Scott J. Kenyon

Planet formation around stars of various masses: the snow line and the frequency of giant planets

We use a semianalytic circumstellar disk model that considers movement of the snow line through evolution of accretion and the central star to investigate how gas giant frequency changes with stellar mass. The snow line distance changes weakly with stellar mass; thus, giant planets form over a wide range of spectral types. The probability that a given star has at least one gas giant increases linearly with stellar mass from 0.4 to 3 -->M?. Stars more massive than 3 -->M? evolve quickly to the main sequence, which pushes the snow line to 10-15 AU before protoplanets form and limits the range of disk masses that form giant planet cores. If the frequency of gas giants around solar mass stars is 6%, we predict occurrence rates of 1% for 0.4 -->M? stars and 10% for 1.5 -->M? stars. This result is largely insensitive to our assumed model parameters. Finally, the movement of the snow line as stars 2.5 -->M? move to the main sequence may allow the ocean planets suggested by L?ger et al. to form without migration.

The symbiotic stars

Symbiotic stars are now commonly accepted is interacting binaries, in which a dwarf star accretes material from its red giant companion. Dr Kenyon has researched and assembled all the existing data for the known symbiotic stars, and in this book he summarizes observational material covering the eruptive and quiescent phases of these objects, emphasizing the important astrophysical problems raised and resolved by recent results at infrared, optical, radio, ultraviolet and X-ray wavelengths. Physical models for the eruptive and quiescent phases, as well as the long-term evolution, of symbiotic stars are also discussed, with the goal of developing observational diagnostics that serve to test the basic theories. The work concludes with a detailed appendix and bibliography that will aid researchers interested in the history of individual symbiotic systems and confirm this volume as an indispensable handbook at any observatory where research on stellar objects is undertaken.

An IRAS survey of the Taurus-Auriga molecular cloud

IRAS data are used to search for young premain-sequence stars not previously associated with molecular cloud cores in the Taurus-Auriga region. NIR photometry and optical spectroscopy suggest that many of the objects are young stars. The sample includes six new embedded sources with luminosities comparable to that of the average T Tauri star, suggesting that surveys for premain-sequence stars in the cloud are essentially complete for luminosities greater than 0.5 solar luminosities. A disagreement is found between accretion rates derived from the duration of the embedded phase and those derived from the bolometric luminosity. It is found that this disagreement may be reconciled if a star accretes most of its mass in a time that is short compared to the duration of the embedded phase or if the ages of T Tauri stars have been underestimated. 62 refs.

Tidally Triggered Star Formation in Close Pairs of Galaxies

We analyze optical spectra of a sample of 502 galaxies in close pairs and N-tuples, separated by ≤50 h-1 kpc. We extracted the sample objectively from the CfA2 redshift survey, without regard to the surroundings of the tight systems; we remeasure the spectra with longer exposures, to explore the spectral characteristics of the galaxies. We use the new spectra to probe the relationship between star formation and the dynamics of the systems of galaxies. The equivalent widths of Hα [EW(Hα)] and other emission lines anticorrelate strongly with pair spatial separation (ΔD) and velocity separation; the anticorrelations do not result from any large-scale environmental effects that we detect. We use the measured EW(Hα) and the starburst models of Leitherer et al. to estimate the time since the most recent burst of star formation began for galaxies in our sample. In the absence of a large contribution from an old stellar population to the continuum around Hα that correlates with the orbit parameters, the observed ΔD-EW(Hα) correlation signifies that starbursts with larger separations on the sky are, on average, older. We also find a population of galaxies with small to moderate amounts of Balmer absorption. These galaxies support our conclusion that the sample includes many aging bursts of star formation; they have a narrower distribution of velocity separations, consistent with a population of orbiting galaxies near apogalacticon. By matching the dynamical timescale to the burst timescale, we show that the data support a simple picture in which a close pass initiates a starburst; EW(Hα) decreases with time as the pair separation increases, accounting for the anticorrelation. Recent N-body/smoothed particle hydrodynamics simulations of interacting pairs suggest a physical basis for the correlation—for galaxies with shallow central potentials, they predict gas infall before the final merger. This picture leads to a method for measuring the duration and the initial mass function of interaction-induced starbursts: our data are compatible with the starburst models and orbit models in many respects, as long as the starburst lasts longer than ~108 yr and the delay between the close pass and the initiation of the starburst is less than a few times 107 yr. If there is no large contribution from an old stellar population to the continuum around Hα, the Miller-Scalo and cutoff (M ≤ 30 M☉) Salpeter initial mass functions (IMFs) fit the data much better than a standard Salpeter IMF.

Discovery of an unbound hypervelocity star in the Milky Way halo

We have discovered a star, SDSS J090745.0+024507, leaving the Galaxy with a heliocentric radial velocity of 853 ± 12 km s-1, the largest velocity ever observed in the Milky Way halo. The star is either a hot blue horizontal-branch star or a B9 main-sequence star with a heliocentric distance of 39 or 71 kpc, respectively. Corrected for the solar reflex motion and to the local standard of rest, the Galactic rest-frame velocity is 709 km s-1. We suggest that this star is the first example of a hypervelocity star ejected from the Galactic center, as predicted by Hills and later discussed by Yu & Tremaine. The stars radial velocity vector points 174° from the Galactic center. The star has [Fe/H] ~ 0, consistent with a Galactic center origin, and a travel time of 80 Myr from the Galactic center, consistent with its stellar lifetime. If the star is indeed traveling from the Galactic center, it should have a proper motion of ~0.3 mas yr-1 observable with the Space Interferometry Mission or the Global Astrometric Interferometer for Astrophysics. Identifying additional hypervelocity stars throughout the halo will constrain the production rate history of hypervelocity stars at the Galactic center.

On the spatial distribution of pre-main-sequence stars in Taurus

We derive characteristic properties of the nonrandom spatial distribution of pre-main-sequence stars in the Taurus-Auriga molecular cloud by applying several different statistical techniques. We find that a power-law form for the two-point angular correlation function (with index −1.2) reproduces the overall shape of the actual pre-main-sequence distribution in Taurus at small angular scales (∼0.3°). This result is consistent with the existence of real clustering in the T Tauri distribution. With the aid of the nearest-neighbor distribution technique, we determine a median projected separation of −0.3 pc for young stars in this cloud, even after eliminating close pairs with separations less than 20″ (−3000 AU at the distance of Taurus) from our sample

The embedded young stars in the Taurus-Auriga molecular cloud. I - Models for spectral energy distributions

We describe radiative transfer calculations of infalling, dusty envelopes surrounding pre-main-sequence stars and use these models to derive physical properties for a sample of 21 heavily reddened young stars in the Taurus-Auriga molecular cloud. The density distributions needed to match the FIR peaks in the spectral energy distributions of these embedded sources suggest mass infall rates similar to those predicted for simple thermally supported clouds with temperatures about 10 K. Unless the dust opacities are badly in error, our models require substantial departures from spherical symmetry in the envelopes of all sources. These flattened envelopes may be produced by a combination of rotation and cavities excavated by bipolar flows. The rotating infall models of Terebey et al. (1984) models indicate a centrifugal radius of about 70 AU for many objects if rotation is the only important physical effect, and this radius is reasonably consistent with typical estimates for the sizes of circumstellar disks around T Tauri stars.

A Spectroscopic survey of subarcsecond binaries in the Taurus-Auriga dark cloud with the Hubble Space Telescope

We report the results of a spectroscopic survey of 20 close T Tauri binaries in the Taurus-Auriga dark cloud where the separations between primaries and their secondaries are less than the typical size of a circumstellar disk around a young star. Analysis of low- and medium-resolution Space Telescope Imaging Spectrograph spectra yields the stellar luminosities, reddenings, ages, masses, mass accretion rates, IR excesses, and emission-line luminosities for each star in each pair. We examine the ability of IR color excesses, Hα equivalent widths, [O I] emission, and veiling to distinguish between weak emission and classical T Tauri stars. Four pairs have one classical T Tauri star (CTTS) and one weak-lined T Tauri star (WTTS); the CTTS is the primary in three of these systems. This frequency of mixed pairs among the close T Tauri binaries is similar to the frequency of mixed pairs in wider young binaries. Extinctions within pairs are usually similar; however, the secondary is more heavily reddened than the primary in some systems, where it may be viewed through the primarys disk. Mass accretion rates of primaries and secondaries are strongly correlated, and Hα luminosities, IR excesses, and ages also correlate within pairs. Primaries tend to have somewhat larger accretion rates than their secondaries do and are typically slightly older than their secondaries according to three different sets of modern pre-main-sequence evolutionary tracks. Age differences for XZ Tau and FS Tau, systems embedded in reflection nebulae, are striking: the secondary in each pair is less massive but more luminous than the primary. The stellar masses of the UY Aur and GG Tau binaries measured from their rotating molecular disks are about 30% larger than the masses inferred from the spectra and evolutionary tracks. This discrepancy can be resolved in several ways, among them a 10% closer distance for the Taurus-Auriga dark cloud.

Variations on Debris Disks: Icy Planet Formation at 30-150 AU for 1-3 M☉ Main-Sequence Stars

We describe calculations for the formation of icy planets and debris disks at 30-150 AU around 1-3 M☉ stars. Debris disk formation coincides with the formation of planetary systems. As protoplanets grow, they stir leftover planetesimals to large velocities. A cascade of collisions then grinds the leftovers to dust, forming an observable debris disk. Stellar lifetimes and the collisional cascade limit the growth of protoplanets. The maximum radius of icy planets, -->rmax ≈ 1750 km, is remarkably independent of initial disk mass, stellar mass, and stellar age. These objects contain 3%-4% of the initial mass in solid material. Collisional cascades produce debris disks with maximum luminosity ~ -->2 × 10−3 times the stellar luminosity. The peak 24 μm excess varies from ~1% times the stellar photospheric flux for 1 M☉ stars to ~50 times the stellar photospheric flux for 3 M☉ stars. The peak 70-850 μm excesses are ~30-100 times the stellar photospheric flux. For all stars, the 24-160 μm excesses rise at stellar ages of 5-20 Myr, peak at 10-50 Myr, and then decline. The decline is roughly a power law, -->f t−n with -->n ≈ 0.6–1.0. This predicted evolution agrees with published observations of A-type and solar-type stars. The observed far-IR color evolution of A-type stars also matches model predictions.

Accretion in the Early Kuiper Belt II. Fragmentation

We describe new planetesimal accretion calculations in the Kuiper Belt that include fragmentation and velocity evolution. All models produce two power law cumulative size distributions, NC ∝ r −2.5 for radii ∼ 0.3–3 km and NC ∝ r −3 for radii ∼ 1–3 km. The power law indices are nearly independent of the initial mass in the annulus, M0; the initial eccentricity of the planetesimal swarm, e0; and the initial size distribution of the planetesimal swarm. The transition between the two power laws moves to larger radii as e0 increases. The maximum size of objects depends on their intrinsic tensile strength, S0; Pluto formation requires S0 ∼ 300 erg g −1 . The timescale to produce Pluto-sized objects, τP, is roughly proportional to M −1 0 and e0, and is less sensitive to other input parameters. Our models yield τP ≈ 30–40 Myr for planetesimals with e0 = 10 −3 in a Minimum Mass Solar Nebula. The production of several ‘Plutos’ and ∼ 10 5 50 km radius Kuiper Belt objects leaves most of the initial mass in 0.1–10 km radius objects that can be collisionally depleted over the age of the solar system. These results resolve the puzzle of large Kuiper Belt objects in a small mass Kuiper Belt.