A new optical reddening model for the Solar neighborhood is presented. The model makes use of the... more A new optical reddening model for the Solar neighborhood is presented. The model makes use of the large-scale properties of the dust layer in the Galaxy, and of the observed clumpiness in its distribution, and it is used to compute starcounts in the plane of the Galaxy, which are compared to the observed counts from the Guide Star Catalogue (GSC) for a few representative regions having a range of reddening values. These comparisons provide a good test case in a wide variety of conditions with regards to both the amount and distribution of the reddening material along the line-of-sight. This is the first systematic study of low-Galactic latitude starcounts from the perspective of a Galactic structure and reddening model, and it is demonstrated that the model can be used to obtain meaningful starcount estimates in the plane of the Milky-Way. We find that it is possible to predict starcounts in the range 8 ≤ V < 13.7 with a mean accuracy of 15% or better for total samples of several thousand stars at |b| < 10 • , and for total reddening at 1 kpc of up to E(B − V) ∼ 0.7 mag. Self-consistency checks indicate that a differential optical absorption of 0.5 mag/kpc is adequate to reproduce, with our model, the available reddening maps in the distance range 2 ≤ r ≤ 6 kpc. Our best-fit model to three (out of six) selected regions yields a distance of the Sun from the symmetry plane of the Galaxy of Z ⊙ = 27 ± 3 (3σ) pc, while the disk's scale-length is found to be H Disk = 6 ± 2 (3σ) kpc. While the derived value for Z ⊙ is quite sensitive to the adopted scale-height of disk stars, no big dependency of H Disk on this parameter is found. These values should be taken, of course, with caution because of the inherent uncertainties in modeling optical starcounts in the presence of patchy reddening material. The accuracy of the reddening and starcounts model opens the possibility of using it to estimate and compare observations of diffuse starlight to constrain the major structural parameters of the Galaxy, and to compute the distance distribution of stars self-consistently with the starcounts, in order to derive corrections to absolute parallax.
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.
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...
The results of speckle interferometric observations at the SOAR telescope in 2015 are given, tota... more The results of speckle interferometric observations at the SOAR telescope in 2015 are given, totaling 1303 measurements of 924 resolved binary and multiple stars and non-resolutions of 260 targets. The separations range from 12 mas to 3. ′′ 37 (median 0. ′′ 17); the maximum measured magnitude difference is 6.7 mag. We resolved 27 pairs for the first time, including 10 as inner or outer subsystems in previously known binaries, e.g. the 50-mas pair in ǫ Cha. Newly resolved pairs are commented upon. We discuss three apparently non-hierarchical systems discovered in this series, arguing that their unusual configuration results from projection. The resolved quadruple system HIP 71510 is studied as well.
Revista Mexicana De Astronomia Y Astrofisica, 2006
Presentamos las principales motivaciones, objetivos, métodos y primeros resultados de un programa... more Presentamos las principales motivaciones, objetivos, métodos y primeros resultados de un programa astrométrico que tiene por finalidad la determinación de paralajes trigonométricas de estrellas cercanas (D < 25 pc) del Hemisferio Sur. Las observaciones se están llevando a cabo en el Observatorio Inter-Americano de Cerro Tololo (CTIO), Chile.
The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research ... more The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research Telescope (SOAR) in 2018 are given, totaling 3097 measurements of 2427 resolved pairs with separations from 11 mas to 5. ′′ 9 (median 0. ′′ 15, magnitude difference up to 7 mag) and non-resolutions of 624 targets. This work continues our long-term speckle program. Its main goal is to monitor orbital motion of close binaries, including members of high-order hierarchies and Hipparcos pairs in the solar neighborhood. Also, pre-main-sequence stars in the Orion OB1 association were surveyed, resolving 26 out of 118 targets. In addition, we report discovery of 35 new companions among field visual multiples (some of which are likely optical) and first-time resolutions of another 31 pairs. By combining the measurements given here with the published ones, we computed 76 orbits for the first time and updated orbital elements of 34 visual binaries. Their periods range from 0.65 to 1100 years, and their quality varies from first tentative solutions of grade 5 to accurate elements of grades 1 and 2. Finally, a list of 53 spurious pairs discovered by various techniques and unresolved at SOAR is given.
The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research ... more The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research Telescope (SOAR) in 2018 are given, totaling 3097 measurements of 2427 resolved pairs with separations from 11 mas to 5. ′′ 9 (median 0. ′′ 15, magnitude difference up to 7 mag) and non-resolutions of 624 targets. This work continues our long-term speckle program. Its main goal is to monitor orbital motion of close binaries, including members of high-order hierarchies and Hipparcos pairs in the solar neighborhood. Also, pre-main-sequence stars in the Orion OB1 association were surveyed, resolving 26 out of 118 targets. In addition, we report discovery of 35 new companions among field visual multiples (some of which are likely optical) and first-time resolutions of another 31 pairs. By combining the measurements given here with the published ones, we computed 76 orbits for the first time and updated orbital elements of 34 visual binaries. Their periods range from 0.65 to 1100 years, and their quality varies from first tentative solutions of grade 5 to accurate elements of grades 1 and 2. Finally, a list of 53 spurious pairs discovered by various techniques and unresolved at SOAR is given.
We present the results of radial velocity measurements of 770 thick disk red giants toward the So... more We present the results of radial velocity measurements of 770 thick disk red giants toward the South Galactic Pole, vertically distributed from 0.5 kpc to 5 kpc with respect to the Galactic plane. We find a small gradient in the vertical velocity dispersion (sigma_{W}) of 3.8+/-0.8 km s^{-1} kpc^{-1}. Even more noteworthy, our values of sigma_{W} are small compared to
We have determined the proper motion of the Large Magellanic Cloud (LMC) relative to a background... more We have determined the proper motion of the Large Magellanic Cloud (LMC) relative to a background quasi-stellar object, using observations carried out in seven epochs (six years of base time). Our proper motion value agrees well with most results obtained by other authors and indicates that the LMC is not a member of a proposed stream of galaxies with similar
Using data derived from the deepest and finest angular resolution images of the universe yet acqu... more Using data derived from the deepest and finest angular resolution images of the universe yet acquired by astronomers at optical wavelengths using the Hubble Space Telescope (HST) in two postage-stamp sections of the sky (Williams et al. 1996a,b), plus simple geometrical and scaling arguments, we demonstrate that the faint blue population of point-source objects detected on those two fields (Méndez et al. 1996) could actually be ancient halo white dwarfs at distances closer than about 2 kpc from the Sun. This finding has profound implications, as the mass density of the detected objects would account for about half of the missing dark matter in the Milky-Way (Bahcall and Soneira 1980), thus solving one of the most controversial issues of modern astrophysics (Trimble 1987, Ashman 1992). The existence of these faint blue objects points to a very large mass locked into ancient halo white dwarfs. Our estimate indicates that they could account for as much as half of the dark matter in our Galaxy, confirming the suggestions of the MACHO microlensing experiment (Alcock et al. 1997). Because of the importance of this discovery, deep followup observations with HST within the next two years would be needed to determine more accurately the kinematics (tangential motions) for these faint blue old white dwarfs.
Eighteen RR Lyrae variables (RRLs) that lie in the "12 h .4 clump" identified by the Quasar Equat... more Eighteen RR Lyrae variables (RRLs) that lie in the "12 h .4 clump" identified by the Quasar Equatorial Survey Team (QUEST) have been observed spectroscopically to measure their radial velocities and metal abundances. Ten blue horizontal branch (BHB) stars identified by the Sloan Digital Sky Survey (SDSS) were added to this sample. Six of the nine stars in the densest region of the clump have a mean radial velocity in the Galactic rest frame () of 99.8 and km s Ϫ1 , which is slightly smaller than the average error of the measurements. V j p 17.3 gsr The whole sample contains eight RRLs and five BHB stars that have values of suggesting membership in V gsr this stream. For seven of these RRLs, the measurements of [Fe/H], which have an internal precision of 0.08 dex, yield and. These values suggest that the stream is a tidally disrupted dwarf spheroidal A[Fe/H]S p Ϫ1.86 j p 0.40 galaxy of low luminosity. Photometry from the database of the SDSS indicates that this stream covers at least 106 deg 2 of the sky in the constellation Virgo. The name Virgo stellar stream is suggested.
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.
Astrometric measurements for 25 red dwarf systems are presented, including the first definitive t... more Astrometric measurements for 25 red dwarf systems are presented, including the first definitive trigonometric parallaxes for 20 systems within 10 pc of the Sun, the horizon of the RECONS sample. The three nearest systems that had no previous trigonometric parallaxes (other than perhaps rough preliminary efforts) are SO 0253+1652 (3.84 ± 0.04 pc, the 23rd nearest system), SCR 1845-6357 AB (3.85 ± 0.02 pc, 24th), and LHS 1723 (5.32 ± 0.04 pc, 56th). In total, seven of the systems reported here rank among the nearest 100 stellar systems. Supporting photometric and spectroscopic observations have been made to provide
White dwarf cooling sequences provides a useful indicator of the evolutionary time scales involve... more White dwarf cooling sequences provides a useful indicator of the evolutionary time scales involved in the chronometry and star formation history of the galactic disk and, for this reason, the luminosity function of white dwarfs has become a powerful tool for studies of the evolution and formation of the Milky Way. We examine the faint-end ($M_V > +14$) behavior of the disk white dwarf luminosity function using the $1/V_{\rm max}$ method, but, for the first time, fully including the effects of realistic observational errors in the derived luminosity function. We find that observational errors, mostly in the bolometric corrections and trigonometric parallaxes, play a major role in obliterating (real or artificial) small scale fluctuations in the luminosity function. A better estimator of the true luminosity function seems to be the median over simulations, rather than the mean. When using the latter, an age of 10 Gyr or older can not be ruled out from the sample of Leggett, Ruiz, a...
We have begun an investigation at Plaut`s low extinction window, located at (l,b)=(0,-8), ie, at ... more We have begun an investigation at Plaut`s low extinction window, located at (l,b)=(0,-8), ie, at the interface between the bulge and the halo. We use 43 scanned photographic plates which cover around 250 250. These plates were taken through five different telescopes over 21 ...
... William F. van Altena and Terrence Girard Yale University Observatory New Haven, CT 06511 USA... more ... William F. van Altena and Terrence Girard Yale University Observatory New Haven, CT 06511 USA Carlos E. L6pez Observatorio Astronomico "Felix Aguilar" Universidad Nacional de SanJuan and Yale Southern Observatory San Juan, Argentina 1. INTRODUCTION ...
To explore the complex halo substructure that has been reported in the direction of the Virgo con... more To explore the complex halo substructure that has been reported in the direction of the Virgo constellation, radial velocities and metallicities have been measured for 82 RR Lyrae stars (RRLS) that were identified by the QUEST survey. These stars are distributed over 90 square degrees of the sky, and lie from 4 to 23 kpc from the Sun. Using an algorithm for finding groups in phase space and modeling the smooth halo component in the region, we identified the 5 most significant RRLS groups, some of which were previously known or suspected. We have examined the SEKBO and the Catalina catalog of RRLS (with available spectroscopic measurements), as well as the bright QUEST RRLS sample, the catalog of Red Giant stars from the Spaghetti survey, and three recent catalogs of blue horizontal branch (BHB) stars, for stars that may be related to the QUEST RRLS groups. The most significant group of RRLS is the Virgo stellar stream (VSS) identified here as group A, which is composed of at least 10 RRLS and 3 BHB stars. It has a mean distance of 19.6 kpc and a mean radial velocity V gsr = 128 km s −1 , as estimated from its RRLS members. With the revised velocities reported here, there is no longer an offset in velocity between the RRLS in the VSS and the prominent peak in the velocities of main-sequence turnoff stars reported by other researchers in the same direction and at a similar distance (known as S297+63-20.5). The location in phase space of two other groups (F and H) suggests a possible connection with the VSS, which cannot be discarded at this point, although the turnoff colors of the VSS and group H, as identified from other works, suggest they might be composed of different populations. Two more groups, B and D, are found at mean distances of 19.0 and 5.7 kpc, and mean radial velocities of V gsr = −94 and 32 km s −1. The latter is the more numerous in terms of total members, as well as the more extended in RA. A comparison with the latest model of the disruption of the Sagittarius dwarf, indicates that none of the above groups is related to it. Rather than being the result of a single accretion event, the excess of stars observed in Virgo appears to be composed of several halo substructures along the same line of sight.
Three nearby star candidates were selected as bright 2MASS point sources without associated optic... more Three nearby star candidates were selected as bright 2MASS point sources without associated optical sources, i.e. with potentially large proper motions, subsequently confirmed by multi-epoch optical data from the SuperCOSMOS Sky Surveys. All three objects are listed in the NLTT catalogue of high proper motion stars. Follow-up spectroscopic observations allowed us to classify L 449-1 as M 4.0e dwarf at 5.7 pc, L 43-72 as M 4.5e dwarf at 7.2 pc, and LP 949-15 as M 5.0e dwarf at 6.1 pc, using known absolute JHK s magnitudes of dwarfs with corresponding spectral types, respectively. All three stars exhibit Hα emission lines, and all three can be identified with bright X-ray sources. The available ROSAT light curves of two of the objects show typical flare-like variability. Thus, all three stars are active, very close potential neighbours of the Sun, previously not mentioned as such, certainly deserving further attention. In particular, these are very promising candidates for trigonometric parallax programs.
A new optical reddening model for the Solar neighborhood is presented. The model makes use of the... more A new optical reddening model for the Solar neighborhood is presented. The model makes use of the large-scale properties of the dust layer in the Galaxy, and of the observed clumpiness in its distribution, and it is used to compute starcounts in the plane of the Galaxy, which are compared to the observed counts from the Guide Star Catalogue (GSC) for a few representative regions having a range of reddening values. These comparisons provide a good test case in a wide variety of conditions with regards to both the amount and distribution of the reddening material along the line-of-sight. This is the first systematic study of low-Galactic latitude starcounts from the perspective of a Galactic structure and reddening model, and it is demonstrated that the model can be used to obtain meaningful starcount estimates in the plane of the Milky-Way. We find that it is possible to predict starcounts in the range 8 ≤ V < 13.7 with a mean accuracy of 15% or better for total samples of several thousand stars at |b| < 10 • , and for total reddening at 1 kpc of up to E(B − V) ∼ 0.7 mag. Self-consistency checks indicate that a differential optical absorption of 0.5 mag/kpc is adequate to reproduce, with our model, the available reddening maps in the distance range 2 ≤ r ≤ 6 kpc. Our best-fit model to three (out of six) selected regions yields a distance of the Sun from the symmetry plane of the Galaxy of Z ⊙ = 27 ± 3 (3σ) pc, while the disk's scale-length is found to be H Disk = 6 ± 2 (3σ) kpc. While the derived value for Z ⊙ is quite sensitive to the adopted scale-height of disk stars, no big dependency of H Disk on this parameter is found. These values should be taken, of course, with caution because of the inherent uncertainties in modeling optical starcounts in the presence of patchy reddening material. The accuracy of the reddening and starcounts model opens the possibility of using it to estimate and compare observations of diffuse starlight to constrain the major structural parameters of the Galaxy, and to compute the distance distribution of stars self-consistently with the starcounts, in order to derive corrections to absolute parallax.
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.
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...
The results of speckle interferometric observations at the SOAR telescope in 2015 are given, tota... more The results of speckle interferometric observations at the SOAR telescope in 2015 are given, totaling 1303 measurements of 924 resolved binary and multiple stars and non-resolutions of 260 targets. The separations range from 12 mas to 3. ′′ 37 (median 0. ′′ 17); the maximum measured magnitude difference is 6.7 mag. We resolved 27 pairs for the first time, including 10 as inner or outer subsystems in previously known binaries, e.g. the 50-mas pair in ǫ Cha. Newly resolved pairs are commented upon. We discuss three apparently non-hierarchical systems discovered in this series, arguing that their unusual configuration results from projection. The resolved quadruple system HIP 71510 is studied as well.
Revista Mexicana De Astronomia Y Astrofisica, 2006
Presentamos las principales motivaciones, objetivos, métodos y primeros resultados de un programa... more Presentamos las principales motivaciones, objetivos, métodos y primeros resultados de un programa astrométrico que tiene por finalidad la determinación de paralajes trigonométricas de estrellas cercanas (D < 25 pc) del Hemisferio Sur. Las observaciones se están llevando a cabo en el Observatorio Inter-Americano de Cerro Tololo (CTIO), Chile.
The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research ... more The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research Telescope (SOAR) in 2018 are given, totaling 3097 measurements of 2427 resolved pairs with separations from 11 mas to 5. ′′ 9 (median 0. ′′ 15, magnitude difference up to 7 mag) and non-resolutions of 624 targets. This work continues our long-term speckle program. Its main goal is to monitor orbital motion of close binaries, including members of high-order hierarchies and Hipparcos pairs in the solar neighborhood. Also, pre-main-sequence stars in the Orion OB1 association were surveyed, resolving 26 out of 118 targets. In addition, we report discovery of 35 new companions among field visual multiples (some of which are likely optical) and first-time resolutions of another 31 pairs. By combining the measurements given here with the published ones, we computed 76 orbits for the first time and updated orbital elements of 34 visual binaries. Their periods range from 0.65 to 1100 years, and their quality varies from first tentative solutions of grade 5 to accurate elements of grades 1 and 2. Finally, a list of 53 spurious pairs discovered by various techniques and unresolved at SOAR is given.
The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research ... more The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research Telescope (SOAR) in 2018 are given, totaling 3097 measurements of 2427 resolved pairs with separations from 11 mas to 5. ′′ 9 (median 0. ′′ 15, magnitude difference up to 7 mag) and non-resolutions of 624 targets. This work continues our long-term speckle program. Its main goal is to monitor orbital motion of close binaries, including members of high-order hierarchies and Hipparcos pairs in the solar neighborhood. Also, pre-main-sequence stars in the Orion OB1 association were surveyed, resolving 26 out of 118 targets. In addition, we report discovery of 35 new companions among field visual multiples (some of which are likely optical) and first-time resolutions of another 31 pairs. By combining the measurements given here with the published ones, we computed 76 orbits for the first time and updated orbital elements of 34 visual binaries. Their periods range from 0.65 to 1100 years, and their quality varies from first tentative solutions of grade 5 to accurate elements of grades 1 and 2. Finally, a list of 53 spurious pairs discovered by various techniques and unresolved at SOAR is given.
We present the results of radial velocity measurements of 770 thick disk red giants toward the So... more We present the results of radial velocity measurements of 770 thick disk red giants toward the South Galactic Pole, vertically distributed from 0.5 kpc to 5 kpc with respect to the Galactic plane. We find a small gradient in the vertical velocity dispersion (sigma_{W}) of 3.8+/-0.8 km s^{-1} kpc^{-1}. Even more noteworthy, our values of sigma_{W} are small compared to
We have determined the proper motion of the Large Magellanic Cloud (LMC) relative to a background... more We have determined the proper motion of the Large Magellanic Cloud (LMC) relative to a background quasi-stellar object, using observations carried out in seven epochs (six years of base time). Our proper motion value agrees well with most results obtained by other authors and indicates that the LMC is not a member of a proposed stream of galaxies with similar
Using data derived from the deepest and finest angular resolution images of the universe yet acqu... more Using data derived from the deepest and finest angular resolution images of the universe yet acquired by astronomers at optical wavelengths using the Hubble Space Telescope (HST) in two postage-stamp sections of the sky (Williams et al. 1996a,b), plus simple geometrical and scaling arguments, we demonstrate that the faint blue population of point-source objects detected on those two fields (Méndez et al. 1996) could actually be ancient halo white dwarfs at distances closer than about 2 kpc from the Sun. This finding has profound implications, as the mass density of the detected objects would account for about half of the missing dark matter in the Milky-Way (Bahcall and Soneira 1980), thus solving one of the most controversial issues of modern astrophysics (Trimble 1987, Ashman 1992). The existence of these faint blue objects points to a very large mass locked into ancient halo white dwarfs. Our estimate indicates that they could account for as much as half of the dark matter in our Galaxy, confirming the suggestions of the MACHO microlensing experiment (Alcock et al. 1997). Because of the importance of this discovery, deep followup observations with HST within the next two years would be needed to determine more accurately the kinematics (tangential motions) for these faint blue old white dwarfs.
Eighteen RR Lyrae variables (RRLs) that lie in the "12 h .4 clump" identified by the Quasar Equat... more Eighteen RR Lyrae variables (RRLs) that lie in the "12 h .4 clump" identified by the Quasar Equatorial Survey Team (QUEST) have been observed spectroscopically to measure their radial velocities and metal abundances. Ten blue horizontal branch (BHB) stars identified by the Sloan Digital Sky Survey (SDSS) were added to this sample. Six of the nine stars in the densest region of the clump have a mean radial velocity in the Galactic rest frame () of 99.8 and km s Ϫ1 , which is slightly smaller than the average error of the measurements. V j p 17.3 gsr The whole sample contains eight RRLs and five BHB stars that have values of suggesting membership in V gsr this stream. For seven of these RRLs, the measurements of [Fe/H], which have an internal precision of 0.08 dex, yield and. These values suggest that the stream is a tidally disrupted dwarf spheroidal A[Fe/H]S p Ϫ1.86 j p 0.40 galaxy of low luminosity. Photometry from the database of the SDSS indicates that this stream covers at least 106 deg 2 of the sky in the constellation Virgo. The name Virgo stellar stream is suggested.
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.
Astrometric measurements for 25 red dwarf systems are presented, including the first definitive t... more Astrometric measurements for 25 red dwarf systems are presented, including the first definitive trigonometric parallaxes for 20 systems within 10 pc of the Sun, the horizon of the RECONS sample. The three nearest systems that had no previous trigonometric parallaxes (other than perhaps rough preliminary efforts) are SO 0253+1652 (3.84 ± 0.04 pc, the 23rd nearest system), SCR 1845-6357 AB (3.85 ± 0.02 pc, 24th), and LHS 1723 (5.32 ± 0.04 pc, 56th). In total, seven of the systems reported here rank among the nearest 100 stellar systems. Supporting photometric and spectroscopic observations have been made to provide
White dwarf cooling sequences provides a useful indicator of the evolutionary time scales involve... more White dwarf cooling sequences provides a useful indicator of the evolutionary time scales involved in the chronometry and star formation history of the galactic disk and, for this reason, the luminosity function of white dwarfs has become a powerful tool for studies of the evolution and formation of the Milky Way. We examine the faint-end ($M_V > +14$) behavior of the disk white dwarf luminosity function using the $1/V_{\rm max}$ method, but, for the first time, fully including the effects of realistic observational errors in the derived luminosity function. We find that observational errors, mostly in the bolometric corrections and trigonometric parallaxes, play a major role in obliterating (real or artificial) small scale fluctuations in the luminosity function. A better estimator of the true luminosity function seems to be the median over simulations, rather than the mean. When using the latter, an age of 10 Gyr or older can not be ruled out from the sample of Leggett, Ruiz, a...
We have begun an investigation at Plaut`s low extinction window, located at (l,b)=(0,-8), ie, at ... more We have begun an investigation at Plaut`s low extinction window, located at (l,b)=(0,-8), ie, at the interface between the bulge and the halo. We use 43 scanned photographic plates which cover around 250 250. These plates were taken through five different telescopes over 21 ...
... William F. van Altena and Terrence Girard Yale University Observatory New Haven, CT 06511 USA... more ... William F. van Altena and Terrence Girard Yale University Observatory New Haven, CT 06511 USA Carlos E. L6pez Observatorio Astronomico "Felix Aguilar" Universidad Nacional de SanJuan and Yale Southern Observatory San Juan, Argentina 1. INTRODUCTION ...
To explore the complex halo substructure that has been reported in the direction of the Virgo con... more To explore the complex halo substructure that has been reported in the direction of the Virgo constellation, radial velocities and metallicities have been measured for 82 RR Lyrae stars (RRLS) that were identified by the QUEST survey. These stars are distributed over 90 square degrees of the sky, and lie from 4 to 23 kpc from the Sun. Using an algorithm for finding groups in phase space and modeling the smooth halo component in the region, we identified the 5 most significant RRLS groups, some of which were previously known or suspected. We have examined the SEKBO and the Catalina catalog of RRLS (with available spectroscopic measurements), as well as the bright QUEST RRLS sample, the catalog of Red Giant stars from the Spaghetti survey, and three recent catalogs of blue horizontal branch (BHB) stars, for stars that may be related to the QUEST RRLS groups. The most significant group of RRLS is the Virgo stellar stream (VSS) identified here as group A, which is composed of at least 10 RRLS and 3 BHB stars. It has a mean distance of 19.6 kpc and a mean radial velocity V gsr = 128 km s −1 , as estimated from its RRLS members. With the revised velocities reported here, there is no longer an offset in velocity between the RRLS in the VSS and the prominent peak in the velocities of main-sequence turnoff stars reported by other researchers in the same direction and at a similar distance (known as S297+63-20.5). The location in phase space of two other groups (F and H) suggests a possible connection with the VSS, which cannot be discarded at this point, although the turnoff colors of the VSS and group H, as identified from other works, suggest they might be composed of different populations. Two more groups, B and D, are found at mean distances of 19.0 and 5.7 kpc, and mean radial velocities of V gsr = −94 and 32 km s −1. The latter is the more numerous in terms of total members, as well as the more extended in RA. A comparison with the latest model of the disruption of the Sagittarius dwarf, indicates that none of the above groups is related to it. Rather than being the result of a single accretion event, the excess of stars observed in Virgo appears to be composed of several halo substructures along the same line of sight.
Three nearby star candidates were selected as bright 2MASS point sources without associated optic... more Three nearby star candidates were selected as bright 2MASS point sources without associated optical sources, i.e. with potentially large proper motions, subsequently confirmed by multi-epoch optical data from the SuperCOSMOS Sky Surveys. All three objects are listed in the NLTT catalogue of high proper motion stars. Follow-up spectroscopic observations allowed us to classify L 449-1 as M 4.0e dwarf at 5.7 pc, L 43-72 as M 4.5e dwarf at 7.2 pc, and LP 949-15 as M 5.0e dwarf at 6.1 pc, using known absolute JHK s magnitudes of dwarfs with corresponding spectral types, respectively. All three stars exhibit Hα emission lines, and all three can be identified with bright X-ray sources. The available ROSAT light curves of two of the objects show typical flare-like variability. Thus, all three stars are active, very close potential neighbours of the Sun, previously not mentioned as such, certainly deserving further attention. In particular, these are very promising candidates for trigonometric parallax programs.
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Papers by René Méndez