The Discovery of an Embedded Cluster of High-Mass Stars Near SGR 1900+14

Deep I-band imaging to approximately I = 26.5 of the soft gamma-ray repeater SGR 1900+14 region has revealed a compact cluster of massive stars located only a few arcseconds from the fading radio source thought to be the location of the SGR (Frail, Kulkarni, & Bloom 1999). This cluster was previously hidden in the glare of the pair of M5 supergiant stars (whose light was removed by PSF subtraction) proposed by Vrba et al. (1996) as likely associated with the SGR 1900+14. The cluster has at least 13 members within a cluster radius of approximately 0.6 pc, based on an estimated distance of 12-15 kpc. It is remarkably similar to a cluster found associated with SGR 1806-20 (Fuchs et al. 1999). That similar clusters have now been found at or near the positions of the two best-studied SGRs suggests that young neutron stars, thought to be responsible for the SGR phenomenon, have their origins in proximate compact clusters of massive stars.Comment: 5 pages, 3 figures, accepted Astrophysical Journal Letter

Parallax and Luminosity Measurements of an L Subdwarf

We present the first parallax and luminosity measurements for an L subdwarf, the sdL7 2MASS J05325346+8246465. Observations conducted over three years by the USNO infrared astrometry program yield an astrometric distance of 26.7+/-1.2 pc and a proper motion of 2.6241+/-0.0018"/yr. Combined with broadband spectral and photometric measurements, we determine a luminosity of log(Lbol/Lsun) = -4.24+/-0.06 and Teff = 1730+/-90 K (the latter assuming an age of 5-10 Gyr), comparable to mid-type L field dwarfs. Comparison of the luminosity of 2MASS J05325346+8246465 to theoretical evolutionary models indicates that its mass is just below the sustained hydrogen burning limit, and is therefore a brown dwarf. Its kinematics indicate a ~110 Myr, retrograde Galactic orbit which is both eccentric (3 <~ R <~ 8.5 kpc) and extends well away from the plane (Delta_Z = +/-2 kpc), consistent with membership in the inner halo population. The relatively bright J-band magnitude of 2MASS J05325346+8246465 implies significantly reduced opacity in the 1.2 micron region, consistent with inhibited condensate formation as previously proposed. Its as yet unknown subsolar metallicity remains the primary limitation in constraining its mass; determination of both parameters would provide a powerful test of interior and evolutionary models for low-mass stars and brown dwarfs.Comment: Accepted to ApJ 10 September 2007; 13 pages, 5 figures, 3 tables, formatted in emulateapj styl

Trigonometric Parallaxes of Central Stars of Planetary Nebulae

Trigonometric parallaxes of 16 nearby planetary nebulae are presented, including reduced errors for seven objects with previous initial results and results for six new objects. The median error in the parallax is 0.42 mas, and twelve nebulae have parallax errors less than 20 percent. The parallax for PHL932 is found here to be smaller than was measured by Hipparcos, and this peculiar object is discussed. Comparisons are made with other distance estimates. The distances determined from these parallaxes tend to be intermediate between some short distance estimates and other long estimates; they are somewhat smaller than estimated from spectra of the central stars. Proper motions and tangential velocities are presented. No astrometric perturbations from unresolved close companions are detected.Comment: 24 pages, includes 4 figures. Accepted for A

Trigonometric Parallaxes for Two Late-Type Subdwarfs: LSR1425+71 (sdM8.0) and the Binary LSR1610-00 (sd?M6pec)

Trigonometric parallax astrometry and BVI photometry are presented for two late-type subdwarf candidates, LSR1425+71 (sdM8.0) and LSR1610-00 (sd?M6pec). For the former we measure an absolute parallax of 13.37+/-0.51 mas yielding Mv=15.25+/-0.09. The astrometry for LSR1610-00 shows that this object is an astrometric binary with a period of 1.66+/-0.01 yr. The photocentric orbit is derived from the data; it has a moderate eccentricity (e ~ 0.44+/-0.02) and a semi-major axis of 0.28+/-0.01 AU based on our measured absolute parallax of 31.02+/-0.26 mas. Our radial velocity measure of -108.1+/-1.6 km/s for LSR1610-00 at epoch 2006.179, when coupled with the observation of -95+/-1 km/s at epoch 2005.167 by Reiners & Basri, indicates a systemic radial velocity of -101+/-1 km/s for the LSR1610-00AB pair. The galactic velocity components for LSR1425+71 and LSR1610-00AB -- (U,V,W)=(84+/-6, -202+/-13, 66+/-14) km/s and (U,V,W)=(36+/-2, -232+/-2, -61+/-2) km/s, respectively. For both stars, the velocities are characteristic of halo population kinematics. However, modeling shows that both stars have orbits around the galaxy with high eccentricity that pass remarkably close to the galactic center. LSR1425+71 has a luminosity and colors consistent with its metal-poor subdwarf spectral classification, while LSR1610-00 has a luminosity and most colors indicative of being only mildly metal-poor, plus a uniquely red B-V color. The companion to LSR1610-00 must be a low-mass, substellar brown dwarf. We speculate on the paradoxical nature of LSR1610-00 and possible sources of its peculiarities.Comment: Accepted for ApJ. 37 pages, including 8 figure

The USNO-B Catalog

USNO-B is an all-sky catalog that presents positions, proper motions, magnitudes in various optical passbands, and star/galaxy estimators for 1,042,618,261 objects derived from 3,643,201,733 separate observations. The data were obtained from scans of 7,435 Schmidt plates taken for the various sky surveys during the last 50 years. USNO-B1.0 is believed to provide all-sky coverage, completeness down to V = 21, 0.2 arcsecond astrometric accuracy at J2000, 0.3 magnitude photometric accuracy in up to five colors, and 85% accuracy for distinguishing stars from non-stellar objects. A brief discussion of various issues is given here, but the actual data are available from http://www.nofs.navy.mil and other sites.Comment: Accepted by Astronomical Journa

Astrometry and Photometry for Cool Dwarfs and Brown Dwarfs

Trigonometric parallax determinations are presented for 28 late type dwarfs and brown dwarfs, including eight M dwarfs with spectral types between M7 and M9.5, 17 L dwarfs with spectral types between L0 and L8, and three T dwarfs. Broadband photometry at CCD wavelengths (VRIz) and/or near-IR wavelengths (JHK) are presented for these objects and for 24 additional late-type dwarfs. Supplemented with astrometry and photometry from the literature, including ten L and two T dwarfs with parallaxes established by association with bright, usually HIPPARCOS primaries, this material forms the basis for studying various color-color and color-absolute magnitude relations. The I-J color is a good predictor of absolute magnitude for late-M and L dwarfs. M_J becomes monotonically fainter with I-J color and with spectral type through late-L dwarfs, then brightens for early-T dwarfs. The combination of zJK colors alone can be used to classify late-M, early-L, and T dwarfs accurately, and to predict their absolute magnitudes, but is less effective at untangling the scatter among mid- and late-L dwarfs. The mean tangential velocity of these objects is found to be slightly less than that for dM stars in the solar neighborhood, consistent with a sample with a mean age of several Gyr. Using colors to estimate bolometric corrections, and models to estimate stellar radii, effective temperatures are derived. The latest L dwarfs are found to have T_eff ~ 1360 K.Comment: 48 pages, including 7 figures and 6 tables. Accepted for A

IC 1590, a young cluster embedded in the nebulosity of NGC 281

Publisher's version/PDFPhotoelectric and CCD photometry (to a limit of V=17.0) is presented for 61 and 243 stars, respectively--a total of 279 individual stars--in and about IC 1590, a young galactic cluster embedded in the nebulosity of NGC 281, the H II region S184. New spectroscopic observations are also presented for 20 stars in or near the cluster. A small amount of differential reddening is observed across the face of IC 1590, and is described by a visual reddening relation of slope E[subscript U-B]/E[subscript B-V]=0.735. From near infrared photometry of bright cluster members in the I and K bands and a variable-extinction analysis of ZAMS members, the value of R = A[subscript V]/E[subscript B-V], the ratio of total to selective absorption, is found to be 3.44[plus or minus]0.07, slightly larger than what has been found for other nearby galactic fields. Most of the cluster reddening appears to originate in dust clouds which are ~0.66 kpc and 2.0-2.2 kpc distant, the latter cloud lying in the immediate foreground of the cluster, which has a derived distance of 2.94[plus or minus]0.15 kpc (V[subscript 0]-M[subscript V] = 12.34[plus or minus]0.11 s.d., [plus or minus]0.03 s.e.). IC 1590 is extremely young and has an estimated age for its 63 identified probable members of 3.5([plus or minus]0.2) × 10[superscript 6] years, with very little evidence for age spread. Its well-defined sequence of 22 gravitationally-contracting stars, the choice of which is supported by star counts and reddening information, provides evidence for the extreme youth of the cluster's central trapezium system, HD 5005. According to the luminosity distribution of likely cluster members, the initial mass function for IC 1590 has a slope of [Gamma] = -1.00[plus or minus]0.21, similar to what has been found for other very young clusters and associations. Given that the photometric study terminates at a magnitude limit marginally brighter than where embedded protostars and T Tauri variables belonging to the cluster are expected to be found, IC 1590 would make an excellent target for Hubble Space Telescope observations

The Discovery of an Embedded Cluster of High-Mass Starts near SGR 1900+14

Deep I-band imaging to of the soft gamma-ray repeater SGR 1900ϩ14 region has revealed a compact I ≈ 26.5 cluster of massive stars located only a few arcseconds from the fading radio source thought to be the location of the soft gamma-ray repeater (SGR). This cluster was previously hidden in the glare of the pair of M5 supergiant stars (whose light was removed by point-spread function subtraction) proposed by Vrba et al. as likely associated with SGR 1900ϩ14. The cluster has at least 13 members within a cluster radius of ≈0.6 pc based on an estimated distance of 12–15 kpc. It is remarkably similar to a cluster found associated with SGR 1806Ϫ20. That similar clusters have now been found at or near the positions of the two best studied SGRs suggests that young neutron stars, which are thought to be responsible for the SGR phenomenon, have their origins in proximate compact clusters of massive stars