260 research outputs found
A New Multiple Stellar System in the Solar Neighborhood
Adaptive optics corrected images obtained with the CIAO instrument at the
Subaru 8.2-meter telescope show the presence of two subarsecond companions to
the nearby (d=19.3 pc) young star GJ 900, which was previously classified as a
single member of the IC 2391 supercluster. The two companions share the same
proper motion as the primary and are redder. Their projected separations from
the primary are 10 AU and 14.5 AU for B and C, respectively. The estimated
masses for the two new companions depend strongly on the age of the system. For
the range of ages found in the literature for IC 2391 supercluster members
(from 35 Myr to 200 Myr), the expected masses range from 0.2 M to 0.4
M for the B component, and from 0.09 M to 0.22 M for
the C component. The determination of the dynamical mass of the faintest
component of GJ 900 will yield the age of the system using theoretical
evolutionary tracks. The apparent separations of the GJ 900 system components
meet the observational criterion for an unstable Trapezium-type system, but
this could be a projection effect. Further observations are needed to establish
the nature of this interesting low-mass multiple system.Comment: Scheduled for publication in the Astronomical Journal (August 2003
The Solar Neighborhood XVII: Parallax Results from the CTIOPI 0.9m Program -- Twenty New Members of the RECONS 10 Parsec Sample
Astrometric measurements for 25 red dwarf systems are presented, including
the first definitive trigonometric parallaxes for 20 systems within 10 pc of
the Sun, the horizon of the RECONS sample. The three nearest systems that had
no previous trigonometric parallaxes (other than perhaps rough preliminary
efforts) are SO 0253+1652 (3.84 +/- 0.04 pc, the 23rd nearest system), SCR
1845-6357 AB (3.85 +/- 0.02 pc, 24th), and LHS 1723 (5.32 +/- 0.04 pc, 56th).
In total, seven of the systems reported here rank among the nearest 100 stellar
systems. Supporting photometric and spectroscopic observations have been made
to provide full characterization of the systems, including complete VRIJHK
photometry and spectral types. A study of the variability of 27 targets reveals
six obvious variable stars, including GJ 1207, for which we observed a flare
event in the V band that caused it to brighten by 1.7 mag.
Improved parallaxes for GJ 54 AB and GJ 1061, both important members of the
10 pc sample, are also reported. Definitive parallaxes for GJ 1001 A, GJ 633,
and GJ 2130 ABC, all of which have been reported to be within 10 pc, indicate
that they are beyond 10 pc. From the analysis of systems with (previously) high
trigonometric parallax errors, we conclude that parallaxes with errors in
excess of 10 mas are insufficiently reliable for inclusion in the RECONS
sample. The cumulative total of new additions to the 10 pc sample since 2000 is
now 34 systems -- 28 by the RECONS team and six by other groups. This total
represents a net increase of 16% in the number of stellar systems reliably
known to be nearer than 10 pc.Comment: 33 pages, including 3 figures and 3 table
Measuring the Magnetic Field on the Classical T Tauri Star TW Hydrae
We present infrared (IR) and optical echelle spectra of the Classical T Tauri
star TW Hydrae. Using the optical data, we perform detailed spectrum synthesis
to fit atomic and molecular absorption lines and determine key stellar
parameters: Teff = 4126 \pm 24 K, log g = 4.84 \pm 0.16, [M/H] = -0.10 \pm
0.12, vsini = 5.8 \pm 0.6 km/s. The IR spectrum is used to look for Zeeman
broadening of photospheric absorption lines. We fit four Zeeman sensitive Ti I
lines near 2.2 microns and find the average value of the magnetic field over
the entire surface is 2.61 \pm 0.23 kG. In addition, several nearby
magnetically insensitive CO lines show no excess broadening above that produced
by stellar rotation and instrumental broadening, reinforcing the magnetic
interpretation for the width of the Ti I lines. We carry out extensive tests to
quantify systematic errors in our analysis technique which may result from
inaccurate knowledge of the effective temperature or gravity, finding that
reasonable errors in these quantities produce a 10% uncertainty in the mean
field measurement.Comment: The tar file includes one Tex file and four .eps figures. The paper
is accepted and tentatively scheduled for the ApJ 1 December 2005, v634, 2
issue. ApJ manuscript submission # 6310
Confronting the Superbubble Model with X-ray Observations of 30 Dor C
We present an analysis of XMM-Newton observations of the superbubble 30 Dor C
and compare the results with the predictions from the standard wind-blown
bubble model. We find that the observed X-ray spectra cannot be fitted
satisfactorily with the model alone and that there is evidence for nonthermal
X-ray emission, which is particularly important at > 4 keV. The total
unabsorbed 0.1-10 keV luminosities of the eastern and western parts of the
bubble are ~3 10^36 erg/s and ~5 10^36 erg/s, respectively. The unabsorbed
0.1-10 keV luminosity of the bubble model is 4 10^36 erg/s and so the power-law
component contributes between 1/3 and 1/2 to the total unabsorbed luminosity in
this energy band. The nature of the hard nonthermal emission is not clear,
although recent supernovae in the bubble may be responsible. We expect that
about one or two core-collapse supernovae could have occured and are required
to explain the enrichment of the hot gas, as evidenced by the overabundance of
alpha-elements by a factor of 3, compared to the mean value of 0.5 solar for
the interstellar medium in the Large Magellanic Cloud. As in previous studies
of various superbubbles, the amount of energy currently present in 30 Dor C is
significantly less than the expected energy input from the enclosed massive
stars over their lifetime. We speculate that a substantial fraction of the
input energy may be radiated in far-infrared by dust grains, which are mixed
with the hot gas because of the thermal conduction and/or dynamic mixing.Comment: 25 pages, 4 figures. To appear in The Astrophysical Journal, August
20, 2004 issu
A Candidate Substellar Companion to HR 7329
We present the discovery of a candidate substellar companion from a survey of
nearby, young stars with the NICMOS coronagraph on the Hubble Space Telescope.
The H ~ 12 mag object was discovered approximately 4" from the young A0V star
HR 7329. Using follow-up spectroscopy from STIS, we derive a spectral type
between M7V and M8V with an effective temperature of ~ 2600 K. We estimate that
the probability of a chance alignment with a foreground dwarf star of this
nature is ~ 10^(-8) and therefore suggest the object (HR 7329B) is physically
associated with HR 7329 with a projected separation of 200 AU. Current brown
dwarf cooling models indicate a mass of less than 50 Jupiter masses for HR
7329B based on age estimates of < 30 Myr for HR7329A.Comment: 8 pages LATEX, 5 ps figures, accepted for Ap
Gauge transformations for higher-order lagrangians
Noether's symmetry transformations for higher-order lagrangians are studied.
A characterization of these transformations is presented, which is useful to
find gauge transformations for higher-order singular lagrangians. The case of
second-order lagrangians is studied in detail. Some examples that illustrate
our results are given; in particular, for the lagrangian of a relativistic
particle with curvature, lagrangian gauge transformations are obtained, though
there are no hamiltonian gauge generators for them.Comment: 22 pages, LaTe
Frequency of Debris Disks around Solar-Type Stars: First Results from a Spitzer/MIPS Survey
We have searched for infrared excesses around a well defined sample of 69 FGK
main-sequence field stars. These stars were selected without regard to their
age, metallicity, or any previous detection of IR excess; they have a median
age of ~4 Gyr. We have detected 70 um excesses around 7 stars at the 3-sigma
confidence level. This extra emission is produced by cool material (< 100 K)
located beyond 10 AU, well outside the ``habitable zones'' of these systems and
consistent with the presence of Kuiper Belt analogs with ~100 times more
emitting surface area than in our own planetary system. Only one star, HD
69830, shows excess emission at 24 um, corresponding to dust with temperatures
> 300 K located inside of 1 AU. While debris disks with Ld/L* > 10^-3 are rare
around old FGK stars, we find that the disk frequency increases from 2+-2% for
Ld/L* > 10^-4 to 12+-5% for Ld/L* > 10^-5. This trend in the disk luminosity
distribution is consistent with the estimated dust in our solar system being
within an order of magnitude, greater or less, than the typical level around
similar nearby stars.Comment: 11 figure
The decay of optical emission from the Gamma-Ray Burst GRB 970228
We present the R_c band light curve of the optical transient (OT) associated
with GRB970228, based on re-evaluation of existing photometry. Data obtained
until April 1997 suggested a slowing down of the decay of the optical
brightness. However, the HST observations in September 1997 show that the light
curve of the point source is well represented by a single power law, with a
``dip'', about a week after the burst occured. The exponent of the power law
decay is = --1.10 0.04. As the point source weakened it also
became redder.Comment: 5 pages, latex, to appear in Gamma-Ray Bursts, 4-th Huntsville
Symposium, eds Meegan, Preece, Koshu
Star Counts in the Globular Cluster Omega Centauri. I. Bright Stellar Components
We present an extensive photometry on HB, RGB, and MSTO stars in Omega Cen.
The central regions of the cluster were covered with a mosaic of F435W, F625W,
and F658N-band data collected with ACS/HST. The outer reaches were covered with
a large set of U,B,V,I-band data collected with the [email protected] ESO/MPI telescope.
The final catalogue includes ~1.7 million stars. We identified ~3,200 likely HB
stars and ~12,500 stars brighter than the subgiant branch and fainter than the
RGB bumps. The HB morphology changes with the radial distance. The relative
number of extreme HB stars decreases from ~30% to ~21% when moving from the
center toward the outer regions of the cluster, while the fraction of less hot
HB stars increases from ~62% to ~72%. We performed a detailed comparison
between observed ratios of different stellar tracers and predictions based on
canonical evolutionary models with a primordial helium (Y=0.23) content and
metal abundances (Z=0.0002,0.001) that bracket the observed spread in
metallicity of Omega Cen stars. We found that the empirical star counts of HB
stars are on average larger (30%-40%) than predicted. Moreover, the rate of HB
stars is 43% larger than the MSTO rate. The discrepancy between the rate of HB
compared with the rate of RG and MSTO stars supports the evidence that we are
facing a true excess of HB stars. The same comparison was performed by assuming
a mix of stellar populations made with 70% of canonical stars and 30% of
He-enhanced stars. The discrepancy between theory and observations decreases by
a factor of two when compared with rates predicted by canonical He content
models, but still 15%-25% (Y=0.42) and 15%-20% (Y=0.33) higher than observed.
Furthermore, the ratio between HB and MSTO star counts are ~24% (Y=0.42) and
30% (Y=0.33) larger than predicted lifetime ratios.Comment: 54 pages, 17 figures,to be published in ApJ, see link at
http://stellari.wiki.zoho.co
The LAOG-Planet Imaging Surveys
With the development of high contrast imaging techniques and infrared
detectors, vast efforts have been devoted during the past decade to detect and
characterize lighter, cooler and closer companions to nearby stars, and
ultimately image new planetary systems. Complementary to other observing
techniques (radial velocity, transit, micro-lensing, pulsar-timing), this
approach has opened a new astrophysical window to study the physical properties
and the formation mechanisms of brown dwarfs and planets. I here will briefly
present the observing challenge, the different observing techniques, strategies
and samples of current exoplanet imaging searches that have been selected in
the context of the LAOG-Planet Imaging Surveys. I will finally describe the
most recent results that led to the discovery of giant planets probably formed
like the ones of our solar system, offering exciting and attractive
perspectives for the future generation of deep imaging instruments.Comment: 6 pages, 5 figures, Invited talk of "Exoplanets and disks: their
formation and diversity" conference, 9-12 March 200
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