977 research outputs found

    Proper motions and velocity asymmetries in the RW Aur jet

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    We present adaptive optics spectro-imaging observations of the RW Aur jet in optical forbidden lines, at an angular resolution of 0.4 arcsec. Comparison with HST data taken 2 years later shows that proper motions in the blueshifted and redshifted lobes are in the same ratio as their radial velocities, a direct proof that the velocity asymmetry in this jet is real and not an emissivity effect. The inferred jet inclination to the line of sight is i = 46 +/- 3 degrees. The inner knot spacing appears best explained by time variability with at least two modes: one irregular and asymmetric (possibly random) on timescales of <3-10 yr, and another more regular with ~ 20 yr period. We also report indirect evidence for correlated velocity and excitation gradients in the redshifted lobe, possibly related to the blue/red velocity and brightness asymmetry in this system.Comment: 4 pags, 3 figure

    NICMOS2 hubble space telescope observations of the embedded cluster associated with Mon R2: Constraining the substellar initial mass function

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    We have analyzed Hubble Space Telescope NICMOS2 F110W-, F160W-, F165M-, and F207M-band images covering the central 1' × 1' region of the cluster associated with Mon R2 in order to constrain the initial mass function (IMF) down to 20M_J. The flux ratio between the F165M and F160W bands was used to measure the strength of the water-band absorption feature and select a sample of 12 out of the total sample of 181 objects that have effective temperatures between 2700 and 3300 K. These objects are placed in the H-R diagram together with sources observed by Carpenter et al. to estimate an age of ~1 Myr for the low-mass cluster population. By constructing extinction-limited samples, we are able to constrain the IMF and the fraction of stars with a circumstellar disk in a sample that is 90% complete for both high- and low-mass objects. For stars with estimated masses between 0.1 and 1.0 M_☉ for a 1 Myr population with A_V ≤ 19 mag, we find that 27% ± 9% have a near-infrared excess indicative of a circumstellar disk. The derived fraction is similar to or slightly lower than the fraction found in other star-forming regions of comparable age. We constrain the number of stars in the mass interval 0.08-1.0 M_☉ to the number of objects in the mass interval 0.02-0.08 M_☉ by forming the ratio R^(**) = N(0.08-1 M_☉)/N(0.02-0.08 M_☉) for objects in an extinction-limited sample complete for A_V ≤ 7 mag. The ratio is found to be R^(**) = 2.2 ± 1.3, assuming an age of 1 Myr, consistent with the similar ratio predicted by the system IMF proposed by Chabrier. The ratio is similar to the ratios observed toward the Orion Nebula Cluster and IC 348, as well as the ratio derived in the 28 deg^2 survey of Taurus by Guieu et al

    Modeling T Tauri Winds from He I 10830 Profiles

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    The high opacity of He I 10830 makes it an exceptionally sensitive probe of the inner wind geometry of accreting T Tauri stars. In this line blueshifted absorption below the continuum results from simple scattering of stellar photons, a situation which is readily modeled without definite knowledge of the physical conditions and recourse to multi-level radiative transfer. We present theoretical line profiles for scattering in two possible wind geometries, a disk wind and a wind emerging radially from the star, and compare them to observed He I 10830 profiles from a survey of classical T Tauri stars. The comparison indicates that subcontinuum blueshifted absorption is characteristic of disk winds in ~30% of the stars and of stellar winds in ~40%. We further conclude that for many stars the emission profile of helium likely arises in stellar winds, increasing the fraction of accreting stars inferred to have accretion-powered stellar winds to ~60%. Stars with the highest disk accretion rates are more likely to have stellar wind than disk wind signatures and less likely to have redshifted absorption from magnetospheric funnel flows. This suggests the possibility that when accretion rates are high, disks can extend closer to the star, magnetospheric accretion zones can be reduced in size and conditions arise that favor radially outflowing stellar winds.Comment: 41 pages, 11 figures. Accepted by Astrophysical Journa

    Origin of the wide-angle hot H2 in DG Tauri: New insight from SINFONI spectro-imaging

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    We wish to test the origins proposed for the extended hot H2 at 2000K around the atomic jet from the T Tauri star DGTau, in order to constrain the wide-angle wind structure and the possible presence of an MHD disk wind. We present flux calibrated IFS observations in H2 1-0 S(1) obtained with SINFONI/VLT. Thanks to spatial deconvolution by the PSF and to accurate correction for uneven slit illumination, we performed a thorough analysis and modeled the morphology, kinematics, and surface brightness. We also compared our results with studies in [FeII], [OI], and FUV-pumped H2. The limb-brightened H2 emission in the blue lobe is strikingly similar to FUV-pumped H2 imaged 6yr later, confirming that they trace the same hot gas and setting an upper limit of 12km/s on any expansion proper motion. The wide-angle H2 rims are at lower blueshifts than probed by narrow long-slit spectra. We confirm that they extend to larger angle and to lower speed the onion-like velocity structure observed in optical atomic lines. The latter is shown to be steady over more/equal than 4yr but undetected in [FeII] by SINFONI, probably due to strong iron depletion. The H2 rim thickness less/equal than 14AU rules out excitation by C-shocks, and J-shock speeds are constrained to 10km/s. We find that explaining the H2 wide-angle emission with a shocked layer requires either a recent outburst (15yr) into a pre-existing ambient outflow or an excessive wind mass flux. A slow photoevaporative wind from the dense irradiated disk surface and an MHD disk wind heated by ambipolar diffusion seem to be more promising and need to be modeled in more detail

    PdBI sub-arcsecond study of the SiO microjet in HH212 - Origin and collimation of Class 0 jets

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    The bipolar HH 212 outflow has been mapped in SiO using the extended configuration of the Plateau de Bure Interferometer (PdBI), revealing a highly collimated SiO jet closely associated with the H2 jet component. We study at unprecedented resolution (0.34" across the jet axis) the properties of the innermost SiO ``microjet'' within 1000 AU of this young Class 0 source, to compare it with atomic microjets from more evolved sources and to constrain its origin. The SiO channel maps are used to investigate the microjet collimation and velocity structure. A large velocity gradient analysis is applied to SiO (2-1), (5-4) and (8-7) data from the PdBI and the Submillimeter Array to constrain the SiO opacity and abundance. The HH212 Class 0 microjet shows striking similarities in collimation and energetic budget with atomic microjets from T Tauri sources. Furthermore, the SiO lines appear optically thick, unlike what is generally assumed. We infer T(kin) ~ 50-500 K and an SiO/H2 abundance greater than 4 10(-8)-6 10(-5) for n(H2) = 10(7)-10(5) cm(-3), i.e. 0.05-90% of the elemental silicon. This similar jet width, regardless of the presence of a dense envelope, definitely rules out jet collimation by external pressure, and favors a common MHD self-collimation (and possibly acceleration) process at all stages of star formation. We propose that the more abundant SiO in Class 0 jets could mainly result from rapid (less than 25 yrs) molecular synthesis at high jet densities

    The inner environment of Z~CMa: High-Contrast Imaging Polarimetry with NaCo

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    Context. Z\,CMa is a binary composed of an embedded Herbig Be and an FU Ori class star separated by ∼100\sim100 au. Observational evidence indicate a complex environment in which each star has a circumstellar disk and drives a jet, and the whole system is embedded in a large dusty envelope. Aims. We aim to probe the circumbinary environment of Z\,CMa in the inner 400 au in scattered light. Methods. We use high contrast imaging polarimetry with VLT/NaCo at HH and KsK_s bands. Results. The central binary is resolved in both bands. The polarized images show three bright and complex structures: a common dust envelope, a sharp extended feature previously reported in direct light, and an intriguing bright clump located 0\farcs3 south of the binary, which appears spatially connected to the sharp extended feature. Conclusions.We detect orbital motion when compared to previous observations, and report a new outburst driven by the Herbig star. Our observations reveal the complex inner environment of Z\,CMa with unprecedented detail and contrast.Comment: Accepted for publication in A&A Letter

    Four Brown Dwarfs in the Taurus Star-Forming Region

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    We have identified four brown dwarfs in the Taurus star-forming region. They were first selected from RR and II CCD photometry of 2.29 square degrees obtained at the Canada-France-Hawaii Telescope. Subsequently, they were recovered in the 2MASS second incremental data release point source catalog. Low-resolution optical spectra obtained at the William Herschel telescope allow us to derive spectral types in the range M7--M9. One of the brown dwarfs has very strong Hα\alpha emission (EW=-340 \AA). It also displays Brγ\gamma emission in an infrared spectrum obtained with IRCS on the Subaru telescope, suggesting that it is accreting matter from a disk. The \ion{K}{1} resonance doublet and the \ion{Na}{1} subordinate doublet at 818.3 and 819.5 nm in these Taurus objects are weaker than in field dwarfs of similar spectral type, consistent with low surface gravities as expected for young brown dwarfs. Two of the objects are cooler and fainter than GG Tau Bb, the lowest mass known member of the Taurus association. We estimate masses of only 0.03 M⊙_\odot for them. The spatial distribution of brown dwarfs in Taurus hints to a possible anticorrelation between the density of stars and the density of brown dwarfs.Comment: ApJ Letters (in press
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