A second planetesimal collision in the Fomalhaut system.

A variety of interesting objects such as Wolf–Rayet stars, tight OB associations, planetary nebulae, X-ray binaries, etc., can be discovered as point or compact sources in Hα surveys. How these objects distribute through a galaxy sheds light on the galaxy star formation rate and history, mass distribution, and dynamics. The nearby galaxy M33 is an excellent place to study the distribution of Hα-bright point sources in a flocculant spiral galaxy. We have reprocessed an archived WIYN continuum-subtracted Hα image of the inner 6.′5 × 6.′5 of M33 and, employing both eye and machine searches, have tabulated sources with a flux greater than approximately 10−15 erg cm−2s−1. We have effectively recovered previously mapped H ii regions and have identified 152 unresolved point sources and 122 marginally resolved compact sources, of which 39 have not been previously identified in any archive. An additional 99 Hα sources were found to have sufficient archival flux values to generate a Spectral Energy Distribution. Using the SED, flux values, Hα flux value, and compactness, we classified 67 of these sources.

We observed supernova 1987A (SN 1987A) with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) in 1999 September and again with the Advanced Camera for Surveys (ACS) on the HST in 2003 November. Our spectral observations cover ultraviolet (UV) and optical wavelengths from 1140 to 10266 A, and our imaging observations cover UV and optical wavelengths from 2900 to 9650 A. No point source is observed in the remnant. We obtain a limiting flux of Fopt<=1.6×10-14 ergs s-1 cm-2 in the wavelength range 2900-9650 A for any continuum emitter at the center of the supernova remnant (SNR). This corresponds to an intrinsic luminosity of Lopt<=5×1033 ergs s-1. It is likely that the SNR contains opaque dust that absorbs UV and optical emission, resulting in an attenuation of ~35% due to dust absorption in the SNR. Correcting for this level of dust absorption would increase our upper limit on the luminosity of a continuum source by a factor of 1.54. Taking into account dust absorption in the remnant, we find a limit of Lopt<=8×1033 ergs s-1. We compare this upper bound with empirical evidence from point sources in other supernova remnants and with theoretical models for possible compact sources. We show that any survivor of a possible binary system must be no more luminous than an F6 main-sequence star. Bright young pulsars such as Kes 75 or the Crab pulsar are excluded by optical and X-ray limits on SN 1987A. Other nonplerionic X-ray point sources have luminosities similar to the limits on a point source in SN 1987A; RCW 103 and Cas A are slightly brighter than the limits on SN 1987A, while Pup A is slightly fainter. Of the young pulsars known to be associated with SNRs, those with ages <=5000 yr are all too bright in X-rays to be compatible with the limits on SN 1987A. Examining theoretical models for accretion onto a compact object, we find that spherical accretion onto a neutron star is firmly ruled out and that spherical accretion onto a black hole is possible only if there is a larger amount of dust absorption in the remnant than predicted. In the case of thin-disk accretion, our flux limit requires a small disk, no larger than 1010 cm, with an accretion rate no more than 0.3 times the Eddington accretion rate. Possible ways to hide a surviving compact object include the removal of all surrounding material at early times by a photon-driven wind, a small accretion disk, or very high levels of dust absorption in the remnant. It will not be easy to improve substantially on our optical-UV limit for a point source in SN 1987A, although we can hope that a better understanding of the thermal infrared emission will provide a more complete picture of the possible energy sources at the center of SN 1987A.

Future direct observations of extrasolar Earth-sized planets in the habitable zone could be hampered by a worrisome source of noise, starlight-reflecting exozodiacal dust. Mid-infrared surveys are currently underway to constrain the amount of exozodiacal dust in the habitable zones around nearby stars. However, at visible wavelengths another source of dust, invisible to these surveys, may dominate over exozodiacal dust. For systems observed near edge-on, a cloud of dust with face-on optical depth 10^-7 beyond ~5 AU can mimic the surface brightness of a cloud of exozodiacal dust with equal optical depth if the dust grains are sufficiently forward-scattering. We posit that dust migrating inward from cold debris belts via Poynting-Robertson drag could produce this "pseudo-zodiacal" effect, potentially making it ~50% as common as exozodiacal clouds. We place constraints on the disk radii and scattering phase function required to produce the effect.

Planetary debris disks around other stars are analogous to the asteroid and Kuiper belts in the Solar System. Their structure reveals the configuration of small bodies and provides hints for the presence of planets. The nearby star Fomalhaut hosts one of the most prominent debris disks, resolved by the Hubble Space Telescope, Spitzer, Herschel and the Atacama Large Millimeter Array. Images of this system at mid-infrared wavelengths using JWST/MIRI not only show the narrow Kuiper belt-analogue outer ring, but also that (1) what was thought from indirect evidence to be an asteroid-analogue structure is instead broad, extending outward into the outer system, and (2) there is an intermediate belt, probably shepherded by an unseen planet. The newly discovered belt is demarcated by an inner gap, located at ~78 au, and it is misaligned relative to the outer belt. The previously known collisionally generated dust cloud, Fomalhaut b, could have originated from this belt, suggesting increased dynamical stirring and collision rates there. We also discovered a large dust cloud within the outer ring, possible evidence of another dust-creating collision. Taken together with previous observations, Fomalhaut appears to be the site of a complex and possibly dynamically active planetary system. JWST mid-infrared images of the nearby star Fomalhaut reveal a complex system of dusty rings and disks, created as debris from planetesimal collisions. These structures suggest the presence of a complex and probably dynamically active planetary system.

At 7.7 pc, the A-type star Fomalhaut hosts a bright debris disc with multiple radial components. The disc is eccentric and misaligned, strongly suggesting that it is sculpted by interaction with one or more planets. Compact sources are now being detected with JWST, suggesting that new planet detections may be imminent. However, to confirm such sources as companions, common proper motion with the star must be established, as with unprecedented sensitivity comes a high probability that planet candidates are actually background objects. Here, Atacama Large Millimetre/Submillimetre Array and Keck observations of Fomalhaut are found to show significant emission at the same sky location as multiple compact sources in JWST Mid-Infrared Instrument coronagraphic observations, one of which has been dubbed the ‘Great Dust Cloud’ because it lies within the outer belt. Since the ground-based data were obtained between 6 and 18 yr prior to the JWST observations, these compact sources are unlikely to be common proper motion companions to Fomalhaut. More generally, this work illustrates that images collected at a range of wavelengths can be valuable for rejecting planet candidates uncovered via direct imaging with JWST.

Context.The discovery of planets orbiting at less than 1 au from their host star and less massive than Saturn in various exoplanetary systems revolutionized our theories of planetary formation. The fundamental question is whether these close-in low-mass planets could have formed in the inner disk interior to 1 au, or whether they formed further out in the planet-forming disk and migrated inward. Exploring the role of additional giant planet(s) in these systems may help us to pinpoint their global formation and evolution.Aims.We searched for additional substellar companions by using direct imaging in systems known to host close-in small planets. The use of direct imaging complemented by radial velocity and astrometric detection limits enabled us to explore the giant planet and brown dwarf demographics around these hosts to investigate the potential connection between both populations.Methods.We carried out a direct imaging survey with SPHERE at VLT to look for outer giant planets and brown dwarf companions in 27 systems hosting close-in low-mass planets discovered by radial velocity. Our sample is composed of very nearby (&lt;20 pc) planetary systems, orbiting G-, K-, and M-type mature (0.5–10 Gyr) stellar hosts. We performed homogeneous direct imaging data reduction and analysis to search for and characterize point sources, and derived robust statistical detection limits. The final direct imaging detection performances were globally considered together with radial velocity and astrometric sensitivity.Results.Of 337 point-source detections, we do not find any new bound companions. We recovered the emblematic very cool T-type brown dwarf GJ 229 B. Our typical sensitivities in direct imaging range from 5 to 30MJupbeyond 2 au. The non-detection of massive companions is consistent with predictions based on models of planet formation by core accretion. Our pilot study opens the way to a multi-technique approach for the exploration of very nearby exoplanetary systems with future ground-based and space observatories.

The inner solar system, where the terrestrial planets formed and evolve, is populated by small grains of dust produced by collisions of asteroids and outgassing comets. At visible and infrared wavelengths, this dust cloud is in fact the most luminous component in the solar system after the Sun itself and the Earth may appear similar to a clump of zodiacal dust to an external observer. Hence, the presence of large amounts of dust in the habitable zone around nearby main-sequence stars is considered as a major hurdle toward the direct imaging of exoEarths with future dedicated space-based telescopes. In that context, we address in this paper the detectability of exoEarths embedded in structured debris disks with future space-based visible coronagraphs and mid-infrared interferometers. Using a collisional grooming algorithm, we produce models of dust clouds that simultaneously and self-consistently handle dust grain dynamics, including resonant interactions with planets, and grain-grain collisions. Considering various viewing geometries, we also derive limiting dust densities that can be tolerated around nearby main-sequence stars in order to ensure the characterization of exoEarths with future direct imaging missions.

Black holes start their lives as big dead stars in the depth of the universe. As they move towards the outer universe they gather cosmic dust, asteroids, planets, eventually they get bigger. The gravitational force increases and continues this process for billions of years. When an asteroid enters the gravitational field of an ever-growing dead star, it is pulled in and gets smashed against the core’s surface, into fundamental particles. The gravitational pull has grown to such an extent, the protons and neutrons get packed into the core so tightly that electrons cannot penetrate the core’s surface to form atoms with the firmly embedded protons and neutrons of the core. All the stars in a galaxy have rotation and revolution. How do they get it? Galaxies are rather flat. Why? This research aims to understand facts of this theory on the evolution of the barred spiral galaxies. As the cores of the black sphere blast out the stars, huge quantity of atomic dust and gas are also released. These dust and gas clouds speed along with the stars, enter, and illuminate the Arms. The open end of the Arm presents its open end and gives these orbiting dust clouds and the stars a passageway into the space. The dust clouds and the stars enter the space and create the Spiral Arms. These atomic dust and gas particles reflect and scatter the light rays from the nearby stars.

We present recent polarimetric images of the highly variable star R CrB using ExPo and archival WFPC2 images from the HST. We observed R CrB during its current dramatic minimum where it decreased more than 9 mag due to the formation of an obscuring dust cloud. Since the dust cloud is only in the line-of-sight, it mimics a coronograph allowing the imaging of the star’s circumstellar environment. Our polarimetric observations surprisingly show another scattering dust cloud at approximately 1.3′′ or 2000 AU from the star. We find that to obtain a decrease in the stellar light of 9 mag and with 30% of the light being reemitted at infrared wavelengths (from R CrB’s SED) the grains in R CrB’s circumstellar environment must have a very low albedo of approximately 0.07%. We show that the properties of the dust clouds formed around R CrB are best fitted using a combination of two distinct populations of grains size. The first are the extremely small 5 nm grains, formed in the low density continuous wind, and the second population of large grains (~0.14 μm) which are found in the ejected dust clouds. The observed scattering cloud, not only contains such large grains, but is exceptionally massive compared to the average cloud.

The centre of our Galaxy has been observed extensively at infrared and radio wavelengths. The compact radio source at the nucleus (Sgr A*) may be a weaker form of those in active galaxies. The region is obscured in the visible band by ~30 magnitudes of absorption but the intervening dust and gas clouds become transparent again for X-rays of energy greater than a few keV. We report here observations made with a coded mask X-ray telescope flown on the Spacelab 2 mission (29 July to 6 August 1985), yielding for the first time images of the galactic centre in high-energy X-rays up to 30 keV. Components detected include a region of diffuse emission 2° in diameter and several new point sources. At the higher energies emission from the nucleus is weak and the region is dominated by one of the surrounding point sources.

Recent observations suggest that galactic line emission at 511 keV results from the superposition of contributions from a variable, compact source and an interstellar distribution of positrons resulting from the decay of radionuclei produced by thermonuclear burning supernovae. The compact point source could have turned on as recently as 1977 and has not been seen since 1979. Photon‐photon pair production in the vicinity of a relatively small black hole (<103 M⊙) could be the source of the annihilating positrons in the point source. It is not known whether this compact object lies exactly at the Galactic Center.

The Hubble Space Telescope (HST) is widely viewed as one of the most important scientific and technological achievements of modern times, comparable in its impact to Galileo's first use of the telescope for fundamental astronomical research in 1610. Although it is not the first astronomical observatory to exploit the benefits of viewing the Universe from outside Earth's atmosphere, it is the first to realize fully the gain inclarityof astronomical images that results from the absence of atmospheric turbulence. Without having to contend with the atmosphere's rapidly fluctuating refraction and transmission, the HST's angular resolution is limited primarily by light diffraction at the entrance aperture of its 2.4‐meter telescope.Earth's atmosphere glows from the emission of light by excited atoms and molecules. It is opaque at ultraviolet wavelengths below about 300 nm and strongly absorbs in broad intervals of the near‐infrared band above 1100 nm. Outside the atmosphere, the optics of the Hubble telescope and its scientific instruments provide sharply focused and remarkably stable images against a very dark sky at wavelengths that span approximately 4.5 octaves—110 to 2500 nm. The ability to concentrate light from a point or compact source into a tightly focused image superposed on a dark, low‐noise background allows the relatively small‐aperture HST to detect extremely faint astronomical objects in its direct imaging mode—fainter by as much as 1.5 stellar magnitudes (four times fainter) than current 8–10 meter mountaintop telescopes.This unique combination of capabilities has made HST one of the most productive scientific tools of modern times—and one of the most sought after. Observing time on Hubble is allocated by a process of competitive peer review on the basis of scientific research proposals submitted yearly by astronomers from all over the world. The demand for using HST exceeds the available time typically by a factor of 6:1. The result is an almost continuous stream of amazing scientific accomplishments; many were unanticipated before Hubble's launch. These include the deepest view of the Universe ever acquired that revealed protogalaxies whose light was emitted when the Universe was less than 10% of its present age, the first demographic census of supermassive black holes at the centers of galaxies, accurate calibration of the age and expansion rate of the Universe, strong evidence acquired in partnership with ground‐based observatories that cosmic expansion is accelerating, and frequent observation of dusty disks containing complex structures of rings and gaps possibly indicative of planet formation around other stars.The HST is essentially unique among robotic space missions because it was designed for a long lifetime in space, enabled by regular orbital visits by crews aboard the Space Shuttle who implement technological upgrades of Hubble's instruments and other systems and perform a variety of maintenance and repair tasks on the spacecraft. This concept of preplanned, periodic servicing missions by Shuttle astronauts allowed the correction of a serious optical flaw in Hubble's telescope that was discovered shortly after it was first deployed in 1990. The first HST servicing mission in 1993 demonstrated that humans can carry out arduous and complex work during a period of many days, encumbered by bulky spacesuits in the severe environment of low Earth orbit. Without the intervention of the Human Space Flight Program, the unmanned Hubble observatory would undoubtedly have come to be viewed by history as an embarrassing failure. Instead, Hubble became a national icon.

A 0.5-3-keV X-ray map of the Perseus cluster of galaxies is presented. The map shows a region of strong emission centered near NGC 1275 plus a highly elongated emission region which lies along the line of bright galaxies that dominates the core of the cluster. The data are compared with various models that include point and diffuse sources. One model which adequately represents the data is the superposition of a point source at NGC 1275 and an isothermal ellipsoid resulting from the bremsstrahlung emission of cluster gas. The ellipsoid has a major core radius of 20.5 arcmin and a minor core radius of 5.5 arcmin, consistent with the values obtained from galaxy counts. All acceptable models provide evidence for a compact source (less than 3 arcmin FWHM) at NGC 1275 containing about 25% of the total emission. Since the diffuse X-ray and radio components have radically different morphologies, it is unlikely that the emissions arise from a common source, as proposed in inverse-Compton models.

GRB 970228 is the first gamma ray burst for which prolonged post-burst transient x-ray, optical, and infrared emission has been detected. Recent Hubble Space Telescope observations show that the transient consists of two components: a point source, which is known to be fading, and an extended source, which is possibly fading. I fit standard fireball remnant models to the first month of x-ray, optical, and infrared measurements, which may be done without assuming a GRB distance scale. I show that its emission is consistent with that of the remnant of a relativistically expanding impulsive fireball in which a forward shock dominates the emission of the GRB event: the piston model. However, two discrepant measurements may indicate that the post-burst flux varies by factors of approximately 3 on timescales of days or weeks. Furthermore, using the HST observations and the fitted model, I show that the extended object probably is fading, which may place GRB 970228 at galactic halo distances.

The effect of Nix and Hydra on the putative Pluto–Charon dust cloud