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ABSTRACT: We try to constrain the Equation-of-State (EoS) of supra-nuclear-density
matter in neutron stars (NSs) by observations of nearby NSs. There are seven
thermally emitting NSs known from X-ray and optical observations, the so-called
Magnificent Seven (M7), which are young (up to few Myrs), nearby (within a few
hundred pc), and radio-quiet with blackbody-like X-ray spectra, so that we can
observe their surfaces. As bright X-ray sources, we can determine their
rotational (pulse) period and their period derivative from X-ray timing. From
XMM and/or Chandra X-ray spectra, we can determine their temperature. With
precise astrometric observations using the Hubble Space Telescope, we can
determine their parallax (i.e. distance) and optical flux. From flux, distance,
and temperature, one can derive the emitting area - with assumptions about the
atmosphere and/or temperature distribution on the surface. This was recently
done by us for the two brightest M7 NSs RXJ1856 and RXJ0720. Then, from
identifying absorption lines in X-ray spectra, one can also try to determine
gravitational redshift. Also, from rotational phase-resolved spectroscopy, we
have for the first time determined the compactness (mass/radius) of the M7 NS
RBS1223. If also applied to RXJ1856, radius (from luminosity and temperature)
and compactness (from X-ray data) will yield the mass and radius - for the
first time for an isolated single neutron star. We will present our
observations and recent results.
11/2011;
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M Moualla,
T ~O ~B Schmidt,
R Neuhäuser, V ~V Hambaryan,
R Errmann,
L Trepl,
C Broeg,
T Eisenbeiss,
M Mugrauer,
C Marka, [......],
T Pribulla,
S Rätz,
J Schmidt,
A Berndt,
G Maciejewski,
T Röll,
M ~M Hohle,
N Tetzlaff,
S Fiedler,
S Baar
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ABSTRACT: We present a new flare star, which was discovered during our survey on a
selected field at the edge of the Pleiades cluster. The field was observed in
the period 2007 - 2010 with three different CCD-cameras at the University
Observatory Jena with telescopes from 25 to 90 cm. The flare duration is almost
one hour with an amplitude in the R-band of about 1.08 mag. The location of the
flare star in a color-magnitude diagram is consistent with age and distance of
the Pleiades. In the optical PSF of the flare star there are two 2MASS objects
(unresolved in most images in the optical Jena PSF), so it is not yet known
which one of them is responsible for this flare. The BVRIJHK colors yield
spectral types of M1 and M2 with extinction being A_V=0.231+/-0.024 mag and
A_V=0.266+/-0.020 for those two stars, consistent with the Pleiades cluster.
Astronomische Nachrichten 08/2011; 332:661. · 1.01 Impact Factor
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R. Neuhäuser,
R. Errmann,
A. Berndt,
G. Maciejewski,
H. Takahashi,
W.P. Chen,
D.P. Dimitrov,
T. Pribulla,
E.H. Nikogossian,
E.L.N. Jensen, [......],
E. Schmidt,
M.M. Hohle,
M. Kitze,
N. Chakrova,
C. Gräfe,
K. Schreyer, V.V. Hambaryan,
C.H. Broeg,
J. Koppenhoefer,
A.K. Pandey
[show abstract]
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ABSTRACT: We present the Young Exoplanet Transit Initiative (YETI), in which we use several 0.2 to 2.6-m telescopes around the world to monitor continuously young (≤100 Myr), nearby (≤1 kpc) stellar clusters mainly to detect young transiting planets (and to study other variability phenomena on time-scales from minutes to years). The telescope network enables us to observe the targets continuously for several days in order not to miss any transit. The runs are typically one to two weeks long, about three runs per year per cluster in two or three subsequent years for about ten clusters. There are thousands of stars detectable in each field with several hundred known cluster members, e.g. in the first cluster observed, Tr-37, a typical cluster for the YETI survey, there are at least 469 known young stars detected in YETI data down to R = 16.5 mag with sufficient precision of 50 millimag rms (5 mmag rms down to R = 14.5 mag) to detect transits, so that we can expect at least about one young transiting object in this cluster. If we observe ∼10 similar clusters, we can expect to detect ∼10 young transiting planets with radius determinations. The precision given above is for a typical telescope of the YETI network, namely the 60/90-cm Jena telescope (similar brightness limit, namely within ±1 mag, for the others) so that planetary transits can be detected. For targets with a periodic transit-like light curve, we obtain spectroscopy to ensure that the star is young and that the transiting object can be sub-stellar; then, we obtain Adaptive Optics infrared images and spectra, to exclude other bright eclipsing stars in the (larger) optical PSF; we carry out other observations as needed to rule out other false positive scenarios; finally, we also perform spectroscopy to determine the mass of the transiting companion. For planets with mass and radius determinations, we can calculate the mean density and probe the internal structure. We aim to constrain planet formation models and their time-scales by discovering planets younger than ∼100 Myr and determining not only their orbital parameters, but also measuring their true masses and radii, which is possible so far only by the transit method. Here, we present an overview and first results (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Astronomische Nachrichten 06/2011; 332(6):547 - 561. · 1.01 Impact Factor
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R. Neuhäuser,
R. Errmann,
A Berndt,
G. Maciejewski,
H Takahashi,
W P Chen,
D. P. Dimitrov,
T. Pribulla,
E. H. Nikogossian,
E. L. N. Jensen, [......],
E Schmidt,
M. M. Hohle,
M. Kitze,
N. Chakrova,
C. Gräfe,
K. Schreyer, V. V. Hambaryan,
C. H. Broeg,
J. Koppenhoefer,
A. K. Pandey
[show abstract]
[hide abstract]
ABSTRACT: We present the Young Exoplanet Transit Initiative (YETI), in which we use
several 0.2 to 2.6m telescopes around the world to monitor continuously young
(< 100 Myr), nearby (< 1 kpc) stellar clusters mainly to detect young
transiting planets (and to study other variability phenomena on time-scales
from minutes to years). The telescope network enables us to observe the targets
continuously for several days in order not to miss any transit. The runs are
typically one to two weeks long, about three runs per year per cluster in two
or three subsequent years for about ten clusters. There are thousands of stars
detectable in each field with several hundred known cluster members, e.g. in
the first cluster observed, Tr-37, a typical cluster for the YETI survey, there
are at least 469 known young stars detected in YETI data down to R=16.5 mag
with sufficient precision of 50 milli-mag rms (5 mmag rms down to R=14.5 mag)
to detect transits, so that we can expect at least about one young transiting
object in this cluster. If we observe 10 similar clusters, we can expect to
detect approximately 10 young transiting planets with radius determinations.
The precision given above is for a typical telescope of the YETI network,
namely the 60/90-cm Jena telescope (similar brightness limit, namely within
+/-1 mag, for the others) so that planetary transits can be detected. For
planets with mass and radius determinations, we can calculate the mean density
and probe the internal structure. We aim to constrain planet formation models
and their time-scales by discovering planets younger than 100 Myr and
determining not only their orbital parameters, but also measuring their true
masses and radii, which is possible so far only by the transit method. Here, we
present an overview and first results. (Abstract shortened)
06/2011;
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ABSTRACT: 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. 4
05/2010;
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ABSTRACT: Since the first optical detection of RX J0720.4–3125 various observations have been performed to determine astrometric and photometric data. We present the first detection of the isolated neutron star in the V Bessel filter to study the spectral energy distribution and derive a new astrometric position. At ESO Paranal we obtained very deep images with FORS 1 (three hours exposure time) of RX J0720.4–3125 in the V Bessel filter in January 2008. We derive the visual magnitude by standard star aperture photometry. Using sophisticated resampling software we correct the images for field distortions. Then we derive an updated position and proper motion value by comparing its position with FORS 1 observations of December 2000. We calculate a visual magnitude of V = 26.81 ± 0.09 mag, which is seven times in excess of what is expected from X-ray data, but consistent with the extant U, B, and R data. Over about a seven year epoch difference we measured a proper motion of μ = 105.1 ± 7.4 mas yr–1 towards θ = 296.951° ± 0.0063° (NW), consistent with previous data (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Astronomische Nachrichten 02/2010; 331(3):243 - 249. · 1.01 Impact Factor
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ABSTRACT: The analysis of the proper motions of bright members of the Per OB2 association on the basis of Hipparcos high-accuracy astromethc
observations is presented. The existence of two different subgroups of OB-stars in the Per OB2 association that are in the
state of expansion is considered. The expansion ages are calculated as 1.3 and 1.9×106 years. The membership of the observed OB-stars to these subgroups is considered.
Astrophysics 06/1999; 42(3):247-254. · 0.47 Impact Factor
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ABSTRACT: The X-ray data of a sample of 104 flare stars (FSs) in the Pleiades cluster region obtained by Stauffer et al. [1] on the
basis of deep ROSAT PSPC observations are analyzed. If we divide the X-ray emission detected in late-type stars of the Pleiades
cluster into FSs and non-FSs, we find that X-ray luminosities of stars of both groups can be considered as coming from the
same parent population. Moreover, in order to classify stars in a sample of 23 late-type Pleiades stars of unknown nature
discriminant analysis in a four-dimensional parameter space (log (Lx, log (Lx/Lbol), and ROSAT hardness ratios HR1 and HR2) has been used. It can be shown that the majority of these stars (16) are very likely
FSs rather than non-FSs.
Astrophysics 04/1998; 40(4):354-362. · 0.47 Impact Factor
