Arcadio Poveda

Dynamical Decay of a Massive Multiple System in Orion KL

We present absolute astrometry of 35 radio sources in the Orion Trapezium and Becklin-Neugebauer/Kleinman-Low (BN/KL) regions, obtained from Very Large Array archival observations collected over a period of 15 years. By averaging the results for all the sources, we estimate the mean absolute proper motion of Orion to be—in Galactic coordinates—μl cos b = +2.1 ± 0.2 mas yr-1, μb = -0.1 ± 0.2 mas yr-1. These values agree remarkably well with those expected from the differential rotation of the Milky Way. Subtraction of this mean motion from the individual measurements allows us to register all proper motions to the rest frame of the Orion Nebula and to identify radio sources with large residual velocities. In the KL region, we find three sources in this situation: the BN object, the radio source I, and the radio counterpart of the infrared source n. All three objects appear to be moving away from a common point where they must all have been located about 500 years ago. This suggests that all three sources were originally part of a multiple massive stellar system that recently disintegrated as a result of a close dynamical interaction.

Proper Motions of the BN Object and the Radio Source I in Orion: Where and When Did the BN Object Become a Runaway Star?

We present absolute astrometry of the core of the Orion molecular cloud, made with Very Large Array archive data taken over the last two decades. Our analysis reveals that both the BN object and the radio source I have proper motions: the BN object has a proper motion of 12.6 ± 0.6 mas yr-1 (corresponding to a velocity of 27 ± 1 km s-1 at an adopted distance of 450 pc) to the northwest, while the radio source I has a proper motion of 5.6 ± 0.7 mas yr-1 (corresponding to a velocity of 12 ± 2 km s-1) to the southeast. The motion of the two sources is nearly antiparallel, diverging from a point in between them, where they were located about 500 years ago. These results suggest that the BN object and the radio source I were part of a multiple young stellar system that disintegrated in the recent past.

A fourth planet orbiting υ Andromedae

We present a 4-planet Keplerian fit for the radial velocity curve of the F8V star υ Andromeda, indicating the presence of a fourth planet in the system. We detect an additional fifth coherent signal in the radial velocity curve which we attribute to stellar activity. The discovery of a new planet around υ Andromedae makes this system the fifth to contain, at least, four planets. These four planets have minimum masses of 0.69, 1.98, 4.13 and 1.06 MJup and orbital periods of 4.62, 241.26, 1276.46 and 3848.9 days, respectively. We have numerically integrated the orbital solution for these four planets and find that the system is stable for at least 10 Myr. The orbit of the fourth planet coincides with an island of stability reported by Rivera & Haghighipour (2007, MNRAS, 374, 599). We find that the characteristics of the new fourth planet are very similar to those of Jupiter and that the planets in this system have very strong interactions with each other. As previously found, υ And− ba ndυ And−c are in apsidal alignment, while the orbit of the new planet (υ And−e) is close to an external 3:1 resonance with υ And−c.

The expected frequency of collisions of smallmeteorites with cars and aircraft

Abstract The cumulative distribution cf2>Ncf1>(cf2>dcf1>) of diameters of Earth-CrossingAsteroids (ECAs) derived by Poveda et al. ( Poveda et al., 1998 , submitted) is used to estimatethe frequency of collisions of meteoroids with cars and with aircraft. The expected frequency ofcollisions of a car with a meteorite larger than 10 cm in diameter turns out to be one impact every16 years. This frequency is consistent with the known incidence of such events ( Lewis, 1996 ).The expected frequency of collisions of a cruising airplane with a meteorite larger than 1 cm indiameter turns out to be one impact every 30 years. Such an impactor hitting an airplane at avelocity of several hundreds of meters per second could cause a serious accident. 1999 ElsevierScience Ltd. All rights reserved.

G 112-29 (=NLTT 18149): A VERY WIDE COMPANION TO GJ 282 AB WITH A COMMON PROPER MOTION, COMMON PARALLAX, COMMON RADIAL VELOCITY, AND COMMON AGE

We have made a search for common proper motion (CPM) companions to the wide binaries in the solar vicinity. We found that the binary GJ 282AB has a very distant CPM companion (NLTT 18149) at a separation

THE DIAMETER DISTRIBUTION OF EARTH-CROSSING ASTEROIDS

s=1.09 \arcdeg

On the Reproducibility of Run-Away Stars Formed in Collapsing Clusters

. Improved spectral types and radial velocities are obtained, and ages determined for the three components. The Hipparcos trigonometric parallaxes and the new radial velocities and ages turn out to be very similar for the three stars, and provide strong evidence that they form a physical system. At a projected separation of 55733AU from GJ 282AB, NLTT 18149 ranks among the widest physical companions known.

The Frequency Distribution of Semimajor Axes of Wide Binaries: Cosmogony and Dynamical Evolution

Abstract The knowledge of the diameter distribution of Earth-Crossing Asteroids (ECAs) is important because it gives information about the evolutionary processes that have affected the asteroid population in the main belt as well as their ejection into Earth-crossing orbits. On a more practical sense it is also of interest to know the frequency of collisions with the earth of asteroids of different diameters. In the present investigation we briefly discuss the luminosity functions of van Houten et al. (1970) (hereafter VH) for main belt asteroids (MBAs) and of Shoemaker et al., 1990 (hereafter SH) for earth-crossing asteroids (ECAs). The luminosity function of VH is well represented by an exponential N (⩽ H ) ∼ e α H in the interval 11.25 H ⩽ 16.25, while that of SH is represented, in the same magnitude interval, by two exponentials with different values of the constant α . The exponential behavior of VH luminosity function is consistent with the mass distribution function derived theoretically by Dohnanyi (1969) for the fragments of a population of objects in collisional equilibrium. Specifically, Dohnanyi shows that the power law n ( m )d m ∼ m − β d m holds in principle for masses down to sizes of centimeters, where the Poynting–Robertson effect depopulates the distribution. The apparent inconsistency between the distributions of VH and SH, and the expectations that follow from Dohnanyis work, led us to study the problem of the distribution of diameters of ECAs, taking advantage of the much larger number of currently known earth-crossers (457, as of February, 1998, in the WEB page of the Minor Planet Center, http://cfa-www.harvard.edu:80/ ∼graff/lists/Unusual.htm). In the first section of this paper we derive the luminosity functions of MBAs and of ECAs. For MBAs we use the data of VH in the absolute magnitude interval 11.25 H ⩽ 16.25, and for ECAs we use the data given in the Web page of the Minor Planet Center, in the magnitude interval 12.0 H ⩽ 15.5. We find that in both cases the constants α are very similar to the value 1.1513 predicted by Dohnanyis theory. In the second section we use the known values of the mean albedos to transform the magnitude distribution N (⩽ H ) into the equivalent diameter-frequency distribution n ( d ). This distribution is then transformed to a mass-frequency distribution n ( m ) by means of the relevant densities, assuming that the asteroids are spherical. Finally, we use the distribution of masses and the kinetic energy for a given encounter velocity v with the earth to obtain the frequency distribution of impact energies n ( E ). In the third section we find the expected frequency of impacts with the earth of objects larger than a given diameter d . We also exhibit the frequency of impacts of energies equal or larger than E . Finally, our frequencies are compared with those obtained using the distribution of SH. We find that for small asteroids (∼10 m), the frequency of impacts predicted from our distribution is an order of magnitude larger than the one obtained from the distribution of SH. It is to be noted, on the other hand, that our frequency distribution n ( d ) of diameters is reasonably consistent, within the uncertainties, with those of Rabinowitz et al., 1994 and Menichella et al. (1996) .

HST Images Do Not Support the Presence of Three High-Velocity, Low-Mass Runaway Stars in the Core of the Orion Nebula Cluster*

To test the stability of the trajectories of run-away stars we present the results of a comparative study of 26 star clusters involving very strong encounters. Each one of these clusters was computed with different time steps, different techniques of integration and on different machines. We confirmed that, in general, the values of the energies, velocities and positions for the individual stars are not reproducible from run to run. We found, however, that in all the cases that have been tested, the run-away star preserved its energy, velocity and position from one run to another. It was also found that escapers are more reproducible than stars with negative energy.

The Distribution of Separations of Wide Binaries

The frequency distribution f(a) of semi-major axis of double and multiple systems, as well as their eccentricities and mass ratios, contain valuable fossil information about the process of star formation and the dynamical history of the systems. In order to advance in the understanding of these questions, we have made an extensive analysis of the frequency distribution f (a) for wide binaries (a>25 AU) in the various published catalogues, as well as in our own (Poveda et al., 1994; Allen et al., 2000; Poveda & Hernandez, 2003). Based upon all these studies we have established that the frequency f(a) is function of the age of the system and follows Oepiks distribution f(a) ~ 1/a in the range of 100 AU < a < a[c](t), where a[c](t) is a critical semi-major axis beyond which binaries have dissociated by encounters with massive objects. We argue that the physics behind the distribution f(a) ~ 1/a is a process of energy relaxation, analogous to that present in stellar clusters (secular relaxation) or in spherical galaxies (violent relaxation). The frequency distribution of mass ratios in triple systems as well as the existence of runaway stars, indicate that both types of relaxation are important in the process of binary and multiple star formation.