arXiv subjects

Sebastien Lepine

Publications and source records attributed to Sebastien Lepine.

At least 37 records · Page 2Linked to original sources

arXiv2014

Optical BVRI Photometry of Common Proper Motion F/G/K+M Wide Separation Binaries

We present optical (BVRI) photometric measurements of a sample of 76 common proper motion wide separation main sequence binary pairs. The pairs are composed of a F-, G-, or K-type primary star and an M-type secondary. The sample is selected from the revised NLTT catalog and the LSPM catalog. The photometry is generally precise to 0.03 mag in all bands. We separate our sample into two groups, dwarf candidates and subdwarf candidates, using the reduced proper motion (RPM) diagram constructed with our improved photometry. The M subdwarf candidates in general have larger $V-R$ colors than the M dwarf candidates at a given $V-I$ color. This is consistent with an average metallicity difference between the two groups, as predicted by the PHOENIX/BT-Settl models. The improved photometry will be used as input into a technique to determine the metallicities of the M-type stars.

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arXiv2014

Prospecting in late-type dwarfs: a calibration of infrared and visible spectroscopic metallicities of late-K and M dwarfs spanning 1.5 dex

Knowledge of late K and M dwarf metallicities can be used to guide planet searches and constrain planet formation models. However, the determination of metallicities of late-type stars is difficult because visible wavelength spectra of their cool atmospheres contain many overlapping absorption lines, preventing the measurement of equivalent widths. We present new methods, and improved calibrations of existing methods, to determine metallicities of late-K and M dwarfs from moderate resolution (1300 < R < 2000) visible and infrared spectra. We select a sample of 112 wide binary systems that contain a late-type companion to a solar-type primary star. Our sample includes 62 primary stars with previously published metallicities, as well as 50 stars with metallicities determined from our own observations. We use our sample to empirically determine which features in the spectrum of the companion are best correlated with the metallicity of the primary. We derive metallicity calibrations for different wavelength ranges, and show that it is possible to get metallicities reliable to < 0.10 dex using either visible, J, H, or K band spectra. Our calibrations are applicable to dwarfs with metallicities of -1.04 < [Fe/H]< +0.56 and spectral types from K7 to M5. Lastly, we use our sample of wide binaries to test and refine existing calibrations to determine M dwarf metallicities. We find that the zeta parameter, which measures the ratio of TiO can CaH bands, is correlated with [Fe/H] for super-solar metallicities, and zeta does not always correctly identify metal-poor M dwarfs. We also find that existing calibrations in the K and H band are quite reliable for stars with [Fe/H] > -0.5, but are less useful for more metal-poor stars.

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arXiv2014

The First Hypervelocity Star from the LAMOST Survey

We report the first hypervelocity star (HVS) discovered from the LAMOST spectroscopic survey. It is a B-type star with a heliocentric radial velocity about 620 km/s, which projects to a Galactocentric radial velocity component of ~477 km/s. With a heliocentric distance of ~13 kpc and an apparent magnitude of ~13 mag, it is the nearest bright HVS currently known. With a mass of ~9Msun, it is one of the three most massive HVSs discovered so far. The star is clustered on the sky with many other known HVSs, with the position suggesting a possible connection to Galactic center structures. With the current poorly-determined proper motion, a Galactic center origin of this HVS remains consistent with the data at the 1sigma level, while a disk run-away origin cannot be excluded. We discuss the potential of the LAMOST survey to discover a large statistical sample of HVSs of different types.

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arXiv2013

Nearby Planetary Systems As Lenses During Predicted Close Passages to Background Stars

The Einstein rings and proper motions of nearby stars tend to be large. Thus, every year some foreground stars within a few hundred parsecs of Earth induce gravitational lensing events in background stars. In some of these cases, the events may exhibit evidence of planets orbiting the nearby star. In fact, planets can even be discovered during relatively distant passages. Here, we study the lensing signatures associated with planets orbiting nearby high-proper-motion stars. We find the following. (1) Wide-orbit planets can be detected for all distances of closest approach between the foreground and background stars, potentially producing independent events long before and/or after the closest approach. (2) Close-orbit planets can be detected for intermediate distances of closest approach, producing quasiperiodic signatures that may occur days or weeks before and after the stellar-lens event. (3) Planets in the so-called 'zone for resonant lensing' can significantly increase the magnification when the distance of closest approach is small, making the stellar-lens event easier to detect, while simultaneously providing evidence for planets. Because approaches close enough to allow planets to be detected can be predicted, we can plan observing strategies to take advantage of the theoretical framework built in this paper, which describes the sequence of expected effects in terms of a sequence of detection regimes.

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arXiv2013

A Spectroscopic Catalog of the Brightest (J<9) M Dwarfs in the Northern Sky

We present a spectroscopic catalog of the 1,564 brightest (J<9) M dwarf candidates in the northern sky, as selected from the SUPERBLINK proper motion catalog. Observations confirm 1,408 of the candidates to be late-K and M dwarfs with spectral subtypes K7-M6. From the low μ>40 mas/yr proper motion limit and high level of completeness of the SUPERBLINK catalog in that magnitude range, we estimate that our spectroscopic census most likely includes >90% of all existing, northern-sky M dwarfs with apparent magnitude J<9. Only 682 stars in our sample are listed in the Third Catalog of Nearby Stars (CNS3); most others are relative unknowns and have spectroscopic data presented here for the first time. Spectral subtypes are assigned based on spectral index measurements of CaH and TiO molecular bands. A comparison of spectra from the same stars obtained at different observatories however reveals that spectral band index measurements are dependent on spectral resolution, spectrophotometric calibration, and other instrumental factors. After systematic corrections and a recalibration of the subtype-index relationships, we find that we can consistently and reliably classify all our stars to a half-subtype precision. The use of corrected spectral indices further requires us to recalibrate the ζparameter, from which we estimate metallicities +/-0.5dex accuracy in dwarfs of subtypes M2 and earlier, and +/-0.2dex accuracy for later (M3-M5) subtypes. Existing geometric parallax measurements are extracted from the literature for 624 stars, and are used to determine spectroscopic and photometric distances for all the other stars. Our catalog will be most useful to guide the selection of the best M dwarf targets for exoplanet searches, in particular those using high-precision radial velocity measurements.

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arXiv2012

Likely Members of the Beta Pictoris and AB Doradus Moving Groups in the North

We present first results from follow-up of targets in the northern hemisphere Beta Pictoris and AB Doradus moving group candidate list of Schlieder, Lepine, and Simon (2012). We obtained high-resolution, near-infrared spectra of 27 candidate members to measure their radial velocities and confirm consistent group kinematics. We identify 15 candidates with consistent predicted and measured radial velocities, perform analyses of their 6-dimensional (U,V,W,X,Y,Z) Galactic kinematics, and compare to known group member distributions. Based on these analyses, we propose that 7 Beta Pic and 8 AB Dor candidates are likely new group members. Four of the likely new Beta Pic stars are binaries; one a double lined spectroscopic system. Three of the proposed AB Dor stars are binaries. Counting all binary components, we propose 22 likely members of these young, moving groups. The majority of the proposed members are M2 to M5 dwarfs, the earliest being of type K2. We also present preliminary parameters for the two new spectroscopic binaries identified in the data, the proposed Beta Pic member and a rejected Beta Pic candidate. Our candidate selection and follow-up has thus far identified more than 40 low-mass, likely members of these two moving groups. These stars provide a new sample of nearby, young targets for studies of local star formation, disks and exoplanets via direct imaging, and astrophysics in the low-mass regime.

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arXiv2012

Wide companions to Hipparcos stars within 67 pc of the Sun

A catalog of common-proper-motion (CPM) companions to stars within 67 pc of the Sun is constructed based on the SUPERBLINK proper-motion survey. It contains 1392 CPM pairs with angular separations 30" < ρ< 1800", relative proper motion between the two components less than 25 mas/yr, magnitudes and colors of the secondaries consistent with those of dwarfs in the (M_V,V-J) diagram. In addition, we list 21 candidate white-dwarf CPM companions with separations under 300", about half of which should be physical. We estimate a 0.31 fraction of pairs with red-dwarf companions to be physical systems (about 425 objects), while the rest (mostly wide pairs) are chance alignments. For each candidate companion, the probability of a physical association is evaluated. The distribution of projected separations s of the physical pairs between 2 kAU and 64 kAU follows f(s) ~ s^{-1.5}, which decreases faster than Öpik's law. We find that Solar-mass dwarfs have no less than 4.4% +/- 0.3% companions with separations larger than 2 kAU, or 3.8% +/- 0.3% per decade of orbital separation in the 2 to 16 kAU range. The distribution of mass ratio of those wide companions is approximately uniform in the 0.1<q<1.0 range, although we observe a dip at q=0.5 which, if confirmed, could be evidence of bimodal distribution of companion masses. New physical CPM companions to two exoplanet host stars are discovered.

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arXiv2012

Discovery of a Very Low Mass Triple with Late-M and T Dwarf Components: LP 704-48/SDSS J0006-0852AB

We report the identification of the M9 dwarf SDSS J000649.16-085246.3 as a spectral binary and radial velocity variable with components straddling the hydrogen burning mass limit. Low-resolution near-infrared spectroscopy reveals spectral features indicative of a T dwarf companion, and spectral template fitting yields component types of M8.5\pm0.5 and T5\pm1. High-resolution near-infrared spectroscopy with Keck/NIRSPEC reveals pronounced radial velocity variations with a semi-amplitude of 8.2\pm0.4 km/s. From these we determine an orbital period of 147.6\pm1.5 days and eccentricity of 0.10\pm0.07, making SDSS J0006-0852AB the third tightest very low mass binary known. This system is also found to have a common proper motion companion, the inactive M7 dwarf LP 704-48, at a projected separation of 820\pm120 AU. The lack of H-alpha emission in both M dwarf components indicates that this system is relatively old, as confirmed by evolutionary model analysis of the tight binary. LP 704-48/SDSS J0006-0852AB is the lowest-mass confirmed triple identified to date, and one of only seven candidate and confirmed triples with total masses below 0.3 Msun currently known. We show that current star and brown dwarf formation models cannot produce triple systems like LP 704-48/SDSS J0006-0852AB, and we rule out Kozai-Lidov perturbations and tidal circularization as a viable mechanism to shrink the inner orbit. The similarities between this system and the recently uncovered low-mass eclipsing triples NLTT 41135AB/41136 and LHS 6343ABC suggest that substellar tertiaries may be common in wide M dwarf pairs.

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arXiv2012

The LEGUE Input Catalogue for Dark Night Observing in the LAMOST Pilot Survey

We outline the design of the dark nights portion of the LAMOST Pilot Survey, which began observations in October 2011. In particular, we focus on Milky Way stellar candidates that are targeted for the LEGUE (LAMOST Experiment for Galactic Understanding and Exploration) survey. We discuss the regions of sky in which spectroscopic candidates were selected, and the motivations for selecting each of these sky areas. Some limitations due to the unique design of the telescope are discussed, including the requirement that a bright (V < 8) star be placed at the center of each plate for wavefront sensing and active optics corrections. The target selection categories and scientific goals motivating them are briefly discussed, followed by a detailed overview of how these selection functions were realized. We illustrate the difference between the overall input catalog - Sloan Digital Sky Survey (SDSS) photometry - and the final targets selected for LAMOST observation.

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arXiv2012

The LEGUE High Latitude Bright Survey Design for the LAMOST Pilot Survey

We describe the footprint and input catalog for bright nights in the LAMOST Pilot Survey, which began in October 2011. Targets are selected from two stripes in the north and south Galactic Cap regions, centered at $α$= 29$^\circ$, with 10$^\circ$ width in declination, covering right ascension of 135$^\circ-290^\circ$ and -30$^\circ$ to 30$^\circ$ respectively. We selected spectroscopic targets from a combination of the SDSS and 2MASS point source catalogs. The catalog of stars defining the field centers (as required by the Shack-Hartmann wavefront sensor at the center of the LAMOST field) consists of all V < 8m stars from the Hipparcos catalog. We employ a statistical selection algorithm that assigns priorities to targets based on their positions in multidimensional color/magnitude space. This scheme overemphasizes rare objects and de-emphasizes more populated regions of magnitude and color phase space, while ensuring a smooth, well-understood selection function. A demonstration of plate design is presented based on the Shack-Hartmann star catalog and an input catalog that was generated by our target selection routines.

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arXiv2012

An Algorithm for Preferential Selection of Spectroscopic Targets in LEGUE

We describe a general target selection algorithm that is applicable to any survey in which the number of available candidates is much larger than the number of objects to be observed. This routine aims to achieve a balance between a smoothly-varying, well-understood selection function and the desire to preferentially select certain types of targets. Some target-selection examples are shown that illustrate different possibilities of emphasis functions. Although it is generally applicable, the algorithm was developed specifically for the LAMOST Experiment for Galactic Understanding and Exploration (LEGUE) survey that will be carried out using the Chinese Guo Shou Jing Telescope. In particular, this algorithm was designed for the portion of LEGUE targeting the Galactic halo, in which we attempt to balance a variety of science goals that require stars at fainter magnitudes than can be completely sampled by LAMOST. This algorithm has been implemented for the halo portion of the LAMOST pilot survey, which began in October 2011.

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arXiv2012

On the detectability of a predicted mesolensing event associated with the high proper motion star VB 10

Extrapolation of the astrometric motion of the nearby low-mass star VB 10 indicates that sometime in late December 2011 or during the first 2-3 months of 2012, the star will make a close approach to a background point source. Based on astrometric uncertainties, we estimate a 1 in 2 chance that the distance of closest approach ρ_{min} will be less than 100mas, a 1 in 5 chance that ρ_{min}<50mas, and a 1 in 10 chance that ρ_{min}<20mas. The latter would result in a microlensing event with a 6% magnification in the light from the background source, and an astrometric shift of 3.3 mas. The lensing signal will however be significantly diluted by the light from VB 10, which is 1.5mag brighter than the background source in B band, 5 mag brighter in I band, and 10 mag brighter in K, making the event undetectable in all but the bluer optical bands. However, we show that if VB 10 happens to harbor a \sim 1 M_J planet on a moderately wide (\approx 0.18AU-0.84AU) orbit, there is a chance (1% to more than 10%, depending on the distance of closest approach and orbital period and inclination) that a passage of the planet closer to the background source will result in a secondary event of higher magnification. The detection of secondary events could be made possible with a several-times-per-night multi-site monitoring campaign.

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arXiv2012

Searching for Planets During Predicted Mesolensing Events: II. PLAN-IT: An Observing Program and its Application to VB 10

The successful prediction of lensing events is a new and exciting enterprise that provides opportunities to discover and study planetary systems. The companion paper investigates the underlying theory. This paper is devoted to outlining the components of observing programs that can discover planets orbiting stars predicted to make a close approach to a background star. If the time and distance of closest approach can be well predicted, then the system can be targeted for individual study. In most cases, however, the predictions will be imprecise, yielding only a set of probable paths of approach and event times. We must monitor an ensemble of such systems to ensure discovery, a strategy possible with observing programs similar to a number of current surveys, including PTF and Pan-STARRS; nova searches, including those conducted by amateurs; ongoing lensing programs such as MOA and OGLE; as well as MEarth, Kepler and other transit studies. If well designed, the monitoring programs will be guaranteed to either discover planets in orbits with semi-major axes smaller than about two Einstein radii, or else to rule out their presence. Planets on wider orbits may not all be discovered, but if they are common, will be found among the events generated by ensembles of potential lenses. We consider the implications for VB 10, the first star to make a predicted approach to a background star that is close enough to allow planets to be discovered. VB 10 is not an ideal case, but it is well worth studying. A more concise summary of this work, and information for observers can be found at https://www.cfa.harvard.edu/~jmatthews/vb10.html.

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arXiv2012

The Na 8200 Angstrom Doublet as an Age Indicator in Low-Mass Stars

We investigate the use of the gravity sensitive neutral sodium (NaI) doublet at 8183 Angstroms 8195 Angstroms (Na 8200 Angstrom doublet) as an age indicator for M dwarfs. We measured the Na doublet equivalent width (EW) in giants, old dwarfs, young dwarfs, and candidate members of the Beta Pic moving group using medium resolution spectra. Our Na 8200 Angstrom doublet EW analysis shows that the feature is useful as an approximate age indicator in M-type dwarfs with (V-K_s) >= 5.0, reliably distinguishing stars older and younger than 100 Myr. A simple derivation of the dependence of the Na EW on temperature and gravity supports the observational results. An analysis of the effects of metallicity show that this youth indicator is best used on samples with similar metallicity. The age estimation technique presented here becomes useful in a mass regime where traditional youth indicators are increasingly less reliable, is applicable to other alkali lines, and will help identify new-low mass members in other young clusters and associations.

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arXiv2011

On the Nature of Small Planets around the Coolest Kepler Stars

We constrain the densities of Earth- to Neptune-size planets around very cool (Te =3660-4660K) Kepler stars by comparing 1202 Keck/HIRES radial velocity measurements of 150 nearby stars to a model based on Kepler candidate planet radii and a power-law mass-radius relation. Our analysis is based on the presumption that the planet populations around the two sets of stars are the same. The model can reproduce the observed distribution of radial velocity variation over a range of parameter values, but, for the expected level of Doppler systematic error, the highest Kolmogorov-Smirnov probabilities occur for a power-law index alpha ~ 4, indicating that rocky-metal planets dominate the planet population in this size range. A single population of gas-rich, low-density planets with alpha = 2 is ruled out unless our Doppler errors are >= 5m/s, i.e., much larger than expected based on observations and stellar chromospheric emission. If small planets are a mix of gamma rocky planets (alpha = 3.85) and 1-gamma gas-rich planets (alpha = 2), then gamma > 0.5 unless Doppler errors are >=4 m/s. Our comparison also suggests that Kepler's detection efficiency relative to ideal calculations is less than unity. One possible source of incompleteness is target stars that are misclassified subgiants or giants, for which the transits of small planets would be impossible to detect. Our results are robust to systematic effects, and plausible errors in the estimated radii of Kepler stars have only moderate impact.

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arXiv2011

M2K: II. A Triple-Planet System Orbiting HIP 57274

Doppler observations from Keck Observatory have revealed a triple planet system orbiting the nearby mid-type K dwarf, HIP 57274. The inner planet, HIP 57274b, is a super-Earth with \msini\ = 11.6 \mearth (0.036 \mjup), an orbital period of 8.135 $\pm$ 0.004 d, and slightly eccentric orbit $e=0.19 \pm 0.1$. We calculate a transit probability of 6.5% for the inner planet. The second planet has \msini\ = 0.4 \mjup\ with an orbital period of 32.0 $\pm 0.02$ d in a nearly circular orbit, and $e = 0.05 \pm 0.03$. The third planet has \msini\ = 0.53 \mjup\ with an orbital period of 432 $\pm 8$ d (1.18 years) and an eccentricity $e = 0.23 \pm 0.03$. This discovery adds to the number of super Earth mass planets with $\msini < 12 \mearth\ $ that have been detected with Doppler surveys. We find that 56 $\pm 18$% super-Earths are members of multi-planet systems. This is certainly a lower limit because of observational detectability limits, yet significantly higher than the fraction of Jupiter mass exoplanets, $20 \pm 8$%, that are members of Doppler-detected, multi-planet systems.

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arXiv2011

Eclipses During the 2010 Eruption of the Recurrent Nova U Scorpii

The eruption of the recurrent nova U Scorpii on 28 January 2010 is now the all-time best observed nova event. We report 36,776 magnitudes throughout its 67 day eruption, for an average of one measure every 2.6 minutes. This unique and unprecedented coverage is the first time that a nova has any substantial amount of fast photometry. With this, two new phenomena have been discovered: the fast flares in the early light curve seen from days 9-15 (which have no proposed explanation) and the optical dips seen out of eclipse from days 41-61 (likely caused by raised rims of the accretion disk occulting the bright inner regions of the disk as seen over specific orbital phases). The expanding shell and wind cleared enough from days 12-15 so that the inner binary system became visible, resulting in the sudden onset of eclipses and the turn-on of the supersoft X-ray source. On day 15, a strong asymmetry in the out-of-eclipse light points to the existence of the accretion stream. The normal optical flickering restarts on day 24.5. For days 15-26, eclipse mapping shows that the optical source is spherically symmetric with a radius of 4.1 R_sun. For days 26-41, the optical light is coming from a rim-bright disk of radius 3.4 R_sun. For days 41-67, the optical source is a center-bright disk of radius 2.2 R_sun. Throughout the eruption, the colors remain essentially constant. We present 12 eclipse times during eruption plus five just after the eruption.

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arXiv2011

Spectroscopic confirmation of UV-bright white dwarfs from the Sandage Two-Color Survey of the Galactic Plane

We present spectroscopic observations confirming the identification of hot white dwarfs among UV-bright sources from the Sandage Two-color Survey of the Galactic plane and listed in the Lanning (Lan) catalog of such sources. A subsample of 213 UV bright Lan sources have been identified as candidate white dwarfs based on the detection of a significant proper motion. Spectroscopic observations of 46 candidates with the KPNO 2.1m telescope confirm 30 sources to be hydrogen white dwarfs with subtypes in the DA1-DA6 range, and with one of the stars (Lan 161) having an unresolved M dwarf as a companion. Five more sources are confirmed to be helium white dwarfs, with subtypes from DB3 to DB6. One source (Lan 364) is identified as a DZ 3 white dwarf, with strong lines of calcium. Three more stars are found to have featureless spectra (to within detection limits), and are thus classified as DC white dwarfs. In addition, three sources are found to be hot subdwarfs: Lan 20 and Lan 480 are classified as sdOB, and Lan 432 is classified sdB. The remaining four objects are found to be field F star interlopers. Physical parameters of the DA and DB white dwarfs are derived from model fits.

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