663 research outputs found
The Chandra LETGS high resolution X-ray spectrum of the isolated neutron star RX J1856.5-3754
We present the Chandra LETGS X-ray spectrum of the nearby (~60 pc) neutron
star RX J1856.5-3754. Detailed spectral analysis of the combined X-ray and
optical data rules out the nonmagnetic neutron star atmosphere models with
hydrogen, helium, iron and solar compositions. We also conclude that strongly
magnetized atmosphere models are unable to represent the data. The data can be
explained with a two-component blackbody model. The harder component with
temperature of kT_bb~63 eV and a radius R_bb~2.2 km of the emitting region well
fits the X-ray data and can be interpreted as radiation from a hot region on
the star's surface.Comment: 4 pages, 3 color figures; acceped by A&A Letters;
http://www.xray.mpe.mpg.de/~burwitz/burwitz_refereed.htm
Orbital motion of the young brown dwarf companion TWA 5 B
With more adaptive optics images available, we aim at detecting orbital
motion for the first time in the system TWA 5 A+B. We measured separation and
position angle between TWA 5 A and B in each high-resolution image available
and followed their change in time, because B should orbit around A. The
astrometric measurement precision is about one milli arc sec. With ten year
difference in epoch, we can clearly detect orbital motion of B around A, a
decrease in separation by ~ 0.0054 arc sec per year and a decrease in position
angle by ~ 0.26 degrees per year. TWA 5 B is a brown dwarf with ~ 25 Jupiter
masses (Neuh\"auser et al. 2000), but having large error bars (4 to 145 Jupiter
masses, Neuh\"auser et al. 2009). Given its large projected separation from the
primary star, ~ 86 AU, and its young age ~ 10 Myrs), it has probably formed
star-like, and would then be a brown dwarf companion. Given the relatively
large changes in separation and position angle between TWA 5 A and B, we can
conclude that they orbit around each other on an eccentric orbit. Some evidence
is found for a curvature in the orbital motion of B around A - most consistent
with an elliptic (e=0.45) orbit. Residuals around the best-fit ellipse are
detected and show a small-amplitude (~ 18 mas) periodic sinusoid with ~ 5.7 yr
period, i.e., fully consistent with the orbit of the inner close pair TWA 5
Aa+b. Measuring these residuals caused by the photocenter wobble - even in
unresolved images - can yield the total mass of the inner pair, so can test
theoretical pre-main sequence models.Comment: 6 pages, 4 figures, accepted for publication in A&A; corrected typo
in amplitude below Fig.
Direct detection of exoplanet host star companion Îł Cep B and revised masses for both stars and the sub-stellar object
Context. The star Îł Cep is known as a single-lined spectroscopic triple system at a distance of 13.8 pc, composed of a K1 III-IV primary star with V = 3.2 mag, a stellar-mass companion in a 66-67 year orbit (Torres 2007, ApJ, 654, 1095), and a substellar companion with M_p sin i = 1.7 M_(Jup) that is most likely a planet (Hatzes et al. 2003, ApJ, 599, 1383).
Aims. We aim to obtain a first direct detection of the stellar companion, to determine its current orbital position (for comparison with the spectroscopic and astrometric data), its infrared magnitude and, hence, mass.
Methods. We use the Adaptive Optics camera CIAO at the Japanese 8 m telescope Subaru on Mauna Kea, Hawaii, with the semi-transparent coronograph to block most of the light from the bright primary γ Cep A, and to detect at the same time the faint companion B. In addition, we also used the IR camera Ω Cass at the Calar Alto 3.5 m telescope, Spain, to image γ Cep A and B by adding up many very short integrations (without AO).
Results. γ Cep B is clearly detected on our CIAO and Ω Cass images. We use a photometric standard star to determine the magnitude of B after PSF subtraction in the Subaru image, and the magnitude difference between A and B in the Calar Alto images, and find an average value of K = 7.3 ± 0.2 mag. The separations and position angles between A and B are measured on 15 July 2006 and 11 and 12 Sept. 2006, B is slightly south of west of A.
Conclusions. By combining the radial velocity, astrometric, and imaging data, we have refined the binary orbit and determined the dynamical masses of the two stars in the Îł Cep system, namely 1.40 ± 0.12 M_â for the primary and 0.409 ± 0.018 M_â for the secondary (consistent with being a M4 dwarf). We also determine the minimum mass of the sub-stellar companion to be M_p sin i = 1.60 ± 0.13 M_(Jup)
Astrometric proof of companionship for the L dwarf companion candidate GJ 1048B
Gizis et al. (2001) reported a companion candidate of spectral type L1 near
the K2 dwarf GJ 1048 using the Two Micron All-Sky Survey (2MASS). At that time
it was not possible to verify companionship astrometrically using the 2MASS
data alone due to the small proper motion of GJ 1048. We now show that both
objects share the same proper motion by using data from the UK Schmidt
Telescope Near-infrared (IVN) Southern Survey as the first epoch and data from
2MASS as the second epoch. Our technique of subtracting the PSF of the primary
from the SuperCOSMOS I scans of the Southern Survey enables the astrometry of
the companion candidate to be measured directly.Comment: Accepted to A&A 2004/03/14, 3 pages, 4 figure
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