25 research outputs found
Discovery of "isolated" comoving T Tauri stars in Cepheus
During the course of a large spectroscopic survey of X-ray active late-type
stars in the solar neighbourhood, we discovered four lithium-rich stars packed
within just a few degrees on the sky. These very young stars are projected
several degrees away from the Cepheus-Cassiopea clouds, in front of an area
void of interstellar matter. As such, they are very good "isolated" T Tauri
star candidates. We acquired high-resolution optical spectra as well as
photometric data allowing us to investigate in detail their nature and physical
parameters with the aim of testing the "runaway" and "in-situ" formation
scenarios. We derive accurate radial and rotational velocities and perform an
automatic spectral classification. The spectral subtraction technique is used
to infer chromospheric activity level in the H-alpha line core and clean the
spectra of photospheric lines before measuring the equivalent width of the
lithium absorption line. Both physical (lithium content, magnetic activity) and
kinematical indicators show that all stars are very young (ages in the range
10-30 Myr). In particular, the spectral energy distribution of TYC4496-780-1
displays a strong near- and far-infrared excess, typical of T Tauri stars still
surrounded by an accretion disc. They also share the same Galactic motion,
proving that they form a homogeneous moving group of stars with the same
origin. The most plausible explanation of how these "isolated" T Tauri stars
formed is the "in-situ" model, although accurate distances are needed to
clarify their connection with the Cepheus-Cassiopeia complex
An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 um) and the coronagraph on the
Near-InfraRed Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space
Telescope (HST) to survey 45 single stars with a median age of 0.15 Gyr, an
average distance of 30 pc, and an average H-magnitude of 7 mag. For the median
age we were capable of detecting a 30 M_Jup companion at separations between 15
and 200 AU. A 5 M_Jup object could have been detected at 30 AU around 36% of
our primaries. For several of our targets that were less than 30 Myr old, the
lower mass limit was as low as a Jupiter mass, well into the high mass planet
region. Results of the entire survey include the proper motion verification of
five low-mass stellar companions, two brown dwarfs (HR7329B and TWA5B) and one
possible brown dwarf binary (Gl 577B/C).Comment: 11 figures, accepted by A
X-Ray Spectroscopy of II Pegasi: Coronal Temperature Structure, Abundances, and Variability
We have obtained high resolution X-ray spectra of the coronally active
binary, II Pegasi (HD 224085), covering the wavelength range of 1.5-25
Angstroms. For the first half of our 44 ksec observation, the source was in a
quiescent state with constant X-ray flux, after which it flared, reaching twice
the quiescent flux in 12 ksec, then decreasing. We analyze the emission-line
spectrum and continuum during quiescent and flaring states. The differential
emission measure derived from lines fluxes shows a hot corona with a continuous
distribution in temperature. During the non-flare state, the distribution peaks
near log T = 7.2, and when flaring, near 7.6. High-temperature lines are
enhanced slightly during the flare, but most of the change occurs in the
continuum. Coronal abundance anomalies are apparent, with iron very deficient
relative to oxygen and significantly weaker than expected from photospheric
measurements, while neon is enhanced relative to oxygen. We find no evidence of
appreciable resonant scattering optical depth in line ratios of iron and
oxygen. The flare light curve is consistent with Solar two-ribbon flare models,
but with a very long reconnection time-constant of about 65 ks. We infer loop
lengths of about 0.05 stellar radii, to about 0.25 in the flare, if the flare
emission originated from a single, low-density loop.Comment: 25 pages, 5 figures, 3 tables, accepted by ApJ (scheduled for the
v559 n2 p1 Oct 1, 2001 issue