181 research outputs found

    On the Interferometric Sizes of Young Stellar Objects

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    Long-baseline optical interferometers can now detect and resolve hot dust emission thought to arise at the inner edge of circumstellar disks around young stellar objects (YSOs). We argue that the near-infrared sizes being measured are closely related to the radius at which dust is sublimated by the stellar radiation field. We consider how realistic dust optical properties and gas opacity dramatically affect the predicted location of this dust destruction radius, an exercise routinely done in other contexts but so far neglected in the analysis of near-infrared sizes of YSOs. We also present the accumulated literature of near-infrared YSO sizes in the form of a ``size-luminosity diagram'' and compare with theoretical expectations. We find evidence that large (>~ 1 micron) dust grains predominate in the inner disks of T Tauri and Herbig Ae/Be stars, under the assumption that the inner-most gaseous disks are optically-thin at visible wavelengths.Comment: Accepted by Astrophysical Journa

    Co-phasing the Large Binocular Telescope: status and performance of LBTI/PHASECam

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    The Large Binocular Telescope Interferometer is a NASA-funded nulling and imaging instrument designed to coherently combine the two 8.4-m primary mirrors of the LBT for high-sensitivity, high-contrast, and high-resolution infrared imaging (1.5-13 um). PHASECam is LBTI's near-infrared camera used to measure tip-tilt and phase variations between the two AO-corrected apertures and provide high-angular resolution observations. We report on the status of the system and describe its on-sky performance measured during the first semester of 2014. With a spatial resolution equivalent to that of a 22.8-meter telescope and the light-gathering power of single 11.8-meter mirror, the co-phased LBT can be considered to be a forerunner of the next-generation extremely large telescopes (ELT).Comment: 8 pages, 5 figures, SPIE Conference proceeding

    Exoplanet science with the LBTI: instrument status and plans

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    The Large Binocular Telescope Interferometer (LBTI) is a strategic instrument of the LBT designed for high-sensitivity, high-contrast, and high-resolution infrared (1.5-13 μ\mum) imaging of nearby planetary systems. To carry out a wide range of high-spatial resolution observations, it can combine the two AO-corrected 8.4-m apertures of the LBT in various ways including direct (non-interferometric) imaging, coronagraphy (APP and AGPM), Fizeau imaging, non-redundant aperture masking, and nulling interferometry. It also has broadband, narrowband, and spectrally dispersed capabilities. In this paper, we review the performance of these modes in terms of exoplanet science capabilities and describe recent instrumental milestones such as first-light Fizeau images (with the angular resolution of an equivalent 22.8-m telescope) and deep interferometric nulling observations.Comment: 12 pages, 6 figures, Proc. SPI

    Study of molecular layers in the atmosphere of the supergiant star µ Cep by interferometry in the K band

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    Infrared interferometry of supergiant and Mira stars has recently been reinterpreted as revealing the presence of deep molecular layers. Empirical models for a photosphere surrounded by a simple molecular layer or envelope have led to a consistent interpretation of previously inconsistent data. The stellar photospheres are found to be smaller than previously understood, and the molecular layer is much higher and denser than predicted by hydrostatic equilibrium. However, the analysis was based on spatial observations with medium-band optical filters, which mixed the visibilities of different spatial structures. This paper reports spatial interferometry with narrow spectral bands, isolating near-continuum and strong molecular features, obtained for the supergiant µ Cep. The measurements confirm strong variation of apparent diameter across the K-band. A layer model shows that a stellar photosphere of angular diameter 14.11±0.60 mas is surrounded by a molecular layer of diameter 18.56±0.26 mas, with an optical thickness varying from nearly zero at 2.15 µm to >1 at 2.39 µm. Although µ Cep and α Ori have a similar spectral type, interferometry shows that they differ in their radiative properties. Comparison with previous broad-band measurements shows the importance of narrow spectral bands. The molecular layer or envelope appears to be a common feature of cool supergiants

    First-light LBT nulling interferometric observations: warm exozodiacal dust resolved within a few AU of eta Corvi

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    We report on the first nulling interferometric observations with the Large Binocular Telescope Interferometer (LBTI), resolving the N' band (9.81 - 12.41 um) emission around the nearby main-sequence star eta Crv (F2V, 1-2 Gyr). The measured source null depth amounts to 4.40% +/- 0.35% over a field-of-view of 140 mas in radius (~2.6\,AU at the distance of eta Corvi) and shows no significant variation over 35{\deg} of sky rotation. This relatively low null is unexpected given the total disk to star flux ratio measured by Spitzer/IRS (~23% across the N' band), suggesting that a significant fraction of the dust lies within the central nulled response of the LBTI (79 mas or 1.4 AU). Modeling of the warm disk shows that it cannot resemble a scaled version of the Solar zodiacal cloud, unless it is almost perpendicular to the outer disk imaged by Herschel. It is more likely that the inner and outer disks are coplanar and the warm dust is located at a distance of 0.5-1.0 AU, significantly closer than previously predicted by models of the IRS spectrum (~3 AU). The predicted disk sizes can be reconciled if the warm disk is not centrosymmetric, or if the dust particles are dominated by very small grains. Both possibilities hint that a recent collision has produced much of the dust. Finally, we discuss the implications for the presence of dust at the distance where the insolation is the same as Earth's (2.3 AU).Comment: 9 pages, 6 figures, accepted for publication in Ap

    Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models

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    We present K'-band observations of five Mira stars with the IOTA interferometer. The interferograms were obtained with the FLUOR fiber optics beam combiner, which provides high-accuracy visibility measurements in spite of time-variable atmospheric conditions. For the M-type Miras X Oph, R Aql, RU Her, R Ser, and the C-type Mira V CrB we derived the uniform-disk diameters 11.7mas, 10.9mas, 8.4mas, 8.1mas, and 7.9mas (+/- 0.3mas), respectively. Simultaneous photometric observations yielded the bolometric fluxes. The derived angular Rosseland radii and the bolometric fluxes allowed the determination of effective temperatures. For instance, the effective temperature of R Aql was determined to be 2970 +/- 110 K. A linear Rosseland radius for R Aql of (250 +100/-60) Rsun was derived from the angular Rosseland radius of 5.5mas +/- 0.2mas and the HIPPARCOS parallax of 4.73mas +/- 1.19mas. The observations were compared with theoretical Mira star models of Bessel et al. (1996) and Hofmann et al. (1998). The effective temperatures of the M-type Miras and the linear radius of R Aql indicate fundamental mode pulsation.Comment: 12 pages, 4 postscript figure

    The near-infrared size-luminosity relations for Herbig Ae/Be disks

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    We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes using the 85-m baseline Keck Interferometer. Targets were chosen to span the maximum range of stellar properties to probe the disk size dependence on luminosity and effective temperature. For most targets, the measured near-infrared sizes (ranging from 0.2 to 4 AU) support a simple disk model possessing a central optically-thin (dust-free) cavity, ringed by hot dust emitting at the expected sublimation temperatures (T_sub~1000-1500K). Furthermore, we find a tight correlation of disk size with source luminosity R propto L^(1/2) for Ae and late Be systems (valid over more than 2 decades in luminosity), confirming earlier suggestions based on lower-quality data. Interestingly, the inferred dust-free inner cavities of the highest luminosity sources (Herbig B0-B3 stars) are under-sized compared to predictions of the optically-thin cavity model, likely due to optically-thick gas within the inner AU.Comment: Accepted by Astrophysical Journal; 24 pages, 4 figures, 4 table

    Aperture synthesis using multiple facilities: Keck aperture masking and the IOTA interferometer

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    As the number of optical interferometers increase, multi-facility observations become both feasible and scientifically interesting. For imaging of complex sources, the capability of increasing (u,v) coverage by using multiple arrays may be necessary for accurately interpreting the fringe visibility and closure phase data. Toward this end, coordinated observations with the IOTA interferometer and Keck aperture masking have been carried out to test techniques for synthesizing images using data from heterogeneous arrays with sparse (u,v) coverage. In particular, we will focus on how the image prior in the Maximum Entropy Method can be used to efficiently incorporate very high spatial frequency information with "low-resolution" data for imaging the generic prototype "Star + Dust Shell" image morphology. Preliminary results using real data for a few dusty evolved stars are presented

    The HOSTS Survey for Exozodiacal Dust: Preliminary results and future prospects

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    [abridged] The presence of large amounts of dust in the habitable zones of nearby stars is a significant obstacle for future exo-Earth imaging missions. We executed an N band nulling interferometric survey to determine the typical amount of such exozodiacal dust around a sample of nearby main sequence stars. The majority of our data have been analyzed and we present here an update of our ongoing work. We find seven new N band excesses in addition to the high confidence confirmation of three that were previously known. We find the first detections around Sun-like stars and around stars without previously known circumstellar dust. Our overall detection rate is 23%. The inferred occurrence rate is comparable for early type and Sun-like stars, but decreases from 71% [+11%/-20%] for stars with previously detected mid- to far-infrared excess to 11% [+9%/-4%] for stars without such excess, confirming earlier results at high confidence. For completed observations on individual stars, our sensitivity is five to ten times better than previous results. Assuming a lognormal luminosity function of the dust, we find upper limits on the median dust level around all stars without previously known mid to far infrared excess of 11.5 zodis at 95% confidence level. The corresponding upper limit for Sun-like stars is 16 zodis. An LBTI vetted target list of Sun-like stars for exo-Earth imaging would have a corresponding limit of 7.5 zodis. We provide important new insights into the occurrence rate and typical levels of habitable zone dust around main sequence stars. Exploiting the full range of capabilities of the LBTI provides a critical opportunity for the detailed characterization of a sample of exozodiacal dust disks to understand the origin, distribution, and properties of the dust.Comment: To appear in SPIE Astronomical Telescopes + Instrumentation 2018 proceedings. Some typos fixed, one reference adde
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