David J. Wilner

Evidence for a Developing Gap in a 10 Myr Old Protoplanetary Disk

We have developed a physically self-consistent model of the disk around the nearby 10 Myr old star TW Hya that matches the observed spectral energy distribution and 7 mm images of the disk. The model requires both significant dust-size evolution and a partially evacuated inner disk region, as predicted by theories of planet formation. The outer disk, which extends to at least 140 AU in radius, is very optically thick at infrared wavelengths and quite massive (~0.06 M☉) for the relatively advanced age of this T Tauri star. This implies long viscous and dust evolution timescales, although dust must have grown to sizes of the order of ~1 cm to explain the submillimeter and millimeter spectral slopes. In contrast, the negligible near-infrared excess emission of this system requires that the disk be optically thin inside 4 AU. This inner region cannot be completely evacuated; we need ~0.5 lunar mass of ~1 μm particles remaining to produce the observed 10 μm silicate emission. Our model requires a distinct transition in disk properties at ~4 AU separating the inner and outer disks. The inner edge of the optically thick outer disk must be heated almost frontally by the star to account for the excess flux at mid-infrared wavelengths. We speculate that this truncation of the outer disk may be the signpost of a developing gap due to the effects of a growing protoplanet; the gap is still presumably evolving because material still resides in it, as indicated by the silicate emission, the molecular hydrogen emission, and the continued accretion onto the central star (albeit at a much lower rate than typical of younger T Tauri stars). Thus, TW Hya may become the Rosetta stone for our understanding of the evolution and dissipation of protoplanetary disks.

Resolved Images of Large Cavities in Protoplanetary Transition Disks

Circumstellar disks are thought to experience a rapid transition phase in their evolution that can have a considerable impact on the formation and early development of planetary systems. We present new and archival high angular resolution (03 40-75 AU) Submillimeter Array (SMA) observations of the 880 μm (340 GHz) dust continuum emission from 12 such transition disks in nearby star-forming regions. In each case, we directly resolve a dust-depleted disk cavity around the central star. Using two-dimensional Monte Carlo radiative transfer calculations, we interpret these dust disk structures in a homogeneous, parametric model framework by reproducing their SMA continuum visibilities and spectral energy distributions. The cavities in these disks are large (R cav = 15-73 AU) and substantially depleted of small (~μm-sized) dust grains, although their mass contents are still uncertain. The structures of the remnant material at larger radii are comparable to normal disks. We demonstrate that these large cavities are relatively common among the millimeter-bright disk population, comprising at least 1 in 5 (20%) of the disks in the bright half (and ≥26% of the upper quartile) of the millimeter luminosity (disk mass) distribution. Utilizing these results, we assess some of the physical mechanisms proposed to account for transition disk structures. As has been shown before, photoevaporation models do not produce the large cavity sizes, accretion rates, and disk masses representative of this sample. A sufficient decrease of the dust optical depths in these cavities by particle growth would be difficult to achieve: substantial growth (to meter sizes or beyond) must occur in large (tens of AU) regions of low turbulence without also producing an abundance of small particles. Given those challenges, we suggest instead that the observations are most commensurate with dynamical clearing due to tidal interactions with low-mass companions—very young (~1 Myr) brown dwarfs or giant planets on long-period orbits.

COHERENCE IN DENSE CORES. II. THE TRANSITION TO COHERENCE

After studying how line width depends on spatial scale in low-mass star-forming regions, we propose that dense cores (Myers & Benson 1983) represent an inner scale of a self-similar process that characterizes larger scale molecular clouds. In the process of coming to this conclusion, we define four distinct types of line width-size relation (ΔvRai), which have power-law slopes a1, a2, a3, and a4, as follows: Type 1—multitracer, multicloud intercomparison; Type 2—single-tracer, multicloud intercomparison; Type 3—multitracer study of a single cloud; and Type 4—single-tracer study of a single cloud. Type 1 studies (of which Larson 1981 is the seminal example) are compendia of Type 3 studies which illustrate the range of variation in the line width-size relation from one region to another. Using new measurements of the OH and C18O emission emanating from the environs of several of the dense cores studied in NH3 by Barranco & Goodman (1998; Paper I), we show that line width increases with size outside the cores with a4 ~ 0.2. On scales larger than those traced by C18O or OH,12CO and 13CO observations indicate that a4 increases to ~0.5 (Heyer & Schloerb 1997). By contrast, within the half-power contour of the NH3 emission from the cores, line width is virtually constant, with a4 ~ 0. We interpret the correlation between increasing density and decreasing Type 4 power-law slope as a transition to coherence. Our data indicate that the radius Rcoh at which the gas becomes coherent (i.e., a4 → 0) is of order 0.1 pc in regions forming primarily low-mass stars. The value of the nonthermal line width at which coherence is established is always less than but still of order of the thermal line width of H2. Thus coherent cores are similar to, but not exactly the same as, isothermal balls of gas. Two other results bolster our proposal that a transition to coherence takes place at ~0.1 pc. First, the OH, C18O, and NH3 maps show that the dependence of column density on size is much steeper (N R-0.9) inside Rcoh than outside of it (N R-0.2), which implies that the volume filling factor of coherent cores is much larger than in their surroundings. Second, Larson (1995) has recently found a break in the power law characterizing the clustering of stars in Taurus at 0.04 pc, just inside of Rcoh. Larson and we interpret this break in slope as the point at which stellar clustering properties change from being determined by the (fractal) gas distribution (on scales greater than 0.04 pc) to being determined by fragmentation processes within coherent cores (on scales less than 0.04 pc). We speculate that the transition to coherence takes place when a dissipation threshold for the MHD turbulence that characterizes the larger scale medium is crossed at the critical inner scale Rcoh. We suggest that the most likely explanation for this threshold is the marked decline in the coupling of the magnetic field to gas motions due to a decreased ion/neutral ratio in dense, high filling factor gas.

A Search for Infall Motions toward Nearby Young Stellar Objects

We report observations of 47 candidate protostars in two optically thick lines [H2CO (212-111) and CS (2-1)] and one optically thin line [N2H+ (1-0)] using the IRAM 30 m, SEST 15 m, and Haystack 37 m radio telescopes. The sources were selected for the redness of their spectra (Tbol < 200 K) and their near distance (d < 400 pc). Most of the sources have asymmetric, optically thick lines. The observed distribution of velocity differences, ?V = (Vthick - Vthin)/?Vthin, is skewed toward negative (blueshifted) velocities for both the H2CO and CS samples. This excess is much more significant for class 0 than for class I sources, suggesting that we detect infall motions toward class 0 and not toward class I sources. This indicates a difference in the physical conditions in the circumstellar envelopes around class I and class 0 sources, but it does not rule out the presence of infall onto class I sources by, for example, lower opacity gas. Bipolar outflows alone, or rotation alone, cannot reproduce these statistics if the sample of sources has randomly oriented symmetry axes. We identify 15 spectroscopic infall candidates, six of which are new. Most of these infall candidates have primarily turbulent rather than thermal motions and are associated with clusters rather than being isolated.

Flows of gas through a protoplanetary gap.

The formation of gaseous giant planets is thought to occur in the first few million years after stellar birth. Models predict that the process produces a deep gap in the dust component (shallower in the gas). Infrared observations of the disk around the young star HD 142527 (at a distance of about 140 parsecs from Earth) found an inner disk about 10 astronomical units (au) in radius (1 au is the Earth–Sun distance), surrounded by a particularly large gap and a disrupted outer disk beyond 140 au. This disruption is indicative of a perturbing planetary-mass body at about 90 au. Radio observations indicate that the bulk mass is molecular and lies in the outer disk, whose continuum emission has a horseshoe morphology. The high stellar accretion rate would deplete the inner disk in less than one year, and to sustain the observed accretion matter must therefore flow from the outer disk and cross the gap. In dynamical models, the putative protoplanets channel outer-disk material into gap-crossing bridges that feed stellar accretion through the inner disk. Here we report observations of diffuse CO gas inside the gap, with denser HCO+ gas along gap-crossing filaments. The estimated flow rate of the gas is in the range of 7 × 10−9 to 2 × 10−7 solar masses per year, which is sufficient to maintain accretion onto the star at the present rate.1. Departamento de Astronomı́a, Universidad de Chile, Casilla 36-D, Santiago, Chile 2. Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura 763-0355, Santiago Chile 3. European Southern Observatory (ESO), Casilla 19001, Vitacura, Santiago, Chile 4. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA 5. Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, 1290, Versoix, Switzerland 6. Departamento de Astronomı́a y Astrofı́sica, Pontificia Universidad Católica de Chile, Santiago, Chile 7. UMI-FCA, CNRS / INSU France (UMI 3386) , and Departamento de Astronomı́a, Universidad de Chile, Santiago, Chile. 8. CNRS / UJF Grenoble 1, UMR 5274, Institut de Planétologie et dAstrophysique de Grenoble (IPAG), France 9. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 USA 10. Department of Astronomy, U. C. Berkeley, 601 Campbell Hall, Berkeley, CA 94720 11. Departamento de Fı́sica y Astronomı́a, Universidad Valparaiso, Av. Gran Bretana 111, Valparaiso, Chile. 12. University Observatory, Ludwig-Maximillians University, Munich.

Intense star formation within resolved compact regions in a galaxy at z = 2.3

Massive galaxies in the early Universe have been shown to be forming stars at surprisingly high rates. Prominent examples are dust-obscured galaxies which are luminous when observed at sub-millimetre wavelengths and which may be forming stars at a rate of 1,000 solar masses (M⊙) per year. These intense bursts of star formation are believed to be driven by mergers between gas-rich galaxies. Probing the properties of individual star-forming regions within these galaxies, however, is beyond the spatial resolution and sensitivity of even the largest telescopes at present. Here we report observations of the sub-millimetre galaxy SMMJ2135-0102 at redshift z = 2.3259, which has been gravitationally magnified by a factor of 32 by a massive foreground galaxy cluster lens. This magnification, when combined with high-resolution sub-millimetre imaging, resolves the star-forming regions at a linear scale of only 100 parsecs. We find that the luminosity densities of these star-forming regions are comparable to the dense cores of giant molecular clouds in the local Universe, but they are about a hundred times larger and 107 times more luminous. Although vigorously star-forming, the underlying physics of the star-formation processes at z ≈ 2 appears to be similar to that seen in local galaxies, although the energetics are unlike anything found in the present-day Universe.

Large dust particles in disks around T Tauri stars

We present 7-mm continuum observations of 14 low-mass pre-main-sequence stars in the Taurus-Auriga star-forming region obtained with the Very Large Array with ∼1.5 resolution and ∼0.3 mJy rms sensitivity. For 10 objects, the circumstellar emission has been spatially resolved. The large outer disk radii derived suggest that the emission at this wavelength is mostly optically thin. The millimetre spectral energy distributions are characterised by spectral indices α mm = 2.3 to 3.2. After accounting for contributions from free-free emission and corrections for optical depth, we determine dust opacity indices β in the range 0.5 to 1.6, which suggest that millimetre-sized dust aggregates are present in the circumstellar disks. Four of the sources with β > 1 may be consistent with submicron-sized dust as found in the interstellar medium. Our findings indicate that dust grain growth to millimetre-sized particles is completed within less than 1 Myr for the majority of circumstellar disks.

PROSAC: a submillimeter array survey of low-mass protostars II. The mass evolution of envelopes, disks, and stars from the Class 0 through I stages

Context. The key question about early protostellar evolution is how matter is accreted from the large-scale molecular cloud, through the circumstellar disk onto the central star. Aims. We constrain the masses of the envelopes, disks, and central stars of a sample of low-mass protostars and compare the results to theoretical models for the evolution of young stellar objects through the early protostellar stages. Methods. A sample of 20 Class 0 and I protostars has been observed in continuum at (sub)millimeter wavelengths at high angular resolution (typically 2 �� ) with the submillimeter array. Using detailed dust radiative transfer models of the interferometric data, as well as single-dish continuum observations, we have developed a framework for disentangling the continuum emission from the envelopes and disks, and from that estimated their masses. For the Class I sources in the sample HCO + 3–2 line emission was furthermore observed with the submillimeter array. Four of these sources show signs of Keplerian rotation, making it possible to determine the masses of the central stars. In the other sources the disks are masked by optically thick envelope and outflow emission. Results. Both Class 0 and I protostars are surrounded by disks with typical masses of about 0.05 M� , although significant scatter is seen in the derived disk masses for objects within both evolutionary stages. No evidence is found for a correlation between the disk . .. . ..

A ∼ 0.2-solar-mass protostar with a Keplerian disk in the very young L1527 IRS system

In their earliest stages, protostars accrete mass from their surrounding envelopes through circumstellar disks. Until now, the smallest observed protostar-to-envelope mass ratio was about 2.1 (ref. 1). The protostar L1527 IRS is thought to be in the earliest stages of star formation. Its envelope contains about one solar mass of material within a radius of about 0.05 parsecs (refs 3, 4), and earlier observations suggested the presence of an edge-on disk. Here we report observations of dust continuum emission and 13CO (rotational quantum number J = 2 → 1) line emission from the disk around L1527 IRS, from which we determine a protostellar mass of 0.19 ± 0.04 solar masses and a protostar-to-envelope mass ratio of about 0.2. We conclude that most of the luminosity is generated through the accretion process, with an accretion rate of about 6.6 × 10−7 solar masses per year. If it has been accreting at that rate through much of its life, its age is approximately 300,000 years, although theory suggests larger accretion rates earlier, so it may be younger. The presence of a rotationally supported disk is confirmed, and significantly more mass may be added to its planet-forming region as well as to the protostar itself in the future.

Imaging of the CO Snow Line in a Solar Nebula Analog

Solar Snow Lines Models of the formation of our solar system have suggested that condensation lines, or snow lines—the distance from a star beyond which a gas or a liquid can condense into the solid phase—are favorable locations for planet formation. Taking advantage of the increase of N2H+ abundance in cold regions where CO condenses out of the gas phase, Qi et al. (p. 630, published online 18 July) used the Atacama Large Millimeter/Submillimeter Array to image the CO snow line in the disk around TW Hya, an analog of the solar nebula from which the solar system formed. This disks snow line corresponds to Neptunes orbit in our solar system. Millimeter-wavelength observations locate the carbon monoxide condensation line within the disk around a young planet-forming star. 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 (N2H+), a reactive ion present in large abundance only where CO is frozen out. The N2H+ 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.