Dave Lommen

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 . .. . ..

Probing dust grain evolution in IM Lupi's circumstellar disc Multi-wavelength observations and modelling of the dust disc

Aims. We present a panchromatic study, involving a multiple technique approach, of the circumstellar disc surrounding the T Tauri star IM Lupi (Sz 82). Methods. We have undertaken a comprehensive observational study of IM Lupi using photometry, spectroscopy, millimetre interferometry and multi-wavelength imaging. For the first time, the disc is resolved from optical and near-infrared wavelengths in scattered light, to the millimetre regime in thermal emission. Our data-set, in conjunction with existing photometric data, provides an extensive coverage of the spectral energy distribution, including a detailed spectrum of the silicate emission bands. We have performed a simultaneous modelling of the various observations, using the radiative transfer code MCFOST, and analysed a grid of models over a large fraction of the parameter space via Bayesian inference. Results. We have constructed a model that can reproduce all of the observations of the disc. Our analysis illustrates the importance of combining a wide range of observations in order to fully constrain the disc model, with each observation providing a strong constraint only on some aspects of the disc structure and dust content. Quantitative evidence of dust evolution in the disc is obtained: grain growth up to millimetre-sized particles, vertical stratification of dust grains with micrometric grains close to the disc surface and larger grains which have settled towards the disc midplane, and possibly the formation of fluffy aggregates and/or ice mantles around grains.

Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths

Received ?? ; Accepted ?? Abstract. Low-mass stars form with disks in which the coagulation of grains may eventually lead to the formation of planets. It is not known when and where grain growth occurs, as models that explain the observations are often degenerate. A way to break this degeneracy is to resolve the sources under study.

SMA observations of young disks: separating envelope, disk, and stellar masses in class I YSOs

Context. Young stars are born with envelopes, which in the early stages obscure the central (proto)star and circumstellar disk. In the Class I stage, the disks are still young, but the envelopes are largely dispersed. This makes the Class I sources ideal targets for studies of the early stages of disks. Aims. We aim to determine the masses of the envelopes, disks, and central stars of young stellar objects (YSOs) in the Class I stage. Methods. We observed the embedded Class I objects IRS 63 and Elias 29 in the ρ Ophiuchi star-forming region with the Submillimeter Array (SMA) at 1.1 mm. Results. IRS 63 and Elias 29 are both clearly detected in the continuum, with peak fluxes of 459 and 47 mJy/beam, respectively. The continuum emission toward Elias 29 is clearly resolved, whereas IRS 63 is consistent with a point source down to a scale of 3 ��

Grain growth signatures in the protoplanetary discs of Chamaeleon and Lupus

We present Australia Telescope Compact Array results of a 3 and 7 mm continuum survey of 20 T Tauri stars in the Chamaeleon and Lupus star-forming regions. This survey aims to identify protoplanetary discs with signs of grain growth. We detected 90 per cent of the sources at 3 and 7 mm, and determined the spectral slopes, dust opacity indices and dust disc masses. We also present temporal monitoring results of a small subset of sources at 7, 15 mm and 3+6 cm to investigate grain growth to centimetre (cm) sizes and constrain emission mechanisms in these sources. Additionally, we investigated the potential correlation between grain growth signatures in the infrared (10 μm silicate feature) and millimetre (1–3 mm spectral slope, α). Eleven sources at 3 and 7 mm have dominant thermal dust emission up to 7 mm, with seven of these having a 1–3 mm dust opacity index less than unity, suggesting grain growth up to at least mm sizes. The Chamaeleon sources observed at 15 mm and beyond show the presence of excess emission from an ionized wind and/or chromospheric emission. Long-time-scale monitoring at 7 mm indicated that cm-sized pebbles are present in at least four sources. Short-time-scale monitoring at 15 mm suggests that the excess emission is from thermal free–free emission. Finally, a weak correlation was found between the strength of the 10 μm feature and α, suggesting simultaneous dust evolution of the inner and outer parts of the disc. This survey shows that grain growth up to cm-sized pebbles and the presence of excess emission at 15 mm and beyond are common in these systems, and that temporal monitoring is required to disentangle these emission mechanisms.

Grain growth across protoplanetary discs: 10 μm silicate feature versus millimetre slope

Context. Young stars are formed with dusty discs around them. The dust grains in the disc are originally of the same size as interstellar dust, i.e., of the order of 0.1 μm. Models predict that these grains will grow in size through coagulation. Observations of the silicate features around 10 and 20 μm are consistent with growth from submicron to micron sizes in selected sources whereas the slope of the spectral energy distribution (SED) at mm and cm wavelengths traces growth up to mm sizes and larger. Aims. We here look for a correlation between these two grain growth indicators. Methods. A large sample of T-Tauri and Herbig-Ae/Be stars, spread over the star-forming regions in Chamaeleon, Lupus, Serpens, Corona Australis, and the Gum nebula in Vela, was observed with the Spitzer Space Telescope at 5–13 μm, and a subsample was observed with the SMA, ATCA, CARMA, and VLA at mm wavelengths. We complement this subsample with data from the literature to maximise the overlap between μm and mm observations and search for correlations in the grain-growth signatures. Synthetic spectra are produced to determine which processes may produce the dust evolution observed in protoplanetary discs. Results. Dust disc masses in the range <1 to 7 × 10^(-4) M_☉ are obtained. The majority of the sources have a mm spectral slope consistent with grain growth. There is a tentative correlation between the strength and the shape of the 10-μm silicate feature and the slope of the SED between 1 and 3 mm. The observed sources seem to be grouped per star-forming region in the 10-μm-feature vs. mm-slope diagram. The modelling results show that, if only the maximum grain size is increased, first the 10-μm feature becomes flatter and subsequently the mm slope becomes shallower. To explain the sources with the shallowest mm slopes, a grain size distribution shallower than that of the interstellar medium is required. Furthermore, the strongest 10-μm features can only be explained with bright (L ~ 6 L_☉), hot (T_(eff) = 4000 K) central stars. Settling of larger grains towards the disc midplane results in a stronger 10-μm feature, but has a very limited effect on the mm slope. Conclusions. A tentative correlation between the strength of the 10-μm feature and the mm slope is found, which would imply that the inner and outer disc evolve simultaneously. Dust with a mass dominated by large, ~mm-sized, grains is required to explain the shallowest mm slopes. Other processes besides grain growth, such as the clearing of an inner disc by binary interaction, may also be responsible for the removal of small grains. Observations with future telescopes with larger bandwidths or collecting areas are required to provide the necessary statistics to study these processes of disc and dust evolution.

Resolving structure of the disc around HD100546 at 7 mm with ATCA

There is much evidence that planet formation is occurring in the disc around the Herbig Be star HD100546. To learn more about the processes occurring in this disc, we conducted high-resolution imaging at 43/45 GHz with the Australia Telescope Compact Array. Multiple array configurations were used, providing a best spatial resolution of ∼0.15 arcsec, or 15 au at HD100546s distance of ∼100 pc. Significant structure is revealed, but its precise form is dependent on the u − v plane sampling used for the image reconstruction. At a resolution of ≤30 au, we detected an inner gap in the disc with a radius of ∼25 au and a position angle approximately along the known disc major axis. With different weighting, and an achieved resolution of ∼15 au, emission appears at the centre and the disc takes on the shape of an incomplete ring, much like a horseshoe, again with a gap radius of ∼25 au. The position angle of the disc major axis and its inclination from face-on are determined to be 140° ± 5° and 40° ± 5°, respectively. The ∼25 au gap radius is confirmed by a null in the real part of the binned visibilities at 320 ± 10 kλ, whilst the non-axisymmetric nature is also confirmed through significant structure in the imaginary component. The emission mechanism at the central peak is most likely to be free–free emission from a stellar or disc wind. Overall our data support the picture of at least one, but probably several, giant planets orbiting HD100546 within 25 au.

Radio Monitoring of Protoplanetary Discs

Protoplanetary disc systems observed at radio wavelengths often show excess emission above that expected from a simple extrapolation of thermal dust emission observed at short millimetre wavelengths. Monitoring the emission at radio wavelengths can be used to help disentangle the physical mechanisms responsible for this excess, including free-free emission from a wind or jet, and chromospheric emission associated with stellar activity. We present new results from a radio monitoring survey conducted with Australia Telescope Compact Array over the course of several years with observation intervals spanning days, months and years, where the flux variability of 11 T Tauri stars in the Chamaeleon and Lupus star forming regions was measured at 7 and 15 mm and 3 and 6 cm. Results show that for most sources are variable to some degree at 7 mm, indicating the presence of emission mechanisms other than thermal dust in some sources. Additionally, evidence of grain growth to cm-sized pebbles was found for some sources that also have signs of variable flux at 7 mm. We conclude that multiple processes contributing to the emission are common in T Tauri stars at 7 mm and beyond, and that a detection at a single epoch at radio wavelengths should not be used to determine all processes contributing to the emission.

Resolving structure in the HD100546 disk - signatures of planet building?

Several lines of evidence suggest that planet formation may be well underway within the circumstellar disk of the enigmatic Herbig Be star HD100546, including a cleared inner cavity, spiral structure, and similar dust mineralogy as seen in our own solar system. To learn more about the processes occurring in this disk we have conducted a multi-frequency observing program with the Australia Telescope Compact Array (ATCA). We find the millimetre slope of the spectral energy distribution (SED) is α ∼ 2.3 (using fluxes integrated over the entire extent of the emission), suggesting a dust opacity index β ∼ 0.3. From spatially resolved and temporally stable emission over 3 years, we find that the flux at 3, 7 and 16 mm is dominated by thermal emission from dust grains up to several tens of centimetre in size. At all millimetre wavelengths the peak emission is centered 0.5′′ west of the optical stellar position. There is however structure in the emission, especially at 3 mm where we detect a discrete clump about an arcsecond to the south-west coincident with features seen in HST ACS images, and a second clump about 2.5 arcsec to the north. Subarcsecond-resolution 3 mm images show a deficit of emission at the centre of the disk, in agreement with the inner cavity seen in infrared observations. We determine a dust disk FWHM of ∼50–100 AU and mass of ∼ 10 MEarth . The longer wavelength 3.5 and 6.2 cm emission is relatively stable on the time scale of months and years, and we find the centimetre SED is consistent with emission arising from free-free processes. The 3.5 cm emission is elongated orthogonal to the millimetre disk emission, suggestive of a wind, with the data indicating a wind mass loss rate of ∼ 10−8 M /yr. We also present HCO+(1→0) line data which demonstrates the presence of dense molecular gas in the disk, supported by our 12CO (4→3) and (7→6) data from NANTEN2. The line profile is double-peaked, with component velocities at 3.5 and 7.0 km/s, in agreement with APEX 12CO(3→2) data. Each component is coincident with the position of HD100546, but with a slight spatial offset approximately along the disk major axis. If interpreted as Keplerian rotation, the radius of the molecular gas emission is ∼350 AU, with the SE side approaching and the NW side receding from Earth. Our new results show that the pebble-sized grains in the disk of HD100546 are amongst the largest yet observed, and combined with the cleared inner dust cavity might suggest that planet formation is indeed well underway, or that grain sizes are so large that they are becoming invisible at millimetre wavelength. The detection of a large molecular gas disk and a wind, along with ultraviolet accretion signatures, would suggest that the system is still quite immature. Perhaps the gas is needed to allow grains to grow to such large sizes in the inner disk.