Active Star-forming Regions Research Articles

Astrometry of Galactic Star-Forming Region G48.61+0.02 with VERA

Abstract We performed the astrometry of H$_2$O masers in the Galactic star-forming region G48.61$+$0.02 with the VLBI Exploration of Radio Astrometry (VERA). We derived a trigonometric parallax of 199$\ \pm\ $7$\ \mu$as, which corresponds to a distance of 5.03$\ \pm\ $0.19 kpc. The distance to G48.61$+$0.02 is about half of its far kinematic distance, which was often assumed previously. This distance places G48.61$+$0.02 in the Sagittarius–Carina arm, and near to the active star-forming region and the supernova remnant W 51. We also obtained the three-dimensional motion of G48.61$+$0.02, and found that it has a large peculiar motion of 40$\ \pm\ $5 km s$^{-1}$. This peculiar motion would have originated along with multiple supernovae explosions in W 51, or streaming motion across the Sagittarius–Carina arm.

12CO, 13CO and C18O observations along the major axes of nearby bright infrared galaxies

We present simultaneous observations of 12CO, 13CO and C18O J = 1-0 emission in 11 nearby (cz < 1000km s−1) bright infrared galaxies. Both 12CO and 13CO are detected in the centers of all the galaxies, except for 13CO in NGC 3031. We have also detected C18O, CS J = 2−1 and HCO+ J = 1−0 emission in the nuclear regions of M82 and M51. These are the first systematical extragalactic detections of 12CO and its isotopes from the PMO 14m telescope. We have conducted half-beam-spaced mapping of M82 over an area of 4′ × 2.5′ and major axis mapping of NGC 3627, NGC 3628, NGC 4631 and M51. The radial distributions of 12CO and 13 CO in NGC 3627, NGC 3628 and M51 can be well fitted by an exponential profile. The 12CO/13CO intensity ratio, ℛ, decreases monotonically with the galactocentric radius in all mapped sources. The average ℛ in the center and disk of the galaxies are 9.9±3.0 and 5.6±1.9, respectively, much lower than the peculiar ℛ(∼24) found in the center of M82. The intensity ratios of 13CO/C18O, 13CO/HCO+ and 13CO/CS (either our or literature data) show little variation with galactocentric radius, in sharp contrast with the greatly varied ℛ. This supports the notion that the observed gradient in ℛ could be the result of the variations of the physical conditions across the disks. The H2 column density derived from C18O shows that the Galactic standard conversion factor (X-factor) overestimates the amount of the molecular gas in M82 by a factor of ∼2.5. These observations suggest that the X-factor in active star-forming regions (i.e., nuclear regions) should be lower than that in normal star-forming disks and the gradient in ℛ can be used to trace the variations of the X-factor.

GRAVITATIONAL LENS CANDIDATES IN THE E-CDFS

We report ten lens candidates in the E-CDFS from the GEMS survey. Nine of the systems are new detections and only one of the candidates is a known lens system. For the most promising five systems including the known lens system, we present results from preliminary lens mass modelling, which tests if the candidates are plausible lens systems. Photometric redshifts of the candidate lens galaxies are obtained from the COMBO-17 galaxy catalog. Stellar masses of the candidate lens galaxies within the Einstein radius are obtained by using the $z$-band luminosity and the $V-z$ color-based stellar mass-to-light ratios. As expected, the lensing masses are found to be larger than the stellar masses of the candidate lens galaxies. These candidates have similar dark matter fractions as compared to lenses in SLACS and COSMOS. They also roughly follow the halo mass-stellar mass relation predicted by the subhalo abundance matching technique. One of the candidate lens galaxies qualifies as a LIRG and may not be a true lens because the arc-like feature in the system is likely to be an active region of star formation in the candidate lens galaxy. Amongst the five best candidates, one is a confirmed lens system, one is a likely lens system, two are less likely to be lenses and the status of one of the candidates is ambiguous. Spectroscopic follow-up of these systems is still required to confirm lensing and/or for more accurate determination of the lens masses and mass density profiles.

The VLT-FLAMES Tarantula Survey

VFTS 682 is located in an active star-forming region, at a projected distance of 29 pc from the young massive cluster R136 in the Tarantula Nebula of the Large Magellanic Cloud. It was previously reported as a candidate young stellar object, and more recently spectroscopically revealed as a hydrogen-rich Wolf-Rayet (WN5h) star. Our aim is to obtain the stellar properties, such as its intrinsic luminosity, and to investigate the origin of VFTS 682. To this purpose, we model optical spectra from the VLT-FLAMES Tarantula Survey with the non-LTE stellar atmosphere code CMFGEN, as well as the spectral energy distribution from complementary optical and infrared photometry. We find the extinction properties to be highly peculiar (RV ~4.7), and obtain a surprisingly high luminosity log(L/Lsun) = 6.5 \pm 0.2, corresponding to a present-day mass of ~150Msun. The high effective temperature of 52.2 \pm 2.5kK might be explained by chemically homogeneous evolution - suggested to be the key process in the path towards long gamma-ray bursts. Lightcurves of the object show variability at the 10% level on a timescale of years. Such changes are unprecedented for classical Wolf-Rayet stars, and are more reminiscent of Luminous Blue Variables. Finally, we discuss two possibilities for the origin of VFTS 682: (i) the star either formed in situ, which would have profound implications for the formation mechanism of massive stars, or (ii) VFTS 682 is a slow runaway star that originated from the dense cluster R136, which would make it the most massive runaway known to date.

The Spitzer Surveys of the Small Magellanic Cloud: Insights into the Life-Cycle of Polycyclic Aromatic Hydrocarbons

We present the results of modeling dust spectral energy distributions (SEDs) across the Small Magellanic Cloud (SMC) with the aim of mapping the distribution of polycyclic aromatic hydrocarbons (PAHs) in a low-metallicity environment. Using Spitzer Survey of the SMC (S 3 MC) photometry from 3.6 to 160 µm over the main star-forming regions of the Wing and Bar of the SMC along with spectral mapping observations from 5 to 38 µm from the Spitzer Spectroscopic Survey of the Small Magellanic Cloud (S 4 MC) in selected regions, we model the dust spectral energy distribution and emission spectrum to determine the fraction of dust in PAHs across the SMC. We use the regions of overlaping photometry and spectroscopy to test the reliability of the PAH fraction as determined from SED fits alone. The PAH fraction in the SMC is low compared to the Milky Way and variable– with relatively high fractions (qPAH� 1 − 2%) in molecular clouds and low fractions in the diffuse ISM (average hqPAHi = 0.6%). We use the map of PAH fraction across the SMC to test a number of ideas regarding the production, destruction and processing of PAHs in the ISM. We find weak or no correlation between the PAH fraction and the distribution of carbon AGB stars, the location of supergiant H I shells and young supernova remnants, and the turbulent Mach number. We find that the PAH fraction is correlated with CO intensity, peaks in the dust surface density and the molecular gas surface density as determined from 160 µm emission. The PAH fraction is high in regions of active star-formation, as predicted by its correlation with molecular gas, but is supressed in H II regions. Because the PAH fraction in the diffuse ISM is generally very low–in accordance with previous work on modeling the integrated SED of the SMC–and the PAH fraction is relatively high in molecular regions, we suggest that PAHs are destroyed in the diffuse ISM of the SMC and/or PAHs are forming in molecular clouds. We discuss the implications of these observations for our understanding of the PAH life cycle, particularly in low-metallicity and/or primordial galaxies. Subject headings: dust, extinction — infrared: ISM — Magellanic Clouds

The Ap 2-1 nebula and the surrounding molecular cloud G35.2 − 0.74: an active star-forming region

Using data from large-scale surveys: 2MASS, GLIMPSE, MIPSGAL, VGPS, GRS, and IPHAS, we performed a multiwavelength study of the ISM in a region of about 20' x 20' towards the molecular cloud G35.2-0.74. Additionally, the Ap 2-1 nebula, that is seen in projection over the molecular cloud, was studied using optical data obtained with the 2.15 m telescope at CASLEO, Argentina. From the HI absorption study we estimate a distance of ~2 kpc for Ap 2-1 confirming that the nebula is embedded in the south portion of the molecular cloud G35.2-0.74. Performing a photometric study and analysing the spectral energy distributions of the sources likely embedded in the cloud, we confirm that this region is very active in star formation, mainly towards the north, where we discover a cluster of young stellar objects. From the H_alpha and [NII] lines we obtain a radial velocity of v_LSR ~ 31 km/s for the Ap 2-1 nebula, in coincidence with the velocity of the molecular cloud. Finally, we conclude that Ap 2-1 is an HII region probably excited by an early B-type star.

Integral-field near-infrared spectroscopy of two blue dwarf galaxies: NGC 5253 and He 2-10

We present integral field spectroscopy in the near infrared (NIR) of He 2-10 and NGC 5253, two well known nearby dwarf irregular galaxies showing high star-formation rates. Our data provide an unprecedented detailed view of the interstellar medium and star formation in these galaxies, allowing us to obtain spatially resolved information from the NIR emission and absorption line tracers. We study the spatial distribution and kinematics of different components of the interstellar medium (ISM) mostly through the Bracket series lines, the molecular hydrogen spectrum, [FeII] emission, and CO absorptions. Although the ISM is mostly photo-excited, as derived by the [FeII]/Br<i>γ<i/> and line ratios, some regions corresponding to non-thermal radio sources show a [FeII]/Br<i>γ<i/> excess due to a significant contribution of SN driven shocks. In He 2-10 we find that the molecular gas clouds, as traced by CO(2-1) and infrared line, show consistent morphologies and velocities when studied with the two different tracers. Moreover, there is a clear association with the youngest super star clusters as traced by the ionized gas. In the same galaxy we observe a cavity depleted of gas, which is surrounded by some of the most active regions of star formation, that we interpret as a signature of feedback-induced star formation from older episodes of star formation. Finally, we measured high turbulence in the ISM of both galaxies, <i>σ<i/> ~ 30–80 km s<sup>-1<sup/>, driven by the high star-formation activity.

A submillimetre survey of the kinematics of the Perseus molecular cloud - II. Molecular outflows

We present a census of molecular outflows across four active regions of star formation in the Perseus molecular cloud (NGC 1333, IC348/HH211, L1448 and L1455), totalling an area of over 1000 arcmin2. This is one of the largest surveys of outflow evolution in a single molecular cloud published to date. We analyse large-scale, sensitive CO J= 3 → 2 data sets from the James Clerk Maxwell Telescope, including new data towards NGC 1333. Where possible we make use of our complementary 13CO and C18O data to correct for the 12CO optical depth and measure ambient cloud properties. Of the 65 submillimetre cores in our fields, we detect outflows towards 45. 24 of these are marginal detections where the outflow's shape is unclear or could be confused with the other outflows. We compare various parameters between the outflows from Class 0 and I protostars, including their mass, momentum, energy and momentum flux. Class 0 outflows are longer, faster, more massive and have more energy than Class I outflows. The dynamical time-scales we derive from these outflows are uncorrelated to the age of the outflow driving source, computed from the protostar's bolometric temperature. We confirm the results of Bontemps et al. that outflows decrease in force as they age. There is a decrease in momentum flux from the Class 0 to I stage: 〈FCO〉= (0.8 ± 0.3) × 10−4 compared to (1.1 ± 0.3) × 10−5 M⊙ km s−1 yr−1, suggesting a decline in the mass accretion rate assuming the same entrainment fraction for both classes of outflow. If Frad=Lbol/c is the flux expected in radiation from the central source, then FCO(Class I) ∼ 100Frad and FCO(Class 0) ∼ 1000Frad. Furthermore, we confirm there are additional sources of mass loss from protostars. If a core's mass is only lost from outflows at the current rate, cores would endure a few million years, much longer than current estimates for the duration of the protostellar stage. Finally, we note that the total energy contained in outflows in NGC 1333, L1448 and L1455 is greater than the estimated turbulent energy in the respective regions, which may have implications for the regions’ evolution.

A CO LINE AND INFRARED CONTINUUM STUDY OF THE ACTIVE STAR-FORMING COMPLEX W51

We present the results of an extensive observational study of the active star-forming complex W51 that was observed in the J=2-1 transition of the 12CO and 13CO molecules over a 1.25 deg x 1.00 deg region with the University of Arizona Heinrich Hertz Submillimeter Telescope. We use a statistical equilibrium code to estimate physical properties of the molecular gas. We compare the molecular cloud morphology with the distribution of infrared (IR) and radio continuum sources, and find associations between molecular clouds and young stellar objects (YSOs) listed in Spitzer IR catalogs. The ratios of CO lines associated with HII regions are different from the ratios outside the active star-forming regions. We present evidence of star formation triggered by the expansion of the HII regions and by cloud-cloud collisions. We estimate that about 1% of the cloud mass is currently in YSOs.

Galactic parameters from masers with trigonometric parallaxes

Spatial velocities of all currently known 28 masers having trigonometric parallaxes, proper motion and line-of-site velocities are reanalyzed using Bottlinger's equations. These masers are associated with 25 active star-forming regions and are located in the range of galactocentric distances 3<R<14 kpc. To determine the Galactic rotation parameters, we used the first three Taylor expansion terms of angular rotation velocity {\Omega} at the galactocentric distance of the Sun R0=8 kpc. We obtained the following solutions: {\Omega}o=-31.0 +/- 1.2 km/s/kpc, {\Omega}o'=4.46 +/- 0.21 km/s/kpc^2, {\Omega}o"=-0.876 +/- 0.067 km/s/kpc^3, Oort constants: A=17.8 +/- 0.8 km/s/kpc, B=-13.2 +/- 1.5 km/s/kpc and circular velocity of the Solar neighborhood rotation Vo=248 +/- 14 km/s. Fourier analysis of galactocentric radial velocities of masers VR allowed us to estimate the wavelength {\lambda}=2.0 +/- 0.2 kpc and peak velocity f_R=6.5 +/- 2 km/s of periodic perturbations from the density wave and velocity of the perturbations 4 +/- 1 km/s near the location of the Sun. Phase of the Sun in the density wave is estimated as {\chi}o ~ -130^o +/- 10^o. Taking into account perturbations evoked by spiral density wave we obtained the following non-perturbed components of the peculiar Solar velocity with respect to the local standard of rest (LSR) (Uo,Vo,Wo)LSR=(5.5,11,8.5) +/- (2.2,1.7,1.2) km/s.

Galactic cold cores:Herschelstudy of firstPlanckdetections

Context. We present the first results from the project Galactic cold cores, where the cold interstellar clouds detected by the Planck satellite are studied with Herschel photometric observations. The final Planck catalogue is expected to contain several thousand sources. The Herschel observations during the science demonstration phase provided the first glimpse into the nature of these sources. Aims. The main goal of the project is to derive the physical properties of the cold core population revealed by Planck. We examine three fields and confirm the Planck detections with Herschel data, which we also use to establish the evolutionary stage of the identified cores. Methods. We study the morphology and spectral energy distribution of the sources using the combined wavelength coverage of Planck and Herschel. The dust colour temperatures and emissivity indices are determined. The masses of the cores are determined with distance estimates which are taken from the literature and are confirmed by kinematic and extinction information. Results. The observations reveal extended regions of cold dust with dust colour temperatures down to T_(dust) ~ 11 K. The fields represent different evolutionary stages ranging from a quiescent, cold filament inMusca to regions of active star formation in Cepheus. Conclusions. The Herschel observations confirm that the all-sky survey of Planck is capable of making a large number of new cold core detections. Our results suggest that many of the sources may already have left the pre-stellar phase or are at least closely associated with active star formation. High-resolution Herschel observations are needed to establish the true nature of the Planck detections.

NANTEN Survey of Molecular Clouds toward the Galactic Center; Association of Compact Astronomical Objects

Abstract In order to better understand molecular clouds and their properties toward the Galactic center region, we have analyzed the NANTEN database of $^{12}$CO ($J =$ 1–0) and $^{13}$CO ($J =$ 1–0) to search for associations with candidates for young high-mass star-forming regions, such as IRAS point sources, radio continuum sources, recombination line sources, maser line sources, and other molecular line sources. We have also compared the data with TeV $\gamma$-ray sources. The analyzed region covers $-$12$^{\circ} \leq l \leq$ 12$^{\circ}$ and $-$1${\rlap{\hskip.75pt{^\circ}}\hskip.75pt.\hskip1pt}$5 $\leq b \leq$ 1${\rlap{\hskip.75pt{^\circ}}\hskip.75pt.\hskip1pt}$5 for $^{12}$CO, and $-$6$^{\circ} \leq l \leq$ 8$^{\circ}$ and $-$1$^{\circ} \leq b \leq$ 1$^{\circ}$ for $^{13}$CO. As a result, we identified 167 IRAS point sources, 73 recombination line sources, 58 maser sources, 107 radio continuum sources, and 77 molecular line sources associated with 169 positions with the CO emission. The associations among the objects with known velocity are fairly certain (68%), while those with only positional coincidence are less reliable (32%). We present a catalog of these CO clouds in a form useful for future follow-up studies. As specific examples of usage of the catalog, we highlight three outstanding regions of active star formation: W 28, W 30, and W 31. In particular, we have discovered that the W 30 region shows a striking correlation between the extended TeV $\gamma$-ray source and the molecular gas. This is a second case of such a good correlation identified from the NANTEN dataset subsequent to W 28 (Aharonian et al. 2008, A&amp;A, 481, 401). We suggest that the pulsar PSR J1803$-$2137 near the molecular features may play a role in $\gamma$-ray production. The W 31 region exhibits another active star formation, while no $\gamma$-ray source is yet known.

A Photometric Catalog of 77 Newly Recognized Star Clusters in M31

ABSTRACTThis paper describes the results of a Hubble Space Telescope (HST) WFPC2 search for star clusters in active star-formation regions of M31. Nine of the clusters were previously cataloged and 77 are new. Our 23 fields cover key areas of the galaxy’s recent star-formation activity. We provide a catalog of positions and integrated magnitudes in four colors, taken with the 336W, 439W, 555W, and 814W filters of the HST. A future article will discuss the results of stellar photometry in some of the clusters in six colors, including two additional uv colors. The integrated magnitudes and colors of the clusters show a range of characteristics, but the mean color is bluer than for previous surveys, reflecting the concentration of our sample on active star-forming regions. Absolute magnitudes range from M555 = -10.3 to -3.5. The observed luminosity function shows a nearly Gaussian distribution with a peak value at M555 = -5.4 and a shoulder of unusually bright clusters. We look in detail at two of these unusually bright examples, cluster 45 (C410) and cluster 10 (BH05). C410 lies at the core of a bright H II region. Its absolute magnitude is M555 = -10.3. BH05 is a similar object, with an absolute magnitude of M555 = -8.9. These two clusters are among the most luminous young clusters in M31.

THE COMPLETE SURVEY OF OUTFLOWS IN PERSEUS

We present a study on the impact of molecular outflows in the Perseus molecular cloud complex using the COMPLETE survey large-scale 12CO(1-0) and 13CO(1-0) maps. We used three-dimensional isosurface models generated in RA-DEC-Velocity space to visualize the maps. This rendering of the molecular line data allowed for a rapid and efficient way to search for molecular outflows over a large (~ 16 sq. deg.) area. Our outflow-searching technique detected previously known molecular outflows as well as new candidate outflows. Most of these new outflow-related high-velocity features lie in regions that have been poorly studied before. These new outflow candidates more than double the amount of outflow mass, momentum, and kinetic energy in the Perseus cloud complex. Our results indicate that outflows have significant impact on the environment immediately surrounding localized regions of active star formation, but lack the energy needed to feed the observed turbulence in the entire Perseus complex. This implies that other energy sources, in addition to protostellar outflows, are responsible for turbulence on a global cloud scale in Perseus. We studied the impact of outflows in six regions with active star formation within Perseus of sizes in the range of 1 to 4 pc. We find that outflows have enough power to maintain the turbulence in these regions and enough momentum to disperse and unbind some mass from them. We found no correlation between outflow strength and star formation efficiency for the six different regions we studied, contrary to results of recent numerical simulations. The low fraction of gas that potentially could be ejected due to outflows suggests that additional mechanisms other than cloud dispersal by outflows are needed to explain low star formation efficiencies in clusters.

THESPITZERSURVEY OF THE SMALL MAGELLANIC CLOUD (S3MC): INSIGHTS INTO THE LIFE CYCLE OF POLYCYCLIC AROMATIC HYDROCARBONS

We present the results of modeling dust SEDs across the SMC with the aim of mapping the distribution of PAHs in a low-metallicity environment. Using Spitzer Survey of the SMC (S3MC) photometry from 3.6-160 um over the main star-forming regions of the Wing and Bar along with spectral mapping from 5-38 um from the Spitzer Spectroscopic Survey of the SMC (S4MC) in selected regions, we model the dust SED and emission spectrum to determine the fraction of dust in PAHs across the SMC. We use the regions of overlapping photometry and spectroscopy to test the reliability of the PAH fraction as determined from SED fits alone. The PAH fraction in the SMC is low compared to the Milky Way and variable--with relatively high fractions (q_PAH~1-2%) in molecular clouds and low fractions in the diffuse ISM (<q_PAH>=0.6%). We use the map of PAH fraction across the SMC to test a number of ideas regarding the production, destruction and processing of PAHs in the ISM. We find weak or no correlation between the PAH fraction and the distribution of carbon AGB stars, the location of supergiant H I shells and young SN remnants, or the turbulent Mach number. We find that the PAH fraction is correlated with CO intensity, peaks in the dust surface density and the molecular gas surface density as determined from 160 um emission. The PAH fraction is high in regions of active star-formation, as predicted by its correlation with molecular gas, but is suppressed in H II regions. Because the PAH fraction in the diffuse ISM is generally very low--in accordance with previous work on modeling the SED of the SMC--and the PAH fraction is relatively high in molecular regions, we suggest that PAHs are destroyed in the diffuse ISM of the SMC and/or PAHs are forming in molecular clouds. We discuss the implications of these observations for our understanding of the PAH life cycle, particularly in low-metallicity galaxies.

Outflows and Turbulence in Young Stellar Clusters — An Observer's View

AbstractRecent numerical studies have focused their interest on the impact outflows have on the cloud's turbulence. The contradictory results obtained by these studies indicate that it is essential for observers to provide the required data to constrain the models. Here we discuss the impact of outflows on the environment surrounding clusters of young stellar objects, from an observer's point of view. We have conducted several studies of outflows in different active star-forming regions. In all cases it is clear that outflows have the power to sustain the observed turbulence in the gas surrounding protostellar clusters. We investigate whether there is a correlation between outflow strength and star formation efficiency, as predicted by numerical simulations, for six different regions in the Perseus molecular cloud complex. We argue that results of other recent studies that use CO line maps to study the turbulence driving length should not be used to discard outflows as major drivers of turbulence in clusters.

AN X-RAY VIEW OF STAR FORMATION IN THE CENTRAL 3 kpc OF NGC 2403

Archival Chandra observations are used to study the X-ray emission associated with star formation in the central region of the nearby SAB(s)cd galaxy NGC 2403. The distribution of X-ray emission is compared to the morphology visible at other wavelengths using complementary Spitzer, Galaxy Evolution Explorer, and ground-based Hα imagery. In general, the brightest extended X-ray emission is associated with H ii regions and to other star-forming structures but is more pervasive, existing also in regions devoid of strong Hα and UV emission. This X-ray emission has the spectral properties of diffuse hot gas (kT ∼ 0.2 keV) whose likely origin is in gas shock heated by stellar winds and supernovae with ≲20% coming from faint unresolved X-ray point sources. This hot gas may be slowly cooling extra-planar remnants of past outflow events, or a disk component that either lingers after local star formation activity has ended or that has vented from active star-forming regions into a porous interstellar medium.

3D spectroscopy of local luminous compact blue galaxies: kinematics of NGC 7673

The kinematic properties of the ionized gas of local Luminous Compact Blue Galaxy (LCBG) NGC 7673 are presented using three dimensional data taken with the PPAK integral field unit at the 3.5-m telescope in the Centro Astron\'omico Hispano Alem\'an. Our data reveal an asymmetric rotating velocity field with a peak to peak difference of 60 km s$^{-1}$. The kinematic centre is found to be at the position of a central velocity width maximum ($\sigma=54\pm1$ km s$^{-1}$), which is consistent with the position of the luminosity-weighted centroid of the entire galaxy. The position angle of the minor rotation axis is 168$^{\circ}$ as measured from the orientation of the velocity field contours. At least two decoupled kinematic components are found. The first one is compact and coincides with the position of the second most active star formation region (clump B). The second one is extended and does not have a clear optical counterpart. No evidence of active galactic nuclei activity or supernovae galactic winds powering any of these two components has been found. Our data, however, show evidence in support of a previously proposed minor merger scenario in which a dwarf galaxy, tentatively identified with clump B, is falling into NGC 7673. and triggers the starburst. Finally, it is shown that the dynamical mass of this galaxy may be severely underestimated when using the derived rotation curve or the integrated velocity width, under the assumption of virialization.

EMBEDDED YOUNG STELLAR OBJECT CANDIDATES IN THE ACTIVE STAR-FORMING COMPLEX W51: MASS FUNCTION AND SPATIAL DISTRIBUTION

We present 737 candidate Young Stellar Objects (YSOs) near the W51 Giant Molecular Cloud (GMC) over an area of 1.25 deg x 1.00 deg selected from Spitzer Space Telescope data. We use spectral energy distribution (SED) fits to identify YSOs and distinguish them from main-sequence or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses > 5 Msun (corresponding to mid-to early-B main-sequence spectral types) 46 of which are located in the central active star forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 HII region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 Msun and 18 Msun. The derived slopes of the MF are -1.26 and -2.36 in the active star-forming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

S 235 B explained: an accreting Herbig Be star surrounded by reflection nebulosity

The intent of this study is to determine the nature of the star and associated nebulosity S 235 B, which are located in a region of active star formation still heavily obscured by the parent molecular cloud. Low-resolution (R = 400) long-slit spectra of the star and nebulosity, and medium- (R = 1800) and high-resolution (R = 60 000) spectra of the central star are presented along with the results of Fabry-Perot interferometric imaging of the entire region. Based on the long-slit and Fabry-Perot observations, the nebulosity appears to be entirely reflective in nature, with the stellar component S 235 B★ providing most of the illuminating flux. The stellar source itself is classified here as a B 1 V star, with emission-line profiles indicative of an accretion disc. S 235 B* thus belongs to the relatively rare class of early-type Hebrig Be stars. Based on the intensity of the reflected component, it is concluded that the accretion disc must be viewed nearly edge-on. Estimates of the accretion rate of S 235 B★ from the width of the Hα profile at 10 per cent of maximum intensity, a method which has been used lately for T Tauri stars and Brown Dwarfs, appear to be inconsistent with the mass outflow rate and accretion rate implied from previous infrared observations by Felli et al., suggesting this empirical law does not extend to higher masses.