Search

Polarimetric observations of about 50 stars located in a close vicinity of the Bok globule CB67 having significantly nonspherical shape and rapid rotation are performed. The data obtained are compared with the available observations of this globule at radio and submillimeter wavelengths as well as some theoretical calculations. It is found that the elongation and the rotation moment of CB67 are oriented rather perpendicular to the magnetic fields, which is unusual for Bok globules and is difficult to be explained from the theoretical point of view., 8 pages, 6 figures, 2 tables

We performed an observational study of the relation between the interstellar magnetic field alignment and star formation in twenty (20) sky regions containing Bok Globules. The presence of young stellar objects in the globules is verified by a search of infrared sources with spectral energy distribution compatible with a pre main-sequence star. The interstellar magnetic field direction is mapped using optical polarimetry. These maps are used to estimate the dispersion of the interstellar magnetic field direction in each region from a Gaussian fit, sigma_B. In addition to the Gaussian dispersion, we propose a new parameter, eta, to measure the magnetic field alignment that does not rely on any function fitting. Statistical tests show that the dispersion of the magnetic field direction is different in star forming globules relative to quiescent globules. Specifically, the less organised magnetic fields occur in regions having young stellar objects., Submitted contribution to the proceedings of IAU Symposium 302: Magnetic Fields Throughout Stellar Evolution

The NH3(1, 1) line emission was detected at −120 arcsec, +60 arcsec offset from the star Cyg OB2 No. 12. The region had been initially mapped in 13 CO(1–0) and was called clump B. The subsequent detection of the CS(2–1) line by the same authors led them to suggest that the clump could be an object similar to a Bok globule. The present detection and analysis confirm what was proposed before, that is clump B is a newly discovered starless Bok globule.

We present optical and submillimetre polarimetry data of the Bok globule CB3 and optical polarimetry data of the Bok globule CB246. We use each set of polarimetry data to infer the B-field orientation in each of the clouds. The optical data can only be used in the low density, low extinction edge regions of clouds. The submillimetre data can only be used in the high column-density, central regions of the clouds. It has previously been found that near-infrared polarisation mapping of background stars does not accurately trace the magnetic field in dense cloud regions. This may be due to a lack of aligned grains in dense regions. We test this by comparing the field orientations measured by our two independent polarimetry methods. We find that the field orientation deduced from the optical data matches up well with the orientation estimated from the submillimetre data. We therefore claim that both methods are accurately tracing the same magnetic field in CB3. Hence, in this case, there must be significant numbers of aligned dust grains in the high density region, and they do indeed trace the magnetic field in the submillimetre. We find an offset of 40$\pm$14 degrees between the magnetic field orientation and the short axis of the globule. This is consistent with the mean value of 31$\pm$3 degrees found in our previous work on prestellar cores, even though CB3 is a protostellar core. Taken together, the six prestellar cores that we have now studied in this way show a mean offset between magnetic field orientation and core short axis of $\sim30\pm$3 degrees, in apparent contradiction with some models of magnetically dominated star formation., Comment: 8 pages, 3 figures, accepted for publication in MNRAS

We report the most sensitive water maser survey towards Bok globules to date, using NASA's 70m antenna in Robledo de Chavela (Spain). We observed 207 positions within the CB catalog with a higher probability of harboring a young star, using as selection criteria the presence of radio continuum emission (from submillimeter to centimeter wavelengths), geometrical centers of molecular outflows, peaks in maps of high-density gas tracers (NH3 or CS), and IRAS point sources. We have obtained 7 maser detections, 6 of which (in CB 34, CB 54, CB 65, CB 101, CB 199, and CB 232) are reported for the first time here. Most of the water masers we detected are likely to be associated with young stellar objects (YSOs), except for CB 101 (probably an evolved object) and CB 65 (uncertain nature). The water maser in CB 199 shows a relatively high shift (~30 km/s) of its velocity centroid with respect to the cloud velocity, which is unusual for low-mass YSOs. We speculate that high-velocity masers in this kind of object could be related with episodes of energetic mass-loss in close binaries. Alternatively, the maser in CB 199 could be pumped by a protoplanetary or a young planetary nebula. CB 232 is the smallest Bok globule (~0.6 pc) known to be associated with water maser emission, although it would be superseded by the cases of CB 65 (~0.3 pc) and CB 199 (~0.5 pc) if their association with YSOs is confirmed. All our selection criteria have statistically compatible detection rates, except for IRAS sources, which tend to be a somewhat worse predictor for the presence of maser emission., Comment: 42 pages, 7 figures. Accepted by The Astronomical Journal. Corrected typos in Tables 1 & 3

A method for determining distances to dark clouds and Bok globules based on broad-band optical and near-infrared photometry is presented. In this method, intrinsic colour indices of stars projected towards the direction of a cloud are computed by dereddening the observed colour indices using various trial values of extinction AV and a standard extinction law. The computed intrinsic colour indices for a star are then compared with the intrinsic colour indices of normal main-sequence stars and a spectral type is assigned to the star for which the computed colour indices best match the standard intrinsic colour indices. Distances (d )t othe stars are determined using the AV and absolute magnitudes (MV ) corresponding to the spectral types thus obtained. A plot of AV against d undergoes a sharp rise at a distance corresponding to the distance to the cloud. Using this method, we have determined a distance of 550 pc to the cometary globule CG 12. The distance of 550 pc and a Galactic latitude of b = 21 ◦ imply that CG 12 is at a height of ∼200 pc above the galactic mid-plane. The star formation efficiency in this cloud is found to be relatively high (16 per cent, to as large as ∼33 per cent). The existence of an H I shell centred at l = 315 ◦ , b = 30 ◦ with CG 12 near its boundary and its tail pointing away from the centre of the shell supports the suggestion of a supernova explosion, near the centre of the H I shell, being responsible for the cometary morphology and the triggering of star formation. Thus CG 12 is a rare example of triggered high-mass star formation at relatively large galactic height. Ke yw ords: stars: distances ‐ stars: formation ‐ ISM: clouds ‐ dust, extinction ‐ ISM: globules ‐ ISM: individual: CG 12.

We study the influence and structure of the magnetic field in the early phases of low-mass star formation using polarization maps of Bok globules at a wavelength of 850micron, obtained with the Submillimeter Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT). We discuss observations of the following sources: CB26 - a small globule with a nearly dispersed dense core and a young and large circumstellar disk, CB54 - a large globule with a massive dense core and a deeply embedded young stellar cluster, and B335, CB230, and CB244 - three nearby, relatively isolated small globules with low-mass protostellar cores. We find strongly aligned polarization vectors in the case of CB26, B335, and CB230, while the vector orientations in the case of CB54 and CB244 are more or less randomly distributed. The degree of polarization, amounting to several percent, was found to decrease toward the center in each source. Assuming dichroic emission by aligned non-spherical grains as the polarization mechanism, where the magnetic field plays a role in the alignment process, we derive magnetic field strengths and structures from the observed polarization patterns. We compare the magnetic field topology with the morphology and outflow directions of the globules. In the Class 0 sources B335, CB230, and CB244, the magnetic field is oriented almost perpendicular to the ouflow direction. In contrast, the inclination between outflow axis and magnetic field direction is much more moderate (36deg) in the more evolved Class I source CB26., to be published in Conf. Proc. "Magnetic Fields and Star Formation"

We derive an exact solution to the inverse absorption problem to calculate the density distribution in spherical symmetry of absorbing particles from the intensity pattern obtained for homogeneous illumination. We illustrate the capabilities of the method by the simple example of a constant density core and find the required numerical effort to be negligible. The applicability is discussed for physical problems where unknown absorption coefficients, particle size or density distributions can be determined from multi-frequency measurements of the transmission coefficient. The applications range from targets being evaporated by laser pulses to Bok globules in astrophysics.

The analysis of the sample of southern globules investigated by Scappini et al. in the CO (4−3) transition has been extended using a statistical equilibrium–radiative transfer model and making use of the results of Bourke et al. and Henning & Launardt for those globules which are in common among these samples. CO column densities and excitation temperatures have been calculated and the results compared with a chemical model representative of the chemistry of a spherical dark cloud. In a number of cases the gas kinetic temperatures have been constrained.

A new method for determining distances to dark clouds and small Bok globules based on broadband optical photometry has been developed. The method exploits the fact that M dwarf stars do not redden into the colors of other main-sequence stars. Consequently, from analysis of color-color diagrams of V-I versus B-V, spectral types and extinctions of faint M dwarf stars located in front of and behind the molecular clouds can be established. For sufficiently deep observations (B ≈ 20–24 mag), distances to clouds closer than about 1 kpc can be bracketed to a higher degree of precision (10–40 pc) than previously possible. Photometric data for the Bok globule CB 24 are used in this paper to demonstrate this method, yielding a maximum distance of 360 pc.

SEEN in silhouette against the numerous ionized H II regions in the Milky Way are groups of small dark clouds of neutral gas and dust known as Bok globules. In some cases the stellar wind from a nearby star strips material from the globules, creating a tail pointing away from the source of the wind. Here I report the use of a photographic enhancement technique to reveal a family of four such 'cometary' globules in the southeast quadrant of the Rosette nebula, NGC2237 – 2246. The globules are not visible on R-glass copies of the National Geographic/ Palomar Observatory Sky Survey. The tails of the globules all point away from the IRAS (Infrared Astronomy Satellite) source 06314 + 0427, the position of which coincides with the peak of CO emission1. The far-infrared luminosity of 06314 + 0427 is estimated to be a factor of at least 880 greater than the Sun's luminosity, with a total energy output enough to drive a stellar wind that could produce the observed globules.

Astronomy![Figure][1] CREDIT: M. L. MCCOLLOUGH, R. K. SMITH, L.A. VALENCIC, THE ASTROPHYSICAL JOURNAL 762 , 2 (1 JANUARY 2013) © 2013 THE AMERICAN ASTRONOMICAL SOCIETY. REPRODUCED BY PERMISSION OF AAS Bok globules, named after astronomer Bart Bok, are dense clouds of cold gas and dust that

In this work we report the most sensitive water maser survey towards Bok globules to date, using NASA's 70 m antenna in Robledo de Chavela (Spain). We observed 207 positions within the Clemens & Barvainis catalog that show indications of possible star formation or with a high probability of harboring a young stellar object. With this survey we have increased the number of Bok globules known to present water maser emission from three to nine. We have complemented these results with interferometric high-angular resolution observations towards some of our detections.

We study the instantaneous virial balance of clumps and cores (CCs) in 3D simulations of driven, MHD, isothermal molecular clouds (MCs). The models represent a range of magnetic field strengths in MCs from subcritical to non-magnetic regimes. We identify CCs at different density thresholds, and for each object, we calculate all the terms that enter the Eulerian form of the virial theorem (EVT). A CC is considered gravitationally bound when the gravitational term in the EVT is larger than the amount for the system to be virialized, which is more stringent than the condition that it be large enough to make the total volume energy negative. We also calculate, quantities commonly used in the observations to indicate the state of gravitational boundedness of CCs such as the Jeans number J_c, the mass-to magnetic flux ratio mu_c, and the virial parameter alpha_vir. Our results show that: a) CCs are dynamical out-of-equilibrium structures. b) The surface energies are of the same order than their volume counterparts c) CCs are either in the process of being compressed or dispersed by the velocity field. Yet, not all CCs that have a compressive net kinetic energy are gravitationally bound. d) There is no 1-to-1 correspondence between the state of gravitational boundedness of a CC as described by the virial analysis or as implied by the classical indicators. In general, in the virial analysis, we observe that only the inner regions of the objects are gravitationally bound, whereas J_c, alpha_vir, and mu_c estimates tend to show that they are more gravitationally bound at the lowest threshold levels and more magnetically supercritical. g) We observe, in the non-magnetic simulation, the existence of a bound core with structural and dynamical properties that resemble those of the Bok globule Barnard 68 (B68)., Accepted to ApJ. Discussion substantially enlarged, a few corrections and additional figures. Main conclusions unchanged

It is pointed out that the isotope of carbon monoxide, CO-17, has appreciable hyperfine structure caused by the electric quadrupole and the magnetic dipole interactions of the O-17 nucleus which has a spin of 5/3. During a radioastronomical study of the structure and dynamics of cold interstellar clouds, it was found that the Bok globule B335 had an extremely small velocity dispersion such that the hyperfine components are clearly resolved. A graph is provided which shows the antenna temperature (a measure of intensity) of the CO-17 emission as a function of frequency. The hyperfine constants and line frequencies were redetermined for the CO-17 J=1 yields 0 rational transition. The observation of CO-17 was carried out with a 7 meter Cassegrain antenna during 1979 and 1980. The CO-17 molecular line parameters are listed in a table.