arXiv subjects

Sebastien Lepine

Publications and source records attributed to Sebastien Lepine.

At least 55 records · Page 3Linked to original sources

arXiv2010

A Census of White Dwarfs Within 40 Parsecs of the Sun

Our aim is to compile a catalog of white dwarfs within 40 parsecs of the Sun, in which newly discovered objects would significantly increase the completeness of the current census. White dwarf candidates are identified from the SUPERBLINK proper motion database (Lepine & Shara 2005), which allows us to investigate stars down to a proper motion limit as low as 40 mas yr-1. The selection criteria and distance estimates are based on a combination of color-magnitude and reduced proper motion diagrams. Candidates with distances less than 50 parsecs are selected for spectroscopic follow-up. We present our preliminary sample of spectroscopically confirmed white dwarfs, as well as their atmospheric parameters. These parameters are obtained using the spectroscopic technique developed in Bergeron et al.(1992) for DA stars. DB, DQ, and DZ stars are also analyzed spectroscopically. For featureless spectra as well as those showing only Halpha, we perform a detailed photometric analysis of their energy distribution.

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arXiv2009

Nearby young stars selected by proper motion. I. Four new members of the Beta Pictoris moving group from the Tycho-2 catalog

We describe a procedure to identify stars from nearby moving groups and associations out of catalogs of stars with large proper motions. We show that from the mean motion vector of a known or suspected moving group, one can identify additional members of the group based on proper motion data and photometry in the optical and infrared, with minimal contamination from background field stars. We demonstrate this technique by conducting a search for low-mass members of the Beta Pictoris Moving Group in the Tycho-2 catalog. All known members of the moving group are easily recovered, and a list of 51 possible candidates is generated. Moving group membership is evaluated for 33 candidates based on X-ray flux from ROSAT, Halpha line emission, and radial velocity measurement from high-resolution infrared spectra obtained at Infrared Telescope Facility. We confirm three of the candidates to be new members of the group: TYC 1186-706-1, TYC 7443-1102-1, and TYC 2211-1309-1 which are late-K and early-M dwarfs 45-60pc from the Sun. We also identify a common proper motion companion to the known Beta Pictoris Moving Group member TYC 7443-1102-1, at a 26".3 separation; the new companion is associated with the X-ray source 1RXS J195602.8-320720. We argue that the present technique could be applied to other large proper motion catalogs to identify most of the elusive, low-mass members of known nearby moving groups and associations.

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arXiv2009

New Neighbors: Parallaxes of 18 nearby stars selected from the LSPM-north catalog

We present astrometric parallaxes for 18 suspected nearby stars selected from the LSPM-north proper motion catalog. Sixteen objects are confirmed to be main sequence M dwarfs within 16 parsecs of the Sun, including three stars (LSPM J0011+5908, LSPM J0330+5413, LSPM J0510+2714) which lie just within the 10 parsec horizon. Two other targets (LSPM J1817+1328, LSPM J2325+1403) are confirmed to be nearby white dwarfs at distances of 14 and 22 parsecs, respectively. One of our targets, the common proper motion pair LSPM J0405+7116E + LSPM J0405+7116W, is revealed to be a triple system, with the western component resolved into a pair of 16th magnitude stars (LSPM J0405+7116W-A and LSPM J0405+7116W-B) with a 0.7"+/-0.1" angular separation. We find two stars (LSPM J1314+1320 and LSPM J1757+7042) to be significantly overluminous for their colors, and suggest that these may be unresolved doubles/multiples.

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arXiv2009

"An Era of Precision Astrophysics: Connecting Stars, Galaxies and the Universe," an Astro2010 Science White Paper

Abridged: The golden age of astrophysics is upon us with both grand discoveries (extra-solar planets, dark matter, dark energy) and precision cosmology. Fundamental understanding of the working of stars and galaxies is within reach, thanks to newly available precision measurements. We highlight the importance of distances and model independent distances and masses. Distances are fundamental in astrophysics and their knowledge can change our perception of phenomena dramatically: e.g., in antiquity, the Heliocentric model was rejected because the predicted stellar parallaxes were not observed. Distance measurements are directly related to the history & fate of the uni- verse as they provide 2 of 3 methods available to date the universe. The 1st method is based on the ages of stars, which can be ascertained if their lumi- nosities (distances) are accurately known. The 2nd method relies on cosmolo- gical methods. To 1st order, the age of the universe is the inverse H_0. As stressed by the previous decadal report, "the fundamental goal of ... astrophysics is to understand how the universe ... galaxies [and] stars ... formed, how they evolved, and what their destiny will be." These questions can be answered partly by micro-arcsecond astrometry: 1) Galactic archeology: a detailed reconstruction of the formation history of the Milky Way and other Local Group galaxies, 2) the very oldest stars in the Milky Way and the age of the Universe, and 3) H_0 and concordance cosmology. These goals are achievable by combining muas-arcsecond astrometry from the proposed SIM-Lite mission supplemented with ground-based spectroscopy. The results of our proposed project will force the biggest reassessment of stellar astrophysics in 50 years, which will affect most branches of astrophysics.

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arXiv2008

A New All-Sky Catalog of Stars with Large Proper Motions

A new all-sky catalog of stars with proper motions pm>0.15"/yr is presented. The catalog is largely a product of the SUPERBLINK survey, a data-mining initiative in which the entire Digitized Sky Surveys are searched for moving stellar sources. Findings from earlier proper motions surveys are also incorporated. The new all-sky catalog supersedes the great historic proper motion catalogs assembled by W. J. Luyten (LHS, NLTT), and provides a virtually complete (>98%) census of high proper motion stars down to magnitude R=19.

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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

Twenty-three new ultra-cool subdwarfs from the Sloan Digital Sky Survey

A search of the Sloan Digital Sky Survey spectroscopic database has turned up 23 new ultra-cool subdwarfs, low-mass metal-poor stars of spectral subtype M 7.0 or later. Spectra from these red objects all show very strong molecular bands of CaH but relatively weak bands of TiO, indicative of a cool, metal-poor atmosphere. Five of the stars are formally classified as M subdwarfs (sdM7.0-sdM8.5), 13 as more metal-poor extreme subdwarfs (esdM7.0-esdM8.0), and five as extremely metal-poor ultra subdwarfs (usdM7.0-usdM7.5). In the [H_r,r-z] reduced proper motion diagram, these subdwarfs clearly populate the locus of low-luminosity stars with halo kinematics. It is argued that the objects are all very low-mass, metal-poor stars from the Galactic halo (Population II). These new discoveries more than double the census of spectroscopically confirmed ultra-cool subdwarfs. We show that the stars stand out remarkably in the [g-r,r-i] color-color diagram; a proposed color and proper motion selection scheme is expected to be extremely efficient in identifying more of these old, very low mass stars in the vicinity of the Sun.

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arXiv2008

New High Proper Motion Stars from the Digitized Sky Survey. IV. Completion of the Southern Survey and 170 Additional Stars with pm>0.45 arcsec/yr

Completion of the SUPERBLINK proper motion survey in the southern celestial hemisphere has turned up 170 new stars with proper motion 0.45 arcsec/yr < pm < 2.0 arcsec/yr. This fourth and final installment completes the all-sky, data-mining of the Digitized Sky Surveys for stars with large proper motions. The areas investigated in this final installment comprise 11,600 square degrees in the declination range -30 degrees < Decl. <0 degrees, and in low galactic latitude areas south of Decl.= -30 degrees which had not been covered in earlier data releases. Astrometric and photometric data are provided for the 170 new stars, along with finder charts. Most of the new discoveries are found in densely populated fields along the Milky Way, towards the Galactic bulge/center. The list of new discoveries includes 4 stars with proper motion pm >1.0 arcsec/yr. The total list of high proper motion stars recovered by SUPERBLINK in the southern sky now includes 2,228 stars with proper motions 0.45 arcsec/yr < pm <2.0 arcsec/yr.

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arXiv2008

Direct spectroscopic observations of clumping in O-star winds

We report the detection and monitoring of transient substructures in the radiation-driven winds of five massive, hot stars in different evolutionary stages. Clumping in the winds of these stars shows up as variable, narrow subpeaks superposed on their wide, wind-broadened (optical) emission lines. Similar patterns of emission-line profile variations are detected in the Of stars zeta Puppis and HD93129A, in the more evolved hydrogen-rich, luminous, Of-like WN stars HD93131 and HD93162, and in the more mass-depleted WC star in gamma2 Velorum. These observations strongly suggest that stochastic wind clumping is a universal phenomenon in the radiation-driven, hot winds from all massive stars, with similar clumping factors in all stages of mass depletion.

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arXiv2007

An astrometric companion to the nearby metal-poor, low-mass star LHS 1589

We report the discovery of a companion to the high proper motion star LHS 1589, a nearby high-velocity, low-mass subdwarf. The companion (LHS 1589B) is located 0.224+/-0.004" to the southwest of the primary (LHS 1589A), and is 0.5 magnitude fainter than the primary in the K_s band. The pair was resolved with the IRCAL infrared camera at Lick Observatory, operating with the Laser Guide Star Adaptive Optics system. A low-resolution spectrum of the unresolved pair obtained at the MDM observatory shows the source to be consistent with a cool subdwarf of spectral subtype sdK7.5. A photometric distance estimate places the metal-poor system at a distance d=81+/-18 parsecs from the Sun. We also measure a radial velocity V_rad=67+/-8 km/s which, together with the proper motion and estimated distance, suggests that the pair is roaming the inner Galactic halo on a highly eccentric orbit. With a projected orbital separation s=18.1+/-4.8 AU, and a crude estimate of the system's total mass, we estimate the orbital period of the system to be in the range 75 yr < P < 500 yr. This suggests that the dynamical mass of the system could be derived astrometrically, after monitoring the orbital motion over a decade or so. The LHS 1589AB system could thus provide a much needed constraint to the mass-luminosity relationship of metal-poor, low-mass stars.

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arXiv2007

Revised metallicity classes for low-mass stars: dwarfs (dM), subdwarfs (sdM), extreme subdwarfs (esdM), and ultra subdwarfs (usdM)

The current classification system of M stars on the main sequence distinguishes three metallicity classes (dwarfs - dM, subdwarfs - sdM, and extreme subdwarfs - esdM). The spectroscopic definition of these classes is based on the relative strength of prominent CaH and TiO molecular absorption bands near 7000A, as quantified by three spectroscopic indices (CaH2, CaH3, and TiO5). We re-examine this classification system in light of our ongoing spectroscopic survey of stars with proper motion μ> 0.45 "/yr, which has increased the census of spectroscopically identified metal-poor M stars to over 400 objects. Kinematic separation of disk dwarfs and halo subdwarfs suggest deficiencies in the current classification system. Observations of common proper motion doubles indicates that the current dM/sdM and sdM/esdM boundaries in the [TiO5,CaH2+CaH3] index plane do not follow iso-metallicity contours, leaving some binaries inappropriately classified as dM+sdM or sdM+esdM. We propose a revision of the classification system based on an empirical calibration of the TiO/CaH ratio for stars of near solar metallicity. We introduce the parameter ζ_{TiO/CaH} which quantifies the weakening of the TiO bandstrength due to metallicity effect, with values ranging from ζ_{TiO/CaH}=1 for stars of near-solar metallicity to ζ_{TiO/CaH}~0 for the most metal-poor (and TiO depleted) subdwarfs. We redefine the metallicity classes based on the value of the parameter ζ_{TiO/CaH}; and refine the scheme by introducing an additional class of ultra subdwarfs (usdM). We introduce sequences of sdM, esdM, and usdM stars to be used as formal classification standards.

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arXiv2007

The Space Motion of the Globular Cluster NGC 6397

As a by-product of high-precision, ultra-deep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly point-like. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and find that the cluster has made frequent passages through the Galactic disk.

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arXiv2007

New Distant Companions to Known Nearby Stars. II. Faint companions of Hipparcos stars and the frequency of wide binary systems

We perform a search for faint, common proper motion companions of Hipparcos stars using the recently published LSPM-north catalog of stars with proper motion mu>0.15 arcsec/yr. Our survey uncovers a total of 521 systems with angular separations 3 arcsec < Delta theta < 1500 arcsec, with 15 triples and 1 quadruple. Our new list of wide systems with Hipparcos primaries includes 130 systems identified here for the first time, including 44 in which the secondary star has V>15.0. Our census is statistically complete for secondaries with angular separations 20 arcsec < Delta theta < 300 arcsec and apparent magnitudes V<19.0. Overall, we find that at least 9.5 % of nearby (d<100 pc) Hipparcos stars have distant stellar companions with projected orbital separations s > 1,000 AU. We observe that the distribution in orbital separations is consistent with Opik's law f(s) ds ~ s^{-1} ds only up to separation s \approx 3,000 AU, beyond which it follows a more steeply decreasing power law f(s) ds ~ s^{-l} ds with l=1.6+/-0.1. We also find that the luminosity function of the secondaries is significantly different from that of the single stars field population, showing a relative deficiency in low-luminosity (8<M_V<14) objects. The observed trends suggest either a formation mechanism biased against low-mass companions, or a disruption over time of systems with low gravitational binding energy.

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arXiv2006

The Unusual Cataclysmic Binary Star RBS 0490 and the Space Density of Cataclysmics

RBS (Rosat Bright Source) 0490 is a cataclysmic variable star (CV) with unusually strong emission lines. The strength of the emission lines led to a suggestion that the object is intrinsically faint and correspondingly nearby (approx. 33 pc), which, if true, would strongly affect estimates of the CV space density. Here we report astrometry, filter photometry, and time-series spectroscopy of this object. The astrometry gives an absolute parallax pi(abs) = 4.5 +- 1.5 mas and a relative proper motion of 102 mas/yr. A Bayesian procedure gives a very uncertain distance estimate of d approx. 300 pc, and the small parallax alone implies d > 133 pc (at two standard deviations). The mean V magnitude is 17.4, which implies M_V = 10.9 - 5 log (d/200 pc), neglecting extinction. At 200 pc, the space velocity would be over 90 km/s with respect to the LSR. The time-series spectroscopy shows a possible emission-line radial-velocity period near 46 min. This would be unusually short for an orbital period and it may represent some other clock in the system.

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arXiv2005

Discovery of a nearby halo white dwarf with proper motion mu=2.55 arcsec/yr

We report the discovery of PM J13420-3415, a faint (V=17) white dwarf with a very high proper motion mu = 2.55 arcsec/yr. The star was found in the southern sky extension of the SUPERBLINK proper motion survey. A red spectrum shows the classical signature of a DA white dwarf, with a weak H-alpha line in absorption as the only prominent feature. The star is also found to have a large radial velocity V_rad = +212+/-15 km/s. At the adopted distance of 18 pc, the star has a very large space motion of 313 km/s relative to the Sun. An integration of the space motion shows that the star is on a nearly polar Galactic orbit, and is thus an unambiguous member of the Galactic halo. However, with an estimated effective temperature 5,000K < T_eff < 5,500K, the white dwarf appears to be much younger than expected for a denizen of the halo. The apparent paradox can be explained if the white dwarf is the relatively young (~2 Gyr) remnant of a longer-lived (10-14 Gyr) main sequence star, in which case the object is predicted to be a low-mass white dwarf with M ~ 0.45 M_sun.

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arXiv2005

New High Proper Motion Stars from the Digitized Sky Survey. III. Stars with proper motions 0.45<mu<2.0''/yr south of Decl.=-30

We report the discovery of 182 new southern stars with proper motion larger than 0.45''/yr. The stars were found in an expansion of the SUPERBLINK proper motion survey to 8,980 square degrees south of Decl.=-30. The new high proper motion stars include 123 objects with mu>0.5''/yr, and 5 with mu>1.0''/yr. These new stars consist in a variety of nearby red dwarfs and white dwarfs, and (slightly more distant) red halo subdwarf, and are all prime targets for follow-up spectroscopic and astrometric (parallax) observations. Comparison with previous proper motion surveys in the southern sky suggests that SUPERBLINK has a recovery rate between 80% and 90% at southern declinations for stars with red magnitude 10<R<19 and proper motion in the range 0.5<mu<2.0''/yr. This survey makes a significant addition to the census of high proper motion stars at southern declinations.

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arXiv2005

Nearby stars from the LSPM-north Proper Motion Catalog. I. Main Sequence Dwarfs and Giants Within 33 Parsecs of the Sun

A list of 4,131 dwarfs, subgiants, and giants located, or suspected to be located, within 33 parsecs of the Sun is presented. All the stars are drawn from the new LSPM-north catalog of 61,976 stars with annual proper motions larger than 0.15''/yr$. Trigonometric parallax measurements are found in the literature for 1,676 of the stars in the sample; photometric and spectroscopic distance moduli are found for another 783 objects. The remaining 1,672 objects are reported here as nearby star candidates for the first time. Photometric distance moduli are calculated for the new stars based on the (M_V,V-J) relationship, calibrated with the subsample of stars which have trigonometric parallaxes. The list of new candidates includes 539 stars which are suspected to be within 25 parsecs of the Sun, including 63 stars estimated to be within only 15 parsecs. The current completeness of the census of nearby stars in the northern sky is discussed in light of the new candidates presented here. It is estimated that 32% (18%) of nuclear burning stars within 33 parsecs (25 parsecs) of the Sun remain to be located. The missing systems are expected to have proper motions below the 0.15''/yr limit of the LSPM catalog.

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arXiv2004

A Catalog of Northern Stars With Annual Proper Motions Larger Than 0.15 Seconds of Arc (LSPM catalog - North)

The LSPM-north catalog is a comprehensive list of 61,977 stars north of the J2000 celestial equator that have proper motions larger than 0.15 seconds of arc per year (local-background-stars frame). The catalog has been generated primarily as a result of our systematic search for high proper motion stars in the Digitized Sky Surveys using our SUPERBLINK software. At brighter magnitudes, the catalog incorporates stars and data from the Tycho-2 Catalogue and also, to a lesser extent, from the All-sky Compiled Catalogue of 2.5 million stars. The LSPM catalog expands considerably over the old Luyten (LHS, NLTT) catalogs, superseding them for northern declinations. Corrections to the local-background-stars proper motions have been calculated, and absolute proper motions in the extragalactic frame are given. The catalog is estimated to be over 99% complete at high Galactic latitudes (|b>15|), and over 90% complete at low Galactic latitudes (|b<15|), down to a magnitude V=19.0, and has a limiting magnitude V=21.0. All the northern stars listed in the LHS and NLTT catalogs have been re-identified, and their positions, proper motions and magnitudes re-evaluated. The catalog also lists a large number of completely new objects, which promise to expand very significantly the census of red dwarfs, subdwarfs, and white dwarfs in the vicinity of the Sun.

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