51 research outputs found
Polarimetry of Li-rich giants
Protoplanetary nebulae typically present non-spherical envelopes. The origin
of such geometry is still controversial. There are indications that it may be
carried over from an earlier phase of stellar evolution, such as the AGB phase.
But how early in the star's evolution does the non-spherical envelope appear?
Li-rich giants show dusty circumstellar envelopes that can help answer that
question. We study a sample of fourteen Li-rich giants using optical
polarimetry in order to detect non-spherical envelopes around them. We used the
IAGPOL imaging polarimeter to obtain optical linear polarization measurements
in V band. Foreground polarization was estimated using the field stars in each
CCD frame. After foreground polarization was removed, seven objects presented
low intrinsic polarization (0.19 - 0.34)% and two (V859 Aql and GCSS 557)
showed high intrinsic polarization values (0.87 - 1.16)%. This intrinsic
polarization suggests that Li-rich giants present a non-spherical distribution
of circumstellar dust. The intrinsic polarization level is probably related to
the viewing angle of the envelope, with higher levels indicating objects viewed
closer to edge-on. The correlation of the observed polarization with optical
color excess gives additional support to the circumstellar origin of the
intrinsic polarization in Li-rich giants. The intrinsic polarization correlates
even better with the IRAS 25 microns far infrared emission. Analysis of
spectral energy distributions for the sample show dust temperatures for the
envelopes tend to be between 190 and 260 K. We suggest that dust scattering is
indeed responsible for the optical intrinsic polarization in Li-rich giants.
Our findings indicate that non-spherical envelopes may appear as early as the
red giant phase of stellar evolution.Comment: to be published in A&A, 15 pages, 10 figures. Fig. 3 is available in
ftp://astroweb.iag.usp.br/pub/antonio/4270/4270.fig3.pd
Discovery of a Luminous Quasar in the Nearby Universe
In the course of the Pico dos Dias survey (PDS), we identified the stellar
like object PDS456 at coordinates alpha = 17h 28m 19.796s, delta = -14deg 15'
55.87'' (epoch 2000), with a relatively nearby (z = 0.184) and bright (B =
14.69) quasar. Its position at Galactic coordinates l_II = 10.4deg, b_II =
+11.2deg, near the bulge of the Galaxy, may explain why it was not detected
before. The optical spectrum of PDS456 is typical of a luminous quasar, showing
a broad (FWHM ~ 4000 km/s) H_\beta line, very intense FeII lines and a weak
[OIII]\lambda5007 line. PDS456 is associated to the infrared source IRAS
17254-1413 with a 60 \mum infrared luminosity L_{60} = 3.8 x 10^{45} erg/s. The
relatively flat slopes in the infrared (\alpha(25,60) = -0.33 and \alpha(12,25)
= -0.78) and a flat power index in the optical (F_{\nu} \propto \nu^{-0.72})
may indicate a low dust content. A good match between the position of PDS456
and the position of the X-ray source RXS J172819.3-141600 implies an X-ray
luminosity L_x = 2.8 x 10^{44} erg/s. The good correlation between the strength
of the emission lines in the optical and the X-ray luminosity, as well as the
steep optical to X-ray index estimated (\alpha_{ox} = -1.64) suggest that
PDS456 is radio quiet. A radio survey previously performed in this region
yields an upper limit for radio power at ~ 5 GHz of ~ 2.6 x 10^{30} erg/s/Hz.
We estimate the Galactic reddening in this line-of-sight to be A_B \simeq 2.0,
implying an absolute magnitude M_B = -26.7 (using H_0 = 75 km s^{-1} Mpc^{-1}
and q_0 = 0). In the optical, PDS456 is therefore 1.3 times more luminous than
3C 273 and the most luminous quasar in the nearby (z \leq 0.3) Universe.Comment: 12 pages, LaTeX (aasms4.sty) + 3 figures; accepted for publication in
the Astrophysical Journal Letter
Internal Structure of Stellar Clusters: Geometry of Star Formation
The study of the internal structure of star clusters provides important clues
concerning their formation mechanism and dynamical evolution. There are both
observational and numerical evidences indicating that open clusters evolve from
an initial clumpy structure, presumably a direct consequence of the formation
in a fractal medium, toward a centrally condensed state. This simple picture
has, however, several drawbacks. There can be very young clusters exhibiting
radial patterns maybe reflecting the early effect of gravity on primordial gas.
There can be also very evolved cluster showing fractal patterns that either
have survived through time or have been generated subsequently by some
(unknown) mechanism. Additionally, the fractal structure of some open clusters
is much clumpier than the average structure of the interstellar medium in the
Milky Way, although in principle a very similar structure should be expected.
Here we summarize and discuss observational and numerical results concerning
this subject.Comment: 8 pages including 4 figures and 2 tables. Conference proceedings of
IAU Symposium 270: Computational Star Formatio
SACY - a Search for Associations Containing Young stars
The scientific goal of the SACY (Search for Associations Containing
Young-stars) was to identify possible associations of stars younger than the
Pleiades Association among optical counterparts of the ROSAT X-ray bright
sources. High-resolution spectra for possible optical counterparts later than
G0 belonging to HIPPARCOS and/or TYCHO-2 catalogs were obtained in order to
assess both the youth and the spatial motion of each target. More than 1000
ROSAT sources were observed, covering a large area in the Southern Hemisphere.
The newly identified young stars present a patchy distribution in UVW and XYZ,
revealing the existence of huge nearby young associations. Here we present the
associations identified in this survey.Comment: 8 pages, 2 figures, to appear in the Proceedings of Open Issues in
Local Formation and Early Stellar Evolution, Ouro Preto, Brazi
A 2MASS Analysis of the Stability of Southern Bok Globules
We used near-infrared 2MASS data to construct visual extinction maps of a
sample of Southern Bok globules utilizing the NICE method. We derived radial
extinction profiles of dense cores identified in the globules and analyzed
their stability against gravitational collapse with isothermal Bonnor-Ebert
spheres. The frequency distribution of the stability parameter xi_max of these
cores shows that a large number of them are located in stable states, followed
by an abrupt decrease of cores in unstable states. This decrease is steeper for
globules with associated IRAS point sources than for starless globules.
Moreover, globules in stable states have a Bonnor-Ebert temperature of T = 15
+- 6 K, while the group of critical plus unstable globules has a different
temperature of T = 10 +- 3 K. Distances were estimated to all the globules
studied in this work and the spectral class of the IRAS sources was calculated.
No variations were found in the stability parameters of the cores and the
spectral class of their associated IRAS sources. On the basis of 13CO J = 1-0
molecular line observations, we identified and modeled a blue-assymetric line
profile toward a globule of the sample, obtaining an upper limit infall speed
of 0.25 km/s.Comment: 53 pages, 15 figures, accepted for publication in Ap
On a Rapid Lithium Enrichment and Depletion of K Giant Stars
A model scenario has recently been introduced to explain the presence of very
strong Li lines in the spectra of some low mass K giant stars (de la Reza et
al. 1996). In this scenario all ordinary, Li poor, K giants become Li rich
during a short time () when compared to the red giant phase of 5
10^7 yr. In this ``Li period'', a large part of the stars are associated with
an expanding thin circumstellar shell supposedly triggered by an abrupt
internal mixing mechanism resulting in a surface new ^7Li enrichment. This
letter presents near 40 Li rich K giants known up to now. The distribution of
these Li rich giants, along with other 41 observed K giants that have shell,
but are not Li rich, in a color-color IRAS diagram confirms this scenario,
indicating, also as a new result, that a rapid Li depletion takes place on a
time scale of between and 10^5 yr. This model explains the problem
of the presence of K giants with far infrared excesses presented by Zuckerman
et al. (1995). Other present and future tests of this scenario are briefly
discussed.Comment: 14 pages, 2 figure
Identifying birth places of young isolated neutron stars
Young isolated radio-quiet neutron stars are still hot enough to be
detectable at X-ray and optical wavelengths due to their thermal emission and
can hence probe cooling curves. An identification of their birth sites can
constrain their age. For that reason we try to identify the parent associations
for four of the so-called Magnificent Seven neutron stars for which proper
motion and distance estimates are available. We are tracing back in time each
neutron star and possible birth association centre to find close encounters.
The associated time of the encounter expresses the kinematic age of the neutron
star which can be compared to its characteristic spin-down age. Owing to
observational uncertainties in the input data, we use Monte-Carlo simulations
and evaluate the outcome of our calculations statistically. RX J1856.5-3754
most probably originated from the Upper Scorpius association about 0.3 Myr ago.
RX 0720.4-3125 was either born in the young local association TWA about 0.4 Myr
ago or in Tr 10 0.5 Myr in the past. Also RX J1605.3+3249 and RBS 1223 seem to
come from a nearby young association such as the Sco-Cen complex or the
extended Corona-Australis association. For RBS 1223 also a birth in Sct OB2 is
possible. We also give constraints on the observables as well as on the radial
velocity of the neutron star. Given the birth association, its age and the
flight time of the neutron star, we estimate the mass of the progenitor star.
Some of the potential supernovae were located very nearby (<100pc) and thus
should have contributed to the 10Be and 60Fe material found in the Earth's
crust. In addition we reinvestigate the previously suggested neutron star/
runaway pair PSR B1929+10/ zeta Ophiuchi and conclude that it is very likely
that both objects were ejected during the same supernova event.Comment: 14 figures, 13 table
Search for Associations Containing Young stars (SACY): I. Sample & Searching Method
We report results from a high-resolution optical spectroscopic survey aimed
to search for nearby young associations and young stars among optical
counterparts of ROSAT All-Sky Survey X-ray sources in the Southern Hemisphere.
We selected 1953 late-type (B-V >= 0.6), potentially young, optical
counterparts out of a total of 9574 1RXS sources for follow-up observations. At
least one high-resolution spectrum was obtained for each of 1511 targets. This
paper is the first in a series presenting the results of the SACY survey. Here
we describe our sample and our observations. We describe a convergence method
in the (UVW) velocity space to find associations. As an example, we discuss the
validity of this method in the framework of the BetaPic Association.Comment: Accepted for publication in A&
NMDA Receptor Hypofunction Leads to Generalized and Persistent Aberrant γ Oscillations Independent of Hyperlocomotion and the State of Consciousness
International audienceNMDAr antagonists acutely produces, in the rodent CNS, generalized aberrant gamma oscillations, which are not dependent on hyperlocomotion-related brain state or conscious sensorimotor processing. These findings suggest that NMDAr hypofunction-related generalized gamma hypersynchronies represent an aberrant diffuse network noise, a potential electrophysiological correlate of a psychotic-like state. Such generalized noise might cause dysfunction of brain operations, including the impairments in cognition and sensorimotor integration seen in schizophrenia
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