Abstracts (not all papers listed)

STIS Coronagraphic Imaging of Fomalhaut: Main Belt Structure and the Orbit of Fomalhaut b

Paul Kalas, James R. Graham, Michael Fitzgerald, & Mark Clampin
2013, The Astrophysical Journal

We present new optical coronagraphic data of Fomalhaut obtained with the HST/STIS in 2010 and 2012. Fomalhaut b is recovered at both epochs to high significance. The observations include the discoveries of tenuous nebulosity beyond the main dust belt detected to at least 209 AU projected radius, and a ~50 AU wide azimuthal gap in the belt northward of Fomalhaut b. The two epochs of STIS photometry exclude optical variability greater than 35%. A MCMC analysis demonstrates that the orbit of Fomalhaut $b$ is highly eccentric, with e = 0.8+/-0.1, a = 177+/-68 AU, and q = 32+/-24 AU. Fomalhaut b is apsidally aligned with the belt and 90% of allowed orbits have mutual inclination $\leq36\degr$. Fomalhaut b's orbit is belt-crossing in the sky plane projection, but only 12% of possible orbits have ascending or descending nodes within a 25 AU wide belt annulus. The high eccentricity invokes a dynamical history where Fomalhaut b may have experienced a significant dynamical interaction with a hypothetical planet Fomalhaut c, and the current orbital configuration may be relatively short-lived. The Tisserand parameter with respect to a hypothetical Fomalhaut planet at 30 AU or 120 AU lies in the range 2-3, similar to highly eccentric dwarf planets in our solar system. We argue that Fomalhaut b's minimum mass is that of a dwarf planet in order for a circumplanetary satellite system to remain bound to a sufficient radius from the planet to be consistent with the dust scattered light hypothesis. In the coplanar case, Fomalhaut b will collide with the main belt around 2032, and the subsequent emergent phenomena may help determine its physical nature.

Optical images of an exosolar planet 25 light-years from Earth

Paul Kalas, James R. Graham, Eugene Chiang, Michael Fitzgerald, Mark Clampin, Edwin S. Kite, Karl Stapelfeldt, John Krist
2008, Science, Vol. 322, p. 1345 - 1347

Fomalhaut, a bright star 7.7 parsecs (25 light-years) from Earth, harbors a belt of cold dust with a structure consistent with gravitational sculpting by an orbiting planet. Here, we present optical observations of an exoplanet candidate, Fomalhaut b. Fomalhaut b lies about 119 astronomical units (AU) from the star and 18 AU from the dust belt, matching predictions of its location. Hubble Space Telescope observations separated by 1.73 years reveal counterclockwise orbital motion. Dynamical models of the interaction between the planet and the belt indicate that the planet’s mass is at most three times that of Jupiter; a higher mass would lead to gravitational disruption of the belt. The flux detected at 0.8 micron is also consistent with that of a planet with mass no greater than a few times that of Jupiter. The brightness at 0.6 micron and the lack of detection at longer wavelengths suggest that the detected flux may include starlight reflected off a circumplanetary disk, with dimension comparable to the orbits of the Galilean satellites. We also observe variability of unknown origin at 0.6 micron.

Discovery of extreme asymmetry in the debris disk surrounding HD 15115

Paul Kalas, Michael P. Fitzgerald and James R. Graham
2007, The Astrophysical Journal, Vol. 661, p. L85 - L88

We report the first scattered light detection of a dusty debris disk surrounding the F2 V star HD 15115 using the Hubble Space Telescope in the optical and Keck adaptive optics in the near-infrared. The most remarkable property of the HD 15115 disk relative to other debris disks is its extreme length asymmetry. The east side of the disk is detected to ~315 AU radius, whereas the west side of the disk has radius >550 AU. We find a blue optical to near-infrared scattered light color relative to the star that indicates grain scattering properties similar to the AU Mic debris disk. The existence of a large debris disk surrounding HD 15115 adds further evidence for membership in the β Pic moving group, which was previously argued based on kinematics alone. Here we hypothesize that the extreme disk asymmetry is due to dynamical perturbations from HIP 12545, an M star east of HD 15115 that shares a common proper motion vector, heliocentric distance, galactic space velocity, and age.


First scattered light images of debris disks around HD 53143 and HD 139664

Paul Kalas, James R. Graham, Mark Clampin, and Michael Fitzgerald
2006, The Astrophysical Journal, Vol. 637, p. L57 - L60

We present the first scattered light images of debris disks around a K star (HD 53143) and an F star (HD 139664) using the coronagraphic mode of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). With ages of 0.3 - 1 Gyr, these are among the oldest optically detected debris disks. HD 53143, viewed ~45 degrees from edge-on, does not show radial variation in disk structure and has a width >55 AU. HD 139664 is seen close to edge-on and has a beltlike morphology with a dust peak 83 AU from the star and a distinct outer boundary at 109 AU. We discuss evidence for significant diversity in the radial architecture of debris disks that appears unconnected to stellar spectral type or age. HD 139664 and possibly the solar system belong in a category of narrow belts 20-30 AU wide. HD 53143 represents a class of wide-disk architecture with characteristic width >50 AU.


A planetary system as the origin of structure in Fomalhaut's dust belt

Paul Kalas, James R. Graham, and Mark Clampin
2005, Nature, Vol. 435, p. 1067

The Sun and >15 percent of nearby stars are surrounded by dusty debris disks that must be collisionally replenished by asteroids and comets, as the dust would otherwise be depleted on <10 Myr timescales (ref. 1). Theoretical studies show that disk structure can be modified by the gravitational influence of planets (ref. 2-4), but the observational evidence is incomplete, at least in part because maps of the thermal infrared emission from disks have low linear resolution (35 AU in the best case; ref. 5). Optical images provide higher resolution, but the closest examples (AU Mic and Beta Pic) are edge-on (ref. 6,7), preventing the direct measurement of azimuthal and radial disk structure that is required for fitting theoretical models of planetary perturbations. Here we report the detection of optical light reflected from the dust grains orbiting Fomalhaut (HD 216956). The system is inclined 24 degrees away from edge-on, enabling the measurement of disk structure around its entire circumference, at a linear resolution of 0.5 AU. The dust is distributed in a belt 25 AU wide, with a very sharp inner edge at a radial distance of 133 AU, and we measure an offset of 15 AU between the belt's geometric centre and Fomalhaut. Taken together, the sharp inner edge and offset demonstrate the presence of planet-mass objects orbiting Fomalhaut.

 

Discovery of a large dust disk around the nearby star AU Microscopii

Paul Kalas, Michael Liu, and Brenda Matthews 2004, Science, Vol. 303, p. 1990

We present the discovery of a circumstellar dust disk surrounding AU Microscopii (AU Mic, GJ 803, HD 197481). This young M star at 10 parsec has the same age and origin as beta Pictoris, another nearby star surrounded by a dust disk. The AU Mic disk is detected between 50 AU and 210 AU radius, a region where dust lifetimes exceed the present stellar age. Thus, AU Mic is the nearest star where we directly observe the solid material required for planet formation. Since 85% of stars are M-type, the AU Mic disk provides new clues on how the majority of planetary systems might form and evolve.


Discovery of Reflection Nebulosity Around Five Vega-like Stars

Paul Kalas, James R. Graham, Steven V.W. Beckwith, David C. Jewitt and James P. Lloyd
2002, The Astrophysical Journal, Vol. 567, p. 999

Coronagraphic optical observations of six Vega-like stars reveal reflection nebulosities, five of which were previously unknown. The nebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, and HD 49662 resemble that of the Pleiades, indicating an interstellar origin for dust grains. The reflection nebulosity around HD 123160 has a double-arm morphology, but no disk-like feature is seen as close as 2.5 arcsec from the star in K-band adaptive optics data. We demonstrate that uniform density dust clouds surrounding HD 23362, HD 23680 and HD 123160 can account for the observed 12-100 micron spectral energy distributions. For HD 4881, HD 26676, and HD 49662 an additional emission source, such as from a circumstellar disk or non-equilibrium grain heating, is required to fit the 12-25 micron data. These results indicate that in some cases, particularly for Vega-like stars located beyond the Local Bubble (>100 pc), the dust responsible for excess thermal emission may originate from the interstellar medium rather than from a planetary debris system.

"Stellar Encounters with the Beta Pictoris Planetesimal System"

Paul Kalas, Jean-Marc Deltorn, and John Larwood
2001, The Astrophysical Journal, Vol. 553, p. 410

We use data from the Hipparcos Catalog and the Barbier-Brosat and Figon (2000) catalog of stellar radial velocities to test the hypothesis that the Beta Pic planetesimal disk was disrupted by a close stellar encounter. We trace the space motions of 21,497 stars and discover 18 that have passed within 5 pc of Beta Pic in the past 1 Myr. Beta Pic's closest encounter is with the K2III star HIP 27628 (0.6 pc), but dynamically the most important encounter is with the F7V star HIP 23693 (0.9 pc). We calculate the velocity and eccentricity changes induced by the 18 perturbations and conclude that they are dynamically significant if planetesimals exist in a Beta Pic Oort cloud. We provide a first-order estimate for the evolutionary state of a Beta Pic Oort cloud and conclude that the primary role of these stellar perturbations would be to help build a comet cloud rather than destroy a pre-existing structure. The stellar sample is 20% complete and motivates future work to identify less common close interactions that would significantly modify the observed circumstellar disk. For future radial velocity study we identify 6 stars in the Hipparcos Catalog that may have approached Beta Pic to within 0.1 pc and therefore remain as candidate disk perturbers.

"Rings in the Planetesimal Disk of Beta Pictoris"

Paul Kalas, John Larwood, Brad Smith, and Al Schultz, 2000, ApJlett, Vol. 530, p. 133

The nearby main-sequence star beta Pictoris is surrounded by an edge-on disk of dust produced by the collisional erosion of larger planetesimals. Here we report the discovery of substructure within the northeast extension of the disk midplane that may represent an asymmetric ring system around beta Pic. We present a dynamical model showing that a close stellar flyby with a quiescient disk of planetesimals can create such rings, along with previously unexplained disk asymmetries. We infer that beta Pic's planetesimal disk was highly disrupted by a stellar encounter in the last hundred thousand years.

 

A Candidate Dust Disk Surrounding the Binary Stellar System BD +31 643

Paul Kalas and David Jewitt 1997, Nature, Vol. 386, p. 52

...To date, only one main-sequence star - beta Pictoris - has been shown
to have a dust disk that can be resolved optically. Here we report the optical
image of a candidate dust disk surrounding a main-sequence binary stellar system, BD +31 643. If the existence of this dust disk is confirmed by future
observations, it would imply that binary stars may possess stable environments
for planetesimal formation.

 

"The Detectability of Beta Pic-like Circumstellar Disks Around Nearby Main Sequence Stars"

Paul Kalas and David Jewitt, 1996, The Astronomical Journal, Vol. 111, p. 1347

We model scattered light from a circumstellar disk and assess its detectability in ground-based coronagraphic observations of Beta Pic, Vega and Fomalhaut. The model is fitted to the observed Beta Pic disk, adjusted to reflect different physical and observational parameters, and inserted into our raw data, which we subsequently reduce and evaluate for circumstellar nebulosity. We find that the prominence of the Beta Pic disk is primarily a result of its large scattering cross-section, rather than its edge-on inclination or close proximity to the Sun. Non-detections of disks in our coronagraphic observations of Vega and Fomalhaut imply that the total scattering cross-section of dust around these two nearby stars is not greater than a tenth of Beta Pic's. Our results indicate that coronagraphic surveys for circumstellar disks are unlikely to produce positive results unless one order of magnitude improvement is made in the suppression of stellar light.

 

"Asymmetries in the Beta Pictoris Dust Disk"

Paul Kalas and David Jewitt
1995, The Astronomical Journal, Vol. 110, p. 794.

Five types of asymmetry are identified and measured in the circumstellar dust disk of Beta Pictoris using new R-band coronagraphic data. Models of axisymmetric dust disks show that the observed tilt of the midplane may result from a small inclination of the disk to our line of sight combined with a non-isotropic scattering phase function. The remaining four asymmetries indicate a non-axisymmetric distribution of orbiting dust particles between 150 and 800 AU projected radius. The disk may have been gravitationally perturbed in the past 103 to 104 years, though a perturbing agent has not been detected. The statistical probability of a stellar close-approach is very small and no field stars have been uniquely identified as having passed near Beta Pictoris recently. Planets are unlikely candidates due to the large scale of the asymmetries, while a brown dwarf search has yielded negative results.

 

"The 3.28 micron Emission Feature in NGC 253"

Paul Kalas and C.G. Wynn-Williams
1994, The Astrophysical Journal, Vol. 434, p. 546

The 3.28 micron polycyclic aromatic hydrocarbon (PAH) emission feature in the galaxy NGC 253 comes from an extended star formation region about 100 pc across. The brightest mid-infrared source in the galaxy, which is displaced about 45 pc from the probable nucleus, does not show the PAH feature; it may be a dust-enshrouded recent supernova.

 

"A Coronagraphic Survey for Circumstellar Disks"

Paul Kalas and David Jewitt
1993, B.A.A.S., Vol. 25, p. 1353

We present results from a coronagraphic survey at visible wavelengths for circumstellar disks. Candidates include both main-sequence and pre-main- sequence stars selected on the basis of proximity, infrared excess, submillimeter emission, or spectral features indicative of solid, circumstellar material. While our goal is to image over 100 stars, we have not detected evidence for circumstellar material around the 55 stars observed to date, including extensive observations of 68 Oph and alpha PsA. New findings include the discovery of faint stars near circumstellar disk candidate stars lambda Boo (R=+17.2 mag, sep.=15.7", P.A.=315 degrees), HD 98800 (R=+19.5, sep.=9.9", P.A.=343 degrees), and HR 4796 (R=+16.3 mag, sep=4.9", P.A.=315 degrees). Detailed imaging of the environment near DG Tau shows an oval cavity around the star with a major axis oriented at P.A.=135 degrees and length=20", which is bounded tothe northeast by bright arcs most likely due to scattering by dust. The jet producing the HH object 8" to the southwest is seen for the first time as it emanates from near the star.