arXiv subjects

Gil Nachmani

Publications and source records attributed to Gil Nachmani.

6 recordsLinked to original sources

arXiv2026

MESS: Multi-Epoch Spectroscopic Solver for Detecting Double-Lined Systems

We present MESS, a fully automated algorithm for identifying and characterizing double-lined spectroscopic binaries (SB2) in large databases of multi-epoch spectra. MESS extends the two-dimensional TODCOR approach to a global multi-epoch formalism, deriving the radial velocities (RVs) of both components at each epoch while optimizing the templates jointly across all observations. Template optimization searches a continuous synthetic-spectra manifold spanning an eight-dimensional parameter space: effective temperature, surface gravity, and rotational broadening for each star, together with a common metallicity and the flux ratio. Single-lined spectroscopic binaries (SB1) and single stars (S1) are handled within the same framework by fitting one optimized template, with either epoch-dependent RVs (SB1) or a single shared RV (S1). Model selection among S1/SB1/SB2 uses the Bayesian information criterion with an effective sample size that accounts for intra-spectrum correlations, and is complemented by the Wilson relation between the two RVs to infer the mass ratio and systemic velocity without a full orbital solution. We validate MESS on 1500 simulated LAMOST MRS systems (SNR=50), with primary RV semi-amplitudes predominantly below the instrumental resolution, achieving an overall classification accuracy of ~95%. We also derive full orbital solutions for two SB2 systems detected in our LAMOST analysis, including a faint-secondary case with flux ratio ~0.1, and present example outputs for one SB1 and three constant-velocity stars. A companion paper will report the survey-wide application to LAMOST DR11 and the resulting SB1/SB2 catalogs.

astro-ph.SR↗

arXiv2022

$\it COD:$ An Algorithm for Shape Reconstruction of Transiting Celestial Bodies through Topological Optimization

We introduce a novel algorithm, $\textit{COD}$ -- Compact Opacity Distribution, for shape reconstruction of a celestial body that has been observed to occult a star, using the photometric time-series observations of the occultation. $\textit{COD}$ finds a solution to the light-curve inversion problem for an optically thick occulter having an approximately convex shape, together with an estimate of its size, impact parameter and velocity, relative to the occulted star. The algorithm is based on an optimization scheme that uses topological constraints and an objective function for the geometry of the occulter. The constraints of the problem follow linear relations, which enable the use of linear programming optimization as the mathematical framework. Multiple tests of the algorithm were performed, all of which resulted in high correlations between the simulated and obtained shapes of the occulting objects, with errors within $5\%$ in their projected velocities and horizontal sizes, and within $0.1$ in their impact parameters. These tests include a video of a solar eclipse by Phobos, as seen by NASA's Curiosity rover, which was collapsed into its corresponding light curve and reconstructed afterwards. We applied $\textit{COD}$ to the mysterious case of VVV-WIT-08 -- a single deep occultation ($\sim 96 \%$) of a giant star lasting for over 200 days. The analysis, which did not assume any specific shape of the occulter, suggested an object with a projected opacity distribution resembling an ellipse with an eccentricity of $\sim 0.5$, tilted at $\sim 30$ degrees relative to the direction of motion, with a semi-minor axis similar to the stellar radius.

astro-ph.IM↗

arXiv2016

Transit Timing Observations from Kepler. IX. Catalog of the Full Long-Cadence Data Set

We present a new transit timing catalog of 2599 Kepler Objects of Interest (=KOIs), using the PDC-MAP long-cadence light curves that include the full seventeen quarters of the mission (ftp://wise- ftp.tau.ac.il/pub/tauttv/TTV/ver_112). The goal is to produce an easy-to-use catalog that can stimulate further analyses of interesting systems. For 779 KOIs with high enough SNRs, we derived the timing, duration and depth of 69,914 transits. For 1820 KOIs with lower SNR, we derived only the timing of 225,273 transits. After removal of outlier timings, we derived various statistics for each KOI that were used to indicate significant variations. Including systems found by previous works, we have detected 260 KOIs which showed significant TTVs with long-term variations (>100 day), and another fourteen KOIs with periodic modulations shorter than 100 day and small amplitudes. For five of those, the periodicity is probably due to the crossing of rotating stellar spots by the transiting planets.

astro-ph.EP↗

arXiv2015

Time variation of Kepler transits induced by stellar spots - a way to distinguish between prograde and retrograde motion. II. Application to KOIs

Mazeh, Holczer, and Shporer (2015) have presented an approach that can, in principle, use the derived transit timing variation (TTV) of some transiting planets observed by the $Kepler$ mission to distinguish between prograde and retrograde motion of their orbits with respect to their parent stars' rotation. The approach utilizes TTVs induced by spot-crossing events that occur when the planet moves across a spot on the stellar surface, looking for a correlation between the derived TTVs and the stellar brightness derivatives at the corresponding transits. This can work even in data that cannot temporally resolve the spot-crossing events themselves. Here we apply this approach to the $Kepler$ KOIs, identifying nine systems where the photometric spot modulation is large enough and the transit timing accurate enough to allow detection of a TTV-brightness-derivatives correlation. Of those systems five show highly significant prograde motion (Kepler-17b, Kepler-71b, KOI-883.01, KOI-895.01, and KOI-1074.01), while no system displays retrograde motion, consistent with the suggestion that planets orbiting cool stars have prograde motion. All five systems have impact parameter $0.2\lesssim b\lesssim0.5$, and all systems within that impact parameter range show significant correlation, except HAT-P-11b where the lack of a correlation follows its large stellar obliquity. Our search suffers from an observational bias against detection of high impact parameter cases, and the detected sample is extremely small. Nevertheless, our findings may suggest that stellar spots, or at least the larger ones, tend to be located at a low stellar latitude, but not along the stellar equator, similar to the Sun.

astro-ph.SR↗

arXiv2013

Transit Timing Observations from Kepler. VIII Catalog of Transit Timing Measurements of the First Twelve Quarters

Following Ford et al. (2011, 2012) and Steffen et al. (2012) we derived the transit timing of 1960 Kepler KOIs using the pre-search data conditioning (PDC) light curves of the first twelve quarters of the Kepler data. For 721 KOIs with large enough SNRs, we obtained also the duration and depth of each transit. The results are presented as a catalog for the community to use. We derived a few statistics of our results that could be used to indicate significant variations. Including systems found by previous works, we have found 130 KOIs that showed highly significant TTVs, and 13 that had short-period TTV modulations with small amplitudes. We consider two effects that could cause apparent periodic TTV - the finite sampling of the observations and the interference with the stellar activity, stellar spots in particular. We briefly discuss some statistical aspects of our detected TTVs. We show that the TTV period is correlated with the orbital period of the planet and with the TTV amplitude.

astro-ph.EP↗

arXiv2011

On the Mass of CoRoT-7b

The mass of CoRoT-7b, the first transiting superearth exoplanet, is still a subject of debate. A wide range of masses have been reported in the literature ranging from as high as 8 M_Earth to as low as 2.3 M_Earth. Although most mass determinations give a density consistent with a rocky planet, the lower value permits a bulk composition that can be up to 50% water. We present an analysis of the CoRoT-7b radial velocity measurements that uses very few and simple assumptions in treating the activity signal. By only analyzing those radial velocity data for which multiple measurements were made in a given night we remove the activity related radial velocity contribution without any a priori model. We demonstrate that the contribution of activity to the final radial velocity curve is negligible and that the K-amplitude due to the planet is well constrained. This yields a mass of 7.42 +/- 1.21 M_Earth and a mean density of rho = 10.4 +/- 1.8 gm cm^-3. CoRoT-7b is similar in mass and radius to the second rocky planet to be discovered, Kepler-10b, and within the errors they have identical bulk densities - they are virtual twins. These bulk densities lie close to the density - radius relationship for terrestrial planets similar to what is seen for Mercury. CoRoT-7b and Kepler-10b may have an internal structure more like Mercury than the Earth.

astro-ph.EP↗