330 research outputs found
The First Direct Distance and Luminosity Determination for a Self-Luminous Giant Exoplanet: The Trigonometric Parallax to 2MASS1207334-393254Ab
We present the first trigonometric parallax and distance for a young
planetary mass object. A likely TW Hya cluster member, 2MASSW J1207334-393254Ab
(hereafter 2M1207Ab) is an M8 brown dwarf with a mid to late L type planetary
mass companion. Recent observations of spectral variability have uncovered
clear signs of disk accretion and outflow, constraining the age of the system
to <10 Myr. Because of its late spectral type and the clearly youthful nature
of the system, 2M1207b is very likely a planetary mass object. We have measured
the first accurate distance and luminosity for a self-luminous planetary mass
object. Our parallax measurements are accurate to <2 mas (1sigma) for 2M1207Ab.
With 11 total epochs of data taken from January 2006 through April 2 007 (475
images for 2M1207Ab), we determine a distance of 58.8+-7.0 pc (17.0{+2.3}{-1.8}
mas, 1.28sigma) to 2M1207Ab and a calculated luminosity of 0.68-2.2x10^-5 Lsun
for 2M1207b. Hence 2M1207Ab is a clear member of the TW Hya cluster in terms of
its distance, proper motions, and youthful nature. However, as previously noted
by Mohanty and co-workers, 2M1207b's luminosity appears low compared to its
temperature according to evolutionary models.Comment: 12 pages, 3 figures, accepted to ApJ Letter
Image Analysis for Facility Siting: a Comparison of Lowand High-altitude Image Interpretability for Land Use/land Cover Mapping
For two test sites in Pennsylvania the interpretability of commercially acquired low-altitude and existing high-altitude aerial photography are documented in terms of time, costs, and accuracy for Anderson Level II land use/land cover mapping. Information extracted from the imagery is to be used in the evaluation process for siting energy facilities. Land use/land cover maps were drawn at 1:24,000 scale using commercially flown color infrared photography obtained from the United States Geological Surveys' EROS Data Center. Detailed accuracy assessment of the maps generated by manual image analysis was accomplished employing a stratified unaligned adequate class representation. Both 'area-weighted' and 'by-class' accuracies were documented and field-verified. A discrepancy map was also drawn to illustrate differences in classifications between the two map scales. Results show that the 1:24,000 scale map set was more accurate (99% to 94% area-weighted) than the 1:62,500 scale set, especially when sampled by class (96% to 66%). The 1:24,000 scale maps were also more time-consuming and costly to produce, due mainly to higher image acquisition costs
Short-term spectroscopic monitoring of two cool dwarfs with strong magnetic fields
Context: There is now growing evidence that some brown dwarfs (BDs) have very
strong magnetic fields, and yet their surface temperatures are so low that the
coupling is expected to be small between the matter and the magnetic field in
the atmosphere. In the deeper layers, however, the coupling is expected to be
much stronger. Aims: This raises the question of whether the magnetic field
still leads to the formation of structures in the photosphere. Methods: We
carried out a spectroscopic monitoring campaign of two ultracool dwarfs that
have strong magnetic fields: the BD LP944-20 and 2MASSW J0036159+182110.
LP944-20 was observed simultaneously in the optical and in the near infrared
regime, 2MASSW J0036159+182110 only in the infrared. Results: Both dwarfs
turned out to be remarkably constant. In the case of LP944-20, the
Teff-variations are <50K, and the rms-variations in the equivalent widths of
Halpha small. We also find that the equivalent widths of photospheric lines are
remarkably constant. We did not find any significant variations in the case of
2MASSW J0036159+182110 either. Thus the most important result is that no
significant variability was found at the time of our observations. When
comparing our spectra with spectra taken over the past 11 years, we recognize
significant changes during this time. Conclusions: We interpret these results
as evidence that the photosphere of these objects are remarkably homogeneous,
with only little structure in them, and despite the strong magnetic fields.
Thus, unlike active stars, there are no prominent spots on these objects.Comment: 7 pages, 9 figures. Astronomy and Astrophysics (accepted
Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. II. Mixed Trends in VB10 and LSR1835+32 and the Possible Role of Rotation
[Abridged] As part of our on-going investigation of magnetic activity in
ultracool dwarfs we present simultaneous radio, X-ray, UV, and optical
observations of LSR1835+32 (M8.5), and simultaneous X-ray and UV observations
of VB10 (M8), both with a duration of about 9 hours. LSR1835+32 exhibits
persistent radio emission and H-alpha variability on timescales of ~0.5-2 hr.
The detected UV flux is consistent with photospheric emission, and no X-ray
emission is detected to a deep limit of L_X/L_bol<10^-5.7. The H-alpha and
radio emission are temporally uncorrelated, and the ratio of radio to X-ray
luminosity exceeds the correlation seen in F-M6 stars by >2x10^4. Similarly,
L_Halpha/L_X>10 is at least 30 times larger than in early M dwarfs, and
eliminates coronal emission as the source of chromospheric heating. The lack of
radio variability during four rotations of LSR1835+32 requires a uniform
stellar-scale field of ~10 G, and indicates that the H-alpha variability is
dominated by much smaller scales, <10% of the chromospheric volume. VB10, on
the other hand, shows correlated flaring and quiescent X-ray and UV emission,
similar to the behavior of early M dwarfs. Delayed and densely-sampled optical
spectra exhibit a similar range of variability amplitudes and timescales to
those seen in the X-rays and UV, with L_Halpha/L_X~1. Along with our previous
observations of the M8.5 dwarf TVLM513-46546 we conclude that late M dwarfs
exhibit a mix of activity patterns, which points to a transition in the
structure and heating of the outer atmosphere by large-scale magnetic fields.
We find that rotation may play a role in generating the fields as evidenced by
a tentative correlation between radio activity and rotation velocity. The X-ray
emission, however, shows evidence for super-saturation at vsini>25 km/s.Comment: Submitted to Ap
Irrigated lands assessment for water management: Technique test
A procedure for estimating irrigated land using full frame LANDSAT imagery was demonstrated. Relatively inexpensive interpretation of multidate LANDSAT photographic enlargements was used to produce a map of irrigated land in California. The LANDSAT and ground maps were then linked by regression equations to enable precise estimation of irrigated land area by county, basin, and statewide. Land irrigated at least once in California in 1979 was estimated to be 9.86 million acres, with an expected error of less than 1.75% at the 99% level of confidence. To achieve the same level of error with a ground-only sample would have required 3 to 5 times as many ground sample units statewide. A procedure for relatively inexpensive computer classification of LANDSAT digital data to irrigated land categories was also developed. This procedure is based on ratios of MSS band 7 and 5, and gave good results for several counties in the Central Valley
Wall Pressure Unsteadiness and Side Loads in Overexpanded Rocket Nozzles
Surveys of both the static and dynamic wall pressure signatures on the interior surface of a sub-scale, cold-flow and thrust optimized parabolic nozzle are conducted during fixed nozzle pressure ratios corresponding to FSS and RSS states. The motive is to develop a better understanding for the sources of off-axis loads during the transient start-up of overexpanded rocket nozzles. During FSS state, pressure spectra reveal frequency content resembling SWTBLI. Presumably, when the internal flow is in RSS state, separation bubbles are trapped by shocks and expansion waves; interactions between the separated flow regions and the waves produce asymmetric pressure distributions. An analysis of the azimuthal modes reveals how the breathing mode encompasses most of the resolved energy and that the side load inducing mode is coherent with the response moment measured by strain gauges mounted upstream of the nozzle on a flexible tube. Finally, the unsteady pressure is locally more energetic during RSS, albeit direct measurements of the response moments indicate higher side load activity when in FSS state. It is postulated that these discrepancies are attributed to cancellation effects between annular separation bubbles
Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic and HST WFPC2 Data
We have measured the absolute proper motion of the Fornax dwarf spheroidal
galaxy from a combination of photographic plate material and HST WFPC2 data
that provide a time baseline of up to 50 years. The extragalactic reference
frame consists of 8 QSO images and 48 galaxies. The absolute proper motion is
mu_alpha cos(delta) = 0.59 +-0.16 mas/yr and mu_delta = -0.15 +- 0.16 mas/yr.
The corresponding orbit of Fornax is polar, with an eccentricity of 0.27, and a
radial period of 4.5 Gyr. Fornax's current location is near pericenter. The
direction of the motion of Fornax supports the notion that Fornax belongs to
the Fornax-LeoI-LeoII-Sculptor-Sextans stream as hypothesized by Lynden-Bell
(1976, 1982) and Majewski (1994).
According to our orbit determination, Fornax crossed the Magellanic plane
\~190 Myr ago, a time that coincides with the termination of the star-formation
process in Fornax. We propose that ram-pressure stripping due to the passage of
Fornax through a gaseous medium denser than the typical intragalactic medium
left behind from the LMC may have caused the end of star formation in Fornax.
The excess, anomalous clouds within the South Galactic pole region of the
Magellanic Stream whose origin has long been debated in the literature as
constituents of either the Magellanic Stream or of the extragalactic Sculptor
group, are found to lie along the orbit of Fornax. We speculate that these
clouds are stripped material from Fornax as the dwarf crossed the Magellanic
Clouds' orbit.Comment: Accepted for publication in Astronomical Journal. The version with
high resolution figures can be found at
ftp://pegasus.astro.yale.edu/pub/dana/paper
The Near-Infrared and Optical Spectra of Methane Dwarfs and Brown Dwarfs
We identify the pressure--broadened red wings of the saturated potassium
resonance lines at 7700 \AA as the source of anomalous absorption seen in the
near-infrared spectra of Gliese 229B and, by extension, of methane dwarfs in
general. This conclusion is supported by the recent work of Tsuji {\it et al.}
1999, though unlike them we find that dust need not be invoked to explain the
spectra of methane dwarfs shortward of 1 micron. We find that a combination of
enhanced alkali abundances due to rainout and a more realistic non-Lorentzian
theory of resonant line shapes may be all that is needed to properly account
for these spectra from 0.5 to 1.0 microns. The WFPC2 measurement of Gliese
229B is also consistent with this theory. Furthermore, a combination of the
blue wings of this K I resonance doublet, the red wings of the Na D lines at
5890 \AA, and, perhaps, the Li I line at 6708 \AA can explain in a natural way
the observed WFPC2 band flux of Gliese 229B. Hence, we conclude that the
neutral alkali metals play a central role in the near-infrared and optical
spectra of methane dwarfs and that their lines have the potential to provide
crucial diagnostics of brown dwarfs. We speculate on the systematics of the
near-infrared and optical spectra of methane dwarfs, for a given mass and
composition, that stems from the progressive burial with decreasing \teff of
the alkali metal atoms to larger pressures and depths.Comment: Revised and accepted to Ap.J. volume 531, March 1, 2000, also
available at http://jupiter.as.arizona.edu/~burrows/papers/BMS.p
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