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Posts: 131433
Date:
Epsilon Eridani's Debris Ring
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Title: The Northern Arc of epsilon Eridani's Debris Ring as Seen by ALMA
Author: Mark Booth, William R. F. Dent, Andrés Jordán, Jean-François Lestrade, Antonio S. Hales, Mark C. Wyatt, Simon Casassus, Steve Ertel, Jane S. Greaves, Grant M. Kennedy, Luca Matrà, Jean-Charles Augereau, Eric Villard

We present the first ALMA observations of the closest known extrasolar debris disc. This disc orbits the star epsilon Eridani, a K-type star just 3.2pc away. Due to the proximity of the star, the entire disc cannot fit within the ALMA field of view. Therefore, the observations have been centred 18" North of the star, providing us with a clear detection of the northern arc of the ring, at a wavelength of 1.3mm. The observed disc emission is found to be narrow with a width of just 11-13AU. The fractional disc width we find is comparable to that of the Solar System's Kuiper Belt and makes this one of the narrowest debris discs known. If the inner and outer edges are due to resonances with a planet then this planet likely has a semi-major axis of 48AU. We find tentative evidence for clumps in the ring, although there is a strong chance that at least one is a background galaxy. We confirm, at much higher significance, the previous detection of an unresolved emission at the star that is above the level of the photosphere and attribute this excess to stellar chromospheric emission.

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Posts: 131433
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RE: Epsilon Eridani
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SOFIA Confirms Nearby Planetary System is Similar to Our Own

NASA's flying observatory, the Stratospheric Observatory for Infrared Astronomy, SOFIA, recently completed a detailed study of a nearby planetary system. The investigations confirmed that this nearby planetary system has an architecture remarkably similar to that of our solar system.
Located 10.5 light-years away in the southern hemisphere of the constellation Eridanus, the star Epsilon Eridani, eps Eri for short, is the closest planetary system around a star similar to the early sun. It is a prime location to research how planets form around stars like our sun, and is also the storied location of the Babylon 5 space station in the science fictional television series of the same name.

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Posts: 131433
Date:
HD 22049
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Title: The Inner 25 AU Debris Distribution in the epsilon Eri System
Author: Kate Y. L. Su (1), James M. De Buizer (2), George H. Rieke (1), Alexander V. Krivov (3), Torsten Lohne (3), Massimo Marengo (4), Karl R. Stapelfeldt (5), Nicholas P. Ballering (1), William D. Vacca (2) ((1) Steward Observatory, University of Arizona, (2) SOFIA-USRA, NASA Ames Research Center, (3) Astrophysikalisches Institut und Universitatssternwarte, Friedrich-Schiller-Universitat Jena, (4) Department of Physics & Astronomy, Iowa State University, (5) Jet Propulsion Laboratory, Caltech)

Debris disk morphology is wavelength dependent due to the wide range of particle sizes and size-dependent dynamics influenced by various forces. Resolved images of nearby debris disks reveal complex disk structures that are difficult to distinguish from their spectral energy distributions. Therefore, multi-wavelength resolved images of nearby debris systems provide an essential foundation to understand the intricate interplay between collisional, gravitational, and radiative forces that govern debris disk structures. We present the SOFIA 35 um resolved disk image of epsilon Eri, the closest debris disk around a star similar to the early Sun. Combining with the Spitzer resolved image at 24 um and 15-38 um excess spectrum, we examine two proposed origins of the inner debris in epsilon Eri: (1) in-situ planetesimal belt(s) and (2) dragged-in grains from the cold outer belt. We find that the presence of in-situ dust-producing planetesmial belt(s) is the most likely source of the excess emission in the inner 25 au region. Although a small amount of dragged-in grains from the cold belt could contribute to the excess emission in the inner region, the resolution of the SOFIA data is high enough to rule out the possibility that the entire inner warm excess results from dragged-in grains, but not enough to distinguish one broad inner disk from two narrow belts.

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Posts: 131433
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RE: Epsilon Eridani
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Title: The Epsilon Eridani System Resolved by Millimeter Interferometry
Author: Meredith A. MacGregor, David J. Wilner, Sean M. Andrews, Jean-Francois Lestrade, Sarah Maddison

We present observations of Epsilon Eridani from the Submillimeter Array (SMA) at 1.3 millimeters and from the Australia Telescope Compact Array (ATCA) at 7 millimeters that reach an angular resolution of ~4" (13 AU). These first millimeter interferometer observations of Epsilon Eridani, which hosts the closest debris disk to the Sun, reveal two distinct emission components: (1) the well-known outer dust belt, which, although patchy, is clearly resolved in the radial direction, and (2) an unresolved source coincident with the position of the star. We use direct model-fitting of the millimeter visibilities to constrain the basic properties of these two components. A simple Gaussian shape for the outer belt fit to the SMA data results in a radial location of 64.4^{+2.4}_{-3.0} AU and FWHM of 20.2^{+6.0}_{-8.2} AU (fractional width Delta R/R=0.3. Similar results are obtained taking a power law radial emission profile for the belt, though the power law index cannot be usefully constrained. Within the noise obtained (0.2 mJy/beam), these data are consistent with an axisymmetric belt model and show no significant azimuthal structure that might be introduced by unseen planets in the system. These data also limit any stellocentric offset of the belt to <9 AU, which disfavors the presence of giant planets on highly eccentric (>0.1) and wide (10's of AU) orbits. The flux density of the unresolved central component exceeds predictions for the stellar photosphere at these long wavelengths, by a marginally significant amount at 1.3 millimeters but by a factor of a few at 7 millimeters (with brightness temperature 13000±1600K for a source size of the optical stellar radius). We attribute this excess emission to ionized plasma from a stellar corona or chromosphere.

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Title: Magnetic Activity Cycles in the Exoplanet Host Star epsilon Eridani
Authors: T.S. Metcalfe, A.P. Buccino, B.P. Brown, S. Mathur, D.R. Soderblom, T.J. Henry, P.J.D. Mauas, R. Petrucci, J.C. Hall, S. Basu

The active K2 dwarf epsilon Eri has been extensively characterised, both as a young solar analogue and more recently as an exoplanet host star. As one of the nearest and brightest stars in the sky, it provides an unparalleled opportunity to constrain stellar dynamo theory beyond the Sun. We confirm and document the 3 year magnetic activity cycle in epsilon Eri originally reported by Hatzes and coworkers, and we examine the archival data from previous observations spanning 45 years. The data show coexisting 3 year and 13 year periods leading into a broad activity minimum that resembles a Maunder minimum-like state, followed by the resurgence of a coherent 3 year cycle. The nearly continuous activity record suggests the simultaneous operation of two stellar dynamos with cycle periods of 2.95±0.03 years and 12.7±0.3 years, which by analogy with the solar case suggests a revised identification of the dynamo mechanisms that are responsible for the so-called "active" and "inactive" sequences as proposed by Bohm-Vitense. Finally, based on the observed properties of epsilon Eri we argue that the rotational history of the Sun is what makes it an outlier in the context of magnetic cycles observed in other stars (as also suggested by its Li depletion), and that a Jovian-mass companion cannot be the universal explanation for the solar peculiarities.

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Title: Confirming Fundamental Parameters of the Exoplanet Host Star epsilon Eridani Using the Navy Optical Interferometer
Authors: Ellyn K. Baines, J. Thomas Armstrong

We measured the angular diameter of the exoplanet host star epsilon Eridani using the Navy Optical Interferometer. We determined its physical radius, effective temperature, and mass by combining our measurement with the star's parallax, photometry from the literature, and the Yonsei-Yale isochrones (Yi et al. 2001), respectively. We used the resulting stellar mass of 0.82 ± 0.05 M_Sun plus the mass function from Benedict et al. (2006) to calculate the planet's mass, which is 1.53 ± 0.22 M_Jupiter. Using our new effective temperature, we also estimated the extent of the habitable zone for the system.

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Title: The cold origin of the warm dust around epsilon Eridani
Authors: Martin Reidemeister, Alexander V. Krivov, Christopher C. Stark, Jean-Charles Augereau, Torsten Loehne, Sebastian Mueller

Context: The K2V star eps Eri hosts one known inner planet, an outer Kuiper belt analog, and an inner disk of warm dust. Spitzer/IRS measurements indicate that the warm dust is present at distances as close as a few AU from the star. Its origin is puzzling, since an "asteroid belt" that could produce this dust would be unstable because of the known inner planet. Aims: Here we test the hypothesis that the observed warm dust is generated by collisions in the outer belt and is transported inward by Poynting-Robertson (P-R) drag and strong stellar winds.
Methods: We simulated a steady-state distribution of dust particles outside 10AU with a collisional code and in the inner region (r<10AU) with single-particle numerical integrations. By assuming homogeneous spherical dust grains composed of water ice and silicate, we calculated the thermal emission of the dust and compared it with observations. We investigated two different orbital configurations for the inner planet inferred from RV measurements, one with a highly eccentric orbit of e=0.7 and another one with a moderate one of e=0.25. We also produced a simulation without a planet.
Results: Our models can reproduce the shape and magnitude of the observed SED from mid-IR to sub-mm wavelengths, as well as the Spitzer/MIPS radial brightness profiles. The best-fit dust composition includes both ice and silicates. The results are similar for the two possible planetary orbits and without a planet.
Conclusions: The observed warm dust in the system can indeed stem from the outer belt and be transported inward by P-R and stellar wind drag. The inner planet has little effect on the distribution of dust, so that the planetary orbit could not be constrained. Reasonable agreement between the model and observations can only be achieved by relaxing the assumption of purely silicate dust and assuming a mixture of silicate and ice in comparable amounts.

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Posts: 131433
Date:
Fomalhaut and epsilon Eridani
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Title: Spitzer/IRAC Limits to Planetary Companions of Fomalhaut and epsilon Eridani
Authors: M. Marengo, K. Stapelfeldt, M. W. Werner, J. L. Hora, G. G. Fazio, M. T. Schuster, J. C. Carson, S. T. Megeath

Fomalhaut and epsilon Eridani are two young, nearby stars that possess extended debris disks whose structures suggest the presence of perturbing planetary objects. With its high sensitivity and stable point spread function, Spitzer/IRAC is uniquely capable of detecting cool, Jupiter-like planetary companions whose peak emission is predicted to occur near 4.5 um. We report on deep IRAC imaging of these two stars, taken at 3.6 and 4.5 um using subarray mode and in all four channels in wider-field full array mode. Observations acquired at two different telescope roll angles allowed faint surrounding objects to be separated from the stellar diffraction pattern. No companion candidates were detected at the reported position of Fomalhaut b with 3 sigma model-dependent mass upper limits of 3 MJ (for an age of 200 Myr). Around epsilon Eridani we instead set a limit of 4 and <1 MJ (1 Gyr model age) at the inner and outer edge of the sub-millimetre debris ring, respectively. These results are consistent with non-detections in recent near-infrared imaging searches, and set the strongest limits to date on the presence of planets outside epsilon Eridani sub-millimetre ring.

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Posts: 131433
Date:
RE: Epsilon Eridani
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Title: Epsilon Eridani's Planetary Debris Disk: Structure and Dynamics based on Spitzer and CSO Observations
Authors: D. Backman, M. Marengo, K. Stapelfeldt, K. Su, D. Wilner, C. D. Dowell, D. Watson, J. Stansberry, G. Rieke, T. Megeath, G. Fazio, M. Werner

Spitzer and Caltech Submillimeter Observatory (CSO) images and spectrophotometry of epsilon Eridani at wavelengths from 3.5 to 350 um reveal new details of its bright debris disk. The 350 um map confirms the presence of a ring at r = 11-28 arcsec (35-90 AU) observed previously at longer sub-mm wavelengths. The Spitzer mid- and far-IR images do not show the ring, but rather a featureless disk extending from within a few arcsec of the star across the ring to r ~ 34 arcsec (110 AU). The spectral energy distribution (SED) of the debris system implies a complex structure. A model constrained by the surface brightness profiles and the SED indicates that the sub-mm ring emission is primarily from large (a ~ 135 um) grains, with smaller (a ~ 15 um) grains also present in and beyond the ring. The Spitzer IRS and MIPS SED-mode spectrophotometry data clearly show the presence of spatially compact excess emission at lambda > 15 um that requires the presence of two additional narrow belts of dust within the sub-mm ring's central void. The innermost belt at r ~ 3 AU is composed of silicate dust. A simple dynamical model suggests that dust produced collisionally by a population of about 11 M_Earth of planetesimals in the sub-mm ring could be the source of the emission from both in and beyond the sub-mm ring. Maintaining the inner belts and the inner edge to the sub-mm ring may require the presence of three planets in this system including the candidate radial velocity object.

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Spitzer has discovered that the system also has dual asteroid belts. One sits at approximately the same position as the one in our solar system. The second, denser belt, most likely also populated by asteroids, lies between the first belt and the comet ring. The presence of the asteroid belts implies additional planets in the Epsilon Eridani system.

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