50 research outputs found
A Search for Planets Transiting the M Dwarf Debris Disk Host, AU Microscopii
We present high cadence, high precision multi-band photometry of the young,
M1Ve, debris disk star, AU Microscopii. The data were obtained in three
continuum filters spanning a wavelength range from 4500\AA to 6600\AA, plus
H, over 28 nights in 2005. The lightcurves show intrinsic stellar
variability due to starspots with an amplitude in the blue band of 0.051
magnitudes and a period of 4.847 days. In addition, three large flares were
detected in the data which all occur near the minimum brightness of the star.
We remove the intrinsic stellar variability and combine the lightcurves of all
the filters in order to search for transits by possible planetary companions
orbiting in the plane of the nearly edge-on debris disk. The combined final
lightcurve has a sampling of 0.35 minutes and a standard deviation of 6.8
millimags (mmag). We performed Monte Carlo simulations by adding fake transits
to the observed lightcurve and find with 95% significance that there are no
Jupiter mass planets orbiting in the plane of the debris disk on circular
orbits with periods, P days. In addition, there are no young
Neptune-like planets (with radii 2.5 smaller than the young Jupiter) on
circular orbits with periods, P days.Comment: accepted to MNRA
The HD 163296 Circumstellar Disk in Scattered Light: Evidence of Time-Variable Self-Shadowing
We present the first multicolor view of the scattered light disk of the Herbig Ae star HD 163296, based on coronagraphic observations from the Hubble Space Telescope Advanced Camera for Surveys (HST ACS). Radial profile fits of the surface brightness along the disk's semimajor axis indicate that the disk is not continuously flared, and extends to ~540 AU. The disk's color (V â I) = 1.1 at a radial distance of 3.5'' is redder than the observed stellar color (V â I) = 0.15. This red disk color might be indicative of either an evolution in the grain size distribution (i.e., grain growth) and/or composition, both of which would be consistent with the observed nonflared geometry of the outer disk. We also identify a single ansa morphological structure in our F435W ACS data, which is absent from earlier epoch F606W and F814W ACS data, but corresponds to one of the two ansae observed in archival HST Space Telescope Imaging Spectrograph (STIS) coronagraphic data. Following transformation to similar bandpasses, we find that the scattered light disk of HD 163296 is 1 mag arcsec^(â2) fainter at 3.5'' in the STIS data than in the ACS data. Moreover, variations are seen in (1) the visibility of the ansa(e) structures, (2) the relative surface brightness of the ansa(e) structures, and (3) the (known) intrinsic polarization of the system. These results indicate that the scattered light from the HD 163296 disk is variable. We speculate that the inner disk wall, which Sitko et al. suggests has a variable scale height as diagnosed by near-IR SED variability, induces variable self-shadowing of the outer disk. We further speculate that the observed surface brightness variability of the ansa(e) structures may indicate that the inner disk wall is azimuthally asymmetric
The Very Low Mass Component of the Gliese 105 System
Multiple-epoch, multicolor images of the astrometric binary Gliese 105A and
its very low mass companion Gliese 105C have been obtained using the Hubble
Space Telescope's Wide Field Planetary Camera 2 (WFPC2) and Near-Infrared
Camera and Multi-Object Spectrometer (NICMOS). The optical and near-infrared
colors of Gl 105C strongly suggest a spectral type of M7V for that star.
Relative astrometric measurements spanning 3 yr reveal the first evidence of Gl
105C's orbital motion. Previous long-term astrometric studies at Sproul and
McCormick Observatories have shown that the period of Gl 105A's perturbation is
approximately 60 yr. To satisfy both the observed orbital motion and Gl 105A's
astrometric period, Gl 105C's orbit must have an eccentricity of approximately
0.75 and a semimajor axis of approximately 15 AU. Measurements of Gl 105A's
radial velocity over 12 yr show a linear trend with a slope of 11.3 m/s/yr,
which is consistent with these orbital constraints and a nearly face-on orbit.
As no other faint companions to Gl 105A have been detected, we conclude that Gl
105C is probably the source of the 60-yr astrometric perturbation.Comment: 16 pages, 6 figures; to be published in the October 2000 issue of The
Astronomical Journa
Five Debris Disks Newly Revealed in Scattered Light from the HST NICMOS Archive
We have spatially resolved five debris disks (HD 30447, HD 35841, HD 141943,
HD 191089, and HD 202917) for the first time in near-infrared scattered light
by reanalyzing archival Hubble Space Telescope (HST)/NICMOS coronagraphic
images obtained between 1999 and 2006. One of these disks (HD 202917) was
previously resolved at visible wavelengths using HST/Advanced Camera for
Surveys. To obtain these new disk images, we performed advanced point-spread
function subtraction based on the Karhunen-Loeve Image Projection (KLIP)
algorithm on recently reprocessed NICMOS data with improved detector artifact
removal (Legacy Archive PSF Library And Circumstellar Environments Legacy
program). Three of the disks (HD 30447, HD 35841, and HD 141943) appear
edge-on, while the other two (HD 191089 and HD 202917) appear inclined. The
inclined disks have been sculpted into rings; in particular, the disk around HD
202917 exhibits strong asymmetries. All five host stars are young (8-40 Myr),
nearby (40-100 pc) F and G stars, and one (HD 141943) is a close analog to the
young sun during the epoch of terrestrial planet formation. Our discoveries
increase the number of debris disks resolved in scattered light from 19 to 23
(a 21% increase). Given their youth, proximity, and brightness (V = 7.2 to
8.5), these targets are excellent candidates for follow-up investigations of
planet formation at visible wavelengths using the HST/STIS coronagraph, at
near-infrared wavelengths with the Gemini Planet Imager (GPI) and Very Large
Telescope (VLT)/SPHERE, and at thermal infrared wavelengths with the James Webb
Space Telescope NIRCam and MIRI coronagraphs.Comment: 6 pages, 1 figure, 1 tabl
Our Nearest 15 Million Neighbors: The Field Low-Mass Stellar Luminosity and Mass Functions
We report on a new measurement of the luminosity function (LF) and mass
function (MF) of field low-mass dwarfs using Sloan Digital Sky Survey (SDSS)
photometry. The final catalog is composed of ~15 million low-mass stars (0.1
Msun < M < 0.8 Msun), spread over 8,400 square degrees. Distances to the stars
are estimated using new photometric parallax relations, constructed from ugriz
photometry of nearby low-mass stars with trigonometric parallaxes. The LF is
measured with a novel technique, which simultaneously measures Galactic
structure and the stellar LF. The resulting LF is compared to previous studies
and converted to a MF. The MF is well-described by a log-normal distribution,
with Mo = 0.27 Msun.Comment: 4 pages, 3 figures. Included in the proceedings of Cool Stars 1
A MASSIF Effort To Determine The Mass-Luminosity Relation for Stars of Various Ages, Metallicities, and Evolution States
The MASSIF (Masses and Stellar Systems with Interferometry) Team will use SIM to investigate the mass content of the Galaxy - from huge stars to barely glimmering brown dwarfs, and from hot white dwarfs to exotic black holes. We will target various samples of the Galactic population to determine and relate the fundamental characteristics of mass, luminosity, age, composition, and multiplicity - attributes that together yield an extensive understanding of the stars. Our samples will include distant clusters that span a factor of 5000 in age, and commonplace stars and substellar objects that lurk near the Sun. The principal goals of the MASSIF Key Project are to (1) define the mass-luminosity relation for main sequence stars in five fundamental clusters so that effects of age and metallicity can be mapped (Trapezium, TW Hydrae, Pleiades, Hyades, and M67), and (2) determine accurate masses for representative examples of nearly every type of star, stellar descendant or brown dwarf in the Galaxy
Discovery of an 86 AU Radius Debris Ring Around HD 181327
HST/NICMOS PSF-subtracted coronagraphic observations of HD 181327 have
revealed the presence of a ring-like disk of circumstellar debris seen in 1.1
micron light scattered by the disk grains, surrounded by a di use outer region
of lower surface brightness. The annular disk appears to be inclined by 31.7
+/- 1.6 deg from face on with the disk major axis PA at 107 +/-2 deg . The
total 1.1 micron flux density of the light scattered by the disk (at 1.2" < r <
5.0") of 9.6 mJy +/- 0.8 mJy is 0.17% +/- 0.015% of the starlight. Seventy
percent of the light from the scattering grains appears to be confined in a 36
AU wide annulus centered on the peak of the radial surface brightness (SB)
profile 86.3 +/- 3.9 AU from the star, well beyond the characteristic radius of
thermal emission estimated from IRAS and Spitzer flux densities assuming
blackbody grains (~ 22 AU). The light scattered by the ring appears bilaterally
symmetric, exhibits directionally preferential scattering well represented by a
Henyey-Greenstein scattering phase function with g = 0.30 +/- 0.03, and has an
azimuthally medianed SB at the 86.3 AU radius of peak SB of 1.00 +/- 0.07 mJy
arcsec^-2. No photocentric offset is seen in the ring relative to the position
of the central star. A low surface brightness diffuse halo is seen in the
NICMOS image to a distance of ~ 4" Deeper 0.6 micron HST/ACS PSF-subtracted
coronagraphic observations reveal a faint outer nebulosity, asymmetrically
brighter to the North of the star. We discuss models of the disk and properties
of its grains, from which we infer a maximum vertical scale height of 4 - 8 AU
at the 87.6 AU radius of maximum surface density, and a total maximum dust mass
of collisionally replenished grains with minimum grain sizes of ~ 1 micron of ~
4 M(moon).Comment: 45 pages, 15 figures, accepted for publication in Ap
SDSS J1534+1615AB: A Novel T Dwarf Binary Found with Keck Laser Guide Star Adaptive Optics and the Potential Role of Binarity in the L/T Transition
We have resolved the newly discovered T dwarf SDSS J1534+1615 into a 0.11''
binary using the Keck sodium laser guide star adaptive optics system. With an
integrated-light spectral type of T3.5, this binary provides a new benchmark
for studying the distinctive J-band brightening previously noted among early
and mid-T dwarfs, using two brown dwarfs with different spectral types but
having a common metallicity and age and very similar surface gravities. We
estimate spectral types of T1.5+/-0.5 and T5.5+/-0.5 for the two components
based on their near-IR colors, consistent with modeling the integrated-light
spectrum as the blend of two components. The observed near-IR flux ratios are
unique compared to all previously known substellar binaries: the component that
is fainter at H and K' is brighter at J. This inversion of the near-IR fluxes
is a manifestation of the J-band brightening within this individual binary
system. Therefore, SDSS 1534+1615 demonstrates that the brightening can be
intrinsic to ultracool photospheres (e.g., arising from cloud disruption and/or
rapid increase in cloud sedimentation) and does not necessarily result from
physical variations among the observed ensemble of T dwarfs (e.g., a range in
masses, ages and/or metallicities). We suggest that the apparently large
amplitude of the J-band brightening may be due to a high incidence of
unresolved binaries and that the true amplitude of the phenomenon could be more
modest. This scenario would imply that truly single objects in these spectral
subclasses are relatively rare, in agreement with the small effective
temperature range inferred for the L/T transition.Comment: ApJ, in press, 26 page
The Discovery of a Second Field Methane Brown Dwarf from Sloan Digital Sky Survey Commissioning Data
We report the discovery of a second field methane brown dwarf from the
commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS
J134646.45-003150.4 (SDSS 1346-00), was selected because of its very red color
and stellar appearance. Its spectrum between 0.8-2.5 mic is dominated by strong
absorption bands of H_2O and CH_4 and closely mimics those of Gliese 229B and
SDSS 162414.37+002915.6 (SDSS 1624+00), two other known methane brown dwarfs.
SDSS 1346-00 is approximately 1.5 mag fainter than Gliese 229B, suggesting that
it lies about 11 pc from the sun. The ratio of flux at 2.1 mic to that at 1.27
mic is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which
suggests that SDSS 1346-00 has a slightly higher effective temperature than the
others. Based on a search area of 130 sq. deg. and a detection limit of z* =
19.8, we estimate a space density of 0.05 pc^-3 for methane brown dwarfs with
T_eff ~ 1000 K in the 40 pc^3 volume of our search. This estimate is based on
small-sample statistics and should be treated with appropriate caution.Comment: 9 pages, 3 figures, AASTeX, to appear in ApJ Letters, authors list
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