arXiv subjects

Wei-Chun Jao

Publications and source records attributed to Wei-Chun Jao.

72 records · Page 4Linked to original sources

arXiv2011

The Solar Neighborhood XXIV. Parallax Results from the CTIOPI 0.9-m Program: Stars with $μ$ $\ge$ 1\farcs0 yr$^{-1}$ (MOTION Sample) and Subdwarfs

We present 41 trigonometric parallaxes of 37 stellar systems, most of which have proper motions greater than 1\farcs0 yr$^{-1}$. These are the first trigonometric parallaxes for 24 systems. Overall, there are 15 red dwarf systems and 22 red subdwarf systems in the sample. Five of the systems are multiples with directly detected companions, and we have discovered perturbations caused by unseen companions in two additional cases, the dwarf LHS 501 and the subdwarf LHS 440. The latter system may eventually provide important dynamical mass points on the subdwarf mass-luminosity relation. Two additional stars of note are LHS 272, the third closest M-type subdwarf at a distance of only 13.6 pc, and LHS 2734AB, a high velocity subdwarf binary with $V_{tan}>$ 700 km/sec, which likely exceeds the escape velocity of the Milky Way. We also report the first long term variability study of cool subdwarfs indicating that cool subdwarfs are less photometrically variable than their main sequence counterparts.

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arXiv2011

Astrophysical Parameters and Habitable Zone of the Exoplanet Hosting Star GJ 581

GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline interferometric measurements from the CHARA Array, coupled with trigonometric parallax information, to directly determine its physical radius to be $0.299 \pm 0.010 R_{\odot}$. Literature photometry data are used to perform spectral energy distribution fitting in order to determine GJ 581's effective surface temperature $T_{\rm EFF}=3498 \pm 56$ K and its luminosity $L=0.01205 \pm 0.00024 L_{\odot}$. From these measurements, we recompute the location and extent of the system's habitable zone and conclude that two of the planets orbiting GJ 581, planets d and g, spend all or part of their orbit within or just on the edge of the habitable zone.

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arXiv2010

The Solar Neighborhood XXIII CCD Photometric Distance Estimates of SCR Targets -- 77 M Dwarf Systems within 25 Parsecs

We present CCD photometric distance estimates of 100 SCR (SuperCOSMOS RECONS) systems with $μ$ $\geq$ 0$\farcs$18/yr, 28 of which are new discoveries previously unpublished in this series of papers. These distances are estimated using a combination of new $VRI$ photometry acquired at CTIO and $JHK$ magnitudes extracted from 2MASS. The estimates are improvements over those determined using photographic plate $BRI$ magnitudes from SuperCOSMOS plus $JHK$, as presented in the original discovery papers. In total, 77 of the 100 systems investigated are predicted to be within 25 pc. If all 77 systems are confirmed to have $π$$_{trig}$ $\ge$ 40 milliarcseconds, this sample would represent a 23% increase in M dwarf systems nearer than 25 pc in the southern sky.

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arXiv2009

Cool Subdwarf Investigations II: Multiplicity

Cool subdwarfs of types K and M are the fainter counterparts of cool main sequence dwarfs that dominate the Galactic population. In this paper we present the results of an optical speckle survey of 62 confirmed cool subdwarf systems within 60 pc. We have resolved two new companions and confirmed two previously known companions with separations 0\farcs13 to 3\farcs29. After including previously known wide companions and all known spectroscopic binaries, we determine the multiplicity rate of cool subdwarfs to be 26$\pm$6%, which is somewhat lower than comparable main sequence stars, which have a multiplicity rate of 37$\pm$5%. We find that only 3% of the cool subdwarfs surveyed have companions within 10 AU, 3% have companions between 10 and 100 AU, and 14% have companions beyond 100 AU. The other 6% of cool subdwarfs are spectroscopic binaries. This is very different from K/M dwarfs that have most companions (13%) at separations closer than 10 AU. However, because a search for close binaries among a large sample of nearby cool subdwarfs remains elusive, it is not yet settled whether or not the multiplicity rates are significantly different. Nonetheless, several different observational results and theories pointing to a possible dearth of subdwarf multiples are discussed.

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arXiv2009

The Solar Neighborhood. XXI. Parallax Results from the CTIOPI 0.9m 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 sample as part of the DENSE project (Discovery and Evalution of Nearby Stellar Embers, http://www.DenseProject.com). Previously, there were a total of 112 white dwarf 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 white dwarfs accurately known to be within 25 pc. In addition, we present updated parallaxes for seven known white dwarfs within 10 pc that have been observed as part of the ASPENS initiative (Astrometric Search for Planets Encircling Nearby Stars) to monitor nearby southern red and white dwarfs for astrometric perturbations from unseen companions. Including a few white dwarf companions and white dwarfs beyond 25 pc, we present a total of 33 trigonometric parallaxes. We perform atmospheric modeling for white dwarfs to determine physical parameters (i.e., effective temperature, log g, mass, and white dwarf age). Finally, a new ZZ Ceti pulsating white dwarf was identified and revised constraints are placed on two mixed H/He atmosphere cool white dwarfs that display continuum absorption in the near-infrared.

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arXiv2008

Ultracool Subdwarfs: The Halo Population Down to the Substellar Limit

Ultracool subdwarfs are low luminosity, late-type M and L dwarfs that exhibit spectroscopic indications of subsolar metallicity and halo kinematics. Their recent discovery and ongoing investigation have led to new insights into the role of metallicity in the opacity structure, chemistry (e.g. dust formation) and evolution of low-temperature atmospheres; the long-term evolution of magnetic activity and angular momentum amongst the lowest-mass stars; the form of the halo luminosity and mass functions down to the hydrogen-burning mass limit; and even fundamental issues such as spectral classification and absolute brightness scales. This Splinter Session was devoted to bringing advances in observational and theoretical ultracool subdwarf research to the attention of the low-mass stellar and brown dwarf communities, as well as to share results among ultracool subdwarf enthusiasts.

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arXiv2008

Cool Subdwarf Investigations (CSI) I: New Thoughts for the Spectral Types of K and M Subdwarfs

Using new spectra of 88 K and M-type subdwarfs, we consider novel methods for assigning their spectral types and take steps toward developing a comprehensive spectral sequence for subdwarf types K3.0 to M6.0. The types are assigned based on the overall morphology of spectra covering 6000Åto 9000Å. The types and sequence presented link the spectral types of cool subdwarfs to their main sequence counterparts, with emphasis on the relatively opacity-free region from 8200--9000Å. When available, supporting abundance, kinematic, and trigonometric parallax information is used to provide more complete portraits of the observed subdwarfs. We find that the CaHn (n $=$ 1--3) and TiO5 indices often used for subdwarf spectral typing are affected in complicated ways by combinations of subdwarfs' temperatures, metallicities, and gravities, and we use model grids to evaluate the trends in all three parameters. Because of the complex interplay of these three characteristics, it is not possible to identify a star as an ``extreme'' subdwarf simply based on very low metallicity, and we suggest that the modifiers ``extreme'' or ``ultra'' only outline locations on spectroscopic indices plots, and should not be used to imply low or very low metallicity stars. In addition, we propose that ``VI'' be used to identify a star as a subdwarf, rather than the confusing ``sd'' prefix, which is also used for hot O and B subdwarfs that are unrelated to the cool subdwarfs discussed in this paper.

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arXiv2008

Angular Diameters of the G Subdwarf $μ$ Cassiopeiae A and the K Dwarfs $σ$ Draconis and HR 511 from Interferometric Measurements with the CHARA Array

Using the longest baselines of the CHARA Array, we have measured the angular diameter of the G5 V subdwarf $μ$ Cas A, the first such determination for a halo population star. We compare this result to new diameters for the higher metallicity K0 V stars, $σ$ Dra and HR 511, and find that the metal-poor star, $μ$ Cas A, has an effective temperature ($T_{\rm eff}=5297\pm32$ K), radius ($R=0.791\pm0.008 R_{\rm \odot}$), and absolute luminosity ($L=0.442\pm0.014 L_{\rm \odot}$) comparable to the other two stars with later spectral types. We show that stellar models show a discrepancy in the predicted temperature and radius for $μ$ Cas A, and we discuss these results and how they provide a key to understanding the fundamental relationships for stars with low metallicity.

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arXiv2007

The Trigonometric Parallax of the Brown Dwarf Planetary System 2MASSW J1207334-393254

We have measured a trigonometric parallax to the young brown dwarf 2MASSW J1207334-393254. The distance [54.0 (+3.2,-2.8) pc] and space motion confirm membership in the TW Hydrae Association. The primary is a ~25 M_jup brown dwarf. We discuss the "planetary mass" secondary, which is certainly below the deuterium-burning limit but whose colors and absolute magnitudes pose challenges to our current understanding of planetary-mass objects.

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arXiv2007

The Solar Neighborhood. XVIII. Discovery of new proper motion stars with 0.40 arcsec yr^-1 > mu >= 0.18 arcsec yr^-1 between declinations -90 degrees and -47 degrees

We report 1606 new proper motion systems in the southern sky (declinations -90 degrees to -47 degrees with 0.40 arcsec yr^-1 > mu >= 0.18 yr^-1. This effort is a continuation of the SuperCOSMOS-RECONS (SCR) proper motion search to lower proper motions than reported in Papers VIII, X, XII, and XV in this series. Distance estimates are presented for the new systems, assuming that all stars are on the main sequence. We find that 31 systems are within 25 pc, including two systems -- SCR 0838-5855 and SCR 1826-6542 -- we anticipate to be within 10 pc. These new discoveries constitute a more than ten-fold increase in new systems found in the same region of sky searched for systems with mu >= 0.40 arcsec yr^-1, suggesting a happy hunting ground for new nearby slower proper motion systems in the region just north (declinations -47 degrees to 0 degrees, much of which has not been rigorously searched during previous efforts.

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arXiv2006

The Solar Neighborhood XVII: Parallax Results from the CTIOPI 0.9m Program -- Twenty 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), 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 full characterization of the systems, including complete VRIJHK 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.

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arXiv2006

Two Suns in The Sky: Stellar Multiplicity in Exoplanet Systems

We present results of a reconnaissance for stellar companions to all 131 radial-velocity-detected candidate extrasolar planetary systems known as of July 1, 2005. CPM companions were investigated using the multi-epoch DSS images, and confirmed by matching the trigonometric parallax distances of the primaries to companion distances estimated photometrically. We also attempt to confirm or refute companions listed in the Washington Double Star Catalog, the Catalogs of Nearby Stars, in Hipparcos results, and in Duquennoy & Mayor (1991). Our findings indicate that a lower limit of 30 (23%) of the 131 exoplanet systems have stellar companions. We report new stellar companions to HD 38529 and HD 188015, and a new candidate companion to HD 169830. We confirm many previously reported stellar companions, including six stars in five systems that are recognized for the first time as companions to exoplanet hosts. We have found evidence that 20 entries in the Washington Double Star Catalog are not gravitationally bound companions. At least three, and possibly five, of the exoplanet systems reside in triple star systems. Three exoplanet systems have potentially close-in stellar companions ~ 20 AU away from the primary. Finally, two of the exoplanet systems contain white dwarf companions. This comprehensive assessment of exoplanet systems indicates that solar systems are found in a variety of stellar multiplicity environments - singles, binaries, and triples; and that planets survive the post-main-sequence evolution of companion stars.

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arXiv2005

Ages of White Dwarf-Red Subdwarf Systems

We provide the first age estimates for two recently discovered white dwarf-red subdwarf systems, LHS 193AB and LHS 300AB. These unusual systems provide a new opportunity for linking the reliable age estimates for the white dwarfs to the (measurable) metallicities of the red subdwarfs. We have obtained precise photometry in the $V_{J}R_{KC}I_{KC}JH$ bands and spectroscopy covering from 6000Åto 9000Åfor the two new systems, as well as for a comparison white dwarf-main sequence red dwarf system, GJ 283 AB. Using model grids available in the literature, we estimate the cooling age as well as temperature, surface gravity, mass, progenitor mass and {\it total} lifetimes of the white dwarfs. The results indicate that the two new systems are probably ancient thick disk objects with ages of at least 6-9 Gyr. We also conduct searches of red dwarf and white dwarf compendia from SDSS data and the L{é}pine Shara Proper Motion (LSPM) catalog for additional common proper motion white dwarf-red subdwarf systems. Only seven new candidate systems are found, which indicates the rarity of these systems.

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arXiv2005

The Solar Neighborhood XV: Discovery of New High Proper Motion Stars with mu >= 0.4"/yr between Declinations -47 degrees and 00 degrees

We report the discovery of 152 new high proper motion systems (mu >= 0.4"/yr) in the southern sky (Declination = -47 degrees to 00 degrees) brighter than UKST plate R_{59F} =16.5 via our SuperCOSMOS-RECONS (SCR) search. This paper complements Paper XII in The Solar Neighborhood series, which covered the region from Declination = -90 degrees to -47 degrees and discussed all 147 new systems from the southernmost phase of the search. Among the total of 299 systems from both papers, there are 148 (71 in Paper XII, 77 in this paper) new systems moving faster than 0.5"/yr that are additions to the classic ``LHS'' (Luyten Half Second) sample. These constitute an 8% increase in the sample of all stellar systems with mu >= 0.5"/yr in the southern sky. As in Paper XII, distance estimates are provided for the systems reported here based upon a combination of photographic plate magnitudes and 2MASS photometry, assuming all stars are on the main sequence. Two SCR systems from the portion of the sky included in this paper are anticipated to be within 10 pc, and an additional 23 are within 25 pc. In total, the results presented in Paper XII and here for this SCR sweep of the entire southern sky include five new systems within 10 pc and 38 more between 10 and 25 pc. The largest number of nearby systems have been found in the slowest proper motion bin, 0.6"/yr > mu >= 0.4"/yr, indicating that there may be a large population of low proper motion systems very near the Sun.

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arXiv2005

The Solar Neighborhood XIII: Parallax Results from the CTIOPI 0.9-m Program -- Stars with mu >= 1"/year (MOTION Sample)

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 $VRIJHK_s$ photometry are presented for 36 red and white dwarf stellar systems with proper motions faster than 1\farcs0/yr. Of these, thirty three systems have the first ever trigonometric parallaxes, which comprise 41% of MOTION systems (those with proper motions greater than 1\farcs0/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 $\pm$ 0.03 pc), GJ 1128 (LHS 271, 6.53 $\pm$ 0.10 pc), GJ 1068 (LHS 22, 6.97 $\pm$ 0.09 pc), and GJ 1123 (LHS 263, 9.02 $\pm$ 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.

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arXiv2004

The Solar Neighbourhood XI: The trigonometric parallax of SCR 1845--6357

We present a trigonometric parallax for the nearby star SCR1845--6357, an extremely red high proper motion object discovered by Hambly et al. (2004) and identified via accurate photoelectric photometry and spectroscopy to be an M8.5 dwarf with a photometric parallax of $4.6\pm0.8$ pc by Henry et al. (2004). Using methods similar to those described in Deacon & Hambly (2001) we have derived a full astrometric solution from SuperCOSMOS scans of eight survey and non--survey Schmidt photographs held in the United Kingdom Schmidt Telescope Unit plate library. We calculate the trigonometric parallax to be $π=282\pm23$ mas yielding a distance of $3.5\pm0.3$ pc which implies an absolute K$_{s}$ magnitude of 10.79. This distance calculation places SCR1845--6357 as the 16th closest stellar system to the Sun.

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arXiv2004

The Solar Neighborhood XII: Discovery of New High Proper Motion Stars with 1.0"/yr > mu >= 0.4"/yr between Declinations -90 degrees and -47 degrees

We report the discovery of 141 new high proper motion systems (1.0"/yr > mu >= 0.4"/yr) in the southern sky (Declination = -90 degrees to -47 degrees) brighter than UKST plate R_{59F} = 16.5 via our SuperCOSMOS-RECONS (SCR) search. When combined with the nine systems having mu >= 1.0"/yr and/or late spectral type from the initial phases of this effort (Hambly et al. 2004, Henry et al. 2004), we find that 73 of the 150 total systems are moving faster than 0.5"/yr, and are therefore new members of the classic "LHS" (Luyten Half Second) sample. These constitute a 21% increase in the sample of stars with mu >= 0.5"/yr in the declination region searched, thereby comprising an important addition to this long-neglected region of the sky. Distance estimates are provided for the entire sample, based upon a combination of photographic plate magnitudes and 2MASS photometry, using the relations presented in Hambly et al. (2004) for the presumed main sequence stars. Three systems are anticipated to be within 10 pc, and an additional 15 are within 25 pc. Eight of these 18 nearby systems have proper motions falling between 0.4"/yr and 0.6"/yr, hinting at a large population of nearby stars with fast, but not extremely high, proper motions that have not been thoroughly investigated.

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arXiv2004

The Solar Neighborhood X: New Nearby Stars in the Southern Sky and Accurate Photometric Distance Estimates for Red Dwarfs

Photometric VRI and spectroscopic (6000-9500 A) observations of high proper motion stars discovered during the first phase of the SuperCOSMOS RECONS (SCR) search are used to estimate accurate distances to eight new nearby red dwarfs, including probable 10 pc sample members SCR 1845-6357 (M8.5V at 4.6 pc), the binary SCR 0630-7643AB (M6.0VJ at 7.0 pc), and SCR 1138-7721 (M5.0V at 9.4 pc). Distance estimates are determined using a suite of new photometric color-Mk relations defined using a robust set of nearby stars with accurate VRIJHK photometry and trigonometric parallaxes. These relations are utilized, with optical and infrared photometry, to estimate distances on a uniform system (generally good to 15%) for two additional samples of red nearby star candidates: several recently discovered members of the solar neighborhood, and known faint stars with proper motions in excess of 1.0"/yr south of DEC = -57.5. Of those without accurate trigonometric parallax measurements, there are five stars in the first sample and three in the second that are likely to be within 10 pc. The two nearest are SO 0253+1652 (M7.0V at 3.7 pc) and DEN 1048-3956 (M8.5V at 4.5 pc). When combined with SCR 1845-6357, these three stars together represent the largest increase in the 5 pc sample in several decades. Red spectra are presented for the red dwarfs and types are given on the RECONS standard spectral system. Red spectra are also given for two new nearby white dwarfs for which we estimate distances from the photometry of less than 20 pc -- WD 0141-675 (LHS 145, 9.3 pc) and SCR 2012-5956 (17.4 pc). WD 0141-675 brings the total number of systems nearer than 10 pc discussed in this paper to 12.

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