We discuss possible sources of biases in the determination of the initial mass function (IMF) introduced by the binning of the data, the uncertainty in the determinations of masses, and the existence ...of unresolved multiple systems. Those three effects tend to produce IMFs that are flatter than the real one. We analyze the importance of each effect and suggest techniques that minimize or eliminate the biases. We also report the detection of the first astrometric binary system composed of two very-early O-type stars, HD 93129 A.
We are using HST GO programs 10205, 10602, and 10898 to test the stellar upper mass limit in the solar vicinity by attempting to detect optical close companions, thus lowering the calculated ...evolutionary masses. We have observed with ACS/HRC all the known (as of early 2005) Galactic O2/3/3.5 stars. We also have observations with HST/FGS and ground-based spectroscopy from LCO and CASLEO. Here we discuss our results for Pismis 24 and HD 93129A.
The accuracy in determining the spatial-kinematical parameters of open clusters makes them ideal tracers of the Galactic structure. Young open clusters (YOCs) are the main representative of the ...clustered star formation mode, which identifies how most of the stars in the Galaxy form. We apply the Kriging technique to a sample of Gaia YOCs within a 3.5 kpc radius around the Sun and log(age) \(\leq\) 7.5, age in years, to obtain the \(Z(X,Y)\) and \(V_Z(X,Y)\) maps. The previous work by Alfaro et al. (1991) showed that Kriging can provide reliable results even with small data samples (\(N \sim 100\)). We approach the 3D spatial and vertical velocity field structure of the Galactic disk defined by YOCs and analyze the hierarchy of the stellar cluster formation, which shows a rich hierarchical structure, displaying complexes embedded within each other. We discuss the fundamental characteristics of the methodology used to perform the mapping and point out the main results obtained in phenomenological terms. Both the 3D spatial distribution and the vertical velocity field reveal a complex disk structure with a high degree of substructures. Their analysis provides clues about the main physical mechanisms that shape the phase space of the clustered star formation in this Galactic area. Warp, corrugations, and high local deviations in \(Z\) and \(V_Z\), appear intimately connected in a single but intricate scenario.
(Abridged) We have used Gaia EDR3 data, together with spectra of a large sample of luminous stars in the field surrounding Westerlund 1, to explore the extent of the cluster. We carry out a ...non-parametric analysis of proper motions and membership determination. We investigate the reddening and proper motions of several dozen OB stars and red supergiants < 1 deg away from Wd 1. We identify a population of kinematic members of Wd 1 that largely includes the known spectroscopic members. From their EDR3 parallaxes, we derive a distance to the cluster of \(4.23^{+0.23}_{-0.21}\)kpc. Extinction in this direction increases by a large amount around 2.8 kpc, due to dark clouds associated to the Scutum-Crux arm. As a consequence, we hardly see any stars at distances comparable to that of the cluster. The proper motions of Wd 1, however, are very similar to those of stars in the field surrounding it, but distinct. We find a second, astrometrically well-defined population in the foreground (\(d\approx2\:\)kpc), likely connected to the possible open cluster BH197. Wd 1 is very elongated, an effect not driven by the very heavy extinction to the East and South. We find a low-density halo extending up to 10' from the cluster centre, mainly in the NW quadrant. A few OB stars at larger distances from the cluster, most notably the LBV MN48, share its proper motions, suggesting that Wd 1 has little or no peculiar motion with respect to the field population of the Norma arm. However, we are unable to find any red supergiant that could belong to an extended population related to the cluster, although we observe several dozen such objects in the foreground. We find a substantial population of luminous OB members obscured by several more magnitudes of extinction than most known members. These objects, mostly located in the central region of the cluster, increase the population of OB supergiants by about 25%.
In this study, we test the theoretically predicted mass-loss behaviour as a
function of stellar effective temperature across the so-called `bi-stability'
jump (BSJ). We gathered radio observations of ...30 OB supergiants (O8-B3). We
derived the radio mass-loss rates and wind efficiencies, and compared our
results with Halpha mass-loss rates and predictions based on radiation-driven
wind models. he wind efficiency shows the possible presence of a local maximum
around an effective temperature of 21~000 K -- in qualitative agreement with
predictions. We also find that the radio mass-loss rates show good agreement
with empirical Halpha rates. However, the empirical mass-loss rates are larger
than the predicted rates from radiation-driven wind theory for objects above
the BSJ temperature, whilst they are smaller for the rest. A new wind
momenta-luminosity relation for O8-B0 stars has been derived.
We present the results of a first campaign of radio continuum observations
toward supergiants with spectral types in the range O8 - B3. Three targets out
of twelve were detected with the ATCA and the ...VLA. The measured fluxes and the
upper limits yielded values of the stellar mass-loss rates dM/dt and the wind
efficiencies. The comparison of predicted and derived values of dM/dt shows a
good agreement.
Mon.Not.Roy.Astron.Soc.366:739-757,2006 A detailed study of the Hourglass Nebula in the M8 star forming region is
presented. The study is mainly based on recent subarcsec-resolution JHKs images
taken ...at Las Campanas Observatory and complemented with archival HST images and
longslit spectroscopy retrieved from the ESO Archive Facility. Using the new
numerical code CHORIZOS, we estimate the distance to the earliest stars in the
region to be 1.25 kpc. Infrared photometry of all the sources detected in the
field is given. From analysis of the JHKs colour-colour diagrams, we find that
an important fraction of these sources exhibit significant infrared excess.
These objects are candidates to be low- and intermediate-mass pre-main sequence
stars. Based on HST observations, the spatial distribution of gas, dust and
stars in the region is analyzed. The morphological analysis of these images
also reveals a rich variety of structures related to star formation (proplyds,
jets, bow shocks), similar to those observed in M16 and M42, along with the
detection of the first four Herbig-Haro objects in the region. Furthermore, a
longslit spectrum obtained with NTT confirms the identification of one of them
(HH 870) in the core of the Hourglass nebula, providing the first direct
evidence of active star formation by accretion in M8.
In this study, we test the theoretically predicted mass-loss behaviour as a function of stellar effective temperature across the so-called `bi-stability' jump (BSJ). We gathered radio observations of ...30 OB supergiants (O8-B3). We derived the radio mass-loss rates and wind efficiencies, and compared our results with Halpha mass-loss rates and predictions based on radiation-driven wind models. he wind efficiency shows the possible presence of a local maximum around an effective temperature of 21~000 K -- in qualitative agreement with predictions. We also find that the radio mass-loss rates show good agreement with empirical Halpha rates. However, the empirical mass-loss rates are larger than the predicted rates from radiation-driven wind theory for objects above the BSJ temperature, whilst they are smaller for the rest. A new wind momenta-luminosity relation for O8-B0 stars has been derived.
We present the results of a first campaign of radio continuum observations toward supergiants with spectral types in the range O8 - B3. Three targets out of twelve were detected with the ATCA and the ...VLA. The measured fluxes and the upper limits yielded values of the stellar mass-loss rates dM/dt and the wind efficiencies. The comparison of predicted and derived values of dM/dt shows a good agreement.
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