Runaway OB Stars Research Articles

Runaway OB Stars in the Small Magellanic Cloud. III. Updated Kinematics and Insights into Dynamical versus Supernova Ejections

We use the kinematics of field OB stars to estimate the frequencies of runaway stars generated by the dynamical ejection scenario (DES), the binary supernova scenario (BSS), and the combined two-step mechanism. We update the proper motions for field OB and OBe stars in the Small Magellanic Cloud (SMC) using Gaia DR3. Our sample now contains 336 stars from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC, and we update our algorithm to calculate more accurate velocities compared to those obtained previously from DR2. We find a decrease in median velocity from 39 to 29 km s−1, implying that the proper motions from our previous work were systematically overestimated. We present the velocity distribution for OBe stars and quantitatively compare it to those of noncompact binaries and high-mass X-ray binaries. We confirm that OBe stars appear to be dominated by the BSS and are likely post-SN binary systems, further supporting the mass-transfer model to explain the origin of their emission-line disks. In contrast, normal OB stars may show a bimodal velocity distribution, which may be expected from different processes that occur with dynamical ejections. The kinematics of fast-rotating OB stars are similar to those of normal OB stars rather than OBe stars, suggesting that the origin of their high vrsini is different from that of OBe stars. We update our model parameters describing the kinematic origins of the SMC field population, still confirming that for runaway stars, the DES mechanism dominates, and two-step ejections seem comparable in frequency to pure BSS ejections.

Наиболее интересные результаты изучения Галактики по OB-звездам

A review of the main results of studying the structure and kinematics of the Galaxy obtained by various research teams using OB stars is given. The main attention is paid to the results of the authors of this work. The following questions are considered: a) the nature of the spatial distribution of OB stars in the Galaxy; b) the use of OB stars to refine the parameters of the spiral pattern of the Galaxy; c) refinement of the Galactic rotation parameters and perturbation velocities caused by the influence of the Galactic spiral density wave using OB stars; d) spatial and kinematic relationship of OB stars with galactic warp; e) analysis of the motion of some runaway OB stars.

Kinematics of the Central Stars Powering Bowshock Nebulae and the Large Multiplicity Fraction of Runaway OB Stars

OB stars powering stellar bowshock nebulae (SBNe) have been presumed to have large peculiar velocities. We measured peculiar velocities of SBN central stars to assess their kinematics relative to the general O-star population using Gaia EDR3 data for 267 SBN central stars and a sample of 455 Galactic O stars to derive projected velocities v 2D. For a subset of each sample, we obtained new optical spectroscopy to measure radial velocities and identify multiple-star systems. We find a minimum multiplicity fraction of 36% ± 6% among SBN central stars, consistent with >28% among runaway Galactic O stars. The large multiplicity fraction among runaways implicates very efficient dynamical ejection rather than binary-supernova origins. The median v 2D of SBN central stars is v 2D = 14.6 km s−1, larger than the median v 2D = 11.4 km s−1 for non-bowshock O stars. Central stars of SBNe have a runaway (v 2D > 25 km s−1) fraction of 24%, consistent with the % for control-sample O stars. Most (76%) SBNe central stars are not runaways. Our analysis of alignment (ΔPA) between the nebular morphological and v 2D kinematic position angles reveals two populations: a highly aligned (σ PA = 25°) population that includes stars with the largest v 2D (31% of the sample) and a random (nonaligned) population (69%). SBNe that lie within or near H ii regions comprise a larger fraction of this latter component than SBNe in isolated environments, implicating localized ISM flows as a factor shaping their orientations and morphologies. We outline a new conceptual approach to computing the solar local standard of rest motion, yielding [U ⊙, V ⊙, W ⊙] = [5.5, 7.5,4.5] km s−1.

3D MHD astrospheres: applications to IRC-10414 and Betelgeuse

ABSTRACT A significative fraction of all massive stars in the Milky Way move supersonically through their local interstellar medium (ISM), producing bow shock nebulae by wind-ISM interaction. The stability of these observed astrospheres around cool massive stars challenges precedent 2D (magneto-)hydrodynamical (MHD) simulations of their surroundings. We present 3D MHD simulations of the circumstellar medium of runaway M-type red supergiant stars moving with velocity $v_{\star }=50\, \rm km\, \rm s^{-1}$. We treat the stellar wind with a Parker spiral and assume a $7\, \rm \mu G$ magnetization of the ISM. Our free parameter is the angle θmag between ISM flow and magnetization, taken to 0°, 45°, and 90°. It is found that simulation dimension, coordinate systems, and grid effects can greatly affect the development of the modelled astrospheres. Nevertheless, as soon as the ISM flow and magnetization directions differs by more than a few degrees (θmag ≥ 5°), the bow shock is stabilized, most clumpiness and ragged structures vanishing. The complex shape of the bow shocks induce important projection effects, e.g. at optical H α line, producing complex of astrospheric morphologies. We speculate that those effects are also at work around earlier-type massive stars, which would explain their diversity of their observed arc-like nebula around runaway OB stars. Our 3D MHD models are fitting well observations of the astrospheres of several runaway red supergiant stars. The results interpret the smoothed astrosphere of IRC-10414 and Betelgeuse (αOri) are stabilized by an organized non-parallel ambient magnetic field. Our findings suggest that IRC-10414 is currently in a steady state of its evolution, and that Betelgeuse’s bar is of interstellar origin.

Resolved Kinematics of Runaway and Field OB Stars in the Small Magellanic Cloud

Abstract We use Gaia Data Release 2 proper motions of field OB stars from the Runaways and Isolated O-Type Star Spectroscopic Survey of the Small Magellanic Cloud (SMC) to study the kinematics of runaway stars. The data reveal that the SMC Wing has a systemic peculiar motion relative to the SMC Bar of and relative radial velocity . This unambiguously demonstrates that these two regions are kinematically distinct: the Wing is moving away from the Bar, and towards the Large Magellanic Cloud with a 3D velocity of . This is consistent with models for a recent, direct collision between the Clouds. We present transverse velocity distributions for our field OB stars, confirming that unbound runaways comprise on the order of half our sample, possibly more. Using eclipsing binaries and double-lined spectroscopic binaries as tracers of dynamically ejected runaways, and high-mass X-ray binaries (HMXBs) as tracers of runaways accelerated by supernova kicks, we find significant contributions from both populations. The data suggest that HMXBs have lower velocity dispersion relative to dynamically ejected binaries, consistent with the former group corresponding to less energetic supernova kicks that failed to unbind the components. Evidence suggests that our fast runaways are dominated by dynamical, rather than supernova, ejections.

On the observability of bow shocks of Galactic runaway OB stars

Massive stars that have been ejected from their parent cluster and supersonically sailing away through the interstellar medium (ISM) are classified as exiled. They generate circumstellar bow shock nebulae that can be observed. We present two-dimensional, axisymmetric hydrodynamical simulations of a representative sample of stellar wind bow shocks from Galactic OB stars in an ambient medium of densities ranging from n_ISM=0.01 up to 10.0/cm3. Independently of their location in the Galaxy, we confirm that the infrared is the most appropriated waveband to search for bow shocks from massive stars. Their spectral energy distribution is the convenient tool to analyze them since their emission does not depend on the temporary effects which could affect unstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks generated by high-mass (~40 Mo) runaway stars yield H$\alpha$ fluxes which could be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The brightest bow shock nebulae are produced in the denser regions of the ISM. We predict that bow shocks in the field observed at Ha by means of Rayleigh-sensitive facilities are formed around stars of initial mass larger than about 20 Mo. Our models of bow shocks from OB stars have the emission maximum in the wavelength range 3 <= lambda <= 50 micrometer which can be up to several orders of magnitude brighter than the runaway stars themselves, particularly for stars of initial mass larger than 20 Mo.

ESCAPE FRACTION OF IONIZING PHOTONS DURING REIONIZATION: EFFECTS DUE TO SUPERNOVA FEEDBACK AND RUNAWAY OB STARS

The fraction of hydrogen ionizing photons escaping from galaxies into the intergalactic medium is a critical ingredient in the theory of reionization. We use two zoomed-in, high-resolution (4 pc), cosmological radiation hydrodynamic simulations with adaptive mesh refinement to investigate the impact of two physical mechanisms (supernova feedback and runaway OB stars) on the escape fraction (f_esc) at the epoch of reionization (z>7). We implement a new, physically motivated supernova feedback model that can approximate the Sedov solutions at all (from the free expansion to snowplow) stages. We find that there is a significant time delay of about ten million years between the peak of star formation and that of escape fraction, due to the time required for the build-up and subsequent destruction of the star-forming cloud by supernova feedback. Consequently, the photon number-weighted mean escape fraction for dwarf galaxies in halos of mass 10^8-10^10.5 Msun is found to be <fesc>~11%, although instantaneous values of f_esc>20% are common when star formation is strongly modulated by the supernova explosions. We find that the inclusion of runaway OB stars increases the mean escape fraction by 22% to <fesc>~14%. As supernovae resulting from runaway OB stars tend to occur in less dense environments, the feedback effect is enhanced and star formation is further suppressed in halos with Mvir>10^9 Msun in the simulation with runaway OB stars compared with the model without them. While both our models produce enough ionizing photons to maintain a fully ionized universe at z>7 as observed, a still higher amount of ionizing photons at z>9 appears necessary to accommodate the high observed electron optical depth inferred from cosmic microwave background observations.

IRC −10414: a bow-shock-producing red supergiant star★

Most runaway OB stars, like the majority of massive stars residing in their parent clusters, go through the red supergiant (RSG) phase during their lifetimes. Nonetheless, although many dozens of massive runaways were found to be associated with bow shocks, only two RSG bow-shock-producing stars, Betelgeuse and \mu Cep, are known to date. In this paper, we report the discovery of an arc-like nebula around the late M-type star IRC-10414 using the SuperCOSMOS H-alpha Survey. Our spectroscopic follow-up of IRC-10414 with the Southern African Large Telescope (SALT) showed that it is a M7 supergiant, which supports previous claims on the RSG nature of this star based on observations of its maser emission. This was reinforced by our new radio- and (sub)millimeter-wavelength molecular line observations made with the Atacama Pathfinder Experiment (APEX) 12 meter telescope and the Effelsberg 100 m radio telescope, respectively. The SALT spectrum of the nebula indicates that its emission is the result of shock excitation. This finding along with the arc-like shape of the nebula and an estimate of the space velocity of IRC-10414 (\approx 70\pm20 km/s) imply the bow shock interpretation for the nebula. Thus, IRC-10414 represents the third case of a bow-shock-producing RSG and the first one with a bow shock visible at optical wavelengths. We discuss the smooth appearance of the bow shocks around IRC-10414 and Betelgeuse and propose that one of the necessary conditions for stability of bow shocks generated by RSGs is the ionization of the stellar wind. Possible ionisation sources of the wind of IRC-10414 are proposed and discussed.

Runaway stars as progenitors of supernovae and gamma-ray bursts

When a core-collapse supernova occurs in a binary system, the surviving star as well as the compact remnant emerging from the supernova may reach a substantial space velocity. With binary population synthesis modelling at solar and one-fifth of solar metallicity, we predict the velocities of such runaway stars or binaries. We compile predictions for runaway OB stars, red supergiants and Wolf–Rayet stars, either isolated or with a compact companion. For those stars or binaries which undergo a second stellar explosion we compute their further evolution and the distance travelled until a Type II or Type Ibc supernova or a long or short gamma-ray burst (GRB) occurs. We find our predicted population of OB runaway stars broadly matches the observed population of stars but, to match the fastest observed Wolf–Rayet runaway stars, we require that black holes receive an asymmetric kick upon formation. We find that at solar metallicity Type Ic supernova progenitors travel shorter distances than the progenitors of other supernova types because they are typically more massive and thus have shorter lifetimes. Those of Type IIP supernovae can fly farthest about 48 pc on average at solar metallicity, with about 8 per cent of them reaching 100 pc. In considering the consequences of assuming that the progenitors of long GRBs are spun-up secondary stars that experience quasi-homogeneous evolution, we find that such evolution has a dramatic effect on the population of runaway Wolf–Rayet stars and that some 30 per cent of GRBs could occur a hundred parsec or more from their initial positions. We also consider mergers of double compact object binaries consisting of neutron stars and/or black holes. We find the most common type of visible mergers are neutron star–black hole mergers that are roughly 10 times more common than neutron star–neutron star mergers. All compact mergers have a wide range of merger times from years to Gyr and are predicted to occur 300 times less often than supernovae in the Milky Way. We also find that there may be a population of low-velocity neutron stars that are ejected from a binary rather than by their own natal kick. These neutron stars need to be included when the distribution of neutron star kicks is deduced from observations.

Identification of blue high proper motion objects in the Tycho-2 and 2MASS catalogues using Virtual Observatory tools

With available Virtual Observatory tools, we looked for new bright blue high proper motion objects in the entire sky: white dwarfs, hot subdwarfs, runaway OB stars, and early-type stars in nearby young moving groups. We performed an all-sky cross-match between the optical Tycho-2 and near-infrared 2MASS catalogues with Aladin, and selected objects with proper motions >50mas/yr and colours Vt-Ks<-0.5mag with TOPCAT. We also collected multi-wavelength photometry, constructed the spectral energy distributions and estimated effective temperatures from fits to atmospheric models with VOSA for the most interesting targets. We assembled a sample of 32 bright blue high proper motion objects, including ten sdO/B subdwarfs, nine DA white dwarfs, five young early-type stars (two of which are runaway stars), two blue horizontal branch stars, one star with poor information, and five objects reported for the first time in this work. These last five objects have magnitudes Bt~11.0-11.6mag, effective temperatures ~24,000-30,000K, and are located in the region of known white dwarfs and hot subdwarfs in a reduced proper motion-colour diagram. We confirmed the hot subdwarf nature of one of the new objects, Albus 5, with public far-ultraviolet spectroscopic data obtained with FUSE.

Massive runaway stars in the Small Magellanic Cloud

Using archival Spitzer Space Telescope data, we identified for the first time a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the detection of their bow shocks. The geometry of detected bow shocks allows us to infer the direction of motion of the associated stars and to determine their possible parent clusters and associations. One of the identified runaway stars, AzV 471, was already known as a high-velocity star on the basis of its high peculiar radial velocity, which is offset by ~40 km/s from the local systemic velocity. We discuss implications of our findings for the problem of the origin of field OB stars. Several of the bow shock-producing stars are found in the confines of associations, suggesting that these may be "alien" stars contributing to the age spread observed for some young stellar systems. We also report the discovery of a kidney-shaped nebula attached to the early WN-type star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure created owing to the interaction between the stellar wind and the ambient interstellar medium.

Getting a Kick out of the Stellar Disk(s) in the Galactic Center

AbstractRecent observations of the Galactic center revealed a nuclear disk of young OB stars, in addition to many similar outlying stars with higher eccentricities and/or high inclinations relative to the disk (some of them possibly belonging to a second disk). Binaries in such nuclear disks, if they exist in non-negligible fractions, could have a major role in the evolution of the disks through binary heating of this stellar system. We suggest that interactions with/in binaries may explain some (or all) of the observed outlying young stars in the Galactic center. Such stars could have been formed in a disk, and later on kicked out from it through binary related interactions, similar to ejection of high velocity runaway OB stars in young clusters throughout the galaxy.

The Fraction of Runaway OB Stars in the SMC Field

AbstractThe fraction of field OB stars that originate from clusters can help probe the dynamical evolution of clusters. Field stars represent a significant fraction (20-30%) of the OB population in galaxies, and estimates for the fraction of field OB stars that are runaways range from the classical value of &lt;10% (Blaauw 1961) to contemporary results suggesting &gt;90% (de Wit et al. 2005). We obtained Magellan IMACS observations on the kinematics of field OB stars in the SMC to examine the line-of-sight velocities of this population. Using these observations, we will estimate the fraction of runaways to serve as a probe of cluster evolution.

A Radial Velocity Survey of the Cyg OB2 Association

We conducted a radial velocity survey of the Cygnus OB2 Association over a 6 year (1999 - 2005) time interval to search for massive close binaries. During this time we obtained 1139 spectra on 146 OB stars to measure mean systemic radial velocities and radial velocity variations. We spectroscopically identify 73 new OB stars for the first time, the majority of which are likely to be Association members. Spectroscopic evidence is also presented for a B3Iae classification and temperature class variation (B3 - B8) on the order of 1 year for Cygnus OB2 No. 12. Calculations of the intial mass function with the current spectroscopic sample yield Gamma = -2.2 +/- 0.1. Of the 120 stars with the most reliable data, 36 are probable and 9 are possible single-lined spectroscopic binaries. We also identify 3 new and 8 candidate double-lined spectroscopic binaries. These data imply a lower limit on the massive binary fraction of 30% - 42%. The calculated velocity dispersion for Cygnus OB2 is 2.44 +/- km/s, which is typical of open clusters. No runaway OB stars were found.

A Spectroscopic Study of Field and Runaway OB Stars

Identifying binaries among runaway O- and B-type stars offers valuable insight into the evolution of open clusters and close binary stars. Here we present a spectroscopic investigation of 12 known or suspected binaries among field and runaway OB stars. We find new orbital solutions for five single-lined spectroscopic binaries (HD 1976, HD 14633, HD 15137, HD 37737, and HD 52533), and we classify two stars thought to be binaries (HD 30614 and HD 188001) as single stars. In addition, we reinvestigate their runaway status using our new radial velocity data with the UCAC2 proper-motion catalogs. Seven stars in our study appear to have been ejected from their birthplaces, and at least three of these runaways are spectroscopic binaries and are of great interest for future study.

Searching for Runaway OB Stars in Supernova Remnants

Searching for runaway stars in Supernova remnants gives us the possibility to estimate the mass ratio in binary systems in which there occur Supernova explosion. Such a method also gives information on portions of spectroscopic and close binaries among the stars on the main sequence with mass > 7–8 M⊙. More importantly, we can learn more about space velocities and spin periods of pulsars by this way. We have searched for runaway OB stars in central parts of 48 Supernova remnants with distances less than about 3 kpc. In 16 of the remnants in our sample, there is no candidate O or B type star and we have shown that pulsars (point sources) might be born not only in spectroscopic and close binaries. We have represented a list of stars which are candidates for runaway B type stars located in Supernova remnants. Spectroscopic investigations on these candidates could provide solutions for the problems mentioned above.

The Physical Properties and Effective Temperature Scale of O‐Type Stars as a Function of Metallicity. II. Analysis of 20 More Magellanic Cloud Stars and Results from the Complete Sample

In order to determine the physical properties of the hottest and most luminous stars and understand how these properties change as a function of metallicity, we have analyzed HST/UV and high-S/N optical spectra of an additional 20 Magellanic Cloud stars, doubling the sample presented in the first paper in this series. Our analysis uses non-LTE line-blanketed models that include spherical extension and the hydrodynamics of the stellar wind. In addition, our data set includes FUSE observations of O VI and HST near-UV He I and He II lines to test for consistency of our derived stellar properties for a few stars. The results from the complete sample are as follows: (1) We present an effective temperature scale for O stars as a function of metallicity. We find that the SMC O3-7 dwarfs are 4000 K hotter than Galactic stars of the same spectral type. The difference is in the sense expected due to the decreased significance of line blanketing and wind blanketing at the lower metallicities that characterize the SMC. The temperature difference between the SMC and Milky Way O dwarfs decreases with decreasing temperature, becoming negligible by spectral type B0, in accord with the decreased effects of stellar winds at lower temperatures and luminosities. The temperatures of the LMC stars appear to be intermediate between that of the Milky Way and SMC, as expected based on their metallicities. Supergiants show a similar effect but are roughly 3000-4000 K cooler than dwarfs for early O stars, also with a negligible difference by B0. The giants appear to have the same effective temperature scale as dwarfs, consistent with there being little difference in the surface gravities. When we compare our scale to other recent modeling efforts, we find good agreement with some CMFGEN results, while other CMFGEN studies are discordant, although there are few individual stars in common. WM-BASIC modeling by others has resulted in significantly cooler effective temperatures than what we find, as does the recent TLUSTY/CMFGEN study of stars in the NGC 346 cluster, but our results lead to a far more coeval placement of stars in the H-R diagram for this cluster. (2) We find that the wind momentum of these stars scales with luminosity and metallicity in the ways predicted by radiatively driven wind theory, supporting the use of photospheric analyses of hot luminous stars as a distance indicator for galaxies with resolved massive star populations. (3) A comparison of the spectroscopic masses with those derived from stellar evolutionary theory shows relatively good agreement for stars with effective temperatures below 45,000 K; however, stars with higher temperatures all show a significant mass discrepancy, with the spectroscopic masses a factor of 2 or more smaller than the evolutionary masses. This problem may in part be due to unrecognized binaries in our sample, but the result suggests a possible systematic problem with the surface gravities or stellar radii derived from our models. (4) Our sample contains a large number of stars of the earliest O types, including those of the newly proposed O2 subtype. We provide the first quantitative descriptions of their defining spectral characteristics and investigate whether the new types are a legitimate extension of the effective temperature sequence. We find that the N III/N IV emission line ratio used to define the new classes does not, by itself, serve as an effective temperature indicator within a given luminosity class: there are O3.5 V stars that are as hot or hotter than O2 V stars. However, the He I/He II ratio does not fair much better for stars this hot, as we find that He I λ4471/He II λ4542, usually taken primarily as a temperature indicator, becomes sensitive to both the mass-loss rate and surface gravities for the hottest stars. This emphasizes the need to rely on all of the spectroscopic diagnostic lines, and not simply N III/N IV or even He I/He II, for these extreme objects. (5) The two stars with the most discordant radial velocities in our sample happen to be O3 field stars, i.e., found far from the nearest OB associations. This provides the first compelling observational evidence as to the origin of the field O stars in the Magellanic Clouds, i.e., that these are classic runaway OB stars, ejected from their birthplaces.

The origin of massive O-type field stars

In two papers we try to confirm that all Galactic high-mass stars are formed in a cluster environment, by excluding that O-type stars found in the Galactic field actually formed there. In de Wit et al. (2004) we presented deep K-band imaging of 5 arcmin fields centred on 43 massive O-type field stars that revealed that the large majority of these objects are single objects. In this contribution we explore the possibility that the field O stars are dynamically ejected from young clusters, by investigating their peculiar space velocity distribution, their distance from the Galactic plane, and their spatial vicinity to known young stellar clusters. We (re-)identify 22 field O-type stars as candidate runaway OB-stars. The statistics show that 4 ± of all O-type stars with can be considered as formed outside a cluster environment. Most are spectroscopically single objects, some are visual binaries. The derived percentage for O-type stars that form isolated in the field based on our statistical analyses is in agreement with what is expected from calculations adopting a universal cluster richness distribution with power index of , assuming that the cluster richness distribution is continuous down to the smallest clusters containing one single star.

X‐Ray Counterparts of Runaway Stars

An X-ray search for possible compact companions of runaway OB stars has been conducted using pointed ROSAT observations. Of a list of 71 runaway stars, ROSAT exposures were available for 24, of which 13 are detected. These numbers are nearly 3 times larger than for a previously studied Einstein sample, and spectral information is exploited as well. Luminosities, hardness ratios, and long-term variability are as for normal OB stars and do not suggest the presence of collapsed companions. A result like this is often interpreted as support for dynamical ejection from a dense group rather than a supernova event in a binary as a production process for runaway stars. There are, however, several circumstances that may adversely affect the observability of a compact companion, or after a supernova explosion systems may be disrupted because of the large natal kick velocity imparted to the neutron star as a result of asymmetries in the explosions. It is noted that there is actually evidence for both of these production routes and that they may be expected to occur sequentially in the evolution of OB associations.

Subarcsecond mid-infrared and radio observations of the W3 IRS5 protocluster

Observations at mid-infrared (4.8-17.65 μm) and radio (0.7-1.3 cm) wavelengths are used to constrain the structure of the high-mass star-forming region W3 IRS5 on 0.1 (200 AU) scales. Two bright mid-infrared sources are detected, as well as diffuse emission. The bright sources have associated compact radio emission and probably are young high-mass stars. The measured sizes and estimated temperatures indicate that these sources together can supply the observed far-infrared luminosity. However, an optically thick radio source with a possible mid-infrared counterpart may also contribute significant luminosity; if so, it must be extremely deeply embedded. The infrared colour temperatures of 350-390 K and low radio brightness suggest gravitational confinement of the H II regions and ongoing accretion at a rate of a few 10 -8 M ○. yr -1 or more. Variations in the accretion rate would explain the observed radio variability. The low estimated foreground extinction suggests the existence of a cavity around the central stars, perhaps blown by stellar winds. At least three radio sources without mid-infrared counterparts appear to show proper motions of ∼100 km s -1 , and may be deeply embedded young runaway OB stars, but more likely are clumps in the ambient material which are shock-ionized by the OB star winds.