is supported by the “inside-out” scenario of galaxy formation. We find a positive abundance gradient of 0.26 dex in the inner region. There are possible reasons causing the positive gradient, including the freezing of the chemical enrichment due to the star-forming quenching in the bulge and the gas infall and dilution due to the pseudobulge growth and/or galactic interaction.

Internal kinematics of giant H ii regions in M101 with the Keck Cosmic Web Imager

ABSTRACT We study the kinematics of the giant H ii regions NGC 5455 and NGC 5471 located in the galaxy M101, using integral field observations that include the H β and [O iii]$\, \lambda$ 5007 emission lines, obtained with the Keck Cosmic Web Imager. We analyse the line profiles using both single and multiple Gaussian curves, gathering evidence for the presence of several expanding shells and moving filaments. The line decomposition shows that a broad (σ ≃ 30–50 km s−1) underlying component is ubiquitous, extending across hundreds of pc, while a large fraction of the narrow components have subsonic line widths. The supersonic turbulence inferred from the global line profiles is consistent with the velocity dispersion of the individual narrow components, i.e. the global profiles likely arise from the combined contribution of discrete gas clouds. We confirm the presence of very extended (400–1200 km s−1) low-intensity line components in three bright star-forming cores in NGC 5471, possibly representing kinematic signatures of supernova remnants. For one of these, the known supernova remnant host NGC 5471 B, we find a significantly reduced [O iii]/H β line ratio relative to the surrounding photoionized gas, due to the presence of a radiative shock at low metallicity. We explore the systematic width discrepancy between H i and [O iii] lines, present in both global and individual spaxel spectra. We argue that the resolution of this long-standing problem lies in the physics of the line-emitting gas rather than in the smearing effects induced by the different thermal widths.

Features of Structure and Absorption in the Jet-Launching Region of M87

M87 is one of the best available source for studying the AGN jet-launching region. To enrich our knowledge of this region, with quasi-simultaneous observations using VLBA at 22, 43 and 86 GHz, we capture the images of the radio jet in M87 on a scale within several thousand R s . Based on the images, we analyze the transverse jet structure and obtain the most accurate spectral-index maps of the jet in M87 so far, then for the first time, we compare the results of the two analyses and find a spatial association between the jet collimations and the local enhancement of the density of external medium in the jet-launching region. We also find the external medium is not uniform, and greatly contributes to the free-free absorption in this region. In addition, we find for the jet in M87, its temporal morphology in the launching region may be largely affected by the local, short-lived kink instability growing in itself.

Elemental Abundances in M31: First Alpha and Iron Abundance Measurements in M31's Giant Stellar Stream∗

Abstract We present the first measurements of [Fe/H] and abundances, obtained using spectral synthesis modeling, for red giant branch stars in M31's giant stellar stream (GSS). The spectroscopic observations, obtained at a projected distance of 17 kpc from M31's center, yielded 61 stars with [Fe/H] measurements, including 21 stars with measurements, from 112 targets identified as M31 stars. The [Fe/H] measurements confirm the expectation from photometric metallicity estimates that stars in this region of M31's halo are relatively metal rich compared to stars in the Milky Way’s inner halo: more than half the stars in the field, including those not associated with kinematically identified substructure, have [Fe/H] abundances . The stars in this field are α-enhanced at lower metallicities, while decreases with increasing [Fe/H] above metallicities of [Fe/H] ≳ −0.9. Three kinematical components have been previously identified in this field: the GSS, a second kinematically cold feature of unknown origin, and M31's kinematically hot halo. We compare probabilistic [Fe/H] and distribution functions for each of the components. The GSS and the second kinematically cold feature have very similar abundance distributions, while the halo component is more metal poor. Although the current sample sizes are small, a comparison of the abundances of stars in the GSS field with abundances of M31 halo and dSph stars from the literature indicate that the progenitor of the stream was likely more massive, and experienced a higher efficiency of star formation, than M31's existing dSphs or the dEs NGC 147 and NGC 185.

Diffuse interstellar bands in the H II region M17

Context. Diffuse interstellar bands (DIBs) are broad absorption features measured in sightlines probing the diffuse interstellar medium. Although large carbon-bearing molecules have been proposed as the carriers producing DIBs, their identity remains unknown. DIBs make an important contribution to the extinction curve; the sightline to the young massive star-forming region M17 shows anomalous extinction in the sense that the total-to-selective extinction parameter (RV) differs significantly from the average Galactic value and may reach values RV > 4. Anomalous DIBs have been reported in the sightline towards Herschel 36 (RV = 5.5), in the massive star-forming region M8. Higher values of RV have been associated with a relatively higher fraction of large dust grains in the line of sight. Aims. Given the high RV values, we investigate whether the DIBs in sightlines towards young OB stars in M17 show a peculiar behaviour. Methods. We measure the properties of the most prominent DIBs in M17 and study these as a function of E(B–V) and RV. We also analyse the gaseous and dust components contributing to the interstellar extinction. Results. The DIB strengths in M17 concur with the observed relations between DIB equivalent width and reddening E(B–V) in Galactic sightlines. For several DIBs we discover a linear relation between the normalised DIB strength EW/AV and RV−1. These trends suggest two groups of DIBs: (i) a group of ten moderately strong DIBs that show a sensitivity to changes in RV that is modest and proportional to DIB strength, and (ii) a group of four very strong DIBs that react sensitively and to a similar degree to changes in RV, but in a way that does not appear to depend on DIB strength. Conclusions. DIB behaviour as a function of reddening is not peculiar in sightlines to M17. Also, we do not detect anomalous DIB profiles like those seen in Herschel 36. DIBs are stronger, per unit visual extinction, in sightlines characterised by a lower value of RV, i.e. those sightlines that contain a relatively large fraction of small dust particles. New relations between extinction normalised DIB strengths, EW/AV, and RV support the idea that DIB carriers and interstellar dust are intimately connected. Furthermore, given the distinct behaviour of two groups of DIBs, different types of carriers do not necessarily relate to the dust grains in a similar way.

The Magnetized Disk-Halo Transition Region of M51

AbstractAn excellent laboratory for studying large scale magnetic fields is the grand design face-on spiral galaxy M51. Due to wavelength-dependent Faraday depolarization, linearly polarized synchrotron emission at different radio frequencies gives a picture of the galaxy at different depths: Observations at L-band (1 – 2 GHz) probe the halo region while at C- and X-band (4 – 8 GHz) the linearly polarized emission probe the disk region of M51. We present new observations of M51 using the Karl G. Jansky Very Large Array (VLA) at S-band (2 – 4 GHz), where previously no polarization observations existed, to shed new light on the transition region between the disk and the halo. We discuss a model of the depolarization of synchrotron radiation in a multilayer magneto-ionic medium and compare the model predictions to the multi-frequency polarization data of M51 between 1 – 8 GHz. The new S-band data are essential to distinguish between different models. Our study shows that the initial model parameters, i.e. the total regular and turbulent magnetic field strengths in the disk and halo of M51, need to be adjusted to successfully fit the models to the data.

Chemical Abundances of Planetary Nebulae in the Substructures of M31. II. The Extended Sample and a Comparison Study with the Outer-disk Group*

Abstract We report deep spectroscopy of 10 planetary nebulae (PNe) in the Andromeda Galaxy (M31) using the 10.4 m Gran Telescopio Canarias (GTC). Our targets reside in different regions of M31, including halo streams and the dwarf satellite M32, and kinematically deviate from the extended disk. The temperature-sensitive [O iii] λ4363 line is observed in all PNe. For four PNe, the GTC spectra extend beyond 1 μm, enabling the explicit detection of the [S iii] λ6312 and λλ9069, 9531 lines and thus determination of the [S iii] temperature. Abundance ratios are derived and generally consistent with AGB model predictions. Our PNe probably all evolved from low-mass (<2 M ☉) stars, as analyzed with the most up-to-date post-AGB evolutionary models, and their main-sequence ages are mostly ∼2–5 Gyr. Compared to the underlying, smooth, metal-poor halo of M31, our targets are uniformly metal rich ([O/H] ≳ −0.4), and seem to resemble the younger population in the stream. We thus speculate that our halo PNe formed in the Giant Stream’s progenitor through extended star formation. Alternatively, they might have formed from the same metal-rich gas as did the outer-disk PNe but were displaced into their present locations as a result of galactic interactions. These interpretations are, although speculative, qualitatively in line with the current picture, as inferred from previous wide-field photometric surveys, that M31's halo is the result of complex interactions and merger processes. The behavior of the N/O of the combined sample of the outer-disk and our halo/substructure PNe signifies that hot bottom burning might actually occur at <3 M ☉ but careful assessment is needed.

Red Supergiants as Cosmic Abundance Probes: Massive Star Clusters in M83 and the Mass–Metallicity Relation of Nearby Galaxies

Abstract We present an abundance analysis of seven super star clusters in the disk of M83. The near-infrared spectra of these clusters are dominated by red supergiants, and the spectral similarity in the J-band of such stars at uniform metallicity means that the integrated light from the clusters may be analyzed using the same tools as those applied to single stars. Using data from VLT/KMOS, we estimate metallicities for each cluster in the sample. We find that the abundance gradient in the inner regions of M83 is flat, with a central metallicity of relative to a solar value of Z ⊙ = 0.014, which is in excellent agreement with the results from an analysis of luminous hot stars in the same regions. Compiling this latest study with our other recent work, we construct a mass–metallicity relation for nearby galaxies based entirely on the analysis of RSGs. We find excellent agreement with the other stellar-based technique—that of blue supergiants—as well as with temperature-sensitive (“auroral” or “direct”) H ii-region studies. Of all the H ii-region strong-line calibrations, those that are empirically calibrated to direct-method studies (N2 and O3N2) provide the most consistent results.

Unique probe of dark matter in the core of M87 with the Event Horizon Telescope

We demonstrate the unprecedented capabilities of the Event Horizon Telescope (EHT) to image the innermost dark matter profile in the vicinity of the supermassive black hole at the center of the M87 radio galaxy. We present the first model of the synchrotron emission induced by dark matter annihilations from a spiky profile in the close vicinity of a supermassive black hole, accounting for strong gravitational lensing effects. Our results show that the EHT should readily resolve dark matter spikes if present. Moreover, the photon ring surrounding the silhouette of the black hole is clearly visible in the spike emission, which introduces observable small-scale structure into the signal. We find that the dark matter-induced emission provides an adequate fit to the existing EHT data, implying that in addition to the jet, a dark matter spike may account for a sizable portion of the millimeter emission from the innermost (subparsec) region of M87. Regardless, our results show that the EHT can probe very weakly annihilating dark matter. Current EHT observations already constrain very small cross sections, typically down to a few 10^{-31} cm^{3} s^{-1} for a 10 GeV candidate, close to characteristic values for p-wave-suppressed annihilation. Future EHT observations will further improve constraints on the DM scenario.

Spectroscopic Observation and Analysis of H ii Regions in M33 with MMT: Temperatures and Oxygen Abundances

Abstract The spectra of 413 star-forming (or H ii) regions in M33 (NGC 598) were observed using the multifiber spectrograph of Hectospec at the 6.5 m Multiple Mirror Telescope. Using this homogeneous spectra sample, we measured the intensities of emission lines and some physical parameters, such as electron temperatures, electron densities, and metallicities. Oxygen abundances were derived via the direct method (when available) and two empirical strong-line methods, namely, O3N2 and N2. At the high-metallicity end, oxygen abundances derived from the O3N2 calibration were higher than those derived from the N2 index, indicating an inconsistency between O3N2 and N2 calibrations. We present a detailed analysis of the spatial distribution of gas-phase oxygen abundances in M33 and confirm the existence of the axisymmetric global metallicity distribution that is widely assumed in the literature. Local variations were also observed and subsequently associated with spiral structures to provide evidence of radial migration driven by arms. Our O/H gradient fitted out to 1.1 R 25 resulted in slopes of −0.17 ± 0.03, −0.19 ± 0.01, and −0.16 ± 0.17 dex , utilizing abundances from O3N2, N2 diagnostics, and a direct method, respectively.

A dearth of short-period massive binaries in the young massive star forming region M 17

The formation of massive stars remains poorly understood and little is known about their birth multiplicity properties. Here, we investigate the strikingly low radial-velocity dispersion measured for a sample of 11 massive pre- and near-main-sequence stars (sigma_rv = 5.6 +/- 0.2 km/s) in the young massive star forming region M17 to obtain first constraints on the multiplicity properties of young massive stellar objects. Methods: We compute the RV dispersion of synthetic populations of massive stars for various multiplicity properties and we compare the simulated sigma_rv distributions to the observed value. We specifically investigate two scenarios: a low binary fraction and a dearth of short-period binary systems. Results: Simulated populations with low binary fractions (f_bin = 0.12_{-0.09}^{+0.16}) or with truncated period distributions (P_cutoff > 9 months) are able to reproduce the low sigma_rv observed within their 68%-confidence intervals. Parent populations with f_bin > 0.42 or P_cutoff < 47 d can however be rejected at the 5%-significance level. Both constraints are contrast with the high binary fraction and plethora of short-period systems found in few Myr-old, OB-type populations. To explain the difference, the first scenario requires a variation of the outcome of the massive star formation process. In the the second scenario, compact binaries must form later on, and the cut-off period may be related to physical length-scales representative of the bloated pre-main-sequence stellar radii or of their accretion disks. Conclusions: If the obtained constraints are representative of the overall properties of massive young stellar objects, our results may provide support to a formation process in which binaries are initially formed at larger separations, then harden or migrate to produce the typical (untruncated) power-law period distribution observed in few Myr-old OB binaries.

SPATIAL VARIATIONS OF PAH PROPERTIES IN M17SW REVEALED BY SPITZER/IRS SPECTRAL MAPPING

ABSTRACT We present Spitzer/IRS mid-infrared spectral maps of the Galactic star-forming region M17 as well as IRSF/SIRIUS Brγ and Nobeyama 45 m/FOREST 13CO (J = 1–0) maps. The spectra show prominent features due to polycyclic aromatic hydrocarbons (PAHs) at wavelengths of 6.2, 7.7, 8.6, 11.3, 12.0, 12.7, 13.5, and 14.2 μm. We find that the PAH emission features are bright in the region between the H ii region traced by Brγ and the molecular cloud traced by 13CO, supporting that the PAH emission originates mostly from photo-dissociation regions. Based on the spatially resolved Spitzer/IRS maps, we examine spatial variations of the PAH properties in detail. As a result, we find that the interband ratio of PAH 7.7 μm/PAH 11.3 μm varies locally near M17SW, but rather independently of the distance from the OB stars in M17, suggesting that the degree of PAH ionization is mainly controlled by local conditions rather than the global UV environments determined by the OB stars in M17. We also find that the interband ratios of the PAH 12.0 μm, 12.7 μm, 13.5 μm, and 14.2 μm features to the PAH 11.3 μm feature are high near the M17 center, which suggests structural changes of PAHs through processing due to intense UV radiation, producing abundant edgy irregular PAHs near the M17 center.

Massive Stars in M31

AbstractMassive stars are intrinsically rare and therefore present a challenge to understand from a statistical perspective, especially within the Milky Way. We recently conducted follow-up observations to the Panchromatic Hubble Andromeda Treasury (PHAT) survey that were designed to detect more than 10,000 emission line stars, including WRs, by targeting regions in M31 previously known to host large numbers of young, massive clusters and very young stellar populations. Because of the existing PHAT data, we are able to derive an effective temperature, bolarimetric luminosity, and extinction for each of our detected stars. We report on preliminary results of the massive star population of our dataset and discuss how our results compare to previous studies of massive stars in M31.