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«The Solar Neighborhood XIII: Parallax Results from the CTIOPI ′′ 0.9-m Program – Stars with µ ≥ 1. 0/year (MOTION Sample) Wei-Chun Jao1, ...»

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to appear in the Astronomical Journal

The Solar Neighborhood XIII: Parallax Results from the CTIOPI

′′

0.9-m Program – Stars with µ ≥ 1. 0/year (MOTION Sample)

Wei-Chun Jao1, Todd J. Henry1, John P. Subasavage1 and Misty A. Brown1

arXiv:astro-ph/0502167v2 15 Feb 2005

Georgia State University, Atlanta, GA 30302-4106

jao@chara.gsu.edu, thenry@chara.gsu.edu, subasavage@chara.gsu.edu,

brown@chara.gsu.edu

Philip A. Ianna1, Jennifer L. Bartlett1

University of Virginia, Charlottesville, VA 22903 pai@virginia.edu, jlb2j@virginia.edu and Edgardo Costa1, Ren´ A. M´ndez1 e e Universidad de Chile, Santiago, Chile costa@das.uchile.cl, rmendez@das.uchile.cl ABSTRACT We present the first set of definitive trigonometric parallaxes and proper motions from the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI). Full astrometric reductions for the program are discussed, including methods of reference stars selection, differential color refraction corrections, and conversion of relative to absolute parallax. Using data acquired at the 0.9-m at CTIO, full astrometric solutions and V RIJHKs photometry are presented for ′′ 36 red and white dwarf stellar systems with proper motions faster than 1. 0/yr.

Of these, thirty three systems have the first ever trigonometric parallaxes, w

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′′ 1. 0/yr) 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 HR diagram.

Subject headings: astrometry — stars: distance — photometry — solar neighborhood — stars: high proper motion stars — white dwarfs

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The first ever stellar trigonometric parallax was reported by F. Bessel in 1838 for 61 Cygni, after a “race” in which he narrowly defeated F. G. Wilhelm Struve and T. Henderson, who published the parallaxes for Vega and α Centauri, respectively, in the next year.

Since then, trigonometric parallax measurements have provided one of the most important parameters for understanding stellar astronomy — distance — and provide one of the sturdiest rungs on the cosmic distance ladder. Trigonometric parallaxes are used to derive the intrinsic luminosities of stars, calculate accurate masses for binary system components, and answer questions about stellar populations and Galactic structure. In addition, the solar neighborhood is mapped out via trigonometric parallaxes, and these nearby objects provide the brightest examples of many stellar types, supplying the benchmarks to which more distant stars are compared.

Two of the most important parallax references are the Yale Parallax Catalog (van Altena et al. 1995) and Hipparcos Catalog (ESA 1997). Combined, they offer ∼120,000 parallaxes both from space and ground observations. Of these, 92% of trigonometric parallaxes are from the Hipparcos mission. However, because of the relatively bright magnitude limit of Hipparcos, many nearby stars candidates were excluded. Consequently, the faint members of the solar neighborhood are under-presented, and these faint red, brown, and white dwarfs are the objects targeted by recent trigonometric parallax efforts, including the one discussed in this paper. Recent results for nearby red and brown dwarfs include the efforts of Ianna et al. (1996), Tinney et al. (1995, 2003), Dahn et al. (2002), and Vrba et al. (2004), which together have provided ∼130 total ground-based parallaxes since 1995.

–3–

2. The CTIOPI Effort and Sample

In order to recover “missing” members in the solar neighborhood, the Research Consortium On Nearby Stars (RECONS) group is currently carrying out a southern sky parallax survey known as Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI).

The primary goals of CTIOPI are to discover and characterize nearby red, brown, and white dwarfs that remain unidentified in the solar neighborhood. This program was selected as a NOAO Survey Program, and observations commenced in 1999 August. CTIOPI used both 0.9-m and 1.5-m telescopes during the NOAO Survey Program, and has continued on the 0.9-m as part of the SMARTS (Small and Moderate Aperture Research Telescope System) Consortium beginning in 2003 February. The RECONS team at Georgia State University is responsible for data reduction for the 0.9-m program, while data from the 1.5-m program is being analyzed by E. Costa and R. M´ndez of the Universidad de Chile in Santiago. The e extended 0.9-m program has recently surpassed 400 systems on the observing list, while the final 1.5-m program included ∼50 systems that were fainter (and for which less observing time was awarded).

Most of the target stars (hereafter, the “pi stars”) are selected for CTIOPI because available astrometric (e.g, high proper motion), photometric, or spectroscopic data indicate that they might be closer than 25 pc. In the 0.9-m program, roughly 95% of the pi stars are red dwarfs and the remainder are white dwarfs. The fainter brown dwarf candidates were included in the 1.5-m program. In all, ∼30% of the 0.9-m targets are members of what we ′′ call the MOTION sample — stellar systems having proper motions of at least 1. 0/yr. This paper describes the first definitive astrometric results of CTIOPI, focusing on the results for 36 MOTION systems.





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The 0.9-m telescope is equipped with a 2048 × 2048 Tectronix CCD camera with ′′ 0. 401/pixel plate scale (Jao et al. 2003). All observations were made using the central quar ter of the chip, yielding a 6.8 square field of view, through VJ, RKC and IKC filters (hereafter, the subscripts are not given)1. The dewar containing the CCD camera is mounted on the telescope with columns oriented in the north-south direction. A slight rotation relative to the

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sky is possible because of instrument flexure and repositioning during telescope maintenance.

This rotation angle can be calibrated, as discussed in section 4.1.

The observing procedures employed during CTIOPI mimic those used in University of Virginia southern parallax program at the Siding Spring Observatory in Australia, led by P.

Ianna, who is a member of the CTIOPI team. When a star is observed for the first time, exploratory exposures are taken in the V RI filters to find a suitable set of reference stars in the field. The parallax filter and position of the field are selected to balance the brightness of the pi star with available potential reference stars, and that filter is used for all subsequent parallax frames. Because most of our pi stars are nearby star candidates, they are brighter than most of the field stars. We attempt to place the pi star on the chip so that 5 to 15 field stars of adequate flux can be included. Typically, a good reference star is not more than twice as bright as the pi star (in the few cases when the pi star is not the brightest star in the field), but has at least 1000 peak counts during a typical parallax exposure.

Bias frames and dome flats are taken at the beginning of each night to allow for basic data reduction calibration. Parallax observations are usually made within ±30 minutes of a pi star’s transit in order to minimize the corrections required for differential color refraction (DCR) corrections, which are discussed in section 4.2. A few faint pi stars are observed with a wider hour angle tolerance because frame acquisition takes longer. Exposure times for parallax frames typically provide a peak of ∼50,000 counts for the pi star (saturation occurs at 65,535 counts), in an effort to maximize the number of counts available for pi star and reference star centroiding. Usually, 3–10 frames are taken in each visit, depending primarily on the exposure time required. Multiple frames are taken to reduce the errors on the pi star and reference star positions at each observation epoch. The typical set of observations required to determine a final parallax and proper motion includes four seasons of observations carried out over at least 2.5 years (further details in section 6.1).

3.2. V RI Photometry Observations

The V RI photometry reported here was acquired at the CTIO 0.9-m utilizing the same instrument setup used for the astrometry frames. All of the results are from observations taken between November 1999 and September 2004. As with the astrometry observations, bias and dome flat images were acquired nightly and used for basic science frame calibration.

Most pi stars were observed at sec z 1.8 or less (a few were between 1.8 and 2.0 airmasses because of extreme northern or southern declinations). Various exposure times were used to reach S/N 100 for pi stars in each of the V RI filters. Standard star fields –5– with averagely total 10 stars from Landolt (1992) and/or E-regions from Graham (1982) were observed several times each night to derive transformation equations and extinction curves. In addition, one or two very red standard stars with V − I 3.7 were observed in order to derive extinction coefficients for stars with a wide range of colors. Typically, a total of 4–5 standard star fields were observed 2–3 times each per night.

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The basic data reduction for the astrometry CCD frames includes overscan correction, bias subtraction and flat-fielding, performed using a customized IRAF package called redpi (because our pi stars are primarily red dwarfs). After processing the raw data, frames are sorted into storage directories by object until there are enough parallax frames and time coverage to derive a reliable astrometric solution, typically at least 40 frames over at least two years. When sufficient frames are available, SExtractor (Bertin & Arnouts 1996) is used to determine centroids for each pi star and a set of reference stars that is chosen using the

following general guidelines:

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2. Five to 15 reference stars in the field are selected that evenly surround the pi star in order to determine reliable plate rotation, translation, and scaling coefficients.

3. Each reference star must have a complete set of V RI photometry, which is required for DCR corrections and the conversion of relative to absolute parallax.

4. Using the IRAF imexam task, each reference star is checked to make sure that it is not a resolved binary or galaxy.

5. Ideally, all of the reference stars are have peak counts above 1000, although some fields require the selection of fainter stars in order to have a sufficient number of reference stars.

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In order to calculate the parallax factors, accurate coordinates and a value for the Earth to Solar System barycenter distance need to be known. The coordinates used for parallax factor calculations are extracted from the Two Micron All Sky Survey (2MASS) All-Sky Point Source Catalog via OASIS. Because these objects are high proper motion stars, all of them were manually identified by comparison with finding charts instead of retrieving data blindly by setting a searching radius around a given RA and DEC. The coordinates listed in Table 2 for the pi stars have been shifted to epoch 2000 using available proper motion measurements, primarily Luyten (1979), instead of the epoch at which the images were taken by 2MASS. To compute an accurate distance from the Earth to the Solar System barycenter at the time of observation, the JPL ephemeris DE405 is used.

Before calculating the parallax and proper motion of the pi star using frames taken at many epochs, a single “trail plate” is selected as a fundamental reference frame to which all other images are compared. This trail plate is used to remove any rotation, translation, and scaling changes between frames. A customized program organizes the set of frames used during the reductions for a particular field, is run to calculate the hour angle, parallax factors, and FWHM of images for each frame, and a trail plate is selected using the results,

using the following criteria:

1. All reference stars and pi star(s) have peak counts less than 65500 and greater than 100.

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4. The hour angle is within 2 minutes of the meridian at the midpoint of the integration.

A frame taken very near the meridian provides a trail plate with minimal DCR.

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′′ (but larger than 0. 6 to avoid centroiding on cosmic rays and bad pixels) are used. Once the rotation angle is determined, the parallax factors and centroids for all reference stars and pi stars on the trail plate are recalculated and used as the fundamental reference frame.

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DCR corrections are required because the pi star and reference stars are not of identical color; therefore, their positions as seen from underneath Earth’s atmosphere shift relative to one another because of different, but calibrateable amounts of refraction. Although most of our parallax observations suffer minimal DCR because they are made within 30 minutes of the meridian, sometimes frames are taken far enough from the meridian that it is advantageous to make DCR corrections, e.g. for important targets observed in non-ideal observing seasons and in cases when the total number of available frames can be boosted by utilizing photometry frames taken in the parallax filter. Different observing and reduction methods used to measure DCR have been discussed by Monet et al. (1992), Tinney (1993), Stone (1996), and Stone (2002). Here we use both the theoretical methods proposed by Stone (1996) and the empirical methodology proposed by Monet et al. (1992) to measure DCR for the CTIOPI program, and to make final corrections during astrometric reductions.



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