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Tsevi Mazeh

Publications and source records attributed to Tsevi Mazeh.

143 records · Page 8Linked to original sources
arXiv2005

Correcting systematic effects in a large set of photometric lightcurves

We suggest a new algorithm to remove systematic effects in a large set of lightcurves obtained by a photometric survey. The algorithm can remove systematic effects, like the ones associated with atmospheric extinction, detector efficiency, or PSF changes over the detector. The algorithm works without any prior knowledge of the effects, as long as they linearly appear in many stars of the sample. The approach, which was originally developed to remove atmospheric extinction effects, is based on a lower rank approximation of matrices, an approach which was already suggested and used in chemometrics, for example. The proposed algorithm is specially useful in cases where the uncertainties of the measurements are unequal. For equal uncertainties the algorithm reduces to the Principal Components Analysis (PCA) algorithm. We present a simulation to demonstrate the effectiveness of the proposed algorithm and point out its potential, in search for transit candidates in particular.

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arXiv2004

An intriguing correlation between the masses and periods of the transiting planets

We point out an intriguing relation between the masses of the transiting planets and their orbital periods. For the six currently known transiting planets, the data are consistent with a decreasing linear relation. The other known short-period planets, discovered through radial-velocity techniques, seem to agree with this relation. We briefly speculate about a tentative physical model to explain such a dependence.

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arXiv2003

A Possible Correlation between Mass Ratio and Period Ratio in Multiple Planetary Systems

We report on a possible correlation between the mass ratio and period ratio of pairs of adjacent planets in extra-solar planetary systems. Monte-Carlo simulations show that the effect is significant to level of 0.7%, as long as we exclude two pairs of planets whose periods are at the 1:2 resonance. Only the next few multiple systems can tell if the correlation is real.

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arXiv2002

Fine Structure in the Circumstellar Environment of a Young, Solar-like Star: the Unique Eclipses of KH 15D

Results of an international campaign to photometrically monitor the unique pre-main sequence eclipsing object KH 15D are reported. An updated ephemeris for the eclipse is derived that incorporates a slightly revised period of 48.36 d. There is some evidence that the orbital period is actually twice that value, with two eclipses occurring per cycle. The extraordinary depth (~3.5 mag) and duration (~18 days) of the eclipse indicate that it is caused by circumstellar matter, presumably the inner portion of a disk. The eclipse has continued to lengthen with time and the central brightness reversals are not as extreme as they once were. V-R and V-I colors indicate that the system is slightly bluer near minimum light. Ingress and egress are remarkably well modeled by the passage of a knife-edge across a limb-darkened star. Possible models for the system are briefly discussed.

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arXiv2002

On the Mass-Period Correlation of the Extrasolar Planets

We report on a possible correlation between the masses and periods of the extrasolar planets, manifested as a paucity of massive planets with short orbital periods. Monte-Carlo simulations show the effect is significant, and is not solely due to an observational selection effect. We also show the effect is stronger than the one already implied by published models that assumed independent power-law distributions for the masses and periods of the extrasolar planets. Planets found in binary stellar systems may have an opposite correlation. The difference is highly significant despite the small number of planets in binary systems. We discuss the paucity of short-period massive planets in terms of some theories for the close-in giant planets. Almost all models can account for the deficit of massive planets with short periods, in particular the model that assumes migration driven by a planet-disk interaction, if the planet masses do not scale with their disk masses.

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arXiv2002

A Statistical Analysis of The Extrasolar Planets and The Low-Mass Secondaries

We show that the astrometric Hipparcos data of the stars hosting planet candidates are not accurate enough to yield statistically significant orbits. Therefore, the recent suggestion, based on the analysis of the Hipparcos data, that the orbits of the sample of planet candidates are not randomly oriented in space, is not supported by the data. Assuming random orientation, we derive the mass distribution of the planet candidates and show that it is flat in log M, up to about 10 Jupiter masses. Furthermore, the mass distribution of the planet candidates is well separated from the mass distribution of the low-mass companions by the 'brown-dwarf desert'. This indicates that we have here two distinct populations, one which we identify as the giant planets and the other as stellar secondaries. We compare the period and eccentricity distributions of the two populations and find them surprisingly similar. The period distributions between 10 and 1650 days are flat in log period, indicating a scale-free formation mechanism in both populations. We further show that the eccentricity distributions are similar - both have a density distribution peak at about 0.2-0.4, with some small differences on both ends of the eccentricity range. We present a toy model to mimic both distributions. We found a significant paucity of massive giant planets with short orbital periods. The low frequency of planets is noticeable for masses larger than about 1 Jupiter Mass and periods shorter than 30 days. We point out how, in principle, one can account for this paucity.

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arXiv2001

Analysis of the Hipparcos Observations of the Extrasolar Planets and the Brown-Dwarf Candidates

We analyzed the Hipparcos astrometric observations of 47 stars that were discovered to harbor giant planets and 14 stars with brown-dwarf secondary candidates. The Hipparcos measurements were used together with the corresponding stellar radial-velocity data to derive an astrometric orbit for each system. To find out the significance of the derived astrometric orbits we applied a "permutation" technique by which we analyzed the permuted Hipparcos data to get false orbits. The size distribution of these false orbits indicated the range of possibly random, false orbits that could be derived from the true data. These tests could not find any astrometric orbit of the planet candidates with significance higher than 99%, suggesting that most if not all orbits are not real. Instead, we used the Hipparcos data to set upper limits on the masses of the planet candidates. The lowest derived upper limit is that of 47UMa - 0.014 solar mass, which confirms the planetary nature of its unseen companion. For 13 other planet candidates the upper limits exclude the stellar nature of their companions, although brown-dwarf secondaries are still an option. These negate the idea that all or most of the extrasolar planets are disguised stellar secondaries. Of the 14 brown-dwarf candidates, our analysis reproduced the results of Halbwachs et al., who derived significant astrometric orbits for 6 systems which imply secondaries with stellar masses. We show that another star, HD164427, which was discovered only very recently, also has a secondary with stellar mass. Our findings support Halbwachs et al. conclusion about the possible existence of the "brown-dwarf desert" which separates the planets and the stellar secondaries.

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arXiv2001

Derivation of the Mass Distribution of Extrasolar Planets with MAXLIMA - a Maximum Likelihood Algorithm

We construct a maximum-likelihood algorithm - MAXLIMA, to derive the mass distribution of the extrasolar planets when only the minimum masses are observed. The algorithm derives the distribution by solving a numerically stable set of equations, and does not need any iteration or smoothing. Based on 50 minimum masses, MAXLIMA yields a distribution which is approximately flat in log M, and might rise slightly towards lower masses. The frequency drops off very sharply when going to masses higher than 10 Jupiter masses, although we suspect there is still a higher mass tail that extends up to probably 20 Jupiter masses. We estimate that 5% of the G stars in the solar neighborhood have planets in the range of 1-10 Jupiter masses with periods shorter than 1500 days. For comparison we present the mass distribution of stellar companions in the range of 100--1000 Jupiter masses, which is also approximately flat in log M. The two populations are separated by the "brown-dwarf desert", a fact that strongly supports the idea that these are two distinct populations. Accepting this definite separation, we point out the conundrum concerning the similarities between the period, eccentricity and even mass distribution of the two populations.

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arXiv2001

On the Statistical Significance of the Hipparcos Astrometric Orbit of rho Coronae Borealis

Recently Gatewood, Han & Black presented an analysis of the Hipparcos stellar positions and their own ground-based measurements of rho CrB, suggesting an astrometric orbit of 1.5 mas with an extremely small orbital inclination of 0.5 degrees. This indicates that the planet-candidate secondary might be a late M star. We used the Hipparcos data of rho CrB together with the individual radial velocities of Noyes et al. to independently study the stellar orbit and to assess its statistical significance. Our analysis yielded the same astrometric orbit. However, a permutation test we performed on the Hipparcos measurements indicated that the statistical significance of the astrometric orbit is only 2-sigma. Therefore, we can expect about one out of 40 systems with a similar data set to show a false astrometric orbit.

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arXiv2001

Studies of multiple stellar systems - IV. The triple-lined spectroscopic system Gliese 644

We present a radial-velocity study of the triple-lined system Gliese 644 and derive spectroscopic elements for the inner and outer orbits with periods of 2.9655 and 627 days. We also utilize old visual data, as well as modern speckle and adaptive optics observations, to derive a new astrometric solution for the outer orbit. These two orbits together allow us to derive masses for each of the three components in the system: M_A = 0.410 +/- 0.028 (6.9%), M_Ba = 0.336 +/- 0.016 (4.7%), and $M_Bb = 0.304 +/- 0.014 (4.7%) M_solar. We suggest that the relative inclination of the two orbits is very small. Our individual masses and spectroscopic light ratios for the three M stars in the Gliese 644 system provide three points for the mass-luminosity relation near the bottom of the Main Sequence, where the relation is poorly determined. These three points agree well with theoretical models for solar metallicity and an age of 5 Gyr. Our radial velocities for Gliese 643 and vB 8, two common-proper-motion companions of Gliese 644, support the interpretation that all five M stars are moving together in a physically bound group. We discuss possible scenarios for the formation and evolution of this configuration, such as the formation of all five stars in a sequence of fragmentation events leading directly to the hierarchical configuration now observed, versus formation in a small N cluster with subsequent dynamical evolution into the present hierarchical configuration.

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arXiv2000

A 10-micron Search for Inner-Truncated Disks Among Pre-Main-Sequence Stars With Photometric Rotation Periods

We use mid-IR (primarily 10 $μ$m) photometry as a diagnostic for the presence of disks with inner cavities among 32 pre-main sequence stars in Orion and Taurus-Auriga for which rotation periods are known and which do not show evidence for inner disks at near-IR wavelengths. Disks with inner cavities are predicted by magnetic disk-locking models that seek to explain the regulation of angular momentum in T Tauri stars. Only three stars in our sample show evidence for excess mid-IR emission. While these three stars may possess truncated disks consistent with magnetic disk-locking models, the remaining 29 stars in our sample do not. Apparently, stars lacking near-IR excesses in general do not possess truncated disks to which they are magnetically coupled. We discuss the implications of this result for the hypothesis of disk-regulated angular momentum. Evidently, young stars can exist as slow rotators without the aid of present disk-locking, and there exist very young stars already rotating near breakup velocity whose subsequent angular momentum evolution will not be regulated by disks. Moreover, we question whether disks, when present, truncate in the manner required by disk-locking scenarios. Finally, we discuss the need for rotational evolution models to take full account of the large dispersion of rotation rates present at 1 Myr, which may allow the models to explain the rotational evolution of low-mass pre-main sequence stars in a way that does not depend upon braking by disks.

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arXiv2000

Studies of multiple stellar systems - III. Modulation of orbital elements in the triple-lined system HD 109648

The triple-lined spectroscopic triple system HD 109648 has one of the shortest periods known for the outer orbit in a late-type triple, 120.5 days, and the ratio between the periods of the outer and inner orbits is small, 22:1. With such extreme values, this system should show orbital element variations over a timescale of about a decade. We have monitored the radial velocities of HD 109648 with the CfA Digital Speedometers for eight years, and have found evidence for modulation of some orbital elements. While we see no definite evidence for modulation of the inner binary eccentricity, we clearly observe variations in the inner and outer longitudes of periastron, as well as in the radial velocity amplitudes of the three components. The observational results, combined with numerical simulations, allow us to put constraints on the orientation of the orbits.

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arXiv2000

Analysis of The Hipparcos Measurements of HD10697 - A Mass Determination of a Brown-Dwarf Secondary

HD10697 is a nearby main-sequence star around which a planet candidate has recently been discovered by means of radial-velocity measurements (Vogt et al. 1999, submitted to ApJ). The stellar orbit has a period of about three years, the secondary minimum mass is 6.35 Jupiter masses and the minimum semi-major axis is 0.36 milli-arc-sec (mas). Using the Hipparcos data of HD10697 together with the spectroscopic elements of Vogt et al. (1999) we found a semi-major axis of 2.1 +/- 0.7 mas, implying a mass of 38 +/- 13 Jupiter masses for the unseen companion. We therefore suggest that the secondary of HD10697 is probably a brown dwarf, orbiting around its parent star at a distance of 2 AU.

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arXiv1999

Analysis of The Hipparcos Measurements of Upsilon Andromedae - A Mass Estimate of Its Outermost Known Planetary Companion

We present an analysis of Hipparcos astrometric measurements of Upsilon Andromedae, a nearby main-sequence star around which three planet candidates have recently been discovered by means of radial-velocity measurements. The stellar orbit associated with the outermost candidate has a period of 1269 +/- 9 days and a minimum semi-major axis of 0.6 milli-arc-sec (mas). Using the Hipparcos data together with the spectroscopic elements we found a semi-major axis of 1.4 +/- 0.6 mas. This implies a mass of 10.1 (+4.7, -4.6) Jupiter masses for that planet of Upsilon Andromedae.

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arXiv1998

The Mass Distribution of Extrasolar Planet-Candidates and Spectroscopic-Binary Low-Mass Companions

We consider the mass distribution of the nine unseen companions orbiting solar-type stars with minimum possible masses in the planetary mass range reported as of March 1998. We compare the mass distribution of these nine extrasolar planet candidates with the distribution of low-mass secondaries in spectroscopic binaries. We choose to use a logarithmic scale to study the combined mass distribution, because of the large range of masses, 0.5-300 Jupiter masses, involved. Although the results are based on a very small number of systems, the combined distribution looks different at the high- and low-ends. At the high-mass end the distribution drops steeply from 200 to 20 Jupiter masses. At the planetary range of masses the distribution is flat, and might even rise mildly from, say, 20 to 0.6 Jupiter masses, depending on the assumed detection threshold for the precise surveys. The transition region between the two slopes is at about 10-30 Jupiter masses. One possible interpretation of this result is that we have here two different populations. Maybe the lower-mass population was formed like planets, out of an accretion disc, while the high-mass population was formed like binary stars. If the shape of the combined distribution can be verified by many more detections, and if the planetary-mass objects prove to be extrasolar planets, this can give us the long-sought clue for how to distinguish planets from low-mass stellar companions.

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arXiv1996

The High Eccentricity of the Planet Around 16 Cyg B

We consider the high eccentricity, 0.66, of the newly discovered planet around 16 Cyg B, using the fact that the parent star is part of a wide binary. We show that the high eccentricity of the planet could be the result of tidal forces exerted on 16 Cyg B and its planet by 16 Cyg A, the distant companion in the system. By following stellar triple systems with parameters similar to those of 16 Cyg, we have established that the orbital eccentricity of the planet could have gone through strong modulation, with an amplitude of 0.8 or even larger, with typical timescale of tens of millions years. The amplitude of the planet eccentricity strongly depends on the relative inclination between the plane of motion of the planet and that of the wide binary 16 Cyg AB. To account for the present eccentricity of the planet we have to assume that the angle between the two planes of motion is large, at least 60 deg. We argue that this assumption is reasonable for wide binaries like 16 Cyg AB.

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