Thais Idiart | Universidade de São Paulo (original) (raw)

Papers by Thais Idiart

Astronomy and Astrophysics, 2010

Astrophys J, 2000

Abundances of α-elements such as Ca and Mg in disk and halo stars are usually derived from equiva... more Abundances of α-elements such as Ca and Mg in disk and halo stars are usually derived from equivalent widths lines measured on high resolution spectra, and assuming Local Thermodynamic Equilibrium (LTE). In this paper, we present non-LTE differential abundances derived by computing the statistical equilibrium of CaI and MgI atoms, using high resolution equivalent widths available in the literature for 252 dwarf to subgiant stars. These non-LTE abundances combined with recent determination of non-LTE abundances of iron, seem to remove the dispersion of the [Ca/Fe] and [Mg/Fe] ratios in the galactic halo and disk phases, revealing new and surprising structures. These results have important consequences for chemical evolution models of the Galaxy. In addition, non-LTE abundance ratios for stars belonging to the M92 cluster apparently have the same behavior. More high resolution observations, mainly of globular clusters, are urgently needed to confirm our results.

Proceedings of the International Astronomical Union

In this work we analyze the beryllium-iron chemical diagram from the point of view of non-LTE eff... more In this work we analyze the beryllium-iron chemical diagram from the point of view of non-LTE effects. Be abundances were re-calculated by considering non-LTE corrections in ionization equilibrium (logg) and Fe abundances ([Fe/H]). These corrections seem do not affect the linear relation between Be-Fe for metal-poor stars already found in the literature for LTE derived abundances.

Proceedings of The International Astronomical Union, 2009

We have obtained a large sample of PN with accurately determined helium abundances, as well as ab... more We have obtained a large sample of PN with accurately determined helium abundances, as well as abundances of several heavy elements. The nebulae are located in the solar neighbourhood, in the galactic bulge, disk and anticentre, and in the Magellanic Clouds. The abundances are analyzed both in terms of the nucleosynthesis of intermediate mass stars and the chemical evolution of

ABSTRACT In this work we obtain chemical diagrams for the Small Magellanic Cloud (SMC) using plan... more ABSTRACT In this work we obtain chemical diagrams for the Small Magellanic Cloud (SMC) using planetary nebulae (PNe) data. We collected a large number of measured spectral fluxes for a sample of SMC PNe, in order to derive accurate physical parameters and abundances. Here we present new spectral data for SMC PNe obtained between 1999 and 2002. These data are used together with all data available in the literature to improve the accuracy of the fluxes for each PN spectral line.

ABSTRACT We investigate the chemical evolution of the Small Magellanic Cloud (SMC) based on abund... more ABSTRACT We investigate the chemical evolution of the Small Magellanic Cloud (SMC) based on abundance data of planetary nebulae (PNe). The main goal is to investigate the time evolution of the oxygen abundance in this galaxy by deriving an age-metallicity relation. Such a relation is of fundamental importance as an observational constraint for chemical evolution models of the SMC. We have used high quality PNe data to derive the properties of the progenitor stars, so that the stellar ages could be estimated. We collected a large number of measured spectral fluxes for each nebula and derived accurate physical parameters and nebular abundances. New spectral data for a sample of SMC PNe obtained between 1999 and 2002 are also presented. These data are used with data available in the literature to improve the accuracy of the fluxes for each spectral line. (4 data files).

Planetary nebulae (PN) are an essential tool in the study of the chemical evolution of the Milky ... more Planetary nebulae (PN) are an essential tool in the study of the chemical evolution of the Milky Way and galaxies of the Local Group, particularly the Magellanic Clouds. In this work, we present some recent results on the determination of chemical abundances from PN in the Large and Small Magellanic Clouds, and compare these results with data from our own Galaxy and other galaxies in the Local Group. As a result of our continuing long term program, we have a large database comprising about 300 objects for which reliable abundances of several elements from He to Ar have been obtained. Such data can be used to derive constraints to the nucleosynthesis processes in the progenitor stars in galaxies of different metallicities. We also investigate the time evolution of the oxygen abundances in the SMC by deriving the properties of the PN progenitor stars, which include their masses and ages. We have then obtained an age-metallicity relation taking into account both oxygen and [Fe/H] abundances. We show that these results have an important consequence on the star formation rate of the SMC, in particular by suggesting a star formation burst in the last 2-3 Gyr.

ABSTRACT We discuss three methods to estimate the age distribution of central stars of planetary ... more ABSTRACT We discuss three methods to estimate the age distribution of central stars of planetary nebulae (CSPN) and their progenitor stars. These methods include the use of an age-metallicity relation that also depends on the galactocentric distance, an age-metallicity relation for the Galactic disk, and the determination of ages from the central star masses obtained from the observed nitrogen abundances. We conclude that most CSPN in the Galactic disk have ages under 6 Gyr, and their age distribution is peaked around 2-4 Gyr. These results have been successfully compared with the age distributions of white dwarf stars based on their mass distribution, and on available mass determinations of CSPN.

The determination of accurate chemical abundances of planetary nebulae (PN) in different galaxies... more The determination of accurate chemical abundances of planetary nebulae (PN) in different galaxies allows us to obtain important constraints of chemical evolution models for these systems. We have a long term program to derive abundances in the galaxies of the Local Group, particularly the Large and Small Magellanic Clouds. In this work, we present our new results on these objects and discuss their implications in view of recent abundance determinations the literature. In particular, we obtain distance-independent correlations involving He, N, O, Ne, S, and Ar, and compare the results with data from our own Galaxy and other galaxies in the Local Group. As a result of our observational program, we have a large database of PN in the Galaxy and the Magellanic Clouds, so that we can obtain reliable constraints to the nucleosynthesis processes in the progenitor stars in galaxies of different metallicities.

International Journal of Modern Physics D - INT J MOD PHYS D, 2010

Elliptical galaxies are the best systems to study the early star formation activity in the univer... more Elliptical galaxies are the best systems to study the early star formation activity in the universe. This work aims to understand the formation and evolution of these objects through the study of the integrated properties of their stellar populations. Here an evolutionary model is developed and their predicted spectrophotometric properties are presented.

In this work, the chemical history of the SMC is described through an observed sample of planetar... more In this work, the chemical history of the SMC is described through an observed sample of planetary nebulae (PNe). Age-abundance relations were derived through the [o-elements/H] (oxygen, argon, neon and sulphur) ratios estimated from PNe of different progenitor masses. In particular, oxygen abundances seem do not decrease due to ON cycle operation.

Monthly Notices of The Royal Astronomical Society - MON NOTIC ROY ASTRON SOC, 2007

New calibrations of spectrophotometric indices of elliptical galaxies as functions of spectrophot... more New calibrations of spectrophotometric indices of elliptical galaxies as functions of spectrophotometric indices are presented, permitting estimates of mean stellar population ages and metallicities. These calibrations are based on evolutionary models including a two-phase interstellar medium, infall and a galactic wind. Free parameters were fixed by requiring that models reproduce the mean trend of data in the colour-magnitude diagram as well as in the plane of indices Hbeta-Mg2 and Mg2-. To improve the location of faint ellipticals (MB > -20) in the Hbeta-Mg2 diagram, downsizing was introduced. An application of our calibrations to a sample of ellipticals and a comparison with results derived from single stellar population models are given. Our models indicate that mean population ages span an interval of 7-12 Gyr and are correlated with metallicities, which range from approximately half up to three times solar.

With an emphasis on our polarization monitoring program, we present our observations to date of A... more With an emphasis on our polarization monitoring program, we present our observations to date of AG CARINAE, the prototype Luminous Blue Variable (LBV). The star is experiencing an active phase, increasing in brightness since 1990. The variability, which is semi-periodic on time scales 10-15 years, may culminate in an eruptive episode. Although AG Car is a well-studied LBV, the geometry and physics of its environment are extremely complex. To obtain a better description of AG Car, the team has been collecting many observations during the last year: UBVRI photo-polarimetry and visual spectroscopy (Brazilian National Observatory), UV spectropolarimetry with the Hubble Space Telescope Faint Object Spectrograph, UBV photometry (Port Alegre Observatory), IUE and H-alpha spectroscopy (LaSilla). UV polarization observations were selected as part of the campaign in order to probe for asymmetric structures ejected from the star in the innermost unresolved region (within 100 stellar radii). The ground based photo-polarization monitoring program enables us to track the polarimetric variability of the entire complex and to assist in the difficult task of separating the interstellar and intrinsic polarization components. We have found that the UV and optical polarization and position angle are strongly variable in time and structure is evident across the UV resonance scattering lines. The UV and optical spectroscopy show that the Teff is decreasing, the bolometric luminosity is remaining constant, and the structure and ionization state of the wind is evolving. We will discuss how these data support models which incorporate circumstellar disks, jets, and/or plumes to explain the mass loss characteristics.

Astrophysical Journal, 1994

Planetary and Space Science, 2008

The Gaia Space Mission [Mignard, F., 2005. The three-dimensional universe with Gaia. ESA/SP-576; ... more The Gaia Space Mission [Mignard, F., 2005. The three-dimensional universe with Gaia. ESA/SP-576; Perryman, M., 2005. The three-dimensional universe with Gaia. ESA/SP-576] will observe several transient events as supernovae, microlensing, gamma ray bursts and new Solar System objects. The satellite, due to its scanning law, will detect these events but will not be able to monitor them. So, to take these events into consideration and to perform further studies it is necessary to follow them with Earth-based observations. These observations could be efficiently done by a ground-based network of well-equipped telescopes scattered in both hemispheres.Here we focus our attention at the new Solar System objects to be discovered and observed by the Gaia satellite [Mignard, F., 2002. Observations of Solar System objects by Gaia I. Detection of NEOS. Astron. Astrophys. 393, 727] mainly asteroids, NEOs and comets. A dedicated ground-based network of telescopes as proposed by Thuillot [2005. The three-dimensional universe with Gaia. ESA/SP-576] will allow to monitor those events, to avoid losing them and to perform a quick characterization of some physical properties which will be important for the identification of these objects in further measurements by Gaia.We present in this paper, the beginning of the organization of a Latin-American ground-based network of telescopes and observers joining several institutions in Argentina, Bolivia, Brazil and other Latin-American countries aiming to contribute to the follow-up of Gaia science alerts for Solar System objects.

The nature of the observed activity in many galactic nuclei is not well established yet. Differen... more The nature of the observed activity in many galactic nuclei is not well established yet. Different models have been proposed, like a more gradual process of star formation than those in "normal" bulges or a bulge "rejuvenated" by accretion of gas clouds. One way of tackle this problem is to study the stellar populations in these objects, by performing the analysis of their integrated spectra. A sample of galaxies with nuclear activity classified as LINERS or HII galaxies was observed spectroscopically, and mean abundances as well as mean population ages were derived from the procedure developed by Idiart et al (1996). Some clues on the chemical enrichment processes will be given.

Astrophysical Journal, 2000

Abundances of α-elements such as Ca and Mg in disk and halo stars are usually derived from equiva... more Abundances of α-elements such as Ca and Mg in disk and halo stars are usually derived from equivalent widths lines measured on high resolution spectra, and assuming Local Thermodynamic Equilibrium (LTE) . In this paper, we present non-LTE differential abundances derived by computing the statistical equilibrium of CaI and MgI atoms, using high resolution equivalent widths available in the literature for 252 dwarf to subgiant stars. These non-LTE abundances combined with recent determination of non-LTE abundances of iron, seem to remove the dispersion of the [Ca/Fe] and [Mg/Fe] ratios in the galactic halo and disk phases, revealing new and surprising structures. These results have important consequences for chemical evolution models of the Galaxy. In addition, non-LTE abundance ratios for stars belonging to the M92 cluster apparently have the same behavior. More high resolution observations, mainly of globular clusters, are urgently needed to confirm our results.

New colour distributions have been derived from wide field UBVRI frames for 36 northern bright el... more New colour distributions have been derived from wide field UBVRI frames for 36 northern bright elliptical galaxies and a few lenticulars. The classical linear representations of colours against log r were derived, with some improvements in the accuracy of the zero point colours and of the gradients. The radial range of significant measurements was enlarged both towards the galaxy center and towards the outskirts of each object. Thus, the "central colours", integrated within a radius of 3 ′′ , and the "outermost colours" averaged near the µ V = 24 surface brightness could also be obtained. Some typical deviations of colour profiles from linearity are described. Colourcolour relations of interest are presented. Very tight correlations are found between the U−V colour and the M g 2 line-index, measured either at the galaxian center or at the effective radius.

Astronomy and Astrophysics, 2010

Astrophys J, 2000

Abundances of α-elements such as Ca and Mg in disk and halo stars are usually derived from equiva... more Abundances of α-elements such as Ca and Mg in disk and halo stars are usually derived from equivalent widths lines measured on high resolution spectra, and assuming Local Thermodynamic Equilibrium (LTE). In this paper, we present non-LTE differential abundances derived by computing the statistical equilibrium of CaI and MgI atoms, using high resolution equivalent widths available in the literature for 252 dwarf to subgiant stars. These non-LTE abundances combined with recent determination of non-LTE abundances of iron, seem to remove the dispersion of the [Ca/Fe] and [Mg/Fe] ratios in the galactic halo and disk phases, revealing new and surprising structures. These results have important consequences for chemical evolution models of the Galaxy. In addition, non-LTE abundance ratios for stars belonging to the M92 cluster apparently have the same behavior. More high resolution observations, mainly of globular clusters, are urgently needed to confirm our results.

Proceedings of the International Astronomical Union

In this work we analyze the beryllium-iron chemical diagram from the point of view of non-LTE eff... more In this work we analyze the beryllium-iron chemical diagram from the point of view of non-LTE effects. Be abundances were re-calculated by considering non-LTE corrections in ionization equilibrium (logg) and Fe abundances ([Fe/H]). These corrections seem do not affect the linear relation between Be-Fe for metal-poor stars already found in the literature for LTE derived abundances.

Proceedings of The International Astronomical Union, 2009

We have obtained a large sample of PN with accurately determined helium abundances, as well as ab... more We have obtained a large sample of PN with accurately determined helium abundances, as well as abundances of several heavy elements. The nebulae are located in the solar neighbourhood, in the galactic bulge, disk and anticentre, and in the Magellanic Clouds. The abundances are analyzed both in terms of the nucleosynthesis of intermediate mass stars and the chemical evolution of

ABSTRACT In this work we obtain chemical diagrams for the Small Magellanic Cloud (SMC) using plan... more ABSTRACT In this work we obtain chemical diagrams for the Small Magellanic Cloud (SMC) using planetary nebulae (PNe) data. We collected a large number of measured spectral fluxes for a sample of SMC PNe, in order to derive accurate physical parameters and abundances. Here we present new spectral data for SMC PNe obtained between 1999 and 2002. These data are used together with all data available in the literature to improve the accuracy of the fluxes for each PN spectral line.

ABSTRACT We investigate the chemical evolution of the Small Magellanic Cloud (SMC) based on abund... more ABSTRACT We investigate the chemical evolution of the Small Magellanic Cloud (SMC) based on abundance data of planetary nebulae (PNe). The main goal is to investigate the time evolution of the oxygen abundance in this galaxy by deriving an age-metallicity relation. Such a relation is of fundamental importance as an observational constraint for chemical evolution models of the SMC. We have used high quality PNe data to derive the properties of the progenitor stars, so that the stellar ages could be estimated. We collected a large number of measured spectral fluxes for each nebula and derived accurate physical parameters and nebular abundances. New spectral data for a sample of SMC PNe obtained between 1999 and 2002 are also presented. These data are used with data available in the literature to improve the accuracy of the fluxes for each spectral line. (4 data files).

Planetary nebulae (PN) are an essential tool in the study of the chemical evolution of the Milky ... more Planetary nebulae (PN) are an essential tool in the study of the chemical evolution of the Milky Way and galaxies of the Local Group, particularly the Magellanic Clouds. In this work, we present some recent results on the determination of chemical abundances from PN in the Large and Small Magellanic Clouds, and compare these results with data from our own Galaxy and other galaxies in the Local Group. As a result of our continuing long term program, we have a large database comprising about 300 objects for which reliable abundances of several elements from He to Ar have been obtained. Such data can be used to derive constraints to the nucleosynthesis processes in the progenitor stars in galaxies of different metallicities. We also investigate the time evolution of the oxygen abundances in the SMC by deriving the properties of the PN progenitor stars, which include their masses and ages. We have then obtained an age-metallicity relation taking into account both oxygen and [Fe/H] abundances. We show that these results have an important consequence on the star formation rate of the SMC, in particular by suggesting a star formation burst in the last 2-3 Gyr.

ABSTRACT We discuss three methods to estimate the age distribution of central stars of planetary ... more ABSTRACT We discuss three methods to estimate the age distribution of central stars of planetary nebulae (CSPN) and their progenitor stars. These methods include the use of an age-metallicity relation that also depends on the galactocentric distance, an age-metallicity relation for the Galactic disk, and the determination of ages from the central star masses obtained from the observed nitrogen abundances. We conclude that most CSPN in the Galactic disk have ages under 6 Gyr, and their age distribution is peaked around 2-4 Gyr. These results have been successfully compared with the age distributions of white dwarf stars based on their mass distribution, and on available mass determinations of CSPN.

The determination of accurate chemical abundances of planetary nebulae (PN) in different galaxies... more The determination of accurate chemical abundances of planetary nebulae (PN) in different galaxies allows us to obtain important constraints of chemical evolution models for these systems. We have a long term program to derive abundances in the galaxies of the Local Group, particularly the Large and Small Magellanic Clouds. In this work, we present our new results on these objects and discuss their implications in view of recent abundance determinations the literature. In particular, we obtain distance-independent correlations involving He, N, O, Ne, S, and Ar, and compare the results with data from our own Galaxy and other galaxies in the Local Group. As a result of our observational program, we have a large database of PN in the Galaxy and the Magellanic Clouds, so that we can obtain reliable constraints to the nucleosynthesis processes in the progenitor stars in galaxies of different metallicities.

International Journal of Modern Physics D - INT J MOD PHYS D, 2010

Elliptical galaxies are the best systems to study the early star formation activity in the univer... more Elliptical galaxies are the best systems to study the early star formation activity in the universe. This work aims to understand the formation and evolution of these objects through the study of the integrated properties of their stellar populations. Here an evolutionary model is developed and their predicted spectrophotometric properties are presented.

In this work, the chemical history of the SMC is described through an observed sample of planetar... more In this work, the chemical history of the SMC is described through an observed sample of planetary nebulae (PNe). Age-abundance relations were derived through the [o-elements/H] (oxygen, argon, neon and sulphur) ratios estimated from PNe of different progenitor masses. In particular, oxygen abundances seem do not decrease due to ON cycle operation.

Monthly Notices of The Royal Astronomical Society - MON NOTIC ROY ASTRON SOC, 2007

New calibrations of spectrophotometric indices of elliptical galaxies as functions of spectrophot... more New calibrations of spectrophotometric indices of elliptical galaxies as functions of spectrophotometric indices are presented, permitting estimates of mean stellar population ages and metallicities. These calibrations are based on evolutionary models including a two-phase interstellar medium, infall and a galactic wind. Free parameters were fixed by requiring that models reproduce the mean trend of data in the colour-magnitude diagram as well as in the plane of indices Hbeta-Mg2 and Mg2-. To improve the location of faint ellipticals (MB > -20) in the Hbeta-Mg2 diagram, downsizing was introduced. An application of our calibrations to a sample of ellipticals and a comparison with results derived from single stellar population models are given. Our models indicate that mean population ages span an interval of 7-12 Gyr and are correlated with metallicities, which range from approximately half up to three times solar.

With an emphasis on our polarization monitoring program, we present our observations to date of A... more With an emphasis on our polarization monitoring program, we present our observations to date of AG CARINAE, the prototype Luminous Blue Variable (LBV). The star is experiencing an active phase, increasing in brightness since 1990. The variability, which is semi-periodic on time scales 10-15 years, may culminate in an eruptive episode. Although AG Car is a well-studied LBV, the geometry and physics of its environment are extremely complex. To obtain a better description of AG Car, the team has been collecting many observations during the last year: UBVRI photo-polarimetry and visual spectroscopy (Brazilian National Observatory), UV spectropolarimetry with the Hubble Space Telescope Faint Object Spectrograph, UBV photometry (Port Alegre Observatory), IUE and H-alpha spectroscopy (LaSilla). UV polarization observations were selected as part of the campaign in order to probe for asymmetric structures ejected from the star in the innermost unresolved region (within 100 stellar radii). The ground based photo-polarization monitoring program enables us to track the polarimetric variability of the entire complex and to assist in the difficult task of separating the interstellar and intrinsic polarization components. We have found that the UV and optical polarization and position angle are strongly variable in time and structure is evident across the UV resonance scattering lines. The UV and optical spectroscopy show that the Teff is decreasing, the bolometric luminosity is remaining constant, and the structure and ionization state of the wind is evolving. We will discuss how these data support models which incorporate circumstellar disks, jets, and/or plumes to explain the mass loss characteristics.

Astrophysical Journal, 1994

Planetary and Space Science, 2008

The Gaia Space Mission [Mignard, F., 2005. The three-dimensional universe with Gaia. ESA/SP-576; ... more The Gaia Space Mission [Mignard, F., 2005. The three-dimensional universe with Gaia. ESA/SP-576; Perryman, M., 2005. The three-dimensional universe with Gaia. ESA/SP-576] will observe several transient events as supernovae, microlensing, gamma ray bursts and new Solar System objects. The satellite, due to its scanning law, will detect these events but will not be able to monitor them. So, to take these events into consideration and to perform further studies it is necessary to follow them with Earth-based observations. These observations could be efficiently done by a ground-based network of well-equipped telescopes scattered in both hemispheres.Here we focus our attention at the new Solar System objects to be discovered and observed by the Gaia satellite [Mignard, F., 2002. Observations of Solar System objects by Gaia I. Detection of NEOS. Astron. Astrophys. 393, 727] mainly asteroids, NEOs and comets. A dedicated ground-based network of telescopes as proposed by Thuillot [2005. The three-dimensional universe with Gaia. ESA/SP-576] will allow to monitor those events, to avoid losing them and to perform a quick characterization of some physical properties which will be important for the identification of these objects in further measurements by Gaia.We present in this paper, the beginning of the organization of a Latin-American ground-based network of telescopes and observers joining several institutions in Argentina, Bolivia, Brazil and other Latin-American countries aiming to contribute to the follow-up of Gaia science alerts for Solar System objects.

The nature of the observed activity in many galactic nuclei is not well established yet. Differen... more The nature of the observed activity in many galactic nuclei is not well established yet. Different models have been proposed, like a more gradual process of star formation than those in "normal" bulges or a bulge "rejuvenated" by accretion of gas clouds. One way of tackle this problem is to study the stellar populations in these objects, by performing the analysis of their integrated spectra. A sample of galaxies with nuclear activity classified as LINERS or HII galaxies was observed spectroscopically, and mean abundances as well as mean population ages were derived from the procedure developed by Idiart et al (1996). Some clues on the chemical enrichment processes will be given.

Astrophysical Journal, 2000

Abundances of α-elements such as Ca and Mg in disk and halo stars are usually derived from equiva... more Abundances of α-elements such as Ca and Mg in disk and halo stars are usually derived from equivalent widths lines measured on high resolution spectra, and assuming Local Thermodynamic Equilibrium (LTE) . In this paper, we present non-LTE differential abundances derived by computing the statistical equilibrium of CaI and MgI atoms, using high resolution equivalent widths available in the literature for 252 dwarf to subgiant stars. These non-LTE abundances combined with recent determination of non-LTE abundances of iron, seem to remove the dispersion of the [Ca/Fe] and [Mg/Fe] ratios in the galactic halo and disk phases, revealing new and surprising structures. These results have important consequences for chemical evolution models of the Galaxy. In addition, non-LTE abundance ratios for stars belonging to the M92 cluster apparently have the same behavior. More high resolution observations, mainly of globular clusters, are urgently needed to confirm our results.

New colour distributions have been derived from wide field UBVRI frames for 36 northern bright el... more New colour distributions have been derived from wide field UBVRI frames for 36 northern bright elliptical galaxies and a few lenticulars. The classical linear representations of colours against log r were derived, with some improvements in the accuracy of the zero point colours and of the gradients. The radial range of significant measurements was enlarged both towards the galaxy center and towards the outskirts of each object. Thus, the "central colours", integrated within a radius of 3 ′′ , and the "outermost colours" averaged near the µ V = 24 surface brightness could also be obtained. Some typical deviations of colour profiles from linearity are described. Colourcolour relations of interest are presented. Very tight correlations are found between the U−V colour and the M g 2 line-index, measured either at the galaxian center or at the effective radius.