There is currently no accepted theoretical framework for the formation of the most massive stars, and the manner in which protostars continue to accrete and grow in mass beyond ∼10 M⊙ is still a ...controversial topic. In this study we use several prescriptions of stellar accretion and a description of the Galactic gas distribution to simulate the luminosities and spatial distribution of massive protostellar population of the Galaxy. We then compare the observables of each simulation to the results of the Red MSX Source (RMS) survey, a recently compiled data base of massive young stellar objects (YSO). We find that the observations are best matched by accretion rates which increase as the protostar grows in mass, such as those predicted by the turbulent core and competitive accretion (i.e. Bondi-Hoyle) models. These 'accelerating accretion' models provide very good qualitative and quantitative fits to the data, though we are unable to distinguish between these two models on our simulations alone. We rule out models with accretion rates which are constant with time, and those which are initially very high and which fall away with time, as these produce results which are quantitatively and/or qualitatively incompatible with the observations. To simultaneously match the low- and high-luminosity YSO distribution we require the inclusion of a 'swollen-star' pre-main-sequence phase, the length of which is well-described by the Kelvin-Helmholz time-scale. Our results suggest that the lifetime of the YSO phase is ∼105 yr, whereas the compact H ii region phase lasts between ∼2 and 4 × 105 yr depending on the final mass of the star. Finally, the absolute numbers of YSOs are best matched by a globally averaged star formation rate for the Galaxy of 1.5-2 M⊙.
ABSTRACT
Close to 100 per cent of massive stars are thought to be in binary systems. The multiplicity of massive stars seems to be intrinsically linked to their formation and evolution, and massive ...young stellar objects (MYSOs) are key in observing this early stage of star formation. We have surveyed three samples totalling hundreds of MYSOs ($\gt 8\, \mathrm{M}_\odot$) across the Galaxy from the Red MSX Source (RMS) catalogue, using United Kingdom Infra-Red Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) and Vista Variables in the Via Lactea (VVV) point source data, and UKIRT K-band imaging to probe separations between 0.8 and 9 arcsec (approx 1000–100 000 au). We have used statistical methods to determine the binary statistics of the samples, and we find binary fractions of 64 ± 4 per cent for the UKIDSS sample, 53 ± 4 per cent for the VVV sample, and 49 ± 8 per cent for the RMS imaging sample. Also, we use the J- and K-band magnitudes as a proxy for the companion mass, and a significant fraction of the detected systems have estimated mass ratios >0.5, suggesting a deviation from the capture formation scenario which would be aligned with random IMF sampling. Finally, we find that YSOs located in the outer Galaxy have a higher binary fraction than those in the inner Galaxy. This is likely due to a lower stellar background density than observed towards the inner Galaxy, resulting in higher probabilities for visual binaries to be physical companions. It does indicate a binary fraction in the probed separation range of close to 100 per cent without the need to consider selection biases.
We present 1.3 mm Submillimeter Array (SMA) observations at ∼3 arcsec resolution towards the brightest section of the intermediate/massive star-forming cluster NGC 2264-C. The millimetre continuum ...emission reveals ten 1.3 mm continuum peaks, of which four are new detections. The observed frequency range includes the known molecular jet/outflow tracer SiO (5−4), thus providing the first high-resolution observations of SiO towards NGC 2264-C. We also detect molecular lines of 12 additional species towards this region, including CH3CN, CH3OH, SO, H2CO, DCN, HC3N, and 12CO. The SiO (5−4) emission reveals the presence of two collimated, high-velocity (up to 30 km s−1 with respect to the systemic velocity) bipolar outflows in NGC 2264-C. In addition, the outflows are traced by emission from 12CO, SO, H2CO, and CH3OH. We find an evolutionary spread between cores residing in the same parent cloud. The two unambiguous outflows are driven by the brightest mm continuum cores, which are IR-dark, molecular line weak, and likely the youngest cores in the region. Furthermore, towards the Red MSX Source AFGL 989-IRS1, the IR-bright and most evolved source in NGC 2264-C, we observe no molecular outflow emission. A molecular line rich ridge feature, with no obvious directly associated continuum source, lies on the edge of a low-density cavity and may be formed from a wind driven by AFGL 989-IRS1. In addition, 229 GHz class I maser emission is detected towards this feature.
We present a determination of the luminosity functions of massive young stellar objects (MYSOs) and compact (C) H II regions within the Milky Way using the large, well-selected sample of these ...sources identified by the Red MSX Source survey. The MYSO luminosity function decreases monotonically such that there are few with L105 L , while the CH II regions are detected up to ~106 L . The lifetimes of these phases are also calculated as a function of luminosity by comparison with the luminosity function for local main-sequence OB stars. These indicate that the MYSO phase has a duration ranging from 4X105 yr for 104 L to ~7X104 yr at 105 L , while the CH II region phase lasts of order 3X105 yr or ~3%-10% of the exciting star's main-sequence lifetime. MYSOs between 104 L and ~105 L are massive but do not display the radio continuum or near-IR H I recombination line emission indicative of an H II region, consistent with being swollen due to high ongoing or recent accretion rates. Above ~105 L the MYSO phase lifetime becomes comparable to the main-sequence Kelvin-Helmholtz timescale, at which point the central star can rapidly contract onto the main sequence even if still accreting, and ionize a CH II region, thus explaining why few highly luminous MYSOs are observed.
Context. There are a number of methods that identify stellar sub-structure in star forming regions, but these do not quantify the degree of association of individual stars – something which is ...required if we are to better understand the mechanisms and physical processes that dictate structure. Aims. We present the new novel statistical clustering tool “INDICATE” which assesses and quantifies the degree of spatial clustering of each object in a dataset, discuss its applications as a tracer of morphological stellar features in star forming regions, and to look for these features in the Carina Nebula (NGC 3372). Methods. We employ a nearest neighbour approach to quantitatively compare the spatial distribution in the local neighbourhood of an object with that expected in an evenly spaced uniform (i.e. definitively non-clustered) field. Each object is assigned a clustering index (“I”) value, which is a quantitative measure of its clustering tendency. We have calibrated our tool against random distributions to aid interpretation and identification of significant I values. Results. Using INDICATE we successfully recover known stellar structure of the Carina Nebula, including the young Trumpler 14-16, Treasure Chest and Bochum 11 clusters. Four sub-clusters contain no, or very few, stars with a degree of association above random which suggests these sub-clusters may be fluctuations in the field rather than real clusters. In addition we find: (1) Stars in the NW and SE regions have significantly different clustering tendencies, which is reflective of differences in the apparent star formation activity in these regions. Further study is required to ascertain the physical origin of the difference; (2) The different clustering properties between the NW and SE regions are also seen for OB stars and are even more pronounced; (3) There are no signatures of classical mass segregation present in the SE region – massive stars here are not spatially concentrated together above random; (4) Stellar concentrations are more frequent around massive stars than typical for the general population, particularly in the Tr14 cluster; (5) There is a relation between the concentration of OB stars and the concentration of (lower mass) stars around OB stars in the centrally concentrated Tr14 and Tr15, but no such relation exists in Tr16. We conclude this is due to the highly sub-structured nature of Tr16. Conclusions. INDICATE is a powerful new tool employing a novel approach to quantify the clustering tendencies of individual objects in a dataset within a user-defined parameter space. As such it can be used in a wide array of data analysis applications. In this paper we have discussed and demonstrated its application to trace morphological features of young massive clusters.
We present a power-spectrum analysis of the final 2dF Galaxy Redshift Survey (2dFGRS), employing a direct Fourier method. The sample used comprises 221 414 galaxies with measured redshifts. We ...investigate in detail the modelling of the sample selection, improving on previous treatments in a number of respects. A new angular mask is derived, based on revisions to the photometric calibration. The redshift selection function is determined by dividing the survey according to rest-frame colour, and deducing a self-consistent treatment of k-corrections and evolution for each population. The covariance matrix for the power-spectrum estimates is determined using two different approaches to the construction of mock surveys, which are used to demonstrate that the input cosmological model can be correctly recovered. We discuss in detail the possible differences between the galaxy and mass power spectra, and treat these using simulations, analytic models and a hybrid empirical approach. Based on these investigations, we are confident that the 2dFGRS power spectrum can be used to infer the matter content of the universe. On large scales, our estimated power spectrum shows evidence for the ‘baryon oscillations’ that are predicted in cold dark matter (CDM) models. Fitting to a CDM model, assuming a primordial ns= 1 spectrum, h= 0.72 and negligible neutrino mass, the preferred parameters are Ωmh= 0.168 ± 0.016 and a baryon fraction Ωb/Ωm= 0.185 ± 0.046 (1σ errors). The value of Ωmh is 1σ lower than the 0.20 ± 0.03 in our 2001 analysis of the partially complete 2dFGRS. This shift is largely due to the signal from the newly sampled regions of space, rather than the refinements in the treatment of observational selection. This analysis therefore implies a density significantly below the standard Ωm= 0.3: in combination with cosmic microwave background (CMB) data from the Wilkinson Microwave Anisotropy Probe (WMAP), we infer Ωm= 0.231 ± 0.021.
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, H2 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 object's 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.
Context. Better understanding of star formation in clusters with high-mass stars requires rigorous dynamical and spatial analyses of star-forming regions. Aims. We seek to demonstrate that “INDICATE” ...is a powerful spatial analysis tool which when combined with kinematic data from Gaia DR2 can be used to probe star formation history in a robust way. Methods. We compared the dynamic and spatial distributions of young stellar objects (YSOs) at various evolutionary stages in NGC 2264 using Gaia DR2 proper motion data and INDICATE. Results. The dynamic and spatial behaviours of YSOs at different evolutionary stages are distinct. Dynamically, Class II YSOs predominately have non-random trajectories that are consistent with known substructures, whereas Class III YSOs have random trajectories with no clear expansion or contraction patterns. Spatially, there is a correlation between the evolutionary stage and source concentration: 69.4% of Class 0/I, 27.9% of Class II, and 7.7% of Class III objects are found to be clustered. The proportion of YSOs clustered with objects of the same class also follows this trend. Class 0/I objects are both found to be more tightly clustered with the general populous/objects of the same class than Class IIs and IIIs by a factor of 1.2/4.1 and 1.9/6.6, respectively. An exception to these findings is within 0.05° of S Mon where Class III objects mimic the behaviours of Class II sources across the wider cluster region. Our results suggest (i) current YSOs distributions are a result of dynamical evolution, (ii) prolonged star formation has been occurring sequentially, and (iii) stellar feedback from S Mon is causing YSOs to appear as more evolved sources. Conclusions. Designed to provide a quantitative measure of clustering behaviours, INDICATE is a powerful tool with which to perform rigorous spatial analyses. Our findings are consistent with what is known about NGC 2264, effectively demonstrating that when combined with kinematic data from Gaia DR2 INDICATE can be used to study the star formation history of a cluster in a robust way.
ABSTRACT
We analyse the observed correlation between galaxy environment and Hα emission‐line strength, using volume‐limited samples and group catalogues of 24 968 galaxies at 0.05 < z < 0.095, drawn ...from the 2dF Galaxy Redshift Survey (MbJ< −19.5) and the Sloan Digital Sky Survey (Mr < −20.6). We characterize the environment by: (1) Σ5, the surface number density of galaxies determined by the projected distance to the fifth nearest neighbour; and (2) ρ1.1 and ρ5.5, three‐dimensional density estimates obtained by convolving the galaxy distribution with Gaussian kernels of dispersion 1.1 and 5.5 Mpc, respectively. We find that star‐forming and quiescent galaxies form two distinct populations, as characterized by their Hα equivalent width, W0(Hα). The relative numbers of star‐forming and quiescent galaxies vary strongly and continuously with local density. However, the distribution of W0(Hα) amongst the star‐forming population is independent of environment. The fraction of star‐forming galaxies shows strong sensitivity to the density on large scales, ρ5.5, which is likely independent of the trend with local density, ρ1.1. We use two differently selected group catalogues to demonstrate that the correlation with galaxy density is approximately independent of group velocity dispersion, for σ= 200–1000 km s‐1. Even in the lowest‐density environments, no more than ∼70 per cent of galaxies show significant Hα emission. Based on these results, we conclude that the present‐day correlation between star formation rate and environment is a result of short‐time‐scale mechanisms that take place preferentially at high redshift, such as starbursts induced by galaxy–galaxy interactions.
We compute the bispectrum of the 2dF Galaxy Redshift Survey (2dFGRS) and use it to measure the bias parameter of the galaxies. This parameter quantifies the strength of clustering of the galaxies ...relative to the mass in the Universe. By analysing 80 × 106 triangle configurations in the wavenumber range 0.1 < k < 0.5 h Mpc−1 (i.e. on scales roughly between 5 and 30 h−1 Mpc) we find that the linear bias parameter is consistent with unity: b1= 1.04 ± 0.11, and the quadratic (non-linear) bias is consistent with zero: b2=−0.054 ± 0.08. Thus, at least on large scales, optically selected galaxies do indeed trace the underlying mass distribution. The bias parameter can be combined with the 2dFGRS measurement of the redshift distortion parameter β≃Ωm0.6/b1, to yield Ωm= 0.27 ± 0.06 for the matter density of the Universe, a result that is determined entirely from this survey, independent of other data sets. Our measurement of the matter density of the Universe should be interpreted as Ωm at the effective redshift of the survey (z= 0.17).
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