A. Banzatti

Space Telescope Science Institute, Baltimore, Maryland, United States

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Publications (7)23.32 Total impact

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    ABSTRACT: We investigate new and archival 3-5 $\mu$m high resolution ($\sim3$ km s$^{-1}$) spectroscopy of molecular gas in the inner disk of the young solar-mass star EX Lupi, taken during and after the strong accretion outburst of 2008. The data were obtained using the CRIRES spectrometer at the ESO Very Large Telescope in 2008 and 2014. In 2008, emission lines from CO, H$_{2}$O, and OH were detected with broad profiles tracing gas near and within the corotation radius (0.02-0.3 AU). In 2014, the spectra display marked differences. The CO lines, while still detected, are much weaker, and the H$_{2}$O and OH lines have disappeared altogether. At 3 $\mu$m a veiled stellar photospheric spectrum is observed. Our analysis finds that the molecular gas mass in the inner disk has decreased by an order of magnitude since the outburst, matching a similar decrease in the accretion rate onto the star. We discuss these findings in the context of a rapid depletion of material accumulated beyond the disk corotation radius during quiescent periods, as proposed by models of episodic accretion in EXor type young stars.
    12/2014; 798(1). DOI:10.1088/2041-8205/798/1/L16
  • Andrea Banzatti, M. Meyer, K. Pontoppidan
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    ABSTRACT: Recent analyses of mid-infrared spectra have shown that warm molecular gas (mainly water, OH, and simple organic molecules) is commonly detected in the inner regions of T Tauri disks and might be an important tracer for the chemical and physical evolution of the terrestrial planet formation region. Many studies suggest that the composition of gas and dust in circumstellar disks (inherited from the ISM and evolved through the protostellar phase) can be further altered by several processes relevant for planet formation. The outcome of this evolution may have important implications concerning the diversity of planetary systems, the composition of planetary surfaces and atmospheres, and on planet habitability. Molecular abundances in the inner disk are a privileged tracer of both the local irradiation environment and the radial transport of icy bodies that evaporate after crossing the snowline, the building blocks of rocky planets. Water and other molecules in the gas phase are therefore strongly connected to our understanding of disk evolution and planet formation processes. In this presentation I will show the highlights from the research I have done during my PhD, using mid-infrared spectroscopy from Spitzer and VLT-VISIR to address: 1) the effect of variable UV radiation in shaping the properties of the molecular gas in inner disks, during accretion phenomena in the T Tauri phase, and 2) the abundance of water vapor inward of the snowline as indicative of its origin through chemical (gas-phase reactions) and/or physical processes (ongoing inward migration of icy solids), and a potential tracer of disk evolution and planet formation processes.
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    ABSTRACT: Young stars are known to show variability due to non-steady mass accretion rate from their circumstellar disks. Accretion flares can produce strong energetic irradiation and heating that may affect the disk in the planet formation region, close to the central star. During an extreme accretion outburst in the young star EX Lupi, the prototype of EXor variables, remarkable changes in molecular gas emission from $\sim1$ AU in the disk have recently been observed (Banzatti et al. 2012). Here, we focus on water vapor and explore how it is affected by variable accretion luminosity in T Tauri stars. We monitored a young highly variable solar-mass star, DR Tau, using simultaneously two high/medium-resolution ESO-VLT spectrographs: VISIR at 12.4 $\mu$m to observe water lines from the disk, and X-shooter covering from 0.3 to 2.5 $\mu$m to constrain the stellar accretion. Three epochs spanning timescales from several days to several weeks were obtained. Accretion luminosity was estimated to change within a factor $\sim2$, and no change in water emission was detected at a significant level. In comparison to EX Lupi and EXor outbursts, DR Tau suggests that the less long-lived and weaker variability phenomena typical of T Tauri stars may leave water at planet-forming radii in the disk mostly unaffected. We propose that these systems may provide evidence for two processes that act over different timescales: UV photochemistry in the disk atmosphere (faster) and heating of the disk deeper layers (slower).
    The Astrophysical Journal 10/2013; 780(1). DOI:10.1088/0004-637X/780/1/26 · 6.28 Impact Factor
  • The Astrophysical Journal 06/2012; 751(2):160. DOI:10.1088/0004-637X/751/2/160 · 6.28 Impact Factor
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    ABSTRACT: We present a comparison of archival Spitzer spectra of the strongly variable T Tauri EX Lupi, observed before and during its 2008 outburst. We analyze the mid-infrared emission from gas-phase molecules thought to originate in a circumstellar disk. In quiescence the emission shows a forest of H2O lines, highly excited OH lines, and the Q branches of the organics C2H2, HCN, and CO2, similar to the emission observed toward several T Tauri systems. The outburst emission shows instead remarkable changes: H2O and OH line fluxes increase, new OH, H2, and HI transitions are detected, and organics are no longer seen. We adopt a simple model of a single-temperature slab of gas in local thermal equilibrium, a common approach for molecular analyses of Spitzer spectra, and derive the excitation temperature, column density, and emitting area of H2O and OH. We show how model results strongly depend on the selection of emission lines fitted, and that spectrally-resolved observations are essential for a correct interpretation of the molecular emission from disks, particularly in the case of water. Using H2O lines that can be approximated as thermalized to a single temperature, our results are consistent with a column density decrease in outburst while the emitting area of warm gas increases. A rotation diagram analysis suggests that the OH emission can be explained with two temperature components, which remarkably increase in column density in outburst. The relative change of H2O and OH emission suggests a key role for UV radiation in the disk surface chemistry.
    The Astrophysical Journal 11/2011; 745(1). DOI:10.1088/0004-637X/745/1/90 · 6.28 Impact Factor
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    ABSTRACT: 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 01/2011; 525. DOI:10.1051/0004-6361/201015206 · 4.48 Impact Factor
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    ABSTRACT: 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. 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. We present new high angular resolution (0.3-0.9 arcsec) observations at wavelengths from 850um to 3.6cm 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. We detect the presence of evolved dust by constraining the disk averaged dust opacity coefficient beta (computed between 1.3 and 7mm) 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. Comment: 9 pages, A&A in press