Edgardo Costa

THE SOLAR NEIGHBORHOOD. XVII. PARALLAX RESULTS FROM THE CTIOPI 0.9 m PROGRAM: 20 NEW MEMBERS OF THE RECONS 10 PARSEC SAMPLE

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 nearest), and LHS 1723 (5.32 ± 0.04 pc, 56th nearest). 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 full characterization of the systems, including complete VRIJHKs photometry and spectral types. A study of the variability of 27 targets reveals six obvious variable stars, including GJ 1207, for which we observed a flare event in the V band that caused it to brighten by 1.7 mag. Improved parallaxes for GJ 54 AB and GJ 1061, both important members of the 10 pc sample, are also reported. Definitive parallaxes for GJ 1001 A, GJ 633, and GJ 2130 ABC, all of which have been reported to be within 10 pc, indicate that they are beyond 10 pc. From the analysis of systems with (previously) high trigonometric parallax errors, we conclude that parallaxes with errors in excess of 10 mas are insufficiently reliable for inclusion in the RECONS sample. The cumulative total of new additions to the 10 pc sample since 2000 is now 34 systems: 28 by the RECONS team and six by other groups. This total represents a net increase of 16% in the number of stellar systems reliably known to be nearer than 10 pc.

THE CHEMICAL ENRICHMENT HISTORY OF THE SMALL MAGELLANIC CLOUD AND ITS GRADIENTS

We present stellar metallicities derived from Ca II triplet spectroscopy in over 350 red giant branch stars in 13 fields distributed in different positions in the Small Magellanic Cloud, ranging from ~1° to ~4° from its center. In the innermost fields, the average metallicity is [Fe/H] ~–1. This value decreases when we move away toward outermost regions. This is the first detection of a spectroscopic metallicity gradient in this galaxy. We show that the metallicity gradient is related to an age gradient, in the sense that more metal-rich stars, which are also younger, are concentrated in the central regions of the galaxy.

The Solar Neighborhood. XIII. Parallax Results from the CTIOPI 0.9 Meter Program: Stars with μ ≥ 10 yr−1 (MOTION Sample)

We present the first set of definitive trigonometric parallaxes and proper motions from the Cerro Tololo Inter-American Observatory Parallax Investigation. Full astrometric reductions for the program are discussed, including methods of reference star selection, differential color refraction corrections, and conversion of relative to absolute parallax. Using data acquired at the 0.9 m telescope at CTIO, full astrometric solutions and VRIJHKs photometry are presented for 36 red and white dwarf stellar systems with proper motions faster than 10 yr-1. Of these, 33 systems have their first ever trigonometric parallaxes, which comprise 41% of MOTION systems (those reported to have proper motions greater than 10 yr-1) south of δ = 0° that have no parallaxes. Four of the systems are new members of the RECONS 10 pc sample for which the first accurate trigonometric parallaxes are published here: DENIS J1048-3956 (4.04 ± 0.03 pc), GJ 1128 (LHS 271, 6.53 ± 0.10 pc), GJ 1068 (LHS 22, 6.97 ± 0.09 pc), and GJ 1123 (LHS 263, 9.02 ± 0.16 pc). In addition, two red subdwarf–white dwarf pairs, LHS 193AB and LHS 300AB, are identified. The white dwarf secondaries fall in a previously uncharted region of the H-R diagram.

THE SOLAR NEIGHBORHOOD. XXI. PARALLAX RESULTS FROM THE CTIOPI 0.9 m PROGRAM: 20 NEW MEMBERS OF THE 25 PARSEC WHITE DWARF SAMPLE

We present accurate trigonometric parallaxes for 20 new members of the 25 pc white dwarf (WD) sample as part of the Discovery and Evalution of Nearby Stellar Embers (DENSE) project.7 http://www.DenseProject.com Previously, there were a total of 112 WD systems with trigonometric parallaxes placing them within 25 pc and of these, 99 have trigonometric parallaxes known to better than 10%. Thus, the 20 new members presented in this work represent a 20% increase in the number of WDs accurately known to be within 25 pc. In addition, we present updated parallaxes for seven known WDs within 10 pc that have been observed as part of the Astrometric Search for Planets Encircling Nearby Stars initiative to monitor nearby southern red dwarfs and WDs for astrometric perturbations from unseen companions. Including a few WD companions and WDs beyond 25 pc, we present a total of 33 trigonometric parallaxes. We perform atmospheric modeling for WDs to determine physical parameters (i.e., T eff, log g, mass, and WD age). Finally, a new ZZ Ceti pulsating WD was identified and revised constraints are placed on two mixed H/He atmosphere cool WDs that display continuum absorption in the near-infrared.

The Solar Neighborhood. XIV. Parallaxes from the Cerro Tololo Inter-American Observatory Parallax Investigation—First Results from the 1.5 m Telescope Program

Trigonometric parallaxes, proper motions, and VJ(RI)KC photometry are presented for 31 stars targeted by the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI), a program of wide scope aimed at discovering and characterizing nearby stars. The data given are the first that have been obtained with the CTIO 1.5 m telescope, targeting a fainter subset of the CTIOPI input list. We present the first trigonometric parallaxes for 21 systems, of which one is within 10 pc (LP 647-013 at 9:59 � 0:22 pc) and six are between 10 and 25 pc. Concurrently with our Cerro Tololo 0.9 m program, we have determined parallaxes for DEN 1048� 3956 and LTT 6933 that place them at 4:00 � 0:03 and 16:24 � 0:43 pc from the Sun, respectively. We also present an improved parallax for the important nearby triple system GJ 2005ABC, placing it at 7:72 � 0:15 pc from the Sun. The remaining seven parallaxes are for calibration stars, whose values indicate that our results agree well with other parallax determinations. We present color-magnitude and color-color diagrams that, in combination with theoretical isochrones from the literature and other derived properties of the observed sample, have aided the identification of the general nature of each of our targets. We have in this way discovered five new subdwarfs and several very low mass stars, a few of which may be brown dwarfs.

THE SOLAR NEIGHBORHOOD. XVI. PARALLAXES FROM CTIOPI: FINAL RESULTS FROM THE 1.5 m TELESCOPE PROGRAM

Trigonometric parallaxes, proper motions and VJ(RI)KC photometry are given for 25 stars (of which one is a zero-parallax control field) targeted by the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI), a widely scoped program aimed at discovering and characterizing nearby stars. The trigonometric parallaxes and proper motions presented are the last that were obtained with the CTIO 1.5 m telescope, which targeted the fainter subset of the CTIOPI input list. First trigonometric parallaxes are given for 22 systems, of which one is within 10 pc (DENIS 0255-4700), and 10 of which are between 10 and 25 pc. At a distance of 4.97 ± 0.10 pc, and with a spectral type of L7.5 V, DENIS 0255-4700 is now the closest known L dwarf. In addition, with MV = 24.44, it is the faintest dwarf with a measured absolute visual magnitude. We present preliminary trigonometric parallaxes for five additional systems worthy of follow-up, and VRIJHKS photometry and photometric distance estimates for four of them. We also give photometry and distance estimates for 21 other promising targets in our input list for which definitive trigonometric parallaxes were not possible; 13 are likely to be closer than 25 pc. We also present color-magnitude and color-color diagrams, which, in combination with theoretical isochrones from the literature, tangential velocities, and MR and MJ, have aided to identify the general nature of each of our targets. We have in this way discovered one new (spectroscopically confirmed) subdwarf and two suspected extreme subdwarfs that could be among the most extreme cases of these objects. We have also identified several very low mass stars, a few of which could be brown dwarfs. This concludes the CTIOPI 1.5 m program, from which we have derived a total of 69 trigonometric parallaxes (55 definitive, 6 preliminary, and 8 calibration).

THE EDGE OF THE YOUNG GALACTIC DISK

In this work, we report and discuss the detection of two distant diffuse stellar groups in the third Galactic quadrant. They are composed of young stars, with spectral types ranging from late O to late B, and lie at galactocentric distances between 15 and 20 kpc. These groups are located in the area of two cataloged open clusters (VdB-Hagen 04 and Ruprecht 30), projected toward the Vela-Puppis constellations, and within the core of the Canis Major overdensity. Their reddening and distances have been estimated by analyzing their color-color and color-magnitude diagrams, derived from deep UBV photometry. The existence of young star aggregates at such extreme distances from the Galactic center challenges the commonly accepted scenario in which the Galactic disk has a sharp cutoff at about 14 kpc from the Galactic center and indicates that it extends to much greater distances (as also supported by the recent detection of CO molecular complexes well beyond this distance). While the groups we find in the area of Ruprecht 30 are compatible with the Orion and Norma-Cygnus spiral arms, respectively, the distant group we identify in the region of VdB-Hagen 04 lies in the external regions of the Norma-Cygnus arm, at a galactocentric distance (~20 kpc) where no young stars have been detected so far in the optical.

THE PROPER MOTION OF THE MAGELLANIC CLOUDS. I. FIRST RESULTS AND DESCRIPTION OF THE PROGRAM

We present the first results of a ground-based program to determine the proper motion of the Magellanic Clouds (MCs) relative to background quasars (QSO), being carried out using the Irenee du Pont 2.5 m telescope at Las Campanas Observatory, Chile. Eleven QSO fields have been targeted in the Small Magellanic Cloud (SMC) over a time base of six years, and with seven epochs of observation. One quasar field was targeted in the Large Magellanic Cloud (LMC), over a time base of five years, and with six epochs of observation. The shorter time base in the case of the LMC is compensated by the much larger amount of high-quality astrometry frames that could be secured for the LMC quasar field (124 frames), compared to the SMC fields (an average of roughly 45 frames). In this paper, we present final results for field Q0557–6713 in the LMC and field Q0036–7227 in the SMC. From field Q0557–6713, we have obtained a measured proper motion of μαcos δ = +1.95 ± 0.13 mas yr–1, μδ = +0.43 ± 0.18 mas yr–1 for the LMC. From field Q0036–7227, we have obtained a measured proper motion of μα cosδ = +0.95 ± 0.29 mas yr–1, μδ = –1.14 ± 0.18 mas yr–1 for the SMC. Although we went through the full procedure for another SMC field (QJ0036–7225), on account of unsolvable astrometric difficulties caused by blending of the QSO image, it was impossible to derive a reliable proper motion. Current model rotation curves for the plane of the LMC indicate that the rotational velocity (V rot) at the position of LMC field Q0557–6713 can be as low as 50 km s–1, or as high as 120 km s–1. A correction for perspective and rotation effects leads to a center of mass proper motion for the LMC of μα cosδ = +1.82 ± 0.13 mas yr–1, μδ = +0.39 ± 0.15 mas yr–1 (V rot = 50 km s–1), and to μα cosδ = +1.61 ± 0.13 mas yr–1, μδ = +0.60 ± 0.15 mas yr–1 (V rot = 120 km s–1). Assuming that the SMC has a disk-like central structure, but that it does not rotate, we obtain a center of mass proper motion for the SMC of μα cosδ = +1.03 ± 0.29 mas yr–1, μδ = –1.09 ± 0.18 mas yr–1. Our results are in reasonable agreement with most previous determinations of the proper motion of the MCs, including recent Hubble Space Telescope measurements. Complemented with published values of the radial velocity of the centers of the LMC and SMC, we have used our proper motions to derive the galactocentric (gc) velocity components of the MCs. For the LMC, we obtain V gc,t = +315 ± 20 km s–1, V gc,r = +86 ± 17 km s–1 (V rot = 50 km s–1), and V gc,t = +280 ± 24 km s–1, V gc,r = +94 ± 17 km s–1 (V rot = 120 km s–1). For the SMC, we obtain V gc,t = +258 ± 50 km s–1, V gc,r = +20 ± 44 km s–1. These velocities imply a relative velocity between the LMC and SMC of 84 ± 50 km s–1, for V rot,LMC = 50 km s–1, and 62 ± 63 km s–1 for V rot,LMC = 120 km s–1. Albeit our large errors, these values are not inconsistent with the standard assumption that the MCs are gravitationally bound to each other.

The Solar Neighborhood. VII. Discovery and Characterization of Nearby Multiples in the CTIO Parallax Investigation

We report the discovery of eight new multiple star systems among 191 stellar systems targeted for parallax determinations in the RECONS (Research Consortium on Nearby Stars) southern parallax program, CTIOPI (Cerro Tololo Inter-American Observatory Parallax Investigation). The eight new companions have separations of 1>42 to 14>90 and instrumental magnitude differences at VRI of 0.06–6.07 mag. Orbital motion has not been detected in any of these systems. These new companions increase the multiplicity fraction of this sample, made up primarily of nearby (less than 25 pc) M dwarfs, to 15%. Comparison with samples that have been more completely scrutinized for companions indicates that probably only half of all multiples have so far been discovered. Given the large number of frames acquired for the astrometric series, the eight new systems and 16 known multiples have been searched for variability at VRI during the 3 year duration of CTIOPI. A flare has been detected in the secondary of the RX J1132� 264 system, while at least one component in the GJ 2006 system is a probable long-term variable. Variables were detected in only 9% of the systems searched, primarily as a result of the restrictive 0.05 mag threshold required for variability confirmation.

NGC 6404 and 6583: two neglected intermediate-age open clusters located in the Galactic Centre direction

We report on VI charge-coupled device photometry of two fields centred in the region of the open clusters NGC 6404 and 6583 down to V = 22.0. So far these clusters have never been studied, and we provide for the first time estimates of their fundamental parameters, namely, radial extent, age, distance and reddening. We find that the radius of NGC 6404 is 2.0 arcmin, as previously proposed, while the radius of NGC 6583 is 1.0 arcmin, significantly lower than previous estimates. Both clusters turn out to be of intermediate age (0.5-1.0 Gyr old), and located inside the solar ring, at a Galactocentric distance of about 6.5 kpc. These results make these objects very interesting targets for spectroscopic follow-up to measure their metallicity. In fact, they might allow us to enlarge by more than 1 kpc the baseline of the radial abundance gradient in the Galactic disc towards the Galactic Centre direction. This baseline is currently rather narrow especially for clusters of this age.