We present a Mass-Luminosity Relation (MLR) for red dwarfs spanning a range of masses from 0.62 M to the end of the stellar main sequence at 0.08 M . The relation is based on 47 stars for which dynamical masses have been determined, primarily using astrometric data from Fine Guidance Sensors (FGS) 3 and 1r, white-light interferometers on the Hubble Space Telescope (HST), and radial velocity data from McDonald Observatory. For our HST/FGS sample of 15 binaries, component mass errors range from 0.4% to 4.0% with a median error of 1.8%. With these and masses from other sources, we construct a V -band MLR for the lower main sequence with 47 stars, and a K-band MLR with 45 stars with fit residuals half of those of the V -band.We use GJ 831 AB as an example, obtaining an absolute trigonometric parallax, π abs = 125.3 ± 0.3 milliseconds of arc, with orbital elements yielding M A = 0.270 ± 0.004M and M B = 0.145 ± 0.002M . The mass precision rivals that derived for eclipsing binaries.

The Kuiper belt is a remnant of the primordial Solar System. Measurements of its size distribution constrain its accretion and collisional history, and the importance of material strength of Kuiper belt objects (KBOs)(1; 2; 3; 4). Small, sub-km sized, KBOs elude direct detection, but the signature of their occultations of background stars should be detectable (5; 6; 7; 8; 9). Observations at both optical(10) and Xray(11) wavelengths claim to have detected such occultations, but their implied KBO abundances are inconsistent with each other and far exceed theoretical expectations. Here, we report an analysis of archival data that reveals an occultation by a body with a ∼500 m radius at a distance of 45 AU. The probability of this event to occur due to random statistical fluctuations within our data set is about 2%. Our survey yields a surface density of KBOs with radii larger than 250 m of 2.1 +4.8 −1.7 × 10 7 deg −2 , ruling out inferred surface densities from previous claimed detections by more than 5 σ. The fact that we detected only one event, firmly shows a deficit of sub-km sized KBOs compared to a population extrapolated from objects with r > 50 km. This implies that sub-km-sized KBOs are undergoing collisional erosion, just like debris disks observed around other stars.A small KBO crossing the line of sight to a star will partially obscure the stellar light, an event which can be detected in the star's light curve. For visible light, the characteristic scale of diffraction effects, known as the Fresnel scale, is given by (λa/2) 1/2 ∼ 1.3 km, where a ∼ 40 AU is the distance to the Kuiper belt and λ ∼ 600 nm is the wavelength of our observations. Diffraction effects will be apparent in the star's light curve due to occulting KBOs provided that both star and the occulting object are smaller than the Fresnel scale (12; 13). Occultations by objects smaller than the Fresnel scale are in the Fraunhofer regime. In this regime the diffraction pattern is determined by the size of the KBO and its distance -2 -to the observer, the angular size of the star, the wavelength range of the observations and the impact parameter between the star and the KBO (see Supplementary Information for details). The duration of the occultation is approximately given by the ratio of the Fresnel scale to the relative velocity perpendicular to the line of sight between the observer and the KBO. Since the relative velocity is usually dominated by the Earth's velocity around the Sun, which is 30 km s −1 , typical occultations only last of order of a tenth of a second.Extensive ground based efforts have been conducted to look for optical occultations (10; 9; 14; 15). To date, these visible searches have announced no detections in the region of the Kuiper belt (30-60 AU), but one of these quests claims to have detected some events beyond 100 AU and at about 15 AU (10). Unfortunately, ground based surveys may suffer from a high rate of false-positives due to atmospheric scintillation, and lack the stability of space based platforms. The ground ...

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We present absolute trigonometric parallaxes and relative proper motions for three members of the Pleiades, obtained with the Hubble Space Telescope's Fine Guidance Sensor 1r, a white-light interferometer. We estimate spectral types and luminosity classes of the stars comprising the astrometric reference frame from R % 2000 spectra, VJHK photometry, and reduced proper motions. From these we derive estimates of absolute parallaxes and introduce them into our model as observations with error. We constrain the three cluster members to have a 1 dispersion in distance less than 6.4 pc and find an average abs ¼ 7:43 AE 0:17 AE 0:20 mas, where the second error is systematic due to member placement within the cluster. This parallax corresponds to a distance of 134:6 AE 3:1 pc or a distance modulus of m À M ¼ 5:65 AE 0:05 for these three Pleiades stars, presuming a central location. This result agrees with three other independent determinations of the Pleiades distance. Presuming that the cluster depth systematic error can be significantly reduced because of the random placement of these many members within the cluster, these four independent measures yield a best-estimate Pleiades distance of abs ¼ 7:49 AE 0:07 mas, corresponding to a distance of 133:5 AE 1:2 pc or a distance modulus of m À M ¼ 5:63 AE 0:02. This resolves the dispute between the main-sequence fitting and the Hipparcos distance moduli in favor of main-sequence fitting. Key words: astrometry -distance scale -open clusters and associations: general -stars: distancestechniques: interferometric 1. THE PROBLEM Our knowledge of the life histories of stars relies on models whose fidelity is ultimately tested by appeal to real stars. The Sun provides the most basic calibration of these models, of course, because it is only for the Sun that an accurate age exists and for which the mass, temperature, composition, and structure are known with precision, accuracy, and completeness. Clusters of stars are also fundamental for constructing models because we can assume that all the cluster's members are of the same age and composition, even if other parameters are more loosely constrained.Preeminent among clusters is the Pleiades, and much effort has gone into determining the absolute parallax of this cluster. ESA's Hipparcos mission brought the benefits of space observing to astrometry to produce precise positions, proper motions, and parallaxes for nearly all stars brighter than V % 9. Before Hipparcos, the distance to the Pleiades was too large for ground-based parallaxes to yield a good distance, so the best estimates were derived by comparing the main sequence of the Pleiades with a main sequence constructed from nearby stars with large parallaxes. A small correction for evolution is necessary ( the Pleiades is about 100 Myr old [ Pinsonneault et al. 1998], whereas the nearby field stars are typically as old as the Sun), but the Pleiades appears to have essentially the same elemental abundances as the Sun ( Boesgaard & Friel 1990), obviating a need for a me...

We identified 46 unresolved source candidates in the Hubble Ultra Deep Field, down to i775 = 29.5. Unresolved objects were identified using a parameter S, which measures the deviation from the curve-of-growth of a point source. Extensive testing of this parameter was carried out, including the effects of decreasing signal-to-noise and of the apparent motions of stars, which demonstrated that stars brighter than i775 = 27.0 could be robustly identified. Low resolution grism spectra of the 28 objects brighter than i775 = 27.0 identify 18 M and later stellar type dwarfs, 2 candidate L-dwarfs, 2 QSOs, and 4 white dwarfs. Using the observed population of dwarfs with spectral type M4 or later, we derive a Galactic disk scale height of 400 \pm 100 pc for M and L stars. The local white dwarf density is computed to be as high as (1.1 \pm 0.3) x10^(-2) stars/pc^3. Based on observations taken 73 days apart, we determined that no object in the field has a proper motion larger than 0.027"/year (3 sigma detection limit). No high velocity white dwarfs were identified in the HUDF, and all four candidates appear more likely to be part of the Galactic thick disk. The lack of detected halo white dwarfs implies that, if the dark matter halo is 12 Gyr old, white dwarfs account for less than 10% of the dark matter halo mass.Comment: 35 pages, 11 figures, accepted by Ap