Publications (11)43.58 Total impact
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Article: Remnant gas in evolved circumstellar disks: Herschel PACS observations of 10-100 Myr old disk systems
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ABSTRACT: We present Herschel PACS spectroscopy of the [OI] 63 micron gas-line for three circumstellar disk systems showing signs of significant disk evolution and/or planet formation: HR 8799, HD 377 and RX J1852.3-3700. [OI] is undetected toward HR 8799 and HD 377 with 3 sigma upper limits of 6.8 x 10^-18 W m^-2 and 9.9 x 10^-18 W m^-2 respectively. We find an [OI] detection for RX J1852.3-3700 at 12.3 +- 1.8 x 10^-18 W m^-2. We use thermo-chemical disk models to model the gas emission, using constraints on the [OI] 63 micron, and ancillary data to derive gas mass upper limits and constrain gas-to-dust ratios. For HD 377 and HR 8799, we find 3 sigma upper limits on the gas mass of 0.1-20 Mearth. For RX J1852.3-3700, we find two distinct disk scenarios that could explain the detection of [OI] 63 micron and CO(2-1) upper limits reported from the literature: (i) a large disk with gas co-located with the dust (16-500 AU), resulting in a large tenuous disk with ~16 Mearth of gas, or (ii) an optically thick gas disk, truncated at ~70 AU, with a gas mass of 150 Mearth. We discuss the implications of these results for the formation and evolution of planets in these three systems.06/2012; -
Article: Spitzer Spectroscopy of the Transition Object TW Hya
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ABSTRACT: We report sensitive Spitzer IRS spectroscopy in the 10-20 μm region of TW Hya, a nearby T Tauri star. The unusual spectral energy distribution of the source, that of a "transition object," indicates that the circumstellar disk in the system has experienced significant evolution, possibly as a result of planet formation. The spectrum we measure is strikingly different from that of other classical T Tauri stars reported in the literature, displaying no strong emission features of H2O, C2H2, or HCN. The difference suggests that the inner planet formation region (5 AU) of the gaseous disk has evolved physically and/or chemically away from the classical T Tauri norm. Nevertheless, TW Hya does show a rich spectrum of emission features of atoms (H I, [Ne II], and [Ne III]) and molecules (H2, OH, CO2, HCO+, and possibly CH3), some of which are also detected in classical T Tauri spectra. The properties of the neon emission are consistent with an origin for the emission in a disk irradiated by X-rays (with a possible role for additional irradiation by stellar EUV). The OH emission we detect, which also likely originates in the disk, is hot, arising from energy levels up to 23,000 K above ground, and may be produced by the UV photodissociation of water. The H I emission is surprisingly strong, with relative strengths that are consistent with case B recombination. While the absence of strong molecular emission in the 10-20 μm region may indicate that the inner region of the gaseous disk has been partly cleared by an orbiting giant planet, chemical and/or excitation effects may be responsible instead. We discuss these issues and how our results bear on our understanding of the evolutionary state of the TW Hya disk.The Astrophysical Journal 02/2010; 712(1):274. · 6.02 Impact Factor -
Article: Time-Evolution of Viscous Circumstellar Disks due to Photoevaporation by FUV, EUV and X-ray Radiation from the Central Star
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ABSTRACT: We present the time evolution of viscously accreting circumstellar disks as they are irradiated by ultraviolet and X-ray photons from a low-mass central star. Our model is a hybrid of a 1D time-dependent viscous disk model coupled to a 1+1D disk vertical structure model used for calculating the disk structure and photoevaporation rates. We find that disks of initial mass 0.1M_o around 1M_o stars survive for 4x10^6 years, assuming a viscosity parameter $\alpha=0.01$, a time-dependent FUV luminosity $L_{FUV}~10^{-2}-10^{-3}$ L_o and with X-ray and EUV luminosities $L_X \sim L_{EUV} ~ 10^{-3}$L_o. We find that FUV/X-ray-induced photoevaporation and viscous accretion are both important in depleting disk mass. Photoevaporation rates are most significant at ~ 1-10 AU and at >~ 30 AU. Viscosity spreads the disk which causes mass loss by accretion onto the central star and feeds mass loss by photoevaporation in the outer disk. We find that FUV photons can create gaps in the inner, planet-forming regions of the disk (~ 1-10 AU) at relatively early epochs in disk evolution while disk masses are still substantial. EUV and X-ray photons are also capable of driving gaps, but EUV can only do so at late, low accretion-rate epochs after the disk mass has already declined substantially. Disks around stars with predominantly soft X-ray fields experience enhanced photoevaporative mass loss. We follow disk evolution around stars of different masses, and find that disk survival time is relatively independent of mass for stars with M <~ 3M_o; for M >~ 3M_o the disks are short-lived(~10^5 years). Comment: Accepted to ApJ, Main Journal09/2009; -
Article: Diagnostic Line Emission from EUV and X-ray Illuminated Disks and Shocks around Low Mass stars
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ABSTRACT: Extreme ultraviolet (EUV, 13.6 eV $< h\nu \lta 100$ eV) and X-rays in the 0.1-2 keV band can heat the surfaces of disks around young, low mass stars to thousands of degrees and ionize species with ionization potentials greater than 13.6 eV. Shocks generated by protostellar winds can also heat and ionize the same species close to the star/disk system. These processes produce diagnostic lines (e.g., [NeII] 12.8 $\mu$m and [OI] 6300 \AA) that we model as functions of key parameters such as EUV luminosity and spectral shape, X-ray luminosity and spectral shape, and wind mass loss rate and shock speed. Comparing our models with observations, we conclude that either internal shocks in the winds or X-rays incident on the disk surfaces often produce the observed [NeII] line, although there are cases where EUV may dominate. Shocks created by the oblique interaction of winds with disks are unlikely [NeII] sources because these shocks are too weak to ionize Ne. Even if [NeII] is mainly produced by X-rays or internal wind shocks, the neon observations typically place upper limits of $\lta 10^{42}$ s$^{-1}$ on the EUV photon luminosity of these young low mass stars. The observed [OI] 6300 \AA line has both a low velocity component (LVC) and a high velocity component. The latter likely arises in internal wind shocks. For the former we find that X-rays likely produce more [OI] luminosity than either the EUV layer, the transition layer between the EUV and X-ray layer, or the shear layer where the protostellar wind shocks and entrains disk material in a radial flow across the surface of the disk. Our soft X-ray models produce [OI] LVCs with luminosities up to $10^{-4}$ L$_\odot$, but may not be able to explain the most luminous LVCs. Comment: 51 pages, 10 figures, accepted to ApJ08/2009; -
Article: Photoevaporation of Circumstellar Disks Due to External Far-Ultraviolet Radiation in Stellar Aggregates
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ABSTRACT: When stars form within small groups (with N* ≈ 100-500 members), their circumstellar disks are exposed to relatively little extreme-ultraviolet (EUV; hν > 13.6 eV) radiation but a great deal of far-ultraviolet (FUV; 6 eV < hν < 13.6 eV) radiation (~103 times the local interstellar FUV field) from the most massive stars in the group. This paper calculates the mass-loss rates and evaporation timescales for circumstellar disks exposed to external FUV radiation. Previous work treated large disks and/or intense radiation fields in which the disk radius rd exceeds the critical radius rg where the sound speed in the FUV heated surface layer exceeds the escape speed; it has often been assumed that photoevaporation occurs for rd > rg and is negligible for rd < rg. Since rg 100 AU for FUV heating, this would imply little mass loss from the planet-forming regions of a disk. In this paper we focus on systems in which photoevaporation is suppressed because rd < rg and show that significant mass loss still takes place as long as rd/rg 0.1-0.2. Some of the gas extends beyond the disk edge (or above the disk surface) to larger distances where the temperature is higher, the escape speed is lower, and an outflow develops. The resulting evaporation rate is a sensitive function of the central stellar mass and disk radius, which determine the escape speed, and the external FUV flux, which determines the temperature structure of the surface layers and outflowing gas. Disks around red dwarfs, low-mass stars with M* 0.5 M☉, are evaporated and shrink to disk radii rd 15 AU on short timescales t 10 Myr when exposed to moderate FUV fields with G0 = 3000 (where G0 = 1.7 for the local interstellar FUV field). The disks around solar-type stars are more durable. For intense FUV radiation fields with G0 = 30,000, however, even these disks shrink to rd 15 AU on timescales t ~ 10 Myr. Such fields exist within about 0.7 pc of the center of a cluster with N* ≈ 4000 stars. If our solar system formed in the presence of such strong FUV radiation fields, this mechanism could explain why Neptune and Uranus in our solar system are gas-poor, whereas Jupiter and Saturn are relatively gas-rich. This mechanism for photoevaporation can also limit the production of Kuiper Belt objects and can suppress giant planet formation in sufficiently large clusters, such as the Hyades, especially for disks associated with low-mass stars.The Astrophysical Journal 12/2008; 611(1):360. · 6.02 Impact Factor -
Article: The Dust, Planetesimals, and Planets of HD 38529
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ABSTRACT: HD 38529 is a post-main-sequence G8 III/IV star (3.5 Gyr old) with a planetary system consisting of at least two planets having M sin i of 0.8 and 12.2 MJup, semimajor axes of 0.13 and 3.74 AU, and eccentricities of 0.25 and 0.35, respectively. Spitzer observations show that HD 38529 has an excess emission above the stellar photosphere, with a signal-to-noise ratio (S/N) at 70 μm of 4.7, a small excess at 33 μm (S/N = 2.6), and no excess <30 μm. We discuss the distribution of the potential dust-producing planetesimals from the study of the dynamical perturbations of the two known planets, considering in particular the effect of secular resonances. We identify three dynamically stable niches at 0.4-0.8, 20-50, and beyond 60 AU. We model the spectral energy distribution (SED) of HD 38529 to find out which of these niches show signs of harboring dust-producing planetesimals. The secular analysis, together with the SED modeling results, suggest that the planetesimals responsible for most of the dust emission are likely located within 20-50 AU, a configuration that resembles that of the Jovian planets + Kuiper Belt in our solar system. Finally, we place upper limits (8 × 10-6 lunar masses of 10 μm particles) to the amount of dust that could be located in the dynamically stable region that exists between the two planets (0.25-0.75 AU).The Astrophysical Journal 12/2008; 668(2):1165. · 6.02 Impact Factor -
Article: The moth : an unusual circumstellar structure associated with HD 61005
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ABSTRACT: We present the discovery of an unusual spatially resolved circumstellar structure associated with the ≈90 Myr, nearby, G dwarf star HD 61005. Observations from the FEPS Spitzer Legacy Science survey reveal thermal emission in excess of expected stellar photospheric levels. Follow-up 0.1" resolution HST NICMOS coronagraphic images reveal scattered starlight ≤7" (~240 AU) from the occulted star (1.1 μm flux density =18 ± 3.3 mJy; and 0.77% ± 0.16% of the starlight). The extremely high near-IR scattering fraction and IR excess luminosity f = L_(IR)/L_* ≈2 × 10^(−3) suggests scattering particle sizes of order a ~<1.1 μm/2π ~ 0.2 μm , comparable to the blowout size (a ≈ 0.3 μm) due to radiation pressure from the star. Dust-scattered starlight is traced inward to an instrumental limit of ~10 AU. The structure exhibits a strong asymmetry about its morphological major axis but is mirror-symmetric about its minor axis.The Astrophysical Journal 12/2008; · 6.02 Impact Factor -
Article: Are Debris Disks and Massive Planets Correlated?
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ABSTRACT: Using data from the Spitzer Space Telescope Legacy Science Program Formation and Evolution of Planetary Systems (FEPS), we have searched for debris disks around nine FGK stars (2-10 Gyr), known from radial velocity (RV) studies to have one or more massive planets. Only one of the sources, HD 38529, has excess emission above the stellar photosphere; at 70 μm the signal-to-noise ratio in the excess is 4.7, while at λ < 30 μm there is no evidence of excess. The remaining sources show no excesses at any Spitzer wavelengths. Applying survival tests to the FEPS sample and the results for the FGK survey recently published in Bryden et al., we do not find a significant correlation between the frequency and properties of debris disks and the presence of close-in planets. We discuss possible reasons for the lack of a correlation.The Astrophysical Journal 12/2008; 658(2):1312. · 6.02 Impact Factor -
Article: Formation and Evolution of Planetary Systems (FEPS): Properties of Debris Dust around Solar-type Stars
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ABSTRACT: We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRS low resolution) observations for 314 stars in the Formation and Evolution of Planetary Systems (FEPS) Legacy program. These data are used to investigate the properties and evolution of circumstellar dust around solar-type stars spanning ages from approximately 3 Myr to 3 Gyr. We identify 46 sources that exhibit excess infrared emission above the stellar photosphere at 24um, and 21 sources with excesses at 70um. Five sources with an infrared excess have characteristics of optically thick primordial disks, while the remaining sources have properties akin to debris systems. The fraction of systems exhibiting a 24um excess greater than 10.2% above the photosphere is 15% for ages < 300 Myr and declines to 2.7% for older ages. The upper envelope to the 70um fractional luminosity appears to decline over a similar age range. The characteristic temperature of the debris inferred from the IRS spectra range between 60 and 180 K, with evidence for the presence of cooler dust to account for the strength of the 70um excess emission. No strong correlation is found between dust temperature and stellar age. Comparison of the observational data with disk models containing a power-law distribution of silicate grains suggest that the typical inner disk radius is > 10 AU. Although the interpretation is not unique, the lack of excess emission shortwards of 16um and the relatively flat distribution of the 24um excess for ages <300~Myr is consistent with steady-state collisional models. Comment: 85 pages, 18 figures, 4 tables; accepted for publication in ApJSThe Astrophysical Journal Supplement Series 10/2008; · 13.46 Impact Factor -
Article: The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer
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ABSTRACT: We provide an overview of the Spitzer Legacy Program ``Formation and Evolution of Planetary Systems" (FEPS) which was proposed in 2000, begun in 2001, and executed aboard the Spitzer Space Telescope between 2003 and 2006. This program exploits the sensitivity of Spitzer to carry out mid-infrared spectrophotometric observations of solar-type stars. With a sample of ~ 328 stars ranging in age from ~ 3 Myr to ~ 3 Gyr, we trace the evolution of circumstellar gas and dust from primordial planet-building stages in young circumstellar disks through to older collisionally generated debris disks. When completed, our program will help define the time scales over which terrestrial and gas giant planets are built, constrain the frequency of planetesimal collisions as a function of time, and establish the diversity of mature planetary architectures. In addition to the observational program, we have coordinated a concomitant theoretical effort aimed at understanding the dynamics of circumstellar dust with and without the effects of embedded planets, dust spectral energy distributions, and atomic and molecular gas line emission. Together with the observations, these efforts will provide astronomical context for understanding whether our Solar System - and its habitable planet - is a common or a rare circumstance. Additional information about the FEPS project can be found on the team website: feps.as.arizona.edu02/2007; -
Article: Models of Chemistry, Thermal Balance and Infrared Spectra from Intermediate-Aged Disks around G and K stars
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ABSTRACT: We model gas and dust emission from regions 0.3-20 AU from a central low mass star in intermediate-aged(~10Myr) disks with optically thin dust. The models treat thermal balance and chemistry self-consistently and calculate the vertical density temperature structure of the gas in a disk. The gas and dust temperatures are calculated separately. We focus on IR gas emission lines and find that the [SI]25.23 micron line is the strongest emission line for a wide range of disk and stellar parameters, followed by emission from [SiII]34.8 um, [FeII]26 um and [OI]63um. [FeI]24um and rotational lines of OH and H2O are strong when gas masses are high (>0.1 M_J). Emission from the rotational lines of H2 is difficult to detect, unless disk gas masses are substantial (>0.1 M_J). Comment: To appear in the Astrophysical Journal05/2004;
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2010
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SETI Institute
Mountain View, CA, USA
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