A comprehensive census of the stellar content of the OB associations within 1 kpc from the Sun is presented, based on Hipparcos positions, proper motions, and parallaxes. It is a key part of a long-term project to study the formation, structure, and evolution of nearby young stellar groups and related star-forming regions. OB associations are unbound moving groups, which can be detected kinematically because of their small internal velocity dispersion. The nearby associations have a large extent on the sky, which traditionally has limited astrometric membership determination to bright stars (V 6 mag), with spectral types earlier than ~B5. The Hipparcos measurements allow a major improvement in this situation. Moving groups are identified in the Hipparcos Catalog by combining de Bruijne's refurbished convergent point method with the Spaghetti method of Hoogerwerf & Aguilar. Astrometric members are listed for 12 young stellar groups, out to a distance of ~650 pc. These are the three subgroups Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2, α Persei (Per OB3), Cas–Tau, Lac OB1, Cep OB2, and a new group in Cepheus, designated as Cep OB6. The selection procedure corrects the list of previously known astrometric and photometric B- and A-type members in these groups and identifies many new members, including a significant number of F stars, as well as evolved stars, e.g., the Wolf-Rayet stars γ2 Vel (WR 11) in Vel OB2 and EZ CMa (WR 6) in Col 121, and the classical Cepheid δ Cep in Cep OB6. Membership probabilities are given for all selected stars. Monte Carlo simulations are used to estimate the expected number of interloper field stars. In the nearest associations, notably in Sco OB2, the later-type members include T Tauri objects and other stars in the final pre–main-sequence phase. This provides a firm link between the classical high-mass stellar content and ongoing low-mass star formation. Detailed studies of these 12 groups, and their relation to the surrounding interstellar medium, will be presented elsewhere. Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive. OB associations do exist in many of these regions, but they are either at distances beyond ~500 pc where the Hipparcos parallaxes are of limited use, or they have unfavorable kinematics, so that the group proper motion does not distinguish it from the field stars in the Galactic disk. The mean distances of the well-established groups are systematically smaller than the pre-Hipparcos photometric estimates. While part of this may be caused by the improved membership lists, a recalibration of the upper main sequence in the Hertzsprung-Russell diagram may be called for. The mean motions display a systematic pattern, which is discussed in relation to the Gould Belt. Six of the 12 detected moving groups do not appear in the classical list of nearby OB associations. This is sometimes caused by the absence of O stars, but in other cases a previously known open cluster turns out to be (part of) an extended OB association. The number of unbound young stellar groups in the solar neighborhood may be significantly larger than thought previously.

/pdf/a-hipparcos-census-of-the-nearby-ob-associations-44xw838axn.pdf

A Hipparcos Census of the Nearby OB Associations

We have carried out 21 cm radio continuum observations of 16 UC HII regions using the VLA (D-array) in search of associated extended emission. We have also observed H76$_\alpha$ recombination line towards all the sources and He76$_\alpha$ line at the positions with strong H76$_\alpha$ line emission. The UC HII regions have simple morphologies and large (>10) ratios of single-dish to VLA fluxes. Extended emission was detected towards all the sources. The extended emission consists of one to several compact components and a diffuse extended envelope. All the UC HII regions but two are located in the compact components, where the UC HII regions always correspond to their peaks. The compact components with UC HII regions are usually smaller and denser than those without UC HII regions. Our recombination line observations indicate that the ultracompact, compact, and extended components are physically associated. The UC HII regions and their associated compact components are likely to be ionized by the same sources on the basis of the morphological relations mentioned above. This suggests that almost all of the observed UC HII regions are not `real' UC HII regions and that their actual ages are much greater than their dynamical age (<10000 yr). We find that most of simple UC HII regions previously known have large ratios of single-dish to VLA fluxes, similar to our sources. Therefore, the `age problem' of UC HII regions does not seem to be as serious as earlier studies argued. We present a simple model that explains extended emission around UC HII regions. Some individual sources are discussed.

Radio Continuum and Recombination Line Study of 16 UC HII Regions with Extended Envelopes

New UBVI and Hα photometry has been performed for the young open cluster NGC 2244. We classified 30 OB stars as being members of the cluster using proper-motion data and spectral types from previous investigators, along with photometric diagrams obtained in this study. We measured Hα emission strength of the stars by Hα photometry and set up a selection criterion to select pre–main-sequence (PMS) stars with Hα emission. Fourteen PMS stars and seven PMS candidates were found using the criterion. In addition, six stars found near the positions of ROSAT HRI X-ray sources were assumed to be PMS stars and the optical counterparts of these X-ray sources. We determined a reddening of E(B-V) = 0.47 ± 0.04 for the cluster and a total-to-selective extinction ratio of RV = 3.1 ± 0.2. The distance modulus derived was V0 - MV = 11.1. By comparing our photometric results with theoretical evolution models, we derived a main-sequence turnoff age of 1.9 Myr and a PMS age spread of about 6 Myr. The slope of the initial mass function, Γ, calculated in the mass range 0.5 ≤ log m ≤ 2.0 could be flat (Γ = -0.7 ± 0.1).

https://iopscience.iop.org/article/10.1086/338643/pdf

UBVRI and Hα Photometry of the Young Open Cluster NGC 6231

NGC 1333, a highly active star formation region within the Perseus molecular cloud complex, has been observed with the ACIS-I detector on board the Chandra X-Ray Observatory In our image with a sensitivity limit of ~1028 ergs s-1, we detect 127 X-ray sources, most with subarcsecond positional accuracy While 32 of these sources appear to be foreground stars and extragalactic background, 95 X-ray sources are identified with known cluster members The X-ray luminosity function of the discovered young stellar object (YSO) population spans a range of log LX 280-315 ergs s-1 in the 05-8 keV band, and absorption ranges from log NH 20 to 23 cm-2 Most of the sources have plasma energies between 06 and 3 keV, but a few sources show higher energies up to ~7 keV Comparison with K-band source counts indicates that we detect all of the known cluster members with K 12 K 11, the peak of the K-band luminosity function, corresponds to 02-04 M☉ stars for a cluster age of ~1 Myr We detect seven of the 20 known YSOs in NGC 1333 producing jets or molecular outflows as well as one deeply embedded object without outflows No evident difference in X-ray emission of young stars with and without outflows is found Based on the complete subsample of T Tauri stars, we also find no difference in X-ray properties and X-ray production mechanism of stars with and without K-band excess disks Several other results are obtained We suggest that the X-ray emission from two late B stars that illuminate the reflection nebula originates from unresolved late-type companions Two T Tauri stars are discovered in the ACIS images as previously unknown components of visual binaries A good correlation LX J is seen, which confirms the well-known relation LX Lbol found in many star-forming regions Based on spectral analysis for the X-ray counterpart of SVS 16, we establish that the column density NH is much lower than that expected from near-IR photometry so that its X-ray luminosity, log LX 306 ergs s-1, is not unusually high

https://iopscience.iop.org/article/10.1086/341219/pdf

Chandra study of young stellar objects in the NGC 1333 star-forming cloud

We have conducted a survey of HCN and HNC (two rotational transitions each) in our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores whose structures and chemistry have been studied earlier in this series. Both species are seen in all 38 objects. HCNH+ has been searched in three objects. These results are modeled in terms of our previous hydrostatic equilibrium and n ~ r-α structures together with other chemical and physical properties derived earlier. A detailed program has been written to handle the complex radiative transfer of the hyperfine splitting (hfs) of HCN. It is shown that serious errors are made in deriving HCN abundances by methods that ignore the hfs. Both HCN and HNC abundances are high, typically 1(-8) in most sources. The chemically important ratio HCN/HNC is found to be ~2.5 if these species are spatially centrally peaked and ~6 if not. Both species abundances increase monotonically with increasing extinction in the 1.2-2.7 mag range (edge to center), thus displaying the same characteristic transition between diffuse and dense cloud chemistry as do most other species we have studied. HCN/HNC decreases with increasing extinction to a value of 1.3 at Av0 ~ 10, approaching the expected value of 1.0 for dense clouds. Two types of ion-molecule chemistry models have been carried out: a full model using the Standard Model rate file and comprising 409 species (by Lee and Herbst), and a simplified model comprising 21 nitrogen-bearing species for conditions relevant to translucent clouds. Good agreement between observations and chemistry models is achieved throughout the translucent extinction range. Important conclusions are that (1) neutral-neutral reactions such as N + CH2 dominate the chemistry of HCN; (2) low ion-polar reaction rates are strongly favored over high ones; (3) the reaction C+ + NH3 → H2NC+ → HNC is unimportant, thus largely uncoupling the CN and NH chemistries; (4) the ratio HCN/HNC is not a particularly important diagnostic of the CN chemistry; (5) model NH3 abundances are at least a factor 100 lower than observed in translucent clouds, even if the reaction N+H+3→NH+2 is permitted at Langevin rate.

http://iopscience.iop.org/article/10.1086/304228/pdf

The Physics and Chemistry of Small Translucent Molecular Clouds. VIII. HCN and HNC

High-quality J=1$Yields$0 H$sup 12$CN and H$sup 13$CN line profiles at 3.4 mm have been obtained in M17, NGC 2264, Ori A, and NGC 1333. Values for the ratios of the hyperfine-split intensities and the ratio of the integrated intensities of H$sup 12$CN and H$sup 13$CN are given. Collapsing- or expanding- sphere and turbulent-slab cloud models are used to explain the observed line brightness temperatures, the anomalous ratios of the hyperfine-split components, and the abundances of the isotopes. Doppler overlap and Doppler shifting of hyperfine-split components at 1.7 and 1.1 mm are crucial to the results of radiative transport. Comparison of calculations with the data indicates that the parts of the collapsing or the turbulent cloud responsible for observed line widths of greater than 1 or 2 km s$sup -1$ cannot be interacting radiatively via these Doppler effects. No such restrictions are important for the expanding sphere. Conditions of high hydrogen density and moderate HCN column density can be found that are consistent with both CS data and the terrestrial ratio of abundances H$sup 12$CN/H$sup 13$CN, but not with the observed H$sup 13$CN hyperfine-split intensity ratios. An isotope ratio smaller then terrestrial by a factor of 4 or more is required.more » However, parameters remain sufficiently uncertain that further HCN work, especially at shorter wavelengths, is indicated. (AIP)« less

Observations of millimeter-wave HCN in four prototype clouds

Molecular studies of two dark nebulae associated with Herbig-Haro objects

Millimeter-wave observations made toward the NGC 6530-M8 star-forming complex are compared with high-quality optical interference-filter photographs of that region taken with the new Kitt Peak 4-m Mayall telescope and the Kitt Peak No. 192-cm telescope. Extensive CO observations reveal a large molecular cloud in this direction, within which three bright spots of CO emission are found. Two bright spots are associated in angle with prominent optical features. One of these is coincident with the star Herschel 36 and the well known Hourglass Nebula, and the second appears coincident in angle with a bright-rimmed structure shown to be an ionization front southeast of Herschel 36. Emission from H2CO at 140.8 GHZ and H2O at 22.2 GHz has been detected from this last bright spot but not from the other two. The third bright spot does not seem to be associated with any visible nebular feature. Carbon monoxide, radio continuum, and optical Hα emission are found to be very similar in angular distribution throughout M8. The kinematics of the molecular cloud are studied, and it is found that the radial velocity of CO emission varies smoothly across the cloud, but it is difficult to interpret this in terms of coherent mass motionmore » of the entire cloud. Analysis of available radial-velocity data for the H ii region shows that the ionized gas is approaching the Earth at a velocity of 7 km s-1 with respect to the molecular cloud. Analyses of CO, radio continuum, and, especially, optical data indicate that Herschel 36 is the most likely ionizing source for most of the M8 H ii region.« less

Optical and millimeter-wave observations of the M8 region

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Observations of millimeter-wave HCN in four prototype clouds

Molecular studies of two dark nebulae associated with Herbig-Haro objects

Optical and millimeter-wave observations of the M8 region