462 research outputs found
Spectral Energy Distributions for Disk and Halo M--Dwarfs
We have obtained infrared (1 to 2.5 micron) spectroscopy for 42 halo and disk
dwarfs with spectral type M1 to M6.5. These data are compared to synthetic
spectra generated by the latest model atmospheres of Allard & Hauschildt.
Photospheric parameters metallicity, effective temperature and radius are
determined for the sample. We find good agreement between observation and
theory except for known problems due to incomplete molecular data for metal
hydrides and water. The metal-poor M subdwarfs are well matched by the models
as oxide opacity sources are less important in this case. The derived effective
temperatures for the sample range from 3600K to 2600K; at these temperatures
grain formation and extinction are not significant in the photosphere. The
derived metallicities range from solar to one-tenth solar. The radii and
effective temperatures derived agree well with recent models of low mass stars.Comment: 24 pages including 13 figures, 4 Tables; accepted by Ap
Measurements of Wood Fiber
Many of the properties of wood depend upon the character and arrangement of the fiber cells which enter into its composition. This is especially true of those properties which determine its strength, hardness and adaptability for the manufacture of paper pulp. With a view to throwing light on these properties, we have made measurements of the length and diameter of the fibers in forty-one species of trees. No attempt was made to segregate the various types of fiber that occur in different species. We have taken any of the slender, fusiform, non-septate, thick walled members of the xylem. The maximum and minimum figures indicate that this lack of discrimination has not introduced any serious error. Were very critical discrimination attempted, the comparison of species would prove unduly complex, if not impossible
The Mosses of Southwestern Iowa
The territory of southwestern Iowa in this report includes Harrison, Shelby, Audubon, Guthrie, Pottawatamie, Cass, Adair, Mills, Montgomery, Adams, Union, Fremont, Page, Taylor and Ringgold counties. Four of these border the Missouri state line and four are along the Missouri river. About half the region is prairie, unmodified by erosion or the deposition of loess in hills. The mosses were collected on the wooded banks of streams or in cultivated land. The collections were made mostly by Dr. Conard during 1930-1933. They are obviously incomplete. In some counties only one stop was made for collecting, in others the work was done more thoroughly
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
The Spectra of T Dwarfs I: Near-Infrared Data and Spectral Classification
We present near-infrared spectra for a sample of T dwarfs, including eleven
new discoveries made using the Two Micron All Sky Survey. These objects are
distinguished from warmer (L-type) brown dwarfs by the presence of methane
absorption bands in the 1--2.5 \micron spectral region. A first attempt at a
near-infrared classification scheme for T dwarfs is made, based on the
strengths of CH and HO bands and the shapes of the 1.25, 1.6, and 2.1
\micron flux peaks. Subtypes T1 V through T8 V are defined, and spectral
indices useful for classification are presented. The subclasses appear to
follow a decreasing T scale, based on the evolution of CH and
HO bands and the properties of L and T dwarfs with known distances.
However, we speculate that this scale is not linear with spectral type for cool
dwarfs, due to the settling of dust layers below the photosphere and subsequent
rapid evolution of spectral morphology around T 1300--1500 K.
Similarities in near-infrared colors and continuity of spectral features
suggest that the gap between the latest L dwarfs and earliest T dwarfs has been
nearly bridged. This argument is strengthened by the possible role of CH as
a minor absorber shaping the K-band spectra of the latest L dwarfs. Finally, we
discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue
near-infrared colors (J-K = 0.24) due to suppression of the 2.1
\micron peak. The feature is likely caused by enhanced collision-induced
H absorption in a high pressure or low metallicity photosphere.Comment: 74 pages including 26 figures, accepted by ApJ v563 December 2001;
full paper including all of Table 3 may be downloaded from
http://www.gps.caltech.edu/~pa/adam/classification ;also see submission
010844
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