arXiv subjects

M. Manteiga

Publications and source records attributed to M. Manteiga.

26 records · Page 2Linked to original sources

arXiv2016

On the estimation of stellar parameters with uncertainty prediction from Generative Artificial Neural Networks: application to Gaia RVS simulated spectra

Aims. We present an innovative artificial neural network (ANN) architecture, called Generative ANN (GANN), that computes the forward model, that is it learns the function that relates the unknown outputs (stellar atmospheric parameters, in this case) to the given inputs (spectra). Such a model can be integrated in a Bayesian framework to estimate the posterior distribution of the outputs. Methods. The architecture of the GANN follows the same scheme as a normal ANN, but with the inputs and outputs inverted. We train the network with the set of atmospheric parameters (Teff, logg, [Fe/H] and [alpha/Fe]), obtaining the stellar spectra for such inputs. The residuals between the spectra in the grid and the estimated spectra are minimized using a validation dataset to keep solutions as general as possible. Results. The performance of both conventional ANNs and GANNs to estimate the stellar parameters as a function of the star brightness is presented and compared for different Galactic populations. GANNs provide significantly improved parameterizations for early and intermediate spectral types with rich and intermediate metallicities. The behaviour of both algorithms is very similar for our sample of late-type stars, obtaining residuals in the derivation of [Fe/H] and [alpha/Fe] below 0.1dex for stars with Gaia magnitude Grvs<12, which accounts for a number in the order of four million stars to be observed by the Radial Velocity Spectrograph of the Gaia satellite. Conclusions. Uncertainty estimation of computed astrophysical parameters is crucial for the validation of the parameterization itself and for the subsequent exploitation by the astronomical community. GANNs produce not only the parameters for a given spectrum, but a goodness-of-fit between the observed spectrum and the predicted one for a given set of parameters. Moreover, they allow us to obtain the full posterior distribution...

astro-ph.IM↗

arXiv2015

Stellar parametrization from Gaia RVS spectra

Among the myriad of data collected by the ESA Gaia satellite, about 150 million spectra will be delivered by the Radial Velocity Spectrometer (RVS) for stars as faint as G_RVS~16. A specific stellar parametrization will be performed for most of these RVS spectra. Some individual chemical abundances will also be estimated for the brightest targets. We describe the different parametrization codes that have been specifically developed or adapted for RVS spectra within the GSP-spec working group of the analysis consortium. The tested codes are based on optimization (FERRE and GAUGUIN), projection (MATISSE) or pattern recognition methods (Artificial Neural Networks). We present and discuss their expected performances in the recovered stellar atmospheric parameters (Teff, log(g), [M/H]) for B- to K- type stars. The performances for the determinations of [alpha/Fe] ratios are also presented for cool stars. For all the considered stellar types, stars brighter than G_RVS~12.5 will be very efficiently parametrized by the GSP-spec pipeline, including solid estimations of [alpha/Fe]. Typical internal errors for FGK metal-rich and metal-intermediate stars are around 40K in Teff , 0.1dex in log(g), 0.04dex in [M/H], and 0.03dex in [alpha/Fe] at G_RVS=10.3. Similar accuracies in Teff and [M/H] are found for A-type stars, while the log(g) derivation is more accurate. For the faintest stars, with G_RVS>13-14, a spectrophotometric Teff input will allow the improvement of the final GSP-spec parametrization. The reported results show that the contribution of the RVS based stellar parameters will be unique in the brighter part of the Gaia survey allowing crucial age estimations, and accurate chemical abundances. This will constitute a unique and precious sample for which many pieces of the Milky Way history puzzle will be available, with unprecedented precision and statistical relevance.

astro-ph.GA↗

arXiv2013

An approach to the analysis of SDSS spectroscopic outliers based on Self-Organizing Maps

Aims. A new method is applied to the segmentation, and further analysis of the outliers resulting from the classification of astronomical objects in large databases is discussed. The method is being used in the framework of the Gaia satellite DPAC (Data Processing and Analysis Consortium) activities to prepare automated software tools that will be used to derive basic astrophysical information that is to be included in Gaia final archive. Methods. Our algorithm has been tested by means of simulated Gaia spectrophotometry, which is based on SDSS observations and theoretical spectral libraries covering a wide sample of astronomical objects. Self-Organizing Maps (SOM) networks are used to organize the information in clusters of objects, as homogeneous as possible, according to their spectral energy distributions (SED), and to project them onto a 2-D grid where the data structure can be visualized. Results. We demonstrate the usefulness of the method by analyzing the spectra that were rejected by the SDSS spectroscopic classification pipeline and thus classified as "UNKNOWN". Firstly, our method can help to distinguish between astrophysical objects and instrumental artifacts. Additionally, the application of our algorithm to SDSS objects of unknown nature has allowed us to identify classes of objects of similar astrophysical nature. In addition, the method allows for the potential discovery of hundreds of novel objects, such as white dwarfs and quasars. Therefore, the proposed method is shown to be very promising for data exploration and knowledge discovery in very large astronomical databases, such as the upcoming Gaia mission.

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arXiv2013

The Gaia astrophysical parameters inference system (Apsis). Pre-launch description

The Gaia satellite will survey the entire celestial sphere down to 20th magnitude, obtaining astrometry, photometry, and low resolution spectrophotometry on one billion astronomical sources, plus radial velocities for over one hundred million stars. Its main objective is to take a census of the stellar content of our Galaxy, with the goal of revealing its formation and evolution. Gaia's unique feature is the measurement of parallaxes and proper motions with hitherto unparalleled accuracy for many objects. As a survey, the physical properties of most of these objects are unknown. Here we describe the data analysis system put together by the Gaia consortium to classify these objects and to infer their astrophysical properties using the satellite's data. This system covers single stars, (unresolved) binary stars, quasars, and galaxies, all covering a wide parameter space. Multiple methods are used for many types of stars, producing multiple results for the end user according to different models and assumptions. Prior to its application to real Gaia data the accuracy of these methods cannot be assessed definitively. But as an example of the current performance, we can attain internal accuracies (RMS residuals) on F,G,K,M dwarfs and giants at G=15 (V=15-17) for a wide range of metallicites and interstellar extinctions of around 100K in effective temperature (Teff), 0.1mag in extinction (A0), 0.2dex in metallicity ([Fe/H]), and 0.25dex in surface gravity (logg). The accuracy is a strong function of the parameters themselves, varying by a factor of more than two up or down over this parameter range. After its launch in November 2013, Gaia will nominally observe for five years, during which the system we describe will continue to evolve in light of experience with the real data.

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arXiv2013

Detection of a multi-shell planetary nebula around the hot subdwarf O-type star 2MASS J19310888+4324577

(Abridged) The origin of hot subdwarf O-type stars (sdOs) remains unclear since their discovery in 1947. Among others, a post-Asymptotic Giant Branch (post-AGB) origin is possible for a fraction of sdOs. We are involved in a comprehensive ongoing study to search for and to analyze planetary nebulae (PNe) around sdOs with the aim of establishing the fraction and properties of sdOs with a post-AGB origin. We use deep Halpha and [OIII] images of sdOs to detect nebular emission and intermediate resolution, long-slit optical spectroscopy of the detected nebulae and their sdO central stars. These data are complemented with other observations for further analysis of the detected nebulae. We report the detection of an extremely faint, complex PN around 2MASS J19310888+4324577 (2M1931+4324), a star classified as sdO in a binary system. The PN shows a bipolar and an elliptical shell, whose major axes are oriented perpendicular to each other, and high-excitation structures outside the two shells. WISE archive images show faint, extended emission at 12 and 22 microns in the inner nebular regions. The internal nebular kinematics is consistent with a bipolar and a cylindrical/ellipsoidal shell, in both cases with the main axis mainly perpendicular to the line of sight. The nebular spectrum only exhibits Halpha, Hbeta and [OIII]4959,5007 emission lines, but suggests a very low-excitation ([OIII]/Hbeta = 1.5), in strong contrast with the absence of low-excitation emission lines. The spectrum of 2M1931+4324 presents narrow, ionized helium absorptions that confirm the previous sdO classification and suggest an effective temperature >= 60000 K. The binary nature of 2M1931+4324, its association with a complex PN, and several properties of the system provide strong support for the idea that binary central stars are a crucial ingredient in the formation of complex PNe.

astro-ph.SR↗

arXiv2012

Rule or exception? Planetary nebulae around hot subdwarf stars

In this work, we present the first results of an ongoing survey to search for planetary nebulae (PNe) around hot subdwarf stars (sdOs). Deep images and intermediate-resolution long-slit spectra of RWT 152, the only confirmed PN+sdO system in the northern hemisphere, as well as preliminary results for other sdO+PN candidate are presented.

astro-ph.SR↗

arXiv2011

IRAS 17423-1755 (Hen 3-1475) revisited: an O-rich high-mass post-Asymptotic Giant Branch star

The high-resolution (R=600) Spitzer/IRS spectrum of the bipolar proto-planetary nebula (PPN) IRAS 17423-1755 is presented in order to clarify the dominant chemistry (C-rich versus O-rich) of its circumstellar envelope as well as to constrain its evolutionary stage. The high quality Spitzer/IRS spectrum shows weak 9.7 um absorption from amorphous silicates. This confirms for the first time the O-rich nature of IRAS 17423-1755 in contradiction to a previous C-rich classification, which was based on the wrong identification of the strong 3.1 um absorption feature seen in the Infrared Space Observatory (ISO) spectrum as due to acetylene (C2H2). The high-resolution Spitzer/IRS spectrum displays a complete lack of C-rich mid-IR features such as molecular absorption features (e.g., 13.7 um C2H2, 14.0 um HCN, etc.) or the classical polycyclic aromatic hydrocarbon infrared emission bands. Thus, the strong 3.1 um absorption band toward IRAS 17423-1755 has to be identified as water ice. In addition, a [Ne II] nebular emission line at 12.8 um is clearly detected, indicating that the ionization of its central region may be already started. The spectral energy distribution in the infrared (2-200 um) and other observational properties of IRAS 17423-1755 are discussed in comparison with the similar post-asymptotic giant branch (AGB) objects IRAS 19343+2926 and IRAS 17393-2727. We conclude that IRAS 17423-1755 is an O-rich high-mass post-AGB object that represents a link between OH/IR stars with extreme outflows and highly bipolar PN.

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arXiv2006

A spectroscopic atlas of post-AGB stars and planetary nebulae selected from the IRAS Point Source Catalogue

Aims: We study the optical spectral properties of a sample of stars showing far infrared colours similar to those of well-known planetary nebulae. The large majority of them were unidentified sources or poorly known in the literature at the time when this spectroscopic survey started, some 15 years ago. Methods: We present low-resolution optical spectroscopy, finding charts and improved astrometric coordinates of a sample of 253 IRAS sources. Results: We have identified 103 sources as post-AGB stars, 21 as ``transition sources'', and 36 as planetary nebulae, some of them strongly reddened. Among the rest of sources in the sample, we were also able to identify 38 young stellar objects, 5 peculiar stars, and 2 Seyfert galaxies. Up to 49 sources in our spectroscopic sample do not show any optical counterpart, and most of them are suggested to be heavily obscured post-AGB stars, rapidly evolving on their way to becoming planetary nebulae. Conclusions: An analysis of the galactic distribution of the sources identified as evolved stars in the sample is presented together with a study of the distribution of these stars in the IRAS two-colour diagram. Finally, the spectral type distribution and other properties of the sources identified as post-AGB in this spectroscopic survey are discussed in the framework of stellar evolution.

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