Joseph J. Stead

The slope of the near-infrared extinction law

We determine the slope of the near-infrared extinction power law (A λ α λ -α ) for eight regions of the Galaxy between l ∼ 27° and ∼ 100° . UKIDSS Galactic Plane Survey data are compared, in colour-colour space, with Galactic population synthesis model data reddened using a series of power laws and convolved through the UKIDSS filter profiles. Monte Carlo simulations allow us to determine the best-fitting value of α and evaluate the uncertainty. All values are consistent with each other giving an average extinction power law of α = 2.14 +0.04 -0.05 . This is much steeper than most laws previously derived in the literature from colour excess ratios, which are typically between 1.6 and 1.8. We show that this discrepancy is due to an inappropriate choice of filter wavelength in conversion from colour excess ratios to α and that effective rather than isophotal wavelengths are more appropriate. In addition, curved reddening tracks, which depend on the spectral type and filter system, should be used instead of straight vectors.

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 RMS Survey: Distribution and properties of a sample of massive young stars

The Red MSX Source (RMS) survey has identified a large sample of massive young stellar objects and ultra compact H II regions from a sample of ~2000 MSX and Two Micron All Sky Survey (2MASS) colour selected sources. Using a recent catalogue of molecular clouds derived from the Boston University-Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS), and by applying a Galactic scaleheight cut-off of 120 pc, we solve the distance ambiguity for RMS sources located within 18° 〈|l|〉 54°. These two steps yield kinematic distances to 291 sources out of a possible 326, located within the GRS longitude range. Combining distances and integrated fluxes derived from spectral energy distributions, we estimate luminosities to these sources and find that >90 per cent are indicative of the presence of a massive star. We find the completeness limit of our sample is ~10 4 L ⊙ , which corresponds to a zero-age main-sequence star with a mass of ~12 M ⊙ . Selecting only these sources, we construct a complete sample of 196 sources. Comparing the properties of the sample of young massive stars with the general population, we find the RMS clouds are generally larger, more massive, and more turbulent. We examine the distribution of this subsample with respect to the location of the spiral arms and the Galactic bar and find them to be spatially correlated. We identify three significant peaks in the source surface density at Galactocentric radii of approximately 4, 6 and 8 kpc, which correspond to the proposed positions of the Scutum, Sagittarius and Perseus spiral arms, respectively. Fitting a scaleheight to the data we obtain an average value of ~29 ± 0.5 pc, which agrees well with other reported values in the literature, however we note a dependence of the scaleheight on galactocentric radius with it increases from 30 to 45 pc between 2.5 and 8.5 kpc.

The RMS survey: H2O masers towards a sample of southern hemisphere massive YSO candidates and ultra compact HII regions

Context. The red MSX source (RMS) survey has identified a large sample of candidate massive young stellar objects (MYSOs) and ultra compact (UC) HII regions from a sample of ∼2000 MSX and 2MASS colour selected sources. Aims. To search for H 2 O masers towards a large sample of young high mass stars and to investigate the statistical correlation of H 2 O masers with the earliest stages of massive star formation. Methods. We have used the Mopra Radio telescope to make position-switched observations towards ∼500 UCHII regions and MYSOs candidates identified from the RMS survey and located between 190° < l < 30°. These observations have a 4σ sensitivity of ∼1 Jy and a velocity resolution of ∼0.4 km s -1 . Results. We have detected 161 H 2 O masers, approximately 75% of which were previously unknown. Comparing the maser velocities with the velocities of the RMS sources, determined from 13 CO observations, we have identified 135 RMS-H 2 O maser associations, which corresponds to a detection rate of ∼27%. Taking into account the differences in sensitivity and source selection we find our detection rate is in general agreement with previously reported surveys. Conclusions. We find similar detection rates for UCHII regions and MYSOs candidates, suggesting that the conditions needed for maser activity are equally likely in these two stages of the star formation process. Looking at the detection rate as a function of distance from the Galactic centre we find it significantly enhanced within the solar circle, peaking at ∼37% between 6-7 kpc, which is consistent with previous surveys of UC HII regions, possibly indicating the presence of a high proportion of more luminous YSOs and HII regions.

DISTANCES TO DARK CLOUDS: COMPARING EXTINCTION DISTANCES TO MASER PARALLAX DISTANCES

We test two different methods of using near-infrared extinction to estimate distances to dark clouds in the first quadrant of the Galaxy using large near-infrared (Two Micron All Sky Survey and UKIRT Infrared Deep Sky Survey) surveys. Very long baseline interferometry parallax measurements of masers around massive young stars provide the most direct and bias-free measurement of the distance to these dark clouds. We compare the extinction distance estimates to these maser parallax distances. We also compare these distances to kinematic distances, including recent re-calibrations of the Galactic rotation curve. The extinction distance methods agree with the maser parallax distances (within the errors) between 66% and 100% of the time (depending on method and input survey) and between 85% and 100% of the time outside of the crowded Galactic center. Although the sample size is small, extinction distance methods reproduce maser parallax distances better than kinematic distances; furthermore, extinction distance methods do not suffer from the kinematic distance ambiguity. This validation gives us confidence that these extinction methods may be extended to additional dark clouds where maser parallaxes are not available.

The RMS survey: far-infrared photometry of young massive stars

Context. The Red MSX Source (RMS) survey is a multi-wavelength campaign of follow-up observations of a colour-selected sample of candidate massive young stellar objects (MYSOs) in the galactic plane. This survey is returning the largest well-selected sample of MYSOs to date, while identifying other dust contaminant sources with similar mid-infrared colours including a large number of new ultra-compact (UC) H ii regions. Aims. To measure the far-infrared (IR) flux, which lies near the peak of the spectral energy distribution (SED) of MYSOs and UCH ii regions, so that, together with distance information, the luminosity of these sources can be obtained. Methods. Less than 50% of RMS sources are associated with IRAS point sources with detections at 60 μm and 100 μm, though the vast majority are visible in Spitzer MIPSGAL or IRAS Galaxy Atlas (IGA) images. However, standard aperture photometry is not appropriate for these data due to crowding of sources and strong spatially variable far-IR background emission in the galactic plane. A new technique using a 2-dimensional fit to the background in an annulus around each source is therefore used to obtain far-IR photometry for young RMS sources. Results. Far-IR fluxes are obtained for a total of 1113 RMS candidates identified as young sources. Of these 734 have flux measurements using IGA 60 μm and 100 μm images and 724 using MIPSGAL 70 μm images, with 345 having measurements in both data sets.

Determining the age of young embedded clusters

A new Monte Carlo method has been developed in order to derive ages of young embedded clusters in massive star-forming regions where there is strong differential reddening. After foreground and infrared excess source candidates are removed, each cluster candidate star is individually dereddened. Simulated clusters are constructed using isochrones, an initial mass function, realistic photometric errors, simulated background field populations and extinction distributions. These synthetic clusters are then dereddened in the same way as the real data, obtained from a deep near-infrared survey, and used to derive the ages of three embedded clusters. Results were found to be consistent with those determined using spectrophotometric methods. This new method provides way to determine the ages of embedded clusters when only photometric data are available and there is strong differential reddening.

Molecular cloud distance determination from deep NIR survey extinction measurements

Using near-infrared United Kingdom Infrared Deep Sky Survey (UKIDSS) Galactic Plane Survey data, we make extinction measurements to individual stars along the same line of sight as molecular clouds. Using an existing 3D extinction map of the inner Galaxy, that provides line of sight specific extinction-distance relationships, we convert the measured extinction of molecular clouds to a corresponding distance. These distances are derived independently from kinematic methods, typically used to derive distances to molecular clouds, and as such they have no near/far ambiguity. The near/far distance ambiguity has been resolved for 27 clouds, and distances have been derived to 20 clouds. The results are found to be in good agreement with kinematic measurements to molecular clouds where the ambiguity has already been resolved, using H I self-absorption techniques.

The Red MSX Source survey: distribution and properties of a sample of massive young stars: Properties of sites of massive star formation

The Red MSX Source (RMS) survey has identified a large sample of massive young stellar objects and ultra compact H II regions from a sample of ~2000 MSX and Two Micron All Sky Survey (2MASS) colour selected sources. Using a recent catalogue of molecular clouds derived from the Boston University-Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS), and by applying a Galactic scaleheight cut-off of 120 pc, we solve the distance ambiguity for RMS sources located within 18° 〈|l|〉 54°. These two steps yield kinematic distances to 291 sources out of a possible 326, located within the GRS longitude range. Combining distances and integrated fluxes derived from spectral energy distributions, we estimate luminosities to these sources and find that >90 per cent are indicative of the presence of a massive star. We find the completeness limit of our sample is ~10 4 L ⊙ , which corresponds to a zero-age main-sequence star with a mass of ~12 M ⊙ . Selecting only these sources, we construct a complete sample of 196 sources. Comparing the properties of the sample of young massive stars with the general population, we find the RMS clouds are generally larger, more massive, and more turbulent. We examine the distribution of this subsample with respect to the location of the spiral arms and the Galactic bar and find them to be spatially correlated. We identify three significant peaks in the source surface density at Galactocentric radii of approximately 4, 6 and 8 kpc, which correspond to the proposed positions of the Scutum, Sagittarius and Perseus spiral arms, respectively. Fitting a scaleheight to the data we obtain an average value of ~29 ± 0.5 pc, which agrees well with other reported values in the literature, however we note a dependence of the scaleheight on galactocentric radius with it increases from 30 to 45 pc between 2.5 and 8.5 kpc.

The Red MSX Source survey: distribution and properties of a sample of massive young stars

The Red MSX Source (RMS) survey has identified a large sample of massive young stellar objects and ultra compact H II regions from a sample of ~2000 MSX and Two Micron All Sky Survey (2MASS) colour selected sources. Using a recent catalogue of molecular clouds derived from the Boston University-Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS), and by applying a Galactic scaleheight cut-off of 120 pc, we solve the distance ambiguity for RMS sources located within 18° 〈|l|〉 54°. These two steps yield kinematic distances to 291 sources out of a possible 326, located within the GRS longitude range. Combining distances and integrated fluxes derived from spectral energy distributions, we estimate luminosities to these sources and find that >90 per cent are indicative of the presence of a massive star. We find the completeness limit of our sample is ~10 4 L ⊙ , which corresponds to a zero-age main-sequence star with a mass of ~12 M ⊙ . Selecting only these sources, we construct a complete sample of 196 sources. Comparing the properties of the sample of young massive stars with the general population, we find the RMS clouds are generally larger, more massive, and more turbulent. We examine the distribution of this subsample with respect to the location of the spiral arms and the Galactic bar and find them to be spatially correlated. We identify three significant peaks in the source surface density at Galactocentric radii of approximately 4, 6 and 8 kpc, which correspond to the proposed positions of the Scutum, Sagittarius and Perseus spiral arms, respectively. Fitting a scaleheight to the data we obtain an average value of ~29 ± 0.5 pc, which agrees well with other reported values in the literature, however we note a dependence of the scaleheight on galactocentric radius with it increases from 30 to 45 pc between 2.5 and 8.5 kpc.