arXiv subjects

I. Neill Reid

Publications and source records attributed to I. Neill Reid.

At least 73 records · Page 4Linked to original sources

arXiv2003

Meeting the Cool New Neighbors V: A 2MASS-Selected Sample of Ultracool Dwarfs

We present initial results of our effort to create a statistically robust, volume-limited sample of ultracool dwarfs from the 2MASS Second Incremental Data Release. We are engaged in a multifaceted search for nearby late-type dwarfs and this is the first installment of our search using purely photometric selection. The goal of this work is a determination of the low-mass star and brown dwarf luminosity function in the infrared. Here, we outline the construction of the sample, dubbed 2MU2, and present our first results, including the discovery of 186 M7--L6 dwarfs--47 of these are likely to be within 20 pc of the Sun. These results represent 66% of the ultracool candidates in our sample yet constitute an 127% increase in the number of ultracool dwarfs known within the volume searched (covering 40% of the sky out to 20 pc). In addition, we have identified 10 M4--M6.5 objects that are likely to be within 20 pc (or within 1$σ$). Finally, based on these initial data, we present a preliminary luminosity function and discuss several interesting features of the partial sample presented here. Once our sample is complete, we will use our measured luminosity function to constrain the mass function of low-mass stars and brown dwarfs.

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arXiv2003

The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics

We present the discovery of the first L-type subdwarf, 2MASS J05325346+8246465. This object exhibits enhanced collision-induced H$_2$ absorption, resulting in blue NIR colors ($J-K_s = 0.26{\pm}0.16$). In addition, strong hydride bands in the red optical and NIR, weak TiO absorption, and an optical/J-band spectral morphology similar to the L7 DENIS 0205$-$1159AB imply a cool, metal-deficient atmosphere. We find that 2MASS 0532+8246 has both a high proper motion, $μ$ = 2$\farcs60\pm0\farcs$15 yr$^{-1}$, and a substantial radial velocity, $v_{rad} = -195{\pm}11$ km s$^{-1}$, and its probable proximity to the Sun (d = 10--30 pc) is consistent with halo membership. Comparison to subsolar-metallicity evolutionary models strongly suggests that 2MASS 0532+8246 is substellar, with a mass of 0.077 $\lesssim$ M $\lesssim$ 0.085 M$_{\sun}$ for ages 10--15 Gyr and metallicities $Z = 0.1-0.01$ $Z_{\sun}$. The discovery of this object clearly indicates that star formation occurred below the Hydrogen burning mass limit at early times, consistent with prior results indicating a flat or slightly rising mass function for the lowest-mass stellar subdwarfs. Furthermore, 2MASS 0532+8246 serves as a prototype for a new spectral class of subdwarfs, additional examples of which could be found in NIR proper motion surveys.

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arXiv2003

The Discovery of Two Nearby Carbon Dwarfs

The comparison of optical and 2MASS near-infrared photometry for large samples of catalogued proper motion stars has the potential to discover previously unrecognized nearby objects of rare type. In this paper we present the discovery of two new carbon dwarfs, LSR 2105+2514 and LP 758-43, which were drawn from proper motion lists and which lie in a sparsely populated part of optical/near-IR color-color space. Their optical spectra, exhibiting absorptions by C_2 and/or CN, are discussed. LSR 2105+2514 is believed to lie within 200 pc and would have M_(K_s) < 6.7, making it lower in luminosity than any carbon dwarf with a measured trigonometric parallax. LP 758-43, which is not as red but still probably cooler than the best studied carbon dwarfs, is believed to lie within 360 pc. Using our optical/near-infrared selection technique on published lists of proper motion stars, we hope in the near future to expand further the current sample of carbon dwarfs, which numbers only 31 objects at this writing.

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arXiv2002

Binarity in Brown Dwarfs: T Dwarf Binaries Discovered with the Hubble Space Telescope WPFC2

(abridged) We present the discovery of two T dwarf binaries, 2MASS 1225-2739AB and 2MASS 1534-2952AB, identified in a sample of ten T dwarfs imaged with the Hubble Space Telescope Wide Field Planetary Camera 2. The separations of the two binary systems are 0{\farcs}282$\pm$0{\farcs}005 and 0{\farcs}065$\pm$0{\farcs}007, implying projected separations of 3.17$\pm$0.14 and 1.0$\pm$0.3 AU, respectively. The observed binary fraction of our HST sample, 20$^{+17}_{-7}$%, is consistent with results obtained for late-M and L field dwarfs, and implies a bias-corrected binary fraction of 9$^{+15}_{-4}$% for $a \gtrsim 1$ AU and $q \gtrsim 0.4$, significantly lower than the binary fractions of F--G and early-type M dwarf stars. Neither of the T binaries have separations $a \gtrsim 10$ AU, consistent with results from other brown dwarf binary searches. We conclude that tidal disruption by passing stars or Giant Molecular Clouds, which limits the extent of wide stellar binaries, plays no role in eliminating wide brown dwarf binaries, implying either disruption very early in the formation process (ages $\lesssim 1-10$ Myr) or a formation mechanism which precludes such systems. We find that the maximum binary separation in the brown dwarf regime appears to scale as M$_{total}^2$, a possible clue to the physical mechanism which restricts wide substellar systems.

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arXiv2002

A Flaring L5 Dwarf: The Nature of HαEmission in Very Low Mass (Sub)Stellar Objects

Time series spectrophotometry of the L5 dwarf 2MASS 01443536-0716142 showed strong Hαemission which declined by nearly 75% in four consecutive exposures. The line was not detected in emission on a spectrum obtained eleven months later. This behavior constrasts with that of 2MASSI J1315309-264951, an L5 dwarf which has shown even stronger Hαemission on four separate occasions. The observational database suggests that L dwarfs can be found in such strong flares only occasionally, with a duty cycle of order 1%. In contrast, the few, continuously-strong Hαemitters, including PC 0025+0447 and 2MASSI J1237392+652615, must either be (1) objects no older than 10-100 Myrs with continuously-active accretion and/or chromospheres, but which apparently formed in isolation from known young stellar clusters and associations, or (2) objects empowered by a different and unknown mechanism for the Hαenergy.

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arXiv2002

Meeting the Cool Neighbors III: Spectroscopy of Northern NLTT Stars

We present initial results of an all-sky search for late-type dwarfs within 20 pc of the Sun using the New Luyten Two-Tenths (NLTT) catalog cross-referenced with the 2-Micron All Sky Survey (2MASS) database. The results were obtained with low-resolution optical spectroscopic follow-up of candidate nearby-stars as a preliminary test of our methodology. M_J, derived using spectral indices, and 2MASS J are used to estimate distances. Out of the 70 objects observed, 28 are identified as previously unrecognized objects within 25 pc of the Sun, and up to 19 of these are within 20 pc. One, LP 647-13 is an M9-type dwarf at 10.5 pc making it one of the four closest M9 dwarfs currently known. We also discuss the chromospheric activity of the observed dwarfs.

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arXiv2002

High resolution spectroscopy of ultracool M dwarfs

(abridged) We present high-resolution echelle spectroscopy of a photometricaly-selected sample if thirty-nine dwarfs with spectral types between M6.5 and L0.5. Two stars, 2MASSI 0253202+271333 and 2MASSW 0952219-192431, are double-lined spectroscopic binaries. We have used our observations to search for Li I 6708 A absorption, characteristic of sub-stellar mass; estimate the level of chromospheric activity through measurement of H-alpha emission fluxes; measure rotational velocities via line broadening; and determine radial velocities and Galactic space motions. Two dwarfs have strong lithium absorption, the previously-known brown dwarf, LP 944-20, and 2MASSI J0335020+234235, which we identify as a probable 0.06 M_sun brown dwarf, age ~1 Gyr. We have investigated the prospect of using the observed frequency of lithium absorption amongst ultracool M dwarfs (M7 to M9.5) as a probe of the initial mass function. The available observations are difficult to reconcile with Salpeter-like power-law mass functions (alpha > 2) for masses below 0.1M_Sun. A comparison between the rotational velocities and -alpha fluxes shows no evidence for significant correlation. Velocity dispersions are significantly lower than those measured for nearby M dwarfs, but show remarkable similarity to results for earlier-type emission-line (dMe) dwarfs. The latter are generally assigned ages of less than ~3 Gyrs.

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arXiv2002

Meeting the Cool Neighbours I: Nearby stars in the NLTT Catalogue - Defining the sample

We are currently undertaking a program aimed at identifying previously-unrecognised late-type dwarfs within 20 parsecs of the Sun. As a first step, we have cross-referenced Luyten's NLTT proper motion catalogue against the second incremental release of the 2MASS Point Source Catalogue, and use optical/infrared colours, derived by combining Luytens's m(r) estimates with 2MASS data, to identify candidate nearby stars. This paper describes the definition of a reference sample of 1245 stars, and presents a compilation of literature data for over one-third of the sample. Only 274 stars have trigonometric parallax measurements, but we have used data for nearby stars with well-determined trigonometric parallaxes to compute colour-magnitude relations in the (M(V), (V-K)), (M(V), (V-I)) and (M(I), (I-J)) planes, and use those relations to determine photometric parallaxes for NLTT stars with optical photometry. Based on the 2MASS JHK data alone, we have identified a further 42 ultracool dwarfs ((J-K > 0.99) and use (J-K) colours to estimate photometric parallaxes. Combining these various techniques, we identify 308 stars with formal distances of less than 20 parsecs, while a further 46 have distance estimates within 1-sigma of our survey limit. Of these 354 stars, 75, including 39 of the ultracool dwarfs. are new to nearby star catalogues. Two stars with both optical and near-infrared photometry are potential additions to the immediate Solar Neighbourhood, with formal distance estimates of less than 10 parsecs.

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arXiv2002

Meeting the Cool Neighbours, II: Photometry of southern NLTT stars

We present BVRI photometry of 180 bright, southern nearby-star candidates. The stars were selected from the New Luyten Two-Tenths proper motion catalogue based on optical/infrared colours, constructed by combining Luytens's m(r) estimates with near-infrared photometry from the 2-Micron All Sky Survey. Photometric parallaxes derived from (V-K), (V-I) and (I-J) colours, combined with the limited available astrometry, show that as many as 108 stars may lie within 20 parsecs of the Sun. Of these, 53 are new to nearby star catalogues, including three within 10 parsecs of the Sun.

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arXiv2001

On the Nature of Stars with Planets

We consider the metallicities and kinematics of nearby stars known to have planetary-mass companions in the general context of the overall properties of the local Galactic Disk. We have used Stromgren photometry to determine abundances for both the extrasolar-planet host stars and for a volume-limited sample of 486 F, G and K stars selected from the Hipparcos catalogue. The latter data show that the Sun lies near the modal abundance of the disk, with over 45% of local stars having super-solar metallicities. Twenty of the latter stars (4.1%) are known to have planetary-mass companions. Using that ratio to scale data for the complete sample of planetary host stars, we find that the fraction of stars with extrasolar planets rises sharply with increasing abundance, confirming previous results. However, the frequency remains at the 3-4% level for stars within 0.15 dex of solar abundance, and falls to ~1% only for stars with abundances less than half solar. Given the present observational constraints, both in velocity precision and in the available time baseline, these numbers represent a lower limit to the frequency of extrasolar planetary systems. A comparison between the kinematics of the planetary host stars and a representative sample of disk stars suggests that the former have an average age which is ~60% of the latter.

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arXiv2001

The Spectra of T Dwarfs I: Near-Infrared Data and Spectral Classification

We present near-infrared spectra for a sample of T dwarfs, including eleven new discoveries made using the Two Micron All Sky Survey. These objects are distinguished from warmer (L-type) brown dwarfs by the presence of methane absorption bands in the 1--2.5 $\micron$ spectral region. A first attempt at a near-infrared classification scheme for T dwarfs is made, based on the strengths of CH$_4$ and H$_2$O bands and the shapes of the 1.25, 1.6, and 2.1 $\micron$ flux peaks. Subtypes T1 V through T8 V are defined, and spectral indices useful for classification are presented. The subclasses appear to follow a decreasing T$_{eff}$ scale, based on the evolution of CH$_4$ and H$_2$O bands and the properties of L and T dwarfs with known distances. However, we speculate that this scale is not linear with spectral type for cool dwarfs, due to the settling of dust layers below the photosphere and subsequent rapid evolution of spectral morphology around T$_{eff}$ $\sim$ 1300--1500 K. Similarities in near-infrared colors and continuity of spectral features suggest that the gap between the latest L dwarfs and earliest T dwarfs has been nearly bridged. This argument is strengthened by the possible role of CH$_4$ as a minor absorber shaping the K-band spectra of the latest L dwarfs. Finally, we discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infrared colors (J-K$_s$ = $-0.89\pm$0.24) due to suppression of the 2.1 $\micron$ peak. The feature is likely caused by enhanced collision-induced H$_2$ absorption in a high pressure or low metallicity photosphere.

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arXiv2001

High-velocity white dwarfs: thick disk, not dark matter

We present an alternative interpretation of the nature of the extremely cool, high-velocity white dwarfs identified by Oppenheimer et al (2001) in a high-latitude astrometric survey. We argue that the velocity distribution of the majority of the sample is more consistent with the high-velocity tail of a rotating population, probably the thick disk, rather than a pressure-supported halo system. Indeed, the observed numbers are well matched by predictions based on the kinematics of a complete sample of nearby M dwarfs. Analysing only stars showing retrograde motion gives a local density close to that expected for white dwarfs in the stellar (R^-3.5) halo. Under our interpretation, none of the white dwarfs need be assigned to the dark-matter, heavy halo. However, luminosity functions derived from observations of these stars can set important constraints on the age of the oldest stars in the Galactic Disk.

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arXiv2001

Three newly-discovered M-dwarf companions of Solar Neighbourhood stars

We present low-resolution spectroscopy of newly-discovered candidate companions to three stars in the Solar Neighbourhood. All three companions are M dwarfs, with spectral types ranging from M4 to M9.5. In two cases, G85-55`B' (M6) and G87-9`B' (M4), we have circumstantial evidence from spectroscopy, photometry and limited astrometry that the systems are physical binaries; in the third, G216-7B (M9.5), comparison of POSS II IIIaF plate material and the 2MASS image indicates common proper motion. The primary star in this system, G216-7A (M0), appears itself to be an unresolved, nearly equal-mass binary. All three low-mass companions are highly likely to be stellar in nature, although G216-7B lies very close to the hydrogen-burning limit.

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arXiv2001

Analysis of Keck HIRES spectra of early L-type dwarfs

We present analyses of high resolution and medium resolution spectra of early L dwarfs. We used our latest set of model atmospheres to reproduce and analyze the observed features. We can model the optical flux and the atomic line profiles with the best accuracy to date. The models used to reproduce the observations include dust condensation and dust opacities. Compared to previous studies using older models we find that our dust treatment is much improved. The derived parameters for the objects are well in the expected range for old very low mass objects. This is also supported by the absence of Li in most of the objects. For the objects showing Li we can be almost certain that those are brown dwarfs. However, a spectral analysis in general, and this one in particular can only very roughly determine mass and age.

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arXiv2001

Substellar Companions to Main Sequence Stars: No Brown Dwarf Desert at Wide Separations

We use three field L and T dwarfs which were discovered to be wide companions to known stars by the Two Micron All-Sky Survey (2MASS) to derive a preliminary brown dwarf companion frequency. Observed L and T dwarfs indicate that brown dwarfs are not unusually rare as wide (Delta >1000 A.U.) systems to F-M0 main-sequence stars (M>0.5M_sun, M_V<9.5), even though they are rare at close separation (Delta <3 A.U.), the ``brown dwarf desert.'' Stellar companions in these separation ranges are equally frequent, but brown dwarfs are >~ 10 times as frequent for wide than close separations. A brown dwarf wide-companion frequency as low as the 0.5% seen in the brown dwarf desert is ruled out by currently-available observations.

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arXiv2001

Brown Dwarf Companions to G-type Stars. I: Gliese 417B and Gliese 584C

We present astrometric and spectroscopic observations confirming that two nearby G dwarf systems (Gliese 417 = BD+36 2162 and Gliese 584AB = eta CrB AB) have a widely separated, L dwarf, substellar companion. Using age estimates of the G dwarf primaries, we estimate masses for these L dwarfs from theoretical evolutionary tracks. For the L4.5 dwarf Gl 417B we estimate an age of 0.08-0.3 Gyr and a mass of 0.035+/-0.015 M_sun. For the L8 dwarf Gl 584C we estimate an age of 1.0-2.5 Gyr and a mass of 0.060+/-0.015 M_sun. This latter object also shows evidence of spectrum variability, which may be due to surface inhomogeneities rotating into and out of view. These new companions are also compared to six other L dwarf and T dwarf companions previously recognized. For the L dwarf companions, ages implied by the presence or absence of lithium are consistent with ages inferred from the primaries alone.

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arXiv2001

Discovery of a Magnetic DZ White Dwarf with Zeeman-Split Lines of Heavy Elements

A spectroscopic survey of previously-unstudied Luyten Half Second proper motion stars has resulted in the discoveries of two new cool magnetic white dwarfs. One (LHS 2273) is a routine DA star, T= 6,500K, with Zeeman-split H alpha and H beta, for which a simple model suggests a polar field strength of 18.5 MG viewed close to equator-on. However, the white dwarf LHS 2534 proves to be the first magnetic DZ showing Zeeman-split Na I and Mg I components, as well as Ca I and Ca II lines for which Zeeman components are blended. The Na I splittings result in a mean surface field strength estimate of 1.92 MG. Apart from the magnetic field, LHS 2534 is one of the most heavily-blanketed and coolest DZ white dwarfs at T ~ 6,000K.

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arXiv2000

A search for previously unrecognised metal-poor subdwarfs in the Hipparcos astrometric catalogue

We have identified 317 stars included in the Hipparcos astrometric catalogue which have parallaxes measured to a precision of better than 15%, and whose location in the (M_V, (B-V)_T) diagram implies a metallicity comparable to or less than that of the intermediate-abundance globular cluster, M5. We have undertaken an extensive literature search to locate Stromgren, Johnson/Cousins and Walraven photometry for over 120 stars. In addition, we present new UBVRI photometry of 201 of these candidate halo stars, together with similar data for a further 14 known metal-poor subdwarfs. Those observations provide the first extensive dataset of RI photometry of metal-poor, main-sequence stars with well-determined trigonometric parallaxes. Finally, we have obtained intermediate-resolution optical spectroscopy of 175 stars. We are able to estimate abundances for 270 stars. The overwhelming majority have near-solar abundance, with their inclusion in the present sample stemming from errors in the colours listed in the Hipparcos catalogue. Only 44 stars show consistent evidence of abundances below [Fe/H]= -1.0. Nine are additions to the small sample of metal-poor subdwarfs with accurate photometry. We consider briefly the implication of these results for cluster main-sequence fitting.

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