Bok Globules Research Articles

Investigating the extinction and magnetic field geometry in the Bok globule CB26

Investigating the extinction and magnetic field geometry in the Bok globule CB26

Jeans analysis in fractional gravity

It has recently been demonstrated (Giusti in Phys Rev D 101:124029, 2020, https://doi.org/10.1103/PhysRevD.101.124029) that characteristic traits of Milgrom’s modified Newtonian dynamics (MOND) can be replicated from an entirely distinct framework: a fractional variant of Newtonian mechanics. To further assess its validity, this proposal needs to be tested in relevant astrophysical scenarios. Here, we investigate its implications on Jeans gravitational instability and related phenomena. We examine scenarios involving classical matter confined by gravity and extend our analysis to the quantum domain, through a Schrödinger–Newton approach. We also derive a generalized Lane–Emden equation associated with fractional gravity. Through comparisons between the derived stability criteria and the observed stability of Bok globules, we establish constraints on the theory’s parameters to align with observational data.

A resolved rotating disk wind from a young T Tauri star in the Bok globule CB 26

Context. The disk-outflow connection plays a key role in extracting excess angular momentum from a forming protostar. Although indications of jet rotation have been reported for a few objects, observational constraints of outflow rotation are still very scarce. We have previously reported the discovery of a small collimated molecular outflow from the edge-on T Tauri star-disk system in the Bok globule CB 26 that shows a peculiar velocity pattern, reminiscent of an outflow that corotates with the Keplerian disk. However, we could not ultimately exclude possible alternative explanations for the origin of the observed velocity field. Aims. We report new, high angular resolution millimeter-interferometric observations of CB 26 with the aim of revealing the morphology and kinematics of the outflow at the disk-outflow interface to unambiguously discriminate between the possible alternative explanations for the observed peculiar velocity pattern. Methods. The IRAM PdBI array and the 30 m telescope were used to observe HCO+(1–0) and H13CO+(1–0) at 3.3 mm and 12CO(2–1) at 1.3 mm in three configurations plus zerospacing, resulting in spectral line maps with angular resolutions of 3.″5 and 0.″5, respectively. The SMA was used to observe the HCO+(3–2) line at 1.1 mm with an angular resolution of 1.″35. Additional earlier observations of 13CO(1-0) at 2.7mm with an angular resolution of 1.″0, obtained with OVRO, are also used for the analysis. Using a physical model of the disk, which was derived from the dust continuum emission, we employed chemo-dynamical modeling combined with line radiative transfer calculations to constrain kinematic parameters of the system and to construct a model of the CO emission from the disk that allowed us to separate the emission of the disk from that of the outflow. Results. Our observations confirm the disk-wind nature of the rotating molecular outflow from CB 26 - YSO 1. The new high-resolution data reveal an X-shaped morphology of the CO emission close to the disk, and vertical streaks extending from the disk surface with a small half-opening angle of ≈7°, which can be traced out to vertical heights of ≈500 au. We interpret this emission as the combination of the disk atmosphere and a well-collimated disk wind, of which we mainly see the outer walls of the outflow cone. The decomposition of this emission into a contribution from the disk atmosphere and the disk wind allowed us to trace the disk wind down to vertical heights of ≈40 au, where it is launched from the surface of the flared disk at radii of RL ≈ 20–45 au. The disk wind is rotating with the same orientation and speed as the Keplerian disk and the velocity structure of the cone walls along the flow is consistent with angular momentum conservation. The observed CO outflow has a total gas mass of ≈ 10−3 M⊙, a dynamical age of τdyn ≈ 740 yr, and a total momentum flux of ṖCO ≈ 1.0 × 10−5 M⊙ km s−1 yr−1, which is nearly three orders of magnitude larger than the maximum thrust that can be provided by the luminosity of the central star. Conclusions. We conclude that photoevaporation cannot be the main driving mechanism for this outflow, but it must be predominantly a magnetohydrodynamic disk wind. It is thus far the best-resolved rotating disk wind observed to be launched from a circumstellar disk in Keplerian rotation around a low-mass young stellar object (YSO), albeit also the one with the largest launch radius. It confirms the observed trend that disk winds from Class I YSOs with transitional disks have much larger launch radii than jets ejected from Class 0 protostars.

A self-gravitating system composed of baryonic and dark matter analysed from the post-Newtonian Boltzmann equations

We study the Jeans gravitational instability for a mixture of baryonic and dark matter particles, in the post-Newtonian approximation. We adopt a kinetic model consisting of a coupled system of post-Newtonian collisionless Boltzmann equations, for each species, coupled to the post-Newtonian Poisson equations. We derive the stability criterion, accounting for both post-Newtonian corrections and the presence of dark matter. It is shown that both effects give rise to smaller Jeans masses, in comparison with the standard Jeans criterion, meaning that a smaller mass is needed to begin the gravitational collapse. Taking advantage of that, we confront the model with the observational stability of Bok globules, and show that the model correctly reproduces the data.

The magnetic fields around the cometary globules L328, L323, and L331

ABSTRACT This work presents the magnetic field geometry in a complex of three cometary (with head–tail morphology) globules, namely LDN 323, LDN 328, and LDN 331, using R-band polarization measurements of background stars. These observations were combined with a Planck sky survey to study the large-scale morphology of the magnetic fields in the region. The distances of the target stars were adopted from the Gaia catalogue. The variation of the degree of polarization and polarization position angle with distances of stars is analysed. The field geometry is mostly found to follow the cometary shape of the cloud, with some randomness at certain locations. For studying the correlation between cloud morphology and magnetic field orientations, a modified version of the histogram of relative orientation (HRO) analysis was employed.

ALMA ACA study of the H2S/OCS ratio in low-mass protostars

Context. The identification of the main sulfur reservoir on its way from the diffuse interstellar medium to the cold dense star-forming cores and, ultimately, to protostars is a long-standing problem. Despite sulfur’s astrochemical relevance, the abundance of S-bearing molecules in dense cores and regions around protostars is still insufficiently constrained. Aims. The goal of this investigation is to derive the gas-phase H2S/OCS ratio for several low-mass protostars, which could provide crucial information about the physical and chemical conditions in the birth cloud of Sun-like stars. This may also shed new light onto the main sulfur reservoir in low-mass star-forming systems. Methods. Using Atacama Large Millimeter/submillimeter Array (ALMA) Atacama Compact Array (ACA) Band 6 observations, we searched for H2S, OCS, and their isotopologs in ten Class 0/I protostars with different source properties such as age, mass, and environmental conditions. The sample contains IRAS 16293-2422 A, IRAS 16293-2422 B, NGC 1333-IRAS 4A, RCrA IRS7B, Per-B1-c, BHR71-IRS1, Per-emb-25, NGC 1333-IRAS4B, Ser-SMM3, and TMC1. A local thermal equilibrium (LTE) model is used to fit synthetic spectra to the detected lines and to derive the column densities based solely on optically thin lines. Results. The H2S and OCS column densities span four orders of magnitude across the sample. The H2S/OCS ratio is found to be in the range from 0.2 to above 9.7. IRAS 16293-2422 A and Ser-SMM3 have the lowest ratio, while BHR71-IRS1 has the highest. Only the H2S/OCS ratio of BHR71-IRS1 is in agreement with the ratio in comet 67P/Churyumov–Gerasimenko within the uncertainties. Conclusions. The determined gas-phase H2S/OCS ratios can be below the upper limits on the solid-state ratios by as much as one order of magnitude. The H2S/OCS ratio depends in great measure on the environment of the birth cloud, such as UV-irradiation and heating received prior to the formation of a protostar. The highly isolated birth environment (a Bok globule) of BHR71-IRS1 is hypothesized as the reason for its high gaseous H2S/OCS ratio that is due to lower rates of photoreactions and more efficient hydrogenation reactions under such dark, cold conditions. The gaseous inventory of S-bearing molecules in BHR71-IRS1 appears to be the most similar to that of interstellar ices.

On the origin of de‐polarization in CB54

AbstractWe present polarimetric observations of the Bok globule CB54 in the far‐infrared via SOFIA/HAWC+ bands D (154 m) and E (214 m). We detect polarization with mean polarization degrees of 5.4 and 5.4 at 154 and 214 m, respectively. The polarization degree decreases toward the inner region of CB54, revealing a “polarization hole”. This finding can be explained by the impact of dichroic absorption counteracting the effect of polarized emission. The same effect allows us to explain the observed wavelength‐dependent orientation of the linear polarization in the dense, central region of CB54. The polarization pattern appears uniform at core scales but un‐ordered at larger scales—similar to what was found in previous studies of this object. The mean polarization angle amounts to = 62.4 44.5° and = 60.0° at 154 and 214 m, respectively.

A Disk Wind Driving the Rotating Molecular Outflow in CB 26

We present the 12CO (J = 2–1) sensitive molecular line and 1.3 mm continuum observations from the Submillimeter Array (SMA) of the bipolar outflow associated with the young star located in the Bok globule known as CB 26. The SMA observations were carried out in its extended configuration allowing us to study the kinematics and structure of the outflow with about 1″ or 140 au resolution. We find that the dusty and edge-on circumstellar disk related to the outflow has a projected spatial (deconvolved) size of 196 ± 31 × 42 ± 29 au with a total (gas+dust) mass of 0.031 ± 0.015 M ⊙. We estimated a dynamical mass for the central object of 0.66 ± 0.03 M ⊙, and the mass of the molecular outflow of 5 ± 1.5 × 10−5 M ⊙. All these values are consistent with recent estimations. The observations confirm that the outflow rotation has a similar orientation to that of the edge-on disk. For the outflow, we find that the following quantities: the rotation velocity (∼1–3 km s−1), specific angular momentum (∼200–700 au km s−1), and launching radius (∼15–35 au) decrease with the height above the midplane, as observed in other molecular rotating outflows. The radius (∼180–280 au) and expansion velocity (∼2–4 km s−1) also increase with the height above the disk midplane for z < 0 au; however, for z > 0 au these quantities do not exhibit this behavior. Estimations of the outflow linear momentum rate, outflow angular momentum rate, and accretion luminosity seem to be well explained by the presence of a disk wind in CB 26.

Quantum kinetic theory of Jeans instability in non-minimal matter-curvature coupling gravity

We present a quantum treatment of the Jeans gravitational instability in the Newtonian limit of the non-minimal matter-curvature coupling gravity model. By relying on Wigner functions, allowing for the representation of quantum states in a classical phase space, we formulate a quantum kinetic treatment of this problem, generalizing the classical kinetic approach (Gomes in Eur Phys J C 80:633, 2020). This allows us to study the interplay between non-minimal matter-curvature coupling effects, quantum effects, and kinetic (finite-temperature) effects, on the Jeans criterion. We study in detail special cases of the model (general relativity, f(R) theories, pure non-minimal coupling, etc.) and confront the model with the observed stability of Bok globules.

Chemical and Physical Characterization of the Isolated Protostellar Source CB68: FAUST IV

The chemical diversity of low-mass protostellar sources has so far been recognized, and environmental effects are invoked as its origin. In this context, observations of isolated protostellar sources without the influence of nearby objects are of particular importance. Here, we report the chemical and physical structures of the low-mass Class 0 protostellar source IRAS 16544−1604 in the Bok globule CB 68, based on 1.3 mm Atacama Large Millimeter/submillimeter Array observations at a spatial resolution of ∼70 au that were conducted as part of the large program FAUST. Three interstellar saturated complex organic molecules (iCOMs), CH3OH, HCOOCH3, and CH3OCH3, are detected toward the protostar. The rotation temperature and the emitting region size for CH3OH are derived to be 131 ± 11 K and ∼10 au, respectively. The detection of iCOMs in close proximity to the protostar indicates that CB 68 harbors a hot corino. The kinematic structure of the C18O, CH3OH, and OCS lines is explained by an infalling–rotating envelope model, and the protostellar mass and the radius of the centrifugal barrier are estimated to be 0.08–0.30 M ⊙ and <30 au, respectively. The small radius of the centrifugal barrier seems to be related to the small emitting region of iCOMs. In addition, we detect emission lines of c-C3H2 and CCH associated with the protostar, revealing a warm carbon-chain chemistry on a 1000 au scale. We therefore find that the chemical structure of CB 68 is described by a hybrid chemistry. The molecular abundances are discussed in comparison with those in other hot corino sources and reported chemical models.

Magnetic fields and outflows in the large Bok globule CB 54

ABSTRACT We have observed the large Bok globule CB 54 in 850-$\mu$m polarized light using the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We find that the magnetic field in the periphery of the globule shows a significant, ordered deviation from the mean-field direction in the globule centre. This deviation appears to correspond with the extended but relatively weak 12CO outflow emanating from the Class 0 sources at the centre of the globule. Energetics analysis suggests that if the outflow is reshaping the magnetic field in the globule’s periphery, then we can place an upper limit of $\lt 27\, \mu$G on the magnetic field strength in the globule’s periphery. Comparison with archival Planck and CARMA measurements shows that the field in the centre of the globule is consistent over several orders of magnitude in size scale, and oriented parallel to the density structure in the region in projection. We thus hypothesize that while non-thermal motions in the region may be sub-Alfvénic, the magnetic field is subdominant to gravity over a wide range of size scales. Our results suggest that even a relatively weak outflow may be able to significantly reshape magnetic fields in star-forming regions on scales &amp;gt;0.1 pc.

An extended scattered light disk around AT Pyx

Aims. To understand how the multitude of planetary systems that have been discovered come to be, we need to study systems at different evolutionary stages, with different central stars but also in different environments. The most challenging environment for planet formation may be the harsh UV radiation field of nearby massive stars which quickly erodes disks by external photo-evaporation. We observed the AT Pyx system, located in the head of a cometary globule in the Gum Nebula, to search for signs of ongoing planet formation. Methods. We used the extreme adaptive optics imager VLT/SPHERE in Dual Beam Polarization Imaging Mode in H-band as well as in IRDIFS Extended mode (K12-band imaging and Y-H integral field spectroscopy) to observe AT Pyx in polarized light and total intensity. Additionally, we employed VLT/NACO to observe the system in the L-band. Results. We resolve the disk around AT Pyx for the first time in scattered light across multiple wavelengths in polarized light and total intensity. We find an extended (≥126 au) disk, with an intermediate inclination of between 35° and 42°. The disk shows a complex substructure and we identify two or possibly three spiral-like features. Depending on the precise geometry of the disk (which we cannot unambiguously infer from our data), the disk may be eccentric with an eccentricity of ~0.16 or partially self-shadowed. The spiral features and possible eccentricity are both consistent with signatures of an embedded gas giant planet with a mass of ~1 MJup. Our own observations can rule out brown dwarf companions embedded in the resolved disk, but are nevertheless not sensitive enough to confirm or rule out the presence of a gas giant. Conclusions. AT Pyx is the first disk to be spatially resolved in a cometary globule in the Gum Nebula. By comparison with disks in the Orion Nebula Cluster we note that the extension of the disk may be exceptional for this environment if the external UV radiation field is indeed comparable to other cometary globules in the region. The signposts of ongoing planet formation are intriguing and need to be followed up with either higher sensitivity or at different wavelengths.

Gravitational instability with a dark matter background: exploring the different scenarios

We study the Jeans-type gravitational instability for a self-gravitating medium composed of two species, baryonic (bright) and dark matter particles, using a hybrid quantum-classical fluid approach. Baryonic matter is treated classically, which is appropriate for most astrophysical environments, e.g., Bok globules, while dark matter is treated through a quantum hydrodynamic approach allowing for possible nonlinearities. These nonlinearities may arise in bosonic dark matter due to attractive or repulsive short-range self-interaction (attractive interaction being more relevant for axions) or from the Pauli exclusion principle for fermionic dark matter, e.g., massive neutrinos. This allows us to explore, in a very broad context, the impact of a dark matter background on the Jeans process for different scenarios discussed in the literature. We confront the established stability criterion with Bok globule stability observations and show that the model adequately accounts for the data with dark matter parameters close to those predicted independently from numerical simulations.

Magnetic fields and young stellar objects in cometary cloud L1616

ABSTRACT The Lynds’ Dark Nebula (LDN) 1615/1616 and CB 28 (hereafter L1616) together form a cometary globule located at an angular distance of about 8° west of the Orion OB1 association, aligned roughly along the east–west direction, and showing a distinct head–tail structure. The presence of massive stars in the Orion belt has been considered to be responsible for the radiation-driven implosion mode of star formation in L1616. Based on the latest Gaia Early Data Release 3 (EDR3) measurements of the previously known young stellar objects (YSOs) associated with L1616, we find the distance to this cloud to be 384 ± 5 pc. We present optical polarimetry towards L1616 that maps the plane-of-sky component of the ambient magnetic field (BPOS) geometry. Based on the proper motion of the YSOs associated with L1616, we investigate their plane-of-sky motion relative to the exciting star ϵ Ori. Using the Gaia EDR3 measurements of the distances and proper motions of the YSOs, we find two additional sources comoving with the known YSOs. One comoving source is HD 33056, a B9 star, and the other might be a young pre-main-sequence star not reported in previous studies. The mean direction of BPOS is found to follow the cloud structure. This could be the effect of dragging of the magnetic field lines by the impact of the ionizing radiation from ϵ Ori. Based on the pressure exerted on L1616, and the ages of the associated YSOs, we show that it could possibly be the main source of ionization in L1616, and thus the star formation in it.

VLA and NOEMA Views of Bok Globule CB 17: The Starless Nature of a Proposed First Hydrostatic Core Candidate

Abstract We use 3 mm continuum NOrthern Extended Millimeter Array and NH3 Very Large Array observations toward the First Hydrostatic Core (FHSC) candidate CB 17 MMS in order to reveal the dust structure and gas properties to 600–1100 au scales and to constrain its evolutionary stage. We do not detect any compact source at the previously identified 1.3 mm point source, despite expecting a minimum signal-to-noise ratio of 9. The gas traced by NH3 exhibits subsonic motions, with an average temperature of 10.4 K. A fit of the radial column density profile derived from the ammonia emission finds a flat inner region of radius ∼1800 au and a central density of ∼6 × 105 cm−3. Virial and density structure analysis reveals the core is marginally bound (α vir = 0.73). The region is entirely consistent with that of a young starless core, hence ruling out CB 17 MMS as an FHSC candidate. Additionally, the core exhibits a velocity gradient aligned with the major axis, showing an arc-like structure in the position–velocity diagram and an off-center region with high velocity dispersion, caused by two distinct velocity peaks. These features could be due to interactions with the nearby outflow, which appears to deflect due to the dense gas near the NH3 column density peak. We investigate the specific angular momentum profile of the starless core, finding that it aligns closely with previous studies of similar radial profiles in Class 0 sources. This similarity to more evolved objects suggests that motions at 1000 au scales are determined by large-scale dense cloud motions, and may be preserved throughout the early stages of star formation.

What’s Behind the Elephant’s Trunk? Identifying Young Stellar Objects on the Outskirts of IC 1396*

Abstract Empirically, the estimated lifetime of a typical protoplanetary disk is &lt;5–10 Myr. However, the disk lifetimes required to produce a variety of observed exoplanetary systems may exceed this timescale. Some hypothesize that this inconsistency is due to estimating disk fractions at the cores of clusters, where radiation fields external to a star–disk system can photoevaporate the disk. To test this, we have observed a field on the western outskirts of the IC 1396 star-forming region with XMM-Newton to identify new Class III YSO cluster members. Our X-ray sample is complete for YSOs down to 1.8 M ⊙. We use a subset of these X-ray sources that have near- and mid-infrared counterparts to determine the disk fraction for this field. We find that the fraction of X-ray-detected cluster members that host disks in the field we observe is 17 − 7 + 10 % (1σ), comparable with the 29 − 3 + 4 % found in an adjacent field centered on the cometary globule IC 1396A. We reevaluate YSO identifications in the IC 1396A field using Gaia parallaxes compared to previous color-cut-only identifications, finding that incorporating independent distance measurements provides key additional constraints. Given the existence of at least one massive star producing an external radiation field in the cluster core, the lack of a statistically significant difference in disk fraction in each observed field suggests that disk lifetimes remain consistent as a function of distance from the cluster core.

Feedback from γ Cassiopeiae: Large Expanding Cavity, Accelerating Cometary Globules, and Peculiar X-Ray Emission

Abstract We present wide-field multiwavelength observations of γ Cassiopeiae (or γ Cas for short) in order to study its feedback toward the interstellar environment. A large expanding cavity is discovered toward γ Cas in the neutral hydrogen (H i) images at a systemic velocity of about −10 km s−1. The measured dimension of the cavity is roughly 2.°0 × 1.°4 (or 6.0 pc × 4.2 pc at a distance of 168 pc), while the expansion velocity is ∼5.0 ± 0.5 km s−1. The CO observations reveal systematic velocity gradients in IC 63 (∼20 km s−1 pc−1) and IC 59 (∼30 km s−1 pc−1), two cometary globules illuminated by γ Cas, proving fast acceleration of the globules under stellar radiation pressure. The gas kinematics indicate that the cavity is opened by strong stellar wind, which has high potential to lead to the peculiar X-ray emission observed in γ Cas. Our result favors a new scenario that emphasizes the roles of stellar wind and binarity in the X-ray emission of the γ Cas stars.

Star Formation Triggered by Shocks

Abstract Star formation can be triggered by compression from shock waves. In this study, we investigated the interaction of hydrodynamic shocks with Bonnor–Ebert spheres using 3D hydrodynamical simulations with self-gravity. Our simulations indicated that the cloud evolution primarily depends on two parameters: shock speed and initial cloud radius. Stronger shocks can compress clouds more efficiently, and when the central region becomes gravitationally unstable, a shock triggers cloud contraction. However, if it is excessively strong, it shreds the cloud more violently and the cloud is destroyed. From simple theoretical considerations, we derived the condition of triggered gravitational collapse, which agreed with the simulation results. Introducing sink particles, we followed the further evolution after star formation. Since stronger shocks tend to shred the cloud material more efficiently, the stronger the shock is, the smaller the final (asymptotic) masses of the stars formed (i.e., sink particles) become. In addition, shocks accelerate clouds, promoting mixing of shock-accelerated interstellar medium gas. As a result, the separation between sink particles and shocked clouds center and their relative speeds increase over time. We also investigated the effect of cloud turbulence on shock–cloud interaction. We observed that cloud turbulence prevents rapid cloud contraction; thus, turbulent clouds are destroyed more rapidly than thermally supported clouds. Therefore, the masses of stars formed become smaller. Our simulations provide a general guide to the evolutionary process of dense cores and Bok globules impacted by shocks.

Higher-order generalized uncertainty principle corrections to the Jeans mass

The Jeans instability is regarded as an important tool for analyzing the dynamics of a self-gravitating system. However, this theory is challenging since astronomical observation data show some Bok globules, whose masses are less than the Jeans mass and still have stars or at least undergo the star formation process. To explain this problem, we investigate the effects of the higher-order generalized uncertainty principle on the Jeans mass of the collapsing molecular cloud. The results in this paper show that the higher order generalized uncertainty principle has a very significant effect on the canonical energy and gravitational potential of idea gas, and finally leads to a modified Jeans mass lower than the original case, which is conducive to the generation of stars in small mass Bok globules. Furthermore, we estimate the new generalized uncertainty principle parameter gamma _0 by applying various data of Bok globules, and find that the range of magnitude of gamma _0 is {10^{11}} {-} {10^{12}}.

On the photoevaporation, dust polarization and kinematics of two nebulae in Sh2-236

In the work presented here, the impact of magnetic field on the dynamical evolution of cometary globules Sim 129 and Sim 130 in the Sh2-236 HII region and the ionized gas streaming out of their surfaces is investigated. The magnetic field morphology in the region associated with these globules is inferred using optical polarization measurements with the Sampurnanand Telescope at ARIES. The nebular emission is probed through radio continuum mapping at 1.4 GHz with the archival National Radio Astronomy Observatory (NRAO) Very Large Array (VLA) Sky Survey (NVSS) data. The correlation of these measurements suggests that photoevaporated gas from the surfaces of Sim 129 and Sim 130 accumulates in clouds and starts streaming along the magnetic field lines. The 12CO (J=1–0) molecular line observations are performed towards NGC 1893 from the 14-m single dish radio telescope in Taeduk Radio Astronomy Observatory (TRAO). The velocity dispersion in the 12CO (J=1–0) molecular line and the dispersion in polarization angles are considered in the Davis-Chandrasekhar-Fermi formulation to estimate the magnetic field strength towards two Sim nebulae. The average value of field strength is found to be ∼60 μG with uncertainty of 0.5 times the estimated value.