84 research outputs found

    Calculation and observation of x-ray-sensitive molecules in envelopes of young stellar objects

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    The Atomic and Molecular Content of Disks Around Very Low-mass Stars and Brown Dwarfs

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    There is growing observational evidence that disk evolution is stellar-mass dependent. Here, we show that these dependencies extend to the atomic and molecular content of disk atmospheres. We analyze a unique dataset of high-resolution Spitzer/IRS spectra from 8 very low-mass star and brown dwarf disks. We report the first detections of Ne+, H2, CO2, and tentative detections of H2O toward these faint and low-mass disks. Two of our [NeII] 12.81 micron emission lines likely trace the hot (>5,000 K) disk surface irradiated by X-ray photons from the central stellar/sub-stellar object. The H2 S(2) and S(1) fluxes are consistent with arising below the fully or partially ionized surface traced by the [NeII] emission, in gas at about 600 K. We confirm the higher C2H2/HCN flux and column density ratio in brown dwarf disks previously noted from low-resolution IRS spectra. Our high-resolution spectra also show that the HCN/H2O fluxes of brown dwarf disks are on average higher than those of T Tauri disks. Our LTE modeling hints that this difference extends to column density ratios if H2O lines trace warm > 600 K disk gas. These trends suggest that the inner regions of brown dwarf disks have a lower O/C ratio than those of T Tauri disks which may result from a more efficient formation of non-migrating icy planetesimals. A O/C=1, as inferred from our analysis, would have profound implications on the bulk composition of rocky planets that can form around very low-mass stars and brown dwarfs.Comment: Accepted to Ap

    Continuum Variability of Deeply Embedded Protostars as a Probe of Envelope Structure

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    Stars may be assembled in large growth spurts, however the evidence for this hypothesis is circumstantial. Directly studying the accretion at the earliest phases of stellar growth is challenging because young stars are deeply embedded in optically thick envelopes, which have spectral energy distributions that peak in the far-IR, where observations are difficult. In this paper, we consider the feasibility of detecting accretion outbursts from these younger stars by investigating the timescales for how the protostellar envelope responds to changes in the emission properties of the central source. The envelope heats up in response to an outburst, brightening at all wavelengths and with the emission peak moving to shorter wavelengths. The timescale for this change depends on the time for dust grains to heat and re-emit photons and the time required for the energy to escape the inner, optically-thick portion of the envelope. We find that the dust response time is much shorter than the photon propagation time and thus the timescale over which the emission varies is set by time delays imposed by geometry. These times are hours to days near the peak of the spectral energy distribution and weeks to months in the sub-mm. The ideal location to quickly detect continuum variability is therefore in the mid- to far-IR, near the peak of the spectral energy distribution, where the change in emission amplitude is largest. Searching for variability in sub-mm continuum emission is also feasible, though with a longer time separation and a weaker relationship between the amount of detected emission amplitude and change in central source luminosity. Such observations would constrain accretion histories of protostars and would help to trace the disk/envelope instabilities that lead to stellar growth.Comment: 25 pages, 6 figures, accepted for publication in the Astrophysical Journa

    Testing protostellar disk formation models with ALMA observations

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    Abridged: Recent simulations have explored different ways to form accretion disks around low-mass stars. We aim to present observables to differentiate a rotationally supported disk from an infalling rotating envelope toward deeply embedded young stellar objects and infer their masses and sizes. Two 3D magnetohydrodynamics (MHD) formation simulations and 2D semi-analytical model are studied. The dust temperature structure is determined through continuum radiative transfer RADMC3D modelling. A simple temperature dependent CO abundance structure is adopted and synthetic spectrally resolved submm rotational molecular lines up to Ju=10J_{\rm u} = 10 are simulated. All models predict similar compact components in continuum if observed at the spatial resolutions of 0.5-1"" (70-140 AU) typical of the observations to date. A spatial resolution of \sim14 AU and high dynamic range (>1000> 1000) are required to differentiate between RSD and pseudo-disk in the continuum. The peak-position velocity diagrams indicate that the pseudo-disk shows a flatter velocity profile with radius than an RSD. On larger-scales, the CO isotopolog single-dish line profiles are similar and are narrower than the observed line widths of low-JJ lines, indicating significant turbulence in the large-scale envelopes. However a forming RSD can provide the observed line widths of high-JJ lines. Thus, either RSDs are common or a higher level of turbulence (b0.8 km s1b \sim 0.8 \ {\rm km \ s^{-1}} ) is required in the inner envelope compared with the outer part. Multiple spatially and spectrally resolved molecular line observations are needed. The continuum data give a better estimate on disk masses whereas the disk sizes can be estimated from the spatially resolved molecular lines observations. The general observable trends are similar between the 2D semi-analytical models and 3D MHD RSD simulations.Comment: 16 pages, 14 figures, accepted for publication, A&

    Protoplanetary disk masses from CO isotopologues line emission

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    One of the methods for deriving disk masses relies on direct observations of the gas, whose bulk mass is in the outer cold (T30T\lesssim30K) regions. This zone can be well traced by rotational lines of less abundant CO isotopologues, that probe the gas down to the midplane. The total CO gas mass is then obtained with the isotopologue ratios taken to be constant at the elemental isotope values found in the local ISM. This approach is however imprecise, because isotope selective processes are ignored. The aim of this work is an isotopologue selective treatment of CO isotopologues, in order to obtain a more accurate determination of disk masses. The isotope-selective photodissociation, the main process controlling the abundances of CO isotopologues in the CO-emissive layer, is properly treated for the first time in a full disk model (DALI, Bruderer et al. 2012; Bruderer 2013). The chemistry, thermal balance, line and continuum radiative transfer are all considered together with a chemical network that treats 13^{13}CO, C18^{18}O, C17^{17}O, isotopes of all included atoms, and molecules, as independent species. Isotope selective processes lead to regions in the disk where the isotopologues abundance ratios are considerably different from the elemental ratio. The results of this work show that considering CO isotopologue ratios as constants can lead to an underestimate of disk masses by up to almost two orders of magnitude if grains have grown to larger sizes. This may explain observed discrepancies in mass determinations from different tracers. The dependence of the various isotopologues emission on stellar and disk parameters is investigated. Including CO isotope selective processes is crucial to determine the gas mass of the disk accurately (through ALMA observations) and thus to provide the amount of gas which may eventually form planets or change the dynamics of forming planetary systems.Comment: Accepted for publication in A&A, 16 pages, 10 figures, 4 table

    Probing the protoplanetary disk gas surface density distribution with 13^{13}CO emission

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    It is key to constrain the gas surface density distribution, Sigma_gas, as function of disk radius in protoplanetary disks. In this work we investigate if spatially resolved observations of rarer CO isotopologues may be good tracers of Sigma_gas. Physical-chemical models with different input Sigma_gas(R) are run. The input disk surface density profiles are compared with the simulated 13CO intensity radial profiles to check if and where the two follow each other. There is always an intermediate region in the disk where the slope of the 13CO radial emission profile and Sigma_gas(R) coincide. At small radii the line radial profile underestimates Sigma_gas, as 13CO emission becomes optically thick. The same happens at large radii where the column densities become too low and 13CO is not able to efficiently self-shield. If the gas surface density profile is a simple power-law of the radius, the input power-law index can be retrieved within 20% uncertainty if one choses the proper radial range. If instead Sigma_gas(R) follows the self-similar solution for a viscously evolving disk, retrieving the input power-law index becomes challenging, in particular for small disks. Nevertheless, it is found that the power-law index can be in any case reliably fitted at a given line intensity contour around 6 K km/s, and this produces a practical method to constrain the slope of Sigma_gas(R). Application of such a method is shown in the case study of the TW Hya disk. Spatially resolved 13CO line radial profiles are promising to probe the disk surface density distribution, as they directly trace Sigma_gas(R)profile at radii well resolvable by ALMA. There, chemical processes like freeze-out and isotope selective photodissociation do not affect the emission, and, assuming that the volatile carbon does not change with radius, no chemical model is needed when interpreting the observations.Comment: 14 pages, 10 figures, A&A accepte

    Combining strong and weak lensing estimates in the Cosmos field

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    We present a combined cosmic shear analysis of the modeling of line-of-sight distortions on strongly lensed extended arcs and galaxy shape measurements in the COSMOS field. We develop a framework to predict the covariance of strong lensing and galaxy shape measurements of cosmic shear on the basis of the small scale matter power-spectrum. The weak lensing measurement is performed using data from the COSMOS survey calibrated with a cloning scheme using the Ultra Fast Image Generator UFig (Berge 2013). The strong lensing analysis is performed by forward modeling the lensing arcs with a main lensing deflector and external shear components from the same Hubble Space Telescope imaging data set. With a sample of three strong lensing shear measurements we present a 2-sigma detection of the cross-correlation signal between the two complementary measurements of cosmic shear along the identical line of sight. With large samples of lenses available with the next generation ground and space based observatories, the covariance of the signal of the two probes with large samples of lenses allows for systematic checks, cross-calibration of either of the two measurement and the measurement of the small scale shear power-spectrum.Comment: 27 pages, 7 figures, 4 table

    Resolved gas cavities in transitional disks inferred from CO isotopologues with ALMA

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    Transitional disks around young stars are promising candidates to look for recently formed, embedded planets. Planet-disk interaction models predict that planets clear a gap in the gas while trapping dust at larger radii. Other physical mechanisms could be responsible for cavities as well. Previous observations have revealed that gas is still present inside these cavities, but the spatial distribution of this gas remains uncertain. We present high spatial resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of 13CO and C18O lines of four well-studied transitional disks. The observations are used to set constraints on the gas surface density, specifically cavity size and density drop inside the cavity. The physical-chemical model DALI is used to analyze the gas images of SR21, HD135344B, DoAr44 and IRS48. The main parameters of interest are the size, depth and shape of the gas cavity. CO isotope-selective photodissociation is included to properly constrain the surface density in the outer disk from C18O emission. The gas cavities are up to 3 times smaller than those of the dust in all four disks. Model fits indicate that the surface density inside the gas cavities decreases by a factor of 100-10000 compared with the surface density profile derived from the outer disk. A comparison with an analytical model of gap depths by planet-disk interaction shows that the disk viscosities are likely low, with a<1E-3 for planet masses <10 MJup. The resolved measurements of the gas and dust in transition disk cavities support the predictions of models that describe how planet-disk interactions sculpt gas disk structures and influence the evolution of dust grains. These observed structures strongly suggest the presence of giant planetary companions in transition disk cavities, although at smaller orbital radii than is typically indicated from the dust cavity radii alone.Comment: Accepted by A&A; version after language-editin

    Warm formaldehyde in the Oph IRS 48 transitional disk

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    Simple molecules like H2CO and CH3OH in protoplanetary disks are the starting point for the production of more complex organic molecules. So far, the observed chemical complexity in disks has been limited due to freeze out of molecules onto grains in the bulk of the cold outer disk. Complex molecules can be studied more directly in transitional disks with large inner holes, as these have a higher potential of detection, through UV heating of the outer disk and the directly exposed midplane at the wall. We use Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 (~680 GHz) line data of the transitional disk Oph IRS 48, previously shown to have a large dust trap, to search for complex molecules in regions where planetesimals are forming. We report the detection of the H2CO 9(1,8)-8(1,7) line at 674 GHz, which is spatially resolved as a semi-ring at ~60 AU radius centered south from the star. The inferred H2CO abundance is ~10^{-8} derived by combining a physical disk model of the source with a non-LTE excitation calculation. Upper limits for CH3OH lines in the same disk give an abundance ratio H2CO/CH3OH>0.3, which points to both ice formation and gas-phase routes playing a role in the H2CO production. Upper limits on the abundances of H13CO+, CN and several other molecules in the disk are also derived and found to be consistent with full chemical models. The detection of the H2CO line demonstrates the start of complex organic molecules in a planet-forming disk. Future ALMA observations should be able to push down the abundance detection limits of other molecules by 1-2 orders of magnitude and test chemical models of organic molecules in (transitional) disks.Comment: Updated references and minor changes to text, approved by language edito
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