Search

Canis Major OB1 (CMa OB1) is a Galactic stellar association with a very intriguing star-formation scenario. There are more than two dozen known star clusters in its line of sight, but it is not clear which ones are physically associated with CMa OB1. We use a clustering code that employs 5-dimensional data from the Gaia DR2 catalogue to identify physical groups and obtain their astrometric parameters and, in addition, we use two different isochrone-fitting methods to estimate the ages of these groups. We find 15 stellar groups with distances between 570 pc and 1650 pc, including 10 previously known and 5 new open cluster candidates. Four groups, precisely the youngest ones ($, Comment: 23 pages, 28 figures, 7 tables

In this study we define a new sample of distant powerful radio galaxies in order to study their host-galaxy properties and provide targets for future observations of HI absorption with new radio telescopes and to understand the fuelling and feedback from such sources. We have cross-matched the Sydney University Molonglo Sky Survey (SUMSS) radio catalogue at 843 MHz with the VISTA Hemisphere Survey (VHS) near-infrared catalogue using the Likelihood Ratio technique. Photometric redshifts from the Dark Energy Survey are then used to assign redshifts to the radio source counterparts. We found a total of 249 radio sources with photometric redshifts over a 148 deg^2 region. By fitting the optical and near-infrared photometry with spectral synthesis models we determine the stellar mass and star-formation rates of the radio sources, finding typical stellar masses of 10^{11} - 10^{12}M$_{\odot}$ for the powerful high-redshift radio galaxies. We also find a population of low-mass blue galaxies. However, by comparing the derived star-formation rates to the radio luminosity, we suggest that these sources are false positives in our likelihood ratio analysis. Follow up, higher-resolution (, 11 pages, 9 figures. Accepted for publication in MNRAS

An exotic environment surrounds the young stellar groups associated with the Canis Major (CMa) OB1/R1 region, which probably was formed under feedback from at least three supernova events having occurred a few million years ago. We use astrometric data from the Gaia-DR2 to confirm the membership of the stars in CMa R1, based on proper motion and parallax, which revealed 514 new members and candidates. The mean age of 5 Myr estimated from the color-magnitude diagram characterizes the sources as likely pre-main sequence candidates. In total, a sample of 694 stars detected with the T80-South telescope was analyzed according to different color-color diagrams, which were compared with theoretical colors from evolutionary models, aiming to reveal the objects that exhibit color excess due to accretion processes. Accretion and magnetic activity were also explored on the basis of empirical flux-flux relation, such as F660 and F861 that are related to Halpha and Ca II triplet emission, respectively. A low fraction (3 percent) of the sample have Halpha excess and other colors expected for stars exhibiting chromospheric activity. The number of Class I and Class II objects, identified by the infrared (WISE) colors, indicates a disk fraction of 6 percent, which is lower than the expected for stellar clusters with similar age. A such large sample of objects associated with CMa R1 without evidences of circumstellar accretion can be interpreted as a lack of disk-bearing stars, unusual for young star-forming regions. However, this may be explained as the result of supernova events., Comment: 29 pages, 9 tables, 11 figures, accepted for publication in The Astronomical Journal

The relationship between young stellar clusters and respective parental molecular clouds is still an open issue: for instance, are the similarities between substructures of clouds and clusters just a coincidence? Or would they be the indication of a physical relationship? In order to address these issues, we have studied the CMa OB1/R1 region that shows evidence for a complex star formation history. We obtained molecular clouds mapping with the IRAM-30 metre telescope to reveal the physical conditions of an unexplored side of the CMa region aiming to compare the morphology of the clouds with the distribution of the young stellar objects (YSOs). We also study the clouds kinematics searching for gradients and jet signatures that could trace different star formation scenarios. The YSOs were selected on the basis of astrometric data from Gaia EDR3 that characterise the moving groups. The distance of 1099$_{-24}^{+25}$ pc was obtained for the sample, based on the mean error-weighted parallax. Optical and near-infrared photometry is used to verify the evolutionary status and circumstellar characteristics of the YSOs. Among the selected candidates we found 40 members associated with the cloud: 1 Class I, 11 Class II, and 28 Class III objects. Comparing the spatial distribution of the stellar population with the cores revealed by the 13CO map, we verify that peaks of emission coincide with the position of YSOs confirming the association of these objects to their dense natal gas. Our observations support the large-scale scenario of the CMa shell-like structure formed as a relic of successive supernova events., A&A (accepted): 11 pages with 8 figures and 4 Tables

The Southern Photometric Local Universe Survey (S-PLUS) is imaging similar to 9300 deg(2) of the celestial sphere in 12 optical bands using a dedicated 0.8mrobotic telescope, the T80-South, at the Cerro Tololo Inter-american Observatory, Chile. The telescope is equipped with a 9.2k x 9.2k e2v detector with 10 mu m pixels, resulting in a field of view of 2 deg(2) with a plate scale of 0.55 arcsec pixel-1. The survey consists of four main subfields, which include two non-contiguous fields at high Galactic latitudes (vertical bar b vertical bar > 30 degrees, 8000 deg(2)) and two areas of the Galactic Disc and Bulge (for an additional 1300 deg(2)). S-PLUS uses the Javalambre 12-band magnitude system, which includes the 5 ugriz broad-band filters and 7 narrow-band filters centred on prominent stellar spectral features: the Balmer jump/[OII], Ca H + K, Hd, G band, Mg b triplet, H alpha, and the Ca triplet. S-PLUS delivers accurate photometric redshifts (dz /(1 + z) = 0.02 or better) for galaxies with r < 19.7 AB mag and z < 0.4, thus producing a 3D map of the local Universe over a volume of more than 1 (Gpc/h)(3). The final S-PLUS catalogue will also enable the study of star formation and stellar populations in and around the Milky Way and nearby galaxies, as well as searches for quasars, variable sources, and low-metallicity stars. In this paper we introduce the main characteristics of the survey, illustrated with science verification data highlighting the unique capabilities of S-PLUS. We also present the first public data release of similar to 336 deg(2) of the Stripe 82 area, in 12 bands, to a limiting magnitude of r = 21, available at datalab.noao.edu/splus.© 2019 The Author(s).Published by Oxford University Press on behalf of the Royal Astronomical Society, The S-PLUS project, including the T80S robotic telescope and the S-PLUS scientific survey, was founded as a partnership between the Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP), the Observatorio Nacional (ON), the Federal University of Sergipe (UFS), and the Federal University of Santa Catarina (UFSC), with important financial and practical contributions from other collaborating institutes in Brazil, Chile (Universidad de La Serena), and Spain (Centro de Estudios de Fisica del Cosmos de Aragon, CEFCA). The members of the collaboration are grateful for the support received from the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq; grants 312333/2014-5, 306968/2014-2, 142436/2014-3, 459553/2014-3, 400738/2014-7, 302037/2015-2, 312307/2015-2, 300336/2016-0, 304184/2016-0, 304971/2016-2, 401669/2016-5, 308968/2016-6, 309456/2016-9, 421687/2016-9, 150237/2017-0, 311331/2017-3, 304819/2017-4, and 200289/2017-9), FAPESP (grants 2009/54202-8, 2011/51680-6, 2014/07684-5, 2014/11806-9, 2014/13723-3, 2014/18632-6, 2016/17119-9, 2016/12331-0, 2016/21532-9, 2016/21664-2, 2016/23567-4, 2017/01461-2, 2017/23766-0, 2018/02444-7, and 2018/21661-9), the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES; grants 88881.030413/2013-01 and 88881.156185/2017-01), the Fundacao de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ; grants 202.876/2015, 202.835/2016, and 203.186/2016), the Financiadora de Estudos e Projetos (FINEP; grants 1217/13-01.13.0279.00 and 0859/10-01.10.0663.00), the Direccion de Investigacion y Desarrollo de la Universidad de La Serena (DIDULS/ULS; projects PR16143 and PTE16146 and the Programa de Investigadores Asociados), and the Direccion de Postgrado y Postitulo. TCB, VMP, and DDW acknowledge the support from the Physics Frontier Center for the Evolution of the Elements (JINA-CEE) through the US National Science Foundation (grant PHY 14-30152). JLNC is grateful for financial support received from the Southern Office of Aerospace Research and development (SOARD; grants FA9550-15-1-0167 and FA9550-18-1-0018) of the Air Force Office of the Scientific Research International Office of the United States (AFOSR/IO). YJT and RAD acknowledge support from the Spanish National Research Council (CSIC) I-COOP + 2016 program (grant COOPB20263), and the Spanish Ministry of Economy, Industry, and Competitiveness (MINECO; grants AYA2013-48623-C2-1-P and AYA2016-81065-C2-1-P). RAOM acknowledges support from the Direccion General de Asuntos del Personal Academico of the Universidad Nacional Autonoma de Mexico (DGAPA-UNAM) through a post-doctoral fellowship from the Programa de Becas Posdoctorales en la UNAM. This work has made use of data from the Sloan Digital Sky Survey. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Enenergy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/.The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), the New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington. This publication makes use of data products from the Widefield Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. We are grateful for the contributions of CTIO staff in helping in the construction, commissioning, and maintenance of the telescope and camera and we are particularly thankful to the CTIO director, Steve Heathcote, for his support at every phase, without which this project would not have been completed. We thank Cesar Iniguez for making the 2D measurements of the filter transmissions at CEFCA. We warmly thank David Cristobal-Hornillos and his group for helping us to install and run the reduction package JYPE version 0.9.9 in the S-PLUS computer system in Chile. We warmly thank Mariano Moles, Javier Cenarro, Tamara Civera, Sergio Chueca, Javier Hernandez Fuertes, Antonio Marin Franch, Jesus Varella, and Hector Vazquez Ramio -the success of the S-PLUS project relies on the dedication of these and other CEFCA staff members in building OAJ and running J-PLUS and J-PAS. We deeply thank Rene Laporte and INPE, as well as Keith Taylor, for their contributions to the T80S camera.

The origin of the arc-shaped Sh2-296 nebula is still unclear. Mainly due to its morphology, the nebula has been suggested to be a 0.5 Myr-old supernova remnant (SNR) that could be inducing star formation in the CMa OB1 association. We aim to show, for the first time, that the nebula is part of a large, shell-like structure, which we have designated the ``CMa shell'', enclosing a bubble created by successive supernova (SN) explosions. We identified three runaway stars, associated with bow-shock structures, in the direction of the CMa shell and we investigate the possibility that they have originated in the center of the shell. By analyzing images of the CMa OB1 association at several wavelengths, we clearly see that the Sh2-296 nebula is in fact part of a large structure, which can be approximated by a large (with a diameter of ~60 pc) elliptical shell. Using the recent Gaia-DR2 astrometric data, we trace back the path of the three runaway stars, in order to find their original position in the past, with relation to the CMa shell. We also revise the heating and ionization of the Sh2-296 nebula, by comparing the photon budget provided by the O stars in the region with results from radio observations. We find that the runaway stars have likely been ejected from a Trapezium-like progenitor cluster on three successive SN explosions having taken place ~6, ~2 and ~1 Myr ago. We also show that the few late-type O stars in the region cannot explain the ionization of the Sh~2-296 nebula and other mechanisms need to be at work. We argue that, though we now have evidence for several SNe events in the CMa OB1 association, the SNe probably played a minor role in triggering star formation in these clouds. In contrast, the CMa OB1 association, as it is now, likely testifies to the last stages of a star-forming region., 15 pages, 10 figures, accepted by A&A

The Canis Major OB1 Association has an intriguing scenario of star formation, especially in the Canis Major R1 (CMa R1) region traditionally assigned to a reflection nebula, but in reality an ionized region. This work is focused on the young stellar population associated to CMa R1, for which our previous results from ROSAT, optical and near-infrared data had revealed two stellar groups with different ages, suggesting a possible mixing of populations originated from distinct star-formation episodes. The X-ray data allow the detected sources to be characterized according to hardness ratios, light curves and spectra. Estimates of mass and age were obtained from the 2MASS catalogue, and used to define a complete subsample of stellar counterparts, for statistical purposes. A catalogue of 387 XMM-Newton sources is provided, 78% being confirmed as members or probable members of the CMa R1 association. Flares were observed for 13 sources, and the spectra of 21 bright sources could be fitted by a thermal plasma model. Mean values of fits parameters were used to estimate X-ray luminosities. We found a minimum value of log(L$_X$[erg/s]) = 29.43, indicating that our sample of low-mass stars (M$_\star$ $\leq$ 0.5 M$_\odot$), being faint X-ray emitters, is incomplete. Among the 250 objects selected as our complete subsample (defining our best sample), 171 are found to the East of the cloud, near Z CMa and dense molecular gas, 50% of them being young (< 5 Myr) and 30% being older (> 10 Myr). The opposite happens to the West, near GU CMa, in areas lacking molecular gas: among 79 objects, 30% are young and 50% are older. These findings confirm that a first episode of distributed star formation occurred in the whole studied region ~10 Myr ago and dispersed the molecular gas, while a second, localized episode (< 5 Myr) took place in the regions where molecular gas is still present., 38 pages, 21 figures, accepted for A&A

The scenario of star cluster formation can be better understood based on the detailed study of the dynamical conditions of the associated gas, clustering properties and effects of ionizing sources, among others. Some of these characteristics are explored in our ongoing work based on observations with the SOAR and T80-S telescopes. With these data we have obtained a complete multi-band photometric catalogue of selected clusters that we characterize using color-color diagrams and flux ratios. In particular, this work is focused on SOAR/Spartan (near-infrared) observations of the Canis Major star-forming region.

Our team is carrying out a multi-year observing program to directly image and characterize young extrasolar planets using the Near-Infrared Coronagraphic Imager (NICI) on the Gemini-South 8.1-meter telescope. NICI is the first instrument on a large telescope designed from the outset for high-contrast imaging, comprising a high-performance curvature adaptive optics system with a simultaneous dual-channel coronagraphic imager. Combined with state-of-the-art observing methods and data processing, NICI typically achieves ~2 magnitudes better contrast compared to previous ground-based or space-based programs, at separations inside of ~2 arcsec. In preparation for the Campaign, we carried out efforts to identify previously unrecognized young stars, to rigorously construct our observing strategy, and to optimize the combination of angular and spectral differential imaging. The Planet-Finding Campaign is in its second year, with first-epoch imaging of 174 stars already obtained out of a total sample of 300 stars. We describe the Campaign's goals, design, implementation, performance, and preliminary results. The NICI Campaign represents the largest and most sensitive imaging survey to date for massive (~1 Mjup) planets around other stars. Upon completion, the Campaign will establish the best measurements to date on the properties of young gas-giant planets at ~5-10 AU separations. Finally, Campaign discoveries will be well-suited to long-term orbital monitoring and detailed spectrophotometric followup with next-generation planet-finding instruments., Proceedings of the SPIE, vol 7736 (Advances in Adaptive Optics, San Diego, CA, June 2010 meeting), in press

We report the discovery of a tight substellar companion to the young solar analog PZ Tel, a member of the Beta Pictoris moving group observed with high contrast adaptive optics imaging as part of the Gemini NICI Planet-Finding Campaign. The companion was detected at a projected separation of 16.4 +/- 1.0 AU (0.33 +/- 0.01") in April 2009. Second-epoch observations in May 2010 demonstrate that the companion is physically associated and shows significant orbital motion. Monte Carlo modeling constrains the orbit of PZ Tel B to eccentricities > 0.6. The near-IR colors of PZ Tel B indicate a spectral type of M7+/-2 and thus this object will be a new benchmark companion for studies of ultracool, low-gravity photospheres. Adopting an age of 12 +8 -4 Myr for the system, we estimate a mass of 36 +/- 6 Mjup based on the Lyon/DUSTY evolutionary models. PZ Tel B is one of few young substellar companions directly imaged at orbital separations similar to those of giant planets in our own solar system. Additionally, the primary star PZ Tel A shows a 70 um emission excess, evidence for a significant quantity of circumstellar dust that has not been disrupted by the orbital motion of the companion., 15 pages, 4 figures, to appear in ApJ Letters

A large sample of Herbig Ae/Be (HAeBe) candidates, distributed in different Galactic regions south to declination +30°, were identified by the Pico dos Dias Survey (a search for young stellar objects based on IRAS colors). Most of the candidates are nearby or associated with star-forming clouds, but several others are considered isolated objects. Aiming to verify the young nature of 93 HAeBe candidates, we searched for additional information that could be useful to confirm if they are pre-main-sequence (PMS) stars or evolved objects, which coincidentally show similar IRAS colors. By adopting a spectral index that is related to the amount of infrared excess and the shape of the spectral energy distribution, we have classified the sample according to three groups, which are analyzed on the basis of (1) circumstellar luminosity; (2) spatial distribution; (3) optical polarization; (4) near-infrared colors; (5) stellar parameters (mass, age, effective temperature); and (5) intensity of emission lines. Our analysis indicates that only 76% of the studied sample, mainly the group with intermediate to low levels of circumstellar emission, can be more confidently considered PMS stars. The nature of the remaining stars, which are in the other group that contains the highest levels of infrared excess, remains to be confirmed. They share the same characteristics of evolved objects, requiring complementary studies in order to correctly classify them. At least seven objects show characteristics typical of post-asymptotic giant branch or proto-planetary nebulae.

This was a very pleasant and interesting meeting on star formation. The debate run on freely and contributions were of a very high level, including the oral contributions of four exceptional graduate students.

We present a study of the correlation between the direction of the symmetry axis of the circumstellar material around intermediate mass young stellar objects and that of the interstellar magnetic field. We use CCD polarimetric data on 100 Herbig Ae/Be stars. A large number of them shows intrinsic polarization, which indicates that their circumstellar envelopes are not spherical. The interstellar magnetic field direction is estimated from the polarization of field stars. There is an alignment between the position angle of the Herbig Ae/Be star polarization and that of the field stars for the most polarized objects. This may be an evidence that the ambient interstellar magnetic field plays a role in shaping the circumstellar material around young stars of intermediate mass and/or in defining their angular momentum axis., ApJ accepted

A detailed study was performed for a sample of low-mass pre-main-sequence (PMS) stars, previously identified as weak-line T Tauri stars, which are compared to members of the Tucanae and Horologium Associations. Aiming to verify if there is any pattern of abundances when comparing the young stars at different phases, we selected objects in the range from 1 to 100 Myr, which covers most of PMS evolution. High-resolution optical spectra were acquired at European Southern Observatory and Observatdrio do Pico dos Dias. The stellar fundamental parameters effective temperature and gravity were calculated by excitation and ionization equilibria of iron absorption lines. Chemical abundances were obtained via equivalent width calculations and spectral synthesis for 44 per cent of the sample, which shows metallicities within 0.5 dex solar. A classification was developed based on equivalent width of Li I 6708 A and Ha lines and spectral types of the studied stars. This classification allowed a separation of the sample into categories that correspond to different evolutive stages in the PMS. The position of these stars in the Hertzsprung-Russell diagram was also inspected in order to estimate their ages and masses. Among the studied objects, it was verified that our sample actually contains seven weak-line T Tauri stars, three are Classical T Tauri, 12 are Fe/Ge PMS stars and 21 are post-T Tauri or young main-sequence stars. An estimation of circumstellar luminosity was obtained using a disc model to reproduce the observed spectral energy distribution. Most of the stars show low levels of circumstellar emission, corresponding to less than 30 per cent of the total emission.

The protoplanetary discs of T Tauri and Herbig Ae/Be stars have been studied by using geometric disc models to fit their spectral energy distribution (SED). The simulations provide means to reproduce the signatures of different circumstellar structures, which are related to different levels of infrared excess. Aiming to improve our previous model that assumed a simple flat disc configuration, in the present work we adopt a reprocessing flared disc model that considers hydrostatic, radiative equilibrium (Dullemond et al. 2001). We developed a method to optimise the parameters estimation based on genetic algorithms (GA). This paper is dedicated to describe the implementation of the new code, which has been applied for Herbig stars from the Pico dos Dias Survey catalogue, in order to illustrate the quality of the fitting for a variety of SED shapes. The star AB Aur was used as a test of the GA parameters estimation, demonstrating that the new code reproduces successfully a canonical example of the flared disc model. The GA method gives good quality of fittings, but the range of input parameters must be chosen with caution, since unrealistic disc parameters can be derived. The flared disc model is confirmed to fit the flattened SEDs typical from Herbig stars, however the embedded objects (increasing SED slope) and debris discs (steeper SED) are not well fitted with this configuration. Even considering the limitation of the derived parameters, the automatic process of SED fitting provides an interesting tool for the statistical analysis of the circumstellar luminosity of large samples of young stars., Comment: 12 pages, 12 figures. MNRAS Accepted

A large sample of young stellar groups is analysed aiming to investigate their clustering properties and dynamical evolution. A comparison of the Q statistical parameter, measured for the clusters, with the fractal dimension estimated for the projected clouds shows that 52% of the sample has substructures and tends to follow the theoretically expected relation between clusters and clouds, according to calculations for artificial distribution of points. The fractal statistics was also compared to structural parameters revealing that clusters having radial density profile show a trend of parameter s increasing with mean surface stellar density. The core radius of the sample, as a function of age, follows a distribution similar to that observed in stellar groups of Milky Way and other galaxies. They also have dynamical age, indicated by their crossing time that is similar to unbound associations. The statistical analysis allowed us to separate the sample into two groups showing different clustering characteristics. However, they have the same dynamical evolution, since the whole sample has been revealed as expanding objects, for which the substructures seem to have not been erased. These results are in agreement with simulations that adopt low surface densities and models under supervirial conditions., 11 pages, 8 figures, accepted by MNRAS

We studied a sample of stars associated with the Sh 2-296 nebula, part of the reflection nebulae complex in the region of Canis Major (CMa R1). Our sample corresponds to optical counterparts of X-ray sources detected from observations with the XMM-Newton satellite, which revealed dozens of possible low-mass young stars not yet known in this region. A sample of 58 young star candidates were selected based on optical spectral features, mainly H{\alpha} and lithium lines, observed with multi-objects spectroscopy performed by the Gemini South telescope. Among the candidates, we find 41 confirmed T Tauri and 15 very likely young stars. Based on the H{\alpha} emission, the T Tauri stars were distinguished between classical (17%) and weak-lined (83%), but no significant difference was found in the age and mass distribution of these two classes. The characterization of the sample was complemented by near- and mid-infrared data, providing an estimate of ages and masses from the comparison with pre-main-sequence evolutionary models. While half of the young stars have an age of 1-2 Myrs or less, only a small fraction (~25%) shows evidence of IR excess revealing the presence of circumstellar discs. This low fraction is quite rare compared to most young star-forming regions, suggesting that some external factor has accelerated the disc dissipation.

We present BVRI CCD aperture polarization and near-infrared photometry of the proto-planetary nebula Hen 3-1475. Its intrinsic polarization is high and shows a strong spectral dependence. The position angles in all bands are perpendicular to the axis of the observed bipolar structure. A Monte Carlo code is used to model the intrinsic polarization of \hhe. Using disk dimensions and other constraints suggested by previous works, we are able to reproduce the observations with an optically thick disk composed by grains with a power-law size distribution ranging from 0.06 to 0.22 um. We also reliably estimate the foreground polarization from hundreds of stars contained in the CCD images. It is parallel to the intrinsic polarization of Hen 3-1475. Possible implications of this result are discussed. From IR observations, we estimate a interstellar reddening, A(V), of about 3.2.

We studied a sample of 27 T Tauri stars (TTs) showing a 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 in order to reproduce their observed infrared excess and to evaluate the individual contribution of the circumstellar components (dust disc and/or envelope) to the total emitted flux. The objects were separated into different categories of young stars, searching for an agreement with an evolutionary sequence. The classification is based on the stellar characteristics as spectral type; equivalent width of the Li 670.8-nm line; optical and X-ray properties; and infrared excess. We conclude that only 41 per cent of our sample actually correspond to weak TTs, and the remaining objects are mainly classical TTs or young main-sequence stars. Most of the stars have a near-infrared index typical of Class II objects and show an important fraction of the observed emission (∼30 per cent) being generated by the circumstellar material.

Aiming to contribute to the understanding of star formation and evolution in the Canis Major (CMa R1) Molecular Clouds Complex, we analyze the spectral characteristics of a population of young stars associated with the arc-shaped nebula Sh2-296. Our XMM/Newton observations detected 109 X-ray sources in the region and optical spectroscopy was performed with Gemini telescope for 85 optical counterparts. We identified and characterized 51 objects that present features typically found in young objects, such as Hα emission and strong absorption on the Li I line.

We present intermediate-resolution (FWHM∼1.5–2.6 A) optical spectra of 106 candidate optical counterparts of 77 X-ray sources detected in four pointed (t exp ≥ 8700 s) ROSAT PSPC observations of the ρ Ophiuchi star-forming region and vicinity. Using the spectral types and equivalent widths of Hα and Li iλ670.8 nm obtained from our spectra, we applied spectroscopic criteria in order to classify our sample in different pre-main-sequence subtypes: ‘classical’ T Tauri stars (CTTS); ‘weak’ T Tauri stars (WTTS), and ‘post’ T Tauri stars (PTTS). A total of 10 CTTS, 43 WTTS and 6 PTTS were found among the PSPC-selected stars. Our results more than double the number of pre-main-sequence stars spectroscopically identified in the ρ Ophiuchi region.We considered regions with different molecular cloud properties: the central core and the outer ring of the ρ Ophiuchi dark cloud (L1688); the ‘streamers’ (L1709); the R7 clump; and the ‘smoke rings’. In the inner field of L1688, the ratio of WTTS over CTTS is ∼1:1, significantly smaller than in the other regions (4:1 in the outer ring of L1688 and 5:1 in the smoke rings). The WTTS/CTTS ratio in the R7 field is the highest of our survey (10:1). We argue that this could be a result of the UV radiation from the nearby massive binary ρ Oph AB, and/or of winds from the Upper Sco OB association, which might shorten the lifetime of the circumstellar discs of the low-mass stars.We find no PTTS in the inner field of our L1688 PSPC image, and only 3 PTTS in the outer ring despite the high sensitivity of our X-ray observations in this region. This result confirms that the central region of L1688 is extremely young (age < 5 Myr), as suggested by near-infrared surveys. The presence of a small number of PTTS scattered around the ρ Ophiuchi molecular clouds suggests that star formation may have been going on for 10 to 30 Myr, very slowly at first, but at a much higher rate for the last ∼10 Myr. We provide rough estimates of the star formation rate for the main ρ Ophiuchi molecular cloud complex.

In a previous work we studied the IRAS colours of known Li-rich red giants and showed that they have flux ratios F12=F25 and F25=F60 in well dened ranges. By using this result as a selection criterion, we prepared a list of 280 IRAS Point Source candidates to be Li-rich giant stars. Up to the present we have obtained spectra for 57% of our target list. We identied ve stars showing a strong LiI 670.079 nm line and six ones with a Li line of medium strength. Most of the candidates show features typical of normal giants having circumstellar dust, as indicated by their IRAS colours.

The results of an optical monitoring campaign on the active nucleus in the luminous Seyfert 1 galaxy Fairall 9 are presented. This campaign was undertaken in parallel with ultraviolet spectroscopic moni- toring with the IUE satellite which is described in a separate paper. The primary purpose of this program is to measure the response times (or "" lags II) of the emission lines to continuum variations and thus to extend the range in luminosity of active galactic nuclei (AGNs) for which such measurements have been made. The main conclusions of this work are as follows: 1. Continuum (at 5340 variations of amplitude D12% are detected on timescales as short as D20 Ae ) days. These variations are much larger than the typical uncertainties in the measurements, which are of order D2%. Over D94 days, a factor of 2 change in the nuclear continuum was observed. 2. The optical continuum light curve resembles that of the UV continuum, showing two "" events II of low-amplitude variations with a duration of D70 days and with no measurable lag between the UV and optical continuum light curves. The UV data show a third larger amplitude event that occurred after the optical monitoring had terminated and unfortunately went unobserved in the optical. 3. The Hb emission-line Nux also underwent signi-cant, low-amplitude ("20%) variations. Cross- correlation analysis reveals that Hb lags behind the UV continuum by about 23 days, a value much smaller than what was previously suggested by earlier variability studies. However, this small lag is con- sistent with the lags for the UV lines during this campaign in the sense that the Hb lag is approximately 50% larger than that of Ly aj 1216, as it has been found for lower luminosity AGNs. 4. The Hb di†erence pro-le produced by subtracting the low-state from the high-state data can be described as a two-component structure with blue and red components of similar width (D2500 km s~1) and that appear to vary in phase. Subject headings: galaxies: active E galaxies: individual (Fairall 9) E galaxies: nuclei E galaxies: Seyfert

We have completed a high-contrast direct imaging survey for giant planets around 57 debris disk stars as part of the Gemini NICI Planet-Finding Campaign. We achieved median H-band contrasts of 12.4 mag at 0.5" and 14.1 mag at 1" separation. Follow-up observations of the 66 candidates with projected separation < 500 AU show that all of them are background objects. To establish statistical constraints on the underlying giant planet population based on our imaging data, we have developed a new Bayesian formalism that incorporates (1) non-detections, (2) single-epoch candidates, (3) astrometric and (4) photometric information, and (5) the possibility of multiple planets per star to constrain the planet population. Our formalism allows us to include in our analysis the previously known Beta Pictoris and the HR 8799 planets. Our results show at 95% confidence that 5MJup planet beyond 80 AU, and 3MJup planet outside of 40 AU, based on hot-start evolutionary models. We model the population of directly-imaged planets as d^2N/dMda ~ m^alpha a^beta, where m is planet mass and a is orbital semi-major axis (with a maximum value of amax). We find that beta < -0.8 and/or alpha > 1.7. Likewise, we find that beta < -0.8 and/or amax < 200 AU. If we ignore the Beta Pic and HR 8799 planets (should they belong to a rare and distinct group), we find that < 20% of debris disk stars have a > 3MJup planet beyond 10 AU, and beta < -0.8 and/or alpha < -1.5. Our Bayesian constraints are not strong enough to reveal any dependence of the planet frequency on stellar host mass. Studies of transition disks have suggested that about 20% of stars are undergoing planet formation; our non-detections at large separations show that planets with orbital separation > 40 AU and planet masses > 3 MJup do not carve the central holes in these disks., Accepted to ApJ on June 24, 2013. 67 pages, 17 figures, 12 tables

We have carried out high contrast imaging of 70 young, nearby B and A stars to search for brown dwarf and planetary companions as part of the Gemini NICI Planet-Finding Campaign. Our survey represents the largest, deepest survey for planets around high-mass stars (~1.5-2.5 M_sun) conducted to date and includes the planet hosts beta Pic and Fomalhaut. We obtained follow-up astrometry of all candidate companions within 400 AU projected separation for stars in uncrowded fields and identified new low-mass companions to HD 1160 and HIP 79797. We have found that the previously known young brown dwarf companion to HIP 79797 is itself a tight (3 AU) binary, composed of brown dwarfs with masses 58 (+21, -20) M_Jup and 55 (+20, -19) M_Jup, making this system one of the rare substellar binaries in orbit around a star. Considering the contrast limits of our NICI data and the fact that we did not detect any planets, we use high-fidelity Monte Carlo simulations to show that fewer than 20% of 2 M_sun stars can have giant planets greater than 4 M_Jup between 59 and 460 AU at 95% confidence, and fewer than 10% of these stars can have a planet more massive than 10 M_Jup between 38 and 650 AU. Overall, we find that large-separation giant planets are not common around B and A stars: fewer than 10% of B and A stars can have an analog to the HR 8799 b (7 M_Jup, 68 AU) planet at 95% confidence. We also describe a new Bayesian technique for determining the ages of field B and A stars from photometry and theoretical isochrones. Our method produces more plausible ages for high-mass stars than previous age-dating techniques, which tend to underestimate stellar ages and their uncertainties., Comment: 84 pages, 26 figures, 10 tables. ApJ in press

A model with two dust components is used do explain the circumstellar structure of weak-T Tauri stars. The IR-excess was calculated and compared to spectroscopic criteria in order to classify the objects according an evolutionary sequence. About 46% of the sample correspond to young main sequence stars showing dust distribution consistent with a disrupted disk, that could be possibly caused by the formation of a planetary system.

We present the discovery of a wide (67 AU) substellar companion to the nearby (21 pc) young solar-metallicity M1 dwarf CD-35 2722, a member of the ~100 Myr AB Doradus association. Two epochs of astrometry from the NICI Planet-Finding Campaign confirm that CD-35 2722 B is physically associated with the primary star. Near-IR spectra indicate a spectral type of L4\pm1 with a moderately low surface gravity, making it one of the coolest young companions found to date. The absorption lines and near-IR continuum shape of CD-35 2722 B agree especially well the dusty field L4.5 dwarf 2MASS J22244381-0158521, while the near-IR colors and absolute magnitudes match those of the 5 Myr old L4 planetary-mass companion, 1RXS J160929.1-210524 b. Overall, CD-35 2722 B appears to be an intermediate-age benchmark for L-dwarfs, with a less peaked H-band continuum than the youngest objects and near-IR absorption lines comparable to field objects. We fit Ames-Dusty model atmospheres to the near-IR spectra and find T=1700-1900 K and log(g) =4.5\pm0.5. The spectra also show that the radial velocities of components A and B agree to within \pm10 km/s, further confirming their physical association. Using the age and bolometric luminosity of CD-35 2722 B, we derive a mass of 31\pm8 Mjup from the Lyon/Dusty evolutionary models. Altogether, young late-M to mid-L type companions appear to be over-luminous for their near-IR spectral type compared to field objects, in contrast to the under-luminosity of young late-L and early-T dwarfs., 35 pages, 11 figures, 4 tables. Accepted to ApJ on January 10, 2011. Minor typo correction

International audience; We present some preliminary results based on the CoRoT observations of the young distant cluster Dolidze 25. The light curve for the 28 stars observed by the satellite have been analyzed and the relevant periodogram presented. As a result we have found two binary, three Pulsating Be, three SPB, two Spotted and one ellipsoidal variables. Noteworthy, we have also identified one PMS delta Scuti candidate.

A sample of 27 sources, catalogued as pre-main sequence stars by the Pico dos Dias Survey (PDS), is analyzed to investigate a possible contamination by post-AGB stars. The far-infrared excess, due to dust present in the circumstellar envelope, is typical for both categories: young stars and objects that have already left the main sequence and are suffering a severe mass-loss. The presence of two known post-AGB stars in our sample inspired us to seek for other very likely or possible post-AGB objects among PDS sources previously suggested to be Herbig Ae/Be stars, by revisiting the observational database of this sample. In a comparative study with well known post-AGBs, several characteristics were evaluated: (i) parameters related to the circumstellar emission; (ii) spatial distribution to verify the background contribution from dark clouds; (iii) spectral features, and (iv) optical and infrared colors. These characteristics suggest that 7 objects of the studied sample are very likely post-AGBs, 5 are possible post-AGBs, 8 are unlikely post-AGBs, and the nature of 7 objects remains unclear., Comment: 12 pages, 9 figures

The CMa R1 star-forming region contains several compact clusters as well as many young early-B stars. It is associated with a well-known bright rimmed nebula, the nature of which is unclear (fossil HII region or supernova remnant). To help elucidate the nature of the nebula, our goal was to reconstruct the star-formation history of the CMa R1 region, including the previously unknown older, fainter low-mass stellar population, using X-rays. We analyzed images obtained with the ROSAT satellite, covering ~5 sq. deg. Complementary VRI photometry was performed with the Gemini South telescope. Colour-magnitude and colour-colour diagrams were used in conjunction with pre-main sequence evolutionary tracks to derive the masses and ages of the X-ray sources. The ROSAT images show two distinct clusters. One is associated with the known optical clusters near Z CMa, to which ~40 members are added. The other, which we name the "GU CMa" cluster, is new, and contains ~60 members. The ROSAT sources are young stars with masses down to M_star ~0.5 M_sun, and ages up to 10 Myr. The mass functions of the two clusters are similar, but the GU CMa cluster is older than the cluster around Z CMa by at least a few Myr. Also, the GU CMa cluster is away from any molecular cloud, implying that star formation must have ceased; on the contrary (as already known), star formation is very active in the Z CMa region., 19 pages, 15 figures, accepted by A&A

We have studied the IRAS colours of Li-rich giants, concluding that they define a locus on the IRAS colours diagram (Gregorio-Hetem et al. 1993), within a box of colours −0.7

Class I protostars1 are very young, low-mass stellar objects that are, according to current models, composite: they include a central star (still in the process of formation) surrounded by an accretion disk ~10-10OAu in radius and embedded in an extended, infalling envelope of gas and dust up to ~104 AU in size2. X-ray emission from such protostars has recently been reported3,4, suggesting that X-ray ionization of gas and heating of dust could profoundly influence the physical and chemical properties of young stellar systems. But these observations did not have the resolution necessary to rule out a non-protostar origin for the emissions. Here we report X-ray observations of one of the nearest star-forming regions—the ρ Ophiuchi cloud—that clearly show an intense X-ray flare associated with a deeply embedded protostar. The peak X-ray luminosity, after correcting for extinction, is ≥10-100 times the Sun's bolometric luminosity. The behaviour and intensity of the flare can be modelled as arising from a magnetically confined, low-density plasma bubble ~0.050-0.3 Au in diameter (much larger than the star itself), and the X-ray luminosity equals or exceeds the bolometric luminosity of the forming star. Taken together, the evidence suggests that the X-rays are not created by the type of magnetic activity seen on the Sun or on other young low-mass stars, but rather are associated with processes in the circumstellar accretion disk or within the envelope.

The X-ray emission detected by ROSAT is used to search for unidentified young stars in the Canis Majoris (CMa R1) cloud, which distance is ~ 1kpc. We studied the properties of 61 X-ray sources detected in the PSPC image. Most of them have optical and infrared counterparts, which probably are low- and intermediate-mass young stars, as indicated by the strong correlation of the X-ray emission and other stellar characteristics. CMa R1 shows a ring emission nebula that coincides with an expanding H I region, suggesting that a supernova remnant (SNR) could be responsible for inducing the star formation in this region. The ROSAT image shows an extended feature that could be due either to an unresolved stellar cluster (the PSF is very degraded towards the edge of the PSPC field) or truly diffuse emission, which would correspond to the optical diffuse feature considered a possible old SNR. The nature of the extended features in CMa R1 remains an open question that will be investigated using recently approved observations with XMM-Newton.

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. We adopted a simple model to fit the spectral energy distribution (SED) of 99 PDS stars (80 candidates and 19 well-known HAeBe stars). From this model, we estimated Sc, the contribution of the circumstellar components (dust disk and/or envelope) to the total emitted flux. We classified the sample stars in 3 groups based on the shape of their SEDs. We analyzed this classification and Sc in relation to other properties of the these stars.

Stellar Groups and Associations.- Major Unsolved Problems in Star Formation.- The Stellar Mass Function of Galactic Clusters and Its Evolution.- Encounters and Close Fly-Bys of Galactic Clusters and Associations in the Past 50 Myr.- Solar-Type Post-T Tauri Stars in the Nearest OB Subgroups.- NGC 2362: The Shape of the Pre-Main-Sequence from A-Stars to Brown Dwarfs.- Young Stars and Their Circumstellar Disks in the ? Orionis Cluster.- The Oph-Sco-Lup-Cen-Cru-Mus-Cha Star-formation Region.- The Gas-to-Dust Ratio and Metallurgy of Nearby Dark Clouds Probed by X-ray Absorption Measurements.- Sacy - A Search for Associations Containing Young Stars.- Age Determination of the Ursa Major Association.- Young Stellar Objects.- Accretion Powered Emission in Young Stellar Objects.- The Pre-Main Sequence Spectroscopic Binary AK Sco.- Survey of Young Stellar Objects Associated with Molecular Clouds.- The Stellar Population of Embedded Galactic Massive Star Clusters.- Spectroscopic Analysis of 131 Herbig Ae/Be Candidate Stars.- Classification of the Pico Dos Dias Survey Herbig Ae/Be Stars.- Chemical Composition Study of an Accretion Episode in the Herbig Candidate Star PDS080.- Magnetically Channeled Accretion in T Tauri Stars.- Properties of Young Stellar Objects from High Resolution Near Infrared Spectroscopy.- Probing the Circumstellar Structure of Pre-main Sequence Stars.- Accretion Signatures in the X-Ray Spectrum of TW Hya.- Post-T Tauri Stars Rotation in Associations.- Star Formation in Canis Majoris.- X-ray Emission from NGC 2362.- Accretion and Ejection.- The Origin of Jets in Young Stars.- Non Coeval Young Multiple Systems?.- Brown Dwarfs.- Brown Dwarfs in Young Open Clusters.- Brown Dwarf Companions.- Observational Clues to Brown Dwarf Origins.- Surface Gravity & Mass in Young Brown Dwarfs and Planemos.- Disks, Outflows and Jets.- Fu Orionis Eruptions and the Formation of Close Binaries.- Seds of Flared Dust Disks.- On the Alignment of T Tauri Stars with the Local Magnetic Field.- Temporal Evolution of Main Sequence Dusty Disks.- Numerical Simulations of Young Stellar Jets: From One Tenth to Parsec Scales.- Alfven Waves in Disks, Outflows and Jets.- Early Stages of Star Formation.- Submillimeter Studies of Protostellar Cores.- Detection of a Collimated Jet Towards the High-Mass Protostar IRAS 16547-4247.- Disks and Halos in Pre-Main-Sequence Stars.- Radiative Transfer in Prestellar Cores: A Monte Carlo Approach.- The ISM Conditions for Star Formation.- The Initial Conditions for Star Formation.- Chemical Signatures of New Star Formation Toward Young Stellar Clusters.- Collapse of Molecular Clouds Leading to Star Formation.- Star Formation in Globules.- Simba Survey Toward High-Mass Star Forming Regions in the Southern Hemisphere.- Conference Summary and Conclusions.

In recent years our knowledge of star, brown dwarf and planet formation has progressed immensely due to new data in the IR domain (Spitzer telescope), new X-ray campaigns such as the Chandra Orion Ultradeep Project (COUP) and the X-ray Emission Survey of Taurus (XEST), with XMM-Newton, as well as adaptive optics results and synoptic studies of young stellar and substellar objects.

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 Letters

We report results of a direct imaging survey for giant planets around 80 members of the Beta Pic, TW Hya, Tucana-Horologium, AB Dor, and Hercules-Lyra moving groups, observed as part of the Gemini NICI Planet-Finding Campaign. For this sample, we obtained median contrasts of \Delta H=13.9 mag at 1" in combined CH4 narrowband ADI+SDI mode and median contrasts of \Delta H=15.1 mag at 2" in H-band ADI mode. We found numerous (>70) candidate companions in our survey images. Some of these candidates were rejected as common-proper motion companions using archival data; we reobserved with NICI all other candidates that lay within 400 AU of the star and were not in dense stellar fields. The vast majority of candidate companions were confirmed as background objects from archival observations and/or dedicated NICI campaign followup. Four co-moving companions of brown dwarf or stellar mass were discovered in this moving group sample: PZ Tel B (36+-6 MJup, 16.4+-1.0 AU, Biller et al. 2010), CD -35 2722B (31+-8 MJup, 67+-4 AU, Wahhaj et al. 2011), HD 12894B (0.46+-0.08 MSun, 15.7+-1.0 AU), and BD+07 1919C (0.20+-0.03 MSun, 12.5+-1.4 AU). From a Bayesian analysis of the achieved H band ADI and ASDI contrasts, using power-law models of planet distributions and hot-start evolutionary models, we restrict the frequency of 1--20 MJup companions at semi-major axes from 10--150 AU to, Comment: 96 pages, 23 figures, accepted for publication in ApJ

We determined temperatures and Lithium 6707 A resonance line equivalent width of a sample of 62 T Tauri stars. Lithium abundances were then estimated by using a grid of curves of growth. The lithium abundance is shown to decrease with the distance of the stars to the nearest dense core of a molecular cloud. This effect is interpreted as being due to the ages of the stars, the youngest ones being closer to still active star formation regions.

We have carried out a survey to detect Lithium-rich giants (LRGs). In Castilho et al. (1998) we reported the discovery of five red giants showing a strong Li I (λ670.78 nm) line and six other ones showing a moderate Li I line. In this work we present the main results of a detailed analysis of 10 stars from our survey and 6 LRGs selected from the literature.

The molecular cloud Canis Major R1 (CMa R1) contains several embedded stellar clusters associated to a ring of nebular emission, which is an expanding shell suggested to be a supernova remnant (SNR) inducing the star formation in this region (Herbst & Assousa 1977, Comeron et al. 1998). However, there are alternatives to the SNR hypothesis, since the shell-like structure could be produced by strong stellar winds or an evolving HII region, as suggested by Reynolds & Ogden (1978), Blitz (1980), and Pyatunina & Taraskin (1986), for example. Two main challenges have motivated us to investigate this interesting region: (i) to conduct a stellar population study, from 7 to 0.4 solar masses, and (ii) to verify the evolutionary status of embedded cluster members. This contribution is dedicated to report VRI data obtained with Gemini South telescope in the direction of six X-ray sources that are probably unresolved. The results reveal several faint candidates that could be multiple counterparts of X-ray emitters detected by ROSAT as single sources (Gregorio-Hetem, Montmerle & Marciotto 2003). These fields have not been observed in more recent X-ray surveys. The V-R and R-I colours were estimated for the objects associated with the position of the X-ray emission, aiming to distinguish between field stars and members of the cloud. For each ROSAT source, it has been detected the following number of candidates, which we suggest to be stellar groups: src15 has 7 possible optical counterparts (86% of them are NIR sources); src17 has 14 counterparts (71% are NIR sources); src37 has 11 (73% NIR); src42 has 16 (56% NIR); src44 has 10 (80% NIR); and src55 has 6 (67% NIR). Investigating the evolutionary scenario of the embedded stellar clusters associated to X-ray emitters, which are probably very young, is a unique opportunity to better understand the star formation process in CMa R1 and to test SNR models, verifying the hypothesis of induced star formation in this region. Figure 1. Optical image showing the contours of the X-Ray emission in the fields src15 and src17. The 2MASS catalogue was inspected searching for near-infrared (NIR) counterparts (crosses) related to the optical candidates (squares). Colour-magnitude diagrams have been constructed to evaluate the evolutionary status of the stellar groups.

We have developed geometric disk models to study the circumstellar geometries by fitting the spectral energy distribution (SED) of T Tauri and Herbig Ae/Be stars. The simulations provide means to recognize the signatures of different disk structures, including the effects due to external UV photoevaporation. Following Chiang & Goldreich (1997) and Dullemond et al. (2001), we used hydrostatic, radiative equilibrium models for passive, reprocessing flared disks. The grains in the surface of the disk are directly exposed to the radiation from the star and the interior of the disk is heated by diffusion from the surface. Adopting this two-layers disk structure, our disk model was improved in order to optimize the parameters estimated by using a calculation technique based on genetic algorithms presented by Bentley & Corne (2002). In the present work, we apply the code to model the SED of protoplanetary disks, which have being destroyed by photoevaporation due to the presence of ionizing OB stars, as the example of Trapezium region in the Orion Nebula. We compare geometric disk characteristics and physical conditions evaluated by our method to those obtained to the “proplyds” studied by Scally & Clarke (2001), Robberto et al. (2002) and Smith et al. (2005), among others. We also conclude that the parameter estimation by genetic algorithms assures accurate and efficient calculations. Figure 1. Fitting procedure sample session: the first panel shows the evolution of a solution; as the generation evolves, a best set of parameters is achieved. The right panel presents the SED for the best solution.