arXiv subjects

Christine Allen

Publications and source records attributed to Christine Allen.

17 recordsLinked to original sources

arXiv2021

Échelle Spectroscopy of the Chemically Peculiar Star $θ^1$ Ori F

We analyse Échelle spectra of $θ^1$ Ori F obtained by us on six nights unevenly distributed along six years; we identify several hundred spectral lines and measure, for the first time, the star's heliocentric radial velocity. We also collect and discuss previously published photometry of $θ^1$ OriF . We find that $θ^1$ Ori F is a Chemically Peculiar (CP) star with overabundant silicon and phosphorus, and possibly other elements as well. From the singly ionised Fe, Cr and Ti lines we estimate its spectral type to be between B7 and B8. The radial velocity of $θ^1$ Ori F is possibly marginally variable, with an average of $24 \pm 4.2$ kms, (standard deviation), in good agreement with the mean radial velocity of the Orion Nebula Cluster members, and about 5 kms smaller than the average of the other Trapezium components. We cast doubt on the coeval nature of this star relative to the other Trapezium components, and present arguments that almost certainly exclude its membership to the Orion Trapezium. $θ^1$ Ori F turns out to be enigmatic in several respects, and is probably an important link for understanding the evolutionary stage at which the CP phenomenon sets on.

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arXiv2020

The reliability of the Titius-Bode relation and its implications for the search for exoplanets

The major semiaxes of the planets in our Solar System obey a simple geometric progression known as the Titius-Bode Relation (TBR), whose physical origin remains disputed. It has been shown that the exoplanetary systems follow a similar (but not identical) progression of the form a_n= a_0 e^(bn), where a_0, b are constants to be determined for each system. Since its formulation, the Titius-Bode Relation has proved to be highly predictive in our Solar System. Using data from 27 exoplanetary systems with 5 or more planets and applying a proposed method, we conclude that reliable TB-like fits can be obtained for systems with at least 4 planets and that the precision of the TBR is 78%. By means of a statistical test we show that the periods of planets in real exoplanetary systems are not consistent with a random distribution. Rather, they show signs that their configuration is shaped by their mutual interactions.

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arXiv2019

The Importance of Historical Measures for Dynamical Models of the Evolution of Trapezium-type Multiple Systems

As an illustration of the value of historical measures we present some examples of the dynamical evolution of multiple systems resembling the Orion Trapezium. We constructed models by combining carefully selected historical measures of the separations among components of young massive stellar systems with modern observations. By computing large numbers of fictitious systems resembling real trapezia we were able to simulate the dynamical evolution of such systems. Our results on the dynamical fate of the Orion Trapezium and of ten additional young clusters resembling the Orion Trapezium show extremely short dynamical lifetimes for these systems.

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arXiv2019

Variable stars in Palomar 13; an evaporating globular cluster

We present new CCD $VI$ photometry of the distant globular cluster Pal 13. Fourier decomposition of the light curves of the three cluster member RRab stars lead to estimations of [Fe/H]=-1.65, and a distance of 23.67$\pm$0.57 kpc. Light and colour near minimum phases for RRab stars leads to an estimate of $E(B-V)$=0.104 $\pm$ 0.001. A $V/(V-I)$ colour-magnitude diagram, built exclusively with likely star members, shows consistency with the above parameters and an age of 12 Gyrs. A search of variable stars in the field of view of our images revealed the variability of a red giant cluster member and of three probably non-member stars; two RRab stars and one W Virginis star or CW. The GAIA proper motions of member stars in Pal 13 show a significant scatter, consistent with the scenario of the cluster being tidally stripped.

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arXiv2018

The Dynamical Evolution of Multiple Systems of Trapezium Type

We have selected archival observational data for several O and B trapezia in the Milky Way. For each of the main components of the trapezia we obtained transverse velocities from the historical separation data. With this information, and with the stellar masses of the main components, we studied the dynamical evolution of ensembles of multiple systems mimicking each one of the trapezia. For this purpose we conducted numerical $N-$body integrations using the best available values for the masses, the observed positions and {\bf transverse} velocities, randomly generated radial velocities, and random line-of-sight ($z$) positions for all components. Random perturbations were assigned to the observed quantities, compatible with the observational errors. A large fraction of the simulated systems ({\bf between 70 and 90 percent}) turned out to be unbound. The properties of the evolving systems are studied at different values of the evolution time. We find that the dynamical lifetimes of both the bound and unbound seems to be quite short, of less than 10~000 yr for the unbound systems, and of 10~000 to 20~000 yr for the bound systems. The end result of the simulations is usually a binary {\bf with semiaxes of a few hundred AU} sometimes a triple of hierarchical on non-hierarchical type. Non-hierarchical triples formed during the integrations (which are dynamically unstable) were found to have much longer lifetimes, of 250~000 to 500~000 yr. The frequency distributions of the major semi-axes and eccentricities of the resulting binaries are discussed and compared with observational properties of binary systems from the literature.

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arXiv2014

An improved catalog of halo wide binary candidates

We present an improved catalog of halo wide binaries, compiled from an extensive literature search. Most of our binaries stem from the common proper motion binary catalogs by Allen et al. (2004), and Chanamé \& Gould. (2004) but we have also included binaries from the lists of Ryan (1992) and Zapatero-Osorio \& Martin (2004). All binaries were carefully checked and their distances and systemic radial velocities are included, when available. Probable membership to the halo population was tested by means of reduced proper motion diagrams for 251 candidate halo binaries. After eliminating obvious disk binaries we ended up with 211 probable halo binaries, for 150 of which radial velocities are available. We compute galactic orbits for these 150 binaries and calculate the time they spend within the galactic disk. Considering the full sample of 251 candidate halo binaries as well as several subsamples, we find that the distribution of angular separations (or expected major semiaxes) follows a power law $f(a) \sim a^{-1}$ (Oepik's relation) up to different limits. For the 50 most disk-like binaries, those that spend their entire lives within $z = \pm 500$~pc, this limit is found to be 19,000 AU (0.09 pc), while for the 50 most halo-like binaries, those that spend on average only 18\% of their lives within $z = \pm 500$~pc, the limit is 63,000 AU (0.31 pc). In a companion paper we employ this catalog to establish limits on the masses of the halo massive perturbers (MACHOs).

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arXiv2014

The end of the MACHO era- revisited: new limits on MACHO masses from halo wide binaries

In order to determine an upper bound for the mass of the massive compact halo objets (MACHOs) we use the halo binaries contained in a recent catalog (Allen \& Monroy-Rodr\'ıguez 2013). To dynamically model their interactions with massive perturbers a Monte Carlo simulation is conducted, using an impulsive approximation method and assuming a galactic halo constituted by massive particles of a characteristic mass. The results of such simulations are compared with several subsamples of our improved catalog of candidate halo wide binaries. In accordance with Quinn et al. (2009) we also find our results to be very sensitive to the widest binaries. However, our larger sample, together with the fact that we can obtain galactic orbits for 150 of our systems, allows a more reliable estimate of the maximum MACHO mass than that obtained previously. If we employ the entire sample of 211 candidate halo stars we obtain an upper limit of $112 M_\sun$. However, using the 150 binaries in our catalog with computed galactic orbits we are able to refine our fitting criteria. Thus, for the 100 most halo-like binaries we obtain a maximum MACHO mass of $21-68 M_\sun$. Furthermore, we can estimate the dynamical effects of the galactic disk using binary samples that spend progressively shorter times within the disk. By extrapolating the limits obtained for our most reliable -albeit smallest- sample we find that as the time spent within the disk tends to zero the upper bound of the MACHO mass tends to less than $5 M_\sun$. The non-uniform density of the halo has also been taken into account, but the limit obtained, less than $5 M_\sun$, does not differ much from the previous one. Together with microlensing studies that provide lower limits on the MACHO mass, our results essentially exclude the existence of such objects in the galactic halo.

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arXiv2012

The Sun was not born in M 67

Using the most recent proper-motion determination of the old, Solar-metallicity, Galactic open cluster M 67, in orbital computations in a non-axisymmetric model of the Milky Way, including a bar and 3D spiral arms, we explore the possibility that the Sun once belonged to this cluster. We have performed Monte Carlo numerical simulations to generate the present-day orbital conditions of the Sun and M 67, and all the parameters in the Galactic model. We compute 3.5 \times 10^5 pairs of orbits Sun-M 67 looking for close encounters in the past with a minimum distance approach within the tidal radius of M 67. In these encounters we find that the relative velocity between the Sun and M 67 is larger than 20 km/s. If the Sun had been ejected from M 67 with this high velocity by means of a three-body encounter, this interaction would destroy an initial circumstellar disk around the Sun, or disperse its already formed planets. We also find a very low probability, much less than 10^-7, that the Sun was ejected from M 67 by an encounter of this cluster with a giant molecular cloud. This study also excludes the possibility that the Sun and M 67 were born in the same molecular cloud. Our dynamical results convincingly demonstrate that M67 could not have been the birth cluster of our Solar System.

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arXiv2008

Monitoring the Large Proper Motions of Radio Sources in the Orion BN/KL Region

We present absolute astrometry of four radio sources in the Becklin-Neugebauer/Kleinman-Low (BN/KL) region, derived from archival data (taken in 1991, 1995, and 2000) as well as from new observations (taken in 2006). All data consist of 3.6 cm continuum emission and were taken with the Very Large Array in its highest angular resolution A configuration. We confirm the large proper motions of the BN object, the radio source I (GMR I) and the radio counterpart of the infrared source n (Orion-n), with values from 15 to 26 km/s. The three sources are receding from a point between them from where they seem to have been ejected about 500 years ago, probably via the disintegration of a multiple stellar system. We present simulations of very compact stellar groups that provide a plausible dynamical scenario for the observations. The radio source Orion-n appeared as a double in the first three epochs, but as single in 2006. We discuss this morphological change. The fourth source in the region, GMR D, shows no statistically significant proper motions. We also present new, accurate relative astrometry between BN and radio source I that restrict possible dynamical scenarios for the region. During the 2006 observations, the radio source GMR A, located about 1' to the NW of the BN/KL region, exhibited an increase in its flux density of a factor of ~3.5 over a timescale of one hour. This rapid variability at cm wavelengths is similar to that previously found during a flare at millimeter wavelengths that took place in 2003.

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arXiv2007

Six New Galactic Orbits of Globular Clusters in a Milky-Way-Like Galaxy

Absolute proper motions for six new globular clusters have recently been determined. This motivated us to obtain the Galactic orbits of these six clusters both in an axisymmetric Galactic potential and in a barred potential, such as the one of our Galaxy. Orbits are also obtained for a Galactic potential that includes spiral arms. The orbital characteristics are compared and discussed for these three cases. Tidal radii and destruction rates are also computed and discussed.

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arXiv2006

Halo Wide Binaries and Moving Clusters as probes of the Dynamical and Merger History of our Galaxy

Wide or fragile pairs are sensitive probes of the galactic potential, and they have been used to provide information about the galactic tidal field, the density of GMC and the masses of dark matter perturbers present in both the disk and the halo. Halo wide binaries and moving clusters, since they are likely to be the remains of past mergers or of dissolved clusters, can provide information on the dynamical and merger history of our Galaxy. Such remnants should continue to show similar motions over times of the order of their ages. We have looked for phase space groupings among the low-metallicity stars of Schuster et al. (2006) and have identified a number of candidate moving clusters. In several of the moving clusters we found a wide CPM binary already identified in our catalogue of wide binaries among high-velocity and metal-poor stars (Allen et al 2000a). Spectroscopic follow-up studies of these stars would confirm the physical reality of the groups, as well as allow us to distinguish whether their progenitors are dissolved clusters or accreted extragalactic systems

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arXiv2006

The orbits of 48 globular clusters in a Milky-Way-Like Barred Galaxy

The effect of a barred potential (such as the one of the Milky Way) on the galactic orbits of forty-eight globular clusters for which absolute proper motions are known is studied. The orbital characteristics are compared with those obtained for the case of an axisymmetric galactic potential. Tidal radii are computed and discussed for both the better known axisymmetric case and that including a bar. The destruction rates due to bulge and disk shocking are calculated and compared in both galactic potentials.

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arXiv2005

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

A recent article has employed the determination from groundbased images of high proper motions in the Orion Nebula Cluster to argue that JW 349, JW 355, and JW 451 are high velocity (38 km/s, 89 km/s, and 69 km/s, respectively) low mass runaway stars. We report on measurement of the proper motions of these stars using images made by the Hubble Space Telescope's WFPC2 imager and find that there is no evidence for motions above 6.2 km/s for JW 349 and 7.9 km/s for JW 355, while the motion of 5.5 km/s for JW 451 is only slightly larger than the measurement uncertainty of 3.9 km/s. We conclude that there is no observational support for these stars being high velocity runaway stars.

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arXiv2005

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 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-- $μ_\ell \cos b$ = +2.1 $\pm$ 0.2 mas yr$^{-1}$; $μ_b$ = $-$0.1 $\pm$ 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 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

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arXiv2005

Low-Mass Runaway Stars from the Orion Trapezium Cluster

In the course of a search for common proper motion binaries, in the Jones & Walker (JW) catalogue of proper motions in the Orion Nebula Cluster, we came across several faint stars with proper motions larger than one arcsecond per century and probabilities of membership P larger than 0.90. Such stars are interesting because they could be low-mass runaway stars recently accelerated by n-body interactions in compact multiple systems. Of particular interest among these stars is JW 451, which has a P = 0.98, the largest transverse velocity among all the stars with P larger or equal than 0.5 (69 km/s), and a proper motion vector which suggests that it was accelerated by the component C of the Trapezium some 1000 years ago. A closer examination of those JW stars with proper motion larger than one arcsec per century revealed that two other stars, JW 349 and JW 355 (with transverse velocities of 38 and 90 km/s respectively), in spite of being listed with P = 0 by JW, should also be considered part of the cluster, because these objects are also externally ionized proplyds. In fact, Hillenbrand (1997) assigns to them probabilities of membership of 0.99. Moreover, the proper motion errors of these two stars are relatively small, and so they are good candidates to be runaway stars recently accelerated in the Orion Nebula Cluster.

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arXiv2005

Proper Motions of the BN Object and the I Radio Source in Orion: Where and When Did BN 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 $12.6 \pm 0.6$ mas yr$^{-1}$ (corresponding to a velocity of $27 \pm 1$ km s$^{-1}$ at an adopted distance of 450 pc) to the northwest, while the radio source I has a proper motion of $5.6 \pm 0.7$ mas yr$^{-1}$ (corresponding to a velocity of $12 \pm 2$ 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.

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arXiv2002

Young stars at large distances from the galactic plane: mechanisms of formation

We have collected from the literature a list of early-type stars, situated at large distances from the galactic plane, for which evidence of youth seems convincing. We discuss two possible formation mechanisms for these stars: ejection from the plane by dynamical interactions within small clusters, and formation away from the plane, via induced shocks created by spiral density waves. We identify the stars that could be explained by each mechanism. We conclude that the ejection mechanism can account for about two thirds of the stars, while a combination of star formation at z = 500-800 pc from the plane and ejection, can account for 90 percent of these stars. Neither mechanism, nor both together, can explain the most extreme examples.

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