Stellar Groups & Associations: PMSs surveys; galactic dynamics; star forming regions

• "A search for T Tauri stars based on the IRAS PSC (Pico dos Dias Survey - PDS)"
  (I) Gregorio-Hetem, Lépine, Quast, Torres, de la Reza (1992) AJ 103, 549-563
  (II) Torres, Quast, de la Reza, Gregorio-Hetem, Lépine (1995) AJ 105, 2146-2154
  We present the results of a survey for new T Tauri stars based on spectroscopic and photometric observations of candidates selected in the IRAS Catalog. Among other interesting objects, PDS has revealed 71 new TTs and 108 HAeBes candidates.

• "X-rays and star formation: ROSAT observations of the Monoceros and Rosette molecular clouds"
  Gregorio-Hetem, Montmerle, Casanova, Feigelson (1998) A&A 331, 193-210
  We used the X-ray emission as a tracer of star formation at distances ~1kpc and detected 62 sources in two molecular clouds previously unstudied in the X-ray band. Comparison with near-infrared data supports the broad correspondence, also found in nearby clouds, between X-ray and near-IR sources associated with young stellar objects.

• "Classification of a selected sample of weak-T Tauri Stars"
  Gregorio-Hetem & Hetem (2002) MNRAS 336, 197
  We studied a sample of 27 T Tauri stars (Tts) showing weak Ha line, selected from the Pico dos Dias Survey, a search for young stellar objects. A model to fit the spectral energy distribution is used to reproduce their observed infrared excess and to evaluate the individual contribution of the circumstellar components (dust disk and/or envelope) to the total emitted flux. We conclude that only 41% of our sample corresponds actually to weak-TTs. Most of the stars has near-IR excess typical of Class II objects and show an important fraction of the observed emission (~30%) due to the circumstellar material.

• "The Origin of the ß Pictoris Moving Group"
  Ortega, V. G.; de la Reza, R.i; Jilinski, E.; Bazzanella, B., 2002, The Astrophysical Journal, Volume 575, Issue 2, pp. L75-L78.
  It is applied here a 3D Galactic orbital treatment for a young association. Using a general Galactic potential and by retracing the orbits of the Beta Pictoris moving group and looking for the first maximum orbital concentration, we found a dynamical age of 11.5 Myr for this group. This age is similar to the one (12 Myr) obtained by means of a HR diagram indicating that the group formed unbound. Aproximate calculations of the past orbits of the SCo-Cen OB association oldest subgroups indicates that supernovae from these sub-groups could triggered the formation of the Beta Pic group.

• "New Aspects of the formation of the Beta Pictoris Moving Group"
  Ortega, de la Reza, Jilinski, Bazzanella, 2004, The Astrophysical Journal, Volume 609, pp.243-246
  A new treatment of the dynamics of the Beta Pic moving group, this time calculating the Sco-Cen OB with the same treatment as the Beta group, leave to the following conclusions: One part of the group have its past orbits confined at the same age of 11.4 Myr (see paper before) in the considered birth region. A second group is found more dispersed around the first one for the same age. By retracing the orbit of one selected runaway B star we were able to identify the supernova that could triggered the Beta Pictoria group. The Sedov explosion equations are discussed for these conditions.

• "On the Origin of the Young Groups Eta and Epsilon Chamaeleontis"
  Jilinski, E.; Ortega, V. G.; de la Reza, R.i 2004, in revision for publication in The Astrophysical Journal
  By retracing back selected members (those with Hipparcos distances) we found that both groups were formed together 7 Myr ago.

• " The Origin of the TW Hydra Moving Group"
  de la Reza, R.; Ortega, V. G.; Jilinski, E. 2004, to be submitted to The Astronomical Journal

• "A method to Search for Associations of Young Stars"
  Quast, Torres, Melo, Sterzik, de la Reza, da Silva (2003) in Open Issues in Local Star Formation, eds. J. Lépine & J. Gregorio-Hetem (Kluwer)

• "Radial Velocities of B Stars in the Scorpio-Centaurus Association"
  E. Jilinski, R. de la Reza and K. Cunha, 2004, to appear in the Astronomical Journal october issue.
  A group of B stars belonging in general to the three subgroups of Sco-Cen (LCC, UCL and US) have been observed at ESO with the FEROS spectrograph. Direct radial velocities have been measured from high resolution spectra to be used in Galactic dynamics studies.

T Tauri stars: circumstellar structure; disks; accretion; binaries; rotation

• "The pre-main sequence spectroscopic binary AK Scorpii revisited" 
  Alencar, Melo,  Dullemond, Andersen,  Batalha, Vaz, Mathieu (2003) A&A 409, 1037
  We analysed high-resolution echelle spectra and light curves of the pre-main sequence spectroscopic binary AK Sco. Disk models to fit the spectral energy distribution of AK Sco indicate that dynamical effects due to tidal interaction of the binary system are responsible for pushing the inner disk radius outwards. Combining simultaneous photometry and spectroscopy we computed the dust obscuration in front of each star. The results demonstrate the existence of substructure at scales of just a single stellar diameter, and also that one side of the orbit is more heavily obscured than the other.

• "Eclipses by circumstellar material in the T Tauri star AA Tau. II. Evidences for non-stationary magnetospheric accretion"
  Bouvier, Grankin, Alencar, et al. (2003) A&A 409, 169 
  We present the results of a synoptic study of the photometric and spectroscopic variability of the classical T Tauri star AA Tau on timescales ranging from a few hours to several weeks. Our results show the system is dynamic and presents non-stationary variability both in the photometry and spectroscopy. We see evidence of a temporary disruption of the magnetic configuration at the disk inner edge and of large scale variations of the magnetic configuration on a timescale of a month. In addition, we report periodic radial velocity variations of the photospheric spectrum which might point to the existence of a 0.02 Msun object orbiting the star at a distance of 0.08 AU.

• "Testing Pre-Main Sequence Evolution Theory: Discovery and Analysis of a Young Low-Mass Eclipsing Binary"
  Stassun,Vaz, Mathieu, Stroud (2003) in Open Issues in Local Star Formation, eds. J. Lépine & J. Gregorio-Hetem (Kluwer)
  SM 790 is another eclipsing binary with pre-Main Sequence (PMS) components discovered in the M42 region. With a period of 2.63d,   SM 790 was observed and we obtained both light curves (Strömgren intermediate bands v, b and y, and Johnson-Cousins broad bands V and I) and radial velocity curves, which were analysed to derive absolute dimensions presented in this work. The components show clear signs of being in the PMS phase: their location in M42, the presence of Li lines in the spectra, the relatively large sizes for thei masses (1.3 Rsun and 0.94 Rsun for 1.01 Msun  and 0,73 Msun, respectively). Besides, the system presents a circular orbit and a light curve modulated by the presence of star spots on both components, which cause significant out-of-eclipse variations with time. We present preliminary absolute dimensions for the components and a discussion on their evolutionary stage.

• "T Tauri Disk Model: Confidence Levels for Parameter Estimation"
  Hetem, Gregorio-Hetem (2003) in Open Issues in Local Star Formation, eds. J. Lépine & J. Gregorio-Hetem (Kluwer)
  A simple model to explain the circumstellar structure of T Tauri stars (TTS) by a c² fitting of the observed Spectral Energy Distribution (SED) has been used by us to evaluate the disk and/or envelope contribution to the total luminosity of the system. The assumed structure considers two dust components and the main fitting parameters are: disk and envelope radius, optical depth and angle of view. The method was applied to a selected sample of TTS recently studied by us (Gregorio-Hetem & Hetem 2002). The present work contains a more detailed description of the method to estimate the permissible range of parameters by considering volumes of confidence levels. The c² values are mapped in a projection of the parameter space and the confidence levels are identified. Some results of calculating and plotting out such confidence limits are presented. The main conclusion of this study is that the problem of multiple solutions in the SED fit can be solved by a statistical treatment of the parameter sets and verifying the discrepancies among them when compared with a common solution.

• "Heating of magnetic tubes of young low mass stars driven by damped Alfven waves"
  Vasconcelos, Jatenco-Pereira, Opher (2003) in Open Issues in Local Star Formation, eds. J. Lépine & J. Gregorio-Hetem (Kluwer)

Herbig Ae/Be stars

• "Investigation of 131 Herbig Ae/Be candidate stars"
  Vieira, Corradi, Alencar, Mendes, Guimarães, Torres, Quast, da Silva (2003) AJ 126, 2971
  We present a new catalog of 131 Herbig Ae/Be candidate stars based on low and/or medium resolution spectroscopy, complemented with high resolution spectra of 39 stars. The presence of  OI and SII forbidden lines and preliminary spectral energy distributions are used together with the Halpha line profiles to discuss the circumstellar environment of the candidates. The large variety of line profiles in the Balmer series and the distribution of  the forbidden lines favor the hypothesis that the stars are observed on different evolutionary stages.

• "Classification of the PDS HAeBe stars"  
  Sartori, Gregorio-Hetem, Hetem (2003), in Open Issues in Local Star Formation, ed. J. Lépine & J. Gregorio-Hetem (Kluwer), p. 133
  In this work we analyzed the circumstellar matter distribution of a large sample  of Herbig Ae/Be (HAeBe) stars identified by the Pico dos Dias Survey (PDS). We  adopted a simple model to fit the spectral energy distribution (SED) of 99 PDS  stars (80 candidates and 19 well-known HAeBe stars). We classified the sample  stars in 3 groups based on the spectral index measured in the visible to mid-IR range because their SEDs show very different shapes, although they all have  similar IRAS colors. We analyzed this classification in relation to other properties of these stars.

• "Chemical composition study of an accretion episode in the Herbig candidate star PDS080"
  Guimarães, Vieira, Alencar, Corradi (2003) in Open Issues in Local Star Formation, ed. J. Lépine & J. Gregorio-Hetem, ASS 299, 141 (Kluwer)
  We present an investigation of an accretion episode in the Herbig candidate PDS080, based in spectra obtained with the 1.52m ESO telescope and FEROS echelle spectrograph at La Silla (Chile) during three nights in May 2002. Redshifted absorption patterns, due to infall of circumstellar matter, were observed in the Balmer lines, and other metallic lines such as Na I D and Ca II. In a preliminary chemical study of the infalling structure we analysed these line profiles to determine whether the redshifted components are due to a gaseous structure or the evaporation of a solid body.

• "Anusual phenomena in the circunstellar envelope of an unique magnetic Ae star HD190073"
  Pogodin, Franco, Lopes, (2003) in Open Issues in Local Star Formation, eds. J. Lépine & J. Gregorio-Hetem (Kluwer)

PMS stars: metallicity; abundance; rotation

• "The Metallicity of Post-T Tauri Stars: A preliminary approach to the understanding of the metal enrichment of stars harboring planets"
  de la Reza, Almeida, Oliveira, Torres, Quast (2003) in the IAU Symp. 219 “Stars and Suns: Activity Evolution and Planets", eds. A. Benz & K. Dupree, in press

• "Chemical Abundances of Weak T Tauri Stars"
  Rojas, Gregorio-Hetem (2003) in Open Issues in Local Star Formation, eds. J. Lépine & J. Gregorio-Hetem (Kluwer)

• "On the Rotation of Post-T Tauri Stars in Associations"
  R. de la Reza, G. Pinzon, 2004, to appear in the Astronomical Journal october issue.
  For the first time is analysed the behavior of stellar rotation in associations with ages between 8 and 30 Myr. By means of a mathematical integral equation, mean distributions of the equatorial rotation velocities are obtained from the observed distributions of vsini velocities. A net spin up is found for the large mass component (1.4< M (Mo)< 2.6) for three associations between 8 and 30 Myr as predicted by stellar models. It is explained why an X- rays desaturation appears only at 30 Myr.

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