Search

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

Four Bok globules were studied in the Near-Infrared, through narrow-band filters, centered at the 1.644 micron line of [FeII], the H2-line at 2.122 micron, and the adjacent continuum. We report the discovery of [FeII] and H2 protostellar jets and knots in the globules CB3 and CB230. The [FeII]-jet in CB230 is defined by a continuous elongated emission feature, superimposed on which two knots are seen; the brighter one lies at the tip of the jet. The jet is oriented in the same direction as the large-scale CO outflow, and emerges from the nebulosity in which a Young Stellar Object is embedded. The H2 emission associated with this jet is fainter and wider than the [FeII] emission, and is likely coming from the walls of the jet-channel. In CB3 four H2 emission knots are found, all towards the blue-shifted lobe of the large-scale outflow. There is a good correspondence between the location of the knots and the blue-shifted SiO(5-4) emission, confirming that SiO emission is tracing the jet-like flow rather well. No line emission is found in the other two targets, CB188 and CB205, although in CB205 faint line emission may have been hidden in the diffuse nebulosity near the IRAS position. Around this position a small group of (> 10) stars is found, embedded in the nebula. A diffuse jet-like feature near this group, previously reported in the literature, has been resolved into individual stars., 7 pages, 8 figures, to be published in A&A

(Abrigded) We discuss the column density profiles of "cores" in 3D SPH numerical simulations of turbulent molecular clouds. The SPH scheme allows us to perform a high spatial resolution analysis of the density maxima (cores) at scales between ~0.003 and 0.3pc. We analyze simulations in three different physical conditions: large scale driving, small scale driving, and random Gaussian initial conditions without driving; each one at two different timesteps: just before self-gravity is turned-on, and when gravity has been operating such that 5% of the total mass in the box has been accretted into cores. For this dataset, we perform Bonnor-Ebert fits to the column density profiles of cores. We find that 65% of the cores can be matched to Bonnor-Ebert profiles, and of these, 47% correspond to stable equilibrium configurations with xi_max < 6.5, even though the cores analyzed in the simulations are not in equilibrium, but instead are dynamically evolving. We also find in some cases substantial superposition effects when we analyze the projection of the density structures. As a consequence, different projections of the same core may give very different values of the BE fits. Finally, we briefly discuss recent results claiming that Bok globule B68 is in hydrostatic equilibrium, stressing that they imply that this core is unstable by a wide margin. We conclude that fitting BE profiles to observed cores is not an unambiguous test of hydrostatic equilibrium, and that fit-estimated parameters of the BE sphere may differ significantly from the actual values in the cores., ApJ accepted. 11 pages, 8 ps figures, 6 gif figures

We present high-resolution polarization maps, obtained with near-infrared instruments such as ISAAC at the VLT, SOFI at the NTT, and SCUBA at the JCMT. While we use the near-infrared polarization maps to determine the structure of the optical reflection nebula Cederblad 110 IRS 4 and to investigate the alignment of circumstellar disks around T Tauri binary stars, submillimeter polarization maps are used to derive the magnetic field structure and strength in Bok globules. Furthermore we show that near-infrared polarimetry represents a powerful tool to distinguish between different polarization models developed for active galactic nuclei.

Using the Submillimeter Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT), we obtained submillimeter polarization maps of the Bok globules B335, CB230, and CB244 at 850micron. We find strongly aligned polarization vectors in the case of B335 and CB230, indicating a strong coupling of the magnetic field to the dust grains. Based on the distribution of the orientation and strength of the linear polarization we derive the magnetic field strengths in the envelopes of the globules. In agreement with previous submillimeter polarization measurements of Bok globules we find polarization degrees of several percent decreasing towards the centers of the cores. Furthermore, we compare the magnetic field topology with the spatial structure of the globules, in particular with the orientation of the outflows and the orientation of the nonspherical globule cores. In case of the globules B335 and CB230, the outflows are oriented almost perpendicular to the symmetry axis of the globule cores. The magnetic field, however, is aligned with the symmetry axis of the prolate cores in the case of the Bok globules B335 and CB230, while it is slightly aligned with the outflow axis in the case of the Bok globules CB26 and CB54. We discuss the possibility that the different orientations of the magnetic field relative to the outflow directions reflect different evolutionary stages of the single globules. The complete version of this article (containing all figures) can be downloaded from http://spider.ipac.caltech.edu/staff/swolf/homepage/public/preprints/mfe.ps.gz, Comment: ApJ, in press

A complete sample of 169 small and opaque southern Bok globules is currently surveyed at 10 and 1200 μm to investigate the formation of individual stars independent of violent environmental effects. According to the association with IRAS sources, the sample has been divided into FIR-loud and FIR-quiet objects. Preliminary results for 36 FIR-loud sources suggest different classes of globules as witnessed by their ratio of bolometric luminosity to gas mass and — as a consequence — by the number and mass of embedded objects. Nine FIR-quiet globules show signs of fragmentation and seem to be at the border of gravitational instability. The present study indicates that about 75% of the FIR-loud and 30% of the FIR-quiet globules do actively form stars.

We have modelled chemical abundances and line profiles in the Bok globule B335. The chemical characteristics of this star-forming core are largely determined by gas-grain interaction in the inflowing material. By comparing high resolution observational data with our model it should be possible to determine the evolutionary status of B335 and to establish the role that surface chemistry plays in protostellar clouds.

Abundances for 5 molecules (C18O, CS, NH3, H2CO, and C3H2) and 1 molecular ion (N2H+) and upper limits for the abundances of 1 molecule (13CO) and 1 molecular ion (HCO+) are derived for gas within the Bok globule Barnard 68 (B68). The abundances were determined using our own BIMA millimeter interferometer data and single-dish data gathered from the literature, in conjunction with a Monte Carlo radiative transfer model. Since B68 is the only starless core to have its density structure strongly constrained via extinction mapping, a major uncertainty has been removed from these determinations. All abundances for B68 are lower than those derived for translucent and cold dense clouds, but perhaps only significantly for N2H+, NH3, and C3H2. Depletion of CS toward the extinction peak of B68 is hinted at by the large offset between the extinction peak and the position of maximum CS line brightness. Abundances derived here for C18O and N2H+ are consistent with other, recently determined values at positions observed in common., 16 pages, 1 figure, accepted by AJ, typo corrected, reference removed in Section 4.2

We describe a near-infrared imaging survey of Globule 2 in the Coalsack. This Bok globule is the highest density region of this southern hemisphere molecular cloud and is the most likely location for young stars in this complex. The survey is complete for K < 14.0, H < 14.5, and J < 15.5, several magnitudes more sensitive than previous observations of this globule. From the large number of background stars, we derive an accurate near-infrared extinction law for the cloud. Our result, E_{J-H}/E_{H-K} = 2.08 \pm 0.03, is significantly steeper than results for other southern clouds. We use the J-H/H-K color-color diagram to identify two potential young stars with K < 14.0 in the region. We apply H-band star counts to derive the density profile of the Coalsack Globule 2 and use a polytropic model to describe the internal structure of this small cloud. For a gas temperature T \sim 15 K, this globule is moderately unstable., 19 pages, manuscript format; 6 figures -- Accepted by AJ

We present the results of the first detailed millimeter and submillimeter molecular line survey of bright rimmed clouds, observed at FCRAO in the CO (J=1-0), C18O (J=1-0), HCO+ (J=1-0), H13CO+ (J=1-0), and N2H+ (J=1-0) transitions, and at the HHT in the CO (J=2-1), HCO+ (J=3-2), HCO+ (J=4-3), H13CO+ (J=3-2), and H13CO+ (J=4-3) molecular line transitions. The source list is composed of a selection of bright rimmed clouds from the catalog of such objects compiled by Sugitani et al. (1991). We also present observations of three Bok globules done for comparison with the bright rimmed clouds. We find that the appearance of the millimeter CO and HCO+ emission is dominated by the morphology of the shock front in the bright rimmed clouds. The HCO+ (J=1-0) emission tends to trace the swept up gas ridge and overdense regions which may be triggered to collapse as a result of sequential star formation. Five of the seven bright rimmed clouds we observe seem to have an outflow, however only one shows the spectral line blue-asymmetric signature that is indicative of infall, in the optically thick HCO+ emission. We also present evidence that in bright rimmed clouds the nearby shock front may heat the core from outside-in thereby washing out the normally observed line infall signatures seen in isolated star forming regions. We find that the derived core masses of these bright rimmed clouds are similar to other low and intermediate mass star forming regions., 67 pages, including 35 figures and 6 tables. Accepted for publication in ApJ. Version with embedded full-resolution figures available at http://www.astro.umass.edu/~devries/brc1/

BHR 71 is a well isolated Bok globule located at ~200 pc, which harbours a highly collimated bipolar outflow. The outflow is driven by a very young Class 0 protostar with a luminosity of ~9 L ⊙. It is one of a very small number that show enhanced abundances of a number of molecular species, notably SiO and CH3OH, due to shock processing of the ambient medium. In this paper the properties of the globule and outflow are discussed. “In the darkness, there’ll be hidden worlds that shine” — Bruce Springsteen, Candy’s Room 1977

Context: The role of magnetic fields in the star formation process is a contentious matter of debate. In particular, no clear observational proof exists of a general influence by magnetic fields during the initial collapse of molecular clouds. Aims: Our aim is to examine magnetic fields and their influence on a wide range of spatial scales in low-mass star-forming regions. Method: We trace the large-scale magnetic field structure on scales of 10^3-10^5 AU in the local environment of Bok globules through optical and near-infrared polarimetry and combine these measurements with existing submillimeter measurements, thereby characterizing the small-scale magnetic field structure on scales of 10^2-10^3 AU. Results: For the first time, we present polarimetric observations in the optical and near-infrared of the three Bok globules B335, CB68, and CB54, combined with archival observations in the submillimeter and the optical. We find a significant polarization signal (P>=2%, P/sigma(P)>3) in the optical and near-infrared for all three globules. Additionally, we detect a connection between the structure on scales of 10^2-10^3 AU to 10^3-10^4 AU for both B335 and CB68. Furthermore, for CB54, we trace ordered polarization vectors on scales of ~10^5 AU. We determine a magnetic field orientation that is aligned with the CO outflow in the case of CB54, but nearly perpendicular to the CO outflow for CB68. For B335 we find a change in the magnetic field oriented toward the outflow direction, from the inner core to the outer regions. Conclusion: We find strongly aligned polarization vectors that indicate dominant magnetic fields on a wide range of spatial scales., 9 pages, 7 figures, accepted by A&A

New AAT near-infrared and SEST 12CO J=2-1 observations are combined with existing ISO mid-infrared and ATCA cm radio continuum observations to examine the protostellar content of the Bok globule BHR 71. Together with observations of Herbig-Haro objects, these data show: (1) Two protostellar sources, IRS1 and IRS2, with a separation of ~17 arcsec (3400 AU) are located within BHR 71. (2) Each protostar is driving its own molecular outflow. The outflow from IRS1 is much larger in extent, is more massive, and dominates the CO emission. (3) Both protostars are associated with Herbig-Haro objects and shock excited 2.122 micron H2 v=1-0S(1) emission, which coincide spatially with their CO outflows. (4) IRS1 is associated with cm continuum emission, with a flat or rising spectrum which is consistent with free-free emission, a signpost of protostellar origin., 12 pages, 2 figures (colour), accepted by ApJ Letters. See also http://cfa-www.harvard.edu/~bourke/

We decipher intrinsic three-dimensional shape distributions of molecular clouds, cloud cores, Bok globules, and condensations using recently compiled catalogues of observed axis ratios for these objects mapped in carbon monoxide, ammonia, through optical selection, or in continuum dust emission. We apply statistical techniques to compare assumed intrinsic axis ratio distributions with observed projected axis ratio distributions. Intrinsically triaxial shapes produce projected distributions which agree with observations. Molecular clouds mapped in $^{12}$CO are intrinsically triaxial but more nearly prolate than oblate, while the smaller cloud cores, Bok globules, and condensations are also intrinsically triaxial but more nearly oblate than prolate., Comment: 12 pages, 11 figures. Version with color figures can be found at http://www.astro.uwo.ca/~cjones/ or http://www.astro.uwo.ca/~basu/. To appear in ApJ, 10 April 2002, v. 569, no. 1

In order to study the in—uence and structure of the magnetic —eld in the early phases of low-mass star formation, we obtained polarization maps of three Bok globules at a wavelength of 850 km, using the Submillimeter Common-User Bolometer Array at the James Clerk Maxwell Telescope. We observed the following sources: CB 26, a globule with a nearly dispersed dense core containing a source with a circumstellar disk; CB 54, a deeply embedded young stellar cluster; and DC 253[1.6 (CG 30), a protostellar double core. We —nd strongly aligned polarization vectors in the case of CB 26 and DC 253[1.6, while the vector orientations in the case of CB 54 are more or less randomly distributed. The degree of polarization, amounting to several percent, was found to decrease toward the center in each source. In the case of CB 54 and DC 253[1.6, the degree of polarization similarly depends on the corresponding intensity. Assuming dichroic emission by aligned nonspherical grains as the polarization mechanism, where the magnetic —eld plays a role in the alignment process, we derive magnetic —eld strengths and structures from the observed polarization patterns. In the case of the double core DC 253[1.6, we discuss the correlation between the fragmentation process and the magnetic —eld direction.

We model the polarized thermal dust emission from protostellar cores that are assembled by super-sonic turbulent flows in molecular clouds. Self-gravitating cores are selected from a three dimensional simulation of super-sonic and super-Alfvenic magneto-hydrodynamic (MHD) turbulence. The polarization is computed in two ways. In model A it is assumed that dust properties and grain alignment efficiency are uniform; in model B it is assumed that grains are not aligned at visual extinction larger than 3 mag. The main results of this work are: i) Values of the degree of polarization P between 1 and 10% are typical, despite the super-Alfvenic nature of the turbulence; ii) A steep decrease of P with increasing values of the sub-mm dust continuum intensity I is always found in self--gravitating cores selected from the MHD simulations, if grains are not aligned above a certain value of visual extinction (model B); iii) The same behavior is hard to reproduce if grains are aligned independently of visual extinction (model A); iv) The Chandrasekhar-Fermi formula, corrected by a factor f=0.4, provides an approximate estimate of the average magnetic field strength in the cores. Sub-mm dust continuum polarization maps of quiescent protostellar cores and Bok globules always show a decrease in P with increasing value of I consistent with the predictions of our model B. We therefore conclude that sub-mm polarization maps of quiescent cores do not map the magnetic field inside the cores at visual extinction larger than approximately 3 mag. There is no inconsistency between the results from optical and near-IR polarized absorption of background stars, and the observed polarization of sub-mm dust continuum from quiescent cores. In both cases, grains at large visual extinction appear to be virtually unaligned., Comment: 21 pages, color figures, submitted to ApJ

The detection of a new water maser source on the 6 16 → 5 23 transition in the Bok globule LBN 594 is presented. Besides the main feature, a number of velocity components were detected. Spectra taken within a few days interval suggest possible maser variability. Maser emission was also confirmed for two Ae/Be Herbig stars. Upper limits (3σ) are given for the 79 observed globules and 12 Herbig stars

Magnetic fields can play an important role in many astrophysical problems. At the present time, sufficient observational data have been gathered about the magnetic field of the interstellar medium, molecular clouds, star formation regions, Bok globules, and young stellar objects (Heiles et all, 1993; Dudorov, 1995; Vallee, 1997). This shows that star formation takes place in rotating magnetized interstellar clouds.

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.

In order to investigate the formation of stars independent of violent environmental effects we surveyed a large sample of 128 small and opaque southern Bok globules at 1.2 mm continuum with SIMBA at SEST. In addition, most of these globules were and will yet be observed at 10 µm by TIMMI2 and in the far-infrared (FIR) regime by SIRTF. According to the association with IRAS sources, the sample has been divided into FIR-loud and FIR-quiet objects. Preliminary results for 36 FIR-loud sources suggest different classes of globules as witnessed by their ratio of bolometric luminosity to gas mass and — as a consequence — by the number and mass of embedded objects. At least nine FIR-quiet globules show signs of fragmentation and seem to be at the border of gravitational instability. The present study indicates that about 75% of the FIR-loud and 30% of the FIR-quiet globules do actively form stars.

Fifteen small, apparently starless Bok globules were mapped at high spatial and spectral (0.007 km s−1 channel−1) resolution in the (J=1→0) rotational line of 13CO using the 14 m radio telescope of the Five College Radio Astronomy Observatory, and the fifteen-element 3 mm array receiver QUARRY. From 120 to 360 positions per globule, sampled with half-beam spacing, were observed in the 13CO line. Gaussian fitting of the emission lines was used to establish mean radial velocities and uncertainties. Each globule radial velocity distribution on the sky was fit to a plane (solid body rotation) to yield mean velocity gradients with position, and rotation axis directions. The globules are rotating at rates about 30 times faster than velocity shifts attributable to local differential Galactic rotation. For globule assumed mean distances of 600 pc, the gradients range in a distinctly bimodal distribution, from 0.089 km s−1 pc−1 (ω∼3×10−15 s−1) to 0.950 km s−1 pc−1 (ω∼3×10−14 s−1). Detailed examination of the globu...

We present recent results of near-infrared imaging and mm-continuum mapping of a sample of star-forming cores in Bok globules. The circumstellar masses of the embedded YSOs are in most of the cases high enough to fulfil the formal criterion of beeing class 0 protostars. Most of the objects, however, were classified as typical class I objects according to their NIR-MIR spectral energy distributions. This apparent contradiction will be discussed in this paper.

The shapes of isolated Bok globules and embedded dense cores of molecular clouds are analyzed using a nonparametric method, under the alternate hypotheses that they are randomly oriented prolate objects or that they are randomly oriented oblate objects. In all cases, the prolate hypothesis gives a better fit to the data. If Bok globules are oblate, they must be very flat; the average axis ratio is b/a = 0.3, and few or no globules can have b/a > 0.7. If Bok globules are prolate, then the mean axis ratio is b/a = 0.5. For most data samples of dense cores, the randomly-oriented oblate hypothesis can be rejected at the 99% confidence level. If the dense cores are prolate, their mean axis ratio is approximately 0.4 to 0.5. Dense cores are significantly different in shape from the clouds in which they are embedded; clouds have flatter apparent shapes, and are inconsistent with a population of randomly oriented axisymmetric objects., 26 pages (LaTeX) including 8 postscript figures; to appear in ApJ

We present the 3D image and temporal evolution of a remarkable dark cloud: B164. This large Bok globule (Bok & Cordwell 1973) possesses a seldom shared property: it rotates. The rotation curve presents a clear ‘S’ shape but this cloud is unique today because a third of the rotation curve is missing. Indeed all the observations show that the cloud ends up abruptly on one side suggesting that a part of the cloud is missing. Taking the present rotational axis as the original symmetry axis we can deduce some important information such as the original size and mass of the cloud and an upper limit on the time at which the perturbation took place. The rotation curve also allows to give a 3D picture of the cloud which we explore with a Monte-Carlo model: the closest solution indicates a large envelope of increasing density from 100 cm−3 to 1000 cm−3 and a relatively small core of density of the order of 5000 cm−3. CO isotopomers have a relatively low abundance in this model with the 13CO abundance peaking at 2–3×10−6 for Av=0.5 to 1 mag. and C18O being one to two orders of magnitude less abundant, as a function of depth. Next, we show that if Azelfafage (HD 206672), a Be3-III star is at the same distance from us and thus only at 3 pc from the cloud, it could be the progenitor of the observed perturbation. Using a chemico- hydro-dynamical model (CHD) (Breart de Boisanger et al. 1992) we show that the most probable explanation is that the star has just arrived and that its main action is to compress the cloud via the heating of the diffuse interstellar medium and/or the increase in radiation pressure rather than by photodissociating the molecular gas. Preliminary HI data and 12CO observations show some evidence that the general contraction of the molecular envelope would be due to a direct pressure of the HI halo. It would seem that under the action of the star the whole molecular cloud is moving backwards and is getting compressed against its HI halo. The HD model to describe this effect is being currently developed and also more HI observations are underway to confirm this idea.

Using the unique X-ray imaging capabilities of the Chandra observatory, a 2006 observation of Cygnus X-3 has provided insight into a singular feature associated with this well-known microquasar. This extended emission, located ~16 arcseconds from Cygnus X-3, varies in flux and orbital phase (shifted by 0.56 in phase) with Cygnus X-3, acting like a celestial X-ray "mirror". The feature's spectrum, flux and time variations allow us to determine the location, size, density, and mass of the scatterer. We find that the scatterer is a Bok globule located along our line of sight, and discuss its relationship to Cygnus X-3. This is the first time such a feature has been identified with the Chandra X-ray Observatory., 35 pages, 14 figures, 1 movie, accepted for publication by ApJ

We present a hierarchical Bayesian method for fitting infrared spectral energy distributions (SEDs) of dust emission to observed fluxes. Under the standard assumption of optically thin single temperature (T) sources, the dust SED as represented by a power-law-modified blackbody is subject to a strong degeneracy between T and the spectral index β. The traditional non-hierarchical approaches, typically based on χ2 minimization, are severely limited by this degeneracy, as it produces an artificial anti-correlation between T and β even with modest levels of observational noise. The hierarchical Bayesian method rigorously and self-consistently treats measurement uncertainties, including calibration and noise, resulting in more precise SED fits. As a result, the Bayesian fits do not produce any spurious anti-correlations between the SED parameters due to measurement uncertainty. We demonstrate that the Bayesian method is substantially more accurate than the χ2 fit in recovering the SED parameters, as well as the correlations between them. As an illustration, we apply our method to Herschel and submillimeter ground-based observations of the star-forming Bok globule CB244. This source is a small, nearby molecular cloud containing a single low-mass protostar and a starless core. We find that T and β are weakly positively correlated—in contradiction with the χ2 fits, which indicate a T-β anti-correlation from the same data set. Additionally, in comparison to the χ2 fits the Bayesian SED parameter estimates exhibit a reduced range in values.

We present high angular resolution Submillimeter Array (SMA) and Spitzer observations toward the Bok globule CB 17. SMA 1.3 mm dust continuum images reveal within CB 17 two sources with an angular separation of {approx}21'' ({approx}5250 AU at a distance of {approx}250 pc). The northwestern continuum source, referred to as CB 17 IRS, dominates the infrared emission in the Spitzer images, drives a bipolar outflow extending in the northwest-southeast direction, and is classified as a low-luminosity Class 0/I transition object (L{sub bol} {approx} 0.5 L{sub Sun }). The southeastern continuum source, referred to as CB 17 MMS, has faint dust continuum emission in the SMA 1.3 mm observations ({approx}6{sigma} detection; {approx}3.8 mJy), but is not detected in the deep Spitzer infrared images at wavelengths from 3.6 to 70 {mu}m. Its bolometric luminosity and temperature, estimated from its spectral energy distribution, are {

Star formation in galaxies is a major astrophysical problem which can be investigated in several ways. The distribution and loci of all kinds of young objects, including OB associations, young clusters, HII regions, GMCs, Bok globules, dark clouds, dust lanes, protostars, as well as YSOs detected in NIR and FIR surveys constitute the principal signposts for this investigation. The individual nature of all these objects has been and is still continously studied. However it is also extremely interesting to associate the coexistence of these objects, and their relation to the structure of the parent galaxy. Such studies have been carried out by several investigators and are frequently summarized when star formation processes are examined.

We have studied a sample of seven dark clouds (Bok globules) and eight Hergib Ae/Be stars in the J=1 → 0 transition of HCO+, H13CO+, HCN, and H13CN. The most abundant isotopomers were found in almost all the sources (detection rate 70–90%).From the derived physical parameters and column densities it seems that a quite similar scenario applies for the dark clouds and the gas around the Herbig stars. These similarities, together with the already known features from literature, suggest that the presently studied globules are likely to be sites of low‐mass star formation.

Almost 50 years ago, the famous astronomer Bart Bok revealed that the Milky Way was filled with dense concentrations of gas that could be the birthplaces of new stars. Although these Bok globules, as they are now known, are faint and esoteric objects, they captured the public imagination, thanks to a 1957 science fiction story by astronomer Fred Hoyle, called The Black Cloud. As the story opens, astronomers using Palomar’s 18-inch Schmidt telescope (the same one used in real life to discover all the Shoemaker comets) spot a big black cloud approaching the Earth. “Such globules are not uncommon in the Milky Way,” the astronomer says, “but usually they’re tiny things. My God, look at this! It’s huge, it must be the best part of two and a half degrees across!”1 As the drama unfolds, the globule appears to get bigger as it closes in on the Earth. Finally one morning the assistant appears in a great panic: “It’s not there, sir, it’s not there!” “What isn’t there?” “The day, sir! There’s no Sun!”

We have modeled chemical abundances and line profiles in the Bok globule B335. The chemical characteristics of this star‐forming core are largely determined by gas‐grain interaction in the inflowing material. As with L1498, we aim to identify the molecular species which have broad velocity distributions. The resulting line profile broadening for these species can be used to directly diagnose the characteristics of star‐formation. By comparing high resolution observational data with our model it should be possible to determine the evolutionary status of B335 and to establish the role that surface chemistry plays in protostellar clouds.

We pursue the investigation of a previously proposed correlation between chemical properties and physical evolutionary stage of isolated low-mass star-forming regions. In the past, the NH3/CCS abundance ratio was suggested to be a potentially useful indicator for the evolutionary stage of cloud cores. We aim to study its applicability for isolated Bok globules. A sample of 42 Bok globules with and without signs of current star formation was searched for CCS(2-1) emission, the observations were complemented with NH3 measurements available in the literature and own observations. The abundance ratio of both molecules is discussed with respect to the evolutionary stage of the objects and in the context of chemical models. The NH3/CCS ratio could be assessed for 18 Bok globules and is found to be moderately high and roughly similar across all evolutionary stages from starless and prestellar cores towards internally heated cores harbouring protostars of Class 0, Class I or later. Bok globules with extremely high CCS abundance analogous to carbon-chain producing regions in dark cloud cores are not found. The observed range of NH3/CCS hints towards a relatively evolved chemical state of all observed Bok globules., Comment: 12 pages, 6 figures, accepted for publication in Astronomy & Astrophysics

The Bok globule CB238 has been investigated in 13CO (220.4 GHz), C18O (109.7 GHz), CS (97.9 and 146.7 GHz), and SO (99.3 GHz) lines, and mapped with half-power beamwidths of 11'', 22'', 38'', 16'', and 36'', respectively. The two dense cores previously found in NH3 are presently confirmed in CS. The cores exhibit similar physical and chemical characteristics. The lack of outflows, the limited chemical content, and the very modest depletion in the ammonia cores provide evidence that CB238 is not an evolved object. Molecular abundances and density ratios constrain chemical modeling whose results provide a chemical age close to the free-fall timescale.

The Bok globule B335 is a small molecular cloud in the solar neighbourhood near the galactic plane. The aim for this three-paper-study is to construct and analyze the extinction for this globule. The method we apply is to use the light from field stars behind the cloud in broadband filters ranging from UV to the mid-infrared. We have observations performed at the ESO telescopes at La Silla and Paranal as well as at the Nordic 2.5 m telescope at La Palma. Together with images and spectra from 2MASS-, ISO- and Spitzer-archives we are able to cover the wavelength range from 0.35 to 24 μm. An important tool to analyze these observations results in order to get the extinction is the grid of synthetic stellar atmospheric spectra provided by Hauschildt (2005). The extinction so received is a result in itself. From the analysis of the extinction wavelength dependence we derive properties of the dust, especially its composition and grain size distribution. By modeling the grain size distribution we are able to find the extinction from the reddening of the stars. We find that the extinction in the optical wavelength 0.35 to 2 μm range nicely follows the functional form described by Cardelli et al. (1989). Our result from the wavelength range redward of 2 μm show an extinction dependent on the part of the cloud examined. For the rim of the cloud we get an extinction similar to that reported earlier for the diffuse interstellar medium. From the central parts of the cloud, however, a higher extinction was found. Our grain size model contains a carbonaceous particle distribution and a silicate one. The result can be explained by depletion of carbon onto carbonaceous grains and also by carbon onto all grains including the silicates. Our modeling of the extinction and our classification of the background stars allow us to - determine the distance to the globule - estimate the gas column density ratio - estimate the mass of globule - get a handle on the dust conversion processes through the grain size distribution From the water- and CO-ice spectra we are able to estimate the ice column densities. We find similar ice column densities for the two ices. The estimates differ, when calculated from band strengths or from Lorenz-Mie calculations of ice mantles on the grain size distribution, by a factor of two.

Context. The small compact isolated dark clouds also known as “Bok globules” are believed to be ideal sites for low-mass star formation. Some of these clouds are undergoing gravitational collapse, and the ambient magnetic field plays a key role in collapse dynamics. The background star polarimetry is generally accepted as a good tool to map the magnetic field, which is responsible for the alignment of dichroic grains that produce polarization. Aims. The background star polarization when studied together with extinction is expected to help us to understand various grain properties and the role of polarimetry as a tracer of magnetic field in these star-forming clouds. With this idea, polarization and colour excess E(B − V) values for a set of background stars have been studied together to understand various astrophysical process in some star-forming dark clouds. Methods. Optical photometric observations of the three clouds CB3, CB25, and CB39 were carried out at the 2 m H.C. Telescope, India, to determine the colour excess E(B − V) of the background stars by following a technique adopted by Bernabei & Polacaro (2001, A&A, 371, 123). These three clouds were selected from a set of eight clouds previously observed by us in optical polarimetry (Sen et al. 2000, A&AS, 141, 175). Further independent spectroscopic measurements of a few selected sample stars were recently carried out during February and March 2010 from 1.52 m Cassini Telescope, Loinao, Italy, to confirm the correctness of estimated E(B − V) values obtained by this photometric technique. Results. The colour excess E(B − V) values so obtained were compared with optical polarization values obtained for the same set of stars. It was found that the measured extinction values increase with the increase in percentage polarization for the cloud CB39 and to some extent for CB25. However, for cloud CB31 no such correlation was observed. It is normally expected that the grains causing extinction should also cause polarization of the light from background stars. Any possible deviation from this under different circumstances here has been discussed in the light of the ongoing physical processes in the star-forming clouds.

If the grains in pre-collapsing clouds are due to the contribution of winds from previous generations of stars and supernovae ejecta, they already contain a fraction if not all, of the heavy elements. The far infrared spectrum of Bok globules, which are at relatively early contraction stages, has been attributed to dusty material.

The stellar population of dark molecular clouds and isolated Bok globules consist mainly of low mass stars. The physical conditions of these clouds from their largest to their smallest structures are reviewed and the different methods used to derive them are evaluated. The interaction of the newly born stars with the ambient gas is analyzed on the basis of new results obtained from high angular resolution molecular line observations.

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

We present photometric and spectroscopic evidence of two new T Tauri stars formed in the conditions of isolated small Bok globules. The spectral energy distributions of these objects display excess infrared emission, they are associated with optical reflection nebulae, and their optical spectra reveal Balmer emission lines and the Li I λ6707 Å absorption line. Additionnally, we report the discovery of what is likely to be a new FU Orionis star seen towards Bok globule CB34. The star is about 4 magnitudes brighter than it appears on the Palomar plates and is associated with the aggregate of young stellar objects forming in Bok globule CB34.