We report on the discovery of HAT-P-11b, the smallest radius transiting extrasolar planet (TEP) discovered from the ground, and the first hot Neptune discovered to date by transit searches. HAT-P-11b orbits the bright (V=9.587) and metal rich ([Fe/H] = +0.31 ± 0.05) K4 dwarf star GSC 03561-02092 with P = 4.8878162 ± 0.0000071 days and produces a transit signal with depth of 4.2 mmag; the shallowest found by transit searches that is due to a confirmed planet. We present a global analysis of the available photometric and radial-velocity data that result in stellar and planetary parameters, with simultaneous treatment of systematic variations. The planet, like its near-twin GJ 436b, is somewhat larger than Neptune (17M ⊕ , 3.8R ⊕ ) both in mass M p = 0.081 ± 0.009 M J (25.8 ± 2.9 M ⊕ ) and radius R p = 0.422 ± 0.014 R J (4.73 ± 0.16 R ⊕ ). HAT-P-11b orbits in an eccentric orbit with e = 0.198 ± 0.046 and ω = 355.2 ± 17.3 • , causing a reflex motion of its parent star with amplitude 11.6 ± 1.2 m s −1 , a challenging detection due to the high level of chromospheric activity of the parent star. Our ephemeris for the transit events is T c = 2454605.89132 ± 0.00032 (BJD), with duration 0.0957 ± 0.0012 d, and secondary eclipse epoch of 2454608.96 ± 0.15 d (BJD). The basic stellar parameters of the host star are M ⋆ = 0.809 +0.020 −0.027 M ⊙ , R ⋆ = 0.752 ± 0.021 R ⊙ and T eff⋆ = 4780 ± 50 K. Importantly, HAT-P-11 will lie on one of the detectors of the forthcoming Kepler mission; this should make possible fruitful investigations of the detailed physical characteristic of both the planet and its parent star at unprecedented precision. We discuss an interesting constraint on the eccentricity of the system by the transit light curve and stellar parameters. This will be particularly useful for eccentric TEPs with low amplitude RV variations in Kepler's field. We also present a blend analysis, that for the first time treats the case of a blended transiting hot Jupiter mimicing a transiting hot Neptune, and proves that HAT-P-11b is not such a blend.

Over the duration of the Kepler mission, KIC 8462852 was observed to undergo irregularly shaped, aperiodic dips in flux of up to ∼20 per cent. The dipping activity can last for between 5 and 80 d. We characterize the object with high-resolution spectroscopy, spectral energy distribution fitting, radial velocity measurements, high-resolution imaging, and Fourier analyses of the Kepler light curve. We determine that KIC 8462852 is a typical main-sequence F3 V star that exhibits no significant IR excess, and has no very close interacting companions. In this paper, we describe various scenarios to explain the dipping events observed in the Kepler light curve. We confirm that the dipping signals in the data are not caused by any instrumental or data processing artefact, and thus are astrophysical in origin. We construct scenario-independent constraints on the size and location of a body in the system that are needed to reproduce the observations. We deliberate over several assorted stellar and circumstellar astrophysical scenarios, most of which have problems explaining the data in hand. By considering the observational constraints on dust clumps in orbit around a normal main-sequence star, we conclude that the scenario most consistent with the data in hand is the passage of a family of exocomet or planetesimal fragments, all of which are associated with a single previous break-up event, possibly caused by tidal disruption or thermal processing. The minimum total mass associated with these fragments likely exceeds 10 −6 M ⊕ , corresponding to an original rocky body of >100 km in diameter. We discuss the necessity of future observations to help interpret the system.

By searching the IRAS and ISO databases, we compiled a list of 60 debris disks that exhibit the highest fractional luminosity values (f d > 10 À4) in the vicinity of the Sun (d < 120 pc). Eleven out of these 60 systems are new discoveries. Special care was taken to exclude bogus disks from the sample. We computed the fractional luminosity values using available IRAS, ISO, and Spitzer data and analyzed the Galactic space velocities of the objects. The results revealed that stars with disks of high fractional luminosity often belong to young stellar kinematic groups, providing an opportunity to obtain improved age estimates for these systems. We found that practically all disks with f d > 5 ; 10 À4 are younger than 100 Myr. The distribution of the disks in the fractional luminosity versus age diagram indicates that (1) the number of old systems with high f d is lower than was claimed before, (2) there exist many relatively young disks of moderate fractional luminosity, and (3) comparing the observations with a current theoretical model of debris disk evolution, a general good agreement could be found.

No abstract

The 30 Myr old A3-type star HD 21997 is one of the two known debris dust disks having a measurable amount of cold molecular gas. With the goal of understanding the physical state, origin, and evolution of the gas in young debris disks, we obtained CO line observations with the Atacama Large Millimeter/submillimeter Array (ALMA). Here we report on the detection of 12 CO and 13 CO in the J=2-1 and J=3-2 transitions and C 18 O in the J=2-1 line. The gas exhibits a Keplerian velocity curve, one of the few direct measurements of Keplerian rotation in young debris disks. The measured CO brightness distribution could be reproduced by a simple star+disk system, whose parameters are r in < 26 AU, r out = 138 ± 20 AU, M * = 1.8 +0.5 −0.2 M ⊙ , and i = 32. • 6 ± 3. • 1. The total CO mass, as calculated from the optically thin C 18 O line, is about (4-8)×10 −2 M ⊕ , while the CO line ratios suggest a radiation temperature on the order of 6-9 K. Comparing our results with those obtained for the dust component of the HD 21997 disk from the ALMA continuum observations by Moór et al., we conclude that comparable amounts of CO gas and dust are present in the disk. Interestingly, the gas and dust in the HD 21997 system are not co-located, indicating a dust-free inner gas disk within 55 AU of the star. We explore two possible scenarios for the origin of the gas. A secondary origin, which involves gas production from colliding or active planetesimals, would require unreasonably high gas production rates and would not explain why the gas and dust are not co-located. We propose that HD 21997 is a hybrid system where secondary debris dust and primordial gas coexist. HD 21997, whose age exceeds both the model predictions for disk clearing and the ages of the oldest T Tauri-like or transitional gas disks in the literature, may be a key object linking the primordial and the debris phases of disk evolution.

According to the current paradigm of circumstellar disk evolution, gas-rich primordial disks evolve into gas-poor debris disks composed of second-generation dust. To explore the transition between these phases, we searched for 12 CO, 13 CO, and C 18 O emission in seven dust-rich debris disks around young A-type stars, using ALMA in Band 6. We discovered molecular gas in three debris disks. In all these disks, the 12 CO line was optically thick, highlighting the importance of less abundant molecules in reliable mass estimates. Supplementing our target list by literature data, we compiled a volume-limited sample of dust-rich debris disks around young A-type stars within 150 pc. We obtained a CO detection rate of 11/16 above a 12 CO J=2-1 line luminosity threshold of ∼1.4×10 4 Jy km s −1 pc 2 in the sample. This high incidence implies that the presence of CO gas in bright debris disks around young A-type stars is likely more the rule than the exception. Interestingly, dust-rich debris disks around young FG-type stars exhibit, with the same detectability threshold as for A-type stars, significantly lower gas incidence. While the transition from protoplanetary to debris phase is associated with a drop of dust content, our results exhibit a large spread in the CO mass in our debris sample, with peak values comparable to those in protoplanetary Herbig Ae disks. In the particularly CO-rich debris systems the gas may have primordial origin, characteristic of a hybrid disk.

The radii of debris disks and the sizes of their dust grains are important tracers of the planetesimal formation mechanisms and physical processes operating in these systems. Here we use a representative sample of 34 debris disks resolved in various Herschel Space Observatory a programs to constrain the disk radii and the size distribution of their dust. While we modeled disks with both warm and cold components, and identified warm inner disks around about two-thirds of the stars, we focus our analysis only on the cold outer disks, i.e. Kuiper-belt analogs. We derive the disk radii from the resolved images and find a large dispersion for host stars of any spectral class, but no significant trend with the stellar luminosity. This argues against ice lines as a dominant player in setting the debris disk sizes, since the ice line location varies with the luminosity of the central star. Fixing the disk radii to those inferred from the resolved images, we model the spectral energy distribution to determine the dust temperature and the grain size distribution for each target. While the dust temperature systematically increases towards earlier spectral types, the ratio of the dust temperature to the blackbody temperature at the disk radius decreases with the stellar luminosity. This is explained by a clear trend of typical sizes increasing towards more luminous stars. The typical grain sizes are compared to the radiation pressure blowout limit s blow that is proportional to the stellar luminosityto-mass ratio and thus also increases towards earlier spectral classes. The grain sizes in the disks of G-to A-stars are inferred to be several times s blow at all stellar luminosities, in agreement with collisional models of debris disks. The sizes, measured in the units of s blow , appear to decrease with the luminosity, which may be suggestive of the disk's stirring level increasing towards earlier-type stars. The dust opacity index β ranges between zero and two, and the size distribution index q varies between three and five for all the disks in the sample.

Gas-rich primordial disks and tenuous gas-poor debris disks are usually considered as two distinct evolutionary phases of the circumstellar matter. Interestingly, the debris disk around the young mainsequence star 49 Ceti possesses a substantial amount of molecular gas, and possibly represents the missing link between the two phases. Motivated to understand the evolution of the gas component in circumstellar disks via finding more 49 Ceti-like systems, we carried out a CO J=3−2 survey with Atacama Pathfinder EXperiment, targeting 20 infrared-luminous debris disks. These systems fill the gap between primordial and old tenuous debris disks in terms of fractional luminosity. Here we report on the discovery of a second 49 Ceti-like disk around the 30 Myr old A3-type star HD21997, a member of the Columba Association. This system was also detected in the CO(2−1) transition, and the reliable age determination makes it an even clearer example of an old gas-bearing disk than 49 Ceti. While the fractional luminosities of HD21997 and 49 Ceti are not particularly high, these objects seem to harbor the most extended disks within our sample. The double-peaked profiles of HD21997 were reproduced by a Keplerian disk model combined with the LIME radiative transfer code. Based on their similarities, 49 Ceti and HD21997 may be the first representatives of a so far undefined new class of relatively old ( 8 Myr), gaseous dust disks. From our results, neither primordial origin nor steady secondary production from icy planetesimals can unequivocally explain the presence of CO gas in the disk of HD21997.