MISALIGNED DISKS IN THE BINARY PROTOSTAR IRS 43 Brinch, Christian; Jørgensen, Jes K.; Hogerheijde, Michiel R. ...
Astrophysical journal. Letters,
10/2016, Letnik:
830, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT Recent high angular resolution ( ) ALMA observations of the 1.1 mm continuum and of HCO+ J = 3-2 and HCN J = 3-2 gas toward the binary protostar IRS 43 reveal multiple Keplerian disks that ...are significantly misaligned ( ), both in inclination and position angle and also with respect to the binary orbital plane. Each stellar component has an associated circumstellar disk while the binary is surrounded by a circumbinary disk. Together with archival VLA measurements of the stellar positions over 25 years, and assuming a circular orbit, we use our continuum measurements to determine the binary separation, , and its inclination, . The misalignment in this system suggests that turbulence has likely played a major role in the formation of IRS 43.
Imaging of the CO Snow Line in a Solar Nebula Analog Qi, Chunhua; Öberg, Karin I.; Wilner, David J. ...
Science (American Association for the Advancement of Science),
08/2013, Letnik:
341, Številka:
6146
Journal Article
Recenzirano
Odprti dostop
Planets form in the disks around young stars. Their formation efficiency and composition are intimately linked to the protoplanetary disk locations of "snow lines" of abundant volatiles. We present ...chemical imaging of the carbon monoxide (CO) snow line in the disk around TW Hya, an analog of the solar nebula, using high spatial and spectral resolution Atacama Large Millimeter/Submillimeter Array observations of diazenylium (N 2 H + ), a reactive ion present in large abundance only where CO is frozen out. The N 2 H + emission is distributed in a large ring, with an inner radius that matches CO snow line model predictions. The extracted CO snow line radius of ∼30 astronomical units helps to assess models of the formation dynamics of the solar system, when combined with measurements of the bulk composition of planets and comets.
ABSTRACT An abundance decrease in carbon- and oxygen-bearing species relative to dust has been frequently found in planet-forming disks, which can be attributed to an overall reduction of gas mass. ...However, in the case of TW Hya, the only disk with gas mass measured directly with HD rotational lines, the inferred gas mass ( solar mass) is significantly below the directly measured value ( solar mass). We show that this apparent conflict can be resolved if the elemental abundances of carbon and oxygen are reduced in the upper layers of the outer disk but are normal elsewhere (except for a possible enhancement of their abundances in the inner disk). The implication is that in the outer disk, the main reservoir of the volatiles (CO, water, ...) resides close to the midplane, locked up inside solid bodies that are too heavy to be transported back to the atmosphere by turbulence. An enhancement in the carbon and oxygen abundances in the inner disk can be caused by inward migration of these solid bodies. This is consistent with estimates based on previous models of dust grain dynamics. Indirect measurements of the disk gas mass and disk structure from species such as CO will thus be intertwined with the evolution of dust grains, and possibly also with the formation of planetesimals.
Abstract
The thermal structure of protoplanetary disks is a fundamental characteristic of the system that has wide-reaching effects on disk evolution and planet formation. In this study, we constrain ...the 2D thermal structure of the protoplanetary disk TW Hya structure utilizing images of seven CO lines. This includes new ALMA observations of
12
CO
J
= 2–1 and C
18
O
J
= 2–1 as well as archival ALMA observations of
12
CO
J
= 3–2,
13
CO
J
= 3–2 and 6–5, and C
18
O
J
= 3–2 and 6–5. Additionally, we reproduce a Herschel observation of the HD
J
= 1–0 line flux and the spectral energy distribution and utilize a recent quantification of CO radial depletion in TW Hya. These observations were modeled using the thermochemical code RAC2D, and our best-fit model reproduces all spatially resolved CO surface brightness profiles. The resulting thermal profile finds a disk mass of 0.025
M
⊙
and a thin upper layer of gas depleted of small dust with a thickness of ∼1.2% of the corresponding radius. Using our final thermal structure, we find that CO alone is not a viable mass tracer, as its abundance is degenerate with the total H
2
surface density. Different mass models can readily match the spatially resolved CO line profiles with disparate abundance assumptions. Mass determination requires additional knowledge, and, in this work, HD provides the additional constraint to derive the gas mass and support the inference of CO depletion in the TW Hya disk. Our final thermal structure confirms the use of HD as a powerful probe of protoplanetary disk mass. Additionally, the method laid out in this paper is an employable strategy for extraction of disk temperatures and masses in the future.
We present arcsecond-scale Submillimeter Array observations of the CO(3-2) line emission from the disks around the young stars HD 163296 and TW Hya at a spectral resolution of 44 m s--1. These ...observations probe below the ~100 m s--1 turbulent linewidth inferred from lower-resolution observations, and allow us to place constraints on the turbulent linewidth in the disk atmospheres. We reproduce the observed CO(3-2) emission using two physical models of disk structure: (1) a power-law temperature distribution with a tapered density distribution following a simple functional form for an evolving accretion disk, and (2) the radiative transfer models developed by D'Alessio et al. that can reproduce the dust emission probed by the spectral energy distribution. Both types of models yield a low upper limit on the turbulent linewidth (Doppler b-parameter) in the TW Hya system (40 m s--1) and a tentative (3 Delta *s) detection of a ~300 m s--1 turbulent linewidth in the upper layers of the HD 163296 disk. These correspond to roughly <=10% and 40% of the sound speed at size scales commensurate with the resolution of the data. The derived linewidths imply a turbulent viscosity coefficient, Delta *a, of order 0.01 and provide observational support for theoretical predictions of subsonic turbulence in protoplanetary accretion disks.
Icy bodies may have delivered the oceans to the early Earth, yet little is known about water in the ice-dominated regions of extrasolar planet-forming disks. The Heterodyne Instrument for the ...Far-Infrared on board the Herschel Space Observatory has detected emission lines from both spin isomers of cold water vapor from the disk around the young star TW Hydrae. This water vapor likely originates from ice-coated solids near the disk surface, hinting at a water ice reservoir equivalent to several thousand Earth oceans in mass. The water's ortho-to-para ratio falls well below that of solar system comets, suggesting that comets contain heterogeneous ice mixtures collected across the entire solar nebula during the early stages of planetary birth.
We present Submillimeter Array observations of several deuterated species in the disk around the classical T Tauri star TW Hydrae at arcsecond scales, including detections of the DCN image and DCO ...super(+) image lines and upper limits to the HDO image, ortho-image, and para-image transitions. We also present observations of the HCN image, HCO super(+) image, and H super(13)CO super(+) image lines for comparison with their deuterated isotopologues. We constrain the radial and vertical distributions of various species in the disk by fitting the data using a model where the molecular emission from an irradiated accretion disk is sampled with a two-dimensional Monte Carlo radiative transfer code. We find that the distribution of DCO super(+) differs markedly from that of HCO super(+). The D/H ratios inferred change by at least 1 order of magnitude (0.01-0.1) for radii <30 to >=70 AU, and there is a rapid falloff of the abundance of DCO super(+) at radii larger than 90 AU. Using a simple analytical chemical model, we constrain the degree of ionization, image, to be image10 super(-7) in the disk layer(s) where these molecules are present. Provided the distribution of DCN follows that of HCN, the ratio of DCN to HCN is determined to be image ; however, this ratio is very sensitive to the poorly constrained vertical distribution of HCN. The resolved radial distribution of DCO super(+) indicates that in situ deuterium fractionation remains active within the TW Hydrae disk and must be considered in the molecular evolution of circumstellar accretion disks.
HD 100546 is a well-studied Herbig Be star-disk system that likely hosts a close-in companion with compelling observational evidence for an embedded protoplanet at 68 AU. We present Atacama Large ...Millimeter/Submillimeter Array observations of the HD 100546 disk which resolve the gas and dust structure at (sub)millimeter wavelengths. The CO emission (at 345.795 GHz) originates from an extensive molecular disk (390 + or - 20 AU in radius) whereas the continuum emission is more compact (230 + or - 20 AU in radius ), suggesting radial drift of the millimeter-sized grains. The CO emission is similar in extent to scattered light images indicating well-mixed gas and micrometer-sized grains in the disk atmosphere. Assuming azimuthal symmetry, a single-component power-law model cannot reproduce the continuum visibilities. The visibilities and images are better reproduced by a double-component model: a compact ring with a width of 21 AU centered at 26 AU and an outer ring with a width of 75 + or - 3 AU centered at 190 + or - 3 AU. The influence of a companion and protoplanet on the dust evolution is investigated. The companion at 10 AU facilitates the accumulation of millimeter-sized grains within a compact ring, asymptotically =20-30 AU, by asymptotically = 10 Myr. The injection of a protoplanet at 1 Myr hastens the ring formation (asymptotically =1.2 Myr) and also triggers the development of an outer ring (asymptotically =100-200 AU). These observations provide additional evidence for the presence of a close-in companion and hint at dynamical clearing by a protoplanet in the outer disk.
Context. The abundance of deuterated molecules in a star-forming region is sensitive to the environment in which they are formed. Deuteration fractions, in other words the ratio of a species ...containing D to its hydrogenated counterpart, therefore provide a powerful tool for studying the physical and chemical evolution of a star-forming system. While local low-mass star-forming regions show very high deuteration ratios, much lower fractions are observed towards Orion and the Galactic centre. Astration of deuterium has been suggested as a possible cause for low deuteration in the Galactic centre. Aims. We derive methanol deuteration fractions at a number of locations towards the high-mass star-forming region NGC 6334I, located at a mean distance of 1.3 kpc, and discuss how these can shed light on the conditions prevailing during its formation. Methods. We use high sensitivity, high spatial and spectral resolution observations obtained with the Atacama Large Millimeter/ submillimeter Array to study transitions of the less abundant, optically thin, methanol-isotopologues: 13CH3OH, CH318OH, CH2DOH 13 CH 3 OH , CH 3 18 OH , CH 2 DOH $ {}^{13}{\mathrm{CH}}_3\mathrm{OH},\mathrm{CH}_3^{18}\mathrm{OH},{\mathrm{CH}}_2\mathrm{DOH} $ and CH3OD, detected towards NGC 6334I. Assuming local thermodynamic equilibrium (LTE) and excitation temperatures of ~120–330 K, we derive column densities for each of the species and use these to infer CH2DOH/CH3OH and CH3OD/CH3OH fractions. Results. We derive column densities in a range of (0.8–8.3) × 1017 cm−2 for 13CH3OH, (0.13–3.4) × 1017 cm−2 for CH318OH CH 3 18 OH $ \mathrm{CH}_3^{18}\mathrm{OH} $ , (0.03–1.63) × 1017 cm−2 for CH2DOH and (0.15–5.5) × 1017 cm−2 for CH3OD in a ~1″ beam. Interestingly, the column densities of CH3OD are consistently higher than those of CH2DOH throughout the region by factors of 2–15. We calculate the CH2DOH to CH3OH and CH3OD to CH3OH ratios for each of the sampled locations in NGC 6334I. These values range from 0.03% to 0.34% for CH2DOH and from 0.27% to 1.07% for CH3OD if we use the 13C isotope of methanol as a standard; using the 18 O-methanol as a standard, decreases the ratios by factors of between two and three. Conclusions. All regions studied in this work show CH2DOH/CH3OH as well as CH2DOH/CH3OD values that are considerably lower than those derived towards low-mass star-forming regions and slightly lower than those derived for the high-mass star-forming regions in Orion and the Galactic centre. The low ratios indicate a grain surface temperature during formation ~30 K, for which the efficiency of the formation of deuterated species is significantly reduced. Therefore, astration of deuterium in the Galactic centre cannot be the explanation for its low deuteration ratio but rather the high temperatures characterising the region.
PDF
