No abstract

▪ Abstract  We summarize the properties of FU Orionis variables, and show how accretion disk models simply explain many peculiarities of these objects. FU Ori systems demonstrate that disk accretion in early stellar evolution is highly episodic, varying from ∼ 10−7[Formula: see text] yr−1 in the low (T Tauri) state to 10−4 [Formula: see text] yr−1 in the high (FU Ori) state. This variability in mass accretion is matched by a corresponding variability in mass ejection, with mass loss rates reaching ∼ 10−1 of the mass accretion rates in outburst. It appears that the FU Ori phenomenon is restricted to early phases of stellar evolution, probably with infall still occuring to the disk, which may help drive repetitive outbursts. Thermal instabilities are a promising way to produce FU Ori disk outbursts, although many uncertainties remain in the theory; triggering by interactions with companion stars on eccentric orbits may also play a role.

No abstract

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 Y15 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 k2.5 M move to the main sequence may allow the ocean planets suggested by Léger et al. to form without migration.

We analyze optical spectra of a sample of 502 galaxies in close pairs and n-tuples, separated by ≤ 50h −1 kpc. We extracted the sample objectively from the CfA2 redshift survey, without regard to the surroundings of the tight systems; we re-measure 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 anti-correlate strongly with pair spatial separation (∆D) and velocity separation; the anti-correlations 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 ∆D -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 suport 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 anti-correlation. Recent n-body/SPH 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 ∼10 8 years and the delay between the close pass and the initiation of the starburst is less than a few×10 7 years. 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 fit the data much better than a standard Salpeter IMF.

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 ∼55 kpc. Corrected for the solar reflex motion and to the local standard of rest, the Galactic rest-frame velocity is +709 km s −1 . Because its radial velocity vector points 173.8 • from the Galactic center, we suggest that this star is the first example of a hyper-velocity star ejected from the Galactic center as predicted by Hills and later discussed by Yu & Tremaine. 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 GAIA. Identifying additional hyper-velocity stars throughout the halo will constrain the production rate history of hyper-velocity stars at the Galactic center.

No abstract

No abstract