A. Isella | Rice University (original) (raw)
Papers by A. Isella
Astronomy & Astrophysics, 2016
Astronomy & Astrophysics, 2015
Eso Astrophysics Symposia, 2008
AMBER is the first near infrared focal instrument of the VLTI. It combines three telescopes and p... more AMBER is the first near infrared focal instrument of the VLTI. It combines three telescopes and produces spectrally resolved interferometric measures. This paper discusses some preliminary results of the first scientific observations of AMBER with three Unit Telescopes at medium (1500) and high (12000) spectral resolution. We derive a first set of constraints on the structure of the circumstellar material around the Wolf Rayet γ 2 Velorum and the LBV η Carinae.
Proceedings of the International Astronomical Union, 2013
ABSTRACT For over a decade, the structure of the inner ``hole'' in the transition... more ABSTRACT For over a decade, the structure of the inner ``hole'' in the transition disk around TW Hydrae has been a subject of debate. To probe the innermost regions of the protoplanetary disk, observations at the highest possible spatial resolution are required. We present new interferometric data of TW Hya from near-infrared to millimeter wavelengths. We confront existing models of the disk structure with the complete data set and develop a new, detailed radiative-transfer model. This model is characterized by: 1) a spatial separation of the largest grains from the small disk grains; and 2) a smooth inner rim structure, rather than a sharp disk edge.
The first steps toward planet formation involve the coagulation of small microscopic grains into ... more The first steps toward planet formation involve the coagulation of small microscopic grains into larger and larger pebbles and rocks in gas-rich disks around young stars and brown dwarfs. Observations in the sub-millimeter can trace mm/cm-sized pebbles in the outer disks, and investigate the mechanisms of coagulation/fragmentation and radial migration of these solids. These represent key, yet not fully understood ingredients for our understanding of the formation of planetesimals, the building blocks of planets. Here we present the first results from an observational program using the ALMA and CARMA submm/mm interferometers aimed at characterizing the dust properties and disk structure of young disks around brown dwarfs and very low mass stars. Given the physical conditions expected for these disks, they represent critical test beds for the models of the early stages of planet formation in proto-planetary disks.
The Astrophysical Journal, 2014
Eso Astrophysics Symposia, 2008
The young stellar object MWC 297 is a B1.5Ve star exhibiting strong hydrogen emission lines. This... more The young stellar object MWC 297 is a B1.5Ve star exhibiting strong hydrogen emission lines. This object has been observed by the AMBER/VLTI instrument in 2-telescope mode in a sub-region of the K spectral band centered around the Brγ line at 2.1656µm. The object has not only been resolved in the continuum with a visibility of 0.50 ± 0.10, but also in the Brγ line, where the flux is about twice larger, with a visibility about twice smaller (0.33 ± 0.06). The continuum emission is consistent with the expectation of an optically thick thermal emission from dust in a circumstellar disk. The hydrogen emission can be understood by the emission of a halo above the disk surface. It can be modelled as a latitudinal-dependant wind model and it explains the width, the strength and the visibibility through the emission lines. The AMBER data associated with a high resolution ISAAC spectrum constrains the apparent size of the wind but also its kinematics.
The Astrophysical Journal, 2014
We present ALMA continuum and spectral line data at 0.89 mm and 3.2 mm for three disks surroundin... more We present ALMA continuum and spectral line data at 0.89 mm and 3.2 mm for three disks surrounding young brown dwarfs and very low mass stars in the Taurus star forming region. Dust thermal emission is detected and spatially resolved for all the three disks, while CO(J = 3 − 2) emission is seen in two disks. We analyze the continuum visibilities and constrain the disks physical structure in dust. The results of our analysis show that the disks are relatively large, the smallest one with an outer radius of about 70 AU. The inferred disk radii, radial profiles of the dust surface density and disk to central object mass ratios lie within the ranges found for disks around more massive young stars. We derive from our observations the wavelength dependence of the millimeter dust opacity. In all the three disks data are consistent with the presence of grains with at least millimeter sizes, as also found for disks around young stars, and confirm that the early stages of the solid growth toward planetesimals occur also around very low mass objects. We discuss the implications of our findings on models of solids evolution in protoplanetary disks, on the main mechanisms proposed for the formation of brown dwarfs and very low mass stars, as well as on the potential of finding rocky and giant planets around very low mass objects.
Proceedings of the International Astronomical Union, 2007
We investigate the behaviour of dust in protoplanetary disks under the action of gas drag using o... more We investigate the behaviour of dust in protoplanetary disks under the action of gas drag using our 3D, two-fluid (gas+dust) SPH code. We present the evolution of the dust spatial distribution in global simulations of planetless disks as well as of disks containing an already formed planet. The resulting dust structures vary strongly with particle size and planetary gaps are much sharper than in the gas phase, making them easier to detect with ALMA than anticipated. We also find that there is a range of masses where a planet can open a gap in the dust layer whereas it doesn't in the gas disk. Our dust distributions are fed to the radiative transfer code MCFOST to compute synthetic images, in order to derive constraints on the settling and growth of dust grains in observed disks.
The Astrophysical Journal, 2014
We present sub-arcsecond resolution observations of continuum emission associated with the GG Tau... more We present sub-arcsecond resolution observations of continuum emission associated with the GG Tau quadruple star system at wavelengths of 1.3, 2.8, 7.3, and 50 mm. These data confirm that the GG Tau A binary is encircled by a circumbinary ring at a radius of 235 AU with a FWHM width of ∼60 AU. We find no clear evidence for a radial gradient in the spectral shape of the ring, suggesting that the particle size distribution is spatially homogeneous on angular scales 0. ′′ 1. A central point source, likely associated with the primary component (GG Tau Aa), exhibits a composite spectrum from dust and free-free emission. Faint emission at 7.3 mm is observed toward the low-mass star GG Tau Ba, although its origin remains uncertain. Using these measurements of the resolved, multifrequency emission structure of the GG Tau A system, models of the far-infrared to radio spectrum are developed to place constraints -2on the grain size distribution and dust mass in the circumbinary ring. The non-negligible curvature present in the ring spectrum implies a maximum particle size of 1-10 mm, although we are unable to place strong constraints on the distribution shape. The corresponding dust mass is 30-300 M ⊕ , at a temperature of 20-30 K. We discuss how this significant concentration of relatively large particles in a narrow ring at a large radius might be produced in a local region of higher gas pressures (i.e., a particle "trap") located near the inner edge of the circumbinary disk.
Monthly Notices of the Royal Astronomical Society, 2012
The intermediate-to high-mass star-forming region IRAS 20343+4129 is an excellent laboratory to s... more The intermediate-to high-mass star-forming region IRAS 20343+4129 is an excellent laboratory to study the influence of high-and intermediate-mass young stellar objects on nearby starless dense cores, and investigate for possible implications in the clustered star formation process. We present 3 mm observations of continuum and rotational transitions of several molecular species (C 2 H, c-C 3 H 2 , N 2 H + , NH 2 D) obtained with the Combined Array for Research in Millimetre-wave Astronomy, as well as 1.3 cm continuum and NH 3 observations carried out with the Very Large Array, to reveal the properties of the dense gas. We confirm undoubtedly previous claims of an expanding cavity created by an ultracompact Hii region associated with a young B2 zero-age main sequence (ZAMS) star. The dense gas surrounding the cavity is distributed in a filament that seems squeezed in between the cavity and a collimated outflow associated with an intermediate-mass protostar. We have identified 5 millimeter continuum condensations in the filament. All of them show column densities consistent with potentially being the birthplace of intermediate-to high-mass objects. These cores appear different from those observed in low-mass clustered environments in sereval observational aspects (kinematics, temperature, chemical gradients), indicating a strong influence of the most massive and evolved members of the protocluster. We suggest a possible scenario in which the B2 ZAMS star driving the cavity has compressed the surrounding gas, perturbed its properties and induced the star formation in its immediate surroundings.
The Astrophysical Journal, 2013
We present CARMA 1.3 mm continuum data of the disk surrounding the young brown dwarf 2MASS J04442... more We present CARMA 1.3 mm continuum data of the disk surrounding the young brown dwarf 2MASS J044427+2512 in the Taurus molecular cloud. The high angular resolution of the CARMA observations (0.16 ′′ ) allows us to spatially resolve for the first time the thermal emission from dust around a brown dwarf. We analyze the interferometric visibilities and constrain the disk outer radius adopting disk models with power-law radial profiles of the dust surface density. In the case of a power-law index ≤ 1, we obtain a disk radius in the range of about 15 -30 AU, while larger disks are inferred for steeper radial profiles. By combining this information on the disk spatial extent with the sub-mm spectral index of this source we find conclusive evidence for mm-sized grains, or larger, in this brown dwarf disk. We discuss the implications of our results on the models of dust evolution in proto-planetary disks and brown dwarf formation.
Astronomy & Astrophysics, 2014
Context. Transition disks are protoplanetary disks with inner depleted dust cavities that are exc... more Context. Transition disks are protoplanetary disks with inner depleted dust cavities that are excellent candidates for investigating the dust evolution when there is a pressure bump. A pressure bump at the outer edge of the cavity allows dust grains from the outer regions to stop their rapid inward migration towards the star and to efficiently grow to millimetre sizes. Dynamical interactions with planet(s) have been one of the most exciting theories to explain the clearing of the inner disk. Aims. We look for evidence of millimetre dust particles in transition disks by measuring their spectral index α mm with new and available photometric data. We investigate the influence of the size of the dust depleted cavity on the disk integrated millimetre spectral index. Methods. We present the 3-millimetre (100 GHz) photometric observations carried out with the Plateau de Bure interferometer of four transition disks: LkHα 330, UX Tau A, LRLL 31, and LRLL 67. We used the available values of their fluxes at 345 GHz to calculate their spectral index, as well as the spectral index for a sample of twenty transition disks. We compared the observations with two kinds of models. In the first set of models, we considered coagulation and fragmentation of dust in a disk in which a cavity is formed by a massive planet located at different positions. The second set of models assumes disks with truncated inner parts at different radii and with power-law dust-size distributions, where the maximum size of grains is calculated considering turbulence as the source of destructive collisions. Results. We show that the integrated spectral index is higher for transition disks (TD) than for regular protoplanetary disks (PD) with mean values ofᾱ TD mm = 2.70 ± 0.13 andᾱ PD mm = 2.20 ± 0.07 respectively. For transition disks, the probability that the measured spectral index is positively correlated with the cavity radius is 95%. High angular resolution imaging of transition disks is needed to distinguish between the dust trapping scenario and the truncated disk case.
Astronomy & Astrophysics, 2011
Context. Circumstellar disks surrounding young stars are known to be the birthplaces of planetary... more Context. Circumstellar disks surrounding young stars are known to be the birthplaces of planetary systems, and the innermost astronomical unit is of particular interest. Near-infrared interferometric studies have revealed a complex morphology for the close environment surrounding Herbig Ae stars. Aims. We present new long-baseline spectro-interferometric observations of the Herbig Ae star, HR 5999, obtained in the H and K bands with the AMBER instrument at the VLTI, and aim to produce near-infrared images at the sub-AU spatial scale. Methods. We spatially resolve the circumstellar material and reconstruct images in the H and K bands using the MiRA algorithm. In addition, we interpret the interferometric observations using models that assume that the near-infrared excess is dominated by the emission of a circumstellar disk. We compare the images reconstructed from the VLTI measurements to images obtained using simulated model data.
Astronomy & Astrophysics, 2011
Context. Grain growth in circumstellar disks is expected to be the first step towards the formati... more Context. Grain growth in circumstellar disks is expected to be the first step towards the formation of planetary systems. There is now evidence for grain growth in several disks around young stars. Aims. Radially resolved images of grain growth in circumstellar disks are believed to be a powerful tool to constrain the dust evolution models and the initial stage for the formation of planets. In this paper we attempt to provide these constraints for the disk surrounding the young star CQ Tau. This system was already suggested from previous studies to host a population of grains grown to large sizes. Methods. We present new high angular resolution (0. 3−0. 9) observations at wavelengths from 850 μm to 3.6 cm obtained at the SMA, IRAM-PdBI and NRAO-VLA interferometers. We perform a combined analysis of the spectral energy distribution and of the high-resolution images at different wavelengths using a model to describe the dust thermal emission from the circumstellar disk. We include a prescription for the gas emission from the inner regions of the system. Results. We detect the presence of evolved dust by constraining the disk averaged dust opacity coefficient β (computed between 1.3 and 7 mm) to be 0.6 ± 0.1. This confirms the earlier suggestions that the disk contains dust grains grown to significant sizes and puts this on firmer grounds by tightly constraining the gas contamination to the observed fluxes at mm-cm wavelengths. We report some evidence of radial variations in dust properties, but current resolution and sensitivity are still too low for definitive results.
Astronomy and Astrophysics, 2007
We present new multi-wavelength millimeter interferometric observations of the Herbig Ae star HD ... more We present new multi-wavelength millimeter interferometric observations of the Herbig Ae star HD 163296 obtained with the IRAM/PBI, SMA and VLA arrays both in continuum and in the 12 CO, 13 CO and C 18 O emission lines. Gas and dust properties have been obtained comparing the observations with self-consistent disk models for the dust and CO emission. The circumstellar disk is resolved both in the continuum and in CO. We find strong evidence that the circumstellar material is in Keplerian rotation around a central star of 2.6 M⊙. The disk inclination with respect to the line of sight is 46 o ±4 o with a position angle of 128 o ±4 o . The slope of the dust opacity measured between 0.87 and 7 mm (β = 1) confirms the presence of mm/cm-size grains in the disk midplane. The dust continuum emission is asymmetric and confined inside a radius of 200 AU while the CO emission extends up to 540 AU. The comparison between dust and CO temperature indicates that CO is present only in the disk interior. Finally, we obtain an increasing depletion of CO isotopomers from 12 CO to 13 CO and C 18 O. We argue that these results support the idea that the disk of HD 163296 is strongly evolved. In particular, we suggest that there is a strong depletion of dust relative to gas outside 200 AU; this may be due to the inward migration of large bodies that form in the outer disk or to clearing of a large gap in the dust distribution by a low mass companion.
Astronomy and Astrophysics, 2008
We investigate the origin of the near-infrared emission of the Herbig Ae star MWC 758 on sub-astr... more We investigate the origin of the near-infrared emission of the Herbig Ae star MWC 758 on sub-astronomical unit (AU) scales using spectrally dispersed low resolution (R = 35) AMBER/VLTI interferometric observations in the H (1.7 µm) and K (2.2 µm) bands. We find that the K band visibilities and closure phases are consistent with the presence of a dusty disk inner rim located at the dust evaporation distance (0.4 AU) while the bulk of the H band emission arises within 0.1 AU from the central star. Comparing the observational results with theoretical model predictions, we suggest that the H band emission is dominated by an hot gaseous accretion disk.
Astronomy & Astrophysics, 2013
ABSTRACT Grain growth in protoplanetary disks is the first step towards the formation of the rock... more ABSTRACT Grain growth in protoplanetary disks is the first step towards the formation of the rocky cores of planets. Models predict that grains grow, migrate, and fragment in the disk and predict varying dust properties as a function of radius, age, and physical properties. High-angular resolution observations at more than one (sub-)mm wavelength are the essential tool for constraining grain growth and migration on the disk midplane. We developed a procedure to analyze self-consistently multi wavelength (sub-)mm continuum interferometric observations of protoplanetary disks to constrain the radial distribution of dust properties. We apply this technique to existing multi frequency continuum mm observations of the disk around CQ Tau, a A8 pre-main sequence star with a well-studied disk. We demonstrate that our models can be used to simultaneously constrain the disk and dust structure. In CQ Tau, the best-fitting model has a radial dependence of the maximum grain size, which decreases from a few cm in the inner disk (<40 AU) to a few mm at 80 AU. Nevertheless, the currently available dataset does not allow us to exclude the possibility of a uniform grain size distribution at a 3sigma level.
Astronomy and Astrophysics, 2010
Context. Accretion is a fundamental process in star formation. Although the time evolution of acc... more Context. Accretion is a fundamental process in star formation. Although the time evolution of accretion remains a matter of debate, observations and modelling studies suggest that episodic outbursts of strong accretion may dominate the formation of the central protostar. Observing young stellar objects during these elevated accretion states is crucial to understanding the origin of unsteady accretion. Aims. Z CMa is a pre-main-sequence binary system composed of an embedded Herbig Be star, undergoing photometric outbursts, and a FU Orionis star. This system therefore provides a unique opportunity to study unsteady accretion processes. The Herbig Be component recently underwent its largest optical photometric outburst detected so far. We aim to constrain the origin of this outburst by studying the emission region of the HI Br γ line, a powerful tracer of accretion/ejection processes on the AU-scale in young stars. Methods. Using the AMBER/VLTI instrument at spectral resolutions of 1500 and 12 000, we performed spatially and spectrally resolved interferometric observations of the hot gas emitting across the Br γ emission line, during and after the outburst. From the visibilities and differential phases, we derive characteristic sizes for the Br γ emission and spectro-astrometric measurements across the line, with respect to the continuum. Results. We find that the line profile, the astrometric signal, and the visibilities are inconsistent with the signature of either a Keplerian disk or infall of matter. They are, instead, evidence of a bipolar wind, maybe partly seen through a disk hole inside the dust sublimation radius. The disappearance of the Br γ emission line after the outburst suggests that the outburst is related to a period of strong mass loss rather than a change of the extinction along the line of sight. Conclusions. Apart from the photometric increase of the system, the main consequence of the outburst is to trigger a massive bipolar outflow from the Herbig Be component. Based on these conclusions, we speculate that the origin of the outburst is an event of enhanced mass accretion, similar to those occuring in EX Ors and FU Ors.
Astronomy & Astrophysics, 2016
Astronomy & Astrophysics, 2015
Eso Astrophysics Symposia, 2008
AMBER is the first near infrared focal instrument of the VLTI. It combines three telescopes and p... more AMBER is the first near infrared focal instrument of the VLTI. It combines three telescopes and produces spectrally resolved interferometric measures. This paper discusses some preliminary results of the first scientific observations of AMBER with three Unit Telescopes at medium (1500) and high (12000) spectral resolution. We derive a first set of constraints on the structure of the circumstellar material around the Wolf Rayet γ 2 Velorum and the LBV η Carinae.
Proceedings of the International Astronomical Union, 2013
ABSTRACT For over a decade, the structure of the inner ``hole'' in the transition... more ABSTRACT For over a decade, the structure of the inner ``hole'' in the transition disk around TW Hydrae has been a subject of debate. To probe the innermost regions of the protoplanetary disk, observations at the highest possible spatial resolution are required. We present new interferometric data of TW Hya from near-infrared to millimeter wavelengths. We confront existing models of the disk structure with the complete data set and develop a new, detailed radiative-transfer model. This model is characterized by: 1) a spatial separation of the largest grains from the small disk grains; and 2) a smooth inner rim structure, rather than a sharp disk edge.
The first steps toward planet formation involve the coagulation of small microscopic grains into ... more The first steps toward planet formation involve the coagulation of small microscopic grains into larger and larger pebbles and rocks in gas-rich disks around young stars and brown dwarfs. Observations in the sub-millimeter can trace mm/cm-sized pebbles in the outer disks, and investigate the mechanisms of coagulation/fragmentation and radial migration of these solids. These represent key, yet not fully understood ingredients for our understanding of the formation of planetesimals, the building blocks of planets. Here we present the first results from an observational program using the ALMA and CARMA submm/mm interferometers aimed at characterizing the dust properties and disk structure of young disks around brown dwarfs and very low mass stars. Given the physical conditions expected for these disks, they represent critical test beds for the models of the early stages of planet formation in proto-planetary disks.
The Astrophysical Journal, 2014
Eso Astrophysics Symposia, 2008
The young stellar object MWC 297 is a B1.5Ve star exhibiting strong hydrogen emission lines. This... more The young stellar object MWC 297 is a B1.5Ve star exhibiting strong hydrogen emission lines. This object has been observed by the AMBER/VLTI instrument in 2-telescope mode in a sub-region of the K spectral band centered around the Brγ line at 2.1656µm. The object has not only been resolved in the continuum with a visibility of 0.50 ± 0.10, but also in the Brγ line, where the flux is about twice larger, with a visibility about twice smaller (0.33 ± 0.06). The continuum emission is consistent with the expectation of an optically thick thermal emission from dust in a circumstellar disk. The hydrogen emission can be understood by the emission of a halo above the disk surface. It can be modelled as a latitudinal-dependant wind model and it explains the width, the strength and the visibibility through the emission lines. The AMBER data associated with a high resolution ISAAC spectrum constrains the apparent size of the wind but also its kinematics.
The Astrophysical Journal, 2014
We present ALMA continuum and spectral line data at 0.89 mm and 3.2 mm for three disks surroundin... more We present ALMA continuum and spectral line data at 0.89 mm and 3.2 mm for three disks surrounding young brown dwarfs and very low mass stars in the Taurus star forming region. Dust thermal emission is detected and spatially resolved for all the three disks, while CO(J = 3 − 2) emission is seen in two disks. We analyze the continuum visibilities and constrain the disks physical structure in dust. The results of our analysis show that the disks are relatively large, the smallest one with an outer radius of about 70 AU. The inferred disk radii, radial profiles of the dust surface density and disk to central object mass ratios lie within the ranges found for disks around more massive young stars. We derive from our observations the wavelength dependence of the millimeter dust opacity. In all the three disks data are consistent with the presence of grains with at least millimeter sizes, as also found for disks around young stars, and confirm that the early stages of the solid growth toward planetesimals occur also around very low mass objects. We discuss the implications of our findings on models of solids evolution in protoplanetary disks, on the main mechanisms proposed for the formation of brown dwarfs and very low mass stars, as well as on the potential of finding rocky and giant planets around very low mass objects.
Proceedings of the International Astronomical Union, 2007
We investigate the behaviour of dust in protoplanetary disks under the action of gas drag using o... more We investigate the behaviour of dust in protoplanetary disks under the action of gas drag using our 3D, two-fluid (gas+dust) SPH code. We present the evolution of the dust spatial distribution in global simulations of planetless disks as well as of disks containing an already formed planet. The resulting dust structures vary strongly with particle size and planetary gaps are much sharper than in the gas phase, making them easier to detect with ALMA than anticipated. We also find that there is a range of masses where a planet can open a gap in the dust layer whereas it doesn't in the gas disk. Our dust distributions are fed to the radiative transfer code MCFOST to compute synthetic images, in order to derive constraints on the settling and growth of dust grains in observed disks.
The Astrophysical Journal, 2014
We present sub-arcsecond resolution observations of continuum emission associated with the GG Tau... more We present sub-arcsecond resolution observations of continuum emission associated with the GG Tau quadruple star system at wavelengths of 1.3, 2.8, 7.3, and 50 mm. These data confirm that the GG Tau A binary is encircled by a circumbinary ring at a radius of 235 AU with a FWHM width of ∼60 AU. We find no clear evidence for a radial gradient in the spectral shape of the ring, suggesting that the particle size distribution is spatially homogeneous on angular scales 0. ′′ 1. A central point source, likely associated with the primary component (GG Tau Aa), exhibits a composite spectrum from dust and free-free emission. Faint emission at 7.3 mm is observed toward the low-mass star GG Tau Ba, although its origin remains uncertain. Using these measurements of the resolved, multifrequency emission structure of the GG Tau A system, models of the far-infrared to radio spectrum are developed to place constraints -2on the grain size distribution and dust mass in the circumbinary ring. The non-negligible curvature present in the ring spectrum implies a maximum particle size of 1-10 mm, although we are unable to place strong constraints on the distribution shape. The corresponding dust mass is 30-300 M ⊕ , at a temperature of 20-30 K. We discuss how this significant concentration of relatively large particles in a narrow ring at a large radius might be produced in a local region of higher gas pressures (i.e., a particle "trap") located near the inner edge of the circumbinary disk.
Monthly Notices of the Royal Astronomical Society, 2012
The intermediate-to high-mass star-forming region IRAS 20343+4129 is an excellent laboratory to s... more The intermediate-to high-mass star-forming region IRAS 20343+4129 is an excellent laboratory to study the influence of high-and intermediate-mass young stellar objects on nearby starless dense cores, and investigate for possible implications in the clustered star formation process. We present 3 mm observations of continuum and rotational transitions of several molecular species (C 2 H, c-C 3 H 2 , N 2 H + , NH 2 D) obtained with the Combined Array for Research in Millimetre-wave Astronomy, as well as 1.3 cm continuum and NH 3 observations carried out with the Very Large Array, to reveal the properties of the dense gas. We confirm undoubtedly previous claims of an expanding cavity created by an ultracompact Hii region associated with a young B2 zero-age main sequence (ZAMS) star. The dense gas surrounding the cavity is distributed in a filament that seems squeezed in between the cavity and a collimated outflow associated with an intermediate-mass protostar. We have identified 5 millimeter continuum condensations in the filament. All of them show column densities consistent with potentially being the birthplace of intermediate-to high-mass objects. These cores appear different from those observed in low-mass clustered environments in sereval observational aspects (kinematics, temperature, chemical gradients), indicating a strong influence of the most massive and evolved members of the protocluster. We suggest a possible scenario in which the B2 ZAMS star driving the cavity has compressed the surrounding gas, perturbed its properties and induced the star formation in its immediate surroundings.
The Astrophysical Journal, 2013
We present CARMA 1.3 mm continuum data of the disk surrounding the young brown dwarf 2MASS J04442... more We present CARMA 1.3 mm continuum data of the disk surrounding the young brown dwarf 2MASS J044427+2512 in the Taurus molecular cloud. The high angular resolution of the CARMA observations (0.16 ′′ ) allows us to spatially resolve for the first time the thermal emission from dust around a brown dwarf. We analyze the interferometric visibilities and constrain the disk outer radius adopting disk models with power-law radial profiles of the dust surface density. In the case of a power-law index ≤ 1, we obtain a disk radius in the range of about 15 -30 AU, while larger disks are inferred for steeper radial profiles. By combining this information on the disk spatial extent with the sub-mm spectral index of this source we find conclusive evidence for mm-sized grains, or larger, in this brown dwarf disk. We discuss the implications of our results on the models of dust evolution in proto-planetary disks and brown dwarf formation.
Astronomy & Astrophysics, 2014
Context. Transition disks are protoplanetary disks with inner depleted dust cavities that are exc... more Context. Transition disks are protoplanetary disks with inner depleted dust cavities that are excellent candidates for investigating the dust evolution when there is a pressure bump. A pressure bump at the outer edge of the cavity allows dust grains from the outer regions to stop their rapid inward migration towards the star and to efficiently grow to millimetre sizes. Dynamical interactions with planet(s) have been one of the most exciting theories to explain the clearing of the inner disk. Aims. We look for evidence of millimetre dust particles in transition disks by measuring their spectral index α mm with new and available photometric data. We investigate the influence of the size of the dust depleted cavity on the disk integrated millimetre spectral index. Methods. We present the 3-millimetre (100 GHz) photometric observations carried out with the Plateau de Bure interferometer of four transition disks: LkHα 330, UX Tau A, LRLL 31, and LRLL 67. We used the available values of their fluxes at 345 GHz to calculate their spectral index, as well as the spectral index for a sample of twenty transition disks. We compared the observations with two kinds of models. In the first set of models, we considered coagulation and fragmentation of dust in a disk in which a cavity is formed by a massive planet located at different positions. The second set of models assumes disks with truncated inner parts at different radii and with power-law dust-size distributions, where the maximum size of grains is calculated considering turbulence as the source of destructive collisions. Results. We show that the integrated spectral index is higher for transition disks (TD) than for regular protoplanetary disks (PD) with mean values ofᾱ TD mm = 2.70 ± 0.13 andᾱ PD mm = 2.20 ± 0.07 respectively. For transition disks, the probability that the measured spectral index is positively correlated with the cavity radius is 95%. High angular resolution imaging of transition disks is needed to distinguish between the dust trapping scenario and the truncated disk case.
Astronomy & Astrophysics, 2011
Context. Circumstellar disks surrounding young stars are known to be the birthplaces of planetary... more Context. Circumstellar disks surrounding young stars are known to be the birthplaces of planetary systems, and the innermost astronomical unit is of particular interest. Near-infrared interferometric studies have revealed a complex morphology for the close environment surrounding Herbig Ae stars. Aims. We present new long-baseline spectro-interferometric observations of the Herbig Ae star, HR 5999, obtained in the H and K bands with the AMBER instrument at the VLTI, and aim to produce near-infrared images at the sub-AU spatial scale. Methods. We spatially resolve the circumstellar material and reconstruct images in the H and K bands using the MiRA algorithm. In addition, we interpret the interferometric observations using models that assume that the near-infrared excess is dominated by the emission of a circumstellar disk. We compare the images reconstructed from the VLTI measurements to images obtained using simulated model data.
Astronomy & Astrophysics, 2011
Context. Grain growth in circumstellar disks is expected to be the first step towards the formati... more Context. Grain growth in circumstellar disks is expected to be the first step towards the formation of planetary systems. There is now evidence for grain growth in several disks around young stars. Aims. Radially resolved images of grain growth in circumstellar disks are believed to be a powerful tool to constrain the dust evolution models and the initial stage for the formation of planets. In this paper we attempt to provide these constraints for the disk surrounding the young star CQ Tau. This system was already suggested from previous studies to host a population of grains grown to large sizes. Methods. We present new high angular resolution (0. 3−0. 9) observations at wavelengths from 850 μm to 3.6 cm obtained at the SMA, IRAM-PdBI and NRAO-VLA interferometers. We perform a combined analysis of the spectral energy distribution and of the high-resolution images at different wavelengths using a model to describe the dust thermal emission from the circumstellar disk. We include a prescription for the gas emission from the inner regions of the system. Results. We detect the presence of evolved dust by constraining the disk averaged dust opacity coefficient β (computed between 1.3 and 7 mm) to be 0.6 ± 0.1. This confirms the earlier suggestions that the disk contains dust grains grown to significant sizes and puts this on firmer grounds by tightly constraining the gas contamination to the observed fluxes at mm-cm wavelengths. We report some evidence of radial variations in dust properties, but current resolution and sensitivity are still too low for definitive results.
Astronomy and Astrophysics, 2007
We present new multi-wavelength millimeter interferometric observations of the Herbig Ae star HD ... more We present new multi-wavelength millimeter interferometric observations of the Herbig Ae star HD 163296 obtained with the IRAM/PBI, SMA and VLA arrays both in continuum and in the 12 CO, 13 CO and C 18 O emission lines. Gas and dust properties have been obtained comparing the observations with self-consistent disk models for the dust and CO emission. The circumstellar disk is resolved both in the continuum and in CO. We find strong evidence that the circumstellar material is in Keplerian rotation around a central star of 2.6 M⊙. The disk inclination with respect to the line of sight is 46 o ±4 o with a position angle of 128 o ±4 o . The slope of the dust opacity measured between 0.87 and 7 mm (β = 1) confirms the presence of mm/cm-size grains in the disk midplane. The dust continuum emission is asymmetric and confined inside a radius of 200 AU while the CO emission extends up to 540 AU. The comparison between dust and CO temperature indicates that CO is present only in the disk interior. Finally, we obtain an increasing depletion of CO isotopomers from 12 CO to 13 CO and C 18 O. We argue that these results support the idea that the disk of HD 163296 is strongly evolved. In particular, we suggest that there is a strong depletion of dust relative to gas outside 200 AU; this may be due to the inward migration of large bodies that form in the outer disk or to clearing of a large gap in the dust distribution by a low mass companion.
Astronomy and Astrophysics, 2008
We investigate the origin of the near-infrared emission of the Herbig Ae star MWC 758 on sub-astr... more We investigate the origin of the near-infrared emission of the Herbig Ae star MWC 758 on sub-astronomical unit (AU) scales using spectrally dispersed low resolution (R = 35) AMBER/VLTI interferometric observations in the H (1.7 µm) and K (2.2 µm) bands. We find that the K band visibilities and closure phases are consistent with the presence of a dusty disk inner rim located at the dust evaporation distance (0.4 AU) while the bulk of the H band emission arises within 0.1 AU from the central star. Comparing the observational results with theoretical model predictions, we suggest that the H band emission is dominated by an hot gaseous accretion disk.
Astronomy & Astrophysics, 2013
ABSTRACT Grain growth in protoplanetary disks is the first step towards the formation of the rock... more ABSTRACT Grain growth in protoplanetary disks is the first step towards the formation of the rocky cores of planets. Models predict that grains grow, migrate, and fragment in the disk and predict varying dust properties as a function of radius, age, and physical properties. High-angular resolution observations at more than one (sub-)mm wavelength are the essential tool for constraining grain growth and migration on the disk midplane. We developed a procedure to analyze self-consistently multi wavelength (sub-)mm continuum interferometric observations of protoplanetary disks to constrain the radial distribution of dust properties. We apply this technique to existing multi frequency continuum mm observations of the disk around CQ Tau, a A8 pre-main sequence star with a well-studied disk. We demonstrate that our models can be used to simultaneously constrain the disk and dust structure. In CQ Tau, the best-fitting model has a radial dependence of the maximum grain size, which decreases from a few cm in the inner disk (<40 AU) to a few mm at 80 AU. Nevertheless, the currently available dataset does not allow us to exclude the possibility of a uniform grain size distribution at a 3sigma level.
Astronomy and Astrophysics, 2010
Context. Accretion is a fundamental process in star formation. Although the time evolution of acc... more Context. Accretion is a fundamental process in star formation. Although the time evolution of accretion remains a matter of debate, observations and modelling studies suggest that episodic outbursts of strong accretion may dominate the formation of the central protostar. Observing young stellar objects during these elevated accretion states is crucial to understanding the origin of unsteady accretion. Aims. Z CMa is a pre-main-sequence binary system composed of an embedded Herbig Be star, undergoing photometric outbursts, and a FU Orionis star. This system therefore provides a unique opportunity to study unsteady accretion processes. The Herbig Be component recently underwent its largest optical photometric outburst detected so far. We aim to constrain the origin of this outburst by studying the emission region of the HI Br γ line, a powerful tracer of accretion/ejection processes on the AU-scale in young stars. Methods. Using the AMBER/VLTI instrument at spectral resolutions of 1500 and 12 000, we performed spatially and spectrally resolved interferometric observations of the hot gas emitting across the Br γ emission line, during and after the outburst. From the visibilities and differential phases, we derive characteristic sizes for the Br γ emission and spectro-astrometric measurements across the line, with respect to the continuum. Results. We find that the line profile, the astrometric signal, and the visibilities are inconsistent with the signature of either a Keplerian disk or infall of matter. They are, instead, evidence of a bipolar wind, maybe partly seen through a disk hole inside the dust sublimation radius. The disappearance of the Br γ emission line after the outburst suggests that the outburst is related to a period of strong mass loss rather than a change of the extinction along the line of sight. Conclusions. Apart from the photometric increase of the system, the main consequence of the outburst is to trigger a massive bipolar outflow from the Herbig Be component. Based on these conclusions, we speculate that the origin of the outburst is an event of enhanced mass accretion, similar to those occuring in EX Ors and FU Ors.