We report detection with the Spitzer Space Telescope of cool dust surrounding solar type stars. The observations were performed as part of the Legacy Science Program, "Formation and Evolution of Planetary Systems" (F EP S). From the overall F EP S sample of 328 stars having ages ∼0.003-3 Gyr we have selected sources with 70 µm flux densities indicating excess in their spectral energy distributions above expected photospheric emission. Six strong excess sources are likely primordial circumstellar disks, remnants of the star formation process. Another 25 sources having ≥ 3σ excesses are associated with dusty debris disks, generated by collisions within planetesimal belts that are possibly stirred by existing planets. We draw attention to six additional sources with ≥ 2σ excesses which require confirmation as debris disks. In our analysis, most (>80%) of the debris disks identified via 70 µm excesses have ≥ 3σ excesses at 33 µm as well, while only a minority (<40%) have ≥ 3σ excesses at 24 µm.The rising spectral energy distributions towards -and perhaps beyond -70 µm imply dust temperatures T dust <45-85 K for debris in equilibrium with the stellar radiation field. We infer bulk properties such as characteristic temperature, location, fractional luminosity, and mass of the dust from fitted single temperature blackbody models. For >1/3 of the debris sources we find that multiple temperature components are suggested, implying a spatial distribution of dust extending over many tens of AU. Because the disks are dominated by collisional processes, the parent body (planetesimal) belts may be extended as well. Preliminary assessment of the statistics of cold debris around sun-like stars shows that ∼10% of F EP S targets with masses between 0.6 and 1.8 M ⊙ and ages between 30 Myr and 3 Gyr exhibit 70 µm emission in excess of the expected photospheric flux density. We find that fractional excess amplitudes appear higher for younger stars and that there may be a trend in 70 µm excess frequency with stellar mass.The F EP S program utilized all three Spitzer science instruments -IRAC, IRS, and MIPS -to observe 328 solar-type stars. Meyer et al. (2006) provides a description of the F EP S observing strategy. Among the F EP S sample are 15 previously suspected (based on IRAS or ISO literature) debris or long-lived primordial disk systems, only 11 of which are in fact confirmed by Spitzer. Ten of these 15 were observed by F EP S for the purpose of probing primordial gas disk dissipation (e.g. Pascucci et al 2006Pascucci et al , 2007 while the others were either serendipitously on our lists or discovered as excess sources after the F EP S program was submitted. The sources selected ab initio because they were claimed to exhibit infrared excess emission can not be included in statistical analyses of F EP S Spitzer data for debris characteristics as a function of e.g. stellar age, stellar mass, stellar metallicity, stellar rotation, etc. However, we do include them in this paper which presents disk detections and simple dust models....

Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.

We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRS low resolution) observations for 314 stars in the Formation and Evolution of Planetary Systems (FEPS) Legacy program. These data are used to investigate the properties and evolution of circumstellar dust around solar-type stars spanning ages from approximately 3 Myr to 3 Gyr. We identify 46 sources that exhibit excess infrared emission above the stellar photosphere at 24µm, and 21 sources with excesses at 70µm. Five sources with an infrared excess have characteristics of optically thick primordial disks, while the remaining sources have properties akin to debris systems. The fraction of systems exhibiting a 24µm excess greater than 10.2% above the photosphere is 15% for ages < 300 Myr and declines to 2.7% for older ages. The upper envelope to the 70µm fractional luminosity appears to decline over a similar age range. The characteristic temperature of the debris inferred from the IRS spectra range between 60 and 180 K, with evidence for the presence of cooler dust to account for the strength of the 70µm excess emission. No strong correlation is found between dust temperature and stellar age. Comparison of the observational data with disk models containing a powerlaw distribution of silicate grains suggest that the typical inner disk radius is > ∼ 10 AU. Although the interpretation is not unique, the lack of excess emission shortwards of 16µm and the relatively flat distribution of the 24µm excess for ages < ∼ 300 Myr is consistent with steady-state collisional models.

We report the discovery of T dwarf companions to the nearby stars HN Peg (G0V, 18.4 pc, τ ∼ 0.3 Gyr) and HD 3651 (K0V, 11.1 pc, τ ∼ 7 Gyr). During an ongoing survey of 5 ′ ×5 ′ fields surrounding stars in the solar neighborhood with the Infrared Array Camera aboard the Spitzer Space Telescope, we identified these companions as candidate T dwarfs based on their mid-infrared colors. Using near-infrared spectra obtained with SpeX at the NASA Infrared Telescope Facility, we confirm the presence of methane absorption that characterizes T dwarfs and measure spectral types of T2.5±0.5 and T7.5±0.5 for HN Peg B and HD 3651 B, respectively. By comparing our Spitzer data to images from the Two-Micron All-Sky Survey obtained several years earlier, we find that the proper motions of HN Peg B and HD 3651 B are consistent with those of the primaries, confirming their companionship. HN Peg B and HD 3651 B have angular separations of 43. ′′ 2 and 42. ′′ 9 from their primaries, which correspond to projected physical separations of 795 and 476 AU, respectively. A comparison of their luminosities to the values predicted by theoretical evolutionary models implies masses of 0.021 ± 0.009 and 0.051 ± 0.014 M ⊙ for HN Peg B and HD 3651 B. In addition, the models imply an effective temperature for HN Peg B that is significantly lower than the values derived for other T dwarfs at similar spectral types, which is the same behavior reported by Metchev & Hillenbrand for the young late-L dwarf HD 203030 B. Thus, the temperature of the L/T transition appears to depend on surface gravity. Meanwhile, HD 3651 B is the first substellar companion directly imaged around a star that is known to harbor a close-in planet from radial velocity surveys. The discovery of this companion supports the notion that the high eccentricities of close-in planets like the one near HD 3651 may be the result of perturbations by low-mass companions at wide separations.

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ∼10 in amplitude) flare in γ-rays towards the end of a 3-week pointing by CGRO, in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ∼8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with RXTE, ASCA, ROSAT and IUE and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of ≃3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ∼1 day. The optical-UV light curves, which are not well sampled during the high energy flare, exhibit more modest variations (factor of ∼2) and a lower degree of correlation. The flux at millimetric wavelengths was near an historical maximum during the γ-ray flare peak and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical Stanford, CA 94305