Cluster Origin of Triple Star HD 188753 and its Planet

The recent discovery by M. Konacki of a ``hot Jupiter'' in the hierarchical triple star system HD 188753 challenges established theories of giant-planet formation. If the orbital geometry of the triple has not changed since the birth of the planet, then a disk around the planetary host star would probably have been too compact and too hot for a Jovian planet to form by the core-accretion model or gravitational collapse. This paradox is resolved if the star was initially either single or had a much more distant companion. It is suggested here that a close multi-star dynamical encounter transformed this initial state into the observed triple, an idea that follows naturally if HD 188753 formed in a moderately dense stellar system--perhaps an open cluster--that has since dissolved. Three distinct types of encounters are investigated. The most robust scenario involves an initially single planetary host star that changes places with the outlying member of a pre-existing hierarchical triple.Comment: Accepted by ApJL; minor changes from origina

The GL 569 Multiple System

We report the results of high spectral and angular resolution infrared observations of the multiple system GL 569 A and B that were intended to measure the dynamical masses of the brown dwarf binary believed to comprise GL 569 B. Our analysis did not yield this result but, instead, revealed two surprises. First, at age ~100 Myr, the system is younger than had been reported earlier. Second, our spectroscopic and photometric results provide support for earlier indications that GL 569 B is actually a hierarchical brown dwarf triple rather than a binary. Our results suggest that the three components of GL 569 B have roughly equal mass, ~0.04 Msun.Comment: 29 pages, 10 figures, accepted for publication in the Astrophysical Journal; minor corrections to Section 5.1; changed typo in 6.

Planets in triple star systems--the case of HD188753

We consider the formation of the recently discovered ``hot Jupiter'' planet orbiting the primary component of the triple star system HD188753. Although the current outer orbit of the triple is too tight for a Jupiter-like planet to have formed and migrated to its current location, the binary may have been much wider in the past. We assume here that the planetary system formed in an open star cluster, the dynamical evolution of which subsequently led to changes in the system's orbital parameters and binary configuration. We calculate cross sections for various scenarios that could have led to the multiple system currently observed, and conclude that component A of HD188753 with its planet were most likely formed in isolation to be swapped in a triple star system by a dynamical encounter in an open star cluster. We estimate that within 500pc of the Sun there are about 1200 planetary systems which, like Hd188753, have orbital parameters unfavorable for forming planets but still having a planet, making it quite possible that the HD188753 system was indeed formed by a dynamical encounter in an open star cluster.Comment: ApJ Letters in pres

Precision radial velocities of double-lined spectroscopic binaries with an iodine absorption cell

A spectroscopic technique employing an iodine absorption cell (I_2) to superimpose a reference spectrum onto a stellar spectrum is currently the most widely adopted approach to obtain precision radial velocities of solar-type stars. It has been used to detect ~80 extrasolar planets out of ~130 know. Yet in its original version, it only allows us to measure precise radial velocities of single stars. In this paper, we present a novel method employing an I_2 absorption cell that enables us to accurately determine radial velocities of both components of double-lined binaries. Our preliminary results based on the data from the Keck I telescope and HIRES spectrograph demonstrate that 20-30 m/s radial velocity precision can be routinely obtained for "early" type binaries (F3-F8). For later type binaries, the precision reaches ~10 m/s. We discuss applications of the technique to stellar astronomy and searches for extrasolar planets in binary systems. In particular, we combine the interferometric data collected with the Palomar Testbed Interferometer with our preliminary precision velocities of the spectroscopic double-lined binary HD 4676 to demonstrate that with such a combination one can routinely obtain masses of the binary components accurate at least at the level of 1.0%.Comment: Accepted for publication in The Astrophysical Journa

High Orbital Eccentricities of Extrasolar Planets Induced by the Kozai Mechanism

One of the most remarkable properties of extrasolar planets is their high orbital eccentricities. Observations have shown that at least 20% of these planets, including some with particularly high eccentricities, are orbiting a component of a wide binary star system. The presence of a distant binary companion can cause significant secular perturbations to the orbit of a planet. In particular, at high relative inclinations, a planet can undergo a large-amplitude eccentricity oscillation. This so-called "Kozai mechanism" is effective at a very long range, and its amplitude is purely dependent on the relative orbital inclination. In this paper, we address the following simple question: assuming that every host star with a detected giant planet also has a (possibly unseen, e.g., substellar) distant companion, with reasonable distributions of orbital parameters and masses, how well could secular perturbations reproduce the observed eccentricity distribution of planets? Our calculations show that the Kozai mechanism consistently produces an excess of planets with very high (e >0.6) and very low (e < 0.1) eccentricities. The paucity of near-circular orbits in the observed sample cannot be explained solely by the Kozai mechanism, because, even with high enough inclinations, the Kozai mechanism often fails to produce significant eccentricity perturbations when there are other competing sources of orbital perturbations on secular timescales, such as general relativity. On the other hand, the Kozai mechanism can produce many highly eccentric orbits. Indeed the overproduction of high eccentricities observed in our models could be combined with plausible circularizing mechanisms (e.g., friction from residual gas) to create more intermediate eccentricities (e=0.1-0.6).Comment: 24 pages, 6 figures, ApJ, in press, minor changes to reflect the accepted versio

Effect of Binary Source Companions on the Microlensing Optical Depth Determination toward the Galactic Bulge Field

Currently, gravitational microlensing survey experiments toward the Galactic bulge field utilize two different methods of minimizing blending effect for the accurate determination of the optical depth \tau. One is measuring \tau based on clump giant (CG) source stars and the other is using `Difference Image Analysis (DIA)' photometry to measure the unblended source flux variation. Despite the expectation that the two estimates should be the same assuming that blending is properly considered, the estimates based on CG stars systematically fall below the DIA results based on all events with source stars down to the detection limit. Prompted by the gap, we investigate the previously unconsidered effect of companion-associated events on $\tau$ determination. Although the image of a companion is blended with that of its primary star and thus not resolved, the event associated with the companion can be detected if the companion flux is highly magnified. Therefore, companions work effectively as source stars to microlensing and thus neglect of them in the source star count could result in wrong \tau estimation. By carrying out simulations based on the assumption that companions follow the same luminosity function of primary stars, we estimate that the contribution of the companion-associated events to the total event rate is ~5f_{bi}% for current surveys and can reach up to ~6f_{bi}% for future surveys monitoring fainter stars, where f_{bi} is the binary frequency. Therefore, we conclude that the companion-associated events comprise a non-negligible fraction of all events. However, their contribution to the optical depth is not large enough to explain the systematic difference between the optical depth estimates based on the two different methods.Comment: 4 pages, 1 figure, 1 table, ApJ, submitte

A discontinuity in the low-mass initial mass function

The origin of brown dwarfs (BDs) is still an unsolved mystery. While the standard model describes the formation of BDs and stars in a similar way recent data on the multiplicity properties of stars and BDs show them to have different binary distribution functions. Here we show that proper treatment of these uncovers a discontinuity of the multiplicity-corrected mass distribution in the very-low-mass star (VLMS) and BD mass regime. A continuous IMF can be discarded with extremely high confidence. This suggests that VLMSs and BDs on the one hand, and stars on the other, are two correlated but disjoint populations with different dynamical histories. The analysis presented here suggests that about one BD forms per five stars and that the BD-star binary fraction is about 2%-3% among stellar systems.Comment: 14 pages, 11 figures, uses emulateapj.cls. Minor corrections and 1 reference added after being accepted by the Ap

Planetary Companions Around Two Solar Type Stars: HD 195019 and HD 217107

We have enlarged the sample of stars in the planet search at Lick Observatory. Doppler measurements of 82 new stars observed at Lick Observatory, with additional velocities from Keck Observatory, have revealed two new planet candidates. The G3V/IV star, HD 195019, exhibits Keplerian velocity variations with a period of 18.27 d, an orbital eccentricity of 0.03 +/- 0.03, and M sin i = 3.51 M_Jup. Based on a measurement of Ca II H&K emission, this star is chromospherically inactive. We estimate the metallicity of HD 195019 to be approximately solar from ubvy photometry. The second planet candidate was detected around HD 217107, a G7V star. This star exhibits a 7.12 d Keplerian period with eccentricity 0.14 +/- 0.05 and M sin i = 1.27 M_Jup. HD 217107 is also chromospherically inactive. The photometric metallicity is found to be [Fe/H] = +0.29 +/- 0.1 dex. Given the relatively short orbital period, the absence of tidal spin-up of HD 217107 provides a theoretical constraint on the upper limit of the companion mass of < 11 M_Jup.Comment: 15 pages, plus 6 figures. To appear in Jan 1999 PAS

Binaries in star clusters and the origin of the field stellar population

Many, possibly most, stars form in binary and higher-order multiple systems. Therefore, the properties and frequency of binary systems provide strong clues to the star-formation process, and constraints on star-formation models. However, the majority of stars also form in star clusters in which the birth binary properties and frequency can be altered rapidly by dynamical processing. Thus, we almost never see the birth population, which makes it very difficult to know if star formation (as traced by binaries, at least) is universal, or if it depends on environment. In addition, the field population consists of a mixture of systems from different clusters which have all been processed in different ways.Comment: 16 pages, no figures. To appear as invited review article in a special issue of the Phil. Trans. Royal Soc. A: Ch. 8 "Star clusters as tracers of galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed. LaTeX, requires rspublic.cls style fil

The role of planets in shaping planetary nebulae

In 1997 Soker laid out a framework for understanding the formation and shaping of planetary nebulae (PN). Starting from the assumption that non-spherical PN cannot be formed by single stars, he linked PN morphologies to the binary mechanisms that may have formed them, basing these connections almost entirely on observational arguments. In light of the last decade of discovery in the field of PN, we revise this framework, which, although simplistic, can still serve as a benchmark against which to test theories of PN origin and shaping. Within the framework, we revisit the role of planets in shaping PN. Soker invoked a planetary role in shaping PN because there are not enough close binaries to shape the large fraction of non-spherical PN. In this paper we adopt a model whereby only ~20% of all 1-8 solar mass stars make a PN. This reduces the need for planetary shaping. Through a propagation of percentages argument, and starting from the assumption that planets can only shape mildly elliptical PN, we conclude, like in Soker, that ~20% of all PN were shaped via planetary and other substellar interactions but we add that this corresponds to only ~5% of all 1-8 solar mass stars. This may be in line with findings of planets around main sequence stars. PN shaping by planets is made plausible by the recent discovery of planets that have survived interactions with red giant branch (RGB) stars. Finally, we conclude that of the ~80% of 1-8 solar mass stars that do not make a PN, about one quarter do not even ascend the AGB due to interactions with stellar and substellar companions, while three quarters ascend the AGB but do not make a PN. Once these stars leave the AGB they evolve normally and can be confused with post-RGB, extreme horizontal branch stars. We propose tests to identify them.Comment: 23 pages, accepted by PAS