Observations from the Herschel Space Observatory have more than doubled the number of wide debris disks orbiting Sunlike stars to include over 30 systems with R > 100 AU. Here we present new Herschel PACS and re-analyzed Spitzer MIPS photometry of five Sunlike stars with wide debris disks, from Kuiper belt size to R > 150 AU. The disk surrounding HD 105211 is well resolved, with an angular extent of >14" along the major axis, and the disks of HD 33636, HD 50554, and HD 52265 are extended beyond the PACS PSF size (50% of energy enclosed within radius 4.23"). HD 105211 also has a 24 mu m infrared excess that was previously overlooked because of a poorly constrained photospheric model. Archival Spitzer IRS observations indicate that the disks have small grains of minimum radius a(min) similar to 3 mu m, although the a(min) is larger than the radiation pressure blowout size in all systems. If modeled as single-temperature blackbodies, the disk temperatures would all be <60 K. Our radiative transfer models predict actual disk radii approximately twice the radius of model blackbody disks. We find that the Herschel photometry traces dust near the source population of planetesimals. The disk luminosities are in the range 2 x 10(-5) <= L/L-circle dot <= 2 x 10(-4) , consistent with collisions in icy planetesimal belts stirred by Pluto-size dwarf planets.

We present a spectral analysis of the spectrum from the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) of the southern VY Scl nova-like variable BB Doradus, obtained as part of a Cycle 20 HST/COS survey of accreting white dwarfs (WDs) in cataclysmic variables. BB Dor was observed with COS during an intermediate state with a low mass accretion rate, thereby allowing an estimate of the WD temperature. The results of our spectral analysis show that the WD is a significant far-ultraviolet (FUV) component of the spectrum with a temperature of about 35,000-50,000 K, assuming a WD mass of 0.80 M-circle dot (log(g) = 8.4). The disk, with a mass accretion rate of approximate to 10(-10) M-circle dot yr(-1), contributes about 1/5 to 1/2 of the FUV flux.

Open clusters have been the focus of several exoplanet surveys, but only a few planets have so far been discovered. The Kepler spacecraft revealed an abundance of small planets around small cool stars, therefore, such cluster members are prime targets for exoplanet transit searches. Kepler's new mission, K2, is targeting several open clusters and star-forming regions around the ecliptic to search for transiting planets around their low-mass constituents. Here, we report the discovery of the first transiting planet in the intermediate-age (800 Myr) Beehive cluster (Praesepe). K2-95 is a faint (Kp = 15.5 mag) M3.0 +/- 0.5 dwarf from K2's Campaign 5 with an effective temperature of 3471 +/- 124 K, approximately solar metallicity and a radius of 0.402 +/- 0.050 R-circle dot. We detected a transiting planet with a radius of 3.47(-0.53)(+0.78)R(circle plus) and an orbital period of 10.134 days. We combined photometry, medium/high-resolution spectroscopy, adaptive optics/speckle imaging, and archival survey images to rule out any false-positive detection scenarios, validate the planet, and further characterize the system. The planet's radius is very unusual as M-dwarf field stars rarely have Neptune-sized transiting planets. The comparatively large radius of K2-95b is consistent with the other recently discovered cluster planets K2-25b (Hyades) and K2-33b (Upper Scorpius), indicating systematic differences in their evolutionary states or formation. These discoveries from K2 provide a snapshot of planet formation and evolution in cluster environments and thus make excellent laboratories to test differences between field-star and cluster planet populations.

Condensate clouds fundamentally impact the atmospheric structure and spectra of exoplanets and brown dwarfs, but the connections between surface gravity, cloud structure, dust in the upper atmosphere, and the red colors of some brown dwarfs remain poorly understood. Rotational modulations enable the study of different clouds in the same atmosphere, thereby providing a method to isolate the effects of clouds. Here, we present the discovery of high peak-to-peak amplitude (8%) rotational modulations in a low-gravity, extremely red (J-K-s = 2.55) L6 dwarf WISEP J004701.06+680352.1 (W0047). Using the Hubble Space Telescope (HST) time-resolved grism spectroscopy, we find a best-fit rotational period (13.20 +/- 0.14 hr) with a larger amplitude at 1.1 mu m than at 1.7 mu m. This is the third-largest near-infrared variability amplitude measured in a brown dwarf, demonstrating that large-amplitude variations are not limited to the L/T transition but are present in some extremely red L-type dwarfs. We report a tentative trend between the wavelength dependence of relative amplitude, possibly proxy for small dust grains lofted in the upper atmosphere, and the likelihood of large-amplitude variability. By assuming forsterite as a haze particle, we successfully explain the wavelength-dependent amplitude with submicron-sized haze particle sizes of around 0.4 mu m. W0047 links the earlier spectral and later spectral type brown dwarfs in which rotational modulations have been observed; the large amplitude variations in this object make this a benchmark brown dwarf for the study of cloud properties close to the L/T transition.

We present infrared spectroscopy of the classical nova V339 Del, obtained over an similar to 2-yr period. The infrared emission lines were initially symmetrical, with half width half-maximum velocities of 525 km s(-1). In later (t greater than or similar to 77 d, where t is the time from outburst) spectra, however, the lines displayed a distinct asymmetry, with a much stronger blue wing, possibly due to obscuration of the receding component by dust. Dust formation commenced at approximately day 34.75 at a condensation temperature of 1480 +/- 20 K, consistent with graphitic carbon. Thereafter, the dust temperature declined with time as T-d alpha t(-0.346), also consistent with graphitic carbon. The mass of dust initially rose, as a result of an increase in grain size and/or number, peaked at approximately day 100, and then declined precipitously. This decline was most likely caused by grain shattering due to electrostatic stress after the dust was exposed to X-radiation. The appendix summarizes Planck means for carbon and the determination of grain mass and radius for a carbon dust shell.

WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age (< 1 Gyr), the star presents an anomalously low stellar activity level: the measured log R'(HK) activity parameter lies slightly below the basal level; there is no significant time-variability in the log R'(HK) value; there is no detection of the star in the X-rays. We present results of far-UV observations of WASP-18 obtained with COS on board of Hubble Space Telescope aimed at explaining this anomaly. From the star's spectral energy distribution, we infer the extinction (E(B-V) approximate to 0.01 mag) and then the interstellar medium (ISM) column density for a number of ions, concluding that ISM absorption is not the origin of the anomaly. We measure the flux of the four stellar emission features detected in the COS spectrum (C II, C III, C IV, Si IV). Comparing the C II/C IV flux ratio measured for WASP-18 with that derived from spectra of nearby stars with known age, we see that the far-UV spectrum of WASP-18 resembles that of old (> 5 Gyr), inactive stars, in stark contrast with its young age. We conclude that WASP-18 has an intrinsically low activity level, possibly caused by star-planet tidal interaction, as suggested by previous studies. Re-scaling the solar irradiance reference spectrum to match the flux of the Si IV line, yields an XUV integrated flux at the planet orbit of 10.2 erg s(-1) cm(-2). We employ the rescaled XUV solar fluxes to models of the planetary upper atmosphere, deriving an extremely low thermal mass-loss rate of 10(-20) M-J Gyr(-1). For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.

HD 100453AB is a 10 +/- 2 Myr old binary whose protoplanetary disk was recently revealed to host a global two-armed spiral structure. Given the relatively small projected separation of the binary (1.'' 05, or similar to 108 au), gravitational perturbations by the binary seemed to be a likely driving force behind the formation of the spiral arms. However, the orbit of these stars remained poorly understood, which prevented a proper treatment of the dynamical influence of the companion on the disk. We observed HD. 100453AB between 2015 and 2017, utilizing extreme adaptive optics systems on the Very Large Telescope and the Magellan Clay Telescope. We combined the astrometry from these observations with published data to constrain the parameters of the binary's orbit to a = 1.'' 06 +/- 0.'' 09, e = 0.17 +/- 0.07, and i = 32 degrees.5 +/- 6 degrees.5. We utilized publicly available ALMA (CO)-C-12 data to constrain the inclination of the disk, i(disk) similar to 28 degrees, which is relatively coplanar with the orbit of the companion and consistent with previous estimates from scattered light images. Finally, we input these constraints into hydrodynamic and radiative transfer simulations to model the structural evolution of the disk. We find that the spiral structure and truncation of the circumprimary disk in HD 100453 are consistent with a companion-driven origin. Furthermore, we find that the primary star's rotation, its outer disk, and the companion exhibit roughly the same direction of angular momentum, and thus the system likely formed from the same parent body of material.

We present the first scattered-light images of two debris disks around the F8 star HD. 104860 and the F0V star HD. 192758, respectively similar to 45 and similar to 67. pc away. We detected these systems in the F110W and F160W filters through our reanalysis of archival Hubble Space Telescope (HST) NICMOS data with modern starlight-subtraction techniques. Our image of HD. 104860 confirms the morphology previously observed by Herschel in thermal emission with a well-defined ring at a radius of similar to 114. au inclined by similar to 58 degrees. Although the outer edge profile is consistent with dynamical evolution models, the sharp inner edge suggests sculpting by unseen perturbers. Our images of HD. 192758 reveal a disk at radius similar to 95. au inclined by similar to 59 degrees, never resolved so far. These disks have low scattering albedos of 10% and 13%, respectively, inconsistent with water ice grain compositions. They are reminiscent of several other disks with similar inclination and scattering albedos: Fomalhaut, HD. 92945, HD. 202628, and HD. 207129. They are also very distinct from brighter disks in the same inclination bin, which point to different compositions between these two populations. Varying scattering albedo values can be explained by different grain porosities, chemical compositions, or grain size distributions, which may indicate distinct formation mechanisms or dynamical processes at work in these systems. Finally, these faint disks with large infrared excesses may be representative of an underlying population of systems with low albedo values. Searches with more sensitive instruments on HST or on the James Webb Space Telescope and using state-of-the art starlight-subtraction methods may help discover more of such faint systems.

GJ 436b is a prime target for understanding warm Neptune exoplanet atmospheres and a target for multiple James Webb Space Telescope (JWST) Guaranteed Time Observation programs. Here, we report the first space-based optical transmission spectrum of the planet using two Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) transit observations from 0.53 to 1.03 mu m. We find no evidence for alkali absorption features, nor evidence of a scattering slope longward of 0.53 mu m. The spectrum is indicative of moderate to high metallicity (similar to 100-1000x solar), while moderate-metallicity scenarios (similar to 100x. solar) require aerosol opacity. The optical spectrum also rules out some highly scattering haze models. We find an increase in transit depth around 0.8 mu m in the transmission spectra of three different sub-Jovian exoplanets (GJ 436b, HAT-P-26b, and GJ 1214b). While most of the data come from STIS, data from three other instruments may indicate this is not an instrumental effect. Only the transit spectrum of GJ 1214b is well fit by a model with stellar plages on the photosphere of the host star. Our photometric monitoring of the host star reveals a stellar rotation rate of 44.1 days and an activity cycle of 7.4 years. Intriguingly, GJ 436 does not become redder as it gets dimmer, which is expected if star spots were dominating the variability. These insights into the nature of the GJ 436 system help refine our expectations for future observations in the era of JWST, whose higher precision and broader wavelength coverage will shed light on the composition and structure of GJ 436b's atmosphere.

It has recently been discovered that some, if not all, classical novae emit GeV gamma-rays during outburst, but the mechanisms involved in the production ofgamma-rays are still not well understood. We present here a comprehensive multiwavelength data set-from radio to X-rays-for the most gamma-ray-luminous classical nova to date, V1324 Sco. Using this data set, we show that V1324 Sco is a canonical dusty Fe II-type nova, with a maximum ejecta velocity of 2600 km s(-1) and an ejecta mass of a few x 10(-5) M-circle dot. There is also evidence for complex shock interactions, including a double-peaked radio light curve which shows high brightness temperatures at early times. To explore why V1324. Sco was so gamma-ray luminous, we present a model of the nova ejecta featuring strong internal shocks and find that higher gamma-ray luminosities result from higher ejecta velocities and/or mass-loss rates. Comparison of V1324. Sco with other gamma-ray-detected novae does not show clear signatures of either, and we conclude that a larger sample of similarly well-observed novae is needed to understand the origin and variation of gamma-rays in novae.