We characterize the kinematic and chemical properties of 589 Galactic anticenter substructure sta... more We characterize the kinematic and chemical properties of 589 Galactic anticenter substructure stars (GASS) with K/M giants in integrals-of-motion space. These stars likely include members of previously identified substructures such as Monoceros, A13, and the Triangulum-Andromeda cloud. We show that these stars are in nearly circular orbits on both sides of the Galactic plane. We can see a velocity (V Z ) gradient along Y-axis especially for the south GASS members. Our GASS members have similar energy and angular momentum distributions to thin-disk stars. Their location in [α/M] versus [M/H] space is more metal-poor than typical thin-disk stars, with [α/M] lower than that of the thick disk. We infer that our GASS members are part of the outer metal-poor disk stars and that the outer disk extends to 30 kpc. Considering the distance range and α-abundance features, GASS could be formed after the thick disk was formed due to the molecular cloud density decreasing in the outer disk where ...
We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from... more We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from the plane,by means of the kinematics of ~400 thick disk stars. The results match the expectations for the visible mass only, and no dark matter is detected in the volume under analysis. The current models of dark matter halo are excluded with a significance higher than 5sigma, unless a highly prolate halo is assumed, very atypical in cold dark matter simulations. The resulting lack of dark matter at the solar position challenges the current models.
In this work, we present new catalogs of M giant and M dwarf stars from the LAMOST DR5. In total,... more In this work, we present new catalogs of M giant and M dwarf stars from the LAMOST DR5. In total, 39,796 M giants and 501,152 M dwarfs are identified from the classification pipeline. The template-fitting results contain M giants with 7 temperature subtypes from M0 to M6, M dwarfs with 18 temperature subtypes from K7.0 to M8.5 and 12 metallicity subclasses from dMr to usdMp. We cross-matched our M-type catalog with the 2MASS and W ISE catalog to obtain infrared magnitude and colors. Adopting the distances derived from the parallaxes in Gaia DR2, the M G vs. (G bp − G rp) 0 diagram shows that there are also early-type stars and white dwarf-M dwarf binaries included in our M type stars sample, with a contamination rate of about 4.6% for M giants and 0.48% for M dwarfs. We found that CaH spectral indices are an efficient selection criteria for carbon stars. A total of 289 carbon stars were identified from the M giants sample, and further confirmed by LAMOST spectra.
We explore the origins of the young B-type stars found by Casetti-Dinescu et al. (2014) at the ou... more We explore the origins of the young B-type stars found by Casetti-Dinescu et al. (2014) at the outskirts of the Milky-Way disk in the sky region of Leading Arm of the Magellanic Stream. High-resolution spectroscopic observations made with the MIKE instrument on the Magellan Clay 6.5m telescope for nine stars are added to the previous sample analyzed by Zhang et al. (2017). We compile a sample of fifteen young stars with well-determined stellar types, ages, abundances and kinematics. With proper motions from Gaia DR2 we also derive orbits in a realistic Milky-Way potential.We find that our previous radial-velocity selected LA candidates have substantial orbital angular momentum. The substantial amount of rotational component for these stars is in contrast with the near-polar Magellanic orbit, thus rendering these stars unlikely members of the LA. There are four large orbital-energy stars in our sample. The highest orbital-energy one has an age shorter than the time to disk crossing, with a birthplace z = 2.5 kpc and R GC ∼ 28 kpc. Therefore, the origin of this star is uncertain. The remaining three stars have disk runaway origin with birthplaces between 12 and 25 kpc from the Galactic center. Also, the most energetic stars are more metal poor
En este proyecto de tesis se propone un estudio de las subestructuras conocidas del halo Galáctic... more En este proyecto de tesis se propone un estudio de las subestructuras conocidas del halo Galáctico, así como el hallazgo de nuevas subestructuras a partir de datos procedentes de la colaboración QUEST. Se cree que tales subestructuras son remanentes de cúmulos globulares o galaxias enanas acrecionadas, similares a la galaxia esferoidal enana de Sagitario. Las estrellas RR Lyrae serán empleadas para trazar el halo debido a que son fácilmente detectables ya que constituyen excelentes luminarias estándar, cuyas distancias están mejor determinadas que trazadores alternativos, e.g. gigantes M. Parte del proyecto comprende la observación espectroscópica de estrellas RR Lyrae en la corriente generada por la galaxia enana de Sagitario a fin de determinar sus dimensiones espaciales, estructura, velocidad radial y velocidad de dispersión. Tales propiedades permiten restringir modelos de la corriente que contempla diferentes geometrías del halo de materia oscura.
We present preliminary results from the deepest and largest photographic proper-motion survey eve... more We present preliminary results from the deepest and largest photographic proper-motion survey ever undertaken of the Galactic bulge. Our first-epoch plate material (from 1972-3) goes deep enough (V_lim = 22) to reach below the bulge main-sequence turnoff. These plates cover an area of approximately 25 arc-min X 25 arc-min of the bulge in the low-extinction (A_v = 0.8 mag) Plaut
Publications of the Astronomical Society of the Pacific, 2021
Under certain rather prevalent conditions (driven by dynamical orbital evolution), a hierarchical... more Under certain rather prevalent conditions (driven by dynamical orbital evolution), a hierarchical triple stellar system can be well approximated, from the standpoint of orbital parameter estimation, as two binary star systems combined. Even under this simplifying approximation, the inference of orbital elements is a challenging technical problem because of the high dimensionality of the parameter space, and the complex relationships between those parameters and the observations (astrometry and radial velocity). In this work we propose a new methodology for the study of triple hierarchical systems using a Bayesian Markov-Chain Monte Carlo-based framework. In particular, graphical models are introduced to describe the probabilistic relationship between parameters and observations in a dynamically self-consistent way. As information sources we consider the cases of isolated astrometry, isolated radial velocity, as well as the joint case with both types of measurements. Graphical models provide a novel way of
We present orbital elements, orbital parallaxes, and individual component masses for 14 spatially... more We present orbital elements, orbital parallaxes, and individual component masses for 14 spatially resolved double-line spectroscopic binaries derived doing a simultaneous fit of their visual orbit and radial velocity curve. This was done by means of a Markov Chain Monte Carlo code developed by our group that produces posterior distribution functions and error estimates for all of the parameters. Of this sample, six systems had high-quality previous studies and were included as benchmarks to test our procedures, but even in these cases, we could improve the previous orbits by adding recent data from our survey of southern binaries being carried out with the HRCam and ZORRO speckle cameras at the SOAR 4.1 m and Gemini South 8.1 m telescopes, respectively. We also give results for eight objects that did not have a published combined orbital solution, one of which did not have a visual orbit either. We could determine mass ratios with a typical uncertainty of less than 1%, mass sums wit...
We present trigonometric parallaxes of 64 stellar systems with proper motions between 0. ′′ 5 yr ... more We present trigonometric parallaxes of 64 stellar systems with proper motions between 0. ′′ 5 yr −1 and 1. ′′ 0 yr −1 from the ongoing RECONS (Research Consortium On Nearby Stars) parallax program at CTIO (the Cerro Tololo Interamerican Observatory). All of the systems are south of DEC = +30, and 58 had no previous trigonometric parallaxes. In addition to parallaxes for the systems, we present proper motions, Johnson-Kron-Cousins V RI photometry, variability measurements, and spectral types. Nine of the systems are multiple; we present results for their components, three of which are new astrometric detections. Of the 64 systems, 56 are within 25 parsecs of the Sun and 52 of those are in the southern hemisphere, comprising 5.7% of the total number of known southern 25 parsec systems.
Communications of the Byurakan Astrophysical Observatory, 2021
During several surveys of high-quality film-copies of the ESO B, R, ESO/SRC J, and EJ plates, exc... more During several surveys of high-quality film-copies of the ESO B, R, ESO/SRC J, and EJ plates, except other young unstable objects (HH objects, star-forming regions, cometary nebulae) were found also two types of other unstable objects. 1. Bright condensations, connected with stars by bright filaments. 2. Pairs of stars, connected with each other by bright filaments. The objects of first type are mostly seen in infrared, on 2MASS K images. Due to their infrared colours, two of condensations and half of all stars can have thick dust discs.
We present results from Speckle inteferometric observations of 15 visual binaries and one double-... more We present results from Speckle inteferometric observations of 15 visual binaries and one double-line spectroscopic binary, carried out with the HRCam Speckle camera of the SOAR 4.1 m telescope. These systems were observed as a part of an on-going survey to characterize the binary population in the solar vicinity, out to a distance of 250 pc. We obtained orbital elements and mass sums for our sample of visual binaries. The orbits were computed using a Markov Chain Monte Carlo algorithm that delivers maximum likelihood estimates of the parameters, as well as posterior probability density functions that allow us to evaluate their uncertainty. Their periods cover a range from 5 yr to more than 500 yr; and their spectral types go from early A to mid M, implying total system masses from slightly more than 4M⊙ down to 0.2M ⊙. They are located at distances between approximately 12 and 200 pc, mostly at low Galactic latitude. For the double-line spectroscopic binary YSC8, we present the fir...
Presentamos los resultados de una inspección sistemática de las placas ESO/SRC del hemisferio sur... more Presentamos los resultados de una inspección sistemática de las placas ESO/SRC del hemisferio sur realizada con el propósito de descubrir nuevos sistemas radiales de glóbulos oscuros. Durante esta inspección encontramos 16 sistemas radiales de tipo 1 y 6 sistemas de tipo 2. Con esto, se duplica el número de sistemas radiales conocidos. En la zona central de los sistemas de tipo 1 se encuentran estrellas de tipo espectral O−B2, mientras que los sistemas de tipo 2 no exhiben estrellas de tipo temprano en sus centros. Proponemos una interpretación tentativa de los grupos de cuerpos condensados submilimétricos que no poseen una contraparte estelar como sistemas radiales de glóbulos oscuros situados detrás de nubes oscuras gruesas, lo que explicaría por qué estos glóbulos se observan solamente en longitudes de onda submilimétricas.
Proceedings of the International Astronomical Union, 2007
The purpose of this work is to do a systematic, deep search for sub-stellar objects orbiting near... more The purpose of this work is to do a systematic, deep search for sub-stellar objects orbiting nearby white dwarfs (WDs). The scientific interest spans testing specific predictions of models of common envelope phase, as well as providing constraints to planetary system evolution in advanced stages of its parent star. Additionally, we seek to explore the hypothesis about the origin of metal lines in hydrogen WDs, produced by the accretion of tidal disturbed asteroidal or cometary material. This could be linked to the presence of a planetary object. Here, we show preliminary results of our near-infrared astrometric project.
In this paper we use the Cramér-Rao lower uncertainty bound to estimate the maximum precision tha... more In this paper we use the Cramér-Rao lower uncertainty bound to estimate the maximum precision that could be achieved on the joint simultaneous (or 2D) estimation of photometry and astrometry of a point source measured by a linear CCD detector array.
Proceedings of the International Astronomical Union, 2006
We report the results of our project devoted to study the chemical enrichment history of the fiel... more We report the results of our project devoted to study the chemical enrichment history of the field population in the Magellanic Clouds using Ca II triplet spectroscopy.
Proceedings of the International Astronomical Union, 2008
We report the results of our project devoted to study the chemical enrichment history of the fiel... more We report the results of our project devoted to study the chemical enrichment history of the field population in the Magellanic Clouds using Ca ii triplet spectroscopy.
Proceedings of the International Astronomical Union, 2006
We present deep ground-based B and R observations of 12 fields in the Small Magellanic Cloud (SMC... more We present deep ground-based B and R observations of 12 fields in the Small Magellanic Cloud (SMC). The resulting color-magnitude diagrams (CMDs) reach the oldest mainsequence (MS) turnoff at MR∼3.5 and reveal the stellar population differences between the part of the galaxy facing the Large Magellanic Cloud (LMC) and an area on the opposite side. In the Southern part of the galaxy, we found that there are still intermediate-age stars as far as 4 kpc from the SMC center. The Chemical Enrichment History (CEH) in one of our SMC fields is also presented.
Trigonometric parallax observations made with the Hubble Space Telescope (HST) Fine Guidance Sens... more Trigonometric parallax observations made with the Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) 3 of seven Hyades members in six fields of view have been analyzed along with their proper motions to determine the distance to the cluster. Knowledge of the convergent point and mean proper motion of the Hyades is critical to the derivation of the distance to the center of the cluster. Depending on the choice of the proper-motion system, the derived cluster center distance varies by 9%. Adopting a reference distance of 46.1 pc or m Ϫ M ϭ 3.32, which is derived from the ground-based parallaxes in the General Catalogue of Trigonometric Stellar Parallaxes (1995 edition), the FK5/PPM proper-motion system yields a distance 4% larger, while the Hanson system yields a distance 2% smaller. The HST FGS parallaxes reported here yield either a 14% or 5% larger distance, depending on the choice of the proper-motion system. Orbital parallaxes (Torres et al.) yield an average distance 4% larger than the reference distance. The variation in the distance derived from the HST data illustrates the importance of the proper-motion system and the individual proper motions to the derivation of the distance to the Hyades center; therefore, a full utilization of the HST FGS parallaxes awaits the establishment of an accurate and consistent proper-motion system.
We characterize the kinematic and chemical properties of 589 Galactic anticenter substructure sta... more We characterize the kinematic and chemical properties of 589 Galactic anticenter substructure stars (GASS) with K/M giants in integrals-of-motion space. These stars likely include members of previously identified substructures such as Monoceros, A13, and the Triangulum-Andromeda cloud. We show that these stars are in nearly circular orbits on both sides of the Galactic plane. We can see a velocity (V Z ) gradient along Y-axis especially for the south GASS members. Our GASS members have similar energy and angular momentum distributions to thin-disk stars. Their location in [α/M] versus [M/H] space is more metal-poor than typical thin-disk stars, with [α/M] lower than that of the thick disk. We infer that our GASS members are part of the outer metal-poor disk stars and that the outer disk extends to 30 kpc. Considering the distance range and α-abundance features, GASS could be formed after the thick disk was formed due to the molecular cloud density decreasing in the outer disk where ...
We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from... more We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from the plane,by means of the kinematics of ~400 thick disk stars. The results match the expectations for the visible mass only, and no dark matter is detected in the volume under analysis. The current models of dark matter halo are excluded with a significance higher than 5sigma, unless a highly prolate halo is assumed, very atypical in cold dark matter simulations. The resulting lack of dark matter at the solar position challenges the current models.
In this work, we present new catalogs of M giant and M dwarf stars from the LAMOST DR5. In total,... more In this work, we present new catalogs of M giant and M dwarf stars from the LAMOST DR5. In total, 39,796 M giants and 501,152 M dwarfs are identified from the classification pipeline. The template-fitting results contain M giants with 7 temperature subtypes from M0 to M6, M dwarfs with 18 temperature subtypes from K7.0 to M8.5 and 12 metallicity subclasses from dMr to usdMp. We cross-matched our M-type catalog with the 2MASS and W ISE catalog to obtain infrared magnitude and colors. Adopting the distances derived from the parallaxes in Gaia DR2, the M G vs. (G bp − G rp) 0 diagram shows that there are also early-type stars and white dwarf-M dwarf binaries included in our M type stars sample, with a contamination rate of about 4.6% for M giants and 0.48% for M dwarfs. We found that CaH spectral indices are an efficient selection criteria for carbon stars. A total of 289 carbon stars were identified from the M giants sample, and further confirmed by LAMOST spectra.
We explore the origins of the young B-type stars found by Casetti-Dinescu et al. (2014) at the ou... more We explore the origins of the young B-type stars found by Casetti-Dinescu et al. (2014) at the outskirts of the Milky-Way disk in the sky region of Leading Arm of the Magellanic Stream. High-resolution spectroscopic observations made with the MIKE instrument on the Magellan Clay 6.5m telescope for nine stars are added to the previous sample analyzed by Zhang et al. (2017). We compile a sample of fifteen young stars with well-determined stellar types, ages, abundances and kinematics. With proper motions from Gaia DR2 we also derive orbits in a realistic Milky-Way potential.We find that our previous radial-velocity selected LA candidates have substantial orbital angular momentum. The substantial amount of rotational component for these stars is in contrast with the near-polar Magellanic orbit, thus rendering these stars unlikely members of the LA. There are four large orbital-energy stars in our sample. The highest orbital-energy one has an age shorter than the time to disk crossing, with a birthplace z = 2.5 kpc and R GC ∼ 28 kpc. Therefore, the origin of this star is uncertain. The remaining three stars have disk runaway origin with birthplaces between 12 and 25 kpc from the Galactic center. Also, the most energetic stars are more metal poor
En este proyecto de tesis se propone un estudio de las subestructuras conocidas del halo Galáctic... more En este proyecto de tesis se propone un estudio de las subestructuras conocidas del halo Galáctico, así como el hallazgo de nuevas subestructuras a partir de datos procedentes de la colaboración QUEST. Se cree que tales subestructuras son remanentes de cúmulos globulares o galaxias enanas acrecionadas, similares a la galaxia esferoidal enana de Sagitario. Las estrellas RR Lyrae serán empleadas para trazar el halo debido a que son fácilmente detectables ya que constituyen excelentes luminarias estándar, cuyas distancias están mejor determinadas que trazadores alternativos, e.g. gigantes M. Parte del proyecto comprende la observación espectroscópica de estrellas RR Lyrae en la corriente generada por la galaxia enana de Sagitario a fin de determinar sus dimensiones espaciales, estructura, velocidad radial y velocidad de dispersión. Tales propiedades permiten restringir modelos de la corriente que contempla diferentes geometrías del halo de materia oscura.
We present preliminary results from the deepest and largest photographic proper-motion survey eve... more We present preliminary results from the deepest and largest photographic proper-motion survey ever undertaken of the Galactic bulge. Our first-epoch plate material (from 1972-3) goes deep enough (V_lim = 22) to reach below the bulge main-sequence turnoff. These plates cover an area of approximately 25 arc-min X 25 arc-min of the bulge in the low-extinction (A_v = 0.8 mag) Plaut
Publications of the Astronomical Society of the Pacific, 2021
Under certain rather prevalent conditions (driven by dynamical orbital evolution), a hierarchical... more Under certain rather prevalent conditions (driven by dynamical orbital evolution), a hierarchical triple stellar system can be well approximated, from the standpoint of orbital parameter estimation, as two binary star systems combined. Even under this simplifying approximation, the inference of orbital elements is a challenging technical problem because of the high dimensionality of the parameter space, and the complex relationships between those parameters and the observations (astrometry and radial velocity). In this work we propose a new methodology for the study of triple hierarchical systems using a Bayesian Markov-Chain Monte Carlo-based framework. In particular, graphical models are introduced to describe the probabilistic relationship between parameters and observations in a dynamically self-consistent way. As information sources we consider the cases of isolated astrometry, isolated radial velocity, as well as the joint case with both types of measurements. Graphical models provide a novel way of
We present orbital elements, orbital parallaxes, and individual component masses for 14 spatially... more We present orbital elements, orbital parallaxes, and individual component masses for 14 spatially resolved double-line spectroscopic binaries derived doing a simultaneous fit of their visual orbit and radial velocity curve. This was done by means of a Markov Chain Monte Carlo code developed by our group that produces posterior distribution functions and error estimates for all of the parameters. Of this sample, six systems had high-quality previous studies and were included as benchmarks to test our procedures, but even in these cases, we could improve the previous orbits by adding recent data from our survey of southern binaries being carried out with the HRCam and ZORRO speckle cameras at the SOAR 4.1 m and Gemini South 8.1 m telescopes, respectively. We also give results for eight objects that did not have a published combined orbital solution, one of which did not have a visual orbit either. We could determine mass ratios with a typical uncertainty of less than 1%, mass sums wit...
We present trigonometric parallaxes of 64 stellar systems with proper motions between 0. ′′ 5 yr ... more We present trigonometric parallaxes of 64 stellar systems with proper motions between 0. ′′ 5 yr −1 and 1. ′′ 0 yr −1 from the ongoing RECONS (Research Consortium On Nearby Stars) parallax program at CTIO (the Cerro Tololo Interamerican Observatory). All of the systems are south of DEC = +30, and 58 had no previous trigonometric parallaxes. In addition to parallaxes for the systems, we present proper motions, Johnson-Kron-Cousins V RI photometry, variability measurements, and spectral types. Nine of the systems are multiple; we present results for their components, three of which are new astrometric detections. Of the 64 systems, 56 are within 25 parsecs of the Sun and 52 of those are in the southern hemisphere, comprising 5.7% of the total number of known southern 25 parsec systems.
Communications of the Byurakan Astrophysical Observatory, 2021
During several surveys of high-quality film-copies of the ESO B, R, ESO/SRC J, and EJ plates, exc... more During several surveys of high-quality film-copies of the ESO B, R, ESO/SRC J, and EJ plates, except other young unstable objects (HH objects, star-forming regions, cometary nebulae) were found also two types of other unstable objects. 1. Bright condensations, connected with stars by bright filaments. 2. Pairs of stars, connected with each other by bright filaments. The objects of first type are mostly seen in infrared, on 2MASS K images. Due to their infrared colours, two of condensations and half of all stars can have thick dust discs.
We present results from Speckle inteferometric observations of 15 visual binaries and one double-... more We present results from Speckle inteferometric observations of 15 visual binaries and one double-line spectroscopic binary, carried out with the HRCam Speckle camera of the SOAR 4.1 m telescope. These systems were observed as a part of an on-going survey to characterize the binary population in the solar vicinity, out to a distance of 250 pc. We obtained orbital elements and mass sums for our sample of visual binaries. The orbits were computed using a Markov Chain Monte Carlo algorithm that delivers maximum likelihood estimates of the parameters, as well as posterior probability density functions that allow us to evaluate their uncertainty. Their periods cover a range from 5 yr to more than 500 yr; and their spectral types go from early A to mid M, implying total system masses from slightly more than 4M⊙ down to 0.2M ⊙. They are located at distances between approximately 12 and 200 pc, mostly at low Galactic latitude. For the double-line spectroscopic binary YSC8, we present the fir...
Presentamos los resultados de una inspección sistemática de las placas ESO/SRC del hemisferio sur... more Presentamos los resultados de una inspección sistemática de las placas ESO/SRC del hemisferio sur realizada con el propósito de descubrir nuevos sistemas radiales de glóbulos oscuros. Durante esta inspección encontramos 16 sistemas radiales de tipo 1 y 6 sistemas de tipo 2. Con esto, se duplica el número de sistemas radiales conocidos. En la zona central de los sistemas de tipo 1 se encuentran estrellas de tipo espectral O−B2, mientras que los sistemas de tipo 2 no exhiben estrellas de tipo temprano en sus centros. Proponemos una interpretación tentativa de los grupos de cuerpos condensados submilimétricos que no poseen una contraparte estelar como sistemas radiales de glóbulos oscuros situados detrás de nubes oscuras gruesas, lo que explicaría por qué estos glóbulos se observan solamente en longitudes de onda submilimétricas.
Proceedings of the International Astronomical Union, 2007
The purpose of this work is to do a systematic, deep search for sub-stellar objects orbiting near... more The purpose of this work is to do a systematic, deep search for sub-stellar objects orbiting nearby white dwarfs (WDs). The scientific interest spans testing specific predictions of models of common envelope phase, as well as providing constraints to planetary system evolution in advanced stages of its parent star. Additionally, we seek to explore the hypothesis about the origin of metal lines in hydrogen WDs, produced by the accretion of tidal disturbed asteroidal or cometary material. This could be linked to the presence of a planetary object. Here, we show preliminary results of our near-infrared astrometric project.
In this paper we use the Cramér-Rao lower uncertainty bound to estimate the maximum precision tha... more In this paper we use the Cramér-Rao lower uncertainty bound to estimate the maximum precision that could be achieved on the joint simultaneous (or 2D) estimation of photometry and astrometry of a point source measured by a linear CCD detector array.
Proceedings of the International Astronomical Union, 2006
We report the results of our project devoted to study the chemical enrichment history of the fiel... more We report the results of our project devoted to study the chemical enrichment history of the field population in the Magellanic Clouds using Ca II triplet spectroscopy.
Proceedings of the International Astronomical Union, 2008
We report the results of our project devoted to study the chemical enrichment history of the fiel... more We report the results of our project devoted to study the chemical enrichment history of the field population in the Magellanic Clouds using Ca ii triplet spectroscopy.
Proceedings of the International Astronomical Union, 2006
We present deep ground-based B and R observations of 12 fields in the Small Magellanic Cloud (SMC... more We present deep ground-based B and R observations of 12 fields in the Small Magellanic Cloud (SMC). The resulting color-magnitude diagrams (CMDs) reach the oldest mainsequence (MS) turnoff at MR∼3.5 and reveal the stellar population differences between the part of the galaxy facing the Large Magellanic Cloud (LMC) and an area on the opposite side. In the Southern part of the galaxy, we found that there are still intermediate-age stars as far as 4 kpc from the SMC center. The Chemical Enrichment History (CEH) in one of our SMC fields is also presented.
Trigonometric parallax observations made with the Hubble Space Telescope (HST) Fine Guidance Sens... more Trigonometric parallax observations made with the Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) 3 of seven Hyades members in six fields of view have been analyzed along with their proper motions to determine the distance to the cluster. Knowledge of the convergent point and mean proper motion of the Hyades is critical to the derivation of the distance to the center of the cluster. Depending on the choice of the proper-motion system, the derived cluster center distance varies by 9%. Adopting a reference distance of 46.1 pc or m Ϫ M ϭ 3.32, which is derived from the ground-based parallaxes in the General Catalogue of Trigonometric Stellar Parallaxes (1995 edition), the FK5/PPM proper-motion system yields a distance 4% larger, while the Hanson system yields a distance 2% smaller. The HST FGS parallaxes reported here yield either a 14% or 5% larger distance, depending on the choice of the proper-motion system. Orbital parallaxes (Torres et al.) yield an average distance 4% larger than the reference distance. The variation in the distance derived from the HST data illustrates the importance of the proper-motion system and the individual proper motions to the derivation of the distance to the Hyades center; therefore, a full utilization of the HST FGS parallaxes awaits the establishment of an accurate and consistent proper-motion system.
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Papers by Rene Mendez