A polarisation study of spiral galaxies

Optical polarimetry results are presented for four spiral galaxies: NGC 5194 (M51), NGC 1068, NGC 4565 and NGC 4594 (Ml04). M51 and NGC 1068 show spiral polar isation patterns which are interpreted as indicating a spiral magnetic field in each case. NGC 4565 and M104 show polar isations in their dust lanes which are parallel to their galactic planes, and which are interpreted in terms of a magnetic field in the plane of each. It is hypothesised that the observed magnetic fields may be linked to galactic shocks. A discussion of the origin of galactic magnetic fields concludes that there is no evidence which necessitates a primordial magnetic field

BIMA N2H+ 1-0 mapping observations of L183 -- fragmentation and spin-up in a collapsing, magnetized, rotating, pre-stellar core

We have used the Berkeley-Illinois-Maryland Array (BIMA) to make deep N2H+ 1-0 maps of the pre-stellar core L183, in order to study the spatial and kinematic substructure within the densest part of the core. Three spatially and kinematically distinct clumps are detected, which we label L183-N1, L183-N2 and L183-N3. L183-N2 is approximately coincident with the submillimetre dust peak and lies at the systemic velocity of L183. Thus we conclude that L183-N2 is the central dense core of L183. L183-N1 and 3 are newly-discovered fragments of L183, which are marked by velocity gradients that are parallel to, but far stronger than, the velocity gradient of L183 as a whole, as detected in previous single-dish data. Furthermore, the ratio of the large-scale and small-scale velocity gradients, and the ratio of their respective size-scales, are consistent with the conservation of angular momentum for a rotating, collapsing core undergoing spin-up. The inferred axis of rotation is parallel to the magnetic field direction, which is offset from its long axis, as we have seen in other pre-stellar cores. Therefore, we propose that we have detected a fragmenting, collapsing, filamentary, pre-stellar core, rotating about its B-field, which is spinning up as it collapses. It will presumably go on to form a multiple protostellar system.Comment: 7 figures, 1 table, 21 pages, accepted for publication in Ap

Interferometric Identification of a Pre-Brown Dwarf

It is not known whether brown dwarfs (stellar-like objects with masses less than the hydrogen-burning limit, 0.075 Msun) are formed in the same way as solar-type stars or by some other process. Here we report the clear-cut identification of a self-gravitating condensation of gas and dust with a mass in the brown-dwarf regime, made through millimeter interferometric observations. The level of thermal millimeter continuum emission detected from this object indicates a mass ~ 0.02-0.03 Msun, while the small radius < 460 AU and narrow spectral lines imply a dynamical mass of 0.015-0.02 Msun. The identification of such a pre-brown dwarf core supports models according to which brown dwarfs are formed in the same manner as hydrogen-burning stars.Comment: 23 pages, 6 figures, Supporting Online Material included. Published in Science (Vol. 337, 6 july 2012); http://www.sciencemag.org/cgi/content/full/337/6090/69?ijkey=ODfDOFPXGRc9E&keytype=ref&siteid=sc

Stellar clustering and the kinematics of stars around Collinder 121 using Gaia DR3

We study the region around Collinder 121 (Cr 121) using newly available 6-dimensional data from the Gaia DR3 catalogue. Situated in the third quadrant, near the galactic plane, Collinder 121 lies in the region of Canis Major centred around l = 236 degrees, b = -10 degrees. Previous studies have suggested that the stellar associations in this region comprise an OB association (CMa OB2) lying at about 740 pc with a more distant open cluster (Cr 121) at approximately 1170 pc. Despite these studies, the precise nature of Collinder 121 remains uncertain. This study investigates the region bounded by the box l = 225 to 245 degrees, b = 0.00 to -20.00 degrees to a depth of 700 pc from 500 to 1200 pc which fully encompasses the region discussed in the literature. Using Gaia DR3 data, we do not find associations at the distances given in the literature. Instead, using the HDBSCAN machine learning algorithm, we find a major association of OB stars centred around 803 pc. Within this association we find four smaller subgroups that may be indicative of a larger association and which are located at a mean distance of 827 pc. Proper motion studies find coherence between these four subgroups and show a distinctive east to west increase in the size of the velocity vectors which supports contemporary studies that show similar trends in OB populations in Cygnus and within the Carina spiral Arm. Therefore, we hypothesize that Cr 121 and CMa OB2 are the same cluster, consistent with the 1977 study by Hoogerwerf

¹³CO and C¹⁸O observations of S140: delineation of the outflow structure, a study of fractionation effects and comparison with CI observations

The outflow and photon-dominated region (PDR) associated with the S140 complex have been observed at high resolution (~14 arcsec) in the 13CO and C18O J=3→2 lines. The C18O map confirms earlier C17O J=3→2line observations (Minchin et al. 1994) that show an `arc' of emission observed to the south of the peak, and also reveals a similar (and more prominent) arc feature to the east, a region not covered by the C17O map. This is a particularly fine example of the classic `tuning fork' morphology, where emission at the ambient cloud velocity is tracing the outflow cavity wall of the blueshifted lobe. The N(13CO)/N(C18O) ratio has been plotted against extinction and fits the power law relation N(13CO)/N(C18O)=21Av-0.35. The highest values, as expected, occur for observed positions towards the PDR, with N(13CO)/N(C18O) exceeding the terrestrial value (5.5) for Av ≤40 magnitudes. In the outermost parts of the cloud (Av ≤10 magnitudes) the N(13CO)/N(C18O) ratio is largest, up to 20. The increased fractionation may be due to higher photoionization of the optically thinner isotope, C18O. There is a close correlation between N(CI)/N(CO) and visual extinction over a wide extinction range (Av=3-100 mags.). The best fit power law is N(CI)/N(CO)=4.2Av0.9. For positions toward the outflow (Av~50-100) N(CI)/N(CO) ~0.1(0.07-0.12). N(CI)/N(CO) increases with decreasing extinction to ~1 for Av≤5 mags., corresponding to positions near the edge of the cloud. A detailed comparison of antenna temperatures and linewidths for the 13CO, C18O and CI lines is presented. The 13CO and C18O antenna temperatures and linewidths are closely correlated, and imply the emission, for both isotopes, emanates from gas that is in LTE and is well mixed. The CI emission from the PDR implies that here the atomic carbon is in LTE, but occupies a different volume of gas than the isotopic CO. Towards the outflow the CI linewidths are systematically broadened relative to those for the isotopic CO lines. This is interpreted as evidence that atomic carbon is produced by the effect of shocks on the chemical and physical processes at the interface between a stellar wind and the outflow cavity wall

Detailed Studies of Cloud Cores: Probing the Initial Conditions for Protostellar Collapse

Improving our understanding of the earliest stages of star formation is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. We discuss recent advances made in this area thanks to detailed mapping observations at infrared and (sub)millimeter wavelengths. Although ambipolar diffusion appears to be too slow to play a direct role in the formation of dense cores, there is nevertheless good evidence that the gravitational collapse of isolated protostellar cores is strongly magnetically controlled. We also argue that the beginning of protostellar collapse is much more violent in cluster-forming clouds than in regions of distributed star formation.Comment: 10 pages, 4 figures, to appear in the proceedings of the JENAM2003 minisymposium "Early Stages of Star Formation" (special issue of Baltic Astronomy - M. Kun & J. Eisloeffel Eds.

Carbon monoxide and far-infrared observations of the S 155-Cepheus B region

We present maps of the CO J = 3-2 and 13CO J = 2→1 molecular line and mid- to far-infrared continuum emission of the interface between the Cepheus B molecular cloud and the S155 H II region. Far-infrared dust color temperature and optical depth maps show the molecular cloud to be externally heated and that the edge of the cloud is compressed by the expansion of S155. The data are compared with current models, and various dust grain parameters are derived. A hotspot is observed in the CO J = 3→2 emission line, at a position coincident with the radio continuum and infrared emission peaks. The infrared, radio continuum, and molecular line emission from the hotspot are all consistent with it being a compact H II region, ionized by an embedded B1-B0.5 star. The position of the compact H II region, adjacent to the northwestern edge of Cep B, suggests it is the product of a phase of sequential OB star formation, which has already been responsible for the youngest subgroup of the Cepheus OB3 association. The mass of the cloud is estimated to be ~100-200M⊙</sub

A submillimetre continuum study of S 140/L 1204: the detection of three new submillimetre sources and a self-consistent model for the region

We present submillimetre continuum observations of the L 1204/S 140 complex in broad bands centred at 450, 800 and 1100μm. The morphology of the region is similar at all three wavelengths, with the emitting region compact, about 90 arcsec in diameter, and centrally peaked around the cloud core. Three new submillimetre continuum sources are observed which are not coincident with any previously known near or mid-infrared sources. We designate the sources S 140-SMM1-3. SMM1 is roughly coincident with a previously known NH3 clump and 2.7mm source, and near-IR reflection nebulosity from the surface of SMM2 has previously been seen. The three submillimetre continuum sources may be protostellar in nature, although it is not possible to determine whether they are gravitationally bound, since virial mass estimates are disrupted by the presence of an energetic bipolar outflow. For this reason, earlier claims that the 2.7mm source in SMM1 is collapsing appear somewhat premature. The observation that SMM1 and SMM2 lie either side of the infrared sources, in a line roughly perpendicular to the direction of the bipolar outflow, imply they may be the remnants of a large-scale disk. Comparison of the continuum emission with previous high resolution CS, NH3 and CI observations provides evidence that, for the first time, demonstrates the photon-dominated region and outflow are intimately linked. The only scenario that is able to explain all of the available molecular and atomic emission line data and our submillimetre continuum data, is one in which the outflow has expanded towards the edge of the molecular cloud and the edge of the blueshifted outflow lobe is now bounded by the expanding HII region. The NH3 and continuum emission emanate from the inner edge of the outflow lobe, shielded from the external UV field. A plot of the 800μm flux against N(C18O) implies that the dust/gas mass ratio is close to the canonical value (~1%) at the lower end of the observed extinction range (Av≤70), but for the highest observed extinctions (Av=70-100) the continuum flux density increases rapidly, implying a higher dust/gas mass ratio is appropriate (~2-5%), possibly indicating freeze-out of gas onto dust grains

From Filamentary Networks to Dense Cores in Molecular Clouds: Toward a New Paradigm for Star Formation

Recent studies of the nearest star-forming clouds of the Galaxy at submillimeter wavelengths with the Herschel Space Observatory have provided us with unprecedented images of the initial and boundary conditions of the star formation process. The Herschel results emphasize the role of interstellar filaments in the star formation process and connect remarkably well with nearly a decade's worth of numerical simulations and theory that have consistently shown that the ISM should be highly filamentary on all scales and star formation is intimately related to self-gravitating filaments. In this review, we trace how the apparent complexity of cloud structure and star formation is governed by relatively simple universal processes - from filamentary clumps to galactic scales. We emphasize two crucial and complementary aspects: (i) the key observational results obtained with Herschel over the past three years, along with relevant new results obtained from the ground on the kinematics of interstellar structures, and (ii) the key existing theoretical models and the many numerical simulations of interstellar cloud structure and star formation. We then synthesize a comprehensive physical picture that arises from the confrontation of these observations and simulations.Comment: 24 pages, 15 figures. Accepted for publication as a review chapter in Protostars and Planets VI, University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C. Dullemond, Th. Hennin