Low-mass Prestellar Cores Research Articles

Chemistry in infrared dark clouds

Massive stars play an important role in shaping the structure of galaxies. Infrared dark clouds (IRDCs), with their low temperatures and high densities, have been identified as the potential birthplaces of massive stars. In order to understand the formation processes of massive stars the physical and chemical conditions in infrared dark clouds have to be characterized. The goal of this paper is to investigate the chemical composition of a sample of southern infrared dark clouds. One important aspect of the observations is to check, if the molecular abuncances in IRDCs are similar to the low-mass pre-stellar cores, or whether they show signatures of more evolved evolutionary stages. We performed observations toward 15 IRDCs in the frequency range between 86 and 93 GHz using the 22-m Mopra radio telescope. We detect HNC, HCO$^+$ and HNC emission in all clouds and N$_2$H$^+$ in all IRDCs except one. In some clouds we detect SiO emission. Complicated shapes of the HCO$^+$ emission line profile are found in all IRDCs. Both signatures indicates the presence of infall and outflow motions and beginning of star formation activity, at least in some parts of the IRDCs. Where possible, we calculate molecular abundances and make a comparison with previously obtained values for low-mass pre-stellar cores and high-mass protostellar objects (HMPOs). We show a tendency for IRDCs to have molecular abundances similar to low-mass pre-stellar cores rather than to HMPOs abundances on the scale of our single-dish observations.

YOUNG STARLESS CORES EMBEDDED IN THE MAGNETICALLY DOMINATED PIPE NEBULA

The Pipe Nebula is a massive, nearby dark molecular cloud with a low star-formation efficiency which makes it a good laboratory to study the very early stages of the star formation process. The Pipe Nebula is largely filamentary, and appears to be threaded by a uniform magnetic field at scales of few parsecs, perpendicular to its main axis. The field is only locally perturbed in a few regions, such as the only active cluster forming core B59. The aim of this study is to investigate primordial conditions in low-mass pre-stellar cores and how they relate to the local magnetic field in the cloud. We used the IRAM 30-m telescope to carry out a continuum and molecular survey at 3 and 1 mm of early- and late-time molecules toward four selected starless cores inside the Pipe Nebula. We found that the dust continuum emission maps trace better the densest regions than previous 2MASS extinction maps, while 2MASS extinction maps trace better the diffuse gas. The properties of the cores derived from dust emission show average radii of ~0.09 pc, densities of ~1.3x10^5 cm^-3, and core masses of ~2.5 M_sun. Our results confirm that the Pipe Nebula starless cores studied are in a very early evolutionary stage, and present a very young chemistry with different properties that allow us to propose an evolutionary sequence. All of the cores present early-time molecular emission, with CS detections toward all the sample. Two of them, Cores 40 and 109, present strong late-time molecular emission. There seems to be a correlation between the chemical evolutionary stage of the cores and the local magnetic properties that suggests that the evolution of the cores is ruled by a local competition between the magnetic energy and other mechanisms, such as turbulence.

Protostellar discs formed from rigidly rotating cores

Abridged: We use three-dimensional SPH simulations to investigate the collapse of low-mass prestellar cores and the formation and early evolution of protostellar discs. The initial conditions are slightly supercritical Bonnor-Ebert spheres in rigid rotation. The core mass and initial radius are held fixed at M_O=6.1 M_sun and R_O=17,000 AU, and the only parameter that we vary is the initial angular speed \Omega_O. Protostellar discs forming from cores with \Omega_O<1.35 10d-13 1/s have radii between 100 and 300 AU and are quite centrally concentrated; due to heating by gas infall onto the disc and accretion onto the central object, they are also quite warm, T>100 K, and therefore stable against gravitational fragmentation. In contrast, more rapidly rotating cores form discs which are less concentrated and cooler, and have radii between 400 and 1000 AU; as a consequence they are prone to gravitational fragmentation and the formation of multiple systems. We derive a criterion that predicts whether a rigidly rotating core having given M_O, R_O and \Omega_O will produce a protostellar disc which fragments whilst material is still infalling from the core envelope. We then apply this criterion to core samples for which M_O, R_O and \Omega_O have been estimated observationally. We conclude that the observed cores are stable against fragmentation at this stage, due to their low angular speeds and the heat delivered at the accretion shock where the infalling material hits the disc.

Brown dwarf formation by binary disruption

Context.The principal mechanism by which brown dwarfs form, and its relation to the formation of higher-mass (i.e. hydrogen-burning) stars, is poorly understood. Aims.We advocate a new model for the formation of brown dwarfs. Methods.In this model, brown dwarfs are initially binary companions, formed by gravitational fragmentation of the outer parts ( $R\ga 100\,{\rm AU}$) of protostellar discs around low-mass hydrogen-burning stars. Most of these binaries are then gently disrupted by passing stars to create a largely single population of brown dwarfs and low-mass hydrogen-burning stars. Results.This idea is consistent with the excess of binaries found in low-density pre-main sequence populations, like that in Taurus, where they should survive longer than in denser clusters. Conclusions.If brown dwarfs form in this way, as companions to more massive stars, the difficulty of forming very low-mass prestellar cores is avoided. Since the disrupted binaries will tend to be those involving low-mass components and wide orbits, and since disruption will be due to the gentle tides of passing stars (rather than violent N-body interactions in small-N sub-clusters), the liberated brown dwarfs will have velocity dispersions and spatial distributions very similar to higher-mass stars, and they will be able to retain discs, and thereby to sustain accretion and outflows. Thus the problems associated with the ejection and turbulence mechanisms can be avoided. This model implies that most, possibly all, stars and brown dwarfs form in binary or multiple systems.

Searching for massive pre-stellar cores through observations of N$_\mathsf{2}$H+and N$_\mathsf{2}$D+

Aims. We have measured the deuterium fractionation and the CO depletion factor (ratio between expected and observed CO abundance) in a sample of high-mass protostellar candidates, in order to understand whether the earliest evolutionary stages of high-mass stars have chemical characteristics similar to those of low-mass ones. It has been found that low-mass starless cores on the verge of star formation have large values both of the column density ratio $N({\rm N_2D^+})/N({\rm N_2H^+})$ and of the CO depletion factor. Methods. With the IRAM-30 m telescope and the JCMT we have observed two rotational lines of N 2 H + and N 2 D + , the (2–1) line of C 17 O and DCO + , and the sub-millimeter continuum towards a sample of 10 high-mass protostellar candidates. Results. We have detected N 2 D + emission in 7 of the 10 sources of our sample, and found an average value $N({\rm N_2D^+})/N({\rm N_2H^+})\sim 0.015$. This value is ~3 orders of magnitude larger than the interstellar D/H ratio, indicating the presence of cold and dense gas, in which the physical-chemical conditions are similar to those observed in low-mass pre-stellar cores. The integrated CO depletion factors show that in the majority of the sources the expected CO abundances are larger than the observed values, with a median ratio of 3.2. Conclusions. In principle, the cold gas that generates the N 2 D + emission can be the remnant of the massive molecular core in which the high-mass (proto-)star was born, not yet heated up by the central object. If so, our results indicate that the chemical properties of the clouds in which high-mass stars are born are similar to their low-mass counterparts. Alternatively, this cold gas could be located in one (or more) starless core (cores) near the protostellar object. Due to the poor angular resolution of our data, we cannot distinguish between the two scenarios.

The formation of brown dwarfs

We review four mechanisms for forming brown dwarfs: (i) turbulent fragmentation (producing very low-mass prestellar cores); (ii) gravitational instabilities in discs; (iii) dynamical ejection of stellar embryos from their placental cores; and (iv) photo-erosion of pre-existing cores in HII regions. We argue (a) that these are simply the mechanisms of low-mass star formation , and (b) that they are not mutually exclusive. If, as seems possible, all four mechanisms operate in nature, their relative importance may eventually be constrained by their ability to reproduce the binary statistics of brown dwarfs, but this will require fully 3-D radiative magneto-hydrodynamic simulations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Detection of D 2 H + in the Dense Interstellar Medium

The 692 GHz para ground-state line of D2H+ has been detected at the Caltech Submillimeter Observatory toward the prestellar core 16293E. The derived D2H+ abundance is comparable to that of H2D+, as determined by observations of the 372 GHz line of ortho-H2D+. This is an observational verification of recent theoretical predictions, developed to explain the large deuteration ratios observed in cold, high-density regions of the interstellar medium associated with low-mass prestellar cores and protostars. This detection confirms expectations that the multiply deuterated forms of H were missing factors of earlier models. The inclusion of D2H+ and D in the models leads to predictions of higher values of the D/H ratio in the gas phase.