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Register a new userThe (w)hole survey: an unbiased sample study of transition disk candidates based on Spitzer catalogs
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Added by
Nienke van der Marel
Publication date
2016
fields
Physics
and research's language is
English
Authors
Nienke van der Marel
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Understanding disk evolution and dissipation is essential for studies of planet formation. Transition disks, i.e., disks with large dust cavities and gaps, are promising candidates of active evolution. About two dozen SED-selected candidates have been confirmed to have dust cavities through millimeter interferometric imaging, but this sample is biased towards the brightest disks. The Spitzer surveys of nearby low-mass star forming regions have resulted in more than 4000 Young Stellar Objects (YSOs). Using color criteria we have selected a sample of ~150 candidates, and an additional 40 candidates and known transition disks from the literature. The Spitzer data were complemented by new observations at longer wavelengths, including new JCMT and APEX submillimeter photometry, and WISE and Herschel-PACS mid and far-infrared photometry. Furthermore, optical spectroscopy was obtained and stellar types were derived for 85% of the sample, including information from the literature. The SEDs were fit to a grid of RADMC-3D disk models with a limited number of parameters: disk mass, inner disk mass, scale height and flaring, and disk cavity radius, where the latter is the main parameter of interest. A large fraction of the targets possibly have dust cavities based on the SED. The derived cavity sizes are consistent with imaging/modeling results in the literature, where available. Trends are found with Ldisk/Lstar and stellar mass and a possible connection with exoplanet orbital radii. A comparison with a previous study where color observables are used (Cieza et al. 2010) reveals large overlap between their category of planet-forming disks and our transition disks with cavities. A large number of the new transition disk candidates are suitable for follow-up observations with ALMA.
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Although the final observations of the Spitzer Warm Mission are currently scheduled for March 2019, it can continue operations through the end of the decade with no loss of photometric precision. As we will show, there is a strong science case for extending the current Warm Mission to December 2020. Spitzer has already made major impacts in the fields of exoplanets (including microlensing events), characterizing near Earth objects, enhancing our knowledge of nearby stars and brown dwarfs, understanding the properties and structure of our Milky Way galaxy, and deep wide-field extragalactic surveys to study galaxy birth and evolution. By extending Spitzer through 2020, it can continue to make ground-breaking discoveries in those fields, and provide crucial support to the NASA flagship missions JWST and WFIRST, as well as the upcoming TESS mission, and it will complement ground-based observations by LSST and the new large telescopes of the next decade. This scientific program addresses NASAs Science Mission Directives objectives in astrophysics, which include discovering how the universe works, exploring how it began and evolved, and searching for life on planets around other stars.
Context. Detecting and characterizing circumstellar dust is a way to study the architecture and evolution of planetary systems. Cold dust in debris disks only traces the outer regions. Warm and hot exozodiacal dust needs to be studied in order to trace regions close to the habitable zone. Aims. We aim to determine the prevalence and to constrain the properties of hot exozodiacal dust around nearby main-sequence stars. Methods. We search a magnitude limited (H < 5) sample of 92 stars for bright exozodiacal dust using our VLTI visitor instrument PIONIER in the H-band. We derive statistics of the detection rate with respect to parameters such as the stellar spectral type and age or the presence of a debris disk in the outer regions of the systems. We derive more robust statistics by combining our sample with the results from our CHARA/FLUOR survey in the K-band. In addition, our spectrally dispersed data allows us to put constraints on the emission mechanism and the dust properties in the detected systems. Results. We find an over-all detection rate of bright exozodiacal dust in the H-band of 11% (9 out of 85 targets) and three tentative detections. The detection rate decreases from early type to late type stars and increases with the age of the host star. We do not confirm the tentative correlation between the presence of cold and hot dust found in our earlier analysis of the FLUOR sample alone. Our spectrally dispersed data suggest that either the dust is extremely hot or the emission is dominated by the scattered light in most cases. The implications of our results for the target selection of future terrestrial planet finding missions using direct imaging are discussed.
We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm continuum observations of the SR 24S transition disk with an angular resolution $lesssim0.18$ (12 au radius). We perform a multi-wavelength investigation by combining new data with previous ALMA data at 0.45 mm. The visibilities and images of the continuum emission at the two wavelengths are well characterized by a ring-like emission. Visibility modeling finds that the ring-like emission is narrower at longer wavelengths, in good agreement with models of dust trapping in pressure bumps, although there are complex residuals that suggest potentially asymmetric structures. The 0.45 mm emission has a shallower profile inside the central cavity than the 1.3 mm emission. In addition, we find that the $^{13}$CO and C$^{18}$O (J=2-1) emission peaks at the center of the continuum cavity. We do not detect either continuum or gas emission from the northern companion to this system (SR 24N), which is itself a binary system. The upper limit for the dust disk mass of SR 24N is $lesssim 0.12,M_{bigoplus}$, which gives a disk mass ratio in dust between the two components of $M_{mathrm{dust, SR,24S}}/M_{mathrm{dust, SR,24N}}gtrsim840$. The current ALMA observations may imply that either planets have already formed in the SR 24N disk or that dust growth to mm-sizes is inhibited there and that only warm gas, as seen by ro-vibrational CO emission inside the truncation radii of the binary, is present.
We report the discovery of TOI-561, a multi-planet system in the galactic thick disk that contains a rocky, ultra-short period planet (USP). This bright ($V=10.2$) star hosts three small transiting planets identified in photometry from the NASA TESS mission: TOI-561 b (TOI-561.02, P=0.44 days, $R_b = 1.45pm0.11,R_oplus$), c (TOI-561.01, P=10.8 days, $R_c=2.90pm0.13,R_oplus$), and d (TOI-561.03, P=16.3 days, $R_d=2.32pm0.16,R_oplus$). The star is chemically ([Fe/H]$=-0.41pm0.05$, [$alpha$/H]$=+0.23pm0.05$) and kinematically consistent with the galactic thick disk population, making TOI-561 one of the oldest ($10pm3,$Gyr) and most metal-poor planetary systems discovered yet. We dynamically confirm planets b and c with radial velocities from the W. M. Keck Observatory High Resolution Echelle Spectrometer. Planet b has a mass and density of $3.2pm0.8,M_oplus$ and $5.5^{+2.0}_{-1.6},$g$,$cm$^{-3}$, consistent with a rocky composition. Its lower-than-average density is consistent with an iron-poor composition, although an Earth-like iron-to-silicates ratio is not ruled out. Planet c is $7.0pm2.3,M_oplus$ and $1.6pm0.6,$g$,$cm$^{-3}$, consistent with an interior rocky core overlaid with a low-mass volatile envelope. Several attributes of the photometry for planet d (which we did not detect dynamically) complicate the analysis, but we vet the planet with high-contrast imaging, ground-based photometric follow-up and radial velocities. TOI-561 b is the first rocky world around a galactic thick-disk star confirmed with radial velocities and one of the best rocky planets for thermal emission studies.
The $sigma$ Orionis cluster is important for studying protoplanetary disk evolution, as its intermediate age ($sim$3-5 Myr) is comparable to the median disk lifetime. We use ALMA to conduct a high-sensitivity survey of dust and gas in 92 protoplanetary disks around $sigma$ Orionis members with $M_{ast}gtrsim0.1 M_{odot}$. Our observations cover the 1.33 mm continuum and several CO $J=2-1$ lines: out of 92 sources, we detect 37 in the mm continuum and six in $^{12}$CO, three in $^{13}$CO, and none in C$^{18}$O. Using the continuum emission to estimate dust mass, we find only 11 disks with $M_{rm dust}gtrsim10 M_{oplus}$, indicating that after only a few Myr of evolution most disks lack sufficient dust to form giant planet cores. Stacking the individually undetected continuum sources limits their average dust mass to 5$times$ lower than that of the faintest detected disk, supporting theoretical models that indicate rapid dissipation once disk clearing begins. Comparing the protoplanetary disk population in $sigma$ Orionis to those of other star-forming regions supports the steady decline in average dust mass and the steepening of the $M_{rm dust}$-$M_{ast}$ relation with age; studying these evolutionary trends can inform the relative importance of different disk processes during key eras of planet formation. External photoevaporation from the central O9 star is influencing disk evolution throughout the region: dust masses clearly decline with decreasing separation from the photoionizing source, and the handful of CO detections exist at projected separations $>1.5$ pc. Collectively, our findings indicate that giant planet formation is inherently rare and/or well underway by a few Myr of age.
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