Ben Davies

THE RED MSX SOURCE SURVEY: THE MASSIVE YOUNG STELLAR POPULATION OF OUR GALAXY

We present the Red MSX Source survey, the largest statistically selected catalog of young massive protostars and H II regions to date. We outline the construction of the catalog using mid- and near-infrared color selection. We also discuss the detailed follow up work at other wavelengths, including higher spatial resolution data in the infrared. We show that within the adopted selection bounds we are more than 90% complete for the massive protostellar population, with a positional accuracy of the exciting source of better than 2 arcsec. We briefly summarize some of the results that can be obtained from studying the properties of the objects in the catalog as a whole; we find evidence that the most massive stars form: (1) preferentially nearer the Galactic center than the anti-center; (2) in the most heavily reddened environments, suggestive of high accretion rates; and (3) from the most massive cloud cores.

The RMS survey - 6 cm continuum VLA observations towards candidate massive YSOs in the northern hemisphere

Context. The Red MSX Source (RMS) survey is an ongoing multi-wavelength observational programme designed to return a large, well-selected sample of massive young stellar objects (MYSOs). We have identified ∼2000 MYSO candidates located throughout the Galaxy by comparing the colours of MSX and 2MASS point sources to those of known MYSOs. The aim of these follow-up observations is to identify other objects with similar colours such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe) and distinguish between genuine MYSOs and nearby low-mass YSOs. Aims. To identify the populations of UCHII regions and PNe within the sample and examine their Galactic distribution. Methods. We have conducted high resolution radio continuum observations at 6 cm towards 659 MYSO candidates in the northern hemisphere (10 ◦ < l < 250 ◦ ) using the Very Large Array (VLA). These observations have a spatial resolution of ∼1–2 �� and typical image rms noise values of ∼0.22 mJy – sensitive enough to detect a HII region powered by B0.5 star at the far side of the Galaxy. In addition to these targeted observations we present archival data towards a further 315 RMS sources extracted from a previous VLA survey of the inner Galaxy. Results. We present the results of radio continuum observations made towards 974 MYSO candidates, 272 (∼27% of the observed sample) of which are found to be associated with radio emission above a 4σ detection limit (∼1 mJy). Using results from other parts of our multi-wavelength survey we separate these RMS-radio associations into two distinct types of objects, classifying 51 as PNe and a further 208 as either compact or UC HII regions. Including all HII regions and PNe identified either from the literature or from the multi-wavelength data these numbers increase to 391 and 79, respectively. Using this well selected sample of HII regions we estimate their Galactic scale height to be 0.6 ◦ . In addition to the RMS-radio associations we are able to set upper limits on the radio emission of ≤1 mJy for the 702 non-detections, which is below the level expected if they had already begun to ionise their surroundings. Conclusions. Using radio continuum and archival data we have identified 79 PNe and 391 HII regions within the northern RMS catalogue. We estimate the total fraction of contamination by PNe in the RMS sample is of order 10%. The sample of HII regions is probably the best representation to date of the Galactic population of HII regions as a whole.

Constraining Globular Cluster Formation Through Studies of Young Massive Clusters: I. A lack of ongoing star formation within young clusters

We present a survey of 130 Galactic and extragalactic young massive clusters (YMCs, 10 4 < M/M⊙ < 10 8 , 10 < t/Myr < 1000) with integrated spectroscopy or resolved stellar photometry (40 presented here and 90 from the literature) and use the sample to search for evidence of ongoing star-formation within the clusters. Such episodes of secondary (or continuous) star-formation are predicted by models that attempt to explain the observed chemical and photometric anomalies observed in globular clusters as being due to the formation of a second stellar population within an existing first population. Additionally, studies that have claimed extended star-formation histories within LMC/SMC intermediate age clusters (1-2 Gyr), also imply that many young massive clusters should show ongoing star-formation. Based on visual inspection of the spectra and/or the colour-magnitude diagrams, we do not find evidence for ongoing star-formation within any of the clusters, and use this to place constraints on the above models. Models of continuous star-formation within clusters, lasting for hundreds of Myr, are ruled out at high significance (unless stellar IMF variations are invoked). Models for the (nearly instantaneous) formation of a secondary population within an existing first generation are not favoured, but are not formally discounted due to the finite sampling of age/mass-space.

The RMS Survey: Ammonia and water maser analysis of massive star forming regions. ⋆

The Red MSX Source (RMS) survey has identified a sample of∼1200 massive young stellar objects (MYSOs), compact and ultra compact Hii regions from a sample of∼2000 MSX and 2MASS colour selected sources. We have used the 100-m Green Bank telescope to search for 22-24 GHz water maser and ammonia (1,1), (2,2) and (3,3) emission towards∼600 RMS sources located within the northern Galactic plane. We have identified 308 H 2O masers which corresponds to an overall detection rate of∼50 per cent. We find no significant di fference in the detection rate for Hii regions and MYSOs which would suggest that the conditions required to produce maser emission are equally likely in both phases. Comparing the detection rates as a function of luminosity we find the H 2O detection rate has a positive dependence on the source luminosity, with the detection rate increasing with increasing luminosity. We detect ammonia emission towards 479 of these massive young stars, which corresponds to∼80 per cent. Ammonia is an excellent probe of high density gas allowing us to measure key parameters such as gas temperatures, opacities, and column densities, as well as providing an insight into the gas kinematics. The average kinetic temperature, FWHM line width and total NH3 column density for the sample are approximately 22 K, 2 km s −1 and 2× 10 15 cm −2 , respectively. We find that the NH 3 (1,1) line width and kinetic temperature are correlated with luminosity and finding no underlying dep endence of these parameters on the evolutionary phase of the embedded sources, we conclude that the observed trends in the derived parameters are more likely to be due to the energy output of the central source and/or the line width-clump mass relationship. The velocities of the peak H2O masers and the NH3 emission are in excellent agreement with each other, which would strongly suggest an association between the dense gas and the maser emission. Moreover, we find the bolometric luminosity of the embedded source and the isotropic luminosity of the H2O maser are also correlated. We conclude from the correlations of the cloud and water maser velocities and the bolometric and maser luminosity that there is a strong dynamical relationship between the embedded young massive star and the H2O maser.

The Red MSX Source survey: ammonia and water maser analysis of massive star-forming regions

The Red MSX Source (RMS) survey has identified a sample of∼1200 massive young stellar objects (MYSOs), compact and ultra compact Hii regions from a sample of∼2000 MSX and 2MASS colour selected sources. We have used the 100-m Green Bank telescope to search for 22-24 GHz water maser and ammonia (1,1), (2,2) and (3,3) emission towards∼600 RMS sources located within the northern Galactic plane. We have identified 308 H 2O masers which corresponds to an overall detection rate of∼50 per cent. We find no significant di fference in the detection rate for Hii regions and MYSOs which would suggest that the conditions required to produce maser emission are equally likely in both phases. Comparing the detection rates as a function of luminosity we find the H 2O detection rate has a positive dependence on the source luminosity, with the detection rate increasing with increasing luminosity. We detect ammonia emission towards 479 of these massive young stars, which corresponds to∼80 per cent. Ammonia is an excellent probe of high density gas allowing us to measure key parameters such as gas temperatures, opacities, and column densities, as well as providing an insight into the gas kinematics. The average kinetic temperature, FWHM line width and total NH3 column density for the sample are approximately 22 K, 2 km s −1 and 2× 10 15 cm −2 , respectively. We find that the NH 3 (1,1) line width and kinetic temperature are correlated with luminosity and finding no underlying dep endence of these parameters on the evolutionary phase of the embedded sources, we conclude that the observed trends in the derived parameters are more likely to be due to the energy output of the central source and/or the line width-clump mass relationship. The velocities of the peak H2O masers and the NH3 emission are in excellent agreement with each other, which would strongly suggest an association between the dense gas and the maser emission. Moreover, we find the bolometric luminosity of the embedded source and the isotropic luminosity of the H2O maser are also correlated. We conclude from the correlations of the cloud and water maser velocities and the bolometric and maser luminosity that there is a strong dynamical relationship between the embedded young massive star and the H2O maser.

The Temperatures of Red Supergiants

We present a re-appraisal of the temperatures of red supergiants (RSGs) using their optical and near-infrared spectral energy distributions (SEDs). We have obtained data of a sample of RSGs in the Magellanic Clouds using VLT+XSHOOTER, and we fit MARCS model atmospheres to different regions of the spectra, deriving effective temperatures for each star from (1) the TiO bands, (2) line-free continuum regions of the SEDs, and (3) the integrated fluxes. We show that the temperatures derived from fits to the TiO bands are systematically lower than the other two methods by several hundred kelvin. The TiO fits also dramatically overpredict the flux in the near-IR, and imply extinctions which are anomalously low compared to neighboring stars. In contrast, the SED temperatures provide good fits to the fluxes at all wavelengths other than the TiO bands, are in agreement with the temperatures from the flux integration method, and imply extinctions consistent with nearby stars. After considering a number of ways to reconcile this discrepancy, we conclude that three-dimensional effects (i.e., granulation) are the most likely cause, as they affect the temperature structure in the upper layers where the TiO lines form. The continuum, however, which forms at much deeper layers, is apparently more robust to such effects. We therefore conclude that RSG temperatures are much warmer than previously thought. We discuss the implications of this result for stellar evolution and supernova progenitors, and provide relations to determine the bolometric luminosities of RSGs from single-band photometry.

A third red supergiant rich cluster in the Scutum-Crux arm

Aims. We aim to characterise the properties of a third massive, red supergiant dominated galactic cluster. Methods. To accomplish this we utilised a combination of near/mid- IR photometry and spectroscopy to identify and classify the properties of cluster members, and statistical arguments to determine the mass of the cluster. Results. We found a total of 16 strong candidates for cluster membership, for which formal classification of a subset yields spectral types from K3–M4 Ia and luminosities between log(L/L� ) ∼ 4.5–4.8 for an adopted distance of 6 ± 1 kpc. For an age in the range of 16–20 Myr, the implied mass is 2–4×10 4 M� , making it one of the most massive young clusters in the Galaxy. This discovery supports the hypothesis that a significant burst of star formation occurred at the base of Scutum-Crux arm between 10–20 Myr ago, yielding a

The Red MSX Source survey: the bolometric fluxes and luminosity distributions of young massive stars

Context. The Red MSX Source (RMS) survey is returning a large sample of massive young stellar objects (MYSOs) and ultra-compact (UC) H II regions using follow-up observations of colour-selected candidates from the MSX point source catalogue. Aims. We obtain the bolometric fluxes and, using kinematic distance information, the luminosities for young RMS sources with far-infrared fluxes. Methods. We use a model spectral energy distribution (SED) fitter to obtain the bolometric flux for our sources, given flux data from our work and the literature. The inputs to the model fitter were optimised by a series of investigations designed to reveal the effect varying these inputs had on the resulting bolometric flux. Kinematic distances derived from molecular line observations were then used to calculate the luminosity of each source. Results. Bolometric fluxes are obtained for 1173 young RMS sources, of which 1069 have uniquely constrained kinematic distances and good SED fits. A comparison of the bolometric fluxes obtained using SED fitting with trapezium rule integration and two component greybody fits was also undertaken, and showed that both produce considerable scatter compared to the method used here. Conclusions. The bolometric flux results allowed us to obtain the luminosity distributions of YSOs and UCH II regions in the RMS sample, which we find to be different. We also find that there are few MYSOs with L ≥ 10 5 L ⊙ , despite finding many MYSOs with 10 4 L ⊙ ≥ L ≥ 10 5 L ⊙ .

The circumstellar disc, envelope and bipolar outflow of the massive young stellar object W33A

The young stellar object (YSO) W33A is one of the best known examples of a massive star still in the process of forming. Here we present Gemini North Altitude conjugate Adaptive optics for the Infrared (ALTAIR)/Near-Infrared Integral Field Spectrograph laser-guide star adaptive-optics assisted K-band integral-field spectroscopy of W33A and its inner reflection nebula. In our data, we make the first detections of a rotationally flattened outer envelope and fast bipolar jet of a massive YSO at near-infrared wavelengths. The predominant spectral features observed are Br γ, H 2 and a combination of emission and absorption from CO gas. We perform a 3D spectro-astrometric analysis of the line emission, the first study of its kind. We find that the objects Br γ emission reveals evidence for a fast bipolar jet on sub-milliarcsecond scales, which is aligned with the larger scale outflow. The hybrid CO features can be explained as a combination of hot CO emission arising in a disc close to the central star, while cold CO absorption originates in the cooler outer envelope. Kinematic analysis of these features reveals that both structures are rotating and consistent with being aligned perpendicular to both the ionized jet and the large-scale outflow. Assuming Keplerian rotation, we find that the circumstellar disc orbits a central mass of ≳10 M ⊙ , while the outer envelope encloses a mass of ~ 15 M⊙. Our results suggest a scenario of a central star accreting material from a circumstellar disc at the centre of a cool extended rotating torus, while driving a fast bipolar wind. These results therefore provide strong supporting evidence for the hypothesis that the formation mechanism for high-mass stars is qualitatively similar to that of low-mass stars.

Chemical Abundance Patterns in the Inner Galaxy: The Scutum Red Supergiant Clusters

The location of the Scutum Red Supergiant (RSG) clusters at the end of the Galactic Bar makes them an excellent probe of the Galaxys secular evolution, while the clusters themselves are ideal testbeds in which to study the predictions of stellar evolutionary theory. To this end, we present a study of the RSGs surface abundances using a combination of high-resolution Keck/NIRSPEC H-band spectroscopy and spectral synthesis analysis. We provide abundance measurements for elements C, O, Si, Mg, Ti, and Fe. We find that the surface abundances of the stars studied are consistent with CNO burning and deep, rotationally enhanced mixing. The average ?/Fe ratios of the clusters are solar, consistent with a thin-disk population. However, we find significantly subsolar Fe/H ratios for each cluster, a result which strongly contradicts a simple extrapolation of the Galactic metallicity gradient to lower Galactocentric distances. We suggest that a simple one-dimensional parameterization of the Galaxys abundance patterns is insufficient at low Galactocentric distances, as large azimuthal variations may be present. Indeed, we show that the abundances of O, Si, and Mg are consistent with independent measurements of objects in similar locations in the Galaxy. In combining our results with other data in the literature, we present evidence for large-scale (~ kpc) azimuthal variations in abundances at Galactocentric distances of 3-5?kpc. While we cannot rule out that this observed behavior is due to systematic offsets between different measurement techniques, we do find evidence for similar behavior in a study of the barred spiral galaxy NGC 4736 which uses homogeneous methodology. We suggest that these azimuthal abundance variations could result from the intense but patchy star formation driven by the potential of the central bar.