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This page provides links to preprints and published papers resulting from the various projects being undertaken by our NStars group.
Abstract: We present observations of 2MASSI J1835379+325954, a previously unrecognised late-type M dwarf within 6 parsecs of the Sun. Identified based analysis of the Two-Micron All Sky Survey, optical spectroscopy and photometry indicate a spectral type of M8.5. The star has a proper motion of 0.759 \pma and is clearly visible on both POSS I and POSS II photographic plate material, but may have escaped detection in previous surveys through its proximity to the Galactic Plane. We discuss potential implications for the completeness of the local stellar census.
A2. Reid, I.N., Gizis, J.E., Hawley, S.L. 2002, Astr. J., 124, 2721 Abstract:
We have used new astrometric and spectroscopic observations to refine
the volume-complete sample of M dwarfs defined in previous
papers in this series. With the addition of Hipparcos astrometry, our revised
VC2 sample includes 558 main-sequence stars in 448 systems. Analysis of that dataset shows
no evidence for any systematic kinematic bias. Combining those data with an
Hipparcos-based sample of AFGK dwarfs within 25 parsecs of the Sun, we have derived
the Solar Neighbourhood luminosity function, Phi(MV), for stars with absolute magnitudes
between -1 and +17. Using empirical and semi-empirical mass-MV relations, we
transform $\Phi$(M$_V$) to the present-day mass function, psi(M) (dN\dM). Depending on the
mass-luminosity calibration adopted, psi(M) can be represented by either a two-component
or three-component power-law. In either case, the power-law
index, alpha, has a value of ~1.3 at low masses (0.1 < M < 0.7, in solar masses),
and the local mass density of main sequence stars is ~0.031 MSun pc-3.
We have converted psi(M) to an estimate of the initial mass function, Psi(M),
by allowing for stellar evolution, the density law perpendicular to the Plane and
the local mix of stellar populations.
The results give alpha = 1.1 to 1.3 at low masses, and alpha = 2.5 to 2.8 at high masses,
with the change in slope lying between 0.7 and 1.1 MSun.
Finally, the (U, W) velocity distributions of both the VC2 sample and
the fainter (MV > 4) stars in the Hipparcos 25-pc sample are
well-represented by two-component
Gaussian distributions, with ~10% of the stars in the higher
velocity-dispersion component. We suggest that the latter component
is the thick disk, and offer a possible
explanation for the relatively low velocity dispersions shown by
ultracool dwarfs.
Abstract: 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-sigma). 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.
A4. I. N. Reid 2003, Astr. J., in pressAbstract: Surveys for nearby low-luminosity dwarfs tend to avoid the crowded regions of the Galactic Plane. We have devised near-infrared colour-magnitude and colour-colour selection criteria designed to identify late-type M and early-type L dwarfs within 12 parsecs of the Sun. We use those criteria to search for candidates within the regions of the Galactic Plane (|b| < 10^o) covered by the Second Incremental Release of data from the Two-Micron All Sky Survey. Detailed inspection of the available photographic images of the resulting 1299 candidates confirms only two as ultracool dwarfs. Both are known proper motion stars, identified in the recent survey by Lepine et al (2002). Despite the low numbers, the inferred surface density is consistent with comparable surveys at higher latitudes. We discuss the implications for the luminosity function, and consider means of improving the efficiency and scope of photometric surveys in the Plane.
Abstract: 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 mr 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 (MV, (V-K)), (MV, (V-I)) and (MI, (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.
B2. Reid, I.N., Kilkenny, D., Cruz, K.L. 2002, Astr. J., 123, 2822:Abstract: 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 mr 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.
B3. Cruz, K.L., Reid, I.N. 2002, Astr. J., 123, 2828:Abstract: We present initial results of an all sky search for late-type dwarfs within 20 parsecs of the Sun using the New Luyten Two-Tenths (NLTT) catalogue cross-referenced with the 2-Micron All Sky Survey (2MASS) database. The results presented here were obtained with low-resolution optical spectroscopic follow-up of candidate nearby-stars as a preliminary test of our methodology. MJ, 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 parsecs of the Sun and up to 19 of these are within 20 parsecs. One, LP647-13 is an M9-type dwarf at 10.5 parsecs making it one of the three closest M9 dwarfs currently known. We also discuss the chromospheric activity of the observed dwarfs.
B4. Lowrance, P.J., Kirkpatrick, J.D., Reid, I.N., Cruz, K.L., Liebert, J. 2003, Astrophys. J., 584, L95Abstract: 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.
B5. I. N. Reid, Cruz, K.L., Allen, P., Mungall, F., Kilkenny, D., Liebert, J., Hawley, S.L., Fraser, O.J., Covey, K.R., Lowrance, P.J. 2003, Astr. J., in pressAbstract: We present low-resolution optical spectroscopy and BVRI photometry of 453 candidate nearby stars drawn from the NLTT proper motion catalogue. The stars were selected based on optical/near-infrared colours, derived by combining the NLTT photographic data with photometry from the 2MASS Second Incremental Data Release. Based on the derived photometric and spectroscopic parallaxes, we identify 111 stars as lying within 20 parsecs of the Sun, including 9 stars with formal distance estimates of less than 10 parsecs. A further 53 stars have distance estimates within 1-sigma of our 20-parsec limit. Almost all of those stars are additions to the nearby star census. In total, our NLTT-based survey has so far identified 496 stars likely to be within 20 parsecs, of which 195 are additions to nearby-star catalogues. Most of the newly-identified nearby stars have spectral types between M4 and M8.
Abstract: 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), is itself 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.
C2. Lowrance, P.J., Kirkpatrick, J.D., Beichman, C.A. 2002, Astrophys. J., 572, L79Abstract: Upsilon Andromedae is an F8 V star known to have an extrasolar system of at least three planets in orbit around it. Here we report the discovery of a low-mass stellar companion to this system. The companion shares common proper motion, lies at a projected separation of 750 AU, and has a spectral type of M4.5 V. The effect of this star on the radial velocity of the brighter primary is negligible, but this system provides an interesting test bed for stellar planetary formation theory and understanding dynamical stability since it is the first multiple planetary system known in a multiple stellar system.
Abstract: ime series spectrophotometry of the L5 dwarf 2MASS J01443536-0716142 showed strong Hα emission that declined by nearly 75% in four consecutive exposures. The line was not detected in emission on a spectrum obtained 11 months later. This behavior contrasts with that of 2MASSI J1315309-264951, an L5 dwarf that has shown even stronger Hα emission on four separate occasions. The observational database suggests that L dwarfs can be found in such strong flares only occasionally, with a duty cycle on the order of 1%. In contrast, the few, continuously strong Hα emitters, including PC 0025+0447 and 2MASSI J1237392+652615, must either be (1) objects no older than 10-100 Myr 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 but unknown mechanism for the Hα energy.
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