Sori Research Articles

X-ray emission near the substellar limit: TheσOrionis and Taurus star forming regions

We have carried out an extensive search for X-ray emission from young, very low-mass objects near and beyond the substellar limit, making use of archived ROSAT PSPC and HRI observations pointed at Brown Dwarfs and Brown Dwarf candidates in the young sigma Orionis and Taurus-Auriga associations. In sigma Ori we identify three Brown Dwarf candidates with X-ray sources; in Taurus-Auriga we add one further X-ray detection of a Brown Dwarf to the list published earlier. We combine this data with all previously X-ray detected Brown Dwarfs and Brown Dwarf candidates in young stellar associations and star forming regions to perform a study of stellar activity parameters on the as yet largest sample of young, very low mass objects. A similar relation between X-ray and bolometric luminosity, and H-alpha emission, respectively, as is known for T Tauri stars seems to hold for young objects down to the substellar limit, too. No signs for a change in X-ray activity are found on the transition to substellar masses.

Optical spectroscopy of isolated planetary mass objects in the $\mathsf{\sigma}$ Orionis cluster

We have obtained low resolution optical spectra of 15 isolated planetary mass objects (IPMOs) in the sigma Orionis cluster, and derived spectral types by comparison with nearby M and L dwarfs. The spectral types are in the range late M - mid L, in agreement with our expectations based on colors and magnitudes for bona fide members. Therefore, most of these objects have masses below the deuterium burning limit. About 2/3 show H(alpha) in emission at our spectral resolution. From our spectroscopic and photometric data, we infer that three IPMOs in this sample may be binaries with components of similar masses. These results confirm that the substellar mass function of the sigma Orionis cluster, in the form dN/dM, keeps rising in the planetary domain.

The Substellar Mass Function in σ Orionis

We combine results from imaging searches for substellar objects in the sigma Orionis cluster and follow-up photometric and spectroscopic observations to derive a census of the brown dwarf population in a region of 847 arcmin^2. We identify 64 very low-mass cluster member candidates in this region. We have available three color (IZJ) photometry for all of them, spectra for 9 objects, and K photometry for 27% of our sample. These data provide a well defined sequence in the I vs I-J, I-K color magnitude diagrams, and indicate that the cluster is affected by little reddening despite its young age (~5 Myr). Using state-of-the-art evolutionary models, we derive a mass function from the low-mass stars (0.2 Msol) across the complete brown dwarf domain (0.075 Msol to 0.013 Msol), and into the realm of free-floating planetary-mass objects (<0.013 Msol). We find that the mass spectrum (dN/dm ~ m^{-alpha}) increases toward lower masses with an exponent alpha = 0.8+/-0.4. Our results suggest that planetary-mass isolated objects could be as common as brown dwarfs; both kinds of objects together would be as numerous as stars in the cluster. If the distribution of stellar and substellar masses in sigma Orionis is representative of the Galactic disk, older and much lower luminosity free-floating planetary-mass objects with masses down to about 0.005 Msol should be abundant in the solar vicinity, with a density similar to M-type stars.

A New Species of Adiantum (Pteridaceae) from Bahia, Brazil

A new species,Adiantum discolor Prado (Pteridaceae), known only from the wet forests of eastern coastal Brazil, in Bahia, is described and illustrated. It is distinguished principally by its 2-pinnate fronds with a long stipe, 2 or 3 pairs of lateral pinnae, discolorous laminae, and glabrous indusia. Uma nova especie,Adiantum discolor Prado (Pteridaceae), conhecida somente das florestas umidas da costa leste do Brasil, no Estado da Bahia, e descrita e ilustrada. Distingue-se princi-palmente pelas frondes 2-pinadas com um longo peciolo, 2 ou 3 pares de pinas laterais, lâminas discolores e indusios glabros.

Cyathea concordia (Cyatheaceae), a New Pinnate-Pinnatifid Tree Fern from the Peruvian/Ecuadorian Border

Cyathea concordia, described herein, resembles C. palaciosii but can be distinguished by its indusiate sori, pinnae with hairs restricted to the veins, and nearly concolorous stem scales.

The magnetic field of Sigma Orionis E

We have detected a magnetic field in the peculiar variable helium-rich B2 star sigma Ori E. The field varies between -2300 and +3100 gauss with the 1/sup d/.19 period of the spectroscopic and light variations of the star. We suggest that the detection of this field allows all the variable phenomena of sigma Ori E to be understood in terms of an oblique rotator model that has hot gas trapped in a magnetosphere above the magnetic equator, and atmospheric helium enhancement which has occurred preferentially in a zone around the magnetic equator.

The polarization of Sigma Orionis E - A curious eclipsing binary

The polarization of Sigma Orionis E - A curious eclipsing binary

On the period and luminosity stability of sigma Orionis E

On the period and luminosity stability of sigma Orionis E

Sigma Orionis E as a mass-transfer binary system

SIGMA Orionis E, the first He-rich B star discovered1,2, was recently found3,4 to have an extremely broad (∼1,800 km s−1), rapidly varying Hα-emission feature. From independent spectroscopic, spectrophotometric, and photometric observations on 4 consecutive nights in December 1974, we have shown4 that it is a variable, with P ≃ 1.19 d; however, incomplete phase coverage in those data, as well as uncertainty about the nature of the periodicity, has hampered the development of a model to account for the wealth of observed phenomena. Here we briefly communicate results of new continuous uvbyβ photometry carried out during the 11 consecutive nights December 28, 1975–January 7, 1976 (UT), together with some possible interpretations.

Recent Results Pertaining to the Helium-Rich Stars

The term helium-rich stars here refers to those stars with apparent spectral types near B2 ana helium lines which are enhanced, but not stronger than the hydrogen lines. The previously known helium-rich star HD 184927 has been found to be a spectrum variable; the equivalent widths of the helium lines changed by an average of 46% between two observations. HD 186205 has been newly recognized as a member of the helium-rich class. Further work toward understanding the rapidly variable Ha emission in the prototype helium-rich star, Sigma Orionis E, is in progress in co-operation with Dr J. E. Hesser at CTIO.

The evolution of the helium star sigma Orionis E

view Abstract Citations (6) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The evolution of the helium star Sigma Orionis E. Odell, A. P. Abstract The helium-rich star sigma Ori E is a member of the quintuple star sigma Ori. The visual binary AB has recently been studied by Heintz, and a solution to its orbit and a distance was determined. It is shown that this new distance to the system resolves the problem of too large a mass for the helium star, and allows helium-rich models to approximate the present star. The upper age to the star is 10 m.y., and with a mass of less than 5 solar masses the star must have been formed with a large helium abundance. It is unlikely that component E is the result of binary mass transfer. It is difficult to understand the star in terms of mass loss from a very massive star. The idea that an original single star broke up into five stars is attractive to the time scale, but the angular momentum of the present system is very large. Alternatively, the system might have resulted from the capture of E, an old star which has been completely mixed through its evolution. Publication: The Astrophysical Journal Pub Date: December 1974 DOI: 10.1086/153285 Bibcode: 1974ApJ...194..645O Keywords: Abundance; Astronomical Models; Helium; Stellar Evolution; Angular Momentum; Mass Transfer; Orion Constellation; Stellar Mass Ejection; Stellar Rotation; Astrophysics full text sources ADS | data products SIMBAD (2)

High-Dispersion Spectroscopy of the Sigma Orionis System

High-Dispersion Spectroscopy of the Sigma Orionis System

A Cellular Slime Mold with Flagellate Cells

(1974). A Cellular Slime Mold with Flagellate Cells. Mycologia: Vol. 66, No. 4, pp. 685-690.

A New Phenomenon in the Spectrum of Sigma Orionis E

view Abstract Citations (77) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A New Phenomenon in the Spectrum of Sigma Orionis E Walborn, Nolan R. Abstract A broad, variable emission has been found at Ha in the spectrum of the helium-rich star cr Ori E. The feature is a Woff-Rayet-like band of maximum width near 1800 km 1, which fills in the deep Ha absorption and reaches maximum intensities of nearly 30 percent above the continuum on either side. The emission intensity undergoes large variations with a time scale of a few hours. Radial-velocity measures do not establish binary motion, although a low-mass companion cannot be excluded. The nature of the energetic circumstellar activity reported, and its relationship to the helium anomaly, remain to be determined. SnUect headings: circumstellar shells - early-type stars - spectrum variables Publication: The Astrophysical Journal Pub Date: July 1974 DOI: 10.1086/181558 Bibcode: 1974ApJ...191L..95W full text sources ADS | data products SIMBAD (7)

Gymnosporangium Gaeumannii in North America

(1969). Gymnosporangium Gaeumannii in North America. Mycologia: Vol. 61, No. 2, pp. 401-404.

The Radial Velocity of Sigma Orionis

The Radial Velocity of Sigma Orionis

The Helium-Rich Star, Sigma Orionis E.

The Helium-Rich Star, Sigma Orionis E.