Fragmentation and mass segregation in the massive dense cores of Cygnus X

We present Plateau de Bure interferometer observations obtained in continuum at 1.3 and 3.5 mm towards the six most massive and young (IR-quiet) dense cores in Cygnus X. Located at only 1.7 kpc, the Cygnus X region offers the opportunity of reaching small enough scales (of the order of 1700 AU at 1.3 mm) to separate individual collapsing objects. The cores are sub-fragmented with a total of 23 fragments inside 5 cores. Only the most compact core, CygX-N63, could actually be a single massive protostar with an envelope mass as large as 60 Msun. The fragments in the other cores have sizes and separations similar to low-mass pre-stellar and proto-stellar condensations in nearby protoclusters, and are probably of the same nature. A total of 9 out of these 23 protostellar objects are found to be probable precursors of OB stars with envelope masses ranging from 6 to 23 Msun. The level of fragmentation is globally higher than in the turbulence regulated, monolithic collapse scenario, but is not as high as expected in a pure gravo-turbulent scenario where the distribution of mass is dominated by low-mass protostars/stars. Here, the fractions of the total core masses in the high-mass fragments are reaching values as high as 28, 44, and 100 % in CygX-N12, CygX-N53, and CygX-N63, respectively, much higher than what an IMF-like mass distribution would predict. The increase of the fragmentation efficiency as a function of density in the cores is proposed to be due to the increasing importance of self-gravity leading to gravitational collapse at the scale of the dense cores. At the same time, the cores tend to fragment into a few massive protostars within their central regions. We are therefore probably witnessing here the primordial mass segregation of clusters in formation.Comment: 14 pages, 16 figures, submitted for publication in A&

Evidence of triggered star formation in G327.3-0.6. Dust-continuum mapping of an infrared dark cloud with P-ArT\'eMiS

Aims. Expanding HII regions and propagating shocks are common in the environment of young high-mass star-forming complexes. They can compress a pre-existing molecular cloud and trigger the formation of dense cores. We investigate whether these phenomena can explain the formation of high-mass protostars within an infrared dark cloud located at the position of G327.3-0.6 in the Galactic plane, in between two large infrared bubbles and two HII regions. Methods: The region of G327.3-0.6 was imaged at 450 ? m with the CEA P-ArT\'eMiS bolometer array on the Atacama Pathfinder EXperiment telescope in Chile. APEX/LABOCA and APEX-2A, and Spitzer/IRAC and MIPS archives data were used in this study. Results: Ten massive cores were detected in the P-ArT\'eMiS image, embedded within the infrared dark cloud seen in absorption at both 8 and 24 ?m. Their luminosities and masses indicate that they form high-mass stars. The kinematical study of the region suggests that the infrared bubbles expand toward the infrared dark cloud. Conclusions: Under the influence of expanding bubbles, star formation occurs in the infrared dark areas at the border of HII regions and infrared bubbles.Comment: 4 page

Rice plants respond to ammonium‐stress by adopting a helical root growth pattern

High levels of ammonium nutrition reduce plant growth and different plant species have developed distinct strategies to maximize ammonium acquisition while alleviate ammonium toxicity through modulating root growth. Up to now, the mechanism underlying plant tolerance or sensitivity towards ammonium remain unclear. Rice uses ammonium as its main N source. Here we show that ammonium supply restricts rice root elongation and induces a helical growth pattern, which is attributed to root acidification resulting from ammonium uptake. Ammonium-induced low pH triggers asymmetric auxin distribution in rice root tips through changes in auxin signaling, thereby inducing a helical growth response. Blocking auxin signaling completely inhibited this root response. In contrast, this root response is not activated in ammonium-treated Arabidopsis. Acidification of Arabidopsis roots leads to the protonation of IAA, and dampening the intracellular auxin signaling levels that are required for maintaining root growth. Our study suggests a different mode of action by ammonium on the root pattern and auxin response machinery in rice versus Arabidopsis, and the rice-specific helical root response towards ammonium is an expression of the ability of rice in moderating auxin signaling and root growth to utilize ammonium while confronting acidic stress

Some empirical estimates of the H2 formation rate in photon-dominated regions

We combine recent ISO observations of the vibrational ground state lines of H2 towards Photon-Dominated Regions (PDRs) with observations of vibrationally excited states made with ground-based telescopes in order to constrain the formation rate of H2 on grain surfaces under the physical conditions in the layers responsible for H2 emission. We use steady state PDR models in order to examine the sensitivity of different H2 line ratios to the H2 formation rate Rf. We show that the ratio of the 0-0 S(3) to the 1-0 S(1) line increases with Rf but that one requires independent estimates of the radiation field incident upon the PDR and the density in order to infer Rf from the H2 line data. We confirm the earlier result of Habart et al. (2003) that the H2 formation rate in regions of moderate excitation such as Oph W, S140 and IC 63 is a factor of 5 times larger than the standard rate inferred from UV observations of diffuse clouds. On the other hand, towards regions of higher radiation field such as the Orion Bar and NGC 2023, we derive H2 formation rates consistent with the standard value. We find also a correlation between the H2 1-0 S(1) line and PAH emission suggesting that Rf scales with the PAH abundance. With the aim of explaining these results, we consider some empirical models of the H2 formation process. Here we consider both formation on big (a~0.1 microns) and small (a~10 Angstroms) grains by either direct recombination from the gas phase or recombination of physisorbed H atoms with atoms in a chemisorbed site. We conclude that indirect chemisorption is most promising in PDRs. Moreover small grains which dominate the total grain surface and spend most of their time at relatively low temperatures may be the most promising surface for forming H2 in PDRs.Comment: A&A in press, 16 pages, 5 figure

Density structure of the Horsehead nebula photo-dissociation region

We present high angular resolution images of the H$_2$ 1-0 S(1) line emission obtained with the Son of ISAAC (SOFI) at the New Technology Telescope (NTT) of the Horsehead nebula. These observations are analysed in combination with H$\alpha$ line emission, aromatic dust, CO and dust continuum emissions. The Horsehead nebula illuminated by the O9.5V star $\sigma$ Ori ($\chi \sim$ 60) presents a typical photodissociation region (PDR) viewed nearly edge-on and offers an ideal opportunity to study the gas density structure of a PDR. The H$_2$ fluorescent emission observations reveal extremely sharp and bright filaments associated with the illuminated edge of the nebula which spatially coincides with the aromatic dust emission. Analysis of the H$_2$ fluorescent emission, sensitive to both the far-UV radiation field and the gas density, in conjunction with the aromatic dust and H$\alpha$ line emission, brings new constraints on the illumination conditions and the gas density in the outer PDR region. Furthermore, combination of this data with millimeter observations of CO and dust continuum emission allows us to trace the penetration of the far-UV radiation field into the cloud and probe the gas density structure throughout the PDR. From comparison with PDR model calculations, we find that i) the gas density follows a steep gradient at the cloud edge, with a scale length of 0.02 pc (or 10'') and $n_H\sim 10^4$ and $10^5$ cm$^{-3}$ in the H$_2$ emitting and inner cold molecular layers respectively, and ii) this density gradient model is essentially a constant pressure model, with $P\sim$4 $10^6$ K cm$^{-3}$. The constraints derived here on the gas density profile are important for the study of physical and chemical processes in PDRs and provide new insight into the evolution of interstellar clouds.Comment: To be published in A&

Molecular Inventories and Chemical Evolution of Low-mass Protostellar Envelopes

This paper presents the first substantial study of the chemistry of the envelopes around a sample of 18 low-mass pre- and protostellar objects for which physical properties have previously been derived from radiative transfer modeling of their dust continuum emission. Single-dish line observations of 24 transitions of 9 molecular species (not counting isotopes) including HCO+, N2H+, CS, SO, SO2, HCN, HNC, HC3N and CN are reported. The line intensities are used to constrain the molecular abundances by comparison to Monte Carlo radiative transfer modeling of the line strengths. An empirical chemical network is constructed on the basis of correlations between the abundances of various species. For example, it is seen that the HCO+ and CO abundances are linearly correlated, both increasing with decreasing envelope mass. Species such as CS, SO and HCN show no trend with envelope mass. In particular no trend is seen between ``evolutionary stage'' of the objects and the abundances of the main sulfur- or nitrogen-containing species. Among the nitrogen-bearing species abundances of CN, HNC and HC3N are found to be closely correlated, which can be understood from considerations of the chemical network. The CS/SO abundance ratio is found to correlate with the abundances of CN and HC3N, which may reflect a dependence on the atomic carbon abundance. An anti-correlation is found between the deuteration of HCO+ and HCN, reflecting different temperature dependences for gas-phase deuteration mechanisms. The abundances are compared to other protostellar environments. In particular it is found that the abundances in the cold outer envelope of the previously studied class 0 protostar IRAS16293-2422 are in good agreement with the average abundances for the presented sample of class 0 objects.Comment: Accepted for publication in A&A. 29 pages, 23 figures. Abstract abridge

Carbon budget and carbon chemistry in Photon Dominated Regions

We present a study of small carbon chains and rings in Photon Dominated Regions (PDRs) performed at millimetre wavelengths. Our sample consists of the Horsehead nebula (B33), the rho,Oph L1688 cloud interface, and the cometary-shaped cloud IC63. Using the IRAM 30-m telescope, the SEST and the Effelsberg 100-m teles cope at Effelsberg., we mapped the emission of \cch, c-C3H2 and C4H, and searched for heavy hydrocarbons such as c-C3H, l-C3H, l-C3H2, l-C4H2 and C6H. The large scale maps show that small hydrocarbons are present until the edge of all PDRs, which is surprising as they are expected to be easily destroyed by UV radiation. Their spatial distribution reasonably agrees with the aromatic emission mapped in mid-IR wavelength bands. Their abundances relative to H2 are relatively high and comparable to the ones derived in dark clouds such as L134N or TMC-1, known as efficient carbon factories. In particular, we report the first detection of C6H in a PDR. We have run steady-state PDR models using several gas-phase chemical networks (UMIST95 and the New Standard Model) and conclude that both networks fail in reproducing the high abundances of some of these hydrocarbons by an order of magnitude. The high abundance of hydrocarbons in the PDR may suggest that the photo-erosion of UV-irradiated large carbonaceous compounds could efficiently feed the ISM with small carbon clusters or molecules. This new production mechanism of carbon chains and rings could overcome their destruction by the UV radiation field. Dedicated theoretical and laboratory measurements are required in order to understand and implement these additional chemical routes.Comment: 18 pages, 12 figure

Variability in the stellar initial mass function at low and high mass: 3-component IMF models

Three component models of the IMF are made to consider possible origins for the observed relative variations in the numbers of brown dwarfs, solar-to-intermediate mass stars, and high mass stars. Three distinct physical processes are noted. The characteristic mass for most star formation is identified with the thermal Jeans mass in the molecular cloud core, and this presumably leads to the middle mass range by the usual collapse and accretion processes. Pre-stellar condensations (PSCs) observed in mm-wave continuum studies presumably form at this mass. Significantly smaller self-gravitating masses require much larger pressures and may arise following dynamical processes inside these PSCs, including disk formation, tight-cluster ejection, and photoevaporation as studied elsewhere, but also gravitational collapse of shocked gas in colliding PSCs. Significantly larger stellar masses form in relatively low abundance by normal cloud processes, possibly leading to steep IMFs in low-pressure field regions, but this mass range can be significantly extended in high pressure cloud cores by gravitationally-focussed gas accretion onto PSCs and by the coalescence of PSCs. These models suggest that the observed variations in brown dwarf, solar-to-intermediate mass, and high mass populations are the result of dynamical effects that depend on environmental density and velocity dispersion. They accommodate observations ranging from shallow IMFs in cluster cores to Salpeter IMFs in average clusters and whole galaxies to steep and even steeper IMFs in field and remote field regions. They also suggest how the top-heavy IMFs in some starburst clusters may originate and they explain bottom-heavy IMFs in low surface brightness galaxies.Comment: 10 pages, 2 figures, accepted by Monthly Notices of the Royal Astronomical Societ

Large Scale CO and [CI] emission in the rho Ophiuchi Molecular Cloud

We present a comprehensive study of the rho Ophiuchi molecular cloud that addresses aspects of the physical structure and condition of the molecular cloud and its photodissociation region (PDR) by combining far-infrared and submillimeter-wave observations with a wide range of angular scale and resolution. We present 40'x40' maps in CO(4-3) and [CI](3P1-3P0) line emission from the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), and pointed observations in the CO(7-6) and [CI](3P2-3P1) lines. Within, smaller spectral line maps of the cloud in CO, CS, HCO+ and their rare isotopomers are made at the Heinrich Hertz Submillimeter Telescope Observatory (HHT) in Arizona. Comparison with far-infrared and submillimeter continuum emission, and near-infrared H2 emission allows clearer determination of the physical and chemical structure of the rho Oph photodissociation region (PDR). The excitation conditions needed to produce the observed HCO+ and [OI] emission directly imply inhomogeneous structure. Strong chemical gradients are observed in HCO+ and CS; the former is ascribed to a local enhancement in the H2 ionization rate, the latter is principally due to shocks. The distribution of [CI] is very similar to C18O, and generally consistent with illumination from the 'far' side of the cloud. A notable exception is found at the the western edge of the cloud, where UV photons create a PDR viewed `edge-on'. The abundance of atomic carbon is accurately modeled using a radiation field that decreases with increasing projected distance from the exciting star HD147889. In contrast to conclusions of other studies, we find that no non-equilibrium chemistry is needed to enhance the atomic carbon abundance.Comment: 17 pages, 21 figures. To be published in the Astrophysical Journal. High resolution color version (PS, PDF formats) available at http://loke.as.arizona.edu/~ckulesa/research/publications/rhooph