http://hdl.handle.net/2117/1128 2013-05-20T01:51:23Z http://hdl.handle.net/2117/6777 Title: Classification of local stellar populations: the improved MEMPHIS algorithm - Part II Authors: Cubarsí Morera, Rafael; Alcobé López, Santiago Abstract: Discontinuities of the local velocity distribution which are associated with stellar populations are studied from the improved statistical method MEMPHIS (Maximum Entropy of the Mixture Probability from HIerarchical Segregation), by combining a sampling parameter, optimisation of the mixture approach, and maximum partition entropy of populations composing the stellar sample. The sampling parameter is associated with isolating integrals of the star motion and it is used to build a hierarchical family of subsamples. An accurate characterisation of the entropy graph is given where a local maximum of entropy takes place simultaneously with a local minimum 2 error. By working from different sampling parameters the method is applied to samples from HIPPARCOS and Geneva-Copenhagen survey (GCS) to obtain kinematic parameters and mixture proportions of thin disk, thick disk and halo. The sampling parameter P = |(U, V,W)|, absolute heliocentric velocity, allows to build an optimal subsample containing thin and thick disk stars, by leaving aside most of the halo population. The sampling parameter P = |W|, absolute perpendicular velocity, is able to build an optimal subsample containing a mixture of total disk and halo stars, although it does not allow an optimal segregation of thin and thick disks. Other sampling parameters like P = |(U,W)| or P = |V | are found to be less population informative. By comparing both samples, HIPPARCOS provides more accurate estimates for thick disk and halo, while GCS does for the total disk. In particular, the radial velocity dispersion of the halo fits perfectly into the empirical Titius-Bode like law U = 6.6 ( 4 3 )3n+2, which was previously proposed for discrete kinematic components, where the values n = 0, 1, 2, 3 stands for early-type stars, thin disk, thick disk, and halo populations. Population statistics are used to segregate thin disk, thick disk, and halo, and to obtain a more accurate bayesian estimation of the population fractions. 2010-03-23T11:10:08Z http://hdl.handle.net/2117/6775 Title: Partition entropy and chi-squared error: the improved MEMPHIS algorithm - Part I Authors: Cubarsí Morera, Rafael; Alcobé López, Santiago Abstract: The entropy of the population partition is studied as a function of the sampling parameter, so that within a particular interval of its graph, the plateau region, it is possible to get a stable estimation of the mixture parameters. The optimal estimation is associated with a local maximum of entropy. Alter natively, the $\chi^2$ error of the mixture approach may also be used to obtain an optimal segregation. The relationship between the fitting error and the population entropy has been analysed in detail. We have proved that, by using an appropriate sampling parameter, within a plateau region of the entropy graph, a local entropy maximum takes place simultaneously with a local minimum of the $\chi^2$ error. Therefore, the combined statistical method provides the best approximation mixture, as well as the less informative partiti on, to estimate the kinematic parameters of populations. 2010-03-23T10:56:41Z http://hdl.handle.net/2117/6773 Title: Structure of the velocity distribution of the Galactic disc: a maximum entropy statistical approach - Part I Authors: Cubarsí Morera, Rafael Abstract: The maximum entropy approach is proposed to describe the local structures of the veloc- ity distribution, which are collected through its sample moments. The method is used with several samples from the HIPPARCOS and Geneva-Copenhagen survey catalogues. For the large-scale distribution, the phase density function may be obtained by fitting moments up to sixth order as a product of two exponential functions, one giving a background ellipsoidal shape of the distribution and the other accounting for the skewness and for the slight shift in the ellipsoidal isocontours in terms of the rotation velocity. The small-scale distribution can be deduced from truncated distributions, such as velocity-bounded samples with |V| ≤ 51 km s−1, which contain a complex mixture of early-type and young disc stars. By fitting up to ten-order moments, the maximum entropy approach gives a realistic portrait of actual asymmetries, showing a clear bimodal pattern: (i) around the Hyades-Pleiades stream, with negative radial mean velocity and (ii) around the Sirius-UMa stream, with slightly positive radial mean velocity. The “U-anomaly” along the radial direction is estimated straightfor- wardly 30 − 35 km s−1 from the contour plots. 2010-03-23T10:45:48Z http://hdl.handle.net/2117/6710 Title: Disc and halo kinematic populations from HIPPARCOS and Geneva-Copenhagen surveys of the solar neighbourhood Authors: Cubarsí Morera, Rafael; Alcobé López, Santiago; Vidojevic, S.; Ninkovic, S. Abstract: Discontinuities in the local velocity distribution associated with stellar populations are studied using the Maximum Entropy of the Mixture Probability from HIerarchical Segregation (MEMPHIS) improved statistical method, by combining a sampling parameter, an optimisation of the mixture approach, and a maximisation of the partition entropy for the constituent populations of the stellar sample. The sampling parameter is associated with isolating integrals of the stellar motion and is used to build a hierarchical family of subsamples. We provide an accurate characterisation of the entropy graph, in which a local maximum of entropy takes place simultaneously with a local minimum of the χ2 error. By analysing different sampling parameters, the method is applied to samples from the HIPPARCOS and Geneva-Copenhagen survey (GCS) to determine the kinematic parameters and the stellar population mixture of the thin disc, thick disc, and halo. The sampling parameter P = |(U, V,W)|, which is the absolute heliocentric velocity, allows us to build an optimal subsample containing both thin and thick disc stars, omitting most of the halo population. The sampling parameter P = |W|, which is absolute perpendicular velocity, allows us to create an optimal subsample of all disc and halo stars, although it does not allow an optimal differentiation of thin and thick discs. Other sampling parameters, such as P = |(U,W)| or P = |V|, are found to provide less information about the populations. By comparing both samples, HIPPARCOS provides more accurate estimates for the thick disc and halo, and GCS for the total disc. In particular, the radial velocity dispersion of the halo fits perfectly into the empirical Titius-Bode-like law σU = 6.6 (43 )3n+2, previously proposed for discrete kinematical components, where the values n = 0, 1, 2, 3 represent early-type stars, thin disc, thick disc, and halo populations, respectively. The kinematic parameters are used to segregate thin disc, thick disc, and halo stars, and to obtain a more accurate Bayesian estimation of the population fractions. To check the reliability of our results, an alternative segregation approach is used. GCS stars are classified into different kinematical populations in terms of their orbital parameters. The population fractions and velocity moments obtained by both methods are in excellent agreement. 2010-03-19T11:35:30Z http://hdl.handle.net/2117/6707 Title: Large and small-scale structures of the local Galactic disc: a maximum entropy approach to the stellar velocity distribution Authors: Cubarsí Morera, Rafael Abstract: An analytical model based on the maximum entropy approach is proposed to describe the eventual asymmetries of the velocity distribution, which are collected through its sample moments. If an extended set of moments is available, the current method provides a linear algorithm, associated with a Gramian system of equations, that leads to a fast and suitable estimation of the velocity distribution. In particular, it could be used to model multimodal distributions that cannot be described through Gaussian mixtures. The method is used with several samples from the HIPPARCOS and Geneva-Copenhagen survey catalogues. For the large-scale distribution, the phase density function may be obtained by fitting moments up to sixth order as a product of two exponential functions, one giving a background ellipsoidal shape of the distribution and the other accounting for the skewness and for the slight shift in the ellipsoidal isocontours in terms of the rotation velocity. The small-scale distribution can be deduced from truncated distributions, such as velocity-bounded samples with |V| ≤ 51 km s−1, which contain a complex mixture of early-type and young disc stars. By fitting up to ten-order moments, the maximum entropy approach gives a realistic portrait of actual asymmetries, showing a clear bimodal pattern: (i) around the Hyades-Pleiades stream, with negative radial mean velocity and (ii) around the Sirius-UMa stream, with slightly positive radial mean velocity. Among metallicity, colour, and other star properties, the eccentricity of the star’s orbit behaves as a very good sampling parameter to find a more detailed structure for the disc velocity distribution, allowing distinctions between different eccentricity layers. For subsamples with eccentricities e < 0.15, star velocities are approximately symmetrically distributed around the LSR in the radial direction, with a dearth of stars at the LSR. For e = 0.15, the core distribution of the thin disc is supported by two major stellar groups with opposite radial velocities. Several simulations confirm that such a double-peaked distribution comes from the lognormal distribution of the velocity amplitudes. For maximum eccentricity 0.3 and maximum distance to the Galactic plane 0.5 kpc a representative thin disc sample is obtained. The “U-anomaly” along the radial direction is estimated straightforwardly 30−35 km s−1 from the contour plots. An explanation of the apparent vertex deviation of the disc from the swinging of those major kinematic groups around the LSR is then possible, which predicts a continuously changing orientation of the disc’s pseudo ellipsoid. 2010-03-19T08:59:31Z http://hdl.handle.net/2117/6700 Title: Small-scale structure of the disc velocity distribution. A maximum entropy statistical approach - Part II Authors: Cubarsí Morera, Rafael Abstract: Among metallicity, colour, and other star properties, the eccentricity of the star’s orbit behaves as a very good sampling parameter to find a more detailed structure for the disc velocity distribution, allowing distinctions between different eccentricity layers. For subsam- ples with eccentricities e < 0.15, star velocities are approximately symmetrically distributed around the LSR in the radial direction, with a dearth of stars at the LSR. For e = 0.15, the core distribution of the thin disc is supported by two major stellar groups with opposite radial velocities. Several simulations confirm that such a double-peaked distribution comes from the lognormal distribution of the velocity amplitudes. For maximum eccentricity 0.3 and maximum distance to the Galactic plane 0.5 kpc a representative thin disc sample is obtained. An explanation of the apparent vertex deviation of the disc from the swinging of those major kinematic groups around the LSR is possible, which predicts a continuously changing orientation of the disc’s pseudo ellipsoid. 2010-03-18T12:49:37Z http://hdl.handle.net/2117/3046 Title: On the maximum entropy and the problem of moments: an application to stellar kinematics Authors: Cubarsí Morera, Rafael Abstract: The maximum entropy approach is used to solve the classical moment problem of stellar kinematics. If an extended set of moments is available, the current method provides a linear estimation algorithm, which is given by a Gramian system of equations, that leads to a fast and suitable estimation of the velocity distribution. In particular, it can be used as an alternative approach for modelling multimodal distributions that can not be described through gaussian mixtures. 2009-08-24T09:04:26Z http://hdl.handle.net/2117/3014 Title: Open source precise point positioning with GNSS data structures and the GPSTk Authors: Salazar Hernández, Dagoberto José; Hernández Pajares, Manuel; Juan Zornoza, José Miguel; Sanz Subirana, Jaume 2009-06-12T06:53:05Z http://hdl.handle.net/2117/2821 Title: Methods and other considerations to correct higher order ionospheric delay terms in GNSS Authors: Hernández Pajares, Manuel; Fritsche, M.; Hoque, M.M.; Jakowski, Norbert; Juan Zornoza, José Miguel; Kedar, S.; Krankowski, A.; Petrie, E.; Sanz Subirana, Jaume 2009-03-31T18:44:25Z http://hdl.handle.net/2117/2650 Title: Direct determination of electron density profiles from GNSS occulttion data Authors: Aragón Ángel, María Ángeles; Hernández Pajares, Manuel; Miguel Juan, Miguel; Sanz Subirana, Jaume; Ramos Bosch, Pedro 2009-03-12T08:12:22Z http://hdl.handle.net/2117/2617 Title: Improved Abel transform inversion: application to COSMIC/FORMOSAT-3 ionospheric occultations Authors: Aragón Ángel, María Ángeles; Hernández Pajares, Manuel; Miguel Juan, Miguel; Sanz Subirana, Jaume; Altadill, David; Liou, Yuei­An; Lee, Chien­Chih 2009-03-02T11:36:47Z http://hdl.handle.net/2117/2615 Title: The User Domain Integrity Assessment Technique Authors: Sanz Subirana, Jaume; Ramos Bosch, Pedro; Aragón Ángel, María Ángeles; Ciollaro, Massimo; Toran, Felix; Ventura Travesset, Javier; López, C. Abstract: This paper presents a study performed for the detailed analysis of user domain and pseudorange integrity on Satellite Based Augmentation Systems. Experimental data of EGNOS pre-operational service was used for some specific tests, aiming at identifying for instance the good reaction of the EGNOS check-set subsystem and the way pseudorange errors are translated into the user domain. From this analysis, a new technique for User Domain Integrity Assessment (UDIA) was developed. With the UDIA technique, the integrity is checked for all possible geometries from 4 to all satellites in view from a network of stations. When no MI is found (from a wide and sufficiently dense stations network), it can be assured that the system is safe, and it becomes the best experimental guarantee at the User Domain Level. If a NMI/MI appears from any geometry of satellites from the stations in the network, thence a search algorithm is applied to identify the satellite, or combinations of satellites, responsible of such NMI/MIs (i.e., their exclusion leads to the disappearance of the NMI/MIs). After identifying such satellites, the reaction of the EGNOS Check Set (CS) is evaluated by checking if they were set as DU or NM within a predefined time interval after the NMI/MI is detected. 2009-03-02T11:17:56Z http://hdl.handle.net/2117/2614 Title: High accuracy positioning using carrier-phases with the opensource GPSTK software Authors: Salazar Hernández, Dagoberto José; Hernández Pajares, Manuel; Miguel, J.; Sanz Subirana, Jaume 2009-03-02T10:59:57Z http://hdl.handle.net/2117/2613 Title: Mejora de la transformada inversa de Abel: aplicación ocultaciones ionosféricas de COSMIC/FORMOSAT-3 Authors: Aragón Ángel, María Ángeles; Hernández Pajares, Manuel; Miguel, J.; Sanz Subirana, Jaume 2009-03-02T09:42:21Z http://hdl.handle.net/2117/2515 Title: Open source precise point positioning with GNSS data structures and the GPSTk Authors: Salazar Hernández, Dagoberto José; Hernández Pajares, Manuel; Juan Zornoza, José Miguel; Sanz Subirana, Jaume 2009-01-21T10:12:35Z