An idea of Lyubarskii (ref) concerning x-ray variability in low-mass x-ray binaries due
to stochastic variations of the 
-disk parameter, leading to an avalanche within the accretion
disk and stochastically enhanced mass accretion on the central body was investigated in
order to verify if this might be a viable mechanism for flickering in CVs. A numerical
model based on a realistic disk model (albeit containing significant simplifications!) was
elaborated which is able to follow the effects which a local variation of the -parameter
has on the mass flow in the disk, the accretion on the central body and the luminosity of
the system. First results show that even drastic variations of have only a local effect and
trigger very little additional mass transfer on the central body. Thus, the total effect
on the system luminosity is small, suggesting that this mechanism cannot explain the
strong flickering variations in CVs. It is planned to persue the model further and the
publish the results. However, due the the negative preliminary results the project has
reduced priority (A. Bruch).
A photometric study of the pre-cataclysmic variable MT Ser, the central star of the
planetary nebula Abell 41, begun in previous years was continued and finished. The
periodic modulations detected by Grauer & Bond (ApJ 271, 295, 1983) are confirmed,
thus firmly establishing the binary nature of MT Ser. The significantly enlarged time
base permits to derive more accurate ephemeris. The orbital period is either P1=0.113226533
days or twice that value, P2=0.226453066 days. We analyze the light curve
(after a careful subtraction of the nebular contribution) with the Wilson-Devinney light
curve synthesis routine. Since it is not a priori clear which is the true orbital period
of MT Ser, two radically different models, one based on P1 the other on P2
are considered: (1) A low temperature component orbiting around a hot sub-dwarf. The
variability is then due to a reflection effect together with ellipsoidal variations of one
or both components. (2) Two hot sub-dwarfs of similar temperature and luminosity,
partially eclipsing each other and exhibiting ellipsoidal variations. In both models, the
primary as well as the secondary component are required to almost fill their respective
Roche lobes. A contact configuration is possible. Pros and cons can be found for either of
the two models. A final decision between them has to await the observations of a radial
velocity curve. The orbital period is currently decreasing at a rate of 
. Interpreting this as
due to mass loss via a stellar winds permits to estimate mass loss rates depending on the
different model assumptions. These results were published in A&A. (A. Bruch)
Spectroscopy of the dwarf nova AT Ara obtained at the 1.6-m telescope of the OPD
in the previous year was complemented by some photometric observations with high time
resolution (5 sec) obtained in June/July 2001 at the OPD 60-cm Zeiss telescope. The system
is a double lined spectroscopic binary and thus permits to measure radial velocity curves
of both, emission lines from the accretion disk (in this particular case of H) and of
the secondary star. An orbital period of 0.37490 days is derived, placing AT Ara
among the cataclysmic variables with the longest revolution periods. The RV-amplitudes
yield a mass ratio of 
. The spectral type of the secondary is K2. a photometric
parallax of 
pc
is derived. AT Ara has a low orbital inclination of 
and a secondary star mass of 
. Its radius is 
, considerably larger
than that of a main sequence star of the same mass. The mass of the secondary star turns
out to be considerably less than that of a main sequence star of the same spectral type,
making it not only oversized aut also underweighted. (A. Bruch)
Photometric observations of the 
-Sct star V1162 Ori, obtained in the previous year as
part of an international campain were analyzed. The purpose of the observations was to
investigate amplitude and period variability previously observed in this star, and to
search for low-amplitude frequencies. Apart from the main frequency and its harmonics,
four additional frequencies were found in the light curves, all with low amplitudes
(1-3 mmag). Combining this data set with previous observations confirms the new
frequencies and reveals the probable presence of yet another pulsational frequency. All
five low-amplitude frequencies are statistically significant in the data, but at least one
of them suffers from uncertainty due to aliasing. Using colour photometry, evidence is
found for a radial main frequency, while most of the low-amplitude frequencies are likely
non-radial. The main frequency has variable amplitude and period/phase, the latter being
also displayed in the 
diagram of the light extrema. The amplitude variability is
cyclic with a period of 282 days and a range of nearly 20 mmag, but earlier amplitude
values quoted in the literature cannot be explained by this cyclic variation. analysis including
data from the literature shows that the period of V1162 Ori displays a linear period
change as well as sudden cyclic variations on a time scale similar to that of the
amplitude variations. These results were published in A&A. (A. Bruch; in
collaboration with T. Arenthoft, Brussels, et al.).
Photometric observations of the star HD 209295 obtained in the previous year as
part of an international campaign were analyzed. Both, intermediate-order gravity mode and
low-order pressure mode pulsation were detected in this star. It is therefore both a 
Doradus and a Scuti
star, which makes it the first confirmed member of two classes of pulsating star. The
analysis of our 135 h of multisite spectroscopic observations carried out over two seasons
reveals that the star is a single-lined spectroscopic binary with an orbital period of
3.10575 ± 0.00010 d and an eccentricity of 0.352 ± 0.011. Only weak
pulsational signals are found in both the radial velocity and line-profile variations, but
we succeeded in showing the two highest-amplitude Doradus pulsation modes being
consistent with 
and 
. These two
modes dominated the 280 h of BVI
multisite photometry, also obtained over two seasons.
However, altogether eleven frequencies were detected in the light variations, two in the Scuti
regime and nine in the Doradus domain. Five of the Doradus frequencies are exact
integer multiples of the orbital frequency, which which are probalby tidally excited.
Attempts to derive mode identifications from the multicolour photometry failed. A
stability analysis of the pulsations could well reproduce the range in which the Scuti
modes are excited. Doradus pulsations cannot be excited in the models, but a region of
smaller damping in the range of the observed modes is found. The case for tidal excitation
of
Doradus modes was also investigated. It was found that some of the observed harmonics of
the orbital period could be excited, but not all. The observed orbital harmonics not
excited in the models can however be explained by linear combinations of the excited ones.
The secondary component of the system could not be detected in infrared photometry, which
means it may not be a main-sequence star. A strong ultraviolet excess was found in TD-1
measurements, but are unable to explain its origin. The orbit of the primary is consistent
with the secondary being a neutron star. The results of this study were submitted for
publication in A&A. (A. Bruch; in collaboration with G. Handler, SAAO, et
al.).
Spectra of the region between He I 
5876 and H of the classical Nova Sgr 2001 (V4643 Sgr)
in early decline were obtained on two occasion in april and may 2001 at the OPD 1.6-m
telescope. The data were combined with visual estimates of the brightness obtained by
members of the Variable Star Section of the Royal Astronomical Society of New Zealand
(kindly provided by F.M. Bateson). 
(
) times of 4.8 (8.6) days make V4643 Sgr a very fast
nova. The maximum magnitude - rate of decline relation suggests an (upper limit) to the
distance of 
.
The profile of the H emission line consists of a very broad, almost flat component and a
narrower central emission (the equivalent width of the latter decreasing more rapidly than
that of the former) suggesting a geometry of the ejecta consisting of an equatorial ring
and polar blobs and a high orbital inclination of the underlying binary system. The
results of this study were published as an Information Bulletin on Variable Stars.
(A. Bruch)
A limited amount of time-resolved spectroscopy of the Novalike Variable KQ Mon in
the range of H was obtained at the 1.6-m telescope of the OPD. Preliminary results
indicate a possible orbital period right in the middle of the CV period gap, making this
object particularly interesting. Moreover, the system exhibits intermittent P Cyg
profiles in H, possibly repeating on the orbital period. Further observations of this
interesting CV are scheduled for early 2002. (A. Bruch)
Further high time-resolution photometry (5 sec) of this dwarf nova were obtained at the 60-cm Zeiss telescope of the OPD. This data serve to increase the time base for the period determination via eclipse timing and the enlarge the already available photometric data base for a detailed study of eclipses, the orbital variations and the flickering in this system. (A. Bruch)
High time-resolution (5 sec) photometry of this AM Her type CV were obtained at the 60-cm Zeiss telescope of the OPD in order to increase the data base of light curves of AM Her star for an encompassing study of the flickering in these systems (A. Bruch)
A two-week international observing campain of the Sct star HD 21190
involving the OPD 60-cm Zeiss telescope and telescopes in South Africa and Australia met
with little success: While during the entire period of the campain the weather was bad on
OPD and in South Africa, instrumental problems impeded observations in Australia.
(A. Bruch)
We have investigated a group of stars known to have low chromospheric ages, but high kinematical ages. Isochrone, chemical and lithium ages are estimated for them. The majority of stars in this group show lithium abundances much smaller than expected for their chromospheric ages, which is interpreted as an indication of their old age. Radial velocity measurements in the literature also show that they are not close binaries. The results suggest that they can be formed from the coalescence of short-period binaries. Coalescence rates, calculated taking into account several observational data and a maximum theoretical time scale for contact, in a short-period pair, predict a number of coalesced stars similar to what we have found in the solar neighbourhood. (B.V. Castilho)
A small fraction of the low mass red giants presents Li abundances 100 times (or more) higher than the observed mean for the giants. New evidences, both observational and theoretical, suggests that the observed Li is not the star original one, but that this one was almost completely destroyed and the observed one was produced inside the star after the first dredge up stage and lifted to stellar surface by convection. But the details of this process and the exact stage(s) when it occur are unknown. As CNO are good tracers of the nucleosynthesis processes, their abundance analysis will help in the solution of this problem. In previous works we performed the detailed analysis of a sample of red giants with strong Li 6707 Ĺ lines, from what many could be classified as Li-rich giants. In this work we present recent CNO abundance and 12C/13C ratio determinations for this sample. The abundances were obtained from high resolution and high S/N spectra (ESO CES / FEROS, LNA Coudé) though spectrum synthesis. Our results, together with published ones, confirm our previous proposal that most of the Li-rich giants have destroyed their initial Li (by convection) and that the observed one is produced in the stars. Besides this, these results could be used to constrain the Li production models so we can finally understand this process. (B.V. Castilho)
Li abundances are derived for a sample of bulgelike stars with isochronal ages of 10-11
Gyr. These stars have orbits with pericentric distances, 
, as small as 2-3 kpc and 
kpc. The sample
comprises G and K dwarf stars in the metallicity range 
. Few data of Li abundances in old
turn-off stars (> 4.5 Gyr) within the present metallicity range are available. M67 (4.7
Gyr) and NGC 188 (6 Gyr) are the oldest studied metal-rich open clusters with late-type
stars. Li abundances have also been studied for few samples of old metal-rich field stars.
In the present work a high dispersion in Li abundances is found for bulgelike stars for
all the metallicity range, comparable with values in M67. The role of metallicity and age
on a Li depletion pattern is discussed. The possible connection between Li depletion and
oxygen abundance due to atmospheric opacity effects is investigated. (B.V. Castilho)
We performed an empirical synthesis of the blue integrated spectrum of the metal-rich
globular cluster 47 Tucanae, based directly on the color-magnitude diagram of the cluster
coupled to a moderately high-resolution spectral library. Freed from any significant
dependence on theory, we are able to perform a fundamental test of the adequacy of the
spectral library and its associated stellar parameters. Excellent fits are achieved for
almost all absorption-line indices studied, provided the computations are corrected for
two limitations of the spectral library, namely, the lack of a representative set of
metal-poor giants and the absence of CN-strong stars. The latter effect is corrected by
means of spectrum synthesis from model photospheres, considering the abundance pattern of
CN-strong and CN-normal stars. We also need to perform a slight correction of the
metallicity of the cluster (by -0.05 dex in relation to the standard value [Fe/H]=-0.7) in
order to match the metal-line index measurements in the cluster spectrum. After these
relatively small adjustments, the overall spectral agreement is good. Good fits are
achieved for H
, H, Mgb,
<Fe>, Ca 4227 and Fe 4383, and only H 
is overpredicted. Thus, ages inferred from H are slightly older
than the ages based on the other Balmer lines, by ~3 Gyrs. The success of this exercise
suggests that previous failures to synthesize the spectrum of 47 Tuc must have arisen from
inadequacies in the theoretical evolutionary isochrones and/or luminosity functions. Such
a possibility is considered in a companion paper. (B.V. Castilho)
Origin of young stars in nearby star-forming regions: The distribution of known young stars, both of low and of high mass, in the region of the molecular cloud complexes of Ophiuchus, Lupus, and Chamaeleon, and of the Scorpius OB2 association, suggests that they belong to a single large structure. The presence of young stars closer than 100 pc, based on Hipparcos parallaxes, suggests that this large structure extends closer to the Sun than the molecular clouds. The analysis of the motion of pre-main sequence stars and of related groups of young stars provides essential tests of star formation models. The pre-main sequence stars are supposed to be sufficiently young to be very close to their birthplaces and to have velocities still very similar to the initial ones, so that one can get clear constraints on the birth mechanism. The analysis of the space velocities and of the age distributions of the pre-main sequence stars and those of the young early-type stars has shown that they are also very similar. The conclusion is that the two stellar populations have a common global origin, possibly in a nearby spiral arm (Sartori M.J. 2000, Ph.D. Thesis). (M.J. Sartori, in collaboration with J. Lépine and W.S. Dias, IAG/USP)
Historically, T Tauri (TT) stars have been found closely associated with molecular
clouds, where they have formed. However, TT stars are occasionally also found to be
isolated from molecular clouds, as in the case of the stars AS 216 and AS 218,
which are, respectively, 8 and 10 degrees far from the core of the 
Ophiuchi dark cloud. The search for
other young stars close to AS 216 and AS 218 is important to understand the
origin of these isolated stars and to determine their role in the global star formation
history in the solar neighborhood. Possible new pre-main sequence stars have been selected
via their X-ray emission. As most TT stars in a star-forming region typically share the
same radial velocity and proper motion, the membership of a TT star to a particular T
association can be investigated using kinematical data. A project for the observed X-ray
sources proper motions determination has been developed. (M.J. Sartori, in
collaboration with C. Ducourant, Observatoire de Bordeaux, and R. Teixeira,
IAG/USP)
After the detection of a nearby young association in Horologium (HorA) by Torres et al. (2000) and, independently, of a similar one in Tucana (TucA), a survey was begun to search for nearby associations containing young stars (SACY) in the Southern Hemisphere. The candidate stars are selected from a correlation between the ROSAT Bright Source Catalogue and TYCHO-2 or Hipparcos catalogues. The candidate stars are being observed with FEROS at the 1.52m La Silla telescope, mainly to measure their Li equivalent widths and radial velocities. Until now about 650 stars were observed covering nearly 60stars are probably young by their relative Li abundances. In the velocity space there are some concentrations showing the presence of possible young nearby associations. The more proeminent is GAYA, but all those associations are large, occupying vaste regions in the sky.
GAYAI is nearby, about 60 pc away, and spread over about 100 pc. Its mean space velocities are (UVW) = (-9.5 ± 0.9, -21.6 ± 1.1, -0.9 ± 0.7). There is no detected SB among its 30 proposed members.
GAYAII is at about 80 pc and somewhat larger than GAYAI. Its mean space velocities are (UVW) = (-10.2 ± 0.6, -22.1 ± 0.8, -4.6 ± 1.1) and it differs from GAYAI mainly in W velocities.
Oph, forming a unique large young
association at about 120 pc and spread over at least 100 pc. The mean space
velocities of the proposed 23 members are (UVW) = (-3.9 ± 1.4, -13.3 ± 0.8,
-8.3 ± 1.3). The Project SACY has obtained 16 nights in 3 missions at the ESO 1.5-m telescope in 2001. Quast observed in June and Torres in December. (C.A. Torres, G. Quast, in collaboration with L. da Silva and R. de la Reza, Observatório Nacional, Rio de Janeiro).
A study of the spectral properties and of the circumstellar matter distribution in a
large sample of probable new Herbig Ae/Be stars (HAeBe) discovered by the Pico dos Dias
Survey (a search for T Tauri stars based on IRAS colors) is being done. The strong 
emission, spectral
type earlier than F5 and infrared excess similar to T Tauri stars were first used to
classify the candidates by the Pico dos Dias Survey. In order to confirm their young
nature, analysis of low-resolution spectra and of optical and infrared photometry for the
program stars has been performed. A model to fit the spectral energy distribution is being
used in order to determine stellar parameters, and the contribution of different
components in the total emitted flux. As the first results had shown that the near and
middle infrared data are essential to the correct determination of the spectral energy
distribution components, a search in the literature for complementary data and the
organization of these data have been done. (M.J. Sartori, in collaboration with
J. Gregorio-Hetem, IAG/USP)
Some aspects of binary star formation are not quite undertood. Is the binary frequency a function of time or of the initial conditions? The associations detected in the project SACY for, their proximity, could help answer this question and shed some light on orbital evolution. About half of the proposed GAYA members and some other young stars have been observed for radial velocity variations, with FEROS, CORALIE, and the coude spectrograph at Pico dos Dias.
This project has obtained 4 nights with FEROS (ESO time), 3 nights with ADONIS (not used by bad weather) and some nights with CORALIE. (C.A. Torres, G. Quast, in collaborations with C.H. de Figueiredo Melo, Observatoire de Genčve, and L. da Silva, Observatório Nacional, Rio de Janeiro).